Mn-Based Molecular Catalysts for the Electrocatalytic Disproportionation of CO2 into CO and CO32–

trans-Di-bromido-tetra-kis-(5-methyl-1H-pyrazole-κN2)manganese(II)

The title compound, trans-di-bromido-tetra-kis-(5-methyl-1H-pyrazole-κN 2)manganese(II), [MnBr2(C4H6N2)4] or [Mn(3-MePzH)4Br2] (1) crystallizes in the triclinic P space group with the cell parameters a = 7.6288 (3), b = 8.7530 (4), c = 9.3794 (4) Å and α = 90.707 (4), β = 106.138 (4), γ = 114.285 (5)°, V = 542.62 (5) Å3, T = 120 K. The asymmetric unit contains only half the mol-ecule with the manganese atom is situated on a crystallographic inversion center. The 3-MePzH ligands are present in an AABB type manner with two methyl groups pointing up and the other two down. The supra-molecular architecture is characterized by several inter-molecular C-H⋯N, N-H⋯Br, and C-H⋯π inter-actions. Earlier, a polymorphic structure of [Mn(3-MePzH)4Br2] (2) with a similar geometry and also an AABB arrangement for the pyrazole ligands was described [Reedijk et al. (1971 ▸). Inorg. Chem. 10, 2594-2599; a = 8.802 (6), b = 9.695 (5), c = 7.613 (8) Å and α = 105.12 (4), β = 114.98 (4), γ = 92.90 (3)°, V = 558.826 (5) Å3, T = 295 K]. A varying supra-molecular pattern was reported, with the structure of 1 featuring a herringbone type pattern while that of structure 2 shows a pillared network type of arrangement along the a axis. A nickel complex [Ni(3-MePzH)4Br2] isomorphic to 1 and the analogous chloro derivatives of FeII, CoII and CuII are also known.

Inept N2 Activation of Tri-Nuclear Nickel Complex with Labile Sulfur Ligands Facilitates Selective N2 H4 Formation in Electrocatalytic Conversion of N2

Conversion of N2 to the energy vector N2 H4 under benign conditions is highly desirable. However, such N2 fixation processes are extremely rare. It has been recently reported that N2 to N2 H4 conversion can be achieved electrochemically by using a trinuclear [Ni3 (S2 C3 H6 )4 ]2- complex (named as [Ni3 S8 ]2- ). There are hardly any precedents of Nitrogen Reduction Reaction (NRR) by molecular catalysts having Ni and the highly unusual selectivity for N2 H4 over NH3 makes this electrochemical reduction unique. A systematic theoretical study employing calibrated Density Functional Theory to unearth the mechanisms of NRR (4e- /4H+ ) and Hydrogen Evolution Reaction (2e- /2H+ ) was conducted for the aforementioned trinuclear Ni complex. Our findings unravel a curious case of ligand lability working in tandem with metal centers in facilitating this unprecedented electrocatalytic activity. Furthermore, it is shown that the poor N-N bond activation property of Ni is responsible for this unusual selectivity. Additionally, the Hydrogen Evolution Reaction (HER) mechanistic pathways have also been delineated in this report. The mechanistic intricacies thus unearthed in this study may assist in developing more efficient electrocatalysts for N2 H4 production through NRR.

On the Temperature Sensitivity of Electrochemical Reaction Thermodynamics

Herein, we report a method to estimate the thermodynamic potentials of electrochemical reactions at different temperatures. We use a two-term Taylor series approximation of thermodynamic potential as a function of temperature, and we calculate the temperature sensitivity for a family of twenty seven known half reactions. We further analyze pairs of cathode and anode half-cells to pinpoint optimal voltage matches and discuss implications of changes in temperature on overall cell voltages. Using these observations, we look forward to increased interest in temperature and idealized half-reaction pairing as experimental choices for the optimization of electrochemical processes.

Impacts of anions on activated persulfate oxidation of Fe(II) - Rich potassium doped magnetic biochar

The potassium-doped magnetic biochar (KMBC) preparation was inevitably introduced the different anions in the process of modifying magnetic biochar (MBC) with different potassium salts, but the effect and mechanism of different anion on KMBC activation properties has not been reported. Therefore, in this paper, five different KMBCs were prepared using several common potassium salts under the same dosage of K⁺ and Fe²⁺, and then was added in the presence of persulfate (PS) for the removal of metronidazole (MNZ). The removal rate of metronidazole was ordered as KMBC_K2SO4 (98.40%) > KMBC_KNO3 (76.84%) > KMBC_KCl (20.79%) > KMBC_K2CO3 (19.02%) > KMBC_K2C2O4 (14.23%). However, the semi-quantitative of Fe(II) experiments results confirmed that the effectively increase of Fe(II) content by potassium salts modification played the dominant role in improvement of KMBC activation performance. The Fe(II) content of KMBC were ordered as KMBC_K2CO3 > KMBC_K2SO4 > KMBC_KNO3 > KMBC_KCl > KMBC_K2C2O4, with the Fe(II) content of KMBC of 36.74, 17.70, 8.79, 5.24 and 4.85 mg/g, respectively. The indicated that the introduction of different anions would lead to different optimal Fe(Ⅱ) content in KMBC modified with different potassium salts, which was most directly reflected in ¹O₂ content in different KMBC/PS systems, and account for the difference in MNZ degradation efficiency. Meanwhile, when the Fe(II) content in KMBC reached the range of 13.7-28.8 mg/g, KMBC had the better performance of activating PS.

Catalysis of CO2 reduction by diazapyridinophane complexes of Fe, Co, and Ni: CO2 binding triggered by combined frontier MO associations involving a SOMO

Our previous study on the photochemical CO₂ reduction into CO catalyzed by the diazapyridinophane complexes of Fe, Co, and Ni revealed that (i) the Co catalyst shows the highest TOF but degrades rapidly, (ii) the Fe catalyst exhibits a lower TOF relative to Co but shows higher robustness, giving a higher TON, and (iii) the Ni complex shows no activity (Sakaguchi et al., Chem. Commun., 2019, 55, 8552). Here we show our DFT results unveiling that the Fe and Co catalysts can utilize multiple sets of frontier MO associations at the CO₂ binding by including one of the SOMOs in a high-spin d⁷ Fe(I) and d⁸ Co(I) center, respectively, giving an increased driving force for these oxidative addition steps. Remarkably, two-electron reduction of CO₂ to CO₂^2- at the binding step is driven by the two electrons transferred from different d-based orbitals. The Co^I species binds CO₂ at the rate-limiting step with an activation barrier of 15.0 kcal mol^-1, rationalizing the high initial TOF observed. However, the Co^I(CO) species is given as a dead-end product, consistent with its relatively rapid deactivation. The Fe catalyst possesses a slightly higher barrier in CO₂ binding (ΔG^‡ = 15.8 kcal mol^-1) but does not stabilize the Fe^I(CO) species which readily releases CO (ΔG = 3.5 kcal mol^-1). The Ni catalyst has the smallest barrier in CO₂ binding (ΔG^‡ = 11.5 kcal mol^-1) but the CO release is largely prohibited by the dead-end Ni^I(CO) species, consistent with its inactive character towards CO₂ reduction. The combined results all satisfactorily explain the observed catalytic behaviors.

Reaction of a Molybdenum Bis(dinitrogen) Complex with Carbon Dioxide: A Combined Experimental and Computational Investigation

Refluxing Mo(CO)₆ in the presence of the phosphine-functionalized α-diimine ligand ^Ph2PPrDI allowed for substitution and formation of the dicarbonyl complex, (^Ph2PPrDI)Mo(CO)₂. Oxidation with I₂ followed by heating resulted in further CO dissociation and isolation of the corresponding diiodide complex, (^Ph2PPrDI)MoI₂. Reduction of this complex under a N₂ atmosphere afforded the corresponding bis(dinitrogen) complex, (^Ph2PPrDI)Mo(N₂)₂. The solid-state structures of all three compounds were found to feature a tetradentate chelate and cis-monodentate ligands. Notably, the addition of CO₂ to (^Ph2PPrDI)Mo(N₂)₂ is proposed to result in head-to-tail CO₂ coupling to generate the corresponding metallacycle and ultimately a mixture of (^Ph2PPrDI)Mo(CO)₂ and the bis(oxo) dimer, [(κ³-^Ph2PPrDI)Mo(O)(μ-O)]₂. Computational studies have been performed to gain insight into the reaction and evaluate the importance of cis-coordination sites for selective head-to-tail CO₂ reductive coupling, CO deinsertion, disproportionation, and stepwise CO₂ deinsertion.

Transition Metal Complexes as Catalysts for the Electroconversion of CO2 : An Organometallic Perspective

The electrocatalytic transformation of carbon dioxide has been a topic of interest in the field of CO2 utilization for a long time. Recently, the area has seen increasing dynamics as an alternative strategy to catalytic hydrogenation for CO2 reduction. While many studies focus on the direct electron transfer to the CO2 molecule at the electrode material, molecular transition metal complexes in solution offer the possibility to act as catalysts for the electron transfer. C1 compounds such as carbon monoxide, formate, and methanol are often targeted as the main products, but more elaborate transformations are also possible within the coordination sphere of the metal center. This perspective article will cover selected examples to illustrate and categorize the currently favored mechanisms for the electrochemically induced transformation of CO2 promoted by homogeneous transition metal complexes. The insights will be corroborated with the concepts and elementary steps of organometallic catalysis to derive potential strategies to broaden the molecular diversity of possible products.

Mechanistic Insights into Co and Fe Quaterpyridine-Based CO2 Reduction Catalysts: Metal-Ligand Orbital Interaction as the Key Driving Force for Distinct Pathways

Both [Co^II(qpy)(H₂O)₂]²⁺ and [Fe^II(qpy)(H₂O)₂]²⁺ (with qpy = 2,2':6',2″:6'',2‴-quaterpyridine) are efficient homogeneous electrocatalysts and photoelectrocatalysts for the reduction of CO₂ to CO. The Co catalyst is more efficient in the electrochemical reduction, while the Fe catalyst is an excellent photoelectrocatalyst ( ACS Catal. 2018, 8, 3411-3417). This work uses density functional theory to shed light on the contrasting catalytic pathways. While both catalysts experience primarily ligand-based reductions, the second reduction in the Co catalyst is delocalized onto the metal via a metal-ligand bonding interaction, causing a spin transition and a distorted ligand framework. This orbital interaction explains the experimentally observed mild reduction potential and slow kinetics of the second reduction. The decreased hardness and doubly occupied d_z²-orbital facilitate a σ-bond with the CO₂-π* in an η¹-κC binding mode. CO₂ binding is only possible after two reductions resulting in an EEC mechanism (E = electron transfer, C = chemical reaction), and the second protonation is rate-limiting. In contrast, the Fe catalyst maintains a Lewis acidic metal center throughout the reduction process because the metal orbitals do not strongly mix with the qpy-π* orbitals. This allows binding of the activated CO₂ in an η²-binding mode. This interaction stabilizes the activated CO₂ via a π-type interaction of a Fe-t_2g orbital and the CO₂-π* and a dative bond of the oxygen lone pair. This facilitates CO₂ binding to a singly reduced catalyst resulting in an ECE mechanism. The barrier for CO₂ addition and the second protonation are higher than those for the Co catalyst and rate-limiting.

MnI complex redox potential tunability by remote lewis acid interaction

In this work we provide direct experimental evidence on the correlation of remote interactions between a newly synthesized MnI-complex (1) and different alkali cations with redox potential tuning. Furthermore we report the electrochemical behavior of 1 towards carbon dioxide, including the effects of added alkali salts using cyclic voltammetry.

Redox-Active Manganese Pincers for Electrocatalytic CO2 Reduction

The decrease of total amount of atmospheric CO2 is an important societal challenge in which CO2 reduction has an important role to play. Electrocatalytic CO2 reduction with homogeneous catalysts is based on highly tunable catalyst design and exploits an abundant C1 source to make valuable products such as fuels and fuel precursors. These methods can also take advantage of renewable electricity as a green reductant. Mn-based catalysts offer these benefits while incorporating a relatively cheap and abundant first-row transition metal. Historically, interest in this field started with Mn(bpy-R)(CO)3X, whose performance matched that of its Re counterparts while achieving substantially lower overpotentials. This review examines an emerging class of homogeneous Mn-based electrocatalysts for CO2 reduction, Mn complexes with meridional tridentate coordination also known as Mn pincers, most of which contain redox-active ligands that enable multi-electron catalysis. Although there are relatively few examples in the literature thus far, these catalysts bring forth new catalytic mechanisms not observed for the well-established Mn(bpy-R)(CO)3X catalysts, and show promising reactivity for future studies.

Chemical and Electrochemical Recycling of End-Use Poly(ethylene terephthalate) (PET) Plastics in Batch, Microwave and Electrochemical Reactors

This work describes new methods for the chemical recycling of end-use poly(ethylene terephthalate) (PET) in batch, microwave and electrochemical reactors. The reactions are based on basic hydrolysis of the ester moieties in the polymer framework and occur under mild reaction conditions with low-cost reagents. We report end-use PET depolymerization in refluxing methanol with added NaOH with 75% yield of terephthalic acid in batch after 12 h, while yields up to 65% can be observed after only 40 min under microwave irradiation at 85 °C. Using basic conditions produced in the electrochemical reduction of protic solvents, electrolytic experiments have been shown to produce 17% terephthalic acid after 1 h of electrolysis at −2.2 V vs. Ag/AgCl in 50% water/methanol mixtures with NaCl as a supporting electrolyte. The latter method avoids the use of caustic solutions containing high-concentration NaOH at the outset, thus proving the concept for a novel, environmentally benign method for the electrochemical recycling of end-use PET based on low-cost solvents (water and methanol) and reagents (NaCl and electricity).

Metalloradical intermediates in electrocatalytic reduction of CO2 to CO: Mn versus Re bis-N-heterocyclic carbene pincers

This work examines the relative reactivities of Re^I and Mn^I tricarbonyl pyridine-2,6-bis-N-heterocyclic carbene pincers M(CO)₃CNC^BnX (M = Re, Mn and X = Cl and Br) towards catalysis for the electrochemical conversion of CO₂ to CO. Unlike prior well-studied group VII catalysts, Mn(CO)₃CNC^BnX is extraordinarily active, while the new Re(CO)₃CNC^BnX complex surprisingly does not exhibit catalytic response. DFT calculations shed light on this puzzling behavior and show that the redox-active pyridine-2,6-bis-N-heterocyclic carbene ligand facilitates the reduction of the ground-state complexes; however, the extent of electronic delocalization in the reduced intermediates differs in the degree of metalloradical character. The highly-active Mn(CO)₃CNC^BnX complex proceeds through an intermediate with nucleophilic metalloradical character in which 66% of the unpaired electron spin resides on Mn. In contrast, Re(CO)₃CNC^BnX reduction proceeds through an intermediate with less metalloradical character in which only 38% of the unpaired spin is localized on Re with the remainder delocalized over the ligand. The energetic penalty of the electron delocalization of an electron on the ligand affects the M-CO bond strengths and related kinetic barriers. We discuss these observations in the context of turnover-enabling effects in CO₂ reductions mediated by group VII NHC pincer molecular electrocatalysts.

Transition metal-based catalysts for the electrochemical CO2 reduction: from atoms and molecules to nanostructured materials

An overview of the main strategies for the rational design of transition metal-based catalysts for the electrochemical conversion of CO₂, ranging from molecular systems to single-atom and nanostructured catalysts.