Polynuclear oxomolybdenum(VI) complexes of dihydroxybenzoic acids: Synthesis, spectroscopic and structure characterization of a tetranuclear catecholato-type coordinated 2,3-dihydroxybenzoate and a novel tridentate salicylato-type coordinated 2,5-dihydroxybenzoate trinuclear complex

Abstract

In order to study the reaction of molybdenum(VI) with dihydroxybenzoic acids (DHBAs), compounds (PPh 4) 2 · [Mo 4O 6(μ-O) 4(μ 3-OCH 3) 2(2,3-DHBA) 2] ( 1) and (PPh 4) 2 · [Mo 3O 6(μ-O) 2(2,5-DHBA) 2] · CH 3CN ( 2) were isolated and determined by X-ray crystallography. The structure of complex 1 consists of four distorted MoO 6 octahedra with two cis- MoO 2 2 + units and two molybdenum atoms having each ligand coordinated and one terminal oxygen atom, while two triple bridged methanol molecules are present. The ligand 2,3-dihydroxybenzoic acid (2,3-DHBA) prefers the catecholate-type coordination with strong hydrogen bonding between the carboxylic hydrogen and the ortho-phenolic oxygen. The structure of complex 2 consists of three distorted MoO 6 octahedra. The 2,5-dihydroxybenzoic acid (2,5-DHBA) acts as a tridentate ligand using both carboxyl oxygen atoms, giving thus an unusual trinuclear molybdenum complex with a unique coordination mode of the ligand. The compounds were further investigated using IR, UV–Vis, ESR and NMR spectroscopy, elemental and thermogravimetric analysis, as well as cyclic voltammetry.