Molecular tweezers as synthetic receptors: Molecular recognition of electron-deficient aromatic substrates by chemically bonded stationary phases

The synthesis and chromatographic properties of novel chemically-bonded stationary phases CBSP-1 and CBSP-2, containing substituted molecular tweezers with benzene and naphthalene spacer-units, are described, These phases selectively retain electron-deficient aromatic and quinoid analytes of appropriate size and topography, such as 1,4-dinitrobenzene, 1,2-, 1,3-, and 1,4-dicyanobenzenes, and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ), in HPLC studies, The good qualitative correlation between the capacity factors k' derived from the HPLC retention times and the association constants K(a) obtained from binding studies in solution using molecular tweezers 1 and 2 as receptors, indicates that the mechanism of retention involves selective complexation by the molecular tweezers on the silica surface, As expected from the solution experiments, higher capacity factors and selectivities were obtained with CBSP-2 than with CBSP-1 because of a better structural fit of the naphthalene-spaced receptor with the aromatic analytes. Capacity factors, k', and enthalpies of retention, ΔHR, were measured for four different aromatic analytes in 15 solvents. Chromatographic separation factors, a, were determined for seven structurally-related nitroaromatic compounds. The results of these measurements allow for the conclusion that the electrostatic nature and steric complementarity of the receptors and analytes is most important in determining selectivities.