10012
J. Phys. Chem. B 2004, 108, 10012-10018
Theoretical Prediction of a New Dinitrogen Reduction Process: Utilization of Four Dihydrogen Molecules and a Zr2Pt2 Cluster Djamaladdin G. Musaev* Cherry L. Emerson Center for Scientific Computation, Emory UniVersity, 1515 Dickey DriVe, Atlanta, Georgia 30322 ReceiVed: April 20, 2004
The mechanisms of dinitrogen reduction to hydrazine by four dihydrogen molecules and a Zr2Pt2, I, cluster have been extensively studied at the density functional level. It was shown that the reaction starts from the coordination of the N2 molecule to the Zr centers of I. The resulting complex, Zr2Pt2(µ-1,2-N2), II, activates four dihydrogen molecules and produces the (µ-1-H)2ZrPt(µ-1,2-N2H4)PtZr(µ-1-H)2, X complex. The activation barriers corresponding to the first, second, third, and fourth H2 molecules are predicted to be ∆H ) 7.0 (∆G ) 15.6), 10.6 (19.3), 18.3 (27.4), and 25.8 (34.6) kcal/mol, respectively. The calculated dissociation energy of the hydrazine molecule from the product X is 19.2 (6.5) kcal/mol. The entire reaction Zr2Pt2 + N2 + 4H2 f (µ-1-H)2ZrPt2Zr(µ-1-H)2 + N2H4 is found to be exothermic by 52.6 (21.7) kcal/mol and proceed via a 25.8 (34.6) kcal/mol rate-determining barrier corresponding to the activation of the fourth H2 molecule. The dinitrogen reduction to hydrazine by four dihydrogen molecules and a Zr2Pt2 cluster is predicted to be feasible at the modern experimental conditions.
I. Introduction Activation of the NtN triple bond of dinitrogen and its chemical transformations under mild conditions is one of the most fascinating tasks of the modern chemical and biochemical sciences, and has challenged the scientists for over a century.1 The reduced nitrogen finds applications in fuels, fertilizers, and dyes. However, the major part of all nitrogen required in human nutrition is still derived from biological nitrogen fixation.2 An industrial analogue of the biological nitrogenation is the HaberBosch process,3 which accounts for almost all the industrial dinitrogen fixation. However this process operates at a very high temperature and pressure, and is economically less effective. Therefore, the search for the more economical and alternative methods of nitrogen fixation in mild conditions continues. The extensive studies have led to the discovery of many fascinating new classes of processes and elementary reactions. Currently, much more accurate fundamental and practical knowledge has accumulated about the bonding modes and reactivity of the coordinated dinitrogen molecule. Many of these reactions have been extensively reviewed,4 and therefore will not be discussed in this paper. However, the recent reactions of the transitionmetal coordinated dinitrogen molecule with electrophiles,5 coordinated,6 and free7,8 dihydrogen molecules need to be briefly mentioned. Reaction of the coordinated N2 molecule with electrophiles is a core process of the many known nitrogen fixation processes. The mechanism of this process is reasonably well-established, especially with single (η1-manner, end-on) bound dinitrogen complexes. It is believed that this process occurs via a stepwise mechanism (known as the Chatt mechanism) by the protonation of the coordinated dinitrogen and reduction with the electrons flowing from the metal ion.9 Recently, Yandulov and Schrock5 reported the catalytic reduction of the N2 molecule coordinated * E-mail:
[email protected].
to the triamidoamine-Mo(III) complex with the electrophiles. It was demonstrated that N2 reduction occurs at a stericallyprotected single Mo center that cycled from Mo(III) through Mo(VI) states. The fixation of the coordinated-N2 molecule with an H2 molecule is even more challenging. Hidai et al.6 have shown that the dinitrogen molecule of the long-known [W(N2)2(PMe2Ph)4] and [W(N2)2(Ph2PCH2CH2CH2PPh2)2] complexes could be easily transformed to ammonia and/or hydrazine in the presence of the complex [RuCl(Ph2PCH2CH2CH2PPh2)2]+ and a dihydrogen molecule. Although the mechanism of this process still remains obscure, the fascinating aspect of this process is that in order to reduce the dinitrogen by dihydrogen the authors have utilized two different and well-known complexes, metaldinitrogen and metal-dihydrogen. In this process, the tungsten acts as an N2 coordinating and stabilizing center, while the ruthenium acts as an H-H bond coordinating and activating center for the further transfer of H (or H+) to the coordinated N2. Thus, this work demonstrated that combining two (and more) transition metal centers with different functionality could be indispensable for the dinitrogen reduction by dihydrogen. It is important to point out that the two other recently reported N2 hydrogenation reactions, by Fryzuk and co-workers,7 and Chirik et al.,8 are also based on the binuclear transition metal complexes. The major difference of these reactions is that in the Hidai process6 the two active sites (N2-coordination/ activation and H2 coordination/activation) are located on the two different complexes (W and Ru complexes, respectively), whereas in the reactions suggested by Fryzuk and co-workers7 and Chirik et al.,8 both the N2 coordination/activation and H2 coordination/activation sites are located within the same binuclear complex. Indeed, Fryzuk and co-workers7 have reported that the dihydrogen molecule added to the binuclear metal-N2 complex [P2N2]Zr(µ2-η2,η2-N2)Zr[P2N2], FI, where [P2N2] ) PhP(CH2SiMe2NSiMe2CH2)2PPh, reacts with the dinitrogen and leads
10.1021/jp0482767 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/10/2004
Theoretical Prediction of a New Reduction Process SCHEME 1: Schematic Presentation of “Donation” and “Back-donation” Contributions on M-N2 Interaction
to a new complex with bridging Zr‚‚‚H‚‚‚Zr and N-H bonds: [P2N2]Zr(µ2-η2,η2-N2H)Zr[P2N2](µ-H), FII. Our extensive computational studies10 of the mechanism of this reaction on the model complex [p2n2]Zr(µ2-η2,η2-N2)Zr[p2n2], where [p2n2] ) (PH3)2(NH2)2, F1, showed that the reaction of the hydrogen molecule proceeds via a 21 kcal/mol barrier at the “metathesislike” four-center transition state (involving the two H-atoms, and one N center, and one Zr center), and produces the diazenido-µ-hydride complex, [p2n2]Zr(µ2-η2,η2-N2H)(µ-1,2H)Zr[p2n2], F2, in agreement with the experiment.7 However, the calculations clearly demonstrated that the experimentally observed diazenido-µ-hydride complex, F2, should not be the only product of the reaction (at least for the model complex!). Indeed, the calculations show that the H2 molecule (second one) could be added to the complex F2 with almost the same (in fact, with slightly less, 19.5 kcal/mol) energy barrier. The transition state for this process is another “metathesis-like” fourcenter structure, involving the H-atoms from the second H2 molecule and an unutilized N/Zr pair. The product of the second H2 addition is the complex [p2n2](µ-H)Zr(µ2-η2,η2-N2H2)Zr(µH)[p2n2], F3, with two bridging NH units and two terminal Zr-H bonds. Since the addition of the first H2 molecule to F1 is known to occur at laboratory conditions, it was predicted that the addition of the second hydrogen molecule to F1 (or the addition of the H2 molecule to F2) should occur as easily as the addition of the first H2 molecule to F1. Furthermore, the calculations suggested that the addition of the third H2 molecule to F1 is kinetically less favorable than the first two, but it is still feasible at the appropriate dihydrogen pressure and temperature. Based on these results, we concluded that the model complex [p2n2]Zr(µ2-η2,η2-N2)Zr[p2n2], where [p2n2] ) (PH3)2(NH2)2, can easily react with two (and, perhaps, three) hydrogen molecules under the appropriate laboratory conditions. Recently, Chirik et al.8 have beautifully demonstrated that the dizirconium complex indeed can reduce dinitrogen to ammonia by utilizing dihydrogen molecules upon the proper regulation of the ligand environment of the Zr centers. The authors have demonstrated that the reduction of (η5-C5Me4H)2ZrCl2 with sodium amalgam under 1 atm of N2 produces [(η5C5Me4H)2Zr]2(µ2,η2,η2-N2) complex C1, which easily adds two dihydrogen molecules and produces [(η5-C5Me4H)2ZrH]2(µ2,η2,η2-N2H2), C2. Heating solutions of the resulted complex C2 in boiling heptane for 5 min resulted in the loss of one equivalent of H2 and cleavage of the N-N bond to form the complex [(η5-C5Me4H)2Zr]2(µ2-NH2)(µ2-N), C3. Strikingly, warming the heptane solution of the complex C2 resulted in the formation of ammonia. Thus, the nitrogen fixation has been accomplished under mild conditions using H2 and variation of the ligand environment of the dizirconium. Since previous studies have demonstrated that one of the most important steps of all nitrogen fixation processes is the dinitrogen coordination to the transition metal center, the nature of the M-N2 interaction needs to be briefly discussed. It is generally accepted that the M-N2 interaction could be described in terms of donation and back-donation11 (see Scheme 1). Since N2 has a filled σ orbital it could be considered as a σ-donor.
J. Phys. Chem. B, Vol. 108, No. 28, 2004 10013 SCHEME 2: Possible Coordination Modes of the Dinitrogen Molecule in the Mono- and Dinuclear Transition Metal Complexes
This weak σ-donation is expected to favor, linear η1-fashion, the coordination of N2 to transition metal centers. N2 also has an empty π* orbital for the backbonding. The backdonation is expected to favor side-on, η2-fashion, coordination of N2 to transition metal centers that is more important for NtN triple bond activation. Therefore, to efficiently functionalize the Nt N bond one should find ways to stabilize the side-on coordination of N2 (The most common coordination modes of the N2 molecule to the transition metal centers are presented in Scheme 2). Based on the above-presented discussions it could be expected that in the mononuclear transition metal complexes, the end-on coordination of N2 would be the most favorable for the early transition metals, where the transition metal center has empty (or partially empty) s and dσ orbitals, but no doubly occupied dπ orbitals. The side-on coordination of dinitrogen, in principle, could only be possible for late transition metal complexes. It has to be noted that, in almost all the existing mononuclear transition metal dinitrogen complexes, the N2 molecule is in end-on coordination mode, to my best knowledge. In fact, this is one of the reasons why all known (except Hidai process6) activations of the coordinated N2 molecule in mononuclear transition metal complexes occur via the Chatt mechanism. The desirable side-on coordination of N2, as well as its activation and hydrogenation, could be achieved by utilizing the multiple transition metal centers. For example, Cummins and co-workers,12 Fryzuk and co-workers,7 and Chirik et al.8 have demonstrated that the coordination of N2 molecules between the two transition metal centers facilitate the NtN triple bond cleavage and hydrogenation. However, the transition metal clusters (or nanostructures), where the N2 molecule could coordinate to two transition metal atoms at the same time with its N-ends (formally side-on fashion) might be even more attractive. We believe that the “cooperative” action of the transition metal atoms might significantly facilitate the NtN triple bond activation and functionalization. In this paper we continue our efforts to elucidate the dinitrogen hydrogenation and validate the idea of utilizing the transition metal clusters (nanostructures) for the N2 reduction. Here we utilize the M2M2′ cluster for N2 reduction to hydrazine by four H2 molecules. Our preliminary screening for the different M and M′ (such as Zr, Ta, Pd, Pt, and Ru) indicates that only Zr2Pt2 and Zr2Pd2 clusters coordinate the N2 molecule by the µ-1,2-N2 manner, which is a desirable coordination mode of N2 for its hydrogenation. Furthermore, we have found that N2
10014 J. Phys. Chem. B, Vol. 108, No. 28, 2004
Musaev
TABLE 1: Calculated (in kcal/mol) Energies (Relative to the Corresponding Reactants) of the Reactions Zr2Pt2 + N2 + 4H2 f Zr2Pt2(µ-1,2-N2) + 4H2 f TS1 f (µ-1-H)ZrPt(µ-1,2-N2H)PtZr + 3H2 f TS2 f (µ-1-H)ZrPt(µ-1,2-N2H2)PtZr(µ-1-H) + 2H2 f TS3 f (µ-1-H)ZrPt(µ-1,2-N2H3)PtZr(µ-1-H)2 + H2 f TS4 f (µ-1-H)2ZrPt(µ-1,2-N2H4)PtZr(µ-1-H)2 f (µ-1-H)2ZrPtPtZr(µ-1-H)2 + N2H4a structures Zr2Pt2 + N2 Zr2Pt2(µ-1,2-N2) Zr2Pt2(µ-1,2-N2) + H2 Zr2Pt2(µ-1,2-N2)(H2), TS1 (µ-1-H)ZrPt(µ-1,2-N2H)PtZr (µ-1-H)PtZr + H2 (µ-1-H)ZrPt(µ-1,2-N2H)PtZr(H2), TS2 (µ-1-H)ZrPt(µ-1,2-N2H)PtZr(µ-1-H) (µ-1-H)ZrPt(µ-1,2-N2H2)PtZr(µ-1-H) + H2 (µ-1-H)ZrPt(µ-1,2-N2H2)PtZr(µ-1-H)(H2), TS3 (µ-1-H)ZrPt(µ-1,2-N2H3)PtZr(µ-1-H)2 (µ-1-H)ZrPt(µ-1,2-N2H3)PtZr(µ-1-H)2+ H2 (µ-1-H3)ZrPt(µ-1,2-N2H3)PtZr(µ-1-H)2(H2), TS4 (µ-1-H)2ZrPt(µ-1,2-N2H4)PtZr(µ-1-H)2 (µ-1-H)2ZrPt(µ-1,2-N2H4)PtZr(µ-1-H)2 (µ-1-H)2ZrPt2Zr(µ-1-H)2 + N2H4 a
I II III VI VI VI VII VIII IX X X XI
∆E
∆E + ZPC
∆H
∆G
0.0 -27.1 0.0 7.2 -32.2 0.0 10.8 -26.8 0.0 17.8 -6.6 0.0 25.1 7.3 0.0 20.9
0.0 -26.8 0.0 8.7 -28.4 0.0 12.3 -22.3 0.0 20.1 -0.9 0.0 27.6 13.0 0.0 18.7
0.0 -27.7 0.0 7.0 -29.6 0.0 10.6 -23.7 0.0 18.3 -2.3 0.0 25.8 11.5 0.0 19.2
0.0 -16.1 0.0 15.6 -21.6 0.0 19.3 -16.1 0.0 27.4 6.0 0.0 34.6 19.6 0.0 6.5
Zero-point corrections (ZPC), enthalpy (∆H) and entropy (∆G) corrections to the energy were calculated at 1 atm and 278.15 K.
functionalization by four H2 molecules occurs via same mechanism for both Zr2Pt2 and Zr2Pd2 clusters, while the Zr2Pt2 shows slightly more activity. Therefore, below we only discuss the mechanism of the reaction Zr2Pt2 + N2 + 4H2 in more detail. II. Calculation Procedures All calculations were performed using the quantum chemical package GAUSSIAN-2003.13 The geometries, vibrational frequencies, and energetics of all the reactants, intermediates, transition states, and products were calculated using a hybrid density functional theory employing the B3LYP method.14 In these calculations, we used the Stutgard group’s pseudopotentials15 and associated SDD basis sets for transition metal atoms and the standard 6-311 + G(d,p) split-valence basis set for H and N atoms. The nature of all calculated intermediates and transition states, as well as reactants and products, were confirmed by performing normal-mode analysis. The performed IRC calculations allowed the clear assignment of the obtained transition states. The energetics presented below include unscaled zero-point energy corrections at 298.73 K and 1 atm. III. Results and Discussion Let us start our discussion with the electronic and geometrical structures of the reactant, Zr2Pt2, structure I. Geometrical structures of its lower-lying singlet and triplet states, as well as their relative energies, are given in Figure 1. As seen from this figure, the singlet and triplet states of this cluster are almost degenerated with only an 0.80 (1.41) kcal/ mol difference (here and below ∆H and ∆G values will be given without and with parenthesis, respectively). Despite the triplet state being slightly lower in energy than the singlet state, their geometrical structures are very different; the Zr-Zr distance is 2.519 Å for the singlet state, and elongated to 2.725 Å in the triplet state. The Zr-Pt bond distances are 2.615 and 2.574 Å for singlet and triplet states, respectively. In other words, during the singlet-to-triplet transition the Zr-Zr bond converts from the double bond to the single bond. This picture is also consistent with the calculated spin distributions, it is found that in the triplet Zr2Pt2 each Zr centers have 0.89e unpaired spins. The quintet state of this cluster is very high in energy and will not be discussed further.
The first step of the reaction is the coordination of the N2 molecule, which leads to the most favorable structure Zr2Pt2(µ-1,2-N2), II, given in Figure 1, and is exothermic by 27.7 (16.1) kcal/mol. Calculations show that the coordination of the dinitrogen molecule stabilizes the singlet state more than the triplet state and makes it slightly,1.24 (0.33) kcal/mol, more favorable in II. This trend (i.e. the stabilization of the singlet states over the triple ones) becomes even more pronounced in the next stages of the studied processes (see the data given in the Available Supporting Materials). Therefore, below we will mainly discuss the ground singlet state of the calculated intermediates, transition states, and products. As shown in Figure 1, in II the dinitrogen molecule is coordinated to Zr centers via its N ends. The calculated N-N bond distance is 1.292 (1.233) Å (here and below, geometrical parameters presented without parentheses correspond to the singlet state, while those given in parentheses are for the triplet state), which is 0.196 (0.147) Å longer than the N-N bond distance, 1.096Å, in the free N2 molecule. This value is even larger than the N-N bond distance of 1.237 Å in the free N2H2 molecule. In the other words, the coordination of the N2 molecule to Zr2Pt2 activates one of the three components of the NtN bond and makes the nitrogen-nitrogen bond a doublebond character in the resulting complex II. The comparison of the geometries of II and I shows that their Zr-Pt distances are very similar, while the Zr-Zr bond distance is significantly longer in II than in I: Upon going from structure I to structure II the Zr-Zr bond distance is elongated from 2.519 (2.725) to 2.855 (2.878) Å. The observed Zr-Zr bond elongation (as well as calculated energetics) correlates with the calculated trends in the Zr-N bond distances, which are slightly shorter for the singlet state structures than the triplet state one. Thus, upon going from the singlet to the triplet state in II, the N-N and Zr-Pt bond distances become shorter, while the Zr-N and Zr-Zr bonds become longer. These trends are consistent with the calculated spin distributions. In the triplet state of II the two unpaired spins are distributed as following: 0.94 (Zr1), 0.31 (Zr2), 0.08 (N2), 0.42 (N1), 0.13 (Pt1), and 0.13 (Pt2). Thus, the largest unpaired spins are located at the Zr1 and N1 centers, which is consistent with the larger elongation of the Zr1-N1 bond distance upon going from the singlet to the triplet state.
Theoretical Prediction of a New Reduction Process
J. Phys. Chem. B, Vol. 108, No. 28, 2004 10015
Figure 1. Calculated important geometrical parameters (distances in Å) of the reactants, intermediates, and transition states of the reaction Zr2Pt2 + N2 + 4H2 f (µ-1-H)2ZrPt2Zr(µ-1-H)2 + N2H4.
10016 J. Phys. Chem. B, Vol. 108, No. 28, 2004 Another distinct feature of this complex is the asymmetric character of the Zr-N bonds; the Zr1-N1 bond is slightly longer than the Zr2-N2 bond. This structural data also indicate that the N2 molecule is (partially) side-on coordinated to Zr1, but end-on coordinated to the Zr2 center, implying that most of the required electrons for the reduction of N2 come from the Zr1. This is consistent with the analysis of the charge distribution in the complex II: Mulliken atomic charges of the N2 unit, Zr1, and Zr2 centers are calculated to be -0.72, +1.09, and +0.89e, respectively. A similar structural feature has been reported by Fryzuk et al.16 for the complexes {(NPN)Ta(µ-H)2}2(N2) (where NPN ) (PhNSiMe2CH2)2PPh) and {(PH3)(NH2)2Ta(µ-H)2}2(N2). One should note that in the reported organometallic dizirconium complexes, [P2N2]Zr(µ2,η2,η2-N2)Zr[P2N2], where [P2N2] ) PhP(CH2SiMe2NSiMe2CH2)2PPh and [(η5-C5Me4H)2Zr]2(µ2,η2,η2-N2), the N2 unit is located between the two Zr centers and is inserted into the Zr-Zr bond. However, in our system, as well as in the complex {(NPN)Ta(µ-H)2}2(N2),16 it is not inserted into the Zr-Zr bond, mainly because of the geometrical constraint imposed by the Zr-Pt and Ta-H interactions, respectively. We also searched for the structures with the side-on and/or end-on coordinated N2 molecule to one of the Zr centers. All of these calculations led to the structure II, indicating that these types of structures do not exist. The next step of the reaction is the coordination of the dihydrogen molecule to the structure II. In general, H2 molecule can coordinate to the Zr2Pt2(µ-1,2-N2) complex from several different positions: (1) side-on manner to one of the Pt centers, (2) simultaneously to the Zr1 and N1 centers, (3) simultaneously to the Zr2 and N2 centers, and (4) side-on manner to one of the Zr centers. The calculations indicate the existence of several Zr2Pt2(µ-1,2-N2)(H2) complexes with extremely small (
