Mechanism of Coupling of Methylidene to Ethylene Ligands in Dimetallic Assemblies; Computational Investigation of a Model for a Key Step in Catalytic C 1 Chemistry.

Methylidene complexes often couple to ethylene complexes, but the mechanistic insight is scant. The path by which two cations [(η 5 -C 5 H 5 )Re(NO)(PPh 3 )(═CH 2 )] + ( 5 + ) transform (CH 2 Cl 2 /acetonitrile) to [(η 5 -C 5 H 5 )Re(NO)(PPh 3 )(H 2 C═CH 2 )] + ( 6 + ) and [(η 5 -C 5 H 5 )Re(NO)(PPh 3 )(NCCH 3 )] + 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-CH 2 ) 2 Re cores represent energy minima, they are not accessible by 2 + 2 cycloadditions of 5 + . Transition states leading to ReCH 2 CH 2 Re linkages are prohibitively high in energy. However, 5 + can give non-covalent S Re / S Re or S Re / R Re dimers with π interactions between the PPh 3 ligands but long ReCH 2 ···H 2 CRe and H 2 CRe···H 2 CRe distances (3.073-3.095 Å and 3.878-4.529 Å, respectively). In rate-determining steps, these afford [(η 5 -C 5 H 5 )Re(NO)(PPh 3 )(μ-η 2 :η 2 -H 2 C ··· CH 2 )(Ph 3 P)(ON)Re(η 5 -C 5 H 5 )] 2+ ( 13 2+ ), 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 S Re / R Re adduct, Dewar-Chatt-Duncanson optimization leads to unfavorable PPh 3 /PPh 3 contacts. Ligand interactions are further dissected in the preceding transition states via component analyses, and ΔΔ G ‡ (1.2 kcal/mol, CH 2 Cl 2 ) favors the S Re / S Re pathway, in accordance with the experiment. Acetonitrile then displaces 6 + from the more weakly bound rhenium of 13 2+ . The formation of similar μ-H 2 C ··· CH 2 intermediates is found to be rate-determining for varied coordinatively saturated M═CH 2 species [M = Fe(d 6 )/Re(d 4 )/Ta(d 2 )], establishing generality and enhancing relevancy to catalytic CH 4 and CO/H 2 chemistry.