Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX
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Ammonia Borane Acid−Base Complex: An Experiment Involving 1H and 11B NMR Spectroscopy and the Gas Laws Navamoney Arulsamy* Department of Chemistry, University of Wyoming, Dept. 3838, 1000 East University Avenue, Laramie, Wyoming 82071, United States
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S Supporting Information *
ABSTRACT: An experiment involving a moisture-sensitive synthesis and spectroscopic characterization suitable for the advanced inorganic laboratory course is presented. The acid−base complex ammonia borane (AB) is synthesized from the heterogeneous reaction between sodium borohydride and finely ground ammonium sulfate in tetrahydrofuran solvent. AB is isolated as a fluffy white solid. The 1H and 11B NMR spectra of AB, separately measured in the solvents D2O and CD3CN, provide an excellent opportunity to introduce some of the basic concepts of multinuclear NMR spectroscopy. The triplet and quartet peaks observed for the NH3 and BH3 groups in the 1H spectrum measured in CD3CN can be used to discuss the nature of JNH and JBH coupling. The former coupling is rarely observed because the quadrupolar 14N in nonspherical structures experiences rapid relaxation and large J coupling, leading to broad peaks for −NH and −NH2 protons. Although such coupling due to 11B is more commonly observed, a comparison of the 1H NMR spectra of AB and NaBH4 further illustrates the effect of spherical symmetry on the NMR behavior of quadrupolar nuclei. AB is also valued in the field of energy research as a potential chemical H2 storage material and has assumed new significance as a potential portable energy source. The efficacy of H2 generation from AB is examined, and the purity of student-synthesized samples is determined by applying the gas laws. The experiment can be performed in two 3-hour lab periods. KEYWORDS: Upper-Division Undergraduate, Inorganic Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Gases, Hydrogen Bonding, Instrumental Methods, IR and NMR Spectroscopy, Lewis Acids/Bases
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INTRODUCTION The three major theories of acids and bases, namely, the Arrhenius, Brønsted-Lowry, and Lewis theories, are taught in chronological order in general chemistry courses.1 The Lewis acid−base definition stands out from the rest by its farreaching scope in explaining a number of closely related concepts and offering a much broader description of acids and bases. The Lewis acid−base theory defines an acid as a substance that accepts an electron pair and a base as a substance that donates an electron pair. The acid−base reaction, therefore, is readily understood as an exchange of an electron pair between two substances. This satisfactorily explains the acidic and basic properties of both protic and aprotic compounds. The Lewis concept also lays the foundation for an understanding of coordination chemistry, as it explains why an electron-deficient metal ion (Lewis acid) and electrondonating neutral molecules or anionic ligands (Lewis bases) would form stable compounds. Although the importance of emphasizing Lewis acid−base theory in the undergraduate chemistry curriculum is recognized,2,3 only two experiments involving simple adducts formed from molecular Lewis acids and bases are available for the undergraduate laboratory.4,5 One of them describes the adduct formed between alkylamines and toxic boron trifluoride, whose synthesis requires a vacuum line,4 and the other describes a similar adduct synthesized from tert-butylamine hydrochloride and sodium borohydride.5 The latter experiment is suitable for sophomore-level students and © XXXX American Chemical Society and Division of Chemical Education, Inc.
is limited to the synthesis, IR spectroscopic characterization, and computational analysis of the adduct. Recognizing the value of a laboratory experiment that would illustrate the Lewis acid−base theory while also introducing some of the basics of multinuclear NMR spectroscopy, we have developed a new experiment suitable for the advanced inorganic chemistry laboratory. We have chosen the classic Lewis acid−base complex ammonia borane (AB) for this experiment. AB is stable, nontoxic, and readily synthesized. Undergraduate texts often employ the example of the ammonia−boron trifluoride adduct (NH3BF3) while discussing the Lewis acid−base theory (e.g., p 818 in ref 1), and we believe that AB would be a better choice for a laboratory experiment as it can be prepared from less expensive, nontoxic reagents. AB is experiencing a resurgence of research interest because of its potential application as a chemical H2 storage material and solid H2 carrier.6 It has a higher hydrogen density than liquid H2. A general point of interest with respect to bonding is that although AB and ethane are isoelectronic, their melting points are significantly different (104 and −181 °C, respectively). Whereas AB is a solid, ethane is a gas. The bond between NH3 and BH3 is dative or coordinate, but the Received: January 16, 2018 Revised: June 16, 2018
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DOI: 10.1021/acs.jchemed.8b00038 J. Chem. Educ. XXXX, XXX, XXX−XXX
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bond between the two C atoms in ethane is covalent. They are equally hydrogen-rich, but AB releases H2 gas more readily than ethane, and the higher reactivity of AB is attributed to the protic (H+) and hydridic (H−) natures of its NH3 and BH3 hydrogens, respectively. Interestingly, the two sets of H atoms are strongly H-bonded with short intermolecular N−H···H−B distances. The H···H distance of 2.02(3) Å in AB is much shorter than the van der Waals distance of 2.4 Å, and the strong interaction is explained as dihydrogen bonding.7 Although the molecule is quite stable under ambient conditions, it can be decomposed catalytically in acidic aqueous solutions. The decomposition is quantitative, with each molecule of AB releasing three molecules of H2. In fact, the decomposition reaction is utilized to estimate the purity of AB.8 We employ the gas laws to verify the stoichiometry of the decomposition reaction, which also serves to reinforce student understanding of the fundamental concept.
The synthesis must be carried out in a well-ventilated fume hood. Oven-dried glassware and maintenance of the temperature at 40 °C are critically required for both high yield and purity of the product. In our lab, a hot water bath maintained at 38−40 °C on a stirring hot plate is used. The reaction is ventilated through a mineral oil bubbler. The progress of the reaction is observed by the steady release of H2 gas through the bubbler. The reaction is stopped when the gas release subsides (1 h), and the mixture is filtered using a Celite-padded filter. Oven-dried filter funnels and filter flasks are used. The filtrate is rotary-evaporated to dryness with strict exclusion of water. Even drops of water inadvertently admitted into the filtrate lead to sticky and impure product. Although the reaction between NaBH4 and (NH4)2SO4 powder does not go to completion in the 1 h reaction time, the unreacted reagents can be filtered off because both reagents are poorly soluble in THF whereas AB is freely soluble. The samples are dried in a vacuum oven at room temperature for 30 min. AB is saved in screw-cap scintillation vials and shows no evidence of decomposition for several days. Student yields are in the range of 40−60%. The isolation of pure AB requires the use of (1) oven-dried glassware, (2) f inely ground ammonium sulfate and dry, powdery, f ree-flowing NaBH4 reagents, and (3) a crack-free Celite-padded f ilter to filter of f unreacted reagents. While the reaction is in progress, we measure the 1H and 11B NMR spectra of NaBH4. Basified D2O solvent (0.005 N in NaOH) containing 4,4-dimethyl-4-silapentane-1-sulfonic acid (DSS) (0.05% w/v) is used as the solvent. The 11B spectrum is referenced externally to BF3·Et2O. Typically, the spectra are measured by the instructor as a demonstration for the students. A brief introduction to multinuclear NMR spectroscopy, in particular 1H and 11B NMR spectroscopy, is given while the spectra are collected. Complete details of the synthetic procedure are given in the Supporting Information. In the second laboratory period, the 1H and 11B NMR spectra for the student-synthesized AB samples are measured by students with assistance from the instructor. D2O containing DSS (0.05% w/v) is used by half of the class, while the other half use acetonitrile-d3 as the solvent. Since the two spectra can be measured for the same solution, it usually takes less than 30 min for each group, and there is sufficient time for the six or seven samples generated in the lab. Students also measure IR spectra for their products as neat powders using an FTIR spectrometer equipped with an attenuated total reflectance (ATR) accessory. In addition, students are asked to estimate the purity of their product by the acid-catalyzed decomposition of AB in aqueous solution (eq 2):
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LEARNING GOALS The experiment is designed with three pedagogic goals of student learning. The primary goal is to demonstrate the usefulness of multinuclear NMR spectroscopy in the characterization of inorganic materials. The other goals are to train students in carrying out moisture-sensitive synthetic reactions with dry reagents and oven-dried glassware and to apply the principles of the gas laws in determining the purity of their product samples. Specifically, the laboratory introduces the basic principles of 1H and 11B NMR spectroscopy,9 including factors that influence J coupling by quadrupolar nuclei, and also provides an opportunity to apply the gas laws (usually taught in the first-year undergraduate general chemistry courses, e.g., p 207 in ref 1) in a laboratory setting. Upon completion of the experiment, students will be able to choose the appropriate parameter files for the measurement of 1 H and 11B NMR spectra and recognize J coupling by quadrupolar nuclei in the spectra measured, and they will also gain a clear understanding of the natural occurrence of the two boron isotopes (10B and 11B) and its impact on the 11B NMR spectra of boron compounds. Our advanced inorganic laboratory course includes 10 experiments, each of which is performed in two 3 h lab periods per week throughout the semester. The present experiment is offered in the first half of the semester with the intended purpose of familiarizing students with the basic concepts of NMR spectroscopy. We allow students work in pairs but require them to write independent lab reports. The synthesis of AB and the measurement of 1H and 11B NMR spectra for sodium borohydride are performed in the first lab period, and the IR and NMR characterization of the product and estimation of its purity are carried out in the second lab period.
BH3NH3 + H+ + 3H 2O → NH4 + + B(OH)3 + 3H 2 (2)
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Details are given in the Supporting Information.
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EXPERIMENTAL OVERVIEW In the first 3 h lab period, the students are asked to synthesize AB by the procedure well-optimized by Ramachandran and Gagare.8 The reaction between finely powdered ammonium sulfate and sodium borohydride in tetrahydrofuran (THF) solvent at 40 °C forms NH3BH3 with the release of H2 gas (eq 1):
HAZARDS Gloves and eye protection must be worn all the time. THF is highly flammable and must be stored in tightly closed containers to avoid the formation of explosive peroxides. It is not toxic but is capable of penetrating the skin even through acrylonitrile gloves. In the event that it is accidentally spilled on the skin, the affected area must be washed with plenty of water. Sodium borohydride is also flammable. It forms toxic gases when suspended in organic solvents such as THF, so it must be handled in well-ventilated fume hood. Hydrogen gas is
40° C
(NH4)2 SO4 + 2NaBH4 ⎯⎯⎯⎯→ 2BH3NH3 + Na 2SO4 + 2H 2 THF
(1) B
DOI: 10.1021/acs.jchemed.8b00038 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Figure 1. Experimental setup for the acid-catalyzed decomposition of AB and collection of H2 gas.
isolated as a fluffy white powder by rotary evaporation of the filtrate. Students measure IR spectra for their products. The product samples are further characterized by 1H and 11B NMR spectra. The IR spectrum for pure AB exhibits two sets of strong absorptions at ca. 3310, 3250, and 3200 cm−1 and 2320, 2280, and 2210 cm−1 due to N−H and B−H stretching vibrations, respectively (Figure 1 in the instructor’s notes). The spectrum also contains two additional pairs of peaks at ca. 1600 and 1370 cm−1 and 1150 and 1050 cm−1 due to the deformation vibrations of the two bonds.10 The spectra of the product samples synthesized in the lab, however, may contain additional peaks due to THF and water impurities as well as any AB decomposition products. Further evidence for contamination can be observed in their 1H and 11B NMR spectra. Whereas pure samples of AB dissolve completely in acetonitrile-d3, some of the student samples do not dissolve completely because of the presence of NaBH4 contaminant. The filtration of the reaction mixture on a Celite-padded filter is a critical step, as unreacted reagents can slip into the filtrate through cracks in the Celite pad, leading to impure products. Both pure and impure samples of AB dissolve in D2O completely, but those containing NaBH4 form gas bubbles, possibly as a result of decomposition of the contaminant. The
highly flammable, and the second part of the experiment must also be performed in the fume hood. Boron trifluoride etherate is toxic and corrosive. An NMR tube containing boron trifluoride etherate in CDCl3 (15% v/v) must be prepared and flame-sealed by the instructor. BF3·Et2O can be absorbed through skin contact and breathing. If skin contact occurs, the affected area must be washed with water immediately, and the lab should be vacated to a well-ventilated area. Storage of impure AB samples in closed containers and desiccators poses a major explosion hazard, as they can decompose, releasing H2 gas and building pressure. Therefore, student-generated AB samples must be destroyed in acidic water at the completion of the experiment. The AB samples can be safely decomposed in the fume hood by slow addition of ca. 0.1 g portions with stirring to ca. 1 L of 0.1 N aqueous H2SO4 in a beaker.
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RESULTS AND DISCUSSION The synthesis of ammonia borane is a straightforward metathesis reaction between ammonium sulfate and sodium borohydride (eq 1) using oven-dried glassware.8 The use of excess ammonium sulfate leads to near complete consumption of NaBH4, and the poor solubility of the reagents in THF ensures the formation of pure AB. Unreacted reagents are filtered off using a Celite-padded fritted funnel, and AB is C
DOI: 10.1021/acs.jchemed.8b00038 J. Chem. Educ. XXXX, XXX, XXX−XXX
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materials are handled through the synthesis of AB. The molecule synthesized is special: AB is the simplest example of an acid−base complex. Though structurally similar to ethane gas, it differs in its bonding and is one of the first examples of dihydrogen bonding. Despite the presence of very few peaks, the NMR spectra are very valuable in introducing concepts such as the frequently observed H/D exchange of the −NH and −OD protons, the J coupling between 11B and 1H nuclei, and the rarely observed J coupling between 14N and 1H nuclei. The acid-catalyzed decomposition of AB with the quantitative release of 3 equiv of H2 provides an excellent opportunity to apply the gas laws, which the students would have studied in first-year general chemistry. Finally, the lab also highlights AB’s potential application as a chemical storage material for H2, directly introducing one of the challenges of current chemical research.
NMR spectra measured in the two solvents are helpful to explain the occurrence of rapid H/D exchange between the NH3 group and D2O solvent.11 The 1H NMR spectrum measured in D2O (Figure 2 in the instructor’s notes) exhibits a quartet at ca. 1.3 ppm with an approximately 1:1:1:1 intensity ratio due to the BH3 hydrogens, which is readily explained to arise from the J coupling between 1H and 11B (with nuclear spin I = 3/2) nuclei. The spectrum measured in acetonitrile-d3 also exhibits a well-resolved triplet at 3.55 ppm due to the NH3 protons in addition to the quartet at 1.3 ppm due to the BH3 group. The triplet arises from the J coupling of the NH3 protons with the 14N nucleus (I = 1). Such 14N−1H couplings are rarely observed because of the significantly larger J coupling constants and very fast relaxation of the quadrupolar 14N nuclei (see p 269 in ref 9). In AB, however, the near-spherical symmetry of the N atom results in a smaller coupling constant and longer relaxation time, leading to the observation of its J coupling with the 1H nuclei. Some of the spectra for the student samples also contain peaks due to THF. The impact of spherical symmetry on the relaxation time of quadrupolar nuclei can also be seen in the 1H spectrum of NaBH4, as described in a laboratory experiment previously published in this Journal.12 The four equivalent hydrogens in the tetrahedral BH4− anion exhibit an overlapping pattern of quartet and septet peaks. The relative intensities of the peaks at ca. 4:1 are consistent with the isotopic abundance percentages of 80.1 and 19.9% for 11B and 10B, respectively, and the splitting patterns are consistent with their nuclear spins (3/2 and 3, respectively). The 11B spectra measured in the two solvents are similar and exhibit a well-resolved quartet due to the BH3 boron (Figure 3 in the instructor’s notes). The quartet is readily explained to arise from the J coupling of the three H atoms of the BH3 group with the 11B nucleus. The intensity ratio of the quartet is attributed to the 1:3:3:1 ratio expected from Pascal’s triangle (see p 243 in ref 9). Some of the student spectra (measured in D2O) also contain a weak quintet at ca. −0.12 ppm due to NaBH4 impurity. As students take turns measuring the NMR spectra, they will have sufficient time to study the decomposition of their AB samples in acidic aqueous solutions. As a prelab exercise, the students are asked to determine the amount of analytically pure AB needed to obtain ca. 50 mL of H2 gas. A gas receiver assembly as shown in Figure 1 is set up for the collection of gases released during the decomposition reaction. The decomposition proceeds rapidly,13 and the progress of the reaction and its eventual completion (in less than 2 min) are observed by means of the formation of gas bubbles in the water-filled buret. H2 gas is nearly insoluble in water, and therefore, the gas collected is assumed to be H2. We observe the release of 40−49 mL from 0.67 mmol of studentsynthesized AB samples. Students note the barometric pressure on the lab barometer14 and apply the gas laws to determine the exact molar amount of H2 gas released from their sample. Subsequently, they compare the calculated volume to the measured volume and determine the percent purity of their sample. The purity of the AB samples is generally observed to be in the range of 90−98%.
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ASSESSMENT Anhydrous NaBH4 and finely powdered NH4SO4 are used in the preparation of AB, and pure products are obtained only when the reaction is carried out with the strict exclusion of moisture. We typically observe that one or two of the six pairs of student groups experience problems with the synthesis and obtain poor yields or sticky impure products. The use of coarse ammonium sulfate reagent leads to poor yield, and even traces of moisture in the glassware or cracks in the Celite-padded filter allow unreacted NaBH4 to slip into the filtrate, resulting in impure product. Such errors in lab technique can be readily seen in the IR and NMR spectra and can be used to assess student proficiency in laboratory techniques. The simplicity of the IR spectrum, particularly the uniqueness of the B−H stretching vibrations, is helpful in illustrating the usefulness of IR spectroscopy. Similarly, the NMR spectra contain either one or two peaks due to the compound. However, the splitting pattern observed requires a thorough understanding of J coupling by quadrupolar nuclei. We observed that after we began offering this experiment, students performed significantly better in regard to their ability to interpret more complex NMR spectra for organometallic compounds that contain multiple nuclei such as 1H, 31P, 13C, and Ag (107Ag and 109 Ag). Although this is a general observation, we are confident that this experiment is greatly useful in the teaching of basic concepts of NMR spectroscopy. The experiment can be valuable even if the NMR spectra cannot be measured. The spectra provided in the instructor’s notes can be used to demonstrate all of the highlighted concepts. The experiment seamlessly integrates two other fundamental concepts of chemistry, namely, the Lewis acid−base theory and the gas laws. Students are able to see the usefulness of the Lewis structures of BH3 and AB in explaining their relative reactivity. They are also excited to recall and apply the gas laws. While preparing for the lab and answering the prelab questions, they often ask what particular gas law can be used, and they quickly figure out that the Boyle’s law is suitable. Some students do miss the correction needed for the partial vapor pressure of water (Dalton’s law of partial pressure). Nevertheless, they learn and appreciate the practical value of the gas laws. Students can be asked to compare the data from the acidic decomposition experiment with the IR spectra (and NMR spectra when measured) and to comment on the purity of their AB samples. Similarly, they can also be asked to compare the 11 B NMR spectra for NaBH4 and AB in their lab reports. Our
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SUMMARY The ammonia borane experiment presented here consists of several components suitable for the undergraduate lab. Students are taught how dry glassware and hygroscopic D
DOI: 10.1021/acs.jchemed.8b00038 J. Chem. Educ. XXXX, XXX, XXX−XXX
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(7) Klooster, W. T.; Koetzle, T. F.; Siegbahn, P. E. M.; Richardson, R. B.; Crabtree, R. H. Study of the N-H···H-B Dihydrogen Bond Including the Crystal Structure of BH3NH3 by Neutron Diffraction. J. Am. Chem. Soc. 1999, 121 (27), 6337−6343. (8) Ramachandran, P. V.; Gagare, P. D. Preparation of Ammonia Borane in High Yield and Purity, Methanolysis, and Regeneration. Inorg. Chem. 2007, 46 (19), 7810−7817. (9) Drago, R. S. Physical Methods for Chemists, 2nd ed.; Surfside Scientific Publishers: Gainesville, FL, 1992; pp 211−289. (10) Smith, J.; Seshadri, K. S.; White, D. Infra-red Spectra of Matrix Isolated BH3-NH3, BD3-ND3 and BH3-ND3. J. Mol. Spectrosc. 1973, 45 (3), 327−337. (11) Hu, M. G.; Van Paasschen, J. M.; Geanangel, R. A. New Synthetic Approaches to Ammonia-Borane and Its Deuterated Derivatives. J. Inorg. Nucl. Chem. 1977, 39 (12), 2147−2150. (12) Zanger, M.; Moyna, G. Measurement of the Isotopic Ratio of 10 B/11B in NaBH4 by 1H NMR. J. Chem. Educ. 2005, 82 (9), 1390− 1392. (13) Kelly, H. C.; Marriott, V. B. Re-examination of the AcidCatalyzed Amine-Borane Hydrolysis. The Hydrolysis of NH3·BH3. Inorg. Chem. 1979, 18 (10), 2875−2878. (14) If a lab barometer is not available, atmospheric pressure could be taken as 1 atm or data available on the Internet for a nearby location at the time of the experiment could be used in the calculation to obtain an approximate purity value.
experience shows that these exercises lead students to recognize the less than spherically symmetric geometry of the NH3 protons of AB and the attendant broadness of the peak due to the NH3 protons in the spectrum measured in acetonitrile-d3. Student understanding of the basics of NMR spectroscopy and interpretation of spectra can be judged from their lab reports. Proficiency in lab technique is generally reflected in percent yield and purity of their products. Therefore, students can be graded on the basis of (i) percent yield and purity of products isolated by each of the groups; (ii) discussion of IR and NMR spectral data, which includes assignment of peaks and explanation of all of the important spectral features; and (iii) how accurately they have applied the gas laws in determining the purity of their AB samples.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00038. Student handout including detailed procedures for the synthesis and measurement of spectra (PDF, DOCX) Notes for instructors including hazards, desired experimental conditions, interpretation of spectra, and selected NMR spectra and assessment of how the experiment improved student learning (PDF, DOCX)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Navamoney Arulsamy: 0000-0001-5274-0906 Notes
The author declares no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the University of Wyoming in the form of a Faculty Grant-In Aid and the Wyoming NASA Space Grant Consortium (NASA Grant NNX15AI08H) is gratefully acknowledged. I am also grateful for the feedback from students of the CHEM 4100 advanced inorganic chemistry course over the past three semesters.
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REFERENCES
(1) Silberberg, M. S. Chemistry: The Molecular Nature of Matter and Change, 5th ed.; McGraw-Hill Higher Education: New York, 2009; Chapters 5 and 18. (2) Shaffer, A. A. Let Us Give Lewis Acid−Base Theory the Priority It Deserves. J. Chem. Educ. 2006, 83 (12), 1746−1749. (3) Paik, S.-H. Understanding the Relationship Among Arrhenius, Brønsted-Lowry, and Lewis Theories. J. Chem. Educ. 2015, 92 (9), 1484−1489. (4) Girolami, G. S.; Rauchfuss, T. B.; Angelici, R. J. Synthesis and Technique in Inorganic Chemistry, 3rd ed.; University Science Books: Sausalito, CA, 1999; pp 47−53. (5) Nataro, C.; Ferguson, M. A.; Bocage, K. M.; Hess, B. J.; Ross, V. J.; Swarr, D. T. Lewis Acid−Base, Molecular Modeling, and Isotopic Labeling in a Sophomore Inorganic Chemistry Laboratory. J. Chem. Educ. 2004, 81 (5), 722−724. (6) Stephens, F. H.; Pons, V.; Baker, R. T. Ammonia-borane: The hydrogen source par excellence. Dalton Trans. 2007, 2613−2626. E
DOI: 10.1021/acs.jchemed.8b00038 J. Chem. Educ. XXXX, XXX, XXX−XXX
