A family of vanadate esters of monoionized and diionized aromatic 1,2-diols: synthesis, structure, and redox activity

Abstract

The concerned diols (general abbreviation, H₂L) are catechol (H₂L¹) and its 3,5-Bu^t₂ derivative (H₂L²). Esters of the type VO(xsal)(HL), 2, are obtained by reacting H₂L with VO(xsal)(H₂O) or VO(xsal)(OMe)(HOMe), where xsal^2- is the diionized salicylaldimine of glycine (x = g), l-alanine (x = a), or l-valine (x = v). The reaction of VO(acac)₂ with H₂L and the salicylaldimine (Hpsal) of 2-picolylamine has furnished VO(psal)(L), 3. In the structures of VO(gsal)(HL¹), 2a, and VO(vsal)(HL²), 2f, the HL^- ligand is O,O-chelated, the phenolic oxygen lying trans to the oxo oxygen atom. The xsal^2- coligand has a folded structure and the conformation of 2f is exclusively endo. In both 2a and 2f the phenolic oxygen atom is strongly hydrogen bonded (O···O, 2.60 Å) to a carboxylic oxygen atom of a neighboring molecule. In VO(psal)(L²)·H₂O, 3b, the diionized diol is O,O-chelated to the metal and the water molecule is hydrogen bonded to a phenoxidic oxygen atom (O···O, 2.84 Å). The C-O and C-C distances in the V(diol) fragment reveal that 2 is a pure catecholate and 3 is a catecholate-semiquinonate hybrid. In solution each ester gives rise to a single ⁵¹V NMR signal (no diastereoisomers), which generally shifts downfield with a decrease in the ester LMCT band energy. The V(V)/V(IV) and catecholate-semiquinonate reduction potentials lie near -0.75 and 0.35, and 1.10 and 0.70 V vs SCE for 2 and 3, respectively. Molecular oxygen reacts smoothly with 2 quantitatively furnishing the corresponding o-quinone, and in the presence of H₂L the reaction becomes catalytic. In contrast, type 3 esters are inert to oxygen. The initial binding of O₂ to 2 is proposed to occur via hydrogen bonding with chelated HL-.