Reversible Dehydration in Polyoxometalate-Based Hybrid Compounds: A Study of Single-Crystal to Single-Crystal Transformations in Keggin-Type Germanotungstates Decorated with Copper(II) Complexes of Tetradentate N‑Donor Ligands

Hydrothermal reaction of the Keggin-type
[α-GeW11O39]8– anion
with copper­(II) complexes
of the tetradentate N-donor ligands N,N′-dimethyl-N,N′-bis-(pyridine-2-ylmethyl)-1,2-diaminoethane
(bpmen) or N,N′-dimethyl-N,N′-bis-(pyridine-2-ylmethyl)-1,3-diaminopropane
(bpmpn) results in three new hybrid compounds [Cu­(bpmen)­(H2O)]­[GeW12O40­{Cu­(bpmen)}]­·3.5H2O (1), [GeW12O40­{Cu­(bpmpn)­(H2O)}2]­·3H2O (2), and [Cu­(bpmpn)­(H2O)]­[GeW12O40­{Cu­(bpmpn)}]­·8H2O (3). All compounds have been characterized by elemental analyses and
infrared spectroscopy, and their structures have been established
by single-crystal X-ray diffraction. Both 1 and 3 contain monodecorated polyanions and additional complex
cations, but their layered structures show noticeable differences.
For 1, the polyanions are arranged in rows whose stacking
generates intralamellar cavities where complex cations and lattice
water molecules are hosted, whereas double-layers of hybrid anions
pillared by the cations occupying the interlamellar space are observed
for 3. In the case of 2, the packing of
trans-didecorated clusters generates honeycomb-like metalorganic layers
and perpendicular hexagonal channels where Keggin moieties are nested.
A combination of thermogravimetric and variable temperature powder
X-ray diffraction analyses show that dehydration of 1 and 2 results in thermally stable, crystalline phases.
Dehydrations are fully reversible upon exposure to air and proceed
via single-crystal to single-crystal transformations implying rupture/formation
processes in the copper­(II) bonding and consequent geometrical and
conformational modifications in the complexes. Transformations have
been followed by single-crystal X-ray diffraction, allowing for the
structures of the anhydrous phases [GeW12O40­{Cu­(bpmen)}2] (1a) and [GeW12O40­{Cu­(bpmpn)}2] (2a) to
be determined. Dehydration of 1 into 1a with
consequent formation of neutral, didecorated clusters does not result
in the collapse of the intralamellar cavities but rather in an open-framework
structure with slit-shaped micropores. Structural variations between 1 and 2 and their corresponding anhydrous phases
are reflected in the electron paramagnetic resonance spectra.