Platov South-Russian State Polytechnic University (NPI), Prosveschenya 132, Novocherkassk, 346428, Russia.
Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospect 47, Moscow, 119991, Russia.
Dalton Trans. 2019 Dec 7;48(45):17052-17062. doi: 10.1039/c9dt03266e. Epub 2019 Nov 7.
The complexes of Ni, Pd, and Pt with N-heterocyclic carbenes (NHCs) catalyze numerous organic reactions via proposed typical M/M catalytic cycles comprising intermediates with the metal center in (0) and (II) oxidation states. In addition, M/M catalytic cycles have been proposed for a number of reactions. The catalytic intermediates in both cycles can suffer decomposition via R-NHC coupling and the side reductive elimination of the NHC ligand and R groups (R = alkyl, aryl, etc.) to give [NHC-R] cations. In this study, the relative stabilities of (NHC)M(R)(X)L and (NHC)M(R)(X)L intermediates (X = Cl, Br, I; L = NHC, pyridine) against R-NHC coupling and other decomposition pathways via reductive elimination reactions were evaluated theoretically. The study revealed that the R-NHC coupling represents the most favorable decomposition pathway for both types of intermediates (M and M), while it is thermodynamically and kinetically more facile for the M complexes. The relative effects of the metal M (Ni, Pd, Pt) and ligands L and X on the R-NHC coupling for the M complexes were significantly stronger than that for the M complexes. In particular, for the (NHC)M(Ph)(Br) complexes, Ph-NHC coupling was facilitated dramatically from Pt (ΔG = -36.9 kcal mol, ΔG = 37.5 kcal mol) to Pd (ΔG = -61.5 kcal mol, ΔG = 18.3 kcal mol) and Ni (ΔG = -80.2 kcal mol, ΔG = 4.7 kcal mol). For the M oxidation state of the metal, the bis-NHC complexes (L = NHC) were slightly more kinetically and thermodynamically stable against R-NHC coupling than the mono-NHC complexes (L = pyridine). An inverse relation was observed for the M oxidation state of the metal as the (NHC)M(R)(X) complexes were kinetically (4.3-15.9 kcal mol) and thermodynamically (8.0-23.2 kcal mol) significantly less stable than the (NHC)M(R)(X)L (L = pyridine) complexes. For the Ni and Pd complexes, additional decomposition pathways via the reductive elimination of the NHC and X ligands to give the [NHC-X] cation (X-NHC coupling) or reductive elimination of the X-X molecule were found to be thermodynamically and kinetically probable. Overall, the obtained results demonstrate significant instability of regular Ni/NHC and Pd/NHC complexes (for example, not additionally stabilized by chelation) and high probability to initiate "NHC-free" catalysis in the reactions comprising M intermediates.
镍、钯和铂与 N-杂环卡宾 (NHC) 的配合物通过建议的典型 M/M 催化循环催化许多有机反应，该循环包括具有金属中心处于 (0) 和 (II) 氧化态的中间体。此外，已经提出了许多反应的 M/M 催化循环。两个循环中的催化中间体都可以通过 R-NHC 偶联和 NHC 配体和 R 基团（R = 烷基、芳基等）的侧还原消除而分解，生成 [NHC-R]+阳离子。在这项研究中，理论上评估了 (NHC)M(R)(X)L 和 (NHC)M(R)(X)L 中间体（X = Cl、Br、I；L = NHC、吡啶）与 R-NHC 偶联和其他通过还原消除反应进行的分解途径的相对稳定性。研究表明，R-NHC 偶联是两种类型的中间体（M 和 M）最有利的分解途径，而对于 M 配合物，它在热力学和动力学上更有利。金属 M（Ni、Pd、Pt）和配体 L 和 X 对 M 配合物的 R-NHC 偶联的相对影响明显强于 M 配合物。特别是对于 (NHC)M(Ph)(Br)配合物，Ph-NHC 偶联从 Pt（ΔG = -36.9 kcal mol，ΔG = 37.5 kcal mol）到 Pd（ΔG = -61.5 kcal mol，ΔG = 18.3 kcal mol）和 Ni（ΔG = -80.2 kcal mol，ΔG = 4.7 kcal mol）显著促进。对于金属的 M 氧化态，双 NHC 配合物（L = NHC）在热力学和动力学上对 R-NHC 偶联的稳定性略高于单 NHC 配合物（L = 吡啶）。对于金属的 M 氧化态，观察到一种相反的关系，因为 (NHC)M(R)(X) 配合物在动力学上（4.3-15.9 kcal mol）和热力学上（8.0-23.2 kcal mol）明显不如 (NHC)M(R)(X)L（L = 吡啶）配合物稳定。对于 Ni 和 Pd 配合物，发现通过还原消除 NHC 和 X 配体以生成 [NHC-X]+阳离子（X-NHC 偶联）或还原消除 X-X 分子的额外分解途径在热力学和动力学上是可能的。总体而言，所得结果表明常规 Ni/NHC 和 Pd/NHC 配合物（例如，没有通过螯合进一步稳定）极不稳定，并且在包含 M 中间体的反应中很可能引发“无 NHC”催化。
