Cyclodisilazanes

There are many four-membered metallacycles containing short metal—metal nonbonded distances. Cyclodisilazanes (Scheme 12a) isoelectronic to 1,3-cyclodis-iloxanes also have short Si—Si distances [136, 137]. 

Short nonbonded Si—Si distances have been observed in four membered metallacycles (Scheme 12b) with a Pt, Ir, W, or Nb atom [138-142] in place of one of the oxygen (nitrogen) atoms of 1,3-cyclodisilazanes (1,3-cyclodisilazanes) and in U-silylene-bridged dinuclear platinum complexes (Scheme 12c) [143, 144]. Electron donating occupied orbitals are expected to be on the platinum atoms like lone pair orbitals on the oxygen atoms in cyclodisiloxanes. 

The formation of the cyclodisilazane ring system can be explained by dimerization of the unstable iminosilane in the cold trap.7... 

Polycondensation of difunctional oligomeric cyclodisilazanes with a,co-dihydroxypolysiloxanes proved to be another successful attempt to synthesize silazane modified polysiloxanes of improved thermostability. The cyclodisilazane oligomers IHa (see scheme below) are readily available from common and cheap materials, dichlorodimethylsilane and ammonia (fi). The chlorine atoms in these compounds can be easily exchanged to other functional groups, such as amino, hydroxyl, etc. 

The susceptibility of cyclodisilazanes to nucleophilic attack by aromatic amines has also been used to prepare silazane containing polymers. Polysilazane cyclo-linear chains with aromatic spacing groups, synthesized by polycondensations of difunctional cyclodisilazanes with bis-phenols and N.N -diorganosilane diamines, have been reported (13). 

With m-phenylenediamine, the unsymmetrical linear disilazane IVb is formed which undergoes recyclization to the unsymmetrical cyclodisilazane mb The following set of equilibria is thus established. 

Perfluoro-substituted four- and six-membered (Si-N) ring systems (3, 5) can be obtained by reacting dilithiated silylamines with SiF4. Cyclodisilazanes of the type 3 are the smallest known four-membered (Si-N) rings. Transannular Si Si ring distances of 237 pm are found [1,3]. For the first time cyclotrisilazane crystallizes in twist conformation [1]. 

By contrast, the preparation of the l,3-bis(dimethylchlorosilyl)cyclodisilazane 379 can be achieved in one step by the condensation of the cyclotetrasilazane (Me2SiNH)4 380 with Me2SiCl2 (Scheme 54).405 The reduction of 379 with sodium has given rise to formation of the polycyclodisilazane 381, an interesting precursor for SiN-based ceramics (Scheme 54).405... 

The cyclization of allyl silyl amine 697 by hydrosilylation led to silaazetidine 698, which was subjected to flash vacuum thermolysis at 700-900°C at 10-4 hPa313. The silanimines 699 and 700 themselves were too reactive to be observed by high resolution mass spectrometry of the reaction mixture, but their cyclic dimers, the cyclodisilazane 701 and 702 and a trapping product with t-BuOH 703, were definitely confirmed... 

The compounds resulting from the reaction of 748 were characterized by HRMS directly coupled to the reactor. The stable products 750 and 751 were analysed by GC and 111 NMR spectroscopy. The formation of the cyclodisilazane 750 is explained by dimerization of the unstable silanimine 749 only in the cold trap, as the reaction is carried out under high dilution conditions (equation 247). It was also shown that the hydrogen chloride elimination did not occur in the ion source of the mass spectrometer. 

The addition of donors to the Lewis acidic sp2-hybridized silicon in Si=N systems is a general method to stabilize the multiple bond, even when non-bulky substituents are used. In most cases the donor adduct reacts just like the free Si=N compound, obviously by initial dissociation of the donor molecule. In the absence of trapping agents and when the steric bulk allows it, silanimines dimerize300,308,311, e.g. 672 gives cyclodisilazane 789 (equation 265). 

Cyclodisilazane anions and cyclodisilazanes in cw-conformation are found in the reaction of dilithiated bis(silylamino)fluorosilanes with chlorotrimethylsilane. The dilithium salt of the corresponding bis(silylamino)chlorosilane is obtained. LiCl elimination in the presence of THF leads to the formation of silaamidides (which are isolated as dimers),19,26 four-membered cyclodisilazane anions and (thf)3Li-Cl-Li(thf)36 or Li(thf)/ cations (Scheme 10).19,27 Hydrolysis of dimeric silaamidides is a facile synthesis leading to cyclodisilazanes in the cw-conformation. Examples are shown in Figs. 1 and 2 (Scheme 11).19,27... 

The Si-N bonds in the cyclodisilazane (174.7 pm) are significantly shorter than in the dication (179.3 pm), i.e., they are weakened in the course of the reaction because of the sp3-hybridization of the nitrogen atoms in the resulting cations. 

A possible isomer of the cyclotrisilazane (Me2Si-NH)3, A, is a cyclodisilazane HN(SiMe2)2NSiMe2NH2, B, with an exocyclic SiMe2NH2 group. 

B. Isomeric Cyclotri- and Cyclodisilazane-Anions and -Dianions, DFT Calculations... 

While theoretically the anionic HMCTS (A ) has two possible cyclodisilazane isomers (B, Bl ), experimental results give evidence that only the isomers A and Bl are of importance. For example, it was found that the anion of HMCTS (A ) always reacts with Me3SiCl, fluorosilanes and -boranes in a molar ratio 1 1 with retention of the ring size. 

Equilibrium between Silyl-Substituted Cyclotri- and Isomeric Cyclodisilazane-Anions... 

In 1969, Fink found that a compound that had been described as a bissilyl-substituted six-membered ring was in fact an isomeric cyclodisilazane (Scheme 17).40,41... 

Another approach to silyl-coupled cyclodisilazanes starts with a fluorosilyl-substituted cyclotrisilazane and half an equivalent of BuLi. In this case, only half of... 

Using bulkier groups and higher reaction temperatures, the lithium salt of 1-borazinyl-cyclotrisilazane contracts to give the four-membered cyclodisilazane 15... 

The cyclodisilazane is not precisely planar. The sum of the angles around the nitrogen atom N(l) is 352.1°. Both of the borazine rings have a boat conformation.49... 

The Si(3)-N(2) bond of the four-membered cyclodisilazane isomer has the length of an Si-N double bond. 

So far, no tris(silyl)-substituted cyclotetrasilazane is known. Starting with the isomeric unsymmetrically substituted cyclodisilazane, the next substituent R is bonded exocyclically at the four-membered ring yielding 25 (Scheme 33).31 56 63 64 66... 

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