Structural characterization of possible intermediates in vanadium-catalysed sulfide oxidation

Abstract

Reaction between 3-methoxysalicylaldehyde and (R,R)-1,2-diaminocyclohexane with VO²⁺ as a template yielded the racemic Schiff-base complex [VO(L¹)][H₂L¹=N,N′-bis(3-methoxysalicylidene)cyclohexane-1,2-diamine] which is converted into trans-[VCl₂(L¹)] with SOCl₂. The crystal structures of both have been determined. The former is a tetragonal-pyramidal complex, whereas the structure of the latter is best described as an octahedron distorted towards a trigonal prism. The complex [VO(acac)₂](acac = acetylacetonate) reacted with HN(CH₂CH₂OH)₂ in the presence of oxygen to yield [VO(acac){PhN(CH₂CH₂O)₂}], the crystal structure of which reveals a co-ordination geometry in-between tetragonal-pyramidal and octahedral (provided by a weak vanadium-to-nitrogen bonding interaction). Aeration of a reaction mixture consisting of [VO(acac)₂] and the Schiff base (H₂L²) formed between H₂NCH₂CH₂OH and o-hydroxynaphthaldehyde afforded [{VO(L²)}₂O] having a bent and slightly asymmetric oxo-bridge, and an additional weak bridging interaction between one of the V–O (ethanolate) centres and the second vanadium centre (crystal structure). In solution, the inequivalency of the two vanadium centres is offset. The reactivity of the complexes towards Me₂SO and/or PhSMe has been studied by EPR and ⁵¹V NMR spectroscopy.