Electrocatalytic Reduction of CO2 by Copper (II) Cyclam Derivatives

This study investigates Cu(II) complexes of cyclam, propylene cross-bridged cyclam (PCB-cyclam), and propylene crossbridged cyclam diacetate (PCB-TE2A) as homogeneous electrocatalysts for CO2 reduction in comparison with Ni(II)- cyclam. It is found that Cu(II)-cyclam can catalyze CO2 reduction at the potential close to its thermodynamic value (0.75 V vs. Ag/AgCl) in tris-HCl buffer (pH 8.45) on a glassy carbon electrode. Cu(II)-cyclam, however, suffers from severe demetalation due to the insufficient stability of Cu(I)-cyclam. Cu(II)-PCB-cyclam and Cu(II)-PCB-TE2A are revealed to exhibit much less demetalation behavior, but poor CO2 reduction activities as well. The inferior electrocatalytic ability of Cu(II)-PCB-cyclam is ascribed to its redox potential that is too high for CO2 reduction, and that of Cu(II)-PCB-TE2A to the steric hindrance preventing facile contact with CO2 molecules. This study suggests that in addition to the redox potential and chemical stability, the stereochemical aspect has to be considered in designing efficient electrocatalysts for CO2 reduction. © 2015, Korean Electrochemical Society. All rights reserved.

Synthesis and Evaluation of New Generation Cross-Bridged Bifunctional Chelator for Cu-64 Radiotracers

Bifunctional chelators have been successfully used to construct 64Cu-labeled radiopharmaceuticals. Previously reported chelators with cross-bridged cyclam backbones have various essential features such as high stability of the copper(II) complex, high efficiency of radiolabeling at room temperature, and good biological inertness of the radiolabeled complex, along with rapid body clearance. Here, we report a new generation propylene-cross-bridged chelator with hybrid acetate/phosphonate pendant groups (PCB-TE1A1P) developed with the aim of combining these key properties in a single chelator. The PCB-TE1A1P was synthesized from cyclam with good overall yield. The Cu(II) complex of our chelator showed good robustness in kinetic stability evaluation experiments, such as acidic decomplexation and cyclic voltammetry studies. The Cu(II) complex of PCB-TE1A1P remained intact under highly acidic conditions (12 M HCl, 90 C) for 8 d and showed quasi-reversible reduction/oxidation peaks at -0.77 V in electrochemical studies. PCB-TE1A1P was successfully radiolabeled with 64Cu ions in an acetate buffer at 60 C within 60 min. The electrophoresis study revealed that the 64Cu-PCB-TE1A1P complex has net negative charge in aqueous solution. The biodistribution and in vivo stability study profiles of 64Cu-PCB-TE1A1P indicated that the radioactive complex was stable under physiological conditions and cleared rapidly from the body. A whole body positron emission tomography (PET) imaging study further confirmed high in vivo stability and fast clearance of the complex in mouse models. In conclusion, PCB-TE1A1P has good potential as a bifunctional chelator for 64Cu-based radiopharmaceuticals, especially those involving peptides. (Chemical Equation Presented). © 2015 American Chemical Society.