Complex three-dimensional lanthanide metal–organic frameworks with variable coordination spheres based on pyrazine-2,3,5,6-tetracarboxylate

Abstract

Metal–organic frameworks {[Ln₄(pztc)₃(H₂O)₁₁]·10(H₂O)}_n (Ln = Gd(1), Tb(2); pztc = pyrazine-2,3,5,6-tetracarboxylate) containing variable coordination spheres and with a complex and unusual three dimensional structure, were synthesized by the reaction of H₄pztc with the respective Ln(III) salt in water under hydrothermal conditions. The compounds were characterized by single crystal X-ray crystallography, elemental and thermal analysis, and FTIR spectroscopy. The asymmetric units in these compounds have four symmetry-independent Ln(III) ions and these are octa- and nona-coordinate centers, with irregular coordination polyhedra from [Ln(pztc)₂(H₂O)₆], [Ln(pztc)₂(H₂O)₄], [Ln(pztc)₃(H₂O)₃], [Ln(pztc)₃(H₂O)], and [Ln(pztc)₄] cluster units. The fully deprotonated ligand, pztc, coordinates to the Ln³⁺ ions through seven or through ten of its atoms (i.e., the maximum coordination number for this ligand). The three-dimensional open framework contains irregular channels along the [001] crystallographic direction. The channels are approximately 12 Å wide at their largest dimension and contain strongly hydrogen bonded water molecules of crystallization which further stabilize the structure. The solvent accessible volume is 20% of the total volume. The structures exhibit magnetic behavior that is characteristic of the respective isolated paramagnetic lanthanide ions.