Intramolecular B/N frustrated Lewis pairs and the hydrogenation of carbon dioxide

Article abstract of DOI:10.1039/c5cc03072b

The FLP species 1-BR₂-2-NMe₂-C₆H₄ (R = 2,4,6-Me₃C₆H₂1, 2,4,5-Me₃C₆H₂2) reacts with H₂ in sequential hydrogen activation and protodeborylation reactions to give (1-BH₂-2-NMe₂-C₆H₄)₂3. While 1 reacts with H₂/CO₂ to give formyl, acetal and methoxy-derivatives, 2 reacts with H₂/CO₂ to give C₆H₄(NMe₂)(B(2,4,5-Me₃C₆H₂)O)₂CH₂4. The mechanism of CO₂ reduction is considered.

Full text of DOI:10.1039/c5cc03072b

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Intramolecular B/N frustrated Lewis pairs and the
hydrogenation of carbon dioxide†
Cite this: Chem. Commun., 2015,
51, 9797
a
b
a
´
Marc-Andre Courtemanche, Alexander P. Pulis, Etienne Rochette,
´
Marc-Andre Legare,a Douglas W. Stephan*b and Frederic-Georges Fontaine*a
´ ´
Received 13th April 2015,
Accepted 13th May 2015
´
´
´
DOI: 10.1039/c5cc03072b
www.rsc.org/chemcomm
The FLP species 1-BR2-2-NMe2-C6H4 (R = 2,4,6-Me3C6H2 1, 2,4,5- the reduction of CO2 using ambiphilic FLP Ph2PC6H4B(O2C6H4),
Me3C6H2 2) reacts with H2 in sequential hydrogen activation generating methoxyboranes with TOF exceeding 900 hꢀ1 at
and protodeborylation reactions to give (1-BH2-2-NMe2-C6H4)2 3. 70 1C.12 In related work, Stephan and co-workers have also
While 1 reacts with H2/CO2 to give formyl, acetal and methoxy- described the use of C3H2(NPR2)2BC8H14 and phosphines14 to
13
derivatives, 2 reacts with H2/CO2 to give C6H4(NMe2)(B(2,4,5- catalyze the hydroboration of CO2 affording mixtures of
Me3C6H2)O)2CH2 4. The mechanism of CO2 reduction is considered. HCO2(B(C8H14)), H2C(OB(C8H14))2 and MeOB(C8H14).
Although hydroboration and hydrosilylation of CO2 to
General concerns regarding global warming, climate changes, methanol are academically interesting, only the hydrogenation
and the need for renewable fuels have prompted researchers of CO2 could be industrially viable. Ashley and O’Hare15 have
from around the world to target methodologies to utilize CO2 as reported the only metal-free system in which CO2 is hydroge-
a C1 source.1 Transition metal catalysts have been uncovered nated. Employing the FLP TMP/B(C6F5)3 (TMP = tetramethylpi-
that either hydrogenate,2 hydrosilylate,3 or hydroborate CO2 to peridine), CH3OH was generated after 6 days at 160 1C under
formic acid, methanol, methane, CO and methoxide derivatives.4 CO2 and H2. While this precedent establishes the concept, the
An alternative strategy for the reduction of CO2 which is gaining development of an efficient FLP catalyst requires attention to
attention is based on non-metal catalysts. While strong Lewis the entropic challenge associated with bringing all reagents
bases can reduce CO2 using either hydrosilanes or hydro- together. In addition, since the transformation of CO2 to
boranes,3b,5 our research groups have been exploring the utility methanol is a 6-electron process generating formic acid and
of frustrated Lewis pairs (FLPs) for the capture and the formaldehyde as intermediates, thus involving three very dis-
reduction of CO2. Since the original report by Stephan, Erker, tinct reduction steps, the Lewis acidity of the electrophilic
and co-workers on the capture of CO2 by FLPs,6 a number of boron center must be judiciously designed to facilitate hydride
inter- or intramolecular FLP variants have been employed to delivery. To address these issues, we are exploring intra-
sequester CO2 and much of this chemistry has been recently molecular FLP systems which incorporate tri-aryl boron centers
reviewed.7 Beyond capture, FLP mediated CO2 reductions have that are significantly less Lewis acidic than the ubiquitous
been probed. The reaction of Al/P FLPs with CO2 and ammonia- B(C6F5)3.16 In this fashion, the proximity of the Lewis acid
borane was shown to give methanol,8 while an alternative and base reduces the entropic barrier, while the reduced Lewis
reaction pathway affords CO.9 In a related study, Piers and acidity at B is expected to promote hydride delivery. In this
coworkers used Et3SiH as a reductant to catalytically generate communication, we describe the reactivity of these B/N FLPs
CH4 and (Et3Si)2O.10 While Stephan and coworkers have also with H2 and the subsequent hydrogenation of CO2 at ambient
reported the catalytic reduction of CO2 using phosphine/CH2I2 temperature.
and ZnBr2 to give CO and phosphine oxide,11 Fontaine and
The fluorescence properties related to compounds 1-BR2-2-
coworkers described one of the most efficient systems to date for NMe2-C6H4 (R = 2,4,6-Me3C6H2 (1), 2,4,5-Me3C6H2 (2)) have
been previously studied, although in our hands the reported
a
´
´
´
Departement de Chimie, Universite Laval, 1045 Avenue de la Medecine,
synthetic route proved problematic.17 Nonetheless, 1 was pre-
pared in 72% yield by the stoichiometric reaction of 1-Li-2-
NMe2-C6H4 with (2,4,6-Me3C6H2)2BF in toluene. In a similar
fashion, the corresponding reaction with (2,4,5-Me3C6H2)2BCl
yielded 2 in 64% yield, following crystallization from a satu-
rated solution in cold hexanes (Scheme 1).
´
´
Quebec (Quebec), Canada, G1V 0A6. E-mail: Frederic.fontaine@chm.ulaval.ca
b Department of Chemistry, University of Toronto, 80 St. George Street, Toronto,
Ontario M5S 3H6, Canada. E-mail: dstephan@chem.utoronto.ca
† Electronic supplementary information (ESI) available. Spectral and computa-
tional data. CDCC 1059372. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c5cc03072b
This journal is ©The Royal Society of Chemistry 2015
Chem. Commun., 2015, 51, 9797--9800 | 9797

View Article Online
Communication
ChemComm
Scheme 1 Preparation of 1–2.
The reactivity of these bright green compounds with both H2
and CO2 was investigated. When a benzene-d6 solution of 1 was
exposed to either 1 atm of CO2 or 4 atm of H2, no change was
evidenced by 1H NMR spectroscopy. However, heating for 24 h a
solution of 1 at 80 1C under 1 atm of HD led to isotopic
scrambling as evidenced by the observation of H2 and HD by
1H NMR spectroscopy. In addition, new signals at 6.72 and
2.16 ppm were observed and assigned to free mesitylene,
suggesting that protodeborylation occurred after the activation
of H2. Indeed, protodeborylation reactions have been shown to
occur before in related systems.18 Monitoring of this protode-
borylation with the use of cyclohexane as an internal standard,
revealed that 1 releases both of its mesityl groups after 72 hours
at 80 1C affording (1-BH2-2-NMe2-C6H4)2 (3). The nature of the
aryl group impacts the facility of protodeborylation as the
species 2, with one less methyl in ortho position than 1, was
converted to 3 after 72 hours at room temperature or after
6 hours at 80 1C. Compound 3 was prepared on a larger scale
from 1 at 80 1C under 4 atm of H2 for 48 hours and was
ultimately isolated in 54% yield. The broad signal at 3.55 ppm
in the 1H NMR spectrum was attributed to B–H protons, which
became sharper with 11B decoupling. The presence of the B–H
bonds was further confirmed by the broad 11B NMR signal at
2.5 ppm. The HRMS data suggest that compound 3 is dimeric
(m/z: 265 = [M–H]), which is further supported by the observation
of inequivalent methyl groups on nitrogen (see ESI†). This view was
further supported by computational studies, in which a number of
isomeric forms of 3 were considered and where the dimeric form
which adopts a ‘‘boat’’ shaped 8-membered ring was computed
to be 9.2 kcal molꢀ1 more stable than the monomeric form
Scheme 2 DFT study of isomers 3. Level of theory (oB97XD/6-31++G**,
solvent = benzene, SMD). DG (DH) are reported in kcal molꢀ1
.
Scheme 3 DFT study of H2 activation and protodeborylation at the
(oB97XD/6-31++G**, solvent = benzene, SMD) level of theory. DG (DH)
are reported in kcal molꢀ1, X0 refers to R = 2,4,6-Me3C6H2, X00 refers to
2,4,5-Me3C6H2.
(Scheme 2). It is noteworthy that Repo and coworkers have acetals and methoxides (Table 1). Heating a benzene-d6 solution
recently described 1-BH2-2-TMP-C6H4, which is also a dimer; of 1 to 80 1C for 24 hours under 4 atm of H2 and 1 atm of 13CO2,
however in this case, structural characterization confirmed that resulted in the appearance of doublets arising from the coupling
the steric congestion favors dimerization via the B–H bonds.19 with the 13C atom for the formate (HCOO at ca. 8.5 ppm ( JCH
B
DFT calculations were also employed to shed light on the 210 Hz)) and acetal derivatives in 1H NMR spectrum (ca. 5.2 ppm
mechanism of this transformation (Scheme 3). The activation (JCH B 165 Hz)). It was found that CO2 was transformed into
of H2 by 1 or 2 proceeds through TS1 to generate A0 and A00, 0.89 equivalents of boron bound formates relative to the amount
respectively, in a slightly endothermic process. Subsequent of 1 at the start of the reaction, and 0.31 equivalents of boron
protodeborylation can occur through TS2, eliminating the bound acetals. Repeating the experiment with a reduced CO2
B-bound aryl substituent to give the ambiphilic hydroboranes pressure (0.5 atm) led to similar conversions to formates and
B0 and B00, respectively.20 Further activation of H2 via TS3 to acetals, but in addition 0.1 equivalent boron bound methoxides
give C0 and C00, prompts a second protodeborylation reaction were formed (ca. 3.5 ppm, JCH B 140 Hz). Further reduction of the
pathway via TS4 affording the primary amino-borane product 3. CO2 pressure to 0.1 atm resulted in the formation of methoxides
While the computed energies for these reactions of 1 and 2 and traces of 13CH4. An experiment under 1 atm of CO2 and 4 atm
follow the same trends, the reduced steric demands of 2 leads H2 in bromobenzene-d5 yielded 0.75, 0.21 and 0.07 equivalents of
to significant lowering of the activation barriers.
formate, acetal and methoxide species respectively after only
The hydrogenation of CO2 with 1 and 2 was investigated and 24 hours at 130 1C. In contrast, 3 did not react in the presence
in general was found to produce several boron bound formates, of H2 and CO2, even after prolonged heating at 80 1C. This lack of
9798 | Chem. Commun., 2015, 51, 9797--9800
This journal is ©The Royal Society of Chemistry 2015

View Article Online
ChemComm
Communication
Table 1 Hydrogenation of carbon dioxide by 1 and 2
Fig. 2 Geometry of TS for reaction of C with CO2 as calculated by
oB97XD/6-31++G** level of theory; (solvent = benzene, SMD).
Equivalentsa of
CO2
H2
FLP (atm) T (h) T (1C) HCOO OCH2O CH3O consumed
was found to be only 24.4 kcal molꢀ1 for R = 2,4,6-Me3C6H2 and
22.1 kcal molꢀ1 for R = 2,4,5-Me3C6H2. The transition state of
interest (Fig. 2) illustrates a concerted interaction of the proton
on N with one of the O of CO2 with the simultaneous inter-
action of the boron-bound hydride with the C atom, thus
directing the hydride delivery to the carbon atom. This TS is
reminiscent of that proposed for the bifunctional Noyori-type
catalysts for metal-based ketone reduction and a similar transi-
tion state was proposed by Musgrave, Zhang and Zimmerman21
for CO2 reduction using ammonia borane as a model reduc-
tant.22 Subsequent reductions of formic acid are thought to
proceed either via similar hydride delivery to formate or by
simple hydroboration, generating acetal derivatives. It is also
interesting that the minor variation in the steric demands of
the substituent on B provide a mixture of reduction products in
the case of reactions of 1 yet allow the isolation of 4 at room
temperature in the reaction of 2.
While previous reports have described conceptually impor-
tant metal-free catalytic hydrosilylation or hydroboration of
CO2, the present report is a rare example of direct FLP hydro-
genation of CO2 as only the earlier report by O’Hare and
Ashley15 described the use of H2 in the metal-free reduction
of CO2. Nonetheless, the present intramolecular FLPs effect this
reduction under much milder conditions (ambient temperature).
The reactions of the present N/B intramolecular FLPs with
H2 demonstrate a rare case where weakly Lewis acidic boron
centres participate in H2 activation. Such systems offer
increased facility for hydride delivery and thus provide an
avenue to CO2 reduction. Moreover, the reaction with CO2
is facilitated by the concurrent interaction of NH and BH
fragments with CO2 affording formate, acetal and methoxy-
derivatives. While the present systems are generated by proto-
deborylation, the reactivity suggests that judicious substituent
selection could provide an avenue to the design of intra-
molecular FLPs catalysts for H2/CO2 chemistry. Efforts towards
such metal-free catalysts for CO2 hydrogenation are the subject
of current work in our laboratories.
1
2
3
4
5
6
1b
1
216
216
216
24
72
3
80
80
80
130
23
80
0.89
0.84
0
0.75
0
0.31
0.34
0
0.21
0.37
0.30
0
1.5
1.8
0.25
1.4
0.74
0.81
1b
0.5
0.1
1
1
1
0.1
0.08
0.07
0
1b
1b,c
2d,e
2
0.21
0
Conditions: 0.014 mmol 1 or 2, 0.4 mL C6D6, 4 atm H2. Yields were
determined by NMR integration with respect to an internal standard
(cyclohexane). a Equivalents of the indicated hydrogenation moiety relative
b
to the amount of starting aminoborane. A white precipitate crashed out
of the solution so 0.1 mL of CD3CN was added before taking the spectra.
c
d
Reaction was carried out in bromobenzene-d5. Reaction was carried out
e
under 1 atm of H2. Compound 4 was exclusively formed.
reactivity is consistent with its dimeric nature that provides a
stabilization of 13.4 kcal molꢀ1
.
Interestingly, the analogous reactions of 2 gave a single
acetal species after 72 hours at room temperature in the presence
of 1 atm H2 and 1 atm CO2 as evidenced by 1H NMR spectroscopy.
On the other hand, higher temperature gave some additional
formate species. When carried out on a larger scale, product 4 was
isolated in 60% yield. The NMR data and the crystallographic
structure (Fig. 1) supported the formulation of 4 as C6H4(NMe2)-
(B(2,4,5-Me3C6H2)O)2CH2. Based on these observations, the first
protodeborylation step is believed to be required prior to CO2
reduction while complete protodeborylation inhibits the
reduction processes due to dimerization of 3.
The initial steps in reaction of 1 and 2 with H2/CO2 were probed
using DFT computations. The reactions of the products of H2
activation A–C (Scheme 3) with CO2 were considered. The barriers
to reduce CO2 with A and B were computed to range between
27.2 and 34.7 kcal molꢀ1 whereas the transition state with C
This work was supported by the National Sciences and
Engineering Research Council of Canada (NSERC, Canada)
and the Centre de Catalyse et Chimie Verte (Quebec). M.-A.
C., E. R. and M.-A. L. would like to thank NSERC and FQRNT for
scholarships. DWS is grateful for the award of a Canada
Research Chair. We acknowledge W. Bi for the resolution of
the crystal structure. The authors wish to acknowledge the
Fig. 1 ORTEP depiction of 4, 50% thermal ellipsoids are shown, N: blue,
C: black, O: red, B: orange. H-atoms are omitted for clarity.
This journal is ©The Royal Society of Chemistry 2015
Chem. Commun., 2015, 51, 9797--9800 | 9799

View Article Online
Communication
ChemComm
¨
¨
6 C. M. Momming, E. Otten, G. Kehr, R. Frohlich, S. Grimme, D. W.
Stephan and G. Erker, Angew. Chem., Int. Ed., 2009, 48, 6643–6646.
7 (a) D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2015, DOI:
10.1002/anie.201409800; (b) D. W. Stephan, Acc. Chem. Res., 2015,
48, 306–316; (c) F.-G. Fontaine, M.-A. Courtemanche and M.-A.
Canadian Foundation for Innovation, project number 19119,
and the Ontario Research Fund for funding of the Centre for
Spectroscopic Investigation of Complex Organic Molecules and
Polymers.
´
´
Legare, Chem. – Eur. J., 2014, 20, 2990–2996; (d) A. E. Ashley and
D. O’Hare, Top. Curr. Chem., 2013, 334, 191–217.
Notes and references
´
8 G. Menard and D. W. Stephan, J. Am. Chem. Soc., 2010, 132,
1796–1797.
1 (a) M. Aresta, Carbon Dioxide as Chemical Feedstock, Wiley-VCH,
Weinheim, 2010; (b) A. Correa and R. Martin, Angew. Chem., Int. Ed.,
2009, 48, 6201–6204; (c) C. Federsel, A. Boddien, R. Jackstell,
R. Jennerjahn, P. J. Dyson, R. Scopelliti, G. Laurenczy and M. Beller,
Angew. Chem., Int. Ed., 2010, 49, 9777–9780; (d) D. M. Dalton and
T. Rovis, Nat. Chem., 2010, 2, 710–711.
2 (a) T. J. Schmeier, G. E. Dobereiner, R. H. Crabtree and N. Hazari,
J. Am. Chem. Soc., 2011, 133, 9274–9277; (b) R. Langer, Y. Diskin-
Posner, G. Leitus, L. J. W. Shimon, Y. Ben-David and D. Milstein,
Angew. Chem., Int. Ed., 2011, 50, 9948–9952; (c) C. A. Huff and M. S.
Sanford, J. Am. Chem. Soc., 2011, 133, 18122–18125; (d) S. Wesselbaum,
T. V. Stein, J. Klankermayer and W. Leitner, Angew. Chem., Int. Ed., 2012,
51, 7499–7502.
´
9 (a) G. Menard and D. W. Stephan, Angew. Chem., Int. Ed., 2011, 50,
´
8396–8399; (b) G. Menard, T. M. Gilbert, J. A. Hatnean, A. Kraft,
I. Krossing and D. W. Stephan, Organometallics, 2013, 32, 4416–4422.
10 A. Berkefeld, W. E. Piers and M. Parvez, J. Am. Chem. Soc., 2010, 132,
10660–10661.
11 R. Dobrovetsky and D. W. Stephan, Angew. Chem., Int. Ed., 2013, 52,
2516–2519.
´
´
12 (a) M. A. Courtemanche, M. A. Legare, L. Maron and F. G. Fontaine,
J. Am. Chem. Soc., 2013, 135, 9326–9329; (b) M.-A. Courtemanche,
´
´
M.-A. Legare, L. Maron and F.-G. Fontaine, J. Am. Chem. Soc., 2014,
136, 10708–10717; (c) M.-A. Courtemanche, J. Larouche, M.-A.
´
´
Legare, W. Bi, L. Maron and F.-G. Fontaine, Organometallics, 2013,
32, 6804–6811; (d) R. Declercq, G. Bouhadir, D. Bourissou, M.-A.
3 (a) K. Motokura, D. Kashiwame, A. Miyaji and T. Baba, Org. Lett.,
2012, 14, 2642–2645; (b) F. Huang, G. Lu, L. Zhao, H. Li and Z. Wang,
J. Am. Chem. Soc., 2010, 132, 12388–12396; (c) T. Matsuo and
H. Kawaguchi, J. Am. Chem. Soc., 2006, 128, 12362–12363;
´
´
Legare, M.-A. Courtemanche, K. S. Nahi, N. Bouchard, F.-G.
Fontaine and L. Maron, ACS Catal., 2015, 5, 2513–2520.
13 T. Wang and D. W. Stephan, Chem. – Eur. J., 2014, 20, 3036–3039.
14 T. Wang and D. W. Stephan, Chem. Commun., 2014, 50, 7007–7010.
15 A. E. Ashley, A. L. Thompson and D. O’Hare, Angew. Chem., Int. Ed.,
2009, 48, 9839–9843.
16 The highly Lewis acidic analogues of 1 and 2 where R = C6F5 have
been reported to activate H2, see: (a) K. Chernichenko, M. Nieger,
´
(d) S. Park, D. Bezier and M. Brookhart, J. Am. Chem. Soc., 2012,
134, 11404–11407; (e) M. Khandelwal and R. Wehmschulte, Angew.
Chem., Int. Ed., 2012, 51, 7323–7326; ( f ) S. J. Mitton and L. Turculet,
Chem. – Eur. J., 2012, 18, 15258–15262.
4 (a) S. Chakraborty, J. Zhang, J. A. Krause and H. R. Guan, J. Am.
Chem. Soc., 2010, 132, 8872–8873; (b) F. Huang, C. Zhang, J. Jiang,
Z. Wang and H. Guan, Inorg. Chem., 2011, 50, 3816–3825;
(c) S. Chakraborty, J. Zhang, Y. J. Patel, J. A. Krause and H. Guan,
Inorg. Chem., 2013, 52, 37–47; (d) S. Bontemps, L. Vendier and
S. Sabo-Etienne, Angew. Chem., Int. Ed., 2012, 51, 1671–1674;
(e) M. J. Sgro and D. W. Stephan, Angew. Chem., Int. Ed. Engl.,
2012, 51, 11343–11345; ( f ) R. Shintani and K. Nozaki, Organome-
tallics, 2013, 32, 2459–2462.
5 (a) O. Jacquet, X. Frogneux, C. Das Neves Gomes and T. Cantat,
Chem. Sci., 2013, 4, 2127–2131; (b) O. Jacquet, C. D. Gomes,
M. Ephritikhine and T. Cantat, J. Am. Chem. Soc., 2012, 134,
2934–2937; (c) C. D. N. Gomes, O. Jacquet, C. Villiers, P. Thuery,
M. Ephritikhine and T. Cantat, Angew. Chem., Int. Ed., 2012, 51,
¨
M. Leskela and T. Repo, Dalton Trans., 2012, 41, 9029–9032;
´
´
¨
(b) K. Chernichenko, A. Madarasz, I. Papai, M. Nieger, M. Leskela
and T. Repo, Nat. Chem., 2013, 5, 718–723.
17 K. Albrecht, V. Kaiser, R. Boese, J. Adams and D. E. Kaufmann,
J. Chem. Soc., Perkin Trans., 2000, 2, 2153–2157.
18 (a) S. Moebs-Sanchez, N. Saffon, G. Bouhadir, L. Maron and
D. Bourissou, Dalton Trans., 2010, 39, 4417–4420; (b) Z. M.
Heiden, M. Schedler and D. W. Stephan, Inorg. Chem., 2011, 50,
1470–1479; (c) M. Devillard, R. Brousses, K. Miqueu, G. Bouhadir
and D. Bourissou, Angew. Chem., Int. Ed., 2015, 127, 5814–5818.
´
´
19 K. Chernichenko, B. Kotai, I. Papai, V. Zhivonitko, M. Nieger,
¨
M. Leskela and T. Repo, Angew. Chem., Int. Ed., 2015, 54, 1749–1753.
20 B could not be isolated as the second H2 activation/protodeboryla-
tion events are faster than the first. Nonetheless, B was intercepted
by reacting 1 with H2 and benzaldehyde to give a borinic ester.
´
´
´
187–190; (d) M.-A. Courtemanche, M.-A. Legare, E. Rochette and F.-G.
Fontaine, Chem. Commun., 2015, 51, 6858–6861; (e) S. N. Riduan,
Y. G. Zhang and J. Y. Ying, Angew. Chem., Int. Ed., 2009, 48, 21 P. Zimmerman, Z. Zhang and C. Musgrave, Inorg. Chem., 2010, 49,
´
´
3322–3325; ( f ) M.-A. Legare, M.-A. Courtemanche and F.-G.
Fontaine, Chem. Commun., 2014, 50, 11362–11365.
8724–8728.
22 R. Noyori, Angew. Chem., Int. Ed., 2002, 41, 2008–2022.
9800 | Chem. Commun., 2015, 51, 9797--9800
This journal is ©The Royal Society of Chemistry 2015