| Summary: | The halogen-bond (X-bond) donors 1,3- and 1,4-diiodotetrafluorobenzene (<b>1,3-di-I-tFb</b> and <b>1,4-di-I-tFb</b>, respectively) form cocrystals with <i>trans</i>-1,2-bis(2-pyridyl)ethylene (<b>2,2′-bpe</b>) assembled by N···I X-bonds. In each cocrystal, 2(<b>1,3-di-I-tFb</b>)·2(<b>2,2′-bpe</b>) and (<b>1,4-di-I-tFb</b>)·(<b>2,2′-bpe</b>), the donor molecules support the C=C bonds of <b>2,2′-bpe</b> to undergo an intermolecular [2+2] photodimerization. UV irradiation of each cocrystal resulted in stereospecific and quantitative conversion of <b>2,2′-bpe</b> to <i>rctt</i>-tetrakis(2-pyridyl)cyclobutane (<b>2,2′-tpcb</b>). In each case, the reactivity occurs via face-to-face <i>π</i>-stacked columns wherein nearest-neighbor pairs of <b>2,2′-bpe</b> molecules lie sandwiched between X-bond donor molecules. Nearest-neighbor C=C bonds are stacked criss-crossed in both cocrystals. The reactivity was ascribed to the olefins undergoing pedal-like motion in the solid state. The stereochemistry of <b>2,2′-tpcb</b> is confirmed in cocrystals 2(<b>1,3-di-I-tFb</b>)·(<b>2,2′-tpcb</b>) and (<b>1,4-di-I-tFb</b>)·(<b>2,2′-tpcb</b>).
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