Generation and trapping of a superelectrophilic terminal fluorophosphinidene complex

Article abstract of DOI:10.1021/om050870v

Comparative DFT calculations on terminal phosphinidene complexes [MeP-Cr(CO)₅] and [FP-Cr(CO)₅] show a huge increase of the positive charge at P when replacing the methyl by the fluorine substituent (NBO charges +0.60 and +0.94). The electrophilic fluorination of the (3,4-dimethylphospholyl)pentacarbonylmolybdenum anion by Selectfluor gives a straightforward access to the corresponding 1-fluoro-3,4-dimethylphosphole molybdenum complex. This complex reacts as a conjugated diene with dimethyl acetylenedicarboxylate to give the corresponding 7-fluoro-7-phosphanorbornadiene complex. This bicyclic species is a precursor of [FP-Mo(CO)₅] at 120°C in xylene, as shown by trapping reactions with diphenylacetylene and 2,3-dimethylbutadiene.

Full text of DOI:10.1021/om050870v

540
Organometallics 2006, 25, 540-543
Generation and Trapping of a Superelectrophilic Terminal
Fluorophosphinidene Complex
Carine Compain, Bruno Donnadieu, and Franc¸ois Mathey*
UCR-CNRS Joint Research Chemistry Laboratory, Department of Chemistry, UniVersity of California
RiVerside, RiVerside, California 92521-0403
ReceiVed October 7, 2005
Summary: ComparatiVe DFT calculations on terminal phos-
phinidene complexes [MeP-Cr(CO)5] and [FP-Cr(CO)5] show
a huge increase of the positiVe charge at P when replacing the
methyl by the fluorine substituent (NBO charges +0.60 and
+0.94). The electrophilic fluorination of the (3,4-dimethylphos-
pholyl)pentacarbonylmolybdenum anion by Selectfluor giVes a
straightforward access to the corresponding 1-fluoro-3,4-
dimethylphosphole molybdenum complex. This complex reacts
as a conjugated diene with dimethyl acetylenedicarboxylate to
giVe the corresponding 7-fluoro-7-phosphanorbornadiene com-
plex. This bicyclic species is a precursor of [FP-Mo(CO)5] at
120 °C in xylene, as shown by trapping reactions with
diphenylacetylene and 2,3-dimethylbutadiene.
comparative DFT calculations7 on [MeP-Cr(CO)5] and [FP-Cr-
(CO)5] using the B3LYP functional8,9 with 6-31G(d) basis sets
for all atoms except Cr (lanl2dz). Our results on [MeP-Cr(CO)5]
are close to those of Nguyen and co-workers on the same
species.10 The replacement of the methyl by the fluorine
substituent induces some significant changes in the geometry
of the complex. The R-P-Cr angle decreases from 112.5° (Me)
to 109.4° (F). Similarly, the P-Cr bond is shortened from 2.270
Å (Me) to 2.207 Å (F). This does not necessarily mean that the
strength of the P-Cr bond has increased since the out-of-phase
P-Cr-trans-C(O) vibration is displaced at the same time toward
lower frequencies (from 440.8 to 435.8 cm-1). The absence of
correlation between bond lengths and bond strengths has already
been noticed in [R3P-M(CO)5] complexes.11 Curiously the
LUMO (essentially the phosphorus py orbital perpendicular to
the Cr-P-R plane) is not affected by the replacement of Me
by F, but the HOMO (essentially the in-plane lone pair at P) is
lowered by 0.49 eV. Drastic changes are also observed on the
NBO charges. The positive charge at P increases from 0.60 to
0.94 e. It is thus clear that [FP-Cr(CO)5] will behave as a
superelectrophile.
Introduction
The η1-coordination of phosphinidenes to transition metals
yields terminal complexes whose chemistry heavily depends on
the nature of the coordinated metal. With low-valent metals at
the right of the periodic table, phosphorus behaves as an elec-
trophilic center, whereas, with high-valent metals at the left of
the periodic table, phosphorus behaves as a nucleophilic center.
Typical examples include [RP-M(CO)5] (M ) Cr, Mo, W)
(unstable, electrophilic),1,2 [RP-Ru(CO)2Cp*]+ (stable, electro-
philic),3 and [RP-Zr(PMe3)Cp2] (stable, nucleophilic).4 This
dichotomy has been studied in some depth from a theoretical
standpoint.5,6 Although, intuitively, the substituent at P might
also play a significant role in the chemical reactivity of these
complexes, this question has not received any attention until
now. In the case of the widely used [RP-M(CO)5] complexes,
it was more precisely quite interesting to see whether it was
possible to enhance the phosphorus electrophilicity that governs
all of their chemistry. From this standpoint, the choice of the
fluorine substituent was quite obvious. In this report, we com-
pare [MeP-Cr(CO)5] and [FP-Cr(CO)5] from a theoretical stand-
point. In so doing, we show that the fluorophosphinidene com-
plex displays a much higher electrophilicity than its methyl
analogue. Besides, we describe the generation and trapping of
the molybdenum analogue [FP-Mo(CO)5], thus opening the
possibility to study the chemistry of these superelectrophilic
species.
From a practical standpoint, building an appropriate precursor
of [FP-M(CO)5] is not trivial since the fluorine substituent is
known to drastically enhance the stability of the rings whose
thermal cycloreversion normally generates the phosphinidene
species. This is especially true for the 7-phosphanorbornadi-
ene system. A (7-fluoro-7-phosphanorbornadiene)pentacarbonyl-
tungsten complex has been shown to be stable up to 160 °C.12
(5) Frison, G.; Mathey, F.; Sevin, A. J. Organomet. Chem. 1998, 570,
225.
(6) Ehlers, A. W.; Baerends, E. J.; Lammertsma, K. J. Am. Chem. Soc.
2002, 124, 2831.
(7) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
Revision B.05; Gaussian, Inc.: Pittsburgh, PA, 2003.
Results and Discussion
To evaluate the effect of the introduction of a fluorine
substituent in [RP-M(CO)5], we first decided to perform
(8) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(1) Mathey, F.; Tran Huy, N. H.; Marinetti, A. HelV. Chim. Acta 2001,
84, 2938.
(2) Lammertsma, K.; Vlaar, M. J. M. Eur. J. Org. Chem. 2002, 4, 1127.
(3) Sterenberg, B. T.; Udachin, K. A.; Carty, A. J. Organometallics 2001,
20, 2657.
(9) Lee, C.; Yang, W.; Parr. R. G. Phys. ReV. B 1988, 37, 785.
(10) Creve, S.; Pierloot, K.; Nguyen, M. T.; Vanquickenborne, L. G.
Eur. J. Inorg. Chem. 1999, 107.
(11) Frenking, G.; Wichmann, K.; Fro¨hlich, N.; Grobe, J.; Golla, W.;
Le Van, D.; Krebs, B.; La¨ge, M. Organometallics 2002, 21, 2921.
(12) Alcaraz, J.-M.; Svara, J.; Mathey, F. New J. Chem. 1986, 10, 321.
(4) Breen, T. L.; Stephan, D. W. J. Am. Chem. Soc. 1995, 117, 11914.
10.1021/om050870v CCC: $33.50 © 2006 American Chemical Society
Publication on Web 12/14/2005

Notes
Organometallics, Vol. 25, No. 2, 2006 541
We reasoned that a stronger P-M bond is likely to induce
a stronger stabilization of the phosphorus ligand. In the
[R3P-M(CO)5] series, the P-M bond dissociation energies
always vary in the order W-P > Cr-P > Mo-P.11 For
example, for R ) Me, the computed data are as follows: W-P
43.8, Cr-P 41.2, and Mo-P 37.9 kcal mol-1 11
. Since we were
looking for a less stable 7-phosphanorbornadiene complex, it
was thus tempting to investigate the synthesis and thermal
splitting of a (7-fluoro-7-phosphanorbornadiene)pentacar-
bonylmolybdenum complex. Due to the higher reactivity of the
Mo(CO)5 group toward a strong chlorinating reagent such as
SO2Cl2, it was difficult to transpose the previously described
synthesis of the (1-fluoro-3,4-dimethylphosphole)pentacarbon-
yltungsten starting product, which implies a chlorination of the
complexed phospholide, followed by a chlorine to fluorine
exchange.12 Our new, easier, and simpler approach involves the
direct electrophilic fluorination of the (3,4-dimethylphospholyl)-
pentacarbonylmolybdenum anion (1) by Selectfluor at low
temperature (eq 1).13
Figure 1. ORTEP drawing of one molecule of 3. Ellipsoids are
scaled to enclose 50% of the electronic density. Main bond lengths
(Å) and angles (deg): P(1)-Mo(1) 2.4521(5), P(1)-F(1) 1.5998-
(10), P(1)-C(1) 1.8605(17), P(1)-C(4) 1.8616(16); C(1)-P(1)-
C(4) 80.29(7), F(1)-P(1)-C(1) 103.65(7), F(1)-P(1)-C(4) 104.23-
(6), F(1)-P(1)-Mo(1) 109.02(4).
7-phosphanorbornadiene complex.14 The P-C bonds are also
significantly shorter than in the normal case: 1.8605-1.8616-
(16) vs 1.877-1.878(3) Å.14
Despite these negative indications, we decided to study the
decomposition of 3 in the presence of trapping reagents. The
first experiments were carried out with an excess of diphenyl-
acetylene. At 120 °C overnight in xylene, the 31P resonance of
3 disappears and is replaced by a new resonance at -16.7 ppm
(1JP-F ) 1127.5 Hz). The product proved to be very sensitive
to hydrolysis and even more difficult to purify than the other
P-F compounds. The most significant clue to establish its
structure was the exact mass as measured by FAB: found:
465.9326; calcd for C19H10FMoO5P (98Mo): 465.930395. The
huge upfield shift of the phosphorus resonance when compared
to 2 and 3 was also a strong indication in favor of the three-
membered ring structure. The recorded 31P chemical shift is
compatible with the data on free 1-fluorophosphirenes.15 Thus
the formulation of this product as a fluorophosphirene complex
(4) is highly likely, but we have no X-ray crystal structure
analysis to definitively prove it. Another reaction was thus
performed with an excess of 2,3-dimethybutadiene. The reaction
is complete after 11 h at 120 °C (eq 3).
The reaction is easily monitored by 31P NMR spectroscopy.
The 31P resonance of the starting complexed anion (δ31P(1) -35
ppm) is replaced by the resonance of the fluorophosphole
complex (2), which appears as a doublet at 191.5 ppm in CDCl3
(1JP-F ) 914.4 Hz). This fluoro complex tends to spontaneously
dimerize to give a [4+2] dimer (1 h at 50 °C in toluene). This
dimerization competes with the [4+2] cycloaddition with
dimethyl acetylenedicarboxylate, which was carried out in dilute
solution with a 4-fold excess of the acetylenic reagent (eq 2).
The cycloadduct 3 is characterized by the low-field shift of
its 31P resonance: δ31P(3) 239, 1JP-F ) 1016 Hz (CDCl3). It is
accompanied by ca. 10% of an isomer, 3a, which likely
corresponds to the cycloaddition of the CC triple bond on the
1
other face of phosphole 2: δ31P(3a) 269, JP-F ) 1031 Hz
(CDCl3). The mass spectrum (FAB) shows the molecular peak
of 3 at m/z 511 (M + 1, 98Mo) and the M - 5CO peak at m/z
370. A striking demonstration of the enhanced stability provided
to the bicyclic structure by the fluorine substituent is the total
absence of peaks corresponding to the terminal phosphinidene
complex [FP-Mo(CO)5] at m/z 288 (molecular peak) and 260
(M - CO, base peak), contrary to what happens with other
7-phosphanorbornadiene complexes.14 The stereochemistry of
3 was established by an X-ray crystal structure analysis (Figure
1). As usual, the cycloaddition takes place on the least hindered
side of the phosphole ring corresponding to the fluorine
substituent. At 80.3°, the C(1)-P(1)-C(4) bridge angle is
somewhat larger than that (79.0°) found in a more classical
The product of the reaction (5) shows a 31P resonance at 217.2
ppm in C6D6 (1JP-F ) 852.3 Hz). Its phospholene structure was
established by X-ray crystal structure analysis (Figure 2). The
phosphanorbornadiene complex 3 is thus a genuine precursor
of the fluorophosphinidene complex [FP-Mo(CO)5].
(14) Marinetti, A.; Mathey, F.; Fischer, J.; Mitschler, A. J. Chem. Soc.,
Chem. Commun. 1982, 667.
(13) Sankar Lal, G.; Pez, G. P.; Syvret, R. G. Chem. ReV. 1996, 96,
1737.
(15) Wagner, O.; Ehle, M.; Birkel, M.; Hoffmann, J.; Regitz, M. Chem.
Ber. 1991, 124, 1207.

542 Organometallics, Vol. 25, No. 2, 2006
Notes
(CDCl3): δ 239 (1JP-F ) 1016 Hz). H NMR (CDCl3): δ 2.07 (s,
1
2
3
6H, CH3), 3.87 (s, 6H, OCH3), 3.90 (pseudo-t, 2H, JH-P ) JH-F
) 12.3 Hz, P-CH). 13C NMR (CDCl3): δ 16.1 (s, CH3), 52.82 (s,
1
2
O-CH3), 62.17 (pseudo-t, JC-P ) JC-F ) 8.1 Hz, C-P), 134.28
2
(d, JC-P ) 20.7 Hz, dCMe), 141.96 (s, dC-CO2Me), 164.72 (s,
COO), 203.39 (cis-CO). EIMS (98Mo): m/z: 510 (M+, 30%), 426
(M - 3CO, 58%), 370 (M - 3CO, 100%).
(1-Fluoro-2,3-diphenylphosphirene)pentacarbonylmolyb-
denum (4). Precursor 3 (0.45 g, 9 × 10-4 mol) and tolan (0.64 g,
3.3 × 10-3 mol) in xylene (4 mL) were heated overnight at 120
°C in a Schlenk tube. After evaporation of the solvent, the residue
was chromatographed on Florisil with hexane as the eluent. Yield
of white crystals: 0.037 g, 9%. 31P NMR (CDCl3): δ -16.7 (1JP-F
) 1127 Hz). 13C NMR (CDCl3): δ 149.4 (pseudo-t, 1JC-P ) 2JC-F
2
) 19 Hz, dCPh), 204.2 (d, JC-P ) 12.7 Hz, CO cis), 208.6 (d,
2JC-P ) 44.6 Hz, CO trans). Exact mass: calcd for C19H10FMoO5P
(98Mo) 465.930395; found (FAB) 465.9326.
(1-Fluoro-3,4-dimethylphosphol-3-ene)pentacarbonylmolyb-
denum (5). Precursor 3 (0.32 g, 6.3 × 10-4 mol) and 2,3-
dimethylbutadiene (0.52 g, 6.3 × 10-3 mol) in xylene (8 mL) were
heated for 11 h at 120 °C in a Schlenk tube. After evaporation of
the solvent, the residue was chromatographed on Florisil with
hexane/dichloromethane (4:1) as the eluent. Yield of white crys-
Figure 2. ORTEP drawing of one molecule of 5. Ellipsoids are
scaled to enclose 50% of the electronic density. Main bond lengths
(Å) and angles (deg): P(1)-Mo(1) 2.4456(5), P(1)-F(1) 1.5975-
(10), P(1)-C(1) 1.8201(15), P(1)-C(4) 1.8207(15); C(1)-P(1)-
C(4) 93.91(7), F(1)-P(1)-C(1) 99.34(7), F(1)-P(1)-C(4) 99.38-
(7), F(1)-P(1)-Mo(1) 113.44(4).
1
tals: 0.1 g, 44%. 31P NMR (CDCl3): δ 217 (1JP-F ) 857 Hz). H
NMR (C6D6): δ 1.22 (s, 6H, Me), 2.33 (m, 4H, CH2). 13C NMR
3
1
(C6D6): δ 15.7 (d, JC-P ) 8.1 Hz, CH3), 49.8 (dd, JC-P ) 16.1
2
2
Hz, JC-F ) 13.8 Hz, P-CH2), 128.3 (s, CdC), 204.9 (d, JC-P
10.3 Hz, CO cis), 209.1 (d, JC-P ) 28.8 Hz, CO trans).
)
Experimental Section
2
General Procedures. Reactions were performed under nitrogen
using oven-dried glassware. Dry tetrahydrofuran was obtained by
distillation from Na/benzophenone. Due to their high hydrolytic
sensitivity, the products were purified by chromatography on
Florisil. The modest isolated yields do not reflect the intrinsic yields
of the reactions. Elemental analyses were precluded by the
instability of the products. Nuclear magnetic resonance spectra were
obtained on a Bruker Avance 3000 and Varian Inova spectrometer
operating at 300.13 MHz for 1H, 75.45 MHz for 13C, and 121.496
MHz for 31P. Chemical shifts are expressed in parts per million
downfield from external TMS (1H and 13C) and external 85%
H3PO4 (31P). Mass spectra were obtained on VG 7070 and Hewlett-
Packard 5989A GC/Ms spectrometers. Selectfluor was bought from
Aldrich and used as received.
X-ray Structure Characterizations of 3 and 5. Measurements
were carried out using a low-temperature device at T ) 100(2) K
on a Bruker X8 APEX KAPPA-CCD X-ray diffractometer system16
using Mo radiation (λ ) 0.71073 Å). The automated strategy
determination program COSMO17 was used to find diffraction
experiments on the basis of phi and omega scans. Frames were
integrated using the Bruker SAINT version 7.06A software18 and
using a narrow-frame integration algorithm. The integrated frames
yielded the following.
For C17H14FO9PMo (3) a total of 18 350 reflections were
collected at a maximum 2θ angle of ) 57.22° (4888 independent
reflections, Rint ) 0.0202, Rsig ) 0.0207, completeness ) 92.8%)
and 4327 (88.52%) reflections were found greater than 2σ(I). Space
group P2(1)/c. The unit cell parameters were a ) 7.8918(6) Å, b
) 32.555(3) Å, c ) 8.3793(7) Å, R ) 90.0°, â ) 107.2600(10)°,
γ ) 90.0°, V ) 2055.8(3) Å3, Z ) 4, calculated density Dc ) 1.642
Mg/m3.
For C11H10FO5PMo (5) a total of 10 783 reflections were
collected at a maximum 2θ angle of 56.56° (3382 independent
reflections, Rint ) 0.0214, Rsig ) 0.0236, completeness ) 98.0%)
and 3000 (88.70%) reflections were found greater than 2σ(I). Space
group P2(1)/n. The unit cell parameters were a ) 11.0324(17) Å,
b ) 6.8608(11) Å, c ) 18.504(3) Å R ) 90.0°, â ) 97.287(2)°, γ
) 90.0°, V ) 1252.9(2) Å3, Z ) 4, calculated density Dc ) 1.760
Mg/m3.
Absorption corrections were applied for all data using the
SADABS program included in the SAINTPLUS software pack-
age.18 Direct methods using the Sir92 program19 were used for
resolution. Direct methods of phase determination followed by a
subsequent difference Fourier map led to an electron density map
from which most of the non-hydrogen atoms were identified in the
asymmetry unit. With subsequent isotropic refinement and some
Fourier differences synthesis, all non-hydrogen atoms were identi-
(1-Fluoro-3,4-dimethylphosphole)pentacarbonylmolyb-
denum (2). To a THF solution (40 mL) of 3,4-dimethyl-1-phen-
ylphosphole (3 g, 16 × 10-3 mol) was added an excess of lithium
wire (with 1% of Na). The mixture was stirred overnight at RT.
The excess of lithium was removed, and AlCl3 (0.5 g) was added
t
(45 min stirring), then BuCl (0.4 mL, 30 min stirring). To the
resulting solution was slowly added molybdenum hexacarbonyl (4.2
g, 16 × 10-3 mol) at 0 °C over 35 min. After 45 min, the solution
was monitored by 31P NMR and cooled at -70 °C. A solution of
Selectfluor (4.2 g, 12 × 10-3 mol) in acetonitrile (70 mL) was
added and the mixture stirred for 30 min at -70 °C, then warmed
to RT. After evaporation of the solvents, the residue was extracted
with pentane. The evaporation of pentane left yellow crystals of 2.
Yield: 2.3 g (40%). 31P NMR (CDCl3): δ 191.5 (1JP-F ) 914 Hz).
4
1H NMR (CDCl3): δ 2.09 (d, 6H, JH-P ) 2.7 Hz, CH3), 6.17 (d,
2H, 2JH-P ) 39.2 Hz, dCH). 13C NMR (CDCl3): δ 16.9 (d, 3JC-P
1
2
) 11.5 Hz, CH3), 125.3 (dd, JC-P ) 36.8 Hz, JC-F ) 10.3 Hz,
2
2
dCH), 151.0 (d, JC-P ) 11.5 Hz, Me-Cd), 204.1 (d, JC-P
10.3 Hz, CO cis), 208.0 (d, JC-P ) 26.5 Hz, CO trans).
)
2
7-Fluoro-7-phosphanorbornadiene Pentacarbonylmolybde-
num Complex (3). A mixture of fluorophosphole complex 2 (0.2
g, 5.5 × 10-4 mol) and dimethyl acetylenedicarboxylate (0.31 g,
2.2 × 10-3 mol) in toluene (2 mL) was heated at 70 °C for 1 h.
After evaporation of the solvent, the residue was purified by
chromatography on Florisil with hexane/dichloromethane (1:1) as
the eluent. Yield: 0.089 g (32%) of maroon crystals of 3. 31P NMR
(16) APEX 2 version 1.0-22; Bruker AXS Inc.: Madison, WI, 2004.
(17) COSMO NT version 1.40; Bruker AXS Inc.: Madison, WI.
(18) SAINTPLUS Software Reference Manual, Version 6.02A; Bruker
Analytical X-Ray System, Inc.: Madison, WI, 1997-1998.
(19) SIR92-A program for crystal structure solution. Altomare, A.;
Cascarano, G.; Giacovazzo, C.; Guagliardi, A. J. Appl. Crystallogr. 1993,
26, 343.

Notes
Organometallics, Vol. 25, No. 2, 2006 543
fied, and atomic coordinates and isotropic and anisotropic displace-
ment parameters of all the non-hydrogen atoms were refined by
means of a full matrix least-squares procedure on F2, using
SHELXTL software.20 Hydrogen atoms were included in the
refinement in calculated positions, riding on the carbons atoms.
Isotropic thermal parameters of H atoms were fixed 20% and 50%
higher than Csp2 and Csp3 atoms, respectively, to which they were
connected, and torsion angles for methyl groups were refined. The
refinement converged at R1 ) 0.0231, wR2 ) 0.0512, with intensity
I > 2σ(I), and largest peak/hole in the final difference map were
found to be 0.441 and -0.274 e Å-3 for C17H14FO9PMo. R1 )
0.0180, wR2 ) 0.0432, with intensity I > 2σ(I), and largest peak/
hole in the final difference map were found to be 0.411 and -0.335
e Å-3 for 6. Drawings of molecules were achieved using ORTEP32.21
Acknowledgment. The authors thank the University of
California Riverside and the CNRS for the financial support of
this work.
Supporting Information Available: X-ray crystal structure
1
analysis of compounds 2 and 6. Copies of the H and 13C NMR
spectra of 2-5. This material is available free of charge via the
Internet at http://pubs.acs.org.
OM050870V
(20) SHELXTL Software Reference Manual, Version 6.10; Bruker
Analytical X-Ray System, Inc.: Madison, WI, 2000.
(21) ORTEP 3 for Windows. Farrugia, L. J. J. Appl. Crystallogr. 1997,
30, 565.