Supersized Aromatic Me

Philip Andrews – Mon, 08 Jun 2015 16:05:00 GMT

I recently came across a report about the largest known aromatic molecule, to date,¹ and it got me thinking about similar entries in the CSD. I know from editing structures into the database that a significant proportion of new entries contain an aromatic system. The structures also seem to be getting bigger and bigger but I had never really connected these two features in any detail. First of all, I was delighted to learn that the supersized structures from the report were already in the CSD. Our new automated systems could deal with them without a problem and one of my colleagues had already cast their expert eye over them. In the report Dongho Kim and co-workers synthesised a [50]dodecaphyrin (KUHHIG, see below), the full name of which is in our curated entry but would have taken up half this blog so I decided not to include it! If you are wondering how to make your own supersized aromatic then the structure was synthesised to contain Hückel aromaticity by oxidising a non-aromatic [52]dodecaphyrin with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Dongho Kim and co-workers appear to be record breakers in the world of these structures; the largest known aromatic prior to this was a [46]decaphyrin palladium(II) complex (DONZEN), also synthesised by the same research groups.²

Did you know that large expanded porphyrins commonly lose planarity and exhibit twisted conformations due to increased flexibility and intramolecular hydrogen bonding? Protonation is an effective strategy for cleaving these intramolecular H-bonds to afford a planar, aromatic structure. Subsequent protonation of the [50]dodecaphyrin with methanesulfonic acid led to a tetraprotonated dodecaphyrin (KUHHUS), which has a coplanar cyclic conformation and a corresponding increase in aromaticity (see below).

After looking at these supersized aromatics I went on to search for porphyrinoid structures in the CSD and, as I suspected, these have been increasing steadily over the past 45 years (see chart below), with the last two decades seeing a rapid growth in the number of database entries. Currently, there are over 9,300 porphyrinoid structures in the database. The last two decades have also seen increased interest in studies of expanded porphyrin systems and their application in nonlinear optics, coordination chemistry and anion sensing.

Lastly, I looked to see if my suspicion of an increasing number of aromatics was true and I was actually astounded at the number – there are now over 582,000 entries in the CSD that contain aromatic moieties, which is a whopping 75% of the whole database! The importance of aromatic molecules is also highlighted by their widespread use in bestselling pharmaceutical products. Of the top 200 drugs by US retail sales in 2012, 150 were small organic or organometallic compounds, and 126 of these contained aromatic moieties.³

I wonder if the recent publication has also prompted attempts worldwide to synthesise much larger expanded porphyrins? I am looking forward to seeing even more aromatics added into the CSD in the future and I hope I see a few more record breakers along the way - so why not have a stab at breaking the record?

Soya, T., Kim, W., Kim, D. and Osuka, A. Chemistry - A European Journal, 2015, DOI: 10.1002/chem.201500650. Stable [48]-, [50]-, and [52]Dodecaphyrins(1.1.0.1.1.0.1.1.0.1.1.0): The Largest Hückel Aromatic Molecules.
Yoneda, T.; Sung, Y. M.; Lim, J. M.; Kim, D. and Osuka, A. Angewandte Chemie International Edition, 2014, 53, 13169, DOI: 10.1002/anie.201408506. PdII Complexes of [44]- and [46]Decaphyrins: The Largest Hückel Aromatic and Antiaromatic, and Möbius Aromatic Macrocycles.
http://www.pharmacytimes.com/publications/issue/2013/July2013/Top-200-Drugs-of-2012

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