A trio of nanoswitches in redox-potential controlled communication

Article abstract of DOI:10.1039/c4cc05773b

A potential-controlled two-step bidirectional communication protocol between the nanoswitches [Cu(1)]⁺, 2 and 3 is set up, in which ligand followed by metal-ion oxidation drives two subsequent metal ion translocations (self-sorting) changing th

Full text of DOI:10.1039/c4cc05773b

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A trio of nanoswitches in redox-potential
controlled communication†
Cite this: Chem. Commun., 2014,
50, 13254
Susnata Pramanik, Soumen De and Michael Schmittel*
Received 24th July 2014,
Accepted 3rd September 2014
DOI: 10.1039/c4cc05773b
www.rsc.org/chemcomm
A
potential-controlled two-step bidirectional communication
protocol between the nanoswitches [Cu(1)]+, 2 and 3 is set up, in
which ligand followed by metal-ion oxidation drives two subsequent
metal ion translocations (self-sorting) changing the switching state
at each switch. The communication is reset to its starting point by a
two-electron reduction.
Life processes are sustained through intricate communication and
signaling networks involving recognition, processing and amplifi-
cation of information, sometimes through cascading pathways.1
While multistep signaling, such as in the metallothionein protein,2
is often encountered in biology, artificial switches3 or machines4
are rarely networked by chemical communication,5 and if, they are
engaged in single-step protocols.6 Intermolecular communication
involving major rearrangement at two switches has been reported
by the group of Aprahamian6c and more recently by us.6d In the
former case, being reminiscent of a biological proton relay, bind-
ing of Zn2+ ions induced a toggling at the first switch triggering
proton release, which stimulated a change in the switching state at
the second switch. Our recent work6d demonstrated bidirectional
Scheme 1 Reversible cascade switching among three nanoswitches.
reversible communication between the nanoswitches 2 and 3 one-electron redox inputs. Despite integrating an already reported
using redox equivalents as input signals (Scheme 1). The strategy redox-controlled communication scenario, i.e. between switches 2
was based on two orthogonal7 coordination scenarios developed and 3,6d the realisation of two-step communication between the
by us over the years. Switch 2 equipped with its intramolecular trio 1–3 adds a new level of complexity requiring to devise a
bipyridine and phenanthroline (HETPHEN8) coordination site perfectly matching nanoswitch 1 and a potential control for the
prefers Cu+ ions, whereas 3 with its terpyridine and phenanthro- two-step translocation. The key criteria for our design were: (1) the
line (HETTAP8) binding motif favours Cu2+ or Zn2+.9 Due to these two oxidation steps have to occur at separate potentials each
preferences, the nanoswitch duo [Cu(2)]+ and 3 experienced a followed by a clean self-sorting10 propelling the communication
copper translocation generating 2 and [Cu(3)]2+ upon oxidation, cascade. (2) The new electron-rich switch 1 should have a higher
while the starting point was reset by reduction.6d
affinity toward Cu+ than the known HETPHEN- and HETTAP-
Here, we describe a two-step bidirectional communication based switches 2 and 3 and (3) the lowest oxidation potential.
protocol between the three nanoswitches 1–3 using two subsequent (4) After oxidation, nanoswitch 1 should be the weakest binding
ligand for Cu+ and (5) also should rapidly release Cu+.
Taking all the requirements into account, we considered
Center of Micro and Nanochemistry and Engineering, Organische Chemie I,
2-ferrocenyl-[1,10]-phenanthroline and 6-ferrocenyl-2,20-bipyridine
¨
Universitat Siegen, Adolf-Reichwein-Str. 2, D-57068 Siegen, Germany.
as possible binding motifs in 1, expecting that the electron-
donating ferrocenyl group11 next to the chelate site would not
only provide a sizeable gain in binding strength but also ensure
E-mail: schmittel@chemie.uni-siegen.de
† Electronic supplementary information (ESI) available: Synthesis, characterisa-
tion, and switching studies. See DOI: 10.1039/c4cc05773b
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release of the metal ion upon oxidation of the ferrocene. In an
initial screening, 2,9-dimesityl-[1,10]-phenanthroline provided
a stronger binding heteroleptic copper(I) complex in combination
with 2-ferrocenylphenanthroline, but after oxidation copper remained
strongly bound. As a result, we turned to 6-ferrocenyl-2,20-bipyridine
despite its lower binding quality for Cu+.
Based on these considerations, we devised a new nanoswitch
1 following our successful triangular design12 (Scheme 1), expecting
the kinetics of metal-ion release to be fast. Its main competitor for
Cu+, i.e. nanoswitch 2, should be at disadvantage in the competition,
because copper(I) complexation at 2 goes along with loss of the
pyrimidine-zinc(II) porphyrin interaction.
While the preparation of nanoswitches 212b and 36d has been
reported previously, nanoswitch 1 was synthesised in 60% yield
from its direct precursor via a Sonogashira coupling (for details, see
the ESI†). Upon reacting switch 1 with Cu+, the ferrocene protons at
the substituted cyclopentadienyl ring lost their symmetry and shifted
Fig. 1 Partial 1H NMR spectra (400 MHz, CD2Cl2, 298 K) of switches 1–3
in the presence of metal ions after equilibration. The traces represent:
(a) Formation of a 90 : 10 ratio of [Cu(1)]+ and [Cu(2)]+ from a 1 : 1 : 1 mixture
of 1, 2 and Cu+. (b) Sole formation of [Cu(1)]+ from a 1 : 1 : 1 mixture of 1,
3 and Cu+. (c) Exclusive formation of [Zn(1)]2+ from a 1 : 1 : 1 mixture of 1,
2 and Zn2+. (d) Complete formation of [Zn(3)]2+ from a 1 : 1 : 1 mixture of 1,
3 and Zn2+. (e) Mixing of 1, 2, 3 and Cu+ (1 : 1 : 1 : 1) generates 90% of
[Cu(1)]+ and 10% of [Cu(2)]+ leaving 3 free in solution. (f) Exclusive
formation of [Zn(3)]2+ when 1, 2, 3 and Zn2+ (1 : 1 : 1 : 1) were mixed.
1
upfield to 3.32, 3.67, 4.54 and 4.92 ppm in the H NMR spectrum
due to the proximity of the shielding phenanthroline. Additionally,
the mesityl protons shifted upfield to 6.35 and 6.43 ppm (Fig. S5,
ESI†). The ESI-MS spectrum displays a single peak at m/z = 1155.5 Da
with its isotopic distribution exactly matching the theoretical one of
complex [Cu(1)]+ (Fig. S7, ESI†).
The exclusive selectivity for [Cu(1)]+ over switch 3 is warranted
The cyclic voltammograms show fully reversible redox Fc0/+ due to the fact that 2 has a much higher binding constant for
waves for 10/+ at E1/2 = 450 mVSCE (Fig. S3, ESI†) and for [Cu(1)]+/2+ Cu+ than switch 3 (Fig. S14, ESI†). For a kinetic analysis, we used
at E1/2 = 610 mVSCE, while Cu+/2+ oxidation in [Cu(1)]2+/3+ UV-Vis spectroscopy (Fig. S15, ESI†) at c = 0.1 mM, because the
emerged at E1/2 = 940 mVSCE (Fig. S6, ESI†). On the basis of metal-ion transfer is readily detectable due to the reorganisation
the data, we found that Cu+ is less strongly bound in [Cu(1)]2+ of the azaterpyridine arm from the phenanthroline to the zinc
than in [Cu(1)]+ by Dlog K = 2.7.
porphyrin station. Addition of 1 to a solution of [Cu(3)]+ at room
As a model for paramagnetic [Cu(1)]2+, we characterised temperature entailed a shift of the Q band absorption from 550
diamagnetic [Zn(1)]2+, prepared from stoichiometric amounts of 1 to 561 nm within 3 min in dichloromethane (DCM).
and Zn(OTf)2. The splitting patterns of the diagnostic protons are
We chose Zn2+ as a diamagnetic input to check the face-to-face
similar as for [Cu(1)]+ but they appear at different positions in the preferences of the switches for Cu2+. Although in the competition
1H NMR spectrum. For instance, protons from the substituted cyclo- between nanoswitches 1 and 2, it was complex [Zn(1)]2+ that formed
pentadienyl ring of ferrocene appear at 3.35, 3.84, 4.37 and 5.48 ppm. exclusively with switch 2 remaining untouched (Fig. 1c and Fig. S16,
Moreover, mesityl protons are shifted to 6.03 and 6.43 ppm (Fig. S8, ESI†), the pentacoordinated complex [Zn(3)]2+ was afforded as a
ESI†). Formation of the complex was also proved from ESI-MS data single product (Fig. 1d and Fig. S17, ESI†) from a mixture of 1, 3
with a peak at m/z = 578.6 Da and an isotopic distribution matching and Zn2+ (1 : 1 : 1). The latter result is quite surprising because
the one expected for [Zn(1)]2+ (Fig. S9, ESI†).
formation of [Zn(3)]2+ goes along with the loss of a azaterpyridine-
To address the first step of the new communication protocol, zinc(II)porphyrin interaction. The outcome thus reflects the low
switches 1 and 2 were treated with Cu+ (1: 1 : 1). After equili- stability of tetracoordinated zinc(II) ions. Indeed, the typical
bration at 40 1C for a few minutes, 90% of copper(I) was found on 1H NMR signals of the free switch 1 were preserved at 4.06, 4.32
1
[Cu(1)]+ (Fig. 1a). The selectivity was ascertained from H NMR and 5.57 ppm (from the ferrocene unit), whereas the diagnostic
and UV-Vis data. Reciprocally, addition of switch 1 to a solution mesityl protons of switch 3 shifted upfield and splitted into four
of [Cu(2)]+ (2.70 mM) led to the same results as documented by sets appearing at 6.07, 6.18, 6.24, and 6.27 ppm. The same set of
the appearance of new signals at 2.87 and 3.31 ppm being typical signals is seen in the separately prepared [Zn(3)]2+ complex.6d Thus,
for the free nanoswitch 2 with its pyrimidine ring being immersed Cu2+, due to its similar coordination demands as Zn2+, should bind
into the shielding current of the zinc porphyrin ring (Fig. S11, selectively to nanoswitch 3 over 1 and even more over 1+.
ESI†). At the same time, emergence of signals at 3.32, 3.67, 4.54 and
Because of the rather slow rate preventing a kinetic study using
4.92 ppm along with upfield shifted mesityl protons (6.35 and 6.43 cyclic voltammetry, we initiated redox-triggered communication by
ꢁ
ppm) suggests formation of [Cu(1)]+. For the communication step adding TBPA+ SbCl6 (tris(4-bromophenyl)aminium hexachloro-
[Cu(2)]+ + 1, the NMR spectrum shows that full equilibrium was antimonate) for oxidation and dmfc (decamethylferrocene) for
reached 58 min after mixing at room temperature (Fig. S12, ESI†). reduction. To evaluate the subsequent step, a solution of [Cu(1)]+
At a lower concentration (c = 0.1 mM) metal translocation from and 2 (at 1 : 1 ratio, 0.1 mM) in DCM was treated with one
[Cu(2)]+ to 1, followed by UV-Vis spectroscopy at room temperature, equivalent of TBPA+ꢀ SbCl6ꢁ. As a result, the Q band of 2 shifted
ꢀ
took over a day to reach the equilibrium (Fig. S13, ESI†).
from 561 to 550 nm and became constant after 4 min at room
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Fig. 2 (a) A solution of [Cu(CH3CN)4]B(C6F5)4, 1 and 2 = 1 : 1 : 1 at 10ꢁ4 M in
dry DCM was treated with 1 equiv. of TBPA+ SbCl6ꢁ (black trace), then with
ꢀ
1 equiv. of dmfc (red trace, also see the text). (b) UV-Vis changes at the
Q band showing the communication between 1–3 and Cu+ (1: 1 : 1 : 1; at
2 ꢂ 10ꢁ5 M) as a result of two oxidation and two reduction steps (see text).
temperature (Fig. 2a, black trace). The change is in full agree-
ment with a Cu+-induced swing of the nanoarm in 2 from the
zinc(II) porphyrin to the phenanthroline station and thus
formation of [Cu(2)]+ and 1+. This interpretation was further
corroborated by ESI-MS peaks at m/z = 1093.4 and 1799.2 Da,
indicating the presence of switches 1+ and [Cu(2)]+, respectively
(Fig. S20, ESI†). For a reversal of the process, the above solution was
reacted with dmfc (one equiv.). The absorption peak shifted back
to 561 nm and became constant after 18 h (Fig. 2a, red trace). The
ESI-MS spectrum of the solution confirmed the reverse transloca- Fig. 3 ESI-MS signature (a) of a solution containing 1, 2, 3 and Cu+ (1 : 1 : 1)
tion of Cu+ as it displays mainly the peak at m/z = 1155.5 Da for
in DCM. The solution after oxidation with (b) one or (c) two equiv. of
TBPA+ SbCl6ꢁ. (d) After reduction of (c) with two equiv. of dmfc. Full
ꢀ
[Cu(1)]+ and a small one at 1799.2 Da for [Cu(2)]+, reflecting the
starting situation (Fig. S21, ESI†). To monitor the copper ion
transfer by NMR, we chose a soluble BFD13 derivative for reduction
spectra, see Fig. S28 and S31–S34 (ESI†).
1
of [Cu(2)]+ and 1+ as it produces a diamagnetic oxidation product. each addition recorded by H NMR. After the first addition,6d
a
1
The H NMR spectrum shows the presence of complexes [Cu(1)]+ 90 : 10 ratio between [Cu(2)]+ and [Cu(3)]+ was established (Fig. S26,
and [Cu(2)]+ in a ratio of 90 : 10 (Fig. S18, ESI†).
ESI†). Finally, after adding 1, a new equilibrium emerged with
Reversible communication between [Cu(1)]+ and 3 upon oxida- [Cu(1)]+ being formed predominantly in 90% yield, whereas [Cu(2)]+
tion is more complicated because on the product side the redox was present in only 10% yield and switch 3 was completely devoid
potentials, i.e. E1/2(10/+) = 450 mVSCE and E1/2([Cu(3)]+/2+) = of Cu+ ions (Fig. 1e and Fig. S27, ESI†). Both 1H NMR and ESI-MS
250 mVSCE, invert.6d As a result, a follow-up redox disproportiona- spectra, the latter (Fig. 3a) displaying signals at 1155.5 Da for
tion of 1+ and [Cu(3)]+ furnished 1 and [Cu(3)]2+. In the experi- [Cu(1)]+ and at 1799.2 Da for [Cu(2)]+, thus fully match with the
ment, a 1 : 1 solution of [Cu(1)]+ and 3 (0.1 mM) in DCM was data obtained from individual metal complexation studies invol-
reacted with one equiv. of TBPA+ SbCl6ꢁ and the metal exchange ving two switches only. A similar protocol was employed for the
ꢀ
process was monitored at 561 nm (Q band of the free nano- Zn2+ translocation. As expected, in the presence of all switches,
switch 3). The absorption gradually shifted to 550 nm with full the metal ion distributions were similar to those found in the
completion after 3 min (Fig. S23, ESI†). Metal translocation was individual dual-switch studies. As a matter of fact, nanoswitches
ascertained from ESI-MS peaks at 1093.4 and 1876.2 Da attributed 1 and 2 remained completely uncomplexed in the presence of
to 1+ and [Cu(3)]+, respectively, but equally a signal at 938.6 Da Zn2+ with [Zn(3)]2+ being the only coordination complex (Fig. 1f
suggestive of [Cu(3)]2+ was noticed (Fig. S24, ESI†). After reduction and Fig. S29, ESI†), also proven from the ESI-MS data showing
with dmfc (1 equiv.) the Q band was restored at 561 nm within only one signal at 939.1 Da (Fig. S30, ESI†).
3 min (Fig. S23, ESI†). ESI-MS supported the reversal by showing a
Lastly, to investigate multistep communication by using only
signal at 1155.5 Da revealing the formation of [Cu(1)]+ (Fig. S25, electron transfer as the input, chemical oxidation and reduction
1
ESI†). Similarly, H NMR data, now using the BFD13 derivative steps were implemented (vide supra). For characterisation, we
for reduction, indicate complete reverse translocation of the applied the ESI-MS and UV-Vis techniques as each step produced
Cu+ from switch [Cu(3)]+ to 1 (Fig. S22, ESI†). Thus, the battery paramagnetic component(s). The equilibrium mixture of switches
of results is clearly in full accord with the required selectivity 1, 2 and 3 in the presence of Cu+ (ratio 1ꢁ:1:1:1, Fig. 3a) was
changes upon oxidation/reduction.
treated with one equiv. of TBPA+ SbCl6 and the resulting
ꢀ
Finally, we focused on stepwise switching with all three nano- solution was injected into the ESI-MS instrument. The spectrum
switches being present in solution. In both directions of metal (Fig. 3b) shows the presence of 1+, [Cu(2)]+, and [Cu(2)(H2O)2]+,
translocation, at first the least stable complex was prepared and which were assigned from their signals at 1093.4, 1799.2, and
then the other switches with increasing binding ability were 1834.5 Da. Only traces of [Cu(1)]+ and [Cu(3)]+ were visible at
added sequentially. Thus, to test Cu+ transfer, complex [Cu(3)]+ 1155.5 and 1876.2 Da, respectively. The relative abundance
was prepared first, then switches 2 and 1 were added sequentially, pattern clearly implies a translocation of Cu+ from switch 1+
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to 2, which goes along with a reorganisation of the arm in 2 Notes and references
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the UV-Vis spectrum (Fig. 2b, black - red trace). Subsequently,
the mixture was reacted with one more equivalent of TBPA+
ꢀ
SbCl6ꢁ to oxidise [Cu(2)]+, which triggered Cu2+ translocation as
seen from ESI-MS signals at m/z = 1093.4 and 938.6 Da for 1+
and [Cu(3)]2+, respectively (Fig. 3c). Despite the reorganisation at both
nanoswitches 2 and 3, the amount of N - zinc(II) porphyrin
coordination remains in essence constant (Fig. 2b, red - green
trace). The backward process was initiated by adding one equiv. of
dmfc as a reducing agent, which, however, should reduce 1+ and not
[Cu(3)]2+ by oxidation potential considerations. Indeed, no reorganisa-
tion of the arm and thus no severe changes in the UV-Vis spectrum
were observed (Fig. 2b, green - blue trace). Finally, after treatment
with one additional equivalent of dmfc to reduce [Cu(3)]2+, Cu+
translocation from [Cu(3)]+ to 1 generated [Cu(1)]+ as demonstrated
by the ESI-MS spectrum (Fig. 3d). Diagnostically, the process was
accompanied by a large nanomechanical reorganisation of the
moving arm as evidenced from UV-Vis data at the Q band (Fig. 2b,
blue - cyano trace). Thus, the trio of nanoswitches has been reset to
its initial position after two oxidation and two reduction steps.
Herein, we establish an unprecedented two-step communication
cascade between three nanoswitches by programming both metal
ion and ligand binding properties depending on their redox states
(self-sorting). Ligand oxidation in [Cu(1)]+ at E1/2 = 610 mVSCE
triggered the release of Cu+ acting as an input signal for the
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750 mVSCE),6d translocation of Cu2+ changed the switching state of
3 - [Cu(3)]2+. A two-electron reduction reversed the system to its
initial state. Thus, this example not only introduces multistep
communication in systems chemistry14 but also advances the inter-
connection of molecular gates and machinery.
13 3-(11-Bromoundecyl)-1,10-biferrocenylene is used in this study
as a reductant. E1/2(BFD) = 0.09 VSCE and E1/2(BFD+ꢀ) = 0.76
VSCE (see (a) R. Breuer and M. Schmittel, Organometallics, 2012,
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¨
We are grateful to the DFG (Schm 647/19-1) and the Universitat
Siegen for financial support as well as to Kun Chen for the biferro-
cenylene BFD and Dr T. Paululat for NMR measurements.
14 J. R. Nitschke, Nature, 2009, 462, 736.
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