Preparation of closo-decaborate by pyrolysis of tetraethylammonium borohydride: revision and optimization for large-scale production

Several methods for preparation of closo-decaborate (B10H102- as polyalkylammonium salts) – a key compound for synthesis of carborane – by pyrolysis of tetraethylammonium borohydride were examined. The studied reactions included variation of decomposition temperature, liquid medium, pressure and reaction time. The products were analyzed by 11B and 1H NMR spectroscopy, and, in some cases, by IR-spectroscopy. It was found that pyrolysis of Et4NBH4 in solid state as well as in suspension in silicone fluid led to the mixtures of (Et4N)2B10H10 and (Et4N)2B12H12. Pyrolysis of Et4NBH4 in n-decane or decalin resulted in Et4NB3H8 as a primary product; increase of reaction time in n-decane or both time and temperature in decalin yielded the mixture of (Et4N)2B10H10, (Et4N)2B12H12 and other compounds. It was shown that the degree of conversion of Et4NBH4 into Et4NB3H8 on time in n-decane at 174 °C and decalin at 180 °C linearly depended on time. Several methods for purification of (Et4N)2B10H10 or (Bu3NH)2B10H10 from other pyrolysis products were checked. A synthetic procedure for preparation of closo-decaborate from 60 g of Et4NBH4 was proposed, yielding 2.85 g of (Bu3NH)2B10H10 in one run.