Mixed-Valent Diiron μ-Carbyne, μ-Hydride Complexes: Implications for Nitrogenase

A Diiron(I/II) μ-1,2-Carbonyl Complex Relevant to CO Binding on Fe(111)

Fischer-Tropsch conversion of syngas to hydrocarbons is proposed to begin with CO binding to the iron surface of the catalyst. CO adsorption on various iron facets of relevance to the Fischer-Tropsch process suggest that the Fe(111) surface is the most active for catalysis, and that CO bound to the penultimate layer of Fe atoms or the b-state is the resting state during catalysis. Notably, a μ-1,2 mode was discarded for the b-state due to a lack of exemplar molecular species and expectation that such a mode would have a higher energy infrared (IR) absorption than observed experimentally (viz. 1735-1860 cm-1). Here, we report the synthesis of a diiron(I/II) complex in which CO binds μ-1,2: (Fe(OTf))(Fe(THF)(μ-1,2-CO))L where L2- is a bis(β-diketiminate) cyclophane (1). Surprisingly, the observed νCO at 1763 cm-1 for 1 compares well with that reported for b-state. Electron paramagnetic resonance (EPR), Mössbauer, and density functional theory (DFT) results support a weakly coupled s = 3/2 iron(I) and s = 2 iron(II) pair. Reduction of 1 results in C-O cleavage and C-C bond formation to yield a ketenylidene (CCO) complex as a major product observed spectroscopically.

Molybdenum-Iron-Sulfur Cluster with a Bridging Carbide Ligand as a Partial FeMoco Model: CO Activation, EPR Studies, and Bonding Insight

Nitrogenase enzymes catalyze the reduction of N2 to NH3 at a complex Fe-M (M = Mo, Fe, or V) cofactor (FeMco), which displays eight metal centers and sulfide and carbide bridges with a MFe7S8C composition. The role of the unusual μ6-carbide ligand and its effects on the metal centers remain unclear. Here, we describe the transfer of a carbide ligand to a MoFe3S3 cluster supported by a bisphenoxide ligand from a previously reported terminal Mo carbide complex to yield a pentametallic cluster of MoS3Fe3CMo composition, which also displays a bridging CO that resembles the lo-CO form of nitrogenase. This cluster has an S = 1/2 spin state amenable to studies by pulse EPR spectroscopy, revealing a significantly larger carbide 13C hyperfine interaction (aiso(13C) = 12.5 MHz) than any observed for various states of FeMoco studied by EPR thus far (|aiso(13C)| = 0.89 to 2.7 MHz). This report provides a strategy for the synthesis of carbide-containing iron-sulfur clusters relevant to nitrogenase cluster modeling, as well as benchmarking information for the metal-carbon interactions by EPR methods.

Spectroscopic and Computational Interrogation of a High-Valent Nickel-Dialkyl Complex Indicates Electronic Structure Asymmetry Drives C-C Bond Formation Reactivity

The study of high-valent organometallic nickel compounds has gained considerable interest recently, primarily driven by the development of nickel-catalyzed alkyl-alkyl cross-coupling reactions that are proposed to employ such high-valent intermediates. In that regard, we have recently reported a formal Ni(III)-dimethyl intermediate supported by the ligand N,N',N″-triisopropyl-1,4,7-triazacyclononane (iPr3tacn) that can undergo rapid C-C reductive elimination and catalyze alkyl-alkyl Kumada cross-coupling reactions. The bulky nature of this tridentate ligand was suggested to lead to two geometrically and electronically inequivalent alkyl groups bound to the five-coordinate Ni center. Herein, we have employed pulsed electron paramagnetic resonance techniques such as electron nuclear double resonance, hyperfine sublevel correlation, and electron spin echo envelope modulation to provide strong experimental evidence for the geometrically and electronically inequivalent nature of the two methyl groups in which one methyl ligand can be better described as a methyl radical. These experimental results were supported by density functional theory computational methods used to probe the covalent nature of the Ni-C bonds and the formal Ni oxidation state assignment for this catalytically relevant, high-valent Ni intermediate. Moreover, computational investigation of a series of related methyl/alkyl analogs reveals that the radical character of an alkyl group increases for a tertiary vs a secondary vs a primary alkyl group, with direct relevance for alkyl-alkyl cross-coupling catalysis. Overall, this study provides valuable insights into the nature of organometallic Ni-dialkyl species that undergo efficient reductive elimination, likely through an SH2-type mechanism.

Low-Coordinate Iron Hydride Chemistry at an N,N,C-Heteroscorpionate Platform

Locally high-spin iron hydrides are proposed to play a critical role as intermediates in iron-molybdenum cofactor (FeMoco)-catalyzed N2 fixation. Inspired by these biological systems, we report herein our initial investigations into low-coordinate iron hydride chemistry supported by our N,N,C-heteroscorpionate ligands. Those ligands with smaller steric profiles are unable to completely suppress the formation of a binuclear [Fe(μ2-H)]2 complex; however, the incorporation of more substantial steric bulk allows for the isolation of a rare example of a terminal, high-spin (S = 2) Fe2+ hydride. Fourier transform infrared spectroscopy suggests an unusually weak Fe-H bond in the latter molecule. Mössbauer spectroscopies, coupled with density functional theory calculations, highlights the substantial influence of the terminal hydride ligand on 57Fe isomer shift.

Terminal Hydride Complex of High-Spin Mn

The iron-molybdenum cofactor of nitrogenase (FeMoco) catalyzes fixation of N2 via Fe hydride intermediates. Our understanding of these species has relied heavily on the characterization of well-defined 3d metal hydride complexes, which serve as putative spectroscopic models. Although the Fe ions in FeMoco, a weak-field cluster, are expected to adopt locally high-spin Fe2+/3+ configurations, synthetically accessible hydride complexes featuring d5 or d6 electron counts are almost exclusively low-spin. We report herein the isolation of a terminal hydride complex of four-coordinate, high-spin (d5; S = 5/2) Mn2+. Electron paramagnetic resonance and electron-nuclear double resonance studies reveal an unusually large degree of spin density on the hydrido ligand. In light of the isoelectronic relationship between Mn2+ and Fe3+, our results are expected to inform our understanding of the valence electronic structures of reactive hydride intermediates derived from FeMoco.

Highly Activated Terminal Carbon Monoxide Ligand in an Iron-Sulfur Cluster Model of FeMco with Intermediate Local Spin State at Fe

Nitrogenases, the enzymes that convert N2 to NH3, also catalyze the reductive coupling of CO to yield hydrocarbons. CO-coordinated species of nitrogenase clusters have been isolated and used to infer mechanistic information. However, synthetic FeS clusters displaying CO ligands remain rare, which limits benchmarking. Starting from a synthetic cluster that models a cubane portion of the FeMo cofactor (FeMoco), including a bridging carbyne ligand, we report a heterometallic tungsten-iron-sulfur cluster with a single terminal CO coordination in two oxidation states with a high level of CO activation (νCO = 1851 and 1751 cm-1). The local Fe coordination environment (2S, 1C, 1CO) is identical to that in the protein making this system a suitable benchmark. Computational studies find an unusual intermediate spin electronic configuration at the Fe sites promoted by the presence the carbyne ligand. This electronic feature is partly responsible for the high degree of CO activation in the reduced cluster.

Valence Delocalization and Metal-Metal Bonding in Carbon-Bridged Mixed-Valence Iron Complexes

The carbide ligand in the iron-molybdenum cofactor (FeMoco) in nitrogenase bridges iron atoms in different oxidation states, yet it is difficult to discern its ability to mediate magnetic exchange interactions due to the structural complexity of the cofactor. Here, we describe two mixed-valent diiron complexes with C-based ketenylidene bridging ligands, and compare the carbon bridges with the more familiar sulfur bridges. The ground state of the [Fe2 (μ-CCO)2 ]+ complex with two carbon bridges (4) is S=1 / 2 ${{ 1/2 }}$ , and it is valence delocalized on the Mössbauer timescale with a small thermal barrier for electron hopping that stems from the low Fe-C force constant. In contrast, one-electron reduction of the [Fe2 (μ-CCO)] complex with one carbon bridge (2) affords a mixed-valence species with a high-spin ground state (S=7 / 2 ${ 7/2 }$ ), and the Fe-Fe distance contracts by 1 Å. Spectroscopic, magnetic, and computational studies of the latter reveal an Fe-Fe bonding interaction that leads to complete valence delocalization. Analysis of near-IR intervalence charge transfer transitions in 5 indicates a very large double exchange constant (B) in the range of 780-965 cm-1 . These results show that carbon bridges are extremely effective at stabilizing valence delocalized ground states in mixed-valent iron dimers.

Synthesis of a Fe3-Carbyne Motif by Oxidation of an Alkyl Ligated Iron-Sulfur (WFe3S3) Cluster

The presence of a carbide ligand in the active site of nitrogenases remains an unusual example of organometallic chemistry employed by a protein. Carbide incorporation into the MFe7S9C cluster involves complex biosynthesis, but analogous synthetic methodologies are limited. Herein, we present a new synthetic strategy for incorporating carbon based bridging ligands into iron-sulfur clusters. Starting from a halide precursor, a WFe3S3 cluster displaying three terminal alkyl ligands and an open Fe3 face was prepared. Oxidation results in loss of alkane and formation of a μ3-carbyne. Characterization of these clusters and mechanistic studies are presented.

Probing Redox Non-Innocence in Iron-Carbene Complexes {Fe=C(H)Ar}10-11 by 1,2 H and 13 C Pulse Electron Paramagnetic Resonance

We report the synthesis and spectroscopic characterization of a series of iron-carbene complexes in redox states {Fe=C(H)Ar}^10-11 . Pulse EPR studies of the ^1,2 H and ¹³ C isotopologues of {Fe=C(H)Ar}¹¹ reveal the high covalency of the Fe-carbene bonding, leading to a more even spin distribution than commonly observed for reduced Fischer carbenes.

Partial synthetic models of FeMoco with sulfide and carbyne ligands: Effect of interstitial atom in nitrogenase active site

Nitrogen-fixing organisms perform dinitrogen reduction to ammonia at an Fe-M (M = Mo, Fe, or V) cofactor (FeMco) of nitrogenase. FeMco displays eight metal centers bridged by sulfides and a carbide having the MFe₇S₈C cluster composition. The role of the carbide ligand, a unique motif in protein active sites, remains poorly understood. Toward addressing how the carbon bridge affects the physical and chemical properties of the cluster, we isolated synthetic models of subsite MFe₃S₃C displaying sulfides and a chelating carbyne ligand. We developed synthetic protocols for structurally related clusters, [Tp*M'Fe₃S₃X]^n-, where M' = Mo or W, the bridging ligand X = CR, N, NR, S, and Tp* = Tris(3,5-dimethyl-1-pyrazolyl)hydroborate, to study the effects of the identity of the heterometal and the bridging X group on structure and electrochemistry. While the nature of M' results in minor changes, the chelating, μ₃-bridging carbyne has a large impact on reduction potentials, being up to 1 V more reducing compared to nonchelating N and S analogs.

Terminal, Open-Shell Mo Carbide and Carbyne Complexes: Spin Delocalization and Ligand Noninnocence

Open-shell compounds bearing metal-carbon triple bonds, such as carbides and carbynes, are of significant interest as plausible intermediates in the reductive catenation of C₁ oxygenates. Despite the abundance of closed-shell carbynes reported, open-shell variants are very limited, and an open-shell carbide has yet to be reported. Herein, we report the synthesis of the first terminal, open-shell carbide complexes, [K][1] and [1][BAr^F₄] (1 = P2Mo(≡C:)(CO), P2 = a terphenyl diphosphine ligand), which differ by two redox states, as well as a series of related open-shell carbyne complexes. The complexes are characterized by single-crystal X-ray diffraction and NMR, EPR, and IR spectroscopies, while the electronic structures are probed by EPR studies and DFT calculations to assess spin delocalization. In the d₁ complexes, the spin is primarily localized on the metal (∼55-77% Mo d_xy) with delocalization on the triply bonded carbon of ∼0.05-0.09 e^-. In the reduced carbide [K][1], a direct metal-arene interaction enables ancillary ligand reduction, resulting in reduced radical character on the terminal carbide (⩽0.02 e^-). Reactivity studies with [K][1] reveal the formation of mixed-valent C-C coupled products at -40 °C, illustrating how productive reactivity manifolds can be engendered through the manipulation of redox states. Combined, the results inform on the electronic structure and reactivity of a new and underrepresented class of compounds with potential significance to a wide array of reactions involving open-shell species.

Structure, reactivity, and spectroscopy of nitrogenase-related synthetic and biological clusters

The reduction of dinitrogen (N2) is essential for its incorporation into nucleic acids and amino acids, which are vital to life on earth. Nitrogenases convert atmospheric dinitrogen to two ammonia molecules (NH3) under ambient conditions. The catalytic active sites of these enzymes (known as FeM-cofactor clusters, where M = Mo, V, Fe) are the sites of N2 binding and activation and have been a source of great interest for chemists for decades. In this review, recent studies on nitrogenase-related synthetic molecular complexes and biological clusters are discussed, with a focus on their reactivity and spectroscopic characterization. The molecular models that are discussed span from simple mononuclear iron complexes to multinuclear iron complexes and heterometallic iron complexes. In addition, recent work on the extracted biological cofactors is discussed. An emphasis is placed on how these studies have contributed towards our understanding of the electronic structure and mechanism of nitrogenases.