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

Abstract

A B S T R A C T : T h e 3 D c r y s t a l s t r u c t u r e o f {Ni 2 (H 2 O) 2 (Bpa) 2 }(V 6 O 17 ) (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-(H 2 O)(Bpa)} metal−organic and (V 3 O 8.5 ) vanadate chains. The connectivity of these one-dimensional units generates a three-dimensional inorganic substructure, and also a threedimensional 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, {Ni 2 (Bpa) 2 }(V 6 O 17 ) (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 VO 4 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 VO 4 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.

Water-induced phase transformation of a CuII coordination framework with pyridine-2,5-dicarboxylate and di-2-pyridyl ketone: synchrotron radiation analysis

“…Crystal engineering of solid coordination frameworks (SCFs) [1][2][3][4] has received a lot of interest during the past decade, not only for their potential applications as functional materials in gas storage, [5][6][7] gas separation, [8][9][10] heterogeneous catalysis, [11][12][13] drug delivery, [14][15][16] chemical sensing, 17,18 nonlinear optics 19 and biomedical imaging, 20,21 but also for their fascinating variety of structural topologies and flexible response to external stimuli. 22,23 Thus, many interesting solid-state transformations 24 of SCFs can be induced by light, heat, guest removal, uptake or exchange, expansion or reduction of the coordination sphere, oxidation of metal centers, condensation, or reactions between the ligands. 25,26 Due to changes in the coordination sphere, it is worth noting that removal/uptake of a ligand can induce local distortions that can extend along the whole framework.…”

Hybrid vanadates constructed from extended metal–organic arrays: crystal architectures and properties

“…In {Ni 2 IJH 2 O) 2 IJBpa) 2 }IJV 6 O 17 ), 59 the Ni(II) octahedra are directly coordinated to the Bpa ligands, generating {NiBpa} n chains parallel to the vanadium oxide chains. Fig.…”

“…27(b)). 59 The structural transformation also involves changes in the cell parameters and the reorientation of the Bpa ligand and the {V 4 O 12 } cycles as well as the {V 2 O 7 } dimers that form the vanadate chains (see Fig. 12, section 3.1).…”

“…The rational design of the crystal structures and more specifically the rational choice of initial structural building blocks with enough degree of freedom to establish new weak interactions and/or bonds are essential tasks to avoid crystalline to pseudoamorphous transformations after the loss of coordinated species. Different possible strategies can be adopted, such as: (i) the synthesis of crystal structures containing non-rigid inorganic substructures formed by corner-sharing polyhedra (for more information about the hydrothermal synthesis of HV, the reader is referred to the studies by Fernández de Luis et al, 59,89 Livaje et al 90 and Chirayil et al); 91 (ii) the choice of secondary metal centres that could adopt different coordination environments, such as Co(II) and Cu(II); and (iii) the use of flexible organic ligands that not only could adopt different conformations, but also do not incorporate extra donor atoms that could complete the coordination sphere of the secondary metal centres.…”

“…(b) Connectivity between the {V 4 O 12 } cycles and {V 2 O 7 } dimers and the metal-organic chains in {Ni 2 (H 2 O) 2 (Bpa) 2 }(V 6 O 17). The Bpa ligands have been simplified as lines 59.…”