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A bio‐inspired FeFe hydrogenase model which catalyses hydrogen evolution reaction (HER) in acidic solutions is immobilized in polyaniline (PANI)‐based nanotubes. A combination of analytical techniques reveals that this construct maintains both the molecular signatures of the bio‐inspired complex and the material properties of PANI. The amine and imine‐rich environment of the PANI chain amplifies the inherent HER activity of the bio‐inspired complex, allowing electrocatalytic HER at neutral pH, with lower overpotentials and higher current densities compared to the bio‐inspired complex alone. This construct retains the oxygen stability of the bio‐inspired complex and remains stable through several hours of aerobic electrolysis, producing only 6.5 % H₂O₂ from the competing oxygen reduction reaction (ORR). [ABSTRACT FROM AUTHOR]

Mimicking hydrogenases with synthetic complexes is a promising strategy for the design of Earth-abundant electrocatalysts for H2 evolution as alternative to platinum. Here, we describe a bio-inspired FeFe electrocatalyst, with a semi-bridging µ-CO ligand, active and stable for H2 evolution in acidic aqueous solutions after its physiadsorption onto carbon-based electrodes. [ABSTRACT FROM AUTHOR]

Reduction of oxides and oxoanions of carbon and nitrogen are of great contemporary importance as they are crucial for a sustainable environment. Substantial research has been dedicated to these areas in the last few decades. These reductions require both electrons and protons and their thermodynamic potentials often make them compete with hydrogen evolution reaction i.e., the reaction of protons and electrons to generate H2. These reactions are abundant in the environment in microorganisms and are facilitated by naturally occurring enzymes. This review brings together the state-of-the-art knowledge in the area of enzymatic reduction of CO2, NO2− and H+ with those of artificial molecular electrocatalysis. A simple ligand field theory-based design principle for electrocatalysts is first described. The electronic structure considerations developed automatically yield the basic geometry required and the 2nd sphere interactions which can potentially aid the activation and the further reduction of these small molecules. A systematic review of the enzymatic reaction followed by those reported in artificial molecular electrocatalysts is presented for the reduction of CO2, NO2− and H+. The review is focused on mechanism of action of these metalloenzymes and artificial electrocatalysts and discusses general principles that guide the rates and product selectivity of these reactions. The importance of the 2nd sphere interactions in both enzymatic and artificial molecular catalysis is discussed in detail. [ABSTRACT FROM AUTHOR]

A synthetic mimic of Fe–Fe hydrogenase (H2ase) is reported which bears a terminal alkyne group in the ligand. Using a terminal azide bearing organic linkers, this complex could be covalently attached to various electrode surfaces (e.g. edge plane graphite, reduced graphene oxide, etc.). The electrocatalytic hydrogen evolution (HER) efficiency of these constructs is investigated and the results show that the EPG–H2ase mimic construct is able to produce H2 from acidic water efficiently with over 90% selectivity. [ABSTRACT FROM AUTHOR]

Reduction of nitrate is an essential, yet challenging chemical task required to manage this relatively inert oxoanion in the environment and biology. We show that thiols, ubiquitous reductants in biology, convert nitrate to nitric oxide at a Cu(II) center under mild conditions. The β-diketiminato complex [Cl 2 NN F6 ]Cu(κ 2 -O 2 NO) engages in O-atom transfer with various thiols (RSH) to form the corresponding copper(II) nitrite [Cu II ](κ 2 -O 2 N) and sulfenic acid (RSOH). The copper(II) nitrite further reacts with RSH to give S -nitrosothiols RSNO and [Cu II ] 2 (μ-OH) 2 en route to NO formation via [Cu II ]-SR intermediates. The gasotransmitter H 2 S also reduces nitrate at copper(II) to generate NO, providing a lens into NO 3 - /H 2 S crosstalk. The interaction of thiols with nitrate at copper(II) releases a cascade of N- and S-based signaling molecules in biology.

Reduction of oxides and oxoanions of carbon and nitrogen are of great contemporary importance as they are crucial for a sustainable environment. Substantial research has been dedicated to these areas in the last few decades. These reductions require both electrons and protons and their thermodynamic potentials often make them compete with hydrogen evolution reaction i.e., the reaction of protons and electrons to generate H 2 . These reactions are abundant in the environment in microorganisms and are facilitated by naturally occurring enzymes. This review brings together the state-of-the-art knowledge in the area of enzymatic reduction of CO 2 , NO 2 - and H + with those of artificial molecular electrocatalysis. A simple ligand field theory-based design principle for electrocatalysts is first described. The electronic structure considerations developed automatically yield the basic geometry required and the 2 nd sphere interactions which can potentially aid the activation and the further reduction of these small molecules. A systematic review of the enzymatic reaction followed by those reported in artificial molecular electrocatalysts is presented for the reduction of CO 2 , NO 2 - and H + . The review is focused on mechanism of action of these metalloenzymes and artificial electrocatalysts and discusses general principles that guide the rates and product selectivity of these reactions. The importance of the 2 nd sphere interactions in both enzymatic and artificial molecular catalysis is discussed in detail.

The chemical and electrochemical reduction of CO 2 to value added chemicals entails the development of efficient and selective catalysts. Synthesis, characterization and electrochemical CO 2 reduction activity of a air-stable cobalt(III) diphenylphosphenethano-bis(2-pyridinethiolate)chloride [{Co(dppe)(2-PyS) 2 }Cl, 1-Cl ] complex is divulged. The complex reduces CO 2 under homogeneous electrocatalytic conditions to produce CO with high Faradaic efficiency (FE > 92%) and selectivity in the presence of water. Through detailed electrochemical investigations, product analysis, and mechanistic investigations supported by theoretical calculations, it is established that complex 1-Cl reduces CO 2 in its Co(I) state. A reductive cleavage leads to a dangling protonated pyridine arm which enables facile CO 2 binding through a H-bond donation and facilitates the C-O bond cleavage via a directed protonation. A systematic benchmarking of this catalyst indicates that it has a modest overpotential (∼180 mV) and a TOF of ∼20 s -1 for selective reduction of CO 2 to CO with H 2 O as a proton source.

[NiFe]-hydrogenase enzymes are efficient catalysts for H 2 evolution but their synthetic models have not been reported to be active under aqueous conditions so far. Here we show that a close model of the [NiFe]-hydrogenase active site can work as a very active and stable heterogeneous H 2 evolution catalyst under mildly acidic aqueous conditions. Entry in catalysis is a Ni I Fe II complex, with electronic structure analogous to the Ni-L state of the enzyme, corroborating the mechanism modification recently proposed for [NiFe]-hydrogenases., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The instability of [FeFe]-H 2 ases and their biomimetics toward O 2 renders them inefficient to implement in practical H 2 generation (HER). Previous investigations on synthetic models as well as natural enzymes proved that reactive oxygen species (ROS) generated on O 2 exposure oxidatively degrades the 2Fe subcluster within the H-cluster active site. Recent electrochemical studies, coupled with theoretical investigations on [FeFe]-H 2 ase suggested that selective O 2 reduction to H 2 O could eliminate the ROS, and hence, tolerance against oxidative degradation could be achieved ( Nat. Chem. 2017, 9, 88-95). We have prepared a series of 2Fe subsite mimics with substituted arenes attached to bridgehead N atoms in the S to S linker, (μ-S 2 (CH 2 ) 2 NAr)[Fe(CO) 3 ] 2 . Structural analyses find the nature of the substituent on the arene offers steric control of the orientation of bridgehead N atoms, affecting their proton uptake and translocation ability. The heterogeneous electrochemical studies of these complexes physiadsorbed on edge plane graphite (EPG) electrode show the onset of HER activity at ∼180 mV overpotential in pH 5.5 water. In addition, bridgehead N-protonation and subsequent H-bonding capability are established to facilitate the O-O bond cleavage resulting in selective O 2 reduction to H 2 O. This allows a synthetic [FeFe]-H 2 ase model to reduce protons to H 2 unabated in the presence of dissolved O 2 in water at nearly neutral pH (pH 5.5); i.e., O 2 -tolerant, stable HER activity is achieved.

Reduction of CO 2 holds the key to solving two major challenges taunting the society-clean energy and clean environment. There is an urgent need for the development of efficient non-noble metal-based catalysts that can reduce CO 2 selectively and efficiently. Unfortunately, activation and reduction of CO 2 can only be achieved by highly reduced metal centers jeopardizing the energy efficiency of the process. A carbon monoxide dehydrogenase inspired Co complex bearing a dithiolato ligand can reduce CO 2 , in wet acetonitrile, to CO with ∼95% selectivity over a wide potential range and 1559 s -1 rate with a remarkably low overpotential of 70 mV. Unlike most of the transition-metal-based systems that require reduction of the metal to its formal zerovalent state for CO 2 reduction, this catalyst can reduce CO 2 in its formal +1 state making it substantially more energy efficient than any system known to show similar reactivity. While covalent donation from one thiolate increases electron density at the Co(I) center enabling it to activate CO 2 , protonation of the bound thiolate, in the presence of H 2 O as a proton source, plays a crucial role in lowering overpotential (thermodynamics) and ensuring facile proton transfer to the bound CO 2 ensuring facile (kinetics) reactivity. A very covalent Co(III)-C bond in a Co(III)-COOH intermediate is at the heart of selective protonation of the oxygen atoms to result in CO as the exclusive product of the reduction.

A synthetic mimic of Fe-Fe hydrogenase (H 2 ase) is reported which bears a terminal alkyne group in the ligand. Using a terminal azide bearing organic linkers, this complex could be covalently attached to various electrode surfaces (e.g. edge plane graphite, reduced graphene oxide, etc.). The electrocatalytic hydrogen evolution (HER) efficiency of these constructs is investigated and the results show that the EPG-H 2 ase mimic construct is able to produce H 2 from acidic water efficiently with over 90% selectivity.