Syntheses, structures and antimicrobial activities of bis(imino) acenaphthene (BIAN) imidazolium salts

Article abstract of DOI:10.3390/molecules16043168

The syntheses of four new bis(imino)acenaphthene (BIAN) imidazolium chlorides are reported, three of which have been structurally characterized. The synthesis of a new, structurally authenticated BIAN ligand is also described. We report the results of the

Full text of DOI:10.3390/molecules16043168

Molecules 2011, 16, 3168-3178; doi:10.3390/molecules16043168
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Syntheses, Structures and Antimicrobial Activities of
bis(Imino)acenaphthene (BIAN) Imidazolium Salts
Rachel R. Butorac 1, Salem S. Al-Deyab 2 and Alan H. Cowley 1,*
1
Department of Chemistry and Biochemistry, University of Texas at Austin, 1 University Station,
A5300, Austin, TX 78712, USA
2
Chemistry Department, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451,
Saudi Arabia
* Author to whom correspondence should be addressed; E-Mail: cowley@mail.utexas.edu;
Tel.: +1-512-471-7478.
Received: 25 March 2011; in revised form: 11 April 2011 / Accepted: 12 April 2011 /
Published: 15 April 2011
Abstract: The syntheses of four new bis(imino)acenaphthene (BIAN) imidazolium
chlorides are reported, three of which have been structurally characterized. The synthesis
of a new, structurally authenticated BIAN ligand is also described. We report the results of
the use of these BIAN imidazolium salts as antimicrobials against the pathogens S. aureus,
B. subtilis, E. coli and P. aeruginosa. The antimicrobial efficacies were particularly high
for the N-(2,6-diisopropylphenyl)- and N-(mesityl)- substituted BIAN imidazolium salts
(MIC values < 0.6 μg/mL).
Keywords: imidazolium chloride; BIAN; synthesis; X-ray structure; antimicrobial activity
1. Introduction
It has been known for over a century that quaternary ammonium compounds (QACs) possess
antibacterial properties [1]. More recently [2] it has been recognized that an important aspect of the
initial attack of a bacterial cell by the QAC involves an electrostatic interaction with the phospholipid
bilayer. In turn, this interaction can compromise the integrity of the cytoplasmic membrane, thus
causing respiratory inhibition and intracellular coagulation [3]. Given the electrostatic nature of the
initial phase of the biocidal process, interest has been generated in the synthesis and antimicrobial

Molecules 2011, 16
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properties of other cationic nitrogen containing molecules and polymers [4]. Generally speaking, small
molecule antimicrobial agents are excellent in terms of cell wall diffusion capability [3].
The present work is focused on the antimicrobial behavior of imidazolium salts. Much of the early
work in this area was descrbed by Pernak et al. [5]. For example, these authors noted a quantitative
correlation between the minimum inhibitory concentration (MIC value) [6], the critical micelle
concentration and hydrophobicity in the case of 3-alkylthiomethyl-1-ethylimidazolium chlorides. In
subsequent work [7] with 3- alkoxymethyl-1-methylimidazolium salts, it was shown that the
compounds with 10, 11, 12, and 14 carbon atoms associated with the alkoxy group exhibited high
antimicrobial activities. The MIC values ranged from 1.13–11.9 μg/mL for the C10H21-substituted salt
to 0.69–46 μg/mL for the C12H25 analogue (for reference, the MIC value for AgNO3 is between 0.1 and
0.05 μg/mL) [8]. The foregoing results were independent of the type of anion employed.
More recently, Lee et al. [9] synthesized and studied two comparable series of antimicrobial
compounds, namely 1-alkyl-3-hydroxyethyl-3-methylimidazolium chlorides and 1-alkyl-3-methyl-
imidazolium halides. The chloride and bromide salts of the latter class of compounds were found to be
the most effective antibacterial agents and the preferred alkyl chain lengths fell in the range C12 – C16.
For example, the MIC values for the C14H29-substituted chloride and bromide salts were 4 μg/mL
against E. coli, S. aureus and B. subtilis. In contrast to the foregoing, the antimicrobial activity of the
functional polymer poly[1-vinyl-3-(2-sulfoethylimidazolium betaine)] (Figure 1) was found to be
sensitive to the nature of the counteranion [10]. The anion dependence in this case is attributable to the
anion-dependent changes in the alignment of the polymer chains.
Figure 1. Structure of poly[1-vinyl-3-(2-sulfoethylimidazolium betaine)].
-
-
X- = OH-, SH-, SCN-, Cl-, F-, BF4 , CH3COO-, NO3
Çetinkaya et al. [11] have examined the antimicrobial activities of several saturated analogues of
imidazolium salts, namely 1,3-diorganylimidazolidinium salts. These authors discovered that, while
there was a dependence of activity on the length of the n-alkyl chains, the attachment of mesityl or
mesitylmethyl substituents to the N atoms resulted in the most significant increases of activity towards
both Gram-positive and Gram-negative bacteria. Interestingly, however, in contrast to the work of
Pernak et al. [2] the activities of these derivatives were found to depend on the identity of the counter
anion. For example, the MIC value for the mesitylmethyl chloride salt was 3.12 μg/mL.
In the present manuscript, we describe a new class of potential antimicrobial agent, namely
bis(imino)acenaphthene (BIAN) imidazolium salts. The BIAN class of ligand, which features the
fusion of a naphthalene moiety to a diimine, has a number of distinctive properties, one of which is
facile redox behavior. To the best of our knowledge, there is only one reference [12] to the use of

Molecules 2011, 16
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BIAN-supported antimicrobial compounds, namely the neutral compounds bis[N-(2,6-
diisopropylphenyl)imino]acenaphthene (Pr-BIAN), [Co(Pr-BIAN)Cl2], [Co(OAc)2(Pr-BIAN)2][ClO4],
[Ni(Pr-BIAN)(NO3)2] and [Ni(Pr-BIAN)2][ClO4]2. The most active compounds against S. aureus, E.
coli, and C. albicans were found to be [Co(Pr-BIAN)Cl2], [Co(OAc)2(Pr-BIAN)2] and Pr-BIAN. The
MIC values for these compounds fell within the range 1.7–2.6 mg/mL. Interestingly, bis[N-(p-
tolylphenyl)imino]acenaphthene, the other BIAN ligand that was tested, was found to be inactive, thus
suggesting that the antimicrobial activity is dependent upon the steric bulk of the N-substituents. In a
previous publication we have discussed the antimicrobial behavior of BIAN-N-heterocyclic carbene
complexes of silver and gold [13].
2. Results and Discussion
2.1. Syntheses of L, 2, 3, 4 and 5
The new 4-n-hexyl(BIAN) ligand (L) shown in Figure 2 was prepared by treatment of
acenaphthenequinone with 4-n-hexylaniline. Ligand L, along with the known ligands dipp(BIAN),
mes(BIAN), p-MeO(BIAN) and p-F(BIAN) were converted into the corresponding imidazolium salts
by treatment with methoxy(methyl)chloride. Details of these syntheses are provided in the
Experimental Section.
2.2. X-ray Crystal Structures of L, 2, 4 and 5
The molecular structures of L, 2, 4 and 5 are presented as ORTEP diagrams in Figures 2-5. A
summary of X-ray data collection details appears in Table 1 and a selection of pertinent metrical
parameters for 2, 4 and 5 is available in Table 2. Unfortunately, despite several attempts, it was not
possible to grow suitable crystals of the p-MeO-substituted analogue 3. As expected, given the rigidity
of the BIAN framework, the bond distances and angles for 2, 4 and 5 (Table 2) are very similar to each
other and, in turn, to those already reported for IPr(BIAN) (1) [14]. There are, however, differences in
the counter-anions in the sense that 2 and 5 crystallize with a hydrogen dichloride anion rather than a
chloride anion. Although somewhat rare, salts of this anion have been prepared by e.g. the reaction of
crown ethers with hydrogen chloride, water and 18-crown-6 in toluene and the reaction of an alkali-
metal chloride with HCl in aqueous media [15,16].
Figure 2. ORTEP diagram of L with 50% probability thermal ellipsoids. Hydrogen atoms
are omitted for clarity. L lies on a two-fold axis (-x, y, 3/2-z) and as a result, N1i is
equivalent to N1.

Molecules 2011, 16
Figure 3. ORTEP diagram of 2 with 50% probability thermal ellipsoids. Hydrogen atoms
3171
except H1 and H34 are omitted for clarity.
Figure 4. ORTEP diagram of 4 with 50% probability thermal ellipsoids. Hydrogen atoms
except H1 are omitted for clarity.

Molecules 2011, 16
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Figure 5. ORTEP diagram of 5 with 40% probability thermal ellipsoids. Hydrogen atoms
except H1 are omitted for clarity. The hydrogen atom of the anion [HCl2]- was not detected
in the difference map.
Table 1. Selected crystal data, data collection and refinement parameters for compounds L, 2, 4 and 5.
L
2
4
5
Formula
Formula weight
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
C36H40N2
500.70
Monoclinic
C2/c
31.987(3)
9.0070(9)
10.3906(10)
90.0
C33H35Cl3N2O
581.98
Monoclinic
P21/c
12.7377(12)
14.4165(13)
16.2893(14)
90.0
C26H17Cl3F2N2
501.77
Monoclinic
P21/c
10.104(3)
24.514(7)
9.243(2)
90.0
C37H42Cl2N2
585.63
Monoclinic
P21/c
17.047(4)
12.711(3)
14.823(4)
90.0
β/°
γ/°
105.809(4)
90.0
91.280(3)
90.0
94.103(3)
90.0
90.142(5)
90.0
Z
4
4
4
4
Dc/g cm-3
F(000)
1.155
1080
1.293
1224
1.459
1024
1.211
1248
Crystal size/nm
Θ range/°
Collected reflections 29622
0.24 × 0.18 × 0.06 0.16 × 0.14 × 0.11 0.26 × 0.16 × 0.14 0.23 × 0.20 × 0.09
3.00–27.48
3.00–25.00
24514
2.02–27.50
25427
2.99–25.00
19946
Independent reflns
Rint
R1 [I>2σ(I)]
wR2 (all data)
3304
5172
5213
5644
0.0602
0.0502
0.1226
0.1143
0.0529
0.1375
0.0583
0.0567
0.1785
0.0808
0.0878
0.2421

Molecules 2011, 16
Table 2. Selected bond distances (Å) and angles (°) for compounds 2, 4 and 5.
3173
Bond distances/Å 2
4
5
C(1)-N(1)
C(1)-N(2)
C(2)-C(3)
C(2)-N(1)
1.351(3) 1.347(3)
1.346(3) 1.345(3)
1.368(4) 1.371(3)
1.375(3) 1.383(3)
1.390(3) 1.383(3)
1.339(5)
1.341(5)
1.350(6)
1.386(5)
1.385(5)
5
C(3)-N(2)
Bond angles/°
N(1)-C(1)-N(2)
N(1)-C(2)-C(3)
C(2)-C(3)-N(2)
C(1)-N(1)-C(2)
C(1)-N(2)-C(3)
2
4
109.5(2) 109.08(18) 110.0(4)
107.8(2) 107.45(19) 107.2(4)
107.3(2) 107.10(19) 108.3(4)
107.8(2) 108.03(18) 107.5(4)
107.6(2) 108.33(17) 106.9(4)
2.3. Antimicrobial Testing of Compounds 1-5
In the present work, we report the results of the use of BIAN imidazolium salts as antimicrobials.
As mentioned in the Introduction, the previous use of BIAN ligands was focused on neutral species
and N-heterocyclic carbene derivatives thereof [13]. Since the initial phase of the attack of bacterial
cell walls by antimicrobials is believed to be electrostatic in nature [3], it was thought that the use of
cationic BIAN species might prove to be advantageous.
It is clear from the work of El-Ayaan et al. [12] and Çetinkaya et al. [11] that the nature of the
nitrogen substituents on the cation can play an important role in terms of antimicrobial activity.
Accordingly, we chose to compare the activities of IPr(BIAN) imidazolium chloride 1 and mes(BIAN)
imidazolium chloride 2. The p-MeO(BIAN) and p-F(BIAN) imidazolium chlorides 3 and 4 were
selected in order to explore the sensitivity of antimicrobial activity to the presence of electron donating
and electron withdrawing groups, respectively on the nitrogen substituents. Finally, the 4-n-
hexyl(BIAN) imidazolium chloride 5 was chosen in order to probe the possible influence of the
presence of long alkyl chains on the para positions of the N-aryl groups.
Table 3. Minimum inhibitory concentrations (MIC) for imidazolium salts 1-5 in μg/mL.
Microbial spp.
Compound
S. aureus B. subtilis E. coli
P. aeruginosa
39.06
39.06
1
2
3
4
5
<0.6
<0.6
1.22
19.53
9.76
<0.6
<0.6
4.88
39.06
9.76
39.06
78.12
312.55
>312.55 >312.55
>312.55 >312.55
>312.55
Based on the MIC values presented in Table 3, the relative activities of the five imidazolium salts
against the four pathogens tested can be summarized a follows:
S. aureus
B. subtilis
E. coli
1 = 2 > 3 > 5 > 4
1 = 2 > 3 > 5 > 4
1 > 2 > 3 > 4 = 5
1 = 2 > 3 = 4 = 5
P. aeruginosa

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The most obvious features of the antimicrobial efficacy data presented in Table 3 are the high
activities of dipp(BIAN) imidazolium chloride 1 and mes(BIAN) imidazolium chloride 2 toward the
Gram-positive bacteria S. aureus and B. subtilis. In both cases, the MIC values are < 0.6 μg/mL. It is
also noteworthy that, while the MIC values are larger for the Gram-negative bacteria E. coli and P.
aeroginosa, 1 and 2 are still the most active imidazolium salts. The lower bactericidal activity toward
the Gram-negative bacteria is presumably a consequence of their more complex, less porous cell wall
structures [3]. There is also an interesting parallel with the results of Çetinkaya et al. [11] in the sense that
these authors noted that the attachment of mesityl or methylmesityl groups to both nitrogen atoms of
imidazolidinium salts resulted in high activities against both Gram-positive and Gram-negative bacteria.
The importance of a positive charge on BIAN-supported antimicrobials is highlighted by the report
that the MIC values for the neutral compound Pr-BIAN against S. aureus, E. coli and C. albicans are
2.1, 2.0 and 2.3 mg/mL, respectively [12]. The fact that the MIC values for p-MeO(BIAN) 3 against S.
aureus and B. subtilis are less than those for p-F(BIAN) 4 may be due to the stabilization of 3 by
electron donation from the methoxy substituent.
3. Experimental
3.1. General
All glassware was oven-dried before use. All reagents were obtained commercially and used as
received. The neutral ligands mes(BIAN) [17], p-F(BIAN) [18], and p-MeO(BIAN) [19] and the salt
IPr(BIAN) imidazolium chloride (1) [14] were prepared according to literature procedures.
3.2. Physical measurements
Low-resolution CI mass spectra were obtained on a Thermo Scientific TSQ Quantum GC mass
spectrometer and high-resolution CI mass spectra were recorded on a magnetic sector Waters Autospec
1
13
Ultima instrument. H- and C{1H}-NMR spectra were recorded at 295 K on a Varian INOVA 500
13
spectrometer (1H, 500 MHz; C, 125 MHz) immediately following sample preparation. Deuterated
chloroform, deuterated methylene chloride and deuterated dimethyl sulfoxide were obtained from
Cambridge Isotopes and stored over 4 Å molecular sieves prior to use. The 1H- and 13C{1H} chemical
shift values are reported in parts per million (ppm) relative to SiMe4 (δ 0.00), using the solvent
resonance as the internal standard.
3.3. Preparations
4-n-hexyl(BIAN) ligand (L). Acenaphthenequinone (0.2 g, 1.098 mmol) and 4-n-hexylaniline
(0.47 mL, 2.41 mmol) were added to a 100 mL round bottom flask. Glacial acetic acid (20 mL) was
added slowly. The yellow-orange suspension was refluxed for 2 h and then cooled to room
temperature. The mixture was filtered and washed with glacial acetic acid (2 × 15 mL) and finally
diethyl ether (3 × 20 mL). The resulting orange solid was added to a 100 mL round bottom flask and
refluxed for 2 h with 15 mL of a K2CO3 solution (10 g of K2CO3 in 15 mL H2O). The mixture was
cooled to room temperature and the aqueous layer was extracted with methylene chloride (3 × 20 mL).
The organic layer was washed with water (3 × 15 mL), following which the resulting solution was

Molecules 2011, 16
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dried over MgSO4 and evaporated to dryness to give a yellow-orange crystalline analytically pure
powder (0.398 g, 72.4%). Crystals of L suitable for X-ray diffraction experiments were obtained from
a saturated THF/hexanes solution that had been stored at ambient temperature for 7 days. MS (CI+,
CH4): m/z 501 [M+H]+ (+ H); HRMS (CI+, CH4): calcd for C36H40N2 m/z 500.3191; found, 500.3171;
1H-NMR (CDCl3): δ 0.90 (t, 6H, -CH3), 1.32–1.40 (m, 12H, -CH2-CH2CH2-), 1.69 (quin, 4H, -CH2),
2.67 (t, 4H, -CH2), 6.86 (d, 2H, Naph-H), 7.02 (d, 4H, Ar-H), 7.24 (d, 4H, Ar-H), 7.33 (t, 2H, Naph-
13
H), 7.85 (d, 2H, Naph-H); C-NMR (CDCl3): δ 14.07, 22.63, 28.95, 31.47, 31.72, 35.50, 118.13,
123.81, 127.51, 128.70, 128.73, 129.23, 131.18, 139.01, 141.65, 149.42, 161.29; m.p.: 136–138 °C.
Mes(BIAN) imidazolium chloride (2). Mes(BIAN) (0.200 g, 0.48 mmol) and methoxy(methyl)chloride
(3.22 g, 40.0 mmol) were added to an argon-flushed thick-walled reaction vessel. The vessel was
sealed and the reaction mixture was stirred at 100 °C for 16 h, during which time the appearance of the
mixture changed from a murky black suspension to a clear dark-red solution. Cooling of the reaction
mixture to ambient temperature and the subsequent addition of 10 mL of diethyl ether resulted in the
formation of a yellow precipitate. The resulting solid was filtered off and washed with 50 mL of
diethyl ether and dried in vacuo to afford 2 as an analytically pure yellow powder (0.186 g, 83.4%).
Crystals of 2 suitable for X-ray diffraction experiments were isolated from the reaction mixture and
stored in anhydrous diethyl ether. MS (CI+, CH4): m/z 429 [M+H]+ (-Cl); HRMS (CI+, CH4): calcd for
C31H29N2 m/z 429.2331; found, 429.2325; 1H-NMR (CDCl3): δ 2.30 (s, 12H, Ar-CH3), 2.40 (s, 6H, Ar-
CH3), 7.12 (s, 4H, Ar-H), 7.27 (d, 2H, Naph-H), 7.54 (d of d, 2H, Naph-H), 7.95 (d, 2H, Naph-H),
11.37 (s, 1H, C-H); 13C-NMR (CDCl3): δ 17.89, 21.26, 122.81, 123.50, 128.14, 129.74, 129.83,
130.09, 130.21, 134.13, 136.58, 141.37, 142.52; m.p. (decomp): 256–259 °C.
p-MeO(BIAN) imidazolium chloride (3). Compound 3 was prepared in a similar fashion to that
described for 2 starting with p-MeO(BIAN) (0.216 g, 0.55 mmol) and methoxy(methyl)chloride
(3.22 g, 40.0 mmol). The resulting olive green precipitate was filtered off and washed with 50 mL of
diethyl ether and dried in vacuo to afford 3 as an analytically pure olive green powder (0.220 g,
90.5%). MS (CI+, CH4): m/z 405 [M+H]+ (-Cl); HRMS (CI+, CH4): calcd for C27H21N2O2 m/z
405.1603; found, 405.1600; 1H-NMR [(CD3)2SO]: δ 3.93 (s, 6H, -CH3), 7.36 (d, 4H, Ar-H), 7.71 (d of
d, 2H, Naph-H), 7.84 (d, 2H, Naph-H), 8.04 (d, 4H, Ar-H), 8.15 (d, 2H, Naph-H), 10.11 (s, 1H, C-H);
13C-NMR [(CD3)2SO]: δ 55.83, 115.53, 122.86, 123.57, 125.22, 126.72, 128.27, 129.40, 129.50,
130.06, 135.05, 138.43, 160.70; m.p. (decomp): 193–195 °C.
p-F(BIAN) imidazolium chloride (4). Compound 4 was prepared in a similar fashion to that described
for 2 starting with p-F(BIAN) (0.211 g, 0.572 mmol) and methoxy(methyl)chloride (3.22 g,
40.0 mmol). The resulting yellow precipitate was filtered off and washed with 50 mL of diethyl ether
and dried in vacuo to afford 4 as an analytically pure yellow powder (0.229 g, 95.8%). Crystals of 4
suitable for X-ray diffraction experiments were grown from a saturated CH2Cl2/hexanes solution
stored at −40 °C. MS (CI+, CH4): m/z 381 [M+H]+ (-Cl); HRMS (CI+, CH4): calcd for C25H15N2F2 m/z
1
381.1203; found, 381.1195; H-NMR (CD2Cl2): δ 7.49 (d, 4H, Ar-H), 7.68 (t, 2H, Naph-H), 7.84 (d,
2H, Naph-H), 8.08 (d, 2H, Naph-H), 8.38 (d, 4H, Ar-H), 11.23 (s, 1H, C-H); 13C-NMR [(CD3)2SO]: δ

Molecules 2011, 16
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117.55, 117.74, 122.63, 123.93, 126.36, 126.43, 128.35, 129.43, 129.46, 130.34, 135.15, 139.31,
161.89, 163.87; m.p. (decomp): 274–276 °C.
4-n-hexyl(BIAN) imidazolium chloride (5). Compound 5 was prepared in a similar fashion to that
described for 2 starting with p-n-hexyl(BIAN) (0.213 g, 0.425 mmol) and methoxy(methyl)chloride
(3.22 g, 40.0 mmol). The resulting olive green/brown precipitate was filtered off and washed with
50 mL of diethyl ether and dried in vacuo to afford 5 as an analytically pure olive green powder
(0.167 g, 71.7%). Crystals of 5 suitable for X-ray diffraction experiments were grown from a saturated
CH2Cl2/hexanes solution stored at −40 °C. MS (CI+, CH4): m/z 513 [M+H]+ (-Cl); HRMS (CI+, CH4):
1
calcd for C37H41N2 m/z 513.3270; found, 513.3260; H-NMR (CD2Cl2): δ 0.92 (t, 6H, -CH3),
1.34–1.44 (m, 12H, -CH2-CH2-CH2-), 1.72 (quint, 4H, -CH2), 2.77 (t, 4H, -CH2), 7.54 (d, 4H, Ar-H),
7.60 (d of d, 2H, Naph-H), 7.78 (d, 2H, Naph-H), 8.00 (d, 2H, Naph-H), 8.17 (d, 2H Ar-H), 11.16 (s,
13
1H, C-H); C-NMR (CD2Cl2): δ 14.19, 22.93, 29.34, 31.54, 32.00, 35.99, 123.55, 123.67, 124.01,
128.20, 130.18, 130.66, 130.69, 132.07, 135.78, 138.68, 146.81; m.p. (decomp): 218–221 °C.
3.4. X-ray crystallography
For compounds L, 2, 4 and 5 a crystal of suitable quality was covered with mineral oil and mounted
on a nylon thread loop. The X-ray diffraction data were collected on either a Nonius Kappa CCD
diffractometer equipped with an Oxford Cryostream low-temperature device cooled to 153 K or a
Rigaku AFC-12 Saturn 724+ CCD diffractometer with a Rigaku XStream low temperature device
cooled to 100 K. Both instruments were equipped with a graphite-monochromated Mo Kα radiation
source (λ = 0.71073 Å). Corrections were applied for Lorentz and polarization effects. The structures
were solved by direct methods and refined by full-matrix least-squares cycles on F2 using the Siemens
SHELXTL PLUS 5.0 (PC) software package [20] and PLATON[21]. All non-hydrogen atoms were refined
anisotropically, and hydrogen atoms were placed in fixed, calculated positions using a riding model.
The crystallographic data for this paper have been deposited at the Cambridge Crystallographic
Data Centre. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html
(or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033; e-mail:
deposit@ccdc.cam.ac.uk). The CCDC reference numbers for compounds L, 2, 4 and 5 are 813271,
811058, 811059 and 811060, respectively.
3.5. Antimicrobial studies
Antimicrobial activities of compounds 1-5 were determined using the Microtiter-Based Minimum
Inhibitory Concentration (MIC) Test. S. aureus ATCC 6538, B. subtilis ATCC 19659, E. coli ATCC
11229 and P. aeruginosa ATCC 15442 were grown to approximetly 105 CFU/mL in Mueller-Hinton
broth. The stock solutions of compounds 1-5 were prepared with DI water supplemented with dimethyl
sulfoxide (DMSO). The concentrations of the tested compounds ranged from 0.03125% to 0.000006%
using the Mueller-Hinton broth as the diluent, supplemented with DMSO to a final concentration of
2%. The plates were incubated at 36.0 ± 1 °C for 18–24 h. The MIC was taken to be the last well in the
dilution series that did not exhibit growth as determined on the basis of turbidity.

Molecules 2011, 16
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4. Conclusions
In summary, four new bis(imino)acenaphthene (BIAN) imidazolium chlorides have been prepared
and structurally authenticated. Along with the known N(2,6-diisopropylphenyl)imidazolium chloride,
these salts have been tested for their antimicrobial activities against the Gram-positive bacteria S.
aureus and B. subtilis and the Gram-negative bacteria E. coli and P. aerginosa. The MIC values of
< 0.6 μg/mL for the N(2,6-diisopropylphenyl)- and N(mesityl)-substituted salts against S. aureus and
B. subtilis imply that these compounds are particularly effective antimicrobials. Although the MIC
values for these imidazolium salts are higher against the Gram-negative bacteria, they are still the most
active of the imidazolium salts that were tested.
Acknowledgements
The authors are grateful to the Robert A. Welch Foundation (Grant F-0003) and the National
Science Foundation (Grant 0741973) for financial support of this work.
References and Notes
1. For an early account of QACs as germicides, see Shelton R.S.; VanCampen, M.G.; Tilford, C.H.;
Lang, H.C.; Nisonger, L.; Bandelin, F.J.; Rubenkoenig, H.L. Quaternary ammonium salts as
germicides. I. Nonacylated quaternary ammonium salts derived from aliphatic amines. J. Am.
Chem. Soc. 1946, 68, 753 and references therein.
2. For more recent work on QACs, see Skzrypczak, A.; Brycki, B.; Mirsha I.; Pernak, J. Synthesis
and antimicrobial activities of new quats. Eur. J. Med. Chem. 1997, 32, 661-668.
3. Denyer, S.P.; Stewart, G.S.A.B. Mechanisms of action of disinfectants. Int. Biodeterior.
Biodegrad. 1998, 41, 261-268.
4. Hindi, K.M.; Panzer, M.J.; Tessier, C.A.; Cannon, C.L.; Youngs, W.J. The medicinal applications
of imidazolium carbene-metal complexes. Chem. Rev. 2009, 109, 3859-3884.
5. Pernak, J.; Skrzypczak, A. 3-Alkylthiomethyl-l-ethylimidazolium chlorides. Correlation between
critical micelle concentrations and minimum inhibitory concentrations. Eur. J. Med. Chem. 1996,
31, 901-903.
6. The minimal inhibitory concentration (MIC) is defined as the lowest concentration of an
antimicrobial that will inhibit the visible growth of a microorganism after overnight incubation.
See Andrews, J.M. Determination of minimum inhibitory concentrations. J. Antimicrob.
Chemother. 2001, 48 (Suppl. 1), S5.
7. Pernak, J.; Sobaszkiewicz, K.; Mirska, I. Anti-microbial activities of ionic liquids. Green Chem.
2003, 5, 52-56.
8. Butkus, M.A.; Edling, L.; Labare, M.P. The efficacy of silver as a bactericidal agent: Advantages,
limitations and considerations for future use. J. Water Supply Res. T. 2003, 52, 407-416.
9. Demberelnyamba, D.; Kim, K.-S.; Choi, S.; Park, S.-Y.; Lee, H.; Kim, C.-J.; Yoo, J.-D. Synthesis
and antimicrobial properties of imidazolium and pyrrolidinonium salts. Bioorg. Med. Chem. 2004,
12, 853-857.

Molecules 2011, 16
3178
10. Garg, G.; Chauhan, G.S.; Gupta, R.; Ahn, J.-H. Anion effects on anti-microbial activity of poly[1-
vinyl-3-(2-sulfoethyl imidazoliumbetaine)]. J. Colloid Interface Sci. 2010, 344, 90-96.
11. Çetinkaya, E.; Denizci, A.; Özdemir, I.; Oxturk, H.T.; Karaboz, I.; Çetinkaya, B. Remarkable
substituent effects on antimicrobial activities of 1,3-diorganylimidazolidinium salts. J. Chemother.
2002, 14, 241-245.
12. El-Ayaan, U.; Abdel-Aziz, A.-A.-M. Synthesis, antimicrobial activity and molecular modeling of
cobalt
and
nickel
complexes
containing
the
bulky
ligand:
bis[N-(2,6-
diisopropylphenyl)imino]acenaphthene. Eur. J. Med. Chem. 2005, 40, 1214-1221.
13. Butorac, R.R.; Al-Deyab S.S.; Cowley, A.H. Antimicrobial properties of some
bis(iminoacenaphthene (BIAN)-supported N-heterocyclic carbene complexes of silver and gold.
Molecules 2011, 16, 2285-2292.
14. Vasudevan, K.V., Butorac, R.R., Abernethy, C.D.; Cowley, A.H. Synthesis and coordination
compounds of a bis(imino)acenaphthene (BIAN)-supported N-heterocyclic carbene. Dalton
Trans. 2010, 39, 7401.
15. Atwood, J.L.; Bott, S.G.; Means, M.; Coleman, A.W.; Zhang, H.; May, M.T. Synthesis of salts of
the hydrogen dichloride anion in aromatic solvents. 2. Syntheses and crystal structures of [Kyl8-
crown-6][C1-H-C1], [Mgy18-crown-6][Cl-H-C1]2, [H3Oy18-crown-6][CI-H-C1], and the related
[H3Oy18-crown-6][Br-H-Br]. Inorg. Chem. 1990, 29, 467-470.
16. Tuck, D.G. Structures and properties of HX2- and HXY- anions. Prog. Inorg. Chem. 1968, 9,
161-194.
17. El-Ayaan, U.; Murata, F.; El-Derby, S.; Fukuda, Y. Synthesis, structural and solvent influence
studies on solvatochromic mixed-ligand copper(II) complexes with the rigid nitrogen ligand:
bis[N-(2,4,6-trimethylphenyl)imino]acenaphthene. J. Mol. Struct. 2004, 692, 209-216.
18. Mak, C.S.K.; Wong, H.L.; Leung, Q.Y.; Tam, W.Y.; Chan, W.K.; Djurišić, A.B. The use of
sublimable chlorotricarbonyl bis(phenylimino)acenaphthene rhenium(I) complexes as
photosensitizers in bulk-heterojunction photovoltaic devices J. Organomet. Chem. 2009, 694,
2770-2776.
19. Coventry, D.N.; Batsanov, A.S.; Goeta, A.E.; Howard, J.A.K.; Marder, T.B. Synthesis and
molecular structures of α-diimines and their zinc and palladium dichloride complexes. Polyhedron
2004, 23, 2789-2795.
20. Sheldrick, G.M. SHELXTL-PC, version 5.03, Siemens Analytical X-ray Instruments, Inc.,
Madison, WI, 1994. See also Sheldrick, G.M. Acta Crystallogr. Sect. A 2008, A 64, 112.
21. Spek, A.L. Structure validation in chemical crystallography. Acta Crystallogr. Sect D. 2009, D65,
148-155.
Sample Availability: Samples of the compounds are available from the authors.
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