New Insight into the Reactivity of Pyridine-Functionalized Phosphine Complexes of Ruthenium(II) with Respect to Olefin Metathesis and Transfer Hydrogenation

The present paper deals with the synthesis and full characterization of a series of pyridine-functionalized phosphine complexes of Ru(II), namely, RuCl2(Lnx)(PPh3) (Lnx = R2PCH2(C5H2R′R′′N)), differing in the nature of the substituents on the phosphorus (superscript label n in Lnx defined as n = 1 for R = Ph, n = 2 for R = Cy) and/or on the pyridyl group (superscript label x in Lnx defined as x = a for picolyl, noted pic, and x = b for quinolyl, noted quin) and discloses new aspects of their reactivity with respect to catalysis. The ligands 2-[(diphenylphosphino)methyl]-6-methylpyridine, L1a, 2-[(diphenylphosphino)methyl]quinoline, L1b, 2-[(dicyclohexylphosphino)methyl]-6-methylpyridine, L2a, and 2-[(dicyclohexylphosphino)methyl]quinoline, L2b, were prepared and respectively reacted with RuCl2(PPh3)3 under optimized experimental conditions. In a preliminary test, the reaction of RuCl2(PPh3)3 with L1a using a stoichiometric 1/1 metal/ligand ratio gave three complexes, namely, [RuCl2(PPh3)2]2 (1), [(PPh3)2ClRu(μ-Cl)3Ru(L1a)(PPh3)] (21a), and RuCl2(L1a)2 (31a). These were isolated by fractional crystallization and, at that stage, identified only by single-crystal X-ray diffraction. The formation of 1 and 21a reflects the existence of the elusive 14 e− fragment “RuCl2(PPh3)2”, which tends to relieve its unsaturation by intermolecular association. By contrast, controlled addition of 2-(phosphinomethyl)pyridine type ligands Lnx to RuCl2(PPh3)2 leads selectively to the desired 16 e− species RuCl2(Lnx)(PPh3) (4nx). For example, with L1b, the green complex RuCl2(L1b)(PPh3) (41b-trans-Cl) was identified as the kinetic product of ligand addition. It slowly and irreversibly converts into the more stable isomer RuCl2(L1b)(PPh3) (41b-cis-Cl), representing the thermodynamic product. Both isomers were fully characterized by NMR spectroscopy and X-ray diffraction. Similar transformations, taking place at different rates, were observed within the ligand series examined here. All isomeric forms of type 4na complexes react cleanly with a terminal alkyne-like phenylacetylene to give a new complex identified by NMR spectroscopy as the vinylidene species RuCl2(L)(CCHPh)(PPh3) (5na). The reaction of 4nb-cis-Cl with an excess of ethyl diazoacetate at –60 °C gives the novel complex RuCl2(Lna){cis-EtO(O)C(H)CC(H)C(O)OEt} (6na) with concomitant elimination of the phosphonium ylide, Ph3PC(H)C(O)OEt. Whereas 1 equiv of diazoalkane thus serves as phosphine scavenger, the uptake of two more carbene units by the remaining 14 e− fragment “RuCl2(L1a)” results in their coupling, providing diethyl maleate, intercepted in 6na as a coordinated ligand. Preliminary catalytic tests indicate that the complexes 4nx act as catalyst precursors for the ROMP of norbornene in the presence of trimethylsilyldiazomethane as the carbene source. The same compounds 4nx are also used as catalyst precursors in the transfer hydrogenation of a series of ketone substrates using alcohol as the hydrogen source. For example, the hydrogenation of cyclohexanone is achieved in 99% yield within 45 s with only 0.01 mol (0.1 mol %) of the precatalyst RuCl2(Ph2PCH2pic)(PPh3)-trans-Cl (41a), representing a turnover frequency of 272 571 h−1. The X-ray structure analyses of 1, 21a, 31a, 41b (both trans-Cl and cis-Cl isomers), and 61a are reported.

本论文针对一系列吡啶功能化膦配位钌(II)配合物的合成与全面表征展开研究，该系列配合物通式为RuCl₂(Lₙₓ)(PPh₃)（其中Lₙₓ = R₂PCH₂(C₅H₂R′R′′N)），其磷原子上的取代基（Lₙₓ的上标n定义为：R为苯基（Ph）时n=1，R为环己基（Cy）时n=2）与/或吡啶基上的取代基（Lₙₓ的上标x定义为：x=a为吡啶甲基（picolyl，简写为pic），x=b为喹啉基（quinolyl，简写为quin））存在差异，并揭示了该类配合物在催化反应中的全新反应特性。

本文合成了四种配体：2-[(二苯基膦基)甲基]-6-甲基吡啶（L¹ᵃ）、2-[(二苯基膦基)甲基]喹啉（L¹ᵇ）、2-[(二环己基膦基)甲基]-6-甲基吡啶（L²ᵃ）以及2-[(二环己基膦基)甲基]喹啉（L²ᵇ），并分别在优化的实验条件下与三(三苯基膦)二氯化钌（RuCl₂(PPh₃)₃）进行反应。初步实验中，当RuCl₂(PPh₃)₃与L¹ᵃ按照金属/配体1:1的化学计量比进行反应时，生成三种配合物：[RuCl₂(PPh₃)₂]₂（1）、[(PPh₃)₂ClRu(μ-Cl)₃Ru(L¹ᵃ)(PPh₃)]（2¹ᵃ）以及二(2-[(二苯基膦基)甲基]-6-甲基吡啶)二氯化钌（RuCl₂(L¹ᵃ)₂，3¹ᵃ）。通过分步结晶分离得到上述产物，并仅通过单晶X射线衍射完成了初步结构鉴定。产物1与2¹ᵃ的生成，印证了难以分离的14电子片段“RuCl₂(PPh₃)₂”的存在——该片段倾向于通过分子间缔合来缓解自身的不饱和状态。

与之相对，将2-(膦甲基)吡啶类配体Lₙₓ可控地添加至RuCl₂(PPh₃)₂中时，可选择性得到目标16电子物种RuCl₂(Lₙₓ)(PPh₃)（4ₙₓ）。例如，以L¹ᵇ为配体时，绿色配合物RuCl₂(L¹ᵇ)(PPh₃)（4¹ᵇ-trans-Cl）被鉴定为配体加成的动力学产物，其会缓慢且不可逆地转化为更稳定的异构体RuCl₂(L¹ᵇ)(PPh₃)（4¹ᵇ-cis-Cl），即热力学产物。两种异构体均通过核磁共振波谱（NMR）与X射线衍射完成了全面表征。在所研究的配体系列中，均观察到了以不同速率进行的类似转化过程。

所有4ₙₓ型异构体均可与端炔类底物苯乙炔发生干净的反应，生成经NMR波谱鉴定为亚乙烯基配合物的新型产物RuCl₂(L)(CCHPh)(PPh₃)（5ₙₓ）。在–60 ℃条件下，4ⁿᵇ-cis-Cl与过量的重氮乙酸乙酯反应，得到新型配合物RuCl₂(Lₙₐ){cis-EtO(O)C(H)C=C(H)C(O)OEt}（6ₙₐ），同时消除鏻叶立德Ph₃P=C(H)C(O)OEt。1当量重氮烷烃可作为膦捕获剂，而剩余的14电子片段“RuCl₂(L¹ᵃ)”可再捕获两个卡宾单元并发生偶联，生成马来酸二乙酯，该产物以配位配体的形式被截留在配合物6ⁿᵃ中。

初步催化实验结果表明，配合物4ₙₓ可作为降冰片烯开环易位聚合（ROMP）的催化剂前驱体，以三甲基硅基重氮甲烷作为卡宾源。上述4ₙₓ配合物同样可用作一系列酮类底物转移氢化反应的催化剂前驱体，以醇作为氢源。例如，仅使用0.01 mol（即0.1 mol%）的预催化剂RuCl₂(Ph₂PCH₂pic)(PPh₃)-trans-Cl（4¹ᵃ），即可在45秒内以99%的产率完成环己酮的氢化反应，其转化频率（TOF）可达272571 h⁻¹。

本文报道了配合物1、2¹ᵃ、3¹ᵃ、4¹ᵇ（包含trans-Cl与cis-Cl两种异构体）以及6¹ᵃ的X射线晶体结构分析结果。