Variable coordination modes of benzaldehyde thiosemicarbazones - synthesis, structure, and electrochemical properties of some ruthenium complexes

Abstract

Reaction of benzaldehyde thiosemicarbazones [H₂LR, where H₂ stands for the two protons, the hydrazinic proton, and the phenyl proton at the ortho position, with respect to the imine function and R (R = OCH₃, CH₃, H, Cl, and NO₂) for the para substituent] with [Ru(PPh₃)₂(CO)₂Cl₂], carried out in refluxing ethanol, afforded monomeric complexes of type [Ru(PPh₃)₂(CO)(HLR)(H)]. The crystal structure of the [Ru(PPh₃)₂(CO)(HLNO₂)(H)] complex was determined. The thiosemicarbazone ligand is coordinated to the ruthenium center as a bidentate N,S-donor ligand forming a four-membered chelate ring. When the reaction of the thiosemicarbazones with [Ru(PPh₃)₂(CO)₂Cl₂] was carried out in refluxing toluene, a family of dimeric complexes of type [Ru2(PPh₃)₂(CO)₂(LR)₂] were obtained. The crystal structure of [Ru2(PPh₃)₂(CO)₂(LCl)₂] was determined. Each thiosemicarbazone ligand is coordinated to one ruthenium atom, by dissociation of the two protons, as a dianionic tridentate C,N,S-donor ligand, and at the same time the sulfur atom is also bonded to the second ruthenium center. ¹H NMR spectra of the complexes of both types are in excellent agreement with their compositions. All the dimeric and monomeric complexes are diamagnetic (low-spin d⁶, S = 0) and show intense absorptions in the visible and ultraviolet regions. Cyclic voltammetry of the [Ru(PPh₃)₂(CO)(HLR)(H)] and [Ru₂(PPh₃)₂(CO)₂(LR)₂] complexes show the ruthenium(II)-ruthenium(III) oxidation within 0.48-0.73 V vs. SCE followed by a ruthenium(III)-ruthenium(IV) oxidation within 1.09-1.47 V vs. SCE. Potentials of both the oxidations are found to correlate linearly with the electron-withdrawing character of the substituent R.