Synthesis and molecular structure of a palladium complex containing λ5-phosphinine-based SPS pincer ligand

Article abstract of DOI:10.1021/om0200961

2,6-Bis(diphenylphosphino)-3,5-diphenylphosphinine (1) reacts with elemental sulfur to yield the corresponding 2,6-bis(diphenylphosphine sulfide)-3,5-diphenylphosphinine (2). Reaction of ligand 2 with [Pd(COD)Cl₂] affords the square planar complex 4 resulting from the displacement of one chloride ligand. An X-ray crystal study reveals that complex 4 features a λ⁴-1-P-chlorophosphinine ligand which is bound to the palladium atom through the phosphorus atom. Reaction of 4 with methanol gives the corresponding P-OMe complex resulting from the nucleophilic substitution of the P-Cl bond. DFT calculations suggest that the formation of 4 results from the internal attack of the chloride counteranion onto the highly electrophilic P atom of the transient λ³-phosphinine complex [Pd(2)Cl]⁺ (3).

Full text of DOI:10.1021/om0200961

Organometallics 2002, 21, 2785-2788
2785
Syn th esis a n d Molecu la r Str u ctu r e of a P a lla d iu m
Com p lex Con ta in in g a λ5-P h osp h in in e-Ba sed SP S P in cer
Liga n d
Marjolaine Doux, Claire Bouet, Nicolas Me´zailles, Louis Ricard, and
Pascal Le Floch*
Laboratoire “He´teroe´le´ments et Coordination”, UMR CNRS 7653, Ecole Polytechnique,
91128 Palaiseau Cedex, France
Received February 8, 2002
Summary: 2,6-Bis(diphenylphosphino)-3,5-diphenylphos-
phinine (1) reacts with elemental sulfur to yield the
corresponding 2,6-bis(diphenylphosphine sulfide)-3,5-
diphenylphosphinine (2). Reaction of ligand 2 with [Pd-
(COD)Cl2] affords the square planar complex 4 resulting
from the displacement of one chloride ligand. An X-ray
crystal study reveals that complex 4 features a λ4-1-P-
chlorophosphinine ligand which is bound to the pal-
ladium atom through the phosphorus atom. Reaction of
4 with methanol gives the corresponding P-OMe com-
plex resulting from the nucleophilic substitution of the
P-Cl bond. DFT calculations suggest that the formation
of 4 results from the internal attack of the chloride
counteranion onto the highly electrophilic P atom of the
transient λ3-phosphinine complex [Pd(2)Cl]+ (3).
pendant diphenylphosphino groups. Thus, reaction of
2,6-bis(diphenylphosphino)-3,5-diphenylphosphinine (1)
with 2 equiv of elemental sulfur in toluene under reflux
cleanly affords the corresponding bis(diphenylphosphine
sulfide) phosphinine 2 in good yields (eq 1). The prefer-
ence for the sulfurization of the two phosphino groups
over that of the phosphorus atom lone pair of phosphi-
nine is obvious if we consider the respective basicity of
the two groups. Indeed, the weaker basicity of sp2-
hybridized phosphorus ligands relative to classical
tertiary phosphines is now well established.4a Thus, in
most cases, high temperatures and long heating periods
are usually required to achieve sulfurization of the
phosphorus atom lone pair of phosphinines.5 The for-
mulation of 2 was easily established on the basis of its
NMR data and elemental analysis.
In tr od u ction
Pincer ligands play an increasing role in coordination
chemistry and catalysis,1 where their utility has been
recently emphasized in some processes of importance
such as the activation of alkanes.2 Beside their intrinsic
structural rigidity, which provides a significant ther-
modynamic stability to their complexes, one of their
advantages relies on the possibility to finely tune the
reactivity of the metal center by adjusting the nature
and thus the electronic properties of the different pincer
ligands. This allows for the continuous interest that has
been devoted to the synthesis of various mixed systems
incorporating different heteroatoms such as O, S, N, and
P.1 As part of a continuing program aimed at developing
the use of sp2-hybridized phosphorus ligands in coordi-
nation chemistry and catalysis we recently focused our
attention on a mixed species featuring a central phos-
phinine unit and two pendant PPh2dS ligands. Herein,
we report on the surprising reactivity of this ligand
toward the [PdCl2] metal fragment.
To test the capability of 2 to act as a pincer ligand,
we attempted to react it with [Pd(COD)Cl2]. The reac-
tion proceeds cleanly in dichloromethane at room tem-
perature to yield the moisture-sensitive complex 4,
which exhibits an AB2 spin system in 31P NMR spec-
troscopy (eq 2). Though this pattern suggests that
coordination through the three binding sites occurred,
the upfield shift of the A part of the spectrum, δ (CD2-
Cl2) ) 94.9 (AB2, t, PA), cannot be assigned to a
phosphinine ring. Indeed, chemical shifts of free phos-
phinines and their corresponding complexes usually fall
in the range from 160 to 250 ppm.4b Interesting insights
1
on the structure of 4 are given by the H and 13C NMR
data, which reveal the presence of a strongly shielded
(3) See for example: (a) Le Floch, P.; Carmichael, D.; Ricard, L.;
Mathey, F. J . Am. Chem. Soc. 1993, 115, 10665-10670. (b) Wasch-
bu¨sch, K.; Le Floch, P.; Mathey, F. Organometallics 1996, 15, 1597-
1603. (c) Avarvari, N.; Le Floch, P.; Ricard, L.; Mathey, F. Organo-
metallics 1997, 16, 4089-4098.
(4) (a) Me´zailles, N.; Mathey, F.; Le Floch, P. Progress in Inorganic
Chemistry; Karlin, K. D., Ed.; J ohn Wiley and Sons: New York, 2001;
Vol. 49, pp 455-550. (b) Le Floch, P. In Phosphorus-Carbon Hetero-
cyclic Chemistry: The Rise of a New Domain; Mathey, F., Ed.;
Pergamon: New York, 2001; pp 485-533.
Resu lts a n d Discu ssion
Some years ago, we developed two synthetic ap-
proaches toward 2,6-bis(diphenyl)phosphino phosphin-
ines.3 These easily available phosphinines can be used
as convenient precursors for the synthesis of mixed
S-P-S pincer ligands through sulfurization of the two
(5) See for example: (a) Alcaraz, J .-M.; Mathey, F. J . Chem. Soc.,
Chem. Commun. 1984, 508-509. (b) Holah, D. G.; Hugues, A. N. J .
Chem. Soc., Chem. Commun. 1988, 493-495. (c) Le Floch, P.; Car-
michael, D.; Ricard, L.; Mathey, F. J . Am. Chem. Soc. 1991, 113, 667-
669.
(1) Albrecht A.; Van Koten G. Angew. Chem., Int. Ed. 2001, 40,
3750-3781.
(2) See for example: Liu, F.; Pak, E. B.; Singh, B.; J ensen, C. M.;
Goldman, A. S. J . Am. Chem. Soc. 1999, 121, 4086-4087.
10.1021/om0200961 CCC: $22.00 © 2002 American Chemical Society
Publication on Web 05/31/2002

2786 Organometallics, Vol. 21, No. 13, 2002
Notes
not totally preserved, and the phosphorus atom deviates
from the plane defined by the carbocyclic system by 8.3°.
Apart from this, the overall geometry around palladium
is square planar, as expected for a d8 center, and the
four ligands are nearly located in the same plane. On
the other hand, no significant distortion can be noted
on examining Pd-S bond distances, which are very close
to those recorded in phosphine sulfide palladium com-
plexes.8
Complex 4 may be used as precursor in the synthesis
of λ4-1-R-P functional derivatives of complex 4 through
the nucleophilic substitution of the chloride atom. Thus,
when methanol is added onto a solution of freshly
prepared 4, an immediate reaction takes place leading
to the formation of complex 5 and the concomitant
release of HCl (eq 3). Complex 5, which was recovered
as a very stable yellow solid, was characterized by
means of NMR spectroscopy and elemental analysis. In
31P NMR spectroscopy, the substitution of the chlorine
atom is only evidenced by a very small downfield shift
of ∆δ ) 2.4 ppm, but the presence of a methoxy group
in the H NMR spectrum, at 3.84 ppm with a J (H-PA)
) 14.4 Hz coupling constant, confirms the formulation
proposed. Apart from this, NMR data of 5, which are
very similar to those of precursor 4, do not deserve
further comments.
F igu r e 1. ORTEP view of one molecule of complex 4.
Ellipsoids are scaled to enclose 50% of the electron density.
The numbering is arbitrary and different from that used
in the assignment of NMR spectra. Relevant distances (Å)
and bond angles (deg): P1-C1, 1.743(3); C1-C2, 1.409-
(3); C2-C3, 1.402(3); C3-C4, 1.405(3); C4-C5, 1.398(3);
C5-P1, 1.754(2); C1-P2, 1.779(2); C5-P3, 1.777(3); P2-
S1, 2.036(1); P3-S2, 2.036(1); P1-Cl1, 2.139(1); P1-Pd1,
2.1748(8); S1-Pd1, 2.317(1); S2-Pd1, 2.3277(8); Pd1-Cl2,
2.3720(8). P1-C1-C2, 119.5(2); C1-C2-C3, 122.6(2); C2-
C3-C4, 126.2(2); C3-C4-C5, 122.4(2); C4-C5-P1, 119.9-
(2); C5-P1-C1, 105.7(1); P1-C1-P2, 110.3(1); P1-C5-
P3, 11.7(1); P1-Pd1-S1, 85.33(3); P1-Pd1-S2, 90.13(3);
S1-Pd1-Cl2, 92.97(3); S2-Pd1-Cl2, 91.73(3); P1-Pd1-
Cl2, 177.34(2); S1-Pd1-S2, 173.36(2).
1
3
H4 proton (δ (CD2Cl2) ) 5.84) and C2 (δ (CD2Cl2) ) 94.3)
and C4 (δ ) 119.9) carbon atoms. Usually, such a
shielding is encountered in λ5-phosphinines.4b
Now it is worth discussing the formation of complex
4. Phosphinines are relatively soft donors but display a
very strong π-accepting power.4a As we recently showed,
they logically behave as very efficient ligands for the
stabilization of reduced transition metal centers.9 On
the other hand, their stability is very sensitive to the
π-donating capability of the metal fragment to which
they are coordinated. Though some complexes have been
successfully characterized, coordination to Pd and Pt-
(II) fragments usually provokes a dearomatization of the
ring which makes the PdC double bond very sensitive
toward nucleophilic attack. Some 1,2-dihydrophosphi-
nine complexes resulting from the attack of water or
alcohols onto the PdC bond have been characterized.10
Another important factor that must not be neglected is
the substitution scheme of the ring. Indeed, studies on
P-W(CO)5 complexes have shown that the presence of
strong acceptor groups, at the R-position at phosphorus,
The formulation of 4 was definitively established
through an X-ray crystal structure study. An ORTEP
view of one molecule of 4 is presented in Figure 1, and
relevant bond distances and angles are listed below this.
As suggested by NMR data, the aromaticity of the
phosphinine ring has been destroyed and a chloride
atom is now bound to the phosphorus atom. Thus
complex 4 can be regarded as the coordination of a λ4-
1-P-chlorophosphinine anionic ligand onto a [PdCl]+
fragment. Though a λ4-phosphinine iron cyclopentadi-
enyl complex has already been synthesized and spec-
troscopically identified,6 4 is the first structurally
characterized complex.7 As expected, the phosphorus
atom is nearly tetrahedral and the chlorine atom points
out of the mean plane defined by the ring. Curiously,
the loss of aromaticity does not significantly modify the
internal bond distances and angles within the ring,
which roughly compare with that of a complexed λ3-
phosphinine. Nevertheless, the planarity of the ring is
(8) See for example: (a) Phillips J . R.; Poat J . C.; Slawin A. M. Z.;
Williams D. J .; Wood P. T.; Woolins J . D. J . Chem. Soc., Dalton Trans.
1995, 2369-2375. (b) Fornie´s J .; Navarro R.; Sicilia V.; Thomas M.
Inorg. Chim. Acta 1990, 168, 201-207.
(9) See for example: (a) Choua, S.; Sidorenkova, H.; Berclaz, T.;
Geoffroy, M.; Rosa, P.; Me´zailles, N.; Ricard, L.; Mathey, F.; Le Floch,
P. J . Am. Chem. Soc. 2000, 122, 12227-12234. (b) Rosa, P.; Me´zailles,
N.; Ricard, L.; Mathey, Le Floch, P.; J ean, Y. Angew. Chem., Int. Ed.
2001, 40, 1251-1253. (c) Rosa, P. Me´zailles, N. Ricard, L.; Mathey, F.
Le Floch, P. Angew. Chem., Int. Ed. 2000, 39, 1823-1826
(10) (a) Schmid, B.; Venanzi, L.; Albinati, A.; Mathey, F. Inorg.
Chem. 1991, 30, 4693-4699. (b) Carmichael, D.; Le Floch, P.; Mathey,
F. Phosphorus, Sulfur, Silicon 1993, 77, 255.
(6) Dave, T.; Berger, S.; Bilger, E.; Kaletsch, H.; Pebler, J .; Knecht,
J .; Dimroth, K. Organometallics 1985, 4, 1565-1572.
(7) Some complexes in which the phosphinine ring behaves as a
bridging bidentate ligand between two metallic fragments were also
characterized; see for example: (a) Schmid, B.; Venanzi, L.; Gerfin, T.;
Gramlich, V.; Mathey, F. Inorg. Chem. 1992, 31, 5117-5122. (b) Reetz,
M.; Bohres, E.; Goddard, R.; Holthausen, M. C.; Thiel, W. Chem. Eur.
J . 1999, 7, 2101-2108.

Notes
Organometallics, Vol. 21, No. 13, 2002 2787
hexanes were obtained by distillation from Na/benzophenone,
and dry CH2Cl2 and CDCl3 from P2O5. Dry CD2Cl2 was distilled
and stored, like CDCl3, on 4 Å Linde molecular sieves. Nuclear
magnetic resonance spectra were recorded on a Bruker Avance
300 spectrometer operating at 300 MHz for 1H, 75.5 MHz for
13C, and 121.5 MHz for 31P. Solvent peaks are used as internal
reference relative to Me4Si for 1H and 13C chemical shifts
(ppm); 31P chemical shifts are relative to a 85% H3PO4 external
reference, and coupling constants are expressed in hertz. The
following abbreviations are used: b; broad, singlet; d, doublet;
t, triplet; m, multiplet; p, pentuplet; sext, sextuplet; sept,
septuplet; v, virtual. Mass spectra were obtained at 70 eV with
a HP 5989B spectrometer coupled to a HP 5980 chromatograph
by the direct inlet method. Elemental analyses were performed
by the “Service d’analyse du CNRS”, at Gif sur Yvette, France.
Phosphinine 1 was prepared according to a published pro-
cedure.3c
Sch em e 1
also activate the reactivity of the PdC system.11 To
estimate the importance of these two effects, a series of
DFT calculations were carried out. As can be shown in
Scheme 1, the introduction of phosphino groups at the
C2 and C6 positions significantly enhances the positive
charge (NBO charges) at phosphorus, which ranges from
+0.66 in the parent compound I to +0.72 in the 2,6-
bis(diphosphino)phosphinine III. Furthermore, this ef-
fect is slightly reinforced when the phosphino groups
are replaced by the sulfide derivatives such as in IV
(+0.75). As expected, coordination to the [Pd-Cl]+
fragment is another important destabilizing factor
which also enhances the polarization of the ring. Thus,
in the theoretical structure [Pd(IV)Cl]+ V, the NBO
charge at phosphorus is very high (+0.95).
On the basis of these data, a tentative mechanism,
which is depicted in eq 4, can be proposed to explain
the formation of complex 4. In a first step, coordination
of ligand 2 leads to the formation of the transient
cationic complex 3, which undergoes a nucleophilic
attack of the chloride counteranion at phosphorus to
finally yield complex 4. It must be noted that additional
experiments aimed at converting complex 4 into the
phosphinine complex 3 by abstraction of the chloride
substituents were also attempted using AgBF4 or TlPF6
as chloride abstractor. Whatever the experimental
conditions used (solvent, amount and nature of the salt),
these experiments exclusively led to complicated mix-
tures of compounds that could not be identified.
2,6-Bis(d ip h en ylp h osp h in e su lfid e)-3,5-d ip h en ylp h os-
p h in in e (2). A solution of 2,6-bis(diphenylphosphino)-3,5-
diphenylphosphinine (1) (4.00 g, 6.5 mmol) and elemental
sulfur (0.415 g, 13 mmol) were heated in toluene for 12 h at
120 °C. The reaction mixture was cooled to room temperature
and filtered, and the white solid collected on the frit was
washed with toluene. After drying, sulfide 2 was recovered as
a pale yellow powder. Yield: 4.23 g (96%). 31P NMR (CDCl3):
δ 43.4 (AB2, d, 2J (PA-PB) ) 115, PBPh2), 253.1 (AB2, t, PA). 1H
NMR (CDCl3): δ 6.94 (t, 4J (H-PB) ) 3, 1H, H4), 6.96-7.76
(m, 30H, CH of C6H5). 13C NMR (CDCl3): δ 127.1-131.6 (m,
CH of C6H5), 132.5 (ABX, dd, 1J (C-PB) ) 48.9, 3J (C-PA) )
5.4, Cipso of PPh2), 133.0-133.3 (m, CH of C6H5), 140.2 (t, 3J (C-
PB) ) 4, C4), 140.6 (m, C3), 154.3 (d, 3J (C-PA) ) 9, Cipso of
1
1
C6H5), 159.7 (ABB′X, ddd, J (C-PA) ) 85.3, J (C-PB) ) 66.2,
3J (C-PB′) ) 12.4, C2). MS (CH2Cl2): m/z 683 (M)+, 464 (M -
PPh2S)+. Anal. Calcd for C41H31P3S2: C, 72.34; H, 4.59.
Found: C, 72.08; H, 4.48.
Syn th esis of Com p lex 4. In the glovebox, a mixture of [Pd-
(COD)Cl2] (125 mg, 0.44 mmol) and 2,6-bis(diphenylphosphine
sulfide)-3,5-diphenylphosphinine (2) (300 mg, 0.44 mmol) was
stirred for 5 min in CH2Cl2 (5 mL). After the evaporation of
the solvent, the yellow solid obtained was washed several times
with hexanes (3 × 5 mL). After drying, complex 4 was
recovered as a yellow powder. Yield: 370 mg (98%). 31P NMR
(CD2Cl2): δ 52.6 (AB2, d, 2J (PA-PB) ) 115.3, PBPh2) 94.9 (AB2,
t, PA). 1H NMR (CD2Cl2): δ 5.84 (AB2X, dt, 4J (H-PB) ) 4.8,
4J (H-PA) ) 3.3, 1H, H4), 6.60-7.63 (m, 30H, CH of C6H5).
13C NMR (CD2Cl2): δ 94.3 (ABB′X, ddd, 1J (C-PA) ) 91.2, 1J (C-
PB) ) 55.2, 3J (C-PB′) ) 7.6, C2), 119.9 (AB2X, dt, 4J CA ) 18.2,
4J (C-PB) ) 9.1, C4), 127.8-129.0 (m, CH of C6H5), 129.7 (ABX,
dd, 1J (C-PB) ) 86.1, 3J (C-PA) ) 10.2, Cipso of PPh2), 130.5
(ABX, dd, 1J (C-PB) ) 86.7, 3J (C-PA) ) 4.7, Cipso of PPh2),
2
132.3-132.9 (m, CH of C6H5), 139.3 (ABB′X, dt, J (C-PA) )
8.0, 2J (C-PB) ) 4J (C-PB′) ) 3.0, C3), 157.8 (d, 3J (C-PA) )
6.3, Cipso of C6H5). Complex 4 turns out to be too moisture
sensitive to give satisfactory elemental data.
Syn th esis of Com p lex 5. A solution of [Pd(COD)Cl2] (125
mg, 0.44 mmol) and 2,6-bis(diphenylphosphine sulfide)-3,5-
diphenylphosphinine 2 (300 mg, 0.44 mmol) was stirred for 5
min in CH2Cl2 (10 mL), and methanol (200 µL, 0.49 mmol)
was added. After stirring for 10 min, the solvent was removed
under vacuum, and the yellow powder obtained was washed
several times with hexanes (3 × 5 mL). After drying, complex
5 was recovered as a yellow powder. Yield: 365 g (97%). 31P
NMR (CH2Cl2): δ 51.1 (AB2, t, 2J (PA-PB) ) 102.3, PBPh2), 97.3
(AB2, t, PA); 1H NMR (CD2Cl2): δ 3.84 (d, 3J (H-PA) ) 14.4,
CH3O), 5.59 (AB2X, dt, J (H-PB) ) 8.9, J (H-PA) ) 4.7, 1H,
H4), 6.70-7.71 (m, 30H, CH of C6H5). 13C NMR (CD2Cl2): δ
30.0 (s, CH3), 93.5 (ABB′X, ddd, 1J (C-PA) ) 91.3, 1J (C-PB) )
67.2, 3J (C-PB′) ) 7.4, C2), 115.6 (m, C4), 127.6-128.8 (m, CH
of C6H5), 130.6 (ABX, dd, 1J (C-PB) ) 84.8, 3J (C-PA) ) 7.5,
Cipso of PPh2), 131.6 (ABX, dd, 1J (C-PB) ) 78.5, 3J (C-PA) )
4.5, Cipso of PPh2), 132.1-132.6 (m, CH of C6H5), 140.1 (ABB′X,
In conclusion, we have established that the presence
of π-accepting groups at the R-positions at phosphorus
as well as coordination to a Pd(II) fragment strongly
dearomatize the phosphinine nucleus, which becomes
highly sensitive toward nucleophilic attacks. This method
provides an easy entry to λ4-1-R-P functional phosphi-
nine complexes.
4
4
Exp er im en ta l Section
All reactions were routinely performed under an inert
atmosphere of argon or nitrogen by using Schlenk and glovebox
techniques and dry deoxygenated solvents. Dry THF and
(11) Carmichael, D. Le Floch, P.; Mathey, F. Organometallics 1991,
10, 2432-2436.

2788 Organometallics, Vol. 21, No. 13, 2002
Notes
dt, 2J (C-PA) ) 8.2, 2J (C-PB) ) 4J (C-PB′) ) 3.2, C3), 156.8 (d,
3J (C-PA) ) 2.3, Cipso of C6H5). Anal. Calcd for C42H34ClOP3-
PdS2: C, 59.09; H, 4.01. Found: C, 58.85; H, 4.10.
Ta ble 1. Cr ysta l Da ta a n d Str u ctu r a l Refin em en t
Deta ils for th e Str u ctu r e of Com p ou n d 4
emp formula
fw
C41H31Cl2P3PdS2.CH2Cl2
942.91
Th eor etica l Meth od s. Geometry optimizations were car-
ried out by means of a pure gradient-corrected exchange
functional and the Lee-Yang-Parr nonlocal correlation func-
tional BLYP as implemented in the Gaussian 98 program.12,13
A 6-31G(d) basis set was used for H, C, P, and S atoms. The
Hay-Wadt small core relativistic effective core-potential with
a valence shell of double-ú quality (441/2111/41) was used on
Pd,14 and the Stuttgart pseudopotential with a (31/31/1) basis
set was used for Cl in calculations of complex V.15 This type
of basis set derives from the basis set II successfully used by
Frenking and co-workers in many studies.16 Vibrational
frequencies of the stationary points were calculated at B3LYP
with numerical second derivatives of the energy with respect
to the coordinates. The structures calculated were located at
minima on the potential-energy surface. The bonding situation
of the optimized structures was analyzed using the natural
bond orbital (NBO) method developed by Weinhold.17
temp, K
wavelength, Å
cryst syst
space group
a, Å
150.0(10)
0.71069
monoclinic
P21/c
14.316(5)
22.241(5)
14.356(5)
117.510(5)
4054(2)
4
1.545
0.974
b, Å
c, Å
â, deg
V, Å3
Z
D
calcd, g/cm3
µ cm-1
h,k,l ranges
-18, 18; -18, 26; -18, 18
cryst size, mm3
cryst color and habit
2θmax, deg
0.20 × 0.20 × 0.20
orange cube
27.48
16 947
9282
7399
0.0369
0.1025
1.056
no. of reflns measd
no. of ind reflns
no. of reflns used
R1a [I>2σ(I)]
X-r a y Str u ctu r a l Deter m in a tion . Single crystals of com-
pound 4 suitable for X-ray crystallography were obtained by
diffusing hexane into a dichloromethane solution of the
compound. Data were collected on a Nonius Kappa CCD
diffractometer using an Mo KR (λ ) 0.71069 Å) X-ray source
and a graphite monochromator. Experimental details are
described in Table 1. The crystal structures were solved using
wR2b [I>2σ(I)]
GOF on F2
largest diff peak, e Å-3
1.290 (0.090)/-0.960 (0.090)
a
b
2
2
R1 ) ∑|Fo| - |Fc||/ ∑|Fo|. wR2 ) (∑w||Fo| - |Fc|| /∑w|Fo| )1/2
.
SIR 9718 and SHELXL-97.19 ORTEP drawings were made
using ORTEP III for Windows.20
(12) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098-3108. (b) Perdew,
J . P. Phys. Rev. B 1986, 33, 8822-8832.
Ack n ow led gm en t. The authors thank the CNRS
and the Ecole Polytechnique for supporting this work.
(13) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J r. J .
A.; Stratmann, R. E.; Burant, J . C.; Dapprich, S.; Millam, J . M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J .;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J .; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J . B.; Cioslowski, J .; Ortiz, J . V.; Baboul, A. G.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.;
Martin, R. L.; Fox, D. J .; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; J ohnson, B.; Chen,
W.; Wong, M. W.; Andres, J . L.; Gonzalez, C.; Head-Gordon, M.;
Replogle, E. S.; Pople, J . A. Gaussian 98, Revision A.9; Gaussian,
Inc.: Pittsburgh, PA, 1998.
Su p p or tin g In for m a tion Ava ila ble: Listings of crystal
data, atomic coordinates, including H atoms and equivalent
isotropic displacement parameters, bond lengths, and bond
angles. Listings of bond distances, bond angles, and NBO
charges for calculated structures I-V. This material is avail-
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