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

Methylidene complexes often couple to ethylene complexes, but the mechanistic insight is scant. The path by which two cations [(η-CH)Re(NO)(PPh)(═CH)] () transform (CHCl/acetonitrile) to [(η-CH)Re(NO)(PPh)(HC═CH)] () and [(η-CH)Re(NO)(PPh)(NCCH)] is studied by density functional theory. Experiments provide a number of constraints such as the second-order rate in ; no prior ligand dissociation/exchange; a faster reaction of ()- with ()- than with ()- ("enantiomer self-recognition"). Although dirhenium dications with Re(μ-CH)Re cores represent energy minima, they are not accessible by 2 + 2 cycloadditions of . Transition states leading to ReCHCHRe linkages are prohibitively high in energy. However, can give non-covalent / or / dimers with π interactions between the PPh ligands but long ReCH···HCRe and HCRe···HCRe distances (3.073-3.095 Å and 3.878-4.529 Å, respectively). In rate-determining steps, these afford [(η-CH)Re(NO)(PPh)(μ-η:η-HCCH)(PhP)(ON)Re(η-CH)] (), 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 / adduct, Dewar-Chatt-Duncanson optimization leads to unfavorable PPh/PPh contacts. Ligand interactions are further dissected in the preceding transition states via component analyses, and ΔΔ (1.2 kcal/mol, CHCl) favors the / pathway, in accordance with the experiment. Acetonitrile then displaces from the more weakly bound rhenium of . The formation of similar μ-HCCH intermediates is found to be rate-determining for varied coordinatively saturated M═CH species [M = Fe(d)/Re(d)/Ta(d)], establishing generality and enhancing relevancy to catalytic CH and CO/H chemistry.