Control over Phase Transformations in a Family of Flexible Double Diamondoid Coordination Networks through Linker Ligand Substitution

In this work, we present the first metal-organic framework (MOF) platform with a self-penetrated double diamondoid (ddi) topology that exhibits switching between closed (nonporous) and open (porous) phases induced by exposure to gases. A crystal engineering strategy, linker ligand substitution, was used to control gas sorption properties for CO2 and C3 gases. Specifically, bimbz (1,4-bis(imidazol-1-yl)benzene) in the coordination network X-ddi-1-Ni ([Ni2(bimbz)2(bdc)2(H2O)]n , H2bdc = 1,4-benzenedicarboxylic acid) was replaced by bimpz (3,6 b i s ( i m i d a z o l-1-y l ) p y r i d a z i n e ) in X-d d i-2-N i ([Ni2(bimpz)2(bdc)2(H2O)]n). In addition, the 1:1 mixed crystal X-ddi-1,2-Ni ([Ni2(bimbz)(bimpz)(bdc)2(H2O)]n) was prepared and studied. All three variants form isostructural closed (beta) phases upon activation which each exhibited different reversible properties upon exposure to CO2 at 195 K and C3 gases at 273 K. For CO2, X-ddi-1-Ni revealed incomplete gate-opening, X-ddi-2-Ni exhibited a stepped isotherm with saturation uptake of 3.92 mol center dot mol-1, and X-ddi-1,2-Ni achieved up to 62% more gas uptake and a distinct isotherm shape vs the parent materials. Single-crystal X-ray diffraction (SCXRD) and in situ powder X-ray diffraction (PXRD) experiments provided insight into the mechanisms of phase transformation and revealed that the beta phases are nonporous with unit cell volumes 39.9, 40.8, and 41.0% lower than the corresponding as-synthesized alpha phases, X-ddi-1-Ni-alpha, X-ddi-2-Ni-alpha, and X-ddi-1,2-Ni-alpha, respectively. The results presented herein represent the first report of reversible switching between closed and open phases in ddi topology coordination networks and further highlight how ligand substitution can profoundly impact the gas sorption properties of switching sorbents.

Automated summary

In this work, a metal-organic framework (MOF) platform with a self-penetrated double diamondoid (ddi) topology that exhibits switching between closed and open phases induced by exposure to gases was presented. A crystal engineering strategy, linker ligand substitution, was used to control gas sorption properties. Three variants with different ligands were studied, and they all showed reversible properties upon exposure to CO2 and C3 gases.