A Transient Iron Carbide Generated by Cyaphide Cleavage

Despite their potential relevance as molecular models for industrial and biological catalysis, well-defined mononuclear iron carbide complexes are unknown, in part due to the limited number of appropriate C₁ synthons. Here, we show the ability of the cyaphide anion (C≡P^–) to serve as a C₁ source. The high spin (S = 2) cyaphide complex PhB(^tBuIm)₃Fe–C≡P (PhB(^tBuIm)₃^– = phenyl(tris(3-tert-butylimidazol-2-ylidene)borate) is readily accessed using the new cyaphide transfer reagent [Mg(^DippNacNac)(CP)]₂ (^DippNacNac = CH{C(CH₃)N(Dipp)}₂ and Dipp = 2,6-di(iso-propyl)phenyl). Phosphorus atom abstraction is effected by the three-coordinate Mo(III) complex Mo(N^tBuAr)₃ (Ar = 3,5-Me₂C₆H₃), which produces the known phosphide (^tBuArN)₃Mo≡P along with a transient iron carbide complex PhB(^tBuIm)₃Fe≡C. Electronic structure calculations reveal that PhB(^tBuIm)₃Fe≡C adopts a doublet ground state with nonzero spin density on the carbide ligand. While isolation of this complex is thwarted by rapid dimerization to afford the corresponding diiron ethynediyl complex, the carbide can be intercepted by styrene to provide an iron alkylidene.