Molecular Tweezers for Hydrogen: Synthesis, Characterization, and Reactivity
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- 1 October 2008
- journal article
- research article
- Published by American Chemical Society (ACS) in Journal of the American Chemical Society
- Vol. 130 (43), 14117-14119
- https://doi.org/10.1021/ja806627s
Abstract
The first ansa-aminoborane N-TMPN-CH2C6H4B(C6F5)2 (where TMPNH is 2,2,6,6-tetramethylpiperidinyl) which is able to reversibly activate H2 through an intramolecular mechanism is synthesized. This new substance makes use of the concept of molecular tweezers where the active N and B centers are located close to each other so that one H2 molecule can fit in this void and be activated. Because of the fixed geometry of this ansa-ammonium-borate it forms a short N−H···H−B dihydrogen bond of 1.78 Å as determined by X-ray analysis. Therefore, the bound hydrogen can be released above 100 °C. In addition, the short H···H contact and the N−H···H (154°) and B−H···H (125°) angles show that the dihydrogen interaction in N-TMPNH-CH2C6H4BH(C6F5)2 is partially covalent in nature. As a basis for discussing the mechanism, quantum chemical calculations are performed and it is found that the energy needed for splitting H2 can arise from the Coulomb attraction between the resulting ionic fragments, or “Coulomb pays for Heitler−London”. The air- and moisture-stable N-TMPNH-CH2C6H4BH(C6F5)2 is employed in the catalytic reduction of nonsterically demanding imines and enamines under mild conditions (110 °C and 2 atm of H2) to give the corresponding amines in high yields.Keywords
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