[acs symposium series] inorganic and organometallic polymers volume 360 (macromolecules containing...

Chapter 32

Boron Nitride and Its Precursors

K. J. L. Paciorek 1, W. Krone-Schmidt 1, D. H. Harris 1, R. H. Kratzer 1, and Kenneth J. Wynne 2

1Ultrasystems Defense and Space, Inc., Irvine, CA 92714 2Office of Naval Research, Arlington, VA 22217

Stepwise condensation of a series of borazines was investigated. The compounds studied were B-tri-chloro-N-triphenylborazine, B-triamino-N-triphenyl-borazine, B-trianilinoborazine, B-tris[di(trimethyl-silyl)amino]-borazine, B-trichloro-N-tris(trimethyl-silyl)borazine, and B-triamino-N-tris(trimethyl-silyl)borazine. The formation of preceramic polymers occurs by ring opening mechanism which leads to singly and doubly joined borazine rings. This process requires the presence of a combined total of six protons on the ring and exocyclic nitrogens. Compounds where this arrangement is absent, B-trichloro-N-triphenylborazine, B-tris[di(trimethylsilyl)amino]borazine, and B-trichloro-N-tris(tri-methylsilyl)borazine, failed to undergo condensation to any significant degree. Materials obtained from the phenyl-substituted borazines gave essentially nonprocessible polymers on pyrolysis; from B-tri-amino-N-tris(trimethylsilyl)borazine, preceramic polymers amenable to fiber manufacture were obtained. Investigation of potential linear precursor to boron-nitrogen polymers resulted in the synthesis of μ-imido-bis[bis(trimethylsilyl)-aminotrimethylsilylamino]borane (I) and μ-οxο-bis-[bis(trimethylsilyl)aminotrimethylsilylamino]borane (II). The crystal structures of the two compounds were essentially identical with the exception of the bridging atoms and their immediate environments. The same applied to mass spectral breakdown patterns wherein the major fragments differed by one amu, with the exception of the ion 332+ which is characteristic to the oxygen-bridged compound.

R e f r a c t o r i e s such as boron n i t r i d e , s i l i c o n n i t r i d e , s i l i c o n carbide, and boron carbide are of great importance f o r the product i o n or pr o t e c t i o n of systems which can be operated i n very high

0097-6156/88/0360-0392506.00/0 © 1988 American Chemical Society

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In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

32. PACIOREK ET AL. Boron Nitride and Its Precursors 393

temperature environments. The a p p l i c a b i l i t y of these materials as coatings, f i b e r s , as well as bulk items depends on the a b i l i t y to produce a r e a d i l y p r o c e s s i b l e polymer, which can be formed in t o the desired shape of the f i n a l item p r i o r to exhaustive p y r o l y s i s and transformation into a ceramic. The technology to provide such precursors f o r s i l i c o n carbide, s i l i c o n n i t r i d e , and graphite, i n p a r t i c u l a r the l a t t e r , has been developed and although work i s p r o c e e d i n g on improvements, the premises are r e l a t i v e l y well e s t a b l i s h e d Q ) . The s i t u a t i o n i n the case of boron n i t r i d e i s e n t i r e l y d i f f e r e n t . The extensive work performed i n the s i x t i e s was di r e c t e d at thermally s t a b l e polymers. The products were poorly characterized and the l i t e r a t u r e data are c o n f l i c t i n g (2). More recently, claims have been made to the synthesis of BN preceramic polymers amenable to f i b e r production using the p y r o l y s i s of B-triamino-N-triphenylborazine (3). Unfortunately, t h i s work could not be reproduced (4,5).

Boron n i t r i d e , i n view of i t s unique p r o p e r t i e s , namely absence of e l e c t r i c a l c o n d u c t i v i t y , oxidation r e s i s t a n c e , o p t i c a l transparency, and high neutron capture c r o s s - s e c t i o n f o r s p e c i a l a p p l i c a t i o n s , o f f e r s advantages over other ceramics. Thus, f o r the past several years the group at Ultrasystems has been a c t i v e l y involved i n i n v e s t i g a t i n g routes leading to preceramic BN polymers using both c y c l i c and l i n e a r s t a r t i n g materials. Some of the find i n g s generated w i l l be discussed.

Results and Discussion

Borazine synthesis. The ease of borazine r i n g condensation would be expected to depend on the nature of the substituents. To i n v e s t i gate these aspects, a s e r i e s of compounds were synthesized, namely B - t r i c h l o r o - N - t r i phenyl b o r a z i n e , B-triamino-N-triphenylborazine, B - t r i a n i l i n o b o r a z i n e , B - t r i s [ d i ( t r i m e t h y l s i l y l ) a m i n o ] b o r a z i n e , B - t r i c h l o r o - N - t r i s ( t r i m e t h y l s i l y l ) b o r a z i n e , and B-triamino-N-tris-( t r i m e t h y l s i l y l ) b o r a z i n e .

B - t r i c h l o r o - N - t r i p h e n y l b o r a z i n e , mp 290-292°C, was obtained i n 86* y i e l d f o l l o w i n g the procedure of Groszos and S t a f i e j (6). T h i s m a t e r i a l was then transformed i n t o B-triamino-N-triphenylborazine i n 67% y i e l d using the method of Toeniskoetter and Hall (1) · B - t r i a n i l i n o b o r a z i n e and the novel B - t r i s [ d i ( t r i m e t h y l s i l y l ) -aminojborazine (mp, 131.5-132°C.; characterized by GC/MS, molecular ion 558 amu, and elemental a n a l y s i s ) were synthesized i n 76 and 71* y i e l d s , r e s p e c t i v e l y , by i n t e r a c t i o n of a n i l i n e and hexamethyl-d i s i l a z a n e with chloroborazine i n the presence of triethylamine.

The s y n t h e s i s of B - t r i c h l o r o - N - t r i s ( t r i m e t h y l s i l y l ) b o r a z i n e was much more complicated than that of the other borazines. The o v e r a l l scheme i s given below:

BC1 3 + E t 3 N • B C l 3 ' N E t 3

CI

Me QSiN NSiMe Q 3 I I 3

CIB^

HN(SiMe.) 2

+ NEt 3

^ B C l « P y r o l y s i s

I SiMe„

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394 INORGANIC AND ORGANOMETALLIC POLYMERS

Boron t r i c h l o r i d e - t r i e t h y l a m i n e adduct, mp 89-91°C, was obtained i n 80* y i e l d f o l l o w i n g e s s e n t i a l l y the procedure of Ohashi, et a l . (8). The product was washed w i t h methanol, but not c r y s t a l l i z e d from i t . I t was found that c r y s t a l l i z a t i o n from d i l u t e e t h a n o l r e p o r t e d by G e r r a r d , L a p p e r t , and Pearce (9) caused extensive degradation . B i s ( t r i m e t h y l s i l y l ) a m i n o d i c h l o r o b o r a n e was prepared f o l l o w i n g the procedure of Wells and C o l l i n s (10).

The p y r o l y s i s of b i s ( t r i m e t h y l s i l y l )aminodichloroborane to B - t r i c h l o r o - N - t r i s ( t r i m e t h y l s i l y l ) b o r a z i n e , contrary to l i t e r a t u r e (11^, d i d not take place i n b o i l i n g xylene. Temperatures above 150 C were necessary f o r t r i m e t h y l c h l o r o s i l a n e e l i m i n a t i o n . The highest y i e l d of the r e l a t i v e l y pure product was around 20*. The transformation of the B - t r i c h l o r o - N - t r i s ( t r i m e t h y l s i l y l J b o r a z i n e to B - t r i a m i n o - N - t r i s ( t r i m e t h y l s i l y l Jborazine proceeded r e a d i l y using l i q u i d ammonia.

B o r a z i n e P y r o l y s i s Studies. In a s i m p l i s t i c manner, one can v i s u a l i z e the formation of boron n i t r i d e v i a stepwise e l i m i n a t i o n s between r i n g s , i . e . ,

R I

X MONOMER PRECERAMIC POLYMER

\

BORON NITRIDE CROSSLINKED POLYMER

g i v i n g i n i t i a l l y the " l i n e a r " preceramic polymer, then the l i g h t l y c r o s s l i n k e d s t r u c t u r e B, followed by a p a r t i a l B-N system, C, and f i n a l l y pure boron n i t r i d e . The p y r o l y s i s techniques u t i l i z e d were f u l l y described p r e v i o u s l y (4J and thus w i l l not be r e i t e r a t e d here. The p o t e n t i a l borazine candidates can be broadly d i v i d e d i n t o two c l a s s e s of m a t e r i a l s , one where the s u b s t i t u e n t on the r i n g boron i s n i t r o g e n - f r e e , e.g., a halogen, and the second where the su b s t i t u e n t i s an amino moiety. With respect to the s u i t a b i l i t y of any given borazine, the important aspects are the ease of formation of the l e a v i n g molecule and i t s v o l a t i l i t y . In t h i s respect, B - t r i c h l o r o - N - t r i s ( t r i m e t h y l s i l y l ) b o r a z i n e would seem an i d e a l candidate. Unfortunately, p y r o l y s i s at ,^60 C l i b e r a t e d only 6.1* of the expected t r i m e t h y l c h l o r o s i l a n e ; 75* of the s t a r t i n g m a t e r i a l was recovered. Heating at 360°C f o r 19 h r e s u l t e d i n 56.0* y i e l d of t r i m e t h y l c h l o r o s i l a n e and a gl a s s y residue. Further heating at

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360°C f o r 72 h afforded only an a d d i t i o n a l 7.3% of the expected t r i m e t h y l c h l o r o s i l a n e . This r e s u l t shows c l e a r l y that t h i s type of borazine does not lead to a d e s i r a b l e preceramic polymer, inasmuch as at 260 C the condensation process proceeds only to a l i m i t e d degree, with the s t a r t i n g m a t e r i a l being l a r g e l y recovered and the p o l y m e r i c p r o d u c t formed b e i n g both i n s o l u b l e and i n f u s i b l e . B - t r i c h l o r o - N - t r i p h e n y l b o r a z i n e would not be expected to undergo condensation r e a d i l y inasmuch as the formation of chlorobenzene i s not a f a v o r e d p r o c e s s . The experimental data supported t h i s p o s t u l a t i o n ; e s s e n t i a l l y no degradation occurred at 300°C.

B-triamino-N-triphenylborazine, and i n p a r t i c u l a r i t s isomer, B - t r i a n i l i n o b o r a z i n e , the l a t t e r e i t h e r alone or i n co - r e a c t i o n w i t h B-triamino-N-triphenylborazine, gave f u s i b l e , <200 C, and organic solvent s o l u b l e preceramic polymers a f t e r exposure to temperatures below 250 C (1,5). However, i n p a r t i c u l a r using B - t r i a m i n o - N - t r i phenylborazine alone, the process was i r r e p r o d u c i b l e ; n o n f u s i b l e p r o d u c t s were obtained. In t h i s system, carbon r e t e n t i o n was i n v a r i a b l y observed a f t e r exposure to 1000°C, contrary to the Japanese claims (3).

The thermal degradation of B - t r i a n i l i n o b o r a z i n e , when conducted i n ammonia even below 215°C, produced 87.7* of the a v a i l a b l e a n i l i n e . This was brought up to 92.3* by f u r t h e r p y r o l y s i s at 275-299 C which compares wit h the maximum of 53* observed under the same heating regime i n ni t r o g e n atmosphere (4). Thus, i t i s obvious that the presence of ammonia f a c i l i t a t e s the e l i m i n a t i o n process. However, since 31* of the a n i l i n e evolved was replaced by ammonia, based on the weight of the resi d u e , the degree of condensat i o n was not as high as would appear from the a n i l i n e c o l l e c t e d . Due t o the m a t e r i a l s ' i n s o l u b i l i t y i n organic s o l v e n t s , the molecular weights could not be determined.

From the i n v e s t i g a t i o n s of the phenylaminoborazines i t was e s t a b l i s h e d that the condensation does occur v i a r i n g opening (4), p o s t u l a t e d e a r l i e r by Toeniskoetter and H a l l (.7). followed by e l i m i n a t i o n and bridge formation. Isomerization i s i n h e r e n t l y a s s o c i a t e d w i t h t h i s mechanism, as i l l u s t r a t e d i n Scheme 1. For t h i s process to take place, i t i s necessary that the boron subs t i t u e n t i s e i t h e r a NH^ or NHR moiety. Namely, the r i n g opening mechanism requires the presence of a combined t o t a l of s i x protons on the r i n g and e x o c y c l i c nitrogens. In agreement wit h the above, B - t r i s [ d i ( t r i m e t h y l s i l y l ) a m i n o ] b o r a z i n e , which contains only three hydrogens, was recovered unchanged a f t e r exposure to 410 C f o r 89 h. One could v i s u a l i z e here the int e r m o l e c u l a r condensation to occur, i n the absence of r i n g opening, w i t h e l i m i n a t i o n of hexa-m e t h y l d i s i l a z a n e , e.g.,

N ( S i M e 3 ) 2

ΗΝ NH 2 I I

( M e 3 S i ) 2 N B ^ / B N ( S i M e 3 ) 2 Ν I Η

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( M e 3 S i ) 2 N

HN I

(Me S i ) NB >

H I

Β

N ( S i M e 3 ) 2

I HN

BN(SiMeJ I NH

I N(SiMe 3),

Ύ 2

This apparently i s not the case. On prolonged exposure i n ammonia at 250-260°c some l i b e r a t i o n of hexamethyldisilazane took place, but only to a very low degree.

The behavior of B - t r i a m i n o - N - t r i s ( t r i m e t h y l s i l y l ) b o r a z i n e was i n good agreement with the r i n g opening mechanism. This compound was much more r e a c t i v e than i t s phenyl analogues; thus, the pure monomer could not be i s o l a t e d . The product obtained from the i n t e r a c t i o n of ammonia and B - t r i c h l o r o - N - t r i s ( t r i m e t h y l s i l y l )-borazine co n s i s t e d of a 1:1 mixture of the monomer and a s i n g l y -bridged dimer, x=l.

Me 3Si-H2N-

NH„

N-I

I NH, SiMe,

I 3

2 M e 3 S i ι

NH 2

•SiMe n

NH n

Both the monomer and dimer were composed most l i k e l y of isomers w i t h some p r o t o n s r e s i d i n g on r i n g nitrogens and with the NHSiMe^ moiety r e p l a c i n g the amino group on some of the boron r i n g atoms i n accordance w i t h the Scheme 1. This assumption i s supported by the l i q u i d n a t u r e of the product and i t s i n f r a r e d spectrum which e x h i b i t e d two broad bands centered at 3430 and 3530 cm" . The i n f r a r e d spectrum of pure B - t r i a m i n o - N - t r i s ( t r i m e t h y l s i l y l ) b o r a z i n e would be expected to be s i m i l a r to that of B - t r i a m i n o - N - t r i s ( t r i -phenyl)borazine where three sharp bands at 3430, 3505, and 3530 were observed (4). In the l a t t e r case, once the condensation process was i n i t i a t e d , the sharp bands disappeared {4).

To remove v o l a t i l e i m p u r i t i e s , the m a t e r i a l was heated i n vacuo a t 135 ° C This treatment r e s u l t e d i n f u r t h e r condensation. Thus, the d i s t i l l a t i o n residue was composed of a mixture of doubly-bridged dimers and tetramers, χ = 1 and 2 as determined from the molecular weight, boron and nitrogen analyses and the v o l a t i l e condensibles produced.

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φ I

Η I

Η I

Η I

Φ I

Β >H) ι 1 ^ · -

Η Β Η

^N^ / N v Η Β Β Η I I

Η Β Φ I

Η Η

Η Η I I

Φ Β Β I I

— Ν Ν I I

/ Β ^ /Φ Φ I Ν < /Ν^ \ Η

Φ Η Η Η Η

/Ν Ν Η Β

I

Scheme Î.

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H2N-Me 3Si-f-N

SiMe Q I 3

-Β Ε I I

*B" I NH 0

NH 2

J N-I I

I SiMe„

•SlMe 3

NH 0

The c o n d e n s a t i o n p r o c e s s a p p a r e n t l y i n v o l v e s e l i m i n a t i o n of t r i m e t h y l s i l y l a m i n e , ( C H 3 ) S i N H ^ f w h i c h , b e i n g u n s t a b l e , d i s p r o p o r t i o n a t e s i n t o hexamethyldisnazane and ammonia. In the condensable v o l a t i l e s c o l l e c t e d , these m a t e r i a l s were present i n a 1:1 r a t i o supporting the postulated process. Further thermolysis at 200°C (52 h ) , f o l l o w e d by 4 h at 250-260°C, r e s u l t e d i n a mixture of doubly-bridged tetramers and octamers, x=2 and 4. At t h i s stage, 53* of the p o t e n t i a l l e a v i n g groups were l i b e r a t e d . From the tetramer/octamer mixture, f i b e r s with 10-20 μ diameter could be melt drawn as shown i n Figures 1 and 2. Gradual heat treatment i n ammonia, from 65-950°C, r e s u l t e d i n white, pure boron n i t r i d e f i b e r s . The f i b e r s e x h i b i t e d no weight l o s s under thermogravimetric c o n d i t i o n s , both i n a i r and i n nit r o g e n up to 1000°C (12).

P o t e n t i a l Non-Cyclic Precursors of Preceramic Polymers. Boranes such as b i s ( t r i m e t h y l s i l y l ) a m i n o t r i m e t h y l s i l y l a m i n o c h l o r o b o r a n e s can be viewed as monomers f o r preceramic polymer and, u l t i m a t e l y , boron n i t r i d e p r o d u c t i o n . Intermolecular dehydrohalogenation of t h i s borane would be thus expected to y i e l d e i t h e r the dimer or the polymeric system.

CI I

(Me 3Si) 2NBNHSiMe 3 (Me.Si) 0NB — NSiKe.

Me 3S IN BN (S iMe 3) 2

and/or

[(Me 3Si) 2NB-NSiMe 3] x

B i s ( t r i m e t h y l s i l y l ) a m i n otr i m e t h y l s i l y l a m i n o c h l o r o b o r a n e was obtained i n a 70* y i e l d f o l l o w i n g the procedure of Wells and C o l l i n s ( 10). No t r i e t h y l a m i n e hydrochloride was formed and the s t a r t i n g m a t e r i a l was recovered unchanged a f t e r prolonged r e f l u x i n g w i t h a f i v e - f o l d excess of s p e c i a l l y d r i e d t r i e t h y l a m i n e . However, when the t r i e t h y l a m i n e employed was not so r i g o r o u s l y d r i e d and 10- to 20 - f o l d excess was used, an oxygen-bridged compound,

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Figure 1. Melt spun BN precursor f i b e r s , m a g n i f i c a t i o n 500x.

Figure 2. Melt spun BN precursor f i b e r s , m a g n i f i c a t i o n 5,000x.

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(Me 3Si) 2NBNHSlMe 3

0 1


I I was obtained i n a 35% y i e l d . Conducting the r e a c t i o n i n benzene using the required q u a n t i t y of water i n the presence of t r i e t h y l amine gave 55% y i e l d of t r i e t h y l a m i n e h y d r o c h l o r i d e , but only a 7% y i e l d of the oxygen-bridged product. The s u r p r i s i n g f i n d i n g was the r e l a t i v e l y high y i e l d ( M8%) of m a t e r i a l which i s e i t h e r ( M e 3 S i N H ) B N ( S i M e ) o r [ ( M e S i ) Ν ] BNH ( t h e d i f f e r e n t i a t i o n could not Be made from the mass S p e c t r a l f r a g mentation p a t t e r n ) , together w i t h hexamethyldisilazane and amino-b i s ( t r i m e t h y l s i l y l ) ami n o t r i me thy Is i l y l a m i n o b o r a n e , ( M e 3 S i ) -NB(NH 2)NHSiMe 3, or i t s isomer, Β(NHSiMe 3) 3. These produces were a l s o observed, but i n low concentrations, i n the r e a c t i o n where the high y i e l d of the oxygen-bridged compound was r e a l i z e d . The major d i f f e r e n c e between these r e a c t i o n s was the r e l a t i v e r a t i o of t r i e t h y l a m i n e and the presence of benzene. Based on these r e s u l t s , i t i s c l e a r that the process i n v o l v i n g hydrogen c h l o r i d e e l i m i n a t i o n and oxygen s u b s t i t u t i o n ,

P 1 (Me .Si).NBNHS iMe . I NEt. I 3

2 (Me 3Si) 2NBNHSiMe 3 + H 20 ^ 0 + 2 Et 3N-HCl (Me 3Si) 2NBNHSiMe 3

i s not the only one o c c u r r i n g . Apparently, a more extensive h y d r o l y s i s of the chloroborane takes place c o n c u r r e n t l y , i . e . ,

(Me.Si) 0NBNHSiMe. = •(Me.Si) 0NH + B(0H). + [H.NSiMe.] + Et.N-HCl j l à NEt 3 J Ζ j ζ J J

Inasmuch as t r i m e t h y l s i l y l a m i n e i s unstable, i t w i l l decompose i n t o ammonia and hexamethyldisilazane. The ammonia thus formed could be v i s u a l i z e d t o f o r m t h e amino compound ( M e ^ s i ) 2 N B ( N H . ) -NHSiMe . S i n c e (Me S i ) NB(NHSiMe ). was p r e p a r e d by Wells and C o l l i n s (j_3) v i a i n t e r a c t i o n of tne chloroborane with hexam e t h y l d i s i l a z a n e , i t s production here could f o l l o w the same path.

The n i t r o g e n - b r i d g e d analogue, [ ( M e 3 S i ) NBNHSiMe^NH, Compound I, was formed i n a 57% y i e l d from b i s t r i m e t h y l s i l y l ) -a m i n o t r i m e t h y l s i l y l a m i n o c h l o r o b o r a n e and aminobis ( t r i m e t h y l s i l y l )-aminotrimethylsilylaminoborane i n the presence of t r i e t h y l a m i n e by heating at 100°C over an 18 h p e r i o d , i . e .

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NH 0 CI (Me 3Si) 2NBNHSiMe + (Me 3Si) 2NBNHSiMe 3

J E t 3 N (Me 3Si) 2NBNHSiMe 3

NH + Et QN-HCl 1 3


I The nitrogen-bridged compound e x h i b i t e d p h y s i c a l and s p e c t r a l

c h a r a c t e r i s t i c s very s i m i l a r to those of the oxygen-bridged compound. This i s i l l u s t r a t e d by the c l o s e melting points (169-171°c and 158-160°C, r e s p e c t i v e l y ) and e s s e n t i a l l y i d e n t i c a l IR spectra (the spectrum of y - i m i d o - b i s [ b i s ( t r i m e t h y l s i l y l ) a m i n o t r i m e t h y l s i l y l -amino]borane i s given i n Figure 3). Thermal behavior of I and I I i s a l s o s i m i l a r as shown by DSC t r a c e s , where i n the oxygen-bridged compound two strong endotherms were observed at 70 and 75°C and i n the nitrogen-bridged m a t e r i a l at 85 and 105°C. No phase t r a n s i t i o n r e s p o n s i b l e f o r these endotherms could be v i s u a l l y observed on heating. Both m a t e r i a l s , on r e m e l t i n g i n a i r , e x h i b i t e d the same melt i n g points as those obtained under i n e r t - c o n d i t i o n s p o i n t i n g to t h e i r thermal, o x i d a t i v e , and h y d r o l y t i c s t a b i l i t i e s . I n v e s t i g a t i o n of the c o n d e n s a t i o n r e a c t i o n s of Compound I are c u r r e n t l y i n progress.

The mass s p e c t r a l breakdown patterns f o r the two bridged compounds were very s i m i l a r , with the exception that the peaks were one mass u n i t higher i n the case of the oxygen-bridged m a t e r i a l . Both compounds produced strong molecular ions at 533 (I) and 534 ( I I ) . The processes r e s p o n s i b l e f o r s e v e r a l of the major fragments could be i d e n t i f i e d from the metastables. In the case of μ - i m i d o - b i s [ b i s ( t r i m e t h y l s i l y l ) a m i n o t r i m e t h y l s i l y l a m i n o ] b o r a n e , these are:

533 + (Μ) • δίδ* + 15 [Me] m* 503.4

518 + • 429 + + 89 [H^NSiMe^] m* 355.3

429^ • 341 + + 88 [HNSiMe^] m* 271.0

429 + • 3 1 6 + + 113 [BNHNSiMe^] m* 232.8

In view of the absence of the a d d i t i o n a l proton, the equivalent of the 341 i o n , namely the 342 + i o n , would not be expected to be formed to any s i g n i f i c a n t extent i n the case of the oxygen-bridged compound. The fragmentation p a t t e r n confirms i t and no metastable f o r t h i s p r o c e s s was found. In the oxygen-bridged compound, metastables were observed at m/e 504.5, 357.5, 213, and 82.5. The processes responsible have been i d e n t i f i e d as:

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Ο w ο > ο > ζ ο ο ο > ο w

Η > r η Ο r κ;

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534" (Μ) •δίθ"1" + 15 [Me] m* 504.4

519 + • 430 + + 89 [H^NSiMe^] m* 356.3

519 + • 332 + + 187 [B(NHSiMe^)g] m* 212.4

259" • 1 4 6 + + 113 m* 82.3

The 332* ion i s s p e c i f i c to the oxygen-bridged product. I t i s speculated that i t i s formed by the f o l l o w i n g path from the 519 io n :

( M e Q S i ) 0 N v (Me.Si) 9N 3 2 ; Β — 0 — B — N H S i K e J Z — 0 Β NHSiMe^

Me^SiHN \ ^ | Me~SiHN | | 5 M e — S i — Ν — S i M e 0 ^ M e - S i +N — SiMe.

H2C — H CH 2 Η

519 + 332 +

In order to obtain i n s i g h t i n t o the nature of bonding i n c a t e n a t e d BN s p e c i e s , the c r y s t a l and molecular s t r u c t u r e s of Compounds I and I I were obtained. Both compounds c r y s t a l l i z e isomorphously i n the monoclinic space group P2 /n. For I. a = 12.876(2) 3Â, b = 14.828(3) \t c = 18.628(4) Â; 1 β - 97.07(2) ; V = 3529(1) Â ; Ζ = 4, agd the molecular weight based on a c a l c u l a t e d d e n s i t y of 1.005 g/cm was 533.85, i n e x c e l l e n t agreement wit h the molecular ion observed i n the mass spectrum (533 m/e). Refinement le d to R f = 4.1% and R w f = 4.2% f o r 5093 independent r e f l e c t i o n s . The s t r u c t u r e of I, given i n Figure 4, confirms the presence of a c e n t r a l Ν BNBN framework that i s e s s e n t i a l l y planar w i t h a B(1)-N(1)-B(2J angle of 135.9(2)°. The remaining B-N-B angles are close to 120 . The B-N bond distances r e f l e c t a competition f o r T T-electron d e n s i t y . The sho r t e s t B-N bonds, (B(2)-N(5) 1.405(3) % and B ( ( l ) - N ( 3 ) 1.423(3) Â, are to nitrogens bonded to one s i l i c o n , while the longest B-N bonds B(2)-N(4) 1.485(3) λ and B(l)-N(2) 1.487(3) Â are to nitrogens bonded to two s i l i c o n s . For I I , a = 12.826(3) Â, b = 14.822(4) Â, and c - 18.557(4) λ; β - 95.98(2)°; V = 3509(1) Â ; Ζ = 4, F.W. = 534.8 amu, and the c a l c u l a t e d d e n s i t y i s 1.012 g/cm . Refinement l e d to R f = 4.3% and R w f * 4.0% f o r 4458 independent r e f l e c t i o n s . S t r u c t u r e I I , given i n Figure 5, d i f f e r s from I only i n the existence of a c e n t r a l B-O-B u n i t wherein the boron oxygen distances [ B ( l ) - 0 ( 1 ) 1.380(3) and B ( 2 ) - 0 ( l ) 1.389(3)] are somewhat shorter than the corresponding B-N distances f o r I [ B ( l ) - N ( l ) 1.428(3) and B(2)-N(l) 1.446(3)].

Acknowledgment Support of t h i s research from the S t r a t e g i c Defense Sciences O f f i c e through Contract N00014-85-C-0659 from the O f f i c e of Naval Research and the c r y s t a l s t r u c t u r e a n a l y s i s by C. S. Day of C r y s t a l y t i c s Co. are g r a t e f u l l y acknowledged.

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F i g u r e 4. A p e r s p e c t i v e drawing of C H ^ N g S i Β w i t h nonhydrogen atoms represented by thermal v i b r a t i o n e l l i p s o i d s drawn to encompass 50* of t h e i r e l e c t r o n d e n s i t y ; hydrogen atoms are represented by a r b i t r a r i l y small spheres which are i n no way re p r e s e n t a t i v e of t h e i r true thermal motion.

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F i g u r e 5. A p e r s p e c t i v e drawing of c i a H 5 6 N 4 0 S i 6 , B 2 , w i t h nonhydrogen atoms represented by thermal v i o r a t i o n e l l i p s o i d s drawn to encompass 50* of t h e i r e l e c t r o n d e n s i t y ; hydrogen atoms are represented by a r b i t r a r i l y small spheres f o r purposes of c l a r i t y .

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Literature Cited 1. Wynne, K. J., Rice, R. W. Ann. Rev. Mater. Sci. 1984, 14, 297. 2. Steinberg, H., Brotherton, R. J. Organoboron Chemistry, Vol. 2, pp. 175-434; J. Wiley, New York, 1966. 3. Taniguchi, I., Harada, Κ., Maeda, T. Jpn. Kokai 76 53,000, 11 May 1976. 4. Paciorek, K. J. L., Harris, D. H., Kratzer, R. H. J. Polym. Sci. 1986, 24, 173. 5. Paciorek, K. J. L., Kratzer, R. Η., Harris, D. H., Smythe, M. E. Polymer Preprints 1984, 25, 15. 6. Groszos, S. J., Stafiej, S. F. J. Am. Chem. Soc. 1958, 80, 1357. 7. Toeniskoetter, R. H., Hall F. R. Inorg. Chem. 1963, 2, 29. 8. Ohashi, O., Kurita, Y., Totani, T., Watanabe, H., Nakagawa, T., Kubo, M. Bull. Chem. Soc. Japan 1962, 35, 1317. 9. Gerrard, W., Lappert, M. F., Pearce, C. A. J. Chem. Soc. 1957, 381. 10. Wells, R. L., Collins, A. L. Inorg. Chem. 1966, 5, 1327. 11. Geymayer, P., Rochow, E. G. Monatsh. 1966, 97, 429. 12. Without additional heat treatment above 1000°C (annealing), the fibers are amorphous based on transmission electron microscopy (TEM) with islands of microfibrils or microcrystallites. Selected area electron diffraction (SAED) of the microcrystallites indicates that they could be BN, B2O3, or graphite. Auger electron spectroscopy showed the fibers to be essentially free of carbon which is supported by their colorless appearance. X-ray diffraction produces a weak signal corresponding to hexagonal BN. The fact that the fibers do not change in shape when heated at 1000°C rules out the presence of significant amounts of B2O3, which melts at 460°C. Further characterization of the fibers in regard to composition, structure, susceptibility to "graphitization", as well as strength and density is in progress. 13. Wells, R. L., Collins, A. L. Inorg. Chem. 1968, 7, 419.

RECEIVED September 1, 1987

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[acs symposium series] inorganic and organometallic polymers volume 360 (macromolecules containing...

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