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

(η4-Tetrafluorobenzobarrelene)-η1-((tri-4-fluorophenyl)­phosphine)-η1-(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 103Rh and 31P-103Rh­{1H, 103Rh} 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. 103Rh and 2D 31P-103Rh­{1H} 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 π-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 η2–π interactions and different steps in the catalyst initiation process.