Synthesis and structural characterisation of lanthanide coordination polymers featuring 4,4′,6,6′-tetra-carboxy-2,2′-bipyridine and rare network topology

Abstract

Reported herein is a synthetic and structural study of the coordination chemistry of 4,4′,6,6′-tetracarboxylic acid-2,2′-bipyridine (H₄tcbp) with some lanthanide ions. The reaction of H₄tcbp under acidic hydrothermal conditions with Ln(III) (Ln = La, Eu and Gd) in 1 : 1 stoichiometric ratio led to the formation of crystalline {[La(Htcbp)(H₂O)₃]·H₂O}, 1, {[La(Htcbp)(H₂O)]}, 2, {[Eu(Htcbp)(H₂O)]·2 H₂O}, 3 and {[Gd(Htcbp)(H₂O)]·2 H₂O}, 4; which were structurally characterised. Using a 1 : 1 H₂O/DMF-solution in combination with Eu(III) and H₄tcbp gave {[Eu(tcbp)(H₂O)]·(Me₂NH₂)}, 5, which was also structurally characterised. The acidic conditions in 1–4 yielded 1 : 1 neutral {Ln : Htcbp} complexes whereas in 5 the H₄tcbp ligand is fully deprotonated and charge neutrality is brought about by the presence of a dimethylammonium cation (arising through the hydrolysis of the DMF solvent). Complex 1 forms a 1D coordination polymer which cross-links through extensive hydrogen bonding to form a complicated 3D network. Each {Ln(Htcbp)} moiety in 2 acts as a four-connecting node and forms a 2D sheet with (4,4)-topology, which is different to the 2D sheets with (4,4)-topology formed in isostructural 3 and 4. Within 5 each [Eu(tcbp)]⁻ moiety acts as a 5-connecting node forming a heretofore unprecedented 2D 5-connected network with (3³.4³.5⁴) topology. Furthermore, each 2D sheet in 2–5 features an array of hydrogen bonded contacts both within and between neighbouring sheets to give complicated 3D networks.