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-((CN)-N-boolean AND)(L)] [1(PS): (CN)-N-boolean AND = 2-phenylpyridinate (ppy), L = SMe2; 1(BS): (CN)-N-boolean AND = benzo[h]quinolate (bhq), L = SMe2; 1(PP): (CN)-N-boolean AND = ppy, L = PPh3; and 1(BP): (CN)-N-boolean AND = 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((CN)-N-boolean AND)(CX3CO2)(SMe2)]. When L = PPh3, the selectivity of the reaction is reversed. In the reaction of [PtMe((CN)-N-boolean AND)(PPh3)] with CF(3)CO2(H), the Pt-(CN)-N-boolean AND bond is cleaved rather than the PtMe bond. The latter reaction gave [PtMe(kappa N-1-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-sp(3) bond versus the Pt-C-sp(2) bond. Two pathways including the direct one-step acid attack at the Pt-C bond (S(E)2) 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(E)2) 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 ((CN)-N-boolean AND) is also discussed.

Automated summary

This study investigated the reactivity of cyclometalated platinum(II) complexes with different acids, revealing that the selectivity of the reaction can be influenced by the nature of the ancillary ligand, potentially affecting the cleavage of Pt-C bond. Computational discussions and experimental results suggested that a one-step electrophilic attack at the Pt-C bond is the most likely mechanism for these complexes, with the potential to change the selectivity in Pt-C bond cleavage.