Asymmetric synthesis of a (P-chiral) as-P bidentate ligand via an organopalladium complex promoted asymmetric diels-alder reaction between Ph2AsCH=CH2 and 1-phenyl-3,4-dimethylphosphole

Article abstract of DOI:10.1021/om9600351

Convenient access to the enantiomerically pure rigid bidentate ligand (-)-[5-(diphenylarsino)-2,3-dimethyl-7-phenyl-7(S)phosphabicyclo[2.2.1]hept-2- ene is established via an asymmetric [4 + 2] cycloaddition between diphenylvinylarsine and 1-phenyl-3,4-dimethylphosphole using the chiral organopalladium(II) complex containing ortho-metalated dimethyl[1-(2-naphthyl)ethyl]amine as the reaction promoter. The absolute configurations of the four newly generated stereocenters have been assigned by single-crystal X-ray analysis.

Full text of DOI:10.1021/om9600351

3640
Organometallics 1996, 15, 3640-3643
Articles
Asym m etr ic Syn th esis of a (P -Ch ir a l) As-P Bid en ta te
Liga n d via a n Or ga n op a lla d iu m Com p lex P r om oted
Asym m etr ic Diels-Ald er Rea ction betw een
P h 2AsCHdCH2 a n d 1-P h en yl-3,4-d im eth ylp h osp h ole
Beng-Hwee Aw,1a Pak-Hing Leung,*,1a Andrew J . P. White,1b and
David J . Williams1b
Department of Chemistry, National University of Singapore, Kent Ridge, Singapore 0511, and
Department of Chemistry, Imperial College, London, SW7 2AY, United Kingdom
Received J anuary 19, 1996X
Convenient access to the enantiomerically pure rigid bidentate ligand (-)-[5-(dipheny-
larsino)-2,3-dimethyl-7-phenyl-7(S)-phosphabicyclo[2.2.1]hept-2-ene is established via an
asymmetric [4 + 2] cycloaddition between diphenylvinylarsine and 1-phenyl-3,4-dimeth-
ylphosphole using the chiral organopalladium(II) complex containing ortho-metalated
dimethyl[1-(2-naphthyl)ethyl]amine as the reaction promoter. The absolute configurations
of the four newly generated stereocenters have been assigned by single-crystal X-ray analysis.
Sch em e 1
In tr od u ction
In keeping with our studies concerning the stereo-
chemistry of chelating arsine and phosphine ligands
with resolved chiral donor atoms, we have resolved
several asymmetric As-S2,3 and P-S3,4 bidentate ligands
by the metal complexation technique. Recently, we
have also reported the asymmetric syntheses of a
sulfinyl-substituted P-chiral phosphine5 and a rigid
P-chiral diphosphine.6 These stereochemically well-
defined metal sequesters have been shown to have a rich
coordination chemistry.7 Here we present the first
asymmetric synthesis of a rigid As-P hetero-bidentate
species with one phosphorus and three carbon stereo-
genic centers. This class of unsymmetrical bidentates
has important implication in enantioselective synthe-
sis.8 The only two documented optically active As-P
bidentate ligands have been obtained by optical resolu-
tions, however.9
X Abstract published in Advance ACS Abstracts, J uly 1, 1996.
(1) (a) National University of Singapore. (b) Imperial College.
(2) (a) Leung, P. H.; McLaughlin, G. M.; Martin, J . W. L.; Wild, S.
B. Inorg. Chem. 1986, 25, 3392. (b) Kerr, P. G.; Leung, P. H.; Wild, S.
B. J . Am. Chem. Soc. 1987, 109, 4321.
(3) Chooi, S. Y. M.; Siah, S. Y.; Leung, P. H.; Mok, K. F. Inorg. Chem.
1993, 32, 4812.
(4) Leung, P. H.; Willis, A. C.; Wild, S. B. Inorg. Chem. 1992, 31,
1406.
Resu lts a n d Discu ssion
Asym m etr ic Syn th esis. Diphenylvinylarsine is ob-
tained in 92% isolated yield from the thermolysis of
[2-(methylsulfinyl)ethyl]diphenylarsine.3 As illustrated
in Scheme 1, the tertiary arsine is a powerful ligand
which splits the chloro bridges in the chiral dimeric
complex (+)-1 regiospecifically10 to give the monomeric
neutral compound (+)-2 in 88% yield: [R]D +75.2° (CH2-
Cl2). The asymmetric [4 + 2] cycloaddition reaction is
achieved by first treating (+)-2 with a stoichiometric
quantity of silver perchlorate in dichloromethane; upon
removal of silver chloride, the cyclic diene 1-phenyl-3,4-
(5) Siah, S. Y.; Leung, P. H.; Mok, K. F. J . Chem. Soc., Chem.
Commun. 1995, 1747.
(6) Aw, B. H.; Leung, P. H. Tetrahedron: Asymmetry 1994, 5, 1167.
(7) (a) Leung, P. H.; Martin, J . W. L.; Wild, S. B. Inorg. Chem. 1986,
25, 3396. (b) Chooi, S. Y. M.; Ranford, J . D.; Leung, P. H.; Mok, K. F.
Tetrahedron: Asymmetry 1994, 5, 1805. (c) Chooi, S. Y. M.; Leung, P.
H.; Sim, K. Y.; Tan, K. S.; Kon, O. L. Tetrahedron: Asymmetry 1994,
5, 49. (d) Chooi, S. Y. M.; Leung, P. H.; Mok, K. F. Inorg. Chim. Acta
1993, 205, 245.
(8) Wild, S. B. In The Chemistry of Arsenic, Antimony and Bismuth
Compounds; Patai, S., Ed.; Wiley: Chichester, U.K., 1994; Chapter 3,
and references cited therein.
(9) (a) Salem, G.; Wild, S. B. Inorg. Chem. 1983, 22, 4049. (b) Doyle,
R. J .; Salem, G.; Willis, A. C. J . Chem. Soc., Dalton Trans. 1995, 1867.
(10) (a) Loh, S. K.; Mok, K. F.; Leung, P. H.; White, A. J . P.;
Williams, D. J . Tetrahedron: Asymmetry 1996, 7, 45. (b) Leung, P.
H.; Loh, S. K.; Mok, K. F.; White, A. J . P.; Williams, D. J . J . Chem.
Soc., Chem. Commun. 1996, 591 and references cited therein.
S0276-7333(96)00035-0 CCC: $12.00 © 1996 American Chemical Society

Synthesis of a (P-Chiral) As-P Bidentate Ligand
Organometallics, Vol. 15, No. 17, 1996 3641
F igu r e 1. Crystal structure of (-)-4. Hydrogen atoms,
other than those at chiral centers, have been omitted for
clarity, and the thermal ellipsoids are drawn at the 50%
probability level.
Ta ble 1. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for (-)-4
F igu r e 2. Interchelate interactions (a) in (+)-3 and (b) in
Pd-As
2.349(1)
2.345(2)
1.950(7)
1.920(3)
1.849(7)
Pd-P
2.218(2)
2.360(2)
1.921(3)
1.830(4)
1.834(7)
the unfavored diastereomer.
Pd-Cl(1)
As-C(11)
As-C(26)
P-C(7)
Pd-Cl(2)
As-C(20)
P-C(6)
trans influence of the phosphorus donor atom. There
is a noticeable contraction from tetrahedral of both the
Pd-As-C(11) (101.9(2)°) and C(20)-As-C(26) (103.4-
(2)°) angles. The angles at P are similar to those in a
related racemic complex.11
P-C(10)
As-Pd-P
83.1(1)
93.6(1)
175.0(1)
80.8(3)
113.8(2)
101.9(2)
120.6(1)
98.1(5)
As-Pd-Cl(1)
P-Pd-Cl(1)
Cl(1)-Pd-Cl(2)
C(7)-P-Pd
168.3(1)
89.1(1)
As-Pd-Cl(2)
P-Pd-Cl(2)
C(7)-P-C(10)
C(10)-P-Pd
C(11)-As-Pd
C(26)-As-Pd
C(11)-C(10)-P
C(9)-C(10)-P
Treatment of a dichloromethane solution of (-)-4 with
aqueous potassium cyanide liberates the optically active
bidentate ligand (-)-5 as a white solid in 82% isolated
yield: [R]D -54.9° (CH2Cl2). Significantly, the 31P NMR
spectrum of (-)-5 in CDCl3 exhibits a sharp singlet at
δ 95.3. This low-field signal confirms that the exo-syn
stereochemical relationship is retained.11 Owing to the
configurational instability of the uncoordinated bridge-
head phosphorus stereogenic center,12 the liberated
ligand cannot be stored for longer than ca. 30 min and
was therefore recoordinated immediately to selected
metal ions. These optically active ligand complexes are
stereodynamically stable.
Or igin s of Ster eoselectivity. It is well established
that the five-membered (S)-metalated naphthylamine
ring adopts a λ absolute conformation with the methyl
substituent on the chiral carbon invariably taking up
the axial position above the PdCN ring.7,10 This axial
rather than equatorial geometry for the methyl group
is attributed to the extreme steric congestion that would
otherwise be present between the proximal H(8) naph-
thyl proton and the methyl group.13 Due to the fixed
ring conformation, the prochiral N-Me groups are
locked into the stereochemically nonequivalent axial
and equatorial positions (Figure 2). The N-Me(ax)
group projects perpendicularly below the square plane,
and N-Me(eq) is somewhat above the plane and quite
close to the phosphorus donor atom. It is important to
94.78(9)
115.3(2)
120.4(2)
117.6(2)
103.4(2)
100.9(5)
100.7(5)
C(6)-P-Pd
C(20)-As-Pd
C(20)-As-C(26)
C(12)-C(7)-P
C(8)-C(7)-P
101.2(5)
dimethylphosphole (DMPP) is then added directly into
the complex solution. The Diels-Alder reaction is
complete in 3 days at room temperature to give (+)-3
as pale yellow microcrystals in 50% isolated yield: [R]D
+136.0° (CHCl3). It should be note that, prior to
purification, the 31P NMR spectrum of the crude Diels-
Alder product in CDCl3 exhibits a sharp singlet at δ
118.1. No other 31P resonance signals can be detected
in the 202 MHz NMR spectra, thus indicating that only
a single diastereomer is formed in the Diels-Alder
reaction. The chiral amine auxiliary on (+)-3 can be
removed from the template complex chemoselectively
using concentrated hydrochloric acid. The resultant
dichloro complex (-)-4 is thus obtained as yellow prisms
in 91% isolated yield: [R]D -34.1° (CH2Cl2).
The X-ray analysis of (-)-4 confirms that the desired
As-P coordination complex has been formed (Figure 1)
and establishes the absolute stereochemistries of the
four chiral centers created: R at P, C(7), and C(10) and
S at C(11). Selected bond lengths and angles are listed
in Table 1. With reference to the 31P NMR spectra, this
X-ray analysis also established the absolute stere-
ochemistries for the four equivalent chiral centers in
(+)-3. In (-)-4 the geometry at Pd is slightly distorted
(tetrahedrally) square-planar with angles at Pd in the
ranges 83.1(1)-94.8(1) and 168.3(1)-175.0(1)°, the small-
est of these former angles being associated with the bite
of the diphenylarsino-phosphole ligand. The Pd-P
(2.218(2) Å) and Pd-As (2.349(1) Å) distances are
unexceptional. The two Pd-Cl distances differ signifi-
cantly, with that trans to As (2.345(2) Å) being shorter
than that trans to P (2.360(2) Å), reflecting the greater
(11) Wilson, W. L.; Rahn, J . A.; Alcock, N. W.; Fischer, J .; Frederick,
J . H.; Nelson, J . H. Inorg. Chem. 1994, 33, 109. J i, H. L.; Nelson, J .
H.; De Cian, A.; Fischer, J .; Solujic, L.; Milosavljevic, E. B. Organo-
metallics 1992, 11, 1840. Vac, R.; Nelson, J . H.; Milosavljevic, E. B.;
Solujic, L.; Fischer, J . Inorg. Chem. 1989, 28, 4132. Rahn, J . A.; Holt,
M. S.; O’Neil-J ohnson, M.; Nelson, J . H. Inorg. Chem. 1988, 27, 1316.
Holt, M. S.; Nelson, J . H.; Savignac, P.; Alcock, N. W. J . Am. Chem.
Soc. 1985, 107, 6396 and references cited therein.
(12) (a) Mathey, F.; Mercier, F. Tetrahedron Lett. 1981, 22, 391. (b)
Mesch, K. A.; Quin, L. D. Tetrahedron Lett. 1980, 21, 4791.
(13) (a) Chooi, S. Y. M.; Leung, P. H.; Mok, K. F. Inorg. Chem. 1994,
33, 3096. (b) Alcock, N. W.; Hulmes, D. I.; Brown, J . M. J . Chem. Soc.,
Chem. Commun. 1995, 395.

3642 Organometallics, Vol. 15, No. 17, 1996
Aw et al.
Ta ble 2. Cr ysta llogr a p h ic Da ta for (-)-4
note that, due to their relative proximities, equatorially
located donor substituents in rigid five-membered rings
generally experience much more direct and severe
interchelate repulsive forces than their axial counter-
parts. On the other hand, due to the rigid skew ring
conformation and the strict planarity of the naphthyl
ring, the H(3) naphthyl proton protrudes invariably
toward the space just below the arsenic donor. This
naphthyl proton, together with the stereochemically
well-defined N-Me groups, are powerful chirality in-
ducers and, in many instances, are able to control the
stereochemistry of their neighboring coordination sites.13a
mol formula
mol wt
C26H26AsCl2PPd‚Me2CO
679.7
cryst syst
triclinic
space group
P1
a, Å
b, Å
c, Å
8.790(2)
8.830(2)
10.422(2)
R, deg
97.02(1)
112.63(1)
100.75(2)
293(2)
â, deg
γ , deg
T, K
V, Å3
716.6(2)
Z
1
1.58
342
d(calcd), g cm-3
F(000), e
It has been well documented that Diels-Alder reac-
tions involving DMPP require both the cyclic diene and
the selected dienophiles to be coordinated simulta-
neously on a transition-metal template during the
course of a cycloaddition reaction.11 Interestingly, the
thermodynamically unstable exo-syn products are in-
variably produced from these processes. Figure 2a
shows the interchelate interactions in the cationic
complex (+)-3. In this structure, the As-C-C-P link-
age may be viewed as part of the rigid five-membered
As-Pd-P chelate ring which adopts the rigid λ confor-
mation. Hence, As-Ph occupies a pseudoequatorial
position above the CNAsP square plane and As-Ph′ is
in an axial position below the plane. The bridgehead
substituent, P-Ph, projects toward the space below the
plane. Model studies indicate that there is no major
steric repulsion between the two metal chelates. Figure
2b, on the other hand, shows the structure of the
unfavored diastereomer that was not formed in the
Diels-Alder reaction. In this unfavored complex, the
absolute configuration at P is S and the As-C-C-P
ring adopts the δ conformation. Accordingly, As-Ph
occupies the axial position above the square plane and
As-Ph′ is equatorially disposed below it. In contrast to
its counterpart in (+)-3, P-Ph in this unfavored dias-
tereomer projects to the space above the plane. Model
studies clearly indicate that two major interchelate
repulsions exist within this isomer: one exists between
the sterically protruding H(3) naphthyl proton and the
equatorial As-Ph′ group; another severe steric con-
straint is observed between the proximal N-Me(eq)
steric group and P-Ph. Interestingly, both repulsive
forces are effective at equatorial positions. We believe
that these interchelate repulsive forces are the discrimi-
nating factors that hinder the formation of this unfa-
vored diastereomer in the Diels-Alder reaction.
cryst size, mm
total no. of observns
total no. of unique observns
no. of data used in refinement
no. of params
λ(Mo KR), Å
m, cm-1
0.23 × 0.23 × 0.23
2705
2705
2534
280
0.7107 3
20.6
hkl limits
0-10, -10 to 10, -12 to 11
0.035
0.085
R1a
wR2b
a
R1 ) Σ||Fo| - |Fc||/Σ|Fo|. b wR2 ) {Σ[w(Fo2 - Fc2)2]/Σ[w(Fo2)2]}1/2
,
w-1 ) σ2(Fo2) + (aP)2 + bP.
dipalladium(II) dichloromethane solvate ((()-1)14 and diphe-
nylvinylarsine3 were prepared as previously described.
Elemental analyses were performed by the microanalytical
laboratory of the Department of Chemistry at the National
University of Singapore.
Ch lor o{(S )-1-[1-(d im e t h yla m in o)e t h yl]-2-n a p h t h yl-
C2,N}(d ip h en ylvin yla r sin e-As)p a lla d iu m (II) ((+)-2).
A
mixture of diphenylvinylarsine (2.0 g) and (+)-1 (3.0 g) in
dichloromethane (200 mL) was stirred at room temperature
until all the reaction promoter had dissolved (ca. 1 h). The
solvent was removed from the reaction mixture, and the
residue was recrystallized from a dichloromethane-ethanol
mixture, forming bright yellow prisms: yield 4.1 g (88%); mp
1
200 °C dec; [R]D +75.2° (c 1.0, CHCl3); H NMR (CDCl3, 300
MHz) δ 2.02 (d, 3H, 3J HH ) 6.3 Hz, CHMe), 2.84 (s, 3H, NMe),
2.98 (s, 3H, NMe), 4.35 (q, 1H, 3J HH ) 6.3 Hz, CHMe), 5.54 (d,
3
3
1H, J HH ) 18.2 Hz, cis AsCCH), 6.14 (d, 1H, J HH ) 11.2 Hz,
3
3
trans AsCCH), 7.10 (dd, 1H, J HH ) 18.2 Hz, J HH′ ) 11.2 Hz,
AsCH), 6.70-7.90 (m, 16H, aromatics). Anal. Calcd for
C
28H29AsClNPd: C, 56.4; H, 4.9; N, 2.3; Cl, 5.9. Found: C,
56.3; H, 4.8; N, 2.2; Cl, 5.8.
{(S )-1-[1-(d i m e t h y la m i n o )e t h y l]n a p h t h y l-C 2 ,N }-
{(1r,4r,5r,7R)-[5-(d ip h en yla r sin o)-2,3-d im eth yl-7-p h en -
yl-7-p h osp h a bicyclo[2.2.1]h ep t-2-en e-As5,P 7}p a lla d iu m -
(II) P er ch lor a te ((+)-3). A solution of (+)-2 (3.0 g) in
dichloromethane (100 mL) was stirred for 2 h in the presence
of a solution of silver perchlorate (1.0 g) in water (1 mL). The
colorless organic layer, after the removal of AgCl and drying
(MgSO4), was treated with DMPP (1.0 g) at room temperature
for 3 days. Removal of the solvent gave (+)-3 as a colorless
oil, which was then crystallized from dichloromethane-diethyl
ether to give the complex as pale yellow crystals: yield 2.1 g
Finally, it is noteworthy that no Diels-Alder reaction
is observed between free diphenylvinylarsine and free
DMPP. Interestingly, the chiral palladium complex
activated cycloaddition is found to be at least 15 times
slower than the analogous process involving diphenylvi-
nylphosphine.6 To our knowledge, this is the first
example of a metal ion activated reaction involving
tertiary arsine ligands.
1
(50%); mp 188 °C dec; [R]D +135.5° (c 0.6, CH2Cl2); H NMR
(CDCl3, 300 MHz) δ 1.39 (s, 3H, CdCMe), 1.98 (s, 3H, CdCMe),
2.08 (d, 3H, 3J HH ) 6.2 Hz, CHMe), 2.30 (ddd, 1H, 3J PH ) 28.0
Hz, 2J HH ) 13.4 Hz, 3J HH ) 8.9 Hz, H6,endo), 2.70 (d, 3H, 4J PH
)
2
1.3 Hz, NMe), 2.78 (s, 1H, H1), 2.88 (d, 1H, J HH ) 13.4 Hz,
4
3
Exp er im en ta l Section
H
6,exo), 3.06 (d, 3H, J PH ) 3.6 Hz, NMe), 3.10 (dd, 1H, J PH )
3
38.0 Hz, J HH ) 8.9 Hz, H5), 3.77 (s, 1H, H4), 4.42 (qn, 1H,
3J HH ) 4J PH ) 6.2 Hz, CHMe), 6.80-7.80 (m, 21H, aromatics);
Reactions involving air-sensitive compounds were performed
under a positive pressure of purified nitrogen. NMRs were
recorded at 25 °C on Bruker ACF 300 and AMX 500 spectrom-
eters. Optical rotations were measured on the specified
solutions in a 1-dm cell at 25 °C with a Perkin-Elmer model
341 polarimeter. The enantiomerically pure form of bis(µ-
chloro)bis{(S)-1-[1-(dimethylamino)ethyl]-2-naphthyl-C,N}-
31P NMR (CDCl3, 36 MHz)
δ
118.1. Anal. Calcd for
C
40H42AsClNO4PPd: C, 56.6; H, 5.0; N, 1.7; Cl, 4.2; P, 3.7.
Found C, 56.3; H, 5.1; N, 1.9; Cl, 4.4; P, 3.7.
(14) Allen, D. G.; McLaughlin, G. M.; Robertson, G. B.; Steffen, W.
L.; Salem, G.; Wild, S. B. Inorg. Chem. 1982, 21, 1007.

Synthesis of a (P-Chiral) As-P Bidentate Ligand
Organometallics, Vol. 15, No. 17, 1996 3643
Dich lor o-{[1r,4r,5r(S ),7R ]-[5-(d ip h e n yla r sin o)-2,3-
d im et h yl-7-p h en yl-7-p h osp h a b icyclo[2.2.1]h ep t -2-en e-
As5,P 7}p a lla d iu m (II) ((-)-4). The Diels-Alder template
complex (+)-3 (1.5 g) was redissolved in acetone (40 mL) and
was treated with hydrochloric acid (10 M, 2 mL). The reaction
mixture was then refluxed for 15 min. The bright yellow
microcrystals of (-)-4 precipitated out during this period. The
product was then filtered and recrystallized from dichlo-
romethane-diethyl ether: Yield 1.0 g (91%); mp >280 °C dec;
× 0.23 × 0.23 mm obtained from an acetone solution of the
complex was used. The compound crystallized as an acetone
solvate. Crystallographic details are given in Table 2. Data
were measured on a Siemens P4/PC diffractometer with Mo
KR radiation (graphite monochromator) using ω scans. The
structure was solved by direct methods, and the non-hydrogen
atoms were refined anisotropically using full-matrix least
squares based on F2 to give R1 ) 0.035, and wR2 ) 0.085 for
2534 independent, observed reflections (|Fo| > 4σ(|Fo|), 2θ e
50°) and 280 parameters.16 The absolute stereochemistry of
(-)-4 was determined unambiguously by both an R-factor test
and the Flack parameter: R+ ) 0.035, R- ) 0.043, x ) 0.04-
(2). Atomic coordinates, bond lengths and angles, and thermal
parameters have been deposited at the Cambridge Crystal-
lographic Data Centre.
[R]D -34.1° (c 0.6, CH2Cl2); 1H NMR (CDCl3, 300 MHz) δ 1.49
3
(s, 3H, CdCMe), 1.60 (s, 3H, CdCMe), 1.95 (ddd, 1H, J PH
)
2
3
23.0 Hz, J HH ) 13.8 Hz, J HH ) 9.1 Hz, H6,endo), 2.47 (d, 1H,
3
3
2J HH ) 13.4 Hz, H6,exo), 3.04 (dd, 1H, J PH ) 45.8 Hz, J HH
)
9.1 Hz, H5), 3.07 (s, 1H, H1), 3.45 (s, 1H, H4), 7.40-8.10 (m,
15H, aromatics); 31P NMR (CDCl3, 36 MHz) δ 129.4. Anal.
Calcd for C26H26AsCl2PPd: C, 50.3; H, 4.2; P, 5.0. Found: C,
50.3; H, 4.2; P, 5.0. Treatment of a dichloromethane solution
of the dichloro complex with aqueous potassium cyanide for 2
h liberated the free ligand (S)-(-)-5 in 82% yield. Due to its
stereodynamic instability, the free ligand was not isolated and
was immediately used for recomplexation reactions. The
apparent inversion of configuration of the tertiary phosphine
that takes place when it is displaced from the metal is
consistent with the specification of Cahn et al. for absolute
configurations.15
Ack n ow led gm en t. This work was supported by the
National University of Singapore (Research Grant No.
920606) and we thank the EPSRC for the diffractome-
ter.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal-
lographic data, positional and thermal parameters, and bond
distances and angles for (-)-4 (5 pages). Ordering information
is given on any current masthead page.
Cr ysta l Da ta for (-)-4. A yellow prism of dimensions 0.23
OM9600351
(15) Cahn, R. S.; Ingold, C. K.; Prelog, V. Angew. Chem., Int. Ed.
Engl. 1966, 5, 385.
(16) SHELXTL PC version 5.03, Siemens Analytical X-Ray Instru-
ments, Inc., Madison, WI, 1994.