Phosphinoalkylidene and -alkylidyne Complexes of Titanium: Intermolecular C–H Bond Activation and Dehydrogenation Reactions

The ethylene complex
(PNP)­Ti­(η2-H2CCH2)­(CH2tBu) or (PNP)­TiCHtBu­(CH2tBu) (PNP– = N­[2-P­(CHMe2)2-4-methylphenyl]2) reacts with H2CPPh3 to form the κ2-phosphinoalkylidene (PNP)­TiCHPPh2(Ph) (1). Compound 1 activates benzene
via the transient intermediate [(PNP)­TiCPPh2] (C). By treatment of (PNP)­TiCHtBu­(OTf) with LiCH2PPh2, 1 or its isotopologue (PNP)­TiCDPPh2(C6D5) (1-d6) can
be produced by an independent route involving intermediate C, which activates benzene or benzene-d6 and dehydrogenates cyclohexane-d12.
Addition of MeOTf to 1 results in elimination of benzene
concomitant with the formation of the phosphonioalkylidyne complex,
[(PNP)­TiCPPh2Me­(OTf) (2). Theoretical
studies of 2 suggest a resonance structure having dominant
Ti–C triple-bond character with some contribution also from
a C–P multiple bond.