Challenges in reduction of dinitrogen by proton and electron transfer.

Electrochemical ammonia synthesis: Mechanistic understanding and catalyst design

Catalytic reduction of acetylene and dinitrogen was carried out by sodium, zinc, and europium amalgams in the presence of polymolybdenum clusters and the iron-molybdenum cofactor of nitrogenase isolated from the enzyme. The activity of both catalysts toward acetylene changes in the sequence Zn(Hg)<Eu(Hg)<Na(Hg), increasing as the redox potential of the reducing agent is shifted to the negative region. The catalytic reduction of N2 occurs only by the action of sodium and europium amalgams and only in the presence of synthetic polymolybdenum complexes; in the case of Na(Hg), the main product is hydrazine; in the case of Eu(Hg), it is ammonia.

Molybdenum complexes supported by tridentate pincer ligands are exceptional catalysts for dinitrogen fixation using chemical reductants, but little is known about their prospects for electrochemical reduction of dinitrogen. The viability of electrochemical N2 binding and splitting by a molybdenum(III) pincer complex, (pyPNP)MoBr3 (pyPNP = 2,6-bis(tBu2PCH2)-C5H3N)), is established in this work, providing a foundation for a detailed mechanistic study of electrode-driven formation of the nitride complex (pyPNP)Mo(N)Br. Electrochemical kinetic analysis, optical and vibrational spectroelectrochemical monitoring, and computational studies point to two concurrent reaction pathways: In the reaction-diffusion layer near the electrode surface, the molybdenum(III) precursor is reduced by 2e- and generates a bimetallic molybdenum(I) Mo2(μ-N2) species capable of N-N bond scission; and in the bulk solution away from the electrode surface, over-reduced molybdenum(0) species undergo chemical redox reactions via comproportionation to generate the same bimetallic molybdenum(I) species capable of N2 cleavage. The comproportionation reactions reveal the surprising intermediacy of dimolybdenum(0) complex trans,trans-[(pyPNP)Mo(N2)2](μ-N2) in N2 splitting pathways. The same "over-reduced" molybdenum(0) species was also found to cleave N2 upon addition of lutidinium, an acid frequently used in catalytic reduction of dinitrogen.

Molybdenum complexes that contain a new TREN-based ligand [(3,5-(2,5-diisopropyl-pyrrolyl)(2)C(6)H(3)NCH(2)CH(2))(3)N](3-) ([DPPN(3)N](3-)) that are relevant to the catalytic reduction of dinitrogen have been prepared. They are [Bu(4)N]{[DPPN(3)N]MoN(2)}, [DPPN(3)N]MoN(2), [DPPN(3)N]MoN=NH, {[DPPN(3)N]MoN=NH(2)}[BAr(f)(4)], [DPPN(3)N]Mo[triple bond]N, {[DPPN(3)N]Mo[triple bond]NH}[BAr(f)(4)], and {[DPPN(3)N]MoNH(3)}[BAr(f)(4)]. NMR and IR data for [Bu(4)N]{[DPPN(3)N]MoN(2)} and [DPPN(3)N]MoN(2) are close to those reported for the analogous [HIPTN(3)N](3-) compounds (HIPT = hexaisopropylterphenyl), which suggests that the degree of reduction of dinitrogen is virtually identical in the two systems. However, X-ray studies and several exchange studies support the conclusion that the apical pocket is less protected in [DPPN(3)N]Mo complexes than in [HIPTN(3)N]Mo complexes. For example, (15)N/(14)N exchange studies showed that exchange in [DPPN(3)N]MoN(2) is relatively facile (t(1/2) approximately 1 h at 1 atm) and depends upon dinitrogen pressure, in contrast to the exchange in [HIPTN(3)N]MoN(2). Several of the [DPPN(3)N]Mo complexes, e.g., the [DPPN(3)N]MoN(2) and [DPPN(3)N]MoNH(3) species, are also less stable in solution than the analogous "parent" [HIPTN(3)N]Mo complexes. Four attempted catalytic reductions of dinitrogen with [DPPN(3)N]MoN yielded 2.53 +/- 0.35 equiv of total ammonia. These studies reveal more than any other just how sensitive a successful catalytic reduction is to small changes in the triamidoamine supporting ligand.

[HIPTN3N]WCl (WCl) can be synthesized readily by adding H3[HIPTN3N] to WCl4(DME) followed by LiN(SiMe3)2 ([HIPTN3N]3– = [(HIPTNCH2CH2)3N]3– where HIPT = 3,5-(2,4,6-i-Pr3C6H2)2C6H3 = HexaIsoPropylTerphenyl). Reduction of WCl with KC8 in benzene under N2 yields WN=NK. WN=NK is readily oxidized in THF by ZnCl2 to yield zinc metal and WN2. Reduction of WN2 to [WN2]– is reversible at –2.27 V vs. FeCp2+/0 in 0.1 mol/L [Bu4N][BAr′4]/PhF electrolyte (Ar′ = 3,5-(CF3)2C6H3), while oxidation of WN2 to [WN2]+ is also reversible at –0.66 V. Protonation of WN=NK by [Et3NH][OTf] in benzene yields WN=NH essentially quantitatively. Protonation of WN=NH at Nβ with [H(OEt)2][BAr′4] in ether affords [W=NNH2][BAr′4] quantitatively. Electrochemical reduction of [W=NNH2][BAr′4] in 0.1 mol/L [Bu4N][BAr′4]/PhF is irreversible at scan rates of up to 1 V/s. Addition of NaBAr′4 and NH3 to WCl in PhF yields [W(NH3)][BAr′4]. Electrochemical reduction of [W(NH3)][BAr′4] in 0.1 mol/L [Bu4N][BAr′4]/PhF is irreversible at –2.06 V vs. FeCp2+/0 at a scan rate of 0.5 V/s. Treatment of [W(NH3)][BAr′4] with triethylamine and [FeCp2][PF6] in C6D6, followed by LiN(SiMe3)2, yielded W≡N. Treatment of [W(NH3)][BAr′4] with LiBHEt3 (1 mol/L in THF) results in formation of WH, which is converted to WH3 upon exposure to an atmosphere of H2. Attempts to prepare WN=NH by treating WN2 with [2,6-LutH][BAr′4] and CoCp2 yielded only [W=NNH2][BAr′4]. [W=NNH2][BAr′4] is reduced to W=NNH2 by CoCp*2, but this species disproportionates to yield WN=NH, W≡N, and ammonia. Reduction of [W(NH3)][BAr′4] with CoCp*2 does not yield any observable W(NH3). Attempted catalytic reduction of dinitrogen using WN2 as the catalyst under conditions identical or similar to those employed for catalytic reduction of dinitrogen by MoN2 and related Mo complexes failed. Single crystal X-ray studies were carried out on W-N=NK, WN2, W-N=NH, [W=NNH2][BAr′4], and [W(NH3)][BAr′4].Key words: dinitrogen, reduction, tungsten, ammonia.

Abstract

Three new tetramines, (ArNHCH(2)CH(2))(3)N, have been synthesized in which Ar = 3,5-(2,4,6-t-Bu(3)C(6)H(2))(2)C(6)H(3) (H(3)[HTBTN(3)N]), 3,5-(2,4,6-Me(3)C(6)H(2))(2)C(6)H(3) (H(3)[HMTN(3)N]), or 4-Br-3,5-(2,4,6-i-Pr(3)C(6)H(2))(2)C(6)H(2) (H(3)[pBrHIPTN(3)N]). The diarylated tetramine, [3,5-(2,4,6-t-Bu(3)C(6)H(2))(2)C(6)H(3)NHCH(2)CH(2)](2)NCH(2)CH(2)NH(2), has also been isolated, and the "hybrid" tetramine [3,5-(2,4,6-t-Bu(3)C(6)H(2))(2)C(6)H(3)NHCH(2)CH(2)](2)NCH(2)CH(2)NH(4-t-BuC(6)H(4)) has been prepared from it. Monochloride complexes, [(TerNCH(2)CH(2))(3)N]MoCl, have been prepared, as well as a selection of intermediates that would be expected in a catalytic dinitrogen reduction such as [(TerNCH(2)CH(2))(3)N]Mo[triple bond]N and [[(TerNCH(2)CH(2))(3)N]Mo(NH(3))][BAr'(4)] (Ter = HTBT, HMT, or pBrHIPT and Ar' = 3,5-(CF(3))(2)C(6)H(3))). Intermediates that contain the new terphenyl-substituted ligands are then evaluated for their efficiency for the catalytic reduction of dinitrogen under conditions where analogous [HIPTN(3)N]Mo species give four turnovers to ammonia under "standard" conditions with an efficiency of approximately 65%. Only [pBrHIPTN(3)N]Mo compounds are efficient catalysts for dinitrogen reduction. The reasons are explored and discussed.

Ammonia (NH3) is essential to all life on our planet. Until about 100 years ago, NH3 produced by reduction of dinitrogen (N2) in air came almost exclusively from bacteria containing the enzyme nitrogenase.. DOE convened a roundtable of experts on February 18, 2016. Participants in the Roundtable discussions concluded that the scientific basis for sustainable processes for ammonia synthesis is currently lacking, and it needs to be enhanced substantially before it can form the foundation for alternative processes. The Roundtable Panel identified an overarching grand challenge and several additional scientific grand challenges and research opportunities: -Discovery of active, selective, scalable, long-lived catalysts for sustainable ammonia synthesis. -Development of relatively low pressure (<10 atm) and relatively low temperature (<200 C) thermal processes. -Integration of knowledge from nature (enzyme catalysis), molecular/homogeneous and heterogeneous catalysis. -Development of electrochemical and photochemical routes for N2 reduction based on proton and electron transfer -Development of biochemical routes to N2 reduction -Development of chemical looping (solar thermochemical) approaches -Identification of descriptors of catalytic activity using a combination of theory and experiments -Characterization of surface adsorbates and catalyst structures (chemical, physical and electronic) under conditions relevant to ammonia synthesis.

A potentially useful trianionic ligand for the reduction of dinitrogen catalytically by molybdenum complexes is one in which one of the arms in a [(RNCH(2)CH(2))(3)N](3-) ligand is replaced by a 2-mesitylpyrrolyl-alpha-methyl arm, that is, [(RNCH(2)CH(2))(2)NCH(2)(2-MesitylPyrrolyl)](3-) (R = C(6)F(5), 3,5-Me(2)C(6)H(3), or 3,5-t-Bu(2)C(6)H(3)). Compounds have been prepared that contain the ligand in which R = C(6)F(5) ([C(6)F(5)N)(2)Pyr](3-)); they include [(C(6)F(5)N)(2)Pyr]Mo(NMe(2)), [(C(6)F(5)N)(2)Pyr]MoCl, [(C(6)F(5)N)(2)Pyr]MoOTf, and [(C(6)F(5)N)(2)Pyr]MoN. Compounds that contain the ligand in which R = 3,5-t-Bu(2)C(6)H(3) ([Ar(t-Bu)N)(2)Pyr](3-)) include {[(Ar(t-Bu)N)(2)Pyr]Mo(N(2))}Na(15-crown-5), {[(Ar(t-Bu)N)(2)Pyr]Mo(N(2))}[NBu(4)], [(Ar(t-Bu)N)(2)Pyr]Mo(N(2)) (nu(NN) = 2012 cm(-1) in C(6)D(6)), {[(Ar(t-Bu)N)(2)Pyr]Mo(NH(3))}BPh(4), and [(Ar(t-Bu)N)(2)Pyr]Mo(CO). X-ray studies are reported for [(C(6)F(5)N)(2)Pyr]Mo(NMe(2)), [(C(6)F(5)N)(2)Pyr]MoCl, and [(Ar(t-Bu)N)(2)Pyr]MoN. The [(Ar(t-Bu)N)(2)Pyr]Mo(N(2))(0/-) reversible couple is found at -1.96 V (in PhF versus Cp(2)Fe(+/0)), but the [(Ar(t-Bu)N)(2)Pyr]Mo(N(2))(+/0) couple is irreversible. Reduction of {[(Ar(t-Bu)N)(2)Pyr]Mo(NH(3))}BPh(4) under Ar at approximately -1.68 V at a scan rate of 900 mV/s is not reversible. Ammonia in [(Ar(t-Bu)N)(2)Pyr]Mo(NH(3)) can be substituted for dinitrogen in about 2 h if 10 equiv of BPh(3) are present to trap the ammonia that is released. [(Ar(t-Bu)N)(2)Pyr]Mo-N=NH is a key intermediate in the proposed catalytic reduction of dinitrogen that could not be prepared. Dinitrogen exchange studies in [(Ar(t-Bu)N)(2)Pyr]Mo(N(2)) suggest that steric hindrance by the ligand may be insufficient to protect decomposition of [(Ar(t-Bu)N)(2)Pyr]Mo-N=NH through a variety of pathways. Three attempts to reduce dinitrogen catalytically with [(Ar(t-Bu)N)(2)Pyr]Mo(N) as a "catalyst" yielded an average of 1.02 +/- 0.12 equiv of NH(3).

The design of electrocatalysts for the oxidation and production of H2 is important for the development of alternative energy sources. This Article focuses on the [Ni(P2RN2R′)2]2+ electrocatalysts, where P2RN2R′ denotes 1,5-diaza-3,7-diphosphacyclooctane ligands with substituent groups R and R′ covalently bound to the phosphorus and nitrogen atoms, respectively. Theoretical methods are used to investigate the mechanism of the step in the catalytic cycle corresponding to [HNiII(P2N2)2]+ – e– → [NiI(P2HN2)(P2N2)]2+ for H2 oxidation and the reverse reaction for H2 production. This step involves electron transfer (ET) between the Ni complex and the electrode as well as proton transfer (PT) between the Ni and the N. The sequential mechanisms, PT–ET and ET–PT, are investigated for the following (R,R′) substituents: (Me,Me), (Ph,Ph), and (Ph,Bz), where Me, Ph, and Bz denote methyl, phenyl, and benzyl substituents. Density functional theory is used to calculate reduction potentials, pKa values, and PT pathways, and the inner- and outer-sphere reorganization energies for electrochemical ET are calculated within the framework of Marcus theory. For the (Ph,Ph) and (Ph,Bz) systems, the sequential PT–ET mechanism for H2 production would require surmounting a large free energy barrier for the initial PT step, followed by thermodynamically favorable ET. The sequential ET–PT mechanism for these systems would require a moderate initial applied overpotential, followed by a PT reaction with a relatively low free energy barrier. Consistent with experimental data, the calculated overpotential required for the initial reduction in the ET–PT mechanism is lower for the (Ph,Bz) system than for the (Ph,Ph) system for H2 production. The concerted mechanism, in which the electron and proton transfer simultaneously without a stable intermediate, may be thermodynamically favorable and is a direction of future research.

In this paper we explore several issues surrounding the catalytic reduction of dinitrogen by molybdenum compounds that contain the [(HIPTNCH2CH2)3N]3- ligand (where HIPT = 3,5-(2,4,6-i-Pr3C6H2)2C6H3). Four additional plausible intermediates in the catalytic dinitrogen reduction have now been crystallographically characterized; they are MoN= NH (Mo = [(HIPTNCH2CH2)3N]Mo), [Mo=NNH2][BAr'4] (Ar' = 3,5-(CF3)2C6H3), [Mo=NH][BAr'4], and Mo(NH3). We also have crystallographically characterized a 2,6-lutidine complex, Mo(2,6-Lut)+, which is formed upon treatment of MoH with [2,6-LutH][B(C6F5)4]. We focus on the synthesis of compounds that have not yet been isolated, which include Mo=NNH2, Mo=NH, and Mo(NH2). Mo=NNH2, formed by reduction of [Mo=NNH2]+, has not been observed. It decomposes to give mixtures that contain two or more of the following: MoN=NH, Mo triple bond N, Mo(NH3)+, Mo(NH3), and ammonia. Mo=NH, which can be prepared by reduction of [Mo=NH]+, is stable for long periods in the presence of a small amount of CrCp*2, but in the absence of CrCp*2, and in the presence of Mo=NH+ as a catalyst, Mo=NH is slowly converted into a mixture of Mo triple bond N and Mo(NH2). Mo(NH2) can be produced independently by deprotonation of Mo(NH3)+ with LiN(SiMe3)2 in THF, but it decomposes to Mo triple bond N upon attempted isolation. Although catalytic reduction of dinitrogen could involve up to 14 intermediates in a "linear" sequence that involves addition of "external" protons and/or electrons, it seems likely now that several of these intermediates, along with ammonia and/or dihydrogen, can be produced in several reactions between intermediates that themselves behave as proton and/or electron sources.

End-on binding of dinitrogen to low valent metal centres is common in transition metal chemistry but remains extremely rare in f-elements chemistry. In particular, heterobimetallic end-on N2 bridged complexes of lanthanides are unprecedented despite their potential relevance in catalytic reduction of dinitrogen. Here we report the synthesis and characterization of a series of N2 bridged heterobimetallic complexes of U(iii), Ln(iii) and Ln(ii) which were prepared by reacting the Fe dinitrogen complex [Fe(depe)2(N2)] (depe = 1,2-bis(diethylphosphino)-ethane), complex A with [MIII{N(SiMe3)2}3] (M = U, Ce, Sm, Dy, Tm) and [LnII{N(SiMe3)2}2], (Ln = Sm, Yb). Despite the lack of reactivity of the U(iii), Ln(iii) and Ln(ii) amide complexes with dinitrogen, the end-on dinitrogen bridged heterobimetallic complexes [{Fe(depe)2}(μ-η1:η1-N2)(M{N(SiMe3)2}3)], 1-M (M = U(iii), Ce(iii), Sm(iii), Dy(iii) and Tm(iii)), [{Fe(depe)2}(μ-η1:η1-N2)(Ln{N(SiMe3)2}2)], 1*-Ln (Ln = Sm(ii), Yb(ii)) and [{Fe(depe)2(μ-η1:η1-N2)}2{SmII{N(SiMe3)2}2}], 3 could be prepared. The synthetic method used here allowed to isolate unprecedented end-on bridging N2 complexes of divalent lanthanides which provide relevant structural models for the species involved in the catalytic reduction of dinitrogen by Fe/Sm(ii) systems. Computational studies showed an essentially electrostatic interaction of the end-on bridging N2 with both Ln(iii) and Ln(ii) complexes with the degree of N2 activation correlating with their Lewis acidity. In contrast, a back-bonding covalent contribution to the U(iii)–N2Fe bond was identified by computational studies. Computational studies also suggest that end-on binding of N2 to U(iii) and Ln(ii) complexes is favoured for the iron-bound N2 compared to free N2 due to the higher N2 polarization.

The activation and reduction of dinitrogen (N2) to ammonia (NH3) presents one of the essential conversions in sustainable energy production as ammonia holds various industrial applications from fertilizers to food production to hydrogen energy storage. Most of the ammonia supply is fulfilled by the traditional energy-intensive Haber-Bosch (HB) process that has limitations of high operational temperature (400°C–500°C) and pressure (~200 bar) along with the emission of greenhouse gas (CO2). To overcome these limitations and for a sustainable energy future, we need to explore alternative pathways for ammonia synthesis that are eco-friendly and carry out production under ambient conditions. Photoelectrochemical ammonia production offers a renewable energy production route that combines the merits of both light energy (photons) and charge transfer (electrical bias) with moderate energy consumption. This chapter summarizes and analyzes the fundamental aspect of nitrogen reduction reaction (NRR), potential catalysts, and instrumentation setup to carry out N2 reduction followed by ammonia quantification techniques. The recent advances in designing effective catalytic systems with computational efforts for better fundamental understanding are also discussed.

Catalytic reduction of molecular dinitrogen (N2) to ammonia (NH3) is one of the most important and challenging industrial reactions. Electrochemical reduction is considered as an energy‐saving technology for artificial ambient nitrogen fixation, which is emerging as an optimal potential sustainable strategy to substitute for the Haber–Bosch process. However, this process demands efficient catalysts for the N2 reduction reaction (NRR). Here, by means of first‐principles calculations, we systematically explored the potential electrocatalytic performance of single transition metal atoms (Pd, Ag, Rh, Cu, Ti, Mo, Mn, Zn, Fe, Co, Ru, and Pt) embedded in monolayer defective boron phosphide (TMs/BP) monolayer with a phosphorus monovacancy for ambient NH3 production. Among them, the Mo/BP exhibits the best catalytic performance for ambient reduction of N2 through the typical enzymatic and consecutive reaction pathways with an activation barrier of 0.68 eV, indicating that Mo/BP is an efficient catalyst for N2 fixation. We believe that this work could provide a new avenue of ambient NH3 synthesis by using the designed single‐atom electrocatalysts.

There is a growing appreciation of the important role that H-bond complexes can play in the mechanism of proton-coupled electron transfer (PCET) reactions. Until relatively recently it had been thought that PCET reactions always proceeded step-wise with sequential electron and proton transfer. Much of the recent fundamental interest in PCET stems from the realization that a third option is available, concerted electron and proton transfer or CPET in which the electron and proton both move in a single kinetic step. This interest in the concerted process has increased awareness of H-bonding states in PCET, since the concerted reaction occurs within a H-bonded intermediate. However, even if the proton and electron transfer is not concerted, the H-bonded complex formed in the process of proton transfer can play an important role in the PCET mechanism if it is sufficiently long-lived.Recently we have introduced a generally useful mechanistic framework with which to include H-bonding steps within an overall PCET pathway. This scheme, which for obvious reasons we call a “wedge”, is shown in Scheme 1 for the generic 1e−, 1H+ oxidation, AH + B = A + HB+ + e−. The front face in the wedge (in bold) is the standard electron transfer/proton transfer square scheme, with the two possible electron transfer reactions on the top and bottom edges, and the two possible proton transfers on the left and right edges. However, proton transfer reactions actually go through a H-bond intermediate, so a more accurate description of the proton transfer follows the dashed lines on the triangular sides of the wedge to and from the H-bond intermediates, A-H-B or A-H-B+, which are meant to represent the thermodynamically most stable H-bond complex in each oxidation state. If the H-bonded intermediate has sufficient lifetime, then electron transfer to/from the H-bond complex is also possible, represented by the rear edge of the wedge (thin solid line). If the proton moves from being more attached to A in A-H-B to being more attached to B in A-H-B+, then E° of this reaction is that of the CPET step, if the proton doesn’t move then the E° is simply that of oxidation of the H-bond complex. Either way, it is straightforward to show that E°(A-H-B0/+) has to have a value in between E°(AH0/+) and E°(A−/0). Thus the possibility of electron transfer through the H-bond complex opens up a pathway of intermediate potential for the overall reaction AH + B = A + HB+ + e−.The usefulness of the wedge scheme is demonstrated by its ability to explain the unusual electrochemistry of the phenylenediamine-based urea, U(H)H, which we have shown undergoes a self proton transfer upon oxidation to give half equivalent of the doubly oxidized quinoidal cation and half-equivalent of the electroinactive, protonated reduced urea, Scheme 2. The reaction gives chemically irreversible voltammetry in acetonitrile as would be expected given that the quinoidal cation is harder to reduce than the initially formed radical cation. However, it gives reversible voltammetry in methylene chloride, which can be explained by the greater stability of the H-bonded intermediate in this solvent. In addition, in methylene chloride, we are able to clearly observe a concentration and scan rate dependent conversion between two different reduction pathways on the return scan. This behavior cannot be explained by a simple square scheme, but is readily explained by the wedge scheme.In this presentation, we will report recent results on the voltammetry of U(H)H in the presence of guest molecules that H-bond to the starting, reduced state. We will show that their effect on the voltammetry can be explained in terms of two interlinked wedge schemes, one representing the electron transfer / H-bonding / proton transfer reactions of U(H)H with itself and the other representing the reactions with the added guest.