Unusual (μ-aqua)bis(μ-carboxylate) bridge in homometallic M(II) (M = mn, co and ni) two-dimensional compounds based on the 1,2,3,4-butanetetracarboxylic acid:: Synthesis, structure, and magnetic properties

The first coordination compounds of 1,2,3,4-butanetetracarboxylate anion (butca(4-)) of the formula [M-2(butca)(H2O)(5)](n)center dot 2nH(2)O [M = Mn(II) (1), Co(II) (2), and Ni(II) (3)] were prepared and their X-ray crystal structures and magnetic properties investigated. The three complexes have a very similar two-dimensional structure which consists of (4,4) networks, 1 and 2 being isostructural. The tetracarboxylate ligand acts as a 4-fold connector leading to two-dimensional (4,4) networks of metal atoms, this topology being possible because of its planar conformation. The nodes of these networks are formed by dinuclear motifs which exhibit the unusual (mu-aqua)bis(mu-carboxylate) bridging unit which is analogous to that observed in some molecules of biological interest. The variable-temperature magnetic susceptibility measurements of 1-3 show that 1 and 2 are antiferromagnetically coupled systems whereas 3 exhibits a ferromagnetic behavior. The analysis of the magnetic data of 1-3 through a simple dinuclear model allowed the determination of the values of the magnetic coupling (J) -3.6 (1), -1.2 (2), and +1.47 cm(-1) (3) with the Hamiltonian being defined as H = -JS(A)center dot S-B. The countercomplementarity between the two bridges (aqua and syn-syn carboxylate) accounts for the trend exhibited by the values of the magnetic coupling in this family.