Molybdenum 17- and 18-Electron Bis- and Tris(Butadiene) Complexes: Electronic Structures, Spectroscopic Properties, and Oxidative Ligand Substitution Reactions

New results on the
electronic structures, spectroscopic properties, and reactivities
of the molybdenum tris­(butadiene) and tris­(2,3-dimethylbutadiene)
complexes [Mo­(bd)3] (1bd) and [Mo­(dmbd)3] (1dmbd), respectively, are reported. Importantly, the metal ligand
bonding interaction can be weakened by oxidizing the metal center
with ferrocenium salts. The addition of the bidentate phosphine ligand
1,2-bis­(diphenylphosphino)­ethane then leads to a new type of stable
17-electron complex, [Mo­(dmbd)2(dppe)]­(X) (2; X = BF4–, PF6–, BPh4–), where one of the butadiene
ligands is exchanged by a chelating phosphine. Reduction of the cationic
complexes 2 generates the corresponding 18-electron complex
[Mo­(dmbd)2(dppe)] (3), thus establishing a
new strategy for ligand substitution reactions in [Mo­(bd)3] complexes via one-electron oxidized intermediates. The new heteroleptic
molybdenum complexes are characterized by X-ray structure analysis;
vibrational, NMR, and EPR spectroscopy; and electrochemistry. DFT
calculations are performed to explain the structural and specroscopic
trends observed experimentally. For compound 1bd, a normal coordinate analysis is presented, providing
additional information on the bonding situation in this type of complex.

本研究报道了三（丁二烯）合钼[Mo(bd)₃]（1bd）与三（2,3-二甲基丁二烯）合钼[Mo(dmbd)₃]（1dmbd）的电子结构、光谱性质及反应活性的最新研究结果。尤为关键的是，通过二茂铁盐（ferrocenium salts）对金属中心进行氧化，可弱化该类配合物的金属-配体键相互作用。随后向体系中加入双齿膦配体1,2-双（二苯基膦）乙烷（1,2-bis(diphenylphosphino)ethane，简称dppe），可得到一类新型稳定的17电子配合物[Mo(dmbd)₂(dppe)]⁺X⁻（2；X⁻=BF₄⁻、PF₆⁻、BPh₄⁻），其中一个丁二烯配体被螯合膦配体取代。对阳离子配合物2进行还原反应，可得到对应的18电子配合物[Mo(dmbd)₂(dppe)]（3），由此为[Mo(bd)₃]类配合物的配体取代反应开辟了一条基于单电子氧化中间体的全新策略。本研究通过X射线晶体结构分析（X-ray structure analysis）、振动光谱、核磁共振（Nuclear Magnetic Resonance, NMR）与电子顺磁共振（Electron Paramagnetic Resonance, EPR）光谱，以及电化学测试手段，对这类新型杂配钼配合物进行了完整表征。本研究开展了密度泛函理论（Density Functional Theory, DFT）计算，以解释实验中观测到的结构与光谱变化规律。针对配合物1bd，本文还开展了简正坐标分析，为该类配合物的成键特征提供了额外的理论依据。