Charge Transfer Properties of Triarylamine Integrated Dimolybdenum Dyads

Three quadruply bonded dimolybdenum complexes equipped with a triarylamine pendant, [(DAniF)₃Mo₂(μ-O₂CC₆H₄N(C₆H₄CH₃)₂] (DAniF = N,N'-di(p-anisyl)formamidinate; [OO-ph-N]), [(DAniF)₃Mo₂(μ-OSCC₆H₄N(C₆H₄CH₃)₂] ([OS-ph-N]), and [(DAniF)₃Mo₂(μ-S₂CC₆H₄N(C₆H₄CH₃)₂] ([SS-ph-N]), have been synthesized and characterized by single crystal X-ray diffraction. In electrochemical measurements, the redox couple for the organic amine group becomes irreversible, reflecting the substantially strong electronic interaction between the dimetal center and organic redox site. The potential difference for the two successive redox events, ca. ΔE_1/2(E_1/2(2)(N/N^•+) - E_1/2(1)(Mo₂^IV/V)), falls in the range of 0.5-0.8 V as estimated from the differential pulse voltammograms. For the monocation radicals [OO-ph-N]⁺, [OS-ph-N]⁺, and [SS-ph-N]⁺, obtained by chemical oxidation of the neutral precursor, a broad ligand (amine) to metal (Mo₂) charge transfer (LMCT) absorption band is observed in the near-IR region. Interestingly, analogous to the intervalence charge transfer (IVCT) bands for mixed-valence complexes, the LMCT absorption bands, which are solvent dependent, decrease in energy and bandwidth as the electronic coupling between the two redox sites increases in an order of increasing S content in the chelating group. The electronic coupling matrix elements (H_ab) are determined by optical analyses from the generalized Mulliken-Hush (GMH) theory, falling in the range of 400-800 cm^-1 in CH₂Cl₂. These results indicate that in these radical cations the charge is localized. Time-dependent DFT calculations show that the frontier molecular orbitals for these asymmetrical donor-acceptor systems have unbalanced distribution of electron density, and the LMCT bands arise from an electronic transition from the pendant ligand-based to metal-based molecular orbitals, corresponding to donor (N)-acceptor (Mo₂) charge transfer.