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. 2022 Dec 6;61(50):20216–20221. doi: 10.1021/acs.inorgchem.2c03330

Mechanism Insights into the Iridium(III)- and B(C6F5)3-Catalyzed Reduction of CO2 to the Formaldehyde Level with Tertiary Silanes

Jefferson Guzmán †, Asier Urriolabeitia ‡, Marina Padilla †, Pilar García-Orduña †, Víctor Polo ‡, Francisco J Fernández-Alvarez †,*
PMCID: PMC10468102  PMID: 36472385

Abstract

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The catalytic system [Ir(CF3CO2)(κ2-NSiMe)2] [1; NSiMe = (4-methylpyridin-2-yloxy)dimethylsilyl]/B(C6F5)3 promotes the selective reduction of CO2 with tertiary silanes to the corresponding bis(silyl)acetal. Stoichiometric and catalytic studies evidenced that species [Ir(CF3COO-B(C6F5)3)(κ2-NSiMe)2] (3), [Ir(κ2-NSiMe)2][HB(C6F5)3] (4), and [Ir(HCOO-B(C6F5)3)(κ2-NSiMe)2] (5) are intermediates of the catalytic process. The structure of 3 has been determined by X-ray diffraction methods. Theoretical calculations show that the rate-limiting step for the 1/B(C6F5)3-catalyzed hydrosilylation of CO2 to bis(silyl)acetal is a boron-promoted Si–H bond cleavage via an iridium silylacetal borane adduct.

Short abstract

The study of the iridium(III) (κ2-NSiMe)2/B(C6F5)3-catalyzed selective reduction of CO2 to the formaldehyde level with tertiary silanes is reported. Mechanistic studies support a boron-assisted Si−H cleavage involving an iridium silylacetal borane adduct as the rate-limiting step of the process.

The potential of CO2 as a renewable and cheap C1 carbon source has received increasing attention over recent years.1 The major difficulties to achieve this goal are the kinetic and thermodynamic stability of CO2, which hampers most of its chemical transformations. In this regard, catalysis has proven to be an essential tool for transforming CO2 into value-added chemicals. Although great advances have been made in the field of the catalytic transformation of CO2, there are still many challenges to overcome for its utilization as a raw material on an industrial scale.2,3

Formic acid, formaldehyde, methanol, and methane are C1 chemicals that can be obtained from the reduction of CO2. In this work, we focus on formaldehyde, which is obtained industrially by the partial oxidation of methanol and has an annual demand of 30 million tons.4 The catalytic hydrogenation of CO2 to formaldehyde has been scarcely reported.5 However, several examples of the catalytic reduction of CO2 to the formaldehyde level with hydrosilanes6−14 or hydroboranes15 have been reported. Catalytic systems based on Zr,6 Re,7 Ru,8 Co,9 Ni,10 Pd,11 Pt,11 Sc,12 Mg,13 and Zn13 complexes and germylene-B(C6F5)3 adducts14 have proven to be effective for the selective reduction of CO2 with hydrosilanes to the corresponding bis(silyl)acetal. It is noteworthy that all of these catalytic systems require the use of a Lewis acid, such as B(C6F5)3, to selectively achieve the formation of the corresponding bis(silyl)acetal.16 The selectivity of these processes depends on the metal/B(C6F5)3 ratio. Thus, with an excess of borane, the formation of methane is facilitated.6−14 Although the effectivity of B(C6F5)3 as a hydrosilylation catalyst is well-known,17 B(C6F5)3 alone cannot catalyze the hydrosilylation of CO2.6a,18

It has recently been proven that bis(silyl)acetal, H2C(OSiPh3)2, provides a means to incorporate CHn (n = 1 or 2) moieties into organic molecules.19 Therefore, developing catalytic systems effective for the reduction of CO2 to the bis(silyl)acetal level using hydrosilanes is of great interest.

To date, few studies have been reported on the mechanism of these processes. Indeed, the mechanistic discussion remains open. For example, two different mechanisms have been proposed for the bis(phosphino)borylnickel hydride/B(C6F5)3-catalyzed reduction of CO2 to the formaldehyde level with hydrosilanes. Thus, while Rodriguez et al. proposed a boron-promoted Si–H activation mechanism,10a,10b Ke et al. proposed a nickel-promoted Si–H mechanism.10c

Understanding the mechanisms that operate in different transition-metal-catalyzed processes to reduce CO2 with hydrosiloxanes is one of our aims.20 We have recently reported that species [Ir(CF3CO2)(κ2-NSiMe)2] [1; NSiMe = (4-methylpyridin-2-yloxy)dimethylsilyl] catalyzes the selective reduction of CO2 with HSiMe(OSiMe3)2 to the corresponding methoxysilane, CH3OSiMe(OSiMe3)2, or silylformate, HCO2SiMe(OSiMe3)2, under mild reaction conditions. The selectivity of this catalytic system can be easily tuned by controlling the pressure of CO2.21 It is noteworthy that the two active positions of the catalytic systems based on 1 are trans located to two silyl groups; in addition, the Ir–Si bond in such species is stronger than would be expected for a traditional Ir–silyl bond.22 Hence, the positions trans to the Ir–Si bonds in Ir(κ2-NSiMe)2 complexes are highly activated.

We now report that using 1 as a catalyst precursor in the presence of catalytic amounts of B(C6F5)3 allow achievement of the selective formation of bis(silyl)acetals by the reaction of CO2 with hydrosilanes (Scheme 1).

Scheme 1. 1-Catalyzed (1.0 mol %) Reactions of CO2 with Tertiary Silanes in the Presence of B(C6F5)3 (1.0 mol %).

Scheme 1

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1H NMR studies of the 1/B(C6F5)3 (1:1 ratio; 1.0 mol %)-catalyzed reaction of CO2 (1 bar) with HSiMe(OSiMe3)2 (HMTS) in C6D6 at 323 K show the slow and selective formation of H2C{OSiMe(OSiMe3)2}2 (2a; Table 1, entry 1). To explore the scope of this catalytic process, we performed the reaction of CO2 with different silicon hydrides (HSiMe2Ph, HSiMePh2, HSiEt3, and HSiMe(OSiMe3)2) in the presence of 1/B(C6F5)3 (1:1) in C6D6. The best reaction conditions were found to be CO2 (1 bar) and 323 K. The reactions are highly selective to the formation of the corresponding bis(silyl)acetal (Table 1, entries 1, 2, 4, and 5). The nature of silane influences the reaction performance. The best reaction rates were obtained using HSiMe2Ph and HSiMePh2 (Table 1). The reactions with HMTS and HSiEt3 were slower, which can be attributable to the higher hindrance of the Si–H bond in such compounds.

Table 1. Results from the 1 (1.0 mol %)- and BR3-Catalyzed Reaction of CO2 with Hydrosilanes in C6D6 at 323 K.

              ratio of the reaction products
entry silane borane equiv of BR3 CO2 (bar) time (h) conversion (%)a OCHO (%)b OCH2O (%)b CH3O (%)b CH4 (%)
1 HMTS B(C6F5)3 1 1 16 28   >99 <1  
          40 74   >99 <1  
2 HSiMe2Ph B(C6F5)3 1 1 16 93   >99 <1  
          40 >99   >99    
3 HSiMe2Ph BPh3 1 1 24 73 81 12 7  
4 HSiMePh2 B(C6F5)3 1 1 16 78   >99 <1  
          40 >99   >99 <1  
5 HSiEt3 B(C6F5)3 1 1 16 12   >99 <1  
          40 25   >99 <1  
6 HSiMe2Ph B(C6F5)3 1 3 8 >99 83 13 4  
7 HSiMe2Ph B(C6F5)3 2 1 24 48       >99c
8 HSiMe2Ph B(C6F5)3 0.5 1 24 93 82 18    
9 HSiMe2Ph     1 24 50 90 2 8  
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a

Conversion and selectivity percentages are based on 1H NMR integration using hexamethylbenzene (0.0525 mmol) as an internal standard.

b

Composition of the mixture of products.

c

On the basis of the 1H NMR integral of O(SiMe2Ph)2, 12% CH4 was formed.

1H NMR studies of the 1/B(C6F5)3 (1:1; 1.0 mol %)-catalyzed reaction of CO2 with HSiMe2Ph in C6D6 at 323 K demonstrate the influence of CO2 pressure on the reaction performance; at 3 bar, the reactions are faster but less selective than those at at 1 bar (Table 1, entries 2 and 6). The stoichiometry of borane is a key factor in the selectivity of these catalytic processes. Within the range of 1–3 bar of CO2, if the load of B(C6F5)3 is increased from 1.0 to 2.0 mol %, the reactions are selective toward the formation of methane23 and O(SiMe2Ph)2, albeit at a lower rate (Table 1, entry 7). While reducing the amount of B(C6F5)3 to 0.5 mol % does not alter the activity, the selectivity is affected, resulting in the formation of silylformate (82%) and bis(silyl)acetal (18%) as secondary products (Table 1, entry 8). In the absence of additives, the catalyst precursor 1 promotes the reduction of CO2 (1 bar) with HSiMe2Ph to give silylformate (90%) as major reaction product (Table 1, entry 9).

1H NMR studies of the 1-catalyzed (1.0 mol %) reaction of CO2 (1 bar) with HSiMe2Ph in the presence of BPh3 (1.0 mol %), instead of B(C6F5)3, show a slower and less selective reaction. After 24 h, a 73% conversion of hydrosilane is reached to give a mixture of the corresponding silylformate (81%), bis(silyl)acetal (12%), and methoxysilane (7%) (Table 1, entry 3). Therefore, BPh3 plays a role in the activity and selectivity of the process, although to a lesser degree than B(C6F5)3, which can be correlated to its lower Lewis acidic character.24

1H NMR studies of the reaction of 1 with B(C6F5)3 evidenced the quantitative formation of [Ir(CF3COO-B(C6F5)3)(κ2-NSiMe)2] [3 (CCDC 2218258); Scheme 2]. Contrarily, no reaction is observed between 1 and BPh3 under the same conditions. The molecular structure of 3 has been confirmed by X-ray diffraction studies (Figure S38). The geometrical parameters of the [Ir(κ2-NSiMe)2] fragment (see the Supporting Information, SI) agree with those of 1, with short Ir–Si bond lengths [2.2526(11) and 2.2599(11) Å]. The Ir–O bond length in 3 [2.285(3) Å] is shorter than those found in 1 [2.363(3) and 2.418(3) Å].22a

Scheme 2. NMR Monitoring of the Stepwise Stoichiometric Reaction.

Scheme 2

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The 11B{1H} NMR spectra of 3 show a singlet at δ = −1.7 ppm (Figure S18), in agreement with what is expected for the O–B(C6F5)3 fragment25 (Scheme 2). The absolute value of the difference between δpara and δmeta of the fluorine atoms Δ(δm,p) in the 19F NMR spectra of 3 is 6.3 ppm (Figure S21), which agrees with the presence of a tetracoordinated borate anion.26,27

The addition of 1 equiv of HSiMe(OSiMe3)2, at room temperature (RT), to C6D6 solutions of 3 gives [Ir(κ2-NSiMe)2][HB(C6F5)3] (4) and CF3CO2SiR3. The 11B NMR spectra of 4 show a doublet resonance at δ = −15.1 ppm (1JB–H = 53 Hz),12b,15a which in the 11B{1H} NMR spectra appears as a singlet (Figures S24 and S25). Moreover, the 19F NMR spectra show a Δ(δm,p) value of ∼6 ppm, which is higher than the characteristic values found for noncoordinating [HB(C6F5)3]− anions [Δ(δm,p) < 3 ppm], which indicates a certain degree of coordination of the [HB(C6F5)3]− anion to the metallic center.17b

The addition of an excess of HSiMe(OSiMe3)2 to C6D6 solutions of 3 produces 4 and CF3CH{OSiMe(OSiMe3)2}2. Note that the overreduced product CF3CH2OSiMe(OSiMe3)2 is not obtained, which is reminiscent of the 1/B(C6F5)3 system selectivity toward the bis(silyl)acetal species. This evidences the effective entrapment of B(C6F5)3 in the form of a hydridoborate ion pair because the free borane might promote activation of the Si–H bond toward reduction of the bis(silyl)acetal derivatives, as well as the direct participation of 4 in the catalytic reaction, because 4 not only promotes hydrosilylation of the TFA ligand or CO2 but also catalyzes reduction of the R′COOSiR3 species (R′ = H, CF3).

The 1H NMR spectra of 4 in C6D6 show no changes when pressurized with CO2 (3 bar) at RT. However, after the reaction mixture is heated at 323 K, the formation of complex [Ir(HCOOB(C6F5)3)(κ2-NSiMe)2] (5) is observed. The presence of a IrOC(H)OB(C6F5)3 moiety in 5 has been demonstrated by means of 1H, 13C, 11B, and 19F NMR spectroscopies (Figures S32–S36). The addition of 2 equiv of HSiMe(OSiMe3)2 to a solution of 5, in the absence of CO2, produces the formation of 2a and the regeneration of 4 within 1 h at RT (Scheme 2). Exposure of 5 to 13CO2 (2.7 bar) at 353 K for 48 h did not result in the partial substitution of [Ir]OC(H)OB(C6F5)3 to the 13C-enriched [Ir]O13C(H)OB(C6F5)3, which suggests that, different from that reported for analogous MOC(H)OB(C6F5)3 (M = Re,8 Ni,10 Pd,11 Pt11) species, the CO2 insertion step to give 5 is irreversible under the catalytic conditions.

Density functional theory (DFT) studies at the M06L(SMD)/def2-TZVP//B3LYP-D3(BJ)/def2-SVP level have been performed to study in detail the reaction mechanism of CO2 hydrosilylation catalyzed by 3 (see the SI). HSiMe3 has been selected as a model system for the silanes. The Gibbs free energy energetic profile for the catalyst activation process, from 3 (A) to 4 (D) (Figure S39), is exoergic by 10.8 kcal mol–1. Si–H bond activation occurs via boron-promoted Si–H cleavage TSBC (9.0 kcal mol–1), which corresponds to a linear SN2 nucleophilic attack of the terminal oxygen of the trifluoroacetate ligand to the silicon atom in which the leaving hydride is transferred to the boron moiety. A similar type of activation mechanism has been proposed for Lewis acid PBP–Ni hydrosilylation of CO2 based on DFT calculations.10b An alternative mechanism for the Si–H activation step based on a nickel-promoted Si–H cleavage has been proposed.10c In our case, the iridium-promoted Si–H cleavage is energetically disfavored (see Figures S40 and S41 for a comparison of both pathways). Intermediate D can be described as a hydroborate moiety and a cationic metallic complex rather than a metal hydride interacting with the Lewis acid. Inspection of the natural bond orbitals reveals a σ(B–H) bonding orbital with an electron population of 1.76 electrons (Figure S42).

The coordination of CO2 to D leads to the beginning of the catalytic cycle. The Gibbs free energy profile for this process is reported in Figure 1. The first step corresponds to hydride transfer from HB(C6F5)3 to CO2 via TSEF at an energy barrier of 22.2 kcal mol–1 from intermediate D. The obtained intermediate F is thermodynamically favored (−13.1 kcal mol–1) and corresponds to complex 5 experimentally detected by NMR. Following that, the addition of silane leads to σ1-H-(HSiMe3) coordination to F, yielding G. Then, activation of the Si–H bond takes place via TSGH, like the previously reported TSBC, consisting of the linear SN2 nucleophilic attack of the terminal oxygen atom of the formate to the silicon atom and transfer of the leaving hydride to the boron atom of the Lewis acid. The activation barrier of TSGH is 15.9 kcal mol–1, leading to intermediate H. The subsequent hydride transfer from the hydroborate to the carbon atom of the silylformate coordinated to the metal takes place through TSHI, yielding intermediate I. Upon reaction with another molecule of HSiMe3, the silylformate develops into the final bis(silyl)acetal product via TSIJ, with the activation energy for this step being 23.2 kcal mol–1. This activation barrier for the boron-promoted Si–H bond cleavage is higher than those of the previously related processes, TSBC (9.0 kcal mol–1) and TSGH (15.9 kcal mol–1). It should be noted that, for TSIJ, the nucleophilic attack to the silane is performed by an alkoxy group,10,17c,28 in contrast with previous steps, where the nucleophilic attack was performed by trifluoroacetate and formate groups.

Figure 1.

Figure 1

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DFT-calculated Gibbs free energy profile for the catalytic formation of bis(silyl)acetal from E (kcal mol–1) relative to A.

The catalytic process is strongly exergonic (−30.9 kcal mol–1), and the rate-limiting step is boron-promoted Si–H cleavage by the iridium silylacetal borane adduct I (23.2 kcal mol–1) characterized by TSIJ. This activation barrier agrees with the experimental finding that the reaction proceeds slowly at RT. Indeed, the reaction of 4 with CO2 (3 bar) to give 5 requires heating at 323 K. It should be noted that the intermediates proposed in the DFT-calculated catalytic cycle match the experimentally detected species (4 and 5; Scheme 2).

In conclusion, this is the first example of an iridium-based catalytic system effective for the selective reduction of CO2 to the formaldehyde level with hydrosilanes. The selectivity of this catalytic system to the formation of bis(silyl)acetals is determined by the interaction between the active species and the Lewis acid B(C5F6)3. In fact, any factor that affects that interaction influences the selectivity of the process. Thus, using a borane with a lower Lewis acidity such as BPh3, high temperature, or CO2 pressure higher than 1.0 bar inhibit the selectivity toward the bis(silyl)acetal. DFT calculations support a boron-promoted Si–H cleavage mechanism, with the rate-limiting step being boron-promoted Si–H cleavage by the iridium silylacetal borane adduct I.

Acknowledgments

Financial support from Projects PGC2018-099383-B-I00 (MCIU/AEI/FEDER, UE) and DGA/FSE Project E42_20R is gratefully acknowledged. A.U. thankfully acknowledges the Spanish MECD for a FPU fellowship (FPU 2017/05417). The authors acknowledge the resources from the supercomputers “Memento” and “Cierzo” and technical expertise and assistance provided by BIFI-ZCAM (Universidad de Zaragoza, Spain). M.P. thankfully acknowledges the resources cofinanced with the European Social Fund (ESF) and the Youth Employment Initiative assigned to the CSIC in the Youth Employment Operational Program ESF 2014-2020.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.2c03330.

  • Experimental details, NMR spectra, crystallographic, and theoretical calculation data (PDF)

Author Contributions

J.G., M.P., and F.J.F.-A., experimental studies; P.G.-O., X-ray diffraction; A.U. and V.P., theoretical calculations.

The authors declare no competing financial interest.

Supplementary Material

ic2c03330_si_001.pdf (2.7MB, pdf)

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