Five related metal–organic frameworks constructed from [Ln2(SO4)2(H2O)n]2+ units and oxalate or acetate ligands

Abstract

We have synthesized a series of lanthanide-based metal–organic solids and characterized them through structural, magnetic and luminescence analyses. The nine compounds in the series NH₄[Ln(SO₄)(H₂O)(C₂O₄)] [Ln = Ce, Nd, Eu, Gd (×2), Tb, Dy, Er, Yb] belong to four new three-dimensional (3D) structural groups, which we label CKMOF-4a, -4b, -5a, and -5b. The CKMOF-4a (Ce·1) and -4b (Gd·5, Tb·6, Dy·7, Er·8, Yb·9) structures feature [Ln₂(SO₄)₂(H₂O)₂]²⁺ units and C₂O₄²⁻ ligands having the same symmetry operations, but their SO₄²⁻ anions feature different connective modes. These units are extended into one-dimensional ribbons fused together by oxalate ligands to form the two 3D open frameworks. The structures of CKMOF-5a (Nd·2) and -5b (Eu·3, Gd·4) comprise networks of c-glide-arranged [Ln₂(SO₄)₂(H₂O)₂]²⁺ units, in which the oxalate ligands are encapsulated within eight-membered rings. The networks are supported by distinct sites of oxalate ligands to form the two types of 3D open frameworks, the structural topologies of which are distinguished by the versatile connective modes of the SO₄²⁻ anions. The polymeric phases CKMOF-4b and -5b exist in the Gd analogues. For the Dy analogues, varying the size of the cation used as the template led to the selective precipitation of each of these two phases. The layer structure of [Gd(SO₄)(H₂O)₂(CH₃CO₂)] (Gd·10), labelled CKMOF-6, assembled from [Gd₂(SO₄)₂(H₂O)₄]²⁺ units and acetate ligands; the geometries of the [Gd₂O₂] rhombic units may induce ferromagnetic coupling at low temperature. Eu·3 and Tb·6 were luminescent, displaying red and green emissions, respectively.