A DFT Study of the Conversion of CO 2 in Dimethylcarbonate Catalyzed by Sn ( IV ) Alkoxides

Foram realizados cálculos de teoria do funcional da densidade (DFT) de intermediários e estados de transição da reação entre o CO2 e metanol sobre catalisadores do tipo R2Sn(OCH3)2. A interação da molécula de CO2 com o catalisador de estanho é controlada pelo termo entrópico, sendo desfavorecida à temperatura ambiente e pressão atmosférica. Por outro lado, a inserção da molécula de CO2 na ligação Sn–OCH3 é termodinamicamente favorecida para todos os catalisadores estudados. A energia livre de ativação calculada varia com a natureza do substituinte R. Grupos fenila apresentam a menor barreira, enquanto que os átomos de halogênio, as mais elevadas. Os grupos alquila apresentam barreiras intermediárias. Os cálculos estão de acordo com resultados experimentais recentes, que indicaram uma maior frequência de rotação da reação (TON) para a formação de dimetil carbonato (DMC) quando Ph2SnO foi usado como catalisador. O esquema mecanístico completo foi calculado para os substituintes fenila e metila, considerando uma espécie de estanho dimérica.


Introduction
Carbon dioxide (CO 2 ) is essential to maintain optimal temperature conditions for life on our planet, because it is the main responsible for the greenhouse effect.However, due to the burning of fossil fuels, the concentration of CO 2 in the atmosphere is dramatically increasing, causing problems related with the global warming and climate changes.If no action is made to control or reduce the emission of CO 2 to the atmosphere, it is expected that the Earth temperature may increase by up to 6 °C until the end of this century. 1here are many studies for capturing, storing and using the CO 2 emitted from the burning of fossil fuels. 2 One alternative is the use of CO 2 as feedstock in sustainable processes to produce fuels and chemicals. 3Today, the industrial utilization of carbon dioxide is limited.It is used in the production of urea and salicylic acid.Methanol can Vol.25, No. 12, 2014 also be produced from CO 2 , through hydrogenation over Cu and Zn based catalysts. 4An industrial plant using this route has been recently started up in Iceland, using hydrogen produced from geothermal source.Cyclic organic carbonates can be produced through the reaction of CO 2 with epoxides. 5thylene carbonate is industrially produced by the reaction of ethylene oxide with carbon dioxide.This route avoids the emission of 1730 ton of CO 2 to the atmosphere for each 10000 ton of polymer produced. 6The cyclic carbonates can react with methanol to produce dimethylcarbonate (DMC), an important and versatile chemical.
DMC is mainly used as a polar solvent in the manufacture of pharmaceuticals, agrochemicals, paints, coatings and fragrances. 6It is also used 7 as a carbonylation and alkylation reagent in organic synthesis, substituting toxic reagents such as COCl 2 (phosgene), CH 3 OCOCl (methyl chloroformate), (CH 3 ) 2 SO 4 (dimethylsulfate) and methyl halides (MeX).Yet, DMC is used in the production of polycarbonates and polyurethanes, which are important polymers of versatile applications.DMC can also be used as oxygenated additive in gasoline, improving the octane number and reducing the emission of pollutants.
DMC can be prepared by the reaction of methanol with phosgene. 8However, this method has been gradually phased out, mainly due to the high toxicity of phosgene.The oxidative carbonylation 9 of methanol and transesterification routes are, today, the most used pathways to DMC, but also have environmental concerns.
One alternative for a green synthesis of DMC is the direct carbonylation of methanol with CO 2 (Scheme 1).Apart from being a cleaner route, this process may also contribute to a reduction in the amount of carbon dioxide released into the atmosphere.
The conversion of CO 2 to DMC has been extensively investigated. 10Organometallic compounds, such as Sn(IV) alkoxides, are capable of catalyzing the reaction. 11ifferent pathways may be envisaged to describe the role of Sn(IV) alkoxides in catalyzing the reaction of CO 2 with methanol to afford DMC and water.The most accepted 12 mechanistic pathway is shown on Scheme 2, involving a tin dimer species as catalyst.The dimerization increases the nucleophilicity of the Sn-OCH 3 oxygen atom and the electrophilicity of the tin atom.Kinetic studies show that the DMC yield increases smoothly and does not depend on the presence of methanol in the reaction medium.In fact, the alcohol can react with the product of the reaction between CO 2 and the catalyst (distannoxane), after formation of the DMC, to regenerate the catalyst.
The initial step of the mechanism is the insertion of the CO 2 molecule into the Sn-OCH 3 bonds of the catalyst, which involves a nucleophilic attach at the carbon atom of the CO 2 molecule by the oxygen atom of the tin alkoxide, affording a methoxy carbonate (2).This intermediate has already been isolated from the reaction medium, supporting the mechanistic pathway. 13The formation of the methoxy carbonate ( 2) is followed by an isomerization step, to afford an intermediate where the tin atom has an expanded valence shell, which then decomposes into DMC and regenerates the methoxyalkyltin species (1) upon reaction with methanol, restarting the catalytic cycle.In this last step, H 2 O is released and may contribute to the deactivation of the catalyst, as well as it may react with the formed DMC to yield CO 2 and methanol.In all transition states, the Sn atom provides electrophilic assistance, binding to the oxygen atom of the CO 2 molecule or the carbonate group.Therefore, electron withdrawing or releasing groups may influence the Lewis acidity of the Sn atom, affecting the eletrophilic assistance which may modify the kinetic profile of the reaction.
Our aim in this contribution is to carry out a theoretical study to understand the electronic effect of the R 1 and R 2 substituents on the thermodynamics and kinetic parameters of the conversion of CO 2 and methanol into DMC.

Computational methodology
Although the reaction of CO 2 and methanol to afford DMC and water is proposed to involve a dimeric Sn(IV) alkoxide species, we considered only the monomer, is always in equilibrium with the dimer, to reduce the computational costs.This procedure may give a trend of the electronic effect of the substituents (R 1 and R 2 ) on the thermodynamics and kinetic parameters of the first step (formation of the methoxy carbonate 2 through insertion of CO 2 molecule into Sn-OCH 3 bond).In addition, steric effects are more significant in the dimeric form, which, in principle, may override the electronic effects in the presence of bulky substituents.Based on these results, we, subsequently, evaluated the thermodynamics and kinetic parameters for all steps of those catalysts that present the best performance in the activation of the CO 2 molecule, using the same methodology, but considering a dimeric form of the catalysts.For comparison purposes, we also performed the same calculations for the methyl-substituted catalyst, as a reference case.
Geometry optimizations were performed with the Gaussian 09 package 14 using the meta-GGA functional M06-2x developed by Truhlar. 15The double-zeta polarized basis set 6-31G(d,p) was used to describe all atoms except Sn, for which the LANL2DZ effective core potential and basis set were applied.Vibrational analysis in the harmonic approximation (HO) was performed for all optimized structures at same level to correct for the zero-point energy (ZPE) and thermal effects at 298.15 K and 1 atm.Furthermore, all transition state structures were checked using the intrinsic reaction coordinate (IRC) method.
The rate constants k(T) were calculated using transition state theory with in which c 0 is the inverse of the reference volume assumed in translational partition function calculation, k b is the Boltzmann constant, T is temperature, h is plank's constant, R is the universal gas constant, m is the molecularity of the reaction and ∆S ≠ and ∆H ≠ are entropy and enthalpy of activation, respectively.∆H ≠ is given by ∆H ≠ = (E 0 + ZPVE + ∆∆H) TS-R , where ∆∆H is a temperature correction; ZPVE is the difference in zero-point vibrational energy between the transition state and the reactants; and E 0 is the difference in electronic energy of the transition state and the reactants.We also calculated the activation energy with: E a = ∆H ≠ + mRT and frequency factor with .

Results and Discussion
Figure 1 shows the structures of the transition state and intermediates for the insertion of the CO 2 molecule in the Sn-OCH 3 bond of the (CH 3 ) 2 Sn(OCH 3 ) 2 monomer.The calculated thermodynamics and kinetic data are also reported in Figure 1 at 298.15 K and 1 atm (Gibbs free energies with the enthalpic and entropic contributions).The structures for the same species in the presence of other catalysts do not present significant differences.
Table 1 shows the thermodynamic and kinetic parameters for all catalysts studied.According to the results, the interaction between the CO 2 molecule and the tin monomer to afford the interaction complex is not thermodynamically favorable for all the substituents (R 1 and R 2 ) at room temperature and atmospheric pressure.This results show the weak interaction between the species, not capable of overcoming the entropic term at room temperature.On the other hand, the CO 2 insertion in the Sn-OCH 3 bond is thermodynamically favorable for all catalysts and involves energy barriers in the range of 30 to 50 kJ mol −1 .The weak interaction between CO 2 and the tin complex may explain the use of high pressures in the experimental studies related with the formation of DMC.The interaction is basically of non-dispersive nature (VDW), with carbon atom faced to the oxygen atom of the OCH 3 moiety, whereas the tin atom interacts with the oxygen atom of the CO 2 molecule.
Figure 2 shows the effect of the substituents on the kinetic parameter (∆G ‡ ) for the insertion of CO 2 molecule in the Sn-OCH 3 bond of the (CH 3 ) 2 Sn(OCH 3 ) 2 monomer.Phenyl substituents are the most effective groups to enhance nucleophilic/electrophilic assistance of the catalyst in the activation of the CO 2 molecule, because the reaction in the presence of Ph 2 Sn(OCH 3 ) 2 has the lowest energy barrier.The Sn-OCO distance on the Ph 2 Sn(OCH 3 ) 2 catalyst is shorter (d Sn-OCO = 2.56 Å) than on (CH 3 ) 2 Sn(OCH 3 ) 2 (d Sn−OCO = 2.70 Å), showing that CO 2 is more strongly bounded with the phenyl-substituted catalyst.Butyl groups also present quite similar results, with activation parameters close to what was calculated for the phenyl substituents.Although the difference is not significant, the others alkyl groups (methyl, ethyl and n-propyl) present higher activation energy compared to the n-butyl group.The results of calculations with the phenyl and butyl groups are in agreement with recent experimental results, that showed that these catalysts presented higher turnover frequencies (TON) for DMC formation in the reaction between methanol and CO 2 , supporting the present calculation model. 16he free-energy of activation of fluorine, chlorine and bromine substituents are larger, compared with the alkyl groups.The order among the halogen atoms may Table 1.Thermodynamic (kinetic) parameters for the insertion of CO 2 into Sn-OCH 3 bond, computed at M062x/6-31G(d,p) level of theory Step a ∆H (∆H ‡ ) / (kJ mol -1 ) −T∆S (−T∆S ‡ ) / (kJ mol -1 ) be explained by the electronegativity, which affect the electrophilicity of the Sn atom.As the electronegativity of the halogen decreases, the free-energy of activation increases.
Based on these results, we evaluated the thermodynamics and kinetic parameters for the whole mechanistic pathway considering the dimeric form of Ph 2 Sn(OCH 3 ) 2 and (CH 3 ) 2 Sn(OCH 3 ) 2 , as a reference case.Figure 3 shows the structure of the transition states and intermediates involved in the reaction of CO 2 and methanol to afford DMC with the [(CH 3 ) 2 Sn(OCH 3 ) 2 ] 2 dimer as catalyst.The structures are similar when considering the [Ph 2 Sn(OCH 3 ) 2 ] dimer.
The potential energy surfaces for both catalysts are shown in Figure 4.It comes from the results that the steric hindrance, due to the presence of bulky phenyl groups, overrides the electronic effects in the dimer catalyst, which present a similar kinetic profile.Indeed, the Sn-OCO distance in TS1   slightly increases from 2.56 Å to 2.60 Å when considering the dimeric form of the Ph 2 Sn(OCH 3 ) 2 catalyst.This may be explained to the steric hindrance, whist keeping the same value (d Sn-OCO = 2.70 Å) for the dimeric form of the (CH 3 ) 2 Sn(OCH 3 ) 2 catalyst.Entropic contributions do not significantly modify the kinetic profile of the reaction, which still shows an unfavorable thermodynamic energy.This may explain the low yields and conversions observed in the synthesis of DMC from CO 2 and methanol.The use of water suppressor is normally required 17 to shift equilibrium and achieve higher yields of DMC.The calculated kinetic parameters for the reaction steps involving the TS are shown in the Table 2.According to the results, the activation energy of the slowest step (step C→D) for the dimeric form of the Ph 2 Sn(OCH 3 ) 2 catalyst is slightly higher compared to the (CH 3 ) 2 Sn(OCH 3 ) 2 catalyst, probably due to steric reasons.

Conclusions
Calculations on the thermodynamic and kinetic profile of the insertion of CO 2 in the R 1 R 2 Sn(OCH 3 ) 2 catalysts were carried out at M062x/6-31G(d,p) level of theory.Regardless of the substituents on the tin complex, the interaction with the CO 2 molecule is not favored at room temperature and atmospheric pressure due to the entropic term.The insertion into the Sn-OCH 3 bond is thermodynamic favorable for all the catalyst, but the free energy of activation depends on the nature of the substituents.Phenyl groups showed the lowest barrier, whereas halogen atoms the highest, supporting the dependence of the kinetics on the electrophilicity/ nucleophilicity of the Sn atom, caused by the substituents.
The entire mechanistic scheme was then calculated for the dimeric Ph 2 Sn(OCH 3 ) 2 catalyst, showing that steric factors become predominant when considering the dimer as catalysts.

Figure 1 .
Figure 1.Calculated structure of the transition state and intermediates of the insertion of CO 2 into Sn-OCH 3 bond of (CH 3 ) 2 Sn(OCH 3 ) 2 (thermodynamic and kinetic parameters are in kJ mol −1 and refer to 298.25 K and 1 atm).

Figure 2 .
Figure 2. Effect of the substituent (R) on the kinetic parameter for the insertion of CO 2 molecule in the Sn-OCH 3 bond of the R 2 Sn(OCH 3 ) 2 monomer.

Figure 3 .
Figure 3. Calculated structures of the transition states and intermediates in the reaction of CO 2 with methanol to DMC, catalyzed by the (CH 3 ) 2 Sn(OCH 3 ) 2 dimer.

Figure 4 .
Figure 4. Thermodynamic and kinetic profile for the reaction of CO 2 with methanol to afford DMC, catalyzed by the R 1 R 2 Sn(OCH 3 ) 2 dimer, according to the mechanism depicted in Figure 3. Dashed lines (enthalpy) and solid lines (Gibbs free energy).
Simplified mechanistic pathway for the conversion of CO 2 and CH 3 OH into DMC catalyzed by Sn (IV) alkoxides.