The df–d Dative Bonding in a Uranium–Cobalt Heterobimetallic

May 22, 2019 - Single transition-metal site catalysts with s-, p-, or d-block atom anchor for nitrogen fixation have been extensively studied, and yet...
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Cite This: Inorg. Chem. 2019, 58, 7433−7439

The df−d Dative Bonding in a Uranium−Cobalt Heterobimetallic Complex for Efficient Nitrogen Fixation Jun-Bo Lu,†,‡ Xue-Lu Ma,†,⊥,‡ Jia-Qi Wang,† Ya-Fei Jiang,† Yong Li,† Han-Shi Hu,† Hai Xiao,*,† and Jun Li*,†,§ †

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Department of Chemistry and Key Laboratory of Organic Optoelectronics & Molecular Engineering of the Ministry of Education, Tsinghua University, Beijing 100084, China ⊥ School of Chemical & Environmental Engineering, China University of Mining & Technology, Beijing 100083, China § Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, China S Supporting Information *

ABSTRACT: Single transition-metal site catalysts with s-, p-, or d-block atom anchor for nitrogen fixation have been extensively studied, and yet the studies of the f-block atom anchor are rarely reported. Thus, we investigate the feasibility of using a newly synthesized U−Co complex featuring a single CoI site coordinated by tetrakis(phophinoamide) and an UIV anchor for N2to-NH3 conversion by theoretical modeling. We characterize the evolution of oxidation states of U and Co along the reaction pathways from ab initio density matrix renormalization group (DMRG) calculations, and we find that the variation of the Co → U dative bond is correlated with the changes of oxidation states. Both uranium and cobalt can serve as electron reservoirs to facilitate breaking the N−N bond. Our study demonstrates the viability of metal → metal dative bonds, particularly the df−d one, for the reduction of N2 to NH3, and thus, this opens up a new avenue to the rational design of efficient catalyst for nitrogen fixation.

1. INTRODUCTION As the heaviest naturally abundant element, uranium plays an important role in both modern science and society. Besides applications in nuclear energy, its usefulness has been extended to design of novel catalysts, owing to its unique properties, comparatively large ionic radius, and the availability of forbitals for bonding interactions.1−13 A notable example is the early heterogeneous catalyst used in the Haber−Bosch process for nitrogen reduction, one of the most important industrial catalytic reactions, which utilized uranium.4,14 Over the past few decades, more and more uranium dinitrogen complexes were synthesized, and they all engage 5f orbitals of uranium in bonding with dinitrogen.15−19 In addition to acting as a binding site, uranium has been suggested to serve as a Z-type metalloligand for small molecule activation.20,21 The hemilabile bonding between a transition metal (TM) center and an ancillary p-block or d-block atom has been extensively studied for facilitating N2 to NH3 conversion.22−29 Recently, a novel df−d dative-bonding paradigm has been found to exist between later TMs and actinides.30,31 However, such bonding between a TM center and a f block atom (e.g., uranium) for nitrogen fixation remains underexplored and thus poorly understood, although complexes with U−Fe,32 U−Ru,33 U−Rh,34 U−Co,20,21 or U−Ni35 dative bonds have been extensively synthesized. In particular, the 5f orbitals of uranium were shown to participate in covalent bonding,34 and the dative df−d heterobimetallic © 2019 American Chemical Society

bonding deems to present a fascinating and promising strategy to facilitate dinitrogen activation. It is especially noteworthy that a heterobimetallic tetra(phophinoamide) U−Co complex ICo(Ph2PNiPr)3U[η2Ph2PNiPr] (abbreviated as [UCo]I) was synthesized and characterized,20 in which the monovalent CoI single site bonds to three phosphine donor arms tethering a tetravalent UIV anchor. The complex features a short U−Co bond, and the df−d intermetallic interaction provides an ideal model to examine its potential for nitrogen fixation. Different from the high-temperature and high-pressure industrial Haber−Bosch process that involves a direct dissociative pathway for N2 activation, alternative associative pathways allow N2 activation at mild conditions.36,37 In the present work, we investigate the geometries and electronic structures of reaction intermediates along the associative distal and hybrid pathways for tetrakis(phophinoamide) U−Co complex catalyzed N2 reduction using density functional theory (DFT) and ab initio density matrix renormalization group (DMRG) methods. We find that uranium participates in the initial electron steps, and the flexible U−Co bond serves as an electron reservoir to stabilize different oxidation states of uranium and cobalt through the catalytic cycle. By comparing the [UCo] complex with two cobalt-containing complexes Received: February 28, 2019 Published: May 22, 2019 7433

DOI: 10.1021/acs.inorgchem.9b00598 Inorg. Chem. 2019, 58, 7433−7439

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Inorganic Chemistry i n c l u d i n g d − d i n t e r a c t i o n ( T i C o [N (o - ( N C H 2 P (iPr)2(C6H4)3],25 abbreviated as [TiCo]) and p−d interaction ((TPB)Co,28 abbreviated as [BCo]) dative bonds as shown in Scheme 1, we find that the [UCo] complex with a df−d dative bond is the most efficient catalyst for nitrogen fixation among them.

Mn4Ca and Fe−S metal clusters.55,56 Reiher and co-workers also used DMRG to calculate the Cu2O22+ and CUO systems.57,58 We utilize DMRG here to predict the spin state for each intermediate during the catalytic cycle. In order to understand the bonding between the local U− Co bond and adsorbates, we adopt the full valence orbitals of U, Co and adsorbate molecules as the active space. We first performed unrestricted BP86 calculations by PySCF, and then split localized α orbitals to do DMRG calculations. We choose Co 3d, 4s orbitals, U 5f, 6d orbitals, N 2s, 2p orbitals, and H 1s orbital as the active space. For the intermediates on the [CoB] complex, we choose Co 3d, 4s orbitals, B 2s, 2p orbitals, N 2s, 2p orbitals, and H 1s orbital as the active space. For the intermediates on [CoTi] complex, we choose Co 3d, 4s orbitals, Ti 3d, 4s orbitals, N 2s, 2p orbitals, and H 1s orbital as the active space. In order to balance the accuracy and computational cost, we test the dimension M in DMRG calculation for the [UCo]N2 complex as shown in Figure S5, and show that M = 2000 is a sufficiently reliable choice. Thus, for the following DMRG calculations, we set M = 2000 and 1 × 10−8 Hartree energy threshold. Electron populations are derived from the natural orbitals and the corresponding occupation numbers, ranging from 0 (unoccupied) to 2 (occupied), based on DMRG calculations. The determination of formal oxidation states of U and Co is based on the results of natural orbital occupation numbers (NOONs) generated by DMRG calculations.

Scheme 1. Geometry Structures of [XCo]N2 (X = U, B, Co) with Different Ancillary Elements

2. THEORETICAL METHODOLOGY Relativistic quantum chemical calculations were performed using density functional theory (DFT) and wave function theory (WFT) methods. To highlight the heterobimetallic df− d dative interaction of U−Co unit, the simplified model Co(Me2PNMe)3U[η2-Me2PNMe] was employed, which is abbreviated as [UCo] here. Geometry optimizations of all intermediates [UCo]NxHy were carried out at PBE level as implemented in ADF2016 code.38,39 We used uncontracted Slater basis sets of DZP quality for C, H, N, and P elements and TZP basis sets for U and Co.40 The scalar relativistic (SR) effects were taken into account by ZORA formalism.41−44 The PBE functional was used for geometry optimization because it can reproduce well the geometry structures from experimental characterization.20 Vibrational frequency analysis within the harmonic approximation was performed at the optimized geometries to validate the nature of the stationary points on the potential energy surface. The single point M06-2X and ab initio density matrix renormalization group (DMRG) calculations of optimized structures were performed by Gaussian 09, PySCF, and BLCOK code, using cc-pVDZ Gaussian basis sets for C, H, N, and P and basis sets and pseudopotentials ECP10MDF for Co and ECP60MWB for U.45−53 The hybrid meta-GGA functional M06-2X was used for the energy profile as it can match well the DMRG results for predicting the true actual spin states, as shown in Tables S1 and Table S3. The energy levels of intermediates are referenced to the [UCo]N2 complex by the redox (proton + electron) potentials. A key problem for elucidating the N2 fixation mechanism on the proposed catalyst is to determine the electronic structures of ground states for all intermediates along the catalytic pathways. Previous work revealed that several intermediates with d−p dative interactions during the catalytic cycle are characteristic multireference systems.54 However, it is a longstanding challenge for DFT methods to predict the true spin state for multireference systems. Ab initio density matrix renormalization group (DMRG) method is a powerful tool to handle the multireference problem to get reliable results with huge active space. Chan and Yanai have applied DMRG to predict the ground states and low energy excited states for

3. RESULTS AND DISCUSSION The associative distal and hybrid pathways for N2 reduction have been well characterized on the (TPB)Fe catalyst,23,27,54 based on which we investigate the feasible reaction pathways for N2 reduction via [UCo]N2 as shown in Figure S1. Figure S3 shows the corresponding reaction energy profiles predicted at the DFT M06-2X level, and the ground states of each intermediate are identified with the ab initio DMRG method (details in Table S1). The likely hydrogen evolution reaction (HER) pathway from [UCo]N2 to [UCo]N2− is less favored; we instead focus on the dinitrogen reduction reaction (NRR) without considering the HER as a side reaction. The initial conversion from precursor to [UCo]NNH2+ undergoes a series of two-electron and two-proton (2e−2H+) steps. Due to the remarkable endothermicity (87.3 kcal/mol) of protonating [UCo]N2 to produce [UCo]NNH+, successive two electron reduction steps are predicted to take place instead, followed by two proton steps that lead to [UCo]NNH2+. The same 2e−2H+ series occurs in the N2-to-NH3 conversion catalyzed by the (TPB)Fe complex and others.27,59 After a further electron reduction step to deliver [UCo]NNH2, the reaction pathway branches into the hybrid and distal ones. Figure S3 shows that on the [UCo] catalyst the hybrid pathway is preferred, while the distal pathway is excluded because of a high barrier. The associative hybrid pathway was also suggested for the (TPB)Fe system.54 The overall reaction energy from [UCo]N2 to [UCo]NH3+ is exothermic by 300.8 kcal/mol, consistent with the chemical overpotential of 291.6 kcal/mol calculated from the effective bond dissociation free energy of the acid-reductant pair.60 Along the 6e−6H+ reduction and protonation steps of hybrid pathway, the N2 activation proceeds with its two antibonding π* orbitals filled one by one, and at last with the high-lying antibonding σ* orbital filled to break the N−N bond, as shown in Figure 1 and Figure S4 (where the π-orbitals 7434

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Inorganic Chemistry of N2-species are align for comparison). The π* orbitals and σ* orbital of N2 are thus stabilized gradually with the 6e−6H+ process.

electron steps. However, in the protonation steps where electrons are consumed, electrons are first depleted from uranium and then transferred to cobalt in [UCo]NNH, and then from cobalt to adsorbent in [UCo]NNH2+. This bottomup electron flow process from uranium to cobalt to NxHy for both electron injection and electron consumption highlights the remarkable role of uranium as an extra electron reservoir in facilitating the gradual breakage of the triple bond of N2. The correlation between df−d dative interaction and oxidation states of uranium and cobalt from the electronic structure analysis can provide further insight into how the U− Co bond modulates the nitrogen fixation. Our previous work on the (TPB)Fe system has revealed that the Fe−B bond length decreases at the electron steps but increases at the proton steps.54 Peters and co-workers reported that the Fe−C bond length in the [CSiP3Ph]Fe−Cl complex increases for the initial two electron steps.63 On the basis of this work, the bond length variation is expected to be monotonic with respect to the monotonic change of formal oxidation state of reaction site. However, the [UCo] catalyst does not follow the same rule because of the intricate variation of oxidation states of uranium and cobalt. Figure 2 plots the U−Co bond lengths of the intermediates along the whole associative hybrid pathway. For the first two electron steps, the U−Co bond length is about 3.215 Å and barely changes. However, a significantly shorter U−Co bond length of 2.935 Å is observed in [UCo]NNH. Previous studies of dative bonds between uranium and 3d TMs found that the 3d TMs tend to be at relatively low oxidation states when stable U-TM bonds are formed.20,30,34,35 Indeed, the formal oxidation states of uranium and cobalt in [UCo]N2 and [UCo]N2−, as assigned based on DMRG natural orbitals, are UIII−Co0 and UIII−Co−I, respectively, while the oxidation states of uranium and cobalt in [UCo]NNH and [UCo]NNH+ are UIV−Co−I and UIV−Co0, respectively. The short U−Co bond length in [UCo]NNH may be then owing to stabilization from the interaction between low valent Co−I and high valent UIV. There is a different pattern of U−Co bond length variation after [UCo]NNH: the U−Co bond length decreases in the electron steps and increases in the proton steps as shown in

Figure 1. Bottom-up electron flow process from uranium to cobalt to NxHy for both electron injection and electron consumption. The oxidation state changes are shown with the reaction intermediates.

Especially noteworthy is the uranium anchor participation in the initial electron step that serves as an electron reservoir. Indeed, early actinide elements have rich oxidation states, especially for uranium, and besides the common UVI and UV, even the rare UII,III complex has been synthesized.61,62 For the initial electron step, uranium obtains one electron, reduced from UIV in [UCo]I to UIII in [UCo]N2. This scenario also occurs in the [TiCo] system, in which one electron occupies a 3d orbital of Ti, leading to Ti reduction.25 After uranium acquires an electron to produce [UCo]N2, cobalt then obtains an electron to generate [UCo]N2−, which raises the chemical potential and reducing ability of the bimetallic site. Thus, the following electron steps occur with the NxHy units obtaining electrons to form [UCo]NHNH2+ as shown in Figure 1. Thus, the electrons are injected starting from the bottom uranium up to the adsorbents gradually in the

Figure 2. U−Co and N−N bond length variations along the associative hybrid pathway. 7435

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Figure 3. Energy profile of N2 reduction on the [XCo] (X = U, B, Ti) complex along the hybrid pathway in the M06-2X level.

important factor for buffering the evolution of oxidation states of active site.25,28 Table 1 shows that the bond order of Ti−Co

Figure 2. This can also be well interpreted by the charge analysis of uranium and cobalt. Table S5 shows that cobalt tends to have low oxidation state in the electron steps and high oxidation state in the proton steps from [UCo]NNH to [UCo]NH2NH2+. However, the oxidation state of uranium does not change during these steps. Therefore, after [UCo]NNH, cobalt serves as the electron reservoir, and the U−Co bond buffers and thus stabilizes the evolution of oxidation state of cobalt. We further compare different-natured anchor atoms, by investigating the energetics, electronic structures, and geometries of intermediates along the hybrid pathway on the reported [BCo], [TiCo], and [UCo] complexes as shown in Scheme 1. We choose [XCo]N2, [XCo]N2−, [XCo]NNH, [XCo]NNH2+, [XCo]NHNH2+, [XCo]NH2NH2+, and [XCo]NH3+ (X = B, Ti, U) as essential intermediates to map the whole energy profile as shown in Figure 3, because they possess crucial electron occupations of the π* orbitals of N2, i.e., [XCo]N2 and [XCo]N2− with π*0, [XCo]NNH with π*1, [XCo]NNH2+ with π*2, [XCo]NHNH2+ with π*3, and [XCo]NH2NH2+ with π*4.54 Figure 3 shows that the steps from [XCo]N2 to [XCo]NNH2+ in [BCo], [TiCo], and [UCo] systems are significantly different from each other, resulting from the different anchor atom X, which plays a critical role as electron reservoir. The most striking difference is in the initial electron step from [XCo]N2 to [XCo]N2−. The reaction energies are endothermic for [BCo] and [TiCo], by 2.4 and 29.3 kcal/mol, respectively, but it is exothermic by 17.3 kcal/mol for [UCo], owing to the uranium anchor being a better electron reservoir. However, the difference is small in the energy profiles from [XCo]NNH2+ to [XCo]NH3+, in which cobalt plays the key role as electron reservoir. This matches the conclusion from the variation of electronic structures of cobalt and anchor atom from [XCo]NNH2+ to [XCo]NH3+. It is clear from Figure 3 that the [TiCo] and [BCo] complexes are less efficient for N2 reduction to NH3 than the [UCo] complex. This can be well explained from a few perspectives. First, the flexibility of hemilabile interaction is an

Table 1. Calculated Bond Orders of X−Co in a Few Key Intermediates on the [XCo] (X = U, Ti, B) Complexes from Mayer and Gophinatan−Jug (G−J) Analyses complex X=U X = Ti X=B

Mayer G−J Mayer G−J Mayer G−J

[XCo]N2

[XCo]N2−

[XCo]NNH

[XCo]NNH2+

0.33 0.24 0.45 0.39 0.60 0.55

0.33 0.26 0.60 0.51 0.61 0.54

0.61 0.47 0.41 0.34 0.51 0.45

0.42 0.34 0.42 0.33 0.42 0.37

is not flexible, giving rise to the poor performance of the [TiCo] complex. Second, the capability of anchor atom to serve as an electron reservoir is a key factor, and this relies on the flexibility of its oxidation states. Figure 4 shows that the energy gap between the d band of cobalt and the f band of uranium is smaller than the counterparts in [TiCo] and [BCo], implying easier electron transfer in the [UCo] system. Third, the d band of cobalt in [UCo] is higher in energy than those in

Figure 4. Comparison of orbital energy level diagrams for [XCo]N2 (X = U, B, Ti). 7436

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Laboratory for Information Science and Technology and the Computational Chemistry Laboratory of Department of Chemistry under the Tsinghua Xuetang Talents Program.

[BCo] and [TiCo] and thus is energetically closer to the antibonding orbitals of N2, which results in better activation of the N2 triple bond. All these consolidate our proposal that the [UCo] complex is a promising candidate catalyst for efficient conversion of N2 to NH3.

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4. CONCLUSIONS In summary, we investigate thoroughly the N2 reduction catalyzed by the [UCo] complex through both the associative hybrid and distal pathways. We find that the uranium anchor serves well as an electron reservoir at the initial stage, and the U−Co bonding buffers and thus stabilizes the evolution of oxidation states of cobalt at a later stage with Co acting as electron reservoir. Our results extend the choice of effective anchor for N2 reduction from the extensively studied p-block and d-block elements to f-block elements, which are likely better options, owing to the unique properties of f orbitals. The accumulation of nearly nonradioactive depleted uranium (DU) in nuclear industry makes it particularly interesting to use DU compounds and materials as useful catalysts. We expect that the bimetallic systems with lanthanide and late 3d TM are also potentially efficient catalysts for N2 reduction. Further extension of such a concept to heterogeneous catalysis (e.g., using metal borides and metal−organic frameworks) may be a novel and promising design strategy toward efficient catalysts for industrial N2-to-NH3 conversion under ambient conditions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00598. Geometric structure, electronic structure, and energies of several intermediates during the catalytic cycle (Tables S1−S5 and Figure S2), proposed mechanism for N2 reduction (Figure S1), energy profile of N2 reduction path (Figure S3), DMRG natural orbital and corresponding occupation numbers (Figures S4−S23) (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*(J.L.) E-mail: [email protected]. *(H.X.) E-mail: [email protected]. ORCID

Xue-Lu Ma: 0000-0002-2654-3092 Hai Xiao: 0000-0001-9399-1584 Jun Li: 0000-0002-8456-3980 Author Contributions ‡

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Science Challenge Project (JCKY2016212A504) and National Natural Science Foundation of China (Nos. 21433005, 91645203, and 21590792) to J.L., and the Beijing Natural Science Foundation (2184105) and the Fundamental Research Funds for the Central Universities (2019QH01) to X.-L.M. The calculations were performed using the supercomputers at Tsinghua National 7437

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.9b00598 Inorg. Chem. 2019, 58, 7433−7439

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DOI: 10.1021/acs.inorgchem.9b00598 Inorg. Chem. 2019, 58, 7433−7439