Reversible Solid-State Transformation in {Ni2(H2O)2(Bpa)2}(V6O17) Proved by Synchrotron Radiation: Color and Magnetic Properties Change

Abstract

The 3D crystal structure of {Ni2(H2O)2(Bpa)2}(V6O17) (NiBpaRT), were Bpa is 1,2-bis(4-pyridil)ethane, was determined by single crystal X-ray diffraction. The crystal structure is constructed from {Ni(H2O)(Bpa)} metal–organic and (V3O8.5) vanadate chains. The connectivity of these one-dimensional units generates a three-dimensional inorganic substructure, and also a three-dimensional inorganic–organic framework. The asymmetric unit contains two crystallographically pseudoequivalent vanadate and metal–organic chains. The stacking of these crystallographically pseudoequivalent units seems to be nearly related to the twinning law of the crystals, and with the diffuse scattering observed in the diffraction pictures. Several models of local disorders for the packing of these one-dimensional units have been proposed to explain the origin of the diffuse scattering observed in the diffraction images. The obtained single crystals are systematically twinned. The origin of this twining is clearly related to the packing of the crystallographically independent chains along the [100] direction. The topology of the crystal structure is a new self-catenated three nodal net. The simplified structure can be described also as the natural tiling of two different tiles. NiBpaRT shows a reversible solid state transformation due to the loss of coordinated water molecules at 180 °C. The high temperature compound, {Ni2(Bpa)2}(V6O17) (NiBpaHT), maintains the crystallinity, but the solid state transformation involves a single crystal to polycrystalline reaction. So, the determination of the high temperature structure have been carried out by rigid body Rietveld refinement of the room temperature crystal structure, from synchrotron X-ray diffraction radiation. The loss of coordinated water molecules of the nickel cations is compensated by the incorporation of the terminal oxygen atoms, belonging to the adjacent VO4 tetrahedra, into the coordination environment of the nickel cations. We have referred to this mechanism of reorganization of the structural units as padlock solid state transformation. The UV–vis spectra corroborate the presence of octahedrally coordinated nickel cations in the high temperature crystal structure. The IR and Raman spectra show strong changes in the absorption maxima related to the stretching vibration of V═O terminal bonds, in good agreement with the proposed padlock mechanism. The dimeric ferromagnetic coupling of the nickel cations through the VO4 tetrahedra is similar for NiBpaRT and NiBpaHT, showing also an antiferromagnetic coupling at low temperatures. However, the distortion of the nickel coordination environment during the transformation implies an important difference in the g values for NiBpaRT and NiBpaHT.