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

Abstract

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.