New Organoborane Compounds Are Stable Cage structure of C2BioHio unit resists attack by heat and wide range of chemicals Some detailed chemistry of a new class of organoboranes (called carboranes) is being made public this month [Inorg. Chem., 2, 1089-1128, 1317 (1963)]. The compounds, which may contain an icosahedral CoB 10 H 10 cage-type nucleus, are much more stable to heat, oxidation, and hydrolysis than boranes generally. Research on the new compounds has been done by Dr. Murray S. Cohen, Dr. Marvin M. Fein, Joseph Green, and co-workers at Reaction Motors Division, Thiokol Chemical Corp., Denville, N.J., and by Dr. Theodore L. Heying, Dr. Hansjuergen Schroeder, and their co-workers at Olin Mathieson Chemical Corp., New Haven, Conn. The first observation of an unusual borane species was made about seven years ago at Reaction Motors, Inc., then an Olin subsidiary, when scien tists from both companies were in
vestigating the use of boron hydrides in propellant applications for the U.S. Air Force. The work was continued at Olin and Reaction Motors after the latter became a division of Thiokol in 1958. Results of some of this re search have recently been declassified. But additional work at Rohm & Haas, Hunts ville, Ala., is still classified. Current emphasis at Olin for the Office of Naval Research and at Reaction Motors for the Bureau of Ships is on the use of carboranes in making ther mally stable polymers. The carboranes have the general formula RR'C 2 B 10 H 10 , where R and R' are substituent groups on the two carbon atoms of the C2Bl0H10 nucleus. Carborane is a trivial name given to the compound where R and R' are hydrogens, as well as to the general class of these compounds. The structure of the carborane nucleus is probably an icosahedron of the boron and carbon atoms in which a carbon-to-carbon sigma bond is oriented at right angles to the plane described by the boron atoms B-6, B-4, B-9, and B-10 of the decaborane nucleus. The two carbon atoms are usually C-l and C-2. The remainder of the valence electrons are involved in three-centered bonds with the remaining six boron atoms. The carborane polyhedron is char acterized by:
Carborane Nucleus May Have Icosahedral Structure
Q
Boron
Ο Hydrogen
φ
Carbon
The open polyhedral structure of decaborane (left) is probably transformed to an icosahedron (in which two carbon and 10 boron atoms form the cage) when acety lene is passed through a solution of decaborane and diethylsulfide forming dicarbaclovododecaborane(12); the trivial name is carborane 62
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DEC.
9, 196 3
• A strong electron-withdrawing power which inductively deactivates olefinic substituents to ionic addition. • Inhibition of nucleophilic dis placement reactions of halomethyl derivatives. • Metallation of polyhedral C—H, and enhancement of the acid strength of carboxylic acid substituents. • Remarkable stability to oxidizing and reducing reagents, permitting classical reactions of organo-functional substituents. • Electron deficiency, allowing formation of stable diadducts with strongly basic amines. The first member of the new family of stable organoboranes was prepared at Reaction Motors by treating 6,9bis ( acetonitrile ) decaborane with isopropenylacetylene in benzene at 80° C. under autogeneous pressure. The solid product, isopropenylcarborane, melts at 46.7° to 47.1° C. after several recrystallizations. It is soluble in polar and nonpolar organic solvents, and it is stable to 350° C. Following this initial observation, chemists at Olin suspected an icosa hedral structure analogous to that pre dicted for the then unknown B 12 H 12 ~ 2 . Careful elemental and mass spectral analysis confirmed the empirical for mula of the parent compound to be B 1 0 C 2 H l 2 . Proton nuclear magnetic resonance analysis of suitable deriva tives established that one hydrogen atom is attached to each of the 10 boron and two carbon atoms. This was later supported by chemical be havior of various derivatives. More recently, the general conformation of carborane and its meta isomer, neocarborane (prepared by thermal rear rangement), has been supported by n B NMR analysis of some chloro de rivatives, the Olin chemists say. Stability. Compounds having the carborane nucleus have far greater chemical and thermal stability than do compounds which have a more open boron hydride structure. For example, isopropenylcarborane was refluxed in methanol for 24 hours and recovered unchanged. In fact, it was refluxed in water with no hydrolysis, and can even be steam distilled quan titatively.
The remarkable stability of the carborane nucleus is further shown by the fact that isopropenylcarborane can be recrystallized from hot (150° C.) 100% sulfuric acid. Fur thermore, when the carborane is treated with alkaline permanganate in acetone, the alkenyl group is oxidized but the carborane nucleus remains intact, Dr. Fein says. General Reaction. A wide variety of terminal and internal mono- and diacetylenes react with substituted decaboranes such as 6,9-bis ( acetoni trile )decaborane or 6,9-bis (diethylsulfide) decaborane. Thus, conversion of 1-pentyne to 1-n-propylcarborane gives a crystalline product (m.p. 68° to 69° C ) . Treatment of diethyldipropargyl malonate with (CH 3 CN)2B10H12 gives pure diethylbis(l-carboranylmethyl)malonate in 5 3 % yield. By contrast, treatment of diacetylenes such as dibenzoate and diacetate esters of 2,4-hexadiyne-l,6-diol results in formation of monocarborane as the major product (along with traces of the dicarborane) in spite of an excess of (CH 3 CN) 2 B 1 0 H 1 2 . The saponification of the esters of the carboranylcarboxylic acids is pos sible but difficult. Transesterification of diesters of the carborane 1,2-diols proceeds as easily as it does with a monoester such as 1-acetoxymethylcarborane. Examples of diesters studied are 1,2-bis ( acetoxymethyl ) carborane, l-acetoxymethyl-2- ( acetoxypropynyl ) carborane, and l-benzoxymethyl-2benzoxypropynylcarborane. Transes terification of the latter two com pounds results in the same product, 1hydroxymethyl-2- ( γ-hydroxy-a-propynyl) carborane. The symmetrical ester is hydrolyzed under basic or acidic conditions to the crystalline diol, 1,2bis ( hydroxymethyl ) carborane. The carboranvlation reaction is general for compounds of the type R C A C H except when R contains a functional group (such as —CH 2 OH or —C0 2 H) capable of destroying the borane precursor. However, esterification of these reactive groups followed by treatment with 6,9-bis ( acetoni trile) decaborane, for example, con verts R C A C H to the corresponding carborane derivative. For instance, CH 3 COOCH 2 C A CCH 2 COOCH 3 adds Bi 0 Hi 2 (CH 3 CN) 2 at the triple bond to give the carborane derivative. This carborane diester is then hydrolyzed to the dialcohol. The coexistence of a —CHoOH group and a boron hy dride group in the same molecule is
Carboranes May Have Ortho and Meta Configurations
Meta
Ortho
Para (Unknown)
When carborane, BwHiuCsHs, is heated at 500° C. for four to six hours, it rearranges to an isomer. Called neocarborane, this new isomer is probably l,7-dicarbaclovododecaborane(12). 1,12-Dicarbaclovodode caborane(12), the para isomer, has not been made 12
11
15
14
13
100
B H 1100 C C 22 H H B 1100 H
80
, o C l 1 0 C22H H22 '•.CI,„C ;
60 40
Λ Π
AΛ U 1y
V
r^
ι
/
3,
20
r •—I
\
ι 2*- , 'β^- / 2**2
100 21,5 32.0
neo-B10H10C2H2
16.8 28.0
neo-B1QCl10C2H2
Λ
80 Φ
1 ε I
60
*
20
40
r ι| /
ν
Ι1V Iι Γ A AJ
\\
λ
ΕioC lioC22r~ r" 2
I1
>Λ
" 'M(H2CO2
Β|οΗι6, 0 -W=P-C6H4-P=
B^H,,,
0
0J *
* = 3
( H C H ^ ^ B T 0H10)
from the corresponding acetylenes, are difficult laboratory preparations. 1-Bromomethylcarborane and 1,2bis(acetoxymethyl) carborane are useful starting materials. Treatment of 1-bromomethylcarborane under typical Grignard conditions gives 1-carboranylmethylmagnesium bromide. This Grignard reagent is capable of many of the standard reactions. Condensations with ketones, allyl halides, and alkyl halides go easily. Hydrolysis of the Grignard reagent gives methylcarborane, identical to the product prepared from propyne, decaborane, and a Lewis base. However, hydrolysis of the Grignard is a more convenient laboratory method. The hydrogens on the polyhedral carbon atoms are labile, as shown by some Grignard reactions. In diethyl ether, allyl bromide and 1-bromomethylcarborane give 4-(l-carboranyl)-l-butene. When this same condensation is done in tetrahydrofuran, the product is l-allyl-2-methylcarborane. Thus the Grignard isomerizes to l-methyl-2-carboranylmagnesium bromide. The alkylated isomers are distinguished in the infrared spectrum mainly by the presence of the polyhedral C—H absorption at 3060 cm. - 1 in the 1-butène compound and the absence of this absorption in the methylcarborane isomer. Labile Hydrogens. This unusual rearrangement was an early indication to the chemists that the hydrogen atoms attached to polyhedral carbon are labile enough to permit metallation. The polyhedral C—H functions in carborane or 1-methylcarborane are metallated in ether by alkyl- and aryllithium reagents such as CH 8 Li or C 6 H 5 Li. The carboranyllithium compounds produced this way frequently provide the most satisfactory intermediates to carboranyl acids, alcohols, and halides. For example, 1,2-carboranyldicarboxylic acid, l,2-bis(hydroxy ethyl ) carborane, 1 -methyl-2-carDEC.
9, 196 3 C & E N
65
perhaps this MILESAMINE CH 3
^S-CH2-N-CH2/ / \S
N, N-DIBENZYLMETHYLAMINE \ . jtf one of our many aralkyl amines ^ir
Description — Colorless to light yellow liquid. Assay 9 5 % minimum. Specific gravity 0.99 at 25°C. Refrac tive index 1.5560-1.5590 at 25°C. Distilling range — 9 5 % distills — 152 to 158°C at 11mm Hg. Residue on evaporation 0.2% maximum.
i$ jUSt
what you need
SYNTHESIS. Dr. Hansjuergen Schroeder (top) and Dr. T. L. Heying check solvent distillation during a preparation of a carborane derivative
Other uses — As an intermediate for chemical synthesis. Its insolubility in water and its solubility in organic liquids suggest its use as an oil-soluble rust in hibitor or as an ingredient in cutting oils, hydraulic fluids, specialty lubricants and similar application to block corrosive ac tivity of acids produced by oxidation. S-5-3
MILES
CHEMICAL COMPANY DIVISION OF MILES LABORATORIES. INC. · ELKHART. INDIANA General Sales Offices: Elkhart. Ind.. COngress 4 - 3 1 1 1 ; Clifton. N.J.. 772-4800; New York. N Y . , M U r r a y Hill 2 7970.
66
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9, 196 3
boranylcarboxylic acid, and 1-iodocarborane, were each prepared via a carboranyllithium intermediate. Simple carboranyl compounds (car borane and 1-methylcarborane, for example) are inert to mild nucleophilic reagents such as alcohols, ethers, nitriles, sulfides, and weakly basic amines. However, they add two equivalents of alkali metal in ammonia or amine solution to give reactive, solvated salts. Addition products are also obtained when carborane is treated with η-propylamine, piperi dine, or hydrazine. But weaker bases such as aniline, pyridine, and aqueous ammonia do not react. Steric factors may be the reason, Dr. Cohen says, since diethylamine (a strong base) also does not react. These amine
compounds probably are coordinated adducts and not lower boron-contain ing species, he says. The n-propylamine product is liquid at room tem perature, and both it and the solid piperidine product (m.p. 171° to 176° C.) are benzene soluble. Symmetrical ethers such as bis ( 1 carboranylmethyl)ether and /?,/?'-bis(lcarboranylethyl)ether have been syn thesized from carborane in acetonitrile and the appropriate acetylenic ether. The boranylethers are soluble in aliphatic and aromatic hydrocar bons, ethers, and other nonpolar sol vents, but are insoluble in water. The two labile hydrogen atoms attached to the polyhedral carbon atoms of the ethers (like those of carborane) are readily displaced with butyllithium
STRUCTURE. Joseph Green, Dr. Marvin M. Fein, and Dr. Murray S. Cohen (left to right) discuss the evidence for the icosahedral structure for the C2B10H10 nucleus
or phenyllithium to form 2,2'-dilithium derivatives which are useful for syn thesizing other compounds. For ex ample, carbonation of the lithium salt of bis ( 1 -carboranylmethyl ) ether in diethyl ether gives a salt; acidifica tion of the salt forms bis(2-carboxy-lcarboranylmethyl) ether in 55.5% yield. A similar reaction run in tetrahydrofuran gives the carboxy ether in 9 1 % yield. Dr. Daniel Grafstein and co-workers at Thiokol have shown that when bis( 2-carboxy-1 -carboranylmethyl ) ether is heated (250° to 270° C ) , it con verts to the lactone of 1-hydroxymethyl-2-carboranylcarboxylic acid. The crude lactone sublimes from the mixture in 75% yield. The formation of the lactone illustrates the ease with which 1,2-disubstituted carboranes form exocycles containing three or more atoms. The pure lactone melts at 253° to 254.5° C. It shows prominent in frared absorptions at 3.90 microns (Β—Η), 5.55 microns (lactone C—O), and a characteristic lactone triplet at 9.30 microns, 9.68 microns, and 10.18 microns. The lactone is soluble in benzene, carbon tetra chloride, tetrahydrofuran, dioxane, and diethyl ether. It is moderately soluble in pentane, hexane, and isopropyl ether, and insoluble in water. A suspension of the lactone hydrolyzes slowly with aqueous alkali at 18° C. Acidification and concentra tion of the mixture form crystals of 1 -hydroxy-2-carboranylcarboxylic acid in 9 3 % yield. The acid does not melt, but decomposes at temperatures
above 180° C. IR spectra of the decomposition product shows absorp tions at 2.95 microns ( O H ) , 3.80 microns (Β—Η), and 5.82 microns, 7.18 microns, and 7.98 microns (COOH). But there are no sig nificant absorptions at 5.55 microns (lactone C = 0 ) , or at 8.95 microns (CH2-0-CH2). The acid and its lactone were the first mixed difunctional carboranes made which are capable of homopolymerization. Chloro Derivatives. Carborane in an inert solvent such as carbon tetra chloride reacts rapidly at about 25° C. with chlorine. The reaction proceeds with the stepwise replacement of hy drogen atoms. The use of carbon tetrachloride as solvent was fortuitous, Dr. Schroeder says. Carborane is soluble in carbon tetrachloride, but the chlorocarboranes are less soluble; several of them precipitated, even from the refluxing solution. Thus, it is possible to regulate the degree of substitution by adjusting the amount of solvent, the reaction time, and the temperature. The Olin chemists iso lated chlorination products containing two, three, four, six, eight, and 10 chlorine atoms per carborane unit. IR spectra of these compounds show that the hydrogens replaced had been attached to boron, Dr. Heying says. Carborane shows typical C—H and Β—Η IR stretching frequencies at 3.25 microns and 3.92 microns, re spectively. While the relative in tensity of the 3.25-micron absorption remains unchanged in all compounds, the 3.92-micron absorption gradually decreases as the chlorine content in-
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DEC.
9, 1963
C&EN
67
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68
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DEC.
9, 196 3
creases. The latter absorption is no longer seen in the spectrum of the decachloro compound, which must be B 10 Cl 10 C 2 Ho, according to Dr. Schroeder. When the reaction goes longer (seven hours compared to less than three), one additional hydrogen is replaced, giving B 10 C1 10 C 2 HC1 as the final product in 80 to 90% yield. B 10 Cl 10 C 2 Clo has not been made, Dr. Heying adds. The relatively sharp melting points of decachlorocarborane and undecachlorocarborane suggest that they are each single compounds and not mix tures of isomers, Dr. Schroeder says. However, isomers have been isolated for the dichloro- and the tetrachlorocarboranes. The highest melting (and also least soluble) member of this series is tetrachlorocarborane. It melts at 352° C ; the lowest melting (and most soluble) derivative, dichlorocarborane, melts at 230° C. Besides the melting points, the most reliable method of differentiating the chlorocarboranes is IR. All of these compounds show two strong characteristic bands in the 8- to 9micron region, their exact position being essentially unaffected by the number of chlorine atoms present. Some interesting chemical charac teristics of these chlorocarboranes have been observed, Dr. Schroeder notes. For example, when B 10 Cl ]n CoHo was treated with SbF 3 Clo at 240° C. (to attempt halo gen exchange) and subsequently refluxed with water to remove the fluorinating agent, B 10 C1 10 C 2 H2 was quan titatively recovered. However, in an attempt to induce B 10 H 2 C1 8 C 2 H 2 to participate in a Friedel-Crafts reaction by refluxing it in benzene with A1C13, no reaction occurred. The fact that Friedel-Crafts alleviation does not oc cur easily under any conditions indi cates that the carborane polyhedron is too electron deficient to promote substitution by electrophilic reagents. The Cl—B bonds in chlorocarbor anes are strong, but the chlorine atoms can be removed by heating at 100° C. in 50% aqueous potassium hydroxide in the presence of hydrogen peroxide. Polymers. An interest which con tinues to stimulate research in carboranes is their possible use as semiorganic, high-temperature materials. The carborane nucleus imparts thermal stability to polymers, Mr. Green says. For example, commercial dimethyl silicone polymers have a thermal stability in the order of 225° C. But
Functionally Substituted Carboranes Prepared by Direct Synthesis Using Decaborane in Acetonitrile and the Appropriate Substituted Acetylene
—CH 2 Br
R—C(Bi0H] o)C-R' % Yield R' 70 Η
M.P., ° C. 30
—COOCH3
Η
35
73
—
—CH 2 CI
—CH2CI
74
119-120
—
-CH2OOCCH3
Η
83
42-43
82-84/0.2 mm.
—11C3H7
Br
66
44-45
107/1
—11C4H9
Br
37
—
-(CH2)200CCH3
Η
72
61-63
-CH(OOCCH,)CH
Hb
66
—
85-95/0.2
—ΟΗ2Ν(02Η5)2
Hc
d
33-35
—
-CH200CCH3
—CH2OOCCH3 89
43-44
—
—COOCH 3
—COOCH3
54
66-67
—
B.P., ° C.
85-90/0.5 146/1.6
«
nD 23 = 1.5500.
c
Hydrochloride m.p. > 300° C.
6
nD 2 1 = 1.5291.
d
> 50%, but not all was recovered.
when a carborane nucleus is intro duced into the structure, the ther mal stability increases to about 400° to 450° C. (measured by thermal gravimetric analysis). Besides their thermal stability, the carborane-substituted silicones are fusible and are soluble in organic solvents. These properties make them more useful than the usual inorganic-type poly mers, Mr. Green says. Metallocarboranes are useful in making silylcarboranes, according to Dr. Stelvio Papetti at Olin. Chlorosilylcarboranes can be prepared by treating dilithiocarboranes with R 2 SiCl 2 , where R is - C H 3 or - C e H 5 . Corresponding tetrachloro and hexachloro derivatives can be made using dilithiocarborane and CH 3 SiCl 3 or SiCl4. When bis ( chlorodimethylsilyl ) car borane is treated with dilithiocarbo rane, a product forms which has a sixmembered ring composed of the four carbon atoms of two carborane nuclei and two silicon atoms. When bis(trichlorosilyl) carborane was similarly treated with an equimolar quantity of dilithiocarborane, the analogous reac tion occurred to give the cyclic tetra chloro derivative. Hydrolysis of bis( chlorodimethylsilyl ) carborane yields cyclic tetramethyldisilylcarboranyl
oxane. Olin chemists assigned the cyclic structure on the basis of ele mental analysis, molecular weight, IR, and mass spectral data. When bis ( chlorodimethylsilyl ) car borane is treated with ammonia, a similar reaction occurs, giving the cyclic tetramethyldisilylcarboranyl azane. Unlike organic silicon-nitro gen compounds which readily react with moisture, these cyclic carboranes do not hydrolyze. Furthermore, these compounds can be recovered quanti tatively after heating to 500° C. But above this temperature, some methane is evolved. Thermally stable poly mers have been synthesized from some of the silyl carboranes, Dr. Heying and Dr. Papetti add. Polymers prepared from [HC 2 B 10 H 1 0 (CH 2 ) n Si(CH 3 ) 2 X 2 ] (X = CI or —OC2H5 and η = 3 or 4) have prop erties different from their all-carbon analogs. For example, carborane polymers are stable to 450° C. and are soluble in aromatic solvents. These differences in properties result, in part, from the large, three-dimen sional carborane group (about 8 A. in diameter) which may sterically affect polymerization, Mr. Green says. Thiokol chemists tried to polymer ize vinyl carboranes (isopropenylcarborane, for example) through the
We're now producing it in semi-com mercial quantities for use as a chemi cal intermediate in making synthetic rubbers (bycopolymerization with pro pylene oxide). And in preparing bac tericides, fungicides, leather tanning agents, and a variety of polymeric materials. Other suggested uses are included in our technical bulletin on AGE. Along w i t h c h e m i c a l reactions, physical properties, and an extensive bibliog raphy. Write for a copy.
Olin ORGANICS DIVISION SPECIALTY CHEMICALS, 100 McKEE RD., ROCHESTER, N.Y.
DEC.
9, 196 3 C & E N
69
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C&EN
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DEC. 9, 196 3
double bond. Unlike a-methylstyrene, which polymerizes to high mo lecular weights, the carborane deriva tives do not. Mr. Green attributes the inhibition of polymerization to steric hindrance of the carborane nucleus. However, acrylate derivatives such as H 2 C=CHCOOCH 2 HC 2 B 10 H 10 polym erize easily. These polymers have softening temperatures as high as 160° to 165° C , which is significantly greater than for the usual organic acrylate polymers. Condensation polymerizations of adipic acid with an α,ω-aliphatic diols generally give products which are crystalline or waxes because of the tendency toward close-packing. However, the carborane polyadipates prepared by Thiokol chemists are amorphorus even after long-term storage. Mr. Green believes that the large carborane group prevents close packing; it functions as an internal plasticizer. The carborane content of polymers can be increased by condensing car borane diols with such compounds as [ HOOC ( C H 2 ) n C 2 B 1 0 H 1 0 CH 2 ] 2 0 , provided the carborane units are sep arated sufficiently. No polymeriza tion takes place if η = 0 or 1. Formaldehyde condenses with bis(hydroxymethyl) carborane to form a seven-membered ring which is ther mally and chemically stable. Carbon polymers of this type form but cleave readily. If the distance between the carborane nucleus and the — CH 2 OH group is increased [using (HOCH 2 CH 2 C 2 B 1 0 H 1 0 CH 2 ) 2 O], linear poly mers form which show an increase of 50° to 100° C. in thermal stability over their corresponding noncarborane analogs. The carborane nucleus probably shields the backbone of polymers of this type, Mr. Green says, or else makes adjacent bonds more ionic, which would also increase ther mal stability. Olin chemists have made linear polymers with P—N=P links between carborane nuclei. For example, dilithiocarborane with phosphorus tri chloride forms a compound having two phosphorus atoms and two car borane nuclei in a six-membered ring. When this compound is treated with sodium azide, a cyclic diazide forms which then polymerizes with p-[P(C (( H 5 ) 2 ] 2 C 6 H 4 to form a P - N = P bonded polymer containing three units. These polymers also show unusual thermal and hydrolytic stability, Dr. Schroeder says.
