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.

本论文研究了一系列吡啶功能化膦配位的二价钌(Ru(II))配合物RuCl₂(Lₙₓ)(PPh₃)的合成与完整表征，其中Lₙₓ = R₂PCH₂(C₅H₂R′R′′N)，配合物的差异源于磷原子上（Lₙₓ的上标n定义为：n=1时R为苯基(Ph)，n=2时R为环己基(Cy)）和/或吡啶基上（Lₙₓ的下标x定义为：x=a为皮考林基(picolyl，记为pic)，x=b为喹啉基(quinolyl，记为quin)）取代基的性质，并揭示了其在催化反应中活性的新维度。本文合成了配体2-[(二苯基膦基)甲基]-6-甲基吡啶(L₁ₐ)、2-[(二苯基膦基)甲基]喹啉(L₁ᵦ)、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(L₂ᵦ)，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，按金属与配体1:1的化学计量比使RuCl₂(PPh₃)₃与L₁ₐ反应，得到三种配合物：[RuCl₂(PPh₃)₂]₂(1)、[(PPh₃)₂ClRu(μ-Cl)₃Ru(L₁ₐ)(PPh₃)](2₁ₐ)以及RuCl₂(L₁ₐ)₂(3₁ₐ)。通过分级结晶分离得到这些产物，当时仅通过单晶体X射线衍射(single-crystal X-ray diffraction)对其进行了结构鉴定。配合物1和2₁ₐ的生成，反映出难以捕捉的14电子(14 e⁻)片段“RuCl₂(PPh₃)₂”的存在，该片段倾向于通过分子间缔合来缓解不饱和状态。与之相反，向RuCl₂(PPh₃)₂中可控加入2-(膦甲基)吡啶类配体Lₙₓ，可选择性得到目标的16电子(16 e⁻)物种RuCl₂(Lₙₓ)(PPh₃)(4ₙₓ)。例如，以L₁ᵦ为配体时，绿色配合物RuCl₂(L₁ᵦ)(PPh₃)(41b-trans-Cl)被鉴定为配体加成反应的动力学产物，其会缓慢且不可逆地转化为更稳定的异构体RuCl₂(L₁ᵦ)(PPh₃)(41b-cis-Cl)，即热力学产物。两种异构体均通过核磁共振(NMR)光谱与X射线衍射完成了完整表征。在本次研究的配体系列中，观察到了类似的转化过程，只是反应速率有所差异。所有4ₙₓ型异构体均可与端炔类底物苯乙炔(phenylacetylene)发生专一且无副产物的反应，生成一种新的配合物，经核磁共振光谱鉴定为亚乙烯基(vinylidene)物种RuCl₂(L)(CCHPh)(PPh₃)(5ₙₓ)。在–60 ℃下，将4nb-cis-Cl与过量的重氮乙酸乙酯(ethyl diazoacetate)反应，得到新型配合物RuCl₂(Lₙₐ){cis-EtO(O)C(H)C=C(H)C(O)OEt}(6na)，同时伴随鏻叶立德(phosphonium ylide)Ph₃P=C(H)C(O)OEt的消除。虽然1当量的重氮烷烃可作为膦捕获剂，但剩余的14电子片段“RuCl₂(L₁ₐ)”可再结合两个卡宾单元并发生偶联，生成马来酸二乙酯，该产物作为配位配体被捕获于配合物61a中。初步催化测试表明，配合物4ₙₓ可作为降冰片烯(norbornene)开环易位聚合(ROMP)的催化剂前驱体，反应以三甲基硅基重氮甲烷(trimethylsilyldiazomethane)作为卡宾源。同样的4ₙₓ类化合物也可用作一系列酮类底物转移氢化(transfer hydrogenation)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%的收率实现环己酮(cyclohexanone)的氢化，周转频率(turnover frequency)可达272571 h⁻¹。本文还报道了配合物1、21a、31a、41b（包括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。