Vanadium(II) Salts in Pyridine and Acetonitrile Solvents

Article abstract of DOI:10.1021/ic00127a013

Earlier work on the interaction of V(II) salts with pyridine and acetonitrile as solvents has been extended. The structure of the compound VPy4(O3SCF3)2, 1, an intermediate in many of the preparations, has been determined by X-ray diffraction. Analogous solids with I(1-), Br(1-), Cl(1-), SCN(1-), N3(1-), PhS(1-), EtS(1-), and BH4(1-) as counterions have been prepared. Attempts to prepare solids with PhO(1-), HO(1-), MeO(1-), Ph(1-), CN(1-), AlH4(1-), and H(1-) failed, but the tetrapyridine complexes were prepared by titrating a solution of 1 in pyridine with the lithium salt of each anion (in the case of H(1-), the anion was Et3BH(1-)) to an end point observed at the 1:2 ratio. Comparisons in a number of casesof the absorption spectra and of the cyclovoltammetric behavior of the tetrapyridine salts in pyridine and in CH2Cl2 show the species in the two solvents to be the same and to correspond to the composition of the solids. Intercomparisons of the absorption spectra in pyridine suggest that the complexes with the aforementioned anions all have similar structures: four pyridine molecules situated equatorially - on the basis of crystal structure determinations, in a propeller arrangement - and the anions occupying axial positions. The initial absorption spectrum of a solution of VPy4(PF6)2 in pyridine changes to that shown by a solution of VPy6(PF6)2 or of VPy6(BPh4)2, and we conclude that with these weakly nucleophilic anions VPy6(2+) is the dominant form of V(II) in pyridine solution. In every case, the prominent feature of the absorption spectrum is a band envelope consisting of a maximum that ranges from 586 to 400 nm accompanied by a shoulder which usually lies on the low-energy side of the dominant peak and which in these cases we assign to νCT and ν1'', respectively. The latter is the higher energy component of the two transitions ν1' and ν1'', which arise from ν1 (octahedral) when the symmetry is reduced to axial. Energy correlations suggest that νCT always lies at higher energy than ν1'', so that when the shoulder lies on the high-energy side of the dominant peak, ε(ν1'') exceeds ε(νCT). In a number of cases ν1' is observed. It is of much lower intensity than ν1'', and the wavelengths of the transitions lie in the range of 600 nm and higher and vary with the nature of ligandsas observed in other cases of axial symmetry. The absorption characteristics of VPy6(2+) resemble those of the tetrapyridine complexes, suggesting that the propeller-like arrangement is retained in this species, twopyridines replacing the anions in the axial positions; i.e., the axial and equatorial pyridines are inequivalent.