Proton-Coupled Reduction of an Iron Cyanide Complex to Methane and Ammonia

Catalytic Reduction of Cyanide to Ammonia and Methane at a Mononuclear Fe Site

Nitrogenase enzymes catalyze nitrogen reduction (N2R) to ammonia and also the reduction of non-native substrates, including the 7H+/6e- reduction of cyanide to CH4 and NH3. CN- and N2 are isoelectronic, and it is hence fascinating to compare the mechanisms of synthetic Fe catalysts capable of both CN- and N2 reduction. Here, we describe the catalytic reduction of CN- to NH3 and CH4 by a highly selective (P3Si)Fe(CN) catalyst (P3Si represents a tris(phosphine)silyl ligand). Catalysis is driven in the presence of excess acid ([Ph2NH2]OTf) and reductant ((C6H6)2Cr), with turnover as high as 73 demonstrated. This catalyst system is also modestly competent for N2R and structurally related to other tris(phosphine)Fe-based N2R catalysts. The choice of catalyst and reductant is important to observe high yields. Mechanistic studies elucidate several intermediates of CN- reduction, including iron isocyanides (P3SiFeCNH+/0) and terminal iron aminocarbynes (P3SiFeCNH2+/0). Aminocarbynes are isoelectronic to iron hydrazidos (Fe═N-NH2+/0), which have been invoked as selectivity-determining intermediates of N2R (NH3 versus N2H4 products). For the present CN- reduction catalysis, reduction of aminocarbyne P3SiFeCNH2+ is proposed to be rate but not selectivity contributing. Instead, by comparison with the reactivity of a methylated aminocarbyne analogue (P3SiFeCNMe2), and associated computational studies, formation of a Fischer carbene (P3SiFeC(H)(NH2)+) intermediate that is on path for either CH4 and NH3 (6 e-) or CH3NH2 (4 e-) products is proposed. From this carbene intermediate, pathways to the observed CH4 and NH3 products (distinct from CH3NH2 formation) are considered to compare and contrast the (likely) mechanism/s of CN- and N2 reduction.

A Triplet Iron Carbyne Complex

Tuning the spin state of metal carbynes, which have broad applications in organic synthesis and material science, presents a formidable challenge for modern chemists as the strong field nature of carbyne ligands dictates low-spin ground spin states (S = 0 or 1/2) for known metal carbynes. Through the oxidative addition reaction of a low-coordinate iron(0) N-heterocyclic carbene complex with the C-S bond of a thioazole-2-ylidene, we synthesized the first triplet (S = 1) metal terminal carbyne, an iron cyclic carbyne complex. Different from the classical metal carbynes, the triplet complex features an LXZ-type carbyne ligand and a weak Fe≡C triple bond, which endow it with the unique reactivity pattern of facile carbyne coupling, weak affinity toward nucleophiles, and facial addition reactions with electrophiles.

Ammonia from dinitrogen at ambient conditions by organometallic catalysts

Fixation of atmospheric dinitrogen in plants by [Mo-Fe] cofactor of nitrogenase enzyme takes place efficiently under atmospheric pressure and normal temperature. In search for an alternative methodology for the highly energy intensive Haber-Bosch process, design and synthesis of highly efficient inorganic and organometallic complexes by mimicking the structure and function of [Mo-Fe] cofactor system is highly desirable for ammonia synthesis from dinitrogen. An ideal catalyst for ammonia synthesis should effectively catalyse the reduction of dinitrogen in the presence of a proton source under mild to moderate conditions, and thereby, significantly reducing the cost of ammonia production and increasing the energy efficacy of the process. In the light of current research, it is evident that there is a plenty of scope for the development and enhanced performance of the inorganic and organometallic catalysts for ammonia synthesis under ambient temperature and pressure. The review furnishes a comprehensive outlook of numerous organometallic catalysts used in the synthesis of ammonia from dinitrogen in the past few decades.

Organometallic Chemistry of Transition Metal Alkylidyne Complexes Centered at Metathesis Reactions

Transition metals form a variety of alkylidyne complexes with either a d⁰ metal center (high-valent) or a non-d⁰ metal center (low-valent). One of the most interesting properties of alkylidyne complexes is that they can undergo or mediate metathesis reactions. The most well-studied metathesis reactions are alkyne metathesis involving high-valent alkylidynes. High-valent alkylidynes can also undergo metathesis reactions with heterotriple bonded species such as N≡CR, P≡CR, and N≡NR⁺. Metathesis reactions involving low-valent alkylidynes are less known. Highly efficient alkyne metathesis catalysts have been developed based on Mo(VI) and W(VI) alkylidynes. Catalytic cross-metathesis of nitriles with alkynes has also been achieved with M(VI) (M = W, Mo) alkylidyne or nitrido complexes. The metathesis activity of alkylidyne complexes is sensitively dependent on metals, supporting ligands and substituents of alkylidynes. Beyond metathesis, metal alkylidynes can also promote other reactions including alkyne polymerization. The remaining shortcomings and opportunities in the field are assessed.

Formation of thiolate-bridged diiron complexes featuring anionic isocyanide originating from the activation of counterions in the outer sphere

Transition metal isocyanide complexes have attracted increasing attention owing to their versatile applications in catalytic organic transformations. Compared with metal complexes with neutral isocyanide ligands, those featuring anionic isocyanide groups are relatively rare and poorly understood. So far, there has been no report on structurally characterized metal anionic isocyanopentafluorophosphate complexes that may have potential for the development of some unique polymerization reactions. In this paper, we adopt a dicationic thiolate-bridged diiron complex as the reaction platform for the coordination activation and functionalization of cyanide. When treating with KCN, a facile salt metathesis with hexafluorophosphate anions occurred to generate monocyanide or dicyanide species. However, using trimethylsilyl cyanide as the substrate, an unsymmetrical diiron complex bearing a terminal [CNSiMe₃] ligand and an anionic [NCPF₅]^- group derived from the activation of one non-coordinating anion PF₆^- was obtained in a high yield. Interestingly, due to the lability of the N-Si bond in the [CNSiMe₃] ligand, it can play the role of an active site for the interaction with counter anions in the outer sphere. On one hand, this labile ligand can facilitate the activation of the P-F bond in PF₆^- and the C-B bond in BPh₄^- to afford structurally characterized thiolate-bridged diiron anionic isocyanopentafluorophosphate and isocyanotriphenylborate complexes, respectively. On the other hand, it can also interact with Lut·HCl to convert into a cyanide ligand stabilized by a hydrogen bonding interaction. This work represents a new synthetic pathway to furnish metal anionic isocyanide complexes.

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.

Multiple N-H and C-H Hydrogen Atom Abstractions Through Coordination-Induced Bond Weakening at Fe-Amine Complexes

We report the use of the reported Fe-phthalocyanine complex, PcFe (1; Pc = 1,4,8,11,15,18,22,25-octaethoxy-phthalocyanine), to generate PcFe-amine complexes 1-(NH₃)₂, 1-(MeNH₂)₂, and 1-(Me₂NH)₂. Treatment of 1 or 1-(NH₃)₂ to an excess of the stable aryloxide radical, 2,4,6-tritert-butylphenoxyl radical (^tBuArO^•), under NH₃ resulted in catalytic H atom abstraction (HAA) and C-N coupling to generate the product 4-amino-2,4,6-tritert-butylcyclohexa-2,5-dien-1-one (2) and ^tBuArOH. Exposing 1-(NH₃)₂ to an excess of the trityl (CPh₃) variant, 2,6-di-tert-butyl-4-tritylphenoxyl radical (^TrArO^•), under NH₃ did not lead to catalytic ammonia oxidation as previously reported in a related Ru-porphyrin complex. However, pronounced coordination-induced bond weakening of both α N-H and β C-H in the alkylamine congeners, 1-(MeNH₂)₂ and 1-(Me₂NH)₂, led to multiple HAA events yielding the unsaturated cyanide complex, 1-(MeNH₂)(CN), and imine complex, 1-(MeN═CH₂)₂, respectively. Subsequent C-N bond formation was also observed in the latter upon addition of a coordinating ligand. Detailed computational studies support an alternating mechanism involving sequential N-H and C-H HAA to generate these unsaturated products.

A computational study of the electrochemical cyanide reduction for ambient ammonia production on a nickel cathode

Electroreduction of cyanide can proceed through one of two pathways. The first one results in the generation and desorption of methylamine as major product, whereas the second path the formation of this species, yielding methane and ammonia.

The Spectroscopy of Nitrogenases

Nitrogenases are responsible for biological nitrogen fixation, a crucial step in the biogeochemical nitrogen cycle. These enzymes utilize a two-component protein system and a series of iron-sulfur clusters to perform this reaction, culminating at the FeMco active site (M = Mo, V, Fe), which is capable of binding and reducing N2 to 2NH3. In this review, we summarize how different spectroscopic approaches have shed light on various aspects of these enzymes, including their structure, mechanism, alternative reactivity, and maturation. Synthetic model chemistry and theory have also played significant roles in developing our present understanding of these systems and are discussed in the context of their contributions to interpreting the nature of nitrogenases. Despite years of significant progress, there is still much to be learned from these enzymes through spectroscopic means, and we highlight where further spectroscopic investigations are needed.

Mononuclear Fe(I) and Fe(II) Acetylene Adducts and Their Reductive Protonation to Terminal Fe(IV) and Fe(V) Carbynes

The activity of nitrogenase enzymes, which catalyze the conversion of atmospheric dinitrogen to bioavailable ammonia, is most commonly assayed by the reduction of acetylene gas to ethylene. Despite the practical importance of acetylene as a substrate, little is known concerning its binding or activation in the iron-rich active site. "Fischer-Tropsch" type coupling of non-native C1 substrates to higher-order C≥2 products is also known for nitrogenase, though potential metal-carbon multiply bonded intermediates remain underexplored. Here we report the activation of acetylene gas at a mononuclear tris(phosphino)silyl-iron center, (SiP3)Fe, to give Fe(I) and Fe(II) side-on adducts, including S = 1/2 FeI(η2-HCCH); the latter is characterized by pulse EPR spectroscopy and DFT calculations. Reductive protonation reactions with these compounds converge at stable examples of unusual, formally iron(IV) and iron(V) carbyne complexes, as in diamagnetic (SiP3)Fe≡CCH3 and the paramagnetic cation S = 1/2 [(SiP3)Fe≡CCH3]+. Both alkylcarbyne compounds possess short Fe-C triple bonds (approximately 1.7 Å) trans to the anchoring silane. Pulse EPR experiments, X-band ENDOR and HYSCORE, reveal delocalization of the iron-based spin onto the α-carbyne nucleus in carbon p-orbitals. Furthermore, isotropic coupling of the distal β-CH3 protons with iron indicates hyperconjugation with the spin/hole character on the Fe≡CCH3 unit. The electronic structures of (SiP3)Fe≡CCH3 and [(SiP3)Fe≡CCH3]+ are discussed in comparison to previously characterized, but heterosubstituted, iron carbynes, as well as a hypothetical nitride species, (SiP3)Fe≡N. Such comparisons are germane to the consideration of formally high-valent, multiply bonded Fe≡C and/or Fe≡N intermediates in synthetic or biological catalysis by iron.

O-Functionalization of a cobalt carbonyl generates a terminal cobalt carbyne

Despite efforts toward extending multiple bonding motifs to late metal systems, examples of late transition metal carbynes remain scarce. Herein, we describe the synthesis of a series of L3Co(CO) complexes supported by a trisphosphine ligand framework, with the most reduced of these complexes being amenable to O-functionalization. This transformation provides access to the second reported example of a terminal Co-carbyne complex, in this case stabilized in a pseudotetrahedral geometry (i.e., L3Co[triple bond, length as m-dash]C-OSiR3). Its geometry makes its electronic structure suitable for comparison to structurally-related examples of terminal Co-imido and oxo species.

Platinum(ii) as an assembly point for carbide and nitride ligands

The sequential treatment of (Cy₃P)₂Cl₂RuC with [PtCl₂(C₂H₄)]₂ and (dbm)₂CrN affords a platinum(ii) center coordinated by both carbide and nitride ligands.

A Highly-Reduced Cobalt Terminal Carbyne: Divergent Metal- and α-Carbon-Centered Reactivity

Reported here is the isolation of a dianionic cobalt terminal carbyne derived from chemical reduction of an encumbering isocyanide ligand. Crystallographic, spectroscopic and computational data reveal that this carbyne possesses a low-valent cobalt center with an extensively filled d-orbital manifold. This electronic character renders the cobalt center the primary site of nucleophilicity upon reaction with protic substrates and silyl electrophiles. However, reactions with internal alkynes result in [2+2] cycloaddition with the carbyne carbon to form a new C-C bond.

N-H Bond Dissociation Enthalpies and Facile H Atom Transfers for Early Intermediates of Fe-N2 and Fe-CN Reductions

Fe-mediated biological nitrogen fixation is thought to proceed via either a sequence of proton and electron transfer steps, concerted H atom transfer steps, or some combination thereof. Regardless of the specifics and whether the intimate mechanism for N₂-to-NH₃ conversion involves a distal pathway, an alternating pathway, or some hybrid of these limiting scenarios, Fe-N_xH_y intermediates are implicated that feature reactive N-H bonds. Thermodynamic knowledge of the N-H bond strengths of such species is scant, and is especially difficult to obtain for the most reactive early stage candidate intermediates (e.g., Fe-N═NH, Fe═N-NH₂, Fe-NH═NH). Such knowledge is essential to considering various mechanistic hypotheses for biological (and synthetic) nitrogen fixation and to the rational design of improved synthetic N₂ fixation catalysts. We recently reported several reactive complexes derived from the direct protonation of Fe-N₂ and Fe-CN species at the terminal N atom (e.g., Fe═N-NH₂, Fe-C≡NH, Fe≡C-NH₂). These same Fe-N₂ and Fe-CN systems are functionally active for N₂-to-NH₃ and CN-to-CH₄/NH₃ conversion, respectively, when subjected to protons and electrons, and hence provide an excellent opportunity for obtaining meaningful N-H bond strength data. We report here a combined synthetic, structural, and spectroscopic/analytic study to estimate the N-H bond strengths of several species of interest. We assess the reactivity profiles of species featuring reactive N-H bonds and estimate their homolytic N-H bond enthalpies (BDE_N-H) via redox and acidity titrations. Very low N-H bond dissociation enthalpies, ranging from 65 (Fe-C≡NH) to ≤37 kcal/mol (Fe-N═NH), are determined. The collective data presented herein provide insight into the facile reactivity profiles of early stage protonated Fe-N₂ and Fe-CN species.

N-H Bond Dissociation Enthalpies and Facile H Atom Transfers for Early Intermediates of Fe-N2 and Fe-CN Reductions

Fe-mediated biological nitrogen fixation is thought to proceed via either a sequence of proton and electron transfer steps, concerted H atom transfer steps, or some combination thereof. Regardless of the specifics and whether the intimate mechanism for N2-to-NH3 conversion involves a distal pathway, an alternating pathway, or some hybrid of these limiting scenarios, Fe-NxHy intermediates are implicated that feature reactive N-H bonds. Thermodynamic knowledge of the N-H bond strengths of such species is scant, and is especially difficult to obtain for the most reactive early stage candidate intermediates (e.g., Fe-N═NH, Fe═N-NH2, Fe-NH═NH). Such knowledge is essential to considering various mechanistic hypotheses for biological (and synthetic) nitrogen fixation and to the rational design of improved synthetic N2 fixation catalysts. We recently reported several reactive complexes derived from the direct protonation of Fe-N2 and Fe-CN species at the terminal N atom (e.g., Fe═N-NH2, Fe-C≡NH, Fe≡C-NH2). These same Fe-N2 and Fe-CN systems are functionally active for N2-to-NH3 and CN-to-CH4/NH3 conversion, respectively, when subjected to protons and electrons, and hence provide an excellent opportunity for obtaining meaningful N-H bond strength data. We report here a combined synthetic, structural, and spectroscopic/analytic study to estimate the N-H bond strengths of several species of interest. We assess the reactivity profiles of species featuring reactive N-H bonds and estimate their homolytic N-H bond enthalpies (BDEN-H) via redox and acidity titrations. Very low N-H bond dissociation enthalpies, ranging from 65 (Fe-C≡NH) to ≤37 kcal/mol (Fe-N═NH), are determined. The collective data presented herein provide insight into the facile reactivity profiles of early stage protonated Fe-N2 and Fe-CN species.

Multidentate silyl ligands in transition metal chemistry

This review presents and discusses the use of multidentate silanide ligands in transition metal chemistry depending on their ligand architecture.