Dae-Wi Yoon

Benzene-, pyrrole-, and furan-containing diametrically strapped calix[4]pyrroles - An experimental and theoretical study of hydrogen-bonding effects in chloride anion recognition

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.

Real-time determination of chloride anion concentration in aqueous-DMSO using a pyrrole-strapped calixpyrrole anion receptor

The use of a pyrrole-strapped calix[4]pyrrole () permits the determination of chloride anion concentrations in mixed aqueous DMSO-d(6)-H(2)O environments via proton NMR spectroscopy.

Calix[6]pyrroles Capped with 1,3,5-Trisubstituted Benzene

One side of the calix[6]pyrrole, capped with 1,3,5-trisubstituted benzene, have been synthesized for the first time and fully characterized. The synthesis of the target systems were accomplished by acid-catalyzed cyclization of ester/ether-functionalized tripodal dipyrromethanes. The solution state binding studies of the capped calix[6]pyrroles revealed a slow complexation/decomplexation kinetics for fluoride anion and chloride anion. The results indicated that fluoride anion binds inside the cavity with 1/1 binding stoichiometry. On the other hand, only four pyrrolic N-Hs participate in chloride anion binding.

Electron-transfer and acid-base properties of a two-electron oxidized form of quaterpyrrole that acts as both an electron donor and an acceptor.

Electron-transfer interconversion between the four-electron oxidized form of a quaterpyrrole (abbreviated as P4 for four pyrroles) and the two-electron oxidized form (P4H2) as well as between P4H2 and its fully reduced form (P4H4) bearing analogous substituents in the alpha- and beta-pyrrolic positions was studied by means of cyclic voltammetry and UV-visible spectroelectrochemistry combined with ESR and laser flash photolysis measurements. The two-electron oxidized form, P4H2, acts as both an electron donor and an electron acceptor. The radical cation (P4H2*+) and radical anion (P4H2*-) are both produced by photoinduced electron transfer from dimeric 1-benzyl-1,4-dihydronicotinamide to P4H2, whereas the cation radical form of the compound is also produced by electron-transfer oxidation of P4H2 with [Ru(bpy)3]3+. The ESR spectra of P4H2*+ and P4H2*- were recorded at low temperature and exhibit spin delocalization over all four pyrrole units. Thus, the two-electron oxidized form of the quaterpyrrole (P4H2) displays redox and electronic features analogous to those seen in the case of porphyrins and may be considered as a simple, open-chain model of this well-studied tetrapyrrolic macrocycle. The dynamics of deprotonation from P4H2*+ and disproportionation of P4H2 were examined by laser flash photolysis measurements of photoinduced electron-transfer oxidation and reduction of P4H2, respectively.