Solution-based routes to ammine metal borohydrides: formation of ammonia-borane.

Ammine metal borohydrides (AMBs) have recently commanded attention as low-temperature hydrogen sources. As an alternative to widely used mechanochemical synthesis that affords mixtures with salt co-products, we have been investigating solution synthesis routes to obtain pure AMBs. Here we show that reactions of MCln + nNaBH4 with ammonia in thf afford ammonia-borane (AB) via borane abstraction from M-coordinated borohydride. The amount of AB formed correlates roughly with the metal ion electronegativity and AMB thermal stability, except for reducible metals such as Ti, which affords nearly 3 equiv. of AB per Ti.