Synthesis and Characterization of O,N-Chelated Vanadium(IV) Oxo Phenolate Complexes:  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 [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 (dE ox /dσ ) 170 mV (per phenoxy ligand)) on the Hammett constant. Crystal data: 1a, orthorombic, Pbca, a ) 9.4321 (7)

O, N‐Chelated Vanadium(IV) Oxo Aminophenolate Complexes: the Effect of Steric Bulk on the Vanadium Coordination Geometry. Can this Influence be detected Spectroscopically?

“…The three structures are quite similar to the structure of the related compound [VO(OC 6 H 4 (CH 2 NMe 2 )-2] (1) [6]. The molecules are approximately C 2 symmetric and in all four compounds the geometry around the vanadium atom is best described as a distorted trigonal bipyramid with the three oxygen atoms in the equatorial plane and the two nitrogen donor atoms placed at the axial positions.…”

“…The difference in HOMO-LUMO transition, ∆(HOMO-LUMO), between 11 and 12 is 64 nm. For the vanadyl complexes with different parasubstituents, [VO(OC 6 H 3 (CH 2 NMe 2 ) 2 -R-4) 2 ] (R ϭ MeO, NϭNPh), the ∆(HOMO-LUMO) is 30 nm [6], which means that the HOMO-LUMO transition is as sensitive to electronic influences caused by inductive and/or mesomeric effects, as it is to geometrical changes. Since it is not possible to modify the steric requirements of the ligand system without changing the electronic influences, it turns out to be impossible to use the HOMO-LUMO transition for determining geometrical changes.…”

“…The anisotropic A and g values were obtained by spectral simulation [18] and the results are listed in Table 3 (for exemplary frozen state and simulated spectra and a more detailed description of the spectra, see ref. [6]). The spectra of 1, 11 and 12 are rhombic with the rhombicity increasing with THC N .…”

“…This type of ligands has been used previously in our group in aluminium [1], iron [2], zinc [3], tungsten [4], and molybdenum [5] chemistry and they proved to be versatile in the stabilization of metal atoms in various oxidation states. Earlier we have reported that substituent variation at the meta or para positions can have a significant electronic influence on the vanadium atom [6]. In the field of catalytic α-olefin polymerization, variation in the size of substituents present in the ligands can have large effects.…”

“…To study the effect of steric bulk at the three different positions on the geometry around the vanadium nucleus, various vanadium(IV) oxo (bis)-phenolates have been prepared, in which either R, RЈ, RЉ or R * has been varied. The parent compound [VO(OC 6 H 4 (CH 2 NMe 2 )-2) 2 ] (1) was used for comparison [6]. For qualitative comparison of the structures we have used the angular structural parameter τ, which is defined in Figure 2 [10].…”