Abstract
Hexa-ammine complexes, Mn(NH3)6X2 (X = Cl, Br), have been synthesized by ammoniation of the corresponding transition metal halide and characterized by Powder X-ray diffraction (PXRD) and Raman spectroscopy. The hexa-ammine complexes are isostructural (Cubic, Fm-3m, Z = 4; a = 10.2742(6) Å and 10.527(1) Å for X = Cl, Br respectively). Temperature programmed desorption (TPD) demonstrated that ammonia release from Mn(NH3)6X2 complexes occurred in three stages corresponding to the release of 4, 1 and 1 NH3 equivalents respectively. The chloride and bromide both exhibit a deammoniation onset temperature below 323 K. The di-ammoniates from the first desorption step were isolated during TPD measurements and their crystal structures determined by Rietveld refinement against PXRD data (X = Cl: orthorhombic Cmmm, a = 8.1991(9) Å, b = 8.2498(7) Å, c = 3.8212(4) Å, Z = 2; X = Br: orthorhombic Pbam, a = 6.0109(5) Å, b = 12.022(1) Å, c = 4.0230(2) Å, Z = 2).
Highlights
Fossil fuel depletion and societal demands are driving the need for more sustainable fuels.Hydrogen, a plausible high energy density alternative to fossil fuels, must comply with stringentCrystals 2012, 2 performance requirements in order to be economically viable and safe for commercial and public use [1].It may be safely stored in a solid material and subsequently exploited to drive fuel cells, which form the basis of many automotive power systems [2]
We describe here a systematic study of the deammoniation behavior of the manganese chloride and bromide hexa-ammines
From the results presented here we can establish that this is the case at least for Mn(NH3)6Br2 owing to the degenerate band observed in the Raman spectra at cm−1 and the large H-H bond lengths suggested from XRD refinements (Tables 3–6)
Summary
Fossil fuel depletion and societal demands are driving the need for more sustainable fuels. Crystals 2012, 2 performance requirements in order to be economically viable and safe for commercial and public use [1] It may be safely stored in a solid material and subsequently exploited to drive fuel cells, which form the basis of many automotive power systems [2]. Structural characterization of metal ammines and their decomposition products is required to understand fully the mechanisms by which ammonia release occurs and establish where improvements may be made for such systems. The complexation of transition metals with ammonia is well known and transition metal ammines, such as Fe(NH3)nCl2 and Ni(NH3)nCl2 [15,16], offer flexibility in the design of systems with tunable ammonia release thermodynamics Such compounds are at the point of being understood and developed.
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