Metal–organic coordination polymers based on a flexible tetrahydrofuran-2,3,4,5-tetracarboxylate ligand: syntheses, crystal structures, and magnetic/photoluminescent properties

Abstract

The coordination possibilities of flexible heteroalicyclic ligands with transition metals have been explored. Five Cu^II, Mn^II, and Cd^II coordination polymers with the fully deprotonated tetrahydrofuran-2,3,4,5-tetracarboxylate (L) ligand were synthesized and characterized, sometimes incorporating different auxiliary ligands: {[Cu₂(L)(H₂O)](H₂O)_1.42}_∞ (1), {[Cu₂(L)(2bpy)(H₂O)₂](H₂O)₂}_∞ (2), {[Cu₂(L)(phen)(H₂O)₂](H₂O)_1.5(CH₃OH)_0.25}_∞ (3), {[Mn₂(L)(phen)₂(H₂O)](H₂O)₂}_∞ (4), and {[Cd₂(L)(phen)₂(H₂O)](H₂O)₂}_∞ (5) (2bpy = 2,2′-bipyridine and phen = 1,10-phenanthroline). The structural analysis related to 1–5 reveals that the fully deprotonated L ligand displays versatile coordination modes, which result in the generation of a fairly complicated three dimensional (3D) framework for 1 with a Schläfli symbol of (4³·6⁷)(4⁶·6⁴), two isostructural two dimensional (2D) puckered layers for 2 and 3 with the decorated binodal (3,5)-connected (3·5²)₂(3²·5³·6⁴·7) topology, and two isostructural one dimensional (1D) ribbon-like chains for 4 and 5, respectively. The present work indicates that, in comparison with the rigid aromatic multicarboxylate ligands, tetrahydrofuran-2,3,4,5-tetracarboxylate (L), a heteroalicyclic multicarboxylate ligand, has richer coordination modes and a more flexible ring skeleton, which may remain largely unexpected in the process of constructing metal–organic frameworks (MOFs) and make it more difficult to predict and control the final coordination polymers. Magnetic susceptibility measurements demonstrate that complexes 1 and 4 exhibit antiferromagnetic coupling, whereas 2 and 3 show ferromagnetic coupling, with the corresponding J values of −23.62 cm⁻¹ for 1, 5.18 cm⁻¹ for 2, 3.90 cm⁻¹ for 3, and −0.79 cm⁻¹ for 4. Moreover, complex 5 displays solid blue emission originating from an intraligand π → π* transition of the auxiliary phen ligand.