A Multiweek Upper-Division Inorganic Laboratory Based on

Apr 29, 2013 - ... conduct a literature search, develop characterization parameters based on departmental resources, interpret experimental results an...
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Laboratory Experiment pubs.acs.org/jchemeduc

A Multiweek Upper-Division Inorganic Laboratory Based on Metallacrowns Brian J. Sirovetz, Nicole E. Walters, Collin N. Bender, Christopher M. Lenivy, Anna S. Troup, Daniel P. Predecki, John N. Richardson, and Curtis M. Zaleski* Department of Chemistry, Shippensburg University, Shippensburg, Pennsylvania 17257-2200, United States S Supporting Information *

ABSTRACT: Metallacrowns are a versatile class of inorganic compounds with uses in several areas of chemistry. Students engage in a multiweek, upperdivision inorganic laboratory that explores four different metallacrown compounds: Fe III (O 2 CCH 3 ) 3 [9-MC Fe I I I (N)shi -3](CH 3 OH) 3 ·3CH 3 OH, MnII(O2CCH3) 2[12-MCMnIII(N)shi-4](C3H7NO)6·2C3H7NO, [(CH3)4N]2{CuII[12-MCCuII(N)shi-4]}·C3H7NO, and MnII(O2CCH3)2[15-MCMnIII(N)shi-5](C5H5N)6·CH3OH. The syntheses are straightforward, and the compounds are characterized by cyclic voltammetry, solid-state magnetic susceptibility, FT-IR, UV−vis, and paramagnetically shifted 1H NMR. Students are provided with a research-like experience in which they can successfully synthesize a metallacrown, conduct a literature search, develop characterization parameters based on departmental resources, interpret experimental results and compare them to the primary literature, analyze the crystallographic structures of the compounds, and finally present an oral seminar detailing their results and analysis. KEYWORDS: Upper-Division Undergraduate, Inorganic Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Coordination Compounds, Crystal Field/Ligand Field Theory, Electrochemistry, IR Spectroscopy, NMR Spectroscopy, UV-Vis Spectroscopy

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ince their recognition in 1989 by Pecoraro,1 metallacrowns have grown into a versatile class of compounds. Initially investigated as the structural and functional inorganic analogues to crown ethers,2 the applications of the molecules have spread to the fields of single-molecule magnetism, MRI contrast agents, catalysis, molecular recognition, liquid crystals, and molecular frameworks.3 The classic metallacrown contains a metal−nitrogen−oxygen cyclic repeat unit, where the metal ion and nitrogen replace the methylene carbon atoms of a crown ether (Figure 1). Similar to crown ethers, metallacrowns can bind a central metal ion and display preferential binding of alkali metal ions.2 However, unlike crown ethers, metallacrowns do not exist as a simple ring, but a ligand scaffold must be built around each metal to stabilize the molecule (Figure 2). Due to

the versatility of metallacrowns, the molecules make excellent candidates for a multiweek, upper-division inorganic laboratory. The topic of metallacrowns allows an instructor to combine a variety of inorganic principles, such as Lewis acid−base chemistry, coordination chemistry, and crystal field theory, in one laboratory. While hundreds of metallacrowns have been reported,3 four molecules were chosen from the literature: Fe III (O 2 CCH 3 ) 3 [9-MC Fe III (N)shi -3](CH 3 OH) 3 ·3CH 3 OH, 4,5 MnII(O2CCH3)2[12-MCMnIII(N)shi-4](C3H7NO)6·2C3H7NO,5−8 [(CH 3 ) 4 N] 2 {Cu II [12-MC Cu I I (N)shi -4]}·C 3 H 7 NO, 8,9 and MnII(O2CCH3)2[15-MCMnIII(N)shi-5](C5H5N)6·CH3OH.10 The laboratory sequence described revolves around five articles from the primary literature and two theses.4−10 The syntheses of chosen molecules are facile, high yielding, and the products have been extensively characterized by a variety of methods (e.g., cyclic voltammetry, solid-state magnetic susceptibility, FT-IR, UV−vis, and paramagnetically shifted 1H NMR). However, not all of the methods have been used to characterize these four metallacrowns, which is advantageous for this student experiment. The starting reagents are relatively inexpensive with only one commercially available ligand, salicylhydroxamic acid (Figure 3), required for the synthesis of all four metallacrowns. The purpose of the experiment

Figure 1. Comparison of 12-metallacrown-4 and 12-crown-4 ring structures. © 2013 American Chemical Society and Division of Chemical Education, Inc.

Published: April 29, 2013 782

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inorganic chemistry laboratory course with an average of 10 students per year. Table 1. Experiment Outline and Skills Utilized Week

Synthesis of the metallacrowns

2

Isolation of the metallacrowns and analysis of a crystal structure Characterization of the metallacrowns

3 and 4 5

Figure 2. Atomic displacement ellipsoid plot of [(CH3)4N]2{CuII[12MCCuII(N)shi-4]}·C3H7NO at 50% probability with the central atom and atoms of the metallacrown ring labeled. The counter cations, lattice DMF molecule, hydrogen atoms, and disordered atoms have been eliminated for clarity. Color scheme: carbon, gray; oxygen, red; nitrogen, blue; copper, orange.

Activity

1

Metallacrown presentations

Skills and Techniques Stoichiometric ratios in synthesis, crystal growth, and Lewis acid−base coordination chemistry Mercury and CIF files

Cyclic voltammetry, solid-state magnetic susceptibility, FT-IR, UV− vis, and paramagnetic 1H NMR Oral presentation skills and data analysis

The premise of the project is to mimic a research laboratory atmosphere in a controlled teaching environment. Students are provided with the name and syntheses of each metallacrown. Then each student is randomly assigned one metallacrown to investigate over a four-week period. The objective of the experiment is to allow students to explore the primary literature and to develop characterization conditions for their particular metallacrown. No articles are provided by the instructor. Following a literature search, each student devises characterization conditions and consults with the instructor. Students may have to modify their experimental conditions depending on the chemicals, supplies, and instrumentation that are available in the department. For instance, one of the major factors for characterization is the stability of the metallacrowns in various solvents. A metallacrown may not be stable in the solvent in which it was synthesized. Thus, students must be aware of these factors. Complete experimental details for the synthesis and characterization of the metallacrowns are provided in the Supporting Information. The first week of the experiment consists of an introduction to metallacrowns and the synthesis of the compounds. A sample student handout is provided in the Supporting Information. Students are reminded about the importance of stoichiometric ratios in synthesis as the ratio of the metal to the ligand in the reactants controls the type of metallacrown produced. In addition, students are familiarized with Lewis acid−base coordination chemistry and are introduced to crystal growth techniques. Prior to this laboratory, students have had experience isolating crystals in organic laboratory; however, most have not been instructed how to grow single crystals of Xray quality, which require longer periods of time (3−7 days). Students are instructed in the techniques of solvent evaporation, solvent layering, and vapor diffusion. Students are also informed about temperature variations of these three techniques. Because each metallacrown requires a specific crystal growth technique, this is provided to the student in the synthetic scheme. However, each student is encouraged to synthesize a second batch and to experiment with other crystal growth techniques. The second week of the experiment consists of isolation of the product, a resynthesis if the product was not isolated, and analysis of the crystal structure of a different, but related, metallacrown. The instructor typically chooses a compound from the recent literature. Using the crystallographic information file (CIF) of the related metallacrown, students are instructed on the use of the software Mercury, which is

Figure 3. Structure of salicylhydroxamic acid.

sequence is to provide students with a research-like experience as they investigate the metallacrowns. To achieve this goal, students are required to synthesize a metallacrown, conduct a literature search, develop characterization parameters based on departmental resources, interpret experimental results and compare them to the primary literature, analyze the crystallographic structures of the compounds, and finally present an oral seminar detailing their results and analysis. Student inorganic synthetic lab experiments have been reported,11−13 but to the best of our knowledge, this is the first report utilizing metallacrowns for an undergraduate experiment.

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DESCRIPTION OF STUDENT POPULATION Students enrolled in the inorganic chemistry lecture−laboratory course are typically second-semester fourth-year chemistry majors, although some second-semester third-year chemistry majors enroll (1−2 per semester). The average enrollment is 10 students. Besides general and organic chemistry, students typically have completed one year of physical chemistry, a onesemester instrumental analysis course, and other upper-division elective chemistry courses. Thus, students in the course have previous experience with a vast majority of the departmental instrumentation.

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DESIGN OF THE EXPERIMENT The inorganic laboratory (one session per week for 4 h) is concurrent with the inorganic lecture, and the sequence of topics in lecture and laboratory is coordinated as best as possible. The metallacrown experiment serves as a capstone to the course as multiple lecture topics and laboratory techniques are combined in the last weeks of the semester (Table 1). The metallacrown experiment has been used for six years in the 783

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structural features using the software Mercury. Using the CIF file and the program Mercury, students can determine important bond angles, metal−ligand bond distances, and oxidation states of the metal ions through bond valence sum analysis.15−17 This enables students to critically analyze the crystallographic data of their metallacrown and of related metallacrowns from the primary literature. Furthermore, students are expected to compare their results to the literature reports and then discuss implications of their data. Students are assessed on their presentations skills, interpretation of their results, and their ability to answer questions from the audience. A presentation rubric can be found in the Supporting Information. Following the conclusion of the presentations, the class as a whole compares and contrasts the different properties of the metallacrowns and how the properties are dependent upon the metal type, number of metal ions, metal oxidation states, and structural features.

available free of charge from the Cambridge Crystallographic Data Centre.14 Students are later expected to use this program to investigate their individual metallacrown. Crystal structures have been reported for each of the investigated metallacrowns.4−10 CIF files for each metallacrown can be obtained from the Cambridge Crystallographic Data Centre, and the CIF files of the four presented metllacrowns have been included in the Supporting Information. The metallacrown structures provide an excellent opportunity to reinforce topics covered earlier in the semester. For example, the structures contain chelate rings, Jahn−Teller axes, and the absolute stereoconfigurations Λ and Δ. In addition, the structural parameters can be used to discuss the oxidation states of the metal ions. Furthermore, the CIF file is used to discuss previous lecture and laboratory topics, such as space groups, symmetry, and unit cell parameters. Weeks 3 and 4 are dedicated to the characterization of the metallacrowns using the methods of cyclic voltammetry (CV), solid-state magnetic susceptibility, FT-IR spectroscopy, UV−vis spectroscopy, and paramagnetically shifted 1H NMR spectroscopy. Students must design their own experiments based on the literature and the resources of the department. Students are expected to consult with the instructor prior to the third laboratory. (For large class sizes, it may be beneficial to employ a teaching assistant to meet with the students on a regular basis, or the instructor may limit the number of possible metallacrowns that the students may synthesize to one or two compounds.) Although literature experimental conditions exist for most of the techniques, some compounds have not been characterized by all of the methods used in this laboratory, for example, the FT-IR spectrum of [(CH3) 4N]2{CuII[12MCCuII(N)shi-4]}·C3H7NO has not been previously reported. For instances where there are no literature conditions for comparison, students are instructed to develop experimental conditions based on the other metallacrowns of the experiment. Again, it is imperative that the students consult with the instructor. The selected methods cover a wide variety of techniques that are designed to reinforce what students have learned earlier in the semester and in previous courses. Students are expected to relate the data that they collect with previous topics from the course, such as ligand field theory, coordination chemistry, magnetism, symmetry, and electronic structure of d-metal complexes. Students are then expected to compare their experimental values to the literature values4−10 or the values reported in the Supporting Information. The Supporting Information contains full experimental details and spectra for the characterization techniques of each metallacrown. This information is very limited in the primary literature. Moreover, some of the reported characterization conditions deviate from the primary literature. Experimental materials were chosen based on practicality and the availability of equipment in most chemistry departments. The results presented in the Supporting Information are representative of data acquired by students. The last week of the experiment is devoted to student presentations (30 min per presentation). To minimize repetition, students who investigated the same metallacrown are grouped together for the presentations (2−3 students). In addition, students are encouraged to investigate the applications of their molecule and closely related metallacrowns. This requires students to complete a limited literature review. Students are expected to provide a detailed experimental procedure and to analyze the crystal structure for important

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HAZARDS

Care should be taken to minimize exposure to chemicals in the laboratory. Goggles, gloves, and a laboratory coat are required, and all reactions should be performed in a fume hood. Students must consult the MSDS for each chemical prior to use. In addition, the instructor and students should consult the MSDS regarding the hazards of each chemical before any laboratory work is begun. Due to the large number of chemicals used in this laboratory, a complete list of hazards is included in the Instructor Notes of the Supporting Information. All waste should be collected and properly disposed according to safety regulations.

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CONCLUSION Using the class of molecules known as metallacrowns, students experienced a research-like laboratory environment. Students successfully synthesized their appropriate metallacrown in yields ranging from 46 to 80% with limited difficulty. Using available department resources, students designed, performed, and interpreted characterization studies. Student results from cyclic voltammetry, solid-state magnetic susceptibility, FT-IR, UV−vis, and paramagnetically shifted 1H NMR were in agreement with the reported literature values. The primary assessment of students was an oral presentation that required them to explain the synthesis and to interpret results based on their knowledge of a variety of inorganic themes such as crystal field theory, coordination chemistry, and Lewis acid−base chemistry. Review of the oral presentations and interactions with students over the course of the multiweek experiment indicated that the students achieved the goals of the experiment. This research-like experience permitted students to critically analyze articles from the primary literature and discuss them with the instructor and fellow students. The oral presentations allowed students to demonstrate their ability to develop their own characterization studies and properly assimilate their results with their knowledge of inorganic chemistry. Course evaluations from six years indicated that students enjoyed the experimental experience as it required them to learn new synthetic techniques and to go beyond the textbook to develop their own characterization studies. Students also commented on the evaluations that they were apprehensive about the oral presentations; however, most stated that the presentation was a valuable experience as this is one of the few courses in the chemistry curriculum that 784

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(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) Boffa, V.; Yue, Y.; He, W. Sol-Gel Synthesis of a Biotemplated Inorganic Photocatalyst: A Simple Experiment for Introducing Undergraduate Students to Materials Chemistry. J. Chem. Educ. 2012, 89, 1466−1469. (14) Mercury, version 3.0; Cambridge Crystallographic Data Centre: Cambridge, United Kingdom, 2012. The software is available at http://beta-www.ccdc.cam.ac.uk/pages/Home.aspx (accessed Apr 2013). (15) Brown, I. D.; Altermatt, D. Bond-Valence Parameters Obtained from a Systematic Analysis of the Inorganic Crystal Structure Database. Acta Crystallogr. 1985, B41, 244−247. (16) Thorp, H. H. Bond Valence Sum Analysis of Metal-Ligand Bond Lengths in Metalloenzymes and Model Complexes. Inorg. Chem. 1992, 31, 1585−1588. (17) Liu, W.; Thorp, H. H. Bond Valence Sum Analysis of MetalLigand Bond Lengths in Metalloenzymes and Model Complexes. 2. Inorg. Chem. 1993, 32, 4102−4105.

required an oral presentation as opposed to a poster presentation.

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ASSOCIATED CONTENT

* Supporting Information S

Guide to metallacrown nomenclature; complete experimental details and data sets; student metallacrown handout; metallacrown presentation rubric; crystallographic information files for the presented metallacrowns. 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 The authors thank Dimitris P. Kessissoglou of the Aristotle University of Thessaloniki for sharing FT-IR spectra and Joseph W. Shane of Shippensburg University for a careful reading of the manuscript.

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REFERENCES

(1) Pecoraro, V. L. Structural Characterization of [VO(salicylhydroximate)(CH3OH)]3: Applications to the Biological Chemistry of Vanadium(V). Inorg. Chim. Acta 1989, 155, 171−173. (2) Pecoraro, V. L.; Stemmler, A. J.; Gibney, B. R.; Bodwin, J. J.; Wang, H.; Kampf, J. W.; Barwinski, A. Metallacrowns: A New Class of Molecular Recognition Agents. In Progress in Inorganic Chemistry; Karlin, K. D., Ed.; John Wiley and Sons: New York, 1997; Vol. 45, pp 83 − 177. (3) Mezei, G.; Zaleski, C. M.; Pecoraro, V. L. Structural and Functional Evolution of Metallacrowns. Chem. Rev. 2007, 107, 4933− 5003. (4) Lah, M. S.; Kirk, M. L.; Hatfield, W.; Pecoraro, V. L. The Tetranuclear Cluster Fe III [Fe III (salicylhydroximato)(MeOH)(acetate)]3 is an Analogue of M3+ (9-crown-3). J. Chem. Soc., Chem. Commun. 1989, 1606−1608. (5) Lah, M. S. Development of Metallacrowns and Structural Characterization of Manganese Structures. Ph.D. Dissertation, University of Michigan, Ann Arbor, MI, 1991. (6) Lah, M. S.; Pecoraro, V. L. Isolation and Characterization of {MnII[MnIII(salicylhydroximate)]4(acetate)2(DMF)6}·2DMF: An Inorganic Analogue of M2+(12-crown-4). J. Am. Chem. Soc. 1989, 111, 7258−7259. (7) Gibney, B. R.; Wang, H.; Kampf, J. W.; Pecoraro, V. L. Structural Evolution and Solution Integrity of Alkali Metal Salt Complexes of the Manganese 12-Metallacrown-4 (12-MC-4) Structural Type. Inorg. Chem. 1996, 35, 6184−6193. (8) Gibney, B. R. 12-Metallacrown-4: A Structural and Functional Inorganic Analogue of 12-C-4. Ph.D. Dissertation, University of Michigan, Ann Arbor, MI, 1994. (9) Gibney, B. R.; Kessissoglou, D. P.; Kampf, J. W.; Pecoraro, V. L. Copper(II) 12-Metallacrown-4: Synthesis, Structure, Ligand Variability, and Solution Dynamics in the 12-MC-4 Structural Motif. Inorg. Chem. 1994, 33, 4840−4849. (10) Kessissoglou, D. P.; Kampf, J.; Pecoraro, V. L. Compositional and Geometrical Isomers of 15-Metallacrown-5 Complexes. Polyhedron 1994, 13, 1379−1391. (11) Rood, J. A.; Henderson, K. W. Synthesis and Small Molecule Exchange Studies of a Magnesium Bisformate Metal-Organic Framework: An Experiment in Host-Guest Chemistry for the Undergraduate Laboratory. J. Chem. Educ. 2013, 90, 379−382. 785

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