1,2-CF bond activation of perfluoroarenes and alkylidene isomers of titanium. DFT analysis of the C-F bond activation pathway and rotation of the titanium alkylidene moiety

Isomeric alkylidene complexes syn- and anti-(PNP)Ti[C^tBu(C ₆F₅)](F) (1) and (PNP)Ti[C^tBu(C ₇F₇)](F) (2) have been generated from C-F bond addition of hexafluorobenzene (C₆F₆) and octafluorotoluene (C ₇F₈) across the alkylidyne ligand of transient (PNP)Ti≡C^tBu (A) (PNP^-N[2-P(CHMe₂) ₂-4-methylphenyl]₂), which was generated from the precursor (PNP)TiCH^tBu(CH₂^tBu). Two mechanistic scenarios for the activation of the C-F bond by A are considered: 1,2-CF addition and [2 + 2]-cycloaddition/β-fluoride elimination. Upon formation of the alkylidenes 1 and 2, the kinetic and thermodynamic alkylidene product is the syn isomer, which gradually isomerizes to the corresponding anti isomer to ultimately establish an equilibrium mixture (when using 1, 65/35) if the solution is heated in benzene to 105 °C for 1 h. Single crystal X-Ray crystallographic data obtained for the two isomers of 2 (and syn isomer of 1) are in good agreement with computed DFT-optimized models. Our calculations suggest convincingly that the isomerization process proceeds via a concerted rotation involving a heterolytic bond cleavage about the alkylidene bond. The two rotamers are thermodynamically very close in energy and interconvert with an estimated barrier of ∼26 kcal/mol. The electronic reason for this unexpectedly low barrier is investigated.