Properties of monovalent nickel complexes with tetraaza-macrocyclic ligands in aqueous solutions

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

An overview of the main strategies for the rational design of transition metal-based catalysts for the electrochemical conversion of CO₂, ranging from molecular systems to single-atom and nanostructured catalysts.

Sulfur substitution in a Ni(cyclam) derivative results in lower overpotential for CO2 reduction and enhanced proton reduction

The replacement of the opposing nitrogen atoms in 1,4,8,11-tetraazacyclotetradecane (cyclam) with two sulfur atoms in 1,8-dithia-4,11-diazacyclotetradecane (dithiacyclam) enables the electrochemical reduction of protons and CO2via the corresponding nickel(ii) complex at more positive potentials. In addition, a 10-fold enhancement in the proton reduction rate of [Ni(dithiacyclam)]2+ relative to [Ni(cylcam)]2+ was observed. The study provides vital insight into Nature's choice of employing predominantly sulfur based ligand platforms in achieving biological proton and CO2 reductions.

Pendant Hydrogen-Bond Donors in Cobalt Catalysts Independently Enhance CO2 Reduction

The bioinspired incorporation of pendant proton donors into transition metal catalysts is a promising strategy for converting environmentally deleterious CO₂ to higher energy products. However, the mechanism of proton transfer in these systems is poorly understood. Herein, we present a series of cobalt complexes with varying pendant secondary and tertiary amines in the ligand framework with the aim of disentangling the roles of the first and second coordination spheres in CO₂ reduction catalysis. Electrochemical and kinetic studies indicate that the rate of catalysis shows a first-order dependence on acid, CO₂, and the number of pendant secondary amines, respectively. Density functional theory studies explain the experimentally observed trends and indicate that pendant secondary amines do not directly transfer protons to CO₂, but instead bind acid molecules from solution. Taken together, these results suggest a mechanism in which noncooperative pendant amines facilitate a hydrogen-bonding network that enables direct proton transfer from acid to the activated CO₂ substrate.

Homogeneous CO2 reduction by Ni(cyclam) at a glassy carbon electrode

The homogeneous CO(2) reduction activity of several nickel cyclam complexes was examined by cyclic voltammetry and controlled potential electrolysis. CO production with high efficiency from unsubstituted Ni(cyclam) was verified, while the activity was found to be attenuated with methyl substitution of the amines on the cyclam ring. Reactivity with CO(2) was also probed using density functional theory (DFT) calculations. The relative CO(2) binding energies to the Ni(I) state obtained from DFT were found to match well with the experimental results and shed light on the possible importance of the isomeric form of Ni(cyclam) in determining the catalytic activity.

Remarks on catalytic reduction of CO2, H+ and H2 by monovalent Ni

Prompted by catalysis of CO2 electroreduction by a tetraazamacrocyclic Ni(i)(cyclam) complex (cyclam = 1,4,8,11-tetraazacyclotetradecane), we examine theoretically the possibility of H2 reduction by this molecule. We show that the process 2 Ni(i) + H2 --> 2 [Ni(ii)(H-)] is thermodynamically facile, and that H2 could be reduced by a binuclear Ni(i) complex in two concerted 1e- processes. Our calculations also indicate that hydride complexes of Ni(iii)(cyclam) are significantly unstable thermodynamically and therefore they are unlikely to serve as intermediates in process of H2 evolution from water.

Reduction of the Allylic Substituents in NiI(1,8-dipropenyl-1,4,8,11-tetraazacyclotetradecane)+ by the Central Ni(I) in Aqueous Solutions

The complex Ni(II)(1,8,-di-2-propenyl-1,4,8,11-tetraazacyclotetradecane)(2+), (NiL(1))(2+), was synthesized. X-ray crystallography demonstrates that the complex obtained is the trans-III isomer. The allylic substituents shift the redox couples (NiL(1))(3+/2+) and (NiL(1))(2+/+) anodically relative to the corresponding couples for Ni(II)(1,4,8,11-tetraazacyclotetradecane)(2+), (NiL(2))(2+), as expected. Surprisingly, the lifetime of (NiL(1))(+) in neutral aqueous solutions is shorter than that of (NiL(2))(+). Pulse radiolysis experiments reveal that the allylic substituents are reduced by the central Ni(I) ion. The first step in this reduction is a general acid catalyzed process. The results suggest that this step involves schematically the reaction Ni(I)[bond]NCH(2)CH[double bond]CH(2)(+) + H(+) --> Ni(III)[bond]NCH2CH2CH(2)(2+). The latter transient decomposes slowly with a half-life time of several minutes. Preliminary results support the suggestion that (NiL(2))(+), or other Ni(I)L complexes of this family, might reduce many alkenes present in the solution.

Solvation largely accounts for the effect of N-alkylation on the properties of nickel(II/I) and chromium(III/II) cyclam complexes

The source of the effect of N-alkylation on the redox properties of Ni(II/I) and Cr(III/II) cyclam complexes has been investigated using DFT calculations. The structures of the anhydrous and hydrated complexes were optimized in the gas phase, and single point calculations were performed in a polarized continuum. The main results are the following: the decrease in outer sphere solvation upon N-alkylation is the major source of the relative stabilization of the lower oxidation state complexes by the tertiary amine ligands; tertiary amine nitrogen donors are stronger sigma-donors than the secondary amines, as predicted from the inductive effect of alkyls; steric strain elongates the metal-nitrogen bonds in the tertiary complexes and decreases the ligand strain energies; and the site of water binding to the complexes differs because of their different electronic structures (i.e., in the Ni complexes, the water molecules bind to the M[bond]N[bond]H sites, whereas in the Cr complexes they bind to the central metal cation). Outer sphere hydrogen bonding of water to the ligands in the coordination sphere lowers the ionization potentials by charge delocalization.

High-pressure pulse radiolysis as a tool in the study of transition metal reaction mechanisms

The unique combination of high-pressure and pulse radiolysis kinetic techniques enables detailed mechanistic insight for a large variety of chemical processes to be gained. Typical examples for transition metal reactions are presented. These include ligand substitution, electron transfer, reactions of complexes with uncommon oxidation states, and reactions with radicals, including the formation and decomposition reactions of complexes with metal-carbon sigma bonds. The volumes of activation and volume profiles obtained form the basis of a critical analysis of different plausible reaction mechanisms.