Dimolybdenum dimers spaced by phenylene groups: The experimental models for study of electronic coupling

•The advantages of δ bonded Mo2 units for electron transfer study are illustrated.•Electronic coupling is evaluated by Hush model and CNS superexchange formalism.•Transition from Class II to Class II/III borderline for the MV systems is observed.•Both electron- and hole-hopping pathways are confirmed optically.•Kinetic parameters for electron transfer reactions are quantitatively determined.

This account reviews our recent investigation on the electronic coupling and intramolecular electron transfer in D–B–A systems constructed by employing covalently bonded [Mo2] units as the electron donor (D) and acceptor (A) and phenylene groups as the bridges (B). In these complexes, the δ electrons are transferring from the donor to the acceptor and thus, exclusively responsible for the electrochemical and optical behaviors. The unique electronic structures for the donor and acceptor sites as well as the D–B–A molecules make them excellent experimental models for validating and refining the existing electronic coupling and electron transfer theories. By designed syntheses of the complex systems, the impact of coordinating atoms and length of bridging ligands and electronic property of the ancillary ligands on the donor–acceptor interaction have been examined by means of electrochemical, spectroscopic, magnetic techniques and theoretic calculations. The widely accepted Hush vibronic model and the latest CNS superexchange formalism have been employed to determine the electronic coupling matrix elements (H), which yield remarkably consistent results. By modulation of the structural variables, the electronic coupling interaction in the mixed-valence complexes varies from weakly to moderately strong coupling; the MV systems span a transition from Class II to the Class II/III borderline in terms of Robin-Day’s classification. On this basis, the intramolecular electron transfer kinetics has been studied under the Marcus–Hush semi-classical framework. The adiabatic electron transfer rate constants are determined in the range of 108–1012 s−1, depending on the nature of bridge. The results also offer new mechanistic insights into electron transfer theory, as both electron-hopping and hole-hopping pathways are confirmed to be effective for certain compounds.

Graphical abstractVarious dimolybdenum dimers are synthesized to study the impacts of bridging and ancillary ligands on electronic coupling between the two Mo2 units. The coupling parameters (H) calculated from Hush model conforms with those from CNS formalism. The kinetic parameters (ΔG∗ and ket) for the electron transfer reactions are optically determined.Figure optionsDownload full-size imageDownload as PowerPoint slide