Differentiation of Oxygen Atom Chirality in Copper(II) Complexes with Dipicolylamine‐Derived Ligands

Evaluation of oxygen-containing pentadentate ligands with pyridine/quinoline/isoquinoline binding sites via the structural and electrochemical properties of mononuclear copper(II) complexes

Eighteen mononuclear copper(II) complexes with oxygen-containing N4O1 pentadentate ligands were prepared. The ligand library consists of 2-aminoethanol derivatives ((Ar1CH2)(Ar2CH2)NCH2CH2OCH2Ar3) bearing three nitrogen-containing heteroaromatics (Ars) including pyridine, quinoline and isoquinoline via a methylene linker. Systematic replacements of pyridine binding sites with quinolines and isoquinolines reveal the general trends in the perturbation of bond distances and angles, the redox potential and the absorption maximum wavelength of the copper(II) complexes, depending on the position and number of (iso)quinoline heteroaromatics. The small effect on the redox potentials resulting from quinoline substitution at the Ar3 position (near oxygen) of the ligand comes from the steric hindrance of the peri hydrogen atom in the quinoline moiety at this position, which removes the counter anion to enhance the coordination of quinoline nitrogen and ether oxygen atoms to the metal centre. In the absorption spectra of copper(II) complexes in the d-d transition region, the quinoline substitution at this site (Ar3) exhibits an opposite effect to those at the Ar1 and Ar2 sites. The electronic and steric contributions of the heteroaromatic binding sites to the ligand properties are comprehensively discussed.

Control of stereogenic oxygen in a helically chiral oxonium ion

The control of tetrahedral carbon stereocentres remains a focus of modern synthetic chemistry and is enabled by their configurational stability. By contrast, trisubstituted nitrogen¹, phosphorus² and sulfur compounds³ undergo pyramidal inversion, a fundamental and well-recognized stereochemical phenomenon that is widely exploited⁴. However, the stereochemistry of oxonium ions-compounds bearing three substituents on a positively charged oxygen atom-is poorly developed and there are few applications of oxonium ions in synthesis beyond their existence as reactive intermediates^5,6. There are no examples of configurationally stable oxonium ions in which the oxygen atom is the sole stereogenic centre, probably owing to the low barrier to oxygen pyramidal inversion⁷ and the perception that all oxonium ions are highly reactive. Here we describe the design, synthesis and characterization of a helically chiral triaryloxonium ion in which inversion of the oxygen lone pair is prevented through geometric restriction to enable it to function as a determinant of configuration. A combined synthesis and quantum calculation approach delineates design principles that enable configurationally stable and room-temperature isolable salts to be generated. We show that the barrier to inversion is greater than 110 kJ mol^-1 and outline processes for resolution. This constitutes, to our knowledge, the only example of a chiral non-racemic and configurationally stable molecule in which the oxygen atom is the sole stereogenic centre.

Designing a robust recyclable tricopolymer poly(ionic liquid) macroligand for copper-mediated atom transfer radical polymerization in non-aqueous biphasic systems

Herein, a robust thermoregulated poly(ionic liquid) macroligand was designed, synthesized and applied in an ICAR-based ATRP-TPSC system with efficient recycling/reuse.