Magnesium Versus Zinc Coordination to Multidentate Schiff Base Ligands

The reactions of two mononuclear zinc complexes, [Zn(HL1)(H2O)2], 1, and [ZnL4(H2O)], 7, (H3L1 = Schiff base derived from 3-formylsalicylic acid and glycine, H2L4 = Schiff base derived from 2,6-diformyl-4-methylphenol and 1,2-diaminoethane), with Mg2+ has been studied. These complexes contain binucleating Schiff base ligands and have vacant coordination sites. For both compounds, substitution of zinc by magnesium has been observed in DMSO. Likewise, the Zn complex of H2L2 (Schiff base derived from salicylaldehyde and glycine) can be converted into the corresponding Mg compound. The following complexes were characterized by single-crystal X-ray analysis: [MgL3(H2O)3], 4, [MgL2(H2O)2]n, 5, [Mg(sal)2(H2O)2], 6, and [MgL4(H2O)(CH3OH)], 8 (H2L3 = Schiff base derived from 5-bromosalicylaldehyde and glycine, Hsal = salicylaldehyde). The tri- and tetradentate N,O ligands H2L3 and H2L4 form discrete complex molecules in the solid state, while a carboxylate-bridged polymeric structure is formed by H2L2. In 4 and 8 the aqua ligands hydrogen bond with the deprotonated phenolate and carboxylate oxygen atoms, resulting in the formation of dimers. In 6 two aqua ligands, two phenolate oxygens and two aldehyde oxygens give rise to an O6 coordination sphere.

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