The synthesis and structural characterisation of the mercury (II) halide complexes of the phosphorus ylide carbethoxymethylenetriphenylphosphorane

Article abstract of DOI:10.1016/j.jorganchem.2006.11.002

The reaction of the ylide carbethoxymethylenetriphenylphosphorane (EPPY), Ph₃PCHCOOEt, with mercury (II) halides has been investigated. The resulting dimeric mercury (II)-ylide complexes are isostructural and of the form [(EPPY)(HgX₂)]₂ where X is either bromine (1), chlorine (2), or iodine (3). These complexes have been characterised by spectroscopic techniques and X-ray diffraction. The ylide ligands have been shown to be C-coordinated to the mercury (II) atom.

Full text of DOI:10.1016/j.jorganchem.2006.11.002

Journal of Organometallic Chemistry 692 (2007) 1081–1086
www.elsevier.com/locate/jorganchem
The synthesis and structural characterisation of the mercury (II)
halide complexes of the phosphorus ylide
carbethoxymethylenetriphenylphosphorane
a
b
a,
*
Elinor C. Spencer , Mahimaidoss Baby Mariyatra , Judith A.K. Howard
,
a
b
Alan M. Kenwright , Krishnaswamy Panchanatheswaran
a
Department of Chemistry, Durham University, South Rd, Durham DH1 3LE, United Kingdom
b
Department of Chemistry, Bharathidasan University, Tiruchirappalli – 620 024, India
Received 10 October 2006; received in revised form 2 November 2006; accepted 2 November 2006
Available online 9 November 2006
Abstract
The reaction of the ylide carbethoxymethylenetriphenylphosphorane (EPPY), Ph3PCHCOOEt, with mercury (II) halides has been
investigated. The resulting dimeric mercury (II)-ylide complexes are isostructural and of the form [(EPPY)(HgX2)]2 where X is either
bromine (1), chlorine (2), or iodine (3). These complexes have been characterised by spectroscopic techniques and X-ray diffraction.
The ylide ligands have been shown to be C-coordinated to the mercury (II) atom.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Mercury complexes; Phosphorus ylide; Mercury (II); EPPY; Carbethoxymethylenetriphenylphosphorane
1. Introduction
From consideration of the HSAB rules [3] it can be
hypothesised that the EPPY ligand will coordinate to the
‘soft’ Hg(II) ion via the ‘soft’ ylidic carbon atom as
opposed to the ‘hard’ carbonyl oxygen atom. Navarro
and Urriolabeitia have published an excellent study of
the bonding modes of a-stabilised phosphorus ligands [4],
and the reader is directed to this article and the reference
therein for a full appreciation of the flexible nature of the
ligating properties of such ylides.
The utility of metalated phosphorus ylides in synthetic
chemistry has been well documented [1,2]. Herein we report
on the preparation and structural elucidation of the mer-
cury halide complexes of the ylide EPPY.
The juxtaposition of the keto group and carbanion in
phosphorus ylide EPPY allows for the resonance delocali-
sation of the ylidic electron density providing additional
stabilisation to the ylide species (Scheme 1). This so-called
a-stabilisation provides EPPY with the potential to act as
an ambi-dentate ligand and thus bond to an Hg(II) centre
through either the carbanion (ii) or the enolate oxygen (iii).
The enolate form (iii) may assume either a cis or trans
arrangement, the geometry of which will be retained upon
bonding to Hg(II).
2. Experimental
2.1. Sample preparation
2.1.1. EPPY [5]
The reaction of a-chloroethyl acetate with triphenyl-
phosphine at 343–348 K gave the chloride salt of EPPY
as a white crystalline solid; this was washed with diethyl
ether and dried (m.p. 363 K, yield: 81.9%). To prepare
the free ylide, a cold aqueous solution of KOH was added
*
Corresponding author.
E-mail addresses: j.a.k.howard@dur.ac.uk (J.A.K. Howard), panch_45
@yahoo.co.in (K. Panchanatheswaran).
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.11.002

1082
E.C. Spencer et al. / Journal of Organometallic Chemistry 692 (2007) 1081–1086
solutions were via Direct Methods. Refinement was on F2
+
+
(Ph)3P
(Ph)3P
(Ph)3P
using full-matrix least-squares techniques. All data were
corrected for absorption using SADABS [7]. All hydrogen
atoms are placed at calculated positions, and have been
refined with a riding model. The exception being the hydro-
gen atoms residing on the ylide carbon, these were located
in the difference Fourier map and their positional parame-
ters were refined (Uiso fixed at 120% of the parent atom).
These were then fixed for the final round of least-squares
refinement.
-
-
O
OEt
O
OEt
O
OEt
i
ii
Scheme 1. The canonical forms of EPPY.
iii
to an aqueous solution of the prepared phosphonium salt.
A thick pasty solid separated out from this solution, which
was subsequently refrigerated to give a white solid. This
solid was then re-crystallised from benzene via the vapour
diffusion method using petroleum ether as the diffusing sol-
vent. m.p. 126–127 ꢁC, yield: 89.5%.
3. Results and discussion
3.1. X-ray crystallography
Table 1 provides crystallographic results and refinement
information for all complexes. The molecular structures of
1, 2, and 3 are shown in Figs. 1–3, respectively. The dimeric
complexes crystallise in the centrosymmetric space group
2.1.2. Complexes 1, 2 and 3
A solution of the appropriate mercury (II) halide
(0.05 mmol) in methanol was added to a methanolic solu-
tion of EPPY (0.20 g, 0.05 mmol) at room temperature
and stirred overnight. The solvent was then removed in
vacuo. White crystals were obtained by crystallisation of
the crude product from chloroform using the vapour diffu-
sion method using petroleum ether as the diffusing solvent.
1: m.p. 431–433 K, yield: 87.2%. Anal. Found: C,
35.48%; H, 2.84%. C22H21O2PHgBr2 Anal. Calc.: C,
37.29%; H, 2.99%.
2: m.p. 423–425 K, yield: 92.6%. Anal. Found: C,
41.42%; H, 3.34%. C22H21O2PHgCl2 Anal. Calc.: C,
42.63%; H, 3.42%.
3: m.p. 425–427 K, yield: 80.1%. Anal. Found: C,
33.10%; H, 2.63%. C22H21O2PHgI2 Anal. Calc.: C,
32.92%; H, 2.64%.
ꢀ
P1, and straddle the crystallographic inversion centre.
Monomeric material was found to be present as a minor
component, approximately 5%, of the crystalline state of 2
(Fig. 2). The monomer is of the form [(EPPY)(HgCl)] in
which EPPY is C-coordinated to the Hg(II) centre. The
chlorine atom of this second component could not be
resolved from the disordered CHCl3 solvent molecule.
The crystallographic model for 3 also includes a disordered
component. In this case, approximately 5% of un-reacted
(HgI2)2 had crystallised with the complex. The positional
and atomic displacement parameters for the disordered
components in 2 and 3 were refined freely. The occupancy
values were fixed at 0.05 for the final round of least-squares
refinement. Further discussion of complexes 2 and 3 will be
confined to consideration of the principal components
only.
In all three complexes the mercury atom is sp3 hybri-
dised and resides in a distorted tetrahedral environment.
The inter-nuclear distances between mercury atoms of the
dimer units are greater than twice the ionic radii for 4-coor-
2.2. Instrumentation
Solution-state 1H, 31P and 13C NMR spectra at ambient
probe temperature were obtained on solutions in CD2Cl2
using a Varian Mercury-400 spectrometer operating at
˚
dinate Hg (II) (2.20 A) [8], indicating the absence of signif-
399.97 MHz for H, 161.91 MHz for 31P and 100.57 MHz
1
icant bonding interactions between the mercury atoms in
these complexes.
for 13C. Solution-state H and 31P spectra at 193 K were
1
obtained in CD2Cl2 using a Varian Inova-500 spectrometer
The dimeric structure adopted by complexes 1, 2 and 3 is
in contrast to the C-coordinated trinuclear mercury (II)
complex of the phosphorus ylide Ph3PCHCOPh [9], but
are similar to the structure of trans-Di-l-iodo- diiod-
obis(triphenylphosphoniumcyclopentadienylide)dimercury
(II) reported by Baenziger et al. [10] and the C-coordinated
dinuclear mercury (II) halide complexes of Ph3PCHCOPh
(BPPY) [11].
operating at 499.77 MHz for H, and 202.31 MHz for 31P.
1
All spectra were referenced to the relevant IUPAC stan-
dards via the frequency of the lock signal.
Solid-state 31P NMR spectra at ambient probe tempera-
ture were obtained using a Varian Inova-300 spectrometer
operating at 121.37MHz for 31P by cross-polarisation with
a contact time of 3 ms and a recycle delay of 120 s.
Tables 2–4 list key bond lengths and angles for com-
plexes 1, 2 and 3, respectively. The Hg–C, Hg–Cl bond
lengths are notably shorter in 2 in comparison with the cor-
2.3. X-ray crystallography
˚
All data were collected at 120 K using graphite mono-
chromated X-radiation (k = 0.71073 A) on a Bruker
SMART 6 K diffractometer. Data processing was per-
formed using standard Bruker software [6]. Structure
responding BPPY complex (Hg–C: 2.208(8) A, Hg–Cl(l):
˚
˚
˚
2.710(2) A, and Hg–Cl: 2.417(3) A.). A similar scenario is
found for complex 3 and the equivalent BPPY-Hg(II)
dimer, with the exception being the terminal Hg–I bond

E.C. Spencer et al. / Journal of Organometallic Chemistry 692 (2007) 1081–1086
1083
Table 1
Crystallographic data and refinement details for complexes 1–3
1
2
3
Crystal size (cm3)
Crystal colour
Crystal habit
Chemical formula
Formula weight (g)
Crystal system
Space group
0.19 · 0.17 · 0.16
Colourless
Block
0.17 · 0.15 · 0.14
Colourless
Block
C44H42Cl4Hg2O4P2(CHCl3)2
1715.15
Triclinic
0.19 · 0.17 · 0.16
Colourless
Block
C44H42I4Hg2O4P2
1603.48
a
a
C
44H42Br4Hg2O4P2(CHCl3)2
1652.24
Triclinic
Triclinic
ꢀ
ꢀ
ꢀ
P1
P1
P1
Z
1
1
1
˚
˚
˚
a (A)
b (A)
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
10.6974(4)
11.1019(4)
12.5683(4)
76.161(2)
68.698(2)
81.658(2)
1347.49(8)
9.052
11.2465(6)
11.8297(7)
12.0170(7)
82.602(2)
80.642(2)
78.368(2)
1537.5(2)
5.774
10.7970(5)
11.0485(5)
11.3649(5)
67.813(2)
81.098(2)
69.550(2)
1175.82(9)
9.253
3
˚
V (A )
l (Mo Ka) (mmꢀ1
)
Collected reflections
Unique reflections
Data completeness
No. parameters
Rint
8945
5257
99% (2hmax = 26ꢁ)
283
0.0204
8546
5361
99% (2hmax = 25ꢁ)
329
0.0478
6922
4556
99% (2hmax = 26ꢁ)
272
0.0240
R1 [I > 2r(I)]
wR2 [I > 2r(I)]
a
0.0349
0.0837
0.0538
0.1239
0.0318
0.0659
The exact formula units reported in the CIF for these structures do not have integer values for the number of atoms, this is because of the site disorder
observed in these structures.
˚
˚
(Hg–C: 2.31(1) A, Hg–I(l): 3.010(1) A, and Hg–I:
The C-coordination of EPPY is in stark contrast to the
O-coordination of the phosphorus ylide Ph3PC(COMe)
(COPh) (ABPPY) to the Hg(II) centre [12]. The difference
in coordination mode between ABPPY and EPPY to
Hg(II) can be rationalised in terms of the electronic prop-
erties and steric requirements of the ylides. The nucleophi-
licity of the carbanion in ABPPY is less than for EPPY,
this is due to the additional delocalisation of the ylide elec-
tron density in ABPPY which is facilitated by the second
carbonyl group. This will reduce the ability of ABPPY to
bind via the ylidic carbon. Belluco et al. have studied steric
˚
2.705(1) A. These disparities can be attributed to the
greater electron withdrawing properties of the EPPY
ligand with respect to BPPY.
In all three complexes the ylide ligands are C-coordi-
nated to the Hg(II) atoms in a trans arrangement within
the dimer unit. The coordination of the EPPY molecules
via the ylide carbon (C19) introduces chiral centre to the
complexes, and as a centre of inversion is intrinsic to the
molecules, the complexes are meso-isomers (achiral
diasteroisomers).
Fig. 1. The molecular structure of 1. The disordered CHCl3 solvent molecule and hydrogen atoms have been excluded for clarity. Symmetry related atoms
are shown for completeness.

1084
E.C. Spencer et al. / Journal of Organometallic Chemistry 692 (2007) 1081–1086
Fig. 2. Structure of complex 2. Hydrogen atoms and the second solvent molecule are excluded for clarity. The minor monomer component is displayed
using dashed bonds and disorder in the phenyl rings is shown using white bonds. Symmetry related atoms are shown for completeness.
Fig. 3. Structure of complex 3. Hydrogen atoms are omitted for clarity. The disordered component is represented with dashed bonds. Symmetry related
atoms are shown for completeness.
influences on the coordination modes of ylide molecules to
Pt(II) systems [13]. These authors concluded that the pre-
Table 2
ferred coordination mode is via the ylidic carbon, but that
steric hindrance around the metal centre or the ylidic car-
bon will necessitate O-coordination. Indeed, this trend is
reflected here, both BPPY and EPPY are slightly less steri-
cally demanding than ABPPY, and are both C-coordinated
to Hg(II).
Selected molecular parameters for complex 1
˚
Bond
Length (A)
Atoms
Angle (ꢁ)
P1–C1
P1–C7
1.806(6)
1.796(5)
1.798(5)
1.788(5)
1.482(7)
1.207(7)
1.344(7)
1.455(7)
1.52(1)
P1–C19–C20
C19–C20–O1
C19–C20–O2
O1–C20–O2
O2–C21–C22
P1–C19–Hg1
Hg1–C19–C20
C19–Hg1–Br1
C19–Hg1–Br2
Br2–Hg1–Br20
P1–C19–C20–O1
113.5(4)
124.5(5)
111.8(5)
123.8(5)
111.9(5)
111.2(2)
104.5(3)
117.8(1)
110(1)
P1–C13
P1–C19
C19–C20
O1–C20
O2–C20
O2–C21
C21–C22
Hg1–C19
Hg1–Br1
Hg1–Br2
The significant elongation of the P1–C19 and C19–C20
bonds in complexes 1, 2 and 3 with respect to the free ylide
˚
˚
[14] (1.706(2) A and 1.408(2) A, respectively) is consistent
with the loss of resonance delocalisation within the ylide
ligands upon complexation. The non-bonding distance
between the phosphorus and oxygen atoms of the ylide
2.224(5)
2.5784(6)
2.8088(6)
87.85(2)
35.3(7)
˚
˚
molecules are 3.048(4) A (1), 3.155(8) A (2), and

E.C. Spencer et al. / Journal of Organometallic Chemistry 692 (2007) 1081–1086
1085
Table 3
(21.0 ppm), free trans-ylide (21.2 ppm), cis-ylide in a com-
plex containing two mercury atoms (34.3 ppm) and a peak
at 26.5 ppm which we believe corresponds to both the trans
ylide in the complex containing two mercury atoms, and
trans ylide in a complex containing only one mercury atom,
a possible monomer complex. At low temperature (193 K)
the peak at 26.5 ppm exhibits satellites due to coupling to
199Hg (8% intensity each, 2JP–Hg ꢁ 210 Hz). These mercury
satellites are also apparent in the corresponding solid-state
31P spectra at ambient temperature, but cannot be resolved
in the solution-state 31P spectra at ambient temperature,
presumably because the faster exchange at ambient temper-
ature causes exchange decoupling. The presence of peaks
due to free ylide and of peaks corresponding to a complex
containing only one mercury atom in the spectrum of com-
pound 2 suggests an excess of ylide and this is supported by
the results of elemental analysis.
Selected molecular parameters for complex 2
˚
Bond
Length (A)
Atoms
Angle (ꢁ)
P1–C1
P1–C7
1.787(9)
1.79(1)
1.796(9)
1.78(1)
1.49(1)
1.22(1)
1.32(1)
1.47(1)
1.38(2)
2.159(9)
2.648(2)
2.352(3)
P1–C19–C20
C19–C20–O1
C19–C20–O2
O1–C20–O2
O2–C21–C22
P1–C19–Hg1
Hg1–C19–C20
C19–Hg1–Cl1
C19–Hg1–Cl2
Cl1–Hg1–Cl10
P1–C19–C20–O1
115.7(7)
124.6(9)
111.2(8)
124.1(9)
111(1)
111.2(5)
107.2(6)
104.4(3)
148.2(3)
90.08(7)
ꢀ44(1)
P1–C13
P1–C19
C19–C20
O1–C20
O2–C20
O2–C21
C21–C22
Hg1–C19
Hg1–Cl1
Hg1–Cl2
P1–C19–Hg2
C19–Hg2–Cl8
C20–C19–Hg2
113.5(5)
130.8(4)
105.5(7)
Hg2–C19
Hg2–Cl8
2.25(1)
2.569(9)
Comparison of the above with solution-state 31P spectra
of the three complexes at ambient temperature (Fig. S2)
and solid-state 31P spectra of the three complexes at ambi-
ent temperature (Fig. S3) shows the same basic pattern, but
indicates that the amount of excess ylide present decreases
in the order 2 > 1 > 3. This is again in agreement with the
results from elemental analysis. The apparent rates of
exchange between the various species also changes across
the series. In particular, the relatively sharp peak due to
cis-ylide in a complex containing two mercury atoms at
33.6 ppm in the ambient temperature spectrum of com-
pound 2 shows considerable broadening and shifts to lower
frequency (32.4 ppm) in the spectrum of 1, and is absent
from the spectrum of compound 3 but an additional sharp
peak is apparent in the region of the main peak (25.4 ppm)
possibly the result of complete exchange averaging between
free cis-ylide and cis-ylide in a complex containing two
mercury atoms. However, it must be remembered that
the observed effects may also depend on the relative pro-
portions of the components so it is not possible to make
firm conclusions about relative rates of exchange from
the limited data available.
The dynamic nature of the mixture was further con-
firmed in a solution-state 31P NMR experiment where a
small amount of free ylide was added to a sample of 2
(CD2Cl2, ambient temperature) in an attempt to confirm
the assignment for free trans-ylide. Instead of an increase
in the signal assigned to free trans-ylide, an increase in
the relative intensity of the peak around 27 ppm was
observed, together with a broadening of the signal assigned
to cis-ylide in complex containing two mercury atoms. Our
interpretation of this observation is that the presence of
additional free ylide shifts the equilibrium in favour of
the formation of complexes containing only one mercury
atom.
Table 4
Selected molecular parameters for complex 3
˚
Bond
Length (A)
Atoms
Angle (ꢁ)
P1–C1
P1–C7
1.805(7)
1.811(6)
1.799(6)
1.786(6)
1.472(9)
1.216(8)
1.348(7)
1.460(8)
1.45(1)
P1–C19–C20
C19–C20–O1
C19–C20–O2
O1–C20–O2
O2–C21–C22
P1–C19–Hg1
Hg1–C19–C20
C19–Hg1–I1
C19–Hg1–I2
I1–Hg1–I10
114.0(5)
125.0(6)
111.4(6)
123.6(6)
114.1(7)
111.3(3)
102.3(4)
128.8(2)
110.8(2)
90.46(1)
ꢀ24.0(8)
P1–C13
P1–C19
C19–C20
O1–C20
O2–C20
O2–C21
C21–C22
Hg1–C19
Hg1–I1
Hg1–I2
2.275(7)
2.8030(5)
2.7349(5)
P1–C19–C20–O1
Hg2–I1
Hg2–I3
2.820(7)
2.70(1)
I1–Hg2–I3
I1–Hg2–I10
110.3(3)
91.2(2)
˚
3.013(5) A (3). These values are considerably less than the
sum of the van der Waals radii for these atoms (3.3 A)
[15] and imply the presence of significant interactions
between these two atoms in all three complexes. This type
of short contact is a well-known feature of ylide chemistry
[16,17].
˚
3.2. Spectroscopy
Solution-state spectra of the free ylide were obtained for
reference purposes. The 31P spectrum of the free ylide indi-
cated that the trans-ylide was the major species [18].
1H, 31P, and 13C NMR spectra of the three complexes
(provided as electronic supplementary information) were
consistent with the determined crystal structure being the
major component present in each sample, but also indi-
cated that the major component was present as part of a
dynamic mixture. This can be most easily seen in the 31P
spectrum of compound 2 at 193 K (Fig. S1), where
exchange between the species present has been substan-
tially slowed. In this spectrum it is possible to distinguish
peaks which we believe correspond to free cis-ylide
Acknowledgements
E.C.S. thanks the EPSRC for funding and J.A.K.H.
thanks the EPSRC for a Senior Research Fellowship.

1086
E.C. Spencer et al. / Journal of Organometallic Chemistry 692 (2007) 1081–1086
[4] R. Navarro, E.P. Urriolabeitia, Dalton Trans. (1999) 4111–4122.
[5] A. Michaelis, H.V. Gimborn, Ber. Deut. Chem. Ges. 27 (1894) 272.
[6] Bruker, SMART-NT Version 5.0 (Data Collection), SAINT Version 6.04
(Data Processing), and SHELXTL Version 6.1 (Structure Solution,
Refinement and Graphics), Bruker AXS Inc., Madison, WI, USA,
1998.
E.C.S. thanks Drs. D. Yufit and A.S. Batsanov for assis-
tance and advice. We also thank D.C. Apperley of the
EPSRC Solid-State NMR Service for providing the solid-
state NMR spectra. K.P. thanks the Department of Science
and Technology, New Delhi, India for financial assistance
(SERC-SR/S1/IC-29/2003).
[7] A.L. Spek, J. Appl. Cryst. 36 (2003) 7–13.
[8] R.D. Shannon, Acta Crystallogr., Sect. A 32 (1976) 751–767.
[9] B. Kalyanasundari, K. Panchanatheswaran, V. Parthasarathi, W.T.
Robinson, Bull. Chem. Soc. Jpn. 72 (1999) 33–36.
Appendix A. Supplementary material
[10] N.C. Baenziger, R.M. Flynn, D.C. Swenson, Acta Crystallogr., Sect.
B 34 (1978) 2300–2301.
[11] M. Kalyanasundari, K. Panchanatheswaran, W.T. Robinson, H.
Wen, J. Organomet. Chem. 491 (1995) 103–109.
[12] P. Laavanya, U. Venkatasubramanian, K. Panchanatheswaran, J.A.
Krause Bauer, Chem. Commun. (2001) 1660–1661.
[13] U. Belluco, R.A. Michelin, R. Bertani, G. Facchin, G. Pace, L.
Zanotto, M. Mozzon, M. Furlan, E. Zangrando, Inorg. Chim. Acta
(1996) 355–366.
[14] E.C. Spencer, B. Kalyanasundari, M. Baby Mariyatra, J.A.K.
Howard, K. Panchanatheswaran, Inorg. Chim. Acta (2006) 35–43.
[15] J.D. Dunitz, X-ray Analysis and the Structure of Organic Molecules,
Cornell University Press, Ithaca, 1979, p. 339.
CCDC 263302, 263303 and 263304 contain the supple-
mentary crystallographic data for 1, 2 and 3. These data can
be obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ,UK;fax:(+44)1223-336-033;ore-mail:deposit@ccdc.-
cam.ac.uk. CIFs for complexes 1, 2 and 3 are deposited with
the Cambridge Structural Database. Supplementary data
associatedwiththisarticlecanbefound,intheonlineversion,
at doi:10.1016/j.jorganchem.2006.11.002.
´
´
[16] A. Lledos, J.J. Carbo, E.P. Urriolabeitia, Inorg. Chem. 40 (2001)
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