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 Beeke-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-mu-oxo bridged dimers [(MnMnIV)-Mn-III(mu-O)(2)(H2O)(2)(terpy)(2)](3+) (terpy = 2,2':6,2-terpyridine) and [(MnMnIV)-Mn-III(mu-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. (c) 2006 Elsevier Inc. All rights reserved.