Effects of a strong π-accepting ancillary ligand on the water oxidation activity of weakly coupled binuclear ruthenium catalysts

Abstract

Significant differences were found in the proton-coupled redox chemistry and catalytic behavior of the binuclear [{Ru(H₂O)(bpz)}₂(tpy₂ph)](PF₆)₄ complex [bpz = 2,2′-bipyrazine; tpy₂ph = 1,3-bis(4′-2,2′:6′,2′′-terpyridin-4-yl)benzene] as compared with the structurally analogous derivative with 2,2′-bipyridine (bpy) instead of bpz. The differences were assigned to the stronger π-accepting character of bpz relative to bpy as the ancillary ligand. The expectation of a positive shift for the Ru-centered redox potentials was confirmed for the lower oxidation state species, but that trend was reversed in the formation of the high-valence catalytic active species as shown by a negative shift of 0.14 V for the potential of the [Ru^IV/VO] process. Moreover, DFT calculations indicated a significant decrease of about 15% on the spin density and oxyl character of the [Ru^VO]³⁺ fragment. The significantly lower k_cat(O₂) for the bpz system was attributed to these combined electronic effects.