Optimized nanospace of coordination isomers with selenium sites for acetylene separation

Assisted by functional Cu and Se sites, a new group of porous isomers demonstrated systematically tunable C2H2/CO2 separation.

Protonation of carboxyl groups in EuDOTA-tetraamide complexes results in catalytic prototropic exchange and quenching of the CEST signal

The CEST properties of EuDOTA-tetraamide complexes bearing pendant carboxylate and carboxyl ethyl esters were measured as a function of pH. The CEST signal from the Eu 3+ -bound water molecule decreased in intensity between pH 8.5 and 4.5 while the proton exchange rates ( k ex ) increased over this same pH range. In comparison, the CEST signal in the corresponding carboxyl ester derivatives was nearly constant. Both observations are consistent with stepwise protonation of the four carboxylic acid groups over this same pH range. This indicates that negative charges on the carboxyl groups above pH 6 facilitate the formation of a strong hydrogen-bonding network in the coordination second sphere above the single Eu 3+ -bound water molecule, thereby decreasing prototropic exchange of protons on the bound water molecule with bulk water protons. The percentage of square antiprismatic versus twisted square antiprism coordination isomers also decreased as the appended carboxylic acid groups were positioned further away from the amide. The net effect of lowering the pH was an overall increase in k ex and a quenching of the CEST signal. This article is part of the themed issue ‘Challenges for chemistry in molecular imaging’.

Supramolecular silver(i) coordination isomers of different dimensionalities assembled using a conformationally flexible multidentate oligo-α-sulfanylpyrazinyl ligand

Three silver(i) supramolecular coordination isomers that display different dimensionalities and one homologue are derived from the conformationally flexible multidentate ligand 2,6-bis(pyrazin-2-ylthio)pyrazine (L).

Reaction of 2-Pyridylmethylthiourea Derivatives with [(en)2Co(OSO2CF3)2]+ Induces Hypodentate Coordination of an Ethylenediamine Ligand

Pyridylmethylthiourea derivatives coordinate with [(en)2Co(OSO2CF3)2]+ in a tridentate manner resulting in the formation of a hypodentate ethylenediamine ligand. Four ligands were studied: N-(R)phenyl-N′-2-pyridylmethylthiourea (R = H (1a), CH3 (1b), OCH3 (1c)) and N-benzyl-N′-2-pyridylmethylthiourea (2). These bind through the sulfur, a deprotonated exo nitrogen, and the pyridyl nitrogen atoms forming four and five-membered rings, respectively. The ligand also coordinates in a bidentate manner through the sulfur and deprotonated endo or exo nitrogen atoms, forming two additional coordination isomers. The solid state structure (X-ray) of one of the bidentate isomers of Co-1b2+ (endo isomer) shows that the coordinated thiourea sulfur induces a structural trans effect of 0.035 Å on the trans Co–N bond while that of the tridentate isomer of Co-1a3+ confirms the coordination mode of the ligand and the presence of a protonated hypodentate ethylenediamine ligand as suggested by 1H and 13C NMR spectroscopy.

Synthesis and Characterisation of [(en)2Co]3+ Complexes Coordinated by Substituted Thiourea Ligands

Substituted thiourea ligands bind in a bidentate manner forming a four-membered ring through the sulfur atom and a deprotonated thiourea nitrogen atom when reacted with [(en)2Co(OSO2CF3)2]+ in tetramethylene sulfone. Reaction of unsymmetrical ligands results in the formation of coordination isomers, some of which can be separated by column chromatography using Sephadex SPC-25. Coordination isomers are easily distinguishable based on visible and 1H NMR spectroscopy . Twelve para-substituted and one meta-substituted ligands were studied: N,N′-dibenzylthiourea (1a); N-(R)phenyl-N′-benzylthiourea {R = H (2a), NO2 (2b), CH3 (2c)}; N-(R)phenyl-N′-(R′)phenylthiourea {R, R′: H, H (3a), H, CH3 (3b), OCH3, NO2 (3c), CH3, NO2 (3d)}; N-methyl-N′-(R)phenylthiourea {R = H (4a), CH3 (4b), OCH3 (4c), NO2 (4d), 3-CH3 (4e)}. The solid state structure (X-ray) of one isomer of Co-4a as its perchlorate salt confirms the coordination mode suggested by 1H NMR spectroscopy and shows that the Co–N bond trans to the coordinated thiourea sulfur induces a structural trans effect of 0.019 Å.