{"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":"2017-04-09T13:37:01","modified_gmt":"2017-04-09T12:37:01","slug":"cyclopropenium-cyclopentadienide-a-strangely-neutral-ion-pair","status":"publish","type":"post","link":"https:\/\/rzepa.net\/blog\/2017\/04\/09\/cyclopropenium-cyclopentadienide-a-strangely-neutral-ion-pair\/","title":{"rendered":"Cyclopropenium cyclopentadienide: a strangely neutral ion-pair?"},"content":{"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?
\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","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":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[6,11],"tags":[167,209,392,542,713,1308,1309,1962,2243],"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"],"_links":{"self":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/18058","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/comments?post=18058"}],"version-history":[{"count":0,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/18058\/revisions"}],"wp:attachment":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/media?parent=18058"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/categories?post=18058"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/tags?post=18058"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}