Chemical insight from crystallographic disorder-structural studies of supramolecular photochemical systems. Part 3. The beta-cyclodextrin-7-hydroxy-4-methylcoumarin inclusion complex: direct observation of photodimerization by X-ray crystallography

As part of an ongoing structural study of supramolecular photochemical beta -cyclodextrin(beta-CD)-coumarin derivatives systems the crystal structure o f the beta-CD-7-hydroxy-4-methylcoumarin complex has been determined for th e unreacted crystal and for a crystal exposed to UV light for 6 days. The 7 -hydroxy-4-methylcoumarin molecules form a crystalline 2:2 host-guest (H-G) complex with beta-CD. The guest molecules are disordered, with pairs of gu est molecules similarly oriented with respect to each other distributed ove r three sites. The guest molecules are oriented with their reactive double bonds separated by about 3.5-3.7 Angstrom and oriented parallel to each oth er. Crystallographic analysis of the photoreacted crystal shows the reactio n proceeds within the crystal to produce the topochemically predicted anti- HT photodimer. The system is appropriately described as a "reaction nano-ve ssel", where the inter-molecular interactions of import to the outcome of t he reaction are confined to a single beta-CD dimer cavity. Ab initio molecu lar orbital calculations of the conformation of the anti-HT 7-hydroxy-4-met hylcoumarin photodimer show that the preferred gas-phase geometry for the c yclobutyl ring is planar, the same as that observed in the beta-CD complex. This provides further evidence for classifying the beta-CD dimer environme nt as non-constraining.