Compelling mechanistic data and identification of the active species in tungsten-catalyzed alkyne polymerizations: conversion of a trianionic pincer into a new tetraanionic pincer-type ligand

Complex [tBuOCO]WC(tBu)(THF)2 (1) {where tBuOCO = [2,6-(tBuC6H3O)2C6H3]3−, THF = tetrahydrofuran} polymerizes acetylenes (R-phenylacetylene (R = H, p-OMe, p-F, 3,5-diCF3), 1-decyne, 3,3-dimethyl-1-butyne, and trimethylsilylacetylene) to form π-conjugating polymers. Upon treating 1 with 2 equiv. of phenylacetylene in toluene-d8 at −35 °C, two isolable products form. These two products are [O2C(tBuC)W(η2-HCCPh)] (2-tttBu) and [O2C(PhC)W(η2-HCCtBu)] (2-Ph) {where OC(tBuC)O = [2,6-(tBuC6H3O)2C6H3(tBuC)]4−, OC(PhC)O = [2,6-(tBuC6H3O)2C6H3(PhC)]4−} and derived from an apparent reductive alkylidyne migratory insertion into a metal–arene bond. Complexes 2-tttBu and 2-Ph polymerize acetylene and a wide variety of monosubstituted acetylenes including phenylacetylene derivatives, 1-decyne, 3,3-dimethyl-1-butyne and trimethylsilylacetylene. With a substrate to catalyst loading ratio of 25 000 : 1, complex 2-tttBu polymerizes phenylacetylene with a turnover number (TON) of 17 233. Additionally, 2-tttBu polymerizes phenylacetylene and 1-decyne with catalytic activities up to 5.64 × 106 gPPA mol−1 h−1 and 7.98 × 106 gPA mol−1 h−1, respectively. 2-tttBu also polymerizes the disubstituted acetylene, 1-phenyl-1-propyne. NMR spectroscopic and single crystal X-ray structural studies provide compelling evidence for polymer chain growth via an insertion ring-expansion mechanism.