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Coordination Frameworks Containing Magnetic Single Chain of Imidazoledicarboxylate-Bridged Cobalt(II)/Nickel(II): Syntheses, Structures, and Magnetic Properties

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journal contribution
posted on 14.05.2018, 19:20 by Wenbo Wang, Ruiying Wang, Lina Liu, Benlai Wu
Five new magnetic coordination frameworks with formulas of {[Co2(HL)2(H2O)4]·4H2O}n (1), {[Ni2(HL)2(H2O)4]·4H2O}n (2), [Co­(HL)­(bpy)]n (3), [Co­(HL)­(pbim)]n (4), and [Ni­(HL)­(pbim)]n (5) [H3L = 2-(5-bromo-pyridin-3-yl)-1H-imidazole-4,5-dicarboxylic acid, bpy = 4,4′-bipyridine, and pbim = 1,1′-(5-methyl-1,3-phenylene)­bis­(1H-imidazole)] have been synthesized and structurally characterized by elemental, thermogravimetric, and X-ray diffraction analyses. In all compounds, the doubly deprotonated (HL)2– ligands adopt the same μ-kN,O:kN′,O′ coordination mode, showing very interesting coordination orientation in the construction of those coordination polymers. Complexes 1 and 2 feature (HL)2–-bridged linear chain structures. Complex 3 is a 2D network with (4,4) grids built up from the (HL)2–-bridged linear chains observed in 1 being further linked by rod-like bpy bridges replacing the coordination of water molecules. Those almost or completely linear chains formed by the bis-N,O-chelation of the imidazoledicarboxylate of (HL)2– in 13 are rarely observed in the complexes of 4,5-imidazoledicarboxylic acid and its derivatives, giving a typical structure model to investigate the magnetic exchange-coupling mediated by imidazoledicarboxylate. Isostructural complexes 4 and 5 are 2D helicates where (HL)2–-bridged left- and right-handed helixes are doubly linked into the mesolayer structures with 63 topology through the further coordination of angular spacers pbim. Remarkably, the formation of (HL)2–-bridged linear chains or helical chains is mainly dependent on the trans- or cis-chelation of two (HL)2– ligands around the same metal center, which could be finely tuned by the structural features of auxiliary ligands used. Magnetostructural analyses disclose that the bis-N,O-chelating imidazoledicarboxylates of (HL)2– transmit antiferromagnetic interactions along the (HL)2–-bridged metal–organic chains with the spin-coupling constants of −4.89, −17.8,–6.55, −7.35, and −19.6 cm–1 for 15, respectively. Notably, the μ-kN,O:kN′,O′ coordination mode of (HL)2– ligand dominates the assembly of metal–organic frameworks as well as the magnetic exchange between the paramagnetic ions in those magnetic chains, but the magnetic exchange-coupling depends strongly on the nature of the metal center.