Automated Image Analysis for Single-Atom Detection in Catalytic Materials by Transmission Electron Microscopy.

Single-atom catalytic sites may have existed in all supported transition metal catalysts since their first application. Yet, interest in the design of single-atom heterogeneous catalysts (SACs) only really grew when advances in transmission electron microscopy (TEM) permitted direct confirmation of metal site isolation. While atomic-resolution imaging remains a central characterization tool, poor statistical significance, reproducibility, and interoperability limit its scope for deriving robust characteristics about these frontier catalytic materials. Here, we introduce a customized deep-learning method for automated atom detection in image analysis, a rate-limiting step toward high-throughput TEM. Platinum atoms stabilized on a functionalized carbon support with a challenging irregular three-dimensional morphology serve as a practically relevant test system with promising scope in thermo- and electrochemical applications. The model detects over 20,000 atomic positions for the statistical analysis of important properties for establishing structure-performance relations over nanostructured catalysts, like the surface density, proximity, clustering extent, and dispersion uniformity of supported metal species. Good performance obtained on direct application of the model to an iron SAC based on carbon nitride demonstrates its generalizability for single-atom detection on carbon-related materials. The approach establishes a route to integrate artificial intelligence into routine TEM workflows. It accelerates image processing times by orders of magnitude and reduces human bias by providing an uncertainty analysis that is not readily quantifiable in manual atom identification, improving standardization and scalability.

Design of Iron Coordination Complexes as Highly Active Homogenous Water Oxidation Catalysts by Deuteration of Oxidation-Sensitive Sites.

The nature of the oxidizing species in water oxidation reactions with chemical oxidants catalyzed by α-[Fe(OTf)2(mcp)] (1α; mcp = N, N'-dimethyl- N, N'-bis(pyridin-2-ylmethyl)cyclohexane-1,2-diamine, OTf = trifluoromethanesulfonate anion) and ß-[Fe(OTf)2(mcp)] (1ß) has been investigated. Mössbauer spectroscopy provides definitive evidence that 1α and 1ß generate oxoiron(IV) species as the resting state. Decomposition paths of the catalysts have been investigated by identifying and quantifying ligand fragments that form upon degradation. This analysis correlates the water oxidation activity of 1α and 1ß with stability against oxidative damage of the ligand via aliphatic C-H oxidation. The site of degradation and the relative stability against oxidative degradation are shown to be dependent on the topology of the catalyst. Furthermore, the mechanisms of catalyst degradation have been rationalized by computational analyses, which also explain why the topology of the catalyst enforces different oxidation-sensitive sites. This information has served in creating catalysts where sensitive C-H bonds have been replaced by C-D bonds. The deuterated analogues D4-α-[Fe(OTf)2(mcp)] (D4-1α), D4-ß-[Fe(OTf)2(mcp)] (D4-1ß), and D6-ß-[Fe(OTf)2(mcp)] (D6-1ß) were prepared, and their catalytic activity has been studied. D4-1α proves to be an extraordinarily active and efficient catalyst (up to 91% of O2 yield); it exhibits initial reaction rates identical with those of its protio analogue, but it is substantially more robust toward oxidative degradation and yields more than 3400 TON ( n(O2)/ n(Fe)). Altogether this evidences that the water oxidation catalytic activity is performed by a well-defined coordination complex and not by iron oxides formed after oxidative degradation of the ligands.

Acid-Triggered O-O Bond Heterolysis of a Nonheme FeIII (OOH) Species for the Stereospecific Hydroxylation of Strong C-H Bonds.

A novel hydroperoxoiron(III) species [FeIII (OOH)(MeCN)(PyNMe3 )]2+ (3) has been generated by reaction of its ferrous precursor [FeII (CF3 SO3 )2 (PyNMe3 )] (1) with hydrogen peroxide at low temperatures. This species has been characterized by several spectroscopic techniques and cryospray mass spectrometry. Similar to most of the previously described low-spin hydroperoxoiron(III) compounds, 3 behaves as a sluggish oxidant and it is not kinetically competent for breaking weak C-H bonds. However, triflic acid addition to 3 causes its transformation into a much more reactive compound towards organic substrates that is capable of oxidizing unactivated C-H bonds with high stereospecificity. Stopped-flow kinetic analyses and theoretical studies provide a rationale for the observed chemistry, a triflic-acid-assisted heterolytic cleavage of the O-O bond to form a putative strongly oxidizing oxoiron(V) species. This mechanism is reminiscent to that observed in heme systems, where protonation of the hydroperoxo intermediate leads to the formation of the high-valent [(Porph. )FeIV (O)] (Compound I).

Trapping of superoxido cobalt and peroxido dicobalt species formed reversibly from CoII and O2.

The formation and spectroscopic characterization of a superoxido cobalt(iii) and a peroxido dicobalt(iii) species formed in the temperature dependent reversible reaction of a cobalt(ii) precursor with O2 is described. The electronic nature of each species is explored in their reactivity with organic substrates.

A CuI/CuIII prototypical organometallic mechanism for the deactivation of an active pincer-like CuI catalyst in Ullmann-type couplings.

Unraveling the mechanistic details of copper-catalyzed arylation of nucleophiles (Ullmann-type couplings) is a very challenging task. It is a matter of intense debate whether it is a radical-based process or an organometallic redox-based process. The ancillary ligand choice in Ullmann-type couplings plays a key role in such transformations and can strongly influence the catalytic efficiency as well as the mechanism. Here, we show how a predesigned tridentate pincer-like catalyst undergoes a deactivation pathway through a CuI/CuIII prototypical mechanism as demonstrated by helium-tagging infrared photodissociation (IRPD) spectroscopy and DFT studies, lending a strong support to the existence of an aryl-CuIII species in the Ullmann couplings using this tridentate ligand.

Dual cobalt-copper light-driven catalytic reduction of aldehydes and aromatic ketones in aqueous media.

We present an efficient, general, fast, and robust light-driven methodology based on earth-abundant elements to reduce aryl ketones, and both aryl and aliphatic aldehydes (up to 1400 TON). The catalytic system consists of a robust and well-defined aminopyridyl cobalt complex active for photocatalytic water reduction and the [Cu(bathocuproine)(Xantphos)](PF6) photoredox catalyst. The dual cobalt-copper system uses visible light as the driving-force and H2O and an electron donor (Et3N or iPr2EtN) as the hydride source. The catalytic system operates in aqueous mixtures (80-60% water) with high selectivity towards the reduction of organic substrates (>2000) vs. water reduction, and tolerates O2. High selectivity towards the hydrogenation of aryl ketones is observed in the presence of terminal olefins, aliphatic ketones, and alkynes. Remarkably, the catalytic system also shows unique selectivity for the reduction of acetophenone in the presence of aliphatic aldehydes. The catalytic system provides a simple and convenient method to obtain α,ß-deuterated alcohols. Both the observed reactivity and the DFT modelling support a common cobalt hydride intermediate. The DFT modelled energy profile for the [Co-H] nucleophilic attack to acetophenone and water rationalises the competence of [CoII-H] to reduce acetophenone in the presence of water. Mechanistic studies suggest alternative mechanisms depending on the redox potential of the substrate. These results show the potential of the water reduction catalyst [Co(OTf)(Py2Tstacn)](OTf) (1), (Py2Tstacn = 1,4-di(picolyl)-7-(p-toluenesulfonyl)-1,4,7-triazacyclononane, OTf = trifluoromethanesulfonate anion) to develop light-driven selective organic transformations and fine solar chemicals.

Rapid Hydrogen and Oxygen Atom Transfer by a High-Valent Nickel-Oxygen Species.

Terminal high-valent metal-oxygen species are key reaction intermediates in the catalytic cycle of both enzymes (e.g., oxygenases) and synthetic oxidation catalysts. While tremendous efforts have been directed toward the characterization of the biologically relevant terminal manganese-oxygen and iron-oxygen species, the corresponding analogues based on late-transition metals such as cobalt, nickel or copper are relatively scarce. This scarcity is in part related to the "Oxo Wall" concept, which predicts that late transition metals cannot support a terminal oxido ligand in a tetragonal environment. Here, the nickel(II) complex (1) of the tetradentate macrocyclic ligand bearing a 2,6-pyridinedicarboxamidate unit is shown to be an effective catalyst in the chlorination and oxidation of C-H bonds with sodium hypochlorite as terminal oxidant in the presence of acetic acid (AcOH). Insight into the active species responsible for the observed reactivity was gained through the study of the reaction of 1 with ClO- at low temperature by UV-vis absorption, resonance Raman, EPR, ESI-MS, and XAS analyses. DFT calculations aided the assignment of the trapped chromophoric species (3) as a nickel-hypochlorite species. Despite the fact that the formal oxidation state of the nickel in 3 is +4, experimental and computational analysis indicate that 3 is best formulated as a NiIII complex with one unpaired electron delocalized in the ligands surrounding the metal center. Most remarkably, 3 reacts rapidly with a range of substrates including those with strong aliphatic C-H bonds, indicating the direct involvement of 3 in the oxidation/chlorination reactions observed in the 1/ClO-/AcOH catalytic system.

Oxidant-Free Au(I)-Catalyzed Halide Exchange and C(sp2)-O Bond Forming Reactions.

Au has been demonstrated to mediate a number of organic transformations through the utilization of its π Lewis acid character, Au(I)/Au(III) redox properties or a combination of both. As a result of the high oxidation potential of the Au(I)/Au(III) couple, redox catalysis involving Au typically requires the use of a strong external oxidant. This study demonstrates unusual external oxidant-free Au(I)-catalyzed halide exchange (including fluorination) and Csp2-O bond formation reactions utilizing a model aryl halide macrocyclic substrate. Additionally, the halide exchange and Csp2-O coupling reactivity could also be extrapolated to substrates bearing a single chelating group, providing further insight into the reaction mechanism. This work provides the first examples of external oxidant-free Au(I)-catalyzed carbon-heteroatom cross-coupling reactions.

H2 oxidation versus organic substrate oxidation in non-heme iron mediated reactions with H2O2.

Herein we show that species generated upon reaction of α-[Fe(CF3SO3)2(BPMCN)] (BPMCN = N,N'-bis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane) with H2O2 (putatively [Fe(V)(O)(OH)(BPMCN)]) is able to efficiently oxidize H2 to H2O even in the presence of organic substrates, while species formed in the presence of acetic acid (putatively [Fe(V)(O)(OAc)(BPMCN)]) prefer organic substrate oxidation over H2 activation. Mechanistic implications have been analysed with the aid of computational methods.

Reactivity of a Nickel(II) Bis(amidate) Complex with meta-Chloroperbenzoic Acid: Formation of a Potent Oxidizing Species.

Herein, we report the formation of a highly reactive nickel-oxygen species that has been trapped following reaction of a Ni(II) precursor bearing a macrocyclic bis(amidate) ligand with meta-chloroperbenzoic acid (HmCPBA). This compound is only detectable at temperatures below 250 K and is much more reactive toward organic substrates (i.e., C-H bonds, C=C bonds, and sulfides) than previously reported well-defined nickel-oxygen species. Remarkably, this species is formed by heterolytic O-O bond cleavage of a Ni-HmCPBA precursor, which is concluded from experimental and computational data. On the basis of spectroscopy and DFT calculations, this reactive species is proposed to be a Ni(III) -oxyl compound.

Aryl-copper(III)-acetylides as key intermediates in Csp2-Csp model couplings under mild conditions.

The mechanism of copper-mediated Sonogashira couplings (so-called Stephens-Castro and Miura couplings) is not well understood and lacks clear comprehension. In this work, the reactivity of a well-defined aryl-Cu(III) species (1ClO4) with p-R-phenylacetylenes (R = NO2, CF3, H) is reported and it is found that facile reductive elimination from a putative aryl-Cu(III)-acetylide species occurs at room temperature to afford the Caryl-Csp coupling species (IR), which in turn undergo an intramolecular reorganisation to afford final heterocyclic products containing 2H-isoindole (P NO2, P CF3, PHa) or 1,2-dihydroisoquinoline (PHb) substructures. Density Functional Theory (DFT) studies support the postulated reductive elimination pathway that leads to the formation of C sp2-Csp bonds and provide the clue to understand the divergent intramolecular reorganisation when p-H-phenylacetylene is used. Mechanistic insights and the very mild experimental conditions to effect Caryl-Csp coupling in these model systems provide important insights for developing milder copper-catalysed Caryl-Csp coupling reactions with standard substrates in the future.

Direct observation of two-electron Ag(I)/Ag(III) redox cycles in coupling catalysis.

Silver is extensively used in homogeneous catalysis for organic synthesis owing to its Lewis acidity, and as a powerful one-electron oxidant. However, two-electron redox catalytic cycles, which are most common in noble metal organometallic reactivity, have never been considered. Here we show that a Ag(I)/Ag(III) catalytic cycle is operative in model C-O and C-C cross-coupling reactions. An aryl-Ag(III) species is unequivocally identified as an intermediate in the catalytic cycle and we provide direct evidence of aryl halide oxidative addition and C-N, C-O, C-S, C-C and C-halide bond-forming reductive elimination steps at monometallic silver centres. We anticipate our study as the starting point for expanding Ag(I)/Ag(III) redox chemistry into new methodologies for organic synthesis, resembling well-known copper or palladium cross-coupling catalysis. Furthermore, findings described herein provide unique fundamental mechanistic understanding on Ag-catalysed cross-coupling reactions and dismiss the generally accepted conception that silver redox chemistry can only arise from one-electron processes.

Theoretical study of the water oxidation mechanism with non-heme Fe(Pytacn) iron complexes. Evidence that the Fe(IV)(O)(Pytacn) species cannot react with the water molecule to form the O-O bond.

Recent studies have shown that non-heme iron complexes [Fe(L(N4))X2], where L(N4) stands for a tetradentate nitrogen based aminopyridine ligand (L(N4) = Pytacn, mcp or mep, Pytacn = 1-(2-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane, mcp = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)cyclohexane-trans-1,2-diamine, mep = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)ethylendiamine), and X are monodentate ligands (X = Cl, CH3CN, CF3SO3(-), or H2O), catalyze the oxidation of water using cerium(IV) ammonium nitrate (CAN) as oxidant. Spectroscopic monitoring of catalytic water oxidation with [Fe(CF3SO3)2(Pytacn)] established [Fe(IV)(O)(OH2)(Pytacn)](2+) as an intermediate along the catalytic pathway, raising the question if these high valent species could be directly responsible for the O-O bond formation event. Herein, this question is addressed by a computational analysis of the thermodynamic and kinetic parameters associated with the reaction of non-heme iron complexes [Fe(IV)(O)(OH)(Pytacn)](+), [Fe(IV)(O)(OH2)(Pytacn)](2+), and [Fe(IV)(OH)(OH)(Pytacn)](2+) with water. Two different mechanisms have been studied for [Fe(IV)(O)(OH)(Pytacn)](+); the nucleophilic water attack assisted by the hydroxyl group as internal base, which is the lowest energy path, and the external nucleophilic water attack. For [Fe(IV)(OH)(OH)(Pytacn)](2+), only the attack assisted by the internal base has been studied, while in the case of [Fe(IV)(O)(OH2)(Pytacn)](2+), the only viable mechanism is the external nucleophilic water attack. Up to four water molecules were needed to be included in modeling of the O-O bond formation event for a proper description of the external nucleophilic water attack. The lowest Gibbs energy barrier and reaction free energy found for the direct water nucleophilic attack to the oxo ligand are of 32.2 and 28.3 kcal·mol(-1) for [Fe(IV)(O)(OH)(Pytacn)](+), 52.0 and 40.5 kcal·mol(-1) for [Fe(IV)(O)(OH2)(Pytacn)](2+), and 28.3 and 28.3 kcal·mol(-1) for [Fe(IV)(OH)(OH)(Pytacn)](2+), respectively. These energy barriers and endergonic reaction energies are too high for the reaction to proceed and inconsistent with the relatively rapid reaction rates determined experimentally (ΔG(‡)(exp.) = 17.6 kcal·mol(-1)). Therefore, this study provides strong evidence against the O-O bond formation by these species. The energetic accessibilities of Fe(V)(O) and Fe(VI)(O) intermediates have also been investigated, showing that Fe(V) is the higher oxidation state accessible under catalytic conditions, consistent with our previous results.

Photo- and electrocatalytic H2 production by new first-row transition-metal complexes based on an aminopyridine pentadentate ligand.

The synthesis and characterisation of the pentadentate ligand 1,4-di(picolyl)-7-(p-toluenesulfonyl)-1,4,7-triazacyclononane (Py2(Ts)tacn) and their metal complexes of general formula [M(CF3SO3)(Py2(Ts)tacn)][CF3SO3], (M = Fe (1Fe), Co (1Co) and Ni (1Ni)) are reported. Complex 1Co presents excellent H2 photoproduction catalytic activity when using [Ir(ppy)2(bpy)]PF6 (PSIr) as photosensitiser (PS) and Et3N as electron donor, but 1Ni and 1Fe result in a low activity and a complete lack of it, respectively. On the other hand, all three complexes have excellent electrocatalytic proton reduction activity in acetonitrile, when using trifluoroacetic acid (TFA) as a proton source with moderate overpotentials for 1Co (0.59 V vs. SCE) and 1Ni (0.56 V vs. SCE) and higher for 1Fe (0.87 V vs. SCE). Under conditions of CH3CN/H2O/Et3N (3:7:0.2), 1Co (5 µM), with PSIr (100 µM) and irradiating at 447 nm gives a turnover number (TON) of 690 (n H2/n1Co) and initial turnover frequency (TOF) (TON×t(-1)) of 703 h(-1) for H2 production. It should be noted that 1Co retains 25 % of the catalytic activity for photoproduction of H2 in the presence of O2. The inexistence of a lag time for H2 evolution and the absence of nanoparticles during the first 30 min of the reaction suggest that the main catalytic activity observed is derived from a molecular system. Kinetic studies show that the reaction is -0.7 order in catalyst, and time-dependent diffraction light scattering (DLS) experiments indicate formation of metal aggregates and then nanoparticles, leading to catalyst deactivation. By a combination of experimental and computational studies we found that the lack of activity in photochemical water reduction by 1Fe can be attributed to the 1Fe (II/I) redox couple, which is significantly lower than the PSIr (III/II) , while for 1Ni the pKa value (-0.4) is too small in comparison with the pH (11.9) imposed by the use of Et3N as electron donor.

Unraveling the mechanism of water oxidation catalyzed by nonheme iron complexes.

Density functional theory (DFT) is employed to: 1) propose a viable catalytic cycle consistent with our experimental results for the mechanism of chemically driven (Ce(IV) ) O2 generation from water, mediated by nonheme iron complexes; and 2) to unravel the role of the ligand on the nonheme iron catalyst in the water oxidation reaction activity. To this end, the key features of the water oxidation catalytic cycle for the highly active complexes [Fe(OTf)2 (Pytacn)] (Pytacn: 1-(2'-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane; OTf: CF3 SO3 () ) (1) and [Fe(OTf)2 (mep)] (mep: N,N'-bis(2-pyridylmethyl)-N,N'-dimethyl ethane-1,2-diamine) (2) as well as for the catalytically inactive [Fe(OTf)2 (tmc)] (tmc: N,N',N'',N'''-tetramethylcyclam) (3) and [Fe(NCCH3 )((Me) Py2 CH-tacn)](OTf)2 ((Me) Py2 CH-tacn: N-(dipyridin-2-yl)methyl)-N',N''-dimethyl-1,4,7-triazacyclononane) (4) were analyzed. The DFT computed catalytic cycle establishes that the resting state under catalytic conditions is a [Fe(IV) (O)(OH2 )(LN4 )](2+) species (in which LN4 =Pytacn or mep) and the rate-determining step is the OO bond-formation event. This is nicely supported by the remarkable agreement between the experimental (ΔG(≠) =17.6±1.6 kcal mol(-1) ) and theoretical (ΔG(≠) =18.9 kcal mol(-1) ) activation parameters obtained for complex 1. The OO bond formation is performed by an iron(V) intermediate [Fe(V) (O)(OH)(LN4 )](2+) containing a cis-Fe(V) (O)(OH) unit. Under catalytic conditions (Ce(IV) , pH 0.8) the high oxidation state Fe(V) is only thermodynamically accessible through a proton-coupled electron-transfer (PCET) process from the cis-[Fe(IV) (O)(OH2 )(LN4 )](2+) resting state. Formation of the [Fe(V) (O)(LN4 )](3+) species is thermodynamically inaccessible for complexes 3 and 4. Our results also show that the cis-labile coordinative sites in iron complexes have a beneficial key role in the OO bond-formation process. This is due to the cis-OH ligand in the cis-Fe(V) (O)(OH) intermediate that can act as internal base, accepting a proton concomitant to the OO bond-formation reaction. Interplay between redox potentials to achieve the high oxidation state (Fe(V) O) and the activation energy barrier for the following OO bond formation appears to be feasible through manipulation of the coordination environment of the iron site. This control may have a crucial role in the future development of water oxidation catalysts based on iron.

Electronic effects on single-site iron catalysts for water oxidation.

Getting in tune: Systematic tuning of the electronic properties of modular non-heme iron coordination complexes can be used to extract important information on the reaction mechanism and intermediates, which, in turn, help to explain the activity of these systems as water oxidation catalysts.