Gold(i) complexes bearing a PNP-type pincer ligand: photophysical properties and catalytic investigations

Abstract

The synthesis and characterization of two dinuclear and five tetranuclear gold(I) complexes bearing the 2,6-bis(diphenylphosphinomethyl)pyridine diphosphane ligand (DPPMPY) are herein reported. The reaction between the dinuclear complexes, DPPMPY(AuCl)₂ (1) or DPPMPY(AuBr)₂ (2), with 1 or 2 equivalents of Ag salts yielded five tetranuclear gold(I) complexes, DPPMPY₂Au₄X₂ (3–7), differing in the terminal ancillary ligands (X = Cl, Br, acetonitrile) and the counter ions (SbF₆⁻ or BF₄⁻). The structures of complexes 1, 2, 3, and 5 were confirmed by single-crystal X-ray diffraction studies. The Au⋯Au distances found in complexes 3 and 5 are in the range of aurophilic interactions and the arrangement of the Au atoms varies from a linear arrangement in complex 3 to a zigzag arrangement in complex 5. The photophysical characterization of the compounds was performed both in solution and in the solid state. Very high emission quantum yields were observed for the acetonitrile complexes 4 and 6 in the solid state. The use of this family of gold(I) complexes as catalysts for lactone synthesis via oxidative heteroarylation of alkenes was investigated and yields up to ca. 65% were obtained. Dicationic halide complexes 3 and 5 showed a slight enhancement of the yield of the catalytic reaction, indicating that there is no influence of the counter ion employed on the reaction outcome. Luminescence techniques have been also used to follow the progress of the catalytic reaction.