Ein Dimetalloxycarben-Bindungsmodus und der reduktive Kupplungsmechanismus zur Bildung von Oxalat ausgehend von CO2

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.

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.

Selective Transformation of Nickel-Bound Formate to CO or C-C Coupling Products Triggered by Deprotonation and Steered by Alkali-Metal Ions

The complexes [LtBu Ni(OCO-κ2 O,C)]M3 [N(SiMe3 )2 ]2 (M=Li, Na, K), synthesized by deprotonation of a nickel formate complex [LtBu NiOOCH] with the corresponding amides M[N(SiMe3 )2 ], feature a NiII -CO2 2- core surrounded by Lewis-acidic cations (M+ ) and the influence of the latter on the behavior and reactivity was studied. The results point to a decrease of CO2 activation within the series Li, Na, and K, which is also reflected in the reactivity with Me3 SiOTf leading to the liberation of CO and formation of a Ni-OSiMe3 complex. Furthermore, in case of K+ , the {[K3 [N(SiMe3 )2 ]2 }+ shell around the Ni-CO2 2- entity was shown to have a large impact on its stabilization and behavior. If the number of K[N(SiMe3 )2 ] equivalents used in the reaction with [LtBu NiOOCH] is decreased from 3 to 0.5, the deprotonated part of the precursor enters a complex reaction sequence with formation of [LtBu NiI (μ-OOCH)NiI LtBu ]K and [LtBu Ni(C2 O4 )NiLtBu ]. The same reaction at higher concentrations additionally led to the formation of a unique hexanuclear NiII complex containing both oxalate and mesoxalate ([O2 C-CO2 -CO2 ]4- ) ligands.

Aromatic Ester-Functionalized Ionic Liquid for Highly Efficient CO2 Electrochemical Reduction to Oxalic Acid

Electrochemical reduction of CO₂ into valuable chemicals is a significant route to utilize CO₂ resources. Among various electroreduction products, oxalic acid (H₂ C₂ O₄ ) is an important chemical for pharmaceuticals, rare earth extraction, and metal processing. Here, an aprotic aromatic ester-functionalized ionic liquid (IL), 4-(methoxycarbonyl) phenol tetraethylammonium ([TEA][4-MF-PhO]), was designed as an electrolyte for CO₂ electroreduction into oxalic acid. It exhibited a large oxalic acid partial current density of 9.03 mA cm^-2 with a faradaic efficiency (FE) of 86 % at -2.6 V (vs. Ag/Ag⁺ ), and the oxalic acid formation rate was as high as 168.4 μmol cm^-2  h^-1 , which is the highest reported value to date. Moreover, the results of density functional theory calculations demonstrated that CO₂ was efficiently activated to a -COOH intermediate by bis-active sites of the aromatic ester anion via the formation of a [4-MF-PhO-COOH]^- adduct, which finally dimerized into oxalic acid.

Reduced Arene Complexes of Scandium

Alkali metal naphthalenide or anthracenide reacted with scandium(III) anilides [Sc(X){N(tBu)Xy}2 (thf)] (X=N(tBu)Xy (1); X=Cl (2); Xy=C6 H3 -3,5-Me2 ) to give scandium complexes [M(thf)n ][Sc{N(tBu)Xy}2 (RA)] (M=Li-K; n=1-6; RA=C10 H8 2- (3-Naph-K) and C14 H10 2- (3-Anth-M)) containing a reduced arene ligand. Single-crystal X-ray diffraction revealed the scandium(III) center bonded to the naphthalene dianion in a σ2 :π-coordination mode, whereas the anthracene dianion is symmetrically attached to the scandium(III) center in a σ2 -fashion. All compounds have been characterized by multinuclear, including 45 Sc NMR spectroscopy. Quantum chemical calculations of these intensely colored arene complexes confirm scandium to be in the oxidation state +3. The intense absorptions observed in the UV/Vis spectra are due to ligand-to-metal charge transfers. Whereas nitriles underwent C-C coupling reaction with the reduced arene ligand, the reaction with one equivalent of [NEt3 H][BPh4 ] led to the mono-protonation of the reduced arene ligand.

Linear End-On Coordination Modes of CO2

The second case of linear end-on and evidence for an unprecedented bridging end-on coordination mode of CO₂ have been discovered for vanadium aryloxide complexes of the tetradentate ligand system (ONNO)^2- (ONNO=2,4-Me₂ -2-(OH) C₆ H₂ CH₂ ]₂ N(CH₂ )₂ NMe₂ ). The reaction of divalent (ONNO)V^II (TMEDA) with CO₂ and under the appropriate reaction conditions affords the trivalent (ONNO)V^III (OH)(η¹ -CO₂ ) resulting from an intermediate CO₂ deoxygenation pathway followed by H-atom abstraction from the aromatic solvent, and CO₂ fixation. In contrast, the reduction of trivalent (ONNO)V^III Cl(THF) with K, followed by exposure to CO₂ in ethereal solvent, afforded the dinuclear [(ONNO)V^II ]₂ (μ_, η¹ -CO₂ ).

Trapping of a Highly Bent and Reduced Form of 2-Phosphaethynolate in a Mixed-Valence Diuranium-Triamidoamine Complex

The chemistry of 2-phosphaethynolate is burgeoning, but there remains much to learn about this ligand, for example its reduction chemistry is scarce as this promotes P-C-O fragmentations or couplings. Here, we report that reduction of [U(Tren^TIPS )(OCP)] (Tren^TIPS =N(CH₂ CH₂ NSiPrⁱ ₃ )₃ ) with KC₈ /2,2,2-cryptand gives [{U(Tren^TIPS )}₂ {μ-η² (OP):η² (CP)-OCP}][K(2,2,2-cryptand)]. The coordination mode of this trapped 2-phosphaethynolate is unique, and derives from an unprecedented highly reduced and highly bent form of this ligand with the most acute P-C-O angle in any complex to date (P-C-O ∡ ≈127°). The characterisation data support a mixed-valence diuranium(III/IV) formulation, where backbonding from uranium gives a highly reduced form of the P-C-O unit that is perhaps best described as a uranium-stabilised OCP^2-. radical dianion. Quantum chemical calculations reveal that this gives unprecedented carbene character to the P-C-O unit, which engages in a weak donor-acceptor interaction with one of the uranium ions.

Reaction of CO2 with a Vanadium(II) Aryl Oxide: Synergistic Activation of CO2 /Oxo Groups towards H-Atom Radical Abstraction

Treatment of divalent (ONNO)V(TMEDA) (1; ONNO=[2,4-Me₂ -2-(OH)C₆ H₂ CH₂ ]₂ N(CH₂ )₂ NMe₂ ) with CO₂ afforded [(ONNO)V]₂ (μ-OH)(μ-formate) (2). Whereas the bridging hydroxo and formate groups both originated from CO₂ , the H atoms present on the two residues were obtained through H-atom radical abstraction from the solvent. DFT calculations revealed an initially linear CO₂ bonding mode, followed by deoxygenation, and highlighted a synergistic effect between the so-formed oxo group and an additional bridging CO₂ residue in promoting radical behavior.

Metal-Free Reduction of CO2 to Methoxyborane under Ambient Conditions through Borondiformate Formation

An abnormal N-heterocyclic carbene (aNHC) based homogeneous catalyst has been used for the reduction of carbon dioxide to methoxyborane in the presence of a range of hydroboranes under ambient conditions and resulted in the highest turnover number of 6000. A catalytically active reaction intermediate, [aNHC-H⋅9BBN(OCOH)₂ ] was structurally characterized and authenticated by NMR spectroscopy. A detailed mechanistic cycle of this catalytic process via borondiformate formation has been proposed from tandem experimental and computational experiments.

Concomitant Carboxylate and Oxalate Formation From the Activation of CO2 by a Thorium(III) Complex

Improving our comprehension of diverse CO₂ activation pathways is of vital importance for the widespread future utilization of this abundant greenhouse gas. CO₂ activation by uranium(III) complexes is now relatively well understood, with oxo/carbonate formation predominating as CO₂ is readily reduced to CO, but isolated thorium(III) CO₂ activation is unprecedented. We show that the thorium(III) complex, [Th(Cp′′)₃] (1, Cp′′={C₅H₃(SiMe₃)₂‐1,3}), reacts with CO₂ to give the mixed oxalate‐carboxylate thorium(IV) complex [{Th(Cp′′)₂[κ²‐O₂C{C₅H₃‐3,3′‐(SiMe₃)₂}]}₂(μ‐κ²:κ²‐C₂O₄)] (3). The concomitant formation of oxalate and carboxylate is unique for CO₂ activation, as in previous examples either reduction or insertion is favored to yield a single product. Therefore, thorium(III) CO₂ activation can differ from better understood uranium(III) chemistry.