Synthesis, Crystal Structure, and Magnetic Characterization of the Three-Dimensional Compound [Co2(cbut)(H2O)3]n (H4cbut = 1,2,3,4-Cyclobutanetetracarboxylic Acid)

A novel cobalt­(II) complex of formula [Co2(cbut)­(H2O)3]n (1) (H4cbut = 1,2,3,4-cyclobutanetetracarboxylic acid) has been synthesized under hydrothermal conditions and its crystal structure has been determined by means of synchrotron radiation and neutron powder diffraction. The crystal structure of 1 consists of layers of cobalt­(II) ions extending in the bc-plane which are pillared along the crystallographic a-axis through the skeleton of the cbut4– ligand. Three crystallographically independent cobalt­(II) ions [Co(1), Co(2), and Co(3)] occur in 1. They are all six-coordinate with four carboxylate-oxygens [Co(1)–Co(3)] and two cis-[Co­(1)] or trans-water molecules [Co(2) and Co(3)] building distorted octahedral surroundings. Regular alternating double oxo­(carboxylate) [between Co(1) and Co­(1a)] and oxo­(carboxylate) plus one aqua and a syn–syn carboxylate bridges [between Co(1) and Co(2)] occur along the crystallographic b-axis, the values of the cobalt–cobalt separation being 3.1259(8) and 3.1555(6) Å, respectively. These chains are connected to the Co(3) atoms through the OCO carboxylate along the [01̅1] direction leading to the organic–inorganic bc-layers with Co(1)–OCO(anti–syn)–Co­(3) and Co(2)–OCO(anti–anti)–Co­(3) distances of 5.750(2) and 4.872(1) Å. The shortest interlayer cobalt–cobalt separation through the cbut4– skeleton along the crystallographic a-axis is 7.028(2) Å. Variable-temperature magnetic susceptibility measurements show the occurrence of antiferromagnetic ordering with a Néel temperature of 5.0 K, followed by a field-induced ferromagnetic transition under applied dc fields larger than 1500 Oe. The magnetic structure of 1 has been elucidated at low temperatures in zero field by neutron powder diffraction measurements and was found to be formed by ferromagnetic chains running along the b-axis which are antiferromagnetically coupled with the Co(3) ions through the c-axis giving rise to noncompensated magnetic moments within each bc-layer (ferrimagnetic plane). The occurrence of an antitranslation operation between these layers produces a weak interlayer antiferromagnetic coupling along the a-axis which is overcome by dc fields greater than 1500 Oe resulting in a phase transition toward a ferromagnetic state (metamagnetic behavior).