{"id":18058,"date":"2017-04-09T13:37:01","date_gmt":"2017-04-09T12:37:01","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=18058"},"modified":"2017-04-12T13:33:32","modified_gmt":"2017-04-12T12:33:32","slug":"cyclopropenium-cyclopentadienide-a-strangely-neutral-ion-pair","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=18058","title":{"rendered":"Cyclopropenium cyclopentadienide: a strangely neutral ion-pair?"},"content":{"rendered":"
\n

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?
\n \"\"<\/a><\/p>\n

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, private communication to CSD) where the two sub-structures are common. The pyridinium-cyclopropenium fragment is actually a di-cation stabilized with dimethylamino substituents, with these charges balanced\u00a0by two cyclopentadienide anions stabilized with ester substituents. The stacking distance between the ion-pairs is ~3.5-3.6\u00c5, a bit larger than normal\u00a0\u03c0-\u03c0 stacking distances of 3.2-3.3\u00c5<\/p>\n

\"\"<\/p>\n

So could a “pure” cyclopropenium cyclopentadienide ion-pair exist, and if so what would its\u00a0\u03c0-\u03c0 stacking distance be? A\u00a0\u03c9B97XD\/Def2-TZVPPD\/SCRF=water calculation (DOI: 10.14469\/hpc\/2442<\/a>)\u00a0provides one answer to this question; 2.57\u00c5!‡<\/sup> It is a true minimum in the potential energy surface (all +ve force constants) with a calculated dipole moment of only 7.57D. This species is “only” 27.1 kcal\/mol higher in \u0394G than the neutral hydrocarbon (DOI: 10.14469\/hpc\/2443<\/a>), a difference which is as low as it is because of the gain in aromatic stabilization of two rings upon ion-pair formation.<\/p>\n

\"\"<\/p>\n

A few posts back, I was considering candidates for the most polar neutral compound synthesized<\/a>\u00a0and I suggested a candidate with a dipole moment of ~22D, based as it happens on\u00a0cyclopropenium\u00a0and\u00a0cyclopentadienide rings directly connected by a bond. So when this bond is removed and the two rings are allowed to stack one above the other, we now have an interesting inversion of the original challenge: what is the least-polar ionic organic compound<\/strong><\/span> (ionic in the sense of being an unconnected ion-pair)?<\/p>\n

Here are some more properties of this intriguing “neutral” ion-pair.<\/p>\n

    \n
  1. It has a number of low-frequency modes with correspond to the two rings moving with respect to each other (\u03bd 216 cm-1<\/sup>)
    \n \"\"<\/li>\n
  2. The molecular electrostatic potential illustrates the sense of polarization, with negative region (orange) residing on the 5-membered ring:\"\"<\/li>\n
  3. The most stable \u03c0-type molecular orbital (below) reminds of the \u03c0-complex formed in the benzidine rearrangement<\/a>\u00a0and that in fact modelling this ion-pair may require a multi-reference (CASSCF) wavefunction, with the single-determinantal one used here only being a first approximation.\"\"<\/li>\n
  4. A QTAIM analysis of the electron density topology shows only weak “bond” connectors between the two rings, with \u03c1(r) being typical of weak interactions such as hydrogen bonds.\"\"<\/li>\n
  5. An ELF (electron localisation function) analysis also holds no surprises, with all the electron density basins (purple) confined to the two rings, just as expected of an ion-pair.\"\"<\/li>\n
  6. I will leave one further question to a future discussion; what happens to the aromaticity and ring currents<\/a> of the two individual rings as they combine to form this ion-pair? Might this property be connected to the very close separation between the two rings?<\/li>\n<\/ol>\n

    So we have a remarkably “neutral” ionic hydrocarbon to match the “ionic” neutral organic molecules previously discussed. This ion-pair may yet prove to have interesting properties, even if is unlikely to be synthesized without the addition of stabilising substituents.<\/p>\n

    \n

    ‡<\/sup> For example, the stacking distance in graphite<\/a> is 3.35Å.<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    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, private communication to CSD) where […]<\/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":[1745,4],"tags":[1830,2117,2118,1395,2116,1714,2050,142,1619],"ppma_author":[2661],"class_list":["post-18058","post","type-post","status-publish","format-standard","hentry","category-crystal_structure_mining","category-interesting-chemistry","tag-anions","tag-aromatization","tag-cation-pi-interaction","tag-chemistry","tag-cyclopentadienyl-anion","tag-ion","tag-ion-association","tag-potential-energy-surface","tag-simple-aromatic-rings"],"yoast_head":"\nCyclopropenium cyclopentadienide: a strangely neutral ion-pair? - 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=18058\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Cyclopropenium cyclopentadienide: a strangely neutral ion-pair? - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"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, private communication to CSD) where […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=18058\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2017-04-09T12:37:01+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2017-04-12T12:33:32+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2017\/04\/ion-pair-page001.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":"Cyclopropenium cyclopentadienide: a strangely neutral ion-pair? - 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=18058","og_locale":"en_GB","og_type":"article","og_title":"Cyclopropenium cyclopentadienide: a strangely neutral ion-pair? - Henry Rzepa's Blog","og_description":"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, private communication to CSD) where […]","og_url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=18058","og_site_name":"Henry Rzepa's Blog","article_published_time":"2017-04-09T12:37:01+00:00","article_modified_time":"2017-04-12T12:33:32+00:00","og_image":[{"url":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2017\/04\/ion-pair-page001.svg","type":"","width":"","height":""}],"author":"Henry Rzepa","twitter_card":"summary_large_image","twitter_misc":{"Written by":"Henry Rzepa","Estimated reading time":"3 minutes"},"schema":{"@context":"https:\/\/schema.org","@graph":[{"@type":"Article","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=18058#article","isPartOf":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=18058"},"author":{"name":"Henry Rzepa","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281"},"headline":"Cyclopropenium cyclopentadienide: a strangely neutral ion-pair?","datePublished":"2017-04-09T12:37:01+00:00","dateModified":"2017-04-12T12:33:32+00:00","mainEntityOfPage":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=18058"},"wordCount":522,"commentCount":6,"image":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=18058#primaryimage"},"thumbnailUrl":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2017\/04\/ion-pair-page001.svg","keywords":["Anions","Aromatization","Cation\u2013pi interaction","Chemistry","Cyclopentadienyl anion","Ion","Ion association","potential energy surface","Simple aromatic rings"],"articleSection":["crystal_structure_mining","Interesting chemistry"],"inLanguage":"en-GB","potentialAction":[{"@type":"CommentAction","name":"Comment","target":["https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=18058#respond"]}]},{"@type":"WebPage","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=18058","url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=18058","name":"Cyclopropenium cyclopentadienide: a strangely neutral ion-pair? 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All is not what it seems with this Sn2 reaction!","author":"Henry Rzepa","date":"April 4, 2019","format":false,"excerpt":"Previously, I explored the Graham reaction to form a diazirine. The second phase of the reaction involved an Sn2' displacement of N-Cl forming C-Cl. Here I ask how facile the simpler displacement of C-Cl by another chlorine might be and whether the mechanism is Sn2 or the alternative Sn1. The\u2026","rel":"","context":"In "reaction mechanism"","block_context":{"text":"reaction mechanism","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1086"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":7964,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7964","url_meta":{"origin":18058,"position":1},"title":"Text-books and the bromination of ethene.","author":"Henry Rzepa","date":"October 14, 2012","format":false,"excerpt":"There is often a disconnect between how a text-book (schematically) represents a reaction and a more quantitive \"reality\" revealed by quantum mechanics. Is the bromination of ethene to give 1,2-dibromoethane one such example? Text-books will show how ethene interacts with bromine to form a cyclic bromonium cation, which with the\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.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/10\/Br2b2.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":7027,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7027","url_meta":{"origin":18058,"position":2},"title":"Joining up the pieces. Peroxidation of ethyne.","author":"Henry Rzepa","date":"July 9, 2012","format":false,"excerpt":"Sometimes, connections between different areas of chemistry just pop out (without the help of semantic web tools, this is called serendipity). So here, I will try to join up some threads which emerge from previous posts. I had noted that antiaromaticity in cyclopropenium anion is lessened by the system adopting\u2026","rel":"","context":"In "Curly arrows"","block_context":{"text":"Curly arrows","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2327"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":2973,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=2973","url_meta":{"origin":18058,"position":3},"title":"(anti)aromaticity avoided: a tutorial example","author":"Henry Rzepa","date":"December 7, 2010","format":false,"excerpt":"More inspiration from tutorials. 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":17692,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17692","url_meta":{"origin":18058,"position":4},"title":"What is the (calculated) structure of a norbornyl cation anion-pair in water?","author":"Henry Rzepa","date":"April 1, 2017","format":false,"excerpt":"In a comment appended to an earlier post, I mused about the magnitude of the force constant relating to the interconversion between a classical and a non-classical structure for the norbornyl cation. Most calculations indicate the force constant for an \"isolated\" symmetrical cation\u00a0is +ve, which means it is a true\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\/03\/b3lypd3bj.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":3003,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=3003","url_meta":{"origin":18058,"position":5},"title":"Janus mechanisms (the past and the future): Reactions of the diazonium cation.","author":"Henry Rzepa","date":"December 11, 2010","format":false,"excerpt":"Janus was the mythological Roman god depicted as having two heads facing opposite directions, looking simultaneously into the past and the future. Some of the most ancient (i.e. 19th century) known reactions can be considered part of a chemical mythology; perhaps it is time for\u00a0a Janus-like look into their future.\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\/diazonium.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\/18058","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=18058"}],"version-history":[{"count":20,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/18058\/revisions"}],"predecessor-version":[{"id":18120,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/18058\/revisions\/18120"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=18058"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=18058"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=18058"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=18058"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}