Snapshots of an Alkylidyne for Nitride Triple-Bond Metathesis

The transient titanium alkylidyne complex (PNP)Ti⋮CtBu (PNP = N[2-P(CHMe2)2-4-methylphenyl]2-), [2+2] cycloadds sterically bulky nitriles such as NCR to afford unique examples of kinetically stable azametallacyclobutadienes, (PNP)Ti(NCRCtBu) (R = tBu, 1; Ad, 2). Complexes 1 and 2 can further react with electrophiles such as ClSi(CH3)3 or Al(CH3)3 to extrude the alkyne RC⋮CtBu (R = tBu, 1; R = Ad, 2) and form neutral or zwitterionic titanium imide species, respectively. Imide formation for the latter species results from complete tBuC3- for N3- metathetical exchange. Isotopic labeling studies applying 15N enriched nitrile N⋮CAd clearly reveal the nitrogen atom transfer results from incomplete and complete metathesis of a titanium alkylidyne with a nitrile.