Esterification and isolation of the carboxylic acid with salicyl-bis-hydrazide via coordination of iron(III) 18-metallacrown-6 complex

Abstract

The trianionic heptadentate ligand, (Z)-3-(5′-bromosalicylhydrazinocarbonyl) propenoic acid ((Z)-H4bshcpa), has been synthesized in good yield and reacted with FeCl3 · 6H2O to produce [FeIII 6(C12H8N2O5Br)6(H2O)2(CH3OH)4] · 8H2O · 8CH3OH. The complex has been characterized by single-crystal X-ray diffraction. In the self-assembly process the ligand was esterified and transferred into (Z)-methyl 3-(5′-bromosalicylhydrazinocarbonyl) propenoate ((Z)-H3mbshcp). In the crystal structure, the neutral Fe(III) complex contains an 18-membered metallacrown ring consisting of six Fe(III) and six trianionic ligands. The 18-membered metallacrown ring is formed by six structural moieties of the type [Fe(III)–N–N]. Due to the meridional coordination of the ligands to Fe3+, the ligands enforce stereochemistry of the Fe3+ ions as a propeller configuration with alternating Λ/Δ forms. The metallacrown can be treated with SnCl2 to obtain purified ester. In addition, we have also obtained reduced esterified ligand, methyl 3-(5′-bromosalicylhydrazinocarbonyl) propanoate (H3mbshcp), with Zn powder as reductant.