An Electron-Rich Calix[4]arene-Based Receptor with Unprecedented Binding Affinity for Nitric Oxide

Calixarenes have found widespread application as building blocks for the design and synthesis of functional materials in host-guest chemistry. The ongoing desire to develop a detailed understanding of the nature of NO bonding to multichromophoric pi-stacked assemblies led us to develop an electron-rich methoxy derivative of calix[4]arene (3), which we show exists as a single conformer in solution at ambient temperature. Here, we examine the redox properties of this derivative, generate its cation radical (3(+center dot)) using robust chemical oxidants, and determine the relative efficacy of its NO binding in comparison with model calixarenes. We find that 3/3(+center dot) is a remarkable receptor for NO+/NO, with unprecedented binding efficacy. The availability of precise experimental structures of this calixarene derivative and its NO complex, obtained by X-ray crystallography, is critically important both for developing novel functional NO biosensors, and understanding the role of stacked aromatic donors in efficient NO binding, which may have relevance to biological NO transport.