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.
Highlights
Cyclobutane rings appended with n-pyridyl (n = 2, 3 or 4) groups are useful building blocks to construct metal-organic assemblies and materials [1,2,3,4]
We show that in two of these cocrystals, the ditopic X-bond donor serves to support nearest-neighbor unsymmetrical alkenes in a geometry conducive to topochemical [2+2] photodimerization in the solid state
Work by our group has demonstrated that the unsymmetrical cyclobutanes rctt-bis(npyridyl)-bis(n0 -pyridyl)cyclobutanes (n 6= n0, n = 2 or 4, n0 = 2 or 4) can be constructed in the solid state by way of hydrogen-bond (H-bond) mediated self-assembly
Summary
Cyclobutane rings appended with n-pyridyl (n = 2, 3 or 4) (pyr) groups are useful building blocks to construct metal-organic assemblies and materials [1,2,3,4]. Many such molecules have been accessed via template-directed, topochemical [2+2] photodimerizations of alkenes within cocrystals. Of particular and recent interest to our group have been cyclobutanes derived from photodimerization of unsymmetrical alkenes. These photoproducts are appended with two pairs of differently substituted pyr groups. Given that covalent-bond-forming reactions performed in the solid state are extremely sensitive to molecular packing, it is imperative to identify diverse and robust classes of template molecules capable of directing photodimerizations in crystals
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