Mechanism of Coupling of Methylidene to Ethylene Ligands in Dimetallic Assemblies; Computational Investigation of a Model for a Key Step in Catalytic C1 Chemistry

Methylidene complexes often couple to ethylene complexes, but the mechanistic insight is scant. The path by which two cations [(η5-C5H5)Re(NO)(PPh3)(CH2)]+ (5+) transform (CH2Cl2/acetonitrile) to [(η5-C5H5)Re(NO)(PPh3)(H2CCH2)]+ (6+) and [(η5-C5H5)Re(NO)(PPh3)(NCCH3)]+ is studied by density functional theory. Experiments provide a number of constraints such as the second-order rate in 5+; no prior ligand dissociation/exchange; a faster reaction of (S)-5+ with (S)-5+ than with (R)-5+ (“enantiomer self-recognition”). Although dirhenium dications with Re(μ-CH2)2Re cores represent energy minima, they are not accessible by 2 + 2 cycloadditions of 5+. Transition states leading to ReCH2CH2Re linkages are prohibitively high in energy. However, 5+ can give non-covalent SRe/SRe or SRe/RRe dimers with π interactions between the PPh3 ligands but long ReCH2···H2CRe and H2CRe···H2CRe distances (3.073–3.095 Å and 3.878–4.529 Å, respectively). In rate-determining steps, these afford [(η5-C5H5)Re(NO)(PPh3)(μ-η2:η2-H2C···CH2)(Ph3P)(ON)Re(η5-C5H5)]2+ (132+), in which one rhenium binds the bridging ethylene more tightly than the other (2.115–2.098 vs 2.431–2.486 Å to the centroid). In the SRe/RRe adduct, Dewar–Chatt–Duncanson optimization leads to unfavorable PPh3/PPh3 contacts. Ligand interactions are further dissected in the preceding transition states via component analyses, and ΔΔG‡ (1.2 kcal/mol, CH2Cl2) favors the SRe/SRe pathway, in accordance with the experiment. Acetonitrile then displaces 6+ from the more weakly bound rhenium of 132+. The formation of similar μ-H2C···CH2 intermediates is found to be rate-determining for varied coordinatively saturated MCH2 species [M = Fe(d6)/Re(d4)/Ta(d2)], establishing generality and enhancing relevancy to catalytic CH4 and CO/H2 chemistry.

亚甲基配合物（methylidene complexes）通常可与乙烯配合物发生偶联反应，但相关的机理认知仍较为有限。本文采用密度泛函理论（density functional theory），对两种[(η⁵-环戊二烯基)Re(NO)(三苯基膦(PPh₃))(亚甲基)]⁺（记为5+）阳离子在二氯甲烷/乙腈混合溶剂体系中转化为[(η⁵-环戊二烯基)Re(NO)(三苯基膦(PPh₃))(乙烯)]⁺（记为6+）与[(η⁵-环戊二烯基)Re(NO)(三苯基膦(PPh₃))(乙腈)]⁺的反应路径展开研究。实验结果给出了多项约束条件：该反应对5+呈二级动力学行为，无预先的配体解离或配体交换过程；(S)-5+与(S)-5+的反应速率快于(S)-5+与(R)-5+，即“对映体自识别”现象。尽管具有Re(μ-亚甲基)₂Re核的二铼双正离子属于能量极小值结构，但该物种无法通过5+的[2+2]环加成得到。生成ReCH₂CH₂Re键联的过渡态能垒极高。不过，5+可形成非共价的SRe/SRe或SRe/RRe二聚体，其中三苯基膦配体间存在π相互作用，但ReCH₂···H₂CRe与H₂CRe···H₂CRe的距离分别长达3.073~3.095 Å和3.878~4.529 Å。在决速步中，此类二聚体可转化为[(η⁵-环戊二烯基)Re(NO)(三苯基膦(PPh₃))(μ-η²:η²-H₂C···CH₂)(三苯基膦(PPh₃))(ON)Re(η⁵-环戊二烯基)]²+（记为13²+）。在该中间体中，一个铼原子对桥联乙烯的结合能力强于另一个（与桥联乙烯质心的键长分别为2.115~2.098 Å和2.431~2.486 Å）。对于SRe/RRe二聚体，杜瓦-查特-邓坎森（Dewar–Chatt–Duncanson）构型优化会导致不利的三苯基膦配体间空间接触。通过组分分析对前述过渡态中的配体相互作用进行进一步拆解，吉布斯自由能活化差ΔΔG‡为1.2 kcal/mol（二氯甲烷溶剂体系），该结果有利于SRe/SRe反应路径，与实验观测结果一致。随后，乙腈会从13²+中结合能力较弱的铼原子位点上取代出6+。研究发现，对于一系列配位饱和的M=CH₂物种[M分别为Fe(d⁶)、Re(d⁴)、Ta(d²)]，类似的μ-H₂C···CH₂中间体的生成均为决速步，这确立了该反应的普适性，并提升了其与催化甲烷及合成气（CO/H₂）化学的相关性。