Chelating and Bridging Roles of 2-(2-Pyridyl)benzimidazole and Bis(diphenylphosphino)acetylene in Stabilizing a Cyclic Tetranuclear Platinum(II) Complex

Ligand-Controlled Csp2-H versus Csp3-H Bond Formation in Cycloplatinated Complexes: A Joint Experimental and Theoretical Mechanistic Investigation

The cyclometalated platinum(II) complexes [PtMe(C^∧N)(L)] [1_PS: C^∧N = 2-phenylpyridinate (ppy), L = SMe₂; 1_BS: C^∧N = benzo[h]quinolate (bhq), L = SMe₂; 1_PP: C^∧N = ppy, L = PPh₃; and 1_BP: C^∧N = bhq, L = PPh₃] containing two different cyclometalated ligands and two different ancillary ligands have been investigated in the reaction with CX₃CO₂H (X = F or H). When L = SMe₂, the Pt-Me bond rather than the Pt-C bond of the cycloplatinated complex is cleaved to give the complexes [Pt(C^∧N)(CX₃CO₂)(SMe₂)]. When L = PPh₃, the selectivity of the reaction is reversed. In the reaction of [PtMe(C^∧N)(PPh₃)] with CF₃CO₂H, the Pt-C^∧N bond is cleaved rather than the Pt-Me bond. The latter reaction gave [PtMe(κ¹N-Hppy)(PPh₃)(CF₃CO₂)] as an equilibrium mixture of two isomers. For L = PPh₃, no reaction was observed with CH₃CO₂H. The reasons for this difference in selectivity for complexes 1 are computationally discussed based on the energy barrier needed for the protonolysis of the Pt-C_sp³ bond versus the Pt-C_sp² bond. Two pathways including the direct one-step acid attack at the Pt-C bond (S_E2) and stepwise oxidative-addition on the Pt(II) center followed by reductive elimination [S_E(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 (S_E2) at the Pt-C bond is the most likely mechanism for complexes 1, 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 (C^∧N) is also discussed.