Shape Selectivity by Guest-Driven Restructuring of a Porous Material

A flexible metal‐organic framework selectively sorbs para‐ (pX) over meta‐xylene (mX) by synergic restructuring around pX coupled with generation of unused void space upon mX loading. The nature of the structural change suggests more generally that flexible structures which are initially mismatched in terms of fit and capacity to the preferred guest are strong candidates for effective molecular separations. Nobody′s perfect: The metal‐organic framework Ce(HTCPB) is structurally mismatched to both the para and meta isomers of xylene. It restructures synergically around the para isomer to give a high‐quality structural and functional fit, but rearrangement around the meta isomer requires larger host distortions and gives a poorer fit than in the para case. This “off‐target” host thus has high selectivity for the sorption of para‐ over meta‐xylene.