A new bis(phenol-armed) pyridazine-containing Schiff base ligand: synthesis, complexation and reduction to the amine ligand analogue

Abstract

A bis(phenol-armed) acyclic Schiff base ligand H 2L has been synthesised from 3,6-diformylpyridazine and two equivalents of o-aminophenol. The H 2L ligand has been structurally characterised and is shown to be close to planar (maximum twist of pyridazine ring vs. phenyl ring planes is just 17.7°) with both intramolecular and intermolecular hydrogen-bonding interactions present. The latter hydrogen-bonding interactions in combination with offset parallel π–π stacking interactions are instrumental in the formation of columns of a stepped criss-cross type array. The ligand undergoes an irreversible oxidation in dimethylformamide at E pa=+0.91 V vs. 0.01 mol l −1 AgNO 3/Ag (s), to form an unstable new compound which exhibits closely spaced quasi-reversible processes at E 1/2≈−0.77 V. In addition, there is a process at −0.25 V which is associated with an irreversible reduction process at E pc=−1.52 V. Chemical reduction of the two imine bonds of H 2L was achieved by reaction with NaBH 4 to give the diamine analogue H 6L′ as the dihydrochloride salt: no evidence of pyridazine ring reduction was observed. Three complexes of L 2−, Co 2L(CH 3COO) 2, Ni 2L(CH 3COO) 2(H 2O) and Cu 2L(CH 3COO) 2(H 2O) 2(CH 3CN), have been prepared and partially characterised.