Homoleptic β-Diketiminato Complexes of the Alkaline-Earth Metals:  Trends in the Series Mg, Ca, Sr, and Ba

Four homoleptic β-diketiminato complexes were prepared and structurally characterized: (DIPP-nacnac)2M with M = Mg, Ca, Sr, and Ba (DIPP-nacnac = (2,6-iPr2C6H3)NC(Me)C(H)C(Me)N(2,6-iPr2C6H3)). The crystal structures show very similar C2-symmetric monomeric species in which the ligand backbones are nearly planar and the metals are situated out-of-plane. Several C−H···π 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···π 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 sp2 electron pair into the aryl ring increases along the row Mg < Ca < Sr < Ba.