Spectroscopic Study of δ Electron Transfer between Two Covalently Bonded Dimolybdenum Units via a Conjugated Bridge: Adequate Complex Models to Test the Existing Theories for Electronic Coupling

Abstract

Three symmetrical and one unsymmetrical dimolybdenum dimers, namely, [Mo₂(DAniF)₃]₂(E₂CC₆H₄CE₂) (DAniF = <i>N</i>,<i>N</i>′-di­(<i>p</i>-anisyl)­formamidinate and E = O or S), are structurally and electronically closely related. The mixed-valence cation radicals display well-defined metal to ligand (ML), ligand to metal (LM), and metal to metal (MM) charge transfer absorption bands. Successive thiolations of the complexes result in steady increases of the electronic coupling between the two [Mo₂] units. The electronic coupling matrix elements (<i>H</i>_ab) calculated from the Hush model fall in the range of 600–900 cm^–1, which are remarkably consistent with the results from the CNS superexchange formalism. Spectroscopic analyses suggest that the intramolecular electron transfer occurs by electron-hopping and hole-hopping in concert. The rate constants (<i>k</i>_et) are estimated in the range of 10¹¹–10¹² s^–1 for the symmetrical analogues and 10⁷ s^–1 for the unsymmetrical species. The ultrafast electron transfer in such a weakly coupled system (<i>H</i>_ab < 1000 cm^–1) is attributed to the d­(δ)–p­(π) conjugation between the dimetal centers and the bridge