Cyclometalated N‑Heterocyclic Carbene Complexes of Ruthenium for Access to Electron-Rich Silylene Complexes That Bind the Lewis Acids CuOTf and AgOTf

The synthesis of
the cyclometalated complexes Cp*Ru­(IXy-H) (2) [IXy =
1,3-bis­(2,6-dimethylphenyl)­imidazol-2-ylidene; IXy-H
= 1-(2-CH2C6H3-6-methyl)-3-(2,6-dimethylphenyl)­imidazol-2-ylidene-1-yl
(the deprotonated form of IXy); Cp* = η5-C5Me5] and Cp*Ru­(IXy-H)­(N2) (3)
was achieved by dehydrochlorination of Cp*Ru­(IXy)Cl (1) with KCH2Ph. Complexes 2 and 3 activate primary silanes (RSiH3) to afford the silyl
complexes Cp*­(IXy-H)­(H)­RuSiH2R [R = p-Tol
(4), Mes (5), Trip (6)]. Density
functional theory studies indicated that these complexes are close
in energy to the corresponding isomeric silylene species Cp*­(IXy)­(H)­RuSiHR.
Indeed, reactivity studies indicated that various reagents trap the
silylene isomer of 6, Cp*­(IXy)­(H)­RuSiHTrip (6a). Thus, benzaldehyde reacts with 6 to give
the [2 + 2] cycloaddition product 7, while 4-bromoacetophenone
reacts via C–H bond cleavage and formation of the enolate Cp*­(IXy)­(H)2RuSiH­[OC­(CH2)­C6H4Br]­Trip (8). Addition of the O–H bond of 2,6-dimethylphenol
across the RuSi bond of 6a gives Cp*­(IXy)­(H)2RuSiH­(2,6-Me2C6H3O)­Trip (9). Interestingly, CuOTf and AgOTf also react with 6 to provide unusual Lewis acid-stabilized silylene complexes in which
MOTf bridges the Ru–Si bond. The AgOTf complex, which was crystallographically
characterized, exhibits a structure similar to that of [Cp*­(iPr3P)­Ru­(μ-H)2SiHMes]+, with a three-center, two-electron Ru–Ag–Si
interaction. Natural bond orbital analysis of the MOTf complexes supported
this type of bonding and characterized the donor interaction with
Ag (or Cu) as involving a delocalized interaction with contributions
from the carbene, silylene, and hydride ligands of Ru.

本研究通过苄基钾（KCH₂Ph）对配合物Cp*Ru(IXy)Cl（1）进行脱氯化氢反应，成功合成了环金属化配合物Cp*Ru(IXy-H)（2）与Cp*Ru(IXy-H)(N₂)（3），其中IXy为1,3-双(2,6-二甲基苯基)咪唑-2-亚基，IXy-H为IXy的去质子化形式，即1-(2-CH₂C₆H₃-6-甲基)-3-(2,6-二甲基苯基)咪唑-2-亚基-1-基，Cp*为η⁵-五甲基环戊二烯基。配合物2与3可活化伯硅烷（RSiH₃），生成硅基配合物Cp*(IXy-H)(H)RuSiH₂R，其中R分别为对甲苯基（p-Tol，4）、均三甲苯基（Mes，5）、三异丙基苯基（Trip，6）。密度泛函理论研究表明，上述配合物与其对应的异构体硅烯物种Cp*(IXy)(H)Ru=SiHR能量相近。反应性研究证实，多种试剂可捕获配合物6的硅烯异构体Cp*(IXy)(H)Ru=SiHTrip（6a）：苯甲醛与6发生[2+2]环加成反应生成产物7；4-溴苯乙酮则通过C-H键断裂过程，生成烯醇盐类配合物Cp*(IXy)(H)₂RuSiH[OC(=CH₂)C₆H₄Br]Trip（8）。将2,6-二甲基苯酚的O-H键对6a的Ru=Si键进行加成，可得到Cp*(IXy)(H)₂RuSiH(2,6-Me₂C₆H₃O)Trip（9）。值得注意的是，三氟甲磺酸铜（CuOTf）与三氟甲磺酸银（AgOTf）也可与6反应，生成罕见的路易斯酸稳定的硅烯配合物，其中MOTf（M为Cu或Ag）桥连Ru-Si键。经单晶X射线衍射表征的AgOTf配合物，其结构与[Cp*(iPr₃P)Ru(μ-H)₂SiHMes]⁺相似，存在三中心两电子Ru-Ag-Si相互作用。对该类MOTf配合物的自然键轨道分析验证了该成键模式，并将其与Ag（或Cu）的给体相互作用表征为：源自Ru连接的卡宾配体、硅烯配体与氢配体共同参与的离域相互作用。