Heterolytic dihydrogen activation by a frustrated carbene-borane Lewis pair

Article abstract of DOI:10.1002/anie.200802705

Relief from frustration: The sterically demanding carbene 1,3-di-tert-butyl-imidazolin-2-ylidene and B(C₆F₅) ₃, a frustrated Lewis pair, is a viable system for the activation of C-O, H-H, and C-H bonds. However, slow rearrangement to an abnormal carbene-borane adduct allows the irreversible formation of a strong B-C bond and enables this system to circumvent frustration at the expense of its activity. (Chemical Equation Presented)

Full text of DOI:10.1002/anie.200802705

Communications
DOI: 10.1002/anie.200802705
H2 Activation
Heterolytic Dihydrogen Activation by a Frustrated Carbene–Borane
Lewis Pair**
Dirk Holschumacher, Thomas Bannenberg, Cristian G. Hrib, Peter G. Jones, and
Matthias Tamm*
The Lewis acid–base concept is fundamental for the under-
standing and the prediction of chemical structure and
reactivity.[1] For instance, boranes (BR3) and phosphines
(PR’3) represent prototypical Lewis acids and bases, respec-
tively, and as such, they are prone to form stable donor–
acceptor adducts[2,3] in which the unshared pair of the base
forms a covalent bond with the vacant orbital of the acid.
Steric factors can suppress this inherent behavior, leading to
so-called “frustrated Lewis pairs” (FLPs).[4] Only recently, the
group of D. W. Stephan was able to demonstrate that FLPs
consisting of boranes, in particular tris(pentafluorophenyl)-
borane, B(C6F5)3,[5] and sterically demanding phosphines are
excellent non-metallic systems for the activation of small
molecules such as dihydrogen, olefins, and tetrahydrofuran
under mild conditions.[6–8] Activation of dihydrogen proceeds
heterolytically, cleanly affording phosphonium borate salts of
the type [R’3PH][HBR3].[6b,c] For the P(tBu)3/B(C6F5)3 pair, a
recent theoretical study has arrived at a novel mechanistic
proposal that involves the preorganization of the donor–
acceptor site, providing the possibility of bifunctional coop-
Scheme 1. Reactions of the frustrated Lewis pair (FLP) 1/B(C6F5)3.
erativity for a synergistic interaction with an incoming
dihydrogen molecule.[9] There is a striking similarity between
electron-rich organophosphines and nucleophilic carbenes of
the imidazolin-2-ylidene type in terms of their ligand proper-
ties,[10] and this prompted us to attempt the development of
novel carbene-containing FLP systems and to study their use
in small molecule activation. It should be emphasized that
imidazolin-2-ylidenes alone and related di(amino)carbenes
are inert towards dihydrogen,[11] whereas acyclic and cyclic
(alkyl)(amino)carbenes undergo oxidative addition of dihy-
drogen at the carbene carbon atom.[12]
The combination of 1,3-di-tert-butylimidazolin-2-ylidene
(1) with tris(pentafluorophenyl)borane was chosen as an
initial carbene–borane FLP, and mixing of these two com-
pounds in THF afforded the imidazolium–borate zwitterion 2
as a white solid in almost quantitative yield (Scheme 1). The
formation of 2 can be rationalized by nucleophilic attack of 1
at the Lewis acid–base adduct THF–B(C6F5)3, effecting ring-
opening of the THF ligand in a similar fashion to that
described for the formation of phosphonium borates from
tris(pentafluorophenyl)borane and sterically demanding
phosphines.[8] Incorporation of a bridging (CH2)4O moiety
was indicated by the observation of four methylene reso-
nances at 3.40, 3.23, 1.93, and 1.67 ppm in the 1H NMR
spectrum and at 61.3, 31.8, 28.5, and 27.6 ppm in the 13C NMR
spectrum. The 11B NMR resonance at À2.8 ppm is identical to
that observed for [Mes2PH(CH2)4OB(C6F5)3].[8] The zwitter-
ionic nature of 2 was also confirmed by an X-ray crystal
structure analysis,[20] and the molecular structure has C N
À
À
À
bond lengths (C1 N1 1.345(2) Š, C1 N2 1.350(2) Š) and an
N1-C1-N2 angle (107.85(15)8) that are typical for imidazo-
lium cations (Figure 1).[13] The ring-opened THF results in an
À
À
O B bond of 1.456(2) Š and a C1 C12 bond of 1.498(2) Š.
The reactivity of the Lewis pair 1/B(C6F5)3 suggested the
presence of frustration, as the use of sterically less-demanding
carbenes is expected to give conventional carbene–borane
adducts with substitution and liberation of the THF
ligand.[14,15] Therefore, it was expected that 1/B(C6F5)3 would
represent a viable system for the activation of dihydrogen.
Indeed, purging of a toluene solution containing equimolar
amounts of 1 and B(C6F5)3 with H2 at 208C resulted in the
instantaneous formation of a white precipitate, and after
stirring for 10 min, the imidazolium borate salt 3 could be
[*] Dipl.-Chem. D. Holschumacher, Dr. T. Bannenberg, Dr. C. G. Hrib,
Prof. Dr. P. G. Jones, Prof. Dr. M. Tamm
Institut für Anorganische und Analytische Chemie, Technische
Universität Carolo-Wilhelmina
Hagenring 30, 38106 Braunschweig (Germany)
Fax: (+49)531-391-5309
E-mail: m.tamm@tu-bs.de
Homepage: http://www.tu-braunschweig.de/iaac
[**] We wish to thank Priv.-Doz. Jörg Grunenberg and Dipl.-Chem. Kai
Brandhorst for helpful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200802705.
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Chemie
hydrogen atom is symmetrically capped by a tert-butyl group,
with BH···HC contacts of 2.50, 2.55, and 2.57 Š.
The limitations of the FLP system 1/B(C6F5)3 came to light
through the observation that initially yellow solutions of 1 and
B(C6F5)3 turn colorless in toluene, and completely lose their
reactivity towards dihydrogen within two hours at room
temperature. Evaporation of the solvent and suspension in
hexane afforded a white solid in 95% yield, which was
characterized by elemental analysis and mass spectrometry,
clearly indicating the formation of a 1:1 carbene–borane
adduct. Recrystallization from CHCl3/pentane solution
afforded single crystals suitable for an X-ray diffraction
analysis,[20] and the molecular structure reveals the formation
of the adduct 4, which contains an “abnormal” carbene ligand
(Figure 3).[16] The B(C6F5)3 moiety is attached to the 4 posi-
tion of the imidazole heterocycle, which involves migration of
Figure 1. ORTEPof 2, with thermal ellipsoids set at 50% probability.
Selected bond lengths [Š] and angles [8]: C1–N1 1.345(2), C1–N2
1.350(2), C1–C12 1.498(2), B–O 1.456(2), C15–O 1.417(2); N1-C1-N2
107.85(15), O-B-C41 107.13(14), O-B-C31 112.89(14), O-B-C21
108.42(14).
isolated by filtration in 82% yield (Scheme 1). In the
1H NMR spectrum, signals pertaining to the cation are
observed at 8.10 ppm and at 7.37 ppm for the two types of
NCH hydrogen atoms, along with a singlet at 1.57 ppm for the
tert-butyl hydrogen atoms, whereas the anion gives rise to a
broad quartet at 3.58 ppm with 1J(1H,11B) of circa 100 Hz. The
11B NMR resonance is observed at À25.2 ppm, and the
19F NMR spectrum shows three resonances at À132.3,
À162.9, and À166.0 ppm for the ortho, para, and meta fluorine
atoms, respectively.
Single crystals of 3 suitable for X-ray crystal structure
determination[20] were obtained from CH2Cl2/pentane solu-
tion, and the asymmetric unit of 3 is shown in Figure 2. The
structural parameters are in full agreement with the presence
of an imidazolium cation and a tetrahedral [HB(C6F5)3] anion
with the C-B-C angles ranging from 110.48(15)8 to
113.63(15)8. Unlike the BH···HP contact of 2.75 Š observed
Figure 3. ORTEPdiagram of 4, with thermal ellipsoids set at 50%
probability. Selected bond lengths [Š] and angles [8]: B–C2 1.649(3),
C1–N1 1.336(3), C2–N1 1.414(3), C1–N2 1.321(3), C3–N2 1.371(3),
C2–C3 1.350(3); C1-N1-C2 108.62(19), C1-N2-C3 106.92(19), N1-C1-
N2 110.0(2), C3-C2-N1 103.76(19), C2-C3-N2 110.7(2), C2-B-C41
115.10(18), C2-B-C31 108.35(18), C2-B-C21 107.99(18).
for the salt [(tBu)3PH][HB(C6F5)3],[6b] the corresponding C
À
À
H and B H units in 3 are not oriented towards one another.
À
À
Instead, all C H ring hydrogen atoms display C H···F
the corresponding hydrogen atom to the former carbene
carbon atom. Accordingly, the five-membered ring features
structural parameters that are typical for complexes contain-
ing abnormal carbene ligands of the imidazolium-4-yl type.[16]
À
contacts ranging from 2.50 to 2.62 Š, whereas the B H
À
The B C2 distance is 1.649(3) Š and is almost identical to
that found in the normal adduct between tris(pentafluoro-
phenyl)borane and the sterically less-encumbered carbene
1,3,4,5-tetramethylimidazolin-2-ylidene.[15] The tert-butyl
group at N1 is interlocked with two C6F5 rings, suggesting
À
that the rotation around the B C axis might be severely
hindered. Indeed, the 19F NMR spectrum exhibits six signals
each for the ortho and meta fluorine atoms and three signals
for the para fluorine atoms, which can be explained by
À
hindered rotation around all four B C bonds, rendering the
15 fluorine atoms inequivalent at room temperature on the
NMR time scale. In agreement with the abnormal binding
Figure 2. ORTEPdiagram of 3, with thermal ellipsoids set at 50%
probability. Selected bond lengths [Š] and angles [8]: C1–N1 1.331(2),
C1–N2 1.332(2), B–H01 1.18(2), C1–H02 0.95(2); N1-C1-N2
109.26(16), C21-B-C31 110.48(15), C21-B-C41 113.63(15), C41-B-C31
112.38(16).
1
mode, the H and 13C NMR spectra indicate the presence of
À
two different tert-butyl and C H groups.
To gain further insight into the reactivity of the FLP
system 1/B(C6F5)3, we carried out a series of DFT calcula-
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tions,[20] initially applying the popular B3LYP functional. In
contrast to calculations reported for the P(tBu)3/B(C6F5)3
pair, we were able to identify a covalently bound adduct A
as a minimum on the B3LYP potential energy surface (PES),
À
with a B C bond length of 1.74 Š, which is higher in energy
(DE = + 2.3 kcalmolÀ1) with respect to the starting materials
1 and B(C6F5)3 (Figure 4). As it has been shown that B3LYP is
Figure 5. Relative energies of normal (black) and abnormal (red)
carbene–borane adducts. Values correspond to DE=zero-point uncor-
rected M05-2X/6-311G(d,p) electronic energies, DH8298 =enthalpies at
298 K (in round brackets), and DG8298 =Gibbs free energies at 298 K
[in square brackets].
Figure 4. Potential energy curves derived at B3LYP/6-311G(d,p) (blue)
and M05-2X/6-311G(d,p) (red) levels for the interaction between 1,3-
di-tert-butylimidazolin-2-ylidene (1) and tris(pentafluorophenyl)borane.
dihydrogen. For comparison, the corresponding normal and
abnormal Lewis acid–base adducts consisting of the sterically
less demanding 1,3-dimethylimidazolin-2-ylidene and tris-
(pentafluorophenyl)borane were also calculated, revealing
that both the normal (DE = À59.3 kcalmolÀ1) and abnormal
adduct (DE = À51.9 kcalmolÀ1) are significantly more stable
with respect to their di-tert-butyl congeners (Figure 5). In
contrast to the frustrated system 1/B(C6F5)3, the formation of
the normal dimethylcarbene–borane adduct is highly exer-
gonic (DG8298 = À40.2 kcalmolÀ1), and this combination can
thus clearly be classified as non-frustrated. It should be noted
unable to properly describe medium-range dispersion inter-
actions,[17] which also significantly contribute to the overall
binding energy in related phosphine/B(C6F5)3 adducts,[3d] the
M05-2X functional developed by Truhlar, which better
describes noncovalent interactions, was then used,[17b] result-
ing in an appreciably stronger stabilization of the carbene–
À1
À
borane adduct A (DE = À23.8 kcalmol , d( B C) = 1.71 Š).
À
À
Furthermore, a relaxed PES scan with respect to the B C
distance allowed us to identify a second minimum B at d(B
that the corresponding B C(carbene) distance of 1.65 Š is
À
virtually identical to the value that was reported for the
closely related 1,3,4,5-tetramethylimidazolin-2-ylidene-tris-
(pentafluoro)borane complex (1.6407(16) Š).[15]
C) = 4.51 Š with an association energy of DE = À10.9 kcal
molÀ1 (Figure 4), which represents a weakly bound adduct
À
stabilized by a combination of C H···F hydrogen bonds and
In analogy to the reaction pathway theoretically derived
for the heterolytic cleavage of dihydrogen with P(tBu)3/
B(C6F5)3,[9] we were also able to locate a transition state
À
C H···p interactions in a similar fashion to that described for
the P(tBu)3/B(C6F5)3 system (see the Supporting Information
for a detailed presentation of the calculated molecular
structures).[20]
À
associated with the H H bond cleavage (Figure 6), which is
higher in energy by only DE = + 1.1 kcalmolÀ1 relative to the
adduct B + H2 (Figure 4). In B, the carbene and the borane
are ideally preorganized for a synergistic interaction with an
Although the calculations predict the formation of a
stable adduct 1/B(C6F5)3 (A), partial dissociation of this
complex in solution is very likely at room temperature, as the
Gibbs free energy of DG8298 = À2.3 kcalmolÀ1 indicates only a
marginally exergonic reaction and an almost complete
compensation of its exothermic nature by entropy loss
(Figure 5). Optimization of the structure of the abnormal
adduct 4 furnishes structural parameters that are in good
agreement with those experimentally observed by X-ray
À
incoming dihydrogen molecule, and the calculated B C
distances in B (4.51 Š) and in the transition structure
(4.46 Š) are very similar. With C-H-H and B-H-H angles of
À
À
1748 and 1438, the H H bond is not fully aligned with the C B
À
axis, and the H H distance of 0.79 Š indicates the presence of
À
an early transition state with regard to H H bond cleavage.
Full cleavage of this bond and formation of the imidazolium
À
diffraction (see above); for example, d(B C)calc = 1.66 Š and
borate 3 is predicted to be highly exothermic (DE =
d(B C)exp = 1.649(3) Š. The formation of 4 is calculated to be
À60.7 kcalmolÀ1 relative to the free educts 1, B( CF5)3, and
À
6
significantly more exothermic (DE = À44.2 kcalmolÀ1), which
confirms the experimental observation that this adduct is
formed irreversibly with complete loss of reactivity towards
H2; Figure 6). In comparison with the energy calculated for
the formation of [(tBu)3PH][HB(C6F5)3] from P(tBu)3,
B(C6F5)3, and H2 (DE = À26.3 kcalmolÀ1),[9] it is obvious
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DG8298 =Gibbs free energies at 298 K [in square brackets]. TS=transi-
tion state.
that heterolytic dihydrogen cleavage is thermodynamically
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However, the strongly exothermic and exergonic nature of
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reversible dihydrogen activation.
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À
À
À
bonds, such as C O, H H and C H bonds. The ability to
À
cleave C H bonds became apparent by the observation that
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À
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Received: June 9, 2008
Published online: July 29, 2008
Keywords: boranes · carbenes · donor–acceptor systems ·
.
hydrogen activation · quantum chemistry
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