Extent of M2 δ to Ligand π-Conjugation in Neutral and Mixed Valence States of Bis(4-isonicotinate)-bis(2,4,6-triisopropylbenzoate) Dimetal Complexes (MM), Where M = Mo or W, and Their Adducts with Tris(pentafluorophenyl)boron

The reaction between W2(TiPB)4, where TiPB = 2,4,6-triisopropylbenzoate, and 2 equiv of 4-isonicotinic acid (nicH) yields the compound W2(TiPB)2(nic)2, 2, and TiPBH. Compound 2 is related to the previously reported molybdenum analog, Mo2(TiPB)2(nic)2, 1. Compounds 1 and 2 react with 2 equiv of B(C6F5)3 in THF to form the adducts M2(TiPB)2(nic-B(C6F5)3)2, 1B (M = Mo) and 2B (M = W), which have been crystallographically characterized as solvates M2(TiPB)2(nic-B(C6F5)3)2·2THF n-hexane. Compounds 1 and 2 are intensely colored due to M2 δ to π* MLCT transitions, and upon complexation with B(C5F5)3 to give 1B and 2B, these bands shift to lower energy and gain in intensity. Each compound shows two one-electron ligand-based reductions with a ΔE1/2 = 120 (1), 300 (1B), 440 (2), and 650 mV (2B). The larger ΔE1/2 values for the tungsten compounds reflect the greater orbital mixing of the metal 5d-based M2 δ and the nic π* LUMO. Reduction of solutions of 1B and 2B with (C5Me5)2Co leads to the anions 1B− and 2B−, which have been characterized spectroscopically by electron paramagnetic resonance (EPR) and UV−vis−NIR absorption. The EPR spectra of 1B− and 2B− are consistent with ligand-based (i.e., organic) radicals. The electronic spectra contain low-energy narrow charge resonance (IVCT) bands at 3800 (1B−) and 4500 cm−1 (2B−), consistent with fully delocalized mixed valence radical anions. The results are compared with electronic structure calculations and with the spectral features of the metal-centered delocalized mixed valence radical cations [(ButCO2)3M2]2-μ2-(O2C−CO2)+, to which they are remarkably similar, as well as with other organic-based mixed valence systems.以TiPB（2,4,6-三异丙基苯甲酸酯，2,4,6-triisopropylbenzoate）为配体的双核钨配合物W₂(TiPB)₄，与2当量的4-异烟酸（nicH）发生反应，生成产物W₂(TiPB)₂(nic)₂（记为化合物2）与TiPBH。化合物2与此前报道的钼类似物Mo₂(TiPB)₂(nic)₂（记为化合物1）具有结构相关性。化合物1和2均可与2当量的三(五氟苯基)硼烷（B(C₆F₅)₃）在四氢呋喃（tetrahydrofuran, THF）中反应，得到加合物M₂(TiPB)₂(nic-B(C₆F₅)₃)₂（M为Mo时对应1B，为W时对应2B）；该类加合物已通过单晶X射线衍射表征为溶剂化物形式M₂(TiPB)₂(nic-B(C₆F₅)₃)₂·2THF·n-正己烷。化合物1和2因存在M₂ δ轨道至π*配体轨道的金属到配体电荷转移（metal-to-ligand charge transfer, MLCT）跃迁而呈现浓郁色泽；当其与B(C₆F₅)₃络合生成1B和2B后，该吸收带发生红移且强度显著提升。每种配合物均展现出两个基于配体的单电子还原峰，其半波电位差ΔE₁/₂分别为120 mV（化合物1）、300 mV（1B）、440 mV（化合物2）以及650 mV（2B）。钨系配合物更大的ΔE₁/₂值，反映出金属5d轨道构成的M₂ δ轨道与异烟酸配体π*最低未占据分子轨道（LUMO）之间存在更强的轨道混合作用。用二(五甲基环戊二烯基)钴[(C₅Me₅)₂Co]还原1B和2B的溶液，可得到阴离子自由基1B⁻和2B⁻；二者已通过电子顺磁共振（electron paramagnetic resonance, EPR）以及紫外-可见-近红外（ultraviolet-visible-near infrared, UV-vis-NIR）吸收光谱完成表征。1B⁻和2B⁻的EPR光谱结果与配体基（即有机）自由基的特征高度吻合。其电子光谱中存在位于3800 cm⁻¹（1B⁻）与4500 cm⁻¹（2B⁻）的低能量窄带间隔电荷转移（intervalence charge transfer, IVCT）吸收峰，这与完全离域的混合价态自由基阴离子的特征一致。本研究将上述结果与电子结构计算结果、以及金属中心离域混合价态自由基阳离子[(ButCO₂)₃M₂]₂-μ₂-(O₂C−CO₂)⁺的光谱特征进行了对比，二者的光谱特征极为相似，同时也与其他基于有机配体的混合价态体系展开了比较。