Heterolytic activation of H2 using a mechanically interlocked molecule as a frustrated Lewis base

Article abstract of DOI:10.1002/anie.201207783

Frustrating: A sterically unencumbered aniline base (see picture, left) can be transformed into a bulky Lewis base by converting it into a [2]rotaxane (right). This Lewis base donor, which is surrounded by a protective macrocyclic ring, exhibits the reactivity of a frustrated Lewis pair (e.g. activation of H₂(g)) without the need for direct covalent modification to increase its bulk. Copyright

Full text of DOI:10.1002/anie.201207783

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DOI: 10.1002/anie.201207783
Rotaxanes
Heterolytic Activation of H2 Using a Mechanically Interlocked
Molecule as a Frustrated Lewis Base**
Christopher B. Caputo, Kelong Zhu, V. Nicholas Vukotic, Stephen J. Loeb,* and
Douglas W. Stephan*
A frustrated Lewis pair (FLP) is an intra- or intermolecular
combination of a Lewis acid and a Lewis base in which steric
hindrance inhibits the formation of a classical Lewis donor–
acceptor adduct. A consequence of the unquenched Lewis
acidity and basicity is unprecedented reactivity, including the
heterolytic cleavage of H2 molecules[1] and activation of small
molecules, such as CO2,[2] N2O,[3] NO,[4] SO2,[5] alkenes,[6] and
alkynes.[7] The FLP concept[8] has been recently exploited for
the development of stoichiometric reductions of anilines to
cyclohexylamines,[9] and of metal-free catalysts for hydro-
genation of polar substrates[8,10] and 1,1-disubstituted ole-
fins.[11]
would convert a simple Lewis base into a bulky Lewis base
partner for FLP chemistry without the restriction of covalent
modification.
The secondary amine (aniline) 1, which contains 1,3-
dimethylphenyl and 1,3-dimethylbenzyl groups, was selected
as the Lewis basic axle for incorporation into a [2]rotaxane.
Scheme 1 outlines the synthesis of the two neutral [2]rotax-
The steric bulk, which inhibits the formation of Lewis
acid–base adducts in FLP chemistry, has traditionally been
attached directly to the Lewis acidic and basic atoms.[7a] For
example, the phosphines,[1a,b] amines,[12] and boranes[1a,b] that
have been used to elicit FLP reactivity all contain either bulky
alkyl or aryl substituents directly bonded to the heteroatom.
As this requirement can be synthetically challenging and
thereby somewhat restrictive, we sought an alternate strategy
to sterically encumber Lewis bases. To this end, we have
considered the notion of converting a Lewis base that would
normally form an adduct with the Lewis acid B(C6F5)3 into
one that participates as a partner in an FLP.
One avenue to restrict access to the reactivity of
a particular functional group is to bury it inside some sort of
cavity, so as to preclude access from other selected reagents.
This is, of course, one of the tactics Nature utilizes to control
the reactivity and selectivity of enzymes.[13] It occurred to us
that the incorporation of a secondary amine into a mechan-
ically interlocked molecule (MIM),[14] such as a [2]rotaxane,[15]
might be a way to conceptually emulate Nature. This strategy
Scheme 1. Synthesis of [2]rotaxanes [1ꢀ24C6] and [1ꢀ22C6] by the
clipping method converts the aniline base 1 into sterically hindered
MIM bases by restricting access to the Lewis basic nitrogen atom.
[*] C. B. Caputo, Prof. D. W. Stephan
anes [1ꢀ24C6] and [1ꢀ22C6] with differently sized macro-
cyclic wheels (24- and 22-membered) and the axle 1. The key
feature of the synthesis is the ring-closing metathesis reaction
facilitated by Grubbsꢀ first-generation catalyst.[16] This reac-
tion occurs while hydrogen-bonding and ion-dipole templat-
ing interactions between the anilinium ion of the protonated
axle [1-H]+ and the oxygen atoms of the crown ether hold the
axle and wheel in close proximity; thus favoring [2]rotaxane
formation.[17] [2]Rotaxanes [1ꢀ24C6] and [1ꢀ22C6] were
isolated in moderate yields after reduction of the residual
double bonds created from olefin metathesis and subsequent
neutralization with base. This [2]rotaxane design utilizes
a short axle with only two nonhydrogen atoms (NH and CH2)
between the stoppering xylene groups, so that the macrocyclic
wheel can only undergo limited translational motion and
Department of Chemistry, University of Toronto
80 St. George Street, Toronto, ON, M5S 3H6 (Canada)
E-mail: dstephan@chem.utoronto.ca
Dr. K. Zhu, V. N. Vukotic, Prof. S. J. Loeb
Department of Chemistry and Biochemistry
University of Windsor
Windsor, ON, N9B 3P4 (Canada)
E-mail: loeb@uwindsor.ca
[**] This research work was supported by NSERC of Canada Discovery
grants and Canada Research Chair awards (S.J.L. and D.W.S.).
C.B.C. and V.N.V. are grateful to NSERC of Canada for the awarding
of Alexander Graham Bell Graduate Scholarships and V.N.V. to the
International Center for Diffraction Data for a Ludo Frevel Scholar-
ship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207783.
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cannot simply slide along the axle and expose the reactive
nitrogen center. Preparation of [2]rotaxanes with smaller
macrocycles was not possible because of ring strain, and of
those with larger macrocycles was not possible because the
rings are too large to be trapped by the xylyl stoppers. An
additional feature of an anilinium [2]rotaxane is the ease of
deprotonation compared to more commonly utilized secon-
dary ammonium analogues.[15i,18]
The 1H NMR spectra (500 MHz, CD2Cl2, 298 K) of
[1ꢀ24C6] and [1ꢀ22C6] show very significant downfield
shifts for the NH protons (c) as compared to the axle
1 because of NH···O hydrogen bonding between the NH
group and the oxygen atoms of the crown ether. The NH
resonance (c) for 1 is observed at 3.96 ppm (Figure 2) but
shifts to 5.12 and 5.61 ppm for [1ꢀ24C6] and [1ꢀ22C6],
respectively (Figure 3). Other axle resonances that are
affected by rotaxane formation are the benzyl protons (d)
and ortho protons (b) and (e), which are involved in weaker
CH···O interactions. The slightly larger shifts for each
resonance in [1ꢀ22C6] as compared to [1ꢀ24C6] can be
attributed to more significant hydrogen bonding, which might
be expected to occur as a result of the smaller size of the 22-
membered macrocycle.
Scheme 2. Reactions of 1, [1ꢀ24C6], and [1ꢀ22C6] with Lewis acid
B(C6F5)3 in the presence of H2(g).
The X-ray structure of [1ꢀ24C6] was determined and is
shown in Figure 1. The 24C6 macrocycle encircles the central
Figure 1. Left: ball-and-stick representation of the X-ray structure of
[1ꢀ24C6]. Only the amine H atom involved in H bonding is shown, all
others are omitted for clarity. Right: space-filling model of the same
view with all H atoms showing the axle in blue and the macrocyclic
wheel in red. The aniline N atom colored dark blue is barely visible.
Figure 2. 1H NMR spectra (500 MHz, CD2Cl2, 298 K) of a) the aniline
1, and b) the adduct [1-B(C6F5)3] formed from 1 and B(C6F5)3.
sets of resonances, a doublet at 4.76 ppm and a triplet at
4.15 ppm. These observations are indicative of the dissym-
À
NHCH2 unit as designed, and a single NH···O hydrogen bond
metry that is created when the B N bond is formed. The
(N···O 3.27 ꢁ, N H···O 1618) is the only substantial non-
11B NMR spectrum (see the Supporting Information) shows
À
À
covalent interaction between the two interlocked compo-
nents. In the space-filling model of the structure of [1ꢀ24C6]
(Figure 1, right), the nitrogen atom of the aniline is colored
dark blue, but only a small portion of the atom is visible, thus
emphasizing how the macrocycle provides significant steric
hindrance to the close approach of any incoming reagent.
Reaction of the sterically unencumbered N-benzylaniline
1 with one equivalent of B(C6F5)3 in CH2Cl2 results in
formation of the classical Lewis acid–base adduct [1-B(C6F5)3]
in 82% yield (Scheme 2). The 1H NMR spectrum of [1-
B(C6F5)3] (Figure 2b) shows that the NH resonance is shifted
downfield to 7.67 ppm, and that a distinct splitting of the
benzylic CH2 group occurs from a solitary doublet into two
a broad singlet at À2.50 ppm, which is also consistent of B N
bond formation.[19] The signals in the 19F NMR spectrum
(Supporting Information) are broad as a result of restricted
À
rotation around the B N bond, resulting from the steric
congestion created by the large substituents on both the B and
N atoms, and the gap between the 19F resonances of the
fluorine atoms in meta and para position is only 6.38 ppm,
indicative of a four-coordinate borate species.[13] Upon cool-
ing to 233 K, the 19F NMR spectrum resolves, so that 13 of the
15 individual fluorine signals (some overlap) could be
observed, again indicating strong adduct formation. Heating
of [1-B(C6F5)3] to 1008C under 4 atm of H2(g) pressure
overnight resulted in no evidence for hydrogen activation.
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To probe whether the macrocyclic ring of a [2]rotaxane
could impart enough steric bulk to a Lewis base to prevent
adduct formation and potentially produce FLP reactivity,
[1ꢀ24C6] was combined with one equivalent of B(C6F5)3 in
toluene. Initial 1H NMR spectra indicated that the two species
were reacting very slowly to give a complex mixture of
unidentifiable products. This is presumed to be an equilibrium
mixture of weak Lewis adducts resulting from the approach of
B(C6F5)3 to the oxygen atoms of the crown ether. This notion
was further supported by the observation that exposure of this
reaction mixture to 4 atm of H2(g) at 1008C led to the clean
formation of the protonated [2]rotaxane cation [1-Hꢀ24C6]+
and the hydridoborate anion [HB(C6F5)3]À (Figure 3) consis-
indicated clean protonation of the [2]rotaxane. The corre-
sponding 2H NMR spectrum showed signals at 9.06 and
3.47 ppm corresponding to the ND and BD fragments. The
11B NMR spectrum also showed a broad resonance at
À25 ppm, the same region in which the borohydride anion
normally shows up. This data unambiguously demonstrates
that [1ꢀ24C6] is capable of acting as a bulky Lewis base in the
FLP-mediated heterolytic cleavage of molecular hydrogen in
the presence of the bulky Lewis acid B(C6F5)3.
Given this success with [1ꢀ24C6], the impact of a smaller
crown ether macrocycle was of interest, as it might reduce the
flexibility of the ring and concomitantly increase the steric
restrictions around the nitrogen center. The reaction of
[1ꢀ22C6] containing a smaller 22-membered crown ether
ring with B(C6F5)3 without an atmosphere of hydrogen gas led
to similar results as observed with the larger crown ether.
However, once the reaction mixture was placed under an
atmosphere of hydrogen gas at room temperature, an oily
product immediately began to precipitate. The product was
characterized and confirmed to be the hydrogen-activated
product [1-Hꢀ22C6][HB(C6F5)3]. The 1H NMR spectrum
shows the NH2 resonance as a broad singlet at 9.33 ppm as
well as a multiplet for the benzyl CH2 at 4.92 ppm. The
11B NMR spectrum (Supporting Information) is also indica-
tive of H2 cleavage, because a borohydride signal was
1
observed at À25.48 (d, JBH = 90 Hz). The product was also
characterized by high-resolution ESI mass spectrometry (see
the Supporting Information).
In conclusion, the heterolytic activation of H2(g) using
[1ꢀ22C6] or [1ꢀ24C6] and B(C6F5)3 demonstrates that the
concept of incorporating a Lewis base into a mechanically
interlocked molecule (MIM) is a valid methodology for
Figure 3. 1H NMR spectra (500 MHz, CD2Cl2, 298 K) of a) [1ꢀ24C6],
b) [1-Hꢀ24C6]+ (product of the reaction of [1ꢀ24C6] with B(C6F5)3
and H2(g)), c) [1ꢀ22C6], and d) [1-Hꢀ22C6]+ (product of the reaction
of [1ꢀ22C6] with B(C6F5)3 and H2(g)).
generating
a sterically hindered base. Moreover, the
increased reactivity with H2 observed for [1ꢀ22C6] versus
[1ꢀ24C6] confirms that the degree of steric protection
experienced by the Lewis basic nitrogen atom can be fine-
tuned by judicious choice of the encircling macrocycle. Thus,
the present results outline a strategy for the transformation of
a sterically unencumbered base for entry into the FLP
reactivity regime, without covalent modification. The further
exploitation of this approach is the subject of ongoing efforts
in our laboratories.
tent with FLP activation of H2. When this reaction was carried
out in hexanes, the [1-Hꢀ24C6][HB(C6F5)3] salt cleanly
precipitated from the solution as a yellow/orange oil in 60%
yield (Scheme 2). The 1H NMR spectrum showed a broad but
definitive NH2 signal at 9.15 ppm and a benzyl CH2 signal at
4.81 ppm, matching a sample of [1-Hꢀ24C6][BF4]. The
11B NMR spectrum (Supporting Information) showed a dis-
tinctive doublet at À25.46 ppm (1JBH = 92 Hz), corresponding
to formation of the hydridoborate anion [HB(C6F5)3]À, which
was corroborated by the existence of a small meta–para gap of
2.84 ppm in the 19F NMR spectrum (Supporting Information).
Presence of the H2-activated species was also verified by high-
resolution ESI mass spectrometry in positive and negative
mode, which clearly showed m/z values for the cation and
anion, respectively (see the Supporting Information).
Experimental Section
NMR experiments were recorded on Bruker Avance-III 400 MHz
and Bruker Avance 500 MHz NMR spectrometers. Details of the
syntheses and spectroscopic characterization of all new compounds
can be found in the Supporting Information.
X-ray data for [1ꢀ24C6]: C35H57NO6, M = 587.82, colorless
prisms (0.38 ꢂ 0.28 ꢂ 0.20 mm), monoclinic, P21/c, a = 20.613(5), b =
19.349(5), c = 18.562(4) ꢁ, b = 105.105(3)8, U = 7148(3) ꢁ3, Z = 8,
1calcd = 1.092 gcmÀ3, m = 0.073 mmÀ1, min/max trans. = 0.9857, MoKa
l = 0.71073 ꢁ, T= 173.0(2) K, 66297 total reflections (R(int) =
0.0685), R1 = 0.1066, wR2 = 0.1946 [I > 2sI], R1 = 0.2740, wR2 =
0.3549 [all data], GoF(F2) = 1.014, data/variables/restraints = 12574/
757/0. The SHELXTL library of programs[20] was used for X-ray
solutions and figures were drawn with CrystalMaker software.[21]
CCDC 902860 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
The corresponding reaction of [1ꢀ24C6] and B(C6F5)3
under 4 atm of D2 under analogous conditions afforded an oil,
which was isolated and determined to be the corresponding
deuterium-incorporated salt [1-Dꢀ24C6][DB(C6F5)3]. This
1
observation was confirmed by the H NMR spectrum, which
gave an NH2 proton integration of less than 1H, indicating
deuterium incorporation, while the remaining chemical shifts
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Received: September 26, 2012
Published online: November 26, 2012
Keywords: frustrated Lewis pairs · hydrogen activation ·
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interlocked molecules · macrocycles · rotaxanes
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