The annual “molecule of the year” results for 2021 are now available … and the winner is Infinitene.[cite]10.33774/chemrxiv-2021-pcwcc[/cite],[cite]10.1021/jacs.1c10807[/cite] This is a benzocirculene in the form of a figure eight loop (the infinity symbol), a shape which is also called a lemniscate [cite]10.1021/jo801022b[/cite] after the mathematical (2D) function due to Bernoulli. The most common class of molecule which exhibits this (well known) motif are hexaphyrins (hexaporphyrins; porphyrin is a tetraphyrin)[cite]10.1039/b502327k[/cite],[cite]10.1021/ol0521937[/cite],[cite]10.1002/chem.200600158[/cite], many of which exhibit lemniscular topology as determined from a crystal structure. Straightforward annulenes have also been noted to display this[cite]10.1107/S1600536811048604[/cite] (as first suggested here for a [14]annulene[cite]10.1021/ol0518333[/cite]) and other molecules show higher-order Möbius forms such as trefoil knots.[cite]10.1038/NCHEM.1955[/cite],[cite]10.1039/D0CC04190D[/cite] This new example uses twelve benzo groups instead of six porphyrin units to construct the lemniscate. So the motif is not new, but this is the first time it has been constructed purely from benzene rings.
The molecule has D2 chiral symmetry and is shown below (click on the image for the 3D model obtained from the crystal structure).
The authors suggest that the aromaticity in a D2-symmetric [12]-circulene is confined to six “Clar” rings each of six electrons, and is not delocalised around the entire molecule. For a molecule with this topology (defined by a linking number, Lk = 2π[cite]10.1016/j.comptc.2014.09.028[/cite]) the entire system would be defined as aromatic (delocalised) for 4n+2 electrons and antiaromatic for 4n electrons around a continuous annulene loop. In this example outer annulene circuits of either 34 or 38 carbons can be constructed which retain D2-symmetry and which both follow the 4n+2 rule, whilst a small inner circuit of 14 carbons can be also be constructed. There are probably other D2-symmetric circuits that could be constructed.
When I saw the molecule, I asked myself what the calculated chiroptical properties for the molecule might be; the optical rotation of the two (separated) enantiomers of [12]-circulene were reported as +1130° (P,P) and -1112° (M,M). The calculated value (ωB97XD/Def2-TZVPP) is in excellent agreement. I have also included versions of this system with [11] and [10] benzo rings, which will be discussed in a future post.
For good measure, the calculated VCD spectrum
Now to the geometry, as obtained from the crystal structure. The [12]circulene shows in total 12 short lengths of 1.348ű0.014, indicating significant localisation in the system. The D2-symmetric C34 path through the system shows a mean length for each bond of 1.405Å, with a maximum value of 1.443Å and a minimum 1.334Å. For this path, the topology of the system indicates Lw = 2π = 0.393Tw + 1.607Wr[cite]10.1021/ja710438j[/cite] This means that most of the coiling of the molecule that results in that figure eight is actually comprised of a topological property known as writhe (Wr) rather than adjacent twisting (Tw) of the p-orbitals. This retains much p(π)-p(π) overlap and hence stabilisation. The values for the inner C14 route are Lw = 2π = 1.256Tw + 0.744Wr which is more highly twisted than the larger outer pathway and so aromaticity via this route is less favoured due to less favourable p(π)-p(π) overlaps.
I also note that the Lw = 2π is an alternative chiral descriptor to the helical notation of (P,P). The (M,M) form would have Lw = -2π. The linking number is more general for more complex helical forms such as trefoils, cinquefoils, hexafoils etc.
So it turns out that this molecule has a fascinating challenge for trying to describe its extended delocalised aromaticity (rather than localised six-membered Clar rings), since more than one “annulene route” for which the “Hückel/Möbius rules” might apply exists.[cite]10.1016/j.comptc.2014.09.028[/cite] Given that the maximum bond length for one of those routes (the [34]annulene) is 1.443Å, there may well be a contribution from this mode of aromaticity other than that from the Clar rings.
I hope to take a look at the [11] and [10]circulenes in a future post.
‡The explanation for this sign inversion is delightful but too complex to give here.[cite]10.1002/chir.22486[/cite]
This post has DOI: 10.14469/hpc/10036
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