A Divergent Alkyne Diol Directs [2 + 2] Photoreactivity in the Solid State: Cocrystal, Supramolecular Catalysis, and Sublimation Effects

2-butyne-1,4-diol (1,4-bd) is used as a divergent ditopic template that directs trans-1,2-bis (n-pyridyl) ethylene (n,n′-bpe, where n = n′ = 3 or 4) to undergo an intermolecular [2 + 2] photodimerization in the solid state. The components of cocrystals [(1,4-bd)·(4,4′-bpe)]n and [(1,4-bd)·(3,3′-bpe)]n form 1D hydrogen-bonded polymers with n,n′-bpe assembled as infinite parallel stacks. The alkenes undergo [2 + 2] photocycloadditions to form rctt-tetrakis (n-pyridyl) cyclobutane (where n = 3 or 4). We demonstrate that the reactive solid involving 4,4′-bpe exhibits supramolecular catalysis.


Photoreactive
The stereochemistry of the photoproduct 3,3 -tpcb as rcttwas confirmed by treatment with acid and extraction into chloroform (see Figure S6). Single crystals of (rctt-3,3 -tpcb)·(H 2 O) in the form of plates were obtained by slow solvent evaporation over a period of 4 d in chloroform. The components of (rctt-3,3 -tpcb)·(H 2 O) crystallize in the triclinic space group P1 (Table 1) 3)] with the included water molecules bridging adjacent cyclobutanes. As a consequence of the assembly process, the components form 1D hydrogen-bonded chains along the crystallographic b-axis. Adjacent 1D chains stack along the a-axis and interact via edge-to-face C-H···π contacts to form 2D sheets. The 2D sheets interdigitate perpendicular to the c-axis. and extraction into chloroform (see Figure S6). Single crystals of (rctt-3,3'-tpcb)·(H2O) in the form of plates were obtained by slow solvent evaporation over a period of 4 d in chloroform.
Infinitely stacked C = C bonds in the solid state will exhibit a maximum possible theoretical conversion owing to independent photodimerization events taking place [9][10][11][12] We envision that the increase in conversion of 4,4'-bpe to 4,4'-tpcb achieved by supramolecular catalysis relative to the stoichiometric cocrystal [(1,4-bd)·(4,4'-bpe)]n likely involves 'free' diol being available for recrystallization to support reactivity of the C = C bonds between the 1D hydrogen-bonded chains.
Infinitely stacked C = C bonds in the solid state will exhibit a maximum possible theoretical conversion owing to independent photodimerization events taking place [9][10][11][12] We envision that the increase in conversion of 4,4 -bpe to 4,4 -tpcb achieved by supramolecular catalysis relative to the stoichiometric cocrystal [(1,4-bd)·(4,4 -bpe)] n likely involves 'free' diol being available for recrystallization to support reactivity of the C = C bonds between the 1D hydrogen-bonded chains. Molecules 2019, 24, x FOR PEER REVIEW 6 of 9

X-ray Diffraction Experiments
Diffraction data were collected on a Bruker © Nonius © (Billerica, MA, USA) APEX II Kappa single-crystal X-ray diffractometer at room temperature (298.15 K) using graphite-monochromated Mo Kα 1 radiation (λ = 0.71073 Å). Structure solution and refinement were accomplished using ShelXT [14] and ShelXL [15], respectively, in the Olex2 user graphical interface. The structures were solved using direct methods. All non-hydrogen atoms were identified from the difference Fourier map and refined anisotropically. All hydrogen atoms were placed in their calculated positions and were refined using isotropic thermal parameters.

H-NMR Experiments
All 1 H-NMR spectra were obtained on a Bruker © Fourier-300 NMR spectrometer (Billerica, MA, USA) operating at 300 MHz. All data were processed with the MestReNova TM v6.0.2 software program.
For catalysis employing a 1,4-bd template, 4,4 -bpe (116 mg, 0.64 mmol) with 50 or 20 mol.% of 1,4-bd were combined and finely ground using an agate mortar and pestle. The formation of cocrystalline material was confirmed using PXRD. The finely ground crystalline powders were spread thinly between two glass plates and exposed to broadband UV-irradiation. The crystalline materials were exposed to additional dry grinding (20 min in an agate mortar and pestle) after 20 h intervals of UV-irradiation. Solid-state catalysis reactions were monitored using 1 H-NMR spectroscopy and PXRD.