THE BORON HYDRIDES BERNARD SIEGEL and JULIUS I. MACK U. S. Naval
Research and Development Department, Powder Factory, Indian Head, Maryland
Tm
preparation and manipulation of boron hydrides require high vacuum techniques (24) made necessary by the extreme toxicity of boron hydride vapors (23) and the explosion hazard in the presence of oxygen. The latter varies from the spontaneous ignition of the pentaboranes in the presence of oxygen to the explosiveness of decaborane if exposed to oxygen above 100°C. (28). The pioneers of this field were Stock (29) in Germany, and later Schlesinger and Burg in this country (28). The former developed the high vacuum techniques and discovered the presently characterized boron hydrides. Although specific boron hydrides have been known since 1912, it has only been within the past decade that these compounds have received widespread attention. During this period methods of preparation have been developed that make these compounds more readily available for study. The recent increased interest in boron hydrides and their derivatives has been stimulated in part by their potential usefulness as high energy fuels in propulsion engines such as rockets. Compared to conventional fuels such as saturated hydrocarbons, these compounds offer the possibility of much greater energy releases per unit weight of fuel. Thermochemical calculations show that the complete reaction of a stoichiometric mixture of a boron hydride and oxygen should release approximately twice as much energy per pound as an equal weight of a mixture of hydrocarbon and oxygen (12). This is due to the higher heat of oxidation of boron compared to carbon, and the positive heats
Figwe 1.
G..metriul
of formation of boron hydrides compared to the negative values for saturated hydrocarbons. The heats of combustion are as follows (21):
Typical heats of formation are compared in Table 1 (21, 22).
TABLE 1 AH",
Comvound
Fornula
State
Diborane Pentaborane Decaborane Ethane n-Pentane n-Octane
BnHs Bas BmHu CzHa CaHn CaHn
gas liquid solid
(25T., kral./rnole)
gas liquid liquid
KNOWN BORON HYDRIDES
The simplest stable boron hydridc is diborane, B2Hs, a colorless gas a t room temperature. It is believed to be in equilibrium with borane, BHJ, according to the equation, 2 BHaeB2H6. That this equilibrium greatly favors diborane is shown by estimates of 1.73 X loL8 atm.? and 2.34 X 1OI1atm.-' for the equilibrium constants a t 273 aud 373'K. (I). Accordingly, it is not surprising that BH8 has been
configu..tioau
of B-n
Hgdrid-
JOURNAL O F CHEMICAL EDUCATION
detected only indirectly (2, 5, 6, $6). A horon hydride containing three boron atoms per molecule has never been observed, although salts such as NaB3Hs are known (11). Four colorless liquid boron hydrides are known, BIH,~, B5Hs, BsHll and BsHlo, called tetraborane, pentaborane (or pentaborane-9),dihydropeutaborane (or pentaborane-1 1), and hexaborane, respectively. There have also been reports of an unstable hexaborane (I$), BBHI2,but the ideutity of this compound has not been definitely established. A heptaborane has never been reported but evidence exists for both an octa- and nonaborane (17, 25). The formulas of these hydrides have not been definitely established. The highest known boron hydride is decaborane, BIoH1,. It is a volatile white solid, generally soluble in many organic solvents without decomposition (3). Boron hydrides with more than ten boron atoms per molecule have not been identified. However, nonvolatile compounds that are believed to be higher horon hydrides are formed as byproducts in the preparation of the volatile boron hydrides, and upon decomposition of the latter. They have generally been reported to be white, yellow, brown and black solids of indefinite compositions ranging down to (BHo.& and even lower hydrogen-to-boron ratios (13, $9). None of these non-volatile compounds has been characterized and very little is known of their properties. Their nonvolatility and general insolubility have made progress in their investigation very slow. These properties make it impossible to obtain mass spectra, for example.
B,H.:
4 o w d e n t B-H bonds 2 hydrogen bridges
BEHI: 5 oovdent B-H bonds 4 hydrogen bridges 2 covalent B-B bonds 1 three-aentered boron bond
STRUCTURES AND BONDING
The valence configuration of the boron atom can be represented by three valence electrons and four hybridized spa orbitals. When atoms such as boron, with fewer valence electrons (two s and one p electrons) than low energy bonding orbitals (one s and three p orbitals) form compounds with atoms or groups which contain no unshared electron pairs, electron-deficient molecules result. In such molerules bonding is delocalized to make use of all the low energy orbitals. Diborane and higher boron hydrides are in this category. As a result they exhibit extremely interesting structural and bonding properties. They do not seem to form singly connected open chains with strougly directed valences as do electron-balanced or electron-excess hydrides such as the paraffins, amines and ethers. Instead they form close-paeked networks of almost equilateral triangles (19). The geometrical structures, deduced chiefly from X-ray and electron diffraction studies, are shown in Figure 1 (9, 14). They are representations of the relative positions of the boron and hydrogen atoms. The lines between the atoms do not represent ordinary covalent bonds siuce the atoms do not furnish sufficientelectrons for this to be possible. Several different types of bonds are postulated for these structures. Hydrogen atoms bonded to single boron atoms are held by ordinary covalent linkages. The hydrogens bonded to two horon atoms are known as hydrogen bridges. These bridges are best represented as three-centered two-electron bonds formed from the hydrogen orbital, and one hybridized orbital from each of the two boron atoms (7, 15). The remaining electrons and orbitals are involved in VOLUME 34, NO. 7, JULY, 1957
BloHa: 10 covhlent B-H bonds 4 hydrogen bridges 2 covalent B-B bonds G three-centered boron bonds Three oentered boron bonds are shown as
,-I. B B.,'
'.B
Three-centered hydrogen bridges are shown as
B Figure 2.
P10~06.d Bonding for Boron Hydrides
boron-to-boron bonds. A representation of the latter consistent with heat of formation data is sh0u.n in Figure 2 (20). It includes ordinary boron-to-boron covalent bonds and three-centered two-electron bonds involving boron alone. The complete geometrical structures are known for BPHS,B4HI0, BEHB,BSHI1and BlbH1l; only the boron configuration is known for B,H,,. The structures of other boron hydrides have not been determined. The structures in Figure 1 are defective polyhedra, resembling completed regular polyhedra. In this sense they are all open hydrides. Hydrogen bridges are present in the open parts of the molecule but do not close up the holes due to the missing boron atoms. Decaborane has its boron atoms at the corners of an icosahedron which has two adjacent corners lacking; this arrangement is related to the icosahedra found in elemental boron and C3BI2. Pentaborane is related to
the octahedron found in CaB6 and other metal borides, with just one apex lacking. Dihydropentaborane is also related to an octahedron, but one side of its base is somewhat spread and has no hydrogen bridge. Instead of completing the octahedron, hexaborane has a boron configuration that is half an icosahedron. Tetraborane and diborane do not have sufficient boron atoms to distinguish between the icosahedron or octahedron. RELATIVE THERMAL STABILITIES
The thermal stabilities of the characterized boron hydrides seem to be related to the degree to which they approach a closed configuration. Accordingly, the following sequence of decreasing stability has been observed (12, 18): BloHa > BsHo > B2Ha > BSHU> B4Hm For comparable rates of thermal decompasition, in the absence of air and moisture, B4Hlohas to be decomposed at 30°, B6Hll a t 60°, B2Hs a t 100" and B6Hg over 200°C. Decaborane is even more stable than BSH9; it is only slightly decomposed after 48 hours a t 200°C. PREPARATION OF DIBORANE
The preparation of diborane is a key synthesis because it is the principal source of higher boron hydrides which can be prepared from diborane by pyrolytic reactions. Diborane can now be prepared (8, 10, 27) from borohydrides and BF3, or from other hydrides that react with BFa to form borohydrides in an initial step. Unlike the earlier methods, these reactions are comparatively simple, and almost quantitative. They are based on the following principles. Borane, BH3, is a Lewis acid (electron pair acceptor) that forms the borohydride ion, BH4-, by combining with a hydride ion. A Lewis acid stronger than BH3 can thus displace it from the borohydride ion of metal borohydrides. Both BFa and BCla can serve as the stronger acid; BF3 generally is used because of its availability. Diborane is the product of this reaction since two BH2units readily combine to form dihorane. The latter can also be formed from hydrides snch as LiAIH4, LiH, NaH, and LiHB(OCH3)3. These hydrides react with BF3 to form the corresponding borohydride which then reacts with additional BF, to form diborane. For laboratory scale preparations, etherates of BF3 are reacted with etherates of LiAlH. or LiBH4. For larger scale preparations NaH or LiH is used. Typical stoichiometries are given by the following equations: LiAIH,
4 NaH
LiH
+ BF. 5LiBH, + A1F3
+ 4 BF? ethef_ 3 NaBF4 $; NaBH4
+1
. 5 B F 3 3 1,5LiBH, + 4 . 5 L i F promoter
3 LiH
+ 3 B F sether A
3 LiBHR
HIGHER BORANES FROM THE PYROLYSIS OF DIBORANE
Mechanisms. Higher boron hydrides generally have lower hydrogen-to-boron ratios than do lower boranes. Diborane has a H/B ratio of 3.0; pentaborane, 1.8; decaborane, 1.4; and ratios for the non-volatile solids have values down to considerably less than one. The preparation of higher boranes from diborane, in the absence of oxygen and moisture, is thus essentially by means of condensation reactions that eliminate hydrogen. Ultimately the end products of these reactions, at high temperatures, are boron and hydrw gen. The little that is known about the mechanisms of these pyrolytic reactions is restricted to those for the simpler boron hydrides The kinetics (2, 6) of the decomposition of dihorane indicate that it is in equilibrium with BH3and that the latter attacks other diborane molecules to form unstable intermediates which react to form B4Hloand B5Hll. These kinetics also show that the decomposition of BzHs is inhibited by the accumulation of hydrogen in the products. The rapid exchange of boron atoms between isotopically normal diborane and diborane enriched with BID also supports the dihorane-borane equilibrium (26). Isotopic exchange reactions (15) between diborane and dihydropentaborane, and diborane and tetraborane indicate that both BIHlo and B5H11 also readily dissociate into smaller fragments. These conclusions are based on the observations of boron exchange and the kinetics of the exchange reactions. The over-all stoichiometries of the reversible reactions between BsH6,B4Hloand BsH,~are represented by the following equations:
= BlHl0 + H2 2 B,Hm + BIHs = 2 BsHlr + 2 Hz 2 BzHs
Pentaborane on the other hand, appears to be formed irreversibly. A kinetic study has shown that it is formed from BSHI1by a simple first order reaction (2), BsH1,-B6Hs H2. There is no evidence for the formation of lower boranes or B6Hll by the pyrolysis of pentaborane. Isotopic exchange reactions (15, 26) have shown that its boron skeleton does not break down into smaller fragments, and no traces of lower boranes have been found among its decomposition products. Specific Preparations. Because B4H10 and BSHI1are so unstable they have to be prepared by the mildest pyrolytic conditions, snch as relatively low reaction temperatures and short reaction times. If these precautions are not taken, the pyrolysis of diborane results in the formation of more stable boron hydrides. Dihydropentaborane is prepared (4, 15) by decomposing diborane at 115' to 130°C. in an apparatus designed for short exposures of diborane and the reaction products to the reaction temperature. Since the reaction rates are low a t these temperatures, repeated recycling of the diborane is necessary for good yields. Under these conditions the principal contaminant is tetraborane which is easily separable from B5H1,. When diborane is allowed to stand a t room temperature under high pressure, the principal initial product is tetraborane; after nine days higher boron hydrides become major products. However, tetraborane is
+
JOURNAL OF CHEMICAL EDUCATION
more conveniently prepared from BSH1,. By heating the latter with a large excess of hydrogen a t 100' for ten minutes, good yields of BIH,o are obtained (4). Further pyrolysis of these lower boranes leads to the formation of pentaborane and higher boranes. Because the formation of non-volatile solids is inhibited by the presence of hydrogen, while pentaborane and decaborane are not, pentaborane is prepared by pyrolyzing diborane in the presence of initially added hydrogen. Due to the stability of pentaborane it can be prepared a t higher temperatures than are possible for B4Hlo and B6H,,, 225' to 250°C. The higher reaction rates at these temperatures permit single-pass systems rather than recycling systems (16). Between pentaborane and decaborane are several rare hydrides that are difficult to prepare in macroscopic amounts. Of these only hexaborane has been prepared in larger than trace quantities. It can be obtained in approximately 5% yield by pyrolyzing B4Hlo(15, 18). Hexaborane can also he prepared in very small yield by the t,edious acidic decomposition of magnesium boride (29). Historically this was the first method for preparing boron hydrides. Boron hydrides, chiefly tetraborane, are minor products of this reaction, with hexaborane a very minor product. Traces of octa- and uonaborane have been observed among the pyrolysis products of B6Hlo and B6Hn (4, 17, $5).
The extremely stable decaborane is prepared by pyrolyzing diborane for longer periods a t 160°C., heating only one side of a sealed tube (28). The decaborane presumably condenses on the cold side, effectivelv removina it from the reaction zone and thus preventing its further decomposition. Prolonged pyrolysis of the volatile boron hydrides or pyrolysis under extreme conditions invariably results in the formation of the non-volatile boranes. Upon continued pyrolysis, they become darker in color and decrease in hydrogen-to-boron ratio. They do not react (4) with hydrogen to reform lower boranes.
-
POSSIBLE IDENTITIES OF THE NON-VOLATILE BORON HYDRIDES
One of the most intriguing aspects of boron hydride chemistry is the existence of non-volatile substances of boron and hydrogen, about which almost nothing is known. The importance of these substances is underscored by the interest in their structural and bonding properties and the fact that only a small number of boron hydrides have actually been characterized. The characterization of these non-volatile substances could also have important practical applications. Since they have generally been found to be inert compared to the volatile boranes, they would be safer and thus easier to work with. Some interesting speculations can be made about the possible identity of these substances. From the structures and stabilities of the volatile boranes, one might infer that the most stable boron hydrides would be composed of completed pseudo-spherical polyhedra. The most likely of these is the icosahedron
VOLUME 34, NO. 7, JULY, 1957
represented by the hypothetical B11H,2 molecule, although the octahedron represented by B6H6 can also be considered. In these molecules each of the hydrogen atoms would be covalently bonded to boron atoms. Larger polymeric units of chains and lattices could form from these simple molecules by condensations between polyhedra, forming borou-to-boron covalent bonds and eliminating hydrogen. It is also possible that defective polyhedra, present in the volatile boron hydrides, also persist in the non-volatile boranes, either alone or in combination with completed polyhedra. LITERATURE CITED (1) BAUER,S. H., A. SKEPP,AND R. E. MCCOY,J . A m . Chem. Soe., 75, 1003 (1953). J. K., L. V. MCCARTY, and F. J. NORTON, J . Am. (2) BRAGG, Chem. Soe., 73, 2134 (1951). H. R., General Electric Co., Technical Report (3) BROADLEY, 55248, Jan. 1948. J. Am. Chem. Soc., 5 5 , (4) BURG,A. R., AND H. I. SCHLESINGER, 4009 (1933). J . Am. Chem. Soc., 59, (5) BURG,A. B., AND H. I. SCHLESINGER, 780 (1937). R. P., AND R. N. PEASE,J . Am. Chem. Soe., 73, (6) CLARKE, 2132 (19511. . (7) EBERHARDT, W. H., B. CRAWFORD, AND W. N. LIPSCOMB, J . Chem. Phys., 22, 989 (1954). AND G. F. ROEDEL, (8) ELLIOIT,J. R., E. M. BOLDEBUCK, J . Am. Chem. Soc., 74, 5047 (1952). J. Chem. Phys., 22, 754 (9) ERIKG,K., AND W. N . LIPSCOMB, (1954). A. C., A. C. BOND,AND H. I. QCHLESINGER, (10) FINHOLT, J . Am. Chem. Sm., 69, 1199 (1947). AND A. B. MCELROY, J . Am. (11) Howan, W. V., L. J. EDWARDS, Chem. Soc.., 78. 689 ~-~ (1966). (12) HURD,D. T., "Chemistry of the Hydrides," Chapter on Boron, John Wiley & Sons, Ino., New York, 1952. (13) Kosnr, W. S., Johns Hopkins University, personal oommunication. (14) LIPSCOMB, W. N., J. Chem. Phys., 22, 985 (1954). Q ~C., N ~J, . Chem. Phys., 46, 268 (1949). (15) L O N G ~ E T - H ~ GH. L. V.. AND P. A. DIGIORGIO. J . Am. Chem. Soc.. (161. MCCARTY. 73,3138 (1951'). F. J., J . Am. Chem. Soe., 72, 1849 (1950). (17) NORTON, (18) NORTON,F. J., General Electric Co., Technical Report 49A0512, March 1949. (19) PLAIT, J. R., J . Chem. Phys., 22, 1033 (1954). (20) PROBEN, E. J., National Bureau of Standards, personal communication. (21) RossINI, F. D., el al., "Selected Vslues of ChemicdThermodvnamic Prooerties." National Bureau of Standards
.
~
~
~
~~~
~.
.
(22) ROSSINI,F. D., et al., "Selected Values of Properties of Hydrocrtrbons," National Bureau of Standards Circular C461, Nov. 1947. H. M., Arch. Znd. Hyg. Occupational Med., (23) ROZENDAAL, 4, 257 (1951). R. T., "Vacuum Manipulation of Volatile Com(24) SANDERSON, pounds," John Wiley & Sons, Ino., New York, 1948. (25) SHAPIRO, I., AND B. KEIL~N, J . Am. Chem. Soe., 76, 3864 (1954). I., AND B. KEILIN,J . Am. Chrm. Soc., 77, 2663 (26) SHAPIRO,
H. I., AND A. B., BURG,Chem. Revs., 31, 1 (28) SCHLESINGER, (1942). (29) STOCK,A., "Hydrides of Boron and Silicon," Cornell University Press, Ithaca, New York, 1933.
