Oxalate Bridged Triangles Incorporating Mo24+Units. Electronic Structure and Bonding

The reactions between [Mo2L2(CH3CN)6][BF4]2 compounds and [Bun4N]2[O2CCO2] in CH3CN are shown to proceed under kinetic control to the formation of a mixture of molecular triangles and squares. The molecular triangles [L2Mo2(O2CCO2)]3 I (L = DPhF, PhNCHNPh) and II (L = DAniF, p-MeO-C6H4NCHNC6H4-p-OMe) are the major products, and when 0.75 equivalents of [Bun4N]2[O2CCO2] is employed, they are formed to the exclusion of the square. The molecular structure of II is reported based on a single crystal X-ray determination. The molecular triangles do not enter into an equilibrium with their molecular square counterparts in CH2Cl2, in contrast to their perfluoroterephthalate bridged counterparts. The compounds I and II are orange and have a strong electronic transition at λmax 460 nm assignable to metal-to-ligand charge transfer (1MLCT) involving the oxalate bridge. Electronic structure calculations employing density functional theory on model compounds [(HCO2)2Mo2(O2CCO2)]3 and [(HNCHNH)2Mo2(O2CCO2)]3 have been carried out and indicate the frontier occupied molecular orbitals are Mo2 δ combinations e4a2, and the lowest unoccupied are bridge π* for the formamidinates and δ* for formates as ancillary ligands. Compounds I and II show quasi-reversible oxidation waves in their cyclic voltammograms and oxidation of II in 2-methyl-THF by reaction with AgPF6 (1 equivalent) leads to a metal centered EPR signal, g 1.95. The electronic absorption spectrum shows a low-energy broad band centered at 6418 cm−1, which is assigned to an intervalence charge transfer (IVCT) band of a class III mixed valence ion.