Room temperature reduction of CO2 to methanol by Al-based frustrated Lewis pairs and ammonia borane

Article abstract of DOI:10.1021/ja9104792

(Formula Presented) AlX₃ (X = Cl or Br) and PMes₃ (Mes = 2,4,6-C₆H₂Me₃) react to form weak Lewis adducts but also react with CO₂ to give Mes₃P(CO ₂)(AlX₃)

Full text of DOI:10.1021/ja9104792

Published on Web 01/20/2010
Room Temperature Reduction of CO2 to Methanol by Al-Based Frustrated
Lewis Pairs and Ammonia Borane
Gabriel Me´nard and Douglas W. Stephan*
Department of Chemistry, UniVersity of Toronto, 80 St. George Street, Toronto, Ontario, Canada, M5S 3H6
Received December 18, 2009; E-mail: dstephan@chem.utoronto.ca
In the past several decades, increasing levels of carbon dioxide
in the atmosphere have been a major contributing factor to recent
global warming and climate change. With the current global
concentration likely surpassing those of the past 2.1 million years,1
the rapid rise in CO2 levels since the industrial revolution prompts
efforts to limit CO2 emissions to avoid potentially devastating
shown to adopt propeller-like orientations (Figure 1). The P-Al
bond distances in 1 and 2 were found to be 2.5239(11) and
2.5360(10) Å, respectively. This compares to the literature values
of 2.58(2) Å predicted on the basis of a solid state NMR study.20
The inference of equilibria involving the formation of 1 and 2
suggests these classical Lewis pairs can dissociate and exhibit FLP-
type reactivity. To probe this postulate, these solutions were exposed
to 1 atm of CO2 in C6D5Br in sealed J-Young NMR tubes. In both
cases, monitoring of the reactions by NMR spectroscopy showed
the liberation of PMes3 and the formation of new products in
approximate 1:1 ratios. The new species gave rise to 31P{1H} NMR
signals at 20 and 22 ppm for the reactions of 1 and 2, respectively.
1H NMR spectra also showed broad peaks in the methyl and
aromatic regions suggesting the possibility of exchange of free
PMes3 and the newly formed compounds 3 and 4, respectively.
The generation of free phosphine further suggests that the stoichi-
ometry of reaction of CO2 with the adducts 1 and 2 is not 1:1.
Thus, 1:2 mixtures of PMes3/AlX3 were prepared and reacted with
CO2. These reactions yielded the immediate formation of the
previously observed 3 and 4, with no evidence of free PMes3. These
compounds were subsequently isolated in 82 and 83% yields.19
While the spectroscopic data were consistent with the inclusion of
the constituents, the structures of these products were ultimately
identified by X-ray crystallographic studies. These products,
formulated as Mes3P(CO2)(AlX3)2 (X ) Cl 3; Br 4) (Figure 1) were
comprised of phosphine bound to the C-atom of CO2 while AlX3
units are bonded to each of the O-atoms. The resulting P-C bond
lengths in 3 and 4 were found to be 1.927(8) and 1.918(5) Å,
respectively, while the O-Al distances were 1.807(5) and 1.808(6)
Å in 3 and 1.829(4) and 1.803(3) Å in 4. The C-O bond lengths
were determined to be 1.233(8) and 1.251(8) Å in 3 and equivalent
at 1.248(6) Å in 4. The C-O bond lengths are significantly longer
than terminal CdO bonds in tBu3PCO2(B(C6F5)3) (1.2081(15) Å)
and Mes2PCH2CH2B(C6F5)2(CO2) (1.209(4) Å).15 Nonetheless, the
C-O in 3 and 4 are shorter than the 1.2988(15) and 1.284(4) Å of
C-O-B linkages seen in these phosphine borane derivatives.15
This is consistent with the greater Lewis acidity of the borane
fragments in these species in comparison to the aluminum halides
in 3 and 4. The similarity of the two C-O bond distances is
consistent with delocalization of the formal negative charge over
the C(OAlX3)2 fragments. The O-C-O angles are 126.6(7)° and
125.8(4)° in 3 and 4, respectively, while the C-O-Al angles
differed substantially from each other being 141.3(5)° and 165.2(6)°
and 140.0(3)° and 178.7(4)° in 3 and 4, respectively.
impacts on the climate.2 The sequestration of CO2 or its direct
3
use as a C1 feedstock4 are both possible means of reducing the
release of this greenhouse gas. However, a more general statement
of the problem from a chemical perspective is the limited known
reactivity of CO2.
Recently, we have reported the concept of “frustrated Lewis
pairs” (FLPs) where steric congestion precludes combinations of
Lewis acids and Lewis bases from forming classical Lewis
acid-base adducts.5 Such FLPs have been shown to undergo
remarkable reactivity, including the reversible activation of H2,6
B-H7 and N-H8 bond activation, THF ring-opening,9 addition to
olefins10 or alkynes,11 and metal-free hydrogenation of functional
groups such as imines,12 nitriles,12a,b aziridines,12a,b enamines,13
and silylenolethers.14 Most recently, we have also discovered the
15
reactivity of FLPs with CO2 and N2O.16 While the capture of
N2O results in quite robust products, the capture of CO2 by these
phosphine-borane systems is easily reversible. As part of our
ongoing program in FLP chemistry, we have explored related
reactivity of more commonly available Lewis acids. Aluminum
halides (X ) Cl or Br) are indeed readily available and inexpen-
sive.17 Herein, we report the irreversible capture of CO2 using AlX3
(X ) Cl or Br) with PMes3 (Mes ) 2,4,6-C6H2Me3). This
combination of a Lewis acid and base exhibits both classical and
FLP characteristics.18 Moreover these combinations react with CO2
irreversibly in an acid-base ratio of 2:1. Remarkably, these species
also react rapidly with excess ammonia borane at room temperature
to give CH3OH upon quenching with water.
A 1:1 solution of PMes3/AlX3 (X ) Cl 1, Br 2)19 in bromoben-
zene reacts to form weak Lewis adducts. This is evidenced by the
very broad multiplet in the 31P{1H} NMR spectrum in the -10 to
-20 ppm region, which is significantly downfield of the resonance
derived from PMes3 (-35 ppm). In addition, these adducts give
rise to broad doublets in the 27Al NMR spectra (1: 110 ppm, JAl-P
) 258 Hz; 2: 100 ppm, JAl-P 222 Hz). The broadness in these
resonances is attributable to an equilibrium involving free phosphine
and AlX3. Attempts to acquire temperature dependent NMR data
were precluded by the poor solubility in most solvents and the
limited temperature range accessible in bromobenzene. Nonetheless,
these classical Lewis adducts could be isolated in 81 and 84%
yields, respectively. These species were fully characterized spec-
troscopically. In both cases, the 1H NMR spectra show resonances
indicating that the ortho-methyl groups and meta-H atoms of the
mesityl groups are inequivalent, consistent with a significant barrier
to rotation of the mesityl rings. This notion is further supported by
the solid state structures of 1 and 2 in which the mesityl rings are
The binding of both O-atoms to Al in 3 and 4 clearly has an
impact on the C-O bond strength. IR spectra do not reveal a clear
or typical CdO stretch, and the vibration could not be assigned.
This is consistent with the remarkable stability of 3 and 4 to CO2
loss even on heating to 80 °C under a vacuum. These results are at
first glance surprising since these systems employ markedly less
Lewis acidic and basic components than the phosphine/borane
9
1796 J. AM. CHEM. SOC. 2010, 132, 1796–1797
10.1021/ja9104792  2010 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Stoichiometric Reduction of 3 or 4 to CH3OH
adducts tBu3PCO2(B(C6F5)3) and Mes2PCH2CH2B(C6F5)2(CO2)
which rapidly lose CO2 at 80 and -20 °C, respectively.15 The
formation of two Al-O in 3 and 4, as opposed to one B-O bond
in the latter cases, could explain this enhanced stability. This is a
rare example of double activation of CO2, although similar systems
have been proposed.21
In summary, we have described the rapid, room temperature
conversion of FLP-activated CO2 to CH3OH using ammonia borane
as the hydrogen source. The mechanism of this reduction and the
development of new catalytic FLP systems are the subject of current
efforts.
Acknowledgment. D.W.S. gratefully acknowledges the financial
support of NSERC of Canada, the award of a Canada Research
Chair, and a Killam Research Fellowship. G.M. is grateful for the
support of an Ontario Graduate Scholarship.
Supporting Information Available: Experimental, NMR, analytical,
and X-ray crystallographic data. This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 1. Synthesis of 1-4 and POV-ray depictions of 2 and 4; H atoms
are omitted. C, black; P, orange; F, pink; Al, teal; Br, scarlet.
References
(1) Ho¨nisch, B.; Hemming, N. G.; Archer, D.; Siddall, M.; McManus, J. F.
Science 2009, 324, 1551.
The reaction of 3 or 4 with excess ammonia borane (H3NBH3)
(3 equiv) was initially monitored by 31P NMR spectroscopy. In
less than 15 min, the resonance arising from the starting material
was replaced with a new signal at -26 ppm with a P-H coupling
constant of 475 Hz. This was attributable to generation of the cation
[Mes3PH]+. To garner further information on the nature of this
(2) IPCC, Climate Change 2007: Synthesis Report. IPCC; Geneva, 2007; p
52.
(3) Banerjee, R.; Phan, A.; Wang, B.; Knobler, C.; Furukawa, H.; O’Keeffe,
M.; Yaghi, O. M. Science 2008, 319, 939.
(4) Sakakura, T.; Choi, J.-C.; Yasuda, H. Chem. ReV. 2007, 107, 2365.
(5) (a) Stephan, D. W. Dalton Trans. 2009, 3129. (b) Stephan, D. W. Org.
Biomol. Chem. 2008, 6, 1535. (c) Stephan, D. W.; Erker, G. Angew. Chem.,
Int. Ed. 2010, 49, 46–76.
(6) Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W. Science
2006, 314, 1124.
(7) Dureen, M. A.; Lough, A.; Gilbert, T. M.; Stephan, D. W. Chem. Commun.
2008, 4303.
(8) Chase, P. A.; Stephan, D. W. Angew. Chem., Int. Ed. 2008, 47, 7433.
(9) Welch, G. C.; Cabrera, L.; Chase, P. A.; Hollink, E.; Masuda, J. D.; Wei,
P. R.; Stephan, D. W. Dalton Trans. 2007, 3407.
(10) McCahill, J. S. J.; Welch, G. C.; Stephan, D. W. Angew. Chem., Int. Ed.
2007, 46, 4968.
(11) Dureen, M. A.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 8396.
(12) (a) Chase, P. A.; Jurca, T.; Stephan, D. W. Chem. Commun. 2008, 1701.
(b) Chase, P. A.; Welch, G. C.; Jurca, T.; Stephan, D. W. Angew. Chem.,
Int. Ed. 2007, 46, 8050. (c) Sumerin, V.; Schulz, F.; Nieger, M.; Leskela,
M.; Repo, T.; Rieger, B. Angew. Chem., Int. Ed. 2008, 47, 6001.
(13) Spies, P.; Schwendemann, S.; Lange, S.; Kehr, G.; Fro¨hlich, R.; Erker, G.
Angew. Chem., Int. Ed. 2008, 47, 7543.
(14) Wang, H.; Fro¨hlich, R.; Kehr, G.; Erker, G. Chem. Commun. 2008, 5966.
(15) Mo¨mming, C. M.; Otten, E.; Kehr, G.; Fro¨hlich, R.; Grimme, S.; Stephan,
D. W.; Erker, G. Angew. Chem., Int. Ed. 2009, 48, 6643.
(16) Otten, E.; Neu, R. C.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 9918.
(17) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.;
Butterworth-Heinemann: Oxford; Boston, MA, 1997; p 1341.
(18) Geier, S. J.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 3476.
(19) See Supporting Information.
reaction, the isotopologues 13C-3 and 13C-4 were prepared. The 13
C
resonances for these species are observed at 172 and 173 ppm,
each with a JP-C of 123 and 119 Hz, respectively. Upon reaction
of 13C-3 or 13C-4 with H3NBH3, 11B NMR spectral data indicate
dehydrogenation of H3NBH3 to borazine and other products22 while
the 31P NMR spectra indicate the formation of [Mes3PH]+. The
13C NMR spectra showed the loss of the CO2 signal derived from
3/4 and the appearance of approximately 4 quartet resonances
between 50-65 ppm with a JC-H of 146-149 Hz.19 Integration of
these signals shows they account for approximately 60 and 64%
of the 13C in the precursor. These data infer the formation of Al-
methoxy species which are unstable with respect to ligand
redistribution (Scheme 1). This view is supported by the isolation
of crystals of [Mes3PH][AlBr4]19 from a reaction mixture, albeit
in low yield.
Following quenching of the reaction with D2O, the organic phase
was shown to contain free PMes3 by 31P NMR spectroscopy while
the aqueous phase exhibited a 13C resonance as a quartet at 49 ppm
(JC-H ) 142 Hz), identical to an authentic sample of CH3OH in
D2O.19 The yield of CH3OH, quantified by integration relative to
an internal standard, 1,4-dioxane, showed average extracted yields
of 37-51%.19 While the second O-atom is thought to be incorpo-
rated into Al byproducts, its precise fate remains unknown at this
time. Nonetheless these data demonstrate the reduction of the FLP-
activated CO2 to CH3OH. In a very recent report, Ashley et al.23
showed the tetramethylpiperidine/B(C6F5)3 FLP effected conversion
of CO2/H2 to CH3OH in 24% yield in 6 days at 160 °C.
(20) Chu, P. J.; De Mallmann, A.; Lunsford, J. H. J. Phys. Chem. 1991, 95,
7362.
(21) (a) Olah, G. A.; To¨ro¨k, B.; Joschek, J. P.; Bucsi, I.; Esteves, P. M.; Rasul,
G.; Surya Prakash, G. K. J. Am. Chem. Soc. 2002, 124, 11379. (b)
Schumann, H.; Girgsdies, F.; Heymer, B.; Kaufmann, J.; Marschall, C.;
Wassermann, W. Z. Anorg. Allg. Chem. 2007, 633, 2268.
(22) (a) Hamilton, C. W.; Baker, R. T.; Staubitz, A.; Manners, I. Chem. Soc.
ReV. 2009, 38, 279. (b) Marder, T. B. Angew. Chem., Int. Ed. 2007, 46,
8116. (c) Stephens, F. H.; Pons, V.; Baker, R. T. Dalton Trans. 2007, 2613.
(23) See Supporting Information.
(24) Ashley, A. E.; Thompson, A. L.; O’Hare, D. Angew. Chem., Int. Ed. 2009,
48, 9839.
JA9104792
9
J. AM. CHEM. SOC. VOL. 132, NO. 6, 2010 1797