Metal-Metal Quadruple Bonds (M = Mo or W) Supported by 4-[2-(4-Pyridinyl)ethenyl] Benzoates and their Complexes with Tris(pentafluorophenyl)boron

From the reactions between M-2((TPB)-P-i)(4) and two equivalents of the acids LH, the compounds trans-M-2((TPB)-P-i)(2)L-2 were isolated where M = Mo, compound I, and M = W, II, (TPB)-P-i = 2,4,6-triisopropylbenzoate, and LH = 4-[2-(4-Pyridinyl)ethenyl]benzoic acid. In related reactions when tris(pentafluorophenyl)boron, A, was added, the Lewis acid-Lewis base complexes Mo-2((TPB)-P-i)(2)(LA)(2), IB, and W-2((TPB)-P-i)(2)(LA)(2), IIB, were isolated. Compounds I and IB are red and purple, respectively, while II and IIB are green. The new compounds have been characterized by H-1 NMR, UV-visible-NIR absorption spectroscopy, and electrochemical studies, which are tied together with density functional theory, DFT, and time-dependent DFT calculations. Chemical reduction of IB and IIB yields anions where the single electron occupies a ligand-based orbital as indicated by EPR spectroscopy. The LUMO and LUMO+1 are ligand-based, and are close in energy, and upon reduction, no IVCT is observed.