Organometallics 2009, 28, 2633–2636
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Efficient and Reversible Fixation of Carbon Dioxide by NCN-Chelated† Organoantimony(III) Oxide Libor Dosta´l,*,‡ Roman Jambor,‡ Alesˇ Ru˚zˇicˇka,‡ Milan Erben,‡ Robert Jira´sko,§ Eva Cˇernosˇkova´,| and Jaroslav Holecˇek‡ Department of General and Inorganic Chemistry, Faculty of Chemical Technology, UniVersity of Pardubice, na´m. Cˇs. Legiı´ 565, Pardubice 53210, Czech Republic, Department of Analytical Chemistry, Faculty of Chemical Technology, UniVersity of Pardubice, na´m. Cˇs. Legiı´ 565, CZ-532 10, Pardubice, Czech Republic, and Joint Laboratory of Solid State Chemistry of Institute of Macromolecular Chemistry of Academy of Sciences of Czech Republic, VVi, and UniVersity of Pardubice, Studentska´ 84, CZ-532 10 Pardubice, Czech Republic ReceiVed January 29, 2009 Summary: The dimeric organoantimony(III) oxide (ArSbO)2 (Ar ) NCN chelating ligand, C6H3-2,6-(CH2NMe2)2) was obtained by the reaction of the parent ArSbCl2 compound with KOH. This oxide is able to bind carbon dioxide with formation of the monomeric, air-stable carbonate ArSbCO3. In turn, carbon dioxide can be easily eliminated from this carbonate by prolonged heating to 130 °C to recoVer (ArSbO)2. Chemical fixation of carbon dioxide has recently attracted considerable attention in the scientific community. This interest has been mainly caused by a promising potential of carbon dioxide as an inexpensive and relatively simply available C1 feedstock.1 Reduction of the amount of atmospheric carbon dioxide and its important role in many biological processes can be seen as other reasons for research in this field.2 A substantial part of the research in this area has focused on organometallic compounds as species that are able to bind CO2 quite easily and are capable of activating this molecule for further chemical reactions in one step or eliminating CO2 under mild conditions again and in such a way serve as a stable reservoir of this gas. Only a few reports dealing with main-group organometallic compounds that target this field have emerged. These include especially organotin(IV)3 oxides, alkoxides, and hydroxides, and very recently the potential of other chelated organobismuth compounds in CO2 fixation was clearly demonstrated as well.4 * To whom correspondence should be addressed. E-mail:
[email protected] Fax: +420 466037068. Tel: +420 466037163. † NCN is designation for NCN chelating ligand, C6H3-2,6-(CH2NMe2)2 ‡ Department of General and Inorganic Chemistry, University of Pardubice. § Department of Analytical Chemistry, University of Pardubice. | Joint Laboratory of Solid State Chemistry of Institute of Macromolecular Chemistry of Academy of Sciences of Czech Republic,vvi and University of Pardubice. (1) (a) Jessop, P. G.; Ikariya, T.; Noyori, R. Chem. ReV. 1995, 95, 259. (b) Song, C. S. Catal. Today 2006, 115, 2. (c) Sakakura, T.; Choi, J. C.; Yasuda, H. Chem. ReV. 2007, 107, 2365. (2) (a) Cleland, W. W.; Andrews, T. J.; Gutteridge, S.; Hartman, F. C.; Lorimer, G. H. Chem. ReV. 1998, 98, 549. (b) Parkin, G. Chem. ReV. 2004, 104, 699. (3) For example, see: (a) Beckmann, J.; Dakternieks, D.; Duthie, A.; Lewcenko, N. A.; Mitchell, C. Angew. Chem., Int. Ed. 2004, 43, 6683. (b) Ballivett-Tkatchenko, D.; Chermette, H.; Plasseraud, L.; Walter, O. Dalton Trans. 2006, 5167. (c) Choi, J. C.; He, L. N.; Yasuda, H.; Sakakura, T. Green Chem. 2002, 4, 230. (d) Choi, J. C.; Sakakura, T.; Sako, T. J. Am. Chem. Soc. 1999, 121, 3793. (e) Ballivett-Tkatchenko, D.; Douteau, O.; Stutzmann, S. Organometallics 2000, 19, 4563. (f) Padeˇlkova´, Z.; Weidlich, T.; Kola´rˇova´, L.; Eisner, A.; Cı´sarˇova´, I.; Zevaco, T. A.; Ru˚zˇicˇka, A. J. Organomet. Chem. 2007, 692, 5633. (g) Kohno, K.; Choi, J. C.; Ohshima, Y.; Yili, A.; Yasuda, H.; Sakakura, T. J. Organomet. Chem. 2008, 693, 1389.
Scheme 1. Preparation of (ArSbO)2 (2)
Here we report on the successful synthesis of the dimeric organoantimony oxide (ArSbO)2 (2; Ar ) NCN chelating ligand, C6H3-2,6-(CH2NMe2)2), which was shown to be capable of reversible CO2 fixation/release under relatively mild reaction conditions. Reaction of the parent chloride5 ArSbCl2 with KOH in a toluene/water system afforded organoantimony oxide 2 in moderate yield (45%; Scheme 1) as a colorless solid partially soluble in hexane as well as in aromatic, chlorinated (CHCl3 and CH2Cl2), and ethereal solvents. The molecular structure of 2 was unambiguously determined by X-ray diffraction techniques (Figure 1). Compound 2 is built up as a centrosymmetric dimer with a central Sb2O2 core, where both µ-O bridges are nearly symmetrical (Sb(1)-O(1) ) 2.000(3) Å and Sb(1)-O(1a) ) 2.018(3) Å). Both NCN pincer ligands are placed mutually in trans positions with regard to the central four-membered core and are coordinated in a tridentate manner to the antimony atoms through medium-strong Sb-N intramolecular dative connections (Sb(1)-N(1) ) 2.800(3) Å and Sb(1)-N(2) ) 2.641(3) Å). The analogous dimeric organoantimony oxide [2,4,6-[(Me3Si)2CH]3C6H2SbO]2 has been isolated recently.6 This dimeric structure of 2 is also preserved in solution, as proved by the 1H and 13C NMR spectra (Figure 4A; see also the Supporting Information), which contain two sets of signals due to the presence of two possible cis/trans isomers with regard to the position of NCN ligands on the central Sb2O2 ring. Similar results were obtained for analogous dimeric organotin and organoantimony compounds (with the central Sb2S2 or Sn2S2 core) bearing similar pincer type (4) (a) Yin, S. F.; Muruayma, J.; Yamashita, T.; Shimada, S. Angew. Chem., Int. Ed. 2008, 47, 6590. (b) Breunig, H. J.; Ko¨nigsmann, L.; Lork, E.; Nema, M.; Philipp, N.; Silvestru, C.; Soran, A.; Varga, R. A.; Wagner, R. Dalton Trans. 2008, 1831. (5) Atwood, D. A.; Cowley, A. H.; Ruiz, J. Inorg. Chim. Acta 1992, 198-200, 271. (6) Tokitoh, N.; Arai, Y.; Sasamori, T.; Okazaki, R.; Nagase, S.; Uekusa, H.; Ohashi, Y. J. Am. Chem. Soc. 1998, 120, 433.
10.1021/om9000692 CCC: $40.75 2009 American Chemical Society Publication on Web 03/24/2009
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Notes Scheme 2. Reversible Fixation of Carbon Dioxide by 2 To Form ArSbCO3 (3)
Figure 1. ORTEP plot of a molecule of 2 showing 50% probability displacement ellipsoids and the atom numbering scheme (symmetry code: (a) -x + 1, -y, -z + 1). Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Sb(1)-O(1) ) 2.000(3), Sb(1)-O(1a) ) 2.018(3), Sb(1)-C(1) ) 2.171(4), Sb(1)-N(1) ) 2.800(3), Sb(1)-N(2) ) 2.641(6); O(1)-Sb(1)-O(1a) ) 79.67(10), Sb(1)-O(1)-Sb(1a) ) 100.34(11), N(1)-Sb(1)-N(2) ) 121.84(10), C(1)-Sb(1)-O(1) ) 95.83(12), C(1)-Sb(1)-O(1a) ) 95.48(11).
Figure 4. 1H NMR spectra in CDCl3: (A) pure sample of oxide (ArSbO)2 (2) containing two sets of signals due to the cis-trans isomerism; (B) pure sample of ArSbCO3 (3) prepared from 2 via treatment with gaseous CO2; (C) sample of 3 after 7 h of heating to 130 °C, demonstrating partial elimination of CO2; (D) sample of 3 after 10 h of heating to 130 °C, yielding oxide 2 via complete CO2 elimination.
ligands.7,8 Moreover, the dimeric structure is also confirmed by the full scan positive-ion ESI mass spectra, where the most abundant ion corresponds to the protonated molecule [Ar2Sb2O2 + H]+ at m/z 657.
Although compound 2 is stable in air at ambient temperature for a long time (at least several weeks), when it was treated in a toluene solution with gaseous CO2 an immediate reaction took place with precipitation of a white crystalline solid characterized (1H and 13C NMR, IR) as the carbonate ArSbCO3 (3) (Scheme 2). The IR spectra showed strong characteristic bands for the carbonate at 1678 and 1643 cm-1 in a KBr pellet (1676 and 1643 cm-1 in CHCl3 solution), proving entirely the same type of coordination of the carbonate unit in both phases. Also, the position of these bands is indicative of a terminal carbonate (as postulated in Scheme 2), because similar values (1669 and 1634 cm-1)11b were observed in a terminal tin carbonate and, on the other hand, these bands are shifted significantly in the case of bridging carbonates (1458, 1323 cm-1 and 1520, 1283 cm-1 for Bi;4 1586 and 1328 cm-1 for Sb12). The 1H and 13C NMR spectra also revealed one set of signals consistent with the proposed structure of 3 (Scheme 2). The presence of the signal at 161.3 ppm in the 13C NMR spectrum (close to the values reported for other main-group-metal carbonates)3 proved the presence of the CO3 unit as well. Finally, the molecular structure of 3 was unambiguously determined by X-ray studies (Figures 2 and 3). Single crystals of 3 were obtained by slow evaporation of a CH2Cl2 solution in air. The compound 3 was obtained as a solvate with two molecules of water (all attempts to obtain anhydrous single crystals by using dried solvents and an argon atmosphere failed). The most interesting feature of the molecular structure of 3 is coordination of the carbonate moiety as a terminal ligand in a chelating fashion. This coordination mode of the carbonate entity is, to our knowledge, very rare for main-group-metal compounds,11 and among organoantimony and organobismuth compounds, characterized in the solid state by X-ray diffraction, only those with the carbonate group in a bridging position are known.4,12 This coordination of the CO3 moiety to the central Sb(1) atom leads to the formation of a four-membered SbCO2 ring, and the coordination of the carbonate is nearly symmetrical (Sb(1)-O(1) ) 2.126(3) and Sb(1)-O(2) ) 2.151(3) Å, indicating an Sb-O covalent bond, ∑cov(Sb,O) ) 2.14 Å). The (7) (a) Chovancova´, M.; Jambor, R.; Ru˚zˇicˇka, A.; Jira´sko, R.; Cı´sarˇova´, I.; Dosta´l, L. Organometallics 2009, 28, 1934. (b) Dosta´l, L.; Jambor, R.; Ru˚zˇicˇka, A.; Jira´sko, R.; Taraba, J.; Holecˇek, J. J. Organomet. Chem. 2007, 692, 3750. (8) The stabilization of the dimeric structure of 2 with the central fourmembered ring is most probably a consequence of the presence of the NCN chelating ligand, as suggested by one of the reviewers and in accord with a recently published work9 by Jurkschat et al., where the stabilization of small rings was ascribed to an increase in the coordination number of the central atom due to the presence of intramolecular coordinations. However, this phenomenon seems to be a bit more complicated, because a similarly OCO chelated organoantimony oxide is tetrameric10a in the solid state and also various ring systems have been recently obtained in the case of an organotin oxide containing s similarly NC chelating ligand.10b This phenomenon is now being extensively studied by our group. (9) Beckmann, J.; Dakternieks, D.; Lim, A. E. K.; Lim, K. F.; Jurkschat, K. J. Mol. Struct. (THEOCHEM) 2006, 761, 177. (10) (a) Dosta´l, L. Unpublished material. (b) Padeˇlkova´, Z.; Nechaev, ˇ ernosˇek, Z.; Brus, J.; Ru˚zˇicˇka, A. Organometallics 2008, 27, 5303. M. S.; C (12) Lang, G.; Klinkhammer, K. W.; Recker, Ch.; Schmidt, A. Z. Anorg. Allg. Chem. 1998, 624, 689. (11) (a) Hsu, M. H.; Chen, R. T.; Sheu, W. S.; Shieh, M. Inorg. Chem. 2006, 45, 6740. (b) Simo´n-Manso, E.; Kubiak, C. P. Angew. Chem., Int. Ed. 2005, 44, 1125.
Notes
Figure 2. ORTEP plot of a molecule of 3 showing 50% probability displacement ellipsoids and the atom-numbering scheme. Coordinated water molecules are omitted for clarity. Selected bond lengths (Å) and angles (deg): Sb(1)-O(1) ) 2.126(3), Sb(1)-O(2) ) 2.151(3), Sb(1)-C(1) ) 2.121(4), C(13)-O(1) ) 1.314(5), C(13)-O(2) ) 1.312(5), C(13)-O(3) ) 1.223(5), Sb(1)-N(1) ) 2.467(3), Sb(1)-N(2) ) 2.543(3); N(1)-Sb(1)-N(2) ) 132.98(11), N(1)-Sb(1)-O(1) ) 76.92(10), N(2)-Sb(1)-O(2) ) 77.65(11), O(1)-Sb(1)-O(2) ) 61.17(10), O(1)-C(13)-O(2) ) 112.0(3).
bonding angle at the antimony center is significantly more acute (O(1)-Sb(1)-O(2) ) 61.17(10)°) than the angle at the carbon atom (O(1)-C(13)-O(2) ) 112.0(3)°). Comparison of the C-O bond lengths within the carbonate group revealed, as should be expected, a significant difference in the bond lengths between the terminal C(13)-O(3) (1.223(5) Å) and those coordinated to the antimony atom (C(13)-O(1) ) 1.314(5) Å, C(13)-O(2) ) 1.312(5) Å), similarly to the terminal SnCO3 moiety (the C-O bond lengths 1.223(6) vs 1.325(6) Å).11b The NCN ligand is coordinated in a tridentate fashion to the central Sb(1) (Sb(1)-N(1) ) 2.467(3) Å and Sb(1)-N(2) ) 2.543(3)Å) and in this way most probably gives sufficient support for stabilization of the terminal coordination of the carbonate moiety. Due to the presence of the water molecules in the crystal structure, two molecules of carbonate form a hydrate with four water molecules (Figure 3). Two of these water molecules (O(3w) and O(2w)) connect two carbonates via nearly symmetrical bridges involving a terminal oxygen atom from the carbonate groups (O(3)). More interestingly, the remaining two water molecules (O(1w)) reinforce one side of this dimer through two additional bridges between the water molecule (O(2w)) and the carbonate oxygen atoms (O(2)). In all cases, the O · · · O distances describing respective hydrogen bonds (ranging from 2.969(5) to 2.773(5) Å) indicate a medium strength of hydrogen bonding.13 This situation results in the formation of three eight-membered rings, which are only slightly frizzled, as is the whole hydrogen-bonded system. These units are mutually placed in a sheetlike manner along the crystal structure. Similar hydrogen-bond systems have recently been described in the organoantimony(V) hydroxide (CH3)3Sb(OH)2 · H2O and the carbonate [(CH3)3Sb(OH)]2CO3 · 2H2O.12 Carbonate 3 displays remarkable stability in air. Compound 3 is stable in the solid state at -30 °C for several months, and (13) Steiner, T. Angew. Chem., Int. Ed. 2002, 41, 48.
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at ambient temperature both in solution (CHCl3) and in the solid state only negligible elimination of CO2 was observed over a period of several days. However, smooth CO2 elimination occurs after heating to 130 °C (without use of any vacuum) in the solid state (Scheme 2, Figure 4C,D). This reaction proceeds cleanly and quantitatively within 10 h to give oxide 2 without any significant side reactions, as demonstrated by the 1H NMR spectroscopy. In addition, in such a way the obtained oxide 2 can be easily converted back to the carbonate 3 via repeated treatment with CO2(g). This reaction loop (Scheme 2) works without any significant lost of yield for at least 3 runs, demonstrating the interesting utility of oxide 2 in the reversible binding of CO2. The volatility of CO2 fixation in 3 was also demonstrated by ESI-MS spectra (soft ionization technique, see the Supporting Information). The molecular ion is missing in the spectra, and the heaviest ion [Ar2Sb2O(CO3) + H]+ at m/z 701 is most probably formed by loss of one CO2 from compound 3 followed by dimerization. The proposed structure of this ion was further confirmed by MS/MS spectra, and this structure eliminates another CO2 to form the ion [Ar2Sb2O2 + H]+ at m/z 657, indicative of the formation of the oxide 2 (see the Supporting Information). The relative ease of CO2 elimination from the carbonate 3 in comparison to the similar organobismuth carbonate (30% conversion after 10 h, 100 °C/vacuum)4 can be most probably ascribed to the monomeric behavior of 3, where the carbonate moiety is bonded to the central atom as a terminal ligand (vide supra) rather than a bridging ligand between two elements. Finally, the thermal stability of 3 was studied by thermal analysis at four different heating rates. In all cases, it was found that the evolution of CO2 started at 129 °C (close to that in the experiment described above). The enthalpy of carbon dioxide deliberation was found to be 125 J/g, and using the peak-shift method and the Kissinger equation14 the activation energy 147 ( 1 kJ/mol was calculated. In addition, the decomposition of 3 was accompanied by a mass loss of 13% (mean value from all experiments), which correlates well with the theoretical value 11.8% for CO2 elimination from 3, giving the oxide 2. To summarize, the dimeric oxide 2 was prepared and its ability to reversibly bind carbon dioxide under relatively mild physical conditions was demonstrated. The molecular structure of carbonate 3 revealed a rare example of the terminal coordination of the CO3 unit in organometallic main-group chemistry. An investigation concerning the possible utilization of 2 for activation of CO2 and its further reactivity is currently underway.
Experimental Section General Procedures. 1H and 13C NMR spectra were recorded on a Bruker Avance 500 spectrometer, using a 5 mm tunable broadband probe. Appropriate chemical shifts in 1H and 13C NMR spectra were related to the residual signals of the solvents (CDCl3, δ(1H) 7.27 ppm and δ(13C) 77.23 ppm; C6D6, δ(1H) 7.16 ppm and δ(13C) 128.39 ppm). The positive-ion full scan mass spectra measured on an Esquire 3000 ion trap (Bruker Daltonics, Bremen, Germany). The samples were dissolved in acetonitrile and analyzed by the direct infusion at the flow rate of 5 µL/min. Mass spectra were recorded in the range of m/z 50-1500 both in positive- and negative-ion modes. The ion source temperature was 300 °C, and the flow rate and the pressure of nitrogen were 4 L/min and 10 psi, respectively. Infrared spectra were recorded in the range 5000-400 (14) Kissinger, H. E. Anal. Chem. 1957, 29, 1702.
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Notes
Figure 3. PLUTON plot showing hydrogen bonding in 3. Selected O · · · H-O contacts (Å) and angles (deg) characterizing respective hydrogen bonding: O(3) · · · H-O(3w) ) 2.829(5), 166.8; O(3) · · · H-O(2w) ) 2.773(5), 123.4; O(2) · · · H-O(1w) ) 2.969(5), 133.2; O(2w)-H-O(1w) ) 2.833(6), 175.9. cm-1 as KBr pellets or as solutions in CHCl3 on a Bruker IFS 55 FT-IR spectrometer. Thermal analysis was performed with Mettler DSC 12E Heat-flux equipment. Preparation of (ArSbO)2 (2). A solution of KOH (241 mg, 4.3 mmol) in distilled water (20 mL) was added to a solution of ArSbCl25 (716 mg, 1.87 mmol) in 100 mL of toluene and the mixture stirred for additional 4 h. The organic layer was separated, dried (MgSO4), and evaporated in vacuo to give a colorless solid, which was recrystallized from hot hexane. Compound 2 was isolated as colorless crystals after filtration (276 mg, 45%). Mp: 235-238 °C. Anal. Calcd for C24H38N4O4Sb2: C, 43.8; H, 5.8. Found: C, 43.9; H, 6.0. Two sets of signals due to the cis/trans isomers were observed in 1H and 13C NMR (ratio approximately 1:0.5). 1H NMR (CDCl3; 500.13 MHz): δ (ppm) 2.17 (br, N(CH3)2), 2.77 and 4.39 (AX pattern, CH2N), 2.89 and 4.77 (AX pattern, CH2N), 6.55 and 7.00 (br d, Ar H3,5), 6.67 and 7.11 (br-t, Ar H4). 1H NMR (C6D6; 500.13 MHz): δ (ppm) 2.17-2.20 (br s, N(CH3)2), 2.78 and 4.64 (AX pattern, CH2N), 2.90 and 5.11 (AX pattern, CH2N), 6.73 and 7.04 (d, 2H, Ar H3,5), 6.88 and 7.13 (t, 1H, Ar H4). 13C NMR (C6D6; 125.77 MHz): δ (ppm) 44.3 (N(CH3)2), 64.5 and 64.6 (CH2N), 125.3 and 126.2 (Ar C3,5), 127.6 and 128.0 (Ar C4), 146.3 and 147.5 (Ar C2,6), 160.1 and 161.7 (Ar C1). Positive-ion ESI mass spectra: m/z 657 [M + H]+ 100%, 329 [ArSbOH]+. Preparation of ArSbCO3 (3). CO2 gas was blown through a toluene (20 mL) solution of 2 (204 mg, 0.31 mmol) for 30 min, and during this period a white precipitate formed. These colorless crystals of 3 were collected by filtration, washed twice with hexane (5 mL), and dried in vacuo (196 mg, 85%). Anal. Calcd for C13H19N2O3Sb: C, 41.9; H, 5.1. Found: C, 41.6; H, 5.2. 1H NMR (CDCl3; 500.13 MHz): δ (ppm) 2.18 (s, 6H, N(CH3)2), 2.74 (s, 6H, N(CH3)2), 3.45 and 4.36 (AX pattern, 4H, CH2N), 7.13 (d, 2H, Ar H3,5) 7.32 (t, 1H, Ar H4). 13C NMR (CDCl3; 125.77 MHz): δ (ppm) 42.7 (N(CH3)2), 45.4 (N(CH3)2), 63.4 (CH2N), 125.6 (Ar C3,5), 130.9 (Ar C4), 146.5 (Ar C2,6), 149.9 (Ar C1), 161.3 (CO3).
Positive-ion ESI mass spectra: m/z 701 [2M - CO2 + H]+, m/z 657 [2M - 2CO2 + H]+ 100%, 329 [ArSbOH]+. IR (KBr, cm-1): 1678, 1643. IR (CDCl3, cm-1): 1676, 1643. Crystallographic Data. Data for 2: C12H19N2OSb, M ) 329.04, monoclinic, P21/c, colorless plate, a ) 6.5280(4) Å, b ) 17.4160(11) Å, c ) 12.1560(10) Å, β ) 100.706(6)°, V ) 1357.98(16) Å3, Z ) 4, T ) 150(1) K, 13 876 total reflections, 3095 independent reflections (Rint ) 0.086, R1(obsd data) ) 0.034, wR2(all data) ) 0.084. CCDC 717975. Data for 3: C13H23N2SbO5, M ) 409.08, monoclinic, C2/c, colorless block, a ) 11.6248(2) Å, b ) 16.3348(2) Å, c ) 18.3135(5) Å, β ) 106.296(12)°, V ) 3337.8(1) Å3, Z ) 8, T ) 150(2) K, 13 911 total reflections, 3803 independent reflections (Rint ) 0.050, R1(obsd data) ) 0.037, wR2(all data) ) 0.081. CCDC 717976.
ˇ R (project 203/07/ Acknowledgment. We thank the GAC P094, 203/07/P094, and 104/09/0829) and the Ministry of Education of the Czech Republic (projects MSM0021627501 and LC523) for financial support. R. Jira´sko acknowledges the support of grant project No. MSM0021627502, sponsored by the Ministry of Education, Youth and Sports of the Czech ˇ ernosˇek for stimulating Republic. We are indebted to Z. C comments concerning the thermal analysis. Supporting Information Available: Figures, text, tables, and CIF files giving the 1H and 13C NMR spectra of 2 in C6D6, the ESI MS spectrum of 3, an example of the thermal analysis spectrum, all crystal data and structure refinement details, atomic coordinates, anisotropic displacement parameters, and geometric data for compounds 2 and 3. This material is available free of charge via the Internet at http://pubs.acs.org. OM9000692
