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kebochen061金虫 (正式写手)
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thermodynamic sink
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The title compound 1 was synthesized by the hydrothermal reaction of a mixture of concentrated H 3 PO 4 , V 2 O 5 , hydrous niobium oxide (niobic acid), and TMAOH at 130 8C for 16 h, leading to the formation of a clear, yellow solution. In our previous synthesis of V 3 Nb 12 , we found that low temperatures (110 8C) gave better yields because higher temperatures and longer reaction times lead to the formation of tancolored insoluble byproducts, thus lowering the yields.In that system, the tan precipitate was a thermodynamic sink.However, we found that by incorporating H 3 PO 4 into the reaction, these undesired tan precipitates did not form, even at elevated temperatures and much longer reaction times, for example at 150 8C for 60 h, indicating that phosphate in the reaction system stabilizes the cluster and prevents the formation of insoluble precipitate. We characterized the PV 2 Nb 12 cluster using negativemode ESI-MS, which showed a series of signals of 4,3 and 2 charged ions that match calculated peak positions for PV 2 Nb 12 adducts of protons and TMA cation (Figure 1).The peak separations are differentiated by the number of combined charge-balancing TMA and protons. The peaks of PV 2 Nb 12 were at lower m/z value than the V 3 Nb 12 because of the substitution of the central vanadium atom with a phosphorus atom, which has a lower atomic mass. ESI-MS thus allowed us to easily identify PV 2 Nb 12 in mixtures with V 3 Nb 12 and provided qualitative information about purity and redox state. As we observe that even weakly bound TMA can cause changes in chemical shifts in the 51V NMR signals of the two capping VO 5 in 1, we anticipate that the more strongly coordinating alkali cation analogue of PV 2 Nb 12 will have much greater shifted 51V peak positions if the two capping VO 5 have different populations of locally coordinated alkali cations. The pH-dependent 31P and 51V NMR spectra were collected between pH 3.8 and 13.3. The UV/Vis spectra of PV 2 Nb 12 show no significant pH variations in the stable pH region, with an increase in the absorbance at wavelengths at very low and very high pH, re-spectively, which is consistent with our conclusions that the compound is dissociating (Figure 7). Precipitation at low pH may also be associated with protonation and charge-neutral-ization of the cluster, and we observed some small changes in the UV/Vis spectra near 248 nm that might indicate protonation In the lower-energy region, two isosbestic points at about 250 and 260 nm are observed, which is consistent with pH-dependent speciation changes, such as controlled dissociation. The greater pH stability of PV 2 Nb 12 relative to V 3 Nb 12 is evident both in the NMR spectroscopy and pH variation of the UV/Vis absorbance. Another comparison can be made to the isostructural phosphovanadate ion, [PV 14 O 42 ] 9 which is stable in a narrower pH range, 1.5-pH3, than PV 2 Nb 12 . For this PV 14 O 42 molecule, the 17O,31P, and 51VNMR spectra have all been interpreted to indicate steady changes in protonation state. This effect is consistent with the well-known reduced lability of phosphate to substitution at the oxygen sites. We speculate that the phosphate reduces access to lacunary metastable states in the Keggin ion structure that form by dynamic bond-length modulation (or even breaking) being central to the reactivity and dissociation of large metal-oxide clusters. In our view, the rate of equilibration with lacunary metastable states is faster |
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