Bond Dissociation Free Energy Research Articles

Characterization and Reactivity Studies of Mononuclear Tetrahedral Copper(II)-Halide Complexes.

Structures, physicochemical properties, and reactivity of the whole series of copper(II)-halide complexes (1X; X = F, Cl, Br, and I) were examined using a TMG3tach tridentate supporting ligand consisting of cis,cis-1,3,5-triaminocyclohexane (tach) and N,N,N',N'-tetramethylguanidine (TMG). The tach ligand framework with the bulky and strongly electron-donating TMG substituents enforces the copper(II) complexes to take a tetrahedral geometry, as inferred from the electron paramagnetic resonance (EPR) spectra, exhibiting relatively large gz and small Az values. The electronic absorption spectra of 1X agreed with the simulation spectra obtained by time-dependent density functional theory (TD-DFT) calculations on a slightly distorted tetrahedral geometry. 1I and 1Br gradually decomposed to generate the corresponding copper(I) complex and halide radical X•, and in the case of 1Br, intramolecular hydroxylation of a methyl group of the TMG substituent took place under aerobic conditions, which may be caused by the reaction of the generated copper(I) complex and dioxygen (O2), generating a reactive oxygen species. 1X except 1I showed hydrogen atom abstraction (HAA) reactivity toward 1,4-cyclohexadiene (CHD), where 1F exhibited the highest reactivity with a second-order rate constant as 1.4 × 10-3 M-1 s-1 at 25 °C. Such an HAA reactivity can be attributed to the higher basicity of F- and/or large bond dissociation free energy of conjugate acid H-F as well as the unstable copper(II) electronic state in the tetrahedral geometry.

Structure–activity relationship of alkanes and alkane derivatives for the abilities of C(sp3)H bonds toward their H‐atom transfer reactions

AbstractHydrogen atom‐donating ability of alkane is a research hotspot and has been extensively studied. In this article, the second‐order rate constants of 20 hydrogen atom transfer (HAT) reactions between aliphatic, benzylic, and allylic alkanes and alkane derivatives with CumO• in acetonitrile at 298 K were studied. The thermo‐kinetic parameter ΔG≠o(XH), bond dissociation free energy ΔGo(XH), and kinetic intrinsic resistance energy ΔG≠XH/X were determined and used to evaluate the H‐donating abilities of these substrates in thermodynamics, kinetics, and HAT reactions. Structure–activity relationships including the factors (electronic, stereoelectronic, and steric effects), introduction of CH3, Ph, or Cl in alkanes, and introduction of N atom in cycloalkane were discussed carefully. The results show that the order of H‐donating abilities is allylic alkanes > cycloalkanes > chain alkanes ≈ benzylic alkanes > haloalkanes. The introduction of CH3, Ph, or Cl in alkanes and the introduction of N atom to the carbon ring reduce ΔGo(XH) but increase ΔG≠XH/X, and ΔG≠o(XH) is the synthesis result of these two parameters. The reliability of ΔG≠o(XH) was verified, and the accuracy and reliability of the parameters were proved. Through the study of this paper, not only the ΔGo(XH), ΔG≠XH/X, and ΔG≠o(XH) of these alkanes and derivatives in HAT reaction can be quantitatively evaluated but also the structure–activity relationship of alkane is clearly researched.

Enhanced Understanding of Structure-Function Relationships for Oxomanganese(IV) Complexes.

A series of manganese(II) and oxomanganese(IV) complexes supported by neutral, pentadentate ligands with varied equatorial ligand-field strength (N3pyQ, N2py2I, and N4pyMe2) were synthesized and then characterized using structural and spectroscopic methods. On the basis of electronic absorption spectroscopy, the [MnIV(O)(N4pyMe2)]2+ complex has the weakest equatorial ligand field among a set of similar MnIV-oxo species. In contrast, [MnIV(O)(N2py2I)]2+ shows the strongest equatorial ligand-field strength for this same series. We examined the influence of these changes in electronic structure on the reactivity of the oxomanganese(IV) complexes using hydrocarbons and thioanisole as substrates. The [MnIV(O)(N3pyQ)]2+ complex, which contains one quinoline and three pyridine donors in the equatorial plane, ranks among the fastest MnIV-oxo complexes in C-H bond and thioanisole oxidation. While a weak equatorial ligand field has been associated with high reactivity, the [MnIV(O)(N4pyMe2)]2+ complex is only a modest oxidant. Buried volume plots suggest that steric factors dampen the reactivity of this complex. Trends in reactivity were examined using density functional theory (DFT)-computed bond dissociation free energies (BDFEs) of the MnIIIO-H and MnIV ═ O bonds. We observe an excellent correlation between MnIV═O BDFEs and rates of thioanisole oxidation, but more scatter is observed between hydrocarbon oxidation rates and the MnIIIO-H BDFEs.

Spotlights on Recent JACS Publications, Volume 145, Issue 23.

ADVERTISEMENT RETURN TO ISSUESpotlightsNEXTSpotlights on Recent JACS Publications, Volume 145, Issue 23ACS Contributing CorrespondentsACS Contributing CorrespondentsMore by ACS Contributing CorrespondentsCite this: J. Am. Chem. Soc. 2023, 145, 23, 12425Publication Date (Web):June 14, 2023Publication History Received5 June 2023Published online14 June 2023Published inissue 14 June 2023https://doi.org/10.1021/jacs.3c05892Copyright © 2023 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views-Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (703 KB) Get e-AlertscloseSUBJECTS:Annulations,Bond dissociation free energy,Hydrocarbons,Materials,Palladium Get e-Alerts

Experimental and Computational Determination of a M-Cl Homolytic Bond Dissociation Free Energy: Mn(III)Cl-Mediated C-H Cleavage and Chlorination.

This study confirms the hypothesis that [MnCl3(OPPh3)2] (1) and acetonitrile-solvated MnCl3 (i.e., [MnCl3(MeCN)x]) can be used as synthons to prepare Mn(III) chloride complexes with facially coordinating ligands. This was achieved through the preparation and characterization of six new {MnIIICl} complexes using anionic ligands TpH (tris(pyrazolyl)borate) and TpMe (tris(3,5-dimethylpyrazolyl)borate). The MnIII-chloride dissociation and association equilibria (Keq) and MnIII/II reduction potentials were quantified in DCM. These two thermochemical parameters (Keq and E1/2), in addition to the known Cl-atom reduction potential in DCM, enabled the quantification of the Mn-Cl bond dissociation (homolysis) free energy of 21 and 23 ± 7 kcal/mol at room temperature for R = H and Me, respectively. These are in reasonable agreement with the bond dissociation free energy (BDFEM-Cl) of 34 ± 6 kcal/mol calculated using density functional theory. The BDFEM-Cl of 1 was also calculated (25 ± 6 kcal/mol). These energies were used in predictive C-H bond reactivity.

Proton-Coupled Electron Transfer at the Surface of Polyoxovanadate-Alkoxide Clusters.

ConspectusProton-coupled electron transfer (PCET) is a fundamental process involved in all areas of chemistry, with relevance to biological transformations, catalysis, and emergent energy storage and conversion technologies. Early observations of PCET were reported by Meyer and co-workers in 1981 while investigating the proton dependence of reduction of a molecular ruthenium oxo complex. Since that time, this conceptual framework has grown to encompass an enormous scope of charge transfer and compensation reactions. In this Account, we will discuss ongoing efforts in the Matson Laboratory to understand the fundamental thermodynamics and kinetics of PCET processes at the surface of a series of Lindqvist-type polyoxovanadate clusters. This project aims to provide atomistic resolution of net H atom uptake and transfer at the surfaces of transition-metal oxide materials.First, we discuss our efforts aimed at understanding PCET at metal oxide surfaces using the Lindqvist-type polyoxovanadate-alkoxide (POV-alkoxide) cluster [nBu4N]2[V6O13(TRIOLNO2)2]. These clusters reversibly bind H atom equivalents at bridging oxide sites, mirroring the proposed uptake and release of e-/H+ pairs at transition-metal oxide surfaces. Summarized results include the measurement of bond dissociation free energies of surface hydroxide moieties (BDFE(O-H)) as well as mechanistic analyses that verify concerted proton electron transfer as the operative pathway for PCET at the surface of POV-alkoxide clusters.Next, we discuss net proton and H atom uptake at the surface of reduced variants of the Lindqvist-type POV-alkoxide cluster, [V6O7(OR)12]n (R = Me, Et; n = -2, -1, 0, + 1). In the case of these low-valent POV-alkoxide clusters, nucleophilic bridging sites are kinetically inhibited by functionalization of the cluster surface with organic ligands. This molecular modification enables site-selectivity in proton and H atom uptake to terminal oxide sites. The impact of reaction site and cluster electronics on reaction driving force of PCET is explored, with core electron density playing a critical role in dictating thermodynamics of H atom uptake and transfer. Additional work described here contrasts the kinetics of PCET at terminal oxide sites to the reactivity observed at bridging oxides in POV-alkoxide clusters.Overall, this Account summarizes our foundational knowledge regarding the assessment of PCET reactivity at the surfaces of molecular metal oxides. Drawing analogies between POV-alkoxide clusters and nanoscopic metal oxide materials provide design principles for the advancement of materials applications with atomic precision. These complexes are additionally highlighted as tunable redox mediators in their own right; our studies demonstrate how cluster surface reactivities can be optimized by modifying electronic structure and surface functionalities.

Enabling Molecular-Level Computational Description of Redox and Proton-Coupled Electron Transfer Reactions of Samarium Diiodide.

Samarium diiodide (SmI2, Kagan's reagent) is a one-electron reductant with applications ranging from organic synthesis to nitrogen fixation. Highly inaccurate relative energies of redox and proton-coupled electron transfer (PCET) reactions of Kagan's reagent are predicted by pure and hybrid density functional approximations (DFAs) when only scalar relativistic effects are accounted for. Calculations including spin-orbit coupling (SOC) show that the SOC-induced differential stabilization of the Sm(III) versus the Sm(II) ground state is little affected by ligands and solvent, and a standard SOC correction derived from atomic energy levels is thus included in the reported relative energies. With this correction, selected meta-GGA and hybrid meta-GGA functionals predict Sm(III)/Sm(II) reduction free energies to within 5 kcal/mol of the experiment. Considerable discrepancies remain, however, in particular for the PCET-relevant O-H bond dissociation free energies, for which no regular DFA is within 10 kcal/mol of the experiment or CCSD(T). The main cause behind these discrepancies is the delocalization error, which leads to excess ligand-to-metal electron donation and destabilizes Sm(III) versus Sm(II). Fortunately, static correlation is unimportant for the present systems, and the error may be reduced by including information from virtual orbitals via perturbation theory. Contemporary, parametrized double-hybrid methods offer promise as companions to experimental campaigns in the further development of the chemistry of Kagan's reagent.

Gradient Alloyed Quantum Dots for Photocatalytic Lignin Valorization via Proton Coupled Electron Transfer

In the field of photocatalytic lignin valorization, simultaneously fulfilling the requirements of broad light harvesting and strong redox ability has been a key challenge for single binary quantum dot (QD) photocatalyst systems. In order to address the problem, gradient alloyed QDs with funnel-shaped energy structures have been developed. The gradient energy level increases the photon absorption range, while the periphery energy bands at the funnel edge are sufficiently wide to preserve effective redox capacity. In this work, the gradient alloyed QD photocatalyst along with proton donors have been employed in enhancing the efficiency and selectivity of β-O-4 bond cleavage in lignin, realizing yields of target aromatic products phenol and acetophenone to increase up to 70% and 65%, respectively. The mechanism of conversion from the ketone intermediate to target products and how to choose suitable proton donors have been systematically investigated. We propose a proton-coupled electron transfer (PCET) photocatalytic strategy through producing a Cα radical from ketone with the help of acid possessing appropriate bond dissociation free energy (BDFE) and strong electronic transmission facilitating ability. The knowledge acquired in this work is useful for designing photocatalysts and proton donors that can effectively cleave bonds in lignin via the PCET mechanism.

Insights into the Role of Graphitic Carbon Nitride as a Photobase in Proton-Coupled Electron Transfer in (sp3 )C-H Oxygenation of Oxazolidinones.

Graphitic carbon nitride (g-CN) is a transition metal free semiconductor that mediates a variety of photocatalytic reactions. Although photoinduced electron transfer is often postulated in the mechanism, proton-coupled electron transfer (PCET) is a more favorable pathway for substrates possessing X-H bonds. Upon excitation of an (sp2 )N-rich structure of g-CN with visible light, it behaves as a photobase-it undergoes reductive quenching accompanied by abstraction of a proton from a substrate. The results of modeling allowed us to identify active sites for PCET-the 'triangular pockets' on the edge facets of g-CN. We employ excited state PCET from the substrate to g-CN to selectively cleavethe endo-(sp3 )C-H bond in oxazolidine-2-ones followed by trapping the radical with O2 . This reaction affords 1,3-oxazolidine-2,4-diones. Measurement of the apparent pKa value and modeling suggest that g-CN excited state can cleave X-H bonds that are characterized by bond dissociation free energy (BDFE) ≈100 kcal mol-1 .

Structural and Spectroscopic Characterization of Copper(III) Complexes and Subsequent One-Electron Oxidation Reaction and Reactivity Studies.

The formation of Cu(III) species are often invoked as the key intermediate in Cu-catalyzed organic transformation reactions. In this study, we synthesized Cu(II) (1) and Cu(III) (3) complexes supported by a bisamidate-bisalkoxide ligand consisting of an ortho-phenylenediamine (o-PDA) scaffold and characterized them through an array of spectroscopic techniques, including UV-visible, electron paramagnetic resonance, X-ray crystallography, and 1H nuclear magnetic resonance (NMR) and X-ray absorption spectroscopy. The Cu-N/O bond distances in 3 are ∼0.1 Å reduced compared to 1, implying a significant increase in 3's overall effective nuclear charge. Further, a Cu(III) complex (4) of a bisamidate-bisalkoxide ligand containing a trans-cyclohexane-1,2-diamine moiety exhibits nearly identical Cu-N/O bond distances to that of 3, inferring that the redox-active o-PDA backbone is not oxidized upon one-electron oxidation of the Cu(II) complex (1). In addition, a considerable difference in the 1s → 4p and 1s → 3d transition energy was observed in the X-ray absorption near-edge structure data of 3 vs 1, which is typical for the metal-centered oxidation process. Electrochemical measurements of the Cu(II) complex (1) in acetonitrile exhibited two consecutive redox couples at -0.9 and 0.4 V vs the Fc+/Fc reference electrode. One-electron oxidation reaction of 3 further resulted in the formation of a ligand-oxidized Cu complex (3a), which was characterized in depth. Reactivity studies of species 3 and 3a were explored toward the activation of the C-H/O-H bonds. A bond dissociation free energy (BDFE) value of ∼69 kcal/mol was estimated for the O-H bond of the Cu(II) complex formed upon transfer of hydrogen atom to 3. The study represents a thorough spectroscopic characterization of high-valent Cu complexes and sheds light on the PCET reactivity studies of Cu(III) complexes.

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.

Testing the Limits of Imbalanced CPET Reactivity: Mechanistic Crossover in H-Atom Abstraction by Co(III)-Oxo Complexes.

Transition metal-oxo complexes are key intermediates in a variety of oxidative transformations, notably C-H bond activation. The relative rate of C-H bond activation mediated by transition metal-oxo complexes is typically predicated on substrate bond dissociation free energy in cases with a concerted proton-electron transfer (CPET). However, recent work has demonstrated that alternative stepwise thermodynamic contributions such as acidity/basicity or redox potentials of the substrate/metal-oxo may dominate in some cases. In this context, we have found basicity-governed concerted activation of C-H bonds with the terminal CoIII-oxo complex PhB(tBuIm)3CoIIIO. We have been interested in testing the limits of such basicity-dependent reactivity and have synthesized an analogous, more basic complex, PhB(AdIm)3CoIIIO, and studied its reactivity with H-atom donors. This complex displays a higher degree of imbalanced CPET reactivity than PhB(tBuIm)3CoIIIO with C-H substrates, and O-H activation of phenol substrates displays mechanistic crossover to stepwise proton transfer-electron transfer (PTET) reactivity. Analysis of the thermodynamics of proton transfer (PT) and electron transfer (ET) reveals a distinct thermodynamic crossing point between concerted and stepwise reactivity. Furthermore, the relative rates of stepwise and concerted reactivity suggest that maximally imbalanced systems provide the fastest CPET rates up to the point of mechanistic crossover, which results in slower product formation.

Thermodynamic and Kinetic Studies of the Activities of Aldehydic C−H Bonds toward Their H‐Atom Transfer Reactions

AbstractAldehydes can not only accept hydrogen atoms to be reduced to alcohols, showing their oxidation properties; but also can donate hydrogen atoms of the C(sp2)−H bonds as hydrogen atom donors, showing their reduction properties. In this article, the hydrogen atom transfer reactions of eight aldehydes and free radicals in acetonitrile at 298 K were studied. The thermo‐kinetic parameters ΔG≠°(XH) of C(sp2)−H bonds in aldehydes were obtained by ΔG≠XH/Y=ΔG≠°(XH)+ΔG≠°(Y) as the ΔG≠°(Y) of free radicals were available in our previous work. The bond dissociation free energies ΔGo(XH) of C(sp2)−H bonds were calculated by iBonD HM method. The intrinsic resistance energies ΔG≠XH/X were determined as the ΔG≠°(XH) and ΔGo(XH) were available. ΔGo(XH), ΔG≠XH/X, and ΔG≠°(XH) were used to access the H‐donating abilities of C(sp2)−H bonds in aldehydes in thermodynamics, kinetics and actual HAT reactions. The scales and the effect of the structures of aldehydes on ΔGo(XH), ΔG≠XH/X, and ΔG≠°(XH) were discussed carefully. The oxidizing and reducing abilities of aldehydes, and the comparison of H‐donating abilities between C(sp2)−H of aldehydes with C(sp3)−H of the corresponding alcohols were also researched in detail. This paper not only fills the gap in the study of H‐donating activities of aldehydic C−H bonds, but also provides data support for the design and synthesis of more C(sp2)−H type antioxidants.

Cover Feature: Photoelectrochemical C−H Activation Through a Quinacridone Dye Enabling Proton‐Coupled Electron Transfer (ChemSusChem 5/2023)

The Cover Feature shows the photoelectrochemical activation of C−H bonds with mesoporous SnO2 electrodes sensitized with a quinacridone dye (pigment violet 19, used in paints). The key feature of quinacridone is the possibility to operate through a proton-coupled electron transfer mechanism, generating a nitrogen-based radical characterized by a bond dissociation free energy (BDFE) of 80.5 kcal mol−1 and capable of hydrogen atom abstraction from allylic and benzylic C−H bonds. More information can be found in the Research Article by Y. Yang, G. A. Volpato, E. Rossin et al.

Trans Ligand Determines the Stability of Paramagnetic Manganese(II) Hydrides of the Type trans-[MnH(L)(dmpe)2]+ Where L is PMe3, C2H4, or CO.

Paramagnetic metal hydride (PMH) complexes play important roles in catalytic applications and bioinorganic chemistry. 3d PMH chemistry has largely focused on Ti, Mn, Fe, and Co. Various MnII PMHs have been proposed as intermediates in catalysis, but isolated MnII PMHs are limited to dimeric high-spin MnII structures with bridging hydrides. In this paper, a series of the first low-spin monomeric MnII PMH complexes are generated by chemical oxidation of their MnI analogues. This series is of the type trans-[MnH(L)(dmpe)2]+/0 where the trans ligand L is PMe3, C2H4, or CO [dmpe is 1,2-bis(dimethylphosphino)ethane], and the thermal stability of the MnII hydride complexes was found to be strongly dependent on the identity of the trans ligand. When L is PMe3, the complex is the first example of an isolated monomeric MnII hydride complex. In contrast, when L is C2H4 or CO, the complexes are only stable at low temperatures; upon warming to room temperature, the former decomposed to afford [Mn(dmpe)3]+, accompanied by ethane and ethylene, whereas the latter eliminated H2, generating [Mn(MeCN)(CO)(dmpe)2]+ or a mixture of products including [Mn(κ1-PF6)(CO)(dmpe)2], depending on the reaction conditions. All PMHs were characterized by low-temperature electron paramagnetic resonance (EPR) spectroscopy, and stable [MnH(PMe3)(dmpe)2]+ was further characterized by UV-vis and IR spectroscopy, Superconducting Quantum Interference Device magnetometry, and single-crystal X-ray diffraction. Noteworthy spectral properties are the significant EPR superhyperfine coupling to the hydride (∼85 MHz) and an increase (+33 cm-1) in the Mn-H IR stretch upon oxidation. Density functional theory calculations were also employed to gain insights into the acidity and bond strengths of the complexes. MnII-H bond dissociation free energies are estimated to decrease in the series of complexes from 60 (L = PMe3) to 47 kcal/mol (L = CO).

Proton-Coupled Electron Transfer of Cerium Oxide Nanoparticle Thin-Film Electrodes

Metal oxide (MOx) materials are effective catalysts or cocatalysts in a range of electrochemical reactions and energy systems. A key component of their redox chemistry is proton-coupled electron transfer (PCET). Reported here are studies of isolated cerium oxide nanoparticles deposited on fluorine-doped tin oxide (FTO) electrodes with a Langmuir–Blodgett trough and calcined. Cyclic voltammograms of these films showed well-defined, quasi-reversible waves. The E1/2 values moved with pH by −64 ± 4 mV/pH, close to the ideal Nernstian −59 mV/pH for a 1e–/1H+ couple. These results imply that the electroactive CeO–H bonds had an average bond dissociation free energy (BDFE) of 78.6 ± 1.5 kcal mol–1. Integration of the faradaic currents indicates that 0.15 ± 0.04 electrons can be added per cerium atom in the nanoparticle film or ∼20% of the surface cerium atoms. This study shows how electrochemical investigations can elucidate the stoichiometry and thermochemistry of PCET processes of MOx nanoparticle films. Comparison of these results with those for related ceria nanoparticles shows the remarkable range of properties of this useful material.

Photoelectrochemical C-H Activation Through a Quinacridone Dye Enabling Proton-Coupled Electron Transfer.

Dye-sensitized photoanodes for C-H activation in organic substrates are assembled by vacuum sublimation of a commercially available quinacridone (QNC) dye in the form of nanosized rods onto fluorine-doped tin oxide (FTO), TiO2 , and SnO2 slides. The photoanodes display extended absorption in the visible range (450-600 nm) and ultrafast photoinduced electron injection (<1 ps, as revealed by transient absorption spectroscopy) of the QNC dye into the semiconductor. The proton-coupled electron-transfer reactivity of QNC is exploited for generating a nitrogen-based radical as its oxidized form, which is competent in C-H bond activation. The key reactivity parameter is the bond-dissociation free energy (BDFE) associated with the N⋅/N-H couple in QNC of 80.5±2.3 kcal mol-1 , which enables hydrogen atom abstraction from allylic or benzylic C-H moieties. A photoelectrochemical response is indeed observed for organic substrates characterized by C-H bonds with BDFE below the 80.5 kcal mol-1 threshold, such as γ-terpinene, xanthene, or dihydroanthracene. This work provides a rational, mechanistically oriented route to the design of dye-sensitized photoelectrodes for selective organic transformations.

Vanadium-Catalyzed Dinitrogen Reduction to Ammonia via a [V]═NNH2 Intermediate.

The catalytic transformation of N2 to NH3 by transition metal complexes is of great interest and importance but has remained a challenge to date. Despite the essential role of vanadium in biological N2 fixation, well-defined vanadium complexes that can catalyze the conversion of N2 to NH3 are scarce. In particular, a V(NxHy) intermediate derived from proton/electron transfer reactions of coordinated N2 remains unknown. Here, we report a dinitrogen-bridged divanadium complex bearing POCOP (2,6-(tBu2PO)2-C6H3) pincer and aryloxy ligands, which can serve as a catalyst for the reduction of N2 to NH3 and N2H4. Low-temperature protonation and reduction of the dinitrogen complex afforded the first structurally characterized neutral metal hydrazido(2-) species ([V]═NNH2), which mediated 15N2 conversion to 15NH3, indicating that it is a plausible intermediate of the catalysis. DFT calculations showed that the vanadium hydrazido complex [V]═NNH2 possessed a N-H bond dissociation free energy (BDFEN-H) of as high as 59.1 kcal/mol. The protonation of a vanadium amide complex ([V]-NH2) with [Ph2NH2][OTf] resulted in the release of NH3 and the formation of a vanadium triflate complex, which upon reduction under N2 afforded the vanadium dinitrogen complex. These transformations model the final steps of a vanadium-catalyzed N2 reduction cycle. Both experimental and theoretical studies suggest that the catalytic reaction may proceed via a distal pathway to liberate NH3. These findings provide unprecedented insights into the mechanism of N2 reduction related to FeV nitrogenase.

HAA by the first {Mn(iii)OH} complex with all O-donor ligands.

There is considerable interest in MnOHx moieties, particularly in the stepwise changes in those O-H bonds in tandem with Mn oxidation state changes. The reactivity of aquo-derived ligands, {MOHx}, is also heavily influenced by the electronic character of the other ligands. Despite the prevalence of oxygen coordination in biological systems, preparation of mononuclear Mn complexes of this type with all O-donors is rare. Herein, we report several Mn complexes with perfluoropinacolate (pinF)2- including the first example of a crystallographically characterized mononuclear {Mn(iii)OH} with all O-donors, K2[Mn(OH)(pinF)2], 3. Complex 3 is prepared via deprotonation of K[Mn(OH2)(pinF)2], 1, the pKa of which is estimated to be 18.3 ± 0.3. Cyclic voltammetry reveals quasi-reversible redox behavior for both 1 and 3 with an unusually large ΔEp, assigned to the Mn(iii/ii) couple. Using the Bordwell method, the bond dissociation free energy (BDFE) of the O-H bond in {Mn(ii)-OH2} is estimated to be 67-70 kcal mol-1. Complex 3 abstracts H-atoms from 1,2-diphenylhydrazine, 2,4,6-TTBP, and TEMPOH, the latter of which supports a PCET mechanism. Under basic conditions in air, the synthesis of 1 results in K2[Mn(OAc)(pinF)2], 2, proposed to result from the oxidation of Et2O to EtOAc by a reactive Mn species, followed by ester hydrolysis. Complex 3 alone does not react with Et2O, but addition of O2 at low temperature effects the formation of a new chromophore proposed to be a Mn(iv) species. The related complexes K(18C6)[Mn(iii)(pinF)2], 4, and (Me4N)2[Mn(ii)(pinF)2], 5, have also been prepared and their properties discussed in relation to complexes 1-3.

Catalytic reduction of dinitrogen to ammonia using molybdenum porphyrin complexes.

Porphyrin complexes are well-known in O2 and CO2 reduction, but their application to N2 reduction is less developed. Here, we show that oxo and nitrido complexes of molybdenum supported by tetramesitylporphyrin (TMP) are effective precatalysts for catalytic N2 reduction to ammonia, verified by 15N2 labeling studies and other control experiments. Spectroscopic and electrochemical studies illuminate some relevant thermodynamic parameters, including the N-H bond dissociation free energy of (TMP)MoNH (43 ± 2 kcal mol-1). We place these results in the context of other work on homogeneous N2 reduction catalysis.