Improving and Understanding the Hydrogen Evolving Activity of a Cobalt Dithiolene Metal-Organic Framework

Electrocatalytic CO2 reduction to formate by a cobalt phosphino-thiolate complex

Electrochemical conversion of CO2 to value-added products serves as an attractive method to store renewable energy as energy-dense fuels. Selectivity in this type of conversion can be limited, often leading to the formation of side products such as H2. The activity of a cobalt phosphino-thiolate complex ([Co(triphos)(bdt)]+) towards the selective reduction of CO2 to formate is explored in this report. In the presence of H2O, selective production of formate (as high as 94%) is observed at overpotentials of 750 mV, displaying negligible current degradation during long-term electrolysis experiments ranging as long as 24 hours. Chemical reduction studies of [Co(triphos)(bdt)]+ indicates deligation of the apical phosphine moiety is likely before catalysis. Computational and experimental results suggest a metal-hydride pathway, indicating an ECEC based mechanism.

Computational Prediction of Stacking Mode in Conductive Two-Dimensional Metal-Organic Frameworks: An Exploration of Chemical and Electrical Property Changes

Conductive two-dimensional metal-organic frameworks (2D MOFs) have attracted interest as they induce strong charge delocalization and improve charge carrier mobility and concentration. However, characterizing their stacking mode depends on expensive and time-consuming experimental measurements. Here, we construct a potential energy surface (PES) map database for 36 2D MOFs using density functional theory (DFT) for the experimentally synthesized and non-synthesized 2D MOFs to predict their stacking mode. The DFT PES results successfully predict the experimentally synthesized stacking mode with an accuracy of 92.9% and explain the coexistence mechanism of dual stacking modes in a single compound. Furthermore, we analyze the chemical (i.e., host-guest interaction) and electrical (i.e., electronic structure) property changes affected by stacking mode. The DFT results show that the host-guest interaction can be enhanced by the transition from AA to AB stacking, taking H2S gas as a case study. The electronic band structure calculation confirms that as AB stacking displacement increases, the in-plane charge transport pathway is reduced while the out-of-plane charge transport pathway is maintained or even increased. These results indicate that there is a trade-off between chemical and electrical properties in accordance with the stacking mode.

Controlling Intramolecular and Intermolecular Electronic Coupling of Radical Ligands in a Series of Cobaltoviologen Complexes

Controlling electronic coupling between multiple redox sites is of interest for tuning the electronic properties of molecules and materials. While classic mixed-valence (MV) systems are highly tunable, e.g., via the organic bridges connecting the redox sites, metal-bridged MV systems are difficult to control because the electronics of the metal cannot usually be altered independently of redox-active moieties embedded in its ligands. Herein, this limitation was overcome by varying the donor strengths of ancillary ligands in a series of cobalt complexes without directly perturbing the electronics of viologen-like redox sites bridged by the cobalt ions. The cobaltoviologens [1_X-Co]ⁿ⁺ feature four 4-X-pyridyl donor groups (X = CO₂Me, Cl, H, Me, OMe, NMe₂) that provide gradual electronic tuning of the bridging Co^II centers, while a related complex [2-Co]ⁿ⁺ with NHC donors supports exclusively Co^III states even upon reduction of the viologen units. Electrochemistry and IVCT band analysis indicate that the MV states of these complexes have electronic structures ranging from fully localized ([2-Co]⁴⁺; Robin-Day Class I) to fully delocalized ([1_CO2Me-Co]³⁺; Class III) descriptions, demonstrating unprecedented control over electronic coupling without changing the identity of the redox sites or bridging metal. Additionally, single-crystal XRD characterization of the homovalent complexes [1_H-Co]²⁺ and [1_H-Zn]²⁺ revealed radical-pairing interactions between the viologen ligands of adjacent complexes, representing a type of through-space electronic coupling commonly observed for organic viologen radicals but never before seen in metalloviologens. The extended solid-state packing of these complexes produces 3D networks of radical π-stacking interactions that impart unexpected mechanical flexibility to these crystals.

Highly enhanced electrocatalytic OER activity of water-coordinated copper complexes: effect of lattice water and bridging ligand

The use of metal-organic compounds as electrocatalysts for water splitting reactions has gained increased attention; however, a fundamental understanding of the structural requirement for effective catalytic activity is still limited. Herein, we synthesized water-coordinated mono and bimetallic copper complexes (CuPz-H₂O·H₂O, CuPz-H₂O, CuBipy-H₂O·H₂O, and CuMorph-H₂O) with varied intermetallic spacing (pyrazine/4,4'-bipyridine) and explored the structure-dependent oxygen evolution reaction (OER) activity in alkaline medium. Single crystal structural studies revealed water-coordinated monometallic complexes (CuMorph-H₂O) and bimetallic complexes (CuPz-H₂O·H₂O, CuPz-H₂O, CuBipy-H₂O·H₂O). Further, CuPz-H₂O·H₂O and CuBipy-H₂O·H₂O contained lattice water along with coordinated water. Interestingly, the bimetallic copper complex with lattice water and shorter interspacing between the metal centres (CuPz-H₂O·H₂O) showed strong OER activity and required an overpotential of 228 mV to produce a benchmark current density of 10 mA cm^-2. Bimetallic copper complex (CuPz-H₂O) without lattice water but the same intermetallic spacing and bimetallic complex with increased interspacing but with lattice water (CuBipy-H₂O·H₂O) exhibited relatively lower OER activity. CuPz-H₂O and CuBipy-H₂O·H₂O required an overpotential of 236 and 256 mA cm^-2, respectively. Monometallic CuMorph-H₂O showed the lowest OER activity (overpotential 271 mV) compared to bimetallic complexes. The low Tafel slope and charge transfer resistance of CuPz-H₂O·H₂O facilitated faster charge transfer kinetics at the electrode surface and supported the enhanced OER activity. The chronoamperometric studies indicated good stability of the catalyst. Overall, the present structure-electrocatalytic activity studies of copper complexes might provide structural insight for designing new efficient electrocatalysts based on metal coordination compounds.

Broad Electronic Modulation of Two-Dimensional Metal-Organic Frameworks over Four Distinct Redox States

Two-dimensional (2D) inorganic materials have emerged as exciting platforms for (opto)electronic, thermoelectric, magnetic, and energy storage applications. However, electronic redox tuning of these materials can be difficult. Instead, 2D metal-organic frameworks (MOFs) offer the possibility of electronic tuning through stoichiometric redox changes, with several examples featuring one to two redox events per formula unit. Here, we demonstrate that this principle can be extended over a far greater span with the isolation of four discrete redox states in the 2D MOFs Li_xFe₃(THT)₂ (x = 0-3, THT = triphenylenehexathiol). This redox modulation results in 10,000-fold greater conductivity, p- to n-type carrier switching, and modulation of antiferromagnetic coupling. Physical characterization suggests that changes in carrier density drive these trends with relatively constant charge transport activation energies and mobilities. This series illustrates that 2D MOFs are uniquely redox flexible, making them an ideal materials platform for tunable and switchable applications.

Electronically-coupled redox centers in trimetallic cobalt complexes

Synthesis and isolation of molecular building blocks of metal-organic frameworks (MOFs) can provide unique opportunities for characterization that would otherwise be inaccessible due to the heterogeneous nature of MOFs. Herein, we report a series of trinuclear cobalt complexes incorporating dithiolene ligands, triphenylene-2,3,6,7,10,11-hexathiolate (THT) (13+), and benzene hexathiolate (BHT) (23+), with 1,1,1,-tris(diphenylphosphinomethyl)ethane (triphos) employed as the capping ligand. Single crystal X-ray analyses of 13+ and 23+ display three five-coordinate cobalt centers bound to the triphos and dithiolene ligands in a distorted square pyramidal geometry. Cyclic voltammetry studies of 13+ and 23+ reveal three redox features associated with the formation of mixed valence states due to the sequential reduction of the redox-active metal centers (Co^III/II). Using this electrochemical data, the comproportionality values were determined for 1 and 2 (log K_c = 1.4 and 1.5 for 1, and 4.7 and 5.8 for 2), suggesting strong resonance-stabilized coupling of the metal centers, with stronger electronic coupling observed for complex 2 compared to that for complex 1. Cyclic voltammetry studies were also performed in solvents of varying polarity, whereupon the difference in the standard potentials (ΔE_1/2) for 1 and 2 was found to shift as a function of the polarity of the solvent, indicating a negative correlation between the dielectric constant of the electrochemical medium and the stability of the mixed valence species. Spectroelectrochemical studies of in situ generated multi-valent (MV) states of complexes 1 and 2 display characteristic NIR intervalence charge transfer (IVCT) bands, and analysis of the IVCT transitions for complex 2 suggests a weakly coupled class II multi-valent species and relatively large electronic coupling factors (1700 cm^-1 for the first multi-valent state of 22+, and 1400 and 4000 cm^-1 for the second multi-valent state of 2+). Density functional theory (DFT) calculations indicate a significant deviation in relative energies of the frontier orbitals of complexes 13+, 23+, and 3+ that contrasts those calculated for the analogous trinuclear cobalt dithiolene complexes employing pentamethylcyclopentadienyl (Cp*) as the capping ligand (Co3Cp*3THT and Co3Cp*3BHT, respectively), and may be a result of the cationic nature of complexes 13+, 23+, and 3+.

Two-Dimensional Conjugated Metal-Organic Frameworks for Electrocatalysis: Opportunities and Challenges

A highly effective electrocatalyst is the central component of advanced electrochemical energy conversion. Recently, two-dimensional conjugated metal-organic frameworks (2D c-MOFs) have emerged as a class of promising electrocatalysts because of their advantages including 2D layered structure with high in-plane conjugation, intrinsic electrical conductivity, permanent pores, large surface area, chemical stability, and structural diversity. In this Review, we summarize the recent advances of 2D c-MOF electrocatalysts for electrochemical energy conversion. First, we introduce the chemical design principles and synthetic strategies of the reported 2D c-MOFs, as well as the functional design for the electrocatalysis. Subsequently, we present the representative 2D c-MOF electrocatalysts in various electrochemical reactions, such as hydrogen/oxygen evolution, and reduction reactions of oxygen, carbon dioxide, and nitrogen. We highlight the strategies for the structural design and property tuning of 2D c-MOF electrocatalysts to boost the catalytic performance, and we offer our perspectives in regard to the challenges to be overcome.

A Ni(II) coordination polymer with dual electrochemical functions: synthesis, crystal structure, hydrogen evolution reaction and l-ascorbic acid sensing

A two-dimensional Ni(II) coordination polymer (NiCP) of the formula {[NiL(terephthalate)(H2O)2]·2H2O} n (L = bis(1-(pyridin-4-ylmethyl)-benzimidazol-2-ylmethyl)ether), has been obtained from a solvothermal reaction, and characterized by single-crystal X-ray diffraction, elemental analysis, and IR and UV/Vis spectra. The coordinated terephthalate anions and the L ligands connect the Ni(II) ions in two directions, resulting in the construction of a corrugated layered structure. The electrochemical properties of a NiCP-CPE composite electrode supported by this coordination polymer were studied. For the electrocatalytic hydrogen evolution reaction, the required overpotential of this electrode (NiCP-CPE) is −521 mV when the current density reaches 10 mA cm−2. Compared with the solid carbon paste electrode (sCPE, −976 mV), the smaller overpotential proves effective electrocatalysis of the coordination polymer of the hydrogen evolution reaction. The doped electrode also exhibits high-efficiency in the electrochemical sensing of l-ascorbic acid in water, showing a detection limit of 0.28 μM in a linear range of 0.4–4000 μM.

Hydrogen Evolving Activity of Dithiolene-Based Metal-Organic Frameworks with Mixed Cobalt and Iron Centers

Electrocatalytic systems based on metal-organic frameworks (MOFs) have attracted great attention due to their potential application in commercially viable renewable energy-converting devices. We have recently shown that the cobalt 2,3,6,7,10,11-triphenylenehexathiolate (CoTHT) framework can catalyze the hydrogen evolution reaction (HER) in fully aqueous media with Tafel slopes as low as 71 mV/dec and near-unity Faradaic efficiency (FE). Taking advantage of the high synthetic tunability of MOFs, here, we synthesize a series of iron and mixed iron/cobalt THT-based MOFs. The incorporation of the iron and cobalt dithiolene moieties is verified by various spectroscopic techniques, and the integrity of the crystalline structure is maintained regardless of the stoichiometries of the two metals. The hydrogen evolving activity of the materials was explored in pH 1.3 aqueous electrolyte solutions. Unlike CoTHT, the FeTHT framework exhibits minimal activity due to a late catalytic onset [-0.440 V versus reversible hydrogen electrode (RHE)] and a large Tafel slope (210 mV/dec). The performance of the mixed-metal MOFs is adversely affected by the incorporation of Fe, where increasing Fe content results in MOFs with lower HER activity and diminished long-term stability and FE for H₂ production. It is proposed that the FeTHT domains undergo alternative Faradaic processes under catalytic conditions, which alter its local structure and electrochemical behavior, eventually resulting in a material with diminished HER performance.

Cu[Ni(2,3-pyrazinedithiolate)2] Metal-Organic Framework for Electrocatalytic Hydrogen Evolution

The application of metal-organic frameworks (MOFs) as electrocatalysts for small molecule activation has been an emerging topic of research. Previous studies have suggested that two-dimensional (2D) dithiolene-based MOFs are among the most active for the hydrogen evolution reaction (HER). Here, a three-dimensional (3D) dithiolene-based MOF, Cu[Ni(2,3-pyrazinedithiolate)₂] (1), is evaluated as an electrocatalyst for the HER. In pH 1.3 aqueous electrolyte solution, 1 exhibits a catalytic onset at -0.43 V vs the reversible hydrogen electrode (RHE), an overpotential (η_10 mA/cm²) of 0.53 V to reach a current density of 10 mA/cm², and a Tafel slope of 69.0 mV/dec. Interestingly, under controlled potential electrolysis, 1 undergoes an activation process that results in a more active catalyst with a 200 mV reduction in the catalytic onset and η_10 mA/cm². It is proposed that the activation process is a result of the cleavage of Cu-N bonds in the presence of protons and electrons. This hypothesis is supported by various experimental studies and density functional theory calculations.