Lanthanide-based F-MOFs: Structure, hydrolytic stability, spectral and magnetic properties

A series of five novel lanthanide-based fluorinated metal-organic frameworks (Ln-F-MOFs) have been synthesized under solvothermal conditions by the reaction of 3,3 '-bis(trifluoromethyl)-[1,1 '-biphenyl]-4,4 '-dicarboxylic acid (H2L) and lanthanide Ln(III) ions (Ln(III)-La, Ce, Pr, Nd and Eu). Powder X-ray diffraction (PXRD) analysis revealed that all prepared complexes are isostructural. A single-crystal X-ray crystallographic study of one representative of the isostructural group, namely {[La-2(L)(3)(DMF)(2)(H2O)(2)]}(n) showed, that compound crystallizes in a triclinic system with space group P-1. The lattice parameter values are a = 8.563(2) angstrom, b = 13.199(3) angstrom, c = 16.008(4) angstrom, alpha = 104.588(7) degrees, beta = 92.904(7) degrees and gamma = 92.717(7) degrees, with two formula units in the unit cell. The overall structure is formed by 2D polymeric layers, which are arranged into a semi-3D supramolecular structure through hydrogen bonds and other intramolecular interactions. The study of the hydrophobic properties of the complexes showed that the complexes exhibit surface hydrophobicity with a "rose petal effect" and a contact angle of approximately 107 degrees. However, the structures are not hydrolytically stable in the long term and the structure starts to delaminate after two days in water. This is a manifestation of the fact that the complexes do not form a 3D polymer network, it is made up of 2D layers connected only by weak non-bonding interactions. The photoluminescence properties of the Ln(III) complexes are determined by the characteristic 5d-4f or 4f-4f electron transitions for the individual lanthanide ions. The magnetic properties of Nd(III), Pr(III) and Eu(III) variants were studied. The magnetic properties of {[Pr-2(L)(3)(DMF)(2)(H2O)(2)]}(n) are characterized by the presence of a low-lying quasi-doublet with 15.6 cm(-1) energy splitting, whereas Eu(III) variant is nonmagnetic at low temperatures, but the magnetic F-7(1) state is accessible by thermal excitation. For Nd(III) complex, the X-band EPR measurements were performed. Since 1D channels with dimensions of 4.90 x 7.23 angstrom(2) are present within the structure, the adsorption of N-2, CO2 and H-2 gases was also studied.