Synthesis and characterisation of new homoleptic rhenium thiosemicarbazone complexes

A series of new rhenium(III) complexes, [ReL2]+, where LH = a 2-formylpyridine thiosemicarbazone have been synthesised and characterised. The complexes have been synthesised from the Re(V) starting material [ReOCl3 (PPh3)2] and from [ReO4]– in the presence of triphenylphosphine. All of the new compounds have been characterised by X-ray crystallography, NMR and mass spectroscopy. In all cases the Re atom is in a distorted octahedral environment with two tridentate deprotonated thiosemicarbazones binding as monoanionic ligands through the sulfur, pyridyl nitrogen and azamethinic nitrogen in a mer(azomethine nitrogen atoms trans) configuration. Electrochemical measurements show that the complexes undergo two reversible reductions at biologically accessible potentials. Under certain conditions the 2-formylpyridine thiosemicarbazone ligands undergo reductive cleavage of the hydrazinic N–N bond resulting in the formation of a rhenium complex of methyl(2-pyridyl)methyleneimine which has been characterised by X-ray crystallography.