A combined kinetico-mechanistic and computational study on the competitive formation of seven- versus five-membered platinacycles; the relevance of spectator halide ligands

Abstract

The metalation reactions between [Pt₂(4-MeC₆H₄)₄(μ-SEt₂)₂] and 2-X,6-FC₆H₃CHNCH₂CH₂NMe₂ (X = Br, Cl) have been studied. In all cases, seven-membered platinacycles are formed in a process that involves an initial reductive elimination from cyclometallated Pt^IV intermediate compounds, [PtX(4-CH₃C₆H₄)₂(ArCHNCH₂CH₂NMe₂)] (X = Br, Cl), followed by isomerization of the resulting Pt^II complexes and a final cyclometallation step. For the process with X = Br, the final seven-membered platinacycle and two intermediates, isolated under the conditions implemented from parallel kinetic studies, have been characterized by XRD. Contrary to previous results for the parent non-fluorinated imine 2-BrC₆H₄CHNCH₂CH₂NMe₂ the presence of a fluoro substituent prevents the formation of the more stable five-membered platinacycle. Temperature and pressure dependent kinetico-mechanistic and DFT studies indicate that the final cyclometallation step is strongly influenced by the nature of the spectator halido ligand, the overall reaction being much faster for X = Cl. The same DFT study conducted on the previously studied systems with imine 2-BrC₆H₄CHNCH₂CH₂NMe₂ indicates that, when possible, five-membered platinacycles are kinetically preferred for X = Br, while the presence of Cl as a spectator halido ligand leads to a preferential faster formation of seven-membered analogues.