However, the reversible capacity based on the two‐electron reaction mechanism is not satisfactory limited by the inactive M1 lattice sites during the insertion/extraction process. Herein, self‐supporting 3D porous NVP materials with different crystallinity are fabricated on carbon foam substrates by a facile electrostatic spray deposition method. The V5+/V4+ redox couple is effectively activated and the three‐electron reactions are realized in NVP for sodium storage by a proper crystallinity tuning. In a disordered NVP sample, an ultra‐high specific capacity of 179.6 mAh g−1 at 0.2 C is achieved due to the coexistence of redox reactions of the V4+/V3+ and V5+/V4+ couples. Moreover, a pseudocapacitive charge storage mechanism induced by the disordered structure is first observed in the NVP electrode. An innovative model is given to understand the disorder‐induced‐pseudocapacitance phenomenon in this polyanion cathode material.As a promising cathode material of sodium‐ion batteries, Na 5+/V4+ redox couple in Na3V2(PO4)3 (NVP) can be activated through a proper crystal phase controlling method based on the characteristics of NVP. The obtained NVP material has an ultra‐high specific capacity of 179.6 mAh g−1. In addition, the disordered structure in NVP can induce pseudocapacitance behavior, which is caused by the reduced interaction force between the inserted ions.The V Selecting a proper electrode material is especially important in designing high‐performance SIBs. The sodium superionic conductor (NASICON) material always has an open 3D structure, which is suitable for the rapid migration of sodium ions and may be a potential electrode material for Na+ storage. Among the series of NASICON structural materials, Na3V2(PO4)3 (NVP) has drawn intensive attention due to its stable structure, large capacity, and high working potential.In the past few decades, lithium‐ion batteries (LIBs) have been widely used as rechargeable energy storage devices. However, the scarcity of lithium resources severely limited the development of these energy storage systems. The low cost and abundant resources of sodium make sodium‐ion batteries (SIBs) a potential alternative to LIBs, especially in applications of large‐scale energy storage occasion.1 Alth Na+ ions may diffuse through M1‐M2‐M1 zigzag or M2‐M2 direct jump pathway in the material. Previous studies turned out that M1 sites in NVP are inactive during charge–discharge processes. Na+ ion migrates in the material through the M1‐M2 path, which will increase the average Na—O distance. The migration is a strong endothermic process, which is difficult to achieve thermal equilibrium. Moreover, the migration process is also energetically unfavorable with a relatively high activation barrier from the kinetics point. Hence, the reversible sodium (de)intercalation in NVP only occurred at M2 sites. It means that only two Na+ ions per formula could participate in the electrochemical reaction in Na3V2(PO4)3 via V4+/V3+ redox couple (3.4 V vs Na+/Na), delivering a theoretical specific capacity of 117.6 mAh g−1 and yielding an energy density of 396 Wh kg−1 for sodium storage. To elevate the specific capacity of Na3V2(PO4)3, triggering M1 sites and then realizing the V5+/V4+ redox couple is a possible and effective strategy. Lalère et al. successfully realized the V5+/V4+ redox couple in NVP through a proper substitution of aluminum for vanadium. However, the valence of the substituted Al3+ cannot change during this electrochemical window, the specific capacity of the material was not improved. Hu et al. found that Zn2+ could be mixed‐occupied at both the M1 and M2 sites in Na3V2(PO4)3 when worked as cathode material for aqueous Zn ion batteries. Such a mixed occupation could activate the M1 site for Zn2+ (de)intercalation in NVP, leading to the concerted migration of Na+/Zn2+ ions between M1 and M2 sites. Similar problem of blocked ionic pathways also exists in NaFePO4. Xiong et al. tried to improve the electrochemical performance of a sodium storage inactive material (the maricite phase NaFePO4) through the disorder degree tuning by a mechanochemical route. The derived NaFePO4 cathodes containing both amorphous and maricite phases exhibit much improved sodium storage performance with an initial capacity of 115 mA h g−1 at 1 C and an excellent cycling stability.In the rhombohedral NASICON structure NVP, sodium ions occupy two inequivalent Wyckoff sites, 1/3 located at 6b sites (M1) and 2/3 located at 18e sites (M2) with sixfold and eightfold oxygen coordination, respectively.5 Na+ 5+/V4+ couple activated and the third Na+ ion can participate in the redox procedure, bringing a three‐electron reaction process and excellent electrochemical performance. In addition, an exotic disorder‐induced pseudocapacitance behavior was first observed in NVP material.Herein, self‐supporting 3D porous NVP material was prepared by electrostatic spray deposition (ESD) method on a melamine sponge‐derived carbon foam (CF) substrate. The resultant NVP materials have a disordered structure with coexistence of nanocrystalline and amorphous phases, which shows quite different thermodynamic and kinetic behaviors to the well‐crystalline NVP sample. When used as binder‐free cathode of SIBs, the disordered structure can endow the V2Figure Figure R‐3c space group (JCPDS no. 053‐0018). The 700 °C annealed ESD sample NVP‐E700 has the similar rhombohedral NASICON structure with that of NVP‐S700, but only a few main characteristic peaks presented in the XRD pattern. Compared to the NVP‐S700 sample, the peak intensity of NVP‐E700 sample becomes weaker and the peak shape becomes obviously broadened. The average grain size of the NVP‐E700 sample calculated using the Scherrer equation is around 4 nm, indicating the nanocrystalline feature of the NVP‐E700 sample. As the annealing temperature drops, the peak in the XRD pattern will be further broadened even disappeared. There is only one very broaden peak at about 32° in the XRD pattern of the NVP‐E600 sample, consistent with the (116) atomic planes of NVP, as shown in Figure Self‐supporting 3D porous NVP films were prepared on the CF substrate by ESD method. NVP‐E700 and NVP‐E600 were obtained by adjusting the sintering temperature at 700 and 600 °C, respectively. For comparison, NVP‐S700 sample was synthesized by a conventional sol–gel method. The synthesis procedure of the materials is schematically shown in  It can be seen from the cross‐section that the thickness of the deposited film is about 1 µm 3 for the NVP‐E700 sample  measurements were employed to investigate the morphology of the CF substrate and the NVP film supported by CF. Figure 13 It Although FE‐SEM characterization can provide some important surface morphology information, the bulk structure of the material cannot be thoroughly reflected. To further ascertain the local structure of the two materials prepared by the ESD method, high‐resolution transmission electron microscopy (HRTEM) measurement was employed and the results are shown in Figure Figure −1 are relative to the vibration of P—O bonds in PO4 tetrahedra and the peaks located at 632 and 521 cm−1 are attributed to the vibration of V—O bonds in VO6 octahedra. It is worth noting that the peak width gradually increases with the decreasing crystallinity from NVP‐S700, NVP‐E700 to NVP‐E600, indicating the absorbing energy of the vibration will be changed. For the NVP‐S700 sample, a well‐crystallized material, the lengths and angles of the P—O and V—O bonds are single and the FT‐IR peaks are much sharp. As to the NVP‐E700 sample, the atoms of amorphous and the surface atoms of nanocrystalline account for the majority, which causes the distortion of the PO4 tetrahedra and VO6 octahedra in the NASICON‐type structure, perturbing their vibration frequencies. For the amorphous NVP‐E600, its disordered structure causes a larger range of vibration frequency, so the fine structure of FT‐IR spectra is almost lost.Fourier transform infrared (FT‐IR) spectra were recorded to further investigate the composition of the samples as shown in ctahedra.14 It −1, which are typical characteristics of carbon materials, corresponding to D band (disordered carbon sp3‐coordinated behavior) and G band (graphene carbon sp2‐coordinated behavior). The relative intensity ratio between the D and G bands (ID/IG) is commonly utilized to reflect the degree of the graphitization in the material. The calculated ID/IG of NVP‐S700, NVP‐E700, and NVP‐E600 are 1.23, 1.07, and 1.05, respectively. Due to the CF substrate, the sample prepared by ESD method exhibits a high degree of graphitization. In the Raman spectroscopy of the NVP‐S700 sample, there also exists an obvious peak near 962 cm−1, which is induced by the tensile vibration mode of PO4 in Na3V2(PO4)3. Such a characteristic peak becomes weaker and broader in the Raman spectroscopy of the NVP‐E700 and NVP‐E600 samples, indicating that the degree of the crystallinity is weak in the Na3V2(PO4)3 materials prepared by ESD method, which is consistent with that of the above XRD and FT‐IR tests.Raman spectroscopy was tested to further investigate the structure of the studied NVP samples Figure . Two obvehavior).15 The In fact, the formation of amorphous or nanocrystalline state may be contradictory to the high‐temperature annealing process, since the amorphous or nanocrystalline particles with larger surface energy should easily grow up to reduce their energy at high temperatures. Here, we propose a possible model to explain the formation mechanism of such an amorphous or nanocrystalline NVP phase material 3 materials, half‐coin cells were assembled by using the as‐synthesized NVP‐S700, NVP‐E700, and NVP‐E600 as the working cathode and sodium metal as the counter electrode. Figure −1, far beyond than the theoretical capacity value with 2 sodium ions insertion/extraction. The initial capacity of NVP‐E700 is 155% of the NVP‐S700 sample (115.9 mAh g−1) and 109.3% of the NVP‐E600 sample (163.7 mAh g−1). Second, there exists a long voltage plateau at 3.4 V in the charge–discharge curve of the NVP‐S700 sample, corresponding to the V4+/V3+ redox reaction. Such a plateau is diminished obviously for the NVP‐E700 sample, and the charge–discharge curve shows a rather sloping line in the whole potential range. For the NVP‐E600 sample, the plateau almost disappeared and the charge–discharge curve became an inclined profile. In order to eliminate the influence of the CF on electrochemical properties of the NVP‐E700 and NVP‐E600 samples, the charge–discharge curve of the CF substrate was also measured  and V5+ 2p3/2 (518.0 eV), indicating that three sodium ions per formula could be extracted from the sample. As a comparison, we also did XPS test on the NVP‐S700 sample  and V 2p3/2 (517.1 eV) XPS peaks related to V4+ existed in the spectra, meaning that only two sodium ions per formula could be extracted in this voltage range.To interpret the origin of the much higher initial capacity of NVP‐E700, X‐ray photoelectron spectroscopy (XPS) was measured for the electrode before and after charging it to 4.5 V. The wide‐range spectrum . The well‐crystallined NVP‐S700 electrode delivered an initial specific capacity of 115.9 mAh g−1 and the capacity remains 110.7 mAh g−1 after 200 cycles, corresponding to a capacity retention of 95.5%. Relative to NVP‐S700, the NVP‐E700 electrode delivered a much higher initial discharge capacity of 179.6 mAh g−1 and the capacity attenuation is negligible even after 200 cycles with capacity retention of 99.6%, showing an outstanding cycle stability property. The amorphous NVP‐E600 sample also delivers a high initial capacity of 165 mAh g−1 and the capacity increases to 190 mAh g−1 in the first 30 cycles, and then decreases sharply in the following cycles. The NVP‐E600 and NVP‐E700 electrodes show lower Coulombic efficiency than the NVP‐S700 electrode that is caused of defects in the amorphous structure, which will lead to side reactions during charge/discharge process and result in irreversible losses of sodium. Figure 3+/V2+ redox couple also partially participates in the reaction. The excellent performance of NVP‐E700 electrode could be mainly determined by the exotic structure of coexistence of nanocrystalline and amorphous phases as discussed above. More spaces in the amorphous structure can reduce the volume change during the cycle and stable bonding in crystalline phase can keep the structure from collapsing. For an absolute amorphous NVP‐E600 electrode, the sodium storage sites could be exposed in the first few cycles and then the capacity increased. However, due to the lack of stable support, the structure may collapse in the subsequent cycles, resulting in the capacity decrease.5+/V4+ redox couple and the participation of the third sodium ion as discussed before. The NVP‐E700 electrode delivers charge capacities of 179.6, 169.3, 149.2, 129.5, 98.2, 82.8, and 73.5 mAh g−1 at corresponding C‐rates of 0.2 C, 0.5 C, 1 C, 2 C, 5 C, 8 C, and 10 C. When the current density is restored from 10 C to 0.2 C, the reversible capacity recovers to 179.1 mAh g−1, which is almost the same with its initial value, demonstrating an excellent reversibility property. Although the NVP‐E600 electrode also delivered a high capacity of 165 mAh g−1 at low current density of 0.2 C, the capacity decreased fast to 76.1 mAh g−1 when the current density is higher than 5 C, which is 77.5% of NVP‐E700 at the same rate, even lower than NVP‐S700. The rate properties were mainly related to the disorder structure of the materials: on the one hand, the amorphous phase in NVP could weaken the bondage of sodium ions by oxygen, realizing three‐electron transfer in NVP and much higher capacity at low current density; on the other hand, the electron transportation is hindered due to the scattering effect of the disorder structure. Excessive disorder structure will adversely affect the rate performance of the material.The rate performance of the NVP‐S700, NVP‐E700, and NVP‐E600 samples was tested at current densities of 0.2 C to 10 C as shown in Figure tructure.25 ExcRct) and a straight line in the low‐frequency region reflecting the Warburg resistance (ZW), which is associated with Na+ ion diffusion process in NVP. It is found that the semicircle is larger in the NVP‐E600 sample than that in NVP‐E700 and NVP‐S700, indicating a higher charge‐transfer resistance. The EIS plots were fitted based on the equivalent circuit shown in the inset of Figure Rs stands for the bulk resistance and constant phase element (CPE) is related to the double layer capacitance. The fitting results are listed in Table Rct of the NVP‐S700, NVP‐E700, and NVP‐E600 sample are 428, 712, and 827 Ω, respectively. Meanwhile, the diffusion coefficients of sodium ions (DNa+) of all samples are calculated based on the Warburg impedances in the low‐frequency range of the EIS spectra. The equations can be expressed asσ denotes the Warburg factor, R denotes the gas constant, T is the absolute temperature, A is the surface area of the cathode, n is the number of electrons per molecule during oxidization, F is the Faraday constant, and C reflects the concentration of sodium ions (3.47 × 10−3 mol cm−3). The relationship between Z' and ω−1/2 used for the calculations is displayed in Figure DNa+ of NVP‐E600 is 1.05 × 10−15 cm2 s−1, which is lower than that of NVP‐E700 (2.01 × 10−15 cm2 s−1) and NVP‐S700 (9.77 × 10−14 cm2 s−1), which is consistent with the rate performance as discussed above. The result shows that the rate performance and capacity of NVP materials can be well tuned by the crystalline state.To get direct evidence of the influence of amorphous structure on the electrochemical reaction dynamics for the NVP samples, the electrochemical impedance spectra (EIS) measurements were tested Figure . All the−1  curve of the three samples was tested in the voltage range of 2–4.5 V with a scan rate of 0.2 mV s1 Figure . It can −1. The peak intensity of both the CV curves increases with the increasing scan rates. Figure ip) versus log (v) plots of the samples for each redox peak. All the plots show nearly straight line and the parameter b can be obtained by fitting the line. For NVP‐S700, the calculated b values of the reduction (R) and oxidation (O) peaks are 0.54 and 0.58, respectively , R is the ideal gas constant (8.314 J mol−1 K−1), T is the absolute temperature (K), F is the Faraday constant (96 485 C mol−1), x is the filling fraction of the intercalated ions, and J is the interaction parameter between the adjacent ions. The relationship between E, x, and J is shown in Figure J < 4RT/F), the intercalation or deintercalation is a single‐phase process and the E–x curve is an inclined straight line. When the interaction between adjacent ions is strong (J > 4RT/F), the E–x curve has a maximum and a minimum and the intercalation or deintercalation process exhibits a two‐phase reaction. Therefore, for the intercalation compounds material, the voltage curves are mainly affected by the interaction of the inserted ions in the host site network.Based on the above analysis, we can conclude that the electrochemical performance of NVP material can be well tuned by their crystallinity optimization. NVP material with different crystallinity have different voltage curves, which can be well explained by the lattice‐gas model.30 TheJ values are 0.9, −1.9, and −2.1 for the NVP‐S700, NVP‐E700, and NVP‐E600 samples, respectively, as shown in Figure J value between sodium ions. According to the lattice‐gas model, there should exist a strong repulsive force between the successively inserted ions, making the insertion process be a local and ordered diffusion. Therefore, it is a typical NaV2(PO4)3‐Na3V2(PO4)3 phase transition process rather than random and uniform diffusion. Based on the classic Gibbs phase rule, when the two phases with different compositions reach an equilibrium state, the chemical potential should be a constant and a voltage plateau in the discharge curve appeared, as shown in Figure  However, the long‐range order of lattice in the nanocrystalline NVP (surface disorder) and amorphous NVP (bulk disorder) sample is disrupted and the interaction between the inserted sodium ions is weakened, corresponding to a smaller J value. The inserted sodium ions can diffuse randomly and evenly in the electrode material, showing a tendency to a single‐phase reaction in the electrochemical process. Therefore, the voltage plateau in the discharge curve of such materials disappeared and the potential changes linearly with the increase of the sodium ion concentration, as shown in Figure Based on the lattice‐gas model, the voltage curves in Figure iffusion.31 Bas3V2(PO4)3‐NaV2(PO4)3) in crystalline NVP‐S700 happens if the scan voltage (>3.4 V) makes the electric work higher than the Gibbs free energy change (ΔG) between the two phases. In discharge process, the ΔG makes the electrode reaction (NaV2(PO4)3‐Na3V2(PO4)3) happens when the scan voltage was under 3.4 V. Therefore, the electrode reaction current exhibits a pair of maximal values at a certain voltage in the CV curves. While for amorphous NVP, there is no phase transition in the whole voltage window. The disorder structure will diminish the variance of the site energy for the inserted ions, meaning that the diffusivity of the inserted ions is more independent of their concentration. The ions undergo semi‐infinite diffusion in the electrodes, and the diffusion rate  was just related to the voltage scan rate. Thus, the current tends to be constant in the entire voltage range, leading to the rectangular shape CV curve and showing the extrinsic pseudocapacitance behavior.In the charging process, the electrode reaction (Nantration.33 TheJ value (Figure J value. For crystalline NVP, the J value is large and it always exhibits battery‐type charge‐storage behavior. The J value can be reduced through preparing amorphous or nanoscale material. The inserted ions in such a disordered material could diffuse randomly and cause a disorder‐induced‐pseudocapacitance phenomenon. This pseudocapacitance behavior is another contributing factor to the outstanding sodium storage ability of the NVP‐E700 sample. For some battery‐type materials with two‐phase transition mechanism, due to the limiting of the ions diffusion at the phase interface, some regions cannot participate in the electrochemical reaction, resulting in partial capacity loss during the charge–discharge process. However, in a pseudocapacitive‐type material, the Na+ ion diffusion process will not be limited by such a two‐phase interface, and then the material shows an outstanding electrochemical performance, especially the capacity retention. Benefitting from the synergistic effect of the robust structure, the pseudocapacitive contribution, and the activation for V5+/V4+ redox couple, the NVP‐E700 material shows the best electrochemical performance in the three studied samples.From the above analysis, the extrinsic pseudocapacitance in the NVP material is closely related to the e Figure 6 which 35+/V4+ redox couple, realizing three‐electron reaction process with a specific capacity of 179.6 mAh g−1 at 0.2 C. The disordered structure can also change the interaction between the inserted sodium ions in the host network site, leading to the rectangular‐like CV curve and the disappearance of plateau in the charge–discharge curve, behaving as a disorder‐induced‐pseudocapacitance. The pseudocapacitive contribution plays an important role in the charge storage of the NVP‐E700 sample. Our research demonstrates the importance of a disorder structure to the three‐electron reaction process and extrinsic pseudocapacitance in NVP cathodes for sodium‐ion batteries.In summary, self‐supporting 3D porous NVP films were deposited on CF substrate by ESD method. The structural characterizations indicate that the NVP‐E700 film shows a certain degree of disordered structure. The disordered structure could weaken the bondage of oxygen to sodium ions, activating the VThe authors declare no conflict of interest.Supporting InformationClick here for additional data file.


The medically important bacterial aromatic polyketide natural products typically feature a planar, polycyclic core structure. An exception is found for the rubromycins, whose backbones are disrupted by a bisbenzannulated ‐spiroketal pharmacophore that was recently shown to be assembled by flavin‐dependent enzymes. In particular, a flavoprotein monooxygenase proved critical for the drastic oxidative rearrangement of a pentangular precursor and the installment of an intermediate ‐spiroketal moiety. Here we provide structural and mechanistic insights into the control of catalysis by this spiroketal synthase, which fulfills several important functions as reductase, monooxygenase, and presumably oxidase. The enzyme hereby tightly controls the redox state of the substrate to counteract shunt product formation, while also steering the cleavage of three carbon‐carbon bonds. Our work illustrates an exceptional strategy for the biosynthesis of stable chroman spiroketals. Structural snapshots and mechanistic investigation of a key spiroketal synthase provide insights into extensive oxidative backbone rearrangements and spiroketal pharmacophore assembly in rubromycin biosynthesis. Most rubromycins possess antibiotic or anti‐cancer activities and act as protein inhibitors , thus making them possible drug leads.2 biosynthetic gene cluster from Streptomyces sp. JP95 implied conventional early steps that generate a polycyclic aromatic backbone, before further tailoring reactions including ring hydroxylations afford the advanced intermediate collinone (3) as universal pentangular precursor for the ensuing spiroketal forming steps.‐spiroketal  proved to be highly unstable and spontaneously formed 4 or the shunt product 7. GrhO1 presumably mediates hydroquinone oxidation at ring B of 6 and thereby boosts 4 formation while counteracting 7 formation. Finally, GrhO6 hydroxylates phenolic ring B of 4, which facilitates the decarboxylative ring C opening and subsequent contraction to the mature ‐spiroketal of 7,8‐dideoxy‐6‐oxo‐griseorhodin C (8) , yellow rod‐shaped crystals of native GrhO5 (without 3/7) were obtained after ca. 4–6 months that diffracted up to a resolution of 1.75 Å  and enabled structural elucidation by molecular replacement. GrhO5 features three domains similar to other group A FPMOs , is missing domain III entirely. Compared to other well characterized group A FPMOs  including the GXGXXG , the GD (contact of Gly300/Gly308 and Asp301/Asp309 with the O3′ of the ribose moiety) and the DG (conserved Asp172/Asp180 and Gly173/Gly181 both involved in FAD and NAD(P)H binding) motives.To investigate the spiroketal synthases, the crystallization of GrhO5 and RubL was attempted under low‐salt conditions in the absence and presence of native substrate s Figure ; domains2O2 from collapse of the FADC4aOO(H) oxygenating species) in the absence of the substrates.ox in OUT; accepts substrate, no reaction with NADPH), “reducible state”  and the “catalytic state” . Moreover, these spiroketal synthases feature two distinct openings to the protein interior and the active site, denoted by “substrate entry site” and “NADPH entry site” , consistent with the binding of 3 or 5 (possibly formed by X‐ray radiation induced reduction). In this conformation, the O1 and the O3 atoms were positioned in hydrogen‐bonding distance to the C4 carbonyl group and the N3 (only O1) of the isoalloxazine ring of FAD that retained the OUT conformation  represents the reducible state, in which the substrate is tightly coordinated by a cluster of basic amino acid side chains comprising H217, R79, and R366 that are in hydrogen‐bonding distance to O8/O10, O4/O5/O8/O9, and O4/O8 of 3/5, respectively  and indirect (F287 to W289) π‐π interactions , which resulted in the closure of the substrate entry site and a more confined reaction chamber . Overall, these changes lead to the catalytic state of the enzyme. This is analogous to RebC, in which residues at a similar position (i.e. the domain I‐domain II interface) formed an ordered helix to enclose the bulky substrate.Many group A FPMO structures feature an FAD cofactor in “IN” even in the absence of substrate, which may in some cases result from the binding of Clox resting state  has been investigated spectroscopically.e Figure . Presumaox by NADPH was first analyzed, which clearly showed that significant NADPH oxidation rates were only observed in the presence of 5. Hence, in the acceptor state of the spiroketal synthase without substrate, the direct reduction of FAD‐OUT and uncoupling are impeded. To investigate the basis for this behavior, the reduction potential of RubL‐FAD was determined with and without 5 using the xanthine‐xanthine oxidase method.E°: −184 mV5 on the reduction potential, which was calculated at −177±1 mV . This matches well with previously reported values for group A FPMOs , which, interestingly, is in contrast to previously characterized HbpA.+ (rather than NADPH) was added to the reducible state (RubL‐FADox‐5), a significant loss of the long‐wavelength absorption of the CT complex was observed , which likely indicates that the interaction between the W289 side chain and the FAD cofactor becomes weakened by substrate binding. Presumably, this allows the nicotinamide ring to temporarily disrupt the sandwich π‐π interaction by further displacing the flipped W289 side chain and thereby enable hydride transfer to FAD in the reducible state of the enzyme . The now antiaromatic FADred no longer engages in π‐π stacking interaction; instead conformational changes of the side chains of H51 (H43 for GrhO5), F287 (F279) and also W289 (W281) push the FAD into the IN conformation adopted in the catalytic state and likely promote the departure of NADP+ , indicative of 3 reduction to 5 . The observed rate constants reached a plateau at ca. 250 to 300 μM 3 in the presence of 1 mM NADPH, resulting in a kcat(app) of 4.6±0.2 s−1 and a KM(app) of 22±4 μM. Notably, when assaying the effect of the NADPH‐concentration on 3‐reduction activity, RubL showed second‐order kinetics with a kcat(app) that linearly increased with the NADPH‐concentration , suggesting a transient interaction with NADPH H with low KM and relatively high KD values and promptly release NAD(P)+ subsequent to flavin reduction[3‐complex did not cause any obvious changes to the FAD spectrum, implying that enzyme‐bound 3 is reduced via direct hydride transfer (without involvement of FAD). Interestingly, additional HPLC analysis suggested that enzyme‐bound 5 is most likely displaced by remaining 3 before hydroxylation can occur, as conversion of 5 into 4 and 7 was only observed subsequent to virtually full reduction of 3 to 5 . Next, the overall turnover of 3 into 4 was investigated. As saturation in the reduction reaction is observed at >350 μM 3, assays were supplemented with 450 μM 3 and 1.5 mM NADPH  and also shifted the reaction equilibrium toward the reduced products and intermediates , thereby decreasing the number of metabolites in the assay . Overall, these experiments clearly show that 3 reduction by RubL is drastically faster than the ensuing steps under the tested conditions.To gain a detailed understanding of the quinone reductase functionality, the RubL‐ and NADPH‐dependent conversion of 3 binding and presumably catalysis, in particular histidine and arginine residues . These basic side‐chains may stabilize a deprotonated ring D  and thus increase the nucleophilicity of the phenolate moiety toward the electrophilic FADC4aOOH oxygenating species. Moreover, a positive electrostatic microenvironment should also favor the subsequent ring‐cleavage and rearrangement reactions, as further discussed below. To study the role of these residues in catalysis and 3 turnover, different active site variants of RubL were generated; R87 and R374 were replaced by a lysine or a methionine  and H225 was substituted by an alanine (H225A). In addition, two active site variants were produced in which E103  was replaced by either a glutamine or an alanine. All variants were obtained in similar yields compared to the wild type protein and seemed properly folded according to size exclusion chromatography . For all variants, significantly lower 3 reduction rates were observed under anoxic conditions compared to wild type enzyme; however, the most pronounced effect was found for the W289A and the three arginine  variants with relative activities of ca. 5–6 %, 3–7 %, 7–10 %, and 3–7 %, respectively . In addition, the formation of the final products 4 and 7 was strongly diminished in these variants and could only be observed for the variant with the conservative R374K replacement , suggesting important roles of these residues for aromatic hydroxylation and possibly for the succeeding steps and backbone rearrangements. In contrast, despite their strongly impaired quinone reductase activity, 4 and 7 formation was not affected in the E103A/E103Q or H225A variants, implying that both residues are dispensable for the later steps and could rather be important for substrate binding or positioning.Based on the ternary crystal structure of GrhO5, several active site residues were identified that participate in re‐side of the cofactor against NADPH. Following the binding of 3, the reducible state is adopted by flipping of the W289 side chain that exposes FADox for reduction, while still impeding cofactor movement via aromatic sandwich π‐π interaction , before the substrate is reduced (probably independent of the FAD cofactor), as the 1,4‐naphthoquinone moiety of 3 is a better electron acceptor than the N5 of the isoalloxazine ring due to a more positive reduction potential   that has a dual role and furthermore shields the n Figure . NADPH tred‐IN with O2 leads to the FAD semiquinone (FADSQ) and superoxide radicals and is likely controlled by a dedicated O2 reaction site that facilitates short‐range single electron transfer. The observed Cl− adjacent to the FAD cofactor indicates this site, which is likely optimized to trap the transient superoxide long enough to allow spin inversion and collapse of the radicals to the FADC4aOOH   surrounding rings C and D of bound 5 in the confined active site  and adopts a dual role in the resting state of the enzymes by locking FADox in OUT and by acting as gatekeeper that shields FADox from reduction. This is in contrast to FADox of the prototypal PHBH that is generally reducible in OUT but samples between the IN and a possibly exclusive OPEN position to prevent uncoupling in absence of substrate.In this work, we characterized two functional homologs of the key tailoring enzyme involved in the biosynthesis of bacterial rubromycin‐type polyketides, which mediate a drastic oxidative rearrangement of the pentangular precursor 3 without the involvement of FAD. Quinone reduction appears to be a more common strategy for the activation of pathway intermediates in natural product biosynthesis[7.C4aOO(H) species.[− close to the C4a that likely indicates the O2 reaction site in the “catalytic state”  may function as an H‐bond acceptor to stabilize and position the FADC4aOOH intermediate to ensure regiospecific hydroxylation  should be addressed to the authors.Supporting InformationClick here for additional data file.


N‐methylpyrrolidine (NMP) facilitates the substitution of naphthalene with the incoming arene. A series of fully characterized rhenium sandwich complexes with simple arenes, such as aniline, as well as with active compounds like lidocaine and melatonin are presented. With these rhenium compounds in hand, the radioactive sandwich complexes [99mTc(η6‐pharm)2]+  can be unambiguously confirmed. The direct labelling of pharmaceuticals with 99mTc through η6‐coordination to phenyl rings and the confirmation of the structures with the rhenium homologues opens a path into molecular theranostics.Bis‐arene sandwich complexes are generally prepared by the Fischer‐Hafner reaction, which conditions are incompatible with most O‐ and N‐ functional groups. We report a new way for the synthesis of sandwich type complexes [Re2]+ and [Re(η6‐napht)(benzene)]+ have been used to prepare sandwich complexes [Re(η6‐arene)2]+ and [Re(η6‐arene)(η6‐benzene)]+. NMP is used to promote the substitution of naphthalene with simple arenes as well as with active compounds like lidocaine and melatonin. This compound can be used to unambiguously confirm the radioactive sandwich complexes [99mTc(η6‐pharm)2]+. Compounds with no structural changes in the active lead structures are rare since coordination sites in biologically active pharmacophores are often not available or not appropriately arranged for metal coordination. Exceptions are here curcumins, which offer an acetylacetonato‐like coordination site.3] complex of estradiol or amino acids in which the chromium is directly coordinated to the phenyl ring in the organic structure.Bioorganometallic complexes of rhenium and other transition elements have gained a lot of momentum over the past years, complementing the available armory of inorganic medicinal compounds.99mTc in a pharmaceutically active structure with minimal alteration of their functional groups (D in Scheme 3]+ (M=Re/99mTc) instead, is tempting but not an option since the corresponding [Re(η6‐C6H6)(CO)3]+ complexes are labile and loose the arene ring rapidly.Challenged by the question of introducing rhenium or 6‐C6H6)2]+ and long‐lived [99Tc(η6‐C6H6)2]+ along the classical Fischer‐Hafner approach, that is, using Zn/AlCl3 as activation/reduction agents, and adapting an improved preparation by Kudinov et al.99mTc(η6‐arene)2]+ with arene being functionalized phenyls, are accessible in water in one step and directly from [99mTcO4]−.Earlier, we reported about the preparation of [Re2]+ starting from [Re(η6‐napht)2]+  and catalyzed by N‐methyl‐pyrrolidone (NMP) in low to medium yields. We also present the preparation of the corresponding 99mTc homologues. The rhenium and the 99mTc matched‐pairs offer thus a generally applicable path towards the direct labelling of pharmaceuticals through η6‐coordination to phenyl rings, integrated in the lead. This opens a path into molecular theranostics.Obviously, Fischer‐Hafner conditions are not applicable for functionalized arenes such as found in pharmaceuticals. The reactions with 6‐arene)2]+ complexes, Fischer‐Hafner conditions are not appropriate since AlCl3 is not compatible with most functional O‐ or N‐groups. So far, the sole approach was the substitution of ligands in preformed ReI complexes. The mutual exchange of coordinated arenes in sandwich complexes by other incoming ligands, for example other arenes, is quite common for group 8 or group 9 elements iron and ruthenium, iridium or rhodium, but essentially unknown for those of group 7.6‐C6H6)2]+ is extremely stable, any attempts to replace one or both benzene ligands failed under any conditions. This contrasts reactions with for example the corresponding Ru2+ compounds.I starting materials, we attempted to substitute naphthalene in [Re(η6‐napht)2]+ since naphthalene is generally weaker bound than benzene due to the more extended π‐system.For the preparation of functionalized +. In these, the incoming arene substitutes the solvent ligands to yield the products. A second and equal step will replace the 2nd naphthalene to achieve the complex [Re(η6‐pharm)2]+  was the only one that gave satisfying yields  with a broader variety of arenes. A number of ligands/pharmaceuticals that have been investigated along this line are shown together with the respective complexes in Scheme For substituting naphthalene in 2 and 2,4‐dimethylaniline 4, path A has been exemplified before by us but isolation from excess ligand and purification were demanding.4 as solvent gave [Re(η6‐4)2]+ (5+) and its X‐ray structure could now be solved and is shown in Figure 2) and 1+ were directly reacted in dioxane as solvent in the presence of 5 equiv. of NMP to obtain [Re(η6‐2)2]+(3+) in 39 % yield after purification . The reaction without NMP did not give any yield at all. Mequinol , a simple phenyl with two substituents, is a drug used in skin depigmentation  reactive substituents, which cannot be prepared by Fischer‐Hafner methods. The reaction with mequinol also followed procedure B. Mequinol (50 equiv.) and complex 1+ were dissolved in dioxane and the solution was heated to 130 °C for 86 h. Compound [Re(η6‐6)2]+ (7+) was thus obtained in 18 % yield . Lidocaine is a local anesthetic and an even higher functionalized arene. The reaction with 8 (lidocaine) was performed analogously to obtain the corresponding rhenium complex [Re(η6‐8)2]+ (9+) in 4 % yield after purification. To proceed from single arenes to fused ring system, the indole‐based carbazole system is a prototypical ligand since carbazole subunits are present in many pharmaceuticals. Three anti‐cancer drugs based on carbazole structures, Celiptium®, Alecensa®, Rydapt® are already in the market for cancer therapy.6‐10)2]+ (11+) with carbazole was done differently. This complex was synthesized by dropwise addition of a solution of 1+ in NMP over 15 h into a solution of carbazole in dioxane at 120 °C, resulting in two diastereomers (see below) in a collective yield of about 31 %. Melatonin (12), an endogenous hormone regulating the wake‐sleep cycle, is well known to many of us. It comprises the indole subunit and is a derivatized extension of it. As for 11+, a solution of 1+ in NMP was added dropwise to a NMP solution of 12 over 10 h. The complex [Re(η6‐12)2]+ (13+) was isolated in 34 % yield in the form of its two diastereomers (see below). The different synthetic strategies vary mainly due to solubility reasons. Since carbazole and melatonin are only slightly soluble in dioxane, NMP was taken in excess. Experimental details for all syntheses and compound characterizations can be found in the Supporting Information.For aniline 6‐4)2]+ 5+ and its6‐4)2]+ 5+ and its6‐4)2]+ 5+ and its6, this intermediate [Re(η6‐6)(η6‐napht)]+(14+) could be isolated and was structurally characterized ]+4+ could4]− over the long reaction times. During the reactions, we detected by UPLC‐MS in most cases reduced 1+, in which one of the coordinated naphthalenes was reduced to dihydro‐naphthalene. Similarly, we detected mono‐substituted side‐products in which the remaining naphthalene was also reduced to dihydro‐naphthalene. In the synthesis with carbazole to complex 11+ for example, [Re(η6‐10)(η6‐dihydronapht)]+ and further reduced to [Re(η6‐10)]+ were observed along the reaction by UPLC‐MS. Since these corresponding sandwich complexes are essentially benzene complexes, they do not exchange the dihydronaphthalene or the tetraline anymore. The reductive side reactions are one of the main reasons why the yields are low. We emphasize at this point again that any attempt to substitute benzene by any means instead of naphthalene was unsuccessful. Benzene or other aromatic hydrocarbons are extremely stably bound. The X‐ray structures of complexes [5](ReO4), [7](tfa), [9](ReO4), and [14](PF6) could be elucidated and are shown in Figure The yields for the bis‐substituted sandwich complexes are generally low, mainly due to side reactions and formation of + will be in a meso‐form. If coordination occurs from the same sides either U or D, the two enantiomers [Re(η6‐U)2]+and [Re(η6‐D)2]+ will be obtained as shown in Scheme 6‐4)2]+ in Figure 6‐U)2]+ but its enantiomer is also present in the crystal since the space group is P21/c. If the diastereomers are chemically different enough, they can be distinguished by HPLC. Occasionally, they can even be separated from the reaction, as it is the case for the carbazole sandwiches shown in Scheme 11]BF4 could be separated from the meso‐form and be crystallized. Both enantiomers are of course present in the crystal. An ellipsoid displacement plot of one of them is given in Figure Depending on the configuration of the incoming arenes, some interesting stereochemical features become evident. Multi‐substituted arenes such as 2,4‐dimethylaniline (arbazole 0 or mela99mTc (see below). Two discernable peaks are sometimes found in the radio‐HPLC analyses, which can eventually be separated from each other by “preparative” radio‐HPLC. In case of rhenium, 1H NMR analyses display two distinguishable sets of signals. Figure 1H NMR of the melatonin complex 13+ in which the two sets of signals in the aromatic region are nicely separated. The two diastereomers could be isolated from each other and are shown together with the full spectrum in the Supporting Information.Obviously, the same stereochemical considerations apply also to reactions with 6‐coordinated benzene, it is unlikely that it is replaced. Thus, a reaction of [Re(η6‐C6H6)(η6‐napht)]+ (15+) with a pharmacophore should yield the corresponding mixed‐arene sandwich complexes [Re(η6‐C6H6)(η6‐pharm)]+. The preparation of the corresponding starting material 15+ was reported by us earlier.16), a compound that was previously taken for the treatment of certain hormone‐dependent cancers,6‐C6H6)(η6‐16)]+ (17+). The same procedure with mequinol and with melatonin yielded the corresponding complex [Re(η6‐C6H6)(η6‐6)]+ (18+) and [Re(η6‐C6H6)(η6‐12)]+ (19+) ]+ 15+ with a pht)]+ 15+ with a 17+ and 18+ could be elucidated. Ellipsoid displacement plots are shown in Figure The structures of R or S carbon), two enantiomers are formed. In fact, the space group of the crystal structure is P21/n and both enantiomers are thus present in the crystal. Complex 19+ is formed as a mixture of two enantiomers as well, since melatonin can bind the metal from both sides.As before, some stereochemical peculiarities may appear, depending on the nature of the incoming arene. The hexestrol used for the reaction is in its meso form. Depending on which arene binds the rhenium centre 2]+ and [99mTc(η6‐arene1)(η6‐arene2)]+ directly from [99mTcO4]− in water is feasible 2]+‐type. The reaction conditions are obviously non‐Fischer‐Hafner like. We emphasize that complexes of the type [99mTc(η6‐arene)2]+ are extremely stable and do not decompose if exposed to air or water at any pH value.As shown in Scheme e [99mTcη‐arene2]+99mTc, only comparison with rhenium unambiguously assesses the structure. The synthetic approaches reported herein allow us now to synthesize bis‐arene sandwich complexes of multiple functionalized arenes, and to compare their retention times with the ones obtained from the aqueous synthesis with 99mTc. Figure 9+ (rhenium‐lidocaine) together with the homologue [99mTc]9+ and a trace for the carbazole complex 11+ and [99mTc]11+, displaying clearly the two diastereomers as discussed before (bottom). With this, the path to a wide variety of novel radiopharmaceuticals is paved since the compounds with 99mTc can now be characterized with the fully characterized rhenium homologues.Despite getting one peak in the HPLC for sandwich complexes with 6‐arene)2]+ and [Re(η6‐arene)(η6‐benzene)]+ from [Re(η6‐napht)2]+ and [Re(η6‐napht)(η6‐benzene)]+. “Arene” refers to benzene derivatives or active pharmaceuticals such as anesthetics or anticancer drugs. NMP facilitates the substitution of the naphthalenes with incoming arenes under thermal heating. Our previous approach of reacting [Re(η6‐napht)2]+ in neat arenes under harsh conditions is thus extended to solid and highly functionalized arenes, representing pharmaceuticals. Sandwich complexes with pharmaceuticals open the option of investigating them with respect to their bioactivities. The available 99mTc homologues complement therapy with concomitant imaging, enabling molecular theranostics. To confirm a supposed 99mTc structure with the respective rhenium homologue is the scope of this report and this important base for any attempted theranostic approach has been accomplished. Biological studies with selected compounds are ongoing.In conclusion, we report a synthetic pathway towards sandwich complexes [Re(η99mTc labelling‘s are described in the Supporting Information. Deposition Number(s) 5), 2113182 (for 7), 2113180 (for 9), 2113178 (for 11a), 2113177 (for 11b), 2113183 (for 14), 2113184 (for 17), 2113181 (for 18)2113179 (for  contain(s) the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.All experimental details and The authors declare no conflict of interest.As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.Supporting InformationClick here for additional data file.


There was an error in the original publication, in Equation (5). The sign × needs to be changed to ∗ (the sign for convolution). In Section 3.2, Equation (5) appears as:The original publication has also been updated .


The migration rate responds to control by variation of the temperature, water content and triethylamine concentration, while an excess of fuel controls the duration of an induction period before the migration event.A molecular system comprising a cationic zinc complex and an amino acid‐derived ambident ligand having phosphate and carboxylate binding sites undergoes a series of rearrangements in which the metal cation migrates autonomously from one site to another. The location of the metal is identified by the circular dichroism spectrum of a ligated bis(2‐quinolylmethyl)‐(2‐pyridylmethyl)amine (BQPA) chromophore, which takes a characteristic shape at each binding site. Migration is fuelled by the decomposition of trichloroacetic acid to CO Programmed autonomous migration of cations between alternative binding sites of a peptidomimetic ligand is driven by a fuelled change in acidity of the system. The rate of migration, and delays or resets in the execution of the program, may be controlled by the operating conditions of the molecular automaton. Life is sustained by a collection of simultaneously operating autonomous networks of molecular interactions, using chemical energy to drive cyclic, out‐of‐equilibrium reactions.2+ cation to a set of monoprotic ligands of differing pKa may be controlled by addition of acid or base, modulating the protonation states of the various ligands in such a way that the Zn2+ cation associates chemoselectively with the most basic of the available free anions.2+ centre. We now show that regioselective metal‐ligand coordination may be controlled in an out‐of‐equilibrium system in which pH evolves autonomously as a result of a chemically fuelled deacidification process 2+ with an equimolar mixture of 3 and the phosphazene base tBuP1(NMe2)3 (see Supporting Information). 3 displayed 1 : 1 host:guest binding behaviour. The binding isotherm of 2 f was more complex (see Supporting Information).The binding constant of the carboxylate station was estimated to be 2+ must be powered by a fuel, for which we chose TCAH, which decomposes under basic conditions by decarboxylation of its conjugate base TCA−. The kinetics of this reaction have been extensively studied in water, organic solvents, and solvent mixtures.2+ ions (c). Further decomposition of the remaining TCA− deprotonates the carboxylic acid, causing the zinc to migrate to the carboxylate station and returning the system to the resting state (station 2) (a). In this way, a dynamic system emerges in which the binding selectivity of the metal complex is autonomously shifted from one site, or station, of the ligand to another in a manner that is programmed by the time‐course of the deacidification of the solution.We applied the TCAH‐fuelled pH change to the multi‐site binding system using ligand 4 (0.25 mM), 4 (1 equiv), NEt3 (3 equiv.) in MeCN containing 2 % H2O,2 mol−1 at around 4 minutes, as the zinc migrated to the phosphate station 1. The location reporter signal then swung to positive and increased in magnitude over the next 16 minutes, corresponding to carboxylate deprotonation and migration of Zn2+ to the carboxylate station 2.The molecular automaton was constructed by mixing Zn(BQPA) ⋅ 2ClO4 0.2 mM, 4  to 110 seconds (at 5 equiv.). Similarly, it migrated faster to the carboxylate station 2, as the carboxylate signal plateaued earlier following a steeper gradient.Increased amounts of both NEt− decomposition and faster Zn2+ migration. Samples containing Zn(BQPA) ⋅ 2ClO4 , 4 (1 equiv.), NEt3 (2 equiv.) and TCAH (2 equiv.) were monitored over 30 minutes at temperatures between 20–40 °C  ⋅ 2ClO4 (0.25 mM), 4 (1 equiv.), NEt3 (2 equiv.) and TCAH (2 equiv.) were prepared in MeCN containing 1–6 vol % H2O and monitored over 20 minutes at 35 °C , 4 (1 equiv.), NEt3 (2 equiv.) and TCAH (2–6 equiv.) were monitored over 20 minutes at 35 °C , 4 (1 equiv.) and NEt3 (2 equiv.) was consecutively treated with 1.0, 2.0, 3.0, 0.5 and 1.5 equiv. of TCAH, with a 10 minute observation period between fuel pulses  indicated that the operating rate decreased marginally with each cycle, owing to accumulation of the water required to stabilize the fuel.Finally, multiple additions of fuel were used enforce restarts of the migration program. A sample of Zn(BQPA) ⋅ 2ClO co=0.02 mM,2O, 42+ between different sites on an ambidentate peptide ligand 4, in which the location of the metal is controlled by the pH of the medium. Coupling the migration of the metal between sites with TCA− decomposition creates a chemically fuelled molecular automaton, in which autonomous reorganisation of its components followed a time course that could be programmed using five independent variables. The absolute configuration of the ligand binding sites controlled the sign and magnitude of the output signals, while initial TCA− concentration, temperature and water content all influenced the rate of movement between sites. An excess of TCAH fuel allowed the start time of the motion to be delayed. The total operating time of the automaton was typically on the order of hundreds of seconds, a convenient timescale for continuous monitoring by optical techniques. The migration of the metal between stations and the factors controlling fuel decomposition are general and need not necessarily be limited to systems with a CD output; their highly programmable behaviour shows their potential to be used as controllers for more complex supramolecular assemblies.We have demonstrated migration of a cationic metal complex [Zn(BQPA)]Representative sample preparation for a TCAH fuelled system: An initial sample was prepared in a 1 mm path length quartz cuvette from Zn(BQPA) ⋅ 2ClO4 , ligand 4 , NEt3  and MeCN (120 μL). TCAH  was added and the sample was quickly shaken for homogeneity, then placed into the CD spectrometer sample chamber (temperature set to 25 °C) within 10 seconds. The ellipticity output at 239.5 nm (θ239.5) was monitored over 20 minutes, with data points being taken every 10 seconds. See the Supporting Information for tabulated sample compositions for all TCAH fuelled experiments.1H and 13C NMR spectra), miscellaneous CD data (base titrations and binding isotherms) and pKa calculation methodology may be found in the Supporting Information.Synthetic procedures, characterisation of novel compounds  should be addressed to the authors.Supporting InformationClick here for additional data file.


Reaction conditions including synthesis temperature and Ba/Ti precursor ratio have been explored with the aim of producing BaTiO3 with small crystallites and a low concentration of defects. It has been found that the crystallite/particle size and tetragonality of the BaTiO3 samples increase as the synthesis temperature increases; and the crystallite/particle size of BaTiO3 is also affected by the Ba/Ti precursor ratio. The BaTiO3 sample synthesised using a Ba/Ti precursor ratio of 2 : 1 at a reaction temperature of 120 °C exhibited homogeneous crystallites of the smallest size of 107 nm. Additionally, the Ba/Ti precursor ratio of 2 : 1 with synthesis temperature of 220 °C was found to produce a smaller concentration of defects in BaTiO3.Nanocrystalline BaTiO 3 has been prepared via a hydrothermal synthesis. Reaction conditions including synthesis temperature and Ba/Ti precursor ratio/concentration have been systematically explored to produce small crystallites of phase-pure BaTiO3.Nanocrystalline BaTiO Below 120 °C, BaTiO3 experiences a structural distortion: Ti4+ is displaced off its central position in the octahedron, in the direction of one of the O2−; this tetragonal distortion in the structure generates a net dipole moment, and hence the material behaves as a ferroelectric. BaTiO3 exhibits a very high dielectric constant, particularly at temperatures close to the ferroelectric–dielectric transition.Barium titanate , overcurrent protection (self-resetting fuses) and electronic timing circuitry. These devices use the positive temperature coefficient of resistivity (PTCR) property in BaTiO3 materials to control the resistance in a circuit as a function of temperature in the form of thermistors.6,7 This PTCR effect has also recently been shown to be able to prevent thermal runaway in batteries.8The overriding goal in BaTiO3, however manufacturers of multilayer capacitors and PTCR devices have specific requirements for the ceramic material if it is to be used in the preparation of dielectric layers. The method must produce a high purity product with homogeneous composition to ensure stable and consistent dielectric properties. A route that causes strong agglomeration will result in a non-uniform particle size, which is problematic. For the production of multilayer capacitors, in general, solution-based techniques such as hydrothermal synthesis are used to prepare very fine particles, as these techniques achieve a homogeneous, phase-pure stoichiometric BaTiO3 with synthesis temperatures far lower than solid state methods (>600 °C).5,9 Nanosized particles of BaTiO3 can be achieved by controlling reaction conditions such as synthesis temperature and time, Ba and Ti precursor categories and Ba/Ti precursor ratio.4 Zeng et al. prepared nanocrystalline BaTiO3 from barium hydroxide octahydrate and titanium isopropoxide at low temperature (<100 °C) and ambient pressure, and reported the impact of the reaction temperature and Ba/Ti precursor ratio on the morphology and size of BaTiO3.10,11 Then, Nahm et al. used the same precursors and hydrothermal conditions for the synthesis of nanocrystalline BaTiO3, thus producing a one-step reaction process with faster reaction times, but they only employed one temperature (220 °C) and Ba/Ti precursor ratio (4 : 1).12 In this work, we employ the same precursors and hydrothermal conditions for the synthesis of nanocrystalline BaTiO3, but we systematically explore the effect of the reaction temperature (from 80 to 220 °C) and Ba/Ti precursor ratio (from 4 : 1 to 1 : 1), and we characterise the reaction product to identify the optimal reaction conditions in terms of small particle size and low concentration of defects.There are multiple synthesis methods used in industry to produce BaTiO2.2.1.3 samples were synthesized via a hydrothermal method,12 as illustrated in 3)2)4  was added dropwise to 15 mL of ice cold distilled water while being constantly stirred. Upon addition of the Ti(OCH(CH3)2)4, the distilled water became turbid then a white precipitate formed. The reaction mixture was stirred in ice bath for 1 h then at room temperature for a further 2 h to allow the reaction to reach completion. To make up the Ba precursor, an appropriate amount of Ba(OH)2·8H2O  was dissolved in 55 mL of distilled water. The exact concentration of each precursor used is specific to the reaction parameter being investigated and is specified in the results and discussion section. The Ba precursor solution was added to the Ti precursor suspension to make up the reaction mixture. The reaction mixture was then transferred to an autoclave where it was sealed and heated. Each reaction mixture was heated at a specific temperature in an oven for 16 h. The white solid that formed in the reaction was collected and washed in a centrifuge at 4500 rpm for 8 min. The solid was washed with diluted HCl once, then with distilled water 3 times. After that, the solid was dried at 80 °C for 12 h in a vacuum oven to obtain BaTiO3 samples.The BaTiO2.2.13 The crystallite sizes were calculated from the Lorentzian broadening components collected from the Rietveld fits to the XRD patterns, according to  where p is the crystallite size (unit: nm), K is the Scherrer constant, λ is the wavelength of the incident X-rays (unit: nm), and Lx is the refined Lorentzian value.13 Scanning electron microscopy (SEM) micrographs were collected on a JEOL JSM-6500F operated at 10 kV, and images were analysed using the ImageJ software and Gaussian function. Transmission electron microscopy (TEM) used a Hitachi HT7700 (120 kV). Raman spectra were recorded using a Renishaw inVia confocal microscope, and the spectra were analysed using the WiRE 4.1 software. Dynamic light scattering (DLS) measurements were carried out with a Malvern Zetasizer Nano ZS, and three sequential measurements were performed for each sample. Wavelength dispersive X-ray analysis (WDS) used a Thermofisher Magnaray detector with Noran System 7 processing. Thermogravimetric analysis (TGA) was carried out using a NETZSCH TG 209 F1 Libra instrument, with a heating process of at 1 °C min−1 from 25 to 200 °C, then at 10 °C min−1 from 200 to 1000 °C under Ar atmosphere.X-ray diffraction (XRD) spectra were collected using a Bruker D2 Phaser with Cu-Kα radiation. Rietveld analysis was performed using the GSAS package.3.3.1.3. Using a constant Ti precursor concentration of 0.045 mol L−1 and the Ba precursor concentration was varied from 0.045 to 0.18 mol L−1, thus Ba/Ti precursor ratios of 1 : 1, 2 : 1 and 4 : 1 have been achieved. These three reaction mixtures were heated at various temperatures of 80, 120, 150, 180 and 220 °C to obtain different BaTiO3 samples.3 samples 2)4] with higher reactivity is used as Ti precursor. This precipitates as amorphous TiO2 , tetragonality (c/a), Lorentzian values (Lx and Ly) and reliability factors (Rwp and Rp) are listed in Table S1.†3versus the reaction temperature in hydrothermal synthesis with Ba/Ti precursor ratios of 4 : 1, 2 : 1 and 1 : 1. In all three Ba/Ti precursor ratios, there is a general decrease in crystallite size with decreasing synthesis temperature. Using a 4 : 1 Ba/Ti precursor ratio, the crystallite size of the obtained BaTiO3 reduces from 355 nm at 220 °C to 128 nm at 80 °C. At a 2 : 1 ratio the crystallite size falls from 348 nm at 220 °C to 107 nm at 120 °C. The 1 : 1 ratio shows a crystallite size that drops from 371 nm at 220 °C to 334 nm at 180 °C. The synthesis temperature has been found to have a profound effect on the crystallite size of the BaTiO3 samples with a lower synthesis temperature producing crystallites of a smaller size. The strong growth of BaTiO3 crystallites to their final size is typical of an Ostwald ripening process which involves the growth of large particles at the expense of smaller particles. An accelerated Ostwald ripening process with increasing temperature is likely to be the cause of the increase in crystallite size.15–17 BaTiO3 with crystallites as small as 107 nm have been produced using a 2 : 1 Ba/Ti precursor ratio and reacted at 120 °C.From the Lorentzian values collected from the Rietveld fits Fig. S2 to the Xc/a) has been used to denote the degree of tetragonal–cubic distortion,18 and BaTiO3 samples with large tetragonality are essential to obtain multilayer capacitors with large capacitances.123 against the reaction temperature in hydrothermal synthesis with Ba/Ti precursor ratios of 4 : 1, 2 : 1 and 1 : 1. The increasing c/a value suggests that the tetragonality of the crystal structure increases with the increasing synthesis temperature in all three Ba/Ti precursor ratios. In general, the tetragonality of the BaTiO3 is reduced by the presence of hydroxyl defects in the product.19 Therefore, the BaTiO3 samples synthesised at higher temperatures are considered to contain less hydroxyl defects. This can be confirmed by the TGA characterisation presented below.The tetragonality , E (TO) at ∼180 cm−1, A1 (TO) at ∼270 cm−1, B1, E (LO + TO) at ∼307 cm−1, A1 (TO), E (TO) at ∼515 cm−1 and A1 (LO), E (LO) at ∼720 cm−1.18 It has been reported that the sharp peak at ∼307 cm−1 . With WDS they were successfully resolved, however some degree of overlap remained. To determine the areas under the two separate peaks, they were fitted using Origin software 2)4] as the Ti precursor to precipitate amorphous TiO2 with higher reactivity in hydrothermal synthesis to avoid any unreacted TiO2 impurities.14,15As before for the previous set of conditions, the XRD patterns of the obtained BaTiO samples  indicatea and c), tetragonality (c/a), Lorentzian values (Lx and Ly) and reliability factors (Rwp and Rp) are listed in Table S2.†3versus the reaction temperature in hydrothermal synthesis with Ba/Ti precursor ratios of 4 : 1, 2 : 1 and 1 : 1. Similar to the trend observed in 3 reduces from 394 nm at 220 °C to 156 nm at 120 °C. At a 2 : 1 ratio the crystallite size falls from 386 nm at 220 °C to 112 nm at 120 °C. The 1 : 1 ratio shows a crystallite size that drops from 483 nm at 220 °C to 380 nm at 150 °C. The synthesis temperature has been found to have a profound effect on the crystallite size of the BaTiO3 samples with a lower synthesis temperature producing crystallites of a smaller size, in agreement with previous work.15–17 The BaTiO3 samples synthesised using Ba/Ti precursor ratios of 4 : 1 and 2 : 1 exhibit crystallites of smaller sizes, comparing to that using a Ba/Ti precursor ratio of 1 : 1, as a low Ba precursor concentration would result in a smaller amount of BaTiO3 nuclei generated in the initial stage of the reaction and a larger crystallite size, again in agreement with previous work.25From the Lorentzian values collected from the Rietveld fits Fig. S4 to the X3 against the reaction temperature in hydrothermal synthesis with Ba/Ti precursor ratios of 4 : 1, 2 : 1 and 1 : 1 is presented in a value indicates that the tetragonality of the crystal structure increases with the increasing synthesis temperature in all three Ba/Ti precursor ratios.The tetragonality variation of BaTiO3 samples can also be confirmed by Raman characterisation , and the effects of the Ba/Ti precursor ratio and reaction temperature on the size and morphology of BaTiO3 were investigated.10,11 The same precursors have also been applied by Nahm et al. in hydrothermal synthesis to prepare nanocrystalline BaTiO3, but they only employed one reaction temperature (220 °C) and Ba/Ti precursor ratio (4 : 1).12 In this work, we systematically explored the effects of the reaction temperature  and Ba/Ti precursor ratio  in hydrothermal synthesis for preparing nanocrystalline BaTiO3, and we characterised the reaction product by XRD, Raman, SEM, TEM, DLS, TGA and WDS to identify the optimal hydrothermal synthesis conditions in terms of small particle size and low concentration of defects.Hydrothermal synthesis to produce BaTiO4.3 has been prepared via a hydrothermal synthesis. Reaction conditions including synthesis temperature and Ba/Ti precursor ratio have been controlled and adapted with the aim of producing small crystallites of BaTiO3, as well as to investigate the effects of hydrothermal synthesis conditions on the microstructure of BaTiO3. The XRD, SEM, TEM and DLS results show that the crystallite/particle size of the BaTiO3 samples increases with the increasing synthesis temperature; and the crystallite/particle size of BaTiO3 is also affected by the Ba/Ti precursor ratio. The BaTiO3 sample synthesised using a Ba/Ti precursor ratio of 2 : 1 at a reaction temperature of 120 °C exhibited homogeneous crystallites of the smallest size of 107 nm. Moreover, the Raman and XRD results indicate that the tetragonality of the crystal structure increases with the increasing synthesis temperature. In addition, the TGA results suggest that the number of defects in BaTiO3 decreases as the synthesis temperature increases; and the WDS results indicate that the BaTiO3 with less defects was obtained when using a Ba/Ti precursor ratio of 2 : 1.Nanocrystalline BaTiOThere are no conflicts to declare.RA-012-D2RA03707F-s001


Here, a MoO2+x catalyst enriched with surface Mo6+ is reported. The Mo6+ is stabilized by a native oxide layer to prevent corrosion and its speciation is identified as (MoO3)n clusters. This native layer with Mo6+ suppresses the hydrogen evolution significantly and promotes the activation of nitrogen as supported by both experimental characterization and theoretical calculation. The as‐prepared MoO2+x catalyst shows a high ammonia yield of 3.95 µg mgcat−1h−1 with a high Faradaic efficiency of 22.1% at −0.2 V versus reversible hydrogen electrode, which is much better than the MoO2 catalyst with Mo6+ etched away. The accuracy of experimental results for NRR is confirmed by various control experiments and quantitative isotope labeling.The valance of Mo is critical for FeMo cofactor in ambient ammonia synthesis. However, the valence effect of Mo has not been well studied in heterogeneous nanoparticle catalysts for electrochemical nitrogen reduction reaction (NRR) due to the dissolution of Mo as MoO 6+ is designed and stabilized on a corrosion‐resistance MoO2+x catalyst to enable the study of the Mo valence effect on electrochemical nitrogen reduction. The as‐prepared MoO2+x shows a significantly higher ammonia yield than that of the MoO2 with Mo6+ etched away, proving the effectiveness of surface Mo6+ in catalyzing nitrogen reduction.Surface Mo To break the extremely strong N≡N triple bond (46.1 kJ mol−1), energy‐intensive Haber–Bosch process at high pressures and temperatures (200 atm and 400–450 °C) is used for ammonia production. The Haber–Bosch process consumes ≈1.5% of the global annual energy and releases ≈1.9 tons of CO2 per ton of NH3 produced yearly. Electrochemical nitrogen reduction reaction (NRR) is an attractive alternative to the Haber–Bosch process. It can be carried out under ambient pressure and temperature to significantly reduce the energy consumption and CO2 emission, with the utilization of renewable electricity. However, there is lack of catalysts that can induce strong adsorption and efficient activation of N2 molecule, which limits the NH3 production rate. Meanwhile, the competitive reaction, hydrogen evolution reaction (HER), is more likely to occur because of the easy activation of proton, thus leading to poor Faradaic efficiency (FE).Ammonia 2C6H3NCH2CH2}3N]3−). In N2 conversion to NH3, the Mo center was cycled between Mo(III) and Mo (VI). The ability of Mo to adjust its valance is vital to complete the distal pathway to add six electrons and six protons to produce NH3. Many efforts, such as doping, morphological control, single atom design, and vacancy creation  were made by now to improve the NRR performance of Mo‐based catalysts.FeMo cofactor has served as the source of inspiration for ambient nitrogen reduction catalysts for decades due to its highly efficient and selective ammonia conversion capability in aqueous environment.13 Mol42− as a soluble ion that is not suitable for heterogeneous catalysis. It is known in molybdenum metal corrosion that the solid oxidative forms of molybdenum, mainly Mo(IV)O2 in alkaline conditions with co‐existence of Mo(IV)O(OH)2 and Mo(VI)O3, can occasionally form a passive layer to prevent the dissolution of MoO42−. Therefore, it is possible to investigate molybdenum with different valence states as heterogeneous catalysts in alkaline electrolytes if a self‐limiting oxide layer is formed.In aqueous heterogeneous electrocatalysis, it is important to investigate whether a solid form of molybdenum can function in a similar mechanism to catalyze the NRR. Due to the strong competition from HER, alkaline electrolytes are preferred to suppress the HER. However, the thermodynamically stable form of molybdenum in alkaline aqueous electrolyte  is MoO2+x. The direct reduction of Mo(VI)O42−, grants the MoO2 to be enriched with surface Mo6+ (denoted as MoO2+x). X‐ray photoelectron spectroscopy (XPS), open circuit voltage (OCV), and charge transfer resistance measurements all revealed the presence of surface Mo6+. Fourier‐transform infrared spectroscopy (FT‐IR) characterization determined that the existence form of Mo6+ is (MoO3)n clusters. The surface Mo6+ on MoO2 is stable in pH 13 solution while the control Mo(VI)O3 dissolves immediately. These surface Mo6+ can be etched away to form MoO2 in highly concentrated KOH solution (5 m) to allow us to investigate the valance effect of Mo on NRR. In operando voltage, both Mo6+ and Mo4+ showed reduction to Mo3+. While the lack of oxidation peak and after reaction XPS characterization indicates a possible self‐oxidation mechanism allowing Mo3+ to convert back to its original Mo6+ and Mo4+ state, therefore, shuttling between lower and higher valances. When used as an NRR catalyst, the as‐prepared MoO2+x showed a high NH3 yield of 3.95 µg mgcat.−1h−1 with a high FE of 22.1% at −0.2 V versus reversible hydrogen electrode (RHE), which is much better than the control MoO2 catalyst of 1.06 µg mgcat−1h−1 yield and 9.4% FE, indicating that the remarkable NRR activity originated from the high‐valent Mo6+ on the surface. Without surface Mo6+, protons can be transferred to the MoO2 surface readily, and HER activity is much higher 3 cluster, the activation barrier for the first hydrogen addition step is lowered. Additionally, both distal and alternative pathways are possible for ammonia synthesis, with the distal path similar to that of the molecular Mo catalyst. It is worth mentioning that the accurate detection of ammonia and the evaluation of catalytic performance remains one of the major challenges of the NRR field. Generally, ammonia production is too low to be firmly attributed to NRR rather than ammonia contamination from air, electrolyte, reduction of other nitrogen species, or catalyst itself. Strict protocols need to be followed to ensure the measurement accuracy for gaining any fundamental understanding of the catalytic processes. Our ammonia yield is assessed by tracking NH3 production continuously over 4 h, and we determined the average NH3 yield rate by the slope of the yieldNH3–time plot. This method can reduce test errors significantly compared to the single‐point average method. Moreover, besides all the control experiments, quantitative isotope labeling experiments were performed to track the nitrogen source of yielded ammonia, where purified 15N2 was used as the feed gas for electrochemical nitrogen reduction. The yield from the quantitative isotope labeling experiment is 3.91 µg mgcat.−1h−1 which matches the NH3 yield determined by the slope of the yieldNH3–time plot. This work provided solid evidence pointing to high valence Mo6+ being critical for promoting nitrogen reductionIn this work, we adopt a hydrothermal method  image shows that the MoO2+x grows and coats on the carbon cloth  was prepared by etching MoO2+x sample in 5 m KOH solution for 24 h to remove Mo (VI). X‐ray diffraction (XRD) patterns reveal that both MoO2+x and MoO2 possess a monoclinic MoO2 phase (PDF No. 65‐5787) , and the other two obviously stronger peaks at 234.7/231.7 eV are assigned to Mo6+ (3d3/2/3d5/2), revealing Mo6+ species dominated the surface of the catalyst (65.9%). While for MoO2 sample, only Mo 3p3/2 and Mo 3p5/2 peaks of Mo4+ are seen at 233.4 and 230.4 eV, indicating that there is no Mo6+ on the surface of MoO2 after high concentration KOH etch. Simultaneously, we used inductively coupled plasma mass spectrometry (ICP‐MS) to quantify the mass loss of Mo for sample subjected to the long‐term high concentration KOH etch and calculated that the ratio of Mo6+ in sample is 1.83%, which is much lower than the ratio from XPS result, meaning most of Mo6+ mainly distribute on the surface of sample rather than the inside. In parallel, the O 1s spectra  and adsorbed O, respectively, consistent with Raman results in Figure 2+x and the etching time was varied to get the signals from different depths of MoO2+x. Initially, four peaks related to Mo4+ and Mo6+ could be detected, and the intensity of Mo6+ peaks are much higher than those of Mo4+, implying the upmost surface of MoO2+x is mainly composed of Mo6+. As the Ar ion bombarding time increases, the ratio of Mo6+ declines gradually to a negligible level, while Mo4+ becomes dominant in 360 s. These results imply a descending gradient of Mo6+ from the surface to the inside, and the inside of MoO2+x is still composed of Mo4+. In comparison with MoO2, the Mo4+/3+ redox peak of MoO2+x positively shifted by 90 mV  Figure . Moreovea Figure  of MoO2+2 Figure , suggestm KOH electrolyte using an H‐shape electrolysis cell wherein the two compartments of the cell were separated by a proton exchange membrane, as illustrated in Figure 2+x as the working electrode. Before the test, the N2 gas was bubbled through a 1 m H2SO4 aqueous solution (to eliminate the possible NH3 impurity) and a 2 m KOH solution (to eliminate the possible NOx impurity) before being fed into the electrolyte. All the potentials were converted to the RHE. As shown in Figure 2+x were first collected in both N2‐saturated and Ar‐saturated 0.1 m KOH, respectively. The current density of LSV curves in N2‐saturated electrolyte is higher than that in Ar‐saturated electrolyte, indicating that the electrochemical reduction of N2 has occurred. It is worth mentioning that the HER onsite of MoO2 is much earlier than MoO2+x, indicating a stronger HER competition in the MoO2 case. These results show that the surface Mo6+ can help suppress the HER because of the higher charge transfer resistance  was conducted in inert solvent acetonitrile and inert Ar atmosphere. The CV curves . However, the oxidation part of the CV did not show a comparable peak. XPS spectra of post‐NRR MoO2+x sample , further proving the pivotal role and the optimal content of Mo6+ in MoO2+x for NRR n cluster but MoO2 does not. Then we constructed a (MoO3)3 cluster‐MoO2 (011) surface to simulate the MoO2+x  surface model was built to represent MoO2 , compared to the pure MoO2. The free energy change for this step is comparable to those in literature. Note that the effects of dynamic surface charge and explictic solvation were not considered here due to the computational cost; these factors could further lower the formation energy of *NNH. This finding is consistent with the experimental result and explains why the MoO2+x catalyst has higher NRR performance.To gain insight into the nature of the high NRR activity of MoO2 Figure . The NRR32+x enriched with surface Mo6+, which showed a high NH3 yield of 3.95 µg mgcat.−1h−1 with a high FE of 22.1% at −0.2 V versus RHE. Both experimental characterization and DFT calculations revealed that the remarkably enhanced NRR activity originated from the high‐valent Mo6+ on the surface. Moreover, various control experiments and quantitative isotope labeling experiments were performed to ensure the accuracy of experimental results for electrochemical nitrogen reduction. This work demonstrates that creating unique valence states is an efficient approach to design excellent electrocatalysts for nitrogen reductyieldNH3‐timeion.In summary, we prepared MoO4The experiments and methods are described in the Supporting Information.The authors declare no conflict of interest.C.L. and J.W. designed the project. J.W. performed the experiments. G.P. helped with UV–vis and NRR experiments. E.H. helped with TEM and SEM characterization. G.Y. helped with ICP‐MS and XRD characterization. M.W. helped with Raman characterization. J.W. and C.L. performed the experimental data analysis. Z.J. and Y.L. performed the theoretical calculation. J.W. and C.L. wrote the paper. All authors discussed the results and commented on the manuscript. C.L., X.D., and Y.L. supervised the project.Supporting InformationClick here for additional data file.


In particular, the tetracyclic structure formed with IMes displays zwitterionic character and absorption in the visible region. In contrast to the carbene IMes, the heavier carbenoids :Si(NDippCH)2 and :Ga(AmIm) with a two‐coordinate donor atom afford spiro‐type bicyclic compounds, which display four‐coordinate geometry at silicon or gallium. (TD‐)DFT calculations provide deeper insight into the mechanism of formation and the absorption properties of these new compounds.Donor‐acceptor cyclopropanes or cyclobutanes are dipolar reagents, which are widely used in the synthesis of complex organic (hetero)cycles in ring expansion reactions. Applying this concept to boron containing heterocycles, the four‐membered borete  A four‐membered naphthalene‐supported borete is investigated as a masked 1,4‐dipolar donor‐acceptor cyclobutane. Ring expansion reactions confirm such type of reactivity and afford complex cyclic structures including fused and spiro‐type polycyclics. Among many other methods, ring opening reactions of three‐ or four‐membered (hetero)cycles have been widely exploited in the past.1 or boretate 2.peri‐substitution pattern in the naphthalene scaffold essentially stabilises the four‐membered ring architecture with a bridged single boron centre. Interestingly, besides 1 and 2 only a few other examples of boretanes or boretes have been reported,1 and 2, but such behaviour is currently documented only for electrophilic reagents. Examples include the reaction gallium(III)‐chloride with 2 under ring opening towards 4. Ring expansion reactions to afford extended cycles have only been reported for one example, i. e. the reaction of 1 with boron trichloride to give the six‐membered diboracycle 3.The generation of complex cyclic structures is one of the key areas in synthetic chemistry and includes the formation of 1 based on the formal formulation of hypothetical 1' with both electrophilic behaviour at boron and nucleophilic reactivity at the carbanionic centre. With this in mind and in consideration of the fact that the four‐membered boracycles 1 and 2 were exclusively subjected to electrophiles in the past, we set out to investigate the reactivity of borete 1 with reagents, which typically display donor acceptor reactivity, for example in transition metal complexes. Representative examples include carbon monoxide, isonitriles, N‐heterocyclic carbenes, and carbenoid reagents . The availability of both donor and acceptor orbitals in these species lend themselves to be excellent reagents in ring expansion reagents involving the donor acceptor heterocycle concept.We hypothesise that inherent donor‐acceptor reactivity may be operative in 1 rapidly reacted with 2,6‐dimethylphenyl isonitrile 5, Scheme 6 may be considered as an insertion product of 5 into the B−C bond in 1 with concomitant expansion to five‐membered cyclic boraimine 6. The insertion of isonitriles into B−C bonds has occasionally been reported for cases in which the boron atom was highly Lewis acidic and π‐donor substituents were absent.1 with isonitrile 5 is a result of the significant Baeyer strain and underpins the analogy in view of the reactivity of 1 as the donor‐acceptor boracycle.While no reaction with carbon monoxide (pressure up to 10 bar) was observed, borete 6 was unambiguously confirmed by X‐ray crystallo‐graphic analysis. The nitrogen atom N1 at the diisopropylamino group and the boron centre B1 both appear in a trigonal planar geometry, and the lone pair (at N1) is found in π‐conjugation coplanar to the vacant p‐orbital (at B1). The 11B{1H} NMR spectrum of compound 6 displayed a broad singlet at 37.4 ppm  compared to the narrower signal of starting material 1 at 35.8 ppm (ω1/2=408 Hz). The 1H NMR spectrum of 6 showed broad signals at ambient temperature that indicate dynamic behaviour. Variable temperature NMR experiments  gave rise to significant line narrowing and led to the detection of two interconvertible species 6 a and 6 b, Figure 6 a and 6 b. In particular, the ROESY spectra for 6 b revealed cross peaks of the internal proton in Me2CH with the adjacent naphthalene proton. The conformer 6 b was crystallographically observed, and both forms 6 a and 6 b are distinguishable due to a hindered rotation process of the outward‐oriented iPr group around the N−C bond axis. The rotation barrier of 6 can be traced back to the steric repulsion of the methyl or hydrogen iPr substituents with the adjacent proton of the naphthalene scaffold and the second isopropyl substituent, Scheme 6 b, see Supporting Information) reveals a linear correlation. The enthalpy and entropy at standard conditions for the conversion 6 a→6 b were found to be ▵H°=(3.8±1.3) kJ⋅mol−1 and ▵S°=(3.2±1.5) J⋅K−1⋅mol−1, which indicates the energetic preference of 6 a over 6 b at low temperature.The identity of pm s, ω1/=191 Hz c7 (IMes) was found to react with borete 1 in toluene solution, Scheme 8.The N‐heterocyclic carbene 8 can be viewed as a twofold ring expansion reaction involving both the four‐membered borete 1 (C−B bond insertion) and the five‐membered carbene IMes 7 (N−C bond insertion), which concomitantly gives rise to the highly annulated (tetracyclic) ring system. Remarkably compound 8 displays a strong inherent charge separation (zwitterionic nature). The positive formal charge is located at N3 and carbon atom C1 displays trigonal planar geometry [sum of angles 358.9(3)°], while the negative counter charge resides at the four‐coordinate boron atom B1. The iminium type nature of compound 8 is in line with the short N3−C1 bond distance [1.3253(14) Å] due to π‐donation of the lone electron pair located at N3. The four‐coordinate nature of B1 is also corroborated by the narrow 11B{1H} signal at −1.2 . In accordance with the intense red colour of compound 8, the UV/VIS spectrum recorded in dichloromethane displays a strong single absorption band with maximum absorption at λexp=480 nm, Figure calc=475 nm compared to the experimental value of λmax=480 nm. The absorption band can be traced back mainly to electronic excitation from the HOMO‐1 to LUMO level, which corresponds to an S0→S2 transition.The formation of 8, a double ring expansion process in borete 1 and NHC 7 gives rise to a fused ring system with the common site C1−B1. DFT calculations suggest a stepwise mechanism (Scheme 7 to 1 to form the borete‐NHC‐adduct 8 ‐ int 1.8 ‐ ts 2) with a Gibbs free activation energy ▵Gǂ of 50.7 kJ/mol, which is considered low enough to proceed at an ambient temperature, despite being the rate‐limiting step of the entire reaction. In product 8 ‐ int 2, the four‐coordinate geometry of the carbene carbon atom is usually considered unfavourable and a second ring insertion of the boron atoms occurs . The fused ring system 8 is formed in a highly exergonic reaction (▵G°=−164.7 kJ/mol), which is mostly due to ring strain relaxation of the four‐membered borete, see 8 ‐ int 2.Ring expansion reactions in N‐heterocyclic carbenes (NHCs) have been studied in the past, and in their event the insertion of atomic moieties into the C−N bond afforded rings expanded by one vertex atom.7, 8 Im Scheme  with the1 with IMes 7 as a representative member of N‐heterocyclic carbenes stimulated our curiosity to investigate heavier analogues of carbenoids. In contrast to 7, the reaction of borete 1 with N‐heterocyclic silylene 9 or N‐heterocylic gallylene 1111B{1H} NMR spectrum displays broad signals at 44.5  or 53.5 , both downfield shifted and broader compared to starting material 1. This observation is in line with the lower symmetry and the formation of a B−Si or a B−Ga bond, respectively. Compounds 10 and 12 are formed as insertion products of the two‐coordinate silicon or gallium atom into the C−B bond with concomitant ring expansion of borete 1 and shift to a four‐coordinate silicon or gallium centre, respectively. Reported insertions of N‐heterocyclic silylenes into B−X bonds are rare and usually occur only for highly electronegative substituents X.6F5 structural moiety.10 is found facile for the more electropositive naphthalene platform and ring strain relaxation of 1 is again considered to be a significant driving force. On the contrary, the insertion of N‐heterocyclic gallylenes into B−X bonds has never been observed, and 12 provides the first example. In contrast to related 8, the heavier silicon and gallium carbenoids lead to 10 and 12 adopting a spiro‐cyclic structure with the connecting metalloid between two five‐membered rings. In particular, the two angles at silicon or gallium, which enclose the five‐membered cycles, are close to or significantly lower than 90°. In particular, this holds for 12, in which the angle values of 82.98(8)° for N5‐Ga1‐N2 and 88.44(9)° for C7‐Ga1‐B1 are found. DFT calculations support that the higher temperature required for these reactions is a consequence of the relatively high free enthalpy of the borete‐carbenoid adducts with respect to the separated species; see species 10‐int 1 and 12‐int 1 in the Supporting Information.The observed reactivity of spiro‐cyclic structures 10 and 12 in analogy to the formation of compound 8 is thermodynamically unfavourable. The calculated values of compounds 10‐hypothetical (Si) and 12‐hypothetical (Ga) reveal significantly higher free enthalpy (∼120–160 kJ/mol) than for 10 and 12, respectively, see Supporting Information. This finding is also in line with the lack of stability for the three‐coordinate metalloid centre. While compound 8 is well stabilised by the cationic iminium moiety with C1=N3 double bond motive such a scenario would be unlikely for the hypothetical metalloid centres of silicon and gallium, for which π‐bonding is unfavourable.Further conversion of the 1 was hypothesised to undergo ring expansion reactions within the context of donor‐acceptor functionalised cyclobutanes. Reactions of 1 with representative carbon, silicon, and gallium donor ligands confirmed the assumed behaviour and gave rise to complex cyclic systems, including new tetracyclic (8) and spirocyclic  compounds. The Baeyer strain of borete 1 was identified to have a significant contribution to the driving force of the ring expansion reactions, and we propose the donor‐acceptor concept of boracycles to be considered for the construction of complex boron architectures to a broader community.The four‐membered borete 6), 2145546 (for 8), 2145547 (for 10), 2145548 (for 12)2145545  should be addressed to the authors.Supporting InformationClick here for additional data file.