{"id":275,"date":"2009-04-13T16:34:12","date_gmt":"2009-04-13T15:34:12","guid":{"rendered":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=275"},"modified":"2020-04-21T06:25:55","modified_gmt":"2020-04-21T05:25:55","slug":"a-molecule-with-an-identity-crisis-aromatic-or-anti-aromatic","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=275","title":{"rendered":"A molecule with an identity crisis: Aromatic or anti-aromatic?"},"content":{"rendered":"
\n

In 1988, Wilke[1]<\/a><\/span> reported molecule 1<\/strong><\/p>\n

\"A

A 24-annulene. Click for 3D.<\/p><\/div>
\nIt was a highly unexpected outcome of a nickel-catalyzed reaction and was described as a 24-annulene with an unusual 3D shape. Little attention has been paid to this molecule since its original report, but the focus has now returned! The reason is that a 24- annulene belongs formally to a class of molecule with 4n (n=6) \u03c0-electrons, and which makes it antiaromatic<\/strong> according to the (extended) H\u00fcckel rule. This is a select class of molecule, of which the first two members are cyclobutadiene<\/a> and cyclo-octatetraene. The first of these is exceptionally reactive and unstable and is the archetypal anti-aromatic molecule. The second is not actually unstable, but it is reactive and conventional wisdom has it that it avoids the undesirable antiaromaticity by adopting a highly non-planar tub shape and hence instead adopts reactive non-aromaticity. Both these examples have localized double bonds, a great contrast with the molecule which sandwiches them, cyclo-hexatriene (i.e. benzene). The reason for the resurgent interest is that a number of crystalline, apparently stable, antiaromatic molecules have recently been discovered, and ostensibly, molecule 1<\/strong> belongs to this select class!<\/p>\n

So is 1<\/strong> actually anti-aromatic? Let us look at some of the ways in which this might be estimated.<\/p>\n

    \n
  1. One can inspect the bond lengths, measured from X-ray analysis. The longest is 1.463\u00c5, labelled a<\/em> above, and it corresponds to a single bond (value from the crystal structure).<\/li>\n
  2. If the molecule had a bond alternating structure, the adjacent bonds would be expected to be much shorter, in the region of 1.32\u00c5. In fact, they are rather longer, at 1.37\u00c5. Indeed, in the cycloheptatriene part of the molecule, the alternation is much less than one might expect of an anti-aromatic molecule, oscillating between 1.37 and 1.43\u00c5.<\/li>\n
  3. One can also inspect aromaticity via a variety of magnetic indices. The simplest of these is the NICS probe. Placed at a ring centroid, a negative value of this index of around -10ppm indicates aromaticity (this is the value for benzene), whilst a strongly positive value (of up to +20 ppm) indicates anti-aromaticity. Molecule 1<\/strong> has two potential centroids, one placed at the absolute centre of the system, and one placed at the centroid of the ~6-membered ring completed using bond b<\/em> (in reality, the centroids were computed from the positions of ring critical points obtained from an AIM analysis). The NICS values at these two positions are both ~-4.4 ppm (See DOI: 10042\/to-2156<\/a> for details of the calculation). These values does not indicate antiaromaticity! They could even be described as mildly aromatic. So what is going on?<\/li>\n
  4. What about the chemical shifts of the other protons? All the hydrogens attached to sp2 carbons are predicted to resonate at around 6.7ppm (unfortunately Wilke does not report the experimental spectrum), which is typical of an aromatic system (anti-aromatic systems have high upfield shifts for such protons, at around 2ppm or even -2 ppm, see [2]<\/a><\/span> for examples). The two protons of the methylene bridges are also quite different; 2.9 and 0.8 ppm. The latter is the proton endo<\/em> to the cycloheptatrienyl ring, and is typical of a proton placed in the anisotropic magnetic shielding region of e.g. benzene. Thus the cycloheptatrienyl ring is itself behaving as if it were aromatic<\/strong>, whereas the overarching 24-annulene ring is certainly not behaving as if it were antiaromatic<\/strong>.<\/li>\n<\/ol>\n

    One possible explanation involves a concept known as homoaromaticity<\/strong>. The bond marked as b<\/em> could be regarded as completing the 6\u03c0-electron local aromaticity of that ring (it would be formally considered as a 1\u03c0-electron bond, with no underlying \u03c3-framework, see [3]<\/a><\/span> for discussion). So has the case been made for 1<\/strong> being the first clear cut example of a neutral homoaromatic molecule, containing no less than four rings exhibiting this type of aromaticity?<\/p>\n

    There is one further concept that can be introduced. Clar <\/a>(for a discussion, see DOI: [4]<\/a><\/span> proposed that benzenoid 6\u03c0-electron local aromaticity is preferred to less local or more extended cyclic conjugations, if the two compete. Many examples in a type of compound known as polybenzenoid aromatics are known where the most favourable resonance structure is that which maximises the number of Clar rings. More recently, quite a few ostensibly antiaromatic<\/strong> molecules have been shown to attenuate this unfavourable effect by forming instead groups of aromatic Clar islands<\/em> containing delocalized benzene like rings (discussion of this point can be found at [5]<\/a><\/span>. In molecule 1<\/strong>, we could have a new phenomenon; a homoClar<\/strong> ring, formed to avoid antiaromaticity.<\/p>\n

    For further discussion, see the comment posted<\/a> to Steve Bachrach’s blog.<\/p>\n

    References<\/h2>\n
    1. G. Wilke, \"Contributions to Organo\u2010Nickel Chemistry\", Angewandte Chemie International Edition in English<\/i>, vol. 27, pp. 185-206, 1988. https:\/\/doi.org\/10.1002\/anie.198801851<\/a>\n\n<\/li>\n
    2. H.S. Rzepa, \"Lemniscular Hexaphyrins as Examples of Aromatic and Antiaromatic Double-Twist M\u00f6bius Molecules\", Organic Letters<\/i>, vol. 10, pp. 949-952, 2008. https:\/\/doi.org\/10.1021\/ol703129z<\/a>\n\n<\/li>\n
    3. https:\/\/doi.org\/<\/a>\n\n<\/li>\n
    4. A.T. Balaban, P.V.R. Schleyer, and H.S. Rzepa, \"Crocker, Not Armit and Robinson, Begat the Six Aromatic Electrons\", Chemical Reviews<\/i>, vol. 105, pp. 3436-3447, 2005. https:\/\/doi.org\/10.1021\/cr0300946<\/a>\n\n<\/li>\n
    5. C.S.M. Allan, and H.S. Rzepa, \"A computational investigation of the structure of polythiocyanogen\", Dalton Trans.<\/i>, pp. 6925-6932, 2008. https:\/\/doi.org\/10.1039\/b810147g<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

      In 1988, Wilke reported molecule 1 It was a highly unexpected outcome of a nickel-catalyzed reaction and was described as a 24-annulene with an unusual 3D shape. Little attention has been paid to this molecule since its original report, but the focus has now returned! The reason is that a 24- annulene belongs formally to […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"activitypub_content_warning":"","activitypub_content_visibility":"","activitypub_max_image_attachments":5,"activitypub_interaction_policy_quote":"anyone","activitypub_status":"","footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[4],"tags":[50,49,51,2648,20,42],"ppma_author":[2661],"class_list":["post-275","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-anti-aromatic-systems","tag-chemical-shifts","tag-clar-islands","tag-interesting-chemistry","tag-steve-bachrach","tag-x-ray"],"yoast_head":"\nA molecule with an identity crisis: Aromatic or anti-aromatic? - Henry Rzepa's Blog<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=275\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"A molecule with an identity crisis: Aromatic or anti-aromatic? - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"In 1988, Wilke reported molecule 1 It was a highly unexpected outcome of a nickel-catalyzed reaction and was described as a 24-annulene with an unusual 3D shape. 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But\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"Click for 3D","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/10\/dianion.jpeg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":9322,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9322","url_meta":{"origin":275,"position":1},"title":"Anapolar ring currents: a [144]-Annulene.","author":"Henry Rzepa","date":"February 1, 2013","format":false,"excerpt":"This is a recently published (hypothetical) molecule which has such unusual properties that I cannot resist sharing it with you. It is an annulene with 144 all-cis CH groups, being a (very) much larger cousin of (also hypothetical) systems mooted in 2009,. One fascinating novel aspect of Berger's work is\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"A 144-carbon annulene. Click for 3D.","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/02\/C144.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":24503,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=24503","url_meta":{"origin":275,"position":2},"title":"Molecule of the year 2021: Infinitene.","author":"Henry Rzepa","date":"December 16, 2021","format":false,"excerpt":"The annual \"molecule of the year\" results for 2021 are now available ... and the winner is Infinitene., This is a benzocirculene in the form of a figure eight loop (the infinity symbol), a shape which is also called a lemniscate after the mathematical (2D) function due to Bernoulli. The\u2026","rel":"","context":"In "Chiroptics"","block_context":{"text":"Chiroptics","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2644"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2021\/12\/infinitene.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":23360,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=23360","url_meta":{"origin":275,"position":3},"title":"Non-covalent-interaction (NCI) surfaces for two large annulenes (revisited).","author":"Henry Rzepa","date":"February 7, 2021","format":false,"excerpt":"The last post addressed the concept of \"steric clashes\" in a pericyclic reaction transition state as an extension of the time honoured practice of building\u00a0molecular models to analyse reaction outcomes. A modern computer generated model might express this in terms of a NCI (non-covalent-interaction) surface. A few posts ago, I\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":22996,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22996","url_meta":{"origin":275,"position":4},"title":"An interesting aromatic molecule found in Titan’s atmosphere: Cyclopropenylidene","author":"Henry Rzepa","date":"November 7, 2020","format":false,"excerpt":"Cyclopropenylidene must be the smallest molecule to be aromatic due to \u03c0-electrons, with just three carbon atoms and two hydrogen atoms. It has now been detected in the atmosphere of Titan, one of Saturn's moons and joins benzene, another aromatic molecule together with the protonated version of cyclopropenylidene, C3H3+ also\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":399,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=399","url_meta":{"origin":275,"position":5},"title":"The Chirality of Lemniscular Octaphyrins","author":"Henry Rzepa","date":"April 28, 2009","format":false,"excerpt":"In the previous post, \u00a0it was noted that \u00a0M\u00f6bius annulenes are intrinsically chiral, and should therefore in principle be capable of resolution into enantiomers. The synthesis of such an annulene by Herges and co-workers was a racemic one; no attempt was reported at any resolution into such enantiomers. Here theory\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"A 34-Octaphyrin. 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