The Unusual Salts Derived from the Boron Hydrides

The Unusual Salts Derived from the Boron Hydrides

GEORGE W. CAMPBELL, Jr. U. S. Borax Research Corp., Anaheim, Calif.

The existence of salts derived from the boron hydrides has been of interest since Stock reported the formation of "Na B H ." With the development of boron hydride chemistry, a number of salts have been derived from these compounds which can be classified according to the elements they contain. Some of the salts have only a transitory existence or they have been assumed to be highly reactive or unstable intermediates. The chemical properties and the probable structures of these salts are discussed.

Downloaded by CORNELL UNIV on May 17, 2017 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0032.ch021

2

2

6

E a r l y i n the development of the chemistry of the boron hydrides Stock and his coworkers reported the preparation of some solid salts which were thought to be formed b y the addition of electrons from an active metal to the electron-deficient structure of diborane, thus producing an anion with an ethane-like structure (36). Although the early interpretation of the product obtained now appears to have been in error, the evolution of the chemistry of the boron hydrides has shown the existence of a variety of salts with some unusual properties, and a few with surprising structures. The largest and most important class of boron hydride salts is that of the borohydrides. However, because this subject has been treated extensively elsewhere, this paper is concerned primarily with the less common salts. A t the present time, these compounds appear to be primarily of fundamental interest. I n many cases the structures are still uncertain, and the chemical properties have been studied only briefly. However, the possibility of preparing new and unusual boron compounds b y the use of these salts, as well as rinding spécifie uses for them i n chemical processes, should attract increasing attention. F o r convenience, the salts have been separated into three classes: nitrogen-containing salts, oxygen-containing salts, and the boron hydride salts. Nearly all of the salts which contain nitrogen may be considered as derivatives of the boron hydrides and ammonia or its substituted derivatives. I n general, i f the boron atom is found i n the anion portion of the salt, the negative charge may be thought of as coming from the amide ion, or possibly from the imide ion. I n a like manner, the salts containing oxygen may be thought of as acquiring their anionic charge from the hydroxide or the oxide ions which have been coordinately bonded b y the boron hydride. On the other hand, the salts i n the third class appear to acquire their negative ionic charge either b y the addition of electrons to the boron hydride structure directly, or b y the addition of hydride ions to the otherwise neutral boron hydrides. Salts Containing Nitrogen The salts included i n this class are those which have a direct boron to nitrogen bond, and also have one or more boron to hydrogen bonds. 195 BORAX TO BORANES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

ADVANCES IN CHEMISTRY SERIES

196

Salts Derived from Ammonia and Diborane. T h e reaction of diborane with ammonia, at low temperatures, produces the "diammoniate," Β Η · 2 Ν Η (36, 40). This substance was believed to be the diammonium salt ( N H + ) B H - by Stock and Wiberg. T h e evidence included the observation that the electrolysis of solutions of the diammoniate i n liquid ammonia produced hydrogen at the cathode before any nitrogen was liberated at the anode. However, as was pointed out b y Schlesinger and B u r g (30), these results did not provide a basis for an estimation of the amount of ammonium ion furnished by the original salt. On the other hand, the reaction of the diammoniate, Β Η · 2 Ν Η , with sodium i n liquid ammonia was found to produce just one equivalent of hydrogen rapidly, and some additional gas was formed very slowly. The total quantity of hydrogen, however, never exceeded 1.4 equivalents per mole of diborane (29). On this basis, Schlesinger and B u r g proposed that the diammoniate might best be represented as the monoammonium salt, N H + ( B H N H B H ) . I t was further reported that the stable (to 70°C. i n vacuo) salt, N a B H N , was obtained by the reaction of one mole of sodium with one mole of the diammoniate i n liquid ammonia, followed b y the careful removal of the solvent. T o explain the slow secondary reaction, which increased above one the equivalents of hydrogen produced per mole of diborane, an equilibrium was postulated i n accord with the equation 2

β

4

2

6

2

2

4

3

4

3

2


3

2

2

3

_

8

N H + ( B H N H B H " ) + 2 N H ;= 2 N H ( B H N H " ) 4

8

2

8

8

4

+

8

2

I n support of this postulate, Schlesinger and B u r g prepared the salt borine-dimethyletherate, N a N H B H , b y the reaction of borine-dimethyletherate, B H - 0 ( C H ) , with ammonia and sodium, and they further showed that solid N a N H B H absorbed d i borane, presumably to form N a B H N H B H . A recent study by Schaeffer and Basile (28), on the reaction of lithium amide with diborane, indicated that the reactive intermediate L i N H B H was formed, but that i t reacted further with diborane to produce lithium borohydride and polymeric aminoborine. I n this work no evidence for the existence of L i B H N H B H b y the reaction of L i N H B H with diborane could be found. On the basis of x-ray diffraction studies, Schaeffer, Adams, and Koenig (27) have also demonstrated that when the diammoniate of diborane reacted with an alkali metal i n liquid ammonia, followed b y the removal of the solvent, the corresponding boro hydride and polymeric aminoborine were produced. These results i m p l y that the " N a B H N " obtained by Schlesinger and B u r g (80) was actually a mixture of these two compounds. A s a result of these observations Schaeffer et al. argued that the structure of the diammoniate might best be represented as N H + B H N H B H ~ , which would be interpreted as ammonium borohydride stabilized b y association with an amino borine group. I t has also been suggested (26) that further production of hydrogen beyond one equivalent by the reaction of the diammoniate with sodium i n liquid ammonia might well be due to the reaction of trimeric aminoborine [comparable to trimeric Af-methylaminoborine, as described by Bissot and P a r r y (3)] with sodium to produce the hypothetical salt N a + B N H - . On the other hand, it has been argued by P a r r y and his coworkers (24, 81-34), that the existence of the ammonium ion i n the diammoniate of diborane is inconsistent with the chemical properties which have been observed. A m m o n i u m borohydride, N H B H , is unstable above — 2 0 ° C , whereas the properly prepared diammoniate is stable up to 70°C. Ammonium salts, such as ammonium chloride and bromide, react with the diammoniate of diborane to produce hydrogen. Consequently, the structure of the diammoniate of diborane has been suggested to be B H ( N H ) + B H ~ . I n further support of this suggestion, salts of the cation have been prepared and purified, including the salt B H ( N H ) + C 1 - . The diammoniate of tetramethyldiborane shows very analogous chemical proper ties to the diammoniate of diborane, Β Η · 2 Ν Η (6), and hence i t should have a comparable structure. However, b y treating the tetramethyl derivative, ( C H ) 4 B H * 2

3

3

3

2

3

2

3

2

3

3

2

2

2

3

3

8

4

3

4

2

3

3

2

2

4

n

4

2

2

3

3

2

4

2

2

β

3

3

BORAX TO BORANES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

2

2

CAMPBELL

197

Unusual Salts from Boron Hydrides

2 N H , with successive small amounts of trimethylamine i n vacuo; Martincheck (23) displaced up to 1.6 moles of ammonia. This observation is difficult to interpret, i f the addition compound is assumed to be an ammonium salt, either 3

N H [ ( C H ) B H N H H B ( C H , ) ] - or N H ( C H ) B N H [ H B ( C H , ) r ] 4

+

8

2

2

2

4

+

8

2

2

2

However, the displacement of more than one mole of ammonia from ( C H ) B ( N H ) + H B ( C H ) " ~ would not be unexpected. Hence, i t now appears that the interpretation proposed by P a r r y and his coworkers best explains the observed properties of the diammoniates of diborane and of tetramethyldiborane. Ammoniates of Higher Boranes. T h e ammonia complexes of B H and B H may also be considered as salts, i n view of the electrolysis results reported b y Stock (35). I n liquid ammonia these complexes conduct electrolytically i n a manner similar to the diammoniate of diborane. Stock suggested that the ammoniate of tetraborane was best represented as the tetrammonium salt, ( N H ) B H . However, a recent investigation by K o d a m a and P a r r y (20) has raised some question on whether the tetrammoniate exists, although the existence of the diammoniate appears to be well substantiated. Hence i t appears that more work is needed before the structure of the ammoniate of tetraborane can be established. This appears to be true also of the ammoniate of decaborane, although Stock proposed the hexammonium salt formula ( N H ) B H (85). I n a recent investigation, Schlesinger and Henle (29) have found that when pentaborane, B H , was dissolved i n liquid ammonia, and allowed to stand several days at —80° to — 6 4 ° C , the addition of lithium metal caused a reaction which generated two equivalents of hydrogen per mole of pentaborane. O n the basis of this reaction, and the analysis of the final product obtained after the removal of the solvent, Schlesinger and Henle have tentatively proposed that the salt formed was L i B H ( N H ) , although i t was not definitely established that this product was a single substance. T h e reaction of aged solutions of B H i n ethylamine with lithium apparently proceeded i n a similar manner to produce a substance believed to be L i B H ( N R H ) · 4 N R H (where R = C H ) . When this product was warmed i t decomposed to produce lithium borohydride, hydrogen, and bis-ethylaminoborine, H B ( N R H ) . These studies are not yet complete, and further results should be of considerable interest. I n contrast to the behavior of ammonia, the low temperature reaction of diborane with phosphine apparently forms the simple complex, borine phosphine, B H P H (5, 12). I t is likely that the substituted phosphines would react similarly; therefore these complexes cannot be considered to be salts or saltlike. 3

2

3

4


4

4

6

2

3

1 0

4

4

1 0

1 0

1 4

e

8

5

9

2

2

2

2

5

9

2

5

9

2

2

2

5

9

2

5

2

3

3

Salts Containing Oxygen The salts included i n this class are those which appear to have both boron to oxygen and boron to hydrogen bonds. There has been relatively little work reported on them, recently, and the structures which have been proposed for some of them are difficult to accept, i n view of the present understanding of the structures of the boron hydrides. When tetraborane or diborane is treated with concentrated solutions of alkalies, the hypoborates are produced (35). The empirical formula for the potassium salt so produced is K O B H . This substance is diamagnetic (19) ; hence i t has been proposed that the formula should be doubled, and that i t might best be written as K ( B H O H - B H O H ) . However, the existence of a boron-boron bond has not been demonstrated. I n view of the relative difficulty of forming such a bond i n aqueous systems and the fact that no such bond does exist, as such, i n diborane, this interpretation of the structure of the hypoborate ion appears unlikely. I n speculation, i t a p pears likely that the reaction of diborane with hydroxide i n water actually produced the borohydride ion and an anion, possibly trimeric, with the structure 3

2

2

2



198 H

OH

"

ν o

H I

/

o

X

H


|_ΟΗ

H Y

\

/

I

Χ

Χ

Ο Η

J

This postulate appears to be more consistent with the known behavior of the boron hydrides than the earlier proposal. However, additional studies' are needed before any decision can be made. The hypoborates are stable i n dry air, and when heated strongly they decompose to produce some of the free metal, which accentuates their reducing power. I n solu tion the hypoborates reduce salts of silver, bismuth, mercury, arsenic, and antimony to the free elements. A solution of copper sulfate will react with potassium hypoborate to produce a precipitate of what appears to be a hydride of copper. Perhaps the most characteristic reaction of the hypoborates is that with nickel sulfate to precipitate nickel boride, N i B , even from a cold solution. There are a number of other salts which belong in this class. They are the salts of the lower oxyacids of boron—the subborates—derived from H B 0 , and the borohydrates, derived from the two forms of H B 0 . A l l of these compounds, powerful reducing agents, have been supposed to have boron to boron bonds, although there have been no structural studies reported. The acid, H B 0 , appears to have two forms. F r o m one of these forms salts such as K H B 0 may be prepared, whereas from the other both K H B 0 and K H B 0 have been obtained. The structures which have been proposed for the anions H B 0 are 2

4

e

e

2

4

2

2

4

2

2

2

2

4

2

2

2

2

4

2

2

2

4

2

2

- 2

Γ Η—Β—ΟΗ |_Η—^—OH

_ 2

Ί

Γ

Η—Β—ΟΗ-Η

J

—Η [ Η Ο — ΒΒ —

J

trans 0-form

CIS

:-form

I n view of the rarity of boron to boron bonds, and the fact that boron to boron double bonds have not been found elsewhere, these structures are difficult to accept. The acids from which all of these salts are obtained, including the hypoborates, are found i n the hydrolysis products of magnesium boride. I t has been suggested that they may a l l be considered as hydrolysis products of tetraborane. A review by R a y (25) treats these derivatives i n detail, but little recent work has been reported. The existence of another salt which may be included i n this class has been i n d i cated by the work of Guter and Schaeffer (13) i n which decaborane was shown to behave as a strong monoprotic acid i n water. Since this reaction occurred i n a water solution, it would appear that a hydrate of decaborane was first formed and the base removed a proton from the bound water. The salt formed by the neutralization of this acid, therefore would appear to contain the anion B H O H . However, decaborane has been shown to have protonic character (15), so it may well be that the anion produced in this system was B H ~ (14), although if this were the case an association equilibrium would be expected, according to the equation 1 0

1 0

B

1 0

H

1 S

- + H 0 = Β 2

Thus, both the B H ~ and the B H system. However, the ionic species B sarily be the same. 1 0

1 3

1 0

1 0

1 4 1 0

1 4

-

1 3

Η

1 8

ΟΗ -

(or

2

B

1 0

H

1 4

OH-)

O H - ionic species may well be present i n this H -OH and B H O H ~ would not neces 1 3

2

_

1 0

1 4


CAMPBELL

199


The Boron Hydride Salts Salts Produced by Reaction of Diborane with Alkali Metal Amalgams. T h e first preparation of boron hydride salts was reported by Stock and his coworkers {35-39). These salts, produced by the reaction *of diborane with active metal amalgams, and believed to contain the B H - anion, have been shown to contain the borohydride ion, B H - , as a principal component (18). M o r e recently Hough, Edwards, and M c E l r o y (17) have discovered that the reaction of diborane with amalgamated sodium is promoted b y ethers, and i n these studies both simple borohydrides and salts of the triborohydride anion, B H ~ , have been identified. This topic is considered i n great detail by Hough and Edwards (16). Reaction of Diborane and Its Derivatives with Ammonia Solutions of Active Metals. If diborane is carefully added, at low temperatures, to a solution of sodium i n liquid ammonia, a reaction takes place i n which the sodium is used u p , but only an i n significant amount of hydrogen is produced (6, 26). However, i t has not been possible to separate the products of this reaction from the solvent without decomposition. O n the other hand, B u r g and Campbell (6) found that the reaction of tetramethyldiborane with sodium-ammonia produced the compound N a H B ( C H ) according to the equation: 2

e

2

4


3

8

2

2

( C H , ) B H + 2Na + N H 4

2

2

3

2

Na HB(CH,) + (CH ) BHNH

8

2

2

3

2

(3)

8

-75°C.

The salt N a H B ( C H ) is a white solid which dissolves i n liquid ammonia to give a pale yellow solution. I n liquid dimethyl ether i t seems to be highly aggregated. The formation of N a H B ( C H ) appears to take place b y the reaction of the solvated electrons with the boron-hydrogen bridge structure, causing a symmetrical cleavage of tetramethyldiborane, a pair of electrons adding to one H B ( C H ) group, the other dimethylborine forming the ammonia complex ( C H ) B H N H . The formation of the simple dimethylborine ammine complex rather than the diammoniate of tetramethyldiborane was indicated b y the fact that at —75°C. there was no hydrogen produced from this system, even when sodium was present i n great excess. This also indicated that the simple ammonia addition complex of dimethylborine was not readily converted to the diammoniate of tetramethyldiborane. M o r e recently, Shore and P a r r y have prepared borine ammine, B H N H (32), which was converted to the diammoniate of diborane only with difficulty, if indeed it could be converted at a l l . It has been argued that the structure of the H B ( C H ) ~ ion has a pair of unshared electrons on the boron atom, and, therefore, it could be considered as a Lewis base. On the other hand, the availability of the hydride ion from N a H B ( C H ) should exceed that of sodium borohydride. I n support of the dual character to be expected from this salt, i t was found that i n dimethyl ether, i n which it was highly aggregated, N a H B ( C H ) proved to be a ready source of hydride ion, easily converting chlorosilane to silane; while i n liquid ammonia solution N a H B ( C H ) was indeed a Lewis base, bonding trimethylboron to form the compound N a H B ( C H ) B ( C H ) which was stable u p to 100°C. i n vacuo. Other reactions which were interpreted as demonstrating the hydride potential of N a H B ( C H ) were the 1 to 1 addition of trimethylboron to N a H B ( C H ) i n methyl ether to produce a product which released trimethylboron completely at 0° to 20°C. i n vacuo and the addition of diborane to N a H B ( C H ) i n methyl ether yielding products, stable i n vacuo at room temperature, which contained 1 . 4 B H groups per mole of N a H B ( C H ) . Attempts to prepare other alkali metal salts of the H B ( C H ) ion have indicated that i t may be possible to prepare L i H B ( C H ) (7), but only with unusual difficulty. Only the slightest suggestion was found that the potassium salt could exist. On the other hand, the solvated calcium salt, C a H B ( C H ) - N H , was prepared by the same process used for the sodium salt, and with less difficulty. While the hydride activity of C a H B ( C H ) - N H i n liquid methyl ether was 2

3

2

2

3

2

3

3

3

2

3

2

2

3

3

2

2

2

3

3

2

2

3

2

2

3

2

2

3

2

3

3

2

2

2

3

3

2

2

3

2

3

2

3

2

3

3

3

2

2

- 2

2

2

3


3


200

easily demonstrated by the ready conversion of chlorosilane to silane, the base activity of the calcium salt i n liquid ammonia was greatly reduced by comparison with N a H B ( C H ) . Hence, trimethylboron was not observed to add to C a H B ( C H ) * N H in liquid ammonia. However, it appears that the liquid ammonia reactions of both N a H B ( C H ) and C a H B ( C H ) - N H with tetramethyldiborane are best interpreted in terms of the base activity of the H B ( C H ) ~ ion. The reaction of C a H B ( C H ) with tetramethyldiborane, in the ratio of two moles to one, respectively, gave a clear ammonia solution, from which a solid slowly separated at — 75°C. On the basis of the assumed base activity of C a H B ( C H ) , the analysis of the solid ultimately produced, and from which calcium dimethylborohydride, C a [ H B ( C H ) ] , could be separated, the equations proposed for the reactions which occur i n this system are 2

3

2

3

2

3

2

3

2

3

2

2

3

3

2 C a H B ( C H ) + ( C H ) B H -> 2 C a H B ( C H , ) H B ( C H ) 8

4

2

2

2

8


2

8

2

2

8

2

2

2

2

(4)

2

2 C a H B ( C H ) . H B ( C H ) -> C a [ H B ( C H ) ] + C a [ B ( C H ) ] 8

2

2

2

2

3

3

3

8

2

8

2

(5)

2

These same final products were obtained when tetramethyldiborane reacted with calcium i n liquid ammonia, provided that the mole ratio of reactants was 3 to 2, respectively. I n this reaction, exactly half of the calcium was converted to the ammonia-soluble calcium dimethylborohydride. However, when the ratio of tetra methyldiborane to calcium exceeded 2 to 1, all of the calcium was converted to calcium dimethylborohydride. This has been interpreted in terms of the initial formation of C a H B ( C H ) by a reaction comparable to that of Equation 3 followed by that of Equation 4, and finally the reaction of the diammoniate of tetramethyldiborane with the product of Equation 4 as follows: 3

2

2 C a H B ( C H ) . H B ( C H ) + 2 N H + H B ( C H ) - -> 2 C a [ H B ( C H ) ] + 2 N H H B ( C H ) 8

2

8

2

4

2

8

2

2

8

2

2

8

8

2

(6)

The behavior of lithium toward tetramethyldiborane appears to be comparable to that of calcium. L i t h i u m dimethylborohydride has been prepared by the reaction of excess tetramethyldiborane with lithium. I n contrast, however, when either sodium or potassium reacted with tetramethyldiborane (in excess) none of the dimethylborohydride could be identified, and considerable trimethylboron was produced by the hydrolysis of the product. I n fact, i t has been difficult to avoid contamination of sodium dimethylborohydride, N a H B ( C H ) , by trimethylboron i n the preparation of the boron base, and it has been impossible to avoid such contamination of the potassium salt; therefore the latter has not been prepared i n a pure state. Although the reasons for the differences between these systems are not yet clear, it appears that while the dimethylborohydrides of calcium and lithium are both stable, the correspond ing sodium and potassium salts disproportionate easily, and may actually be incapable of a stable existence. The reactivity of the boron base anion, H B ( C H ) ~ , is so high toward tetramethyldiborane that it has not been possible to prepare these salts free of secondary product contamination except at very low concentrations and with great excesses of active metals. As a result, the scale of preparation has not exceeded approximately 50 mg. of product. V e r y little attention has been given the dimethylborohydrides, but i n one study it was found that the reaction of calcium dimethylborohydride with chlorosilane, i n methyl ether, produced two moles of silane per mole of calcium salt, and tetramethyl diborane was a product of this reaction. It appears, therefore, that this reaction may be represented by the equation 2

3

2

3

Ca[H B(CH ) ] + 2SiH Cl -* 2SiH + CaCl + B H ( C H ) 2

8

2

2

8

4

2

2

2

8

2

2

4

The salt N a H B ( C H ) - B ( C H ) , produced by the reaction of sodium dimethyl borohydride, N a H B ( C H ) , with trimethylboron i n liquid ammonia, appears to be the first true case of a salt of the diborohydride type in which at least one Β—Η bond exists. It now seems unlikely that any of the less substituted diborohydrides will be prepared i n view of the apparent formation and disproportionation of the tetramethyldiborohydrides, together with the unsuccessful attempts to prepare the simple diborohydrides by the reaction of diborane with metal amalgams. 2

2

3

3

2

3

3

2


CAMPBELL

201


N o t much is known about the salts of the B ( C H ) " " anion. I n fact, their existence has not been proved, although the evidence for such salts appears to be convincing. Hence, when N a H B ( C H ) reacts with chlorosilane, the formation of silane implies the loss of a hydride ion from the boron base. Thus it appears that the most acceptable equation for this reaction is 3

2

3

2

2

(CH,) 0 2

Na HB(CH ) + SiH Cl 2

8

2

> NaCl + SiH + NaB(CH )

8

4

3

2

However, the product does not hydrolyze to release all of the boron as the dimethylboric acid, suggesting that the B ( C H ) ~ is polymeric. T h e evidence for the existence of the calcium analog, C a [ B ( C H ) ] , is more convincing. When C a H B ( C H ) reacted with chlorosilane, silane was produced i n agreement with the equation 3

3

2

2

2

2CaHB(CH ) + 2SiH Cl 8

2

3

CaCl + Ca[B(CH ) ] + 2SiH

8

2

2

2

4

The residue, presumably C a C l and C a [ B ( C H ) ] , contained 33.6% calcium (cal culated 34.3%). However, the treatment of this solid w i t h aqueous hydrochloric acid produced only a small fraction of the amount of dimethylboric acid potentially available, although 8 0 % of the expected hydrogen was produced. T h e nonvolatile part of the hydrolysis mixture charred when evaporated to dryness with sulfuric acid, indicating that organic carbon—apparently i n the methyl groups—was held back, probably i n a polymer containing C H — Β groups. In view of these results, further attempts to prepare and study the salts of the postulated anion B ( C H ) ~ ~ are of interest. I t should be noted, however, that salts of the dibutyl analog were reported b y Auten and K r a u s (1). According to them, d i butylboron chloride was reduced b y means of a sodium-potassium alloy, i n ether solu tion. The reduction produced, stepwise, dibutylboron and the dibutylboron salt. T h e salt, M B ( C H ) , in contrast to the salts of the B ( C H ) ~ anion, was soluble i n ether. It reacted readily with methyl iodide to form methyl dibutylboron, and with d i b u t y l boron chloride to produce tributylboron and monobutylboron, which was probably poly meric. The reasons for the remarkable differences between the salts of the dimethylboron anion and the dibutylboron anion are not yet clear, and further studies are re quired. The work of Krause and D i t t m a r (21), Bent and Dorfman (2), and C h u and Weissman (8-10) on the preparation of salts containing triarylboron anions should be mentioned in this discussion, although they are not derived from the boron hydrides. In ether solutions, triphenyl boron reacts with sodium to produce the ether-soluble salts N a B ( C H ) and N a B ( C H ) . The monosodium salt has been shown to be diamagnetic (8, 9) thus suggesting the formation of the dimer N a B ( C H ) , or of ion clusters, as proposed b y C h u and Weissman (9). The disodium salt, N a B ( C H ) , would be structurally similar to the boron base, N a H B ( C H ) , although the chemical properties of the triaryl salt have not been studied extensively. When the phenyl group was replaced by the mesityl group, the corresponding monosodium salt N a B ( A r ) was dissociated i n ether solution, thus forming the corresponding free radical ion, or better, the odd-electron salt, because there would be little true ioniza tion i n ether solution. Similar results were observed when tri-/^methylnaphthylboron was used for the preparation of the triarylboron anion. Salt Derived from Sodium Hydride and Diborane. I t has been reported recently b y Brown (4) that, i n the presence of diethylene glycol dimethyl ether ( d i glym), sodium hydride absorbs diborane to form first sodium borohydride and then, with sufficient diborane, the new salt N a B H . I t appears likely that this salt contains one Β—Η—Β three-center bond holding two borine groups together. However, no further information regarding this salt is now available. Salts Produced from the Higher Boranes. The studies of Stock and his coworkers on the reactions of alkali-metal amalgams with boron hydrides included tetraborane and pentaborane (35, 37). I t was reported that K B H and N a B H were pre pared, and that the preparation of dipotassium pentaborine, K B H , was probable^ 2


8

2

3

2

2

3

3

4

9

e

2

2

3

5

3

2

e

5

2

3

2

2

e

5

2

2

3

2

3

2

7

2

4

1 0

2

2

5

4

9


1 0

e

e

5

8


202

although i t was not obtained i n a pure state. I n each of these preparations, the sys tem was heated to approximately 170°C. or higher to distill away the mercury. Further heating to temperatures as high as 450°C. produced some hydrogen, sublimed some salts (apparently the borohydrides), and left behind mixtures that had simple formulas such as K B H , K B H , and K B H . I t was reported that when the salt produced from tetraborane, K B H , was treated with hydrochloric acid tetraborane was re generated b y the reaction 2

4

8

2

4

2

4

4

2

K B H 2


5

3

1 0

4

+ 2HC1 - * 2KC1 + B H

1 0

4

1 0

+ H

2

There is now some question whether these reactions actually produce the salts formulated, because of the work of Kasper, M c C a r t y , and N e w k i r k (18), i n which similar reactions involving diborane produced borohydrides. Some interesting recent work b y Schlesinger and Henle (29) has been concerned with the behavior of pentaborane with lithium i n liquid ammonia and liquid ethylamine. The reaction of lithium with aged ammonia solutions of pentaborane, whereby hydrogen was produced, has been discussed earlier. If, however, pentaborane was added to a m monia solutions of lithium only an insignificant amount of hydrogen was produced. T h e removal of ammonia, first b y evaporation, followed by repeated additions and removals of small quantities of dimethyl ether, yielded a substance which approached the formula L i B H . I t was soluble i n a 1 to 1 mixture of tetrahydrofuran and diethyl ether. However, the work is not yet completed, and there is no definite evidence that this " a p parent" lithium salt was homogeneous. I f this is indeed a salt with the formula L i B H , a study of its structure and chemical properties should be of great interest. A study of the reactions of decaborane with sodium amalgams and with sodium dissolved i n liquid ammonia (41) has shown the existence of salts corresponding to the following formulas: N a B H , N a B H , N a B H , and Ν 8 ^ Β Η . There was some evidence for a salt with the formula N a B H , but the question has not been resolved whether this is a single substance or a mixture of N a B H and N a B H . On the basis of the topological theory of boron hydrides (11), Lipscomb has proposed that i n addition to B H ~ , B H ~ , B H , and B H ~ , the following boron hydride ions should also have some stability: Β Η + , B H ~ , Β Η ~ , B H ~ , and B H + (22). Since both positive and negative ions are included, i t has been suggested that some purely ionic hydrides might be prepared b y metathetical reactions of salts containing these ions. 2

5

9

2

1 0

1 4

2

1 0

1 4

1 0

2

1 0

1 2

2

1 0

1 0

2

1 4

1 3

9

12

1 3

1 0 β

3

10

5

1 3

η

2

1 0

4

7

_

1 2

3

2

1 0

1 4

8

β

2

β

5

1 0

6

Conclusions The salts containing nitrogen and those containing oxygen present a number of still unsolved structure problems. Progress is being made i n resolving these problems for the salts containing nitrogen, but further investigations regarding the structures of the hypoborates and the borohydrates are needed. The reactions of diborane with alkali metal amalgams produce borohydrides and salts such as sodium triborohydride, N a B H , rather than the "diborane salts" as proposed by Stock. However, comparable systems utilizing the higher boron hydrides should be further investigated. The boron hydride salts hold considerable promise for the synthesis of new boron compounds. This is especially true of boron bases such as N a H B ( C H ) , and those salts i n which the boron-containing ion can serve as either a Lewis acid or a Lewis base. The ability of N a 2 H B ( C H ) to establish a firm Β—Β bond by reacting with trimethyl boron i n liquid ammonia implies the possibility of forming previously unknown bonds by donor-acceptor bonding with appropriate acceptor molecules. The preparation of similar boron bases from* the higher boranes should be of interest. Hence, these salts derived from the boron hydrides are of importance from the standpoint of fundamental chemistry and also as reagents potentially useful i n chemical synthesis. 3

8

2

3

3

2

2


CAMPBELL


203

Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15)

Auten, R. W., Kraus, C. Α., J. Am. Chem. Soc. 74, 3398 (1952). Bent, Η. E., Dorfman, Maurice, Ibid., 57, 1259 (1935). Bissot, T. C., Parry, R. W., Ibid., 77, 3481 (1955). Brown, H . C., Stehle, P. F., Tierney, P. Α., Ibid., 79, 2020 (1957). Brumberger, Harry, Marcus, R. Α., J. Chem. Phys. 24, 741 (1956). Burg, A. B., Campbell, G. W., J. Am. Chem. Soc. 74, 3744 (1952). Campbell, G. W., Ibid., 79, 4023 (1957). Chu, T. L., Ibid.,75,1730(1953). Chu, T. L., Weissman, T. J., Ibid., 78, 23 (1956). Ibid., p. 3610. Dickerson, R. E., Lipscomb, W. N., J. Chem. Phys. 27, 212 (1957). Gamble, E . L., Gilmont, Paul, J. Am. Chem. Soc. 62, 717 (1940). Guter, G. Α., Schaeffer, G. W., Ibid., 78, 3546 (1956). Hawthorne, M . F., Ibid., 80, 754 (1958). Hough, W. V., Edwards, L . J., Abstracts, 133rd Meeting, ACS, p. 28L, San Francisco, Calif., April 1958.


(16) Hough, W. V., Edwards, L . J., ADVANCES I N C H E M . SER., No. 32, 184 (1961).

(17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37) (38) (39) (40) (41)

Hough, W. V., Edwards, L. J., McElroy, A. D., J. Am. Chem. Soc. 78, 689 (1956). Kasper, J. S., McCarty, L. V., Newkirk, A. E., Ibid., 71, 2583 (1949). Klemm, LeRoy, Klemm, William, Z. anorg. u. allgem. Chem. 225, 258 (1935). Kodama, Goji, Parry, R. W., J. Am. Chem. Soc. 79, 1007 (1957). Krause, Erick, Dittmar, Paul, Ber. 63, 2347 (1930). Lipscomb, W. N., J. Phys. Chem. 62, 381 (1958). Martincheck, L . Α., M. S. thesis, University of Houston, Texas, 1957. Parry, R. W., Schultz, D. R., Giradot, P. R., J. Am. Chem. Soc. 80, 1 (1958). Ray, R. C., Chem. & Ind. (London) 1946, 322. Schaeffer, G. W., private communication. Schaeffer, G. W., Adams, M. D., Koenig, F. J., J. Am. Chem. Soc. 78, 725 (1956). Schaeffer, G. W., Basile, L. J., Ibid., 77, 331 (1955). Schlesinger, H . I., private communication. Schlesinger, H . I., Burg, A. B., J. Am. Chem. Soc. 60, 290 (1938). Schultz, D. R., Parry, R. W., Ibid., 80, 4 (1958). Shore, S. G., Parry, R. W., Ibid., 77, 6084 (1955). Ibid., 80, 8 (1958). Ibid., p. 12. Stock, Alfred, "The Hydrides of Boron and Silicon," Cornell University Press, Ithaca, N.Y. 1933. Stock, Alfred, Kuss, Ernst, Ber. 56B, 807 (1923). Stock, Alfred, Laudenklos, Hans, Z. anorg. u. allgem. Chem. 228, 178 (1936). Stock, Alfred, Pohland, Erick, Ber. 59B, 2210 (1926). Stock, Alfred, Sütterlein, Walther, Kurzen, Fritz, Z. anorg. u. allgem. Chem. 225, 225 (1935). Stock, Alfred, Wiberg, Egon, Martini, Hans, Nicklas, August, Ber. 65, 1711 (1932). Toeniskoetter, R. H., Schaeffer, G. W., Evers, E . C., Hughes, R. E., Bagley, G. E., Abstracts, 134th Meeting, ACS, p. 23N, Chicago, Ill., September 1958.


The Unusual Salts Derived from the Boron Hydrides

Recommend Documents