Selective carbon dioxide uptake and crystal-to-crystal transformation: porous 3D framework to 1D chain triggered by conformational change of the spacer

Abstract

Α-Po type 3D porous frameworks, {[M(bpe)₂(N(CN)₂)]N(CN)₂·xH₂O}_n, (M = Zn(II) (x = 5) (1)/Co(II) (x = 4) (2)) (bpe = 1,2-bis(4-pyridyl)ethane, N(CN)₂^- = dicyanamide anion) composed of mixed ligand systems have been synthesized and structurally characterized. Upon two-fold interpenetration both 3D frameworks 1 and 2 show a bimodal channel structure; the small channels contain non-coordinated N(CN)²⁻ anions and the bigger channels are occupied by guest water molecules. High framework stability for both compounds was realized by similarity in the PXRD pattern in dehydrated state and even a reversible single-crystal-to-single-crystal transformation for framework 1. Both the frameworks display unprecedented structural transformation from 3D framework to 1D {[M(bpe)(N(CN)₂)₂]}_n (M = Zn(II) (1b)/Co(II) (2b)) coordination chain upon removal of one molecule of bpe and concomitant bridging of non-coordinated N(CN)²⁻ and conformational change (anti to gauche) by another bridging bpe linker. Moreover, the dehydrated solid {[Co(bpe)₂(N(CN)₂)]N(CN)₂}_n (2a) exhibits highly selective CO₂ uptake relative to a number of adsorbates (H₂, N₂, O₂ and Ar) at 195 K.