Strong electronic communication by direct metal-metal interaction in molecules with halide-bridged dimolybdenum pairs

Reactions of [cis-Mo2(DAniF)2(NCCH3)4](BF4)2, DAniF = N,N'-di-p-anisylformamidinate, with an excess of anhydrous Bu(n)4NX (X = Cl, Br, I), produced the halide-bridged tetranuclear clusters, [cis-Mo2(DAniF)2]2(mu-X)4, X = Cl (1), Br (2), and I (3). All three compounds show two reversible one-electron oxidation processes with potential separations (DeltaE(1/2)) between the two oxidation processes of 540, 499, and 440 mV, respectively. These DeltaE(1/2) values show that the strength of the electronic coupling between the dimetal units decreases as the Mo2...Mo2 distance increases from 1 to 2, and then to 3. The structures, EPR spectra, and near-IR (NIR) spectra of the corresponding mixed-valence species (1-PF6, 2-PF6, and 3-PF6) indicate that the clusters are electronically delocalized. Calculations at the DFT level indicate that the strong electronic communication is principally due to a direct overlap between the delta orbitals from the adjacent dimetal units.