Synthesis of Mono- and Bis-(allenylidene) Pt(II) Complexes and the Catalytic Application Thereof

Seven unchelated bisphosphine-based allenylidene Pt(II) complexes, including three mono(allenylidene) Pt(II) complexes (3a, 3c–d) and four cis-bis(allenylidene) Pt(II) complexes (4a–d), along with four chelated bisphosphine-based bis(allenylidene) Pt(II) complexes (6b–d and 8), have been developed by using alkynyl triazole salt as the allenylidene source, in which a silver transmetalation reaction of mono(allenylidene) Ag(I)Cl complex 2 was involved as the key step. All of these mono- and bis(allenylidene) Pt(II) complexes were fully characterized by NMR spectroscopy and ESI-MS analysis. Moreover, the crystal structures of the unchelated mono(allenylidene) Pt(II) complex 3c, bis(allenylidene) Pt(II) complex 4a, and the chelated bis(allenylidene) Pt(II) complexes 6c and 8 were elucidated, revealing a square-planar coordination geometry in 3c and 4a, whereas a slightly twisted square-planar arrangement was observed in 6c and 8. In comparison to unchelated counterparts 3c and 4a, a stronger coordination of Pt(II) with the chelated bisphosphine groups and a weakened Pt–C_l complexation were observed in complexes 6c and 8. The catalytic prowess of these bisphosphine-bearing mono(allenylidene) Pt(II) complexes was then evaluated in Pt(II)-catalyzed 1,6-enyne cycloisomerization and Pt(II)-mediated bis-indolylation of cyclic vinyl amines/ethers, resulting in the formation of five methylene pyrrolidine derivatives (11a–e), four tri- or tetra-substituted cyclopropane derivatives (12a–d), and four ring-opening products (14a–d).