Organometallic diazene synthesized with silicon substituted on both nitrogens An organometallic diazene with silicon atoms substituted on both nitrogen atoms has been synthesized by scientists at the University of Munich. The work represents the first successful synthesis of a doubly metal-substituted diazene. The compound, prepared by Dr. Nils Wiberg and coworkers of the university's institute for inorganic chemistry, is bis ( trimethylsilyl ) -diazene, ( CH 3 ) 3 S i - N = N - S i ( CH 3 ) 3 . The synthesis technique used is a unique low-temperature oxidation reaction that starts with a silyl hydrazide. A blue liquid, the silicon-substituted diazene is unstable and decomposes above — 35° C. It is also a highly reactive compound and very sensitive to oxidizing agents, Dr. Wiberg told the Second International Symposium for Organosilicon Chemistry, meeting in Bordeaux, France, last month. The work of Dr. Wiberg's group will be published soon in Angewandte Chemie. Numerous methods have been developed to synthesize bis(organyl)-diazenes (or azoalkanes) and many of these compounds are very stable. Some azobenzenes are stable to 600° C. So why should it not be possible, research workers have long asked themselves, to substitute silicon atoms for the carbon atoms in azoalkanes to give bis ( silyl ) -diazenes ( or azosilanes ) of similar stability? One approach that has been tried is oxidation of l,2-bis(silyl)-hydrazines by Dr. H. Bock of the University of Munich and by Dr. Robert West and coworkers at the University of Wisconsin. These attempts, however, have always led to products other than the desired diazene. In 1964 Dr. U. Wannagat and Dr. C, Kriiger, at the Inorganic Chemistry Institute in Graz, Austria, reacted the lithium salt of phenyl-silyl-hydrazine with bromine to get a mixed organyl-silyl-diazene with
only one nitrogen substituted with silicon. But this method cannot be used to prepare the completely silicon-substituted diazene. Dr. Wiberg and Dr. W.-Ch. Joo also tried the oxidation method, starting with bis (silyl) -hydrazine. They used a number of oxidizing agents under a variety of reaction conditions—but without success. "Apparently bis ( silyl) diazenes are very sensitive to oxidation and thus are decomposed by the same agent that produces them," Dr. Wiberg told the meeting. "But then while investigating the reactions of tosylazide with silylamides we quite by accident found that this azide is a suitable oxidation medium/' he added. They observed that at low temperatures tosylazide reacts with bis (trimethylsilyl)-amide to give N-trimethylsilyl-tosylamide and trimethylsilylazide. This means that two nitrogen atoms of the tosylazide transfer to the nitrogen atom of the tosylamide. So, the research workers reasoned, the two nitrogen atoms should also transfer to a silylhydrazide in this manner. To determine if this is the case, Dr. Wiberg and his coworkers combined equimolar quantities of tosylazide and N-lithium-tris ( trimethylsilyl ) -hydrazide in ether solution at —78° C. After two hours at this low temperature, the green reaction mixture yields exactly 1 mole of nitrogen gas per mole of tosylazide. Then the ether was removed from the solution via high vacuum at —45° C. The residue is a green solid which under high vacuum at —30° C. gives a blue liquid, leaving a light yellow solid. Since the yellow solid is the lithium salt of N-trimethylsilyl-tosylamide, the blue liquid must be, for stoichiometric reasons, bis (trimethylsilyl) -diazene. Analysis of the blue liquid verified this conclusion. Oxidizing the compound using tellurium tetrachloride
Synthesis gives organometallic diazene
CH3C6H4S02NNN + CcH3)3SiN-N[SiCcri3)3]2
CH3C6H4S02 MSi ((^3)3 + N 2 + CCH375SiN=NSiCCH3)3
DIAZENE. Dr. N. Wiberg (left) and Dr. K. H. Schmid examine bis(trimethylsilyl)-diazenef freshly prepared by a lowtemperature oxidation method
gives 2 moles of nitrogen per mole of tellurium, in agreement with that calculated for bis (trimethylsilyl)-diazene. Mass spectrometer analysis gives a molecular weight that agrees with that calculated; NMR studies show a single sharp signal in the region of the trimethylsilyl protons. The metal-substituted diazene could exist in three isomeric forms—cis, trans, or linear, Dr. Wiberg says. But which of these forms it actually takes hasn't been determined. Besides a 1,2-diazene, there could be a 1,1-diazene and a 1,1,4,4-tetrazene. Mass spectrometric studies rule out the 1,1,4,4-tetrazene while the 1,1-diazene structure is unfavorable because of formal charges on the nitrogen atoms. However, small amounts of both of these, although not measurable at this time, could be present with the 1,2-diazene. A very reactive compound, the diazene hydrolyzes quickly and the end products are nitrogen and hydrazine. It burns in oxygen with a bright flame and decomposes quickly at room temperature. The products of the decompostion reaction are molecular nitrogen and tetrakis(trimethylsilyl)-hydrazine. . Oxidation of bis (trimethylsilyl)-diazene yields bis ( trimethylsilyl ) -peroxide and hexamethyldisiloxane, if the rate of oxidation is carefully controlled. Apparently bis ( silyl ) -diazene reacts first with the oxygen double bond in the same way as with the diazene double bond, by transferring two silyl groups. Then in a secondary reaction the peroxide once again reacts with the bis (silyl)-diazene. In any event, the diazene is very unstable in the presence of an oxidizing agent and, unless suitable precautions are taken, can react with such agents to give violent explosions. AUG. 5, 1968 C&EN 39
