Parahydrogen-Induced Polarization in the Study of Rhodium(I

Laboratory Experiment pubs.acs.org/jchemeduc

Parahydrogen-Induced Polarization in the Study of Rhodium(I)Catalyzed Alkyne Hydrogenation: An Organometallic Undergraduate Laboratory Experiment Meaghan M. Sebeika† and Daniel J. Fox*,‡ Department of Chemistry and Biochemistry, Suffolk University, Boston, Massachusetts 02114, United States S Supporting Information *

ABSTRACT: This upper-level undergraduate chemistry experiment involves the rhodium(I)-catalyzed hydrogenation of phenylacetylene to styrene using two different catalysts, which result in two different mechanisms of hydrogenation. Students were able to determine which of two mechanisms corresponded to two different hydrogenation catalysts using parahydrogen-induced polarization (PHIP), a technique rarely studied in the undergraduate laboratory, and a more traditional kinetic study that followed the uptake of hydrogen gas over time. This experiment also exposes students to the use of Schlenk lines, high-pressure gases, and cryogenic liquids, all of which are important tools in the organometallic chemist’s toolbox. KEYWORDS: Upper-Division Undergraduate, Laboratory Instruction, Inorganic Chemistry, Organic Chemistry, Organometallics, Catalysis, Kinetics, Gases, NMR Spectroscopy, Mechanisms of Reactions

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These reactions can be studied by monitoring the uptake of hydrogen2 or by the observation of PHIP in the 1H NMR spectrum of the product.1 When Rh(PPh3)3Cl (1; also called Wilkinson’s catalyst) is used, the oxidative addition of hydrogen to the rhodium center occurs reversibly in the first step of the catalytic cycle (Scheme 1); when a cationic catalyst, such as [Rh(COD)(PPh3)2]+PF6− (2; COD = cyclooctadiene), is used, the oxidative addition of hydrogen to the rhodium center occurs irreversibly in the second step of the catalytic cycle (Scheme 2).3 A traditional way to study a reaction mechanism involving a gas, such as hydrogen, is to determine the rate of reaction by monitoring the uptake of that gas;2 however, this general method does not always give definitive information about the mechanism of the reaction because rate data are not always sufficient for distinguishing between two or more possible mechanisms. In this work, PHIP was chosen for students to use as an alternative method for the determination of a reaction mechanism involving the pairwise addition of hydrogen to an unsaturated organic substrate.1,4,5 PHIP is a useful technique for the study of reaction mechanisms involving reaction with molecular hydrogen in that it can result in large signal enhancements in the 1H NMR spectrum; this large signal enhancement allows for the observation of species formed in very low concentrations and also provides evidence for pairwise addition of the hydrogen molecule in which the two hydrogen atoms that have added originated in the same H2 molecule.

he determination of reaction mechanisms is a key process for many organic and organometallic chemists, and in many cases more than one technique must be used to elucidate the mechanism of a particular reaction. This undergraduate experiment highlights two different techniques that can be used in mechanistic studies involving the hydrogenation of unsaturated organic substrates. The first technique involves monitoring the consumption of a gaseous reactant (in this case hydrogen) over the course of a reaction, a more traditional technique commonly used in kinetics studies of reactions where one of the reactants is a gas. The second technique takes advantage of parahydrogen-induced polarization (PHIP), a phenomenon that can be seen in the NMR spectrum of a hydrogenation product and is an extremely powerful tool for the study of reactions involving the addition of molecular hydrogen. Rhodium(I)-catalyzed hydrogenation reactions lend themselves well to the use of both techniques;1 in addition, these types of reactions are well suited for demonstrating the use of a Schlenk line, which is an important skill for work in organic, organometallic, and inorganic chemistry. One of two different mechanisms is operative in the hydrogenation of unsaturated organic substrates with a homogeneous catalyst. One such reaction is the hydrogenation of phenylacetylene to styrene using a homogeneous rhodium(I) catalyst (eq 1).

Published: August 15, 2013 © 2013 American Chemical Society and Division of Chemical Education, Inc.

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dx.doi.org/10.1021/ed300775h | J. Chem. Educ. 2013, 90, 1239−1243

Journal of Chemical Education

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Laboratory Experiment

EXPERIMENTAL OVERVIEW During the first of two weeks, half of the students perform the hydrogen-uptake experiment while the other half perform the PHIP experiment; during the second week, the roles are reversed so that all students have experience using both techniques. In the hydrogen-uptake experiment, students prepare samples in a round-bottom flask and connect it to a buret−separatory funnel apparatus similar to the one described by Landgrebe (Figure 1).2a Students prepare solutions

Scheme 1. The Mechanism of the Hydrogenation of Phenylacetylene Catalyzed by 1a

a

Note that the mechanism involves reversible oxidative addition of hydrogen as the first step.

Scheme 2. The Mechanism of the Hydrogenation of Phenylacetylene Catalyzed by 2a

Figure 1. Photograph showing the apparatus for monitoring the uptake of hydrogen gas over time.

containing the catalyst, purge the system with hydrogen, and, after recording an initial volume measurement, inject the phenylacetylene substrate into the solution and monitor the change of hydrogen volume every minute over the course of one hour. In the PHIP experiment, students use a Schlenk line to prepare parahydrogen. After preparing solutions containing the catalyst and substrate in a J. Young NMR tube, students degas the solutions using the freeze−pump−thaw method, after which parahydrogen is transferred into the J. Young NMR tube. After thawing the prepared samples, the tubes are shaken vigorously to dissolve parahydrogen in solution and single-scan 1 H NMR spectra are acquired (a 60 MHz Anasazi Eft-60 NMR spectrometer was used) in rapid succession to observe PHIP effects in the styrene product. Both parts of the experiment can be completed in approximately two-and-a-half hours. Further details of the experiment may be found in the Supporting Information.

a

Note that the mechanism involves irreversible oxidative addition of hydrogen in the second step (S = solvent).

Additionally, the magnitude and persistence of the PHIP effect can be used in the determination of a reaction mechanism; larger polarization effects are indicative of a fast rate of reaction, and the length of time during which the polarization persists is indicative of the reacting hydrogen remaining enriched in the para spin state. In addition to demonstrating methods often used in mechanistic investigations, this experiment also gives students valuable experience with techniques that may not be encountered as part of the undergraduate curriculum at smaller institutions, including the use of Schlenk lines, high-pressure gases, and cryogenic liquids.

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HAZARDS Acetone (CAS 67-64-1) and phenylacetylene (CAS 536-74-3) are volatile and flammable, and they are skin and eye irritants; safety glasses and gloves must be worn. Liquid nitrogen (CAS 7727-37-9) is extremely cold and must be handled carefully in 1240




of hydrogen consumed over time as collected by students; the concentration of hydrogen in solution was assumed to be constant as a result of rapid stirring under constant pressure.) Plots of the concentration of phenylacetylene versus time also did not always yield linear plots (Figure 3; the amount of

order to avoid burns; safety glasses must be worn. Compressed hydrogen gas (CAS 1333-74-0) is flammable and potentially explosive. Wilkinson’s catalyst (1; CAS 14694-95-2) and (1,5cyclooctadiene)bis(triphenylphosphine)rhodium(I) hexafluorophosphate dichloromethane complex (2; CAS 35238-97-2) are skin and eye irritants; safety glasses and gloves must be worn. Additionally, it should be noted that hydrogen at a pressure of 1 atm at liquid nitrogen temperatures (77 K) will increase to a pressure close to 3 atm when warmed to ambient temperature such that the J. Young NMR tubes containing reaction samples will be highly pressurized when they are warmed to room temperature.

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RESULTS AND DISCUSSION

Logistics

In both parts of this experiment, a total of 19 students (from two different advanced laboratory courses: transition metals and physical chemistry laboratories) worked in eight groups of two to three students, with each group using one of the two catalysts. Each group exchanged data for both parts of the experiment with a group that had used the other catalyst. In this way, each group was able to compare data collected for the two catalysts using each of the two methods to reach conclusions about the rates of reaction and mechanisms.

Figure 3. Student data showing the change in concentration of phenylacetylene over time using 1 and 2 as catalysts; k = 0.0062 mol/ (L·min) with 1 and k = 0.0074 mol/(L·min) with 2 at 25 °C.

hydrogen consumed was used to calculate the concentration of phenylacetylene at each time point using the ideal gas law). Although a linear relationship between hydrogen consumed and time was observed in this experiment, indicating a reaction that is zero-order in alkyne, kinetic studies by Esteruelas et al.6 have demonstrated a first-order relationship between substrate concentration and time in a closely related reaction using a catalyst very similar to 2. However, in that case dichloromethane, which was used as the solvent, was implicated in the mechanism of the reaction. Although the hydrogen-uptake experiments did not allow for determination of which mechanism is operative for each catalyst, the experiments clearly allowed for the determination of reaction rates and the order of reaction with respect to the phenylacetylene substrate. Assuming that both reactions are zero order (as was seen in the development phase before students had performed the experiment), students determined that the rate of reaction using 2 was greater than the rate with 1 (representative data are shown in Figure 3). There were significant differences in the rate constant determinations when the experiment was run in two different classes, but for all but one group the rate of hydrogenation was determined to be faster using 2 as the catalyst. In the transition metals laboratory (CHEM L375), the determinations of k ranged from 0.0072 to 0.0075 mol/(L·min) using 1 as catalyst and from 0.0087 to 0.0091 mol/(L·min) using 2 as catalyst. In the physical chemistry laboratory (CHEM L411), the determinations of k ranged from 0.0062 to 0.0068 mol/(L·min) using 1 as catalyst and from 0.0042 to 0.0076 mol/(L·min) using 2 as catalyst.

Hydrogen-Uptake Experiments

The first part of this experiment gave students experience in following the progress of a chemical reaction through the consumption of a starting material (in this case, hydrogen gas) and determining the rate of the reaction. Students reviewed the use of the ideal gas law for the conversion of the volume of a gas (hydrogen, in this case) to moles and used this result to determine how the concentration of phenylacetylene changed over time. It was noted that there was confusion for some students about which volume to use in determining the phenylacetylene concentration; some students suggested that the volume of hydrogen consumed should be used, but upon further discussion during prelab lecture they determined that the volume of solvent (in the round-bottom flask containing acetone, catalyst, and phenylacetylene) should be used in calculating the concentration of the phenylacetylene substrate. The relationship between the volume of hydrogen consumed and time was found to be linear by the authors in developing this experiment. However, some of the data collected by students did not give linear plots. (Figure 2 shows the volume

Parahydrogen-Induced Polarization (PHIP)

After the differences in rates using catalysts 1 and 2 were investigated, parahydrogen-induced polarization was used by students to distinguish between the two reaction mechanisms by which hydrogenation of phenylacetylene can occur using these same catalysts.1 Students were instructed in the proper use of a vacuum line and a compressed gas cylinder and the method of degassing their NMR tube using the freeze−pump− thaw method before the introduction of parahydrogen to the

Figure 2. Student data showing the volume of hydrogen consumed over time using 1 and 2 as catalysts. 1241




tube. Students were also instructed in some details about the PHIP phenomenon, including the fact that the two hydrogen atoms that originated as parahydrogen (p-H2) must be magnetically inequivalent in the product.4,5,7 Although some data were inconsistent, students were able to observe the expected signal enhancement and polarization of PHIP4 in the 1H NMR spectra of their products formed as a result of pairwise addition of parahydrogen to phenylacetylene using both catalysts. NMR spectra were collected in rapid succession so that multiple spectra could be acquired before the polarization had abated.8 Once polarization had subsided, the NMR tube was removed from the spectrometer and shaken to dissolve unreacted p-H2 from the headspace of the tube; this part of the activity demonstrated the slow exchange of a gas into solution, even at high pressures, due to lack of mixing while the sample was in the NMR probe and a small area of liquid−gas interface. In the hydrogenations of phenylacetylene catalyzed by 1 and 2, the strength of the polarization and length of its duration allowed students to distinguish between reaction mechanisms involving reversible and irreversible oxidative addition of hydrogen to the rhodium(I) center (Schemes 1 and 2, respectively). The slower reaction rate using 1 as the catalyst resulted in weaker initial polarization due to the production of a smaller amount of product formed by addition of p-H2 to the substrate over a given period of time. Additionally, the reversible oxidative addition of hydrogen catalyzed the equilibration of the hydrogen spin states resulting in the loss of enrichment in the para spin state. As a result, the polarization was not as long-lived as in the reaction catalyzed by 2, which involved irreversible oxidative addition.4,5,7 Students observed that the initial polarization when using 2 was significantly stronger than with 1 (see Figures 4 and 5 for samples of student

Figure 5. Student-acquired 1H NMR spectra showing the loss of polarization over time in styrene produced using 2 to catalyze the hydrogenation of phenylacetylene (TMS at +0.28 ppm).

polarization was stronger and longer-lived when using compound 2 as the catalyst.

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CONCLUSIONS By monitoring the consumption of hydrogen gas, students were able to determine the rate constants for the hydrogenation of phenylacetylene using two different homogeneous rhodium(I) catalysts. Although the two catalysts could not be definitively correlated with the two possible mechanisms using this method, students were able to compare the rates of reaction with the two catalysts. In the PHIP reaction, students obtained spectral data that allowed them to determine which mechanism was operative for each catalyst by observing the intensity and duration of polarization. Several undergraduate experiments have been published that involve the measurement of hydrogen uptake to determine the rates of reactions, but to our knowledge there are no other experiments designed for undergraduate students that involve the use of parahydrogeninduced polarization. In performing these experiments, students gained valuable new experience using a Schlenk line and compressed gases, they gained additional hands-on experience using an NMR spectrometer, and they also were reminded of the usefulness of the ideal gas law in a common kinetics experiment. It is also notable that the PHIP experiment was possible using a 60 MHz NMR spectrometer, making it a useful technique for programs with many different levels of instrumentation in their teaching laboratories.

Figure 4. Student-acquired 1H NMR spectra showing the loss of polarization over time in styrene produced using 1 to catalyze the hydrogenation of phenylacetylene (TMS at −1.3 ppm).

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ASSOCIATED CONTENT

S Supporting Information *

data). Furthermore, the polarization when using 2 as catalyst persisted for a significantly longer time period. These observations led to students’ conclusions that irreversible oxidative addition of molecular hydrogen was consistent with the reaction when using 2 as a catalyst (Scheme 2). It was further noted that even though some students did not calibrate their NMR spectra correctly, they were still readily able to draw important conclusions from the NMR data, namely, that the

Notes for the instructor, including PHIP NMR data collected by the authors; student handout. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: dfox@suffolk.edu. 1242




Present Addresses †

D.J.F.: Boston College, 2609 Beacon Street, Chestnut Hill, MA 02467-3860; [email protected] ‡ M.M.S.: Northeastern University, 360 Huntington Avenue, Boston, MA 02115; [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge Suffolk University for financial support of this work. We thank Matthew Hamada, Anton Dubarry, and Martin Lyttle for earlier work developing our methodology for PHIP experiments at Suffolk; we thank Melanie Berkmen, Rachael Kipp, and Andrew Dutton for helpful comments and discussion of this work; we thank the students of CHEM L375 (Spring 2012) and CHEM L411 (Fall 2012) for piloting this experiment.

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REFERENCES

(1) Kirss, R. U.; Eisenberg, R. Para-Hydrogen-Induced Polarization in Rhodium Complex-Catalyzed Hydrogenation Reactions. J. Organomet. Chem. 1989, 359, C22−C26. (2) (a) Landgrebe, J. A. Selective and Quantitative Catalytic Hydrogenation: Safe, Inexpensive Experiment for Large Classes. J. Chem. Educ. 1995, 72, A220−A222. (b) Blanchard, D. E. Quantitative Microscale Hydrogenation of Vegetable Oils. J. Chem. Educ. 2003, 80, 544−546. (c) Kittredge, K. W.; Marine, S. S.; Taylor, R. T. Combinatorial Partial Hydrogenation Reactions of 4-Nitroacetophenone: An Undergraduate Organic Laboratory. J. Chem. Educ. 2004, 81, 1494−1496. (d) Amoa, K. Catalytic Hydrogenation of Maleic Acid at Moderate Pressures: A Laboratory Demonstration. J. Chem. Educ. 2007, 84, 1948−1950. (e) O’Conner, K. J.; Zuspan, K.; Berry, L. An Undergraduate Organic Chemistry Laboratory: The Facile Hydrogenation of Methyl trans-Cinnamate. J. Chem. Educ. 2011, 88, 325− 327. (3) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987; pp 529−537. (4) Eisenberg, R. Parahydrogen-Induced Polarization: A New Spin on Reactions with H2. Acc. Chem. Res. 1991, 24, 110−116. (5) Duckett, S. B.; Blazina, D. The Study of Inorganic Systems by NMR Spectroscopy in Conjunction with Parahydrogen-Induced Polarisation. Eur. J. Inorg. Chem. 2003, 2901−2912. (6) Esteruelas, M. A.; González, I.; Herrero, J.; Oro, L. A. Kinetic Studies on the Selective Hydrogenation of Phenylacetylene Catalyzed by [Rh(NBD)(PPh3)2]BF4 (NBD = 2,5-norbornadiene). J. Organomet. Chem. 1998, 551, 49−53. (7) Eisenberg, R.; Fox, D. Experimental Methods and Techniques: Parahydrogen Induced Polarization in Organometallic Chemistry. In Comprehensive Organometallic Chemistry III; Crabtree, R. H., Mingos, M. P., Eds.; Elsevier, Ltd.: Oxford, 2007; pp 429−450. (8) In PHIP experiments, once p-H2 has added to a substrate and an NMR spectrum is acquired, the nuclear spins of the hydrogen atoms relax rapidly and the spin correlation between the two atoms disappears, necessitating the acquisition of single-scan NMR spectra in rapid succession.

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Parahydrogen-Induced Polarization in the Study of Rhodium(I

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