Coordination complexes are important throughout chemistry and biology. They act as catalysts, drugs, oxygen carriers, and electron transfer agents. In this experiment, students metallate tetraphenylporphyrin with zinc using zinc acetate. Students monitor the metallation by observing the changes in the absorption spectrum by using visible spectroscopy. Students observe the Zn2+ that was colorless in solution, forming a blood red complex with the tetraphenylporphyrin ligand. In addition, the porphyrin changes the solubility of the zinc ion. Thus, ligands such as the porphyrin can be used to impart new properties (optical or solubility) to a metal through the formation of a complex.
Reaction of the metal-free porphyrin ligand with the metal salt inserts the metal into the four-fold (tetradentate) binding site (metallation). Typically, halogenated solvents or dimethylformamide are used to metallate porphyrins. In this experiment, students use two, more benign solvents, 1-methyl-2-pyrrolidinone (also called N-methylpyrrolidinone) and dimethylsulfoxide to dissolve both of the reagents and ensure facile reaction.
The solvents used in this metallation offer less health and environmental hazards than the traditionally used solvents such as chlorinated hydrocarbons and N,N-dimethylformamide. Further the reaction is conducted at room temperature instead of the elevated temperatures traditionally used. The link to the laboratory procedures includes pre- and -post lab questions.
Summary prepared May 2005 by Julie A. Haack, Department of Chemistry at the University of Oregon.
Warner, M.; Hutchison, J. E. Greener Approaches to Undergraduate Chemistry Experiments, Print Kirchhoff, M., Ryan, M., Eds.; American Chemical Society: Washington D.C., 2002; pp 32-34.