Recognition with metallo cavitands

Significance Cavitands are nanoscale container molecules widely used in studies of molecular recognition and reactivity. They act as hosts that temporarily confine small molecule guests and much of their action as reaction chambers is due to movable walls allowing the uptake and release of guests. Water-soluble and organic-soluble cavitands featuring quinoxaline walls were prepared and their complexes with alkanes, and amphiphilic guests were characterized by 1H NMR spectroscopy and computational methods. The quinoxalines bound Pd(II) near the rim of the cavitands and distorted the shape of the space, stabilized the receptive forms, and enhanced guest binding. Further development and application of cavitands as reaction vessels depends on closely positioned charges and potentially interactive sites near the cavity and its contents. We describe here the effects of metal complexation on the molecular recognition behavior of cavitands with quinoxaline walls. The nitrogen atoms of the quinoxalines are near the upper rim of the vase-like shape and treatment with Pd(II) gave 2:1 metal:cavitand derivatives. Characterization by 1H, 13C NMR spectroscopy, HR ESI-MS, and computations showed that the metals bridged adjacent quinoxaline panels and gave cavitands with C2v symmetry. Both water-soluble and organic-soluble versions were prepared and their host/guest complexes with alkanes, alcohols, acids, and diols (up to C12) were studied by 1H NMR spectroscopy. Analysis of the binding behavior indicated that the metals rigidified the walls of the receptive vase conformation and enhanced the binding of hydrophobic and even water-soluble guests, compared to related cavitands reported previously. The results demonstrated that the conformational dynamics of the cavitand were slowed by the coordination of Pd(II) and stabilized the host’s complexes.

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