Dichloromethane as C1 Synthon for the Photoredox Catalytic Cyclopropanation of Aromatic Olefins.

Dichloromethane, as a readily available and inexpensive C1 synthon is proposed as a powerful building block for cyclopropanation of alkenes under mild conditions. Herein, we report a highly efficient and versatile dual photoredox system, involving a nickel aminopyridine coordination complex and a photocatalyst, for the cyclopropanation of aromatic olefins using dichloromethane, under visible-light irradiation. The cyclopropanation protocol has been successfully applied at gram scale. Mechanistic studies suggest a Ni(II) pyridyl radical complex as the key intermediate for the homolytic cleavage of the Csp3-Cl bond, generating a chloromethyl radical that is captured by the olefin coupling partner. Our findings also highlight the versatility of this methodology. By directing the radical/polar crossover process, we were able to selectively drive the reaction towards either the formation of cyclopropyl derivatives or the corresponding non-cyclic alkyl chloride products. The methodology also successfully apply to geminal dichloroalkanes, including the formation of spiro[2,2] compounds. Moreover, our methodology extends to the synthesis of deuterium-labelled cyclopropanes, demonstrating its utility in isotopic labelling and broadening its applicability in chemical synthesis and drug development.

An Electrocatalytic Cascade Reaction for the Synthesis of Ketones Using CO2 as a CO Surrogate.

The construction of carbonyl compounds via carbonylation reactions using safe CO sources remains a long-standing challenge to synthetic chemists. Herein, we propose a catalyst cascade Scheme in which CO2 is used as a CO surrogate in the carbonylation of benzyl chlorides. Our approach is based on the cooperation between two coexisting catalytic cycles: the CO2-to-CO electroreduction cycle promoted by [Fe(TPP)Cl] (TPP=meso-tetraphenylporphyrin) and an electrochemical carbonylation cycle catalyzed by [Ni(bpy)Br2] (2,2'-bipyridine). As a proof of concept, this protocol allows for the synthesis of symmetric ketones from good to excellent yields in an undivided cell with non-sacrificial electrodes. The reaction can be directly scaled up to gram-scale and operates effectively at a CO2 concentration of 10 %, demonstrating its robustness. Our mechanistic studies based on cyclic voltammetry, IR spectroelectrochemistry and Density Functional Theory calculations suggest a synergistic effect between the two catalysts. The CO produced from CO2 reduction is key in the formation of the [Ni(bpy)(CO)2], which is proposed as the catalytic intermediate responsible for the C-C bond formation in the carbonylation steps.

Spectroelectrochemical Analysis of the Water Oxidation Mechanism on Doped Nickel Oxides.

Metal oxides and oxyhydroxides exhibit state-of-the-art activity for the oxygen evolution reaction (OER); however, their reaction mechanism, particularly the relationship between charging of the oxide and OER kinetics, remains elusive. Here, we investigate a series of Mn-, Co-, Fe-, and Zn-doped nickel oxides using operando UV-vis spectroscopy coupled with time-resolved stepped potential spectroelectrochemistry. The Ni2+/Ni3+ redox peak potential is found to shift anodically from Mn- < Co- < Fe- < Zn-doped samples, suggesting a decrease in oxygen binding energetics from Mn- to Zn-doped samples. At OER-relevant potentials, using optical absorption spectroscopy, we quantitatively detect the subsequent oxidation of these redox centers. The OER kinetics was found to have a second-order dependence on the density of these oxidized species, suggesting a chemical rate-determining step involving coupling of two oxo species. The intrinsic turnover frequency per oxidized species exhibits a volcano trend with the binding energy of oxygen on the Ni site, having a maximum activity of ∼0.05 s-1 at 300 mV overpotential for the Fe-doped sample. Consequently, we propose that for Ni centers that bind oxygen too strongly (Mn- and Co-doped oxides), OER kinetics is limited by O-O coupling and oxygen desorption, while for Ni centers that bind oxygen too weakly (Zn-doped oxides), OER kinetics is limited by the formation of oxo groups. This study not only experimentally demonstrates the relation between electroadsorption free energy and intrinsic kinetics for OER on this class of materials but also highlights the critical role of oxidized species in facilitating OER kinetics.

Photoredox Activation of Inert Alkyl Chlorides for the Reductive Cross-Coupling with Aromatic Alkenes.

The inertness of chloroalkanes has precluded them as coupling partners for cross-coupling reactions. Herein we disclose a general strategy for the activation of inert alkyl chlorides through photoredox catalysis and their use as coupling partners with alkenes. The catalytic system is formed by [Ni(OTf)(Py2 Ts tacn)](OTf) (1Ni ), which is responsible for the Csp3 -Cl bond activation, and [Ir(NMe2 bpy)(ppy)2 ]PF6, (PCIr NMe2 ), which is the photoredox catalyst. Combined experimental and theoretical studies show an in situ photogenerated NiI intermediate ([Ni(Py2 Ts tacn)]+ ) which is catalytically competent for the Csp3 -Cl bond cleavage via a SN 2 mechanism for primary alkyl chlorides, forming carbon-centered free radicals, which react with the olefin leading to the formation of the Csp3 -Csp3 bond. These results suggest inert alkyl chlorides can be electrophiles for developing new intermolecular strategies in which low-valent aminopyridine nickel complexes act as key catalytic species.

A Unified Electro- and Photocatalytic CO2 to CO Reduction Mechanism with Aminopyridine Cobalt Complexes.

A mechanistic understanding of electro- and photocatalytic CO2 reduction is crucial to develop strategies to overcome catalytic bottlenecks. In this regard, for a new CO2-to-CO reduction cobalt aminopyridine catalyst, a detailed experimental and theoretical mechanistic study is herein presented toward the identification of bottlenecks and potential strategies to alleviate them. The combination of electrochemistry and in situ spectroelectrochemistry together with spectroscopic techniques led us to identify elusive key electrocatalytic intermediates derived from complex [LN4Co(OTf)2] (1) (LN4 = 1-[2-pyridylmethyl]-4,7-dimethyl-1,4,7-triazacyclononane) such as a highly reactive cobalt(I) (1(I)) and a cobalt(I) carbonyl (1(I)-CO) species. The combination of spectroelectrochemical studies under CO2, 13CO2, and CO with DFT disclosed that 1(I) reacts with CO2 to form the pivotal 1(I)-CO intermediate at the 1(II/I) redox potential. However, at this reduction potential, the formation of 1(I)-CO restricts the electrocatalysis due to the endergonicity of the CO release step. In agreement with the experimentally observed CO2-to-CO electrocatalysis at the CoI/0 redox potential, computational studies suggested that the electrocatalytic cycle involves striking metal carbonyls. In contrast, under photochemical conditions, the catalysis smoothly proceeds at the 1(II/I) redox potential. Under the latter conditions, it is proposed that the electron transfer to form 1(I)-CO from 1(II)-CO is under diffusion control. Then, the CO release from 1(II)-CO is kinetically favored, facilitating the catalysis. Finally, we have found that visible-light irradiation has a positive impact under electrocatalytic conditions. We envision that light irradiation can serve as an effective strategy to circumvent the CO poisoning and improve the performance of CO2 reduction molecular catalysts.

Luminescent Rhenium(I)tricarbonyl Complexes Containing Different Pyrazoles and Their Successive Deprotonation Products: CO2 Reduction Electrocatalysts.

Cationic fac-[Re(CO)3(pz*H)(pypzH)]OTf (pz*H = pyrazole, pzH; 3,5-dimethylpyrazole, dmpzH; indazole, indzH; 3-(2-pyridyl)pyrazole, pypzH) were obtained from fac-[ReBr(CO)3(pypzH)] by halide abstraction with AgOTf and subsequent addition of the corresponding pyrazole. Successive deprotonation with Na2CO3 and NaOH gave neutral fac-[Re(CO)3(pz*H)(pypz)] and anionic Na{fac-[Re(CO)3(pz*)(pypz)]} complexes, respectively. Cationic fac-[Re(CO)3(pz*H)(pypzH)]OTf, neutral complexes fac-[Re(CO)3(pz*H)(pypz)], and fac-[Re(CO)3(pypz)2Na] were subjected to photophysical and electrochemical studies. They exhibit phosphorescent decays from a prevalently 3MLCT excited state with quantum yields (Φ) in the range between 0.03 and 0.58 and long lifetimes (τ from 220 to 869 ns). The electrochemical behavior in Ar atmosphere of cationic and neutral complexes indicates that the oxidation processes assigned to ReI → ReII occurs at lower potentials for the neutral complex compared to cationic complex. The reduction processes occur at the ligands and do not depend on the charge of the complexes. The electrochemical behavior in CO2 saturated media is consistent with CO2 electrocatalyzed reduction, where the values of the catalytic activity [icat(CO2)/icat(Ar)] ranged from 2.7 to 11.5 (compared to 8.1 for fac-[Re(CO)3Cl(bipy)] studied as a reference). Controlled potential electrolysis for the pyrazole cationic (3a) and neutral (4a) complexes after 1 h affords CO in faraday yields of 61 and 89%, respectively. These values are higher for indazole complexes and may be related to the acidity of the coordinated pyrazole.

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.

Reductive Cyclization of Unactivated Alkyl Chlorides with Tethered Alkenes under Visible-Light Photoredox Catalysis.

The chemical inertness of abundant and commercially available alkyl chlorides precludes their widespread use as reactants in chemical transformations. Presented in this work is a metallaphotoredox methodology to achieve the catalytic intramolecular reductive cyclization of unactivated alkyl chlorides with tethered alkenes. The cleavage of strong C(sp3 )-Cl bonds is mediated by a highly nucleophilic low-valent cobalt or nickel intermediate generated by visible-light photoredox reduction employing a copper photosensitizer. The high basicity and multidentate nature of the ligands are key to obtaining efficient metal catalysts for the functionalization of unactivated alkyl chlorides.

Spectroscopic and DFT Characterization of a Highly Reactive Nonheme FeV-Oxo Intermediate.

The reaction of [(PyNMe3)FeII(CF3SO3)2], 1, with excess peracetic acid at -40 °C generates a highly reactive intermediate, 2b(PAA), that has the fastest rate to date for oxidizing cyclohexane by a nonheme iron species. It exhibits an intense 490 nm chromophore associated with an S = 1/2 EPR signal having g-values at 2.07, 2.01, and 1.94. This species was shown to be in a fast equilibrium with a second S = 1/2 species, 2a(PAA), assigned to a low-spin acylperoxoiron(III) center. Unfortunately, contaminants accompanying the 2(PAA) samples prevented determination of the iron oxidation state by Mössbauer spectroscopy. Use of MeO-PyNMe3 (an electron-enriched version of PyNMe3) and cyclohexyl peroxycarboxylic acid as oxidant affords intermediate 3b(CPCA) with a Mössbauer isomer shift δ = -0.08 mm/s that indicates an iron(V) oxidation state. Analysis of the Mössbauer and EPR spectra, combined with DFT studies, demonstrates that the electronic ground state of 3b(CPCA) is best described as a quantum mechanical mixture of [(MeO-PyNMe3)FeV(O)(OC(O)R)]2+ (∼75%) with some FeIV(O)(•OC(O)R) and FeIII(OOC(O)R) character. DFT studies of 3b(CPCA) reveal that the unbound oxygen of the carboxylate ligand, O2, is only 2.04 Å away from the oxo group, O1, corresponding to a Wiberg bond order for the O1-O2 bond of 0.35. This unusual geometry facilitates reversible O1-O2 bond formation and cleavage and accounts for the high reactivity of the intermediate when compared to the rates of hydrogen atom transfer and oxygen atom transfer reactions of FeIII(OC(O)R) ferric acyl peroxides and FeIV(O) complexes. The interaction of O2 with O1 leads to a significant downshift of the Fe-O1 Raman frequency (815 cm-1) relative to the 903 cm-1 value predicted for the hypothetical [(MeO-PyNMe3)FeV(O)(NCMe)]3+ complex.

Bioinspired Electro-Organocatalytic Material Efficient for Hydrogen Production.

Commercial carbon fibers can be used as electrodes with high conductive surfaces in reduced devices. Oxidative treatment of such electrodes results in a chemically robust material with high catalytic activity for electrochemical proton reduction, enabling the measurement of quantitative faradaic yields (>95 %) and high current densities. Combination of experiments and DFT calculations reveals that the presence of carboxylic groups triggers such electrocatalytic activity in a bioinspired manner. Analogously to the known Hantzsch esters, the oxidized carbon fiber material is able to transfer hydrides, which can react with protons, generating H2 , or with organic substrates resulting in their hydrogenation. A plausible mechanism is proposed based on DFT calculations on model systems.