{"id":9218,"date":"2013-01-19T08:53:49","date_gmt":"2013-01-19T08:53:49","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=9218"},"modified":"2013-01-19T10:09:09","modified_gmt":"2013-01-19T10:09:09","slug":"aromaticity-in-the-benzidine-%cf%80-complex","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9218","title":{"rendered":"Aromaticity in the benzidine-like \u03c0-complex formed from PhNHOPh."},"content":{"rendered":"
\n

The transient \u03c0-complex formed during the “[5,5]” sigmatropic rearrangement of protonated N,O-diphenyl hydroxylamine can be (formally) represented as below, namely the interaction of a six-\u03c0-electron aromatic ring (the phenoxide anion 2<\/strong>) with a\u00a0four-\u03c0-electron phenyl dication-anion pair 1<\/strong>. Can one analyse this interaction in terms of aromaticity?<\/p>\n

\"pi-complex1\"<\/p>\n

I showed previously<\/a> that the interaction between these two components involves the stabilising overlap (donation) of a filled orbital on\u00a02<\/strong> with an empty (acceptor) orbital on the\u00a0dication-anion pair 1<\/strong>. So what does the interaction of a six-electron (and hence 4n+2 H\u00fcckel aromatic) donor ring with a four-electron (and hence formally a H\u00fcckel anti-aromatic) acceptor ring lead to? To find out, I carried out a QTAIM analysis of the ring- and bond-critical points in the topology of the computed electron density of complex, and then evaluated the NICS (nucleus-independent-chemical shift) NMR probe at these points. First, the QTAIM analysis. Green=bond critical points, red=ring and blue=cage.<\/p>\n

\"pi-QTAIM\"<\/p>\n

The NMR analysis<\/a> (\u03c9B97XD\/6-311G(d,p)\/SCRF=water) is shown below. This is for a closed shell wavefunction, which does not include contributions from any open shell biradical singlet.<\/p>\n

\"Click

Click for 3D.<\/p><\/div>\n

    \n
  1. This is the ring centroid of the phenoxide anion 2<\/strong>, and would normally be expected to show a highly diatropic NICS value indicative of ring aromaticity. The value computed for the complex is -0.9 ppm, which is not aromatic!<\/li>\n
  2. This is the ring centroid of the (nominally) antiaromatic 1<\/strong>, and has a value of -5.9 (mildly aromatic; benzene itself on this scale is about -10 ppm). Neither ring is behaving as might be indicated they should prior to their forming the\u00a0\u03c0-complex.<\/li>\n
  3. The remaining points all lie in plane between the two rings; they are unique to the\u00a0\u03c0-complex itself. Point # 3 has a NICS of -14.0 ppm; it is ~located at the centroid of the C=O and C=N bonds.<\/li>\n
  4. This point has NICS -17.1 ppm, being the most highly diatropic of the seven computed.<\/li>\n
  5. This, -12.1, and<\/li>\n
  6. the next -13.2 are ring points lying between the 3,3′ carbons of either ring.<\/li>\n
  7. This point is the (defining) centroid of the whole complex and has NICS -15.7 ppm.<\/li>\n<\/ol>\n

    This reveals that neither individual ring of the complex sustains a a diatropic ring current, but that the region between the two rings, one that defines the\u00a0\u03c0-complex itself, is very highly diatropic. The most simple way of looking at it is that the two rings coming together has created an aromatic complex (I remind again that this is in the closed-shell picture of this system, allowing partial biradical character may influence this). To illustrate this holistic aspect, I show below the most stable of the \u03c0-MOs (this MO in fact resembles to remarkable degree the lowest \u03c0-MO of ferrocene<\/a>\u00a0 which can be used to illustrate the 18-electron filled shells<\/a> of the iron at the centre).\u00a0\u00a0in fact this is one of seven \u03c0-MOs that can be identified<\/a>, making the system a 14<\/strong> (certainly 10)-\u03c0-electron aromatic (the extra electrons come from the oxygen of 2<\/strong> and the C=N region of\u00a01<\/strong>).<\/p>\n

    \"Click

    Click for 3D.<\/p><\/div>\n

    It is most amusing (which is how Michael Dewar might have stated it) that such an unpretentious molecule as PhNHOPh could reveal such surprises. It is also noteworthy that Dewar championed the concept of using aromaticity to determine selection rules for pericyclic reactions, and so he would perhaps have appreciated that the\u00a0\u03c0-complex he suggested for the benzidine pericyclic rearrangement might have its own unique aromatic character.<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    The transient \u03c0-complex formed during the “[5,5]” sigmatropic rearrangement of protonated N,O-diphenyl hydroxylamine can be (formally) represented as below, namely the interaction of a six-\u03c0-electron aromatic ring (the phenoxide anion 2) with a\u00a0four-\u03c0-electron phenyl dication-anion pair 1. Can one analyse this interaction in terms of aromaticity? I showed previously that the interaction between these two […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_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":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[4],"tags":[27,591],"ppma_author":[2661],"class_list":["post-9218","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-chemical","tag-michael-dewar"],"yoast_head":"\nAromaticity in the benzidine-like \u03c0-complex formed from PhNHOPh. - 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=9218\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Aromaticity in the benzidine-like \u03c0-complex formed from PhNHOPh. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"The transient \u03c0-complex formed during the “[5,5]” sigmatropic rearrangement of protonated N,O-diphenyl hydroxylamine can be (formally) represented as below, namely the interaction of a six-\u03c0-electron aromatic ring (the phenoxide anion 2) with a\u00a0four-\u03c0-electron phenyl dication-anion pair 1. Can one analyse this interaction in terms of aromaticity? I showed previously that the interaction between these two […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9218\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2013-01-19T08:53:49+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2013-01-19T10:09:09+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/01\/pi-complex1.svg\" \/>\n<meta name=\"author\" content=\"Henry Rzepa\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Henry Rzepa\" \/>\n\t<meta name=\"twitter:label2\" content=\"Estimated reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"3 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"Aromaticity in the benzidine-like \u03c0-complex formed from PhNHOPh. - Henry Rzepa's Blog","robots":{"index":"index","follow":"follow","max-snippet":"max-snippet:-1","max-image-preview":"max-image-preview:large","max-video-preview":"max-video-preview:-1"},"canonical":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9218","og_locale":"en_GB","og_type":"article","og_title":"Aromaticity in the benzidine-like \u03c0-complex formed from PhNHOPh. - Henry Rzepa's Blog","og_description":"The transient \u03c0-complex formed during the “[5,5]” sigmatropic rearrangement of protonated N,O-diphenyl hydroxylamine can be (formally) represented as below, namely the interaction of a six-\u03c0-electron aromatic ring (the phenoxide anion 2) with a\u00a0four-\u03c0-electron phenyl dication-anion pair 1. Can one analyse this interaction in terms of aromaticity? 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In a lecture on organic aromaticity, the 4n+2\/4n H\u00fcckel rule was introduced (in fact, neither rule appears to have actually been coined in this form by H\u00fcckel himself!). The simplest examples are respectively the cyclopropenyl cation and anion. The former has 2 \u03c0-electrons exhibiting cyclic delocalisation,\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":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2010\/12\/cyclopropenium.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":18058,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=18058","url_meta":{"origin":9218,"position":1},"title":"Cyclopropenium cyclopentadienide: a strangely neutral ion-pair?","author":"Henry Rzepa","date":"April 9, 2017","format":false,"excerpt":"Both the cyclopropenium cation and the cyclopentadienide anion are well-known 4n+2-type aromatic ions, but could the two together form an ion-pair? A search of the Cambridge structure database reveals 52 instances of the cyclopropenium cation with a variety of counter-anions, 77 cyclopentadienide\u00a0anions with a variety of counter-cations and one (SOWMOG,\u2026","rel":"","context":"In "crystal_structure_mining"","block_context":{"text":"crystal_structure_mining","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1745"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2017\/04\/SOWMOG-1002x1024.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":11421,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11421","url_meta":{"origin":9218,"position":2},"title":"Six vs ten aromatic electrons?","author":"Henry Rzepa","date":"October 20, 2013","format":false,"excerpt":"Homoaromaticity is a special case of\u00a0aromaticity\u00a0in which\u00a0\u03c0-conjugation\u00a0is interrupted by a single sp3\u00a0hybridized\u00a0carbon atom (it is sometimes referred to as a suspended \u03c0-bond with no underlying \u03c3-foundation).\u00a0But consider the carbene shown below. This example comes from a recently published article which was highlighted on Steve Bachrach's blog. Here aromaticity has resulted\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\/B10-sigma.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":24881,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=24881","url_meta":{"origin":9218,"position":3},"title":"A four-atom molecule exhibiting simultaneous compliance with H\u00fcckel 4n+2 and Baird 4n selection rules for ring aromaticity.","author":"Henry Rzepa","date":"March 22, 2022","format":false,"excerpt":"Normally, aromaticity is qualitatively assessed using an electron counting rule for cyclic conjugated rings. The best known is the H\u00fcckel 4n+2 rule (n=0,1, etc) for inferring diatropic aromatic ring currents in singlet-state \u03c0-conjugated cyclic molecules\u2021 and a counter 4n rule which infers an antiaromatic paratropic ring current for the system.\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":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2022\/03\/C2B2-300x212.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":1292,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1292","url_meta":{"origin":9218,"position":4},"title":"Clar islands in a \u03c0 Cloud","author":"Henry Rzepa","date":"December 9, 2009","format":false,"excerpt":"Clar islands are found not so much in an ocean, but in a type of molecule known as polycyclic aromatic hydrocarbons (PAH). One member of this class, graphene, is attracting a lot of attention recently as a potential material for use in computer chips. Clar coined the term in 1972\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":"Clar islands in a polybenzenoid hydrocarbon","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/12\/C42H18-diag.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":4967,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4967","url_meta":{"origin":9218,"position":5},"title":"Hunt the charge: the Cheshire cat of chemistry","author":"Henry Rzepa","date":"September 29, 2011","format":false,"excerpt":"Charges in chemistry, like the grin on Lewis Carroll's cat, can be mysterious creatures. Take for example the following structure, reported by Paul Lickiss and co-workers (DOI: 10.1039\/b513203g). A student of chemistry might be wondering what is going on, since this representation seems to \"break the rules\". Thus there is\u2026","rel":"","context":"In "Chemical IT"","block_context":{"text":"Chemical IT","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/09\/Si.jpg?resize=350%2C200","width":350,"height":200},"classes":[]}],"jetpack_likes_enabled":false,"authors":[{"term_id":2661,"user_id":1,"is_guest":0,"slug":"admin","display_name":"Henry Rzepa","avatar_url":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g","0":null,"1":"","2":"","3":"","4":"","5":"","6":"","7":"","8":""}],"_links":{"self":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/9218","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=9218"}],"version-history":[{"count":29,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/9218\/revisions"}],"predecessor-version":[{"id":9251,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/9218\/revisions\/9251"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=9218"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=9218"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=9218"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=9218"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}