Solvent-dependent selective cation exchange in anionic frameworks based on cobalt(ii) and triphenylamine linkers: reactor-dependent synthesis and sorption properties

Abstract

Two cobalt(II) coordination polymers with anionic networks of formulae {(Me₂NH₂)₂[CoCl(ntb)]}_n (JUMP-2) and {(Me₂NH₂)₂[Co₅(ntb)₄(H₂O)₃(Me₂NH)]}_n (previously reported as MIL-144 by Livage et al., Microporous Mesoporous Mater., 2012, 157, 37) have been obtained via a solvothermal reaction of cobalt chloride and 4,4′,4′′-nitrilotribenzoic acid (H₃ntb) in DMF employing two differently-sized reactors, while using the same absolute amount of reactants. Structure analysis revealed that JUMP-2 crystallized in the monoclinic space group P2₁/n and displays a two-dimensional (2D) network, which by topological analysis was characterized as a layered 3-connected hcb net. The topological analysis of MIL-144 revealed a 3,6-connected net with 3,6T80 topology. The magnetic properties of JUMP-2 are indicative of independent single-ion behavior of the tetrahedral cobalt(II) ions and showed an out-of-phase signal in the alternating-current (ac) magnetic susceptibility below 2.5 K, whereas for MIL-144 an overall antiferromagnetic interaction within the di- and trinuclear secondary building units is observed and no indication for slow magnetization dynamics. The organic cations in both frameworks could successfully be exchanged with inorganic cations under retention of the respective network structure. In the process of exchange, both compounds displayed cation selectivity based on which solvent was utilized for immersing the solids. JUMP-2 shows a preference for europium(III) ions in DMF, whereas MIL-144 preferentially takes up lithium ions when ethanol is used. The N₂ adsorption isotherms were measured before and after exchange and revealed a considerable improvement in the sorption properties of the exchanged samples.