Molecular Catalysis of Energy Relevance in Metal-Organic Frameworks: From Higher Coordination Sphere to System Effects.

The modularity and synthetic flexibility of metal-organic frameworks (MOFs) have provoked analogies with enzymes, and even the term MOFzymes has been coined. In this review, we focus on molecular catalysis of energy relevance in MOFs, more specifically water oxidation, oxygen and carbon dioxide reduction, as well as hydrogen evolution in context of the MOF-enzyme analogy. Similar to enzymes, catalyst encapsulation in MOFs leads to structural stabilization under turnover conditions, while catalyst motifs that are synthetically out of reach in a homogeneous solution phase may be attainable as secondary building units in MOFs. Exploring the unique synthetic possibilities in MOFs, specific groups in the second and third coordination sphere around the catalytic active site have been incorporated to facilitate catalysis. A key difference between enzymes and MOFs is the fact that active site concentrations in the latter are often considerably higher, leading to charge and mass transport limitations in MOFs that are more severe than those in enzymes. High catalyst concentrations also put a limit on the distance between catalysts, and thus the available space for higher coordination sphere engineering. As transport is important for MOF-borne catalysis, a system perspective is chosen to highlight concepts that address the issue. A detailed section on transport and light-driven reactivity sets the stage for a concise review of the currently available literature on utilizing principles from Nature and system design for the preparation of catalytic MOF-based materials.

Fast charging of energy-dense lithium-ion batteries.

Lithium-ion batteries with nickel-rich layered oxide cathodes and graphite anodes have reached specific energies of 250-300 Wh kg-1 (refs. 1,2), and it is now possible to build a 90 kWh electric vehicle (EV) pack with a 300-mile cruise range. Unfortunately, using such massive batteries to alleviate range anxiety is ineffective for mainstream EV adoption owing to the limited raw resource supply and prohibitively high cost. Ten-minute fast charging enables downsizing of EV batteries for both affordability and sustainability, without causing range anxiety. However, fast charging of energy-dense batteries (more than 250 Wh kg-1 or higher than 4 mAh cm-2) remains a great challenge3,4. Here we combine a material-agnostic approach based on asymmetric temperature modulation with a thermally stable dual-salt electrolyte to achieve charging of a 265 Wh kg-1 battery to 75% (or 70%) state of charge in 12 (or 11) minutes for more than 900 (or 2,000) cycles. This is equivalent to a half million mile range in which every charge is a fast charge. Further, we build a digital twin of such a battery pack to assess its cooling and safety and demonstrate that thermally modulated 4C charging only requires air convection. This offers a compact and intrinsically safe route to cell-to-pack development. The rapid thermal modulation method to yield highly active electrochemical interfaces only during fast charging has important potential to realize both stability and fast charging of next-generation materials, including anodes like silicon and lithium metal.

Elemental Depth Profiling of Intact Metal-Organic Framework Single Crystals by Scanning Nuclear Microprobe.

The growing field of MOF-catalyst composites often relies on postsynthetic modifications for the installation of active sites. In the resulting MOFs, the spatial distribution of the inserted catalysts has far-reaching ramifications for the performance of the system and thus needs to be precisely determined. Herein, we report the application of a scanning nuclear microprobe for accurate and nondestructive depth profiling of individual UiO-66 and UiO-67 (UiO = Universitetet i Oslo) single crystals. Initial optimization work using native UiO-66 crystals yielded a microbeam method which avoided beam damage, while subsequent analysis of Zr/Hf mixed-metal UiO-66 crystals demonstrated the potential of the method to obtain high-resolution depth profiles. The microbeam method was further used to analyze the depth distribution of postsynthetically introduced organic moieties, revealing either core-shell or uniform incorporation can be obtained depending on the size of the introduced molecule, as well as the number of carboxylate binding groups. Finally, the spatial distribution of platinum centers that were postsynthetically installed in the bpy binding pockets of UiO-67-bpy (bpy = 5,5'-dicarboxyy-2,2'-bipyridine) was analyzed by microbeam and contextualized. We expect that the method presented herein will be applicable for characterizing a wide variety of MOFs subjected to postsynthetic modifications and provide information crucial for their optimization as functional materials.

Enhancing photovoltages at p-type semiconductors through a redox-active metal-organic framework surface coating.

Surface modification of semiconductors can improve photoelectrochemical performance by promoting efficient interfacial charge transfer. We show that metal-organic frameworks (MOFs) are viable surface coatings for enhancing cathodic photovoltages. Under 1-sun illumination, no photovoltage is observed for p-type Si(111) functionalized with a naphthalene diimide derivative until the monolayer is expanded in three dimensions in a MOF. The surface-grown MOF thin film at Si promotes reduction of the molecular linkers at formal potentials >300 mV positive of their thermodynamic potentials. The photocurrent is governed by charge diffusion through the film, and the MOF film is sufficiently conductive to power reductive transformations. When grown on GaP(100), the reductions of the MOF linkers are shifted anodically by >700 mV compared to those of the same MOF on conductive substrates. This photovoltage, among the highest reported for GaP in photoelectrochemical applications, illustrates the power of MOF films to enhance photocathodic operation.

Transport Phenomena: Challenges and Opportunities for Molecular Catalysis in Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) are appealing heterogeneous support matrices that can stabilize molecular catalysts for the electrochemical conversion of small molecules. However, moving from a homogeneous environment to a porous film necessitates the transport of both charge and substrate to the catalytic sites in an efficient manner. This presents a significant challenge in the application of such materials at scale, since these two transport phenomena (charge and mass transport) would need to operate faster than the intrinsic catalytic rate in order for the system to function efficiently. Thus, understanding the fundamental kinetics of MOF-based molecular catalysis of electrochemical reactions is of crucial importance. In this Perspective, we quantitatively dissect the interplay between the two transport phenomena and the catalytic reaction rate by applying models from closely related fields to MOF-based catalysis. The identification of the limiting process provides opportunities for optimization that are uniquely suited to MOFs due to their tunable molecular structure. This will help guide the rational design of efficient and high-performing catalytic MOF films with incorporated molecular catalyst for electrochemical energy conversion.

Analysis of Electrocatalytic Metal-Organic Frameworks.

The electrochemical analysis of molecular catalysts for the conversion of bulk feedstocks into energy-rich clean fuels has seen dramatic advances in the last decade. More recently, increased attention has focused on the characterization of metal-organic frameworks (MOFs) containing well-defined redox and catalytically active sites, with the overall goal to develop structurally stable materials that are industrially relevant for large-scale solar fuel syntheses. Successful electrochemical analysis of such materials draws heavily on well-established homogeneous techniques, yet the nature of solid materials presents additional challenges. In this tutorial-style review, we cover the basics of electrochemical analysis of electroactive MOFs, including considerations of bulk stability, methods of attaching MOFs to electrodes, interpreting fundamental electrochemical data, and finally electrocatalytic kinetic characterization. We conclude with a perspective of some of the prospects and challenges in the field of electrocatalytic MOFs.

Facile Orientational Control of M2L2P SURMOFs on ⟨100⟩ Silicon Substrates and Growth Mechanism Insights for Defective MOFs.

Layer-by-layer growth of Cu2(bdc)2(dabco) surface-mounted metal-organic frameworks (SURMOFs) was investigated on silicon wafers treated with different surface anchoring molecules. Well-oriented growth along the [100] and [001] directions could be achieved with simple protocols: growth along the [100] direction was achieved by substrate pretreatment with 80 °C piranha, while growth along the [001] direction was enabled by only rinsing silicon with absolute ethanol. Growth along the [001] direction produced more homogeneous SURMOF films. Optimization to enhance [001]-preferred orientation growth revealed that small changes in the SURMOF growth sequence (the number of rinse steps and linker concentrations) have a noticeable impact on the final film quality and the number of misaligned crystals. This new straightforward protocol was used to successfully grow other layer pillar-type SURMOFs, including the growth of Cu2(bdc)2(bipy) with simultaneous suppression of framework interpenetration.

On decomposition, degradation, and voltammetric deviation: the electrochemist's field guide to identifying precatalyst transformation.

What is the identity of the true electrocatalytic species? This fundamental question has plagued the molecular electrocatalysis community during its decades-long search for selective and efficient transition-metal based electrocatalysts for fuel forming reactions. Identifying when the added species is a precatalyst that transforms into the active catalyst in situ is an extraordinarily complex endeavor. Thankfully, the last decade has witnessed a resurgence of interest in understanding and controlling these transformations, leading to an expansion of the experimental toolkit available to probe catalyst identity. In this Tutorial Review, researchers will learn how the nature of the active catalyst can be uncovered using state-of-the-art electrochemical and spectroscopic methods. Analysis of catalytic voltammograms can quickly furnish qualitative evidence of precatalyst transformation and a library of these tell-tale signs is discussed, along with the chemical phenomena underpinning each feature. Complementary electrochemical and spectroscopic methods for identifying in situ generation of heterogeneous catalysts are also presented, outlining the conditions required for correct application with special emphasis on potential pitfalls when studying weakly-adsorbed material. Case studies are presented to showcase how these different probes can be integrated to develop a comprehensive picture of precatalyst transformation.

Decoding Proton-Coupled Electron Transfer with Potential-p Ka Diagrams: Applications to Catalysis.

The applied potential at which [NiII(P2PhN2Bn)2]2+ (P2PhN2Bn = 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane) catalyzes hydrogen production is reported to vary as a function of proton source p Ka in acetonitrile. By contrast, most molecular catalysts exhibit catalytic onsets at p Ka-independent potentials. Using experimentally determined thermochemical parameters associated with reduction and protonation, a coupled Pourbaix diagram is constructed for [NiII(P2PhN2Bn)2]2+. One layer describes proton-coupled electron transfer reactivity involving ligand-based protonation, and the second describes metal-based protonation. An overlay of this diagram with experimentally determined E cat/2 values spanning 15 p Ka units, along with complementary stopped-flow rapid mixing experiments to detect reaction intermediates, supports a mechanism in which the proton-coupled electron transfer processes underpinning the p Ka-dependent catalytic processes involve protonation of the ligand, not the metal center. For proton sources with p Ka values in the range 6-10.6, the initial species formed is the doubly reduced, doubly protonated species [Ni0(P2PhN2BnH)2]2+, despite a higher overpotential for this proton-coupled electron transfer reaction in comparison to forming the metal-protonated isomer. In this complex, each ligand is protonated in the exo position with the two amine moieties on each ligand binding a single proton and positioning it away from the metal center. This species undergoes very slow isomerization to form an endo-protonated hydride species [HNiII(P2PhN2Bn)(P2PhN2BnH)]2+ that can release hydrogen to close the catalytic cycle. Importantly, this slow isomerization does not perturb the initially established proton-coupled electron transfer equilibrium, placing catalysis under thermodynamic control. New details revealed about the reaction mechanism from the coupled Pourbaix diagram and the complementary stopped-flow studies lead to predictions as to how this p Ka-dependent activity might be engendered in other molecular catalysts for multi-electron, multi-proton transformations.

Post synthetic exchange enables orthogonal click chemistry in a metal organic framework.

Biphenyl-4,4'-dicarboxylic acid derivatives containing either azide or acetylene functional groups were inserted into UiO-67 metal organic frameworks (MOFs) via post synthetic linker exchange. Sequential and orthogonal click reactions could be performed on these modified MOFs by incubating the crystals with small molecule substrates bearing azide or acetylene groups in the presence of a copper catalyst. 1H NMR of digested MOF samples showed that up to 50% of the incorporated linkers could be converted to their "clicked" triazole products. Powder X-ray diffraction confirmed that the UiO-67 structure was maintained throughout all transformations. The click reaction efficiency is discussed in context of MOF crystallite size and pore size. As the incorporation of clicked linkers could be controlled by post synthetic exchange, this work introduces a powerful method of quickly introducing orthogonal modifications into known MOF architectures.

Identification of an Electrode-Adsorbed Intermediate in the Catalytic Hydrogen Evolution Mechanism of a Cobalt Dithiolene Complex.

Analysis of a cobalt bis(dithiolate) complex reported to mediate hydrogen evolution under electrocatalytic conditions in acetonitrile revealed that the cobalt complex transforms into an electrode-adsorbed film upon addition of acid prior to application of a potential. Subsequent application of a reducing potential to the film results in desorption of the film and regeneration of the molecular cobalt complex in solution, suggesting that the adsorbed species is an intermediate in catalytic H2 evolution. The electroanalytical techniques used to examine the pathway by which H2 is generated, as well as the methods used to probe the electrode-adsorbed species, are discussed. Tentative mechanisms for catalytic H2 evolution via an electrode-adsorbed intermediate are proposed.

Decoding Proton-Coupled Electron Transfer with Potential-pKa Diagrams.

Aqueous potential-pH diagrams, commonly called Pourbaix diagrams, were originally developed to study metal corrosion in the 1930s and 1940s. Pourbaix diagrams have since been widely adopted for use across chemistry disciplines, particularly for the study of aqueous proton-coupled electron transfer reactions. Despite this enormous versatility, a clear extension of analogous diagrams to nonaqueous solvents is lacking. The problem hinges on the difficulty of defining the nonaqueous solution pH. Here, we address this issue by reporting the development of diagrams based on nonaqueous pKa scales. We experimentally construct diagrams for two transition-metal complexes that undergo proton-coupled electron transfer reactivity by recording their reduction potentials in the presence of acids with varying pKa values. These experimental diagrams validate the potential-pKa theory and provide valuable thermochemical information for proton-coupled electron transfer reactions, including for fleetingly stable species.

Qualitative extension of the EC' Zone Diagram to a molecular catalyst for a multi-electron, multi-substrate electrochemical reaction.

The EC' Zone Diagram, introduced by Savéant and Su over 30 years ago, has been used to classify voltammetric responses for electrocatalytic systems. With a single H2-evolving catalyst, Co(dmgBF2)2(CH3CH)2 (dmgBF2 = difluoroboryl-dimethylglyoxime), and a series of para-substituted anilinium acids, experimental conditions were carefully tuned to access to each region of the classic zone diagram. Close scrutiny revealed the extent to which the kinetic (λ) and excess (γ) factors could be experimentally controlled and used to access a variety of waveforms for this ECEC' catalytic system. It was found that most of the tunable experimental parameters (such as catalyst concentration, scan rate, and substrate concentration) predicted in the EC' Zone Diagram could be extended to a multi-electron system and produced similarly-shaped waveforms with some deviations. Tuning of a single catalyst across every region of the classic zone diagram has previously been prevented due to the seven orders of magnitude that need to be traversed across the kinetic parameter; however, the cobalt catalyst in this study provided unique control of this parameter. By varying the acids used as the proton source, the rate constants for protonation were tuned via a pKa-dependent linear free energy relationship.

Electrode initiated proton-coupled electron transfer to promote degradation of a nickel(ii) coordination complex.

A Ni(ii) bisphosphine dithiolate compound degrades into an electrode-adsorbed film that can evolve hydrogen under reducing and protic conditions. An electrochemical study suggests that the degradation mechanism involves an initial concerted proton-electron transfer. The potential susceptibility of Ni-S bonds in molecular hydrogen evolution catalysts to degradation via C-S bond cleavage is discussed.

Electrochemical hydrogenation of a homogeneous nickel complex to form a surface adsorbed hydrogen-evolving species.

A Ni(II) complex degrades electrochemically in the presence of acid in acetonitrile to form an electrode adsorbed film that catalytically evolves hydrogen. Comparison with a similar compound permitted investigation of the degradation mechanism.

Evaluation of homogeneous electrocatalysts by cyclic voltammetry.

The pursuit of solar fuels has motivated extensive research on molecular electrocatalysts capable of evolving hydrogen from protic solutions, reducing CO2, and oxidizing water. Determining accurate figures of merit for these catalysts requires the careful and appropriate application of electroanalytical techniques. This Viewpoint first briefly presents the fundamentals of cyclic voltammetry and highlights practical experimental considerations before focusing on the application of cyclic voltammetry for the characterization of electrocatalysts. Key metrics for comparing catalysts, including the overpotential (η), potential for catalysis (E(cat)), observed rate constant (k(obs)), and potential-dependent turnover frequency, are discussed. The cyclic voltammetric responses for a general electrocatalytic one-electron reduction of a substrate are presented along with methods to extract figures of merit from these data. The extension of this analysis to more complex electrocatalytic schemes, such as those responsible for H2 evolution and CO2 reduction, is then discussed.

Electrochemical reduction of Brønsted acids by glassy carbon in acetonitrile-implications for electrocatalytic hydrogen evolution.

Molecular catalysts for electrochemically driven hydrogen evolution are often studied in acetonitrile with glassy carbon working electrodes and Brønsted acids. Surprisingly, little information is available regarding the potentials at which acids are directly reduced on glassy carbon. This work examines acid electroreduction in acetonitrile on glassy carbon electrodes by cyclic voltammetry. Reduction potentials, spanning a range exceeding 2 V, were found for 20 acids. The addition of 100 mM water was not found to shift the reduction potential of any acid studied, although current enhancement was observed for some acids. The data reported provides a guide for selecting acids to use in electrocatalysis experiments such that direct electrode reduction is avoided.

Charge Transfer or J-Coupling? Assignment of an Unexpected Red-Shifted Absorption Band in a Naphthalenediimide-Based Metal-Organic Framework.

We investigate and assign a previously reported unexpected transition in the metal-organic framework Zn2(NDC)2(DPNI) (1; NDC = 2,6-naphthalenedicarboxylate, DPNI = dipyridyl-naphthalenediimide) that displays linear arrangements of naphthalenediimide ligands. Given the longitudinal transition dipole moment of the DPNI ligands, J-coupling seemed possible. Photophysical measurements revealed a broad, new transition in 1 between 400 and 500 nm. Comparison of the MOF absorption spectra with that of a charge transfer (CT) complex formed by manual grinding of DPNI and H2NDC led to the assignment of the new band in 1 as arising from an interligand CT. Constrained density functional theory utilizing a custom long-range-corrected hybrid functional was employed to determine which ligands were involved in the CT transition. On the basis of relative oscillator strengths, the interligand CT was assigned as principally arising from π-stacked DPNI/NDC dimers rather than the alternative orthogonal pairs within the MOF.

Halogen oxidation and halogen photoelimination chemistry of a platinum-rhodium heterobimetallic core.

The heterobimetallic complexes, PtRh(tfepma)(2)(CN(t)Bu)X(3) (X = Cl, Br), are assembled by the treatment of Pt(cod)X(2) (cod =1,5-cyclooctadiene) with {Rh(cod)X}(2), in the presence of tert-butylisonitrile (CN(t)Bu) and tfepma (tfepma = bis(trifluoroethoxyl)phosphinomethylamine). The neutral complexes contain Pt-Rh single bonds with metal-metal separations of 2.6360(3) and 2.6503(7) Å between the square planar Pt and octahedral Rh centers for the Cl and Br complexes, respectively. Oxidation of the XPt(I)Rh(II)X(2) cores with suitable halide sources (PhICl(2) or Br(2)) furnishes PtRh(tfepma)(2)(CN(t)Bu)X(5), which preserves a Pt-Rh bond. For the chloride system, the initial oxidation product orients the platinum-bound chlorides in a meridional geometry, which slowly transforms to a facial arrangement in pentane solution as verified by X-ray crystal analysis. Irradiation of the mer- or fac-Cl(3)Pt(III)Rh(II)Cl(2) isomers with visible light in the presence of olefin promotes the photoelimination of halogen and regeneration of the reduced ClPt(I)Rh(II)Cl(2) core. In addition to exhibiting photochemistry similar to that of the chloride system, the oxidized bromide cores undergo thermal reduction chemistry in the presence of olefin with zeroth-order olefin dependence. Owing to an extremely high photoreaction quantum yield for the fac-ClPt(I)Rh(II)Cl(2) isomer, details of the X(2) photoelimination have been captured by transient absorption spectroscopy. We now report the first direct observation of the photointermediate that precedes halogen reductive elimination. The intermediate is generated promptly upon excitation (<8 ns), and halogen is eliminated from it with a rate constant of 3.6 × 10(4) s(-1). As M-X photoactivation and elimination is the critical step in HX splitting, these results establish a new guidepost for the design of HX splitting cycles for solar energy storage.

Turn-on fluorescence in tetraphenylethylene-based metal-organic frameworks: an alternative to aggregation-induced emission.

Coordinative immobilization of functionalized tetraphenylethylene within rigid porous metal-organic frameworks (MOFs) turns on fluorescence in the typically non-emissive tetraphenylethylene core. The matrix coordination-induced emission effect (MCIE) is complementary to aggregation-induced emission. Despite the large interchromophore distances imposed by coordination to metal ions, a carboxylate analogue of tetraphenylethylene anchored by Zn(2+) and Cd(2+) ions inside MOFs shows fluorescence lifetimes in line with those of close-packed molecular aggregates. Turn-on fluorescence by coordinative ligation in a porous matrix is a powerful approach that may lead to new materials made from chromophores with molecular rotors. The potential utility of MCIE toward building new sensing materials is demonstrated by tuning the fluorescence response of the porous MOFs as a function of adsorbed small analytes.