Eugen S. Andreiadis

Molecular engineering of a cobalt-based electrocatalytic nanomaterial for H 2 evolution under fully aqueous conditions

The viability of a hydrogen economy depends on the design of efficient catalytic systems based on earth-abundant elements. Innovative breakthroughs for hydrogen evolution based on molecular tetraimine cobalt compounds have appeared in the past decade. Here we show that such a diimine-dioxime cobalt catalyst can be grafted to the surface of a carbon nanotube electrode. The resulting electrocatalytic cathode material mediates H(2) generation (55,000 turnovers in seven hours) from fully aqueous solutions at low-to-medium overpotentials. This material is remarkably stable, which allows extensive cycling with preservation of the grafted molecular complex, as shown by electrochemical studies, X-ray photoelectron spectroscopy and scanning electron microscopy. This clearly indicates that grafting provides an increased stability to these cobalt catalysts, and suggests the possible application of these materials in the development of technological devices.

Artificial Photosynthesis: From Molecular Catalysts for Light-driven Water Splitting to Photoelectrochemical Cells

Photosynthesis has been for many years a fascinating source of inspiration for the development of model systems able to achieve efficient light‐to‐chemical energetic transduction. This field of research, called “artificial photosynthesis,” is currently the subject of intense interest, driven by the aim of converting solar energy into the carbon‐free fuel hydrogen through the light‐driven water splitting. In this review, we highlight the recent achievements on light‐driven water oxidation and hydrogen production by molecular catalysts and we shed light on the perspectives in terms of implementation into water splitting technological devices.

Remarkable Tuning of the Coordination and Photophysical Properties of Lanthanide Ions in a Series of Tetrazole-Based Complexes

A series of seven new tetrazole-based ligands (L1, L3-L8) containing terpyridine or bipyridine chromophores suited to the formation of luminescent complexes of lanthanides have been synthesized. All ligands were prepared from the respective carbonitriles by thermal cycloaddition of sodium azide. The crystal structures of the homoleptic terpyridine-tetrazolate complexes [Ln(Li)(2)]NHEt(3) (Ln = Nd, Eu, Tb for i = 1, 2; Ln = Eu for i = 3, 4) and of the monoaquo bypyridine-tetrazolate complex [Eu(H(2)O)(L7)(2)]NHEt(3) were determined. The tetradentate bipyridine-tetrazolate ligand forms nonhelical complexes that can contain a water molecule coordinated to the metal. Conversely, the pentadentate terpyridine-tetrazolate ligands wrap around the metal, thereby preventing solvent coordination and forming chiral double-helical complexes similarly to the analogue terpyridine-carboxylate. Proton NMR spectroscopy studies show that the solid-state structures of these complexes are retained in solution and indicate the kinetic stability of the hydrophobic complexes of terpyridine-tetrazolates. UV spectroscopy results suggest that terpyridine-tetrazolate complexes have a similar stability to their carboxylate analogues, which is sufficient for their isolation in aerobic conditions. The replacement of the carboxylate group with tetrazolate extends the absorption window of the corresponding terpyridine- (approximately 20 nm) and bipyridine-based (25 nm) complexes towards the visible region (up to 440 nm). Moreover, the substitution of the terpyridine-tetrazolate system with different groups in the ligand series L3-L6 has a very important effect on both absorption spectra and luminescence efficiency of their lanthanide complexes. The tetrazole-based ligands L1 and L3-L8 sensitize efficiently the luminescent emission of lanthanide ions in the visible and near-IR regions with quantum yields ranging from 5 to 53% for Eu(III) complexes, 6 to 35% for Tb(III) complexes, and 0.1 to 0.3% for Nd(III) complexes, which is among the highest reported for a neodymium complex. The luminescence efficiency could be related to the energy of the ligand triplet states, which are strongly correlated to the ligand structures.

Efficient sensitization of lanthanide luminescence by tetrazole-based polydentate ligands

Tetrazolate groups have been included by a convenient synthetic route in diverse ligand topologies, which have allowed the incorporation of lanthanide ions into highly luminescent double- and triple-helical complexes, demonstrating their potential for the expansion of lanthanide chemistry and the development of lanthanide-based applications.

Combined experimental-theoretical characterization of the hydrido-cobaloxime [HCo(dmgH)2(PnBu3)].

A combined theoretical and experimental approach has been employed to characterize the hydrido-cobaloxime [HCo(dmgH)(2)(PnBu(3))] compound. This complex was originally investigated by Schrauzer et al. [Schrauzer et al., J. Am. Chem. Soc. 1971, 93,1505] and has since been referred to as a key, stable analogue of the hydride intermediate involved in hydrogen evolution catalyzed by cobaloxime compounds [Artero, V. et al. Angew. Chem., Int. Ed. 2011, 50, 7238-7266]. We employed quantum chemical calculations, using density functional theory and correlated RI-SCS-MP2 methods, to characterize the structural and electronic properties of the compound and observed important differences between the calculated (1)H NMR spectrum and that reported in the original study by Schrauzer and Holland. To calibrate the theoretical model, the stable hydrido tetraamine cobalt(III) complex [HCo(tmen)(2)(OH(2))](2+) (tmen = 2,3-dimethyl-butane-2,3-diamine) [Rahman, A. F. M. M. et al. Chem. Commun. 2003, 2748-2749] was subjected to a similar analysis, and, in this case, the calculated results agreed well with those obtained experimentally. As a follow-up to the computational work, the title hydrido-cobaloxime compound was synthesized and recharacterized experimentally, together with the Co(I) derivative, giving results that were in agreement with the theoretical predictions.

A Computational Study of the Mechanism of Hydrogen Evolution by Cobalt(Diimine‐Dioxime) Catalysts

Cobalt(diimine-dioxime) complexes catalyze hydrogen evolution with low overpotentials and remarkable stability. In this study, DFT calculations were used to investigate their catalytic mechanism, to demonstrate that the initial active state was a Co(I) complex and that H2 was evolved in a heterolytic manner through the protonation of a Co(II)-hydride intermediate. In addition, these catalysts were shown to adjust their electrocatalytic potential for hydrogen evolution to the pH value of the solution and such a property was assigned to the presence of a H(+)-exchange site on the oxime bridge. It was possible to establish that protonation of the bridge was directly involved in the H2-evolution mechanism through proton-coupled electron-transfer steps. A consistent mechanistic scheme is proposed that fits the experimentally determined electrocatalytic and electrochemical potentials of cobalt(diimine-dioxime) complexes and reproduces the observed positive shift of the electrocatalytic potential with increasing acidity of the proton source.

Lanthanide Complexes Based on β-Diketonates and a Tetradentate Chromophore Highly Luminescent as Powders and in Polymers

A new type of octacoordinated ternary β-diketonates complexes of terbium and europium has been prepared using the anionic tetradentate terpyridine-carboxylate ligand (L) as a sensitizer of lanthanide luminescence in combination with two β-diketonates ligands 2-thenoyltrifluoroacetyl-acetonate (tta(-)) for Eu(3+) and trifluoroacetylacetonate (tfac(-)) for Tb(3+). The solid state structures of the two complexes [Tb(L)(tfac)2] (1) and [Eu(L)(tta)2] (2) have been determined by X-ray crystallography. Photophysical and (1)H NMR indicate a high stability of these complexes with respect to ligand dissociation in solution. The use of the anionic tetradentate ligand in combination with two β-diketonates ligands leads to the extension of the absorption window toward the visible region (390 nm) and to high luminescence quantum yield for the europium complex in the solid state (Φ = 66(6)%). Furthermore, these complexes have been incorporated in polymer matrixes leading to highly luminescent flexible layers.

Chiral undecagold clusters: synthesis, characterization and investigation in catalysis

Enantiopure undecagold clusters protected by chiral atropisomeric diphosphine ligands (P^P) have been synthesized by the stoichiometric reduction of the corresponding (P^P)(AuCl)(2) complexes with NaBH(4). The molecular mono-disperse [Au(11)(P^P)(4)Cl(2)]Cl species have been thoroughly characterized using an array of analytical techniques. (31)P NMR experiments suggested the presence of a slow intramolecular ligand exchange process. Circular dichroism measurements showed that enantiomeric clusters display mirror-image chiroptical activity. Such undecagold clusters containing two chloride ligands bound to the peripheral Au(I) atoms were expected to display a carbophilic Lewis acidity similar to the well-documented molecular Au(I) complex catalysts. Chloride abstraction, performed to generate active Au(+) sites, induced the Au(11) cluster evolution to larger gold clusters and nanoparticles, together with Au(I) complexes, which, in fact, perform the catalysis. This result was corroborated by running an asymmetric tandem hydroarylation-carbocyclization reaction, for which the enantiomeric excesses obtained with Au(11) clusters are similar to those reported using Au(I) complexes.