Novel flexible frameworks of porous cobalt(II) coordination polymers that show selective guest adsorption based on the switching of hydrogen-bond pairs of amide groups.

Abstract

Four porous crystalline coordination polymers with two-dimensional frameworks of a double-edged axe-shaped motif, [[Co(NCS)(2)(3-pia)(2)] x 2 EtOH.11 H(2)O](n) (1 a), [[Co(NCS)(2)(3-pia)(2)] x 4 Me(2)CO](n) (3 a), [[Co(NCS)(2)(3-pia)(2)] x 4T HF](n) (3 b) and [[Co(NCS)(2)(3-pna)(2)](n)] (5), have been synthesized by the reaction of cobalt(II) thiocyanate with N-(3-pyridyl)isonicotinamide (3-pia) or N-(3-pyridyl)nicotinamide (3-pna). X-ray crystallographic characterization reveals that adjacent layers are stacked such that channels are created, except in 5. The channels form a hydrogen-bonded interior for guest molecules; in practice, 1 a contains ethanol and water molecules as guests in the channels with hydrogen bonds, whereas 3 b (3 a) contains tetrahydrofuran (acetone) molecules. In 1 a, the "double-edged axe-shaped" motifs in adjacent sheets are not located over the top of each other, while the motifs in 3 b stack so perfectly as to overlap each other in an edge-to-edge fashion. This subtle change in the three-dimensional framework is associated with the template effect of the guests. Compound 5 has no guest molecules and, therefore, the amide groups in one sheet are used for hydrogen-bonding links with adjacent sheets. Removal of the guest molecules from 1 a and 3 b (3 a) causes a structural conversion accompanied by a color change. Pink 1 a cannot retain its original framework and changes into a blue amorphous compound. On the other hand, the framework of pink 3 b (3 a) is transformed to a new crystalline framework of violet 4. Interestingly, 4 reverts to the original pink crystals of 3 b (3 a) when it is exposed to THF (or acetone) vapor. Spectroscopic measurements (visible, EPR, and IR) provide a clue to the crystal-to-crystal transformation; on removal of the guests, the amide groups are used to form the beta sheet-type hydrogen bonding between the sheets, and thus the framework withstands significant stress on removal of guest molecules. This mechanism is attributed to the arrangement of the adjacent sheets so suited in regularity that the beta sheet-type structure forms efficiently. The apohost 4 does not adsorb cyclopentane, showing a guest selectivity that, in addition to size, hydrogen-bonding capability is required for the guest molecules. The obtained compound is categorized as a member of a new generation of compounds tending towards functional porous coordination polymers.