Site-selective cation-π interaction as a way of selective recognition of the caesium cation using sumanene-functionalized ferrocenes

Article abstract of DOI:10.1039/c9dt03162f

The first sumanene-ferrocene probes for efficient and selective caesium cation (Cs⁺) recognition are reported. The working mechanism of the sumanene moiety as the sensing unit was based on the site-selective cation-π interaction in its neutral state. The interactions with Cs⁺ were characterized by high association constant values together with low limits of detection.

Full text of DOI:10.1039/c9dt03162f

Volume 48 Number 46 14 December 2019 Pages 17135–17424
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An international journal of inorganic chemistry
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COMMUNICATION
Artur Kasprzak and Hidehiro Sakurai
Site-selective cation– interaction as a way of selective
recognition of the caesium cation using sumanene-
functionalized ferrocenes

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Site-selective cation–π interaction as a way of
selective recognition of the caesium cation
using sumanene-functionalized ferrocenes†
Cite this: Dalton Trans., 2019, 48,
17147
Received 3rd August 2019,
b
Artur Kasprzak *a and Hidehiro Sakurai
Accepted 23rd September 2019
DOI: 10.1039/c9dt03162f
rsc.li/dalton
The first sumanene–ferrocene probes for efficient and selective change in the Fc’s oxidation/reduction potential position is
caesium cation (Cs+) recognition are reported. The working commonly observed.5 All of the mentioned features make Fc
mechanism of the sumanene moiety as the sensing unit was based derivatives ideal candidates for ion recognition materials.
on the site-selective cation–π interaction in its neutral state. The
Sumanene (1) is a bowl-shaped molecule belonging to the
interactions with Cs+ were characterized by high association con- class of so-called buckybowls (Fig. 1a).6 Due to its interesting
stant values together with low limits of detection.
physicochemical properties, the chemistry of sumanene has
been intensively studied since its first synthesis in 2003.7 The
reports covered for example the creation of sumanene-based
supramolecular architectures8 and self-assembly studies with
sumanenylferrocenes.9
The design of selective caesium cation (Cs+) sensors constitu-
tes a great challenge and is of the highest environmental
importance in the modern world, because of the presence of
this cation in nuclear plant waste coming from the reprocessing
of irradiated fuels.1 Several classes of organic compounds have
been reported as Cs+ recognition materials, most commonly
fluorescent probes based on calixarene-templated crown
ethers.2 The working mechanism of these types of sensors was
based on the size-selective recognition and the formation of a
complex between the crown ether unit and Cs+ through the ion–
dipole electrostatic interaction. These compounds exhibited
superior properties toward Cs+ recognition compared to the
other metal cations, such as Na+, Mg2+ or Rb+.
Electrochemical probes can be applied for the real-time
detection of various analytes, including pollutants, such as
metal ions, because of the sensitivity, selectivity, portability
and fast analytical response time of such sensors.3 The use of
ferrocene (Fc) in the construction of ion sensors is of general
interest.4 Fc-based electrochemical probes show reversible and
quantitative reduction–oxidation behaviour, associated with the
presence of the ferrocene/ferrocenium (Fc/Fc+) redox couple.
The ion recognition phenomenon can be easily tracked by
various electrochemical methods, such as the commonly used
cyclic voltammetry (CV) technique. Upon addition of an ion, a
In 2017, the collaborative effort of Hirao, Petrukhina and
Rogachev groups resulted in the preparation of a novel organo-
metallic sumanene-based compound of Cs+.10 The driving
force for the formation of such a unique sandwich complex
(Fig. 1b) is a perfect match of Cs+ with the sumanene’s
concave cavity. The caesium cation was encapsulated by two
sumanenyl monoanions, generated by the reaction of suma-
nene with sodium as a solvent-separated ion pair. Encouraged
by this report, in pursuit of the design of new selective and
efficient Cs+ sensors, we envisaged that ferrocene–sumanene
conjugates may act as caesium cation recognition materials.
To our surprise, the sumanene moiety was found to work as a
sensing unit even in the neutral state only through the
cation–π interaction. Presented herein is our detailed research
on the synthesis and the metal cation binding of novel conju-
gates of ferrocene and sumanene.
The synthetic design involved the conjugation of formylsu-
manene (2) and aminosumanene (7) with ferrocene (Fc) deriva-
tives bearing either the primary amine group (in the case of
aFaculty of Chemistry, Warsaw University of Technology, Noakowskiego Str. 3, 00-664
Warsaw, Poland. E-mail: akasprzak@ch.pw.edu.pl
bDivision of Applied Chemistry Graduate School of Engineering, Osaka University, 2-
1 Yamadaoka, Suita, Osaka 565-0871, Japan
†Electronic supplementary information (ESI) available: Experimental section,
characterization data, and metal binding experiments. See DOI: 10.1039/
C9DT03162F
Fig. 1 (a) Structure of sumanene (1) and (b) structure of the reported
example of the Cs+(sumanene)2-type sandwich complex.10
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and compounds containing a CH2NH-type (3, 8) or an imine-
type (4, 5, and 9) linker.
Formation of the desired products was confirmed spectro-
scopically (NMR, IR, UV-Vis, and MS). 1H NMR spectra showed
the characteristic peaks coming from both structural moieties,
i.e. ferrocene (H of cyclopentadienyl (Cp) rings) and sumanene
(HAr, benzylic Hendo and Hexo), as well from the introduced
linkers, including the imine peak at ca. 8.60–8.55 ppm for
compounds 4, 5, and 9.13 The structural elucidation of 3, 4
and 5 was also supported by 2-D NMR experiments (1H–1H
1
COSY and H–1H TOCSY).14 The presence of the desired struc-
tural motifs in the obtained conjugates was also confirmed by
IR spectroscopy, as the characteristic absorption bands
coming from the Fc, (e.g., 1520–1510 cm−1) and the introduced
linkages (e.g., imine
– –
1660–1630 cm−1 and amine
1560–1530 cm−1) were observed.15 The UV-Vis spectra of the
obtained compounds consisted of the absorption maxima
coming from the sumanene (ca. 280–250 nm) and ferrocene
(ca. 390–360 nm) moieties.16,17
Cyclic voltammetry (CV) experiments revealed the reversible
reduction–oxidation electrode process for all the ferrocene–
sumanene conjugates, due to the presence of the Fc/Fc+ redox
couple. The electrochemical parameters of the studied Fc-
sumanene conjugates are listed in Table 1. The peak position
depended on the type of substituent on the Cp ring18 and was
Scheme 1 Synthesis of ferrocenyl derivatives of sumanene 3–5: (a) see
ref. 12; (b) N-(ferrocenylmethyl)benzene-1,4-diamine 100 mol% in
MeOH, formylsumanene 100 mol% in THF, formic acid (cat.), 27 °C, 24 h,
then NaBH4 300 mol%, 27 °C, 3 h, 75% yield; (c) N-(ferrocenylmethyl)
benzene-1,4-diamine 100 mol% in MeOH, formylsumanene 100 mol% in
THF, formic acid (cat.), 27 °C, 24 h, 94% yield; (d) N1,N1-(1,1’-bis(methyl)
ferrocenyl)bis(benzene-1,4-diamine) 100 mol% in MeOH, formylsuma-
nene 200 mol% in THF, formic acid (cat.), 27 °C, 24 h, 72% yield.
found to be the highest for compound 9 (Fc-CHvN-suma-
ox
nene; E
= 0.751 V) and the lowest for compound 8
1=2
(Fc-CH2NH-sumanene; Eox = 0.577 V).19
1=2
The thus-obtained ferrocene–sumanene conjugates were
applied for the caesium cation (Cs+) recognition. First, the
interactions were studied by fluorescence spectroscopy. The
titration experiment for Cs+ and the representative conjugate 3
is presented in Fig. 2.21,22 All the conjugates exhibited ‘turn-
off’ fluorescence behaviour (lowering of the emission intensity)
after the addition of further portions of Cs+. Such a feature
indicates the ion recognition phenomenon.23 The continuous
variation method (Job’s plot method) revealed the 1 : 1 binding
mode for the di-sumanenyl ferrocene derivative (5), whilst the
2 : 1 mode (conjugate : Cs+) was found for the mono-sumanenyl
ferrocenes.24 Thus, the structures of the complexes were
Scheme 2 Synthesis of ferrocenyl derivatives of sumanene 8–9: (a) see
ref. 12; (b) nitrosumanene 100 mol%, Pd/C 200 wt%, H2, EtOH/AcOEt,
27 °C, 3 h, 99% yield; (c) formylferrocene 100 mol%, aminosumanene
100 mol%, formic acid (cat.), MeOH, reflux 6 h, then NaBH4 300 mol%,
reflux, 1 h, 69% yield; (d) formylferrocene 100 mol%, aminosumanene
100 mol%, formic acid (cat.), MeOH, reflux 6 h, 61% yield.
sumanene derivative 2; Scheme 1) or the formyl functionality hypothesized (Fig. 3). The sandwich complexes are comprised
(in the case of sumanene derivative 7; Scheme 2).11 Both of one caesium cation and (1) two Fc-templated sumanene
mono- and di-substituted Fc derivatives were included in our molecules – compounds 3, 4, 8, and 9 (Fig. 3a) or (2) one Fc-
studies. In brief, formylsumanene (2) was prepared by the aro-
matic electrophilic substitution reaction using sumanene (1)
as the starting material (Scheme 1a), whilst aminosumanene
(7) was prepared by the Pd-catalysed hydrogenation reaction
starting from nitrosumanene (6) (Scheme 2a and b).
Table 1 Data for the electrochemical parameters of the obtained Fc–
sumanene conjugates20
Ferrocene–sumanene conjugates were obtained in good yields
Entry
Compound label
E
a [V]
Electrode process
ox
1=2
by means of the reductive amination reaction (compounds 3
and 8; Schemes 1b and 2c), or condensation for imine for-
mation (compounds 4, 5 and 9; Schemes 1c, d and 2d). As a
result, various mono- or di-substituted ferrocenyl derivatives of
sumanene were prepared, with the difference in the type of
linkage between these motifs, i.e., compounds containing a
p-phenylenediamine-type spacer (3–5) or linked directly (8–9)
1
2
3
4
5
3
4
5
8
9
0.625
0.621
0.674
0.577
0.751
Reversible
Reversible
Reversible
Reversible
Reversible
a Conditions: CH2Cl2, 0.5 mM Fc–sumanene conjugate, 0.1 M tetra-n-
butylammonium perchlorate (TBAP), scan rate; 0.05 V s−1
.
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Table 2 Caesium (Cs+) binding parameters exhibited by the ferrocene–
sumanene conjugates (3–5 and 8–9)
Entry
Compound label
Ka
LOD [M]
1
2
3
4
5
3
4
5
8
9
ca.a 4.5 × 105 M−2
ca.a 4.9 × 104 M−2
8.9 × 104 M−1
1.8 × 10−5
7.0 × 10−5
1.2 × 10−5
1.5 × 10−5
3.8 × 10−5
ca.a 2.9 × 105 M−2
ca.a 1.7 × 104 M−2
a 2 : 1 complex; the approximate Ka value was calculated based on the
I0/I = f(Ccation2) linear dependency following the Stern–Volmer method.
values. The goal at this stage of our research work was to
provide initial information on the influence of the type of
linkage between Fc and sumanene on Cs+ binding exhibited
by the conjugates considering the relative differences between
the binding parameters. All the conjugates exhibited high
association constant values in terms of Cs+ binding (with the
Ka value in the range of 104–105) together with low limits of
detection (LOD = 1.8–7.0 × 10−5 M). A direct comparison
between the Ka values leads to the following conclusions: (1)
the probes comprised of the p-phenylenediamine-type linkage
showed ca. 3- and 1.5-fold higher Ka than the respective conju-
gates without the above-mentioned spacer (3 vs. 8 and 4 vs. 9),
(2) the samples containing the CH2NH-type linkage exhibited
ca. 10-fold higher Ka than the respective conjugates bearing
the imine-type moiety (3 vs. 4, 8 vs. 9), and (3) the di-sumane-
nyl derivative 5 showed ca. 2-fold higher Ka than the respective
mono-sumanenyl derivative 4.
Fig. 2 Caesium (Cs+) binding studies with compound 3 using the fluor-
escence spectra titration method (excitation wavelength: 280 nm).
1H NMR titration provided further insight into the inter-
action pattern. The experiments were performed for the repre-
sentative compounds 3 (CH2NH-type linkage) and 4 (imine-
type spacer). A significant downfield shift for the benzylic endo
protons of the sumanene moiety in these compounds was
observed after the addition of further portions of Cs+.27,28 In
contrast, the corresponding exo protons did not show any sig-
nificant change in their chemical shifts. This observation con-
firmed that the concave face of the sumanene moiety is
strongly involved in the interaction with Cs+. The sumanene–
Cs+ interaction (compounds 3–5 and 8–9) could also be
Fig. 3 Hypothetical structures of the studied complexes: (a) for com-
pounds 3, 4, 8, and 9 and (b) for compound 5.
sumanene molecule ‘wrapping’ Cs+ – compound 5 (Fig. 3b). It tracked by CV, as (1) the lowering of oxidation current intensity
ox
is noteworthy that the sumanene moiety remains neutral and (2) the presence of a new peak at a lower potential (E
=
max
throughout the complexation, therefore the driving force for 0.235–0.075 V) were observed upon the addition of excess
wrapping was the site-selective ‘cation–π interaction’ between Cs+.20 Such trends have been previously demonstrated to be
the concave face of sumanene and Cs+.10,25
the characteristic features of the ion sensing phenomenon.5
The change in the emission intensity differed between the This trial also showed the potential application of the obtained
samples, as a result of different association constant values ferrocene–sumanene conjugates in Cs+ recognition using the
(Ka). The association constant values and the limits of detec- electrochemical methods.
tion (LOD) were calculated using the Stern–Volmer equation.26
To study the significance of the sumanene moiety in com-
The data for compounds 3–5 and 8–9 are summarized in pounds 3–5 and 8–9 in their Cs+ recognition ability, a ferro-
Table 2. It is noteworthy that the Stern–Volmer model is used cene–naphthalene conjugate (10) was synthesized as a refer-
to describe 1 : 1 systems, however, both the I0/I = f(Ccation) and ence for the fragment of the ferrocene–sumanene conjugate 3,
I0/I = f(Ccation2) plots for compounds 3, 4, 8, and 9 (2 : 1 com- with the difference in the structural moiety (sumanene (3) vs.
plexes) were found to be linear. Nevertheless, the Ka values cal- naphthalene (10)) (Fig. 4a).11 The 1H NMR titration pattern27
culated for these systems should be treated as the approximate as well as the emission spectrum of compound 10 (Fig. 4b)
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studied Fc-sumanene probes was observed which means that
the metal ions of similar radii do not interfere in Cs+ reco-
gnition. These results further confirmed the selective inter-
action of the ferrocene–sumanene conjugate with Cs+, due to
the perfect match of Cs+ with the sumanene’s concave site.
Our study also included the comparison between the emission
intensity of (i) the sample containing compound 3 with the
excess of Cs+ and (ii) the sample containing compound 3 with
the excess of the mixture of cations (12 eq. each) (Fig. 4c). No
significant difference between these samples was observed.
This finding provided the evidence that the designed ferro-
cene–sumanene conjugates can be used for the recognition of
Cs+ in ion-rich samples.
In conclusion, our work revealed that the sumanene-based
ferrocene-templated conjugates can be applied as selective and
efficient caesium cation (Cs+) recognition materials. The
novelty of this work included the evidence that the site-selec-
tive ‘cation–π interaction’ with the inclusion of the sumanene’s
concave face enabled the selective binding of Cs+. The inter-
actions can be tracked both spectroscopically (fluorescence
spectra titration and 1H NMR titration) and electrochemically
(cyclic voltammetry). Taking into account high association con-
stant values exhibited by all of the conjugates (in the range of
104–105) and low limits of detection (LOD = 1.8–7.0 × 10−5 M),
our work sheds light on the unexplored applications of com-
pounds bearing a buckybowl motif and the cation–π interaction
phenomenon. This work presents a new application of the
‘cation–π interaction’ phenomenon in the development of
binding sites for the design of selective receptors of metal ions.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Fig. 4 (a) Synthesis of ferrocene–naphthalene conjugate 10 (conditions
a: N-(ferrocenylmethyl)benzene-1,4-diamine 100 mol%, 2-naphthalde-
hyde 100 mol%, formic acid (cat.), MeOH, 27 °C, 27 h, then NaBH4
300 mol%, 27 °C, 3 h, 67% yield.); (b) caesium (Cs+) binding studies with
compound 10 using the fluorescence spectra titration method (exci-
tation wavelength: 280 nm); (c) different cation binding studies with
compound 3 using the fluorescence spectra titration method (excitation
wavelength: 280 nm).30
This study was supported by a Grant-in-Aid for Scientific
Research on Innovative Area ‘π System Figuration’ from the
MEXT (No. JP26102002). A. K. acknowledges the National
Science Centre (Poland) for the ETIUDA scholarship (No. 2018/
28/T/ST5/00018) and the Foundation for Polish Science
(Poland) for the START scholarship. The authors appreciate
the experiments performed by Dr Fong Jiao Hong, Univ.
Malaya toward aminosumanene synthesis.
remained unchanged after the addition of further portions of
Cs+.29 This feature clearly showed that the presence of the
sumanene moiety in compounds 3–5 and 8–9 was essential for
their Cs+ binding ability. The interaction of the representative
ferrocene–sumanene conjugate 3 with other cations, namely
K+, Mg2+, Fe2+, Rb+ and Ba2+, was also studied (fluorescence
spectroscopy assay; Fig. 4c). No change in the emission inten-
sity of compound 3 was observed after the addition of other
cations. This is especially important for trials with Rb+ and
Ba2+, as no interaction between these metal cations and the
Notes and references
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ox
19 In the reference, the Fc oxidation peak was at E = 0.298
1=2
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25 The studied Cs+ recognition phenomenon using the Fc–
11 For experimental details, see the Experimental section in
the ESI.† The reaction scheme dealing with the synthesis
of ferrocene derivatives used is also presented therein
(Scheme S1).†
sumanene conjugates did not utilize the ligation of the
metal ion at the heteroatom. Theoretical evidence for the
observed cation–π interaction with buckybowls was pre-
viously provided, see: (a) U. D. Priyakumar and G. N. Sastry,
12 B. B. Shrestha, S. Karanjit, G. Panda, S. Higashibayashi and
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13 For NMR spectra, see section S2 in the ESI.†
14 1H–1H COSY NMR spectra enabled (1) the confirmation of
the chemical shifts of the benzylic endo and benzylic exo
protons coming from the sumanene moiety, (2) the confir-
mation of the signal overlapping for compound 3 (benzylic
endo + NH–CH2-sumanene), and (3) the assignment of the 26 For the basis of this method, see for example: A. M. Pyle,
respective NH and CH2 protons. The 1H–1H TOCSY NMR
J. P. Rehmann, R. Meshoyrer, C. V. Kumar, N. J. Turro
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and J. K. Barton, J. Am. Chem. Soc., 1989, 111, 3051–3058. 29 Despite the good solubility of compound 10 in MeOH, the
The details of calculations are given in the ESI,
section S9.†
binding studies were performed in a mixed solvent system
(MeOH : CHCl3 = 1/1 v/v), in order to provide a direct
comparison with the results obtained for the sumanene–
ferrocene conjugates 3–5 and 8–9.
27 See section S7 in the ESI† for 1H NMR titration spectra.
28 Studies performed in
CD3OD : CDCl3 = 1/1 v/v.
a
mixed solvent system:
30 See section S5 in the ESI for fluorescence titration spectra.†
17152 | Dalton Trans., 2019, 48, 17147–17152
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