Slow Proton-Transfer Reactions in Organometallic and Bioinorganic Chemistry

Intramolecular Energy Transfer Competing with Light-Driven Intermolecular Proton Transfer in an Iron(II)-NHC Complex? A Query into the Role of Photobasic Ligands and MLCT States

Inorganic photoacids and photobases comprising of photoactive transition metal complexes (TMCs) offer the ability to modulate proton transfer reactions through light irradiation, while utilizing the excellent optical properties of the latter. This provides a powerful tool for precise control over chemical reactions and processes, with implications for both fundamental science and practical applications. In this contribution, we present a novel molecular architecture amending an Fe-NHC complex with a pendant quinoline, as a prototypical photobase, as a representative earth-abundant TMC based inorganic photobase. We characterize the excited-state properties and proton-transfer dynamics using steady-state absorption and emission spectroscopy as well as pump wavelength dependent transient absorption spectroscopy in various protic solvents. The kinetics and thermodynamics of proton transfer in the quinoline moiety are influenced by both the presence of the metal center and the choice of the solvent. Furthermore, we see indications of intramolecular energy transfer from the quinoline to the MLCT state as a limiting factor for panchromatic photobasicity of the complex.

Thermodynamic-Kinetic Comparison of Palladium(II)-Mediated Alcohol and Hydroquinone Oxidation

Palladium(II) catalysts promote oxidative dehydrogenation and dehydrogenative coupling of many organic molecules. Oxidations of alcohols to aldehydes or ketones are prominent examples. Hydroquinone (H2Q) oxidation to benzoquinone (BQ) is conceptually related to alcohol oxidation, but it is significantly more challenging thermodynamically. The BQ/H2Q redox potential is sufficiently high that BQ is often used as an oxidant in Pd-catalyzed oxidation reactions. A recent report (J. Am Chem. Soc. 2020, 142, 19678-19688) showed that certain ancillary ligands can raise the PdII/0 redox potential sufficiently to reverse this reactivity, enabling (L)PdII(OAc)2 to oxidize hydroquinone to benzoquinone. Here, we investigate the oxidation of tert-butylhydroquinone ( t BuH2Q) and 4-fluorobenzyl alcohol (4FBnOH), mediated by (bc)Pd(OAc)2 (bc = bathocuproine). Although alcohol oxidation is thermodynamically favored over H2Q oxidation by more than 400 mV, the oxidation of t BuH2Q proceeds several orders of magnitude faster than 4FBnOH oxidation. Kinetic and mechanistic studies reveal that these reactions feature different rate-limiting steps. Alcohol oxidation proceeds via rate-limiting β-hydride elimination from a PdII-alkoxide intermediate, while H2Q oxidation features rate-limiting isomerization from an O-to-C-bound PdII-hydroquinonate species. The enhanced rate of H2Q oxidation reflects the kinetic facility of O─H relative to C─H bond cleavage.

N-O Bond Activation and Cleavage Reactions of the Nitrosyl-Bridged Complexes [M2Cp2(μ-PCy2)(μ-NO)(NO)2] (M = Mo, W)

The title complexes (1a,b) were prepared in two steps by first reacting the hydrides [M₂Cp₂(μ-H)(μ-PCy₂)(CO)₄] with [NO](BF₄) in the presence of Na₂CO₃ to give dinitrosyls [M₂Cp₂(μ-PCy₂)(CO)₂(NO)₂](BF₄), which were then fully decarbonylated upon reaction with NaNO₂ at 323 K. An isomer of the Mo₂ complex having a cisoid arrangement of the terminal ligands ( cis-1a) was prepared upon irradiation of toluene solutions of 1a with visible-UV light at 288 K. The structure of these trinitrosyl complexes was investigated using X-ray diffraction and density functional theory (DFT) calculations, these revealing a genuine pyramidalization of the bridging NO that might be associated in part to an increase of charge at the N atom and anticipated a weakening of the N-O bond upon reaction with bases or reducing reagents. Complexes 1a,b reacted with [FeCp₂](BF₄) to give first the radicals [M₂Cp₂(μ-PCy₂)(μ-NO)(NO)₂](BF₄) according to CV experiments, which then underwent H-abstraction to yield the nitroxyl-bridged complexes [M₂Cp₂(μ-PCy₂)(μ-κ¹:η²-HNO)(NO)₂](BF₄), alternatively prepared upon protonation with HBF₄·OEt₂. The novel coordination mode of the nitroxyl ligand in these products was thermodynamically favored over its tautomeric hydroximido form, according to DFT calculations, and similar nitrosomethane-bridged cations [M₂Cp₂(μ-PCy₂)( μ-κ¹:η²-MeNO)(NO)₂]⁺ were prepared by reacting 1a,b with CF₃SO₃Me or [Me₃O]BF₄. Complexes 1 reacted with M(Hg) (M = Zn, Na) in tetrahydrofuran to give the amido-bridged derivatives [M₂Cp₂(μ-PCy₂)(μ-NH₂)(NO)₂] with retention of stereochemistry, a transformation also induced by using mild O atom scavengers such as CO and phosphites in the presence of water. In the absence of water, phosphites accomplished a deoxygenation of the bridging NO of the Mo₂ complexes to yield the phosphoraniminato-bridged derivatives [Mo₂Cp₂(μ-PCy₂){μ-NP(OR)₃}(NO)₂] (R = Et, Ph), also with retention of stereochemistry.

Synthetic Fe/Cu Complexes: Toward Understanding Heme-Copper Oxidase Structure and Function

Heme-copper oxidases (HCOs) are terminal enzymes on the mitochondrial or bacterial respiratory electron transport chain, which utilize a unique heterobinuclear active site to catalyze the 4H+/4e- reduction of dioxygen to water. This process involves a proton-coupled electron transfer (PCET) from a tyrosine (phenolic) residue and additional redox events coupled to transmembrane proton pumping and ATP synthesis. Given that HCOs are large, complex, membrane-bound enzymes, bioinspired synthetic model chemistry is a promising approach to better understand heme-Cu-mediated dioxygen reduction, including the details of proton and electron movements. This review encompasses important aspects of heme-O2 and copper-O2 (bio)chemistries as they relate to the design and interpretation of small molecule model systems and provides perspectives from fundamental coordination chemistry, which can be applied to the understanding of HCO activity. We focus on recent advancements from studies of heme-Cu models, evaluating experimental and computational results, which highlight important fundamental structure-function relationships. Finally, we provide an outlook for future potential contributions from synthetic inorganic chemistry and discuss their implications with relevance to biological O2-reduction.

Switching between Stepwise and Concerted Proton-Coupled Electron Transfer Pathways in Tungsten Hydride Activation

Catalytic processes to generate (or oxidize) fuels such as hydrogen are underpinned by multiple proton-coupled electron transfer (PCET) steps that are associated with the formation or activation of metal-hydride bonds. Fully understanding the detailed PCET mechanisms of metal hydride transformations holds promise for the rational design of energy-efficient catalysis. Here we investigate the detailed PCET mechanisms for the activation of the transition metal hydride complex CpW(CO)₂(PMe₃)H (Cp = cyclopentadienyl) using stopped-flow rapid mixing coupled with time-resolved optical spectroscopy. We reveal that all three limiting PCET pathways can be accessed by changing the free energy for elementary proton, electron, and proton-electron transfers through the choice of base and oxidant, with the concerted pathway occurring exclusively as a secondary parallel route. Through detailed kinetics analysis, we define free energy relationships for the kinetics of elementary reaction steps, which provide insight into the factors influencing reaction mechanism. Rate constants for proton transfer processes in the limiting stepwise pathways reveal a large reorganization energy associated with protonation/deprotonation of the metal center (λ = 1.59 eV) and suggest that sluggish proton transfer kinetics hinder access to a concerted route. Rate constants for concerted PCET indicate that the concerted routes are asynchronous. Additionally, through quantification of the relative contributions of parallel stepwise and concerted mechanisms toward net product formation, the influence of various reaction parameters on reactivity are identified. This work underscores the importance of understanding the PCET mechanism for controlling metal hydride reactivity, which could lead to superior catalyst design for fuel production and oxidation.

Hydrogenase Enzymes and Their Synthetic Models: The Role of Metal Hydrides

Hydrogenase enzymes efficiently process H2 and protons at organometallic FeFe, NiFe, or Fe active sites. Synthetic modeling of the many H2ase states has provided insight into H2ase structure and mechanism, as well as afforded catalysts for the H2 energy vector. Particularly important are hydride-bearing states, with synthetic hydride analogues now known for each hydrogenase class. These hydrides are typically prepared by protonation of low-valent cores. Examples of FeFe and NiFe hydrides derived from H2 have also been prepared. Such chemistry is more developed than mimicry of the redox-inactive monoFe enzyme, although functional models of the latter are now emerging. Advances in physical and theoretical characterization of H2ase enzymes and synthetic models have proven key to the study of hydrides in particular, and will guide modeling efforts toward more robust and active species optimized for practical applications.

Mechanism of H2 Production by Models for the [NiFe]-Hydrogenases: Role of Reduced Hydrides

The intermediacy of a reduced nickel-iron hydride in hydrogen evolution catalyzed by Ni-Fe complexes was verified experimentally and computationally. In addition to catalyzing hydrogen evolution, the highly basic and bulky (dppv)Ni(μ-pdt)Fe(CO)(dppv) ([1](0); dppv = cis-C2H2(PPh2)2) and its hydride derivatives have yielded to detailed characterization in terms of spectroscopy, bonding, and reactivity. The protonation of [1](0) initially produces unsym-[H1](+), which converts by a first-order pathway to sym-[H1](+). These species have C1 (unsym) and Cs (sym) symmetries, respectively, depending on the stereochemistry of the octahedral Fe site. Both experimental and computational studies show that [H1](+) protonates at sulfur. The S = 1/2 hydride [H1](0) was generated by reduction of [H1](+) with Cp*2Co. Density functional theory (DFT) calculations indicate that [H1](0) is best described as a Ni(I)-Fe(II) derivative with significant spin density on Ni and some delocalization on S and Fe. EPR spectroscopy reveals both kinetic and thermodynamic isomers of [H1](0). Whereas [H1](+) does not evolve H2 upon protonation, treatment of [H1](0) with acids gives H2. The redox state of the "remote" metal (Ni) modulates the hydridic character of the Fe(II)-H center. As supported by DFT calculations, H2 evolution proceeds either directly from [H1](0) and external acid or from protonation of the Fe-H bond in [H1](0) to give a labile dihydrogen complex. Stoichiometric tests indicate that protonation-induced hydrogen evolution from [H1](0) initially produces [1](+), which is reduced by [H1](0). Our results reconcile the required reductive activation of a metal hydride and the resistance of metal hydrides toward reduction. This dichotomy is resolved by reduction of the remote (non-hydride) metal of the bimetallic unit.

Preparation and Protonation of Fe2(pdt)(CNR)6, Electron-Rich Analogues of Fe2(pdt)(CO)6

The complexes Fe2(pdt)(CNR)6 (pdt(2-) = CH2(CH2S(-))2) were prepared by thermal substitution of the hexacarbonyl complex with the isocyanides RNC for R = C6H4-4-OMe (1), C6H4-4-Cl (2), Me (3). These complexes represent electron-rich analogues of the parent Fe2(pdt)(CO)6. Unlike most substituted derivatives of Fe2(pdt)(CO)6, these isocyanide complexes are sterically unencumbered and have the same idealized symmetry as the parent hexacarbonyl derivatives. Like the hexacarbonyls, the stereodynamics of 1-3 involve both turnstile rotation of the Fe(CNR)3 as well as the inversion of the chair conformation of the pdt ligand. Structural studies indicate that the basal isocyanide has nonlinear CNC bonds and short Fe-C distances, indicating that they engage in stronger Fe-C π-backbonding than the apical ligands. Cyclic voltammetry reveals that these new complexes are far more reducing than the hexacarbonyls, although the redox behavior is complex. Estimated reduction potentials are E1/2 ≈ -0.6 ([2](+/0)), -0.7 ([1](+/0)), and -1.25 ([3](+/0)). According to DFT calculations, the rotated isomer of 3 is only 2.2 kcal/mol higher in energy than the crystallographically observed unrotated structure. The effects of rotated versus unrotated structure and of solvent coordination (THF, MeCN) on redox potentials were assessed computationally. These factors shift the redox couple by as much as 0.25 V, usually less. Compounds 1 and 2 protonate with strong acids to give the expected μ-hydrides [H1](+) and [H2](+). In contrast, 3 protonates with [HNEt3]BAr(F)4 (pKa(MeCN) = 18.7) to give the aminocarbyne [Fe2(pdt)(CNMe)5(μ-CN(H)Me)](+) ([3H](+)). According to NMR measurements and DFT calculations, this species adopts an unsymmetrical, rotated structure. DFT calculations further indicate that the previously described carbyne complex [Fe2(SMe)2(CO)3(PMe3)2(CCF3)](+) also adopts a rotated structure with a bridging carbyne ligand. Complex [3H](+) reversibly adds MeNC to give [Fe2(pdt)(CNR)6(μ-CN(H)Me)](+) ([3H(CNMe)](+)). Near room temperature, [3H](+) isomerizes to the hydride [(μ-H)Fe2(pdt)(CNMe)6](+) ([H3](+)) via a first-order pathway.

Artificial hydrogenases

Decades of biophysical study on the hydrogenase (H(2)ase) enzymes have yielded sufficient information to guide the synthesis of analogs of their active sites. Three families of enzymes serve as inspiration for this work: the [FeFe]-H(2)ases, [NiFe]-H(2)ases, and [Fe]-H(2)ases, all of which feature iron centers bound to both CO and thiolate. Artificial H(2)ases affect the oxidation of H(2) and the reverse reaction, the reduction of protons. These reactions occur via the intermediacy of metal hydrides. The inclusion of amine bases within the catalysts is an important design feature that is emulated in related bioinspired catalysts. Continuing challenges are the low reactivity of H(2) toward biomimetic H(2)ases.

Isomerization of the hydride complexes [HFe2(SR)2(PR3)(x)(CO)(6-x)]+ (x = 2, 3, 4) relevant to the active site models for the [FeFe]-hydrogenases

The stepwise formation of bridging (mu-) hydrides of diiron dithiolates is discussed with attention on the pathway for protonation and subsequent isomerizations. Our evidence is consistent with protonations occurring at a single Fe center, followed by isomerization to a series of mu-hydrides. Protonation of Fe(2)(edt)(CO)(4)(dppv) (1) gave a single mu-hydride with dppv spanning apical and basal sites, which isomerized at higher temperatures to place the dppv into a dibasal position. Protonation of Fe(2)(pdt)(CO)(4)(dppv) (2) followed an isomerization pathway similar to that for [1H](+), except that a pair of isomeric terminal hydrides were observed initially, resulting from protonation at the Fe(CO)(3) or Fe(CO)(dppv) site. The first observable product from low temperature protonation of the tris-phosphine Fe(2)(edt)(CO)(3)(PMe(3))(dppv) (3) was a single mu-hydride wherein PMe(3) is apical and the dppv ligand spans apical and basal sites. Upon warming, this isomer converted fully but in a stepwise manner to a mixture of three other isomeric hydrides. Protonation of Fe(2)(pdt)(CO)(3)(PMe(3))(dppv) (4) proceeded similarly to the edt analogue 3, however a terminal hydride was observed, albeit only briefly and at very low temperatures (-90 degrees C). Low-temperature protonation of the bis-chelates Fe(2)(xdt)(CO)(2)(dppv)(2) produced exclusively the terminal hydrides [HFe(2)(xdt)(mu-CO)(CO)(dppv)(2)](+) (xdt = edt and pdt), which subsequently isomerized to a pair of mu-hydrides. At room temperature these (dppv)(2) derivatives convert to an equilibrium of two isomers, one C(2)-symmetric and the other C(s)-symmetric. The stability of the terminal hydrides correlates with the (C(2)-isomer)/(C(s)-isomer) equilibrium ratio, which reflects the size of the dithiolate. The isomerization was found to be unaffected by the presence of excess acid, by solvent polarity, and the presence of D(2)O. This isomerization mechanism is proposed to be intramolecular, involving a 120 degrees rotation of the HFeL(3) subunit to an unobserved terminal basal hydride as the rate-determining step. The observed stability of the hydrides was supported by DFT calculations, which also highlight the instability of the basal terminal hydrides. Isomerization of the mu-hydride isomers occurs on alternating FeL(3) via 120 degree rotations without generating D(2)O-exchangeable intermediates.

Small molecule mimics of hydrogenases: hydrides and redox

This tutorial review is aimed at chemical scientists interested in understanding and exploiting the remarkable catalytic behavior of the hydrogenases. The key structural features are analyzed for the active sites of the two most important hydrogenases. Reactivity is emphasized, focusing on mechanism and catalysis. Through this analysis, gaps are identified in the synthesis of functional replicas of these fascinating and potentially useful enzymes.

A metalloradical mechanism for the generation of oxygen from water in photosynthesis

In plants and algae, photosystem II uses light energy to oxidize water to oxygen at a metalloradical site that comprises a tetranuclear manganese cluster and a tyrosyl radical. A model is proposed whereby the tyrosyl radical functions by abstracting hydrogen atoms from substrate water bound as terminal ligands to two of the four manganese ions. Molecular oxygen is produced in the final step in which hydrogen atom transfer and oxygen-oxygen bond formation occur together in a concerted reaction. This mechanism establishes clear analogies between photosynthetic water oxidation and amino acid radical function in other enzymatic reactions.