Transition metal complexes (M = Cu, Ni and Mn) of Schiff-base ligands: Syntheses, crystal structures, and inhibitory bioactivities against urease and xanthine oxidase

Article abstract of DOI:10.1016/j.ica.2007.02.019

Six new transition metal complexes (M = Cu(II), Ni(II) and Mn(III)) of tridentate (H₂L¹, HL²) and/or bidentate (HL³, HL⁴) Schiff-base ligands, obtained from the condensation of salicylaldehyde with glycine, N-(2-aminoethyl)morpholine, 4-(2-aminoethyl)phenylic acid and 4-(2-aminoethyl)benzsulfamide, respectively, were synthesized and structurally determined by single-crystal X-ray analysis. Complexes 1-6 were evaluated for their effect on the jack bean urease and xanthine oxidase (XO). Copper(II) complexes 1-3 (IC₅₀ = 0.43-2.25 μM) showed potent inhibitory activity against jack bean urease, comparable with acetohydroxamicacid (IC₅₀ = 42.12 μM), which is a positive reference. And these copper(II) complexes (IC₅₀ = 10.26-15.82 μM) also exhibited strong ability to inhibit activity of XO, comparable to allopurinol (IC₅₀ = 10.37 μM), which was used as a positive reference. Nickel(II) and manganese(III) complexes 4-6 showed weak inhibitory activity to jack bean urease (IC₅₀ = 4.36-8.25 μM) and no ability to inhibit XO (IC₅₀ > 100 μM).

Full text of DOI:10.1016/j.ica.2007.02.019

Inorganica Chimica Acta 360 (2007) 2881–2889
www.elsevier.com/locate/ica
Transition metal complexes (M = Cu, Ni and Mn) of
Schiff-base ligands: Syntheses, crystal structures, and inhibitory
bioactivities against urease and xanthine oxidase
a
a
a,
a
b
Yu-Guang Li , Da-Hua Shi , Hai-Liang Zhu *, Hong Yan , Seik Weng Ng
a
School of Life Sciences & School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR China
b
Department of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia
Received 24 August 2006; received in revised form 31 January 2007; accepted 1 February 2007
Available online 20 February 2007
Abstract
Six new transition metal complexes (M = Cu(II), Ni(II) and Mn(III)) of tridentate (H2L1, HL2) and/or bidentate (HL3, HL4) Schiff-
base ligands, obtained from the condensation of salicylaldehyde with glycine, N-(2-aminoethyl)morpholine, 4-(2-aminoethyl)phenylic
acid and 4-(2-aminoethyl)benzsulfamide, respectively, were synthesized and structurally determined by single-crystal X-ray analysis.
Complexes 1–6 were evaluated for their effect on the jack bean urease and xanthine oxidase (XO). Copper(II) complexes 1–3
(IC50 = 0.43–2.25 lM) showed potent inhibitory activity against jack bean urease, comparable with acetohydroxamicacid
(IC50 = 42.12 lM), which is a positive reference. And these copper(II) complexes (IC50 = 10.26–15.82 lM) also exhibited strong ability
to inhibit activity of XO, comparable to allopurinol (IC50 = 10.37 lM), which was used as a positive reference. Nickel(II) and manga-
nese(III) complexes 4–6 showed weak inhibitory activity to jack bean urease (IC50 = 4.36–8.25 lM) and no ability to inhibit XO
(IC50 > 100 lM).
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Complexes; Schiff bases; Crystal structures; Urease inhibitors; Xanthine oxidase inhibitors
1. Introduction
1D, 2D, 3D complexes [5] together with other coordinat-
ing moieties such as azide ion, thiocyanate and
carboxylic group acting as either a bridging or a terminal
ligand. The use foreground of such metal complexes is
also promising such as acting as single-molecule magnets
(SMMs) [6], as luminescent probes [7], as catalysts for
specific DNA [8,9] and RNA [10] cleavage reactions. In
order to have an insight into the potential application
of complexes of Schiff-base ligands as an enzyme
inhibitor against urease and xanthine oxidase (XO), we
have designed and synthesized six new complexes of
Cu, Ni, Mn containing Schiff-base ligands derived from
the condensation of salicylaldehyde and various primary
amines.
In the past decades, the complexes containing
transition metal ions and various Schiff-base ligands have
been extensively investigated due to their novel structures
and potential applications in many fields [1]. Particularly,
a
large number of transition metal complexes of
Schiff-base ligands derived from the condensation of
salicylaldehyde and its derivatives with various primary
amines become the hot topics of contemporary research
[2]. These Schiff-base ligands may act as a bidentate
N,O- [3] and a tridentate N,O,O-donor ligand [4], and
so on, which can be designed to yield mono-, bi-, dimer,
Urease, the first enzyme crystallized to be shown to
possess nickel ions [11], is an important enzyme in both
agriculture and medicine, which rapidly catalyzes the
*
Corresponding author. Tel.: +0086 25 8359 2572; fax: +0086 25 8359
2672.
E-mail address: zhuhl@nju.edu.cn (H.-L. Zhu).
0020-1693/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2007.02.019

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Y.-G. Li et al. / Inorganica Chimica Acta 360 (2007) 2881–2889
hydrolysis of urea to form ammonia and carbamic acid
[12]. But the end byproduct of such decomposing reactions
also results in a pH increase, responsible for negative effects
of urease activity in human health [13], such as causing
peptic ulcers, stomach cancer, etc., and agriculture, for
example, the efficiency of soil nitrogen fertilization with
urea decreases due to ammonia volatilization and root
damage caused by soil pH increase [14]. It’s interesting to
control the activity of urease through the use of inhibitors
in order to counteract these negative effects. Some matters
possessing inhibitory activity against urease have been
reported such as boric acid [15], fluoride [16], heavy metal
ions [17], a-hydroxyketones [18].
(10 mL, 1:1 v/v). The mixture was stirred for 1 h to give
a yellow solution, which was added to a stirred methanol
of CuSO4 Æ 5H2O (0.5 g, 2 mmol, 10 mL). The mixture
was stirred for another 10 min at room temperature and
then filtered. The filtrate was kept in air for 9d, forming
blue block-shaped crystals of 1. The crystals were isolated,
washed three times with MeOH and dried in a vacuum
desiccator containing anhydrous CaCl2. Yield: 43%. Anal.
Calc. for [(CuL1)2 Æ 1/3(CH3OH)]n: C, 44.59; H, 3.34; N,
5.57. Found: C, 44.60; H, 3.47; N, 5.30%. IR (KBr,
cmꢀ1): 3094, 3030, 1649, 1636, 1614, 1601, 1571, 1534,
1447, 1372, 1339, 1301, 1195, 1128, 1094, 796, 769. UV–
Vis (DMSO-H2O, k/nm): 251, 264, 368.
However, XO catalyzes the hydroxylation of hypoxan-
thine and of xanthine to yield uric acid and superoxide
anions. The latter has been linked to post ischaemic tis-
sue injury and edema as well as to vascular permeability
[19]. XO may reduce pharmacological properties of syn-
thetic purine drugs, such as antileukaemic 6-mercapto-
purine, through oxidization reaction [20]. Controlling
the activities of XO may be helpful to the therapy of
some correlative diseases. Nowadays, as a potent inhibi-
tor of XO allopurinol to treat gout has been known for
a long time [21]. But reports on inhibitory activity against
the XO of Schiff-base complexes are very rare in the lit-
erature [22].
In this paper, we reported the synthesis of six d-transition
metal complexes 1–6 (M = Cu, Ni, Mn) and investigated
their inhibitory activities against urease and XO. The crystal
structures of these complexes with Schiff-base ligands L1–4
derived from the condensation of salicylaldehyde with
glycine, N-(2-aminoethyl)morpholine, 4-(2-amino-ethyl)-
phenylic acid, and 4-(2-aminoethyl)benzsulfamide, respec-
tively, were determined by X-ray analysis: Complex
[(CuL1)2 Æ 1/3(CH3OH)]n (1); [CuL2(Py)(H2O)] (ClO4) Æ
H2O (2); [Cu(L3)2] Æ 2H2O (3); [Ni(L3)2] Æ 2H2O (4);
[Ni(L4)2] (5); [Mn2(L4)2(N3)2] Æ 2CH3OH (6).
2.2.2. Synthesis of [CuL2(Py)(H2O)](ClO4) Æ H2O (2)
Salicylaldehyde (0.24 g, 2.0 mmol) and N-(2-amino-
ethyl)morpholine (0.26 g, 2.0 mmol) were dissolved in pyri-
dine and methanol mixture (10 mL, 10:1 v/v). The mixture
was stirred for 30 min to give a yellow solution, which was
added to a stirred methanol of Cu(ClO4)2 Æ 7H2O (0.78 g,
2.0 mmol, 10 mL). The mixture was stirred for another
10 min at room temperature and then filtered. The filtrate
was kept in air for 7d, forming blue block-shaped crystals
of 2. The crystals were isolated, washed three times with
MeOH and dried in a vacuum desiccator containing
anhydrous CaCl2. Yield: 51%. Anal. Calc. for [(CuL2)(Py)-
(H2O)](ClO4) Æ H2O: C, 42.27; H, 5.12; N, 8.22. Found: C,
42.20; H, 5.37; N, 8.11%. IR (KBr, cmꢀ1): 3513, 3339,
3232, 1643, 1608, 1600, 1540, 1471, 1448, 1343, 1314,
1100, 1090, 1070, 1051, 908, 751. UV–Vis (DMSO-H2O,
k/nm): 256, 268, 301, 370.
2.2.3. Synthesis of [Cu(L3)2] Æ 2H2O (3) and
[Ni(L3)2] Æ 2H2O (4)
Schiff-base ligand HL3 (0.48 g, 2.0 mmol) derived from
condensation of salicylaldehyde and 4-(2-aminoethyl)phe-
nylic acid was dissolved in methanol solution. The mixture
was stirred for 30 min to give an orange solution, which
was added to a stirred methanol of CuCl2 Æ 2H2O (0.34 g,
2.0 mmol, 10 mL). The mixture was stirred for another
10 min at room temperature and then filtered. The filtrate
was kept in air for 7d, forming deep-blue block-shaped
crystals of 3. The crystals were isolated, washed three times
with MeOH and dried in a vacuum desiccator containing
anhydrous CaCl2. Yield: 29%. Anal. Calc. for
(CuL3)2 Æ 2H2O: C, 62.11; H, 5.56; N, 4.83. Found: C,
62.00; H, 5.63; N, 4.81%. IR (KBr, cmꢀ1): 3235(br),
1620, 1598, 1547, 1514, 1473, 1449, 1439, 1385, 1296,
1271, 1229, 1204, 1152, 1127, 1102, 827, 764. UV–Vis
(DMSO-H2O, k/nm): 251, 269, 307, 366.
2. Experimental
2.1. Materials and physical measurements
All the chemicals used in syntheses were of reagent grade
and were used without further purification. HL3 and HL4
ligands were obtained as described in the literature [23,24].
Elemental analyses for C, H and N were carried out on a
Perkin–Elmer 2400 analyzer. IR spectra were recorded
using KBr pellets (4000–400 cmꢀ1) on a Nexus 870 FT-
IR spectrophotometer. Electronic spectra in the 200–
800 nm range were measured using DMSO-H2O (1:1 v/v)
solution on a Shimadzu UV-160 A spectrophotometer.
Schiff-base ligand HL3 (0.48 g, 2.0 mmol) was dissolved
in methanol solution. The mixture was stirred for 30 min to
give an orange solution, which was added to a stirred meth-
anol of NiCl2 Æ 6H2O (0.48 g, 2.0 mmol, 10 mL). The
mixture was stirred for another 10 min at room tempera-
ture and then filtered. The filtrate was kept in air for 7d,
forming yellow block-shaped crystals of 4. The crystals
2.2. Synthesis
2.2.1. Synthesis of [(CuL1)2 Æ 1/3(CH3OH)]n (1)
Salicylaldehyde (0.24 g, 2.0 mmol) and glycine (0.15 g,
2.0 mmol) were dissolved in aqueous methanol solution

Y.-G. Li et al. / Inorganica Chimica Acta 360 (2007) 2881–2889
2883
were isolated, washed three times with MeOH and dried in
a vacuum desiccator containing anhydrous CaCl2. Yield:
71%. Anal. Calc. for (NiL3)2 Æ 2H2O: C, 62.63; H, 5.61;
N, 4.87. Found: C, 62.61; H, 5.70; N, 4.77%. IR (KBr,
cmꢀ1): 3202(br), 1614, 1597, 1550, 1514, 1473, 1442,
1350, 1295, 1270, 1230, 1152, 1127, 1103, 828, 764. UV–
Vis (DMSO-H2O, k/nm): 252, 272, 317, 373.
IR (KBr, cmꢀ1): 3605, 3307, 3252, 2055, 1609, 1544,
1470, 1445, 1405, 1338, 1309, 1222, 1159, 1123, 1093,
1009, 820, 756. UV–Vis (DMSO-H2O, k/nm): 257, 271,
298, 385.
2.3. Inhibitory enzyme bioactivities tests
2.3.1. Measurement of inhibitory activity against jack bean
urease
2.2.4. Synthesis of [Ni(L4)2] (5) and
[Mn2(L4)2(N3)2] Æ 2CH3OH (6)
Jack bean urease was purchased from Sigma–Aldrich
Co. (St. Louis, Mo, USA). The measurement of urease
was carried out according to the literature reported by
Tanaka [25]. Generally, the assay mixture, containing
25 lL of Jack bean urease (10 kU/L) and 25 lL of the
tested complexes of various concentrations (dissolved in
the solution of DMSO:H2O = 1:1 (v/v)), was preincubated
for 1 h at 37 ꢁC in a 96-well assay plate. After preincuba-
tion, 0.2 mL of 100 mM HEPES (N-[2-hydroxy-ethyl]
Schiff-base ligand HL4 (0.61 g, 2.0 mmol) derived from
condensation of salicylaldehyde and 4-(2-aminoethyl)benz-
sulfamide was dissolved in methanol solution. The mixture
was stirred for 30 min to give a yellow solution, which was
added to a stirred methanol of NiCl2 Æ 6H2O (0.48 g,
2.0 mmol, 10 mL). The mixture was stirred for another
10 min at room temperature and then filtered. The filtrate
was kept in air for 7d, forming black block-shaped crystals
of 5. The crystals were isolated, washed three times with
MeOH and dried in a vacuum desiccator containing anhy-
drous CaCl2. Yield: 29%. Anal. Calc. for Ni(L4)2: C, 54.15;
H, 4.54; N, 8.42. Found: C, 53.77; H, 4.85; N, 8.40%. IR
(KBr, cmꢀ1): 3357, 3265, 1613, 1599, 1541, 1470, 1448,
1401, 1335, 1300, 1216, 1152, 1127, 1091, 1031, 876, 758.
UV–Vis (DMSO-H2O, k/nm): 252, 265, 319, 373.
piperazine-N0-[2-ethanesulfonic
acid])
buffer
[26]
pH = 6.8 containing 500 mM urea and 0.002% phenol
red were added and incubated at 37 ꢁC. The reaction time
was measured by micro plate reader (570 nm), which
was required to produce enough ammonium carbonate
to raise the pH of a HEPES buffer from 6.8 to 7.7, the
end-point being determined by the color of phenol red
indicator [27].
Schiff-base ligand HL4 (0.48 g, 2.0 mmol), Mn(AC)2 Æ
4H2O (0.50 g, 2.0 mmol) and sodium azide (0.065 g,
1 mmol) was dissolved in 20 mL methanol solution. The
mixture was stirred for 30 min to give a brown solution.
The mixture was stirred for another 10 min at room tem-
perature and then filtered. The filtrate was kept in air for
8d, forming black block-shaped crystals of 6. The crystals
were isolated, washed three times with MeOH and dried
in a vacuum desiccator containing anhydrous CaCl2. Yield:
44%. Anal. Calc. for [Mn2(L4)4(N3)2] Æ 2CH3OH: C, 50.61;
H, 4.65; N, 13.33. Found: C, 50.71; H, 4.60; N, 13.20%.
2.3.2. Measurement of inhibitory activity against XO
Xanthine oxidase from cow’s milk was purchased from
Sigma–Aldrich Co. (St. Louis, Mo, USA). The XO activi-
ties with xanthine as the substrate were measured spectro-
photometrically, based on the procedure reported by Kong
et al. [28], with modification. The assay was performed in a
final volume of 1 mL 20 mM HEPES buffer pH 7.8 in
quartz cuvette. Generally, the assay mixture, 100 lL
25 mU/mL XO, 50 lL of the various concentrations tested
Table 1
Crystal data for complexes 1–6
Compound
1
2
3
4
5
6
Empirical formula
Molecular weight
Crystal system
Space group
C
18.67H16.67N2O6.67Cu2 C18H26N3O8ClCu C30H32N2O6Cu C30H32N2O6Ni C30H32N4O6S2Ni C62H68N14O14S4Mn2
502.75
monoclinic
R-3
26.991(7)
26.991(7)
6.987(4)
90.00
511.41
580.13
orthorhombic
Pbcn
575.27
orthorhombic
Pbcn
667.43
triclinic
1469.41
triclinic
monoclinic
P2(1)/n
10.123(2)
12.683(3)
18.024(4)
90.00
103.24(3)
90.00
293(2)
ꢀ
ꢀ
P1
P1
˚
a (A)
15.482(2)
10.782(2)
15.957(2)
90.00
90.00
90.00
15.370(2)
10.914(2)
15.659(2)
90.00
90.00
90.00
10.011(2)
10.473(2)
16.499(2)
106.98(2)
90.02(2)
117.46(2)
292(2)
11.912(2)
12.301(2)
13.781(2)
116.06(2)
111.03(2)
91.46(2)
293(2)
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
90.00
120.00
295(2)
4408.5(3)
9
1.704
2286
T (K)
292(2)
293(2)
3
˚
V (A )
2253(8)
4
2663.9(5)
4
2626.9(4)
4
1449.3(4)
2
1652.2(4)
1
Z
qcalc (gcmꢀ3
F (000)
)
1.508
1.402
1.409
1.529
1.477
1060
1172
1168
696
762
l (Mo Ka) (mmꢀ1
)
2.213
1.137
0.863
0.782
0.866
0.584
Data/restraint/parameters 2245/13/133
4835/3/280
0.983
3039/0/179
0.943
2723/0/175
1.061
6277/0/393
1.003
7425/0/436
0.990
Goodness-of-fit on F2
1.121
Final R1, wR2 [I > 2r(I)]
0.031, 0.095
0.063, 0.163
0.049, 0.127
0.060, 0.183
0.054, 0.137
0.050, 0.122

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Y.-G. Li et al. / Inorganica Chimica Acta 360 (2007) 2881–2889
Table 2
addition 200 lL of 84.8 lg/mL xanthine in 50 mM
HEPES. The reaction is monitored for 6 min at 295 nm
at 37 ꢁC and the product is expressed as lmol uric acid
per minute.
˚
Selected bond lengths (A) and angles (ꢁ) in complexes 1–6
Compound 1
Cu1–N1
Cu1–O1
Cu1–O3
Cu1–O3[a]
Cu1–O2[b]
Cu1–Cu1[a]
O2–Cu1[c]
O3–Cu1[a]
N1–Cu1–O1
1.927(2)
1.937(2)
1.947(2)
1.993(2)
2.259(2)
3.020(5)
2.258(2)
1.993(2)
83.85(7)
N1–Cu1–O3
O1–Cu1–O3
92.98(7)
174.68(7)
162.10(7)
101.84(7)
79.95(7)
102.83(8)
93.85(8)
91.03(7)
93.79(7)
N1–Cu1–O3[a]
O1–Cu1–O3[a]
O3–Cu1–O3[a]
N1–Cu1–O2[b]
O1–Cu1–O2[b]
O3–Cu1–O2[b]
O3[a]–Cu1–O2[b]
2.4. Crystal structure determination
Diffraction intensities for the six complexes were col-
lected at 292(2)–295(2) K using a Bruker SMART CCD
˚
area detector with Mo Ka radiation (k = 0.71073 A). The
collected data were reduced using the SAINT program [29],
and empirical absorption corrections were performed using
the SADABS program [30]. The structures were solved by
direct methods and refined against F2 by full-matrix least-
squares methods using the SHELXTL version 5.1 [31]. All of
the non-hydrogen atoms were refined anisotropically. All
other hydrogen atoms were placed in geometrically ideal
positions and constrained to ride on their parent atoms.
The crystallographic data for six complexes are summa-
rized in Table 1. Selected bond lengths and angles are given
in Table 2.
Compound 2
Cu1–O1
Cu1–N1
Cu1–N3
Cu1–N2
Cu1–O3W
O1–Cu1–N1
O1–Cu1–N3
N1–Cu1–N3
1.974(4)
1.978(4)
2.055(5)
2.139(5)
2.495(5)
92.84(2)
87.07(2)
167.56(2)
O1–Cu1–N2
176.44(2)
84.71(2)
94.77(2)
91.55(2)
90.85(2)
101.58(2)
91.07(2)
N1–Cu1–N2
N3–Cu1–N2
O1–Cu1–O3W
N1–Cu1–O3W
N3–Cu1–O3W
N2–Cu1–O3W
Compound 3
Cu1–O1
1.903(2)
1.903(2)
1.995(2)
1.995(2)
180.00(2)
O1–Cu1–N1
89.96(9)
90.04(9)
90.04(9)
89.96(9)
180.00(2)
Cu1–O1[a]
Cu1–N1
O1[a]–Cu1–N1
O1–Cu1–N1[a]
O1[a]–Cu1–N1[a]
N1–Cu1–N1[a]
Cu1–N1[a]
O1–Cu1–O1[a]
3. Results and discussion
Compound 4
Ni1–O1[a]
Ni1–O1
1.834(2)
1.834(2)
1.939(3)
1.939(3)
180.00(1)
O1[a]–Ni1–N1[a]
O1–Ni1–N1[a]
O1[a]–Ni1–N1
O1–Ni1–N1
89.52(2)
90.48(2)
90.48(2)
89.52(2)
180.00(1)
3.1. General aspects
Ni1–N1[a]
Ni1–N1
The complexes have been characterized from the ele-
mental analytical and spectral data, respectively. The IR
spectra of complexes 1–6 exhibit strong absorption at
1609–1649 cmꢀ1, assignable to the m(C@N) absorption
[32]. For 1, the 1614 and 1601 cmꢀ1 bands are assigned
to the masym(COO) and msym(COO), respectively. The fact
that Dm [Dm = masym(COO) ꢀ msym(COO)] is largely less
than 200 cmꢀ1 demonstrates that the carboxylate group
is coordinated as a bidentate ligand [33]. ꢀFor 2, the strong
absorption peak for vibration of ClO4 is observed at
about 1090 cmꢀ1. For 3 and 4, the appearances of the
broad strong absorption at about 3230 cmꢀ1 can be rea-
sonably attributed to the presence of lattice water mole-
cule and/or phenolic OH group. The IR spectra of 5
and 6 show strong bands at 1613 and 1609 cmꢀ1 related
to the m(C@N) of the Schiff-base moiety, respectively,
which are less than the corresponding value reported for
their Schiff-base ligand [24]. This indicates coordination
of the imine nitrogen to Ni2+ and Mn3+. The strong
absorption at 1300–1338 cmꢀ1 is attributed to the stretch-
ing vibration of S@O groups. Peak at 2055 cmꢀ1 in the IR
spectrum of 6 is due to asymmetric stretching of the azide
group [5b]. The UV–Vis spectra for the complexes 1–6
were obtained in assay condition (DMSO:H2O, 1:1 v/v).
All the complexes show a weak d–d band in more than
500 nm region. In addition, the complexes exhibit intense
bands in the 366–385 nm regions, which are attributed to
a charge transfer (CT) transition [8,34]. For complex 2–6,
an absorption band at about 310 nm may also be associ-
ated with charge transfer and/or n ! p* transitions [35].
O1[a]–Ni1–O1
N1[a]–Ni1–N1
Compound 5
Ni1–O1[a]
Ni1–O1
Ni1–N1[a]
1.842(2)
1.842(2)
1.927(3)
1.927(3)
180.00(1)
91.28(2)
88.72(2)
88.72(2)
91.28(2)
180.00(2)
Ni2–O4
1.832(2)
1.832(2)
1.922(2)
1.922(2)
180.0(2)
87.48(2)
92.52(2)
92.52(2)
87.48(2)
180.00(2)
Ni2–O4[b]
Ni2–N3[b]
Ni2–N3
Ni1–N1
O1[a]–Ni1–O1
O1[a]–Ni1–N1[a]
O1[a]–Ni1–N1
O1–Ni1–N1[a]
O1–Ni1–N1
N1–Ni1–N1[a]
O4[b]–Ni2–O4
O4–Ni2–N3[b]
O4[b]–Ni2–N3[b]
O4–Ni2–N3
O4[b]–Ni2–N3
N3[b]–Ni2–N3
Compound 6
Mn1–O1
Mn1–O2
Mn1–N1
Mn1–N2
1.855(2)
1.898(2)
2.038(2)
2.051(2)
2.163(2)
2.519(2)
166.58(8)
90.07(8)
92.74(7)
89.61(7)
87.44(7)
N1–Mn1–N2
O1–Mn1–N5
O2–Mn1–N5
N1–Mn1–N5
N2–Mn1–N5
O1–Mn1–O2[a]
O2–Mn1–O2[a]
N1–Mn1–O2[a]
N2–Mn1–O2[a]
N5–Mn1–O2[a]
179.34(8)
96.43(9)
96.54(9)
92.73(9)
87.88(9)
91.44(7)
75.30(7)
93.28(7)
86.16(6)
170.08(7)
Mn1–N5
Mn1–O2[a]
O1–Mn1–O2
O1–Mn1–N1
O2–Mn1–N1
O1–Mn1–N2
O2–Mn1–N2
Symmetry transformations used to generate equivalent atoms. Complex 1:
[a] 1/3 ꢀ x, 2/3 ꢀ y, 2/3 ꢀ z; [b] ꢀ1/3 ꢀ x + y, 1/3 ꢀ x, 1/3 + z; [c] 1/3 ꢀ y,
2/3 + x ꢀ y, z ꢀ 1/3; 3: [a] ꢀx, 1 ꢀ y, ꢀz; 4: [a] 2 ꢀ x, 1 ꢀ y, 1 ꢀ z; 5: [a]
1 ꢀ x, 1 ꢀ y, 2 ꢀ z; [b] ꢀx, 1 ꢀ y, ꢀz; 6: [a] 2 ꢀ x, 1 ꢀ y, 1 ꢀ z.
complexes (dissolved in the solution of DMSO:H2O = 1:1)
and 700 lL of 50 mM HEPES buffer were preincubated for
1 h at 37 ꢁC. After preincubation, the reaction is started by

Y.-G. Li et al. / Inorganica Chimica Acta 360 (2007) 2881–2889
2885
The intense higher-energy bands at around 260 nm can be
attributed to intraligand p ! p* transitions.
3.2. Synthesis
Although imine bonds are susceptible to hydrolysis,
metal complexes of Schiff bases are usually resistant
towards hydrolytic cleavage, and as a result Schiff-base
ligand H2L1 was obtained from the condensation of sali-
cylaldehyde with glycine in aqueous methanol solution (1:1
v/v) (see Section 2). In order to get better single-crystal,
complex 1 was prepared by the reaction of Schiff-base
ligand H2L1 and CuSO4 Æ 5H2O in aqueous methanol
solution (1:1 v/v) too. Schiff-base ligand HL2 was
obtained from the condensation of salicylaldehyde with
N-(2-aminoethyl)morpholine in anhydrous methanol solu-
tion. However, complex 2 was prepared by the reaction of
Schiff-base ligand HL2 and Cu(ClO4)2 Æ 7H2O in pyridine
and methanol mixture (10:1 v/v). As previously reported
in the literature, Schiff-base ligands HL3 and HL4 were
obtained from the condensations of salicylaldehyde with
4-(2-aminoethyl)phenylic acid and 4-(2-aminoethyl)ben-
zsulfamide, respectively. Complexes 3 and 4, showing
metal ion Cu2+ and Ni2+ exchange were prepared by
the reactions of Schiff-base ligand HL3 and CuCl2 Æ 2H2O
and NiCl2 Æ 6H2O in the same condition, respectively.
Complex 6 were prepared by the reactions of Schiff-base
ligand HL4, Mn(AC)2 Æ 4H2O and NaN3 (molar
ratio = 1:1:05).
Fig. 1. Molecular packing view of [(CuL1)2 Æ 1/3(CH3OH)]n (1) along the
c-axis; the disordered methanol solvent are shown as the red dots. (For
interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)
actions [36]. The N-salicylideneglycinate dianion N,O,O’-
chelates to one Cu atom and the dianion also uses its
hydroxy O atom to form a dative bond to the other Cu
atom so that one N and three O atoms comprise a square.
trans-[(C9H7NO3)Cu]2 can be regarded as a quasi-planar 4-
connected building unit. The assembly of the 4-connected
nodes results in the generation of three- and six-membered
windows to form a desired channel structure along [001]
´
(Fig. 1), giving an overall Kagome lattice topology. The
3.3. Crystal structure description
2
˚
largest channel is about 13.5 · 13.5 A in inner size, compa-
rable to the porous or grid-like coordination polymers
reported so far [37]. The methanol solvate is formally a
dinuclear compound (Scheme 1), lying on a three-fold rota-
tion axis and located in the wider areas of the channel,
exagonal cavities.
3.3.1. [(CuL1)2 Æ 1/3(CH3OH)]n (1)
The structure of complex 1 consists of a three-dimen-
sional network motif which is constructed through the
dinuclear moieties [Cu(L1)]2 and the disordered lattice
methanol molecules as shown Fig. 1. In 1, the moieties
[Cu(L1)]2, which lies on a center-of-inversion, were bridged
through the double-bond carbonyl O atoms into a three-
dimensional network motif whose cavities are occupied
by the disordered lattice methanol. The dinuclear unit
[Cu(L1)]2 is formed by two Cu atoms labeled Cu1 and
Cu1A [symmetry code: 1/3 ꢀ x, 2/3 ꢀ y, 2/3 ꢀ z], bridged
by the two l2-phenolato oxygen atom O3 and O3A of
Schiff-base ligands (L1) (Scheme 1). Each copper atom is
in a square pyramidal geometry and is five-coordinated
by one nitrogen atom and four oxygen atoms from three
independent N-salicylideneglycinate ligands, respectively.
3.3.2. [CuL2(Py)(H2O)](ClO4) Æ H2O (2)
Complex 2 consists of the mononuclear units [CuL2-
(Py)(H2O)], the perchlorate counter ions and the lattice
water molecules as shown in Fig. 2. In the mononuclear
unit of complex2, copper metal ion is in a square-pyrami-
dal coordination with the apical site occupied by water
˚
The copper metal ion locates 0.025 A above the basal
N
O
O
plane, which is constituted via N1, O1, O3, O3A atoms,
toward the apical site of the square pyramidal geometry
which is taken up by the bridging carbonyl O atom of an
O
Cu
Cu
CH OH
0.67n
O
O
3
O
˚
adjacent dimer [Cu1–O2 2.258 (2) A]. The bond lengths
N
in the basal plane are Cu1–N1 1.927(2); Cu1–O1
˚
1.937(2); Cu1–O3 1.947(2); Cu1–O3A 1.993(2) A. The
n
˚
Cu. . .Cu distance of 3.02 A falls in the relatively small
range of 2.93–3.10 A, showing the weak metal–metal inter-
˚
Scheme 1. The structure of complex 1.

2886
Y.-G. Li et al. / Inorganica Chimica Acta 360 (2007) 2881–2889
˚
molecule [Cu1–O3w, 2.495(5) A]. The basal plane of the
square pyramid is occupied by one oxygen atom from phe-
3.3.4. [Ni(L4)2] (5) and [Mn2(L4)2(N3)2] Æ 2CH3OH (6)
The structure of the Schiff-base compound (HL4),
derived from the condensation of salicylaldehyde with
4-(2-aminoethyl)benzsulfamide, was reported previously
[24]. Herein we report two metal complexes 5 and 6 which
were prepared from reactions of Ni2+ and Mn2+ with the
ligand HL4, respectively. The molecular structures of 5
and 6 were ascertained by X-ray crystallography. Perspec-
tive drawings of 5 and 6 are shown in Figs. 5 and 6, respec-
tively, and selected bond lengths and bond angles are given
in Table 2. The former is a mononuclear complex of for-
mula Ni(L4)2, while the latter is a dinuclear complex of
formula Mn2(L4)2(N3)2] Æ 2CH3OH. In the crystal structure
of complex 5, there are two independent half-molecules per
asymmetric unit. Each nickel metal ion lies on inversion
centers and is four-coordinate by two deprotonated
Schiff-base ligands (L4) through the phenolate O atoms
˚
nolate group [Cu1–O1, 1.974(4) A] and three nitrogen
atoms from the imine group [Cu1–N1, 1.978(4) A], the
morpholine group [Cu1–N2, 2.139(5) A], and pyridine
group [Cu1–N3, 2.055(5) A], respectively. The copper
metal ion is 0.0159 A above the basal plane towards the
apical oxygen atom O3w of water molecule.
˚
˚
˚
˚
3.3.3. [Cu(L3)2] Æ 2H2O (3) and [Ni(L3)2] Æ 2H2O (4)
Recently, we have reported a Schiff base (HL3) derived
from the condensation of salicylaldehyde with 4-(2-amino-
ethyl)phenylic acid, which has been structurally character-
ized [23]. Herein we report two metal complex 3 and 4
which were prepared from reactions of Cu2+ and Ni2+ with
the ligand HL3, respectively. The molecular structures of 3
and 4 were ascertained by X-ray crystallography and were
found to be structurally very similar. Perspective drawings
of 3 and 4 are shown in Figs. 3 and 4, respectively, and
selected bond lengths and bond angles are given in Table 2.
Both of them are the mononuclear complex of formula
Cu(L3)2 Æ 2H2O and Ni(L3)2 Æ 2H2O, respectively. Complex
3 affords a square planar trans-[CuN2O2] coordination
geometry, whose central Cu2+ lies on the center of symme-
try. The ligand L3 acts as a bidentate ligand and coordi-
nates to Cu2+ through the phenolate O atom [Cu1–O1,
˚
˚
[Ni1–O1, 1.842(2) A; Ni2–O4, 1.832(2) A] and the
˚
˚
1.903(2) A] and the N-donor [Cu1–N1, 1.995(2) A] from
the imine group. In the crystal structure of complex 4,
Ni2+ lies in the position as Cu2+ of complex 3 does and
is four-coordinate via the phenolate O atom and N-donor
of the Schiff-base moiety. The bond distances of Ni1–O1
˚
and Ni1–N1 are 1.834(2) and 1.939(3) A, respectively,
which are comparable with the corresponding values
reported for nickel(II) species having the same coordinat-
ing atoms [38].
Fig. 3. ORTEP drawing of the molecular structure of [Cu(L3)2] Æ 2H2O
(3). Thermal ellipsoids are drawn at the 30% probability level and
hydrogen atoms are omitted for clarity.
Fig. 2. ORTEP drawing of the molecular structure of [CuL2(Py)(H2O)]
(ClO4) Æ H2O (2). Thermal ellipsoids are drawn at the 30% probability level
and hydrogen atoms are omitted for clarity.
Fig. 4. ORTEP drawing of the molecular structure of [Ni(L3)2] Æ 2H2O (4).
Thermal ellipsoids are drawn at the 30% probability level and hydrogen
atoms are omitted for clarity.

Y.-G. Li et al. / Inorganica Chimica Acta 360 (2007) 2881–2889
2887
Fig. 5. ORTEP drawing of the molecular structure of [Ni(L4)2] (5). Thermal ellipsoids are drawn at the 30% probability level and C atoms not labeled and
hydrogen atoms omitted for clarity.
100 lg) tested against jack bean urease (25 lL, 10 kU/L)
using urea (500 mM) in HEPES buffer (0.2 mL, 100 mM;
pH = 6.8). On reaction with Jack bean urease in the pres-
ence of phenol red, complexes 1–6 show inhibitory enzyme
activity (Fig. 7). Under the same condition the Schiff-base
ligands L1–4 as enzyme inhibitors have no ability to inhibit
urease (see Table 3). This indicates the Schiff-base ligand
have less influence on the activity of jack bean urease.
Metal ions as enzyme inhibitors exhibit different ability
to inhibit urease. Inhibitory efficiency of metal ions toward
urease follows the order: Cu2+ > Ni2+ > Mn2+, which has
been reported in the literature [39]. To my surprise, the
abilities of these complexes to inhibit urease follow the
order: 2 > 3 > 1 > 4 > 6 > 5.
For these transition metal complexes used as urease
Fig. 6. ORTEP drawing of the molecular structure of [Mn2(L4)2(N3)2] Æ
2CH3OH (6). Thermal ellipsoids are drawn at the 30% probability level
and C atoms not labeled and hydrogen atoms omitted for clarity.
inhibitors, copper(II) complexes exhibit stronger ability
to inhibit urease due to the strong Lewis acid properties
of copper metal ions. Among these copper complexes
complex 2 exhibited strongest ability to inhibit urease
probably because of copper metal ion being in the
square–pyramidal coordination and simple molecular
structure. In contrast, complex 1 exhibited weaker ability
˚
˚
N-donors [Ni1–N1, 1.927(3) A; Ni2–N3, 1.922(3) A] from
the imine groups to form a square-planar geometry sur-
rounding Ni2+. Complex 6 consists of a centrosymmetric
dimer with manganese centers assuming a distorted octahe-
dral coordination geometry and two methanol anions. The
dinuclear unit is formed by two Mn atoms labeled Mn1
and Mn1A [symmetry code: 2 ꢀ x, 1 ꢀ y, 1 ꢀ z], bridged
by the phenolic oxygen atoms O2 and O2A of the deproto-
nated Schiff-base ligands (L4), to construct an interesting
square plane consisting of Mn1, O2, Mn1A, O2A. The
˚
Mn. . .Mn distance is 3.517(2) A. At each manganese cen-
ter, the equatorial plane of octahedral geometry is occupied
˚
by two phenolate O atoms [Mn1–O1, 1.855(2) A; Mn1–O2,
˚
1.897(2) A] and two imine nitrogen atoms [Mn1–N1,
˚
˚
2.038(2) A; Mn1–N2, 2.051(2) A] from two deprotonated
ligands (L4). The apical sites of the manganese centers have
anti configuration and are occupied by the bridging oxygen
˚
atom [Mn1–O2A, 2.519(2) A] from phenolate group of
another deprotonated ligand (L4) and the nitrogen atom
˚
[Mn1–N5, 2.163(2) A] from azide group.
3.4. Inhibitory bioactivities against jack bean urease and XO
3.4.1. Inhibitory activity against jack bean urease
The ability of the complexes 1–6 in inhibiting urease has
been studied by the IC50 values of the complexes (25 lL,
Fig. 7. The inhibitory activities of complexes 1–6 against urease.

2888
Y.-G. Li et al. / Inorganica Chimica Acta 360 (2007) 2881–2889
Table 3
The average IC50 values (in lM) of the tested compounds against urease
and XO
Tested materials
Inhibition of urease
activity
Inhibition of
XO activity
H2L1 (C9H9NO3)
HL2 (C13H18N2O2)
HL3 (C15H15NO2)
>100
>100
>100
>100
>100
>100
>100
1.24
HL4 (C15H16N2O3S) >100
Cu2+
Ni2+
Mn2+
0.37
2.87
>100
>100
>100
Positive controls
42.12 (acetohydroxamic acid) 10.37 (allopurinol)
to inhibit urease probably resulting from its molecular
polymerization. The ability of complex 3 to inhibit the
urease exhibits stronger than that of complex 4 although
their crystal structures are very similar. This also shows
that inhibitory efficiency of the complex toward urease:
copper complex > nickel complex. However, complex 4
exhibits stronger ability to inhibit urease than complex 5
due to two different Schiff-base ligands (L3 and L4),
respectively. The former containing soluble phenolic
hydroxy is in favor of inhibitory urease. Complex 5 has
weaker inhibitory activity against urease than complex 6
in despite of containing the same Schiff-base ligand (L4).
This shows that inhibitory efficiency of the complex
toward urease may be influenced not only by the transi-
tion metal ion but also by ligands. To my surprise, com-
plex 6 exhibit strong ability to inhibit the urease although
manganese metal ion has no inhibitory activity of urease
probably due to the reaction of Schiff-base ligand and
Mn3+. Besides azide ligand probably strengthens inhibi-
tory activity of 6.
Fig. 8. The inhibitory activities of complexes 1–6 against XO (the IC50
value that is more than 100 lM were not shown).
substrate of XO. As the inhibitor of XO, complex 1
shows weaker inhibitory activity probably owing to the
molecular polymerization. Both nickel and manganese
complexes 4–6 exhibit no ability to inhibit XO probably
due to their metal character.
4. Conclusion
The present report describes the syntheses, X-ray crys-
tal structures and inhibitory enzyme activity of transition
metal complexes (M = Cu, Ni, Mn) having salicylaldi-
mine Schiff-base ligands. The syntheses of these com-
pounds are intriguing and their properties are strongly
dependent on the kind of metal ions and organic ligands.
Copper complexes 1–3 exhibit strong ability to inhibit
activity of urease and xanthine oxidase. However, nickel
and manganese complexes only exhibit ability to inhibit
activity of urease, which may be potential selective
enzyme inhibitors. Most of these complexes exhibit ideal
ability to inhibit urease and XO. It’s very interesting to
study that as enzyme inhibitors Schiff-base complexes
exhibit potential abilities to inhibit activities of urease
and XO in order to discover novel urease and XO inhib-
itors. We have demonstrated for the first time that Schiff-
base complexes show urease and XO inhibitory activities.
The mechanisms of the inhibitory activity require to be
further investigated.
3.4.2. Inhibitory activity against XO
The ability of the complexes 1–6 in inhibiting xanthine
oxidase has been studied by the IC50 values of the com-
plexes (50 lL, 200 lg) tested against XO (100 lL, 25 mU/
mL) using xanthine (200 lL, 84.8 lg/mL) in HEPES buf-
fer (0.7 mL, 50 mM; pH = 7.8). On reaction with XO in
the presence of uric acid, complexes 1–6 show inhibitory
XO activity (Fig. 8). Under the same condition the Schiff-
base ligands L1–4 as XO inhibitors have no ability to
inhibit XO (see Table 3). This indicates the Schiff-base
ligands as XO inhibitors have no influence on the activity
of XO. Only copper metal ion exhibits ability to inhibit
XO among metal ions. To my surprise, the abilities of
complexes obtained from reactions of Schiff-base ligands
and these metal ions to inhibit XO follow the order:
2 > 3 > 1 > 4 ꢁ 5 ꢁ 6. The ability of three copper com-
plexes to inhibit XO is comparable to that of allopurinol,
a well-known inhibitor of XO. The ability of complex 2
to inhibit XO exhibits the strongest among three copper
complexes probably due to being in the square-pyramidal
coordination and simple molecular structure, which is
better to let inhibitor interact with active site of XO or
Acknowledgement
This work was co-financed by grant (Project 30672516)
from National Natural Science Foundation of China and
by grant (Key Project 03085) from the State Education
Ministry of China.

Y.-G. Li et al. / Inorganica Chimica Acta 360 (2007) 2881–2889
[25] T. Tanaka, M. Kawase, S. Tani, Life Sci. 73 (2003) 2985.
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