Photocatalytic CO2 Reduction Using Mixed Catalytic Systems Comprising an Iron Cation with Bulky Phenanthroline Ligands

This study reports on efficient photocatalytic CO2 reduction reactions using mixed catalytic systems of an Fe ion source and various 1,10-phenanthroline derivatives (R1R2p) as ligands in the presence of triethanolamine (TEOA). As the relatively bulky substituents at positions 2 and 9 of R1R2p weakened the ability to coordinate to the Fe ion, the Fe ion formed TEOA complexes. The free R1R2p accepted an electron from the reduced photosensitizer through proton-coupled electron transfer (PCET) using protons of TEOA dissolved in a CH3CN solution in a CO2 atmosphere as the initial step of the catalytic cycle. Although the mixed system of the nonsubstituted 1,10-phenanthroline generates a stable tris(phenanthroline)-Fe(II) complex in solution, this complex could not function as a CO2 reduction catalyst. The mechanism in which R1R2p interacts with the Fe ion after PCET was proposed for this efficient photocatalytic CO2 reduction. The proposed photocatalytic system using the 2,9-di-sec-butyl-phenanthroline ligand could produce CO with high efficiency (quantum yield of 8.2%) combined with a dinuclear Cu(I) complex as a photosensitizer.

Cleavage of Carbon Dioxide C=O Bond Promoted by Nickel-Boron Cooperativity in a PBP-Ni Complex

The synthesis and characterization of (tBu PBP)Ni(OAc) (5) by insertion of carbon dioxide into the Ni-C bond of (tBu PBP)NiMe (1) is presented. An unexpected CO2 cleavage process involving the formation of new B-O and Ni-CO bonds leads to the generation of a butterfly-structured tetra-nickel cluster (tBu PBOP)2 Ni4 (μ-CO)2 (6). Mechanistic investigation of this reaction indicates a reductive scission of CO2 by O-atom transfer to the boron atom via a cooperative nickel-boron mechanism. The CO2 activation reaction produces a three-coordinate (tBu P2 BO)Ni-acyl intermediate (A) that leads to a (tBu P2 BO)-NiI complex (B) via a likely radical pathway. The NiI species is trapped by treatment with the radical trap (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) to give (tBu P2 BO)NiII (η2 -TEMPO) (7). Additionally, 13 C and 1 H NMR spectroscopy analysis using 13 C-enriched CO2 provides information about the species involved in the CO2 activation process.

Isostructural bridging diferrous chalcogenide cores [FeII(μ-E)FeII] (E = O, S, Se, Te) with decreasing antiferromagnetic coupling down the chalcogenide series

Iron compounds containing a bridging oxo or sulfido moiety are ubiquitous in biological systems, but substitution with the heavier chalcogenides selenium and tellurium, however, is much rarer, with only a few examples reported to date. Here we show that treatment of the ferrous starting material [(^tBupyrpyrr₂)Fe(OEt₂)] (1-OEt₂) (^tBupyrpyrr₂ = 3,5-^tBu₂-bis(pyrrolyl)pyridine) with phosphine chalcogenide reagents E = PR₃ results in the neutral phosphine chalcogenide adduct series [(^tBupyrpyrr₂)Fe(EPR₃)] (E = O, S, Se; R = Ph; E = Te; R = ^tBu) (1-E) without any electron transfer, whereas treatment of the anionic starting material [K]₂[(^tBupyrpyrr₂)Fe₂(μ-N₂)] (2-N₂) with the appropriate chalcogenide transfer source yields cleanly the isostructural ferrous bridging mono-chalcogenide ate complexes [K]₂[(^tBupyrpyrr₂)Fe₂(μ-E)] (2-E) (E = O, S, Se, and Te) having significant deviation in the Fe-E-Fe bridge from linear in the case of E = O to more acute for the heaviest chalcogenide. All bridging chalcogenide complexes were analyzed using a variety of spectroscopic techniques, including ¹H NMR, UV-Vis electronic absorbtion, and ⁵⁷Fe Mössbauer. The spin-state and degree of communication between the two ferrous ions were probed via SQUID magnetometry, where it was found that all iron centers were high-spin (S = 2) Fe^II, with magnetic exchange coupling between the Fe^II ions. Magnetic studies established that antiferromagnetic coupling between the ferrous ions decreases as the identity of the chalcogen is tuned from O to the heaviest congener Te.

Nickel(II) Complexes of Tripodal Ligands as Catalysts for Fixation of Atmospheric CO2 as Organic Carbonates

The fixation of atmospheric CO₂ into value-added products is a promising methodology. A series of novel nickel(II) complexes of the type [Ni(L)(CH₃ CN)₂ ](BPh₄ )₂ 1-5, where L=N,N-bis(2-pyridylmethyl)-N', N'-dimethylpropane-1,3-diamine (L1), N,N-dimethyl-N'-(2-(pyridin-2-yl)ethyl)-N'-(pyridin-2-ylmethyl) propane-1,3-diamine (L2), N,N-bis((4-methoxy-3,5-dimethylpyridin-2-ylmethyl)-N',N'-dimethylpropane-1,3-diamine (L3), N-(2-(dimethylamino) benzyl)-N',N'-dimethyl-N-(pyridin-2-ylmethyl) propane-1,3-diamine (L4) and N,N-bis(2-(dimethylamino)benzyl)-N', N'-dimethylpropane-1,3-diamine (L5) have been synthesized and characterized as the catalysts for the conversion of atmospheric CO₂ into organic cyclic carbonates. The single-crystal X-ray structure of 2 was determined and exhibited distorted octahedral coordination geometry with cis-α configuration. The complexes have been used as a catalyst for converting CO₂ and epoxides into five-membered cyclic carbonates under 1 atmospheric (atm) pressure at room temperature in the presence of Bu₄ NBr. The catalyst containing electron-releasing -Me and -OMe groups afforded the maximum yield of cyclic carbonates, 34% (TON, 680) under 1 atm air. It was drastically enhanced to 89% (TON, 1780) under pure CO₂ gas at 1 atm. It is the highest catalytic efficiency known for CO₂ fixation using nickel-based catalysts at room temperature and 1 atm pressure. The electronic and steric factors of the ligands strongly influence the catalytic efficiency. Furthermore, all the catalysts can convert a wide range of epoxides (ten examples) into corresponding cyclic carbonate with excellent selectivity (>99%) under this mild condition.

Divergent CO2 Activation by Tuning the Lewis Acid in Iron-Based Bimetallic Systems

Bimetallic motifs mediate the selective activation and functionalization of CO₂ in metalloenzymes and some recent synthetic systems. In this work, we build on the nascent concept of bimetallic frustrated Lewis pairs (FLPs) to investigate the activation and reduction of CO₂ . Using the Fe⁰ fragment [(depe)₂ Fe] (depe=1,2-bis(diethylphosphino)ethane) as base, we modify the nature of the partner Lewis acid to accomplish a divergent and highly chemoselective reactivity towards CO₂ . [Au(PMe₂ Ar)]⁺ irreversibly dissociates CO₂ , Zn(C₆ F₅ )₂ and B(C₆ F₅ )₃ yield different CO₂ adducts stabilized by push-pull interactions, while Al(C₆ F₅ )₃ leads to a rare heterobimetallic C-O bond cleavage, and thus to contrasting reduced products after exposure to dihydrogen. Computational investigations provide a rationale for the divergent reactivity, while Energy Decomposition Analysis-Natural Orbital for Chemical Valence (EDA-NOCV) method substantiates the heterobimetallic bonding situation.

Molecular Catalysts for the Reductive Homocoupling of CO 2 towards C 2+ Compounds

The conversion of CO₂ into multicarbon (C₂₊) compounds by reductive homocoupling offers the possibility to transform renewable energy into chemical energy carriers and thereby create “carbon‐neutral” fuels or other valuable products. Most available studies have employed heterogeneous metallic catalysts, but the use of molecular catalysts is still underexplored. However, several studies have already demonstrated the great potential of the molecular approach, namely, the possibility to gain a deep mechanistic understanding and a more precise control of the product selectivity. This Minireview summarizes recent progress in both the thermo‐ and electrochemical reductive homocoupling of CO₂ toward C₂₊ products mediated by molecular catalysts. In addition, reductive CO homocoupling is discussed as a model for the further conversion of intermediates obtained from CO₂ reduction, which may serve as a source of inspiration for developing novel molecular catalysts in the future.

Recent advances in cooperative activation of CO2 and N2O by bimetallic coordination complexes or binuclear reaction pathways

The gaseous small molecules, CO₂ and N₂O, play important roles in climate change and ozone layer depletion, and they hold promise as underutilized reagents and chemical feedstocks. However, productive transformations of these heteroallenes are difficult to achieve because of their inertness. In nature, these gases are cycled through ecological systems by metalloenzymes featuring multimetallic active sites that employ cooperative mechanisms. Thus, cooperative bimetallic chemistry is an important strategy for synthetic systems, as well. In this Perspective, recent advances (since 2010) in cooperative activation of CO₂ and N₂O are reviewed, including examples involving s-block, p-block, d-block, and f-block metals and different combinations thereof.

Revisiting Reduction of CO2 to Oxalate with First-Row Transition Metals: Irreproducibility, Ambiguous Analysis, and Conflicting Reactivity

Construction of higher C_≥2 compounds from CO₂ constitutes an attractive transformation inspired by nature's strategy to build carbohydrates. However, controlled C-C bond formation from carbon dioxide using environmentally benign reductants remains a major challenge. In this respect, reductive dimerization of CO₂ to oxalate represents an important model reaction enabling investigations on the mechanism of this simplest CO₂ coupling reaction. Herein, we present common pitfalls encountered in CO₂ reduction, especially its reductive coupling, based on established protocols for the conversion of CO₂ into oxalate. Moreover, we provide an example to systematically assess these reactions. Based on our work, we highlight the importance of utilizing suitable orthogonal analytical methods and raise awareness of oxidative reactions that can likewise result in the formation of oxalate without incorporation of CO₂. These results allow for the determination of key parameters, which can be used for tailoring of prospective catalytic systems and will promote the advancement of the entire field.

Biomimetic Metal-Free Hydride Donor Catalysts for CO2 Reduction

The catalytic reduction of carbon dioxide to fuels and value-added chemicals is of significance for the development of carbon recycling technologies. One of the main challenges associated with catalytic CO₂ reduction is product selectivity: the formation of carbon monoxide, molecular hydrogen, formate, methanol, and other products occurs with similar thermodynamic driving forces, making it difficult to selectively reduce CO₂ to the target product. Significant scientific effort has been aimed at the development of catalysts that can suppress the undesired hydrogen evolution reaction and direct the reaction toward the selective formation of the desired products, which are easy to handle and store. Inspired by natural photosynthesis, where the CO₂ reduction is achieved using NADPH cofactors in the Calvin cycle, we explore biomimetic metal-free hydride donors as catalysts for the selective reduction of CO₂ to formate. Here, we outline our recent findings on the thermodynamic and kinetic parameters that control the hydride transfer from metal-free hydrides to CO₂. By experimentally measuring and theoretically calculating the thermodynamic hydricities of a range of metal-free hydride donors, we derive structural and electronic factors that affect their hydride-donating abilities. Two dominant factors that contribute to the stronger hydride donors are identified to be (i) the stabilization of the positive charge formed upon HT via aromatization or by the presence of electron-donating groups and (ii) the destabilization of hydride donors through the anomeric effect or in the presence of significant structural constrains in the hydride molecule. Hydride donors with appropriate thermodynamic hydricities were reacted with CO₂, and the formation of the formate ion (the first reduction step in CO₂ reduction to methanol) was confirmed experimentally, providing an important proof of principle that organocatalytic CO₂ reduction is feasible. The kinetics of hydride transfer to CO₂ were found to be slow, and the sluggish kinetics were assigned in part to the large self-exchange reorganization energy associated with the organic hydrides in the DMSO solvent. Finally, we outline our approaches to the closure of the catalytic cycle via the electrochemical and photochemical regeneration of the hydride (R-H) from the conjugate hydride acceptors (R⁺). We illustrate how proton-coupled electron transfer can be efficiently utilized not only to lower the electrochemical potential at which the hydride regeneration takes place but also to suppress the unwanted dimerization that neutral radical intermediates tend to undergo. Overall, this account provides a summary of important milestones achieved in organocatalytic CO₂ reduction and provides insights into the future research directions needed for the discovery of inexpensive catalysts for carbon recycling.

Cooperative Activation of CO2 and Epoxide by a Heterobinuclear Al-Fe Complex via Radical Pair Mechanisms

Activation of inert molecules like CO₂ is often mediated by cooperative chemistry between two reactive sites within a catalytic assembly, the most common form of which is Lewis acid/base bifunctionality observed in both natural metalloenzymes and synthetic systems. Here, we disclose a heterobinuclear complex with an Al-Fe bond that instead activates CO₂ and other substrates through cooperative behavior of two radical intermediates. The complex L^dipp(Me)AlFp (2, L^dipp = HC{(CMe)(2,6-ⁱPr₂C₆H₃N)}₂, Fp = FeCp(CO)₂, Cp = η⁵-C₅H₅) was found to insert CO₂ and cyclohexene oxide, producing L^dippAl(Me)(μ:κ₂-O₂C)Fp (3) and L^dippAl(Me)(μ-OC₆H₁₀)Fp (4), respectively. Detailed mechanistic studies indicate unusual pathways in which (i) the Al-Fe bond dissociates homolytically to generate formally Al^II and Fe^I metalloradicals, then (ii) the metalloradicals add to substrate in a pairwise fashion initiated by O-coordination to Al. The accessibility of this unusual mechanism is aided, in part, by the redox noninnocent nature of L^dipp that stabilizes the formally Al^II intermediates, instead giving them predominantly Al^III-like physical character. The redox noninnocent nature of the radical intermediates was elucidated through direct observation of L^dippAl(Me)(OCPh₂) (22), a metalloradical species generated by addition of benzophenone to 2. Complex 22 was characterized by X-band EPR, Q-band EPR, and ENDOR spectroscopies as well as computational modeling. The "radical pair" pathway represents an unprecedented mechanism for CO₂ activation.

Non-redox reactivity of V(ii) and Fe(ii) formamidinates towards CO2 resulting in the formation of novel M(ii) carbamates

Multiple CO2 insertions into the M–N bonds of V(ii) and Fe(ii) bis(formamidinates) led to the isolation of three novel carbamates. The CO2 insertion effectivity depended on the solvent used and the metal centre's coordination sphere geometry.

Alkali Metal Adducts of an Iron(0) Complex and Their Synergistic FLP-Type Activation of Aliphatic C-X Bonds

We report the formation and full characterization of weak adducts between Li⁺ and Na⁺ cations and a neutral iron(0) complex, [Fe(CO)₃(PMe₃)₂] (1), supported by weakly coordinating [BAr^F₂₀] anions, [1·M][BAr^F₂₀] (M = Li, Na). The adducts are found to synergistically activate aliphatic C-X bonds (X = F, Cl, Br, I, OMs, OTf), leading to the formation of iron(II) organyl compounds of the type [FeR(CO)₃(PMe₃)₂][BAr^F₂₀], of which several were isolated and fully characterized. Stoichiometric reactions with the resulting iron(II) organyl compounds show that this system can be utilized for homocoupling and cross-coupling reactions and the formation of new C-E bonds (E = C, H, O, N, S). Further, we utilize [1·M][BAr^F₂₀] as a catalyst in a simple hydrodehalogenation reaction under mild conditions to showcase its potential use in catalytic reactions. Finally, the mechanism of activation is probed using DFT and kinetic experiments that reveal that the alkali metal and iron(0) center cooperate to cleave C-X via a mechanism closely related to intramolecular FLP activation.

Fixation of atmospheric CO2 as C1-feedstock by nickel(ii) complexes

The development of molecular catalysts for the activation and conversion of atmospheric carbon dioxide (CO₂) into a value-added product is a great challenge. A series of nickel(ii) complexes, [Ni(L)(CH₃CN)₃](BPh₄)₂, 1-4 of diazepane based ligands, 4-methyl-1-[(pyridin-2-yl-methyl)]-1,4-diazepane (L1), 4-methyl-1-[2-(pyridine-2-yl)ethyl]-1,4-diazepane (L2), 4-methyl-1-[(quinoline-2-yl)-methyl]-1,4-diazepane (L3) and 1-[(4-methoxy-3,5-dimethyl-pyridin-2-yl)methyl]-4-methyl-1,4-diazepane (L4), have been synthesized and characterized as catalysts for the activation of atmospheric CO₂. The single-crystal X-ray structure of 1 shows a distorted octahedral geometry with a cis-β configuration around the NiN₆ coordination sphere. All the complexes are used as catalysts for the conversion of atmospheric CO₂ and epoxides into cyclic carbonates at 1 atmosphere (atm) pressure and in the presence of Et₃N. Catalyst 4 was found to be the most efficient catalyst and showed a 31% formation of cyclic carbonates with a TON of 620 under 1 atm air as the CO₂ source. This yield was enhanced to 94% with a TON of 1880 under 1 atm pure CO₂ gas and it is the highest catalytic efficiency known for nickel(ii)-based catalysts. Catalyst 4 enabled the transformation of a wide range of epoxides (eight examples) into corresponding cyclic carbonates with excellent selectivity (>99%) and yields of 59-94% and 11-31% under pure CO₂ and atmospheric CO₂, respectively. The catalytic efficiency is strongly influenced by the electronic nature of the complexes. The CO₂ fixation reactions without an epoxide substrate led to the formation of the carbonate bridged dinuclear nickel(ii) complexes [(LNi^II)₂CO₃](BPh₄)₂1a-4a, which are speculated as catalytically active intermediates. The formation of these species was accompanied by the formation of new absorption bands around 592-681 nm and was further confirmed by the ESI-MS and IR spectral studies. The molecular structures of these carbonate-bridged key intermediates were determined by X-ray analysis. The structures contain two Ni²⁺-centers bridged via a carbonate ion that originated from CO₂. Distorted square pyramidal geometries are adopted around each Ni(ii) center. All these results support that CO₂ fixation reactions occur via CO₂-bound nickel key intermediates.

Opening up the Valence Shell: A T-Shaped Iron(I) Metalloradical and Its Potential for Atom Abstraction

A thermally stable, T-shaped, d7 high-spin iron(I) complex was obtained by reduction of a PNP-supported ferrous chloride. Paramagnetic NMR spectroscopy combined with DFT modeling was used to analyze the electronic structure of the coordinatively highly unsaturated complex. The metalloradical character of the compound was demonstrated by the formation of a benzophenone ketyl radical complex upon addition of benzophenone. Furthermore, the compound displays a rich chemistry as an oxygen-atom abstractor from epoxides, yielding a dinuclear, diferrous [Fe2 O] complex.

Facile C=O Bond Splitting of Carbon Dioxide Induced by Metal-Ligand Cooperativity in a Phosphinine Iron(0) Complex

New iron complexes [Cp*FeL]⁻ (1‐σ and 1‐π, Cp*=C₅Me₅) containing the chelating phosphinine ligand 2‐(2′‐pyridyl)‐4,6‐diphenylphosphinine (L) have been prepared, and found to undergo facile reaction with CO₂ under ambient conditions. The outcome of this reaction depends on the coordination mode of the versatile ligand L. Interaction of CO₂ with the isomer 1‐π, in which L binds to Fe through the phosphinine moiety in an η⁵ fashion, leads to the formation of 3‐π, in which CO₂ has undergone electrophilic addition to the phosphinine group. In contrast, interaction with 1‐σ—in which L acts as a σ‐chelating [P,N] ligand—leads to product 3‐σ in which one C=O bond has been completely broken. Such CO₂ cleavage reactions are extremely rare for late 3d metals, and this represents the first such example mediated by a single Fe centre.

Diiron oxo reactivity in a weak-field environment

Concomitant deprotonation and metalation of a dinucleating cofacial Pacman dipyrrin ligand platform tBudmxH2 with Fe2(Mes)4 results in formation of a diiron complex ( tBudmx)Fe2(Mes)2. Treatment of ( tBudmx)Fe2(Mes)2 with one equivalent of water yields the diiron μ-oxo complex ( tBudmx)Fe2(μ-O) and free mesitylene. A two-electron oxidation of ( tBudmx)Fe2(μ-O) gives rise to the diferric complex ( tBudmx)Fe2(μ-O)Cl2, and one-electron reduction from this FeIIIFeIII state allows for isolation of a mixed-valent species [Cp2Co][( tBudmx)Fe2(μ-O)Cl2]. Both ( tBudmx)Fe2(μ-O) and [Cp2Co][( tBudmx)Fe2(μ-O)Cl2] exhibit basic character at the bridging oxygen atom and can be protonated using weak acids to form bridging diferrous hydroxide species. The basicity of the diferrous oxo ( tBudmx)Fe2(μ-O) is quantified through studies of the pK a of its conjugate acid, [( tBudmx)Fe2(μ-OH)]+, which is determined to be 15.3(6); interestingly, upon coordination of neutral solvent ligands to yield ( tBudmx)Fe2(μ-O)(thf)2, the basicity is increased as observed through an increase in the pK a of the conjugate acid [( tBudmx)Fe2(μ-OH)(thf)2]+ to 26.8(6). In contrast, attempts to synthesize a diferric bridging hydroxide by two-electron oxidation of [( tBudmx)Fe2(μ-OH)(thf)2]+ resulted in isolation of ( tBudmx)Fe2(μ-O)Cl2 with concomitant loss of a proton, consistent with the pK a of the conjugate acid [( tBudmx)Fe2(μ-OH)Cl2]+ determined computationally to be -1.8(6). The foregoing results highlight the intricate interplay between oxidation state and reactivity in diiron μ-oxo units.

Reduction of CO2 by a masked two-coordinate cobalt(i) complex and characterization of a proposed oxodicobalt(ii) intermediate

Fixation and chemical reduction of CO2 are important for utilization of this abundant resource, and understanding the detailed mechanism of C-O cleavage is needed for rational development of CO2 reduction methods. Here, we describe a detailed analysis of the mechanism of the reaction of a masked two-coordinate cobalt(i) complex, L tBuCo (where L tBu = 2,2,6,6-tetramethyl-3,5-bis[(2,6-diisopropylphenyl)imino]hept-4-yl), with CO2, which yields two products of C-O cleavage, the cobalt(i) monocarbonyl complex L tBuCo(CO) and the dicobalt(ii) carbonate complex (L tBuCo)2(μ-CO3). Kinetic studies and computations show that the κN,η6-arene isomer of L tBuCo rearranges to the κ2 N,N' binding mode prior to binding of CO2, which contrasts with the mechanism of binding of other substrates to L tBuCo. Density functional theory (DFT) studies show that the only low-energy pathways for cleavage of CO2 proceed through bimetallic mechanisms, and DFT and highly correlated domain-based local pair natural orbital coupled cluster (DLPNO-CCSD(T)) calculations reveal the cooperative effects of the two metal centers during facile C-O bond rupture. A plausible intermediate in the reaction of CO2 with L tBuCo is the oxodicobalt(ii) complex L tBuCoOCoL tBu, which has been independently synthesized through the reaction of L tBuCo with N2O. The rapid reaction of L tBuCoOCoL tBu with CO2 to form the carbonate product indicates that the oxo species is kinetically competent to be an intermediate during CO2 cleavage by L tBuCo. L tBuCoOCoL tBu is a novel example of a thoroughly characterized molecular cobalt-oxo complex where the cobalt ions are clearly in the +2 oxidation state. Its nucleophilic reactivity is a consequence of high charge localization on the μ-oxo ligand between two antiferromagnetically coupled high-spin cobalt(ii) centers, as characterized by DFT and multireference complete active space self-consistent field (CASSCF) calculations.

Carbon dioxide reduction by dinuclear Yb(ii) and Sm(ii) complexes supported by siloxide ligands

Low-coordinate dinuclear lanthanide complexes supported by siloxides effect the reduction of carbon dioxide to both carbonate and oxalate, but the cooperative binding of CO2 to the two Ln(ii) cations in the dimer favours oxalate formation.

Monomeric Fe(iii) half-sandwich complexes [Cp'FeX2] - synthesis, properties and electronic structure

The half-sandwich complex [Cp'Fe(μ-I)]₂ (1; Cp' = η⁵-1,2,4-(Me₃C)₃C₅H₂) is cleaved when heated in toluene to form a cation-anion pair [{Cp'Fe(η⁶-toluene)}⁺{Cp'FeI₂}^-] (2), in which the two Fe(ii) atoms adopt different spin states, i.e., a low-spin (S = 0) and a high-spin (S = 2) configuration. Upon oxidation of 1 with C₂H₄I₂, the thermally stable 15VE species [Cp'FeI₂] (3) can be isolated, in which the Fe(iii) atom adopts an intermediate spin (S = 3/2) configuration. Complex 3 is an excellent starting material for further functionalizations and it reacts with Mg(CH₂SiMe₃)₂ to form the unprecedented Fe(iii) (S = 3/2) bis(alkyl) complex [Cp'Fe(CH₂SiMe₃)₂] (4). The respective spin states of complexes 2-4 are confirmed by single-crystal X-ray crystallography, zero-field ⁵⁷Fe Mössbauer spectroscopy, and solid-state magnetic susceptibility measurements. In contrast to the related 14VE high-spin (S = 2) Fe(ii) alkyl species [Cp'FeCH(SiMe₃)₂], which resists the reaction with H₂ as a consequence of a spin-induced reaction barrier, complex 4 reacts cleanly with H₂ (8 bar) in cyclohexane to yield iron hydrides [{Cp'Fe}₂(μ-H)₃] (5) and [Cp'Fe(μ-H)₂]₂ (6) in a 1 : 4 ratio. However, when the hydrogenation of 4 is carried out in benzene, a green 19VE [Cp'Fe(η⁶-C₆H₆)] (A) intermediate is formed, which dimerizes to the bis(cyclohexadienyl)-bridged product [(Cp'Fe)₂(μ₂-η⁵:η⁵-C₁₂H₁₂)] (7). Further evidence for the intermediacy of [Cp'Fe(η⁶-C₆H₆)] (A) was gathered by X-band EPR and UV/vis spectroscopy. Interestingly, attempts to oxidize 7 with AgSbF₆ proceeded via C-C bond cleavage instead of metal oxidation to form [Cp'Fe(C₆H₆)][SbF₆] (8).

Catalytic Silylation of Dinitrogen by a Family of Triiron Complexes

A series of triiron complexes supported by a tris(β-diketiminate)cyclophane (L ^3- ) catalyze the reduction of dinitrogen to tris(trimethylsilyl)amine using KC₈ and Me₃SiCl. Employing Fe₃Br₃ L affords 83 ± 7 equiv. NH₄ ⁺/complex after protonolysis, which is a 50% yield based on reducing equivalents. The series of triiron compounds tested evidences the subtle effects of ancillary donors, including halides, hydrides, sulfides, and carbonyl ligands, and metal oxidation state on N(SiMe₃)₃ yield, and highlight Fe₃(μ₃-N)L as a common species in product mixtures. These results suggest that ancillary ligands can be abstracted with Lewis acids under reducing conditions.

Countercations and Solvent Influence CO2 Reduction to Oxalate by Chalcogen-Bridged Tricopper Cyclophanates

One-electron reduction of Cu₃EL (L^3- = tris(β-diketiminate)cyclophane, and E = S, Se) affords [Cu₃EL]^-, which reacts with CO₂ to yield exclusively C₂O₄^2- (95% yield, TON = 24) and regenerate Cu₃EL. Stopped-flow UV/visible data support an A→B mechanism under pseudo-first-order conditions ( k_{obs, 298K} = 115(2) s^-1), which is 10⁶ larger than those for reported copper complexes. The k_obs values are dependent on the countercation and solvent (e.g., k_obs is greater for [K(18-crown-6)]⁺ vs (Ph₃P)₂N⁺, and there is a 20-fold decrease in k_obs in THF vs DMF). Our results suggest a mechanism in which cations and solvent influence the stability of the transition state.

Direct CO2 Addition to a Ni(0)-CO Species Allows the Selective Generation of a Nickel(II) Carboxylate with Expulsion of CO

Addition of CO₂ to a low-valent nickel species has been explored with a newly designed ^acriPNP pincer ligand (^acriPNP^- = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide). This is a crucial step in understanding biological CO₂ conversion to CO found in carbon monoxide dehydrogenase (CODH). A four-coordinate nickel(0) state was reliably accessed in the presence of a CO ligand, which can be prepared from a stepwise reduction of a cationic {(^acriPNP)Ni(II)-CO}⁺ species. All three Ni(II), Ni(I), and Ni(0) monocarbonyl species were cleanly isolated and spectroscopically characterized. Addition of electrons to the nickel(II) species significantly alters its geometry from square planar toward tetrahedral because of the filling of the d_x²-y² orbital. Accordingly, the CO ligand position changes from equatorial to axial, ∠N-Ni-C of 176.2(2)° to 129.1(4)°, allowing opening of a CO₂ binding site. Upon addition of CO₂ to a nickel(0)-CO species, a nickel(II) carboxylate species with a Ni(η¹-CO₂-κC) moiety was formed and isolated (75%). This reaction occurs with the concomitant expulsion of CO(g). This is a unique result markedly different from our previous report involving the flexible analogous PNP ligand, which revealed the formation of multiple products including a tetrameric cluster from the reaction with CO₂. Finally, the carbon dioxide conversion to CO at a single nickel center is modeled by the successful isolation of all relevant intermediates, such as Ni-CO₂, Ni-COOH, and Ni-CO.

Current Status and Bioinspired Perspective of Electrochemical Conversion of CO2 to a Long-Chain Hydrocarbon

Electrocatalytic conversion of CO₂ into a long-chain hydrocarbon represents an important research direction in adding value to CO₂-based chemicals and realizing its practical application. Long-chain hydrocarbons may change the current fossil fuel-based industry in that those chemicals have a similar energy density as gasoline, high compatibility with the current infrastructure, and low hydroscopicity for pipeline distribution. However, most of the electrocatalysts produce C₁, C₂, and C₃ chemicals, and methods for producing long-chain hydrocarbons are not available thus far. Interestingly, nature utilizes many enzymes to generate long-chain hydrocarbons using C₂ building blocks and suggests key mechanisms, inspiring new perspective in the design of electrocatalysts. In this Perspective, we present case studies to demonstrate how CO₂ and its reductive derivatives interact with the electrode surface during C-C bond formation and introduce how these issues are addressed in biological systems. We end this Perspective by outlining possible strategies to translate the natural mechanism into a heterogeneous electrode.

Topologically diverse shape-persistent bis-(Zn-salphen) catalysts: efficient cyclic carbonate formation under mild conditions

By adopting a shape-persistent bimetallic design approach, high initial molecular turnover frequencies (up to 14,800 h(-1)) for coupling of CO2 with epoxides in conjunction with (n)Bu4NI, plus excellent yields under mild conditions (1 bar of CO2, 45 °C) have been achieved for catalysts containing cofacial Zn-salphen units.

Insight into the stereoelectronic parameters of N-triphos ligands via coordination to tungsten(0)

A series of new N-triphos tungsten complexes have been synthesised and structurally characterised.

Highly effective sites and selectivity of nitrogen-doped graphene/CNT catalysts for CO2 electrochemical reduction

The selectivity of CO₂ electrochemical reduction can be tuned for N-doped graphene/CNT catalysts after active sites are determined.

Metal-free catalysts, such as graphene/carbon nanostructures, are highly cost-effective to replace expensive noble metals for CO₂ reduction if fundamental issues, such as active sites and selectivity, are clearly understood. Using both density functional theory (DFT) and ab initio molecular dynamic calculations, we show that the interplay of N-doping and curvature can effectively tune the activity and selectivity of graphene/carbon-nanotube (CNT) catalysts. The CO₂ activation barrier can be optimized to 0.58 eV for graphitic-N doped graphene edges, compared with 1.3 eV in the un-doped counterpart. The graphene catalyst without curvature shows strong selectivity for CO/HCOOH production, whereas the (6, 0) CNT with a high degree of curvature is effective for both CH₃OH and HCHO production. Curvature is also very influential to tune the overpotential for a given product, e.g. from 1.5 to 0.02 V for CO production and from 1.29 to 0.49 V for CH₃OH production. Hence, the graphene/CNT nanostructures offer great scope and flexibility for effective tunning of catalyst efficiency and selectivity, as shown here for CO₂ reduction.

Intermediates Involved in the 2e(-)/2H(+) Reduction of CO2 to CO by Iron(0) Porphyrin

The reduction of CO2 by an iron porphyrin complex with a hydrogen bonding distal pocket involves at least two intermediates. The resonance Raman data of intermediate I, which could only be stabilized at -95 °C, indicates that it is a Fe(II)-CO2(2-) adduct and is followed by an another intermediate II at -80 °C where the bound CO2 in intermediate I is protonated to form a Fe(II)-COOH species. While the initial protonation can be achieved using weak proton sources like MeOH and PhOH, the facile heterolytic cleavage of the C-OH bond in intermediate II requires strong acids.