Characterization of synthetic oxomanganese complexes and the inorganic core of the O 2-evolving complex in photosystem II: Evaluation of the DFT/B3LYP level of theory

Abstract

The capabilities and limitations of the Becke-3–Lee–Yang–Parr (B3LYP) hybrid density functional are investigated as applied to studies of mixed-valent multinuclear oxomanganese complexes. Benchmark calculations involve the analysis of structural, electronic and magnetic properties of di-, tri- and tetra-nuclear Mn complexes, previously characterized both chemically and spectroscopically, including the di-μ-oxo bridged dimers [Mn IIIMn IV(μ-O) 2(H 2O) 2(terpy) 2] 3+ (terpy = 2,2′:6,2″-terpyridine) and [Mn IIIMn IV(μ-O) 2(phen) 4] 3+ (phen = 1,10-phenanthroline), the Mn trimer [Mn 3O 4(bpy) 4(H 2O) 2] 4+ (bpy = 2,2′-bipyridine), and the tetramer [Mn 4O 4L 6] + with L = Ph 2 PO 2 - . Furthermore, the density functional theory (DFT) B3LYP level is applied to analyze the hydrated Mn 3O 4CaMn cluster completely ligated by water, OH −, Cl −, carboxylate and imidazole ligands, analogous to the ‘3+1 Mn tetramer’ of the oxygen-evolving complex of photosystem II. It is found that DFT/B3LYP predicts structural and electronic properties of oxomanganese complexes in pre-selected spin-electronic states in very good agreement with X-ray and magnetic experimental data, even when applied in conjunction with rather modest basis sets. However, it is conjectured that the energetics of low-lying spin-states is beyond the capabilities of the DFT/B3LYP level, constituting a limitation to mechanistic studies of multinuclear oxomanganese complexes where until now the performance of DFT/B3LYP has raised little concern.