Rational Ligand Design for an Efficient Biomimetic Water Splitting Complex.

ABSTRACT

Being an important biomimetic model catalyst for water oxidation, the dimanganese molecular complex [H2O(terpy)MnIII(µ-O)2MnIV(terpy)OH2]3+ (complex 1, terpy = 2,2'6',2″-terpyridine) has been investigated extensively by experimentalists. By carrying out density functional theory calculations, we explore theoretically the oxygen evolution mechanisms of complex 1. On the basis of understandings of the geometric and electronic structural features of complex 1, we explore the possibility of improving its catalytic efficiency through a rational design of ligands coordinated to the manganese ions. Recognizing that the rate-determining step of oxygen evolution is the formation of an O-O bond at a high-valent manganese center, we design a new complex, [H2O(2-bpnp)MnIII(µ-O)2MnIV(2-bpnp)OH2]3+ (complex 2, 2-bpnp = 2-([2,2'-bipyridin]-6-yl)-1,8-naphthyridine). It is verified that the proton-accepting 2-bpnp ligand leads to stabilized hydrogen bonding with surrounding water molecules, and hence, the barrier height associated with O-O bond formation is substantially reduced. Moreover, despite its larger size, the 2-bpnp ligand does not cause steric hindrance for the release of molecular oxygen. Consequently, the proposed complex 2 is expected to outperform the existing complex 1 regarding catalytic efficiency. This work highlights the potential usefulness of rational design toward reaching the high efficiency of the oxygen evolution center in photosystem II.