Efficient Nitrogen Fixation via a Redox-Flexible Single-Fe-Site With

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A: Molecular Structure, Quantum Chemistry, and General Theory

Efficient Nitrogen Fixation via a Redox-Flexible Single-Fe-Site With Reverse-Dative Fe#B # Bonding Jun-Bo Lu, Xue-Lu Ma, Jia-Qi Wang, Jin-Cheng Liu, Hai Xiao, and Jun Li J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b02089 • Publication Date (Web): 12 Apr 2018 Downloaded from http://pubs.acs.org on April 15, 2018

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The Journal of Physical Chemistry

Efficient Nitrogen Fixation via a Redox-flexible Single-Fe-site with Reverse-dative Fe→ →B σ Bonding Jun-Bo Lua‡, Xue-Lu Ma a‡, Jia-Qi Wanga, Jin-Cheng Liua, Hai Xiaoa*, Jun Lia,b* a

Department of Chemistry and Key Laboratory of Organic Optoelectronics & Molecular Engineering of the Ministry of Education, Tsinghua University, Beijing 100084, China b

Institute for Interfacial Catalysis and Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, USA * Corresponding author: [email protected], [email protected]

ABSTRACT Model systems of the FeMo cofactor of nitrogenase have been explored extensively in catalysis to gain insights into their ability for nitrogen fixation that is of vital importance to the human society. Here we investigate the trigonal pyramidal borane-ligand Fe complex by first-principles calculations, and find that the variation of oxidation state of Fe along the reaction path correlates with that of the reverse-dative Fe→B bonding. The redox-flexibility of the reverse-dative Fe→B bonding helps to provide an electron reservoir that buffers and stabilizes the evolution of Fe oxidation state, which is essential for forming the key intermediates of N2 activation. Our work provides insights for understanding and optimizing homogeneous and surface single-atom catalysts with reverse-dative donating ligands for efficient dinitrogen fixation. The extension of this kind of molecular catalytic active center to heterogeneous catalysts with surface single-clusters is also discussed.

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1. INTRODUCTION The biological reduction of N2 to NH3 catalyzed by nitrogenase is an intricate and intriguing, yet poorly understood reaction in nature.1-4 Extensive efforts have been made to investigate the catalytic ability of synthetic Fe and Mo complexes, as it is generally believed that Fe and Mo atoms serve as the active sites of FeMoco (Scheme 1) for the activation and functionalization of N2.5-11 One plausible initiating mechanism of N2 activation is that N2 molecule is at first activated by a transition metal (TM) center through donating electrons from its σ- and π bonding orbitals to TM while concurrently accepting back-donating electrons from TM with its antibonding π-orbitals (π*).12-14 While this process is widely studied in homogeneous catalysis, dinitrogen activation and conversion to ammonia on heterogeneous surface has also aroused significant interest.15-16 The recent emerging single-atom catalysis (SAC) provides a bridge for homogeneous and heterogeneous catalysis,17-18 thus allowing a unified viewpoint of the activation mechanism of dinitrogen. The mechanism in homogeneous catalytic center can be extended to design relevant heterogeneous active sites. A key question in the design of such biomimetic model complex is the role of the interaction between Fe/Mo center and its linked main-group atom in binding, activating, and functionalizing N2, and thus several model systems have been synthesized.19-25 The landmark work on well-defined Fe-based complex with tris(phosphino)alkyl(XPiPr3) (X = C, Si, B) ligand featuring axial C/Si/B atoms has established a modestly effective molecular catalyst for N2-to-NH3 conversion by addition of protons and electrons at low temperatures.19-20, 25-26 These complexes provide a platform to investigate the mimic enzymatic process of nitrogen fixation on a molecular level. The trigonal pyramidal borane-ligand Fe ((TPB)Fe = [Fe]) complex is the most efficient Fe-based catalyst for reduction of N2 to NH3 reported so far,27 in which the sp2 hybridized, Lewis-acidic B atom spares an unoccupied pz orbital to form a redox-flexible, reverse-dative TM→B bond as shown in Scheme 1 (The coordinate system orients z-axis along vertical Fe‧‧‧NN and x-axis in one of the P-Fe‧‧‧N plane). This redox-flexible ligand environment clearly plays a crucial role in enabling a single-Fe-site to catalyze N2 reduction.28 The understanding of the response of such flexible Fe→B axial reverse-dative σ bond and corresponding evolution of the formal oxidation state of Fe center is key to rational design of efficient single-Fe-site based homo- and heterogeneous catalysts for N2 activation and functionalization.


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Therefore, the in-depth theoretical understanding and mechanistic investigation of the N2-to-NH3 conversion on the (TPB)Fe complex is needed.

Scheme 1. N2 binding to Fe single-atom in FeMoco (left) and (TPB)Fe (right) complex. The role of FeB bonding, iron oxidation state, and electron/proton transfer for N2 activation on (TPB)Fe are illustrated.

It is of much importance to model individual electron/proton (e-/H+) transfer steps during the stepwise conversion of [Fe]N≡N to [Fe]NH2NH2+ via a redox-active [Fe]N=NH2+ complex.20,

29

Recently, pentavalent iron intermediate [Fe(IV)]N+ has also been proposed upon further protonation of a neutral [Fe]NNH2 hydrazido intermediate.30 Theoretical work found that [Fe]N=NH2+ complex can form [Fe]N2+ iron-nitrido complex with an proton addition.31 These observations verify the viability of associative hybrid and distal mechanisms for Fe-mediated N2-to-NH3 conversion as shown in Figure 1.20, 25, 29-30 In this work, comprehensive theoretical studies using density functional theory (DFT) and ab initio molecular dynamics (AIMD) under the Born-Oppenheimer approximation are carried out to unveil the characteristic features of the reverse-dative Fe→B bond in the complete catalytic cycle of N2-to-NH3 conversion by (TPB)Fe complex. We further use ab initio wavefunction theory (WFT) to find a patterned variation of reverse-dative Fe→B σ bond and formal oxidation state of Fe center, and thus reveal the crucial role of the flexible Fe→B bond as an electron reservoir to stabilize the intermediates toward NH3 production. This work focusing on molecular catalysis provides insight for design of heterogeneous active site with reverse-dative metal→ligand bonding. 2. THEORETICAL METHODOLOGY Quantum chemical calculations were performed using DFT and WFT methods. Geometry optimizations of all intermediates were carried out at the ωB97XD/BSI level32-33 implemented in 3 ACS Paragon Plus Environment


Gaussian-09.34 We use 6-31G* basis set35-37 for B, C, H, P, and N elements, and LANL2DZ basis set for Fe with the scalar relativistic (SR) effects introduced by LANL2DZ effective core potential, which will be abbreviated as BSI basis sets later.38 The energy level is based on the sum of redox (proton+electron) plus the heat of formation of intermediate. The ωB97XD functional was used because

it was shown in our calculations to reproduce the geometric parameters well for several intermediates that are characterized in recent work30 and it was used for alkene hydrogenation by (TBP)Fe in previous calculations.39 The solvation effects were also included upon the gas-phase optimized structures using the SMD40 solvation model with the formic acid as strong acid solvent. The single-point complete-active-space self-consistent field (CASSCF) calculations41-42 were performed using Pyscf code43 as multi-configurational wavefunction is important to understanding of the spin state and oxidation state of transition metal systems.44 Given the complexity of these systems, choosing active space in the multi-configurational calculations is a key issue. To explore the bonding of local Fe-B bond and adsorption substrate, we first performed spin-unrestricted B3LYP45-46 calculations by Pyscf, and then chose the orbitals as initial guess orbitals. In order to test the active space, we first choose a relatively large active space including Fe: 3d, 4d, B: 2pz, and π anti-bonding MOs of N2 as active space. As depicted in Figure S7 and Figure S8, the natural orbital occupation number of Fe:4d is rather small, so we exclude Fe:4d orbital out of active space. For [Fe]Cl, [Fe]NH2NH2+, [Fe]NH2, [Fe]NH3+, we chose the Fe:3d and B:2pz based MOs as active space. For [Fe]N2, [Fe]N2-, [Fe]NNH, [Fe]NNH2+, we chose Fe:3d and B:2pz based MOs and π anti-bonding MOs of N2 as active space. The determination of oxidation states of Fe is based on the natural orbital occupation numbers (NOONs) generated by CASSCF calculations, which is usually more reliable than using the configuration weight. To study the dynamic behaviors, pre-equilibrium calculation was performed using classical molecular dynamics (MD) simulations in GROMACS 5.1.4 package with the standard AMBER99SB forcefield set.47 The Merz-Kollmann (MK) charges of metal complex were calculated by Guassian-09 at HF/6-31G* level, and transferred to restrained electrostatic potential48 (RESP) charges by AMBERTools16,49 which were used to describe the electrostatic interaction. The metal complex was fixed during the classical MD simulations to prevent structural collapse. At the beginning, the metal complex and 100 water molecules were initially distributed randomly in a 4 × 4 × 4 nm3 box. Before MD simulations, energy minimization was carried with the steepest descent method. Then, 4 ACS Paragon Plus Environment

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NPT-equilibration was performed for 1 ns with a time step of 2 fs at 298.15 K, until the volume of box becomes stable. All ab initio molecular dynamics (AIMD) simulations under the Born-Oppenheimer approximation were carried out with CP2K for more than 2000 fs at a time step of 0.5 fs.50 Canonical (NVT) ensemble and Nose-Hoover thermostats were adopted at 298.15 K.51-52 All initial structures for AIMD simulations were optimized to local minima. The spin-polarized PBE exchange-correlation functional was adopted with double-ζ Gaussian basis sets, which were used for the valence electrons.53 An auxiliary plane-wave basis set with a cutoff set to 350 Ry was used for computing the electrostatic terms.54 The initial configuration of AIMD simulation is obtained from the final configuration of classical MD simulations with one more proton. 3. RESULTS AND DISCUSSIONS Several experimental works are done on the dinitrogen conversion with these Fe-B complexes and DFT calculations were also carried out to explain some of the features.20, 25, 28-31, 55-56 However, the complicated electron correlation in Fe complexes with different iron oxidation state often impedes accurate theoretical predictions. Hinted by the experimental results and with combined DFT and WFT calculations, we consider here both the associative hybrid and distal mechanisms for N2 reduction to NH3, which are identified to initiate with two reduction steps and two protonation steps to generate the common experimental detected cationic intermediate [Fe]NNH2+.20, 29-30 The detailed pathways and energetics are shown in Figure 1 and Figure S1. The addition of an electron to [Fe]N2 species (b) produces anionic [Fe]N2- species (c), which is calculated to be exothermic by 15.25 kcal/mol in gas phase. The following proton transfer to the terminal N atom of anionic species [Fe]N2- (c) results in the neutral [Fe]NNH (d), with endothermicity by 9.72 kcal/mol from [Fe]N2- species (c) in gas phase. Such proton transfer is assisted by the hydrogen bond network that connects the proton to the reactant via solvent water molecules, as suggested by our AIMD results of the first protonation step as shown in Figure S3.



Figure 1. The mechanism for the N2 reduction on (TPB)Fe complex by stepwise addition of protons (H+) and electrons (e-). [Fe] = (TPB)Fe. The experimentally characterized species are shown in blue, while as-yet undetected species are shown in black. As is well known, the current industrial approach of ammonia synthesis involves N2 molecule activation on iron or ruthenium surface via a dissociative mechanism with cleavage of the robust N≡N triple bonds, which requires high pressure and temperature.57-60 The Fe-B complex here activates N2 molecule via an alternative associative mechanism, similar to the process involved in FeMoco of nitrogenase. This kind of associative mechanism for N2-to-NH3 conversion is also seen in heterogeneous surface cluster catalysis.15-16 In our case, the associative mechanism branches at the species [Fe]NNH2 (f) into two channels: the hybrid pathway and the distal pathway. In the hybrid pathway, the sequential proton/electron transfers to the Fe-connected N atom. The proton transfer steps from [Fe]NNH2 (f) to [Fe]NHNH2+ (o) and from [Fe]NHNH2 (p) to [Fe]NH2NH2+ (q) are endothermic by 41.09 kcal/mol and 43.22 kcal/mol, respectively. In contrast, the electron transfer steps from [Fe]NHNH2+ (o) to [Fe]NHNH2 (p) and from [Fe]NH2NH2+ (q) to [Fe]NH2NH2 (r) are exothermic by 137.97 kcal/mol and 93.69 kcal/mol, respectively. Hence, the hydrazine complex has also been detected in [SiPiPr3Fe]+ system.29 At warm temperature, experiment showed that [SiPiPr3Fe]NNH2+ rapidly converts to [SiPiPr3Fe]NH2NH2+ through the hybrid pathway.29


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The other reaction channel branched from [Fe]NNH2 (f) is the distal pathway that continues with protonation at the terminal N atom of [Fe]NNH2 (f) to form [Fe]NNH3+ (g) with larger endothermicity of 62.84 kcal/mol compared to the protonation to form [Fe]NHNH2+ (o) in gas phase. However, the free energy gap between [Fe]NNH3+ (g) and [Fe]NHNH2+ (o) is lowered by 16.13 kcal/mol with the solvent effect. The reaction energy of [Fe]NNH3+ (g) liberating NH3 to form [Fe]N+ (s) is exothermic by 30.43 kcal/mol and 24.10 kcal/mol in gas phase and in strong acid solution, respectively, as shown in Figure S2 and Figure 2, which obviously reduces the gap of distal pathway and hybrid pathway. This [Fe]N+ intermediate has also been characterized in the experiment.30

Figure 2. Calculated free energy profile of the N2 reduction at trigonal pyramidal borane-ligand iron complex by stepwise addition of protons and electrons in formic acid solution. [Fe] = (TPB)Fe. (The experimentally characterized species are shown in blue, while as-yet undetected species are shown in black.) Subsequently, the hybrid and distal pathways drive a late N-N bond cleavage in different manners from [Fe]NH2NH2 (r) and [Fe]NNH3 (h), respectively, which are confluent at [Fe]NH2 (l) with further proton/electron transfer to deliver the liberation of NH3. The final reduction of [Fe]NH2 (l) results in the release of the other NH3 and refill of N2 to recover the initial complex [Fe]N2 (b) anchored by the Lewis-acidic B atom of TPB scaffold. The overall reaction energy from [Fe]N2 (b) to



[Fe]NH3+ (m) is exothermic by 208.69 kcal/mol. The result is consistent with the enormous calculated chemical overpotential of 291.6 kcal/mol from the effective bond dissociation free energy of the acid-reductant pair.61 The energy profiles provide a mechanism understanding of the dinitrogen reduction at (TPB)Fe complex by stepwise addition of protons and electrons. Two fundamental questions are still at hand as following: (1) How does the oxidation state of Fe that is essential for N2 activation and hydrogenation evolve along the reaction pathway? (2) How does the Fe→B interaction affect the activation and functionalization of N2? These questions are critical to elucidating the nature of the active center in the N2 activation and step-by-step conversion to NH3. Based on the CASSCF results shown in Figure S4, the Fe center in the starting species, [Fe]Cl (a), has a Fe(I) oxidation state. In catalysis orbital interaction between the environment or ligands and metal active center dictate the physical charge, oxidation state, and energy levels of the metal, thus manipulating the catalytic center.62 The strong electron donation from the electron-rich bulky [P]3 ligands pushes Fe 3d-based levels to higher energy via orbital interaction, which provides possibility of forming better backbonding with the π-antibonding orbitals of N2. The Fe→B bond length in [Fe]Cl (a) is 2.531 Å, which is much larger than the sum of Pyykkö’s single-bond radius of 2.010 Å for Fe-B, indicating Fe→B dative bonding. As depicted in Figure 3, the Fe center then undergoes a two-electron reduction, leading to an uncommon Fe(-I) oxidation state in [Fe]N2- (c), which is critical for subsequent dinitrogen activation. In [Fe]N2 (b) and [Fe]N2- (c), the Fe→B and Fe-P bonds are generally shorter than those in the starting [Fe]Cl (a), and [Fe]N2- (c) has the shortest Fe→B and Fe-P bonds. Our computed Fe-B bond length in [Fe]N2- is 2.294 Å, which is consistent with reported Fe-B bond length 2.311 Å in [[Fe]N2]--Na+ and 2.293 Å in [[Fe]N2)]--[Na([12]crown-4)2]+.28 Compared with that in [Fe]N2 (b), the Fe-N bond distance is shorter in [Fe]N2- (c) with bond length 1.792 Å, which is shorter than the sum of Pyykkö single-bond radius of 1.870 Å. However, the N-N bond is longer in [Fe]N2- (c) (1.136 Å) than that in [Fe]N2 (b) (1.112 Å), indicating moderate N2 activation.


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Figure 3. The schematic orbital energy levels and natural orbital occupation numbers (NOON, shown with each level. Orbitals with NOONs close to 1 and 2 are represented by arrow and two dots, respectively.) from CASSCF calculations of intermediates for the second N2 reduction step and following protonation steps starting from [Fe]N2 ([Fe] = (TBP)Fe) intermediate. (A) [Fe0]N2. (B) [Fe-I]N2. (C) [Fe0]NNH. (D) [FeI]NNH2+. To further investigate these structural variations during N2 activation, the natural orbital occupation numbers (NOONs) of each compound are analyzed using the CASSCF results. As shown in Figure S4 and Figure 3, NOONs of Fe→B σ dative-bonding are 1.55, 1.69, and 1.84 for [Fe]Cl (a), [Fe]N2 (b) and [Fe]N2- (c), respectively. Accordingly, NOONs of Fe→B anti-bonding counterparts decrease as 0.45, 0.31, and 0.16, respectively. Hence, the effective bond orders (EBO) of Fe→B bonds for [Fe]Cl (a), [Fe]N2 (b) and [Fe]N2- (c) are 0.55, 0.69, and 0.84, respectively, as shown in Table 1, which indicate that [Fe]N2- (c) has the strongest dative Fe→B σ bond. While the NOONs of 9 ACS Paragon Plus Environment


π* orbitals of N2 increase, from 0.08 and 0.09 in [Fe]N2 (b) to 0.14 and 0.13 in [Fe]N2- (c) as shown in Figure 3, which reflect the weakening of N-N bonding and activation of N2. Mulliken charges calculated from CASSCF results in Table 1 show that the largest Fe→B electron transfer occurs in [Fe]N2- (c) with -0.29 charge on B atom. Our results is different from previous observation in [CSiP3Ph]Fe-Cl system,26 in which a decreasing degree of covalency with decreased formal oxidation state at Fe from Fe(II) to Fe(I) to Fe(0). Both CASSCF effective bond order (EBO) and charge analysis suggest that the increase of the Fe→B bond strength promotes the reverse-dative electron transfer from Fe to B. This Fe-B reverse-dative bond buffers and thus stabilizes the evolution of oxidation state of Fe, from 0 to an electron-rich state of –I, which is the key to efficiently activating N2. The facile conversion of Fe oxidation state63 in homogeneous conditions clearly promotes the N2 activation and conversion to NH3, which might also be related to the high performance of iron metal as optimal heterogeneous catalysts for ammonia synthesis. As shown in Figure 3, after the first protonation step, [Fe]NNH (d) has NOONs of 1.61 and 1.55 in (Fe)dπ-π*(N2) bonding orbitals and 0.39 and 0.44 in (Fe)dπ-π*(N2) anti-bonding orbitals in the xand y- direction, respectively, which preludes the cleavage of N2 with breaking one of the π bonds in N2 first. The N-H bond forms with the spontaneous electron transfer from Fe center to N2, resulting in the oxidation of Fe from –I back to 0 oxidation state. In response to such change, the Fe→B donation bond strength decreases, with the bond length increasing from 2.294 Å in [Fe]N2- (c) to 2.481 Å in [Fe]NNH (d) and EBO decreasing from 0.84 to 0.69, thus releasing some of the electrons back to Fe to buffer the electron density due to oxidation. The second protonation step from [Fe]NNH (d) to [Fe]NNH2+ (e) pushes the scenario to an extreme. The Fe center is further oxidized to Fe(I), in order to provide the necessary electron for stabilizing [Fe]NNH2 (f). Correspondingly, the Fe‧‧‧B distance in [Fe]NNH2+ (e) increases remarkably to 3.088 Å, indicating essentially no bonding interaction between Fe and B atom. Clearly, the reverse-dative bonding electrons are withdrawn by Fe to buffer its electron loss, which is consistent with the previous work.28, 30 The triple bond of N2 is significantly weakened with the broken of πy bonding. Moreover, the dxz orbital of Fe has a strong interaction with the unbroken πx bonding of N2, with 1.72 and 0.27 NOONs for bonding and anti-bonding orbitals respectively. The πx bonding can be easily broken by the following protonation. The subsequent reduction and protonation of [Fe]NNH2+ (e) generates [Fe]N+ (s) in distal 10 ACS Paragon Plus Environment

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pathway, leading to the release of the first NH3. [Fe]N+ (s) has a Fe≡N triple bond with Fe(IV) oxidation state as shown in Figure S12. However, the oxidation state of Fe center based on our NOONs analysis is Fe(I) in [Fe]NH2NH2+ (q) through hybrid pathway as shown in Figure S4. [Fe]NH2 (I) is protonated further to [Fe]NH3+ (m), in which the NH3 group has less interaction with the Fe center due to the lower energy levels of NH3 orbitals compared with 3d orbital of Fe. This results in the liberation of the other NH3, and the Fe(I) center in [Fe]NH3+ (m) obtains an environmental electron simultaneously to recover the electron-rich Fe(0). With above geometrical and electronic structure analyses, our theoretical study addresses an important issue in N2 fixation: the redox-flexible single-Fe-site with Fe→B reverse-dative bond functions as an electron reservoir that reserves or lends the donation bonding electrons from Fe to buffer the evolution of oxidation state of Fe along the reaction pathway; the Fe→B bond length decreases in the reduction step and increases in the protonation step as shown in Figure 4, which is necessary for effectively forming all the intermediates, [Fe]NxHy, along the reaction.

Figure 4. The evolution of Fe→B distance and oxidation state of Fe along the N2 activation and protonation during the initial two reduction steps and two protonation steps. [Fe] = (TPB)Fe. The patterned variation of the reverse-dative Fe→B bond distances during the initial two reduction steps and two protonation steps is shown in Figure 4 (complete variation of Fe-B bond during catalytic cycle is depicted in Figure S5), along with the evolution of oxidation state of Fe. The



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Fe→B distance decreases to the minimum with electron enrichment of Fe during the N2 activation. Afterwards the Fe→B dative bonding strength starts to decline to essentially no Fe‧‧‧B bonding, as pronation occurs to form [Fe]NNH2+ (e), which drives the electron transfer from Fe to the N2 group. These initial N2 activation and protonation are the key steps for N2 fixation. Table 1. The Fe→B bond lengths (d(Fe-B)) by ωB97XD calculations, and the effective bond orders (EBO) and Mulliken charges (qFe, qB) calculated by CASSCF for selected intermediates. Complex

a

b

c

d

e

s

q

I

m

d(Fe-B)a

2.531

2.365

2.294

2.481

3.088

2.986

2.525

2.567

2.547

EBOb

0.55

0.69

0.84

0.69

-

-

0.54

0.54

0.55

qFe

1.14

0.70

0.48

0.93

0.79

1.05

1.12

1.16

1.12

qB

-0.18

-0.20

-0.29

-0.17

0.26

0.29

-0.17

-0.20

-0.16

a)

Bond length in Å.

b)

There is no Fe→B dative bond in the [Fe]NNH2+ complex (g) and [Fe]N+. Although these results are obtained for the N2 activation and subsequent hydrogenation to NH3

with molecular catalyst of single-Fe-atom complex with trigonal pyramidal borane-ligand, the principles can provide insights for designing heterogeneous single-cluster catalysts to follow the associative mechanism of N2 activation and protonation/hydrogenation. On the one hand, a redox-flexible metal center such as Mn, Fe, Co, Ni is necessary for adjusting the oxidation state of the active center metal and the redox potentials at different steps. With polyatomic metal cluster, the oxidation state change and thus the redox potential control would be even less an issue because of the synergetic effect among the metal atoms, as has been shown in the Fe3/Al2O3 single-cluster catalyst.16 In nature, complicated redox reactions are indeed accomplished by metal clusters embedded in a homogeneous or heterogeneous environment, the [MoFe7S9C] cluster of nitrogenase (Scheme 1) and the [Mn4O5Ca] cluster of enzyme photosystem II for photosynthesis are typical examples.64 On the other hand, the ligands or anchoring atoms at the active center metal need to be able to work as a buffer for the change of the active center during catalytic reactions. In heterogeneous surfaces of metal oxides, nitrides, borides, sulfurides, and phosphorides, the main-group atoms anchoring the metal atoms might be useful in facilitating catalytic reactions via reverse-dative bonding, defects and


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vacancies. The unique surface cluster confined on the support is advantageous for controlling the reaction selectivity and activity for catalytic conversion of N2 to NH3. Design of heterogeneous surface single-cluster catalysts using these principles seems to be interesting. 4. CONCLUSIONS In summary, we elucidate the associative hybrid and distal pathways of N2 to NH3 for the whole catalytic cycle on the well-defined (TPB)Fe molecular catalyst, and identify that there is a patterned variation of Fe→B bond strength correlated with the evolution of oxidation state of Fe along the catalytic reaction pathway. It is shown that the redox-flexibility of single-Fe-site with reverse-dative Fe→B bonding functions as an electron reservoir that buffers and stabilizes the evolution of oxidation state of Fe during N2-to-NH3 conversion. Our results promote a key insight that the redox-flexible active center with reverse-dative bonding between metal and Lewis-acidic ligands is of crucial importance to enable an efficient catalytic cycle of N2 fixation. The great number of reverse-dative donating ligands65-75 available in coordination chemistry allows the development of a wide range of homogenous and heterogeneous single-atom17-18 or single-cluster15 active sites on the basis of a redox-flexible platform. Recently, a series of novel bimetallic systems with reverse-dative Fe/Co/Ni→ U have been found76-78 and they might provide an opportunity for design of single-site binuclear catalysts for complex catalytic reactions related to dinitrogen fixation. Further extension of such active sites to heterogeneous surface (e.g. in metal borides and metal-organic frameworks) seems to be promising for design effective catalysts for N2-to-NH3 conversion at ambient condition.

Supporting Information Geometry structure, electronic structure and energies of several intermediates during catalytic cycle (Table S1 and S2). Free energy profile in gas phase and formic acid solution (Figure S1 and S2). AIMD simulation of [Fe]N2- in water box (Figure S3). The evolution of Fe→B distances during the catalytic cycle for associative hybrid pathway (Figure S5). The natural orbitals and corresponding occupation numbers of [Fe]Cl, [Fe]N2, [Fe]N2-, [Fe]NNH, [Fe]N2H2+, [Fe]N2H4+, [Fe]NH2 and [Fe]NH3+ (Figure S6 and S15)



Acknowledgments The authors are grateful to Dr. Roger Rousseau for fruitful discussion. This work was supported by the National Natural Science Foundation of China (Nos. 21433005, 91426302, and 21590792) to J.L., China Postdoctoral Science Foundation (No. 2017M610863), Beijing Natural Science Foundation (2184105) to X.L.M. This work was partially supported by the US Department of Energy, the Office of Basic Energy Science, Division of Chemical Sciences, Geosciences and Biosciences (to Roger Rousseau), and performed at the Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for DOE by Battelle. The calculations were performed using the supercomputers at Tsinghua National Laboratory for Information Science and Technology and the Computational Chemistry Laboratory of Department of Chemistry under Tsinghua Xuetang Talents Program.

Author Contributions ‡ These authors contributed equally to this work.


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Efficient Nitrogen Fixation via a Redox-Flexible Single-Fe-Site With

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