The structure of a molecule can change considerably as its energy and thus its electron distribution within the time-domain of dynamic relaxation varies. Based on comparison of approriate measured data of related compounds and supported by quantum chemical calculations, therefore, charge-perturbed and/or sterically overcrowded molecules can be designed. Their preparation, handling, and structural characterization, frequently under extreme and especially largely aprotic conditions, provides some surprises. New structural principles become evident and old-fashioned ones are confirmed. Thus the contact-ion aggregates that form on ultrasonically supported reduction of unsaturated hydrocarbons with sodium metal partly contain dibenzene sodium sandwiches. Vicinal dimethylamino substituents or isoelectronic isopropyl groups cause steric overcrowding and facilitate oxidation to molecular cations by energetically favorable delocalization of the generated positive charge. Molecules and molecular ions in which an even number of II electrons are distributed over a D skeleton containing an odd number of centers preferentially form cyanine subunits. This is demonstrated by the novel ethene dication and dianion salts with central C-C single bonds and molecular halves twisted relative to each other. Altogether in two years well over 50 structures have been determined. Much has been learned from them, especially about electron transfer and contact ion-pair formation in aprotic solvents. Nevertheless, we had to realize that answers to many questions, above all "what crystallizes, how, and why", are still out of reach."ActuuNy, everything is more comjdicuted".calculation of an approximate one-dimensional MNDO energy hypersurface, in which only one of the total of (3n -6) = 30 degrees freedom for the molecule containing n = 12 centers, namely the dihedral angle w(C-SCCS-C) (Scheme 2), is considered. 550@ VCH Verluxsgesellschaft mbH, W-6940 Weinheinz. 1992 OS7O-OS33~92jOSOS-OSSO $3.50+ .2SjO Angew. Chem. Int. Ed. Engl. 31 (1992) 550-581 The Structures of Radical Cations R,NNR;@ in SolutionThe oxidation of silyl-substituted hydrazine derivatives by the oxygen-free and selective one-electron-transfer oxidation reagent AICI,/CH,Cl,[41 yields their radial cations['a' 4 9 61 in their doublet ground states [Eq. (a)].
Die Struktur eines Molekiils kann sich mit seiner Energie und damit seiner Elektronenverteilung innerhalb der Zeitskala dynamischer Relaxation betrlchtlich andern. Ausgehend von Vergleichen geeigneter MeDdaten verwandter Verbindungen und unterstiitzt durch quantenchemische Naherungsrechnungen lassen sich daher ladungsgestorte undloder raumlich iiberfiillte Molekiile entwerfen, deren Synthese, Handhabung und Strukturbestimmung -unter oft extremen und vor allem weitgehend aprotischen Bedingungen -zu manchen Uberraschungen fuhrt. Dabei werden neue Strukturprinzipien deutlich und altbekannte bestatigt. So enthalten einige der bei ultraschallgeforderter Reduktion ungesattigter Kohlenwasserstoffe mit Natriummetall entstehenden Kontaktionenaggregate von Molekiilanionen DibenzolnatriumSandwich-Einheiten. Vicinale Dimethylaminosubstituenten bewirken wie die isoelektronischen Isopropylgruppen sterische Uberfiillung und erleichtern die Oxidation zu Molekiilkationen durch energetisch giinstige Delokalisierung der erzeugten positiven Ladung. Molekiile und Molekiilionen, in denen sich eine gerade Anzahl von .n-Elektronen iiber ein ungeradzahliges cr-Geriist verteilt, bilden bevorzugt Cyanin-Untereinheiten aus; dies belegen auch die erstmals isolierten Salze von Ethen-Dikationen und Ethen-Dianionen mit C-C-Einfachbindung und gegeneinander verdrillten Molekiilhalften. -Insgesamt konnten in zwei Jahren iiber 50 unbekannte Strukturen bestimmt werden. Aus ihnen haben wir vie1 gelernt, insbesondere iiber Elektronentransfer und Kontaktionenpaar-Bildung in aprotischen Losungen. Wir mu& ten aber auch erkennen, daI3 eine Beantwortung vieler Fragen -darunter vor allem ,,was kristallisiert wie und warum?" -noch in weiter Ferne liegt. Eigentlich ist alles vie1 komplizierterVorwort einer Frankfurter Doktorarbeit Ausgangspunkt : Strukturabschatzungen aus MeBdaten und naherungsweisen Energieh yperflachen-BerechnungenAuch der iiberwiegend praparativ engagierte Chemiker profitiert erheblich, wenn er seine Spektren zusatzlich unter dem Gesichtspunkt ,,Molekiilzustands-Fingerabdriicke" betrachtet"]. Je ein Beispiel fur Gasphase und Losung sollen Vorgehen und Erkenntnisgewinn verdeutlichen. Die Struktur von H,CS-CEC-SCH, in der GasphaseDie Gasphasenthermolyse von Bis(methy1thio)cyclobuten-l,2-dion bei 720 K verlauft nach Photoelektronen(PE)-
Single crystals of a radical ion pair salt—the title compound—could be grown after stoichiometric reduction of fluorenone with sodium in aprotic solution under argon at –50°C. The structure determination reveals that a dimeric contact ion pair containing an Na2O2 four‐membered ring is present and shows, for the first time, the bonding between radical anions and countercations: The solvated Na⊕ ion is situated above the plane of the molecule and outside of the πCO plane. This finding further supports the assumption of a π spin delocalization from the radical anion to the metal cation, as derived from the positive ENDOR metal couplings of radical ion pairs in solution.
The redox behaviour of aza-substituted naphtho- and anthraquinones, which offer O=C–C=N– chelate tongs for an advantageous five-membered ring metal cation complexation, is investigated by a combination of cyclovoltammetric and ESR/ENDOR spectroscopic measurements. The formation of paramagnetic contact ion pairs like [(quinoline-5,8-semiquinone)·⊖Me⨁]·, with Me⨁ = Li⨁, Na⨁, Tl⨁, or of triple ion radical cations like [(1,4-diazo-9,10-anthrasemiquinone)eMe⨁]·⨁, with Me⨁ = Li⨁, Na⨁ is corroborated both by shifts of the second reduction potential of up to 0,67 V for e.g. quinoline 5,8-quinone upon addition of Li⨁[B(C6H5)4]⊖ to its DMF solution and by the observation of ESR/ENDOR metal couplings.
Blue zwitterionic 2,6-Di(fert-butyl)-4-(2,4,6-triphenyl-l-pyridinio)phenolate l a can be reduced to its bluegreen radical anion 1using alkaline metals, and oxidized to its colorless radical cation 1 t by Ag(OOCCF,) or electrochemically. ESR/ENDOR spectra of their aprotic THF solutions indicate predominant spin population either in the pyridinium (la-) or in the phenolate ring (la'). Reduction with other alkaline metals Li, Na, or Cs yields no changes in the ESR/ENDOR signal patterns, i.e. provides no indication of radical ion pair formation.The cyclovoltammetrically determined first reduction and oxidation potentials at -1. I 1 V and + 0.26 V, respectively, are both reversible and, in principle, allow to construct a molecular battery. Einleitung. -4-( 1-Pyridinio)phenolat-Betaine 1 sind als farbige Zwitterionen umfangreich untersucht worden [3]. Die bekannte Struktur des [4-(4-Bromophenyl)-2,6-diphenyll-pyridinio)-2,6-diphenyl-phenolates (lc) weist als Besonderheiten einen vermutlich sterisch bedingten Diederwinkel o = 65" zwischen Phenolat-und Pyridinium-Ring sowie einen (C-0)-Abstand dco = 129,l & 0,8 pm aus [4], welcher wie im Fluorenon-Radikalanion (dco = 128,2 pm [5]) zwischen den Werten fur die (C=O)-Bindung im neutralen Fluorenon (dco = 122,O pm [6]) und dem vergleichbaren Benzophenon-Dianion (dco = 140,6 pm [7]) liegt. l a R = t-Bu, X = H b R=Ph,X=H c R=Ph,X=Br
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