Catalytic transformation of dinitrogen into ammonia and hydrazine by iron-dinitrogen complexes bearing pincer ligand

Abstract

Synthesis and reactivity of iron-dinitrogen complexes have been extensively studied, because the iron atom plays an important role in the industrial and biological nitrogen fixation. As a result, iron-catalyzed reduction of molecular dinitrogen into ammonia has recently been achieved. Here we show that an iron-dinitrogen complex bearing an anionic PNP-pincer ligand works as an effective catalyst towards the catalytic nitrogen fixation, where a mixture of ammonia and hydrazine is produced. In the present reaction system, molecular dinitrogen is catalytically and directly converted into hydrazine by using transition metal-dinitrogen complexes as catalysts. Because hydrazine is considered as a key intermediate in the nitrogen fixation in nitrogenase, the findings described in this paper provide an opportunity to elucidate the reaction mechanism in nitrogenase.

Backbone Dehydrogenation in Pyrrole-Based Pincer Ligands

“…Such a compound would be isoelectronic with the related Fe(II) complex, [FeCl( t Bu PNP)]. 43 This compound is a rare example of square-planar iron(II) and possesses an intermediate spin ( S = 1) ground state. 47 Reaction of 1 d -Cl with Ag(SbF 6 ) in methylene chloride led to an immediate color change from burgundy to light orange and formation of a new arene-insoluble species that we assign as [CoCl( t Bu dPNP)](SbF 6 ) ( 1 d -Cl + , eq 6).…”

Catalytic N2-to-NH3 Conversion by Fe at Lower Driving Force: A Proposed Role for Metallocene-Mediated PCET

“…9,20 We had previously reported that catalytic N 2 reduction with KC 8 and HBAr F 4 yielded no detectable hydrazine, but observed that if hydrazine was added at the outset of a catalytic run, it was consumed. 6 When 5 equiv of N 2 H 4 were added at the beginning of a catalytic run (again with 322 equiv of [Ph 2 NH 2 ][OTf] and 162 equiv of Cp* 2 Co), only 0.22 equiv of N 2 H 4 (4.4% recovery) remained after workup.…”

N2‐to‐NH3 Conversion by a triphos–Iron Catalyst and Enhanced Turnover under Photolysis

“…[22] These facts have motivated our group and others to develop single (or multiple) site Fe complexes that can bind and activate dinitrogen. [15, 17, 18, 23–27] To this end, we have reported the catalytic reduction of nitrogen to ammonia using Fe complexes supported by a tetradentate P 3 E ligand scaffold (E = B, C, or Si). [17, 20, 28–32] Using the P 3 B Fe catalyst, significant turnover to generate NH 3 has been demonstrated.…”

“…[20, 32] Other Fe systems supported by carbene and phosphine ligands have also shown efficacy for catalytic N 2 -to-NH 3 conversion in recent reports. [15, 18] Freeze-quenched 57 Fe Mössbauer spectroscopic studies of a catalytic reaction using our P 3 B Fe(N 2 ) − system have shown that a significant amount of the iron is tied up as an iron hydride–borohydride complex, (P 3 B )(μ-H)Fe(N 2 )(H), believed to be an off-path state of the system; [20] this species can presumably convert back into an on-path P 3 B Fe(N 2 ) {0,−1} species via formal H 2 loss under turnover conditions.…”