Homoleptic b-Diketiminato Complexes of the Alkaline-Earth Metals: Trends in the Series Mg, Ca, Sr, and Ba.
Four homoleptic b-diketiminato alk. earth complexes were prepd. and structurally characterized, (DIPP-nacnac)2M with M = Mg, Ca, Sr, and Ba and DIPP-nacnac = (2,6-iPr2C6H3)NC(Me)C(H)C(Me)N(2,6-iPr2C6H3). The crystal structures show very similar C2-sym. monomeric species in which the ligand backbones are nearly planar and the metals are situated out-of-plane. Several C-H...p interactions between the ligands stabilize the complexes substantially and play a role in their soln. 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...p 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 chem. shifts point to increased ionicity of the ligand-metal bond along the row Mg < Ca < Sr < Ba. Delocalization of neg. charge from the N sp2 electron pair into the aryl ring increases along the row Mg < Ca < Sr < Ba.
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