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.