Strong Ligand Stabilization Based on π-Extension in a Series of Ruthenium Terpyridine Water Oxidation Catalysts

The substitution behavior of the monodentate Cl ligand of a series of ruthenium(II) terpyridine complexes (terpyridine (tpy)=2,2':6',2''-terpyridine) has been investigated. ¹ H NMR kinetic experiments of the dissociation of the chloro ligand in D₂ O for the complexes [Ru(tpy)(bpy)Cl]Cl (1, bpy=2,2'-bipyridine) and [Ru(tpy)(dppz)Cl]Cl (2, dppz=dipyrido[3,2-a:2',3'-c]phenazine) as well as the binuclear complex [Ru(bpy)₂ (tpphz)Ru(tpy)Cl]Cl₃ (3 b, tpphz=tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine) were conducted, showing increased stability of the chloride ligand for compounds 2 and 3 due to the extended π-system. Compounds 1-5 (4=[Ru(tbbpy)₂ (tpphz)Ru(tpy)Cl](PF₆ )₃ , 5=[Ru(bpy)₂ (tpphz)Ru(tpy)(C₃ H₈ OS)/(H₂ O)](PF₆ )₃ , tbbpy=4,4'-di-tert-butyl-2,2'-bipyridine) are tested for their ability to run water oxidation catalysis (WOC) using cerium(IV) as sacrificial oxidant. The WOC experiments suggest that the stability of monodentate (chloride) ligand strongly correlates to catalytic performance, which follows the trend 1>2>5≥3>4. This is also substantiated by quantum chemical calculations, which indicate a stronger binding for the chloride ligand based on the extended π-systems in compounds 2 and 3. Additionally, a theoretical model of the mechanism of the oxygen evolution of compounds 1 and 2 is presented; this suggests no differences in the elementary steps of the catalytic cycle within the bpy to the dppz complex, thus suggesting that differences in the catalytic performance are indeed based on ligand stability. Due to the presence of a photosensitizer and a catalytic unit, binuclear complexes 3 and 4 were tested for photocatalytic water oxidation. The bridging ligand architecture, however, inhibits the effective electron-transfer cascade that would allow photocatalysis to run efficiently. The findings of this study can elucidate critical factors in catalyst design.