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.