Subtlety in the reactivity of a diketo phosphorus ylide towards mercuric halides: The unprecedented O-coordination of α-acetyl-α-benzoylmethylenetriphenylphosphorane to Hg(II)

Article abstract of DOI:10.1039/b104082k

The reactions of the title ylide with HgX₂ (X = Cl, Br or I) lead to the regiospecific binding of the acetyl oxygen to soft Hg(II), producing a chloro complex with (2 + 2) coordination and isostructural dimeric bromo and iodo complexes containing halogen bridges with tetrahedral configurations around the metal centres.

Full text of DOI:10.1039/b104082k

Subtlety in the reactivity of a diketo phosphorus ylide towards
mercuric halides: the unprecedented O-coordination of
a-acetyl-a-benzoylmethylenetriphenylphosphorane to Hg(II)†
Parthasarathi Laavanya,a Ulaganathan Venkatasubramanian,a Krishnaswamy
Panchanatheswaran*a and Jeanette A. Krause Bauerb
a Department of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, India.
E-mail: pan@bdu.ernet.in
b Department of Chemistry, University of Cincinnati, OH 45221-0172, USA.
E-mail: Jeanette.Krause@UC.edu
Received (in Cambridge, UK) 9th May 2001, Accepted 20th July 2001
First published as an Advance Article on the web 6th August 2001
The reactions of the title ylide with HgX2 (X = Cl, Br or I)
lead to the regiospecific binding of the acetyl oxygen to soft
Hg(II), producing a chloro complex with (2 + 2) coordination
and isostructural dimeric bromo and iodo complexes con-
taining halogen bridges with tetrahedral configurations
around the metal centres.
undertaken. The solid state structure of 1 shows that it adopts a
square planar geometry with two collinear strong covalent Hg–
Cl bonds referred to as ‘characteristic coordination.’ The
preference for a characteristic coordination number of two for
mercury in its complexes with electronegative ligands, as
observed in 1 has been attributed to relativistic effects.6 The
Hg–O bond lengths are distinctly longer in the chloro complex
and denoted7 as ‘secondary bonds.’ The importance of such
inter-species interactions has been realized in solid state
architecture, molecular recognition, prototype ‘ionic liquids’
and biological chemistry.8 Semiempirical calculations at the
PM3 level9 corroborate the geometry and bond parameters of 1,
discerned from X-ray crystallography. In particular, the calcu-
lated values, 173.9 and 179.2°, for O–Hg–O and Cl–Hg–Cl
angles, respectively, are comparable to the experimental values
(Fig. 1). Mulliken population analysis10 on the title ylide shows
that the oxygen of the COMe group possesses a higher negative
charge (20.47) than the oxygen of the COPh group (20.38).
The linear disposition of secondary bonds in 1 is traceable to the
electrostatic interactions caused by the bulky ylide ligands
precluding any weak covalent interactions involving p orbitals.
The latter type of interactions is indicated by B3LYP11/
LANL2DZ12 calculations for the identical Hg–O bonds in
The ambidentate resonance stabilized ylide, Ph3PCHCOPh
coordinates through carbon and oxygen to Hg(II)
1,2 and U(VI),3
respectively. The diketo ylide, a-acetyl-a-benzoylmethylene-
triphenylphosphorane (ABPPY) has an additional oxygen site
for coordination and can utilize different bonding modes. The
symmetrical bidentate O-coordination of the carbonyl groups
had been recently observed by us in its reaction with uranyl
nitrate.3 We present herein the regiospecific coordination of the
diketo ylide to the soft metal center, Hg(II) via the acetyl
oxygen. This paper represents the first report of O-coordination
of any keto phosphorus ylide to Hg(II).
The parent ylide was prepared by the action of acetic
anhydride on Ph3PCHCOPh and spectrally characterized as
published.4 The action of HgX2 on an equimolar methanolic
solution of the ylide afforded the complexes HgCl2(ABPPY)2
(1), [HgBr2(ABPPY)]2 (2) and [HgI2(ABPPY)]2 (3) which were
formed as crystals, on cooling the concentrated solution.
Elemental analyses of the products revealed their stoichiometry.
In the FTIR spectra of the products, the nCO for the COMe group
observed at 1537 cm21 for ABPPY is shifted to 1526, 1573 and
1575 cm21 in 1, 2 and 3, respectively. The corresponding
frequencies for the COPh group occur at 1566, 1568, 1600 and
1596 cm21 in the above molecules. It is significant that the nCO
values are very similar in 2 and 3 as expected for the identical
environment of their two carbonyl groups. In the 13C{1H} NMR
spectra of the products in CDCl3, the resonance due to the ylidic
carbon at d 86.35 as well as the 1JPC of 101.8 Hz observed for
13
HgCl2(HCHO)2 and likely to be present in the crystal
structure of HgCl2(chd) (chd = cyclohexane-1,4-dione) com-
plex14 with an observed O–Hg–O angle of 86°. The Hg(II) in
each of the molecules 2 and 3 has a tetrahedral coordination
environment with two unsymmetrically bridging Hg–X bonds.
The oxygen of the acetyl group is oriented cis to the phosphorus
the free ylide is not very much shifted in the products. The 31
P
NMR spectra contain a single signal around d 17 for the ylide
and each of the complexes. This suggests the presence of a
single isomer in all the complexes, with the oxygen being
bonded to the metal. In contrast, the C-coordination which
implies a change in the hybridization for the ylidic carbon is
characterized by its upfield 13C chemical shifts5 and also by the
downfield shifts of 31P NMR signals.1 That the bonding of the
ylide to Hg(II) in the chloro complex is much weaker than in the
1
bromo and the iodo complexes is indicative in the H NMR
spectra in which the methyl group resonances appear at d 1.80,
1.81, 1.70 and 1.71 for the free ylide and complexes 1, 2 and 3,
respectively.
In order to establish the region and mode of coordination,
single crystal X-ray analysis‡ of the complexes has been
Fig. 1 ZORTEP view of 1 with 50% probability thermal ellipsoids and
selected atom labelling scheme. The hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angles (°): Hg1–O1 2.707(2), Hg1–O3
2.735(2), Hg1–Cl1 2.283(4), Hg1–Cl2 2.297(3); O1–C2 1.275(11), O2–C3
1.227(11), O3–C6 1.249(12), O4–C7 1.247(11), P1–C1 1.788(8), P2–C5
1.731(10), Cl1–Hg1–Cl2 179.8(2), O1–Hg1–O3 179.0(2), Hg1–O1–C2
118.4(7), Hg1–O3–C6 123.5(7).
† Electronic supplementary information (ESI) available: analytical and
spectroscopic data for 1–3. See http://www.rsc.org/suppdata/cc/b1/
b104082k/
1660
Chem. Commun., 2001, 1660–1661
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b104082k

Notes and references
‡ Crystal data: for 1: C56H46Cl2HgO4P2, a = 13.4399(2), b = 10.0105(3),
c = 17.9576(4) Å, b = 100.525(2)°, V = 2375.37(10) Å3, space group P21,
Z = 2, Dc = 1.561 Mg m23, m(Mo-Ka) = 3.467 mm21, reflections
collected/unique 25177/11404, refinement method: full-matrix least-
squares on F2; data/restraints/parameters 11403/1/254, goodness-of-fit on
F2 1.025. Final R indices [I > 2s(I)] R1 = 0.0267, wR2 = 0.0587; R indices
(all data) R1 = 0.0430, wR2 = 0.0645.
For 2: C28H23Br2HgO2P, a
= 9.5287(6), b = 11.6425(8), c =
12.3992(8) Å, a = 94.835(2)°, b = 98.281(2), g = 102.172(2)°, V =
¯
1321.22(15) Å3, space group P1, Z = 2, Dc = 1.968 Mg m23, m(Mo-Ka)
=
8.933 mm21, reflections collected/unique 13906/6329, refinement
method: full-matrix least-squares on F2, data/restraints/parameters
6329/0/142, goodness-of-fit on F2 1.041. Final R indices [I > 2s(I)] R1
=
0.0310, wR2 = 0.0743; R indices (all data) R1 = 0.0371, wR2 = 0.0766.
For 3: C28H23I2HgO2P, a = 8.9528(2), b = 11.7157(3), c = 13.8223(2)
Å, a = 107.6152(12), b = 91.6914(4), g = 99.4180(4)°, V = 1358.37(5)
¯
Å3, space group P1, Z = 2, Dc = 2.144 Mg m23, m(Mo-Ka) = 8.020
mm21, reflections collected/unique 14475/6554, refinement method: full-
matrix least-squares on F2, data/restraints/parameters 6554/0/142, good-
ness-of-fit on F2 1.019. Final R indices [I > 2s(I)] R1 = 0.0313, wR2
0.0637; R indices (all data) R1 = 0.0401, wR2 = 0.0669.
=
The intensity data, collected at 150 K on a standard Siemens SMART 1K
CCD diffractometer were corrected for decay, Lorentz and polarization
effects. Non-hydrogen atoms anisotropic; hydrogen atoms in idealized
positions. Programs used: SAINT16 (X-ray data processing), SADABS17
(absorption correction), MOPAC6.018 (semiempirical PM3 calculations)
SIR-97, (structure solution) SHELX-97 (structure refinement), PARST96
(geometrical calculations) and ZORTEP (molecular Graphics).
CCDC reference numbers 167767–167769. See http://www.rsc.org/
suppdata/cc/b1/b104082k/ for crystallographic data in CIF or other
electronic format.
Fig. 2 ZORTEP view of 2 and 3 with 50% probability thermal ellipsoids and
selected atom labelling scheme. The hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angles (°): for 2: Hg1–O1 2.397(3), Hg1–Br1
2.4693(5), Hg1–Br2 2.4420(5), Hg1–Br1A 2.9997(5), O1–C2 1.270(5),
O2–C3 1.239(5), P1–C1 1.770(4); O1–Hg1–Br1 100.86(7), O1–Hg1–Br2
105.87(7); O1–Hg1–Br1A 87.67(7), Br1–Hg1–Br1A 90.76(2), Hg1–O1–
C2 128.9(3). For 3: Hg1–O1 2.370(3), Hg1–I1 3.120(1), Hg1–I2 2.615(1),
Hg1–I1A 2.676(1), O1–C2 1.269(5), O2–C3 1.223(5), P1–C1 1.766(4);
O1–Hg1–I1 81.37(8), O1–Hg1–I2 104.05(8), O1–Hg1–I1A 102.80(8), I1–
Hg1–I1A 92.49(1), Hg1–O1–C2 129.6(3).
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torsion angles of 0.0 and 6.1° in 1, 0.8° in 2 and 8.31° in 3.
We conclude that novel bonding modes to Hg(II) could be
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HgCl2 prefers to form the 1:2 complex, albeit with secondary
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P. L. thanks the Council of Scientific and Industrial Research,
Government of India for the award of Senior Research
Fellowship [9/475(92)/99-EMRI]. The authors thank the Ohio
Crystallographic Consortium, funded by the Ohio Board of
Regents 1995 Investment Fund [(CA)-075] located at the
University of Toledo, Instrumentation Center in A&S, Toledo,
OH 43606, USA for X-ray data collection, the Sophisticated
Instruments Facility, Indian Institute of Science, Bangalore,
India, for recording 13C NMR spectra for the samples and Dr
Mariappan Manoharan, Post-Doctoral Fellow, Centre for Com-
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