Commercial LiAlH
4 can be used in catalytic quantities in the hydrogenation of imines to amines with H
2. Combined experimental and theoretical investigations give deeper insight in the mechanism and identifies the most likely catalytic cycle. Activity is lost when Li in LiAlH
4 is exchanged for Na or K. Exchanging Al for B or Ga also led to dramatically reduced activities. This indicates a heterobimetallic mechanism in which cooperation between Li and Al is crucial. Potential intermediates on the catalytic pathway have been isolated from reactions of MAlH
4 (M=Li, Na, K) and different imines. Depending on the imine, double, triple or quadruple imine insertion has been observed. Prolonged reaction of LiAlH
4 with PhC(H)=N
tBu led to a side-reaction and gave the double insertion product LiAlH
2[N]
2 ([N]=N(
tBu)CH
2Ph) which at higher temperature reacts further by
ortho-metallation of the Ph ring. A DFT study led to a number of conclusions. The most likely catalyst for hydrogenation of PhC(H)=N
tBu with LiAlH
4 is LiAlH
2[N]
2. Insertion of a third imine via a heterobimetallic transition state has a barrier of +23.2 kcal mol
−1 (Δ
H). The rate-determining step is hydrogenolysis of LiAlH[N]
3 with H
2 with a barrier of +29.2 kcal mol
−1. In agreement with experiment, replacing Li for Na (or K) and Al for B (or Ga) led to higher calculated barriers. Also, the AlH
4− anion showed very high barriers. Calculations support the experimentally observed effects of the imine substituents at C and N: the lowest barriers are calculated for imines with aryl-substituents at C and alkyl-substituents at N.
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