Metal coordination polymers and organic co-crystals of polybenzoic acids and bipyridines

The background to this study is presented in Chapter 1. The results described in this thesis have been divided into three parts. Chapter 2 is concerned with metal coordination polymers constructed by using bi or tetra-functional polybenzoic acids—terephthalic acid, pyromellitic acid and 2,6 ′-naphthelenedicarboxylic acid. A wide variety of 1-D, 2-D and 3-D polymer frameworks can be formed using these ligands, dependent upon the metal ion, the ligand and the synthetic conditions. The degree of ancillary ligation by water on the metal centers is found to be a key factor controlling framework dimensionality.; Several 3-D open-framework polymers were formed. The most promising of these is [Cu₅(1245)₂(OH)₂(H₂O)(DMF) ₂]_n. The framework of this is stable to solvent exchange in boiling water and also allows ion-exchange of the pendant Cu cations.; Considerably more compositional and structural variety can be introduced by using more than one multi-functional ligand. Chapter 3 describes a number of multi-component metal coordination polymers constructed by various bipyridines, in addition to the polybenzoic acids. A detailed examination of synthetic variables affecting framework architecture, such as temperature, time of heating, solvent, concentration of the starting reagents and the source of metal ions have been explored for several systems. Variation of hydrothermal conditions in the zinc, bipyridine and pyromellitic acid system allows control of kinetic and thermodynamic products. Lower ancillary ligation and higher framework dimensionality are found under more forcing conditions. The ultimate product [Zn₂(4,4′-bipy)(1245)]_n.nH ₂O is a 3-D framework polymer. Solvothermal conditions have been found to produce novel REDOX polymers, such as [Cu₄(4,4′ -bipy)₂(14)₃], which has been shown to have delocalized mixed-valence Cu(I)/Cu(II) dimer units.; During hydrothermal synthesis of the metal coordination polymers, co-crystals of the organic acids and bases were isolated as side-products. Chapter 4 describes the advantages of the hydrothermal method for the production of such mixed crystals. High yield, phase purity and crystal size can be achieved along with thermodynamic control of the co-crystal stoichiometry. For example, using 4,4′-bipyridine and pyromellitic acid, co-crystals of 2:1, 1:1 and 1:2 stoichiometry can be formed. These show variable degrees of proton transfer between the aromatic acid and base. The structures possess N-H…O, N-H-O and N…H-O hydrogen bonds. In the 2:1 salt, [4,4 ′-bipy-H]₂[1245-H₃] an extremely short N-H-O single well hydrogen bond of 2.53Å is found. Neutron diffraction studies show the proton to be located near the mid-point of the hetero-atoms at 200K. After cooling to 20K, it is 0.05Å closer to N and at room temperature its position is ca.0.05Å closer to the oxygen. The variable NHO solid state ‘tautomerism’ found in these salts can be attributed to changes in the solid state pKa of the acid groups. This is affected by the twist angle of the COO with respect to the aromatic plane.