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

The ethylene complex (PNP)Ti(n²-H2C-CH2)(CH2tBu) or (PNP)Ti-CHtBu(CH2tBu) (PNP- = N[2-P(CHMe2)2-4-methylphenyl]2) reacts with H2CPPh3 to form the n²-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. [ABSTRACT FROM AUTHOR]