Silylhydrazines: Lithium Derivatives Isomerism and Rings

Katrin Bode; Uwe Klingebiel    Institute of Inorganic Chemistry, University of Goettingen, D-37077 Goettinqen Germany

I INTRODUCTION

The versatility of nitrogen in its compounds depends in large measure on the existence of a range of oxidation states between -3 and + 5. In its combination with silicon, systems are known in which N has an oxidation state of -3 as in the derivatives of silylamines, of -2 as in the derivatives of silylhydrazines, and of -1 as in the derivatives of silylazenes:

Whereas in these compounds silicon is always tetrahedral, nitrogen has a changing coordination geometry: in silylamines it is planar, but in silylazenes it is linear.

This article reviews silylhydrazines, their preparation, properties, and reactions. Other reviews on Si–N compounds have appeared elsewhere. 1-8 However, as the chemistry of silylhydrazines has developed very fast, especially during the past 5 years, it seems appropriate to write an article that deals only with silylhydrazines.

II PREPARATION

A Synthesis and Properties of Silylhydrazines

The synthesis of the first cyclic and acyclic silylhydrazines were reported by Aylett9 and Wannagat et al.10, 11 in 1956–1958. Two main methods of preparation were developed.

1. The first is the treatment of a hydrazine with a halosilane. The silylhydrazines are formed by intermolecular cleavage of a hydrogen halid [Eq. (1)].

This is the most common method for chloro-, bromo-, or iodosilanes.6 Often, auxilary bases such as triethylamine or pyridine are added. But when hydrazine is treated with fluorosilanes or silanes, no condensation is observed because of the reduced reactivity. Fluorosilanes only form adducts with hydrazines, so that the reaction is stopped at step (a). Because of the extremely strong Si—F bond energy, no cleavage of HF or N2H4-condensation is observed.3, 6 In this case another preparation method must be chosen.

2. The second method of preparation is the treatment of lithiated hydrazine with halosilanes, especially fluorosilanes, with elimination of lithium halide [Eq. (2)]:

3Si—Hal+LiHN—NH2→R3Si—NH—NH2+LiHal

The formation of the silylhydrazines depends on the reactivity and the bulkiness of the halosilanes. The condensation increases with increasing number of the hydrogen atoms. While iodosilane or bromo(methyl)-silane yield the tetrakis(silyl)hydrazines,9, 12

the reaction of hydrazine with chlorodimethylsilane gives only the tris(silyl)hydrazine12:

When three methyl groups are bound to the silicon, isomeric bis(trimethylsilyl)hydrazines are formed.7, 10 Wannagat et al. succeeded in separating the isomeric N,N- and N,N′-bis(trimethylsilyl)hydrazines11:

A monosilylhydrazine can only be obtained when bulky substituents such as triphenylsilyl groups stabilize the hdyrazine10:

B Mono(silyl)hydrazines

1 Synthesis and Properties

The only mono(silyl)hydrazine known until 1993 was the triphenylsilylhydrazine 1, described by Wannagat and Liehr in 195810 [Eq. (3)].

Primary silylhydrazines with R = Me, Et, or Pr cannot be isolated, as they immediately undergo further condensation to bis(silyl)hydrazines with elimination of hydrazine.6, 10 For the triphenylsilylhydrazine this condensation only occurs under more drastic conditions at 90 °C [Eq. (4)].

Some other mono(silyl)hydrazines have been synthesized in the reaction of fluorosilanes with lithiated hydrazine:

This class of compounds is kinetically stabilized by bulky tert -butyl groups (3–6) or amine groups (7,8). In 3, 7, and 8, HF elimination should be possible, but it is hindered by the very strong SiF bond energy. These mono(silyl)hydrazines are very stable molecules and show no tendency to undergo condensation at room temperature.

Their stability allows a directed synthesis of asymmetrical bis(silyl)hy-drazines by the reactions of lithiated mono(silyl)hydrazines with halosilanes (Section B,2). These reactions often lead to the formation of isomeric products.

2 Lithium Derivatives of Mono(silyl)hydrazines

Isomeric products are formed in the reaction of lithiated di-tert-butyl-methylsilylhydrazine with fluorosilanes. The formation of these isomers requires prior coordination of the Li+ ion with the two N atoms of the hydrazine unit. The crystal structure of the lithiated di-tert-butylmethyl-silylhydrazine 4 (9)14 exhibits two different silylhydrazide units I and II, which are bound by six Li+ ions to form a hexameric entity.

The Li+ ions are bound to three different structural units: Lil is coordinated with one N atom and two NH groups; Li2 with one N atom, one N2, and one NH unit; and Li3 with one NH, one N2, and one NH2 unit. Thus, four Li+ ions are bound side-on. (See Fig. 1 and Table I.)

Unlike unit II, in the solid state I does not coordinate Li+ ions side-on. The side-on bond lengths for the atoms Li2 and Li3 were determined to be 192 and 188 pm, respectively. The angles at N5 and N6, which bind Li3 side-on, are nearly identical; the angles at N1 and N2, which coordinate Li2, deviate by 7°. Thus, Li3 is positioned centrosymmetrically above the N—N bond.

Table II shows a comparison of the data for ab initio calculations of the lithium hydrazide system (NHNH2)Li+ (A)18, 19 with that for the side-on units Nl–N2–Li2 and N5–N6–Li3 (B) in 9, derived from the X-ray structure determination:

The structural analysis confirms the ab initio calculations, according to which Li+ ions in hydrazines can be coordinated equally by both N atoms of the hydrazine. This phenomenon also accounts for the isomerizations during the secondary substitutions.

C Bis(silyl)hydrazines

1 Symmetrical Bis(silyl)hydrazines

When mono(silyl)hydrazines are not stable at room temperature, they condensate to give the symmetrical bis(silyl)hydrazines 10–15:

These condensation reactions often lead to the formation of isomers. Structural isomerism of bis(silyl)hydrazines was first observed in 1964.19-24 In the absence of strong steric or electronic constraints, the bis(silyl)hy-drazines such as bis(trimethylsilyl)hydrazine give in a thermoneutral reaction essentially equal amounts of the N,N- and...