Alkaline Earth Metal Aluminates as Catalysts for Imine Hydrogenation

Alkaline earth (Ae) metal complexes with the alanate anion AIH(4)(-) have been prepared by salt metathesis between NaAIH(4) and AeCl(2) in THE and could be isolated as Mg(AlH4)(2)center dot(THF)(4), Ca(AIH(4))(2)center dot(THF)(4), and Sr (AlH4)(2)center dot(THF)(3) . The previously reported crystal structure of the Mg alanate complex shows bonding of Ala i - with one bridging hydride, H3Al-(mu-H)-Mg, while the Ca and Sr alanates show a combination of H3Al-(mu-H)-Ae and H2Al-(mu-H)(2) -Ae bridging. The heteroleptic beta-diketiminate complexes ((BDI)-B-DIPP)Mg(AlH4)center dot(THF)(2) and ((BDI)-B-DIPP)Ca(AlH4)center dot(THF)(2) have been prepared by reaction of the corresponding Ae hydride complexes with AlH3 center dot(THF)(2)[(BDI)-B-DIPP = DIPP-NC(Me)C(H)C(Me)N-DIPP, where DIPP = 2,6-diisopropylphenyl]. Crystal structures show H2Al-(mu-H)(2) -Ae bridging. The Ca complex decomposes at room temperature by reduction of the beta-diketiminate anion. Density functional theory calculations (B3PW91/ def2tzvpp) show that the formation of Ae(AlH4)(2) from AeH(2). and AlH3 is exothermic by Delta H (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.