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William F. Coleman Wellesley College Wellesley, MA 02481
Molecular Models of Phthalocyanine and Porphyrin Complexes William F. Coleman Department of Wellesley College, Wellesley, Massachusetts 02481
[email protected] In their paper describing an NMR experiment on some substituted iron(II) phthalocyanine complexes, Ignacio Fernandez and Jorge Fernando Fernandez Sanchez introduce students to the structures of these complexes (1). For many students, this might be their first encounter with a macrocyclic ligand, while others might be tempted to say that the iron phthalocyanines look like the iron site in hemoglobin or myoglobin. Indeed, the structure of phthalocyanine might be considered as a special case of tetraazaporphyrin, with the pyrroles replaced by 3,4-benzopyrrole groups (Figure 1). We have added examples of each of these classes of molecules to the molecule collection (2). In the case of the neutral ligands, the structures that have been added to the library show them to be planar. This is the prediction of Hartree-Fock and DFT calculations and is also seen in many crystal structures. Semiempirical methods disagree in their prediction of the structure of phthalocyanine. The PM3 and AM1 methods correctly predict a planar structure, while for PM6 model predicts that the macrocycle is puckered. However, the ligands have a certain amount of flexibility and are occasionally found to be nonplanar. The oxo-bridged dimer shown in the collection is a dramatic case of nonplanarity in these ligands (3). Phthalocyanine and porphyrin complexes have a rich chemistry that goes well beyond the complexes described in the featured paper or found in oxygen transport proteins. The Journal of Porphyrins and Phthalocyanines is devoted exclusively to the study of porphyrin and phthalocyanine chemistry. Phthalocyanine complexes of copper(II) with the unsubstituted phthalocyanine ligand are used in artists' pigments as phthalocyanine
blue. If 13-15 chlorines are placed on the six-membered rings, then the color is known as phthalocyanine green. However, what one sees as either of these depends on the state of the compound, the size of the particles if solid, and the distribution medium and opacity if in a pigment. Various phthalocyanine complexes are used throughout the dye industry as well. Other applications of these complexes that students might wish to pursue for research papers in nonmajors courses deal with electron transport, solar energy, new materials development, and the self-assembly of phthalocyanines in condensed phases. Another direction in which the properties of the phthalocyanine ligand can be taken is to move from two-dimensional macrocycles to three-dimensional ones. Two such extensions are crown ethers and cryptands. Several crown ethers and a cryptand were featured here in September 2008 (4), and two mixed crown ether-phthalocyanine complexes are added to the collection this month. Literature Cited 1. Fernandez, I.; Fernandez Sanchez, J. F. J. Chem. Educ. 2010, 87, DOI: 10.1021/ed800077f. 2. Molecular Models of Phthalocyanine Complexes and Other Macrocyclic Ligands; available at the JCE Digital Library, http://www.jce. divched.org/JCEWWW/Features/MonthlyMolecules/2010/Mar. 3. Gorun, S. M.; Rathke, J. W.; Chen, M. J. Dalton Trans. 2009, 38, 1095. 4. Coleman, W. F. J. Chem. Educ. 2008, 86, 1296.
Figure 1. Iron phthalocyanine, phthalocyanine, and tetraazaporphyrin are several of the molecules added to the JCE Featured Molecules collection this month (2). The structure of phthalocyanine is similar to tetraazaporphyrin, with the pyrroles replaced by 3,4-benzopyrrole groups.
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Vol. 87 No. 3 March 2010 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed800117y Published on Web 02/09/2010
