Synthesis and magnetism of oxygen-bridged tetranuclear defect dicubane Co(ii) and Ni(ii) clusters

Abstract

Reaction of aqueous/ethanolic solutions of CoCl₂·6H₂O and nitrilotripropionic acid (H₃ntp = N(CH₂CH₂COOH)₃) in the presence of potassium hydroxide affords the hydroxy-bridged tetranuclear cluster [Co₄(μ₃-OH)₂(H₂O)₆(ntp)₂]·2H₂O (1). The Ni(II) analogue [Ni₄(μ₃-OH)₂(H₂O)₆(ntp)₂]·2H₂O (2) can also be isolated using aqueous solutions and Ni(SO₄)·7H₂O as metal salt. With small changes in reaction conditions the methoxy-bridged analogue, [Ni₄(μ₃-OMe)₂(H₂O)₆(ntp)₂] (3), can also be isolated. In these tetramers the M(II) ions are oxygen-bridged and exhibit a defect dicubane-like core with two missing vertices. The magnetic properties have been studied for all three clusters and reveal competing antiferromagnetic and ferromagnetic interactions between the four Co(II) ions in 1 and ferromagnetic coupling between the four Ni(II) ions in 2 and 3. In all three compounds the individual clusters order antiferromagnetically at Néel temperatures below 1 K.