Structure and thermal decomposition of methylamine borane Mark E. Bowden * , Ian W.M. Brown, Graeme J. Gainsford, Herbert Wong Industrial Research Ltd., P.O. Box 31-310, Lower Hutt, New Zealand Received 7 August 2007; received in revised form 22 October 2007; accepted 22 October 2007 Available online 1 November 2007 Abstract The crystal structure of methylamine borane has been determined and contains parallel chains of dihydrogen-bonded CH 3 NH 2 BH 3 molecules. Thermal decomposition takes place from the melt (DH fusion = 8.5 kJ mol 1 ) and begins with the formation of an ionic boro- hydride. Hydrogen is liberated in two stages, at ca. 100 and 190 °C, with the observed rates during the first stage (DH = 25 kJ mol 1 , E a = 115 kJ mol 1 ) strongly dependent on temperature and time. cis- and trans-N-trimethylcyclotriborazane are formed during the first stage and subsequently cross-link to yield a non-volatile solid. Before this cross-linking, the system exhibited a high degree of volatility, with weight losses in excess of 80% observed in TG experiments using flowing gas. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Methylamine borane; Hydrogen storage; X-ray crystal structure; Thermal analysis 1. Introduction The need for energy technologies with a low environ- mental impact has prompted a search for materials that can store and release significant quantities of hydrogen. Ammonia borane (NH 3 BH 3 ) has attracted considerable interest for this application because of its high hydrogen content [1]. Two of the three moles of H 2 this compound contains can be released under relatively mild conditions by hydrolysis [2,3], or by solid-state [4] or solution [5,6] thermolysis. The maximum gravimetric hydrogen yield is naturally obtained from the neat solid-state material. The product after thermolysis however, is an amorphous (BHNH) n polymer which is not amenable to regeneration back to the parent compound. Substantially higher temper- atures are required to release the final mole of hydrogen, leaving the refractory compound boron nitride [4]. The simplest methyl derivative of ammonia borane (methylamine borane, CH 3 NH 2 BH 3 , 1) also evolves two moles of hydrogen, while maintaining a relatively high storage capacity (9 wt% for compared to 13.1 wt% for NH 3 BH 3 ). Surprisingly little information has been pub- lished detailing the properties and thermolysis of 1. Several workers [7–9] have noted that hydrogen is evolved upon heating and identified the decomposition products as the cis- and trans-N-trimethylcyclotriborazane (CH 3 NHBH 2 ) 3 (2) and N-trimethylborazine (CH 3 NBH) 3 (3). These com- pounds are likely to be soluble in a range of solvents (2 is soluble in chloroform, acetonitrile, monoglyme, methanol, and benzene [10]), affording greater opportunity to chemi- cally regenerate 1. Beachley [11] has provided the most detailed study of its thermal decomposition, using indirect methods to propose a reaction sequence. In this paper, we report on the crystal structure of 1, and provide further information on its decomposition using a combination of thermal analysis, mass spectrometry and nuclear magnetic resonance. 2. Experimental Compound 1 was prepared by reacting methylamine hydrochloride and sodium borohydride in a 1:1 molar ratio in anhydrous THF at room temperature. Complete reac- tion, monitored by the volume of evolved hydrogen, was achieved overnight. Samples appeared phase-pure by 0020-1693/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2007.10.034 * Corresponding author. E-mail address: m.bowden@irl.cri.nz (M.E. Bowden). www.elsevier.com/locate/ica Available online at www.sciencedirect.com Inorganica Chimica Acta 361 (2008) 2147–2153