A Mechanistic Study of the Migratory Insertion Reaction: A

Laboratory Experiment pubs.acs.org/jchemeduc

A Mechanistic Study of the Migratory Insertion Reaction: A Computational Chemistry Exercise Craig D. Montgomery* Department of Chemistry, Trinity Western University, 7600 Glover Road, Langley, British Columbia, Canada, V2Y 1Y1 S Supporting Information *

ABSTRACT: An exercise in computational chemistry is presented whereby the student models the mechanism of migratory insertion. In the case of migratory insertion of CO into the Mn−CH3 bond of [Mn(CH3)(CO)5], semiempirical calculations (PM3) are used to evaluate the concerted mechanism of methyl migration, followed by coordination of CO. The three-centered transition state is determined along with the activation energy, calculated to be 131 kJ/mol. The 16-electron intermediate [Mn(C(O)CH3)(CO)4] is suggested to display an agostic Mn−H interaction as has been postulated previously. The exercise is appropriate as a take-home assignment in a course on inorganic or organometallic chemistry.

KEYWORDS: Upper-Division Undergraduate, Inorganic Chemistry, Laboratory Instruction, Computer-Based Learning, Hands-On Learning/Manipulatives, Coordination Compounds, IR Spectroscopy, Mechanisms of Reactions, Molecular Modeling, Organometallics

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igratory insertion reactions are of immense importance in organometallic chemistry1 and are involved in a variety of catalytic processes, both homogeneous and heterogeneous. For example, hydrogenation by Wilkinson’s catalyst requires the insertion of the olefin into the Rh−H bond, as does hydroformylation. The Monsanto process involves the insertion of CO into a Rh−CH3 bond, and the heterogeneous process of Ziegler−Natta polymerization involves the insertion of the olefin into a Ti−alkyl bond.1 Insight into the mechanism is gained by considering the thermal reaction of [Mn(CH3)(CO)5] and 13CO as in Scheme 1, a case of 1,1-migratory insertion (where, in the product, both

An alternative mechanism would involve migration of the CO (rather than the methyl group) and its insertion into the Mn−CH3 bond in step 1. However, it has been found that the insertion of CO into cis-[Mn(CH3)(13CO)(CO)4] results in a product mixture containing cis and trans products in a 2:1 ratio (as well as [Mn(13C(O)CH3)(CO)5]), as shown in Scheme 2. The presence of trans product eliminates the possibility of intramolecular CO insertion as the operative mechanism, but is consistent with the mechanism of intramolecular methyl migration.2,4 A final experiment supportive of the methyl migration mechanism (versus the intramolecular CO insertion mecha-

Scheme 1. Migratory Insertion of CO in [Mn(CH3)(CO)5]

Scheme 2. Migratory Insertion of CO in cis[Mn(CH3)(13CO)(CO)4]

the Mn and CH3 groups are bound to the same atom of the inserting CO ligand). The observation that the incoming 13CO ligand is not directly inserted into the Mn−CH3 bond but rather occupies a position cis to the newly formed acyl moiety suggests that the mechanism proceeds by intramolecular migration of the CH3 ligand onto a CO ligand in a cis position (step 1), followed by coordination of the labeled 13CO in the position vacated by the methyl group (step 2).2 This methyl migration likely proceeds in a concerted manner and results in an intermediate that features an agostic Mn−H interaction.3 © 2013 American Chemical Society and Division of Chemical Education, Inc.

Published: August 19, 2013 1396

dx.doi.org/10.1021/ed300526k | J. Chem. Educ. 2013, 90, 1396−1400

Journal of Chemical Education

Laboratory Experiment

nism) involves the use of the chiral Fe(II) complex, CpFe(Et)(PPh3)(CO), as the starting material as illustrated in Scheme 3. In this case insertion of CO proceeds with inversion at the Fe(II) center.5 This is again consistent with the mechanism of alkyl migration. Scheme 3. Migratory Insertion of CO in Chiral Metal Complexes

Computational chemistry is increasingly finding application in chemical education as a means of wedding the theory of lecture material with the practical nature of laboratory experimentation.6−31 The purpose of this exercise is to model the mechanism of migratory insertion involving intramolecular migration of the methyl group, followed by coordination of CO. A number of such studies3,32 have been undertaken using DFT calculations; however, such calculations are not practical for an undergraduate exercise, due to the time required (see discussion accompanying Table 1 below). Herein the mechanism is modeled using semiempirical (PM3) calculations. The exercise is appropriate as a take-home assignment in a course on inorganic or organometallic chemistry. The use of computational chemistry can allow students to better visualize the mechanism and deepen their understanding of the process of migratory insertion. Indeed that has been the experience of students who have done this assignment; specifically, using the calculations to construct an energy profile diagram was cited as a significant aid to understanding the mechanism.

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2. Students open the [Mn(CH3)(CO)5] file and rename it as the “Transition State”. The “Guess Transition State” command in Spartan Student (or similar commands in other software) is used to draw a reaction arrow moving the electron pair in the Mn−CH3 bond to the carbon atom of a CO ligand in a cis position. The transition state is calculated using a semiempirical calculation (PM3), including a calculation of the IR spectrum. 3. From the IR spectrum, students animate the atomic motion that would bring about the transition state from the reactant molecule [Mn(CH3)(CO)5]. 4. Students note the energy of each molecule (in kJ/mol) and record the data in a table. 5. From the results, students calculate the activation energy and draw a reaction profile. A figure of the transition state should be included and discussed, and the atomic motions necessary to move from the reactant molecule to the transition state should be identified. Likewise a figure of the intermediate [Mn(C(O)Me)(CO)4] should be included and discussed.

HAZARDS There are no hazards associated with this lab. RESULTS AND DISCUSSION It is always important when employing computational chemistry as a means of illustrating chemical principles that one first determine the accuracy of the models. Table 1 displays Table 1. Comparison of Calculated and Experimental Bond Lengths for [Mn(CH3)(CO)5] Bond Distance in [MnCH3(CO)5]/Å

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PROCEDURE The software employed here is Spartan Student 5.0.0.33 Although it is assumed that the students have a basic understanding of the software and of computational chemistry, more detailed step-by-step instructions are provided in the Supporting Information. It may be possible to employ other computational software; however, the method and step-by-step instructions may have to be adapted, particularly with regard to the determination of the transition state. The exercise can be utilized as a take-home assignment, where the students are asked to calculate the various models along with their energies, construct an energy profile diagram for the reaction, and determine the activation energy as well as the likely transition state and intermediate in the mechanism. The time taken by students (who are somewhat familiar with Spartan) to build the models and perform the calculations is approximately one hour. The steps are as follows: 1. Students build the following molecules and then optimized, first with molecular mechanics, then by semiempirical methods (PM3): [Mn(CH3)(CO)5], [Mn(C(O)CH3)(CO)4], [Mn(C(O)CH3)(CO)5], [Mn(CO)5]+, CO, and CH3−. The final PM3 optimization should include the calculation of molecular orbitals and atomic charges. For purposes of comparison, the calculations are repeated for [Mn(CH3)(CO)5] using DFT (B3LYP-6-31G*). Students save the models and output of the calculations in each case.

Bond Mn− CH3 Mn− COcis Mn− COtrans C−O

Model Calculation (PM3)

Model Calculation DFT (B3LYP/ 6-31G*)

Lit. Model Calculation DFT (B3B86/split valence)3

Experimental34

2.129

2.191

2.17

2.20

1.860−1.863

1.852−1.855

1.84

1.85

1.826

1.829

1.83

1.83

1.157 (cis) 1.166 (trans)

1.148 (cis) 1.152 (trans)

1.15

1.16 (cis) 1.17 (trans)

a comparison of some selected bond lengths for the reactant complex [Mn(CH3)(CO)5] obtained in this study by both semiempirical (PM3) and DFT (B3LYP with 6-31G* basis set) with previous DFT theoretical studies 3,32 as well as experimental values.34 The differences between the semiempirical calculated values and the experimental values range from 0.004 Å in the case of the Mn−COtrans bond length to 0.071 Å in the case of the Mn−CH3 bond. When DFT (B3LYP with 6-31G*) calculations were employed in this study, the error in these calculated values decreased to 0.001 Å and 0.009 Å for the Mn−COtrans and Mn−CH3 bonds, respectively. Although it is clear that more accurate values are obtained by utilizing DFT calculations, nevertheless, this also results in an unnecessary increase in the time required for the exercise. For example, starting from a model previously optimized by molecular mechanics, the CPU time required to optimize the final product [Mn(C(O)CH 3)(CO)5] by semiempirical 1397




Figure 2. These results suggest the activation energy for the three-centered concerted mechanism to be 131 kJ/mol. This compares to an experimental value of 119.3 kJ/mol for ΔG⧧.32,35 Previous DFT calculations resulted in a calculated value of 106 kJ/mol for ΔG⧧.32 Alternately one could propose a dissociative mechanism whereby the Mn−CH3 bond is broken and the methyl reattaches on the cis carbonyl ligand. The activation energy for such a mechanism can be estimated by comparing the energies of the [Mn(CH3)(CO)5] model with that of the [Mn(CO)5]+ and CH3− models; the activation energy in that case would be 1033 kJ/mol. Comparison of these two values calculated here for the activation energy also supports the three-centered concerted mechanism. Of course, the stereochemistry observed in Scheme 1 also eliminates the possibility of the dissociative mechanism, in that such a mechanism would be expected to result in some trans product. It is also of interest to consider the intermediate [Mn(C(O)CH3)(CO)4] as it is a 16-electron species. A number of structures have been postulated previously for this proposed intermediate, including structures displaying an η1-acyl ligand, an η2-acyl ligand, and an η1-acyl ligand featuring an agostic Mn−H interaction, as in Figure 3.

methods was 0.21 s (actual time of approximately 1−2 s) compared with 0.79 s (actual time of approximately 28 min) when DFT methods are employed. Furthermore the activation energy calculated herein as well as the transition state (as reported below) compare very favorably with previous theoretical studies as well as experimental work. Therefore it seems that the semiempirical calculations employed in this study are sufficient for the purposes of an undergraduate exercise. The calculated transition state is shown in Figure 1. From the animations of atomic motion, two vibrational modes are

Figure 1. The transition state for the migratory insertion reaction [Mn(Me)(CO)5] + CO → [Mn(C(O)Me)(CO)5].

identified that are necessary for the molecule to adopt the transition state structure. Obviously the carbon atoms of the methyl ligand and one of the cis carbonyl ligands must move together via a C−Mn−C bending mode, and the methyl group also needs to rotate. The energies of the various models are included in Table 2 and are then used to construct the reaction profile shown in

Figure 3. Proposed structures for the intermediate [Mn(C(O)CH3)(CO)4].

Previous theoretical studies3,32 had proposed the likelihood of the agostic structure for the intermediate, and indeed, when the [Mn(C(O)CH3)(CO)4] model was optimized in this study using PM3 methods, such an agostic interaction was observed (see Figure 4). The Mn−H distance was calculated to be 2.29 Å

Table 2. Calculated (PM3) Energies for the Molecular Models Compound

Energy /(kJ/mol)

[Mn(CH3)(CO)5] CO [Mn(CH3)(CO)5] transition state [Mn(CO)5]+ CH3− [Mn(C(O)CH3)(CO)4] [Mn(C(O)CH3)(CO)5]

−1513.75 −82.61 −1382.68 −695.77 +215.52 −1427.73 −1718.93

Figure 4. Structure of the intermediate [Mn(C(O)CH3)(CO)4] consistent with an agostic Mn−H interaction.

by DFT calculations previously3 and 1.938 Å herein using PM3 calculations. DFT calculations yielded a Cmethyl−Ccarbonyl−Mn bond angle of 83°3 whereas the PM3 calculations in this study yielded a value of 84.80°.

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CONCLUSIONS The computational chemistry exercise described herein uses PM3 calculations to consider the mechanism of migratory insertion. Using the example of [Mn(CH3)(CO)5], the exercise demonstrates that the mechanism is likely a concerted one,

Figure 2. Reaction profile for [MnMe(CO)5] + CO → [Mn(C(O)Me)(CO)5]. 1398




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proceeding by migration of the CH3 ligand to a cis-CO via a three-centered transition state, rather than a dissociative one. The activation energy was calculated to be 131 kJ/mol. The structure of the [Mn(C(O)CH3)(CO)4] intermediate features an agostic Mn−H interaction, as has been suggested previously. Students are readily able to perform the calculations and the exercise and find it helpful in visualizing the mechanism of migratory insertion and confirming the viability of such a mechanism.

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ASSOCIATED CONTENT

S Supporting Information *

A student handout is provided that includes detailed step-bystep instructions. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.D.M. acknowledges the Institute of Chemistry at the Hebrew University of Jerusalem for a Visiting Professorship and is grateful to Trinity Western University and the Natural Sciences and Engineering Research Council of Canada for financial support.

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REFERENCES

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A Mechanistic Study of the Migratory Insertion Reaction: A

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