Synthesis, Spectroscopic and Electrochemical Properties of Ruthenium (II) Polypyridyl Complexes with Dimethyl-bibenzimidazolate Ligands

A family of ruthenium (II) polypyridyl complexes, namely [Ru(bpy)(2)(DMBbimH(x))](y+) (bpy=2,2'-bipyridine, DMBbimH(2)=7,7'-dimethyl-2,2'-bibenzimidazole, 1-A: x=2, y=2; 1-B: x=1, y=1; 1-C: x=0, y=0), were synthesized and determined by H-1 NMR, UV-Vis absorption and electrochemical measurements. With the deprotonation of N-H in DMBbimH(x) ligands, the spectroscopic and electrochemical properties of complexes showed evident differences. Interestingly, the electrochemical properties of mono-deprotonated 1-B behaved very differently in dichloromethane and acetonitrile, which is due to different polarities of the solvents. In dichloromethane, mononuclear complex 1-B underwent a two. step oxidation. This is because [Ru(bpy)(2)(DMBbimH)](+) cations are bonded by hydrogen bonds to form dimers in weakly polar solvent, and proton-coupled electron transfer (PCET) exists between the [Ru(bpy)(2) (DMBbimH)](+) cations. Single crystals of [Ru(bpy)(2)(DMBbimH)]PF6 center dot 2CH(2)Cl(2) (2) were obtained from the solution of complex 1-B in dichloromethane. Crystal structure analysis shows that [Ru(bpy)(2)(DMBbimH)](+) cations are bonded by hydrogen bonds to form dimers, which is consistent with the result of the electrochemical measurements. However, hydrogen-bonded dimers couldn't exist stably in the more polar acetonitrile, therefore only one peak existing in the CV curve.