This invention relates to a method for the cleavage of carboranyl carbon-silicon bonds in compounds and polymers containing same, and in addition, to a method for preparing meta-carborane using this technique.
ORTHO-Carborane, of the empirical formula C.sub.2 B.sub.10 H.sub.12, has an icosahedral structure and was the first icosahedral carborane discovered and prepared in quantity. Subsequently, methods were developed for isomerizing the ortho derivative to the meta-and para-isomers. For example, ortho-carborane may be rearranged or isomerized to meta-carborane in a hot tube flow process at high temperatures (i.e., 600.degree. C) involving short residence times. See S. Papetti, et al, Industrial Engineering Chemical Product Research and Development, 5,334 (1966). At these temperatures, carborane vapors may burn or explode if allowed to contact air and hence such a process is not attractive for industrial operation.
Another process is known for isomerizing carboranes in bulk and is disclosed in U.S. Pat. No. 3,440,265. The process involves the thermal rearrangement of large silyl group-substituted ortho-carboranes to the metal-isomer at significantly lower temperatures than the prior technique. In contrast to the earlier flow process, this bulk isomerization occurs readily at the reflux temperature and can be performed using conventional laboratory equipment. Furthermore, the method is adaptable to large-scale industrial production.
The metal-isomer has become increasingly important as the key ingredient of high performance metal-carborane-siloxane polymers. Such polymers have excellent high temperature characteristics and can be prepared by methods known in the art. For example, linear, high molecular weight carborane-siloxane polymers and methods for their preparation are disclosed in commonly-assigned, copending U.S. Pat. Application Ser. No. 770,509, filed Feb. 22, 1977. These polymers may be compounded with suitable fillers and additives and fabricated into cured elastomeric articles. The excellent high temperature properties of such elastomers make them especially useful for a wide variety of applications, such as gaskets, seals, wire and cable insulation, and the like. Although carborane-siloxane polymers have excellent high temperature and other high performance characteristics, they are extremely expensive to produce. Hence, it would be an advantage to be able to reclaim the carborane portion from scrap, off-grade or otherwise used-up polymer, and also to recover or regenerate carborane monomer by-products used in forming the carborane-siloxane polymers.
It is known in the art that the carborane icosahedral structure is subject to attack by strong bases. See, for example, Papetti, et al, Inorganic Chemistry, 3 1444 (1964) which discloses without detailed explanation that some cyclic silylated carboranes are cleaved. In addition, Schwartz, et al, Inorganic Chemistry, 4, 661-4 (1955) indicate that a very labile diethoxysilylated carborane is cleaved under acidic or basic conditions not specified.
The present invention provides a technique for highly selectively cleaving carboranyl carbon-silicon bonds in monomeric silylated carboranes and carborane polymers. The method taught herein is highly selective in that the carborane cage contained in these materials may be recovered intact which is highly advantageous due to the expensive nature of the materials involved. Further, this novel technique may be employed in a process for the formation of metal-carborane in bulk when used in combination with known techniques. This represents a significant advance since this method for preparing meta-carborane in high yield in bulk may be practiced on a large-scale, industrial level.