Insight into catalytic reduction of CO2 to methane with silanes using Brookhart's cationic Ir(iii) pincer complex

Using density functional theory computations, we investigated in detail the underlying reaction mechanism and crucial intermediates present during the reduction of carbon dioxide to methane with silanes, catalyzed by the cationic Ir-pincer complex ((POCOP)Ir(H)(acetone)+, POCOP = 2,6-bis(dibutylphosphinito)phenyl). Our study postulates a plausible catalytic cycle, which involves four stages, by sequentially transferring silane hydrogen to the CO2 molecule to give silylformate, bis(silyl)acetal, methoxysilane and the final product, methane. The first stage of reducing carbon dioxide to silylformate is the rate-determining step in the overall conversion, which occurs via the direct dissociation of the silane Si–H bond to the C 
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 O bond of a weakly coordinated Ir–CO2 moiety, with a free energy barrier of 29.5 kcal mol−1. The ionic SN2 outer-sphere pathway in which the CO2 molecule nucleophilically attacks at the η1-silane iridium complex to cleave the η1-Si–H bond, followed by the hydride transferring from iridium dihydride [(POCOP)IrH2] to the cation [OC–OSiMe3]+, is a slightly less favorable pathway, with a free energy barrier of 33.0 kcal mol−1 in solvent. The subsequent three reducing steps follow similar pathways: the ionic SN2 outer-sphere process with silylformate, bis(silyl)acetal and methoxysilane substrates nucleophilically attacking the η1-silane iridium complex to give the ion pairs [(POCOP)IrH2] [HC(OSiMe3)2]+, [(POCOP)IrH2] [CH2(OSiMe3)2(SiMe3)]+, and [(POCOP)IrH2] [CH3O(SiMe3)2]+, respectively, followed by the hydride transfer process. The rate-limiting steps of the three reducing stages are calculated to possess free energy barriers of 12.2, 16.4 and 22.9 kcal mol−1, respectively. Furthermore, our study indicates that the natural iridium dihydride [(POCOP)IrH2] generated along the ionic SN2 outer-sphere pathway could greatly facilitate the silylation of CO2, with a potential energy barrier calculated at a low value of 16.7 kcal mol−1.


Introduction
Carbon dioxide is a cheap, nontoxic and readily available carbon resource for the organic synthesis of valuable chemicals and materials. 1,2 Transformation of carbon dioxide into fuels and useful organics such as formic acid, 3 formaldehyde, 4 methanol, 5 and other derivatives 6 is a topic of growing interest. 7,8 Among others, signicant efforts have been devoted to the chemical reduction of carbon dioxide using transitionmetal catalysts. 9-14 These catalysts have been extensively reviewed in the literature. 15 Some representative examples are shown in Scheme 1, including a rhodium complex (RuCl 2 (PTA) 4 , PTA ¼ 1,3,5-triaza-7-phosphaadamantane) reported by Laurenczy et al., which catalyzed the hydrogenation of carbon dioxide affording formic acid as the only product, 16 and an iron complex ([FeF(2)]BF 4 , 2 ¼ tris(o-diphenylphosphinophenyl)phosphine) reported by Beller et al., which catalyzed the hydrogenation of carbon dioxide affording formates and formamides. 17 Recently, a class of transition-metal catalysts supported by pincer ligands have been developed to achieve remarkable catalytic efficiency for carbon dioxide reduction. These include an Ir(III) tri-hydride PNP-ligated complex, iPr(PNP)IrH 3 , reported by Nozaki et al., which was used for the hydrogenation of carbon dioxide to formate, 18 (PNP)RuH 2 CO reported by Pidko et al., 19 Ru(acriphos)(PPh 3 )(Cl)(PhCO 2 ) reported by Leitner et al., 20 (POCOP)IrH 2 (MeCN) reported by Meyer, 21 (PNHP)IrH 3 (PNHP ¼ HN{CH 2 CH 2 (P i Pr 2 )} 2 ), 22 and RhCl(PPh 3 ) 3 , 23 etc. 24 However, the use of hydrogen as the reducing agent to convert carbon dioxide generally requires higher pressures and/or temperatures and also involves the use of strong bases as co-reagents. 25 In this regard, hydrosilane as a reductant has been explored as an alternative methodology, since the formation of the Si-O bond in silyl compounds is a thermodynamically favorable process. 26 For example, carbon dioxide can be reduced to silyl formate, bis(silyl)acetal or silylether under the catalysis of [ReHBr(NO)(PR 3 ) 2 ]/B(C 6 F 5 ) 3 , 27 [Cp* 2 Sc][HB(C 6 F 5 ) 3 ]1 CIP , 28 cis/trans-[RuCl 2 (MeCN) 4 ], 29 (BDP) CuH, 30 and other Rh, Ir, Ru, Cu and Fe complexes, 31 and main group catalysts including the frustrated Lewis acid B(C 6 F 5 ) 3 , 32 and N-heterocyclic carbenes. 33 Notably, the Brookhart group presented a cationic Ir-pincer complex, (POCOP)Ir(H)(acetone) + (POCOP ¼ 2,6-bis(dibutylphosphinito)phenyl), to catalyze the hydrosilylation of carbon dioxide under mild reaction conditions, and achieved a high turnover number of 8300 and moderate turnover frequency (660/h at 60 C). 34 It is worth noting that by using this Ir-pincer catalyst, carbon dioxide could even be reduced to methane in high yields with less sterically hindered silanes.
The reaction mechanism for carbon dioxide transformation mediated by transition-metal complexes has been studied extensively. 35 As exemplied by Scheme 2, the common feature is the insertion of carbon dioxide into the metal-hydrogen bond of a metal hydride. Two general pathways have been identied: (a) via transfer of the hydride directly from the metal complex; and (b) via prior coordination of carbon dioxide to the metal center, followed by carbon dioxide abstracting a hydride from the metal center. Both pathways reduce carbon dioxide to a formate ion (HCOO À ) around the metal center, forming a metal formate complex. Then, the formate ion or its derivative is eliminated as the metal formate intermediate reacts with H 2 . The hydrogenation of carbon dioxide to formic acid via these two modes, especially that involving insertion into the M-H bond of a metal complex, is considered to be the rst elemental step in transition-metal-catalyzed hydrogenation/hydrosilylation reactions, which have been the subject of several reviews. [36][37][38] The reaction mechanisms mediated by transition-metal complexes are usually complex. For the hydrosilylation of carbon dioxide into methane catalyzed by the cationic Ir-pincer complex, Brookhart and co-workers postulated an unconventional pathway. As shown in Scheme 3, the reaction occurs through activation of the Si-H bond of hydrosilane by the electrophilic Ir(III) ion, forming a silane-iridium adduct as the step initiating the catalytic cycle. The silane-iridium adduct acts as an effective catalyst to reduce carbon dioxide to a silylformate (HCOOSiR 3 ) product. Then, the silylformate substrate reacts with the silane-iridium adduct to provide bis(silyl)acetal (R 3 SiOCH 2 -OSiR 3 ), methoxysilane (R 3 SiOCH 3 ) intermediates and nally, methane (CH 4 ). This mechanistic proposal is remarkable and represents a new way of activating carbon dioxide using transition-metal complexes. The corresponding catalytic cycle is an outer-sphere mode, in which insertion of a carbon dioxide molecule into the metal-hydride bond does not occur. 39 However, the detailed underlying reaction mechanism for the complete reduction of carbon dioxide to methane by the cationic Ir-pincer complex has not been investigated computationally, and the reaction mechanism is not yet understood in detail. Indeed, few examples of transition-metal catalyst systems are known to be active for the selective reduction of CO 2 to methane. 40 To better understand the reduction of CO 2 mediated by the cationic Irpincer complex with silanes, we sought to explore the mechanism in more detail by employing DFT calculations. Our purpose was to uncover characteristic features of the electronic process of each elementary step during the catalytic reaction. An in-depth density functional theory (DFT) study of this system allows for further development on the conversion of carbon dioxide under the catalysis of transition-metal complexes.

Computational methodology
All molecular geometries of the model complexes were optimized at the DFT Becke3LYP (B3LYP) 41 level and by using the hybrid meta exchange-correlation M06 functional, 42 which includes a medium-range correlation as implemented in Gaussian 09. 43 The effective core potentials (ECPs) of Hay and Wadt with doublez valence basis sets (LanL2DZ) 44 were used to describe the Ir atom. In addition, polarization functions were added for Ir (z f ¼ 0.938). 45 The 6-311g (d,p) basis set was used for all other atoms, including C, H, P, Si and O. Frequency calculations at the same level of theory were performed to verify all stationary points as minima (zero imaginary frequency) and transition states (one imaginary frequency), as well as to provide free energies at 298.15 K, including entropic contributions. All transition states were veried to connect the respective minima through optimizations following initial intrinsic reaction coordinate calculations. To obtain the relative solvation-free energies, we used a continuum medium to perform single-point calculations for all the species under study using the SMD solvation model (an IEFPCM calculation with radii and non-electrostatic terms for Truhlar and coworkers' SMD solvation model), 46 as implemented in Gaussian 09. CH 2 Cl 2 was used as the solvent.
A model catalyst was used where the large butyl/isopropyl substituents at the carbon atom in the tridentate POCOPpincer ligand were replaced with methyl groups. Trimethylsilane was used as a model silane. The nal Gibbs energies (DG) reported in this article are based on B3LYP energies with Gibbs energy corrections (at 298.15 K), solvation corrections, and corrections for dispersion effects using the method of Grimme. 47 Furthermore, it should be noted that the entropic contribution in a solvent medium is overestimated for a reaction using the ideal gas phase model. To reduce the overestimation of the entropy contribution in the results, we adopted the approximate approach proposed by Martin et al.,53 i.e., a reaction from m-to n-components has an additional correction of (n À m) Â 4.3 kcal mol À1 . Detailed comparisons of the different functionals (B3LYP, B3LYP-D and M06) are listed in the ESI. † The geometries are displayed using CYLview. 48

Results and discussion
Overall catalytic mechanism The following part of the paper is devoted to our theoretical analysis of the cationic Ir-pincer complex catalyzing the hydrosilylation of CO 2 , following a stepwise process comprising four steps. First, CO 2 is reduced to give silylformate (HCOOSiMe 3 ). Then, the silylformate is reduced to give bis(silyl) acetal (H 2 C(OSiMe3) 2 ), methoxysilane (H 3 COSiMe 3 ) and nally, methane (CH 4 ).
Stage I: hydrosilylation of CO 2 to silylformate (HCOOSiMe 3 ). Three different pathways were explored for the hydrosilylation of CO 2 to silylformate under the catalysis of the cationic Irpincer complex: (a) the cationic Ir-pincer complex activating CO 2 rst, followed by the Ir-CO 2 moiety activating a free silane molecule; (b) the cationic Ir-pincer complex activating the silane rst as proposed by Brookhart, by coordination of Me 3 SiH to the iridium atom, followed by the silane-iridium adduct activating a CO 2 molecule; and (c) CO 2 inserting into the iridium-hydride bond of the cationic Ir-pincer complex, generating iridium formate, and reacting with a free silane to generate silylformate.
In Fig. 1, the reaction pathway representing the Ir-CO 2 moiety to activate a free silane is illustrated, together with the optimized structures of key stationary points that were located. Carbon dioxide exhibits poor ligand properties toward the cationic Ir-pincer complex. An h 1 O -coordinated Ir-CO 2 complex is located as CO 2 weakly coordinates to the iridium atom, d(Ir/ O(CO 2 )) ¼ 2.43 A. The adduct formation is endothermic by +6.7 kcal mol À1 . Subsequently, dissociation of a free silane Si-H bond to the C]O bond of the weakly coordinated carbon dioxide would directly generate silylformate. Here, two metathesis transition states were located: TS3a and TS4a. In these two four-membered-ring transition states, the Si and H atoms of free silanes are approaching the C]O bonds of CO 2 , respectively, as shown in Fig bond angles of CO 2 are reduced signicantly to 136 and 134 , respectively. The two transition states show only a small energetic difference, with energies of 30.6 kcal mol À1 and 29.5 kcal mol À1 in solvent. Both transition states yield the Obridged silylformate iridium intermediates (IM4 and IM5). Nevertheless, IM4 can isomerize to the more stable intermediate IM5 by crossing a low barrier of 5.3 kcal mol À1 (TS5a relative to IM4). TS5a represents an h 2 O,O -carbonyl transition state, with Ir/O bond distances of 2.65 and 2.98 A, respectively. Because the two silylformate iridium intermediates can be readily interconverted, only the more stable intermediate IM5 is considered in the subsequent studies. Therefore, TS4a is the highest stationary point along pathway A, wherein the Ir-CO 2 moiety activates free silanes and generates the silylformate iridium complex, whose energy is 29.5 kcal mol À1 lower than that of the reactants ([Ir(H)] + + silane + CO 2 ). The reaction is exergonic by À17.8 kcal mol À1 . Fig. 2 illustrates the pathway for the silane-iridium adduct activating carbon dioxide, together with the energetic results and the optimized structures of key stationary points. This pathway starts with silane h 1 -binding to the iridium atom of the cationic Ir-pincer complex, generating an h 1 -silane iridium complex IM2 (see Table S1 in the ESI † for a comparison of the key structures, with an X-ray single crystal structure of the complex with trimethylsilane coordinated end-on to the iridium center, as reported by Brookhart 49 ). The interaction between iridium and silane activates the Si-H bond, as shown by the elongated Si-H bond distance (1.24 vs. 1.16 A in free silane), and results in a net charge of 1.584e on the silicon atom as compared to 1.345e in free silane, thus making it favorable for CO 2 molecules to attack. Specically, it is worth noting that the silane-iridium complexes have been suggested to be active in the reduction of a series of organic substrates, including alkyl halides, carbonyl compounds and amines, etc. 50 The underlying reduction reaction mechanism has also been computed recently by several groups, and characterized as the ionic outer-sphere mechanism. 51 Aer a thorough investigation into the possibilities of how the silane-iridium complex reduces carbon dioxide, we were able to locate three transition states. In the rst transition state, TS3b (Fig. 2), a carbon dioxide molecule attacks the silicon atom at the face opposite to the iridium atom. TS3b can be viewed as a very late S N 2-Si transition state, in which the silicon atom has a distorted trigonal bipyramidal structure. The Si-H bond is broken and becomes signicantly elongated to be 2.99 A, while Ir-H and Si-O bonds become fully formed (1.65 A and 2.22 A, respectively). The four atoms of O-Si/H-Ir maintain their linear arrangement (179 ). The second transition state, TS4b, involves CO 2 nucleophilically attacking the silicon atom of the h 1 -silane iridium complex from the same side as the iridium atom. At this transition state (Fig. 2) (IM7). The hydride transfer process represents a very small barrier, of a negligible 0.9 kcal mol À1 (TS7b relative to IM6b). As expected, the ratedetermining step for the h 1 -silane iridium complex to activate carbon dioxide, generating the intermediate of silylformate, is the ionic S N 2 outer-sphere transition states. Interestingly, we found three ionic S N 2 transition states possessing similar activation energies: 33.9 kcal mol À1 (TS3b), 33.0 kcal mol À1 (TS4b) and 33.8 kcal mol À1 (TS5b) relative to the reactants ([Ir(H)] + + silane + CO 2 ). The third pathway illustrates carbon dioxide insertion into the Ir-H bond of the cationic Ir-pincer complex. Insertion of carbon dioxide (and other unsaturated organic substrates) into the M-H bond of a metal complex has been suggested as being the key step in catalytic hydrogenation. 52 The insertion step is identied as a four-membered-ring transition state (TS3c), where the Ir-H bond is broken (d(Ir/Ha) ¼ 2.61 A), and the C-H bond is correspondingly formed (d(C/Ha) ¼ 1.13 A). It is worth noting that the apical H atom around the iridium atom deviates considerably from the apical position and is largely elongated when approaching the carbon atom of CO 2 in the transition state TS3c. The barrier for TS3c is calculated to be signicantly high, at 44.5 kcal mol À1 (TS3c relative to the reactants ([Ir(H)] + + silane + CO 2 ). The following steps along pathway C are shown in the ESI (Fig. S3 †) (Fig. 3).
The calculated free energies of three pathways calculated for the cationic Ir-pincer complex catalyzing the hydrosilylation of CO 2 to yield silylformate (HCOOSiMe 3 ) are compared and shown in Fig. 4. Our DFT energetic results indicate that the insertion pathway can be ruled out, because the free energy barrier is as high as 44.3 kcal mol À1 and is $10 kcal mol À1 higher than the other two pathways. Therefore, a carbon dioxide molecule cannot directly insert into the Ir-H bond of the cationic Ir-pincer complex. However, it was found that the energy barrier for the ionic S N 2 outer-sphere pathway (TS4b, 33.0 kcal mol À1 , the highest energy along pathway B) is 3.5 kcal mol À1 higher than the dissociation pathway A (TS4a, 29.5 kcal mol À1 , the highest energy along pathway A) in the solvent. When comparing the gas-phase electronic energies, TS4a was calculated to be 1.6 kcal mol À1 higher than TS4b. Nevertheless, considering the small energy difference of 3.5 kcal mol À1 in solvent and the inaccuracy of the computational method, we speculate that both pathways of A and B could be operating under the working conditions, with pathway A being slightly more favorable.
Stage II: hydrosilylation of silylformate (HCOOSiMe 3 ) to bis(silyl)acetal (H 2 C(OSiMe3) 2 ) or formaldehyde (H 2 C]O). Three similar possible pathways for the cationic Ir-pincer complex catalyzing the hydrosilylation of silylformate substrate were studied. Pathway A, involving dissociation of a second silane Si-H bond to the C-O bond of silylformate, and pathway C, involving silylformate insertion into the Ir-H bond, were calculated to have higher activation free energies of 38.7 kcal mol À1 (TS6a) and 34.1 kcal mol À1 (TS7c), respectively (see Fig. S2 in the ESI † for more details about the attack model). Therefore, only the free energy changes of the favorable pathway B leading to bis(silyl)acetal or formaldehyde, as shown in Fig. 5, are discussed.
Pathway B corresponds to the ionic S N 2 outer-sphere mechanism with the silylformate nucleophilically attacking the h 1silane iridium complex. Initially, silylformate is liberated from the iridium atom. For the next step, we considered two attacking models. We rst considered an activation mode with the exposed oxygen atom of the C]O bond of silylformate acting as the entering group to attack the silicon atom of the h 1 -H-Si bond (Fig. 5). The corresponding transition state featuring the ionic S N 2 outer-sphere process was identied as TS8b, in which the Si2-H2 bond is elongated to 2.49 A, and the O-Si2 and Ir-H2 bonds are shortened to 1.88 A and 1.64 A, respectively.  Alternatively, the exposed oxygen atom of the C-O(SiMe 3 ) bond of the silylformate substrate could also act as the entering group to attack the Si center of the h 1 -H-Si bond. The transition state was identied as TS8bi. At this transition state, accompanying the Si2-H2 bond heterolytic cleavage, the silyl moiety and the silane hydrogen binds to silylformate and iridium, respectively (d(Si2/O1) ¼ 2.06 A, d(Ir/H2) ¼ 1.64 A). Both of the S N 2 transition states yield ionic pair intermediates. TS8b leads to an ion pair comprising iridium dihydride [IrH 2 (POCOP)] and the cation [HC(OSiMe 3 ) 2 ] + . Alternatively, the ionic pair of iridium dihydride [IrH 2 (POCOP)] and the cation [HCOO(SiMe 3 ) 2 ] + are formed from TS8bi. At the next step, the weakly bound moiety in the ion pairs can reorganize, followed by hydride transfer to give the corresponding product. The hydride on the iridium dihydride transfers to the cation [HC(OSiMe 3 ) 2 ] + to give a bis(silyl) acetal-Ir adduct, passing TS10b. Alternatively, the hydride on the iridium dihydride migrates to the carbon atom of the cation [HCOO(SiMe 3 ) 2 ] + passing TS10bi. Through TS10bi, disiloxane O(SiMe 3 ) 2 can easily dissociate to give the Ir-bound formaldehyde adduct. Our DFT results show that both hydride transfer processes are thermodynamically favorable and proceed effectively with a negligibly barrier. TS10b is barrier free. TS10bi was computed to be 0.4 kcal mol À1 relative to the ionic pair of [IrH 2 (POCOP)] and the cation [HCOO(SiMe 3 ) 2 ] + in solvent.
Therefore, the rate-determining steps along the ionic S N 2 outer-sphere pathway are the transition states TS8b and TS8bi. The energy needed for the C]O bond of silylformate to attack the h 1 -H-Si bond is 12.2 kcal mol À1 (TS8b relative to the silylformate iridium complex IM5), which is 17.0 kcal mol À1 lower than that needed for the C-O(SiMe 3 ) bond of silylformate to attack the h 1 -H-Si bond (29.2 kcal mol À1 , TS8bi relative to the silylformate iridium complex IM5). Therefore, the ionic S N 2 outer-sphere pathway leading to the bis(silyl)acetal-Ir adduct is greatly favored relative to the pathway leading to the formaldehyde-Ir adduct. In other words, nucleophilic attack of the C]O double bond is highly favorable, while the nucleophilic attack of the C-O(SiMe 3 ) single bond is highly unlikely. This argument nds support from the structural characteristics calculated for the two transition states. As shown in the optimized structures of TS8b and TS8bi, the Si2-O2 bond distance is 2.49 A in TS8b, which is shorter than the Si2-O1 bond distance of 2.61 A in TS8bi. This indicates a stronger interaction between the oxygen atom of the C]O bond in silylformate and the silicon atom of h 1 -silane, compared to that between the oxygen atom of C-O(SiMe 3 ) in silylformate and the silicon atom of h 1 -silane.
In summary, as shown in Fig. 5, we conclude that the hydrosilylation of silylformate catalyzed by the cationic iridium complex is an exothermic process with a value of À7.5 kcal mol À1 (IM5 to IM10). The most favorable pathway to generate the bis(silyl)acetal substrate is via the ionic S N 2 outersphere pathway, with an activation free energy barrier of only 12.2 kcal mol À1 (TS8b relative to IM5). The generation of formaldehyde was found to be less energetically favorable, associated with an activation free energy of 29.2 kcal mol À1 (TS8bi relative to IM5). Thus, the generation of formaldehyde is prevented from participating in the reaction. Our observation suggests that the bis(silyl)acetal substrate, but not the formaldehyde product, is observed in the hydrosilylation of CO 2 catalyzed by the cationic Ir-pincer complex, which is consistent with the experimental results obtained by Brookhart et al. 34 Third and fourth reduction steps: reducing bis(silyl)acetal (H 2 C(OSiMe 3 ) 2 ) to methoxysilane (H 3 COSiMe 3 ) and reducing methoxysilane (H 3 COSiMe 3 ) to methane (CH 4 ). In the continuous reduction of bis(silyl)acetal to methoxysilane and then to methane, mediated by the cationic Ir-pincer complex, the favorable pathway takes place via two sequential ionic S N 2 outer-sphere processes. Fig. 6 shows the energy proles for the two steps. When the bis(silyl)acetal substrate nucleophilically attacks the h 1 -silane iridium complex, the ionic S N 2 transition state TS11b can be identied. At TS11b, a new Ir-H3 bond is partially formed (1.67 A), accompanied by the signicant elongation of the Si3-H3 bond (2.30 A). Together with silyl binding to bis(silyl)acetal and a silane hydrogen binding to the iridium atom, an ion pair comprising iridium dihydride and the cation [CH 2 (OSiMe 3 ) 2 (SiMe 3 )] + is formed. Next, iridium dihydride transfers a hydride to the carbon atom of the cation [CH 2 (-OSiMe 3 ) 2 (SiMe 3 )] + via the transition state TS13b, giving a methoxysilane-Ir adduct. The rate-determining step is calculated to be associated with an activation barrier of 16.4 kcal mol À1 (TS11b relative to the h 1 -silane iridium complex and bis(silyl)acetal).
Then, the reduction of methoxysilane via the ionic S N 2 outersphere reaction pathway leads to the nal product methane. The ionic S N 2 outer-sphere transition state was identied as TS14b. At TS14b, the new Ir-H4 bond is partially formed (1.66 A), accompanied by the signicant elongation of the Si4-H4 bond (2.33 A). TS14b gives the ion pair comprising iridium dihydride and the cation [CH 3 O(SiMe 3 ) 2 ] + . Subsequently, iridium dihydride transfers a hydride to the carbon of the cation [CH 2 (OSiMe 3 ) 2 (SiMe 3 )] + , leading to the formation of methane, by passing TS15b. The rate-determining step along the ionic S N 2 outer-sphere pathway for the reduction of methoxysilane to methane was calculated to have an activation barrier of 22.9 kcal mol À1 (TS15 relative to the h 1 -silane iridium complex and methoxysilane). Therefore, the third stage of the reduction reaction of bis(silyl)acetal to methoxysilane along TS11b / IM11b / IM12b / TS13b / IM13 and the fourth stage of the reduction of methoxysilane to methane along TS14b / IM14b / TS15b / IM15 have low activation barriers of 16.4 kcal mol À1 and 22.9 kcal mol À1 , respectively.
The iridium dihydride complex [IrH 2 (POCOP)] catalyzing the reduction of CO 2 with silanes. Our DFT results show that the iridium dihydride can be generated in situ along the ionic S N 2 outer-sphere pathways for the hydrosilylation of carbon dioxide catalyzed by the cationic Ir-pincer complex, as detailed in Fig. 4-6. Since metal dihydrides have previously been explored to be effective catalysts in the conversion of carbon dioxide, 54 we investigated the likelihood of the iridium dihydride-catalyzed reduction of CO 2 with silanes. The optimized structures of the relevant mechanism and relative free energy proles are depicted in Fig. 7. According to our calculations, carbon dioxide insertion into the Ir-H bond of [IrH 2 (POCOP)] is very feasible. The activation free energy of the transition state TS17 is only 6.9 kcal mol À1 higher than that of the reacting species ([IrH 2 (-POCOP)] + CO 2 ). It is noteworthy that the equatorial hydride approaches the C]O bond of CO 2 without a large change in the conguration around the iridium center in the optimized structure of the transition state TS17. The generated intermediate IM17 is the starting point for the silylation of formate. First, the h 2 -HCOO moiety rotates around the iridium atom to the h 1 -HCOO moiety to accommodate a free silane (TS18 / IM18). Subsequently, a silane molecule coordinates to the iridium atom, which then undergoes a metathesis process of the four-membered ring transition state (TS20), generating silylformate. This metathesis process is feasible, with an activation free energy of 16.7 kcal mol À1 (relative to IM19). This barrier would be easily surmountable at the temperatures typically used for the silylation reaction. The potential energy surface in Fig. 7 reveals that the reduction of CO 2 by the iridium dihydride with silanes is energetically much more preferable. More importantly, this rate-determining barrier is much lower than that calculated for the reduction of CO 2 to silylformate substrate with the cationic iridium complex (Fig. 4, 29.5 kcal mol À1 ). Thus, we propose that the generation of iridium dihydride [IrH 2 (POCOP)] plays an important role in the reduction of carbon dioxide with silanes catalyzed by the cationic Ir-pincer complex.
The overall catalytic cycle for the cationic Ir-pincer complex catalyzing the reduction of CO 2 with silanes to methane is summarized in Scheme 4. According to our calculations, the whole transformation of CO 2 with silanes to methane by the cationic Ir-pincer can be divided into four reducing steps with silane hydrogen atoms subsequently being transferred to a carbon dioxide molecule: CO 2 / silylformate (HCOOSiMe 3 ) / bis(silyl)acetal (H 2 C(OSiMe 3 ) 2 ) / methoxysilane (H 3 -COSiMe 3 ) / methane (CH 4 ). The results obtained in this study are consistent with the catalytic cycle proposed by Brookhart. The rst step of reducing CO 2 to silylformate is the ratedetermining step of the overall catalytic cycle. Our DFT results identied two competing pathways: a dissociation pathway featuring the silane Si-H bond directly dissociating onto the C]O bond of an Ir-CO 2 moiety (passing TS4a) and an ionic S N 2 outer-sphere pathway of CO 2 nucleophilically attacking the h 1silane iridium complex (passing TS4b and TS7b). Moreover, on the basis of the calculated energy proles, the generation of the iridium dihydride complex was found to effectively promote CO 2 hydrosilylation. Our results reveal that the rate-determining step for the CO 2 activation to silylformate, catalyzed by iridium dihydride, possesses a barrier of 16.7 kcal mol À1 . The subsequent stages of reducing silylformate to bis(silyl)acetal, methoxysilane and nally to methane are all feasible. The ratedetermining activation free energy for stage II of reducing silylformate to bis(silyl)acetal is 12.2 kcal mol À1 (TS8b relative to the iridium-silylformate complex). The rate-determining activation free energy for stage III of reducing bis(silyl)acetal to methoxysilane is 16.4 kcal mol À1 (TS11b relative to the h 1 - Fig. 7 The DFT results for the iridium dihydride complex [IrH 2 (POCOP)] catalyzing the hydrosilylation of CO 2 . Optimized geometries of key stationary points are displayed in Fig. S11. †

Scheme 4
The overall four stages of the reaction mechanism for the catalytic hydrosilylation of CO 2 to methane catalyzed by the cationic iridium complex. silane iridium complex and bis(silyl)acetal). Furthermore, the rate-determining activation free energy for stage IV of reducing methoxysilane to methane is 22.9 kcal mol À1 (TS15b relative to the h 1 -silane iridium complex and methoxysilane).

Conclusions
The mechanisms behind the cationic Ir-pincer complex catalyzing the hydrosilylation of carbon dioxide to methane product were elucidated using DFT calculations. The calculated results indicate that the conversion of carbon dioxide to methane includes four stages. The rst stage of reducing CO 2 to silylformate is the rate-determining step in the overall carbon dioxide conversion process. The present computational study suggests two possible pathways: the dissociation pathway, in which a free silane dissociates into the weak C]O bond of the Ir-CO 2 moiety, and the ionic S N 2 outer-sphere pathway, in which CO 2 nucleophilically attacks the h 1 -silane iridium complex to cleave the Si-H bond, followed by the hydride transfer process.
Reducing carbon dioxide to silylformate via dissociation of the silane Si-H bond to the C]O bond of Ir-CO 2 has a free energy barrier of around 30 kcal mol À1 in solvent. To the best of our knowledge, the pre-coordination of CO 2 to the metal center being the rate-determining step in a dissociation pathway has not been reported before for M-H complexes. In the second stage, silylformate is reduced to bis(silyl)acetal substrate. The rate-limiting step is calculated to have a free energy barrier of around 12.2 kcal mol À1 in solvent. In the third stage, bis(silyl) acetal is reduced to methoxysilane, and the rate-limiting step is calculated to have a free energy barrier of around 16.4 kcal mol À1 in solvent. In the fourth stage, methoxysilane is reduced to methane, and the rate-limiting step is calculated to have a free energy barrier of around 22.9 kcal mol À1 in solvent. Based on the DFT calculations, the three subsequent reduction steps favor the ionic S N 2 outer-sphere pathways. Furthermore, our calculations indicate that formaldehyde is unlikely to be an intermediate in the CO 2 conversion catalyzed by the cationic Irpincer complex. The possible ways to generate formaldehyde are all associated with high free energy barriers: 38.7 kcal mol À1 (TS6a relative to IM5), 29.2 kcal mol À1 (TS8bi relative to IM5), and 34.1 kcal mol À1 (TS7c relative to IM5). Moreover, our results show that the in situ generation of iridium dihydride can greatly promote the silylation of CO 2 . The computed potential energy barrier for iridium dihydride catalyzing the silylation of CO 2 is quite low, with an activation free energy of 16.7 kcal mol À1 .

Conflicts of interest
There are no conicts to declare.