Synthesis and Reactivity of Iron Complexes with a Biomimetic SCS Pincer Ligand

Abstract

Recent experimental evidence suggests that the FeMoco of nitrogenase undergoes structural rearrangement during N 2 reduction, which may result in the generation of coordinatively unsaturated iron sites with two sulfur donors and a carbon donor. In an effort to synthesize and study small-molecule model complexes with a one-carbon/two-sulfur coordination environment, we have designed two new SCS pincer ligands containing a central NHC donor accompanied by thioether-or thiolatefunctionalized aryl groups. Metalation of the thioether ligand with Fe(OTf) 2 gives 6-coordinate complexes in which the SCS ligand binds meridionally. In contrast, metalation of the thiolate ligand with Fe(HMDS) 2 gives a four-coordinate pseudotetrahedral amide complex in which the ligand binds facially, illustrating the potential structural flexibility of these ligands. Reaction of the amide complex with a bulky monothiol gives a four-coordinate complex with a one-carbon/three-sulfur coordination environment that resembles the resting state of nitrogenase. Reaction of the amide complex with phenylhydrazine gives a product with a rare κ 1 -bound phenylhydrazido group which undergoes N−N cleavage to give a phenylamido complex.

Iron Complexes of a Proton-Responsive SCS Pincer Ligand with a Sensitive Electronic Structure

“…[19][20][21] However, CO-free iron complexes with supporting ligands that coordinate through only carbon and sulfur donors are rare. [22][23][24][25][26][27][28] Of these examples, only one multidentate C/S ligand is known to support N2 binding. 26 In addition, Qu has provided important studies on Cp*-supported iron dimers bridged by dithiolates, and this C/S ligand sphere can bind and facilitate the reduction of nitrogenase-relevant NxHy substrates.…”

“…Namely, we synthesized a new SCS pincer comprised of an N-heterocyclic carbene (NHC) ligand with two bulky thiolate arms, and isolated a number of iron complexes with this scaffold. 28 The geometries at iron sites with this supporting ligand were sometimes tetrahedral and sometimes square planar, indicating significant flexibility in the core. However, this NHC ligand was also sensitive to acids, and the ligand synthesis involved time-consuming protection and deprotection steps.…”

“…Despite being in an intermediate spin state, these bond distances are closer to those in resting state FeMoco than the two previously-reported (SCS)iron complexes, which were in lower oxidation states and thus had longer Fe−C/S bonds. 26,28 The final state of the enzyme during catalysis is an iron-ammonia adduct (E8). 101 Because turnover from E8 to E0 is redox neutral, ammonia would be expected to bind to either the iron(II)…”

Coupling of CO2and epoxides catalysed by novelN-fused mesoionic carbene complexes of nickel(ii)

“…1,2 Therefore, a large variety of neutral and monoanionic pincer ligands have been synthesised in the past decades (Figure 1). 2,3 Due to their unique profiles and modular chemical designs, especially regarding the nature of the coordinating donor atoms (ranging from PNP, 4 PCP, 5 POP, 6 OCO 7 , SCS, 8 CNC, 9 or CCC 10 just to mention a few), numerous breakthroughs have been achieved. One moiety of everincreasing focus in this context is the monoanionic carbazole fragment.…”

Catalytic Reduction of Dinitrogen to Ammonia and Hydrazine Using Iron–Dinitrogen Complexes Bearing Anionic Benzene-Based PCP-type Pincer Ligands

“…Iron-dinitrogen complexes, supported by various carbon donor ligands, have been model compounds of the active site of nitrogenases (Figure 1a) to gain insight into the influence of the carbon atom on the reactivity and the structure. [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] Although their reactivity toward nitrogen fixation has been investigated, 25,26,31,32 there are limited examples of the successful application of these complexes as catalysts to reduce dinitrogen into ammonia and hydrazine. [20][21][22] The catalytic transformation of dinitrogen into ammonia and hydrazine using homogeneous transition-metal complexes under mild reaction conditions has been studied intensively to develop efficient catalysts for nitrogen fixation.…”