Solvent-directed assembly in MOFs with linear trinuclear cobalt(<scp>ii</scp>) nodes: formation of inverted Figaro chains

Four new cobalt(II) coordination networks based on the nitrilotribenzoic acid ligand (H₃ntb) with the formula [Co₃(ntb)₂(solv)₂]_<i>n</i> (where solv = dmf, def, dma, and EtOH), denoted as JUMP-4(solv), are reported. The trinuclear cobalt(II) clusters that constitute the three-dimensional network are coordinated by the six carboxylate groups of two bridging, deprotonated ntb^3- ligands. These clusters have two flexible coordination sites on the two terminal cobalt ions, which are partially saturated by the condensation of these clusters, leading to unprecedented inverted Figaro chains. Three of the frameworks were synthesized by combining cobalt(II) chloride and the ligand in a mixture of acetonitrile and the corresponding amide (dmf, def, and dma) in a 9 : 1 ratio, while the ethanol analogue could only be obtained by single-crystal-to-single-crystal transformation starting from the dmf and dma derivatives. The difference in the size of the solvents coordinated to the terminal cobalt(II) ions results in a pore-partitioning effect, as reflected in the argon sorption behavior of the networks. Magnetic measurements reveal antiferromagnetic interactions between the anisotropic cobalt(II) ions along the alternating inverted Figaro chains.