Low-Temperature N_2 Binding to Two-Coordinate L_2Fe^0 Enables Reductive Trapping of L_2FeN_2^- and NH_3 Generation

Abstract

The two-coordinate [(CAAC)_2Fe] complex [CAAC=cyclic (alkyl)(amino)carbene] binds dinitrogen at low temperature (T<−80 °C). The resulting putative three-coordinate N_2 complex, [(CAAC)_2Fe(N_2)], was trapped by one-electron reduction to its corresponding anion [(CAAC)_2FeN_2]^− 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)_2Fe^I][BAr^F_4], [(CAAC)_2Fe^0], and [(CAAC)_2Fe^(−I)N_2]^− were all found to be active for the reduction of dinitrogen to ammonia upon treatment with KC_8 and HBAr^F_4^(⋅2) Et_2O at −95 °C [up to (3.4±1.0) equivalents of ammonia per Fe center]. The N_2 reduction activity is highly temperature dependent, with significant N_2 reduction to NH_3 only occurring below −78 °C. This reactivity profile tracks with the low temperatures needed for N_2 binding and an otherwise unavailable electron-transfer step to generate reactive [(CAAC)_2FeN_2]^−.

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