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[CtBu(C6F5)](F) (1) and (PNP)Ti[CtBu(C7F7)](F) (2) have been generated from C–F bond addition of hexafluorobenzene (C6F6) and octafluorotoluene (C7F8) across the alkylidyne ligand of transient (PNP)Ti≡CtBu (A) (PNP−N[2-P(CHMe2)2-4-methylphenyl]2), which was generated from the precursor (PNP)TiCHtBu(CH2tBu). 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.

Graphical abstractThe mechanism to C–F activation by a transient titanium alkylidyne as well as rotation about a titanium alkylidene has been investigated.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The mechanism to carbon–hydrogen bond activation involving titanium–carbon multiple bonds is studied. ► Carbon–fluoride bond addition is slightly favored over the cycloaddition pathway. ► We examine the mechanism to titanium-alkylidene isomerization. ► Rotation about the titanium–carbon double bond involves small change in the TiC bond.