Benzene‐, Pyrrole‐, and Furan‐Containing Diametrically Strapped Calix[4]pyrroles—An Experimental and Theoretical Study of Hydrogen‐Bonding Effects in Chloride Anion Recognition

Abstract

Weak hydrogen bonds recently have arisen as a topic of interest in supramolecular chemistry.[1] Among the various interactions being studied, the C-H···anion hydrogen bonds have drawn considerable attention. Positively-charged groups, such as imidazolium cations, provide strong C-H donors and have been used extensively in the design of numerous anionophore architectures.[2] There is increasing evidence, however, that even charge-neutral C-H donors may be strong enough to be exploited effectively in anion recognition chemistry. Such interactions, which involve both aliphatic and aryl C-H groups, have been inferred from gas phase studies,[3] deduced from NMR spectroscopic studies via, e.g., chemical shifts changes,[4-7] and observed in solid state structures.[4,5,8] A recent review of anion-arene adducts notes that C-H···anion hydrogen bonding, rather than interaction with the π-system, is by far the most prevalent bonding motif observed in the solid state.[9] These experimental observations are supported by theoretical analyses.[10,11] For instance, Hay and co-workers have calculated that benzene C-H···anion hydrogen bonds are significant,[11a] being roughly half the strength of typical neutral N-H···anion hydrogen bonds. In a subsequent theoretical report, it was noted that the aryl C-H···Cl binding energies in the gas phase can be tuned from –8 to –16 kcal/mol by altering the para substitution from NH2 to NO2.[11b] Although there are a couple examples where complementary aryl C-H anion interactions have been deliberately incorporated into the design of anion receptors,[7,12] to the best of our knowledge, no efforts have been made to date to test theoretical predictions via the synthesis and experimental study of a matched series of anion receptors. We now report efforts along these lines.