Thermal Response, Catalytic Activity, and Color Change of the First Hybrid Vanadate Containing Bpe Guest Molecules

Four isomorphic compounds with formula [{Co2(H2O)2(Bpe)2}­(V4O12)]­·4H2O·Bpe, CoBpe 1; [{CoNi­(H2O)2(Bpe)2}­(V4O12)]­·4H2O·Bpe, CoNiBpe 2; [{Co0.6Ni1.4(H2O)2(Bpe)2}­(V4O12)]­·4H2O·Bpe, NiCoBpe 3; and [{Ni2(H2O)2(Bpe)2}­(V4O12)]­·4H2O·Bpe, NiBpe 4, have
been obtained by hydrothermal synthesis. The crystal structures of
CoBpe 1 and NiBpe 4 were determined by single-crystal
X-ray diffraction (XRD). The Rietveld refinement of CoNiBpe 2 and NiCoBpe 3 XRD patterns confirms that those
are isomorphic. The compounds crystallize in the P1̅ space group, exhibiting a crystal structure constructed
from inorganic layers pillared by Bpe ligands. The crystal structure
contains intralayer and interlayer channels, in which the crystallization
water molecules and Bpe guest molecules, respectively, are located.
The solvent molecules establish a hydrogen bonding network with the
coordinated water molecules. Thermodiffractometric and thermogravimetric
studies showed that the loss of crystallization and coordinated water
molecules takes place at different temperatures, giving rise to crystal
structure transformations that involve important reduction of the
interlayer distance, and strong reduction of crystallinity. The IR,
Raman, and UV–vis spectra of the as-synthesized and heated
compounds confirm that the structural building blocks and octahedral
coordination environment of the metal centers are maintained after
the structural transformations. The color change and reversibility
of the water molecules uptake/removal were tested showing that the
initial color is not completely recovered when the compounds are heated
at temperatures higher than 200 °C. The thermal evolution of
the magnetic susceptibility indicates one-dimensional antiferromagnetic
coupling of the metal centers at high temperatures. For NiCoBpe 3 and NiBpe 4 compounds magnetic ordering is
established at low temperatures, as can be judged by the maxima observed
in the magnetic susceptibilities. CoNiBpe 2 was proved
as catalyst being active for cyanosilylation reactions of aldehydes.