Synthesis, Structure, Bonding, and Ligand Exchange of a Bimetallic Rhodium Dimetallocyclobutane Complex

We report the synthesis of a 2-phosphinoimidazole-scaffolded bimetallic Rh complex (7) that contains a unique dimetallocyclobutane structure resulting from addition to a cyclooctene (COE) ligand. This bridging COE ligand is labile in the presence of 2-butyne and readily transforms into dimetallocyclobutene complex 8. Density functional theory (DFT) calculations and bonding analyses indicate that the dimetallocyclobutane structure 7 has significant Rh–Rh single σ bond character and two Rh–C covalent σ bonds. DFT calculations also suggest that ligand exchange occurs via an associative mechanism without formation of a RhRh cycloreversion intermediate. Complex 7 was oxidized to a halide-bridged bimetallic Rh(III,III) complex 9 by treatment with PhICl2, which resulted in destruction of the Rh–Rh bond.本工作报道了一种2-膦基咪唑支架支撑的双金属铑配合物（7）的合成，该配合物含有独特的双金属环丁烷结构，该结构由其环辛烯（cyclooctene，COE）配体的加成反应构建而成。该桥联环辛烯配体在2-丁炔存在下易发生解离，可快速转化为双金属环丁烯配合物（8）。密度泛函理论（Density functional theory，DFT）计算与成键分析表明，配合物7的双金属环丁烷结构具有显著的铑-铑单σ键特征，以及两条铑-碳共价σ键。DFT计算还显示，配体交换通过缔合机理进行，并未生成铑-铑双键环消除中间体。配合物7经PhICl₂（二氯碘苯）处理后被氧化为卤离子桥联的双金属三价铑(III,III)配合物（9），该过程伴随铑-铑键的断裂。