Global aromaticty

20 July 2019 - PhysChem

rickhaus 2019 aromaticity.JPGHückel's rule predicts that any cyclic conjugated system with 2n+4 electrons is aromatic with marked consequences for reactivity en spectroscopy. The classic examples with an increasing number of participating electrons are the cyclopropenium ion (n=0) benzene (n=1) and the cyclooctatetraenide anion (n=3) . But does the electron count have a limit? Researchers of the University of Oxford propose a new macrocycle with global aromaticity containing no less than 162 electrons (Rickhaus et al. Chemrxiv). The molecule looks like a ferris wheel with a hexaethynylbenzene core and with tetraphenylporphyrin cabins. In the smallest of the studied compounds the 6 porphyrin units are linked via ethynyl bridges and and a central zinc ion connects to the spokes via a zinc - pyridine bond.

The electron count for the neutral compound comes to 6x10 + 6x2 = 72 (not all porphyrin double bonds participate in the ring current). In NMR titration with the hexafluoroantimonate salt of thianthrenium the porphyrin units were oxidized to the +6 state leaving 66 (n = 16) electrons. The aromaticity of the compound in the NMR tube was not difficult to spot: all cabins were purposely fitted with 3,5-bis(trihexylsilyl)phenyl sidegroups and some of its protons were found to be shielded to negative ppm numbers. This effect is also observed in certain cyclophanes where any proton dangling over the ring current experiences the full induced magnetic field. By variation of oxidation states and ring architecture it was possible to create anti-aromatic compounds as well and also larger rings with ultimately 162 electrons participating. The experiments were not without technical challenges: buried in the supplemental information are warnings that rubber septa's will mess everything up. Any temperature above -10 degrees also destroys the compounds.