Article pubs.acs.org/JPCA
Comprehensive Computational Study of Decamethyldizincocene Formation. 1. Reaction of ZnR2 Reagents with Decamethylzincocene Steven S. Hepperle and Yan Alexander Wang* Department of Chemistry, University of British Columbia, Vancouver, Canada V6T 1Z1 S Supporting Information *
ABSTRACT: Computational methods were used to study the surprising 2004 synthesis of decamethyldizincocene, Zn2(η5C5Me5)2, which was the first molecule to have a direct, unbridged bond between two first-row transition metals. The computational results show that the methyl groups of decamethylzincocene, Zn(η5-C5Me5)(η1-C5Me5), affect the transition-state stability of its reaction with ZnEt2 (or ZnPh2) through steric hindrance, and this could possibly allow a counter-reaction, the homolytic dissociation of Zn(η5C5Me5)(η1-C5Me5) into Zn(η5-C5Me5)• and (η1-C5Me5)•, to occur, and because no such steric hindrance occurs when zincocene, Zn(η5-C5H5)(η1-C5H5), is used as a reactant, its dissociation never occurs regardless of what ZnR2 reagent is used.
1. INTRODUCTION Eight years ago, the Carmona group published the unexpected synthesis of decamethyldizincocene (3),1a the first compound to contain a direct, unbridged bond between any two first-row transition metals (Scheme 1). This discovery ignited new interest in dimetallocene chemistry, and since then, six other dizinc compounds have been synthesized.1b The Carmona group accidently discovered 3 while attempting to synthesize the half-sandwich Zn complex with the formula Zn(η5C5Me5)Et (4, R = Et) by reacting decamethylzincocene (1) Scheme 1. Reactions of Zn(η5-C5Me5)(η1-C5Me5) with ZnR2 (Top) and Zn(η5-C5H5)(η1-C5H5) with ZnR2 (Bottom)
Figure 1. HOMO of ZnEt2 (top right) and LUMO of Zn(η5C5H5)(η1-C5H5) (bottom right).
with ZnEt2. The desired molecule 4 does form during this reaction, but it is only the minor product. The authors have noted that this process seldom produces yields of 3 higher than 30%. More often, yields are much less, and sometimes 3 does not form in any detectable amount at all (despite careful efforts Received: November 27, 2012 Revised: May 2, 2013 Published: May 3, 2013 © 2013 American Chemical Society
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Figure 2. Reaction pathway of ZnEt2 reacting with 5. (Zinc atoms appear in red and hydrogens have been removed for clarity.)
Figure 3. Reaction pathway of ZnEt2 reacting with 1. (Zinc atoms appear in red and hydrogens have been removed for clarity.)
There have been a flurry of theoretical papers studying the properties of 3,2a−g but because we are not interested merely in the properties of 3, but rather the mechanism of its formation, the reader is referred to our 2008 paper on this subject, which provides a brief summary of relevant theoretical studies of 3.3 Despite the fact that the original synthesis of 31a has been cited over 100 times, no computational work has ever been published that explains the mechanism of the formation of 3 with the exception of our previous letter on the subject.3 All data from that publication have been included and updated for this comprehensive study. This paper will focus on the formation of 3 via 1 and ZnR2 reagents (Scheme 1), and the possible reasons why certain ZnR2 reagents form 3 whereas others do not. The formation of 3 via Zn(η5-C5Me5)(η1-C5Me5), ZnCl2, and KH will be covered shortly in a future publication.
Scheme 2. Dissociation of Zn(η5-C5R5)(η1-C5R5) into Zn(η5C5R5)• and (C5R5)• Radicals
to reproduce the reaction conditions).1c Later, the authors reported a more efficient syntheses of 3 by reacting 1 with either KH (in a 1:2 ratio) or ZnCl2 and KH (in a 1:1:2 ratio) with the latter being the most efficient procedure.1c Still, the most intriguing aspect of dizinc synthesis is that when zincocenes are used as the starting material, only the decasubstituted zincocenes 1 and Zn(η 5 -C 5 Me 4 Et)(η 1 C5Me4Et) can form dizinc products. If one strips the methyl groups off of 1 and 3, they become zincocene (5) and dizincocene (6), respectively. 5 cannot lead to the formation of 6 regardless of what ZnR2 reagent is used.
2. STRATEGY AND METHODS To fully understand why and how 3 forms, it is necessary to figure out why dizincocene, 6, does not form. Calculations on the reactants, products, intermediates, and transition states 4658
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Figure 4. Simplified reaction pathways of ZnMe2 (top), ZniPr2 (middle), and ZnPh2 (bottom) reacting with 5.
Gaussian 094a was used for all calculations. Geometry optimizations were performed using the popular Kohn−Sham method B3LYP5a−d with Dunning’s cc-pVDZ6a basis set whereas the Pople 6-31G(d)6b set was used exclusively for the Zn atoms. This basis set provides a geometry for 3 that agrees very well with the experimental neutron diffraction geometry, in which the Zn−Zn bond length is 2.292(±1) Å.1d The cc-pVDZ (with 6-31G(d) for Zn) basis set produces a length of 2.293 Å. All transition states were analyzed and confirmed by multiplying the imaginary mode Cartesian displacements by a scale factor (which varied depending on
along the reaction paths of formation of 6 and 3 were performed, and the resultant geometries for the formation of 6 were used (with methyl groups added) to find the intermediates and transition states along the formation path of 3. This facilitates an efficient and thorough comparison of these two processes. The formation reactions of these dizinc products were studied from two hypothetical perspectives: (1) neutral-charge electrostatic attraction and rearrangement of the reactants and (2) radical dissociation and recombination of the reactants. 4659
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Table 1. Energy (in kcal/mol) Compilation of ΔG‡(symmetric) of the Forward and Backward Reactions and the Energy Difference between the Activation Barrier Height of the Forward Symmetric Process and the Homolytic Radical Dissociations of 5 and 1
electron density to the LUMO of 5, the π* orbital on the bottom (η1) Cp ring of 5. Consequently, we have discovered that ZnEt2 can attack zincocene in a sideways fashion, cutting open the sandwich structure while forming an intermediate (Figure 2). The activation barrier for this process is 4.7 kcal/ mol (from the complex to ‡associative). At this point in the reaction, the intermediate can do one of two things. It can overcome a symmetric barrier (‡symmetric) of 3.1 kcal/mol to form two equivalents of symmetric products, Zn(η5-C5H5)Et (4, R = Et), which are considerably more stable than the reactants (−16.7 kcal/mol). On the basis of their experimental data, the Carmona group hypothesized a mechanism like this in 2007.1e Alternatively, the intermediate can overcome an asymmetric barrier (‡asymmetric) with a steep 62.9 kcal/mol activation barrier to form the asymmetric products, 6 and butane. Obviously, the activation barrier for the asymmetric process is much too high relative to the symmetric process to form 6, even though the asymmetric products are much more thermodynamically stable than the reactants (−35.8 kcal/mol). 3.2. Neutral-Charge Electrostatic Decamethyldizincocene Formation via ZnEt2 and Zn(η5-C5Me5)(η1-C5Me5). As one might assume, the reactive species in the analogous formation of 3 are similar in geometry (excluding the methyl groups) compared to the pathway for 6 (Figure 3). Note that the MP2 single-point energies predict that the reactants will reach the intermediate without an associative barrier. However, the absolutely crucial difference when 1 is used in this synthesis is that the symmetric barrier is much higher (13.2 vs 3.1 kcal/ mol when 5 is used). However, this difference is not enough to suggest that 3 forms via the asymmetric barrier. Based on these results, it is reasonable to conclude that 3 does not form via simple neutral-charge electrostatic attraction and rearrangement of 1 and 2 (R = Et). 3.3. Radical Dissociation of Parent Zincocenes. There apparently is a moderate difference in the homolytic dissociation energies of 5 (43.0 kcal/mol) and 1 (36.1 kcal/ mol) due to the methyl groups stabilizing the resulting radicals from the cleavage of Zn(η5-C5R5)(η1-C5R5) (Scheme 2). These data alone are not significant enough for us to suggest that the formation of 6 will not occur, as it does not experimentally. On
the transition state), adding them to the saddle-point geometries, and performing geometry optimizations to connect the transition states to their corresponding minima. All thermodynamic energies reported here are free energies (G263) at the temperature (263 K) of the initial experiment. Møller−Plesset second-order perturbation theory (MP2)7 single-point calculations were performed using the same basis set with the B3LYP optimized geometries. Solvent effects were further incorporated by employing an integral equation formalism polarizable continuum model (PCM), a solvent model appropriate for diethyl ether, with the MP2 single-point energies.8a−e The final energies were derived by adding the B3LYP free energy corrections to the PCM-MP2 single-point energies. In the context of the homolytic dissociation energies of Zn(η5-C5R5)(η1-C5R5) into Zn(η5-C5R5)• and (η1-C5R5)• radicals (R = H, Me), the counterpoise optimization method proposed by Simon et al. was employed.9 These restricted open-shell B3LYP energies were also improved using PCMMP2. The counterpoise correction was obtained by computing counterpoise-corrected single-point MP2 energies with the counterpoise optimized geometries of the parent zincocenes. The exact same procedure was employed to compute the association energy of 3 and 6 from Zn(η5-C5Me5)• and Zn(η5C5H5)• radicals, respectively. Because current computational resources do not allow for the computation of MP2 Hessians for molecules of this size, the B3LYP thermal data and optimized geometries are our best alternative. Ball and stick molecular images throughout this article were generated using Gaussview 5.4b
3. RESULTS AND DISCUSSION Literally any ZnR2 reagent could be used to react with 1 or 5. So for the purposes of this study, we have selected ZnMe2, ZnEt2, ZniPr2, and ZnPh2. Only reactions with ZnEt2 will be analyzed extensively, but all necessary kinetic and thermodynamic data will be shown for the other three ZnR2 reagents. 3.1. Neutral-Charge Electrostatic Dizincocene Formation via ZnEt2 and Zn(η5-C5H5)(η1-C5H5). A natural bond orbital analysis10a−c of 5 and ZnEt2 (Figure 1) shows that the HOMO of ZnEt2, the Zn−C σ-bonding orbital, can donate 4660
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Figure 5. Simplified reaction pathways of ZnMe2 (top left), ZniPr2 (top right), and ZnPh2 (bottom) reacting with 1.
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the other hand, if this difference in dissociation is coupled with the large difference in the symmetric activation barriers in the formation of 4 (R = Et) and 7 (R = Et) (13.2 vs 3.1 kcal/mol), it is possible that during the formation of 3, the homolytic dissociation of 1 competes with the formation of the symmetric products (36.1 vs 13.2 kcal/mol). Further, the formation of 6 would not be possible due to a prohibitive energy difference between these two processes (43.0 vs 3.1 kcal/mol).3 3.4. Reaction of Parent Zincocenes with Other ZnR2 Reagents. Though it has been hypothesized that 3 forms because the homolytic dissociation of 1 competes with the formation of 4 (R = Et),3 the Carmona group has reported that 3 only forms when 1 is reacted with ZnR2 (R = Et, Ph) and 6 will never form regardless of what other ZnR2 reagent is used.1c So the question now becomes, can this “competition” hypothesis be applied to this expanded list of reagents? The reactions of 1 and 5 with ZnR2 (R = Me, iPr, Ph) were investigated and compared with the results for ZnEt2. For each reaction, only ‡symmetric will be discussed because it is the rate limiting step when 1 reacts with ZnEt2 and ZnPh2, and ‡asymmetric is ignored because it is irrelevant. For each reaction of ZnR2 (R = Me, Et, iPr, Ph) with 5, the geometries of ‡symmetric do not change all that much (Figure 4), and in accordance with experimental findings1c and our hypothesis, the barrier heights remain low (1.7−9.9 kcal/mol, Table 1). Even though ‡symmetric for ZnPh2 is relatively high (9.9 kcal/mol), the reaction is very exothermic (−14.8 kcal/ mol) with ‡symmetric being 4.5 kcal/mol below the energy of the reactants (Figure 4). It appears that because of steric reasons, when ZnR2 (R = Me, Et, iPr, Ph) reacts with 1, the geometries of ‡symmetric are relatively consistent (Figure 5), but the energies vary quite dramatically. Predictably, ‡symmetric for ZnMe2 is the lowest (5.2 kcal/mol, Table 1) due to minimal steric interactions between the methyl groups of ZnMe2 and 1. However, one would assume that ‡symmetric for ZniPr2 would be higher than ZnEt2 due to the bulkiness of the isopropyl groups, and that would contradict our hypothesis because ZniPr2 does not lead to the formation of 3 experimentally, but in reality that is not the case. The energy of ‡symmetric for ZniPr2 reacting with 1 is 7.3 kcal/mol whereas it is 13.2 kcal/mol for ZnEt2. ZnPh2, which produces consistently higher experimental yields of 3 when reacted with 1, has the highest ‡symmetric at 17.1 kcal/ mol (Table 1). Intriguingly, the attraction of the right phenyl ring of ZnPh2 to the left zinc necessitates a second ‡symmetric (5.4 kcal/mol) to break this attraction and form the symmetric products (second ‡symmetric, Figure 5). The overall backward reaction barrier height is 20.9 kcal/mol measured from the symmetric products to the first ‡symmetric.
competing reaction channel is the homolytic dissociation (Scheme 2) if the energetics are favorable. Our results show that a possible scenario is that through ‡symmetric, ZnR2 reacts with 1 to form 4 and two units of 4 can also undergo the reverse reaction to reproduce ZnR2. The rates of the forward and backward reactions are dictated by the activation barriers and the equilibrium constant is determined by the energy difference between the products and the reactants. Among the four (ZnR2 + 1) systems considered here, ZnMe2 or ZniPr2 forms 4 most efficiently. After the preequilibrium is reached, 1 survives most in the (ZnPh2 + 1) system, much less in both (ZnMe2 + 1) and (ZnEt2 + 1) systems, and very little in the (ZniPr2 + 1) system. For the competing reaction channel, the homolytic radical dissociation of 1, the (ZnPh2 + 1) system has the lowest extra energy requirement of 19.0 kcal/mol, which is closely followed by the (ZnEt2 + 1) system with 22.9 kcal/mol, whereas all other systems demand much more energy for 1 to undergo radical dissociation (thus unlikely to happen in reality). Experimentally, the remaining 1 in solution could begin to dissociate into Zn(η5-C5Me5)• and (C5Me5)• radicals. Two Zn(η5-C5Me5)• units could readily combine given their considerable association energy (46.5 kcal/mol). This large energy release could then further fuel the radical dissociation of 1 and consequently form more 3. This proposal would support the experimental hypothesis that 3 forms via combination of two Zn(η5C5Me5)• units.1c A further piece of experimental data that supports the above explanation is the yields of 3. It has been found that yields of 3 can be as high as 30% when ZnEt2 is used, but often they are much less (and sometimes no detectable amount of 3 forms at all).1c When ZnPh2 is used, yields are a little higher, but the inconsistency remains.1c Given the lack of ability to control homolytic radical dissociations and reassociations in an experimental setting, it seems logical that the yields of these processes would lack consistency and reproducability. Almost self-evidently, when 5 is used, ‡symmetric for all ZnR2 reagents are more than 33 kcal/mol lower than the energy cost of the homolytic dissociation (Table 1), which would essentially prohibit the homolytic dissociation of 5 and make the formation of 6 impossible via this type of mechanism.
4. CONCLUSION With the energetic data collected in Table 1, we are ready to present kinetic arguments to explain why 3 forms during this process whereas 6 does not. It is well-known that every forward reaction is accompanied by a backward reaction involving the same reactants and products. Because any other processes, i.e., the homolytic radical dissociation (Scheme 2) and the asymmetric pathway (Figures 2 and 3), have much higher activation barriers than the symmetric pathway does (Table 1), we can reasonably assume that a pre-equilibrium is established between the reactants and the symmetric products. During the course of the reaction, for the remaining reactants in the system, the only viable
Corresponding Author
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ASSOCIATED CONTENT
S Supporting Information *
Cartesian geometries for all reactants, products, intermediates, and transition states. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support for this project was provided by a grant from the Natural Sciences and Engineering Research Council (NSERC) of Canada. S.S.H. is grateful to NSERC for PGSM/D funding (2005-2009) and to the Izaak Walton Killam Memorial Foundation for a pre-Doctoral scholarship (20062008). Westgrid and CFI are thanked for providing computational resources. Dr. Roman V. Krems is also thanked for 4662
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allowing the generous use of his group’s computational resources. Finally, Dr. Laurel L. Schafer is thanked for her contributions to the discussion and conclusion.
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