Cyclometalation of the (2-BrC6H4)CHNBzl imine via oxidative addition on [PtII(Ph)2(SMe2)2] to produce the corresponding [PtIVBr(Ph)2(CC5H3CHNBzl)(SMe2)] compound has been achieved, and the X-ray crystal structure of its triphenylphosphine derivative has been determined. The lability of the SMe2 ligand in this complex enables the reductive elimination of a C6H6 molecule and the formal insertion of the other phenyl ligand in the cyclometalated Pt−C bond, producing the first structurally characterized seven-membered cyclometalated platinum complex.

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)(2)X(C-N)(SR(2))], 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 SMe(2); for the bulkier SEt(2) and SBzl(2) 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 DeltaS values that move from +48 to -90 J K(-1) mol(-1). The reaction on [Pt(Me)(2)F(C(5)CF(4)CHNCH(2)Ph) (SMe(2))] has been found to take place via a mechanism that depends strongly on the bulkiness of the substituting phosphine. While for PCy(3) 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 DeltaH and DeltaS 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.

Careful tuning of the reaction conditions has been proved to be essential for the operation of two disjunctive reaction mechanisms occurring on the ortho-metalated platinum compounds [Pt IV Br(Ph) 2 -(C 5 CH 4 CHNZ)(SMe 2 )] (Z ) Me, Bzl, CH 2 (2,4,6-Me 3 C 6 H 2 )). In dilute solutions, where the already established lability of the SMe 2 ligand favors the existence of the pentacoordinated {Pt IV Br(Ph) 2 (C 4 CH 4 -CHNZ)} species, the complex evolves to produce the insertion of one of the phenyl ligands into the cyclometalated Pt IV -C bond to yield the complexes [Pt II Br(C 4 CH 3 C 6 H 4 CHNZ)(SMe 2 )], which contain a seven-membered cyclometalated ring. The process involves the formation of an hydride intermediate, which has been detected via low-temperature proton NMR for Z ) Bzl, prior to the reductive elimination of benzene. In more concentrated solutions, or in the presence of large amounts (200-500 fold) of SMe 2 , the existence of pentacoordinated species is reduced to a minimum and a reductive C-C coupling takes place between the metalated imine carbon and one of the phenyl ligands, yielding the coordination complexes [Pt II Br(Ph)(C 6 H 3 C 6 H 4 CHNZ)(SMe 2 )], which evolve rapidly to trans-[Pt II Br(Ph)(SMe 2 ) 2 ] and free C 5 H 3 C 6 H 4 CHNZ. The validity of the mechanisms proposed has been proved via stoichiometric and reactivity studies carried out under carefully controlled conditions, both on initial Pt IV complex and on the final inserted complex, [Pt II Br(C 4 CH 3 C 6 H 4 CHNZ)(SMe 2 )]. The overall reactivity is rather surprising, given the generally accepted dissociative processes involved in the preliminary steps of reductive elimination reactions on Pt IV complexes. DFT calculations have been carried out in order to check the energetic validity of the proposed disjunctive reaction mechanisms. From the data obtained, it is clear that the formation of the pentacoordinated species {Pt IV Br(Ph) 2 (C 5 CH 4 CHNZ)} could effectively lead to the standard C-C reductive coupling, but in our case the existence of the parallel insertion process is highly favored. As a consequence the observed reductive elimination reaction can only occur via the otherwise less favored direct C-C coupling on the octahedral [Pt IV Br(Ph) 2 (C 4 CH 4 CHNZ)(SMe 2 )] starting material.

The substitution reactions of SMe 2 by phosphines (PMePh 2 , PEtPh 2 , PPh 3 , P(4-MeC 6 H 4 ) 3 , P(3-MeC 6 H 4 ) 3 , PCy 3 ) 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(CkN)(CH 3 ) 2 (X)(SMe 2 )], have been studied as a function of temperature, solvent, and electronic and steric characteristics of the phosphines and the X and CkN ligands. In all cases, a limiting dissociative mechanism has been found, where the dissociation of the SMe 2 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 CkN imine and X ligands produces differences in the dimethyl sulfide dissociation reactions rates, which can be quantified by the corresponding ∆S q values (72, 64, 48, 31, and 78 J K -1 mol -1 for CkN/X being C