Relative energies of dioxo mu-oxo molybdenum complexes from various fragmentation strategies

A thiocyanatomolybdenum(VI) dioxo micro-oxo complex dimer bearing 4,4'-di-tBu-2,2'-bipyridine ligands has shown exceptional oxidizing ability. It exists as a meso form (symmetrical) and a d,l pair (unsymmetrical) in the crystal unit cell and also in equilibrium in solution. Which oxygen atom and which configuration are predominantly involved in the process of oxygen atom transfer is a question whose answer would certainly help experimentalists. From ab initio theoretical calculations we analyzed the electronic differences between the two configurations. The large number of atoms (101) restricts the choice of theoretical methods. We give results for second-order Møller-Plesset (MP2) and DFT with the hybrid functional B3LYP, with and without pseudopotentials, with double-zeta basis sets plus polarization functions. Although the structures of the two types of configurations are quite different, we show they have practically the same energy. Similarly, no significant differences were found for electronic atomic populations on oxygen and surrounding atoms. To facilitate future studies on the process of oxygen atom transfer, we compare the entire molecules to smaller entities obtained by fragmentations or by so-called hybrid methods. We show that the tBu groups, and even sometimes the pyridyl rings, play only a minor role in determining the electronic structure. Concerning the energy difference between the two configurations, the MP2 results appear more consistent than the B3LYP results.