(η4-Tetrafluorobenzobarrelene)-η1-((tri-4-fluorophenyl)phosphine)-η1-(2-phenylphenyl)rhodium(I): A Catalyst for the Living Polymerization of Phenylacetylenes

(eta(4)-Tetrafluorobenzobarrelene)-eta(1)-((tri-4-fluorophenyl) phosphine)-eta(1)-(2-phenylphenyl) rhodium(I), Rh-(tfb)(biph)(PAr3), was prepared and evaluated as a catalyst in the controlled, stereospecific (co)polymerization of arylacetylenes. Following recrystallization, the complex was characterized by single-crystal X-ray diffraction, elemental analysis, and multinuclear NMR spectroscopy, including Rh-103 and P-31-Rh-103{H-1, Rh-103} heteronuclear multiple quantum coherence (HMQC) experiments. Single-crystal X-ray diffraction indicated that Rh(tfb)(biph)(PAr3) adopts a slightly distorted square-planar geometry consistent with previously reported tetracoordinate rhodium(I)-aryl and -vinyl complexes. Rh-103 and 2D P-31-Rh-103{H-1} HMQC NMR spectroscopy confirmed the purity and stability of the new complex in solution. In the presence of excess P(4-FC6H4)(3) as a rate modifier, the Rh(I)-aryl catalyst mediated the homopolymerization of phenylacetylene, PhC2H, in a controlled manner as evidenced from the linearity of the pseudo-first-order kinetic plots, the evolution of molecular weight and dispersity, and the quantitative crossover efficiency in a self-blocking experiment. The broader utility of Rh(tfb)(biph)(PAr3) was demonstrated in the polymerization of a series of functional arylacetylenes, including 4-trifluoromethoxyphenylacetylene and 3,4-dichlorophenylacetylene, as well as in the preparation of well-defined AB diblock copolymers of phenylacetylene with the trifluoromethoxy and dichloro derivatives. Computational studies using density functional theory allowed a quantitative comparison between Rh(tfb)(biph)(PAr3) and the 2,5-norbornadiene (nbd) analogue, Rh(nbd)(biph)(PAr3). Results indicated that the former has a lower HOMO energy compared to the nbd derivative and is consistent with the enhanced p-acidity of the tetrafluorobenzobarrelene ligand species. Calculations similarly indicated that Rh(tfb)(biph)(PAr3) has a slightly higher binding affinity for PhC2H. Finally, we highlight the role the biph aryl ligand plays in stabilizing the rhodium complex through a C-H agostic and eta(2)-p interactions and different steps in the catalyst initiation process.

Automated summary

Rh-(tfb)(biph)(PAr3) is a catalyst for the controlled, stereospecific copolymerization of arylacetylenes, characterized by its slightly distorted square-planar geometry and stability in solution. Computational studies indicated its enhanced p-acidity and binding affinity for phenylacetylene compared to the 2,5-norbornadiene analog.