Voltammetric metal cation sensors based on ferrocenylthiosemicarbazone

Article abstract of DOI:10.1016/j.inoche.2007.09.015

Electrochemical studies on redox-active ferrocenylthiosemicarbazone L_1-4 were carried out in acetonitrile in the presence of various metal cations. Electrochemical investigation have demonstrated that the binding of Cu²⁺, Hg²⁺ and Ni²⁺ guest cations by L_1-4 results in large shifts of respective Fc/Fc⁺ redox couple to more positive potentials. Moreover, for compound L₄, significant fluorescence enhancement was found in the presence of the Cu²⁺, and this suggests it can act as a biresponsive fluorescent and electrochemical chemosensor for Cu²⁺.

Full text of DOI:10.1016/j.inoche.2007.09.015

Available online at www.sciencedirect.com
Inorganic Chemistry Communications 10 (2007) 1485–1488
www.elsevier.com/locate/inoche
Voltammetric metal cation sensors based on
ferrocenylthiosemicarbazone
*
Wei Liu, Xia Li, Zhiyuan Li, Maolin Zhang, Maoping Song
Department of Chemistry, Zhengzhou University, Zhengzhou 450052, PR China
Received 8 July 2007; accepted 13 September 2007
Available online 29 September 2007
Abstract
Electrochemical studies on redox-active ferrocenylthiosemicarbazone L1–4 were carried out in acetonitrile in the presence of various
metal cations. Electrochemical investigation have demonstrated that the binding of Cu2+, Hg2+ and Ni2+ guest cations by L1–4 results in
large shifts of respective Fc/Fc+ redox couple to more positive potentials. Moreover, for compound L4, significant fluorescence enhance-
ment was found in the presence of the Cu2+, and this suggests it can act as a biresponsive fluorescent and electrochemical chemosensor
for Cu2+
.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Ferrocene; Electrochemistry; Fluorescence; Cation sensing
Selective sensing of cations, anions even neutral mole-
cules play important roles in the environmental areas, bio-
logical and chemistry process, which has attracted much
interest in the supramolecular science [1]. It is well docu-
mented in literature that a well-defined chemical sensor
consists two basic parts: the signal report unit and the
binding unit. In the signaling system, redox-active moieties
(e.g. ferrocene, other metallocene) have been well employed
for signaling unit since the binding information can be
readout with the changing of the redox behavior through
the host–guest interactions such as ‘through space’ electro-
static perturbation and/or via various conjugated bond
linkages [1a,1b,1c,2]. In addition, various chromophores
or flurophores have been also successfully engaged in
developing normal optical chemosensors which can trans-
late the binding event into a measurable signal such as fluo-
rescence quenching or enhancement, and/or color changing
through the variety interactions between the host and guest
[3]. On the other hand, various functional groups such as
amine, polyamines, imine, amide, thiourea, guanidinium
and crown-ether, etc. [1a,1c] are well employed as binding
units in developing new cation and/or anion sensors. Gen-
erally, sensing for a specific guest can be achieved through
carefully designing the binding unit and the signal report
unit.
It is known that the shiff base (C@N) and thiourea can
form stable complexes with various metal ions and anions,
respectively [4]. However, to our surprising there has been
limited study about the electrochemical sensing properties
of the ferrocenyl-bearing thiosemicarbazone compounds.
Taking these into consideration, ferrocenyl-thiosemicarba-
zone L1–3 were synthesized and investigated. Moreover, to
achieve multiresponsive sensing of guest, we report the
compound L4 which consists of a redox-active ferrocene
moiety and a photoactive naphthalene group as an electro-
chemical and fluorescent chemosensor. In our study, it is
achieved by electrochemical and fluorescent sensing of
Cu2+
.
Ferrocenylthiosemicarbazone L1–4 were prepared in
high yields by several easy steps (Scheme 1). Their struc-
1
*
tures were well characterized by H NMR, HRMS and
Corresponding author. Tel./fax: +86 371 67767753.
IR [5]. To investigate the solid-state structure of compound
E-mail address: maopingsong@zzu.edu.cn (M. Song).
1387-7003/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2007.09.015

1486
W. Liu et al. / Inorganic Chemistry Communications 10 (2007) 1485–1488
H
H
1.1 EtOH, NH4OH, H2O, CS2, 0.5h, <10°C;1h< 10°C;
S
N
N
1.2 H2O, ClCH2COONa, 0.5h, r.t; 2h, r.t. ;
MeOH,FcCHO,3h. reflux
R
NH2
R
N
Fe
1.3 NH2NH2-H2O, H2O, 10min; 3h, 60°C.
R
N
NHNH2
S
H
L1 R = p-CH3OC6H4
L2 R = m-CH3OC6H4
L3 R = o-CH3OC6H4
L4 R = 1-naphthalene
L1- 4
Scheme 1. Synthesis of the ferrocenylthiosemicarbazone L1–4
.
Table 1
Electrochemical parameters of L1–4 (ca 1.0 · 10À3 mol LÀ1
)
Compounds Epa (mV) Epc (mV) DEp (mV) E1/2 (mV) ipa/ipc
L1
L2
L3
L4
569
569
581
582
493
497
506
510
76
72
75
72
533
533
543
546
1.16
1.13
1.17
1.19
All potentials Data are referred to the saturated calomel electrode (SCE)
at a scan rate of 100 mV sÀ1 in acetonitrile solution using TBAP
(0.1 mol LÀ1) as the supporting electrolyte on a GC working electrode, CV
recorded from 0.0 to 1.0 V.
˚
DEp = (Epa À Epc), E1/2 = (Epa + Epc)/2.
Fig. 1. Molecular structure of L1. Selected bond lengths (A): C10–C11
1.442(4), C12–S1 1.686(3), C11–N1 1.276(4), C12–N2 1.340(4), C12–N3
1.334(4), C13–N3 1.438(4), N1–N2 1.386(3),C16–O1 1.367(3), C19–O1
1.439(4); Selected angles (ꢁ): C11–N1–N2 116.1(3), C12–N2–N1 120.3(3),
C12–N3–C13 124.7(3), C16–O1–C19 117.1(3), N3–C12–N2 116.2(3), N3–
C12–S1 123.1(2), N2–C12–S1 120.7(2), C14–C13–N3 120.3(3), C18–C13–
N3 119.6(3), O1–C16–C17 116.1(3), O1–C16–C15 124.4(3).
Table 2
Electrochemical response for L1–4 vs selected metal cations in acetonitrile
in 0.1 M tetrabutylammonium perchlorate
Receptor DE1/2(mV)
Ca2+ Mg2+ Mn2+ Zn2+ Cu2+ Co2+ Hg2+ Pb2+ Ni2+
L, the single-crystal structure of L1 was determined, which
is in good agreement with the proposed structures [6]. The
crystal structure of compound L1 is shown in Fig. 1. The
bond lengths and angles of the ferrocenyl fragment are
unexceptional in contrast with the report in the literature.
It is clear from the structure that C11N1N2C12S1N3 is
planar (the mean deviation is 0.0227) and almost coplanar
to the cyclopentadienyl ring plane to which it is attached,
and the dihedral angle is 165.2ꢁ. C11N1N2C12S1N3 is ver-
tical (the dihedral angle is 91.2ꢁ) to the phenyl ring. The
hydrogen bonds linkages are the N–HÁ Á ÁS–C which have
L1
L2
L3
L4
<10
<10
<10
<10
<10 <10
<10 <10
<10 <10
<10 <10
17
13
155 45
157 40
102 40
99 39
103 38
103 39
181
182
182
190
<10 155 45
<10 157 42
DE1/2 is define as E1/2 (receptor + cation) À E1/2 (free receptor). Scan rate:
100 mV sÀ1. All potentials Data are referred to the saturated calomel
electrode (SCE) at a scan rate of 100 mV sÀ1 in acetonitrile solution using
TBAP (0.1 mol LÀ1) as the supporting electrolyte on a GC working elec-
trode, CV recorded from 0.0 to 0.9 V.
(1) the growth at a higher positive potential of a new redox
wave at the expense of the original wave, such as Ni2+ and
Cu2+; (2) a gradual positive shift in the Fc/Fc+ potential,
for Hg2+, Pb2+ and Co2+. All the complexes presented
quasi-reversible behavior. DE1/2[E1/2 (receptor + cation) À
E1/2 (free receptor)] was observed by addition of various
metal cations to L1–4 (Table 2), and only in the presence
of Cu2+, Ni2+and Hg2+ large anodic shifts were caused
(e.g. for L4 157 mV, 190 mV and 103 mV, respectively).
Though slightly changes for DE1/2 were noticed with the
various substituent (Ar) of L, the DE1/2 tendency of L with
NÁ Á ÁS distances of 3.389(3) A [N2Á Á ÁS1#1, #1 = Àx + 1,
˚
Ày, Àz + 1] and the strong hydrogen bond which have
˚
N3–HÁ Á ÁN1 distances of 2.628(4) A [N3Á Á ÁN1]. The vital
hydrogen bonds caused the dimer form in the solid state
of L1.
The electrochemical properties of L1–4 were investigated
using CV in acetonitrile. Each compound exhibited a
reversible redox wave typical of a ferrocene derivative
(Table 1). For L1–4 the peak current of the Fc oxidation
peak are proportional to the square root of the scan rate,
indicating a diffusion-controlled process. The addition of
Ni2+, Cu2+, Hg2+, Pb2+ and Co2+ (all cations are added
as perchlorate salts) to acetonitrile solutions of L1–4 led sig-
nificant anodic shifts of the redox potential of the Fc/Fc+
couple (Table 2). Two typical CV behavior for the Fc cen-
ter in L1–4 have been obtained upon addition of increasing
amounts of a given metal ions (see support information):
different metal cations are in the sequence of Ni2+
Cu2+ > Hg2+ > Co2+ > Pb2+. No obvious DE1/2 shifts were
>
noticed in the measure condition by addition of Ca2+
,
Mg2+ to L1–4. Furthermore, when an equimolar mixture
Ni2+, Cu2+, Hg2+, Co2+, Pb2+, Ca2+ and Mg2+ was added
to the solution of L1–4, the extent of the anodic shift of the
redox potentials is approximately equal to that induced by

W. Liu et al. / Inorganic Chemistry Communications 10 (2007) 1485–1488
1487
ion in the region 1090–1140 cmÀ1, which is also conformed
that the ligand L4 takes a keto form in chelating metal ions
not a thioenol form which loses a proton and results in a
deprotonated complex [9]. The evidence described above
suggests that coordination mode is similar to those pre-
pared previously using other thiosemicarbazones [4c,4d,9].
In summary, we developed ferrocenylthiosemicarbazone
L1–4 as a new set of electrochemical sensors for metal ions
such as Cu2+ and Hg2+. The compound L4 presents very
interesting properties and shows a highly sensitive response
in its emission toward Cu2+, which is rarely reported in the
multiresponsive sensors for metal ions. Furthermore, L1–4
presented in this work is a suitable molecular sensor for
the toxic Hg2+ since it presented a unique DE1/2 which is
substantially different from that found for other metal ions.
120000
100000
80000
60000
40000
20000
0
ligand
Ca2+
Mg2+
Mn2+
Ni 2+
Pb2+
Hg2+
Cu2+
Zn2+
Co2+
350
400
450
500
550
600
λ(nm)
Fig. 2. The changes in the fluorescence emission spectra of L4 (ca
2.5 · 10À5 mol LÀ1) upon addition of the perchlorate salt of measured
cations (2 equiv.) in acetonitrile.
Acknowledgements
This work was financially supported by the National
Natural Science Foundation of China (No. 20572102)
and the Natural Science Foundation of Henan province
(No. 0524270054).
Hg2+ alone. This result suggests that in the presence of all
these metal ions L1–4 preferentially coordinates to Hg2+
.
Therefore L1–4 is a good candidate for a sensor to detect
the toxic Hg2+
.
Moreover, to make good use of florescent and electro-
chemical multiresponsive sensing of metal cations, naph-
thalene group was introduced in L4. The fluorescent
spectrum studies (by excitation at 317 nm in acetonitrile
solution) of L4 with selected metal ions (all cations are
added as perchlorate salts) reveal that fluorescence
enhancement was noticed by addition of Ni2+, Cu2+and
Hg2+ from titration study, and this is most stinking for
Cu2+ (Fig. 2). The fluorescent spectrum studies of L1–3 with
different metal cations were also investigated for contrast,
but only L4 has the special property. It is assumed that
the naphthalene group plays an important role in the
MLCT process in resulting large fluorescence emission
enhancement. The result is accordant with the reported
two urea group substituted BMPUN [8]. Up to our known,
it is one of the few reported sensors which the binding of
Cu2+ caused great changes both in fluorescence emission
and potential shift of the redox-center.
To investigate the binding modes of L with different
metal ions, complexes of L4 with Cu2+, Hg2+, Co2+, Ni2+
and Pb2+ were prepared from methanol and finally recrys-
tallized from dichloromethane. Careful analysis of the IR
spectrum of the ligand L4 and its complexes has helped
in identifying the donor points of the ligand involved in
coordination to the cation (II) center. The coordination
of the ligands in their thione form is indicated by the pres-
ence of m(C@S) band in the 850–810 cmÀ1 region. Shift of
the band of the m(C@S) ligand to the 800–840 cmÀ1 region
along with the shift of the m(C@N), m(C@C) band at 1599–
1543 cmÀ1 by 5–20 cmÀ1 in the complexes indicate that the
ligand is attached to the metal ion through the thiocarbon-
yl sulfur and the azomethine nitrogen atom of the thiosem-
icarbazone moiety. Additionally, IR spectrum of the
complexes exhibit characteristic bonds of the perchlorate
Appendix A. Supplementary material
CCDC 647555 for compound L1 contains the supple-
mentary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/
j.inoche.2007.09.015.
References
[1] (a) F.P. Schmidtchen, M. Berger, Chem. Rev. 97 (1997) 1609;
(b) P.D. Beer, P.A. Gale, Angew. Chem. Int. Ed. 40 (2001) 486;
´
´
´
(c) R. Martınez-Manez, F. Sancenon, Chem. Rev. 103 (2003) 4419;
˜
(d) A. Goel, N. Brennan, N. Brady, P.T.M. Kenny, Biosens.
Bioelectro. 22 (2007) 2047;
(e) X.L. Cui, H.M. Carapuca, R. Delgado, M.G.B. Drew, V. Felix,
Dalton Trans. (2004) 1743;
(f) D. Saravanakumar, N. Sengottuvelan, M. Kandaswamy, Inorg.
Chem. Commun. 8 (2005) 386;
´
´
(g) V. Bereau, Inorg. Chem. Commun. 7 (2004) 829;
(h) J. Westwood, S.J. Coles, S.R. Collinson, G. Gasser, S.J. Green,
M.B. Hursthouse, M.E. Light, J.H.R. Tucker, Organometallics 23
(2004) 946;
(i) S.R. Collinson, J.H.R. Tucker, T. Gelbrich, M.B. Hursthouse,
Chem. Commun. 6 (2001) 555.
[2] (a) O. Reynes, F. Maillard, J.C. Moutet, G. Royal, E. Saint-Aman, G.
Stanciu, J.P. Dutasta, I. Gosse, J.C. Mulatier, J. Organomet. Chem.
637–639 (2001) 343;
(b) P.D. Beer, Z. Chen, M.G.B. Drew, J. Kingston, M. Ogden, P.
Spencer, J. Chem. Soc., Chem. Commun. (1993) 1046.
[3] (a) L.J. Kuo, J.H. Liao, C.T. Chen, C.H. Huang, C.S. Chen, J.M.
Fang, Org. Lett. 5 (2003) 1821;
(b) B.G. Zhang, J. Xu, Y.G. Zhao, C.Y. Duan, X. Cao, Q.J. Men,
Dalton Trans. (2006) 1271;
(c) H. Zhang, L.F. Han, K.A. Zachaariasse, Y.B. Jiang, Org. Lett. 7
(2005) 4217;

1488
W. Liu et al. / Inorganic Chemistry Communications 10 (2007) 1485–1488
(d) H. Miyaji, S.R. Collinson, I. Prokesˆ, J.H.R. Tucker, Chem.
Commun. (2003) 64.
C
19H20FeN3OS [M+H]+ 394.0677, found 394.0680; IR mmax (KBr):
3269, 1600, 1551,1241, 754 cmÀ1
;
1H NMR (400 MHz): 3.94(s, 3 H, –
[4] (a) E.J. Cho, J.W. Moon, S.W. Ko, J.Y. Lee, S.K. Kim, J. Yoon, K.C.
Nam, J. Am. Chem. Soc. 125 (2003) 12376;
OCH3), 4.22(s, 5H, cp-H), 4.44(s, 2H, cp-H), 4.63(s, 2H, cp-H), 6.94(d,
J = 8 Hz, 1H, Ar–H), 7.02(t, J = 8 Hz, 1H, Ar–H), 7.15(t, J = 8 Hz,
1H, Ar–H), 7.77(s, 1H, CH@N), 8.75(d, J = 8 Hz, 1H, Ar–H), 9.47(s,
1H, CSN–H), 9.75(s, 1H, Ar–NH). For L4: m.p.: 139–140 ꢁC; HRMS:
Calcd for C22H20FeN3S [M+H]+ 414.0727, found 414.0730; IR mmax
(b) Y.X. Ma, G.S. Huang, Z.Q. Li, X.L. Wu, Synth. React. Inorg.
Met.-Org. Chem. 21 (1991) 859;
(c) J.S. Casas, M.V. Castano, M.C. Cifuentes, J.C. Garcia-Monte-
agudo, A. Sanchez, J. Sordo, U. Abram, J. Inorg. Biochem. 98 (2004)
1009;
(d) R. Carballo, J.S. Casas, E. Garcia-Martinez, G. Pereiras-Gabian,
A. Sanchez, J. Sordo, E.M. Vazquez-Lopez, J.C. Garcia-Monteagudo,
U. Abram, J. Organomet. Chem. 656 (2002) 1;
(e) G. Thorfinnur, P.D. Anthony, E.O. John, G. Mark, Org. Lett. 4
(2002) 2449.
1
(KBr): 3330, 1600, 1542,1498, 744 cmÀ1; H NMR (400 MHz): 4.23(s,
5H, cp-H), 4.44(s, 2H, cp-H), 4.61(s, 2H, cp-H), 7.51–7.58(m, 3H, Ar–
H), 7.84(s, 1H, CH@N), 7.86–8.00(m, 4H, Ar–H), 9.31(s, 1H, CSN–
H), 10.12(s, 1H, Ar–NH).
[6] Date collection and refinement of L1: Red crystals of L1 suitable for X-
ray analysis were obtained by slow evaporation of CH2Cl2/CH3OH
solution. It belongs to monoclinic system P2(1)/n space group,
˚
˚
˚
[5] Synthesis of L:
A
mixture of ferrocenecarboxaldehyde (0.428 g,
a = 12.763(3) A, b = 9.0043(18) A, c = 15.998(3) A, b = 95.12(3)ꢁ.
3
V = 1831.2(6) A , Z = 4, Dc = 1.427 g cmÀ3, R1 [I > 2r(I)] = 0.0452,
˚
2.0 mmol) and aromatic thiosemicarbazide (2.0 mmol) was refluxed
in MeOH (20 ml) with AcOH (0.1 ml) as catalysis for 3 h. The resulting
solution was then cooled at ice-bath. The red precipitate was collected
by filtration and washed with MeOH several times. The crude product
was purified by recrystallization from CHCl3/MeOH to give L as red
crystals. Yield 82–95%. For L1: m.p.: 187–188 ꢁC; HRMS: Calcd for
C19H20FeN3OS [M+H]+ 394.0677, found 394.0680; IR mmax(KBr):
xR2 (all data) = 0.1129. The crystal structure was determined on a
Rigaku RAXIS-IV imaging plate area detector with graphite mono-
˚
chromated MoKa radiation (k = 0.71073 A) at temperature 291(2) K.
Reflections (I > 2r) of 3231 were corrected for Lorentz-polarization
effects. All calculations were performed using the Shelxl-97 crystallo-
graphic software package [7].
3343, 1606, 1541,1241, 1026, 816 cmÀ1
;
1H NMR (400 MHz): 3.83(s,
[7] G.M. Sheldrick, Shelxl-97: Program for Crystal Structure Refinement,
University of Go¨ttingen, Go¨ttingen, Germany, 1997.
[8] H. Yang, Z.Q. Liu, Z.G. Zhou, E.X. Shi, F.Y. Li, Y.K. Du, T. Yi,
C.H. Huang, Tetrahedron Lett. 47 (2006) 2911.
[9] (a) P. Sengupta, R. Dinda, S. Ghosh, W.S. Sheldrick, Polyhedron 22
(2003) 447;
3H, –OCH3), 4.23(s, 5H, cp-H), 4.44(s, 2H, cp-H), 4.61(s, 2H, cp-H),
6.94(d, J = 8 Hz, 2H, Ar–H), 7.49(d, J = 8 Hz, 2H, Ar–H), 7.77(s, 1H,
CH@N), 8.91(s, 1H, CSN–H), 9.63(s, 1H, Ar–NH). For L2: m.p.: 145–
146 ꢁC; HRMS: Calcd for C19H20FeN3OS [M+H]+ 394.0677, found
394.0680; IR mmax (KBr): 3297, 1597, 1549,1291, 1160, 771 cmÀ1 1H
;
NMR (400 MHz): 3.76(s, 3H, –OCH3), 4.24(s, 5H, cp-H), 4.46(s, 2H,
cp-H), 4.84(s, 2H, cp-H), 6.75(d, J = 8 Hz, 2H, Ar–H), 7.20–7.27(m,
2H, Ar–H), 7.35(s, 1H, CH@N), 8.00(s, 1H, Ar–H), 9.73(s, 1H, CSN–
H), 11.61(s, 1H, Ar–NH). For L3: m.p.: 198–199 ꢁC; HRMS: Calcd for
(b) M. Maji, S. Ghosh, S.K. Chattopadhyay, T.C. Mak. Thomas,
Inorg. Chem. 36 (1997) 2938;
(c) X.X. Zhou, Y.M. Liang, F.J. Nan, Y.X. Ma, Polyhedron 11 (1992)
447.