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

The new [2 + 2] Schiff-base macrocyclic ligand L-2, containing pyridazine head units and pyridine pendant arms, was synthesised as [(Ba2L2)-L-II(ClO4)(4)(OH2)] 1 from the barium(II) ion templated condensation reaction of 3,6-diformylpyridazine and N-1-(2-aminoethyl)-N-1-(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 [(Cu2L2)-L-II](ClO4)(4)center dot 2MeCN 2, [(Fe2L2)-L-II(MeCN)(2)](ClO4)(4) 3 and two manganese complexes, 4 and 5, with the same formula, [(Mn2L2)-L-II(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) angstrom 1; Mn(1) (...) Mn(2) 4.520(1) angstrom 4; Mn(1) (...) Mn(2) 4.3707(8) angstrom 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) angstrom 2; Fe(1) (...) Fe(2) 5.7407(12) angstrom 3]. In the cyclic voltammogram of [(Cu2L2)-L-II](ClO4)(4) center dot 2MeCN 2 in MeCN vs. Ag/AgCl two separate reversible one-electron transfer steps are observed [E-1/2 = 0.04 V, Delta E = 0.12 V and E-1/2 = 0.20 V, Delta E = 0.12 V; K-c = 510; in this system E-1/2 ( Fc(+)/Fc(+)) = 0.42 V and Delta E (Fc(+)/ Fc) = 0.08 V]. The other complexes cannot be reversibly reduced/oxidised.