Unprecedented insertion of alkynes into a palladium-phosphine bond. A facile route to palladium-bound alkenyl phosphorus ylides

Article abstract of DOI:10.1021/om990014h

Refluxing a mixture of (dppm)palladium(II) dichloride (where dppm is bis(diphenylphosphino)-methane) and various alkynes in the 1,2-dichloroethane / 1,4-dioxane mixed solvents affords a variety of Pd-bound alkenyl phosphorus ylides in modest to good yields. Detailed spectroscopic data for these Pd-bound alkenyl phosphorus ylides along with the X-ray single-crystal structure of 1c are reported. The mechanism and implications for such an unprecedented apparent insertion of alkynes into the Pd-phosphine bond are also discussed.

Full text of DOI:10.1021/om990014h

2922
Organometallics 1999, 18, 2922-2925
Un p r eced en ted In ser tion of Alk yn es in to a
P a lla d iu m -P h osp h in e Bon d . A F a cile Rou te to
P a lla d iu m -Bou n d Alk en yl P h osp h or u s Ylid es
Aberdeen Allen, J r. and Wenbin Lin*
Department of Chemistry, Brandeis University, Waltham, Massachusetts 02454
Received J anuary 12, 1999
Summary: Refluxing a mixture of (dppm)palladium(II)
dichloride (where dppm is bis(diphenylphosphino)-
methane) and various alkynes in the 1,2-dichloroethane/
1,4-dioxane mixed solvents affords a variety of Pd-bound
alkenyl phosphorus ylides in modest to good yields.
Detailed spectroscopic data for these Pd-bound alkenyl
phosphorus ylides along with the X-ray single-crystal
structure of 1c are reported. The mechanism and impli-
cations for such an unprecedented apparent insertion of
alkynes into the Pd-phosphine bond are also discussed.
alkenyl phosphorus ylide 1a in 49% yield after purifica-
tion by column chromatography (eq 1). The 1H NMR
(1)
Phosphines are among the most important ancillary
ligands for transition-metal catalysts. In particular,
palladium phosphine complexes have been widely used
to catalyze many organic transformations that utilize
unsaturated hydrocarbon substrates.1 For example,
additions of HX to unsaturated hydrocarbons (e.g.,
hydrogenation2 and hydrosilation3 reactions) and carbon-
carbon bond forming reactions4 (e.g., Heck and Stille
reactions) are readily catalyzed by palladium phosphine
complexes. Under these often forcing conditions, ortho-
metalation and carbon-phosphorus bond cleavage of
arylphosphines represent common catalyst deactivation
pathways.5 Herein we wish to report an unprecedented
apparent insertion of alkynes into palladium-phosphine
bonds to result in the first examples of alkenyl phos-
phorus ylides of palladium. Such a phosphorus-carbon
bond forming process will lead to thermodynamically
stable, catalytically inactive alkenyl phosphorus ylides
and thus presents a potential catalyst deactivation
pathway for palladium-catalyzed organic transforma-
tions with unsaturated hydrocarbon substrates.
spectrum of 1a exhibits a characteristic doublet of
doublet for the alkenyl proton at δ 5.88; the alkenyl
proton is coupled to two inequivalent phosphorus nuclei
2
4
with J H-P ) 47.7 Hz and J H-P ) 10.3 Hz. The
methylene protons of the Pd-bound alkenyl phosphorus
ylide ligand also appear as a doublet of doublet at δ 3.69
due to the coupling to two inequivalent phosphorus
nuclei. Consistent with the formation of the phosphorus
ylide, the 31P{1H} NMR spectrum of 1a shows two
downfield doublets at δ 50.1 and 47.1. The characteristic
CtC stretch at ∼2106 cm-1 for phenylacetylene has also
disappeared in the IR spectrum of 1a .
Refluxing a mixture of (dppm)palladium(II) dichlo-
ride6 (where dppm is bis(diphenylphosphino)methane)
and excess phenylacetylene in the 1,2-dichloroethane/
1,4-dioxane mixed solvents results in the Pd-bound
(1) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis, 2nd ed.;
Wiley: New York, 1992.
Other alkynes with either electron-donating or elec-
tron-withdrawing substituents can be used in place of
phenylacetylene in this insertion reaction to result in a
variety of Pd-bound alkenyl phosphorus ylides. Com-
pounds 1b-d were synthesized by refluxing a mixture
of (dppm)palladium dichloride and n-undecyne, (p-
(dimethylamino)phenyl)acetylene,7 and (p-nitrophenyl)-
acetylene7 in the 1,2-dichloroethane/1,4-dioxane mixed
solvents, respectively. These Pd-bound alkenyl phos-
phorus ylides all exhibit characteristic doublets of
(2) Takaya, H.; Ohta, T.; Noyori, R. In Catalytic Asymmetric
Synthesis; Ojima, I., Ed.; VCH: New York, 1993; Chapter 1, pp 1-39.
(3) (a) Marciniec, B.; Gulinski, J . J . Organomet. Chem. 1993, 446,
15-23. (b) Tanke, R. S.; Crabtree, R. H. J . Am. Chem. Soc. 1990, 112,
7984-7989. (c) Brunner, H.; Nishiyama, H.; Itoh, K. In Catalytic
Asymmetric Synthesis; Ojima, I., Ed.; VCH: New York, 1993; Chapter
6, pp 303-322.
(4) (a) Heck, R. F. In Comprehensive Organic Synthesis Vol. 4, Trost,
B. M., Fleming, I., Eds.; Pergamon: Oxford, U.K., 1991. (b) de Meijere,
A.; Meyer, F. E. Angew. Chem., Int. Ed. Engl. 1994, 33, 2379-2411.
(c) Stille, J . K.; Simpson, J . H. J . Am. Chem. Soc. 1987, 109, 2138-
2152. (d) Scott, W. J .; Crisp, G. T.; Stille, J . K. J . Am. Chem. Soc. 1984,
106, 4630-3462.
(5) (a) Garrou, P. E. Chem. Rev. 1985, 85, 171-185. (b) Michman,
M. Isr. J . Chem. 1986, 27, 241-249.
(6) Steffen, W. L.; Palenik, G. J . Inorg. Chem. 1976, 15, 2432-2438.
(7) (a) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N.
Synthesis 1980, 627. (b) Ames, D. E.; Bull, D.; Takundwa, C. Synthesis
1981, 364-365.
10.1021/om990014h CCC: $18.00 © 1999 American Chemical Society
Publication on Web 06/30/1999

Notes
Ta ble 1. Da ta for th e X-r a y Diffr a ction of 1c
Organometallics, Vol. 18, No. 15, 1999 2923
chem formula
cryst size, mm
a, Å
b, Å
C35H33Cl2NP2Pd‚CH2Cl2
0.04 × 0.05 × 0.14
16.7972(5)
14.7025(5)
14.5388(5)
104.167(1)
3481.3(2)
4
c, Å
â, deg
V, Å3
Z
fw
791.86
space group
T, °C
λ(Mo KR), Å
P21/c (No. 14)
-75(1)
0.71073
F
calcd, g/cm3
1.51
13.5
-0.58, 0.56
µ(Mo KR), cm-1
min and max residual
density, e/Å3
no. of rflns
no. of params
R
2184 (I > 2σ)
276
0.049
Rw
0.078
0.71
F igu r e 1. ORTEP drawing of 1c with 30% probability
ellipsoids. The phenyl groups on the phosphorus centers
have been refined isotropically and represented with small
circles. Hydrogen atoms (except H1) have been omitted for
clarity.
goodness of fit
1
doublets for the alkenyl proton in the H NMR spectra
and two downfield doublets for the two inequivalent
phosphorus nuclei in the 31P{1H} NMR spectra. All
these alkenyl phosphorus ylides are extremely ther-
mally stable. They are also air- and moisture-stable and
have in fact all been purified by silica column chroma-
tography with a mixture of chloroform and methanol
as eluents without any sign of decomposition. Com-
pounds 1a -d represent the first examples of alkenyl
phosphorus ylides of palladium.
Ta ble 2. Selected Bon d Dista n ces (Å) a n d Bon d
An gles (d eg) for 1c
Pd1-C2
Pd1-Cl1
P1-C23
C1-P2
N1-C9
P2-C23
C3-C4
C4-C5
C6-C7
C1-H1
2.017(7)
2.378(2)
1.823(6)
1.757(7)
1.425(8)
1.815(6)
1.402(9)
1.391(8)
1.415(9)
0.96(5)
Pd1-P1
Pd1-Cl2
C1-C2
N1-C6
N1-C10
C2-C3
C3-C8
C5-C6
C7-C8
2.215(2)
2.394(2)
1.350(8)
1.399(8)
1.463(8)
1.451(9)
1.405(8)
1.371(8)
1.373(9)
Because no coordinated alkenyl phosphorus ylides of
palladium are known in the literature, we undertook
an X-ray single-crystal structural determination on the
representative compound 1c (Table 1). Dark red crystals
of 1c were obtained by slow diffusion of diethyl ether
into a concentrated solution of 1c in dichloromethane.
An X-ray single-crystal diffraction study of 1c estab-
lishes the phosphonium alkenyl ylide structure (Figure
1). Pd1 coordinates to C2 and P1 of the alkenyl phos-
phorus ylide and to two chloride atoms in a slightly
distorted square planar fashion (Table 2). Pd1 is 0.08
Å out of the plane defined by C2, P1, Cl1, and Cl2 atoms.
The Pd1-P2 distance is very long at 3.42 Å, indicating
that no interaction exists between the Pd1 center and
the ylide phosphorus. The palladium-phosphorus and
palladium-chloride distances are all normal, while the
Pd1-C2 distance of 2.022(6) Å is very close to the sum
of covalent radii of Pd(II) and sp2-hydridized carbon
(1.97 Å) and comparable to other known Pd-alkenyl
bonds.8 The C1-C2 distance of 1.349(8) Å indicates the
double-bond nature, which has been further supported
by the bond angles of near 120° surrounding the C1 and
C2 atoms. These structural data indicate that com-
pounds 1a -d can be described as phosphonium alkenyl
ylides9-11 (i.e., the zwitterionic structure as shown in
C2-Pd1-P1
P1-Pd1-Cl1
P1-Pd1-Cl2
C23-P1-Pd1
C2-C1-P2
C1-P2-C23
C1-C2-Pd1
C11-P2-C17
85.5(2)
169.83(7)
93.38(7)
104.2(2)
126.0(6)
110.5(3)
120.1(6)
105.1(3)
C2-Pd1-Cl1
C2-Pd1-Cl2
Cl1-Pd1-Cl2
C1-C2-C3
C1-P2-C11
C11-P2-C23
C3-C2-Pd1
C2-C1-H1
90.6(2)
178.5(2)
90.69(7)
120.3(6)
108.1(3)
110.5(3)
119.5(5)
124(3)
eq 1) with the Pd center coordinated to one phosphine
ligand, one alkenyl group, and two chloride atoms. The
P2-C1 distance is not significantly different from the
average of P2-C11 and P2-C17 distances. It is also
noteworthy that the insertion of alkynes is highly
regiospecific: only the alkenyl ylides with the terminal
alkyne carbon bonded to the phosphorus center are
formed. This regiospecificity is undoubtedly a conse-
quence of the steric crowdedness of the phosphorus
center in dppm (vs the palladium center).
The description of 1a -d as phosphonium alkenyl
ylides is further supported by the chemical reactivity
of the ylide carbon center (C1). Earlier reports indicated
that ylide carbon centers in rhenium alkenyl phospho-
rus ylides were susceptible to electrophilic attack as a
result of the partial negative charge on ylide carbon
centers (facile deuterium incorporation from methanol-
d4 into the ylide carbons was observed).11 In contrast,
no deuterium incorporation was observed when com-
plexes 1a -d were treated with methanol-d4. This result
strongly supports the phosphonium alkenyl ylide struc-
ture of 1a -d , and the different behavior between
palladium and rhenium alkenyl phosphorus ylides can
(8) Maitlis, P. M.; Espinet, P.; Russell, M. J . H. In Comprehensive
Organometallic Chemistry, Wilkinson, G, Stone, F. G. A., Abel, E. W.,
Eds.; Pergamon: New York, 1982; Vol. 6, p 334.
(9) Analogous phosphonium allyl ylides of palladium have been
reported: (a) Itoh, K.; Nishiyama, H.; Ohnishi, T.; Ishii, Y. J .
Organomet. Chem. 1974, 76, 401-406. (b) Hirai, M.-F.; Miyasaka, M.;
Itoh, K.; Ishii, Y. J . Organomet. Chem. 1978, 160, 25-34.
(10) Rogers, R. D.; Alt, H. G.; Maisel, H. E. J . Organomet. Chem.
1990, 381, 233-238.
(11) (a) Lappas, D.; Hoffman, D. M.; Folting, K.; Huffman, J . C.
Angew. Chem., Int. Ed. Engl. 1988, 27, 587-588. (b) Hoffman, D. M.;
Huffman, J . C.; Lappas, D.; Wierda, D. A. Organometallics 1993, 12,
4312-4320.

2924 Organometallics, Vol. 18, No. 15, 1999
Notes
Sch em e 1
drawing capability of the substituents on the phenyl
groups in 1a -d . Finally, insertion of internal alkynes
does not occur under similar conditions, apparently as
a consequence of the steric hindrance of the dangling
phosphine moiety.
In summary, we have discovered an unprecedented
apparent insertion of alkynes into the Pd-phosphine
bond to result in the first examples of palladium alkenyl
phosphorus ylides. The formation of thermodynamically
stable alkenyl phosphorus ylides could present a poten-
tial catalyst deactivation pathway for palladium-
catalyzed organic transformations using unsaturated
hydrocarbons.
Exp er im en ta l Section
All reactions were performed under an argon atmosphere
using standard Schlenk techniques. 1,4-Dioxane and 1,2-
dichloroethane were dried over molecular sieve (4 Å). (p-
(dimethylamino)phenyl)acetylene,8 (p-nitrophenyl)acetylene,8
and (dppm)PdCl2 were prepared according to literature pro-
cedures. The IR spectra were recorded as Nujol mulls on a
be attributed to the different electronegativity of pal-
ladium vs rhenium.
1
Paragon 1000 FT-IR spectrometer. H, 13C{1H}, and 13P{1H}
NMR spectra were taken on a Varian Unity 400 Plus spec-
trometer at 399.7, 100.5, and 161.8 MHz, respectively. El-
emental analyses were done at the Microanalytical Laboratory
of University of Illinois at Urbana-Champaign.
Insertion of in situ generated carbenes into metal-
phosphine bonds represents an important pathway for
the synthesis of metal-bound phosphorus ylides.12 How-
ever, the insertion of an alkyne molecule into the Pd-
phosphine bond is unprecedented.13 We speculate that
compounds 1a -d may have formed via the nucleophilic
attack on the palladium-coordinated alkynes by the
dangling phosphine moiety of the dppm ligand (Scheme
1). Several lines of evidence support this nucleophilic
attack pathway. First, the insertion reaction critically
depends on the coordinating capability of the solvent.
When only a noncoordinating polar solvent such as 1,2-
dichloroethane or chlorobenzene is used, no reaction is
observed. The strongly coordinating solvent p-dioxane
is needed to generate intermediates 2 and 3. Second,
the insertion reaction only occurred when (dppm)PdCl2
was used. When (dppe)PdCl2 (dppe represents 1,2-bis-
(diphenylphosphino)ethane) was used, no reaction was
observed. The large strain in the four-membered chelate
ring presumably facilitates the formation of intermedi-
ates 2 and 3. Third, the PdII center renders the
coordinated alkynes in 3 susceptible to nucleophilic
attack by the dangling phosphine. For example, PdII-
promoted ethylene oxidation is a key step in the Wacker
process, which converts ethylene into acetaldehyde.1
Moreover, nucleophilic attack of phosphines on coordi-
nated alkynes has been used to synthesize alkenyl
phosphorus ylides of manganese and is proposed to be
the pathway for the formation of alkenyl phosphorus
ylides of rhenium. Although we have not yet carried out
kinetic studies with different alkynes, the isolated yields
for 1a -d are consistent with the proposed mechanism.
The isolated yield under comparable conditions in-
creases in the order 1c < 1b < 1a < 1d ; this order of
isolated yields correlates well with the electron-with-
Syn th esis of 1a . Pd(dppm)Cl2 (250 mg, 0.46 mmol) and
phenylacetylene (70 mg, 0.69 mmol) were dissolved in a
mixture of dry 1,4-dioxane (10 mL) and 1,2-dichloroethane (10
mL) under argon. The solution was heated with stirring at
110 °C for 90 h and evaporated in vacuo to leave a pale orange
solid. The solid was purified by column chromatography (SiO2,
CHCl3/MeOH, 6/1) to afford 150 mg (49%) of pure 1a as a
bright orange solid. Mp: 218-220 °C. 1H NMR (CDCl3): δ
2
8.61-6.98 (25 H, aromatic H), 5.88 (dd, 1H, J H-P ) 47.7 Hz,
2
4J H-P ) 10.3 Hz), 3.69 (dd, 2H, J H-P ) 17.1 Hz). 13C NMR
(CDCl3): δ 134.1, 133.9, 133.6, 133.4, 130.8, 129.9, 129.5, 128.4,
128.3, 128.1, 127.5, 121.9, 121.0, 102.9, 101.9, 22.1. 31P{1H}
2
2
NMR (CDCl3): δ 50.1 (d, J P-P ) 40.3 Hz), 47.1 (d, J P-P
)
40.3 Hz). IR (Nujol, cm-1): 3050.4, 1508.1, 1481.0, 1435.9,
1111.1, 739.2, 689.7. Anal. Calcd for C33H28Cl2P2Pd‚0.5CH2-
Cl2: C, 57.0; H, 4.11. Found: C, 56.5; H, 4.16.
Syn th esis of 1b. A mixture of (dppm)PdCl2 (100 mg, 0.18
mmol) and 1-undecyne (100 mg, 0.72 mmol) in dry 1,4-dioxane
(10 mL) and 1,2-dichloroethane (10 mL) was heated with
stirring at 110 °C for 100 h and evaporated in vacuo to leave
a pale orange solid. The solid was purified by column chro-
matography (SiO2, CHCl3/MeOH, 15/1) to afford 60 mg (48%)
1
of pure 1b as a pale orange solid. Mp: 195-200 °C. H NMR
2
(CDCl3): δ 8.29-7.02 (20 H, aromatic H), 5.54 (dd, 1H, J
H-P
4
2
) 50.7, 1H, J
) 11.6 Hz), 4.45 (dd, 2H, J H-P ) 22.6 Hz),
H-P
3.43 (s, 2H), 1.61-1.59 (m, 2H), 1.37-1.10 (m, 9H), 0.92 (t,
3H, J ) 14.1 Hz). 13C NMR (CDCl3): δ 134.3, 133.6, 133.4,
133.3, 131.0, 130.2, 130.0, 128.6, 128.5, 31.9, 29.7, 29.5, 29.4,
29.3, 22.7. 31P{1H} NMR (CDCl3): δ 51.7 (d, 2J P-P ) 44.5 Hz),
2
47.7 (d, J P-P ) 44.5 Hz). IR (Nujol, cm-1): 3050.9, 2916.0,
2843.4, 1518.9, 1430.7, 1098.6, 787.3, 735.4, 683.5, 507.1. Anal.
Calcd for C36H42Cl2P2Pd‚CH2Cl2: C, 55.6; H, 5.51. Found: C,
56.0; H, 5.99.
Syn th esis of 1c. (dppm)PdCl2 (100 mg, 0.18 mmol) and
4-ethynyl-N,N-dimethylaniline (40 mg, 0.27 mmol) were dis-
solved in a mixture of dry 1,4-dioxane (10 mL) and 1,2-
dichloroethane (10 mL) under argon. The solution was heated
with stirring at 105 °C for 72 h and evaporated in vacuo to
leave a brown solid. The solid was purified by column chro-
matography (SiO2, CHCl3/MeOH, 11/1) to afford orange 1c.
Yield: 40 mg (31%). Mp: 229-232 °C. 1H NMR (CD2Cl2): δ
(12) (a) Alcock, N. W.; Pringle, P. G.; Bergamini, P.; Sostero, S.;
Traverso, O. J . Chem. Soc., Dalton Trans. 1990, 1553-1556. (b) Gong,
J . K.; Peters, T. B.; Fanwick, P. E.; Kubiak, C. P. Organometallics 1992,
11, 1392-1394. (c) Moss, J . R.; Spiers, J . C. J . Organomet. Chem. 1979,
182, C20-C24.
(13) The insertions of alkynes into M-PR2 and M-P(O)R2 bonds
are well-established. See: (a) Han, L.-B.; Choi, N.; Tanaka, M.
Organometallics 1996, 15, 3259-3261. (b) Han, L.-B.; Hua, R.; Tanaka,
M. Angew. Chem., Int. Ed. Engl. 1998, 37, 94-96.
7.83 (d, 2H, J ) 8.9 Hz), 7.73-7.24 (20 H, aromatic H), 6.56
2
(d, 2H, J ) 8.9 Hz), 5.76 (dd, 1H, J H-P ) 49.5 Hz, 4J
)
H-P

Notes
Organometallics, Vol. 18, No. 15, 1999 2925
12.2 Hz), 3.48 (dd, 2H, 2J H-P ) 20.1 Hz), 2.99 (s, 6H). 13C NMR
(CD2Cl2): δ 151.9, 134.8, 133.4, 131.6, 130.0, 128.7, 110.6, 40.3.
in structure solution and refinement. The structure was solved
by direct methods using SHELX-TL14 and refined by full-
matrix least squares using anisotropic displacement param-
eters for all non-hydrogen atoms except the phenyl groups on
the phosphorus centers. The carbon atoms of the phenyl groups
were refined isotropically because of insufficient data points.
H1 was located in an electron density difference map and
refined isotropically, while all the other hydrogen atoms were
located by geometric placing. Final refinement gave R ) 0.049,
Rw ) 0.078, and goodness of fit ) 0.71. Experimental details
for the X-ray data collection of 1c are tabulated in Table 1.
Selected bond distances and angles for 1c are listed in Table
2.
2
31P{1H} NMR (CD2Cl2): δ 49.8 (d, J P-P ) 43.3 Hz), 48.1 (d,
2J P-P ) 43.3 Hz). IR (Nujol, cm-1): 3050.9, 2884.9, 1596.7,
1477.4, 1430.7, 1358.0.
Syn th esis of 1d . A mixture of (dppm)PdCl (200 mg, 0.36
mmol) and 4-ethynylnitrobenzene (60 mg, 0.41 mmol) in dry
1,4-dioxane (10 mL) and 1,2-dichloroethane (10 mL) was
heated with stirring at 105 °C for 48 h and evaporated in vacuo
to leave a dark brown solid. The solid was purified by column
chromatography (SiO2, CHCl3/MeOH, 15/1) to afford 1d .
1
Yield: 185 mg (73%). Mp: 265-270 °C. H NMR (DMSO-d6):
2
δ 7.89-7.20 (24 H, aromatic H), 5.98 (dd, 1H, J H-P ) 46.5
Hz, 4J H-P ) 9.78 Hz), 3.41 (dd, 2H, 2J H-P ) 13.6 Hz). 13C NMR
(DMSO-d6): 146.3, 134.5, 134.0, 133.9, 133.6, 133.5, 131.1,
130.2, 130.0, 128.6, 128.5, 127.9, 122.6, 120.8, 119.9. 31P{1H}
Ack n ow led gm en t. We are indebted to Dr. Scott R.
Wilson and the Materials Chemistry Laboratory at
University of Illinois at Urbana-Champaign for X-ray
data collection and to Dr. Zhiyong Wang for help with
the synthesis of alkynes. We acknowledge the NSF
(Grant No. CHE-9727900) and ACS-PRF for financial
support.
NMR (DMSO-d6): δ 50.5 (d, 2J P-P ) 39.1 Hz), 45.8 (d, 2J P-P
)
39.1 Hz). IR (Nujol, cm-1): 3050.9, 1518.9, 1503.3, 1430.7,
1342.5, 1109.0, 854.7, 740.6, 693.9, 501.9. Anal. Calcd for
C
33H27Cl2NO2P2Pd‚0.5CH2Cl2: C, 53.5; H, 3.73; N, 1.86.
Found: C, 54.0; H, 4.20; N, 1.59.
X-r a y Str u ctu r e Deter m in a tion of 1c. Data collection for
1c was carried out with an orange crystal of dimensions of
0.04 × 0.05 × 0.14 mm on a Siemens SMART system equipped
with a CCD detector using Mo KR radiation. Of the 6129
reflections measured, 2184 reflections with I > 2σ(I) were used
Su p p or tin g In for m a tion Ava ila ble: Tables giving posi-
tional and thermal parameters and bond distances and angles
for 1c. This material is available free of charge via the Internet
at http://pubs.acs.org.
(14) SHELX-TL Version 5.1; Bruker Analytical X-ray Systems, Inc.,
Madison, WI, 1997.
OM990014H