Trapping formaldehyde in the homogeneous catalytic reduction of carbon dioxide

Article abstract of DOI:10.1002/anie.201304025

Formaldehyde detectives: Evidence for the production of formaldehyde during a ruthenium-catalyzed CO₂ reduction process, and for its involvement in the formation of the resulting C₂ compound, is disclosed. Ultimately, formaldehyde can be recovered by methanol trapping. HBPin=pinacolborane. Copyright

Full text of DOI:10.1002/anie.201304025

Angewandte
Chemie
Communications
Reaction Intermediates
S. Bontemps,*
S. Sabo-Etienne*
&&&& — &&&&
Formaldehyde detectives: Evidence for
the production of formaldehyde during
a ruthenium-catalyzed CO2 reduction
process, and for its involvement in the
formation of the resulting C2 compound,
is disclosed. Ultimately, formaldehyde
can be recovered by methanol trapping.
HBPin=pinacolborane.
Trapping Formaldehyde in the
Homogeneous Catalytic Reduction of
Carbon Dioxide
Angew. Chem. Int. Ed. 2013, 52, 1 – 4
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!

.
Angewandte
Communications
DOI: 10.1002/anie.201304025
Reaction Intermediates
Trapping Formaldehyde in the Homogeneous Catalytic Reduction of
Carbon Dioxide**
Sꢀbastien Bontemps* and Sylviane Sabo-Etienne*
The abundance, low cost, and low toxicity of CO2 make it an
attractive carbon source.[1] Nature succeeds in transforming
CO2 into carbohydrates by photosynthesis,[2] but the reduc-
tion of this highly oxidized molecule is a significant challenge
for chemists.[3] Following the pioneering studies of Sadighi
et al.[4] and Kawagushi et al.,[5] it has been demonstrated that
oxygen abstraction from CO2 can be achieved under rather
active system, but precedents exist for acetal motifs in the
reduction of CO2.[5,8a,b,9,18] The “inevitable” formation of
formaldehyde from the observed R3SiOCH2OSiR3 acetal
structure was also highlighted in another computational study
by Wang et al. on the NHC/silane reduction process of CO2 to
afford CH3OSiR3.[19]
We recently reported the catalytic reduction of CO2 by
[RuH2(H2)2(PCy3)2] (1; Cy = cyclohexyl) and pinacolborane
(HBpin) reductant.[20] Complexes 2–6 (Scheme 2) are crucial
to the catalytic cycle affording compounds 7–11 (Scheme 3).
Compound 10 was the first bis(boryl) acetal structure
mild conditions by homogeneous catalysis[6] to yield CO,[4,7]
[5,9]
CH3OH,[8] and CH4
with silanes, boranes, silylboranes or
aldehydes as reducing agents. Recently, silane-based reduc-
tive processes have been used in a so-called diagonal
À
approach for the formylation or methylation of N H
bonds.[10] With dihydrogen as the sole reductant, CH3OH
can also be produced through a cascade reaction involving
three different catalysts,[11] or by using a tridentate phosphine
ruthenium catalyst precursor under harsher conditions.[12] In
the series of C1 molecules derived from CO2, formaldehyde is
the missing link. Despite its importance as a C1 building
block,[13] H2CO has not been isolated, or even observed, in
any homogeneous catalytic reduction of CO2.[14] More than
20 years ago, Cutler et al. reported the synthesis of a hetero-
bimetallic bridging formaldehyde complex from the stoichio-
metric reduction of the corresponding carbon dioxide com-
plex.[15] Moreover, the computational mechanistic study by
Wang et al.[16] on the Ni/borane system described by Guan
et al.,[8c] predicts the reduction of CO2 to formaldehyde, and
finally to CH3OBCat, from the decomposition of an acetal
[Ni]OCH2OBCat species (Scheme 1).[17] The nickel acetal
complex was not experimentally observed in this particularly
Scheme 2. Complexes 2–6 involved in the CO2 reduction by HBpin.
Scheme 3. Compounds 7–11 obtained after 30 min at RT from the
reaction of HBpin with 13CO2 (1 atm) using complex 1 as the catalyst
precursor.
observed in the reduction of CO2, and the formation of the
C2 compound pinBO13CH2O13CHO (11) through the reduc-
tive coupling of two molecules of 13CO2 was unprecedented.
Herein, we provide direct proof of the production of
formaldehyde during the ruthenium-catalyzed CO2 reduction
process, and of its involvement in the formation of the C2
compound 11. Ultimately, formaldehyde can be recovered by
methanol trapping.
In the CO2 reduction reported earlier, HBpin was readily
transformed (< 30 min.) into compounds 7–11.[20] A slow
retransformation of 9–11 into 7–8 was achieved (22 days). We
now observe that any attempt to generate the C2 compound
11 from formaldehyde, HBpin, and any catalyst precursor
(1–6) failed. Notably, no reaction occurred when exposing
a solution of complex 3, the main organometallic species
observed during the catalysis, to formaldehyde (8 h, RT).
However, when conducting the standard reaction (30 min,
Scheme 1. Calculated intermediates in the reduction of CO2 by the
[Ni]-H/HBCat system.
[*] Dr. S. Bontemps, Dr. S. Sabo-Etienne
CNRS, LCC (Laboratoire de Chimie de Coordination)
205 route de Narbonne, 31077 Toulouse (France)
and
Universitꢀ de Toulouse, UPS, INPT
31077 Toulouse (France)
E-mail: bontemps@lcc-toulouse.fr
sylviane.sabo@lcc-toulouse.fr
Homepage: http://www.lcc-toulouse.fr/lcc/spip.php?article433
[**] We thank the ANR (Programme blanc “IRONHYC” ANR-12), the
CNRS for support, and Johnson Matthey Plc for the generous gift of
hydrated ruthenium trichloride.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201304025.
2
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 4
These are not the final page numbers!

Angewandte
Chemie
room temperature, 13CO2) to form labeled compounds 3 and
7–11, and subsequently adding H212CO to the mixture,[21]
1H NMR analysis shows that the added H212CO reacted
completely with pinBOCHO (9) to generate pin-
BO12CH2O13CHO (11*) within 28 min. H212CO and H213CO
could be observed in the mixture by NMR spectroscopy after
a few hours (Figure 1). Within 12 days, only compounds 7 and
Shintani and Nozaki,[22] as well as by Guan et al.[8c] for the
related CatBOCHO compound.
Formaldehyde can be stabilized in alcohol solutions as the
hemiacetal species.[23] Thus, to trap formaldehyde from
compound 11, we added [D4]MeOH to the solution of
compound 11 generated in situ and observed its complete
and clean conversion into CD3OCH2OD.[24] We then added
CD3OD to the catalytic mixture of 7–11. Compounds 8 and 9
were transformed into 13CH3OD and H13COOD, and to our
delight, not only compound 11, but also compound 10, were
readily converted into CD3O13CH2OD, as observed by NMR
spectroscopy (Figure 2). The latter transformation is direct
Figure 1. 1H NMR spectra of the catalytic mixture leading to 7–11
before (top) and after subsequent addition of H212CO.
8 were observed, along with pinBO12CH3 (8*). Thus, formal-
dehyde is formed during the catalysis and readily trapped by
compound 9 to yield the C2 compound 11*. Furthermore, the
characterization of compound 8* confirms that compound 11
is reduced to form compound 8.
Compound 9 could be independently prepared by addi-
tion of an excess of HBpin to formic acid in C6D6 over 20 h;
1 equiv of formaldehyde was then added to the resulting
solution. This readily resulted in total conversion of 9 into the
C2 compound 11. Thus, the insertion of formaldehyde into 9 to
generate 11 does not need to be catalyzed by a metal complex
(Scheme 4). During the synthesis of 9, the excess HBpin does
Figure 2. 1H NMR spectra before (top) and after (bottom) the addition
of CD3OD to the 7–11 mixture.
experimental evidence of the recovery of formaldehyde from
an acetal compound.[16,19] Under the standard catalytic con-
ditions (catalyst 1 (10 mol%), 1 atm CO2, RT, 30 min) the
hemiacetal species CD3O13CH2OD was produced in 10%
yield, as estimated by NMR spectroscopy.[21,25] Upon reducing
the catalyst loading to 1% and increasing the CO2 pressure to
2 atm, 4 h were needed for the complete conversion of HBpin.
The catalyst/HBpin/CO2 ratio better favors the formation of
compound 11, which leads to an increased yield of 37%.
In summary, through detailed NMR studies using labeled
species, we herein disclose direct proof of the formation of
formaldehyde, which is readily trapped by compound 9 to
afford the C2 compound 11, during a ruthenium-catalyzed
CO2 reduction process. The intermediacy of formaldehyde in
the production of 11 could offer new strategies to prepare Cn
compounds. Moreover, formaldehyde was shown to be
recovered not only from 11, but also from the acetal
compound 10 by methanol trapping. Indeed, this important
building block is no longer an elusive molecule in the field of
CO2 homogeneous catalysis.
Scheme 4. Reaction of compound 9 with H2CO.
not react with 9. However, when complex 1 was added to a 1:2
mixture of 9 and HBpin, compounds 7–11 were readily
formed (< 10 min). This observation is consistent with 1) the
need for the ruthenium catalyst in the reduction of compound
9 and 2) the involvement of compound 9 as an intermediate in
the catalyzed reduction process, and not as a side product. In
an attempt to isolate 9, a controlled vacuum was applied to
remove excess HBpin. As a result, formic acid was detected,
along with further reduction of 9.[21] It appears that the excess
of HBpin is required for the stabilization of 9, which might
explain the difficulties in isolating it, as also mentioned by
Received: May 10, 2013
Revised: June 12, 2013
Published online: && &&, &&&&
Keywords: acetals · borane · CO2 reduction · formaldehyde ·
.
ruthenium
Angew. Chem. Int. Ed. 2013, 52, 1 – 4
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3
These are not the final page numbers!

.
Angewandte
Communications
[10] a) C. Das Neves Gomes, O. Jacquet, C. Villiers, P. Thuꢂry, M.
Ephritikhine, T. Cantat, Angew. Chem. 2012, 124, 191 – 194;
Angew. Chem. Int. Ed. 2012, 51, 187 – 190; b) O. Jacquet, C.
Das Neves Gomes, M. Ephritikhine, T. Cantat, J. Am. Chem.
Soc. 2012, 134, 2934 – 2937; c) O. Jacquet, X. Frogneux, C.
Das Neves Gomes, T. Cantat, Chem. Sci. 2013, 4, 2127 – 2131;
d) O. Jacquet, C. Das Neves Gomes, M. Ephritikhine, T. Cantat,
ChemCatChem 2013, 5, 117 – 120; e) Y. Li, X. Fang, K. Junge, M.
Beller, Angew. Chem. 2013, DOI: 10.1002/ange.201301349;
Angew. Chem. Int. Ed. 2013, DOI: 10.1002/anie.201301349.
[11] C. A. Huff, M. S. Sanford, J. Am. Chem. Soc. 2011, 133, 18122 –
18125.
[12] S. Wesselbaum, T. vom Stein, J. Klankermayer, W. Leitner,
Angew. Chem. 2012, 124, 7617 – 7620; Angew. Chem. Int. Ed.
2012, 51, 7499 – 7502.
[13] Z.-C. Wang, N. Dietl, R. Kretschmer, J.-B. Ma, T. Weiske, M.
Schlangen, H. Schwarz, Angew. Chem. 2012, 124, 3763 – 3767;
Angew. Chem. Int. Ed. 2012, 51, 3703 – 3707.
[14] Interestingly, Miller, Goldberg, and co-workers recently
reported evidence of the formation of formaldehyde during
acid formic disproportionation into methanol, water, and carbon
dioxide: A. J. M. Miller, D. M. Heinekey, J. M. Mayer, K. I.
Goldberg, Angew. Chem. 2013, 125, 4073 – 4076; Angew. Chem.
Int. Ed. 2013, 52, 3981 – 3984.
[15] a) C. C. Tso, A. R. Cutler, J. Am. Chem. Soc. 1986, 108, 6069 –
6071; b) B. D. Steffey, J. C. Vites, A. R. Cutler, Organometallics
1991, 10, 3432 – 3435.
[1] H. Arakawa, M. Aresta, J. N. Armor, M. A. Barteau, E. J.
Beckman, A. T. Bell, J. E. Bercaw, C. Creutz, E. Dinjus, D. A.
Dixon, K. Domen, D. L. DuBois, J. Eckert, E. Fujita, D. H.
Gibson, W. A. Goddard, D. W. Goodman, J. Keller, G. J. Kubas,
H. H. Kung, J. E. Lyons, L. E. Manzer, T. J. Marks, K. Moro-
kuma, K. M. Nicholas, R. Periana, L. Que, J. Rostrup-Nielson,
W. M. H. Sachtler, L. D. Schmidt, A. Sen, G. A. Somorjai, P. C.
Stair, B. R. Stults, W. Tumas, Chem. Rev. 2001, 101, 953 – 996.
[2] M. Calvin, A. A. Benson, Science 1948, 107, 476 – 480.
[3] a) I. Omae, Coord. Chem. Rev. 2012, 256, 1384 – 1405; b) W.
Wang, S. Wang, X. Ma, J. Gong, Chem. Soc. Rev. 2011, 40, 3703 –
3727; c) E. A. Quadrelli, G. Centi, J.-L. Duplan, S. Perathoner,
ChemSusChem 2011, 4, 1194 – 1215; d) M. Peters, B. Kçhler, W.
Kuckshinrichs, W. Leitner, P. Markewitz, T. E. Mꢀller, Chem-
SusChem 2011, 4, 1216 – 1240; e) M. Cokoja, C. Bruckmeier, B.
Rieger, W. A. Herrmann, F. E. Kꢀhn, Angew. Chem. 2011, 123,
8662 – 8690; Angew. Chem. Int. Ed. 2011, 50, 8510 – 8537; f) S. N.
Riduan, Y. Zhang, Dalton Trans. 2010, 39, 3347 – 3357; g) C.
Federsel, R. Jackstell, M. Beller, Angew. Chem. 2010, 122, 6392 –
6395; Angew. Chem. Int. Ed. 2010, 49, 6254 – 6257; h) G. Centi, S.
Perathoner, Catal. Today 2009, 148, 191 – 205; i) G. A. Olah, A.
Goeppert, G. K. S. Prakash, J. Org. Chem. 2009, 74, 487 – 498;
j) T. Sakakura, J.-C. Choi, H. Yasuda, Chem. Rev. 2007, 107,
2365 – 2387; k) M. Aresta, A. Dibenedetto, Dalton Trans. 2007,
2975 – 2992; l) P. G. Jessop, F. Joo, C.-C. Tai, Coord. Chem. Rev.
2004, 248, 2425 – 2442; m) W. Leitner, Angew. Chem. 1995, 107,
2391 – 2405; Angew. Chem. Int. Ed. Engl. 1995, 34, 2207 – 2221.
[4] D. S. Laitar, P. Mueller, J. P. Sadighi, J. Am. Chem. Soc. 2005, 127,
17196 – 17197.
[16] F. Huang, C. Zhang, J. Jiang, Z.-X. Wang, H. Guan, Inorg. Chem.
2011, 50, 3816 – 3825.
[17] For computational studies on CO2 reduction, see the review: T.
Fan, X. Chen, Z. Lin, Chem. Commun. 2012, 48, 10808 – 10828.
[18] a) N. E. Schlçrer, S. Berger, Organometallics 2001, 20, 1703 –
1704; b) N. E. Schlçrer, E. J. Cabrita, S. Berger, Angew. Chem.
2002, 114, 114 – 116; Angew. Chem. Int. Ed. 2002, 41, 107 – 109;
c) O. Tardif, D. Hashizume, Z. Hou, J. Am. Chem. Soc. 2004, 126,
8080 – 8081; d) M. A. Rankin, C. C. Cummins, J. Am. Chem. Soc.
2010, 132, 10021 – 10023.
[5] T. Matsuo, H. Kawaguchi, J. Am. Chem. Soc. 2006, 128, 12362 –
12363.
[6] In most cases, at room temperature and 1 atm of CO2.
[7] a) H. Zhao, Z. Lin, T. B. Marder, J. Am. Chem. Soc. 2006, 128,
15637 – 15643; b) L. Gu, Y. Zhang, J. Am. Chem. Soc. 2010, 132,
914 – 915; c) C. Kleeberg, M. S. Cheung, Z. Lin, T. B. Marder, J.
Am. Chem. Soc. 2011, 133, 19060 – 19063; d) R. Dobrovetsky,
D. W. Stephan, Angew. Chem. Int. Ed. 2013, 52, 2516 – 2519.
[8] a) S. N. Riduan, Y. Zhang, J. Y. Ying, Angew. Chem. 2009, 121,
3372 – 3375; Angew. Chem. Int. Ed. 2009, 48, 3322 – 3325;
b) A. E. Ashley, A. L. Thompson, D. OꢁHare, Angew. Chem.
2009, 121, 10023 – 10027; Angew. Chem. Int. Ed. 2009, 48, 9839 –
9843; c) S. Chakraborty, J. Zhang, J. A. Krause, H. Guan, J. Am.
Chem. Soc. 2010, 132, 8872 – 8873; d) S. Chakraborty, Y. J. Patel,
J. A. Krause, H. Guan, Polyhedron 2012, 32, 30 – 34; e) M. J.
Sgro, D. W. Stephan, Angew. Chem. 2012, 124, 11505 – 11507;
Angew. Chem. Int. Ed. 2012, 51, 11343 – 11345.
[19] F. Huang, G. Lu, L. Zhao, H. Li, Z.-X. Wang, J. Am. Chem. Soc.
2010, 132, 12388 – 12396.
[20] S. Bontemps, L. Vendier, S. Sabo-Etienne, Angew. Chem. 2012,
124, 1703 – 1706; Angew. Chem. Int. Ed. 2012, 51, 1671 – 1674.
[21] See the Supporting Information for further experimental details.
[22] R. Shintani, K. Nozaki, Organometallics 2013, 32, 2459 – 2462.
[23] a) M. Maiwald, H. H. Fischer, Y.-K. Kim, K. Albert, H. Hasse, J.
Magn. Reson. 2004, 166, 135 – 146; b) B. Coto, R. Peschla, C.
Kreiter, G. Maurer, Ind. Eng. Chem. Res. 2003, 42, 2934 – 2939.
[24] a) V. M. Nosova, Y. A. Ustynyuk, L. G. Bruk, O. N. Temkin,
A. V. Kisin, P. A. Storozhenko, Inorg. Chem. 2011, 50, 9300 –
9310; b) The characterization of CD3OCH2OD was unambigu-
ously established by its preparation from formaldehyde with
CD3OD, see the Supporting Information.
[9] a) A. Berkefeld, W. E. Piers, M. Parvez, J. Am. Chem. Soc. 2010,
132, 10660 – 10661; b) S. Park, D. Bꢂzier, M. Brookhart, J. Am.
Chem. Soc. 2012, 134, 11404 – 11407; c) S. J. Mitton, L. Turculet,
Chem. Eur. J. 2012, 18, 15258 – 15262; d) A. Berkefeld, W. E.
Piers, M. Parvez, L. Castro, L. Maron, O. Eisenstein, Chem. Sci.
2013, 4, 2152 – 2162.
[25] Yield based on the fact that two equivalents of HBpin are
necessary for the formation of pinBOBpin and H2CO.
4
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 4
These are not the final page numbers!