Pt(II) N-Heterocyclic carbene and cyclometallated complexes: Synthesis, photoluminescence and hydrosilylation

Abstract

The caffeine derived N-heterocyclic carbene (NHC) complex [PtCl(Me4-Xny)(Phpy)] (Phpy = 2-phenylpyridine and Me4-Xny = 1,3,7,9-tetramethylxanthine-8-ylidene), 7 has the opposite stereochemistry and a shorter Pt¿C carbene bond compared to that of a recently reported analogous benzimidazole derived N,N-heterocyclic carbene (NNHC) Pt(Phpy) complex. These suggest a lower trans influence of pyridyl N compared to cyclometallated carbon and an increased Pt-NHC p-backbonding because of decreased p-donation resulting from conjugation to the electron deficient pyrimidine of caffeine. Moreover, 7 has a lower emission quantum yield and is blue-shifted into the green region of the visible spectrum relative to a closely related non-carbene [PtCl2(Phpy)(Me4-XnH)] (Me4-XnH = 1,3,7,9-tetramethylxanthinium) complex 6. For the catalytic hydrosilylation of phenylacetylene with triethylsilane, 6 is more active than 7 giving a higher product conversion over a shorter duration of time. This suggests that the coordination of NHC to Pt might reduce or deccelerate the catalytic activity of Pt(II) cyclometallated complex.