Research Article pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10527-10537
Mechanistic Consequences of Chelate Ligand Stabilization on Nitrogen Fixation by Yandulov−Schrock-Type Complexes Tamara Husch and Markus Reiher* ETH Zürich, Laboratorium für Physikalische Chemie, Vladimir-Prelog-Weg 3, 8093 Zürich, Switzerland
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S Supporting Information *
ABSTRACT: The Yandulov−Schrock catalyst, a mononuclear molybdenum complex with a tetra-coordinate triamidoamine chelate ligand with hexa-iso-propyl-terphenyl groups at the amide nitrogen atoms, catalyzes the reduction of dinitrogen to ammonia. Its turnover number is very low, which may be attributed to the (partial) loss of the chelate ligand. Protonation of an amide nitrogen atom of the ligand and subsequent reduction leads to the formation of a labile amine ligand. We find that this equatorial amine group can detach from the molybdenum center of the Yandulov−Schrock complex with a comparatively small barrier. This decomposition reaction is in direct competition with reactions producing intermediates of the Chatt−Schrock cycle. Clamping the substituents on the amide nitrogen atoms by a calix[6]arene unit (replacing the hexa-iso-propyl-terphenyl groups) successfully suppresses the detachment of a generated equatorial amine group from the molybdenum center. We discuss dinitrogen reduction according to the Chatt−Schrock cycle for a molybdenum complex with such a calix[6]tren ligand. We find that the first protonation step and several reduction steps become thermodynamically less favored compared to the original Yandulov−Schrock catalyst, indicating that even stronger acids and reductants than lutidinium and decamethylchromocene, respectively, might be needed. Also, multiple side reactions can occur that are characterized by moderate to high barriers which can reduce the turnover frequency or even prevent catalytic behavior altogether. Strong acidic conditions are, however, found to induce ether cleavage of methoxy substituents in the calix[6]tren ligand. Upon reduction of a protonated methoxy group, a methyl residue is transferred onto the distal nitrogen atom of the coordinated dinitrogen ligand. It is therefore advantageous to avoid alkoxy substituents at the chelate ligand. KEYWORDS: Nitrogen fixation, Computational chemistry, Homogeneous Catalysis, Nitrogen reduction, Molybdenum, Reaction mechanisms
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INTRODUCTION Dinitrogen (N2) as an inert gas cannot be directly assimilated by most organisms. Instead, N2 must be activated and transformed to less inert species, e.g., through reduction to ammonia (NH3), to make it bioaccessible.1,2 Industrially, NH3 is produced by the Haber−Bosch process at high temperature and pressure, which consumes up to 2% of the annual energy production.3 Biologically, however, N2 is bound and reduced by nitrogenase enzymes under ambient conditions.4−6 For synthetic approaches, dinitrogen fixation still represents a formidable challenge. Only a few homogeneous transitionmetal catalysts have been discovered which are capable of reducing N2 at room temperature and pressure.7−21 All of these catalysts suffer from low turnover numbers which range from four for the Yandulov−Schrock catalyst7 to about 100.16,18 The turnover number specifies the total number of turnovers which a catalyst can achieve before inactivation.22 In this work, we focus on the theoretical rationalization of the low turnover number of Yandulov−Schrock-type catalysts (mononuclear molybdenum complexes with tetra-coordinate triamidoamine chelate ligands) and on the consequences of a clamping strategy to enhance their stability. To reduce N2 to two NH3 molecules, six protons and six electrons must be transferred onto dinitrogen coordinated to © 2017 American Chemical Society
the molybdenum complex. These transfer steps may take place sequentially as in the Chatt−Schrock cycle.7,23 However, individual proton transfer steps may be coupled to an electron transfer in order to reduce potential energy barriers. Still, the sequential electron-follows-proton-transfer picture is very suitable for assessing the catalytic power of a potential nitrogen-reduction system in terms of reaction thermodynamics. Many of the Yandulov−Schrock intermediates in the Chatt−Schrock cycle were characterized experimentally7,23−30 and theoretically.29−45 However, the Chatt−Schrock cycle only represents a small fraction of all possible reactions occurring in the presence of highly reactive species such as strong acids and reductants. We recently identified42 multiple protonation and reduction reactions competing with the ones in the catalytic cycle by application of a network-exploration algorithm. Here, we extend this previous work42 toward decomposition channels. Such competing reaction paths are the key to rationalizing the low stability of the Yandulov−Schrock catalyst. One prominent side reaction, which has been known for several years32,37,46 and which also emerged as an alternative Received: July 24, 2017 Revised: September 6, 2017 Published: September 7, 2017 10527
DOI: 10.1021/acssuschemeng.7b02518 ACS Sustainable Chem. Eng. 2017, 5, 10527−10537
Research Article
ACS Sustainable Chemistry & Engineering reaction path in our network exploration42 is the protonation of an amide nitrogen atom of the chelate ligand instead of the dinitrogen ligand. A protonation of the chelate ligand might ultimately lead to the dissociation of an arm of the Yandulov− Schrock catalyst which can be considered as a likely cause for the low turnover number.47,48 It was therefore suggested47,48 that a ligand in which the substituents bound to the amide nitrogen atoms are connected might prevent ligand loss. Zahim et al. recently reported48 the synthesis of such a ligand, a calix[6]tren ligand, which creates a rigid and sterically protected coordination environment. While Zahim et al. reported a zinc complex with the calix[6]tren chelate ligand,48 the Schrock group is continuing to work on preparing and exploring a molybdenum complex with this ligand.49 In this work, we investigate theoretically whether a molybdenum complex with such a calix[6]tren ligand can sustain nitrogen reduction to ammonia following the Chatt−Schrock cycle. Moreover, we provide a comparative assessment of both complex architectures, also including potential decomposition pathways.
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PBE optimized minimum-energy structures (denoted as PBE0//PBE). We obtained estimates for barrier heights of reactions from constrained PBE/def2-TZVPP structure optimizations along potential reaction coordinates. These barrier heights can therefore be considered as upper bounds for fully optimized barrier heights. The catalytic reduction of N2 to NH3 takes place in heptane solution.7 Apolar solvents such as heptane are known to have negligible dielectric continuum effects,54 and we therefore chose to not employ an empirical continuum solvation model in accord with our previous work.32,33,37−39,42−45 In order to assess the effect of intramolecular dispersion interactions, we considered a comparison of selected PBE optimized structures with PBE-D360 optimized structures in the Supporting Information. We found that dispersion interactions hardly change the molecular structures of the complexes inspected. Furthermore, it is not obvious that the inclusion of dispersion corrections in structure optimizations would lead to an improved structural modeling in this work because we would neglect dispersion interactions with the solvent environment as we do not model explicit solvent molecules. Therefore, we continue to discuss only results obtained without empirical dispersion corrections.
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RESULTS AND DISCUSSION General Structure of Complex Architectures. The Yandulov−Schrock catalyst is a mononuclear molybdenum complex with a tetra-coordinate triamidoamine chelate ligand with hexa-iso-propyl-terphenyl (HIPT) substituents at the amide nitrogen atoms.7 Whereas the HIPT groups are not bound to one another, a tetra-coordinate triamidoamine chelate ligand was recently synthesized by Zahim et al.48 in which the triamidoamine entity is capped by a calix[6]arene unit. This design is supposed to be more stable against ligand dissociation.48 Figure 1 presents the optimized structures of a four-coordinate molybdenum metal fragment with this calix[6]tren chelate ligand (1a) next to the original Yandulov−Schrock metal fragment (1b; see the Supporting Information for Cartesian coordinates of all optimized intermediates). We will also compare to model complexes with methylated amide nitrogen atoms, with the corresponding metal fragment (1c in Figure 1). The labeling of the compounds encodes the type of intermediate and the chelate-ligand scaffold. For instance, 1 denotes the tetra-coordinate metal fragment. We append “a,” “b,” or “c” to this number to denote the calix[6]tren ligand, the triamidoamine ligand carrying HIPT substituents, or methylated amide nitrogen atoms, respectively. The triamidoamine entity in 1a is structurally very similar to the one in 1b. The internuclear distance of the molybdenum atom to the amide nitrogen atoms is 1.98−2.03 Å in 1a and 1b, whereas it is 2.12 Å between the molybdenum atom and the amine nitrogen atom in both compounds. The structures of 1a and 1b, however, differ in the angle ϕ between the molybdenum atom, the amide nitrogen atom, and the substituent. The angle ϕ is significantly smaller in 1a (121− 124°) than in 1b (130−133°). A comparison with ϕ in 1c (ϕ = 130−132°) indicates that the reduced ϕ in 1a may be attributed to strain exerted by the calix[6]arene. The molybdenum atom is pyramidalized in 1a and 1b. Inspection of Dunitz’s pyramidalization measure,61 which is defined in this case as the shortest distance of the molybdenum atom from the plane spanned by the three amide nitrogen atoms, shows that the molybdenum atom in both Yandulov− Schrock-type metal fragments 1a and 1b is located 0.27 Å above this plane. Hence, the decrease of ϕ in the complex with ligand a compared to ϕ in the original Yandulov−Schrock
COMPUTATIONAL METHODOLOGY
In this work, we consider electronic energy differences of isolated systems at 0 K without thermal corrections. Recently, Thimm et al.41 presented a re-examination of the Chatt−Schrock cycle for the Yandulov−Schrock catalyst with thermal corrections. These corrections, which are subject to extensive modeling assumptions, turned out to be small when proton and electron transfer reactions are studied (on average 7 kJ mol−1) and the electronic energy differences clearly govern the reported free energy differences.41 Moreover, the mean absolute error (MAE) of density-functional electronic energy differences often exceeds the contributions of thermal corrections (e.g., for the density functional PBE0,50,51 MAEs of 13 to 27 kJ mol−1 were reported (see ref 54), and a MAE of about 13 kJ mol−1 was found in ref 46 for Yandulov−Schrock-type complexes). Note that large thermal contributions can be found for gas-phase reactions in which the number of molecules changes (e.g., thermal contributions of 127 kJ mol−1 to the gas-phase reaction free energy of N2 + 3H2 → 2NH3).41 This is due to 60 kJ mol−1 introduced by translational and rotational entropy contributions per excess reactant on one side of the reaction arrow,53 which amounts to a roughly 120 kJ mol−1 thermal contribution in the gas-phase reaction that turns one dinitrogen and four dihydrogen molecules into only two ammonia molecules. Whereas this will be sufficiently accurate and reliable for the gas phase, it is a very poor estimate for the condensed phase. However, the actual change in the translational and rotational entropy in solution is difficult to calculate with methods of stationary quantum chemistry.54 In particular for reactions with a different number of reactants on the educt and product sides, the approximation of the condensed-phase translational entropy by a particle-in-a-box consideration, which assumes an ideal gas, and the rotational entropy, which assumes a free rotor, dramatically fails.53 Recently, we assessed the reliability of density functional theory for the prediction of reaction energies within the Chatt−Schrock cycle with Bayesian error estimation.44 Density functional theory turned out to be sufficiently accurate to determine the relevance of alternative reactions to those of the Chatt−Schrock cycle. As the Perdew− Becke−Ernzerhof exchange-correlation functional PBE55,56 and especially its hybrid variant PBE0 may in general be considered comparatively accurate,52 we optimized all structures reported in this work (see the Supporting Information) with PBE and a def2-TZVPP57 basis set on all atoms. A Stuttgart effective core potential represented the relativistic core electrons of molybdenum.58 Additionally, we present results obtained with the BP8655,59 density functional in the Supporting Information to allow for a comparison with our previous results.32,33,37−39 The qualitative conclusions are not affected by choosing PBE and PBE0 in favor of BP86. We calculated single-point energies with the PBE0 functional and a def2-QZVPP basis set57 for 10528
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Figure 2. Chatt−Schrock nitrogen-fixation cycle. Generic intermediates are labeled by blue integers; reactions are labeled in green.
We present selected structural parameters for three key intermediates in the Chatt−Schrock cycle (2, 14, and 8) in Figure 3. We observe hardly any differences ( 160 kJ mol−1 (see Figure 10, for numerical data, see the Supporting Information). The dinitrogen ligand must bend toward the amine nitrogen atom to enable a proton transfer which is an energetically unfavorable situation and requires activation by occupation of an antibonding π*-orbital on the N2 ligand.68,69 We also did not find a viable path for a direct proton transfer from the amine nitrogen atom to Nα due to the large distance between the two nitrogen atoms (>2.97 Å). Hence, all proton transfer reactions from the equatorial amine nitrogen atom are likely to proceed 10533
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Figure 11. PBE/def2-TZVPP optimized structures of a molybdenum complex with ligand a in which an oxygen atom of a methoxy substituent is protonated (left) and subsequently reduced (right). For the sake of clarity, the triamidoamine moiety, the N2 ligand, and the methoxy group are highlighted with bold-printed bonds. Element color code: Molybdenum, cyan; nitrogen, blue; carbon, gray; oxygen, red; hydrogen, white.
Figure 10. Subnetworks of first protonation and reduction reactions of 2a and 2b. The shape of each vertex indicates to which potential energy surface of a Chatt−Schrock intermediate a compound belongs to: Rectangular means “to protonated 3a or 3b;” circular, “to protonated and reduced 4a or 4b.” Each vertex is colored according to the PBE/def2-TZVPP relative energy of the compound with respect to the most stable intermediate on its potential energy surface (ΔEtd [kJ mol−1]). The color of edges between vertices encodes the PBE/ def2-TZVPP barrier height for intramolecular H-transfer steps (ΔEkin [kJ mol−1]). Reactions connecting different potential energy surfaces are represented by dashed arrows (such a Born−Oppenheimer surface is defined by the number of electrons and the number and type of nuclei).
4a. The complex 31a is therefore the most stable intermediate of all intermediates that we optimized on this specific potential energy surface. As a consequence, the presence of alkoxy groups as substituents in the chelate-ligand scaffold might severely affect the catalytic ability of a Yandulov−Schrock-type complex. Dissociation of the Chelate Ligand. Finally, we examine the dissociation of an equatorial amine from the molybdenum center of Yandulov−Schrock-type complexes. In order to enforce this process, we performed constraint optimizations in which we increased the distance between the equatorial amine nitrogen atom and the molybdenum center. For cationic complexes of both architectures, a detachment of this equatorial amine from the molybdenum center is unfeasible because high PBE/def2-TZVPP dissociation energies (>127 kJ mol−1) are required to double the amine nitrogen−molybdenum internuclear distance. Not surprisingly, for neutral complexes with a cramped calix[6]tren ligand, the ligand arm carrying the equatorial amine cannot be easily detached from the molybdenum center, and we did not find a stable intermediate in which the amine−nitrogen molybdenum distance is significantly increased. For the original Yandulov−Schrock catalyst, however, we located a fully relaxed intermediate (protonated at Nβ and carrying an equatorial amine nitrogen atom) in which the amine nitrogen atom is 4.62 Å away from the molybdenum center (see also Figure S6). The detachment increases the PBE/def2-TZVPP electronic energy by only 51 kJ mol−1, and the product is, by 42 kJ mol−1, less stable than the complex in which the chelate ligand with the equatorial amine is bound to the molybdenum center. While this is only one of the possible dissociation channels, it indicates that these dissociation reactions are in direct competition to the reactions of the Chatt−Schrock cycle for the original catalyst, and hence a likely reason for its low turnover number.
via a transfer of the proton to the molybdenum center of the complexes. The transfer of a proton from the amine nitrogen atom to the molybdenum center (ΔEkin > 72 kJ mol−1) and from the molybdenum center to Nα (ΔEkin > 67 kJ mol−1) are viable at room temperature. A transfer of the proton from Nα to Nβ is, however, very slow at room temperature, if possible at all (ΔEkin > 120 kJ mol−1). This implies that species within the Chatt−Schrock cycle cannot easily be reached if the ligated complex has been protonated at Nα, the molybdenum atom, or the amide nitrogen atomat least not through the reactions included in the subnetworks of 2a and 2b. Ether Cleavage and Methyl Group Transfer. Methoxy substituents at the calix[6]tren ligand can be protonated. However, when considering lutidinium as the proton source, this protonation is endothermic by 45 kJ mol−1 (PBE0//PBE). The product 23a is therefore by only 25 kJ mol−1 less stable than the intermediate (3a), in which Nβ is protonated. A transfer of a proton from 3a (or 4a) onto the oxygen atom of a methoxy substituent is unlikely to occur because such a transfer would be associated with a rather large barrier. The barrier is due to the distance of more than 4 Å between donor and acceptor, which formally makes this a deprotonation− reprotonation sequence. Considering the fact that an endothermicity of 45 kJ mol−1 might be accessible in view of the uncertainty of the electronic structure and molecular structure models (see also the discussion above), we are advised to consider possible reactions of 30a. Structure optimization after protonation of a methoxy substituent leads to a distortion of the ligand structure to allow for the formation of a hydrogen bond with a neighboring methoxy substituent (see Figure 11 and Figure S5 in the Supporting Information). If the cationic complex 30a is reduced, the methyl group will be spontaneously transferred to the dinitrogen ligand upon structure optimization (see Figure 11). This spontaneous reaction can be understood as an ether cleavage, in which the nucleophile is the bound dinitrogen ligand that might be assigned a negative formal charge reflecting the fact that the overall structure was reduced after protonation. The resulting structure 31a is by 62 kJ mol−1 more stable than
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CONCLUSIONS The Yandulov−Schrock catalyst, the first of only a few known homogeneous transition-metal complexes capable of reducing dinitrogen under ambient conditions, suffers from a low turnover number. The presence of highly reactive species such as strong acids and reductants can induce a multitude of side reactions. Such competing reaction paths are the key to rationalizing the low stability of the catalyst. The loss of the ligand was identified as a likely reason for the low turnover number of this catalyst. It was speculated47,48 that a catalyst 10534
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might experience no ligand loss when the substituents on the amide nitrogen atoms are connected, which we confirmed here. We investigated the mechanistic consequences of such a molybdenum complex with an interconnected calix[6]tren ligand in comparison to the original Yandulov−Schrock catalyst. We assessed the catalytic propensity of a molybdenum complex with the calix[6]tren ligand in terms of reaction thermodynamics according to the Chatt−Schrock cycle. Our results indicate a potential loss of catalytic capability due to the calix[6]tren ligand because all reactions which are challenging for the original Yandulov−Schrock catalyst become more endothermic. In particular, the first protonation reaction is the most endothermic and hence decisive step in the Chatt− Schrock cycle for both complex architectures. As a consequence, strong acids and reductants, i.e., stronger than lutidinium and decamethylchromocene, might be required. Furthermore, our investigation indicates that the exchange of NH3 for N2 required to close the cycle might be difficult to achieve. Ammonia is less strongly bound to the molybdenum center of the complex with the calix[6]tren ligand than to that of the original Yandulov−Schrock complex, which indicates that the new complex tends to prefer an elimination−addition pathway. An elimination is, however, slightly endothermic. In contrast to the original Yandulov−Schrock catalyst, an addition of dinitrogen to the penta-coordinate complex to form a hexacoordinate intermediate is also endothermic, which renders an addition−elimination pathway unlikely. When strong acids are employed, it is then more likely that alkoxy substituents at the chelate-ligand scaffold suffer from ether cleavage so that alkyl groups are transferred onto the dinitrogen ligand, which can act in a reduced species as a nucleophile (as witnessed in our calculations at the example of methyl-group transfer from cleaved methoxy substituents). It could be advantageous to replace the alkoxy substituents for less reactive groups. We showed that if an equatorial amine nitrogen atom is formed through protonation and reduction of an amide nitrogen atom, a dissociation of this ligand arm from the molybdenum center will be thermodynamically and kinetically feasible for the original Yandulov−Schrock catalyst. However, such a detachment of an amine group from the molybdenum center is not possible for a molybdenum complex with the calix[6]tren ligand. To conclude, the cramped Yandulov−Schrock-type complex is an interesting system with catalytic potential that will, however, require further structural design in order to remove the potentially labile methoxy substituents and to increase its first proton affinity and the electron affinity of most of the protonated intermediates in the Chatt−Schrock cycle.
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Research Article
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Tamara Husch: 0000-0002-2880-2481 Markus Reiher: 0000-0002-9508-1565 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Professor Richard R. Schrock for drawing our attention to the calix[6]tren ligand and for encouraging us to carry out calculations on the molybdenum complex with this ligand. This work was supported by the Schweizerischer Nationalfonds (Project No. 200020_169120).
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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/acssuschemeng.7b02518. A detailed description of the computational methodology, a discussion of selected structures, a comparison of results obtained with different density functionals (PDF) Cartesian coordinates (ZIP) 10535
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