Electronic Coupling and Electron Transfer between Two Dimolybdenum Units Spaced by a Biphenylene Group

Three symmetrical dimolybdenum dimers bridged by 4,4′-biphenyldicarboxylate and the partially and fully thiolated derivatives have been synthesized and studied with respect to electronic coupling and intramolecular electron transfer. As generally denoted by [Mo₂]–(ph)₂–[Mo₂], the complexes are differentiated by the [Mo₂] units but have a biphenylene spacer in common, where [Mo₂] = [Mo₂(DAniF)₃(EE′C)] with auxiliary ligands DAniF (N,N′-di­(p-anisyl)­formamidinate) and donor atoms E and E′ (O or S). The radical cations {[Mo₂]–(ph)₂–[Mo₂]}⁺, prepared by one-electron oxidation of the corresponding neutral precursor, exhibit a characteristic intervalence (IV) charge transfer absorbance in the near-IR spectra. The electronic coupling matrix elements (H_ab) calculated from the Mulliken–Hush expression vary in the range of 245–415 cm^–1 depending on the number of sulfur atoms in the [Mo₂] units. These parameters are also calculated by CNS superexchange formalism, in which only the electron-hopping pathway is taken into account because of the lack of ligand to metal charge transfer absorptions in the spectra. The results show remarkable alignment between the two different methods. Thus, the mixed-valence complexes are assigned to weakly coupled Class II in terms of Robin–Day’s classification. Under the Marcus–Hush theoretical framework, the adiabatic electron transfer rate constants (k_et) are optically determined in the range of 10⁹ – 10¹¹ s^–1. The fastest electron transfer is observed in the fully thiolated species. In comparison with the reported (ph)₁ series, a relatively small attenuation factor, ca. β­(H_ab) 0.17, is also estimated for the tetrathiolated system. Therefore, the introduction of sulfur atoms along the charge transfer axis efficiently enhances the electronic coupling and facilitates the electron transfer between the two dimetal centers.