Mechanistic roles of metal- and ligand-protonated species in hydrogen evolution with [Cp*Rh] complexes.

Protonation reactions involving organometallic complexes are ubiquitous in redox chemistry and often result in the generation of reactive metal hydrides. However, some organometallic species supported by η5-pentamethylcyclopentadienyl (Cp*) ligands have recently been shown to undergo ligand-centered protonation by direct proton transfer from acids or tautomerization of metal hydrides, resulting in the generation of complexes bearing the uncommon η4-pentamethylcyclopentadiene (Cp*H) ligand. Here, time-resolved pulse radiolysis (PR) and stopped-flow spectroscopic studies have been applied to examine the kinetics and atomistic details involved in the elementary electron- and proton-transfer steps leading to complexes ligated by Cp*H, using Cp*Rh(bpy) as a molecular model (where bpy is 2,2'-bipyridyl). Stopped-flow measurements coupled with infrared and UV-visible detection reveal that the sole product of initial protonation of Cp*Rh(bpy) is [Cp*Rh(H)(bpy)]+, an elusive hydride complex that has been spectroscopically and kinetically characterized here. Tautomerization of the hydride leads to the clean formation of [(Cp*H)Rh(bpy)]+. Variable-temperature and isotopic labeling experiments further confirm this assignment, providing experimental activation parameters and mechanistic insight into metal-mediated hydride-to-proton tautomerism. Spectroscopic monitoring of the second proton transfer event reveals that both the hydride and related Cp*H complex can be involved in further reactivity, showing that [(Cp*H)Rh] is not necessarily an off-cycle intermediate, but, instead, depending on the strength of the acid used to drive catalysis, an active participant in hydrogen evolution. Identification of the mechanistic roles of the protonated intermediates in the catalysis studied here could inform design of optimized catalytic systems supported by noninnocent cyclopentadienyl-type ligands.

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+.

Reductive Dynamic and Static Excited State Quenching of a Homoleptic Ruthenium Complex Bearing Aldehyde Groups.

A new homoleptic Ru polypyridyl complex bearing two aldehyde groups on each bipyridine ligand, [Ru(dab)3](PF6)2, where dab is 4,4'-dicarbaldehyde-2,2'-bipyridine, was synthesized, characterized, and utilized for iodide photo-oxidation studies. In acetonitrile (CH3CN) solution, the complex displayed an intense metal-to-ligand charge transfer (MLCT) absorbance maximum at 475 nm (ε = 22,000 M-1 cm-1) and an infrared (IR) band at 1712 cm-1 assigned to the pendent aldehyde groups. Visible light excitation in air-saturated solution resulted in room temperature photoluminescence (PL) with a maximum at 675 nm, a quantum yield, ϕPL = 0.048, and an excited state lifetime, το = 440 ns, from which radiative and nonradiative relaxation rate constants were extracted, kr = 9.1 × 104 s-1 and knr = 1.8 × 106 s-1. Pulsed visible light excitation yielded transient UV-vis and IR absorption spectra consistent with an MLCT excited state; relaxation occurred with the maintenance of two isosbestic points in the visible region, and a lifetime that agreed with that measured by time-resolved PL. Cyclic voltammetry studies in a CH3CN solution with 0.1 M TBAPF6 electrolyte revealed a quasi-reversible oxidation, E°(RuIII/II) = +1.25 V vs. Fc+/0, and three sequential one-electron reductions at -1.10, -1.25, and -1.54 V vs. Fc+/0. An excited state reduction potential of E°(Ru*2+/+) = +0.89 V vs. Fc+/0 was estimated with the Rehm-Weller expression. Titration of tetrabutylammonium iodide, TBAI, into a CD3CN solution of [Ru(dab)3](PF6)2 resulted in significant shifts in the aldehyde H atom and 3,3'-biypridyl resonances that were analyzed with a 1:1 equilibrium model, from which Keq = 460 M-1 was extracted, increasing to 5800 M-1 when the solvent was changed to acetone-d6. Iodide titrations resulted in a significant quenching of the [Ru(dab)3]*2+ lifetime and quantum yield in both CH3CN and acetone solvents. In CH3CN, the quenching was mainly dynamic and well described by the Stern-Volmer model, from which a quenching rate constant, kq, of 4.5 × 1010 M-1 s-1 and an equilibrium constant, Keq, of 8.3 × 103 M-1 were obtained. In acetone, the static quenching pathway by iodide was greatly enhanced, with a Keq of 1.2 × 104 M-1 and a higher kq of 9.2 × 1010 M-1 s-1.

Multi-Electron Transfer at H-Terminated p-Si Electrolyte Interfaces: Large Photovoltages under Inversion Conditions.

Photovoltages for hydrogen-terminated p-Si(111) in an acetonitrile electrolyte were quantified with methyl viologen [1,1'-(CH3)2-4,4'-bipyridinium](PF6)2, abbreviated MV2+, and [Ru(bpy)3](PF6)2, where bpy is 2,2'-bipyridine, that respectively undergo two and three one-electron transfer reductions. The reduction potentials, E°, of the two MV2+ reductions occurred at energies within the forbidden bandgap, while the three [Ru(bpy)3]2+ reductions occurred within the continuum of conduction band states. Bandgap illumination resulted in reduction that was more positive than that measured with a degenerately doped n+-Si demonstrative of a photovoltage, Vph, that increased in the order MV2+/+ (260 mV) < MV+/0 (400 mV) < Ru2+/+ (530 mV) ∼ Ru+/0 (540 mV) ∼ Ru0/- (550 mV). Pulsed 532 nm excitation generated electron-hole pairs whose dynamics were nearly constant under depletion conditions and increased markedly as the potential was raised or lowered. A long wavelength absorption feature assigned to conduction band electrons provided additional evidence for the presence of an inversion layer. Collectively, the data reveal that the most optimal photovoltage, as well as the longest electron-hole pair lifetime and the highest surface electron concentration, occurs when E° lies energetically within the unfilled conduction band states where an inversion layer is present. The bell-shaped dependence for electron-hole pair recombination with the surface potential was predicted by the time-honored SRH model, providing a clear indication that this interface provides access to all four bias conditions, i.e., accumulation, flat band, depletion, and inversion. The implications of these findings for photocatalysis applications and solar energy conversion are discussed.

Understanding the Role of Inter- and Intramolecular Promoters in Electro- and Photochemical CO2 Reduction Using Mn, Re, and Ru Catalysts.

Recycling of carbon dioxide to fuels and chemicals is a promising strategy for renewable energy storage. Carbon dioxide conversion can be achieved by (i) artificial photosynthesis using photoinduced electrons; (ii) electrolysis using electricity produced by photovoltaics; and (iii) thermal CO2 hydrogenation using renewable H2. The focus of our group's research is on molecular catalysts, in particular coordination complexes of transition metals (e.g., Mn, Re, and Ru), which offer versatile platforms for mechanistic studies of photo- and electrochemical CO2 reduction. The interactions of catalytic intermediates with Lewis or Brønsted acids, hydrogen-bonding moieties, solvents, cations, etc., that function as promoters or cofactors have become increasingly important for efficient catalysis. These interactions may have dramatic effects on selectivity and rates by stabilizing intermediates or lowering transition state barriers, but they are difficult to elucidate and challenging to predict. We have been carrying out experimental and theoretical studies of CO2 reduction using molecular catalysts toward addressing mechanisms of efficient CO2 reduction systems with emphasis on those containing intramolecular (or pendent) and intermolecular (solution phase) additives. This Account describes the identification of reaction intermediates produced during CO2 reduction in the presence of triethanolamine or ionic liquids, the benefits of hydrogen-bonding interactions among intermediates or cofactors, and the complications of pendent phenolic donors/phenoxide bases under electrochemical conditions.Triethanolamine (TEOA) is a common sacrificial electron donor for photosensitizer excited state reductive quenching and has a long history of use in photocatalytic CO2 reduction. It also functions as a Brønsted base in conjunction with more potent sacrificial electron donors, such as 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH). Deprotonation of the BIH•+ cation radical promotes irreversible photoinduced electron transfer by preventing charge recombination. Despite its wide use, most research to date has not considered the broader reactions of TEOA, including its direct interaction with CO2 or its influence on catalytic intermediates. We found that in acetonitrile, TEOA captures CO2 in the form of a zwitterionic adduct without any metal catalyst. In the presence of ruthenium carbonyl catalysts bearing α-diimine ligands, it participates in metal hydride formation, accelerates hydride transfer to CO2 to form the bound formate intermediate, and assists in the dissociation of formate anion from the catalyst ( J. Am. Chem. Soc. 2020, 142, 2413-2428).Hydrogen bonding and acid/base promoters are understood to interact with key catalytic intermediates, such as the metallocarboxylate or metallocarboxylic acid during CO2 reduction. The former is a high energy species, and hydrogen-bonding or Lewis acid-stabilization are beneficial. We have found that imidazolium-based ionic liquid cations can stabilize the doubly reduced form of the [ReCl(bpy)(CO)3] (bpy = 2,2'-bipyridine) electrocatalyst through both hydrogen-bonding and π-π interactions, resulting in CO2 reduction occurring at a more positive potential with a higher catalytic current ( J. Phys. Chem. Lett. 2014, 5, 2033-2038). Hydrogen bonding interactions between Lewis basic methoxy groups in the second coordination sphere of a Mn-based catalyst and the OH group of the Mn-COOH intermediate in the presence of a Brønsted acid were also found to promote C-(OH) bond cleavage, enabling access to a low-energy protonation-first pathway for CO2 reduction ( J. Am. Chem. Soc. 2017, 139, 2604-2618).The kinetics of forming the metallocarboxylic acid can be enhanced by internal acids, and its proton-induced C-OH bond cleavage to the metallocarbonyl and H2O is often the rate-limiting step. Therefore, proton movement organized by pendent hydrogen-bonding networks may also accelerate this step. In contrast, during electrolysis, OH groups in the second coordination sphere are deprotonated to the oxyanions, which deter catalytic CO2 reduction by directly binding CO2 to form the carbonate or by making an M-O bond in competition with CO2 binding ( Inorg. Chem. 2016, 55, 4582-4594). Our results emphasize that detailed mechanistic research is critical in discovering the design principles for improved catalysts.

Lewis Acids and Electron-Withdrawing Ligands Accelerate CO Coordination to Dinuclear CuI Compounds.

A series of dinuclear molecular copper complexes were prepared and used to model the binding and Lewis acid stabilization of CO in heterogeneous copper CO2 reduction electrocatalysts. Experimental studies (including measurement of rate and equilibrium constants) and electronic structure calculations suggest that the key kinetic barrier for CO binding may be a σ-interaction between CuI and the incoming CO ligand. The rate of CO coordination can be increased upon the addition of Lewis acids or electron-withdrawing substituents on the ligand backbone. Conversely, Keq for CO coordination can be increased by adding electron density to the metal centers of the compound, consistent with stronger π-backbonding. Finally, the electrochemically measured kinetic results were mapped onto an electrochemical zone diagram to illustrate how these system changes enabled access to each zone.

Reactivity of radiolytically and photochemically generated tertiary amine radicals towards a CO2 reduction catalyst.

Homogeneous solar fuels photocatalytic systems often require several additives in solution with the catalyst to operate, such as a photosensitizer (PS), Brønsted acid/base, and a sacrificial electron donor (SED). Tertiary amines, in particular triethylamine (TEA) and triethanolamine (TEOA), are ubiquitously deployed in photocatalysis applications as SEDs and are capable of reductively quenching the PS's excited state. Upon oxidation, TEA and TEOA form TEA•+ and TEOA•+ radical cations, respectively, which decay by proton transfer to generate redox non-innocent transient radicals, TEA• and TEOA•, respectively, with redox potentials that allow them to participate in an additional electron transfer step, thus resulting in net one-photon/two-electron donation. However, the properties of the TEA• and TEOA• radicals are not well understood, including their reducing powers and kinetics of electron transfer to catalysts. Herein, we have used both pulse radiolysis and laser flash photolysis to generate TEA• and TEOA• radicals in CH3CN, and combined with UV/Vis transient absorption and time-resolved mid-infrared spectroscopies, we have probed the kinetics of reduction of the well-established CO2 reduction photocatalyst, fac-ReCl(bpy)(CO)3 (bpy = 2,2'-bipyridine), by these radicals [kTEA• = (4.4 ± 0.3) × 109 M-1 s-1 and kTEOA• = (9.3 ± 0.6) × 107 M-1 s-1]. The ∼50× smaller rate constant for TEOA• indicates, that in contrast to a previous assumption, TEA• is a more potent reductant than TEOA• (by ∼0.2 V, as estimated using the Marcus cross relation). This knowledge will aid in the design of photocatalytic systems involving SEDs. We also show that TEA can be a useful radiolytic solvent radical scavenger for pulse radiolysis experiments in CH3CN, effectively converting unwanted oxidizing radicals into useful reducing equivalents in the form of TEA• radicals.

Steric and Lewis Basicity Influence of the Second Coordination Sphere on Electrocatalytic CO2 Reduction by Manganese Bipyridyl Complexes.

This study aims to provide a greater insight into the balance between steric (bpy vs (Ph)2bpy vs mes2bpy ligands) and Lewis basic ((Ph)2bpy vs (MeOPh)2bpy vs (MeSPh)2bpy ligands) influence on the efficiencies of the protonation-first vs reduction-first CO2 reduction mechanisms with [MnI(R2bpy)(CO)3(CH3CN)]+ precatalysts, and on their respective transition-state geometries/energies for rate-determining C-OH bond cleavage toward CO evolution. The presence of only modest steric bulk at the 6,6'-diphenyl-2,2'-bipyridyl ((Ph)2bpy) ligand has here allowed unique insight into the mechanism of catalyst activation and CO2 binding by navigating a perfect medium between the nonsterically encumbered bpy-based and the highly sterically encumbered mes2bpy-based precatalysts. Cyclic voltammetry conducted in CO2-saturated electrolyte for the (Ph)2bpy-based precatalyst [2-CH3CN]+ confirms that CO2 binding occurs at the two-electron-reduced activated catalyst [2]- in the absence of an excess proton source, in contrast to prior assumptions that all manganese catalysts require a strong acid for CO2 binding. This observation is supported by computed free energies of the parent-child reaction for [Mn-Mn]0 dimer formation, where increased steric hindrance relative to the bpy-based precatalyst correlates with favorable CO2 binding. A critical balance must be adhered to, however, as the absence of steric bulk in the bpy-based precatalyst [1-CH3CN]+ maintains a lower overpotential than [2-CH3CN]+ at the protonation-first pathway with comparable kinetic performance, whereas an ∼2-fold greater TOFmax is observed at its reduction-first pathway with an almost identical overpotential as [2-CH3CN]+. Notably, excessive steric bulk in the mes2bpy-based precatalyst [3-CH3CN]+ results in increased activation free energies of the C-OH bond cleavage transition states for both the protonation-first and the reduction-first pathways relative to both [1-CH3CN]+ and [2-CH3CN]+. In fact, [3-CH3CN]+ requires a 1 V window beyond its onset potential to reach its peak catalytic current, which is in contrast to the narrower (<0.30 V) potential response window of the remaining catalysts here studied. Voltammetry recorded under 1 atm of CO2 with 2.8 M (5%) H2O establishes [2-CH3CN]+ to have the lowest overpotential (η = 0.75 V) in the series here studied, attributed to its ability to lie "on the fence" when providing sufficient steric bulk to hinder (but not prevent) [Mn-Mn]0 dimerization, while simultaneously having a limited steric impact on the free energy of activation for the rate-determining C-OH bond cleavage transition state. While the methoxyphenyl bpy-based precatalyst [4-CH3CN]+ possesses an increased steric presence relative to [2-CH3CN]+, this is offset by its capacity to stabilize the C-OH bond cleavage transition states of both the protonation-first and the reduction-first pathways by facilitating second coordination sphere H-bonding stabilization.

Structural and Electronic Influences on Rates of Tertpyridine-Amine CoIII -H Formation During Catalytic H2 Evolution in an Aqueous Environment.

In this work, the differences in catalytic performance for a series of Co hydrogen evolution catalysts with different pentadentate polypyridyl ligands (L), have been rationalized by examining elementary steps of the catalytic cycle using a combination of electrochemical and transient pulse radiolysis (PR) studies in aqueous solution. Solvolysis of the [CoII -Cl]+ species results in the formation of [CoII (κ4 -L)(OH2 )]2+ . Further reduction produces [CoI (κ4 -L)(OH2 )]+ , which undergoes a rate-limiting structural rearrangement to [CoI (κ5 -L)]+ before being protonated to form [CoIII -H]2+ . The rate of [CoIII -H]2+ formation is similar for all complexes in the series. Using E1/2 values of various Co species and pKa values of [CoIII -H]2+ estimated from PR experiments, we found that while the protonation of [CoIII -H]2+ is unfavorable, [CoII -H]+ reacts with protons to produce H2 . The catalytic activity for H2 evolution tracks the hydricity of the [CoII -H]+ intermediate.

Unexpected Roles of Triethanolamine in the Photochemical Reduction of CO2 to Formate by Ruthenium Complexes.

A series of 4,4'-dimethyl-2,2'-bipyridyl ruthenium complexes with carbonyl ligands were prepared and studied using a combination of electrochemical and spectroscopic methods with infrared detection to provide structural information on reaction intermediates in the photochemical reduction of CO2 to formate in acetonitrile (CH3CN). An unsaturated 5-coordinate intermediate was characterized, and the hydride-transfer step to CO2 from a singly reduced metal-hydride complex was observed with kinetic resolution. While triethanolamine (TEOA) was expected to act as a proton acceptor to ensure the sacrificial behavior of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole as an electron donor, time-resolved infrared measurements revealed that about 90% of the photogenerated one-electron reduced complexes undergo unproductive back electron transfer. Furthermore, TEOA showed the ability to capture CO2 from CH3CN solutions to form a zwitterionic alkylcarbonate adduct and was actively engaged in key catalytic steps such as metal-hydride formation, hydride transfer to CO2 to form the bound formate intermediate, and dissociation of formate ion product. Collectively, the data provide an overview of the transient intermediates of Ru(II) carbonyl complexes and emphasize the importance of considering the participation of TEOA when investigating and proposing catalytic pathways.

Molecular Catalysts with Intramolecular Re-O Bond for Electrochemical Reduction of Carbon Dioxide.

A new Re bipyridine-type complex, namely, fac-Re(pmbpy)(CO)3Cl (pmbpy = 4-phenyl-6-(2-hydroxy-phenyl)-2,2'-bipyridine), 1, carrying a single OH moiety as local proton source, has been synthesized, and its electrochemical behavior under Ar and under CO2 has been characterized. Two isomers of 1, namely, 1-cis characterized by the proximity of Cl to OH and 1-trans, are identified. The interconversion between 1-cis and 1-trans is clarified by DFT calculations, which reveal two transition states. The energetically lower pathway displays a non-negligible barrier of 75.5 kJ mol-1. The 1e- electrochemical reduction of 1 affords the neutral intermediate 1-OPh, formally derived by reductive deprotonation and loss of Cl- from 1. 1-OPh, which exhibits an entropically favored intramolecular Re-O bond, has been isolated and characterized. The detailed electrochemical mechanism is demonstrated by combined chemical reactivity, spectroelectrochemistry, spectroscopic (IR and NMR), and computational (DFT) approaches. Comparison with previous Re and Mn derivatives carrying local proton sources highlights that the catalytic activity of Re complexes is more sensitive to the presence of local OH groups. Similar to Re-2OH (2OH = 4-phenyl-6-(phenyl-2,6-diol)-2,2'-bipyridine), 1 and Mn-1OH display a selective reduction of CO2 to CO. In the case of the Re bipyridine-type complex, the formation of a relatively stable Re-O bond and a preference for phenolate-based reactivity with CO2 slightly inhibit the electrocatalytic reduction of CO2 to CO, resulting in a low TON value of 9, even in the presence of phenol as a proton source.

Radiolytic formation of the carbon dioxide radical anion in acetonitrile revealed by transient IR spectroscopy.

The solvated electron in CH3CN is scavenged by CO2 with a rate constant of 3.2 × 1010 M-1 s-1 to produce the carbon dioxide radical anion (CO2˙-), a strong and versatile reductant. Using pulse radiolysis with time-resolved IR detection, this radical is unambiguously identified by its absorption band at 1650 cm-1 corresponding to the antisymmetric CO2˙- stretch. This assignment is confirmed by 13C isotopic labelling experiments and DFT calculations. In neat CH3CN, CO2˙- decays on a ∼10 µs time scale via recombination with solvent-derived radicals (R˙) and solvated protons. Upon addition of formate (HCO2-), the radiation yield of CO2˙- is substantially increased due to H-atom abstraction by R˙ from HCO2- (R˙ + HCO2- → RH + CO2˙-), which occurs in two kinetically separated steps. The rapid step involves the stronger H-abstracting CN˙, CH3˙, and possibly, H˙ primary radicals, while the slower step is due to the less reactive, but more abundant radical, CH2CN˙. The removal of solvent radicals by HCO2- also results in over a hundredfold increase in the CO2˙- lifetime. CO2˙- scavenging experiments suggest that at 50 mM HCO2-, about 60% of the solvent-derived radicals are engaged in CO2˙- generation. Even under CO2 saturation, no formation of the radical adduct, (CO2)2˙-, could be detected on the microsecond time scale.

Nitrile Vibration Reports Induced Electric Field and Delocalization of Electron in the Charge-Transfer State of Aryl Nitriles.

An electric field is created upon photoinduced charge separation in electron donor-acceptor (D-A) molecules. The photophysics of a prototypical D-A molecule, 4-(dimethylamino)-benzonitrile (DMABN), has been under extensive investigation for decades. Here, by using the framework of the vibrational Stark effect (VSE), we show that the nitrile vibration quantifies a significant induced electric field in the intramolecular charge-transfer state of DMABN. We further demonstrate that such a phenomenon can be observed in a structurally similar aryl nitrile and that the VSE depends on solvent polarity due to dielectric screening. Our current work shows how the superb sensitivity of the nitrile vibration can be used to identify the nature of electron delocalization and quantify the induced electric field in photoinduced charge transfer processes.

Turning on the Protonation-First Pathway for Electrocatalytic CO2 Reduction by Manganese Bipyridyl Tricarbonyl Complexes.

Electrocatalytic reduction of CO2 to CO is reported for the complex, {fac-MnI([(MeO)2Ph]2bpy)(CO)3(CH3CN)}(OTf), containing four pendant methoxy groups, where [(MeO)2Ph]2bpy = 6,6'-bis(2,6-dimethoxyphenyl)-2,2'-bipyridine. In addition to a steric influence similar to that previously established [Sampson, M. D. et al. J. Am. Chem. Soc. 2014, 136, 5460-5471] for the 6,6'-dimesityl-2,2'-bipyridine ligand in [fac-MnI(mes2bpy)(CO)3(CH3CN)](OTf), which prevents Mn0-Mn0 dimerization, the [(MeO)2Ph]2bpy ligand introduces an additional electronic influence combined with a weak allosteric hydrogen-bonding interaction that significantly lowers the activation barrier for C-OH bond cleavage from the metallocarboxylic acid intermediate. This provides access to the thus far elusive protonation-first pathway, minimizing the required overpotential for electrocatalytic CO2 to CO conversion by Mn(I) polypyridyl catalysts, while concurrently maintaining a respectable turnover frequency. Comprehensive electrochemical and computational studies here confirm the positive influence of the [(MeO)2Ph]2bpy ligand framework on electrocatalytic CO2 reduction and its dependence upon the concentration and pKa of the external Brønsted acid proton source (water, methanol, trifluoroethanol, and phenol) that is required for this class of manganese catalyst. Linear sweep voltammetry studies show that both phenol and trifluoroethanol as proton sources exhibit the largest protonation-first catalytic currents in combination with {fac-MnI([(MeO)2Ph]2bpy)(CO)3(CH3CN)}(OTf), saving up to 0.55 V in overpotential with respect to the thermodynamically demanding reduction-first pathway, while bulk electrolysis studies confirm a high product selectivity for CO formation. To gain further insight into catalyst activation, time-resolved infrared (TRIR) spectroscopy combined with pulse-radiolysis (PR-TRIR), infrared spectroelectrochemistry, and density functional theory calculations were used to establish the v(CO) stretching frequencies and energetics of key redox intermediates relevant to catalyst activation.

Vibrational Stark effects to identify ion pairing and determine reduction potentials in electrolyte-free environments.

A recently developed instrument for time-resolved infrared detection following pulse radiolysis has been used to measure the ν(C≡N) IR band of the radical anion of a CN-substituted fluorene in tetrahydrofuran. Specific vibrational frequencies can exhibit distinct frequency shifts due to ion pairing, which can be explained in the framework of the vibrational Stark effect. Measurements of the ratio of free ions and ion pairs in different electrolyte concentrations allowed us to obtain an association constant and free energy change for ion pairing. This new method has the potential to probe the geometry of ion pairing and allows the reduction potentials of molecules to be determined in the absence of electrolyte in an environment of low dielectric constant.

Electron Localization of Anions Probed by Nitrile Vibrations.

Localization and delocalization of electrons is a key concept in chemistry, and is one of the important factors determining the efficiency of electron transport through organic conjugated molecules, which have potential to act as "molecular wires". This, in turn, substantially influences the efficiencies of organic solar cells and other molecular electronic devices. It is also necessary to understand the electronic energy landscape and the dynamics that govern electron transport capabilities in one-dimensional conjugated chains so that we can better define the design principles for conjugated molecules for their applications. We show that nitrile ν(C≡N) vibrations respond to the degree of electron localization in nitrile-substituted organic anions by utilizing time-resolved infrared detection combined with pulse radiolysis. Measurements of a series of aryl nitrile anions allow us to construct a semiempirical calibration curve between the changes in the ν(C≡N) infrared (IR) shifts and the changes in the electronic charges from the neutral to the anion states in the nitriles; more electron localization in the nitrile anion results in larger IR shifts. Furthermore, the IR line width in anions can report a structural change accompanying changes in the electronic density distribution. Probing the shift of the nitrile ν(C≡N) IR vibrational bands enables us to determine how the electron is localized in anions of nitrile-functionalized oligofluorenes, considered as organic mixed-valence compounds. We estimate the diabatic electron transfer distance, electronic coupling strengths, and energy barriers in these mixed-valence compounds. The analysis reveals a dynamic picture, showing that the electron is moving back and forth within the oligomers with a small activation energy of ≤kBT, likely controlled by the movement of dihedral angles between monomer units. Implications for the electron transport capability in molecular wires are discussed.

The reaction of cobaloximes with hydrogen: products and thermodynamics.

A cobalt hydride has been proposed as an intermediate in many reactions of the Co(dmgBF2)2L2 system, but its observation has proven difficult. We have observed the UV-vis spectra of Co(dmgBF2)2L2 (1) in CH3CN under hydrogen pressures of up to 70 atm. A Co(I) compound (6a) with an exchangeable proton is eventually formed. We have determined the bond dissociation free energy and pK(a) of the new O-H bond in 6a to be 50.5 kcal/mol and 13.4, respectively, in CH3CN, matching previous reports.

Kinetic and mechanistic studies of carbon-to-metal hydrogen atom transfer involving Os-centered radicals: evidence for tunneling.

We have investigated the kinetics of novel carbon-to-metal hydrogen atom transfer reactions, in which homolytic cleavage of a C-H bond is accomplished by a single metal-centered radical. Time-resolved IR spectroscopic measurements revealed efficient hydrogen atom transfer from xanthene, 9,10-dihydroanthracene, and 1,4-cyclohexadiene to Cp(CO)2Os(•) and (η(5)-(i)Pr4C5H)(CO)2Os(•) radicals, formed by photoinduced homolysis of the corresponding osmium dimers. The rate constants for hydrogen abstraction from these hydrocarbons are in the range 1.5 × 10(5) M(-1) s(-1) to 1.7 × 10(7) M(-1) s(-1) at 25 °C. For the first time, kinetic isotope effects for carbon-to-metal hydrogen atom transfer were determined. Large primary deuterium kinetic isotope effects of 13.4 ± 1.0 and 16.8 ± 1.4 were observed for the hydrogen abstraction from xanthene to form Cp(CO)2OsH and (η(5)-(i)Pr4C5H)(CO)2OsH, respectively, at 25 °C. Temperature-dependent measurements of the kinetic isotope effects over a 60 °C temperature range were carried out to obtain the difference in activation energies (E(D) - E(H)) and the pre-exponential factor ratio (A(H)/A(D)). For hydrogen atom transfer from xanthene to (η(5)-(i)Pr4C5H)(CO)2Os(•), the (E(D) - E(H)) = 3.3 ± 0.2 kcal mol(-1) and A(H)/A(D) = 0.06 ± 0.02 values suggest a quantum mechanical tunneling mechanism.

Mechanism of the formation of a Mn-based CO2 reduction catalyst revealed by pulse radiolysis with time-resolved infrared detection.

Using a new technique, which combines pulse radiolysis with nanosecond time-resolved infrared (TRIR) spectroscopy in the condensed phase, we have conducted a detailed kinetic and mechanistic investigation of the formation of a Mn-based CO2 reduction electrocatalyst, [Mn((t)Bu2-bpy)(CO)3]2 ((t)Bu2-bpy = 4,4'-(t)Bu2-2,2'-bipyridine), in acetonitrile. The use of TRIR allowed, for the first time, direct observation of all the intermediates involved in this process. Addition of excess [(n)Bu4N][HCO2] to an acetonitrile solution of fac-MnBr((t)Bu2-bpy)(CO)3 results in its quantitative conversion to the Mn-formate complex, fac-Mn(OCHO)((t)Bu2-bpy)(CO)3, which is a precatalyst for the electrocatalytic reduction of CO2. Formation of the catalyst is initiated by one-electron reduction of the Mn-formate precatalyst, which produces the bpy ligand-based radical. This radical undergoes extremely rapid (τ = 77 ns) formate dissociation accompanied by a free valence shift to yield the five-coordinate Mn-based radical, Mn(•)((t)Bu2-bpy)(CO)3. TRIR data also provide evidence that the Mn-centered radical does not bind acetonitrile prior to its dimerization. This reaction occurs with a characteristically high radical-radical recombination rate (2kdim = (1.3 ± 0.1) × 10(9) M(-1) s(-1)), generating the catalytically active Mn-Mn bound dimer.

Kinetics and thermodynamics of small molecule binding to pincer-PCP rhodium(I) complexes.

The kinetics and thermodynamics of the binding of several small molecules, L (L = N2, H2, D2, and C2H4), to the coordinatively unsaturated pincer-PCP rhodium(I) complexes Rh[(t)Bu2PCH2(C6H3)CH2P(t)Bu2] (1) and Rh[(t)Bu2P(CH2)2(CH)(CH2)2P(t)Bu2] (2) in organic solvents (n-heptane, toluene, THF, and cyclohexane-d12) have been investigated by a combination of kinetic flash photolysis methods, NMR equilibrium studies, and density functional theory (DFT) calculations. Using various gas mixtures and monitoring by NMR until equilibrium was established, the relative free energies of binding of N2, H2, and C2H4 in cyclohexane-d12 were found to increase in the order C2H4 < N2 < H2. Time-resolved infrared (TRIR) and UV-vis transient absorption spectroscopy revealed that 355 nm excitation of 1-L and 2-L results in the photoejection of ligand L. The subsequent mechanism of binding of L to 1 and 2 to regenerate 1-L and 2-L is determined by the structure of the PCP ligand framework and the nature of the solvent. In both cases, the primary transient is a long-lived, unsolvated species (τ = 50-800 ns, depending on L and its concentration in solution). For 2, this so-called less-reactive form (LRF) is in equilibrium with a more-reactive form (MRF), which reacts with L at diffusion-controlled rates to regenerate 2-L. These two intermediates are proposed to be different conformers of the three-coordinate (PCP)Rh fragment. For 1, a similar mechanism is proposed to occur, but the LRF to MRF step is irreversible. In addition, a parallel reaction pathway was observed that involves the direct reaction of the LRF of 1 with L, with second-order rate constants that vary by almost 3 orders of magnitude, depending on the nature of L (in n-heptane, k = 6.7 × 10(5) M(-1) s(-1) for L = C2H4; 4.0 × 10(6) M(-1) s(-1) for L = N2; 5.5 × 10(8) M(-1) s(-1) for L = H2). Experiments in the more coordinating solvent, THF, revealed the binding of THF to 1 to generate 1-THF, and its subsequent reaction with L, as a competing pathway.