Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, USA.
J Phys Chem A. 2013 Jul 25;117(29):5997-6006. doi: 10.1021/jp311830h. Epub 2013 Apr 9.
2-(2-Pyridyl)-4-methylthiazole carboxylic acid (PMT-H) and rhenium tricarbonyl chloride react to form the red crystalline compound fac-Re(PMT-H)(CO)3Cl, I, which is an analog of the well-known Re(bpy)(CO)3Cl molecule, where bpy is 2,2'-bipyridine. The acids PMT-H (2 equiv) and Re(PMT-H)(CO)3Cl (2 equiv) also react with Mo2(T(i)PB)4 (T(i)PB = 2,4,6-triisopropylbenzoate) in toluene to give the red compound trans-Mo2(T(i)PB)2(PMT)2, II, and the royal blue compound trans-Mo2(T(i)PB)2[(PMT)Re(CO)3Cl]2, III, respectively. The X-ray and spectroscopic characterization of I confirms its close relationship with Re(bpy)(CO)3Cl, as does the spectroscopic characterization of compounds II and III as analogs of other compounds of the form trans-M2(TiPB)2L2, where L is a π-acceptor ligand. Electronic structure calculations on model compounds II' and III', where formate ligands substitute for T(i)PB, show that the highest occupied molecular orbital (HOMO) in II is Mo2δ. When the Re(CO)3Cl unit is attached to the PMT ligand to form III, this orbital is stabilized significantly and now becomes associated with a close in energy band of Re d(6), t2g type orbitals. Oxidation of III is shown to be Mo2-based, as evident by EPR spectroscopy, and the lowest-energy electronic absorption corresponds to a Mo2δ-to-PMT π* transition. The S1 states in both II and III are metal-to-ligand charge-transfer (MLCT), and the lowest-energy triplet sate, T1 is (3)MoMoδδ*, as evidenced by its steady state emission spectral features. The excited states of compounds I (T1) and III (S1 and T1) have been investigated by time-resolved infrared spectroscopy (TRIR). The spectral features of I parallel those for Re(bpy)(CO)3Cl, with the lowest-energy T1 state corresponding to Re dπ to PMT-H π* charge transfer, producing higher-energy CO stretching vibrations relative to the ground state. For III, the CO vibrations are shifted to lower energy, consistent with charge being located on the PMT ligand, which enhances Re-to-CO backbonding. In the MoMoδδ* T1 state, however, the backbonding is reduced to the PMT ligand, and the CO stretches are at slightly higher energy relative to the ground state.
2-(2-吡啶基)-4-甲基噻唑羧酸（PMT-H）和铼三羰基氯化物反应生成红色结晶化合物 fac-Re(PMT-H)(CO)3Cl，I，它是众所周知的 Re(bpy)(CO)3Cl 分子的类似物，其中 bpy 是 2,2'-联吡啶。PMT-H（2 当量）和 Re(PMT-H)(CO)3Cl（2 当量）也与 Mo2(T(i)PB)4（T(i)PB = 2,4,6-三异丙基苯甲酸盐）在甲苯中反应，分别得到红色化合物 trans-Mo2(T(i)PB)2(PMT)2，II，和深蓝色化合物 trans-Mo2(T(i)PB)2[(PMT)Re(CO)3Cl]2，III。I 的 X 射线和光谱特征证实了它与 Re(bpy)(CO)3Cl 的密切关系，化合物 II 和 III 的光谱特征也证实了它们是其他形式的 trans-M2(TiPB)2L2 化合物的类似物，其中 L 是一个π-受体配体。对模型化合物 II'和 III'的电子结构计算，其中甲酸盐配体取代 T(i)PB，表明 II 中的最高占据分子轨道（HOMO）是 Mo2δ。当 Re(CO)3Cl 单元连接到 PMT 配体形成 III 时，这个轨道得到了显著的稳定，现在与一个能量相近的 Re d(6)，t2g 类型轨道相关联。电子顺磁共振（EPR）光谱表明，III 的氧化是基于 Mo2 的，最低能量的电子吸收对应于 Mo2δ 到 PMT π跃迁。化合物 II 和 III 中的 S1 态都是金属到配体电荷转移（MLCT），最低能量三重态 T1 是（3）MoMoδδ，这可以从其稳态发射光谱特征中得到证明。化合物 I（T1）和 III（S1 和 T1）的激发态已经通过时间分辨红外光谱（TRIR）进行了研究。I 的光谱特征与 Re(bpy)(CO)3Cl 相似，最低能量的 T1 态对应于 Re dπ 到 PMT-H π电荷转移，产生比基态更高能量的 CO 伸缩振动。对于 III，CO 振动移向较低的能量，这与电荷位于 PMT 配体上一致，这增强了 Re 到 CO 的反馈键。然而，在 MoMoδδ T1 态中，反馈键减少到 PMT 配体上，CO 伸展的能量相对于基态略高。
