Ligand-Controlled C-H versus C-H Bond Formation in Cycloplatinated Complexes: A Joint Experimental and Theoretical Mechanistic Investigation.

The cyclometalated platinum(II) complexes [PtMe(CN)(L)] [: CN = 2-phenylpyridinate (ppy), L = SMe; : CN = benzo[h]quinolate (bhq), L = SMe; : CN = ppy, L = PPh; and : CN = bhq, L = PPh] containing two different cyclometalated ligands and two different ancillary ligands have been investigated in the reaction with CXCOH (X = F or H). When L = SMe, the Pt-Me bond rather than the Pt-C bond of the cycloplatinated complex is cleaved to give the complexes [Pt(CN)(CXCO)(SMe)]. When L = PPh, the selectivity of the reaction is reversed. In the reaction of [PtMe(CN)(PPh)] with CFCOH, the Pt-CN bond is cleaved rather than the Pt-Me bond. The latter reaction gave [PtMe(κN-Hppy)(PPh)(CFCO)] as an equilibrium mixture of two isomers. For L = PPh, no reaction was observed with CHCOH. The reasons for this difference in selectivity for complexes are computationally discussed based on the energy barrier needed for the protonolysis of the Pt-C bond versus the Pt-C bond. Two pathways including the direct one-step acid attack at the Pt-C bond (S2) and stepwise oxidative-addition on the Pt(II) center followed by reductive elimination [S(ox)] are proposed. A detailed density functional theory (DFT) study of these protonations along with experimental UV-vis kinetics suggests that a one-step electrophilic attack (S2) at the Pt-C bond is the most likely mechanism for complexes , and changing the nature of the ancillary ligand can influence the selectivity in the Pt-C bond cleavage. The effect of the nature of the acid and cyclometalated ligand (CN) is also discussed.