Methylmercury(II) and phenylmercury(II) chelated complexes with the ligand tris(2-diphenylphosphinoethyl)phosphine: Synthesis, X-ray diffraction and NMR studies

Article abstract of DOI:10.1016/S0022-328X(98)00984-X

The reaction of methyl and phenylmercury ions with the ligand tris(2-diphenylphosphinoethyl)phosphine, pp₃, allowed the isolation of the complexes [(pp₃)HgR]BF₄ (R=Me, Ph). The X-ray crystal structure of the methyl derivat

Full text of DOI:10.1016/S0022-328X(98)00984-X

Journal of Organometallic Chemistry 575 (1999) 119–125
Methylmercury(II) and phenylmercury(II) chelated complexes with the
ligand tris(2-diphenylphosphinoethyl)phosphine: synthesis, X-ray
diffraction and NMR studies
Franco Cecconi, Carlo A. Ghilardi *, Andrea Ienco, Stefano Midollini, Annabella Orlandini
Istituto per lo Studio della Stereochimica ed Energetica dei Composti di Coordinazione, CNR, Via Jacopo Nardi 39, 50132 Firenze, Italy
Received 13 July 1998
Abstract
The reaction of methyl and phenylmercury ions with the ligand tris(2-diphenylphosphinoethyl)phosphine, pp3, allowed the
isolation of the complexes [(pp3)HgR]BF4 (R=Me, Ph). The X-ray crystal structure of the methyl derivative established that in
the cation the metal atom is coordinated to the central phosphorus atom of the tripod pp3 ligand and to the methyl group. Two
terminal phosphorus atoms of the ligand are additionally linked to the metal throught secondary bonding interactions. 31P- and
199Hg-NMR spectra indicated that the coordination of terminal phosphorus atoms of pp3 to the mercury atom occurs also in
solution. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Organomercurials; Phosphine complexes; Crystal structure; NMR spectroscopy
1. Introduction
chelate RHg(II) ions to give tetrahedral complexes both
in solid state and in solution [5].
Methylmercury(II) and related organomercury(II)
ions have been found preferentially to display linear
dicoordination [1]. Polydentate nitrogen donor ligands
have often been reported to induce higher coordination
numbers in the solid state, although the geometries at
mercury are with only one strong Hg–N interaction [2].
Therefore, there is the convinction that it is not possible
to design suitable sequestering agents for methylmer-
cury(II) because of the impossibility to include
methylmercury into chelated rings [3].
We now report the preparation and the characteriza-
tion of the complexes [(pp3)HgR]BF4 (R=Me, Ph),
where pp3 is the related ligand tris(2-diphenylphosphi-
noethyl)phosphine. The tetradentate phosphine pp3 has
been reported previously to occupy four coordination
sites with various geometries such as the trigonal pyra-
midal [6], trigonal bipyramidal [7], square pyramidal [8]
or octahedral [9]. Sometimes pp3 can also act as a
bidentate or tridentate ligand toward one or two metal
centers [10,11].
However, in dealing with the interaction of MeHg(II)
with biomolecules it has been pointed out recently that
a coordination mode in which mercury has a coordina-
tion number higher than two is of great biological
interest [4]. Thus, we have found that the tripod ligand
tris(2-diphenylphosphinoethyl)amine (np3) can really
2. Experimental
All the solvents and chemicals were reagent grade
and were used as received by commercial suppliers. The
organomercurials MeHgI and HgPh2 were purchased
from Strem and used without further purification. Pro-
ton NMR spectra were recorded at 200.133 MHz, on a
* Corresonding author. Tel.: +39-55-243990; fax: +39-55-
2478366; e-mail: ghilardi@cacdo.issecc.fi.cnr.it.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(98)00984-X

120
F. Cecconi et al. / Journal of Organometallic Chemistry 575 (1999) 119–125
Bruker AC-200 spectrometer. Peak positions are rela-
tive to TMS as external reference. 31P{1H}- and
199Hg{1H}-NMR spectra were recorded on the same
instrument operating respectively at 81.015 and 35.85
MHz. Chemical shifts are relative to external 85%
H3PO4 and external 0.1 mol dm−3 Hg(ClO4)2 in 0.1
mol dm−3 HClO4 respectively, with downfield values
reported as positive.
the filtrate; evaporation of the solvent in a current of
nitrogen afforded colorless crystals. These were filtered
and washed with methanol (yield 797 mg, 77%). Anal.
calc. for C48H47BF4HgP4: C, 55.69; H, 4.58; Hg, 19.38.
Found: C, 55.50; H, 4.70; Hg, 19.25. Selected NMR
data (CD2Cl2, solution 0.175 M): 31P{1H}-NMR (223
K) l 22.76 ppm (q with satellites, 2JPP=115 Hz,
1JHgPap=2255 Hz, Pap), l −5.66 ppm (d with satellites,
1JHgPter=597 Hz, Pter); 199Hg {1H)-NMR (223 K) l
Caution! Organomercurials are extremely toxic, and
all experimentation involving these reagents should be
carried out in a well vented fume hood.
1
1
2004 ppm (dq, JHgPap=2258 Hz, JHgPter=598 Hz).
2.2. X-ray crystallography
2.1. Syntheses
Diffraction data of 1 were collected at room temper-
ature on an Enraf Nonius CAD4 automatic diffrac-
tometer. Unit cell parameters were determined from a
least-squares refinement of the setting angles of 25
carefully centred reflections. Crystal data and data col-
lection details are given in Table 1. During the data
collection the stability of the crystal was checked by
periodically measuring three standard reflections. After
correction for background the intensities I were as-
signed standard deviations |(I) calculated using the
value of 0.03 for the instability factor k [12]. The
intensities were corrected for Lorentz and polarization
effects and for absorption by means of „ scans [13]. All
the calculations were performed on a personal com-
All reactions and manipulations were routinely per-
formed under a nitrogen atmosphere, unless otherwise
noted. The solid compounds were collected on a sin-
tered-glass frit and dried under a stream of nitrogen.
2.1.1. [(pp3)HgMe]BF4, 1
Solid AgBF4 (195 mg, 1 mmol) was added to a
solution of MeHgI (342 mg, 1 mmol) in methanol (20
ml). The mixture was stirred in the dark for 3 h, then
AgI was filtered off. The solvent was removed from the
filtrate under a vacuum, at room temperature, to leave
a
colorless material, which was dissolved in
dichloromethane (25 ml). The solution was filtered and
solid pp3 (670 mg, 1 mmol) was added. Methanol (25
ml) was added to the solution and the solvent was
evaporated under a stream of nitrogen till the volume
was reduced to ca. 15 ml. Then diethyl ether (20 ml)
and n-hexane (40 ml) were added in turn, and the
resultant solution was held in the refrigerator at 253 K
for 5 h. Well shaped colorless crystals precipitated.
These were filtered and washed with diethyl ether (yield
818 mg, 84%). Anal. calc. for C43H45BF4HgP4: C,
53.07; H, 4.66; Hg, 20.61. Found: C, 53.25; H, 4.75;
Hg, 20.50. Selected NMR data (CD2Cl2, solution 0.18
Table 1
Crystal data and structure refinement for 1
Empirical formula
Formula weight
Temperature (K)
C43H45BF4HgP4
973.07
293(2)
˚
Wavelength (A)
0.71070
Space group
P21/n
Unit cell dimensions
˚
a (A)
9.498(7)
25.117(8)
17.890(7)
˚
b (A)
˚
c (A)
1
M): H-NMR (293 K) l 1.29 ppm (s with satellites,
i (°)
V (A )
95.94(6)
4245(2)
2JHgH=180 Hz, CH3Hg); 31P{1H}-NMR (223 K) l
3
˚
2
1
31.17 ppm (q with satellites, JPP=105 Hz, JHgPap
=
Z
4
Densitycalc. (Mg m−3
)
1.523
3.823
1936
1740 Hz, Pap), l −7.98 ppm (d with satellites,
Absorption coefficient (mm−1
F(000)
)
1JHgPter=583 Hz, Pter); 199Hg{1H)-NMR (223 K) l
1
1
2192 ppm (dq, JHgPap=1745 Hz, JHgPter=584 Hz).
Crystal size
Theta range for data collection (°)
Index ranges
0.12×0.15×0.45
2.5–20.0
−95h59, 05k524,
05l517
2.1.2. [(pp3)HgPh]BF4, 2
A solution of HgPh2 (335 mg, 1 mmol) in THF (20
ml) was added to a solution of HgI2 (454 mg, 1 mmol)
in acetone (15 ml). After the solution became colorless
solid AgBF4 (195 mg, 1 mmol) was added. The mixture
was stirred in the dark for 3 h, then AgI was filtered
off. The solvents were removed from the filtrate under
a vacuum, at room temperature, to leave a colorless
material, which was dissolved in dichloromethane (25
ml). Solid pp3 (670 mg, 1 mmol) was added and the
solution was filtered. Methanol (25 ml) was added to
Reflections collected
Independent reflections
Refinement method
3949
3949 [Rint=0.0000]
Full-matrix least-squares on
F2
Data/restraints/parameters
Goodness-of-fit on F2
3949/0/189
1.054
Final R indices [I\2|(I)]
R indices (all data)
R1=0.0427, wR2=0.1005
R1=0.0649, wR2=0.1075
0.649 and −0.455
Largest difference peak and hole
−3
˚
(e A
)

F. Cecconi et al. / Journal of Organometallic Chemistry 575 (1999) 119–125
121
Fig. 1. Perspective view of the complex cation [(pp3)HgMe)]+
. ZORTEP drawing with 30% probability ellipsoids.
˚
puter, using the progams SHELXS86 [14], SHELXL93 [15],
and ZORTEP [16]. Atomic scattering factors were taken
from Ref. [17] and an anomalous dispersion correction,
real and imaginary part, was applied [18]. Patterson
and Fourier maps enabled the location of all the atoms.
Full matrix least-squares refinements on F2 were carried
out with anisotropic thermal parameters assigned to
mercury, phosphorus and fluorine atoms. Hydrogen
atoms were introduced in their calculated positions
riding on their carbon atoms, with thermal parameters
20% larger than those of the respective carbon atoms.
The function minimized during the refinement was
S w(F2o−F2c)2, with w=1/[|2(Fo)2+(0.0443P)2+
18.75P], where P=(max(F2o, 0)+2Fc2)/3.
C(1) 2.087(11) A, the metal is linked to other two
phosphorus donors of the tripod ligand which display
˚
bond distances of 2.879(3) and 3.078(3) A. The fourth
phosphorus atom of the pp3 ligand is definitively not
coordinated to the metal the Hg···P(4) contact being
˚
5.04 A. Therefore, the actual coordination geometry is
somewhat in between the trigonal bipyramidal geome-
try (all the three terminal phosphorus coordinated in
the equatorial plane) and the limit linear geometry
when even P(2) and P(3) are not coordinated to the
metal atom. Indeed the metal atom displays a 2+2
coordination mode which is rather frequent for the
mercury [1]. Bond distances in the Pap–Hg–C fragment
appear normal for covalent bonds and in particular
match for example those reported for [(dpb)Hg(Me)]2
(dpb=diphenylphosphinobenzoate) [19] and for
[(PPh3)HgPh]+ [20]. As concerns the two secondary
bonds, if the sum of the van der Waals radii [21] of
3. Discussion
˚
mercury and phosphorus is taken to be ca. 3.4 A, the
Hg–P distances of 2.879(3) and 3.078(3) A may be
3.1. X-ray structure
˚
considered as weak bond interactions.
The molecular structure of 1 consists of discrete
[(pp3)HgMe]+ cations and BF4− anions. Fig. 1 shows a
perspective view of the cation and Table 2 reports
selected bond distances and angles. In the cation the
metal atom is linearly coordinated (P(1)–Hg–C(1)=
167.5(3)°) to the apical phosphorus atom of the tripod
pp3 ligand and to the methyl group. In addition to the
two strong covalent bonds, Hg–P(1) 2.446(3) and Hg–
The present compound can be compared with the
related reported np3 derivative, where the mercury
atom displays a tetrahedral coordination with the api-
cal nitrogen atom definitively away from the metal
˚
(Hg···N=3.50 A), and the three phosphorus atoms
covalently bonded in the equatorial plane [5]. In the pp3
case the apical phosphorus binds the metal centre,

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F. Cecconi et al. / Journal of Organometallic Chemistry 575 (1999) 119–125
Table 2
˚
Selected bond lengths (A) and angles (°) for 1
˚
Bond lengths (A)
Hg(1)–C(1)
Hg(1)–P(1)
Hg(1)–P(2)
Hg(1)–P(3)
P(1)–C(4)
P(1)–C(2)
P(1)–C(6)
P(2)–C(1,2)
P(2)–C(3)
P(2)–C(1,1)
2.087(11)
2.446(3)
2.879(3)
3.078(3)
1.812(10)
1.826(10)
1.843(10)
1.830(6)
1.834(10)
1.840(6)
P(3)–C(1,3)
P(3)–C(1,4)
P(3)–C(7)
P(4)–C(1,5)
P(4)–C(1,6)
P(4)–C(5)
C(2)–C(3)
C(4)–C(5)
C(6)–C(7)
1.824(5)
1.825(6)
1.838(10)
1.838(8)
1.851(6)
1.859(11)
1.530(13)
1.523(13)
1.513(13)
Bond angles (°)
C(1)–Hg(1)–P(1)
C(1)–Hg(1)–P(2)
P(1)–Hg(1)–P(2)
C(1)–Hg(1)–P(3)
P(1)–Hg(1)–P(3)
P(2)–Hg(1)–P(3)
C(4)–P(1)–C(2)
167.5(3)
109.4(3)
80.66(9)
111.0(3)
74.04(9)
97.46(8)
105.2(5)
107.1(5)
104.3(5)
116.6(3)
108.1(3)
114.5(3)
106.8(4)
105.4(3)
103.6(4)
122.0(3)
95.6(3)
C(7)–P(3)–Hg(1)
C(1,5)–P(4)–C(1,6)
C(1,5)–P(4)–C(5)
C(1,6)–P(4)–C(5)
C(3)–C(2)–P(1)
C(2)–C(3)–P(2)
C(5)–C(4)–P(1)
C(4)–C(5)–P(4)
C(7)–C(6)–P(1)
102.0(3)
101.7(4)
101.6(5)
99.3(5)
113.4(7)
111.6(7)
118.3(7)
114.2(7)
115.3(7)
114.4(7)
117.4(4)
122.5(4)
124.4(4)
115.6(4)
115.9(4)
124.1(4)
115.3(4)
124.7(4)
116.3(6)
123.7(6)
116.9(4)
122.9(4)
C(4)–P(1)–C(6)
C(2)–P(1)–C(6)
C(4)–P(1)–Hg(1)
C(2)–P(1)–Hg(1)
C(6)–P(1)–Hg(1)
C(1,2)–P(2)–C(3)
C(1,2)–P(2)–C(1,1)
C(3)–P(2)–C(1,1)
C(1,2)–P(2)–Hg(1)
C(3)–P(2)–Hg(1)
C(1,1)–P(2)–Hg(1)
C(1,3)–P(3)–C(1,4)
C(1,3)–P(3)–C(7)
C(1,4)–P(3)–C(7)
C(1,3)–P(3)–Hg(1)
C(1,4)—P(3)–Hg(1)
C(6)–C(7)–P(3)
C(6,1)–C(1,1)–P(2)
C(2,1)–C(1,1)–P(2)
C(6,2)–C(1,2)–P(2)
C(2,2)–C(1,2)–P(2)
C(6,3)–C(1,3)–P(3)
C(2,3)–C(1,3)–P(3)
C(6,4)–C(1,4)–P(3)
C(2,4)–C(1,4)–P(3)
C(6,5)–C(1,5)–P(4)
C(2,5)–C(1,5)–P(4)
C(6,6)–C(1,6)–P(4)
C(2,6)–C(1,6)–P(4)
120.4(2)
104.7(3)
100.7(4)
104.4(4)
142.0(2)
98.7(2)
competing with the strong s-donor CH3 group in utiliz-
ing the metal orbitals suitable for axial linkages (i.e. s
and pz). It follows that, since the Hg(II) d orbitals are
all full, only two orbitals (px and py) remain available
for the linkages of the equatorial phosphines. The
different role of the phosphine with respect to the
amine axial donor (i.e. the better s-donor capabilities
of the phosphine) has been again enlighted, in agree-
ment with what has been discussed already for the
series [LHgX]+, where L=N(CH2CH2PPh3)3 X=Me,
p-C6H5S−, halide, CF3SO3−; L=N(CH2CH2NMe2)3,
X=Me, Ph; L=N(CH2CH2SCHMe2)3, X=Cl [22].
solution. Indeed the signals are broad and no metal/
phosphine coupling is observed (Fig. 2). However the
fact that the 31P resonances are shifted to higher fre-
quences and the phosphorus to phosphorus coupling
constant is remarkably higher than in the free pp3
ligand is indicative of some metal/phosphine interac-
tion. This behaviour is consistent with a rapid phos-
phine dissociation as confirmed by the spectra of the
complexes solution containing free pp3 ligand. In fact
these consist of two broad signals the chemical shifts of
which are the weighted averages of those of the corre-
sponding resonances of the complexes and the free pp3.
The variable temperature 31P-NMR spectra of com-
plex 2 are reported in Fig. 3 (the pattern of the corre-
sponding spectra of 1 is quite similar). When the
temperature of the solution decreases both the signals
sharpen and the 199Hg satellites loom out of the base-
line. The slow exchange limit of the two systems is
reached below 230 K in the present concentration con-
ditions, ca. 0.2 M. It is worthwhile to note that a
concentration decrease lowers the slow exchange limit.
The low temperature 199Hg spectra of 1 and 2 (Fig. 4)
3.2. NMR spectra
The 31P- and 199Hg-NMR spectra of the two com-
plexes in CD2Cl2 solution are quite similar indicating
analogous coordination geometries. The room tempera-
ture 31P-NMR spectra of 1 and 2 consist of a down-
field quartet and a high-field doublet, due respectively,
to the apical and the terminal phosphorus atoms. These
spectra show that the two compounds are labile in

F. Cecconi et al. / Journal of Organometallic Chemistry 575 (1999) 119–125
123
Fig. 2. 31P{1H}-NMR spectra (CD2Cl2, 298 K) of 1, 2 and pp3.
are in perfect agreement with the 31P spectra. Then, at
low temperature, all the phosphorus atoms of the lig-
and pp3 appear to be coordinated to the metal, the
a coordination number of the metal higher in solution
than in the solid state. We propose that the complexes
in solution retain the coordination geometry found in
the solid state, the two weakly coordinated terminal
phosphorus atoms rapidly scrambling with the dan-
gling one, also at low temperature. The maintenance
of the 199Hg–31P spin correlation during such a pro-
cess, could likely occur if the time of the Hg–P bond
breaking and forming is short relative to the observed
nucleus relaxation time [24]. In this case the observed
2
terminal ones being magnetically equivalent. The JPP
coupling constants remain practically unchanged in
the temperature range investigated. The values of the
1JHgP coupling constants, which are in the range re-
ported previously for phosphine organo–mercury(II)
complexes [23], increase when the temperature de-
1
creases (1: JHgPap is 1740 Hz at 223 K vs. 1777 at 213
1
1
1
K, JHgPter is 583 at 223 K vs. 591 at 213 K; 2: JHgPap
value of the JHgPter should be an average of the mer-
1
is 2255 at 223 K vs. 2334 at 203 K, JHgPter is 597 at
cury–phosphorus coupling of the two coordinated
atoms and the uncoordinated (J=0) one. This has to
be expected especially in complexes of chelating lig-
ands and has been observed already [24,25].
The results of this work confirm that polydentate
ligands, containing soft donor atoms, engaged in a
suitable skeleton, can chelate the mercury atom of
organomercury(II) ions also in solution. This agrees
with the recent hypothesis that in certain enzymatic
active sites (i.e. organomercurial lyase) the mercury of
the HgMe+ ion has a coordination number higher
than one [4].
223 K vs. 621 at 203 K), and also when the concen-
tration increases, as found previously for the related
1
[(np3)HgMe]CF3SO3 complex [5]. The JHgPap is re-
1
markably higher than the JHgPter suggesting that the
Hg–Pap bond is stronger than the Hg–Pter ones, in
agreement with X-ray structural findings. However a
five-coordinated geometry in solution does not agree
with the results of the solid state structure. Even if the
structure of a complex can be different in the solid
state and in solution, in the case of an organomer-
cury(II) derivative it appears at least surprising to find

124
F. Cecconi et al. / Journal of Organometallic Chemistry 575 (1999) 119–125
Fig. 3. 31P{1H} variable temperature NMR spectra (CD2Cl2) of 2.
4. Supplementary material
Acknowledgements
Supplementary materials consist of coordinates, ther-
mal parameters, complete bond distances and angles
and Fo/Fc listings.
We gratefully acknowledge the financial support of
EU for the HCM program ‘Metals and Environmental
Problems’ (1995–97), through Grant ERBCHRX-
CT94-0632 and of the Ministero dell’Ambiente of Italy
for the contract (PR1/C).
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