Guest-driven structural flexibility of 2D coordination polymers: Synthesis, structural characterizations, and gas sorption properties

Two coordination polymers, {[(NiL_allyl)₂(BuTC)] â̂™2DEFâ̂™2H₂O} (1) and {[(NiL _allyl)₂(BuTC)]â̂™3H₂O} (2), ([NiL_allyl](ClO₄)₂ = [Ni(C₁₄H ₃₀N₆)](ClO₄)₂, H₄BuTC = 1,2,3,4-butanetetracarboxylic acid, DEF = N,N′-diethylformamide), were prepared by the self-assembly of [NiL_allyl](ClO₄) ₂ and H₄BuTC in DEF/H₂O and acetonitrile/H ₂O, respectively. Single crystal X-ray diffraction and X-ray powder diffraction (XRPD) results revealed that 1 and 2 have two-dimensional layered structures, and the layers were stacked infinitely. Further, guest molecules were intercalated between the layers. Since both of coordination polymers 1 and 2 were constructed from the same building blocks, the structure of each layer was identical. However, depending on the size and the nature of guest molecules, layer packing and the interlayer distances were different for each coordination polymer. Interestingly, due to the instability of DEF guest molecules intercalated in the coordination polymer 1, these guest molecules could be easily liberated from the host, consequently resulting in the same XRPD pattern as that of 2 with slightly different relative intensities. Dried compounds of 1 and 2 (1′ and 2′, respectively) also showed the same result as evidenced by the XRPD patterns. The similar but non-identical XRPD patterns were revealed that the dried structure 1′ had the same interlayer distance and the same intralayer structure as 2 and 2′ did, whereas its layer packing remained the same as that of 1. This subtle structural difference of 1′ and 2′ resulted in their different CO₂ uptake behaviors at 195 K.