The present review is devoted to summarizing the recent advances (2015-2017) in the field of metal-catalysed group-directed C-H functionalisation In order to clearly showcase the molecular diversity that can now be accessed by means of directed C-H functionalisation, the whole is organized following the directing groups installed on a substrate Its aim is to be a comprehensive reference work, where a specific directing group can be easily found, together with the transformations which have been carried out with it Hence, the primary format of this review is schemes accompanied with a concise explanatory text, in which the directing groups are ordered in sections according to their chemical structure The schemes feature typical substrates used, the products obtained as well as the required reaction conditions Importantly, each example is commented on with respect to the most important positive features and drawbacks, on aspects such as selectivity, substrate scope, reaction conditions, directing group removal, and greenness The targeted readership are both experts in the field of C-H functionalisation chemistry (to provide a comprehensive overview of the progress made in the last years) and, even more so, all organic chemists who want to introduce the C-H functionalisation way of thinking for a design of straightforward, efficient and step-economic synthetic routes towards molecules of interest to them Accordingly, this review should be of particular interest also for scientists from industrial R&D sector Hence, the overall goal of this review is to promote the application of C-H functionalisation reactions outside the research groups dedicated to method development and establishing it as a valuable reaction archetype in contemporary R&D, comparable to the role cross-coupling reactions play to date

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A comprehensive overview of directing groups applied in metal-catalysed C-H functionalisation chemistry.

Electrocatalysts with single metal atoms as active sites have received increasing attention owing to their high atomic utilization efficiency and exotic catalytic activity and selectivity. This review aims to provide a comprehensive summary on the recent development of such single-atom electrocatalysts (SAECs) for various energy-conversion reactions. The discussion starts with an introduction of the different types of SAECs, followed by an overview of the synthetic methodologies to control the atomic dispersion of metal sites and atomically resolved characterization using state-of-the-art microscopic and spectroscopic techniques. In recognition of the extensive applications of SAECs, the electrocatalytic studies are dissected in terms of various important electrochemical reactions, including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO2RR), and nitrogen reduction reaction (NRR). Examples of SAECs are deliberated in each case in terms of their catalytic performance, structure-property relationships, and catalytic enhancement mechanisms. A perspective is provided at the end of each section about remaining challenges and opportunities for the development of SAECs for the targeted reaction.

Advanced Electrocatalysts with Single-Metal-Atom Active Sites.

The electrochemical reduction of carbon dioxide (CO2) powered by renewable energy is an attractive sustainable approach to mitigate CO2 emissions and to produce fuels or value-added chemicals. In order to tackle the challenges related to selectivity, activity, overpotential and durability, transition metal-based catalysts have been widely investigated in the last decades. In an effort to bridge the gap between the fields of homogeneous and heterogeneous catalysis, this review aims to survey the main strategies explored for the rational design of a wide variety of different metal catalysts, ranging from molecular systems to single-atom and nanostructured catalysts. Transition metal complexes containing heme and non-heme ligands have been selected to discuss the recent advances in the understanding of the structure–function relationship in molecular homogeneous catalysis as well as to summarize the main approaches proposed for the heterogenization or confinement of molecular catalysts on conductive surfaces. The main strategies to minimize catalyst cost are also presented, leading to atomically dispersed molecular-like M–Nx moieties embedded on 2D conducting materials. The superior performances of single-atom catalysts (SACs) and the structural similarity with their molecular analogs, suggest that transition metal catalysts containing well-defined sites may be intrinsically more active and selective towards CO2 conversion than the bulk heterogeneous materials. Finally, design approaches for metal nanoparticles (NPs) based on size, shape, and support tuning are summarized and compared to novel strategies based on the interaction with surface-bonded organic molecules. The studies herein presented show that the basic principles in molecular catalysis and organometallic chemistry can be effectively used to design new efficient and selective heterogeneous catalysts for CO2 reduction.

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Transition metal-based catalysts for the electrochemical CO2 reduction: from atoms and molecules to nanostructured materials

Porphyrin-based frameworks, as specific kinds of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), have been widely used in energy-related conversion processes, including the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and CO2 reduction reaction (CO2RR), and also in energy-related storage technologies such as rechargeable Zn-air batteries. This review starts by summarizing typical crystal structures, molecular building blocks, and common synthetic procedures of various porphyrin-based frameworks used in energy-related technologies. Then, a brief introduction is provided and representative applications of porphyrin-based frameworks in ORR, OER, Zn-air batteries, and CO2RR are discussed. The performance comparison of these porphyrin-based frameworks in each field is also summarized and discussed, which pinpoints a clear structure-activity relationship. In addition to utilizing highly active porphyrin units for catalytic conversions, regulating the porous structures of porphyrin-based frameworks will enhance mass transfer and growing porphyrin-based frameworks on conductive supports will accelerate electron transfer, which will result in the improvement of the electrocatalytic performance. This review is therefore valuable for the rational design of more efficient porphyrin-based framework catalytic systems in energy-related conversion and storage technologies.

Porphyrin-based frameworks for oxygen electrocatalysis and catalytic reduction of carbon dioxide

The electrocatalytic transformation of carbon dioxide has been a topic of interest in the field of CO2 utilization for a long time. Recently, the area has seen increasing dynamics as an alternative strategy to catalytic hydrogenation for CO2 reduction. While many studies focus on the direct electron transfer to the CO2 molecule at the electrode material, molecular transition metal complexes in solution offer the possibility to act as catalysts for the electron transfer. C1 compounds such as carbon monoxide, formate, and methanol are often targeted as the main products, but more elaborate transformations are also possible within the coordination sphere of the metal center. This perspective article will cover selected examples to illustrate and categorize the currently favored mechanisms for the electrochemically induced transformation of CO2 promoted by homogeneous transition metal complexes. The insights will be corroborated with the concepts and elementary steps of organometallic catalysis to derive potential strategies to broaden the molecular diversity of possible products.

Transition Metal Complexes as Catalysts for the Electroconversion of CO 2 : An Organometallic Perspective

Iron porphyrins are potential catalysts for the electrocatalytic and photocatalytic reduction of CO2. It has been recently established that the reduction of CO2 by an iron porphyrin complex with a hydrogen bonding distal pocket involves at least two intermediates: a Fe(II)–CO22− and a Fe(II)–COOH species. A distal hydrogen bonding interaction was found to be key in determining the stability of these intermediates and affecting both the selectivity and rate of CO2 reduction. In this report, a series of iron porphyrins that vary only in the distal H-bonding network are further investigated and these exhibit turnover frequencies (TOFs) ranging from 1.0 s−1 to 103 s−1. The experimental TOFs correlate with the H-bonding ability of the distal superstructure of these iron porphyrin complexes and analysis suggests that H-bonding alone can tune the rate of CO2 reduction by as much as 1000 fold. DFT calculations provide a detailed insight into how the, apparently weak, 2nd sphere interactions lead to efficient CO2 activation for reduction. The ability to tune CO2 reduction rates by changing the H-bonding residue instead of the acid source is a convenient way to tune CO2 reduction electrocatalysis without compromising selectivity by introducing competitive hydrogen evolution reaction or formate generation.

Role of 2nd sphere H-bonding residues in tuning the kinetics of CO2 reduction to CO by iron porphyrin complexes

Herein we present a Pd-catalyzed direct C–H hydroxylation of 2-arylpyridines using molecular oxygen (O2) as the sole oxidant. The key aspects of the method include: (a) the activation of molecular oxygen with a nontoxic and inexpensive aldehyde; (b) an efficient association of the in situ-generated acyl peroxo radical with palladium catalysis; and (c) convenient operating conditions. On the basis of the results obtained in a series of control experiments, a PdII/PdIV catalytic cycle is implicated for the transformations. Furthermore, the method offers an easy access to a broad range of substituted 2-(pyridin-2-yl)phenols in good isolated yields.

Aerobic Direct C(sp2)-H Hydroxylation of 2-Arylpyridines by Palladium Catalysis Induced with Aldehyde Auto-Oxidation

Reduction of CO2 to value-added chemicals is a logical way of fixing the rising levels of CO2. Activation and reduction of CO2 requires low-valent transition metals as catalysts. A major challenge in this chemistry is sensitivity of these low-valent metal sites to more abundant O2. Since O2 is a stronger oxidant than CO2 and isolated from the obvious competitive inhibition of CO2, partial reduction of O2 leads to formation of reactive oxygen species like O2– and H2O2, which are deleterious to the catalyst itself. An iron porphyrin complex appended with four ferrocene groups in its distal site is demonstrated to reduce CO2 unabated in the presence of O2 as it can reduce O2 to benign H2O under the same conditions. Further investigations reveal that iron porphyrins, in general, reduce CO2 selectively in the presence of O2. The aforementioned selectivity is derived from a 500 times faster rate of reaction of CO2 with Fe(0) porphyrin relative to O2 despite a higher driving force for the latter.

Reduction of CO2 to CO by an Iron Porphyrin Catalyst in the Presence of Oxygen

In the current environmental and economic context, there is an urgent need to develop catalytic systems to efficiently activate and transform abundant small molecules while demonstrating selectivit...

Dibyajyoti Saha

Papers

Role of 2nd sphere H-bonding residues in tuning the kinetics of CO2 reduction to CO by iron porphyrin complexes

Aerobic Direct C(sp2)-H Hydroxylation of 2-Arylpyridines by Palladium Catalysis Induced with Aldehyde Auto-Oxidation

Reduction of CO2 to CO by an Iron Porphyrin Catalyst in the Presence of Oxygen

Repurposing a Bio-Inspired NiFe Hydrogenase Model for CO2 Reduction with Selective Production of Methane as the Unique C-Based Product

An [FeFe]-Hydrogenase Mimic Immobilized through Simple Physiadsorption and Active for Aqueous H2 Production