A toggle nanoswitch alternately controlling two catalytic reactions

Article abstract of DOI:10.1002/anie.201408457

Reversible switching between two states of the triangular nanoswitch [Cu(1)]⁺ was accomplished by alternate addition of 2-ferrocenyl-1,10-phenanthroline (2) and copper(I) ions. The two switching states regulate the binding and release of two di

Full text of DOI:10.1002/anie.201408457

Angewandte
Chemie
DOI: 10.1002/anie.201408457
Switchable Catalysis
A Toggle Nanoswitch Alternately Controlling Two Catalytic
Reactions**
Soumen De, Susnata Pramanik, and Michael Schmittel*
Dedicated to Professor Christoph Rꢀchardt on the occasion of his 85th birthday
Abstract: Reversible switching between two states of the
triangular nanoswitch [Cu(1)]+ was accomplished by alternate
addition of 2-ferrocenyl-1,10-phenanthroline (2) and cop-
per(I) ions. The two switching states regulate the binding and
release of two distinct catalysts, piperidine and [Cu(2)]+, in
a fully interference-free manner and allow alternating on/off
switching of two orthogonal catalytic processes. In switching
state I, piperidine is released from the nanoswitch and catalyzes
a Knoevenagel addition between 4-nitrobenzaldehyde and
diethyl malonate (ON-1 and OFF-2), while in state II the
released [Cu(2)]+ catalyzes a click reaction between 4-nitro-
phenylacetylene and benzylazide (OFF-1 and ON-2). Upon
addition of one equivalent of 2 to the (OFF-1 and ON-2)-state,
both catalytically active processes are shut down (OFF-1 and
OFF-2).
In FBPA/P,[3] the morphological changes at the enzymeꢀs
active site are brought about by a flip of the Schiff base loop
while the lid loop and the C-terminal loop close. Extensive
conformational reorganization that is linked to a change in
functionality is reminiscent of artificial nanomechanical
switches (here: nanoswitches) that have emerged only
recently.[6,7] It occurred to us that the new family of triangular
nanoswitches as developed by us over the past three years
with their 2 nm reorganization at their switching arm[8,9] may
be suitable to bring about an analogous dual alternating
catalytic scenario using exclusively artificial components. At
present, only a handful of nanomechanical switches are
known that allow regulation of catalytic processes, for
example, Rebekꢀs light-controlled azobenzene–calixarene
switch modulating a Knoevenagel condensation,[10] Mirkinꢀs
weak-link-controlled Diels–Alder cycloaddition,[11] Shibasa-
kiꢀs acylation,[12] Feringaꢀs motor[13] and Leighꢀs rotaxane both
of which catalyze a conjugate addition,[14] plus our large-
amplitude ON/OFF nanoswitches controlling the catalysis of
a Knoevenagel addition[8] as well as a cis–trans isomeriza-
tion.[9]
Herein we describe how the chemically toggled nano-
switch [Cu(1)]+[9] is able to alternately regulate two different
catalyzed reactions—a Knoevenagel addition and a click
reaction—depending on the respective switching state
(Scheme 1). To the best of our knowledge such dual
alternating catalysis employing an artificial nanoswitch is
unprecedented. As such, it may only be remotely compared
with a recent tandem reaction that was catalyzed by both an
artificial host–guest complex and an enzyme.[15]
If one considers possible options for the design of
a nanoswitch that administrates two catalytic reactions in an
alternating manner, one has to bear in mind that both inputs
for reversible switching between the two states as well as both
catalytic processes have to operate in a fully orthogonal[16]
manner at exactly the same temperature and during the same
time. Quite obviously, the demands for the non-interference
of all participants (two inputs for switching, two catalysts, two
sets of reactants and products) severely limit the number of
options and require high flexibility for the optimization of
a two-state dual-catalytic system. Furthermore, one has to
decide the fundamental issue of whether the switch itself will
act as a catalyst[9] or whether the switch should regulate two
catalytically active species by strong binding (inhibition) and
release into solution (Scheme 2).[8] For the present paper, we
have focused on the latter design because it is much more
flexible toward probing a large variety of catalytic processes.
N
ature has often been a fruitful source of inspiration for
chemists in their quest for promising research topics.[1] For
decades, the mode of action of unusual enzymes, for example,
ribonucleotide reductase,[2] has fascinated chemists spurring
numerous model investigations. More recently, a paper by
Fushinobu and co-workers[3] has drawn attention to another
fascinating biological phenomenon: an enzyme that reorgan-
izes its active site for catalyzing two distinct processes.
Accordingly, the fructose-1,6-biphosphate aldolase/phospha-
tase (FBPA/P) is able to use a lysine residue in the active site
for aldolase activity whereas when an alternative substrate is
bound the enzyme undergoes major conformational rearrang-
ments and the resulting magnesium aspartate complex
displays phosphatase activity. As a result, the mentioned
enzyme operates as a substrate-triggered switch with the
effect that in state I (with substrate I) only catalysis I is ON,
while catalysis II is OFF. Reciprocally, in state II only
catalysis II is ON and catalysis I is OFF. Such clear-cut
alternation between catalytic processes in biology is rare[4]
and only demonstrated to a certain extent by other promis-
cuous enzymes.[5]
[*] Dr. S. De, Dr. S. Pramanik, Prof. Dr. M. Schmittel
Center of Micro and Nanochemistry and Engineering
Organische Chemie I, Universitꢀt Siegen
Adolf-Reichwein-Strasse 2, 57068 Siegen (Germany)
E-mail: schmittel@chemie.uni-siegen.de
[**] We are grateful to the DFG (Schm 647/19-1) and the University of
Siegen for financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201408457.
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Scheme 3. Self-sorting of diimine ligands in the presence of copper(I)
ions. Fc=ferrocenyl.
a heteroleptic copper(I) complex (Scheme 1).[18] Upon addi-
tion of one equivalent of 2-ferrocenylphenanthroline (2), the
azabipyridine arm detaches from the phenanthroline station
and binds to the zinc(II) porphyrin station. At the same time,
2 forms an intermolecular heteroleptic complex at the
phenanthroline station (Scheme 1, step I), most likely driven
by the stabilization gained in realizing the pyridine!zinc(II)
porphyrin interaction. This switching process was docu-
mented by several spectroscopic techniques but it was also
visible to the naked eye since the color of the solution
immediately changed from deep pink to greenish violet. In
Scheme 1. Toggling of nanoswitch 1 upon chemical input.
1
the H NMR spectrum, for example, signals for protons a-H
and b-H of the azabipyridine unit appear in the aliphatic
region at 3.31 and 2.87 ppm, respectively, far upfield from
their original positions at 7.30 ppm[9] in [Cu(1)]+. Addition-
ally, the appearance of new peaks for the mesityl protons 9-H
and 10-H at 6.30, 6.16, 6.15, and 6.11 ppm (Figure 1b) and
their concomitant disappearance at 6.37, 6.26, 6.16, and
5.93 ppm (Figure 1a) are indicative of the new complex. The
involvement of ligand 2 in the complexation is evident as the
1H NMR signals of the ferrocenyl group at 5.24, 4.49, and
4.07 ppm (Figure S1) shifted and split, particularly in the
Scheme 2. The principle of regulating two different catalytic processes
by a nanoswitch with two stations. In switching state I, station II is
used to firmly bind catalyst 2 thereby inhibiting its catalytic activity. At
the same time, catalyst 1 is available in solution to catalyze process 1.
After switching to state II, catalyst 2 separates from station II and thus
reaction 2 is catalyzed in solution. Concurrently, catalyst 1 is inhibited
by binding to station I.
First, we needed adequate chemical inputs suitable for
both switching and catalysis. In our quest for electron-rich and
thus strongly metal-ion-binding ligands we realized that after
addition of 2-ferrocenylphenanthroline (2) to complex
[Cu(3)(4)]+ (Scheme 3),[17] which is isostructural to the
metal complexation site in [Cu(1)]+ (Scheme 1), self-sorting
furnished a 1:1 mixture of [Cu(2)(3)]+ and 4 in 98% yield (see
Supporting Information: Figure S11. LogK2 = 3.9 ꢀ 0.2 for
[Cu(3)]+ + 2). Notably, upon addition of a further equivalent
of Cu+, the mixture reshuffled in a second self-sorting to
[Cu(3)(4)]+ and [(Cu(2)]+ in 90% yield (Figure S6).
Based on the above self-sorting, we selected the known
nanoswitch[9] [Cu(1)]+ as our starting point and phenanthro-
line 2 and Cu+ as inputs to toggle the rotary arm. In
[Cu(1)]+—generated by adding one equivalent of [Cu-
(CH3CN)4]PF6 to 1[9] in [D2]dichloromethane—the azabipyr-
idine arm is anchored to the phenanthroline station in
Figure 1. Partial 1H NMR spectra (400 MHz, CD2Cl2, 298 K) of the
switching process representing: a) [Cu(1)]+ (red peaks); b) [Cu(1)(2)]+
formed after addition of one equivalent of 2 to (a);* c) [Cu(1)]+ and
[Cu(2)]+ (dark blue peaks) generated after addition of one equivalent
of [Cu(CH3CN)4]PF6 to (b); d) [Cu(1)]+ and [Cu(2)2]+ (green peaks)
formed after addition of one more equivalent of 2 to (c); e) addition of
another equivalent of 2 to (d) regenerates spectrum (b) along with
[Cu(2)2]+ as a byproduct. * The colors distinguish the proton signals of
the azabipyridine (cyan), mesityl (magenta), and ferrocenyl (orange)
units.
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cyclopentadienyl ring attached to the phenanthroline, to 4.27,
4.23, 3.73, and 3.65 ppm, respectively (Figure 1b). The UV/
Vis changes were equally conclusive as the Q-band at 550 nm
shifted completely to 562 nm in a titration of [Cu(1)]+ against
2 (Figure 2).
Figure 2. UV/Vis titration of [Cu(1)]+ with 2 (requires 4.5 equiv due to
the low concentration of 10ꢁ5 m). The Q band completely shifts from
550 to 562 nm.
Figure 3. ESI-MS of the mixture obtained in step II of the switching
process. It contains [Cu(1)]+ and [Cu(2)]+. Inset: Experimental and
theoretical (top) isotopic distribution.
Kinetic studies at room temperature on the switching of
[Cu(1)]+ with 2 were conducted at 2.5 ꢁ 10ꢁ6 m with UV/Vis
data being collected at 1 min intervals. The Soret band at
422 nm shifted completely to 429 nm within 26 min (Fig-
ure S12). The data for switching followed a first-order rate
law at a half-life t1/2 = 233 s (Figure S13). Due to the first-
order behavior, the monomolecular breakup of complex
[Cu(1)]+ is the rate-determining step.
experimental distributions. As a result, all data unambigu-
ously indicate the full relocation of the azabipyridine arm
from the zinc(II) porphyrin to the phenanthroline station.
This switching can be performed reversibly by removing Cu+
with a stronger binding ligand such as cyclam (Figure S10).
In step III (Scheme 1), addition of one further equivalent
of 2 in [D2]dichloromethane led to the formation of the
homoleptic complex [Cu(2)2]+ without disturbing the nano-
switch [Cu(1)]+. As expected, the azabipyridine arm did not
change its position as the signals of the mesityl protons
remained unaffected at 6.37, 6.26, 6.21, and 5.93 ppm (Fig-
ure 1d). The new peaks at 4.80, 4.58, and 3.60 ppm are
indicative of the homoleptic copper complex [Cu(2)2]+,
because they are identical to the NMR signals obtained
when complex [Cu(2)2]+ was prepared individually from 2 and
[Cu(CH3CN)4]PF6 (2:1) (Figure S7). Thus, by the alternate
addition of 2 and Cu+ to state 0 = [Cu(1)]+, the two states I
and II of the nanoswitch 1 can be produced quantitatively.
After adding one more equivalent of 2, [Cu(1)]+ (= state 0) is
regenerated, however, with [Cu(2)2]+ as a byproduct (Fig-
ure 1e).
Having managed the reversible switching between the two
states I and II by adding chemical inputs, we investigated the
efficacy of the nanoswitch in regulating two different catalytic
reactions by means of inhibition and release of catalysts. To
set up two catalytic reactions in concurrent presence of all
chemical inputs and reagents, we had to identify two non-
interfering reaction systems compatible with the orthogonal
inputs. After extensive exploration, we finally selected
a [Cu(2)]+-catalyzed click reaction and a piperidine-catalyzed
Knoevenagel addition.
In step II of the switching process (Scheme 1), one equi-
valent of Cu+ was added to complex [Cu(1)(2)]+ in
[D2]dichloromethane, which quantitatively generated
[Cu(1)]+ and [Cu(2)]+. In contrast to step I, this process,
visible by the color change of from greenish violet to deep
pink, was rather slow, so that the solution had to be heated for
a few minutes at 408C. As a further sign of switching, the
1H NMR spectrum provided three types of characteristic
shifts (Figure 1c): First, the signals of protons a-H and b-H of
the azabipyridine arm are now shifted from 3.31 and 2.87 ppm
to 7.30 ppm, clearly indicating that the unit is no longer in the
shielding region of the zinc porphyrin. Second, a new set of
signals for the mesityl protons appeared at 6.37, 6.26, 6.21, and
5.93 ppm, characteristic of the intramolecular complex
[Cu(1)]+.[9] Third, signals for the ferrocenyl protons now
emerged at 5.24, 4.63, and 4.16 ppm, that is, at shifts that are
distinctive for [Cu(2)]+ (Figure S7). The UV/Vis spectrum
showed a Q-band absorption at 550 nm, which precludes the
possibility of axial coordination at the zinc porphyrin, as the
latter would give rise to a band at 562 nm. The kinetics of
switching was evaluated with UV/Vis spectroscopy at 5 ꢁ
10ꢁ6 m. However, even after 3 h at room temperature there
were no changes in the absorption. We then conducted the
kinetic measurements at higher concentration (0.8 mm of
1
both Cu+ and [Cu(1)(2)]+) using H NMR spectroscopy. The
Analyzing the switching as depicted in Scheme 1, it is
apparent that complex [Cu(1)(2)]+ (= state I)—due to its
intramolecular azabipyridine!zinc(II) porphyrin linkage—
should not be able to harbor an additional guest like
piperidine at the zinc(II) porphyrin. Added piperidine (8)
should therefore be available as a catalyst for a Knoevenagel
data revealed that the switching was almost complete (98%)
within 13 min (Figure S9). The ESI-MS of the resultant
complex showed molecular ion peaks at m/z 426.9 for
[Cu(2)]+ and at m/z 1800.4 for [Cu(1)]+ (Figure 3). The
theoretical isotopic distributions for both species match the
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[Cu(2)(3)]+ while piperidine was
released (by mixing 2, 3, 5, 6, 8, 9,
10,
and
Cu+
in
1:1:10:10:1:10:1000:1). Here, the
Knoevenagel addition product 11
formed in (34 ꢀ 4)% yield while the
click product was not detectable in
the NMR spectrum (Figure S18).
Finally, the nanoswitchꢀs poten-
tial to alternately control two cata-
lytic processes had to be evaluated.
For this, an NMR tube loaded with
complex
[Cu(1)]+
(0.8 mm; =
state 0) and compounds 5, 6, 8, 9,
and 10 in a 1:10:10:1:10:1000 ratio
was heated (558C for 3 h). No
detectable amounts of products 7
or 11 were formed as evidenced by
NMR spectroscopy (OFF-1 and
OFF-2) (Figure S20a). After addi-
tion of one equivalent of 2 and thus
formation of [Cu(1)(2)]+, heating
under the same conditions resulted
in the Knoevenagel product 11 in
(35 ꢀ 4)% yield (Figure S20b),
Scheme 4. Reversible switching controls two orthogonal catalytic reactions in two different switching
states.
addition reaction (Scheme 4, left). In contrast, [Cu(2)]+ is
strongly bound in [Cu(1)(2)]+. Upon addition of one equiv-
alent of copper(I) ions to [Cu(1)(2)]+ and generation of
state II, the catalytic activity should switch because now
piperidine is strongly bound to the liberated zinc(II) porphy-
rin station and hence Knoevenagel catalysis should stop. At
the same time, [Cu(2)]+ is detached and should be available to
catalyze the click reaction (Scheme 4, right).
We first confirmed by NMR analysis that the reaction
(558C, 3 h) between 9 (8.2 mm) and 10 (820 mm) in the
presence of piperidine (8) was completely inhibited by zinc
tetraphenylporphyrin (ZnTPP) (both 0.82 mm), whereas in
absence of ZnTPP the Knoevenagel reaction product 11 was
observed in (34 ꢀ 2)% yield (Figure S16). Using the same
concentrations and conditions,[19] we found that the reaction
of 5 with 6 in the presence of [Cu(2)]+ (10:10:1) produced the
click product 7 in (55 ꢀ 2)% yield (Figure S15), whereas the
corresponding hetphen complex [Cu(2)(3)]+ was unable to
drive the above transformation (Figure S17). Furthermore, in
a control experiment, the reaction of [Cu(CH3CN)4]+ with 5
and 6 (1:10:10) afforded the click product (Figure S21), but at
a much lower yield (25%). The latter finding clearly proves
that in concurrent presence of both phenanthroline 2 and
[Cu(CH3CN)4]+ (1:1) only complex [Cu(2)]+ is the active
catalyst.[20]
while the click reaction product 7 was not observed (ON-
1 and OFF-2). After the addition of one more equivalent of
Cu+ to the above mixture,[21] the azabipyridine arm switched
from the zinc porphyrin to the phenanthroline station
releasing [Cu(2)]+ into the solution. Simultaneously,
[Cu(1)]+ removed free piperidine (8) from the solution by
the strong piperidine!zinc(II) porphyrin binding. When this
mixture was heated, again at 558C for 3 h, it furnished the
click reaction product 7 in (50 ꢀ 2)% yield, while no further
Knoevenagel addition reaction product 11 was formed, thus
representing the OFF-1 and ON-2 mode of the dual
alternating catalytic system (Figure S20c). Both catalytic
processes were switched OFF by adding[22] one equivalent
of 2 thus masking [Cu(2)]+ as the homoleptic complex
[Cu(2)2]+, while piperidine remained bound to state
[Cu(1)]+ (Figure S20d).
It can be seen that the complexity of the above system is
far beyond that of previous artificial molecular switching
processes, as in this ten-component reaction system all
switching states, inputs, reactants, and products have to act
orthogonally to one another. For instance, none of the
prominent donor centers in 6, 7, 9, and 10 is allowed to
displace, even partly, the piperidine bound to [Cu(1)]+ as
otherwise the Knoevenagel reaction in state II (Scheme 4,
right) would not remain switched OFF.
To evaluate the mutual compatibility of the Knoevenagel
and click reactions in a shared solution and in the presence of
all reagents (and of the later formed products), we firstly
checked the catalysis triggered in switching state II
(Scheme 4, right). Indeed, the reaction (558C, 3 h) of
compounds 2, 5, 6, 8, 9, 10, Cu+, and ZnTPP in
a 1:10:10:1:10:1000:1:1 ratio gave rise to the click product 7
in (51 ꢀ 2)% yield, but no Knoevenagel addition product
(Figure S19). To evaluate state I, Cu+ was masked as
In conclusion, we present a new strategy to reversibly
address the switching states of a two-state nanoswitch by
adding phenanthroline 2 and copper(I) ions and thus to
obtain control over ON/OFF regulation of two catalytic
reactions. As nanoswitch 1 is able to up- and down-regulate
two different catalytic reactions, one from each state, it
mimics in a remote manner the operating principle of the
FBPA/P aldolase/phosphatase. The reaction system presented
in Scheme 4 serves as proof that ten components can
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Int. Ed. 2011, 50, 6415; Angew. Chem. 2011, 123, 6539; d) S.
cooperate in an interference-free and functional manner
within switching processes. It thus represents a fascinating
example of the regulation of complex artificial systems and
opens up new venues in systems chemistry.
Venkataramani, U. Jana, M. Dommaschk, F. D. Sçnnichsen, F.
Tuczek, R. Herges, Science 2011, 331, 445; e) Y. S. Chong, B. E.
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Received: August 22, 2014
Published online: October 27, 2014
Keywords: homogeneous catalysis · host–guest systems ·
nanomechanics · orthogonal coordination · switching
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