Vanadium Diaminebis(phenolate) Complexes: Syntheses, Structures, and Reactivity in Sulfoxidation Catalysis

Vanadium diaminebis(phenolate) complexes of the general formulas [LVCI(THF)] (L = Me2NCH2CH(R)N(CH2-2-O-3,5-(C6H2Bu2)-Bu-t)(2), where R = H, Me) and [LV(O)X] [X=Cl; R=H (2), Me (3), (OPr)-Pr-i (4), (mu-O)V(O)L (5)] are described. All compounds display octahedral geometry and trans-Oph coordination. For compounds 2, 3, and 5, only one isomer, presenting the V=O ligand trans to the tripodal nitrogen, was formed, while for 4, two isomers were observed by NMR in solution. The UV-vis and circular dichroism spectra of 2 and 3 display very intense charge-transfer transition bands from the phenolate donors to the vanadium, which are in agreement with the V-51 low-field shifts observed. All vanadium(V) complexes were tested as thioanisole sulfoxidation catalysts, revealing very high selectivity when H2O2 was used as the oxidant. However, no enantioselectivity was observed even when enantiopure 3 was used as the catalyst precursor. H-1 and V-51 NMR studies were conducted for the reactions of 2 with aqueous solutions of H2O2 in methanol-d(4) and in acetonitrile-d(3); 2 reacts with the solvents, leading to [LV(O)OMe], by replacement of Cl by MeO in methanol, and to a new vanadium aminebis(phenolate) complex, where the dimethylamine fragment of the original ligand L was replaced by CH3CN. In either case, V-51 NMR spectra suggest the formation of peroxovanadium species upon the addition of a H2O2 solution. The subsequent addition of thioanisole to the methanol-d(4) solution leads to regeneration of the original complex.