Fluorine for hydrogen exchange in the hydrofluorobenzene derivatives C 6HxF(6-x), where x = 2, 3, 4 and 5 by monomeric [1,2,4-(Me3C)3C5H2]2CeH: The solid state isomerization of [1,2,4-(Me3C)3C 5H2]2Ce(2,3,4,5-C6HF4) ...

Article abstract of DOI:10.1021/ja800639f

Full title: Fluorine for hydrogen exchange in the hydrofluorobenzene derivatives C ₆H_xF_(6-x), where x = 2, 3, 4 and 5 by monomeric [1,2,4-(Me₃C)₃C₅H₂]₂CeH: The solid state isomerization of [1,2,4-(Me₃C)₃C ₅H₂]₂Ce(2,3,4,5-C₆HF₄) to [1,2,4-(Me₃C)₃C₅H₂] ₂Ce(2,3,4,6-C₆HF₄). The reaction between monomeric bis(1,2,4-tri-tert-butylcyclopentadienyl) cerium hydride, Cp′₂CeH, and several hydrofluorobenzene derivatives is described. The aryl derivatives that are the primary products, Cp′₂Ce(C₆H_5-xF_x) where x = 1,2,3,4, are thermally stable enough to be isolated in only two cases, since all of them decompose at different rates to Cp′₂CeF and a fluorobenzyne; the latter is trapped by either solvent when C₆D ₆ is used or by a Cp′H ring when C₆D₁₂ is the solvent. The trapped products are identified by GC/MS analysis after hydrolysis. The aryl derivatives are generated cleanly by reaction of the metallacycle, Cp′((Me₃C)₂C₅H ₂C(Me₂)CH₂)Ce, with a hydrofluorobenzene, and the resulting arylcerium products, in each case, are identified by their ¹H and ¹⁹F NMR spectra at 20°C. The stereochemical principle that evolves from these studies is that the thermodynamic isomer is the one in which the CeC bond is flanked by two ortho-CF bonds. This orientation is suggested to arise from the negative charge that is localized on the ipso-carbon atom due to C_o(δ+)F_o(δ-) polarization. The preferred regioisomer is determined by thermodynamic rather than kinetic effects; this is illustrated by the quantitative, irreversible solid-state conversion at 25°C over two months of Cp′₂Ce(2, 3,4,5-C₆HF₄) to Cp′₂Ce(2,3,4,6-C ₆HF₄), an isomerization that involves a CeC(ipso) for C(ortho)F site exchange.

Full text of DOI:10.1021/ja800639f

Published on Web 05/09/2008
Fluorine for Hydrogen Exchange in the Hydrofluorobenzene
Derivatives C6HxF(6-x), where x ) 2, 3, 4 and 5 by Monomeric
[1,2,4-(Me3C)3C5H2]2CeH: The Solid State Isomerization of
[1,2,4-(Me3C)3C5H2]2Ce(2,3,4,5-C6HF4) to
[1,2,4-(Me3C)3C5H2]2Ce(2,3,4,6-C6HF4)
Evan L. Werkema and Richard A. Andersen*
Chemistry Department and Chemical Sciences DiVision of Lawrence Berkeley National
Laboratory, UniVersity of California, Berkeley, California 94720
Received January 30, 2008; E-mail: raandersen@lbl.gov
Abstract: The reaction between monomeric bis(1,2,4-tri-tert-butylcyclopentadienyl)cerium hydride, Cp′2CeH,
and several hydrofluorobenzene derivatives is described. The aryl derivatives that are the primary products,
Cp′2Ce(C6H5-xFx) where x ) 1,2,3,4, are thermally stable enough to be isolated in only two cases, since all
of them decompose at different rates to Cp′2CeF and a fluorobenzyne; the latter is trapped by either solvent
when C6D6 is used or by a Cp′H ring when C6D12 is the solvent. The trapped products are identified by
GC/MS analysis after hydrolysis. The aryl derivatives are generated cleanly by reaction of the metallacycle,
Cp′((Me3C)2C5H2C(Me2)CH2)Ce, with a hydrofluorobenzene, and the resulting arylcerium products, in each
1
case, are identified by their H and 19F NMR spectra at 20 °C. The stereochemical principle that evolves
from these studies is that the thermodynamic isomer is the one in which the CeC bond is flanked by two
ortho-CF bonds. This orientation is suggested to arise from the negative charge that is localized on the
ipso-carbon atom due to Co(δ+)Fo(δ-) polarization. The preferred regioisomer is determined by
thermodynamic rather than kinetic effects; this is illustrated by the quantitative, irreversible solid-state
conversion at 25 °C over two months of Cp′2Ce(2,3,4,5-C6HF4) to Cp′2Ce(2,3,4,6-C6HF4), an isomerization
that involves a CeC(ipso) for C(ortho)F site exchange.
1. Introduction
inconsistent with the calculated potential energy surfaces. The
postulate that CH activation barriers are lower, higher, or
comparable to CF barriers can be tested by experimental studies
of the reaction between Cp′2CeH and judiciously chosen
isomeric hydrofluorobenzenes. The results of these experimental
studies are the subject of this paper.
The fluoride for hydrogen-exchange reactions that resulted
when C6F6 or C6HF5 was added to monomeric [1,2,4-
(Me3C)3C5H2]2CeH, abbreviated as Cp′2CeH, have been de-
scribed recently.1 The initial products of the reaction of C6F6
and Cp′2CeH were Cp′2CeC6F5, Cp′2CeF, and H2; the pen-
tafluoroaryl derivative decomposed to the fluoride and tetrafluo-
robenzyne, which was trapped by either C6D6 (solvent), or the
Cp′-ring of a metallocene. Hydrogen was suggested to be formed
in two sequential reactions illustrated in eqs 1a and 1b. Thus,
the net F-/H-exchange reaction was composed of individual
intermolecular CF and CH activation steps.
2. Results
2.1. Strategy. In general, the products formed in the reaction
of Cp′2CeH, 1, and the hydrofluorobenzene derivatives described
below are not isolated as crystalline materials because of their
thermal instability, as mentioned in earlier papers.1,2 The
1
changes that occur in the H and 19F NMR spectra however
Cp′2CeH + C6F6 f Cp′2CeF + C6F5H
Cp′2CeH + C6F5H f Cp′2CeC6F5 + H2
(1a)
(1b)
are readily observed, and these changes are monitored as a
function of time. Even when the fluoroaryl derivatives are
isolated by crystallization, they do not give satisfactory combus-
tion analysis, as is well-known for fluorocarbon compounds.3
The compounds do not yield molecular ions in their mass
spectra; instead they give fragmentation ions and do not sublime
nor melt without decomposition.
DFT calculations on the reaction between (C5H5)2LaH, used
as a model for the experimental reaction, and either C6F6 or
C6F5H showed that the CH activation barrier, eq 1b, is
substantially lower than the CF activation barrier, eq 1a, but
the elimination of C6F4 from Cp′2CeC6F5 proceeded with a
higher barrier. The experimental studies required that the net
CF and CH activation barriers were comparable since the
products derived from each process were observed, which was
The identity of the products is ascertained by comparison of
1
the H and 19F NMR spectral features obtained by reaction of
(2) Werkema, E. L.; Messines, E.; Perrin, L.; Maron, L.; Eisenstein, O.;
Andersen, R. A. J. Am. Chem. Soc. 2005, 127, 7781–7795.
(3) Banks, R. E. Fluorocarbons and Their DeriVatiVes, 2nd ed.; MacDonald
and Company: London, 1970; p 17.
(1) Maron, L.; Werkema, E. L.; Perrin, L.; Eisenstein, O.; Andersen, R. A.
J. Am. Chem. Soc. 2005, 127, 279–292.
9
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2008 American Chemical Society
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A R T I C L E S
Werkema and Andersen
Scheme 1. Reactions of 1,2,4,5-Tetrafluorobenzene
Scheme 2. Reactions of 1,2,3,5-Tetrafluorobenzene
Scheme 5. Reactions of 1,2,4-Trifluorobenzene
Scheme 6. Reactions of 1,2,3-Trifluorobenzene
Scheme 3. Reactions of 1,2,3,4-Tetrafluorobenzene
Scheme 7. Reactions of 1,4-Difluorobenzene
Scheme 4. Reactions of 1,3,5-Trifluorobenzene
the metallacycle Cp′((Me3C)2C5H2C(Me2)CH2)Ce, 2, Schemes
1-9, and a hydrofluorobenzene, which only yields the fluoroaryl
derivatives resulting from insertion of a CH bond into the CeC
bond of the metallacycle. When isomeric fluoroaryl derivatives
are possible, judicious choice of the hydrofluorobenzene yields
spectra that match those obtained in the reaction of Cp′2CeH.
In this manner, the identity and stereochemistry of the fluoroaryl
derivative is delineated. The 1H and 19F NMR spectra listed in
Table 1 are acquired in this manner. Hydrolysis of the reaction
mixture (H2O) and examination of the hydrolysate by 19F NMR
spectroscopy identifies and quantifies the hydrofluorobenzene
or benzenes formed. In general, this protocol shows that the
reactions are clean when the reaction times are short. The NMR
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Fluorine for H Exchange in Hydrofluorobenzene Derivatives
A R T I C L E S
Scheme 8. Reactions of 1,3-Difluorobenzene
Scheme 9. Reactions of 1,2-Difluorobenzene
spectra of the mixtures in either C6D6 or C6D12 are monitored
over time, at 20 or 60 °C, until the resonances due to Cp′2CeH
disappear and those of Cp′2CeF are the only paramagnetic
resonances remaining. The organic products are identified (after
hydrolysis (H2O)) by GC/MS analysis as derived from trapping
of the fluorobenzyne by either C6H6 [A(H)], C6D6 [A(D)], or
Cp′H (B) when C6H6, C6D6, or C6D12 is the solvent, respectively,
Chart 1. Once the library of 1H and 19F NMR spectra are
acquired, Table 1, the products of the reaction between Cp′2CeH
and C6HxF(6-x) (x ) 5,4,3,2) are readily identified. The first
formed product, called the primary product, is readily identified,
as are the subsequent or secondary products. Only two fluoroaryl
derivatives are isolated as pure solids, and both are characterized
by single crystal X-ray crystallography.
2.2. Solution Studies. 2.2.1. Reaction of 1 and 2 with Isomeric
Tetrafluorobenzenes. 2.2.1.1. 1,2,4,5-C6H2F4, Scheme 1. Addi-
tion of an excess of 1,2,4,5-tetrafluorobenzene to a purple
solution of Cp′2CeH, 1, in C6D6 at 20 °C in an NMR tube results
in an immediate color change to orange and gas evolution (H2).
1
Examination of the solution by H NMR spectroscopy within
20 min shows the presence of two new sets of paramagnetic
resonances, each of which appear in a 2:1 area ratio due to the
Me3C groups, in a 1.5:1 area ratio along with a resonance due
to H2 and tiny resonances due to Cp′2CeF, 3; resonances due to
Cp′2CeH are absent. The identity of one of the two new sets of
resonances is established by adding an excess of 1,2,4,5-C6H2F4
to the metallacycle, 2, in C6D12 in an NMR tube at 20 °C. The
1H NMR spectrum of this mixture identifies this set of
resonanaces as due to 4, Table 1. Evaporating the contents of
the NMR tube to dryness (in order to remove excess 1,2,4,5-
C6H2F4), redissolving the residue in C6D12, and adding a small
quantity of 2 results in appearance of the resonances in the 1H
NMR spectrum due to 5 at the expense of those due to 4.
Addition of more 2 increases the resonances due to 5, again at
the expense of those due to 4. These experiments are sufficient
to identify 4 and 5 as the fluoroaryl derivatives derived by
insertion of the CH bond into the CeC bond of 2, Scheme 1.
1
The H NMR spectrum of 4 also contains a triplet resonance
due to a single proton (J ) 7 Hz) that is assigned to the para-H
in 4. The 19F NMR spectrum of 4 contains two broad resonances
assigned to the ortho-F and the meta-F groups, Table 1.
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A R T I C L E S
Werkema and Andersen
Chart 1. Cycloaddition Products of Fluorobenzynes with C6H6, C6D6, or Cp′H
Variable-temperature NMR spectra of 4 and several other
fluoroaryl derivatives are described later in this article.
when C6D12 is the solvent. Evaporation of C6D12 followed by
hydrolysis and analysis by GC/MS showed three major com-
ponents (along with Cp′H and C6D12), one with an m/z value
of 364 due to B-1 and the other two with m/z values of 307 (M
- CMe3)+ due to the two other possible isomers of B-1, Chart
1, which arise from the [2 + 4] cycloaddition reaction between
3,4,6-trifluorobenzyne and Cp′H. In this reaction, C6D6 or C6H6
is not available to trap the benzyne, and the Cp′-ring is the trap.
A larger amount of X forms when C6D12 is the solvent relative
to 3, suggesting that X is a cerium containing species that
contains one substituted cyclopentadienyl ring, but its identity
is a mystery. It is noteworthy that X forms even when C6D6 is
the solvent, suggesting that the substituted cyclopentadienyl
ligands can trap the fluorobenzyne even when C6D6 is present
in large excess.
The reaction between 1 and 1,2,4,5-C6H2F4 in C6D12 at 20 °C
after 20 min yields resonances due to 4 and 5 in comparable
amounts and H2, as when C6D6 is the solvent. These primary
products are derived from exchange between CeH and CH
groups and therefore by CH activation of the hydrofluoroben-
zene. The resonances due to 4 and 5 diminish, while those due
to 3 appear over time. After one day (20 °C), the resonances
due to 5 are gone, and the resonances due to 4 and 3 are present
in a ratio of 18:1. After 11 days, the ratio is 1:4, and
paramagnetic resonances due to X (see later) begin to appear
as do several resonances in the diamagnetic region (δ ) 0-2).
After 17 days, the resonances due to X increase in intensity, as
do the resonances in the diamagnetic region, at the expense of
those due to 4. Heating to 60 °C for one day yields a 1H NMR
spectrum that contains paramagnetic resonances due only to 3
and X in a 6:1 ratio (assuming that the cyclopentadienyl ring
in X contains three CMe3 groups). These two sets of resonances
account for about 30% of the Cp′ groups originally present in
Cp′2CeH, and therefore the “diamagnetic resonances” have
appreciable intensity.
2.2.1.2. 1,2,3,5-C6H2F4, Scheme 2. The reaction between
1,2,3,5-tetrafluorobenzene and Cp′2CeH at 20 °C in C6D12 in
an NMR tube is qualitatively similar to that observed with
1,2,4,5-tetrafluorobenzene. New paramagnetic resonances appear
1
in the H NMR spectrum due to the cyclopentadienyl Me3C
groups in an area ratio of 1:1:1. In addition, resonances due to
H2 and 3 appear; the ratio of the new resonances to those of 3
is about 4:1. The same three Me3C group resonances appear in
Repeating the reaction of 1 with an excess of 1,2,4,5-C6H2F4
in C6H6 followed by heating to 60 °C for one day, evaporation
of the solvent, and dissolution of the residue in C6D6 shows
1
the H NMR spectrum when an excess of 1,2,3,5-C6H2F4 is
added to the metallacycle 2 in C6D12, Scheme 2. The 1H NMR
spectrum also contains a triplet resonance, J ) 7 Hz, due to a
single proton, and the 19F NMR spectrum shows four equal area
resonances, two of which are broad and two of which are narrow
enough for the coupling pattern to be visible, viz., a doublet (J
) 15 Hz) and a doublet of doublets with J ) 18 and 7 Hz,
Table 1. These data are sufficient to identify the product as 6,
Scheme 2, the product resulting from intermolecular CH
activation. The appearance of three chemically inequivalent
Me3C resonances is expected for 6 with averaged Cs symmetry
as is observed at 20 °C, Table 1; the variable-temperature NMR
spectra are described below.
1
resonances in the H NMR spectrum due to 3 and X in a 7:1
1
ratio and resonances in the H and 19F NMR spectra due to
A(H)-1, Chart 1.4 Hydrolysis (H2O) and analysis by GC/MS
shows a single component with m/z of 208 due to A(H)-1, Chart
1. When the solvent is C6D6, the GC/MS shows a single
component with m/z of 214 due to A(D)-1, Chart 1, and the
19F NMR spectrum contains resonances due to A(D)-1.
Repeating the reaction between 1 and 1,2,4,5-C6H2F4 in C6D12
1
at 20 °C yields H and 19F NMR spectra that are qualitatively
similar to those in C6D6 or C6H6. However, at the end of the
reaction the ratio of 3 to X is about 2:1; more X is formed
When 6 is generated in the reaction of 2 with 1,2,3,5-C6H2F4,
it is stable at 20 °C for days in C6D12. When heated to 60 °C
(4) Harrison, R.; Heaney, H. J. Chem. Soc. C 1968, 889–892.
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Fluorine for H Exchange in Hydrofluorobenzene Derivatives
A R T I C L E S
for two days, all of the resonances due to 6 disappear, and
resonances due to 3 and X appear in approximately equal
amounts as well as a number of other resonances in the
diamagnetic region. Hydrolysis and analysis of the hydrolysate
by GC/MS show six components (in addition to Cp′H), two
with m/z 364 and four with m/z 307 (M - CMe3)+ in an
approximate ratio of 3:11:1:1:6:7, respectively, which are
attributed to the six isomers resulting from the [2 + 4]
cycloaddition of 3,4,6- and 3,4,5-trifluorobenzyne with Cp′H,
viz., B-1 and B-2, respectively. The retention times and isotopic
patterns for the six components matched those obtained from
hydrolysis of the thermal decomposition of 4 and 7 (see later).
2.2.1.3. 1,2,3,4-C6H2F4, Scheme 3. Addition of 1,2,3,4-tetra-
fluorobenzene to a solution of Cp′2CeH in C6D12 in an NMR
tube at 20 °C results in an orange solution that contains two
new sets of paramagnetic Me3C resonances in a net area ratio
of 2:1; the resonances of the former display a 1:1:1 pattern for
the Me3C groups while those of the latter appear in a 2:1 ratio.
The spectrum also contains resonances due to 3 and H2. The
new resonances are identified in the following manner. Addition
of 1,2,3,4-C6H2F4 to a solution of the metallacycle 2 in C6D12
and then disappear over the course of a day as those of 3 appear.
This surprising observation is described later in more detail.
2.2.2. Reaction of 1 and 2 with Isomeric Trifluoroben-
zenes. 2.2.2.1. 1,3,5-C6H3F3, Scheme 4. Addition of 1,3,5-
trifluorobenzene to Cp′2CeH in C6D12 at 20 °C and monitoring
by 1H NMR spectroscopy results in the appearance of two new
Me3C resonances in a 2:1 ratio and a doublet (J ) 9 Hz) due
to two hydrogens. These are the resonances expected for aryl
derivative 9, Scheme 4. Reaction of 1,3,5-C6H3F3 with metal-
1
lacycle 2 generates only resonances due to 9 in the H NMR
spectrum. The 19F NMR spectrum consists of two resonances
in a 2:1 ratio; the former is very broad, while the latter is a
triplet (J ) 10 Hz), Table 1. Over time, the resonances due to
9 are replaced by those due to 3, X, and diamagnetic ones.
2.2.2.2. 1,2,4-C6H3F3, Scheme 5. Addition of 1,2,4-trifluoro-
benzene to a C6D12 solution of Cp′2CeH in an NMR tube at
1
20 °C yields a new set of Me3C resonances in the H NMR
spectrum in a relative ratio of 1:1:1 and two resonances due to
one hydrogen each that are a doublet and an apparent triplet, J
) 8 Hz in each case. Three possible isomers can result from
CeH for CH exchange, but the 1H NMR spectrum is consistent
with the one illustrated as 8 in Scheme 5. The structural
assignment is supported by generating 8 from the metallacycle,
2, and 1,2,4-C6H3F3 and observing the 1H and 19F NMR spectra,
Table 1. The compound 8 has Cs symmetry, assuming that the
Cp′ rings are free to rotate, and therefore the CMe3 groups on
a given cyclopentadienyl ring are chemically inequivalent and
will appear as a 1:1:1 pattern, as observed. Heating 8 to 60 °C
for 12 h results in disappearance of the resonances due to 8
and formation of those due to 3, X, and diamagnetic resonances.
Hydrolysis and analysis of the hydrolysate by GC/MS shows
five components in the mixture in an approximate ratio of 3:3:
3:2:2, four of which exhibit m/z of 346 and one with m/z of
289 (M - CMe3)+ due to the isomers B-3 and B-4.
2.2.2.3. 1,2,3-C6H3F3, Scheme 6. In contrast to the reaction
of the two isomers of C6H3F3 just described, which give single
regioisomers, 8 and 9, 1,2,3-trifluorobenzene gives two aryl
derivatives. Examination of the solution formed upon addition
of 1,2,3-trifluorobenzene to Cp′2CeH in C6D12 in an NMR tube
at 20 °C by 1H NMR spectroscopy shows a pair of overlapping
Me3C resonances in a 2:1 area ratio, along with resonances due
to 1, 3, and H2. After 3 h, the minor set of Me3C resonances
disappears, and those due to the major product consist of Me3C
resonances in a 2:1 area ratio, a triplet (J ) 8 Hz) and a doublet
(J ) 8 Hz) due to one and two hydrogens each, respectively,
Table 1; the 19F NMR spectrum consists of a broad single
resonance. The 1H NMR spectrum of the major product is
identical to that of 12, derived by addition of 1,3-difluorobenzene
to the metallacycle 2, Scheme 8, see below. Thus, the major
product is derived from CF activation. The minor isomer is the
CH activation product 10, which is prepared cleanly from the
metallacycle 2, Scheme 6. As in the reaction of 1,2,3,4-
tetrafluorobenzene, the primary product is that derived by CeH
for CF exchange. After one day at 20 °C the ratio of 3 to 10 is
4:1 and after an additional day at 60 °C, only resonances due
to 3, X, and diamagnetic resonances are present in the 1H NMR
spectrum. Thus, the major product formed in this reaction is
derived from CeH for CF exchange, as in the reaction between
1 and 1,2,3,4-tetrafluorobenzene.
1
generates a H NMR spectrum in which the Me3C resonances
appear in a 2:1 ratio with the same chemical shifts as those
mentioned above. The 19F NMR spectrum contains three equal
area resonances, two of which appear as doublets (J ) 18 Hz)
and one as a triplet (J ) 18 Hz), Table 1. This pattern is
consistent with that expected for 7, Scheme 3, assuming that
the resonance for the ortho-F is broadened into the baseline.
The other resonances are identified as those due to 8, Scheme
1
3, since addition of 1,2,4-trifluorobenzene to 2 generates a H
NMR spectrum whose chemical shifts are identical to the 1:1:1
pattern of resonances with relative area 2, Scheme 5.
The aryl derivative 7 results from a net CeH for CH exchange,
while 8 is derived from a net CeH for CF exchange. Apparently,
the major primary product of the reaction of Cp′2CeH and
1,2,3,4-C6H2F4 is not derived from CH activation but from a
CF activation process. This observation contradicts the postulate
that CH activation proceeds with a lower barrier than CF
activation, a postulate derived from DFT calculations on the
reaction of Cp2LaH and C6HF5. This apparent contradiction
caused us to examine more carefully the NMR spectra obtained
in the reaction of 1 and C6HF5, reported earlier.1 Re-examination
shows that Cp′2CeC6F5 is indeed the major product as described,
but small but not insignificant resonances due to 4, Scheme 1,
are also observed in the 1H and 19F NMR spectra at short
reaction times. Thus, our postulate that net CH activation always
proceeds with a lower activation barrier than CF activation does
is inconsistent with the experimental observations and needs to
be modified.
Over time, the resonances due to 7 and 8 disappear (at
different rates), and the resonances due to 3, X, and those in
the diamagnetic region increase in intensity. Heating at 60 °C
for one day results in only those resonances due to 3, X, and
the “diamagnetic ones.” Hydrolysis of the thermal decomposi-
tion products of 7, prepared from 2, and analysis of the organic
products by GC/MS show three primary components along with
Cp′H, one with m/z of 364 and two with m/z of 307 (M -
CMe3)+ in a 4:1:6 ratio, respectively, due to isomers of B-2,
Chart 1; several isomers of the structure represented by B can
form, depending upon which Cp′-ring carbon atoms participate
in the [4 + 2] cycloaddition reaction.
2.2.3. Reaction of 1 and 2 with Isomeric Difluorobenzenes
and Fluorobenzene. 2.2.3.1. 1,4-C6H4F2, Scheme 7. Addition
of 1,4-difluorobenzene to Cp′2CeH or the metallacycle 2
When the thermal decomposition of 7, prepared from 2, is
monitored closely at 20 °C, small resonances due to 6 appear
1
generates identical H NMR spectra which, along with their
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A R T I C L E S
Werkema and Andersen
19F NMR spectra, are listed in Table 1. The spectra are in accord
with those expected for 11, Scheme 7. With time, the resonances
disappear and are replaced by those due to 3, X, and diamagnetic
ones.
2.2.3.2. 1,3-C6H4F2, Scheme 8. This fluorobenzene derivative
reacts with either Cp′2CeH or the metallacycle 2 to give identical
1H and 19F NMR spectra, Table 1, which are consistent with
an aryl derivative with structure 12, Scheme 8. With time,
resonances due to 12 disappear, and those due to 3, X, and
diamagnetic resonances appear.
2.2.3.3. 1,2-C6H4F2, Scheme 9. Addition of 1,2-difluoroben-
zene to Cp′2CeH at 20 °C in C6D12 yields a red-purple solution,
1
which turns orange over 30 min. After 5 min, the H NMR
spectrum contains Me3C resonances in a 2:1 area ratio and three
resonances due to one hydrogen each that are a singlet, a doublet
(J ) 8 Hz), and a triplet (J ) 8 Hz), Table 1. These resonances
are consistent with those expected for 13, Scheme 9. After 30
min at 20 °C, the ratio of 1, 3, and 13 is 2:4.5:1, and after one
day at 20 °C, the only paramagnetic resonances visible are those
due to 3 and diamagnetic resonances which are attributed to
B-5. Unfortunately, the aryl derivatives formed by the reaction
of 1,2-difluorobenzene with the metallacycle, 2, are not stable,
and the amount of 13 that forms is not sufficient to obtain a
satisfactory 19F NMR spectrum.
Figure 1. ORTEP diagram of Cp′2Ce(2,3,4,6-C6HF4), 6, 50% thermal
ellipsoids. The heavy atoms are refined anisotropically. Hydrogen atoms
(not shown) are placed in calculated positions and not refined. The fluorine
atom F(3) is disordered over two sites, C(37) and C(40), and the disorder
was modeled as F(3) on C(37) 75% and F(5) (not shown) on C(40) 25%.
The asymmetric unit contains one-half of a molecule of pentane, which is
not shown.
2.2.3.4. C6H5F. The reaction of fluorobenzene with Cp′2CeH
is related to that of the difluorobenzenes since it is slow, and
the lifetime of the fluoroaryl product, relative to decomposition
to 3, is insufficient to acquire satisfactory 1H or 19F NMR
spectra. A small amount of 3 is generated initially, along with
diamagnetic resonances of considerable intensity. Hydrolysis
and analysis by GC/MS yields one component with m/z of 310,
due to B-6 (Chart 1).
In summary, these four fluorobenzenes give cerium fluoroaryl
derivatives resulting from CeH for CH exchange.
2.2.4. NMR Spectroscopy. The solution 1H and19F NMR
spectra in either C6D12 or C6D6 at 20 °C of the metallocenece-
rium fluoroaryls generated and/or isolated that are described in
this article are listed in Table 1. The Me3C resonances are readily
assigned on the basis of their relative area ratios; their chemical
shifts lie in a narrow range between δ ) -1.5 to -2.2 ppm
and δ ) -9 to -10 ppm. The Me3C resonances are rather broad,
and the resonances due to the cyclopentadienyl ring methynes
are often very broad or not visible. The 19F NMR spectra are
assigned on the basis of their relative area ratios and spin–spin
coupling patterns. The H-F spin–spin coupling is also observed
in the 1H NMR spectra of those fluoroaryl derivatives that
contain hydrogens. In general, the resonances due to hydrogen
or fluorine atoms in the ortho sites are either very broad or
unobserved, whereas those in the meta or para sites are always
observed and narrow enough that the multiplicities due to
spin–spin coupling are visible. The 19F NMR chemical shifts
of the meta-F and para-F resonances generally lie in the range
of δ ) -140 to -165 ppm. The ortho-F resonances are further
upfield in the range of δ ) -210 to -285; when the ortho-F
sites are inequivalent, the resonances are separated by about
90 ppm as in 6 and 8.
Figure 2. Solid state structure of Cp′2Ce(2,3,4,5-C6HF4), 7, 50% thermal
ellipsoids. The 50/50 disorder of the fluoroaryl ring is simplified to show
one orientation; the Cp′-ring carbons and Ce atom are ordered and refined
anisotropically, but the C6HF4 ring carbon and fluorine atoms are refined
isotropically. The solid contains one-half of a molecule of disordered
pentane, which is not shown.
the fluoroaryl ring is asymmetrically substituted, the C2 axis
and a vertical plane of symmetry are absent, the molecule has
averaged Cs symmetry, and the Me3C group resonances will
appear in a 1:1:1 ratio, as in 6 and 8.
The NMR spectra are averaged spectra, assuming that the
solid state structures of these metallocene derivatives are similar
to those of Cp′2CeC6F51 and the two structures reported in this
article, Figures 1 and 2, which are described in more detail
below, in which there is one short Ce · · ·ortho-F contact and
the molecules have no symmetry. A physical process that
involves the synchronous breaking and making of Ce · · ·ortho-F
bonds is sufficient to account for the observed 1H NMR spectra
at 20 °C. It is important to note that this fluxional motion will
not result in the two ortho-F substituents exchanging sites; the
The Me3C resonances of the 1,2,4-(Me3C)3C5H2 ring on the
metallocene derivatives at 20 °C appear as an A2X or an AMX
pattern, Table 1. Assuming that the Cp′-rings are free to rotate
or oscillate about their C5 axes in the complexes in which the
fluorobenzene ring is symmetrically substituted, the metallocene
will have averaged C2V symmetry, and the Me3C resonances
will appear in a 2:1 area ratio, as found in 4, 9, and 12. When
9
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Fluorine for H Exchange in Hydrofluorobenzene Derivatives
A R T I C L E S
Figure 3. δ vs 1/T plot of the 1H NMR resonances of Cp′2Ce(2,3,5,6-
C6HF4), 4, in C7D14, T in Kelvin.
Figure 4. δ vs 1/T plot of the 1H NMR resonances of Cp′2Ce′(2,4,6-
C6H2F3), 9, in C7D14, T in Kelvin.
physical process of rotation is required for ortho-F/ortho-F site
exchange. For example, the 19F NMR spectrum of 9 at 20 °C
has two resonances, which shows that the chemically inequiva-
lent ortho-F and meta-H groups are averaged and, since the
Me3C groups appear in a 2:1 area ratio, 9 has averaged C2V
symmetry, and oscillation of the 2,4,6-C6H2F3 group about the
Ce-C(ipso) axis is a physical process that accounts for the
averaged spectra. However, the observation that the Me3C
groups in 7, 10, 11, and 13, in which the ortho sites contain a
fluorine and a hydrogen substituent, show an A2X pattern at
20 °C requires that additional fluxions must be occurring.
Rotation about the Ce-C(ipso) bond by π/2 to generate a time-
averaged mirror plane is one such physical process; this motion
can occur with the Ce · · ·ortho-F interaction “in place” or with
the Ce· · ·ortho-F interaction broken. In either case, rotation by
π/2 is sufficient to render the adjacent Me3C groups on the
cyclopentadienyl ring equivalent in 7, 10, 11, and 13.
Figure 5. δ vs 1/T plot of the 19F NMR resonances of Cp′2Ce′(2,3,4,6-
C6HF4), 6, in C7D14, T in Kelvin.
The variable-temperature 1H and 19F NMR spectra in C14D8
of several of the compounds listed in Table 1 were studied in
order to explore the ring dynamics in more detail. A limitation
is that the fluoroaryl derivatives decompose at varying rates at
temperatures above 20 °C, and only the low temperature
behavior is studied. As the temperature is lowered to -50 °C,
the Me3C resonance of area 2 decoalesces into two equal area
resonances for 4, 7, 9, 10, and Cp′2CeC6F5,1 consistent with a
molecule of Cs symmetry. The activation barriers ∆Gq
for
(Tc)
this process are approximately 10 kcal mol-1 in each case. As
the temperature is lowered from -50 to -80 °C, at least one
and often all three Me3C resonances grow another resonance,
the total intensity of which is about one-third that of the original
resonance. This behavior is clearly seen in the low-temperature
1H NMR spectra of 4 and 9. The unequal population shows
that the line-shape is not due to an equal population decoales-
cence phenomenon but is, perhaps, most likely due to the
presence of another rotamer in which the orientation of the
substituted cyclopentadienyl rings in the metallocene are dif-
Figure 6. δ vs 1/T plot of the 19F NMR resonances of Cp′2Ce′(2,3,6-
C6H2F3), 8, in C7D14, T in Kelvin.
8, the more shielded resonance has the strongest dependence
on temperature, while the less shielded resonance is weakly
dependent on temperature, as are the resonances due to meta-F
and para-F, Figures 5 and 6. As the temperature is lowered,
each of the fluorine resonances on the meta and para sites grow
an additional resonance. In each case the total populations are
constant, and warming generates the original spectra. The 19F
NMR line-shape supports the contention, developed from the
1H NMR spectra, that at least two isomers of Cs symmetry are
unequally populated at low temperature and only their relative
populations, but not their identities, are discernible from the
line-shape behavior. The presence of isomers that differ in free
energy as a result of the orientation of the substituted cyclo-
pentadienyl rings in paramagnetic metallocenes has been
postulated earlier on the basis of variable-temperature 1H NMR
spectroscopy.5 The fluxional NMR spectra from 20 to -50 °C
are consistent with a model in which the Cp′ and fluorophenyl
groups are dynamic. The minimum fluxional motion of the Cp′-
ring is an oscillation about the pseudo-C5 axis creating a time-
1
ferent but the averaged symmetry is still Cs. The H NMR
spectrum of 4 is consistent with this interpretation since the
para-H resonance, represented as δ vs T-1 plots in Figures 3
and 4, is a single, sharp resonance down to about -70 °C, when
another resonance whose intensity at -80 °C is about 10% of
the original resonance, appears as shown in Figure 3.
The temperature dependence of the 19F NMR spectra of
several of the compounds listed in Table 1 also show common
features. The most shielded and very broad resonance is
attributed to the ortho-F resonance due to its proximity to the
paramagnetic center, and this resonance is highly temperature
dependent. When the ortho-F sites are inequivalent, as in 6 or
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A R T I C L E S
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Table 2. Comparison of Averaged Bond Lengths (Å) and Angles (deg)
Cp′2CeC6F5a
Cp′2Ce(2,3,4,6-C6HF4), 6
Cp′2Ce(2,3,4,5-C6HF4),b
7
Ce-C(Cp′-ring), ave
Ce-(Cp′-ring centroid)
Ce-C(ipso)
2.82 ( 0.06
2.54
2.621(4)
2.682(2)
147
97.5(3)
149.7(3)
116.4(3)
93.6(1)
2.82 ( 0.05
2.55
2.623(3)
2.711(2)
145
98.9(2)
150.5(3)
115.2(3)
93.8(2)
2.82 ( 0.06
2.54
2.64 ( 0.02
2.863 ( 0.005
144
102.6 ( 0.6
140.5 ( 0.4
115.5 ( 0.5
92.0 ( 0.7
Ce· · ·F
(Cp′-ring centroid)-Ce-(Cp′-ring centroid)
Ce-C(ipso)-C(ortho)c
Ce-C(ipso)-C(ortho)d
C(ipso)-C(ortho)-F
Ce· · ·F-C(ortho)
a From ref 1. b Averaged values for the two disordered molecules in the asymmetric unit, deviations are rms values. c The angle involved in the
Ce· · ·F interaction. d The angle not involved in the Ce · · ·F interaction.
Table 3. Crystal Data
2.3.2. Crystal Structures of 6 and 7. It is important that the
crystal data for 7 are collected within a few days after the
Cp′2Ce(2,3,4,6-C6HF4), 6
Cp′2Ce(2,3,4,5-C6HF4), 7
crystals are isolated, since the crystals of 7 change to those of
6 in the solid state, in a Schlenk tube stored inside the drybox
at 20–25 °C over the time period of approximately 2 months.
The single crystal used for the X-ray structure determination
of 7 was obtained as shown in Scheme 3, and the crystals were
obtained by crystallization from pentane. The 1H NMR spectrum
of several of these crystals dissolved in C6D6 showed only
resonances due to 7 and pentane of crystallization. The single
crystal that was used for the structure determination of 6 was
crystal system
space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/n,Z ) 4
10.2210(6)
22.0625(13)
18.0234(11)
92.369(1)
4060.9(4)
-115
monoclinic
P21/n,Z ) 4
10.1818(5)
22.1638(11)
17.8504(9)
91.591(1)
4026.7(3)
-104
ꢀ (deg)
V (Å3)
T (°C)
1
obtained as shown in Scheme 2. The H NMR spectrum of
several of these crystals dissolved in C6D6 showed only
resonances due to 6 and pentane.
averaged mirror plane rendering the top and bottom rings
equivalent. The motion of the fluorophenyl ring is involved in
generating another symmetry plane that results in time-averaged
Me3C groups, but this motion is not necessarily free rotation;
an oscillation about the Ce-C(ipso) bond is sufficient. The
limitations imposed by the paramagnetic nature of these
compounds render the interpretation of their dynamic behavior
qualitative.
Complexes 6 and 7 crystallize in the monoclinic crystal
system in space group P21/n with Z ) 4. The unit cell contains
one-half of a molecule of disordered pentane in each structure.
The crystal data, collected at -115 °C for 6 and -104 °C for
7, are shown in Table 3, and the packing diagram of 6 is shown
in Figure 7. The packing diagram for 7 is identical to that for
6 and is available as Supporting Information. Inspection of the
packing diagrams shows that the unit cell contains considerable
empty space, some of which is filled by the molecule of pentane.
The closest the individual molecules approach each other is
3.03 Å.
The crystal structures of 6 and 7 are isomorphous, and the
only difference is that the a- and c-dimensions of 6 are slightly
longer, while the b-dimension is slightly shorter, relative to those
of 7, resulting in the unit cell volume of 6 being about 0.8%
larger than that of 7. The similarity in unit cell parameters
precludes monitoring their change as a function of time in order
to determine the rate law and rate constant for the solid state
rearrangement, which means that the mechanism for the
rearrangement is necessarily qualitative.
The small change in unit cell parameters shows that the
rearrangement of 7 to 6 is not driven by a favorable change in
the free energy of the ensemble, but the rearrangement is driven
by a favorable change in ∆G of the individual molecules in the
ensemble, that is, molecules of 6 have lower free energy than
those of 7. The ensemble, however, plays a critical role since it
allows 7 to rearrange cleanly to 6 without formation of
detectable amounts of Cp′2CeF and a benzyne, a pathway that
both complexes follow in the solution at 20 °C, over a much
shorter time period. This behavior implies that the rearrangement
mechanism in the solid state does not proceed by formation of
Cp′2CeF and a “free” benzyne followed by trapping of the
benzyne by Cp′2CeF, i.e., a reversible stepwise process. The
solid state environment either prevents benzyne from escaping
and ensures that the benzyne is trapped by Cp′2CeF, or the “free”
2.3. Solid State Studies. 2.3.1. Molecular Structures of 6 and
7. As mentioned above, the solution 1H NMR spectra at 20 °C
of 7, 10, 11, and 13 indicate that these complexes are either
fluxional or they have structures that are different from 4, 6, 8,
9, 12, and Cp′2CeC6F5; the solid state structure of the latter
complex is available in the literature,1 and the X-ray crystal
structures for 6 and 7 are reported below. An ORTEP diagram
for 6 is shown in Figure 1, and the important bond distances
and angles are listed in Table 2, along with those for 7 and
Cp′2CeC6F5. A partial ORTEP diagram for 7 is shown in Figure
2; the C6HF4-ring is disordered over two equivalent positions,
but only one of the molecules is illustrated in Figure 2. Crystal
data for 6 and 7 are shown in Table 3, and additional details
are available as Supporting Information.
Inspection of the ORTEP diagrams and the geometrical
parameters for 6 and 7, along with these data for Cp′2CeC6F5,
shows that the molecular structures are similar; the Cp′-rings
are staggered with identical averaged Ce-C (Cp′-ring) distances.
The planar fluoroaryl rings lie essentially in a plane perpen-
dicular to the plane defined by the (ring centroid)-Ce-(ring
centroid) construction. The Ce-C(ipso) vector lies essentially
on the idealized molecular C2 axis even though one of the
C(ortho)-F groups in 6 and Cp′2CeC6F5 and the only C(ortho)-F
group in 7 have short Ce· · ·Fe contact distances. Accordingly,
the Ce · · ·F contact bends the entire C6HxF(5-x), x ) 0, 1, ring so
that it does not lie on the C2 axis.
(5) (a) Lukens, W. W.; Beshouri, S. M.; Stuart, A. L.; Andersen, R. A.
Organometallics 1999, 18, 1247–1252. (b) Zi, G.; Blosch, L. L.; Jia,
L.; Andersen, R. A. Organometallics 2005, 24, 4602–4612.
9
7160 J. AM. CHEM. SOC. VOL. 130, NO. 22, 2008

Fluorine for H Exchange in Hydrofluorobenzene Derivatives
A R T I C L E S
Figure 7. Crystal packing diagram for Cp′2Ce(2,3,4,6-C6HF4), 6.
benzyne never forms, implying that the mechanism of the
Cp′2Ce and F site exchange between C(35) and C(36) is
synchronous. Although the rearrangement mechanism can only
be described qualitatively, the rearrangement proceeds quanti-
tatively and irreversibly in the solid state, and the net reaction
is exoergic.
5, with two exceptions, always shows a regiochemistry in
which both of the ortho sites in the fluoroaryl ligand in Cp′2Ce
-C6H5-xFx are occupied by fluorine atoms, symbolized as
Ce-CiCo(F,F). The only exceptions to this generalization are
the reactions between Cp′2CeH and 1,4-difluorobenzene or 1,2-
difluorobenzene, which can only afford isomers in which one
ortho site is occupied by a fluorine atom, viz., Ce-CiCo(F,H).
This general substitution pattern presumably reflects the trend
in CeC bond dissociation enthalpies (bond strengths) which then
lie in the order Ce-CiCo(F,F) > Ce-CiCo(F,H) > Ce-
CiCo(H,H). Unfortunately, these experimental bond dissociation
enthalpies are not known, but the solid state rearrangement of
7 to 6 clearly shows that the inequality is ∆H Ce-CiCo(F,F) >
3. Discussion
The reaction products that form when Cp′2CeH is exposed
to a series of hydrofluorobenzenes, C6H6-xFx, x ) 2 – 5, are
shown in Schemes 1-9 and summarized in Chart 2. The only
product, or the major product, that forms when x ) 2, 3, 4, or
9
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A R T I C L E S
Werkema and Andersen
Chart 2. Fluoroarene Reaction Products with Cp′2CeH
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Fluorine for H Exchange in Hydrofluorobenzene Derivatives
A R T I C L E S
Chart 2. Continued
Chart 3. Charge Distribution in Cp′2CeC6H5-xFx
∆H Ce-CiCo(F,H), since the change in translational entropy is
zero, and therefore ∆H ≈ ∆G , assuming that the Ce-substituted
cyclopentadienyl bond enthalpy is constant. Support for this
inequality is derived from experimental and calculational bond
dissociation enthalpies in CpRe(CO)2C6H5-xFx.6 The experi-
mental values of the Re-C(C6H5-xFx) bond dissociation enthal-
pies (where known) agree with the calculated ones, which lie
in the order Re-CiCo(F,F) > Re-CiCo(F,H) > Re-CiCo(H,H).
The origin of this order parallels the increase in electrostatic
contribution to the net bond dissociation enthalpy, while the
orbital contribution remains essentially constant. This postulate
is supported by the calculated charge on Ci, from a natural
population analysis (NPA), which shows that the charge density
on Ci increases in the order CiCo(F,F) > CiCo(F,H) > CiCo(H,H).
Thus, two fluorine atoms located in the ortho sites of a fluoro-
substituted aryl group increase the negative charge on the ipso
site more than does one fluorine atom, which in turn is higher
than when both of the ortho sites contain hydrogen atoms.
This model should be applicable to the Ce-C bond dissocia-
tion enthalpies for the compounds mentioned above, since
lanthanide-X bond dissociation enthalpies are dominated by
electrostatic contributions, i.e., they are dominated by the
Coulombic attraction between the two charges M(+)-X(-).7
Extending this model to the compounds described in this article
yields the charge distribution at C(ipso) and C(ortho) shown in
Chart 3. The electronegative fluorine atom will induce a larger
positive charge on the carbon atom to which it is bonded than
will the less electronegative hydrogen atom, which in turn
induces a larger negative charge on the ipso-carbon atom. This
thermodynamic model is in accord with the product formed in
the reaction between Cp′2CeH and the hydrofluorobenzenes in
Chart 2 and the solid state rearrangement of 7 to 6. Although
the translational entropy for the solid state rearrangement of 7
to 6 is zero, the entropy content in 6 is greater than that in 7,
since two Ce(ortho)-F interactions are available in 6 but only
one in 7. Thus, the vibrational entropy in the two complexes is
not identical, and the rearrangement is favored enthalpically and
entropically.8
Although the thermodynamics of the rearrangement is clear
the mechanism is not. In the initial paper, the calculated
mechanism of the reactions of Cp′2CeH (modeled by Cp2LaH)
with either C6F6 or C6HF5 proceed by way of σ-bond metathesis
transition states in which the barrier for the CeH for CH
exchange process is about 20 kcal mol-1 lower than that for
CeH for CF exchange, resulting in the generalization that CF
activation products are not observed when CH bonds are present
in the hydrofluorobenzene.1 Extension of this generalization
leads to a conflict with the products formed in the reaction of
Cp′2CeH with 1,2,3,4-tetrafluorobenzene and 1,2,3-trifluoroben-
zene. In both of these reactions, the products derived from CeH
for CF and CH activation are observed; the major product in
both reactions is derived from CF activation. If the CH activation
step occurs with a barrier lower than that of the CF activation
step, the products of the reaction of Cp′2CeH and 1,2,3-
trifluorobenzene would be those shown in eqs 2a and 2b.
(6) (a) Carbo, J. J.; Eisenstein, O.; Higgitt, C. L.; Klahn, A. H.; Maseras,
F.; Oelckers, B.; Perutz, R. N. J. Chem. Soc., Dalton Trans. 2001, 9,
1452–1461. (b) Clot, E.; Besora, M.; Maseras, F.; Megret, C.; Eisenstein,
O.; Oelckers, B.; Perutz, R. N. Chem. Commun. 2003, 4, 490–491.
(7) (a) Johnson, D. A. Some Thermodynamic Aspects of Inorganic
Chemistry, 2nd ed.; Cambridge University Press: Cambridge, 1982;
Chapters 2 and 6. (b) Pankratz, L. B. Thermodynamic Properties of
Halides, Bulletin 674; U.S. Bureau of Mines, 1984. (c) Nolan, S. P.;
Hedden, D.; Marks, T. J. Bonding Energetics in Organolanthanide
Chemistry; Marks, T. J. Ed.; ACS Symposium Series 428; American
Chemical Society: Washington, DC, 1990; p 159.
(8) We thank a reviewer for calling our attention to the fact that the
vibrational entropy of 6 is greater than that of 7.
9
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A R T I C L E S
Werkema and Andersen
spectroscopy. NMR spectra were recorded on Bruker AV-300 or
AV-400 spectrometers at 20 °C in the solvent specified. 19F NMR
chemical shifts are referenced to CFCl3 at 0 ppm. J-Young NMR
tubes were used for all NMR tube experiments. Electron impact
mass spectrometry and elemental analyses were performed by the
microanalytical facility at the University of California, Berkeley.
The abbreviation Cp′ is used for the 1,2,4-tri-tert-butylcyclopen-
tadienyl ligand.
Cp′2CeH + 1,2,3-C6H3F3 f Cp′2Ce(2,3,4-C6H2F3) + H2
Cp′2CeH + 1,2,3-C6H3F3 f Cp′2Ce(3,4,5-C6H2F3) + H2
(2a)
(2b)
Although the C-H isomer illustrated in 2a, 10, is formed,
the major product is the result of CF activation, Cp′2Ce(2,6-
C6H3F2), 12, Schemes 6 and 8. A set of elementary reactions
that account for the formation of 12 are those illustrated in eqs
3a-c. If true, 12 is formed by two pathways, 3a and 3c; 3a is
a CeH/CF exchange while 3c is a CeH/CH exchange. The H/F
interchange reaction, 3b, is likely to occur with a low activation
barrier;1 this process along with the reaction symbolized by 3a
yields Cp′2CeF. Reaction 3c demands that Cp′2CeH is not
depleted in reactions 3a and 3b, which means that the barrier
for the reaction 3c must be comparable with that of 3a.
5.2. General Procedure for NMR Tube Reactions of
C6H6-xFx with Cp′2CeH, 1. Cp′2CeH1 was dissolved in C6D6 or
C6D12 in an NMR tube, and a drop of the desired fluorobenzene
was added. The solution turned from purple to orange, and gas
1
bubbles were evolved. The sample was analyzed by H and 19F
NMR spectroscopy. Decomposition over time was observed by
NMR spectroscopy, first at 20–25 °C over 3–7 days and then at 60
1
°C for one day. Products are summarized in Chart 2 and H and
19F NMR resonances are listed in Table 1. Samples for GC/MS
were prepared by adding a drop of H2O, agitating, and allowing
the samples to stand closed for 10 min. The samples were then
dried over magnesium sulfate, filtered, and diluted 10-fold with
pentane. A 1 µL sample was injected into a HP6890 GC system
with a J&W DB-XLB universal nonpolar column, attached to an
HP5973 Mass Selective Detector. The principle elution peaks
consisted of free Cp′H and the cycloaddition product(s) of the
fluorobenzyne(s) and benzene or Cp′H.
Cp′2CeH + 1,2,3-C6H3F3 f Cp′2Ce(2,6-C6H3F2) + HF
Cp′2Ce(2,6-C6H3F2) + HF f Cp′2CeF + 1,3-C6H4F2 + H2 (3b)
Cp′2CeH + 1,3-C6H4F2 f Cp′2Ce(2,6-C6H3F2) + H2 (3c)
(3a)
A similar contradiction is apparent in the reaction of 1,2,3,4-
tetrafluorobenzene in which the major product is 8, Scheme 3,
the result of Ce-H/CF exchange. Indeed, the contradiction also
extends to C6F6 and C6HF5.1 The contradiction between the
calculational and experimental studies described in this article
may be rationalized in the following ways: (a) the calculational
methodology does not correctly deal with the large amount of
charge reorganization in the transition state in the CeH for CF
exchange, (b) the mechanism of reaction may not proceed by
way of a σ-bond metathesis mechanism, or (c) the bulky
substituted cyclopentadienyl ligands influence the barriers more
than expected, since unsubstituted cyclopentadienyl ligands are
used in the calculations.
5.3. General Procedure for NMR Tube Reactions of C6H6-
xFxwith the Metallacycle Cp′[(Me3C)2C5H2C(Me2)CH2]Ce, 2.
1
Cp′2CeCH2C6H5 was dissolved in C6D12 in an NMR tube and
heated at 60 °C for one day, which yielded the metallacycle. A
drop of the desired fluorobenzene was added, and the solution turned
from purple to orange. Subsequent handling and analysis were
identical to those for reactions with Cp′2CeH. The synthetic details
for two specific reactions that yielded isolated compounds are
described below.
5.4. Cp′2Ce(2,3,4,5-C6HF4), 7. Cp′2CeCH2C6H5 (0.5 g, 0.7
mmol) was dissolved in pentane (10 mL) and stirred at room
temperature for 48 h, producing a solution of Cp′[(Me3C)2C5H2-
C(Me2)CH2]Ce. 1,2,3,4-Tetrafluorobenzene (0.18 mL, 1.7 mmol)
was added via syringe. The purple solution turned orange over 20
min. The solution volume was reduced to 5 mL, and the solution
was cooled to -10 °C, yielding orange crystals. Yield: 0.2 g (0.26
mmol), 37%. The low yield was due to the high solubility of the
compound. 1H NMR (C6D12) δ –1.90 (36H, ν1/2 ) 120 Hz), -9.59
(18H, ν1/2 ) 80 Hz), 19F NMR (C6D12) δ -137.0 (1F, d, J ) 18
Hz), -161.8 (1F, d, J ) 18 Hz), -161.6 (1F, dd, J ) 18, 18 Hz).
The solid material decomposed rapidly above 135 °C, which
precluded analysis by EI-MS. Full crystallographic details are
included as Supporting Information: Monoclinic cell space group,
P21/n: a ) 10.1818(5) Å, b ) 22.164(1) Å, c ) 17.8504(9) Å, ꢀ
) 91.591(1)°, V ) 4026.7(3) Å3.
4. Conclusions
The stereochemical principle that emerges from the experi-
mental studies described in this article is that when a choice of
regioisomers is available, the isomer that is observed exclusively
or in the highest yield is always the one in which the fluoroaryl
group contains fluorine atoms in both of the ortho sites of the
polyfluorophenyl derivative. This thermodynamic result is
postulated to be dictated by the CeC bond dissociation enthalpy
that is controlled by the electronegative fluorine atoms that
induce polarization at the ortho-carbon atoms, Co(δ+)-F(δ-),
which in turn induces a negative charge on the ipso-carbon,
Ce-Ci(δ-). Thus the strongest Ce-Ci is formed when both
ortho carbons of the phenyl ring contain fluorine substituents.
The elementary reactions that comprise the net reaction are
consistent with the postulate that the activation energy for CH
and CF have comparable values, and the stereochemistry of the
product is determined by the change in free energy of the net
reaction rather than the activation energy of the elementary
reactions. The thermodynamic control is dramatically illustrated
by the irreversible solid state rearrangement (25 °C) of 7 to 6,
a CeC(ipso) for C(ortho)F site exchange.
5.5. Cp′2Ce(2,3,4,6-C6HF4), 6. Cp′2Ce(2,3,4,5-C6HF4) (0.2 g,
0.26 mmol) was allowed to stand at 25 °C for 2 months. A sample
was dissolved in C6D12 and analyzed by 1H and 19F NMR
spectroscopy,whichindicatedquantitativeconversiontoCp′2Ce(2,3,4,6-
C6HF4). Yield: 0.2 g (0.26 mmol), 100%. This complex was also
prepared from the metallacycle and 1,2,3,5-tetrafluorobenzene in a
1
procedure analogous to that described above. H NMR (C6D12) δ
0.17 (1H, d, J ) 7 Hz), -1.44 (18H, ν1/2 ) 100 Hz), -2.09 (18H,
ν1/2 ) 90 Hz), -9.58 (18H, ν1/2 ) 70 Hz), 19F NMR (C6D12) δ
-139 (1F, dd, J ) 18, 7 Hz), -151 (1F, ν1/2 ) 200 Hz), -166
(1F, d, J ) 15 Hz), -242 (1F, ν1/2 ) 200 Hz). The solid material
decomposed rapidly above 135 °C, which precluded analysis by
EI-MS. Full crystallographic details are included as Supporting
Information: Monoclinic cell space group, P21/n: a ) 10.2210(6)
Å, b ) 22.063(1) Å, c ) 18.024(1) Å, ꢀ ) 92.369(1)°, V )
4060.9(4) Å3.
5. Experimental Details
5.1. General. All manipulations were performed under an inert
atmosphere using standard Schenk and dry box techniques. All
solvents were dried and distilled from sodium or sodium benzo-
phenone ketyl. Fluoro and hydrofluorobenzenes, obtained from
Aldrich Chemical Co., were dried and vacuum transferred from
calcium hydride; the isomer purity was assayed by 1H and 19F NMR
Crystallographic data for the structures in this paper have been
deposited with the Cambridge Crystallographic Data Centre. Copies
of the data (CCDC 671970 and 671971) can be obtained free of
9
7164 J. AM. CHEM. SOC. VOL. 130, NO. 22, 2008

Fluorine for H Exchange in Hydrofluorobenzene Derivatives
A R T I C L E S
charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge CB2
1EZ, U.K.; fax +44 1223 336033. Structure factor tables are
available from the authors.
(at CHEXRAY the U.C. Berkeley X-ray diffraction facility), and
Odile Eisenstein and Laurant Maron for discussions and comments.
Supporting Information Available: Labeling diagrams, tables
giving atomic positions and anisotropic thermal parameters,
bond distances and angles, and least-squares planes for each
structure. This material is available free of charge via the Internet
at http://pubs.acs.org.
Acknowledgment. This work was partially supported by the
Director of Energy Research Office of Basic Energy Sciences,
Chemical Sciences Division of the U.S. Department of Energy under
Contract No DE-AC02-05CH11234. We thank Dr. Fred Hollander
and Dr. Allen Oliver for their assistance with the crystallography
JA800639F
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J. AM. CHEM. SOC. VOL. 130, NO. 22, 2008 7165