Crystal Structure Determination of the Nonclassical 2-Norbornyl Cation

Scholz, Franziska; Himmel, Daniel; Heinemann, Frank W.; Schleyer, Paul von Ragué; Meyer, Karsten; Krossing, Ingo

doi:10.1126/science.1238849

Cited by 112 publications

(104 citation statements)

References 60 publications

(32 reference statements)

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“…Surprisingly,e ven hexagonalplanar carbon has been considered viable (based on theoretical calculations by Exner and Schleyer) in boron compounds, for example,inC B 6 2À . [14] Recently,t he isolation of the non-classical norbornyl cation and its crystal structure analysis was achieved by Scholz et al [15] In this species,aC À Cd ouble bond (bond length of 1.39 )i nteracts with ac arbocationic methylene group at adistance of 1.80 ,which can be rationalized as a3-center 2-electron bond. These results encouraged us to attempt the isolation of C 6 (CH 3 ) 6 2+ ,w here the hexacoordinated carbon is exclusively bound to carbon atoms.…”

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confidence: 99%

Crystal Structure Determination of the Pentagonal‐Pyramidal Hexamethylbenzene Dication C₆(CH₃)₆²⁺

Malischewski

Seppelt

2016

Angew Chem Int Ed

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In contrast to the well-known 2-norbornyl cation, the structure of which was am atter of long debate until its pentacoordinated nature was recently proven by an X-ray structure,t he pentagonal-pyramidal dication of hexamethylbenzeneh as received considerably less attention. This species was first prepared by Hogeveen in 1973 at low temperatures in magic acid (HSO 3 F/SbF 5 ), for whichh ep roposed an onclassical structure (containing ah exacoordinated carbon) based on NMR spectroscopya nd reactivity studies,b ut no Xray crystal structure has been reported. C 6 (CH 3 ) 6 2+ can be obtained through the dissolution of hexamethyl Dewar benzeneepoxideinHSO 3 F/SbF 5 and crystallized as the SbF 6À salt upon addition of excess anhydrous hydrogen fluoride.T he crystal structure of C 6 (CH 3 ) 6 2+ (SbF 6 À ) 2 ·HSO 3 Fc onfirms the pentagonal pyramidal structure of the dication. The apical carbon is bound to one methyl group (distance 1.479(3) )and to the five basal carbon atoms (distances 1.694(2)-1.715(3) ).The tetravalencyo fc arbon and the hexagonal-planar ring structure of benzene are fundamental axioms of organic chemistry,a nd were developed 150 years ago by KekulØ. [1,2] Chemists have long been fascinated by finding exceptions from these rules.S tarting in the 1960s,v alence isomers of benzene were prepared through elaborate organic syntheses, [3][4][5][6] while hypercoordinated carbon compounds entered the stage through the pioneering work of Olah on superacids and non-classical cations. [7,8] Theb onding situation in protonated methane (CH 5 + ), which is the most prominent representative of pentacoordinated carbon, is best described as a3-center 2-electron bond between H 2 and CH 3 + ,but its isolation in bulk is elusive.The replacement of hydrogen with organogold species lead to the isolation of penta-and hexacoordinated methanium salts by Schmidbaur.T hese carbido clusters,[ (Ph 3 PAu) 5 C]+ and [(Ph 3 PAu) 6 C] 2+ ,a re significantly stabilized by aurophilic interactions. [9,10] Recently,arelated structural motive has been identified in the iron-molybdenum nitrogenase cofactor, which contains hexacoordinated C 4À .[11] Taking ac ompletely different approach, Akiba and co-workers prepared formally hypercoordinated organic species through special ligand design, where an electrophilic carbon atom is intramolecularly coordinated by two/four neighboring methoxy groups. TheC À Odistances are in the range of 2.43(1)-2.45(1) and 2.641(5)-2.750(5) ,r espectively, [12,13] thus indicating only weak interactions.Instark contrast, hypercoordinated carbon with much shorter bonds can be found in carboranes. Nevertheless,t hese common structural motives of boron chemistry are only subtly changed by replacing boron with an isolobal carbon fragment. Surprisingly,e ven hexagonalplanar carbon has been considered viable (based on theoretical calculations by Exner and Schleyer) in boron compounds, for example,inC B 6 2À . [14] Recently,t he isolation of the non-classical norbornyl cation and its crystal structure ana...

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confidence: 99%

Crystal Structure Determination of the Pentagonal‐Pyramidal Hexamethylbenzene Dication C₆(CH₃)₆²⁺

Malischewski

Seppelt

2016

Angew Chem Int Ed

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“…Especially questions centered around classicala nd non-classicalc arbocationsf ocusedo nt his keystructure andonlyveryrecentlythe existenceofpenta-coordinate carboncould be proven experimentally on grounds of diffraction experiments based on single crystals [1,2].T he latter finding motivated us to include bicycloheptane-derived ligands into our research efforts in synthesizing novel coordination compounds of transition metals. As aconvenient starting point for our studies, the title compound was chosen as it is commercially available.…”

Section: Discussionmentioning

confidence: 99%

Redetermination of the crystal structure of endo-2-carboxy-bicyclo[2.2.1]- heptane, C8H12O2

Schalekamp

Hosten

Betz

2015

Zeitschrift Für Kristallographie - New Crystal Structures

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Source of materialThec ompoundw as obtained commercially (Frinton Laboratories). Crystals suitable for the diffraction study were obtained upon sublimation of the product at room temperature. Experimental detailsCarbon-bound Hatomswere placed in calculated positions (C-H 0.99 Åfor methylene groups and C-H 1.00 Åfor methine groups) and were included in the refinement in the riding model approximation, with U iso (H) setto1.2U eq (C). The Hatom of the carboxyl group was allowed to rotate with afixed angle around the C-O bond to best fit the experimental electron density (HFIX 147 in the SHELX program suite [11]), with U iso (H) sett o1 .5U eq (O). Due to very low residual electron densities after refinement of givenstructuralmodel,nodisorder model could be set up. DiscussionThe bicycloheptane scaffold has been afocusofresearch in organic chemistry for many decades. Especially questions centered around classicala nd non-classicalc arbocationsf ocusedo nt his keystructure andonlyveryrecentlythe existenceofpenta-coordinate carboncould be proven experimentally on grounds of diffraction experiments based on single crystals [1,2].T he latter finding motivated us to include bicycloheptane-derived ligands into our research efforts in synthesizing novel coordination compounds of transition metals. As aconvenient starting point for our studies, the title compound was chosen as it is commercially available. The molecular and crystal structure of endonorbornane-2-carboxylic acid have been mentioned in the literature once, however, no 3D coordinates have been provided [3]. As the structures of envisioned target molecules shall be elucidated by means of diffraction experiments on single crystals, the structure of the title compound was redetermined. The title compound is the endo-configured carboxylic acid of norbornane. A puckering analysis according to Cremer &Pople [4]showed the five-membered rings in the norbornane scaffold to adopt an E 3 and an E 5 conformation, respectively [5]. The flap atom in both cases is carbon atom C7. For the six-membered ring, a B 2,5 (B C2,C5 )conformation was found [6]. The C-O bond lengths were measured at 1.2130(14) Åa nd 1.3206(14) Åf or the doublebonded and the single-bonded oxygen atom.The latter two values are in good agreement with other values reported for carboxylic acidsw hose metrical parameters have been deposited with the Cambridge Structural Database [7].Inthe crystal, classical hydrogen bondso ft he O-H×××Ot ype arep redominant. There are C-H×××Oc ontacts whose range falls below the sum of van-derWaals radii of the atomsparticipating in them.While the hydrogen bonds give rise to the formation of centrosymmetric dimers as often observed for carboxylic acids [8], the C-H×××Ocontacts extend thesed imeric unitst os trands along the crystallographic a axis. The latter contacts are supported by one of the hydrogen atomsatabridge-head position of the norbornane scaffold and employ the carbonylic oxygen atom as acceptor thus giving rise to two anti-dromic C-H×××Oc ontact chains. In terms...

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“…A vonatkozó könyv már két kiadást ért meg, 9 2013-ban egy közleményben Krossing, Meyer, Schleyer és munkatársaik leírták a norbornil-kation egy alumínium-bromid sójának a szintézisét (3. ábra). 13 Három krisztallitot sikerült izolálniuk és elkészítették ezek XRD-felvételét 40 K-en. A 4. ábrán bemutatott topológiák szerint a C 6 -C 1 és C 6 -C 2 kötések gyakorlatilag azonos hosszúságúk és a rövidebb C 2 -C 1 kötés világosan jelzi ennek részleges kettõs kötés jellegét.…”

Section: A Könyvekunclassified

“…Ennek kulcsvegyülete a CH 5 + metónium-ion (protonált metán, 10). További érdekesebb képviselõk a metán gyök-kation (11), az eténium-ion (protonált etén, 12) és a sarkon-protonált ciklopropóniumion (13). A gyûrûs kationok közül említhetjük a legegyszerûbb Hückeloid aromás 2ð rendszert (Breslow ciklopropenil-kationja, 14), a benzéniumiont (protonált benzol, 15) és a 6ð aromás tropíliumiont (16).…”

Section: "éGi Jelek"unclassified

Oláh György munkássága a Nobel-díj utáni években

Molnár¹

2017

MKF

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BevezetésOláh professzor mindkét önéletrajzi könyvében szerepel egy fejezet a következõ címmel: "A Nobel-díjat követõ évek". A "nem klasszikus ion"-vita és lezárásaAz 1960-as évektõl kezdve hosszú idõn át folyt a "nem klasszikus ion"-vita néven ismert polémia a 2-norbornil-kation (biciklo[2.2.1]heptil kation, C 7 H 11 + ) szerkezetérõl. Ez a vita még jóval a Nobel-díj elnyerése elõtt történt, tehát szorosan véve nem tartozik a közlemény címében jelzett idõszakhoz. Ugyanakkor egy közelmúltbeli fontos, a vitát véglegesen eldöntõ eredmény szükségessé teszi e témakör rövid tárgyalását is.A vita lényegében Winstein és Trifan kezdeti megfigyeléseitõl indult. 1949 és 1952 között tanulmányozták a 2-norbornil-brozilátok szolvolízisét és azt találták, hogy az endo-izomer kizárólag exo konfiguációjú terméket adott. Ezzel szemben az exo izomer exo szubsztituált termékké alakult és ez utóbbi reakció 350-szer gyorsabban ment végbe (1. ábra).1. Ábra. Izomer 2-norbornil-származékok szolvolízise. Ábra.A 2-norbornil-kation. 1: nem klasszikus, ó-delokalizált ion (két ábrázolási lehetõség); 2: klasszikus, ekvilibráló ionok.Magyarázatul egy ó-delokalizált, áthidalt, szimmetrikus hídszerkezetû, két-elektronos-három-centrumú (2e-3c) norbornil-kation köztiterméket tételeztek fel, amely az exo-prekurzor esetén anchimerikus részvétel révén segíti a folyamatot 10 (2. ábra, 1).123. évfolyam, 4. szám, 2017. Magyar Kémiai Folyóirat -Oláh György emlékülés elõadásai 203

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Crystal Structure Determination of the Nonclassical 2-Norbornyl Cation

Cited by 112 publications

References 60 publications

Crystal Structure Determination of the Pentagonal‐Pyramidal Hexamethylbenzene Dication C₆(CH₃)₆²⁺

Crystal Structure Determination of the Pentagonal‐Pyramidal Hexamethylbenzene Dication C₆(CH₃)₆²⁺

Redetermination of the crystal structure of endo-2-carboxy-bicyclo[2.2.1]- heptane, C8H12O2

Oláh György munkássága a Nobel-díj utáni években

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Crystal Structure Determination of the Nonclassical 2-Norbornyl Cation

Cited by 112 publications

References 60 publications

Crystal Structure Determination of the Pentagonal‐Pyramidal Hexamethylbenzene Dication C6(CH3)62+

Crystal Structure Determination of the Pentagonal‐Pyramidal Hexamethylbenzene Dication C6(CH3)62+

Redetermination of the crystal structure of endo-2-carboxy-bicyclo[2.2.1]- heptane, C8H12O2

Oláh György munkássága a Nobel-díj utáni években

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Crystal Structure Determination of the Pentagonal‐Pyramidal Hexamethylbenzene Dication C₆(CH₃)₆²⁺

Crystal Structure Determination of the Pentagonal‐Pyramidal Hexamethylbenzene Dication C₆(CH₃)₆²⁺