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

The reactions between [Mo2L2(CH3CN)(6)][BF4](2) compounds and [(Bu4N)-N-n](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 ((Bu4N)-N-n](2)[O2CCO2] is employed, they are formed to the exclusion of the square. The molecular structure of ll 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 lambda(max) similar to 460 nm assignable to metal-to-ligand charge transfer ((MLCT)-M-1) involving the oxalate bridge. Electronic structure calculations employing density functional theory on model compounds [(HCO2)(2)Mo-2(O2CCO2)](3) and [HNCHNH)(2)Mo-2(O2CCO2)](3) have been carried out and indicate the frontier occupied molecular orbitals are Mo-2 delta combinations e(4)a(2), and the lowest unoccupied are bridge pi* for the formamidinates and delta* 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 EPA 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.