Isolable dicarbon stabilized by a single phosphine ligand

In contrast to naturally occurring F₂, O₂ and N₂, diatomic C₂ is an intriguing species that has only been observed indirectly in the gas phase, and because of its high reactivity has eluded isolation in the condensed phase. It has previously been stabilized in L→C₂←L compounds but the bonding situation of the central C₂ in this motif differs remarkably from that of free C₂. Here we have prepared and structurally characterized diatomic C₂ as a monoligated complex L→C₂ using a bulky phosphine ligand bearing two imidazolidin-2-iminato groups (L is (NHC^R=N)₂(CH₃)P, where NHC^R is an N-heterocyclic carbene). The compound is stable in solution at ambient temperature and has also been isolated in the solid state. Reactivity studies, in combination with quantum chemical analysis, suggest that the two carbon atoms of the L→C₂ complex both have carbene character. The complex underwent intermolecular C-H bond activation upon thermolysis and exhibited hydroalkoxylation-like reactivity with methanol.