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

The ethylene complex (PNP)Ti(eta(2)-H2C=CH2)((CH2Bu)-Bu-t) or (PNP)Ti=(CHBu)-Bu-t((CH2Bu)-Bu-t) (PNP- = N[2-P(CHMe2)(2)-4-methylphenyl](2)) reacts with H2CPPh3 to form the kappa(2)-phosphinoalkylidene (PNP)Ti= CHPPh2(Ph) (1). Compound 1 activates benzene via the transient intermediate [(PNP)Ti CPPh2] (C). By treatment of (PNP)Ti=(CHBu)-Bu-t(OTO with LiCH2PPh2, 1 or its isotopologue (PNP)Ti=CDPPh2(C6D5) (1-d(6)) can be produced by an independent route involving intermediate C, which activates benzene or benzene-d(6) and dehydrogenates cyclohexane-d(12). 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.