Bis(trimethylsilyl)diimine: Preparation, Structure, and Reactivity

Abstract

Dedicated to my father, Professor Egon Wiberg, on the occasion of his 70th birthdayThe highly reactive compound bis(trimethylsily1)diimine (BSD), which was first prepared by oxidation of lithium tris(trimethylsilyl)hydrazide, is light blue, sensitive to thermolysis and hydrolysis, and ignites spontaneously in air. On the basis of electron transfer, acid-base, or freeradical reactions, it acts in particular as a (preparatively useful) redox system and as an agent for the introduction of azo groups. Redox reactions lead by oxidation or reduction of the other reactant through two oxidation stages to hydrazine derivatives or molecular nitrogen, and in the case of electrochemical reduction, to BSD radical-anions. Azo-group transfers, on the other hand, yield new inorganic azo compounds with no change in the oxidation state of the diimine group. Introductionas the fist diimine completely substituted with silyl groups131, the unexpectedly long-wave light absorption and the resulting radiant blue color of the compound immediately aroused our particular interest. The investigation of this substance, which has lost none of its orig- inal fascination, has proved worth while ; it has revealed a varied, unusual, and interesting chemistry.Various investigators had attempted to prepare a bis-(sily1)diimine. The ability of carbon to fonn azo compounds (Z), which are often surprisingly stable (for example, azobenzene is stable up to 600°C[41), makes the synthesis of "azosilanes" (2) a challenge in view ofthe group relationship between carbon and silicon. A possible preparation method was seen in the oxidation of 1,2-bis(silyl)hydrazines (3).However, several attempts to verify this reactionc5-' I were initially unsuccessful because of failure to find an oxidizing agent that could oxidize bis(sily1)hydrazines (3) only to the stage of the readily oxidizable bis(sily1)-diimines ( 4 ) without oxidizing these further to nitrogen '']. By the action of p-toluenesulfonyl azide on lithium tris-(trimethylsily1)hydrazide (X = Li, Y = SiR, in (3)), we were able to obtain BSD as a readily sublimable product that crystallizes in .the form of needles and melts at -3 OC131. Including BSD and a bis(bory1)diimine that is obtainable from it["], there are now ten classes of isolable diimine derivatives X-N=N-X(counting diimine itself) in which ligands X are linked to the azo group via maingroup elements having various atomic numbers:The arenesulfonyl azide, as in its reactions with CH-acidic methylene derivatives"'], evidently adds first to the silylhydrazide to form a nitrogen chain compound R' SO,-N=N-N-N(SiR,)-N(SiR,),,which can decompose after migration of silyl groups to give the final reaction products.Instead of arenesulfonyl azides, arenesulfonyl chlorides may be used as the oxidizing agents[1g! In this case, however, the reaction no longer follows a single course:R'S0,Li + R,SiClSince BSD is also difficult to separate from trirnethylchlorosilane, the process is less suitable for the preparation of this diimine. However, it may offer advantag...

Isolation of a Labile Homoleptic Diazenium Cation

“…tBu-N = N-tBu > 200 8C, Ph-N = N-Ph > 600 8C), which even lies below its melting point of about À3 8C. [16,17] Since diazene 2 decomposes at ambient temperatures with a half-time of 7.6 h (zero-order reaction, in a 5:1 mixture of npentane/toluene) it must be kept at temperatures below À50 8C in solution or in the solid state (see Figure S14 in the Supporting Information). [18] A suitable low-temperature synthesis had to be found to generate the tris(trimethylsilyl)diazenium ion [(Me 3 Si) 2 3 Si-X with X = Cl, F; CB = [CHB 11 H 5 X 6 ], X = Br, Cl).…”

Bond-Forming Processes Enabled by Silicon-Masked Aryl- and Alkyl-Substituted Diazenes

“…Early endeavors demonstrated that the stability of the parent diimide is significantly increased by replacing the hydrogen atom by a silyl group . In this context, Wiberg originally isolated bis­(trimethylsilyl)­diazene (Me 3 Si–NN–SiMe 3 ) as a light blue liquid, displaying both an improved thermal stability and reactivity in comparison to its hydrogen-substituted counterpart . In line with these findings, aryl- and alkyl-substituted silyldiazenes (R–NN– Si , R = aryl or alkyl and Si = triorganosilyl) were shown to be kinetically stable precursors but their synthetic applicability has been far less investigated in part owing to the lack of general strategies for their synthesis. − In 1964, Wannagat and Krüger isolated the deep blue phenyl- and tolyl-substituted diazenes capped with a range of silyl groups through bis-lithiation and further oxidation of the corresponding silylated hydrazines 7 (Scheme , eq 1). , Later, Watanabe and co-workers developed a procedure for the dehydrogenation of silyl-substituted phenylhydrazines, making use of the oxidizing agent di- tert -butylperoxide ( 8 ) at elevated temperatures, delivering the corresponding azo compounds in moderate yields (Scheme , eq 2) .…”

“…9 In this context, Wiberg originally isolated bis(trimethylsilyl)diazene (Me 3 Si−N�N−SiMe 3 ) as a light blue liquid, displaying both an improved thermal stability and reactivity in comparison to its hydrogen-substituted counterpart. 10 In line with these findings, aryl-and alkyl-substituted silyldiazenes (R−N�N−Si, R = aryl or alkyl and Si = triorganosilyl) were shown to be kinetically stable precursors but their synthetic applicability has been far less investigated in part owing to the lack of general strategies for their synthesis. 11−15 In 1964, Wannagat and Kruger isolated the deep blue phenyl-and tolyl-substituted diazenes capped with a range of silyl groups through bis-lithiation and further oxidation of the corresponding silylated hydrazines 7 (Scheme 2, eq 1).…”

General Synthesis and Optical Properties of N-Aryl-N′-Silyldiazenes

“…In 1968, Wiberg and co-workers isolated and studied the light blue, thermolabile trans -bis­(trimethylsilyl)­diazene (BSD, Me 3 Si–NN–SiMe 3 ), which is comparatively more stable than the corresponding diimide ( t 1/2 = 7.6 h versus a few seconds at room temperature) but shares common reactivity patterns with the latter . The synthesis and the properties of N -aryl- N ′-silyldiazenes (Ar–NN–SiR 3 ) have been comparatively less studied.…”