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.

Diiodoacetylene, C 2 I 2 , is the smallest ditopic halogen bond donor other than I 2 or other dihalogens. A convenient synthesis of diiodoacetylene from the common Sonagashira coupling reagent Me 3 SiChCH, is described. The halogen-bonded adducts of C 2 I 2 with dimethylformamide (DMF), pyrazine (pyz) and 1,4-diazabicyclooctane (dabco) have been characterised by X-ray crystallography. All adopt 1D halogen-bonded chains linked via short C-I/O [I/O 2.834(4)-2.888(4) A; C-I/O > 170 ] or C-I/N [I/N 2.715(3)-2.832(7) A; C-I/N > 175 ] interactions. Attempts to synthesise the adduct of C 2 I 2 with hexamethylenetetramine (hmta) resulted in isolation and crystallographic characterisation of the adduct of C 2 I 4 $hmta, indicating decomposition of C 2 I 2 to yield C 2 I 4 in solution. The adduct comprises two independent C 2 I 4 molecules that act, respectively, as tetratopic and ditopic halogen bond donors forming C-I/N interactions [I/N 2.948(7)-2.999(8) A; C-I/N > 165 ], occupying three of the four nitrogen sites on htma. The remaining nitrogen sites engage in N/C(p) interactions directed orthogonal to the plane of the ditopic C 2 I 4 molecules. Separate surveys of halogen bonds formed by diiodo(poly)alkynes I(ChC) n I (n ¼ 1-3) and by C 2 I 4 molecules indicate that C-I/N halogen bonds are shorter, when normalised for van der Waals radii, and, by inference, stronger than halogen bonds involving other acceptor groups, and demonstrates that C sp -I/N halogen bonds are generally shorter C sp 2-I/N halogen bonds.

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.

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