Isolation of an Elusive Phosphametallacyclobutadiene and Its Role in Reversible Carbon-Carbon Bond Cleavage.

The reactivity of phosphaalkynes, the isolobal and isoelectronic congeners to alkynes, with metal alkylidyne complexes is explored in this work. Treating the tungsten alkylidyne [ t BuOCO]W≡C t Bu(THF) 2 (1) with phosphaalkyne (10) results in the formation of [O 2 C( t BuC=)W{η 2 -(P,C)-P≡C-Ad}(THF)] (13- t Bu THF ) and [O 2 C(AdC=)W{η 2 -(P,C)-P≡C- t Bu}(THF)] (13-Ad THF ); derived from the formal reductive migratory insertion of the alkylidyne moiety into a W-C arene bond. Analogous to alkyne metathesis, a stable phosphametallacyclobutadiene complex [ t BuOCO]W[κ 2 -C( t Bu)PC(Ad)] (14) forms upon loss of THF from the coordination sphere of either 13- t Bu THF or 13-Ad THF . Remarkably, the C-C bonds reversibly form/cleave with the addition or removal of THF from the coordination sphere of the formal tungsten(VI) metal center, permitting unprecedented control over the transformation of a tetraanionic pincer to a trianionic pincer and back. Computational analysis offers thermodynamic and electronic reasoning for the reversible equilibrium between 13- t Bu/Ad THF and 14.