Trapping of a Methanoato Bridge in μ-1,1,3,3 Mode for [Cu4] Aggregate Formation: Synthesis, Steric Control on Nuclearity, Antimicrobial Activity, and DNA-Interaction Properties

Article abstract of DOI:10.1002/ejic.201601092

The tetranuclear coordination aggregate [Cu₄(μ-L)₂(μ_1,1,3,3-O₂CH)](OH)·6H₂O (1) with the 1,3-bis[3-aza-3-(1-methyl-3-oxobut-1-enyl)prop-3-en-1-yl]-2-(2-hydroxyphenyl)-1,3-imidazolidine (H₃L) ligand was isolated in good yield, and the HCO₂^–anion is the central bridging unit bound to the four copper centers in a unique μ-1,1,3,3 coordination mode. On the other hand, the use of bulky RCO₂^–anions (R = CMe₃, CF₃, and Ph) resulted in the dinuclear complexes [Cu₂(μ-L)(μ_1,3-O₂CCMe₃)]·3.5H₂O (2a), [Cu₂(μ-L)(μ_1,3-O₂CCF₃)]·2H₂O (2b), and [Cu₂(μ-L)(μ_1,3-O₂CPh)]·4H₂O (2c), in which the carboxylate anions bridge in μ-1,3 mode. All of the complexes were characterized by several physicochemical techniques and single-crystal X-ray structure analysis. The in vitro antimicrobial activities of the complexes were compared with those of free ligand H₃L and Cu(HCO₂)₂·4H₂O. They showed antibacterial and antifungal activities against various strains of Gram-negative (Escherichia coli and Pseudomonas aeruginosa) and Gram-positive (Bacillus subtilis and Streptomyces aureus) bacteria and a yeast (Candida albicans). Among them, complex 1 showed the highest antimicrobial activity against the above-mentioned strains. The interaction of 1 with calf thymus DNA in tris(hydroxymethyl)aminomethane (Tris) buffer was examined by electronic absorption spectroscopy, fluorescence emission spectroscopy and circular dichroism.

Full text of DOI:10.1002/ejic.201601092

A Journal of
Accepted Article
Title: Trapping of Methanoato (HCO2−) Bridge in µ-1,1,3,3 Mode for [Cu4] Aggrega-
te Formation: Synthesis, Steric Control on Nuclearity, Antimicrobial and DNA-
interacting Properties
Authors: Tufan Singha Mahapatra; Anupam Roy; Susmitnarayan Chaudhury; Swa-
gata Dasgupta; Shanker Lal Shrivastava; Valerio Bertolasi; Debashis Ray
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201601092
Link to VoR: https://doi.org/10.1002/ejic.201601092

European Journal of Inorganic Chemistry
10.1002/ejic.201601092
−
Trapping of Methanoato (HCO2 ) Bridge in μ-1,1,3,3 Mode for [Cu4]
Aggregate Formation: Synthesis, Steric Control on Nuclearity, Antimicrobial
and DNA-interacting Properties
Tufan Singha Mahapatra,[a] Anupam Roy,[b] Susmitnarayan Chaudhury,[a] Swagata Dasgupta,[a]
Shankar Lal Shrivastava,[b] Valerio Bertolasi,[c] and Debashis Ray[a]
[a] Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India Fax:
(+91) 3222-82252; Tel: (+91) 3222-283324; E-mail: dray@chem.iitkgp.ernet.in
http://www1.iitkgp.ac.in/fac-profiles/showprofile.php?empcode=abmaZ&depts_name=CM
[b] Agricultural and Food Engineering Department, Indian Institute of Technology Kharagpur,
West Bengal, India
[c] Dipartimento di Scienze Chimiche e Farmaceutiche and Centro di Strutturistica
Diffrattometrica, Università di Ferrara, via L. Borsari, 46, I44121 Ferrara, Italy.
Abstract
Tetranuclear coordination aggregate [Cu4(μ-L)2(μ1,1,3,3-O2CH)](OH)·6H2O (1) has been
preferentially isolated in good yield from the use of H3L [1,3-bis{3-aza-3-(1-methyl-3-oxobut-1-
enyl)-prop-3-en-1-yl}-2-(2-hydroxyphenyl)-1,3-imidazolidine] and binding of HCO2¯ anion as
the central bridging unit bound to four copper centers in a unique μ-1,1,3,3 coordination mode.
Use of bulky RCO2¯ anions (R = CMe3, CF3 and Ph) on the other hand resulted dinuclear
complexes [Cu2(μ-L)(μ1,3-O2CCMe3)]·3.5H2O (2a), [Cu2(μ-L)(μ1,3-O2CCF3)]·2H2O (2b) and
[Cu2(μ-L)(μ1,3-O2CPh)]·4H2O (2c), where carboxylate anions bridge in μ-1,3 mode. All the
complexes have been characterized by several physicochemical techniques and X-ray single-
crystal structure analysis. In vitro antimicrobial activities of the complexes were compared
against free ligand H3L and Cu(HCO2)2∙4H2O salt. They showed antibacterial and antifungal
activities to a various strains of Gram-negative (E. coli and P. aeruginosa) and Gram-positive (B.
subtilis and S. aureus) bacteria, and a yeast (C. albicans). Among them complex 1 showed
highest antimicrobial efficiency against the above mentioned strains. Interaction of 1 with calf
thymus DNA in Tris buffer was examined by electronic absorption, fluorescence emission and
circular dichroism studies.
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European Journal of Inorganic Chemistry
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Introduction
Carboxylate ions having two angularly-disposed oxygen donors can function as a structural
bridge to multiple metal ions already bound to a ligand system and can control the composition
and structure of the product.1 These were used in stabilizing [Mn12] complexes,2 ferric ion based
wheels3 and other high nuclearity 3d metal ion cages.4 Lone pair of electrons from oxygen atoms
bind the metal ions in syn and/or anti mode.5 As a coordinating and charge neutralizing
component, they adopt numerous coordination modes such as terminal, chelating, bridging in
synanti µ-1,1 mode, synsyn, synanti, and anti-anti µ-1,3 mode, and anti-syn-syn, anti-syn-
anti µ3-1,1,3 mode (Scheme S1, ESI).5,6 Involvement of carboxylates is also fascinating in
metalloproteins, where the mode of carboxylate-binding plays a guiding role in deciding the
coordination affinity and selectivity of metal ion cofactor.7 Acetate and other alkyl group bearing
carboxylates are well known for the growth of coordination clusters. Whereas the smallest one,
¯
HCO2 , is seldom found to play a controlling role in the growth of such species.5a,6b The R
groups on the carboxylate ion can control the assembly of two [Cu2L]+ (L = dinucleating ligand)8
units having vacant coordination sites around the metal ions. Unlike perchlorido,9a azido,9b
chlorido,9c and oxido9d bridging units, carboxylato groups are not known thus far to provide
tetranuclear copper(II)-based cluster from pairs of such [Cu2L]+ units exclusively bridged by an
unsupported carboxylate ligand in μ-1,1,3,3 mode (Scheme 1). In all these cases, the formation
of a five-membered imidazolidine ring on ethylenediamine backbone as a spacer-cum-bridging
unit allows the imidazolidine ring bearing ligand system to bind two metal ion centres to form
[M2L]+ units. The aggregation into tetranuclear entities has been further observed through
–
−
bridging by the exogenous ligands such as ClO4 , N3 , Cl− and O2−. The most studied examples
of imidazolidine-based dinucleating ligands are 2-(2′-hydroxyphenyl)-1,3-bis[4-(2-
hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazolidine9a,10, 2-(2′-hydroxyphenyl)-1,3-bis[4-methyl-
4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazolidine11 and 2-methyl-2-(2′-hydroxyphenyl)-
1,3-bis[4-methyl-4-(2-hydroxyphenyl)-3-azabut-3enyl]-1,3-imidazolidine9b,12
Studies on new tetracopper aggregates of unknown structure are useful to rationalize the role of
the [Cu4] cluster based catalytic site for the reduction of N2O to N2.13 Similarly, [Cu2] complexes
derived from newer organic ligands are useful as chemical model to study magnetic exchange
interactions,14 DNA binding and cleavage behavior,15 phosphate diester hydrolysis16 and
oxidation of catechol and related substrates.17
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European Journal of Inorganic Chemistry
10.1002/ejic.201601092
Scheme 1. Azido, perchlorido, chlorido and oxido supported known [Cu4] clusters 9
In recent years, different strains of bacteria and fungi that are resistant to conventional organic
antimicrobials have become a worldwide public health issue.18 It necessitates the use of more
potent and metal-containing antimicrobial agents. The known antimicrobial action of copper
surfaces and copper ions lead to the examination of new copper(II)-based complexes for
antimicrobial potency inhibiting the growth or virulence of a pathogen.19 Compared to free
ligand or metal ion alone, formation of complex species increases the lipophilicity, which in turn
increases permeability and quite often target the affected site favorably. In recent years
quinolones and fluoroquinolones have shown enhanced antibacterial activity through copper(II)
coordination.20 Another significant area of investigation on metal ion complexes is their use to
explore and manipulate the biological processes in living cells.21 Copper(II) complexes are well
known to bind to DNA with specificity and bring about the cleavage of the DNA strand.22
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European Journal of Inorganic Chemistry
10.1002/ejic.201601092
Figure 1. H3L used in this work
Herein, we report the binding copper(II) ions by H3L (Figure 1), first synthesized by us in
¯
200123, in the presence of four RCO2 (R = H, CMe3, CF3, Ph) ions. Synthesis and structural
studies identified the binding modes of H3L and carboxylate ions as presented in Scheme 2.
Synthesis, isolation, crystallization, characterization, structural features, DNA binding and
antimicrobial properties of [Cu4(μ-L)2(μ1,1,3,3-O2CH)](OH)·6H2O (1), [Cu2(μ-L)(μ1,3-
O2CCMe3)]·3.5H2O (2a), [Cu2(μ-L)(μ1,3-O2CCF3)]·2H2O (2b) and [Cu2(μ-L)(μ1,3-
O2CPh)]·4H2O (2c) have been reported.
Scheme 2. Observed binding modes of L3− and carboxylates
Results and Discussion
Synthetic Considerations
Binding modes different carboxylate anions were examined for their steric control on the
nuclearity of the resulting complexes. In MeOH, reaction of Cu(HCO2)2∙4H2O with H3L in
presence of NEt3 resulted [Cu4(μ-L)2(μ1,1,3,3-O2CH)](OH)·6H2O (1) as a green solid from the
reaction mixture in 80% yield (eq 1; Scheme 3).
2H3L + 4Cu(HCO2)2·4H2O + 7NEt3
[Cu4(μ-L)2(μ1,1,3,3-O2CH)](OH)·6H2O
+ 7(NHEt3)(HCO2) + 9H2O
... (1)
A saturated CH2Cl2 solution of the obtained solid as mentioned above was layered with MeOH
(1:1) and room temperature evaporation of the same provided crystals of 1 suitable for X-ray
structure determination. The elemental analysis of the powder sample and solution molar
conductivity data are consistent with the composition of 1 as [Cu4(μ-L)2(μ1,1,3,3
-
O2CH)](OH)·6H2O. Use of Cu(ClO4)26H2O or externally added NaClO4 in the reaction medium
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European Journal of Inorganic Chemistry
10.1002/ejic.201601092
did not separate out the perchlorate salt of 1 as used base NEt3 resulted hydroxide ions which
were not able to coordinate to copper(II) centers. Use of NH4PF6 as metathetic salt also did not

replace the smaller hydroxide anion by bigger and spherical PF6 ions.
Scheme 3. Synthetic routes for 1-2c
In case of other three reactions the available carboxylate groups (RCO2¯, R = CMe3, CF3 and Ph)
provided [Cu2(μ-L)(μ1,3-O2CCMe3)]·3.5H2O (2a), [Cu2(μ-L)(μ1,3-O2CCF3)]·2H2O (2b) and
[Cu2(μ-L)(μ1,3-O2CPh)]·4H2O (2c) in good yield (eq 2; Scheme 3).
H3L + 2Cu(RCO2)2·H2O + 3NEt3
[Cu2(μ-L)(μ1,3-O2CR)]
+ 3(NHEt3)(RCO2) + 2H2O
... (2)
Elemental analysis and molar conductivity data of the crystalline samples agree well with the
molecular formula of 2a-2c as given above and confirmed by X-ray structure determination (vide
supra).
Spectroscopic and PXRD Identification
The FTIR spectral measurements provide the characteristic feature of CuII-bound azomethine
(νC=N) functions of L3− as sharp single peak at 1597 cm−1 for 1-2c. For H3L the free νC=N appears
at 1605 cm−1. In all four cases the varying numbers of molecules of lattice water are integral part
for the stabilization of the crystal lattice. The presence of these water molecules is manifested by
a single broad band of medium intensity at 3421, 3431, 3488 and 3421 cm-1, respectively for 1,
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European Journal of Inorganic Chemistry
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2a, 2b and 2c. The bridging modes of trapped and bridged carboxylate anions can also be


s(COO)
established in confidence from FTIR data. For 1, asymmetric as(COO) and the symmetric
−
stretching vibrations for HCO2 group in 4-bridging mode are detected at 1515 and 1405 cm-1,
-1




respectively with a difference (Δ
=
as(COO) s(COO)) of 110 cm (Figure S1, ESI). The Δ
value is smallest within the family and is characteristic for the {Cu4(4-HCO2)} core and binding
−
of HCO2 in μ1,1,3,3 mode in 1. It is worth noting at this point that monodentate coordination of
carboxylate ion provides Δ value of ca. 350 cm−1.24 For the remaining three compounds 2a, 2b

stretching vibrations were detected at 1564, 1673 and 1560 cm-l,

and 2c, the
as(COO)
respectively, while s(COO) observed at 1405, 1406 and 1405 cm−1, respectively. The differences,

are 159, 267 and 155 cm−1 are in agreement with the presence of



s(COO)
Δ
=
as(COO)
carboxylato bridges connecting two CuII centers in μ1,3 mode in 2a, 2b and 2c, respectively
25
−
band for CF3CO2 bridge within two CuII ions, moved

(Figure S1). In case of 2b the
as(COO)
−1

considerably to a higher magnitude of 1673 cm thus increasing in the Δ value
Moderate solubility of 1-2c in MeOH allowed us to examine the solutions for electronic spectral
transitions in 200 to 800 nm range. Like many other phenol based Schiff bases, H3L displays the
characteristic n  * transition at 310 nm (ε = 30400 L mol-1 cm-1) (Figure S2, ESI). Binding of
CuII ions within L3− blue shift this band to intense absorptions at 283 nm (ε = 34200 L mol-1 cm-
1), 283 nm (ε = 13600 L mol-1 cm-1), 282 nm (ε = 17400 L mol-1 cm-1) and 282 nm (ε = 14000 L
mol-1 cm-1) for 1, 2a, 2b and 2c, respectively. The band at 318 nm (ε = 27800 L mol-1 cm-1), 318
nm (ε = 11200 L mol-1 cm-1), 320 nm (ε = 14200 L mol-1 cm-1) and 319 nm (ε = 11600 L mol-1
cm-1) respectively, for 1, 2a, 2b and 2c have been assigned to the ligand-to-metal charge transfer
transitions (LMCT).26 The CuII centered ligand-field transition was observed as broad absorption
band (λ), with maxima at 611 nm (ε = 618 L mol-1 cm-1), 610 nm (ε = 275 L mol-1 cm-1), 608 nm
(ε = 473 L mol-1 cm-1) and 610 nm (ε = 294 L mol-1 cm-1) for 1, 2a, 2b and 2c, respectively.
The experimental powder X-ray diffraction (PXRD) patterns of 1-2c were shown in Figure S3.
From the single crystal X-ray data the simulated PXRD patterns were obtained for comparison
purpose (vide supra). The experimentally obtained patterns match well to the simulated patterns
with small differences in the intensity patterns which can be attributed to the orientation of the
crystallites in the powder samples. Both the positions (corresponding to lattice spacing) and the
relative intensity of the lines in the diffraction pattern are indicative of the phase purity of the
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European Journal of Inorganic Chemistry
10.1002/ejic.201601092
compounds, providing a ‘fingerprint’ for comparison between the syntheses of the bulk samples
in different batches.
Description of Crystal Structures
Green needle like single crystals of 1 were obtained after 5 days from a saturated
dichloromethane solution of the compound layered with methanol. For 2a, 2b and 2c, good
quality crystals were available by slow evaporation of the saturated methanol, acetonitrile and
dichloromethane-methanol (1:1) solutions of these compounds after 8, 10 and 4 days,
respectively.
[Cu4(μ-L)2(μ1,1,3,3-O2CH)](OH)·6H2O (1). The molecular structure of 1 is given in Figure 2,
and important bond lengths and angles are collected in Table S1 in the ESI.
Figure 2. a) View of the cationic [Cu4(μ-L)2(μ1,1,3,3-O2CH)]+ unit in 1 with partial atom-numbering scheme; b) view
of the intermetallic trapezoid core. Inset: Cu···Cu separations within trapezoid arrangement. H atoms are omitted for
clarity. Color code: Cu, brown; N, blue; O, red; C, black
Compound 1 crystallizes in the monoclinic Cc space group with Z = 4 and consists of a unique
tetranuclear motif developed from the assembly of two [Cu2L]+ fragments around a central
HCO2¯ group. X-ray crystal structure of H3L23 previously indicated that it has two metal ion
binding pockets in a folded form and further stabilized by intramolecular hydrogen bonding from
two terminal >C=O groups with one trapped lattice water. In the crystalline state the C−O
distances are in normal range for the keto form as against the tautomeric enol form. During CuII
coordination the >C=O functions of L3− tautomerized to enol form with elongation of C−O
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European Journal of Inorganic Chemistry
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bonds (Table S2 and Scheme S3, ESI). The trapped HCO2¯ group in μ-1,1,3,3 binding mode
registered wide O−C−O angle of 132.19°. While placing CuII centers in distorted N2O2 square
plane of L3−, the HCO2¯ clip is provided longer Cu−O bonds (2.289-2.380 Å) in apical direction.
In a folded conformation each {Cu2L} half register a dihedral angle of 47.13. The Cu−O/N
distances from ligand donor atoms are comparatively shorter in 1.898−2.077 Å range. The range
of O/N−Cu−O/N cis- and trans-angles within the coordination squares are 79.5−120.7 and
155.1−177.9 respectively. The flexible enol-imine arms of L3− allow trigonal distortions during
trapping of HCO2¯ clip toward trigonal bipyramidal geometry. The magnitude of such distortion
from square bipyramidal geometry around each CuII center is estimated from the Addison (τ)
parameters (τCu1A = 0.33, τCu2A = 0.35, τCu1B = 0.27 and τCu2B = 0.37).27 The μ-1,1,3,3 mode of
binding of HCO2¯ group can be considered as two linked μ-1,1 connectivity within each {Cu2L}
unit having Cu∙∙∙Cu intradimeric separations of 3.178 and 3.192 Å. Thus μ-1,1 part assumes two
short parallel axes of the [Cu4] trapezium. While the μ-1,3 part is responsible for two longer axes
of the same having interdimeric Cu···Cu distances of 5.117 and 5.988 Å (Figure 2, Inset).
The presence of water of crystallization in H3L·2H2O and resulting hydrogen bonding
interactions with terminal ketone and central phenol moiety is responsible for a folded structure.
Closely in a similar fashion two lattice water molecules (O1W and O2W) in [Cu4(μ-L)2(μ1,1,3,3
-
O2CH)](OH)·6H2O show hydrogen bonding interaction with terminal CuII bound enol oxygen
atoms of L3− (O1A···O1W, 2.726; O2A···O1W, 2.732; O1B···O2W, 2.723 and O2B···O2W,
2.743 Å). Among the four other lattice water molecules, one (O4W) is connected to three others
(O5W, O6W and O7W) through hydrogen bonding interactions within the crystal lattice
(O4W···O7W, 2.884; O4W···O6W, 2.831 and O4W···O5W, 2.748 Å) (Figure S4, ESI).
[Cu2(μ-L)(μ1,3-O2CCMe3)]·3.5H2O (2a); [Cu2(μ-L)(μ1,3-O2CCF3)]·2H2O (2b) and
[Cu2(μ-L)(μ1,3-O2CPh)]·4H2O (2c). The molecular structures of 2a, 2b and 2c are similar with
dimeric entities and depicted in Figure 3. The relevant bond lengths and angles are collected in
Table S3 in the ESI. 2a, crystallizes in the monoclinic P21/n space group (Z = 4), while 2b and
̅
2c crystallize in triclinic P space group (Z values of 4 and 2, respectively). The higher steric
bulk on RCO2¯ groups, compared to HCO2¯, permit only the μ-1,3 mode of binding. The RCO2¯
(R = CMe3, CF3 and Ph) groups show some variation in O−C−O angles of 124.42°, 131.34° and
126.41° and shorter Cu−O separations in 2.163−2.265 Å range. Interestingly these bond lengths
were not responsible for higher degree of distortion from square pyramidal geometry.
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European Journal of Inorganic Chemistry
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Determination of Addison τ parameters27 also support this observation (τCu1 = 0.21, τCu2 = 0.09
for 2a; τCu1 = 0.15, τCu2 = 0.13 for 2b and τCu1 = 0.22, τCu2 = 0.11 for 2c). Another significant
observation with respect to intermetallic separation is that the μ-1,3 mode pushed the Cu∙∙∙Cu
separations to 3.376, 3.429 and 3.382 Å for 2a-2c, respectively, compared to that found for 1
(3.178 and 3.192 Å). This also established the fact that HCO2¯ group is a unique one in
modulating the Cu∙∙∙Cu distances. The central phenoxido oxygen atom of L3− as a result register
wider Cu−O−Cu angles of 117.4, 120.6 and 116.9° in case of 2a, 2b and 2c, respectively,
compared to those found in 1 (105.7-106.1°).
Figure 3. (a to c) View of the neutral [Cu2] units in 2a, 2b and 2c with partial atom-numbering scheme. (d to f)
Corresponding core views in square pyramidal geometry. Hydrogen atoms and solvent molecules are omitted for
clarity. Color code: Cu, brown; N, blue; O, red; F, green; C, gray
In 2a, one lattice water (O1W) molecule shows hydrogen bonding interaction with CuII bound
enol oxygen atoms (O3W and O2W) of L3− registering O···O separation of 2.853 and 2.900 Å,
respectively. O1W further established interaction with O3W (O···O, 2.785 Å), which is already
connected to O2W (O···O, 2.836 Å). O2W in turn is close to O4 of Me3CCO2¯ (O···O, 2.741
Å). The central O4W (0.5 occupancy) show connections with O3W and O3W* (O···O, 2.776,
2.554 Å) which in turn linked to four other lattice water molecules for an aesthetically pleasing
(H2O)7 unit. These four water molecules were responsible for folding four {Cu2L} units through
hydrogen bonding interactions. The presence of heptameric water cluster develops a 2D
hydrogen bonding network within the crystal lattice (Figure 4). Such an extended network of
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European Journal of Inorganic Chemistry
10.1002/ejic.201601092
hydrogen bonds are crucial and known to stabilize nickel(II)-bound molecules of water in the
active site of urease.28
Figure 4. a) 2D hydrogen bonding network found in 2a; b) Side view of the network. Inset: Heptameric water
cluster
In 2b, the lattice water O12 shows hydrogen bonding interaction with CuII-bound enol oxygen
atoms (O1 and O3) from L3− for significant O···O separations (2.944 and 3.016 Å). Whereas in
2c, the lattice water oxygen atom (O6) is involved in hydrogen bonding interactions with enolate
oxygen atoms O1 and O3 (O···O, 2.840 and 2.836 Å) and phenolate oxygen atom (O2) of L3−
(O···O, 2.971 Å). This O6 further established hydrogen bonding interaction with another lattice
water (O7) in the crystal lattice (O···O, 2.633 Å) (Figure S5, ESI). Two other lattice water
molecules (O8 and O9) show hydrogen bonding interaction with O···O separation of 2.858 Å.
(Figure S4, ESI).
Table 1. Crystal parameters and refinement data for 1-2c
Compound
Chemical formula
Formula Mass
Crystal system
Space group
1
C47H76Cu4N8O15
1247.32
2a
C28H47Cu2N4O8.5
702.78
2b
C25H33Cu2N4F3O6
669.66
2c
C30H44Cu2N4O9
731.78
Monoclinic
Cc
Monoclinic
P21/n
Triclinic
Triclinic
̅
P
̅
P
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European Journal of Inorganic Chemistry
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Crystal color
Crystal size/mm3
a/Å
Green
Green
0.43x0.40x0.38
11.5861(2)
18.0660(3)
15.9898(3)
90.00
Green
0.42x0.38x0.35
10.3235(7)
15.1224(11)
18.5059(13)
88.307(2)
86.386(2)
84.202(2)
2867.8(3)
4
Green
0.40x0.37x0.35
9.582(5)
10.670(6)
17.255(9)
90.567(10)
95.922(10)
92.845(10)
1752.4(17)
2
0.31x0.25x0.19
13.6150(7)
32.9437(17)
15.8301(8)
90.00
b/Å
c/Å
α/o
β/o
γ/o
109.0610(16)
90.00
91.5567(6)
90.00
V/Å3
6711.0(6)
4
3345.66(10)
4
Z
Dc/g cm-3
μ (mm-1)
F(000)
1.235
1.395
1.546
1.372
1.308
1.323
1.547
1.267
2600
1476
1368
748
T/oK
293(2)
295
293(2)
293(2)
Total reflns
R(int)
40001
29495
36471
17540
0.0570
0.0457
0.0489
0.0748
Unique reflns
Observed reflns
Parameters
R1 (I > 2σ(I))
wR2(all reflns)
GOF (F2)
Largest diff peak and
hole (e Å−3)
CCDC number
10389
8850
11291
7639
5853
6551
7664
3554
675
403
721
415
0.0494, 0.1282
0.0397, 0.1082
0.0628, 0.1951
0.0722, 0.2713
0.867
1.015
1.025
1.005
0.427, -0.267
0.616, -0.431
1.500, -0.942
1.142, -1.185
1469379
1469380
1469381
1469382
2
R1 = ( FoFc )/Fo. wR2 = [ w(FoFc)2/w(Fo)2]1/2. w = 0.75/(2(Fo) + 0.0010Fo )
In vitro Antimicrobial Assays
In recent years discovery of antibiotic resistant strains has been seriously hindered the treatment
of numerous bacterial infections. The negatively charged bacetrial cell membrane may be
considered for the exclusive target with respect to the cationic and neutral metal complexes. It is
known that membrane permeabilization and disruption for breaking of the bacterial membrane is
one of the mechanistic pathway for cationic antimicrobials.29 In this work complexes 1-2c,
Cu(HCO2)2∙4H2O and H3L, were examined for their ability to inhibit the growth of Gram
positive S. aureus, B. subtilis and Gram negative E. coli, P. aeruginosa bacterial strains and the
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European Journal of Inorganic Chemistry
10.1002/ejic.201601092
fungal strain C. albicans. For the first three strains the availability of antibiotics is rapidly
decreasing. Out of these S. aureus is responsible for bacteremia and infective endocarditis as
well as it causes various skin and soft tissue infections. Gram positive B. subtilis is the well
studied model organism for chromosome replication and cell differentiation. E. coli on the other
hand are rod-shaped and found in the lower intestine of warm-blooded organisms. P. aeruginosa
is also a disease causing rod-shaped bacterium. C. albicans is a diploid fungus and a causal agent
of opportunistic oral and genital infections in humans. The antimicrobial efficiency of 1−2c was
evaluated in vitro in terms of their minimal inhibitory concentrations (MIC; μg mL−1). The
results of the assays and the MIC values are presented in Table 2. The observed MIC values
indicated that 1−2c are more effective in terms of their activity compared to H3L and
Cu(HCO2)2∙4H2O. The enhanced activity of complexes (1−2c) than the ligand H3L can be
explained on the basis of Tweedy’s chelation theory.30 The polarity of the metal ion will be
reduced to a greater extent on chelation because of partial sharing of its positive charge with the
donor groups and the delocalization of π-electrons over the whole chelate ring. Such chelation
enhances the lipophilicity of the complexes. Now, the lipid membrane that surrounds the cell of a
microorganism favours the passage of only the lipid-soluble materials. So, the increased
lipophilicity of the complexes enhances the permeation of the complexes into lipid membranes
of the microorganisms which inhibits the growth of those microorganisms. Cationic complex 1
registers higher activity compared to other neutral complexes (2a−2c) as positive charge density
on complex 1 might facilitate a direct interaction with the negatively charged bacetrial cell
membrane.31 The growth profile of bacterial strains treated with 1 at varying doses have been
shown in Figure 5. The profiles toward four bacterial strains clearly reveal that 1 affected the
growth kinetics significantly compared to the control with increasing concentration (culture was
grown in absence of 1). Complex 1 treated at a concentration of 15.62 μg mL-1 resulted 68.91%
and 77.17% reduction of B. subtilis and E. coli density, respectively, compared to the control.
Treatment of complex 1 at 31.25 μg mL-1 and 62.5 μg mL-1 concentrations inhibited the S.
aureus and P. aeruginosa density (as compared to the control sample) at 86.48% and 68.58%,
respectively. In this regard it is worth noting that in vitro measurements for antimicrobial activity
having MIC values  1000 μg mL-1 are considered useless from clinical perspective.32 Complex
1 thus displayed higher level of antibacterial effects against the four bacterial strains where it
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European Journal of Inorganic Chemistry
10.1002/ejic.201601092
exhibited a higher potency in comparison to H3L and Cu(HCO2)2∙4H2O wherefrom it has been
assembled.
Figure 5. Growth analysis of four different bacterial strains in presence of varying concentration of 1
Table 2. Minimum inhibitory concentration (MIC in μg mL-1) of the complexes 1–2c, H3L and Cu(HCO2)2∙4H2O
against various microorganisms
Compounds
B. subtilis
(Gm +ve)
S. aureus
(Gm +ve)
E. coli
(Gm −ve)
MIC (μg mL-1)
15.62
P. aeruginosa
C. albicans
(fungus)
(Gm −ve)
15.62
31.25
62.5
62.5
250
31.25
62.5
62.5
62.5
125
62.5
31.25
31.25
62.5
125
125
250
250
250
250
250
1
62.5
2a
62.5
2b
31.25
2c
H3L
125
Cu(HCO2)2∙4H2O
125
125
125
125
Based on the obtained values of MICs, E. coli was chosen as model system and the
morphological changes of E. coli cells following exposure to 1 were investigated by scanning
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European Journal of Inorganic Chemistry
10.1002/ejic.201601092
electron microscopy (SEM). The SEM micrograph, as presented in Figure 6, clearly indicated the
rod-shaped smooth and intact cell walls surface of untreated E. coli. Interactions with 1 caused
significant perturbation of the cell walls and exhibited number of blebs resulting from the loss of
membrane integrity. The deformations of the bacterial cell wall is due to the electrostatic
interactions between the positively charged 1 with the negatively charge E. coli strains.31 The
mode of action of 1 against E. coli strains may be considered to be membranolytic.
Figure 6. Effect of 1 on the morphology of E. coli. Cells were incubated for 4 h in absence a) and presence b) of 1,
and the cell morphology was visualized by scanning electron microscopy (SEM). Scale bars represent 2 m. ‘Blebs’
are highlighted with red circles
DNA Binding Potential
The DNA binding potential of cationic 1 (25 μM), causing inhibition of DNA synthesis, was
examined by monitoring the UV–vis absorption titrations with CT-DNA (0–50 µM) in 10 mM
Tris–HCl buffer solution at pH 7.4 containing 10 mM NaCl (Figure 7). The electroneutral
complexes 2a-2c has not been examined in this study. Electronic absorption spectroscopy has
been chosen in this study as it is the most well established technique to determine the binding
efficacy of metal complexes with the DNA helix. The intercalative or partial intercalative mode
of binding involve full or partial stacking interaction between the metal complex fragment and
base pairs of DNA, usually leads to hypochromism.33,36 It depends on the planar structure and
available π-electron cloud for stacking interaction.34 Cationic complexes bearing non planar
ligand system, with non-planar coordination geometry, usually bind to the anionic phosphate
backbone of DNA through electrostatic attraction.35 Incremental addition of CT-DNA to 1 in 10
mM Tris–HCl buffer solution at pH 7.4 containing 10 mM NaCl, caused hypochromism (,
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European Journal of Inorganic Chemistry
10.1002/ejic.201601092
~15%) with a small red-shift in π → π* transition at 279 nm originating from copper(II) bound
imine function of the ligand (Figure 7). This hypochromism is expected for a partial intercalation
of the metal complex fragment within DNA base pairs.36 In order to determine the ground state
association constants (Ka) of the complex with CT-DNA the Benesi-Hildebrand equation (eq 3)
was applied.
Here, εb and εf correspond to the extinction coefficients of the characteristic π → π* transition of
complex 1 in fully bound form and of free complex respectively, for a particular DNA
concentration M. LT is the total complex concentration and ΔA is the change in the absorbance at
a given wavelength. The association constant (Ka) of the complex was obtained from the ratio of
the intercept to the slope from the plot of the reciprocal of ΔA versus the reciprocal of
concentration of CT-DNA (Table 3). The obtained binding constant for 1 is lower than for
ethidium bromide (EB) with CT-DNA (Ka = 1.4 × 106), indicating partial intercalation.37
Table 3. Association constants (Ka) for the interaction of CT-DNA (0–50 µM) with 1 (25 µM) at 298K
System
Ka (L mol-1)
CT-DNA-1
3.37 ± 0.53 × 104
Figure 7. a) Absorption spectra of 1 (25 μM) in absence (black) and presence (colored) of incremental CT-DNA (0–
50 µM) in 10 mM Tris–HCl buffer of pH 7.4; b) Benesi–Hildebrand double reciprocal plot
Fluorescence spectral titration was used to examine the relative binding affinity of 1 with CT-
DNA. Fluorescence emission of ethidiumbromide (EB) is used as an optical probe, as no
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European Journal of Inorganic Chemistry
10.1002/ejic.201601092
emission is seen for 1 in aqueous medium under used experimental condition. When EB
intercalates to DNA, the EB-DNA system shows strong fluorescence at 597 nm upon excitation
at 480 nm. For 1, a substantial hypochromism effect was observed during the fluorescence
titration at 597 nm, which suggests that EB was replaced from the EB–DNA system (Figure 8).
The fluorescence emission intensity of EB–DNA system is diminished due to the partial
intercalation of 1 within EB–DNA system. The modified Stern-Volmer equation is employed to
obtain the binding constant of 1 with CT-DNA. In eq 4, F0 and F are the fluorescence intensity
of EB bound DNA and intensity of EB bound DNA upon addition of 1, respectively. [Q] is the
concentration of the quencher. fa is the fraction of fluorophore accessible to quencher and KA is
the modified Stern-Volmer quenching constant.
The apparent DNA binding constant (Kapp), which expresses the degree of binding affinity of
examined species toward DNA, was calculated from eq 5 following the standard literature
method.35
KEB [EB] = Kapp [complex]
… (5)
Herein [complex] corresponds to 50% reduction of fluorescence intensity of EB bound DNA,
and KEB = 1.0 ×107 M-1 at [EB] = 5 µM. Thus the apparent DNA binding constant for 1 was
found as 16.7 × 104 M-1, whereas the KA value is 4.73± 0.33×104 M-1.
Figure 8 a) Effect of addition of 1 (0 to 25.6 µM) on the emission intensity (λex = 480 nm) of CT-DNA bound EB
(CEB =5 µM; CDNA =50 µM) in 10 mM Tris–HCl buffer solution at pH 7.4 at 298K. b) Modified Stern–Volmer plot
CD spectroscopy further supports the DNA binding characteristics of 1. CD spectroscopy is
probably one of the most sensitive techniques available for monitoring structural changes of
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European Journal of Inorganic Chemistry
10.1002/ejic.201601092
DNA in solution or for determining whether a new or unusual structure is formed by a particular
polynucleotide sequence. The CD spectra of CT-DNA bound to 1 in 1:1 mole ratio
([complex]/DNA) is given in Figure S6 (ESI). Both the positive (~275 nm) and negative (~245
nm) bands decrease in intensity with the increase in complex concentration. This suggests that 1
can unwind the DNA helical structure and lead to an appreciable loss of helicity by perturbing
the secondary structure of DNA.38 The observation at the same time establishes the fact that 1
binds to CT-DNA effectively.
Conclusions
In this work, we have established the characteristic role of HCO2¯ and other three carboxylate
anions for coordination aggregation of copper(II) ions already bound to L3−. In solution L3−
spontaneously yields [Cu2] moieties with the possibility of binding by carboxylate anions at the
vacant coordination sites. Compared to three other cases, using CMe3CO2¯, CF3CO2¯ and
PhCO2¯, the L3− has allowed the incorporation of an unprecedented μ-1,1,3,3 coordination mode
of HCO2¯ anion within a tetranuclear [Cu4] coordination cluster. The entrapment of single
HCO2¯ anion, with two angularly-disposed oxygen atoms, by four copper(II) ions resulted the
isolation of {Cu4(µ4¯HCO2)} core for the first time within a [Cu4] cluster. HCO2¯ anion thus
serves as an anion template for a novel trapezoidal molecular geometry as a new entry within the
[Cu4] family. The steric bulk on the carboxylate anions (R = CMe3, CF3 and Ph) inhibit the
aggregation of two {Cu2} fragments bound to L3−. They only connect two copper(II) centers in
μ1,3-mode. In terms of the functional behavior, this family of complexes showed notable
antimicrobial efficiency against both Gram-negative and Gram-positive bacteria as well as to
fungus. Cationic 1 showed enhanced antibacterial and antifungal activity to five different strains
examined in this work. Absorption and fluorescence results along with circular dichroism (CD)
study established the CT-DNA binding ability of 1 through partial intercalative interaction.
Experimental Section
Starting Materials
All the chemicals and solvents used in this work were of reagent grade and used as received
without further purification. They were obtained from the following sources: copper(II)
carbonate, formic acid, sodium benzoate and triethylamine from E. Merck (India); 2,4-
pentanedione (acetylacetone) and trifluoroacetic acid from SRL (India) and pivalic acid from
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European Journal of Inorganic Chemistry
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Sigma Aldrich. Cu(PhCO2)2·H2O was prepared by reacting Cu(NO3)2·3H2O in MeOH (20 mL)
with an aqueous solution (5 mL) of PhCO2Na, followed by concentrating, filtering and drying in
vacuo. Treatment of powder copper(II) carbonate with formic acid (20 mL, 98-100%) diluted
with water (20 mL) with constant stirring until effervescence ceases, followed by filtration and
concentration on water bath resulted Cu(HCO2)2·4H2O39a as blue solid, which was filtered
through suction and dried in vacuo. Cu(CF3CO2)2·H2O was obtained following a similar method
and Cu(CMe3CO2)2·H2O was prepared by following a literature procedure.39b Calf thymus DNA
(CT-DNA) and ethidium bromide (EB) were purchased from SRL (India). Solution of 1 was
prepared in absolute ethanol and subsequent dilution was made with 10 mM Tris–HCl buffer
containing 10 mM NaCl of pH 7.4 with reduction in concentration below 5%.
Syntheses
of
H3L
[1,3-bis{3-aza-3-(1-methyl-3-oxobut-1-enyl)-prop-3-en-1-yl}-2-(2-
hydroxyphenyl)-1,3-imidazolidine]. Yellow crystalline ligand used in this work was prepared
from a Schiff base condensation and imidazolidine ring formation involving triethylenetetramine
(3.5 g, 23.9 mmol), acetylacetone (4.79 g, 47.8 mmol) and salicylaldehyde (2.92 g, 23.9 mmol)
in MeOH in 78% yield, as reported earlier (Scheme S2, ESI).23
[Cu4(μ-L)2(μ1,1,3,3-O2CH)](OH)·6H2O (1). To a yellow MeOH solution (20 mL) of H3L (0.414
g, 1 mmol), a MeOH solution (5 mL) of triethylamine (0.420 mL, 0.303 g, 3 mmol) was added
drop wise with stirring at ambient temperature in air. Then another MeOH solution (10 mL) of
Cu(HCO2)2·4H2O (0.450 g, 2 mmol) was added slowly to the above ligand solution and the
reaction mixture was allowed to stir for 3 h. The solution was evaporated in air to get a green
solid, which was collected by filtration, washed with cold MeOH followed by water, and dried
under vacuum over P4O10. The green solid was then dissolved in a minimum quantity of DCM
and layered with MeOH in 1:1 v/v ratio at room temperature. Green needle like single crystals
suitable for X-ray data collection were obtained after 5 days. Yield: 1.057 g, 80%. Anal. Calc.
for C47H76Cu4N8O15 (1247.32 g mol-1): C, 45.26; H, 6.14; N, 8.98. Found: C, 45.36; H, 6.28; N,
8.76%. Selected FTIR bands: (KBr, cm-1; br = broad, vs = very strong, s = strong, m = medium)
3421(br), 1597(vs), 1515(s), 1405(s), 1258(m), 1243(m), 944(m), 771(m). Molar conductance,
M (~ 10-3 M solution in MeOH): 95 Ω-1 cm2 mol-1. UV-vis spectra [max, nm (ε, L mol-1 cm-1)]:
(MeOH solution) 611 (618), 318 (27800), 283 (34200).
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European Journal of Inorganic Chemistry
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[Cu2(μ-L)(μ1,3-O2CCMe3)]·3.5H2O (2a), [Cu2(μ-L)(μ1,3-O2CCF3)]·2H2O (2b) and [Cu2(μ-
L)(μ1,3-O2CPh)]·4H2O (2c). A general procedure, similar to that used for 1, was employed for
2a, 2b and 2c using Cu(CMe3CO2)2·H2O, Cu(CF3CO2)2·H2O and Cu(PhCO2)2·H2O, respectively
in place of Cu(HCO2)2·4H2O. The green block shaped crystals of 2a, 2b and 2c suitable for X-
ray structural analysis were obtained from MeOH, MeCN and DCM-MeOH after 8, 10 and 4
days, respectively.
2a. Yield: 0.520 g, 74%. Anal. Calcd. for C28H47Cu2N4O8.5 (702.78 g mol-1) C, 47.85; H, 6.74;
N, 7.97. Found: C, 47.71; H, 6.93; N, 8.07. Selected FTIR bands: (KBr, cm-1; br = broad, vs =
very strong, s = strong, m = medium) 3431(br), 1597(s), 1564(m), 1515(vs), 1457(m), 1405(s),
1337(m), 1261(m), 773(m). Molar conductance, M (~ 10-3 M solution in MeOH): 15 Ω-1 cm2
mol-1. UV-vis spectra [max, nm (ε, L mol-1 cm-1)]: (MeOH solution) 610 (275), 318 (11200), 283
(13600).
2b. Yield: 0.585 g, 83%. Anal. Calcd. for C25H33Cu2N4F3O6 (669.66 g mol-1) C, 44.84; H, 4.97;
N, 8.37. Found: C, 44.63; H, 4.83; N, 8.23. Selected FTIR bands: (KBr, cm-1; br = broad, vs =
very strong, s = strong, m = medium) 3488(br), 1673(vs), 1597(vs), 1508(vs), 1477(m), 1406 (s),
1343(m), 1201(s), 1138(m), 771(m). Molar conductance, M (~ 10-3 M solution in MeOH): 10
Ω-1 cm2 mol-1. UV-vis spectra [max, nm (ε, L mol-1 cm-1)]: (MeOH solution) 608 (473), 320
(14200), 282 (17400).
2c. Yield: 0.622 g, 85%. Anal. Calcd. for C30H44Cu2N4O9 (731.78 g mol-1) C, 49.24; H, 6.06; N,
7.66. Found: C, 49.53; H, 6.18; N, 7.38. Selected FTIR bands: (KBr, cm-1; br = broad, vs = very
strong, s = strong, m = medium) 3421(br), 1597(vs), 1560(s), 1508(vs), 1405(s), 1386(s),
1262(s), 1092(m), 1021(m), 944(m), 769(m), 721(s). Molar conductance, M (~ 10-3 M solution
in MeOH): 8.0 Ω-1 cm2 mol-1. UV-vis spectra [max, nm (ε, L mol-1 cm-1)]: (MeOH solution) 610
(294), 319 (11600), 282 (14000).
Physical Measurements
Elemental analyses (C, H, N) were carried out using a Perkin-Elmer model 240C elemental
analyzer. FTIR spectra were recorded on a Perkin-Elmer 883 spectrometer with samples
prepared as KBr pellets (4000−400 cm−1). The electronic spectra (800−200 nm) in MeOH
solution and the UV–vis titrations were carried out in a Shimadzu UVPC 1800
spectrophotometer using a quartz cuvette (1 cm path). The molar conductivity in MeOH
solutions (10-3 M) was obtained using a Unitech type U131C digital conductivity meter. High-
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European Journal of Inorganic Chemistry
10.1002/ejic.201601092
resolution mass spectra (ESI) were measured using a Xevo G2 TOF mass spectrometer. Powder
X-ray diffraction patterns for all the complexes were performed on a BRUKER AXS X-ray
diffractrometer operated at 40 kV and 20 mA, using monochromated Cu Kα radiation (k =
1.5418 Å) over the 5–50° (2θ) angular range and a fixed-time counting of 4 s at room
temperature. The fluorescence spectra were measured using a Fluoromax-4 (Horiba Jobin Yvon)
spectrofluorometer and a 1 cm quartz cell with the slit width and integration time were fixed at 5
nm and 0.3 s, respectively. Circular Dichroism (CD) spectra of only CT-DNA and CT-DNA with
complex 1 were carried out by using a JASCO J-815 spectropolarimeter equipped with a Peltier
temperature control device.
Antibacterial and Antifungal Tests
Antimicrobial activity was assessed by the minimum inhibitory concentration (MIC) data on
strains of bacteria and fungus derived from the Microbial Type Culture Collection (MTCC) of
IMTEC Chandigarh, India and maintained according to the mentioned specifications. The MIC
(μg mL−1) is defined as the lowest concentration of the compound that fully inhibited the growth
of bacteria or fungi. Four bacterial strains of Escherichia coli (MTCC 443), Pseudomonas
aeruginosa (MTCC 7925), Staphylococcus aureus (MTCC 96), Bacillus subtilis (MTCC 441)
and one fungal stain of Candida albicans (MTCC 227) were used. The MICs of four copper(II)
complexes 1-2c, H3L and Cu(HCO2)2·4H2O have been assessed in MeOH using micro-dilution
plate method mentioned by the Clinical and Laboratory Standard Institute (CLSI).40 The
procedure involved preparation of two-fold dilutions of the synthesized compounds starting from
higher concentration to lower by equally diluting (e.g., 250, 125, 62.5, 31.25, 15.62, 7.81, 3.90,
1.92 μg mL−1) in a liquid growth medium containing standardized microbial suspension of 1–
5×106 CFU/mL (0.1%) dispensed in microplate. The antimicrobial activity was evaluated with
compounds in the concentration range of 250 μg mL−1−1.92 μg mL−1. Inhibition of microbial
growth in the plates containing tested solutions was judged by comparison with growth in blank
control plates. The growth kinetics was further studied by monitoring the optical density (OD) of
the cultures against time using a Shimadzu 200 spectrophotometer at 600 nm.41 The freshly
prepared culture of strains for each experiment was used by inoculating a miniscule amount in 10
mL of sterile medium in a culture tube. The bacteria were allowed to grow overnight in an
incubator conserved at 37 °C and shaken at 220 rpm in a shaker. Appropriate concentrations of 1
(250 μg mL−1−1.92 μg mL−1) were added to the test tube and placed in a shaker at 37 °C. The
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European Journal of Inorganic Chemistry
10.1002/ejic.201601092
OD600 nm of this suspension (containing the bacteria and complex 1) was periodically monitored.
The obtained MIC values for 1 prompted to consider for further study, such as on morphology of
cell membrane of E. coli. A Carl Zeiss Merlin scanning electron microscope (SEM) (accelerating
voltage: 20 kV) was used to monitor the effect of 1 on the morphology of E. coli cell membrane.
After incubating at MIC value of complex 1 for 4 h, E. coli cells were harvested by
centrifugation, the supernatant was removed and cells were washed twice with phosphate-
buffered saline. The cells were fixed with glutaraldehyde (2.5%) and dehydrated ethanol
solutions (30%, 50%, 70%, 90%, 95% and 100%).
DNA Binding Study
The binding study of 1 with calf thymus DNA (CT-DNA) was carried out in 10 mM Tris–HCl
buffer containing 10 mM NaCl (pH 7.4). The concentration of CT-DNA was determined using
molar absorption coefficient (ε) of 6600 M-1 cm-1 at 260 nm.42 A constant concentration of 1 (25
µM) was titrated successively with the addition of CT-DNA (0–50 µM) in the above mentioned
buffer and the scan range was set in 400-200 nm range. The absorbance at 279 nm of 1 was
monitored after successive additions of CT-DNA. All the spectra were corrected by subtracting
the spectra of CT-DNA from the complex-CT-DNA spectra at the measured wavelength.
The relative DNA binding ability of 1 was examined with fluorescence spectroscopy
using an EB-bound CT-DNA (CT-DNA-EB) solution in 10 mM Tris-HCl buffer (pH 7.4)
containing 10 mM NaCl. Keeping excitation wavelength at 480 nm, the fluorescence spectra
were measured maintaining excitation and emission slit width at 2 nm. The scan range was set
within 550-700 nm. The fluorescence emission spectra around 597 nm of CT-DNA-EB system
were recorded by successive addition of 1 (0–25.6 μM), while CT-DNA and EB concentration
were maintained at 50 μM and 5 μM, respectively.
The CD spectra within 220 to 320 nm ranges were obtained using a quartz cuvette of path
length of 5 mm and scan rate of 50 nm min-1. The spectra were measured with solutions of CT-
DNA (100 μM) with the [Complex 1]/[DNA] ratios of 0.0 and 1.0 in 10 mM Tris-HCl buffer
(pH 7.4) containing 10 mM NaCl. The back-ground signal due to the buffer was subtracted.
X-ray Crystallography
Appropriate single crystals of 1, 2b and 2c were mounted on glass fibers and the intensity data
were collected on a Bruker SMART APEX-II CCD diffractometer with graphite monochromated
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European Journal of Inorganic Chemistry
10.1002/ejic.201601092
Mo-Kα radiation (λ = 0.71073 Å). The crystals were placed at 60 mm from the CCD and 360
frames were collected with a counting time of 5 s. Space group determination, data integration
and reduction were performed with XPREP and SAINT software package.43 The structures were
solved using direct methods protocol available through SHELXS-9744 and refined with full-
matrix least squares on F2 using the SHELXL-9745 program package incorporated into WinGX
version 1.80.05.46 Multiscan empirical absorption corrections were applied to the entire data set
in all cases using SADABS.47 The data for 2a was collected at at 293 K using a Nonius Kappa
CCD diffractometer. The data set was integrated with the Denzo-SMN package48 and corrected
for Lorentz, polarization, and absorption effects using SORTAV package.49 This structure was
solved by direct methods protocol from SIR9750 and further calculations were performed using
SHELXL-9744 and PARST51 implemented in WINGX suite of program.52 The majority of atoms
were located from direct methods structure solution protocol, and remaining atoms were
subsequently identified from the difference Fourier maps. All non-hydrogen atoms were refined
anisotropically. The H atoms were introduced in calculated positions and refined with fixed
geometry and riding thermal parameters with respect to their carrier atoms. Crystallographic
diagrams were drawn by operating the DIAMOND software.53 Information concerning X-ray
data collection and crystal structure refinement is summarized in Table 1. Crystallographic data
(excluding structure factors) have been deposited with the Cambridge Crystallographic Data
Centre as supplementary publications 1469379-1469382.
Acknowledgements
TSM and SNC are thankful to the University Grant Commission, New Delhi, India for the
research fellowship. DST, New Delhi is acknowledged for providing the single crystal X-ray
diffractometer facility in the Department of Chemistry, IIT Kharagpur under its FIST program.
Keywords: Coordination modes / Carboxylate ligands / X-ray diffraction / Antimicrobial
efficiency / DNA binding
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Table-of-Contents
Text
Presence of HCO2¯ within a [Cu4] aggregate (1) showed versatile coordination behavior,
antibacterial and antifungal propensity to various strains and exhibits DNA interacting capacity
in partial intercalative mode in comparison to three other higher order carboxylate anions (2a-
2c).
Graphic
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