Electrochemically active phenylenediamine probes for transition metal cation detection

Article abstract of DOI:10.1039/c0nj00638f

A novel family of tetraalkyl-p-phenylenediamine (TAPD)-based ligands has been efficiently prepared by reductive amination of heterocyclic aldehydes. The redox properties of these electrochemical active ligands change dramatically upon complexation of the transition metal cations Zn²⁺, Ni ²⁺ and Cd²⁺ leading to large oxidation potential shifts of up to 950 mV depending on the nature of the ligand. Complexes with a metal to ligand ratio of 12 were formed and ¹¹³Cd NMR revealed an octahedral coordination sphere of the metal. All pyridyl derivatives show a distinct chemoselectivity (Zn²⁺ > Cd²⁺ > Ni²⁺). The thiophenyl containing derivatives display a particularly high selectivity for zinc cations (Zn²⁺ Ni²⁺, Cd²⁺).

Full text of DOI:10.1039/c0nj00638f

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PAPER
Electrochemically active phenylenediamine probes for transition metal
cation detectionw
Rihab Sahli,a Noureddine Raouafi,*a Khaled Boujlel,a Emmanuel Maisonhaute,bd
cd
Bernd Schollhorn* and Christian Amatored
¨
Received (in Gainesville, FL, USA) 13th August 2010, Accepted 25th November 2010
DOI: 10.1039/c0nj00638f
A novel family of tetraalkyl-p-phenylenediamine (TAPD)-based ligands has been efficiently
prepared by reductive amination of heterocyclic aldehydes. The redox properties of these
electrochemical active ligands change dramatically upon complexation of the transition metal
cations Zn2+, Ni2+ and Cd2+ leading to large oxidation potential shifts of up to 950 mV
depending on the nature of the ligand. Complexes with a metal to ligand ratio of 1 : 2 were
formed and 113Cd NMR revealed an octahedral coordination sphere of the metal. All pyridyl
derivatives show a distinct chemoselectivity (Zn2+ > Cd2+ > Ni2+). The thiophenyl
containing derivatives display a particularly high selectivity for zinc cations
(Zn2+ c Ni2+, Cd2+).
the Nobel Prize ‘‘for their development and use of molecules
Introduction
with structure-specific interactions of high selectivity’’, many
types of chemosensors have been reported in the literature.7
Metal ion sensing is regaining interest because of the pivotal
role of such species in many biological processes where the
Among the various ligands for electrochemical ion sensing, in
presence of specific metal cations is essential for the good
particular crown ethers have been frequently used because of
functioning of several organs.1 Particularly, the importance of
their capacity to bind metal cations due to the hard oxygen
transition metals is under focus.2 Zinc, for instance, is the
donor atoms supporting their coordination.8 More recently, it
second most abundant d-block metal ion in human brain.
was reported that tetraalkylated p-phenylenediamines (TAPD)
High levels of nickel ions are lethal to animals and human
attached to suitable ligands are useful probes for the detection
of a variety of alkali, alkaline earth9,10 and transition metal
beings, although small amounts of it are conversely vital. In
ions.11 Their physicochemical properties are modified upon
the biology of living organisms nickel plays numerous roles.
Effectively many enzymes such as urease, methyl-coenzyme M
coordination of a guest which gives the possibility to control
reductase, acetyl coenzyme A ligase etc. rely on the nickel ions
in their corresponding actions.3 On the other hand, cadmium
the ionic effectors. These compounds have been a subject for
spectroscopic studies as typical fluorophores12 and chromo-
phores.13 The redox-active phenylenediamine centres can also
toxicity is well established and is known to provoke severe
pollution.4 In fact, contamination of food and water5 by
be reversibly oxidized in two single electron processes to the
cadmium derivatives is now known to induce several types
radical cation and the dication, respectively, as shown in
of cancer and to be highly neurotoxic.6
Scheme 1.14,15
The monitoring of cationic exchanges remains an important
In this work, we report the synthesis of a series of new
challenge. Since Cram, Lehn and Pedersen have been awarded
electroactive transition metal ligands based on TAPD bearing
pyridyl and thiophenyl binding sites, potentially interesting for
a Laboratoire de Chimie Analytique et d’Electrochimie,
the development of new selective chemosensors. The spectro-
´
´
Departement de Chimie, Faculte des Sciences de Tunis,
Universite El-Manar, 2092 Tunis El-Manar, Tunisia.
scopic and electrochemical properties of these compounds
have been evaluated in the presence of the transition metals
Zn(II), Ni(II) and Cd(II).
´
E-mail: noureddine.raouafi@gmail.com
`
Laboratoire Interfaces et Systemes Electrochimiques – UPR 15,
Universite Pierre et Marie Curie – Paris 06, 75005 Paris, France
b
c
´
´
Laboratoire d’Electrochimie Moleculaire,
Universite Paris Diderot – Paris 07, CNRS UMR 7591,
´
15 rue Jean-Antoine de Baıf, Baˆt. Lavoisier, 75013 Paris, France.
¨
E-mail: bernd.schollhorn@univ-paris-diderot.fr
d Laboratoire )Pasteur* UMR CNRS 8640, Ecole Normale
´
´
Superieure, Universite Pierre et Marie Curie – Paris 6,
24 Rue Lhomond, 75231 Paris cedex 05, France
w Electronic supplementary information (ESI) available: Supporting
electrochemical data. See DOI: 10.1039/c0nj00638f
Scheme 1 Reversible oxidation of N,N0-tetramethyl-p-phenylenediamine
(TMPD).
c
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under vacuum and the residue was purified by column chromato-
graphy on silica (20% ethyl acetate in petroleum ether).
Experimental
General
Compound 1a (C20H22N4). Yield 61%; mp 84–86 1C;
1H NMR (CDCl3, 300 MHz) d/ppm 2.9 (s, 6H, CH3), 4.75
(s, 4H, CH2), 6.67 (m, 4H, CHarom.), 7.15 (t, 2H, 3JH–H = 7.8 Hz,
All reactions were performed with previously dried solvents
under argon atmosphere. The chemicals were purchased from
Sigma/Aldrich and Acros (France) and used as received.
Anhydrous acetonitrile was purchased from LabScan. Thin
layer chromatography (TLC) was performed on Fluka or
Merck aluminium sheets pre-coated with F254 silica gel.
Preparative column chromatography was performed on Fluka
3
CHarom.), 7.26 (d, 2H, JH–H = 7.5 Hz, CHarom.), 7.61
(t, 2H, 3JH–H = 7.8 Hz, CHarom.), 8.56 (d, 2H, 3JH–H = 7.5 Hz,
CHarom.); 13C NMR (CDCl3, 75.5 MHz) d/ppm 41.9, 57.9,
114.4, 115.6, 121.2, 122.23, 136.8, 149.3, 149.3, 149.7, 159.5. MS
(CI+, CH4): m/z(%): 319 (100) [M + H]+; 318 (51) [M+]; 227
(49) (M ꢀ C6H6N + H). IR nmax/cmꢀ1 3048 (CH), 2916 (CH),
1522 (CQN), 1224, 1432.
1
silica gel 60 (0.040–0.063). H, 13C, 113Cd NMR spectra were
recorded on a Bruker Advance 300 MHz apparatus in
1
deuterated solvents. Chemical shifts for H, 13C NMR were
reported using TMS signals as internal references and a
solution of cadmium perchlorate as external reference for
113Cd NMR spectra. IR (ATR) spectra were measured on a
Perkin Elmer Alpha spectrophotometer and UV-visible
spectra were measured in 1 cm quartz cells using a Perkin
Elmer Lambda 2 spectrophotometer.
Compound 2a (C18H20N2S2). Yield 74%; mp 62–64 1C;
1H NMR (CDCl3, 300 MHz) d/ppm 2.84 (s, 6H, CH3), 4.53
(s, 4H, CH2), 6.74 (d, 2H, 3JH–H = 7.5 Hz, CHarom.), 6.87 (d,
3
2H, JH–H = 7.5 Hz, CHarom.), 6.90 (mu, 4H, thiophene),
7.18 (mu, 2H, thiophene); 13C NMR (CDCl3, 75.5 MHz)
d/ppm 41.6, 50.6, 114.7, 118.2, 125.2, 126.2, 127.5, 140.8,
142.4. MS (EI+): m/z(%): 328 (76) [M+]; 231 (100)
[M ꢀ C5H5S]+. IR nmax/cmꢀ1 3066 (CH), 2884 (CH), 1514
(CQN), 1243, 705.
Electrochemistry
The electrochemical experiments were conducted in 0.1 M
tetrabutylammonium tetrafluoroborate acetonitrile solution
in a three-electrode glass cell controlled by a Radiometer
Analytical potentiostat consisting of a Voltalab PST050
equipped with a HBV 100 booster (for cyclic voltammetry)
and a POL 150 with a MED 150 stand (for differential pulse
voltammetry). VoltaMaster 4 (CV) and TraceMaster 5 (DPV)
software was used for data acquisition. A platinum wire was
used as the counter electrode. Ag|Ag+ 0.01 M and Ag|AgCl
(3 M KCl) electrodes were used as reference electrodes in CV
and DPV experiments respectively. A 3 mm-diameter glassy
carbon disk electrode was used as the working electrode. It
was polished prior to each experiment. Tetrabutylammonium
tetrafluoroborate has been recrystallized from methanol and
dried at 60 1C for 12 h. Acetonitrile was stored over an
activated 3 A molecular sieve. Cyclo and differential pulse
voltammograms were recorded at ambient temperature and at
a potential sweep rate of 100 mV sꢀ1. Metal transition ions
were added as a solution of the corresponding perchlorate salts
in acetonitrile. The solutions were dried over activated 3 A
molecular sieves.
Compound 1b (C23H26N4). Yield 76%; mp 144–146 1C;
1H NMR (CDCl3, 300 MHz) d/ppm 1.69 (m, 4H, CH2), 1.92
(m, 2H, CH2), 2.98 (t, 4H, 3JH–H = 7.6 Hz, CH2), 4.76 (s, 4H,
3
CH2), 6.63–6.81 (d, 4H, JH–H = 7.6 Hz, CHarom.), 7.13 (t,
2H, 3JH–H = 7.6 Hz, CHarom.), 7.31 (d, 2H, 3JH–H = 7.9 Hz,
3
CHarom.), 7.58 (t, 2H, JH–H = 7.9 Hz, CHarom.), 8.56 (d,
3
2H, JH–H = 7.9 Hz, CHarom.); 13C NMR (CDCl3, 75.5
MHz) d/ppm 24.0, 25.9, 52.6, 57.7, 113.6, 119.1, 121.9, 136.7,
149.5, 159.2. MS (EI+): m/z(%): 358 (3) [M+]; 266 (43)
[M ꢀ C6H6N]+. IR nmax/cmꢀ1 3070 (CH), 2900 (CH), 1501
(CQN), 1200, 1110.
Compound 2b (C21H24N2S2). Yield 85%; mp 69–71 1C;
1H NMR (CDCl3, 300 MHz) d/ppm 1.52 (m, 4H, CH2), 1.66
(m, 2H,CH2), 3.03 (t, 4H, 3JH–H = 7.6 Hz, CH2), 4.56 (s, 4H,
3
CH2), 6.86 (d, 2H, JH–H = 7.2 Hz, CHarom.), 6.90 (m, 4H,
thiophene), 7.17 (m, 2H, thiophene), 7.25 (d, 2H, 3JH–H = 7.2 Hz,
CHarom.); 13C NMR (CDCl3, 75.1 MHz) d/ppm 24.3, 25.9,
50.6, 52.0, 117.3, 118.4, 124.8, 126.2, 127.0, 141.3, 142.3. MS
(EI+): m/z(%): 368 (93) [M+]; 271 (85) [M ꢀ C5H5S]+. IR
n
max/cmꢀ1 3000 (CH), 2950 (CH), 1500 (CQN), 1210,
1109, 701.
Synthesis
Compound 1c (C22H24N4O). Yield 75%; mp 138–140 1C;
1H NMR (CDCl3, 300 MHz) d/ppm 3.33 (t, 4H, 3JH–H = 7.1 Hz,
CH2), 3.79 (t, 4H, JH–H = 7.1 Hz, CH2), 4.78 (s, 4H, CH2),
The reductive amination reaction was performed under argon
atmosphere at room temperature in the dark (shielded from
the light by an aluminium foil). The primary aromatic amine
(2.0 mmol) was dissolved in 50 mL of methanol. Aldehyde
(8.0 mmol) and acetic acid (10.0 mmol) were added and the
mixture was stirred for 12 h. Sodium cyanoborohydride
(2.0 mmol) was added and the mixture was stirred for 4 h.
The reaction progress was followed by thin layer chromato-
graphy. After completion the reaction mixture was neutralized
by 50 mL of saturated ammonium chloride solution and
extracted twice with 20 mL of dichloromethane. The organic
layer was washed twice with 10 mL of distilled water, and then
dried over magnesium sulfate. The filtrate was concentrated
3
3
6.70–6.83 (d, 4H, JH–H = 7.5 Hz, CHarom.), 7.29 (t, 2H,
3
3JH–H = 7.6 Hz, CHarom.), 7.39 (d, 2H, JH–H = 7.5 Hz,
CHarom.), 7.75 (t, 2H, 3JH–H = 7.6 Hz, CHarom.), 8.50 (d, 2H,
3JH–H = 3 Hz, CHarom.); 13C NMR (CDCl3, 75.5 MHz) d/ppm
49.2, 56.5, 65.8, 112.5, 120.6, 121.1, 140.5, 148.6, 156.7. MS (EI+):
m/z(%): 360 (16) [M+]; 268 (100) [M ꢀ C6H6N]+. IR nmax/cmꢀ1
3050 (CH), 2916 (CH), 1514 (CQN), 1232, 1434, 1182, 1112.
Compound 2c (C20H22N2S2O). Yield 52%; mp 98–99 1C;
1H NMR (CDCl3, 300 MHz) d/ppm 3.01 (t, 4H, 3JH–H = 7.1 Hz,
CH2), 3.81 (t, 4H, 3JH–H = 7.1 Hz, CH2), 4.60 (s, 4H, CH2), 6.81
c
710 New J. Chem., 2011, 35, 709–715
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(d, 4H, 3JH–H = 7.5 Hz, CHarom.), 6.91 (m, 4H, thiophene), 7.21
(m, 2H, thiophene); 13C NMR (CDCl3, 75.5 MHz) d/ppm 52.6,
57.7, 76.5, 113.3, 121.1, 121.9, 136.7, 149.5. MS (CI+, CH4):
m/z(%): 371 (41) [M + H]+. IR nmax/cmꢀ1 3066 (CH), 2843
(CH), 1511 (CQN), 1283, 1161, 1122, 694.
corresponding imine which was then reduced in situ by sodium
cyanoborohydride to the respective secondary amine. This
amine reacted with a second molecule of aldehyde giving,
after final reduction, the corresponding TAPD derivatives 1
and 2 in yields of 52% to 85%. The aldehydes were used in
excess in order to ensure complete transformation of the
starting amine.
Preparation of the [Cd (1a)2]2+ complex. The complex was
prepared by dissolving 1.0 mmol cadmium perchlorate
Cd(ClO4)2 in 10 mL of anhydrous dichloromethane (caution:
perchlorates are explosive in the solid state). After stirring the
solution for 1 h at room temperature, 2.0 mmol of 1a were
added and stirring was maintained for 24 h. The solvent was
evaporated to yield a brownish solid which was filtrated and
washed with diethylether. Yield: 85%; 113Cd NMR (CH2Cl2,
66 MHz, T = 298 K) d/ppm 231 (s); 113Cd NMR (CH2Cl2/
CD2Cl2, 66 MHz, T = 208 K) d/ppm 272 (s).
UV spectroscopy
Evolution of UV spectra is a common method for exploring
the formation of metal complexes.14,18 We examined the effect
of gradual addition of Zn2+ to compounds 1a and 2a on the
UV-visible absorption curves. The absorption spectrum of the
free ligand 1a shows two bands, the major one being located at
262 nm and the weaker band at 325 nm (Fig. 1a). The UV
spectrum of compound 2a revealed two intense absorption
bands (lmax = 235, 265 nm) and a weaker one at 323 nm
(Fig. 1b).
Results and discussion
The observation of a bathochromic shift upon the increase
of solvent polarity (acetonitrile/ethanol) confirmed the
assumption that the absorption bands are related to n - p*
transitions of the aromatic phenylenediamine system. The
addition of Zn2+ to a solution of the free ligands induced
significant changes of the respective UV spectra. The addition
of 0.5 equivalents of Zn2+ to a solution of ligand 1a provoked
an increase in the absorption intensity of the band at 262 nm
and a blue shift of ca. 15 nm of the band at 325 nm. In the case
of compound 2a both strong bands increased in the presence
of zinc cations. The bands at 265 and 323 nm display hypso-
chromic shifts of about 8 and 24 nm respectively. These results
can be explained by the formation of zinc complexes with both
Synthesis
Several methods for the preparation of tetra-substituted
phenylenediamine derivatives have been described in the
literature.16 Reductive amination17 of aldehydes is a straight-
forward procedure that has been chosen to obtain in a one pot
two-steps reaction of the compounds 1 and 2 (Scheme 2).
4-Dimethylaminoaniline (3a), 4-piperidinylaniline (3b) and
4-morpholinoaniline (3c) were used to react with pyridinyl-2-
carboxaldehyde and thiophenyl-2-carboxaldehyde leading to
two series of products 1a–c and 2a–c. The condensation of
the N,N-dialkylated phenylenediamines 3a–c with a hetero-
aromatic aldehyde in methanol/acetic acid provided the
Scheme 2 General route to TAPD derivatives.
Fig. 1 UV absorption spectra before (solid line) and after (dashed line) the addition of Zn(ClO4)2 to an acetonitrile solution of (a) 1a (0.5 equiv. of
Zn2+) and (b) 2a (1 equiv. of Zn2+).
c
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representative ligands 1a and 2a. The participation of the
phenylenediamine nitrogen lone pairs in the zinc coordination
is very likely and probably affects the corresponding transition
energy being responsible for the observed hypsochromic shifts.19
In the case of the pyridyl derivatives 1a–c the second wave is
irreversible (see Fig. 2b) because the second electron oxidation
is coupled to a chemical reaction, probably due to the
enhanced C–H acidity of the CH2 groups linked to the pyridyl
moieties. The influence of the substituents a, b and c
(Scheme 2) on the values of the oxidation potentials in each
series 1 and 2 is relatively small but significant, with peak
potential differences up to 130 mV (Table 1). The oxidation
potentials of each ligand can thus be tuned within the range
of over 100 mV without changing the inherent chemical
behaviour of the free ligands (compare Table 2).
Electrochemistry of the free ligands 1 and 2
The redox potentials of the described electroactive ligands
were determined by cyclic voltammetry in anhydrous
acetonitrile using a glassy carbon disc electrode (Tables 1–2).
The electrochemical behaviour of compound 2a–c is similar
to TMPD, displaying two reversible oxidation waves (cf. Fig. 2a).
Accordingly, these waves are assigned to the formation of the
radical-cation (Ox1/Red1) and the corresponding dication
(Ox2/Red2). The respective peak current ratios Ipa/Ipc are close
to unity (Table 1). Furthermore the linear dependence of the
anodic peak current Ipa on the square root indicates a diffusion
limited signal. Accordingly the peak to peak separation is close
to 60 mV as expected for a Nernstian behaviour.
Electrochemistry of metal complexes of compound 1
The CV studies showed that all studied compounds are
sensitive to the presence of Zn2+ ions. Upon the progressive
addition of Zn2+ to a solution of the dipicolylamine derivative
1c (Fig. 3) both oxidation peaks (Ox1, Ox2) disappeared
progressively and an irreversible peak (Ox3) was observed at
a much higher potential at 516 mV (Fig. 3 and Table 2). The
Table 1 Characteristics of the cyclic voltammetric curves of compounds 1 and 2a
Ist oxidation wave
IInd oxidation wave
Epa/mV
Epc/mV
DEpb/mV
E1/2c/mV
Ipa/Ipc
Epa/mV
Epc/mV
DEpb/mV
E1/2c/mV
Compound
1a
1b
1c
2a
2b
2c
ꢀ63
ꢀ23
72
33
67
ꢀ127
ꢀ99
ꢀ01
ꢀ52
ꢀ21
40
64
76
73
85
88
68
ꢀ95
ꢀ61
35.5
ꢀ09.5
23
0.91
0.97
0.97
0.97
0.99
0.96
302
321
401
628
576
581
—
—
—
551
515
492
—
—
—
77
61
89
—
—
—
589.5
545.5
536.5
108
74
a
0.1 M TBABF4 in acetonitrile, scan rate: 0.1 mV sꢀ1; glassy carbon working electrode (diameter: 3 mm); Pt counter electrode and non-aqueous
Ag|Ag+ (0.01 M) reference electrode. DEp = Epa ꢀ Epc
b
c
. E1/2 = (Epa + Epc)/2.
Table 2 Cyclic voltammetry data of 1 and 2 in the presence of Zn(ClO4)2 and Ni(ClO4)2
Zn2+
Ni2+
Epa (Ox3)/mV
DEpa/mV
Epa (Ox3)/mV
DEpa/mV
Compounds
Epa (Ox1)/mV
1a
1b
1c
2a
2b
2c
ꢀ63
ꢀ23
72
33
67
385
489
516
950
1016
634
448
512
444
917
949
526
500
503
626
33
67
108
563
526
554
0
0
0
108
Fig. 2 Cyclic voltammograms of compounds (a) 4 mM 2a and of (b) 6 mM 1a (first wave: solid line) in 0.1 M TBABF4 acetonitrile solution. Scan
rate: 0.1 V sꢀ1; glassy carbon working electrode (diameter: 3 mm), Pt counter electrode and non-aqueous Ag|Ag+ (0.01 M) reference electrode.
c
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Fig. 3 (a) Cyclic voltammograms of 1c (2 mM) showing the first oxidation wave in the absence of metal ions (—) and in the presence of 0.2 (–m–),
0.5 (- - - -) and 1 equiv. (–K–) of Zn(ClO4)2. Scan rate: 0.1 V sꢀ1. (b) Variation of the peak current Ipa of the first oxidation wave of compound 1c
depending on the relative Zn2+ concentration.
conversion was complete when reaching 0.5 equivalents of zinc
suggesting the formation of a 1 : 2 metal–ligand complex
[Zn(1c)2]2+. The strong anodic potential shift of 444 mV
(DEpa) between the new (Ox3) and the first (Ox1) oxidation
peak indicates a strong interaction between Zn2+ and the
phenylenediamine nitrogen.20,21 In fact, the decrease of the
electronic density onto the redox center makes its oxidation
more difficult.
1a,b showed a similar behaviour with potential shifts of up
to 512 mV indicating relatively strong metal complexes
all with a M1L2 stoichiometry (see Table 2 and ESIw).
These shifts depend strongly on the nature of the transition
metal cation (Fig. 4) and increase in the following order:
Cd2+ o Zn2+ o Ni2+
.
However, we observed a higher selectivity for zinc over
nickel despite a smaller potential shift. Fig. 5 shows the cyclic
voltammograms of the free ligand 1a in the absence and in the
presence of metal cations. The addition of nickel to a
[Zn(1a)2]2+ solution did not affect the initial signal at all.
Upon addition of Zn2+ (0.5 equiv.) to a [Ni(1a)2]2+ solution
the oxidation peak shifted to the signal registered for
[Zn(1a)2]2+ suggesting a complete and rapid metal exchange
reaction towards the thermodynamically more stable complex.
Zn2+ ions can be detected even in the presence of high nickel
concentrations. These results are suggesting that the potential
shift is certainly dominated by the sole electronic charge
delocalisation from the redox center towards the metal,
whereas the selectivity depends on the global interactions with
6 nitrogens forming the coordination sphere.
Fig. 4 Cyclic voltammograms of 1a (2 mM) in the absence of metal
ions (——) and in the presence of 0.5 eq. of Cd2+ (- - -), Zn2+ (–’–)
Owing to its high resolution and sensitivity differential pulse
voltammetry (DPV) can distinguish two electroactive species
and Ni2+ (–m–) respectively. Scan rate: 0.1 V sꢀ1
.
Fig. 5 Cyclic voltammograms (a) and differential pulse voltammograms (b) of (i) metal free 1a (2 mM) (–K–), (ii) 1a in the presence of 1 mM of
Zn2+ (—), (iii) 1a in the presence of 1 mM of Ni2+ (–m–) and (iv) solution in the presence of 1 mM of Ni2+ and 1 mM of Zn2+ (– – –) in
acetonitrile/0.1 M TBABF4 solution. Scan rate: 0.1 V sꢀ1
.
c
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Electrochemistry of metal complexes of compound 2
The thiophenyl derivatives 2 also form zinc complexes in
solution. Upon the addition of Zn2+ to a solution of compound
2a the oxidation peaks Ox1 (33 mV) and Ox2 (628 mV)
disappeared in favour of a new peak Ox3 at higher potential
(950 mV) (Fig. 7, Table 2). The strikingly large peak potential
difference of 917 mV (DEpa = Epa(Ox3) ꢀ Epc(Ox1)) indicated
the formation of a relatively strong complex. On the reverse
scan Red4 (551 mV) was observed at the same potential as
Red2 which proves that a complete decomplexation took place
when the phenylenediamine dication was formed due to
electronic repulsion with the Zn2+ cation. The Red5 (113 mV)
peak appeared at a more anodic potential than Red1 (ꢀ52 mV)
meaning that when the radical-cations were reduced to their
neutral species, fast recomplexation of the zinc occurred. This
EC mechanism explains the facilitated reduction of the radical
cation.21,25 The zinc complexes of the other thiophenyl
derivatives showed also high potential shifts of almost
950 mV for compound 2b and 526 mV for 2c.
Fig. 6 113Cd spectra of the [Cd(1a)2]2+ complex at 298 K and at 208 K
(inset) in CH2Cl2/CD2Cl2.
with peak potential separations of less than 50 mV.22 The
differential pulse voltammograms of compound 1a (Fig. 5b)
confirmed the CV experiments supporting the high selectivity
for zinc cations. A series of similar competition experiments
between Zn/Ni, Zn/Cd and Ni/Cd (see ESIw) clearly showed a
selectivity for the studied metal cations in the following order:
In contrast to the pyridyl derivatives 1, the oxidation of
thiophenyl containing ligands 2 and their metal complexes to
the corresponding dications are fully reversible and thus
particularly interesting for Zn2+ sensing. The lower detection
limit of zinc was observed at 10ꢀ5 M for a concentration of
1 mM of ligand 2a. As detailed above the mechanism involves
ejection and recomplexation of the zinc cation as we have
recently shown for calcium crown ether complexes.10 Unlike
the pyridyl derivatives, compounds 2a–c were found to be
insensitive to Ni2+, even at high nickel concentrations
(Table 2 and Fig. 7b). Consequently these ligands selectively
bind Zn2+ without being influenced by the presence of
Ni2+. The same observations were made for cadmium ions
(see ESIw).
Zn2+ > Cd2+ > Ni2+
.
NMR study of the Cd complex
Among the three studied transition metals cadmium was the
only one suitable to be used for NMR experiments because of
the high natural abundance of the active 113Cd isotope.
Previous studies on 113Cd NMR demonstrated that chemical
shift and multiplicity of the signal strongly depend on the
environment of the metal ion and consequently on the
coordination number and geometry of the coordination
sphere.23 The 113Cd NMR of the complex [Cd(1a)2]2+ was
measured in dichloromethane at two different temperatures
(Fig. 6). At 298 K and 208 K a singlet was observed at
231 ppm and at 272 ppm respectively. The absence of
multiplets and the comparison of the obtained chemical shift
to the literature values24 are suggesting the presence of an
octahedral complex. These results are further evidence for the
formation of M1L2 complexes between the metal ions and
ligand 1 in concordance with the electrochemical behavior.
Conclusions
A novel family of electroactive tetraalkyl-p-phenylenediamine
ligands has been prepared. Reductive amination of hetero-
cyclic aldehydes afforded the desired target molecules in a
clean one pot reaction. The oxidation potentials of the free
ligands can easily be tuned within the range of 100 mV
Fig. 7 Cyclic voltammograms of 2a in the absence (solid line) and presence (dashed line) of 1 equiv. of metal cations in 0.1 M TBABF4 acetonitrile
with a scan rate of 0.1 V sꢀ1. Concentrations: (a) 2a (4 mM) and Zn(ClO4)2 (4 mM); (b) 2a (2 mM) and Ni(ClO4)2 (2 mM).
c
714 New J. Chem., 2011, 35, 709–715
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depending on the alkyl groups linked to the phenylenediamine
nitrogens. M1L2 complexes were formed with zinc, nickel and
cadmium ions. 113Cd NMR revealed an octahedral coordination
sphere of the metal. Extremely high oxidation potential shifts
of up to 950 mV were observed upon complexation of the
transition metal cations depending on the nature of the ligand.
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All ligands show a distinct chemoselectivity (for 1a–c: Zn2+
>
Cd2+ > Ni2+; and for 2a–c: Zn2+ c Ni2+,Cd2+). Contrary
to the pyridyl derivatives 1 the oxidation of the thiophenyl
containing ligands 2 and their metal complexes display a
fully reversible oxidation to the corresponding dications. The
mechanism involves ejection and recomplexation of the
zinc cation. These compounds are thus predestined for the
development of selective Zn2+ sensors as well as for potential
applications as zinc release probes.
Acknowledgements
This work was supported by the DGRST and the CNRS
(cooperation CNRS-DGRST, UMR 8640, LIA XiamENS),
E. Maisonhaute and B. Schollhorn, Tetrahedron Lett., 2009, 50,
1720–1722.
¨
the Ecole Normale Superieure, the Universite Pierre et Marie
´ ´
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