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Synthesis, spectroscopic and X-ray structural characterization of two novel mixed-ligand lead(II) complexes
Accepted Manuscript Synthesis, spectroscopic and X-ray structural characterization of two novel mixedligand lead(II) complexes Gholamhossein Mohammadnezhad, Fariba Nasimpour, Mostafa M. Amini, Ezzatollah Najafi, Helmar Görls, Winfried Plass, Mohammad Reza Sabzalian PII:
S0022-2860(18)30942-6
DOI:
10.1016/j.molstruc.2018.07.115
Reference:
MOLSTR 25520
To appear in:
Journal of Molecular Structure
Received Date: 15 May 2018 Revised Date:
30 July 2018
Accepted Date: 31 July 2018
Please cite this article as: G. Mohammadnezhad, F. Nasimpour, M.M. Amini, E. Najafi, H. Görls, W. Plass, M.R. Sabzalian, Synthesis, spectroscopic and X-ray structural characterization of two novel mixed-ligand lead(II) complexes, Journal of Molecular Structure (2018), doi: 10.1016/ j.molstruc.2018.07.115. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Synthesis, spectroscopic and X-ray structural characterization of two novel
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mixed-ligand lead(II) complexes
Gholamhossein Mohammadnezhada∗, Fariba Nasimpoura, Mostafa M. Aminib, Ezzatollah Najafic,
Department of Chemistry, Isfahan University of Technology, Isfahan, 84156-83111, Iran b
Department of Chemistry, Shahid Beheshti University G.C., Tehran 1983963113, Iran c
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Helmar Görlsd, Winfried Plassd and Mohammad Reza Sabzaliane
Department of Chemistry, Payame Noor University (PNU), Tehran, Iran
Lehrstuhl für Anorganische Chemie II, Institut für Anorganische und Analytische Chemie, Friedrich-SchillerUniversität Jena, Humboldtstr. 8,07743 Jena, Germany
Department of Agronomy and Plant Breeding, College of Agriculture, Isfahan University of Technology, Isfahan
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84156-83111, Iran
∗
Corresponding author. Tel.; +98-31-3391-3279; FAX: +98-31-3391-2350.
[email protected]; [email protected] (G. Mohammadnezhad).
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Abstract Two
novel
mixed-ligand
complexes
of
lead(II),
[Pb(Cup)2(Phen)]2](DPE)
(1)
and
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[Pb(Cup)(ONO2)(Phen)] (2) (Cup = cupferronato, Phen = 1,10-phenanthroline, and DPE = 1,2di(4-pyridyl)ethylene), were synthesized in the presence of common bridging ligands which are usually used in synthesis of lead(II) coordination polymers as well as utilization of Cup and Phen
investigated by 1H and
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as O,O’- and N,N’- chelates, respectively. Structural features of 1 and 2 in solution were 13
C NMR experiments. The accurate assignment of these signals was
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inferred from a series of 1H–1H COSY, 1H–13C HSQC, and 1H–13C HMBC spectra. The solidstate structures of 1 and 2 were also investigated via single crystal XRD. By considering the obtained structural parameters, the coordination of 1 is hemidirected while 2 is suggested to be holodirectional. Additionally, complex 1 consist of dimers which are further associated via π-π
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stacking to a one-dimensional coordination polymer while complex 2 forming a step-like onedimensional chain by an intermolecular interaction between each molecule and two nitrato groups of the two adjacent molecules. Both complexes showed significant antibacterial and
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antifungal characteristics in disc diffusion tests and they could be considered as new potent antimicrobial material.
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Keywords: Lead complex; Mixed ligand; Cupferron; 1,10-Phenanthroline; Crystal structure; antimicrobial activity
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1. Introduction Interest in design and synthesis of novel coordination compounds of heavy p-block elements is
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still considered as a promising issue in basic science and technology [1–6]. The physicochemical properties of different metal complexes not only depend on the nature of the central atom/ion but also related to the structural, spectroscopic and other properties of coordinated ligands. A
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synthetic challenge for tuning the physicochemical properties of the coordination compounds is the preparation of mixed-ligands complexes [6–12]. Usually, such mixed-ligand metal complexes
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show enhanced performance for instance in biological activities [13,14], electroluminescence devices [15], thermal stability [16], and photophysical properties [17,18]. Among the p-block mixed metal complexes, lead(II) coordination compounds are of particular interest due to the rapid developments of their applications in advanced materials [19–21].
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Regardless of the toxic nature and environmental issues, Pb(II) with its large radius and flexible coordination, as well as other heavy p-block ions such as Sn(II), Tl(I), and Bi(III), has unique coordination chemistry due to the role and effect of 6s2 lone pair. This effect has been
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widely studied and historically goes back to 1941 when Pauling suggest the valence shell lone pair influence on the stereochemistry of Pb(II) coordination sphere [22]. As the lone pair is not
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directly detectable, different criteria have been explored both experimentally and theoretically [23–26]. Specifically, Shimoni-Livny et al. rationalized the relation of coordination sphere geometry of Pb(II) complexes in terms of the lone pair [25]. Based on the location of the ligands and their bond lengths in the coordination sphere of the complex, Pb(II) complexes were categorized in two general structural features as hemidirected and holodirected. In hemidirected geometry, an obvious coordination site vacancy or void, as well as unusual asymmetric bond
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length, is observable. While in holodirected geometry, ligands are distributed in coordination sphere nearly uniform and depict considerable symmetrical bond lengths. These two categories
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are not the only acceptable forms of geometries, while in some cases unsymmetrical coordination and vacancy/void does not necessarily is a prove of a stereochemically active lone pair and there is a need to apply more analyses such as considering unusual structural parameters and high-level
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computational chemistry approaches [27–33].
Cupferron, ammonium N-Nitroso-N-phenylhydroxylamine, is a good chelating agent which
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is widely used in analytical chemistry and biology for instance, in separation, precipitation, electrochemistry and colorimetric analysis of metals [34,35]. Also, metal cupferron complexes have been used as precursors for the preparation of nanocrystalline metal oxides [36]. The anion, cupferronato (Cup), could act as a bidentate ligand and binds to Pb(II) via the two oxygen atoms
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and forming a five-membered chelate ring [37–39]. In this report, to investigate the structural parameters imposed by the ligands, two novel mixed-ligand complexes of lead(II) were synthesized and characterized both in solid-state and solution. The syntheses were performed in
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the presence of common bridging ligands which are usually used in the synthesis of lead(II) coordination polymers as well as utilization of Cup and phenanthroline (Phen) as O,O’- and
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N,N’- chelate, respectively.
2. Experimental Section 2.1. Materials and techniques Pb(NO3)2 (Merck, 99.0 % trace metals basis), 1,2-di(4-pyridyl)ethylene (DPE) (Aldrich, 97%), cupferron (N-Nitroso-N-phenylhydroxylamine ammonium salt, Aldrich, 97%, reagent grade), 4
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1,10-Phenanthroline (Phen) (Merck, anhydrous for synthesis 99%) and 4-bpdb (1,4-bis(4pyridyl)-2,3-diaza-1,3-butadiene) [40] were used without further purification. Anhydrous
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methanol was prepared according to a standard procedure [41]. FT-IR spectra were recorded on a Bruker IFS55/Equinox spectrometer utilizing a diamond ATR unit or on a JASCO 680-PLUS spectrometer using KBr pellets from 250-4000 cm-1. 1H, 13C, 1H{1H} COSY, 1H{13C} HSQC and
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HMBC NMR spectra were recorded at room temperature on a Bruker Avance DRX400-MHz
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spectrometer.
2.2. Synthesis of [Pb(Cup)2(Phen)]2](DPE) (1)
DPE (0.2 mmol, 0.036 g), Phen (0.2 mmol, 0.036 g), and cupferron (0.2 mmol, 0.031 g) were mixed at room temperature in a solution of Pb(NO3)2 (0.1 mmol, 0.033 g) in methanol (20 mL), respectively. After the addition of each of the mentioned ligands, the resulting solution was
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stirred adequately until a clear solution was obtained. At last, for complete dissolution of the ingredients, a few drops acetonitrile was added. Suitable single crystals were grown by simple
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slow evaporation technique. FT-IR spectrum (ATR, cm-1): 379 (vs), 413 (m), 478 (s), 541 (w), 553 (s), 572 (m), 584 (w), 631 (m), 688 (vs), 715 (s), 728 (s), 762 (vs), 829 (s), 838 (s), 859 (m),
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910 (s), 927 (w), 979 (m), 999 (m), 1019 (m), 1040 (m), 1056 (m), 1100 (m), 1140 (m), 1163 (w), 1199 (s), 1208 (s), 1225 (s), 1261 (s), 1269 (s), 1288 (s), 1301 (s), 1329 (s), 1348 (w), 1417 (s), 1425 (s), 1460 (m), 1487 (m), 1512 (m), 1558 (w), 1589 (w), 1591 (s), 1598 (s), 1621 (w), 2810 (w), 3045 (w), and 3073 (w). 1H NMR (400 MHz, DMSO-d6, 25 °C, ppm): 7.38-7.52 (m, 6H, H-Cup), 7.54 (s, 1H, H-13), 7.61 (dd, 2H, J = 7.61 Hz, H-12), 7.79-7.89 (m, 6H, H-5, HCup), 8.02 (s, 2H, H-7), 8.49-8.57 (dd, J = 8.54 Hz, 2H, H-6), 8.60 (dd, J = 8.60 Hz, 2H, H-11), 9.13-9.24 (dd, J = 9.17 Hz, 2H, H-4). 13C NMR (100 MHz, DMSO-d6, 25 °C, ppm): 119.3 (C-1), 5
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121.3 (C-12), 123.6 (C-5), 126.8 (C-7), 128.7, 128.8 (C-3 and C-9), 129.3 (C- 2), 130.6 (C-13),
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136.6 (C-6), 142.2 (C-10), 143.3 (C-14), 145.3 (C-8), 149.8 (C-4), 150.2 (C-11).
2.3. Synthesis of [Pb(Cup)(ONO2)(Phen)] (2)
4-bpdb (0.2 mmol, 0.042 g), Phen (0.2 mmol, 0.036 g), and cupferron (0.2 mmol, 0.031 g) were
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mixed at room temperature with a methanolic solution (20 mL) of Pb(NO3)2 (0.1 mmol, 0.033 g). After the addition of each of the mentioned ligands, the resulting solution was stirred adequately
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until a clear solution was obtained. Finally, for complete dissolution of constituents, a few drops dichloromethane was added. Suitable single crystals were grown by simple slow evaporation technique. FT-IR spectrum (ATR, cm-1): 380 (s), 410 (m), 417 (m), 471 (s), 510 (w), 555 (w), 582 (w), 623 (w), 637 (s), 692 (vs), 722 (vs), 774 (s), 782 (m), 808 (w), 822 (m), 849 (s), 863 (w), 908 (s), 824 (w), 985 (m), 1019 (w), 1040 (w), 1061 (m), 1099 (m), 1145 (br, m), 1193 (s),
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1210 (w), 1290 (br, vs), 1374 (s), 1424 (m), 1459 (w), 1493 (m), 1514 (m), 1572 (w), 1592 (w), 1622 (w), 2922 (w), and 3063 (w). 1H NMR (400 MHz, DMSO-d6, 25 °C, ppm): 7.42-7.57 (m, 3H, H-Cup), 7.83-7.89 (m, 2H, H-Cup), 7.91-7.99 (m, 2H, H-5), 8.09 (s, 2H, H-7), 8.61-8.69 (dd,
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J = 865 Hz, 2H, H-6), 9.17-9.25 (dd, J = 9.21 Hz, 2H). 13C NMR (100 MHz, DMSO-d6, 25 °C,
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ppm): 119.4 (C-1), 124.1 (C-5), 127.1 (C-7), 129.27, 129.30 (C-2 and C-3), 129.5 (C-10), 137.7 (C-6), 142.0 (C-9), 144.9 (C-8), 149.8 (C-4).
2.4 Crystallographic Data and Refinement The intensity data were collected on a Nonius Kappa CCD diffractometer, using graphitemonochromated Mo-Kα radiation. Data were corrected for Lorentz and polarization effects; absorption was taken into account on a semi-empirical basis using multiple-scans [42–44]. The 6
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structure was solved by direct methods (SHELXS [45]) and refined by full-matrix least squares techniques against Fo2 (SHELXL-97 [45]). All hydrogen atom positions were included at XP software was used for structure
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calculated positions with fixed thermal parameters. representations [46].
Crystal data for 1: C30.75H25N7.25O4.38Pb, Mr = 773.27 gmol-1, colourless prism, size 0.034 × 0.032 × 0.030 mm3, triclinic, space group Pī, a = 10.4521(3), b = 10.5148(3), c = 27.9560(8) Å, α =
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79.617(1), β = 87.222(1), γ = 83.174(1) °, V = 2999.53(15) Å3, T = -140 °C, Z = 4, ρcalcd. = 1.712 gcm-3, µ (Mo-Kα) = 56.75 cm-1, multi-scan, transmin: 0.5781, transmax: 0.7456, F(000) = 1509,
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32625 reflections in h(-11/13), k(-13/13), l(-36/36), measured in the range 2.22° ≤ Θ ≤ 27.48°, completeness Θmax = 97.4%, 13415 independent reflections, Rint = 0.0332, 11780 reflections with Fo > 4σ(Fo), 781 parameters, 0 restraint, R1obs = 0.0391, wR2obs = 0.0878, R1all = 0.0473, wR2all = 0.0924, GOOF = 1.053, largest difference peak and hole: 1.883 / -1.147 e Å-3. Crystal data for 2: C18H13N5O5Pb, Mr = 586.52 gmol-1, colourless prism, size 0.034 × 0.032 × 0.024 mm3, monoclinic, space group P21/n, a = 6.4932(2), b = 10.0679(3), c = 28.0378(8) Å, β =
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95.079(1)°, V = 1825.72(9) Å3 , T = -140 °C, Z = 4, ρcalcd. = 2.134 gcm-3, µ (Mo-Kα) = 92.84 cm-1, multi-scan, transmin: 0.4854, transmax: 0.7456, F(000) = 1112, 21466 reflections in h(-8/8), k(13/13), l(-36/36), measured in the range 2.49° ≤ Θ ≤ 27.50°, completeness Θmax = 98.7%, 4151
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independent reflections, Rint = 0.0348, 3977 reflections with Fo > 4σ(Fo), 262 parameters, 0 restraint, R1obs = 0.0232, wR2obs = 0.0518, R1all = 0.0247, wR2all = 0.0527, GOOF = 1.142, largest
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difference peak and hole: 1.320 / -1.506 e Å-3.
2.5 Antimicrobial assay
The disc diffusion method was performed in Petri dishes containing solid Nutrient Agar (NA) medium. A microbial suspension (1 ml) of E. coli and B. thuringiensis was spread by a sterile swab, evenly over the face of a sterile nutrient agar plate. Two complexes of 1 and 2 were applied to the center of the agar plate into a disc shapes with 10 mm diameter (10 mg of each) in four
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replicates. The same method was used for antifungal assay in which spores of two saprophytic fungi of Penicillium sp. and Aspergillus sp. were spread over the surface of agar plates after
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placing the complexes as described for antibacterial test. Agar plates were then incubated for 48 h at 25±2 °C and inhibition zone was monitored. The size of inhibition zone (diameter of inhibition) was considered as the level of antimicrobial activity present in the two complexes-the
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larger zone of inhibition, the more potential of antimicrobial activity.
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3. Result and discussion
Syntheses of mixed-ligand complexes allow the variation in physical and chemical properties, including geometrical parameters, hydrophobicity, and intra/inter molecular interactions by systematic variation of complex components. In this regards, two new complexes were synthesized by reaction of Pb(NO3)2 and Phen, cupferron and two different bipyridyl (bpy)
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molecules. The initial molar ratio of Pb(NO3)2:phen:cupferron:bpy was 1:2:2:2 for both of the reactions. As we will see in the following sections, the formula of the isolated complexes, [Pb(Cup)2(Phen)]2(DPE)] (1) and [Pb(Cup)(ONO2)(Phen)] (2), showed that the molar ratio of the
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utilized reactants was different as illustrated in Scheme 1 and 2. Notably, when the reactions were
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repeated based on the new molar ratios and in the absence of bpy molecule for preparation of 2, the reactions ends up to the same products. In this regard, the optimized conditions were used for preparation and characterization of these complexes. The obtained crystals were stable at ambient temperature and can be stored for a long period without any changes.
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Scheme 1. Schematic representation for synthesis of [Pb(Cup)2(Phen)]2(DPE)] (1).
Scheme 2. Schematic representation for synthesis of [Pb(Cup)(ONO2)(Phen)] (2).
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FT-IR spectra of Phen, DPE, and cupferon ligands as well as the synthesized complexes, 1 and 2, have been studied. In the FT-IR spectrum of cupferon, the absorption bands include ring,
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NO, CN and NN vibrations as well as skeletal vibrations of ONNO [47]. The ammonium cation (NH4+) stretching and deformation modes are usually seen as a broad band around 3130 cm–1 and strong band around 1400 cm−1, respectively. Additionally, the typical bands at 1230, 1275 and 1339 cm−1 are generally associated with the NO (vs and vas) and NN stretching, respectively. In addition, the bending mode of the ONNO emerged at 912 cm-1. As expected, upon coordination of Cup to Pb and formation of Pb-O bonds, the related ammonium stretching bands of cupferon
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were omitted in the spectra of 1 and 2 (Figure 1). Additionally, the considerable shift of NO and ONNO bands are in agreement with coordination of Cup to the metal center. The spectrum of 1
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depicts a band at 1591 cm-1 that has been assigned to C=C of DPE. The presence of unidentate nitrato ligand in complex 2 was confirmed by the observed bands at 1374, 1290, and 1145 cm−1 for va(NO2), vs(NO2), and v(NO), respectively [48]. The appearance of the Phen characteristic
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peaks confirmed the existence of this ligand in both complexes. The binding mode of the Cup ligand was normally followed by the vibrational stretching frequencies of M-O bonds which are
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positioned in the range of 440-380 cm-1 [48]. In the fingerprint area of metal chelate (below 500
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cm-1), Pb-O and Pb-N frequencies are appeared at about 380, 410 and 470 cm-1, respectively.
Figure 1. FT-IR spectra (ATR) of a) [Pb(Cup)2(Phen)]2(DPE)] (1) and b) [Pb(Cup)(ONO2)(Phen)] (2)
Structural features of 1 and 2 in solution were investigated by 1H and 13C NMR experiments. The accurate assignment of these signals was inferred from a series of 1H–1H COSY, 1H–13C 10
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HSQC, and 1H–13C HMBC spectra. As mentioned, DPE is present in the crystal structure of 1 as a co-crystal and the general formula is [Pb(Cup)2(Phen)]2(DPE). Based on this formula each DPE
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molecule is co-crystalized with two Pb(II) complex units. In 1 each complex includes 18 hydrogens and each DPE has 10 hydrogens; so, the expected hydrogens are 46 which are consistent with the total integration of resonances in aromatic region (See Figure S1 for 1H NMR
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of 1). As expected, the integration ratio of DPE:Phen:Cup signals in 1H NMR is 1:2:4. For instance, in DPE H-10, ethylene hydrogens, is assigned at 7.42 ppm as a singlet with the
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integration of 2, while the other DPE hydrogens, H-8 and H-9, are positioned at 8.60 and 7.61 ppm as a doublet of doublet signals with the integration 4 for each, respectively. COSY 1H NMR of 1, as well as the assigned hydrogens of each signal, is depicted in Figure 2. In 13C NMR of 1 (see Figure S2) fourteen different signals related to ligands (Phen and Cup) and co-crystallized DPE were observed in the range of 118-151 ppm. The resonances accurately assigned by 1H–13C
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HSQC as depicted in Figure 3. The assignments consistency was further investigated and evaluated by 1H–13C HMBC as depicted in Figure S3. For instance, H-8 is in the neighborhood
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of C-9 and a bit farther to C-14, while there is no correlation observed with its assigned signal. In the structure of complex 2, [Pb(Cup)(ONO2)(Phen)], 13 hydrogens are present which is in (See Figure S4).
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agreement with the integration of observed signals of its 1H NMR spectrum
The integration ratio of Phen:Cup signals in 1H NMR, 1:1, is consistence with the chemical formula. COSY 1H NMR of 2, as well as the assigned hydrogens of each signal, is depicted in Figure 4. In
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C NMR of 2 (see Figure S5) 9 different signals related to Phen and Cup were
observed in the range of 118-150 ppm. The resonances accurately assigned by 1H–13C HSQC as depicted in Figure 5. The assignments consistency was further investigated and evaluated by 1H–
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C-8, while there is no correlation observed with its assigned signal.
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Figure 2. 1H{1H} COSY NMR spectrum of [Pb(Cup)2(Phen)]2(DPE)] (1) in DMSO-d6.
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Figure 3. 1H{13C} HSQC NMR spectrum of [Pb(Cup)2(Phen)]2(DPE)] (1) in DMSO-d6.
Figure 4. 1H{1H} COSY NMR spectrum of [Pb(Cup)(ONO2)(Phen)] (2) in DMSO-d6.
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Figure 5. 1H{13C} HSQC NMR spectrum of [Pb(Cup)(ONO2)(Phen)] (2) in DMSO-d6.
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The two lead(II) complexes, [Pb(Cup)2(Phen)]2(DPE)] (1) and [Pb(Cup)(ONO2)(Phen)] (2), were crystallized in triclinic Pī and monoclinic P21/n space groups, respectively. The molecular structure of 1 contains a six-coordinated distorted octahedral Pb(II) center and formed by an
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overall [N2O4] coordination environment. The Pb(II) coordination sphere in 1 includes three bidentate ligands; two cis Cup ligands and one Phen in a propeller-like positioning around the
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Pb(II) center (Figure 6). The molecular structure of 2 contains a five-coordinated Pb(II) center and formed by an overall [N3O2] coordination environment. The Pb(II) coordination sphere in 2 includes two bidentate ligands; one Cup ligand and one Phen as well as a monodentate coordinated nitrato group (Figure 7).
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Figure 6. Molecular structure of [Pb(Cup)2(Phen)]2(DPE)] (1). The ellipsoids represent a probability of
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30%. Hydrogen atoms are omitted for clarity
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Figure 7. Molecular structure of [Pb(Cup)(ONO2)(Phen)] (2). The ellipsoids represent a probability
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of 30 %. Hydrogen atoms are omitted for clarity
The arrangement of ligands around the lead center determines its coordination geometries
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which itself is also affected by the lone pair of electrons on Pb(II) centers. In both complexes, the N and O atoms arrangement shows that the coordinated bonds are directed to one side of
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coordination sphere. This is known as hemidirected coordination in the chemistry of Pb complexes in which a stereoactive lone pair of electrons on Pb centers is identifiable as a void in the crystal structure of the complexes [25]. Some structural parameters are also required to be checked, including the Pb-O bond length of ligands opposite the void (axial) as compared to the other Pb-O bonds (equatorial) [49]. In the crystal structure of 1, the axial Pb-O bond has the shortest distances (2.35 Å) while the other equatorial Pb-O bonds are longer (ca. 2.5 Å). These
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data are consistent with hemidirectional geometry of complex 1. While in the case of holodirectional geometry all the Pb-O distances are nearly equivalent. Additionally, there is a
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Pb···O interaction (3.072(3) and 3.159(4) Å) between two adjacent molecules which form a dimer structure as depicted in Figure 8. By considering this interaction, the structure of complex 1 will continue to be as hemidirectional based on the crystallographic evidence. The molecular
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packing of 1 is illustrated in Figure 8.
Figure 8. The molecular packing of [Pb(Cup)2(Phen)]2(DPE)] (1)
Generally, Pb(II) complexes with low coordination numbers (2-5) are hemidirected while for intermediate coordination numbers (6-8), cases of either type of stereochemistry are observed. In the case of 2, the coordination number is five, all the coordinated bonds are directed to one side 17
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of coordination sphere, and all the bond angles are considerably below 180 ̊ (the maximum value is 135.66 ̊ ). Thus, 2 may be considered as a hemidirected complex with one of the Phen N atom
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is positioned in the trans position to the void. But by considering the bond distances it has been clear that there is no significant difference between Pb-N bond lengths (Pb(1)-N(3) = 2.523(3) vs. Pb(1)-N(4) = 2.584(3)). Moreover, there is an intermolecular interaction between each molecule
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and two nitrato groups of the two adjacent molecules forming a step-like one-dimensional chain (Figure 9). By considering these structural aspects, the coordination of 2 is suggested to be
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holodirectional.
Figure 9. One dimensional step-like chain formed by intermolecular Pb···O interactions in
[Pb(Cup)(ONO2)(Phen)] (2). The interactions are depicted as dashed lines and hydrogen atoms are omitted for clarity.
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In the crystal structure of 1, two crystallographic independent complex molecules are present with very similar structural parameters as well as a co-crystallized DPE molecule. The notable
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difference observed in these two independent complex molecules originates from the free rotation of the two phenyl group of the coordinated Cup ligands. Similar synthetic procedures have been previously reported by Amini and coworkers for synthesis lead(II) complexes in the presence of
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Cup and 4,4’-bipyridine or 4-bpdb as bridging ligands which lead to novel coordination polymers of Pb4(4,4'-bipy)2(NO3)8(Cup)2]n and [[Pb(4-bpdb)(NO3)(Cup)]2]n, respectively [50,51]. In this
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study, DPE, a common bridging ligand which is usually used in the synthesis of lead(II) coordination polymers [52], and 4-bpdb were used as bridging ligands and additionally Phen is also used as a bidentate ligand. The crystal structure of 1 showed that DPE is not acted as a ligand but is present in the structure as a co-crystal, and consequently no polymeric structure is formed. This observation would be as a result of chelate effect of Phen in competition with DPE
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and/or coordinatively saturated formed complex after coordination of two N from Phen and four N from two Cup ligands. In case of 2, 4-bpdb was not observed even as co-crystal which would
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be as a result of crystallization and/or packing effects. It can be concluded that the addition of Phen can disrupt the polymeric structure and provides the possibility for tuning physical and
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chemical properties of such mixed ligand complexes. The presence of aromatic moieties, specifically in N-containing ligands, could lead to an important non-covalent interaction similar to the known interactions between the sheets of graphite, which is known as π-π stacking. A geometrical investigation on complexes 1 and 2 has been performed, and the result shows that in both this intermolecular interaction is present. In 1 two types of such interactions are involved. A slipped π-π interaction with interplanar distance of 3.609(3) Å between DPE and Phen ligand in co-crystal, as depicted in Figure 10, and interaction 19
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between the two Phen ligands of the two adjacent dimer molecules with a centroid-centroid distances of 3.609(3) - 3.899(3) Å which forms an extended 1D coordination polymer (Figure
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11). In 2 slipped π-π interactions between Phen ligands with interplanar distances of 3.669(2) 3.771(2) Å and slippage of 1.302 - 1.715 Å were observed in the same direction to the 1D chain
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as depicted in Figure 12.
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Figure 10. In 1 the co-crystallized DPE and Phen ligand exhibit slipped π-π interactions with an
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interplanar distance of 3.609(3) Å.
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Figure 11. Slipped π-π interactions between the two Phen ligands of the two adjacent dimer
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molecules in 2 with centroid-centroid distances of 3.669(2) - 3.771(2) Å.
Figure 12. Slipped π-π interactions between Phen ligands In 2 with interplanar distances of 3.669(2) 3.771(2) Å and slippage of 1.302 - 1.715 Å.
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The complexes have also been screened for their antimicrobial activity against bacterial species including Escherichia coli (E. coli) and B. thuringiensis as well as fungal species
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including Aspergillus and Penicillium. The results of antimicrobial activity against both bacterial and fungal species showed that the two complexes exhibited a significant inhibition zone (>20 mm). On average, the growth inhibition ring against E. coli and B. thuringiensis was 25.0 and
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24.0 mm (Table 1), respectively, showing dramatic antibacterial effectiveness of the complexes. The results of antifungal activity and the inhibition ring against the two species of Penicillium sp.
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and Aspergillus sp. also demonstrated highly significant inhibitory effect of the complexes (>20 mm).
The disc diffusion test method is a routine technique to test the sensitivity of microbial isolates in the laboratory that although was originally presented in 1966, is well standardized and
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being widely evaluated. The recorded size of inhibition indicates whether the zone size is susceptible (S), intermediate (I), or resistant (R). However, there are different charts for different micro-organisms to categorize their susceptibility or resistance to anti-microbial
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compounds but generally a zone size greater than 20 mm is considered as microbial susceptibility. The antibacterial activity of other complexes with similar type of ligands and metal
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has also been reported [53,54] and the present results are consistent with the reported values. Therefore, it seems that both complexes could be considered as new potent compounds for drug design.
Table 1. Size of inhibition zone (mm) against bacterial and fungal species after incubation in disc diffusion
test.
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E. coli 28 22
LSD* (0.05)
3.2
Bacterial species B. thuringiensis 25 23 2.1
Fungal species Penicillium Aspergillus >30 >30 >30 >30 ---
* LSD: the least significant difference for statistical inference.
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Conclusion
---
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Sample Complex 1 Complex 2
Two new Pb(II) complexes with mixed-ligands, specifically Cup and 1,10-phenanthroline, have
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been synthesized and fully characterized in solution as well as in solid state. Depending on the utilized bridging molecules, DPE and 4-bpdb, two different Pb(II) complexes can be obtained. In 1, DPE is co-crystallized and has a π-π interaction with coordinated Phen ligands. However, in 2, 4-bpdb is not involved in the crystal structure. Both structures are considered as 1D coordination
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polymer and their distinctions lie on the type of imposed interaction in the formation of such extensive structures. In 1 the dimers are formed via intermolecular interaction between Pb··· O and are further assembled into 1D chains by π-π stacking of pyridine moieties of Phen ligands.
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While, 1D step-like chain was formed by intermolecular Pb··· O interactions in 2 and may consider being further reinforced by π-π interactions. Finally, it can be concluded that the
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addition of Phen can disrupt the polymeric structure and provides the possibility for tuning physical, chemical and biological properties of such mixed ligand complexes.
Acknowledgement We acknowledge the Research Affairs Division of Isfahan University of Technology (IUT), Isfahan, for partial financial support.
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Supporting Information Available: Crystallographic data deposited at the Cambridge Crystallographic Data Centre under CCDC-1571647 for 1, and CCDC-1571648 for 2 contain the
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supplementary crystallographic data excluding structure factors; this data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or
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[email protected]).
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Highlights
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Two novel mixed-ligand complexes of lead(II) were prepared. The structures in solution were investigated by different NMR spectroscopies. The molecular structures showed hemidirected and holodirected coordinations. 1 consists of associated dimers via π-π stacking and forms 1D coordination polymer. Complex 2 forms a step-like one-dimensional chain.
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• • • • •
S0022-2860(18)30942-6
DOI:
10.1016/j.molstruc.2018.07.115
Reference:
MOLSTR 25520
To appear in:
Journal of Molecular Structure
Received Date: 15 May 2018 Revised Date:
30 July 2018
Accepted Date: 31 July 2018
Please cite this article as: G. Mohammadnezhad, F. Nasimpour, M.M. Amini, E. Najafi, H. Görls, W. Plass, M.R. Sabzalian, Synthesis, spectroscopic and X-ray structural characterization of two novel mixed-ligand lead(II) complexes, Journal of Molecular Structure (2018), doi: 10.1016/ j.molstruc.2018.07.115. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Synthesis, spectroscopic and X-ray structural characterization of two novel
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mixed-ligand lead(II) complexes
Gholamhossein Mohammadnezhada∗, Fariba Nasimpoura, Mostafa M. Aminib, Ezzatollah Najafic,
Department of Chemistry, Isfahan University of Technology, Isfahan, 84156-83111, Iran b
Department of Chemistry, Shahid Beheshti University G.C., Tehran 1983963113, Iran c
d
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a
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Helmar Görlsd, Winfried Plassd and Mohammad Reza Sabzaliane
Department of Chemistry, Payame Noor University (PNU), Tehran, Iran
Lehrstuhl für Anorganische Chemie II, Institut für Anorganische und Analytische Chemie, Friedrich-SchillerUniversität Jena, Humboldtstr. 8,07743 Jena, Germany
Department of Agronomy and Plant Breeding, College of Agriculture, Isfahan University of Technology, Isfahan
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e
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84156-83111, Iran
∗
Corresponding author. Tel.; +98-31-3391-3279; FAX: +98-31-3391-2350.
[email protected]; [email protected] (G. Mohammadnezhad).
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Abstract Two
novel
mixed-ligand
complexes
of
lead(II),
[Pb(Cup)2(Phen)]2](DPE)
(1)
and
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[Pb(Cup)(ONO2)(Phen)] (2) (Cup = cupferronato, Phen = 1,10-phenanthroline, and DPE = 1,2di(4-pyridyl)ethylene), were synthesized in the presence of common bridging ligands which are usually used in synthesis of lead(II) coordination polymers as well as utilization of Cup and Phen
investigated by 1H and
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as O,O’- and N,N’- chelates, respectively. Structural features of 1 and 2 in solution were 13
C NMR experiments. The accurate assignment of these signals was
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inferred from a series of 1H–1H COSY, 1H–13C HSQC, and 1H–13C HMBC spectra. The solidstate structures of 1 and 2 were also investigated via single crystal XRD. By considering the obtained structural parameters, the coordination of 1 is hemidirected while 2 is suggested to be holodirectional. Additionally, complex 1 consist of dimers which are further associated via π-π
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stacking to a one-dimensional coordination polymer while complex 2 forming a step-like onedimensional chain by an intermolecular interaction between each molecule and two nitrato groups of the two adjacent molecules. Both complexes showed significant antibacterial and
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antifungal characteristics in disc diffusion tests and they could be considered as new potent antimicrobial material.
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Keywords: Lead complex; Mixed ligand; Cupferron; 1,10-Phenanthroline; Crystal structure; antimicrobial activity
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1. Introduction Interest in design and synthesis of novel coordination compounds of heavy p-block elements is
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still considered as a promising issue in basic science and technology [1–6]. The physicochemical properties of different metal complexes not only depend on the nature of the central atom/ion but also related to the structural, spectroscopic and other properties of coordinated ligands. A
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synthetic challenge for tuning the physicochemical properties of the coordination compounds is the preparation of mixed-ligands complexes [6–12]. Usually, such mixed-ligand metal complexes
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show enhanced performance for instance in biological activities [13,14], electroluminescence devices [15], thermal stability [16], and photophysical properties [17,18]. Among the p-block mixed metal complexes, lead(II) coordination compounds are of particular interest due to the rapid developments of their applications in advanced materials [19–21].
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Regardless of the toxic nature and environmental issues, Pb(II) with its large radius and flexible coordination, as well as other heavy p-block ions such as Sn(II), Tl(I), and Bi(III), has unique coordination chemistry due to the role and effect of 6s2 lone pair. This effect has been
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widely studied and historically goes back to 1941 when Pauling suggest the valence shell lone pair influence on the stereochemistry of Pb(II) coordination sphere [22]. As the lone pair is not
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directly detectable, different criteria have been explored both experimentally and theoretically [23–26]. Specifically, Shimoni-Livny et al. rationalized the relation of coordination sphere geometry of Pb(II) complexes in terms of the lone pair [25]. Based on the location of the ligands and their bond lengths in the coordination sphere of the complex, Pb(II) complexes were categorized in two general structural features as hemidirected and holodirected. In hemidirected geometry, an obvious coordination site vacancy or void, as well as unusual asymmetric bond
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length, is observable. While in holodirected geometry, ligands are distributed in coordination sphere nearly uniform and depict considerable symmetrical bond lengths. These two categories
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are not the only acceptable forms of geometries, while in some cases unsymmetrical coordination and vacancy/void does not necessarily is a prove of a stereochemically active lone pair and there is a need to apply more analyses such as considering unusual structural parameters and high-level
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computational chemistry approaches [27–33].
Cupferron, ammonium N-Nitroso-N-phenylhydroxylamine, is a good chelating agent which
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is widely used in analytical chemistry and biology for instance, in separation, precipitation, electrochemistry and colorimetric analysis of metals [34,35]. Also, metal cupferron complexes have been used as precursors for the preparation of nanocrystalline metal oxides [36]. The anion, cupferronato (Cup), could act as a bidentate ligand and binds to Pb(II) via the two oxygen atoms
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and forming a five-membered chelate ring [37–39]. In this report, to investigate the structural parameters imposed by the ligands, two novel mixed-ligand complexes of lead(II) were synthesized and characterized both in solid-state and solution. The syntheses were performed in
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the presence of common bridging ligands which are usually used in the synthesis of lead(II) coordination polymers as well as utilization of Cup and phenanthroline (Phen) as O,O’- and
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N,N’- chelate, respectively.
2. Experimental Section 2.1. Materials and techniques Pb(NO3)2 (Merck, 99.0 % trace metals basis), 1,2-di(4-pyridyl)ethylene (DPE) (Aldrich, 97%), cupferron (N-Nitroso-N-phenylhydroxylamine ammonium salt, Aldrich, 97%, reagent grade), 4
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1,10-Phenanthroline (Phen) (Merck, anhydrous for synthesis 99%) and 4-bpdb (1,4-bis(4pyridyl)-2,3-diaza-1,3-butadiene) [40] were used without further purification. Anhydrous
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methanol was prepared according to a standard procedure [41]. FT-IR spectra were recorded on a Bruker IFS55/Equinox spectrometer utilizing a diamond ATR unit or on a JASCO 680-PLUS spectrometer using KBr pellets from 250-4000 cm-1. 1H, 13C, 1H{1H} COSY, 1H{13C} HSQC and
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HMBC NMR spectra were recorded at room temperature on a Bruker Avance DRX400-MHz
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spectrometer.
2.2. Synthesis of [Pb(Cup)2(Phen)]2](DPE) (1)
DPE (0.2 mmol, 0.036 g), Phen (0.2 mmol, 0.036 g), and cupferron (0.2 mmol, 0.031 g) were mixed at room temperature in a solution of Pb(NO3)2 (0.1 mmol, 0.033 g) in methanol (20 mL), respectively. After the addition of each of the mentioned ligands, the resulting solution was
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stirred adequately until a clear solution was obtained. At last, for complete dissolution of the ingredients, a few drops acetonitrile was added. Suitable single crystals were grown by simple
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slow evaporation technique. FT-IR spectrum (ATR, cm-1): 379 (vs), 413 (m), 478 (s), 541 (w), 553 (s), 572 (m), 584 (w), 631 (m), 688 (vs), 715 (s), 728 (s), 762 (vs), 829 (s), 838 (s), 859 (m),
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910 (s), 927 (w), 979 (m), 999 (m), 1019 (m), 1040 (m), 1056 (m), 1100 (m), 1140 (m), 1163 (w), 1199 (s), 1208 (s), 1225 (s), 1261 (s), 1269 (s), 1288 (s), 1301 (s), 1329 (s), 1348 (w), 1417 (s), 1425 (s), 1460 (m), 1487 (m), 1512 (m), 1558 (w), 1589 (w), 1591 (s), 1598 (s), 1621 (w), 2810 (w), 3045 (w), and 3073 (w). 1H NMR (400 MHz, DMSO-d6, 25 °C, ppm): 7.38-7.52 (m, 6H, H-Cup), 7.54 (s, 1H, H-13), 7.61 (dd, 2H, J = 7.61 Hz, H-12), 7.79-7.89 (m, 6H, H-5, HCup), 8.02 (s, 2H, H-7), 8.49-8.57 (dd, J = 8.54 Hz, 2H, H-6), 8.60 (dd, J = 8.60 Hz, 2H, H-11), 9.13-9.24 (dd, J = 9.17 Hz, 2H, H-4). 13C NMR (100 MHz, DMSO-d6, 25 °C, ppm): 119.3 (C-1), 5
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121.3 (C-12), 123.6 (C-5), 126.8 (C-7), 128.7, 128.8 (C-3 and C-9), 129.3 (C- 2), 130.6 (C-13),
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136.6 (C-6), 142.2 (C-10), 143.3 (C-14), 145.3 (C-8), 149.8 (C-4), 150.2 (C-11).
2.3. Synthesis of [Pb(Cup)(ONO2)(Phen)] (2)
4-bpdb (0.2 mmol, 0.042 g), Phen (0.2 mmol, 0.036 g), and cupferron (0.2 mmol, 0.031 g) were
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mixed at room temperature with a methanolic solution (20 mL) of Pb(NO3)2 (0.1 mmol, 0.033 g). After the addition of each of the mentioned ligands, the resulting solution was stirred adequately
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until a clear solution was obtained. Finally, for complete dissolution of constituents, a few drops dichloromethane was added. Suitable single crystals were grown by simple slow evaporation technique. FT-IR spectrum (ATR, cm-1): 380 (s), 410 (m), 417 (m), 471 (s), 510 (w), 555 (w), 582 (w), 623 (w), 637 (s), 692 (vs), 722 (vs), 774 (s), 782 (m), 808 (w), 822 (m), 849 (s), 863 (w), 908 (s), 824 (w), 985 (m), 1019 (w), 1040 (w), 1061 (m), 1099 (m), 1145 (br, m), 1193 (s),
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1210 (w), 1290 (br, vs), 1374 (s), 1424 (m), 1459 (w), 1493 (m), 1514 (m), 1572 (w), 1592 (w), 1622 (w), 2922 (w), and 3063 (w). 1H NMR (400 MHz, DMSO-d6, 25 °C, ppm): 7.42-7.57 (m, 3H, H-Cup), 7.83-7.89 (m, 2H, H-Cup), 7.91-7.99 (m, 2H, H-5), 8.09 (s, 2H, H-7), 8.61-8.69 (dd,
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J = 865 Hz, 2H, H-6), 9.17-9.25 (dd, J = 9.21 Hz, 2H). 13C NMR (100 MHz, DMSO-d6, 25 °C,
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ppm): 119.4 (C-1), 124.1 (C-5), 127.1 (C-7), 129.27, 129.30 (C-2 and C-3), 129.5 (C-10), 137.7 (C-6), 142.0 (C-9), 144.9 (C-8), 149.8 (C-4).
2.4 Crystallographic Data and Refinement The intensity data were collected on a Nonius Kappa CCD diffractometer, using graphitemonochromated Mo-Kα radiation. Data were corrected for Lorentz and polarization effects; absorption was taken into account on a semi-empirical basis using multiple-scans [42–44]. The 6
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structure was solved by direct methods (SHELXS [45]) and refined by full-matrix least squares techniques against Fo2 (SHELXL-97 [45]). All hydrogen atom positions were included at XP software was used for structure
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calculated positions with fixed thermal parameters. representations [46].
Crystal data for 1: C30.75H25N7.25O4.38Pb, Mr = 773.27 gmol-1, colourless prism, size 0.034 × 0.032 × 0.030 mm3, triclinic, space group Pī, a = 10.4521(3), b = 10.5148(3), c = 27.9560(8) Å, α =
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79.617(1), β = 87.222(1), γ = 83.174(1) °, V = 2999.53(15) Å3, T = -140 °C, Z = 4, ρcalcd. = 1.712 gcm-3, µ (Mo-Kα) = 56.75 cm-1, multi-scan, transmin: 0.5781, transmax: 0.7456, F(000) = 1509,
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32625 reflections in h(-11/13), k(-13/13), l(-36/36), measured in the range 2.22° ≤ Θ ≤ 27.48°, completeness Θmax = 97.4%, 13415 independent reflections, Rint = 0.0332, 11780 reflections with Fo > 4σ(Fo), 781 parameters, 0 restraint, R1obs = 0.0391, wR2obs = 0.0878, R1all = 0.0473, wR2all = 0.0924, GOOF = 1.053, largest difference peak and hole: 1.883 / -1.147 e Å-3. Crystal data for 2: C18H13N5O5Pb, Mr = 586.52 gmol-1, colourless prism, size 0.034 × 0.032 × 0.024 mm3, monoclinic, space group P21/n, a = 6.4932(2), b = 10.0679(3), c = 28.0378(8) Å, β =
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95.079(1)°, V = 1825.72(9) Å3 , T = -140 °C, Z = 4, ρcalcd. = 2.134 gcm-3, µ (Mo-Kα) = 92.84 cm-1, multi-scan, transmin: 0.4854, transmax: 0.7456, F(000) = 1112, 21466 reflections in h(-8/8), k(13/13), l(-36/36), measured in the range 2.49° ≤ Θ ≤ 27.50°, completeness Θmax = 98.7%, 4151
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independent reflections, Rint = 0.0348, 3977 reflections with Fo > 4σ(Fo), 262 parameters, 0 restraint, R1obs = 0.0232, wR2obs = 0.0518, R1all = 0.0247, wR2all = 0.0527, GOOF = 1.142, largest
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difference peak and hole: 1.320 / -1.506 e Å-3.
2.5 Antimicrobial assay
The disc diffusion method was performed in Petri dishes containing solid Nutrient Agar (NA) medium. A microbial suspension (1 ml) of E. coli and B. thuringiensis was spread by a sterile swab, evenly over the face of a sterile nutrient agar plate. Two complexes of 1 and 2 were applied to the center of the agar plate into a disc shapes with 10 mm diameter (10 mg of each) in four
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replicates. The same method was used for antifungal assay in which spores of two saprophytic fungi of Penicillium sp. and Aspergillus sp. were spread over the surface of agar plates after
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placing the complexes as described for antibacterial test. Agar plates were then incubated for 48 h at 25±2 °C and inhibition zone was monitored. The size of inhibition zone (diameter of inhibition) was considered as the level of antimicrobial activity present in the two complexes-the
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larger zone of inhibition, the more potential of antimicrobial activity.
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3. Result and discussion
Syntheses of mixed-ligand complexes allow the variation in physical and chemical properties, including geometrical parameters, hydrophobicity, and intra/inter molecular interactions by systematic variation of complex components. In this regards, two new complexes were synthesized by reaction of Pb(NO3)2 and Phen, cupferron and two different bipyridyl (bpy)
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molecules. The initial molar ratio of Pb(NO3)2:phen:cupferron:bpy was 1:2:2:2 for both of the reactions. As we will see in the following sections, the formula of the isolated complexes, [Pb(Cup)2(Phen)]2(DPE)] (1) and [Pb(Cup)(ONO2)(Phen)] (2), showed that the molar ratio of the
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utilized reactants was different as illustrated in Scheme 1 and 2. Notably, when the reactions were
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repeated based on the new molar ratios and in the absence of bpy molecule for preparation of 2, the reactions ends up to the same products. In this regard, the optimized conditions were used for preparation and characterization of these complexes. The obtained crystals were stable at ambient temperature and can be stored for a long period without any changes.
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Scheme 1. Schematic representation for synthesis of [Pb(Cup)2(Phen)]2(DPE)] (1).
Scheme 2. Schematic representation for synthesis of [Pb(Cup)(ONO2)(Phen)] (2).
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FT-IR spectra of Phen, DPE, and cupferon ligands as well as the synthesized complexes, 1 and 2, have been studied. In the FT-IR spectrum of cupferon, the absorption bands include ring,
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NO, CN and NN vibrations as well as skeletal vibrations of ONNO [47]. The ammonium cation (NH4+) stretching and deformation modes are usually seen as a broad band around 3130 cm–1 and strong band around 1400 cm−1, respectively. Additionally, the typical bands at 1230, 1275 and 1339 cm−1 are generally associated with the NO (vs and vas) and NN stretching, respectively. In addition, the bending mode of the ONNO emerged at 912 cm-1. As expected, upon coordination of Cup to Pb and formation of Pb-O bonds, the related ammonium stretching bands of cupferon
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were omitted in the spectra of 1 and 2 (Figure 1). Additionally, the considerable shift of NO and ONNO bands are in agreement with coordination of Cup to the metal center. The spectrum of 1
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depicts a band at 1591 cm-1 that has been assigned to C=C of DPE. The presence of unidentate nitrato ligand in complex 2 was confirmed by the observed bands at 1374, 1290, and 1145 cm−1 for va(NO2), vs(NO2), and v(NO), respectively [48]. The appearance of the Phen characteristic
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peaks confirmed the existence of this ligand in both complexes. The binding mode of the Cup ligand was normally followed by the vibrational stretching frequencies of M-O bonds which are
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positioned in the range of 440-380 cm-1 [48]. In the fingerprint area of metal chelate (below 500
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cm-1), Pb-O and Pb-N frequencies are appeared at about 380, 410 and 470 cm-1, respectively.
Figure 1. FT-IR spectra (ATR) of a) [Pb(Cup)2(Phen)]2(DPE)] (1) and b) [Pb(Cup)(ONO2)(Phen)] (2)
Structural features of 1 and 2 in solution were investigated by 1H and 13C NMR experiments. The accurate assignment of these signals was inferred from a series of 1H–1H COSY, 1H–13C 10
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HSQC, and 1H–13C HMBC spectra. As mentioned, DPE is present in the crystal structure of 1 as a co-crystal and the general formula is [Pb(Cup)2(Phen)]2(DPE). Based on this formula each DPE
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molecule is co-crystalized with two Pb(II) complex units. In 1 each complex includes 18 hydrogens and each DPE has 10 hydrogens; so, the expected hydrogens are 46 which are consistent with the total integration of resonances in aromatic region (See Figure S1 for 1H NMR
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of 1). As expected, the integration ratio of DPE:Phen:Cup signals in 1H NMR is 1:2:4. For instance, in DPE H-10, ethylene hydrogens, is assigned at 7.42 ppm as a singlet with the
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integration of 2, while the other DPE hydrogens, H-8 and H-9, are positioned at 8.60 and 7.61 ppm as a doublet of doublet signals with the integration 4 for each, respectively. COSY 1H NMR of 1, as well as the assigned hydrogens of each signal, is depicted in Figure 2. In 13C NMR of 1 (see Figure S2) fourteen different signals related to ligands (Phen and Cup) and co-crystallized DPE were observed in the range of 118-151 ppm. The resonances accurately assigned by 1H–13C
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HSQC as depicted in Figure 3. The assignments consistency was further investigated and evaluated by 1H–13C HMBC as depicted in Figure S3. For instance, H-8 is in the neighborhood
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of C-9 and a bit farther to C-14, while there is no correlation observed with its assigned signal. In the structure of complex 2, [Pb(Cup)(ONO2)(Phen)], 13 hydrogens are present which is in (See Figure S4).
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agreement with the integration of observed signals of its 1H NMR spectrum
The integration ratio of Phen:Cup signals in 1H NMR, 1:1, is consistence with the chemical formula. COSY 1H NMR of 2, as well as the assigned hydrogens of each signal, is depicted in Figure 4. In
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C NMR of 2 (see Figure S5) 9 different signals related to Phen and Cup were
observed in the range of 118-150 ppm. The resonances accurately assigned by 1H–13C HSQC as depicted in Figure 5. The assignments consistency was further investigated and evaluated by 1H–
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13
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C-8, while there is no correlation observed with its assigned signal.
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Figure 2. 1H{1H} COSY NMR spectrum of [Pb(Cup)2(Phen)]2(DPE)] (1) in DMSO-d6.
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Figure 3. 1H{13C} HSQC NMR spectrum of [Pb(Cup)2(Phen)]2(DPE)] (1) in DMSO-d6.
Figure 4. 1H{1H} COSY NMR spectrum of [Pb(Cup)(ONO2)(Phen)] (2) in DMSO-d6.
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Figure 5. 1H{13C} HSQC NMR spectrum of [Pb(Cup)(ONO2)(Phen)] (2) in DMSO-d6.
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The two lead(II) complexes, [Pb(Cup)2(Phen)]2(DPE)] (1) and [Pb(Cup)(ONO2)(Phen)] (2), were crystallized in triclinic Pī and monoclinic P21/n space groups, respectively. The molecular structure of 1 contains a six-coordinated distorted octahedral Pb(II) center and formed by an
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overall [N2O4] coordination environment. The Pb(II) coordination sphere in 1 includes three bidentate ligands; two cis Cup ligands and one Phen in a propeller-like positioning around the
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Pb(II) center (Figure 6). The molecular structure of 2 contains a five-coordinated Pb(II) center and formed by an overall [N3O2] coordination environment. The Pb(II) coordination sphere in 2 includes two bidentate ligands; one Cup ligand and one Phen as well as a monodentate coordinated nitrato group (Figure 7).
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Figure 6. Molecular structure of [Pb(Cup)2(Phen)]2(DPE)] (1). The ellipsoids represent a probability of
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30%. Hydrogen atoms are omitted for clarity
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Figure 7. Molecular structure of [Pb(Cup)(ONO2)(Phen)] (2). The ellipsoids represent a probability
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of 30 %. Hydrogen atoms are omitted for clarity
The arrangement of ligands around the lead center determines its coordination geometries
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which itself is also affected by the lone pair of electrons on Pb(II) centers. In both complexes, the N and O atoms arrangement shows that the coordinated bonds are directed to one side of
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coordination sphere. This is known as hemidirected coordination in the chemistry of Pb complexes in which a stereoactive lone pair of electrons on Pb centers is identifiable as a void in the crystal structure of the complexes [25]. Some structural parameters are also required to be checked, including the Pb-O bond length of ligands opposite the void (axial) as compared to the other Pb-O bonds (equatorial) [49]. In the crystal structure of 1, the axial Pb-O bond has the shortest distances (2.35 Å) while the other equatorial Pb-O bonds are longer (ca. 2.5 Å). These
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data are consistent with hemidirectional geometry of complex 1. While in the case of holodirectional geometry all the Pb-O distances are nearly equivalent. Additionally, there is a
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Pb···O interaction (3.072(3) and 3.159(4) Å) between two adjacent molecules which form a dimer structure as depicted in Figure 8. By considering this interaction, the structure of complex 1 will continue to be as hemidirectional based on the crystallographic evidence. The molecular
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packing of 1 is illustrated in Figure 8.
Figure 8. The molecular packing of [Pb(Cup)2(Phen)]2(DPE)] (1)
Generally, Pb(II) complexes with low coordination numbers (2-5) are hemidirected while for intermediate coordination numbers (6-8), cases of either type of stereochemistry are observed. In the case of 2, the coordination number is five, all the coordinated bonds are directed to one side 17
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of coordination sphere, and all the bond angles are considerably below 180 ̊ (the maximum value is 135.66 ̊ ). Thus, 2 may be considered as a hemidirected complex with one of the Phen N atom
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is positioned in the trans position to the void. But by considering the bond distances it has been clear that there is no significant difference between Pb-N bond lengths (Pb(1)-N(3) = 2.523(3) vs. Pb(1)-N(4) = 2.584(3)). Moreover, there is an intermolecular interaction between each molecule
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and two nitrato groups of the two adjacent molecules forming a step-like one-dimensional chain (Figure 9). By considering these structural aspects, the coordination of 2 is suggested to be
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holodirectional.
Figure 9. One dimensional step-like chain formed by intermolecular Pb···O interactions in
[Pb(Cup)(ONO2)(Phen)] (2). The interactions are depicted as dashed lines and hydrogen atoms are omitted for clarity.
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In the crystal structure of 1, two crystallographic independent complex molecules are present with very similar structural parameters as well as a co-crystallized DPE molecule. The notable
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difference observed in these two independent complex molecules originates from the free rotation of the two phenyl group of the coordinated Cup ligands. Similar synthetic procedures have been previously reported by Amini and coworkers for synthesis lead(II) complexes in the presence of
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Cup and 4,4’-bipyridine or 4-bpdb as bridging ligands which lead to novel coordination polymers of Pb4(4,4'-bipy)2(NO3)8(Cup)2]n and [[Pb(4-bpdb)(NO3)(Cup)]2]n, respectively [50,51]. In this
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study, DPE, a common bridging ligand which is usually used in the synthesis of lead(II) coordination polymers [52], and 4-bpdb were used as bridging ligands and additionally Phen is also used as a bidentate ligand. The crystal structure of 1 showed that DPE is not acted as a ligand but is present in the structure as a co-crystal, and consequently no polymeric structure is formed. This observation would be as a result of chelate effect of Phen in competition with DPE
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and/or coordinatively saturated formed complex after coordination of two N from Phen and four N from two Cup ligands. In case of 2, 4-bpdb was not observed even as co-crystal which would
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be as a result of crystallization and/or packing effects. It can be concluded that the addition of Phen can disrupt the polymeric structure and provides the possibility for tuning physical and
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chemical properties of such mixed ligand complexes. The presence of aromatic moieties, specifically in N-containing ligands, could lead to an important non-covalent interaction similar to the known interactions between the sheets of graphite, which is known as π-π stacking. A geometrical investigation on complexes 1 and 2 has been performed, and the result shows that in both this intermolecular interaction is present. In 1 two types of such interactions are involved. A slipped π-π interaction with interplanar distance of 3.609(3) Å between DPE and Phen ligand in co-crystal, as depicted in Figure 10, and interaction 19
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between the two Phen ligands of the two adjacent dimer molecules with a centroid-centroid distances of 3.609(3) - 3.899(3) Å which forms an extended 1D coordination polymer (Figure
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11). In 2 slipped π-π interactions between Phen ligands with interplanar distances of 3.669(2) 3.771(2) Å and slippage of 1.302 - 1.715 Å were observed in the same direction to the 1D chain
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as depicted in Figure 12.
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Figure 10. In 1 the co-crystallized DPE and Phen ligand exhibit slipped π-π interactions with an
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interplanar distance of 3.609(3) Å.
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Figure 11. Slipped π-π interactions between the two Phen ligands of the two adjacent dimer
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molecules in 2 with centroid-centroid distances of 3.669(2) - 3.771(2) Å.
Figure 12. Slipped π-π interactions between Phen ligands In 2 with interplanar distances of 3.669(2) 3.771(2) Å and slippage of 1.302 - 1.715 Å.
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The complexes have also been screened for their antimicrobial activity against bacterial species including Escherichia coli (E. coli) and B. thuringiensis as well as fungal species
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including Aspergillus and Penicillium. The results of antimicrobial activity against both bacterial and fungal species showed that the two complexes exhibited a significant inhibition zone (>20 mm). On average, the growth inhibition ring against E. coli and B. thuringiensis was 25.0 and
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24.0 mm (Table 1), respectively, showing dramatic antibacterial effectiveness of the complexes. The results of antifungal activity and the inhibition ring against the two species of Penicillium sp.
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and Aspergillus sp. also demonstrated highly significant inhibitory effect of the complexes (>20 mm).
The disc diffusion test method is a routine technique to test the sensitivity of microbial isolates in the laboratory that although was originally presented in 1966, is well standardized and
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being widely evaluated. The recorded size of inhibition indicates whether the zone size is susceptible (S), intermediate (I), or resistant (R). However, there are different charts for different micro-organisms to categorize their susceptibility or resistance to anti-microbial
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compounds but generally a zone size greater than 20 mm is considered as microbial susceptibility. The antibacterial activity of other complexes with similar type of ligands and metal
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has also been reported [53,54] and the present results are consistent with the reported values. Therefore, it seems that both complexes could be considered as new potent compounds for drug design.
Table 1. Size of inhibition zone (mm) against bacterial and fungal species after incubation in disc diffusion
test.
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E. coli 28 22
LSD* (0.05)
3.2
Bacterial species B. thuringiensis 25 23 2.1
Fungal species Penicillium Aspergillus >30 >30 >30 >30 ---
* LSD: the least significant difference for statistical inference.
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Conclusion
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Sample Complex 1 Complex 2
Two new Pb(II) complexes with mixed-ligands, specifically Cup and 1,10-phenanthroline, have
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been synthesized and fully characterized in solution as well as in solid state. Depending on the utilized bridging molecules, DPE and 4-bpdb, two different Pb(II) complexes can be obtained. In 1, DPE is co-crystallized and has a π-π interaction with coordinated Phen ligands. However, in 2, 4-bpdb is not involved in the crystal structure. Both structures are considered as 1D coordination
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polymer and their distinctions lie on the type of imposed interaction in the formation of such extensive structures. In 1 the dimers are formed via intermolecular interaction between Pb··· O and are further assembled into 1D chains by π-π stacking of pyridine moieties of Phen ligands.
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While, 1D step-like chain was formed by intermolecular Pb··· O interactions in 2 and may consider being further reinforced by π-π interactions. Finally, it can be concluded that the
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addition of Phen can disrupt the polymeric structure and provides the possibility for tuning physical, chemical and biological properties of such mixed ligand complexes.
Acknowledgement We acknowledge the Research Affairs Division of Isfahan University of Technology (IUT), Isfahan, for partial financial support.
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Supporting Information Available: Crystallographic data deposited at the Cambridge Crystallographic Data Centre under CCDC-1571647 for 1, and CCDC-1571648 for 2 contain the
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supplementary crystallographic data excluding structure factors; this data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or
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[email protected]).
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Highlights
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Two novel mixed-ligand complexes of lead(II) were prepared. The structures in solution were investigated by different NMR spectroscopies. The molecular structures showed hemidirected and holodirected coordinations. 1 consists of associated dimers via π-π stacking and forms 1D coordination polymer. Complex 2 forms a step-like one-dimensional chain.
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