An unprecedented mixed valence cobalt(III)/cobalt(II) ion-pair complex ([CoCO3(bipy)2]2[Co(DCA)2]⋅16H2O) with two novel water tapes

An unprecedented mixed valence cobalt(III)/cobalt(II) ion-pair complex, [CoCO3(bipy)2]2[Co(DCA)2]⋅16H2O (1) [bipy = 2,2′-bipyridine, DCA = demethylcantharidate, exo-1,4-epoxy-cyclohexyl-2,3-dicarboxylate group, (C8H8O5)2 −], has been synthesized and characterized by elemental analysis, IR, UV–vis, Thermogravimetric analysis (TGA) and single crystal X-ray diffraction. The crystal structure of 1 consists of three independent molecular moieties: [CoCO3(bipy)2]+, [Co(DCA)2]2–, and water molecules of crystallization. Interestingly, two novel water tapes, one comprises of alternate cyclic water tetramer and octamer, the other is constructed by alternate cyclic water hexamer and carboxylate–water hybrid octamer [(H2O)2(COO)2]2 −, have been detected for the first time in the host framework of 1. The two novel water tapes are linked together by the DCA and carbonate ligands via hydrogen bonds into a 2D water layer, and then the water layers assemble complex ions ([CoCO3(bipy)2]+ and [Co(DCA)2]2 −) through hydrogen bonds, π–π interactions and electrostatic interactions to form the overall 3D metal–organic framework structure.

There are nine crystal structure figures, including unit cell packing (Fig. 1), complex units (Fig. 2), two novel types of (H2O)8 clusters and water tapes containing them (Fig. 3, Fig. 4, Fig. 5, Fig. 6 and Fig. 7), π–π interactions (Fig. 8) and 3D metal–organic framework structure of the title complex (Fig. 9). The tenth figure (Fig. 10) is the TGA and DTA curves of the title complex.Figure optionsDownload as PowerPoint slideHighlights
► A mononuclear mixed valence Co(III)/Co(II) ion-pair complex has been synthesized.
► The Co(II) ions are partly oxidized to Co(III) ions during the crystallization.
► The Co(III) ions capture atmospheric CO2 to form the corresponding carbonato complex.
► The crystal water molecules form two water tapes: T4(2)8(2) and T6(0)A0-8(0)A0.