A New Class of Mixed-Valence Systems with Orbitally Degenerate Organic Redox Centers. Examples Based on Hexa-Rhenium Molecular Prisms

The ligand-centered mixed-valence (LCMV) properties of two supramolecular complexes are reported: triangular prisms of the form ([Re(CO)3]2X)3-μ,μ‘,μ‘ ‘-[tPyTz]2, where X is 2,2‘-bisbenzimidazolate (1) or a pair of benzylthiols (2), and tPyTz is tri-(4-pyridyl)-1,3,5-triazine. Cyclic voltammetry demonstrates that the redox-accessible bridging ligands, tPyTz, are reduced in sequential, one-electron reactions. The singly reduced prisms, which represent an unusual type of mixed-valence compound in which the tPyTz ligands themselves are the redox centers, show intense, broad intervalance transfer (IT) bands in the NIR, consistent with highly coupled MV species. Electroabsorption (Stark spectroscopy) measurements reveal small dipole moment changes associated with intervalence excitation (|Δμ12| = 0.30 ± 0.02 eÅ for 1- and 0.48 ± 0.02 eÅ for 2-), as well as noncollinear transition dipole moment (μ12) and dipole moment change vectors (ζ ≈ 45°). DFT electronic structure calculations support this unusual result, along with a through-space electronic interaction mechanism. The neutral complexes (D3h symmetry) possess doubly degenerate, but spatially distinct, LUMO and LUMO+ orbitals. The orbital degeneracy of the tPyTz ligands is lifted in the MV forms, resulting in nonsymmetrical charge redistribution within the molecules upon on optical IT.