Jahn–Teller Distorted Effects To Promote Nitrogen Reduction over

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Jahn−Teller Distorted Effects To Promote Nitrogen Reduction over Keggin-Type Phosphotungstic Acid Catalysts: Insight from Density Functional Theory Calculations Yu Wang,†,‡ Xue-Mei Chen,‡ Li-Long Zhang,‡ and Chun-Guang Liu*,†,‡ †

State Key Laboratory for Chemistry and Molecular Engineering of Medicinal Resources, Ministry of Science and Technology of China, School of Chemistry and Pharmaceutical Sciences, Guangxi Normal University, 15 Yu Cai Road, Guilin 541004, P. R. China ‡ College of Chemical Engineering, Northeast Electric Power University, Jilin City 132012, P. R. China

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ABSTRACT: Molecular geometry, electronic structure, and possible reaction mechanism of a series of mono-transition-metal-substituted Keggin-type polyoxometalate (POM)−dinitrogen complexes [PW11O39M(N2)]n− (M = Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Tc, Ru, Rh, Pd, Ag, Cd, W, Re, Os, Ir, Pt, Au, and Hg) have been investigated by using density functional theory (DFT) calculations with M06L functional. The calculated adsorption energy of N2 molecule, N−N bond length, N−N stretching frequency, and the NBO charge on the coordinated N2 moiety indicate that MoII-, TcII-, WII-, ReII-, and OsII-POM complexes are significant for binding and activation of the inert N2 molecule. The degree of the N2 activation can be classified into the “moderately activated” category according to Tuczek’s sense [J. Comput. Chem. 2006, 27, 1278]. Electronic structure and NBO analysis indicate that the terminal N atom of the coordinated N2 molecule in these POM−dinitrogen complexes possesses more negative charge relative to the bridge N atom because Jahn−Teller distorted effects lead to an effective orbital mixture between σ2s* orbital of N2 and dz2 orbital of transition metal center. And the mono-lacunary Keggin-type POM ligand with five oxygen donor atoms serves as a strong electron donor to the bivalent metal center. Meanwhile, a catalytic cycle for direct conversion of N2 into NH3 has been systematically investigated based on a Re-POM complex along distal, alternating, and enzymatic pathways. The calculated free energy profile of the three catalytic cycles indicates that the distal mechanism is the favorable pathway in the presence of proton and electron donors. In the past five decades, transition-metal−dinitrogen complexes have been extensively investigated because of their potential ability to activate the coordinated N2 molecule. A large number of transition-metal−dinitrogen complexes have been successfully synthesized and structurally characterized.17−33 Molecular geometry, electronic structure, physicochemical property, and reaction mechanism of these transitionmetal−dinitrogen complexes have been reported elsewhere. Several Mo−N2 complexes containing multidenate phosphine ligands have been demonstrated to be successful for direct conversion of N2 into NH3 in the presence of appropriate proton and electron sources.34−44 For example, Chatt et al.45 first found that a cyclic production of NH3 on the basis of a Mo−phosphine complex, and the catalytic cycle corresponding to stepwise protonation and reduction of coordinated N2 through N2H, N2H2, N2H3, etc. intermediates, which has

1. INTRODUCTION Nitrogen fixation, which is the reduction of the dinitrogen (N2) to ammonia (NH3) is an important and crucial reaction in both biology and chemistry.1−7 For more than a century, much effort has been devoted to development of the metalmediated nitrogen fixation schemes. The Haber−Bosch process, which converts dinitrogen and dihydrogen into NH3 on the surface of an iron catalyst under harsh reaction conditions (∼500 °C and >100 atm), is a milestone in the history of artificial nitrogen fixation.8−11 However, the very high energy consumption for the H2 production from fossil fuels becomes serious bottlenecks constraining for the Haber− Bosch process. The development of nitrogen fixation scheme without the use of dihydrogen under ambient reaction conditions is highly desired.12−16 Overcoming the high free energy barrier corresponding to breaking the exceptionally strong NN triple bond about the conversion N2 into NH3 under ambient conditions is thus a quite important problem. © XXXX American Chemical Society

Received: February 23, 2019

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DOI: 10.1021/acs.inorgchem.9b00537 Inorg. Chem. XXXX, XXX, XXX−XXX

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Ir) have been systematically studied theoretically based on density functional theory (DFT) calculations.57 Proust and co-workers reported several POM−nitrido complexes ([PW11O39MN]n−, M = Cr, Mo, Ru, Re, Os, Tc),58−61 and the reactivity of these POM−nitrido complexes has been probed and understood. Experimental studies61 showed that a ruthenium−nitrido derivative, [PW11O39RuVI N]4−, can react with triphenylphosphane to produce the bis(triphenylphosphane) iminium cation [PPh3NPPh3]+ (Ph = phenyl), indicating the potential activity of the ruthenium−nitrido POM derivative in nitrogen transfer reaction. Meanwhile, theoretical studies have been carried out to rationalize the reactivity of [PW11O39MN]n− (M = Cr, Mn, Fe, Mo, Re, Tc, Ru, W, Os) in the nitrogen transfer reaction.62,63 These results indicated that these POM−nitrido complexes seemed to share the same mechanism as metalloporphyrin for the activation of the MN multibonds. All these results showed the potential capability of monotransition-metal-substituted Keggin-type POM complexes in the N2 cleavage and nitrogen transfer processes. However, a direct conversion of N2 into NH3 catalyzed by monotransition-metal-substituted Keggin-type POMs is still lacking. In the present paper, molecular geometry, electronic structure, and possible reaction mechanism of a series of the transition-metal-substituted Keggin-type POM−dinitrogen complexes have been systematically studied using DFT calculations. It is well-known that transition metal complexes can activate N2 to various degrees, which finally determines the activated N2 molecule accessibility to protonation, functionalization, or cleavage. In the present paper, adsorption energy of N2, N−N bond length, N−N stretching frequency, and atomic charge of the N2 moiety have been assigned as the criteria for determining the degree of N2 activation based on Tuczek’s rule. Because the findings on a Re-substituted Keggin-type POM complex suggested its potential as a catalyst for conversion of N2 into NH3, a detailed energy profile was probed using DFT calculations.

been viewed as a paradigm for protonation of coordinated N2 to NH3. In recent years, Schrock39 found that a Mo− triamidoamine complex also can catalyze the direct transformation of N2 to NH3 in the presence of 2,6-lutidinium tetrakis[3,5-bis-(trifluoromethyl)phenylborate] ([LutH]BarF4) as a proton source and decamethylchromocene (CrCp*2) as a reducing reagent under ambient conditions, in which 8 equiv of NH3 was produced by using this catalyst. In another case, Nishibayashi and co-workers46 reported that a dinitrogenbridged dimolybdenum complex bearing a PNP-type pincer ligand can transfer N2 to NH3 in the presence of [LutH]BarF4 and CrCp*2 under ambient conditions, where 23 equiv of NH3 was produced based on this catalyst. Apart from the Mo-based catalysts, Peters and co-workers reported that an Fe− tris(phosphine)borane complex can catalyze direct transformation of N2 into NH3 at −78 °C with production of 7 equiv of NH3.47 Distal and alternating pathways of nitrogenase and their mixed pathway are generally accepted for these classic transition metal−dinitrogen complexes. These findings indicate that the nature of the metal center in dinitrogen complexes is the crucial factor in determination of the reactivity for N2 reduction. Meanwhile, steric and electronic structures of the ligand also affect the coordination, charge, and spin state of the transition metal centers and thus the catalytic activity. Metal−dinitrogen complexes with the right combination of both factors would display good catalytic performance for reduction of N2 to NH3. Polyoxometalates (POMs) are early transition metal−oxo discrete cluster anions. The novel optical, electric, redox, medical, and magnetic properties of POMs have prompted studies of versatile applications in medicine, magnetism, electrochemistry, catalysis, etc.48−54 The majority of the applications of POMs are found in the area of catalysis because their acid-based and redox properties can be tuned in a wide range by changing the chemical composition. POMs can stabilize most transition metal atoms via oxygen donor atoms to form totally inorganic POM complexes. Moreover, POMs possess excellent redox stability because they can accept one or several electrons with minimal structural changes.55 The unique withdrawing properties of POM ligands would possibly modify the reactivity of the transition metal centers. Thus, the totally inorganic POM complexes seem to have particular advantages for conversion of N2 into NH3 with multistep redox processes when compared with transition metal or organometallic complexes. The potential capability of POMs in the field of artificial nitrogen fixation has been demonstrated by both experimental and theoretical studies based on the mono-transition-metalsubstituted Keggin-type POM complexes. Sokolov and coworkers56 reported that a ruthenium-containing POM, [PW11O39RuIINO]4−, can be reduce to a ruthenium− dinitrogen POM complex, [PW11O39RuII(N2)]5−, in the present of hydroxylamine. According to 31P NMR and electrospray ionization mass spectrometry, another ruthenium−dinitrogen POM complex, [PW11O39RuIII(N2)]4−, which is the oxidized product of [PW11O39RuII(N2)]5−, also has been identified in their reaction system. These results provided unambiguous evidence for the presence of the POMbased dinitrogen complex. Molecular geometries, electronic structure, metal−dinitrogen bonding nature, and possible reaction mechanisms of metal−dinitrogen derivatives of Keggin-type POMs [PW11O39MIIN2)]5− (M = Ru, Os, Re,

2. COMPUTATIONAL DETAILS All the calculations were performed by using the Gaussian 09 program.64 Geometric optimization and vibrational frequencies were calculated at the M06L level of theory. The LANL2DZ65−67 relativistic effective core potential (RECP) for all metal atoms and the standard 6-31G(d) basis set for all main group atoms were used. The local meta-GGA exchange correlation functional M06L68 is derived from the M05 correlation functional,69 which is developed from the work of Perdew and Wang, Becke, Perdew, and Stoll et al. M06L is constructed via (i) constraint satisfaction, (ii) modeling the exchange correlation hole, and (iii) empirical fits to incorporate spin kinetic energy density in a balanced way for the exchange and correlation energy. This functional has been proven to have many advantages for calculations of thermochemistry, thermochemical kinetics, noncovalent interactions, and vibrational frequencies for main group and transition metal chemistry. Moreover, previous theoretical investigations confirmed the accuracy of M06L results on the geometric parameters, electronic structure, thermochemistry, thermochemical kinetics, and vibrational frequencies of POMs.63,70 In the present paper, the M06L functional was adopted to evaluate the degree of the N2 activation on the transition-metal-substituted Keggin-type POM complex and to predict the energy profile of N2 reduction over these POM B

DOI: 10.1021/acs.inorgchem.9b00537 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry complexes. Meanwhile, at the same level, nature bond orbital (NBO)71 calculations were carried out to obtain atomic charges and Wiberg bond indices (WBIs), which assist our understanding of the electronic structure. Solvent effects of tetrahydrofuran media have been taken into account via the self-consistent reaction field (SCRF) method, using the integral equation formalism polarizable continuum model (IEFPCM) solvent model.72 The free energy corrected by the translational entropy was calculated using the Whitesides method,73 following the procedure adapted by Sakaki et al.74−86 In this approach, the electronic energy (Esol) with zero-point energy correction in solution was evaluated as defined in the following equation: pot ν0 Esol = Esol + Egas

(1)

where Epot sol is the potential energy including nonelectrostatic energy in solution and Eνgas0 is the zero-point vibrational energy in gas phase. Considering that entropy change must be involved in a bimolecular process, the Gibbs energy (Gsol ° ) has been evaluated as follows:

Figure 1. Calculated adsorption energy of the N2 molecule over the all transition metal−dinitrogen POM complexes studied here (units in kcal mol−1).

N2 adsorption on all the systems listed in Figure 1, the configuration adopts an end-on structure. As expected, our DFT-M06L calculations show that the frequently employed transition metals in NRR catalysis, Mo, Ru, W, Re, and Os, are significantly superior for adsorption of the inert N2 molecule when compared with others. Meanwhile, the unused Tc also displays remarkable adsorption energy. The calculated Ead values for these significant metals lay within the range 10−40 kcal mol−1, implying that adsorption of N2 needs to be considered for these most frequently employed heavier transition metals in POM−dinitrogen complexes. According to the distance of the N−N bond and the N−N stretching vibration (νN−N), Tuczek et al. have divided dinitrogen complexes into weakly, moderately, and strongly activated systems.89 The optimized key geometric parameters and N−N stretching frequency of the dinitrogen−POM complexes with remarkable adsorption energy have been compared in Figure 2. The optimized N−N bond distance decreases in the order of WII-POM (1.178 Å) > ReII-POM (1.168 Å) > MoII-POM (1.163 Å) > TcII-POM (1.158 Å) > OsII-POM (1.157 Å) > RuII-POM (1.145 Å), and the calculated νNN value increases in the order of WII-POM (1887 cm−1) < MoII-POM (1952 cm−1) < ReII-POM (1980 cm−1) < TcII-POM (2022 cm−1) < OsII-POM (2060 cm−1) < RuII-POM (2116 cm−1). Obviously, the Ru-POM complex exhibits poor N2-activating ability because of the relevant short N−N distance and large νN−N value. By contrast, the coordinated N2 molecule in the Mo-, Tc-, W-, Re-, and OsPOM−dinitrogen complexes should be categorized to moderate activation based on Tuczek’s criterion.89 The RuPOM complex should be categorized as weak activation for the coordinated N2 molecule. The relationship between N−N bond distance and N−N stretching vibration has been shown in Figure 2a. It can be found that longer N−N distance leads to a smaller N−N stretching vibration, as expected, and a linear correspondence can be approximated. It is well-known that the surface oxygen atoms of Keggintype POM structure can be divided into three sets according to whether they are in terminal position (Ot), bridging two metal atoms (Ob), or at the corners of the Keggin structure (Oc). For our studied transition metal−dinitrogen POM complexes, they all consist of the same lacunary Keggin-type POM ligand, [PW11O39]7−, which has a defect with five oxygen donor atoms (two Oc atoms, two Ob atoms, one Ot atom). Combination of

° = H0 − T (Sr° + Sv° + St°) Gsol = ET + P ΔV − T (Sr° + Sv° + St°) = Esol + Etherm − T (Sr° + Sv° + St°)

(2)

where ΔV is 0 in solution, Etherm is the thermal correction by translational, vibrational, and rotational movements, Sr°, Sv°, and St° are rotational, vibrational, and translational entropies, respectively. Whitesides’s method was employed to calculate the corrected translational entropy in acetonitrile solvent. The corrected translational entropy (Strans) was calculated by eq 3: ÄÅ ÉÑ ÅÅÅij 10−15/2V yzji 2πMRTe 5/3 zy3/2 ÑÑÑ free zj zz ÑÑÑ zj Strans = R lnÅÅÅÅjjjj 4 zz ÑÑ 2 ÅÅ NA [analyte] zzjj h { ÑÑÑÖ {k (3) ÅÅÇk where R is the ideal gas constant, M is the molecular mass, T is the temperature, h is the Planck constant, NA is Avogadro’s number, and [analyte] represents experimental concentration of analyte. The adsorption energy (Ead) of N2 was calculated by Ead = Ecomplex − (EM ‐ POM + E N2)

(4)

where Ecomplex, EPOM, and EN2 are the free energies of the metal−dinitrogen POM complex, POM fragment, and N2 molecule in tetrahydrofuran media, respectively.

3. RESULTS AND DISCUSSION 3.1. The N2 Adsorption and Electronic Structure. The ability to efficiently capture the inert N2 molecule around the active site is a very important criterion for catalytic performance. Understanding N2 adsorption in the series of transition metal−dinitrogen POM complexes is thus the primary step to elucidate the protonation mechanism of the coordinated N2 molecule. Thus, we first discuss the adsorption energy of the N2 molecule. The calculated adsorption energy of the series of transition-metal-substituted Keggin-type POM dinitrogen complexes is summarized in Figure 1. The catalytically important metals Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Tc, Ru, Rh, Pd, Ag, Cd, W, Re, Os, Ir, Pt, Au, and Hg with low valent state were introduced into our studied system to analyze the adsorption behavior of the inert N2 molecule. For C

DOI: 10.1021/acs.inorgchem.9b00537 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Relationship of the N−N distance (dN−N) and (a) N−N stretching vibration (νN−N) or (b) NBO charge of N2 moiety.

Figure 3. Quasi-octahedron coordination sphere of the transition metal center in the series of transition metal−dinitrogen POM complexes.

the coordinated N2 molecule and [PW11O39]7− ligand forms a quasi-octahedral sphere to stabilize transition metal atoms (see Figure 3). Compared to the standard octahedral structure, the significant distortion of our studied systems comes from a large distance between the transition metal center and the Ot atom of the tetrahedral phosphate group PO43−. This structural effect allows a Jahn−Teller distortion in the electronic structure, in which the doubly degenerate dx2−y2 and dz2 orbitals with eg symmetry in standard octahedral sphere split into two different energy levels (see Figure 4), and the triplet degenerate orbitals with t2g symmetry in standard octahedral environment were divided into two groups, rising energy level of dxy orbital and low-lying energy levels of dxz and dyz orbitals. Thus, Jahn−Teller distortion in our studied transition metal− dinitrogen POM complexes reduces the energy levels of dz2, dxz, and dyz orbitals, which would decrease the energy gaps between symmetry-allowed d orbitals (dz2, dxz, and dyz) of these transition metals and frontier molecular orbitals of N2 and significantly promotes atomic orbital interactions between N2 and the transition metal center. The molecular orbital diagram obtained from the optimized structures of Re-POM system using M06L functional is listed in Figure 4. It can be found that the eight molecular orbitals (orbital nos. 135, 174, 256, and 257 with α and β spins) are metal-based d orbitals and responsible for the bonding interaction between Re atom and N2. For orbitals 256 and 257 with both spins, which are formed by an effective overlap between occupied dxz and dyz orbitals of the Re atom and unoccupied π* antibonding unoccupied orbitals of N2, can be viewed as the π-back bonding orbitals. Orbital 174 with both spins are formed by the overlap between the occupied σ2pz bonding orbital of N2 and the unoccupied dz2 orbital of the Re atom and thus can be viewed as the σ donation orbitals.

Interestingly, the special topology of orbital 135 with both spins, which are made from an overlap of the occupied σ*2s antibonding orbital of N2 with the unoccupied dz2 orbital of the Re atom, represents a new σ donation orbital model for binding N2 in our studied system. This formation of a new metal−N2 bonding orbital is mainly due to the Jahn−Teller distorted effects. As mentioned above, the quasi-octahedral coordinated sphere around the transition metal center shifts the energy level of dz2 orbital downward and reduces the energy gap between the σ*2s antibonding orbital of N2 and the dz2 orbital of the Re center and thus creates an effective overlap of them. In general, the rare ability of transition metal complexes to bind N2 mainly arises from an efficient combination of the unoccupied (dz2) and occupied (dxz and dyz) d orbitals to bind N2. As shown in Figure 5, the efficient overlap between π* antibonding unoccupied orbitals of N2 and symmetry-allowed dxz and dyz occupied orbitals of the transition metal center forms π-back bonding orbitals to donate electron density to N2. Meanwhile, mixture of the occupied σ2pz bonding orbital of N2 and the unoccupied dz2 orbital of the transition metal center results in withdrawing of electron density from N2 to the transition metal center. Besides the above, an effective overlap between the occupied σ*2s antibonding orbital of N2 and the unoccupied dz2 orbital of the transition metal center was first observed in our studied system because of the Jahn−Teller distorted effects. These bonding features indicate that the atomic charge of the N2 molecule coordinated to the transition metal center is determined by a balance between the two contributions: σ donation and π back-donation. As we will see later, the role of the new σ(2s−dz2) donation orbital in determination of the atomic charge of the coordinated N2 moiety is special and important for our studied system. D

DOI: 10.1021/acs.inorgchem.9b00537 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Schematic representation of the Re−N2 σ donation and π back-donation bonding interaction.

and thus the high reactivity of the Nα atom with proton in the primary step for reduction of N2 to NH3. As shown in Table 1, the calculated NBO atomic charge on Nα atom increases in the order of W-POM < Re-POM < Tc-POM < Os-POM < MoPOM. W- and Re-POM systems possess more negative charge relative to others studied here. Why is the charge different on Nα and Nβ atoms? The calculated atomic orbital population of Nα and Nβ atoms has been compared in Table 1. It can be found that the three 2p orbitals (2px, 2py, and 2pz) of Nα and Nβ atoms have approximately equal occupation number for each system. By contrast, the occupation number of the 2s orbital of Nα and Nβ atoms is not equal. The calculated occupation number of the Nα atom is larger than that of the Nβ atom; this result suggests that the different charge on Nα and Nβ atom is mainly due to the different occupation number of the 2s orbital. In line with the charge transfer of the mixing orbitals in Figure 5, the σ(2s− dz2) orbital (no. 135) is responsible for this difference. This orbital mixing will result in some charge transfer from occupied 2s orbitals of Nα and Nβ atoms into the empty dz2 orbitals of the Re center. Moreover, the large contribution from the Nβ

Tuczek et al. proposed that the atomic charge on the N2 moiety is also a key factor in determination of the reactivity of N2 with proton.89 In the present paper, the optimized N−N bond length is plotted against the calculated NBO atomic charges of N2 moiety (see Figure 2b). A linear trend suggests that the N−N bond distance has a considerable effect on the degree of charge transfer from transition metal center to N2, with longer N−N distance leading to more negative charge on the N2 moiety. Thus, the NBO atomic charge on the N2 moiety is also an effective predictor of N2 activation for our studied system. In addition, the calculated NBO atomic charges of the two N atoms of the N2 moiety are not equivalent. For example, in the ReII-POM complex, the terminal nitrogen atom (Nα) has 0.323 negative charge, the bridge nitrogen atom (Nβ) has 0.035 negative charge, and the gross charge of the coordinated N2 molecule is −0.358 e. As shown in Table 1, the calculated results for Mo-, TcII-, WII-, and Os-POM systems all show that the atomic charges on Nα are significant relative to Nβ atom and both N atoms have a negative charge. Importantly, the more negative charge of Nα relative to Nβ atom indicates an enhancement of the basicity E

DOI: 10.1021/acs.inorgchem.9b00537 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Scheme orbital diagram of σ donation and π back-donation interactions.

nL = (nM − n formal) + (nπ − nσ )

Table 1. NBO Charge and Natural Hybrid Orbital Composition and Occupancy of Nα and Nβ Atoms in MPOMa

As shown in Figure 5, the number of electrons on the coordinated N2 molecule is determined by a balance between σ donation and π back-donation, in which the σ donation effect leads to electron transfer from N2 to M and the π backdonation effect results in electron transfer from M to N2. The number of electrons transferred from both effects has been defined as nσ and nπ in eq 5, and thus the excess electrons on the N2 moiety can be roughly estimated by using the value of nπ − nσ. As shown in eq 5, nM is defined as the number of valence d electrons of the M, and nformal is the number of valence electrons of the M with the formal charge. The excess electrons of M are thus expressed by the value of nM − nformal. All these values have been approximately estimated by using natural population analysis (NPA) population and compared in Table 2.

valence shell atomic orbitals complex

atom

charge

2s

2px

2py

2pz

W-POM

Nα Nβ Nα Nβ Nα Nβ Nα Nβ Nα Nβ

−0.352 −0.154 −0.323 −0.035 −0.266 0.026 −0.251 0.020 −0.234 −0.076

1.619 1.356 1.616 1.327 1.613 1.352 1.614 1.319 1.616 1.390

1.263 1.232 1.203 1.190 1.172 1.156 1.177 1.181 1.159 1.159

1.161 1.221 1.207 1.194 1.171 1.155 1.170 1.171 1.148 1.148

1.279 1.306 1.268 1.280 1.283 1.267 1.261 1.261 1.284 1.291

Re-POM Tc-POM Os-POM Mo-POM

(5)

a

M = W, Re, Tc, Os, and Mo.

Table 2. NPA on the Charge Transfer among Metal Center M, N2, and the Mono-lacunary Keggin-type POM Ligand L in M-POM Dinitrogen Complexesa

atom in this orbital would lead to a more significant electron transfer from Nβ atom to the Re atom relative to that from the Nα atom and thus more negative charge on the Nα atom relative to the Nβ atom. In addition, this σ(2s)-donor bonding orbital mixing shifts electrons from the σ*(2s) antibonding orbital of the N2 moiety to the transition metal Re center, and the N2 would be less activated. Thus, the σ(2s)-donor bonding interaction leads to difference charge on Nα and Nβ atoms via deactivation of the N2 molecule. The question with which we are now concerned is the role of POM ligand for activation of the N2 molecule. In general, the activation of the N2 molecule by transition metal complexes mainly arises from a strong electron transfer from the metal center to the N2 molecule. In order to show the unique role of the POM ligand, Yoshizawa’s equation87 has been employed to probe electron transfer among metal center M, N2, and the mono-lacunary Keggin-type POM ligand L in this work. The number of electrons transferred (nL) from L to M can be calculated by the following equation:

nσ(N2→M) nπ(M→N2) (nπ − nσ) nM nformal nL a

W-POM

Re-POM

Tc-POM

Os-POM

Mo-POM

0.378 1.557 1.179 4.053 4 1.232

0.447 1.474 1.027 5.435 5 1.462

0.432 1.334 0.911 5.713 5 1.624

0.483 1.379 0.896 6.470 6 1.366

0.357 1.294 0.937 4.532 4 1.469

M = W, Re, Tc, Os, and Mo.

It can be found that the nπ(M → N2) value is larger than the nσ(N2 → M) value, indicating that the π back-donation effect is the key factor for activation of the N2 molecule in the five MPOM complexes when compared with the σ donation effect, which is well in agreement with the gross charge on the coordinated N2 moiety being negative (see Table 1). We employ the Re-POM system as an example to analyze the F

DOI: 10.1021/acs.inorgchem.9b00537 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. Free energy profile for the distal, alternating, and enzymatic mechanisms in the Re-POM complex calculated at the M06L/6-31G(d) level (LANL2DZ basis sets for metal atoms; units in kcal mol−1).

Figure 7. Optimized structure of the key intermediates in distal pathway at the M06L/6-31G(d) level (LANL2DZ basis sets for metal atoms) (units in Å).

behavior of electron transfer among M, N2, and L. Due to the π back-donation effect, the Re center donates 1.474 e to N2, and the Re center accepts 0.447 e as a result of σ donation from N2. Thus the excess electrons on the N2 moiety are calculated to be 1.027 e. By contrast, the calculated nL value indicates that the Re center receives 1.462 e from the POM ligand, which is larger than the excess electrons on the N2 moiety. All these results indicate that the lacunary Keggin-type POM ligand serves as a strong electron donor to the transition metal center. As shown in Figure 3, the transition metal center was stabilized

by five oxygen donor atoms (O2− anion) of the mono-lacunary Keggin-type POM ligand. Thus, the electron donor ability of the mono-lacunary Keggin-type POM ligand originated from the electron-rich properties of the five oxygen donor atoms. Because of the large M−Ot distance (Ot atom of tetrahedral phosphate group, PO43−, see Figure 4), the electron-donating ability of the Ot atom is weak when compared with those of the Ob and Oc atoms. In conclusion, adsorption of the inert N2 molecule on the transition metal center to form dinitrogen−POM complexes G

DOI: 10.1021/acs.inorgchem.9b00537 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry gives rise to three main effects: (i) MoII-, Tc-, W-, ReII-, and OsII-POM complexes are significantly active for binding and activation of the inert N2 molecule; (ii) the terminal N atom of the coordinated N2 molecule has more negative charge relative to the bridging N atom because Jahn−Teller distortion effects lead to an effective orbital mixing between the σ*2s orbital of N2 and the dz2 orbital of the transition metal center; (iii) the mono-lacunary Keggin-type POM ligand with five oxygen donor atoms serves as a strong electron donor to the bivalent metal center. Therefore, the coordinated N2 molecule has been activated, and the terminal N atom of the coordinated N2 molecule should be the potential reactive atom in the initial step for reduction of N2 to NH3. 3.2. Free Energy Profile for the Catalytic Cycle. On the basis of the large adsorption energy for the N2 molecule, long N−N bond distance, and more negative charge on the N2 moiety, the potential catalytic activity of the Re-POM− dinitrogen complex for the conversion of the N2 to NH3 has been evaluated in this work. Although Mo- and W-POM systems possess the most negative charge on the N2 moiety, they have not been further considered in this work because of the small adsorption energy for the N2 molecule. Three possible reaction pathways, including distal, alternating, and enzymatic mechanisms, have been considered, in which LutH+ and Cp*2Co were chosen as a combination of proton/electron donors to implement six consecutive protonation and reduction processes. For the distal pathway, the calculated free energy profile including solvent effects is shown in Figure 6. At the first stage of N2 reduction, the starting Re-POM complex adsorbs N2 to form dinitrogen−Re-POM complex, as mentioned above; the adsorption of N2 to the Re-POM complex was calculated to be exoergic by 31.28 kcal mol−1. For the first protonation of the coordinated N2 molecule, the calculated reaction free energy is slightly endergonic (0.9 kcal mol−1). This result indicates that the first protonation step is nearly thermoneutral, which is very similar to the result on Schrock’s Mo complex reported by Reiher et al.,88 a slightly negative reaction energy for the first protonation step (−0.7 kcal mol−1). The following reducing step is found to be exoergic by 17.64 kcal mol−1, indicating that this step is thermodynamically favorable. According to our optimized calculations, the N−N bond was elongated to 1.268 Å and the Re−N bond was shortened to 1.737 Å in this step (see Figure 7). For the second protonation of the dinitrogen− POM complex, the calculated free reaction energy indicates that this step is endergonic by 4.99 kcal mol−1. The following reducing step is found to be exoergic by 17.33 kcal mol−1. The N−N bond was further elongated to 1.411 Å. The third protonation step is predicted to be most exoergic among all reaction steps. As with the protonation and subsequent reduction step, the cleavage of the N−N bond leads to generation of the first NH3 molecule and a nitrido complex with a Re−N bond length of 1.662 Å. This step is found to be strongly exoergic by 73.68 kcal mol−1, which is well in agreement with the theoretical value on Schrock’s Mo complex reported by Studt et al. (72.8 kcal mol−1)89 and qualitatively agrees with the value reported by Neese et al. (68.4 kcal mol−1).35 This value is also comparable to the DFT-derived result on the nitrogenase cofactor, FeMoco, in the step of releasing the NH3 molecule via cleavage of the N−N bond (76.46 kcal mol−1).90 It should be stressed that the nitrido− POM complex has been successfully synthesized and characterized,61 indicating that it is a thermodynamically stable

species. DFT calculations on Schrock’s Mo complex also showed that the reduction of the cationic Mo−NNH3 complex immediately cleaves the N−N bond to form the first NH3 molecule and a stable nitrido intermediate.35 In part two of this catalytic cycle, the stable nitrido POM complex was first protonated. This step is calculated to be endergonic by 7.04 kcal mol−1. The subsequent reduction step is found to be exoergic by 11.28 kcal mol−1 and forms an imido complex with a Re−N bond length of 1.807 Å. The next two steps, in which the imido complex is protonated and reduced to the amido complex with a Re−N bond length of 1.911 Å, are both exoergic (21.91 and 14.08 kcal mol−1). Protonation of the amido complex results in a Re−NH3+ intermediate, which is calculated to be approximately thermoneutral with a small reaction free energy of 2.92 kcal mol−1. The Re−NH3+ intermediate accepts one electron to form the second NH3 molecule, which is exoergic by 18.11 kcal mol−1. Finally, the ammine complex requires 25.15 kcal mol−1 to release the second NH3 molecule. Because of the large adsorption energy of N2 to the Re-POM complex (−31.28 kcal mol−1), the substitution of NH3 by N2 is exoergic by 6.13 kcal mol−1. For all protonation and reduction steps in this distal pathway, protonation of the nitrido complex is the limiting step (7.04 kcal mol −1 ). This is mainly due to a good thermodynamic stability of the nitride−Re-POM complex. By the same log, we found that the limiting step for conversion of N2 to NH3 in the alternating mechanism is the protonation of the Nβ atom of the coordinated N2 molecule (17.52 kcal mol−1) and that in the enzymatic mechanism is the first protonation of the coordinated N2 molecule (9.10 kcal mol−1 ). Although the limiting step in the enzymatic mechanism is comparable to that of the distal mechanism, the very weak adsorption of the N2 molecule to the Re-POM complex in the enzymatic configuration leading to this pathway is not considered to be favorable.

4. CONCLUSIONS In this paper, DFT calculations have been implemented to study various transition metal substituted Keggin-type POM− dinitrogen complexes, [PW11O39M(N2)]n− (M = Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Tc, Ru, Rh, Pd, Ag, Cd, W, Re, Os, Ir, Pt, Au, and Hg). The degree of N2 activation in the series of POM−dinitrogen complexes has been evaluated based on the adsorption energy of N2, N−N bond length, N−N vibrational frequency, and NBO partial charge on the coordinated N2 moiety. The results indicate that MoII-, TcII-, WII-, ReII-, and OsII-POM complexes are significantly active for binding and activation of the inert N2 molecule among the POM− dinitrogen complexes. Electronic structure and NBO analysis show that the terminal N atom of the coordinated N2 molecule possesses more negative charge relative to the bridging N atom because Jahn−Teller distortion effects lead to an effective orbital mixing between the σ*2s orbital of N2 and the dz2 orbital of the transition metal center. The mono-lacunary Keggin-type POM ligand with five oxygen donor atoms serves as a strong electron donor to the bivalent metal center for activation of the N2 molecule. Because of the large adsorption energy for the N2 molecule, long N−N bond distance, and more negative charge on the N2 moiety relative to others, the N2 reduction process mediated by the Re-POM complex based on distal, alternating, and enzymatic mechanisms has been systematically investigated. The calculated energy profile of the three catalytic cycles demonstrates that the distal mechanism is the favorable H

DOI: 10.1021/acs.inorgchem.9b00537 Inorg. Chem. XXXX, XXX, XXX−XXX

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pathway, where the Re-POM complex severs as a catalyst for N2 reduction in the presence of appropriate proton and electron donors (LutH+ and Cp*2Co). These findings would be very useful to probe the potential of totally inorganic POMs as catalysts to reduce N2 to NH3.

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ASSOCIATED CONTENT

S Supporting Information *

This materials is available free of charge via the Internet at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00537.

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XYZ coordinates for most relevant structures reported in this paper (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel: 86 0432 64606919. Fax: 86 0432 64606919. E-mail addresses: [email protected] or [email protected]. ORCID

Chun-Guang Liu: 0000-0002-1220-5236 Funding

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (21373043). Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.9b00537 Inorg. Chem. XXXX, XXX, XXX−XXX