Substitution Reactions on Cyclometalated Pt(IV) Complexes. Associative Tuning by Fluoro Ligands and Fluorinated Substituents

The substitution reactions of sulfide by phosphines on Pt(IV) complexes having a cyclometalated imine ligand, two
methyl groups in a cis geometrical arrangement, and a halogen and a sulfide as ligands, [Pt (Me)2X(C⌒N)(SR2)],
have been studied as a function of temperature, solvent, and electronic and steric characteristics of the phosphines,
sulfides, X, and C⌒N. In most of these cases, a limiting dissociative mechanism has been found, where the
dissociation of the sulfide ligand corresponds to the rate-determining step. The intermediate species formed behaves
as a true pentacoordinated Pt(IV) compound in a steady-state concentration only for the systems with SMe2; for
the bulkier SEt2 and SBzl2 leaving ligands the rate constants and activation parameters show an important degree
of solvent dependence, which correlates with the ability of the solvent to form hydrogen bonds. The X-ray crystal
structure of one of the dibenzyl sulfide complexes has been determined, and the geometrical arrangement of the
ligands has been determined by NOE NMR measurements at low temperature. The nature of the solvent, imine,
sulfide, and halogen ligands produces differences in the reaction rates, which can be quantified very well by the
corresponding ΔS⧧ values that move from +48 to −90 J K-1 mol-1. The reaction on [Pt(Me)2F(C5CF4CHNCH2Ph)
(SMe2)] has been found to take place via a mechanism that depends strongly on the bulkiness of the substituting
phosphine. While for PCy3 the reaction is dissociative, for smaller entering ligands the first associatively activated
substitution mechanisms on organometallic Pt(IV) complexes have been established with values of ΔH⧧ and ΔS⧧
in the 28−44 kJ mol-1 and −120 to −83 J K-1 mol-1 ranges. Important intramolecular hydrogen bonding in the
starting material can be held responsible for this difference with the remaining systems.