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 [tBuOCO]W≡CtBu(THF)2 (1) with phosphaalkyne (10) results in the formation of [O2C(tBuC=)W{η2‐(P,C)−P≡C−Ad}(THF)] (13‐tBuTHF) and [O2C(AdC=)W{η2‐(P,C)−P≡C−tBu}(THF)] (13‐AdTHF); derived from the formal reductive migratory insertion of the alkylidyne moiety into a W−Carene bond. Analogous to alkyne metathesis, a stable phosphametallacyclobutadiene complex [tBuOCO]W[κ2‐C(tBu)PC(Ad)] (14) forms upon loss of THF from the coordination sphere of either 13‐tBuTHF or 13‐AdTHF. 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‐tBu/AdTHF and 14. The elusive phosphametallacyclobutadiene intermediate is isolated and characterized. This intermediate reveals insights into rapid and reversible C−C bond formation/cleavage, enabling the cycling between a trianionic pincer and a tetraanionic pincer ligand at will.