Perfluoroterephthalate Bridged Complexes with M−M Quadruple Bonds:  (tBuCO2)3M2(μ-O2CC6F4CO2)M2(O2CtBu)3, where M = Mo or W. Studies of Solid-State, Molecular, and Electronic Structure and Correlations with Electronic and Raman Spectral Data

The compounds [(tBuCO2)3M2(μ-O2CC6F4CO2)M2(O2CtBu)3], M4PFT, where M = Mo or W, are shown by model fitting of the powder X-ray diffraction data to have an infinite “twisted” structure involving M···O intermolecular interactions in the solid state. The dihedral angle between the M2 units of each molecule is 54°. Electronic structure calculations employing density functional theory (Gaussian 98 and ADF2000.01, gradient corrected and time dependent) on the model compounds (HCO2)3M2(μ-O2CC6F4CO2)M2(O2CH)3, where M = Mo or W, reveal that in the gas phase the model compounds adopt planar D2h ground-state structures wherein M2 δ to bridge π* back-bonding is maximized. The calculations predict relatively small HOMO−LUMO gaps of 1.53 eV for M = Mo and 1.22 eV for M = W for this planar structure and that, when the “conjugation” is removed by rotation of the plane of the C6F4 ring to become orthogonal to the M4 plane, this energy gap is nearly doubled to 2.57 eV for M = Mo and 2.18 eV for M = W. The Raman and resonance Raman spectra of solid M4PFT and of Mo4PFT in THF solution are dominated by bands assigned to the bridging perfluoroterephthalate (pft) group. The intensities of certain Raman bands of solid W4PFT are strongly enhanced on changing the excitation line from 476.5 nm (off resonance) to 676.5 nm, which is on resonance with the W2 δ → CO2 (pft) π* transition at ca. 650 nm. The resonance enhanced bands are δs(CO2) (pft) at 518 cm-1 and its first overtone at 1035 cm-1, consistent with the structural change to W4PFT expected on excitation from the ground to this π* excited state. The electronic transitions for solid Mo4PFT (lowest at 410 nm) were not accessible with the available excitation lines (457.9−676.5 nm), and no resonance Raman spectra of this compound could be obtained. For Mo4PFT in THF solution, it is the band at 399 cm-1 assigned to ν(MoMo) which is the most enhanced on approach to resonance with the electronic band at 470 nm; combination bands involving the C6F4 ring-stretching mode, 8a, are also enhanced.