Bursten Bruce E, Chisholm Malcolm H, Clark Robin J H, Firth Steven, Hadad Christopher M, Wilson Paul J, Woodward Patrick M, Zaleski Jeffrey M
Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, USA.
J Am Chem Soc. 2002 Oct 16;124(41):12244-54. doi: 10.1021/ja020248j.
The compounds [((t)BuCO(2))(3)M(2)(mu-O(2)CC(6)F(4)CO(2))M(2)(O(2)C(t)Bu)(3)], M(4)PFT, 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 M(2) units of each molecule is 54 degrees. Electronic structure calculations employing density functional theory (Gaussian 98 and ADF2000.01, gradient corrected and time dependent) on the model compounds (HCO(2))(3)M(2)(mu-O(2)CC(6)F(4)CO(2))M(2)(O(2)CH)(3), where M = Mo or W, reveal that in the gas phase the model compounds adopt planar D(2)(h) ground-state structures wherein M(2) delta to bridge pi 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 C(6)F(4) ring to become orthogonal to the M(4) 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 M(4)PFT and of Mo(4)PFT in THF solution are dominated by bands assigned to the bridging perfluoroterephthalate (pft) group. The intensities of certain Raman bands of solid W(4)PFT are strongly enhanced on changing the excitation line from 476.5 nm (off resonance) to 676.5 nm, which is on resonance with the W(2) delta --> CO(2) (pft) pi transition at ca. 650 nm. The resonance enhanced bands are delta(s)(CO(2)) (pft) at 518 cm(-)(1) and its first overtone at 1035 cm(-)(1), consistent with the structural change to W(4)PFT expected on excitation from the ground to this pi excited state. The electronic transitions for solid Mo(4)PFT (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 Mo(4)PFT in THF solution, it is the band at 399 cm(-)(1) assigned to nu(MoMo) which is the most enhanced on approach to resonance with the electronic band at 470 nm; combination bands involving the C(6)F(4) ring-stretching mode, 8a, are also enhanced.
化合物[((t)BuCO₂)₃M₂(μ - O₂CC₆F₄CO₂)M₂(O₂C(t)Bu)₃]，即M₄PFT，其中M = Mo或W，通过粉末X射线衍射数据的模型拟合表明，在固态中具有涉及M - O分子间相互作用的无限“扭曲”结构。每个分子的M₂单元之间的二面角为54度。对模型化合物(HCO₂)₃M₂(μ - O₂CC₆F₄CO₂)M₂(O₂CH)₃（其中M = Mo或W）采用密度泛函理论（高斯98和ADF2000.01，梯度校正和含时）进行电子结构计算，结果表明在气相中模型化合物采用平面D₂ₕ基态结构，其中M₂δ到桥连π反馈键合最大化。计算预测对于这种平面结构，M = Mo时HOMO - LUMO能隙相对较小，为1.53 eV，M = W时为1.22 eV，并且当C₆F₄环的平面旋转至与M₄平面正交以消除“共轭”时，对于M = Mo，该能隙几乎翻倍至2.57 eV，对于M = W则为2.18 eV。固态M₄PFT以及THF溶液中的Mo₄PFT的拉曼光谱和共振拉曼光谱主要由归属于桥连全氟对苯二甲酸酯（pft）基团的谱带主导。固态W₄PFT的某些拉曼谱带的强度在激发线从476.5 nm（非共振）变为676.5 nm时显著增强，676.5 nm与约650 nm处的W₂δ→CO₂(pft)π跃迁共振。共振增强的谱带是518 cm⁻¹处的δ(s)(CO₂)(pft)及其在1035 cm⁻¹处的一级泛频，这与从基态激发到该π激发态时W₄PFT预期的结构变化一致。固态Mo₄PFT的电子跃迁（最低在410 nm）无法通过可用的激发线（457.9 - 676.5 nm）探测到，因此无法获得该化合物的共振拉曼光谱。对于THF溶液中的Mo₄PFT，在接近与470 nm处的电子能带共振时，归属于ν(MoMo)的399 cm⁻¹处的谱带增强最为明显；涉及C₆F₄环伸缩模式8a的组合谱带也增强。
