DOI: 10.1002/chem.201201342 Large-Scale and Facile Preparation of Pure Ammonia Borane through Displacement Reactions Xuenian Chen, [a, b] Xiaoguang Bao, [b] Beau Billet, [a] Sheldon G. Shore,* [b] and Ji-Cheng Zhao* [a] Introduction Renewable energy sources are an important response to rapidly increasing demand for energy and the need to mini- mize the environmental impacts. [1] Hydrogen has been pro- posed as a potential alternative energy carrier to power the world. [2] However, efficient and safe hydrogen storage re- mains a major challenge to meet the gravimetric and volu- metric targets, especially for onboard vehicular applica- tions. [3] Ammonia borane (NH 3 BH 3 , AB) with a high-hydrogen content (19.6 wt. %) has attracted attention as a solid hydro- gen storage material, [4] especially after the recent report of regeneration of AB from its dehydrogenated products. [5] In addition, AB has been used in various fields of chemistry, such as hydridic reduction of carbonyl groups, [6] preparation of metal nanoparticles, [7] and as a precursor in the synthesis of amine borane polymers [8] and boron nitrides. [9] AB was first prepared more than 50 years ago by metathesis reac- tions involving ammonium salts and borohydrides. [10] Al- though improved versions of the metathesis reaction have recently been developed, [11] a convenient synthesis of AB at a scale large enough to meet the potential demand is re- quired. Attempts to prepare pure AB by displacement reac- tions have not been successful due to the formation of the diammoniate of diborane ([H 2 BACHTUNGTRENNUNG(NH 3 ) 2 ]ACHTUNGTRENNUNG[BH 4 ], DADB), an ionic byproduct. [12] DADB, once formed, is difficult to be separated from AB. NH 3 was bubbled through a Et 2 O solu- tion of dimethylsulfide borane ((CH 3 ) 2 S·BH 3 , DMS·BH 3 ) at 20 8C to produce AB, but the purity is unknown before sublimation. [13] It remains unclear whether pure AB could be produced through simple displacement reactions without the need of a sublimation step. Following the Lewis acid-base theory, borane is transfer- red from a weaker base to a stronger base as tetrahydrofur- an (THF) is displaced by trimethylamine to form more stable trimethylamine borane, as illustrated in Equa- tion (1). [14] Many amine borane species have been synthe- sized according to this principle. [14] However, when ammonia (NH 3 ) acts as the stronger base reacting with tetrahydrofur- an borane (THF·BH 3 ), a mixture of AB [Eq. (2)] and DADB [Eq. (3)] is obtained. [12a] The formation of such a mixture observed in other displacement reactions, such as NH 3 reaction with diborane (B 2 H 6 ) [15] or DMS·BH 3 , [12b] has perplexed the boron chemistry community for decades. [16] THF  BH 3 þðCH 3 Þ 3 N !ðCH 3 Þ 3 N  BH 3 þ THF ð1Þ THF  BH 3 þ NH 3 ! NH 3 BH 3 þ THF ð2Þ 2 THF  BH 3 þ 2 NH 3 !½H 2 BðNH 3 Þ 2 ½BH 4 þ 2 THF ð3Þ We recently deduced a mechanism (Scheme 1) to account for the formation of AB and DADB mixtures obtained in Abstract: Ammonia borane (AB) is the most widely studied hydride for hy- drogen storage in addition to being a useful reducing agent. Attempts to syn- thesize pure AB through simple dis- placement reactions date back to the 1960s; but have been thwarted by the formation of the diammoniate of dibor- ane (DADB), an ionic byproduct. Based on our recent characterization of the formation mechanism of DADB, we have developed a large-scale syn- thesis of pure AB by both increasing the basicity of the Lewis base of the borane carrier and using a nonpolar solvent to limit the formation of an in- termediate, the ammonia diborane (AaDB). Conditions were optimized for the preparation of pure AB by two displacement reactions, either ammonia with dimethylsulfide borane or ammo- nia with dimethylaniline borane in tol- uene at room temperature. These pro- cedures are also suitable for prepara- tion of other amine boranes which had the same problem of forming ionic by- products during displacement reactions. Keywords: boranes · displacement reactions · hydrogen storage · Lewis acid-base · synthetic methods [a] Dr. X. Chen, B. Billet, Prof. J.-C. Zhao Department of Material Science and Engineering The Ohio State University, Columbus OH 43210 (USA) Fax: (+ 1) 614-292-1537 E-mail: zhao.199@osu.edu [b] Dr. X. Chen, X. Bao, Prof. S. G. Shore Department of Chemistry, The Ohio State University Columbus OH 43210 (USA) Fax: (+ 1) 614-292-1685 E-mail: shore.1@osu.edu Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201201342.  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2012, 18, 11994 – 11999 11994