Dinuclear zinc(II) complexes of tetraiminodiphenol macrocycles and their interactions with carboxylate anions and amino acids. Photoluminescence, equilibria, and structure

Article abstract of DOI:10.1021/ic049056a

The reaction equilibria [H₄L]²⁺ + Zn(OAc)₂ ? [Zn(H₂L)]²⁺ + 2HOAc (K₁) and [Zn(H₂L)]²⁺ + Zn(OAc)₂ ? [Zn ₂L]²⁺ + 2HOAc (K₂), involving zinc acetate and the perchlorate salts of the tetraiminodiphenol macrocycles [H₄L ^1-3](ClO₄)₂, the lateral (CH₂) _n chains of which vary between n = 2 and n = 4, have been studied by spectrophotometric and spectrofluorimetric titrations in acetonitrile. The photoluminescence behavior of the complexes [Zn₂L¹] (ClO₄)₂, [Zn₂L²(H₂O) ₂](ClO₄)₂, [Zn₂L²(μ- O₂CR)](ClO₄) (R = CH₃, C₆H ₅, ρ-CH₃C₆H₄, ρ-OCH ₃C₆H₄, ρ-ClC₆H₄, ρ-NO₂C₆H₄), and [Zn₂L ³(μ-OAc)](ClO₄) have been investigated. The X-ray crystal structures of the complexes [Zn₂L²(H ₂O)₂]-(ClO₄)₂, [Zn₂L ³(μ-OAc)](ClO₄), and [Zn₂L ²(μ-OBz)(OBz)(H₃O)](ClO₄) have been determined. The complex [Zn₂L²(μ-OBz)(OBz)(H ₃O)](ClO₄) in which the coordinated water molecule is present as the hydronium ion (H₃O⁺) on deprotonation gives rise to the neutral dibenzoate-bridged compound [Zn₂L ²(μ-OBz)₂]·H₂O. The equilibrium constants (K) for the reaction [Zn₂L²(H₂O) ₂]²⁺ + A^- ? [Zn₂L ²A]⁺ + 2H₂O (K), where A^- = acetate, benzoate, or the carboxylate moiety of the amino acids glycine, L-alanine, L-histidine, L-valine, and L-proline, have been determined spectrofluorimetrically in aqueous solution (pH 6-7) at room temperature. The binding constants (K) evaluated for these systems vary in the range (1-8) × 10⁵.

Full text of DOI:10.1021/ic049056a

Inorg. Chem. 2005, 44, 147
−157
Dinuclear Zinc(II) Complexes of Tetraiminodiphenol Macrocycles and
Their Interactions with Carboxylate Anions and Amino Acids.
Photoluminescence, Equilibria, and Structure
Bula Dutta,† Pradip Bag,† Ulrich Florke,‡ and Kamalaksha Nag*,†
1
Department of Inorganic Chemistry, Indian Association for the CultiVation of Science,
JadaVpur, Kolkata 700032, India, and Anorganische und Analytische Chemie,
UniVersita¨t Paderborn, D-33098 Paderborn, Germany
Received July 16, 2004
+
+
+
+
The reaction equilibria [H4L]2
+
Zn(OAc)2 h [Zn(H2L)]2
+
2HOAc (K1) and [Zn(H2L)]2
+
Zn(OAc)2 h [Zn2L]2
+
2HOAc (K2), involving zinc acetate and the perchlorate salts of the tetraiminodiphenol macrocycles [H4L1 3](ClO4)2,
-
the lateral (CH2)n chains of which vary between n
spectrofluorimetric titrations in acetonitrile. The photoluminescence behavior of the complexes [Zn2L1](ClO4)2,
[Zn2L2(H2O)2](ClO4)2, [Zn2L2(
-O2CR)](ClO4) (R CH3, C6H5, p-CH3C6H4, p-OCH3C6H4, p-ClC6H4, p-NO2C6H4),
and [Zn2L3( -OAc)](ClO4) have been investigated. The X-ray crystal structures of the complexes [Zn2L2(H2O)2]-
(ClO4)2, [Zn2L3( -OAc)](ClO4), and [Zn2L2( -OBz)(OBz)(H3O)](ClO4) have been determined. The complex [Zn2L2(
) 2 and n ) 4, have been studied by spectrophotometric and
µ
)
µ
µ
µ
µ-
OBz)(OBz)(H3O)](ClO4) in which the coordinated water molecule is present as the hydronium ion (H3O+) on
deprotonation gives rise to the neutral dibenzoate-bridged compound [Zn2L2(
µ
-OBz)2]
acetate, benzoate, or the carboxylate
-proline, have been determined
‚H2O. The equilibrium constants
(K) for the reaction [Zn2L2(H2O)2]2
+
A-
h
[Zn2L2A]+
-alanine,
spectrofluorimetrically in aqueous solution (pH 6
these systems vary in the range (1 8)
105.
+
2H2O (K), where A-
)
+
moiety of the amino acids glycine,
L
L-histidine,
L-valine, and
L
−
7) at room temperature. The binding constants (K) evaluated for
−
×
Introduction
complexes are found to have functions in RNA hydrolysis7
and dephosphorylation.8 The role of zinc in neurobiology is
Zinc(II) plays important roles in several biological pro-
cesses.1 Apart from enzymes with one zinc binding site, e.g.,
carbonic anhydrase and carboxypeptidase A, enzymes con-
taining two or three zinc ions at the active sites are also of
particular interest.2 Dinuclear zinc(II) cores are often seen
in biological systems, such as phosphatases3,4 and amino-
peptidases.5,6 In addition, some synthetic dinuclear zinc(II)
a topic of substantial current interest.9-11
To elucidate the role of Zn(II) in such events, detection
methods that exhibit selectivity and sensitivity for Zn(II) and
are suitable for use in biological fluids are required. However,
due to its 3d10 electronic configuration, the common analyti-
cal techniques such as UV-vis, Mo¨ssbauer, NMR, and EPR
spectroscopy cannot be applied to detect zinc(II) ions in
biological systems. Fortunately, fluorescent imaging has been
found to be the most powerful technique for this purpose.12
* Author to whom correspondence should be addressed. E-mail: ickn@
mahendra.iacs.res.in.
† Indian Association for the Cultivation of Science.
‡ Universita¨t Paderborn.
(1) (a) Frau´sto da Silva, J. J.; Williams, R. J. P. The Biological Chemistry
of the Elements; Clarendon Press: Oxford, 1991; p 302. (b) Berg, J.
M.; Shi, Yi. Science 1996, 271, 1081.
(2) (a) Lipscomb, W. N.; Stra¨ter, N. Chem. ReV. 1996, 96, 2375. (b)
Wilcox, D. E. Chem. ReV. 1996, 96, 2435. (c) Stra¨ter, N.; Lipscomb,
W. N.; Klabunde, T.; Crebs, B. Angew. Chem., Int. Ed. Engl. 1996,
35, 2024. (d) Steinhagen, H.; Helmchem, G. Angew. Chem., Int. Ed.
Engl. 1996, 35, 2339.
(5) Burley, S. K.; David, P. R.; Taylor, A.; Lipscomb, W. N. Proc. Natl.
Acad. Sci. U.S.A. 1990, 87, 6878.
(6) Roderick, S.; Mathews, B. W. Biochemistry 1993, 32, 3907.
(7) Ishikubo, A.; Yashiro, M.; Komiyama, M. Nucleic Acids. Symp. Ser.
1995, 34, 85.
(8) Bazzicalupi, C.; Bencini, A.; Bianchi, A.; Fusi, V.; Giorgi, C.; Paoletti,
P.; Valtancoli, B.; Zanchi, D. Inorg. Chem. 1997, 36, 2784.
(9) Vallee, B. L.; Falchuk, K. H. Physiol. ReV. 1993, 73, 79.
(10) Frederickson, C. J.; Moncrieff, D. W. Biol. Signals 1994, 3, 127.
(11) Takeda, A. BioMetals 2001, 14, 343.
(3) Dealwis, C. G.; Chen, L.; Brennan, C.; Mandecki, W.; AbadZaptero,
C. Protein Eng. 1995, 8, 865.
(4) Zhang, Y.; Liang, J. Y.; Huang, H.; Ke, H.; Lipscomb, W. N.
Biochemistry 1993, 32, 7844.
(12) Jiang, P.; Guo, Z. Coord. Chem. ReV. 2004, 248, 205 and references
therein.
10.1021/ic049056a CCC: $30.25
Published on Web 11/25/2004
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 1, 2005 147

Dutta et al.
Chart 1
mL) of Zn(ClO4)2‚6H2O (0.36 g, 1 mmol) and a methanol solution
(20 mL) of 1,2-diaminoethane (0.12 g, 2 mmol). The resulting bright
yellow solution was refluxed for 1 h and then concentrated to a
volume of ca. 20 mL. The yellow crystalline compound that was
deposited on standing was collected by filtration and washed with
methanol and diethyl ether. Yield: 0.54 g (85%). Anal. Calcd for
C22H24N4O10ZnCl2: C, 41.22; H, 3.75; N, 8.74. Found: C, 41.36;
H, 3.68; N, 8.57. Selected IR data on KBr (ν/cm-1): 3413br,
2924m, 2856w, 1644s, 1542s, 1492w, 1442m, 1383w, 1335m,
1230m, 1089s, 993m, 876w, 803w, 775w, 687w, 629m, 564w. 1H
NMR (300 MHz, CD3CN): δ 14.62 (s, 2H, N-H‚‚‚O); 8.61 (d, J
) 13.2 Hz, 2H, CHdN); 8.48 (s, 2H, CHdN); 7.62 (s, 2H, Ar);
7.50 (s, 2H, Ar); 4.13 (m, 4H, CH2); 3.96 (s, 4H, CH2); 2.14 (s,
6H, CH3).
[Zn2L1](ClO4)2‚H2O (2). This compound can be prepared in two
ways.
Method I. To a boiling methanol solution (30 mL) of 4-methyl-
2,6-diformylphenol (0.33 g, 2 mmol) was added a methanol solution
(10 mL) containing Zn(ClO4)2‚6H2O (0.36 g, 1 mmol) and
Zn(OAc)2‚2H2O (0.22 g, 1 mmol). A methanol solution (20 mL)
of 1,2-diaminoethane (0.12 g, 2 mmol) was then added dropwise.
The resulting light yellow solution was refluxed for 1 h, after which
it was concentrated to ca. 5 mL. The light yellow crystals that were
deposited were collected by filtration and washed with methanol
and diethyl ether. Yield: 0.57 g (80%).
Accordingly, several intensity-based small-molecule fluo-
rescent Zn(II) sensors have been studied that include probes
incorporating macrocyclic Zn(II) binding units.13-16
In a recent study,17 we have demonstrated that the
perchlorate salts of the tetraiminodiphenol macrocyclic
ligands [H4L1-3](ClO4)2 (Chart 1) exhibit strong lumines-
cence behavior, albeit there is considerable variation of
luminescence intensities along the series. We herein report
the luminescence spectroscopic properties of the dinuclear
zinc complexes derived from the macrocyclic ligands
[H4L1-3](ClO4)2. Of particular interest to us has been to
examine the fluorophoric behavior of these dizinc complexes
toward carboxylate ion binding and to find out whether this
approach could be extended to study their interactions with
biologically important amino acids. In this context we note
that the macrocyclic dinuclear zinc(II) complex [Zn2L2(H2O)2]-
(ClO4)2 has been recently used as a precursor to produce
interesting structural motifs.18,19
Method II. To a stirred acetonitrile solution (15 mL) of
compound 1 (0.64 g, 1 mmol) was added a second acetonitrile
solution (10 mL) of Zn(OAc)2‚2H2O (0.22 g, 1 mmol). After 1 h
of stirring the solution was concentrated to a small volume, when
a light yellow crystalline compound was deposited. The product
was filtered and washed with diethyl ether. Yield: 0.59 g (87%).
Anal. Calcd for C22H24N4O11Zn2Cl2: C, 36.62; H, 3.33; N, 7.77.
Found: C, 36.85; H, 3.42; N, 7.72. Selected IR data on KBr (ν/
cm-1): 3425br, 3008w, 2926m, 2852w, 1633s, 1547s, 1442m,
1390s, 1327m, 1231m, 1148w, 1089s, 1033m, 992w, 881w, 839m,
1
773w, 673w, 624m, 573w, 532w. H NMR (300 MHz, CD3CN):
δ 8.36 (s, 4H, CHdN); 7.34 (s, 4H, Ar); 4.21 (s, 4H, CH2); 3.67
(s, 4H, CH2); 2.24 (s, 6H, CH3); 2.14 (s, br, H2O).
[Zn2L2(H2O)2](ClO4)2 (3). This compound also can be prepared
Experimental Section
in two ways.
Materials. All chemicals were obtained from commercial sources
and used as received. Solvents were purified and dried according
to standard methods.20 The macrocyclic ligands [H4L1-3](ClO4)2
were prepared as reported previously.17
Caution! The perchlorate salts reported in this study are
potentially explosiVe and therefore should be handled with care.21
Preparation of the Complexes. [Zn(H2L1)](ClO4)2 (1). To a
boiling methanol solution (30 mL) of 4-methyl-2,6-diformylphenol
(0.33 g, 2 mmol) were added successively a methanol solution (10
Method I. To a boiling methanol solution (30 mL) of 4-methyl-
2,6-diformylphenol (0.33 g, 2 mmol) and Zn(ClO4)2‚6H2O (0.72
g, 2 mmol) was added a methanol solution (20 mL) of 1,3-
diaminopropane (0.15 g, 2 mmol). The resulting deep yellow
solution was refluxed for 1 h and then concentrated to a volume of
ca. 15 mL. The yellow crystals that were deposited on standing
were collected by filtration and washed with methanol and diethyl
ether. Yield: 0.62 g (80%).
Method II. To an acetonotrile solution (20 mL) of the macro-
cycle [H4L2](ClO4)2 (0.60 g, 1 mmol) were added successively
acetonitrile solutions (10 mL each) of Zn(ClO4)2‚6H2O (0.72 g, 2
mmol) and Et3N (0.40 g, 4 mmol). The solution was refluxed for
1 h and concentrated to a volume of ca. 15 mL. The yellow
crystalline product that was deposited was filtered off and washed
with acetonitrile and diethyl ether. Yield: 0.64 g (83%). Anal. Calcd
for C24H30N4O12Zn2Cl2: C, 37.51; H, 3.91; N, 7.29. Found: C,
37.38; H, 3.96; N, 7.24. Selected IR data on KBr (ν/ cm-1): 3416br,
2916m, 2859w, 1639s, 1558s, 1439m, 1390m, 1366w, 1330s,
1280m, 1238m, 1118s, 1082s, 982w, 930w, 812m, 629m, 519w.
1H NMR (300 MHz, (CD3)2SO): δ 8.48 (s, 4H, CHdN); 7.52 (s,
4H, Ar); 3.97 (s, 8H, R-CH2); 2.27 (s, 6H, CH3); 2.03 (s, 4H,
â-CH2).
(13) Hirano, T.; Kazuya, K.; Urano, Y.; Higuchi, T.; Nagano, T. Angew.
Chem., Int. Ed. 2000, 39, 1052.
(14) Koike, T.; Watanabe, T.; Aoki, S.; Kimura, E.; Shiro, M. J. Am. Chem.
Soc. 1996, 118, 12696.
(15) Kimura, E.; Koike, T. Chem. Soc. ReV. 1998, 27, 179 and references
therein.
(16) Aoki, S.; Kaido, S.; Fujioka, H.; Kimura, E. Inorg. Chem. 2003, 42,
1023.
(17) Dutta, B.; Bag, P.; Adhikary, B.; Flo¨rke, U.; Nag, K. J. Org. Chem.
2004, 69, 5419.
(18) Huang, W.; Gou, S.; Hu, D.; Chantrapromma, S.; Fun, H.-K.; Meng,
Q. Inorg. Chem. 2001, 40, 1712.
(19) Huang, W.; Gou, S.; Hu, D.; Chantrapromma, S.; Fun, H.-K.; Meng,
Q. Inorg. Chem. 2002, 41, 864.
(20) Perrin, D. D.; Armarego, W. L.; Perrin, D. R. Purification of
Laboratory Chemicals, 2nd ed.; Pergamon: Oxford, 1980.
(21) Wolsey, W. C. J. Chem. Educ. 1973, 50, A335.
148 Inorganic Chemistry, Vol. 44, No. 1, 2005

Zinc(II) Complexes of Tetraiminodiphenol Macrocycles
[Zn2L2(µ-O2CR)](ClO4)‚2H2O (4-9). The following general-
ized procedure was used to prepare these compounds. To a boiling
methanol solution (30 mL) of compound 3 (0.78 g, 1 mmol) was
added dropwise a methanol solution (20 mL) of sodium carboxylate
salt RCO2Na (4 mmol). The solution was refluxed for 2 h and then
allowed to concentrate slowly on a hot plate. In all of the cases the
product was deposited in crystalline form, collected by filtration,
and washed with methanol.
C6H4); 7.32 (s, 4H, Ar); 3.99 (s, 8H, R-CH2); 2.22 (s, 6H, CH3);
2.12 (s, â-CH2 + H2O).
[Zn2L2(µ-OBz)(OBz)(H3O)](ClO4) (10). An acetonitrile solution
(40 mL) of 5 (0.20 g, 0.25 mmol) and benzoic acid (0.12 g, 1 mmol)
was heated under reflux for 4 h. The color of the solution faded
from bright yellow to pale yellow during this period. After the
volume of the solution was reduced to ca. 20 mL, it was kept at
room temperature in a stoppered flask. The pale yellow crystals
that were deposited over a period of 2 days were collected by
filtration and washed with methanol and diethyl ether. Yield: 80
mg (35%). Anal. Calcd for C38H39N4O11Zn2Cl: C, 51.05; H, 4.37;
N, 6.27. Found: C, 51.19; H, 4.41; N, 6.18. Selected IR data on
KBr (ν/cm-1): 3496br, 3420br, 3071w, 2928m, 2866w, 1653s,
1605m, 1568s, 1443m, 1421m, 1372s, 1327m, 1274w, 1198w,
1086s, 979w, 945w, 810m, 767m, 726m, 677m, 626m, 526w, 460w.
1H NMR (300 MHz, CD3CN): δ 8.35 (s, 2H, CHdN); 8.30 (s,
2H, CHdN); 7.75 (d, J ) 7.4 Hz, 4H, o-C6H5); 7.38 (s, 4H, Ar);
7.31 (m, 2H, p-C6H5); 7.25 (m, 4H, m-C6H5); 4.20 (m, 4H, R-CH2);
4.07 (m, 4H, R-CH2); 2.35 (m, 2H, â-CH2); 2.20 (m, 2H, â-CH2);
2.17 (s, 6H, CH3).
Data for [Zn2L2(µ-OAc)](ClO4)‚2H2O (4). Yield: 0.63 g
(85%). Anal. Calcd for C26H33N4O10Zn2Cl: C, 42.95; H, 4.50; N,
7.71. Found: C, 43.01; H, 4.53; N, 7.69. Selected IR data on KBr
(ν/cm-1): 3433br, 2918m, 2866w, 1638s, 1580m, 1560s, 1441m,
1411s, 1408m, 1341s, 1280w, 1238m, 1117s, 1085s, 990w, 943w,
1
813m, 774m, 671w, 629m, 512w. H NMR (300 MHz, CD3CN):
δ 8.28 (s, 4H, CHdN); 7.31 (s, 4H, Ar); 4.01 (s, 4H, R-CH2); 3.82
(s, 4H, R-CH2); 2.23 (s, 6H, CH3); 2.15 (s, â-CH2 + H2O); 1.80
(s, 3H, OAc).
Data for [Zn2L2(µ-OBz)](ClO4)‚2H2O (5). Yield: 0.62 g (79%).
Anal. Calcd for C31H35N4O10Zn2Cl: C, 47.21; H, 4.44; N, 7.10.
Found: C, 47.49; H, 4.60; N, 7.13. Selected IR data on KBr (ν/
cm-1): 3433br, 3071w, 2922m, 2859w, 1638s, 1580m, 1560s,
1443m, 1406s, 1331m, 1279w, 1238w, 1116s, 1084s, 990w, 930w,
[Zn2L2(µ-OBz)2]‚H2O (11). Complex 10 (63 mg, 0.07 mmol)
was dissolved in 10 mL of warm acetonitrile and treated with 0.8
mL of 0.1 mM NaOH solution. After a few minutes of stirring, the
yellow microcrystalline product that was deposited was collected
by centrifugation and washed with methanol and diethyl ether.
Yield: 50 mg (90%). Anal. Calcd for C38H38N4O7Zn2: C, 57.50;
H, 4.79; N, 7.06. Found: C, 57.32; H, 4.86; N, 7.11. Selected IR
data on KBr (ν/cm-1): 3430br, 3071w, 2916m, 2859w, 1638s,
1559s, 1437m, 1411s, 1348s, 1279m, 1237m, 1202w, 1129w,
1
892w, 813m, 726m, 628m, 512w. H NMR (300 MHz, CD3CN):
δ 8.31 (s, 4H, CHdN); 7.96 (d, J ) 7.2 Hz, 2H, o-C6H5); 7.46 (m,
1H, p-C6H5); 7.37 (m, 2H, m-C6H5); 7.31 (s, 4H, Ar); 3.99 (s, 8H,
R-CH2); 2.22 (s, 6H, CH3); 2.14 (s, â-CH2 + H2O).
Data for [Zn2L2(µ-O2C-p-CH3C6H4)](ClO4)‚2H2O (6). Yield:
0.64 g (80%). Anal. Calcd for C32H37N4O10Zn2Cl: C, 47.88; H,
4.61; N, 6.98. Found: C, 48.01; H, 4.54; N, 7.12. Selected IR data
on KBr (ν/cm-1): 3424br, 2922m, 2866w, 1637s, 1587m, 1559s,
1444s, 1405s, 1333s, 1278w, 1239m, 1178s, 1087s, 998w, 937w,
814m, 771m, 625m, 519w. 1H NMR (300 MHz, CD3CN): δ 8.31
(s, 4H, CHdN); 7.85 (d, J ) 8.2 Hz, 2H, o-C6H4); 7.31 (s, 4H,
Ar); 7.15 (d, J ) 8.0 Hz, 2H, m-C6H4); 4.08 (s, 4H, R-CH2); 3.89
(s, 4H, R-CH2); 2.29 (s, 3H, p-CH3); 2.22 (s, 6H, CH3); 2.12 (s,
â-CH2 + H2O).
1
1089w, 937w, 811m, 776m, 721m, 677w, 539w. H NMR (300
MHz, (CD3)2SO): δ 8.44 (s, 4H, CHdN); 7.69 (d, J ) 7.6 Hz,
4H, o-C6H5); 7.41 (s, 4H, Ar); 7.33 (m, 2H, p-C6H5); 7.22 (m, 4H,
m-C6H5); 4.03 (m, 8H, R-CH2); 3.34 (s, br, H2O); 2.23 (s, 6H, CH3);
2.09 (s, 4H, â-CH2).
[Zn2L3(µ-OAc)](ClO4) (12). To an acetonitrile solution (20 mL)
of the macrocycle [H4L3](ClO4)2 (0.31 g, 0.5 mmol) was added
dropwise with stirring an acetonitrile solution (10 mL) of Zn(OAc)2‚
2H2O (0.22 g, 1 mmol). After 2 h, the solution was concentrated
in a water bath to a small volume, when a yellow crystalline
compound separated out. It was collected by filtration and washed
with a little methanol and diethyl ether. Yield: 0.40 g (68%). Anal.
Calcd for C28H33N4O8Zn2Cl: C, 46.76; H, 4.59; N, 7.79. Found:
C, 46.40; H, 4.42; N, 7.74. Selected IR data on KBr (ν/cm-1):
3453br, 2937m, 2877w, 1635s, 1604m, 1566s, 1413m, 1408m,
1343m, 1282w, 1223w, 1189w, 1091s, 1017w, 818m, 774w, 623m,
488w. 1H NMR (300 MHz, CD3CN): δ 8.38 (s, 4H, CHdN); 7.33
(s, 4H, Ar); 3.82-3.64 (m, 8H, R-CH2); 2.25 (s, 6H, CH3); 2.03
(m, 8H, â-CH2); 1.78 (s, 3H, OAc).
Data for [Zn2L2(µ-O2C-p-OCH3C6H4)](ClO4)‚2H2O (7).
Yield: 0.62 g (76%). Anal. Calcd for C32H37N4O11Zn2Cl: C, 46.91;
H, 4.52; N, 6.84. Found: C, 47.02; H, 4.56; N, 6.87. Selected IR
data on KBr (ν/cm-1): 3429br, 3058w, 2928m, 2866w, 1635s,
1604m, 1587m, 1560s, 1450m, 1398s, 1338m, 1248m, 1172m,
1
1090s, 1030w, 816w, 788m, 711w, 623m, 512w. H NMR (300
MHz, CD3CN): δ 8.31 (s, 4H, CHdN); 7.92 (d, J ) 8.7 Hz, 2H,
o-C6H4); 7.32 (s, 4H, Ar); 6.85 (d, J ) 8.7 Hz, 2H, m-C6H4); 4.12
(s, 4H, R-CH2); 3.87 (s, 4H, R-CH2); 3.75 (s, 3H, OCH3); 2.22 (s,
6H, CH3); 2.12 (s, â-CH2 + H2O).
Data for [Zn2L2(µ-O2C-p-ClC6H4)](ClO4)‚2H2O (8). Yield:
0.62 g (75%). Anal. Calcd for C31H34N4O10Zn2Cl2: C, 45.23; H,
4.13; N, 6.80. Found: C, 45.46; H, 4.01; N, 6.87. Selected IR data
on KBr (ν/cm-1): 3433br, 2924m, 2858w, 1637s, 1580m, 1560s,
1445m, 1407s, 1334m, 1278w, 1239w, 1194w, 1086s, 937w, 877w,
816m, 774m, 624m, 559w. 1H NMR (300 MHz, CD3CN): δ 8.31
(s, 4H, CHdN); 7.94 (d, J ) 8.1 Hz, 2H, o-C6H4); 7.36 (s, 2H,
m-C6H4); 7.32 (s, 4H, Ar); 4.12 (s, 4H, R-CH2); 3.88 (s, 4H, R-CH2);
2.22 (s, 6H, CH3); 2.10 (s, â-CH2 + H2O).
Physical Measurements. Elemental (C, H, and N) analyses were
performed in-house on a Perkin-Elmer 2400II elemental analyzer.
Infrared spectra were recorded on an FT-IR Nexus Nicolet
1
spectrometer using KBr disks. The H NMR spectroscopic mea-
surements were performed on a Bruker Avance DPX-300 spec-
trometer in CD3CN or (CD3)2SO solutions. Electronic absorption
spectra were obtained with a Shimadzu UV 2100 spectrophotometer.
The spectrophotometric titrations were carried out by recording a
series of spectra of acetonitrile solutions of the macrocycles
[H4L1-3](ClO4)2 mixed with varying quantities of Zn(OAc)2‚2H2O.
The concentration of the ligand was fixed to 10-5 M, while that of
Zn(OAc)2‚2H2O was varied over a wide range till no further change
in absorbance was noted.
Data for [Zn2L2(µ-O2C-p-NO2C6H4)](ClO4)‚2H2O (9). Yield:
0.60 g (72%). Anal. Calcd for C31H34N5O12Zn2Cl: C, 44.63; H,
4.07; N, 8.39. Found: C, 44.71; H, 4.02; N, 8.56. Selected IR data
on KBr (ν/cm-1): 3417br, 2932m, 2872w, 1635s, 1605m, 1573s,
1556s, 1524m, 1450s, 1409s, 1342s, 1275w, 1248m, 1089s, 1009w,
1
976w, 890w, 822m, 775m, 726m, 623m, 539w, 506w. H NMR
Emission spectra were recorded on a Perkin-Elmer LS55
luminescence spectrometer using methanol and acetonitrile solutions
(300 MHz, CD3CN): δ 8.31 (s, 4H, CHdN); 8.11 (s, 4H, o,m-
Inorganic Chemistry, Vol. 44, No. 1, 2005 149

Dutta et al.
Table 1. Crystallographic Data for 3, 10, and 12
3
10
12
empirical formula
fw
C24H30Cl2N4O12ZN2
768.16
C38H38ClN4O11Zn2
892.91
C28H33ClN4O8Zn2
719.77
T, K
120(2)
203(2)
120(2)
crystal syst, space group
a, Å
b, Å
monoclinic, P21/c
16.7023(5)
12.3131(4)
14.4857(5)
orthorhombic, Pbca
14.970(1)
21.633(3)
23.159(3)
90
90
90
7500.0(15)
8, 1.582
1.418
3672
monoclinic, P21/n
10.3474(5)
19.7720(9)
14.7902(7)
90
104.991(1)
90
2922.9(2)
4, 1.636
c, Å
R, deg
90
â, deg
106.121(1)
90
2861.94(16)
4, 1.783
1.934
1568
γ, deg
V, Å3
Z, Fcalcd, Mg/m3
µ, mm-1
1.790
1480
F(000)
cryst size, mm3
refinement method
no. of data/restraints/params
no. of reflns [I > 2σ(I)]
GOF on F2
0.36 × 0.12 × 0.10
0.38 × 0.12 × 0.06
0.35 × 0.30 × 0.20
a
a
a
7071/4/412
35862
0.990
8610/51/504
10193
0.872
7203/0/391
36599
1.043
final R indices [I > 2σ(I)]
R indices (all data)
R1b ) 0.0334, wR2c ) 0.0797
R1 ) 0.0425, wR2 ) 0.0822
R1b ) 0.0758, wR2c ) 0.1555
R1 ) 0.2588, wR2 ) 0.2524
R1b ) 0.0282, wR2c ) 0.0728
R1 ) 0.0326, wR2 ) 0.0751
a Full-matrix least-squares on F2. b R1(F) ) ∑||Fo| - |Fc||/∑|Fo|. c wR2(F2) ) [∑w(Fo
Fc )2/∑w(Fo )2]1/2
.
2
-
2
2
of the complexes. Quantum yields of complexes 3-9 at room
temperature were determined by a relative method using anthracene
as the standard.22 The quantum yields were calculated using the
relation23
Results and Discussion
Syntheses. As outlined in Scheme 1, the macrocyclic
zinc(II) complexes 1-12 have been synthesized either by
metal template condensation reaction or using the preformed
macrocyclic ligands [H4L1-3](ClO4)2.
æ ) æstd(Astd/A)(I/Istd)(η2/ηstd
)
2
The cavity sizes of the macrocyclic ligands markedly
influence the composition of the zinc(II) complexes formed.
Thus, when 2 equiv each of 4-methyl-2,6-diformylphenol
and 1,2-diaminoethane are reacted in the presence of 1 equiv
of Zn(ClO4)2‚6H2O in methanol, the mononuclear complex
[Zn(H2L1)](ClO4)2 (1) is obtained. A similar mononuclear
complex, however, is not obtained when 1,3-diaminopropane
or 1,4-diaminobutane is used as the reactant. On the other
hand, the reaction involving equimolar amounts of 4-methyl-
2,6-diformylphenol, Zn(ClO4)2‚2H2O, and 1,2-diaminoethane
or 1,3-diaminopropane produces the dizinc(II) complex
[Zn2L1](ClO4)2 (2) or [Zn2L2(H2O)2](ClO4)2 (3), while the
same reaction carried out with 1,4-diaminobutane affords the
metal-free macrocycle [H4L3](ClO4)2. Again, complex 3 is
readily obtained by treating [H4L2](ClO4)2 in acetonitrile with
Zn(ClO4)2‚2H2O and triethylamine in a 1:2:4 ratio, albeit
similar reactions with [H4L1](ClO4)2 and [H4L3](ClO4)2 fail
to produce corresponding dizinc(II) complexes. The dinuclear
complex [Zn2L3(µ-OAc)](ClO4) (12) is obtained by reacting
[H4L3](ClO4)2 with Zn(OAc)2‚2H2O in acetonitrile, even as
the same reaction with [H4L2](ClO4)2 produces the acetate-
free diaqua complex 3. A series of carboxylate-bridged
compounds [Zn2L2(µ-O2CR)](ClO4)‚2H2O (4-9) have been
prepared by reacting complex 3 with an excess of NaO2CR
[R ) CH3 (4), C6H5 (5), p-CH3C6H4 (6), p-OCH3C6H4 (7),
p-ClC6H4 (8), p-NO2C6H4 (9)] in methanol. Interestingly, the
reaction between 5 and benzoic acid in acetonitrile has led
to the isolation of an unusual compound of composition
[Zn2L2(µ-OBz)(OBz)(H3O)](ClO4) (10). This compound can
be deprotonated to produce the dibenzoate-bridged complex
[Zn2L2(µ-OBz)2]‚H2O (11) by treating an acetonitrile solution
of 10 with 1 equiv of aqueous sodium hydroxide.
where æ and æstd are the quantum yields of unknown and standard
samples, A and Astd are the solution absorbances at the excitation
wavelength (λex), I and Istd are the integrated emission intensities,
and η and ηstd are the refractive indices of the solvent.
The equlibrium constants involving the binding of acetate or
benzoate anions and the carboxylate moieties of amino acids
such as glycine, L-alanine, L-histidine, L-valine, and L-proline to
[Zn2L2(H2O)2]2+ were determined in aqueous solution at room tem-
perature by spectrofluorimetric titrations. A series of aqueous solu-
tions containing the same amount (1 × 10-5 M) of [Zn2L2(H2O)2]2+
and varying quantities of sodium acetate, sodium benzoate, and
the amino acids were added till no further change in luminescence
intensity was noted.
Crystal Structure Determination of 3, 10, and 12. Diffraction
data were collected on a Siemens R3m/V diffractometer at 120 K
(for 3 and 12) and at 203 K (for 10) in the ω-2θ scan mode using
graphite-monochromated Mo KR radiation (λ ) 0.71073 Å).
Pertinent crystallographic data are summarized in Table 1. Three
standard reflections were periodically monitored, and no crystal
decay was observed. The intensity data were corrected for Lorentz-
polarization effects, and semiempirical absorption corrections were
made from ψ scans. The structures were solved by direct and
Fourier methods and refined by full-matrix least-squares methods
based on F2 using the SHELXTL software package.24 The non-
hydrogen atoms were refined anisotropically, while the hydrogen
atoms were placed at the geometrically calculated positions with
fixed isotropic thermal parameters.
(22) (a) Dawson, W. R.; Windsor, M. W. J. Phys. Chem. 1968, 72, 3251.
(b) Melhuish, W. H. J. Phys. Chem. 1961, 65, 229. (c) Parker, C. A.;
Rees, W. T. Analyst 1960, 85, 587.
(23) van Houten, J.; Watts, R. J. J. Am. Chem. Soc. 1976, 98, 4853.
(24) Sheldrick, G. M. SHELXTL97-2: Program for the Refinement of
Crystal Structures; University of Go¨ttingen: Go¨ttingen, Germany,
1997.
150 Inorganic Chemistry, Vol. 44, No. 1, 2005

Zinc(II) Complexes of Tetraiminodiphenol Macrocycles
Scheme 1a
Figure 1. ORTEP representation of the structure of the [Zn2L2(H2O)2]2+
cation in 3.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 3
Zn(1)-N(101)
Zn(1)-N(102)
Zn(1)-O(101)
Zn(1)-O(101)#1
Zn(1)-O(102)
O(101)-Zn(1)#1
Zn(1)‚‚‚Zn(1)#1
2.0314(18) Zn(2)-N(201)
2.0236(18) Zn(2)-N(202)
2.0422(15) Zn(2)-O(201)
2.0487(15) Zn(2)-O(201)#2
2.0866(17) Zn(2)-O(202)
2.0487(15) O(201)-Zn(2)#2
2.0291(19)
2.0370(18)
2.0589(14)
2.0291(15)
2.1123(17)
2.0291(15)
3.175(1)
3.171(1)
Zn(2)‚‚‚Zn(2)#2
N(102)-Zn(1)-N(101)
N(102)-Zn(1)-O(101)
N(101)-Zn(1)-O(101)
98.51(7)
90.52(7)
N(201)-Zn(2)-N(202)
N(202)-Zn(2)-N(201)
97.76(7)
89.65(7)
160.73(7)
163.44(6) N(201)-Zn(2)-O(201)
N(102)-Zn(1)-O(101)#1 164.25(7) N(202)-Zn(2)-O(201)#2 163.62(6)
N(101)-Zn(1)-O(101)#1 89.83(7)
O(101)-Zn(1)-O(101)#1 78.38(6)
N(102)-Zn(1)-O(102)
N(101)-Zn(1)-O(102)
O(101)-Zn(1)-O(102)
96.63(7)
102.22(7) N(201)-Zn(2)-O(202)
90.40(6) O(201)-Zn(2)-O(202)
O(101)#1-Zn(1)-O(102) 90.60(6)
O(201)#2-Zn(2)-O(202) 93.54(6)
Crystal Structures. [Zn2L2(H2O)2](ClO4)2 (3). The struc-
ture analysis showed that in 3 there are two independent
molecules per asymmetric unit. An ORTEP representation
for one of the [Zn2L2(H2O)2]2+ cations is shown in Figure
1. Selected bond distances and bond angles are given in Table
2. The cation is centrosymmetric with its center of inver-
sion at the middle of the Zn2(µ-phenoxide)2 plane. In
this compound the flatness of the macrocyclic ligand is
evident from the 0° dihedral angle between the two phenyl
rings.
For the [Zn(1)L2Zn(1A)]2+ cation, the in-plane donor
atoms (N2O2) do not deviate from the mean plane by more
than 0.004(2) Å, while for the [Zn(2)L2Zn(2A)]2+ cation the
deviation is (0.039(2) Å. The metal centers are displaced
from the basal plane in the opposite direction by 0.223(2) Å
for Zn(1)/Zn(1A) and 0.260(3) Å for Zn(2)/Zn(2A). The
square-pyramidal geometry of the metal center is completed
by coordination with a water molecule. The average in-plane
Zn-O and Zn-N distances, which lie in the range 2.024-
2.049 Å, are normal and the axial Zn-OH2 distance, 2.089(2)
Å, is relatively longer. The separations between the non-
bonded metal centers are 3.171 Å for Zn(1)‚‚‚Zn(1A) and
3.175 Å for Zn(2)‚‚‚Zn(2A).
a Reagents and conditions: (a) (i) Zn(ClO4)2‚6H2O (0.5 equiv) +
NH2(CH2)2NH2 (1 equiv)/MeOH. (ii) Zn(ClO4)2‚6H2O (0.5 equiv) +
Zn(OAc)2‚2H2O (0.5 equiv) + NH2(CH2)2NH2 (1 equiv)/MeOH. (iii)
Zn(OAc)2‚2H2O (1 equiv)/MeCN. (b) (i) Zn(ClO4)2‚6H2O (1 equiv) +
NH2(CH2)3NH2 (1 equiv)/MeOH. (ii) Zn(ClO4)2‚6H2O (2 equiv) + Et3N
(4 equiv)/MeCN. (iii) NaOOCR (4 equiv)/MeOH. (iv) C6H5COOH (4
equiv)/MeCN. (c) (i) Zn(OAc)2‚2H2O (2 equiv)/MeCN. (ii) Zn(OAc)2‚2H2O
(1 equiv) + NH2(CH2)4NH2 (1 equiv)/MeOH.
Inorganic Chemistry, Vol. 44, No. 1, 2005 151

Dutta et al.
The most striking feature of compound 10 is its composi-
tion. Although [Zn2L2(µ-OBz)(OBz)(H2O)] should be a
neutral compound, it is, indeed, monocationic, and the charge
is neutralized by the associated perchlorate anion. From the
structural parameters (Table 3) it is evident that there is no
scope for protonation of the macrocyclic ligand donor atoms
and also of the carboxylate moieties. We are, therefore, left
with two possibilities, namely, either the water molecule is
protonated, that is, [Zn2L2(µ-OBz)(OBz)(H3O)](ClO4), or the
proton is associated with the perchlorate anion, that is,
[Zn2L2(µ-OBz)(OBz)(H2O)]‚HOClO3. Interestingly, a mac-
rocyclic zinc(II) complex of composition [Zn{[12]aneN3}-
(OH)]3(ClO4)3‚HClO4 has been structurally characterized.25
With the crystallographic data set of 10 it has not been
possible to determine the hydrogen positions in question.
Nevertheless, with HFIX of SHELX we have tried to fix
three hydrogen atoms to O(7) or one to O(11) (the only
perchlorate oxygen which is not disordered and the only one
that due to its orientation could make hydrogen bridge to
Figure 2. ORTEP representation of the structure of the [Zn2L2(µ-OBz)-
(OBz)(H3O)]+ cation in 10. Hydrogen atoms are not shown for the sake of
clarity.
+
O(7)). So, if we have ZnOH3 , there could be an intra-
Table 3. Selected Bond Lengths (Å) and Angles (deg) for 10
molecular bridge, O(7)-H(7B)‚‚‚O(6), with O(7)-H ) 0.83
Å, H‚‚‚O(6) ) 1.77 Å, and O-H‚‚‚O ) 177°, and also an
intermolecular bridge, O(7)-H(7C)‚‚‚O(11), with O(7)-H
) 0.83 Å, H‚‚‚O(11) ) 2.25 Å, and O-H‚‚‚O ) 170°. On
the other hand, if we have O3ClOH, there could be
intermolecular bridges O(11)-H(11)‚‚‚O(7) with O-H )
0.83 Å, H‚‚‚O(7) ) 2.53 Å, and O-H‚‚‚O ) 123° and
O(11)-H(11)‚‚‚N(4) with O-H ) 0.83 Å, H‚‚‚N(4) ) 2.61
Å, and O-H‚‚‚N ) 118°. In the above-mentioned [12]aneN3
complex,25 the hydrogen positions were not determined but
guessed. The argument for O3ClOH in that case was a larger
Cl-OH bond compared to the other Cl-O bonds. In our
case the Cl-O(H) bond in question is just the shortest bond.
Thus, the geometric parameters for a Zn-OH3+ interaction
seems to be the more likely case, as there is no privilege for
a ClOH option.
Zn(1)-O(1)
Zn(1)-O(2)
Zn(1)-O(3)
Zn(1)-O(5)
Zn(1)-N(1)
Zn(1)-N(2)
Zn(1)‚‚‚Zn(2)
2.020(7)
2.006(7)
2.035(7)
1.987(7)
2.036(8)
2.049(9)
3.098(1)
Zn(2)-O(1)
Zn(2)-O(2)
Zn(2)-O(4)
Zn(2)-O(7)
Zn(2)-N(3)
Zn(2)-N(4)
2.076(7)
2.099(7)
2.109(7)
2.153(7)
2.012(9)
2.031(9)
Zn(1)-O(1)-Zn(2)
O(1)-Zn(1)-N(1)
O(2)-Zn(1)-N(1)
O(1)-Zn(1)-N(2)
O(2)-Zn(1)-N(2)
N(1)-Zn(1)-N(2)
O(5)-Zn(1)-O(3)
98.3(3)
172.0(3)
91.5(3)
90.5(3)
172.8(3)
94.9(4)
173.0(3)
Zn(1)-O(2)-Zn(2)
O(1)-Zn(2)-N(3)
O(2)-Zn(2)-N(3)
O(1)-Zn(2)-N(4)
O(2)-Zn(2)-N(4)
N(3)-Zn(2)-N(4)
O(4)-Zn(2)-O(7)
98.0(3)
90.8(3)
169.6(3)
170.3(3)
91.1(3)
98.9(4)
174.3(3)
[Zn2L2(µ-OBz)(OBz)(H3O)](ClO4) (10). Complex 10
consists of a protonated cation, [Zn2L2(µ-OBz)(OBz)(H2O)]+,
and a ClO4 anion in 1:1 stoichiometry. The perchlorate anion
is disordered with three of its oxygen atoms distributed over
two sites in 2:1 occupancies. The ORTEP diagram of the
cation is shown in Figure 2, and the selected bond distances
and angles are given in Table 3.
The two metal centers in 10 are triply bridged by the two
phenolate oxygens of the macrocyclic ligand and a benzoate
group. Both the metal centers are hexacoordinated; the sixth
coordination position of Zn(1) is occupied by a monodentate
benzoate, while that of Zn(2) is occupied by a water
molecule. Unlike 3, the macrocyclic ligand in 10 adopts a
puckered configuration in which the interplanar angle
between the planes of the two aromatic rings is 30.1°. The
equatorial donor atoms (N2O2) do not deviate by more than
0.025 Å from their metal planes. On the other hand, Zn(1)
is displaced above the basal plane by 0.078 Å, while Zn(2)
lies below the plane by 0.027 Å. The two metal centers are
separated from each other by 3.098 Å. Consideration of the
metrical parameters for the two metal centers will indicate
that, although Zn(1) has a regular octahedral geometry, there
is considerable distortion in the geometry of the six-
coordinate Zn(2).
It may be noted that all the Zn(1)-O distances involving
the phenolate oxygens [Zn(1)-O(1) ) 2.020(7) Å, Zn(1)-
O(2) ) 2.006(7) Å], the bridged benzoate oxygen [Zn(1)-
O(3) ) 2.035(7) Å], and the monodentate benzoate oxygen
[Zn(1)-O(5) ) 1.987(7) Å] are rather short. In contrast, the
Zn(2)-O distances involving the phenolate oxygens [Zn(2)-
O(1) ) 2.076(7) Å, Zn(2)-O(2) ) 2.099(7) Å], the bridged
benzoate oxygen [Zn(2)-O(4) ) 2.109(7) Å], and the water
molecule [Zn(2)-O(7) ) 2.153(7) Å] are relatively longer.
The Zn-N distances for the two metal centers [2.012(9)-
2.049(9) Å], however, do not show much difference. If we
consider that the coordinated oxygen O(7) is associated with
+
OH3 , rather than H2O, migration of electron density from
Zn(1) to Zn(2) through the bridging oxygen atoms should
occur. Clearly, this will lead to shortening of the Zn(1)-O
distances compared to the Zn(2)-O distances. Further, the
carbonyl oxygen O(6) of the monodentate benzoate lies on
the same side of the protonated water molecule. The 2.592
Å separation between O(6) and O(7) atoms indicates the pres-
ence of a moderate to strong hydrogen-bonding interaction.
(25) Kimura, E.; Shiota, T.; Koike, T.; Shiro, M.; Kodama, M. J. Am. Chem.
Soc. 1990, 112, 5805.
152 Inorganic Chemistry, Vol. 44, No. 1, 2005

Zinc(II) Complexes of Tetraiminodiphenol Macrocycles
which observed at 8.61 ppm is a doublet (J ) 13.2 Hz) due
to its trans coupling with the hydrogen-bonded proton.
Further, the aromatic protons are also nonequivalent and
observed at 7.62 and 7.50 ppm. In contrast, the singlets due
to the CHdN and aromatic protons of 2 are observed at 8.36
and 7.34 ppm, respectively. The assignment of the 14.62 ppm
resonance for 1 has been confirmed by a deuteration
experiment. Thus, addition of D2O to the CD3CN solution
of 1 has led to the disappearance of the 14.62 ppm resonance
along with a collapse of the doublet at 8.61 ppm to a broad
singlet. The significant deshielding of this resonance is a
clear indication of the hydrogen-bonding interaction.
The spectral features of the macrocyclic ligand L2 in the
carboxylate-bridged complexes 4-9 remain practically un-
changed for all the compounds. For example, the singlets
due to the CHdN, OAr, and CH3 moieties are all observed
at 8.31, 7.31, and 2.22 ppm, respectively. Of course,
complexes 5-9 exhibit additional peaks due to the aromatic
carboxylate moieties. Of particular interest to us has been
Figure 3. ORTEP representation of the structure of the [Zn2L3(µ-OAc)]+
cation in 12.
1
to compare the H NMR spectral features of 10 and 11
Table 4. Selected Bond Lengths (Å) and Angles (deg) for 12
(shown in Figures S1 and S2 in the Supporting Information).
The observation of two singlets of equal intensities at 8.35
and 8.30 ppm due to the imino protons and two multiplets
of equal areas at 4.20 and 4.07 ppm due to the R-CH2 protons
in 10 is consistent with its asymmetric structure. The more
shielded resonances are for the Zn(2) site. In contrast,
complex 11 exhibits a lone singlet at 8.44 ppm for all the
CHdN protons and a single resonance at 4.03 ppm for all
the R-CH2 protons, indicating that the two metal centers are
in identical environments and both the benzoates are bound
in bridged form.
The formation of 11 via deprotonation of 10 has been
further verified by comparing their IR spectra (shown in
Figures S3 and S4 in the Supporting Information). While
the νCdN vibration in 10 is split into two components at 1653
and 1605 cm-1, in the case of 11 a single band is observed
Zn(1)-O(1)
Zn(1)-O(2)
Zn(1)-O(3)
Zn(1)-N(1)
Zn(1)-N(2)
Zn(1)‚‚‚Zn(2)
1.9875(12) Zn(2)-O(1)
2.1140(12) Zn(2)-O(2)
1.9978(13) Zn(2)-O(4)
2.0376(14) Zn(2)-N(3)
2.0768(14) Zn(2)-N(4)
3.039(1)
2.1511(12)
1.9772(12)
1.9990(13)
2.0141(14)
2.0620(14)
Zn(1)-O(1)-Zn(2) 94.41(5)
Zn(1)-O(2)-Zn(2) 95.88(5)
O(1)-Zn(1)-N(1)
O(2)-Zn(1)-N(1)
O(1)-Zn(1)-N(2)
O(2)-Zn(1)-N(2)
N(1)-Zn(1)-N(2)
O(1)-Zn(1)-O(3)
O(2)-Zn(1)-O(3)
132.07(6)
84.71(5)
90.46(5)
165.03(6)
100.72(6)
103.40(5)
91.61(5)
O(1)-Zn(2)-N(3)
O(2)-Zn(2)-N(3)
O(1)-Zn(2)-N(4)
O(2)-Zn(2)-N(4)
N(3)-Zn(2)-N(4)
O(1)-Zn(2)-O(4)
O(2)-Zn(2)-O(4)
85.15(5)
135.56(6)
163.52(5)
90.60(5)
99.96(6)
91.95(5)
101.20(5)
[Zn2L3(µ-OAc)](ClO4) (12). An ORTEP representation
of the cation [Zn2L3(µ-OAc)]+ is shown in Figure 3. Selected
bond lengths and angles are listed in Table 4.
The structure consists of two five-coordinated zinc centers,
which are bridged by two phenoxide oxygens and the acetate
moiety. Due to puckering of the two C4-linkers, the planes
of the two phenyl rings of the macrocycle subtend an angle
of 28.6°. The equatorial donor atoms O(1), O(2), N(1), and
N(2) for Zn(1) deviate from the mean plane by (0.282(1)
Å, while the atoms O(1), O(2), N(3), and N(4) providing
the basal plane for Zn(2) deviate from the least-squares plane
by (0.233(1) Å. The metal centers Zn(1) and Zn(2), in turn,
are displaced in the same direction from their mean planes
by 0.483(1) and 0.470(1) Å, respectively. The distorted
square-pyramidal geometry of the two metal centers is
reflected in their relevant bond lengths, which show non-
uniformity. The nonbonded Zn(1)‚‚‚Zn(2) separation is
3.0389(3) Å.
1H NMR Spectra. The proton NMR spectrum of all the
complexes, except for 3 and 11, was obtained in CD3CN;
for 3 and 11 their (CD3)2SO solutions were used. The
spectrum of the mononuclear complex 1 differs significantly
from that of the dinuclear complex 2. For example, in 1 a
low-field resonance is observed at 14.62 ppm due to the
hydrogen-bonded N-H‚‚‚O protons. There are two CHdN
resonances at 8.61 and 8.48 ppm for this compound, one of
at 1638 cm-1. Moreover, the asymmetric and symmetric νCOO
-
vibrations in 10 are observed at 1568 and 1372 cm-1, as
against those observed at 1559 and 1348 cm-1 for 11. More
importantly, the ClO4- vibrations observed at 1086 and 626
cm-1 in 10 are absent in 11.
Spectrophotometric Titrations of [H4L1-3](ClO4)2 with
Zn(OAc)2‚2H2O. The reaction equilibria involving the
macrocyclic ligands [H4L1-3](ClO4)2 and Zn(OAc)2‚2H2O in
acetonitrile have been followed spectrophotometrically by
observing the spectral changes that occur on the incremental
addition of the metal ion to the ligand solution till no further
change is noted. As shown in Figure 4, the absorption peak
of the ligand [H4L1](ClO4)2 at 450 nm becomes blue-shifted
with diminution of intensity on the addition of the metal salt.
The successive absorption curves pass through an isosbestic
point at 428 nm with the development of a new band at 400
nm, whose intensity becomes maximized when the ligand-
to-metal ratio reaches 1:0.6. However, with further addition
of the metal salt the intensity of the band at 400 nm becomes
depleted at the expense of the growth of another new band
at 370 nm, the intensity of which increases until the ligand-
to-metal ratio reaches 1:2, beyond which no further change
Inorganic Chemistry, Vol. 44, No. 1, 2005 153

Dutta et al.
nm. During this process the absorption peak at 405 nm
continually diminishes in intensity, and at its expense a new
band at 370 nm grows. When the metal-to-ligand ratio
reaches 1:2, the absorption spectrum resembles that of 3.
Although similar to [H4L1]2+ here also stepwise reaction
equilibrium eqs 3 and 4 are involved, compared to the
previous case the two equilibrium constants are more
overlapping in the present case. Failure to isolate the
mononuclear species [Zn(H2L2)](ClO4)2 is consistent with
this observation.
K1
\
[H4L2]2+ + Zn(OAc)2 y
z [Zn(H2L2)]2+ + 2HOAc (3)
K2
[Zn(H2L2)]2+ + Zn(OAc)2 y z [Zn2L2]2+ + 2HOAc (4)
\
The spectrophotometric titration carried out with [H4L3]2+
(Figure S5 in the Supporting Information) shows that with
increasing amount of zinc(II) added the intensity of the ligand
band at 435 nm diminishes, while a new band at 375 nm
grows progressively, and all the absorption curves pass
through the isosbestic points at 395 and 345 nm. The
spectrum obtained after addition of 2 equiv of Zn(OAc)2‚
2H2O is identical to that of 12. The smooth changeover of
the ligand spectrum to that of 12 on addition of zinc(II)
indicates simultaneous occupation of the two cavity sites of
the macrocycle by the metal ions according to eq 5.
Figure 4. Spectrophotometric titration of the macrocycle [H4L1](ClO4)2
(1 × 10-5 M) with various numbers of equivalents of Zn(OAc)2‚2H2O in
acetonitrile.
[H4L3]2+ + 2Zn(OAc)2 h
[Zn2L3(µ-OAc)]+ + H+ + 3HOAc (5)
Spectrofluorimetric Titrations of [H4L1-3](ClO4)2 with
Zn(OAc)2‚2H2O. We have recently reported17 that the
macrocyclic ligands [H4L](ClO4)2 with the lateral side chains
varying from -(CH2)2- to -(CH2)12- all exhibit photo-
luminescence in solution at room temperature. However,
there are considerable differences in the fluorescence intensi-
ties of these compounds across the series. For example, the
acetonitrile solutions (1 × 10-5 M) of [H4L1-3](ClO4)2 on
excitation at 440-450 nm fluoresce at 500-510 nm, and
their normalized intensities (in arbitrary units) decrease in
the order L3 (960) > L1 (145) > L2 (3).
Spectrofluorimetric titrations were carried out by adding
an incremental amount (0.2 equiv) of an acetonitrile solution
of Zn(OAc)2‚2H2O (1 × 10-3 M) to a solution of the ligand
(1 × 10-5 M) in the same solvent and monitoring either the
growth of the luminescent band due to complex formation/
or the depletion of the ligand emission band due to its
consumption.
Figure 5. Spectrophotometric titration of the macrocycle [H4L2](ClO4)2
(1 × 10-5 M) with various numbers of equivalents of Zn(OAc)2‚2H2O in
acetonitrile.
occurs. During the interval of addition of 0.6-2 equiv of
the metal ion, the absorption curves pass through an
isosbestic point at 384 nm, while for the entire range of
ligand-to-metal ratios all the absorption curves pass through
335 nm. It is important to note that the mononuclear complex
1 in acetonitrile exhibits a band at 400 nm (15600 M-1 cm-1),
whereas for the dinuclear complex 2 the absorption band is
shifted to 370 nm (13900 M-1 cm-1). Clearly, the following
reaction equilibria are established in solution:
The growth of the emission band at 435 nm with the
addition of zinc(II) to [H4L2]2+ using 370 nm (λmax for the
dizinc(II) complex) as the excitation wavelength is shown
in Figure 6. The shape of the curve indicates that, while
complex formation takes place in a stepwise manner,
considerable overlapping occurs between the two steps. The
observation made during spectrophotometric titration is
consistent with this conclusion.
K1
\
[H4L1]2+ + Zn(OAc)2 y
z [Zn(H2L1)]2+ + 2HOAc (1)
K2
[Zn(H2L1)]2+ + Zn(OAc)2 y z [Zn2L1]2+ + 2HOAc (2)
\
The spectral changes that take place on titrating [H4L2]-
(ClO4)2 with Zn(OAc)2‚2H2O in acetonitrile are shown in
Figure 5. Barring the ligand-to-metal ratio of 1:0.2, with
increasing amount of metal ion added all the absorption
curves pass through two isosbestic points at 390 and 327
Figure 7 shows that the luminescence band of [H4L3]2+
observed at 500 nm (with λex ) 440 nm) progressively
154 Inorganic Chemistry, Vol. 44, No. 1, 2005

Zinc(II) Complexes of Tetraiminodiphenol Macrocycles
Figure 8. Variation of emission intensity during titration of the macrocycle
[H4L1](ClO4)2 (1 × 10-5 M) with various numbers of equivalents of
Zn(OAc)2‚2H2O in acetonitrile. Solid circles represent emission at 510 nm
with λex ) 440 nm; open circles represent emission at 450 nm with λex
370 nm.
)
Figure 6. Spectrofluorimetric titration of the macrocycle [H4L2](ClO4)2
(1 × 10-5 M) with various numbers of equivalents of Zn(OAc)2‚2H2O in
acetonitrile. The arrow v indicates the increasing concentration of Zn(OAc)2‚
2H2O added. λex ) 370 nm. The inset shows the variation of luminescence
intensity with the number of equivalents of Zn(OAc)2‚2H2O added.
Table 5. Absorption and Emission Spectral Data for Complexes 1-12
absorption
emissiona
em, nm
compd
solvent
λ
max, nm (ꢀ, M-1 cm-1
)
λ
æ
1
2
3
acetonitrile
acetonitrile
methanol
acetonitrile
methanol
acetonitrile
methanol
acetonitrile
methanol
acetonitrile
methanol
acetonitrile
methanol
acetonitrile
methanol
acetonitrile
acetonitrile
402 (15600), 247 (72600)
370 (13900), 251 (76800)
370 (16900), 251 (78200)
370 (16600), 250 (77300)
370 (15700), 250 (70900)
370 (15900), 251 (75300)
370 (17900), 250 (85300)
370 (17000), 252 (84000)
370 (18400), 251 (85800)
370 (18400), 250 (84900)
370 (18800), 250 (88600)
369 (18300), 254 (88200)
370 (19000), 251 (89000)
371 (18900), 250 (86000)
370 (15400), 252 (73000)
370 (15900), 252 (75100)
369 (18000), 254 (91800)
450
427
431
429
433
428
432
428
433
429
433
428
432
427
433
433
438
0.031
0.027
0.039
0.035
0.046
0.043
0.047
0.048
0.047
0.046
0.044
0.042
0.021
0.018
4
5
6
7
8
9
10
12
acetonitrile 377 (10400), 254 (402000)
a Excitation wavelength 370 nm.
the observed data can be best represented by two sets of
straight lines differing in slope which intersect at the point
when L:M is 1. This observation substantiates why, unlike
L2 and L3, it has been possible to isolate both the mono-
nuclear (1) and dinuclear (2) complexes of L1.
Figure 7. Spectrofluorimetric titration of the macrocycle [H4L3](ClO4)2
(1 × 10-5 M) with various equivalents of Zn(OAc)2‚2H2O in acetonitrile.
The arrow V indicates the increasing concentration of Zn(OAc)2‚2H2O added.
λex ) 440 nm. The inset shows the variation of luminescence intensity with
the number of equivalents of Zn(OAc)2‚2H2O added.
becomes quenched on incremental addition of zinc(II) and
finally vanishes when the ligand-to-metal ratio (L:M)
becomes 1:2. The diminution of the luminescence intensity
plotted against the number of equivalents of zinc(II) added
provides a straight line (shown in the inset of Figure 7) to
indicate that in this case the formation of the dizinc(II)
complex occurs in a single step. Conversely, if the growth
of the luminescence band of the complex observed at 438
nm (with λex ) 370 nm) is monitored as a function of the
amount of zinc(II) again a linear relationship is obtained.
In the case of titration of [H4L1]2+ with zinc acetate, the
decrease in intensity of the emission band of the ligand at
510 nm (with λex ) 440 nm) and the increase in intensity of
the emission band due to complex formation at 450 nm (with
λex ) 370 nm) were monitored. The variation of emission
intensities at 510 and 450 nm with the number of equivalents
of zinc acetate added is shown in Figure 8. In both the cases,
Emission Spectra of 4-9. The luminescence spectra of
the carboxylate-bridged complexes 4-9 were measured
in methanol and acetonitrile solutions (1 × 10-5 M) at
room temperature. Table 5 summarizes the absorption and
emission spectral characteristics of the complexes, including
their quantum yields. Similar to the precursor complex
[Zn2L2(H2O)2]2+, complexes 4-9 all exhibit an absorption
band at 370 nm, the position of which remains the same in
methanol and acetonitrile. Accordingly, with 370 nm as the
excitation wavelength, the emission peak is observed at about
430 nm for all the complexes. For the sake of comparison,
the emission spectra recorded for 3-9 in acetonitrile are
shown in Figure 9. It may be noted that, relative to that of
3, the fluorescence intensity of the acetate complex 4 is
stronger (by a factor of 1.2). In the case of the benzoate
complex 5, further intensification of the emission band occurs
(1.4 times stronger relative to that of 3). Subtle electronic
Inorganic Chemistry, Vol. 44, No. 1, 2005 155

Dutta et al.
Figure 10. Spectrofluorimetric titration of [Zn2L2(H2O)2](ClO4)2 (1 ×
10-5 M) with increasing concentration (v) of L-alanine (1 × 10-6 to 3 ×
10-5 M) in water. λex ) 370 nm.
Figure 9. Relative luminescence intensities observed for the carboxylate-
bridged complexes 4-9 with respect to 3 in acetonitrile (concentration 1
× 10-5 M).
The equilibrium constants (K) were evaluated by linear
regression analysis, and the values thus obtained are 2.8(2)
× 105 for acetate, 1.2(2) × 105 for benzoate, 2.5(3) × 105
for alanine, 3.1(2) × 105 for histidine, 4.4(4) × 105 for valine,
and 8.0(5) × 105 for proline. It is interesting to note that the
binding constants obtained for the amino acids do not differ
much among themselves and also are of the same orders of
magnitudes as those of acetate and benzoate. Clearly, in all
the cases a similar mode of substrate binding is involved.
Conclusion. The binding of zinc(II) with the macrocyclic
ligands [H4L1-3]2+, whose cavity sizes increase progressively,
have been studied by spectrophotometric and spectrofluori-
metric titrations. Stepwise complexation occurs in the case
of L1, making it possible to isolate both the mononuclear
(1) and dinuclear (2) complexes. In the case of L2, the two
successive equilibria overlap considerably, and therefore,
only the dinuclear complex 3 could be isolated. As such, L3
fails to complex with zinc(II); however, in the presence of
acetate the formation of [Zn2L3(µ-OAc)]+ takes place in a
single step. Complexes 3 and 12 have been structurally
characterized. The reaction between 3 and an excess of
acetate/benzoate produces complexes 4-9. Significantly, the
benzoate complex 5 on reaction with benzoic acid produces
a compound of unusual composition, 10. The crystal structure
determination of 10 indicates that protonation occurred to
the coordinated water molecule. The structural consequence
of this unusual binding is the drainage of electron density
from the Zn(1) coordination sites to the Zn(2) sites, and as
a result Zn(1)-O bond distances are shorter relative to
Zn(2)-O distances. Importantly, complex 10 on treatment
with 1 equiv of sodium hydroxide produces the neutral
dibenzoate complex 11, whose formation provides support
to the presence of H3O+ ion in 10. The photoluminescence
behavior studied for complexes 4-9 indicates that the
intensities of the luminescence spectra vary with the donor
ability of the carboxylate; it is maximum for the p-methyl
derivative (6) and minimum for the p-nitro derivative (9).
The equilibrium constants (K) involving the binding of
acetate/benzoate and the carboxylate moiety of five amino
acids with [Zn2L2(H2O)2]2+ have been determined in aqueous
influence of the para-substituents is observed for 6 (Me) >
7 (OMe) > 8 (Cl). Significantly, with the strongly electron-
withdrawing p-nitro group in 9, substantial reduction of
fluorescence intensity occurs (1.5 times less relative to that
of 3).
Equilibrium Constants. The equilibrium constants for the
reaction
K
[Zn2L2(H2O)2]2+ + A- y
\
z [Zn2L2A]+ + 2H2O
(6)
involving the dinuclear zinc(II) complex 3 and the carbox-
ylate anions (A-) derived from sodium acetate and benzoate
and a number of amino acids (such as glycine, L-alanine,
L-histidine, L-valine, and L-proline) which exist in the
zwitterionic form RN+H2DCO2- were determined by spec-
trofluorimetric titrations in aqueous solution. No buffer was
used in these measurements to avoid any possible interfer-
ence by its constituents (for example, carboxylate or phos-
phate). Nevertheless, at the level of concentration of the
solutions (∼10-5 M) used, the pH of the solutions re-
mained between 6.4 and 6.8. Typical changes in the
luminescence spectrum of an aqueous solution (1 × 10-5
M) of [Zn2L2(H2O)2]2+ with varying amounts of L-alanine
(1 × 10-6 to 3 × 10-5 M) are shown in Figure 10. The
emission peak of 3 at 425 nm (λex ) 370 nm) grows in
intensity with increasing amount of alanine and reaches the
maximum when its concentration is 2 × 10-5 M. When 3 is
titrated with sodium benzoate solution, a similar observation
(shown in Figure S6 in the Supporting Information) is made.
Since in this case (also for acetate) 1:1 complex formation
occurs, it is reasonable to expect a similar type of interaction
with the amino acids. Indeed, it has been structurally
established that the dinickel(II) macrocyclic analogue forms
µ-carboxylato complex species with glycine etc. in the
zwitterionic form.26,27
(26) Das, R.; Nanda, K. K.; Venkatsubramanian, K.; Paul, P.; Nag, K. J.
Chem. Soc., Dalton. Trans. 1992, 1253.
(27) Nanda, K. K.; Das, R.; Thompson, L. K.; Venkatsubramanian, K.;
Nag, K. Inorg. Chem. 1994, 33, 5934.
156 Inorganic Chemistry, Vol. 44, No. 1, 2005

Zinc(II) Complexes of Tetraiminodiphenol Macrocycles
solution by spectrofluorimetric titrations. The values of K
lying in the range (1-8) × 105 indicate that the amino acids
are quite effective in binding with the zinc fluorophore
through their carboxylate unit.
[Zn2L2(H2O)2]2+ relative to [H4L2](ClO4)2 is further aug-
mented with the formation of the carboxylate-bridged
complexes [Zn2L2(µ-O2CR)]+.
Acknowledgment. We are thankful to the Department
of Science and Technology, Government of India, for
providing funds to purchase the spectrofluorimeter of the
Department of Inorganic Chemistry of the Indian Association
for the Cultivation of Science.
The significant aspect of this study is the observation of
zinc-induced fluorescence enhancement of the macrocyclic
ligand [H4L2](ClO4)2, which can be considered as a photo-
induced charge-transfer fluorophore. The internal charge
transfer (ICT) that occurs from the phenolate donor site to
the protonated imino acceptor site of the ligand at 440 nm
undergoes a blue shift to 370 nm on formation of the
dizinc(II) complex [Zn2L2(H2O)2]2+. Consequently, the lu-
minescence band of the ligand at 510 nm is also markedly
blue-shifted to 435 nm with considerable enhancement of
intensity. The 70-fold increase in luminescence intensity of
Supporting Information Available: X-ray crystallographic files
in CIF format for compounds 3, 10, and 12 and Figures S1-S6
(PDF). This material is available free of charge via the Internet at
http://pubs.acs.org.
IC049056A
Inorganic Chemistry, Vol. 44, No. 1, 2005 157