Electronic Coupling and Electron Transfer between Two Mo₂ Units through meta- and para-Phenylene Bridges

A series of three Mo₂ dimers bridged by a meta-phenylene group has been studied in terms of electronic coupling (EC) and electron transfer (ET) in comparison with the para isomers. Optical analyses on the mixed-valence complexes indicate that by replacing a para-phenylene bridge with a meta one, the EC between the two Mo₂ centers is dramatically weakened; consequently, the ET rates (k_et ) are lowered by two to three orders of magnitude. In the para series, the EC parameters (H_ab ) and ET rates (k_et ) are greatly affected by O/S atomic alternation of the bridging ligand. However, for the meta analogues, similar EC and ET parameters are obtained, that is, H_ab =300-400 cm^-1 and k_et ≈10⁹  s^-1 . These results suggest that through-σ-bond and/or through-space coupling channels become operative as the π conjugation is disabled. DFT calculations reveal that destructive quantum interference features seen for the meta series arise from the cancellation of two π-conjugated coupling pathways.