Polymerization of phenylacetylene catalyzed by rhodium(i) complexes with N-functionalized N-heterocyclic carbene ligands

A series of neutral [RhX(nbd)(κC-MeIm∩Z)] and cationic [Rh(nbd)(κ2C,N-MeIm∩Z)]+ (X = Cl, Br; MeIm = 3-methylimidazol-2-yliden-1-yl; ∩Z = N-functionalized wingtip; nbd = 2,5-norbornadiene) complexes featuring NHC ligands functionalized with a 1-aminopropyl, 3-dimethylaminopropyl, pyridin-2-ylmethyl, or quinolin-8-ylmethyl substituent have been prepared. These complexes efficiently catalyze the polymerization of phenylacetylene without base as a co-catalyst affording stereoregular polyphenylacetylenes of very high molar mass. Polymers of Mw up to 2 × 106 g mol−1 and moderate dispersity have been prepared with neutral chloro-complexes having aminopropyl wingtips. Catalyst precursors bearing functionalized NHC ligands with a flexible amino-alkyl wingtip are significantly more active than those having a heterocyclic substituent. These complexes are in general much more active than related compounds having N-functionalized phosphine ligands. Polymer characterization by SEC/MALS/DRI analysis has revealed the presence of a fraction of branched polymer of high molar mass in most samples obtained with catalysts having N-heterocyclic substituents at the NHC ligand. The N-donor function at the NHC ligand likely behaves as an internal base for the deprotonation of phenylacetylene to give the initiating alkynyl cationic [Rh(nbd)(CC-Ph)(κC-MeIm∩ZH)]+ species. However, the participation of neutral alkynyl species [Rh(nbd)(CC-Ph)(κC-MeIm∩Z)] should be considered in order to rationalize the notable catalytic activity of some neutral chloro-complexes.