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 cyclometalat ed imine ligand, two methyl groups in a cis-geometrical arrangement, a halo gen, and a dimethyl sulfide as ligands, [Pt(CN)(CH3)(2)(X)(SMe2)], have bee n studied as a function of temperature, solvent, and electronic and steric characteristics of the phosphines and the X and CN 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 pentacoordina ted species formed behaves as a true pentacoordinated Pt-IV compound in a s teady-state concentration, given the solvent independence of the rate const ant. 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 phosph ine ligand can only be estimated as reactivity ratios. In all cases an effe ct of the phosphine size is detected, indicating that an associative step t akes place from the pentacoordinated intermediate. The nature of the (CN) i mine and X ligands produces differences in the dimethyl sulfide dissociatio n reactions rates, which can be quantified by the corresponding DeltaS doub le dagger values (72, 64, 48, 31, and 78 J K-1 mol(-1) for CN/X being C6H4C HNCH2C6H5/Br, C6H4CHNCH2-(2,4,6-(CH3)(3))C6H2/Br, C6H4CHNCH2C6H5/Cl, C6Cl4C HNCH2C6H5/Cl, and C6W4CH2NCHC6H5/ Pr, respectively). As a whole, the donor character of the coordinated C-aromatic and X atoms have the greatest influ ence on the dissociativeness of the rate-determining step.