A zwitterionic carbanion frustrated by boranes - Dihydrogen cleavage with weak lewis acids via an "inverse" frustrated lewis pair approach

Article abstract of DOI:10.1021/ja409330h

The synthesis, structural characterization, and acid-base chemistry of [C(SiMe₂OCH₂CH₂OMe)₃]Na (2), a sterically encumbered zwitterionic organosodium compound, is reported. 2 is a strong Bronsted base that forms frustrated Lewis pairs (FLPs) with a number of boron-containing Lewis acids ranging from weakly Lewis acidic aryl and alkyl boranes to various alkyl borates. These intermolecular FLPs readily cleave H₂, which confirms that even poor Lewis acids can engage in FLP-mediated H₂ cleavage provided that the present bulky base is of sufficiently high Bronsted basicity.

Full text of DOI:10.1021/ja409330h

Communication
pubs.acs.org/JACS
A Zwitterionic Carbanion Frustrated by Boranes − Dihydrogen
Cleavage with Weak Lewis Acids via an “Inverse” Frustrated Lewis
Pair Approach
Hui Li, Adelia J. A. Aquino, David B. Cordes, Fernando Hung-Low, William L. Hase,
and Clemens Krempner*
Texas Tech University, Department of Chemistry & Biochemistry, Box 41061, Lubbock, Texas 79409-1061, United States
S
* Supporting Information
chemistry of the first zwitterionic organosodium compound,
ABSTRACT: The synthesis, structural characterization,
and acid−base chemistry of [C(SiMe2OCH2CH2OMe)3]
Na (2), a sterically encumbered zwitterionic organosodium
compound, is reported. 2 is a strong Brønsted base that
forms frustrated Lewis pairs (FLPs) with a number of
boron-containing Lewis acids ranging from weakly Lewis
acidic aryl and alkyl boranes to various alkyl borates. These
intermolecular FLPs readily cleave H2, which confirms that
even poor Lewis acids can engage in FLP-mediated H2
cleavage provided that the present bulky base is of
sufficiently high Brønsted basicity.
[C(SiMe2OCH2CH2OMe)3]Na. We will demonstrate that this
strongly basic and sterically encumbered zwitterion forms
intermolecular FLPs with weak boron-containing Lewis acids
capable of heterolytically cleaving H2.
Our approach to enforce charge separation in carbanionic
structures involves the incorporation of polydonor groups
directly bound to the carbanion, a strategy that we already
successfully applied to the construction of zwitterionic silyl
anions.10 The synthesis of the new carbanion 2 is outlined in
Scheme 1 and involves the generation of HC(SiMe2Cl)3 via
Scheme 1. Synthesis of Zwitterion 2
he transition-metal-free heterolytic cleavage of H2 via
1
Tfrustrated Lewis pairs (FLPs), pioneered by Stephan,
represents an innovative concept in sustainable chemistry. It has
the potential of replacing expensive, less abundant, and toxic
precious metals in their classical domain, the catalytic hydro-
genation of unsaturated organic species.2 FLPs are sterically
encumbered Lewis pairs unable to form classical Lewis acid−base
complexes due to unfavorable repulsive interactions (frustra-
tion). Key to this unique mode of bond activation is the
unquenched Lewis acidity and basicity of an FLP, which polarizes
the H−H bond and facilitates its heterolytic cleavage.3
A number of Lewis bases ranging from amines, phosphines, to
strongly basic carbenes4 and phosphonium ylides5 have been
successfully utilized in FLP-induced H2 cleavage. In contrast, the
Lewis acidic component with a few exceptions6 has been limited
to expensive, highly fluorinated, but strongly Lewis acidic
boranes such as B(C6F5)3 and related systems. Even modest
reductions in the Lewis acidity of the borane resulted in inactive
FLPs with the commonly used base components.7 Computa-
tional studies regarding the thermodynamic feasibility of FLP
induced H2 cleavage, however, suggested that weaker Lewis
acids, most of them being cheaper and more readily available,
might be active FLP components as well, provided a sufficiently
strong base is used.3c
Zwitterionic carbanions, of which the carbanion is charge
separated from the metal cation by internal donor bridges, are a
rare class of strong Brønsted and Lewis bases.8 As a result of
charge separation the stereochemically active electron pair is
largely localized at the “naked carbanion” and is accessible for
Lewis acid−base chemistry.9 Herein, we report on the synthesis,
structural characterization, and “frustrated” Lewis acid−base
chlorodemethylation of HC(SiMe3)3 with AlCl3/acetyl chloride,
followed by treatment with excess HOCH2CH2OMe/NEt3 to
produce 1 in 94% yield. Deprotonation of 1 with benzylsodium
cleanly generates zwitterionic carbanion 2, which is an air- and
moisture-sensitive colorless solid that dissolves in common
organic solvents including hexanes, benzene, toluene, THF, and
ethers.
2 was fully characterized by NMR spectroscopy, combustion
and X-ray analysis (Figure 1). The X-ray data confirm a tripodal
structure with the three bidentate donor groups of the podand
coordinating in a chelate fashion to the sodium cation whose
coordination sphere is best described as distorted octahedral.
The central C6−Na distance with 3.22 Å is significantly longer
than in other organo sodium compounds and suggests only weak
if any bonding cation−anion interactions.
To have an estimate of the proton affinity of the “naked”
carbanion, the pK of 1 (conjugate acid of zwitterion 2) was
Received: September 9, 2013
© XXXX American Chemical Society
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Scheme 2. Reaction of 2/BR3 with H2
Figure 1. (Left) Solid-state structure of 2 (black = carbon, white =
hydrogen); (Right) HOMO of 2.
determined from acid−base reactions of 2 with substituted
fluorenes using 1H NMR spectroscopy (see Supporting
Information). Evidently, 2 (pK(DMSO) of 1 = 22.5) is a
considerably weaker base than LiC(SiMe3)3,11 which is
attributed to the presence of the alkoxide donors bound to
silicon (Chart 1). These electron-withdrawing groups signifi-
1
3.1 ppm and H resonances at −0.58 and 0.31 pm for the H−
−
−
CSi3 units, respectively. The anions, HBPh3 and H2BMes2 ,
give rise to 11B resonances at −8.5 ppm and −25.8 ppm with B−
H coupling constants of ∼79 Hz (doublet) and ∼74 Hz (triplet),
respectively. Note that also the FLP 2/FBMes2 (2:1 molar ratio)
heterolytically cleaves H2 to cleanly generate salt 5. We assume
that in the first step of this FLP-mediated double hydrogenation
reaction [HFBMes2][HC(SiMe2OCH2CH2OMe)3Na] is
formed, which then readily decays into 1, NaF, and HBMes2.
The latter borane in combination with 2 cleaves H2 to finally
generate 5.
Chart 1
The structures of 4 and 5 were further confirmed by X-ray
analysis (Figures 2 and 3). The structural parameters are in full
cantly reduce the electron density at the anionic carbon as
reflected in the shorter C−Si bonds of 2 [av. 1.79 Å] relative to
LiC(SiMe3)3 [av. 1.83−1.84 Å].12 The notion that the reactive
center is the “naked” carbanion is supported by electronic
structure calculations, which revealed the HOMO of 2 to be
largely located at the central anionic CSi3 unit (Figure 1). It is of
further note that the Brønsted basicitiy of 2 is similar to that of
1,3-di-tert-butylimidazol-2-ylidene13 (Chart 1). This sterically
encumbered carbene in combination with the strong Lewis acid
B(C6F5)3 forms a “metastable” frustrated Lewis pair (FLP)
capable of cleaving H2 irreversibly.4a,b
The favorable properties of 2, high Brønsted basicity
combined with the efficient steric protection of the “naked”
carbanion, prompted us to develop novel carbanion-based FLP
systems with simple boranes and to test their potential as H2
cleaving FLPs. First, 2 was treated with excess BH3 and the
classical Lewis acid−base adduct 3 was isolated as a crystalline
material in 96% yield. When BPh3, FBMes2, HBMes2, BMes3,
and B(OMes)3 were treated, respectively, with 2, no Lewis acid−
base adducts were obtained as a result of steric frustration.
Benzene solutions of these FLPs are stable over prolonged
periods of time. Astonishingly, the FLPs 2/BPh3, 2/HBMes2,
and 2/FBMes2 engage in heterolytic cleavage of H2, while 2/
BMes3 and 2/B(OMes)3 were inactive in C6D6 solutions,
presumably due to extensive repulsive interactions. Note that as
single components neither 2 nor the employed boranes react
with H2 even after longer periods of time and higher pressures (4
atm).
Figure 2. Solid-state structure of 4 (black = carbon, red = oxygen, white
= hydrogen; minor disordered form has been omitted).
agreement with tetrahedral HBPh3− and H2BMes2− anions with
av. C−B−C angles of 114° and 110° and B−H bond lengths of
1.1 and 1.13 Å, respectively. Notably, the cation and anion in 4
Upon adding H2 to 2/BPh3 and 2/HBMes2, respectively,
crystalline solids were isolated from solution and characterized
by NMR spectroscopy as the borate salts 4 and 5 (Scheme 2).
The cationic parts of 4 and 5 exhibit 23Na resonances at 0.3 and
Figure 3. Solid-state structure of 5 (black = cabon, red = oxygen, white =
hydrogen; toluene has been omitted for clarity).
B
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pack such that the CH and BH units are oriented toward each
other with a C−H···H−B distance of ca. 2.18 Å, significantly
shorter than that found in 6 (2.47 and 2.98 Å) and the salts
[But3PH···HB(C6F5)3]1b with 2.75 Å and [But3PH···HB(c-
hexyl)(C6F5)2]14 with 2.63 Å. Indeed, diffusion experiments
(1H DOSY NMR) in C6D6 show that the structure of 4 is
retained in solution nondissociated, while in THF dissociation
into the cation and anion occurs.
engage in H2 cleavage was reported recently by Labinger and
Bercaw.15 Even for BPh3 only one example, the synthesis and
isolation of [But3PH][HBPh3], derived from the reaction of the
intermolecular FLP But3P/BPh3 with H2 and claimed to be stable
at room temperature is reported.1b Its formation via H2-cleavage
has been questioned recently by the Papai group due to the
insufficient Brønsted basicity of the Lewis base component
PBut3. DFT calculations of the Gibbs free energy for the overall
reaction BPh3 + PBut3 + H2 → [But3PH][HBPh3] in toluene as
solvent (ΔGR = +18.2 kcal/mol), indeed, disfavor formation of
the product.3c Our results seem to be in line with Papai’s
Aiming at the limit of FLP-mediated H2-cleavage with 2 as the
base component we employed even weaker Lewis acids such as
BEt3 and B(OMe)3 (Table 1). Again, in both cases no Lewis
calculations, as our base component, carbanion 2 (pK(DMSO)
≈
Table 1. Acceptor Numbers (ANs)17 and Calculated Gas-
Phase Hydride Affinities [ΔHHA/kcal/mol] of Selected
Boranes18
22.5), is a considerably stronger base than PBut3 (pK(H O)
=
2
11.4).16
That strong bases are required to form stable products upon
cleavage of molecular hydrogen with weak Lewis acids is further
supported by calculations of the Gibbs free energies of the H2-
cleavage with the FLPs 2/BPh3, 2/BEt3, and 2/B(OMe)3. The
results are shown in Table 2 for both the gas and solvent phase.
a
borane
AN
−ΔHHA
B(C6F5)3
BH3
78.219
112.020
73.718
74.4
79.0
BPh3
65.619
37.3
HBMes2
FBMes2
BEt3
74.7
a
Table 2. Calculated Gibbs Free Energies [ΔGR in kcal/mol]
16.4
64.7
for Heterolytic H2 Cleavage with 2 and Selected Boranes {8 =
[(MeO)3BH][HC(SiMe2OCH2CH2OMe)3Na]}
30.3
58.5
B(OMe)3
13.2
38.2
a
ΔGR (gas
ΔGR
ΔGR
Measured in C6D6; molar ratio BR3/OPEt3 = 5:1.
reaction
phase)
(hexane)
(benzene)
2 + H2 + BPh3 → 4
2 + H2 + BEt3 → 6
2 + H2 + B(OMe)3 → 8
6.7
21.9
27.5
5.1
14.2
22.0
1.0
9.4
acid−base adducts were obtained with 2, presumably as a result
of both steric and electronic frustration. Treating the FLP 2/
B(OMe)3 with H2 did not lead to H2 cleavage even under forced
conditions, due to the extremely poor Lewis acidity and low
hydride affinity of B(OMe)3. However, exposure of hexanes
solutions of 2/BEt3 under an atmosphere of H2 (2.5 atm, 25 °C)
resulted after 1 h in the formation of a precipitate, which after one
day was isolated from the solution and identified by NMR
spectroscopy as the BEt3-adduct 7. In the 11B NMR, the boron
signal of 7 appears as a broad singlet for the [Et3B−H−BEt3]
unit, rather than as the expected doublet from scalar B−H
coupling. The presence of a B−H bond in 7 was confirmed via
hydride transfer to HBMes2. Inspection of the reaction mixture
by 11B NMR revealed a doublet at ∼26 ppm with a coupling
constant of 74 Hz arising from the H2BMes2− unit. The identity
of 7 was further confirmed by its independent synthesis from the
reaction of 1 with Na[HBEt3] followed by the addition of BEt3
(Scheme 3). Interestingly, intermediate 6 could not be detected
in none of the H2 cleavage reactions.
22.3
a
DFT/B3LYP/SVP.
As expected, the thermodynamic feasibility of these reactions is
in the order 4 > 6 > 8 and correlates well with the order in gas-
phase hydride affinity (ΔHHA) of the individual borane
components that is BPh3 > BEt3 > B(OMe)3 (Table 1). In line
with our experimental results only 4 is predicted to form a stable
product in the solvent phase. Salt 6 (not detected but
independently synthesized) is at least thermally accessible,
while 8 (not observed) is highly unstable. That 7, the BEt3 adduct
of salt 6, was isolated from the H2 cleavage reaction with excess
BEt3 underlines the importance of bridging B−H−B interactions
in stabilizing the borohydride of the final product via dispersion
of the negative charge.21
The inability of 2 to engage in H2 cleavage with the poor Lewis
acid B(OMe)3 encouraged us to employ (Me3Si)3CLi(THF)2 as
a base component, a more than 10 orders in magnitude stronger
Brønsted base than 2. Since B(OMe)3 is known to react with
(Me3Si)3CLi(THF)2 to form (Me3Si)3CB(OMe)2,22 the elec-
tronically similar but bulkier borate B(OPri)3 was chosen as a
Lewis acid component. Indeed, THF solutions of the FLP
(Me3Si)3CLi/B(OPri)3 heterolytically cleave H2 (pressure 2.5
atm) as confirmed by 11B NMR spectroscopic studies, according
to the following equation:
Notably, examples of intermolecular FLP-mediated H2 cleavage
involving medium to weak Lewis acids such as FBMes2, HBMes2,
and BEt3 are without precedence. The only trialkyl borane able to
Scheme 3. Hydrogen Cleavage with the FLP 2/BEt3
(Me3Si)3CLi + B(OPri)3 + H2
→ (Me3Si)3CH + Li[HB(OPri)3]
In conclusion, we have synthesized and structurally characterized
2, the first zwitterionic organosodium compound, and
demonstrated its potential as a strong base component in the
FLP-mediated cleavage of H2. The experimental and computa-
tional results clearly show that even poor Lewis acids engage in
H2 cleavage provided that the present base is sterically
C
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Journal of the American Chemical Society
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ASSOCIATED CONTENT
* Supporting Information
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Experimental procedures, compound characterization data, and
spectra for all new compounds, CIF file for compounds 2, 4, and
5 and computational data. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
clemens.krempner@ttu.edu
■
(9) Meyer, J.; Kuzu, I.; Gonzalez-Gallardo, S.; Breher, F. Z. Anorg. Allg.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
The support of our work by TTU, the Welch Foundation (D-
0005), and the Vienna Scientific Cluster for computer time is
greatly acknowledged. David Purkiss is thanked for carrying out
some of the NMR experiments and the NSF for purchase of a
JEOL ECS-400 Spectrometer (CRIF-MU CHE-1048553).
Support was also provided by the TTU Department of
Chemistry & Biochemistry cluster Robinson whose purchase
was funded by the NSF (CRIF-MU CHE-0840493).
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