Influence of pi-pi Stacking Interactions on the Assembly of Layered Copper Phosphonate Coordination Polymers: Combined Powder Diffraction and Electron Paramagnetic Resonance Study

Three new copper phosphonate coordination polymers have been obtained following very soft synthetic routes, and their structures were determined ab initio from powder X-ray diffraction data. They are based on the two building blocks a, alpha'-m-xylylenediphosphonic and alpha,alpha'-p-xylylenediphosphonic acid, either exclusively or combined with 4,4'-bipyridine as an ancillary ligand. The compound obtained from alpha,alpha'-m-xylylenediphosphonate has the formula Cu(HO3PCH2)(2)C6H4 center dot 3H(2)O and a 2D structure of mixed organic inorganic layers built up by the connection of inorganic chains made of edge-sharing copper octahedra via organic bridging groups. The compound obtained from alpha,alpha'-m-xylylenediphosphonate and 4,4'-bipyridine as a coligand has the formula Cu[HO3PCH2)(2)C6H4][N(CH)(4)CC(CH)(4)N]center dot 2H(2)O and a 2D structure based on the connection, via the diphosphonic ligand, of the linear chains resulting from the bridging of copper atoms by the bipyridine moieties. The compound obtained using alpha,alpha'-p-xylylenediphosphonate and 4,4'-bipyricline as a coligand has the same formula and a 2D structure closely resembling that of the previous compound. The influence of the factors driving the molecular assembly is discussed, pointing out the crucial role played by the 4,4'-bipyridine in promoting the pi-pi stacking interactions. With increasing temperatures, the compounds revealed a loss of water molecules bound to copper; in particular, heating the copper alpha,alpha'-m-xylylenediphosphonate-bipyridine results in a relatively stable phase with unsaturated coordination sites on the metal atoms. This behavior was confirmed by magnetic susceptibility and electron paramagnetic resonance measurements which also provided evidence for a partial reversibility of the dehydration process and of the occurrence of weak antiferromagnetic interactions between Cu(II) ions.