Square‐Planar Ruthenium Alkylidyne Complexes Undergo Stepwise Rather Than Concerted [2 + 2] Cycloadditions with Alkynes

A new entry into square-planar, formally d⁴-configured ruthenium alkylidyne complexes is disclosed, using p-tolyl(trimethylsilyl)diazomethane as a convenient and safe alkylidyne synthon. The method furnished complex 12 supported by an electron-rich PNP-pincer ligand, which undergoes remarkably facile [2 + 2] cycloaddition with electron-rich, electron-deficient, and strained alkynes; these reactions represent the first examples of metallacyclobutadiene formation by a d⁴-configured transition metal alkylidyne complex. Strikingly, however, the cycloadditions proceed by a stepwise mechanism, which stands in marked contrast to the concerted pathway entertained by all prototypical d⁰ and d² Schrock-type alkylidynes, including the molybdenum, tungsten, and rhenium complexes that currently dominate the field of alkyne metathesis. In essence, it is the non-bonding lone pair forming the largely metal-centered HOMO of significant d_z2 character that accounts for this unorthodox behavior. The newly gained insight into this key reactivity determinant also allows the few other known reactions of formally d⁴-configured alkylidene complexes previously described in the literature to be explained and will empower further explorations of their chemistry.