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Highlights HO O N O N N 1. Cu2+ O
Figure 1 In the presence of the copper ion, the lone pair is no longer delocalised into the aromatic π system, but instead is transferred to the metal forming a nitrogen-copper bond
H O reduced form O
upper rim
O N O N
oxygen on upper rim
O NBa2 3
oxygen on upper rim
HO R HO HO R HO 5 R OH OH R OH OH
O NBa2 4
Figure 3 Comparing oxidised (3) and reduced (4) forms of functional groups around the rim of a resourcearene (5). Structure 3 has two hydroquinones and two quinoxaline each spanning the oxygen atoms on adjacent resourcinols to form an alternating pattern. The part structures 3 and 4 show the effect of oxidation of the hydroquinone on the hydrogen bonding around the rim.
Chemistry&Industry • November 2012 55 O O
O H O
hydrogen bonds between structures on the rim
Figure 2 NBa2 A cyclen-based silver acceptor ox O
upper rim
O O O O NBa2 no hydrogen bonding O N N 2.
Ca N
confirm Ag+ interactions between the metal ion and the aromatic rings.
Gripping small organic guest molecules Unlike 2, the four arms on 3 are permanently mounted upwards, but each one can fold upward or outward. Accessible conformational mobility is an important part of the concept. When bent outward, there is plenty of room for guest molecules to come and go, but in the ‘all- upward’ form, the arms crowd round the guest molecule and hold it tight. The amide functional groups on the quinoxaline rings are good hydrogen bond acceptors, and the alternating
pattern hydroquinone and quinoxalines places a ring of hydrogen bonds around the upper perimeter of the structure (Figure 3) so tightening the grip of the inclusion complex. The hydroquinone oxidation state is essential for the hydrogen bonds to form, which makes the structures redox-switchable. The target molecules in this case are small organic structures with relatively little functionality to aid recognition, but despite this they are tightly bound when the structure is in its reduced form (I. Pochorovski, M.-O. Ebert, J.-P. Gisselbrecht, C. Boudon, W. B. Schweizer, F. Diederich; J. Am. Chem. Soc., 2012, 134, 14702)