Reduction of CO to Methanol with Recyclable Organic Hydrides

The reaction steps for the selective conversion of a transition metal carbonyl complex to a hydroxymethyl complex that releases methanol upon irradiation with visible light have been successfully quantified in acetonitrile solution with dihydrobenzimidazole organic hydride reductants. Dihydrobenzimidazole reductants have been shown to be inactive toward H2 generation in the presence of a wide range of proton sources and have been regenerated electrochemically or photochemically. Specifically, the reaction of cis-[Ru(bpy)2(CO)2]2+ (bpy = 2,2'-bipyridine) with one equivalent of a dihydrobenzimidazole quantitatively yields a formyl complex, cis-[Ru(bpy)2(CO)(CHO)]+, and the corresponding benzimidazolium on a seconds time scale. Kinetic experiments revealed a first-order dependence on the benzimidazole hydride concentration and an unusually large kinetic isotope effect, inconsistent with direct hydride transfer and more likely to occur by an electron transfer-proton-coupled electron transfer (EΤ-PCET) or related mechanism. Further reduction/protonation of cis-[Ru(bpy)2(CO)(CHO)]+ with two equivalents of the organic hydride yields the hydroxymethyl complex cis-[Ru(bpy)2(CO)(CH2OH)]+. Visible light excitation of cis-[Ru(bpy)2(CO)(CH2OH)]+ in the presence of excess organic hydride was shown to yield free methanol. Identification and quantification of methanol as the sole CO reduction product was confirmed by 1H NMR spectroscopy and gas chromatography. The high selectivity and mild reaction conditions suggest a viable approach for methanol production from CO, and from CO2 through cascade catalysis, with renewable organic hydrides that bear similarities to Nature's NADPH/NADP+.

Electrocatalytic properties of a novel ruthenium(II) terpyridine-based complex towards CO2 reduction

The electrocatalytic properties of Ru complexes are of great technological interest given their potential application in reactions such water splitting and CO₂ reduction. In this work, a novel terpyridine-based Ru(II) complex, [RuCl(trpy)(acpy)], trpy = 2,2':6',2''-terpyridine, acpy^- = 2-pyridylacetate was synthesized and its spectroscopic, electrochemical and catalytic properties were explored in detail. In dry acetonitrile, the complex exhibits two reduction peaks at -1.95 V and -2.20 V vs. Fc/Fc⁺, attributed to consecutive 1 e^- reduction. Under CO₂ atmosphere, a catalytic wave is observed (E_onset = 2.1 V vs. Fc/Fc⁺), with CO as the main reduction product. Bulk electrolysis reveals a turnover number (TON) of 12 (k_obs = 1.5 s^-1). In the presence of 1% water, an improvement in the catalytic activity is observed (TON_CO = 21 and k_obs = 2.0 s^-1) and, additionally, formate was also detected (TON_HCOO = 7). Spectroelectrochemical experiments allowed the identification of a metallocarboxylate (Ru-COO^-) intermediate under anhydrous conditions, while in water, the partial labilization of the acpy^- ligand was observed in the course of the catalytic cycle. The experimental data was combined with DFT calculations, allowing the proposal of a catalytic cycle. The results establish important relationships between selectivity, ligand structure and reaction conditions.

Synthesis and Surface Attachment of Molecular Re(I) Complexes Supported by Functionalized Bipyridyl Ligands

Eleven 2,2'-bipyridine (bpy) ligands functionalized with attachment groups for covalent immobilization on silicon surfaces were prepared. Five of the ligands feature silatrane functional groups for attachment to metal oxide coatings on the silicon surfaces, while six contain either alkene or alkyne functional groups for attachment to hydrogen-terminated silicon surfaces. The bpy ligands were coordinated to Re(CO)₅Cl to form complexes of the type Re(bpy)(CO)₃Cl, which are related to known catalysts for CO₂ reduction. Six of the new complexes were characterized using X-ray crystallography. As proof of principle, four molecular Re complexes were immobilized on either a thin layer of TiO₂ on silicon or hydrogen-terminated silicon. The surface-immobilized complexes were characterized using X-ray photoelectron spectroscopy, IR spectroscopy, and cyclic voltammetry (CV) in the dark and for one representative example in the light. The CO stretching frequencies of the attached complexes were similar to those of the pure molecular complexes, but the CVs were less analogous. For two of the complexes, comparison of the electrocatalytic CO₂ reduction performance showed lower CO Faradaic efficiencies for the immobilized complexes than the same complex in solution under similar conditions. In particular, a complex containing a silatrane linked to bpy with an amide linker showed poor catalytic performance and control experiments suggest that amide linkers in conjugation with a redox-active ligand are not stable under highly reducing conditions and alkyl linkers are more stable. A conclusion of this work is that understanding the behavior of molecular Re catalysts attached to semiconducting silicon is more complicated than related complexes, which have previously been immobilized on metallic electrodes.

Dye-sensitized solar cells strike back

Dye-sensitized solar cells (DSCs) are celebrating their 30th birthday and they are attracting a wealth of research efforts aimed at unleashing their full potential. In recent years, DSCs and dye-sensitized photoelectrochemical cells (DSPECs) have experienced a renaissance as the best technology for several niche applications that take advantage of DSCs' unique combination of properties: at low cost, they are composed of non-toxic materials, are colorful, transparent, and very efficient in low light conditions. This review summarizes the advancements in the field over the last decade, encompassing all aspects of the DSC technology: theoretical studies, characterization techniques, materials, applications as solar cells and as drivers for the synthesis of solar fuels, and commercialization efforts from various companies.

Proton-Coupled Group Transfer Enables Concerted Protonation Pathways Relevant to Small-Molecule Activation

The mechanistic identification of Nature's use of concerted reactions, in which all bond breaking and bond making occurs in a single step, has inspired rational designs for artificial synthetic transformations via pathways that bypass high-energy intermediates that would otherwise be thermodynamically and kinetically inaccessible. In this contribution we electrochemically activate an organometallic Ruthenium(II) complex to show that, in acetonitrile solutions, the movement of protons from weak Brønsted acids, such as water and methanol, is coupled with the transfer of its negatively charged counterpart to carbon dioxide (CO₂)─a process termed proton-coupled group transfer─to stoichiometrically produce a metal-hydride complex and a carbonate species. These previously unidentified pathways have played key roles in CO₂ and proton reduction catalysis by enabling the generation of key intermediates such as hydrides and metallocarboxylic acids, while their applicability to carbon acids may provide alternative approaches in the electrosynthesis of chemical commodities via alkylation and carboxylation reactions.

Photodriven water oxidation initiated by a surface bound chromophore-donor-catalyst assembly

In photosynthesis, solar energy is used to produce solar fuels in the form of new chemical bonds. A critical step to mimic photosystem II (PS II), a key protein in nature's photosynthesis, for artificial photosynthesis is designing devices for efficient light-driven water oxidation. Here, we describe a single molecular assembly electrode that duplicates the key components of PSII. It consists of a polypyridyl light absorber, chemically linked to an intermediate electron donor, with a molecular-based water oxidation catalyst on a SnO₂/TiO₂ core/shell electrode. The synthetic device mimics PSII in achieving sustained, light-driven water oxidation catalysis. It highlights the value of the tyrosine–histidine pair in PSII in achieving efficient water oxidation catalysis in artificial photosynthetic devices.

We describe a single molecular assembly electrode that mimics PSII. Flash photolysis revealed the electron transfer steps between chromophore light absorption and the creation and storage of redox equivalents in the catalyst for water oxidation.

Mechanistic investigation of the aerobic oxidation of 2-pyridylacetate coordinated to a Ru(II) polypyridyl complex

A new ruthenium polypyridyl complex, [Ru(bpy)₂(acpy)]⁺ (acpy = 2-pyridylacetate, bpy = 2,2'-bipyridine), was synthesized and fully characterized. Distinct from the previously reported analog, [Ru(bpy)₂(pic)]⁺ (pic = 2-pyridylcarboxylate), the new complex is unstable under aerobic conditions and undergoes oxidation to yield the corresponding α-keto-2-pyridyl-acetate (acpyoxi) coordinated to the Ru^II center. The reaction is one of the few examples of C-H activation at mild conditions using O₂ as the primary oxidant and can provide mechanistic insights with important implications for catalysis. Theoretical and experimental investigations of this aerobic oxidative transformation indicate that it takes place in two steps, first producing the α-hydroxo-2-pyridyl-acetate analog and then the final product. The observed rate constant for the first oxidation was in the order of 10^-2 h^-1. The reaction is hindered in the presence of coordinating solvents indicating the role of the metal center in the process. Theoretical calculations at the M06-L level of theory were performed for multiple reaction pathways in order to gain insights into the most probable mechanism. Our results indicate that O₂ binding to [Ru(bpy)₂(acpy)]⁺ is favored by the relative instability of the six-ring chelate formed by the acpy ligand and the resulting Ru^III-OO˙^- superoxo is stabilized by the carboxylate group in the coordination sphere. C-H activation by this species involves high activation free energies (ΔG^‡ = 41.1 kcal mol^-1), thus the formation of a diruthenium μ-peroxo intermediate, [(Ru^III(bpy)₂(O-acpy))₂O₂]²⁺via interaction of a second [Ru(bpy)₂(acpy)]⁺ was examined as an alternative pathway. The dimer yields two Ru^IVO centers with a low ΔG^‡ of 2.3 kcal mol^-1. The resulting Ru^IVO species can activate C-H bonds in acpy (ΔG^‡ = 23.1 kcal mol^-1) to produce the coordinated α-hydroxo-2-pyridylacetate. Further oxidation of this intermediate leads to the α-keto-2-pyridyl-acetate product. The findings provide new insights into the mechanism of C-H activation catalyzed by transition-metal complexes using O₂ as the sole oxygen source.

Plasma-Initiated Graft Polymerization of Acrylic Acid onto Fluorine-Doped Tin Oxide as a Platform for Immobilization of Water-Oxidation Catalysts

The discovery of new and versatile strategies for the immobilization of molecular water-oxidation catalysts (WOCs) is crucial for developing clean energy conversion devices [e.g., (photo)electrocatalytic cells for water splitting]. The traditional approach for surface attachment to transparent conductive oxides [e.g., fluorine doped tin oxide (FTO)] is via synthetic modification of the ligand architecture to incorporate functional groups such as carboxylic acids (-COOH) or phosphonates (-PO₃H₂) prior to immobilization. However, challenges arising from desorption and the cumbersome derivatizations steps have limited the scope and applications of surface-bound WOCs. Herein, we report the successful immobilization of underivatized Ru(II)-based WOCs (Ru-Cat1 = [Ru(tpy) (bpy) (H₂O)]²⁺ (tpy = 2,2':6'2″-terpyridine and bpy = 2,2'-bipyridine) and Ru-Cat2 = [Ru(Mebimpy) (bpy) (H₂O)]²⁺ (Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl) pyridine)) and the Ru(II) polypyridyl chromophore Ru-C3 = [Ru(bpy)₃]²⁺ onto a FTO plasma-grafted poly(acrylic acid) surface (PAA|FTO). Various characterization techniques such as attenuated total reflectance Fourier transform infrared spectroscopy, scanning electron microscopy, atomic force microscopy, and cyclic voltammetry measurements provide evidence for the plasma-induced grafted PAA|FTO film and immobilization. Surface stability and electrocatalytic properties of these new hybrid composite films upon cycling were investigated at different pH values. Immobilized Ru-Cat1 and Ru-Cat2 onto PAA|FTO displayed pH-dependent (Ru^III/Ru^II) couples and onset potentials indicative of PCET (proton-coupled electron transfer) reactions. Based on cyclic voltammetry results and spectroscopic monitoring, the immobilized WOCs Ru-Cat1 and Ru-Cat2 exhibited a higher surface stability in neutral aqueous solutions relative to Ru-C3 upon electrochemical oxidation. We attribute the surface PCET and stability to the presence of a water ligand in the coordination sphere of immobilized Ru-Cat1 and Ru-Cat2 which can H-bond with negatively charged carboxylate groups of the cross-linked PAA brushes. Our findings demonstrate that the plasma-grafted polymeric network onto FTO offers a versatile platform to directly anchor unmodified homogeneous WOCs or chromophores for potential applications in solar-to-fuel energy conversion.

Self-Assembled Bilayers as an Anchoring Strategy: Catalysts, Chromophores, and Chromophore-Catalyst Assemblies

Anchoring strategies for immobilization of molecular catalysts, chromophores, and chromophore-catalyst assemblies on electrode surfaces play an important role in solar energy conversion devices such as dye-sensitized solar cells and dye-sensitized photoelectrosynthesis cells. They are also important in interfacial studies with surface-bound molecules including electron-transfer dynamics and mechanistic studies related to small molecule activation catalysis. Significant progress has been made in this area, but many challenges remain in terms of stability, synthetic complexity, and versatility. We report here a new anchoring strategy based on self-assembled bilayers. This strategy takes advantage of noncovalent interactions between long alkyl chains chemically bound to a metal-oxide electrode surface and long alkyl chains on the molecule being anchored. The new methodology is applicable to the heterogenization of both catalysts and chromophores as well as to the in situ "synthesis" of chromophore-catalyst assemblies on the electrode surface.

O–O bond formation in ruthenium-catalyzed water oxidation: single-site nucleophilic attack vs. O–O radical coupling

A review of water oxidation by ruthenium-based molecular catalysts, with emphasis on the mechanism of O–O bond formation.

Water Oxidation by Ruthenium Complexes Incorporating Multifunctional Bipyridyl Diphosphonate Ligands

We describe herein the synthesis and characterization of ruthenium complexes with multifunctional bipyridyl diphosphonate ligands as well as initial water oxidation studies. In these complexes, the phosphonate groups provide redox-potential leveling through charge compensation and σ donation to allow facile access to high oxidation states. These complexes display unique pH-dependent electrochemistry associated with deprotonation of the phosphonic acid groups. The position of these groups allows them to shuttle protons in and out of the catalytic site and reduce activation barriers. A mechanism for water oxidation by these catalysts is proposed on the basis of experimental results and DFT calculations. The unprecedented attack of water at a neutral six-coordinate [Ru(IV) ] center to yield an anionic seven-coordinate [Ru(IV) -OH](-) intermediate is one of the key steps of a single-site mechanism in which all species are anionic or neutral. These complexes are among the fastest single-site catalysts reported to date.

Mechanism of water oxidation by [Ru(bda)(L)2]: the return of the "blue dimer"

We describe here a combined solution-surface-DFT calculations study for complexes of the type [Ru(bda)(L)2] including X-ray structure of intermediates and their reactivity, as well as pH-dependent electrochemistry and spectroelectrochemistry. These studies shed light on the mechanism of water oxidation by [Ru(bda)(L)2], revealing key features unavailable from solution studies with sacrificial oxidants.

Polypyridyl Ru(ii)-derivatized polypropylacrylate polymer with a terminal water oxidation catalyst. Application of reversible addition–fragmentation chain transfer polymerization

A ruthenium containing poly(propylmethacrylate) derivative was synthesized by RAFT polymerization and end-capped with a catalyst derivative.

New water oxidation chemistry of a seven-coordinate ruthenium complex with a tetradentate polypyridyl ligand

The mononuclear ruthenium(II) complex [Ru](2+) (Ru = Ru(dpp)(pic)2, where dpp is the tetradentate 2,9-dipyrid-2'-yl-1,10-phenanthroline ligand and pic is 4-picoline) reported by Thummel's group (Inorg. Chem. 2008, 47, 1835-1848) that contains no water molecule in its primary coordination shell is evaluated as a catalyst for water oxidation in artificial photosynthesis. A detailed theoretical characterization of the energetics, thermochemistry, and spectroscopic properties of intermediates allowed us to interpret new electrochemical and spectroscopic experimental data, and propose a mechanism for the water oxidation process that involves an unprecedented sequence of seven-coordinate ruthenium complexes as intermediates. This analysis provides insights into a mechanism that generates four electrons and four protons in the solution and a gas-phase oxygen molecule at different pH values. On the basis of the calculations and corroborated substantially by experiments, the catalytic cycle goes through [(2)Ru(III)](3+) and [(2)Ru(V)(O)](3+) to [(1)Ru(IV)(OOH)](3+) then [(2)Ru(III)(···(3)O2)](3+) at pH 0, and through [(3)Ru(IV)(O)](2+), [(2)Ru(V)(O)](3+), and [(1)Ru(IV)(OO)](2+) at pH 9 before reaching the same [(2)Ru(III)(···(3)O2)](3+) species, from which the liberation of the weakly bound O2 might require an additional oxidation to form [(3)Ru(IV)(O)](2+) to initiate further cycles involving all seven-coordinate species.

Controlling ground and excited state properties through ligand changes in ruthenium polypyridyl complexes

The capture and storage of solar energy requires chromophores that absorb light throughout the solar spectrum. We report here the synthesis, characterization, electrochemical, and photophysical properties of a series of Ru(II) polypyridyl complexes of the type [Ru(bpy)2(N-N)](2+) (bpy = 2,2'-bipyridine; N-N is a bidentate polypyridyl ligand). In this series, the nature of the N-N ligand was altered, either through increased conjugation or incorporation of noncoordinating heteroatoms, as a way to use ligand electronic properties to tune redox potentials, absorption spectra, emission spectra, and excited state energies and lifetimes. Electrochemical measurements show that lowering the π* orbitals on the N-N ligand results in more positive Ru(3+/2+) redox potentials and more positive first ligand-based reduction potentials. The metal-to-ligand charge transfer absorptions of all of the new complexes are mostly red-shifted compared to Ru(bpy)3(2+) (λmax = 449 nm) with the lowest energy MLCT absorption appearing at λmax = 564 nm. Emission energies decrease from λmax = 650 nm to 885 nm across the series. One-mode Franck-Condon analysis of room-temperature emission spectra are used to calculate key excited state properties, including excited state redox potentials. The impacts of ligand changes on visible light absorption, excited state reduction potentials, and Ru(3+/2+) potentials are assessed in the context of preparing low energy light absorbers for application in dye-sensitized photoelectrosynthesis cells.

A little spin on the side: solvent and temperature dependent paramagnetism in [RuII(bpy)2(phendione)]2+

Magnetometry, ¹H-NMR, EPR and substituent effects are used to explain solvent and temperature dependent paramagnetism in [Ru^II(bpy)₂(phendione)](PF₆)₂.

Accumulation of multiple oxidative equivalents at a single site by cross-surface electron transfer on TiO2

The photodriven accumulation of two oxidative equivalents at a single site was investigated on TiO2 coloaded with a ruthenium polypyridyl chromophore [Ru(bpy)2((4,4'-(OH)2PO)2bpy)](2+) (Ru(II)P(2+), bpy = 2,2'-bipyridine, ((OH)2PO)2-bpy = 2,2'-bipyridine-4,4'-diyldiphosphonic acid) and a water oxidation catalyst [Ru(Mebimpy) ((4,4'-(OH)2PO-CH2)2bpy)(OH2)](2+) (Ru(II)OH2(2+), Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine, (4,4'-(OH)2PO-CH2)2bpy) = 4,4'-bis-methlylenephosphonato-2,2'-bipyridine). Electron injection from the metal-to-ligand charge transfer (MLCT) excited state of -Ru(II)P(2+) (-Ru(II)P(2+)*) to give -Ru(III)P(3+) and TiO2(e(-)) was followed by rapid (<20 ns) nearest-neighbor -Ru(II)OH2(2+) to -Ru(III)P(3+) electron transfer. On surfaces containing both -Ru(II)P(2+) and -Ru(III)OH2(3+) (or -Ru(III)OH(2+)), -Ru(II)OH2(2+) was formed by random migration of the injected electron inside the TiO2 nanoparticle and recombination with the preoxidized catalyst, followed by relatively slow (μs-ms) non-nearest neighbor cross-surface electron transfer from -Ru(II)OH2(2+) to -Ru(III)P(3+). Steady state illumination of coloaded TiO2 photoanodes in a dye sensitized photoelectrosynthesis cell (DSPEC) configuration resulted in the buildup of -Ru(III)P(3+), -Ru(III)OH(2+), and -Ru(IV)═O(2+), with -Ru(IV)═O(2+) formation favored at high chromophore to catalyst ratios.

Mechanism of Catalytic Water Oxidation by the Ruthenium Blue Dimer Catalyst: Comparative Study in D₂O versus H₂O

Water oxidation is critically important for the development of energy solutions based on the concept of artificial photosynthesis. In order to gain deeper insight into the mechanism of water oxidation, the catalytic cycle for the first designed water oxidation catalyst, cis,cis-[(bpy)₂(H₂O)Ru^IIIORu^III(OH₂)(bpy)₂]⁴⁺ (bpy is 2,2-bipyridine) known as the blue dimer (BD), is monitored in D₂O by combined application of stopped flow UV-Vis, electron paramagnetic resonance (EPR) and resonance Raman spectroscopy on freeze quenched samples. The results of these studies show that the rate of formation of BD[4,5] by Ce(IV) oxidation of BD[3,4] (numbers in square bracket denote oxidation states of the ruthenium (Ru) centers) in 0.1 M HNO₃, as well as further oxidation of BD[4,5] are slower in D₂O by 2.1–2.5. Ce(IV) oxidation of BD[4,5] and reaction with H₂O result in formation of an intermediate, BD[3,4]′, which builds up in reaction mixtures on the minute time scale. Combined results under the conditions of these experiments at pH 1 indicate that oxidation of BD[3,4]′ is a rate limiting step in water oxidation with the BD catalyst.

Photoinduced electron transfer in a chromophore-catalyst assembly anchored to TiO2

Photoinduced formation, separation, and buildup of multiple redox equivalents are an integral part of cycles for producing solar fuels in dye-sensitized photoelectrosynthesis cells (DSPECs). Excitation wavelength-dependent electron injection, intra-assembly electron transfer, and pH-dependent back electron transfer on TiO(2) were investigated for the molecular assembly [((PO(3)H(2)-CH(2))-bpy)(2)Ru(a)(bpy-NH-CO-trpy)Ru(b)(bpy)(OH(2))](4+) ([TiO(2)-Ru(a)(II)-Ru(b)(II)-OH(2)](4+); ((PO(3)H(2)-CH(2))(2)-bpy = ([2,2'-bipyridine]-4,4'-diylbis(methylene))diphosphonic acid); bpy-ph-NH-CO-trpy = 4-([2,2':6',2″-terpyridin]-4'-yl)-N-((4'-methyl-[2,2'-bipyridin]-4-yl)methyl) benzamide); bpy = 2,2'-bipyridine). This assembly combines a light-harvesting chromophore and a water oxidation catalyst linked by a synthetically flexible saturated bridge designed to enable long-lived charge-separated states. Following excitation of the chromophore, rapid electron injection into TiO(2) and intra-assembly electron transfer occur on the subnanosecond time scale followed by microsecond-millisecond back electron transfer from the semiconductor to the oxidized catalyst, [TiO(2)(e(-))-Ru(a)(II)-Ru(b)(III)-OH(2)](4+)→[TiO(2)-Ru(a)(II)-Ru(b)(II)-OH(2)](4+).

Interfacial dynamics and solar fuel formation in dye-sensitized photoelectrosynthesis cells

Dye-sensitized photoelectrosynthesis cells (DSPECs) represent a promising approach to solar fuels with solar-energy storage in chemical bonds. The targets are water splitting and carbon dioxide reduction by water to CO, other oxygenates, or hydrocarbons. DSPECs are based on dye-sensitized solar cells (DSSCs) but with photoexcitation driving physically separated solar fuel half reactions. A systematic basis for DSPECs is available based on a modular approach with light absorption/excited-state electron injection, and catalyst activation assembled in integrated structures. Progress has been made on catalysts for water oxidation and CO(2) reduction, dynamics of electron injection, back electron transfer, and photostability under conditions appropriate for water splitting. With added reductive scavengers, as surrogates for water oxidation, DSPECs have been investigated for hydrogen generation based on transient absorption and photocurrent measurements. Detailed insights are emerging which define kinetic and thermodynamic requirements for the individual processes underlying DSPEC performance.