Low-Temperature N2 Binding to Two-Coordinate L2Fe0 Enables Reductive Trapping of L2FeN2 and NH3 Generation


  • This work was supported by the NIH (GM 070757) and the Gordon and Betty Moore Foundation. We thank Larry Henling and Michael K. Takase for crystallographic assistance.


The two-coordinate [(CAAC)2Fe] complex [CAAC=cyclic (alkyl)(amino)carbene] binds dinitrogen at low temperature (T<−80 °C). The resulting putative three-coordinate N2 complex, [(CAAC)2Fe(N2)], was trapped by one-electron reduction to its corresponding anion [(CAAC)2FeN2] at low temperature. This complex was structurally characterized and features an activated dinitrogen unit which can be silylated at the β-nitrogen atom. The redox-linked complexes [(CAAC)2FeI][BArF4], [(CAAC)2Fe0], and [(CAAC)2Fe−IN2] were all found to be active for the reduction of dinitrogen to ammonia upon treatment with KC8 and HBArF4⋅2 Et2O at −95 °C [up to (3.4±1.0) equivalents of ammonia per Fe center]. The N2 reduction activity is highly temperature dependent, with significant N2 reduction to NH3 only occurring below −78 °C. This reactivity profile tracks with the low temperatures needed for N2 binding and an otherwise unavailable electron-transfer step to generate reactive [(CAAC)2FeN2].