Molecular catalyst coordinatively bonded to organic semiconductors for selective light-driven CO reduction in water.

The selective photoreduction of CO in aqueous media based on earth-abundant elements only, is today a challenging topic. Here we present the anchoring of discrete molecular catalysts on organic polymeric semiconductors via covalent bonding, generating molecular hybrid materials with well-defined active sites for CO photoreduction, exclusively to CO in purely aqueous media. The molecular catalysts are based on aryl substituted Co phthalocyanines that can be coordinated by dangling pyridyl attached to a polymeric covalent triazine framework that acts as a light absorber.

Supramolecular interaction of a molecular catalyst with a polymeric carbon nitride photoanode enhances photoelectrochemical activity and stability at neutral pH.

Polymeric carbon nitride (CN) emerged as an alternative, metal-free photoanode material for water-splitting photoelectrochemical cells (PECs). However, the performance of CN photoanodes is limited due to the slow charge separation and water oxidation kinetics due to poor interaction with water oxidation catalysts (WOCs). Moreover, operation under benign, neutral pH conditions is rarely reported.

Supramolecular Anchoring of Fe(III) Molecular Redox Catalysts into Graphitic Surfaces Via CH-π and π-π Interactions for CO Electroreduction.

Photoelectrochemical devices require solid anodes and cathodes for the easy assembling of the whole cell and thus redox catalysts need to be deposited on the electrodes. Typical catalyst deposition involves drop casting, spin coating, doctor blading or related techniques to generate modified electrodes where the active catalyst in contact with the electrolyte is only a very small fraction of the deposited mass. We have developed a methodology where the redox catalyst is deposited at the electrode based on supramolecular interactions, namely CH-π and π-π between the catalyst and the surface.

Robust Molecular Anodes for Electrocatalytic Water Oxidation Based on Electropolymerized Molecular Cu Complexes.

A multistep synthesis of a new tetra-amidate macrocyclic ligand functionalized with alkyl-thiophene moieties, 15,15-bis(6-(thiophen-3-yl)hexyl)-8,13-dihydro-5H-dibenzo[b,h][1,4,7,10]tetraazacyclotridecine-6,7,14,16(15H,17H)-tetraone, H L, is reported. The reaction of the deprotonated ligand, L , and Cu(II) generates the complex [LCu] , that can be further oxidized to Cu(III) with iodine to generate [LCu] . The H L ligand and their Cu complexes have been thoroughly characterized by analytic and spectroscopic techniques (including X-ray Absorption Spectroscopy, XAS).

Mapping the Ultrafast Mechanistic Pathways of Co Photocatalysts in Pure Water through Time-Resolved X-ray Spectroscopy.

Nanosecond time-resolved X-ray (tr-XAS) and optical transient absorption spectroscopy (OTA) are applied to study 3 multimolecular photocatalytic systems with [Ru(bpy) ] photoabsorber, ascorbic acid electron donor and Co catalysts with methylene (1), hydroxomethylene (2) and methyl (3) amine substituents in pure water. OTA and tr-XAS of 1 and 2 show that the favored catalytic pathway involves reductive quenching of the excited photosensitizer and electron transfer to the catalyst to form a Co square pyramidal intermediate with a bonded aqua molecule followed by a Co square planar derivative that decays within ≈8 μs. By contrast, a Co square pyramidal intermediate with a longer decay lifetime of ≈35 μs is formed from an analogous Co geometry for 3 in H O.

Heterogeneous Electrochemical Ammonia Oxidation with a Ru-bda Oligomer Anchored on Graphitic Electrodes via CH-π Interactions.

Molecular catalysts can promote ammonia oxidation, providing mechanistic insights into the electrochemical N cycle for a carbon-free fuel economy. We report the ammonia oxidation activity of carbon anodes functionalized with the oligomer {[Ru(bda-κ- )(4,4'-bpy)](4,4'-bpy)}, , where bda is [2,2'-bipyridine]-6,6'-dicarboxylate and 4,4'-bpy is 4,4'-bipyridine. Electrocatalytic studies in propylene carbonate demonstrate that the Ru-based hybrid anode used in a 3-electrode configuration transforms NH to N and H in a 1:3 ratio with near-unity faradaic efficiency at an applied potential of 0.

Tandem electrocatalytic CO reduction with Fe-porphyrins and Cu nanocubes enhances ethylene production.

Copper-based tandem schemes have emerged as promising strategies to promote the formation of multi-carbon products in the electrocatalytic CO reduction reaction. In such approaches, the CO-generating component of the tandem catalyst increases the local concentration of CO and thereby enhances the intrinsic carbon-carbon (C-C) coupling on copper. However, the optimal characteristics of the CO-generating catalyst for maximizing the C production are currently unknown.

Metamorphic oxygen-evolving molecular Ru and Ir catalysts.

Today sustainable and clean energy conversion strategies are based on sunlight and the use of water as a source of protons and electrons, in a similar manner as it happens in Photosystem II. To achieve this, the charge separation state induced by light has to be capable of oxidising water by 4 protons and 4 electrons and generating molecular oxygen. This oxidation occurs by the intermediacy of a catalyst capable of finding low-energy pathways proton-coupled electron transfer steps.

Interplay between β-Diimino and β-Diketiminato Ligands in Nickel Complexes Active in the Proton Reduction Reaction.

Two Ni complexes are reported with κ-PN β-diimino (BDI) ligands with the general formula [Ni(XBDI)](BF), where BDI is -(2-(diphenylphosphaneyl)ethyl)-4-((2-(diphenylphosphaneyl)ethyl)imino)pent-2-en-2-amine and X indicates the substituent in the α-carbon intradiimine position, X = H for (BF) and X = Ph for (BF). Electrochemical analysis together with UV-vis and NMR spectroscopy in acetonitrile and dimethylformamide (DMF) indicates the conversion of the β-diimino complexes and to the negatively charged β-diketiminato (BDK) analogues () and () via deprotonation in DMF. Moreover, further electrochemical and spectroscopy evidence indicates that the one-electron-reduced derivatives and can also rapidly evolve to the BDK () and (), respectively, via hydrogen gas evolution through a bimolecular homolytic pathway.

Unravelling the Mechanistic Pathway of the Hydrogen Evolution Reaction Driven by a Cobalt Catalyst.

A cobalt complex bearing a κ-N P ligand is presented (1 or Co (L), where L is (1E,1'E)-1,1'-(pyridine-2,6-diyl)bis(N-(3-(diphenylphosphanyl)propyl)ethan-1-imine). Complex 1 is stable under air at oxidation state Co thanks to the π-acceptor character of the phosphine groups. Electrochemical behavior of 1 reveals a two-electron Co /Co oxidation process and an additional one-electron reduction, which leads to an enhancement in the current due to hydrogen evolution reaction (HER) at E =-1.

Electrocatalytic water oxidation from a mixed linker MOF based on NU-1000 with an integrated ruthenium-based metallo-linker.

A novel tetratopic metallo-linker, [Ru(tda)(py(PhCOOH))], 1, (tda = 2,2':6',2''-terpyridine-6,6''-dicarboxylate; py(PhCOOH) = (4,4'-(pyridine-3,5-diyl)dibenzoic acid), that is structurally based on one of the most active molecular water oxidation catalysts has been prepared and fully characterized, including single crystal X-ray diffraction. 1 bears geometric similarities to HTBAPy (HTBAPy = 4,4',4'',4'''-(pyrene-1,3,6,8-tetrayl)tetrabenzoic acid), the native linker in NU-1000, which offers the possibility to synthesize NU-1000-Ru mixed linker MOFs solvothermally. Mixed linker MOF formation was demonstrated by powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM), and Ru linker incorporation confirmed by FT-IR, energy-dispersive X-ray (EDX) spectroscopy and inductively coupled plasma optical emission spectroscopy (ICP-OES).

Synthesis, Structure, and Ammonia Oxidation Catalytic Activity of Ru-NH Complexes Containing Multidentate Polypyridyl Ligands.

Ammonia (electro)oxidation with molecular catalysts is a rapidly developing topic with wide practical applications ahead. We report here the catalytic ammonia oxidation reaction (AOR) activity using [Ru(tda-κ-NO)(py)], , (tda is 2,2':6',2''-terpyridine-6,6''-dicarboxylate; py is pyridine) as a catalyst precursor. Furthermore, we also describe the rich chemistry associated with the reaction of Ru-tda and Ru-tPa (tPa is 2,2':6',2''-terpyridine-6,6''-diphosphonate) complexes with NH and NH using [Ru(tda-κ-NO)(dmso)Cl] (dmso is dimethyl sulfoxide) and [Ru(tPa-κ-NO)(py)], , as synthetic intermediates, respectively.

Surface-Promoted Evolution of Ru-bda Coordination Oligomers Boosts the Efficiency of Water Oxidation Molecular Anodes.

A new Ru oligomer of formula {[Ru(bda-κ-NO)(4,4'-bpy)](4,4'-bpy)}, (bda is [2,2'-bipyridine]-6,6'-dicarboxylate and 4,4'-bpy is 4,4'-bipyridine), was synthesized and thoroughly characterized with spectroscopic, X-ray, and electrochemical techniques. This oligomer exhibits strong affinity for graphitic materials through CH-π interactions and thus easily anchors on multiwalled carbon nanotubes (CNT), generating the molecular hybrid material . The latter acts as a water oxidation catalyst and converts to a new species, , during the electrochemical oxygen evolution process involving solvation and ligand reorganization facilitated by the interactions of molecular Ru catalyst and the surface.

Consecutive Ligand-Based Electron Transfer in New Molecular Copper-Based Water Oxidation Catalysts.

Water oxidation to dioxygen is one of the key reactions that need to be mastered for the design of practical devices based on water splitting with sunlight. In this context, water oxidation catalysts based on first-row transition metal complexes are highly desirable due to their low cost and their synthetic versatility and tunability through rational ligand design. A new family of dianionic bpy-amidate ligands of general formula H LN (LN is [2,2'-bipyridine]-6,6'-dicarboxamide) substituted with phenyl or naphthyl redox non-innocent moieties is described.

Synthesis, Characterization, and Water Oxidation Activity of Isomeric Ru Complexes.

The synthesis and characterization of the isomeric ruthenium complexes with the general formula and [Ru(trpy)(qc)X] (trpy is 2,2':6',2″-terpyridine, qc is 8-quinolinecarboxylate, and , X = Cl, = 0; and , X=OH, = 1) with respect to the relative disposition of the carboxylate and X ligands are reported. For comparison purposes, another set of ruthenium complexes with general formula and [Ru(trpy)(pic)(OH)] (pic is 2-picolinate (-, -)) have been prepared. The complexes with a qc ligand show a more distorted geometry compared to the complexes with a pic ligand.

High Solar-to-Hydrogen Conversion Efficiency at pH 7 Based on a PV-EC Cell with an Oligomeric Molecular Anode.

In the urgent quest for green energy vectors, the generation of hydrogen by water splitting with sunlight occupies a preeminent standpoint. The highest solar-to-hydrogen (STH) efficiencies have been achieved with photovoltaic-electrochemical (PV-EC) systems. However, most PV-EC water-splitting devices are required to work at extreme conditions, such as in concentrated solutions of HClO or KOH or under highly concentrated solar illumination.

A broad view on the complexity involved in water oxidation catalysis based on Ru-bpn complexes.

A new Ru complex with the formula [Ru(bpn)(pic)]Cl (where bpn is 2,2'-bi(1,10-phenanthroline) and pic stands for 4-picoline) (1Cl) is synthesized to investigate the true nature of active species involved in the electrochemical and chemical water oxidation mediated by a class of N4 tetradentate equatorial ligands. Comprehensive electrochemical (by using cyclic voltammetry, differential pulse voltammetry, and controlled potential electrolysis), structural (X-ray diffraction analysis), spectroscopic (UV-vis, NMR, and resonance Raman), and kinetic studies are performed. 1 undergoes a substitution reaction when it is chemically (by using NaIO) or electrochemically oxidized to Ru, in which picoline is replaced by an hydroxido ligand to produce [Ru(bpn)(pic)(OH)] (2).

Synthetic strategies to incorporate Ru-terpyridyl water oxidation catalysts into MOFs: direct synthesis post-synthetic approach.

Incorporating molecular catalysts into metal-organic frameworks (MOFs) is a promising strategy for improving their catalytic longevity and recyclability. In this article, we investigate and compare synthetic routes for the incorporation of the potent water oxidation catalyst Ru(tda)(pyCO2H)2 (tda = 2,2':6',2''-terpyridine-6,6''-dicarboxylic acid, pyCO2H = iso-nicotinic acid) as a structural linker into a Zr-based UiO-type MOF. The task is challenging with this particular metallo-linker because of the equatorial dangling carboxylates that can potentially compete for Zr-coordination, as well as free rotation of the pyCO2H groups around the HO2CpyRupyCO2H axis.

Water oxidation electrocatalysis using ruthenium coordination oligomers adsorbed on multiwalled carbon nanotubes.

Photoelectrochemical cells that utilize water as a source of electrons are one of the most attractive solutions for the replacement of fossil fuels by clean and sustainable solar fuels. To achieve this, heterogeneous water oxidation catalysis needs to be mastered and properly understood. The search continues for a catalyst that is stable at the surface of electro(photo)anodes and can efficiently perform this reaction at the desired neutral pH.

Redox Metal-Ligand Cooperativity Enables Robust and Efficient Water Oxidation Catalysis at Neutral pH with Macrocyclic Copper Complexes.

Water oxidation catalysis stands out as one of the most important reactions to design practical devices for artificial photosynthesis. Use of late first-row transition metal (TM) complexes provides an excellent platform for the development of inexpensive catalysts with exquisite control on their electronic and structural features via ligand design. However, the difficult access to their high oxidation states and the general labile character of their metal-ligand bonds pose important challenges.

Effect of Ligand Chelation and Sacrificial Oxidant on the Integrity of Triazole-Based Carbene Iridium Water Oxidation Catalysts.

We report the effect of replacing the pyridine group in the chelating trz Ir-water oxidation catalysts by a benzoxazole and a thiazole moiety. We have also evaluated if the presence of bidentate ligands is crucial for high activities and to avoid the decomposition into undesired heterogeneous layers. The catalytic performance of these benzoxazole/thiazole-triazolidene Ir-complexes in water oxidation was studied at variable pH using either CAN (pH = 1) or NaIO (pH = 5.

Synthesis, Electrochemical Characterization, and Water Oxidation Chemistry of Ru Complexes Containing the 2,6-Pyridinedicarboxylato Ligand.

The tridentate meridional ligand pyridyl-2,6-dicarboxylato (pdc) has been used to prepare complexes [Ru(pdc-κ-NO)(DMSO)Cl] (), [Ru(pdc-κ-NO)(bpy)(DMSO)] (), and {[Ru(pdc-κ-NO)(bpy)](μ-O)} (), where bpy = 2,2'-bipyridine. All complexes have been fully characterized through spectroscopic, electrochemical, and single-crystal X-ray diffraction techniques. Compounds and show S → O linkage isomerization of the DMSO ligand upon oxidation from Ru to Ru, and thermodynamic and kinetic data have been obtained from cyclic voltammetry experiments.

Analysis of the Active Species Responsible for Water Oxidation Using a Pentanuclear Fe Complex.

Water splitting with sunlight is today one of the most promising strategies that can be used to start the imperatively needed transition from fossil to solar fuels. To achieve this, one of the key reactions that need to be mastered is the electrocatalytic oxidation of water to dioxygen. Great developments have been achieved using transition metal complexes mainly based on Ru, but for technological applications it is highly desirable to be able to use earth-abundant transition metals.