A new family of metal borohydride guanidinate complexes: Synthesis, structures and hydrogen-storage properties

We report on a new class of complex hydrides: borohydride guanidinate complexes (MBH4 center dot nCN(3)H(5), M=Li, Mg, and Ca). They can be prepared via facile solid-state synthesis routes. Their crystal structures were successfully determined using a combination of X-ray diffraction, first-principles calculations and neutron vibrational spectroscopy. Among these compounds, Mg(BH4)(2)center dot 6CN(3)H(5) is composed of large complex Mg[CN3H5](6)(2+) cations and surrounding BH4- ions, while Ca(BH4)(2)center dot 2CN(3)H(5) possesses layers of corner-sharing Ca[BH4](4)(CN(3)Ff(5))(2) octahedra. Our dehydrogenation results show that approximate to 10 wt% hydrogen can be released from MBH4 center dot nCN(3)H(5) (M=Li, Mg, and Ca) at moderate temperatures with minimal ammonia and diborane contamination thanks to the synergistic effect of C-N bonds from guanidine and hydridic H from borohydrides leading to a weakening of the N-H bonds, thus impeding ammonia gas liberation. Further tuning the dehydrogenation with different cation species indicates that Mg (BH4)(2)center dot nCN(3)H(5) can exhibit the optimum properties with nearly thermally neutral dehydrogenation and very high purity hydrogen release. Published by Elsevier Inc.