Toward new classes of potent antibiotics: Synthesis and antimicrobial activity of novel metallosaldach-imidazolium salts

Article abstract of DOI:10.1016/j.ejmech.2013.09.029

Imidazolium salts (Im⁺-R²R³-Cl ^-) attached to the N,N′-bis(salicylidene)-(±)-trans-1,2- diaminocyclohexane (saldach) backbone (4a-f) have been designed and successfully applied for the synthesis of the corresponding mononuclear complexes with Mn(III) and Fe(III) ions. The molecular structures of the saldach ligands H ₂(R¹)₂saldach(Im⁺-R ²R³-Cl^-)₂ (R¹ = H, tert-Bu, R² = H, Et, n-Bu, R³ = H, Me) and their [M(III)Cl{(R¹)₂saldach(Im⁺-R²R ³-Cl^-)₂}] (M = Mn, Fe) complexes have been established. The free ligands exist as the phenol-OH and not as the zwitterionic (imine)N-H⁺···^-O(phenol) tautomer. Antimicrobial activity of the target compounds revealed higher potent antibacterial activity against Salmonella aureus, B. subtilis while less effective against E. coli and C. albicans and inactivity against A. flavus. Compound (4d) and its Fe(III) complex (6d) exhibit remarkable extra-potent bactericidal activity.

Full text of DOI:10.1016/j.ejmech.2013.09.029

Accepted Manuscript
Toward new classes of potent antibiotics: synthesis and antimicrobial activity of novel
metallo-saldach-imidazolium salts
Reda F.M. Elshaarawy, Christoph Janiak
PII:
S0223-5234(13)00602-8
10.1016/j.ejmech.2013.09.029
EJMECH 6432
DOI:
Reference:
To appear in: European Journal of Medicinal Chemistry
Received Date: 16 May 2013
Revised Date: 17 July 2013
Accepted Date: 9 September 2013
Please cite this article as: R.F.M. Elshaarawy, C. Janiak, Toward new classes of potent antibiotics:
synthesis and antimicrobial activity of novel metallo-saldach-imidazolium salts, European Journal of
Medicinal Chemistry (2013), doi: 10.1016/j.ejmech.2013.09.029.
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ACCEPTED MANUSCRIPT
Toward new classes of potent antibiotics: synthesis and antimicrobial activity of novel
metallo-saldach-imidazolium salts
Reda F.M. Elshaarawy,a,b,* and Christoph Janiakb,*
Graphical abstract:
A novel synthetic strategy has been successfully developed for the synthesis of saldach-
imidazolium derivatives and their complexes. These compounds represent promising
candidates when set in the context of antimicrobial applications.

ACCEPTED MANUSCRIPT
Toward new classes of potent antibiotics: synthesis and antimicrobial activity of novel
metallo-saldach-imidazolium salts
Reda F.M. Elshaarawy,a,b,* and Christoph Janiakb,*
Highlights:
ꢀ N,N`-bis(salicylidene)-(±)-trans-1,2-diaminocyclohexane (saldach) as scaffold.
ꢀ Imidazolium salts (Im+-R2R3-Cl–) covalently attached to H2(R1)2saldach backbone.
ꢀ Compounds were tested against four bacterial pathogens and two fungal strains.
ꢀ Most compounds showed promising antibacterial activity.
ꢀ H2(tBu)2saldach(Im+-HMe-Cl–)2 and its Fe-complex are possible antibiotic candidates.

ACCEPTED MANUSCRIPT
Toward new classes of potent antibiotics: synthesis and antimicrobial activity of novel
metallo-saldach-imidazolium salts
Reda F.M. Elshaarawy,a,b,* and Christoph Janiakb,*
a
Faculty of Science, Suez University, Suez, Egypt. Email: reda_elshaarawi@science.suez.edu.eg;
Reda.El-Shaarawy@uni-duesseldorf.de
b Institut für Anorganische Chemie und Strukturchemie, Heinrich-Heine Universität Düsseldorf, 40204
Düsseldorf, Germany. Email: janiak@uni-duesseldorf.de
Abstract
Imidazolium salts (Im+-R2R3-Cl–) attached to the N,N`-bis(salicylidene)-(±)-trans-1,2-
diaminocyclohexane (saldach) backbone (4a-f) have been designed and successfully applied
for the synthesis of the corresponding mononuclear complexes with Mn(III) and Fe(III) ions.
The molecular structures of the saldach ligands H2(R1)2saldach(Im+-R2R3-Cl–)2 (R1 = H, tert-
Bu,R2 = H, Et, n-Bu, R3 = H, Me) and their [M(III)Cl{(R1)2saldach(Im+-R2R3-Cl–)2}] (M =
Mn, Fe) complexes have been established. The free ligands exist as the phenol-OH and not as
the zwitterionic (imine)N-H+···–O(phenol) tautomer. Antimicrobial activity of the target
compounds revealed higher potent antibacterial activity against S. aureus, B. subtilis while
less effective against E. coli and C. albicans and inactivity against A. flavus. Compound (4d)
and its Fe(III) complex (6d) exhibit remarkable extra-potent bactericidal activity.
Keywords:
N,N`-bis(salicylidene)-1,2-diaminocyclohexane,
Imidazolium
chlorides,
Complexes, Spectral study, Antimicrobial survey.
1. Introduction
Antibiotics have revolutionized the medical care in the 20th century. With the discovery of
antibiotics, people were convinced that infectious diseases might someday be wiped out.
However, the emergence and spread of multidrug-resistant bacteria have made treatment of
infectious diseases difficult and have become
a
serious medical problem
[
1,2]. For example, strains of Salmonella enterica, a leading cause of bacterial gastroenteritis,
are no longer susceptible to front line antibiotics [3] and the distribution of methicillin-
resistant S. aureus (MRSA) strains has increased [4]. Moreover, the high level of inherent
antibiotic resistance in P. aeruginosa makes treatment of these infections problematic [5].
This has given urgency to research to develop a variety of interesting compounds as drugs
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used in many fields. Several approaches for negating antibiotic resistance are currently being
investigated, including inactivation of enzymes in essential metabolic pathways and inhibiting
signal transduction systems [6,7]. These approaches involve the development of new
antimicrobial drugs with modes of action that circumvent current resistance mechanisms
[8,9].
Notably, the salen type compounds prepared by the condensation of o-hydroxyaldehydes
and diamine such as 1,2-diaminocyclohexane (DACH, dach) present versatile steric,
electronic and liphophilic properties [10]. Salen type compounds have tremendous potential
for a host of pharmaceutical applications; their enormous effects on different models have
raised serious possibilities recently for their use as starting materials in the synthesis of
antibiotics, antiallergic, antiphlogistic and antitumor drugs [11,12,13]. Unparalleled attention
has been devoted to these materials due to their low cost, ease of fabrication and their
stability. A tautomeric equilibrium has been recognized in 2-hydroxy Schiff bases due to the
existence of (phenol)O-H…N(imine) and (phenol)O–…+H-N(imine) type hydrogen bonds. This
equilibrium can be shifted to the zwitterionic form due to external electric fields from
additional ionic charges within the molecule and molecular packing (Scheme 1) [14].
Hydrogen bonding interactions play roles in preferential solvation and have been investigated
because they are present in a large variety of chemical, biochemical and pharmacological
events. In salen compounds, the imine nitrogen can acts as intramolecular hydrogen bond
acceptor and the phenolic oxygen derivatives can act as intermolecular hydrogen bond
acceptor. Moreover, the conjugation between metal ions and biologically active Schiff bases
becomes a subject worthy of pursuit and has demonstrated great promise for their extensive
applications in the enhancement of their biological and biomedical activities [13,15,16,17].
At the same time, salt formation of Active Pharmaceutical Ingredients (APIs) is an
attractive and commonly used approach to overcome the problems of solubility and stability
of APIs. There are many well-known examples where biological active cations and anions
combine together and the resulted salt exhibits the therapeutic effects of both of its
components [18]. Therefore, it is quite reasonable to think that imidazolium salt units (as
common in ionic liquids (ILs)) could potentially be included in anti-cancer, anti-viral and
other therapeutic agents/drugs. These ‘Therapeutic Ionic salts’ offer distinctly different
properties than the non-ionic backbone. If there is a therapeutic response then the major
advantage of ionic salts would be in managing/tuning their toxicity while tailoring the
physiochemical and pharmacological properties necessary for desired therapeutic
applications. Recently the in vitro anti-microbial, biodegradability [19] and anti-tumor
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activities [20] of a series of imidazolium based ionic liquids were reported. The imidazolium
salts examined exhibit broad activities against cocci, rods and fungi. Also the structure–
activity relationship showed that chain length of N-3 alkyl substitution plays a significant role
in the anti-tumor activity and cytotoxicity.
In a continuation of our earlier endeavor [21] to design and synthesize novel bioactive
compounds, the present work reports the synthesis and characterization of new saldach-
imidazolium salts (ionic compounds) and their metal complexes (Scheme 1). In view of the
already known independent biological importance of metallosaldach compounds and
imidazolium ILs, we also explore here the in vitro antimicrobial activity of Mn(III)- and
Fe(III) saldach-imidazolium salt systems (Scheme 1) against two Gram-positive, two Gram-
negative bacteria and two different fungi. These ionic salts may promise potential therapeutics
to combat antibiotic resistance. Herein, Mn(III) and Fe(III) species have been chosen in this
study due to their potential importance and useful applications in the biological field.
2. Experimental Section
2.1. Physical measurements
Melting point was measured using a BÜCHI Melting point B-540 apparatus; all melting
points were measured in open glass capillary and values are uncorrected. Elemental analyses
for C, H, N, were performed with a Perkin–Elmer 263 elemental analyzer. FT-IR spectra were
recorded on a BRUKER Tensor-37 FT-IR spectrophotometer in the range 400–4000 cm–1 as
KBr discs or in the 4000-550 cm–1 region with 2 cm–1 resolution with an ATR (attenuated
total reflection) unit (Platinum ATR-QL, Diamond). For signal intensities the following
abbreviations were used: br (broad), sh (sharp), w (weak), m (medium), s (strong), vs (very
strong). UV/Vis spectra were measured at 25 °C in deionized water (H2O) (10–5 mol/L) on a
Shimadzu UV-2450 spectrophotometer using quartz cuvettes (1 cm). NMR-spectra were
obtained with a Bruker Avance DRX200 (200 MHz for 1H) or Bruker Avance DRX500
(500 MHz for 13C) spectrometer with calibration to the residual proton solvent signal in
DMSO-d6 (1H NMR: 2.52 ppm, 13C NMR: 39.5 ppm), CDCl3 (1H NMR: 7.26 ppm, 13C NMR:
77.16 ppm) against TMS with δ = 0.00 ppm. Multiplicities of the signals were specified s
(singlet), d (doublet), t (triplet), q (quartet) or m (multiplet). The mass spectra of the
synthesized sal-imidazolium, saldach-imidazolium chlorides and their complexes were
acquired in the linear mode for positive ions on a BRUKER Ultraflex MALDI-TOF instrument
equipped with a 337 nm nitrogen laser pulsing at a repetition rate of 10 Hz. The MALDI
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matrix material (1,8-dihydroxy-9(10H)-anthracenone (dithranol, DIT) (C14H10O3, M=
226.23)) was dissolved in chloroform at a concentration of 10 mg/mL. MALDI probes were
prepared by mixing compound solutions (1 mg/mL in CH2Cl2) with the matrix solution (1:10,
v/v) in a 0.5 mL Eppendorf® micro tube. Finally 0.5 ꢀL of this mixture was deposited on the
sample plate dried at room temperature and then analyzed. The molar conductance 10-3 M
solution of various salts has been measured at ambient temperature with a digital conductivity
meter (S30 SevenEasy™ conductivity, Mettler-Toledo Electronics, LLC, Polaris Parkway,
Columbus). The overall accuracy of the conductance measurements was found to be ± 0.2%.
Magnetic measurements of target complexes were carried out at room temperature using a
Vibrating Sample Magnetometer (VSM), (Model PAR 155).
2.2. Materials and Syntheses
Chemicals were obtained from the following suppliers and used without further
purification: salicylaldehyde (sal), 2-tert-butylphenol (2-tBuPhOH), 1-ethylimidazole (1-Et-
Im), 2-methylimidazole (2-Me-Im), (±)-trans-1,2-diaminocyclohexane (dach), tetrabutyl-
ammonium bromide (tBu4NBr), anhydrous magnesium dichloride (MgCl2) and manganese(II)
·
acetate tetrahydrate (Mn(CH3COO)2 4H2O) (Sigma–Aldrich), paraformaldehyde ((CH2O)n)
(Roth), 1-butylimidazole (1-nBu-Im) (Alfa Aesar), triethyl amine (Et3N) and anhydrous zinc
·
chloride (ZnCl2) (GRÜSSING GmbH) and iron(III) chloride hexahydrate (FeCl3 6H2O) (Acros).
3-tert-Butyl-2-hydroxybenzaldehyde (1b) was synthesized from 2-tert-butylphenol as
described previously [22] and obtained as a pale yellow oil (82% yield). It becomes dark
1
green on storage. H NMR (200 MHz, CDCl3) δ (ppm): 11.87 (s, 1H, Ph-OH), 9.91 (s, 1H,
HC=O), 7.58 (dd, 1H, JHH = 1.66, 6.02 Hz, Ph-H), 7.44 (dd, 1H, JHH = 1.69, 7.67 Hz, Ph-H),
6.99 (t, 1H, JHH = 7.68 Hz, Ph-H), 1.48 (s, 9H, C(CH3)3).
2.2.1. 5-Chloromethyl-2-hydroxybenzaldehyde (2a):
This compound was synthesized from salicylaldehyde according to the classical
chloromethylation procedure [23] modified with catalyst ZnCl2, and hydrogen chloride gas
atmosphere. In a typical synthesis, (17.5 g, 160 mmol) of salicylaldehyde was treated with 24
ml of 37% aqueous formaldehyde and (1.2 g, 8.8 mmol) of ZnCl2 in 100 ml of concentrated
hydrochloric acid. The mixture was stirred at room temperature under HC1g atmosphere
(HC1g was generated in another system, connected to the reaction vessel, by dropping
concentrated H2SO4 slowly on to solid NaCl) for 24 h. The white solid that separated from the
reaction mixture was filtered, re-dissolved in diethyl ether, and the combined organic phases
were washed several times with milli-Q water, till pH = 7 for washing liquor, and then dried
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over sodium sulfate. Volatiles were distilled off, under inert atmosphere, and the resulting
white crude residue was filtered and further recrystallized from n-hexane to furnish 5-
chloromethyl-2-hydroxybenzaldehyde as white-colored needles (15.2 g, 62.0 % yield). FT-IR
(ATR, cm-1): 3240 (m, br, ν OH), 3120 (m, br, νasym CH, Ph), 3050 (m, br, νsym CH, Ph), 2876
(m, sh, ν CH2), 1659 (vs, sh, ν C=O), 1578, 1489, 1437 (s, sh, ν C=Carom + ν C-H bend), 1338
(m, sh, ν CH2), 1252 (s, sh, ν CH2Cl), 1150 (s, sh, ν HCC, Ph), 772 (s, sh, ν C–Cl). 1H NMR
(200 MHz, CDCl3) δ (ppm): 11.07 (s, 1H, Ph-OH) 9.90 (s, 1H, Ph-HC=O), 7.59-7.55 (m, 2H,
13
2 x Ph-H), 7.02–6.98 (d, 1H, JHH = 8.34 Hz, Ph-H), 4.60 (s, 2H, CH2-Ph). C NMR (500
MHz, CDCl3) δ (ppm): 196.02 (HC=O), 160.90 (C-OH), 137.22 (C, Ph), 133.64 (CH, Ph),
129.86 (CH, Ph), 120.34 (C, Ph), 118.72 (CH, Ph), 45.28 CH2-phenyl).
2.2.2. 3-tert-Butyl-5-chloromethyl-2-hydroxybenzaldehyde (2b):
t
A mixture of 1b (2.7 g, 15.2 mmol), (CH2O)n (1.0 g, 33.3 mmol), and Bu4NBr (0.47 g,
1.46 mmol) in 11 ml of concentrated hydrochloric acid was stirred vigorously at ambient
temperature for 72 h [24]. The resulting emulsion was extracted with diethyl ether (3 × 25
mL). Then the organic phase was washed with saturated brine solution (2 × 10 ml), milli-Q
water (5 × 10 ml) and, then dried over anhydrous magnesium sulfate. This extract was further
concentrated under vacuum to give 3-tert-butyl-5-(chloromethyl)-2-hydroxybenzaldehyde 2b
as a pale yellow crystalline solid (1.99 g, 58% yield). FT-IR (ATR, cm-1): 3428 (m, br, ν OH),
3154 (m, br, νasym CH, Ph), 3070 (m, br, νsym CH, Ph), 2956 (m, sh, ν CH3), 2848 (m, sh, ν
CH2), 1651 (vs, sh, ν C=O), 1612, 1476, 1435 (s, sh, ν C=Carom + ν C-H bend), 1331 (m, sh, ν
CH2), 1263 (s, sh, ν CH2Cl), 1172 (s, sh, ν HCC, Ph), 694 (s, sh, C–Cl). 1H NMR (200 MHz,
CDCl3) δ (ppm): 11.90 (s, 1H, Ph-OH) 9.91 (s, 1H, Ph-HC=O), 7.57-7.56 (d, H, JHH = 2.29,
Ph-H), 7.47-7.46 (d, 1H, JHH = 2.27, Ph-H), 4.63 (s, 2H, CH2-Ph), 1.46 (s, 9H, C(CH3)3). 13C
NMR (500 MHz, CDCl3) δ (ppm): 192.98 (HC=O), 160.27 (C-OH), 136.88 (C, Ph), 133.78
(CH, Ph), 130.36 (C, Ph), 129.00 (C, Ph), 125.17 (CH, Ph), 46.06 (CH2-Ph), 31.89
(C(CH3)3), 30.56 (C(CH3)3).
2.2.3. Preparation of salicylaldehyde-alkylimidazolium chlorides H(R1)sal(Im+-R2R3-Cl–) (3a-f)
General method; To a vigorously stirred solution of imidazole derivatives (23.39 mmol) in
dry toluene (10 mL) at room temperature was added the solution of chloromethyl-
salicylaldehydes 2a,b (19.50 mmol) in dry toluene (10 mL), drop-wise, under nitrogen
atmosphere. The resulting solution was further stirred at 60 °C for 24 h. The product
separated out was washed twice with toluene (2 x 5 mL) and several with ether (5 x 10 mL),
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to remove the unreacted materials, and dried under vacuum to give pale yellow solid which
used for the following preparations without further purification. Samples of the products were
nevertheless isolated and fully characterized, as described below.
3-(3-Formyl-4-hydroxybenzyl)-2-methyl-1H-imidazol-3-ium chloride (3a): Obtained as pale
yellow solid (4.48 g, 91%). FT-IR (KBr, cm-1): 3364 (m, br, ν N(1)-H and O-H), 3161 (m, br,
νasym CH, CH, Im and Ph), 3080 (m, br, νsym CH, Im and Ph), 2979 (m, sh, νasym CH3), 2959
(m, sh, νsym CH3), 2884 (m, sh, ν CH2), 1661 (vs, sh, ν C=O), 1573, 1485, 1455 (s, sh, ν
C=Carom + ν C-H bend), 1338 (m, sh, ν CH2), 1149 (s, sh, HCC, HCN bending, Im), 823 (m,
sh), 750 (m, sh), 682 (m, sh), 555 (m, sh). 1H NMR (200 MHz, CDCl3) δ (ppm): 10.83 (s, 1H,
Ph-HC=O) 10.30 (s, 1H, Ph-OH), 10.15 (s, 1H, N-H Im), 7.86-7.81 (d, 1H, JHH =1.39 Hz,
N(1)CHCH- Im), 7.76-7.74 (d, JHH = 1.41 Hz, 1H, N(1)CHCH- Im), 7.47.7.13 (m, 3H, 3 x
Ph-H), 5.45 (s, 2H, -N(3)-CH2-Ph), 2.57 (s, 3H, C(2)-CH3).13C NMR (500 MHz, CDCl3) δ
(ppm): 196.89 (HC=O), 159.21 (C-OH), 144.8 (-N(1)C(CH3)N(3)-), 136.68 (C-C=O), 136.47
(CH, Ph), 130.99 (CH, Ph), 127.35 (C, Ph), 122.74 ((-N(1)CHCH-), 122.41 (-N(1)CHCH-),
117.88 (CH, Ph), 51.88 (-N(1)CH2-Ph), 22.73 (C(2)-CH3). MALDI-TOF MS, m/z: 217.1 [M -
Cl]+.
1-Ethyl-3-(3-formyl-4-hydroxybenzyl)-1H-imidazol-3-ium chloride (3b): Obtained as yellow
solid (3.56 g, 69 %). FT-IR (KBr, cm-1): 3343 (m, br, ν O-H), 3135 (m, br, νasym CH, Im and
Ph), 3098 (m, br, νsym CH, Im and Ph), 2933 (m, sh, νasym CH3), 2908 (m, sh, νsym CH3), 2843
(m, sh, ν CH2), 1662 (vs, sh, ν C=O), 1568, 1497, 1458 (s, sh, ν C=Carom + ν C-H bend), 1249
(m, sh, δ CH3 + ν N-CH2), 1152 (s, sh, ν HCC + ν HCN bending, Im), 825 (m, sh), 759 (m,
sh), 682 (m, sh), 558 (m, sh). 1H NMR (200 MHz, CDCl3) δ (ppm): 10.99 (s, 1H, Ph-HC=O),
10.29 (s, 1H, Ph-OH), 9.31 (s, 1H, -N(1)CHN(3)- Im), 7.83-7.81 (d, JHH = 1.51 Hz, 1H,
N(1)CHCH- Im), 7.77-7.75 (d, JHH = 1.60 Hz, 1H, N(1)CHCH- Im), 7.69 (s, 1H, Ph-H), 7.59-
7.57 (d, 1H, JHH = 7.02 Hz, Ph-H), 7.18-7.16 (d, 1H, JHH = 8.34 Hz, Ph-H), 5.36 (s, 2H, -
N(3)-CH2-Ph), 4.16-4.12 (q, 2H, -N(1)CH2), 1.27-1.24 (t, JHH = 7.30, 7.34 Hz, 3H, -
CH2CH3). 13C NMR (500 MHz, CDCl3) δ (ppm): 196.16 (HC=O), 158.98 (C-OH), 139.02 (-
N(1)CHN(3)-), 136.17 (C-C=O), 136.06 (CH, Ph), 130.64 (CH, Ph), 126.89 (C, Ph), 123.00
((-N(1)CHCH-), 122.92 (-N(1)CHCH-), 117.32 (CH, Ph), 53.68 (-N(1)CH2-phenyl), 41.54
(N(1)-CH2) , 15.79 (CH3, CH2-CH3). MALDI-TOF MS, m/z: 231.1 [M-Cl]+.
1-Butyl-3-(3-formyl-4-hydroxybenzyl)-1H-imidazol-3-ium chloride (3c): Obtained as yellow
solid (4.90 g, 86%). FT-IR (KBr, cm-1): 3359 (m, br, ν O-H), 3185 (m, br, νasym CH, Im and
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Ph), 3066 (m, br, νsym CH, Im and Ph), 2981 (m, sh, νasym CH3), 2960 (m, sh, νsym CH3), 2877
(m, sh, ν CH2), 1657 (vs, sh, ν C=O), 1569, 1552, 1469 (s, sh, ν C=Carom + ν C-H bend), 1336
(m, sh, δ CH3 + δ CH2), 1245 (m, sh, δ CH3 + ν N-CH2), 1149 (s, sh, ν HCC + ν HCN
bending, Im), 850 (m, sh), 736 (m, sh), 679 (m, sh), 582 (m, sh). 1H NMR (200 MHz, CDCl3)
δ (ppm): 11.37 (s, 1H, Ph-HC=O) 10.30 (s, 1H, Ph-OH), 9.53 (s, 1H, -N(1)CHN(3)- Im),
7.88-7.86 (d, 2H, N(1)CHCH- Im), 7.74 (s, 1H, Ph-H), 7.66-7.64 (d, 1H, JHH =6.99 Hz, Ph-
H), 7.26.7.24 (d, 1H, JHH =8.56 Hz, Ph-H), 5.40 (s, 2H, -N(3)-CH2-Ph), 4.20-4.17 (t, JHH
=
7.06, 7.14 Hz, 2H, -N(1)CH2), 1.79-1.73 (m(5), 2H, -N(1)CH2CH2), 1.27-1.19 (m(6), 2H, -
13
CH2CH3), 0.89-0.86 (t, JHH = 7.33, 7.38 Hz, 3H, -CH2CH3). C NMR (500 MHz, CDCl3) δ
(ppm): 195.81 (HC=O), 159.04 (C-OH), 138.51 (-N(1)CHN(3)-), 136.10 (C-C=O), 135.89
(CH, Ph), 130.96 (CH, Ph), 126.73 (C, Ph), 123.23 (-N(1)CHCH-), 123.02 (-N(1)CHCH-),
116.77 (CH, Ph), 53.74 (-N(1)CH2-phenyl), 48.13 (N(1)-CH2) , 34.79 (N(1)-CH2-CH2), 19.44
(-CH2-CH3), 13.40 (-CH2-CH3). MALDI-TOF MS, m/z: 259.2 [M - Cl]+.
3-(3-(tert-Butyl)-5-formyl-4-hydroxybenzyl)-2-methyl-1H-imidazol-3-ium
chloride
(3d):
1
Obtained as faint yellow semi-solid (3.65 g, 88 %). H NMR (200 MHz, CDCl3) δ (ppm):
11.91 (s, 1H, Ph-HC=O) 10.79 (s, 1H, Ph-OH), 9.91 (s, 1H, -N-H Im), 7.85-7.83 (d, 1H, JHH
=1.60 Hz, N(1)CHCH- Im), 7.55-7.54 (d, JHH = 1.60 Hz, 1H, N(1)CHCH- Im), 7.27 (s, 1H,
Ph-H), 7.01 (s, 1H, Ph-H), 5.53 (s, 2H, -N(3)-CH2-Ph), 2.78 (s, 3H, C(2)-CH3), 1.38 (s, 9H,
13
C(CH3)3). C NMR (500 MHz, CDCl3) δ (ppm): 196.84 (HC=O), 160.26 (C-OH), 145.32 (-
N(1)C(CH3)N(3)-), 135.48 (C-C(CH3)3), 133.88 (CH, Ph), 129.06 (C, C-C=O), 127.24 (CH,
Ph), 124.6 (C, Ph), 123.45 ((-N(1)CHCH-), 122.93 (-N(1)CHCH-), 52.00 (-N(1)CH2-Ph),
34.41 (C-C(CH3)3), 29.97 (C-C(CH3)3), 18.52 (C(2)-CH3), MALDI-TOF MS, m/z: 273.1 [M -
Cl]+.
3-(3-(tert-Butyl)-5-formyl-4-hydroxybenzyl)-1-ethyl-1H-imidazol-3-ium chloride
(
(3e):
1
Obtained as yellow-orange wax (2.98 g, 58 %). H NMR (200 MHz, CDCl3) δ (ppm): 11.93
(s, 1H, Ph-HC=O) 9.98 (s, 1H, Ph-OH), 9.91 (s, 1H, -N(1)CHN(3)- Im), 7.86-7.84 (d, 1H,
JHH =1.59 Hz, N(1)CHCH- Im), 7.56-7.54 (d, JHH = 1.63 Hz, 1H, N(1)CHCH- Im), 7.34 (s,
1H, Ph-H), 7.26 (s, 1H, Ph-H), 5.52 (s, 2H, -N(3)-CH2-Ph), 4.22-4.11 (q, 2H, -N(1)CH2),
1.39 (s, 9H, C(CH3)3), 1.26-1.20 (t, JHH = 7.28, 7.30 Hz, 3H, -CH2CH3). 13C NMR (500 MHz,
CDCl3) δ (ppm): 196.89 (HC=O), 155.73 (C-OH), 137.69 (C, Ph), 135.81 (-N(1)CHN(3)-),
133.23 (CH, Ph), 130.33 (C, C-C=O), 128.57 (C, Ph), 125.06(CH, Ph), 123.43 (-
N(1)CHCH-), 122.02 (-N(1)CHCH-), 54.74 (-N(3)CH2-Ph), 41.13 (N(1)-CH2), 34.40
(C(CH3)3), 29.33 (C(CH3)3), 16.08 (CH2CH3). MALDI-TOF MS, m/z: 287.2 [M - Cl]+.
7

ACCEPTED MANUSCRIPT
3-(3-(tert-Butyl)-5-formyl-4-hydroxybenzyl)-1-butyl-1H-imidazol-3-ium
chloride
(3f):
Obtained as yellow solid (3.88 g, 77%). FT-IR (KBr, cm-1): 3465 (m, br, ν O-H), 3101 (m, br,
νsym CH, Im and Ph), 2976 (m, sh, νasym CH3), 2942 (m, sh, νsym CH3), 2883 (m, sh, ν CH2),
1650 (vs, sh, ν C=O), 1541, 1477, 1445 (s, sh, ν C=Carom + ν C-H bend), 1333 (m, sh, ν CH2),
1
1162 (s, sh, HCC, HCN bending, Im), 851 (m, sh), 749 (m, sh), 679 (m, sh), 558 (m, sh). H
NMR (200 MHz, CDCl3) δ (ppm): 11.88 (s, 1H, Ph-HC=O) 10.11 (s, 1H, Ph-OH), 9.83 (s,
1H, -N(1)CHN(3)- Im), 7.87-7.86 (d, 2H, N(1)CHCH- Im), 7.60-7.57 (d, 1H, JHH =7.01 Hz,
Ph-H), 7.25-7.21 (d, 1H, JHH =8.55 Hz, Ph-H), 5.49 (s, 2H, -N(3)-CH2-Ph), 4.09-3.97 (t, JHH
= 7.02, 6.99 Hz, 2H, -N(1)CH2), 1.69-1.65 (m(5), 2H, -N(1)CH2CH2), 1.31 (s, 9H, C(CH3)3),
1.28-1.20 (m(6), 2H, -CH2CH3), 0.92-0.89 (t, JHH = 7.18, 7.17 Hz, 3H, -CH2CH3). MALDI-
TOF MS, m/z: 315.2 [M - Cl]+.
2.2.4. General procedure for the preparation of salen ligands H2(R1)2saldach(Im+-R2R3-Cl–)2 (4a-
f):
A methanolic solution (10 mL) of (±)-trans-1,2-diaminocyclohexane (dach) (0.23 g, 2.0
mmol), in a Schlenk tube, was added drop wise to a methanolic solution (20 mL) of
substituted salicylaldehyde-imidazolium salt H(R1)sal(Im+-R2R3-Cl–) 3a-f (4.0 mmol) into a 100
ml Schlenk flask under nitrogen atmosphere. The reaction mixture was stirred at 60 °C
overnight. MeOH was partially removed under reduced pressure on a rotary evaporator, and
the yellow products of 4a-f were precipitated by ethyl acetate and kept in the refrigerator
overnight. The precipitate was sonicated with Et2O (3 x 25 mL), collected by filtration and
dried under vacuum.
N,N'-Bis[5-((2-methylimidazolium)methylene)-salicylidene)-trans-1,2-cyclohexanediamine
dichloride (4a): Yellow-orange powder, (2.16 g, 87 %). FT-IR (KBr, cm-1): 3395 (m, br, ν
N(1)-H + ν O-H), 3186 (w, br, νasym CH, CH, Im and Ph), 3090 (m, br, νsym CH, Im and Ph),
2941 (m, sh, ν CH3), 2861 (m, sh, ν CH2), 1630 (vs, sh, ν C=N), 1558, 1475, 1443 (s, sh, ν
C=Carom + ν C-H bend), 1320 (m, sh, ν CH2), 1258 (m, sh, ν Ph-O), 1160 (s, sh, ν HCC + ν
1
HCN bending, Im). H NMR (200 MHz, CDCl3) δ (ppm): 13.30 (br, s, 2H, Ph-OH), 9.48 (s,
2H, Im-NH), 8.62 (s, 2H, 2 x H-C=N), 7.86-7.84 (d, 2H, JHH =1.76 Hz, 2 x N(1)CHCH- Im),
7.63-7.62 (d, JHH = 2.02 Hz, 2H, 2 x N(1)CHCH- Im), 7.58-7.42 (m, 4H, 4 x Ph-H), 6.92-6.84
(m, 2H, 2 x Ph-H), 5.30 (s, 4H, 2 x -N(3)-CH2-Ph), 3.95-3.83 (m, 2H, 2 x Cyhex-H), 3.38 (s,
6H, 2 x Im-CH3), 1.79-1.42 (m, 8H, 8 x Cyhex-H). MALDI-TOF MS, m/z: 659.4
+
+
.
·
.
[M 2H2O+K] ; HRMS (ESI): m/z: calcd for C30H36N6O2 2H2O: 547.3835 [M 2H2O-2Cl] ;
8

ACCEPTED MANUSCRIPT
.
found: 547.3745. Anal. Calcd. for C30H36Cl2N6O2 2H2O (M = 619.58): C, 58.16; H, 6.51; N,
13.56; Found: C, 57.84; H, 6.23; N, 13.51. Conductivity = 284 ꢀS/cm.
N,N'-Bis[5-((1-ethylimidazolium)methylene)-salicylidene)-trans-1,2-cyclohexanediamine
dichloride (4b): Yellow powder, (2.51 g, 81 %). FT-IR (KBr, cm-1): 3434 (s, br, ν O-H),
3133 (w, br, νasym CH, CH, Im and Ph), 3068 (m, br, νsym CH, Im and Ph), 2933 (m, sh, ν
CH3), 2858 (m, sh, ν CH2), 1632 (vs, sh, ν C=N), 1569, 1497, 1447 (s, sh, ν C=Carom + ν C-H
bend), 1349 (m, sh, ν CH2), 1253 (m, sh, ν Ph-O), 1155 (s, sh, HCC, HCN bending, Im), 846,
1
761, 662. H NMR (200 MHz, CDCl3) δ (ppm): 13.48 (br, s, 2H, Ph-OH) 9.53 (s, 2H,
N(1)CHN(3)- Im), 8.42 (s, 2H, 2 x H-C=N), 7.80-7.77 (d, 2H, JHH =1.96 Hz, 2 x
N(1)CHCH- Im), 7.50-7.48 (d, JHH = 2.12 Hz, 2H, 2 x N(1)CHCH- Im), 7.36-7.29 (m, 4H, 4
x Ph-H), 6.98-6.93 (m, 2H, 2 x Ph-H), 5.36 (s, 4H, 2 x -N(3)-CH2-Ph), 4.02-3.93 (q, 4H, 2 x
-N(1)CH2), 3.84-3.75 (m, 2H, 2 x Cyhex-H), 1.85-1.42 (m, 8H, 8 x Cyhex-H), 1.28-1.25 (t,
+
.
JHH = 6.99, 7.01 Hz, 6H, 2 x -CH2CH3). MALDI-TOF MS, m/z: 669.3 [M H2O+K] and
+
.
573.3 [M - Cl] . Anal. Calcd. for C32H40Cl2N6O2 H2O (M = 629.62): C, 61.04; H, 6.72; N,
13.35; Found: C, 60.73; H, 6.33; N, 13.20. Conductivity = 265 ꢀS/cm.
N,N'-Bis[5-((1-n-butylidazolium)methylene)-salicylidene)-trans-1,2-cyclohexane-diamine
dichloride (4c): Pale yellow powder, (2.19 g, 77 %). FT-IR (KBr, cm-1): 3398 (m, br, ν O-H),
3140 (w, br, νasym CH, CH, Im and Ph), 3022 (m, br, νsym CH, Im and Ph), 2959 (m, sh, ν
CH3), 2888 (m, sh, ν CH2), 1629 (vs, sh, ν C=N), 1573, 1487, 1438 (s, sh, ν C=Carom + ν C-H
bend), 1346 (m, sh, ν CH2), 1260 (m, sh, ν Ph-O), 1158 (s, sh, HCC, HCN bending, Im), 843,
679, 561. 1H NMR (200 MHz, CDCl3) δ (ppm): 13.35 (br, s, 2H, 2 x Ph-OH) 9.63 (s, 2H, 2 x
N(1)CHN(3)- Im), 8.28 (s, 2H, 2 x H-C=N), 7.76-7.73 (d, 2H, JHH =1.87 Hz, 2 x N(1)CHCH-
Im), 7.65-7.63 (d, JHH = 1.89 Hz, 2H, 2 x N(1)CHCH- Im), 7.18-6.96 (m, 6H, 6 x Ph-H), 5.34
(s, 4H, 2 x -N(3)-CH2-Ph), 4.24-4.17 (t, JHH = 7.08, 7.09 Hz, 4H, 2 x -N(1)CH2), 4.00-3.95
(m, 2H, 2 x Cyhex-H), 1.92-1.76 (m, 8H, 8 x Cyhex-H), 1.65-1.57 (m(5), 4H, 2 x -
N(1)CH2CH2), 1.29-1.19 (m(6), 4H, 2 x -CH2CH3), 0.98-0.90 (t, JHH = 7.05, 7.14 Hz, 6H, 2 x -
+
+
.
CH2CH3). MALDI-TOF MS, m/z: 750.5 [M 2.5H2O+K] and 631.3 [M - Cl] . Anal. Calcd.
.
for C36H48Cl2N6O2 2.5H2O (M = 712.75): C, 60.66; H, 7.50; N, 10.02; Found: C, 60.62; H,
7.35; N, 9.60. Conductivity = 239 ꢀS/cm.
N,N'-Bis[3-tert-butyl-5-((2-methylimidazolium)methylene)-salicylidene)-trans-1,2-cyclo-
hexandiamine dichloride (4d): Canary yellow solid, (2.17 g, 76 %). FT-IR (KBr, cm-1): 3430
(m, br, ν O-H), 3295 (m, br, ν N(1)-H), 3086 (w, br, νasym CH, CH, Im and Ph), 2997 (m, br,
9

ACCEPTED MANUSCRIPT
νsym CH, Im and Ph), 2943 (m, sh, ν CH3), 2868 (m, sh, ν CH2), 1629 (vs, sh, ν C=N), 1593,
1476, 1443 (s, sh, ν C=Carom + ν C-H bend), 1360 (m, sh, ν CH2), 1281 (m, sh, ν Ph-O), 1149
(s, sh, HCC, HCN bending, Im), 897, 868, 775, 700, 655, 510. 1H NMR (200 MHz, CDCl3) δ
(ppm): 13.75 (br, s, 2H, 2 x Ph-OH) 10.05 (s, 2H, 2 x N-H Im), 8.28 (s, 2H, 2 x H-C=N),
7.65-7.64 (d, 2H, JHH =1.93 Hz, 2 x N(1)CHCH- Im), 7.41-7.39 (d, JHH = 1.99 Hz, 2H, 2 x
N(1)CHCH- Im), 7.16-6.99 (m, 4H, 4 x Ph-H), 5.32 (s, 4H, 2 x -N(3)-CH2-Ph), 3.60-3.56
(m, 2H, 2 x Cyhex-H), 2.93 (s, 6H, 2 x C(2)-CH3), 1.83-1.45 (m, 8H, 8 x Cyhex-H), 1.22 (s,
+
+
.
.
18H, 2 x C(CH3)3). MALDI-TOF MS, m/z: 736.5 [M H2O+Na] and 578.1 [M H2O-Cl] .
.
Anal. Calcd. for C38H52Cl2N6O2 H2O (M = 713.78): C, 63.94; H, 7.63; N, 11.77; Found: C,
64.12; H, 7.56; N, 11.48. Conductivity = 258 ꢀS/cm.
N,N'-Bis[3-tert-butyl-5-((1-ethylimidazolium)methylene)-salicylidene)-trans-1,2-cyclohexane-
diamine dichloride (4e): Pale yellow-orange powder, (2.00 g, 69 %). FT-IR (KBr, cm-1): 3446
(m, br, ν O-H), 3090 (w, br, νasym CH, CH, Im and Ph), 3005 (m, br, νsym CH, Im and Ph),
2954 (m, sh, ν CH3), 2865 (m, sh, ν CH2), 1634 (vs, sh, ν C=N), 1591, 1479, 1439 (s, sh, ν
C=Carom + ν C-H bend), 1362 (m, sh, ν CH2), 1285 (m, sh, ν Ph-O), 1138 (s, sh, HCC, HCN
bending, Im), 939, 828, 772, 711, 644. 1H NMR (200 MHz, CDCl3) δ (ppm): 13.68 (br, s, 2H,
2 x Ph-OH) 10.03 (s, 2H, 2 x N(1)CHN(3)- Im), 8.42 (s, 2H, 2 x H-C=N), 7.58-7.56 (d, 2H,
JHH =1.93 Hz, 2 x N(1)CHCH- Im), 7.39-7.37 (d, JHH = 1.99 Hz, 2H, 2 x N(1)CHCH- Im),
7.26-6.98 (m, 4H, 4x Ph-H), 5.22 (s, 4H, 2 x -N(3)-CH2-Ph), 4.58-4.53 (q, 4H, 2 x
N(1)CH2), 3.55-3.49 (m, 2H, 2 x Cyhex-H), 1.89-1.44 (m, 8H, 8 x Cyhex-H), 1.39 (s, 18H, 2
x C(CH3)3), 1.24-1.20 (t, JHH = 7.28, 7.30 Hz, 6H, 2 x -CH2CH3). MALDI-TOF MS, m/z:
+
.
.
671.4 [M H2O-2Cl] . Anal. Calcd. for C40H56Cl2N6O2 H2O (M = 741.83): C, 64.76; H, 7.88;
N, 11.33; Found: C, 64.45; H, 7.83; N, 11.00. Conductivity = 239 ꢀS/cm.
N,N'-Bis[3-tert-butyl-5-((1-butylimidazolium)methylene)-salicylidene)-trans-1,2-cyclohexane-
diamine dichloride (4f): Yellow-orange solid, (2.07 g, 65 %). FT-IR (KBr, cm-1): 3486 (m, br,
ν O-H), 3055 (w, br, νasym CH, CH, Im and Ph), 2999 (m, br, νsym CH, Im and Ph), 2948 (m,
sh, ν CH3), 2880 (m, sh, ν CH2), 1635 (vs, sh, ν C=N), 1560, 1478, 1444 (s, sh, ν C=Carom + ν
C-H bend), 1366 (m, sh, ν CH2), 1284 (m, sh, ν Ph-O), 1176 (s, sh, HCC, HCN bending, Im),
1284 (m, sh, ν Ph-O), 851, 772, 646. 1H NMR (200 MHz, CDCl3) δ (ppm): 13.73 (br, s, 2H, 2
x Ph-OH) 9.99 (s, 2H, 2 x N(1)CHN(3)- Im), 8.68 (s, 2H, 2 x H-C=N), 7.60-7.59 (d, 2H, JHH
=1.90 Hz, 2 x N(1)CHCH- Im), 7.41-7.39 (d, JHH = 1.89 Hz, 2H, 2 x N(1)CHCH- Im), 7.26-
7.00 (m, 4H, 4x Ph-H), 5.28 (s, 4H, 2 x -N(3)-CH2-Ph), 4.32-4.29 (t, JHH = 6.99, 7.01 Hz,
10

ACCEPTED MANUSCRIPT
4H, 2 x -N(1)CH2), 3.48-3.43 (m, 2H, 2 x Cyhex-H), 1.92-1.70 (m, 8H, 8 x Cyhex-H), 1.63-
1.55 (m(5), 4H, 2 x -N(1)CH2CH2), 1.36 (s, 18H, 2 x C(CH3)3), 1.28-1.19 (m(6), 4H, 2 x -
CH2CH3), 1.00-0.96 (t, JHH = 7.12, 7.12 Hz, 6H, 2 x -CH2CH3). MALDI-TOF MS, m/z: 762.2
+
.
.
[M H2O-Cl] . Anal. Calcd. for C44H64Cl2N6O2 H2O (M = 797.94): C, 66.23; H, 8.34; N,
10.53; Found: C, 66.30; H, 8.33; N, 10.10. Conductivity = 220 ꢀS/cm.
2.2.5. General procedure for the preparation of metallosaldach-imidazolium systems
[M(III)Cl{(R1)2saldach(Im+-R2R3-Cl–)2}] (M = Mn, Fe) (5,6a-f):
2.2.5.1. Synthesis of Mn(III) complexes (5a-f):
A yellow solution of the substituted imidazolium-saldach ligand H2(R1)2saldach(Im+-R2R3-
Cl–)2 4a-f (0.9 mmol) in ethanol (10 mL) was degassed for 15 minutes. An ethanolic solution
·
(5 mL) of Mn(OAc)2 4H2O (269 mg, 1.1 mmol) was then added with the yellow solution
turning dark brown immediately, and the reaction mixture was refluxed for 2 hours under N2.
LiCl (69.9 mg, 1.65 mmol) was then added and the solution was refluxed for an additional 2 h
with air bubbled through the solution. After evaporating the solvent under reduced pressure,
the residue was re-dissolved in CH2Cl2 (3 ml), over-layered ethyl acetate (3 ml) and the
mixture kept in a refrigerator overnight. The precipitated solid was filtered off and washed
with ethyl acetate and diethyl ether. Recrystallization from CH2Cl2/n-hexane yielded pure
[Mn(III)Cl{(R1)2saldach(Im+-R2R3-Cl–)2}] 5a-f.
Chlorido-trans-[[2,2`-][(1,2-cyclohexanediyl) bis(nitrilomethylidyne)] bis[4-((2-methyl-
imidazolium)methylene-phenolato]-[N,N`,O,O`] manganese(III) dichloride (5a): Dark-brown
powder (556 mg, 92 %). FT-IR (KBr, cm-1): 3286 (m, br, ν N(1)-H), 3133 (w, br, νasym CH,
CH, Im and Ph), 2934 (m, sh, ν CH3), 2859 (m, sh, ν CH2), 1618 (vs, sh, ν C=N), 1559, 1472,
1436 (s, sh, ν C=Carom + ν C-H bend), 1348 (m, sh, ν CH2), 1275 (m, sh, ν Ph-O), 1167 (s, sh,
ν HCC + ν HCN bending, Im), 826 (m, sh, ν Mn-N). MALDI-TOF MS, m/z: 710.9 [M+K]+.
Anal. Calcd. for C30H34Cl3MnN6O2 (M = 671.93): C, 53.63; H, 5.10; N, 12.51; Found: C,
53.92; H, 5.02; N, 12.29. Conductivity = 351 ꢀS/cm; µeff = 4.97 µB.
Chlorido-trans-[[2,2`-][(1,2-cyclohexanediyl)
bis(nitrilomethylidyne)]
bis[4-((1-
ethylimidazo-lium)methylene-phenolato]-[N,N`,O,O`]
manganese(III)
dichloride
-1
-
·
monohydrate (5b H2O): Brown powder (575 mg, 89 %). FT-IR (KBr, cm ): FT-IR (KBr, cm
1): 3426 (s, br, ν O-H H2O), 3134 (w, br, νasym CH, CH, Im and Ph), 3078 (m, br, νsym CH, Im
and Ph), 2934 (m, sh, ν CH3), 2858 (m, sh, ν CH2), 1621 (vs, sh, ν C=N), 1558, 1439 (s, sh, ν
11

ACCEPTED MANUSCRIPT
C=Carom + ν C-H bend), 1346 (m, sh, ν CH2), 1273 (m, sh, ν Ph-O), 1152 (s, sh, HCC, HCN
bending, Im), 821 (m, sh, ν Mn-N), 471 (m, sh, ν Mn-O). MALDI-TOF MS, m/z: 722.3
+
.
[M+Na] . Anal. Calcd. for C32H38Cl2MnN6O2 H2O (Mw = 718.00): C, 53.53; H, 5.62; N,
11.70; Found: C, 53.83; H, 5.41; N, 11.68. Conductivity = 335 ꢀS/cm; µeff = 4.95 µB.
Chlorido-trans-[[2,2'-][(1,2-cyclohexanediyl)
bis(nitrilomethylidyne)]
manganese(III)
bis[4-((1-
dichloride
butylimidazo-lium)methylene-phenolato]-[N,N’,O,O’]
-1
·
monohydrate (5c H2O): Reddish-brown powder (599 mg, 86 %). FT-IR (KBr, cm ): 3436 (m,
br, ν O-H H2O), 3129 (w, br, νasym CH, CH, Im and Ph), 2935 (m, sh, ν CH3), 2859 (m, sh, ν
CH2), 1614 (vs, sh, ν C=N), 1547, 1500, 1446 (s, sh, ν C=Carom + ν C-H bend), 1347 (m, sh, ν
CH2), 1272 (m, sh, ν Ph-O), 1152 (s, sh, HCC, HCN bending, Im) , 826 (m, sh, ν Mn-N), 468
+
.
(m, sh, ν Mn-O). MALDI-TOF MS, m/z: 796.8 [M H2O+Na] . Anal. Calcd. for
.
C36H46Cl3MnN6O3 H2O (M = 774.10): C, 55.86; H, 6.25; N, 10.86; Found: C, 56.01; H, 6.26;
N, 10.84. Conductivity = 307 ꢀS/cm; µeff = 4.96 µB.
Chlorido-trans-[[2,2'-][(1,2-cyclohexanediyl) bis(nitrilomethylidyne)]
bis[4-((2-methyl-
dichloride
imidazolium)methylene) -6-(t-Bu-phenolato]-[N,N’,O,O’] manganese(III)
-1
·
monohydrate (5d H2O): Brown solid (657 mg, 91 %). FT-IR (KBr, cm ): 3411 (m, br, ν O-H
H2O), 3294 (m, br, ν N(1)-H), 2995 (w, br, νasym CH, CH, Im and Ph), 2941 (m, sh, ν CH3),
2860 (m, sh, ν CH2), 1616 (vs, sh, ν C=N), 1560, 1475, 1442 (s, sh, ν C=Carom + ν C-H bend),
1356 (m, sh, ν CH2), 1295 (m, sh, ν Ph-O), 1142 (s, sh, ν HCC + ν HCN bending, Im), 820
+
.
(m, sh, ν Mn-N), 465(m, sh, ν Mn-O). MALDI-TOF MS, m/z: 824.9 [M H2O+Na] . Anal.
.
Calcd. for C38H50Cl3MnN6O2 H2O (M = 802.15): C, 56.90; H, 6.53; N, 10.48; Found: C,
57.10; H, 6.43; N, 10.30. Conductivity = 327 ꢀS/cm; µeff = 4.89 µB.
Chlorido-trans-[[2,2'-][(1,2-cyclohexanediyl)
bis(nitrilomethylidyne)]
bis[4-((1-
ethylimidazol- ium)methylene) -6-(t-Bu-phenolato]-[N,N’,O,O’] manganese(III) dichloride
-1
·
monohydrate (5e H2O): Greenish-brown solid (672 mg, 90 %). FT-IR (KBr, cm ): 3388 (m,
br, ν O-H H2O), 2997 (w, br, νasym CH, CH, Im and Ph), 2953 (m, sh, ν CH3), 2860 (m, sh, ν
CH2), 1622 (vs, sh, ν C=N), 1585, 1475, 1440 (s, sh, ν C=Carom + ν C-H bend), 1361 (m, sh, ν
CH2), 1298 (m, sh, ν Ph-O), 1136 (s, sh, ν HCC + ν HCN bending, Im), 819 (m, sh, ν Mn-N),
+
.
463(m, sh, ν Mn-O). MALDI-TOF MS, m/z: 830.6 [M H2O+H] . Anal. Calcd. for
.
C40H54Cl3MnN6O2 H2O (M = 830.21): C, 57.87; H, 6.80; N, 10.12; Found: C, 58.09; H, 6.76;
N, 10.12. Conductivity = 307 ꢀS/cm; µeff = 4.87 µB.
12

ACCEPTED MANUSCRIPT
Chlorido-trans-[[2,2'-][(1,2-cyclohexanediyl)
bis(nitrilomethylidyne)]
bis[4-((1-
butylimidazol- ium)methylene) -6-(t-Bu-phenolato]-[N,N’,O,O’] manganese(III) dichloride
(5f): Faint-brown solid (680 mg, 87 %). FT-IR (KBr, cm-1): 3000 (w, br, ν CH, Im and Ph),
2947 (m, sh, ν CH3), 2877 (m, sh, ν CH2), 1623 (vs, sh, ν C=N), 1558, 1475, 1441 (s, sh, ν
C=Carom + ν C-H bend), 1360 (m, sh, ν CH2), 1293 (m, sh, ν Ph-O), 1178 (s, sh, ν HCC + ν
HCN bending, Im), 825 (m, sh, ν Mn-N), 468 (m, sh, ν Mn-O). MALDI-TOF MS, m/z: 832.3
[M-Cl]+. Anal. Calcd. for C44H62Cl3MnN6O2 (Mw = 868.30): C, 60.86; H, 7.20; N, 9.68;
Found: C, 60.66; H, 7.24; N, 9.55. Conductivity = 291 ꢀS/cm; µeff = 4.86 µB.
2.2.5.2 Synthesis of Fe(III) complexes (6a-f):
A yellow solution of the substituted imidazolium-saldach ligand H2(R1)2saldach(Im+-R2R3-
Cl–)2 4a-f (0.9 mmol) in ethanol (10 mL) was degassed for 15 minutes. An ethanolic solution
.
(5 mL) of FeCl3 6H2O (178 mg, 1.1 mmol) was then added with the yellow solution turning
dark reddish-brown immediately, and the reaction mixture was refluxed for 2 hours under N2.
Then, the solution was concentrated and the residue was kept in a refrigerator overnight. The
precipitated solid was filtered off and washed with cold ethanol (2 x 3mL) and diethyl ether (3
x 3mL) to yield [Fe(III)Cl{(R1)2saldach(Im+-R2R3-Cl–)2}] 6a-f.
Chlorido-trans-[[2,2'-][(1,2-cyclohexanediyl)
bis(nitrilomethylidyne)]
bis[4-((2-
methylimidaz-olium)methylene)-phenolato]-[N,N’,O,O’] iron(III) dichloride dihydrate
-1
·
(6a 2H2O): reddish-brown powder (548, mg, 86 %). FT-IR (KBr, cm ): 3408 (m, br, ν N(1)-
H + ν O-H H2O), 3154 (w, br, νasym CH, CH, Im and Ph), 2933 (m, sh, ν CH3), 2862 (m, sh, ν
CH2), 1617 (vs, sh, ν C=N), 1559, 1441 (s, sh, ν C=Carom + ν C-H bend), 1349 (m, sh, ν CH2),
1279 (m, sh, ν Ph-O), 1166 (s, sh, ν HCC + ν HCN bending, Im), 817 (m, sh, ν Fe-N).
+
.
.
MALDI-TOF MS, m/z: 709.2 [M 2H2O+H] . Anal. Calcd. for C30H34FeN6O2 2H2O (M =
708.86): C, 50.83; H, 5.40; N, 11.86; Found: C, 51.24; H, 5.20; N, 11.48. Conductivity = 349
ꢀS/cm; µeff = 5.84 µB.
Chlorido-trans-[[2,2'-][(1,2-cyclohexanediyl) bis(nitrilomethylidyne)] bis[4-((1-ethylimidaz-
.
olium)methylene)-phenolato]-[N,N’,O,O’] iron(III) dichloride dihydrate (6b 2H2O): Brown
powder (484 mg, 73 %). FT-IR (KBr, cm-1): FT-IR (KBr, cm-1): 3433 (s, br, ν O-H H2O),
3136 (w, br, νasym CH, CH, Im and Ph), 3068 (m, br, νsym CH, Im and Ph), 2935 (m, sh, ν
CH3), 2861 (m, sh, ν CH2), 1622 (vs, sh, ν C=N), 1559, 1507, 1443 (s, sh, ν C=Carom + ν C-H
bend), 1347 (m, sh, ν CH2), 1271 (m, sh, ν Ph-O), 1152 (s, sh, HCC, HCN bending, Im), 832
13

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.
(m, sh, ν Fe-N), 468 (m, sh, ν Fe-O). MALDI-TOF MS, m/z: 758.2 [M 2H2O+Na]. Anal.
.
Calcd. for C32H38Cl3FeN6O2 2H2O (M = 736.92): C, 52.16; H, 5.74; N, 11.40; Found: C,
52.26; H, 5.53; N, 11.11. Conductivity = 331 ꢀS/cm; µeff = 5.80 µB.
Chlorido-trans-[[2,2'-][(1,2-cyclohexanediyl) bis(nitrilomethylidyne)] bis[4-((1-butylimidaz-
·~
olium)methylene)-phenolato]-[N,N’,O,O’] iron(III) dichloride dihydrate (6c 2H2O): Dark-
red powder (507 mg, 71 %). FT-IR (KBr, cm-1): 3432 (m, br, ν O-H H2O), 3132 (w, br, νasym
CH, CH, Im and Ph), 2933 (m, sh, ν CH3), 2861 (m, sh, ν CH2), 1616 (vs, sh, ν C=N), 1545,
1504, 1447 (s, sh, ν C=Carom + ν C-H bend), 1350 (m, sh, ν CH2), 1270 (m, sh, ν Ph-O), 1154
(s, sh, HCC, HCN bending, Im) , 822 (m, sh, ν Fe-N), 463 (m, sh, ν Fe-O). MALDI-TOF MS,
+
.~
.
m/z: 821.5 [M 2H2O+Na] . Anal. Calcd. for C36H46Cl3N6O2 ~2H2O (M = 793.02): C, 54.52;
H, 6.36; N, 10.60; Found: C, 54.11; H, 6.41; N, 10.51. Conductivity = 303 ꢀS/cm; µeff = 5.78
µB.
Chlorido-trans-[[2,2'-][(1,2-cyclohexanediyl)
bis(nitrilomethylidyne)]
bis[4-((2-
methylimidaz-olium)methylene) -6-(t-Bu-phenolato]-[N,N’,O,O’] iron(III) dichloride
-1
·
dihydrate (6d 2H2O): Dark-red solid (584 mg, 79 %). FT-IR (KBr, cm ): 3387 (m, br, ν O-H
H2O), 3293 (m, br, ν N(1)-H), 2995 (w, br, νasym CH, CH, Im and Ph), 2941 (m, sh, ν CH3),
2860 (m, sh, ν CH2), 1619 (vs, sh, ν C=N), 1567, 1478, 1443 (s, sh, ν C=Carom + ν C-H bend),
1358 (m, sh, ν CH2), 1291 (m, sh, ν Ph-O), 1145 (s, sh, ν HCC + ν HCN bending, Im), 801
+
.
(m, sh, ν Fe-N), 442 (m, sh, ν Fe-O). MALDI-TOF MS, m/z: 821.7 [M 2H2O+H] . Anal.
.
Calcd. for C38H50Cl3FeN6O2 2H2O (M = 821.08): C, 55.59; H, 6.63; N, 10.24; Found: C,
56.00; H, 6.71; N, 9.88. Conductivity = 323 ꢀS/cm; µeff = 5.76 µB.
Chlorido-trans-[[2,2'-][(1,2-cyclohexanediyl) bis(nitrilomethylidyne)] bis[4-((1-ethylimidaz-
·
olium)methylene) -6-(t-Bu-phenolato]-[N,N’,O,O’] iron(III) dichloride dihydrate (6e 2H2O):
Dark-red solid (588 mg, 77 %). FT-IR (KBr, cm-1): 3389 (m, br, ν O-H H2O), 3280 (m, br, ν
N(1)-H), 2998 (w, br, νasym CH, CH, Im and Ph), 2953 (m, sh, ν CH3), 2865 (m, sh, ν CH2),
1617 (vs, sh, ν C=N), 1566, 1476, 1445 (s, sh, ν C=Carom + ν C-H bend), 1357 (m, sh, ν CH2),
1296 (m, sh, ν Ph-O), 1144 (s, sh, ν HCC + ν HCN bending, Im), 800 (m, sh, ν Fe-N), 438 (m,
+
.
sh,
ν
Fe-O). MALDI-TOF MS, m/z: 887.6 [M 2H2O+K] . Anal. Calcd. for
.
C40H54Cl3FeN6O2 2H2O (M = 849.13): C, 56.58; H, 6.88; N, 9.90; Found: C, 56.56; H, 6.87;
N, 9.70. Conductivity = 305 ꢀS/cm; µeff = 5.74 µB.
14

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Chlorido-trans-[[2,2'-][(1,2-cyclohexanediyl) bis(nitrilomethylidyne)] bis[4-((1-butylimidaz-
olium)methylene) -6-(t-Bu-phenolato]-[N,N’,O,O’] iron(III) dichloride monohydrate
-1
·
(6f H2O): Dark-red solid, (562 mg, 69 %). FT-IR (KBr, cm ): 3432 (m, br, ν O-H H2O),2997
(w, br, ν CH, Im and Ph), 2948 (m, sh, ν CH3), 2875 (m, sh, ν CH2), 1625 (vs, sh, ν C=N),
1559, 1476, 1443 (s, sh, ν C=Carom + ν C-H bend), 1361 (m, sh, ν CH2), 1291 (m, sh, ν Ph-O),
1179 (s, sh, ν HCC + ν HCN bending, Im), 799 (m, sh, ν Fe-N), 436 (m, sh, ν Fe-O). MALDI-
+
.
.
TOF MS, m/z: 909.9 [M H2O+Na] . Anal. Calcd. for C44H62Cl3FeN6O2 H2O (Mw = 887.22):
C, 59.56; H, 7.27; N, 9.47; Found: C, 59.77; H, 7.65; N, 9.21. Conductivity = 288 ꢀS/cm; µeff
= 5.68 µB.
2.3. Antimicrobial activity
The plate-hole diffusion method was employed for the determination of antimicrobial
activities against the gram (+), gram (−) bacterial and fungal organisms. Broth micro-dilution
method was used to determine the MICs (minimum inhibitory concentrations) for the free
ligands and their complexes in H2O against test organisms. All the tests were performed in
duplicate and repeated twice. Modal values were selected. Each microorganism was seeded in
tube with nutrient broth (NB) (1 cm3) which was then homogenized in the tubes with 9 cm3 of
o
melted (45 C) nutrient Agar (NA). The homogeneous suspensions were poured into Petri
dishes and holes of 4 mm diameter were done in the cool medium. After cooling these holes,
100 ꢀL of the investigated compounds solutions, with serial concentrations, were applied
using a micropipette with the pathogens to be tested against. The plates were incubated for
72h hours at 37 °C for bacteria and 28 °C for fungi, after that the clear zone around the wells
was measured as inhibition zones and the diameter of these zone of inhibition (mm) were
measured accurately. The antibacterial activities were observed and measured using a
transparent meter rule and recorded if the zone of inhibition was ≥10 mm [25]. Ampicillin,
Antibacterial, and Amphotericin B, Antifungal, were employed as standard drugs
3. Results and Discussion
3.1. Chemistry
3.1.1. Ligands Synthesis
The syntheses of the six ligands 4a-f is depicted in Scheme 2. The required starting
materials salicylaldehyde-imidazolium chloride salts 2a,b were synthesized starting from 2-
substituted phenol following modified literature procedures [22,23,26]. Under extra dry
conditions and using anhydrous magnesium dichloride as O-magnesiation reagent, 2-tert-
15

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butylphenol was formylated selectively ortho to the hydroxy group by paraformaldehyde with
triethylamine as base. The substituted salicylaldehydes 1a,b were then one-pot
chloromethylated by reaction with paraformaldehyde and 37% HCl under a stream of HClg in
the presence of catalytic amount of ZnCl2 or tetrabutylammonium bromide at room
temperature to yield 5-chloromethylated salicylaldehydes 2a,b in a high purity. Modifying the
R1-group from hydrogen to a bulky t-butyl group increases the reaction time for
chloromethylation at the 5-position from 1-3 days. Subsequent quarternization of
alkylimidazole with 2a,b generate salicylaldehyde-imidazolium chloride salts 3a-f, which are
the key intermediates in the synthesis of the achiral ligands. The chelating salts, (rac-trans-
H2(R1)2saldach(Im+-R2R3-Cl–)2) 4a-f, containing saldach skeleton were synthesized through
the Schiff-base condensation reaction of compounds 3a-f with (±)-trans-1,2-
diaminocyclohexane (rac-trans dach) in methanolic solution. The rac-trans configuration of
N,N′-bis(salicylidene)-1,2-cyclohexanediamine (saldach) backbone was chosen, because it
forces the ligand into a flat chair (planar) conformation similar to the 1,2-cyclohexanediamine
moiety present in oxaliplatin, a very promising metal-based drug [27]. Ligands were isolated
in high yields, gave satisfactory elemental analysis, and characterized by spectroscopic
techniques: 1H NMR, 13C NMR, FTIR, MALDI-TOF and conductivity measurements.
3.1.2. Synthesis of the Metallosaldach-Imidazolium Chlorides
Mn(III)-saldach-imidazolium chloride, synthesized by reaction of free ligands
H2(R1)2saldach(Im+-R2R3-Cl–)2 (4a-f) and manganese(II) acetate tetrahydrate in 1 : 1 molar ratio
in UV-methanol in a Schlenk line under nitrogen atmosphere followed by aerobic oxidation in
the presence of LiCl, are dark brown glassy solids. Fe(III) complexes 6a-f were obtained by
the reaction of equimolar free saldach ligands with iron(III) chloride hexahydrate under
anaerobic conditions (Scheme 3). All the synthesized compounds are soluble in water and
most organic solvents. The resulting chelate complexes are highly soluble in common organic
solvents and were fully characterized using the elemental analyses and spectroscopic
techniques FTIR, UV-Vis, MALDI-TOF, ESI-MS as well as conductivity and magnetic
measurements.
Unfortunately, all attempts to obtain X-ray diffraction quality single crystals of the
Mn(III)- and Fe(III)-saldach-imidazolium chlorides ((5,6)a-f) were unsuccessful.
Nevertheless, the metal-ligand structures suggested in this work (based upon elemental and
16

ACCEPTED MANUSCRIPT
spectral analysis) match with the structures of reported metal-saldach analogues (Table S1,
supporting information).
3.2. Characterization of free ligands and their complexes
The molar conductance values of the ligands and their complexes ranging from 220-284
and 288-351 ꢀS/cm, respectively, reveal their electrolytic nature.
3.2.1. 1H NMR studies
1
The most remarkable feature of the H NMR spectra for all ligands is the down field
singlet at 13.75-13.30 ppm which is assigned to the resonance of phenol OH [28,29]. The N-H
of the imidazolium moiety in 4a,d appears as a singlet at 9.48 and 10.05 respectively. The
signal for the azomethine (CH=N) proton is observed at δ 8.68-8.28 ppm. Also, all saldach-
imidazolium salts have two sets of resonance signals located in the aromatic region of the
NMR spectrum (7.58-6.84 and 7.79-7.38 ppm) assigned to phenyl and imidazole protons,
respectively. The signal set in the high-field region (4.00-3.43 and 1.85-1.42 ppm) is ascribed
to the aliphatic cyclohexyl protons. To the best of our knowledge, the central saldach
backbone is in the O-protonated tautomeric form not in the N-protonated tautomeric form (cf.
Scheme 1) [32].
3.2.2. FTIR spectra
The selected IR spectra of the Schiff bases and their metal complexes along with their
tentative assignments are reported in Table S2 (supporting information). In order to identify
the IR signatures of the central saldach backbone, the bands caused by the terminal
imidazolium compartments of saldach-imidazolium chloride have to be identified. The
characteristic functionalities of saldach-imidazolium ligands include the hydrogen bonded
hydroxyl groups, the azomethine groups (-C=N-) and the terminal imidazolium
compartments. These functional groups were identified in by comparing the spectroscopic
data with literature values of similar systems. Broad peaks were observed around ~ 3400 cm-1
in their IR spectra which strongly suggested the presence of intramolecular hydrogen bonding
OH group. Additionally, a band at 1280-1253 cm−1 was assigned to a mixed ν(Ar-O).
H2saldach and H2(2-tBu)2saldach exhibit a C=N stretch at 1639 and 1641 cm−1,
respectively [30,31]. Introduction of an imidazolium group in (4a-f) results in a low-energy
shift to 1635-1629 cm−1. This low-energy shift of the azomethine band by introduction of
imidazolium group reflects the C=N bond order reduction induced by the (-I) effect of
17

ACCEPTED MANUSCRIPT
quaternary N-atom and hydrogen bond with o-hydroxyl group [32]. The vibrational bands at
~846 and ~760 cm-1 should be assigned to the in-plane and out-of plane flexural vibration
mode of the imidazolium ring respectively [33].
Formation of the complexes and their possible geometries were assigned on the basis of
the electronic and infrared spectral studies, where in the absence of more powerful techniques
such as X-ray, the FTIR data confirm its usefulness in the characterization of the coordination
environments of these complexes ((5,6)a-f). So the study of the IR spectral data was quite
informative in characterizing the metal-saldach binding modes.
All complexes exhibit very similar infrared spectral features. When comparing the
spectroscopic data of Mn(III) and Fe(III) complexes with those of the free ligands, marked
changes may be noticed in the ligand bands arising from various modes of donor groups
involved in bonding to manganese and iron ions (Figure 1). The diagnostic IR spectral bands
with their assignments of the free ligands and their metal complexes are shown in (cf Table
S1, supporting information). The IR stretching bands due to azomethine ν(HC=N) register a
red-shift of 11-15 and 10-17 cm-1 in the manganese(III)- and iron(III)-saldach-imidazolium
complexes respectively (see Table 1) thus, indicating the coordination of the azomethine
nitrogen to Mn(III) and Fe(III) ions [34], which is further confirmed by the appearance of a
new band in the spectra of chelates at 543–570 cm-1 assigned to a νM(III)–N vibration (M=
Mn, Fe) [35].
Strong bands attributable to phenolic oxygen ν(Ar–O) are evident ; these all show a shift
of 9-17 and 7-21 cm-1 to higher energy in the manganese(III)- and iron(III) complexes from
the free ligand values indicative of co-ordination to Mn(III) and Fe(III) respectively [35a].
The new IR band (456-475 cm-1) is assigned to M-O vibrations [35a,36] and supports the
ligation of the phenolate oxygen to metal ions. Moreover the disappearance of the phenolic
O–H vibration, originally found in the saldach-imidazolium salts, indicates that it is
deprotonated in complexes.
The bands ascribed to the vibration of the imidazole ring remain unchanged in the Mn(III)
and Fe(III) complexes revealing that, the nitrogen atom of the imidazole moiety does not take
part in metal coordination [37]. Finally, very broad bands centered at ca. 3430 and 3386 and
3424 cm-1 in some complexes can be assigned to the ν (OH) modes of lattice water in the
corresponding framework [38].
18

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These IR spectral studies reveal that the ligand coordinates to the metal ion in a
tetradentate fashion through the deprotonated phenolate oxygen and azomethine nitrogen
atoms (i.e with NONO (N2O2) donor sites).
3.2.3. Electronic Absorption Spectroscopy and magnetic susceptibility
The UV/Vis electronic spectra of the M(III)-saldach-imidazolium chloride complexes (M
= Mn, Fe) (5a-f) (Table S3, supporting information) exhibit similar features, where all display
a shoulder and three intense absorption peaks at ranges (266-272, 306-311, 393-395, 500-508)
and (258-263, 299-304, 381-384, 504-511) for Mn(III) and Fe(III) complexes, respectively,
which are assigned to the intra- and inter-ligand charge transfer and d-d transition. The higher
energy band (< 300 nm) is due to intra-ligand π→π* transitions (π→π*, LLCT), (phenyl and
C=N moieties) [39]. Also higher energy bands (300-500 nm) are attributable to n→π*
(phenolic oxygen) intra-ligand transitions. The most important feature in the near-UV region
of the M(III)-saldach-imidazolium chloride is the higher wavelength band at (500-511 nm).
These bands indicate the coordination of metal ion with the ligands, and this can be assigned
to a charge-transfer band (MLCT), a transition from pπ orbitals on the phenolate oxygen atom
to the dπ* orbitals of M(III) [40]. These bands in addition to a low intensity broad absorption
bands (that may appear as shoulders > 600 nm) assigned to the three allowed d–d transitions
expected for complexes with a square pyramidal geometry, (dxz→dx2-y2), (dxy→ dx2-y2, dyz) and
(dz2→dx2-y2) [41]. The magnetic moment values for the complexes (4.86-4.97 µB for Mn(III)
saldach and 5.68-5.84 µB Fe(III) saldach) are also supportive of square pyramidal geometry
with high spin metal centers.
One can clearly observe the effect of the substitution of the H-atom in
[M(III)Cl{(H)2saldach(Im+-R2R3-Cl–)2}] (M= Mn, Fe; (5,6)a-c) with the tert-butyl group
[M(III)Cl{(tBu)2saldach(Im+-R2R3-Cl–)2}] (M= Mn, Fe;(5,6)d-f) upon UV absorption. The
intensity of the two higher energy bands increased considerably as a result of enhancement
the inter- and intra-ligand charge transfers. Also, the substitution shifts the absorption energy
bands gradually to longer wavelengths. This effect is indicative of ligand π-dπ interactions
between the saldach-imidazolium systems and the metal ion, which destabilize the ligand π
molecular orbitals and decrease the π-π* energy gap, thus producing a bathochromic shift.
3.3. Antimicrobial Screening
3.3.1. Antibacterial activity
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The free ligands (saldach-imidazolium chlorides, 4a-f), their metal complexes (Mn(III)-
saldach-imidazolium chlorides, 5a-f and Fe(III)-saldach-imidazolium chlorides, 6a-f) and the
standard drug Ampicillin (C16H19N3O4S, 349.41 g·mol−1) were screened separately for their
antibacterial activity against the bacteria Staphylococus aureus (S. aureus, ATCC-25923) and
Bacillus subtilis (B. subtilis, Re-cultured) (as gram-positive bacteria) and Pseudomonas
aeruginosa (P. aeruginosa, ATCC-27853) and Eschercia coli (E. coli, ATCC-25922) (as
gram-negative bacteria) at 2.5, 5, 10 and 20 mg/mL concentrations. The diffusion agar
technique was used to evaluate the antibacterial activity of the synthesized compounds [42].
As a parameter of antibacterial activity, the MIC values were determined from the
percentages of inhibition at four different concentration levels. From the bactericidal activity
data Table S4 (supporting information), it has been observed that the ligand as well as its
complexes shows a significant degree of antibacterial activity against G+ bacteria (S. aureus,
B. subtilis) and E. coli from G- bacteria while slight activity or no activity toward P.
aeruginosa. Generally it is apparent that, all compounds were more toxic towards gram
positive strains than gram negative strains. This could be ascribed to their cell-wall structural
differences, where the walls of gram negative species are more complex than those of gram
positive cells, so it might be difficult for the compounds to diffuse inside the G--bacterial cell.
The zones of inhibition (ZOI) values are susceptible to the concentration of the compound
used for inhibition (Figure S1, supporting information). The activity is greatly enhanced at the
higher concentration. The ZOI values obtained indicate that the ligands have moderate to high
activities as compared to Ampicillin drug towards the infection organisms (S. aureus, B.
subtilis, E. coli). N,N'-Bis[3-tert-butyl-5-((2-methylimidazolium)methylene)-salicylidene)]-
trans-1,2-cyclohexane-diamine dichloride (4d) and its Fe(III) complex (6d) exhibit
remarkable extra-potent bactericidal activity (Figure 2), higher than the standard drug.
Moreover, their MIC values are quite small. The MIC values Table 2 for ligand (4d) against
S. aureus, B. subtilis and E. coli were 14.88, 12.14, and 13.30 µg/mL while for complex (6d)
assigned 8.74, 10.33, and >250 µg/mL and, thus, can be classified as a new good candidate in
the fight against bacterial infections.
Nature of the cell wall, the ligand, coordination sites, geometry of the compound, the
positive charge density, hydrophilicity, lipophilicity, presence of co-ligand, pharmacokinetic
factors, etc. also play decisive roles in determining antimicrobial activity of the Schiff base
and its metal complexes [43]. These factors may affect the Fight against bacterial infections in
two different ways: (i) interactions of compound with microbial cell wall, whereas the
20

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cationic charges on the imidazolium enhance the bactericidal activity, which is partially
credited to the electrostatic attraction between the positively charged ligand or complex and
negatively charged cells walls [43]. (ii) Ability of tested compound to penetrate cell walls by
simply dissolving into and through the lipophilic cell wall, recent results indicate that
hydrophobic groups may increase the antimicrobial activity [44]. The variation in the
antimicrobial activity of different compounds against different microorganisms depends on
their impermeability of the cell or the differences in ribosomes in microbial cell [45]. The
lipid membrane surrounding the cell favors the passage of any lipid soluble materials and it is
known that lipophilicity is an important factor controlling antimicrobial activity [46]. In the
present study, low activity of the some metal complexes is due to their low lipophilicity,
because of which penetration of the complex through the lipid membrane was decreased and
hence, they could neither block nor inhibit the growth of the microorganism.
From the data it has been also observed that the activity depends upon the type of metal
ion and varies in the following order of the metal ion: Fe > Mn.
3.3.2. Proposed mechanisms of bactericidal action
A major component of a bacterial cell wall is peptidoglycan, that is, polysaccharide chains
of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) molecules that
are cross-linked by peptide bonds between NAM subunits [47].
There are no recent studies of the mechanisms of bactericidal action of Schiff bases.
Earlier work had suggested that the bacterial cell wall was a major target site [43b], especially
the major wall component, peptidoglycan. Where the Schiff base and its complex indulge in
the formation of hydrogen bonded interaction through the coordinated anion, hydroxyl group,
etc. with the active centers of the microbial cell constituents resulting in interference with the
normal cell processes.
Also saldach-imidazolium chlorides and their complexes may inhibit the peptidoglycan
formation by irreversibly binding to the enzymes that cross-link NAM subunits and
preventing the cross-linkage of NAM subunits (Figure 3).
3.3.3. Antifungal activity
The free ligands (4a-f), M(III) complexes ((5,6)a-f) and standard Antifungal drug
(Amphotericin B, C47H73NO17) were subjected for in vitro antifungal studies against
Aspergillus flavus (A. flavus) Candida albicans (C. albicans, NCIM No. 3100) at 2.5, 5, 10
21

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and 20 mg/mL concentration. From the experimental data Table S4 (supporting information),
all saldach-imidazolium chlorides and their Fe(III) salts exhibited moderate antifungal activity
against C. albicans infection, and the ZOI values slight increase as the concentration of tested
compound raised from 2.5 to 20 mg/mL. However, they could not inhibit the growth of the A.
flavus. None of the Mn(III)-saldach-imidazolium chloride salts has shown any observable
activity against each fungal species. All tested compounds are infective against fungal strains.
These limited or lack of fungicidal activity could be attributed to two possibilities: (i) the
complex structure of fungal cell-wall, composed typically of chitin, 1,3-β- and 1,6-β-glucan,
mannan and proteins [48], which could neither diffuse nor decrease the rate of diffusion of
tested compound through. (ii) Fungal fighting proceed by much more complex
mechanisms than bacterial conflict. Compound (4d) was identified as the most active against
C. albicans.
Acknowledgment.
This work was in part supported by DFG grant Ja466/24-1 (initiation of bilateral cooperation
RFME-CJ).
Appendix A. Supplementary material
Supplementary data (FT-IR spectral data, UV/Vis Spectral Data and antimicrobial evaluation)
associated with this article can be found, in the online version, at doi: 10.1016/ j.ejmech.
XXXXXXXX
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Tables Captions
Table 1: Assignment of the vibrations that are Responsible for the FTIR from the free saldach
ligands (4a-f) and their complexes ((5,6)a-f).
Table 2: Minimum inhibitory concentration (MIC) profiles of the saldach-imidazolium
chlorides and their M(III) complexes against different strains
27