Direct in Situ Crystallographic Visualization of a Dual Mechanism for the Uptake of CO₂ Gas by a Flexible Metal–Organic Framework

We report a flexible metal–organic framework, [Co₂(OBA)₂(BPMP)]_n (COB), with a new network topology. COB displays structural flexibility under CO₂ gas pressure at 298 K, and the resultant porous phases have been characterized by in situ X-ray diffraction analysis. We show that activation yields a framework with discrete voids and substantial reduction in guest-accessible volume. Single-crystal X-ray diffraction analysis under controlled CO₂ pressure shows that COB exhibits a breathing mode of flexibility, combined with an overall swelling of the framework. This combination of mechanisms is highly unusual.