Synthesis\nof Mono<i>-</i> and Bis<i>-</i>(allenylidene)\nPt(II) Complexes and the Catalytic Application\nThereof

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