Alkaline Earth Metal Aluminates as Catalysts for Imine Hydrogenation

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.