Copper(II) Complexes of Tetradentate Pyridyl Ligands Supported on Keggin Polyoxometalates: Single-Crystal to Single-Crystal Transformations Promoted by Reversible Dehydration Processes

Two
new hybrid compounds constructed from Keggin type polyoxometalates
and copper­(II) complexes of tetradentate ligands containing amine
and pyridyl groups, namely [Cu­(bpmen)­(H2O)]­[SiW12O40{Cu­(bpmen)}] (1) and [SiW12O40{Cu­(bpmpn)­(H2O)}2]·3H2O (2) (bpmen, N,N′-dimethyl-N,N′-bis-(pyridin-2-ylmethyl)-1,2-diaminoethane;
bpmpn, N,N′-dimethyl-N,N′-bis­(pyridin-2-ylmethyl)-1,3-diaminopropane),
have been synthesized under hydrothermal conditions and characterized
by elemental analyses and infrared and Raman spectroscopy. Thermal
stability of 1 and 2 has been studied by
means of thermogravimetric analyses and variable temperature powder
X-ray diffraction. Both compounds undergo single-crystal to single-crystal
transformations promoted by reversible dehydration processes that
have been followed by single-crystal X-ray diffraction. Structures
of 1 and 2, and also of their corresponding
anhydrous phases 1a and 2a, have been established.
The layered structure of 1 shows rows of monodecorated
polyanions with complex cations occupying intralamellar spaces, whereas trans-didecorated species in 2 lead to stacked
honeycomb-like metal–organic layers forming channels where
Keggin clusters are accommodated. Structural differences relate to
changes in the complex geometry and ligand conformation when going
from bpmen to bpmpn. Dehydration of 1 promotes coordination
of the complex countercation and consequent formation of a cis-didecorated species in 1a, whereas changes
in the structure of 2a are more subtle. Structural variations
upon dehydration are reflected in the electron paramagnetic resonance
spectra.