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

The ethylene complex (PNP)­Ti­(η²-H₂CCH₂)­(CH₂^<i>t</i>Bu) or (PNP)­TiCH^<i>t</i>Bu­(CH₂^<i>t</i>Bu) (PNP^– = N­[2-P­(CHMe₂)₂-4-methylphenyl]₂) reacts with H₂CPPh₃ to form the κ²-phosphinoalkylidene (PNP)­TiCHPPh₂(Ph) (<b>1</b>). Compound <b>1</b> activates benzene via the transient intermediate [(PNP)­TiCPPh₂] (<b>C</b>). By treatment of (PNP)­TiCH^<i>t</i>Bu­(OTf) with LiCH₂PPh₂, <b>1</b> or its isotopologue (PNP)­TiCDPPh₂(C₆D₅) (<b>1</b>-<i>d</i>₆) can be produced by an independent route involving intermediate <b>C</b>, which activates benzene or benzene-<i>d</i>₆ and dehydrogenates cyclohexane-<i>d</i>₁₂. Addition of MeOTf to <b>1</b> results in elimination of benzene concomitant with the formation of the phosphonioalkylidyne complex, [(PNP)­TiCPPh₂Me­(OTf) (<b>2</b>). Theoretical studies of <b>2</b> suggest a resonance structure having dominant Ti–C triple-bond character with some contribution also from a C–P multiple bond