Fast Catalysis at Low Overpotential: Designing Efficient Dicationic Re(bpy2+)(CO)3I Electrocatalysts for CO2 Reduction.

We report a series of isomeric, dicationic Re(bpy2+)(CO)3I complexes with bpy (2,2'-bipyridine) modified by two phenyl-CH2-(NMe3)+ pendants with cations located at variable distances from the active site for electrocatalytic CO2 reduction in CH3CN/2.8 M H2O. The position of the cationic groups dramatically increases the rate of catalysis by ∼800-fold, from 1.2 to 950 s-1, with only a minor increase in overpotential. Acceleration is due to stabilization of the initial CO2 adduct and lowering of ΔG‡ for C-OH bond cleavage by Coulombic stabilization of anionic charges. Performance may be enhanced by accumulation in the electrochemical double layer. Transition state stabilization in the optimized isomer unlocks the low overpotential "protonation-first" pathway, highlighting the sizable effects of subtle structural optimization.

Tuning Energetics of 2 e-/2H+ PCET Properties with Model Ru-bisamido Complexes.

Most redox processes that break/form bonds involve net 2e- changes, and many are coupled to protons. Yet most proton-coupled electron transfer (PCET) studies focus on 1e-/1H+ reactions. Reported here is a family of molecular models that undergo tunable 2e-/2H+ redox changes. Complexes [(X2bpy)RuII(en*)2](PF6)2 and [(X2bpy)RuIV(en*-H)2](PF6)2 have been synthesized with bpy=2,2'-bipyridine with 4,4'-subtitutions X=-NMe2, -OMe, -Me, -H, -CF3; and en*=2,3-dimethyl-2,3-butanediamine. They have been characterized by IR, UV-vis, and NMR spectroscopies, XRD, electrochemistry, mass spectrometry, DFT and (TD)DFT computations. The introduction of electron-withdrawing and donating groups at the 4,4'-position of the bpy ligand affects the complexes' redox potentials, pKa's, and Bond Dissociation Free Energies (BDFEs) of the N-H bonds in the en* ligands. The average BDFEs for the overall 2e-/2H+ PCET span over 5 kcal/mol. Notably, these complexes all show marked potential inversion over an extended range, ΔpKa>25 units and ΔE0>1.4 V. Potential inversion remains despite the electronic influence of bpy's substitutions which regulate N-H properties several bonds apart by trans-effect over dπ-molecular orbitals at the Ru center. The experimental and computational results presented in this work support the presence of strong coupling between electrons and protons, for modelling insights of 2e-/2H+ transfer reactivity.

Hydrogen on Colloidal Gold Nanoparticles.

Colloidal gold nanoparticles (AuNPs) have myriad scientific and technological applications, but their fundamental redox chemistry is underexplored. Reported here are titration studies of oxidation and reduction reactions of aqueous AuNP colloids, which show that the AuNPs bind substantial hydrogen (electrons + protons) under mild conditions. The 5 nm AuNPs are reduced to a similar extent with reductants from borohydrides to H2 and are reoxidized back essentially to their original state by oxidants, including O2. The reactions were monitored via surface plasmon resonance (SPR) optical absorption, which was shown to be much more sensitive to surface H than to changes in solution conditions. Reductions with H2 occurred without pH changes, demonstrating that hydrogenation forms surface H rather than releasing H+. Computational studies suggested that an SPR blueshift was expected for H atom addition, while just electron addition likely would have caused a redshift. Titrations consistently showed a maximum redox change of the 5 nm NPs, independent of the reagent, corresponding to 9% of the total gold or ∼30% hydrogen surface coverage (∼370 H per AuNP). Larger AuNPs showed smaller maximum fractional surface coverages. We conclude that H binds to the edge, corner, and defect sites of the AuNPs, which explains the stoichiometric limitation and the size effect. The finding of substantial and stable hydrogen on the AuNP surface under mild reducing conditions has potential implications for various applications of AuNPs in reducing environments, from catalysis to biomedicine. This finding contrasts with the behavior of bulk gold and with the typical electron-focused perspective in this field.

Photoelectrochemical Proton-Coupled Electron Transfer of TiO2 Thin Films on Silicon.

TiO2 thin films are often used as protective layers on semiconductors for applications in photovoltaics, molecule-semiconductor hybrid photoelectrodes, and more. Experiments reported here show that TiO2 thin films on silicon are electrochemically and photoelectrochemically reduced in buffered acetonitrile at potentials relevant to photoelectrocatalysis of CO2 reduction, N2 reduction, and H2 evolution. On both n-type Si and irradiated p-type Si, TiO2 reduction is proton-coupled with a 1e-:1H+ stoichiometry, as demonstrated by the Nernstian dependence of the Ti4+/3+ E1/2 on the buffer pKa. Experiments were conducted with and without illumination, and a photovoltage of ∼0.6 V was observed across 20 orders of magnitude in proton activity. The 4 nm films are almost stoichiometrically reduced under mild conditions. The reduced films catalytically transfer protons and electrons to hydrogen atom acceptors, based on cyclic voltammogram, bulk electrolysis, and other mechanistic evidence. TiO2/Si thus has the potential to photoelectrochemically generate high-energy H atom carriers. Characterization of the TiO2 films after reduction reveals restructuring with the formation of islands, rendering TiO2 films as a potentially poor choice as protecting films or catalyst supports under reducing and protic conditions. Overall, this work demonstrates that atomic layer deposition TiO2 films on silicon photoelectrodes undergo both chemical and morphological changes upon application of potentials only modestly negative of RHE in these media. While the results should serve as a cautionary tale for researchers aiming to immobilize molecular monolayers on "protective" metal oxides, the robust proton-coupled electron transfer reactivity of the films introduces opportunities for the photoelectrochemical generation of reactive charge-carrying mediators.

Photoelectrochemical CO2 Reduction to CO Enabled by a Molecular Catalyst Attached to High-Surface-Area Porous Silicon.

A high-surface-area p-type porous Si photocathode containing a covalently immobilized molecular Re catalyst is highly selective for the photoelectrochemical conversion of CO2 to CO. It gives Faradaic efficiencies of up to 90% for CO at potentials of -1.7 V (versus ferrocenium/ferrocene) under 1 sun illumination in an acetonitrile solution containing phenol. The photovoltage is approximately 300 mV based on comparisons with similar n-type porous Si cathodes in the dark. Using an estimate of the equilibrium potential for CO2 reduction to CO under optimized reaction conditions, photoelectrolysis was performed at a small overpotential, and the onset of electrocatalysis in cyclic voltammograms occurred at a modest underpotential. The porous Si photoelectrode is more stable and selective for CO production than the photoelectrode generated by attaching the same Re catalyst to a planar Si wafer. Further, facile characterization of the porous Si-based photoelectrodes using transmission mode FTIR spectroscopy leads to highly reproducible catalytic performance.

Polar Effects in Hydrogen Atom Transfer Reactions from a Proton-Coupled Electron Transfer (PCET) Perspective: Abstractions from Toluenes.

Rate constants for hydrogen atom transfer (HAT) reactions of substituted toluenes with tert-butyl, tert-butoxy, and tert-butylperoxyl radicals are reanalyzed here using the free energies of related proton transfer (PT) and electron transfer (ET) reactions, calculated from an extensive set of compiled or estimated pKa and E° values. The Eyring activation energies ΔGHAT‡ do not correlate with the relatively constant ΔG°HAT, but do correlate close-to-linearly with ΔG°PT and ΔG°ET. The slopes of correlations are similar for the three radicals except that the tBu• barriers shift in the opposite direction from the oxyl radical barriers─a clear example of the qualitative "polar effect" in HAT reactions. When cast quantitatively in free energy terms (ΔGHAT‡ vs ΔG°PT/ET), this effect is very small, only 5-10% of the typical Bell-Evans-Polanyi (BEP) effect of changing ΔG°HAT. This analysis also highlights connections between polar effects and the concepts of "asynchronous" or "imbalanced" HAT reactions in which the PT and ET components of ΔG°HAT contribute differently to the barrier. Finally, these observations are discussed in light of the traditional explanations of polar effects and the potential for a rubric that could predict the extent to which contra-thermodynamic selectivity may be achieved in HAT reactions.

A Guide to Tris(4-Substituted)-triphenylmethyl Radicals.

Triphenylmethyl (trityl, Ph3C•) radicals have been considered the prototypical carbon-centered radical since their discovery in 1900. Tris(4-substituted)-trityls [(4-R-Ph)3C•] have since been used in many ways due to their stability, persistence, and spectroscopic activity. Despite their widespread use, existing synthetic routes toward tris(4-substituted)-trityl radicals are not reproducible and often lead to impure materials. We report here robust syntheses of six electronically varied (4-RPh)3C•, where R = NMe2, OCH3, tBu, Ph, Cl, and CF3. The characterization reported for the radicals and related compounds includes five X-ray crystal structures, electrochemical potentials, and optical spectra. Each radical is best accessed using a stepwise approach from the trityl halide, (RPh)3CCl or (RPh)3CBr, by controllably removing the halide with subsequent 1e- reduction of the trityl cation, (RPh)3C+. These syntheses afford consistently crystalline trityl radicals of high purity for further studies.

Independent Tuning of the pKa or the E1/2 in a Family of Ruthenium Pyridine-Imidazole Complexes.

Two series of RuII(acac)2(py-imH) complexes have been prepared, one with changes to the acac ligands and the other with substitutions to the imidazole. The proton-coupled electron transfer (PCET) thermochemistry of the complexes has been studied in acetonitrile, revealing that the acac substitutions almost exclusively affect the redox potentials of the complex (|ΔE1/2| ≫ |ΔpKa|·0.059 V) while the changes to the imidazole primarily affect its acidity (|ΔpKa|·0.059 V ≫ |ΔE1/2|). This decoupling is supported by DFT calculations, which show that the acac substitutions primarily affect the Ru-centered t2g orbitals, while changes to the py-imH ligand primarily affect the ligand-centered π orbitals. More broadly, the decoupling stems from the physical separation of the electron and proton within the complex and highlights a clear design strategy to separately tune the redox and acid/base properties of H atom donor/acceptor molecules.

Bonds over Electrons: Proton Coupled Electron Transfer at Solid-Solution Interfaces.

This Perspective argues that most redox reactions of materials at an interface with a protic solution involve net proton-coupled electron transfer (PCET) (or other cation-coupled ET). This view contrasts with the traditional electron-transfer-focused view of redox reactions at semiconductors, but redox processes at metal surfaces are often described as PCET. Taking a thermodynamic perspective, transfer of an electron is typically accompanied by a stoichiometric proton, much as the chemistry of lithium-ion batteries involves coupled transfers of e- and Li+. The PCET viewpoint implicates the surface-H bond dissociation free energy (BDFE) as the preeminent energetic parameter and its conceptual equivalents, the electrochemical ne-/nH+ potential versus the reversible hydrogen electrode (RHE) and the free energy of hydrogenation, ΔG°H. These parameters capture the thermochemistry of PCET at interfaces better than electronic parameters such as Fermi energies, electron chemical potentials, flat-band potentials, or band-edge energies. A unified picture of PCET at metal and semiconductor surfaces is presented. Exceptions, limitations, implications, and future directions motivated by this approach are described.

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.

Inverse kinetic isotope effects in the oxygen reduction reaction at platinum single crystals.

Although the oxygen reduction reaction (ORR) involves multiple proton-coupled electron transfer processes, early studies reported the absence of kinetic isotope effects (KIEs) on polycrystalline platinum, probably due to the use of unpurified D2O. Here we developed a methodology to prepare ultra-pure D2O, which is indispensable for reliably investigating extremely surface-sensitive platinum single crystals. We find that Pt(111) exhibits much higher ORR activity in D2O than in H2O, with potential-dependent inverse KIEs of ~0.5, whereas Pt(100) and Pt(110) exhibit potential-independent inverse KIEs of ~0.8. Such inverse KIEs are closely correlated to the lower *OD coverage and weakened *OD binding strength relative to *OH, which, based on theoretical calculations, are attributed to the differences in their zero-point energies. This study suggests that the competing adsorption between *OH/*OD and *O2 probably plays an important role in the ORR rate-determining steps that involve a chemical step preceding an electrochemical step (CE mechanism).

Coverage-Dependent Rate-Driving Force Relationships: Hydrogen Transfer from Cerium Oxide Nanoparticle Colloids.

Rate-driving force relationships, known as Brønsted-Evans-Polanyi (BEP) relations, are central to many methods for predicting the performance of heterogeneous catalysts and electrocatalysts. Methods such as Tafel plots and "volcano" analyses often assume that the effect of adsorbate coverage on reaction rates across different materials is constant and known. Here, we use UV-visible spectroscopy to test these assumptions by measuring rates of net hydrogen atom transfer from colloidal cerium oxide nanoparticles (nanoceria) to organic reagents at varying surface CeO-H bond strengths and surface coverages. The resulting rate constants follow a linear BEP relationship, Δlog(k) = αΔlog(Keq), across two sizes of nanoceria, two organic reagents, and a ∼10 kcal mol-1 range of CeO-H bond strengths. Interestingly, the Brønsted slope is only 0.2, demonstrating that the rate constants are far less sensitive to CeO-H bond strength than would commonly be assumed for a heterogeneous nanomaterial. Furthermore, we observe a Brønsted slope >1 when altering the reaction driving force via the organic reagent bond strength instead of that of CeO-H. The implications of these Brønsted slopes for either concerted or stepwise mechanisms are discussed. To our knowledge, these are the first solution-phase measurements of BEP relationships for hydrogen coverage on a (nano)material.

Asymmetric Photochemical [2 + 2]-Cycloaddition of Acyclic Vinylpyridines through Ternary Complex Formation and an Uncontrolled Sensitization Mechanism.

Stereochemical control of photochemical reactions that occur via triplet energy transfer remains a challenge. Suppressing off-catalyst stereorandom reactivity is difficult for highly reactive open-shell intermediates. Strategies for suppressing racemate-producing, off-catalyst pathways have long focused on formation of ground state, substrate-catalyst chiral complexes that are primed for triplet energy transfer via a photocatalyst in contrast to their off-catalyst counterparts. Herein, we describe a strategy where both a chiral catalyst-associated vinylpyridine and a nonassociated, free vinylpyridine substrate can be sensitized by an Ir(III) photocatalyst, yet high levels of diastereo- and enantioselectivity in a [2 + 2] photocycloaddition are achieved through a preferred, highly organized transition state. This mechanistic paradigm is distinct from, yet complementary to current approaches for achieving high levels of stereocontrol in photochemical transformations.

Visible Light-Mediated, Highly Diastereoselective Epimerization of Lactams from the Most Accessible to the More Stable Stereoisomer.

Most known methods to access δ-lactams with stereogenic centers at the α- and ß-positions are highly selective for the contra-thermodynamic syn diastereomer, typically via hydrogenation of the corresponding pyridinones or quinolinones. We describe here the development of a photoredox-mediated hydrogen atom transfer (HAT) approach for the epimerization of δ-lactams to access the more stable anti diastereomers from the contra-thermodynamic syn isomers. The reaction displays broad functional group compatibility, including acid, ester, 1°, 2° and 3° amide, carbamate, and pyridyl groups, and was effective for a range of differently substituted monocyclic and bicyclic lactams. Experimentally observed diastereoselectivities are consistent with the calculated relative stabilities of lactam diastereomers. Convergence to the same diastereomer ratio from the syn- and anti- diastereomers establishes that reversible epimerization provides an equilibrium mixture of diastereomers. Additionally, deuterium labeling and luminescence quenching studies shed further light on the mechanism of the reaction.

Proton-coupled energy transfer in molecular triads.

We experimentally discovered and theoretically analyzed a photochemical mechanism, which we term proton-coupled energy transfer (PCEnT). A series of anthracene-phenol-pyridine triads formed a local excited anthracene state after light excitation at a wavelength of ~400 nanometers (nm), which led to fluorescence around 550 nm from the phenol-pyridine unit. Direct excitation of phenol-pyridine would have required ~330-nm light, but the coupled proton transfer within the phenol-pyridine unit lowered its excited-state energy so that it could accept excitation energy from anthracene. Singlet-singlet energy transfer thus occurred despite the lack of spectral overlap between the anthracene fluorescence and the phenol-pyridine absorption. Moreover, theoretical calculations indicated negligible charge transfer between the anthracene and phenol-pyridine units. We construe PCEnT as an elementary reaction of possible relevance to biological systems and future photonic devices.

Bridge Sites of Au Surfaces Are Active for Electrocatalytic CO2 Reduction.

Prior in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) studies of electrochemical CO2 reduction catalyzed by Au, one of the most selective and active electrocatalysts to produce CO from CO2, suggest that the reaction proceeds solely on the top sites of the Au surface. This finding is worth updating with an improved spectroelectrochemical system where in situ IR measurements can be performed under real reaction conditions that yield high CO selectivity. Herein, we report the preparation of an Au-coated Si ATR crystal electrode with both high catalytic activity for CO2 reduction and strong surface enhancement of IR signals validated in the same spectroelectrochemical cell, which allows us to probe the adsorption and desorption behavior of bridge-bonded *CO species (*COB). We find that the Au surface restructures irreversibly to give an increased number of bridge sites for CO adsorption within the initial tens of seconds of CO2 reduction. By studying the potential-dependent desorption kinetics of *COB and quantifying the steady-state surface concentration of *COB under reaction conditions, we further show that *COB are active reaction intermediates for CO2 reduction to CO on this Au electrode. At medium overpotential, as high as 38% of the reaction occurs on the bridge sites.

Facile conversion of ammonia to a nitride in a rhenium system that cleaves dinitrogen.

Rhenium complexes with aliphatic PNP pincer ligands have been shown to be capable of reductive N2 splitting to nitride complexes. However, the conversion of the resulting nitride to ammonia has not been observed. Here, the thermodynamics and mechanism of the hypothetical N-H bond forming steps are evaluated through the reverse reaction, conversion of ammonia to the nitride complex. Depending on the conditions, treatment of a rhenium(iii) precursor with ammonia gives either a bis(amine) complex [(PNP)Re(NH2)2Cl]+, or results in dehydrohalogenation to the rhenium(iii) amido complex, (PNP)Re(NH2)Cl. The N-H hydrogen atoms in this amido complex can be abstracted by PCET reagents which implies that they are quite weak. Calorimetric measurements show that the average bond dissociation enthalpy of the two amido N-H bonds is 57 kcal mol-1, while DFT computations indicate a substantially weaker N-H bond of the putative rhenium(iv)-imide intermediate (BDE = 38 kcal mol-1). Our analysis demonstrates that addition of the first H atom to the nitride complex is a thermochemical bottleneck for NH3 generation.

Structural and Thermodynamic Effects on the Kinetics of C-H Oxidation by Multisite Proton-Coupled Electron Transfer in Fluorenyl Benzoates.

Our recent experimental and theoretical investigations have shown that fluorene C-H bonds can be activated through a mechanism in which the proton and electron are transferred from the C-H bond to a separate base and oxidant in a concerted, elementary step. This multisite proton-coupled electron transfer (MS-PCET) mechanism for C-H bond activation was shown to be facilitated by shorter proton donor-acceptor distances. With the goal of intentionally modulating this donor-acceptor distance, we have now studied C-H MS-PCET in the 3-methyl-substituted fluorenyl benzoate (2-Flr-3-Me-BzO-). This derivative was readily oxidized by ferrocenium oxidants by initial C-H MS-PCET, with rate constants that were 6-21 times larger than those for 2-Flr-BzO- with the same oxidants. Structural comparisons by X-ray crystallography and by computations showed that addition of the 3-methyl group caused the expected steric compression; however, the relevant C···O- proton donor-acceptor distance was longer, due to a twist of the carboxylate group. The structural changes induced by the 3-Me group increased the basicity of the carboxylate, weakened the C-H bond, and reduced the reorganization energy for C-H bond cleavage. Thus, the rate enhancement for 2-Flr-3-Me-BzO- was due to effects on the thermochemistry and kinetic barrier, rather than from compression of the C···O- proton donor-acceptor distance. These results highlight both the challenges of controlling molecules on the 0.1 Å length scale and the variety of parameters that affect PCET rate constants.