Transition from a Nonbonding to a Bonding Interaction in a Tetranuclear [Mo₂]₂(μ-OR)₄ Cluster

Tetranuclear Mo4 clusters with two quadruply bonded Mo24+ units, [Mo2(cis-DAniF)2] (DAniF = N,N‘-di-p-anisylformamidinate), linked by alkoxides (OCH3 for 1 and OC2H5 for 4) have been prepared. The nonbonding separation between the midpoints of the quadruply bonded units, ca. 3.24 Å, is the shortest among compounds having two linked Mo24+ units. Electrochemical measurements show two redox waves for each compound with large ΔE1/2 values (554 and 587 mV for 1 and 4, respectively) that correspond to KC values on the order of 109. The large electronic communication is attributed to the short separation between dinuclear units that favor direct δ-to-δ orbital interactions between the two dimetal centers. Compound 1 was chemically oxidized using stoichiometric amounts of ferrocenium salts to a one-electron oxidation product 2 (in which the counteranion is PF6-) and a two-electron oxidation product 3 (which contains two BF4- anions). Upon oxidation there are significant decreases in the distance between the two [Mo2] units to 3.100 Å and then to 2.945 Å. The mixed-valence species 2 shows two broad absorption bands at 5900 and 7900 cm-1 in the NIR region which are assigned to the HOMO-1 → SOMO and HOMO-2 → SOMO transitions. Compound 3 is fluxional in solution, as shown by variable-temperature 1H NMR spectra. The sharp signals in the NMR spectrum at −50 °C and the lack of an EPR signal suggest that this species is diamagnetic and that a four-center, two-electron bond is formed in the cyclometallic Mo4 cluster. To a first-order approximation, an average bond order of 0.25 is assigned to the bonding interaction between the two Mo atoms along the long edges of the rectangle defined by the four Mo atoms.