Cleft-type diamidinium receptors for dicarboxylate binding in protic solvents

A series of potential cleft-type receptors for dicarboxylate substrates wer e prepared by attachment of two phenylamidinium ions to either naphthalene or 1,1'-binaphthalene scaffolds. Their synthesis (Schemes 1-4) involved the Pd-0-catalyzed cross-coupling of aryl nitriles to the central scaffold, fo llowed by transformation of the nitrile into amidinium groups using the Gar igipati reaction. The 1,1'-binaphthalene derivative (+/-)-1 with phenylamid inium residues attached to the 6,6'-positions in the major groove was found to be a highly efficient receptor for dicarboxylate guests, such as glutar ate and isophthalates. even in competing protic solvents such as CD3OD ( Ta ble 1). The van't Hoff analysis of variable-temperature H-1-NMR (VT-NMR) ti trations (Table 2 and Fig. 3) and isothermal microcalorimetry (ITC; Table 3 and Fig. 4) revealed that complexation in MeOH is strongly entropically dr iven with an unfavorable enthalpic change, which partially compensates the entropic gain. These thermodynamic quantities are best explained by a parti cularly favorable solvation of the binding partners in the unbound state an d the release of the MeOH molecules, which solvate the free ions into the b ulk upon complexation. Receptor (+/-)-1 binds flexible glutarate and rigid isophthalates with similar association strength. This lack in response to g uest preorganization and reduced guest selectivity is explained with the no n-directionality of the coulombic charge-charge interactions in the complex es.