Synthesis, photophysics, electrochemistry and binding studies of zinc(II) dithiolate crown complexes

Article abstract of DOI:10.1039/b109497c

A series of zinc(II) dithiolate complexes containing ligands with benzo-15-crown-5 (dic), benzo-18-crown-6 (dic-18-6), 10-thiabenzo-15-crown-5 (dic-S), 10-selenabenzo-15-crown-5 (dic-Se) and 7,10,13-trithiabenzo-15-crown-5 (dic-3S) moieties have been synthesized and characterized, and their photophysics and electrochemistry studied. The metal ion-binding properties have been investigated by electronic absorption, emission and ¹H NMR spectroscopy. The X-ray crystal structure of [Zn(SC₆H₄-CH₃-p)₂(dic-3S)] (5) has also been determined.

Full text of DOI:10.1039/b109497c

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Synthesis, photophysics, electrochemistry and binding studies
of zinc(II) dithiolate crown complexes
Vivian Wing-Wah Yam,* Yung-Lin Pui, Kung-Kai Cheung and Nianyong Zhu
Department of Chemistry and Open Laboratory of Chemical Biology of The Institute of
Molecular Technology for Drug Discovery and Synthesis,y The University of Hong Kong,
Pokfulam Road, Hong Kong SAR, People’s Republic of China.
E-mail: wwyam@hku.hk
Received (in Montpellier, France) 11th October 2001, Accepted 8th January 2002
First published as an Advance Article on the web
A series of zinc(II) dithiolate complexes containing ligands with benzo-15-crown-5 (dic), benzo-18-crown-6
(dic-18-6), 10-thiabenzo-15-crown-5 (dic-S), 10-selenabenzo-15-crown-5 (dic-Se) and 7,10,13-trithiabenzo-15-
crown-5 (dic-3S) moieties have been synthesized and characterized, and their photophysics and electrochemistry
studied. The metal ion-binding properties have been investigated by electronic absorption, emission and
1H NMR spectroscopy. The X-ray crystal structure of [Zn(SC6H4-CH3-p)2(dic-3S)] (5) has also been
determined.
Host-guest chemistry has been attracting much attention in the
past two decades, which has led to a revival of interest in crown
ether chemistry. Numerous studies have been performed that
are reported in many papers and summarized in reviews and
books.1,2 Crown ethers with only hard oxygen atoms on the
polyether cavities are used as ‘probes’ for alkali and alkaline
earth metals, some other metal ions, oxonium ions, protonated
amines including amino acids, and some neutral rod-like
molecules such as acetonitrile.3 Recently, increasing attention
has been paid to variation of the donor atoms such as sulfur or
selenium in place of the well-known all-oxygen combination in
order to tune the crown ether cation binding ability.4 Although
there have been numerous reports on the complexation of
alkali and alkaline earth metal ions,2a,3 corresponding studies
on the complexation of transition or late transition metal
cations with acyclic or cyclic polythioethers are relatively less
extensive. However, excellent examples of efficient ligands
designed for these soft metal ions have been reported.4,5
In recent years there has been a growing interest in the study
of fluorescent probes for soft metal ions, in particular the
Scheme 1 Schematic drawing of the zinc(II) dithiolate complexes
with crown ether ligands.
group IIB metal ions such as Zn2þ and Cd2þ 6a–c
The biolo-
.
gical activities of these ions are important. For example, zinc is
a vital component in many cellular processes.6d The Zn2þ ion
has the ability to modulate a variety of ion channels and may
play an important role in gene expression and neuro-
transmission.6e Therefore, the availability of better and more
specific probes for Zn2þ and Cd2þ would provide additional
insights into the role played by these metal ions in neuro-
biology.
As a continuation of our studies on the copper(I) crown
ether containing system,7 herein we report the design and
synthesis of a series of zinc(II) dithiolate crown ether con-
taining complexes (Scheme 1) and the cation-binding proper-
ties of these complexes. Attempts have been made to tune their
binding properties by varying the size as well as the donor
properties of the crown ether cavity. Their electronic absorp-
tion, luminescence, electrochemistry and binding studies are
also described.
Experimental
Reagents and materials
The
ligands
N-(2-pyridylmethylene)-4-aminobenzo-15-
crown-5 (dic)8a and N-(2-pyridylmethylene)-4-aminobenzo-18-
crown-6(dic-18-)6 were synthesized by modification of
literature procedures.8 40-(200-Pyridinecarboxaldimino)benzo-10-
thia-15-crown-5 (dic-S), 40-(200-pyridinecarboxaldimino)benzo-
10-selena-15-crown-5 (dic-Se) and 40-(200-pyridinecarboxal-
dimino)benzo-7,10,13-trithia-15-crown-5 (dic-3S) were prepared
according to previously reported procedures.7
p-Thiocresol was obtained from Lancaster Synthesis Ltd.
Zinc tetrafluoroborate was purchased from Strem Chemicals
Inc. Cadmium nitrate tetrahydrate was purchased from Acros.
Zinc acetate dihydrate was obtained from Peking Reagent.
Tetra-n-butylammonium hexafluorophosphate (nBu4NPF6 ;
y Area of Excellence Scheme, University Grants Committee (Hong
Kong).
536
New J. Chem., 2002, 26, 536–542
DOI: 10.1039/b109497c
This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2002

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Lancaster, 98%) was purified by recrystallization with hot
ethanol three times and vacuum dried for over 24 h before use.
Sodium perchlorate (Aldrich, 98%), barium perchlorate
(Aldrich, 97%), lithium perchlorate (Aldrich, 99.9%) and
potassium hexafluorophosphate (Strem, 99.5%) were recrys-
tallized from hot methanol and vacuum dried before use.
Acetonitrile was distilled over calcium hydride before use.
Dichloromethane, after treatment with concentrated H2SO4
and aqueous NaHCO3 , was distilled over calcium hydride
before use. All other reagents were of analytical grade and
were used as received.
J ¼ 7.6Hz, aryl H ortho to S), 6.85 (d, 1H, J ¼ 7.8 Hz, C6H3),
7.10 (m, 5H, aryl H meta to S, C6H3), 7.38 (d, 1H, J ¼ 4.0 Hz,
C6H3), 7.60 (m, 2H, C5H4N), 8.00 (t, 1H, J ¼ 12.0 Hz,
C5H4N), 8.25 (s, 1H, CH=N), 8.70 (d, 1H, J ¼ 4.0 Hz, C5H4N).
Positive FAB-MS (m=z): 623 {M À SC6H4-CH3-p}þ. Anal.
calcd for C34H38N2O4S2SeZnÁH2O: C, 53.37; H, 5.27; N, 3.66;
found: C, 53.14; H, 5.28; N, 3.46.
[Zn(SC6H4-CH3-p)2(dic-3S)] (5). This complex was pre-
pared similarly to 1 except that dic-3S was used in place of dic
1
to give 5 as red crystals. Yield: 130 mg (65%). H NMR (300
MHz, CDCl3): d 2.15 (s, 6H, CH3), 2.90 (m, 6H, CH2SCH2),
3.05 (m, 6H, CH2SCH2), 4.00 (t, 2H, J ¼ 4.8 Hz, C6H3OCH2),
4.30 (t, 2H, J ¼ 5.2 Hz, C6H3OCH2), 6.70 (d, 4H, J ¼ 7.8 Hz,
aryl H ortho to S), 6.85 (d, 1H, J ¼ 7.9 Hz, C6H3), 7.10 (m, 5H,
aryl H meta to S, C6H3), 7.42 (d, 1H, J ¼ 4.2 Hz, C6H3), 7.59
(m, 2H, C5H4N), 8.00 (t, 1H, J ¼ 12.9 Hz, C5H4N), 8.28
(s, 1H, CH=N), 8.62 (d, 1H, J ¼ 4.2 Hz, C5H4N). Positive
FAB-MS (m=z): 613 {M À SC6H4-CH3-p}þ. Anal. calcd for
C34H38N2O2S5ZnÁH2O: C, 54.42; H, 5.37; N, 3.73; found: C,
54.64; H, 5.21; N, 3.34.
Synthesis
[Zn(SC6H4-CH3-p)2(dic)] (1). This complex was prepared
according to a modified method.9 A solution of dic (100 mg,
0.27 mmol) in MeOH (5 mL) was added dropwise to a mixture
of Zn(OAc)2Á2H2O (60 mg, 0.27 mmol) and p-thiocresol (67
mg, 0.54 mmol) in MeOH (10 mL) and the mixture was stirred
at room temperature for 4 h during which the solution colour
changed from colourless to yellow. After evaporation of the
solvent under reduced pressure, the residue was dissolved in
CH2Cl2 and diffusion of diethyl ether vapour into its con-
centrated solution gave 1 as yellow crystals. Yield: 110 mg
(60%). 1H NMR (300 MHz, CDCl3): d 2.15 (s, 6H, CH3), 3.80
(m, 10H, CH2OCH2), 3.98 (m, 4H, C6H3OCH2 , CH2OCH2),
4.18 (t, 2H, J ¼ 8.5 Hz, C6H3OCH2), 6.78 (d, 4H, J ¼ 7.6Hz,
aryl H ortho to S), 6.82 (d, 1H, J ¼ 8.7 Hz, C6H3), 7.08 (d, 4H,
J ¼ 7.7 Hz, aryl H meta to S), 7.15 (d, 1H, J ¼ 7.7 Hz, C6H3),
7.35 (d, 1H, J ¼ 3.9 Hz, C6H3), 7.58 (m, 2H, C5H4N), 8.00 (t, 1H,
J ¼ 13.6Hz, C 5H4N), 8.25 (s, 1H, CH=N), 8.65 (d, 1H, J ¼ 4.2
Hz, C5H4N). Positive FAB-MS (m=z): 559 {M À SC6H4-CH3-
p}þ. Anal. calcd for C34H38N2O5S2ZnÁ½H2O: C, 58.91; H,
5.67; N, 4.04; found: C, 58.89; H, 5.27; N, 3.61.
Physical measurements and instrumentation
1H NMR spectra were recorded on a Bruker DPX-300
FT-NMR spectrometer in CDCl3 at 298 K and chemical shifts
are reported relative to Me4Si. Positive-ion FAB mass spectra
were recorded on a Finnigan MAT95 mass spectrometer.
Elemental analyses of the new complexes were performed on a
Carlo Erba 1106elemental analyzer at the Institute of Chem-
istry, Chinese Academy of Sciences. UV=vis spectra were
obtained on a Hewlett–Packard 8452A diode array spectro-
photometer, and steady-state excitation and emission spectra
on a Spex Fluorolog 111 spectrofluorimeter. The electronic
absorption spectral titration for binding constant determina-
tions was performed with a Hewlett–Packard 8452A diode
array spectrophotometer at 25 ꢀC, which was controlled by a
Lauda RM6compact low-temperature thermostat. Supporting
electrolyte (0.1 mol dmÀ3 nBu4NPF6) was added to maintain
the ionic strength of the sample solution constant during the
titration in order to avoid any effects arising from a change in
the ionic strength of the medium. This is especially important
for complexes showing a LLCT transition since their absorp-
tion characteristics are usually rather sensitive to the nature of
the solution medium.
Cyclic voltammetric measurements were performed by using
a CH Instruments, Inc. CHI 620 electrochemical analyzer
interfaced to an IBM-compatible PC. The electrolytic cell used
was a conventional two-compartment cell. The salt bridge of the
reference electrode was separated from the working electrode
compartment by a Vycor glass bridge. A Ag=AgNO3 (0.1 mol
dmÀ3 in CH3CN) reference electrode wasused. The ferrocenium-
ferrocene couple (FeCp2þ=0) was used as the internal reference
in the electrochemical measurements.10a The working elec-
trode was a glassy carbon (Atomergic Chemetals V25) electrode
with a platinum foil acting as the counter electrode. Treat-
ment of the electrode surfaces was as reported previously.10b
Binding studies using both UV-vis and emission spectro-
photometric measurements were performed by adding aliquots
of the metal salt to a solution of the zinc(II) dithiolate crown
ether containing complexes in the presence of supporting
electrolyte (0.1 mol dmÀ3 nBu4NPF6) while maintaining the
concentration of the zinc complexes constant. Binding con-
stants for 1 : 1 complexation were obtained by a nonlinear
least-squares fit10c of the absorbance (X) versus the con-
centration of the metal ion added (cM) according to eqn. (1):
[Zn(SC6H4-CH3-p)2(dic-18-6)] (2). Complex 2 was prepared
similarly to 1 except that dic-18-6was used in place of dic to
give yellow crystals. Yield: 125 mg (63%). 1H NMR (300
MHz, CDCl3): d 2.15 (s, 6H, CH3), 3.75 (m, 14H, CH2OCH2),
4.00 (m, 4H, C6H3OCH2 , CH2OCH2), 4.20 (t, 2H, J ¼ 8.2 Hz,
C6H3OCH2), 6.68 (d, 4H, J ¼ 7.6Hz, aryl H ortho to S), 6.83
(d, 1H, J ¼ 8.4 Hz, C6H3), 7.10 (d, 4H, J ¼ 7.5 Hz, aryl H meta
to S), 7.18 (d, 1H, J ¼ 7.4 Hz, C6H3), 7.35 (d, 1H, J ¼ 4.0 Hz,
C6H3), 7.50 (d, 1H, J ¼ 4.2 Hz, C5H4N), 7.62 (t, 1H, J ¼ 12.6
Hz, C5H4N), 8.00 (t, 1H, J ¼ 12.9 Hz, C5H4N), 8.27 (s, 1H,
CH=N), 8.62 (d, 1H, J ¼ 4.5 Hz, C5H4N). Positive FAB-
MS (m=z): 603 {M À SC6H4-CH3-p}þ. Anal. calcd for
C36H42N2O6S2ZnÁH2O: C, 57.94; H, 5.94; N, 3.75; found: C,
58.06; H, 5.96; N, 3.51.
[Zn(SC6H4-CH3-p)2(dic-S)] (3). This complex was prepared
similarly to 1 except that dic-S was used in place of dic to give
3 as yellow crystals. Yield: 118 mg (62%). 1H NMR (300 MHz,
CDCl3): d 2.15 (s, 6H, CH3), 2.85 (m, 4H, CH2SCH2), 3.80
(m, 8H, CH2OCH2), 3.90 (t, 2H, J ¼ 12.6Hz, C 6H3OCH2),
4.15 (t, 2H, J ¼ 6.4 Hz, C6H3OCH2), 6.65 (d, 4H, J ¼ 7.5 Hz,
aryl H ortho to S), 6.85 (d, 1H, J ¼ 7.8 Hz, C6H3), 7.08 (m, 5H,
aryl H meta to S, C6H3), 7.38 (d, 1H, J ¼ 4.0 Hz, C6H3), 7.60
(m, 2H, C5H4N), 8.00 (t, 1H, J ¼ 12.3 Hz, C5H4N), 8.25
(s, 1H, CH=N), 8.62 (d, 1H, J ¼ 4.1 Hz, C5H4N). Positive
FAB-MS (m=z): 575 {M À SC6H4-CH3-p}þ. Anal. calcd for
C34H38N2O4S3ZnÁCH2Cl2: C, 55.79; H, 5.29; N, 3.77; found:
C, 55.96; H, 5.40; N, 3.61.
[Zn(SC6H4-CH3-p)2(dic-Se)] (4). This complex was prepa-
red similarly to 1 except that dic-Se was used in place of dic
to give 4 as orange crystals. Yield: 81 mg (40%). 1H NMR
(300 MHz, CDCl3): d 2.15 (s, 6H, CH3), 2.87 (m, 4H,
CH2SeCH2), 3.85 (m, 8H, CH2OCH2), 3.90 (t, 2H, J ¼ 4.3 Hz,
C6H3OCH2), 4.20 (t, 2H, J ¼ 6.4 Hz, C6H3OCH2), 6.70 (d, 4H,
Xlim À X0
X ¼ X0 þ
[c0 þ cM þ 1=KS
2c0
À [(c0 þ cM þ 1=KS)2 À 4c0cM]1=2
]
(1)
New J. Chem., 2002, 26, 536–542
537

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where X0 and X are the absorbance of the complex at a
selected wavelength in the absence and presence of the metal
cation, respectively, c0 is the concentration of the complex, cM
is the concentration of the metal cation, Xlim is the limiting
value of absorbance in the presence of excess metal ion and KS
is the stability constant.
For emission titration studies, eqn. (1) can be modified to
give eqn. (2), written as:
Ilim À I0
I ¼ I0 þ
[c0 þ cM þ 1=KS
2c0
À [(c0 þ cM þ 1=KS)2 À 4c0cM]1=2
]
(2)
where I0 and I are the emission intensities of the complex at a
selected wavelength l in the absence and presence of the metal
cation, respectively, and Ilim is the limiting value of emission
intensity in the presence of excess metal ion.
For proton NMR titration experiments, a solution of Zn(II)
1
crown ether containing complex was prepared. The initial H
NMR spectrum was recorded and aliquots of metal cations
were added using a micro-syringe. After each addition and
mixing, the spectrum was recorded and changes in the che-
mical shift of certain protons were noted. The results of the
experiment were expressed as a plot of chemical shift as a
function of the amount of cation added, which was subjected
to analysis by curve fitting since the shape of the titration curve
is indicative of the stability constant for complex formation.
The computer program EQNMR10d was used, which requires
a knowledge of the concentration of each component and the
observed chemical shift for each data point.
Fig. 1 Perspective drawing of 5 with the atomic numbering scheme.
Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are
drawn at the 40% probability level.
Crystal structure determination
which have been reported previously.7 Reaction of
Zn(OAc)2Á2H2O with p-thiocresol and crown ether ligands in a
molar ratio of 1 : 2 : 1 afforded a series of zinc(II) dithiolate
crown complexes. All of them gave satisfactory elemental
analyses and were characterized by 1H NMR spectroscopy and
positive-ion FAB mass spectrometry.
Single crystals of 5 were obtained by vapour diffusion of di-
ethyl ether into a concentrated dichloromethane solution of
the complex. A red crystal of dimensions 0.40 Â 0.20 Â 0.10
mm mounted on a glass fibre was used for data collection at
28 ꢀC on a Rigaku AFC7R diffractometer with graphite
+
monochromatized Mo-Ka radiation (l ¼ 0.71069 A) using
o À 2y scans with o-scan angle (0.73 þ 0.35 tan y)ꢀ at a scan
speed of 16.0 deg minÀ1 [up to 6scans for reflections with
I < 15s(I)]. The total number of reflections was 5769, of which
5460 were unique and Rint ¼ 0.034. There were 3871 observed
reflections with I > 3s(I), used in the structural analysis. The
space group was determined based on a statistical analysis of
intensity distribution and the successful refinement of the
structure, which was solved by Patterson methods, expanded
by Fourier methods (PATTY)11a and refined by full-matrix
least-squares using the software package TeXsan11b on a Sili-
con Graphics Indy computer. Convergence for 416variable
X-Ray crystal structure determination
The crystal structure of complex 5 with atomic numbering is
+
depicted in Fig. 1. Selected bond distances (A) and bond
angles (ꢀ) are displayed in Table 1. The Zn(II) centre adopts
a distorted tetrahedral geometry with an S(1)–Zn(1)–S(2)
angle of 129.08(6)ꢀ and a N(1)–Zn(1)–N(2) angle of 78.9(1)ꢀ.
The average bond distances of Zn–S and Zn–N are 2.263(2)
and 2.124(4) A, respectively, which are found to be typical
+
of related systems.9b,12
parameters by least-squares refinement on
F
with
Electronic absorption and photophysical properties
w ¼ 4Fo2=s2(Fo2), where s2(Fo2) ¼ [s2(I) þ (0.050Fo2)2] was
The photophysical data for 1–5 are summarized in Table 2.
All the complexes show low-energy absorption bands at
reached at R ¼ 0.048 and wR ¼ 0.071.
Crystal
data
for
5. [C34H38N2O2S5ZnÁ½CH3OHÁ
-
+
Table 1 Selected bond distances (A) and angles (ꢀ) for 5
½C2H5OH], M ¼ 771.42, triclinic, space group P1(No. 2), a ¼
+
11.270(3), b ¼ 11.754(3), c ¼ 15.670(4) A, a ¼100.07(3), b ¼
+
98.45(3), g ¼ 111.88(3)ꢀ, U ¼ 1844(1) A3, Z ¼ 2, Dc ¼ 1.39 g
Zn(1)–S(1)
Zn(1)–N(1)
S(1)–C(1)
2.268(2)
2.112(4)
1.788(6)
1.813(6)
1.783(6)
1.86(2)
Zn(1)–S(2)
Zn(1)–N(2)
S(2)–C(8)
2.258(2)
2.136(4)
1.773(6)
1.805(5)
1.95(2)
cmÀ3, m(Mo-Ka) ¼ 9.9 cmÀ1, T ¼ 301 K.
CCDC reference number 181895. See http:==www.rsc.org=
suppdata=nj=b1=b109497c= for crystallographic data in CIF or
other electronic format.
S(3)–C(25)
S(4)–C(27)
S(5)–C(29)
S(3)–C(26)
S(4)–C(28)
S(5)–C(30)
1.793(8)
S(1)–Zn(1)–S(2)
S(1)–Zn(1)–N(2)
S(2)–Zn(1)–N(2)
Zn(1)–S(1)–C(1)
Zn(1)–N(1)–C(15)
Zn(1)–N(2)–C(20)
C(25)–S(3)–C(26)
C(29)–S(5)–C(30)
129.08(6)
105.8(1)
113.3(1)
105.2(2)
129.7(3)
112.3(3)
105.1(3)
104.3(5)
S(1)–Zn(1)–N(1)
S(2)–Zn(1)–N(1)
N(1)–Zn(1)–N(2)
Zn(1)–S(2)–C(8)
Zn(1)–N(1)–C(19)
Zn(1)–N(2)–C(21)
C(27)–S(4)–C(28)
115.7(1)
103.1(1)
78.9(1)
104.4(2)
112.9(3)
124.9(3)
94.0(5)
Results and discussion
The ligands dic, dic-18-6, dic-S, dic-Se and dic-3S were syn-
thesized by the condensation reaction of the corresponding
40-aminobenzocrown and 2-pyridinecarboxaldehyde in ethanol,
modified from the procedure reported for N-(2-pyridyl-
methylene)-4-aminobenzo-15-crown-5;8 the procedures of
538
New J. Chem., 2002, 26, 536–542

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Table 2 Photophysical data for complexes 1–5
Medium (T=K)
l
abs=nm (e=dm3 molÀ1 cmÀ1
)
lem=nm (t0=ms)
1
2
3
4
5
Solid (77)
570sh (<0.01)
505
524
520
525sh (<0.01)
500
516
510
554sh (<0.01)
502
496
516
595sh (<0.01)
500
492
508
Glass (77)a
MeCN (298)
CH2Cl2 (298)
Solid (77)
250 (27,070), 376(7,980)
252 (26,140), 388 (8,760)
Glass (77)a
MeCN (298)
CH2Cl2 (298)
Solid (77)
262 (21,860), 394 (9,600)
260 (29,600), 402 (10,630)
Glass (77)a
MeCN (298)
CH2Cl2 (298)
Solid (77)
262 (24,360), 388 (8,010)
262 (29,500), 403 (10,730)
Glass (77)a
MeCN (298)
CH2Cl2 (298)
Solid (77)
Fig. 3 The electronic absorption spectral traces of 1 (9 Â 10À5 mol
dmÀ3) in acetonitrile (0.1 mol dmÀ3 nBu4NPF6) upon addition of li-
thium perchlorate at 298 K. The insert shows a plot of absorbance
262 (19,140), 386 (6,400)
260 (25,080), 403 (11,680)
þ
568sh (<0.01)
&
versus [Li ] monitored at l ¼ 410 nm ( ) and the theoretical fit (——).
Glass (77)a
MeCN (298)
CH2Cl2 (298)
482
490
504
270 (33,820), 390 (9,890)
268 (35,480), 406 (11,780)
binding may be suggestive of the dominant character of the IL
n ! p*(L) transition, since the binding of a metal ion to the
polyether cavity is expected to reduce the availability of the
lone pair electrons on the oxygens, leading to a blue shift in
the n ! p* transition. On addition of a large excess of free dic
or dic-18-6ligands to an acetonitrile solution of 1 or 2, the
spectral changes were reversible because of the competition of
the large excess of free crown ethers with the complexes for the
metal cations. The addition of metal cations to degassed
MeCN solutions of this class of complexes led to almost no
changes in the emission spectra. Therefore, the binding of the
metal cations could not be studied by the emission spectro-
photometric method. The change in absorbance of complex 1
upon addition of lithium ions is shown in Fig. 3. The insert
shows the change of absorbance at 410 nm as a function of
lithium ion concentration. For complex 1, the stability con-
stants for the binding of Liþ and Naþ ions are log Ks ¼ 4.13
and 3.73, respectively. The close agreement of the experimental
data with the theoretical fit is supportive of a 1 : 1 stoichio-
metry, which is in accordance with the size match of Liþ and
Naþ to the crown cavity, resulting in preferential binding
(Table 3). With Kþ and Ba2þ ions, no well-defined isosbestic
points were observed in the UV-visible absorption titration
spectral traces, indicative of the formation of both 1 : 1 and
1 : 2 adducts in the solution mixture. No satisfactory fits to
eqn. (1) were obtained.
a
EtOH : MeOH ¼ 4 : 1 (v=v).
ca. 376–406 nm and a higher energy band at ca. 250–270 nm in
both acetonitrile and dichloromethane solutions. The latter is
tentatively assigned as intraligand (IL) transitions of thiolate
ligands since free RSÀ is found to absorb at similar energy.
With reference to previous spectroscopic work on the related
monomeric [Zn(phen)(SR)2],9,12c,e,13,14 the broad low-energy
band at ca. 376–406 nm is suggested to be a mixture of ligand-
to-ligand charge transfer [LLCT, pp(SRÀ) ! p*(L)] and IL
[p(L) ! p*(L) or n ! p*(L)] transitions, where L ¼ dic, dic-
18-6, dic-S, dic-Se or dic-3S.
Similar to the related mononuclear [Zn(N–N)(SR)2] sys-
tem,13 complexes 1–5 are found to be luminescent with emis-
sion maxima at ca. 490–525 nm both in 77 K glasses and at
room temperature in degassed acetonitrile and dichloro-
methane solutions upon excitation at l > 350 nm, but are
only slightly emissive in the low temperature solid states.
With reference to previous work on the related mononuclear
[Zn(N–N)(SR)2] system,9,12c,14 the emissions are most prob-
ably derived from the lowest energy LLCT triplet excited
state. The emission spectrum of 3 in a 77 K glass is depicted
in Fig. 2.
For complex 2, the Ba2þ, Kþ and Naþ ions all gave a perfect
fit to eqn. (1) in acetonitrile, based on 1 : 1 stoichiometry, with
different log Ks values (Table 3), depending on the size match
between the dic-18-6crown cavity and the various metal
ions.4d,e,15 Barium ions gave the largest stability constant with
log Ks ¼ 4.69, while the stability constants are log Ks ¼ 4.26
and 3.26for potassium and sodium ions, respectively. Since the
ionic diameters of Ba2þ and Kþ match the cavity size of
Cation binding studies
Upon addition of aliquots of alkali and alkaline earth metal
salts such as LiClO4 , NaClO4 , KPF6 or Ba(ClO4)2 to a solu-
tion of 1 or 2, the absorption at 376–394 nm in acetonitrile
shifts to higher energy. The observed blue shift upon cation
Table 3 Binding constants of complexes 1 and 2 for alkaline and alka-
line earth metal cations in MeCN (0.1 mol dmÀ3 nBu4NPF6) at 298 K
log Ks
Liþ
Naþ
Ba2þ
Kþ
c
c
1
2
4.13a
4.06b
1.86a
3.73a
3.53b
3.26a
3.19b
—
—
—
c
—
c
4.69a
4.30b
4.26a
c
c
—
—
a
b
From UV-visible spectrophotometric method. From 1H NMR
c
spectroscopic method. Not determined.
Fig. 2 Emission spectrum of 3 in ethanol–methanol (4 : 1 v=v) glass
at 77 K.
New J. Chem., 2002, 26, 536–542
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Table 4 Binding constants of complexes 3–5 for transition metal
cations in MeCN (0.1 mol dmÀ3 nBu4NPF6) at 298 K
log Ks
Zn2þ
Cd2þ
3
4
3.43a
3.50b
3.62c
4.41a
4.57b
4.22c
3.41a
3.64b
3.06a
2.99b
3.14c
3.80a
3.58b
3.62c
3.56a
3.49b
5
Fig. 4 The electronic absorption spectral traces of 5 (1 Â 10À4 mol
dmÀ3) in acetonitrile (0.1 mol dmÀ3 nBu4NPF6) upon addition of Cd2þ
at 298 K. The insert shows a plot of absorbance versus [Cd2þ] mon-
a
b
From UV-visible spectrophotometric method. From emission
spectrophotometric method. From 1H NMR spectroscopic method.
c
&
itored at l ¼ 350 nm ( ) and the theoretical fit (——).
dic-18-6best, 4d,15 a higher affinity of 2 for these cations relative
to Naþ ion is expected. The larger binding constant for Ba2þ
than Kþ is in line with the higher positive charge on Ba2þ
.
The binding constant of 2 for Liþ ions is the smallest
(log Ks ¼ 1.86) compared to that of Naþ, Kþ and Ba2þ ions,
in line with its smallest size.
The ion-binding experiments have also been carried out
1
using H NMR spectroscopy. A shift in the proton signals of
the polyether cavity has been observed upon metal ion binding
and the stability constants were determined using the EQNMR
program.10d The stability constants for 1 and 2 thus deter-
mined are also given in Table 3.
Unlike complexes 1 and 2, addition of aliquots of alkali and
alkaline earth metal cations to an acetonitrile solution of
complexes 3–5 gave no observable changes in the UV-visible
spectra. However, a blue shift in energy was observed upon
addition of the transition metal ions Zn2þ and Cd2þ. The shift
in absorption energy for zinc(II) crown ether-containing com-
plexes was found to be larger compared to that for a related
copper(I) system.7 The stability constants determined for
complexes 3–5 are collected in Table 4. The electronic
absorption spectra of complex 5 upon addition of Cd2þ ions
are depicted in Fig. 4. The insert shows the change of absor-
bance at 350 nm as a function of Cd2þ ion concentration.
Complexes 3–5 in degassed acetonitrile solution show only
one emission with maxima at ca. 490–496nm upon excitation
at l > 350 nm. Addition of Zn2þ or Cd2þ ions to the degassed
MeCN solution of this class of complexes gave rise to a dra-
matic enhancement of the emission intensities with a shift of
emission maxima from 490 to 560 nm upon excitation at the
isosbestic wavelength. Such an enhancement in the emission
intensity could be explained by the blocking of the reduc-
tive intramolecular electron-transfer quenching mechanism
(Scheme 2). The stability constants obtained using emission
Fig. 5 Emission spectral changes of 5 in acetonitrile (0.1 mol dmÀ3
nBu4NPF6) upon addition of Cd2þ at 298 K. Excitation was at the
isosbestic wavelength of 392 nm. The insert shows a plot of emission
intensity versus [Cd2þ] monitored at l ¼ 558 nm ( ) and the theore-
&
tical fit (——).
and 1H NMR methods are comparable to those determined
using the UV-visible spectrophotometric method, and are also
1
summarized in Table 4. However, the H NMR method could
not be employed for the binding studies of 5 due to the low
solubility of the complex. Fig. 5 displays the changes in the
emission spectra of complex 5 in acetonitrile upon addition of
Cd2þ at 298 K. The insert shows the changes in emission
intensity as a function of Cd2þ ion concentration monitored at
558 nm. The close resemblance of the experimental data to the
theoretical fits using both the UV-visible and emission meth-
ods is supportive of a 1 : 1 complexation model. Similar
findings have also been reported in a related copper(I) system.7
Scheme 2
540
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Table 5 Electrochemical data for complexes 1–5a
of metal ions into the crown cavity would give rise to a per-
turbation of the reduction potential to less cathodic values,
in line with the increased p-accepting ability of the dic-type
ligands upon metal ion binding (Fig. 6).
Oxidation
Reduction
b
E1/2 or Epac=V vs. SCE
(in cation)
E1/2b=V vs. SCE
(in cation)
Conclusion
1
2
3
4
5
þ1.50b (þ1.51)d
þ1.17c (þ1.17)d
þ1.49b (þ1.49)e
þ1.19c (þ1.18)e
þ1.46b (þ1.46) f
þ1.22c (þ1.21) f
þ1.50b (þ1.49) f
þ1.24c (þ1.22) f
þ1.48b (þ1.47) f
þ1.24c (þ1.24) f
À1.06( À0.95)d
À1.11 (À1.03)e
À1.09 (À0.77) f
À1.10 (À0.96) f
À1.08 (À0.86) f
A series of zinc(II) dithiolate crown ether-containing complexes
have been successfully synthesized. The electronic absorption,
emission, electrochemical and binding properties of these sys-
tems have been studied. The crystal structure of [Zn(SC6H4-
CH3-p)2(dic-3S)] (5) has also been determined. The low energy
absorption bands of these complexes are suggested to be
a
mixture of ligand-to-ligand charge transfer [LLCT,
pp(SRÀ) ! p*(L)] and IL [p(L) ! p*(L) or n ! p*(L)] transi-
tions. The emissions are thought to be associated with the
lowest energy LLCT triplet excited state. The ion-binding
In acetonitrile (0.1 mol dmÀ3 nBu4NPF6). Working electrode: glassy
a
carbon; DEp of Fcþ=Fc ranges from 60–64 mV; scan rate,
1
100 mV sÀ1
and cathodic peak potentials of the quasi-reversible reduction and
.
E1=2 ¼ (Epa þ Epc)=2 where Epa and Epc are the anodic
b
stability constants obtained from UV-visible, emission and H
NMR experiments were found to be in good agreement with
each other.
c
oxidation couples. Epa refers to the anodic peak potential of the
d
irreversible oxidation wave. E1=2 vs. SCE in the presence of excess
e
NaClO4 . E1=2 vs. SCE in the presence of excess KPF6 . E1=2 vs.
f
SCE in the presence of excess Zn(BF4)2 .
Acknowledgements
V.W.-W.Y. acknowledges financial support from the Research
Grants Council (HKU 7097=01P) and the Generic Drug Pro-
gram of The University of Hong Kong. Y.-L.P. acknowledges
the receipt of a postgraduate studentship, administered by The
University of Hong Kong. Dr. M. J. Hynes is gratefully
acknowledged for providing us with the EQNMR program.
Electrochemical properties
The electrochemical data for complexes 1–5 in acetonitrile (0.1
mol dmÀ3 nBu4NPF6) are tabulated in Table 5. The cyclic
voltammogram of 5 is shown in Fig. 6. Cyclic voltammetric
studies of complexes 1–5 in acetonitrile show similar cyclic
voltammograms with a quasi-reversible and an irreversible
oxidation wave, tentatively assigned as thiolate ligand-centred
oxidations, and a quasi-reversible reduction couple, probably
attributable to the diimine ligand-centred reduction.12e The
similarity of the oxidation potentials is in accord with an
assignment of a thiolate ligand-centred oxidation since all the
complexes possess the same thiolate ligand. It is interesting to
find that upon addition of metal ions, the crown-based
reduction potential was shifted anodically by about 80–320 mV
while the oxidation waves remain more or less undisturbed.
This further supports the assignment of the reduction process
as a diimine-crown ligand-centred reduction, in which binding
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