Oxorhenium Complexes for Catalytic Hydrosilylation and Hydrolytic

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Laboratory Experiment pubs.acs.org/jchemeduc

Oxorhenium Complexes for Catalytic Hydrosilylation and Hydrolytic Hydrogen Production: A Multiweek Advanced Laboratory Experiment for Undergraduate Students A. Ison,* E. A. Ison, and C. M. Perry Department of Chemistry, North Carolina State University, 2620 Yarborough Drive, Raleigh, North Carolina 27695-8204, United States S Supporting Information *

ABSTRACT: An effective way of teaching undergraduates a full complement of research skills is through a multiweek advanced laboratory experiment. Here we outline a comprehensive set of experiments adapted from current primary literature focusing on organic and inorganic synthesis, catalysis, reactivity, and reaction kinetics. The catalyst, bis(2(2′-hydroxyphenyl)-2-oxazoline)oxorhenium(V) tetrapentafluorophenylborate (1) is isolated through a multistep reaction starting with the formation of the ligand, 2-(2′hydroxyphenyl)-2-oxazoline (2), followed by complexation of a Re−oxo precursor to form chlorobis(2-(2′-hydroxyphenyl)-2-oxazoline)oxorhenium(V) (3). Both Re(V)−oxo complexes are diamagnetic and allow for NMR analysis. Complex 1 is an air-stable and highly active catalyst for two reactions: (1) hydrosilylation of carbonyls and (2) hydrolysis of Et3SiH to form Et3SiOH and H2 gas. Students monitor the evolution of hydrogen gas in the second reaction and use the data to investigate the reaction kinetics in order to obtain the complete rate law and the second-order rate constant for the catalytic reaction. The complete project provides a wealth of opportunities to focus on experimental skills, fundamental concepts in inorganic and organic chemistry, catalysis, reactivity, and experimental kinetics. KEYWORDS: Upper-Division Undergraduate, Laboratory Instruction, Synthesis, Catalysis, Kinetics, NMR Spectroscopy, Organic Chemistry, Inorganic Chemistry, Communication/Writing

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INTRODUCTION There has been much interest in the development of methods for teaching undergraduates critical research skills, as evidenced by a number of related publications in this Journal.1−13 In addition, the ACS Committee on Professional Training14 includes the following as critical skills for B.S. Chemistry graduates: • Problem-solving skills • Knowledge of the chemical literature • Laboratory safety • Communication • Teamwork • Ethics An effective way to teach many of these skills involves the use of multiweek advanced laboratory experiments, where many of these competencies are emphasized. This report outlines a multistep advanced laboratory experiment that is adapted from current primary literature15−17 and focuses on organic and inorganic synthesis, catalysis, and reaction kinetics. The project involves the synthesis of two Re(V)−oxo complexes (Figure 1), where complex 1 is an active catalyst in the hydrosilylation of carbonyls (Scheme 1) and the hydrolysis of Et3SiH to form Et3SiOH and H2 gas (Scheme 2). The hydrosilylation reaction catalyzed by 1 presented in this experiment (Scheme 1) is an example of the reduction of carbonyls to form silyl-protected alcohols or silyl ethers.15,18 Hydrosilylation of olefins is a related process that is very © XXXX American Chemical Society and Division of Chemical Education, Inc.

Figure 1. Re(V)−oxo complexes synthesized by students.

Scheme 1. Hydrosilylation of Carbonyls Catalyzed by 1

important in the industrial synthesis of organosilicon compounds and silicon-based polymers.18 The hydrolysis of Et3SiH catalyzed by 1 (Scheme 2) is an interesting reaction in two respects. First, the catalytic Received: December 11, 2016 Revised: March 23, 2017

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Scheme 2. Hydrolysis of Triethylsilane Catalyzed by 1

Scheme 4. Synthesis of Re−Oxo Complexes 1 and 3

formation of Et3SiOH is an example of selective formation of silanols, which are relevant to the industrial synthesis of silicon polymers.19 Second, the reaction features the production of H2 gas on demand from a liquid fuel (in this case Et3SiH). This reaction has been highlighted in Scientif ic American as a potential means of generating hydrogen gas on demand for use in transportation.20 Although Et3SiH may not ultimately be a viable economic choice for fuel, the reaction represents an innovative strategy for overcoming the hazards associated with the use of hydrogen gas as an on-board fuel in a motor vehicle. This project enables students to accomplish the following: • Develop competence using organic and inorganic synthetic techniques • Interpret complex NMR spectra • Study isomerism, symmetry, and bonding in coordination complexes • Study the catalytic reactivity of a coordination complex for two different reactions • Use pseudo-first-order methods to measure rate constants and determine the complete rate law for a reaction • Develop teamwork skills through data sharing • Develop scientific communication skills by reading primary literature and writing an ACS-style journal article.

loadings of 0.5−2 mol %. Students monitored the reaction kinetics by use of a eudiometer to measure the volume of hydrogen gas evolved as a function of time.

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HAZARDS Eye protection and gloves should be worn at all times. All of the reactions must be done in a fume hood. Thionyl chloride is corrosive to skin and causes serious eye damage through contact with liquid. Thionyl chloride is also toxic by oral, skin, and exhalation exposure. It reacts violently with water to form toxic and corrosive fumes (SO2 and HCl). Ethanolamine (2aminoethanol) is a combustible liquid (vapor) and is harmful if absorbed through the skin or inhaled. Ethyl salicylate is a skin, eye, and lung irritant. Additional information on hazards of the solvents and other reagents is included in the Supporting Information.

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RESULTS AND DISCUSSION At North Carolina State University, we offer two Advanced Synthetic Techniques courses as a two-semester sequence of laboratory courses designed for juniors and seniors. The courses are designed to emphasize both organic and inorganic synthetic skills and thus allow for the exploration of multiweek projects that span a range of chemistry subdisciplines (organic, inorganic, physical) and the development of student independence in the laboratory, chemical literacy, and scientific communication skills. This experiment has been implemented in the second-semester Advanced Synthetic Techniques course. The project in its entirety requires 5−6 weeks for completion. Our advanced lab course meets once per week for 5.5 h per lab meeting. However, the reactions forming compounds 1, 2, and 3 can be completed within 3−4 h or left overnight for workup the next lab meeting. The catalytic reactions are all complete within 1−3 h. Over three semesters, a total of 41 students completed the synthesis of compounds 1, 2, and 3 as well as hydrolysis of triethylsilane. A total of 15 students completed the hydrosilylation reaction. Student data and outcomes are discussed below.

LABORATORY OVERVIEW The overall goal of the project is to synthesize and characterize two Re(V)−oxo complexes and then study the two reactions shown in Schemes 1 and 2 that are catalyzed by bis(2-(2′hydroxyphenyl)-2-oxazoline)oxorhenium(V) tetrapentafluorophenylborate (1). The project begins with a two-step synthesis of the ligand, 2-(2′-hydroxyphenyl)-2-oxazoline (2) (Scheme 3). The catalyst is also formed in two steps. Chlorobis(2-(2′hydroxyphenyl)-2-oxazoline)oxorhenium(V) (3) is isolated first, and the subsequent abstraction of chloride from 3 results in the formation of the active catalyst 1 (Scheme 4). The hydrosilylation reaction (Scheme 1) was performed without any additional solvent with a 0.1 mol % catalyst loading. Students were able to sample the reaction mixture directly and monitor the reaction progress by 1H NMR spectroscopy. Students observed a range of reactivities for a variety of carbonyl substrates with observed relative rates of benzaldehyde > acetophenone > acetone. The hydrolysis of Et3SiH (Scheme 2) was performed in acetonitrile with catalyst Scheme 3. Ligand Synthesis

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Ligand and Complex Syntheses

the catalyst and the low catalyst loadings used in the reaction allow for analysis of the reaction products by preparation of NMR samples directly from the reaction mixture without workup (see the S10−S14 in the Supporting Information). Students can easily sample the reaction at various reaction times in order to compare the reactivities of the different carbonyl substrates, as some substrates react quantitatively within 30 min and others are only partially reacted even after 3 h of reaction. The rate data then lead to a discussion of substrate reactivity.

The two-step synthesis of the ligand (Scheme 3) is straightforward and highlights important organic chemistry reactions such as nucleophilic substitution (step 1) and the use of the chlorinating agent SOCl2 and intramolecular cyclization (step 2). Typical student yields of compound 2 are 48−97%. The isolated product is clean and easily characterized by 1H and 13 C NMR spectroscopy (see the Supporting Information). The facile synthesis of complexes 1 and 3 also yields products of high purity as determined by 1H NMR spectroscopy. Typical student yields of compounds 1 and 3 are 49−93% and 53− 76%, respectively. Both Re(V)−oxo complexes are diamagnetic and are amenable to 1H and 13C NMR analysis. The spectroscopic characteristics of the two Re(V)−oxo complexes provide a challenging and educational component to the experiment. Although both complexes bear two oxazoline ligands, the symmetry of the complexes changes in going from complex 3, where there is no axis of symmetry, to complex 1, where a C2 axis of rotation through the axial oxo and acetonitrile ligands is present (Figure 1).21,22 The analysis of the 1H and 13C spectra provides direct evidence for the change in symmetry (see the Supporting Information).

Catalyzed Hydrolytic Hydrogen Production

Catalyst 1 is also active in the hydrolysis of silanes to form silanol and hydrogen gas (Scheme 2). The rate of hydrolysis of triethylsilane was investigated in this experiment; however, other silanes are active as well and can be utilized as deemed appropriate by the instructor.15 Under the reaction conditions, Et3SiH is quantitatively converted to Et3SiOH at room temperature within 1−1.5 h (see the Supporting Information). The production of hydrogen gas provides a simple and convenient means of monitoring the reaction kinetics using a simple eudiometer (see the Supporting Information). Students performed kinetic studies under pseudo-first-order conditions with water as the excess reagent. Students worked in groups of two or three so that each group varied the initial concentrations of water and/or catalyst. The volume of hydrogen gas evolution over time was monitored and recorded at 5 min intervals. In our experience, the process of plotting and interpreting kinetic data poses a significant challenge for most students. Prior to asking students to evaluate their own data, the basics of solution-phase kinetic analysis were reviewed and students worked through sample data to give a better understanding of how to deal with pseudo-first-order kinetic data. The first step of the kinetic analysis involves expressing the rate of the reaction as a function of reactant disappearance and product formation. As illustrated in Scheme 2, the stoichiometric ratio of reactants and products is 1:1, so the reaction rate can be expressed as shown in eq 1:

Catalytic Hydrosilylation

Catalyst 1 is highly active in the hydrosilylation of carbonyls (Scheme 1) at low catalyst loadings (0.1 mol %). The catalyst is also recyclable, although the recovery of small amounts of material is often difficult (∼15 mg of 1).15 The results obtained by students are shown in Table 1. The diamagnetic nature of Table 1. Student Dataa for the Hydrosilylation of Carbonyls

rate = − a

b

Based on the results for 15 students. 1.0 mmol of carbonyl, 1.5 equiv of Et3SiH, 0.1 mol % catalyst 1, 25 °C. cDetermined by 1H NMR spectroscopy; reported values are averages of three independent runs (with standard deviations in parentheses). d% conversion after 30 min. e % conversion after 3 h.

d[Et3SiH] d[H 2] = dt dt

(1)

Therefore, the rate of hydrogen gas production is equal to the rate of reaction. The generic rate law for the reaction can also be written as shown in eq 2: rate = k[Et3SiH]m [H 2O]n [1]p

(2)

Figure 2. Graphical determination of the order with respect to Et3SiH using linear fits. [1] = 4.27 mM, [Et3SiH]0 = 0.357 M, [H2O]0 = 3.17 M, solvent CH3CN, T = 21 °C. The standard deviation of kobs is given in parentheses. C

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where the superscripts m, n, and p are the orders with respect to the two reactants and catalyst 1, respectively, and must be determined experimentally. Under pseudo-first-order reaction conditions where water is used in excess, we can make the assumption that the initial concentration of H2O does not change significantly over the reaction time. The catalyst concentration is also assumed to remain constant since the catalyst is regenerated with each catalytic cycle and is not consumed. These two assumptions allow for simplification of the rate law by defining an observed rate constant (kobs) as shown in eqs 3 and 4: m

rate = kobs[Et3SiH] n

kobs = k[H 2O] [1]

p

rate = kobs[Et3SiH]

The order with respect to H2O was determined by varying [H2O]i while keeping the Et3SiH and catalyst concentrations constant. Students observed that varying the concentration of water from 3.17 to 6.34 M had no effect on the pseudo-firstorder rate constant, and therefore, the order with respect to water is zero (see the Supporting Information). The pseudofirst-order rate constant is therefore only dependent on [1] raised to some power, as shown in eq 7: kobs = k[1]p

(3)

(7)

These data are consistent with results from the primary literature.15 Finally, the order with respect to catalyst 1 can be determined on the basis of eq 7 by plotting kobs as a function of [1]. As shown in Figure 4 this plot produces a straight line,

(4)

The order with respect to Et3SiH in eq 3 can now be determined by fitting the hydrogen evolution data to characteristic linear forms of the integrated rate equations for zeroth-, first-, and second-order reactions (Figure 2). The reaction is determined to be first-order with respect to Et3SiH since only the first-order plot results in a linear relationship. Although pseudo-first-order rate constants (kobs) can be obtained by generating first-order linear fits of all of the data, we chose to also show students the benefits of using nonlinear fits of the first-order integrated rate law (eq 5): [H 2]t = [Et3SiH]0 (1 − e−kobst )

(6)

(5)

The benefit of nonlinear fitting is that all of the collected data in each run can be fit to the equation, whereas data from the linear plot often show curvature at times greater than three half-lives so some data points must be eliminated from the fit. The linear fit of the data shown in Figure 2b includes data to only three half-lives, whereas the nonlinear fit from the same experiment (Figure 3) includes data up to five half-lives. The average

Figure 4. Plot of kobs as a function of [1]. Conditions; [1] = 1.71−6.85 mM, [Et3SiH]0 = 0.357 M, [H2O]0 = 3.17−6.35 M, solvent CH3CN, T = 21 °C.

which suggests that the reaction is first order with respect to the catalyst, yielding the overall rate law rate = k[Et3SiH][1]

(8)

The slope of the plot gives the second-order rate constant as k = 0.25(3) M−1 s−1.

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CONCLUSIONS The benefits of this comprehensive set of experiments were observed in the substantial development of a number of important research skills by the students, such as • Improvement of experimental skills, i.e., the ability to handle and synthesize transition metal complexes and perform catalytic reactions • Ability to analyze and interpret complex NMR spectra and other spectroscopic data for both transition metal complexes and organic ligands • Ability to access and utilize the primary literature to plan experiments and design synthetic procedures • Ability to compile synthetic and reactivity data • Ability to communicate results using formal scientific writing standards Student success in all of these areas was assessed by evaluation of the quality of data and using a detailed rubric to evaluate the journal articles prepared by the students (see the Supporting Information).

Figure 3. Nonlinear first-order fit of hydrogen gas production. [1] = 4.27 mM, [Et3SiH]0 = 0.357 M, [H2O]0 = 3.17 M, solvent CH3CN, T = 21 °C. The data were fit to the exponential rise equation y = m1 + m2(1 − e−kobsx). The standard deviation of kobs is given in parentheses.

difference between the kobs values obtained from the linear and nonlinear fits was on the order of ca. 20%, with the nonlinear fits giving consistently higher kobs values compared with the linear fits (see the Supporting Information). Although nonlinear fitting is preferred if software is available, the correct orders and consistent kobs values can be obtained from simple linear fits as well. The pseudo-first-order rate law consistent with the data in Figures 2 and 3 is shown in eq 6: D

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Donor Ligand: An Integrated Research Experiment for Undergraduate Students. J. Chem. Educ. 2015, 92, 2140−2145. (7) Albin, T. J.; Fry, M. M.; Murphy, A. R. Synthesis, Characterization, and Secondary Structure Determination of a Silk-Inspired, Self-Assembling Peptide: A Laboratory Exercise for Organic and Biochemistry Courses. J. Chem. Educ. 2014, 91, 1981−1984. (8) Streu, C. N.; Reif, R. D.; Neiles, K. Y.; Schech, A. J.; Mertz, P. S. Drug Synthesis and Analysis on a Dime: A Capstone Medicinal Chemistry Experience for the Undergraduate Biochemistry Laboratory. J. Chem. Educ. 2016, 93, 2084−2088. (9) Tsarevsky, N. V.; Woodruff, S. R.; Wisian-Neilson, P. J. An Undergraduate Chemistry Laboratory: Synthesis of Well-Defined Polymers by Low-Catalyst-Concentration ATRP and Postpolymerization Modification to Fluorescent Materials. J. Chem. Educ. 2016, 93, 1452−1459. (10) Monga, V.; Bussière, G.; Crichton, P.; Daswani, S. Synthesis and Decomposition Kinetic Studies ff Bis(Lutidine)Silver(I) Nitrate Complexes as an Interdisciplinary Undergraduate Chemistry Experiment. J. Chem. Educ. 2016, 93, 958−962. (11) Locock, K.; Tran, H.; Codd, R.; Allan, R. Hands-On Approach To Structure Activity Relationships: The Synthesis, Testing, and Hansch Analysis of a Series of Acetylcholineesterase Inhibitors. J. Chem. Educ. 2015, 92, 1745−1750. (12) Ison, E. A.; Ison, A. Synthesis of Well-Defined Copper NHeterocyclic Carbene Complexes and Their Use as Catalysts for a “Click Reaction”: A Multistep Experiment That Emphasizes the Role of Catalysis in Green Chemistry. J. Chem. Educ. 2012, 89, 1575−1577. (13) Vasquez, T. E., Jr; Saldaña, C.; Muzikar, K. A.; Mashek, D.; Liu, J. M. Searching for Synthetic Antimicrobial Peptides: An Experiment for Organic Chemistry Students. J. Chem. Educ. 2016, 93, 1103−1107. (14) Undergraduate Professional Education in Chemistry: ACS Guidelines and Evaluation Procedures for Bachelor’s Degree Programs, 2015. https://www.acs.org/content/dam/acsorg/about/ governance/committees/training/2015-acs-guidelines-for-bachelorsdegree-programs.pdf (accessed March 2017). (15) Ison, E. A.; Trivedi, E. R.; Corbin, R. A.; Abu-Omar, M. M. Mechanism for Reduction Catalysis by Metal Oxo: Hydrosilation of Organic Carbonyl Groups Catalyzed by a Rhenium(V) Oxo Complex. J. Am. Chem. Soc. 2005, 127, 15374−15375. (16) Ison, E. A.; Corbin, R. A.; Abu-Omar, M. M. Hydrogen Production from Hydrolytic Oxidation of Organosilanes Using a Cationic Oxorhenium Catalyst. J. Am. Chem. Soc. 2005, 127, 11938− 11939. (17) Corbin, R. A.; Ison, E. A.; Abu-Omar, M. M. Catalysis by Cationic Oxorhenium(V): Hydrolysis and Alcoholysis of Organic Silanes. Dalton Trans. 2009, 2850−2855. (18) Hydrosilylation: A Comprehensive Review on Recent Advances; Marciniec, B., Ed.; Advances in Silicon Science, Vol. 1; Springer Science & Business Media: Dordrecht, The Netherlands, 2009. (19) Jeon, M.; Han, J.; Park, J. Catalytic Synthesis of Silanols from Hydrosilanes and Applications. ACS Catal. 2012, 2, 1539−1549. (20) Graham, S. New Catalyst Produces Hydrogen from Water (Scientific American, 2005). http://www.scientificamerican.com/ article/new-catalyst-produces-hyd/ (accessed March 2017). (21) Schachner, J. r. A.; Terfassa, B.; Peschel, L. M.; Zwettler, N.; Belaj, F.; Cias, P.; Gescheidt, G.; Mösch-Zanetti, N. C. Oxorhenium (V) Complexes with Phenolate−Oxazoline Ligands: Influence of the Isomeric Form on the O-Atom-Transfer Reactivity. Inorg. Chem. 2014, 53, 12918−12928. (22) Liu, J.; Wu, D.; Su, X.; Han, M.; Kimura, S. Y.; Gray, D. L.; Shapley, J. R.; Abu-Omar, M. M.; Werth, C. J.; Strathmann, T. J. Configuration Control in the Synthesis of Homo- and Heteroleptic Bis(oxazolinylphenolato/thiazolinylphenolato) Chelate Ligand Complexes of Oxorhenium (V): Isomer Effect on Ancillary Ligand Exchange Dynamics and Implications for Perchlorate Reduction Catalysis. Inorg. Chem. 2016, 55, 2597−2611.

All of the students were able to synthesize and isolate compounds 1, 2, and 3 with high purity (by NMR) in good to excellent yields (vide supra). The kinetic results obtained by pooling data from individual groups performing the experiment under a variety of conditions were consistently in agreement and correlated well with each other. The second-order rate constant (k) obtained by plotting kobs values measured by eight different groups (Figure 4) was found to be 0.25 M−1 s−1 (at 21 °C) with a low standard deviation of 0.03 M−1 s−1. In addition, the percent error of the experimental second-order rate constant obtained from student data is 4% compared with the published value of k = 0.24 M−1 s−1 at 21 °C (calculated from activation parameters).15 Therefore, the experimental data are not only precise across multiple groups of students but also accurate compared to previously published data.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00954. Student handouts, experimental procedures, instructor information, and spectra (PDF, DOCX)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

A. Ison: 0000-0001-9622-4372 E. A. Ison: 0000-0002-2902-2671 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the Department of Chemistry at North Carolina State University for support. We also thank our undergraduate students for their contributions to the development of this experiment.

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