Die Struktur eines Molekuls kann sich mit seiner Energie und damit seiner Elektronenverteilung innerhalb der Zeitskala dynamischer Relaxation betrachtlich andern. Ausgehend von Vergleichen geeigneter Mesdaten verwandter Verbindungen und unterstutzt durch quanten-chemische Naherungsrechnungen lassen sich daher ladungsgestorte und/oder raumlich uber-fullte Molekule 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 Molekulanionen Dibenzolnatrium-Sandwich-Einheiten. Vicinale Dimethylaminosubstituenten bewirken wie die isoelektronischen Isopropylgruppen sterische Uberfullung und erleichtern die Oxidation zu Molekul-kationen durch energetisch gunstige Delokalisierung der erzeugten positiven Ladung. Molekule und Molekulionen, in denen sich eine gerade Anzahl von π-Elektronen uber ein ungeradzahliges σ-Gerust 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 Molekulhalften. — Insgesamt konnten in zwei Jahren uber 50 unbekannte Strukturen bestimmt werden. Aus ihnen haben wir viel gelernt, insbesondere uber Elektronentransfer und Kontaktionenpaar-Bildung in aprotischen Losungen. Wir musten aber auch erkennen, das eine Beantwortung vieler Fragen - darunter vor allem „was kristallisiert wie und warum?” - noch in weiter Ferne liegt.

Verzerrte Moleküle : Störungsdesign, Synthesen und Strukturen

The highly twisted tetrakis(di-tert-butylmethylsilyl)disilene 4 was prepared and reacted with tBuLi in THF, producing disilene anion radical 5 upon one-electron reduction. The anion radical 5 was isolated in the form of its lithium salt as extremely air- and moisture-sensitive red crystals. The molecular structure of 5 was established by X-ray crystallography, which showed a nearly orthogonal structure (twisting angle of 88°) along the central Si−Si bond, with a length of 2.341(5) A, which is 3.6% elongated relative to that of 4. The interesting feature of 5 is that one of the central Si atoms has radical character, whereas the other Si atom has silyl anion character. An electron spin resonance (ESR) study of the hyperfine coupling constants of the 29Si nuclei indicates that rapid spin exchange occurs between these central Si atoms on the ESR time scale.

Isolable Anion Radical of Blue Disilene (tBu2MeSi)2SiSi(SiMetBu2)2 Formed upon One-Electron Reduction: Synthesis and Characterization

Synthesis and Characterization of a Cyclobutadiene Dianion Dilithium Salt: Evidence for Aromaticity

An Unexpected Reaction of Silacyclopropene To Form an Organodilithiosilane: Isolation and Characterization of a 1,1-Dilithiosilane Derivative

The novel complex cis-[(ITMe)2Pd(SiMe3)2 (ITMe=1,3,4,5-tetramethylimidazol-2-ylidene) has been synthesized by mild oxidative cleavage of Me3SiSiMe3 using [(ITMe)2Pd0]. The use of this complex as precatalyst for the cis-bis(silyl)ation of alkynes using unactivated disilanes is reported.

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Synthesis of an [(NHC)2Pd(SiMe3)2] Complex and Catalytic cis‐Bis(silyl)ations of Alkynes with Unactivated Disilanes

Isolation and characterization of 1,2-dilithio[tetrakis(trimethylsilyl)]ethane. The first crystal structure of nonconjugated 1,2-dilithioethane

Phenyl- and silyl-substituted ethylene dianion dilithiums, [1-phenyl-1,2,2-tris(trimethylsilyl)ethylene]{bis[(dimethoxyethane)lithium(I)]} (2) and [1,1-diphenyl-2,2-bis(trimethylsilyl)ethylene][tris(tetrahydrofuran)dilithium(I)] (4), were prepared and characterized by X-ray crystallography as well as NMR spectra. The crystal structures of 2 and 4 revealed interesting structural features. One of the lithium cations was located near to the two central carbon atoms, ipso and ortho carbons of the phenyl ring, while the other one was found above the benzene ring. The central C–C bond is twisted by ca. 90° for 2 and ca. 70° for 4.

Highly Twisted Ethylene Dianion Dilithium Stabilized by Phenyl and Silyl Groups.

6Li and 13C NMR spectra of 1,2-dilithio[tetrakis(trimethylsilyl)]ethane revealed a doubly bridged π-structure in aromatic solvents similar to that observed in the crystals. However, the dianion in THF, which was found to exist as a solvent–separated ion pair of Li+(THF)4 and a species bridged by one Li atom, was reactive to undergo anionic rearrangement.

Structure of 1,2-Dilithio[tetrakis(trimethylsilyl)]ethane in Solution. Observation of Lithium–Bridged Structures and Unexpected Anionic Rearrangement

Phenyl[tris(trimethylsilyl)]ethylene bis(lithium dimethoxyethane) was prepared as black crystals. The molecular structure determined by the X-ray diffraction method shows several interesting features. One of the lithium atoms is located near the central two carbon atoms, while the other is above the benzene ring and the central C–C bond is twisted by 92°.

Tetsuo Nakanishi

Papers

Isolation and characterization of 1,2-dilithio[tetrakis(trimethylsilyl)]ethane. The first crystal structure of nonconjugated 1,2-dilithioethane

Highly Twisted Ethylene Dianion Dilithium Stabilized by Phenyl and Silyl Groups.

Structure of 1,2-Dilithio[tetrakis(trimethylsilyl)]ethane in Solution. Observation of Lithium–Bridged Structures and Unexpected Anionic Rearrangement

Molecular structure of phenyl[tris(trimethylsilyl)]ethylene dilithium complex. An orthogonal dianion