Characterization of synthetic oxomanganese complexes and the inorganic core of the O2-evolving complex in photosystem II: Evaluation of the DFT/B3LYP level of theory

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 [MnIIIMnIV(μ-O)2(H2O)2(terpy)2]3+ (terpy = 2,2′:6,2″-terpyridine) and [MnIIIMnIV(μ-O)2(phen)4]3+ (phen = 1,10-phenanthroline), the Mn trimer [Mn3O4(bpy)4(H2O)2]4+ (bpy = 2,2′-bipyridine), and the tetramer [Mn4O4L6]+ with L=Ph2PO2-. Furthermore, the density functional theory (DFT) B3LYP level is applied to analyze the hydrated Mn3O4CaMn 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.