The Study Of Ion-Paired Assisted H-Bonded Dimers

The study and exploitation of hydrogen bonds has been central to the development of modern supramolecular chemistry.Hydrogen bonding is considered to be the‘master key'of molecular recognition—owing to its relatively predictable strength and directionality—and plays a major role in the assembly of diverse non-covalent assemblies.The main drawback of hydrogen bonds,however,is their limited strength.The more polar the solvent is,the weaker are the hydrogen bonds because of the increasing competitive solvation of donor and acceptor sites by polar solvent molecules.Therefore,purely hydrogen-bonded assemblies possess a considerable binding energy only in aprotic solvents of low polarity and are not stable in water.However,with respect to any future technological applications,non-water-stable systems can only have a limited use as water is omnipresent in our everyday life.In this thesis,we describe a readily accessible AAAA-DDDD ion assisted quadruple hydrogen bonding array that exhibits surprisingly strong binding for a small molecule hydrogen bonded complex in water(K>10⁶ M^-1).The main content of the dissertation includes the following three parts:In the first part,we designed and synthesized a novel DDDD hydrogen-bonded motif?compoud 2.1?.Fortunately,we obtained the single crystal structure from water evaporation.The compound was twisted because of the two cis-hydrogen bonds.The binding constant was up to 10⁶ M^-1 in pure DMSO,but it was almost impossible to detect in pure water.In the second part,we succeeded to synthesize a planar aromatic diguanidine by the method mentioned in the first part.We obtained the single crystal structure of the complex 3.3?2.41 from DMSO and methanol.The structure of the AAAA-DDDD motif show the distance of the quadruple hydrogen bonds,and the six secondary electrostatic interactions between adjacent hydrogen bonds strengthened stability of the complex.The power of?-?stacking is also existed in our system.ITC titration studies in highly polar solvents such as water showed that the complex exhibits exceptionally strong binding constant up to 10⁶ M^-1.In the last part,we desired to finger out the binding constant of the complex without?-?stacking in water.We introduced some steric groups in order to block the complex.Finally we found that the?-?stacking can enhance the complex stability about one order of magnitude.