Riyaz2018.pdf

Accepted Manuscript Computational Design of Boron Doped Lithium (BLin) Cluster-Based Catalyst for N2 Fixation Mohd Riyaz, Neetu Goel PII: DOI: Reference:

S2210-271X(18)30092-6 https://doi.org/10.1016/j.comptc.2018.03.010 COMPTC 2744

To appear in:

Computational & Theoretical Chemistry

Received Date: Revised Date: Accepted Date:

21 February 2018 13 March 2018 14 March 2018

Please cite this article as: M. Riyaz, N. Goel, Computational Design of Boron Doped Lithium (BLin) Cluster-Based Catalyst for N2 Fixation, Computational & Theoretical Chemistry (2018), doi: https://doi.org/10.1016/j.comptc. 2018.03.010

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Computational Design of Boron Doped Lithium (BLin) Cluster-Based Catalyst for N2 Fixation Mohd Riyaza and Neetu Goela,* a

Theoretical & Computational Chemistry Group, Department of Chemistry and Centre of Advanced Studies in Chemistry, Panjab University, Chandigarh, 160014, India ABSTRACT The activation of dinitrogen bond and its conversion to ammonia under ambient condition is one of the most attracted issues in chemistry. Based on first principle investigations, we propose a boron doped lithium (BLin, n=5-7) cluster based catalysts for N2 fixation. Our calculations predict BLi6 to be a suitable catalyst for conversion of N2 to ammonia with a limiting potential of 19.13kcal/mol. Two mechanistic routes i.e. distal and enzymatic pathways have been investigated in detail to understand the feasibility of the reaction. The possibility of grafting the cluster on the support surface such as graphene and boron nitride sheet has also been explored. The study concludes that like the isolated cluster, the supported BLi6 cluster is a promising catalyst for N2 activation.

Graphical abstract

Highlights:  Activation of dinitrogen bond and its conversion to ammonia under ambient condition.  Boron doped lithium (BLin, n=5-7) cluster based catalysts for N2 fixation.  BLi6 found to be suitable catalyst for conversion of N2 to ammonia.  Grafting of the cluster catalyst on graphene and BN-sheet has been explored

1. Introduction Dinitrogen (N2) is one of the most inert chemical elements owing to its extremely strong triple bond. Nearly 78% of atmospheric volume is N2, but its conversion to ammonia (NH3) or nitrides (nitrogen fixation) is necessary for its direct utilization by living organisms [1,2]. The HaberBosch process, that uses Fe catalyst, is the most common industrial method to convert N2 to NH3 and the reaction proceeds in extreme reaction conditions and needs tremendous amount of energy [3-5]. However nature accomplishes this task using nitrogenase enzymes that activate dinitrogen under ambient conditions at transition metal (Fe/Mo, V)-sulfide cluster sites [6]. Understanding the mechanism of nitrogen fixation by these enzymes is crucial to design efficient catalyst for conversion of N2 to active forms of nitrogen. Numerous strategies for N2 fixation have been explored in the past and it is one of the trending topics of discussion in synthetic as well as theoretical chemistry. Since the discovery of dinitrogen complex in 1965, a number of dinitrogen complexes with transition metal exhibiting different binding orientation of N2 (end-on, side-on and bridging) have been reported [7,8]. Owing to these efforts, numerous metal and metal oxides based catalysts have been explored which include homogenous catalysis [9, 10], photochemical [11,12], electrochemical [13-14] and heterogeneous based thermo-catalytic processes [15]. The Fe and Mo-based complex are widely explored catalyst to activate N2 due to its presence in active sites of natural nitrogenase enzymes [8,16-19]. These Fe and Mo complexes with suitable ligands mostly pincer ligands of PPP- and PNP-pincer type [Example of PNP= 2,6-bis(di-tert-butylphosphinomethyl)pyridine) and PPP=bis(di-tert-butylphosphinoethyl)phenylphosphine] show promising catalytic activity for N 2 fixation. Cluster mediated catalytical processes are of particular interest now a days [20-22], as they individually can act as active sites and one can tune their activity and selectivity as a function of size of the cluster. Particularly, the clusters of Li have received special attention for their applications in various fields like electronics, hydrogen storage and electroncatalytic processes. Lithium monoxide (LinO) and Lithium carbide (LinC) are the two most famous heterogeneous Li cluster that are well established theoretically as well as experimentally [23-26]. Recently lithium-boron cluster system has been studied extensively to explore its possible use in

lithium batteries, hydrogen storage and catalytical applications [27-34]. Ma et al. [33] have investigated the activation of CO2 with BLin (n=2-6) clusters for C-H carboxylation of benzene and found that BLi5 and BLi6 easily activate CO2 and their structure remains intact during the reaction. Previously Roy et al. [35] discussed the detailed mechanism for insertion of N2 into model Li-cluster that clearly showed the manner in which N2 can be activated and the N≡N bond gets elongated by electron transfer. It has been proposed that while homoatomic Li cluster cannot act as catalysts for the N≡N bond dissociation, incorporation of some other element in the cluster looks promising strategy to design nitrogen fixing catalyst. In view of the fact that BLin exhibits super-alkali atom characteristics, with the likelihood of charge transfer from BLin to the π* orbital of N2, present work proposes boron-lithium cluster (BLin , n=5-7) as efficient catalyst for N2 fixation. In a number of recent studies, supported catalysts, in which the catalytically active site is anchored to nanosurfaces like graphene and boron nitride sheet (BN-sheet), emerge as a new dimension in heterogeneous catalysis [36-39]. For instance Zhao et al. [36] have reported promising catalytic activity of Mo-embedded BN-sheet for N2 fixation and thoroughly discussed its reaction mechanism using density functional theory (DFT) computations. Using DFT computations Schenk et al. [40] have investigated Schrock Mechanism of Dinitrogen reduction mediated by molybdenum complex with HIPTN3N ligand (HIPT= hexaisopropyl-terphenyl) and deduced the thermodynamically most feasible pathway. Recent advances in using a more systematic scientific approach for catalyst design and developing novel functional materials through molecular aggregation have generated interest among researchers to scrutinize the processes at atomic level. Impressive advances in large scale DFT computations have facilitated not only the design of new and efficient catalysts but also help in deeper understanding of reaction mechanism. In the search for a versatile and efficient synthetic method for the catalytic formation of ammonia from molecular dinitrogen under ambient reaction conditions for constructing a “post-Haber−Bosch process” we propose BLin cluster as a mediator for the conversion of N2 to NH3 using first principle DFT calculations that also help us to elucidate the reaction mechanism in detail. While the reaction route has been traced in gas phase, anchoring of cluster to the graphene and BN-sheet has also been studied with the view to deposit the catalytic cluster on a nanosurface. 2. Computational details

All the calculations have been performed with spin-polarized DFT computations using Gaussian 09 (A.1) [41] package. For geometry optimization and frequency calculation, DFT based B3LYP [42-44] hybrid functional was adopted in conjugation with 6-31G+(d) (split-valance basis set with diffuse and polarization functions). The reliability of employed level of theory for lithiumboron cluster system has been established in previous studies [30, 32]. A mixed basis set of 631G+(d) + LanL2DZ coupled with same hybrid functional was used to optimize the metallocene containing Co transition metal. Long range van der Waals interactions have been incorporated by employing the empirical correction in the Grimme scheme [45-46]. Natural bond orbital (NBO) [47] charge analysis was performed to compute the charge transfer during the course of reaction. There are six net coupled proton and electron transfer steps involved in the catalytic cycle for conversion of N2 to two molecules of NH3 (N2 + 6H+ + 6e- → 2NH3). The energies of electron and proton (E(e ) , E(H+) were calculated using the model of Lutidinium ([2,6LutH]+) as a proton source and a metallocene [CoCp*2] as an electron source [40]. The E(H+) = (ELutH+ ELut) and E(e ) values obtained in the present work are -273.24kcal/mol and -103.49kcal/mol respectively, these are concordant with the previously reported values [38-40]. The adiabatic and vertical ionization energies (AIE, VIE) of the LiBn clusters are also in agreement with the literature values (Table 1). [X] +LutH+→ [XH]+ + Lut (protonation) [XH] + [CoCp*2] →[XH] [CoCp*2]+(reduction)

(1) (2)

Here equation 1 and 2 represent protonation and reduction step in the catalytic cycle and X refers to intermediates. The reaction energy for the first protonation (Eproton) is given in equation 3 and accordingly for other protonation steps Eproton= E(BLin--N2+H+

E(BLin--N2

E(H+)

(3)

Likewise the reaction energy for first reduction step (Ereduct) is given in equation 4 and accordingly for other reduction steps, Ereduct = E(BLin--N2 H

E(BLin--N2+H+

E(e )

(4)

Binding energy (EB) was calculated according to the below equation: EB = E(cluster+N2) E(cluster

E(N2)

(5)

where E(cluster+N2), E(cluster) and E(N2) are the total energies of N2 bonded with the cluster and the energy of cluster and N2 molecule in isolation respectively. In case of cluster supported on surface, adsorption energy (E ads) was calculated as

Eads = Esurface/cluster Esurface Ecluster

(6)

where E surface/cluster, Esurface and Ecluster are total energy of the cluster deposited over the surface and the energy of isolated surface and cluster respectively. The computational model of graphene nanosurface consists of 54 carbon atoms and that of BN-sheet has 27 nitrogen and 27 boron atoms, the dangling atoms of both the model structures were passivated with hydrogen. 3. Result and Discussion Literature reports suggest that BLi5, BLi+6, BLi7 and BLi+8 are the most stable species of BLin family compared to the neighboring cluster species. This can be explained by magic numbers of phenomenological shell model, BLi5 and BLi+6 have noble gas configuration with 8 valence electrons required for shell closure. These clusters have high VIE, large HOMO–LUMO gaps and high degree of aromaticity than the other members of the series [31-32]. The present study considers cluster size of n = 5-7 for BLin clusters because of the stability of the system and previous reports suggest that BLin is catalytically active for n value of 5 or more [33]. 3.1. Interaction of N2 with BLin The BLin cluster with boron trapped inside Li alliance is investigated here as a novel catalyst for nitrogen fixation. The foremost step in the fixation process is the activation of strong N≡N triple bond, interaction of N2 with cluster sizes (n=5-7) is thus investigated to look for possible activation of this bond. The BLi5 cluster has a square pyramidal (C4V) geometry and to study its interaction with N2, various relative configurations of N2 with respect to BLi5 were considered. It was observed that N2 interacts through end-on with one of the Li in BLi5 cluster (Fig. 1a) within a distance of 2.04Å and binding energy of -11.84kcal/mol. A small charge transfer of 0.07e from BLi5 to antibonding orbital N2 (KK(σ2s)2(σ2s*)2(σ2pz)2(ᴫ2px)2 (ᴫ2py)2(ᴫ2px*) (ᴫ2py*)) is noticed with trivial elongation of N≡N bond and no significant change in N≡N stretching frequency. Thus BLi5 binds with N2 but performs inadequately as a catalyst for N2 activation. In a similar fashion, possibility to bind and activate N2 over the most stable geometry of BLi6 (Oh) and BLi7 (D5h) was explored by considering various relative orientations. Our calculations suggest that N2 binds with BLi6 and BLi7 in a side-on fashion, N2 interacts with three Li atoms of BLi6 with binding energy of -18.17kcal/mol (see Fig. 1b) and with four Li atoms of BLi7 with binding energy of -18.90kcal/mol (see Fig. 1c). The BLi6-N2 system shows transfer of 0.66e from BLi6 to N2 with elongation of N≡N bond from 1.10Å (isolated N2) to 1.19Å and transfer of 1.00e charge with elongation up to 1.22Å in case of BLi7-N2 system. Corresponding

decrease in N2 stretching frequencies from 2455cm-1 to the N2 bonded with (BLi6 N2) and BLi7 (BLi7 N2) were 1858cm-1 and1593cm-1 respectively. These observations indicate activation of N2 after its binding with BLi6 and BLi7 clusters. But comparing the structural parameter of BLi6 and BLi7 after binding of N2 (Fig. 1b and 1c), the BLi6 cluster does not show much structural changes, but the pentagonal bipyramidal geometry of BLi7 gets distorted after binding with N2. Though BLi7 activates N2 but it cannot hold its structural integrity after interaction with N 2 making its regeneration difficult. The above discussed facts suggest that BLi6 is a promising catalyst for activation of N2, it has an electronic shell structure closely similar an alkali atom (1s22s22p63s1), its IE (3.84eV) is lower than that of BLi5 (4.27eV), BLi7 (4.23eV) and the most electropositive element Cs (3.90eV). Thus BLi6 can be considered as a super alkali atom with a high charge transfer capacity. The average binding energy (ABE) of the BLi6 cluster ((ABE = nE(Li) + E(B) E(BLin))/n) is higher (1.41eV) than BLi5 (1.31eV) and BLi7 (1.38eV). Since BLi6+ is considered to be the most stable species of BLin family with highest ABE value of 1.65eV, catalytic cycle for nitrogen fixation by the cluster has been initiated with the reduction of BLi6+ (X+) to BLi6(X) (Fig.4). 3.2. Catalytic Reduction of N2 on BLi6 All the key reaction steps in the catalytic cycle of N2 conversion to NH3 through two possible pathways i.e. enzymatic and distal have been investigated in detail. The mechanistic route has seven steps, there are six consecutive protonation-reduction steps followed by release of NH3 (Fig. 2). The distal pathway involves successive hydrogenation at one N till release of first NH 3 molecule, and then hydrogenation of second nitrogen to form second NH 3. In the enzymatic pathway, hydrogenation occurs alternatively on both the N atoms with the release of NH 3 molecules at the last two consecutive steps. The reaction proceeds with reduction of BLi6+ (X+) to neutral BLi6 (X), this step (step 1 in Fig.4) was uphill in the energy profile by 12.62kcal/mol. The activation of N2 after its binding with neutral BLi6 has been discussed above, N2 binds with BLi6 in side-on fashion forming XN2*, (step 2 in Fig.4) with the release of 18.16kcal/mol energy. The activated N 2 * was then hydrogenated by consecutive protonation and reduction via two distinct routes. The distalpathway is first elucidated in detail, structure of all the intermediates are depicted in Fig. 3. The energies of all the species in the catalytic cycle are with reference to the energy of initial state

that has been taken as 0kcal/mol [E(BLi6+) + E(N2) + 7E(e-) + 6E(H+) = 0kcal/mol]. he first hydrogenation step of distal mechanism leads to

N2H*(step 3 in Fig.4) with N-H and N=N

bond lengths 1.05Å and 1.26Å (Fig.3a) respectively. In this step the energy goes downhill by 18.65kcal/mol. The addition of second H+ and e at the same N-atom (step N2H2*(Fig. 3b) with the release of energy 1.34Å. The subsequent addition of H+ and e (step

in ig

leads to

cal mol and elongation of N N bond to in ig

leads to formation of

N--NH3*

cal mol and N N bond elongation to 1.46Å. The next

(Fig.3c) with the release of energy

step involves hydrogenation of 2 nd N atom (step 6D in Fig. 4) and the simultaneous release of 1st NH3 molecule. This step is accompanied with release of large amount of energy (72.01kcal/mol) owing to dissociation of the N N bond, releasing first NH3 molecule. hydrogenation steps (step

and

in

ig

leading to the formation of

he subse uent NH2*and

NH3*(Fig.3 e and f) are also exothermic. In the final step, release of second NH 3 molecule from the cluster species needs to overcome a barrier of 19.31kcal/mol (step 9 in Fig.4). This mechanistic pathway predicts remarkable catalytic activity of BLi6 with a limiting-potential of only 19.31kcal/mol. Since, bond length is used as criteria to decide about bond dissociation, Fig 5. incorporates all the bond length variations in the reaction sequence with successive hydrogenation. The separate energy profile given in the supplementary information (Fig 1S) depicts energy change with each protonation and reduction steps. Further, the reaction route is elucidated according to enzymatic mechanism, with alternate hydrogenation on the N atoms (structure of all the intermediates given in Fig. 3). The first three steps (step 1- 3 in Fig. 4) of this route are the same as discussed above, i.e. reduction of BLi6+, activation of N2 followed by first hydrogenation step (i.e. addition of first H ++ e to the N2 to give

N2H*,Fig. 3a). Next steps involve successive hydrogenation at the two nitrogen

alternately leading to the release of first NH3 molecule in the 7th step (step 7E Fig. 4). The last two steps of enzymatic mechanism (step 8 and 9 in Fig. 4) are again the same as that observed in case of distal route. It is to be noted that both the mechanistic possibilities need the limitingpotential of 19.13kcal/mol, but at the junction of step 3 that offers two different routes to the reaction, addition of 2nd hydrogen via enzymatic pathway (i.e. 4E step in Fig.4) is more exothermic than through distal pathway (step 4D in Fig.4). Moreover, the subsequent steps in enzymatic pathway lie lower than the corresponding steps of the distal mechanism and there is no crossing of two reaction routes. This confirms that the hydrogenation of dinitrogen will

follow the thermodynamically more favorable enzymatic pathway. However, enzymatic mechanism presents the likelihood of hydrazine formation as an unwanted side product [39,48], but this possibility was ruled out when we studied if the hydrazine gets released (at step 6E) or the reaction follows its normal course. The energetics reveal that liberation of hydrazine form the BLi6 cluster needs an energy of 30.10kcal/mol (Fig. 2S) which is very high when compared to the subsequent reaction steps towards ammonia formation, so formation of hydrazine is very unlikely. The catalytic performance of BLi6 has been further interpreted in terms of variation of NBO charge density in each step along the most feasible pathway (Fig. 6). Each intermediate is considered to be a combination of two moieties i.e. BLi6 (moiety 1) and NxHy (moiety 2). The species at step 0 represent the isolated BLi6 cluster and N2 molecule, step 1 (binding of N2 to BLi6) shows gain of 0.66e charge by N2 from BLi6. In the subsequent hydrogenation steps moiety 1 has some net positive charge and moiety 2 has net negative charge, so overall moiety 1 servers as electron reservoir and the moiety 2 acts as electron acceptor. In Step 4, there is abrupt charge variation in which both moieties became almost neutral, this corresponds to the formation of stable hydrazine molecule, and finally NH3 is released in the last step and the neutral cluster moiety BLi6 is ready for activation of more N2 molecules. 3.3. Graphene and BN-Sheet supported BLi6 Present computations provide enough evidence in favor of BLi6 cluster as an efficient and active catalyst for N2 fixation, but it’s important to note here that catalytic activity of the cluster in gas phase may have little relevance for experimental situations, the cluster need to be deposited on a substrate. We have considered graphene and BN-sheet as the possible nanosurfaces to support the cluster. Different orientations of BLi6 over both the surfaces were studied and the most stable ones are given in Fig. 7 I and II, in which three lithium atoms faces the surface with minimum adsorption height of 2.45Å and 2.54Å over graphene and BN-sheet respectively. The adsorption energies of -114.00kcal/mol and -77.54kcal/mol over graphene and BN sheet respectively are sufficient to prevent cluster aggregation [21]. In terms of adsorption energies, both the nanosurfaces are well suited to graft the cluster but it necessary to ensure that stabilizing the cluster on the surface does not affect its capacity to activate the N2. It was noticed that N2 interacts with Li atoms of the graphene supported cluster at the minimum distance of 2.14Å and in the case of BLi6 supported on the BN-sheet, N2 is at the

distance of 1.97Å from the cluster atoms (Fig. 7 III and IV). It was further observed that N-N bond elongation is up to 1.15Å and 1.18Å in the case of BLi6 supported on graphene and BNsheet respectively. It is thus concluded that BN sheet is the compatible nanosurface to graft the BLi6 cluster catalyst as it binds the cluster without changing its catalytic activity towards nitrogen fixation. It is pertinent to mention here that in the quest for efficient N 2 fixation catalysts, recently, number of transition metal catalysts with great activity like single metal catalysts embedded in a support [36-37], transition metal complexes [38-40], carbene based organo-catalysts [5] etc have been designed through first principle calculations. However, heavy metal catalysts present environmental hazards that need to be checked. A controlled size cluster can be cost effective as it acts as individual active site, while in traditional catalysts, reaction occurs at the surface and the bulk atoms often not participate in the reaction leading to significant increase in cost of catalysts for expensive metals. Furthermore, controlled size model cluster can be treated with the highest level quantum mechanical methods to gain fundamental understanding of the molecular level interaction occurring in the catalytic process that is crucial in order to make significant advance in this field. 4. Conclusions In the present study, we investigated the possibility of using BLin (n=5-7) cluster as catalysts for N2 fixation to ammonia. Both the mechanistic pathways i.e. distal and enzymatic routes have been traced on the basis of first principle calculations. Out of the three model cluster, BLi 6 is found to be the best suited catalyst for N2 activation without destroying its structural integrity. With the purpose to mimic the experimental set up, grafting of the cluster catalyst on graphene and BN-sheet has been explored. Our calculations predict that BN-sheet stabilizes the BLi6 without altering its catalytic activity towards N2. It is hoped that present findings will provide a new avenue for the development of nitrogen fixation catalysts and motivate more research in this stimulating field. 5. Acknowledgements MR is grateful to the University Grants Commission (UGC) for the junior research fellowship and NG thanks SERB, DST, India for financial support via grant, SERB/F/8589/2014-2015. 6. Supporting Information

Schematic energy profile corresponding to each protonation and reduction steps through both the

pathways, the energy profile for dissociation of hydrazine from BLi6 and the calculated relative energies of the intermediates involved in all the steps of the catalytic cycle.

7. References [1] M.P. Shaver, M.D. Fryzuk, Activation of molecular Nitrogen: coordination, cleavage and functionalization of N2 mediated by metal complexes, Adv. Synth. Catal. 345 (2003) 1061– 1076. [2] M.B. Peoples, D. F. Herridge, J.K. Ladha, Biological nitrogen fixation: An efficient source of nitrogen for sustainable agricultural production?, Plant Soil 174 (1995) 3-28. [3] M.D. Fryzuk, S.A. Johnson, The continuing story of dinitrogen activation, Coord. Chem. Rev. 200 (2000) 379–409. [4] G.J. Leigh,

he World’s Greatest

ix: A history of nitrogen and agriculture; oxford

university press: New York, 2004. [5] S.W. Park, Y. Chun, S.J. Cho, S. Lee, K.S. Kim, Design of Carbene-based organocatalysts for Nitrogen fixation: Theoretical Study, J. Chem. Theory Comput. 8 (2012) 9 3− 9 [6] E.M. Siegbahn, Model calculations suggest that the central carbon in the FeMo- cofactor of nitrogenase becomes protonated in the process of nitrogen fixation, J. Am. Chem. Soc. 138 (2016) 0

− 0 9 .

[7] M. Hidai, Y. Mizobe, Recent advances in the chemistry of dinitrogen complexes, Chem. Rev. 95 (1995) 1115-1133. [8] F. Studt, F. Tuczek, Theoretical, spectroscopic, and mechanistic studies on transition-metal dinitrogen complexes: Implications to reactivity and relevance to the nitrogenase problem, J. comput. Chem. 27 (2006)1278-1291. [9] K.C. MacLeod, P.L. Holland, Recent developments in homogeneous dinitrogen reduction by Molybdenum and Iron, Nat. Chem. 5 (2013) 559-565. [10] H. Broda, F. Tuczek, Catalytic ammonia synthesis in homogeneous solution—biomimetic at last?, Angew. Chem. Int. Ed. 53 (2014) 632 – 634.

[11] A. Banerjee, B.D. Yuhas, E.A Margulies, Y. Zhang, Y.Shim, M.R. Wasielewski, M.G Kanatzidis, Photochemical Nitrogen conversion to Ammonia in ambient conditions with FeMoS-chalcogels, J. Am. Chem. Soc. 137 (2015) 2030−203 [12] H. Li, J. Shang, Z. Ai, L. Zhang, Efficient visible light nitrogen fixation with BiOBr nanosheets of oxygen vacancies on the exposed {001} facets, J. Am. Chem. Soc. 137 (2015) 393− 399 [13] M.A. Shipman, M.D. Symes, Recent progress towards the electrosynthesis of ammonia from sustainable resources, Catal. Today. 286 (2017) 57–68. [14] S. Giddey, S.P.S. Badwal, A. Kulkarni, Review of electrochemical ammonia production technologies and materials, Int. J. Hydrog. Energy. 38 (2013) 14576 -14594. [15] A. Vojvodic, A.J. Medford, F. Studt, F.A. Pedersen, T.S. Khan, T. Bligaard, J.K. Nørskov, Exploring the limits: A low-pressure, low-temperature Haber–Bosch process, Chem. Phys. Lett. , 598 (2014) 108–112. [16] Y. Nishibayashi, Recent Progress in Transition-Metal catalyzed reduction of Molecular Dinitrogen under ambient reaction conditions, Inorg. Chem. 54 (2015) 923 −92 0 [17] K. Arashiba, E. Kinoshita, S. Kuriyama, A. Eizawa, K. Nakajima, H. Tanaka, K. Yoshizawa, Y. Nishibayash, Catalytic reduction of dinitrogen to Ammonia by use of Molybdenum−Nitride complexes bearing a tridentate Triphosphine as catalysts, J. Am. Chem. Soc. 137 (2015)

−

9

[18] M.J. Chalkley, T.J. Del Castillo, B.D. Matson, J.P. Roddy, J.C. Peters, Catalytic N2-to-NH3 conversion by Fe at lower driving force: A proposed role for metallocene-mediated PCET, ACS Cent. Sci. 3 (2017) 2

−223

[19] H. Tanaka, Y. Nishibayashi, K. Yoshizawa, Interplay between theory and experiment for Ammonia synthesis catalyzed by transition metal complexes, Acc. Chem. Res. 49 (2016) 987−995. [20] J.F. Ma, F. Ma, Z.J. Zhoub, Y.T. Liu, Theoretical investigation of boron-doped lithium clusters BLin (n=3-6) activating CO2: An example of the carboxylation of C-H bond, RSC Adv. 6 (2016) 84042-84049. [21] D. Kumar, S. Pal, S. Krishnamurty, N2 activation on Al metal clusters: catalyzing role of BN-doped graphene support, Phys. Chem. Chem. Phys. 18 (2016) 27721-27727.

[22] Y. Gao, N. Shao, Y. Pei, Z. Chen, X.C. Zeng, Catalytic Activities of Sub nanometer Gold Clusters (Au16-Au18, Au20, andAu27-Au35) for CO Oxidation, ACS Nano. 5 (2011)7818-7829. [23] R.O. Jones, A.I. Lichtenstein, Density Functional study of structure and bonding in lithium clusters Lin and their oxides LinO, J. Chem. Phys. 106 (1997) 4566-4574. [24] P.V. Ragu, E. Schleyer, E.U. Wurthwein, E. Kaufmann, T. Clark, J.A. Pople, Effectively hypervalent molecules. 2. lithium carbide (CLi5), lithium carbide (CLi6), and the related effectively hypervalent first row molecules, CLi5-nHn and CLi6-nHn, J. Am. Chem. Soc. 105 (1983) 5930–5932. [25] C.H. Wu, The stability of the molecules Li4O and Li5O, Chem. Phys. Lett. 139 (1987) 357359. [26] H. Kudo, Observation of hypervalent CLi6 by Knudsen-effusion mass spectrometry, Nature. 355 (1992) 432-434. [27] Y. Li, D. Wu, Z.R. Li, C.C. Sun, Structural and electronic properties of Boron-Doped Lithium clusters: ab Initio and DFT studies, J. Comp. Chem. 28 (2006) 1677-1684. [28] T.B. Tai, P.V. Nath, M.T. Nguyen, S. Li, D.A. Dixon, Electronic structure and thermochemical properties of small neutral and cationic lithium clusters and boron-doped Lithium clusters: Lin0/+ and LinB0/+ (n=1-8), J. Phys. Chem. A 115 (2011)7673-7686. [29] C. Wu, H. Wang, J. Zhang, G. Gou, B. Pan, J. Li, Lithium−Boron (Li−B Monolayers: first-crinciples cluster expansion and possible two-dimensional superconductivity, ACS Appl. Mater. Interfaces 8 (2016) 2 2 −2 32 [30] K.A. Nguyen, K. Lammertsma, Structure, bonding, and stability of small boron-lithium Clusters, J. Phys. Chem. A. 102 (1998)1608-1614. [31] T.B. Tai, M.T. Nguyen, The high stability of boron-doped lithium clusters Li5 B, Li6 B+/ and Li7B: A case of the phenomenological shell model, Chem. Phys. Lett. 489 (2010) 75–80. [32] Y. Li, Y.J. Liu, D. Wu, Z.R. Li, Evolution of the structures and stabilities of boron-doped lithium cluster cations: ab Initio and DFT studies, Phys. Chem. Chem. Phys. 11 (2009) 57035710. [33] J.F. Ma, F. Ma, Z.J. Zhoub, Y. T. Liu, Theoretical investigation of boron-doped lithium clusters BLin (n=3-6) activating CO2: An example of the carboxylation of C-H bond, RSC Adv. 6 (2016) 84042-84049.

[34] S. Bandaru, A. Chakraborty, S. Giri, P.K. Chattaraj, Toward analyzing some neutral and cationic boron–lithium clusters (BxLiy x = 2–6; y = 1, 2) as effective hydrogen storage materials: A conceptual density functional study, Int. J. Quantum. Chem. 112 (2012) 695– 702. [35] D. Roy, A.N. Vazquez, P.V.R. Schleyer, Modeling Dinitrogen activation by lithium: A mechanistic investigation of the cleavage of N2 by stepwise insertion into small lithium clusters, J. Am. Chem. Soc. 131 (2009)13045-13053. [36] J. Zhao, Z. Chen, Single Mo atom supported on defective boron nitride monolayer as an efficient electrocatalyst for nitrogen fixation: A computational study, J. Am. Chem. Soc. 139 (2017) 2

0− 2

[37] A. Magistrato, A. Robertazzi, P. Carloni, Nitrogen fixation by a molybdenum catalyst mimicking the function of the nitrogenase enzyme: a critical evaluation of DFT and solvent effects, J. Chem. Theory Comput. 3 (2007) 1708-1720. [38] X.F. Li, Q.K. Li, J. Cheng, L. Liu, Q. Yan, Y. Wu, X.H. Zhang, Z.Y. Wang, Q. Qiu, Y. Luo, Conversion of Dinitrogen to Ammonia by FeN3-Embedded Graphene, J. Am. Chem. Soc. 138 (2016) 8706–8709. [39] S. Kuriyama, K. Arashiba, K. Nakajima, Y. Matsuo, H. Tanaka, K. Ishii, K. Yoshizawa, Y. Nishibayashi, Catalytic transformation of dinitrogen into ammonia and hydrazine by irondinitrogen complexes bearing pincer ligand, Nat. Commun. 7 (2016) 12181. [40] S. Schenk, B.L. Guennic, B. Kirchner, M. Reiher, First-principles investigation of the Schrock mechanism of dinitrogen reduction employing the full HIPTN3N ligand, Inorg. Chem. 47 (2008) 3634-3650. [41] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A .Montgomery, J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J Austin, R. Cammi, C. Pomelli, , J.W. Ochterski, R.L. Martin, K. Morokuma, V.G.

Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09, revision A.1; Gaussian. Inc., Wallingford. CT (2009). [42] P.J. Stephens, F.J. Devlin, C.F. Chabalowski, M.J. Frisch, Ab Initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields, J. Phys. Chem. 98 (1994) 11623-11627. [43] A.D. Becke, Density-functional thermochemistry. III. The role of exact exchange, J. Chem. Phys. 98 (1993) 5648-5652. [44] C. Lee, W. Yang, R.G. Parr, Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density, Phys. Rev. B. 37 (1988) 785-789. [45] S. Grimme, Accurate description of van der Waals complexes by density functional theory including empirical corrections, J. Comput. Chem. 25 (2004) 1463-1473. [46] S. Grimme, Semiempirical GGA-type density functional constructed with a long-range dispersion correction, J. Comput. Chem. 27 (2006) 1787-1799. [47] E.D. Glendening, J.K. Badenhoop, A.E. Reed, J.E. Carpenter, J.A. Bohmann, C.M. Morales, C.R. Landis, Weinhold, F., NBO 6.0, Theoretical Chemistry Institute, University of Wisconsin, Madison (2013). [48] C.V.S. Kumar, V. Subramanian, Can boron antisites of BNNTs be an efficient metal-free catalyst for nitrogen fixation? – A DFT investigation, Phys. Chem. Chem. Phys. 19 (2017) 15377-15387.

Table 1 Adiabatic and vertical ionization energies (AIE, VIE) of the BLin cluster system compared with previously reported IE values (Ref. 30 and 34). cluster

Literature value

Current value

(in eV)

(in eV)

AIE

VIE

AIE

VIE

BLi5

4.3330

4.27

4.33

BLi6

3.8530

3.84

3.83

BLi7

4.2730

4.5730 4.5834 3.8430 3.7534 4.2830 4.3234

4.23

4.28

Fig. 1. Most plausible structure for interaction of N2 with a) BLi5, b) BLi6 and c) BLi7 with the key bond length in (Å).

Fig. 2. Schematic depiction of the pathways (distal and enzymatic) for conversion of N 2 to NH3 catalyzed by BLi6 (X in the scheme).

Fig. 3. Optimized structures of various intermediates formed along reaction path of nitrogen fixation over BLi6 through distal mechanism (a-f) and enzymatic mechanism (a, g-j and f).

Fig. 4. Energy profile for the nitrogen fixation over BLi6 through distal and enzymatic mechanism, (X=BLi6).

Fig.5 N N bond length variation in each step along both the reaction pathways.

Fig. 6. Charge variation during the reduction reaction along enzymatic pathway. Moieties 1 and 2 are BLi6 and NxHy respectively.

Fig. 7. Most plausible adsorption configuration of BLi6 over I) graphene and II) BN-sheet and III) and IV) figure shows the binding of N2 with adsorbed BLi6 over graphene and BN-Sheet respectively.