Homoleptic β-diketiminato complexes of the alkaline-earth metals:: Trends in the series Mg, Ca, Sr, and Ba

Four homoleptic beta-diketiminato complexes were prepared and structurally characterized: (DIPP-nacnac)(2)M with M = Mg, Ca, Sr, and Ba (DIPP-nacnac = (2,6-iPr(2)C(6)H(3))NC(Me)C-(H)C(Me)N(2,6-iPr(2)C(6)H(3))). The crystal structures show very similar C-2-symmetric monomeric species in which the ligand backbones are nearly planar and the metals are situated out-of-plane. Several C-H...pi interactions between the ligands stabilize the complexes substantially and play a role in their solution structures. A dynamic process, which involves inversion of the puckered six-membered metal-N-C-C-C-N ring, leads to exchange of the aryl rings and a reorganization of the C-H...pi network. The activation energies (kcal/mol) for this process are metal dependent: Mg 17.8(1), Ca 17.6(1), Sr 16.6(1), and Ba 15.2(1). The NMR chemical shifts point to increased ionicity of the ligand-metal bond along the row Mg < Ca < Sr < Ba. Delocalization of negative charge from the nitrogen sp(2) electron pair into the aryl ring increases along the row Mg < Ca < Sir < Ba.