One-, two- and three-dimensional Cu(ii) complexes built via new oligopyrazinediamine ligands: from antiferromagnetic to ferromagnetic coupling

Abstract

Six new pyrazine-modulated N,N′-bis(α-pyridyl)-2,6-diaminopyridine ligands (PMN5) were synthesized and their complexes studied. Reaction of copper(II) with the ligand that contained one pyrazine ring in its terminal position led to formation of a one-dimensional zigzag complex whereas copper(II) reactions with ligands containing three pyrazine rings or one pyrazine ring in its middle position yielded straight one-dimensional complexes. A 2-D complex was produced from the ligand with two pyrazine rings at both terminals. When nickel(II) was introduced, a 3-D network was obtained from the three-pyrazine-modulated ligand. Researches on variable-temperature magnetic susceptibility measurements revealed excellent Heisenberg chains with weak antiferromagnetic interaction of J values from −2 to −3 cm⁻¹via σ and π pathways in straight one-dimensional complexes between the Cu(II) centers separated by 6.8–6.9 Å. The zigzag one-dimensional complex showed very poor magnetic coupling. The two-dimensional compound showed significant ferromagnetic interaction in spite of the Cu–Cu distance of 7.2 Å. Ferromagnetic coupling was discussed and attributed to the unusual coordination mode of in-plane and out-of-plane linkage of bridging pyrazine rings. The three-dimensional heterometal Cu(II)–Ni(II) compound showed weak antiferromagnetic interaction, which was satisfactorily fitted with J = −2.4 cm⁻¹ following a one-dimensional theoretical model including MFA.