In the Laboratory
Synthesis and Symmetry of Two Cobalt(III) Complexes with Tetradentate Ligands An Advanced Inorganic Chemistry Experiment Mark McClure Department of Chemistry and Physics, The University of North Carolina at Pembroke, Pembroke, NC 28372-1510;
[email protected] The study of symmetry in chemical compounds typically falls under the realm of inorganic chemistry. However, when choosing examples of various point groups and explaining the relationship between symmetry and spectroscopy, many discussions resort to organic molecules. In this laboratory students will prepare two cobalt(III) complexes derived from two different tetradentate ligands, and the symmetries of these complexes will be explored using 13C NMR spectroscopy. This experiment was designed for an advanced inorganic chemistry course. It is typically the first lab performed in this course and is intended to accompany lectures on symmetry and group theory. The intended time frame is four weeks, three weeks on synthesis and one week on NMR spectroscopy. A number of important concepts are taught in this laboratory, including the synthetic experience gained in preparing the complexes, an understanding of isomerism in chelate complexes, and the spectroscopic experience gained in acquiring and interpreting the NMR spectra. Experimental The two ligands used in this study are triethylenetetramine (trien) and tris(2-aminoethyl)amine (tren). Both of these reagents are inexpensive and are commercially available. The complexes to be synthesized include [Co(trien)(NO2)2]Cl (1, 2) [Co(tren)(NO2)2]Cl (3–5) and [Co(tren)(en)]Cl3, where en is ethlyenediamine. The dinitro compounds are prepared by air
C1
C4
C2
C3
C2 C2
C3
C1
C3
C1
C2 C1
C2
C1
C3
N
C3
N C2
C1
N Co
N
C1
C2
X
N
X
X
C4
N Co X
C2
N N
C3 C1
Figure 1. Structural illustrations of [Co(trien)X2]+ (left) and [Co(tren) X2]+ (right)
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oxidation of CoCl2⋅6H2O with a stoichiometric quantity of the appropriate ligand and NaNO2; a yield of approximately 80% is typical if pure triethylenetetramine hydrate is used. The dichloro compound [Co(tren)Cl2]Cl is prepared by stirring [Co(tren) (NO2)2]Cl with concentrated hydrochloric acid over a period of several days; a yield of approximately 65% is typical for this reaction. Lastly, the compound [Co(tren)(en)]Cl3 is prepared by refluxing [Co(tren)Cl2]Cl with a stoichiometric quantity of ethylenediamine in methanol or ethanol; a yield of 65% is typical for this reaction. Hazards All of the amines, as well as the 12 M HCl, are corrosive. Ethylenediamine fumes in moist air. Students should wear gloves when handling these reagents. Sodium nitrite is potential carcinogen. Care should be taken to ensure that students do not inadvertently mix the NaNO2 with the 12 M HCl, as this releases toxic NO2 gas. Results and Discussion Structural Illustrations of [Co(trien)X2]+ and [Co(tren) X2]+ are shown in Figure 1. (In actuality, three geometric isomers are possible for [Co(trien)X2]+. However, under the conditions employed in this laboratory, only the symmetrical cis isomer is formed.) The structures of these two series of complexes are different and this is reflected in their 13C NMR spectra. The compound [Co(trien)(NO2)2]Cl possesses a C2 axis that divides the six carbon atoms into three sets of two equivalent atoms; therefore a 13C NMR spectrum containing three resonances of roughly equal intensity is observed. The compound [Co(tren) (NO2)2]Cl possesses a mirror plane that contains one of the chelate arms and bisects the other two; this gives rise to a 13C NMR spectrum containing two large and two small peaks. The compound [Co(tren)(en)]Cl3 gives rise 13C NMR spectrum similar to that of [Co(tren)(NO2)2]Cl but with two additional resonances arising from the ethylenediamine. By comparing spectra, students should be able to assign all of the resonances in [Co(tren)(NO2)2]Cl and partial assignments in [Co(trien)(NO2)2]Cl and [Co(tren)(en)]Cl3. The compound [Co(trien)(NO2)2]Cl gives three peaks of roughly equal intensity at approximately 42.7 ppm, 55.5 ppm, and 57.5 ppm. The compound [Co(tren)(NO2)2]Cl gives four signals at 45.2 ppm (large), 45.3 ppm (small), 61.0 ppm (small) and 63.2 ppm (large). Lastly, the compound [Co(tren)(en)]Cl3 contains six signals at approximately 44.4 ppm (small) 45.0 ppm (small), 45.5 ppm (small), 46.3 ppm (large), 61.9(small), and 63.6 (large). Since the inclusion of ethylenediamine results
Journal of Chemical Education • Vol. 85 No. 3 March 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory Table 1. Some Typical Chemical Shifts
Complex
[Co(trien)(NO2)2]Cl
[Co(tren)(NO2)2]Cl
[Co(tren)(en)]Cl3
Chemical Shift (ppm)
42.7 (C1)a
55.5 (C3)
57.5 (C2)
45.2 (C1)
45.3 (C3)
61.0 (C4)
63.2 (C2)
44.4b
45.0b
45.5b
46.3 (C1)
61.9 (C4)
63.6 (C2)
a The
carbon positions are from Figure 1. b The C3 carbon is indistinguishable from the ethylenediamine carbon atoms without additional data.
C1 carbon atoms. Similarly, for [Co(tren)(NO2)2]Cl the signals at 45.2 ppm and 45.3 ppm are assigned to the C3 and C1 carbon atoms. In this case the relative heights of the signals also aid in the assignments; the signal at 45.2 ppm is larger because it arises as a result of two equivalent carbon atoms and is assigned to C1. These data are summarized in Table 1. Literature Cited 1. Sargeson, A. M.; Searle, G. H. Inorg. Chem. 1965, 4, 45–52. 2. Sargeson, A. M.; Searle, G. H. Inorg. Chem. 1966, 4, 787–796. 3. Toyota, E.; Yamamoto, Y.; Yamamoto, Y. Bull. Chem. Soc. Jpn. 1988, 61, 3175–3180. 4. Massoud, S. S.; Milburn, R. M. Inorg. Chim. Acta. 1988, 154, 115–119. 5. Massoud, S. S.; Milburn, R. M. Polyhedron. 1989, 8, 2389– 2394.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Mar/abs420.html Abstract and keywords Full text (PDF) Links to cited JCE articles
in two additional signals in the region of 44–45 ppm, it can be deduced that signals from carbon atoms adjacent to coordinated NH2 groups fall within this region of the spectrum. Returning to [Co(trien)(NO2)2]Cl, the signal at 42.7 can be assigned to the
Supplement Student handouts icluding post-lab questions
Instructor notes including the answers to the post-lab questions and the 13C NMR spectra
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 3 March 2008 • Journal of Chemical Education
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