A Blueprint for Secondary Coordination Sphere Editing: Approaches Toward Lewis-Acid Assisted Carbon Dioxide Co-Activation

Graphical Abstract

The reduction of carbon dioxide (CO₂) to value-added products is top-of-mind. In this arena, chemists are well-positioned to lead the charge. Herein, we present advances made in homogeneous CO₂ reduction using transition metal/Lewis-acid systems, with a focus on installing Lewis-acids into the primary or secondary coordination sphere of the ligand framework.

Abstract

Carbon dioxide (CO₂) is a potent greenhouse gas of environmental concern. Seeking to offer a solution to the “CO₂-problem”, the chemistry community has turned a focus toward transition metal complexes which can activate, reduce, and convert CO₂ into carbon-based products. The design of such systems involves judicious selection of both metal and accompanying donor ligand; in part, these efforts are motivated by biological metalloenzymes that undertake similar transformations. As a design element, metal-ligand cooperativity, which leverages intramolecular interactions between a transition metal and an adjacent secondary ligand site, has been acknowledged as a vitally important component by the CO₂ activation community. These systems offer a “push-pull” style of activation where electron density is chaperoned onto CO₂ with an accompanying electrophile, such as a Lewis-acid, playing the role of acceptor. This pairing allows for the stabilization of reactive C_xH_yO_z-containing intermediates and can bias CO₂ product selectivity. In the laboratory, chemists can test hypotheses and ideas, enabling rationalization of why a given pairing of transition metal/Lewis-acid leads to selective CO₂ reduction outcomes. This Concept identifies literature examples and highlights key design properties, allowing interested contributors to design, create, and implement novel systems for productive transformations of a small molecule (CO₂) with huge potential impact.

1 Introduction

The global community has long-recognized the importance of reducing global carbon dioxide (CO₂) concentrations.¹ Anthropogenic greenhouse gas emissions have led to an annual increase in global temperature. Of these perpetrators, the greatest contributor to climate change is CO₂, with atmospheric levels reaching unprecedented concentrations in recent history. As such, there is merit not only in focusing on the removal of CO₂,² but also in taking advantage of this abundant C₁ source to construct other chemicals, including fuels.^{1, 3}

To effectively activate CO₂, we must first develop systems which take advantage of a range of molecular interactions to selectively convert CO₂ into a product of interest. Of synthetic chemistry-driven efforts, coordination complexes can be used to probe critical structure-reactivity relationships that allow us to better understand modes of CO₂ activation and subsequent reduction. This topic has inspired fruitful investigations that have sought to design new molecular units for the conversion of CO₂ into value-added product(s). To evolve this space, we can begin to outline key design characteristics to access new reactivity, with the goal of ushering forward systems that offer novel capacity for the productive and efficient reduction of CO₂. We additionally recognize important contributions made by the heterogenous catalysis community, though these works will not be discussed here.^4-9

The stability of CO₂ is derived from its symmetry and high C−O bond strength. CO₂ is a triatomic molecule that possesses two short and equivalent C−O bonds (1.1602(8) Å)¹⁰ (c.f., for formaldehyde (H₂CO), d_C-O=1.209(3) Å)¹¹ arranged linearly from the central carbon atom, which renders CO₂ nonpolar.¹² With a C−O bond strength of 799 kJ mol⁻¹, CO₂ is a stable molecule (c.f., N₂ triple bond=942 kJ mol⁻¹ and CO double bond in H₂CO=748 kJ mol⁻¹),¹³ rendering it difficult to completely deconstruct. One of the most challenging issues with the reduction of CO₂ using homo– or heterogeneous systems is in ensuring product selectivity.¹⁴ This difficulty arises due to the many possible products accessible from CO₂ reduction, such as carbon monoxide (CO), formate ([HCO₂]⁻), methanol (CH₃OH), and methane (CH₄), etc. – all of which have niche applications and uses.¹⁵ To mitigate undesired reaction pathways,¹⁶ there are many variables to consider, including ligand design, metal choice, and co-additives.¹⁷

Carbon monoxide dehydrogenase (CODH; Figure 1A) inspires synthetic design advancements. This system, which catalyzes both the reduction of CO₂-to-CO and the reverse, oxidation of CO-to-CO₂, employs metal-ligand cooperativity (MLC), Lewis-acid/base reactivity, and second coordination sphere effects. In CODH, both an Fe and Ni center act in tandem to bind CO₂.^{18, 19} Given that enzymes are some of the most efficient catalytic entities on the planet, their mechanisms of substrate activation provide insight into methods for efficient chemical transformations through biomimicry.²⁰ Therefore, if we examine the catalytic activity of enzymes and their mechanism of action on substrates such as CO₂,^{21, 22} H₂,²³ N₂,²⁴ and CO,²⁵ (as examples) we can assess the efficacy of related design elements in a laboratory setting (Figure 1B). The enzymatic features outlined above have been imitated using bimetallic complexes, main-group co-catalysts, and secondary coordination sphere effects, including pendant proton donor/acceptor groups, to name a few.^26-38 Bimetallic catalysts, for example, show promising activity for CO₂ reduction as compared to their monometallic counterparts due to cooperativity between metal centers.^{4, 39} Additionally, pre-installed Lewis acids (in the secondary coordination sphere) have been shown to promote the stabilization of transition metal bound-CO₂, leading to activation.⁴⁰

Figure 1

A) Overview of the active site of carbon monoxide dehydrogenase (CODH); B) The concept of “push-pull” imparted throughout this article to activate CO₂ using a transition metal and Lewis-acid.

In this Concept, we employ a “bottom-up” approach to molecular design for the systematic construction of transition metal complexes for CO₂ activation beginning with individual ligand components and moving to completed systems. Of note, Campos and co-workers, along with others, have contributed complementary reviews on this subject.^41-43 This Concept, however, focuses on CO₂ activation using transition metal complexes which feature intramolecularly-positioned Lewis-acids.

2 Establishing Ligand Design Characteristics

By examining the above-mentioned factors and previously established (successful) design elements, we can discern which strategies allow us to target desired reactivity. In doing so, we can access an adapted, and optimized strategy for CO₂ activation using molecular coordination compounds. The following section introduces readers to key components of successful catalytic systems by examining the stepwise integration of ligand design concepts.

2.1 Options in Ligand Scaffold Development

Metal center and accompanying ligand scaffold govern catalyst properties. Changing the metal center, altering ligand characteristics such as thedonor atoms,backbone/tether length, and type used, modifying secondary functional elements, etc. will alter reactivity. When describing ligand design, the following terms become crucial. The primary coordination sphere, bound to the metal center, confers electronic properties onto a metal center by virtue of hard (C, N, O) or soft (P, S) donors that can be varied to tune reactivity. Alternatively, the secondary coordination sphere, which contains functional units that are not directly connected to the transition metal, can be used to elicit differential reactivity at a transition metal center.³² Together, these ligand features determine a complex's steric and electronic profile, changing its reactivity depending on the arrangement of atoms present. Common ligand types have included bipyridine, pincer, tripodal, salen, diphosphine, and macrocyclic ligands – which have been modified to provide an assortment of desired properties.

As an example, bipyridine ligands are a prevalent framework for CO₂ reduction catalysis, whose hard N-based donors have found great appeal with oxidized first-row transition metals.⁴⁴ As such, Kojima and co-workers reported on the synthesis and catalytic efficiency of a Ni(II) complex bearing an {S₂N₂} tetradentate ligand (Figure 2A). This design sought to mitigate difficulties arising from the lability of N-based ligands with reduced metal centers. The complex was able to catalytically reduce CO₂-to-CO with high selectivity (>99 %). Additionally, the catalyst was able to facilitate this reactivity many times over – indicated by a turnover number (TON) over 700.⁴⁵ Due to limitations of the reduced Ni(0) center to coordinate CO₂ favourably, the system exhibited little catalytic activity at low CO₂ concentration. Overall, this report highlights how a first-row transition metal (Ni) can selectively reduce CO₂ where “simple” ligand modification improves CO₂ binding and CO dissociation, leading to high TON.

Figure 2

A) Ni-based catalyst featuring an {S₂N₂} ligand for CO₂ reduction to CO; B) Mn-based catalyst that employed stoichiometric Mg²⁺ to reduce CO₂; C) An improvement on the Ni-based catalyst in (A) whereby Mg²⁺ is directly bound within the ligand framework, enhancing CO₂ reduction turnover frequency (TOF).

Metal-ligand cooperativity can be used to promote and stabilize CO₂ coordination, thereby increasing catalyst efficiency. This characteristic encompasses reactivity where the ligand participates in, or facilities, a reaction without itself appearing stoichiometrically in the products.⁴⁶ Different types of cooperative ligands have been shown to enhance CO₂ reduction catalysis, acting as charge carriers to facilitate multielectron transfers,^{47, 48} hemilabile chelating agents to favour substrate binding,^{49, 50} or directing groups that enhance selectivity.⁵¹

2.2 CO₂: A “Push-pull” of Electron Density

The cooperative reactivity observed in CODH results from a “push-pull” mechanism.⁵² The electron-rich Ni acts as a Lewis-base and activates CO₂ through π-backdonation and Fe acts as a Lewis-acid, anchoring CO₂ at the active site by pulling electron density away from the π-system, promoting further donation from the Ni center.¹⁵ Leveraging this cooperativity in molecular complex design, chemists can similarly employ bimetallic complexes for CO₂ activation,⁵³ where one metal acts as a donor and the other, an acceptor (Lewis-acid). This approach leads to stabilization of C_xH_yO_z-containing intermediates.⁴¹

The incorporation of Lewis-acids can allow for increased reaction rates resulting in superior catalytic activity.⁵⁴ Kubiak and co-workers demonstrated that use of a Mg²⁺ co-additive alongside a Mn bipyridine catalyst provided catalytic rate increases proportional to [Mg²⁺] at low concentrations (Figure 2B).²⁹ The concentration-dependent rate indicated that the Lewis-acid was involved in the rate determining step(s). Moreover, using an intermolecular Lewis-acid resulted in the reductive disproportionation of CO₂-to-CO and carbonate ([CO₃]²⁻). From these results, the authors concluded that Mg²⁺ was integral to both stabilizing electron-rich [CO₂]⁻ intermediates and facilitating C−O bond cleavage. While stabilizing bound CO₂, this system required stochiometric amounts of Mg²⁺ due to the formation of MgCO₃ during turnover, which hampers catalyst stability. This illustrates the efficacy of utilizing Lewis-acids to stabilize {CO₂}-bound transition metal complexes, and how this strategy can be harnessed to improve catalyst activity.

The hampered catalyst activity noted above, due to the formation of insoluble MgCO₃, can be mitigated by employing a soluble chelated Lewis-acid.³⁶ Building off of their previous works, Kubiak and co-workers reported the use of the Lewis-acid complex, [Zn(cyclam)]²⁺, as a co-catalyst alongside the Mn catalyst featured in Figure 2B. This modification aided to disfavor the formation of ZnCO₃ precipitate due to the strongly chelating cyclam ligand, thus the Lewis-acid could be used catalytically. Alternatively, this stoichiometric and insoluble reactivity can be mitigated by instead installing the Lewis-acidic species in the ligand scaffold.

2.3 Lewis-Acid Incorporation into Ligand Scaffolds

The covalent installation of Lewis-acidic groups into a ligand framework jointly provides an entropic benefit, by mitigating the need for cooperative reactivity by two species, and can lead to selective reactivity. Here, appended cooperativity is accessible through numerous ligand scaffolds. The nature of cooperative action depends on the Lewis-acid used and proximity of the acid to the metal center. Lewis-acids can be installed in either of the primary or secondary coordination spheres to impart cooperative functionality.³² Both have been shown to stabilize CO₂ and subsequent reactive intermediates along the CO₂ reduction pathway.^{55, 56}

By stabilizing catalytic intermediates, we ultimately improve a catalyst's activity and can expect to see greater TONs at lower CO₂ concentration. This is exemplified by Kojima and co-workers who reported a new catalyst, featuring a modification of their previous {S₂N₂} framework in Figure 2A, to include peripheral pyridine groups in the secondary coordination sphere (Figure 2C).⁵⁷ Rather than directly adhering a Lewis-acid into the ligand scaffold, these pendant pyridines served as a Lewis-acid binding site, allowing for coordination of various Lewis-acidic moieties in proximity to the active metal center. The authors reported differences in catalyst activity between species with and without the Lewis-acid installed in the secondary coordination sphere. Rate of CO evolution (from CO₂ reduction) for the Mg²⁺-installed catalyst, at 5 % CO₂ atmosphere, surpassed those of the catalyst without Mg²⁺ at 100 % CO₂ atmosphere. These significant deviations indicate greater catalyst activity is afforded by incorporating Lewis-acids into the coordination complex. Moreover, the authors also reported the same drastic difference in rate between the Mg²⁺-installed catalyst and their initial catalyst (discussed previously – Figure 2A) in the presence of Mg²⁺. This shows that installation of an intramolecular Lewis-acid can improve catalyst activity c.f., addition of an intermolecular additive.

2.4 Transition Metal Cooperativity and Lewis-Acid Effects

Methods to quantify Lewis-acidity are well-established and allow for thoughtful choice of Lewis-acid partner for a given catalytic system. Indeed, numerous methods have been developed to quantify relative Lewis-acidity values, including the Gutmann-Beckett method,^{58, 59} fluorescence spectroscopy,^{60, 61} and computational methods such as fluoride ion affinity⁶² and hydride ion affinity.⁶³

When incorporating Lewis-acids into a ligand scaffold, the choice of acid impacts catalyst output. In arriving at an optimized Ni(II)/Mg²⁺ system, Kojima and co-workers evaluated the performance of various cationic Lewis-acids alongside their Ni(II) catalyst.⁵⁷ When using Li⁺, Na⁺, Sc³⁺, or Y³⁺, the ion-bound catalyst species was not observed. For Ca²⁺ and Zn²⁺, the ion-bound catalyst species was observed, with catalyst selectivity for these cations being nearly on-par with the original Mg²⁺ species. Differences in catalyst performance between Mg²⁺, Ca²⁺, and Zn²⁺ have yet to be elucidated. One can postulate, however, that it may result from preferential overlap (by similar energy orbitals) between the nucleophilic regions in CO₂ and the electrophilic orbitals of the 2+ cations. This favourable orbital overlap can alternatively be exploited by featuring borane (BR₃) groups along with transition metals, which may similarly capture and activate CO₂ due to preferential B−O interactions.⁴¹

Turning to main-group Lewis-acids, Piers and co-workers showed that a {Sc(III)−H−B(C₆F₅)₃} complex was able to reduce CO₂ to [HCO₂]⁻ in the solution- or solid-state.⁶⁴ Owing to B(C₆F₅)₃-stabilization of the {Sc(III)−H} moiety, this complex also demonstrated competent activity in the hydrosilation of CO₂ using Et₃SiH as the hydride source. In this case, [HB(C₆F₅)₃]⁻ stabilized a [Cp*₂Sc]⁺ (Cp*=C₅Me₅⁻) Lewis-acid, which has an open coordination site primed for CO₂ binding. Due to the moderate hydricity of [HB(C₆F₅)₃]⁻ (ΔG_H−=270 kJ mol⁻¹)⁶⁵ and Lewis-acidity of Sc(III), this system was found to catalyze the deoxygenative hydrosilation of CO₂-to-CH₄. The importance of the Lewis-acidic boron in this catalytic system showcases that Group 13 elements can equally act to improve catalyst performance. These examples illustrate that varying the Lewis-acid identity can have a profound impact on catalyst activity, and that Group 13 elements are to be considered when incorporating Lewis-acids into catalytic systems.

3 Leveraging the Utility of Intramolecularly Positioned Lewis-Acids

To achieve the cooperativity introduced previously, ligand design principles must be employed to carefully tune the steric and electronic parameters of a molecular system. The Lewis-acid must have frontier orbitals of similar energy to bind the oxygen atom(s) of CO₂ and the transition metal must be substantially electron donating to weaken the C−O bond via dπ-to-π*-backdonation. Alternatively, the transition metal must be capable of promoting hydride transfer to CO₂ and the Lewis-acid must have a similar hydricity (ΔG(H⁻) or H⁻ donor ability) to stabilize M−H−LA (LA=Lewis acid) species. By incorporating a Lewis-acid into the ligand scaffold, we can tune these “push-pull” interactions to impart greater cooperativity. Additionally, designing an intramolecular component into these systems can facilitate substrate activation without the need for further co-additives. In recent years, chemists have taken to installing Group 13 Lewis-acids into transition metal complexes either in the primary or secondary coordination sphere. Advantageously, Group 13 Lewis-acids are modifiable (tuneable Lewis-acidity through the addition of electron withdrawing groups) and can easily bind to substrates such as CO₂ through a vacant p-orbital. As such, these moieties are often featured in notable ligand design advancements - a focus of the following sections.

3.1 Considerations for Installing Lewis-Acids into the Primary Coordination Sphere

The installation of a primary coordination sphere Lewis-acid can stabilize transition metal CO₂ coordination. Indeed, this “push-pull” bond-weakening leads to bent η¹-CO₂ structures, thought to be the first step in electrocatalytic CO₂ reduction.⁷ This design principle is featured in the work of Bourissou and co-workers whereby a geminal {P−Al} type ligand resulted in a T-shaped Pt complex bearing a direct [Pt−Al] interaction (Figure 3A).⁶⁶ This complex reacted with CO₂, forming a [(η¹-CO₂)−Pt] moiety, with one of the CO₂ oxygen atoms binding to Al. This serves as a rare example of a carbon-centered η¹-CO₂ binding mode at a d¹⁰ metal center and showcases the ability of Lewis-acids to cooperatively de-symmetrize CO₂ for further reactivity.

Figure 3

A) a [Pt−Al] complex leads to a η¹-CO₂ Pt complex; B) Al in the primary coordination sphere increases the turnover frequency for hydrosilylation of a Pd complex to 19,300 h⁻¹; C) Ga in the primary coordination sphere stabilizes a d¹⁰ anionic [Ni−H].

Incorporation of a Lewis-acid into the primary coordination sphere can alternatively modify the electronic properties of a transition metal center. As such, ligand design endeavours by Takaya and Iwasawa using Al notably resulted in an unprecedented increase in the activity of a CO₂ hydrosilylation catalyst (Figure 3B). By employing a bonded [Pd−Al] complex, the catalytic hydrosilylation of CO₂ proceeded with a TOF=19,300 h⁻¹ – the highest reported to date for this reaction.⁶⁷ Similar to the previous example, they propose the σ[Pd−Al] bond imparts a trans effect, whereby Al reduces electron density at the trans-coordinated substituent, which facilitates the formation of a highly nucleophilic [Pd−H] during catalysis.⁶⁸ This serves as an instance where the inclusion of a Group 13 Lewis-acid allows for the stabilization of a [M−H] intermediate species, leading to a marked increase in catalytic activity.

Modification of a Group 13 Lewis-acid can impact transition metal reactivity. Lu and co-workers exploit this ligand design concept by synthesizing a [Ni−Ga] complex where the Ga(III) was directly bound to Ni (Figure 3C).⁶⁹ In this instance, the Ga(III) served as a Z-type acceptor, allowing Ni(0) to donate electron density into its empty orbital. Relieving electron-density at the Ni center stabilized a reactive anionic Ni(0) hydride species ([H−Ni(0)]). This catalytically active [H−Ni(0)] was only isolable due to the inclusion of Ga(III) in the primary coordination sphere. The [H−Ni(0)−Ga] complex was able to selectively hydrogenate CO₂ to [HCO₂]⁻ with a TON=3,150 and turnover frequency (TOF)=9,700 h⁻¹.

Collectively, these examples show that inclusion of a Lewis-acid in the primary coordination sphere allows for fine-tuning of a metal center's electronic properties, generally leading to higher turnover for CO₂ reduction or leading to unusual CO₂ binding modes. As a design principle, Lewis-acid inclusion should be considered for any homogeneous system intended for use in CO₂ reduction.

3.2 Secondary Coordination Sphere Systems with a Group 13 Lewis-Acidic Element

Incorporating the Lewis-acid directly into the ligand framework guarantees that the Lewis-acid is available for substrate binding (obviates diffusion/concentration effects). Generally, Lewis-acids can be incorporated into the secondary coordination sphere by virtue of a hydroelementation reaction whereby an E−H unit is added across an area of unsaturation, such as for hydroboration with HBR₂ reagents. Indeed, great effort has been focused on adding boranes into the secondary coordination sphere.³² In the context of CO₂ activation, a secondary coordination sphere Group 13 element could engage with one of the two Lewis-basic oxygen atoms in CO₂, affixing it near a transition metal center, priming it for further reduction.

In a recent study by Drover and co-workers, the stabilizing effect between Fe-bound CO₂ and a secondary coordination sphere dicyclohexylborane group was highlighted by computational means (Figure 4A). This investigation was conducted using the Fe(II) complex, [Fe-(dmpe)(Me₂BCH₂CH₂CH₂P(Me)−CH₂CH₂P(Me)₂(η²-CO₂)] (dmpe=1,2-bis(dimethylphosphino)ethane)⁷⁰ where formation of a B−O bond between one of the nucleophilic CO₂ oxygen atoms and the secondary coordination sphere borane was found to offer stabilization of −23 kJ mol⁻¹. This study provides a theoretical basis for the productive stabilization offered to CO₂ by an intramolecular borane group.

Figure 4

A) Computational investigation showing a favourable interaction between a secondary coordination sphere boron and the oxygen of an Fe(II) bound CO₂ unit; B) Re carbonyl complex with secondary coordination sphere boron leads to coupling of two CO moieties upon hydride addition; C) secondary coordination sphere boron allows the isolation of a Cu(I) hydride and the reduction of CO₂ to [HCO₂]⁻.

While no experimental evidence has yet been reported for an interaction between a secondary coordination sphere Group 13 element and a transition metal bound CO₂ unit, we can draw parallel inspiration from an early report focused on secondary coordination sphere boron engagement with a carbonyl (CO) ligand. Here, Bercaw and co-workers employed a monophosphine ligand scaffold modified with a peripheral 9-borabicyclo[3.3.1]nonane (9-BBN) group on Re (Figure 4B).⁷¹ The resulting [Re(CO)₂(phosphinoborane)] complex underwent reductive coupling of two CO ligands upon exposure to 2 equivalents of hydride (either Na[HBEt₃] or [HPt(dmpe)₂]⁺). This productive reactivity results from a B−O interaction between the secondary coordination sphere borane and an O atom of bound CO. Without this favorable B−O interaction i. e., without the boron in the secondary coordination sphere, reductive coupling was not witnessed. This report illustrates that reduced CO₂ products, such as CO, can be further transformed with the inclusion of a Group 13 Lewis-acid in a coordination compound's secondary coordination sphere.

Szymczak and co-workers show how a secondary coordination sphere borane unit can be used in a CO₂-to-[HCO₂]⁻ transformation. The authors synthesized a pseudo-tetrahedral Cu(I)−OTf bearing a 9-BBN in its secondary coordination sphere (Figure 4C).⁷² Crucially, reacting this complex with a hydride source (K[HBEt₃]) resulted in the formation of an observable Cu(I)−H species. Formation of a B−H bond between the Cu(I)−H and the secondary coordination sphere boron afforded stability, as an analogous Cu(I)−H was inaccessible without a secondary coordination sphere borane. Exposure of this complex to CO₂ resulted in the formation of a μ-[HCO₂]⁻ dimer with two {B-[HCO₂]⁻} bridging units. Here, the importance of boron in the secondary coordination sphere is underscored in its requirement to stabilize hydride formation and thus permit proper CO₂ reduction.

By including a Group 13 element in the secondary coordination sphere of a transition metal complex, reactivity can be thoughtfully tuned. These examples highlight the importance of Lewis-acid incorporation for CO₂ activation and downstream functionalization.

4 Summary and Outlook

Controlling atmospheric CO₂ concentration is critical to ensuring the health and well-being of future generations. This contribution is painted against a climate backdrop that calls for an energy transition moving humanity away from carbon-based energy feedstocks. Chemists are well-positioned to tackle this problem by designing molecular complexes that convert this waste chemical into value-added carbon-based products. By leveraging the inherent properties of transition metals and Lewis-acids, we can begin to achieve cooperative effects that allow for advances to be made in CO₂ reduction chemistry. The following key takeaways are offered:

1. Transition metals bind CO₂, leading to C−O bond weakening in a “push”-type fashion, encouraging downstream functionalization [transition metal choice];

2. Exogenously added Lewis-acids, help weaken CO₂(C−O) bonds by accepting electron density from their Lewis-basic oxygen atoms, in a “pull”-type fashion [exogenous Lewis acids];

3. Lewis-acid installation (into the primary coordination sphere) obviates diffusion and concentration effects, avoiding the use of additional stoichiometric co-reagents and may help to stabilize reactive species such as M−H units, leading to increased catalytic activity for CO₂ reduction [1° sphere Lewis acid effects];

4. Lewis-acid installation (into the secondary coordination sphere) obviates diffusion and concentration effects, avoiding the use of additional stoichiometric co-reagents and may help to stabilize reactive metal-bound C_xH_yO_z intermediates via LA−O interactions (LA=Lewis acid) [2° sphere Lewis acid effects];

In sum, this Concept has served as a walkthrough guide for the entry-level practitioner wishing to take advantage of cooperative CO₂ activation strategies. Through the addition of exogenous Lewis-acids, to inclusion of a Lewis-acid in the primary or secondary coordination sphere, reactivity can be modified to best confer the electronic and steric properties requisite for a CO₂ conversion of choice.

Acknowledgments

The authors are grateful to Western University, the Council of Ontario Universities for a John C. Polanyi award to M.W.D., and the Natural Sciences and Engineering Research Council of Canada for program funding (Discovery Grant, RGPIN-2020-04480) and a CGS−D/NSERC Vanier graduate award to J.A.Z.

Conflict of interests

The authors declare no conflict of interest.

Biographical Information

Marcus W. Drover is an Assistant Professor at Western University. He obtained a PhD from the University of British Columbia with Laurel Schafer and Jennifer Love (2016) and performed postdoctoral work at Caltech with Jonas Peters (2017–2019). During his training, Drover was recipient of a Vanier scholarship (2013–2016), MSFSS Supplement (Oxford with Andrew Weller), and Banting/Resnick Postdoctoral Fellowships (2017–2019). Drover's independent program began in July 2019 – his program intersects the fields of main group and organometallic chemistry. Drover also recently joined the Editorial Advisory Boards of Organometallics and Journal of Coordination Chemistry.

Biographical Information

Connor S. Durfy is a B.Sc. (Hons.) student at Western University in Integrated Sciences with a focus in Chemistry. He first worked with Prof. Drover through the Inorganic Chemistry Exchange (ICE) program in 2022. During this time, he explored novel modes of metal-ligand cooperativity for the activation of CO₂ using an iron bis(diphosphine) compound.  Currently, he is working on developing new synthetic routes to access iron compounds with intramolecular Lewis-acids to promote the cooperative activation of small molecules.

Biographical Information

Joseph A. Zurakowski is a Ph.D. candidate under the supervision of Prof. Marcus Drover at Western University. He obtained a B.Sc. (Hons.) degree from Carleton University (2020) in Ottawa, Canada, before joining the Drover lab to pursue his graduate studies. He was awarded a prestigious Vanier Scholarship to pursue his Ph.D. and has since contributed to the field of secondary coordination sphere (SCS) design. His work focuses on the installation of Lewis-acids into the SCS of Fe, Co, and Ni complexes, with a special interest on small molecule activation.