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.