Effect of Silyl Substitution on the Frontier Orbitals of Diimines

Abstract

Temporary anion formation in diimines bearing CH3, C(CH3)3, and Si(CH3)3 substituents is studied by means of electron transmission spectroscopy and ab initio calculations. The sizable reduction in the HOMO−LUMO transition energy caused by silyl substitution is found to be due not only to destabilization of the HOMO, as known from literature data, but also to stabilization of the LUMO, the mainly π*N  N empty orbital.

Isolation of a Labile Homoleptic Diazenium Cation

“…It was not until 1968 that Wiberg et al succeeded in the isolation of pale blue N,N'-bis(trimethylsilyl)diazene (2) from the reaction of p-tosylazide with lithium tris(trimethylsilyl)hydrazide (1) at low temperatures (Scheme 1). [4] In 1974 Veith and Bärnighausen published a single-crystal structure determination of 2, which confirmed the postulated trans configuration but revealed an extraordinary short N-N distance of 1.171 (7) . [5] This value, however, lies outside the reported range for known trans diazenes (1.219-1.254 ; cf.…”

“…[26,27] Finally, it is interesting to note that the previous structure determination of diazene 2 needs to be revised on the basis of our data. [5] Although the cell data are in agreement (space group P2 1 /c, Z = 2), the previously reported structure of 2 was refined without taking into account that the whole molecule is disordered, which results for example in an unrealistically short NÀN bond length of 1.171 (7) and a large N-N-Si angle of 120.0(4)8. However, if the molecule is refined as disordered over two positions then a significantly longer N-N distance of 1.227(2) (cf.…”

“…Moreover, computational studies predicted longer N À N bonds, which were consistent with experimental and theoretical trends found for other diazenes. [5,7] Reports on the chemistry of diazenium cations [R 2 N=N-R] + (R = alkyl, aryl) are restricted to only a few examples or are even unknown (R = SiMe 3 ) in the literature. [8][9][10] Herein, we focus on the synthesis of highly labile silylated diazenium species.…”

Die Isolierung eines labilen homoleptischen Diazeniumkations

“…Erst 1968 gelang Wiberg die Isolierung des hellblauen N,N'-Bis(trimethylsilyl)diazens (2) nach der Reaktion von p-Tosylazid mit Lithium-tris(trimethylsilyl)hydrazid (1) bei tiefen Temperaturen (Schema 1). [4] 1974 publizierten Veith und Bärnighausen eine Einkristallrçntgenstrukturanalyse von 2, die die postulierte trans-Konfiguration bestätigte, allerdings einen außergewçhnlich kleinen N-N-Abstand von 1.171 (7) zeigte. [5] Dieser Abstand liegt jedoch außerhalb des publizierten Bereiches bekannter trans-Diazene (1.219-1.254 ; vgl.…”

“…Zudem sagten Rechnungen grçßere N-N-Bindungslängen voraus, die im Einklang mit experimentellen und theoretischen Trends anderer Diazene sind. [5,7] Berichte über die Chemie der Diazeniumkationen [R 2 N= N-R] + (R = Alkyl, Aryl) sind auf wenige Bespiele beschränkt oder sogar in der Literatur nicht vorhanden (R = SiMe 3 ). [8][9][10] Hier mçchten wir uns auf die Synthese der sehr instabilen silylierten Diazeniumspezies fokussieren.…”