Alkaline Earth Metal Aluminates as Catalysts for Imine Hydrogenation

Elsen H, Langer J, Wiesinger M, Harder S (2020)

Publication Type: Journal article

Publication year: 2020

Journal

Organometallics American Chemical Society

DOI: 10.1021/acs.organomet.0c00226

Abstract

Alkaline earth (Ae) metal complexes with the alanate anion AlH4 - have been prepared by salt metathesis between NaAlH4 and AeCl2 in THF and could be isolated as Mg(AlH4)2·(THF)4, Ca(AlH4)2·(THF)4, and Sr(AlH4)2·(THF)5. The previously reported crystal structure of the Mg alanate complex shows bonding of AlH4 - with one bridging hydride, H3Al-(μ-H)-Mg, while the Ca and Sr alanates show a combination of H3Al-(μ-H)-Ae and H2Al-(μ-H)2-Ae bridging. The heteroleptic β-diketiminate complexes (DIPPBDI)Mg(AlH4)·THF and (DIPPBDI)Ca(AlH4)·(THF)2 have been prepared by reaction of the corresponding Ae hydride complexes with AlH3·(THF)2 [DIPPBDI = DIPP-NC(Me)C(H)C(Me)N-DIPP, where DIPP = 2,6-diisopropylphenyl]. Crystal structures show H2Al-(μ-H)2-Ae bridging. The Ca complex decomposes at room temperature by reduction of the β-diketiminate anion. Density functional theory calculations (B3PW91/def2tzvpp) show that the formation of Ae(AlH4)2 from AeH2 and AlH3 is exothermic by ΔH (kilocalories per mole): Be, -68.8; Mg, -66.1; Ca, -95.4; Sr, -100.9; Ba, -112.3. Calculations of NPA charges on LiAlH4 and the Ae alanate complexes (Ae = Mg, Ca, or Sr) show that these are highly ionic salts in which the charge on AlH4 - of approximately -0.95 is hardly dependent on the countercation. Compared to LiAlH4, the Ae alanates are very efficient catalysts for imine hydrogenation, clearly extending the substrate scope. In addition to aldimines RC(H)═NR′ (R/R′ = Ph/tBu, tBu/tBu, nPr/tBu, or Ph/Ph), ketimine PhC(Me)═NtBu could be reduced. The salt [Bu4N+][AlH4 -] is catalytically not active, which shows that the s-block metal is crucial. The highest activities were found for the heterobimetallic Ca and Sr alanates.

Authors with CRIS profile

Holger Elsen Lehrstuhl für Anorganische und Metallorganische Chemie Jens Langer Lehrstuhl für Anorganische und Metallorganische Chemie Michael Wiesinger Lehrstuhl für Anorganische und Metallorganische Chemie Sjoerd Harder Lehrstuhl für Anorganische und Metallorganische Chemie

How to cite

APA:

Elsen, H., Langer, J., Wiesinger, M., & Harder, S. (2020). Alkaline Earth Metal Aluminates as Catalysts for Imine Hydrogenation. Organometallics. https://doi.org/10.1021/acs.organomet.0c00226

MLA:

Elsen, Holger, et al. "Alkaline Earth Metal Aluminates as Catalysts for Imine Hydrogenation." Organometallics (2020).

BibTeX: Download