Perfluoroterephthalate Bridged Complexes with M−M Quadruple Bonds:  (^tBuCO₂)₃M₂(μ-O₂CC₆F₄CO₂)M₂(O₂C^tBu)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(O₂C^tBu)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 D_2h 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 δ_s(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.