Guest-Responsive Reversal in Structural Transformation after a [2 + 2] Topochemical Reaction in a 3D Pillared Layer MOF: Uncovering the Role of C-H···O Interaction.

Abstract

Here, we report the influences of the C-H···O interaction, weaker than other conventional noncovalent interactions, on the guest-responsive structural modification of a photoactive metal-organic framework (MOF) and the impact on gas sorption properties. A photoactive pillared-layer three-dimensional MOF {[Cd(pzdc)(bpee)]2·3H2O}n (1) (where bpee = 1,2-bis(4-pyridyl)ethylene and pzdc = 2,3-pyrazinedicarboxylate) was synthesized and characterized. Compound 1 shows guest-responsive structural contraction by the movement of two-dimensional layers supported by the C-H···O interaction between the pillar (bpee) and layer (pzdc) linkers. Further, 1 was postsynthetically modified using light by exploiting the parallel arrangement of the olefinic double bondsof the bpee pillars based on a [2 + 2] cycloaddition reaction to produce {[Cd2(pzdc)2(rctt-tpcb)]·3H2O}n, (1IR) (rctt-tpcb = regio cis,trans,trans-tetrakis(4-pyridyl)cyclobutane) in a single-crystal-to-single-crystal transformation (SCSC) manner. The C-H···O interaction between the two linkers is not possible in the photomodified framework, and thus guest-responsive structural expansion is realized. Such a reversal of the structural transformation facilitates the enhanced CO2 uptake in 1IR with respect to 1 at their dehydrated states. Further, the photomodified compound 1IR does not uptake N2 and CH4 at 273 K and shows high selectivity as realized by an ideal adsorbed solution theory calculation. The facile diffusion of CO2 in the irradiated framework is also supported by the kinetic measurements based on MeOH adsorption isotherms at 293 K. Here, postsynthetic modification by a [2 + 2] photochemical reaction is the key to control the structural change for enhanced CO2 uptake capacity.