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) (<b>1</b>) and Eu­(III) (<b>2</b>)] and [Ln₂Co­(bta)₂(H₂O)₈]_<i>n</i>·6<i>n</i>H₂O [Ln = Eu­(III) (<b>3</b>), Sm­(III) (<b>4</b>), Gd­(III) (<b>5</b>), and Tb­(III) (<b>6</b>)] (H₄bta = 1,2,4,5-benzenetretracaboxylic acid) have been synthesized and characterized via single-crystal X-ray diffraction. <b>1</b> and <b>2</b> are isostructural compounds with a structure composed of anionic layers of [Ln­(bta)­(H₂O)₂]_<i>n</i>^<i>n</i>– 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. <b>3</b>–<b>6</b> are also isostructural compounds, and their structure consists of neutral layers of formula [Ln₂Co­(bta)₂(H₂O)₈]_<i>n</i> 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 <i>c</i>-axis are cross-linked by bta ligands within each layer of <b>3</b>–<b>6</b>. Magnetic susceptibility measurements on polycrystalline samples for <b>3</b>–<b>6</b> 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 <b>3</b> and <b>4</b>, and the occurrence of weak antiferromagnetic interactions within the Gd­(III)−Gd­(III) (<b>5</b>) and Tb­(III)−Tb­(III) (<b>6</b>) dinuclear units through the exchange pathway provided by the double oxo­(carboxylate) and double syn–syn carboxylate bridges