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Keywords:

  • Metal–organic frameworks;
  • Nickel;
  • Isomers;
  • Carboxylate ligands;
  • Adsorption

Abstract

Several metal–organic frameworks of nickel(II) were obtained starting from the molecular complex 1, [Ni(OH2)4(4-pyc)2] (4-pyc = 4-pyridinecarboxylate or isonicotinate). By using different reaction conditions, two polymorphs of the anhydrous complex [Ni(4-pyc)2]n (2a and 2b) and the polynuclear compound [Ni5(OH2)3(4-pyc)10]n (3) were obtained in very high yield. All of the compounds were characterized by elemental analysis and spectroscopic data. The crystal structure determination of compounds 2b and 3 was also carried out. Structural and topological analysis of these new structural types and other related ones were performed. It was found that compounds 2a and 2b are a rare case of coordination framework isomers. Thermogravimetric studies indicate that compounds 2b and 3 lose the pore-allocated solvent molecules at moderate temperatures without any significant structural change, and at higher temperatures, compounds 1 and 3 are transformed into 2a. Textural parameters that characterize the microporous networks for 2b and 3 were obtained by nitrogen-adsorption measurements. For both samples, the micropore size distribution indicates cylindrical pores with diameters of 0.85 (2b) and 0.90 nm (3). The values of Brunauer–Emmett–Teller and Langmuir surface areas are 178 and 185 m2 g–1 for compound 2b, and 995 and 982 m2 g–1 for compound 3. Micropore volumes are 0.065 and 0.350 cm3 g–1 for 2b and 3, respectively. The magnetic properties of compounds 1, 2a, and 2b are consistent with the presence of zero-field splitting and a small antiferromagnetic coupling between the metal atoms. The magnetic properties of the pentanuclear compound 3 were analyzed by considering one monomer with zero-field splitting caused by axially distorted NiII octahedral compounds, and two dimeric fragments with magnetic interactions within each dimetallic unit.