Selective sensing of dihydrogen phosphate anion by a fluorescent tetranuclear pentacoordinated zinc(II) complex

Article abstract of DOI:10.1039/b616451j

A cresolic oxygen bridging ligand, 2,6-bis{[(2-hydroxybenzyl)(2- hydroxyethyl)amino]methyl}-4-methylphenol (L), has been synthesized and characterized. The coordination of Zn^II ions with L gives a novel tetranuclear complex, [Zn^II

Full text of DOI:10.1039/b616451j

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PAPER
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Selective sensing of dihydrogen phosphate anion by a fluorescent
tetranuclear pentacoordinated zinc(II) complexwz
Zhanfen Chen,a Xiaoyong Wang,*b Jingwen Chen,a Xiaoliang Yang,a Yizhi Lia and
Zijian Guo*a
Received (in Montpellier, France) 10th November 2006, Accepted 3rd January 2007
First published as an Advance Article on the web 23rd January 2007
DOI: 10.1039/b616451j
A cresolic oxygen bridging ligand, 2,6-bis{[(2-hydroxybenzyl)(2-hydroxyethyl)amino]methyl}-4-
methylphenol (L), has been synthesized and characterized. The coordination of ZnII ions with L
gives a novel tetranuclear complex, [ZnII4(Lꢀ3H)2](ClO4)2 ꢁ 3.5H2O (I), which crystallizes in a
ꢀ
triclinic system with space group P1, a = 12.261(5) A, b = 13.887(6) A, c = 20.738(8) A, a =
89.996(7)1, b = 88.163(7)1 and g = 85.016(7)1. The cationic core of I is formed by four ZnII
cations bridged by two cresolic and four phenolic oxygen atoms from two ligands. All four ZnII
1
centers are pentacoordinated and adopt a distorted square pyramid geometry. ESMS and H
NMR data indicate that I is stable in solution, and the fluorescence measurement demonstrates
that it has strong fluorescence at lex = 298 nm. The fluorescent complex can selectively sense the
dihydrogen phosphate anion in methanol, and the binding phenomenon can be monitored via
UV-vis absorption changes and fluorescence quenching effects. The potential binding mode of I
ꢀ
1
with H2PO4 has been studied by ESMS and H NMR spectroscopy.
The incorporation of transition metals into the design of
Introduction
anion receptors opened up a new avenue in this area.7 In
contrast to purely organic structures, metal-containing recep-
tors can provide unsaturated coordination sites at the metal
centers and present some geometrical preference for anions of
a given shape.8 Based on this strategy, a variety of metal-based
phosphate anion receptors have been developed in last
decade.9 For these receptors, sensing is often realized by
electrochemical or optical means, especially the latter.10
In this paper, a cresolic oxygen-bridging ligand, 2,6-bis
{[(2-hydroxybenzyl)(2-hydroxyethyl)amino]methyl}-4-methyl-
phenol (L, Scheme 1), and its ZnII complex, [ZnII4(Lꢀ3H)2]
(ClO4)2 ꢁ 3.5H2O (I), were synthesized. The complex shows
The design and synthesis of anion receptors or sensors have
attracted considerable attention due to the importance of
anions in biological and environmental systems.1 Among these
endeavours, the development of selective receptors for phos-
phate anions is of particular interest because they play vital
roles in a wide range of life processes, such as energy storage,
signal transduction and gene construction.2 However, the
intrinsic properties of the phosphate ion render the design of
effective receptors a challenging task.3 Various phosphate
anion receptors have been reported recently, especially those
for the dihydrogen phosphate anion.4 The majority of them
are organic molecules bearing groups such as acylamino and
hydroxyl, which are thought to form hydrogen bonds with
phosphate anions. The design of these receptors is particularly
demanding because a single hydrogen bonding interaction of
non-activated amides or alcohols with anions is weak,5 and the
receptors must rely on the concerted action of many such
donors. Moreover, these receptors usually have very elaborate
structures and require sophisticated synthetic work.6
strong ꢀfluorescence and
a selective sensing ability for
H2PO4 ions over other anions in methanol solution.
Results and discussion
Synthesis of ligand and complex
The phenolate-based ligand was prepared by a similar method
described previously.11 Such types of ligand are commonly
a State Key Laboratory of Coordination Chemistry, School of
Chemistry and Chemical Engineering, Nanjing University, Nanjing
210093, P. R. China. E-mail: zguo@nju.edu.cn;
Fax: +86 25-83314502
b State Key Laboratory of Pharmaceutical Biotechnology, School of
Life Science, Nanjing University, Nanjing 210093, P. R. China.
E-mail: boxwxy@nju.edu.cn
w Electronic supplementary information (ESI) available: Details of the
X-ray structure of I, ESMS and 1H NMR spectra of the ligand and
complex, UV-vis and fluorescence spectra of the ligand and complex in
the absence and presence of H2PO4ꢀ, and ESMS of the complex in the
presence of H2PO4ꢀ. See DOI: 10.1039/b616451j
z The HTML version of this article has been enhanced with colour
Scheme 1 Chemical structure of 2,6-bis{[(2-hydroxybenzyl)(2-hydro-
xyethyl)amino]methyl}-4-methylphenol (L).
images.
ꢂc
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used to form dinuclear complexes as models of metallo-
enzymes in biological systems.12 A phosphodiesterase model
has been derived from an analogue of L in our laboratory.11 In
this work, we substituted the phenolic rings for pyridyl rings in
that ligand and obtained the novel cresolic oxygen bridging
ligand L (Scheme 1). The introduction of terminal phenolate
groups in L facilitated the formation of I. Complex I was
obtained simply by stirring L and Zn(ClO4)2 ꢁ 6H2O in metha-
nol. As characteristic data demonstrated (vide infra), four ZnII
ions can be chelated by two L to form a tetranuclear complex.
The ESMS spectrum of I shows two peaks at m/z 594.3 and
1185.2 (Fig. S1w), which correspond to [ZnII4(Lꢀ3H)2]2+ and
{[ZnII4(Lꢀ3H)2]ꢀH}+, respectively, suggesting the tetranuc-
lear complex is quite stable in solution. The isotopic distribu-
tion pattern perfectly matches the corresponding simulated
result using ISOPRO 3.0 (Fig. S1w).13 The 1H NMR spectrum
of I in CD3CN is shown in Fig. S2,w and all the proton
resonances have been fully assigned. The remarkable shifts of
the proton resonances of L strongly support the formation of
I. Interestingly, I appears to exist in tautomeric equilibria in
solution. The tautomerization may result from the chelation of
ethoxyl groups from different directions and conformational
variations in the chelate rings (Fig. 1). The 1H NMR spectrum
(Fig. S3Aw) shows 4 singlets for the methyl group, indicating
the existence of 4 different species. It is evident that one of the
tautomers is dominant.
Table
1
Crystallographic data and structure refinement for
[ZnII4(Lꢀ3H)2](ClO4)2 ꢁ 3.5H2O
Empirical formula
C54H69N4O21.5Cl2Zn4
1450.51
273
Formula weight
Temperature/K
Crystal size/mm
Crystal system
Space group
a/A
b/A
c/A
0.23 ꢃ 0.25 ꢃ 0.32
Triclinic
ꢀ
P1
12.261 (5)
13.887 (6)
20.738 (8)
89.996 (7)
88.163 (7)
85.016 (7)
3516 (2)
2
1.493
1494
0.71073
a/1
b/1
g1
V/A3
Z
m(Mo-Ka)/mmꢀ1
F(000)
Wavelength (Mo-Ka)/A
r
Reflections collected
Unique reflections (Rint
calc/g cmꢀ3
1.37
19230
)
13 536 (0.025)
9944
1.7–26.0
Observed data [I 4 2s(I)]
y range for data collection (1)
Limiting indices
ꢀ13 r h r 15,
ꢀ17 r k r 11,
ꢀ22 r l r 25
1.047
GOF on F2
Final R indices [I 4 2s(I)]a
R1 = 0.0610,
wR2 = 0.1207
R1 = 0.0884,
wR2 = 0.1268
R indices (all data)a
1
a
2
2 2
R1 = SJFo| ꢀ |FcJ/ S|Fo|, wR2 = [Sw(Fo ꢀ Fc2)2/ Sw(Fo ) ]2.
Crystal structure of complex I
ꢀ
Complex I crystallizes in the triclinic space group P1. The
The crystal structure of the cationic core of complex I is shown
in Fig. 1, with selected bond distances and angles shown in the
caption. The crystal data and refinement results for I are
summarized in Table 1.
tetranuclear cationic core is formed by four ZnII cations,
bridged by two cresolic oxygen atoms and four phenolic
oxygen atoms from two ligands. All four ZnII centers have a
similar pentacoordinate environment. The NO4 donor set at
ZnII is comprised of a tertiary amine nitrogen atom, an
alcoholic oxygen atom, a bridging cresolic oxygen atom and
two bridging phenolic oxygen atoms. The calculated structural
indices, t, for each ZnNO4 unit are as follows: tZn1 = 2.9%,
tZn2 = 4.3%, tZn3 = 6.5% and tZn4 = 4.3%, which indicate
that all four ZnII centers in I are distorted from the perfect
square-pyramidal coordination to a greater or lesser extent.14
The rhombic planes formed by the ZnII cations and phenolic
oxygen atoms (Zn2–O9–Zn3–O8 and Zn1–O5–Zn4–O4) are
basically parallel to each other. The phenolic rings are parallel
in pairs, and form four dihedral angles of 30.01 with the plane
formed of two cresolic oxygen atoms and four ZnII cations.
The distances between ZnII cations bridged by cresolic oxygen
Fig.
1 Crystal structure of the cationic core of complex
[ZnII4(Lꢀ3H)2](ClO4)2 ꢁ 3.5H2O (I). Counterions, water molecules
and hydrogen atoms in the complex are omitted for clarity. Selected
bond distances (A) and bond angles (1): Zn(1)–O(2) 1.971(3),
Zn(1)–O(4) 1.971(3), Zn(1)–O(5) 2.023(3), Zn(1)–O(6) 2.123(3),
Zn(1)–N(4) 2.130(3), Zn(2)–O(2) 2.024(3), Zn(2)–O(7) 2.122(3),
Zn(2)–O(8) 2.008(3), Zn(2)–O(9) 2.023(3), Zn(2)–N(3) 2.141(4),
Zn(3)–O(1) 1.966(3), Zn(3)–O(8) 2.020(3), Zn(3)–O(9) 1.990(3),
Zn(3)–O(10) 2.120(3), Zn(3)–N(2) 2.148(3), Zn(4)–O(1) 2.013(3),
Zn(4)–O(3) 2.172(3), Zn(4)–O(4) 2.022(3), Zn(4)–O(5) 1.961(3),
Zn(4)–N(1) 2.092(3), Zn(1)ꢁ ꢁ ꢁZn(2) 3.637(1), Zn(1)ꢁ ꢁ ꢁZn(4) 3.084(1),
Zn(2)ꢁ ꢁ ꢁZn(3) 3.096(1), Zn(3)ꢁ ꢁ ꢁZn(4) 3.626(1), Zn(3)–O(1)–Zn(4)
atoms are 3.637(1)
A
(Zn1ꢁ ꢁ ꢁZn2) and 3.626(1)
A
(Zn3ꢁ ꢁ ꢁZn4), respectively, and those bridged by phenolic
oxygen atoms are 3.084(1) A (Zn1ꢁ ꢁ ꢁZn4) and 3.096(1) A
(Zn2ꢁ ꢁ ꢁZn3), respectively. The former distances are longer
than those in similar bridged complexes (3.421 A),11 while
the latter are comparable to those for doubly-bridged species
(3.024 A).15 These distances may be crucial for the selective
recognition of phosphate anions (vide infra). The observed
bond distances between ZnII cations and alcoholic oxygen
atoms are in the range of 2.120(3)–2.172(3) A, but the average
distance between ZnII cations and deprotonated phenolic/
cresolic oxygens is 1.999(3) A. The smaller bond lengths in
131.39(14),
101.13(12),
Zn(1)–O(2)–Zn(2)
Zn(1)–O(5)–Zn(4)
130.08(14),
101.45(11),
Zn(1)–O(4)–Zn(4)
Zn(2)–O(8)–Zn(3)
100.44(12) and Zn(2)–O(9)–Zn(3) 100.96(12).
ꢂc
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the latter case may result from the electron donating effect of
the negatively charged oxygen atoms. The Zn1–O2–Zn2 and
Zn3–O1–Zn4 angles are 130.08(14) and 131.39(14)1, respec-
tively, much larger than those in similar ZnII complexes.16
Optical properties of complex I
The optical properties of complex I were investigated by UV-
vis and fluorescence spectroscopy. Two intense UV absorption
bands at 233 and 287 nm (e233 = 28 450 and e287 = 13 110 dm3
molꢀ1 cmꢀ1) were observed in a methanol solution of I
(Fig. S4w). The former could be assigned to a ligand-to-metal
charge transfer (LMCT) band and the latter to spin-allowed
intraligand p - p* transitions.17 The absorption at 210 nm is
the primary band of the phenol groups in L.
A strong fluorescence emission at 330 nm (lex = 298 nm),
with a quantum yield of 0.22, was also observed in a methanol
solution of I (Fig. S5w). Similar to other luminescent poly-
nuclear d10 metal complexes, the fluorescence emission could
be attributed to the chelation of L to the ZnII centers, which
enhances the rigidity of L and thus reduces the loss of energy
through a radiationless pathway.18 Specifically, the formation
of O–ZnII–O units in I may be associated with the interaction
between the highest occupied molecular orbital (HOMO) of
the ligand and the lowest unoccupied molecular orbital
(LUMO) of the ZnII. The HOMO is likely to be a hybrid of
pp orbitals of the cresolic and phenolic oxygen atoms and the
LUMO the symmetric combination of the four ZnII 4s orbi-
tals. The LMCT spin-forbidden transition of pp(O2ꢀ) -
4ss(ZnII) may contribute to the high-energy emission
at 330 nm.17
Fig. 2 Fluorescence emission changes of the tetranuclear complex
[ZnII4(Lꢀ3H)2](ClO4)2 ꢁ 3.5H2O (lex = 298 nm, 45 mM, methanol)
upon addition of H2PO4ꢀ, CO32ꢀ, HCO3ꢀ, NO3ꢀ, NO2ꢀ, Fꢀ, Clꢀ,
Brꢀ, Iꢀ and SO42ꢀ (90 mM, sodium salt, A), and an ascending gradient
ꢀ
of H2PO4 (0–135 mM, sodium salt, B) at room temperature.
The sensing property of complex I
It is known that the photoluminescent properties exhibited by
polynuclear d10 metal complexes are sensitive to structural
and/or environmental changes.19 Therefore, the anion sensing
properties of I were investigated against common anions such
as H2PO4ꢀ, CO32ꢀ, HCO3ꢀ, NO3ꢀ, NO2ꢀ, Fꢀ, Clꢀ, Brꢀ, Iꢀ
and SO42ꢀ. The fluorescence spectra of I in methanol upon
addition of these anions were recordꢀed, respectively. As shown
in Fig. 2A, the addition of H2PO4 (90 mM) to a methanol
solution of I (45 mM) induced a dramatic fluorescence quench-
ing effect on I; under the same conditions, anions such as
The LMCT band at 233 nm and the p - p* transition band at
287 nm decreased in intensity with the addition of H2PO4ꢀ. In
the meantime, the absorption bands at around 253 and 306 nm
increased gradually in intensity. The sharp isosbestic point at
278 nm, and the relatively less sharp ones at 249 and 300 nm,
suggest that more than one equilibrium is involved in the
system. The plot of the absorbance changes vs. [H2PO4ꢀ]/
[complex] at 233 nm (Fig. 3A inset) shows a typical profile for
the formation of a strong 1 : 1 adduct, followed by a weaker 2 :
ꢀ
1 (H2PꢀO4 to I) adduct. The ideal binding stoichiometry of
2ꢀ
CO32ꢀ, HCO3ꢀ, NO3ꢀ, NO2ꢀ, Fꢀ, Clꢀ, Brꢀ, Iꢀ and SO4
H2PO4 to I appears to be 2 : 1; however, to achieve absorp-
only caused moderate fluorescence quenching or enhancing
effects. The detailed concentratiꢀon dependent fluorescence
quenching profile of I by H2PO4 is shown in Fig. 2B. Thꢀe
decrease in fluorescence intensity with the addition of H2PO4
is notable. The quenching of fluorescence ꢀmay result partly
from the electron repelling effect of H2PO4 and pꢀartly from
the decrease of ligand rigidity because of H2PO4 binding.
Both effects can deter LMCT and thus reduce fluorescence
intensity. In contrast to this, L has no fluorescence under the
same conditions, and the addition of H2PO4ꢀ does not induce
any obvious change in the fluorescence spectrum. Therefore, L
tion saturation, more than 2 equivalents of [H2PO4ꢀ] are
ꢀ
required. The binding profile of H2PO4 to I was further
analyzed using a non-linear least-square method (sigmoidal
model), which gave a total association constant of ca. 1.0 ꢃ
ꢀ2 20
The result shows that I has a high affinity for
105
M
.
ꢀ 9d,21
H2PO4
.
The addition of other anions, including the
interfering anion SO42ꢀ, did not cause any significant change
to the UV spectrum of I. In comparison, the addition of
ꢀ
increasing H2PO4 to a methanol solution of L (45 mM) did
not induce any obvious change in UV absorption under the
same cꢀonditions, which indicates L alone does not bind to
H2PO4 (Fig. S7w).ꢀA comparison of spectral changes upon
addition of H2PO4 and HCl was made to ascertain the
formation of anion adducts. As shown in Fig. S8,w HCl only
caused very small changes in the UV spectra and moderate
ꢀ
alone cannot sense H2PO4 (Fig. S6w).
The UV-vis absorption spectra of complex I in methanol
upon addition of the above anions were also recorded. As seen
in Fig. 3A, I showed a sharp UV-vis response to H2PO4ꢀ ion.
ꢂc
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Fig.
4
1H NMR spectra (downfield region) of (A) complex
[ZnII4(Lꢀ3H)2](ClO4)2 ꢁ 3.5H2O (CD3OD/D2O, 5/2; 40 mM, 25 1C)
alone and (B) with H2PO4ꢀ in a 1 : 2 molar ratio. The peaks labelled *
correspond to tautomers of the complex.
within a distance of 3–4 A.22 In complex I, the separations
between ZnII cations fall precisely into thiꢀs range, distances
which are appropriate for accepting H2PO4 ions as bridging
ligands. Considering the accessibility of the metal centers,
Zn1/Zn4 and Zn2/Zn3 are very likely to act as pairs to binꢀd
two H2PO4ꢀ ions, though they are also likely to bind H2PO4ꢀ
ions independently. During the binding process, the H2PO4
ions may attack the ZnII centers and induce the chelated
alcoholic hydroxyl groups to dissociate from the ZnIIs, and
eventually bind to the fifth coordination site of the metal
centers (Scheme 2).23 This presumption is strongly supported
by the 1H NMR data (Fig. 4 and Fig. S3w). The signals for the
phenolic protons (Hb, Hc, Hd and He) of I significantly shifted
downfield (0.14–1.25 ppm) upon addition of H2PO4ꢀ, and
those for the methylene protons were largely simplified. This
suggested that the phenolic rings had been greatly influenced
by the binding of H2PO4ꢀ ions and that the alcoholic hydroxyl
groups had departed from the ZnII centers. Due to the
flexibility of the released alcoholic hydroxyl groups, the ex-
Fig. 3 UV spectra of the tetranuclear complex [ZnII4(Lꢀ3H)2]
ꢀ
(ClO4)2 ꢁ 3.5H2O (45 mM, methanol) upon addition of H2PO4
ꢀ
(0–135 mM, sodium salt, A) and addition of H2PO4ꢀ, CO32ꢀ, HCO3
,
NO3ꢀ, NO2ꢀ, Fꢀ, Clꢀ, Brꢀ, Iꢀ , SO4 (90 mM, sodium salt, B) at
room temperature. Inset in A shows the molar ratio plot against the
absorbance changes at 233 nm.
2ꢀ
changes in the fluorescence spectra. The results suggest that
the variations in spectroscopic properties of I were mainly
caused by the formation of a H2PO4ꢀ–I adduct rather than by
complex protonation brought about by proton transfer from
the anion.
ꢀ
istence of hydrogen bonds between I and H2PO4 cannot be
Potential binding mode
excluded, which may stabilize the complex–H2PO4ꢀ adduct. It
seems unlikely that H2PO4ꢀ ions could enter the central cavity
of I because of steric hindrance from the phenolic rings.
It was believed that the selective association of phosphate
anions could be feasible when two ZnII cations were located
Scheme 2 Proposed 2 : 1 binding mode of H2PO4 to the tetranuclear ZnII complex.
ꢀ
ꢂc
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(m), 1589 (m), 1484 (s), 1362 (m), 1256 (s), 1120 (m), 1047 (m),
869 (m) and 754 (s).
Conclusion
Anion sensors usually contain electrochemically or optically
active groups, allowing the binding of anions at metal centers
to be detected by a physical response. For example, a chromo-
genic pyrophosphate anion (PPi) sensor, based on a system
similar to complex I, has been reported recently,24 where a
color-inducing group was introduced at the para-position of
the phenolic moiety to generate a UV response. The sensor
described in this article does not contain such groups but still
displayed a significant optical response towards H2PO4ꢀ. The
tetranuclear ZnII complexꢀshowed a high affinity and remark-
able selectivity for H2PO4 ion over a variety of other anions
commonly found in biological systems at a concentration of
ca. 10ꢀ5 M. The recognition phenomenon can be easily
detected by both fluorescence quenching effects and UV
absorption changes. This might be an improvement on the
mentioned PPi system. Further modification of the present
complex is required to make it applicable in aqueous media.
The precursor, 2,6-bis(chloromethyl)-4-methylphenol, was
prepared by
a
literature method.29 The intermediate,
2-[(2-hydroxyethylamino)methyl]phenol, was synthesized by
condensing ethanolamine and salicylaldehyde in methanol
under reflux and reducing it with sodium borohydride at
0 1C under stirring.
[ZnII4(Lꢀ3H)2](ClO4)2 3.5H2O (I). Caution! The perchlo-
.
rate salts used in this study are potentially explosive and
therefore should be handled with great care. Ligand L (0.37
g, 0.80 mmol) and Zn(ClO4)2 ꢁ 6H2O (0.60 g, 1.60 mmol) were
dissolved in methanol (20 ml). After stirring at room tempera-
ture for 4 h, the resulting solution was concentrated under
reduced pressure and the white precipitate formed was filtered
off, washed with methanol/diethyl ether and dried under
vacuum (yield: 0.59 g, 72%). The product was dissolved in a
mixed solvent of methanol/ethanol (v/v, 1/1) and stood at
room temperature for two weeks. Colorless block single
crystals suitable for X-ray structure determination were ob-
tained by slow evaporation of the solvent. Elemental anal.
found (calc.) for Zn4C54H71N4O22.5Cl2 (%): C, 44.35 (44.16);
H, 4.68 (4.87); N, 3.84 (3.81). ESMS (m/z, methanol, positive
Experimental section
Materials and measurements
[{ZnII4(Lꢀ3H)2}–H]+
and
594.3,
All chemicals and reagents were of analytical grade and used
as received without further purification. Solvents were purified
and dried according to standard methods.25 IR spectra (as
KBr pellets) were recorded in the range of 500–4000 cmꢀ1 with
a Bruker VECTOR22 spectrometer. Elementary analyses were
carried out on a Perkin-Elmer 240C analytical instrument.
ESMS were obtained using a LCQ spectrometer (Finnigan).
The isotopic distribution patterns for the complex were simu-
lated using the ISOPRO 3.0 program.26 1H NMR experiments
were performed on a Bruker DRX-500 spectrometer at 298 K
using standard pulse sequences. UV-vis absorption spectra
were measured on a Perkin-Elmer Lambda UV-vis spectro-
photometer using matched quartz cuvettes (1.0 cm). Fluores-
cence determination was carried out on an AMINCO
Bowman series 2 spectrofluorophotometer. The quantum yield
of the complex at room temperature was determined using
tryptophan (Ff = 0.13, in aqueous solution) as a fluorescence
standard27 and calculated using the reported formula.28
mode):
1185.2,
[ZnII4(Lꢀ3H)2]2+. H NMR (500 MHz, CD3CN): d = 7.14
(s, 4 H), 7.06 (m, 4 H), 6.49 (t, J = 7.5 Hz, 4 H), 6.32 (d, J =
8.0 Hz, 4 H), 4.65 (d, J = 12.5 Hz, 4 H), 4.47 (d, J = 13 Hz, 4
H), 3.38 (d, J = 13 Hz, 8 H), 3.26 (m, 8 H), 2.91 (t, J = 5 Hz, 8
H), 2.28 (s, 3 H) and 2.11 (s, 3 H). FT-IR spectra (n/cmꢀ1):
3412 (br, m), 2906 (m), 1498 (m), 1481 (vs), 1450 (m), 1274
1
(vs), 1114 (vs), 877 (m) and 754 (m). UV-vis (methanol): lmax
/
nm (e/dm3 molꢀ1 cmꢀ1), 211 (55 600), 233 (28 450) and 287
(13 110).
Crystallographic determinationy
Crystal data were collected on a Bruker Smart Apex CCD area
detector using graphite monochromated Mo-Ka radiation
(l = 0.71073 A) at 273(2) K. The collected data were reduced
using the SAINT program30 and an empirical absorption
correction was carried out using a multi-scan program.31
The structure was solved by direct methods and refined on
F2 by full-matrix least-squares using the SHELXTL-97 pro-
gram.32 All the non-hydrogen atoms were anisotropically
refined. The hydrogen atoms of hydroxyl groups and partial
occupation water molecules were located in the Fourier dif-
ference map and treated as riding, with isotropic thermal
parameters 1.2 times the Ueq of the parent atom. All the
C–H atoms were positioned geometrically and treated as
riding with C–H = 0.93–0.97 A, and Uiso(H) = 1.2–1.5Ueq
of the parent atoms.
Syntheses
2,6-Bis{[(2-hydroxybenzyl)(2-hydroxyethyl)amino]methyl}-4-
methylphenol (L). The compound was prepared via a method
described for a similar ligand, except 2-[(2-hydroxyethyl-
amino)methyl]phenol (4.00 g, 20 mmol) was used instead of
2-pyridylmethyl-2-hydroxyethylamine.11 A yellow oily pro-
duct was obtained, which was further purified by silica gel
chromatography using methanol/chloroform (v/v, 1/99) as the
eluent. A yellow powder of L was obtained in a 76% yield.
Elemental anal. found (calc.) for C27H34N2O5 (%): C, 68.7
(69.50); H, 7.46 (7.34); N, 5.88 (6.00). ESMS (m/z, methanol):
467.1 [L + H]+. 1H NMR (500 MHz, CDCl3): d = 7.25 (m, 2
H), 7.03 (d, J = 7.5 Hz, 2 H), 6.90 (d, J = 8.0 Hz, 2 H), 6.79 (t,
J = 14.5 Hz, 4 H), 3.93 (t, J = 15.0 Hz, 4 H), 3.85 (s, 4 H),
3.83 (s, 4 H), 2.68 (s, 4 H), 2.64 (t, J = 6.0 Hz, 4 H) and 2.19 (s,
3 H). FT-IR (n/cmꢀ1): 3378 (br, s), 2949 (s), 2825 (m), 1611
Spectroscopic titrations
UV-vis titration experiments were carried out by recording a
series of UV-vis absorption spectra of L or complex I in
methanol solution, respectively, in the presence of common
y CCDC reference number 289366. For crystallographic data in CIF
or other electronic format see DOI: 10.1039/b616451j
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New J. Chem., 2007, 31, 357–362 | 361

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anions, including H2PO4ꢀ, CO32ꢀ, HCO3ꢀ, NO3ꢀ, NO2ꢀ, Fꢀ,
Clꢀ, Brꢀ, Iꢀ and SO42ꢀ. The concentration of L and I were
fixed at 45 mM, while those of each anion were changed until
the absorption reached saturation. Fluorescence emission
spectra were recorded under similar titration conditions to
those used in the UV-vis studies.
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Acknowledgements
Financial support from the National Natural Science Founda-
tion of China (nos. 20231010, 20228102 and 30370351) and the
Natural Science Foundation of Jiangsu Province (no. BK
2005209) is gratefully acknowledged.
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362 | New J. Chem., 2007, 31, 357–362