Synthesis, characterization, crystal structure, theoretical studies, and antibacterial activities of P-coordinated mercury(II) complexes containing phosphine-phosphonium salts

Article abstract of DOI:10.1016/j.poly.2015.06.019

The reaction of mercury(II) halides with the phosphine-phosphonium salts [PPh₂(CH₂)₂PPh₂CH₂C(O)C₆H₄R]Br (R = Br (S¹), NO₂ (S²) in methanol affords the zwitterionic mercury(II) complexes {HgX₂Br(PPh₂(CH₂)₂PPh₂CH₂C(O)C₆H₄R)} [R = Br: X = Cl (1), Br (2), I (3); R = NO₂: X = Cl (4), Br (5), I (6)]. These complexes were fully characterized by elemental analysis and spectroscopic techniques such as IR, ¹H, ³¹P, and ¹³C NMR. The structure of complex 4 has been characterized crystallographically. Single crystal X-ray analysis reveals the presence of mononuclear P-coordinated complex containing Hg(II) in a distorted tetrahedral environment. Theoretical studies using density functional theory have been performed on the free ligands (S¹ and S²) and their corresponding complexes (1-6). Electronic and structural properties of latter compounds were examined and general trends were derived. The natural bonding orbital calculations have also been carried out to understand the nature of the Hg-P bond. The results show that the interactions between the metal atom and phosphorus atom of phosphine group are mainly an electrostatic interaction. In addition, there is a decrease in the charge distribution on the ligand reflecting electron transfer from the ligand to the metal and halogens atoms. The in vitro antibacterial activities of the entitled compounds were evaluated against Gram-negative as Escherichia coli and Pseudomonas aeruginosa bacteria and Gram-positive as Bacillus subtilis and Staphylococcus aureus and compared with the standard antibacterial drugs.

Full text of DOI:10.1016/j.poly.2015.06.019

Polyhedron 98 (2015) 120–130
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, characterization, crystal structure, theoretical studies,
and antibacterial activities of P-coordinated mercury(II) complexes
containing phosphine–phosphonium salts
a
b
c
c
Sepideh Samiee a, , Nadieh Kooti , Hossein Motamedi , Robert W. Gable , Fateme Akhlaghi Bagherjeri
⇑
a Department of Chemistry, Faculty of Science, Shahid Chamran University, Ahvaz, Iran
b Department of Biology, College of Sciences, Shahid Chamran University, Ahvaz, Iran
c School of Chemistry, University of Melbourne, Victoria 3010, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of mercury(II) halides with the phosphine–phosphonium salts [PPh2(CH2)2
PPh2CH2C(O)C6H4R]Br (R = Br (S1), NO2 (S2) in methanol affords the zwitterionic mercury(II) complexes
{HgX2Br(PPh2(CH2)2PPh2CH2C(O)C6H4R)} [R = Br: X = Cl (1), Br (2), I (3); R = NO2: X = Cl (4), Br (5), I
(6)]. These complexes were fully characterized by elemental analysis and spectroscopic techniques such
as IR, 1H, 31P, and 13C NMR. The structure of complex 4 has been characterized crystallographically. Single
crystal X-ray analysis reveals the presence of mononuclear P-coordinated complex containing Hg(II) in a
distorted tetrahedral environment. Theoretical studies using density functional theory have been per-
formed on the free ligands (S1 and S2) and their corresponding complexes (1–6). Electronic and structural
properties of latter compounds were examined and general trends were derived. The natural bonding
orbital calculations have also been carried out to understand the nature of the Hg–P bond. The results
show that the interactions between the metal atom and phosphorus atom of phosphine group are mainly
an electrostatic interaction. In addition, there is a decrease in the charge distribution on the ligand reflect-
ing electron transfer from the ligand to the metal and halogens atoms. The in vitro antibacterial activities
of the entitled compounds were evaluated against Gram-negative as Escherichia coli and Pseudomonas
aeruginosa bacteria and Gram-positive as Bacillus subtilis and Staphylococcus aureus and compared with
the standard antibacterial drugs.
Received 31 March 2015
Accepted 12 June 2015
Available online 19 June 2015
Keyword:
Phosphine–phosphonium salt
Mercury(II) complex
Theoretical study
Antibacterial activity
Zwitterionic complex
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
in synthetic chemistry has been well documented [19–27]. Most
of the interest in the coordination properties of resonance stabi-
Zwitterionic metal complexes have wide applications in the
catalysis and biochemistry due to their heightened solubility in
low-polarity media, increased tolerance to polar coordinating sol-
vents, and the avoidance of counteranion effects [1–5]. On the
other hand, phosphonium salts have been extensively studied both
as valuable intermediates in organic synthesis and as interesting
ligands in coordination chemistry [6–13]. It is also well established
that in comparison of sulfonium, pyridinium and ammonium salts,
phosphonium counter parts offer a great variety of reactivates
because of their d-orbital participation [14,15]. Moreover, these
types of compounds can be employed as thermally latent initiators
or photoinitiators in cationic polymerization [16–18]. The phos-
phonium salts are most often converted to phosphorus ylides by
releasing a proton. The efficiency of metalated phosphorus ylides
lized phosphorus ylides stemming from their bond versatility is
due to the presence of different functional groups in their molecu-
lar structure [20]. Therefore, phosphorus ylides are known to
demonstrate rich coordination chemistry.
In 2009, Ebrahim and co-workers reported the reactivity of the
hybrid phosphine–phosphonium salt, [PPh2CH2PPh2CH2COPh]Br,
with mercury(II) halides [8]. Recently, Sabounchei et al. also
reported the synthesis of several zwitterionic complexes of
mercury(II) derived from PPh2CH2PPh2 (dppm) and determining
the formation constants of entitled complexes in dimethylsulfox-
ide [9]. In the context of this paper, we report the reactivity of
phosphine–phosphonium salts derived from bis(diphenylphos-
phino)ethane (dppe) towards mercury(II) halides, along with an
in vitro determination of their antibacterial activity. The X-ray crys-
tal structure of complex 4 demonstrates P-coordination of the
ligand to metal atom. In continuous, theoretical studies on entitled
phosphonium salts and their corresponding complexes were
⇑
Corresponding author. Tel.: +98 6113331042; fax: +98 6113337009.
E-mail addresses: s.samiee@scu.ac.ir, samiee.sepideh@gmail.com (S. Samiee).
http://dx.doi.org/10.1016/j.poly.2015.06.019
0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.

S. Samiee et al. / Polyhedron 98 (2015) 120–130
121
performed by density functional theory (DFT), providing further
aeruginosa (ATCC 9027). Penicillin (10
lg/disk) and Gentamicin
understanding of the chemical behavior of phosphonium salts
and their mercury(II) complexes.
(10 lg/disk) were used as standard antibacterial drugs. All com-
pounds were dissolved in dimethylsulfoxide (DMSO) at 5, 10, 20
and 40 mg/mL concentration. Then sterile blank discs (6.4 mm)
were saturated with these concentrations, so the effective dose
per disc of prepared compounds were as 0.2, 0.4, 0.8 and 1 mg.
Bacterial cultures were adjusted to 0.5 Mc Farland turbidity and
lawn culture was then prepared on nutrient agar (Merck,
Germany) plates. The prepared discs were placed on bacterial cul-
ture. Simultaneously, discs saturated with DMSO were used as
above as negative control. Standard antibiotic discs were also
tested against these bacteria as control. The cultures were incu-
bated at 37 °C for 24 h. The zone of inhibition was calculated in
millimeters.
2. Experimental
2.1. Materials and measurements
All reactions were carried out under dry nitrogen atmospheres
using standard Schlenk tube techniques. Solvents were dried and
distilled using standard methods [28]. Starting materials were pur-
chased from commercial sources and used without further purifi-
cation. The phosphine–phosphonium salts, namely [PPh2(CH2)2
PPh2CH2C(O)C6H4Br]Br (S1) [29] and [PPh2(CH2)2PPh2CH2C(O)
C6H4NO2]Br (S2), were prepared by following the methods
described in the literature [30]. Infrared spectra were collected
on samples as KBr pellets using a FT BOMEM MB102 spectropho-
2.5. Synthesis of compounds
tometer from 400 to 4000 cmꢀ1 31P, 1H and 13C NMR spectra were
.
2.5.1. [PPh2(CH2)2PPh2CH2C(O)C6H4Br]Br (S1)
recorded on 300 MHz Bruker spectrometer in DMSO-d6 as solvent
at room temperature. Chemical shifts (ppm) are reported according
to internal TMS and external 85% phosphoric acid. Coupling con-
stants are given in Hz. Elemental analyses were performed using
a Perkin-Elmer 2400 series analyzer. Melting points were mea-
sured on a SMP3 apparatus.
The ligand was prepared by the reaction of bis(diphenylphos-
phino)ethane (dppe) (0.398 g, 1 mmol) with 2,4-dibromoacetophe-
none (0.291 g, 1.05 mmol) in dry acetone at room temperature
overnight. The resulting solution was filtered off, and the precipi-
tate washed with diethyl ether and dried under vacuum [29].
Yield: 0.59 g, 86%. M.p. 145–147 °C. Anal. Calc. for C34H30Br2OP2:
C, 60.38; H, 4.47. Found: C, 60.83; H, 4.75%. Selected IR absorption
in KBr (cmꢀ1): 1672 (
m
C@O). 1H NMR (CDCl3): dH = 2.23 (m, 2H,
2.2. X-ray crystal structure determination
2
CH2); 3.14 (m, 2H, CH2); 5.99 (d, 2H, PCH2CO, JPH = 12.99); 7.28–
8.16 (m, 24H, Ph). 31P{1H} NMR (CDCl3): dP = ꢀ14.89 (br, PPh2);
Suitable single crystal of [HgCl2Br(PPh2(CH2)2PPh2CH2C(O)
C6H4NO2)] (4) was grown by slow evaporation of dichlorometha-
ne/n-hexane solutions. All measurements were performed on an
Agilent Technologies SuperNova diffractometer using mirror
23.65 (d, PCH2CO, JPP = 44.27). 13C{1H} NMR (CDCl3): dC = 19.84
3
1
(br, CH2); 35.75 (d, PCH2CO, JPC = 58.56); 117.07–135.34 (Ph);
191.59 (s, CO).
mono-chromated Cu
Structure solution and refinement were carried out using SHELXS
Ka radiation (k = 1.54184 Å) at 130 K.
2.5.2. [PPh2(CH2)2PPh2CH2C(O)C6H4NO2]Br (S2)
-
97 and SHELXL-97, respectively [31,32]. The structure was solved
by direct methods and refined by full matrix least-squares meth-
ods on F2 and difference Fourier maps using P212121 space group
with Z = 4. All non-hydrogen atoms were refined anisotropically
using reflections I > 2r(I). Hydrogen atoms were located in ideal
positions.
The title ligand was prepared by a similar procedure to that of
ligand (S1) using dppe and 2-bromo-40-nitroacetophenone
(0.256 g, 1.05 mmol), as reported previously [30]. Yield: 0.53 g,
82%. M.p. 172–173 °C (Reported: 169–171 °C). IR (KBr, cmꢀ1):
1683 (
m
C@O). 1H NMR (CDCl3): dH = 2.24 (m, 2H, CH2); 3.24 (m,
2
2H, CH2); 6.26 (d, 2H, PCH2CO, JPH = 12.27); 7.27–8.42 (m, 24H,
Ph). 31P{1H} NMR (CDCl3): dP = ꢀ15.05 (br, PPh2); 20.93 (d,
PCH2CO, JPP = 43.18). 13C{1H} NMR (CDCl3): dC = 19.20 (br, CH2);
3
2.3. Computational method
1
36.43 (d, PCH2CO, JPC = 58.48); 116.80–150.78 (Ph); 191.52 (d,
2
The geometries of two ligands and their complexes were fully
optimized using DFT with Becke’s three-parameter exchange
potential and the Lee–Yang–Parr correlation functional (B3LYP)
[33] along with the CEP-121G basis set [34]. This basis sets
includes effective core potentials (ECP) for mercury, phosphorus
and halide (Cl, Br and I) atoms. A starting ab initio calculations
for S1 and S2 was obtained using the HYPERCHEM 5.02 program [35].
The observed geometry of complex 4 was used as a basis of DFT
calculations for investigated complexes. To evaluate and ensure
the optimized structures of the molecules, frequency calculations
were performed using analytical second derivatives. Beside, the
natural bonding orbital (NBO) calculations [36] were performed
using the NBO 3.1 program as implemented at the same level of the-
ory. All calculations were carried out using the GAUSSIAN 03 compu-
tational chemistry program [37].
CO, JPC = 6.04).
2.5.3. [HgCl2Br(PPh2(CH2)2PPh2CH2C(O)C6H4Br)] (1)
To a solution of HgCl2 (0.098 g, 0.36 mmol) in methanol (8 mL)
under nitrogen, was added a methanolic solution of S1 (0.202 g,
0.36 mmol). The resultant mixture was stirred at room tempera-
ture for 3 h. The white precipitate was isolated, washed twice with
15 ml methanol and dried in vacuo. Yield: 0.34 g, 71%. M.p. 153–
155 °C. Anal. Calc. for C34H30Br2Cl2HgOP2: C, 43.08; H, 3.19.
Found: C, 42.86; H, 3.07%. IR (KBr, cmꢀ1): 1676 ( C@O). 1H NMR
m
(DMSO-d6): dH = 3.007 (br, 2H, CH2); 3.63 (br, 2H, CH2); 5.78 (d,
2
2H, PCH2CO, JPH = 12.50); 7.51–8.02 (m, 24H, Ph). 31P{1H} NMR
3
(DMSO-d6): dP = 14.39 (d, PPh2, JPP = 63.78); 22.48 (d, PCH2CO,
3JPP = 62.20). 13C{1H} NMR (DMSO-d6): dC = 18.08 (br, CH2); 28.80
(m, PCH2CO); 117.86–135.26 (Ph); 192.14 (s, CO).
2.4. Antibacterial tests
2.5.4. [HgBr3(PPh2(CH2)2PPh2CH2C(O)C6H4Br)] (2)
This complex was prepared in the same way as for 1 using
mercury(II) bromide (0.13 g, 0.36 mmol). Yield: 0.40 g, 78%. M.p.
201–203 °C. Anal. Calc. for C34H30Br4HgOP2: C, 39.39; H, 2.92.
The in vitro antibacterial activity of phosphonium salts as free
ligands (S1 and S2) and their corresponding mercury(II) complexes
(1–6) was evaluated by disc diffusion method against two Gram-
positive bacteria, namely Bacillus cereus (ATCC 6633) and
Staphylococcus aureus (ATCC 6538), and two Gram-negative bacte-
ria, namely Escherichia coli (ATCC 25922) and Pseudomonas
Found: C, 39.52; H, 2.99%. IR (KBr, cmꢀ1): 1676 ( C@O), 1H NMR
m
(DMSO-d6): dH = 2.97 (br, 2H, CH2); 3.60 (m, 2H, CH2); 5.79 (d,
2
2H, PCH2CO, JPH = 12.52); 7.52–8.01 (m, 24H, Ph). 31P{1H} NMR
3
(DMSO-d6): dP = 17.95 (br, PPh2); 26.23 (d, PCH2CO, JPP = 55.96).

122
S. Samiee et al. / Polyhedron 98 (2015) 120–130
13C{1H} NMR (DMSO-d6): dC = 18.44 (br, CH2); 117.91–135.47 (Ph);
2.5.8. [HgI2Br(PPh2(CH2)2PPh2CH2C(O)C6H4NO2)] (6)
192.23 (s, CO); (CH2, was not seen).
Yield: 0.45 g, 82%. M.p. >225 °C (decomposes). Anal. Calc. for
C
34H30BrI2HgNO3P2: C, 37.23; H, 2.76; N, 1.28. Found: C, 37.24;
H, 2.96; N, 1.08%. IR (KBr, cmꢀ1): 1686 ( C@O). 1H NMR (DMSO-
m
2.5.5. [HgI2Br(PPh2(CH2)2PPh2CH2C(O)C6H4Br)] (3)
d6): dH = 2.73 (br, 2H, CH2); 3.08 (br, 2H, CH2); 5.99 (d, 2H,
This complex was prepared in the same way as for 1 using
mercury(II) iodide (0.16 g, 0.36 mmol). Yield: 0.46 g, 80%. M.p.
220–222 °C. Anal. Calc. for C34H30Br2I2HgOP2: C, 36.11; H, 2.67.
2
PCH2CO, JPH = 12.50); 7.62–8.58 (m, 24H, Ph). 31P{1H} NMR
3
(DMSO-d6): dP = 2.61 (br, PPh2); 22.51 (d, PCH2CO, JPP = 58.01).
13C{1H} NMR (DMSO-d6): dC = 18.21 (m, CH2); 33.04 (m,
Found: C, 35.97; H, 2.83%. IR (KBr, cmꢀ1): 1675 ( C@O). 1H NMR
m
PCH2CO); 117.66–151.30 (Ph); 192.12 (s, CO).
(DMSO-d6): dH = 2.86 (br, 2H, CH2); 3.51 (br, 2H, CH2); 5.78 (d,
2
2H, PCH2CO, JPH = 12.50); 7.49–8.01 (m, 24H, Ph). 31P{1H} NMR
3
(DMSO-d6): dP = 3.74 (br, PPh2); 22.60 (d, PCH2CO, JPP = 61.76).
3. Results and discussion
13C{1H} NMR (DMSO-d6): dC = 18.37 (br, CH2); 31.69 (m,
1
PCH2CO); 117.80–135.32 (Ph); 192.15 (d, CO, JPC = 5.91).
3.1. Synthesis
The following complexes were made similarly using the appro-
priate starting ligand S2 and related mercury(II) halides. All reac-
tions were also carried out under an atmosphere of dry nitrogen.
The reaction of phosphine–phosphonium salts (S1 and S2) with
mercury(II) halides in 1:1 molar ratio gave the zwitterionic P-coor-
dinated complexes (1–6) in good yield. The ligands S1 and S2 were
already synthesized way back in 2010 [29,30]. These species are
fairly soluble in halogenated solvents such as chloroform and
dichloromethane but insoluble in non-polar solvents such as n-
pentane and n-hexane. The reactions studied in the present work
are summarized in Scheme 1.
2.5.6. [HgCl2Br(PPh2(CH2)2PPh2CH2C(O)C6H4NO2)] (4)
Yield: 0.33 g, 72%. M.p. >250 °C (decomposes). Anal. Calc. for
C
34H30BrCl2HgNO3P2: C, 43.61; H, 3.76; N, 1.53. Found: C, 43.65;
H, 3.32; N, 1.28%. IR (KBr, cmꢀ1): 1684 ( C@O). 1H NMR (DMSO-
m
d6): dH = 3.05 (br, 2H, CH2); 3.66 (br, 2H, CH2); 5.88 (d, 2H,
2
PCH2CO, JPH = 11.75); 7.56–8.46 (m, 24H, Ph). 31P{1H} NMR
3
(DMSO-d6): dP = 14.68 (d, PPh2, JPP = 63.01); 22.37 (d, PCH2CO,
3.2. Spectroscopic studies
3JPP = 64.87). 13C{1H} NMR (DMSO-d6): dC = 19.90 (br, CH2); 35.77
1
(d, PCH2CO, JPC = 58.71); 117.07–141.39 (Ph); 191.28 (s, CO).
The selected important IR and NMR data for all complexes and
free ligands are presented in Table 1. The m(CO), which is sensitive
2.5.7. [HgBr3(PPh2(CH2)2PPh2CH2C(O)C6H4 NO2)] (5)
to complexation, occurs around 1680 cmꢀ1 in the parent phospho-
nium salts. The comparison of the carbonyl stretch of complexes
(1–6) and free ligands show that this absorption band appears near
to that of the free ligands and close to those observed in other P-co-
ordinated complexes [8,26]. Coordination of the ylide through car-
Yield: 0.38 g, 77%. M.p. >138 °C (decomposes). Anal. Calc. for
C
34H30Br3HgNO3P2: C, 40.72; H, 3.02; N, 1.40. Found: C, 40.33; H,
3.11; N, 1.21%. IR (KBr, cmꢀ1): 1686 ( C@O), 1H NMR (DMSO-d6):
m
dH = 3.04 (br, 2H, CH2); 3.48 (br, 2H, CH2); 5.87 (d, 2H, PCH2CO,
2JPH = 11.25); 7.53–8.46 (m, 24H, Ph). 31P{1H} NMR (DMSO-d6):
bon causes an increase in
m(CO), while for O-coordination a
3
dP = 18.28 (br, PPh2); 22.36 (d, PCH2CO, JPP = 63.59). 13C{1H}
decrease of (CO) is expected [22]. Therefore, the Infrared data
m
NMR (DMSO-d6): dC = 21.57 (br, CH2); 36.02 (d, PCH2CO,
indicates that the interaction of the free phosphonium salts with
mercury(II) halides is only through the PPh2 group to metal center.
1JPC = 58.18); 117.59–134.65 (Ph); 191.45 (s, CO).
O
O
P
P
H2
C
h2
C
P
P
H2
C
h2
C
C
H
H
3O
H R
C6
4
C
P
P
h2
H2
H2
C
H X
g
+
r
B
2
r.t
2h N
,
,
2
C
C
H R
C6
4
P
P
h2
H2
H2
H
X
g
r
B
X
1 : R =
X=
Cl
Cl
,
( )
( )
=
R
S1)
;
N
S2)
O
(
2
Cl
(1): R = Br, X = Cl
(
2 : R =
X= Br
Cl
,
(2): R = Br, X = Br
=
=
I
3 : R
X
Cl
NO
,
( )
(3): R = Br, X = I
=
4 : R =
X
Cl
,
( )
2
(4): R = NO , X = Cl
5 : R = N
X= Br
O
2
,
2
( )
:
=
N
X= I
6
R
(5): R =ONO , X = Br
,
2
( )
2
(6): R = NO2, X = I
Scheme 1. Synthesis route for preparation of zwitterionic mercury(II) halides complexes.
Table 1
Selected IR and NMR spectral data for phosphonium salts and their metal complexes (1–6).
Compound
IR
1H NMR
31P NMR
13C NMR
Reference
m
(CO)
d PCH2 (2JP–H
)
d PPh2 (2JP–P
)
d PCH2 (2JP–P
)
d CO
S1
1
2
1672
1676
1676
1674
1683
1684
1685
1686
5.99(1.99)
ꢀ14.89(44.34)
14.39(63.87)
17.95(br)
23.65(44.27)
22.48(62.20)
26.23(55.96)
22.60(61.76)
20.93(43.18)
22.37(64.87)
22.36(63.59)
22.51(58.01)
191.59
192.14
192.23
192.15
191.52
191.28
191.45
192.12
[29]
5.78(12.50)
5.79(12.52)
5.78(12.59)
6.26(12.27)
5.88(11.75)
5.87(11.25)
5.99(12.50)
This work
This work
This work
[30]
This work
This work
This work
3
3.74(br)
S2
4
ꢀ15.05(br)
14.68(63.01)
18.28(br)
5
6
2.61(br)
m
(cmꢀ1), d (ppm), J (Hz); br, broad.

S. Samiee et al. / Polyhedron 98 (2015) 120–130
123
In the 31P{1H} NMR spectra, the signal due to phosphine group
Table 2
Crystal data and experimental details for complex 4.
appears as a doublet. The significant downfield shift of this signal
to that phosphine of the related phosphonium salts is in agreement
with the P-bonding of the ligands. In this regard, the chemical shift
values of phosphonium group (PCH2CO) have either remained
unaffected or shifted slightly with reference to those of the parent
salts. Moreover, in the 1H NMR spectra of complexes (1–6), the
doublet due to methyl group around 5.80 ppm with a coupling
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C34H30NO3P2Cl1.23Br1.76Hg
947.94
130.01(10)
1.54184
orthorhombic
P212121
9.66483(8)
15.23112(13)
23.75760(18)
3497.27(5)
2
b (Å)
c (Å)
constant JP–H of 12 Hz, appears in the same region as observed
for the free ligands, again in agreement with P-coordination. In
the 13C{1H} NMR spectra of all six complexes, the signals attributed
to the carbonyl group have also remained unaffected due to com-
plexation (see Table 1). Similar behavior have been observed ear-
lier in the case of P-coordinated complexes of mercury(II) halides
[8,9]. No 199Hg satellites were observed in the 31P, 1H and 13C
NMR spectra, which means that the Hg–P bond probably are break-
ing and forming quickly, as previously reported in the ylide com-
plexes of Hg(II) [23] and Ag(I) [21]. Hence, the results of
spectroscopic studies clearly demonstrate that the monodentate
coordination of the phosphine–phosphonium salts towards the
mercury center through the phosphorus atom of phosphine group.
Volume (Å3)
Z
4
l
(mmꢀ1
)
12.267
2h range for data collection (°)
Index ranges
6.894 to 154.218
ꢀ12 6 h 6 9
ꢀ19 6 k 6 19
ꢀ30 6 l 6 25
38589
7358 [Rint = 0.0395]
7358/9/413
1.060
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2
r
(I)]
R1 = 0.0368, wR2 = 0.0893
R1 = 0.0376, wR2 = 0.0899
R indices (all data)
Table 3
3.3. Crystal structure analysis
Selected bond lengths (Å) and bond angles (°) for complex 4.
Bond lengths
Hg(1)–P(1)
Hg(1)–Cl(1)
Hg(1)–Cl(2)
Hg(1)–Cl(3)
Hg(1)–Br(1)
Hg(1)–Br(2)
Hg(1)–Br(3)
O(1)–C(28)
P(1)–C(27)
C(27)–C(28)
[P(2). . .O(1)]
The X-ray crystal structure of [HgCl2(Br)(PPh2(CH2)2
PPh2CH2C(O)C6H4NO2)] (4) are shown in Fig. 1. Relevant parame-
ters concerning data collection and refinement are given in
Table 2 and selected bond distances and angles are listed in
Table 3. Fractional atomic coordinates and equivalent isotropic dis-
placement coefficients (Ueq) for the non-hydrogen atoms of the
complex is available as Supplementary material.
2.460(17)
2.399(7)
2.401(7)
2.399(7)
2.637(2)
2.646(2)
2.640(2)
1.217(10)
1.813(8)
1.515(9)
2.860
The X-ray analysis reveals the P-coordination mode of ligand S2
to metal center in complex 4. The mercury is surrounded by one
phosphorus atom and three halide ligands in a distorted tetrahe-
dral geometry (95.8(8)–120.16(11)°). As can be seen in Fig. 1, the
Brꢀ ion of S2 is bonded to metal center, which can be explained
by both the preference of Hg(II) for four coordination and the sta-
bility of the 18 electron configuration around Hg(II). The Hg–P
Bond angles
Cl(1)–Hg(1)–Cl(2)
Cl(1)–Hg(1)–Cl(3)
Cl(2)–Hg(1)–Cl(3)
Br(1)–Hg(1)–Br(2)
Br(1)–Hg(1)–Br(3)
Br(2)–Hg(1)–Br(3)
Cl(1)–Hg(1)–P(1)
Cl(2)–Hg(1)–P(1)
Cl(3)–Hg(1)–P(1)
Br(1)–Hg(1)–P(1)
Br(2)–Hg(1)–P(1)
Br(3)–Hg(1)–P(1)
95.8(8)
107.8(2)
114.0(6)
101.00(14)
107.88(9)
109.0(2)
118.9(6)
109.6(8)
110.2(4)
120.16(11)
110.81(12)
107.6(2)
distance of 2.460(17) Å in this complex is in agreement with the
values reported for P-coordinated monomeric complexes [8,30,26].
During the refinement it became apparent that there is substi-
tution disorder in each of the halogen atoms attached to the mer-
cury; the final occupancy factors (Br & Cl) were 0.656(8) &
0.344(8), 0.696(9) & 0.304(9) and 0.413(7) & 0.587(7) giving rise
to a 7:5 overall ratio. During the refinement the HgꢀBr and
HgꢀCl distances were restrained to be equal, while the Br. . .Cl sep-
aration was constrained to be 0.3 Å; the anisotropic displacement
parameters of the atoms on each site were also constrained to be
equal. The maximum electron density peak was 2.6 eÅꢀ3, which
is 1.07 Å far from Hg. As can be seen in molecular structure of this
complex, the phosphorus and oxygen atoms are cis oriented due to
strong 1,4-P. . .O intramolecular interactions between the positively
charged P atom and the negatively charged O atom, as described
previously for phosphorus ylides and their mercury(II) complexes
[38] (see Table 3). The nitro group makes a dihedral angle of
6.5(3)° with the aromatic ring, while the orientation of the
Fig. 1. ORTEP view of the X-ray crystal structure of complex 4.

124
S. Samiee et al. / Polyhedron 98 (2015) 120–130
Table 5
The Selected Optimized bond lengths (Å) and bond angles (°) for the free ligands (S1)
and (S2).
Compounds
S1
S2
Bond lengths
P(1)–C(13)
P(2)–C(14)
C(13)–C(14)
C(27)–C(28)
C(28)–O(1)
P(2). . .O(1)
P(2). . .Br
1.940
1.899
1.550
1.527
1.268
3.044
4.201
1.940
1.899
1.550
1.524
1.266
3.056
4.189
Bond angles
P(1)–C(13)–C(14)
P(2)–C(13)–C(14)
C(6)–P(1)–C(7)
C(15)–P(2)–C(21)
111.49
114.11
102.50
114.68
111.44
114.06
102.64
110.96
atoms which, presumably, influence the conformation of phospho-
rus ligand. The molecules are linked together via several other
intermolecular C–H. . .Cl, C–H. . .Br and C–H. . .O interactions to form
a 3D H-bonded network (see Table 4).
Fig. 2. Representation of intermolecular contacts of complex 4. Dashed lines
represent short contacts. Color code: Hg grey, p purple, C black, N blue, O red and
Cl#Br green (for the occupancy ratios refer to Table 3). (Colour online.)
3.4. Theoretical studies
Table 4
Significant non-classical hydrogen bonds (interatomic distance (Å) and bond angles °)
found in the structures of complex 4.
We were interested i) to obtain the geometric and electronic
structures of phosphonium salts as free ligands and ii) to compare
the structure, stability, electronic and thermochemical properties
of zwitterionic mercury(II) complexes. Therefore we launched our
investigation on a number of these types of compounds applying
density functional theory (DFT) calculations. To the best of our
knowledge, the geometries of phosphonium salts (S1 and S2) and
their metal complexes (1–6) were fully optimized at B3LYP/CEP-
121G level of theory. However, the theoretical study of phos-
phine–phosphonium salts derived from bis(diphenylphos-
phino)ethane and corresponding complexes have not been
reported.
D–H. . .
A
D–H
H. . .
A
D. . .
A
<(DHA)
C2–H2. . .Br3
0.95
0.95
0.95
0.99
0.99
0.99
0.99
0.95
0.95
0.99
0.99
0.99
0.95
0.99
3.05
2.91
2.88
2.80
2.90
2.91
2.70
2.73
2.72
2.70
2.82
2.54
2.56
2.34
3.896(13)
3.827(12)
3.77(3)
3.762(9)
3.84(2)
3.874(9)
3.67(2)
3.578(14)
3.57(2)
3.536(8)
3.694(16)
3.132(9)
3.437(11)
3.299(9)
149.1
162.1
157.5
163.5
160.5
163.8
166.3
149.5
148.9
142.6
148.3
118.5
154.3
161.9
C8–H8. . .Br2
C8–H8. . .Cl2
C13–H13. . .Br1i
C13–H13. . .Cl1i
C14–H14. . .Br2
C14–H14. . .Cl2
C20–H20. . .Br3i
C20–H20. . .Cl3i
C27–H27. . .Br1i
C27–H27. . .Cl1i
C14–H14. . .O1
C26–H26. . .O3ii
C27–H27. . .O3ii
3.4.1. Geometry and electronic properties of phosphine–phosphonium
salts
The lowest energy of optimized geometries of S1 and S2 as free
ligands is shown in Fig. 3. The important optimized geometrical
parameters such as bond lengths and angles listed in Table 5 are
in accordance with atom numbering scheme given in ORTEP views
of compound 4.
i, 2 ꢀ X, 1/2 + Y, 3/2 ꢀ Z; ii, 1/2 + X, 3/2 ꢀ Y, 1 ꢀ Z.
nitrobenzoyl group is such that all of the atoms of the benzoyl
group are coplanar with the phosphorus atom (the greatest devia-
tion is related to C27 (0.030 Å)). Crystal packing of complex 4 is
presented in Fig. 2. There are several intramolecular C–H. . .Cl and
C–H. . .Br non-classic hydrogen bond interactions, with hydrogen
atoms from three phenyl rings and one of the aliphatic carbon
The final optimized structure of these ligands indicates that the
bromine ion is located near the phosphorus atom of phosphonium
group (ꢁ4 Å). This is may suggest that there is an intramolecular
Fig. 3. The optimized geometry structures of phosphine–phosphonium salts (a) S1 and (b) S2.

S. Samiee et al. / Polyhedron 98 (2015) 120–130
125
Table 6
atom of phosphonium group, and the [(PPh2(CH2)2PPh2CH2C(O)
C6H4R)Br] formulation is more reliable than [PPh2(CH2)2PPh2CH2
C(O)C6H4R]Br.
HOMO (hartree), LUMO (hartree), gap energy (eV), hardness (eV), NBO charges and
Wiberg bond indices of the [(Ph2P(CH2)2PPh2CHC(O)C6H4R)] (R = Br (S1), NO2 (S2))
free ligands.
Natural bond orbital (NBO) analyses to obtain electronic prop-
erties such as the gap energy, partial charge of atoms and Wiberg
bond index (WBI) were also investigated (see Table 6). As can be
seen in this table, when the R group was changed, the LUMO
energy of ligand S2 decreased comparing to ligand S1. In this
regard, the gap energy was found as S1 > S2, which indicates that
the ligand containing nitro is softer than the ligand containing bro-
mine. Hence, the ligand S2, namely [(PPh2(CH2)2PPh2CH2C(O)
C6H4NO2)Br], can be considered as the more reactive ligand.
Comparing partial atomic charge between S1 and S2, the partial
charge of C(32) is the only difference in these ligands. The consid-
erable variation in the charge distribution of C(32) arises from the
electron donation of the Br atom to the phenyl groups via the p
Compound
S1
S2
HOMO
LUMO
Gap
ꢀ0.196
ꢀ0.090
2.88
ꢀ0.203
ꢀ0.132
1.91
g
1.44
0.95
NBO charges
P(1)
P(2)
Br
O(1)
C(13)
C(14)
C(27)
C(28)
C(32)
Br(1)
N(1)
0.734
1.430
ꢀ0.873
ꢀ0.623
ꢀ0.611
ꢀ0.602
ꢀ0.724
0.577
ꢀ0.046
0.043
–
0.735
1.431
ꢀ0.864
ꢀ0.606
ꢀ0.612
ꢀ0.601
ꢀ0.730
0.578
0.091
orbital into the
p orbital of the phenyl group. Furthermore, due
–
to the partial atomic charges on phosphorus atoms and the above
results of final optimized geometries of ligands, it can be again con-
clude that there is an interaction between bromine ion and P(2)
atom (see Table 6).
0.447
O(2)
O(3)
–
–
ꢀ0.359
ꢀ0.368
WBIs
P(1)–C(13)
P(2)–C(14)
P(2)–C(27)
C(13)–C(14)
C(27)–C(28)
C(28)–O(1)
P(2). . .Br
0.898
0.933
0.933
1.041
1.013
1.661
0.005
0.897
0.942
0.931
1.041
1.019
1.682
0.005
3.4.2. Geometry and electronic properties of zwitterionic mercury(II)
complexes
According to result of spectroscopic section, it can be assumed
that the structure of other synthesized complexes would be similar
to complex 4. Table 7 gives the most important bond lengths and
bond angles that have been computed for six zwitterionic
mercury(II) complexes [HgX2Br(PPh2(CH2)2Ph2CH2C(O)C6H4Br)]
interaction between the phosphonium group and the bromine ion.
We are interested to figure out, the bromine ion acts only as a
counter ion or there is a bonding interaction with phosphonium
group in these ligands. To shed some light about this question,
the bonding energy was calculated according to the following for-
mation reaction (1):
(X = Cl (1), Br (2),
I (3)) and [HgX2Br(PPh2(CH2)2Ph2CH2C(O)
C6H4NO2)] (X = Cl (4), Br (5), I (6)). The final optimized geometries
of investigated complexes (4–6) are shown in Fig. 4, and for other
complexes are available in supplementary data (see Fig. 1S). The
geometrical parameters for latter complexes are in generally good
agreement with corresponding experimental values (see Table 7).
The latter results show that the nature of the halogen atoms
around metal and R group has a slight effect on the Hg–P bond
length. On the other hand, the Hg–P bond length in 1 and 4 com-
plexes (containing two chlorides) is shorter than the other com-
puted complexes (without chloride). In comparison, our results
are in good agreement with the observed experimental values in
similar complexes [8].
½PPh2ðCH2Þ PPh2CH2CðOÞC6H4Rꢂþ þ Brꢀ
ꢀꢀꢀꢀ!
2
½ðPPh2ðCH2Þ2PPh2CH2CðOÞC6H4RÞBrꢂ
ð1Þ
The results demonstrate that the latter energies in compounds
S1 and S2 are ꢀ90.73 and ꢀ94.65 kcal molꢀ1, respectively. Due to
the obtained values for bonding energy, it can be suggested that
the bromine ion possesses a bonding interaction to phosphorus
Table 7
A comparison between the selected calculated bond lengths (Å) and bond angles (°) for complexes 1–6 and corresponding experimental values for 4 and similar complex.a
1
2
3
4
5
6
CEP-121G
X-ray
X-raya
Bond lengths
Hg–P(1)
2.527
2.624
2.552
2.590
1.938
1.553
1.897
1.889
1.534
1.268
2.578
2.731
2.659
2.602
1.938
1.554
1.899
1.889
1.533
1.268
2.602
2.767
2.794
2.821
1.931
1.556
1.904
1.892
1.531
1.269
2.502
2.638
2.573
2.592
1.945
1.542
1.896
1.897
1.534
1.261
2.460
–
–
2.486
2.567
2.687
2.742
2.606
1.947
1.542
1.897
1.897
1.534
1.261
2.583
2.822
2.611
2.756
1.950
1.542
1.897
1.897
1.534
1.261
Hg–X(1)b
2.574
2.579
2.522
1.839
Hg–X(2)b
Hg–Br
–
P(1)–C(13)
C(13)–C(14)
C(14)–P(2)
P(2)–C(27)
C(27)–C(28)
C(28)–O(1)
1.826
1.518
1.806
1.813
1.515
1.217
–
1.816
1.810
1.511
1.230
Bond angles
P(1)–Hg–X(1)
P(1)–Hg–X(2)
P(1)–Hg–Br
X(1)–Hg–Br
X(2)–Hg–Br
X(1)–Hg–X(2)
96.39
92.73
137.83
119.44
99.09
98.96
91.71
135.43
117.93
94.99
111.48
91.04
134.36
111.78
89.59
103.15
95.69
135.46
102.75
101.04
105.84
–
–
–
–
–
–
109.81
100.50
126.85
106.30
106.39
104.90
108.39
97.56
133.51
106.41
97.15
110.27
94.99
132.36
108.76
94.26
103.14
106.62
109.63
109.44
112.37
a
Observed values for complex [HgCl2(Br)(Ph2PCH2PPh2CHC(O)Ph)] [8].
X(1) and X(2) in 1 and 4 is Cl; in 2 and 5 is Br; in 3 and 6 is I.
b

126
S. Samiee et al. / Polyhedron 98 (2015) 120–130
Fig. 4. The optimized geometry structures of studied complexes (a) 4, (b) 5 and (c) 6.
microanalysis data for the latter complexes. Hence, in spite of
Table 8
research down in this field [8,9], for reaction between the phos-
phonium salts and HgI2, our experimental and theoretical data
demonstrated that the 1:1 product forms spontaneously at room
temperature.
The calculated thermodynamic parameters for possible reactions shown in Scheme 2
using the DFT-based B3LYP/CEP-121G method.
R
Br
NO2
1:1
1:1
2:1
2:1
Generally, the stability of complexes is the fundamentally
determined stabilization energy. From the energy point of view,
the more negative stabilization energy of complexes, the more
stable they are. In order to realize the stability of formed com-
plexes, the stabilization energy (Estb) was calculated in terms of
the sum of electronic energies of the structures according to the
following definition as Eq. (1):
Estb
ꢀ8.52
ꢀ9.11
ꢀ1.27
ꢀ5.55
ꢀ6.14
1.45
ꢀ3.37
ꢀ3.96
ꢀ3.51
ꢀ0.017
ꢀ0.61
7.095
D
D
H°
G°
Estb (kcal molꢀ1) is the stabilization energy of the process;
enthalpy change of the process;
the process.
D
H° (kcal molꢀ1) is the
D
G° (kcal molꢀ1) is the Gibbs free energy change of
We were interested that the structures of complexes 3 and 6 are
determined by X-ray crystallography because of the different
reported structures for these types of complexes [8,9]. However,
in spite of all attempts, none of the iodine complexes have been
obtained in crystalline state. Thus, the reaction between free
ligands and HgI2 has been investigated by DFT calculation. We
are considered two possible products (1:1 and 2:1) as following
scheme.
Estb ¼ Ecomplex ꢀ EHgX2 ꢀ Esalt
ð1Þ
where Ecomplex is the energy of complex [(S1,2)MX2], EHgX2 is the
energy of mercury(II) halides and Esalt is the energy of free ligands
(S1,2). The calculated stabilization energies of all complexes can be
found in Table 9. According to the Eq. (1), negative and positive sta-
bilization energies indicate that the formed complexes are stable
and metastable, respectively. These results show that the com-
plexes containing S1 ligand are more stable than the complexes con-
taining S2 ligand (ꢁ4 kcal molꢀ1). Along the same lines, the
sequence of the calculated stabilization energies with different
halogens are found as Cl > Br > I. As a result, it can be concluded that
the formed complex (1) is the most stable among the others.
NBO analyses to obtain electronic properties such as the energy
gap, partial charge of atoms and bond order were also investigated.
The energy gap is an important parameter to characterize the
The thermodynamic parameters such as stabilization energy
(Estb), Gibbs free energy changes (
D
G°), the enthalpy changes
S°) were calculated
(D
H°), and the entropy change of process (D
at B3LYP/CEP-121G level of theory for both 1:1 and 2:1 products
(see Table 8). The stabilization energy indicates the formation of
1:1 product is more stable than 2:1 product. The obtained results
from Table 8 indicate that the reaction of 1:1 product shown in
Scheme 2 is exothermic and spontaneous, and these complexes
are able to be synthesized quickly. This is in agreement with the
chemical reactivity and kinetic stability of the molecule.
A

S. Samiee et al. / Polyhedron 98 (2015) 120–130
127
:
1 1
r
H I B PP
H
PP
H
H R
h2C C O C
]
6
4
h
C
(
g
[
(
)
2 2
( )
2
2
2
r +
H I
g
2
PP
R
H
PP
H
H R B
h
C
h2C C O C
[
]
(
)
( )
6
2
2 2
2
4
=
r
(
B
S1
N
O2
S2
,
)
(
)
:
2 1
r
2 H IB PPh CH PPh CH C O C H R
g
[
]
4
(
2
(
2
)
( )
6
2 2
2
2
+
PP
[
H
PP
H
H R I
h2C C O C
]
6
4
h
C
(
)
2 2
( )
2
2
Scheme 2. The possible products of reactions between ligands and HgI2.
Table 9
molecule with a small energy gap is more polarizable, generally
associated with a high chemical reactivity, low kinetic stability,
and is called a soft molecule [39]. The highest occupied molecular
orbital (HOMO) of one component and the lowest unoccupied
molecular orbital (LUMO) are important parameters of molecular
Calculated energies for HOMO and LUMO molecular orbitals, energy gap between the
HOMO and LUMO, hardness and stabilization energy (Estb) for all studied complexes.
Complex HOMO
(hartree)
LUMO
(hartree)
Gap
(eV)
g
(eV)
Estb
(kcal molꢀ1
)
1
2
3
4
5
6
ꢀ0.2336
ꢀ0.2291
ꢀ0.2177
ꢀ0.2258
ꢀ0.2207
ꢀ0.2106
ꢀ0.100
3.62
3.43
3.07
1.61
1.44
1.17
1.81
1.71
1.53
0.80
0.72
0.58
ꢀ18.85
ꢀ12.54
ꢀ10.02
ꢀ14.65
ꢀ8.57
electronic structure. The hardness
Eq. (1):
g
of a molecule is defined as
ꢀ0.102
ꢀ0.1046
ꢀ0.1664
ꢀ0.1676
ꢀ0.1674
ðI ꢀ AÞ
g
¼
ð2Þ
2
ꢀ4.88
E
LUMO = -0.1674 eV
E
LUMO = -0.1046 eV
Egap = 1.17 eV
Egap = 3.07 eV
EHOMO = -0.2106 eV
EHOMO = -0.2177 eV
(a)
(b)
Fig. 5. Molecular orbital surfaces and energy level of complexes (a) 3 and (b) 6.

128
S. Samiee et al. / Polyhedron 98 (2015) 120–130
Table 10
Wiberg bond indices (WBIs) and natural bond orbital (NBO) charges of all studied complexes.
1
2
3
4
5
6
WBIs
Hg–P(1)
Hg–X(1)
Hg–X(2)
Hg–Br
P(2)–C(27)
C(27)–C(28)
C(28)–O(1)
0.1974
0.4130
0.5078
0.6815
0.9345
1.0063
1.6725
0.1757
0.4770
0.5739
0.6638
0.9339
1.0062
1.6729
0.1571
0.7218
0.7660
0.4025
0.9347
1.0054
1.6725
0.1898
0.4195
0.4818
0.6839
0.9180
1.0034
1.7259
0.1775
0.5392
0.6687
0.4816
0.9178
1.0033
1.7233
0.1671
0.6510
0.7715
0.4699
0.9180
1.0032
1.7234
NBO charges
Hg
0.7699
0.6795
0.4673
0.7614
0.6719
0.5016
X(1)
X(2)
Br
P(1)
ꢀ0.6758
ꢀ0.6196
ꢀ0.5008
0.7819
ꢀ0.5644
ꢀ0.5075
ꢀ0.6220
0.7822
ꢀ0.4363
ꢀ0.4064
ꢀ0.6633
0.8072
ꢀ0.6733
ꢀ0.6418
ꢀ0.5013
0.8214
ꢀ0.5068
ꢀ0.5939
ꢀ0.6235
0.8255
ꢀ0.4943
ꢀ0.4040
ꢀ0.6269
0.8216
P(2)
1.4317
1.4319
1.4326
1.4377
1.4375
1.4364
C(27)
C(28)
O(1)
ꢀ0.7161
ꢀ0.7182
0.57301
ꢀ0.6077
ꢀ0.7110
ꢀ0.7032
ꢀ0.7015
ꢀ0.7017
0.5730
0.5703
0.5637
0.5635
0.5635
ꢀ0.6086
ꢀ0.6088
ꢀ0.5585
ꢀ0.5605
ꢀ0.56021
X(1) and X(2) in 1 and 4 is Cl; in 2 and 5 is Br; in 3 and 6 is I.
on mercury changes in order Cl > Br > I. The effect of halogen atoms
on the charge distribution parallels their electronegativities.
Besides, the analysis of charge transfer between free ligands and
mercury(II) halides is considered. The charge transferred between
ligand and mercury(II) halides is measured as the sum of natural
charges of all atoms related to the phosphonium salt and HgX2
Table 11
Calculated charge transferred in all synthesized here complexes using B3LYP/CEP-
121g level of theory.
Type
Charge
transfer (a.u.)
Type
Charge
transfer (a.u.)
S1 as free ligand
S1 in complex 1
S1 in complex 2
S1 in complex 3
0
HgX2 as free ligand
HgCl2 in complex 1
HgBr2 in complex 2
HgI2 in complex 3
0
0.5255
0.5069
0.3754
ꢀ0.5255
ꢀ0.5069
ꢀ0.3754
Table 12
S2 as free ligand
S2 in complex 4
S2 in complex 5
S2 in complex 6
0
HgX2 as free ligand
HgCl2 in complex 4
HgBr2 in complex 5
HgI2 in complex 6
0
Antibacterial activity data of free ligands and their mercury(II) complexes.
0.5537
0.4288
0.3966
ꢀ0.5537
ꢀ0.4288
ꢀ0.3966
Compound
Ligand S1
Complex 1
Complex 2
Complex 3
Ligand S2
Conc. (mg/
mL)
Inhibition zone (mm)
E. coli
P. aeruginosa
S. aureus
(+)
B. cereus
(+)
(ꢀ)
(ꢀ)
5
n.a.
n.a.
n.a.
n.a.
17
18
18
21
17
17
19
21
17
18
18
21
14
15
18
22
18
24
27
30
26
28
40
44
28
30
32
32
9
12
14
14
21
13
13
15
17
12
15
15
19
15
15
17
17
8
n.a.
n.a.
n.a.
10
8
10
11
14
11
12
14
14
11
12
13
15
n.a.
n.a.
n.a.
n.a.
10
20
40
5
10
20
40
5
10
20
40
5
10
20
40
5
where I and A are the ionization potential and the electron affinity
of the system, respectively. Obviously, the energy gap between the
HOMO and LUMO is equal to (I ꢀ A) [40,41]. Therefore, we can
easily calculate the hardness of the present molecules using equa-
tion Eq. (2):
ðELUMO ꢀ EHOMO
Þ
g
¼
ð3Þ
2
Herein, we consider the gap energy since it is known to be an
index of both kinetic stability (reactivity) and electrical conductiv-
ity. The calculated HOMO and LUMO energies, energy gap and
hardness of all complexes are also summarized in Table 9. Upon
going from complex 1 to 6, the energy gap consistently decrease.
It is interesting to note that the energy gap of complexes including
Br group (1–3) are significantly higher than the complexes includ-
ing NO2 group (4–6). In this regard, for metal complexes with dif-
ferent halogens, the energy gap increases in this order Cl > Br > I.
These results are completely consistent with the results obtained
for the stabilization energy. The distribution and energy levels of
the HOMO and LUMO orbitals for the studied complexes 3 and 6
are shown in Fig. 5. As can be seen, the HOMO orbital is only dis-
tributed over the metal and halogen coligands and LUMO orbital
is only localized on CH2C(O)R moiety of ligand.
The natural charge of some atoms and the WBI values are listed
in Table 10. Note that the total charge of the molecules is zero. The
calculated NBO charge distribution indicates that the phosphorus
and metal atoms always carry positive charge and the oxygen,
halogen and methene carbon atoms convey negative charge. For
metal complexes with different halogen atoms, the positive charge
n.a.
n.a.
n.a.
n.a.
10
20
40
5
10
14
22
9
11
17
Complex 4
Complex 5
Complex 6
5
16
17
19
21
19
19
20
23
18
18
20
22
12
20
25
25
25
29
40
40
30
40
20
23
26
28
n.a.
20
15
16
18
20
18
18
21
26
16
16
18
20
26
16
11
13
15
17
13
15
18
24
10
10
13
18
13
21
10
20
40
5
10
20
40
5
10
20
40
Penicillin
Gentamicin
n.a. = no activity.

S. Samiee et al. / Polyhedron 98 (2015) 120–130
129
obtained at the B3LYP/CEP-121 g level of theory (see Table 11). The
3.5. Biological studies
results indicate that the significant charge transfer (ꢁ0.45 a.u.)
from the ligands to the metal and halogen coligands is occurred.
The WBI value arises from the manipulation of the density
matrix in the orthogonal natural atomic orbital basis derived
through the natural population analysis [42]. The WBI expresses
the sum of squares of density matrix elements (pjk) and equals
twice the charge density in the atomic orbital (pjj) minus the square
of the charge density, and so is mathematically defined as Eq. (3):
The in vitro antibacterial activities of the phosphonium salts and
their mercury(II) complexes have been studied along with three
standard antibacterial drugs, viz, Vancomycin, Streptomycin, and
Gentamycin. The microorganisms used in this work include B. cer-
eus and S. aureus as Gram-positive bacteria and E. coli and P. aerug-
inosa as Gram-negative bacteria. Results of antibacterial
assessments of all compounds are presented in Table 12. The data
displays that the concentration of all compounds plays an impor-
tant role by increasing the degree of inhibition as the concentration
increases. Comparing the antibacterial activities of samples with
standard drugs demonstrate that the metal complexes have
remarkable inhibitory potencies against bacterial species taken in
the study (see Fig. 6).
A comparative study of the growth inhibition zone values
between two free ligands indicate that the activity of free ligand
S1 was higher than the activity of S2, and it is interesting that the
latter ligands only exhibits antibacterial activity against on P.
aeruginosa and S. aureus, while is not active against on E. Coli and
B. cereus. Thus, similar to earlier results, the activity of the ligands
X
WBI ¼
p2jk ¼ 2pjj ꢀ p2jj
ð4Þ
k
Table 3 shows that the WBI values of Hg–P bonds in the zwitte-
rionic complexes are significantly slight (ꢁ0.18). On the other
hand, according to NBO analyses, there is no metal–ligand bonding
orbital for all investigated complexes. Therefore, the electrostatic
interaction plays an important role in the interaction between
the metal atom and the phosphonium-phosphine ligands. This is
in accordance with the above conclusion deduced from the charge
distribution.
Fig. 6. Antibacterial activities of two phosphonium-phosphine salts and their mercury(II) complexes against B. cereus and S. aureus as Gram-positive bacteria and E. coli and P.
aeruginosa as Gram-negative bacteria.

130
S. Samiee et al. / Polyhedron 98 (2015) 120–130
was slightly affected by the nature of the substituent in the para
position of the phenyl ring of benzoyl group, and this can be
explained by the lipophilicity of the ligands and their membrane
permeability, a key factor in determining of their entry inside the
cells [43].
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/10.
1016/j.poly.2015.06.019.
On the other hand, a remarkably high activity of all complexes
(1–6) was observed in comparison with the free ligands. Generally,
the antibacterial activity of compounds is attributed mainly to its
major components. However, today it is known that the synergistic
or antagonistic effect of one compound even when it is a minor
component of mixture has to be considered [44,45]. It seems that
the formation of complexes may facilitate crossing of the lipid
layer of the bacterial cell membrane and in this way may effect
on the mechanisms of growth and development of bacteria
[46,47]. Surprisingly, P. aeruginosa revealed high sensitivity to both
groups of the studied mercury complexes, while standard drugs
were found to have no activity against it. Furthermore, the halide
groups coordinated to metal ions exerts a number of changes on
antibacterial activity of the tested complexes. The results of
antibacterial assay express that the complexes reported herein
may be used for control of pathogenic bacteria.
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Appendix A. Supplementary data
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CCDC 1027987 contains the supplementary crystallographic
data for complex 4. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road,