In the Laboratory
Preparation and Characterization of Solid Co(II) Pyrimidinolates in a Multifaceted Undergraduate Laboratory Experiment Norberto Masciocchi* and Simona Galli Dipartimento di Scienze Chimiche e Ambientali, Università dell’Insubria, via Valleggio 11, 22100 Como, Italy; *
[email protected] Angelo Sironi Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Università di Milano, via Venezian 21, 20133 Milano, Italy Gabriella Dal Monte Istituto di Istruzione Superiore G.Torno, Piazza Don Milani 1, 20022 Castano Primo, Italy Elisa Barea Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Università di Milano, via Venezian 21, 20133 Milano, Italy Juan Manuel Salas and Jorge A. R. Navarro Departamento de Química Inorgánica, Universidad de Granada, Av. Fuentenueva S/N, 18071 Granada, Spain
Coordination chemistry and functional properties of solid-state compounds are usually introduced in the classroom at a later stage than basic inorganic and organic chemistry, and, often, little time is dedicated in laboratory classes to the synthesis, characterization, and discussion of polymeric metal complexes containing polyfunctional ligands. However, there are a number of simple coordination polymers that can be easily prepared and suitably characterized through standard and inexpensive methods, allowing the concurrent use of different techniques and the introduction of important basic, as well as advanced, theoretical, and experimental concepts. This multifaceted laboratory project, although tailored for undergraduate or graduate students, can be easily adopted even in high schools, depending on the availability of equipment and on the depth of the problems discussed in the classroom. The scientific literature has recently witnessed the blooming of the chemistry of metal–organic frameworks, with some important results in the field of microporosity and molecular
magnetism. For example, three dimensional, highly porous metal–carboxylates, capable of storing gases (including H2), have been prepared (1) and chain metal triazolates have been proposed as materials possessing magnetic and hysteretic behavior for sensors and color displays (2). In the last decade, we have been studying polymeric metal complexes containing diazaaromatic anions such as pyrazolates, imidazolates, and pyrimidinolates (3), discovering functional properties such as magnetism, second-harmonic generation, and solid-gas adsorption for gas recognition and storage purposes in porous materials. Many of these promising materials can be easily prepared by addition of a solution of the organic ligand to a solution of a metallic salt and are amenable to a variety of reactions and analyses. As shown in the following sections, among these species, we have identified the Co(II) pyrimidin-4-olate system (Scheme I) as an inexpensive and effective teaching example in the classroom and laboratory; its characterization may be used to illustrate several topics in basic and advanced inorganic chemistry
[Co(4-pymo) 2(H2O)4] 1a 110 °C RT
[Co(4-pymo) 2] 2 amorphous
Co2+ + 2[4-pymo]
5 °C
300 °C
[Co(4-pymo) 2] 3 crystalline
90 °C N
[Co(4-pymo) 2(H2O)4]r2(H2O) 1b
N
N
4-Hpymo
OH
N
4-pymo
O
Scheme I. The Co(II) pyrimidin-4-olate synthesis.
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Journal of Chemical Education • Vol. 85 No. 3 March 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
Task
Method
Preparation (3 h)
Simple addition of the reactants in aqueous solution, with stirring and careful control of temperature and pH
Estimation of water content (1h 30 min)
Residual weight after heating in an oven at 130 °C
Estimation of Co(II) content (3 h)
Titration with EDTA and murexide
Estimation of Co(II) content (3 h)
Colorimetric assay (solution)
Spectroscopic characterization (2 h)
Infrared, UV–vis (solid state)
Estimation of C,H,N content (30 min)
Elemental analysis
Evaluation of thermal stability (2 h)
Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC)
Evaluation of phase purity (4 h)
X-ray diffraction (XRD)
Estimation of Co(II) content (3 h)
X-ray fluorescence (XRF, solution)
5
DSC / (mW/mg)
Table 1. Methods Employed for the Preparation and Characterization of the Cobalt pyrimidin-4-olates
0
5 10 15
exo
20 25
100
mass change 23.23%
90
70 60 50
onset 114.3 pC
mass change 49.31%
40 30 100
200
300
400
500
Temperature / pC
600
DSC / (mW/mg)
5 0
5 10
exo
15 20
100 90
TGA (%)
(such as thermodynamics, metallorganic chemistry, crystal field theory, solid-state properties, and spectroscopy) and a number of analytical methods. Accordingly, we propose the preparation and characterization of selected, but related, Co(II) 4-pyrimidinolate complexes (4) that, depending on the local availability of analytical methods, can be characterized by simple methodologies (titration, UV–vis spectroscopy) or, to a deeper extent, by other less conventional methods. A short article on a similar polymer, nickelbispyrazolate, was recently published by Evans, but was mainly limited to its synthesis (5). Also Otieno et al. (6) addressed the synthesis and thermal characterization of a simple coordination polymer, while Näther et al. (7) nicely assembled a graduate course on modern analytical methods (using a simple inorganic salt). Yet, to our knowledge, this is the first integrated set of accessible laboratory experiments coupling these two important aspects. A summary of the several analytical techniques we propose for the project is gathered in Table 1.
TGA (%)
80
onset 436.8 pC
mass change 29.95%
80 70 60
onset 431.4 pC
onset 96.7 pC
mass change 44.12%
50 40 30
100
200
300
400
Temperature / pC
500
600
Experiment and Discussion
Figure 1. TGA and DSC traces for species 1a (top) and 1b (bottom), heating under a N2 atmosphere at a 10 °C/min rate.
The preparation of the hydrated Co(II) complexes is easily performed by adding dropwise a NaOH-containing 4-hydroxypyrimidine (4-Hpymo) solution to a cobalt acetate (or nitrate) solution with continuous stirring. Depending on the conditions used, a pink powder, [Co(4-pymo)2(H2O)4] (1a), or a peach-colored powder, [Co(4-pymo)2(H2O)4]∙2(H2O) (1b), can be prepared. 1b can be selectively formed if the temperature of the solution is kept below 5 °C with the aid of an ice bath, while 1a normally precipitates at room temperature (see Scheme I). Occasionally, if the synthetic conditions are not cautiously controlled, a 1a/1b mixture can precipitate: this occurs, for example, if the temperature of the reaction solutions varies or
if constant magnetic stirring is not applied. Both 1a and 1b can be dried for 0.5 h at 60 °C in an oven without decomposition, but, if kept for about 1 h at 130 °C, they quantitatively lose all the water molecules (as easily checked by IR), affording the violet, amorphous (XRD evidence) [Co(4-pymo)2] species, 2. When dehydrated completely, the starting material suffers a relative mass loss that can be employed to estimate either the water content per formula unit, if a pure (1a or 1b) phase has been employed, or the specimen purity, that is, the 1a/1b molar
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Intensity / cps
In the Laboratory
tively expected, is observed. However, on storing dry powders of pure hexahydrated 1b at room temperature for several months (during a hot summer!), partial transformation to the tetrahydrated 1a is observed. The determination of the molar masses of 1a and 1b and of their correct stoichiometries can be easily achieved by coupling the relative mass losses measured upon heating to decomposition to the Co(II) content. This can be estimated by one of the following methods, which require dissolution of the solids in a small quantity (ca. 1 mL) of diluted acids:
100 cps
15
20
25
30
2R / deg
Figure 2. XRD traces for species 1a, 1b, 2, and 3 (bottom to top).
1a 19.5% 1b 80.5%
Figure 3. Graphical output of the quantitative analysis of a 1a/1b mixture by the Rietveld method implemented in Quanto (10).
ratio: the relative mass losses for complete dehydration have indeed theoretical values of 22.4 and 30.3%, for pure 1a and 1b, respectively. As a further support to this measurement, TGA and DSC traces may be used, the latter allowing also the evaluation of the dehydration reaction enthalpy, if pure phases are employed. Interestingly, the thermal characterization reported in Figure 1 indicates that 1b decomposes at slightly lower temperatures than 1a and, more importantly, that the amorphous material 2 transforms into a polycrystalline one, 3, without any mass change but with a small exothermic effect (‒21 kJ mol‒1, onset at 280 °C), if heated above 300 °C. The XRD traces (Figure 2) clearly show the transformation of 1a and 1b into 2 and, eventually, into 3 under progressive heating (performed either in- or ex-situ). Worthy of note, no intermediate 1b → 1a → 2 transformation, as it could be intui-
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• complexometric determination of the Co(II) ion (nitric acid solutions are preferred), once the pH is raised to about 6, by direct titration with 0.01 M EDTA (using murexide as indicator) (8) • spectrophotometric determination of the Co(II) content (λmax = 511 nm), after dissolution of the solids in 1 M HCl • quantitation of the cobalt atoms by X-ray fluorescence (XRF), after controlled dilution with deionized water of a nitric acid solution
Theoretical relative mass loss values are 75.0 and 77.5 % for 1a and 1b, respectively, for Co3O4 formation as the final thermal decomposition product (XRD evidence). The availability of a standard C,H,N elemental analyzer might further help in assigning the correct stoichiometry or, in combination with any of the above analyses, to foresee the presence of a mixture of the 1a and 1b phases and quantify it. The experience we have accumulated recently suggests that any of the above methods gives reliable results. While X-ray fluorimeters are not common in an inorganic (or organometallic) chemistry department, the structural and analytical characterization of solids has greatly benefited, in the last years, from the recent advances in instrumentation and methods for X-ray powder diffraction analyses (9). Nowadays, many structural chemists have access to automatic powder diffractometers, which, within minutes, can give an indisputable answer on the mono- or polyphasic nature of the prepared solids. More importantly, even if this would require a thorough knowledge of the crystallographic and methodological aspects of the technique, the availability of simple and shareware software for phase quantification by the Rietveld method greatly facilitates the analytical task. For example, Figure 3 shows a typical result obtained by our students on a polyphasic mixture of 1a and 1b once the automatic fitting procedure, implemented in the software Quanto (10), is employed. These analytical results can then be used to validate, or dismiss, the pertinent figures obtained by the other methods cited above. Obviously, this approach needs a minimal presentation and description of the crystallographic program, which can be taken from standard textbooks (11) or other pertinent references (12), and requires the crystal coordinates of the pure phases 1a and 1b, supplied in the online supplement. Once the metal geometry in phases 1a, 1b, and 3 has been visualized and described (using the appropriate measuring tools provided by the software Mercury; ref 13), a conventional interpretation of the spectroscopic features can be initiated, by introducing simple crystal field theories, aiming at the understanding of the changes and intensification of the sample colors upon heating (for d–d transitions) (14).
Journal of Chemical Education • Vol. 85 No. 3 March 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
Hazards Cobalt salts and their solutions are irritants and potential carcinogenic agents. Do not ingest and inhale any fine powders. 4-Hydroxypyrimidine is an irritant and is harmful if ingested, inhaled, or in contact with skin. Ethanol can react vigorously with oxidizing agents and is flammable. Hydrochloric acid, nitric acid, sodium hydroxide, and ammonia solutions are corrosive and should be handled with care. No extensive safety procedures are required for the chemical EDTA, other than those observed in normal laboratory conditions. Conclusions The experiments presented here are particularly suitable for an integrated laboratory project, where solid-state and coordination chemistry, structural aspects, and a variety of analytical methods can be easily introduced. A non-exhaustive list of possible topics that are linked to the proposed activities includes: thermogravimetric and calorimetric analyses; solidstate reactions and solid–solid phase transitions; complexometric, colorimetric, and fluorescence analyses; spectroscopic aspects (electronic and vibrational spectroscopy); X-ray powder diffraction (qualitative and quantitative analyses); crystal and molecular structures, with their description and visualization properties; coordinative bond and metal geometry; and structure of coordination polymers. The experience we accumulated in the last two years shows that a portion of this project can be successfully performed even at a high-school level, provided that simple glassware, an oven, and, perhaps, a UV–vis spectrophotometer, are available. Performing the synthesis of the complexes and their minimal characterization by complexometry (including standardization of EDTA with MgCl2 solutions), thermal treatment, and colorimetric analysis should take no more than 12 hours of experimental work (see Table 1), which might be slightly reduced if students work in pairs or small groups. Obviously, a few introductory classes should be given well ahead of the experimental work, and a round-table discussion, where the many analytical results of the different groups are merged, must follow. Summarizing, we have found this series of experiments very versatile, in that it was easily adopted at different levels, also sequentially during the students’ career. The interest raised among students, which were facing a newly developed integrated sequence of experiments, favored the presentation of many ancillary topics, ranging from the discussion of analytical and spectroscopic aspects, up to structure evaluation via computer graphics analysis. Finally, the introduction of new subjects, such as chemistry of coordination polymers or their applications in a number of developing technologies (hydrogen storage and gas separation processes), has further augmented the impact of the presented project. Acknowledgments This work was partially supported by the Italian and Spanish Ministries of Investigation, through an Integrated Action
Project 2004-2006 and by “Progetto Lauree Scientifiche”. The Spanish Ministry of Education and Science (CTQ-200500329/BQU, EB Postdoctoral Grant (EX2005-1004)) and Junta de Andalucìa are acknowledged for funding. The courtesy of Valentina Colombo and Angelo Maspero (University of Insubria) is also acknowledged. Literature Cited 1. Dybtsev, D. N.; Chun, H.; Yoon, S. H.; Kim, D.; Kim, K. J. Am. Chem. Soc. 2004, 126, 32–33. Côté, A. P.; Benin, A.; Ockwig, N.; Matzger, A.; O’Keeffe, M.; Yaghi, O. M. Science 2005, 310, 1166–1170. Rood, J. A.; Noll, B. C.; Henderson, K. W. Inorg. Chem. 2006, 45, 5521–5528. 2. Gütlich, P.; Garcia, Y.; Goodvin, H. A. Chem. Soc. Rev. 2000, 29, 419–427. 3. Masciocchi, N.; Galli, S.; Sironi, A. Comm. Inorg. Chem. 2005, 26, 1–37. Navarro, J. A. R.; Barea, E.; Galindo, M. A.; Salas, J. M.; Romero, M. A. Quirós, M.; Masciocchi, N.; Galli, S.; Sironi, A.; Lippert, B. J. Sol. Stat. Chem. 2005, 178, 2436–2451. 4. Masciocchi, N.; Galli, S.; Sironi, A.; Barea, E.; Navarro, J. A. R.; Salas, J. M.; Tabares, L. C. Chem. Mater. 2003, 15, 2153–2160. 5. Evans, W. J. Chem. Educ. 2004, 81, 1191–1192. 6. Otieno, T.; Hutchinson, A. R.; Krepps, M. K.; Atwood, D. A. J. Chem. Educ. 2002, 79, 1355–1357. 7. Näther W.; Jeß, I.; Herzog, S.; Teske, C.; Bluhm, K.; Pausch, H.; Bensch, W. J. Chem. Educ. 2003, 80, 320–325. 8. Mendham, J.; Denney, R. C.; Barnes, J. D.; Thomas, M. J. K. Vogel’s Quantitative Chemical Analysis, 6th ed.; Prentice Hall: Upper Saddle River, NJ, 1999. 9. Jenkins, R. J. Chem. Educ. 2001, 78, 601–606. 10. Altomare, A.; Burla, M. C.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Rizzi, R. J. Appl. Cryst. 2001, 34, 392–397. 11. Jenkins, R.; Snyder, R. L. Introduction to X-ray Diffractometry, 1st ed.; Wiley: New York, 1996. Pecharsky, V.; Zavalij, P. Fundamentals of Powder Diffraction and Structural Characterization of Materials, 1st ed.; Kluwer: Dordrecht, 2003. 12. See for example the special issue of J. Chem. Educ. on “Teaching Crystallography” 1988, 65, 470–512. 13. Macrae, C. F.; Edgington, P. R.; McCabe, P.; Pidcock, E.; Shields, G. P.; Taylor, R.; Towler, M.; Van de Streek, J. J. Appl. Cryst. 2000, 39, 453–457. 14. Lever, A. B. P. Inorganic Electronic Spectroscopy; Elsevier: Amsterdam, 1984.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Mar/abs422.html Abstract and keywords Full text (PDF) with links to cited JCE articles Supplement Student handouts
Instructor notes
Fractional atomic coordinates for compounds 1a, 1b, and 3 in CIF and Quanto formats
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 3 March 2008 • Journal of Chemical Education
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