Iron Dinitrogen Complexes Research Articles

Effect of the coordination environment on the ability of iron to bind/activate N2: A theoretical study with relevance to the nitrogenase mechanism

Activation of molecular dinitrogen to ammonia (and an accompanying metal-hydride formation) occurs in nitrogenases at an iron-sulfur active site. Iron-sulfur centers normally feature Fe(II) and/or Fe(III) oxidation states, which in general are not known to bind N2 or H+. Two peculiarities of the nitrogenase active site may be responsible for their unusual reactivity: an unusually reduced Mo(III) center in some of the nitrogenases (possibly suggesting an ability to also generate super-reduced Fe), and the presence of a central carbide anion, inverse-coordinated by 6 Fe centers within the iron-sulfur cluster. To try and deconvolute the factors that may allow the nitrogenase Fe to bind molecular nitrogen, we report here a density functional theory (DFT) study on the ability of a series of mononuclear Fe complexes to bind and activate N2. Structures of the type Fe(H2O)nR(N2) were examined, with the formal oxidation state of the iron varied from 0 to + 3, with octahedral or tetrahedral geometry, where R = H2O, HO–, CH3–, CH22–, H2S, H−. Iron-dinitrogen complexes only appear straightforward with lower oxidation states (0, +1), and the carbanion ligands appear distinctly better than oxygen or sulfur at facilitating N2 binding as well as activation/weakening of the NN bond. For Fe(II), higher spin states and carbanion ligation may in fact allow N2 binding as well – thus raising the question whether in nitrogenases (where sulfur and carbanion-ligated high-spin Fe(II) is present) do require Fe(I) in the catalytic cycle – at least from the point of view of initial N2 ligation.

CS-Symmetric Pyridine(diimine) Iron Methyl Complexes for Catalytic [2+2] Cycloaddition and Hydrovinylation: Metallacycle Geometry Determines Selectivity.

A series of CS-symmetric (aryl,alkyl)-substituted pyridine(dimine) iron methyl (CyARPDI)FeCH3 complexes have been prepared, characterized, and evaluated as precatalysts for the [2+2]-cycloaddition of butadiene and ethylene. Mixtures of vinylcyclobutane and (Z)-hexa-1,4-diene were observed in each case. By comparison, C2v-symmetric, arylated (PDI) iron catalysts are exclusively selective for reversible [2+2]-cycloaddition to yield vinylcyclobutane. The alteration in the chemoselectivity of the catalytic reaction was investigated through a combination of precatalyst stability studies, identification of catalytic resting state(s), and 2H and 13C isotopic labeling experiments. While replacement of an aryl-imine substituent with an N-alkyl group decreases the stability of the formally iron(0) dinitrogen and butadiene complexes, two diamagnetic metallacycles were identified as catalyst resting states. Deuterium labeling and NOESY/EXSY NMR experiments support 1,4-hexadiene arising from catalytic hydrovinylation involving reversible oxidative cyclization leading to accessible cis-metallacycle. Cyclobutane formation proceeds by irreversible C(sp3)-C(sp3) bond-forming reductive elimination from a trans-metallacycle. These studies provide key mechanistic understanding into the high selectivity of bis(arylated) pyridine(diimine) iron catalysts for [2+2]-cycloaddition, unique, thus far, to this class of iron catalysts.

Metal-Ligand Cooperativity in Iron Dinitrogen Complexes: Proton-Coupled Electron Transfer Disproportionation and an Anionic Fe(0)N2 Hydride.

Metal-ligand cooperativity and redox-active ligands enable the use of open-shell first-row transition metals in catalysis. However, the fleeting nature of the reactive intermediates prevents direct inspection of the relevant catalytic species. By employing phosphine α-iminopyridine (PNN)-based complexes, we show that chemical and redox metal-ligand cooperativity can be combined in the coordination sphere of iron dinitrogen complexes. These systems show dual activation modes either through deprotonation, which triggers reversible core dearomatization, or through reversibly accepting one electron by reducing the imine functionality. (PNN)Fe(N2) fragments can be obtained under mildly reducing conditions. Deprotonation of such complexes induces dearomatization of the pyridine core while retaining a terminally coordinated N2 ligand. This species is nevertheless stable in solution only below -30 °C and undergoes unusual ligand-assisted redox disproportionation through proton-coupled electron transfer at room temperature. The origin of this phenomenon is the significant lability of the α-imine C-H bonds in the dearomatized species, where the calculated bond dissociation free energy is 48.7 kcal mol-1. The dispropotionation reaction yields an overreduced iron compound, demonstrating that the formation of such species can be triggered by mild bases, and does not require harsh reducing agents. Reaction of the dearomatized species with dihydrogen yields a rare anionic Fe hydride that binds dinitrogen and features a rearomatized core.

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

Abstract Among synthetic models of nitrogenases, iron–dinitrogen complexes with an Fe–C bond have attracted increasing attention in recent years. Here we report the synthesis of square-planar iron(I)–dinitrogen complexes supported by anionic benzene-based PCP- and POCOP-type pincer ligands as carbon donors. These complexes catalyze the formation of ammonia and hydrazine from the reaction of dinitrogen (1 atm) with a reductant and a proton source at −78 °C, producing up to 252 equiv of ammonia and 68 equiv of hydrazine (388 equiv of fixed N atom) based on the iron atom of the catalyst. Anionic iron(0)–dinitrogen complexes, considered an essential reactive species in the catalytic reaction, are newly isolated from the reduction of the corresponding iron(I)–dinitrogen complexes. This study examines their reactivity using experiments and DFT calculations.

Iron-Catalyzed Trimerization of Terminal Alkynes Enabled by Pyrimidinediimine Ligands: A Regioselective Method for the Synthesis of 1,3,5-Substituted Arenes

The development of pyrimidine-based analogues of the well-known pyridinediimine (PDI) iron complexes enables access to a functional-group-tolerant methodology for the catalytic trimerization of terminal aliphatic alkynes. Remarkably, in contrast to established alkyne trimerization protocols, the 1,3,5-substituted arenes are the main reaction products. Preliminary mechanistic investigations suggest that the enhanced π-acidity of the pyrimidine ring, combined with the hemilability of the imine groups coordinated to the iron center, facilitates this transformation. The entry point in the catalytic cycle is an isolable iron dinitrogen complex. The catalytic reaction proceeds via a 1,3-substituted metallacycle, which explains the observed 1,3,5-regioselectivity. Such a metallacycle could be isolated and represents a rare 1,3-substituted ferracycle obtained through alkyne cycloaddition.

Oxidative Addition of Aryl and Alkyl Halides to a Reduced Iron Pincer Complex.

The two-electron oxidative addition of aryl and alkyl halides to a reduced iron dinitrogen complex with a strong-field tridentate pincer ligand has been demonstrated. Addition of iodobenzene or bromobenzene to (3,5-Me2MesCNC)Fe(N2)2 (3,5-Me2MesCNC = 2,6-(2,4,6-Me-C6H2-imidazol-2-ylidene)2-3,5-Me2-pyridine) resulted in rapid oxidative addition and formation of the diamagnetic, octahedral Fe(II) products (3,5-Me2MesCNC)Fe(Ph)(N2)(X), where X = I or Br. Competition experiments established the relative rate of oxidative addition of aryl halides as I > Br > Cl. A linear free energy of relative reaction rates of electronically differentiated aryl bromides (ρ = 1.5) was consistent with a concerted-type pathway. The oxidative addition of alkyl halides such as methyl-, isobutyl-, or neopentyl halides was also rapid at room temperature, but substrates with more accessible β-hydrogen positions (e.g., 1-bromobutane) underwent subsequent β-hydride elimination. Cyclization of an alkyl halide containing a radical clock and epimerization of neohexyl iodide-d2 upon oxidative addition to (3,5-Me2MesCNC)Fe(N2)2 are consistent with radical intermediates during C(sp3)-X bond cleavage. Importantly, while C(sp2)-X and C(sp3)-X oxidative addition produces net two-electron chemistry, the preferred pathway for obtaining the products is concerted and stepwise, respectively.

Dinitrogen Activation of Cyclopentadienyl-Phosphine–Iron Complexes of Three Different Valences

A series of structurally well-defined iron–dinitrogen complexes (LFe–N2) bearing cyclopentadienyl-phosphine ligands (L) were synthesized and analyzed by single-crystal X-ray structural analysis, IR...

Tripodal P3XFe-N2 Complexes (X = B, Al, Ga): Effect of the Apical Atom on Bonding, Electronic Structure, and Catalytic N2-to-NH3 Conversion.

Terminal dinitrogen complexes of iron ligated by tripodal, tetradentate P3X ligands (X = B, C, Si) have previously been shown to mediate catalytic N2-to-NH3 conversion (N2RR) with external proton and electron sources. From this set of compounds, the tris(phosphino)borane (P3B) system is most active under all conditions canvassed thus far. To further probe the effects of the apical Lewis acidic atom on structure, bonding, and N2RR activity, Fe-N2 complexes supported by analogous group 13 tris(phosphino)alane (P3Al) and tris(phosphino)gallane (P3Ga) ligands are synthesized. The series of P3XFe-N2[0/1-] compounds (X = B, Al, Ga) possess similar electronic structures, degrees of N2 activation, and geometric flexibility as determined from spectroscopic, structural, electrochemical, and computational (DFT) studies. However, treatment of [Na(12-crown-4)2][P3XFe-N2] (X = Al, Ga) with excess acid/reductant in the form of HBArF4/KC8 generates only 2.5 ± 0.1 and 2.7 ± 0.2 equiv of NH3 per Fe, respectively. Similarly, the use of [H2NPh2][OTf]/Cp*2Co leads to the production of 4.1 ± 0.9 (X = Al) and 3.6 ± 0.3 (X = Ga) equiv of NH3. Preliminary reactivity studies confirming P3XFe framework stability under pseudocatalytic conditions suggest that a greater selectivity for hydrogen evolution versus N2RR may be responsible for the attenuated yields of NH3 observed for P3AlFe and P3GaFe relative to P3BFe.

Synthesis of silyl iron dinitrogen complexes for activation of dihydrogen and catalytic silylation of dinitrogen.

Three novel iron dinitrogen hydrides, [FeH(iPr-PSiMeP)(N2)(PMe3)] (1), [FeH(iPr-PSiPhP)(N2)(PMe3)] (2), and [FeH(iPr-PSiPh)(N2)(PMe3)] (3), supported by a silyl ligand are synthesized for the first time by changing the electronic effect and steric hindrance of the ligands through the reaction of ligands L1-L3 with Fe(PMe3)4 in a nitrogen atmosphere. The ligands containing an electron-donating group with large steric hindrance on the phosphorus atom are beneficial for the formation of dinitrogen complexes. A penta-coordinate iron hydride [FeH(iPr-PSiPh)(PMe3)2] (4) was formed through the reaction of ligand L3 with Fe(PMe3)4 in an argon atmosphere under the same conditions. The reactions between complexes 1-3 with an atmospheric pressure of dihydrogen gas resulted in Fe(II) dihydrides, [(iPr-PSiMe(μ-H)P)Fe(H)2(PMe3)] (5), [(iPr-PSiPh(μ-H)P)Fe(H)2(PMe3)] (6) and [(iPr-PSiPh(μ-H))Fe(H)2(PMe3)2] (7), with an η2-(Si-H) coordination. The isolation of dihydrides 5-7 demonstrates the ability of the dinitrogen complexes 1-3 to realize the activation of dihydrogen under ambient temperature and pressure. The molecular structures of complexes 1-7 were elucidated by single crystal X-ray diffraction analysis. The iron dinitrogen hydrides 1-3 are effective catalysts for the silylation of dinitrogen under ambient conditions and among them 3 is the best catalyst.

Facile H/D Exchange at (Hetero)Aromatic Hydrocarbons Catalyzed by a Stable Trans-Dihydride N-Heterocyclic Carbene (NHC) Iron Complex.

Earth-abundant metal pincer complexes have played an important role in homogeneous catalysis during the last ten years. Yet, despite intense research efforts, the synthesis of iron PCcarbeneP pincer complexes has so far remained elusive. Here we report the synthesis of the first PCNHCP functionalized iron complex [(PCNHCP)FeCl2] (1) and the reactivity of the corresponding trans-dihydride iron(II) dinitrogen complex [(PCNHCP)Fe(H)2N2)] (2). Complex 2 is stable under an atmosphere of N2 and is highly active for hydrogen isotope exchange at (hetero)aromatic hydrocarbons under mild conditions (50 °C, N2). With benzene-d6 as the deuterium source, easily reducible functional groups such as esters and amides are well tolerated, contributing to the overall wide substrate scope (e.g., halides, ethers, and amines). DFT studies suggest a complex assisted σ-bond metathesis pathway for C(sp2)–H bond activation, which is further discussed in this study.

Selective Hydroboration of Alkynes Enabled by a Silylene Iron(0) Dinitrogen Complex

Selective Hydroboration of Alkynes Enabled by a Silylene Iron(0) Dinitrogen Complex

Dinitrogen Activation by a Heterometallic VFe Complex Derived from 1,1'‐Bis(arylamido)vanadocene

For dinitrogen activation mediated by a heterobimetallic VFe complex, synthesis and reduction of a novel iron complex having 1,1'‐bis(mesitylamido)vanadocene were performed. Reaction of the vanadocene ligand 2 with iron dichloride afforded a heterobimetallic VFe complex 3, characterized by X‐ray crystallographic study. The molecular structure of the complex 3 showed the short V–Fe distance of 2.6373(3) Å, indicative of the bonding interaction between two metals. Reduction of the complex 3 with KC8 under a nitrogen atmosphere was found to result in the formation of a three‐coordinate iron dinitrogen complex 4, in which the dinitrogen moiety shows one of the longest N–N bond lengths for iron dinitrogen complexes. Reduction of the complex 3 under an argon atmosphere was performed to isolate a reduced species [K(thf)5][3] as a reactive intermediate, whose structure was also determined by X‐ray crystallography.

Revisiting mechanistic studies on dinitrogen reduction to ammonia by an iron dinitrogen complex as nitrogenase mimic

AbstractConversion of free nitrogen to ammonia is a required chemical reaction for both biologically and industrially but their mechanism, specifically the attachment of electron and proton transfer during the cycle, is still doubtful. In this view, a thorough knowledge of the mechanism is crucial. In this article, we employ a density functional method on [(TPB)FeN2]−, the iron‐dinitrogen complex carrying the tris(phosphine)borone (TPB) ligand, for the ammonia production with the inclusion of electrons and protons. The electronic structures, reactivity, and mechanistic possibilities have been extensively explored using the B3LYP functional. Both asymmetric and symmetric pathways in addition to the possible intermediates species and transition states are considered here. Our results conclude tremendously small energy barrier of 3.5 kJ/mol for the first protonation (S = 1/2) for the N─H bond activation by the [(TPB)FeN2]− species. However, high activation barrier for the third protonation was estimated to be 78.5 kJ/mol, which is explained by the high energy of the unoccupied δx2‐y2 orbital in 1ts4 species. The computed spectroscopic parameters such as absorption, electron paramagnetic resonance, and Mössbauer also established the electronic structure details of the species. The calculated parameters are compatible with the experimental results.

Ammonia and Hydrazine from Coordinated Dinitrogen by Complexes of Iron(0)

The iron(0) dinitrogen complexes [Fe(N2)(PP3R)] (PP3R = P(CH2CH2PR2)3, R = Ph, iPr, Cy) were synthesized by reduction of the precursor chloro complexes with potassium graphite. On reaction with triflic acid, [Fe(N2)(PP3R)] complexes afforded ammonia and hydrazine in yields of up to 23 and 16 % respectively. The complex [Fe(N2)(PP3Ph)] which has only been previously synthesized in situ, has now been isolated and fully characterized by 15N NMR spectroscopy and by X‐ray crystallography.

Exploring the mechanism of alkene hydrogenation catalyzed by defined iron complex from DFT computation.

UB3LYP computation including dispersion and toluene solvation has been carried to elucidate the mechanisms of alkene hydrogenation catalyzed by bis(imino)pyridine iron dinitrogen complex (iPrPDI)Fe(N2)2, which has low stability towards N2 dissociation. The coordinatively unsaturated complexes, (iPrPDI)Fe(N2) and (iPrPDI)Fe(1-C4H8), favor open-shell singlet ground states. On the basis of our computations, we propose a new mechanism of 1-butene coordination and hydrogenation after N2 dissociation. The hydrogenation of 1-butene undergoes a concerted open-shell singlet transition state involving H2 dissociation, C-H bond formation and C=C bond elongation, as well as the subsequent C-H reductive elimination. In the whole alkene hydrogenation, the H-H bond cleavage is the rate-determining step. Graphical abstract The alkene hydrogenation catalyzed by redox-active pyridine(diimine)-chelate iron complex follows the open-shell singlet state path.

Effects of N2 Binding Mode on Iron-Based Functionalization of Dinitrogen to Form an Iron(III) Hydrazido Complex.

Distinguishing the reactivity differences between N2 complexes having different binding modes is crucial for the design of effective N2-functionalizing reactions. Here, we compare the reactions of a K-bridged, dinuclear FeNNFe complex with a monomeric Fe(N2) complex where the bimetallic core is broken up by the addition of chelating agents. The new anionic iron(0) dinitrogen complex has enhanced electron density at the distal N atoms of coordinated N2, and though the N2 is not as weakened in this monomeric compound, it is much more reactive toward silylation by (CH3)3SiI (TMSI). Double silylation of N2 gives a three-coordinate iron(III) hydrazido(2-) complex, which is finely balanced between coexisting S = 1/2 and S = 3/2 states that are characterized by crystallography, spectroscopy, and computations. These results give insight into the interdependence between binding modes, alkali dependence, reactivity, and magnetic properties within an iron system that functionalizes N2.

Isolation and characterization of a high-spin mixed-valent iron dinitrogen complex.

We report a rare example of a mixed-valence iron compound with an FeNNFe core, which gives insight into the structural, spectroscopic, and magnetic influences of single-electron reductions and oxidations. In the new compound, the odd electron is localized as judged from Mössbauer spectra at 80 K and infrared spectra at room temperature, and the backbonding into the N2 unit is intermediate between diiron(i) and diiron(0) congeners. Magnetic susceptibility and relaxation studies on the series of FeNNFe compounds show significant magnetic anisotropy, but through-barrier pathways enable fairly rapid magnetic relaxation.

Iron Dinitrogen Complexes Supported by Tris(NHC)borate Ligand: Synthesis, Characterization, and Reactivity Study

Iron Dinitrogen Complexes Supported by Tris(NHC)borate Ligand: Synthesis, Characterization, and Reactivity Study

Synthesis and reactivity of iron-dinitrogen complexes bearing anionic methyl- and phenyl-substituted pyrrole-based PNP-type pincer ligands toward catalytic nitrogen fixation.

Iron-dinitrogen complexes bearing methyl- and phenyl-substituted pyrrole-based anionic PNP-type pincer ligands are prepared and characterized by X-ray analysis. The former complex is found to work as a more effective catalyst than that bearing a non-substituted PNP-type pincer ligand toward the transformation of nitrogen gas into ammonia and hydrazine under mild reaction conditions.

High-Spin Iron(I) and Iron(0) Dinitrogen Complexes Supported by N-Heterocyclic Carbene Ligands.

The use of 1,3-dicyclohexylimidazol-2-ylidene (ICy) as ligand has enabled the preparation of the high-spin tetrahedral iron(I)- and iron(0)-N2 complexes, namely [(ICy)3 Fe(N2 )][BPh4 ] (1) and [(ICy)3 Fe(N2 )] (2), the electronic structures of which have been established by various spectroscopic characterization and DFT calculations. The frequency of the N-N stretching resonance of the iron(0)-N2 complex is the lowest among the reported terminal N2 complexes of iron, signifying the beneficial roles of strongly σ-donating ligands in combination with the high-spin low-valent iron center in promoting N2 -activation. The iron(0)-N2 complex can convert reversibly to the low-spin iron(II)-N2 hydride complex [(ICy)2 (ICy')Fe(N2 )(H)] (4).