Unusual coordination mode of thiosemicarbazone ligands. A search for the origin

Twelve mixed-ligand thiosemicarbazone complexes of ruthenium and osmium, ten of general formula [M(bpy)2(bztsc-R)]ClO4, (M = Ru, Os; bpy = 2,2'-bipyridine, Hbztsc-R = benzaldehyde thiosemicacbazone) and two of type [M(bpy)2(actsc)]ClO4 (Hactsc = acetonethiosemicarbazone), have been synthesized and characterized. All the complexes are diamagnetic (low-spin d6, S = 0) and in acetonitrile solution show several intense metal-to-ligand charge-transfer (MLCT) transitions in the visible region. Structures of Hbztsc-OMe, [Ru(bpy)2(bztsc-NO2)]ClO4 and [Ru(bpy)2(actsc)]ClO4 have been determined by X-ray crystallography. Benzaldehyde thiosemicarbazone exists in the thione form with the phenyl group trans to the hydrazinic nitrogen. The benzaldehyde thiosemicarbazone ligand coordinates to the metals through the hydrazinic nitrogen and sulfur with a bite angle of approximately 67 degrees, forming a four-membered chelate ring. However, the actsc ligand coordinates through the imine nitrogen and sulfur, forming a five-membered chelate ring with a bite angle of approximately 81 degrees. The difference in coordination modes of two types of thiosemicarbazone ligands, viz., bztsc-R and actsc, appears to result from the difference in steric bulk of the aryl and methyl group trans to the hydrazinic nitrogen. In acetronitrile solution they all show a reversible metal(II)-metal(III) oxidation in the range 0.18-0.58 V vs SCE followed by an irreversible oxidation in the range 1.11-1.60 V vs SCE. Two successive one-electron reductions of the coordinated bipyridine are also observed in the range -1.53 to -1.96 V vs SCE.