Influence of π–π 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 α,α′-m-xylylenediphosphonic
and α,α′-p-xylylenediphosphonic
acid, either exclusively or combined with 4,4′-bipyridine as
an ancillary ligand. The compound obtained from α,α′-m-xylylenediphosphonate has the formula Cu­(HO3PCH2)2C6H4·3H2O 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 α,α′-m-xylylenediphosphonate
and 4,4′-bipyridine as a coligand has the formula Cu­[(HO3PCH2)2C6H4]­[N­(CH)4CC­(CH)4N]·2H2O 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 α,α′-p-xylylenediphosphonate and 4,4′-bipyridine 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 π–π
stacking interactions. With increasing temperatures, the compounds
revealed a loss of water molecules bound to copper; in particular,
heating the copper α,α′-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.