Synthesis, spectra and crystal structures of zwitterionic mercury(II) complexes formed by the ligand, [Ph2PCH2PPh2CH2C(O)Ph]+

Article abstract of DOI:10.1016/j.poly.2009.01.001

The reactions of phosphonium salt, [PPh₂CH₂PPh₂CH₂C(O)Ph]Br with Hg(II) halides in methanol formed the zwitterionic products with the composition [HgCl₂(Br)(PPh₂CH₂PPh₂

Full text of DOI:10.1016/j.poly.2009.01.001

Polyhedron 28 (2009) 1017–1021
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, spectra and crystal structures of zwitterionic mercury(II) complexes
formed by the ligand, [Ph2PCH2PPh2CH2C(O)Ph]+
b
Mothi Mohamed Ebrahim a,b, Krishnaswamy Panchanatheswaran a, , Antonia Neels ,
*
Helen Stoeckli-Evans b,
*
a School of Chemistry, Bharathidasan University, Tiruchirappalli, India
b Institute of Microtechnology, University of Neuchâtel, Neuchâtel, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
The reactions of phosphonium salt, [PPh2CH2PPh2CH2C(O)Ph]Br with Hg(II) halides in methanol formed
the zwitterionic products with the composition [HgCl2(Br)(PPh2CH2PPh2CH2C(O)Ph)] (2),
[HgBr3(PPh2CH2PPh2CH2C(O)Ph)] (3), [HgBr2(I)(PPh2CH2PPh2CH2C(O)Ph)] (4). A product of composition,
[HgCl3(PPh2CH2PPh2CH2C(O)Ph)] (5) was obtained when the ylide, Ph2PCH2PPh2@CHC(O)Ph was treated
with HgCl2 in CH2Cl2 and crystallized under aerobic conditions. The complexes were characterized by ele-
mental analysis, IR, 1H, 31P NMR spectra and also by X-ray crystallography. The absence of significant
coordination chemical shifts in the 31P NMR spectra indicates the presence of formal negative charge
on mercury. The single crystal X-ray structures confirm the presence of Hg–P bond and also reveal a dis-
torted tetrahedral geometry around the mercury atom in all the structures.
Received 17 September 2008
Accepted 5 January 2009
Available online 7 February 2009
Keywords:
Phosphonium salt
Hg(II) complexes
Zwitterionic complexes
Mercurates
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
meric halogen bridged phosphine complexes with dangling ylide
[11]. On the other hand, Ph2PCH2PPh2@C(H)C(O)Ph affords P, C-
Phosphonium salts are useful intermediates in organic synthe-
sis, exhibiting variety of reactivity when compared with the
ammonium or pyridinium salts [1]. In the field of cationic polymer-
ization, phosphonium salts showed promise as latent thermal- or
photoinitiators due to their ability to form the corresponding sta-
ble ylides by releasing a proton [2–4]. Different types of homo-
and heteropolynuclear ylide complexes of Hg(II), Pd(II) and Au(I)
were prepared using the corresponding phosphonium salts, by
deprotonation [5]. In the course of our ongoing research on the
coordination chemistry of keto-stabilized phosphorus ylides, we
have been interested to investigate the different bonding modes
adopted by ylides upon ligation to Hg(II) [6–8] and U(VI) [9]. The
chelate complexes with mercury(II) halides [12]. Trihalomercu-
rates such as [(CH3)4N]HgX3 and [(CH3)4P]HgX3 (where X = Cl, Br,
I) are also useful as novel ferroelectric materials [13]. It is therefore
of interest to investigate the reactivity of the hybrid phosphine–
phosphonium salt, [PPh2CH2PPh2CH2COPh]Br with mercury(II) ha-
lides which can in principle form (i) mercury-ylide complexes by
deprotonation, (ii) phosphonium metalates where the metal re-
mains uncoordinated to the ligand or (iii) zwitterionic complexes
containing Hg–P bond. In this paper, we report the formation of
zwitterionic complexes of mercury(II) and their solution and solid
state structures.
a-keto-stabilized ylides derived from bisphosphines, viz.,
Ph2PCH2PPh2@C(H)C(O)R and Ph2PCH2CH2PPh2@C(H)C(O)R
2. Experimental
(R = Me, Ph or OMe) [10] form an important class of hybrid ligands
containing both phosphine and ylide functionalities, and can exist
in ylidic and enolate forms. We recently observed that the mono-
keto ylide with an ethylenic spacer, Ph2PCH2CH2PPh2@C(H)C(O)Ph,
forms polymeric Hg(II) complexes with HgCl2 via P, C-bridging
mode while HgBr2 and HgI2 react with the same ylide giving poly-
All reactions were carried out in an atmosphere of nitrogen.
Reactants and reagents were obtained from Aldrich Chemical Com-
pany and used without further purification. The solvents were
dried and distilled using standard methods [14]. The 1H and 31P-
{1H}NMR spectra were recorded on a Bruker DPX400 spectrometer
at 400.13 and 161.98 MHz, referenced relative to residual solvent
and external 85% H3PO4, respectively. The chemical shifts (d) and
the coupling constants (J) were expressed in ppm and Hz, respec-
tively. The IR spectra in the interval of 4000–400 cmꢀ1 were re-
corded on a Perkin–Elmer 1720X FT-IR spectrophotometer using
* Corresponding authors. Tel.: +91 431 2407053; fax: +91 431 2407045 (K.
Panchanatheswaran).
E-mail address: panch_45@yahoo.co.in (K. Panchanatheswaran).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.01.001

1018
M.M. Ebrahim et al. / Polyhedron 28 (2009) 1017–1021
KBr pellets. Elemental analyses were performed at the Ecole
d’ingénieurs de Fribourg, Switzerland.
7.27–8.05 (m, 25H, Ph). 31P NMR (162 MHz, DMSO–d6): d ꢀ22.78
2
(br, PPh2), 23.55 (d, PCH2COPh, JP–P = 60.2).
2.2. X-ray crystallography
2.1. Synthesis
All the crystallizations were carried out under aerobic condi-
tions. Single crystals of 2, 3 and 4 were obtained by slow evapora-
2.1.1. [Ph2PCH2PPh2CH2C(O)Ph]Br (1)
The phosphonium salt was prepared following the literature
procedure [10] using BrCH2COPh (1.55 g, 7.8 mmol) and
PPh2CH2PPh2 (3.00 g, 7.8 mmol). Yield: 4.10 g (90%). M.p. 194–
tion
of
dichloromethane,
dichloromethane/hexane
and
dichloromethane/pentane solutions, respectively. Well formed
crystals of 5 were grown by layering pentane over a dichlorometh-
ane solution. The intensity data were collected at 173 K (ꢀ100 °C)
on a Stoe Mark II-Image Plate Diffraction System [15] equipped
196 °C (Reported: 195–197 °C). IR (cmꢀ1): 1667 (
m
C@O). 1H NMR
2
(CDCl3): d 4.32 (d, 2H, PCH2P, JP–H = 14.6), 5.93 (d, 2H, PCH2COPh,
2JP–H = 12.5), 7.20–8.13 (m, 25H, Ph). 31P NMR (CDCl3): d ꢀ26.18 (d,
2
2
with a two-circle goniometer and using Mo Ka graphite monochro-
PPh2, JP–P = 63.2), 24.07 (d, PCH2COPh, JP–P = 63.2).
mated radiation. The structures were solved by direct methods
using the programme SHELXS-97 [16]. The refinement and all further
calculations were carried out using SHELXL-97 [16]. The H-atoms
were included in calculated positions and treated as riding atoms
using SHELXL default parameters. The non-H atoms were refined
anisotropically, using weighted full-matrix least-squares on F2.
Further crystallographic data are given in Table 1. The molecular
structure and crystallographic numbering schemes are illustrated
in ORTEP [17] drawings, Figs. 1–3.
2.1.2. [HgCl2(Br)(PPh2CH2PPh2CH2C(O)Ph)] (2)
A
mixture
of
HgCl2
(0.09 g,
0.34 mmol)
and
[PPh2CH2PPh2CH2C(O)Ph]Br (0.18 g, 0.34 mmol) in methanol
(15 ml) was stirred for 3 h. The white precipitate obtained was iso-
lated, washed twice with 15 ml methanol and recrystallised in
dichloromethane. Yield: 0.25 g (86%). M.p. 158–160 °C. Anal. Calc.
for C33H29BrCl2HgOP2: C, 46.36; H, 3.42. Found. C, 46.34; H,
3.35%. IR (cmꢀ1): 1672 (
m
C@O). 1H NMR (DMSO–d6): d 4.43 (d,
2
2
2H, PCH2P, JP–H = 15.5), 5.68 (d, 2H, PCH2COPh, JP–H = 12.2),
7.26–7.99 (m, 25H, Ph). 31P NMR (DMSO–d6): d ꢀ19.79 (d, PPh2,
3. Results and discussion
2JP–P = 63.2), 23.43 (d, PCH2COPh, JP–P = 56.6).
2
3.1. Synthesis
2.1.3. [HgBr3(PPh2CH2PPh2CH2C(O)Ph)] (3)
The reactions of mercury(II) halides with the phosphine–phos-
phonium salt, [Ph2PCH2PPh2CH2C(O)Ph]Br in 1:1 molar ratio in
methanol afford the zwitterionic complexes 2–4 as shown in
Scheme 1.
While 2 and 3 are simple complexation products, the formation
of 4 involves a halogen exchange followed by complexation as
shown below,
This complex was obtained using the same procedure as
adopted for the preparation of 2 using HgBr2 (0.12 g, 0.34 mmol).
The product was recrystallized using dichloromethane containing
a few drops of hexane. Yield: 0.29 g (90%). M.p. 135–137 °C. Anal.
Calc. for C33H29Br3HgOP2: C, 41.99; H, 3.10. Found. C, 41.49; H,
3.00%. IR (cmꢀ1): 1671 (
m
C@O). 1H NMR (DMSO–d6): d 4.27 (d,
2
2
2H, PCH2P, JP–H = 15.4), 5.58 (d, 2H, PCH2COPh, JP–H = 12.2),
7.26–7.97 (m, 25H, Ph). 31P NMR (DMSO–d6): d ꢀ26.72 (d, PPh2,
2½Ph2PCH2PPh2CH2CðOÞPhꢁBr þ HgI2
! ½Br2ðIÞHgðPPh2CH2PPh2CH2CðOÞPhÞꢁ þ ½Ph2PCH2PPh2CH2CðOÞPhꢁI
2
2JP–P = 65.4), 23.74 (d, PCH2COPh, JP–P = 65.4).
2.1.4. [HgBr2(I)(PPh2CH2PPh2CH2C(O)Ph)] (4)
The stability of 4 consisting of the HgIBr2 moiety can be traced
A
mixture
of
HgI2
(0.08 g,
0.17 mmol)
and
to the softness of Hg(II) and Iꢀ. The non-formation of HgI2Brꢀ or
ꢀ
[PPh2CH2PPh2CH2C(O)Ph]Br (0.10 g, 0.17 mmol) in methanol
(15 ml) was stirred for 3 h. The clear colourless solution was evap-
orated to dryness giving an oily residue. Addition of diethyl ether
(30 ml) resulted in the formation of a pale yellow solid. It was
recrystallized in a mixture of dichloromethane–pentane and dried.
Yield: 0.075 g (44%). M.p. 103–105 °C. Anal. Calc. for
C33H29Br2HgIOP2: C, 40.00; H, 2.95. Found. C, 39.53; H, 2.90%. IR
HgI3 may be ascribed to steric effects caused by Iꢀ ion. Complex
5
has been obtained by the reaction of the ylide,
Ph2PCH2PPh2@C(H)C(O)Ph with HgCl2 in dichloromethane and
subsequent crystallization. The preferential formation of zwitter-
ionic complexes as opposed to the formation of phosphonium
metalates can be ascribed to the strength of Hg–P bond.
(cmꢀ1): 1675 (
m
C@O). 1H NMR (DMSO–d6): d 4.24 (d, 2H, PCH2P,
3.2. Spectroscopy
2
2JP–H = 15.4), 5.57 (d, 2H, PCH2COPh, JP–H = 12.3), 7.24–7.97 (m,
25H, Ph). 31P NMR (DMSO–d6): d ꢀ28.29 (d, PPh2, JP–P = 67.0),
In the IR spectra of complexes 2–5, a strong absorption around
1675 cmꢀ1, which is close to the same frequency in free phospho-
nium salt (1667 cmꢀ1) indicates the non involvement of the
–PCH2C(O)Ph group in the reactions. The 31P NMR spectra of com-
plexes 2–5 exhibit two mutually coupled doublets corresponding
to ‘phosphonium’ and ‘phosphine’ groups. The former peak re-
mains sharp and unaltered, while the latter peak is relatively
broad. In contrast to the 31P NMR of Hg(II)–phosphine complexes
[18], the coordination of phosphine to mercury in the present cases
did not cause significant downfield shifts. Complexes 2 and 5 show
downfield shifts (ꢀ19.79 and ꢀ22.78 ppm, respectively) compared
to that of phosphine of the phosphonium salt (ꢀ26.18 ppm). The
chemical shift for 3 (ꢀ26.72 ppm) appears close to that of the free
ligand whereas 4 (ꢀ28.29 ppm) shows a slight upfield shift. These
data indicate that the presence of a formal negative charge on the
metal may effectively reduce the deshielding experienced by the
phosphorus due to complexation. In the 1H NMR spectra, the dou-
2
2
23.39 (d, PCH2COPh, JP–P = 67.0).
2.1.5. [HgCl3(PPh2CH2PPh2CH2C(O)Ph)] (5)
To a suspension of HgCl2 (0.10 g, 0.36 mmol) in dichlorometh-
ane (10 ml) a solution of Ph2PCH2PPh2@C(H)C(O)Ph [10] (0.21 g,
0.36 mmol) in dichloromethane (10 ml) was added dropwise. The
clear suspension immediately turned turbid. After completion of
the addition the solution became clear and the stirring was contin-
ued for two hours. The solution was reduced to about 5 ml and the
addition of excess n-pentane resulted in a white precipitate. The
solid was dissolved in CH2Cl2 and layered with n-pentane. After
two days colourless diffraction quality crystals were obtained.
Yield: 0.06 g (21% based on ylide). M.p. 184–186 °C. Anal. Calc.
for C33H29Cl3HgOP2: C, 48.90; H, 3.61. Found. C, 48.84; H, 3.58%.
IR (cmꢀ1): 1674 (
2H, PCH2P, JP–H = 15.7), 5.64 (d, 2H, PCH2COPh, JP–H = 12.2),
m
C@O). 1H NMR (400 MHz, DMSO–d6): d 4.34 (d,
2
2

M.M. Ebrahim et al. / Polyhedron 28 (2009) 1017–1021
1019
Table 1
Crystal data and refinement details for compounds 2 ꢂ CH2Cl2, 3 ꢂ CH2Cl2, 4 ꢂ CH2Cl2, 5 ꢂ 2CH2Cl2.
2 ꢂ CH2Cl2
3 ꢂ CH2Cl2
4 ꢂ CH2Cl2
5 ꢂ 2CH2Cl2
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal size (mm)
Crystal system
Space group
a (Å)
C33H29BrCl2HgOP2, CH2Cl2
939.83
173(2)
C33H29Br3HgOP2, CH2Cl2
1028.75
173(2)
0.71073
0.50 ꢃ 0.40 ꢃ 0.30
monoclinic
P21/c
10.2043(4)
21.7893(10)
16.1032(6)
96.698(3)
3556.0(3)
4
C33H29Br2HgIOP2, CH2Cl2
1075.74
173(2)
0.71073
0.40 ꢃ 0.30 ꢃ 0.25
monoclinic
P21/c
10.451(2)
21.771(4)
16.265(3)
96.299(16)
3678.4(12)
4
C33H29Cl3HgOP2, 2(CH2Cl2)
980.29
173(2)
0.71073
0.50 ꢃ 0.15 ꢃ 0.10
monoclinic
P21/c
10.1464(4)
21.7503(10)
15.8516(7)
96.702(3)
3474.3(3)
4
0.71073
0.50 ꢃ 0.50 ꢃ 0.29
monoclinic
P21/n
10.5390(4)
22.6369(8)
16.4398(6)
106.031(3)
3769.5(2)
4
b (Å)
c (Å)
b (°)
V (Å3)
Z
F(000)
l
1824
6.007
1968
7.966
2040
7.454
1920
4.693
(mmꢀ1
)
Reflections measured
Independent reflections
Rint
42556
6175
0.0555
47650
9578
0.0642
19814
6572
0.0695
53038
10137
0.0287
Observed reflections [I > 2
r
(I)]
5272
7728
3974
9669
R1 (I > 2
r(I))
0.0399
0.0339
0.0492
0.0310
wR2 (all data)
0.1107
0.0723
0.1278
0.0674
Fig. 2. Molecular structure of [HgBr3(PPh2CH2PPh2CH2COPh)] (3) with 30% prob-
ability ellipsoids. The dichloromethane solvent has been omitted for clarity.
Fig. 1. Molecular structure of [HgCl2(Br)(PPh2CH2PPh2CH2COPh)] (2) with 30%
probability ellipsoids. The dichloromethane solvent has been omitted for clarity.
blet in the region of 4.30 ppm attributed to PCH2P, and the doublet
in the region of 5.62 ppm attributed to PCH2COPh, remains unaf-
fected due to complexation.
3.3. Molecular structures of complexes 2–5
The crystal structures of 2 ꢂ CH2Cl2, 3 ꢂ CH2Cl2, 4 ꢂ CH2Cl2 and
5 ꢂ 2CH2Cl2 have been determined and the relevant bond parame-
ters are given in Table 2. The molecular structures of 2, 3, 4 are rep-
resented in Figs. 1–3, respectively. The structure of 5 is given as
Supplementary data (Fig. S1). The Hg atom in all four structures
is in a distorted tetrahedral environment with one P, and three ha-
lide ligands. The Hg–P distances in all of these zwitterionic com-
plexes fall within the range of 2.39(1)–2.606(3) Å observed in
Fig. 3. Molecular structure of [HgBr2(I)(PPh2CH2PPh2CH2COPh)] (4) with 30%
probability ellipsoids. The dichloromethane solvent has been omitted for clarity.

1020
M.M. Ebrahim et al. / Polyhedron 28 (2009) 1017–1021
H2
C
H2
C
Ph2P
Hg
P
CH2
Ph2P
P
CH2
Ph
Ph
HgX2
Ph2
Ph2
Br
MeOH
X
(1)
Br
O
O
X
X = Cl (2), Br (3)
HgI2/MeOH
H2
C
Ph2P
Hg
Br
P
CH2
Ph
Ph2
I
Br
O
(4)
Scheme 1. Reactions of mercuric halides with phosphonium salt 1.
Table 2
of any significant additional resonance delocalization in the ylide
moiety.
Selected bond distances (Å) and angles (°) in 2–5.a
2
3
4
5
4. Conclusions
Hg(1)–P(2)
Hg(1)–X(l)
Hg(1)–X(m)
Hg(1)–X(n)
P(1)–C(1)
2.4868(15)
2.5221(8)
2.5743(15)
2.5792(14)
1.810(6)
1.511(9)
1.230(8)
1.816(6)
1.839(6)
2.5077(9)
2.6545(4)
2.6474(4)
2.5504(4)
1.806(4)
1.522(5)
1.220(5)
1.816(3)
1.841(3)
2.550(3)
2.4465(7)
2.5197(8)
2.6210(8)
2.4263(8)
1.803(3)
1.518(4)
1.213(4)
1.815(3)
1.843(3)
2.7030(9)
2.7038(13)
2.7084(13)
1.813(10)
1.510(15)
1.230(12)
1.811(10)
1.836(10)
The reactions of the phosphine–phosphonium salt,
[PPh2CH2PPh2CH2COPh]Br with mercury(II) halides offers an easy
method for the preparation of zwitterionic mixed halogen mercu-
rates. Formation of the above complexes can be ascribed to the
strength of Hg–P bond. In the 31P NMR spectra the absence of sig-
nificant downfield shifts due to complexation could be ascribed to
the presence of formal negative charge on the metal which effec-
tively reduces the desheilding experienced by the phosphine phos-
phorus atom. The crystal structures of the above complexes reveal
a distorted tetrahedral geometry around the mercury atom.
C(1)–C(2)
O(1)–C(2)
P(1)–C(21)
P(2)–C(21)
X(n)–Hg(1)–P(2)
X(n)–Hg(1)–X(1)
P(2)–Hg(1)–X(1)
X(n)–Hg(1)–X(m)
P(2)–Hg(1)–X(m)
X(1)–Hg(1)–X(m)
109.81(5)
106.30(4)
126.85(4)
104.90(5)
100.50(5)
106.39(4)
124.51(2)
107.656(15)
99.77(2)
106.554(14)
109.85(2)
107.300(15)
112.10(7)
108.67(3)
121.83(7)
108.55(4)
95.30(7)
133.53(3)
99.16(3)
113.45(3)
107.90(3)
92.71(2)
108.36(3)
109.05(3)
Acknowledgements
P(1)–C(1)–C(2)–O(1)
21.7(8)
24.7(5)
ꢀ14.6(15)
9.3(4)
a
For 2 read l = Br(1), m = Cl(1), n = Cl(2). For 3 read l = Br(1), m = Br(2), n = Br(3).
For 4 read l = I(1), m = Br(1), n = Br(2). For 5 read l = Cl(1), m = Cl(2), n = Cl(3).
M.M.E. thanks the Swiss Federal Commission for a scholarship
to study at the University of Neuchâtel. K.P. thanks Department
of Science and Technology, New Delhi, India, for financial assis-
tance (SERC-SR/S1/IC-29/2003).
Hg(II)–phosphine complexes [19]. The Hg–Cl distances in 5 and 2
are comparable to those reported in the literature [20,21]. In com-
plex 3, one of the Hg–Br distances is shorter [2.550(1) Å] than the
other two. It is worth mentioning that in 4, both the Hg–Br and
Hg–I [2.703(1) Å] distances are very similar. One of the P–Hg–hal-
ogen bonds in the complexes exhibits a major deviation from an
ideal tetrahedral angle. The most striking feature is the large dis-
tortion shown by P–Hg–Cl bond angle in complex 5 (which con-
tains three chlorides). It has been observed previously [22] that
the weaker bonding of chlorine to mercury compared to that of
bromine and iodine, allows the Hg–P bonding to be strengthened
leading to a linear P–Hg–Cl arrangement. In accordance with this
fact the Hg–P bond length is much shorter in complex 5 (contain-
ing three chlorides) and 2 (containing two chlorides) compared to
that in 3 and 4 (containing no chloride). The structural features in
the phosphonium moiety are very similar in all four complexes.
The significant shortening of the C2–O1 bond, as well as the elon-
gation of the P1–C1 and C1–C2 bonds in all four complexes, com-
pared with ylide, PPh2CH2PPh2CHCOPh [12] indicates the absence
Appendix A. Supplementary data
CCDC 662097, 662098, 662099 and 662096 contain the supple-
mentary crystallographic data for 2 ꢂ CH2Cl2, 3 ꢂ CH2Cl2, 4 ꢂ CH2Cl2
and 5 ꢂ 2CH2Cl2, respectively. 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,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: de-
posit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/
j.poly.2009.01.001.
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