CA1040386A - Process for the synthesis of decaborane(14) - Google Patents

Abstract

PROCESS FOR THE SYNTHESIS
OF DECABORANE (14) Abstract of the Disclosure A process is provided for the synthesis of decaborane(l4) in relatively high yields by an efficient, multi-step process from readily available starting materials. In the first step the B11H14-ion is prepared in situ, for example, by the reaction of controlled amounts of sodium borohydride and boron trifluoride diethyletherate. Thereafter the ion is oxidized and the decaborane(l4) recovered. In contrast to known methods for the preparation of decaborane(l4) the process of this invention is efficient and obviates the need for hazardous and expensive materials such as diborane(6) a starting material previously employed.
The decaborane(14) is a precursor to various boranes including carboranes which themselves are useful monomers for the synthesis of carborane-siloxane polymers and elastomeric gum stocks which have attractive applications as sealants and the like.

S P E C I F I C A T I O N

Description

104~)3~6 Thi5 invention relates in general to the synthesis of decaborane(l4). In one aspect, the invention is ; directed to the preparation of decaborane(l4) in an efficient,multistep process from readily available starting materials. In a further aspect, this invention relates to the preparation of the intermediate BllHl4-ion which can be conveniently oxidized to decaborane(l4)~
In a still further aspect the invention is directed to a process for the preparation of decaborane(l4) from 10 sodium borohydride and boron trlfluoride diethyletherate.
Prior to the present invention decaborane(14) :, , was prepared by the controlled thermolysis of diborane~6) or mixtures of diborane(6) tetraborane(10~ and various diluent gases, such as hydrogen. However, in the prior ?
; art processes, including static systems, hot tube systems l and liquid-containing systems, only complex, hazardous :., mixtures were obtained. Even though yields of BloH14 were reasonably good by some of these methods, technical difficulties experienced precluded efficient scale-up and ' 20 most were not practical for laboratory use. Decaborane(14) has also been prepared in very small quantities by the , thermal decomposition of decaborane(l6) in the presence ~, of I2. The decaborane(l6) was prepared from pentaborane(9).
Thus, prior to the present invention, it was not possible to prepare decaborane(l4) efficiently and in relatively high yields which would be suitable for commercial operations without the need for elaborate equipment and the use of potentially hazardous chemicals.
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Accordingly, one or more of the following objects will be achicved by the practice of the present invention. It is an object of this invention to provide a process for the synthesis of decaborane(14). A
further object is to provide a multi-step process which is efficient and employs readily available starting materials. Another object of the invention is to provide a process for the preparation of decaborane(14) which avoids the use of hazardous and expensive starting materials. A still further object of this invention is to provide a process for the preparation of the BllHl4-ion. Another object i8 to provide a process for the preparation of the BllH14- ion from several different starting materials. A further object of this invention i9 to prepare the BllH14- ion from the B3H8- ion.

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These and other objects will readily become apparent , to those skilled in the art in the light of the teachings ;,ii~ set forth.

~, In its broad aspect, this invention is directed to a process for the synthesis of decaborane(l4). One r: embodiment of the invention is also directed to the ~~l preparation of decaborane(l4) from different starting !
materials, such as lower alkylammonium octahydrotri-borates.
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~' In the first embodiment decaborane(14) is prepared by a process which comprises the steps of:
(a) contacting in an inert atmosphere . . .

~ 0 40 38 6 (i) a metal borohydride of the formula:

wherein M represents a monovalent ion and is a member selected from the group consisting of sodium, potassium, rubidium, cesium, and lithium, with (ii) a boron trifluoride-containing compound to provide the tetradecahydro-undecaborate(-l) ion, the metal borohydride and the borontrifluoride-containing compound being employed in a mole ratlo of from about 5:6 to abou~ 6 respectively.
(b) contacting the tetradecahydroundecaborate(-l) ion with an oxidizing a~ent having an electrode potential (E) of at least about +0.6 volts, and (c) thereafter recovering the decaborane(l4).
The process of this invention provides a convenient and safe method for the synthesis of decaborane(l4) from readily available starting materials. As indicated above, in the first step of the process a metal borohydride is contacted with a compound containing boron trifluoride ., itself. m e process is conducted in an inert atmosphere and within the temperature range and conditions hereinafter indicated.
; In this first step the borohydride a~d boron tri-fluoride-containing compound react to provide the tetra-decahydroundecaborate(-l) ion in accordance with the .. , ........... ..

104!~)386 following general equation wherein sodium borohydride and boron trifluoride diethyletherate are typical reactants:

NaBH4 + BF3 0(C2Hs)2 ~~~~ NaBllH14 The synthesis of the BllH14- ion by the reaction of metal borohydrides with decaborane(14) has been reported in the literature. For example, V.D.
Aftandilian, et al., disclose in Inorganic Chemistry Vol. 1, No. 4, pages 734-737 (1962) the reaction of metal borohydrides with ethereal decaborane at 90C.
; 10 to give M BllH14 . Similarly, H.C. Miller et al.
have reported in Inorganic Chemistry, Vol. 3, No. 10, ;; pages 1456-1462 (1964) that the preparation of polyhedral borane 9 tructures such as BllH14- has been achieved in two basic reactions.
., In the first step of this process it has been found that the mole ratio of metal barohydride to boron trifluoride-containing compound is critical if optimum results are to be obtained. It has been found that the ratio of borohydride to the boron trifluoride compound employed be preferably 5 to 6.
Ratios of 5 to ~6 can also be employed but are less preferred.

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104t~386 As indicated above, a variety of metal boro-hydride compounds can be employed. Similarly, the boron trifluoride-containing compound can be boron trifluoride itself or a boron trifluoride di~lkyletherate ~uch as those wherein the alkyl group can contain 1-4 carbon atoms. The alkyl groups need not be of the same chain length in the same molecule.
It is also desirable to conduct the reaction in a solvent. For example, ethe~s boiling at or above 100C. at the reaction pressure are suitable:
dimethyl ether of diethylene glycol; diethyl "Cellosolve", diethyl "Carbitol", and dimethoxy-1,3-butane are illustrative. Obviously, ethers boiling below 100C.
at atmospheric pressure could be used if the reaction were conducted at superatmosphereic pressure but no obvious advantage is obtained by so doing. The amount of solvent is not narrowly critical but must be present in sufficient quantity to permit adequate mixing of the ' reactants.
;1 2~ The reaction is conducted at a temperature of ll from about 100 to about 120C~
:~l As hereinbefore indicated, the second step of ~ the process of this invention is the oxidation of the . . . ..
BllH14- ion to decaborane(l4). Oxidation can be ; effected by a variety of methods.

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104~)386 Although the electrochemical oxidation of BllH14- ion to decaborane has not been reported in the literature, it has been found that chemical oxidation in solution does produce the desired decaborane(14). The highest yields were obtained when the oxidizing agent was used in excess of that required by the proposed equation.
The excess was presumably required due to competition of BllH14 BllHl4 + 3H2O > BloH14 + B(OH)3 ion, BloH14 or other species for oxidizing agent, In the course of the reaction BloH14 was extracted into a benzene layer which could be used directly in further reactions requiring BloH14 as a starting material.
Clearly one of the most interesting of these i8 in the preparation of dicarba-closo-dodecaborane(12).
In practice, it has been observed that a wide variety of compounds can be employed to effect the chemical oxidation of the BllH14- ion. It has also been found that for optimum results the oxidizing agents should have an electrode potential (E) of at least + 0.6 volts and preferably at least + 1.0 volts.

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1~)4~386 Illustrative oxidizing agents which can be employed include, among others, potassium permanganate, potassium dichromate, hydrogen peroxide, peracetic acid, perbenzoic acid, and the like. Other oxidizing agent~
which could be used include:
Ag+2 /H+
AU+3 /H+
Ce+4 /H
CeOH+3 /H+
HC 10 /H+
C103 /H~

Co+3 Cr /H
Fe(phenanthroline)3 /H+

MnO 2 /H
NiO2 /H+
Np+4 02/H+

' " PbO2/H+
.. ' PbO2/S04 /H+
: Pu+4 ~2 Pu+5 Ru04 :` Ti+3 U+5 V (OH)+

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104~386 In practice it has been found that the oxidation step employed in this invention must be carried out under an inert atmosphere. A wide variety of inert atmospheres can be employed, the only requirement being that they do not adversely affect either the reactants or the decaborane(l4) produced. Illustrative inert gases which can be employed include, among others, nitrogen, the noble gases, such as argon, and the like, methane, ethane, and the like.
It has àlso been observed that the reaction temperature during the oxidation step is preferably from about -10C. to about 50C., and more preferably from about 5C. to about 50C.
The reaction time is not necessarily critical.
The product is, however, subject to slow hydrolysis in the aqueous reaction mixture and thus it is preferred to complete the oxidation and separations within a reasonable time. In some instances, the use of drying agent may be desirable to remove water. Most any dessicant can be employed as long as it is non-reactive. Illustra-tive dessicants include, solid dessicants, such as magnesium sulfate, copper sulfate, anhydrous silica gel, molecular sieves and the like. In addition gaseous dessicants, such as phosgene, can also be employed.
Although azeotropic distillation is satisfactory due to hydrolysis it is less preferred.

_ g _ 104~386 A variety of inert solvents can be employed for`extraction of the decaborane(l4). Preferably, the solvent is an aromatic hydrocarbon stable in the presence of the particular oxidizing agent used, melting below 5C. and boiling below 160C. Illustrative solvents are benzene, toluene, and the like.
Less preferred solvents which can also be employed are aliphatic hydrocarbons, (C4 or higher) which boil below 160C., aliphatic ethers, and the like.
In a further embodiment, this invention is directed to a process for ~he preparation of tetradeca-hydroundecaborate(-l) ion from starting materials other than the metal borohydrides.
In the course of studying the reaction of NaBH4 with BF3 O(C2H5)2 to produce B3H8- ion it was found that on continued addition of BF3 O(C2H5)2 beyond the stoichiometric amount necessary for completion of the ; reaction that the concentration of B3H8- ion in the ; 20 reaction mixture decreased. As the concentration of B3H8-ion decreased, a simultaneous production of BllH14- ion occurred. At approximately 160% of the BF3 O(C2H5)2 required to complete the reaction, the concentration of B3H8- ion was essentially zero and with the exception of ` BllH14 ion only traces of other boron-containing species ; were present. Inasmuch as the B3H8- ion is believed to be formed as an intermediate in the reaction of the metal borohydrides with boron trifluoride . - 10 - .
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. . , 1~)4~)386 -containing compounds, it was found that the octahydro-triborate could be substituted as a reactant for the metal borohydride in the preparation of decaborane(14).
In this embodiment the process comprises the step of:
(a) contacting in an inert atmosphere (i) an octahydrotriborate of the formula:

wherein M represents a monovalent ion and is selected from the group consisting :'1 of sodium, potassium,rubidium, cesium and lithium, and lower alkylammonium ions, with ~ (ii) a boron trifluoride-containing ¦ compound selected from the group I consisting of boron trifluoride and l boron trifluoride dialkyletherate to ii provide the tetradecahydroundecaborate(-l) ,. I
;l~ ion,the octahydrotriborate and the ~, boron trifluoride-containing compound being employed in a mole ratio of from about 1:1 toabout 1:~ 1 respectively.
,. l (b) contacting the tetradecahydroundecaborate-(-1) ion with an oxidizing agent having an . -. electrode potential (E) of at least about . .
+ 0.6 volts, and (c) thereafter recovering the decaborane(l4).
. For the first step of the process of this ~ embodiment, the reaction is conducted in a solvent : I and at a temperature ofl lfrom about 100C. to about 1~0C.

104~386 Both in this embodiment and the previous embo~diment wherein a metal borohydride is employed, the addition of the boron trifluoride compound is critical at least to the extent that if it is added too rapidly B2H6 is evolved from the reaction mixture and the yield of BllH14 is reduced. Thus the reactant8 -are employed in the indicated mole ratios but by the gradual addition of the boron trifluoride compound to either the borohydride or the octahydrotriborate.
The reaction is preferably conducted in a solvent as indicated in the first embodiment of this invention.
As indicated, a variety of octahydrotriborates can be employed as the starting material. In addition to the monovalent metals set forth above, the lower alkylammonium octahydrotriborates can be employed.
Preferred compositions are those within the nitrogen atom contains four alkyl groups of from 1 to 4 carbon atoms and need not necessarily be the same throughout the molecule.
The second step, or oxidation of the B11~14 ion to decaborane(14) employs the same conditions as previously indicated in the first embodiment.
Decaborane(14) prepared by the process of this ;. invention is a useful precursor to various boranes including carboranes. The carboranes themselves are useful for the synthesis of carborane-siloxane polymers and elastomeric gum stocks which have attractive appli-cations as sealants and the like. The carborane derivatives -la-.~ .

104~386 are also useful as rocket propellant additives.
The following examples are illustrative:
EXAMPLE I
Synthesis of Decaborane(14) usin~
Potassium Perman~anate A 2000 milliliter, three neck, round bottom flask was employed for this experiment. The flask was fitted with a dry-ice trap and an ether trap attached to one of the necks. A mechanical stirrer was also connected thru the center neck and the remaining neck was connected to a metering pump which led to a reservoir of boron trifluoride diethyletherate. Means also were provided for temperature measurements in the flask as well as the introduction of nitrogen. Commercial ;
grade sodium borohydride andboron trifluoride diethylether-ate (98 per cent) were employed. Diglyme was heated over sodium and benzophenone until a dark blue color was obtained and then distilled at 67C. at a pressure of 13 mm/ Hg. The flask was charged with 500 ml of diglyme and 60 g (1.59 mol) of NaBH4. The trap was cooled with Dry ice/2-propanol, the mixture was heated to 105C. and boron trifluoride diethyletherate (250ml, 2.04 mol) was added at the rate of approximately 40 ml/hr.
When addition was complete the viscous, yellow mixture was allowed to cool to room temperature. The contents of the flask were filtered in air using a medium frit.
; The solids were washed with two, 50 ml portions of dry diglyme. The combined diglyme solutions were stripped to . ~
, 104~)386 a yellow semi-solid using a rotary evaporator and mechanical pump at approximately 57C. The semi-solid was taken up in ~00 ml of water and added to a cooled solution of 100 ml of water, 100 ml of conc. H2SO4 and 946 ml of benzene ccntained in a 5~000 ml, 3 neck flask which had been equipped with a N2 inlet, mechanical stirrer and dropping funnel. The ~ask and contents were cooled to 10C. in an ice bath and a solution of 53.3g (O.34 mol) of KMnO4 in 1600 ml of water and 160 ml of H2SO4 was added over approximately 40 min. The contents of the flask were poured into a separatory funnel and the benzene layer separated and washed with 700 ml of water.
; The benzene solution was dried over MgS04, filtered and ,. j stripped to a yellow oil. The oil was placed into a sublimer, evacuated and heated to 50C. with the cold--finger cooled with ice. After the residual benzene and diglyme had passed, decaborane(l4), collected on the cold finger. Yiel~j9.13g (0.075 mol, mp. 97.5-98C.). The infrared, llB nmr and mass spectra were identical to authentic samples.
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. ~.;.. , ~, r lU4~386 Synthesis of Decaborane(14) Using Sodium Dichromate Without Removal of Sodium Tetrafluoroborate .~ .
: In a manner similar to that employed in Example 1. The flask was charged with 500 ml of diglyme and 60.0g (1.59 mol) of NaBH4. The trap was cooled with Dry-ice/2-propanol, while the flask and ; contents were heated with stirring to 105C. and ,,; BF3~0(C2H5)2 (250 ml, 2.04 mol) was added over a : 6 hour period. When the addition was complete the ~":
viscous yellow mixture was allowed to cool to room temperature. The trap contained 124g of ethyl ether.
The additional apparatus and trap apparatus were removed and a 10 inch Vigreaux column topped with an alembic ~`; still head was added. The pressure was reduced to approximately 50 mm/Hg and stirred for 0.5 hours. The .:
~' flask was heated using a steam bath and the solvent was distilled. After approximately 400 ml of solvent had been removed the pressure was reduced to approximately "
0.1 mm/Hg and an additional 40 ml of solvent was removed.
;~1 20 The flask was cooled in an ice bath and a cooled solution ,,, ' of 200 ml of water and 200 ml of conc. H2S04 was slowly added. ~enzene, 500 ml, was added followed by the ~;,l slow addition of 118.5g(0.4mol) Na2Cr2O7 2H20 in 60 ml of water (approximately 105 ml total) in 1.0 hours. The tempera-ture of the contents of the flask rose to 30C. When ;~
the addition was complete the organic layer was separated .i,, .: .
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i 104~386 and the aqueous layer was washed with 400 ml of benzene.
The combined benzene layers were dried over MgS04 and . filtered. The dried benzene solution (930 ml total) was analyzed and found to be 0.07 molar (7.99g, 0.65 mol,) ln ~lOH14' . '.
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,:`, '' ~1~)4~3386 nthesis of (C2H5)4NBllH14 from (C2Hs)4NB3H~

The apparatus employed in this experiment was a 1000 ml. 3 neck flask similar to that employed in the previous examples with the exception that the acetone trap wascmitted. The off-gases were passed through a trap which contained 200ml of triethylamine and thence through a wet-test meter. The flask was charged with 18.0g (0.105 mol) of (C2H5~4N B3H8 and 500 ml of diglyme and heated to 105C. Over a period of 2.37 hours 15.0 ml (0.120 mol) of BF3 O(C2Hs)2 was added and 0.248 mols of gas were evolved. The triethylamine was stripped on a rotary evaporator to an oil which was identified by the infrared spectrum as (C2Hs)3N BH3, (2.48g, 0.022 mol).
The contents of the flask were cooled to room temperature and filtered with a medium frit. The solids (16.6g, 0.077 mol) of ~2H5)4N BF4 were washed with two, 50ml portions of diglyme. The combined diglyme fractions were stripped at reduced pressure using a rotary evaporator ` to a yellow oil. The oil was triturated in ethyl ether and filtered. The solids were dissolved in acetone and water was added to the hot solution until cloudiness appeared. The hot solution was filtered and water was added. The solution was allowed to cool and maintained ' at 10C. overnight. me crystals were filtered and dried in vacuo. The yield was 3.55 g (0.013 mol) of (C2H5)4N
BllH14. A second crop (0.75 g, 0.003 mol) was obtained from the liquor.

104C~386 SyntheSis of (C2H5)4NBllH14 from B~HR~ion Prepared in situ ; Into a 2000 ml, 3 neck flask which was equipped ` with a N2 inlet and a mechanical stirrer was placed 500 ml of diglyme and 60.0 g. (1.59 mol) of NaBH4.

The off-gases were passed through an acetone bubbler to destroy any evolved B2H6. A thermometer was placed ;; in the acetone. A temperature rise of greater than 5C.

; ~o indicated that the rate of addition of BF3 0(C2H5)2 was .: , too rapid. The contents of the flask were heated to 105 ~ 2C.. The trap was filled with Dry-ice and

2-propanol, and BF3-0(C2H5)2 was added at 40 ml/hr (250 ml, 2.04 mol). After the addition was complete (6 hr) the flask and contents were allowed to cool to room temperature. Theether trap contained 121.1 g (1.64 mol) of ethyl ether. The contents of the flask were filtered in air using a medium frit. The solids (141.6 g, 1.29 mol, NaBF4) were washed twice with 50 ml ;ij ,, ; 20 of dry diglyme. The combined diglyme solution was ., ' evaporated to an oil using a rotary evaporator (1 mm.Hg, 67C.). The oil was taken up in 500 ml of water and ... ....
added in one portion to a solution of 100 g (0.475 mol) of(C2H5)4NBr in 200 ml of water. The mixture was allowed to stand 15 min. and filtered. The filter cake was dissolved in 300 ml of acetone (sllght degassing) , . . . .
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~, , 16~4V386 and heated to reflux. Water was added until slight clo~diness appeared and the solution was allowed to cool slowly to room temperature then cooled to 10C.
overnight. The pale yellow crystals were filtered and dried in vacuo to yield 25.7 g (0.098 mol,~
o (C2H5)4N BllH14. The liquid was evaporated to about one-half the original volume and water added to cloudiness. The solution was cooled as above ; and the yellow crystals filtered. The crystals were washed with ethyl ether to remove the yellow color to yield 5.1 g (0.019 mol) of tan crystals. The pale yellow and tan materials were indi~tinguishable by llB
nmr and lr spectro8copy. The llB nmr 8pectra were .

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16~14~386 Although the invention has been illustrated by the preceeding examples it is not to be construed as belng limited to the materials employed therein but rather, the invention relates to the generic area as hereinbefore disclosed. Various modifications and embodiments thereof, can be made without departing from the spirit and scope thereof.

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Claims (3)

WHAT IS CLAIMED IS:

1. Process for preparing B10H14 which comprises contacting and oxidizing tetradecahydroundecaborate (-1) ions at a temperature between -10°C and 50°C with an oxidizing agent having an electrode potential (E°) of at least +0.6 volts.

2. Process according to claim 1 wherein the tetradecahydroundecaborate (-1) ions are imparted to the reaction mixture in the form of at least one compound having the formula MB11H14 wherein M represents a cation selected from the group consisting of alkali metals and alkyl-substituted ammonium groups and the oxidation reaction is carried out in an aqueous medium.

3. Process according to claim l wherein the oxidizing agent has an electrode potential (E°) of at least +1Ø