Methylazacalix[4]pyridine: En Route to Zn2+-Specific Fluoresence Sensors

Putting anion-π interactions into perspective

Supramolecular chemistry is a field of scientific exploration that probes the relationship between molecular structure and function. It is the chemistry of the noncovalent bond, which forms the basis of highly specific recognition, transport, and regulation events that actuate biological processes. The classic design principles of supramolecular chemistry include strong, directional interactions like hydrogen bonding, halogen bonding, and cation-π complexation, as well as less directional forces like ion pairing, π-π, solvophobic, and van der Waals potentials. In recent years, the anion-π interaction (an attractive force between an electron-deficient aromatic π system and an anion) has been recognized as a hitherto unexplored noncovalent bond, the nature of which has been interpreted through both experimental and theoretical investigations. The design of selective anion receptors and channels based on this interaction represent important advances in the field of supramolecular chemistry. The objectives of this Review are 1) to discuss current thinking on the nature of this interaction, 2) to survey key experimental work in which anion-π bonding is demonstrated, and 3) to provide insights into the directional nature of anion-π contact in X-ray crystal structures.

Occurrence and Chemistry of Dihydroxyfumaric Acid

The paper summarizes literature data on occurrence of dihydroxyfumaric acid and its role in biological systems, as well as its chemical properties.

Room-temperature aerobic formation of a stable aryl-Cu(III) complex and its reactions with nucleophiles: highly efficient and diverse arene C-H functionalizations of azacalix[1]arene[3]pyridine

Under very mild aerobic conditions, azacalix[1]arene[3]pyridine underwent highly efficient C-H activation with Cu(ClO(4))(2).6H(2)O to form a stable aryl-Cu(III) complex which reacted rapidly with various nucleophiles at ambient temperature to afford diverse functionalized azacalix[1]arene[3]pyridine derivatives in excellent yields.

A Zn2+ fluorescent sensor derived from 2-(pyridin-2-yl)benzoimidazole with ratiometric sensing potential

A fluorescent Zn(2+) sensor based on the 2-(2'-pyridinyl)benzoimidazole (2-PBI) fluorophore has been devised by incorporation with a Zn(2+) ionophore, bis(pyridin-2-ylmethyl)amine. The sensor (PBITA) demonstrates a Zn(2+)-specific emission shift and enhancement with a 1:1 binding ratio. Due to the Zn(2+)-induced coplanation of 2-PBI via reversion/rotation, PBITA is shown to behave as a ratiometric sensor. The intracellular Zn(2+) imaging ability of the sensor has been tested in HeLa cells using a confocal microscope.

Synthesis of large macrocyclic azacalix[n]pyridines (n = 6 - 9) and their complexation with fullerenes C(60) and C(70)

Large methylazacalix[n]pyridines (n = 6-9) were synthesized effectively from the Pd-catalyzed macrocyclic fragment coupling reactions between alpha,omega-dibrominated and alpha,omega-diaminated linear oligomers. As macrocyclic host molecules, they formed a 1:1 complex with fullerenes C(60) and C(70) with association constants ranging from 3 x 10(4) to 1 x 10(5) M(-1).

Highly Selective Recognition of Diols by a Self-Regulating Fine-Tunable Methylazacalix[4]pyridine Cavity: Guest-Dependent Formation of Molecular-Sandwich and Molecular-Capsule Complexes in Solution and the Solid State

The molecular recognition of methylazacalix[4]pyridine (MACP-4; 1) towards various diols was investigated by using (1)H NMR spectroscopic and X-ray diffraction analysis. As a unique macrocyclic host molecule that undergoes conformational inversions very rapidly in solution, MACP-4 has been shown to self-regulate its conformation, through the formation of different conjugations of the four bridging nitrogen atoms with their adjacent pyridine rings, to form a cavity that best fits the guest species through intermolecular hydrogen-bond, C-H...pi, and pi-pi interactions between the host and guest. As a consequence, depending upon the diol structure and geometry, MACP-4 forms a 1:1 molecular sandwich, 2:1 molecular capsule, and 1:2 butterfly-layered complex with the guests. As a result of favorable enthalpy and entropy effects, MACP-4 exhibits excellent selectivity in the recognition of resorcinol, thus resulting in a very stable 1:1 sandwich complex with resorcinol with a binding constant of 6000 (M-1). The dynamic (1)H NMR spectroscopic study demonstrated that the 1,3-alternate conformation of the macrocyclic ring of the MACP-4resorcinol complex (13) is stable at low temperature (T<243 K), and its conformational inversion requires a larger activation energy (DeltaG++=(45.5+/-2.2) kJ mol(-1)). In the presence of an excess amount of resorcinol, however, the conformational inversion of the MACP-4resorcinol complex proceeds more readily with a decreased activation energy (DeltaG++=(33.5+/-1.5) kJ mol(-1)) owing most probably to the favorable enthalpy effect of the [3...1...3]++transition state.