Nov. 5 , 1954
PREPARATION AND PROPERTIES O F
equations 4 and 5 hold between 110 and 1000°K. Acknowledgments.-It is a pleasure to acknowledge the help of Prof. J. 0. Lord, who examined the single-crystal copper sample, and of L. E. Cox,
DIBORON TETRACHLORIDE
5293
Cryogenic Laboratory Shop Foreman, who made helpful suggestions in the design of the vacuumtight gla.ss window. COLUMBUS, OHIO
[CONTRIBUTION FROM THE GEORGE HERBERT JONES LABORATORY OF THE UXIVERSITY OF CHICAGO]
The Preparation and Some of the Properties of Diboron Tetrachloride, B,Cl,I BY GRANTURRY,THOMAS WARTIK,~ R. E. MOORE’AND H. I. SCHLESINGER RECEIVED APRIL22, 1954 The latest procedure for preparing diboron tetrachloride, B2C14, is described as are the physical properties and the chemical behavior of the compound. I t absorbs hydrogen rapidly a t room temperature, forming diborane and boron trichloride. The chlorine atoms of the compound may be replaced by bromine, by alkoxy and dimethylamino groups, but not by methyl radicals. It forms a dietherate stable in excess ether, and a monoetherate which loses ethyl chloride a t room temperature. At low temperatures in ether solution it adds one mole of diborane to form an unstable compound the behavior of which is described. With trimethylamine it forms a stable compound having the molecular formula [B2C4.PU’( CHI)?]4. Hydrogenation of diboron tetrachloride by treatment with lithium borohydride or with aluminum borohydride results in the formation of diborane and tetraborane together with smaller amounts of pentaborane ( B ~ H Q and ) , decaborane. The relative amounts of the boranes obtained depend on the temperature and other conditions under which the reaction is carried out.
In spite of numerous efforts to find more efficient procedures for the preparation of diboron tetrachloride2 (BzC14),e.g. , by the reduction of boron trichloride by metals, metal borides and other reducing agents we have not yet found a method more satisfactory than the electrical discharge procedure described by Wartik, Moore and Schlesinger.‘ We have, however, improved that procedure by making it almost automatic, with the result that 5-10 g. of the desired compound may be produced per week with very little attention, as is described in the Experimental part. Diboron tetrachloride is a colorless liquid which ignites in air,3and which, a t temperatures of 0’ and above, undergoes partial decomposition into the moderately volatile, crystalline, pale yellow tetraboron tetrachloride, B4C14, a much less volatile, viscous red material, and a white, non-volatile solid. The composition of the red material may be represented by the formula (BClo.g)x,whereas that of the white solid corresponds to the formula (BClo.e)y. In spite of the reproducibility of the analytical data obtained from different samples, it is not certain that the red and the white products were homogeneous. The methods of analysis, as well as the analytical data for and physical properties of diboron (1) In view of the name, tetraboron tetrachloride, which we have assigned to the compound B4Ch (see footnote 3), the name diboron tetrachloride for B2Clr is considered more appropriate than tetrachlorodiborine, as originally suggested by T. Wartik, R.E. Moore and H. I. Schlesinger, THISJOURNAL, 71,32135 (1949). Since a committee is now considering boron nomenclature, the new compounds herein described are referred t o only by formula. Some of the subject matter of this paper is taken from the thesis presented t o the Department of Chemistry of the University of Chicago in 1950 by R. E.Moore, in part fulfillment of the requirements for the degree of Doctor of Philosophy. Most of the subject matter is taken from reports submitted to the Office of Naval Resealch on Contract No. N6ori-20, Task Order 10, Project h’R 356-255, during the period July 1, 1948, t o January 31, 1954. (2) First prepared by A. Stock, A. Brandt and H. Fischer, Bcr., 58, 855 (1925). (3) Although the sample used for the reaction with air had been carefully purified, it is nevertheless possible t h a t its inflammability may have been due to the presence of traces of tetraboron tetrachloride, which seems t o react more vigorously with air than does the diboron compound (Grant Urry, Thomas Wartik and H. I . Schlesinger. THIS JOURNAL 74, , 5809 (1952)).
tetrachloride itself are recorded in the Experimental section. The chemical properties of diboron tetrachloride are best discussed under the following headings, although the classification is somewhat arbitrary : reaction with hydrogen, replacement of chlorine by other atoms or groups, hydrogenation and addition reactions. In most cases description of experimental details, yields, analyses and physical properties of the compounds involved are deferred to the Experimental part. Reaction with Hydrogen.-Hydrogen is taken up rapidly a t room temperature and below by diboron tetrachloride in the absence of catalysts. If the reaction is carried out with the liquid tetrachloride, a variety of products is obtained, among them boron trichloride, diborane and mixtures of solid materials with viscous liquids. Results are difficult to reproduce under these conditions. I t was soon recognized that the difficulty is due to secondary reactions of diborane with unchanged diboron tetrachloride, as described in the section devoted to “addition reactions.” If hydrogen and diboron tetrachloride vapor are mixed a t room temperature, the reaction proceeds about SO-SO% according to the equation 3BzC1,
+ 3Hz
--f
4BC13
+ BzHs
(1)
Traces of hydrogen chloride as well as the presence of small quantities of solid material indicate that secondary reactions of diborane with diboron tetrachloride were not completely prevented by the procedure. Some monochlorodiborane was also present, as must be the case in mixtures of diborane and boron trichloride, but a correction for its presence was applied in calculating the yield. The reaction might have been considered as an addition reaction since the first step is probably that represented by the equation B2C14
+ Hz +BsClaH2
(2)
Since chlorodiboranes disproportionate almost completely into diborane and boron trichloride unless a considerable excess of boron trichloride is present, the postulated intermediate would be almost com-
5294
G.
URRY,
'r. br.4RT1K,
K. E. MOORE
pletely transformed to the end-products of equation 1. These observations suggest that the electrical discharge method of Schlesinger and Burg4 for the preparation of diborane may be of similar character. One may postulate that the following steps occur in the discharge procedure for preparing diboron tetrachloride. C1
+
BClr --+ BC12 C1 Hg --f HgCl (or HgC12) 2BC12 +BZClr
+
(3a) (3b) (3c)
In the procedure for preparing diborane, the diboron tetrachloride might react with hydrogen as suggested by equation 2, or the hydrogen might react with the BClz fragment to form HBC12, which would dimerize and disproportionate to give the end-products monochlorodiborane, diborane and boron trichloride. In the actual preparation of diborane by the discharge procedure, a large excess of hydrogen was used. Under these conditions the presence of diboron tetrachloride as an intermediate could probably not have been detected. The interpretation given for the reaction has, however, received support from our recent observation that when a relatively large proportion of boron trichloride is passed through the arc in the presence of hydrogen, diboron tetrachloride is produced. Replacement Reactions.-The chlorine of diboron tetrachloride may be replaced by bromine, by alkoxy groups and by dimethylamino groups as represented by the equations 3B2Clr + 4BBr3 3BzBr4+ 4BC13 + 4ROH +Bz(OR)4 + 4HC1 --f
B2Clr
(4)
(R = CH, or CBHS) ( 5 )
fi2Clr $- 8(CHa)2XH
+
B~[S(CHS)ZI( 4( CH3)2NHzCI (6)
The reactions, which are not quantitative in all cases, and the analyses and physical properties of the new compounds are described in the Experimental part. Closely related to the replacement reactions just mentioned is the behavior of ammonia toward diboron tetrachloride. The products obtained are all solid and have not yet been completely identified. The fact that 6 moles of ammonia are taken up and 4 moles of ammonium chloride are produced per mole of diboron tetrachloride may be represented by the tentative equation5 xBzC14 f 6xXHa --f 4xNH4Cl f 2(BNH)z (7) All of the substituents by which we have thus far been able to replace the chlorine of diboron tetrachloride are or contain atoms having urishared electron pairs. Attempts to replace the chlorine by methyl groups (e.g., by treatment with dimethylzinc) hare led to what appears to be complete decomposition, ie., to the formation of trimethylboron, black solids probably containing elementary boron, and other unidentified substances. This observation suggests that molecules containing boron(1) H. I. Schlesingerand A. B. B u r g , T ~ ~ JOURNAL,^^,^^^^ s (1931). (.5) By analogy with the reaction of diboron tetrachloride with tri-
methylamine (see below), t h e first step in the ammonia reaction may he the formation of B~CI~.ZNHI.Attention is called t o the fact that A. Stock and E. Pohland. B e y . , 69B,2210 (1920). repnrt a compound
of the composition BNII.
AND
H. I. SCHLESINGER
Vol. 76
to-boron bonds may not be capable of existence unless they are stabilized by some type of resonance or by some other stabilizing factor. In the boranes the resonance associated with hydrogen bridges may be the factor; in diboron tetrachloride and its stable derivatives it may be resonance of the type represented by structures such as + +
c1)B-# c1
cl\-
\Cl
CI
' cl/B-B
