Intramolecular heterolytic dihydrogen cleavage by a bifunctional frustrated pyrazolylborane Lewis pair

Article abstract of DOI:10.1039/c0cc03474f

The reaction of bis(pentafluorophenyl)borane, HB(C₆F ₅)₂, with 3,5-di-tert-butyl-1H-pyrazole (3) affords the zwitterionic pyrazolium-borate trans-5 and, after dehydrogenation by use of the frustrated carbene-borane Lewis pair 1/B(C₆F₅) ₃, the bifunctional pyrazolylborane 6, which is able to cleave dihydrogen heterolytically with the formation of a mixture of cis-5 and trans-5.

Full text of DOI:10.1039/c0cc03474f

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Intramolecular heterolytic dihydrogen cleavage by a bifunctional
frustrated pyrazolylborane Lewis pairwz
b
b
a
Eileen Theuergarten,a Danny Schlu
¨
ns, Jorg Grunenberg, Constantin G. Daniliuc,
¨
Peter G. Jonesa and Matthias Tamm*a
Received 25th August 2010, Accepted 6th September 2010
DOI: 10.1039/c0cc03474f
The reaction of bis(pentafluorophenyl)borane, HB(C6F5)2, with
3,5-di-tert-butyl-1H-pyrazole (3) affords the zwitterionic
pyrazolium-borate trans-5 and, after dehydrogenation by use
of the frustrated carbene–borane Lewis pair 1/B(C6F5)3,
the bifunctional pyrazolylborane 6, which is able to cleave
dihydrogen heterolytically with the formation of a mixture of
cis-5 and trans-5.
Scheme 1 Heterolytic dihydrogen cleavage by a frustrated carbene–
borane Lewis pair; DE
= zero-point uncorrected M05-2X/6-
311G(d,p) electronic energies in the gas phase.7a
The activation of small molecules, in particular dihydrogen, by
nonmetals has recently become a flourishing area of research.1
This rapid development was triggered by the observation of
Stephan et al. that sterically frustrated Lewis pairs (FLPs)
exhibit previously unknown mutual reactivity as a result of
their unquenched Lewis acidity and basicity.2,3 Most FLP
systems in use today are formed by the combination of bulky
P- or N-donor Lewis bases with polyfluorinated boranes such
as B(C6F5)3;4 however, a significantly smaller number of
ambiphilic P/B or N/B combinations is known in which the
active centers are located close to each other and are therefore
able to act simultaneously and intramolecularly on other
molecules.5,6 Representative examples are the ethylene-
bridged phosphine–borane (C6H2Me3)2P–CH2CH2–B(C6F5)2
and the o-phenylene-bridged tetramethylpiperidine–borane
TMPN–CH2–C6H4–B(C6F5)2 developed by Erker et al. and
Rieger et al., respectively;5e,6 the latter N/B system is also
capable of reversible activation of H2.
activation is rendered almost impossible. In search of FLP
systems with more suitable thermodynamic properties, we were
inspired by a recent report by Wang et al.,10 who introduced
several computationally designed metal-free hydrogen activa-
tion sites in which the annulation of a bicyclic borane moiety to
carbene, pyridine or pyrazole donor groups resulted in a
coplanar orientation of the donor lone pair and the vacant
boron orbital; this alignment afforded particularly low barriers
for the intramolecular dihydrogen activation.
Although it may seem difficult to exploit these computationally
derived model systems experimentally, the pyrazolylborane 4
reported by Yalpani et al. could be considered as a structurally
closely related and promising candidate for intramolecular
dihydrogen activation; the sterically demanding 3,5-di-tert-butyl-
pyrazolyl ring in 4 was shown to prevent dimerization and might
therefore provide the reactivity that is anticipated for a frustrated
Lewis pair (Scheme 2).11 4 can be synthesised directly from 3,5-di-
tert-butyl-1H-pyrazole (3) and the dimeric borane 9H-9-borabi-
cyclo[3.3.l]nonane (9-BBN) with evolution of H2.11c However,
our attempts to observe the reverse reaction—heterolytic H2
cleavage—proved unsuccessful with this system.
Our own contribution to this area comprised the introduction
of FLPs containing N-heterocyclic carbenes as Lewis bases.7,8
As an example, the combination of 1,3-di-tert-butylimidazolin-
2-ylidene (1) with B(C6F5)3 revealed a particularly strong
propensity for heterolytic dihydrogen cleavage, which can be
ascribed to the strongly exergonic character of the H2 splitting
reaction as a result of an enhanced cumulative acid–base
strength (Scheme 1).3a,7 Whereas this strong H2 binding ability
can be exploited for dehydrogenation reactions,7b,9 the corres-
ponding H2 release as required for reversible dihydrogen
Since the use of polyfluorinated boranes has been shown to
enhance the reactivity of FLP systems,12 we aimed at substitu-
tion of the BBN fragment by the bis(pentafluorophenyl)borane
moiety. Accordingly, the pyrazole 3 was treated with HB(C6F5)2
to afford the zwitterionic pyrazolium-borate 5 as a white solid in
high yield by simple adduct formation. In contrast to the
preparation of 4 from 3 and 9-BBN (vide supra), 5 does not
release H2 upon heating in toluene at 111 1C and can be even
sublimed at 100 1C/0.05 mbar without noticeable decomposi-
a Institut fur Anorganische und Analytische Chemie,
¨
Technische Universitat Carolo-Wilhelmina, Hagenring 30,
¨
D-38106 Braunschweig, Germany. E-mail: m.tamm@tu-bs.de;
Fax: +49 531-391-5309; Tel: +49 531-391-5387
1
tion. The H NMR spectrum of 5 (in C6D6) shows two signals
b Institute fur Organishce Chemie, Technische Universitat
¨
¨
for the two different tBu groups; the C2–H resonance appears as
a doublet at 5.82 ppm (4JH,H = 2.7 Hz), whereas the N2–H and
B–H hydrogen atoms give rise to broad resonances at 10.23 and
5.00 ppm, respectively. The 11B NMR resonance is observed at
ꢀ13.6 ppm, and in agreement with free rotation around the B–C
bonds, three multiplets are found in the 19F NMR spectrum for
the o-, m- and p-fluorine atoms.
Carolo-Wilhelmina, Hagenring 30, D-38106 Braunschweig, Germany
w This article is part of a ChemComm ‘Hydrogen’ web-based themed issue.
z Electronic supplementary information (ESI) available: Experimental
section including the synthesis and characterization of compounds 5
and 6; details of the electronic structure calculations and X-ray crystal
structure determination. CCDC 796425 and 796426. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0cc03474f
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 8561–8563 8561

110.83(8)1/108.73(8)1 and a C12–B–C18 angle of 114.23(8)1. The
bond distances and angles within the five-membered heterocycle
are in good agreement with those reported for pyrazolium salts.14
The carbene–borane FLP 1/B(C6F5)3 was employed to
dehydrogenate 5 in toluene solution; the precipitated imidazolium
borate 2 was filtered off and the pyrazolylborane 6 was isolated as
a yellow oil in almost quantitative yield (Schemes 1 and 2). In a
similar fashion to that described for the monomeric BBN system
4, the 1H NMR and 13C NMR spectra of 6 show only one set of
signals for the two tBu in the temperature range between +25 and
ꢀ100 1C, suggesting either dimerisation or a rapid exchange of the
B(C6F5)2 group between the two nitrogen atoms of the pyrazolyl
ring. Theoretical calculations suggest that this interconversion in 4
and related monomeric species occurs via a C2v-symmetric inter-
mediate with a Z2-pyrazolyl moiety bound to a tetrahedrally
coordinated boron atom. For the rearrangement in 6, we were
able to locate a similar intermediate only 2.1 kcal molꢀ1 higher in
energy than the ground state electronic structure of 6 (see ESI for
additional information).z Additional spectroscopic evidence
against dimerisation stems from the 11B NMR spectrum, which
exhibits a resonance at +43.1 ppm that falls in the low-field range
reported for other monomeric pyrazolylboranes.11c
Scheme
2 Intramolecular heterolytic dihydrogen cleavage by a
frustrated pyrazolylborane Lewis pair; DE = zero-point uncorrected
M06-2X/6-311++G(d,p) electronic energies in the gas phase relative
to 3/HB(C6F5)2.
Single crystals of 6ꢁ0.5C6H14 could be obtained from
hexane solution at ꢀ30 1C, and X-ray diffraction analysis
unequivocally confirmed the presence of a monomer in the
solid state. The overall structural parameters in 6 are very
similar to those determined for 4 and related species, although
the B–N1 bond [1.4281(16) A] is noticeably shorter than in 4
[1.450(6) A].11c,15 The boron atom resides in a slightly distorted
trigonal-planar environment with an angle sum of 359.71. The
torsion angle between this plane, which is defined by boron
and the boron-bound atoms (N1, C12, C18), and the pyrazole
ring plane is 33.81, indicating a pronounced deviation from a
perfectly coplanar and electronically more favourable orienta-
tion. Comparison of the B–N1–N2 [113.26(9)1] and B–N1–C3
angles [136.66(9)1] indicates another structural distortion,
presumably resulting from the steric impact of the C3-bound
tert-butyl group (Fig. 2).
Single crystals of 5 were isolated at ꢀ30 1C from hexane
solution, and the molecular structure was determined by X-ray
diffraction analysis (Fig. 1), revealing that 5 adopts a trans-
conformation in the solid state, in which the N2–H and B–H
bonds point in opposite directions. The observation of only
one set of NMR resonances suggests that this conformation is
also present in solution. The B–N1 distance is 1.5794(13) A,
consistent with B–N single bonding.13 As expected for an
sp3-hybridised borate moiety, the boron atom resides in a slightly
distorted tetrahedral environment with N1–B–C12/C18 angles of
The exposure of a toluene solution of 6 to one atmosphere
of dihydrogen at room temperature resulted in rapid decolora-
tion of the yellowish solution, and NMR spectra after 20 min
indicated complete consumption of 6 and the formation of
trans-5 together with another minor compound that gives rise
to a similar set of resonances, albeit with markedly different
chemical shifts (see ESIz). After stirring for 24 h, these
resonances completely disappear, and full conversion into
trans-5 is observed. Based on our theoretical calculations
(vide infra), we assign the cis-5 structure to this intermediate,
which is 3.2 kcal molꢀ1 less stable than trans-5 and will
therefore rearrange to the energetically more favorable
conformer by rotation around the B–N1 bond. Conversion
involving cleavage of the B–N1 bond can be precluded, since
the formation of trans-5 (DE = ꢀ40.5 kcal molꢀ1) and cis-5
(DE = ꢀ37.3 kcal molꢀ1) from pyrazole 3 and HB(C6F5)2 is
highly exothermic (Scheme 2).
Fig. 1 ORTEP diagram of trans-5 with thermal displacement para-
meters drawn at 50% probability. Selected bond lengths (A) and
angles (1): N1–B 1.5794(13), N1–N2 1.3617(12), N2–C1 1.3407(13),
C1–C2 1.3844(15), C2–C3 1.4012(14), N1–C3 1.3481(13), B–C12
1.6376(15), B–C18 1.6381(15); B–N1–N2 118.02(8), B–N1–C3
134.94(9), N2–N1–C3 106.57(8), N1–N2–C1 111.80(9), N1–B–C12
110.83(8), N1–B–C18 108.73(8), C12–B–C18 114.23(8).
By analogy to the reaction pathway theoretically derived for
annulated carbene–, pyridine– and pyrazole–boranes,10 we were
also able to locate a transition state (TS) associated with
the H–H bond cleavage (Fig. 3), which is higher in energy by
c
8562 Chem. Commun., 2010, 46, 8561–8563
This journal is The Royal Society of Chemistry 2010

the conditions and to extend the scope of such hydro-
genation reactions. Thereby, fine-tuning and balancing of
the Lewis acidity and basicity of pyrazolylboranes related to
6 might lead to further enhancement of their catalytic
performance.
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Fig. 3 Structure of the transition state (CH3 and CF groups were
omitted for clarity) and potential-energy profile for heterolytic H2
cleavage with 6. Values correspond to DE = zero-point uncorrected
M06-2X/6-311++G(d,p) electronic energies, DHo298 = enthalpies at
298 K (in round brackets), DGo298 = Gibbs free energies at 298 K
[in square brackets].
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´
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c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 8561–8563 8563