Halogen versus Hydrogen Bonding in Binary Cocrystals: Novel Conformation a Coformer with [2+2] Photoreactivity of Criss-Crossed C=C Bonds.

A series of cocrystals involving the hydrogen- and halogen-bond donor coformers catechol (cat) and 1,2-diiodotetrafluorobenzene (1,2-di-I-tFb), respectively, is reported. Each coformer forms a cocrystal with each of the three symmetric bipyridines trans-1,2-bis(n-pyridyl)ethylene (n,n'-bpe, where: n=n'=2, 3, 4). Four novel cocrystals (cat) ⋅ (3,3'-bpe), 2(1,2-di-I-tFb) ⋅ (2,2'-bpe), 2(1,2-di-I-tFb) ⋅ (3,3'-bpe), and (1,2-di-I-tFb) ⋅ (4,4'-bpe) comprise components that assemble by either O-H⋅⋅⋅N hydrogen bonds (cat) or N⋅⋅⋅I halogen bonds (1,2-di-I-tFb). In (cat) ⋅ (3,3'-bpe), cat acts as a template to support an intermolecular [2+2] photocycloaddition of 3,3'-bpe. The reactivity occurs via a one-dimensional (1D) hydrogen-bonded tape with stacked and criss-crossed olefins that react stereoselectively and quantitatively to form rctt-tetrakis(3-pyridyl)cyclobutane (3,3'-tpcb). The reactivity of the criss-crossed olefins is facilitated by a hitherto not reported cis-gauche conformation adopted by cat. The stereochemistry of 3,3'-tpcb is confirmed in the cocrystal 2(cat) ⋅ (3,3'-tpcb).