Revisiting CO2 Reduction with NaBH4 under Aprotic Conditions

Apr 27, 2015 - ... as evidenced by the analogous reactivity of [NEt4][BH4]. Even at 1 atm pressure and without using rigorously dry solvent, treatment...
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Revisiting CO2 Reduction with NaBH4 under Aprotic Conditions: Synthesis and Characterization of Sodium Triformatoborohydride Ioana Knopf and Christopher C. Cummins* Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-4307, United States S Supporting Information *

ABSTRACT: The reduction of CO2 to formate using sodium borohydride was originally investigated in the 1950s. Despite this clue from the chemical literature, many recent publications describe catalytic CO2 hydroboration methods leading to formate or methoxide with more expensive and less reactive boranes such as pinacolborane. Herein we describe the uptake of 3 equiv of CO2 by NaBH4, along with full spectroscopic and crystallographic characterization of the resulting triformatoborohydride, Na[HB(OCHO)3]. Conducting the synthesis in acetonitrile under 300 psi of CO2 constitutes a new preparative procedure for generating Na[HB(OCHO)3]. This reaction does not require the presence of a strongly coordinating alkali metal cation, as evidenced by the analogous reactivity of [NEt4][BH4]. Even at 1 atm pressure and without using rigorously dry solvent, treatment of NaBH4 with CO2 and subsequent quenching gave formic acid (1.5 equiv based on B).

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hether metal-catalyzed1,2 or organo-catalyzed,3 CO2 hydroboration methods for producing formate2 or methoxide1,3 have become increasingly popular in recent years. While all of these processes are catalytic, stoichiometric reductants such as pinacolborane or catecholborane are required to effect the desired transformations. As recently reported by the group of Mizuta,4 the key to CO2 reduction lies not in the choice of catalyst, but in the choice of reductant. Commercially available solutions of BH3·THF were found to be effective in reducing CO2 to methoxide (trimethoxyboroxine) in the absence of any additive or catalyst.5 Prior to publication of the work by Mizuta et al., our group had made a similar discovery, namely that NaBH4 reacts with 3 equiv of CO2 in the absence of a catalyst. Herein we report the synthesis and full characterization of the resulting sodium triformatoborohydride, Na[HB(OCHO)3].

conducted at high temperature in the absence of solvent, as well as the uptake of 3 equiv of CO2 with formation of Na[HB(OCHO)3] when the reaction was carried out at low temperature in dimethyl ether. The two compounds were only characterized using information from mass balance and hydrolysis products; no spectroscopic or crystallographic data appear to have been reported. An even earlier report describes the reactivity of LiBH4 with CO2 to yield lithium formate.9 Transition metal borohydride complexes have been studied more extensively and regarded as dually activated hydride sources “by presenting both a basic transition metal atom and a latent Lewis acidic boron center”.10 When treated with CO2, transition metal borohydride complexes produce the corresponding metal formate complexes. While the CO2 reactivity of Cu borohydrides is the most studied,11,12 examples of Rh,10 Ru,13 Cr,14 and Ni15 systems have also been reported. In 1985, LaMonica et al. reported that (phen)Cu(PPh3)BH4 reacts with CO2 in aprotic solvents in the presence of PPh3 to give ionic compounds [L 4 Cu][H 2 B(OCHO) 2 ] and [L 4 Cu][HB(OCHO)3].12 This very different mode of reactivity as compared with that reported for the previously described copper borohydrides is explained by the in situ formation of the salt [(phen)Cu(PPh3)2][BH4] that features an unbound borohydride anion. As hinted at by Wartik and Pearson in their original reports,8 the uptake of CO2 by NaBH4 is highly sensitive to the reaction conditions employed. If trimethylphosphine is present in the reaction mixture when CO2 is added to a THF solution of

Sodium borohydride is an ubiquitous reducing agent found in almost every synthetic lab and produced annually on a scale dwarfing any other complex hydride reductant.6 Given its rich and widely explored reductive carbonyl chemistry, the paucity of literature describing NaBH4 reactivity with CO2 is surprising. While exploring mild hydride sources that could reduce our newly discovered metal oxo carbonate complex to formate,7 a control experiment revealed the underlying reactivity of NaBH4 with CO2 in organic media. An early description of such reactivity dates to 1955 and 1958,8 when Wartik and Pearson described the uptake of 2 equiv of CO2 by NaBH4 with formation of Na[BO(O2CH)(OCH3)] when the reaction was © XXXX American Chemical Society

Received: March 7, 2015

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DOI: 10.1021/acs.organomet.5b00190 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics NaBH4, trimethylphosphine borane is produced along with sodium formate.16 A never-before-cited 1967 note reports the production of sodium formate from NaBH4 and CO2 in aqueous conditions.17 More recent papers also describe the reduction of CO2 with NaBH4 in aqueous conditions,18 while another study examined the reduction in water/ethanol mixtures.19 Although changing the reaction conditions can lead to formation of different products, a 1995 paper goes so far as to claim that there is no reaction between NaBH4 and CO2 in a control experiment.13 Simply sparging a stirring solution of NaBH4 in acetonitrile with a stream of CO2 at room temperature leads to the formation of a mixture of Na[H2B(OCHO)2] and Na[HB(OCHO)3] as assessed by 1H NMR and 11B NMR spectroscopy (Figure 1). Although previous literature claims [HB-

Figure 2. Solid-state structure of [Na(DME)][HB(OCHO)3] with thermal ellipsoids at the 50% probability level and DME hydrogen atoms omitted for clarity. One asymmetric unit is shown in full color, while a second asymmetric unit as well as additional truncated formate groups that fill the sodium ions’ coordination spheres are shown in faded colors. Representative interatomic distances [Å] and angles [°]: B1−O1 1.4863(13), B1−O2 1.4736(13), B1−O3 1.4947(14), C1−O1 1.3060(14), C1−O4 1.2073(14), Na1−O4 2.3327(8), Na1−O6C 2.3598(9), Na1−O6D 2.4555(8), Na1−Na1A 3.7863(8); O2−B1− O1 103.91(8), O2−B1−O3 105.53(8), O1−B1−O3 108.54(8), O4− C1−O1 126.50(10).

coordination sites, while the other four are filled by carbonyl oxygens from proximate formate groups. The overall structure is a three-dimensional polymer in which the sodium ions are disposed in pairs bridged by formate oxygens (see Section 4.2 of the Supporting Information for a packing plot). The Na···Na distance in these diamond cores is 3.7863(8) Å, a value consistent with similar structures reported in the CSD.20 Within each formate unit, the C−O interatomic distances are inequivalent; for example, C1−O1 is 1.3060(14) Å and C1−O4 is 1.2073(14) Å, with an O4−C1−O1 angle of 126.50(10)°. Surprisingly, we were unable to find any structurally characterized tricarboxylate borohydride salts in the CSD, making this structure of triformatoborohydride the first of its kind.21 Very recently, the first structurally characterized bis(formyloxy)borate was described by Cantat et al. as an important species in the organo-catalyzed dehydrogenation of formic acid.22 We were curious whether the sodium cation played an important role in this reduction. In the case of LiAlH4, for example, no reduction23 or a significantly slower reduction24 of organic substrates occurs when the alkali metal cation is sequestered. In order to gauge the effect of the sodium cation on the present CO2 reduction, we employed [NEt4][BH4] as the borohydride source. In this case, the corresponding [NEt4][HB(OCHO)3] was produced in good purity under the same experimental conditions used for NaBH4 (300 psi of CO2 and acetonitrile as the solvent), thus showing that the CO2 reactivity is inherent to the borohydride anion. Although we were interested primarily in developing a preparative procedure for Na[HB(OCHO)3], it was also of interest to know if any formate would be produced under very mild conditions. Thus, we sought to investigate this reaction without applying CO2 pressure and without excluding air and moisture. In air, a solution of NaBH4 prepared with wet acetonitrile (HPLC grade) was sparged with 4 equiv of CO2;

Figure 1. 1H NMR and 11B NMR spectra for the mixture of Na[H2B(OCHO)2] and Na[HB(OCHO)3] obtained at room temperature and atmospheric pressure. For experimental details, see Section 2.1 of the Supporting Information.

(OCHO)3]− to be thermodynamically unstable,4,12 no change in ratio between Na[H2B(OCHO)2] and Na[HB(OCHO)3] was observed after keeping the solid mixture under vacuum for prolonged periods of time. This suggested that accessing Na[HB(OCHO)3] was more likely a matter of overcoming a kinetic barrier. Pure Na[HB(OCHO)3] was obtained by conducting the reaction in acetonitrile under 300 psi of CO2 in a glass lined high pressure vessel at room temperature. Isolation required only solvent removal in vacuo. This compound displays a signal at δ = 8.28 ppm by 1H NMR spectroscopy, this being located in a region characteristic of formyl protons, with 13C satellites corresponding to 1JCH = 210 Hz. The borohydride proton signal at δ = 3.79 ppm shows a characteristic four-line splitting pattern with 1JBH = 131 Hz. The 11 B NMR spectrum is diagnostic, as it displays a clear doublet with a B−H coupling constant of 130 Hz, while the 13C NMR spectrum features a single formate carbon resonance at 165.5 ppm. These spectroscopic data are in good agreement with those obtained by LaMonica et al. for the anion of [L4Cu][HB(OCHO)3].12 Single crystals of [Na(DME)][HB(OCHO)3] were obtained by cooling a saturated DME solution of Na[HB(OCHO)3] to −40 °C. The compound crystallized in the monoclinic space group P21/c, with one molecule of solvent coordinated to the Na cation per asymmetric unit (Figure 2). The boron center is pseudotetrahedral, with an average B−O interatomic distance of 1.48 Å. Each sodium ion resides in a pseudo-octahedral environment with one DME molecule occupying two adjacent B

DOI: 10.1021/acs.organomet.5b00190 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics

(7) Knopf, I.; Ono, T.; Temprado, M.; Tofan, D.; Cummins, C. C. Chem. Sci. 2014, 5, 1772−1776. (8) (a) Wartik, T.; Pearson, R. K. J. Am. Chem. Soc. 1955, 77, 1075; (b) Wartik, T.; Pearson, R. K. J. Inorg. Nucl. Chem. 1958, 7, 404−411. (c) Pearson, R. K.; Wartik, T. U.S. Patent 2872474, 1959. (9) Burr, J. G., Jr.; Brown, W. G.; Heller, H. E. J. Am. Chem. Soc. 1950, 72, 2560−2562. (10) Willis, W.; Nicholas, K. M. Inorg. Chim. Acta 1984, 90, L51− L53. (11) (a) Beguin, B.; Denise, B.; Sneeden, R. P. A. J. Organomet. Chem. 1981, 208, C18−C20;(b) Bianchini, C.; Ghilardi, C. A.; Meli, A.; Midollini, S.; Orlandini, A. J. Organomet. Chem. 1983, 248, C13−C16; (c) Bianchini, C.; Ghilardi, C. A.; Meli, A.; Midollini, S.; Orlandini, A. J. Organomet. Chem. 1983, 255, C27−C30;(d) Bianchini, C.; Ghilardi, C. A.; Meli, A.; Midollini, S.; Orlandini, A. Inorg. Chem. 1985, 24, 924−931;(e) La Monica, G.; Angaroni, M. A.; Franco, C.; Cenini, S. Inorg. Chim. Acta 1988, 143, 239−245;(f) Pandey, K. K.; Garg, K. H.; Tiwari, S. K. Polyhedron 1992, 11, 947−950. (12) La Monica, G.; Ardizzoia, G.; Cariati, F.; Cenini, S.; Pizzotti, M. Inorg. Chem. 1985, 24, 3920−3923. (13) Yi, C. S.; Liu, N. Organometallics 1995, 14, 2616−2617. (14) Dionne, M.; Hao, S.; Gambarotta, S. Can. J. Chem. 1995, 73, 1126−1134. (15) (a) Journaux, Y.; Lozan, V.; Klingele, J.; Kersting, B. Chem. Commun. 2006, 219, 83−84;(b) Chakraborty, S.; Zhang, J.; Patel, Y. J.; Krause, J. A.; Guan, H. Inorg. Chem. 2013, 52, 37−47. (16) Schmidbaur, H.; Weib, E.; Muller, G. Synth. React. Inorg. Met. Chem. 1985, 15, 401−413. (17) Eisenberg, F., Jr.; Bolden, A. H. Carbohydr. Res. 1967, 5, 349− 350. A SciFinder search performed on 03/24/2015 revealed no citations for this reference. (18) (a) Pulidindi, I. N.; Kimchi, B. B.; Gedanken, A. J. CO2 Util. 2014, 7, 19−22;(b) Grice, K. A.; Groenenboom, M. C.; Manuel, J. D. A.; Sovereign, M. A.; Keith, J. A. Fuel 2015, 150, 139−145. (19) Zhao, Y.; Zhang, Z.; Qian, X.; Han, Y. Fuel 2015, 142, 1−8. (20) The median Na−Na distance is 3.665 Å for structures with similar diamond cores reported in the Cambridge Structural Database (CSD); to make a good comparison, a coordination sphere filled by six oxygen-based ligands was specified for each sodium. (21) According to a substructure search performed on 03/24/2015, no anions of the form [HB(OCOR)3]− or [H2B(OCOR)2]− could be found in the Cambridge Structural Database (CSD). (22) Chauvier, C.; Tlili, A.; Das Neves Gomes, C.; Thuéry, P.; Cantat, T. Chem. Sci. 2015, 6, 2938−2942. (23) Pierre, J. L.; Handel, H.; Perraud, R. Tetrahedron 1975, 31, 2795−2798. (24) Wiegers, K. E.; Smith, S. G. J. Org. Chem. 1978, 43, 1126−1131.

after 10 min, the reaction was quenched with aqueous acid. Formic acid (1.5 equiv) was produced using this method, as quantified by 1H NMR spectroscopy using an internal standard (see Supporting Information Section 2.4). As expected, abandoning the use of rigorously anhydrous conditions does not take advantage of the full potential of NaBH4 to reduce 3 equiv of CO2 due to competing hydrolysis. The results reported herein indicate that reduction of CO2 to formate is achieved easily using sodium borohydride without employing expensive boranes or intricate catalysts. Our finding complements the recent report from Mizuta and co-workers4 describing the reduction of CO2 to methoxide with commercial BH3·THF. Furthermore, the reactivity of CO2 with borohydride is unperturbed by the use of a non-coordinating cation in lieu of sodium. Consequently, the intrinsic reactivity of the borohydride anion should always be taken into account and rigorously controlled for when studying borohydride-containing systems for substrate reduction.

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ASSOCIATED CONTENT

S Supporting Information *

Full experimental, crystallographic (CCDC 1047485), and spectroscopic data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +1 617 253 5332. Fax: +1 617 253 7670 E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Saudi Basic Industries Corporation for funding this work. X-ray diffraction data were collected on an instrument purchased with the aid from National Science Foundation under CHE-0946721.

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REFERENCES

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DOI: 10.1021/acs.organomet.5b00190 Organometallics XXXX, XXX, XXX−XXX