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

Abstract

The 2-coordinate (CAAC) 2 Fe complex [CAAC = cyclic (alkyl)(amino)carbene] binds dinitrogen at low temperature (T < −80 °C). The resulting putative 3-coordinate N 2 -complex, (CAAC) 2 Fe(N 2 ), was trapped by one electron reduction to its corresponding anion [(CAAC) 2 FeN 2 ] − at low temperature. This complex was structurally characterized and features an activated dinitrogen unit that can be silylated at the β-nitrogen. KeywordsIron; Nitrogen fixation; Low-coordinate; Carbenes While hundreds of transition-metal-N 2 complexes have been prepared and studied, [1] comparatively few systems afford access to productive N 2 functionalization. [2] This is particularly true for the case of N 2 functionalization by protons and electrons to produce NH 3 (or N 2 H 4 ). Building on extensive early Mo and W model work, [3] Schrock [4] and Nishibayashi [5] have reported Mo-containing coordination complexes (A and B in Figure 1, respectively) that facilitate catalytic N 2 reduction to NH 3 in the presence of suitable acids and reductants. Because Fe is (i) the only transition metal known to be essential to enzymatic nitrogenase function, [6] and (ii) the predominant transition metal catalyst used in the Haber-Bosch ammonia synthesis, [7] studying N 2 reduction chemistry at well-defined Fe model complexes is of interest. [8,9] Recently, our lab demonstrated that Fe coordination complexes (C in Figure 1) are capable of modest catalytic N 2 reduction to NH 3 . [10] ** This work was supported by the N.I.H. (GM 070757) and the Gordon and Betty Moore Foundation. We thank Larry Henling and Michael K. Takase for crystallographic assistance.

Coordination chemistry insights into the role of alkali metal promoters in dinitrogen reduction

“…Ung et al reported a low-coordinate Fe-CAAC system (CAAC = cyclic(alkyl)(amino)carbene) that, when reduced with KC 8 in the presence of 18-crown-6, forms (CAAC) 2 Fe(μ-N 2 )K(18-crown-6) ( 17 ) [74]. In contrast to other examples where addition of a crown ether results in sequestration of the coordinated cation, end-on K + still forms a contact ion pair with the anionic Fe-N 2 complex even when chelated by 18-crown-6, similar to the Mo complex 3 .…”

“…The third system capable of catalytic reduction of N 2 to NH 3 has been explored by the Peters group, utilizing Fe tris(phosphino)borane and -alkyl complexes ( 15 , 16 ), which can produce up to 64 equiv NH 3 per Fe depending on the catalyst [73, 97, 98]. The Peters group also recently reported an Fe-CAAC system ( 17 ) that can catalytically reduce N 2 to generate up to 3.4 equiv NH 3 [74]. …”

Remarkable catalytic activity of dinitrogen-bridged dimolybdenum complexes bearing NHC-based PCP-pincer ligands toward nitrogen fixation

“…In 2003, Yandulov and Schrock29303132 reported the first successful example of the catalytic conversion of dinitrogen gas into ammonia using a molybdenum–dinitrogen complex as a catalyst and in 2013 Peters and co-workers33343536373839 reported the iron-catalysed transformation using an iron–dinitrogen complex as a catalyst. We also found that several dinitrogen-bridged dimolybdenum complexes such as [{Mo(N 2 ) 2 (PNP)} 2 (μ-N 2 )] ( 2 ; PNP=2,6-bis(di- tert -butylphosphinomethyl)pyridine)404142 and molybdenum–nitride complexes bearing PNP-type pincer ligands43 or mer -tridentate triphosphine44 worked as more effective catalysts towards ammonia formation under ambient reaction conditions, where up to 63 equiv.…”

Room‐Temperature Functionalization of N2 to Borylamine at a Molybdenum Complex

“…The imido-hydride and di-hydrideamido Mo IV complexes have been isolated and characterized. [2] Most impressively,s everal Mo, [3] Fe, [4] and Co [5] complexes have proven efficient catalysts for NH 3 and N(SiMe 3 ) 3 synthesis,with TON (turn over number) up to circa 415 and 220, respectively. N 2 functionalization using homogeneous metal complexes under mild conditions is one of the most challenging problems facing chemists.…”

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Intermolecular,s tepwise functionalization by BH bonds of a( triphosphine)Mo IV -nitrido complex generated by N 2 splitting is reported. [2] Most impressively,s everal Mo, [3] Fe, [4] and Co [5] complexes have proven efficient catalysts for NH 3 and N(SiMe 3 ) 3 synthesis,with TON (turn over number) up to circa 415 and 220, respectively. Addition of PinBH to the [Mo(H) 2 (N(BPin) 2 )] + complex at room temperature results in the liberation of borylamines from the metal center.

N 2 functionalization using homogeneous metal complexes under mild conditions is one of the most challenging problems facing chemists.

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