H2 Evolution from Electrocatalysts with Redox-Active Ligands: Mechanistic Insights from Theory and Experiment vis-à-vis Co-Mabiq

Low-Valent Transition Metalate Anions in Synthesis, Small Molecule Activation, and Catalysis

This review surveys the synthesis and reactivity of low-oxidation state metalate anions of the d-block elements, with an emphasis on contributions reported between 2006 and 2022. Although the field has a long and rich history, the chemistry of transition metalate anions has been greatly enhanced in the last 15 years by the application of advanced concepts in complex synthesis and ligand design. In recent years, the potential of highly reactive metalate complexes in the fields of small molecule activation and homogeneous catalysis has become increasingly evident. Consequently, exciting applications in small molecule activation have been developed, including in catalytic transformations. This article intends to guide the reader through the fascinating world of low-valent transition metalates. The first part of the review describes the synthesis and reactivity of d-block metalates stabilized by an assortment of ligand frameworks, including carbonyls, isocyanides, alkenes and polyarenes, phosphines and phosphorus heterocycles, amides, and redox-active nitrogen-based ligands. Thereby, the reader will be familiarized with the impact of different ligand types on the physical and chemical properties of metalates. In addition, ion-pairing interactions and metal-metal bonding may have a dramatic influence on metalate structures and reactivities. The complex ramifications of these effects are examined in a separate section. The second part of the review is devoted to the reactivity of the metalates toward small inorganic molecules such as H2, N2, CO, CO2, P4 and related species. It is shown that the use of highly electron-rich and reactive metalates in small molecule activation translates into impressive catalytic properties in the hydrogenation of organic molecules and the reduction of N2, CO, and CO2. The results discussed in this review illustrate that the potential of transition metalate anions is increasingly being tapped for challenging catalytic processes with relevance to organic synthesis and energy conversion. Therefore, it is hoped that this review will serve as a useful resource to inspire further developments in this dynamic research field.

Exploring the Multifarious Role of the Ligand in Electrocatalytic Hydrogen Evolution Reaction Pathways

In recent years, researchers have shifted their focus towards investigating the redox properties of ancillary ligand backbones for small-molecule activation. Several metal complexes have been reported for the electrocatalytic H2 evolution reaction (HER), providing valuable mechanistic insights. This process involves efficient coupling of electrons and protons. Redox-active ligands stipulate internal electron transfer and promote effective orbital overlap between metal and ligand, thereby, enabling efficient proton-coupled electron transfer reactions. Understanding such catalytic mechanisms requires thorough spectroscopic and computational analyses. Herein, we summarize recent examples of molecular electrocatalysts based on 3d transition metals that have significantly influenced mechanistic pathways, thus, emphasizing the multifaceted role of metal-ligand cooperativity.

Electrocatalytic CO2 Reduction and H2 Evolution by a Copper (II) Complex with Redox-Active Ligand

The process of electrocatalytic CO2 reduction and H2 evolution from water, regarding renewable energy, has become one of the global solutions to problems related to energy consumption and environmental degradation. In order to promote the electrocatalytic reactivity, the study of the role of ligands in catalysis has attracted more and more attention. Herein, we have developed a copper (II) complex with redox-active ligand [Cu(L1)2NO3]NO3 (1, L1 = 2-(6-methoxypyridin-2-yl)-6-nitro-1h-benzo [D] imidazole). X-ray crystallography reveals that the Cu ion in cation of complex 1 is coordinated by two redox ligands L1 and one labile nitrate ligand, which could assist the metal center for catalysis. The longer Cu-O bond between the metal center and the labile nitrate ligand would break to provide an open coordination site for the binding of the substrate during the catalytic process. The electrocatalytic investigation combined with DFT calculations demonstrate that the copper (II) complex could homogeneously catalyze CO2 reduction towards CO and H2 evolution, and this could occur with great performance due to the cooperative effect between the central Cu (II) ion and the redox- active ligand L1. Further, we discovered that the added proton source H2O and TsOH·H2O (p-Toluenesulfonic acid) could greatly enhance its electrocatalytic activity for CO2 reduction and H2 evolution, respectively.