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Diamagnetic Anisotropy: Two Iron Complexes as Laboratory Examples ndez* Ignacio Ferna Department of Organic Chemistry, University of Almería, Carretera de Sacramento s/n, E-04120 Almería, Spain *
[email protected] ndez Sa nchez Jorge Fernando Ferna Department of Analytical Chemistry, University of Granada, C/Fuentenueva s/n, E-18071, Granada, Spain
Nuclear magnetic resonance (NMR) is an important analytical tool in structural analysis, and thus undergraduate chemistry students are trained how to utilize 1H and 13C NMR data for structure elucidation. NMR, however, has many uses that go beyond simple structure analysis. Consequently, many chemistry laboratory courses incorporate experiments that expose the students to these techniques. We have developed a student-friendly experiment using iron(II) phthalocyanine (PcFe) (1) that highlights an important concept of NMR found in almost every sample;diamagnetic anisotropy. It is worth mentioning that 605 articles on NMR have been published in this Journal since 1957 but none involve the NMR features of phthalocyanine complexes (2). Systems with pronounced diamagnetic anisotropy are typically aromatic where the induced ring currents are usually underestimated as a source of information. Ring-current effects produce a shielding or deshielding of protons resulting from their orientation with respect to an applied external magnetic field. The circulating π electrons create an induced magnetic field that opposes or reinforces the applied magnetic field in different magnetic zones. The zone where opposition to the applied magnetic field occurs is the shielding (þ) zone. Protons located in this region have upfield chemical shifts (lower δ values). On the other hand, protons located in the zone of reinforcement are in the deshielded (-) zone and have downfield chemical shifts (higher δ values). A detailed explanation of this topic can be found in any organic chemistry or spectroscopic textbook (3). In this experiment, students synthesize two bis(amine) iron(II) phthalocyanine and examine various NMR spectra.
in 1-2 h. The data processing to yield quality spectra is followed with standardized processing protocols allowing the students to process and print spectra with minor difficulties. This experiment integrates two important laboratory concepts: inorganic coordination synthesis and NMR spectroscopy, which further allowed the instructor to teach basic NMR concepts such as phase correction, integration, spectral window (i.e., large enough to cover all the signals), and spin systems and to demonstrate the versatility of NMR by the combination of standard one- and two-dimensional methods. A class of 12 students working in groups of two can successfully complete the experiment in a single laboratory session divided in two parts: sample preparation and data collection on a previously programmed instrument.
Procedure
The structures of [PcFe(decylamine)2] and [PcFe(benzylamine)2] are shown in Figure 1. The differences between these two diamagnetic complexes are reflected in their NMR spectra. In the 1H NMR spectra, integration of the signals in the phthalocyanine region relative to the amine signals confirms that the new structures contain 2 equiv of amine. Given the wealth of conformations that can exist in solution and that only sharp signals are observed, chemical exchange processes are necessarily fast with respect to the NMR time scale. The interesting low frequency signals for the amines that appear between ca. 0.5 ppm and -7 ppm arise as a consequence of the local diamagnetic anisotropy effects associated with the phthalocyaninato structure (5). The alkyl and benzyl chains are assumed to be extended
Decylamine and benzylamine are used in this experiment. Both of these reagents are inexpensive and are commercially available. The iron(II) phthalocyanine complexes are prepared in a straightforward one-pot manner by reaction of PcFe with 2 equiv of the appropriate amine in deuterated tetrahydrofuran.1 Yields of ca. 98% are typical if pure PcFe (available from Fluka) is used (4). The metal complex can be synthesized in 30-60 min. By using a relatively high metal complex concentration (ca. 30 mg in 0.75 mL THF-d8) and optimized parameters, the NMR experiments (1H, 13C, gCOSY, and gHMQC) can be completed 320
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Hazards The experiment should be performed in a fume hood using standard protective equipment: goggles, gloves, and lab coat. Decylamine is skin irritant and harmful if swallowed. Benzylamine is corrosive and harmful in contact with skin or if swallowed. Tetrahydrofuran is a highly flammable solvent, is an irritant, and is harmful if ingested. HCl methanolic 10% solution is corrosive. Iron(II) phthalocyanine may be irritating to the eyes, skin, and respiratory tract. In proximity to the NMR instrument a strong magnetic field is present that could cause health problems to persons with implanted or attached medical devices. Results and Discussion
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In the Laboratory
Figure 3. 1H,1H gCOSY-45 spectra of [PcFe(benzylamine)2] (300 MHz, at 298 K in THF-d8 solution).
Figure 1. Molecular structures of [PcFe(decylamine)2] and [PcFe(benzylamine)2].
The 1H,13C gHMQC spectra for [PcFe(decylamine)2] and [PcFe(benzylamine)2] allowed the assignment of every C-H pair and proved that the lowest frequency resonance (-6.52 ppm for [PcFe(decylamine)2] and -6.01 ppm for [PcFe(benzylamine)2]) correspond to the NH2 groups. The COSY experiment allowed the students to obtain the spin connectivities revealed by the cross-peaks (see Figures 2 and 3). The 45° pulse reduces the intensity of the diagonal signals with respect to the intensity of the cross-peaks, avoiding or at least diminishing signal overlapping, that is, the region of ca. 0.5 ppm in [PcFe(decylamine)2] complex. By comparing spectra, students should be able to assign all of the resonances in [PcFe(decylamine)2] and [PcFe(benzylamine)2]. These data are summarized in Table 1 in the supporting material. The details of the synthesis, NMR acquisition, processing parameters, student spectra, and some instructor notes are also available in the supporting material. To aid students in the correct assignment of the specific protons and carbons, a set of questions was provided to guide them through their data analysis: • How many distinct proton and carbon environments can you identify in the NMR? • Is there any evidence of coupling between protons? • What information can you obtain from the integrals in your 1H NMR spectra? • Are there any intensity differences between signals in the 13C NMR aromatic region? Are they useful for carbon assignment? • What scalar connectivities can you establish in your gCOSY-45 spectra? • What scalar connectivities can you establish in your gHMQC spectra?
Figure 2. 1H,1H gCOSY-45 spectrum of [PcFe(decylamine)2] (300 MHz, at 298 K in THF-d8 solution).
and therefore not parallel to the aromatic macrocycle, which would be in agreement with the way the anisotropy fades along the molecule and with precedent X-ray studies (6). To support this assignment, 13C NMR spectra show that the aliphatic part of the amine appears in relatively normal positions, with no especially low frequency shifts, because the 13 C NMR is less affected by local anisotropic effects (1, 7).
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Conclusions This laboratory experiment gives the students an excellent feel for the important concept of diamagnetic anisotropy. The NMR data allowed the assignment of every resonance by virtue of the corresponding downfield (phthalocyanine protons) or
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upfield (aliphatic and aromatic aminic protons) shift that would have been impossible if one measured free amines without the presence of iron(II) phthalocyanine. The spectra are acquired without any broadness of the signals, which is a common drawback in paramagnetic shift reagents. Additionally, students acquire hands-on experience in the use of an NMR spectrometer and on general aspects of spectral processing.
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Acknowledgment I.F. thanks the Ramon y Cajal program for financial support, and both authors thank Fernando Lopez-Ortiz for helpful discussions.
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Note 1. The reaction proceeds in chloroform as solvent but with longer reaction times (1 h versus several min with THF).
Literature Cited 1. Moser, F. H.; Thomas, A. L. J. Chem. Educ. 1964, 41, 245. Macrocyclic phthalocyanine compounds have found many technological applications. See the supporting material for details 2. JCE Online Journal Search. http://jchemed.chem.wisc.edu/Journal/ Search/index.html (accessed Dec 2009). 3. (a) Streitweiser, A., Jr.; Heathcock, C. H. Introduction to Organic Chemistry, 3rd ed.; Macmillan: New York, 1985. (b) G€ unther, H. NMR Spectroscopy: Basic Principles, Concepts, and Applications in Chemistry; John Wiley and Sons: Chichester, U.K., 1998.
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(c) Friebolin, H. Basic One- and Two-Dimensional NMR Spectroscopy; Wiley-VCH: Weinheim, Germany, 1998. (d) Claridge, T. D. W. High-Resolution NMR Techniques in Organic Chemistry; Pergamon Press: Oxford, 1999. The spectral data have been reported elsewhere. See: Fernandez, I.; Pregosin, P. S.; Albinati, A.; Rizzato, S.; Spichiger-Keller, U. E.; Nezel, T.; Fernandez-Sanchez, J. F. Helv. Chim. Acta 2006, 89, 1485. (a) Maskasky, J. E.; Mooney, J. R.; Kenney, M. E. J. Am. Chem. Soc. 1972, 94, 2132. (b) Maskasky, J. E.; Kenney, M. E. J. Am. Chem. Soc. 1973, 95, 1443. (c) Choy, C. K.; Mooney, J. R.; Kenney, M. E. J. Magn. Reson. 1979, 35, 1. (d) Keppeler, U.; Kobel, W.; Siehl, H.-U.; Hanack, M. Chem. Ber. 1985, 118, 2095. See ref 4 for the X-ray structure of [PcFe(benzylamine)2]. There are only a few more X-ray examples of 6-fold coordinated PcFe complexes containing nitrogen donors: 4-Methylpyridine: (a) Kobayashi, T.; Kurokoawa, F.; Ashida, T.; Uyeda, N.; Suito, E. J. Chem. Soc., Chem. Commun. 1971, 1631. (b) Cariati, F.; Morazzoni, F.; Zocchi, M. J. Chem. Soc., Dalton Trans. 1978, 1018. 4-Methylpiperidine: (c) Nemykin, V. N.; Kobayashi, N.; Chernii, V. Y.; Belsky, V. K. Eur. J. Inorg. Chem. 2001, 3, 733. (d) Janczak, J.; Kubiak, R. Inorg. Chim. Acta 2003, 342, 64. (a) G€ unther, H.; Schmickler, H.; Konigshofen, H.; Recker, K.; Vogel, E. Angew. Chem., Int. Ed. 1973, 12, 243. (b) Kalinowski, H. O.; Lubosch, W.; Seebach, D. Chem. Ber. 1977, 110, 3733.
Supporting Information Available Details of the synthesis; instructor notes; student spectra; NMR parameters. This material is available via the Internet at http://pubs.acs. org.
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