Halogen-Bond Mediated [2+2] Photodimerizations: À la Carte Access to Unsymmetrical Cyclobutanes in the Solid State

Abstract

The ditopic halogen-bond (X-bond) donors 1,2-, 1,3-, and 1,4-diiodotetrafluorobenzene (1,2-, 1,3-, and 1,4-di-I-tFb, respectively) form binary cocrystals with the unsymmetrical ditopic X-bond acceptor trans-1-(2-pyridyl)-2-(4-pyridyl)ethylene (2,4-bpe). The components of each cocrystal (1,2-di-I-tFb)·(2,4-bpe), (1,3-di-I-tFb)·(2,4-bpe), and (1,4-di-I-tFb)·(2,4-bpe) assemble via N···I X-bonds. For (1,2-di-I-tFb)·(2,4-bpe) and (1,3-di-I-tFb)·(2,4-bpe), the X-bond donor supports the C=C bonds of 2,4-bpe to undergo a topochemical [2+2] photodimerization in the solid state: UV-irradiation of each solid resulted in stereospecific, regiospecific, and quantitative photodimerization of 2,4-bpe to the corresponding head-to-tail (ht) or head-to-head (hh) cyclobutane photoproduct, respectively.

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

Scheme 1

Ditopic components for cocrystals and photoproducts.

Scheme 2

UV-irradiation of (1,2-di-I-tFb)·(2,4-bpe) or (1,3-di-I-tFb)·(2,4-bpe) generates either ht- or hh-2,4-tpcb, respectively.

Figure 1

Perspectives of (1,2-di-I-tFb)∙(2,4-bpe): (a) asymmetric unit (anisotropic displacement ellipsoids at 50% probability; view along a); (b) 1D tapes illustrating ABA’B’ repeat motif (view along a); (c) corrugated layered structure (space-filling); and (d) reactive arrangement of nearest-neighbor alkene pairs.

Figure 1

Perspectives of (1,2-di-I-tFb)∙(2,4-bpe): (a) asymmetric unit (anisotropic displacement ellipsoids at 50% probability; view along a); (b) 1D tapes illustrating ABA’B’ repeat motif (view along a); (c) corrugated layered structure (space-filling); and (d) reactive arrangement of nearest-neighbor alkene pairs.

Figure 2

Perspectives of 2(1,2-di-I-tFb)∙(ht-2,4-tpcb): (a) asymmetric unit (anisotropic displacement ellipsoids at 50% probability); (b) 1D assemblies; and (c) sheets based on C-H∙∙∙F forces.

Figure 2

Perspectives of 2(1,2-di-I-tFb)∙(ht-2,4-tpcb): (a) asymmetric unit (anisotropic displacement ellipsoids at 50% probability); (b) 1D assemblies; and (c) sheets based on C-H∙∙∙F forces.

Figure 3

Perspectives of (1,3-di-I-tFb)∙(2,4-bpe) (minor fraction of alkene disorder omitted for clarity): (a) asymmetric unit (anisotropic displacement ellipsoids at 50% probability); (b) four-component assembly; (c) rhomboidal repeat highlighted (C-H∙∙∙I contacts omitted for clarity); (d) sheets with two assemblies shown as space filling; and (e) nearest-neighbor alkenes between adjacent sheets highlighting closest alkene separation (green dashed arrow).

Figure 3

Perspectives of (1,3-di-I-tFb)∙(2,4-bpe) (minor fraction of alkene disorder omitted for clarity): (a) asymmetric unit (anisotropic displacement ellipsoids at 50% probability); (b) four-component assembly; (c) rhomboidal repeat highlighted (C-H∙∙∙I contacts omitted for clarity); (d) sheets with two assemblies shown as space filling; and (e) nearest-neighbor alkenes between adjacent sheets highlighting closest alkene separation (green dashed arrow).

Figure 4

Perspectives of 2(1,3-di-I-tFb)∙(hh-2,4-tpcb): (a) asymmetric unit (thermal ellipsoids shown at 50% probability); (b) discrete, six-component assembly; and (c) adjacent assemblies illustrating Type II I∙∙∙I X-bonds.

Figure 5

Perspectives of (1,4-di-I-tFb)∙(2,4-bpe): (a) ORTEP (thermal ellipsoids shown at 50% probability; view along b); (b) zig-zag chains illustrating ABA’B repeat; (c) columns highlighting Type II π∙∙∙I X-bonds (red dashed lines) (C-H∙∙∙I contacts omitted for clarity; view along c); and (d) herringbone packing of columns.

Figure 5

Perspectives of (1,4-di-I-tFb)∙(2,4-bpe): (a) ORTEP (thermal ellipsoids shown at 50% probability; view along b); (b) zig-zag chains illustrating ABA’B repeat; (c) columns highlighting Type II π∙∙∙I X-bonds (red dashed lines) (C-H∙∙∙I contacts omitted for clarity; view along c); and (d) herringbone packing of columns.