Mechanism of the competition between phenyl insertion and ligand reductive elimination on a hindered platinum(IV) cyclometalated complex

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 [(PtBr)-Br-IV(Ph)(2)(C(5)CH(4)CHNZ)(SMe2)] (Z = Me, Bzl, CH2(2,4,6-Me3C6H2)). In dilute solutions, where the already established lability of the SMe2 ligand favors the existence of the pentacoordinated {(PtBr)-Br-IV(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 [(PtBr)-Br-II(C(4)CH(3)C(6)H(4)CHNZ)(SMe2)], 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 SMe2, 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 [(PtBr)-Br-II(Ph)(C(6)H(3)C(6)H(4)CHNZ)(SMe2)], which evolve rapidly to trans-[(PtBr)-Br-II(Ph)(SMe2)(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, [(PtBr)-Br-II(C(4)CH(3)C(6)H(4)CHNZ)(SMe2)]. 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 {(PtBr)-Br-IV(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 [(PtBr)-Br-IV(Ph)(2)(C(4)CH(4)CHNZ)(SMe2)] starting material.