Ally Aukauloo

Cobalt Clathrochelate Complexes as Hydrogen‐Producing Catalysts

Developing a hydrogen-based economy is one possible scenario to reach a sustainable energy development and also to put ourselves on a path to cut the carbon emissions for obvious climate issues. However, several inherent problems must be overcome, such as production, storage, transport, and efficiency. 3] We are involved in research towards developing metal complexes for the electrocatalysis of hydrogen production. The challenge is to replace the expensive and limited platinum metal with non-noble metal complexes. A bioinorganic approach to tackling this problem wherein metal complexes, based mainly on iron, are designed to mimic the catalytic site of the Fe-hydrogenases, in the hope of simulating their reactivity patterns, has attracted much attention. A general trend in the electrocatalytic activity of these complexes is high overpotential. More classical metal coordination complexes for electrocatalysis have also been reported to show interesting activities towards hydrogen evolution. Among these examples are the difluoroboryl annulated bis(glyoximato) cobalt derivatives studied by Espenson and Chao. In recent years several groups, including ourselves, have demonstrated that this family of complexes can perform electrocatalysis of proton reduction in organic media. Hexacoordinated cobalt complexes have also been reported to act as catalysts for proton reduction. However, no clear-cut mechanism is known for the catalytic activity of these complexes, which was evidenced only on a mercury electrode. In boron-capped tris(glyoximato) cobalt complexes, the metal ion is both coordinatively saturated and encapsulated by a single macrobicyclic ligand. These coordination compounds are classified as clathrochelate complexes, in which the metal ion is locked in a close-knit structure, inhibiting ligand exchange in the more labile oxidation states of the encapsulated metal ion, and, in turn, explaining why the chemical activity of this family of complexes has been particularly elusive. We have been interested in investigating the electrochemical activity of the boron-capped tris(glyoximato) cobalt complexes towards hydrogen evolution. Herein, we report on the synthesis and characterization of three clathrochelate Co complexes [1], [2], and [3] (Figure 1) together with their involvement in an electrocatalytic hydrogen-forming reaction in solution. X-ray crystallographic data for [1] and for a derivative Co complex [4] are also discussed.

Artificial photosynthetic systems. Using light and water to provide electrons and protons for the synthesis of a fuel

This review covers the progress achieved in the synthesis and characterization of different metal based catalysts designed for the photocatalytic oxidation of water with special focus on molecular designed systems. In those cases where the mechanism of the light-driven catalytic activity of these complexes is not known, we have looked to published mechanisms related to chemically activated catalysts. We also discuss the choice of the chromophores used to sensitize these reactions as well as the recovery of the reaction products and nature of the electron acceptors used.

Click chemistry on a ruthenium polypyridine complex. An efficient and versatile synthetic route for the synthesis of photoactive modular assemblies.

In this Communication, we present the synthesis and use of [Ru(bpy)(2)(bpy-CCH)](2+), a versatile synthon for the construction of more sophisticated dyads by means of click chemistry. The resulting chromophore-acceptor or -donor complexes have been studied by flash photolysis and are shown to undergo efficient electron transfer to/from the chromophore. Additionally, the photophysical and chemical properties of the original chromophore remain intact, making it a very useful component for the preparation of visible-light-active dyads.

Reversible double oxidation and protonation of the non-innocent bridge in a nickel(II) salophen complex.

Substitution on the aromatic bridge of a nickel(II) salophen complex with electron-donating dimethylamino substituents creates a ligand with three stable, easily and reversibly accessible oxidation states. The one-electron-oxidized product is characterized as a nickel(II) radical complex with the radical bore by the central substituted aromatic ring, in contrast to other nickel(II) salen or salophen complexes that oxidize on the phenolate moieties. The doubly oxidized product, a singlet species, is best described as having an iminobenzoquinone bridge with a vinylogous distribution of bond lengths between the dimethylamino substituents. Protonation of the dimethylamino substituents inhibits these redox processes on the time scale of cyclovoltammetry, but electrolysis and chemical oxidation are consistent with deprotonation occurring concomitantly with electron transfer to yield the mono- and dioxidized species described above.

From metal to ligand electroactivity in nickel(II) oxamato complexes

The locus of oxidation in square-planar nickel(ii) oxamato complexes can be continuously shifted from the metal to the ligand by an appropriate choice of electron-donating substituents on the aromatic moiety of the ligand.

A Square‐Planar Dinickel(II) Complex with a Noninnocent Dinucleating Oxamate Ligand: Evidence for a Ligand Radical Species

The new bimetallic nickel(II) compound (PPh4)4[Ni2(2)]·6H2O (3), where H8[2] stands for N,N′,N′′,N′′′-1,2,4,5-benzene-tetrayltetrakis(oxamic acid), has been synthesized and its crystal structure determined by single-crystal X-ray diffraction. The structure of 3 consists of [Ni2(η4:η4-2)]4– anions, tetraphenylphosphonium cations, and water molecules. Facile one-electron oxidation of the square-planar diamagnetic dinickel(II) complex [Ni2(η4:η4-2)]4– generates the metallo-radical species [Ni2(η4:η4-2·+)]3– with characteristic intra-ligand π-cation radical transitions in the visible region (475–550 nm) as well as a typical quasi-isotropic EPR signal at g ≈ 2.0.

Cobalt‐Based Particles Formed upon Electrocatalytic Hydrogen Production by a Cobalt Pyridine Oxime Complex

An open-coordination-sphere cobalt(III) oximato-based complex was designed as a putative catalyst for the hydrogen evolution reaction (HER). Electrochemical alteration in the presence of acid occurs, leading to the formation of cobalt-based particles that act as an efficient catalyst for HER at pH 7. The exact chemical nature of these particles is yet to be determined. This study thus raises interesting issues regarding the fate of molecular-based complexes designed for the HER, and points to the challenging task of identifying the real catalytic species. Moreover, understanding and rationalizing the alteration pathways can be seen as a new route to reach catalytic particulates.

Efficient electron transfer through a triazole link in ruthenium(II) polypyridine type complexes

Spectroscopic, electrochemical and theoretical characterisations of photoactive systems readily assembled via click-chemistry show an efficient bi-directional charge shift through the triazole link.

A Unique Heterotopic Ligand for Sequential Synthesis of Polymetallic Complexes

A new organic ligand bearing four coordination sites that may be metallated in a stepwise manner is described, together with its CuII and FeII derivatives. This may be considered as the first ligand precursor for trimetallic systems.

Intramolecular light induced activation of a Salen–MnIII complex by a ruthenium photosensitizer

We have designed a molecular system consisting of a heteroleptic [Ru(bpy)(2)L](2+) chromophore covalently linked to a Mn(III)-Salen unit. We demonstrate the light induced oxidation of the Mn(III) center in this putative photo-catalyst assembly to a Mn(IV) high spin intermediate. Both oxidation states have been characterized by transient absorption and EPR techniques.