Chelates of tetracycline with first row transition metal perchlorates

2:1 adducts of tetracycline (tc) with 3d metal perchlorates (M = Cr ³⁺, Mn ²⁺, Fe ²+, ^Fe3+, ^Co2+, Ni ²⁺, Cu ²⁺, Zn ²⁺) are synthesized by boiling under reflux mixtures of tc free base and metal salt in ethanol— triethyl orthoformate. Characterization studies suggest that the new complexes are monomeric chelates involving bidentate tetracycline ligands, chelating via the amide group oxygen and the C3O oxygen of the ring A tricarbonylmethane. The complexes also contain unidentate coordinated −OClO ₃ ligands, as weil as ionic ClO ₄^-. The M ³⁺ (Cr, Fe) complexes are hexacoordinated of the [M(tc) ₂(OClO ₃)2 ₃]ClO ₄ type (MO ₆ chromophores), with two bidentate chelating tc and two unidentate perchlorato ligands in the first coordination sphere of the central metal ion. In the M2+ (Mn, Fe, Co, Ni, Cu, Zn) complexes, the inner coordination sphere of the metal ion is occupied by two bidentate chelating tc and only one −OClO ₃ ligand, and the coordination number is five, i.e. [M(tc) ₂OClO ₃]ClO ₄ (MO ₅absorbing species).