Nickel macrocycles with complex hydrides—new avenues for hydrogen storage research

Abstract

As part of an investigation into Ni(macrocycle) complexes as a new class of complex hydride hydrogen store catalysts, two isomers of the unusually stable nickel borohydride, Ni(cyclam)(BH4)2 (cyclam = 1,4,8,11-tetraazacyclotetradecane), have been isolated and characterised by means of single crystal X-ray diffraction, infrared spectroscopy, thermogravimetry and calorimetry. trans-Ni(cyclam)(BH4)2 crystallizes monoclinic (P21/c, a = 7.1397(5) Å, b = 12.8109(7) Å, c = 8.7041(5) Å, β = 109.948(6)°, V = 748.36(8) Å3, Z = 2); cis-Ni(cyclam)(BH4)2 crystallizes orthorhombic (Pnma, a = 14.4512(7) Å, b = 9.4625(6) Å, c = 11.7824(7) Å, V = 1611.18(16) Å3, Z = 4, with static disorder). The compounds were found to be considerably more thermally stable than the unchelated relative, Ni(BH4)2, with the onset of decomposition for the trans isomer at 170 °C, thus nearly 200 °C higher than the decomposition temperature of Ni(BH4)2. This spectacular stabilisation holds promise for related macrocycle complexes of late transition metals to be used as hydrogenation catalysts on an equal footing with their unligated early transition metal counterparts.