Stoichiometric CO2 reductions using a bis-borane-based frustrated Lewis pair

Article abstract of DOI:10.1039/c2cc33301e

The bis-borane 1,2-C₆H₄(BCl₂)₂ forms an adduct with PtBu₃, but is still capable of exhibiting FLP reactivity with THF and CO₂. The resulting CO₂ species is reduced by Me₂NHBH₃ or [C₅H₆Me ₄NH₂]X (X = [HB(C₆F₅)₃], [HB(C₆F₅)₂(C₇H₁₁)]) followed by quenching with water to effect the stoichiometric conversion of CO₂ to methanol.

Full text of DOI:10.1039/c2cc33301e

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Stoichiometric CO2 reductions using a bis-borane-based frustrated
Lewis pairw
¨
Michael J. Sgro, Johannes Domer and Douglas W. Stephan*
Received 8th May 2012, Accepted 28th May 2012
DOI: 10.1039/c2cc33301e
The bis-borane 1,2-C6H4(BCl2)2 forms an adduct with PtBu3,
but is still capable of exhibiting FLP reactivity with THF and
CO2. The resulting CO2 species is reduced by Me2NHBH3 or
[C5H6Me4NH2]X (X = [HB(C6F5)3], [HB(C6F5)2(C7H11)]) followed
by quenching with water to effect the stoichiometric conversion
of CO2 to methanol.
(Mes = 2,4,6-C6H2Me3) react with CO2 to give the species
Mes3P(CO2)(AlX3)2 (X = Cl; Br)) (Chart 1).17 Interestingly,
while treatment of these products with ammonia borane
(H3NBH3) and subsequent hydrolysis resulted in the
stoichiometric reduction of CO2 to methanol, simple prolonged
exposure of the species Mes3P(CO2)(AlI3)2 to CO2 resulted in the
reduction of CO2 to CO with concurrent formation of
Mes3P(CO2)(O(AlI2)2)(AlI3) and [Mes3PI][AlI4].18 In related
work, Piers et al. showed catalytic deoxygenative hydrosilylation
of CO2 employing an FLP system generating methane.19 We have
also explored reduction chemistry with boron-based FLPs,
involving bis-boranes, although in these cases, while binding
was seen, such systems proved to be thermally labile (Chart 1).20
In this manuscript, we demonstrate that while the bis-borane
1,2-C6H4(BCl2)2 forms an adduct with PtBu3 it still behaves as
an FLP. Moreover it reacts with CO2, forming a thermally stable
species that can be stoichiometrically reduced in a sequence of
reactions involving treatment with ammonia borane or
ammonium borohydrides, followed by quenching with water.
Interestingly, the latter transformation exploits FLP activation
of CO2 and H2 to effect the stoichiometric conversion to methanol.
The bis-borane 1,2-C6H4(BCl2)2 (1) was prepared following
literature procedures of Kaufmann and also Williams
et al.21,22 Combination of this species with PtBu3 resulted in
the formation of a new species (2) which could be isolated as a
Since the industrial revolution 200 years ago, the global
concentration of carbon dioxide has risen rapidly. This in turn
has contributed to global warming and climate change sparking
much concern worldwide. Efforts to reduce CO2 emissions have
garnered considerable attention and approaches to this problem
have varied, including an array of creative carbon capture
technologies. However, due to the massive scale on which one
has to implement such protocols, a single solution is unlikely.
Another alternative involves innovative energy technologies,1–6
which focus on strategies to reduce consumption or employ
alternative sources of energy. In that regard, one concept that
addresses both CO2 capture and an alternative energy source is
the notion of the ‘‘methanol economy’’ put forth by Olah.7,8 In
this approach CO2 is used as a C1 source for fuel. This strategy
would result in the ‘‘recycling’’ of CO2, but could in principle be
‘‘carbon neutral’’ if the reduction of CO2 to C1 fuels was
achieved with ‘‘green’’ hydrogen, that is hydrogen derived from
the photo-catalytic splitting of water.9,10 The other missing link
in this scenario is an effective and clean process for the low
temperature reduction of CO2. While transition metal chemistry
involving CO2 has been studied, a number of recent efforts have
exploited the notion of ‘‘frustrated Lewis pairs’’ (FLPs) for the
activation of a variety of small molecules,11–14 and CO2 in
particular. For example, we have reported the reversible capture
of CO2 employing the combination of sterically demanding
phosphines and boranes (Chart 1).15 Shortly thereafter,
Ashley and O’Hare demonstrated the conversion of amine-
borane based FLP-CO2 species to methanol upon heating to
160 1C for 6 days.16 Subsequently, we showed that FLPs
derived from aluminum halides (X = Cl or Br) and PMes3
1
white solid in 84% yield. The H NMR data were consistent
with the formation of a 1 : 1 adduct with phosphine. The
31P{1H} NMR spectrum showed a single, but broadened peak
at 28.5 ppm, while the 11B NMR spectrum revealed two broad
resonances at 22.3 and 12.9 ppm. These data were consistent
with the formulation of 2 as 1,2-C6H4(BCl2)2ꢀ(PtBu3). The
broadening of signals in the NMR spectra suggests phosphine
Department of Chemistry, University of Toronto,
80 St. George Street, Toronto, Ontario, Canada M5S 3H6.
E-mail: dstephan@chem.utoronto.ca
w Electronic supplementary information (ESI) available. CCDC
881300 and 881301. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2cc33301e
Chart 1 FLP complexes of CO2.
c
This journal is The Royal Society of Chemistry 2012
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Fig. 1 POV-ray depiction of 3, C: black, B: light-pink, Cl: green;
P: orange, O: red. H-atoms are omitted for clarity.
binding to boron is in slow exchange between the two
boron sites.
Fig. 2 POV-ray depiction of 4, C: black, B: light-pink, Cl: green;
P: orange, O: red. H-atoms are omitted for clarity.
Despite the formation of an adduct, the ability of 2 to act as
an FLP was queried. We have previously shown that borane/
donor FLP systems exhibit the ability to effect THF ring opening.
Thus, in an initial test, the species 2 was exposed to THF. This
resulted in the formation of a new species which was ultimately
obtained as colorless needles of 3 in 97% yield. The 31P {1H} and
11B NMR spectrum showed singlets at 48.80 and 12.00 ppm,
respectively. The corresponding 1H NMR spectrum revealed
resonances consistent with the ring-opening of THF and
thus the formulation of 3 as 1,2-C6H4(BCl2)2(O(CH2)4PtBu3).
consistent with the thermal stability of 4. The O-C-O angle
was found to be 127.0(4)1. It is interesting to note that despite
the asymmetry of the CO2 binding, both boron centers are four
coordinate. This is achieved via bridging of one chlorine atom
between the B centers. The bridging B-Cl distances are found to
be 1.999(5) and 2.038(5) A, while the terminal B-Cl bond
distances averaged 1.838(5) A.
Compound 4 proved to be remarkably stable with respect to
loss of CO2. No decomposition was observed on heating to
80 1C for 24 h. This stands in marked contrast to the CO2
products derived from the FLPs tBu3P/B(C6F5)3 and
(C6H2Me3)2PCH2CH2B(C6F5)2. These species were seen to
lose CO2 at 80 1C and ꢁ20 1C, respectively. Similarly, 4
is significantly more thermally robust than the symmetric
FLP-CO2 complexes derived from the bis-boranes Me2C =
C(BX2)2 (X = Cl, C6F5) and PtBu3 as these latter species were
observed to lose CO2 at 15 1C. The enhanced thermal stability
is thought to arise from the bridging chlorine atom between
the boron centers of 4 as this serves to enhance the Lewis
acidity of the boron bound to the oxygen of CO2. This
enhanced acidity results in a strong B–O bond and explains
the improved stability of the CO2 activation product.
This formulation of
3 was subsequently confirmed by
X-ray crystallography (Fig. 1).z The structural data affirm
the zwitterionic nature resulting from the ring-opening of
THF. The oxygen atom bridges the two boron centers giving
rise to B-O bond distances of 1.532(4) and 1.533(4) A with a
B-O-B angle of 112.2(2)1. It is noteworthy that the constraint
of the five-membered ring imposes a B-O-B angle that is
markedly less than that seen for unrestricted cases such as
O(B(C6F5)2)2 (B-O-B 139.5(2)1). The remaining metric para-
meters are unexceptional. The ability of 2 to activate THF is
consistent with the notion that this species provides access to a
reactive FLP.22–24
Having affirmed the ability of 2 to act as an FLP, a solution
of 2 in toluene was exposed to an atmosphere of CO2 and
allowed to stir overnight. Subsequent work-up afforded a white,
microcrystalline solid 4 in 77% yield. The 31P{1H} NMR
spectrum of 4 exhibits a resonance at 48.4 ppm, while the
1H NMR data are consistent with inclusion of phosphine and
bis-borane in a 1 : 1 ratio. A 13C{1H} NMR signal is observed at
156.3 ppm. This signal also exhibits phosphorus-carbon cou-
pling of 94.6 Hz, suggesting the inclusion of CO2. This was
further supported by the observation of an IR absorption at
1719 cmꢁ1. The 11B NMR spectrum reveals a single peak at
18.0 ppm. Although this latter observation infers a molecular
symmetry that would arise from the CO2 spanning the two
boron centers, X-ray diffraction data reveal the molecular
structure is in fact dissymmetric (Fig. 2). In the solid state,
the CO2 fragment is captured between the phosphine and one of
the boron centers. This gives rise to P-C, O-B, C-O and CQO
bond lengths of 1.894(4), 1.477(5), 1.320(4) and 1.177(5) A,
respectively. While the P-C bond in 4 is similar to that seen in
tBu3P(CO2)B(C6F5)3 (1.8931(12) A), Mes2PCH2CH2B(C6F5)2-
(CO2) (1.900(3) A)15 and Mes3PCO2(AlCl3)2 (1.886(3) A), the
O-B distance in 4 is significantly shorter tBu3P(CO2)B(C6F5)3:
1.5474(15) A, Mes2PCH2CH2B(C6F5)2(CO2): 1.550(4) A and
Mes3PCO2(AlCl3)2: 1.550(3) A). This latter observation is
Given the stability of 4, efforts were undertaken to reduce
the captured CO2. Treatment with the amine-borane
Me2NHBH3 for 15 min followed by quenching with D2O led
to the generation of methanol-d1 in 34% yield (Scheme 1).
The FLP derived from 2 failed to activate hydrogen. Treatment
of compound 4 with three equivalents of [C5H6Me4NH2]-
[HB(C6F5)3], a known salt derived from the activation of H2
by the FLP C5H6Me4NH/B(C6F5)3, reacted slowly. After
stirring for 24 h, subsequent treatment with D2O afforded
methanol in 15% yield. However, altering the salt to
[C5H6Me4NH2][HB(C6F5)2(C7H11)] (5), allowed the reaction
time to be reduced to 1 h, and boosted the yield of methanol
to 57%.
In conclusion, we have demonstrated that the bis-borane
1,2-C6H4(BCl2)2 in combination with PtBu3, reacts as an FLP
despite formation of a phosphorus-boron adduct. Moreover
this FLP is shown to bind CO2 strongly in an asymmetric
fashion, allowing further stoichiometric reactions with reducing
agents. Of particular interest is the ability to reduce this species
employing a product of FLP activation of H2. Although in
this case, B-O bond cleavage requires the addition of water,
this finding further demonstrates the potential of FLPs
to concurrently activate CO2 and H2 for CO2 reduction.
c
Chem. Commun.
This journal is The Royal Society of Chemistry 2012

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a = 11.3108(5) A, b = 14.2805(5) A, c = 14.9742(7) A, V =
2418.69(18) A3, data = 5551, var = 262, R (>2s): 0.0557, Rw (all) =
0.1223, GOF = 0.990.
1 M. Bojic, AIP Conf. Proc., 2010, 1239, 12–21.
2 W. D. Grossmann, I. Grossmann and K. Steininger, Environ. Sci.
Technol., 2010, 44, 4849–4855.
3 R. Kothari, V. V. Tyagi and A. Pathak, Renewable Sustainable
Energy Rev., 2010, 14, 3164–3170.
4 A. M. Omer, Energy Costs, Int. Dev. New Dir., 2009, 39–77.
5 A. M. Omer, Int. J. Curr. Chem., 2010, 1, 31–62.
6 J. Wadsworth, Metall. Mater. Trans. A, 2010, 41A, 1047–1062.
7 G. A. Olah, Angew. Chem., Int. Ed., 2005, 44, 2636–2639.
8 G. A. Olah, G. K. S. Prakash and A. Goeppert, J. Org. Chem.,
2009, 74, 487–498.
9 J. M. Lehn, Int. Congr. Catal., [Proc.], 8th., 1984, 1, 63–83.
10 M. Navarro Yerga Rufino, M. C. Alvarez Galvan, F. del Valle,
A. Villoria de la Mano Jose and L. G. Fierro Jose, ChemSusChem,
2009, 2, 471–485.
11 D. W. Stephan, Org. Biomol. Chem., 2008, 6, 1535–1539.
12 D. W. Stephan, Dalton Trans., 2009, 3129–3136.
13 D. W. Stephan, Chem. Commun., 2010, 46, 8526–8533.
14 D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2010, 49,
46–76.
15 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.
16 A. Ashley, A. Thompson and D. O’Hare, Angew. Chem., Int. Ed.,
2009, 48, 9839–9843.
´
17 G. Menard and D. W. Stephan, J. Am. Chem. Soc., 2010, 132,
Scheme 1 Synthesis of 2–4 and reduction of 4.
We are presently pursuing conceptually similar but less oxophilic
FLP systems in the quest for an FLP system capable of catalytic
reduction of CO2 in the presence of H2.
1796–1797.
18 G. Me
´
nard and D. W. Stephan, Angew. Chem., Int. Ed., 2011, 50,
8396–8399.
19 A. Berkefeld, W. E. Piers and M. Parvez, J. Am. Chem. Soc., 2010,
132, 10660–10661.
20 X. Zhao and D. W. Stephan, Chem. Commun., 2011, 47,
1833–1835.
21 D. Kaufmann, Chem. Ber., 1987, 120, 901–905.
22 V. C. Williams, W. E. Piers, W. Clegg, M. R. J. Elsegood,
S. Collins and T. B. Marder, J. Am. Chem. Soc., 1999, 121,
3244–3245.
DWS gratefully acknowledges the financial support of
NSERC of Canada and the award of a Canada Research
Chair. MJS is grateful for the award of an NSERC of Canada
postgraduate scholarship.
Notes and references
%
23 G. C. Welch, J. D. Masuda and D. W. Stephan, Inorg. Chem.,
2006, 45, 478–480.
24 S. J. Geier and D. W. Stephan, J. Am. Chem. Soc., 2009, 131,
3476–3477.
z Crystallographic data. 3: CCDC: 881300, P1, a = 8.9031(5) A, b =
11.5796(7) A, c = 13.1403(8) A, a = 93.817(4)1, b = 99.079(4)1, g =
92.330(4)1, V = 1332.95(14) A3, data = 4689, var = 280, R (>2s):
0.0447, Rw (all) = 0.0945, GOF = 1.012. 4: CCDC 881301, P212121,
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun.