Supramolecular Sandwiches: Halogen-Bonded Coformers Direct [2+2] Photoreactivity in Two-Component Cocrystals

Abstract

The halogen-bond (X-bond) donors 1,3- and 1,4-diiodotetrafluorobenzene (1,3-di-I-tFb and 1,4-di-I-tFb, respectively) form cocrystals with trans-1,2-bis(2-pyridyl)ethylene (2,2'-bpe) assembled by N···I X-bonds. In each cocrystal, 2(1,3-di-I-tFb)·2(2,2'-bpe) and (1,4-di-I-tFb)·(2,2'-bpe), the donor molecules support the C=C bonds of 2,2'-bpe to undergo an intermolecular [2+2] photodimerization. UV irradiation of each cocrystal resulted in stereospecific and quantitative conversion of 2,2'-bpe to rctt-tetrakis(2-pyridyl)cyclobutane (2,2'-tpcb). In each case, the reactivity occurs via face-to-face π-stacked columns wherein nearest-neighbor pairs of 2,2'-bpe 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 2,2'-tpcb is confirmed in cocrystals 2(1,3-di-I-tFb)·(2,2'-tpcb) and (1,4-di-I-tFb)·(2,2'-tpcb).

Keywords: cocrystal; crystal engineering; cyclobutane; halogen bonding; pedal motion.; photodimerization.

Scheme 1

Components for cocrystals.

Scheme 2

UV-irradiation of 2(1,3-di-I-tFb)·2(2,2′-bpe) and (1,4-di-I-tFb)·(2,2′-bpe) to form 2,2′-tpcb.

Figure 1

Perspectives of 2(1,3-di-I-tFb)·2(2,2′-bpe): (a) asymmetric unit (olefin disorder omitted for clarity); (b) 1D chain based on N1/N2 illustrating ABAB repeat unit; (c) 1D chain based on N3/N4 illustrating ABAB repeat unit; (d) offset, face-to-face π-stacks (highlighted in red) between chains; (e) face-to-face π-stacked sandwiches (space-filling) illustrating ABB’A’ repeat unit; and (f) π-stacked sandwiches highlighting criss-crossed C=C bonds (olefin disorder omitted for clarity).

Figure 2

Perspectives of 2(1,3-di-I-tFb)·(2,2′-tpcb): (a) asymmetric unit highlighting π···π and π···F forces (cyclobutane disorder omitted for clarity) and (b) 1D chains (H atoms omitted for clarity).

Figure 3

Perspectives of (1,4-di-I-tFb)·(2,2′-bpe): (a) asymmetric unit; (b) 1D chain along crystallographic bc-plane illustrating the ABAB repeat unit; (c) offset, face-to-face π-stacks (highlighted in red) between chains; (d) face-to-face π-stacked sandwiches (space-filling) illustrating the ABBA repeat unit; and (e) π-stacked sandwiches highlighting criss-crossed C=C bonds (olefin disorder omitted for clarity).

Figure 4

Perspectives of (1,4-di-I-tFb)·(2,2′-tpcb): (a) asymmetric unit (cyclobutane disorder omitted for clarity); (b) infinite zig-zag chains (space-filling, view along c); (c) herringbone pattern resulting from interactions between zig-zag chains; and (d) 1D chains of 2,2′-tpcb generated along the crystallographic c-axis, highlighting offset, face-to-face π···π forces.