Two-Dimensional 3d–4f Heterometallic Coordination Polymers: Syntheses, Crystal Structures, and Magnetic Properties of Six New Co(II)–Ln(III) Compounds

Six new heterometallic cobalt­(II)-lanthanide­(III) complexes of formulas [Ln­(bta)­(H₂O)₂]₂[Co­(H₂O)₆]·10H₂O [Ln = Nd­(III) (1) and Eu­(III) (2)] and [Ln₂Co­(bta)₂(H₂O)₈]_n·6nH₂O [Ln = Eu­(III) (3), Sm­(III) (4), Gd­(III) (5), and Tb­(III) (6)] (H₄bta = 1,2,4,5-benzenetretracaboxylic acid) have been synthesized and characterized via single-crystal X-ray diffraction. 1 and 2 are isostructural compounds with a structure composed of anionic layers of [Ln­(bta)­(H₂O)₂]_n^n– sandwiching mononuclear [Co­(H₂O)₆]²⁺ cations plus crystallization water molecules, which are interlinked by electrostatic forces and hydrogen bonds, leading to a supramolecular three-dimensional network. 3–6 are also isostructural compounds, and their structure consists of neutral layers of formula [Ln₂Co­(bta)₂(H₂O)₈]_n and crystallization water molecules, which are connected through hydrogen bonds to afford a supramolecular three-dimensional network. Heterometallic chains formed by the regular alternation of two nine-coordinate lanthanide­(III) polyhedra [Ln­(III)­O₉] and one compressed cobalt­(II) octahedron [Co­(II)­O₆] along the crystallographic c-axis are cross-linked by bta ligands within each layer of 3–6. Magnetic susceptibility measurements on polycrystalline samples for 3–6 have been carried out in the temperature range of 2.0–300 K. The magnetic behavior of these types of Ln­(III)–Co­(II) complexes, which have been modeled by using matrix dagonalization techniques, reveals the lack of magnetic coupling for 3 and 4, and the occurrence of weak antiferromagnetic interactions within the Gd­(III)−Gd­(III) (5) and Tb­(III)−Tb­(III) (6) dinuclear units through the exchange pathway provided by the double oxo­(carboxylate) and double syn–syn carboxylate bridges.