Alvarez M Angeles, Casado-Ruano Melodie, García M Esther, García-Vivó Daniel, Ruiz Miguel A
Departamento de Química Orgánica e Inorgánica/IUQOEM, Universidad de Oviedo , E-33071 Oviedo, Spain.
Inorg Chem. 2017 Sep 18;56(18):11336-11351. doi: 10.1021/acs.inorgchem.7b01773. Epub 2017 Aug 31.
A high-yield synthetic route for the preparation of the unsaturated anion [MoCp(μ-PBu)(μ-CO)] (2) was implemented, via two-electron reduction of the chloride complex [MoCp(μ-Cl)(μ-PBu)(CO)] (1). Reaction of 2 with [NH][PF] led to the formation of the 30-electron complex [MoCp(H)(μ-PBu)(CO)] (3), in which the hydride ligand adopts an uncommon terminal disposition. DFT analysis of the electronic structure of 3 gave support to the presence of a M≡M triple bond in this complex following from a σδδ configuration, a view also supported by the high electron density accumulated at the corresponding Mo-Mo bond critical point. In contrast, reactions of 2 with IMe or ClCHPh gave the alkyl-bridged complexes [MoCp(μ-κ:η-CHR)(μ-PBu)(CO)] (R = H (4a), Ph (4b)), which in solution display agostic Mo-H-C interactions. Decarbonylation of 4a took place rapidly under photochemical conditions to give the 30-electron complex [MoCp(μ-κ:η-CH)(μ-PBu)(μ-CO)] (7), with a stronger agostic coordination of its methyl ligand. In contrast, irradiation of 4b led to the formation of the benzylidyne derivative [MoCp(μ-CPh)(μ-PBu)(μ-CO)] (9), following from fast decarbonylation and dehydrogenation of the bridging benzyl ligand. Low-temperature photochemistry allowed for the NMR characterization of an intermediate preceding the hydrogen elimination, identified as the carbene hydride [MoCp(H)(μ-CHPh)(μ-PBu)(CO)] (10), a product which evolves slowly by H elimination to the benzylidyne derivative. Analogous dehydrogenation of the methyl ligand in 7 could be accomplished upon moderate heating, to yield the corresponding methylidyne derivative [MoCp(μ-CH)(μ-PBu)(μ-CO)] (9). A complete reaction mechanism accounting for these photochemical reactions was elaborated, based on the reaction intermediates identified experimentally and on extensive DFT calculations. Surprisingly, for both systems the C-H bond activation steps are relatively easy thermal processes occurring with modest activation energies after photochemical ejection of CO, with a rate-determining step involving the formation of agostic carbenes requiring also a strong structural reorganization of the central MoPC rings of these molecules.
通过对氯配合物[MoCp(μ-Cl)(μ-PBu)(CO)] (1)进行双电子还原，实现了制备不饱和阴离子[MoCp(μ-PBu)(μ-CO)] (2)的高产率合成路线。2与[NH][PF]反应生成30电子配合物[MoCp(H)(μ-PBu)(CO)] (3)，其中氢化物配体采取不常见的端基构型。对3的电子结构进行密度泛函理论(DFT)分析，支持了该配合物中存在源于σδδ构型的M≡M三键的观点，相应的Mo-Mo键临界点处积累的高电子密度也支持了这一观点。相比之下，2与IMe或ClCHPh反应生成烷基桥联配合物[MoCp(μ-κ:η-CHR)(μ-PBu)(CO)] (R = H (4a)，Ph (4b))，它们在溶液中表现出螯合的Mo-H-C相互作用。4a在光化学条件下迅速发生脱羰基反应，生成30电子配合物[MoCp(μ-κ:η-CH)(μ-PBu)(μ-CO)] (7)，其甲基配体具有更强的螯合配位作用。相比之下，4b的辐照导致苄叉衍生物[MoCp(μ-CPh)(μ-PBu)(μ-CO)] (9)的形成，这是由于桥连苄基配体快速脱羰基和脱氢所致。低温光化学使得能够对氢消除之前的中间体进行核磁共振表征，该中间体被鉴定为卡宾氢化物[MoCp(H)(μ-CHPh)(μ-PBu)(CO)] (10)，该产物通过氢消除缓慢演变为苄叉衍生物。7中的甲基配体在适度加热时也可发生类似的脱氢反应，生成相应的亚甲基衍生物[MoCp(μ-CH)(μ-PBu)(μ-CO)] (9)。基于实验鉴定的反应中间体和广泛的DFT计算，阐述了一个完整的解释这些光化学反应的反应机理。令人惊讶的是，对于这两个体系，C-H键活化步骤是相对容易的热过程，在光化学脱除CO后以适度的活化能发生，速率决定步骤涉及形成螯合卡宾，这也需要这些分子的中心MoPC环进行强烈的结构重组。
