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

Reaction of aqueous/ethanolic solutions of CoC(l)2.6H(2)O and nitrilotripropionic acid (H(3)ntp=N(CH2CH2COOH)(3)) in the presence of potassium hydroxide affords the hydroxy-bridged tetranuclear cluster [Co-4(mu(3)-OH)(2)(H2O)(6)(ntp)(2)].2H(2)O (1). The Ni(II) analogue [Ni-4(mu(3)-OH)(2)(H2O)(6)(ntp)(2)].2H(2)O (2) can also be isolated using aqueous solutions and Ni(SO4).7H(2)O as metal salt. With small changes in reaction conditions the methoxy-bridged analogue, [Ni-4(mu(3)-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 Neel temperatures below 1 K.