A study of asymmetrical mixed-valent Mo-Mo complexes in the class III regime.

Three novel asymmetrical dimolybdenum dimers, Mo(DAniF) (DAniF = N,N'-di(p-anisyl)formamidinate) ([OO-OS]), Mo(DAniF) ([SS-OO]), and Mo(DAniF) ([SS-OS]), have been synthesized and characterized by either single-crystal X-ray crystallography or H NMR spectroscopy. The structural asymmetry for these compounds gives rise to a redox asymmetry, which enlarges the potential separation (ΔE) between the two [Mo] units. The mixed-valance (MV) species [OO-OS], [SS-OO] and [SS-OS], prepared by one-electron chemical oxidation of the neutral precursors, exhibit an intense and symmetrical intervalence charge transfer (IVCT) absorption band in the near-IR region, along with the high energy metal (δ) to ligand (π*) (ML) and ligand (π) to metal (δ) charge transfer (LMCT) absorptions. The LMCT band, which is absent in the neutral precursors, is reflective of the cationic [Mo] unit in the MV species; therefore, it is evidenced that in the MV complexes optical electron transfer from the electron donor to acceptor occurs, while the thermal process is energetically unfavorable. The C(1)-C(2) bonds (1.44-1.48 Å) that connect the two [Mo] units are significantly shorter than a C-C single bond, showing that the two Mo centers are strongly coupled. For the series, TD-DFT calculations show that the molecular orbitals have an unsymmetrical charge density distribution over the two dimolybdenum sites. For each of the complex systems, the calculated orbital energy gaps, SOMO(δ - δ)-LUMO(bridging ligand π*), HOMO-8(bridging ligand π)-SOMO(δ - δ) and SOMO(δ - δ)-HOMO-1(δ + δ), are in good agreement with the observed MLCT, LMCT and IVCT absorption band energies, respectively. The consistency in energy between the IVCT band and the SOMO(δ - δ)-HOMO-1(δ + δ) gap permits assignment of the MV complexes to Class III in the Robin-Day scheme.