The ligand, the metal and the ‘Holey’-host: Synthesis, structural and magnetic characterisation of Co(ii), Ni(ii) and Mn(ii) metal–organic frameworks incorporating 4,4′-dicarboxy-2,2′-bipyridine

Abstract

We report herein the single crystal structures of four metal–organic framework complexes incorporating the 4,4′-dicarboxy-2,2′-bipyridine ligand, H₂dcbp: α-[Co(dcbp)(H₂O)₂], 1; β-[Co(dcbp)(H₂O)₂], 2, [Ni(dcbp)(H₂O)₂], 3 and {[Mn(dcbp)]·½DEF}, 4 (DEF = diethylformamide). In each complex the ligand is deprotonated giving neutral species with 1 ∶ 1 stoichiometry that form three-dimensional coordination polymers. Supramolecular isomerism (polymorphism) in 1 and 2 arises from the different ligand connectivity around the octahedral Co(II) centres. The two coordinated water molecules in 1 occupy cis positions, which are trans to the chelating bipyridine nitrogen atoms, leaving the carboxylate oxygen atoms in axial trans positions. In 2 all like donors occupy cis positions. Different modes of carboxylate coordination in 1 and 2 give dissimilar network topologies. A rare example of two interpenetrating 6⁴8²-b (quartz-like) chiral networks in 1 results from both dcbp carboxylate groups coordinating in a monodentate fashion to adjacent Co(II) centres, whereas in 2 only one carboxylate group bridges between adjacent Co(II) centres giving rise to a single chiral (10,3)-a net. In 1 and 2 the coordinated water molecules hydrogen bond to the non-coordinated carboxylate oxygen atoms. These interactions give rise to water–carboxylate double helices in 1, and support the coordination network in 2. Strikingly for a pair of dimorphs the crystal densities of 1 and 2 differ by ca. 0.3 g cm⁻³ (1.654 vs. 1.940 g cm⁻³, respectively). Compound 3 is isomorphous with 1 and likewise features two chiral interpenetrating nets of quartz topology. In 4, chelating bipyridine nitrogen atoms and four carboxylate oxygen atoms from a total of five adjacent dcbp ligands provide distorted octahedral geometry around Mn(II). The carboxylate groups bridge adjacent Mn(II) centres to produce bis-carboxylato chains which cross-link and generate a 3D network that is perforated with channels. The channels are occupied with disordered DEF molecules. The network topology in 4 is quite different to 1–3 and has a (4.6²)(4².6)(4³.6⁶.8⁶) Schläfli notation. Magnetic susceptibility studies performed on 1, 2, {[Mn(dcbp)]·½DMF} 5 (DMF = dimethylformamide) and {[Mn(dcbp)]·2H₂O} 6 reveal very weak antiferromagnetic coupling between the metal centres in each case.