A Sulfide-Bridged Diiron(II) Complex with a cis-N2H4Ligand

Isostructural bridging diferrous chalcogenide cores [FeII(μ-E)FeII] (E = O, S, Se, Te) with decreasing antiferromagnetic coupling down the chalcogenide series

Iron compounds containing a bridging oxo or sulfido moiety are ubiquitous in biological systems, but substitution with the heavier chalcogenides selenium and tellurium, however, is much rarer, with only a few examples reported to date. Here we show that treatment of the ferrous starting material [(^tBupyrpyrr₂)Fe(OEt₂)] (1-OEt₂) (^tBupyrpyrr₂ = 3,5-^tBu₂-bis(pyrrolyl)pyridine) with phosphine chalcogenide reagents E = PR₃ results in the neutral phosphine chalcogenide adduct series [(^tBupyrpyrr₂)Fe(EPR₃)] (E = O, S, Se; R = Ph; E = Te; R = ^tBu) (1-E) without any electron transfer, whereas treatment of the anionic starting material [K]₂[(^tBupyrpyrr₂)Fe₂(μ-N₂)] (2-N₂) with the appropriate chalcogenide transfer source yields cleanly the isostructural ferrous bridging mono-chalcogenide ate complexes [K]₂[(^tBupyrpyrr₂)Fe₂(μ-E)] (2-E) (E = O, S, Se, and Te) having significant deviation in the Fe-E-Fe bridge from linear in the case of E = O to more acute for the heaviest chalcogenide. All bridging chalcogenide complexes were analyzed using a variety of spectroscopic techniques, including ¹H NMR, UV-Vis electronic absorbtion, and ⁵⁷Fe Mössbauer. The spin-state and degree of communication between the two ferrous ions were probed via SQUID magnetometry, where it was found that all iron centers were high-spin (S = 2) Fe^II, with magnetic exchange coupling between the Fe^II ions. Magnetic studies established that antiferromagnetic coupling between the ferrous ions decreases as the identity of the chalcogen is tuned from O to the heaviest congener Te.

A Molecular Low-Coordinate [Fe-S-Fe] Unit in Three Oxidation States

A [Fe-S-Fe] subunit with a single sulfide bridging two low-coordinate iron ions is the supposed active site of the iron-molybdenum co-factor (FeMoco) of nitrogenase. Here we report a dinuclear monosulfido bridged diiron(II) complex with a similar complex geometry that can be oxidized stepwise to diiron(II/III) and diiron(III/III) complexes while retaining the [Fe-S-Fe] core. The series of complexes has been characterized crystallographically, and electronic structures have been studied using, inter alia, 57 Fe Mössbauer spectroscopy and SQUID magnetometry. Further, cleavage of the [Fe-S-Fe] unit by CS2 is presented.

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

Recent experimental evidence suggests that the FeMoco of nitrogenase undergoes structural rearrangement during N₂ 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 thiolate-functionalized 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 κ¹-bound phenylhydrazido group which undergoes N-N cleavage to give a phenylamido complex.

A [2Fe-1S] Complex That Affords Access to Bimetallic and Higher-Nuclearity Iron-Sulfur Clusters

Small, coordinatively unsaturated iron-sulfur clusters are conceived as building blocks for the diverse set of shapes of iron-sulfur clusters in biological and synthetic chemistry. Here we describe a synthetic method for preparing [2Fe-1S] clusters containing two iron(II) ions, which are supported by a relatively unhindered β-diketiminate supporting ligand. The [2Fe-1S] cluster can be isolated in the presence of trimethylphosphine, and the compound with one PMe₃ on each iron(II) ion has been crystallographically characterized. The PMe₃ ligands may be removed with B(C₆F₅)₃ to give a spectroscopically characterized species with solvent ligands. This species is a versatile synthon for [2Fe-2S], [4Fe-3S], and [10Fe-8S] clusters. Crystallographic characterization of the 10Fe cluster shows that it has all iron(II) ions, and the core has two [4Fe-4S] cubes that share a face in a novel arrangement. This cluster also has two iron sites that are coordinated to solvent donors, suggesting the potential for using this type of cluster for reactivity in the future.

A three-coordinate Fe(ii) center within a [3Fe-(μ3-S)] cluster that provides an accessible coordination site

A μ3-sulfide bridged triiron cluster(ii,ii,iii) supported by a cyclophane ligand undergoes metal-based reduction to yield an all-ferrous species. The latter complex incorporates a three-coordinate iron center that provides an accessible coordination site to a solvent molecule.

Insight into the Iron-Molybdenum Cofactor of Nitrogenase from Synthetic Iron Complexes with Sulfur, Carbon, and Hydride Ligands

Nitrogenase enzymes are used by microorganisms for converting atmospheric N2 to ammonia, which provides an essential source of N atoms for higher organisms. The active site of the molybdenum-dependent nitrogenase is the unique carbide-containing iron-sulfur cluster called the iron-molybdenum cofactor (FeMoco). On the FeMoco, N2 binding is suggested to occur at one or more iron atoms, but the structures of the catalytic intermediates are not clear. In order to establish the feasibility of different potential mechanistic steps during biological N2 reduction, chemists have prepared iron complexes that mimic various structural aspects of the iron sites in the FeMoco. This reductionist approach gives mechanistic insight, and also uncovers fundamental principles that could be used more broadly for small-molecule activation. Here, we discuss recent results and highlight directions for future research. In one direction, synthetic iron complexes have now been shown to bind N2, break the N-N triple bond, and produce ammonia catalytically. Carbon- and sulfur-based donors have been incorporated into the ligand spheres of Fe-N2 complexes to show how these atoms may influence the structure and reactivity of the FeMoco. Hydrides have been incorporated into synthetic systems, which can bind N2, reduce some nitrogenase substrates, and/or reductively eliminate H2 to generate reduced iron centers. Though some carbide-containing iron clusters are known, none yet have sulfide bridges or high-spin iron atoms like the FeMoco.