Microscale Syntheses, Reactions, and 1H NMR ... - ACS Publications

Chemical Education Today edited by

NSF Highlights

Susan H. Hixson National Science Foundation Arlington, VA 22230

Projects Supported by the NSF Division of Undergraduate Education

Richard F. Jones Sinclair Community College Dayton, OH 45402-1460

Microscale Syntheses, Reactions, and 1H NMR Spectroscopic Investigations of Square Planar Macrocyclic Tetraamido-N Co(III) Complexes Relevant to Green Chemistry

W

by Erich S. Uffelman,* Jonathan R. Doherty, Carl Schulze, Amy L. Burke, Kristen R. Bonnema, Tanya T. Watson, and Daniel W. Lee, III

Over the past six years, we have developed a series of new NMR experiments across our undergraduate curriculum based on the acquisition of a 400 MHz multinuclear FT NMR with the assistance of the former NSF-ILI program (1). Our two-credit inorganic synthesis and spectroscopy laboratory, Chem 252, is geared to junior and senior chemistry majors. It has been created as a problem-based learning environment that integrates lab and lecture and builds on foundations laid in lower-level chemistry courses. Chem 252 is described to students as a “lab of patents,” since all of the experiments involve recently patented ligands, complexes, or processes (1b, 1d, 2). We present here the microscale preparation, characterization, and reactivity of a square planar Co(III) complex that has grown out of our program to introduce experiments of relevance to green chemistry into the undergraduate curriculum. Now that there are two well-established two-step routes in the literature to the macrocyclic tetraamide ligands described here (3), and given that these ligands are commercially available (4), we believe that this lab and an analogous one involving square planar Cu(III) (1d) are ready for dissemination, especially since these experiments only require 6 mg of macrocycle per student. The class of macrocyclic ligands shown in Figure 1 has attracted considerable attention because it is remarkably robust to oxidative and hydrolytic decomposition and thus: (i) makes it possible to isolate and study unusually high oxida-

a X

NH HN

NH HN NH HN

2. O2

O

O

ⴚ O

ⴙ

N N Co N N NaCN

O

O

RR

[Ph 4P][Co(1)]

2−

O O

N N CN Co N CN N

O

O O

O O H N N Co N N O

O

N

tion states of first row transition metals (5); (ii) permits unusual coordination geometries of the first row transition metals to be studied (6); (iii) is crucial to new, industrially viable catalytic processes in which the relevant macrocyclic iron complexes activate hydrogen peroxide (7). In addition, a derivative complex of one that students prepare for this lab has been studied as a phase-separating electron transfer oxidizing agent applicable to green chemistry within the “use-quantitative recovery paradigm” (7c). The set of experiments and spectra described below takes two or three, four-hour lab periods to complete and involves the transformations shown in Figure 2. These experiments illustrate the remarkable redox

HCl

Br2

N

O

RR

O N N Co N N

R' R'

Figure 1. (a) Tetraamide macrocycles protected against oxidative decomposition. When tetradeprotonated, these tetraanionic macrocycles are powerful sigma electron donors to metal ions. (b) This family of iron complexes serves as catalytic activators of hydrogen peroxide for industrially important, environmentally friendly oxidations (7).

[Ph 4P]Cl Ce(IV)

H41

N

Fe X

NH HN

O

O

N

Cl

O

Li

O

X

R' X

2ⴚ

R O

R'

O 1. CoCl 2 THF LDA

O

R

O

R O

O

Figure 2. Chemical transformations in these cobalt experiments.

b

R

O

O

N N C N Co N N

3− O

O O

O

www.JCE.DivCHED.org

•

Vol. 81 No. 3 March 2004

•

Journal of Chemical Education

325

Chemical Education Today

NSF Highlights and aqueous acid–base stability that make the macrocycles so useful when applied to iron-based green oxidations, and the experiments introduce students to classic aspects of octahedral d6 transition metal chemistry as well as the novel characteristics of square planar d6 intermediate spin Co(III). Procedure H41 (3, 4) (6–7.5 mg) is weighed into a 3 mL 14/10 conical vial on an analytical balance, and a magnetic stir vane is added. A septum cap is loosely applied to the vial so that the gases inside the vial may be readily evacuated. All of the students’ vials with loose septum caps are placed into the mini-antechamber of a glove box (glove bag manipulations may be substituted for all of the inert atmosphere procedures described here). Each student transfers anhydrous CoCl2 (a few mgs) into his/her vial and then tightly closes the vial cap. The vials are removed from the glove box and connected to a N2 gas manifold by needle and hose. Students take turns syringing dry THF (0.6 mL) from an Aldrich Sureseal bottle into their vials. The solutions are stirred (1 min), and then LDA solution (1.5 M, 0.15 mL) (Caution! Caustic! ) is syringed into each reaction; the students wear disposable gloves for this step. A yellow-green precipitate forms as the reaction stirs (10 min). The septum cap is removed from the vial, exposing the solution to air, and a dark purple color develops (1 min). After stirring (5 min), TLC (silica gel/acetone) reveals a purple spot moving near the solvent front and cobalt byproducts stuck at the origin. The reaction solution/ suspension is filtered through a 1.5 cm silica gel pad in a glass pipet. Acetone (1–2 mL) is used to elute the purple Li[Co(1)] product, being careful to leave the byproducts trapped on the silica gel. The purple acetone/THF solution is evaporated to dryness under vacuum. The purple residue is washed with CH2Cl2 (2 × 1 mL) and dissolved in acetone. The solution/suspension is filtered through another 1.5 cm silica gel pad in a glass pipet, using acetone (1–2 mL) to elute Li[Co(1)]. The acetone solution is evaporated to dryness and the purple residue is dissolved in D2O to give a brown solution. This D2O solution is filtered into an NMR tube and an NMR spectrum is obtained, making sure that the sweepwidth, pulse delay time, and number of accumulations are modified appropriately for a paramagnetic sample. The D2O solution of Li[Co(1)] is split into two portions. One portion is left in the NMR tube. The instructor supplies each student with a vial that contains 1 mg of NaCN (Caution! Toxic! Students are never given access to the NaCN supply! ) which the students dissolve using three or four drops of D2O, wearing disposable gloves. This NaCN/D2O solution is added to the Li[Co(1)] solution; only 1–2 drops is sufficient to cause the solution to transform from brown to colorless. Students each take a 1H NMR of their now diamagnetic solutions. The other portion of Li[Co(1)] in D2O is treated with a minimum of gaseous HCl generated by adding H2SO4 (1– 2 mL) to NaCl (75–200 mg) in a micro flask equipped with a micro Claisen head, a septum screw cap, and a micro gas outlet hose. The HCl gas evolved (Caution! ) is added to the D2O solution of Li[Co(1)], stopping when the suspension 326


•

of H[Co(1)] precipitates from solution. To this suspension is added sufficient aqueous NaOH solution to dissolve the precipitate and restore the original color, being careful not to use a large excess. A few drops of an aqueous solution of concentrated [Ph4P]Cl is added. Either with centrifuging, pipet filtration, or both, the precipitate is washed with H2O (2 × 0.5 mL). The [Ph4P][Co(1)] is dissolved in CH2Cl2, the purple solution is dried over MgSO4 or Na2SO4, and the solution is filtered into two small screw cap vials. To the first CH2Cl2 portion of [Ph4P][Co(1)] is added a whiff of Br2 vapor (Caution!) obtained by placing a pipet into the head space of a container of liquid Br2 and drawing up some of the brown gas. Several color changes occur rapidly in succession. If the vapor has been added carefully, the dark intense blue color of square planar neutral Co(1) is transiently observed. To the second portion of [Ph4P][Co(1)] is added solid (NH4)4[Ce(SO4)4]. Agitation of this solution transiently generates the same dark intense blue color of square planar neutral Co(1). A Problem-Based Learning Environment In addition to the lab manuals, the students have access to a microscale laboratory manual for general procedures (8). Three papers (9) from the literature on green chemistry and high-valent transition metal chemistry are also given to students as background reading. The cobalt experiments described here follow immediately after a series of copper experiments that are also based on H41 (1d). The eight points below are the key focus of student–student and student–professor discussions and questions during the lab lectures, and they are also an integral component of the student lab writeups. 1. Brønsted acid–base chemistry. In pre-lab lecture, students recall that the pKa of diisopropylamine is 36 and that the pKa of an organic amide is 18–22. They then decide if LDA is capable of significantly deprotonating H41. The students recall the pKa of water and explain why it is excluded from the initial metallation reaction. In their lab notebooks, they estimate what the pKa of [Co(1)]⫺ would have to be for less than 1% of it to be protonated if 3.5 mg of the ion were dissolved in 0.5 mL of water. The students explain why HCl gas is evolved when H2SO4 is added to NaCl. They discuss the protonation of [Co(1)]⫺ with HCl and the deprotonation of H[Co(1)] with NaOH. 2. Sterics. Students consider in pre-lab if NaNH2 would be a good substitute for LDA in the procedure. They evaluate both attack at the Co2⫹ starting material and attack at the amide carbonyl groups of the macrocycle for NaNH2 versus LDA, thus reviewing the organic concept of basicity versus nucleophilicity. 3. Redox. Because CoCl2 is Co(II) and [Co(1)]⫺ is Co(III), students are asked why a Co(III) complex is not chosen as a starting material (it is typically less substitutionally labile, and the Co(III) could oxidize the deprotonated ligand system prior to complexation). Also, given that the color change on exposure to air is so dramatic, students readily


•

www.JCE.DivCHED.org


identify O2 as the oxidizing agent, but they are asked to consider the fate of the superoxide byproduct generated. One key element of discussion in the lab manual and the lab lecture is the viability or non-viability of a clean oxidation number assignment to the dark blue neutral [Co(1)] species produced. X-ray crystal structures of [Co(1)]⫺ and [Co(1)] are compared (X-ray data are included in the student handout portion of the supplementary material), and students are shown that the bond distances in the benzene ring of [Co(1)]⫺ do not vary much, consistent with an unperturbed pi-system, whereas the bond distances in the benzene ring of [Co(1)] do vary in a manner that is analogous to the bonding pattern of an orthoquinone. Although [Co(1)] is only transiently stable under the conditions students use to generate it, [Co(1)] is indefinitely stable in the absence of moisture (moisture does not decompose it, but rather reduces it to H[Co(1)]). Students note that although Br2 can act as an atom-transfer oxidizing agent, it acts as an electron transfer oxidizing agent in this experiment, because it produces the same product as the Ce(IV). Students explain, using the background reading (5f ), the design features of the macrocycle that protect it from decomposition. 4. Aqueous transition metal chemistry. During the experiment, depending on how much excess CoCl2 and LDA is used, students filter off black cobalt oxides formed after they expose the basic reaction mixture to air and moisture. However, the students are not told about this black solid in advance, and must determine what it might be and how it got into the reaction. 5. Solubility product constants. The students are asked to explain the different aqueous and non-aqueous solubilities of the different salts employed and formed in these experiments based on the “organic” or “inorganic” character of the individual ions. Students are asked to estimate values of Ksp that would lead to 90% versus 99% versus 99.9% precipitation of the [Ph4P][Co(1)] salt. 6. Nernst equation. When time permits, the cyclic voltammogram of [Ph4P][Co(1)] can be run in CH2Cl2, affording the students experience with this important technique. The [Co(1)]/[Co(1)]⫺ is reversible, and occurs at ⫹0.385 V versus ferrocenium/ferrocene. Students are asked to estimate how far the reaction:

[Co(1)] ⫹ ferrocene → [Co(1)]⫺ ⫹ ferrocenium⫹ goes to completion, using the Nernst equation. 7. Paramagnetic versus diamagnetic NMR spectroscopy. [Co(1)]⫺ is a square planar d6 complex with two unpaired electrons. Students run the 1H NMR spectrum, but change the following parameters: (a) Sweep width—as a paramagnetic complex, the paramagnetic shift causes the signals to appear between 10 ppm and –50 ppm. (b) Pulse delay—given that T1 is much shorter for the signals in a paramagnetic environment, no pulse delay is needed to prevent signal saturation, thus significantly accelerating the accumulation of scans. (c) Number of scans—since the paramagnetic line broadening causes noise to interfere more with observation

www.JCE.DivCHED.org

•

of the signal, students are told to run about 250 scans on their sample, assuming roughly 2 mg of [Co(1)]⫺ is present. With the sweep width at 60 ppm and no pulse delay, this typically takes only a few minutes. Students interpret their paramagnetic NMR of [Co(1)]⫺ by using the integral values. Students discover they have to ignore much taller peaks from diamagnetic impurities in order to correctly interpret the spectrum. Once CN⫺ is added, the students can form the diamagnetic [Co(1)(CN)]2⫺, [Co(1)(CN)2]3⫺, or both. Students explain how the strong field cyanide ligand changes the magnetic properties of the complexes. Because [Co(1)(CN)]2⫺ has Cs symmetry and [Co(1)(CN)2]3⫺ has C2v symmetry, and because students often make different mixtures of both complexes, the NMR interpretation gives them much to consider. Students often ask, “What did I do wrong? My spectrum does not look like my hood partner’s.” They analyze the problem and realize that nobody did anything wrong—that they should expect to see the differences they observe. 8. IR and UV-vis spectroscopies. If time permits, students can take the FT-IR spectrum of [Co(1)]⫺ and determine that the amide carbonyls (1648, 1635, 1590, 1572 cm⫺1) are at lower frequencies compared to H41 (1702, 1680, 1652, 1635 cm⫺1), as expected based on amide resonance structures consistent with more electron density at the carbonyl oxygen in the anionic complex versus the neutral ligand. If time permits students can measure the UV–vis spectrum of [Co(1)]⫺. They can estimate the molar extinction coefficient (>2000 M⫺1cm⫺1, λmax ⫽ 426, 516, 630 nm) and determine that they are observing a ligand to metal charge transfer transition.

Assessment and Conclusions The students record their procedures, observations, and calculations in a lab notebook, as well as answers to pre-lab and post-lab questions concerning the eight points discussed above. The students are also responsible for this material during pre-lab lectures and on their end of term cumulative lab final exam. The synthesis and spectroscopy of these complexes is challenging compared to most undergraduate laboratory experiments, but is more reliable than most air and moisture sensitive syntheses. These procedures thus constitute an excellent advanced microscale experiment, and we run these cobalt laboratories after two to three weeks of student work with a less demanding microscale copper experiment with H41 (1d). The students appreciate having an experience relevant to green chemistry. After the lab is concluded, the students enjoy seeing how much of their chemistry background is relevant to understanding this work. The laboratory final exam at the end of the term indicates that students generally absorb most of the key points of this laboratory experience. Some of the procedures in this experiment may be altered or omitted depending on the ability to oversee safety (such as student/teacher ratio in the lab). For instance, glass pipets in a glove bag may be used to administer LDA, if the instructor wants to omit syringes and needles. Although we


•


327


NSF Highlights are rigorously careful to give our students access to only trace quantities of cyanide and carefully oversee their work, the cyanide step in these procedures may be omitted. For instance, at low temperatures, phosphine ligands will bind [Co(1)]⫺.

2.

Hazards Sodium cyanide is extremely toxic (LD50 orally in rats: 15 mg/kg) and reacts with strong acid to generate highly toxic hydrogen cyanide gas (average fatal dose 50–60 mg); each student is given ⱕ 1 mg of NaCN. Needles represent a puncture hazard. LDA is a very caustic base. Standard precautions involving organic solvents and glassware under vacuum should be taken. HCl gas and Br2 gas are both toxic and irritating. Students are given a list of possible health hazards associated with strong magnetic fields (e.g., pacemakers, metal implants, etc.) and told to inform the instructor if they should be kept a safe distance from the NMR magnet. W

3.

Supplementary Materials

Full documentation of these materials is available in this issue of JCE Online. Acknowledgements We thank the National Science Foundation ILI program (Grant No. DUE-9650033) for support in purchasing our JEOL Eclipse+ 400 MHz spectrometer. We thank Charles Steinmetz for facilitating a Hewlett-Packard University Equipment Gift of a diode array UV-vis spectrophotometer (HP 8453 UV–vis system, gift number 37353). ESU thanks the Washington and Lee University Glenn Grant program for summer support. ESU also thanks the Washington and Lee Class of 1965 for two Excellence in Teaching Awards that supported development of new NMR experiments across the Chemistry Department curriculum. Related work that supported these experiments was funded by the Research Corp. (Grant No. CC3870), and the donors of the Petroleum Research Fund, administered by the American Chemical Society (Grant No. 29495-GB3). JRD, CS, ALB, KB, TTW, and DWL thank Washington and Lee for Summer Research Student Fellowships.

4.

5.

Literature Cited 1. (a) France, M. B.; Alty, L. T.; Earl, T. M. J. Chem. Educ. 1999, 76, 659; (b) France, M. B.; Uffelman, E. S. J. Chem. Educ. 1999, 76, 661; (c) Uffelman, E. S.; Cox, E. H.; Davis, C. M.; Goehring, J. B.; Lorig, T. An NMR-Smell Module for the First Semester General Chemistry Laboratory. J. Chem. Educ. 2003, 80, 1368–1372; (d) Uffelman, E. S.; Doherty, J. R.; Schulze, C.; Burke, A. L.; Bonnema, K.; Watson, T. T.; Lee, D.W., III. Microscale Syntheses, Reactions, and 1H NMR Spectroscopic Investigations of Square Planar Macrocyclic Tetraamido-N Cu(III) Complexes Relevant to Green Chemistry. J. Chem. Educ. 2004, 81, 182–185; (e) Alty, L. T. Ter-

328


•

6. 7.

pene Unknowns Identified Using IR, 1H NMR, 13C NMR, DEPT, COSY and HETCOR. J. Chem. Educ., submitted for publication, 2003. (a) Szafran, Z.; Pike, R. M.; Singh, M. M. Microscale Inorganic Chemistry: A Comprehensive Laboratory Experience; Wiley: New York, 1991; pp 176–180; (b) Szafran, Z.; Pike, R. M.; Singh, M. M. Microscale Inorganic Chemistry: A Comprehensive Laboratory Experience; Wiley: New York, 1991; pp 337– 341; (c) Jacobsen, E. N.; Zhang, W.; Muci, A. R.; Ecker, J. R.; Deng, L. J. Am. Chem. Soc. 1991, 113, 7063–7064. The original four-step synthesis may be found in (3a) and (3b), while the two different two-step syntheses may be found in (3c), (3d), and (3e). (a) Uffelman, E. S. “Macrocyclic Tetraamido-N Ligands that Stabilize High-Valent Complexes of Chromium, Manganese, Iron, Cobalt, Nickel, and Copper,” Ph. D. Dissertation, California Institute of Technology, Pasadena, CA, 1991; (b) Collins, T. J.; Powell, R. D.; Slebodnick, C.; Uffelman, E. S. J. Am. Chem. Soc. 1991, 113, 8419–8425; (c) Gordon-Wylie, S. W. “High-Valent Manganese and Cobalt Complexes of Oxidatively Robust N and O Donor Ligands,” Ph. D. Dissertation, Carnegie Mellon University, Pittsburgh, PA, 1995; (d) Collins, T. J.; Gordon-Wylie, S. W.; Woomer, C. G.; Horwitz, C. P.; Uffelman, E. S. Homogeneous oxidation catalysis using metal complexes. PCT Int. Appl. (1998), 62 pp; (e) Rorrer, L. C.; Hopkins, S. D.; Connors, M. K.; Lee, D. W., III; Smith, M. V.; Rhodes, H. J.; Uffelman, E. S. Organic Letters 1999, 1, 1157–1159. Visit the Web site at http://www.chem.cmu.edu/groups/collins/ (accessed Jan 2004). Information on the production of the macrocycles is updated on this page. To obtain samples of macrocycle for this laboratory experiment, use the information on the Web site and contact Terrence J. Collins or Colin Horwitz for pricing and quantities. These macrocycles may also be synthesized by methods published in the accompanying laboratory documentation materials for this paper. (a) Collins, T. J.; Powell, R. D.; Slebodnick, C.; Uffelman, E. S. J. Am. Chem. Soc. 1990, 112, 899–901; (b) Collins, T. J.; Kostka, K. L.; Münck, E.; Uffelman, E. S. J. Am. Chem. Soc. 1990, 112, 5637–5639; (c) Collins, T. J.; Nichols, T. R.; Uffelman, E. S. J. Am. Chem. Soc. 1991, 113, 4708–4709; (d) Collins, T. J.; Slebodnick, C.; Uffelman, E. S. Inorg. Chem. 1990, 29, 3432–3436; (e) Collins, T. J.; Kostka, K. L.; Uffelman, E. S.; Weinberger, T. Inorg. Chem. 1991, 30, 4204– 4210; (f ) Collins, T. J. Acc. Chem. Res. 1994, 27, 279–285; (g) Bartos, M. J.; Gordon-Wylie, S. W.; Fow, B. G.; Wright, L. J.; Weintraub, S. T.; Kauffmann, K. E.; Münck, E.; Kostka, K. L.; Uffelman, E. S.; Rickard, C. E. F.; Noon, K. R.; Collins, T. J. Coord. Chem. Rev. 1998, 174, 361–390; (h) Miller, C. G.; Gordon-Wylie, S. W.; Horwitz, C. P.; Strazisar, S. A.; Peraino, D. K.; Clark, G. R.; Weintraub, S. T.; Collins, T. J. J. Am. Chem. Soc. 1998, 120, 11540–11541; (i) Horwitz, C. P.; Fooksman, D. R.; Vuocolo, L. D.; Gordon-Wylie, S. W.; Cox, N. J.; Collins, T. J. J. Am. Chem. Soc. 1998, 120, 4867– 4868. Collins, T. J.; Uffelman, E. S. Angew. Chem. Int. Ed. Engl. 1989, 28, 1509–1511. (a) Gupta, S. S.; Stadler, M.; Noser, C. A.; Ghosh, A.; Steinhoff, B. A.; Lenoir, D.; Horwitz, C. P.; Schramm, K. W.;


•

www.JCE.DivCHED.org


Collins, T. J. Science 2002, 296, 326–328; (b) Collins, T. J. Acc. Chem. Res. 2002, 35, 782–790; (c) Patterson, R. E.; Gordon-Wylie, S. W.; Woomer, C. G.; Norman, R. E.; Weintraub, S. T.; Horwitz, C. P.; Collins, T. J. Inorg. Chem. 1998, 37, 4748–4750; (d) Collins, T. J.; Gordon-Wylie, S. W.; Bartos, M. J.; Horwitz, C. P.; Woomer, C. G.; Williams, S. A.; Patterson, R. E.; Vuocolo, L. D.; Paterno, S. A.; Strazisar, S. A.; Peraino, D. K.; Dudash, C. A. The Design of Green Oxidants. In Green Chemistry: Frontiers in Benign Chemical Syntheses and Processes; Anastas, P. T., Williamson, T. C., Eds.; Oxford University Press: Oxford, 1998; pp 46–71. 8. Szafran, Z.; Pike, R. M.; Singh, M. M. Microscale Inorganic

www.JCE.DivCHED.org

•

Chemistry: A Comprehensive Laboratory Experience; Wiley: New York, 1991. 9. We currently use references 5f, 7a, and 7b. In previous years we have used reference 5f and 7d.

In the NSF Highlights column, recipients of NSF CCLI grants share their project plans and preliminary findings. Erich S. Uffelman is in the Department of Chemistry, Washington and Lee University, Lexington, VA 24450; uffelmane@ wlu.edu. Jonathan R. Doherty, Carl Schulze, Amy L. Burke, Kristen R. Bonnema, Tanya T. Watson, and Daniel W. Lee, III are former students who participated in this project.


•


329

Microscale Syntheses, Reactions, and 1H NMR ... - ACS Publications

Recommend Documents