Synthesis and Characterization of O,N-Chelated Vanadium (IV) Oxo Phenolate Complexes : the Electronic Effect of meta- and para Substituents on the Vanadium Center

Abstract

A series of O,N-chelated vanadium(IV) oxo bis(phenolate) complexes (1a-i) have been prepared from [VOCl2(THF)2] and several ortho-amino-functionalized phenols in the presence of a base. The intermediates in the synthesis of these compounds are mono(phenolato)vanadate complexes, as was shown by the reaction of [VOCl2(THF)2] with 1 equiv of HOC6H2(CH2NMe2)2-2,6-Me-4 in the absence of base. This yielded [VOCl2(OC6H2(CH2NMe2)-2-Me-4-(CH2NHMe2)-6)] (2), in which the second amine function acts as an internal base, assisting in binding the formed equivalent of HCl. Complex 2 exists in the solid state as the dichlorovanadate(IV) species with the protonated amine function forming a three-centered intramolecular hydrogen bridge in which both a chloride atom and the oxygen atom of the phenolate ligand participate. EPR, UV-vis, and cyclic voltammetry analysis of the complexes with meta or para substituents (1a-g) on the aryloxy ring showed the hyperfine coupling constant, the HOMO-LUMO transition, and the oxidation potential, respectively, to be linearly related to the Hammett constants of the substituents on the monoanionic aryloxy ring. The oxidation potential shows a large dependence (dEox/d = 170 mV (per phenoxy ligand)) on the Hammett constant. Crystal data: 1a, orthorombic, Pbca, a = 9.4321(7) Å, b = 14.1919(14) Å, c = 26.5484(14) Å, V = 3553.8(5), Å3, Z = 8; 2, monoclinic, C2/c, a = 17.9977(15) Å, b = 15.7445(9) Å, c = 14.4986(6) Å, = 113.206(5), V = 3776.0(4) Å3, Z = 4.