Pb, Sr and Ba calix[6]arene hexacarboxylic acid octahedral complexation: a dramatic effect of dealkylation

<div><p>Calix[6]arene hexacarboxylic acid binds instantly and with low symmetry to Pb, Sr and Ba. Later a highly symmetric up-down alternating conformation emerges. The solution structures are identical to their p-tert-butylcalix[6]arene hexacarboxylic acid counterparts. With either receptor, an octahedral cage is formed around the metal. The transformation from low to high symmetry however proceeds at significantly faster rates for the de-t-butylated host.</p></div

Synthesis of 1,2-Bis(2-aryl‑1<i>H</i>‑indol-3-yl)ethynes via 5-<i>exo</i>-Digonal Double Cyclization Reactions of 1,4-Bis(2-isocyanophenyl)buta-1,3-diyne with Aryl Grignard Reagents

New π-conjugated 1,2-bis­(2-aryl-1<i>H</i>-indol-3-yl)­ethynes <b>1a</b>–<b>j</b> having various substituents on the two aryl groups were efficiently synthesized via unusual 5-<i>exo</i>-digonal double isocyanide-acetylene cyclization reactions of 1,4-bis­(2-isocyanophenyl)­buta-1,3-diyne <b>3</b> and aryl Grignard reagents (R-MgBr, R = C₆H₅ (<b>1a</b>), 4-H₃CC₆H₄ (<b>1b</b>), 2-H₃CC₆H₄ (<b>1c</b>), 3-MeOC₆H₄ (<b>1d</b>), 3-(CH₃)₂NC₆H₄ (<b>1e</b>), 4-F₃CC₆H₄ (<b>1f</b>), 4-FC₆H₄ (<b>1g</b>), 3-FC₆H₄ (<b>1h</b>), 4-PhOC₆H₄ (<b>1i</b>), and 2-Naph (<b>1j</b>)) in 19–85% yields. The UV–vis spectra were rationalized in detail using time-dependent DFT and single point calculations. The fluorescence emission peaks for <b>1a</b>–<b>j</b> were observed at around 450 nm. Especially for <b>1f</b> and <b>1j</b>, those spectra displayed broad emission bands and relatively large Stokes shifts (3977–4503 cm^–1), indicating the contribution of an intramolecular charge transfer. The absolute quantum yields (0.50–0.62) of <b>1a</b>–<b>j</b> were higher than those of parent <b>8</b> (0.19) and 2-phenyl-1<i>H</i>-indole (0.11). The electrochemical features for <b>1a</b>–<b>j</b> were investigated by cyclic voltammetry. The frontier molecular orbital levels for <b>1a</b>–<b>j</b> were estimated based on the combination of oxidation potentials, UV–vis, and DFT calculated data. The structural property of 1,2-bis­(2-phenyl-1<i>H</i>-indol-3-yl)­ethyne <b>1a</b> was characterized by several spectroscopic methods and finally determined by X-ray analysis of a single crystal of <b>1a</b> recrystallized from ethyl acetate. The structural features of <b>1a</b>–<b>j</b> were also supported by DFT calculations