R E A C T I O N OF D I B O R A N E W I T H P O T A S S I U M ,

R U B I D I U M , A N D C E S I U M B O R O H Y D R I D E S

IN D I G L Y M E

O. V. S h e i k o , N . N. M a l ' t s e v a , a n d V . I . M i k h e e v a

UDC 546.271 : 546.32'271 :

546.35'271

According to [1], the dissolving of B2H G in diglyme (DG) is accompanied by the formation of solvates and slight H 2 evolution. However, in [2] chemical react ion was observed only above 30~ and at p res su res above 5-6 atm when B2H 6 is dissolved in DG. The lithium and sodium borohydrides in DG absorb B2H 6 in an amount of 0 .5 mole per mole of borohydride [3-6], possibly with the formation of B2H 7- ion in solution, which contains a single-bonded hydrogen bridge B . . . H . . . B [3, 4]. NaB2H 7 �9 DG [5], in which the DG solvates the cation, was isolated f rom the solution. KB2H ? could not be obtained [7], while the diborohy- drides of the substituted ammonium cations are descr ibed in [8, 9].

The react ion of B2H 6 with KBH4, RbBH4, and CsBH 4 in DG was studied in the present paper .

E X P E R I M E N T A L M E T H O D

The s tar t ing borohydrides were obtained by the exchange react ion of the corresponding hydroxides with NaBH 4 [10, 11]. The puri ty of the employed borohydrides exceeded 99~. B2H 6 was obtained by the react ion of NaBH 4 with BF 3 �9 OEt 2 in DG [12]. The DG was dried over Na metal .

In a typical exper iment 2-4 moles of B2H 6 per mole of MBH 4 was displaced f rom the react ion flask with dry argon and passed for 12-24 h through a suspension of the borohydride . The MBH 4 goes into solu- tion during the passage of the B2H 6. The unreacted B2H 6 was quantitatively absorbed in acetone. The com- pleteness of absorption was judged by the constancy of the solution composit ion. The obtained solutions were centrifuged and analyzed.

The amount of hydrogen was determined by the amount of H 2 that was evolved when the solution was decomposed f i r s t with water , and then with I : 1 HCI solution, in which connection a special setup was used to determine the hydride hydrogen in the solutions [13]. The amount of boron was determined by t i trating with alkali in the presence of mannitol and using a-naphtholphthalein as the indicator, the potassium was determined by flame photometry, while the rubidium and ces ium were determined gravimet r ica l ly by p re - cipitation with H2PtC16.

TABLE 1. Cesium Borohydr ides in Diglyme

I M "

K Rb Cs Cs Cs

T a k e n

Absorption of Diborane by Potass ium, Rubidium, and

Amount of solution 70

M B H

- - 0 , t 0 - -

0 , 5 7 0,48 0,13 2,48 0,8t 0,28 1,12 0,28 0,09 3,36 0,66 0,19 8,80 t,72 0,54

MBH,, g DG.ml

- - 50

t , t l 50 2,65 60 1,50 60 2,69 60

t2,20 60

A t o m i c ~atio

M:B:H Remarks

1 :3 , i :9 ,7 1:2,6:9,5 ~ :3,0:t0,6

:2,4:7,6 t : 2 ,4 :8 , t

8olub i l i ty of BzH s 0.03 /100

hacomp'lete solution The same

Incomplete solution

N. S. Kurnakov Institute of General and Inorganic Chemis t ry , Academy of Sciences of the USSR, Moscow. Trans la ted f rom Izvest iya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2111-2112, September , 1975. Original ar t ic le submitted J anua ry 29, 1975.

�9 76 Plenum Pubhshing Corporation, 22 7 West 17th Street, New York, N. Y. 10011. No part o f this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission o f the publisher. A copy o f this article is available from the publisher for $15.00.

1996

DISCUSSION OF RESULTS

Under the experimental conditions the solubility of B2H 6 in DG is slight, but its absorption increases

in the presence of MBH 4. The dissolving of the MBH 4 is also observed at the same time, which increases

from K to Cs. In the obtained saturated solutions (Table i) about two BH 3 groups correspond to one MBH 4

molecule.

The IR spectra of the obtained solutions are the same for all of the borohydrides andare characterized by a system of absorption bands in the 2000-2500 cm -i region. The presence o3 bands at 2045 and 2410 cm -I in the Ill spectrum of the solutions testifies to the presence of two types of hydrogen, attached to the boron atoms, whereas the spectrum of NaBH 4 in DG is characterized by absorption bands at ~2300 cm -I, which are inherent to the tetrahedral BH 4- anion, where all of the hydrogen atoms are equivalent [14].

The solid compounds were isolated from the solutions by salting out with diethyl ether, in which medium, as the preliminary experiments disclosed, the B2HG does not react with the K, Rb, and Cs boro- hydrides. The isolated crystalline compounds are the alkali metal borohydrides, which was confirmed by chemical analysis and the IR absorption spectra�9

B2H 6 in DG forms solvates of composition BH 3 �9 xDG [i]. The solubilizing action of B2H 6 on MBH 4 can be explained by the formation of complexes by the following scheme:

BHs.xDG + MBH~ = M [BH~.BH3] § x !)G . (1)

Since BH 3 is an electron acceptor, while the BH 4- anion and ethers are donors, it is possible to re- gard this process as the displacement of a weaker donor by a stronger one. A single-bonded hydrogen

bridge H3B ... H ... BH 3 is apparently formed here. In turn, the B2H 7- anion adds the BH 3 group to form a more complex anion. The atomic amount of M : B : H in the solution apparently depends on the ratios of the salts with the B2H 7- and B3H 8- anions. It is possible to postulate the formation in individual experi- ments of compounds with the B3HI0- anion, whose possible existence was predicted in [15].

The compounds in solution are unstable and decompose easily when isolated from the solution. Pre- cipitation with ether leads to cleavage of the complex and the formation of MBH 4.

The authors express their gratitude to N. A. Chumaevskii and N. A o Minaeva for taking the IR ab- sorption spectra.

CONCLUSIONS

I. Diborane is absorbed by suspensions of the borohydrides of potassium, rubidium, and cesium in

diglyme, with their simultaneous transition into solution. The solubility of the MBH 4 increases in the order K to Cs. The ratio of the dissolved diborane and borohydride molecules corresponds to the formula MBH 4 �9 xBH3, where x ~ 2.

2. The borohydrides separate from solution when precipitated with diethyl ether.

LITERATURE CITED

i. S. Jerazunis,J. W. Mullen, andR. Steginsky, J. Chem. Eng. Data, 7, 337 (1962). 2. A.I. Gorbunov, G. S. Solov'eva, I. S. Antonov, and M. S. Kharson, Zh. Neorgan. Khim., i__00,

1931 (1965).

3. H.C. Brown, P. F. Stehle, and P. A. Tierney, J. Am. Chem. Soc., 7__99, 2020 (1957). 4. H.C. Brown and P. A. Tierney, J. Am. Chem. Soc., 80, 1552 (1958). 5. E.B. Baker, R. B. Ellis, andW. S. Wilcox, J. Inorg. Nuc[. Chem., 2_33, 41 (1961). 6. E.B. Baker, R. B. Ellis, and W. S. Wilcox, U. S. Patent 2921963 (1960); Chemo Abstrs., 6_44,

11412 (1960).

7. W.G. Evans and C. E. Hollowag, Inorg. Chem., _7, 1746 (1968).

8. R.K. Pearson, L. L. Levis, and L. G. Edwards, Nuc[. Sci. Abstrs., I_22, 4069 (1958). 9. L.V. Titov, V. D. Sosnovskaya, and V. Ya. Rosolovskii, Zh. Neorgan. Khim., 1_-8, 2952 (1973).

I0. V.I. Mikheeva and M. S. Selivokhina, Zh. Neorgan. Khimo, _8, 439 (1963). II. V.I. Mikheeva and S. M. Arkhipov, Zh. Neorgan. Khim., i_ii, 1506 (1966). 12. E.C. Ashby andWo E. Foster, J. Am. Chem. Soc., 8_~8, 3248 (1966).

1997

13. A . I . Golovanova, M. E. Kost, and V. I. Mikheeva, Izv. Akad. Nauk SSSR, Ser. Khim.,1448 (1973). 14. A . E . Shirk and D. F. Shriker, J , Am. Chem. Soc. , 95, 5901 (1973). 15. W . N . Lipscomb, Boron Hydrides, W. A. Benjamin, New York--Amsterdam (1963).

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