Mechanisms of Substitution Reactions on Cyclometallated Platinum(IV) Complexes:  “Quasi-labile” Systems

The substitution reactions of SMe₂ by phosphines (PMePh₂, PEtPh₂, PPh₃, P(4-MeC₆H₄)₃, P(3-MeC₆H₄)₃, PCy₃) on Pt^IV complexes having a cyclometalated imine ligand, two methyl groups in a cis-geometrical arrangement, a halogen, and a dimethyl sulfide as ligands, [Pt(C⌒N)(CH₃)₂(X)(SMe₂)], have been studied as a function of temperature, solvent, and electronic and steric characteristics of the phosphines and the X and C⌒N ligands. In all cases, a limiting dissociative mechanism has been found, where the dissociation of the SMe₂ ligand corresponds to the rate-determining step. The pentacoordinated species formed behaves as a true pentacoordinated Pt^IV compound in a steady-state concentration, given the solvent independence of the rate constant. The X-ray crystal structures of two of the dimethyl sulfide complexes and a derivative of the pentacoordinate intermediate have been determined. Differences in the individual rate constants for the entrance of the phosphine ligand can only be estimated as reactivity ratios. In all cases an effect of the phosphine size is detected, indicating that an associative step takes place from the pentacoordinated intermediate. The nature of the C⌒N imine and X ligands produces differences in the dimethyl sulfide dissociation reactions rates, which can be quantified by the corresponding ΔS^⧧ values (72, 64, 48, 31, and 78 J K^-1 mol^-1 for C⌒N/X being C₆H₄CHNCH₂C₆H₅/Br, C₆H₄CHNCH₂(2,4,6-(CH₃)₃)C₆H₂/Br, C₆H₄CHNCH₂C₆H₅/Cl, C₆Cl₄CHNCH₂C₆H₅/Cl, and C₆H₄CH₂NCHC₆H₅/Br, respectively). As a whole, the donor character of the coordinated C^aromatic and X atoms have the greatest influence on the dissociativeness of the rate-determining step.