β-Diketiminato Calcium and Magnesium Amides; Model Complexes for Hydroamination Catalysis

In a study relevant to group 2-mediated hydroamination catalysis, the reaction of the β-diketiminato magnesium alkyl complex [{ArNC(Me)CHC(Me)NAr}Mg(n/sBu)] (Ar = 2,6-iPr2C6H3) with benzylamine, 2-methoxyethylamine, pyrrolidine, and 2-methyl-4,4-diphenylpyrrolidine has been shown to yield the corresponding magnesium amide complexes [{ArNC(Me)CHC(Me)NAr}Mg(NR1R2)] (R1 = H, R2 = CH2Ph, CH2CH2OMe; R1 = R2 = -(CH2)4-, -CH(Me)CH2CPh2CH2-) within the first point of analysis (30 min) at room temperature in near quantitative yield as monitored by 1H NMR spectroscopy. Reactions proceeded non-reversibly, and the products have been characterized in both solution and the solid state. While single crystal X-ray diffraction analysis demonstrated that the magnesium amides are dimeric in the solid state, with aggregation occurring via μ2-coordinated amide ligands, NMR studies suggest that for more sterically crowded amide ligands discreet monomeric species exist in solution. In contrast, the calcium complex [{ArNC(Me)CHC(Me)NAr}Ca{N(SiMe3)2}(THF)] reacted reversibly with benzylamine at room temperature to form an equilibrium mixture of a calcium benzylamide and bis(trimethylsilyl)amide. A series of Pulsed-Gradient Spin−Echo NMR studies upon β-diketiminato calcium amides were consistent with the formation of a dimer in solution. A van’t Hoff analysis performed on this mixture allowed ΔH° = −51.3 kJ mol−1 and ΔS° = −134 J mol−1 of the protonolysis/dimerization reaction to be derived and the Gibbs’ free energy to be calculated as ΔG° (298 K) = −11.4 kJ mol−1.