Article pubs.acs.org/JPCA
Comprehensive Computational Study of Decamethyldizincocene Formation. 2. Reaction of KH/ZnCl2 with Decamethylzincocene Steven S. Hepperle and Yan Alexander Wang* Department of Chemistry, University of British Columbia, Vancouver V6T 1Z1, Canada S Supporting Information *
ABSTRACT: Computational methods were used to study the experimental finding that forming decamethyldizincocene is more efficient when using a reducing agent (e.g., KH) and ZnCl2 as opposed to a sole ZnR2 reagant. The results show that the methyl groups of decamethylzincocene have an indirect effect on the reaction. When zincocene is used as a reactant, the reaction with KH favors the route that results in the formation of the zincate, K+[Zn(η1-C5H5)3]−. However, the path of formation for the zincate K+[Zn(η1-C5Me5)3]− is simply not favorable kinetically or thermodynamically, so the formation of decamethyldizincocene is the only option when decamethylzincocene is used.
1. INTRODUCTION As interesting as our recent theoretical studies on the formation of decamethyldizincocene Zn2(η5-C5Me5)2 via ZnEt2 and ZnPh2 may be,1,2 that reaction is not at all efficient at forming the dizinc product.3−7 The Carmona group found that any number of reducing agents can be reacted with decamethylzincocene Zn(η5-C5Me5)(η1-C5Me5) to form Zn2(η5-C5Me5)2: potassium hydride (KH) has been found to be the most convenient.3−7 The most effective synthesis of 3 involves reacting 1 with KH and ZnCl2 in a 1:2:1 stoichiometric ratio (Scheme 1). At some point during the formation of 3, the Zn2+ ions are reduced by KH to Zn+; but because 6 does not form when 5 is used (rather the zincate salt, 9), the mechanisms needed to be investigated in detail. The reactions of 1 and 5 with KH were studied first, and then the addition of ZnCl2 was taken into consideration. In order to maintain an unambiguous link between this article and Part 1 of this study,2 the chemical numbering in Scheme 1 of this paper continues the ordering set of the previous study.2
for the reactants, products, intermediates, and transition states were rendered using Gaussview9 and are included in the Supporting Information. Solvent effects were considered by utilizing an integral equation formalism polarizable continuum model (PCM)17−21 with Møller−Plesset second-order perturbation theory (MP2)22 single-point energies based on the B3LYP geometries, but the basis set was upgraded to 6-31++G(d,p) for all atoms for these calculations. Also, since these reactions take place in tetrahydrofuran at 293 K,3−7 the B3LYP thermal corrections at this temperature were computed and added to the PCM-MP2 energies to arrive at the Gibbs free energies at 293 K (G293), which are the exclusive energy quantities reported in this article. For the homolytic dissociation and association energies, the counterpoise-optimized23 restricted open-shell B3LYP geometries were again improved by using counterpoise-corrected MP2 single-point energies.
3. RESULTS AND DISCUSSION 3.1. Reactions of 1 and 5 with KH. By following the reaction scheme (Scheme 2), KH reacts with 1 to form a reactant complex (ΔG = −22.2 kcal/mol, Figure S1 (Supporting Information), Table 1). It appears that the complex could go through Step 1a to form Me5CpZnK and Me5CpH (ΔG = −10.2 kcal/mol, Figure S2, Supporting Information). Me 5 CpZnK could then dissociate into Me5CpZn• and K• radicals (Step 2a, ΔG = 37.4 kcal/mol). Two Me5CpZn• radical units could subsequently combine to form 3 (Step 3a, ΔG = −38.5 kcal/mol). However, forming Me5CpZnK directly (Step 1a) carries a significant energy cost (with an activation barrier height ΔG‡ = 24.2 kcal/mol).
2. STRATEGY AND METHODS Gaussian 098,9 was used for all calculations. Geometry optimizations were performed using B3LYP,10−13 with Pople’s 6-31+G(d,p)14 basis set, whose additional diffuse functions are more suitable for modeling the anionic character of the mechanisms. For steps involving KH, diffuse functions were added to the hydride hydrogens by using 6-31++G(d,p).14 Transition state structures were confirmed using the imaginary mode displacement method (used in Part I of this study).2 However, because of the redox nature of some of the reactions in this work, some transition states were able to relax to different minima with only minor differences in the scalar value, so these structures were more rigorously confirmed using the intrinsic reaction coordinate (IRC) method.15,16 Ball and stick molecular images (Figures S1−S8) and Cartesian geometries © 2013 American Chemical Society
Received: August 6, 2013 Revised: November 22, 2013 Published: November 25, 2013 13161
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Scheme 1. Reactions of Zn(η5-C5Me5)(η1-C5Me5) with KH/ZnCl2 (Top) and Zn(η5-C5H5)(η1-C5H5) with KH/ZnCl2 (Bottom)
Scheme 2. Reaction Scheme of 1 (R = Me) or 5 (R = H) Reacting with KH
On balance, the most likely scenario is that Step 1a prevails over Step 1b despite its higher barrier (ΔG‡ = 24.2 vs 14.7 kcal/mol, respectively). The reason for this is that the Me5CpH generated in Step 1a can react with an additional equivalent of KH to form Me5CpK and hydrogen gas (Step 2b), releasing 28.6 kcal/mol (with a nearly zero barrier), which can be thought of as the thermodynamic driving force for Step 1a. If 5 is used in place of 1, the reaction scheme (Scheme 2, Table 1) is analogous, but a new variable to the process is
Conversely, the complex could go through Step 1b to form Me5CpZnH and Me5CpK (Figure S2 (Supporting Information), ΔG‡ = 14.7 kcal/mol) with an energy benefit of −24.4 kcal/mol. Me5CpZnH could then react with an additional equivalent of KH to form the desired Me5CpZnK (Step 2d, Figure S3, Supporting Information), which would go through Step 2a to produce the final product 3. Although Step 2d is exergonic by −16.7 kcal/mol, its barrier is 39.1 kcal/mol. 13162
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Table 1. Reaction Gibbs Free Energies (in kcal/mol) of 1 and 5 Reacting with KH (Scheme 2) and with KH and ZnCl2 (Schemes 3 and 4) at 293 Ka step
ΔG (complex)
1a 1b 1c 2a 2b 2c 2d 2e 2f 3a 3b
−22.2 −22.2 n/a 0.0 29.4 −1.9 −16.8 −27.9 −2.3 0.0 −9.1
(−30.0) (−30.0) (−3.1) (0.0) (19.9) (1.6) (−18.9) (−28.2) (−0.1) (0.0) (1.3)
ΔGf‡ 24.2 14.7 n/a 37.4 −0.7 14.7 39.1 11.1 34.2 0.0 2.5
(37.8) (11.1) (6.6) (35.2) (0.1) (3.7) (39.4) (17.7) (34.0) (0.0) (2.6)
ΔG (‡activation) 2.0 −7.5 n/a 37.4 28.7 12.8 22.3 −16.8 31.9 0.0 −6.6
(7.8) (−18.9) (3.5) (35.2) (20.0) (5.3) (20.5) (−10.5) (33.9) (0.0) (3.9)
ΔGb‡ 12.2 16.9 n/a 0.0 57.3 11.1 39.0 17.1 19.9 38.5 2.8
(24.0) (12.2) (7.3) (0.0) (63.2) (6.3) (44.4) (15.3) (21.0) (44.5) (8.3)
ΔG (products) −10.2 −24.4 −3.6 37.4 −28.6 1.7 −16.7 −33.9 12.0 −38.5 −3.8
(−16.2) (−31.1) (−3.8) (35.2) (−43.2) (−1.0) (−23.9) (−25.8) (12.9) (−44.5) (−4.4)
ΔG (complex), ΔG (‡activation), and ΔG (products) are measured relative to separated reactants. ΔGf‡ and ΔGb‡ are the activation energies of the forward and backward reactions, respectively. Numerical values in the parentheses are for the reactions involving 5. Entries with n/a mean that the relevant structures were not found.
a
Scheme 3. Reaction Scheme for 1 (R = Me) or 5 (R = H) Reacting with KH and Then ZnCl2
Although theoretically the zincate salt 8 could form when 1 is reacted with KH (Scheme 2), the steric interactions between the methyl groups cause the formation of 8 to be endergonic (ΔG = 1.7 kcal/mol) with a much higher activation barrier (ΔG‡ = 14.7 kcal/mol), making its formation unfavorable both thermodynamically and kinetically. Figure S4 (Supporting Information) shows the geometries for zincate formation from 5 and 1. 3.2. Reactions of 1 and 5 with KH and ZnCl2. 3.2.1. Reactions of 1 and 5 with KH and then ZnCl2. Our
introduced. Step 1a has an even higher barrier relative to Step 1b (ΔG‡ = 37.8 vs 11.1 kcal/mol, respectively), and the resulting CpK product can react with more 5 (Step 2c) to form a zincate salt, 9. The thermodynamic stability of 9 (ΔG = −1.0 kcal/mol) does not exactly make it a strong driving force, but considering that Step 1a has a prohibitive barrier in this case (ΔG‡ = 37.8 kcal/mol) and the formation of 9 has a barrier of just 3.7 kcal/mol, it is understandable that when 5 reacts with KH, the major product will be 9 as opposed to dizincocene, 6. 13163
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Scheme 4. Reaction Scheme for 1 (R = Me) or 5 (R = H) Reacting with ZnCl2 and Then KH
Nonetheless, it is worthwhile to point out that, while Step 1c requires little activation energy, it yields only a very small thermodynamic benefit (ΔG = −3.6 kcal/mol), and the reaction will not produce any significant amount of 3 or 6 without the involvement of KH. Of course, once KH is introduced into the system, the product of Step 1c, Me5CpZnCl, can certainly react with KH to form either Me5CpZnK or Me5CpZnH, and curiously, no transition state was found in this process. Theoretically, Steps 1a and 1b could be bypassed through this alternative, lowenergy-cost route, Step 1c, with a pre-equilibrium established among all reactants and the intermediates produced through Steps 1a, 1b, 1c, 1a′, and 1b′. However, at this point, the system is back to the beginning of all subsequent steps (Steps 2a−3b) in Schemes 2 and 3; in the end, the essential conclusion stays unchanged.
proposed formation route of 3 from 1 and KH is fairly straightforward to understand, but it still involves free radicals; radical processes are difficult to control, and yields can be inconsistent.1−7 The addition of one equivalent of ZnCl2 into the reaction mixture does not change the basic reaction scheme all that much since Steps 1a, 1b, 2b, and 2c remain, but there is one major difference: the radical dissociation and reassociation steps (Steps 2a and 3a in Scheme 2) become highly unfavorable and are bypassed by the reaction of Me5CpZnK with ZnCl2 to form Me5CpZn2Cl and KCl (Step 2e, Scheme 3). The complex before Step 2e (Figure S5, Supporting Information) is extremely stable (ΔG = −27.9 kcal/mol), and Step 2e overall is exergonic by −33.9 kcal/mol (Table 1), but most importantly, the activation barrier is a mere 11.1 kcal/mol. The resulting Me5CpZn2Cl can react with Me5CpK from Step 1b to form 3 and KCl (Step 3b, Figure S6, Supporting Information). This final step is slightly exergonic (ΔG = −3.8 kcal/mol) with a barrier of 2.5 kcal/mol. Although this final ΔG value (−3.8 kcal/mol) for Step 3b may seem low, it should be noted that most of the thermodynamic benefits of forming 3 is already realized in the preceding steps. Conversely, via Step 2f (Figure S7, Supporting Information), the desired Me5CpZn2Cl could also form by reacting ZnCl2 with Me5CpZnH (one of the products of Step 1b), but Step 2f carries a barrier of 34.2 kcal/ mol and the reaction is endergonic (ΔG = 12.0 kcal/mol), so the formation of 3 is unlikely to proceed through this route. It is still theoretically possible to form 6 from 5 since Steps 2e (ΔG = −25.8 kcal/mol) and 3b (ΔG = −4.4 kcal/mol) are relatively exergonic (Scheme 3). However, the formation of CpZnK via Step 1a is quite prohibitive (ΔG‡ = 37.8 kcal/mol) since ZnCl2 in this proposed reaction mechanism does not affect neither Step 1a nor 1b, so it is not surprising that 9 remains the dominate product. 3.2.2. Reactions of 1 and 5 with ZnCl2 and then KH. Though 1 reacts with ZnCl2 and KH in a 1:1:2 stoichiometric ratio (and Scheme 3 assumes that KH reacts with 1 first), the possibility of 1 reacting with ZnCl2 initially to form two equivalents of Me5CpZnCl was investigated as Step 1c (Scheme 4, Table 1). In retrospect, the results regarding Step 1c are consistent with the conclusion of Part 1 of this study,2 where various ZnR2 reagents were employed to react with 1 and 5. Therefore, there is no need to fully repeat the same study here.
4. CONCLUSIONS It appears that the formation of 3 can be achieved much more efficiently through the use of KH and ZnCl2. Although 3 can be formed without the use of ZnCl2, the use of the latter provides a much more favorable reaction pathway, makes the contribution of the free radical steps negligible, and therefore creates a more stable (and controllable) reaction. The formation of 6 is not competitive because the production of the necessary precursor, CpZnK (Step 1a), is not favorable with a very high activation barrier, ΔG‡ = 37.8 kcal/mol.
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ASSOCIATED CONTENT
* Supporting Information S
Ball and stick molecular images and Cartesian geometries for selected 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
Corresponding Author
*(Y.A.W.) E-mail:
[email protected]. Tel: 1-604-8226773. Notes
The authors declare no competing financial interest. 13164
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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 (2006− 2008). Westgrid and CFI are thanked for providing computational resources. Dr. Roman V. Krems is also thanked for allowing the generous use of his group’s computational resources. Finally, Dr. Laurel L. Schafer is thanked for her expertise and input regarding the mechanistic possibilities for this study.
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