RESEARCH
Ν MR spectra light up carborane structures Reactions and structures of polyhedral boranes and borane polymers described at Richmond meeting Boron hydrides, once known chiefly for spontaneous flammability and the careful techniques needed to handle them, are rapidly becoming reagents of routine organic synthesis and ar ticles of commerce. Twenty years ago the extreme reactivity of boranes made them seem like the next step in high-energy fuels. Today the high stability of polyhedral boranes is lead ing to efforts to incorporate them into almost completely inert polymers. At Richmond, Va., chemists gath ered for the Southeastern Regional Meeting of the American Chemical Society and heard reports of progress in three areas of borane chemistry. • New polyhedral systems are com ing from work of Dr. Russell N. Grimes, University of Virginia, Char lottesville, who is also extending knowledge of long known systems. • Successful vinyl polymerization of a carborane monomer has been achieved by Dr. Theodore J. Klingen, University of Mississippi, located in University, Miss. The polymeriza tion of such a simple carborane may mark a crossover from exotic to com monplace boron monomers, and may lead to commercialization of carborane polymers. • The resonance of n B nuclei has
been explored by Dr. B. F. Spielvogel of Army Research Office in work at Duke University, Durham, N.C., to shed light on structures of boron com pounds. Electron deficient. It would seem that boron chemistry could never be very extensive. With its ordinary valence limited to three by the three electrons of its valence shell, boron can only form a sextet rather than the favored octet of shared electrons. With this deficiency of electrons, it would seem, boron hydrides must al ways be curios in a vacuum line, safely out of the way of air, moisture, and Lewis bases. The larger, complex polyhedral boranes are quite stable, however, re sisting oxidation, hydrolysis, and heat degradation (C&EN, May 9, 1966, page 88). In these compounds, bo ron atoms are bonded to four to seven other boron and hydrogen atoms. The ability to do this in the face of such a lack of bonding electrons indicates that the electrons must be highly delocalized, making their bonding ex tremely efficient. This efficiency is similar to that which occurs in aro matic hydrocarbons, where electron derealization gives stability in spite of a surplus of electrons.
JCQSAHEORON, i-vmyi-o-earborane has caged structure with boron atoms bonded to five other boron and carbon atoms (black) by covaieht bonds and bridging Jwdrôaens (not shown) 36 C&EN NOV. 10, 1969
Small carboranes. Most progress has been in the area of icosahedral boranes, like the B 1 2 H 1 2 2 - anion and the carboranes HCB 1 0 H 1 0 CH. In work with Ronald Olsen supported by the Office of Naval Research, Dr. Grimes describes chemistry and bond ing of the small carborane 2,4-dicarbacfoso-heptaborane ( 7 ), HCB 5 H 5 CH (see illustration). In the nomencla ture of boranes [Inorg. Chem., 7, 1945 (1968)], closo means a completely closed polyhedron, and the number in parentheses gives the number of hy drogen atoms. Dr. Grimes finds that the C—H hy drogen atoms of HCB 5 H 5 CH are acidic in reaction with n-butyllithium, but not as acidic as the hydrogen atoms of HCB 1 0 H 1 0 CH. He thinks this is due to the number of electron-deficient boron atoms in each molecule. In the dicarbadodecaborane there are 10 boron atoms, while in the dicarbaheptaborane of his study there are five. He thinks that the minus charge gen erated by the loss of two hydrogen ions is more easily absorbed by the greater number of boron atoms in the dodecaborane, making it the more acidic compound. n-Butyllithium gives the dilithio derivative of HCB 5 H 5 CH, according to
PEHimOHkL BïPYfcAMfD. 2,4~Dicarba~ c/oso-heptaborane (7) is a pyramidal structure of two carbon {black} and five boron atoms
Dr. Grimes. He says that attempts to make the monolithio compound fail, from disproportionation to the dilithio derivative and the heptaborane. Derivative chemistry. From the dilithio intermediate he makes the di methyl compound, CH 3 -CB 5 H 5 C-CH3, by treatment with 2 moles of methyl iodide. The methyl compound is produced by reaction with 1 mole of methyl iodide followed by hydrol ysis with aqueous HC1. Treatment of the dilithio derivative with trimethylsilyl chloride followed by aqueous HC1 hydrolysis gives the trimethylsilyl compound, (CH 3 ) 3 SiCB 5 H 5 CH. Carbonation of the dilithio intermediate with carbon dioxide pro duces the dilithium salt of the dicarboxylic acid. Dr. Grimes' efforts to generate the free acid with aqueous HC1 degrade the compound to boric acid. This decomposition stands in contrast to the stability of HCB 5 H 5 CH itself to aqueous acid, says Dr. Grimes, as well as the success of the attempt when applied to making the diacid HO 2 C-CB 1 0 H 1 0 C-CO 2 H from the dicarbadodecaborane. Group III elements other than bo ron have not been incorporated into polyhedral borane cages, note Dr. Grimes and Dr. William J. Rademaker. They heat 2,3-dicarba-rado-hexaborane (8) and trimethylgallium at 215° C. in a sealed tube to give a product they think is 1-methyl-lgalla-2,3-dicarba-cZoso-heptaborane 7 (see illustration). Nido indicates an incompletely closed polyhedral struc ture. Boron resonance. The Charlottes ville team sees three doublets in the n B magnetic resonance spectrum, taken at 32.1 MHz. relative to boron
trifluoride-etherate as internal stand ard. The boron signals have relative areas in a ratio 2 : 1 : 1 , in line with the proposed structure, since three dif ferent boron atom types are attached to protons. In n B resonance spectroscopy, bo ron signals are split by attached pro tons, but adjacent n B nuclei do not appear to split one another. Naturally occurring boron is 19.6% 1 0 B, which has a nuclear magnetic moment of 3, and 80.4% n B , with a moment of 3 / 2 . The number of spin states of a nucleus is 2S + 1, where S is the magnetic moment. Ή with moment has two spin states and splits the boron signals into doublets. The relatively small amount of 10 B leads to negligible contribution to a spec trum, thus simplifying interpretations. Protons attached to i r B atoms in boranes are split into quartets by the four spin states of the boron nuclei. Further evidence for four atoms in the gallium compound comes from examination of the high m/e end of the mass spectrum, says Dr. Grimes. Relative peak intensities there appear as expected, according to the known ratio of 1 1 B/ 1 0 B in the natural distribution of boron isotopes. The University of Virginia team finds PMR signals corresponding to methyl and cage C—H protons, with the proper areas. They think they see three kinds of Β—Η proton signals, split into quartets by adjacent n B nuclei.
Metallocene analogs.
In another
synthesis, Dr. Grimes and James W. Howard combine 2-methyltricarbarado-hexaborane 7 (with dimanganese) decacarbonyl in a sealed tube at 175° to 215° C. to give a compound they
write as ^-2-methyl-2,3,4-tricarbahexaboranylmanganese (I) tricarbonyl (see illustration). The product carbonyl has stretching frequencies in the infrared spectrum close to those of the analogous com pound with the cyclopentadienyl anion—the maganocene. Mass spectral analysis shows three boron atoms in the molecule, according to the effect of the known 1 1 B/ 1 0 B ratio of na tural boron on the high m / e end of the spectrum. n B resonance spectra show three different kinds of borons of equal intensities, split into dou blets by their protons. The formation of this boranyl com pound is reminiscent of the manganocene formed with M n 2 ( C O ) 1 0 and the isoelectronic cyclopentadienyl anion, and further points up the electron delocalization of boranes and possible resonance properties, Dr. Grimes and Mr. Howard believe. Successful polymerization. Reac tion of decaborane(14) with dimethyl sulfide, followed by treatment with vinylacetylene, gives 1-vinyl-l, 2-dicarba-cZoso-dodecaborane, the mono mer used by Dr. Klingen and J. R. Wright at Mississippi, in work sup ported by the U.S. Atomic Energy Commission. The Mississippi chemists say that the monomer has a mesomor phic "waxy" state between 11° and 79° C., and this is the state in which they initiate polymerization by gamma radiolysis with a 60 Co source. Polymer yields of 40 to 50% are obtained by this method. Dimer, trimer, and tetramer fractions are sep arated by thin-layer chromatography from the mixture, and Dr. Klingen and Mr. Wright examine these fractions for structural determination. They claim
QH$
l-MethyH-galIa«2,3-dicarba-g| c/oso-heptaborane (7) has a group Ml ele~P* ment other than boron on the cage \ GALLIUM,
MANGANESE COMPOUND. (*~2-Methy 1-2,3, 4-triearbahexab0ranyi)mangane$e (!) tri carbonyl is a boranolog of the familiar metallocenes NOV. 10, 1969 C&EN
37
that the polymerization is propagated through the vinyl group and through the carborane cage, possibly in a headto-tail mode. Using relative intensities of PMR signals, the Mississippi chemists eliminate head-to-head, tail-to-tail, and cyclohexane-type structures involving the vinyl group alone. The decision between head-to-tail polymerization through the vinyl group only and polymerization from the vinyl group to the carborane cage is made from the 11 B resonance spectrum. Doublets assigned to boron atoms of positions 9 and 12 in the monomer carborane cage collapse to singlets, says Dr. Klingen, indicating that a proton has been lost from either of those positions. Ultraviolet data. Although the 12 B resonance spectrum leads Dr. Klingen to believe that the carborane cage functions in the polymerization, the ultraviolet spectrum gives evidence that the cage is not disrupted. A maximum at 252 m/x. is characteristic of the 1,2-dicarbadodecaborane cage, he says, and he notes that this maximum survives with a shift due to alkyl substitution in the polymer. One difficulty in using n B resonance to assign structures is the scant data on n B chemical shifts. This lack can
be overcome, say Dr. Spielvogel and Dr. J. M. Purser, North Carolina Wesleyan College, Rocky Mount, by their equation (j,k) B _ 1 = 0.41 (j,k) c _ 1 3 - 2.98 which relates the predicted chemical shift ( j , k ) B _ n of n B to the known chemical shift (j,k) c _ 1 3 of 1 3 C. Reaction product. At the Richmond meeting, Dr. Purser describes work with Dr. Spielvogel and Dr. Raymond Bratton at Duke University, supported by Army Research Office, National Aeronautics and Space Administration, and Petroleum Research Fund. The Durham chemists examine the product of the reaction between lithium trihydrocyanoborate, LiBH 3 ( C N ) , and hydrogen chloride in anhydrous tetrahydrofuran ( T H F ) . Faced with a choice among lithium dihydrochlorocyanoborate, LiBEUCl ( C N ) , and the T H F complex with cyanoborane, BH 2 CN, Dr. Spielvogel and Dr. Purser calculate the 11 B chemical shift of BH 2 Cl(CN) as 32 p.p.m. from the BF 3 etherate standard. The actual chemical shift of the reaction mixture is 23.8 p.p.m., they find, which they feel is too far off the mark. The triplet n B signal indicates two protons attached to a boron atom, however, and so the Durham team postulates the BH 2 CN-THF complex as the product.
RESEARCH IN BRIEF
Free high-resolution mass spectrometry service will be provided by the National Institutes of Health to scientists engaged in timely biomedical research, says Dr. Michael A. Oxman of NIH. Preference will be given to investigators supported by NIH, who should contact the high-resolution mass spectrometry centers at either Battelle Memorial Institute, Columbus, Ohio, or Arthur D. Little, Inc., Cambridge, Mass. Those not supported by NIH should apply to Dr. Oxman at NIH, Bethesda, Md., for approval. An experimental vaccine is at least 9 0 % effective in preventing one form of bacterial meningitis, reports Dr. Malcolm Artenstein of Walter Reed Army Institute of Research, Washington, D.C. The vaccine—a highmolecular-weight (above 200,000) polysaccharide—has been tested in more than 20,000 army recruits during the past two years. Dr. Artenstein doesn't foresee immediate licensing of the vaccine, however, because there is no experimental animal that forms antibodies in response to the substance. He hopes to correlate physicochemical data with antibody reactions in humans.
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