Catalytic Reduction of Dinitrogen into Ammonia and Hydrazine by Using Chromium Complexes Bearing PCP-Type Pincer Ligands

Theoretical Studies on the Reduction of N2 to NH3/N2H4 Catalyzed by Chromium Complexes

Nitrogen fixation catalyzed by transition metal complexes provides an attractive alternative to the Haber-Bosch process and has received widespread attention. In this work, the reaction pathways of N2 to NH3/N2H4 catalyzed by dinuclear and mononuclear Cr-N2 complexes bearing cyclopentadienyl-phosphine ligands and mediated by LutH+ (as proton donors) and CrCp*2 (as electron donors) were investigated systematically using density functional theory calculations. The key step of the reactions was clarified as the first hydrogenation. The effect of different combinations of six proton sources (three pyridine acids and three anilino acids) and three electron sources (CrCp*2, CoCp*2, and CoCp2) on the reduction of N2 to NH3/N2H4 was also explored by calculating the critical step of the reactions. Based on the calculations, the dinuclear Cr-N2 complex is expected to be an effective catalyst for the reduction of N2 to NH3/N2H4 when using the combinations of CrCp*2 with each of the six proton sources and of CoCp*2 with anilino acids. Our work provides insights into understanding and optimizing Cr catalytic systems for efficient dinitrogen fixations.

Diverse Behaviors of N2 on Mo Centers Bearing POCOP-Pincer Ligands and the Role of π-Electron Configuration in Regulating the Pathway of N2 Activation

Activation of N2 through transition-metal complexes has emerged as a powerful strategy for N2 fixation under mild conditions. Dissociative route and associative route are considered as two major routes for N2 transformation on transition-metal complexes. Homolysis of N2 between two metal fragments is the crucial step of the dissociative route and has been proven to be an efficient approach to the terminal metal nitride, which is the key intermediate for both routes. Hence, the conditions for N2 cleavage have attracted much interest and discussion. Herein, we investigated the reactivity of N2 when coordinated on Mo centers bearing POCOP-pincer ligands and isolated and characterized many novel N2-related intermediates such as [(POCOPCy)MoI]2(μ-N2) (2Cy), (POCOPCy)Mo(N)(μ-N)MoI (5Cy), {[(POCOPCy)Mo(N2)2]2(μ-N2)}[Na(crypt-222)] (6Cy-crypt), and [(POCOPCy)Mo(N2)2(μ-N2)Mo(N)]Na (8Cy). The influences of the oxidation state of the metal centers, π electrons, reaction conditions, etc., on the N2-reactivity were also studied both experimentally and theoretically. Accordingly, some fundamental understanding of the regulation of N2 activation pathways was proposed: an N2-bridged Mo dimer without ligand trans to the bridging N2 is a preferred structure for N2 cleavage; having adequate electrons to be transferred into the σ-σ*-σ related orbital in the {MoNNMo} manifold is the key; and heating or electron excitation is advantageous to the dissociative route.

Synthesis and electrochemical properties of molybdenum nitrido complexes supported by redox-active NHC and MIC ligands

We report the synthesis of a series of molybdenum nitrido complexes supported by bis-phenolate N-heterocyclic and mesoionic carbenes (NHC & MIC). The reaction between MoN(OtBu)3 and the corresponding azolium salts [H3L1]Cl and [H3L2]Cl (with L1 = bis-phenolate triazolylidene and L2 = bis-phenolate benzimidazolylidene) gives clean access to the corresponding NHC/MIC complexes 1-Cl and 2-Cl. Electrochemical investigations of these complexes showed that they can be reversibly reduced at potentials of -1.13 and -1.01 V vs. Fc/[Fc]+ and the reduced complexes [1-Cl]- and [2-Cl]- can be cleanly isolated after chemical reduction with one equivalent of decamethylcobaltocene. Exchange of the halide atoms is furthermore reported to give a series of nitrido complexes supported by tert-butanolate (1-OtBu and 2-OtBu), perfluoro-tert-butanolate (1-OtBuF9 and 2-OtBuF9), tritylate (1-OCPh3 and 2-OCPh3), mesitolate (1-OMes and 2-OMes), thio-tert-butanolate (1-StBu), thiotritylate (1-SCPh3 and 2-SCPh3) and thiomesitolate complexes (1-SMes). The electrochemical properties of all complexes were evaluated and compared. All isolated complexes were characterized by multinuclear and multidimensional NMR spectroscopy and (if applicable) by EPR spectroscopy. Furthermore, the reactivity of 1-Cl and 2-Cl in the presence of protons and decamethylcobaltocene was investigated, which shows facile extrusion of ammonia, yielding diamagnetic bis-molybdenum(III) complexes 3 and 4.

Photodriven Ammonia Synthesis from Manganese Nitrides: Photophysics and Mechanistic Investigations

Ammonia synthesis from N,N,O,O-supported manganese(V) nitrides and 9,10-dihydroacridine using proton-coupled electron transfer and visible light irradiation in the absence of precious metal photocatalysts is described. While the reactivity of the nitride correlated with increased absorption of blue light, excited-state lifetimes determined by transient absorption were on the order of picoseconds. This eliminated excited-state manganese nitrides as responsible for bimolecular N-H bond formation. Spectroscopic measurements on the hydrogen source, dihydroacridine, demonstrated that photooxidation of 9,10-dihydroacridine was necessary for productive ammonia synthesis. Transient absorption and pulse radiolysis data for dihydroacridine provided evidence for the presence of intermediates with weak E-H bonds, including the dihydroacridinium radical cation and both isomers of the monohydroacridine radical, but notably these intermediates were unreactive toward hydrogen atom transfer and net N-H bond formation. Additional optimization of the reaction conditions using higher photon flux resulted in higher rates of the ammonia production from the manganese(V) nitrides due to increased activation of the dihydroacridine.

Catalytic Nitrogen Fixation Using Well-Defined Molecular Catalysts under Ambient or Mild Reaction Conditions

Ammonia (NH3) is industrially produced from dinitrogen (N2) and dihydrogen (H2) by the Haber-Bosch process, although H2 is prepared from fossil fuels, and the reaction requires harsh conditions. On the other hand, microorganisms have fixed nitrogen under ambient reaction conditions. Recently, well-defined molecular transition metal complexes have been found to work as catalyst to convert N2 into NH3 by reactions with chemical reductants and proton sources under ambient reaction conditions. Among them, involvement of both N2-splitting pathway and proton-coupled electron transfer is found to be very effective for high catalytic activity. Furthermore, direct electrocatalytic and photocatalytic conversions of N2 into NH3 have been recently achieved. In addition to catalytic formation of NH3, selective catalytic conversion of N2 into hydrazine (NH2NH2) and catalytic silylation of N2 into silylamines have been reported. Catalytic C-N bond formation has been more recently established to afford cyanate anion (NCO-) under ambient reaction conditions. Further development of direct conversion of N2 into nitrogen-containing compounds as well as green ammonia synthesis leading to the use of ammonia as an energy carrier is expected.

Dinitrogen activation at chromium by photochemically induced CrII-C bond homolysis

The synthesis of the organochromium(II) complexes [POCOPtBu]Cr(R) (R = p-Tol, Bn) is reported. Exposure of [POCOPtBu]Cr(Bn) to visible light promoted homolytic Cr-CBn bond cleavage and formed {[POCOPtBu]Cr}2(η1:η1μ-N2) via a putative [POCOPtBu]Cr(I) species.

(Electro)chemical N2 Splitting by a Molybdenum Complex with an Anionic PNP Pincer-Type Ligand

Molybdenum(III) complexes bearing pincer-type ligands are well-known catalysts for N2-to-NH3 reduction. We investigated herein the impact of an anionic PNP pincer-type ligand in a Mo(III) complex on the (electro)chemical N2 splitting ([LMoCl3]-, 1 -, LH = 2,6-bis((di-tert-butylphosphaneyl)methyl)-pyridin-4-one). The increased electron-donating properties of the anionic ligand should lead to a stronger degree of N2 activation. The catalyst is indeed active in N2-to-NH3 conversion utilizing the proton-coupled electron transfer reagent SmI2/ethylene glycol. The corresponding Mo(V) nitrido complex 2H exhibits similar catalytic activity as 1H and thus could represent a viable intermediate. The Mo(IV) nitrido complex 3 - is also accessible by electrochemical reduction of 1 - under a N2 atmosphere. IR- and UV/vis-SEC measurements suggest that N2 splitting occurs via formation of an "overreduced" but more stable [(L(N2)2Mo0)2μ-N2]2- dimer. In line with this, the yield in the nitrido complex increases with lower applied potentials.

Two chromium(II) acetate complexes with N-heterocyclic carbene (NHC) coligands

Tetra-kis(μ-acetato-κ2 O:O')bis-{[1,3-bis-(2,6-diiso-propyl-phen-yl)imidazol-2-yl-idene-κC 2]chromium(II)} tetra-hydro-furan disolvate, [Cr2(C2H3O2)4(C27H36N4)2]·2C4H8O or [Cr2(OAc)4(IDipp)2]·2C4H8O (1), and tetra-kis-(μ-acetato-κ2 O:O')bis-{[1,3-bis-(2,4,6-tri-methyl-phen-yl)imidazol-2-yl-idene-κC 2]chromium(II)},{Cr2(C2H3O2)4(C21H24N2)2] or [Cr2(OAc)4(IMes)2] (2), were synthesized from anhydrous chromium(II) acetate [Cr2(OAc)4] and the corresponding NHC (NHC = N-heterocyclic carbene) in toluene as solvent. Both complexes crystallize in the triclinic system, space group P. The mol-ecular structures consist of Cr2(OAc)4 paddle-wheels that carry two terminal NHC ligands. This leads to a square-pyramidal coordination of the chromium atoms.

Catalytic dinitrogen reduction to hydrazine and ammonia using Cr(N2)2(diphosphine)2 complexes

The synthesis, characterization of trans-[Cr(N2)2(depe)2] (1) is described. 1 and trans-[Cr(N2)2(dmpe)2] (2) catalyze the reduction of N2 to N2H4 and NH3 in THF using SmI2 and H2O or ethylene glycol as proton sources. 2 produces the highest total fixed N for a molecular Cr catalyst to date.

Alkaline earth metal-assisted dinitrogen activation at nickel

Rare examples of trinuclear [Ni-N2-M-N2-Ni] core (M = Ca, Mg) with linear bridged dinitrogen ligands are reported in this work. The reduction of [iPr2NN]Ni(μ-Br)2Li(thf)2 (1) (iPr2NN = 2,4-bis-(2,6-diisopropylphenylimido)pentyl) with elemental Mg or Ca in THF under an atmosphere of dinitrogen yields the complex {iPr2NNNi(μ-N2)}2M (thf)4 (M = Mg, complex 2 and M = Ca, complex 3). The bridging end-on (μ-N2)2M(thf)4 moiety connects the two [iPr2NNNi]- nickelate fragments. A combination of X-ray crystallography, solution and solid-state spectroscopy have been applied to characterize complexes 2 and 3, and DFT studies have been used to help explain the bonding and electronic structure in these unique Ni-N2-Mg and Ni-N2-Ca complexes.

Dinitrogen Activation and Functionalization Affording Chromium Diazenido and Hydrazido Complexes

ConspectusThe activation and functionalization of N2 to form nitrogen-element bonds have long posed challenges to industrial, biological, and synthetic chemists. The first transition-metal dinitrogen complex prepared by Allen and Senoff in 1965 provoked researchers to explore homogeneous N2 fixation. Despite intensive research in the last six decades, efficient and quantitative conversion of N2 to diazenido and hydrazido species remains problematic. Relative to a plethora of reactions to generate N2 complexes, their functionalization reactions are rather rare, and the yields are often unsatisfactory, emphasizing the need for systematic investigations of the reaction mechanisms.In this Account, we summarize our recent work on the synthesis, spectroscopic features, electronic structures, and reactivities of several Cr-N2 complexes. Initially, a series of dinuclear and trinuclear Cr(I)-N2 complexes bearing cyclopentadienyl-phosphine ligands were accessed. However, they cannot achieve N2 functionalization but undergo oxidative addition reactions with phenylsilane, azobenzene, and other unsaturated organic compounds at the low-valent Cr(I) centers rather than at the N2 unit. Further reduction of these Cr(I) complexes leads to the formation of more activated mononuclear Cr(0) bis-dinitrogen complexes. Remarkably, silylation of the cyclopentadienyl-phosphine Cr(0)-N2 complex with Me3SiCl afforded the first Cr hydrazido complex. This process follows the distal pathway to functionalize the Nβ atom twice, yielding an end-on η1-hydrazido complex, Cr(III)═N-N(SiMe3)2. In contrast, upon substitution of the phosphine ligand in the Cr(0)-N2 complex with a N-heterocyclic carbene (NHC) ligand, the corresponding reaction with Me3SiCl proceeds via the alternating pathway; the silylation occurs at both Nα and Nβ atoms and generates a side-on η2-hydrazido complex, Cr(III)(η2-Me3SiN-NSiMe3). Both silylation reactions are inevitably accompanied by the formation of Cr(III) hydrazido complexes and Cr(II) chlorides with a 2:1 ratio. These processes exhibit a peculiar '3-4-2-1' stoichiometry (i.e., treating 3 equiv of Cr(0)-N2 complexes with 4 equiv of Me3SiCl yields 2 equiv of Cr(III) disilyl-hydrazido complexes and 1 equiv of Cr(II) chloride). Upon replacing the monodentate phosphine and/or NHC ligand with a bisphosphine ligand, a monodinitrogen Cr(0) complex, instead of the bis-dinitrogen Cr(0) complexes, is obtained; consequently, the silylation reactions progress via the normal two-electron route, which passes through Cr(II)-N═N-R diazenido species as an intermediate and furnishes [Cr(IV)═N-NR2]+ hydrazido as the final products. More importantly, this type of Cr(0)-N2 complex can be not only silylated but also protonated and alkylated proficiently. All of the second-order reaction rates of the first and second transformations are determined along with the lifetimes of the intervening diazenido species. Based on these findings, we have successfully carried out nearly quantitative preparations of the Cr(IV) hydrazido species with unmixed or hybrid substituents.The studies of Cr-N2 systems provide effective approaches for the activation and functionalization of N2, deepening the understanding of N2 electrophilic attack. We hope that this Account will inspire more discoveries related to the transformation of gaseous N2 to high-value-added nitrogen-containing organic compounds.

Design, synthesis and reactivity of dimolybdenum complex bearing quaterphenylene-bridged pyridine-based PNP-type pincer ligand

Dimolybdenum complexes bearing 3,3'''-(1,1':3',1'':3'',1'''-quaterphenylene)-bridged pyridine-based PNP-type pincer ligand are designed and prepared according to DFT calculations on the cleavage step of dinitrogen-bridged dimolybdenum complexes bearing polyphenylene-bridged pyridine-based PNP-type pincer ligands. The dimolybdenum complexes are found to work as effective catalysts toward ammonia formation from dinitrogen with samarium diiodide as a reductant and water as a proton source under ambient reaction conditions.

Snapshots of Early-Stage Quantitative N2 Electrophilic Functionalization

Electrophilic functionalization of N₂ moieties in metal dinitrogen complexes typically initiates the catalytic synthesis of N-containing molecules directly from N₂. Despite intensive research in the last six decades, how to efficiently and even quantitatively convert N₂ into diazenido and hydrazido species still poses a great challenge. In this regard, systematic and comprehensive investigations to elucidate the reaction intricacies are of profound significance. Herein, we report a kinetic dissection on the first and second electrophilic functionalization steps of a new Cr⁰-N₂ system with HOTf, MeOTf, and Me₃SiOTf. All reactions pass through fleeting diazenido intermediates and furnish long-lived final hydrazido products, and both steps are quantitative conversions at low temperatures. All of the second-order reaction rates of the first and second transformations were determined as well as the lifetimes of the intervening diazenido species. Based on these findings, we succeeded in large-scale and near-quantitative preparation of all hydrazido species.

Syntheses, Characterizations, Crystal Structures, and Protonation Reactions of Dinitrogen Chromium Complexes Supported with Triamidoamine Ligands

A novel dinitrogen-dichromium complex, [{Cr(L^Bn)}₂(μ-N₂)] (1), has been prepared from reaction of CrCl₃ with a lithiated triamidoamine ligand (Li₃L^Bn) under dinitrogen. The X-ray crystal structure analysis of 1 revealed that it is composed of two independent dimeric Cr complexes bridged by N₂ in the unit cell. The bridged N-N bond lengths (1.188(4) and 1.185(7) Å) were longer than the free dinitrogen molecule. The elongations of N-N bonds in 1 were also supported by the fact that the ν(N-N) stretching vibration at 1772 cm^-1 observed in toluene is smaller than the free N₂. Complex 1 was identified to be a 5-coordinated high spin Cr(IV) complex by Cr K-edge XANES measurement. The ¹H NMR spectrum and temperature dependent magnetic susceptibility of 1 indicated that complex 1 is in the S = 1 ground state, in which two Cr(IV) ions and unpaired electron spins of the bridging N₂^2- ligand are strongly antiferromagnetically coupled. Reaction of complex 1 with 2.3 equiv of Na or K gave chromium complexes with N₂ between the Cr ion and the respective alkali metal ion, [{CrNa(L^Bn)(N₂)(Et₂O)}₂] (2) and [{CrK(L^Bn)(N₂)}₄(Et₂O)₂] (3), respectively. Furthermore, the complexes 2 and 3 reacted with 15-crown-5 and 18-crown-6 to form the respective crown-ether adducts, [CrNa(L^Bn)(N₂)(15-crown-5)] (4) and [CrK(L^Bn)(N₂)(18-crown-6)] (5). The XANES measurements of complexes 2, 3, 4, and 5 revealed that they are high spin Cr(IV) complexes like complex 1. All complexes reacted with a reducing agent and a proton source to form NH₃ and/or N₂H₄. The yields of these products in the presence of K⁺ were higher than those in the presence of Na⁺. The electronic structures and binding properties of 1, 2, 3, 4, and 5 were evaluated and discussed based on their DFT calculations.

Dinitrogen Functionalization Affording Chromium Diazenido and Side-on η2-Hydrazido Complexes

Isolation of key intermediate complexes in dinitrogen functionalization is crucial for elucidating the mechanistic details and further investigation. Herein, the synthesis and characterization of (μ-η¹:η¹-N₂)(η¹-N₂)-Cr(I) 3 and (η¹-N₂)₂-Cr(0) complexes 4 supported by Cp* (Cp* = C₅Me₅) and NHC ligands were reported. Further functionalization of Cr(0)-N₂ complex 4 with silyl halides delivered the key intermediates in the alternating pathway, the chromium diazenido complex 5 and the chromium side-on η²-hydrazido complex 6. Protonation of 6 led to the quantitative formation of N₂H₄. Moreover, the [η²-Me₃SiNNSiMe₃]^2- unit in 6 enabled N-C bond formation reactions with CO₂ and ^tBuNCO, giving the corresponding N,O-chelating hydrazidochromium complexes 7 and 8, respectively.

Vanadium-Catalyzed Dinitrogen Reduction to Ammonia via a [V]═NNH2 Intermediate

The catalytic transformation of N₂ to NH₃ by transition metal complexes is of great interest and importance but has remained a challenge to date. Despite the essential role of vanadium in biological N₂ fixation, well-defined vanadium complexes that can catalyze the conversion of N₂ to NH₃ are scarce. In particular, a V(N_xH_y) intermediate derived from proton/electron transfer reactions of coordinated N₂ remains unknown. Here, we report a dinitrogen-bridged divanadium complex bearing POCOP (2,6-(^tBu₂PO)₂-C₆H₃) pincer and aryloxy ligands, which can serve as a catalyst for the reduction of N₂ to NH₃ and N₂H₄. Low-temperature protonation and reduction of the dinitrogen complex afforded the first structurally characterized neutral metal hydrazido(2-) species ([V]═NNH₂), which mediated ¹⁵N₂ conversion to ¹⁵NH₃, indicating that it is a plausible intermediate of the catalysis. DFT calculations showed that the vanadium hydrazido complex [V]═NNH₂ possessed a N-H bond dissociation free energy (BDFE_N-H) of as high as 59.1 kcal/mol. The protonation of a vanadium amide complex ([V]-NH₂) with [Ph₂NH₂][OTf] resulted in the release of NH₃ and the formation of a vanadium triflate complex, which upon reduction under N₂ afforded the vanadium dinitrogen complex. These transformations model the final steps of a vanadium-catalyzed N₂ reduction cycle. Both experimental and theoretical studies suggest that the catalytic reaction may proceed via a distal pathway to liberate NH₃. These findings provide unprecedented insights into the mechanism of N₂ reduction related to FeV nitrogenase.

Synthesis and Reactivity of Manganese Complexes Bearing Anionic PNP- and PCP-Type Pincer Ligands toward Nitrogen Fixation

A series of manganese complexes bearing an anionic pyrrole-based PNP-type pincer ligand and an anionic benzene-based PCP-type pincer ligand is synthesized and characterized. The reactivity of these complexes toward ammonia formation and silylamine formation from dinitrogen under mild conditions is evaluated to produce only stoichiometric amounts of ammonia and silylamine, probably because the manganese pincer complexes are unstable under reducing conditions.