Reversible CO2 fixation by N-heterocyclic imines forming water-stable zwitterionic nitrogen-base-CO2 adducts

Article abstract of DOI:10.1039/c8gc02952k

Zwitterionic Lewis base adducts between nitrogen bases and CO₂ play a pivotal role as transient intermediates in many projects aiming at CO₂ capture, storage and utilization. Yet, fundamental questions about the required parameters for the formation of isolable adducts remain and only a single adduct (TBD-CO₂) has been characterized unequivocally. Using a combination of NMR spectroscopy, single-crystal X-ray diffraction, IR spectroscopy and quantum chemistry, we systematically explore the formation and properties of the CO₂ adducts with amidines (DBN), guanidines (MTBD) and N-heterocyclic imines. Spectroscopic and theoretical results show that the stability of the NHI-CO₂ adducts is largely governed by the nature of the N-heterocycle and the substituent at the exocyclic nitrogen atom. The surprising stability of some NHI-CO₂ adducts towards hydrolysis can be ascribed to the hydrophobic CO₂ binding site of the nitrogen bases and offers new opportunities in the field of CO₂ capture and utilization.

Full text of DOI:10.1039/c8gc02952k

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ARTICLE
Reversible CO2 fixation by N-heterocyclic imines forming water-
stable zwitterionic nitrogen-base–CO2 adducts
Lukas F. B. Wilm,a Tobias Eder,a Christian Mück-Lichtenfeld,b Paul Mehlmann,a Marius Wünsche,a
Received 00th January 20xx,
Accepted 00th January 20xx
Florenz Buß,a Fabian Dielmann*,a
DOI: 10.1039/x0xx00000x
ABSTRACT: Zwitterionic Lewis base adducts between nitrogen bases and CO2 play a pivotal role as transient intermediates
in many projects aiming at CO2 capture, storage and utilization. Yet, fundamental questions about the required parameters
for the formation of isolable adducts remain and only a single adduct (TBD-CO2) has been characterized unequivocally. Using
a combination of NMR spectroscopy, single-crystal X-ray diffraction, IR spectroscopy and quantum chemistry, we
systematically explore the formation and properties of the CO2 adducts with amidines (DBN), guanidines (MTBD) and N-
heterocyclic imines. Spectroscopic and theoretical results show that the stability of the NHI-CO2 adducts is largely governed
by the nature of the N-heterocycle and the substituent at the exocyclic nitrogen atom. The surprising stability of some NHI-
CO2 adducts towards hydrolysis can be ascribed to the hydrophobic CO2 binding site of the nitrogen bases and offer new
opportunities in the field of CO2 capture and utilization.
www.rsc.org/
Nitrogen bases play a particular prominent role in the field of
CO2 capture and utilization owing to their low cost and inertness
towards moisture and air. In this respect, it is surprising that
Introduction
Selective and reversible capture of carbon dioxide (CO2) either
from flue gas or directly from air and its transformation into
value-added chemicals, materials and fuels is one of the most
important challenges to mitigate CO2 accumulation in our
atmosphere.1–3 A key step in many capture-and-release
applications as well as in the catalytic conversion of CO2 is the
reversible binding of the CO2 molecule through low-energy,
easily-broken, bonds.4 Among the many methods for CO2
activation the nucleophilic interaction on the electrophilic
carbon atom is a main pathway. This mode of CO2 activation is
associated with the bending of the CO2 molecule and results in
the formation of zwitterionic adducts between the Lewis base
and CO2. Apart from some low-valent transition metal
complexes containing CO2 as a η1 or η2 ligand,5–10 a variety of
organic bases have been reported to form stable adducts of this
type.11 Carbon bases involving N-heterocyclic carbenes
(NHCs),12–14 N-heterocyclic olefins (NHOs)15,16 and phosphorus
ylides17,18 are well known to form Lewis base adducts with CO2
and have proven to be useful in promoting a variety of catalytic
transformations involving CO2.19–23 We recently discovered that
electron rich phosphines24 are also capable of forming
reversible adducts with CO2 and can even induce the cleavage
of a CO bond of the CO2 molecule.25–27
only little is known about the required parameters for the
formation of isolable nitrogen base-CO2 adducts. Zwitterionic
CO2 adducts with amines were proposed as transient
intermediates in amine-based CO2 capture technologies,28 and
would also occur between CO2 and amino groups of proteins,29
but have never been detected experimentally.30 Similarly,
pyridines only form weak van der Waals complexes with
CO2.31,32 Bicyclic amidines and guanidines such as DBU, DBN,
TBD and MTBD (see Scheme 1) have been proven to be
particularly efficient catalysts for the chemical fixation of CO2
and are also widely used in CO2 capture systems.19,33–43 It has
been suggested that zwitterionic nitrogen base-CO2 adducts are
involved in the reactions, and much effort has been made to
isolate and characterize them. However, bicarbonate salts
-
[baseH+][HCO3 ] were generally obtained due to adventitious
amounts of water. For instance, contrary to the postulated
formation of DBU–CO2 in previous reports,44–46 Jessop and
coworkers showed that DBU does not form an isolable
zwitterionic Lewis base adduct with CO2 under strictly
−
anhydrous conditions, but the salt [DBUH+][HCO3 ] is formed in
the presence of water.47 First experimental hints on the
formation of CO2 adducts with amidines and guanidines were
obtained by the group of Pérez, when they detected the
zwitterions DBN–CO2 and TBD–CO2 in the mixture with their
respective carbonates by using 13C solid-state NMR
spectroscopy.48,49 In 2010 Villiers and coworkers isolated and
fully characterized the zwitterion TBD–CO2 from the reaction of
TBD with CO2 under strictly anhydrous conditions, thus
providing unequivocal experimental proof that the zwitterion
can be isolated.50 It is important to note that TBD–CO2 is
a. Institut für Anorganische und Analytische Chemie, Westfälische Wilhelms-
Universität Münster, Corrensstraße 30, 48149 Münster, Germany.
b. Organisch-Chemisches Institut and Center for Multiscale Theory and
Computation, Westfälische Wilhelms-Universität Münster, Corrensstraße 40,
48149 Münster, Germany
* Corresponding Author, dielmann@uni-muenster.de
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extremely sensitive to hydrolysis and shows fluxional behavior identical to those recorded without CO2 atmosphVeierwe,ArwtichleeOrnelianes
DOI: 10.1039/C8GC02952K
in solution. The stability of TBD–CO2 in the solid state (up to 40 Villiers TBD–CO2 adduct was readily formed under these
°C) was attributed to the high basicity of TBD and multiple conditions, as evident by its characteristic CO2 carbon signal at
hydrogen bonding interactions with the carboxylate O atoms, 154.4 ppm in the 13C NMR spectrum. For DBN, a broad signal
including the intramolecular hydrogen-bonding interaction appears at 131.3 ppm in the 13C NMR spectrum, shifted to
between the N–H of TBD and O-atom of the carboxylate group higher frequency compared to free CO2 (125.9 ppm)51, which
(see Scheme 1).
indicates an equilibrium between the free base DBN and its CO2
Despite the importance of nitrogen base-CO2 adducts in the adduct DBN–CO2 (1) in solution. Pressurizing a pentane solution
field of CO2 capture and utilization, water-stable adducts are of DBN with 2 bar CO2 led to the immediate precipitation of an
unknown and the structural parameters governing the stability off-white crystalline material of 1 in quantitative yield. Single
of such adducts have not yet been identified. Zwitterionic crystals of 1 were obtained from Et2O solutions. An X-ray
nitrogen base-CO2 adducts could provide a source of dry CO2, diffraction study revealed that the molecule is almost planar
because water is not involved in their formation. Additionally, with an amidine-CO2 torsion angle of 9.1° (Figure 1). Similar to
there is no energy penalty associated with the desorption of TBD-CO2,50,52 the DBN-CO2 molecules are arranged in columns
water during the decomplexation of CO2, which would enhance parallel to the c axis with alternating orientation of the dipole
the efficiency of CO2 capture systems. Herein, we systematically moment (Figure S104). The rather long N–CO2 bond (1.513 Å)
explore the stereoelectronic requirements of nitrogen bases for and large O–C–O angle (132.2°) compared to Villiers adduct
the formation of isolable CO2 adducts and unveil their TBD–CO2 (C–N: 1.480 Å, O–C–O: 128.6°)50 indicate that CO2 is
geometries by X-ray single crystal structure analysis. Based on weakly bound to DBN in the solid state.
these results, the first water-stable nitrogen base CO2 adducts
were prepared.
Scheme 1. Three major resonance structures of CO2 adducts with amidine or
guanidine bases.
O
N
O
N
O
N
O
C
O
X
N
O
O
O
X
N
X
N
X
N
N
+
X = CH2, NR
A
B
C
FIGURE 1. Molecular structure of 1. Thermal ellipsoids are set at 50% probability.
Selected bond lengths [Å] and angles [°]: N1–C1 1.513(3), N1–C2 1.330(3), N2–C2
1.322(3), C1–O1 1.233(3), C1–O2 1.228(3), O1–C1–O2 132.2(2), C1–N1–C2 123.6(2), N1–
C2–N2 122.2(2), N2–C2–N1–C1 9.1(2).
I
II
Type
(resonance structures
Type
A B C
A B
,
,
)
(resonance structures
,
)
O
H
N
O
N
O
N
Me
N
O
N
O
O
O
O
N
N
N
N
N
The adduct 1 can be stored under an atmosphere of dry CO2,
but slowly decarboxylates under argon atmosphere. Rapid loss
of CO2 was observed upon evacuation of 1 or by the attempt to
dissolve it in MeCN. Analysis of the thermal stability of 1 by FT-
IR spectroscopy reveals that the characteristic νas(CO) stretching
band at 1727 cm-1 disappears immediately at temperatures
higher than 30 °C. Similar to Villiers TBD–CO2 adduct, DBN–CO2
rapidly hydrolyzes in the presence of moisture to forms the
TBD-CO2
DBN-CO2
MTBD-CO2
DBU-CO2
Jessop 2005
Villiers 2010 Pérez González 2011
this work
this work
isolable
no stable adducts
III
Type
A C
,
(resonance structures
)
steric
control
O
N
R
-
dicarbonate salt [DBNH+][HCO3 ] as evident by the characteristic
O
N
13C NMR signal of the bicarbonate ion at 161.9 ppm and the
asymmetric CO3 stretch at 1592 cm-1 in the IR spectrum. A single
crystal X-ray diffraction study reveals a dimeric assembly that is
associated by hydrogen bonding interactions, which is a typical
structural motif for amidinium and guanidinium bicarbonates
(Figure 2).44,50,53
N
control of
R
reverse tuning
of adduct stability
adduct stability
and steric properties
R
this work
isolable, water-stable
Results and Discussion
CO2 binding with bicyclic amidines and guanidines. We started
our investigation on the formation of CO2 adducts with nitrogen
bases by pressurizing acetonitrile solutions of commercially
available bicyclic amidines (DBU, DBN) and guanidines (MTBD,
TBD) with 2 bar CO2 pressure under strictly anhydrous
conditions. The 1H and 13C NMR spectra of DBU and MTBD were
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base CO2 adducts; c) The stability of the CO2 adVdieuwctAsrticcleaOnnlbinee
DOI: 10.1039/C8GC02952K
controlled in a reverse fashion by the choice of the N-
heterocyclic backbone without affecting the shape of the
hydrophobic pocket.
Scheme 2. Synthesis of NHIs 2-5.a
R'
N
R'
N
R'
N
X
X
a)
b)
-
FIGURE 2. Molecular structure of [DBNH+][HCO3 ] with ellipsoids set at 50% probability.
Selected bond lengths [Å] and angles [°]: N1–C2 1.304(2), N2–C2 1.313(2), C1–O1
1.260(2), C1–O2 1.250(2), C2–N1–O2 113.59(9), N1–C2–N2 124.27(13), N2–C2–N1–O2
1.09(11).
H
Cl
N
N
R'
N
R'
N
R'
R
79-86%
tBu
92-96%
2 5
-
iPr
R'
tBu
Collectively, these results show that zwitterionic adducts
between amidines and CO2 can be isolated although the basicity
of DBN (pKa(MeCN) = 23.79)54 is significantly lower than that of
TBD (pKa(MeCN) = 26.03)55 and TBD–CO2 additionally contains a
stabilizing intramolecular hydrogen-bonding interaction
between the NH of TBD and the oxygen atom of the carboxylate
group. Carneiro showed that the strength of the interaction
energy of CO2 with amines correlate with their basicity in a
nearly linear relationship.56 As a consequence, DBU [pKa(MeCN)
= 24.34] and MTBD [pKa(MeCN) = 25.49],55 being more basic
than DBN, should form isolable CO2 adducts as well. However,
similar to Jessops observation for DBU,47 no evidence for the
zwitterionic CO2 adducts was observed even upon pressurizing
an n-hexane solution of MTBD with 14 bar CO2 pressure, while
N
N
N
N
N
N
R
N
R
N
Me
N
iPr
N
tBu
N
R'
N
tBu
R
(BiPr)NR
(ItBu)NR
(I''R')NR
(sItBu)NMe
2a
2b
2c
3a
3b
3c
3d
4a
4b
4c
4d
4e
5a
: R = Me
: R = tBu
: R = H
: R = Me
: R = tBu
: R = nPr
: R = nOc
: R' = iPr, R = Me
: R' = iPr, R = tBu
: R' = iPr, R = H
: R' = Me, R = H
: R' = Mes, R = H
aReagents and conditions: a) Base, C2Cl6, THF b) RNH2, KF, MeCN, RT, 3-7 days, then
KOtBu, THF.
Synthesis of N-heterocyclic imines 2-5. Following a synthetic
approach by Kunetskiy et al.,57 we prepared a series of NHIs (2-
5) starting from readily available azolium salts (Scheme 2).
Deprotonation and treatment of the free carbenes with
hexachloroethane afforded the corresponding 2-chloroazolium
salts in very good yields. The KF-mediated coupling of the 2-
chloroazolium salts with primary amines in acetonitrile and
subsequent deprotonation of the iminium salts gave the NHIs 2-
5 in excellent yields. The protic NHIs 2c, 4c, 4d and 4e were
synthetized as reported previously via the methanolysis of the
corresponding trimethylsilyl compounds.58,59
–
the salt [MTBDH+][HCO3 ] rapidly forms in the presence of
water (see the Supporting Information for details).
A reasonable explanation for this phenomenon can be derived
from the resonance structures of CO2 adducts with amidine or
guanidine bases (Scheme 1). For bicyclic amidines and
guanidines, two types of CO2 adducts can be distinguished: In
TBD–CO2 and DBN–CO2 (type I) the CO2 atoms and the
amidine/guanidine atoms are nearly coplanar, which enables
extended π delocalization according to resonance structures A,
B and C. Note that resonance structure C contains two formal
negative and two formal positive charges and should therefore
contribute less to the resonance hybrid. In adducts of type II,
steric effects lead to larger C–N–C–O torsion angles up to 90°
and therefore to reduced N to CO2 π donation (resonance
structure C), which explains why DBU and MTBD do not form
isolable CO2 adducts, despite their high basicity. In contrast, CO2
adducts adopting a geometry with an N–C–N–CO2 torsion angle
of 90° (type III) should have an enhanced N=CO2 double bond
character owing to the reduced NCO2 to Cguanidine π bonding.
Given these considerations, we envisaged that N-heterocyclic
imines (NHIs) should be excellent candidates for the
complexation of CO2 owing to the following beneficial factors:
a) CO2 adducts with NHIs should adopt type III geometry driven
by the steric repulsion between the R substituents. Kunetskiy et
al. showed that monocyclic imidazolin-2-imines are significantly
more basic than bicyclic guanidines owing to the synergistic
effect of aromatization of the imidazolium ring and the release
of steric strain upon protonation.57 The same effect should
enhance the stability of CO2 adducts; b) Bulky R groups at the
nitrogen atoms can generate a hydrophobic pocket around the
exocyclic N atom, which might give rise to water-stable nitrogen
Scheme 3. Synthesis of NHI-CO2 adducts 6a-d and 7a-d.
tBu
N
R'
N
O
N
O
O
O
CO2
CO2
N
3a-d
4a-d
N
R'
R
N
tBu
t
R
i
6a
6b
6c
6d
7a
7b
7c
7d
(I Bu)NMeCO2
(I'' Pr)NMeCO2
t
t
i
t
(I Bu)N BuCO2
(I'' Pr)N BuCO2
t
n
i
(I Bu)N PrCO2
(I'' Pr)NHCO2
t
n
(I Bu)N OcCO2
(I''Me)NHCO2
Synthesis and characterization of NHI-CO2 adducts. The
formation of NHI-CO2 adducts was explored by pressurizing
MeCN-d3 solutions of the NHIs 2-5 with 2 bar 13C-enriched 13CO2
under strictly anhydrous conditions. The room-temperature 1H
and 13C NMR spectra of the benzimidazolin-2-imines 2a-c,
imidazolin-2-imine 4e and imidazolidin-2-imine 5a were
identical to those recorded without CO2 atmosphere. In
contrast, obvious differences in the chemical shift were
observed in the H and 13C NMR spectra when using 3a-d and
4a-d. In addition to the slightly shifted signals of the NHI, the
1
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characteristic carboxylate carbon signal of the NHI-CO2 adduct atoms (deviation < 0.1 Å). In contrast, the exocyclic nitrogen
View Article Online
DOI: 10.1039/C8GC02952K
appears in the range of 155.7 ppm to 157.8 ppm (Table 1). The atom of 6b is dislocated by 0.3 Å, seemingly resulting from the
CO2 resonances of 7c (138.9 ppm) and 7d (140.6 ppm) are bulkyness of the tert-butyl substituents. Very similar C1–N1
broadened and had a similar chemical shift to that of DBN–CO2 bond lengths (1.432–1.464 Å) were found that are significantly
(131.3 ppm), indicating fluxional behavior in solution. shorter than those in 1 (1.513 Å) or TBD–CO2 (1.480 Å) which
Moreover, the signal for the N–H protons [8.93 ppm (7c) and agrees with the expected enhanced double bond character of
7.75 ppm (7d)] are significantly shifted to higher frequency the C1–N1 bond. The O–C–O angles of the NHI-CO2 adducts
compared to the free bases 4c (3.76 ppm) and 4d (3.74 ppm). (129.1–131.4°) are slightly larger (by ca. 2°) than in NHO-CO2
13,14
Notably, low-temperature NMR experiments provided adducts15 and are in the range of NHC-CO2
spectroscopic evidence for the formation of CO2 adducts with CO2
or phosphine-
25,27
adducts. Hydrogen bonding intaractions between the
2a and 2b. At 240 K, the carboxylate carbon signals appear at carboxylate O atoms and acidic NH protons, as present in 7c, 7d
154.9 ppm (for 2a) and 154.4 ppm (for 2b), respectively. and TBD–CO2, seem to induce smaller O–C–O angles.
Isolation of the CO2 adducts 6a-d and 7a-d was conveniently Thermal stability. The thermal stability of the NHI-CO2 adducts
accomplished by diffusion of CO2 into a n-pentane solution of 6a-d and 7a-d were examined by FT-IR monitoring of the
the NHI. Owing to the poor solubility in aliphatic solvents, the asymmetric CO2 stretching band of the solids kept under an
zwitterionic adducts immediately precipitate as a white solid in argon atmosphere, while increasing the temperature in steps of
quantitative yield. The asymmetric CO2 stretching band of the 5 °C (Table 1). This analysis revealed that the substituent at the
NHI-CO2 adducts at 1654–1673 cm−1 (Table 1) are in the range exocyclic nitrogen atom has a profound influence on the
of NHC-CO2 adducts (1629–1683 cm−1)13,14 but appear at decarboxylation temperature of the NHI-CO2 adduct, showing
significantly lower frequency than that of the bicyclic nitrogen the trend that an increased size of the alkyl substituent leads to
bases DBN–CO2 (1727 cm−1) and TBD–CO2 (1712 cm−1).
a decrease in decarboxylation temperature. The CO2 adducts of
the methyl substituted NHIs (6a and 7a: >120 °C) decompose at
higher temperature than those carrying n-alkyl (6c and 6d: 75
°C) or tert-butyl (7b: 100 °C) substitutents at the exocyclic
nitrogen atoms. In this respect, complete decarboxylation and
recovery of the free nitrogen bases was observed after heating
solid samples of 6c and 6d for 5 minutes at 75 °C or upon
evacuating the solids for 15 minutes at room temperature. The
thermally more stable adducts 6a, 7a and 7b can be shortly
dried under vacuum without noticeable loss of CO2. The lowest
decarboxylation temperatures were found for the protic NHI-
CO2 adducts 7c (30 °C) and 7d (35 °C) which can be rationalized
by the significantly lower basicity of the protic NHIs compared
to the NHIs with alkyl substituents (vide infra).
Thermal fragmentation of 6b. Examination of the thermal
stability of NHI-CO2 adduct 6b revealed the unexpected
fragmentation of 6b into 1,3-di-tert-butyl-2-imidazolinone and
tert-butyl-isocyanate. NMR monitoring showed that the
fragmentation of 6b in MeCN-d3 solution is slow at room
temperature (29% conversion within 4 h). Complete conversion
was observed after heating the solution to 50 °C for 3 hours
(Figure S94-S99). The unusual reactivity of the sterically
encumbered CO2 adduct 6b is likely to be driven by the release
of steric strain as indicated in the solid-state structure of 6b. The
decomposition process was further probed by DFT calculations
including implicit solvation. The results from the study of NHI-
CO2 adducts 6a and 6b are summarized in Figure 3. The CO2
binding energy for 6a is by -2.8 kcal mol-1 more exergonic than
that of 6b. Moreover, the reaction energy for the fragmentation
of 6b (-24.2 kcal mol-1) is significantly more exergonic than that
of 6a (-10.0 kcal mol-1). According to the IRC scan the
fragmentation into the corresponding urea and isocyanate
derivates is a one-step reaction. In the transition-state structure
one oxygen atom of the CO2 fragment is connected to the
imidazoline C2 atom forming an OCNC four-membered ring,
which contains an elongated C2–N1 bond of 1.74 Å. The
reaction barrier associated with this step is significantly higher
Table 1. Characteristic spectroscopic data and decarboxylation temperatures (T) of
nitrogen-base–CO2 adducts.
13C NMR
[ppm]a
IR
T [°C]
Entry
[cm-1]
TBD–CO2
154.4b
131.3
157.8
156.9
157.0
157.1
156.3
155.7
138.9
140.6
1712b
1727
1666
1654
1665
1666
1663
1673
1671
1662
40b
30
DBN–CO2 (1)
(ItBu)NMeCO2 (6a)
(ItBu)NtBuCO2 (6b)
(ItBu)NnPrCO2 (6c)
(ItBu)NnOcCO2 (6d)
(I"iPr)NMeCO2 (7a)
(I"iPr)NtBuCO2 (7b)
(I"iPr)NHCO2 (7c)
(I"Me)NHCO2 (7d)
>120
n.d.
75
75
>120
100
30
35
a recorded in MeCN-d3, b literature values.50
The molecular structures of compounds 6a, 6c, 7a, 7b, 7c and
7d were established using single-crystal X-ray crystallography
(Figure 4). Selected bond length and angles are collected in
Table 2. Short intermolecular O∙∙∙HC distances around 2.2 Å to
the imidazolium ring hydrogen atoms were found for 6a and 6c.
Such hydrogen-bonding interactions have been postulated to
be vital for efficient catalysis in the context of imidazolium-
promoted CO2 transformations.60 The CO2 adducts of the protic
NHIs 7c and 7d are stabilized by O∙∙∙HN hydrogen-bonding
interactions forming dimeric (7c) and polymeric (7d)
arrangements. For the NHI-CO2 adducts with alkyl substituents
at the exocyclic nitrogen atom, the plane defined by the CO2
moiety is essentially perpendicular to the plane spaned by the
imidazolin atoms ranging from 85° (6c) to 89° (6a). Larger
deviation from this orientation of type III was observed for the
protic NHIs (7c: 76°, 7d: 69°). The exocyclic nitrogen atom of the
CO2 adducts are located in the plane spaned by the imidazoline
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for 6a (ΔG298 = 35.3 kcal mol-1) compared to that of 6b
(ΔG298 = 26.0 kcal mol-1) and explains the slow fragmentation of
6b at room temperature. Phosphazene superbases have been
found to react with CO2 in a similar manner to give the
respective phosphine oxides and isocyanates.61,62
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DOI: 10.1039/C8GC02952K
FIGURE 3. Computed Gibbs free energy profile for the thermal fragmentation of 6a
(black) and 6b (red) along with optimized transition state structure (bond lengths in Å).
Energies were obtained as ΔG298 in solution (MeCN) with density functional theory at the
PW6B95-D3// TPSS-D3/def2-TZVP + COSMO-RS(MeCN) level.
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6a
6b
6c
7a
7b
7c
7d
FIGURE 4. Solid-state structures of NHI-CO2 adducts 6a-c and 7a-d. CH-hydrogen atoms are omitted for clarity. Thermal ellipsoids are set at 50% probability.
Table 2. Selected bond lengths [Å] and angles [deg] of nitrogen base CO2 adducts.
Entry
TBD–CO2
DBN-CO2
6a
C1–N1
1.480(3)a
1.513(3)
C2–N1
1.369(3)a
1.330(3)
C2–N2
C2–N3
C1–O1
C1–O2
O1–C1–O2
128.6(2)a
132.2(2)
130.56(11)
130.0(3)b
130.5b
imidazole-CO2 angle
4.4(2)a,c
1.332(2)a
1.338(3)a
1.322(3)
1.257(3)a
1.229(2)a
1.233(3)
9.1(2)c
1.228(3)
1.439(2)
1.3729(14)
1.381(4)b
1.3701(19)b
1.376(2)
1.3552(14)
1.349(2)
1.2487(14)
1.2432(14)
1.249(4)b
1.237(4)b
1.242(2)b
1.246(2)b
1.244(3)
88.9(2)
6b
1.456(4)b
1.441(2)b
1.443(3)
1.361(4)b
86.1(4)b
84.8(3)b
87.8(3)
1.363(4)b
6c
1.356(2)b
1.377(2)b
7a
1.340(3) 1.343(3)
131.4
1.239(3)
7b
1.4637(10)
1.4386(13)
1.4317(11)
1.3779(10)
1.3668(14)
1.3679(11)
1.3475(10)
1.3450(10)
1.3441(14)
1.3506(13)
1.3384(11)
1.3925(11)
1.2476(11)
1.2385(11)
1.2437(13)
1.2536(13)
1.2506(11)
1.2430(11)
130.39(8)
129.78(10)
129.09(9)
87.3(9)
7c
76.2(1)
7d
68.9(1)
a literature values.50 baverage values. ctorsion angle C1-N1-C2-N2
6 | J. Name., 2012, 00, 1-3
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Computational Studies. To better understand the
influence of the substitution pattern at the N-
heterocycle on the stability of the NHI-CO2 adducts, the
gas-phase basicities (GB) and free energies for binding
of CO2 (ΔG298) were determined for a series of nitrogen
superbases with PW6B95-D3//TPSS-D3/def2-TZVP
(Figure 5). The COSMO-RS solvation model for
acetonitrile was used for the determination of the ΔG298
values. The weak CO2 binding energies of the bicyclic
nitrogen bases, showing the trend DBU < MTBD < DBN
< TBD, agree with the experimental observation that
DBN–CO2 and TBD–CO2 are fluxional in solution, while
no interaction with CO2 was detected for DBU and
MTBD. The fact that DBN–CO2 is isolable despite its
endergonic binding energy can be attributed to
stabilizing intermolecular interactions in the solid state
(see Figure S104). The CO2 binding energies of the NHIs
agree very well with the experiment and illustrate that
the stability of the CO2 adducts is largely governed by
the nature of the N-heterocycle showing the general
gas-phase basicity and the magnitude of the CO2
binding energy increase with the +I effect of the alkyl
substituents at the nitrogen atoms, showing the highest
values for 3a-d and 4b. This trend is particularly
prominent for the substituents at the exocyclic N
atoms, as evident by the series 2c, 2a, 2b and the series
4c, 4a, 4b. However, it should be mentioned that the
calculated values do not take account of the additional
stabilization via hydrogen bonding interactions as
observed in the solid-state structures of 7c and 7d,
respectively. Steric strain around the exocyclic N atom
in 5a and 4y might be responsible for their deviation
from the general trend.
The geometric parameters of the calculated NHI-CO2
adducts show linear correlations with the CO2 binding
energies (Figure S117–S120). With increasing CO2
binding energy the O–C–O angles get smaller, the C1–
N1 bond distances get shorter and the C2–N1 as well as
the average C1–O distances get longer. Noticeable
deviations from this trend are observed for 5a, which
show unusually long C2–N1 bond and short C1–N1
bond lengths, indicating steric strain around the
exocyclic N atom.
trend: benzimidazoline (2)
< imidazolidine (5) <
imidazoline (3) < 4,5-dimethylimidazoline (4), while it is
also sensitive to the steric profile of the NHIs. Both, the
GB [kcal mol-1]
N
N
N
N
270
265
260
255
250
245
N
N
N
N
N
N
N
N
4b -9.86
2x
2a
2y
2b
2c
3x
3y
3a
3b
3c
3d
4x
4a
4y
4b
4c
4d
5x
5y
5a
: (BMe)NMe
: (IMe)NMe
: (I''Me)NMe
: (sIMe)NMe
i
i
i
i
: (B Pr)NMe
: (I Pr)NMe
: (I'' Pr)NMe
: (sI Pr)NMe
3d -9.33
t
t
t
t
: (B Bu)NMe
: (I Bu)NMe
: (I'' Bu)NMe
: (sI Bu)NMe
3c -9.75
i
t
t
t
i
t
3b -7.41
4a -7.80
4y -6.99
: (B Pr)N Bu
: (I Bu)N Bu
: (I'' Pr)N Bu
i
t
n
i
3a -10.23
: (B Pr)NH
: (I Bu)N Pr
: (I'' Pr)NH
t
n
: (I Bu)N Oc
: (I''Me)NH
2b -0.66
3y -3.53
4c -0.26
4d 1.34
4x -5.83
MTBD 2.93
TBD -4.05
5y 2.11
DBU 5.43
3x -2.12
2y 0.17
2a 2.48
5a 3.60
DBN 1.44
5x 2.22
2x 2.75
2c 8.79
ΔG298 [kcal mol-1]
15
10
5
0
-5
-10
-15
FIGURE 5. Plot of calculated gas-phase basicities (GB) versus free energies for CO2 binding (ΔG298) of selected nitrogen superbases determined with PW6B95-D3//TPSS-
D3/def2-TZVP. The COSMO-RS solvation model for acetonitrile was used for the ΔG298 values. Color code of ΔG298 values according to experimental observations: CO2
adduct isolable (green), not isolable (red), not determined (black).
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7c, 7d), showing fluxional behaviour, have been identified in
solution and the first CO2 adducts with amidines (1) and N-
heterocyclic imines (6a-d, 7a-d) have been isolated and
characterized, including by X-ray crystallography. The NHIs are
easily acessible from cheap and commercially available starting
materials and allow for the facile variation of the steric and
electronic properties. DFT calculations reveal that the CO2
binding energy of the NHIs linearly correlates with the gas-
phase basicity, and that it is strongly governed by the nature of
the N-heterocycle and the substitution pattern. FT-IR analysis of
the NHI-CO2’s thermal stability shows that the decarboxylating
ability largely depends on the substituent at the exocyclic
nitrogen atom. For the sterically overloaded NHI 3b, the
thermal decomposition of the NHI-CO2 adduct 6b into 1,3-di-
tert-butyl-2-imidazolinone and tert-butyl isocyanate was
observed. Computational studies confirm that the
fragmentation reaction is driven by the release of steric strain
leading to a rather low energy barrier of 26.0 kcal mol-1.
Examination of the reactivity with water shows that the NHI-
CO2 adducts are surprisingly stable towards hydrolysis, while
the stability largely depends on the size of the alkyl substituent
at the exocyclic nitrogen atom. This unprecedented property for
nitrogen bases can be attributed to the hydrophobic nature of
the CO2 binding site and offer new prospects for the field of CO2
capture and utilization. Follow-up studies on the utilization of
NHIs as organocatalysts and their application in direct air CO2
capture systems are under current investigation in our
laboratories.
FIGURE 6. Hydrolysis study of DBU–CO2, TBD–CO2 and NHI-CO2 adducts (6a-d, 7a-d) in
MeCN-d3 at room temperature.
Stability towards Water. Based on the structural parameters of
the NHI-CO2 adducts we were curious how the substitution
pattern would influence the stability of the NHI-CO2 adducts
towards hydrolysis. The stability of the isolable nitrogen base–
CO2 adducts towards water was evaluated by adding portions
of water to MeCN-d3 solutions of TBD–CO2, DBN-CO2, 6a, 6c, 6d,
and 7a-d at room temperature. Thirty minutes after the
addition and short mixing, the ratio between the CO2 adduct
-
and the bicarbonate salt [baseH+][HCO3 ] was determined by 1H
NMR analysis of the reaction mixture (Figure 6). As expected,
the addition of 2.7 equivalents of water to TBD–CO2 and DBN–
CO2 led to the quantitative formation of the bicarbonate salts.
Similar behaviour was observed for the protic NHIs 7c and 7d.
In contrast, the addition of 2.7 equivalents of water to the
aprotic NHI-CO2 adducts resulted in the incomplete hydrolysis
of 6b (86%), 6c (19%), 6d (27%) and 7b (78%). Subsequent
changes among those ratios were not observed according to
Conflicts of interest
There are no conflicts to declare.
1
prolonged H NMR-monitoring of the solutions (up two three
Acknowledgements
weeks). However, the addition of further equivalents of water
to the mixture resulted in increased hydrolysis. Surprisingly, 6a
and 7a did not react with water even in the presence of a large
excess and upon heating the solutions at 70 °C for 24 h (see
Figure S101 and Figure S102). Collectively, these results show
that among the NHI–CO2 adducts with alkyl groups at the
exocyclic nitrogen atom the stability towards hydrolysis largly
depends on the size of the alkyl substituent, indicating the trend
of decreasing stability with larger alkyl groups.
Financial support was generously provided by the DFG (Emmy
Noether program: DI 2054/1-1, IRTG 2027, SFB 858). Thanks are
due to the FCI and the German Academic Scholarship
Foundation for a PhD fellowship (L.W.). We thank Prof. F. E.
Hahn for his generous support.
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