Concerning the Electronic Coupling of MoMo Quadruple Bonds Linked by 4,4‘-Azodibenzoate and Comparison with t2g 6-Ru(II) Centers by 4,4‘-Azodiphenylcyanamido Ligands

From the reactions between Mo2(O2C t Bu)4 and each of terephthalic acid and 4,4‘-azodibenzoic acid, the compounds [Mo2(O2C t Bu)3]2(μ-O2CC6H4CO2) (1) and [Mo2(O2C t Bu)3]2(μ-O2CC6H4N2C6H4CO2) (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 t BuCO2. On the basis of these studies, the two Mo2 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 t2g 6-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.