Computational Investigation of Dichloromethane Ligand Substitution in the Enantiopure Cation [(η5‑C5H5)Re(NO)(PPh3)(ClCH2Cl)]+, a Functional Equivalent of a Chiral Lewis Acid

The dichloromethane ligand of [(η5-C5H5)­Re­(NO)­(PPh3)­(ClCH2Cl)]+ X– (1+ X–) can be displaced by a variety of ligands (:L), always with retention of configuration at rhenium. Reactions are first order in 1+ X– and first order in :L, but experimental data fail to support several substitution pathways established for eighteen-valence-electron complexes (e.g., η5-cyclopentadienyl slippage and nitrosyl bending). Thus, density functional theory studies have been undertaken with :L = cyclohexanone, dimethyl sulfide, ethyl chloride, and dichloromethane. The stronger nucleophiles favor concerted frontside displacements of dichloromethane (retention; interchange mechanism), whereas with the weaker nucleophiles, a PCCH moiety of the PPh3 ligand can provide anchimeric assistance, displacing dichloromethane from the frontside (neighboring group participation). The π CC unit is in turn displaced from the frontside by the incoming :L. The transition states are analyzed by a distortion/interaction protocol, and the analogous indenyl complexes are found to favor interchange mechanisms.