Ruthenium(III) Complexes with a Phenolate-O, Imine-N, and Amide-O Coordinating Ligand: Syntheses, Structures, Properties, and Protonation Studies of Coordinated Amide

Abstract

The reaction of cis-[RuCl2(dmso)4], N-(acetyl)-N′-(salicylidene)hydrazine (H2acs), and KOH (1:2:2 molar ratio) in methanol under aerobic conditions produces a ruthenium(III) complex, [Ru(acs)(Hacs)]·H2O. Addition of one mol equivalent of HClO4 to this complex in methanol affords [Ru(Hacs) 2]ClO4, On the other hand, reaction of one mol equivalent of KOH with [Ru(acs)(Hacs)]·H2O in methanol produces K[Ru(acs)2]. All three complexes have been characterised by elemental analyses, magnetic, spectroscopic, and electrochemical techniques. In solution, except for [Ru(acs)(Hacs)]·H2O, the other two complexes are 1:1 electrolytes. Solid state magnetic moments (at 298 K) of the complexes are in the range 1.91-2.15 μB. These values reflect a spin state S = 1/2 and hence low-spin ruthenium(III) within each complex is observed. X-ray structures of [Ru(acs)]·H2O and the tetraphenylphosphonium salt of [Ru(acs)2]- have been determined. In both complexes, the ligands bind to the metal ion meridionally through the phenolate-O, the imine-N and the amide-O atoms. In [Ru(acs)(Hacs)]·H2O, the amide functionality of one of the ligands is protonated, and in [PPh4][Ru(acs)2] the amide functionalities of both ligands are deprotonated. Electronic spectra of the complexes display ligand-to-metal charge-transfer bands in the range 626-699 nm. Cyclic voltammetry reveals a RuIII → RuIV oxidation change in the potential range of 0.56-0.84 V (vs. SCE) for these complexes. The charge-transfer band positions and the oxidation potentials are significantly influenced by the protonation state of the O-coordinating amide functionality present in each ligand. The pKa values of the coordinated amide functionalities have been determined by spectrophotometric titration. © Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.