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 sta tionary phases CBSP-1 and CBSP-2, containing substituted molecular tweezers with benzene and naphthalene spacer-units, are described. These phases sel ectively retain electron-deficient aromatic and quinoid analytes of appropr iate size and topography, such as 1,4-dinitrobenzene, 1,2-, 1,3-, and 1,4-d icyanobenzenes, and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ), in HPLC stu dies. The good qualitative correlation between the capacity factors k' deri ved from the HPLC retention times and the association constants K-a obtaine d from binding studies in solution using molecular tweezers 1 and 2 as rece ptors, indicates that the mechanism of retention involves selective complex ation by the molecular tweezers on the silica surface. As expected from the solution experiments, higher capacity factors and selectivities were obtai ned 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, Delta H-R, were measured for four different aromatic analytes in 15 solvents. Chromatographic separation factors, alph a, were determined for seven structurally-related nitroaromatic compounds. The results of these measurements allow for the conclusion that the electro static nature and steric complementarity of the receptors and analytes is m ost important in determining selectivities.