Cobaloxime Catalyst Research Articles

Cover Feature: Cobaloxime Complex Salts: Synthesis, Patterning on Carbon Nanomembranes and Heterogeneous Hydrogen Evolution Studies (Chem. Eur. J. 68/2021)

The setup for operando measurement of hydrogen evolution under illumination is shown in the cover feature picture. Various cobaloxime catalysts were deposited onto solid support (carbon nanomembranes) and evaluated with respect to their hydrogen evolution reaction by using Pd microsensors. The structure of the most efficient cobalt-based double salt complex is depicted in the foreground. More information can be found in the Full Paper by M. von Delius, C. Kranz, et al. (DOI: 10.1002/chem.202102778). Photo credit: Elvira Eberhardt, Ulm University.

Generation of α‐Boryl Radicals by H. Transfer and their Use in Cycloisomerizations

AbstractCarbon‐centered radicals can be stabilized by delocalization of their spin density into the vacant p orbital of a boron substituent. α‐Vinyl boronates, in particular pinacol (Bpin) derivatives, are excellent hydrogen atom acceptors. Under H2, in the presence of a cobaloxime catalyst, they generate α‐boryl radicals; these species can undergo 5‐exo radical cyclizations if appropriate double bond acceptors are present, leading to densely functionalized heterocycles with tertiary substituents on Bpin. The reaction shows good functional group tolerance with wide scope, and the resulting boronate products can be converted into other useful functionalities.

Generation of α-Boryl Radicals by H. Transfer and their Use in Cycloisomerizations.

Carbon-centered radicals can be stabilized by delocalization of their spin density into the vacant p orbital of a boron substituent. α-Vinyl boronates, in particular pinacol (Bpin) derivatives, are excellent hydrogen atom acceptors. Under H2 , in the presence of a cobaloxime catalyst, they generate α-boryl radicals; these species can undergo 5-exo radical cyclizations if appropriate double bond acceptors are present, leading to densely functionalized heterocycles with tertiary substituents on Bpin. The reaction shows good functional group tolerance with wide scope, and the resulting boronate products can be converted into other useful functionalities.

Dual Photoredox/Cobaloxime Catalysis for Cross-Dehydrogenative α-Heteroarylation of Amines.

We report a dual-catalytic platform for the cross-dehydrogenative-coupling between (benzo-)thiazoles and amines which combines low loadings of an iridium photoredox catalyst and a cobaloxime catalyst under blue light irradiation. This transformation occurs without stoichiometric oxidants, giving products in moderate to excellent yields. DFT calculations support the key role of Co(II) for rearomatization of the radical-addition intermediate to generate the product.

A cross-dehydrogenative C(sp3)\u2212H heteroarylation via photo-induced catalytic chlorine radical generation

Hydrogen atom abstraction (HAT) from C(sp3)–H bonds of naturally abundant alkanes for alkyl radical generation represents a promising yet underexplored strategy in the alkylation reaction designs since involving stoichiometric oxidants, excessive alkane loading, and limited scope are common drawbacks. Here we report a photo-induced and chemical oxidant-free cross-dehydrogenative coupling (CDC) between alkanes and heteroarenes using catalytic chloride and cobalt catalyst. Couplings of strong C(sp3)–H bond-containing substrates and complex heteroarenes, have been achieved with satisfactory yields. This dual catalytic platform features the in situ engendered chlorine radical for alkyl radical generation and exploits the cobaloxime catalyst to enable the hydrogen evolution for catalytic turnover. The practical value of this protocol was demonstrated by the gram-scale synthesis of alkylated heteroarene with merely 3 equiv. alkane loading.

Fundamental Characterization, Photophysics and Photocatalysis of a Base Metal Iron(II)-Cobalt(III) Dyad.

A new base metal iron‐cobalt dyad has been obtained by connection between a heteroleptic tetra‐NHC iron(II) photosensitizer combining a 2,6‐bis[3‐(2,6‐diisopropylphenyl)imidazol‐2‐ylidene]pyridine with 2,6‐bis(3‐methyl‐imidazol‐2‐ylidene)‐4,4′‐bipyridine ligand, and a cobaloxime catalyst. This novel iron(II)‐cobalt(III) assembly has been extensively characterized by ground‐ and excited‐state methods like X‐ray crystallography, X‐ray absorption spectroscopy, (spectro‐)electrochemistry, and steady‐state and time‐resolved optical absorption spectroscopy, with a particular focus on the stability of the molecular assembly in solution and determination of the excited‐state landscape. NMR and UV/Vis spectroscopy reveal dissociation of the dyad in acetonitrile at concentrations below 1 mM and high photostability. Transient absorption spectroscopy after excitation into the metal‐to‐ligand charge transfer absorption band suggests a relaxation cascade originating from hot singlet and triplet MLCT states, leading to the population of the 3MLCT state that exhibits the longest lifetime. Finally, decay into the ground state involves a 3MC state. Attachment of cobaloxime to the iron photosensitizer increases the 3MLCT lifetime at the iron centre. Together with the directing effect of the linker, this potentially makes the dyad more active in photocatalytic proton reduction experiments than the analogous two‐component system, consisting of the iron photosensitizer and Co(dmgH)2(py)Cl. This work thus sheds new light on the functionality of base metal dyads, which are important for more efficient and sustainable future proton reduction systems.

Conjugated Linkers Improve the Photoelectrocatalytic H2‐Evolution Activity of Cobaloxime‐Modified Silicon Photocathodes by Largely Suppressing Charge Recombination

AbstractFor molecular catalyst‐modified semiconductor hybrid photoelectrodes, the linker that tethers a molecular catalyst to an electrode surface is one of the important structural factors to influence the photoelectrocatalytic (PEC) H2‐evolution activity of the photoelectrode. A better understanding about the impact of different linkers on charge transfer kinetics of hybrid photoelectrodes will shed light on the design of more efficient photoelectrodes for the selected molecular catalyst and semiconductor. In this work, a series of cobaloxime catalysts, CoPy‐4‐L‐HA (CoPy = cobaloxime; HA = hydroxamate anchor; L = none, CH2, CHCH, and (CHCH)2), are immobilized to the surface of p‐Si|TiO2 with different linkers. Comparative studies on PEC performance of these hybrid photocathodes reveal that the p‐Si|TiO2|CoPy‐4‐(CHCH)2‐HA) electrode exhibits the highest PEC activity among the tested photocathodes, despite the longer linker of (CHCH)2 compared to other ones. The j(0 V) of −0.68 mA cm−2 for p‐Si|TiO2|CoPy‐4‐(CHCH)2‐HA is higher than those reported for planar Si‐based photocathodes decorated with molecular catalysts. The impacts of the length and conjugation property of linkers on the kinetics of surface charge transfer and recombination are probed by transient absorption spectroscopy, and the results well clarify the origin for the disparity in the PEC activities of p‐Si|TiO2|CoPy‐4‐L‐HA photocathodes with different linkers.

Photoredox/Cobalt-Catalyzed C(sp3)-H Bond Functionalization toward Phenanthrene Skeletons with Hydrogen Evolution.

The first example of photoredox strategy for synthesis of phenanthrene skeletons through C(sp3)-H functionalization under external oxidant-free conditions is achieved. This transformation relies on the keto-enol tautomerism of 1,3 dicarbonyl moiety, i.e., the enol form of 1,3-dicarbonyl derivatives with relatively lower oxidation potential can be activated by the excited acridinium photocatalyst. The electron and proton eliminated from the substrate are immediately captured by a cobaloxime catalyst to exclusively afford a-carbonyl radical for highly substituted 10-phenanthrenols in good to excellent yields.

Photophysics of graphene quantum dot assemblies with axially coordinated cobaloxime catalysts.

We report a study of chromophore-catalyst assemblies composed of light harvesting hexabenzocoronene (HBC) chromophores axially coordinated to two cobaloxime complexes. The chromophore-catalyst assemblies were prepared using bottom-up synthetic methodology and characterized using solid-state NMR, IR, and x-ray absorption spectroscopy. Detailed steady-state and time-resolved laser spectroscopy was utilized to identify the photophysical properties of the assemblies, coupled with time-dependent DFT calculations to characterize the relevant excited states. The HBC chromophores tend to assemble into aggregates that exhibit high exciton diffusion length (D = 18.5 molecule2/ps), indicating that over 50 chromophores can be sampled within their excited state lifetime. We find that the axial coordination of cobaloximes leads to a significant reduction in the excited state lifetime of the HBC moiety, and this finding was discussed in terms of possible electron and energy transfer pathways. By comparing the experimental quenching rate constant (1.0 × 109 s-1) with the rate constant estimates for Marcus electron transfer (5.7 × 108 s-1) and Förster/Dexter energy transfers (8.1 × 106 s-1 and 1.0 × 1010 s-1), we conclude that both Dexter energy and Marcus electron transfer process are possible deactivation pathways in CoQD-A. No charge transfer or energy transfer intermediate was detected in transient absorption spectroscopy, indicating fast, subpicosecond return to the ground state. These results provide important insights into the factors that control the photophysical properties of photocatalytic chromophore-catalyst assemblies.

Cobaloxime Catalysis for Enamine Phosphorylation with Hydrogen Evolution.

Direct phosphorylation of enamine and enamide with hydrogen evolution was realized via cobaloxime catalysis under visible-light irradiation. Control experiments and spectroscopic studies demonstrated a reductive quenching pathway of cobaloxime catalyst to produce phosphinoyl radical, which underwent cross-coupling with various enamines (and enamides) to give diverse β-phosphinoyl products in good to excellent yields. More interestingly, Z/E mixture of acyclic enamines could convert into single Z-products with good reactivity.

Understanding the Performance of NiO Photocathodes with Alkyl-Derivatized Cobalt Catalysts and a Push-Pull Dye.

Mesoporous NiO photocathodes containing the push–pull dye PB6 and alkyl-derivatized cobaloxime catalysts were prepared using surface amide couplings and analyzed for photocatalytic proton reduction catalysis. The length of the alkyl linker used to derivatize the cobalt catalysts was found to correlate to the photocurrent with the highest photocurrent observed using shorter alkyl linkers but the lowest one for samples without linker. The alkyl linkers were also helpful in slowing dye–NiO charge recombination. Photoelectrochemical measurements and femtosecond transient absorption spectroscopic measurements suggested electron transfer to the surface-immobilized catalysts occurred; however, H2 evolution was not observed. Based on UV–vis, X-ray fluorescence spectroscopy (XRF), and X-ray photoelectron spectroscopy (XPS) measurements, the cobalt catalyst appeared to be limiting the photocathode performance mainly via cobalt demetallation from the oxime ligand. This study highlights the need for a deeper understanding of the effect of catalyst molecular design on photocathode performance.

Mild Isomerization of Conjugated Dienes Using Co-Mediated Hydrogen Atom Transfer.

A mild and high yielding rearrangement of 1,3-disubstituted-1,3-dienes to 1,1,4-trisubstituted-1,3-dienes using a cobaloxime catalyst and a silane cocatalyst is reported. Chiral centers near the conjugated diene were not racemized. Deuterium labeling studies are consistent with a hydrogen atom transfer mechanism, and radical intermediates were found to be accessible due to the observed ring opening of a cyclopropane ring.

Electrochemistry of Polymeric Cobaloxime-Containing Assemblies in Organic and Aqueous Solvents

We report on the structural and electrochemical properties of a heterogeneous-homogeneous assembly composed of molecular cobaloxime catalysts immobilized onto graphite electrodes via an intervening polyvinylpyridine surface coating. When these modified electrodes are immersed in an organic solvent (propylene carbonate containing 0.1 M tetrabutylammonium perchlorate as a supporting electrolyte) or basic aqueous solutions (0.1 M NaOH), cyclic voltammetry measurements enable determination of the CoIII/II peak potentials and CoII/I midpoint potentials of cobaloximes embedded within the polymeric architectures. Additionally, voltammetry measurements recorded using pH neutral aqueous solutions (0.1 M phosphate buffer) confirm the immobilized cobaloximes remain catalytically active for hydrogen production and operate at a turnover frequency of 1.6 s−1 when polarized at –0.35 V vs the H+/H2 equilibrium potential. Waveform analysis of redox features associated with immobilized cobaloximes indicates more repulsive interactions within the polymer film at pH neutral vs basic conditions, which is attributed to the increased fraction of pyridinium species at lower pH values. Our measurements also show the number of electrochemically active sites changes when measured in different solvent environments, indicating that electroactive loadings determined under non-catalytic solvent conditions are not necessarily representative of those under catalytic conditions and could thereby lead to misrepresentations of catalytic turnover frequencies.

Selective photoredox decarboxylation of α-ketoacids to allylic ketones and 1,4-dicarbonyl compounds dependent on cobaloxime catalysis.

A photoredox/cobaloxime co-catalyzed coupling reaction of α-ketoacids and methacrylates to obtain allylic ketones is described. Without the cobaloxime catalyst, 1,4-dicarbonyl compounds are generated. The cobaloxime catalyst enables dehydrogenation to generate the formation of new olefins. The generality, good substrate scope and mild conditions are good features in the photoredox/cobaloxime catalysis protocol, and this method will provide new opportunities for the functionalization of more olefins.

Design of a [FeFe] macrocyclic metallotecton for light-driven hydrogen evolution reaction

Combining the two most fascinating fields of chemistry in the development of novel metal-organic architectures viz. macrocyclic chemistry and metallotectons, herein, we report the one-pot synthesis of a binuclear Fe(II) macrocyclic metallotecton 2via coordination-driven assembly in excellent yield and purity. The single crystal structure of 2 indicates two units of cationic iron complex with ligand 1 (6- pyridin-2-yl- [1,3,5]-triazine-2,4-diamine) acting as metallotectons connected to each other by two [μ-PHO3CH3]- bridges forming a [FeFe] macrocycle embedded in two metallotectons. The pyridine and DAT rings of both ligands 1 are nearly coplanar in 2 and each iron(II) has a distorted octahedral geometry. Diaminotriazine groups incorporated in 2 direct multiple hydrogen bonding according to predictable motifs II to produce a final three-dimensional structure. Thermal stability, electronic properties and redox properties of 2 have been studied by thermogravimetric analysis, UV–Vis and cyclic voltammetry respectively. The hydrogen evolution reaction (HER) study of 2 reveals it as an efficient photocatalyst displaying a high turnover number (TON) and turnover frequency (TOF), which is much higher than other Fe based molecular catalysts reported in literature. Also, its photocatalytic activity outperforms the most reported standard reference cobaloxime catalyst (CoIII(dmgH)2(py)Cl). This is by far, the only report showcasing a macrocyclic metallotecton for photocatalytic HER. In addition, complex 2 can be regarded as the next level in supramolecular chemistry as it bridges the gap between macrocycles and metallotectons as well as it can be a future option for our deteriorating environment.

Expanding the Redox Range of Surface-Immobilized Metallocomplexes Using Molecular Interfaces.

Rationally designed material interfaces offer opportunities to control matter and energy across multiple length scales, yet remain challenging to synthetically prepare. Inspired by nature, where amino acid residues and soft-material coordination environments regulate the midpoint potentials of metals in proteins, thin-film polymeric coatings have been developed to assemble molecular components, including catalysts, onto solid-state (semi)conducting surfaces. In this report, we describe the immobilization of metalloporphyrins onto transparent conductive oxide supports using either direct grafting to the oxide surface or coordination to an initially applied thin-film polypyridyl coating. The composite materials enable direct measurements of electrochemical and optical properties associated with the surface-immobilized components. Despite the similarity of the core cobalt porphyrin units used in assembling these hybrid architectures, the redox potentials assigned to the CoIII/II relays span a 350 mV range across the distinct constructs. This range in redox potential is extended to 960 mV when including comparisons to constructs utilizing polymer-immobilized cobaloxime catalysts in place of cobalt porphyrins, where reduction of the cobaloximes requires significantly more-negative bias potentials. This work illustrates the use of soft-material interfaces for assembling molecular-modified electrodes where the nanoscale connectivity of the surface coatings determines the electrochemical properties of the macroscopic assemblies.

Electrocatalytic Hydrogen Evolution from a Cobaloxime-Based Metal–Organic Framework Thin Film

Molecular hydrogen evolution catalysts (HECs) are synthetically tunable and often exhibit high activity, but they are also hampered by stability concerns and practical limitations associated with their use in the homogeneous phase. Their incorporation as integral linker units in metal–organic frameworks (MOFs) can remedy these shortcomings. Moreover, the extended three-dimensional structure of MOFs gives rise to high catalyst loadings per geometric surface area. Herein, we report a new MOF that exclusively consists of cobaloximes, a widely studied HEC, that act as metallo-linkers between hexanuclear zirconium clusters. When grown on conducting substrates and under applied reductive potential, the cobaloxime linkers promote electron transport through the film as well as function as molecular HECs. The obtained turnover numbers are orders of magnitude higher than those of any other comparable cobaloxime system, and the molecular integrity of the cobaloxime catalysts is maintained for at least 18 h of electrocatalysis. Being one of the very few hydrogen evolving electrocatalytic MOFs based on a redox-active metallo-linker, this work explores uncharted terrain for greater catalyst diversity and charge transport pathways.

Influence of Anchoring Groups on the Charge Transfer and Performance of p-Si/TiO2/Cobaloxime Hybrid Photocathodes for Photoelectrochemical H2 Production.

Although hybrid photocathodes built by immobilizing molecular catalysts to the surface of semiconductors through chemical linkages have been reported in recent years, systematic and comparative studies remain scarce about the impact of various anchoring groups on the performance, stability, and charge-transfer kinetics of molecular catalyst-decorated hybrid photocathodes for photoelectrochemical (PEC) H2 production. In this study, the molecular cobaloxime catalysts, CoPy-4-X (Py = pyridine, X = PO3H2, COOH, and CONH(OH)), bearing different anchoring groups were synthesized and covalently immobilized to the surface of the porous TiO2 layer coated on a p-Si plate or a fluorine-doped tin oxide glass. The influence of the anchoring groups on the performance of p-Si/TiO2/CoPy-4-X photocathodes was comparatively studied for PEC H2 evolution. Among the tested hybrid photocathodes, the one with a hydroxamate as an anchoring group displayed higher activity and lower charge-transfer resistance than that observed for the electrode with a carboxylate or a phosphonate as the anchoring group. Notably, the catalytic current of p-Si/TiO2/CoPy-4-CONH(OH) was attenuated only by 2.9% in the controlled potential photoelectrolysis tests in borate buffer solution at pH 9 at 0 V versus a reversible hydrogen electrode over 6 h. Moreover, the influence of anchoring groups on the interfacial electron transfer from the TiO2 layer to the immobilized cobaloxime catalyst and electron-hole recombination was studied by transient absorption spectroscopy. These results revealed that the hydroxamate as an anchoring group is superior to the carboxylate and phosphonate groups for speeding up the interfacial electron transfer and firmly immobilizing the molecular catalysts to the metal oxide semiconductors to build efficient and stable hybrid photoelectrodes.

Cobaloxime Catalysis: Selective Synthesis of Alkenylphosphine Oxides under Visible Light.

Direct activation of H-phosphine oxide to react with an unsaturated carbon-carbon bond is a straightforward approach for accessing alkenylphosphine oxides, which shows significant applications in both synthetic and material fields. However, expensive metals and strong oxidants are typically required to realize the transformation. Here, we demonstrate the utility of earth-abundant cobaloxime to convert H-phosphine oxide into its reactive radical species under visible light irradiation. The radical species thus generated can be utilized to functionalize alkenes and alkynes without any external photosensitizer and oxidant. The coupling with terminal alkene generates E-alkenylphosphine oxide with excellent chemo- and stereoselectivity. The reaction with terminal alkyne yields linear E-alkenylphosphine oxide via neutral radical addition, while addition with internal ones generates cyclic benzophosphine oxides and hydrogen. Mechanistic studies on radical trapping experiments, electron spin resonance studies, and spectroscopic measurements confirm the formation of phosphinoyl radical and cobalt intermediates that are from capturing the electron and proton eliminated from H-phosphine oxide. The highlight of our mechanistic investigation is the dual role played by cobaloxime, viz., both as the visible light absorber to activate the P(O)-H bond as well as a hydrogen transfer agent to influence the reaction pathway. This synergetic feature of the cobaloxime catalyst preforming multiple functions under ambient condition provides a convergent synthetic approach to vinylphosphine oxides directly from H-phosphine oxides and alkenes (or alkynes).

Earth-Abundant Molecular Z-Scheme Photoelectrochemical Cell for Overall Water-Splitting

A push-pull organic dye and a cobaloxime catalyst were successfully cografted on NiO and CuGaO2 to form efficient molecular photocathodes for H2 production with >80% Faradaic efficiency. CuGaO2 is emerging as a more effective p-type semiconductor in photoelectrochemical cells and yields a photocathode with 4-fold higher photocurrent densities and 400 mV more positive onset photocurrent potential compared to the one based on NiO. Such an optimized CuGaO2 photocathode was combined with a TaON|CoO x photoanode in a photoelectrochemical cell. Operated in this Z-scheme configuration, the two photoelectrodes produced H2 and O2 from water with 87% and 88% Faradaic efficiency, respectively, at pH 7 under visible light and in the absence of an applied bias, equating to a solar to hydrogen conversion efficiency of 5.4 × 10-3%. This is, to the best of our knowledge, the highest efficiency reported so far for a molecular-based noble metal-free water splitting Z-scheme.