Nanoporous Cobalt(Ⅱ) MOF Exhibiting Four Magnetic Ground States and Changes in Gas Sorption upon Post-Synthetic Modification

摘要

We present the syntheses, structural characterization, gas sorption, I_2 uptake, and magnetic properties of a double-walled porous metal-organic framework, [Co~Ⅱ_3(lac)_2(pybz)_2]·3DMF (1·3DMF, purple, where pybz = 4-pyridyl benzoate, lac = D- and L-lactate) and of its post-synthetic modified (PSM) congeners, [Co~Ⅱ_3(lac)_2(pybz)_2]·xGuest (xGuest = 6MeOH, purple; 4.5EtOH, purple; 3PrOH, purple; 2C_6H_6, purple; 2.7I_2, black), [Co~Ⅱ_3(lac)_2(pybz)_2] (1, purple), [Co~Ⅱ_3(pybz)_2(lac)_2(H_2O)_2]·7H_2O (la·7H_2O, green), and [Co~ⅢCo~Ⅱ_2(pybz)_2(lac)_2(H_2O)_2]I·2H_2O·1.5DMSO (1b·I~-·2H_2O·1.5DMSO, yellow, DMSO = dimethyl sulfoxide). Crystallography shows that the framework is not altered by the replacement of DMF by different solvents or by the removal of the solvent molecules during the single-crystal to single-crystal (SC-SC) transformations, while upon exchange with H_2O or partial oxidation by molecular iodine, the crystallinity is affected. 1 absorbs N_2, H_2, CH_4, CH_3OH, C_2H_5OH, PrOH, C_6H_6, and I_2, but once it is in contact with H_2O the absorption efficiency is drastically reduced. Upon PSM, the magnetism is transformed from a canted antiferromagnet (1·3DMF and 1·xGuest) to single-chain magnet (1), to a ferrimagnet (la·7H_2O), and to a ferromagnet (1b·I~-2H_2O·1.5DMSO). Raman spectroscopy suggests the color change (purple to green la·7H_2O or yellow 1b·I~-·2H_2O·1.5DMSO) is associated with a change of geometry from a strained octahedron due to the very acute chelating angle (～60°) of the lactate of a cobalt center to a regular octahedron with a monodentate carboxylate and one H_2O. The magnetic transformation is explained by the different interchain exchanges (J'), antiferromagnetic for 1·3DMF and 1·xSolvent (J' ＜ 0), SCM for 1 (J' verge to 0), and ferromagnetic for la·7H_2O (J' ＞ 0), between homometal topological ferrimagnetic chains (two octahedral and one tetrahedral Co~Ⅱ ions) connected by the double walls of pybz at 13.3 A (shortest Co…Co). For 1b·I~-·2H_2O·1.5DMSO the moment of the tetrahedral site is turned off, thus stabilizing a ferromagnetic state (J' ＞ 0). The present stabilization of four magnetic ground states is unique in the field of metal-organic frameworks as well as the electrical conductivity of 1·2.7I_2.