In the Laboratory
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Preparation and Investigation of Monodentate and Bridging Pyrazole Complexes Wynne Evans School of Applied Sciences, University of Glamorgan, Pontypridd, CF37 1DL, United Kingdom;
[email protected] Complexes of pyrazole-derived ligands have attracted much interest in recent years (1–2), because of the ability of the pyrazolato anion to form bridged polymetallic compounds in which the metals are held close enough to react with small molecules or facilitate magnetic exchange (3–4). Derivatives of these nitrogen heterocyclic ligands may find applications in the synthesis of new catalytic systems. The nitrogen heterocycle pyrazole (Hpz) and its deprotonated derivative, pyrazolato (pz), can readily coordinate to transition metals. Two principal modes of coordination are: η1-coordination of Hpz via the pyridine-type nitrogen of the ring (Figure 1) and µ2-pz between two metals, coordinating in an exobidentate (N, N′ ) fashion (Figure 1). Pyrazole is a weak acid (5) and coordination of the nitrogen with the lone pair considerably increases this acidity (6). It is therefore possible to deprotonate a metal–pyrazole complex, generating a metal-containing ligand. Base removal of the proton of complexed pyrazole results in the formation of polymeric species in which pyrazolato acts as bridging ligand (Figure 2). Description of the Experiment The preparation of monodentate pyrazole and 3,5dimethylpyrazole (DMHpz) nickel species and the investigation of their reactions with triethylamine are described in detail in the Supplemental Material.W For example, for pyrazole, the reactions are:
[NiCl2(Hpz)4]
NiCl2 + 4Hpz
[NiCl2(Hpz)4] + 2Et3N
(1)
[Ni(pz)2]
+ 2Et3NH+Cl − + 2Hpz (2)
The products, the pseudo-octahedral [NiCl2(Hpz)4] (7) and polymeric [Ni(pz)2]n, (where the Ni(II) ions are in a squareplanar environment) are then investigated using EDTA titration, infrared, UV–vis, and magnetochemistry measurements to provide information concerning formulas, structure, and the modes of coordination of the ligand. Similar reactions involving 3,5-dimethylpyrazole instead of pyrazole are then attempted and the results compared.
Hazards Nickel salts and their solutions are irritants and suspected carcinogens. Do not ingest or inhale any fine powders. Use gloves when handling. Pyrazole is an irritant and is harmful if ingested, inhaled, or in contact with skin. Wear gloves when handling. 3,5-Dimethylpyrazole is an irritant and is harmful if ingested, inhaled, or in contact with skin. Wear gloves when handling. Ethanol can react vigorously with oxidizing agents and is flammable. Methanol is flammable. Triethylamine is harmful by ingestion and is an irritant. Use in a fume hood and do not inhale. Diethyl ether is volatile and flammable. Concentrated nitric acid is corrosive and a powerful oxidizing agent. Ammonia solution is extremely damaging to eyes and is harmful if ingested, inhaled, or in contact with skin. It is also corrosive. Discussion We use this experiment as part of a course providing an introduction to coordination chemistry. It involves the straightforward preparation of relatively pure, inexpensive nickel pyrazole complexes and gives students an appreciation of how the mode of coordination of a ligand bound to a metal ion can affect the properties of the complex. A ratio of at least a 6:1 molar excess of pyrazole to hydrated nickel chloride results in the formation of [NiCl 2 (Hpz) 4 ]; while slightly less than 2:1 yields (8) [NiCl2(Hpz)2]. The laboratory handout describes the preparation of the six coordinate complex because the subsequent reaction with base gives a better yield of product—however both of the nickel pyrazole complexes will go on to form the bridged polymer when reacted with base. Triethylamine is the preferred choice for base; aqueous solutions of sodium hydroxide or ammonia have been used to extract the proton from pyrazole (5), but use of triethylamine in an organic solvent results in a “cleaner” reaction and decreases drying time. There is also a tendency of some of these species to exist in a number of hydrated forms, for that reason minimal exposure to water is better. While different derivatives of monodentate 3,5-dimethylpyrazole (DMHpz) can also be synthesized, depending upon the molar ratio of dimethylpyrazole to the nickel salt,
−
N
N
N
N
N
N
N
N
M
H M 1
M
M
N
N
N
N
M
N
N
N
N
M
M
2
Figure 1. Structure and metal binding of pyrazole, 1, and the pyrazolato ion, 2.
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Figure 2. Structure and metal binding of the bridging pyrazolato.
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In the Laboratory
the six coordinate complex displays an interesting variation versus [NiCl2(Hpz)4]; here [Ni(H2O)2(DMHpz)4]Cl2 is the product. These complexes also do not form the bridged polymer on reaction with base (9) and the students are invited to come up with reasons as to why this should be. The bulk of coordinated 3,5-dimethylpyrazolato and the pKa of coordinated 3,5-dimethlypyrazole (as compared with coordinated pyrazole) provide possible answers. It is interesting to note, however, that the polymer has been prepared by two other routes: condensation of a bis-(acetylacetonato) complex with hydrazine hydrate (10) and, more recently, a route involving nickelocene and 3,5-dimethylpyrazole (4). Possible Extensions The exercise can be extended and could take the form of an individual or class project. Suggestions for variations upon the described experiment include: Variation of stoichiometry: The formula of the product often depends upon the molar ratio of pyrazole to metal salt. Analysis of the metal will allow students to differentiate between the two species. Substitution of pyrazoles, use of imidazoles: There are many substituted forms of pyrazole or imidazole that are cheap and commercially available. Use of these can lead to interesting variations, for example 3-methylpyrazole (3MeHpz) yields [NiX2(3MeHpz)4] only (7), where X = Cl, Br. Imidazole complexes are also readily prepared (11). Variation of metal ion or halogen: A wide variety of complexes have been reported in the literature and substitution of Ni(II) with selected first row transition metals and Cl− with another halide or anion, provides plenty of scope to investigate species with different formulas and geometries (12–13).
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Supplemental Material
Instructions for the students and notes for the instructor, including spectra, are available in this issue of JCE Online. Literature Cited 1. Sadimenko, A. P.; Basson, S. S. Coord. Chem. Rev. 1996, 147, 247–297. 2. La Monica, G.; Attilio Ardizzoia, G. Prog. Inorg. Chem. 1997, 46, 151–238. 3. Pons, J.; López, X.; Benet, E.; Casabó, J. Polyhedron 1990, 23, 2839–2845. 4. Storr, A.; Summers, D. A.; Thompson, R. C. Can. J. Chem. 1998, 76, 1130–1137. 5. Vos, J. G.; Groeneveld, W. L. Inorg. Chim. Acta. 1977, 24, 123–126. 6. Catalan, J.; Abboud, J. L. M.; Elguero, J. Adv. Heterocycl. Chem. 1987, 41, 187–274. 7. Hemholdt, R. B.; Hinrichs, W.; Reedijk, J. Acta Crystallogr., Sect C: Cryst. Struct. Commun. 1987, 43, 226–229. 8. Nicholls, D.; Warburton, B. A. J. Inorg. Nucl. Chem. 1970, 32, 3871–3874. 9. Vos, J. G.; Groeneveld, W. L. Trans. Met. Chem. 1979, 4, 137–141. 10. Singh, C. B.; Satpathy, S.; Sahoo, B. J. Inorg. Nucl. Chem. 1973, 35, 3950–3954. 11. Masciocchi, N.; Ardizzoia, G. A.; LaMonica, G.; Maspero, A.; Galli, S.; Sironi, A. Inorg. Chem. 2001, 40, 6983–6989. 12. Guichelar, M. A.; Van Hest, J. A. M.; Reedijik, J. Delft Progress Report 1976, 2, 51–63 and references therein. 13. Villa, R. A.; Peter, B.; Raptis, R. G. Abstracts of Papers of the Am. Chem. Soc. 2001, 221, Part 1, 387-CHED.
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