Calcium-Amidoborane-Ammine Complexes: Thermal Decomposition of Model Systems

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 B-H...HN interactions (range: 1.97(4)2.39(4) angstrom) 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 degrees 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)]infinity, 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 degrees 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 wcalcium-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.