Catalyst Control of Selectivity in CO2 Reduction Using a Tunable Heterobimetallic Effect

A Co-Doping Materials Design Strategy for Selective Ozone Electrocatalysts

Catalysts for electrochemical ozone production (EOP) face inherent selectivity challenges stemming from thermodynamic constraints. This work establishes a design strategy for minimizing these limitations and inducing EOP activity in tin oxide, which is an intrinsically EOP-inactive material. We propose that selective ozone production using tin oxide catalysts can be broadly achieved by co-doping with two elements: first, n-type dopants to enhance electrical conductivity, and second, transition metal dopants that leach and homogeneously generate essential hydroperoxyl radical intermediates. Synthesizing tantalum, antimony, and tungsten n-type dopants with nickel, cobalt, and iron as transition metal dopants confirms that properly co-doping tin oxide yields EOP-active catalysts. This study offers a robust framework for advancing EOP catalyst design and serves as a case study for the application of fundamental co-catalysis and solid-state physics principles to induce catalytic activity in inert materials.

Enhancement Effect Induced by the Second Metal to Promote Ozone Catalytic Oxidation of VOCs

It is a promising research direction to develop catalysts with high stability and ozone utilization for low-temperature ozone catalytic oxidation of VOCs. While bimetallic catalysts exhibit excellent catalytic activity compared with conventional single noble metal catalysts, limited success has been achieved in the influence of the bimetallic effect on the stability and ozone utilization of metal catalysts. Herein, it is necessary to systematically study the enhancement effect in the ozone catalytic reaction induced by the second metal. With a simple continuous impregnation method, a platinum-cerium bimetallic catalyst is prepared. Also highlighted are studies from several aspects of the contribution of the second metal (Ce) to the stability and ozone utilization of the catalysts, including the "electronic effect" and "geometric effect". The synergistic removal rate of toluene and ozone is nearly 100% at 30 °C, and it still shows positive stability after high humidity and a long reaction time. More importantly, the instructive significance, which is the in-depth knowledge of enhanced catalytic mechanism of bimetallic catalysts resulting from a second metal, is provided by this work.

Bimetallic H2 Addition and Intramolecular Caryl-H Activation Mediated by an Iron-Zinc Hydride

Heteronuclear Fe(μ-H)Zn hydride Cp*Fe(1,2-Cy2PC6H4)HZnEt (3) undergoes reversible intramolecular Caryl-H reductive elimination through coupling of the cyclometalated phosphinoaryl ligand and the hydride, giving rise to a formal Fe(0)-Zn(II) species. Addition of CO intercepts this equilibrium, affording Cp*(Cy2PPh)(CO)Fe-ZnEt that features a dative Fe-Zn bond. Significantly, this system achieves bimetallic H2 addition, as demonstrated by the transformation of the monohydride Fe(μ-H)Zn to a deuterated dihydride Fe-(μ-D)2-Zn upon reaction with D2.

Controlling the Activation at NiII -CO2 2- Moieties through Lewis Acid Interactions in the Second Coordination Sphere

Nickel complexes with a two-electron reduced CO2 ligand (CO2 2- , "carbonite") are investigated with regard to the influence alkali metal (AM) ions have as Lewis acids on the activation of the CO2 entity. For this purpose complexes with NiII (CO2 )AM (AM=Li, Na, K) moieties were accessed via deprotonation of nickel-formate compounds with (AM)N(i Pr)2 . It was found that not only the nature of the AM ions in vicinity to CO2 affect the activation, but also the number and the ligation of a given AM. To this end the effects of added (AM)N(R)2 , THF, open and closed polyethers as well as cryptands were systematically studied. In 14 cases the products were characterized by X-ray diffraction and correlations with the situation in solution were made. The more the AM ions get detached from the carbonite ligand, the lower is the degree of aggregation. At the same time the extent of CO2 activation is decreased as indicated by the structural and spectroscopic analysis and reactivity studies. Accompanying DFT studies showed that the coordinating AM Lewis acidic fragment withdraws only a small amount of charge from the carbonite moiety, but it also affects the internal charge equilibration between the LtBu Ni and carbonite moieties.

Cooperative activation of carbon-hydrogen bonds by heterobimetallic systems

The direct activation of C-H bonds has been a rich and active field of organometallic chemistry for many years. Recently, incredible progress has been made and important mechanistic insights have accelerated research. In particular, the use of heterobimetallic complexes to heterolytically activate C-H bonds across the two metal centers has seen a recent surge in interest. This perspective article aims to orient the reader in this fast moving field, highlight recent progress, give design considerations for further research and provide an optimistic outlook on the future of catalytic C-H functionalization with heterobimetallic complexes.

Donor-Acceptor Activation of Carbon Dioxide

The activation and functionalization of carbon dioxide entails great interest related to its abundance, low toxicity and associated environmental problems. However, the inertness of CO2 has posed a challenge towards its efficient conversion to added-value products. In this review we discuss one of the strategies that have been widely used to capture and activate carbon dioxide, namely the use of donor-acceptor interactions by partnering a Lewis acidic and a Lewis basic fragment. This type of CO2 activation resembles that found in metalloenzymes, whose outstanding performance in catalytically transforming carbon dioxide encourages further bioinspired research. We have divided this review into three general sections based on the nature of the active sites: metal-free examples (mainly formed by frustrated Lewis pairs), main group-transition metal combinations, and transition metal heterobimetallic complexes. Overall, we discuss one hundred compounds that cooperatively activate carbon dioxide by donor-acceptor interactions, revealing a wide range of structural motifs.

Small molecule activation with bimetallic systems: a landscape of cooperative reactivity

There is growing interest in the design of bimetallic cooperative complexes, which have emerged due to their potential for bond activation and catalysis, a feature widely exploited by nature in metalloenzymes, and also in the field of heterogeneous catalysis. Herein, we discuss the widespread opportunities derived from combining two metals in close proximity, ranging from systems containing multiple M-M bonds to others in which bimetallic cooperation occurs even in the absence of M⋯M interactions. The choice of metal pairs is crucial for the reactivity of the resulting complexes. In this context, we describe the prospects of combining not only transition metals but also those of the main group series, which offer additional avenues for cooperative pathways and reaction discovery. Emphasis is given to mechanisms by which bond activation occurs across bimetallic structures, which is ascribed to the precise synergy between the two metal atoms. The results discussed herein indicate a future landscape full of possibilities within our reach, where we anticipate that bimetallic synergism will have an important impact in the design of more efficient catalytic processes and the discovery of new catalytic transformations.

Recent developments on the transformation of CO2 utilising ligand cooperation and related strategies

A portfolio of value-added chemicals, fuels and building block compounds can be envisioned from CO₂ on an industrial scale. The high kinetic and thermodynamic stabilities of CO₂, however, present a significant barrier to its utilisation as a C1 source. In this context, metal-ligand cooperation methodologies have emerged as one of the most dominant strategies for the transformation of the CO₂ molecule over the last decade or so. This review focuses on the advancements in CO₂ transformation using these cooperative methodologies. Different and well-studied ligand cooperation methodologies, such as dearomatisation-aromatisation type cooperation, bimetallic cooperation (M⋯M'; M' = main group or transition metal) and other related strategies are also discussed. Furthermore, the cooperative bond activations are subdivided based on the number of atoms connecting the reactive centre in the ligand framework (spacer/linker length) and the transition metal. Several similarities across these seemingly distinct cooperative methodologies are emphasised. Finally, this review brings out the challenges ahead in developing catalytic systems from these CO₂ transformations.

Hydroboration of CO2 to Methyl Boronate Catalyzed by a Manganese Pincer Complex: Insights into the Reaction Mechanism and Ligand Effect

The conversion of carbon dioxide to fuels, polymers, and chemicals is an attractive strategy for the synthesis of high-value-added products and energy-storage materials. Herein, the density functional theory method was employed to investigate the reaction mechanism of CO₂ hydroboration catalyzed by manganese pincer complex, [Mn(Ph₂PCH₂SiMe₂)₂NH(CO)₂Br]. The carbonyl association and carbonyl dissociation mechanisms were investigated, and the calculated results showed that the carbonyl association mechanism is more favorable with an energetic span of 27.0 kcal/mol. Meanwhile, the solvent effect of the reaction was explored, indicating that the solvents could reduce the catalytic activity of the catalyst, which was consistent with the experimental results. In addition, the X ligand effect (X = CO, Br, H, PH₃) on the catalytic activity of the manganese complex was explored, indicating that the anionic complexes [Mn^I - Br]^- and [Mn^I - H]^- have higher catalytic activity. This may not only shed light on the fixation and conversion of CO₂ catalyzed by earth-abundant transition-metal complexes but also provide theoretical insights to design new transition-metal catalysts.

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.

The transition metal-catalysed hydroboration reaction

This review covers the development of the transition metal-catalysed hydroboration reaction, from its beginnings in the 1980s to more recent developments including earth-abundant catalysts and an ever-expanding array of substrates.

MOF Encapsulating N-Heterocyclic Carbene-Ligated Copper Single-Atom Site Catalyst towards Efficient Methane Electrosynthesis

The exploitation of highly efficient carbon dioxide reduction (CO₂ RR) electrocatalyst for methane (CH₄ ) electrosynthesis has attracted great attention for the intermittent renewable electricity storage but remains challenging. Here, N-heterocyclic carbene (NHC)-ligated copper single atom site (Cu SAS) embedded in metal-organic framework is reported (2Bn-Cu@UiO-67), which can achieve an outstanding Faradaic efficiency (FE) of 81 % for the CO₂ reduction to CH₄ at -1.5 V vs. RHE with a current density of 420 mA cm^-2 . The CH₄ FE of our catalyst remains above 70 % within a wide potential range and achieves an unprecedented turnover frequency (TOF) of 16.3 s^-1 . The σ donation of NHC enriches the surface electron density of Cu SAS and promotes the preferential adsorption of CHO* intermediates. The porosity of the catalyst facilitates the diffusion of CO₂ to 2Bn-Cu, significantly increasing the availability of each catalytic center.

Theoretical insights on boron reducing agent for the reduction of carbonyl compounds

In this perspective, we present computational progress in the reduction of carbonyl compounds using boron reducing agents, such as L·BH₃, HBcat, HBpin, and 9-BBN. For the catalytic reduction reactions, establishing a catalytic mechanism will provide an important theoretical basis for the improvement of a more efficient combination of reducing agents and catalysts. Current computational studies reveal that the mechanisms of reactions are different due to the various combinations of electrophilic boron reducing agents and catalysts (transition-metal catalyst, main group metal catalysts, and metal-free frustrated Lewis pair). We discuss the role of boron reducing agents on the efficiency of reactions and believe that possible Lewis acid-base interaction between B^δ+, M^δ+ and O^δ-, H^δ- existing in boron reducing agent, unsaturated substances, and catalyst should be considered fully. A tentative outlook on future opportunities of this research field is proposed.

H2 and carbon-heteroatom bond activation mediated by polarized heterobimetallic complexes

The field of heterobimetallic chemistry has rapidly expanded over the last decade. In addition to their interesting structural features, heterobimetallic structures have been found to facilitate a range of stoichiometric bond activations and catalytic processes. The accompanying review summarizes advances in this area since January of 2010. The review encompasses well-characterized heterobimetallic complexes, with a particular focus on mechanistic details surrounding their reactivity applications.

Catalytic Reactions by Heterobimetallic Carbonyl Complexes with Polar Metal–Metal Interactions

Heterobinuclear catalysts capable of bimetallic cooperative bond activation provide an alternative pathway to approach the discovery of novel and unique reactivity and selectivity in catalytic transformations, complementing more traditional mononuclear precious metal catalysts. This review summarizes recent advances in homogenous catalysis using heterobimetallic carbonyl catalysts with polar metal–metal interactions.

1 Introduction

2 Hydrogenation and Hydrofunctionalization

3 Carbonylation and Carboxylation

4 Oxidative Transformations

5 Conclusion and Outlook

Catalytic Activation of N2O at a Low-Valent Bismuth Redox Platform

Herein we present the catalytic activation of N₂O at a Bi^I⇄Bi^III redox platform. The activation of such a kinetically inert molecule was achieved by the use of bismuthinidene catalysts, aided by HBpin as reducing agent. The protocol features remarkably mild conditions (25 °C, 1 bar N₂O), together with high turnover numbers (TON, up to 6700) and turnover frequencies (TOF). Analysis of the elementary steps enabled structural characterization of catalytically relevant intermediates after O-insertion, namely a rare arylbismuth oxo dimer and a unique monomeric arylbismuth hydroxide. This protocol represents a distinctive example of a main-group redox cycling for the catalytic activation of N₂O.

Bimetallic cooperation across the periodic table

Organometallic chemistry and its applications in homogeneous catalysis have been dominated by mononuclear transition-metal complexes. The catalytic performance and physico-chemical properties of these mononuclear complexes can be rationally tuned by ligand modification, which has also led to the discovery of new reactions. There is a growing body of evidence implicating the participation of two metals in catalytic processes originally believed to follow monometallic mechanisms. Moreover, the deliberate preparation of bimetallic structures has proven popular because these preorganized structures have many tunable features, such as metal-metal bond order and polarity. These structures can exhibit metal-metal complementarity and allow for multisite activation - reactivity unattainable with truly mononuclear species. This Perspective summarizes the features that are exclusive to bimetallic systems and their roles in substrate activation.

Synthesis and characterization of heterometallic complexes involving coinage metals and isoelectronic Fe(CO)5, [Mn(CO)5]- and [Fe(CO)4CN]- ligands

The chemistry of coinage metal ions with Fe(CO)₅, [Mn(CO)₅]^- and [Fe(CO)₄CN]^- has been explored using Mes₃P and N-heterocyclic carbene supporting ligands. A comparison of [(SIPr)Au-Fe(CO)₅][SbF₆], [(^Et2CAAC)Au-Fe(CO)₅][SbF₆] and [(Mes₃P)Au-Fe(CO)₅][SbF₆] shows that the ligand donor strength towards Au(i) follows the order Mes₃P > ^Et2CAAC > SIPr. These Fe(CO)₅ complexes show significant blue shifts in [small nu, Greek, macron]_CO bands relative to those observed for free Fe(CO)₅ as a result of it serving as a net electron donor to Au(i). Au(i) is a much stronger acceptor in (SIPr)Au-Mn(CO)₅ compared to Ag(i) in (SIPr)Ag-Mn(CO)₅. The structural details of Mes₃PAu-Mn(CO)₅ are also presented. [Fe(CO)₄CN]^- afforded CN bridged coinage metal complexes with (IPr*)Au⁺, (SIPr)Ag⁺ and (SIPr)Cu⁺ moieties, rather than molecules with direct Fe/coinage metal bonds. The computed total interaction energies indicate that both [Mn(CO)₅]^- and [Fe(CO)₄CN]^- are stronger donors toward Au(i) than Fe(CO)₅. A detailed analysis of the bonding interactions between the coinage metal ions and Fe(CO)₅, [Mn(CO)₅]^- and [Fe(CO)₄CN]^- suggests that the largest contribution comes from electrostatic attraction, while the covalent component follows the Dewar-Chatt-Duncanson model. The σ-donor interactions of these organometallic ligands with coinage metal ions are considerably stronger than the π-backbonding from the coinage metal ions.

Mononuclear Cu Complexes Based on Nitrogen Heterocyclic Carbene: A Comprehensive Review

During the last decade, organometallic, coordination, and catalytic chemistry of the three-dimensional metals such as copper (Cu) has been greatly affected by the emergence of nitrogen heterocyclic carbene (NHC) complexes. The NHCs, and in particular the mononuclear Cu^I-based ones, have been proven vastly useful in several applications such as in biosynthesis, catalysis, photochemistry, etc. This review tries to thoroughly describe a series of mononuclear Cu^I NHC complexes and their subcategories such as heteroleptics, and bidentate and tridentate heteroatom complexes, and give some detailed insights on their development, emergence, and applications. A brief outlook is also disclosed to enable other researchers to further develop a platform for future advances and studies in the field of Cu^I-based NHCs.

N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt

The emergence of N-heterocyclic carbenes as ligands across the Periodic Table had an impact on various aspects of the coordination, organometallic, and catalytic chemistry of the 3d metals, including Cu, Ni, and Co, both from the fundamental viewpoint but also in applications, including catalysis, photophysics, bioorganometallic chemistry, materials, etc. In this review, the emergence, development, and state of the art in these three areas are described in detail.

Preparation and characterisation of heterobimetallic copper-tungsten hydride complexes

The preparation and structural characterisation of three new heterobimetallic hydride complexes containing 3-centre,2-electron W-H-Cu bonds is reported. These complexes have been characterised by single crystal X-ray crystallography and multinuclear NMR spectroscopy. The bonding in these complexes has been analysed by DFT calculations.

Cobalt catalysed reduction of CO2via hydroboration

We report an operationally convenient reduction of CO2 to methanol via cobalt catalysed hydroboration which occurs under mild reaction conditions. Addition of NaHBEt3 to Co(acac)3 generates an active hydroboration catalyst, which is proposed to be a "Co-H" species on the basis of infrared spectroscopy. The reduction of CO2 in the presence of various boranes showed that BH3·SMe2 afforded near quantitative conversion (98% NMR yield) to methanol upon hydrolysis.

Zwitterionic indenylammonium with carbon-centred reactivity towards reversible CO2 binding and catalytic reduction

We report the synthesis and characterization of a zwitterionic indenylammonium compound and its carbon-centred reactivity towards reversible CO₂ binding at ambient temperature through its formal insertion into a C-H bond as well as the catalytic hydroboration of CO₂ to methanol derivatives.

Interrogating heterobimetallic co-catalytic responses for the electrocatalytic reduction of CO2 using supramolecular assembly

The use of hydrogen-bonding interactions to direct the non-covalent assembly of a heterobimetallic supramolecular system with Re and Mn bipyridine-based electrocatalysts is reported. Under catalytic conditions, the formation of hydrogen bonds generates a catalyst system which passes ∼10% more current than the individual current responses of the respective Re and Mn complexes for the reduction of CO₂ to CO and H₂O. Infrared spectroelectrochemical studies indicate that the Re and Mn metal centers interact during the reduction mechanism, even forming heterobimetallic bonds under reducing conditions in the absence of substrate. These findings demonstrate that non-covalent assembly is a powerful method for generating new co-catalyst systems with greater reactivity and efficiency for transformations of interest.

Heptanuclear Fe5Cu2-Phenylgermsesquioxane containing 2,2'-Bipyridine: Synthesis, Structure, and Catalytic Activity in Oxidation of C-H Compounds

A new representative of an unusual family of metallagermaniumsesquioxanes, namely the heterometallic cagelike phenylgermsesquioxane (PhGeO₂)₁₂Cu₂Fe₅(O)OH(PhGe)₂O₅(bipy)₂ (2), was synthesized and structurally characterized. Fe(III) ions of the complex are coordinated by oxa ligands: (i) cyclic (PhGeO₂)₁₂ and acyclic (Ph₂Ge₂O₅) germoxanolates and (ii) O^2- and (iii) HO^- moieties. In turn, Cu(II) ions are coordinated by both oxa (germoxanolates) and aza ligands (2,2'-bipyridines). This "hetero-type" of ligation gives in sum an attractive pagoda-like molecular architecture of the complex 2. Product 2 showed a high catalytic activity in the oxidation of alkanes to the corresponding alkyl hydroperoxides (in yields up to 30%) and alcohols (in yields up to 100%) and in the oxidative formation of benzamides from alcohols (catalyst loading down to 0.4 mol % in Cu/Fe).

Diverse bimetallic mechanisms emerging from transition metal Lewis acid/base pairs: development of co-catalysis with metal carbenes and metal carbonyl anions

The rational development of catalytic reactions involving cooperative behavior between two catalytic reactive sites represents a frontier area of research from which novel reactivity and selectivity patterns emerge.

Hydroboration of carbon dioxide with catechol- and pinacolborane using an Ir–CNP* pincer complex. Water influence on the catalytic activity

Lutidine-derived CNP*–Ir complexes catalyze the hydroboration of CO₂ to methoxyborane and formoxyborane in the presence of small amounts of water.

Heterobimetallic Silver-Iron Complexes Involving Fe(CO)5 Ligands

Iron(0) pentacarbonyl is an organometallic compound with a long history. It undergoes carbonyl displacement chemistry with various donors (L), leading to molecules of the type Fe(CO)_x(L)_5-x. The work reported here illustrates that Fe(CO)₅ can also act as a ligand. The reaction between Fe(CO)₅ with the silver salts AgSbF₆ and Ag[B{3,5-(CF₃)₂C₆H₃}₄] under appropriate conditions resulted in the formation of [(μ-H₂O)AgFe(CO)₅]₂[SbF₆]₂ and [B{3,5-(CF₃)₂C₆H₃}₄]AgFe(CO)₅, respectively, featuring heterobimetallic {Ag-Fe(CO)₅}⁺ fragments. The treatment of [B{3,5-(CF₃)₂C₆H₃}₄]AgFe(CO)₅ with 4,4'-dimethyl-2,2'-bipyridine (Me₂Bipy) and Fe(CO)₅ afforded a heterobimetallic [(Me₂Bipy)AgFe(CO)₅][B{3,5-(CF₃)₂C₆H₃}₄] species with a Ag-Fe(CO)₅ bond and a heterotrimetallic [{Fe(CO)₅}₂(μ-Ag)][B{3,5-(CF₃)₂C₆H₃}₄] with a (CO)₅Fe-Ag-Fe(CO)₅ core, respectively, illustrating that it is possible to manipulate the coordination sphere at silver while keeping the Ag-Fe bond intact. The chemistry of [B{3,5-(CF₃)₂C₆H₃}₄]AgFe(CO)₅ with Et₂O and PMes₃ (Mes = 2,4,6-trimethylphenyl) has also been investigated, which led to [(Et₂O)₃Ag][B{3,5-(CF₃)₂C₆H₃}₄] and [(Mes₃P)₂Ag][B{3,5-(CF₃)₂C₆H₃}₄] with the displacement of the Fe(CO)₅ ligand. X-ray structural and spectroscopic data of new molecules as well as results of computational analyses are presented. The Fe-Ag bond distances of these metal-only Lewis pairs range from 2.5833(4) to 2.6219(5) Å. These Ag-Fe bonds are of primarily an ionic/electrostatic nature with a modest amount of charge transfer between Ag⁺ and Fe(CO)₅. The ν̅(CO) bands of the molecules with Ag-Fe(CO)₅ bonds show a notable blue shift relative to those observed for free Fe(CO)₅, indicating a significant reduction in Fe→CO back-bonding upon its coordination to silver(I).