Heterogenization of a macrocyclic cobalt complex for photocatalytic CO₂ reduction

<p>Heterogenization could potentially improve the stability and recyclability of molecular catalysts. In this study, a Co(III) cyclam complex, where cyclam is 1,4,8,11-tetraazacyclotetradecane, was grafted on a mesoporous silica surface via two different covalent linkages. The resulting heterogenized catalysts were characterized with a variety of techniques and tested in photocatalytic CO₂ reduction in the presence of <i>p</i>-terphenyl as a molecular photosensitizer. Linking strategies were shown to be important for the preparation of highly active surface Co(III) sites. In particular, derivatizing one of the amines on the cyclam ligand resulted in a detrimental effect on the activity of the molecular Co(III) catalyst. Improved activity was achieved by reacting the Co(III) catalyst with surface silanol groups, forming Si–O–Co linkages. A catalyst loading effect was observed, where the best catalytic activity was achieved when the surface Co(III) sites likely formed a monolayer in silica mesopores. Our results also indicate that the cyclam ligand was essential for the observed activity using the heterogenized Co(III) catalysts.</p

Three-Dimensional Graphene–TiO₂ Nanocomposite Photocatalyst Synthesized by Covalent Attachment

We report the synthesis of a three-dimensional graphene (3DG)–TiO₂ nanocomposite by covalently attaching P25 TiO₂ nanoparticles onto pristine 3DG through a perfluorophenyl azide-mediated coupling reaction. The TiO₂ nanoparticles were robustly attached on the 3DG surface, with minimal particle agglomeration. In photocatalytic CO₂ reduction, the 3DG–TiO₂ nanocomposite demonstrated excellent activity, about 11 times higher than that of the P25 TiO₂ nanoparticles. The enhanced activity can be partially attributed to the highly dispersed state of the P25 TiO₂ nanoparticles on the 3DG substrate. This 3DG-based system offers a new platform for fabricating photocatalytic materials with enhanced activities

Re(I) NHC Complexes for Electrocatalytic Conversion of CO₂

The modular construction of ligands around an <i>N</i>-heterocyclic carbene building block represents a flexible synthetic strategy for tuning the electronic properties of metal complexes. Herein, methylbenzimidazolium-pyridine and methylbenzimidazolium-pyrimidine proligands are constructed in high yield using recently established transition-metal-free techniques. Subsequent chelation to ReCl­(CO)₅ furnishes ReCl­(<i>N</i>-methyl-<i>N</i>′-2-pyridylbenzimidazol-2-ylidine)­(CO)₃ and ReCl­(<i>N</i>-methyl-<i>N</i>′-2-pyrimidylbenzimidazol-2-ylidine)­(CO)₃. These Re­(I) NHC complexes are shown to be capable of mediating the two-electron conversion of CO₂ following one-electron reduction; the Faradaic efficiency for CO formation is observed to be >60% with minor H₂ and HCO₂H production. Data from cyclic voltammetry is presented and compared to well-studied ReCl­(2,2′-bipyridine)­(CO)₃ and MnBr­(2,2′-bipyridine)­(CO)₃ systems. Results from density functional theory computations, infrared spectroelectrochemistry, and chemical reductions are also discussed