Model studies of bioinorganic molybdenum

Abstract

Oxomolybdenum(V) complexes have been prepared and used in various solvent systems for the reduction of nitrate and acetylene as models for the enzymes nitrate reductase and nitrogenase. With photolysed riboflavin as electron source molybdenum(V) and (VI) oxo-ions were found to reduce nitrate to nitrite catalytically at rates comparable to the enzymic system. A reduction mechanism showing electron transfer from flavin → Mo → NO3- and involving monomeric oxoMo(V) and (VI) was deduced and its relation to the enzymic reaction and other models is discussed. Complexes of Mo(V) with (amino) thiocarboxylic acid ligands were prepared and characterised by I.R. analysis. Tentative assignments of ligand and oxomolybdenum bands indicate the presence of the dioxo bridged (oxo-di-μ-oxo) dimer unit Mo2O42+. The use of riboflavin as an electron carrier with these complexes was found to enhance the rate of reduction of acetylene to ethylene, compared to those shown by other model systems for nitrogenase. Deeply coloured oxomolybdenum(V) compounds were also prepared using a series of π-conjugated Schiff base ligands, containing N, S and O donors. These compounds were characterised by I.R. and visible spectra as being of the monooxo bridged (oxo-μ-oxo) dimer type, Mo2O34+. Kinetic experiments indicated that these complexes disproportionate in organic solvents into oxoMo(IV) and oxoMo(VI) monomers and in doing so produce a species capable of rapid acetylene uptake.