Synthesis, structure and electrochemical properties of ruthenium complexes with some tridentate O,N,O-donor ligands

Reaction of four phenolate ligands, viz. N-(2-hydroxyphenyl)salicylaldimine (H2L1),2,2'-dihydroxyazobenzene (H2L2), N-(2-carboxyphenyl)salicylaldimine (H2L3) and 2-carboxy-2'-hydroxy-5'-methylazobenzene (H2L4) (abbreviated in general as H2L, where H stands for the acidic protons), has been carried out with [Ru(trpy)Cl-3] (trpy = 2,2',2"-terpyridine). The H2L1 and H2L2 ligands form complexes of the type [Ru(trpy)(L)]. Reaction with H2L3 does not afford any ruthenium complex. The H2L4 ligand forms a complex of different type, viz., [Ru(trpy)(HL4)(CH3CN)](+), isolated as the perchlorate salt. Structure of the [Ru(trpy)(HL4)(CH3CN)]ClO4 Complex has been determined by X-ray crystallography. The HL4 ligand is bound to ruthenium as a bidentate N,O-donor and the carboxylate oxygen does not participate in coordination. All the three complexes are diamagnetic (low-spin d(6), S=0) and show intense MLCT transitions in the visible region. Cyclic voltammetric studies on these complexes show Ru(II)-Ru(III) oxidation within 0.00-0.19 V vs SCE and a Ru(III)-Ru(IV) oxidation within 1.08-1.48 V vs SCE. A one-electron reduction of the coordinated trpy ligand is. also observed in all the complexes near -1.4 V vs SCE.