Review on activation of small molecules (dihydrogen,carbon dioxide,and dinitrogen) by various transition metal complexes

Abstract

Small molecules such as dihydrogen (H₂), carbon dioxide (CO₂), and dinitrogen (N₂) are ubiquitous in nature and have important industrial and environmental applications. This review article discusses the activation of these small molecules utilizing transition metal complexes because the activation and transformation of these molecules into value added products is a topic of current research due to economic importance. The review also discusses the interesting interaction between small molecules and metal complexes with their important activation mechanisms, such as the formation of hydride complexes for H₂, formate complexes for CO₂, and ammonia production from N₂. The activation of these compounds using metal complexes recently considered as a more affordable method of synthesizing new organic molecules through catalytic processes. Therefore, this article aspires to provide insights into boosting metal complex design for improved reactivity and greater practicality in industrial and environmental situations by combining the most recent knowledge.

Keywords

activation of small molecules transition metal complexes

Introduction

Small molecules include molecules and molecular ions such as H₂,¹ CO, N₂, H₂O, HCN, O₂, CO₂, CH₄, C₂H₄, C₂H₂, CN, N₃,^2,3 N₂O, and SO₂,⁴ P₄⁵ that include two, three, or four atoms each carrying a small number of electrons. These compounds can be produced in large quantities via industrial processes or are found throughout nature. Small molecules need a bond-activation for transformation since they usually involve one or more very inert bonds and are thermodynamically stable. This purpose has long been served by the utilization of transition metals. In scholarly and societal settings, the importance of small molecule activation is increasing.^6,7 Recently, there has been a lot of interest in the activation of such compounds by transition metal complexes as a more cost-effective way to create novel organic molecules via catalytic reactions.^8,9 Each activation process has a distinct set of requirements according to the desired outcome, which could include changing the reaction pathway, selectivity, efficacy, yields, new processes or reactions that are not allowed thermodynamically or kinetically from the substrate ground state, a lower energy input, and so on. Thermal or photochemical methods may be used to directly or indirectly accomplish these goals in homogeneous or heterogeneous systems.¹⁰

To fulfill the demands of the expanding energy needs, the chemical energy stored in these molecules can be converted into electrical energy. For example, hydrogen can be employed as fuel in fuel cells in order to generate sufficient electricity to run cars and appliances in the house ^11-13 and also The process of CO₂ conversion by electro or photocatalysis to value-added fuels and chemical products is an exciting possibility under investigation for climate change control and sustainable energy storage.^14-16 In their small molecule activation study William and Tolman¹⁷ reported the activation of N₂O at transition metal centers and the homogeneous functionalization of CH₄ was also reported by Gunsalus et al.¹⁸ additionally the reductive cyclotrimerization of carbon monoxide (CO) to the deltate dianion was also reported by Summerscales et al.¹⁹ another small molecule CO₂ activation was reported by Liu et al.²⁰ Zamorano et al.²¹ reported the activation of C₂H_4. In addition, frustrated Lewis pair (FLP) chemistry is also well studied in the activation of small molecules and in the activation processes FLPs utilized as effective catalysts.^22-25 However, the primary objective of this review article is the activation of H₂, CO₂, and N₂ using various transition metal complexes.

Activation of dihydrogen (H₂) by metal complexes

The activation of H₂ by transition metal centers has drawn a lot of interest over the years.^26,27 The breaking the H–H bond by transition metal complexes to form mono and dihydride complexes has been extensively studied because of its importance in catalysis.²⁸ The importance of hydrogen interaction with transition metals for both stoichiometric and catalytic reactions in organometallic chemistry has been extensively studied.^29-31 Molecular hydrogen can be activated in several ways upon interaction with a metal core.^32,33 The first method is that homolytic cleavage (the model of oxidative addition) separates H₂ into two hydrogen atoms, each with one electron. Actually, oxidative addition involves the addition of H₂ to the metal center of a metal complex, increasing its oxidation state. An increase in oxidation state results from the breaking of the H–H bond and each hydrogen atom binding to the metal. The second one is through heterolytic cleavage, H₂ can split into a proton and a hydride anion with two electrons because in this method one hydrogen remains H⁺ and the other hydrogen becomes H⁻ and remains with the metal.³⁴ Homolytic H₂ cleavage is promoted by low oxidation state compounds with nucleophilic metal cores.³⁵ As reported by kubas and coworkers, H₂ forms a nonclassical 3-center 2-electron (3c-2e) bond with the metal center in a side-on (η²) mode by donating its two electrons to a vacant metal d orbital. This lengthens the H–H bond ( Scheme 1 ). The strength of the electron back donation from a filled d orbital of the metal center to the δ* orbital of H₂ determines the stability of the η²-H₂ complex. As a result, the strong backdonation can occasionally break dihydrogen into complexes with a long H–H bond distance ( Scheme 2 ).^27,28,33,36

Scheme 1.

Pathways for H–H bond cleavage (this scheme also shows bonding mode of η²-H₂ complex (B) and dihydride complex (A)).

Scheme 2.

Crystallography H–H bond lengths.

Kubas,²⁷ Henderson,³⁷ and Saillard and Hoffmann³⁸ state that the reactions must include a dihydrogen complex because it can be easily understood as the oxidative addition of dihydrogen to a coordinatively unsaturated complex or as it is opposite, the reductive elimination ( Scheme 3 ).

Scheme 3.

Oxidative addition of dihydrogen to a metal center.

Vaska and DiLuzio³⁹ also reported the stereochemistry of H₂ oxidative addition by the simple and reversible reaction of H₂ with trans-IrCl(CO)(PPh₃)₂ (Vaska’s complex) to create [IrH₂C1(CO)(PPh₃)₂ ( Scheme 4(A) ). H₂ oxidative addition to complexes containing phosphine ligand was reported by Crabtree and co-workers⁴⁰ the reaction of H₂ to Ir(Ph)(CO)(PMe₃)₂ at 80 °C produces [IrH₂(Ph)(CO)(PMe₃)], and other dihydride isomer was formed when the solution was warmed to room temperature ( Scheme 4 (B)). Oxidative addition of dihydrogen to a Cobalt (I) Cation supported by a strong-field pincer ligand has also been reported by Rummelt et al.⁴¹ In their study Cationic cobalt (I) dinitrogen complexes with a strong-field tridentate pincer ligand were prepared, and the oxidative addition of polar and nonpolar bonds was studied. Addition of H₂ to [(^iPrPNP)Co(N₂)]⁺ (^iPrPNP = 2,6-bis((diisopropylphosphaneyl)me-thyl)pyridine) resulted in oxidative addition and formation of the cis-Co(III) dihydride complex, cis-[(iPrPNP)Co(H)₂N₂]⁺ ( Scheme 5 ). Cobalt complexes [Co(CN)₅]^3-and [Co₂(CO)₈] are also important for hydrogen activation to provide the formation of monohydride cobalt complexes rather than dihydride complexes ( Scheme 6 ).⁴²

Scheme 4.

Oxidative addition of dihydrogen to Iridium center.

Scheme 5.

Oxidative addition of dihydrogen to cationic (PNP) Co(I) complex.

Scheme 6.

Homolytic H₂ Cleavage of [Co(CN)₅]^3- and Co₂(CO)₈.

In 2014, Co-hydride complexes’ unique reactivity and characteristics were reported by Rozenel et al.⁴³ within the research In order to produce [(HPNP)CoCl(H)₂] (B) ( Scheme 7 ), the Co(I) chloride species [(HPNP)CoCl] (A) oxidatively adds H₂. According to the result, the produced molecule is exclusively stable in hydrogen atmosphere; even in the solid state, it rapidly regenerates A when removed. Their result revealed that both species A and B in solution are in equilibrium.

Scheme 7.

Reactivity of Co(I) complexes [(HPNP)CoCl] (A) with H₂ to produce [(HPNP)CoCl(H)₂] (B).

Heinekey et al.⁴⁴ synthesized PMe₃ analogues of Kubas complexes [(PCy₃)₂ W (CO)₃H₂] and [(PiPr₃)₂W(CO)₃H₂] ( Scheme 8 ). PCy₃ stands for tricyclohexylphosphine ligand, and PiPr₃ for triisopropylphosphine ligand. According to Matthews et al.⁴⁵ photolysis of dichloromethane solutions of M(CO)₆ (M = Cr, W) results in the production of dihydrogen complexes, W (CO)₅(H₂) and Cr(CO)₅(H₂).

Scheme 8.

Equilibrium existence of (PR₃)₂W (CO)₃(H₂) and (PR₃)₂W (CO)₃H₂ in solution.

Several examples of well-characterized CoI-dihydrogen complexes with pincer ligands have been published recently.^46,47 Suess et al.⁴⁸ synthesized Co-dihydrogen complexes supported by tris-(o-diisopropylphosphinophenyl)silyl ligation (SiP₃) by treating a Co^I-N₂ adduct with H₂ ( Scheme 9 (A)). However, (SiP₃)Co(H₂) exhibits thermal stability. Subsequently, Tokmic and colleagues⁴⁹ discovered a comparable reactivity when they synthesized Co-dihydrogen complexes of the type (CCC)Co(H)(PR) (R = Me, Ph) ( Scheme 9 (B)) using the bis(carbene) pincer ligand bis(mesityl-benzimidazol-2-ylidene)phenyl (CCC).

Scheme 9.

Examples of five-coordinate nonclassical Co-dihydrogen complexes.

Heterolytic cleavage is a more common activation process in which the coordinated H–H bond becomes polarized and splits into a hydride (H^-) legate to the metal center and also resulting in the migration of a proton (H⁺) to either an external Lewis base B (intermolecular heterolytic cleavage) or an ancillary ligand or anion (intramolecular heterolytic cleavage). In the end, this leads to H⁺ to migrate to the base and H^- to coordinate with the metal center to produce metal monohydride ( Scheme 10 ).^33,35

Scheme 10.

Two mechanisms for heterolytic cleavage of H₂.

Complexes with an electrophilic metal core facilitate H₂’s heterolytic cleavage. The electron-poor metal center’s capacity to return electrons to H₂ is limited. Therefore, the stability of the M-H₂ bond is determined by the capacity of both the metal acceptor from the hydride and the proton acceptor from the Lewis base to receive electrons. Adding more basic ligands, such as H₂O or Cl, can enhance hydrogen’s capacity to polarize its electrons and cause the heterolytic cleavage of H₂.³⁵ Crabtree and Lavin,⁵⁰ for instance, showed how a cationic molecule might cause heterolytic η²-H₂ cleavage. The cationic dihydrogen iridium complex [IrH(H₂)(PPh₃)₂(bq)]+ (B) was created by treating the corresponding aqua iridium complexes [IrH(H₂O)(bq)(PPh₃)₂] (A) with hydrogen. intermolecular heterolysis of η²-H₂ on [IrH(H₂)(PPh₃)₂(bq)]⁺ occurred by deprotonation of H₂ in the presence of a strong external base (MeLi), where bq = 7,8=benzoquinoline (Scheme 11). Gilbertson et al.⁵¹ reported H₂ activation with water soluble iron phosphine complex trans-Fe(DMeOPrPE)Cl₂ to produce η²H₂ metal hydride complex, trans[Fe(DMeOPrPE)₂H(H₂)]⁺and H⁺ through heterolysis of H₂ in water, where DMeOPrPE = 1,2-bis(bis(methoxypropyl)phosphino)ethane ( Scheme 12 ).

Scheme 11.

Some reactions of the dihydrogen complex.

Scheme 12.

Activation of H₂ in water soluble iron phosphine complex.

Ni complexes are also known to activate H₂, and it is only recently that thermally stable H₂ adducts of Ni have been identified and structurally characterized.^52,53 The few molecular Ni-(H₂) complexes that have been examined thus far are composed of cationic Ni^II species. It is confirmed that oxidative addition of H₂ to a single Ni core is essentially unknown by the lack of stable nickel dihydride complexes. Furthermore, there are no examples of H₂ adducts of zero-valent Ni centers that might subsequently undergo oxidative addition are known to exist.^54-57 Harman and colleagues⁵⁸ demonstrated H₂ heterolytic cleavage utilizing nickel-based phosphano-borane complexes. A weak bond between the arene and nickel is rapidly broken when nickel reacts with H₂ in the Ni complex shown by ( Scheme 13 ). They speculate that H₂ adds across the Ni-B bond, protonating the Ni core and causing the borane to pick up the hydride, which explains this strange reaction. Heterolysis of H₂ is reversible in the reaction system. A Pt-B combination that induces reversible H₂ cleavage was also discovered by Cowie and Emslie.⁵⁹ After being exposed to H₂, the bis(phosphano)ferrocene-borane ligated platinum complex easily inserts H₂ through the Pt-B bond ( Scheme 14 ).

Scheme 13.

Heterolytic cleavage of H₂ using Nickel based phosphano-borane complex.

Scheme 14.

Heterolytic cleavage of H₂ using a Pt-B complex.

Transition metal complexes are also widely used as hydrogen oxidation reactions to promote the activation of hydrogen.⁶⁰ The H–H bond chemical energy is transformed into electric energy via the hydrogen oxidation reaction, which splits molecular hydrogen into two protons and two electrons. The two most extensively researched active hydrogenases are [FeFe] and [NiFe] hydrogenases. The enzyme matrix that surrounds the organometallic active site of hydrogenase has a significant impact on the hydrogen activation reactivity. The choice of ligand has a significant impact on the H₂ activation’s reactivity. Scientists employ a fundamental ligand, such as a pendant amine group, to enable proton transport for transition metal complexes, drawing inspiration from the nature of biological systems. Transition metals such Ni, Fe, and Mn, are utilized to catalyze the hydrogen oxidation reaction.³⁵ For example; Liu et al.⁶¹ reported an Iron complex with pendant amines as a molecular electro catalyst for oxidation of hydrogen. A mononuclear iron complex, [Cp^C₆^F₅Fe(P^tBu₂N^Bn₂)H], was employed in the investigation and demonstrated to be an effective molecular catalyst for hydrogen oxidation process (1.0 atm, 22 °C) at the highest turnover frequencies of 2.0 s^-1 as presented in ( Scheme 15 ).

Scheme 15.

The Proposed mechanism for electrocatalytic oxidation of H₂.

In the catalytic cycle, a pendant amine group serves as a σ donor ligand to facilitate intramolecular H₂ cleavage. Tert-butyl groups can prevent exogenous amine bases from competitively binding to the cycle and promote dihydrogen coordination. In this catalytic cycle, the pro-ton is transferred to the nearby pendant amine group after Fe (II)-H is first oxidized to Fe (III)-H cation complex. Fe (II)-H cation is reduced to neutral Fe (I) by the intermolecular deprotonation of exogenous amine base, which is afterwards electrochemically oxidized to Fe (II) complex. All through H₂ uptake and subsequent H₂ heterolytic cleavage, which is facilitated by the pendant amine group, Fe (II)-H₂ is produced as an intermediate. Exogenous amine base deprotonates once more to complete the catalytic cycle and regenerate Fe(II)-H complex in the final stage of this catalytic cycle.⁶¹

H₂ activation using a silylenes was also reported. Amido(boryl)silylenes [Si(B(NDippCH)₂)(N(SiMe₃)Dipp] (where Dipp = 2,6-ⁱPr₂C₆H₃), activate H₂ under mild conditions and providing the dihydrosilane [H₂Si(B(NDippCH)₂)(N(SiMe₃)Dipp] ( Scheme 16(A) ).⁶² Similar to this, am-ido(silyl)silylene: Si(Si(SiMe₃)₃)(N(SiMe₃)-Dipp] has demonstrated reactivity with H₂ at room temperature to produce dihydrosilane [H₂Si(Si(SiMe₃)₃) N(SiMe₃)-Dipp] ( Scheme 16(B) ).⁶³

Scheme 16.

Activation of H₂ by acyclic silylenes.

Transition metal hydrides usually act as intermediates in the catalytic cycle.^64,65 Wilkinson created the first extremely active homogeneous hydrogenation catalyst, RhCl(PPh₃)₃.⁶⁶ Catalysis is the process by which alkene can be hydrogenated to alkane ( Scheme 17 ). In this catalytic cycle, the 16 electron RhClH₂(PPh₃)₂ complex is formed by oxidatively adding one PPh3 ligand, which was easily lost from the RhCl(PPh₃)₃ complex to the RhCl(PPh₃)₂ complex. RhH(alkyl)Cl(PPh₃)₂ is created when alkene is coupled to rhodium by π donation after intramolecular hydride transfer. Reductive elimination then yields an alkane product, which is used to renew RhCl(PPh3)2.

Scheme 17.

Catalytic hydrogenation of alkene by Wilkinson’s catalyst.

Generally, the activation of H₂ by utilizing transition metal complexes has numerous significant uses in a variety of domains, most notably industrial chemistry and energy. For example, one of the well-known importances is industrial production of Ammonia. Because ammonia is used as fertilizer, its production throughout the past century has been essential to sustaining population growth. Over the past century, fertilizers that contain ammonia as a fixed nitrogen source have sustained about 27% of the global population. The German chemist Fitz Haber established a technique in 1908 for the production of ammonia from H₂ and N₂ under extreme temperatures and pressures employing reuse in order to meet the enormous growth in demand. Carl Bosch commercialized this method; a prototype ammonia synthesis facility was constructed in 1911. As a result, the Haber-Bosch process is an effective way for producing ammonia from H₂ and N₂ at high temperatures and pressures.⁶⁷ The other one is hydrogenation of alkene as discussed above by Wilkinson catalyst⁶⁶ and additionally Lapointe et al.⁶⁸ reported cobalt complexes containing bulky PNP pincer ligand for H₂ Activation and catalytic hydrogenation of alkenes and alkynes. The activation of H₂ is also important in fuel cells specifically anion exchange membrane fuel cells (AEMFCs) and proton-exchange membrane fuel cells (PEMFCs) in which metal complexes-based electrocatalysts, were used for effective energy conversion from H₂ to electricity, because these complexes are essential for promoting the hydrogen oxidation reaction.^69-71

Activation of carbon dioxide (CO₂) by metal complexes

One of the main worries in the context of climate change is the startling increase in atmospheric CO₂. Globally, the construction, energy, and petroleum industries are the main sources of CO₂ emissions ⁷²,⁷³. Thus, one possible solution to this issue is the conversion of CO2 into a number of beneficial chemical compounds.^74-79 The process of CO₂ transformation is difficult because of its high kinetic and thermodynamic stability. Methanol, for instance, is an important renewable energy source that is produced when hydrogen reduces CO₂.²¹ Strong C=O bonds in CO₂ mean that overcoming the thermodynamic and kinetic stability frequently requires a highly reactive metal core.^74,80 CO₂ can only attach to metals in a few different ways before becoming activated. The bonding mode is determined by the metal center’s basicity and electrophilicity.^80,81 Numerous studies have been conducted on CO₂ activation techniques as well as stoichiometric and catalytic procedures using carbon dioxide and transition metal complexes due to its potential for activation through interaction with these complexes.^82-84 Different interactions with the metal center are caused by the CO₂ molecule’s one Lewis acid site (carbon atom) and two Lewis base sites (oxygen atoms). The four primary coordination modes in monometallic complexes are usually produced by the molecule’s initial linear form bending during coordination due to the presence of the LUMO orbitals.⁸² Therefore, Figure 1 shows the coordination mode of CO₂ at monometallic transition metal centers.

Figure 1.

CO₂ and proposed coordination modes at a monometallic center of a transition metal.

In addition to the η¹-C and η²-CO coordination modes, there are number of bridging modes, catalytic insertions, reductive coupling to oxalate, reductive deoxygenation to CO and O^2-, reductive disproportionation to CO and CO₃^2-, species have all been reported to occur in the presence of transition metal complexes.⁸⁵ For example, Fachinetti et al.⁸⁶ investigated deoxygenation and disproportionation of CO₂ promoted by transition metal complex bis(cyclopentadienyl) titanium and zirconium derivatives. In this interesting study when Cp₂Ti(CO)₂ reacts with CO₂ to generate CO and the carbonate bridged [(Cp₂Ti)₂(CO₃)]₂, complex and the corresponding Zr derivative Cp₂Zr(CO)₂ reacts with CO₂ to produce CO and the oxo bridged trimer (Cp₂ZrO)₃.

According to the review of Yin and Moss⁸⁷ the triatomic CO₂ molecule is linear. Because oxygen and carbon have different electronegativities, the oxygen atom is negatively polarized, and the carbon atom has a partial positive charge. Furthermore, in its ground state, CO₂ has two orthogonal sets of π molecular orbitals. As seen above in Figures 1 and 2, the molecule thus exhibits a number of different sites that could interact in different ways with a metal center.

Figure 2.

CO₂ and proposed coordination modes at a polymetallic center of transition metals.

For instance as reported by Calabrese et al.⁸⁸ side-on coordination modes, as depicted in Scheme 18(A) have been confirmed by X-ray crystal structural investigations of metal CO₂ and complexes η¹Coordination mode of the complex [Rh(diars)₂(Cl)(η¹CO₂)], where diars = 1,2 bis(dimethylarsino)benzene. Langer et al.⁸⁹ also reported the η¹-CO₂ complex of Iridium (Ir-η¹-CO₂) as illustrated in Scheme 18(B) . The earliest documented η2-2CO complex, which is Aresta’s complex, reported as Ni(CO₂)(PCy₃)₂, was serves as an example of the most prevalent η²-CO₂ coordination mechanism as illustrated in Scheme 19(A) .⁹⁰ The synthesis and X-ray structure of the d² 18-electron new CO₂ complex [Nb(η-C₅H₄Me)₂ (CH₂SiMe₃)(η²-CO₂)] were reported by Bristow and his coworker⁹¹ in their study [Nb(η-C₅H₄Me)₂(CH₂SiMe₃)Cl were reduced with sodium amalgam in tetrahydrofuran under 1 atm of CO₂ to produce the final product as depicted in Scheme 19(B) . The complex [Fe(PMe3)4(η2-CO2)] was also reported by Karsch⁷⁶ as depicted in Scheme 19(C) .

Scheme 18.

η¹-CO₂ coordination mode of [Ru-η¹-CO₂] and Ir-η¹-CO₂ complexes.

Scheme 19.

η²-CO₂ coordination mode of different complexes.

Insertion reactions of carbon dioxide

Inserting CO₂ into a metal–element σ-bond is an essential step in many catalytic cycles for CO₂ utilization. Nevertheless, there are few kinetic research studies and information regarding the reaction process has mostly been clarified by computational investigations, even though there have been numerous demonstrations of CO₂ insertion. As reported by Hazari the kinetic research on CO₂ insertion into late transition metal–element σ-bonds along with their catalytic implications.⁹³ Inserting the CO₂ molecule into transition metal-element σ-bonds such as M-H, M-NR₂, M–OR, and M–CR₃ at the beginning of production of chemical compounds of the type M–OC(O)E, where E may be H, OR, NR₂, or CR₃, as shown in Scheme 20 , is one promising method for activating and transforming CO₂.^87,93,94

Scheme 20.

Insertion of CO₂ into the M–E bond.

Insertion of CO₂ to metal hydride (M–H) bonds

As reported in activation above H₂ molecule is typically activated to create the hydride species, CO₂ is inserted into the TM–H bond, formate is released, and the catalyst is regenerated in the transition metal (TM) complex-catalyzed homogeneous hydrogenation of CO₂ to formate. The capture and utilization of CO₂ by the transition metal hydride complex through CO₂ insertion into the transition metal hydride bond has continued to draw the interest of chemists as a crucial step in the catalytic hydrogenation of CO₂ to formate. Research on the CO₂ insertion into the TM–H bond has been used as a model to comprehend the role of hydrogen activated as hydride in the hydrogenation of CO₂.^95,96 Here, Scheme 21 shows that the insertion of CO₂ into a M-H bond results in the formation of either a formate complex or a metallocarboxylic acid.^87,93,94

Scheme 21.

The insertion of CO₂ in M-H bond.

Two general methods for CO₂ insertion into transition metal hydrides have been postulated based on computational investigations.^97-100 The most typical mechanism involves two steps:

The nucleophilic attack of the metal hydride on CO₂ to produce an H-bound formate.

Scheme 22(A) shows the rearrangement of the H-bound formate to provide the O-bound formate product. Researchers adopted the rule that reactions in which the first step is rate-determining are considered to be outer-sphere reactions since there is no direct interaction between CO₂ and the metal center in this transition state. However, inner-sphere reactions are those where rate-determining occurs in the second step.^93,101

A single concerted pathway is also involved in the second common mechanism as shown in Scheme 22(B) . The transition state for this pathway is comparable to the transition state for the H-bound formate to O-bound formate product rearrangement in the second step of stepwise CO₂ insertion. As a result, single-step concerted pathway is also called an inner-sphere process.

Scheme 22.

Common Mechanisms for CO₂ Insertion into Metal-Hydrides bond.

The first CO₂ insertion into a transition metal hydride was reported by Lyong and colleagues.⁹⁷ It is referred to as the CO₂ insertion reaction into the Co-H Bond of Nitrogentris (tripheny1phosphine) cobalt Hydride. The formate complex [(PPh₃)₃Co(OC(O)H)] is produced when (PPh₃)₃ Co(N₂)(H) and H₃Co[(PPh₃)]₃ react with CO₂, as shown in Scheme 23(A) and Scheme 23(B) .

Scheme 23.

CO₂ insertion reaction into the Co-H Bond of Nitrogentris (tripheny1phosphine) cobalt Hydride.

According to Darensbourg and Ash¹⁰² the species nucleophilicity, which is influenced by the ligands capacity to donate electrons, determines the rates of CO₂ insertion into the M-H bond of the anionic complexes [CrH(CO)₄L], [MoH(CO)₄L], and [WH(CO)₄L] (where L=CO, PR₃). The ^tBu₂(PCP)Ni-H (PCP = 2,6-bis((phosphaneyl)methyl)phenyl)) was produced by the insertion of CO₂ within minutes at room temperature, as reported by Schmeier et al.¹⁰³ and recent study by Zhang et al.⁹⁵ and shown in Scheme 24 . This ^tBu₂(PCP) Ni-(O)-formate was characterized by X-ray crystallography.

Scheme 24.

Reaction of CO₂ with ^tBu₂(PCP)Ni-H.

Furthermore, stable formato complexes produced by Ni-H insertion of CO₂ were described by Chakraborty et al.¹⁰⁴ When a CO₂ molecule interacts with [Ni^IIH(POCOP)] Pincer Complexes, complex (A), and corresponding formate (B) as a solid was produced ( Scheme 25 ). According to the X-ray structure, the formato ligand exhibits η¹-coordination. It is interesting to note that the hydrido complex can be regenerated in a manner similar to that of formato complex of iridium which is reported by Kang and coworkers¹⁰⁵ in which dihydridoiridium(III) complex (A) was reacted to CO₂ (1 atm) η² formato adduct was produced (B) ( Scheme 26 ) by heating a solid sample of the formate under vacuum. As a result, this complex exhibits reversible behavior. Recently, Oren et al.¹⁰⁶ examined how the dearomatized hydride complexes A and C combine with CO₂ to form the aromatized complexes B and D via a rare CO₂ η^-1 coordination mode ( Scheme 27 ). According to their study, complex C reacts with substoichiometric levels of CO₂ using just 0.5 bar of CO₂, making complex D in 82% yield (per ³¹PNMR) after 8 h at room temperature. However, the conversion of A to B necessitates the more demanding conditions of 8 bar, 80 °C, and 72 h, producing 60% of the product. According to earlier research by this group, there can be significant variations in reactivity due to the steric difference between the ^tBu-PNP and ⁱPr analogues. Therefore, the Ni−CO₂ bond length can be shortened by a greater degree of π backdonation from the Ni center to the CO₂ which caused by the lower steric hindrance the ⁱPr-PNP. Recently, density functional theory (DFT) simulations are used to thoroughly examine the catalytic function of the hydride intermediate in the reduction of CO₂ to formate (HCOO–) by Ni^II-NHC complexes. A Ni^II-hydride is discovered to be sufficiently hydridic to enable the effective transfer of hydride to the carbon center of CO₂, resulting in the creation of HCOO–. Crucially, the traditional insertion of CO₂ into a metal–hydride bond is avoided by the direct hydride transfer pathway suggested here.⁹⁶

Scheme 25.

Reactivity of Ni^II-POCOP complex with CO₂.

Scheme 26.

Reactivity of dihydridoiridium (III) complex A with CO₂.

Scheme 27.

The reaction of dearomatized hydride complexes A and C with CO₂ to give the aromatized complexes B and D with a rare η¹-coordination mode of CO₂.

The insertion of CO₂ into Ir–H bonds has received significant attention in the past decades, for example, Tanaka et al.¹⁰⁷ seminar reported that (^iPrPN^PyrP)Ir(H)₃ is a highly active catalyst for the hydrogenation of CO₂ to formate, where ^iPrPN^PyrP = 2,6-NC₅H₃(CH₂PⁱPr₂)₂. Under 1 atm of CO₂ at room temperature, (^iPrPN^PyrP)Ir(H)₃ (C) is in equilibrium with the dihydrido formate species (^iPrPN^PyrP)Ir(H)₂(OC(O)H) (D)( Scheme 28 ).

Scheme 28.

Insertion of CO₂ into (^iPrPN^PyrP)Ir(H)₃.

Mitton and Turculet also reported CO₂ insertion into Pincer-supported palladium hydrides [(^CyPSiP)Pd(H)] complex (A) to form palladium formates [(^CyPSiP)Pd(OC(O)H](B), where ^CyPSiP = Si(Me)(2-PCy₂-C₆H₄)₂ as shown in ( Scheme 29 ).¹⁰⁸

Scheme 29.

Pincer-supported palladium hydride reacts with CO₂ to form palladium formats.

The homogeneous catalytic hydrogenation of supercritical carbon dioxide, or Ruthenium (II)-Catalyzed hydrogenation of CO₂, was reported by Noyori and colleagues.¹⁰⁹ In the study heterolytic α-bond activation phase is crucial in catalytic reactions. The hydrogenation of CO₂ into formic acid by a Ruthenium (II) complex is one fascinating example. The overall reaction takes place via CO₂ insertion into the Ru^II-H bond of cis-Ru(H)₂(PMe₃)₃ to form a ruthenium(II) η¹-formate intermediate, Ru(H)(η¹-OCOH)-(PMe₃)₃, H₂ coordination to the ruthenium(II) η¹-formate complex, isomerization of the intermediate, and the reaction of η¹-formate with a H₂ molecule to afford formic acid^110,111 as illustrated in Scheme 30 .

Scheme 30.

The catalytic cycle for Ruthenium (II) Catalyzed Hydrogenation of CO₂.

Insertion of CO₂ into the M–C bond

It is believed that the insertion of CO₂ into transition M–C bonds is the first important step in CO₂ dependent catalytic reactions.^112-116 It is commonly recognized that certain organometallic reagents, with or without catalysts, have the ability to add CO2 into their M–C bonds.^93,117,118

There are two different ways of inserting CO₂ in M–C bond of a complex:

1. Normal insertion, this result in the formation of a carboxylate complex.

2. Abnormal insertion, this result in the formation of a 1-metalated formate ester or alkoxycarbonyl complex as shown in Scheme 31 . As a result a new carbon-carbon bond is created when carbon dioxide is inserted into a transition metal-carbon bond.^87,112,116

Scheme 31.

The two different ways of inserting CO₂ in M–C bond of a metal complex.

Schemes 32 and 33 illustrate how the actual mechanism for CO₂ insertion into M–C bonds varies depending on the properties of the carbon-based ligand. For example, there are differences in the mechanism for introducing CO₂ into a metal-allyl bond compared to a metal-alkyl bond. The rate-determining step in these reactions, however, is typically the initial creation of the C–C bond through nucleophilic assault of the carbon ligand on CO₂. The subsequent rearrangement of a zwitterionic intermediate into a metal carboxylate product typically requires very little energy. As CO₂ does not interact with the metal at the rate-determining transition state, these reactions are typically classified as outer-sphere processes.⁹³

Scheme 32.

General mechanism for CO₂ insertion into Metal-Methyl bonds.

Scheme 33.

General Mechanism for CO₂ insertion into Metal-Allyl Bonds.

For instance, Johansson et al.¹¹⁹ described a Pd-CH₃ bond that is incredibly long and present in a (PCP)Pd-CH₃ complex that includes PCP, the most prevalent pincer ligand. Scheme 34(A) illustrates how CO₂ reacts when it is inserted, yielding the right quantity of acetate complex.

Scheme 34.

Insertion of CO₂ into Pd-CH₃ (A and B) and Ni-CH₃ (C) bond.

Recently, Deziel et al.¹²⁰ also reported the insertion of carbon dioxide into Pd-CH₃ complexes that contain pincer ligand (PBP) of the type (^tBuPBP)M(CH₃), where (^tBuPBP=B(NCH₂P^tBu₂)₂C₆H^4-, and M=Ni or Pd) to synthesize η¹-acetate complexes of the form (^tBuPBP)M(OC(O)CH₃). They conducted a rare kinetic research on carbon dioxide insertion into a late transition metal alkyl species using (^tBuPBP)Pd CH₃) since the conditions for CO₂ insertion were mild as shown in Scheme 34(B) for Palladium and Scheme 34(C) for Nickel.

As already stated, the insertion of CO₂ into metal-alkyl bond, it is also possible to insert CO₂ into both metal-allyl and metal-aryl bond. For example, Brouwer et al.¹²¹ reported that the 16-valence electron diaryl complex [CpM(NO)Ph₂] undergoes CO₂ insertion to give the η²-carboxylate complex [CpM(NO)(η²O₂CPh)Ph] Scheme 35(A) . A coordinatively unsaturated acetylide complex [CpRu(PPh₃)(CCPh)], has also been reported to react with CO₂ to produce interesting carboxylate complex [CpRu(PPh₃)(η²-O₂CCCPh)]. The formation of the carboxylate complex can be explained by migratory insertion of the nucleophilic acetylide ligand to a weakly coordinated carbonyl carbon of CO₂, Scheme 35(B) .¹²² In order to create tin carboxylates, which are commonly employed in industry as stabilizers, Pd-catalyzed CO₂ insertion into Sn-allyl bonds was studied by Shi and Nicholas.¹²³ Johnson and colleagues¹²⁴ also conducted a series of experiments to determine how CO₂ inserted into Pd-η¹allyl bond of square planar complexes (PCP)Pd(η¹allyl), where (PCP = 2,6 bis[(ditert butylphosphino)methyl]-phenyl).¹²⁴

Scheme 35.

Insertion of CO₂ into [CpM(NO)Ph₂] (A) and [CpRu(PPh₃)(CCPh)] (B).

The insertion of CO₂ into Rhodium (I) alkyl and aryl complexes was reported by Darensbourg et al.¹²⁵ Their investigation revealed that the insertion of CO₂ is into (PPh₃)₃Rh(Ph)(A) is slower than [(PMe₃)₃Rh(Ph)] (C) to create [(PMe₃)₃Rh(OC)Ph)] (D) as depicted in Scheme 36 (1 and 2). In particular, the total transformation of A into B takes 24 h under a 20-atm CO₂, whereas the total transformation of (C) into (D) just needs 3 h under a 10-atm CO₂. Despite being indefinitely stable complex (B) transformed from the first produced complex (D) into the η²acetate complex [(PMe₃)₂Rh(η²O₂CPh)] (E), which simultaneously releases ligand PMe₃. In this study they also reported the insertion of CO₂ into [(PMe₃)₃Rh(Me)](F) complex and it was slowly converted to the η¹acetate complex [(PMe₃)₃Rh(OC(O)Me)](G) at ambient temperature and 1 atm of CO₂, as shown in Scheme 36 (3) and Complex (G) consequently rearranges to [(PMe₃)₃Rh(η²O₂CMe)] complex (H). Johansson and Wendt¹²⁶ also investigated the insertion of CO₂ into a palladium allyl bond. In this study a very fast insertion of CO₂ has been reported in the complex 2,6-bis[(di-t-butylphosphino)methyl]phenyl allyl palladium (PCP^tBuPd-allyl (A) and yields an equivalent butenoate complex (C). The cyclic six-membered transition state (B), in which the CO₂-carbon is bonded to the C-carbon of the allyl group was assumed to be the mechanism by which the reaction takes place as illustrated in Scheme 37 . Wu et al.¹²⁷ also reported the reaction of CO₂ with Palladium-allyl bonds. Through the reaction of (2-methylallyl)₂Pd with the suitable free ligand, a family of palladium allyl complexes of the type (2-methylallyl)₂Pd(L) where L= PMe₃, PEt₃, PPh₃ and NHC (NHC=1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene) have been synthesized at low temperature. This investigation revealed that the reaction of palladium η¹-allyls with CO₂ takes place through nucleophilic attack of the terminal olefin on electrophilic CO₂, followed by an associative substitution at palladium to give the O-bound product, rather than through direct insertion of CO₂ into the Pd-C bond. Reaction of excess CO₂ led to fast and quantitative formation of the unidentate carboxylate complexes and Scheme 38 shows the mechanistic production of the unidentate carboxylate complexes. Wu and coworkers¹²⁸ also reported the same result by changing the metal center only. In this study, the reactions of complexes (allyl)₂Ni(L) with CO₂ showed that all of the complexes reacted with CO₂ at low temperature to produce the unidentate carboxylate species efficiently.

Scheme 36.

The insertion of CO₂ into Rhodium (I) alkyl and aryl complexes.

Scheme 37.

The insertion of CO₂ into Pd-allyl bond.

Scheme 38.

The possible mechanistic formation of unidentate carboxylate complexes.

In general, the chemistry of Pd complexes, η¹ and η³-allyls was well understood; η¹-allyls behave with electrophiles as they are nucleophilic, while η³-allyls react first with nucleophiles.¹²⁹ In addition, researchers understand the chemistry of metal complexes with bridging allyl ligands.^130-133 When studying η¹-allyl Pd systems, Hruszkewycz et al.¹³⁴ investigated that Pd(I)-bridging allyl dimers undergo CO₂ insertion to produce a bridging carboxylate. They found that bridging allyls react differently from η³-allyls in that the reaction is more likely terminal allyls of the η¹-allyls. Future studies will examine the reactivity of bridging allyls with terminal η¹-allyls to gain more insight into the mechanism of CO₂ insertion into bridging allyls. Wang et al.¹³³ study was the first examples of a reaction between CO₂ and a bridging allyl on Pd metal.

Insertion of CO₂ into the M–O bond

Many transition metal compounds with M-OR groups undergo insertion reactions with CO_2.¹³⁵ When R is hydrogen, hydrogen carbonate species are created; whereas, alkyl or aryl carbonate complexes are generated when R is an organic component.^93,136,137 For instance, the reactions of CO₂ with metal-hydroxides and alkoxides to produce metallobicarbonates (A) and alkyl carbonates (B), as shown in Scheme 39 , have been reported by Darensbourg et al.¹³⁶

Scheme 39.

The insertion of CO₂ into metal-hydroxides and alkoxide bond.

According to computational studies of researchers, the general mechanism for CO₂ insertion into metal-alkoxides and hydroxides follows a pathway in which the lone pair on the alkoxide or hydroxide acts as a nucleophile first and attacks CO₂ to form a zwitterionic intermediate that leads to the formation of the C–O bond as shown in Scheme 40 .¹³⁸ In most cases, this method has no barriers. Therefore, the zwitterion then undergoes change in structure to form a new M–O connection in the rate-determining step, and fully cleaves the metal alkoxide or hydroxide bond, and produces the bicarbonate product.

Scheme 40.

General mechanism for CO₂ insertion into Metal-Hydroxide or Alkoxide bond.

Schmeier et al.¹³⁹ reported the synthesis of PCP-supported nickel complexes and their CO₂ reactivity According to Scheme 41 . It was found that inserting CO₂ into Ni hydroxide bonds ([(PCP)Ni(OH)]) was easy to do both kinetically and thermodynamically, where PCP is bis-2,6-di-tert-butylphosphinomethylbenzene. Palmer and Harris¹⁴⁰ investigated [(NH₃)₅ MOC(O)O]⁺ carbonate complexes, where [M = Rh or Ir)] by treating the complex [(NH₃)₅ M (OH)]²⁺ with CO₂ in basic aqueous environments. According to this study, insertion of CO₂ into the M–OH bond would first result in the formation of bicarbonate species, which would then deprotonate to yield carbonate products. Scheme 42 illustrates the reversible insertion of CO₂ into the Pt-OH bond, which was additionally investigated by Lohr et al.¹⁴¹ In this work the reaction with bis-hydroxide gives an isolable η²-carbonato compound with elimination of water.

Scheme 41.

The insertion of CO₂ into Ni hydroxide bonds.

Scheme 42.

The insertion of CO₂ into pt-OH bond.

Activation of dinitrogen (N₂) by metal complexes

For microbes, plants, and animals, nitrogen is one of the most important and essential elements.^142-144 78% of the earth’s atmosphere is made up primarily of it.^143,145,146 One of the open problems in chemistry is how to effectively fictionalize molecular nitrogen under homogeneous conditions to create high-value nitrogen-containing molecules.^147-150 Dinitrogen is a linear, triply-bonded diatomic molecule that is extremely difficult to activate due to its high bond dissociation enthalpy (945 kJ /mol), negative electron affinity (−1.8 eV), and high ionization potential (15.058 eV).^151,152 However, there are other variables contributing to N2 inertness besides those mentioned above. Furthermore, the lack of a dipole moment and the large gap between the HOMO and LUMO (22.9 eV) that results in N₂’s inertness prevent it from being affected by Lewis acid/base reactions and electron transfer.^128,142 For d-block metals, nitrogen chemistry is well researched, and reports of stoichiometric or catalytic conversion of nitrogen to ammonia have already been recorded.¹⁵³ The only large-scale industrial N₂ reaction that has been commercialized to far is the Haber-Bosch process for ammonia synthesis.¹⁵⁴ Because both N₂ and CO possess triple bonds, their methods of bonding to transition metals are similar. There are numerous ways for N₂ to bind with metals to generate complexes.^155,156

Depending on if two extra metal centers share the N₂ molecules, the complexes can be classified as bridging or mononuclear. The geometric interaction between the metal center and the N₂ molecule allows the complexes to be further classified as end-on or side-on forms.⁹³ One important factor affecting the potential and activation mechanism of N₂ is the bonding mode. The end-on bridging (μ-η:η-N₂) and end-on terminal (η) modes are the most commonly reported types.^145,157

In spite of the fact that choosing the mode that would be used has historically proven difficult. It has also been reported that the type of metal and steric factors influence which bridging mode is being used.^155,158 Side-on bridging N₂ coordination is also known to occur. Transition metal–based N2 activation requires not only the δ−donation of N₂’s lone pair of electrons into vacant d orbitals, but also the π-backdonation of the metal center’s filled d orbitals into the unoccupied π* orbital of N₂ (Dewar–Chatt–Duncanson bonding model). Figure 3 illustrates the side-on and end-on bonding types of N₂. The back donation weakens the NN bond, which is essential for N₂ activation.^{145,146,153,157-162} Figure 4 shows the different binding modes of N₂ over the metal, including end-on, side-on, side-on/side-on, end-on/end-on, and end-on/side-on.^163-165

Figure 3.

End-on (left) and side-on (right) bonded metal–dinitrogen complexes represent the bonding interactions between a transition metal and N_2.

Figure 4.

Various binding modes of dinitrogen with metals centers.

The most common methods for figuring out the level of N₂ activation are infrared (IR) spectroscopy and X-ray crystallography.^165,166 The degree of π-backbonding after N₂ coordination is calculated based on differences in bond lengths (r) and stretching vibrational frequencies (v).¹⁴⁵ It has been observed that N₂ complexes with weaker and longer N–N bonds are often produced by the “early” transition metals in groups 4-6, while N₂ complexes with shorter and stronger N–N bonds (rNN < 1.15 Å, vNN > 1900 cm^-1) are produced by the “late” transition metals in groups 8–10.¹⁶⁷ The triple bond of free N₂ is 1.10 Å, and extension to 1.15 Å is assumed to be evidence of a singly reduced N₂ ligand. Crystallographic studies can shed insights on the N₂ bond order. N₂ activation is reported using IR spectroscopy using the N–N stretching frequency (vNN). Terminal N₂ ligands exhibit vNN in the region of 1850–2200 cm^-1.^168,169 For example, Fajardo and colleagues conducted studies on the terminal N₂ complex of ruthenium and osmium (Ru (νNN = 1960 cm^-1), and Os (νNN = 1931 cm^-1).¹⁷⁰ Low energy stretching frequency of Mo (νNN = 1859 cm^-1) indicates activated N₂ activity for catalytic NH₃ synthesis, according to Yandulov and Schrock.¹⁷¹

Figure 5.

Historic transition metal–dinitrogen complexes.

The first metal–dinitrogen complex [Ru(N₂)(NH₃)₅]²⁺ (1), which demonstrates the end-on in a terminal binding mode of dinitrogen to ruthenium metal, was synthesized by Allen and Senoff,¹⁷² but they were unable to further activate the complex. Cobalt N₂ compound [Co(N₂)H(PPh₃)₃] was first reported by Yamamoto and colleagues¹⁷³ (2). Its nitrogen gas was used to produce the N₂ ligand. The first molybdenum-N₂ compound is Trans [Mo(N₂)₂(dppe)₂] (3) (dppe =1,2 bis(diphenylphosphino)ethane), as reported by Hidai et al.¹⁷⁴ As was previously mentioned, the most typical bonding mechanism for transition metal N₂ complexes is by far end-on in a terminal mode.

Coordinated N₂ has been known to bond in clever ways.^155,158 On the basis of spectroscopic evidence, the side-on mode was first hypothesized in a mononuclear zirconocene complex [Zr(η-C₅H₄R')₂(η²-N₂)R] and [{Zr (η-C₅H₅)₂R}₂N₂], R=(Me₃Si)₂CH, R'= H OR Me].¹⁸⁰ The reaction of solvent-free decamethylsamarocene with dinitrogen to form the dinuclear complex ((η²-C₅Me₅)₂Sm)₂(µ-η²:η²-N₂) was the first structurally described side-on dinitrogen bridging samarium complex(Figure 7(A)).¹⁸¹ Lv et al.¹⁸² reported (N₂)³⁻ side-on bridged discandium complex Figure 7(B). A side-on bridging N₂ ligand with a binuclear, U(III) triamidoamine complex that has a relatively short N–N bond length has been also described by Roussel and Scott.¹⁸³ Dinitrogen Binding and Activation by U(OAr)₃ Complexes was reported by Mansell et al.¹⁸⁴ In this complex N₂ binding is reversible as shown in Scheme 43(A) . The synthesis of the mixed-sandwich U(III) complex [U(η-Cp)(η-C8H4SiiPr3-1,4)2)] was reported by Cloke and Hitchcock.¹⁸⁵ In addition to its ability to reversibly bind dinitrogen and reduce it to produce a binuclear U(IV) complex that has a bridging, sideways-bound N₂^2- ligand. The reversible formation of the complex involves a formal oxidation of two U(III) centers to U(IV) with concomitant reduction of N₂ to N₂^2- Scheme 43(B) .

Figure 6.

Some of reported N₂ complexes that display end-on mode coordination in both terminal and bridging modes.

Figure 7.

Structurally described side-on dinitrogen bridging complex.

Scheme 43.

Reversible coordination of dinitrogen by U(III) complexes: side-on coordination two electron reduction to afford the side-on N₂^2- bridged diuranium(IV) complexes with aryl oxide ligands (A) and Cp ligands (B).

It is well known that one source for the synthesis of NH₃ is dinitrogen but the transformation of molecular dinitrogen to NH₃ in ambient conditions is one of the major problems in biological, inorganic, and organometallic chemistry.¹⁸⁶ One of a process that takes N₂ from air in a usable form into the biosphere and this process is known as nitrogen fixation and this process is performed by nitrogenases enzymes.^142,187-189 At the center of one of the most prevalent of these kinds of enzymes is the Iron and molybdenum cofactor (FeMoco) active site, where dinitrogen binds and is eventually reduced to form NH₃^190,191 as shown in equation below. Iron and molybdenum, iron and vanadium, and iron solely are the three different forms of nitrogenase enzymes that differ according to the metal content of the active site. All three nitrogenases are identical in their structures and reactivity, however the latter two are usually only generated in situations where molybdenum is insufficient.^142,192 As reported by Chauhan et al.¹⁴³ the molybdenum (Mo)-deficient free soil-dwelling Azotobacter vinelandii has an alternative in the form of the FeVco cofactor of nitrogenase (VFe₇S₈(CO₃)C). At low temperatures, FeVco reduces N₂ to NH₃ at a rate that is a few times faster than FeMoco’s (MoFe₇S₉C). It supplies the soil with ammonium ions, which are a source of nitrogen. An alternative to the industrial, energy-intensive Haber-Bosch process for producing NH₃ is this biological synthesis.¹⁴³

Using a molybdenum-dinitrogen complex with a tetradentate triamidoamine ligand as a catalyst, Yandulov and Schrock¹⁵³ reported that dinitrogen could be converted to ammonia at ambient temperature and atmospheric pressure. 2,6-lutidiniumtetrakis[3,5-bis(trifluoromethyl) phenylborate] ([LutH]BArF₄) as a proton source and decamethylchromocene (CrCp*₂) as a reducing agent reacted with the catalyst in the presence of each other to produce 8 equiv. of ammonia with a 66% efficiency ( Scheme 44 ) and they have also isolated the possible intermediated in the catalytic reaction to determine the possible catalytic cycle for the process as shown in Scheme 45 .

Scheme 44.

Catalytic formation of ammonia using the Schrock system.

Scheme 45.

Structural determination of Yandulov and Schrock cycle for the conversion of dinitrogen into ammonia.

Scheme 46.

Nitrogen fixation Catalyzed by a Dinitrogen-Bridged Dimolybdenum Complex.

The conversion of dinitrogen to ammonia at room temperature was examined by Nishibayashi and associates¹⁷⁶ in a different investigation. They used the dinitrogen-bridged dimolybdenum complex to perform Mo-catalyzed nitrogen fixation. They used the cobaltocene (CoCp₂) complex as a reducing reagent and [LutH]OTf (OTf = CF₃SO₃) as a proton source to convert dinitrogen into ammonia. Depending on the catalyst, 23 equiv. of NH₃ were produced as shown in scheme. To consider a potential catalytic cycle, they also extracted a few facts from experimental data.^192,193

In order to study homogeneous ammonia synthesis, Chatt and Hidai synthesized group 6 metal complexes using the bidentate phosphine ligands dppe (bis(diphenylphosphino)ethane) and depe (bis(diethylphosphino)ethane)^148,194-196 ( Scheme 47 ). The cis- and trans-forms of the [M(N₂)₂(PMe₂Ph)₄] complex, where M = Mo or W, react with sulfuric acid in methanol as the solvent at 20 °C to produce 1.9 and 0.7 NH₃ per W and Mo atom, respectively, and a small quantity of hydrazine for (M = W). From there, two points may be speculated about: (1) dinitrogen reduction to ammonia can occur at room temperature in the presence of monotertiaryphosphines; and (2) oxygen containing solvents or oxoanions may be helpful in the reduction. By isolating the intermediates produced and understanding the diazenido, hydrazido, and hydrazinium complexes produced from the N₂ complexes, the reaction pathway can be clearly identified. In addition, the other complexes that were produced, including nitride (MN), amido (MNH₂), imido (MNH), and ammine (MNH₃), were differentiated from each other and identified. The Chatt cycle,^197-199 which rotates between the oxidation states of Mo(0) and Mo(IV), was called after these intermediates because they ultimately resulted in the production of a catalytic quantity of ammonia. The zero-valent Mo and W metals provided all of the electrons necessary for catalytic conversion into ammonia. It is one of the first examples of using metal-based catalysis to transform dinitrogen into ammonia at normal temperature and pressure. The Schrock cycle ( Scheme 48 ), in which the reaction proceeds by the stepwise addition of protons and electrons leading to the release of two ammonia molecules, was explained by results of the isolation of reactive intermediates and theoretical studies.^{153,171,200,201}

Scheme 47.

Conversion of molybdenum and tungsten dinitrogen complexes to form ammonia.

Scheme 48.

The proposed Schrock cycle for synthesis of Ammonia.

Figure 8.

Cofactor FeMo at the center of nitrogenase enzyme.

By reacting with Brønsted acids, other mononuclear transition metal like Fe-dinitrogen complexes can also form ammonia or hydrazine, but the yield of ammonia is lower than that reported for molybdenum and tungsten–dinitrogen complexes.^202-204 For instance Tyler and colleagues studied the reaction of an iron-dinitrogen complex with trifluoromethanesulfonic acid (HOTf) to produce a mixture of ammonia and hydrazine, and they isolated a number of possibilities for reaction intermediates of this reaction.^202,203 These results showed two potential reaction pathways, which are depicted in ( Scheme 49 ).

Scheme 49.

Two possible reaction mechanisms for the formation of ammonia and hydrazine using Fe-N₂ complex.

Conclusion

Due to the fact that they are not reactive, small molecules such as H₂, CO₂, N₂, and numerous others remain a particularly difficult problem for sustainable transformation. To promote the activation of these small molecules under the correct conditions, metal complexes are commonly used. As a less expensive way to create novel organic molecules through catalytic processes, the activation of these compounds by transition metal complexes has garnered a lot of interest lately.

In hydrogenation and other catalytic reactions, hydrogen activation is crucial. H₂ can be activated in a variety of ways upon engagement with a metal center, including oxidative addition and homolytic or heterolytic breaking of the H–H bond. The steric effect of the ligand substantially facilitates metal H₂ ligand stability. Therefore, a large ligand that also inhibits dihydrogen cleavage can stabilize the H₂ ligand. The process of CO₂ transformation is challenging due to its high levels of kinetic and thermodynamic stability. Since CO₂ has strong C=O bonds, overcoming the thermodynamic and kinetic stability would frequently necessitate a highly reactive metal core. Thus, a potential activation method is its interaction with transition-metal complexes. Incorporating the CO₂ molecule into transition metal-element linkages has been proposed as a possible method for activating and converting CO₂. When an insertion occurs in an M-H bond, formate or metallocarboxilate is formed; when it occurs in an M–OH bond, bicarbonate species are first produced. Chemists have been working for years to functionalize molecular nitrogen, a diatomic molecule that is plentiful yet intrinsically unreactive. However, when industrial and biological processes fix nitrogen, ammonia can be produced. One of the processes that takes N₂ from air in a usable form into the biosphere and this process is known as nitrogen fixation and this process is performed by nitrogenases enzymes. Recently, a few molecular electrocatalysts have been reported to have advanced in the production of molecular catalysts capable of reducing dinitrogen to ammonia using chemical reductants and proton donors. Thus, various metal complexes are employed in this review to increase the activation of these compounds.

Footnotes

Acknowledgements

The authors acknowledge the help received from individuals and colleagues.

Author contributions

Investigation, Conceptualization, and Writing Original Draft, A.M.; review and editing, W.W. and A.H.; and Visualization and Supervision, A.T. and K.M. All authors have read and agreed to the published version of this manuscript.

Data availability

No new data were created in this study. Data sharing is not applicable.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Aklilu Melese

Ayalew Temesgen

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Review on activation of small molecules (dihydrogen,carbon dioxide,and dinitrogen) by various transition metal complexes

Abstract

Keywords

Introduction

Activation of dihydrogen (H2) by metal complexes

Activation of carbon dioxide (CO2) by metal complexes

Insertion reactions of carbon dioxide

Insertion of CO2 to metal hydride (M–H) bonds

Insertion of CO2 into the M–C bond

Insertion of CO2 into the M–O bond

Activation of dinitrogen (N2) by metal complexes

Conclusion

Footnotes

Acknowledgements

Author contributions

Data availability

Declaration of conflicting interests

Funding

ORCID iDs

References

Activation of dihydrogen (H₂) by metal complexes

Activation of carbon dioxide (CO₂) by metal complexes

Insertion of CO₂ to metal hydride (M–H) bonds

Insertion of CO₂ into the M–C bond

Insertion of CO₂ into the M–O bond

Activation of dinitrogen (N₂) by metal complexes