Heavy Alkaline-Earth Metal Hydride Complexes: Syntheses, Structures and Applications as Super Reducing Agents
Final Report Abstract
Our approach to synthesize Ae metal hydride clusters by self-assembly can be generalized. We show that a large array of metal hydride clusters can be formed by simple variations of the anionic amide ligand, the neutral ligand or co-solvent, or the metal. Mixed Ae metal amide-hydride clusters are shown to be highly effective catalysts for alkene hydrogenation. The amine formed during these reactions has been shown to be beneficial in preventing alkene polymerization, a side-reaction generally observed in alkene hydrogenation. Catalyst activities and substrate scope continuously improved during the time of this project. When using very bulky amides, even arenes like benzene could be hydrogenated to cyclohexane. This challenging reduction is generally reserved for precious transition metal catalysts. Surprisingly, it was discovered that the best hydrogenation catalysts are the zerovalent metals itself. Although they need to be activated by Metal-Vapor-Synthesis, the simplicity of this method is highly attractive. It is proposed that interaction of the alkene (or arene substrate) with the Ae0 surface is essential for the high activities of this system. d-Orbitals arguably could play an important role in substrate activation. Slightly lowering the temperature during arene hydrogenation, and exchanging H 2 for D2, led to catalytic H/D exchange. Activities for benzene deuteration generally increase with metal size Ca < Sr < Ba. With TOF’s > 250 h-1, the activity of the best Ba catalysts is comparable to that of the most active Ir and Pt catalysts. Well-defined Ae metal hydride complexes or in situ generated less defined AexHy(NR2)z mixtures are also very efficient for hydrodehalogenation reactions. The halogen X in Ph-X is rapidly substituted by H. Even unactivated C-F bonds were cleaved without difficulty and the reduction of SF6 to sulfur was demonstrated. Cationic clusters were obtained by three methods: (1) Larger multidentate neutral ligands were able to substitute anionic ligands, resulting in a cationic cluster. (2) Using superbulky amide ligands also led to cationic clusters. (3) Finally, decreasing the size of the metal gave cationic clusters. These charged clusters are not more reactive than neutral clusters. Ligand design led to isolation of the first well-defined dimeric Sr hydride complex which, being free from a neutral Lewis base, is highly reactive. Addition of ethylene gave the first Sr-Et complex which we consider the most reactive s-block metal species to date. Even at low temperature it attacks benzene to exchange Sr-Et for Sr-H, forming EtC6H5. It is thus a catalyst for the insertion of ethylene in benzene’s C-H bond. A surprise in this project was the fact that cationic clusters were not more reactive or catalytically active than neutral ones. This was disappointing, however, in order to create the most reactive species, we turned to ligand design. This allowed us to prepare the first highly reactive Lewis base-free Sr hydride and, more important, Sr-ethyl complexes. The reactivity of these complexes is currently unsurpassed. Another surprise was the fact that, although superbulky Ba(NR2)2 was found to be the best hydrogenation catalyst, simple (ligand-free) Ba0 metal is even more active. We explain this by substrate (alkene or arene) activation at the bare Ba0 metal surface. It is believed that d-orbitals on Ba0 play a critical role in this process. This concept, which is shown in Scheme 6b, has an enormous future potential which we currently can hardly oversee. There is ample room for follow-up studies.
Publications
- Angewandte Chemie, 2018, 130, 15397-15402; Angewandte Chemie, International Edition, 2018, 57, 15177-15182: Simple Alkaline-Earth Metal Catalysts for Effective Alkene Hydrogenation
H. Bauer, M. Alonso, C. Fischer, B. Rösch, H. Elsen, S. Harder
(See online at https://doi.org/10.1002/ange.201810026 https://doi.org/10.1002/anie.201810026) - Angewandte Chemie, 2019, 131, 4292-4297; Angewandte Chemie, International Edition, 2019, 58, 4248-4253: Alkene Transfer Hydrogenation with Alkaline-Earth Metal Catalysts
H. Bauer, K. Thum, M. Alonso, C. Fischer, S. Harder
(See online at https://doi.org/10.1002/ange.201813910 https://doi.org/10.1002/anie.201813910) - Angewandte Chemie, 2019, 131, 5450-5455 Angewandte Chemie, International Edition, 2019, 58, 5396-5401: Nucleophilic Aromatic Substitution at Benzene with Powerful Strontium Hydride and Alkyl Complexes
B. Rösch; T. X. Gentner; H. Elsen; C. A. Fischer; J. Langer; M. Wiesinger; S. Harder
(See online at https://doi.org/10.1002/ange.201901548 https://doi.org/10.1002/anie.201901548) - European Journal of Inorganic Chemistry, 2019, 41, 4433-4439: Stabilizing Magnesium Hydride Complexes with Neutral Ligands
M. Wiesinger, B. Maitland, H. Elsen, J. Pahl; S. Harder
(See online at https://doi.org/10.1002/ejic.201900936) - ACS Catal. 2020, 10, 7792–7799: Hydrogen Isotope Exchange with Superbulky Alkaline Earth Metal Amide Catalysts
J. Martin, J. Eyselein, S. Grams, S. Harde
(See online at https://doi.org/10.1021/acscatal.0c01359) - Angewandte Chemie, 2020, 132, 9187 –9197; Angewandte Chemie International Edition, 2020, 59, 9102-9112: Highly Active Superbulky Alkaline Earth Metal Amide Catalysts for Hydrogenation of Challenging Alkenes and Aromatic Rings
J. Martin, C. Knüpfer, J. Eyselein, C. Farber, S. Grams, J. Langer, K. Thum, M. Wiesinger, S. Harder
(See online at https://doi.org/10.1002/ange.202001160 https://doi.org/10.1002/anie.202001160) - Chem. Commun. 2020, 56, 9178-9181: Large Decanuclear Calcium and Strontium Hydride Clusters
J. Martin, J. Eyselein, J. Langer, H. Elsen, S. Harder
(See online at https://doi.org/10.1039/d0cc04330c) - Angewandte Chemie 2021, 133, 4298-4304; Angewandte Chemie International Edition 2021, 60, 4252- 4258: Metallic barium: a versatile and efficient hydrogenation catalyst
P. Stegner, C. Färber, U. Zenneck, C. Knüpfer, J. Eyselein, M. Wiesinger, S. Harder
(See online at https://doi.org/10.1002/ange.202014326 https://doi.org/10.1002/anie.202014326) - Carbon-Halogen Bond Activation with Powerful Heavy Alkaline Earth Metal Hydrides. European Journal of Inorganic Chemistry 2021, 36, 3731-3741
M. Wiesinger, B. Rösch, C. Knüpfer, J. Mai, J. Langer, S. Harder
(See online at https://doi.org/10.1002/ejic.202100529) - ChemCatChem 2021, 13, 4567-4577: Heterometallic Mg-Ba Hydride Clusters in Hydrogenation Catalysis
M. Wiesinger, C. Knüpfer, H. Elsen, J. Mai, J. Langer, S. Harder
(See online at https://doi.org/10.1002/cctc.202101071)
