Concerning the Electronic Coupling of MoMo Quadruple Bonds Linked by 4,4‘-Azodibenzoate and Comparison with t2g6-Ru(II) Centers by 4,4‘-Azodiphenylcyanamido Ligands

From the reactions between Mo2(O2CtBu)4 and each of terephthalic acid and 4,4‘-azodibenzoic acid, the compounds [Mo2(O2CtBu)3]2(μ-O2CC6H4CO2) (1) and [Mo2(O2CtBu)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 tBuCO2. 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 t2g6-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

Studies of oxalate-bridged MM quadruple bonds and their radical cations (M = Mo or W): on the matter of linkage isomers

Electronic structure calculations employing density functional theory (DFT) and time-dependent density functional theory (TD-DFT) have been carried out on the model complexes {[(HCO2)3M2]2(μ-O2CCO2)}0/+ (M = Mo or W) in D2h symmetry, where the oxalate bridge forms either five- or six-membered rings with the M2 centres; the complexes are hereafter referred to as μ(5,5)0/+ and μ(6,6)0/+, respectively. The calculations predict that the neutral complexes should exist as the μ(5,5) linkage isomer, while the radical cations favour the μ(6,6) isomer by ca. 4–6 kJ mol−1. For the μ(5,5) isomers, the rotational barriers about the oxalate C–C bond have been calculated to be 15.9 and 27.2 kJ mol−1 for M = Mo and W, respectively. For the cationic μ(5,5)+ isomers the barrier is higher, being 36.8 and 50.6 kJ mol−1 for M = Mo and W, respectively. The calculated Raman and visible near-IR spectra for the μ(5,5)0/+ and μ(6,6)0/+ are compared with experimental data obtained for the {[(tBuCO2)3M2]2(μ-O2CCO2)}0/+ complexes, hereafter referred to as M4OXA0/+ (M = Mo or W). The experimental data more closely correlate with that calculated for the μ(5,5)0/+ linkage isomers, and the 13C-NMR spectrum of the mixed metal complex Mo2W2OXA indicates the presence of the 5-membered oxalate-bridged species (JCC = 100 Hz)