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

We present the syntheses,
structural characterization, gas sorption,
I2 uptake, and magnetic properties of a double-walled porous
metal–organic framework, [CoII3(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, [CoII3(lac)2(pybz)2]·xGuest (xGuest = 6MeOH, purple; 4.5EtOH, purple; 3PrOH, purple; 2C6H6, purple; 2.7I2, black), [CoII3(lac)2(pybz)2] (1,
purple), [CoII3(pybz)2(lac)2(H2O)2]·7H2O (1a·7H2O, green), and [CoIIICoII2(pybz)2(lac)2(H2O)2]­I·2H2O·1.5DMSO (1b·I–·2H2O·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 H2O or partial oxidation by molecular iodine, the crystallinity
is affected. 1 absorbs N2, H2,
CH4, CH3OH, C2H5OH, PrOH,
C6H6, and I2, but once it is in contact
with H2O 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
(1a·7H2O), and to a ferromagnet (1b·I–·2H2O·1.5DMSO). Raman spectroscopy suggests the color
change (purple to green 1a·7H2O or yellow 1b·I–·2H2O·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 H2O. 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 1a·7H2O
(J′ > 0), between homometal topological
ferrimagnetic
chains (two octahedral and one tetrahedral CoII ions) connected
by the double walls of pybz at 13.3 Å (shortest Co···Co).
For 1b·I–·2H2O·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 1·2.7I2.