Dimetallic complexes of a structurally versatile pyridazine-containing Schiff-base macrocyclic ligand with pendant pyridine arms

Abstract

The new [2 + 2] Schiff-base macrocyclic ligand L², containing pyridazine head units and pyridine pendant arms, was synthesised as [Ba^II₂L²(ClO₄)₄(OH₂)] 1 from the barium(II) ion templated condensation reaction of 3,6-diformylpyridazine and N¹-(2-aminoethyl)-N¹-(methylene-2-pyridyl)-ethane-1,2-diamine. Subsequent transmetallation reactions of 1 with copper(II), iron(II) and manganese(II) perchlorates led to the formation of [Cu^II₂L²](ClO₄)₄·2MeCN 2, [Fe^II₂L²(MeCN)₂](ClO₄)₄3 and two manganese complexes, 4 and 5, with the same formula, [Mn^II₂L²(MeCN)(OH₂)](ClO₄)₄, but slightly different crystal structures, respectively. Single-crystal X-ray structural analyses reveal the variety of structures which can be supported by L² in order to meet the coordination environment preferences of the incorporated metal ions. The barium(II) ions in 1 have an irregular ten-coordinate geometry whereas the copper(II) ions in 2 have a square pyramidal geometry and the iron(II) ions in 3 have an octahedral geometry, while in 4 and 5 every manganese(II) ion is seven-coordinate and the environment can be best described as distorted pentagonal bipyramidal. In 1, 4 and 5 the pyridazine moieties bridge the metal centres [Ba(1)⋯Ba(2) 4.9557(3) Å 1; Mn(1)⋯Mn(2) 4.520(1) Å 4; Mn(1)⋯Mn(2) 4.3707(8) Å 5] but this is not observed in the copper(II) and iron(II) complexes, 2 and 3, in which the metal ions are well separated [Cu(1)⋯Cu(2) 5.9378(6) Å 2; Fe(1)⋯Fe(2) 5.7407(12) Å 3]. In the cyclic voltammogram of [Cu₂^IIL²](ClO₄)₄·2MeCN 2 in MeCN vs. Ag/AgCl two separate reversible one-electron transfer steps are observed [E_1/2 = 0.04 V, ΔE = 0.12 V and E_1/2 = 0.20 V, ΔE = 0.12 V; K_c = 510; in this system E_1/2 (Fc⁺/Fc) = 0.42 V and ΔE (Fc⁺/Fc) = 0.08 V]. The other complexes cannot be reversibly reduced/oxidised.