Molecular structure and antimicrobial activity of a luminescent dinuclear silver(I) complex of phenyl-bis(2-pyridyl)phosphine

Article abstract of DOI:10.1002/zaac.201100235

Phenyl bis(2-pyridyl)phosphine [PhP(2-py)₂] and its silver nitrate complex of formula [Ag₂(μ-(PhP(2-py)₂) ₂(NO₃)₂] (1) were synthesized and characterized by elemental analysis, IR spectrosc

Full text of DOI:10.1002/zaac.201100235

ARTICLE
DOI: 10.1002/zaac.201100235
Molecular Structure and Antimicrobial Activity of a Luminescent Dinuclear
Silver(I) Complex of Phenyl-bis(2-pyridyl)phosphine
Ali Nemati Kharat,*[a] Abolghasem Bakhoda,[a] Sahar Foroutannejad,[b] and
Cina Foroutannejad[a]
Keywords: Phenyl-bis(2-pyridyl)phosphine; Silver; Crystal structure; QTAIM analysis; Antimicrobial activity
Abstract. Phenyl bis(2-pyridyl)phosphine [PhP(2-py)2] and its silver
silver atoms in same environments. Biological studies, carried out in
nitrate complex of formula [Ag2(μ-(PhP(2-py)2)2(NO3)2] (1) were syn- vitro against wide range of gram positive and gram negative bacteria,
thesized and characterized by elemental analysis, IR spectroscopy, sin- have shown that the free [PhP(2-py)2] ligand and its silver complex
gle-crystal X-ray diffraction, and computational studies. The silver(I) show distinct differences in the biological properties.
complex 1 with a 1:1 metal-ligand molar ratio is dinuclear cluster with
Introduction
Results and Discussion
Synthesis
The chemistry of pyridylphosphines and their transition
metal complexes have received considerable attention, primar-
ily because of their significant coordination properties.[1,2] In
addition, pyridylphosphines are versatile ligands, which are
able to coordinate as neutral ligands and are flexible spacers
The new complex was synthesized with the ligand phenyl-
bis(2-pyridyl)phosphine as illustrated in Scheme 1. AgNO3 re-
acted with the ligand in an ethanol:water mixture (90:10). The
complex separated as colorless block-shaped crystals from the
with potential multiple binding sites. Pyridylphosphines can be mother liquor after a few days in the refrigerator.
used to construct metal complexes with various topologies.[3–
7]
However, their coordinative behavior markedly depends on
the transition metal ions employed.
In recent years, the design of silver(I) complexes has
attracted particular attention due to their interesting structures,
photoluminescent properties, biological, and pharmacological
activities, such as anticancer, antibacterial and antifungal
properties.[8–15] To the best of our knowledge, the biological
properties of metal complexes of pyridylphosphines are
ignored. As part of our ongoing studies, the synthesis and crys-
tal structure determination of
a
silver(I) complex
[Ag2(μ-(PhP(2-py)2)2(NO3)2] (1) of phenyl-bis(2-pyridyl)phos-
phine [PhP(2-py)2] and its spectroscopic and computational in-
vestigation are described herein. Biological studies of the free
ligand and the title complex were performed in vitro against a
wide range of human pathogens.
Scheme 1. Synthesis of pyridylphosphine ligand and its silver nitrate
dimer complex.
* Dr. A. Nemati Kharat
Fax: +98-21-66495291
E-Mail: alnema@khayam.ut.ac.ir
[a] School of Chemistry, University College of Science
University of Tehran
Spectroscopic Characterization
Tehran, Iran
[b] Department of Basic Sciences, Faculty of Allied Medicine
Hemmat Campus
In the IR spectra of the ligand and the complex the ν(C=N)
band is found at 1594 and 1566 cm–1, respectively. The de-
crease in the frequency of this band in the spectrum of the
Tehran University of Medical Sciences
Tehran, Iran
2260
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2011, 637, (14-15), 2260–2264

Antimicrobial Activity of a Luminescent Dinuclear AgI Complex
complex is an evidence for the coordination by the pyridyl rus atom of the neighboring ligand. Within the structure of the
nitrogen atom.[16] In the free ligand, a band at 694 cm–1 is complex, the Ag···Ag distance is 3.216(9) Å, this distance is
assigned to ν(C–P), whereas in complex 1 this band is found shorter than the sum of the van der Waals radii of two silver
to be shifted to lower frequency (662 cm–1), indicating the co- atoms (3.44 Å),[18] suggesting the existence of some metal–
ordination of the phosphorus atom to the silver ion. The bands metal interaction between the silver atoms.
between 3069–2957 cm–1 could be explained as νas and νs(CH)
vibrations, which is in agreement with earlier results.[16] The
presence of nitrate anions was confirmed by sharp bands,
which appeared in the range of 1425–1460 cm–1.[16]
In the 31P NMR spectrum of [Ag2(μ-(PhP(2-py)2)2(NO3)2]
(1), a single broad signal was observed at δ = 3.15 at room
temperature in [D3]MeCN. It confirmed that in solution the
complex may keep the same structure as in the solid state.
Electronic Studies
The UV/Vis spectrum of compound 1 shows absorbancea at
226, 263, and 548 nm. The first two bonds are assigned to
inter-ligand transitions (nǞπ*, πǞ*π), whereas the ab-
sorbance at 548 nm could correspond to charge transfer pro-
Figure 2. Molecular structure of the complex 1. All the hydrogen
atoms have been omitted for clarity. Selected bond lengths /Å and
angles /°: Ag(1)–N(1) 2.365(5), Ag(1)–N(2) 2.319(5), Ag(1)–P(1a)
cesses between silver atom and ligand.[17] The emission spec-
tra of compound 1 and the free ligand at room temperature in
liquid state show fascinating blue luminescence. Figure 1
shows the luminescence spectrum of complex 1 in MeCN.
When excited at λex = 285 nm, it exhibited maximal emission
peaks at 449 nm. The possible origin of these peaks is tenta-
tively attributed to ligand-to-ligand transitions from the pyr-
idylphosphine ligand, which is in agreement with silver(I) co-
ordination of P/N-containing multi-topic ligands reported by
others.[17] In addition, the relatively efficient π–π interactions
2.3607(17), Ag(1)–O(1) 2.444(5), P(1)–C(5) 1.849(6), P(1)–C(6)
1.857(6), P(1)–C(11) 1.804(6), N(3)–O(1) 1.265(7), N(3)–O(2)
1.240(7), N(3)–O(3) 1.244(7), Ag(1)···Ag(1a) 3.2164(9) and N(1)–
Ag(1)–N(2) 83.42(18), O(1)–Ag(1)–N(1) 83.79(16), O(1)–Ag(1)–N(2)
95.73(18), O(1)–Ag(1)–P(1a) 119.06(11), N(2)–Ag(1)–Ag(1a),
82.62(14),
N(2)–Ag(1)–P(1a)
131.34(15),
N(1)–Ag(1)–P(1a)
129.90(13).
The Ag–N average bond length is 2.342(5) Å, similar to
between phenyl and pyridyl rings in 1 may be a reason for those found in the complexes of silver(I) and nitrogen-contain-
the red shift of the emission peak in the spectrum of the free ing heterocyclic ligands.[19] Each phosphorus atom bridges
ligand.
symmetrically silver atoms with a Ag–P bond length of
2.361(2) Å, which is in the same range as other poly-nuclear
silver(I) complexes.[20,21] As depicted in Figure 3, the free
oxygen atoms of nitrate ligands interact with hydrogen atoms
from the pyridyl groups to form the hydrogen bonding.
Figure 1. Emission spectrum of 1 in MeCN (c = 10–5 mol·l–1) at ambi-
ent temperature (λex = 285 nm).
X-ray Crystallography
Single-crystal X-ray analysis reveals that complex 1 crys-
tallizes in monoclinic system, with space group P21/n. Figure 2
shows that the structure of complex 1 consists of two silver
atoms as well as two ligand molecules that act as tridentate
ligands besides two nitrate ions. Each silver atom is considered
to be tetracoordinated by two nitrogen atoms of pyridyl the
Figure 3. Unit cell contents of AgNO3/PhP(2-py)2 (1:1) along the
crystallographic a axis. C–H···O and C–H···π interactions are presented
rings, one oxygen atom of the nitrate anion and one phospho- as dashed line.
Z. Anorg. Allg. Chem. 2011, 2260–2264
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A. Nemati Kharat et. al.
ARTICLE
The separations for C(2)···O(3), C(3)···O(2), C(8)···O(1), and Table 2. Antimicrobial activities of complex 1 and free ligand evalu-
ated by MIC /μg·mL–1.
C(10)···O(3) (symmetry code: 1/2+x, 3/2–y, 1/2+z, 5/2–x,
–1/2+y, 3/2–z and –1+x, y, z) are 3.369(8), 3.355(8), 3.313(8),
and 3.332(8) Å with C–H···O angles of 171.00, 165.00, 149.00,
and 143.00°, respectively.
These observations were also confirmed by single-crystal X-
ray structure analysis. As it was mentioned above, the Ag···Ag
distance in the studied complex is less than the van der Waals
radii of silver. In order to verify the presence of silver–silver
intramolecular bonding, the electron density of the synthesized
complex was studied in the context of Quantum Theory of
Atoms in Molecules (QTAIM).[22]
1
Ligand SS
Gram+bacteriaBacillus sp.
16
8
8
4
4
32
32
32
16
16
16
16
8
16
8
16
8
Bacillus cereus
Bacillus megaterium
Micrococcus luteus.
Staphylococcus aureus
Stomatococcus mucilag-
inosus
8
Gram– bac- Escherichia coli
teria
8
16
8
Burkholderia mallei
Neisseria polysaccharea
Corynebacterium minutissi-
num
Corynebacterium ure-
alyticum
8
16
8
32
32
4
64
16
8
Computational Study
16
16
16
A bond path connects two silver atoms to each other; the
characteristics of the (3, –1) critical points on the bond paths
were widely used for defining the type of bonding among dif-
ferent molecules in particular metallic complexes.[23–25]
Table 1 represents the characteristics of the (3, –1) cp and the
electron delocalization between two silver atoms in the com-
plex as well as the optimized Ag2 molecule in its singlet elec-
tronic state [at B3LYP/DZVP (DFT orbitals)].
The electron density, Laplacian of electron density, and Lag-
rangian kinetic energy of the (3, –1) cp in the complex are less
than the Ag2 molecule; this is in line with the increase of bond
length between two silver atoms in the complex compared with
the Ag2 molecule. In the Ag2 molecule the electron delocaliza-
tion between two atoms is considerable and denotes presence
of covalent bonding but in the complex, Ag–Ag delocalization
is about ten times less than the free Ag2 and shows that the
bond is much weaker. The ratio of Lagrangian kinetic energy
to the electron density also confirms covalent bonding (less
than unity, which is usual for shared or covalent bonds); this
ratio is similar for both species in Table 2. On the other hand,
the amount of electron density is not much considerable and
its Laplacian is positive, which is characteristic of metallic
bonding in many dinuclear metallic complexes.[26]
Capnocytophaga cynodegmi 32
Pasteurella lymphangitidis 16
16
16
16
8
Table 2). Based on the minimum inhibitory concentration
(MIC), the title compound displays good antibacterial effect
and more inhibitory properties against Gram positive bacteria
than against Gram negative bacteria. The highest activity in-
vestigated for 1 and PhP(2-py)2 in this test are against Gram
positive Staphylococcus aureus, Micrococcus luteus, and Gram
negative Corynebacterium minutissinum, respectively. In ad-
dition, the free ligand showed lower activity compared to its
silver complex. The distinct difference in the antibacterial po-
tency of the free ligand and the AgI complex 1 further justifies
the purpose of this study. A thorough investigation regarding
the biological effects of the title silver complex is essential for
medical practice as a metal-based drug.
Conclusions
A silver(I) complex (1) with σ donor (nitrogen atoms of
pyridyl rings) and π acceptor (phosphorus atoms) Lewis bases
in PhP(2-py)2 ligand was synthesized. Its selective antimicro-
bial activities against selected Gram positive bacteria were
confirmed. The crystal and molecular structures of 1 were suc-
cessfully determined. Moreover, its electronic as well as IR
and NMR spectroscopic properties were carefully investigated.
The QTAIM analysis demonstrates that a weak bond con-
nects two silver nuclei and characteristics of this bond are in
the normal range of metal–metal bonds in dinuclear com-
plexes.[20]
Antimicrobial Properties
Experimental Section
In view of the antimicrobial activity, we have tested the abil-
ity of the ligand and its silver complex against representative
bacteria by the disc diffusion method,[27] compared to a well-
General Procedures: Dichlorophenyl phosphine, 2-bromopyridine,
butyllithium (15% solution in hexane), silver nitrate, and the solvents
were purchased from Merck and Fluka chemicals. The solvents were
known antibacterial agent silver sulphadizine (SS) (please see of analytical grade and purified prior to use base on standard methods.
Table 1. The electron density ρ(r), Laplacian of the electron density ᭞2ρ(r), Lagrangian kinetic energy G(r), the ratio of the Lagrangian kinetic
energy to the electron density G(r)/ρ(r), electron delocalization between silver atoms δ(Ag, Ag), in the atomic units and the bond length BL /Å
between silver atoms in the complex and the Ag2 molecule.
ρ(r)
᭞2ρ(r)
G(r)
G(r)/ρ(r)
δ (Ag, Ag)
BL
Ag2
Complex
0.0410
0.0160
0.1048
0.0492
0.0366
0.0114
0.8927
0.7125
1.1882
0.1170
2.695
3.216
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Z. Anorg. Allg. Chem. 2011, 2260–2264

Antimicrobial Activity of a Luminescent Dinuclear AgI Complex
Dichlorophenyl phosphine was purified to remove any impurities.
Melting points are uncorrected and were obtained with an Electrother-
mal 9200 melting point apparatus. Infrared spectra from 250 to
4000 cm–1 of solid samples were taken as 1% dispersion in CsI pellets
with a Shimadzu-470 spectrometer. 1H and 31P NMR spectra were
recorded at room temperature with a Bruker AVANCE 300 MHz spec-
trometer. The NMR spectra were referenced to Me4Si and H3PO4
(85%) as external standards. Elemental analyses were performed with
a Heraeus CHN–O Rapid analyzer. UV/Vis spectra were recorded with
a Shimadzu 2100 spectrometer using a 1 cm path length cell. Corrected
luminescence spectra were obtained with a Perkin–Elmer LS45 lumi-
nescence spectrometer.
Table 3. Crystal data and structure refinement for complex 1.
Formula
C32H26Ag2N6O6P2
Formula weight
868.27
Temperature /K
150(2)
Wavelength λ /Å
0.71073
Crystal system
Space group
monoclinic
P21/n
Crystal size /mm
a /Å
b /Å
c /Å
0.20ϫ0.20ϫ0.14
8.9974(2)
12.3756(5)
15.4667(5)
90
α /°
β /°
97.422(2)
90
1707.76(10)
2
1.689
2.66–27.5
864
The ligand PhP(2-py)2 was synthesized by reaction of 2-lithiopyridine,
generated from treatment of n-butyllithium and 2-bromopyridine, and
dichlorophenylphosphine (2:1 molar ratio) in oxygen and moisture-
free Et2O.[28]
γ /°
Volume /Å3
Z
Density (calcd.) /g·cm–1
θ ranges /° for data collection
F(000)
Synthesis of [Ag2(μ-(PhP(2-py)2)2(NO3)2] (1): AgNO3 (2 mmol,
0.34 g) in H2O/EtOH (5:1 mixture, 15 mL) was treated for 15 min in
darkness with an equimolar quantity of PhP(2-py)2 (0.529 g) in EtOH
(5 cm3) to produce 1. Afterwards, the solvent was removed by rotary
evaporator in a vacuum and the precipitate was washed twice with
diethyl ether (2ϫ15 mL). The resulting white residue was recrys-
tallized from H2O/EtOH to give colorless crystals (1.23 g, yield
72.3%). M. p. = 229 °C (dec.). IR (CsI): ν˜ = 3047 (w), 1653 (s), 1380
(s), 1233 (w), 987 (w), 956 (w), 694 (w) cm–1. 31P{1H} NMR [(CD3)
2CO]: δ = 7.93. Anal. data for C32H26Ag2N6O6P2: calcd. C 44.27; H
3.02; N 9.68%; found: C 44.33; H 3.04; N 9.65%.
Absorption coefficient /mm–1
Index ranges
1.293
–11 Յ h Յ 11
–15 Յ k Յ 15
–20 Յ l Յ 20
20951
3904, (0.1058)
217, 0
0.0560, 0.1244
0.1151, 0.1577
1.097
Data collected
Unique data (Rint
Parameters, restrains
Final R1, wR2 (obs. data)
)
a)
a)
Final R1, wR2 (all data)
Goodness of fit on F2 (S)
Largest diff peak and hole /
e·Å–3
2.573, –1.176
2
2 2
2 2 1/2.
Crystal Structure Determination and Refinement: The crystallo-
graphic data was collected with a Nonius Kappa CCD diffractometer,
using graphite-monochromated Mo-Kα radiation of 0.71073 Å. A col-
orless block with a dimension of 0.20ϫ0.20ϫ0.14 mm was useed.
Cell constants and an orientation matrix for data collection were ob-
tained by least-squares refinement of diffraction data from 3904 for
unique reflections. Data were collected at a temperature of 150(1) K
to a maximum 2θ value of 55°. The numerical absorption coefficients,
μ, for Mo-Kα radiation are 1.293 mm–1. The data frames were inte-
grated and scaled using the DENZO-SMN package.[29] The structure
was solved and refined using the SHELXTL/PC V5.1 package.[30] The
structure was solved by direct methods and refinement was by full-
matrix least-squares on F2 using all data (negative intensities in-
cluded). The hydrogen atom parameters were calculated and atoms
were constrained as riding atoms with U isotropic 20% larger than the
corresponding carbon atoms for the phenyl hydrogen atoms and 50%
larger for the methyl hydrogen atoms. A summary of the crystal data,
experimental details, and refinement results is given in Table 3.
a) R1 = Σ||Fo|–|Fc||/Σ|Fo|, wR2 = [Σ (w (Fo –Fc ) )/Σ w(Fo ) ]
mal inhibitory concentrations (MIC, mg·mL–1) were estimated by the
disk diffusion method.[31] The final concentration of all cultures in
Mueller-Hinon agar (MHA) was adjusted to106 cfu·mL–1 and used for
inoculation in the MIC test. Serial dilutions of the test compounds,
previously dissolved in DMSO were prepared at concentrations of 0–
512 μg·mL–1. To each plate was inoculated with 0.1 mL of the pre-
pared bacterial cultures. Similarly, each plate carried a blank disc, with
solvent DMSO only in the center to serve as negative control. A final
concentration of DMSO (5%) was present in all assays. The inoculated
plates were incubated at 37 °C for 18–20 h. The minimal inhibitory
concentration (MIC) was detected as the lowest concentration of drug
in plate, for which no visible growth took place by macroscopic evalu-
ation. All determinations were performed in triplicate and confirmed
by three separate experiments.
Theoretical Study: The electron density of the complex was obtained
from a single point energy calculation on the X-ray resolved crystal
structure by the Win-GAMESS.08 program package[32] at the B3LYP
level[33–36] combined with three different basis sets for different ele-
ments; 6-31G for carbon and hydrogen atoms, 6-31+G(d) for nitrogen,
oxygen, and phosphor and DZVP(DFT orbitals) for silver atoms.[37]
The electron density of the molecule, obtained from DFT computation,
was analyzed by the AIMAll program package.[38] Accuracy of
QTAIM analysis was checked by keeping the integral of Laplacian for
each atomic basin below 10–4 au.
Crystallographic data (excluding structure factors) for the structure
in this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained free of charge on quoting the depository
number
CCDC-826766
(Fax:
+44-1223-336-033;
E-Mail:
deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).
Antimicrobial Activity: The in vitro antibacterial activity of the free
ligand PhP(2-py)2 and its silver nitrate complex 1 was investigated
against several representative Gram positive bacteria (Bacillus sp., Ba-
cillus cereus, Bacillus megaterium, Micrococcus luteus., Staphylococ-
cus aureus and Stomatococcus mucilaginosus) and Gram negative bac-
teria (Escherichia coli, Burkholderia mallei, Neisseria polysaccharea,
Corynebacterium minutissinum, Corynebacterium urealyticum, Cap-
nocytophaga cynodegmi, and Pasteurella lymphangitidis). The mini-
Acknowledgement
We would like to thank the University of Tehran (UT) for financial
support.
Z. Anorg. Allg. Chem. 2011, 2260–2264
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A. Nemati Kharat et. al.
ARTICLE
[18] A. Castiñeiras, I. García-Santos, S. Dehnen, P. Sevillano, Polyhe-
dron 2006, 25,3653.
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Received: May 24, 2011
Published Online: August 31, 2011
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Z. Anorg. Allg. Chem. 2011, 2260–2264