Transformation of a Mother Crystal to a Daughter Crystal through Amorphous Phase: De-assembly of Coordination Helices upon Heating and Re-assembly through Aquation

The nonlinear optical active chiral
complex 1 [{Co­(2,5-pdc)­(H2O)2}­H2O]n (2,5-pdc = 2,5-pyridine dicarboxylate)
has been synthesized via
a solvothermal technique using the achiral 2,5-pdc ligand. Complex 1 (phase 1), a two-dimensional coordination polymer,
undergoes crystalline to amorphous (phase 2) transformation
upon deaquation, which under reaquation generates a new microcrystalline
phase (phase 3). The crystal structure of phase 3 has been determined by powder X-ray diffraction analysis
(PXRD), which reveals that the resultant microcrystalline phase 3 is an achiral complex consisting of one-dimensional coordination
chains. Phase 3 undergoes reversible structural transformation
via amorphous phase (phase 2) upon dehydration and subsequent
rehydration. This amorphous phase shows selective adsorption of water
from a water–DMF mixture and water–CCl4 mixture.
Phase 1 to phase 3 structural transformation
proceeds through selective bond breaking. The magnetic studies of
the two crystalline and the amorphous phase reveal that phase 1 behaves as a canted antiferromagnet, while both amorphous
phase 2 and phase 3 show antiferromagnetism.