Single-Crystal to Single-Crystal Reversible Transformations Induced by Thermal Dehydration in Keggin-Type Polyoxometalates Decorated with Copper(II)-Picolinate Complexes: The Structure Directing Role of Guanidinium

摘要

Three new hybrid inorganic-metalorganic compounds containing Keggin-type polyoxometalates, neutral copper(II)-picolinate complexes and guanidinium cations have been synthesized in bench conditions and characterized by elemental analysis, infrared spectroscopy and single-crystal X-ray diffraction: the isostructural [C(NH₂)₃]₄[{XW₁₂O₄₀}{Cu₂(pic)₄}]·[Cu₂(pic)₄(H₂O)]₂·6H₂O [X = Si (1), Ge (3)] and [C(NH₂)₃]₈[{SiW₁₂O₄₀}₂{Cu(pic)₂}₃{Cu₂(pic)₄(H₂O)}₂]·8H₂O (2). The three compounds show a pronounced two-dimensional character owing to the structure-directing role of guanidinium. In 1 and 3, layers of [{XW₁₂O₄₀}{Cu₂(pic)₄}]_n4n− hybrid POM chains and layers of [Cu₂(pic)₄(H₂O)] complexes and [C(NH₂)₃]+ cations pack alternately along the z axis. The hydrogen-bonding network established by guanidinium leads to a trihexagonal tiling arrangement of all copper(II)-picolinate species. In contrast, layers of [C(NH₂)₃]+-linked [{SiW₁₂O₄₀}₂{Cu(pic)₂}₃]_n8n− double chains where each Keggin cluster displays a {Cu₂(pic)₄(H₂O)} moiety pointing at the intralamellar space are observed in 2. The thermal stability of 1–3 has been studied by thermogravimetric analyses and variable temperature powder X-ray diffraction. Compounds 1 and 3 undergo single-crystal to single-crystal transformations promoted by reversible dehydration processes and the structures of the corresponding anhydrous phases 1a and 3a have been established. Despite the fact that the [Cu₂(pic)₄(H₂O)] dimeric complexes split into [Cu(pic)₂] monomers upon dehydration, the packing remains almost unaltered thanks to the preservation of the hydrogen-bonding network established by guanidinium and its associated Kagome-type lattice. Splitting of the dimeric complexes has been correlated with the electron paramagnetic resonance spectra.