ABSTRACT
Simple and green conditions for copper-catalysed ATRA reactions in water have been developed. Firstly, [Cu(ADPA)(H2O)(ClO4)2] (1b, ADPA = 9-[(2,2'-dipicolylamino)methyl]anthracene) was demonstrated to be capable of selectively catalysing the ATRA of CCl4 to styrene using L-ascorbic acid (AsH2) as a reducing agent in organic solvent mixtures under ambient atmosphere. Mechanistic investigation suggested that our ATRA reaction proceeded via a single-electron transfer (SET) mechanism through an inner-sphere complex, which is consistent with the widely accepted mechanism for copper-catalysed ATRA. To perform the reaction in water as a sole solvent, a biocompatible surfactant (2 wt% Tween 20 or Tween 80) was added to improve solubility and increase the local concentration of organic reagents and the copper catalyst. Without the need for a complicated oxygen-free set-up, the ATRA reaction catalysed by this simple aqueous-dispersed system can be performed at a mild temperature (60 °C) and a relatively short reaction time (6 h) using 1 mol% of the catalyst. Furthermore, this facile protocol is also applicable for other alkene substrates demonstrated in this work, resulting in satisfactory to excellent substrate conversion and product yields.
ABSTRACT
A new and facile approach to stabilise copper(i) complexes in aqueous solution by the addition of zinc(ii) ions in combination with acetate ions (OAc-) was demonstrated. This stability enhancement toward the aerobic oxidation of copper(i) species was investigated by various techniques including UV-vis spectroscopy, 1H-NMR, FT-IR, and ESI-MS. Our experimental results together with DFT calculations led to a proposed structure of [(adpa)Cu-OAc-Zn(OAc)(H2O)2]+/2+. It was also postulated that zinc(ii) with its Lewis acidity may attract electrons from the Cu centre through the bridging ligands (OAc-), resulting in the lower reactivity of Cu(i) with O2. In addition, this strategy was shown to be applicable to ascorbic acid detection by monitoring a change in the redox states of copper complexes using fluorescence spectroscopy. Moreover, it was demonstrated that the method was sensitive and accurate for the quantitative analysis of ascorbic acid in vitamin C tablets.
ABSTRACT
A series of copper complexes bearing polypyridyl tridentate ligands have been prepared to fine tune their reactivity toward the oxygen reduction reaction (ORR). During the process of preparation of our copper complexes, we successfully obtained two new crystal structures which are [Cu2(µ-Cl)2(adpa)2](ClO4)2 (2b) and [Cu2(addpa)(CH3CN)2(ClO4)2](ClO4)2 (3a) and a new structure [Cu2(addpa)(CH3CN)2(H2O)2](ClO4)4 (3b) captured after the catalytic ORR. Electrochemical studies and stoichiometric chemical reduction of copper(ii) complexes by ascorbic acid indicated that the presence of an anthracene unit helps to facilitate the reduction of Cu(ii) as well as the stabilisation of Cu(i) species. Regarding oxygen activation, the dinuclear Cu(i) complex 3a showed significantly higher ORR activity than its analogous mononuclear complex 2a. Complex 3a was also found to be relatively robust and competent in catalytic O2 reduction. The observed H2O2 product after this catalysis, together with the data obtained from DFT calculations supported that 3a exhibited a 2H+, 2e- catalytic activity towards the ORR as opposed to the expected 4H+, 4e- process usually found in copper complexes with tridentate ligands. The proton (H+) source for this process was expected from ascorbic acid which also serves as a reducing agent in this reaction. This work highlighted an approach for tuning the ORR activity of the copper complexes by the introduction of a conjugated-π moiety to the supporting ligand.
ABSTRACT
In this research, the dinuclear Zn(ii) complex of anthracene based tripodal tetramine Zn2L was synthesized, and its sensing abilities towards anions was investigated using the indicator displacement assay (IDA) approach with four complexometric indicators: pyrocatechol violet (PV), bromopyrogallol red (BPG), methylthymol blue (MTB) and xylenol orange (XO). UV-vis spectrophotometry results indicated that the Zn2L-MTB ensemble sensor could discriminate the pyrophosphate anion (PPi) from other phosphate containing anions. (1)H and (31)P NMR spectroscopy as well as DFT calculations confirmed that PPi bound to Zn2L in a 2 : 2 manner. Both NMR spectroscopy and UV-vis spectrophotometry suggested that the two bulky tripodal tetramine units in Zn2L played an important role to provide the ensemble cleft for MTB, giving rise to an ensemble that could be displaced exclusively by PPi. The detection limit of PPi for the reported IDA system was 0.3 µM in 20% (v/v) water-acetonitrile buffered at pH 7.4 with HEPES.
