Concerning the electronic coupling of MoMo quadruple bonds linked by 4,4′-azodibenzoate and comparison with t_2g⁶-Ru(II) centers by 4,4′-azodiphenylcyanamido ligands

From the reactions between Mo₂(O₂C ^tBu)₄ and each of terephthalic acid and 4,4′-azodibenzoic acid, the compounds [Mo₂(O₂C ^tBu)₃]₂(μ-O₂CC₆H ₄CO₂) (1) and [Mo₂(O₂C ^tBu)₃]₂(μ-O₂CC₆H ₄N₂C₆H₄CO₂) (2) have been made and characterized by spectroscopic and electrochemical methods. Their electronic structures have been examined by computations employing density functional theory on model compounds where HCO₂ substitutes for ^tBuCO₂. On the basis of these studies, the two Mo ₂ units are shown to be only weakly coupled and the mixed-valence ions 1⁺ and 2⁺ to be valence-trapped and Class II and I, respectively, on the Robin-Day classification scheme for mixed-valence compounds. These results are compared to t_2g⁶-Ru centers linked by 1,4-dicyanamidobenzene and azo-4,4′-diphenylcyanamido bridges for which the mixed-valence ions [Ru-bridge-Ru]⁵⁺ have been previously classified as fully delocalized, Class III [Crutchley et al. Inorg. Chem. 2001, 40, 1189; Inorg. Chem. 2004, 43, 1770], and on the basis of results described herein, it is proposed that the latter complex ion is more likely a mixed-valence organic radical where the bridge is oxidized and not the Ru(2+) centers.