Calcium–Amidoborane–Ammine Complexes: Thermal Decomposition of Model Systems |
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| Authors: | Prof?Dr Sjoerd Harder Dr Jan Spielmann Briac Tobey |
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| Institution: | 1. Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG, Groningen (Netherlands), Fax: (31)?50‐3634296;2. Anorganische Chemie, Universit?t Duisburg‐Essen, Universit?tsstra?e 5, 45117 Essen (Germany) |
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| Abstract: | Hydrocarbon‐soluble model systems for the calcium–amidoborane–ammine complex Ca(NH2BH3)2 ? (NH3)2 were prepared and structurally characterized. The following complexes were obtained by the reaction of RNH2BH3 (R=H, Me, iPr, DIPP; DIPP=2,6‐diisopropylphenyl) with Ca(DIPP‐nacnac)(NH2) ? (NH3)2 (DIPP‐nacnac=DIPP? NC(Me)CHC(Me)N? DIPP): Ca(DIPP‐nacnac)(NH2BH3) ? (NH3)2, Ca(DIPP‐nacnac)(NH2BH3) ? (NH3)3, Ca(DIPP‐nacnac)NH(Me)BH3] ? (NH3)2, Ca(DIPP‐nacnac)NH(iPr)BH3] ? (NH3)2, and Ca(DIPP‐nacnac)NH(DIPP)BH3] ? NH3. The crystal structure of Ca(DIPP‐nacnac)(NH2BH3) ? (NH3)3 showed a NH2BH3? unit that was fully embedded in a network of BH???HN interactions (range: 1.97(4)–2.39(4) Å) that were mainly found between NH3 ligands and BH3 groups. In addition, there were N? H???C interactions between NH3 ligands and the central carbon atom in the ligand. Solutions of these calcium–amidoborane–ammine complexes in benzene were heated stepwise to 60 °C and thermally decomposed. The following main conclusions can be drawn: 1) Competing protonation of the DIPP‐nacnac anion by NH3 was observed; 2) The NH3 ligands were bound loosely to the Ca2+ ions and were partially eliminated upon heating. Crystal structures of Ca(DIPP‐nacnac)(NH2BH3) ? (NH3)]∞, Ca(DIPP‐nacnac)(NH2BH3) ? (NH3) ? (THF), and Ca(DIPP‐nacnac){NH(iPr)BH3}]2 were obtained. 3) Independent of the nature of the substituent R in NH(R)BH3, the formation of H2 was observed at around 50 °C. 4) In all cases, the complex Ca(DIPP‐nacnac)(NH2)]2 was formed as a major product of thermal decomposition, and its dimeric nature was confirmed by single‐crystal analysis. We proposed that thermal decomposition of calcium–amidoborane–ammine complexes goes through an intermediate calcium–hydride–ammine complex which eliminates hydrogen and Ca(DIPP‐nacnac)(NH2)]2. It is likely that the formation of metal amides is also an important reaction pathway for the decomposition of metal–amidoborane–ammine complexes in the solid state. |
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| Keywords: | alkaline earth metals boranes calcium hydrogen storage structure elucidation |
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