Thermally induced interconversions of metal-pyrimidine-4,6-dicarboxylate polymers: a structural, spectroscopic, and...

Continuing our work on the structural and magnetic aspects of the one-dimensional (1-D) coordination polymers of the [M(pmdc)(H(2)O)(2)] x H(2)O kind (M = Fe, Co, Ni, Cu, Zn; pmdc = pyrimidine-4,6-dicarboxylate), we have combined ab initio X-ray powder diffraction methods with in situ thermodiffractometry and thermal analyses to characterize the selective and reversible transformation of the [M(pmdc)(H(2)O)(2)] x H(2)O compounds (M = Fe, Co, Ni, Cu) into the bis-hydrated [M(pmdc)(H(2)O)(2)] counterparts by moderate heating, which is followed by an irreversible transformation into two-dimensional (2-D) anhydrous species. The structural features of the transient bis-hydrated species and of the completely dehydrated one are described for M = Cu. Remarkably, the first dehydration process does not alter the 1-D nature of the [M(pmdc)(H(2)O)(2)] chains; on the contrary, the second dehydration gives rise to the loss of the axially coordinated water molecules with a concomitant condensation of the 1-D chains into 2-D layers through ancillary carboxylate bridging groups. The magnetic properties of the anhydrous [M(pmdc)] species (M = Co, Ni, Cu) have been investigated, showing that these phases behave as 1-D antiferromagnets with interchain interactions. Notably, in the case of the [Ni(pmdc)] system, a weak ferromagnetic ordering, arising from a spin canting phenomenon with a blocking temperature of 13 K, is observed.