Porphyrins are fun!

Hello from the Chemistry Department and Dr. Grzybowski’s lab! My name is Abby Flanagan-Frankl and I am a rising senior chemistry and psychology double major working with porphyrins and clathrochelates. These are two very important classes of compounds that have a lot of very interesting electrochemical and spectroscopic properties.

Porphyrin (Fig 1) is a highly conjugated macrocycle that is commonly found in nature in important molecules such as hemoglobin and chlorophyll. Buried amidst protein chains, the porphyrin structure is at the active site within these molecules.

 

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Fig 1. General porphyrin structure

A simple porphyrin that can be readily synthesized in the lab is tetraphenylporphyrin. In general, there are two different ways to synthesize tetraphenylporphyrin (Scheme 1), the first of which is essentially a brute force method and the other way is more refined. Both methods start with benzaldehyde and pyrrole, the first method involves combining these two reagents in refluxing propionic acid for 30 minutes. This method gives you a good yield of about 78%, but the product is impure and needs to go through a purification process. Method two involves the same two reagents and is conducted under a nitrogen atmosphere. An oxidizing agent is added near the end of the reaction to complete the formation of the conjugated porhyrin ring. This method produces a much lower yield of about 20% and takes longer, but the product is purer.

 

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Scheme 1. Tetraphenylporphyrin synthesis

The other class of compound that I am working with is clathrochelates, which is a metal complex where a metal is encased in a ligand structure. The specific clathrochelate compound (shown at the end of Scheme 2) that I am commonly using was developed in Dr. Grzybowski’s lab and is based on an oxime-hydrazone ligand system. This type of clathrochelate can be functionalized by using various boronic acids as capping agents. The clathrochelate complex I am using incorporates 4-formylphenylboronic acid into the clathrochelate cage and produces a complex that can be substituted for the benzaldehyde used in the tetraphenylporphyrin synthesis. When it is combined with pyrrole, it produces a porphyrin ring with four clathrochelate complexes appended to it. We call this very large molecule tetraclathroporphyrin (Fig 2). Tetraclathroporphyrin contains four clathrochelate metal centers and a fifth metal binding site in the porphyrin ring. We plan to vary the metal ions in these sites and study the spectroscopic and electrochemical properties of these new species.

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Fig 2. Tetraclathroporphyrin

We still have a lot of work to do with porphyrins and clathroaldehyde and we are looking at different ways to attach clathrochelates to the porphyrin ring as well as ways to change the metal encased in the clathrochelate, but so far it has been tons of fun here at Gettysburg!

 

Abby1Me with the rotary evaporator

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