Modulating the M~(II)/Pyrimidine-4,6-dicarboxylato System by Metal, Solvent and Temperature Variation

摘要

We report herein the synthesis and physicochemical characterisation of seven new compounds employing divalent metal ions and pyrimidine-4,6-dicarboxylato ligand (pmdc): {[Cd(μ-pmdc)(H_2O)_3]·2H_2O}_n (1), [Cd(phen)(μ_3-pmdc)]_n (2), [Cd(μ_5-pmdc)]_n (3), {[Ca(μ_3-pmdc)(H_2O)_3]·H_2O}_n (4), {[Mn(μ-pmdc)-(form)(H_2O)]·2H_2O}_n (5), [Cu(μ-pmdc)(DMSO)_2]_n (6), and {[Zn(μ-pmdc)(DMF)(H_2O)]·DMF}_n (7) (where phen = 1,10-phenanthroline, form = formamide, DMSO = dimethyl sulfoxide, DMF = dimethylformamide). These compounds are the result of three different approaches that share a common aim: to replace/eliminate coordination solvent molecules to force unusual coordination modes of pmdc that render MII-pmdc polymers with different structural motifs. To this end, we have employed either metal ions that permit high coordination numbers or organic aprotic solvents that can also act as terminal ligands, while different thermal treatments have been applied to both the reaction mixtures as well as solid compounds. The crystal structure of compound 1 contains zig-zag polymeric chains in which mer-[Cd(H_2O)_3]~(2+) units are sequentially joined by means of bis-chelating pmdc ligands. Compound 2 is made up of stacked neutral layers that are built up from the fusion of two independent centrosymmetric [Cd_2(phen)_2(μ_3-pmdc)_2] dinuclear units. Hydrothermal treatment at higher temperature led to compound 3, which presents a three-dimensional framework in which the pmdc ligand exhibits an unprecedented μ_5-κ~2N,O:κ~2N',O':κO': κO'':κO''' coordination mode. Moreover, thermogravimetric and thermodiffractometric analyses of 1 have also provided access to an alternative route to obtain higher dimensional frameworks with crystal structures that differ from those crystallised in dissolution. Compound 4 consists of neutral 2D [Ca(μ_3-pmdc)(H_2O)_3] sheets that are formed by the junction of calcium-pmdc chains through carboxylate oxygen bridges, exhibiting a herringbone pattern. On the other hand, formamide, dimethyl sulfoxide, and dimethylformamide have been employed in compounds 5-7 to evaluate their effect on the resulting coordination polymer. A common feature in these compounds is the one-dimensional structure displayed and the fact that at least one solvent molecule is coordinated to the metal atoms. Whereas the manganese and zinc compounds exhibit corrugated M~(II)-pmdc chains, the copper compound adopts the more typical planar arrangement. The disposition of the chains of compounds 5-7 has been rationalised on the basis of the steric hindrance imposed by the coordination of the ligands, which determines the more stable geometrical isomer in each case.