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 beta-diketiminato magnesium alkyl complex [{ArNC(Me)CHC(Me)NAr}Mg(Bu-n/s)] (Ar = 2,6-(Pr2C6H3)-Pr-i) 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((NRR2)-R-1)] (R-1 = H, R-2 = CH2Ph, CH2CH2OMe; R-1 = R-2 = -(CH2)(4)-, -CH(Me)-CH2CPh2CH2-) within the first point of analysis (30 min) at room temperature in near quantitative yield as monitored by H-1 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 mu(2)-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 beta-diketiminato calcium amides were consistent with the formation of a dimer in solution. A van't Hoff analysis performed on this mixture allowed Delta H degrees = -51.3 U mol(-1) and Delta S degrees = -134 J.mol(-1) of the protonolysis/dimerization reaction to be derived and the Gibbs' free energy to be calculated as AGO (298 K) = - 11.4 kJ mol(-1).