Variable Coordination Modes of Benzaldehyde Thiosemicarbazones – Synthesis, Structure, and Electrochemical Properties of Some Ruthenium Complexes

Reaction of benzaldehyde thiosemicarbazones [H 2 LR, where H 2 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 = OCH 3 , CH 3 , H, Cl, and NO 2 ) for the para substituent] with [Ru(PPh 3 ) 2 (CO) 2 Cl 2 ], carried out in refluxing ethanol, afforded monomeric complexes of type [Ru(PPh 3 ) 2 (CO)(HLR)(H)]. The crystal structure of the [Ru(PPh 3 ) 2 (CO)(HLNO 2 )(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(PPh 3 ) 2 (CO) 2 Cl 2 ] was carried out in refluxing toluene, a family of dimeric complexes of type [Ru 2 (PPh 3 ) 2 (CO) 2 (LR) 2 ] were obtained. The crystal structure of [Ru 2 (PPh 3 ) 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. 1 H 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 d 6 , S = 0) and show intense absorptions in the visible and ultraviolet regions. Cyclic voltammetry of the [Ru(PPh 3 ) 2 (CO)(HLR)(H)] and [Ru 2 (PPh 3 ) 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)