Chemical safety: LiAlH4 reduction of SiCl4
Jonathan Veinot, Enrico Fok,
Chemical & Engineering News (19 Dec 2005) Vol. 83, No. 51, pp. 4-7.
We would like to report a potential hazard for the synthetic procedure outlined in Physical Review B (1999, 60, 2704). This contribution is widely referenced and describes a procedure for preparing hydride surface-terminated silicon nanoparticles via lithium aluminum hydride reduction of SiCl4 in micellular tetrahydrofuran (THF) solutions. The authors incorrectly report that this reaction produces hydrogen gas that is presumably released into the "controlled-atmosphere glove box." There is no mention of pyrophoric by-products.
We recently attempted to reproduce this reported procedure and performed all our steps on a double-vacuum manifold equipped with argon working gas and an oil bubbler. We used standard experimental techniques for handling air-sensitive compounds. With our experimental design, gas by-products are carried out of the reaction vessel by flowing working gas and are released from the oil bubbler into the ambient atmosphere of the fume hood.
From the article cited, there is no reason to expect any adverse events from the procedural difference of interchanging a glove box for a double manifold. The release of hydrogen gas from the oil bubbler into the atmosphere should have been inconsequential. Furthermore, synthetic chemists well-versed in manipulating air-sensitive compounds often find a double manifold more effective than using a glove box.
Upon addition of a THF LiAlH4 solution to a THF solution of SiCl4, we observed vigorous bubbling consistent with the literature report. To our surprise, a bright orange flame was released from our oil bubbler. It was impossible for the authors of the aforementioned contribution to make this observation because a gaseous reaction by-product was doubtless captured by the glove box circulation system and was never released into the atmosphere. Fortunately, no one was injured when this "controlled burn" occurred in our laboratory.
Subsequent literature searches have revealed that the gas produced is incorrectly identified as hydrogen in the 1999 reference. In fact, the reaction LiAlH4 + SiCl4 t LiCl + AlCl3 + SiH4 occurs in ethereal solutions and has been known for some time to produce pyrophoric silane (J. Am. Chem. Soc. 1947, 69, 1199). While we did isolate some Si nanoparticles from the reaction mixture and confirmed that they were indeed hydride-terminated, we caution other researchers from carrying out this reaction without exercising extreme care.
This page last revised December 26, 2006