SOLVENT EFFECTS ON THE REACTIVITY OF VANADIUM(II)

Absorption spectra of V(II), covering a range wide enough to encompass the expected three absorption bands arising from d-d transitions, were registered in nine solvents. The source of V(II) in most cases was V(DME)2(O3SCF3)2 (DME is 1,2-dimethoxyethane), but in some cases V(II) was generated by reducing V(O3SCF3)3 with Zn/Hg. Only in the case of the weakly nucleophilic solvent CH2Cl2 is there evidence that the DME introduced with the solid affects the spectrum. Striking changes are registered in passing from a solvent such as H2O to solvents, the molecules of which have pi-acid character. In the case of acetone, pyridine, and acetonitrile, strong MLCT absorption sets in, accompanied by hypsochromic shifts of those d-d transitions that are not obscured by the charge-transfer bands. Thus the long-wavelength bands upsilon-1, which measured 10Dq, for H2O, acetone, CH3CN, and pyridine appear at 843, 820, 614, and 490 nm respectively. There are, as well, marked increases in the intensities of the d-d absorptions for the last three solvents, compared to that for H2O. The reactivity of V(II) to epoxides and to O2 is a sensitive function of the nature of the solvent. In some solvents, with either kind of oxidant, [VOV]4+ is the first observable vanadium-containing product. In certain solvents, on continued oxygenation, VO2+ is formed, and there is evidence that V(V) can also be produced. In the case of epoxides, the gross features of the differences in rate can be understood in terms of the accessibility of a normal coordination position on V(II) to the epoxides. When O2 reacts, substitution in a normal coordination site is not necessarily a prerequisite to reaction.