Hydrogen Storage in Magnesium and Zinc Hydride: the Molecular Approach
Final Report Abstract
New bridged bis-ß-diketiminate ligands have been prepared but in most cases the organometallic products were found to be fully insoluble. This is possibly due to formation of coordination polymers. A pyridine-bridged bis-ß-diketiminate ligand stabilized a tetranuclear Mg hydride cluster. Thermal decomposition, however, involved nucleophilic attack and dearomatization of the pyridine ring. This reactivity has been further exploited in catalytic hydroboration and hydrosilylation of pyridine substrates. A Zn hydride cluster with a bis-ß-diketiminate ligand could be isolated and structurally charactzerized and was found to be isostructural to its Mg analogue. Thermal decomposition studies show that in contrast to H2 elimination in Mg hydride clusters, the temperature for H2 desorption decreases with increasing cluster size. A small mononuclear Zn hydride complex is much more stable than a tetranuclear Zn hydride complex. Possibly, larger clusters show closer resemblance with bulk ZnH2 which already decomposes at room temperature. The most important conclusion is that the trend for H2 release in Zn hydride clusters is opposite to that observed for Mg hydride clusters. Well-defined mixed Zn-Mg hydride clusters could not be obtained. This is partially due to irreproducibility of early work on Mg-Zn hydride salts by Ashby et al. Low valent mixed ZnI-MgI complexes could also not be obtained and were published during the course of the project by Jones and coworkers. As part of the project, heavier alkaline earth metal hydrides (Ca, Sr) have been isolated. Systematic studies of solvent effects on the stability of ß-diketiminate Ca hydride clusters led to a variety of new complexes and also the first amidinate Ca hydride complexes were introduced. Most important, a simple method was found for a high yield synthesis of a larger Ca hydride cluster and the very first Sr hydride cluster. The heavier alkaline earth metal hydrides do not as easily lose H2 as Mg or Zn hydride clusters, which is in agreement with the more negative reduction potentials of Ca and Sr compared to Mg and Zn. A surprise in this project was the fact that many of the bridged bis-ß-diketiminate ligands gave organometallic complexes that were completely insoluble (likely on account of formation of coordination polymers). This stands in stark contrast with our earlier work on such ligand systems in which we did not encounter these problems. Another surprise is the fact that Ashby’s early work on Mg-Zn hydride salts was irreproducible and that the synthesis of mixed Mg-Zn hydride complexes is an enormous challenge.
Publications
- Chemistry: A European Journal 2014, 20, 11204- 11213. Comparison of Hydrogen Elimination from Molecular Zinc and Magnesium Hydride Clusters
J. Intemann, P. Sirsch, S. Harder
(See online at https://doi.org/10.1002/chem.201402141) - J. Intemann Dalton Transactions 2014, 43, 14284-14290. Synthesis and thermal decomposition of a pyridylene-bridged bis-β-diketiminate magnesium hydride cluster
S. Harder, J. Spielmann
(See online at https://doi.org/10.1039/c4dt00835a) - Chemistry: A European Journal 2015, 21, 11452-11461. Calcium Hydride Catalyzed Highly 1,2-Selective Pyridine Hydrosilylation
J. Intemann, H. Bauer, J. Pahl, L. Maron, S. Harder
(See online at https://doi.org/10.1002/chem.201501072) - Chemistry: A European Journal 2016, 22, 17425-17435. A Soft Grip: Magnesium Complexes with a Phosphine- Modified Phosphonium Diylidic Lewis Base
J. Langer, I. Kosygin, R. Puchta, J. Pahl, S. Harder
(See online at https://doi.org/10.1002/chem.201603775) - Organometallics 2016, 35, 3350-3360. Stabilization of Calcium Hydride Complexes by Fine Tuning of Amidinate Ligands
A. Causero, G. Ballmann, J. Pahl, H. S. Zijlstra, C. Färber, S. Harder
(See online at https://doi.org/10.1021/acs.organomet.6b00566) - Angewandte Chemie, International Edition 2017, 56, 11880-11884. A Simple Route to Calcium and Strontium Hydride Clusters
B. Maitland, M. Wiesinger, J. Langer, G. Ballmann, J. Pahl, H. Elsen, C. Färber, S. Harder
(See online at https://doi.org/10.1002/anie.201706786) - Angewandte Chemie, International Edition 2017, 56, 5021-5025. Self-Assembly of Magnesium Hydride Clusters Driven by Chameleon-Type Ligands
J. Langer, B. Maitland, S. Grams, A. Ciucka, J. Pahl, H. Elsen, S. Harder
(See online at https://doi.org/10.1002/anie.201700719)
