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

A series of O,N-chelated vanadium(IV) oxo bis(phenolate) complexes ( 1a − i ) have been prepared from [VOCl 2 (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 [VOCl 2 (THF) 2 ] with 1 equiv of HOC 6 H 2 (CH 2 NMe 2 ) 2 -2,6-Me-4 in the absence of base. This yielded [VOCl 2 (OC 6 H 2 (CH 2 NMe 2 )-2-Me-4-(CH 2 NHMe 2 )-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 σ constants of the substituents on the monoanionic aryloxy ring. The oxidation potential shows a large dependence (d E ox /dσ = 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), Å 3, Z = 8; 2, monoclinic, C 2/ c, a = 17.9977(15) Å, b = 15.7445(9) Å, c = 14.4986(6) Å, β = 113.206(5)°, V = 3776.0(4) Å 3, Z = 4.