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

The reaction of cis ‐[RuCl 2 (dmso) 4 ], N ‐(acetyl)‐ N′ ‐(salicylidene)hydrazine (H 2 acs), and KOH (1:2:2 molar ratio) in methanol under aerobic conditions produces a ruthenium( III ) complex, [Ru(acs)(Hacs)]·H 2 O. Addition of one mol equivalent of HClO 4 to this complex in methanol affords [Ru(Hacs) 2 ]ClO 4 . On the other hand, reaction of one mol equivalent of KOH with [Ru(acs)(Hacs)]·H 2 O 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)]·H 2 O, 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 μ B . 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)(Hacs)]·H 2 O 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)]·H 2 O, the amide functionality of one of the ligands is protonated, and in [PPh 4 ][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 Ru III ⇄ Ru IV 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 p K a values of the coordinated amide functionalities have been determined by spectrophotometric titration. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)