Dicopper(ii) complexes of a new pyrazolate-containing Schiff-base macrocycle and related acyclic ligand

Abstract

Dicopper(II) complexes of two new 3,5-disubstituted-pyrazole-based ligands, bis(quadridentate) macrocyclic ligand (L1)²⁻ and bis(terdentate) acyclic ligand (L2)⁻, were synthesised by Schiff base condensation of 3,5-diformylpyrazole and either one equivalent of 1,3-diaminopropane or two equivalents of 2-(2-aminoethyl)pyridine in the presence of one or two equivalents of copper(II) ions, respectively. Copper(II) acetate monohydrate was employed in the synthesis of [Cu₂(L1)(OAc)₂], [Cu₂(L2)(H₂O)₂(OAc)₃] and [Cu^II₂(L1)(NCS)₂]; in the last of these one equivalent of NaNCS per copper(II) ion was also added. The fourth complex, [Cu₂(L2)(NCS)₂(DMF)]BF₄, was prepared using copper(II) tetrafluoroborate hexahydrate, along with two equivalents of NaOH and six of NaSCN. All four of these dimetallic complexes have been characterised by single crystal X-ray diffraction: the two macrocyclic complexes are the first such Schiff base complexes to be so characterised. A feature common to all four of the structures is bridging of the two copper(II) centres by the pyrazolate moiety/moieties. The structure determinations show that the coordination mode of the acetate groups in both [Cu₂(L1)(OAc)₂]·2MeOH·H₂O and [Cu₂(L2)(H₂O)₂(OAc)₃] is unidentate as had been tentatively predicted by analysis of the infrared spectra (ΔOCO of 199 and 208 cm⁻¹, respectively). The magnetochemical studies of the macrocyclic complexes, over the temperature range 4–300 K, revealed strong antiferromagnetic coupling with J = −169 and −213 cm⁻¹ for [Cu₂(L1)(OAc)₂]·2H₂O and [Cu^II₂(L1)(NCS)₂]·DMF respectively. The J values have been discussed in relation to a published correlation involving the CuN_pyrazolateN_pyrazolate angles.