Nitrosobenzene dimerizations as a model system for studying solid-state reaction mechanisms

Article abstract of DOI:10.1021/jo049537b

Thermal dimerization of nitroso compounds in the solid state was investigated by using para-substituted nitrosobenzenes as model compounds. A mechanism that includes the interplay of topochemical reaction trajectories and phase transfer was proposed on the basis of FT-IR spectroscopic kinetics, time-resolved powder diffraction, and low-temperature X-ray structure determination. From shapes of the kinetic curves analyzed on the basis of the Avrami model, it was found that phase transfer could be triggered by a dimerization reaction of para-substituted nitrosobenzene to azodioxide, which, in turn, can be caused by different packing factors such as disorder in the starting nitroso monomer crystals. Since the represented model can be extended to a broad series of compounds, we propose it as a general method for investigations of solid-state reaction mechanisms.

Full text of DOI:10.1021/jo049537b

Nitr osoben zen e Dim er iza tion s a s a Mod el System for Stu d yin g
Solid -Sta te Rea ction Mech a n ism s
Hrvoj Vancik,* Vesna Simunic-Meznaric, Ernest Mestrovic,* and Ivan Halasz
Department of Chemistry, Faculty of Science, University of Zagreb,
Strossmayerov trg 14, 10000 Zagreb, Croatia
vancik@rudjer.irb.hr
Received March 22, 2004
Thermal dimerization of nitroso compounds in the solid state was investigated by using para-
substituted nitrosobenzenes as model compounds. A mechanism that includes the interplay of
topochemical reaction trajectories and phase transfer was proposed on the basis of FT-IR
spectroscopic kinetics, time-resolved powder diffraction, and low-temperature X-ray structure
determination. From shapes of the kinetic curves analyzed on the basis of the Avrami model, it
was found that phase transfer could be triggered by a dimerization reaction of para-substituted
nitrosobenzene to azodioxide, which, in turn, can be caused by different packing factors such as
disorder in the starting nitroso monomer crystals. Since the represented model can be extended to
a broad series of compounds, we propose it as a general method for investigations of solid-state
reaction mechanisms.
In tr od u ction
required for photochromism by which the reactant and
product should absorb at as different as possible wave-
lengths.2 Because coloration of the dissolved nitroso
compounds has been routinely used even as a visual
indication of the equilibrium, the photodissociation can
be easily followed.4 However, thermal dimerization that
starts after photolysis by warming the sample above 170
K proceeds too fast to be measured accurately, perhaps
because monomers obtained by photolysis remain the
mode of packing of dimeric precursors. For that reason
it was necessary to find another way to measure dimer-
ization reaction rates. The best approach must be based
on finding such a crystal phase in which monomers
survive long enough that reaction rates could be mea-
sured. As can be seen later in this paper, such phases
can be obtained simply by sublimation.
Although most of the nitroso compounds appear as
dimers in the solid state, compounds such as 4-iodoni-
trosobenzene (1), 4-bromonitrosobenzene (2), and 4-me-
thylniytrosobenzene (3) are also known as more or less
stable crystalline monomers.5 However, while 4-iodoni-
trosobenzene immediately after its sublimation exists as
a long-living monomer crystalline species,5e solid 4-bro-
monitrosobenzene and 4-methylnitrosobenzene mono-
mers are unstable under the similar conditions. After
sublimation, 2 and 3 form crystals of nitroso monomers,
A search for new thermochromic and/or photochromic
systems is of great importance in the design of intelligent
materials and molecular electronic devices.1 However,
only a limited number of reaction types are known to
afford efficient photochromism2 or thermochromism in
the solid state.
In our recent paper, we reported a new, very simple
chemical system, the solid-state nitroso dimer-monomer
interconversion, which is based on low-temperature
photodissociation of azodioxides to nitroso compounds
and their thermal dimerization above 170 K.3 The
advantage of this novel photochromic system is the
formation and/or breaking of the one-atom-to-one-atom
chemical bond, i.e., the chemical “off-on switch”, because
most of the previously described photochromic reactions
included formation or breaking of more than two bonds
between reactive atoms (for instance, cycloadditions or
cyclizations, etc.).
The large difference in color between the monomer
(blue, absorbs near 660 nm) and the dimer (colorless,
absorbs in 290 nm region) satisfies the main condition
(1) Irie, M., Ed. Photochromism. Memories and Switches. Chem. Rev.
2000, 100 (No. 5).
(2) (a) Du¨rr, H. Angew. Chem., Int. Ed. Engl. 1989, 28, 413. (b) Du¨rr,
H. Pure Appl. Chem. 1990, 62, 1477. (c) Irie, M.; Lifka, T.; Kobatake,
S.; Kato, N. J . Am. Chem. Soc. 2000, 122, 4871. (d) Becker, H. G. O.;
Bo¨ttcher, H.; Dietz, F.; Rehorek, D.; Roewer, G.; Schiller, K.; Timpe,
H.-J . Einfu¨hrung in die Photochemie; Deutscher Verlag der Wissen-
schaft: Berlin, 1991; p 419. (e) Keating, A. E.; Garcia-Garibay, M. A.
In Photochemical Solid-to-Solid Reactions. Molecular and Supramo-
lecular Photochemistry; Ramamurthy, V., Schanze, K., Eds.; Marcel
Dekker: New York, 1998; Vol. 2, pp 195-248.
(3) (a) Vancik, H.; Simunic-Meznaric, V.; Caleta, I.; Mestrovic, E.;
Milovac, S.; Mlinaric Majerski, K.; Veljkovic, J . J . Phys. Chem. B 2002,
106, 1576-1580. (b) Orrell, G.; Stephenson, D.; Velarque, J . H. J .
Chem. Soc., Perkin Trans. 2 1990, 1297.
(4) (a) Wajer, Th. A. J .; De Boer, Th. J . Recueil 1972, 91, 565. (b)
Greene, F. D.; Gilbert, K. E. J . Org. Chem. 1975, 40, 1409. (c) Greer,
M. L.; Sarker, H.; Medicino, M. E.; Blackstock, S. C. J . Am. Chem.
Soc. 1995, 117, 10460.
(5) (a) Gowenlock, B. G.; Lu¨tke, W.; Quart. Rev. 1958, 12, 321. (b)
Snyder, J . P.; Heyman, M. H.; Suciu, E. J . Org. Chem. 1975, 40, 1395.
(c) Singh, P. J . Org. Chem. 1975, 40, 1405. (d) Fletcher, D. A.;
Gowenlock, B. G.; Orrell, K. G. J . Chem. Soc., Perkin Trans. 2 1998,
797. (e) Fletscher, D. A.; Gowenlock, B. G.; Orrell, K. G.; Sik, V.; Hibbs,
D. E.; Hursthouse, M. B.; Malik, A. K. M. J . Chem. Soc., Perkin Trans.
2 1996, 191.
10.1021/jo049537b CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/18/2004
J . Org. Chem. 2004, 69, 4829-4834
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Vancik et al.
SCHEME 1
which survive at room temperature for a couple of
minutes before their dimerization to azodioxides. This
rate of transformation (Scheme 1) is experimentally
convenient for measurements and affords good opportu-
nity for the study of kinetics of dimerization in the solid
state as well as the study some of general principles of
solid-state reaction mechanisms.
Chemical reactions in the solid state are in different
ways more or less accompanied and coupled with phase
changes in the crystal lattice.6 Since both processes, i.e.,
the change in covalent topology (chemical reaction) and
conversion of supramolecular self-organization (e.g., phase
transfer) occur in the more or less rigid condensed
systems, our concept of the reaction mechanism could be
extended in such a way that it includes both processes.
To find out how these two processes (chemical reaction
and phase conversion) are coupled, it is appropriate to
measure the chemical process and the corresponding
phase changes by different and independent experimen-
tal methods. While the rate of chemical reaction can be
best determined by spectroscopy, the solid-state phase
transition kinetics can well be studied by time-resolved
X-ray powder diffraction methods.7
F IGURE 1. Kinetics of the solid-state dimerization if 2
measured in IR at 25 °C.
obtained after the photolysis of the dimer at cryogenic
temperatures. At 180 K, this reaction was so fast that
under these experimental conditions we were unable to
measure the reaction rate accurately. This difference in
the dimerization rates is evidently topochemically con-
trolled because the dimers in two experiments (produced
by sublimation and cryogenic photolysis, respectively)
were self-organized in different phases.
To follow crystal packing reorganization triggered by
dimerization, we have measured the kinetics of the phase
transfer of sublimed 2 by time-resolved X-ray powder
diffraction at 15 °C (Figure 2). Another advantage of this
experiment is that the sublimed sample of monomer was
not exposed to high pressure, as has been the case for
preparation of a KBr pellet for IR spectroscopy. Conse-
quently, the possible pressure- induced reaction (and/or
phase transformation) is excluded in this experiment. On
the suggestion of a reviewer, we made kinetic measure-
ments by using reflectance IR spectroscopy (see the
Supporting Information). The rate constant that was
obtained (k ) 3.17 × 10-3 s-1) is in principle the same as
in the measurements on the basis transmittance spectra.
In time resolved X-ray diffraction kinetics we found that
different reflections change their intensity following
different kinetic laws. However, satisfactory accuracy of
measurement was obtained only for two most intensive
peaks. The signal at Θ ) 27.5° that belong to the reactant
monomer phase disappears during the reaction and
follows the first-order kinetics (Figure 3) with the “phase
change rate constant” k2 ) 1.31 × 10-3 (15 °C). Because
k2 is very similar to the spectroscopically measured
chemical reaction rate constant k1, the disappearing
crystal plane at Θ ) 27.5° can tentatively be identified
as the critical crystal plane for the dimerization reaction
trajectory, i.e., the plane that intersects the reactive
nitroso groups! If these nitroso groups dimerize to azo-
dioxides, positions of N and O atoms are changed, and
the crystal plane in which these atoms were positioned
must disappear.
Resu lts a n d Discu ssion
Our experimental method is based on findings that
freshly sublimed nitroso compounds appear in monomer
forms. The process of thermally induced dimerization
that affords a new phase was followed by time-resolved
powder X-ray diffraction and by FT-IR spectroscopy.
Typical ONdNO stretching signals appear in the 1250
cm-1 spectral region5,8 as very intensive absorbance, and
the change in its intensity can serve as accurate method
for reaction rate measuring.
4-Br om on itr osoben zen e. Dimerization kinetics of
the chemical reaction M f D (Scheme 1) was measured
for the freshly sublimed 2 by following the change in
intensity of the IR absorption assigned to azodioxide
ONNO asymmetric stretching at 1260 cm-1. The reaction
can be visually observed as a disappearance of the blue
color of the starting monomer. The reaction affords first-
order kinetics with the rate constant k1 ) 3.48 × 10-3
s-1 (25 °C), which corresponds to t1/2 ) 3.32 min (Figure
1). By application of the Arrhenius plot based on mea-
surements at three different temperatures, we have
estimated the activation energy 35 kcal mol-1 (145 kJ
mol-1), which is higher than for similar reactions in
solution.4a This reaction is relatively slow in comparison
with the same reaction observed earlier with monomers
To confirm this assertion, we have determined the
crystal structure of the sublimed monomer 2. The ob-
tained monomer crystals are blue and must be selected
very quickly by catching them directly from the subli-
mator coldfinger. By standing at room temperature,
crystals change their color and shape. After nearly 1 h,
only white polycrystalline powder of nitroso dimers
(6) Schmidt, G. M. J ., et al. Solid State Photochemistry; Ginsburg,
D., Ed.; Verlag Chemie, Weinheim, New York, 1976; p 80.
(7) Kim, J . H.; Hubig, S. M.; Lindeman, S. U.; Kochi, J . K. J . Am.
Chem. Soc. 2001, 123, 87-95.
(8) Fletscher, D. A.; Gowenlock, B. G.; Orrell, K. G. Ibid. 1997, 2201.
4830 J . Org. Chem., Vol. 69, No. 14, 2004

Nitrosobenzene Dimerizations
F IGURE 2. Time-resolved X-ray diffractogram for the A f B phase transfer of the nitroso dimer of 2. Characteristic signals for
the first-order kinetics at Θ ) 27.5° and for the sigmoid phase transition kinetics at Θ ) 25.8°.
F IGURE 3. Disappearance of the reflection at Θ ) 27.5° that
affords the first-order kinetics.
remained. Consequently, this reaction is unfortunately
F IGURE 4. Crystal packing of 2. 50% of nitroso groups are
reactive for dimerization; i.e., they are close to each other.
not a single-crystal-to-single-crystal transformation. The
compound crystallizes in space group P-1 as monomer
with one molecule in an asymmetric cell. Molecules are
packed within normal van der Waals distances but with
50% disorder in respect to in-plane orientation of Br and
NO groups (Figure 4). The observed NdO distance [Nd
O 1.124(3)Å and N*dO* 1.124(3) Å] is slightly shorter
than in other reported structures of nitroso compounds
(1.22 Å).9 The geometry of benzene ring is in the expected
frame for this class of structures (Figure 5).
Two structural features are important for considering
the mechanism of dimerization. First, as can be seen from
Figure 4, since the monomer molecules are statistically
50% disordered, maximally half of monomer pairs are
oriented so that nitroso groups are close enough to enable
dimerization. Second, as we have predicted, the critical
crystal plane, which is from the single-crystal diffraction
identified as 2.0.0. plane (with powder X-ray reflection
at 27.5°) really intersects reactive nitroso groups (Figure
6). Since these topochemically well-oriented pairs of
nitroso groups that are candidates for dimerization are
distributed randomly, the phase transfer (measured as
disappearance of this critical 2.0.0. X-ray reflection) must
follow the same first-order kinetics as the chemical
reaction (measured by IR spectroscopy). In other words,
first-order kinetics of both, chemical reaction (k1) and the
loss of the [200] diffraction signal (k2) indicates that the
dimerization within the crystal lattice proceeds in a
statistical fashion and forms a metastable phase.
On the other hand, the reflection at Θ ) 25.8° follows
quite a different kinetic law (Figure 7) and starts to
change its intensity only when the chemical reaction is
(9) (a) Webster, M. S. J . Chem. Soc. 1956, 2841. (b) Talberg, H. J .
Acta Chem. Scand. A 1979, 33, 289.
J . Org. Chem, Vol. 69, No. 14, 2004 4831

Vancik et al.
F IGURE 5. Molecular structure of p-bromnitrosobenzene 2
showing 50% probability displacement ellipsoids and the
disorder of atoms O, N, and Br.
F IGURE 8. Logarithmic “Sharp-Hanock plot” for the sig-
moid-type kinetics obtained for the crystal plane at Θ ) 25.8°.
normalized relative to the maximal intensity obtained
when the overall process is complete.
a(t) ) IΘ(t)/IΘ (t ) ∞)
In solid-state kinetics based on the Avrami-Erofeyev11
equations it is also convenient to convert the sigmoid
curve to the logarithmic function (Figure 8) to obtain the
so-called “Sharp-Hancock plot”.12 The slope of the line,
the gradient m, can be used as an indication for the
mechanism of the phase transfer. A series of m values
associated to corresponding mechanism of phase trans-
formations were tabulated systematically on the basis of
their solid-state kinetics models.13 In our case, the best
line fitting was obtained for m ) 2.01, the value that is
proposed to be the characteristics of the one-dimensional
phase transfer (lineal growth). Avrami also proposed an
another convenient representative index for estimating
the type of phase growth. It is the ratio of times for 75%
and 25% of extent of transformation.11 In our case this
ratio is 2.25, and it represents the limiting value between
platelike growth and lineal growth.
F IGURE 6. Crystal packing of 2. The extra labeled critical
plane 2.2.0 at Θ ) 27.5° (blue), which disappears during the
dimerization following the first-order kinetics. The plane
intersects reactive nitroso groups (red) of monomers.
Comparing the kinetics in Figures 3 and 7, it could be
concluded that changes of different crystal planes during
and after chemical reaction belong to different phase
transformations. There is one crystal plane, which follows
the chemical reaction by its first order kinetics, but there
is also another crystal plane that develops quite inde-
pendently and affords the slow “autocatalytic” sigmoid
phase transfer. The most important observation is that
this second transition does not begin before the chemical
reaction is almost complete (after 40 min). Consequently,
dimerization of the sublimed 2 proceeds by forming the
metastable phase within the starting monomer crystal
lattice. After all, it seems to be very difficult to answer
the “chicken or egg problem” question, i.e., what is first
event, chemical reaction that triggers observed phase
transition, or packing arrangements such as disorder of
F IGUR E 7. Sigmoid kinetics of the phase transition in 2
observed for the reflection at Θ ) 25.8°.
almost complete. Its sigmoid shape is typical for an
autocatalytic type phase transition. In the diagram in
Figure 7 the degree of the transformation (a) is shown
as function of time. The value a is defined10 as intensity
(11) (a) Avrami, M. J . Chem. Phys. 1939, 7, 1103-1112. (b) Avrami,
M. J . Chem. Phys. 1940, 8, 212-224. (c) Avrami, M. J . Chem. Phys.
1941, 9, 177-184. (d) Erofeev, B. V. Compt. Rend. Acad. Sci. U.S.S.R.
1946, 52, 511-514.
(12) Hancock, J . D.; Sharp, J . H. J . Am. Ceram. Soc. 1972, 55, 74-
(10) Evans, J . S. O.; Price, S. J .; Wong, H.-W.; O’Hare, D. J . Am.
Chem. Soc. 1998, 120, 10837-10846.
77.
(13) Hulbert, S. F. J . Brit. Ceram. Soc. 1969, 6, 11-20.
4832 J . Org. Chem., Vol. 69, No. 14, 2004

Nitrosobenzene Dimerizations
F IGURE 9. Crystal packing of 1.9 Nitroso groups are close to the iodine atom and not to each other, and their dimerization is
impossible.
monomer, which is responsible for instability that leads
to dimerization. This system is much more complex than
for example the previously observed mechanism of solid-
state Diels-Alder reaction where chemical reaction and
phase transition occur simultaneously.7
Th e Ca se of 4-Iod o- a n d 4-Meth yln itr osoben zen e
(1 a n d 3). As can be seen from the structure and packing
of 4-iodonitrosobenzene monomer 1 (Figure 9, based on
literature diffraction data9), the in-plane molecules are
arranged without any disorder, and they are oriented so
that the nitroso group lies always close to the iodine
atom, and there are no neighboring pairs of nitroso
groups, which could react with each other. Consequently,
in such regular packing, dimerization is topochemically
impossible, and the compound remains in form of stable
F IGURE 10. Sigmoid kinetics of the phase transition in 3
observed for the reflection at Θ ) 11.4°.
monomers in the crystal. From exactly the same reason,
p-methoxynitrosobenzene survives as stable monomer in
with stack structure, for the unit cell of 3 we have
preliminary indications that it has different structure.14
Unfortunately, we were not able to solve the structure
of monomer 3 completely, mainly because of thermal
instability of its single crystal, but work on this is in
progress.
the crystal because its nitroso groups are far from each
other and close to the methoxy groups.9b
It could be speculated that halogen-nitrogen nonbond-
ing interactions14b play more important role in stabiliza-
tion of crystals of 1 than in stabilization of crystals of 2,
where disorder has been observed.
We were also able to measure solid-state dimerization
kinetics of 4-methylnitrosobenzene 3 starting with the
freshly sublimed monomer. The best kinetics we found
for the signal at Θ ) 11.4° (Figure 10). From its typical
sigmoid shape we estimated the time for the complete
transformation 13 min, i.e., faster than in the case of
4-bromonitrosobenzene (nearly 40 min). A logarithmic
“Sharp-Hanock plot” for this sigmoid (Figure 11) pro-
vides much higher slope m ) 3.04, the value that can be
characterized for the three-dimensional growth. There-
fore, such phase transfer is different than in 4-bromoni-
trosobenzerne (m ) 2.01) because of different topochem-
ical conditions. While molecules of 1 and 2 are packed
Con clu sion
We have demonstrated that 4-substituted nitrosoben-
zenes form crystalline monomers after their sublimation.
Stability of obtained monomer in the solid state is such
that it spontaneously dimerizes at room temperature
with the reaction rate that can be accurately measured
either by spectroscopy or by time resolved powder X-ray
diffraction. This property of nitrosobenzenes gives op-
portunity for using the described procedure as a more
general method for detailed experimental study of solid-
state reaction mechanisms, simply by variation of the
group R in para or meta position (Scheme 1). In such a
way, correlation between molecular structure, packing,
and reactivity can be established.
(14) (a) Desiraju, G. R. Crystal Engineering. The Design of Organic
Solids; Elsevier: Amsterdam, 1989; pp 92. (b) Desiraju, G. R. Crystal
Engineering. The Design of Organic Solids; Elsevier: Amsterdam,
1989; p 197.
From the comparison of the dimerization rate and
corresponding phase transition rate with the way on
J . Org. Chem, Vol. 69, No. 14, 2004 4833

Vancik et al.
The structures were solved (direct methods) and refined
(difference Fourier synthesis) using the SHELXS16 and SHELX-
TL17 packages. The structural refinement was performed on
F2 using all data. The carbons of the benzene rings were
refined anisotropically without any restriction. Br, N, and O
atoms were refined anisotropically with some restrictions on
thermal and geometrical parameters. After determination of
all position of non-hydrogen atoms the hydrogen atoms were
placed geometrically, with C-H distance in the range 0.98-
0.99 Å and allowed to refine using UisoH ) 1.2Ueq(C). All
calculations were performed using WINGX18 crystallographic
software package. Crystal data and details of data collection
and structure analysis are summarized in Table 1 (Supporting
Information). The ORTEP19 diagram of the molecular structure
are present in Figure 5, and the unit cell packing diagrams
prepared by SHACAL20 are present in Figures 4 and 6.
P ow d er X-r a y Diffr a ction . Data were collected on Philips
PW 1700 automated diffractometer with control unit PW3710
using the scanning method with the following parameters:
monochromatic Cu KR1 radiation (λ ) 1.5406 Å), observation
range 5°-2θ-35°, continuous scan mode, step scan with
0.025°, counting time per step 0.5 s. With this choice of
parameters one circle lasts 10 min. We collect data in period
of 4 h. The data collection was performed by Expert Software
suite 1.2 (Program package for measuring and analysis dif-
fraction data on Philips X-ray diffraction equipment; Analyti-
cal, Lamely: The Netherlands, 1999). For background cor-
recting, Ka2 striping and integration of intensity were
performed using X’Pert Plus 1.0 (Program for Crystallography
and Rietveld Analysis; Panalytical, Almelo: The Netherlands
1999).
F IGURE 11. Logarithmic “Sharp-Hanock plot” for the sig-
moid type kinetics for dimerization of 3, measured as decrease
in the intensity of the reflection assigned to the crystal plane
at Θ ) 11.4°.
which monomer molecules were packed in crystal lattice,
it becomes evident that topochemical environment is an
important factor, which can trigger the dimerization
reaction. Moreover, in the case of 4-bromonitrosobenzene
(2), the critical crystal plane, which intersects the reac-
tive center (nitroso groups) responsible for dimerization
was located on the basis of powder diffraction kinetics
combined with the single-crystal structure analysis.
Ack n ow led gm en t. We thank Professor Emeritus D.
E. Sunko, and Professor Z. Mihalic for helpful discus-
sions and comments. We also thank Tomislav Biljan for
help in IR measurements. The financial support of the
Ministry for Science and Technology, Republic of Croat-
ia, through Grant Nos. 0119611 and 0119630 is grate-
fully acknowledged.
Exp er im en ta l Section
Nitr osoben zen es were prepared by standard methods.3a,b
F T-IR Kin etics. The solid-state dimerization rate was
measured by following the temporal change in transmittance
of the ONNO asymmetric stretching signal at 1260 cm-1. This
signal obeys Lambert-Beer’s law. Bruker FT-IR spectrometer
was used for this measurement. The freshly sublimed sample
was prepared as standard KBr pellet.
Sin gle-Cr ysta l X-r a y Diffr a ction . A single crystal of
monomer for diffraction experiment was prepared by sublima-
tion of dimers precursor under 10-2 Torr pressure at 60 °C.
The sublimed monomer is unstable at room temperature
because it dimerizes within few minutes, and this transforma-
tion destroys the single crystal. Consequently, the crystal must
be mounted in diffractometer very quickly, and the measure-
ment must be performed at 100 K. For better handling with
so unstable sample, the temperature of cooling water for
sublimator was never above 10 °C. The crystal of satisfactory
quality has been selected directly from the coldfinger, but after
many repetitions of the experiment. An Oxford Diffraction
Xcaliburg Kappa CCD X-ray diffractometer with graphite-
monochromated Mo KR radiation was used to collect diffraction
data.15 The data sets were collected using the ω scan mode
over the 2θ range of 4-60°.
Su p p or tin g In for m a tion Ava ila ble: Change in reflec-
tance IR spectra of 2 with time and details of low-temperature
single-crystal experiments and summary of crystallographic
parameters. This material is available free of charge via the
Internet at http://pubs.acs.org.
J O049537B
(15) Oxford Diffraction CrysAlisCCD and CrysAlisRED, Version
1.170; Oxford Diffraction: Wroclaw, Poland and Oxford U.K., 2003.
(16) Sheldrick, G. M. SHELXS97, Programs for Crystal Structure
Analysis (Release 97-2); Institut fu¨r Anorganische Chemie der Uni-
versita¨t, Tammanstrasse 4, D-3400 Go¨ttingen, Germany, 1998.
(17) Sheldrick, G. M. SHELXL93, Program for Crystal Structure
Refinement; Institu¨t fu¨r Anorganische Chemie der Universita¨t, Tam-
manstrasse 4, D-3400 Go¨ttingen, Germany, 1993.
(18) Farrugia, L. J . J . Appl. Crystallogr. 1999, 32, 837-838.
(19) Farrugia, L. J . J . Appl. Crystallogr. 1997, 30, 565.
(20) Keller, E. J . Appl. Crystallogr. 1989, 22, 12-22.
4834 J . Org. Chem., Vol. 69, No. 14, 2004