Imidazole Schiff base ligands: Synthesis, coordination complexes and biological activities

Article abstract of DOI:10.1016/j.poly.2013.03.023

1-(3-Aminopropyl)imidazole (Apim) reacts with salicylaldehyde and a selection of imidazole aldehydes and the resulting Schiff base ligands readily coordinate to Zn(II), Cu(II) and Ag(I) centres. X-ray crystal structures were obtained for two of the free l

Full text of DOI:10.1016/j.poly.2013.03.023

Polyhedron 55 (2013) 169–178
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Imidazole Schiff base ligands: Synthesis, coordination complexes and
biological activities
a
a
a
b
c
John McGinley a, , Malachy McCann , Kaijie Ni , Theresa Tallon , Kevin Kavanagh , Michael Devereux ,
⇑
Xiaomei Ma d, Vickie McKee d
a Department of Chemistry, National University of Ireland Maynooth, Maynooth, Co. Kildare, Ireland
b Department of Biology, National University of Ireland Maynooth, Maynooth, Co. Kildare, Ireland
c The Inorganic Pharmaceutical and Biomimetic Research Centre, Focas Research Institute, Dublin Institute of Technology, Camden Row, Dublin 8, Ireland
d Department of Chemistry, Loughborough University, Loughborough, Leicestershire LE11 3TU, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history:
1-(3-Aminopropyl)imidazole (Apim) reacts with salicylaldehyde and a selection of imidazole aldehydes
and the resulting Schiff base ligands readily coordinate to Zn(II), Cu(II) and Ag(I) centres. X-ray crystal
structures were obtained for two of the free ligands and also the Ag(I) complex of the Apim-salicylalde-
hyde ligand. Encouragingly, all of free ligands and most of their metal complexes are relatively non-toxic,
in vivo, towards Galleria mellonella. Although the free ligands and the Cu(II) and Zn(II) complexes are inac-
tive, in vitro, against a selection of microbial pathogens, most of the Ag(I) complexes exhibit moderate
anti-bacterial activity and good anti-fungal activity.
Received 21 January 2013
Accepted 13 March 2013
Available online 21 March 2013
Keywords:
Imidazoles
Schiff bases
Metal complexes
X-ray
Ó 2013 Elsevier Ltd. All rights reserved.
Antimicrobial
Galleria mellonella
1. Introduction
2. Experimental
In recent years, metal complexes comprising Schiff base
ligands have been shown to exhibit significant anticancer,
antiviral and antimicrobial activity [1–4]. Commercially available
1-(3-aminopropyl)imidazole (Apim) (Fig. 1) has recently been
complexed to the Ag(I) ion and the structurally characterised
complexes, [Ag(Apim)]ClO4 [5] and [Ag(Apim)](9-aca)ÁH2O
(9-acaH = 9-anthracenecarboxylic acid) [6], exhibit high antimi-
crobial activities, in vitro. In addition, metal complexes comprising
Schiff base ligands derived from Apim have also attracted atten-
tion. For example, polymeric Ag(I) complexes of 3 (Fig. 1) show
interesting structural and luminescent properties [7], whilst
Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) complexes of 2 have been
screened for their ability to inhibit the growth of bacterial and
fungal pathogens [8].
2.1. Reagents and instrumentation
1H and 13C NMR (d ppm; J Hz) spectra were recorded on a
BrukerAvance 300 MHz NMR spectrometer using saturated
d6-DMSO solutions with Me4Si reference, unless indicated otherwise,
with resolutions of 0.18 Hz and 0.01 ppm, respectively. Infrared
spectra (cmÀ1) were recorded as KBr discs using a Perkin Elmer
System2000 FT-IR spectrometer. UV–vis spectra were run on a
Unicam UV 540 spectrometer. Melting point analyses were car-
ried out using a Stewart Scientific SMP 1 melting point apparatus
and are uncorrected. Electrospray (ESI) mass spectra were col-
lected on an Agilent Technologies 6410 Time of Flight LC/MS.
Compounds were dissolved in acetonitrile–water (1:1) solutions
containing 0.1% formic acid, unless stated otherwise. The inter-
pretation of mass spectra was made with the help of the pro-
gram, ‘‘Agilent Masshunter Workstation Software’’. Magnetic
susceptibility measurements were carried out at room tempera-
ture using a Johnson Matthey Magnetic Susceptibility Balance
with [HgCo(SCN)4] as reference. Microanalytical data were car-
ried out using an Electron Corporation Thermo FlashEA 1112 Ser-
ies analyser. Starting materials were commercially obtained and
used without further purification.
In the present paper we detail the synthesis, coordination
chemistry, in vivo toxicity and antimicrobial activities of the unco-
ordinated Apim-derived Schiff base ligands 2–6 (Fig. 1) and their
Ag(I), Cu(II) and Zn(II) complexes.
⇑
Corresponding author. Tel.: +353 1 708 4615; fax: +353 1 708 3815.
E-mail address: john.mcginley@nuim.ie (J. McGinley).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.03.023

170
J. McGinley et al. / Polyhedron 55 (2013) 169–178
N
N
N
N
N
N
NH2
3
6
HN
2
N
1
N
N
HO
N
H
N
N
N
N
N
N
N
N
N
N
HN
N
N
5
4
N
Fig. 1. Structures of Apim (1) and its Schiff base derivatives, 2–6.
Caution! Although not encountered in our experiments, per-
chlorate salts of metal ions are potentially explosive and should
be manipulated with care and used only in small quantities.
2.2.3. N-[(E)-1H-Imidazol-2-ylmethylidene]-N-[3-(1H-imidazol-1-
yl)propyl]amine (4)
To a solution of Apim (2.81 g, 22.5 mmol) in dry methanol
(17 mL) was added imidazole-2-carboxaldehyde (2.17 g,
22.5 mmol). The resulting solution was heated to reflux for 3 h
and then stirred overnight at room temperature. The solvent was
removed under reduced pressure to give a brown oil, which, on
standing for 2 months, yielded the orange-brown solid (4)
(3.69 g, yield 86%). M.p. 44–46 °C. C10H13N5Á0.5H2O (212.15): Calc.
C, 56.56; H, 6.59; N, 33.01. Found: C, 56.08; H, 6.32; N, 32.87%. 1H
NMR (d6-DMSO): d = 8.17 (s, 1H), 7.65 (s, 1H), 7.19 (s, 1H), 7.13 (s,
1H), 7.11 (s, 1H), 6.91 (s, 1H), 4.06 (t, 2H, J = 6.9 Hz), 3.43 (t, 2H,
J = 6.0 Hz), 2.01 (t, 2H, J = 6.9 Hz) ppm. 13C NMR (d6-DMSO):
d = 152.3, 144.4, 137.2, 128.2, 119.4, 56.7, 43.9, 31.7 ppm. IR
2.2. Synthesis of ligands
2.2.1. 2-([3-(1H-imidazol-1-
yl)propyl]aminopropyliminomethyl)phenol (2)
This compound was prepared using a method similar to those
previously reported [8,9]. To a solution of Apim (5.43 g, 43.4 mmol)
in dry methanol (30 mL) was added salicylaldehyde (5.30 g,
43.4 mmol) with constant stirring. The resulting yellow solution
was refluxed for 3 h and then stirred overnight at room tempera-
ture. The solvent was then removed under reduced pressure to give
a yellow oil, which, on standing for 4 h, yielded yellow crystals of 2.
The crystals were recrystallised from toluene, filtered, washed with
cold, dry methanol and air-dried (9.5 g, yield 95%). M.p. 83–84 °C
(lit. 78–80 °C). C13H15N3O (229.28): Calc. C, 68.10; H, 6.59; N,
18.33. Found: C, 68.23; H, 6.40; N, 18.24%. 1H NMR (d6-DMSO):
d = 13.36 (br s, 1H), 8.54 (s, 1H), 7.64 (s, 1H), 7.43 (d, 1H,
J = 6.3 Hz), 7.34 (t, 1H, J = 6.6 Hz), 7.21 (s, 1H), 6.91 (m, 3H), 4.05
(t, 2H, J = 6.9 Hz), 3.54 (t, 2H, J = 6.9 Hz), 2.10 (p, 2H, J = 6.9,
7.2 Hz) ppm. 13C NMR (d6-DMSO): d = 166.3, 160.5, 137.2, 132.3,
131.6, 128.5, 119.2, 118.6, 118.5, 116.4, 55.4, 43.8, 31.6 ppm. IR
(KBr):
m = 3114, 1650, 1513, 1446, 1397, 1232, 1109, 1083,
771 cmÀ1. LC/TCOF-MS: Calc. for C10H14N5 [M+1]+ 204.2. Found:
204.1%.
2.2.4. 3-(1H-Imidazol-1-yl)-N-[(E)-(5-methyl-1H-imidazol-4-
yl)methylidene]-1-propanamine (5)
To a solution of Apim (2.81 g, 22.5 mmol) in dry methanol
(17 mL) was added 4-methyl-5-imidazolecarboxaldehyde (2.17 g,
22.5 mmol). The resulting light-yellow suspension was heated to
reflux for 3 h. The suspension was dissolved completely after
5 min. The solution was then stirred overnight at room tempera-
ture. After 0.5 h the solution turned orange in colour. The solvent
was removed under reduced pressure to give an orange oil, which,
on standing for 3 weeks, yielded the orange solid (5) (3.10 g, yield
76%). M.p. 113–115 °C. C11H15N5 (217.27): Calc. C, 60.81; H, 6.96;
N, 32.23. Found: C, 60.60; H, 6.72; N, 31.35%. 1H NMR (d6-DMSO):
d = 8.23 (s, 1H), 7.64 (s, 1H), 7.61 (s, 1H), 7.18 (s, 1H), 6.91 (s, 1H),
4.04 (t, 2H, J = 7.2 Hz), 3.42 (t, 2H, J = 6.6 Hz), 2.32 (s, 3H), 2.03 (p,
2H, J = 6.9, 6.6 Hz) ppm. 13C NMR (d6-DMSO): d = 137.2, 128.3,
(KBr):
1079, 887, 808, 760 cmÀ1
[M+1]+ 230.3. Found: 230.1%.
m
= 3434, 3101, 1632, 1576, 1491, 1401, 1276, 1225, 1151,
.
LC/TCOF-MS: Calc. for C13H16N3O
2.2.2. N-[(E)-1H-Imidazol-5-ylmethylidene]-N-[3-(1H-imidazol-1-
yl)propyl]amine (3)
This compound was prepared using a method similar to those
previously reported [7]. To a solution of Apim (2.83 g, 22.6 mmol)
in dry methanol (17 mL) was added 4(5)-imidazolecarboxaldehyde
(2.17 g, 22.6 mmol). The resulting light-yellow suspension was
heated to reflux for 3 h and the yellow solution was then stirred
overnight at room temperature. The solvent was removed under
reduced pressure to give an orange-yellow oil, which, on standing
for 2 h, yielded the orange-yellow solid (3). The solid was recrystal-
lised from hot ethanol, filtered, washed with cold, dry ethanol and
air-dried (4.3 g, yield 94%). M.p. 178–180 °C (lit. 180–182 °C).
119.3, 57.3, 48.5, 43.9, 32.1 ppm. IR (KBr):
m = 3113, 1643, 1529,
1452, 1395, 1354, 1233, 1110, 1082, 826, 747 cmÀ1. LC/TCOF-MS:
Calc. for C10H14N5 [M+1]+ 218.3. Found 218.1%.
2.2.5. 3-(1H-Imidazol-1-yl)-N-[(E)-(1-methyl-1H-imidazol-2-
yl)methylidene]-1-propanamine (6)
To a solution of Apim (2.81 g, 22.5 mmol) in dry methanol
(17 mL) was added 1-methyl-2-imidazolecarboxaldehyde (2.47 g,
22.5 mmol). The resulting light-yellow solution was refluxed for
3 h and then stirred overnight at room temperature. After 0.5 h
the solution turned orange in colour. The solvent was removed un-
der reduced pressure to give the yellow oil (6) (3.11 g, yield 73%).
C
10H13N5(203.24): Calc. C, 59.09; H, 6.45; N, 34.46. Found: C,
59.14; H, 6.28; N, 34.46%. 1H NMR (d6-DMSO): d = 12.59 (br s,
1H), 8.19 (s, 1H), 7.74 (s, 1H), 7.63 (s, 1H), 7.19 (s, 1H), 6.90 (s,
1H), 4.03 (t, 2H, J = 6.9 Hz), 3.42 (t, 2H, J = 6.6 Hz), 2.01 (p, 2H,
J = 6.9, 6.6 Hz) ppm. 13C NMR (d6-DMSO): d = 137.2, 128.4, 119.2,
C
11H15N5 (217.27): Calc. C, 60.81; H, 6.96; N, 32.23. Found: C,
57.2, 43.9, 31.9 ppm. IR (KBr):
m
= 3436, 3137, 3103, 2844, 1647,
60.20; H, 6.35; N, 31.83%. 1H NMR (d6-DMSO): d = 8.25 (s, 1H),
7.64 (s, 1H), 7.30 (s, 1H), 7.20 (s, 1H), 7.04 (s, 1H), 6.90 (s, 1H),
4.07 (t, 2H, J = 7.5 Hz), 3.92 (s, 3H), 3.51 (t, 2H, J = 6.9 Hz), 2.05
(p, 2H, J = 6.9, 6.9 Hz) ppm. 13C NMR (d6-DMSO): d = 153.6, 142.3,
1509, 1429, 1355, 1307, 1236, 1220, 1111, 1081, 1026, 827,
749 cmÀ1. LC/TCOF-MS: Calc. for C10H14N5 [M+1]+ 204.2. Found:
204.1%.

J. McGinley et al. / Polyhedron 55 (2013) 169–178
171
137.2, 128.5, 128.3, 125.6, 119.3, 57.6, 43.9, 34.9, 31.8 ppm. IR
Calc. for C11H15AgN5[Ag(5)]+ 324.1. Found: 324.0%. Solubility:
(KBr):
m
= 3371, 3112, 1651, 1509, 1480, 1439, 1289, 1231, 1151,
DMSO.
1082, 774 cmÀ1. LC/TCOF-MS: Calc. for C10H14N5 [M+1]+ 218.3.
Found: 218.2%.
2.3.5. [Ag(6)]ClO4
White solid (1.84 g, yield 75%). C11H15AgClN5O4 (424.59): Calc.
C, 31.12; H, 3.56; N, 16.49. Found: C, 31.42; H, 3.45; N, 16.03%.
1H NMR (d6-DMSO): d = 8.67 (s, 1H), 7.95 (s, 1H), 7.64 (s, 1H),
7.43 (s, 1H), 7.32 (s, 1H), 7.11 (s, 1H), 4.12 (t, 2H, J = 7.5 Hz), 3.95
(s, 3H), 3.75 (t, 2H, J = 5.7 Hz), 2.08 (t, 2H, J = 7.2 Hz) ppm. 13C
NMR (d6-DMSO): d = 150.2, 142.8, 138.7, 129.3, 129.2, 126.1,
2.3. General synthesis of silver(I) complexes
All of the Ag(I) complexes were synthesised with the exclusion
of light and samples were stored in the dark. To a solution of the
appropriate ligand (6.07 mmol) in dry methanol (47 mL) was
added, dropwise, a solution of AgClO4 (2.89 mmol) in dry methanol
(5 mL) with constant stirring. The white suspension was then
stirred for 2 h at room temperature. The solid was removed by
filtration and washed with cold methanol to yield either white or
pale-coloured solids, which were recrystallised from either hot
acetonitrile or hot ethanol and air-dried.
120.5, 55.9, 44.8, 32.5, 31.9 ppm. IR (KBr):
m = 3448, 3123, 2940,
1635, 1516, 1492, 1449, 1426, 1327, 1292, 1248, 1074, 948, 818,
780, 701 cmÀ1. LC/TCOF-MS: Calc. for C11H15AgN5 [Ag(6)]+ 324.1.
Found: 324.0%. Solubility: DMSO.
2.4. General synthesis of copper(II) complexes
2.3.1. [Ag(2)2]ClO4
To
a
solution of the appropriate ligand (2.18 mmol) in
solution of
Pale yellow solid recrystallised from hot acetonitrile (1.48 g,
yield 77%). C26H30AgClN6O6 (665.88): Calc. C, 46.90; H, 4.54; N,
12.62. Found: C, 46.34; H, 4.37; N, 13.14%. 1H NMR (d6-DMSO):
d = 13.28 (br s, 2H), 8.55 (s, 2H), 8.07 (s, 2H), 7.50 (d, 2H,
J = 1.2 Hz), 7.46 (s, 2H), 7.34 (m, 2H), 7.12 (m, 2H), 6.91 (m, 2H),
4.18 (t, 4H, J = 6.9 Hz), 3.58 (t, 4H, J = 6.6 Hz), 2.16 (t, 4H,
J = 6.9 Hz) ppm. 13C NMR (d6-DMSO): d = 166.4, 160.4, 139.1,
132.3, 131.6, 129.1, 120.2, 118.6, 118.5, 116.4, 55.4, 44.8,
dry methanol (17 mL) was added, dropwise,
a
Cu(ClO4)2Á6H2O (0.55 mmol) in dry methanol (2 mL) with constant
stirring. The green suspension was then stirred for 3 h at room
temperature. The resulting solid was removed by filtration and
washed with cold ethanol and air-dried to yield either a green or
a blue solid.
2.4.1. [Cu(2)4](ClO4)2
Dark green solid (0.22 g, yield 48%). C52H60Cl2CuN12O12
(1179.56): Calc. C, 52.95; H, 5.13; N, 14.25. Found: C, 53.19; H,
31.3 ppm. IR (KBr):
m = 3432, 3138, 1633, 1579, 1499, 1393, 1278,
1240, 1089, 837, 766 cmÀ1. LC/TCOF-MS: Calc. for C26H30AgN6O2
[Ag(2)2]+ 567.2, found 567.1; Calc. for C13H15AgN3O [Ag(2)]+
336.1. Found: 336.0%. Solubility: MeOH (hot), EtOH (hot), MeCN,
CHCl3, DMSO.
4.90; N, 13.69%. IR (KBr):
m = 3446, 3130, 1622, 1539, 1450, 1404,
1329, 1278, 1237, 1203, 1092, 833, 759 cmÀ1. LC/TCOF-MS: Calc.
for C26H29CuN6O2 [Cu(2)2]+ 521.1. Found: 521.2%.
leff: 2.53 B.M.
Solubility: MeCN (hot), DMSO.
2.3.2. [Ag(3)]ClO4
White solid (0.66 g, yield 28%). C10H13AgClN5O4 (410.56): Calc.
C, 29.25; H, 3.19; N, 17.06. Found: C, 29.29; H, 3.05; N, 17.23%.
1H NMR (d6-DMSO): d = 13.16 (br s, 1H), 8.52 (s, 1H), 8.24 (s,
1H), 7.96 (s, 1H), 7.88 (s, 1H), 7.44 (s, 1H), 7.11 (s, 1H), 4.12 (t,
2H, J = 7.8 Hz), 3.67 (t, 2H, J = 5.7 Hz), 2.06 (t, 2H, J = 8.1 Hz) ppm.
13C NMR (d6-DMSO): d = 155.2, 138.9, 138.6, 136.4, 129.2, 122.3,
2.4.2. [Cu(3)2](ClO4)2ÁH2O
Green solid (0.42 g, yield 50%). C20H28Cl2CuN10O9 (686.95): Calc.
C, 34.97; H, 4.10; N, 20.39. Found: C, 34.51; H, 3.93; N, 19.79%. IR
(KBr):
m
= 3132, 2945, 1643, 1521, 1456, 1346, 1297, 1239, 1092,
830, 752 cmÀ1
.
LC/TCOF-MS: Calc. for C20H26CuN10 [Cu(3)2]+
120.5, 55.5, 44.8, 32.4 ppm. IR (KBr):
m = 3252, 3130, 2952,
469.2. Found: 469.2%. leff: 1.93 B.M. Solubility: DMSO.
2854, 1648, 1519, 1447, 1298, 757 cmÀ1. LC/TCOF-MS: Calc. for
C
10H13AgN5 [Ag(3)]+ 310.1, found 310.0; Calc. for C20H26AgN10
2.4.3. [Cu(4)2](ClO4)2
Light-green solid (0.42 g, yield 51%). C20H26Cl2CuN10O8
(668.94): Calc. C, 35.91; H, 3.91; N, 20.94. Found: C, 36.13; H,
[Ag(3)2]+ 513.3. Found: 513.1%. Solubility: DMSO.
2.3.3. [Ag(4)]ClO4
3.88; N, 20.60%. IR (KBr):
1455, 1237, 1089, 1027, 951, 762 cmÀ1. LC/TCOF-MS: Calc. for
20H27Cl2CuN10O8 [Cu(4)2](ClO4)2+H]+ 669.8. Found: 669.1%. leff
m = 3384, 3123, 2939, 1621, 1524,
Pale-orange solid (0.56 g, yield 22%). C10H13AgClN5O4 (410.56):
Calc. C, 29.25; H, 3.19; N, 17.06. Found: C, 29.56; H, 3.54; N, 16.41%.
1H NMR (d6-DMSO): d = 8.39 (s, 1H), 7.94 (s, 1H), 7.50 (s, 1H), 7.47
(s, 1H), 7.42 (s, 1H), 7.17 (s, 1H), 4.12 (t, 2H, J = 7.2 Hz), 3.70 (t, 2H,
J = 6.6 Hz), 2.06 (t, 2H, J = 7.4 Hz) ppm. 13C NMR (d6-DMSO):
d = 181.1, 152.3, 145.5, 144.4, 137.3, 129.1, 128.2, 119.4, 56.7,
C
:
1.97 B.M. Solubility: DMSO.
2.4.4. [Cu(5)2](ClO4)2
Blue solid (0.48 g, yield 60%). C22H30Cl2CuN10O8 (696.99): Calc.
C, 37.91; H, 4.34; N, 20.10. Found: C, 38.09; H, 4.33; N, 19.67%. IR
43.9, 31.7 ppm. IR (KBr):
m = 3313, 3129, 2939, 1645, 1520, 1448,
1240, 1108, 834, 766 cmÀ1. LC/TCOF-MS: Calc. for C10H13AgN5
[Ag(4)]+ 310.2, found 310.0; Calc. for C20H26AgN10 [Ag(4)2]+
513.3. Found: 513.1%. Solubility: DMSO.
(KBr):
m = 3267, 3139, 2946, 1635, 1579, 1518, 1451, 1352, 1314,
1280, 1239, 1099, 956, 827, 735 cmÀ1. LC/TCOF-MS: Calc. for
C
11H15CuN5 [Cu(5)]+ 280.8, found 280.1; Calc. for C22H30CuN10
[Cu(5)2]+ 497.2. Found: 497.2%.
leff: 1.90 B.M. Solubility: DMSO.
2.3.4. [Ag(5)]ClO4
White solid (1.26 g, yield 52%). C11H15AgClN5O4 (424.59): Calc.
C, 31.12; H, 3.56; N, 16.49. Found: C, 30.50; H, 3.30; N, 16.52%.
1H NMR (d6-DMSO): d = 12.99 (br s, 1H), 8.58 (s, 1H), 8.09 (s, 1H),
7.96 (s, 1H), 7.43 (s, 1H), 7.11 (s, 1H), 4.11 (t, 2H, J = 8.4 Hz), 3.67
(t, 2H, J = 6.6 Hz), 2.39 (s, 3H), 2.07 (p, 2H, J = 6.9, 7.4 Hz) ppm.
13C NMR (d6-DMSO): d = 154.6, 138.6, 137.8, 132.2, 129.2, 120.5,
2.4.5. [Cu(6)2](ClO4)2ÁH2O
Light-blue solid (0.52 g, yield 65%). C22H32Cl2CuN10O9 (715.0):
Calc. C, 36.95; H, 4.51; N, 19.58. Found: C, 36.35; H, 4.31; N,
18.85%. IR (KBr):
m = 3436, 3144, 2950, 1633, 1523, 1485, 1452,
1418, 1359, 1289, 1239, 1095, 950, 827, 769 cmÀ1. LC/TCOF-MS:
55.5, 44.8, 32.5, 8.6 ppm. IR (KBr):
m
= 3259, 2928, 1643, 1519,
Calc. for C22H30CuN10 [Cu(6)2]+ 497.2. Found: 497.2%.
leff: 1.91
1453, 1343, 1240, 1107, 996, 840, 804, 738 cmÀ1. LC/TCOF-MS:
B.M. Solubility: DMSO.

172
J. McGinley et al. / Polyhedron 55 (2013) 169–178
Table 1
2.5. General synthesis of zinc(II) complexes
Crystallographic data for 2, 3 and [Ag(2)2]ClO4.
To
a
solution of the appropriate ligand (2.18 mmol) in
solution of
2
3
[Ag(2)2]ClO4
dry methanol (17 mL) was added, dropwise,
a
Formula
C13H15N3O
229.28
monoclinic
P21/c
150(2)
12.3194(11)
5.8438(5)
16.3752(15)
90
97.963(2)
90
1167.52(18)
4
C10H13N5
203.25
monoclinic
P21/n
150(2)
8.6001(10)
10.1425(12)
12.3976(14)
90
103.131(2)
90
1053.1(2)
4
C26H30AgClN6O6
665.88
M (g molÀ1
)
Zn(ClO4)2Á6H2O (0.55 mmol) in dry methanol (5 mL) with constant
stirring. The suspension was then stirred for 2 h at room tempera-
ture. The resulting solid was removed by filtration and washed
with cold ethanol and air-dried to yield either stiff oils or solids.
Crystal system
Space group
T (K)
a (Å)
b (Å)
triclinic
ꢀ
P1
150(2)
5.915(7)
14.369(17)
16.897(19)
79.644(16)
83.636(16)
88.369(17)
1404(3)
2
1.575
5143
5143
0.0000
2701
2.5.1. [Zn(2)(2À)]ClO4
c (Å)
a
(°)
Stiff, yellow oil (0.22 g, yield 65%). C26H29ClN6O6Zn (622.38):
Calc. C, 50.17; H, 4.70; N, 13.50. Found: C, 50.27; H, 4.77; N,
12.78%. 1H NMR (d6-DMSO): d = 13.29 (br s, 1H), 8.54 (s, 2H),
8.02 (s, 2H), 7.43 (d, 2H, J = 6.6 Hz), 7.34 (t, 2H, J = 7.8 Hz), 7.06
(s, 2H), 6.89 (m, 4H), 4.14 (t, 4H, J = 7.2 Hz), 3.57 (t, 4H,
b (°)
c
(°)
V (Å3)
Z
Dcalc (g cmÀ3
)
1.303
9584
2753
0.0260
2004
1.282
7985
2077
0.0267
1593
Reflections collected
Unique reflections
Rint
Obs. reflections [I > 2
No. of parameters
No. of restraints
J = 6.6 Hz), 2.14 (t, 4H, J = 6.9 Hz) ppm. IR (KBr):
m = 3430, 3102,
2935, 1633, 1578, 1492, 1459, 1401, 1277, 1225, 1151, 1080,
809, 760 cmÀ1. LC/TCOF-MS: Calc. for C26H29ZnN6O2 [Zn(2)(2À)]+
521.3. Found: 521.2%. Solubility: MeCN, DMSO.
r(I)]
157
0
136
0
365
353
R1 [I > 2
wR2 (all data)
Goodness-of-fit (GOF)
r
(I)]
0.0389
0.1128
1.006
0.0359
0.0885
1.091
0.0723
0.1804
0.972
2.5.2. [Zn(3)2](ClO4)2
White solid (0.54 g, yield 66%). C20H26Cl2N10O8Zn (670.77):
Calc. C, 35.81; H, 3.91; N, 20.88. Found: C, 36.20; H, 3.86; N,
20.16%. 1H NMR (d6-DMSO): d = 8.47 (s, 2H), 7.91(s, 2H), 7.63 (s,
2H), 7.14 (s, 2H), 6.86 (s, 2H), 3.88 (t, 4H, J = 7.5 Hz), 3.30 (t, 2H,
were refined with anisotropic atomic displacement parameters
and the hydrogen atoms bonded to carbon were inserted at calcu-
lated positions using a riding model. The hydrogen atom bonded to
O1 in compound 2 was located from difference maps and refined
with a fixed isotropic thermal parameter. The structure of [Ag(2)2]-
ClO4 was solved by direct methods and found to be twinned. The
twin components were identified (CELL_NOW and TWINABS) and
the structure refined on F2 using all the reflections (HKLF 5 data)
[10]. All non-hydrogen atoms were refined with anisotropic atomic
displacement parameters and the hydrogen atoms were inserted at
calculated positions using a riding model. Data collection and
refinement parameters for all three structures are summarized in
Table 1.
J = 6.6 Hz), 1.70 (t, 2H, J = 6.8 Hz) ppm. IR (KBr):
m = 3332, 3144,
2954, 2867, 1645, 1527, 1452, 1299, 1236, 1111, 847, 759 cmÀ1
.
LC/TCOF-MS: Calc. for C10H14N5 [ligand+1]+ 204.2. Found: 204.1%.
Solubility: DMSO.
2.5.3. [Zn(4)2](ClO4)2ÁH2O
Light-orange solid (0.31 g, yield 60%).
C20H28Cl2N10O9Zn
(688.81): Calc. C, 34.87; H, 4.10; N, 20.33. Found: C, 35.16; H,
3.71; N, 19.63%. 1H NMR (d6-DMSO): d = 8.23 (s, 2H), 7.89 (s, 2H),
7.33 (s, 2H), 7.29 (s, 2H), 7.00 (s, 2H), 4.01 (t, 4H, J = 7.2 Hz), 3.40
(t, 2H, J = 6.8 Hz), 1.83 (t, 2H, J = 7.0 Hz) ppm. IR (KBr):
2940, 1644, 1527, 1455, 1241, 1101, 954, 767 cmÀ1. LC/TCOF-MS:
Calc. for
20H27Cl2ZnN10O8 [Zn(4)2](ClO4)2+1]+ 671.1. Found:
m = 3134,
C
671.1%. Solubility: DMSO.
2.7. Biology
2.5.4. [Zn(5)2](ClO4)2ÁMeOH
Sterilisation of microbiological equipment and media was car-
ried out at 394 K and 100 kPa for 15 min. All worktops and benches
were sterilised by washing with 70% (v/v) ethanol–water prior to
use. Flat-bottomed microtitre plates were read using a Bio-Tek
Synergy HT plate reader at 540 nm. Fungal cell density was mea-
sured using a Neubauer hemocytometer under a light microscope
at a magnification of 400Â. Bacterial cell density was recorded at
an optical density of 600 nm using an Eppendorf Biophotometer.
Galleria mellonella (larvae of the greater wax moth), in the sixth
developmental stage, were obtained from Livefoods Direct Ltd.,
Sheffield, S25 4JJ, UK. The larvae were stored at 288 K in wood
shavings and used within 3 weeks of delivery. Significance of the
larvae survival rates was analysed at 72 h using the log rank (Man-
tel–Cox) method utilising GraphPad Prismsoftware (version 5).
Three categories of significance were used (Ã = p < 0.05,
ÃÃ = p < 0.01 and ÃÃÃ = p < 0.001). Pseudomonas aeruginosa ATCC
27853, Methicillin-resistant Staphylococcus aureus (MRSA) and Can-
dida albicans ATCC 10231 were obtained from the American Type
Culture Collection (Manassas, VA, USA). S. aureus and Escherichia
coli were supplied as clinical isolates by the Clinical Microbiology
Laboratory, St. James’s Hospital, Dublin, Ireland.
White solid (0.53 g, yield 66%).
C
22H30Cl2N10O8ZnÁMeOH
(730.87): Calc. C, 37.80; H, 4.69; N, 19.16. Found: C, 37.86; H,
4.26; N, 19.19%. 1H NMR (d6-DMSO): d = 13.18 (br s, 1H), 8.53 (s,
2H), 7.83 (s, 2H), 7.59 (s, 2H), 7.10 (s, 2H), 6.86 (s, 2H), 3.84 (t,
4H, J = 6.0 Hz), 3.29 (t, 4H, J = 5.7 Hz), 2.40 (s, 6H), 1.68 (t, 2H,
J = 7.0 Hz) ppm. IR (KBr):
m = 3246, 3135, 1644, 1597, 1528, 1456,
1362, 1301, 1241, 1103, 967, 839, 749 cmÀ1. LC/TCOF-MS: Calc.
for C11H16N5 [ligand+1]+ 218.3. Found: 218.1%. Solubility: DMSO.
2.5.5. [Zn(6)2](ClO4)2ÁH2O
White solid (0.53 g, yield 65%). C22H30Cl2N10O8ZnÁH2O (716.84):
Calc. C, 36.86; H, 4.50; N, 19.54. Found: C, 37.59; H, 4.25; N, 19.02%.
1H NMR (d6-DMSO): d = 8.54 (s, 2H), 7.75 (s, 2H), 7.60 (s, 2H), 7.26
(s, 2H), 7.05 (s, 2H), 6.91 (s, 2H), 4.10 (t, 4H, J = 7.2 Hz), 3.96 (s, 6H),
3.34 (t, 2H, J = 6.6 Hz), 1.78 (t, 2H, J = 7.2 Hz) ppm. IR (KBr):
m
= 3607, 3135, 2958, 1639, 1531, 1496, 1451, 1418, 1381, 1291,
1241, 1094, 944, 847 cmÀ1. LC/TCOF-MS: Calc. for C22H30ZnN10
[Zn(6)2]+ 498.3. Found: 497.2%. Solubility: DMSO.
2.6. X-ray crystallography
Data for 2, 3 and [Ag(2)2]ClO4 were collected at 150 K on a Bru-
ker APEX II CCD diffractometer. The structures of 2 and [Ag(2)2]-
ClO4 were solved by direct methods and refined by full-matrix
least-squares on F2 using all data [10]. All non-hydrogen atoms
2.7.1. In vivo toxicity assessment
Larvae of the insect G. mellonella were employed to assess the
in vivo cytotoxic tolerance of the test compounds and AgNO3. Such

J. McGinley et al. / Polyhedron 55 (2013) 169–178
173
larvae have been widely used as a convenient and inexpensive
in vivo screening model to assess the toxicity of potential new
drugs [11,12]. Ten healthy larvae, weighing between 0.2–0.4 g
and with no cuticle discolouration, were used for each compound.
Fresh suspensions or solutions of the free ligands and their Ag(I),
Zn(II) and Cu(II) complexes were prepared immediately prior to
testing under sterile conditions. Each sample (0.02 g) was sus-
pended or dissolved in DMSO (1 mL) and sterile water (9 mL)
(100
media with no fungal cells). Column 2 was the positive control
(media and fungal cells only). 100 L of the above complex solu-
tion (200 g/mL) was added to every well in column 3. Serial dilu-
tions (1:1) were made from columns 3–12 to produce a test
concentration range of 100–0.2 g/mL.
lL) was added to column 1 of the plate (negative control,
l
l
l
C. albicans was grown to the stationary phase overnight at 310 K
on YEPD media. The cells were washed with PBS solution and
re-suspended in MM at a density of 5 Â 105 cells/mL. The cell sus-
was added to yield a stock solution of concentration 2000
Each sample was tested across the concentration range 2000–
100 g/mL. Using serial dilutions with water, each sample was
tested across the concentration range 2000–100 g/mL and the fi-
nal concentration of DMSO was ca.1% v/v. Test solution (20 L) was
lg/mL.
pension (100 lL) was added to every well in columns 2–12. The
l
completed plate was then covered to prevent dehydration. The
plate was incubated at 310 K with continuous shaking for 24 h.
The optical density (k = 540 nm) of each well was then recorded.
MIC80 value ranges were then determined and expressed in terms
l
l
injected into the larvae through the last pro-leg and the larvae
were then placed in sterile petri dishes and incubated for 72 h at
30 °C. Death was assessed by the lack of movement in response
to stimulus, together with discolouration of the cuticle. Three dif-
ferent controls were used in this assay: the first was untouched lar-
vae; the second was larvae with the pro-leg pierced with a needle
but no solution injected; the third was larvae injected with sterile
of lM concentration.
3. Results and discussion
3.1. Ligand synthesis
distilled water (20 lL).
The Schiff base ligands 2–6 (Fig. 1) were formed in high yield by
the reaction of 1-(3-aminopropyl)imidazole (Apim) with salicylal-
dehyde and the various imidazole aldehydes in dry methanol. Li-
gand composition was determined by a combination of elemental
analyses, mass spectrometry and 1H and 13C NMR and IR spectros-
copy. The formation of the Schiff base products was confirmed by
1H NMR spectroscopy (imine proton singlet at ca. 8.2 ppm) and IR
spectroscopy (imine C@N band at ca.1650 cmÀ1).
2.7.2. In vitro antimicrobial assessment
In vitro antimicrobial screening: Minimal growth media (MM)
was prepared as previously described [13]. Yeast extract peptone
dextrose (YEPD) media was composed of 2% (w/v) glucose, 2%
(w/v) bacteriological peptone and 1% (w/v) yeast extract. To solid-
ify the media 2% (w/v) bacteriological agar was added when re-
quired. Nutrient broth and phosphate buffered saline (PBS) were
made up according to the manufacturer’s instructions (Scharlau
Microbiology and Aldrich, respectively).
In vitro bacterial susceptibility testing: Bacteria were grown on
nutrient broth agar plates at 310 K and maintained at 277 K for
short-term storage. All assays were run in triplicate and on three
independent occasions. Complexes with low solubility were tested
as fine suspensions. Fresh solutions/suspensions of the test sam-
ples were prepared immediately prior to testing. Samples
(0.020 g) were added to DMSO (1 mL) and then water (9 mL) was
3.2. X-ray structures of ligands 2 and 3
Although the synthesis of 2 has previously been documented
[8,9] its X-ray crystal structure was not reported. Crystals of 2
and 3 were grown from saturated methanol solutions of both com-
pounds. The asymmetric units for each compound are shown in
Figs. 2 and 4 and selected bond lengths and angles are given in
Table 2. In compound 2, there is an intramolecular hydrogen bond
between the phenolic hydrogen atom and the imine nitrogen atom
(N3).
added to give
2000 g/mL). The stock solution (1 mL) was added to water
(9 mL) to yield a solution/suspension with a concentration of
200 g/mL. Nutrient broth (100 L) was added to each well of a
96-well, flat-bottomed microtitre plate. Water (100 L) was added
a
stock solution/suspension (concentration
l
The packing interactions of 2 are illustrated in Fig. 3. There is a
l
l
C–HÁ Á Á
p bonding interaction between the centroid of each imidaz-
l
ole ring and a hydrogen of the C4 methylene group of the spacer
chain of a neighbouring molecule (centroid–H 2.59 Å). There is
an interplanar angle of 88.76(4)° between the imidazole and phe-
to column 1 of the plate (negative control with no bacterial cells).
Column 2 was the positive control (media with bacterial cells).
100
every well in column 3. Serial dilutions (1:1) were made from col-
umn 3–12 to produce a test concentration range of 100–0.2 g/mL.
lL of the above complex solution (200 lg/mL) was added to
nol rings but there are no striking p–p stacking interactions.
In compound 3, the imine bond is clearly located between N3
and C7. There is a hydrogen bond linking N4 to the imine nitrogen
of a neighbouring molecule (2.991(2) Å under ½ À x, ½ + y, ½ À z),
and a weaker bond from the same NH to N5 of the same neighbour
(3.079(2) Å). These interactions link the molecules into chains.
There is also a weak C–HÁ Á ÁN interaction between C10 and N1
(the imidazole lone pair) of a second neighbour (3.38(2) Å under
l
Bacteria were grown overnight to the stationary phase in nutrient
broth at 310 K and 200 rpm. The cells were diluted to give an opti-
cal density of 0.1 at k = 600 nm. The cell suspension (100 lL) was
added to every well in columns 2–12. The completed plates were
incubated at 310 K in a static incubator and the final optical den-
sity recorded. MIC50 values (minimum concentration required to
inhibit 50% of cell growth) were then determined and expressed
in terms of lM concentration.
In vitro fungal susceptibility testing: C. albicans was grown on
YEPD agar plates at 310 K and maintained at 277 K for short-term
storage. All assays were run in triplicate and on three independent
occasions. Fresh solutions/suspensions of test samples were pre-
pared immediately prior to testing. Samples (0.020 g) were dis-
solved/suspended in DMSO (1 mL) and added to water (9 mL) to
give a stock solution (concentration 2000
tion/suspension (1 mL) was added to water (9 mL) to give a solu-
tion with a concentration of 200 g/mL. MM (100 L) was added
to each well of a 96-well, round bottomed microtitre plate. Water
lg/mL). The stock solu-
l
l
Fig. 2. Asymmetric unit of 2 showing intramoleular hydrogen bonding.

174
J. McGinley et al. / Polyhedron 55 (2013) 169–178
Table 2
Selected bond lengths [Å] and angles [°] for 2, 3 and [Ag(2)2]ClO4.
2
3
[Ag(2)2]ClO4
C(1)–N(1)
C(1)–N(2)
N(1)–C(2)
C(3)–N(2)
N(2)–C(4)
C(6)–N(3)
N(3)–C(7)
C(9)–N(4)
N(4)–C(10)
C(10)–N(5)
C(13)–O(1)
Ag(1)–N(1)
1.3172(19)
1.3501(17)
1.372(2)
1.3683(16)
1.4609(17)
1.4613(17)
1.2756(16)
–
1.317(2)
1.350(2)
1.371(2)
1.366(2)
1.462(2)
1.4617(19)
1.272(2)
1.350(2)
1.350(2)
1.314(2)
–
1.318(15)
1.326(15)
1.368(14)
1.347(14)
1.473(14)
1.476(15)
1.244(14)
–
–
–
–
–
1.3531(16)
–
1.349(15)
2.109(10)
–
N(1)–C(1)–N(2)
C(1)–N(1)–C(2)
C(1)–N(2)–C(3)
C(1)–N(2)–C(4)
C(3)–N(2)–C(4)
C(7)–N(3)–C(6)
C(10)–N(4)–C(9)
N(5)–C(10)–N(4)
C(10)–N(5)–C(8)
112.66(13)
104.04(12)
106.65(12)
126.95(12)
126.22(12)
109.95(11)
–
112.39(15)
104.33(14)
106.41(14)
127.34(14)
126.23(14)
118.10(13)
107.01(12)
112.59(14)
104.38(13)
113.5(12)
103.1(11)
106.1(11)
124.5(10)
129.4(11)
118.2(11)
–
–
–
–
–
The IR spectra of all of the present complexes contained bands
associated with perchlorate anions (ca. 1100 and 625 cmÀ1). How-
ever, it was difficult to see in many of the spectra of the metal com-
plexes if there was a shift in the imine band (ca. 1650 cmÀ1) due to
the complexity of the bands in the region. Several major differ-
ences were observed between the 1H NMR spectra of the free li-
gands in d6-DMSO and those of the Ag(I) and Zn(II) complexes of
the same ligands. In all cases, the H–C@N proton signal corre-
sponding to the imine of the chain was significantly shifted in
the metal complexes, suggesting that binding of the metal ion at
this N atom was occurring. Furthermore, there were also shifts in
the signals associated with the imidazole ring systems, again indi-
cating possible binding at the imidazole pyridine-type N atom.
Interestingly, there was no loss of the phenolic proton signal
(13.28 ppm) in the spectrum of [Ag(2)2]ClO4, implying that depro-
tonation of the phenol group did not occur. The signal for the imid-
azole ring N–H proton in the spectra of the free ligands 3–5
becomes sharper and gets shifted upfield in the spectra of the
Ag(I) or Zn(II) complexes containing these ligands.
Fig. 3. Packing diagram for 2.
Fig. 4. Asymmetric unit of 3.
Crystals of [Ag(2)2]ClO4 suitable for X-ray crystallographic stud-
ies were grown from a hot acetonitrile solution of the complex. The
complex contains two independent silver ions, each on a centre of
symmetry, so the asymmetric unit contains two half-molecules
and one perchlorate anion (Fig. 7 and Table 2). The Ag(I) ion is
bonded, with linear coordination, to the imidazole nitrogen donors
and there is also intramolecular hydrogen bonding between the
phenolic hydrogen atoms and the imine nitrogen atoms in the
chains (N3A and N3B). Fig. 8 shows the resulting structure; alter-
nating independent [Ag(2)2]+ cations are linked into chains by
the perchlorate anions which are weakly bonded to the metal ions.
The anion acts as a bidentate ligand to Ag1 (O13–Ag1 and O14–Ag1
3.424(10) and 3.277(7) Å, respectively) and as a monodentate li-
gand to Ag2 (O12–Ag2, 2.895(8) Å). The N1A–Ag1–Ag2–N1B tor-
sion angle is 113.6°. Between the chains there are weak
hydrogen bonds between O1a and its equivalent under symmetry
operation.
The X-ray structures of three Ag(I) complexes of 3 have previ-
ously been reported [7]. [Ag2(3)2](NO3)2 formed as an infinite 1D
chain polymer where each of the two Ag(I) ions have different
coordination environments, with one metal chelated by the imine
N in the linker chain between the two imidazoles and the pyridine-
type N on the imidazole ring closest to it, with the second metal
bonded to the pyridine-type N on the imidazole ring at the further
x, y, z + 1) and including this interaction gives sheets of molecules
in the plane perpendicular to the a axis (Figs. 5, 6 and Table 2).
3.3. Metal complex syntheses and characterisation
The Schiff base ligands (2–6) reacted with Ag(I), Cu(II) and Zn(II)
perchlorate salts at room temperature to give the respective metal
complexes in high yield. The five Ag(I) complexes were reasonably
soluble in DMSO. Four of the complexes formulated with a 1:1
Ag:ligand ratio and the remaining complex, [Ag(2)2]ClO4, had a
1:2 ratio. In the case of the Cu(II) complexes, four formed with a
1:2 Cu:ligand ratio and the remaining complex, [Cu(2)4](ClO4)2,
had a 1:4 ratio. Repeated attempts to grow crystals of the latter
complex proved unsuccessful. All of the Zn(II) complexes had a
1:2 Zn:ligand ratio, with [Zn(2)(2À)](ClO4) being unusual in that
this was the only complex in which one of the Schiff base ligands
deprotonated(2À) at the salicylate O-H moiety. Kalanithi et al. [8]
previously reported the synthesis of Cu(II), Zn(II), Co(II), Ni(II)
and Cd(II) complexes of 2, formed by reacting the appropriate me-
tal chloride salt with 2 in a 1:1 M ratio. Their Cu(II) and Zn(II) com-
plexes formulated as [Cu(2À)Cl] and [Zn(2À)Cl(H2O)2], showing
that there was deprotonation of the salicylate OH moiety of the
single Schiff base ligand and coordination of the chloride ion.

J. McGinley et al. / Polyhedron 55 (2013) 169–178
175
Fig. 5. Partial packing diagram of 3 showing hydrogen bonds linking the molecules into chains.
Fig. 6. Packing diagram of 3 showing sheets perpendicular to a.
end of the ligand and the N from the same ring on an adjacent li-
istered dosage of the free ligands (ca. 0.2
lmol), none of the larvae
gand molecule. In the 2D polymeric network of [Ag2(3)2](BF4)2
there are two crystallographically independent Ag(I) centres hav-
ing the same coordination environment, with each metal coordi-
nated by three N atoms from two different molecules of 3
(chelation by the imine N in the linker chain between the two imi-
dazoles and the pyridine-type N on the closest imidazole ring, and
by the pyridine-type N on the imidazole ring at the opposite end of
an adjacent ligand molecule). [Ag2(3)2](CH3CO2)2 features a dis-
crete [Ag2(3)2]2+ molecular ring with each metal triangularly coor-
dinated by three N atoms from two different molecules of 3 (same
set of donor atoms as in [Ag2(3)2](BF4)2). In the electrospray mass
spectrum of [Ag2(3)2](NO3)2 the authors report signals for [Ag(3)]+
(310.3), [Ag(3)2]+ (512.8), [Ag2(3)2](NO3)2+(683.6). In the mass
spectrum of the present complex, [Ag(3)]ClO4, we observe a large
peak for the [Ag(3)]+ species and a much smaller peak for [Ag(3)2]+.
died, except in the case of ligand 6, where 10% of the larvae ex-
pired. These findings suggest that the ligands do not pose signifi-
cant toxicity issues to the insect. In addition, all of the Cu(II) and
Zn(II) complexes displayed minimal toxicity. [Ag(2)2]ClO4 and
[Ag(3)]ClO4 were less well tolerated, and both of these complexes
proved to be more destructive than AgNO3, where significant
toxicity (30%) was only evident at the much higher dosage of
0.235
lmol. It is interesting to note that, whereas [Ag(3)]ClO4
was quite toxic at the 0.197
l
mol dosage level (90%
mortality), the complex containing the isomeric ligand, 4 (i.e.
[Ag(4)]ClO4ÁMeOH), did not cause any fatalities at this dose.
3.4.2. Antimicrobial activity
The free ligands and the Cu(II) and Zn(II) complexes were essen-
tially inactive against all of the test microbes. The lack of activity
found for uncoordinated ligand 2 and its Cu(II) and Zn(II) com-
plexes is similar to that previously reported [8] for [Cu(2)Cl] and
[Zn(2)Cl(H2O)2] against the same organisms (except MRSA, which
was not tested). In contrast to the relative inactivity of 2, a related
Schiff base, formed from Apim and 2-formylphenoxyacetic acid,
was found to be quite active in its uncoordinated form against
3.4. Biology
3.4.1. Toxicity profiling
Larvae of the insect, G. mellonella, were used to assess the in vivo
toxicity of the test compounds (Table 3). At the maximum admin-

176
J. McGinley et al. / Polyhedron 55 (2013) 169–178
Fig. 7. Asymmetric unit of [Ag(2)2]ClO4 containing two half-molecules and one anion.
Fig. 8. Partial packing diagram of [Ag(2)2]ClO4 showing interactions between Ag(I) cations and perchlorate anions.
strains of S. aureus and E. Coli (MIC100 value of 6.25
case), indicating the importance of functionalization of the salicy-
late phenyl ring [14]. In addition, the lack of antimicrobial potency
l
g/ml in each
of the present Cu(II) complexes differs markedly from that of
[CuL(H2O)3]ClÁ2H2O (LH is a Schiff base ligand derived from -his-
tidine and pyrrole-2-carboxaldehyde) [15], which proved to be
L

J. McGinley et al. / Polyhedron 55 (2013) 169–178
177
Table 3
Mortality (%) of G. mellonella larvae 72 h post injection with various amounts of the test compounds.
Compound
lg/larvae
Administered amount/% mortality
40
20
10
2
2
0.175/0
0.060/80
0.034/0
0.034/0
0.197/0
0.097/90
0.058/20
0.060/10
0.189/0
0.097/0
0.060/10
0.060/0
0.184/0
0.094/20
0.057/10
0.055/0
0.184/10
0.094/0
0.057/0
0.056/0
0.235/30
0.087/0
0.030/10
0.017/0
0.017/0
0.098/0
0.049/0
0.029/10
0.030/10
0.094/0
0.049/0
0.030/0
0.030/0
0.092/0
0.047/10
0.029/0
0.027/0
0.092/0
0.047/0
0.029/0
0.028/10
0.118/10
0.044/0
0.015/0
0.009/0
0.009/0
0.049/0
0.024/0
0.015/0
0.015/0
0.047/0
0.024/0
0.015/0
0.015/0
0.046/0
0.024/0
0.014/0
0.014/0
0.046/0
0.024/0
0.014/0
0.014/0
0.059/0
0.009/0
0.003/0
0.002/0
0.002/0
0.010/0
0.005/0
0.003/0
0.003/0
0.009/0
0.005/0
0.003/0
0.003/0
0.009/0
0.005/0
0.003/0
0.003/0
0.009/0
0.005/0
0.003/0
0.003/0
0.012/0
[Ag(2)2]ClO4
[Cu(2)4](ClO4)2
[Zn(2)(2À)](ClO4)
3
[Ag(3)]ClO4
[Cu(3)2](ClO4)2ÁH2O
[Zn(3)2](ClO4)2
4
[Ag(4)]ClO4
[Cu(4)2](ClO4)2
[Zn(4)2](ClO4)2ÁH2O
5
[Ag(5)]ClO4
[Cu(5)2](ClO4)2
[Zn(5)2](ClO4)2ÁMeOH
6
lmol
lmol
lmol
lmol
lmol
lmol
lmol
lmol
lmol
lmol
lmol
lmol
lmol
lmol
lmol
lmol
lmol
lmol
lmol
lmol
[Ag(6)]ClO4
[Cu(6)2](ClO4)2ÁH2O
[Zn(6)2](ClO4)2ÁH2O
AgNO3
Table 4
Antimicrobial activitiy ranges of the Ag(I) complexes and AgNO3. MIC50 (bacteria) and MIC80 (fungi) values are expressed in
l
M concentrations of the samples.
Complex
MIC50
(l
M)
MIC80 (lM)
S. aureus
MRSA
E. coli
P. aeruginosa
C. albicans
[Ag(2)2]ClO4
[Ag(3)]ClO4
[Ag(4)]ClO4
[Ag(5)]ClO4
[Ag(6)]ClO4
AgNO3
37.5–75
60.9–121.8
56.5–113
58.9–117.8
58.9–117.8
36.8–73.6
37.5–75
37.5–75
60.9–121.8
56.5–113
58.9–117.8
58.9–117.8
73.6–147.2
37.5–75
9.4–18.8
7.6–15.2
7.6–15.2
14.7–29.4
58.9–117.8
9.2–18.4
60.9–121.8
28.3–56.6
58.9–117.8
58.9–117.8
36.8–73.6
30.5–60.9
14.2–28.3
29.5–58.9
58.9–117.8
18.4–36.8
quite active against E. coli and C. albicans (MIC100 = 11.9 and
19.0 M, respectively). The Zn(II) complex of the same ligand,
4. Conclusions
l
[ZnL(H2O)]Cl, was essentially inactive.
Schiff base ligands form readily and in high yield upon reacting
Apim with salicylaldehyde and imidazole aldehydes. These Schiff
bases readily coordinate to Ag(I), Cu(II) and Zn(II) centres. Encour-
agingly, all of free ligands and most of their metal complexes are
relatively non-toxic, in vivo, towards G. mellonella. Whereas the
uncoordinated ligands and the Cu(II) and Zn(II) complexes are
inactive against a selection of microbial pathogens, most of the
Ag(I) complexes, as well as AgNO3, exhibit moderate anti-bacterial
activity and good anti-fungal activity.
The anti-bacterial and anti-fungal activities of the new Ag(I)
complexes and the positive control, AgNO3, are listed in Table 4.
[Ag(6)]ClO4 was the least active across all of the microbial species,
and the activities of the other water-insoluble Ag(I) complexes
were similar to that observed for water-soluble AgNO3. In these
cases, activity against the fungal pathogen, C. albicans, was greater
than that for bacterial pathogens, which is a trend similar to that
previously reported for other Ag(I) complexes [5,6,16]. Although
there was no clear discriminatory trends observed in activities be-
tween Gram-positive (S. aureus and MRSA) and Gram-negative
(E. coli and P. aeruginosa) bacteria, most of the complexes, and
especially [Ag(4)]ClO4ÁMeOH, were more cytotoxic against
P. aeruginosa. The growth inhibitory effects of the present Ag(I)
imidazole Schiff base complexes against MRSA and E. coli are sim-
ilar to those recently documented for a series of Ag(I) complexes of
9-anthracenecarboxylic acid containing non-Schiff base imidazole
ligands and also the clinically used compound, silver sulphadiazine
[6]. However, some of the latter complexes displayed significantly
greater anti-Candida activity compared to the present complexes.
In addition, the present Ag(I) complexes had activities similar
to that of the prescription drug, miconazole (MIC90 against
Appendix A. Supplementary data
CCDC 916653–916655 contain the supplementary crystallo-
graphic data for 2, 3 and [Ag(2)2]ClO4, respectively. These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/con-
ts/retrieving.html, or from the Cambridge Crystallographic Centre,
12 Union Road, Cambridge CB2 1EZ, UK: fax: (+44) 1223-336-033;
or e-mail: deposit@ccdc.cam.ac.uk.
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