DFT Study on Activation of Carbon Dioxide by Dimethytin Dimethoxide

Feb 9, 2010 - Lorenzo Ferro , Peter B. Hitchcock , Martyn P. Coles , Hazel Cox ... Sarah R. Whittleton , Alfred J. Rolle , Russell J. Boyd , and T. Br...
0 downloads 0 Views 2MB Size
Download PDF
1290

Organometallics 2010, 29, 1290–1295 DOI: 10.1021/om901094z

DFT Study on Activation of Carbon Dioxide by Dimethytin Dimethoxide for Synthesis of Dimethyl Carbonate Kan Wakamatsu,*,† Akihiro Orita,‡ and Junzo Otera*,‡ †

Department of Chemistry, ‡Department of Applied Chemistry, Okayama University of Science, Ridai-cho, Kita-ku, Okayama 700-0005, Japan Received December 20, 2009

DFT calculations put forth theoretical grounds for a smooth reaction between methoxy and carbonate groups on the organotin template to afford dimethyl carbonate (DMC). The strong association of the Sn-O bond is the driving force for facile synthesis of DMC with organotin alkoxides. On the dimeric template, the methoxy and carbonate groups can be arranged in a proximate position to acquire an entropic gain only when they are bonded to separate tin atoms. The strong Sn-O bond association also plays a crucial role in driving the process to the final stage. The great stability of the distannoxane dimer formed in the final stage can outweigh all thermodynamic disadvantages involved in the previous steps.

Introduction Activation of carbon dioxide is a long-standing, yet unsolved issue.1 In particular, direct reaction of carbon dioxide with methanol is an ultimate goal in the synthesis of dimethyl carbonate (DMC), one of the most important raw materials in chemical industry, employed for the production of polycarbonates and isocyanates.2 Conventionally, polycarbonates are produced by using phosgene as a carbonyl source, but this process suffers from serious problems: high toxicity of phosgene, use of dichloromethane as solvent, involvement of corrosive materials, etc. A number of attempts have been made to replace this environmentally unfavorable process with greener ones. The following two processes are now under operation for commercial production *Corresponding authors. E-mail: [email protected]; otera@ high.ous.ac.jp. (1) (a) Shaikh, A.-A. G.; Sivaram, S. Chem. Rev. 1996, 96, 951. (b) Sakakura, T.; Choi, J.-C.; Yasuada, H. Chem. Rev. 2007, 107, 2365. (c) Darensbourg, D. J. Chem. Rev. 2007, 107, 2388. (2) Tundo, P.; Selva, M. Acc. Chem. Res. 2002, 35, 706. (3) Romano, U.; Tesei, R.; Masuri, M. M.; Rebora, F. Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 396. (4) Fukuoka, S. Chem. Ind. 1997, 6 (Oct.), 757. (5) Sn: (a) Bloodworth, A. J.; Davies, A. G.; Vasishtha, S. C. J. Chem. Soc. (C) 1967, 1309. (b) Davies, A. G.; Harrison, P. G. J. Chem. Soc. (C) 1967, 1313. (c) Zheng, G.-L.; Ma, J.-F.; Yang, J.; Li, Y.-Y.; Hao, X.-R. Chem.;Eur. J. 2004, 10, 3761. (d) Beckmann, J.; Dakternieks, D.; Duthie, A.; Lewcenko, N. A.; Mitchell, C. Angew. Chem., Int. Ed. 2004, 43, 6683. Zn: (e) Darensbourg, D. J.; Holtcamp, M. W.; Struck, G. E.; Zimmer, M. S.; Niezgoda, S. A.; Rainey, P.; Robertson, J. B.; Draper, J. D.; Reibenspies, J. H. J. Am. Chem. Soc. 1999, 121, 107. Pb: (f) Tam, E. C. Y.; Johnstone, N. C.; Ferro, L.; Hitchcock, P. B.; Fulton, J. R. Inorg. Chem. 2009, 48, 8971. Al and Cr: (g) Luinstra, G. A.; Haas, G. R.; Molnar, F.; Bernhart, V.; Eberhardt, R.; Rieger, B. Chem.;Eur. J. 2005, 11, 2698. Ti: (h) Kizlink, J.; Pastucha, I. Collect. Czech. Chem. Commun. 1995, 60, 687. (i) Kohno, K.; Choi, J.-C.; Ohshima, Y.; Yasuda, H.; Sakakura, T. ChemSusChem 2008, 1, 186. Cr, Mo, W: (j) Darensbourg, D. J.; Sanchez, K. M.; Reibenspies, J. H.; Rheingold, A. L. J. Am. Chem. Soc. 1981, 111, 7049. (k) Darensbourg, D. L.; Mueller, B. L.; Bischoff, C. J.; Chojnacki, S. S.; Reibenspies, J. H. Inorg. Chem. 1991, 30, 2418. Bi: (l) Yin, S.-F.; Maruyama, J.; Yamashita, T.; Shimada, S. Angew. Chem., Int. Ed. 2008, 47, 6590. pubs.acs.org/Organometallics

Published on Web 02/09/2010

of DMC: coupling of carbon monoxide and methanol under oxidative conditions,3 and reaction of ethylene oxide with carbon dioxide followed by methanolysis of the resulting ethylene carbonate.4 Nevertheless, the ideal direct condensation between carbon dioxide and methanol has not been realized yet. Utilization of metal alkoxides, which are readily available from a variety of metal compounds and alcohol, is a strategy of great promise for the above purpose, because it has long been known that some metal alkoxides readily absorb carbon dioxide to form metal alkyl carbonates.5 Although subsequent conversion of these species into the desired dialkyl carbonates is not easy to achieve, organotin alkoxides among others are unique in that they can afford dialkyl carbonates with some ease.6,7 The tin-oxygen bond in dialkyltin dimethoxide undergoes smooth insertion of carbon dioxide in toluene or methanol to give dialkyl(methoxy)stannyl methyl carbonate, from which DMC eliminates, although the process is not catalytic because the elimination leaves relatively inert organotin oxide. (6) (a) Sakai, S.; Fujinami, T.; Furusawa, S. Nippon Kagaku Kaishi 1975, 10, 1789; Chem. Abstr. 1976, 84, 5090y. (b) Yamazaki, N.; Nakahara, S. Ind. Eng. Chem., Prod. Dev. 1979, 18, 249. (c) Kizlink, J.; Pastucha, I. Collect. Czech. Chem. Commun. 1994, 59, 2116. (d) Ballivet-Tkatchenko, D.; Jerphagnon, T.; Ligabue, R.; Plasseraud, L.; Poinsot, D. Appl. Catal., A 2003, 255, 93. (e) Ballivet-Tkatchenko, D.; Burgat, R.; Chambrey, S.; Plasseraud, L.; Richard, P. J. Organomet. Chem. 2006, 691, 1498. (f) Ballivet-Tkatchenko, D.; Douteau, O.; Stutzmann, S. Organometallics 2000, 19, 4563. (7) (a) Ballivet-Tkatchenko, D.; Chermette, H.; Jerphagnon, T. In Environmental Challenges and Greenhouse Gas Control for Fossil Fuel Utilization in the 21st Century; Maroto-Vater, et al., Eds.; Kluwer Academic/Plenum Publishers: New York, 2002; pp 371-384. (b) BallivetTkatchenko, D.; Chermette, H.; Plasserauda, L.; Walter, O. Dalton Trans. 2006, 5167. (8) Dimethyl acetals were used for dehydration in supercritical carbon dioxide: (a) Choi, J.-C.; Sakakura, T.; Sako, T. J. Am. Chem. Soc. 1999, 121, 3793. (b) Sakakura, T.; Choi, J.-C.; Saito, Y.; Masuda, T.; Sako, T.; Oriyama, T. J. Org. Chem. 1999, 64, 4506. (c) Choi, J.-C.; He, L.-N.; Yasuda, H.; Sakakura, T. Green Chem. 2002, 4, 230. (d) Kohno, K.; Choi, J.-C.; Ohshima, Y.; Yili, A.; Yasuda, H.; Sakakura, T. J. Organomet. Chem. 2008, 693, 1389. r 2010 American Chemical Society

Article

Organometallics, Vol. 29, No. 5, 2010

Hydrolysis of the catalyst and DMC by the water coproduct is also responsible for the incomplete catalytic cycle.8 Despite these drawbacks, we considered that the mechanistic scrutiny of the organotin alkoxide process would provide us with a useful clue to designing more efficient alternatives. The present study has its foundation in our previous ab initio molecular orbital calculations of relevant organotin alkoxides.9 In this context, we have performed a theoretical treatment on the reaction of dimethyltin dimethoxide with carbon dioxide. It is noted that Ballivet-Tkatchenko et al. reported DFT-based optimization of tri- and dimethyltin carbonates derived from reactions between methyltin methoxides and CO2.7 In this paper, we investigate the whole reaction path in more detail.

Method All calculations are carried out with the Gaussian 03 program package in this study.10 We treated a series of the reactions in the gas phase because we first investigated model reactions for the experiments in nonpolar solvent (i.e., toluene).7 The calculations including solvent effects by polar solvent such as methanol will be discussed elsewhere. Previously,9 we found that density functional theory (DFT) with the popular B3LYP functional was not reliable in terms of energetics for organotin alkoxides, and the Moeller-Plesset second-order perturbation theory (MP2) was employed instead. However, this method is too computationally intensive for calculations of reaction paths. Thus, we choose here DFT with the recently proposed M05-2X functional by Truhlar’s group, which was found to be suitable for thermochemistry and noncovalent interactions at reasonable calculation cost.11 Because of significant relativistic effects and the unavailability of a complete basis set for heavy atoms, a basis set including the effective core potential (ECP) was selected for the tin atom. In consideration for a balance between reliability and calculation efficiency, we used a combination of the LANL2DZ ECP basis set12 for tin atoms and 6-311þG* for other atoms. We refer to this basis set as “6-311þG*-LANL2DZ(Sn)” hereafter. Additionally, instead of LANL2DZ we also examined the recently proposed basis set LANL2DZdp,13 which includes polarization and diffuse functions with the same valence basis set and ECP as in LANL2DZ. In the course of discussion of the reaction mechanism, little difference of relative energies was found between LANL2DZ and LANL2DZdp (see Table 1). Therefore, we describe only LANL2DZ values in the following text except as specifically noted. To check the validity of the combination of the M05-2X functional and the 6-311þG*-LANL2DZ(Sn) basis set, we calculated the thermochemical energy change (298.15 K, 1 atm) for dimerization of dibutyltin diisopropoxide, for which an experimental value had been reported. From a 119Sn NMR study the experimental ΔH for dibutyltin diisopropoxide was reported as -24 ( 4 kcal mol-1.14 The present calculation gave ΔH and ΔG as -23.44 and -3.79 kcal mol-1 after BSSE (basis set superposition error) correction, respectively. From the comparison between the calculated and experimental values, we thus confirmed that M05-2X/6-311þG*-LANL2DZ(Sn) affords (9) Wakamatsu, K.; Orita, A.; Otera, J. Organometallics 2008, 27, 1092. (10) Frisch, M. J.; et al. Gaussian 03, Revision E.01; Gaussian, Inc.: Wallingford, CT, 2004. Full author list is shown in the Supporting Information. (11) Zhao, Y.; Schultz, N. E.; Truhlar, D. G. J. Chem. Theory Comput. 2006, 2, 364. (12) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (13) Check, C. E.; Faust, T. O.; Bailey, J. M.; Wright, B. J.; Gilbert, T. M.; Sunderlin, L. S. J. Phys. Chem. A 2001, 105, 8111. (14) Smith, P. J.; White, R. F. M. J. Organomet. Chem. 1972, 40, 341.

1291

Table 1. Relative Free Energies ΔG/kcal mol-1a

(a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l) (m) (n) (o) (p) (q)

1 þ 2CO2 2 þ CO2 TS A þ CO2 3 (form A) þ CO2 3 (form B) þ CO2 4 TS B 5 (form A) 5 (form B) 6 TS C 7 TS D 8 9 þ DMC 1/2 10 þ DMC 1/2 11 þ DMC þ CO2

LANL2DZb

LANL2DZdpb

0 0.97 10.99 -2.38 -5.38 -1.89 7.74 -5.67 -7.53 -2.11 31.17 18.08 21.29 9.20 13.49 -14.49 -10.69

0 2.50 12.26 -2.12 -3.72 -0.79 9.47 -4.80 -6.64 -1.27 32.03 18.82 21.30 9.55 11.75 -12.63 -9.61

a Calculated energies are at 298.15 K and 1 atm. b All data were calculated using the M05-2X functional. LANL2DZ and LANL2DZdp were used for tin only, and 6-311þG* was used for other atoms.

relative energies sufficiently reliable for discussion of the reaction mechanism. Similar results were obtained from M05-2X/6311þG*-LANL2DZdp(Sn) (ΔH = -21.50 kcal mol-1, ΔG = -1.29 kcal mol-1), whereas B3LYP/6-311þG*-LANL2DZ(Sn) apparently underestimates the stability of the dimeric form (ΔH = þ1.27 kcal mol-1, ΔG = þ19.09 kcal mol-1). Full geometry optimizations were done with no symmetry restrictions, and the characterization of the stationary points was verified by the subsequent vibrational frequency analyses (no imaginary frequencies for an energy minimum point and a single imaginary frequency for a transition state). We also carried out intrinsic reaction coordinate (IRC) calculations for all transition states. All thermochemical parameters were evaluated at 298.15 K and 1 atm. Atomic charges were determined by natural population analysis (NPA).15

Results and Discussion The whole process derived from DFT calculations is shown in Scheme 1. The relative free energies (ΔG) of intermediates and transition states (TS) are also collected in Table 1 and Figure 1. The initial step is insertion of carbon dioxide into tin-oxygen bonds in dimeric dimethyltin dimethoxide (1) in a stepwise manner to afford methoxy carbonate 5, which was isolated by Sakakura et al.8a and calculated using DFT with the PW91 functional by BallivetTkatchenko et al.7a The monomeric species 12 of dimethyltin dimethoxide does not virtually contribute to the CO2 insertion process because the monomer-dimer equilibrium is overwhelmingly shifted to the dimer side (Scheme 2): ΔG for dimerization (after BSSE correction) is calculated to be -11.17 kcal mol-1 by M05-2X/6-311þG*-LANL2DZ(Sn) and -9.95 kcal mol-1 by M05-2X/6-311þ G*-LANL2DZdp(Sn), respectively.16 In addition, the dimerization enhances both nucleophilicity of the methoxy oxygen atom and electrophilicity of the tin atom: atomic charges of the (nonbridging) methoxy oxygen atom are (15) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO Version 3.1 as part of Gaussian 03. (16) At the MP2 level with LANL2DZ for tin atoms and 6-311þG* for other atoms, ΔG for dimerization was calculated as -7.37 kcal mol-1 after BSSE correction.9 Similar results by using DFT calculations with the PW91 functional were reported.7

1292

Organometallics, Vol. 29, No. 5, 2010

Wakamatsu et al.

Scheme 1. The Whole Process to Produce DMC and the Distannoxane from Dimeric Dimethyltin Dimethoxide and CO2a

a

The optimized graphical structures by M05-2X/6-311þG*-LANL2DZ(Sn) are also shown here. Hydrogen atoms are omitted for clarity.

-1.00 (1) and -0.97 (12), and those of the tin atom are 2.51 (1) and 2.40 (12), respectively. For the tin isopropoxide, the following two reaction paths were suggested: (i) dimerization followed by CO2 insertion and (ii) monocarbonation of the monomer followed by dimerization.7b However, it is apparent from the above arguments that the reaction originates from the dimer 1 for the tin methoxide. With the

substantially low activation free energy (ΔGq = 10.02 kcal mol-1),17 the initial complex 2, in which the nonbridging (17) In the insertion of CO2 to the monomer 12, ΔG and ΔGq (the difference between the transition state and the monomer-CO2 complex) values were obtained as 0.53 and 10.99 kcal mol-1, respectively (Scheme 2).

Article

Organometallics, Vol. 29, No. 5, 2010

1293

Figure 1. Energy diagram for the reaction of dimethyltin dimethoxide dimer (1) with carbon dioxide affording DMC and distannoxane dimer (11). All energies were calculated by M05-2X/6-311þG*-LANL2DZ(Sn). Alphabetical symbols correspond to those in Table 1. Scheme 2. Monomer-Dimer Equilibrium of Dimethyltin Dimethoxide and the insertion of CO2 into the Monomer

methoxy oxygen atom of 1 interacts with the CO2 carbon, changes to the corresponding monocarbonate 3, initially as form A, followed by isomerization to form B. As shown in Figure 2, the insertion process takes place in a concerted manner because of no intermediate appearance, but the C-O bond formation between the CO2 carbon atom and methoxy oxygen atom (1.632 A˚ in TS A) slightly precedes the bond formation between the tin atom and CO2 oxygen atom (2.593 A˚ in TS A). Then, the second insertion of carbon dioxide proceeds more smoothly on the monocarbonateCO2 complex 4 (ΔGq = 9.63 kcal mol-1) to give 5 first in form A and then in form B. In 5 (form B) the carbonyl oxygen atom of the carbonate group interacts with the tin center (Sn-O: 2.951 A˚), and the Sn-O-Sn-O ring and the carbonate group are almost in the same plane. This agrees with the previously reported X-ray structure of 5 (Sn-O: 2.822 A˚).8a The reactions are exothermic as expected (ΔG = -5.38 and -2.15 kcal mol-1 for the first and second insertion steps, respectively). Also, the insertion of carbon dioxide is essentially reversible, judging from the relatively low activation free energies (ΔGq). This agrees with the previous

Figure 2. Changes of relative energy and some atomic distances along the reaction coordinate of the first CO2 insertion process from the IRC calculation using M05-2X/6-311þG*LANL2DZ(Sn).

experimental findings that 5 easily releases carbon dioxide at room temperature.6d,8a Subsequently, 5 (form B) isomerizes to 6, in which one of the carbonyl groups locates in the apical position in a trigonal-bipyramidal structure. This event possibly proceeds through either dissociation into the corresponding monomer 14 or cleavage of one of the Sn-O coordination bonds followed by rotation of the remaining Sn-OCH3 bond. The latter process seems more probable on the basis of the following calculations using M05-2X/6-311þG*LANL2DZ(Sn). Electronic energy difference (ΔE, no zeropoint correction) between two of 14 and dimer 5 (form B) are

1294

Organometallics, Vol. 29, No. 5, 2010

Wakamatsu et al.

Scheme 3. Other Possible Pathways to Produce DMC

calculated to be 36.20 kcal mol-1 after BSSE (basis set superposition error) correction.18 On the other hand, the energy scan varying the bond length of one of the Sn-O coordination bonds reveals that isomerization from 5 to 6 needs only about 22 kcal mol-1 for activation (electronic energy base).19 Although 6 is less stable than 5 (vs form B, 5.42 kcal mol-1), close approach of the carbonyl carbon to the bridging methoxy oxygen in 6 (3.034 A˚) allows transformation of 6 to 7 via transition state C, following the B€ urgi-Dunitz trajectory.20 Intramolecular reaction between the methoxy and carbonate groups on the putative monomeric species 14, if generated, never occurs because of unfavorable relative situations of the two groups to interact with each other (Scheme 3a). In 14, the methoxy oxygen is far from the carbonyl carbon (4.162 A˚) and is placed in the same plane formed by the carbonate group. Moreover, the intramolecular attack to produce an intermediate 15 needs extremely high activation energy (ΔGq = 55.98 kcal mol-1, ΔG = 36.62 kcal mol-1). The intermolecular reaction between the monomers (14 þ 14 in Scheme 3b) also could not occur because the dimeric complex 14 þ 14 is considerably unstable (23.49 kcal mol-1) compared with 5. Moreover, the reaction is uphill: ΔG values for 14 þ 14, 16, and 17 relative to the sum of 1 and 2 mol of CO2 are 15.96, 26.60, and 26.12 kcal mol-1, respectively (compare the values listed in Table 1). Complex 18 could not be fully optimized, but its energy is found to be comparable to the preceding intermediates (20.85 kcal mol-1).21 (18) ΔG for dimerization of 14 affording 5 (form B) was calculated as -19.57 kcal mol-1 after BSSE correction. The comparison with ΔG for dimerization of 12 (-11.17 kcal mol-1) suggests stronger association of the bridged Sn-OMe bond in 5 (bond length: 2.207 A˚) than in 1 (bond length: 2.219 A˚). (19) For further discussion, see the Supporting Information. (20) B€ urgi, H. B.; Dunitz, J. D. Acc. Chem. Res. 1983, 16, 153. (21) One imaginary vibration mode (12.0i cm-1) of methyl rotation was found by frequency analysis.

Figure 3. Optimized structure of the dyad from 5 by M05-2X/6311þG*-LANL2DZ(Sn). Hydrogen atoms are omitted for clarity.

The possibility of an intermolecular reaction between two molecules of the dimeric form 5 also can be ruled out. That is, although the dyad formation from 5 as shown in Figure 3 was found to be endothermic (ΔG = 7.64 kcal mol-1 after BSSE correction), the carbonyl carbon is far away (4.050 A˚) from the methoxy group of another dimer. Apparently, the strong association of organotin alkoxides is primarily important, allowing the methoxy and carbonyl groups to locate in proximate positions on the organotin template. The intermediate 7 exothermically affords the complex of distannoxane and DMC 8 via transition state D (Scheme 1). The subsequent decomplexation delivering DMC and monomeric distannoxane 9 is endothermic (ΔG = 4.29 kcal mol-1); thus the free energy difference relative to 5 (form B) is still positive (ΔG = 21.02 kcal mol-1). However, this enthalpic disadvantage is overwhelmed by spontaneous dimerization of 9 to give 10: ΔG for conversion of 9 to 10 = -27.98 kcal mol-1 (per 1 mol of 9); ΔG relative to 5 (form B) = -6.96 kcal mol-1. The analogous carbonate tetrabutyldistannoxane dimer was isolated by Ballivet-Tkatchenko et al.6d,7b The association of Sn-O bonds again serves as a driving force of the process.

Article

Organometallics, Vol. 29, No. 5, 2010

Finally, decarboxylation of 10 affords the alkoxy distannoxane 11. This event is slightly endothermic, and therefore it proceeds only under forced conditions.6d,7b

Conclusions The strong association of the Sn-O bond is the driving force of facile synthesis of DMC with organotin alkoxides. DFT calculations put forth theoretical grounds for a smooth reaction between methoxy and carbonate groups on the organotin template to afford DMC. While increase of nucleophilicity of the alkoxy oxygen is no doubt a prime requirement in the metal alkoxide-promoted CO2 activation,22 the strong association of Sn-O bonds is equally or even more important in the present protocol. On the dimeric template, the methoxy and carbonate groups can be arranged in proximate positions to acquire an entropic gain (22) Atomic charge of the bridged methoxy oxygen atom in 6 is -1.06, while that of methanol is -0.74. The charge of the carbonyl carbon in 6 is almost the same as in DMC (þ1.05).

1295

only when they are bonded to separate tin atoms. The strong Sn-O bond association also plays a crucial role for driving the process to the final stage. The great stability of the distannoxane dimer formed in the final stage can outweigh all thermodynamic disadvantages involved in the previous steps. In summary, the present theoretical treatment could have put forth a rationale for the effectiveness of organotin alkoxides for carbonate formation.

Acknowledgment. This work was supported by matching fund subsidy for private universities from MEXT (Ministry of Education, Culture, Sports, Science and Technology). Supporting Information Available: List of calculated energies and thermochemical parameters, list of optimized Cartesian coordinates of molecules, further discussion of the isomerization from 5 to 6, and a complete ref 10 containing a full author list. This material is available free of charge via the Internet at http://pubs.acs.org.