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A Family of Vanadate Esters of Monoionized and Diionized Aromatic 1,2-Diols: Synthesis, Structure, and Redox Activity

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posted on 01.08.2002, 00:00 authored by Bharat Baruah, Samir Das, Animesh Chakravorty
The concerned diols (general abbreviation, H2L) are catechol (H2L1) and its 3,5-But2 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 xsal2- 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 xsal2- 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-.