Same Not the Same: Thermally Driven Transformation of Nickel Phosphinate-Bipyridine One-Dimensional Chains into Three-Dimensional Coordination Polymers

Three one-dimensional nickel coordination polymers (CPs) based on P,P′-diphenylethylenediphosphinic acid and three different bis-pyridine coligands, namely, 4,4′-bipyridine (bipy), 1,2-bis(4-pyridyl)ethane (bpy-ane), and 1,2-bis(4-pyridyl)ethylene (bpy-ene), were prepared in mild hydrothermal conditions from water solutions containing the dissolved reagents. The CPs have the formula [Ni(H2O)4(bipy)·pc2p]n (1), [Ni(H2O)4(bpy-ane)·pc2p]n (2), and [Ni(H2O)4(bpy-ene)·pc2p]n (3), and their structural features were investigated by single crystal X-ray diffraction, UV-vis, Fourier transform infrared spectroscopies, and magnetic measurements. They are constituted of infinite Ni(H2O)4(bis-pyridine) one-dimensional rows connected, through hydrogen bonds, with the phosphinic acids placed among adjacent rows. Although the formulas and the structural topologies of the three compounds are almost identical, they behave in different manners upon heating. Compound 1 yields an amorphous phase when water molecules are thermally removed, whereas compound 3 undergoes interesting phase transformations derived from the connection of Ni atoms with the phosphinates oxygen atoms, increasing the dimensionality to three-dimensional and maintaining crystallinity. The behavior of compound 2 has some analogies to that of 3, although a complete structural characterization was not performed because of a significant crystallinity loss of the heated phase. The structural features were studied by means of a combination of variable temperature single crystal and powder X-ray diffraction and thermogravimetric analysis. The reason for these different behaviors was ascribed to both the length and the flexibility degree of the nitrogenated coligands.