Snapshots of an Alkylidyne for Nitride Triple-Bond Metathesis

The transient titanium alkylidyne complex (PNP)Ti⋮C^tBu (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(NCRC^tBu) (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⋮C^tBu (R = ^tBu, 1; R = Ad, 2) and form neutral or zwitterionic titanium imide species, respectively. Imide formation for the latter species results from complete ^tBuC^3- for N^3- 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.