The reaction of bis(1,2,4-tri-t-butylcyclopentadienyl)ceriumbenzyl, Cp′2CeCH2Ph, with methylhalides: A metathesis reaction that does not proceed by a metathesis transition state

Article abstract of DOI:10.1039/b918103b

The experimental reaction between [1,2,4-(Me₃C) ₃C₅H₂]₂CeCH₂Ph and CH₃X, X = F, Cl, Br, and I, yields the metathetical exchange products, [1,2,4-(Me₃C)₃C₅H₂] ₂CeX and CH₃CH₂Ph. The reaction is complicated by the equilibrium between the benzyl derivative and the metallacycle [1,2,4-(Me₃C)₃C₅H₂][(Me ₃C)₂C₅H₂C(CH₃) ₂CH₂]Ce, plus toluene since the metallacycle reacts with CH₃X. Labelling studies show that the methyl group of the methylhalide is transferred intact to the benzyl group. The mechanism, as revealed by DFT calculations on (C₅H₅) ₂CeCH₂Ph and CH₃F, does not proceed by way of a four-center mechanism, a σ-bond metathesis, but by a lower barrier process involving a haptotropic shift of the Cp₂Ce fragment so that at the transition state the para-carbon of the benzene ring is attached to the Cp₂Ce fragment while the CH₂ fragment of the benzyl group attacks CH₃F that is activated by coordination to the metal ion. As a result the mechanism is classified as an associative interchange process. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b918103b

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The reaction of bis(1,2,4-tri-t-butylcyclopentadienyl)ceriumbenzyl,
Cp¢2CeCH2Ph, with methylhalides: a metathesis reaction that does not
proceed by a metathesis transition state†
Evan L. Werkema,a Richard A. Andersen,*a Laurent Maron*b and Odile Eisenstein*c
Received 2nd September 2009, Accepted 1st December 2009
First published as an Advance Article on the web 10th February 2010
DOI: 10.1039/b918103b
The experimental reaction between [1,2,4-(Me3C)3C5H2]2CeCH2Ph and CH3X, X = F, Cl, Br, and I,
yields the metathetical exchange products, [1,2,4-(Me3C)3C5H2]2CeX and CH3CH2Ph. The reaction is
complicated by the equilibrium between the benzyl derivative and the metallacycle
[1,2,4-(Me3C)3C5H2][(Me3C)2C5H2C(CH3)2CH2]Ce, plus toluene since the metallacycle reacts with
CH3X. Labelling studies show that the methyl group of the methylhalide is transferred intact to the
benzyl group. The mechanism, as revealed by DFT calculations on (C5H5)2CeCH2Ph and CH3F, does
not proceed by way of a four-center mechanism, a s-bond metathesis, but by a lower barrier process
involving a haptotropic shift of the Cp2Ce fragment so that at the transition state the para-carbon of the
benzene ring is attached to the Cp2Ce fragment while the CH2 fragment of the benzyl group attacks
CH3F that is activated by coordination to the metal ion. As a result the mechanism is classified as an
associative interchange process.
The calculated free energy barriers (called hereafter activation
barriers) for the C–H activation step are relatively low, DG‡
Introduction
ranges from 18 kcal mol-1 (F) to 14 kcal mol-1 (OMe), but the
activation barrier for the second step is higher in all cases studied.
The calculated activation barrier for a synchronous process that
proceeds by way of a metathesis transition state is higher, by about
6 to 8 kcal mol-1, than the second, rate determining step. The
physical picture that emerges from the calculations is that the
metathesis transition state A has negative charge accumulation
on H and X and positive charges on Cp2Ce and on CH3. It has
a higher activation barrier since it resembles Cp2Ce+CH4X- and
CH4X- is a high energy species. Accordingly, the reactants choose a
two-step pathway in which the transition state for C–H activation,
B, forms Cp2CeCH2X. This is followed by a step in which CH2
inserts into H2 with cleavage of the C–X bond forming Cp2CeX.
Experimental evidence for the two-step mechanism was derived
by observing that (a) the Me3C-groups on the Cp¢-rings can act
as an intramolecular trap for CH2, when H2 is absent, as can
added cyclohexene, which formed norcarane, and cyclohexane-d12
solvent, which formed methylcyclohexane-d12, (b) NMR evidence
was obtained for Cp¢2CeCH2X, X = Cl, Br, I and (c) when X =
The preparation and reactions of [1,2,4-(Me3C)3C5H2]2CeH, ab-
breviated as Cp¢2CeH, with aliphatic and aromatic hydrofluoro-
carbons CH3F and C6H6-nFn, n = 1–6, respectively, have been
published.1,2,3 These studies were extended to other methylhalides
and related compounds, CH3X, X = Cl, Br, I, OMe, NMe2
recently.4 The reactions of these methyl-derivatives with Cp¢2CeH,
illustrated by the net reaction in eqn (1), are at first glance a
simple metathetical H for X exchange reaction but the reaction
mechanism does not proceed by a four-center metathesis transition
state.
Cp¢2CeH + CH3X → Cp¢2CeX + CH4 X = F, Cl, Br, I, OMe (1)
The combined experimental and computational studies1,4
showed that the reaction proceeds by a two-step process, the first
of which is an intermolecular C–H activation, eqn (2a), that is
followed by ejection of CH2 and trapping by H2, eqn (2b).
Cp¢2CeH + CH3X → Cp¢2CeCH2X + H2
Cp¢2CeCH2X + H2 → Cp¢2CeX + CH4
(2a)
(2b)
2
OMe, Cp¢2Ce(h -CH2OMe) was isolated.1,4 Thus, the combined
computational and experimental studies showed that the two-step
pathway proceeding by way of a carbenoid intermediate is general
for the CH3X derivatives studied.
aDepartment of Chemistry and Chemical Sciences Division of Lawrence
Berkeley National Laboratory, University of California, Berkeley, Califor-
nia, 94720-1460
bLPCNO, Universite´ de Toulouse, INSA, UPS, LPCNO, 135 avenue de
Rangueil, F- 31077 Toulouse, France, and CNRS, LPCNO, F-31077,
Toulouse, France
cInstitut Charles Gerhardt, Universite´ Montpellier 2, CNRS 5253, cc 1501
Place E. Bataillon, F-34095, Montpellier, France
† Electronic supplementary information (ESI) available: X-ray crystallo-
graphic data (CIF), d vs. 1/T plots, optimized structures, E and G (in a.
u.) for all stationary points. CCDC reference numbers 745664 for [1,2,4-
(Me3C)3C5H2]2CeCH2Ph, and 745665 for [1,2,4-(Me3C)3C5H2]2Ce(4-
methylbenzene). For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b918103b
The cerium metallocenes used in these studies, Cp¢2CeCH2Ph,
Cp¢2CeH and Cp¢2CeF are monomeric in the solid state and pre-
sumably in solution as well. Hence, they are excellent experimental
6648 | Dalton Trans., 2010, 39, 6648–6660
This journal is
The Royal Society of Chemistry 2010
©

models for the computational studies, which were carried out using
the C5H5 metallocenes in the gas phase.
In this article, a combined experimental and computational
study of the reaction of Cp¢2CeCH2Ph, along with its chemical and
physical properties, with CH3X, X = F, Cl, Br, or I, a stoichiometric
C–C bond forming reaction, is described, eqn (3).
are not observed. In order to confirm this assignment, Cp¢2Ce(4-
methylbenzyl) is prepared and isolated as outlined in the Exper-
1
imental section. The variable temperature H NMR spectrum in
C7D8 of Cp¢2CeCH2Ph shows that the Me3C-resonances are in
a 2 : 1 area ratio at temperatures greater than 300 K but as the
temperature is lowered they decoalesce and reappear as a 1 : 1 : 1
area ratio pattern, see ESI for the d vs. T-1 plots for this and the 4-
methylbenzyl derivative.† The spectra show that Cp¢2CeCH2Ph is a
fluxional molecule with average C2v symmetry at high temperature
but Cs symmetry at lower temperature. In addition, a resonance
assigned to the meta-H’s of the benzyl group is observed at -80 ◦C
as a single resonance in the benzyl and 4-methylbenzyl derivatives,
implying that the phenyl ring is either in the plane of symmetry,
perpendicular to it, or oscillating about the CeCH2–C(ipso) bond
generating a time averaged plane of symmetry.
In the solid state, two molecules are found in the asymmetric unit
of Cp¢2CeCH2Ph in equal amounts as shown in Fig. 1. The
two molecules in the asymmetric unit, a and b in Fig. 1 have
different orientations of their Cp¢ rings and CH2Ph groups. Since
the population of each conformer in the unit cell is equal, their
individual free energies are equal, see ESI for crystallographic
details.† In conformer a, the Cp¢ ring carbon atoms are eclipsed,
which results in two of the Me3C groups at the back of the wedge
avoiding each other as much as possible resulting in the four
Me3C groups in the front of the wedge being pairwise eclipsed.
The Cp¢(ring centroid)–Ce(2)–Cp¢(ring centroid) angle is 149◦ and
the Ce(2)–C(76)–C(77) angle is 130.4(3)◦; the open Ce(2)–C(76)–
Cp¢2CeCH2Ph + CH3X → Cp¢2CeX + CH3CH2Ph
(3)
Results
Synthetic studies. Solution and solid state properties of
Cp¢2CeCH2Ph
The synthesis and some physical properties of Cp¢2CeCH2Ph
are reported in an earlier article2 and additional ones are
reported here. Some of the chemical reactions described in
this article, along with those reported earlier, are shown in
Scheme 1. The benzyl derivative is an excellent precursor to
two useful derivatives, the hydride and the metallacycle [1,2,4-
(Me3C)3C5H2][(Me3C)2C5H2C(CH3)2CH2]Ce, and their deuter-
ated analogues. Although the benzyl and the 4-methylbenzyl
derivatives are isolable, these are the only alkyl derivatives that are
thermally stable at room temperature; all attempts to make Me,
CH2CMe3 or CH2SiMe3 derivatives result in alkane elimination
and formation of the metallacycle at low temperature.
˚
C(77) angle results in a Ce(2) ◊ ◊ ◊ C(77) distance greater than 3.7 A.
The other conformer, b, is rather different, since the Cp¢-ring
carbons atoms are staggered with a closed Cp¢(ring centroid)–
Ce(1)–C¢(ring centroid) angle of 138◦, 11◦ less than in a. In
addition, the Ce(1)–C(35)–C(36) angle of 93.1(4)◦ is some 37◦ less
than the equivalent angle in a, which results in a short Ce(1)–C(36)
˚
distance of 3.023(4) A. The flat phenyl ring is orientated more or
less perpendicular to the open wedge of the Cp¢2Ce fragment, and
˚
the Ce(1)–C(37) and Ce(1)–C(41) distances are 3.996 and 3.253 A,
respectively, thus, the classification of the benzyl group bonding
2
3
2
3
as h or h is ambiguous; the representation Ce(h /h –CH2Ph)
seems appropriate.
The solid state molecular structure of Cp¢2Ce(4-methylbenzyl)
is shown in Fig. 2. The important distances and angles are given
in the caption to Fig. 2. The geometrical parameters for Cp¢2Ce(4-
2
3
methylbenzyl) are very similar to those of Cp¢2Ce(h /h -CH2Ph),
Fig. 1b, including the orientation of the cyclopentadienyl rings in
the Cp¢2Ce fragment and the orientation of the planar benzene
ring.
2
3
2
3
The geometry of the Ce(h /h -CH2Ph) and Ce(h /h -4-
methylbenzyl) fragments, Fig. 1b and 2 respectively, is similar to
3
that found in (C5Me5)2Ce(h -CH2Ph).5 In the latter example, the
Ce–CH2–C(ipso) angle of 86.0(3)◦ is even more acute than the
equivalent angle in the molecule shown in Fig. 1b, resulting in a
˚
Scheme 1
Ce ◊ ◊ ◊ C(ipso) and one Ce ◊ ◊ ◊ C(ortho) distance of 2.885(5) A and
˚
˚
2.882(6) A, respectively. The Ce–CH2Ph distance of 2.596(5) A is
identical, to within 3s, of the equivalent distances in the molecules
shown in Fig. 1a, 1b and 2. The benzyl group in the C5Me5
derivative has three Ce–C distances less than 3 A and therefore is
classified as a h -benzyl. The differing classification clearly is the
1
The H NMR spectrum of the benzyl derivative in C6D6 at
19 ◦C was mentioned previously.2 The spectrum shows the Me3C
group resonances as three broad singlets in a 1 : 1 : 1 area ratio,
inequivalent Cp¢-ring CH resonances and one other resonance of
area 1 that is assumed to be the para-H resonance of the benzyl
group; the CH2 and the benzene ring ortho and meta resonances
˚
3
result of intramolecular steric effects in the metallocene fragments.
The solid state crystal structures of metal-benzyl derivatives often
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6648–6660 | 6649
©

1
Fig. 1 ORTEP of Cp¢2CeCH2Ph showing the two molecules in the asymmetric unit, a, the conformer with Ce(h -CH2Ph) and, b, the conformer with
2
3
Ce(h /h -CH2Ph). The non-hydrogen atoms are refined anisotropically and shown as 50% ellipsoids; the hydrogen atoms are not shown, but they are
˚
placed in idealized positions and not refined. Selected bond distances (A) and angles (deg) are a: Ce(2)–C(Cp¢) = 2.83 0.07 (ave), range 2.752(4)–2.932(4),
Ce(2)–Cp¢(ring centroid) = 2.54, Cp¢(ring centroid)–Ce–Cp¢(ring centroid) = 149, Ce(2)–C(76) = 2.577(4), Ce(2)–C(76)–C(77) 130.4(3). b: Ce(1)–C(Cp¢)=
2.87 0.07(ave), range 2.761(4)–3.010(4), Ce(1)–Cp¢(ring centroid) = 2.54, Cp¢(ring centroid)–Ce(1)–Cp¢(ring centroid) = 138, Ce(1)–C(35) = 2.584(4),
Ce(1)–C(36) = 3.023(4), Ce(1)–C(37) = 3.996, Ce(1)–C(41) = 3.253, Ce(1)–C(35)–C(36) = 93.1(4).
Solid state behavior of Cp¢2CeCD2C6D5
Since the studies described in this article are aimed at un-
derstanding the mechanism of the C–C bond formation, eqn
(3), it is important to study the deuterated benzyl derivative
in order to show whether or not the methyl group of the
methylhalide is transferred intact in the ethylbenzene product. The
deuterated derivative, Cp¢2CeCD2C6D5 is prepared as described in
the Experimental section; the d7-derivative behaves similarly to
the d0-derivative in solution. However, on prolonged storage in
the solid state at 20–25 ◦C in the absence of air and moisture for
about three years, the deuteria in the a-CD2 sites are replaced by
hydrogen and one of the Me3C-groups is enriched with deuteria.
The extent of the H/D exchange is determined by solution 1H and
2H NMR spectroscopy. Thus, when first prepared and isolated
the 1H NMR spectrum in C6D6 consists of three equal area
resonances due to the inequivalent Me3C groups at d -0.53,
-1.80 and -13.19 in addition to the ring methyne resonances.
Fig. 2 ORTEP of Cp¢2Ce(4-methylbenzyl), the half molecule of pentane
in the asymmetric unit is not shown. The non-hydrogen atoms are refined
anisotropically and shown as 50% ellipsoids, the hydrogen atoms are not
refined but they are placed in idealized positions and not shown. Selected
2
The H NMR spectrum shows only two resonances assigned to
deuteria on the para and meta positions. After standing for about
three years, a small portion of the solid was dissolved in C6D6 and
examined by 1H NMR spectroscopy. The spectrum is qualitatively
˚
distances (A) and angles (deg): Ce–C(Cp¢) = 2.86 0.08, range 2.743(8)
2
the same as that obtained originally, but the H NMR spectrum
to 2.995(7), Ce–Cp¢(ring centroid) = 2.60, Cp¢(ring centroid)–Ce–Cp¢(ring
contains a resonance at d -13.2 with an area of approximately
2 relative to the para and meta D-resonances. The resonance at
d -13.2 is identical to one of the cyclopentadienyl ring Me3C
groups mentioned above. Examination of the 1H NMR spectrum
shows that the intensity ratio of the three Me3C resonances is
no longer 9 : 9 : 9 but approximately 9 : 9 : 6.5. In order to confirm
the result, the sample was hydrolyzed (H2O) and the resulting 1H
NMR spectrum shows that the liberated toluene was a mixture of
CH3C6D5 and CH2DC6D5. Hydrolysis (H2O) of another sample
centroid) = 138.5, Ce–C(35) = 2.576(7), Ce–C(36) = 2.969(7), Ce–C(37) =
3.933, Ce–C(41) = 3.228, Ce–C(35)–C(36) = 91.7(4◦).
have structures in which the M–CH2–C(ipso) angle is much
less than 120◦, resulting in short M–C(ipso) and M–C(ortho)
distances. This type of structure was first observed in the crystal
structure of Ti(CH2Ph)4,6,7 Zr(CH2Ph)4 and in f-block benzyl
8
derivatives.9,10,11
6650 | Dalton Trans., 2010, 39, 6648–6660
This journal is
The Royal Society of Chemistry 2010
©

2
and examination of the H NMR spectrum in C6D12 shows the
order to minimize complications due to H/D exchange reactions.
Thus in pentane or in cyclohexane, the metallacycle is isolated
or generated in pure form. In benzene, the phenyl derivative may
be isolated or generated in pure form.2 Although the equilibrium
between the benzyl and the metallacycle is, on one hand, useful as it
allows access to labeled cyclopentadienyl compounds, on the other
hand, it complicates the reactions studied in this article, since the
metallacycle reacts with the methylhalides4 and deuterium labeling
is essential to unravel these pathways.
ortho, meta, and para resonances of toluene in a ratio of 2 : 2 : 1.
Since the freshly liberated (Me3C)3C5H3 is a mixture of two isomers
that isomerizes to◦a single isomer on heating,12 the sample in C6D6
1
was heated at 60 C for two days. Examination of the H NMR
spectrum shows the Me3C resonances in a 9 : 6.5 : 9 area ratio
showing that all of the deuteria are on a single Me3C group. The 13C
NMR spectrum shows three Me3C resonances, only one of which
has a shifted 1 : 1 : 1 resonance associated with it, that is clearly
due to a Me2CCH2D group. Examination of the GC-MS shows
the presence of CH3-xDxC6D5 isotopologues and Cp¢H molecular
ion is an envelope that contains the d0, d1, d2 and d3 isotopologues.
Thus, over a prolonged period of time, both deuteria on the
a-CD2 of the benzyl group exchange with one of the CMe3
groups on the Cp¢ ring in the solid state. The solid state behavior
of Cp¢2Ce(CH2Ph) is rather different from that in solution; in
solution equilibration occurs between the benzyl derivative and the
metallacycle and toluene, Scheme 1. Over reasonably short periods
of time in the solid state the equilibration is not observed, but,
H/D exchange occurs over long periods of time. The difference
is clearly a solid state effect, where the ensemble prevents the
equilibrium with metallacycle and toluene and guides the H for D
exchange into the unique Me3C group of the cyclopentadienyl ring;
unfortunately, we do not know the molecularity or the mechanism
of the exchange. A somewhat related solid state isomerization of
Cp¢2Ce(2,3,4,5-C6HF4) was observed earlier.3
Reactions of Cp¢2CeCH2Ph with CH3X, X = F, Cl, Br, I
Addition of CH3F to a solution of Cp¢2CeCH2Ph in C6D6 at 20 ◦C
in an NMR tube results in a decrease in the resonances due to
the benzyl and the appearance of the resonances due to Cp¢2CeF.
After one day the ratio of Cp¢2CeCH2Ph to Cp¢2CeF is about
2 : 1 and after two days, the resonances due to Cp¢2CeCH2Ph
are absent and those due to Cp¢2CeF and Cp¢Cp¢¢CeF [Cp¢¢ =
(Me3C)2(EtMe2C)C5H2] are present in comparable amounts. The
appearance of Cp¢Cp¢¢CeF is a clear indication that Cp¢2CeCH2F
forms, as illustrated in Scheme 1.1,4 The 1H NMR spectrum
also contains resonances due to CH3CH2Ph and CH3Ph in an
approximate area ratio of 1 : 4 and the total amount of these
two aromatic hydrocarbons corresponds to that expected from
the amount of Cp¢2CeCH2Ph originally present. Qualitatively, the
ratio of CH3CH2Ph to CH3Ph (1 : 4) shows that the reaction of
Cp¢2CeCH2Ph with CH3F is slower than the benzyl to metallacycle
and toluene equilibrium.
Solution behavior of Cp¢2CeCH2Ph
In order to be sure that ethylbenzene is formed from
Cp¢2CeCH2Ph, the reaction of Cp¢2Ce(CD2C6D5) and CH3F in
C6D6 was followed by NMR spectroscopy. After two days at 20 ◦C,
all of the resonances due to Cp¢2Ce(CD2C6D5) have disappeared
and the only resonances observed in the 2H NMR spectrum are due
to CH3CD2C6D5 and CHD2C6D5. Hydrolysis (H2O) and analysis
of the hydrolysates by GC-MS shows the presence of Cp¢H, Cp¢¢H,
CHD2C6D5 and CH3CD2C6D5. These results clearly show that the
methyl group in CH3F is transferred intact to the benzyl group
and that the toluene is derived from the equilibrium reaction of
the benzyl and the metallacycle.
Heating a toluene-h8 solution of Cp¢2CeCH2Ph at 60 ◦C for a day,
then storing the sample at 20 ◦C for an additional day while
monitoring the 1H NMR spectrum, shows resonances due to
the benzyl derivative and the metallacycle in a ratio of 12 : 1, in
addition to four new paramagnetic resonances due, presumably,
to the Me3C groups of at least two Cp¢2Ce(Ar) metallocenes,
where Ar represents the isomeric xylyl groups. Hydrolysis (D2O)
and examination of the 2H NMR spectrum shows singlets in
the aromatic region and a triplet (JHD = 2.1 Hz) in the methyl
region in an area ratio of 2.5 : 1 : 1 : 6. This result is consistent
with the presence of the four possible toluene-d1 isotopomers
in which deuteria are in the four possible sites, implying that
all possible xylyl metallocenes as well as the benzyl metallocene
are formed in toluene solvent. The 4-methylbenzyl complex,
though stable in the solid state, is also in equilibrium with
the metallacycle and p-xylene analogous to that of the benzyl
complex.
The reaction of Cp¢2CeCH2Ph with MeX, X = Cl, Br and I
in C6D12 proceeds in a manner similar to that of CH3F, except
that resonances due to Cp¢2CeCH2X appear and disappear over
time. As shown previously, the formation of Cp¢2CeCH2X is due
to reaction of the metallacycle with CH3X, which reacts further
forming Cp¢2CeX and Cp¢Cp¢¢CeX.4 In addition, CH3CH2Ph is
observed in each case. In order to show that the ethylbenzene is
formed by direct reaction of Cp¢2CeCH2Ph with CH3X, rather
than insertion of CH2 from Cp¢2CeCH2X into toluene formed
along with the metallacycle, the reactions with CD3Br and CD3I
were studied. Addition of either CD3Br or CD3I to Cp¢2CeCH2Ph
in an NMR tube in C6D12 proceeds similar to that of CH3Br
or CH3I. After hydrolysis (H2O), analysis by GC-MS shows the
presence of Cp¢H, Cp¢¢H-d2, CH3Ph-d0 and CH3CH2Ph-d3. The
2H NMR spectrum of the solution, before hydrolysis, shows that
all of the deuteria in the ethylbenzene are on the terminal carbon,
CD3CH2Ph, and no CHD2CH2Ph or CHD2CHDPh are detected.
Repeating the reaction between Cp¢2CeCH2Ph and CD3Br in C6H6
This deduction is strengthened by allowing the deuterated
metallacycle-d53 to rea◦ct with an excess of C7D8 in an NMR
◦
tube for one day at 60 C and an additional day at 20 C. After
removing the C7D8 and redissolving the residue in C6D12, the 2H
NMR spectrum shows resonances due to (Cp¢-d27)2CeCD2C6D5
and four additional resonances at the same chemical shift and
with comparable intensities as above. Upon hydrolysis (H2O),
1
the H NMR spectrum in C6D12 shows two new aromatic and
one aliphatic resonances of toluene. These results strengthen the
deduction reached above, viz. the metallacycle reacts with the
aromatic and aliphatic C–H bonds of toluene; a similar H for D
exchange was shown earlier for methane.1 Accordingly, solvents
for reactions of the benzyl derivative must be chosen carefully in
2
solvent and monitoring the reaction by H NMR spectroscopy
shows only resonances due to CD3CH2Ph. Thus, the methyl group
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The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6648–6660 | 6651
©

of either CD3Br or CD3I are transferred intact and the toluene,
formed from the equilibrium between the benzyl derivative and
the metallacycle, does not trap the CD2 fragment nor does the
benzyl group accept a deuteron from CD3X to form CH2DPh that
traps the CD2 fragment, Scheme 1.
The reaction of Cp¢2Ce(4-methylbenzyl) with CH3F in C6D12
is similar to that of the benzyl, viz., Cp¢2CeF and Cp¢Cp¢¢CeF
are formed along with p-xylene and 4-ethyltoluene; the latter
two products are formed from the equilibrium between the 4-
methylbenzyl derivative and the metallacycle, and direct reaction
with CH3F, respectively. The reactions of Cp¢2Ce(4-methylbenzyl)
with CH3X, X = Cl, Br, and I, proceed with similar rates and
products as with Cp¢2CH2Ph.
3
Fig. 3 The optimized DFT (B3PW91) structure of Cp2Ce(h -CH2Ph)
˚
with, top, distances in A, and, bottom, NBO charges in the CH or CH2
groups of the benzyl. [Ce] represents Cp2Ce. The red color indicates the
most negatively charged atoms in the benzyl group. Numbering of carbon
atoms in the benzyl group is in blue.
Reactions of Cp¢2CePh with CH3F
During the studies described above, some of the reactions of
Cp¢2CeCH2Ph with methylhalides are studied in C6D6 or C6H6 sol-
vents rather than in C6D12. A potential complication of reactions
in C6H6 is exchange that results in the formation of Cp¢2CePh and
toluene, as noted previously.2 If the phenyl derivative reacts with
CH3X then another route for formation of toluene is available.
In order to examine this possibility, Cp¢2CePh and Cp¢2Ce(C6D5)
were prepared and allowed to react with CH3F. When Cp¢2CePh
and CH3F are mixed in an NMR tube in C6H6, resonances due to
Cp¢2CeF and Cp¢Cp¢¢CeF appear within an hour. After two days
at 20 ◦C, the ratio of Cp¢2CePh to the fluorides (Cp¢2CeF and
Cp¢Cp¢¢CeF) is 6 : 5 and after five days the ratio is 1 : 4. Hydrolysis
(H2O) and analysis by GC-MS shows the presence of toluene as
well as Cp¢H and Cp¢¢H. Repeating the reaction of CH3F with
Cp¢2Ce(C6D5) in C6D6 shows that the toluene formed is CH3C6D5
and the two ortho carbon atoms, C(3) and C(7), of the phenyl ring
◦
˚
(3.0 and 3.4 A). This leads to an acute Ce–C(1)–C(2) angle of 80 ,
which compares reasonably well with the experimental angle of
92–93◦. As a consequence of the more acute Ce–C(1)–C(2) angle
in the calculated structure, the distances between Ce and the ortho
carbons are significantly shorter than in the crystal structures.
The modeling of the Cp¢ ligand by the less bulky Cp ligand is
most likely the origin of the smaller Ce–C(1)–C(2) angle in the
1
calculated structure. The benzyl group is not an h -benzyl since
the benzene ring is orientated with its flat, open-face towards the
open wedge of the Cp2Ce fragment. This orientation is the result
of the acute Ce–C(1)–C(2) angle and the negative charge density
distribution around the benzene ring. Since this stereochemistry
is observed in one of the two molecules in the crystal structure of
Cp¢2Ce(benzyl) and in Cp¢2Ce(4-methylbenzyl), this orientation
is not due to steric effects. The calculated structure of Cp2Ce(4-
methylbenzyl), is similar to that of the benzyl, including the acute
Ce–C(1)–C(2) angle of 80◦. The other molecule in the solid state
2
by H NMR spectroscopy and examination of the hydrolysate
(H2O) by GC-MS after the resonances due to Cp¢2Ce(C6D5) have
disappeared (42 days). Thus, the reaction of Cp¢2CePh with CH3F
to give CH3Ph is much slower than that of Cp¢2CeCH2Ph. In
addition these two experiments show that benzene does not trap
the CH2-fragment resulting from Cp¢2CeCH2F.
1
crystal structure of Cp¢2Ce(benzyl) shows an h -benzyl group with
a Ce–C(1)–C(2) angle of 130◦. No calculated structure with a large
Ce–C(1)–C(2) angle is located as a minimum on the potential
3
energy surface, an additional indication that the h -benzyl is an
Computational studies
energy minimum.
The calculated structure, with a Ce–C(1)–C(2) angle of 80◦, can
be understood by considering how the Cp2Ce+ fragment interacts
with a benzyl anion. In an isolated benzyl anion, the p density
is mostly on C(1), the ortho C(3) and C(7) and the para carbon
C(5). Interaction between the Cp2Ce+ fragment and the benzyl
ligand polarizes the p electron density. In the optimal structure, the
electron p density of the benzyl ligand is mainly localized on C(1),
C(2) and an ortho carbon C(3) with only a small amount localized
on the other ortho carbon C(7), the meta and para carbons C(4,
5 and 6). According to the NBO analysis, the charge (sum of the
charges on carbon and adjacent hydrogens) on C(1) is -0.48 and
on each ipso C(2) and ortho C(3) it is -0.13. The other carbon
atoms have charges smaller than 0.04. Therefore the bonding
between the Cp2Ce fragment and the benzyl group becomes allylic
Model
The mechanism for the reaction of Cp¢2CeCH2Ph and CH3F was
analyzed by DFT(B3PW91) calculations with the methodology
used in all our previous studies on the reactivity of lanthanide
complexes with a variety of small molecules, including CH3X.1,2,4,13
Cp¢2CeCH2Ph is modeled by Cp2CeCH2Ph, which decreases
significantly the steric effects of the metallocene fragment. This
model was appropriate in the study of the reaction of CH3X (X =
F, Cl, Br, I, OMe, NMe2) with Cp¢2CeH.
Structure of Cp2Ce(CH2Ph)
The calculated structure of the benzyl complex shown in Fig. 3 is
in good agreement with the structure shown in Fig. 1b, one of the
two molecules found in the crystal structure of Cp¢2CeCH2Ph and
in Fig. 2, the 4-methylbenzyl structure. The C(1)–C(2) distance of
3
in character and the most stable structure has an h -benzyl as is
3
found in (C5Me5)2Ce(h -CH2Ph).5
˚
1.44 A, is between that expected for a single and a double bond.
Finally the good agreement between the calculated structure
and one of the two molecules in the crystal structures validates the
choice of Cp2CeCH2Ph as a model for Cp¢2CeCH2Ph.
The cerium is bonded to the benzyl group by way of C(1) at a
˚
˚
distance of 2.64 A but the cerium atom is also close to C(2) (2.78 A)
6652 | Dalton Trans., 2010, 39, 6648–6660
This journal is
The Royal Society of Chemistry 2010
©

Pathways for the reaction of Cp2CeCH2Ph with CH3F
36.2 kcal mol-1. While the activation energy barriers for this
two-step process are not low, they are significantly lower than
that of the s-bond metathesis. However, the methyl group is
not transferred intact in this process, which disagrees with the
experimental result and another physical process needs to be
discovered.
The s-bond metathesis pathway is considered first. In this one-
step concerted reaction, the transition state has the usual kite-
shaped structure. The CH2 group of the benzyl and F of CH3F
occupy the a-positions of the four-membered ring and the CH3
group occupies the b-site. The four-membered ring has angles
which are close to 90◦ and a long C ◊ ◊ ◊ C distance. The Gibbs free
energy of this transition state is 50.6 kcal mol-1, a barrier that is
significantly higher than the 31.1 kcal mol-1 free energy barrier
for the metathesis transition state in the H for F exchange in the
reaction of Cp2Ce–H and CH3–F. The structure of the metathesis
transition state is shown in Fig. 4. The high activation barrier
makes a s-bond mechanism improbable.
A transition state that connects directly Cp2CeCH2Ph and
CH3F forming Cp2CeF and ethylbenzene and agrees with the
labeling experiment has a Gibbs free energy barrier of 33.3 kcal
-1
˚
mol . In this transition state, the Cp2Ce fragment is 2.95 A from
˚
C(5), the para carbon of the benzyl group and 3.15 A from the
˚
two meta carbons C(4) and C(6), and 4.55 A from the methylene
carbon, C(1) (Fig. 6). Thus, the Cp2Ce fragment is not bonded
to the CH2 group of the benzyl fragment but is attached by way
˚
of C(5). The C(1)–C(2) distance of 1.39 A is shorter than in the
˚
benzyl complex where it is 1.44 A (Fig. 3). The CH3F molecule
˚
is bonded to Ce by way of F, and the CH3 group is 2.47 A
from the unbound CH2 group of the benzyl fragment with an
C(CH2)–C(CH3)–F angle of 160◦. Thus the CH2 group is ideally
orientated for a nucleophilic attack on the CH3 group of CH3F
whose electrophilicity is increased by the coordination of F to the
positively charged Cp2Ce fragment, an associative interchange,
IA, mechanism.14 This transition state forms the C–C bond and
cleaves the C–F bond without rearranging the hydrogen atoms
and therefore the CH3 group of CH3F is transferred intact to form
the ethylbenzene in accord with experiment.
Fig. 4 Transition state for s-bond metathesis for the reaction between
Cp2CeCH2Ph and CH3F, distances in A, angles in degrees.
˚
The two-step pathway, which begins by a proton transfer
from the methyl group of CH3F to the benzyl ligand, is the
next alternative considered. The starting structure for the proton
transfer step is the adduct between Cp2CeCH2Ph and CH3F; the
interaction is rather weak and the binding energy of 5.6 kcal
mol-1 does not compensate fully the loss of entropy, which leads
to an adduct 4.8 kcal mol-1 above the separated Cp2CeCH2Ph
and CH3F reactants. This adduct is not shown in Fig. 7. The
free energy of the proton transfer transition state is 32.4 kcal
mol-1, which is again significantly higher than the value of 18 kcal
mol-1 found for Cp2CeH and CH3F and higher than the value of
21.6 kcal mol-1 calculated for the proton transfer in the reaction of
Cp2CeCH3 and CH3F (Fig. 5). The proton transfer leads to toluene
and Cp2CeCH2F, which is 4.1 kcal mol-1 higher in energy than
Cp2CeCH2Ph and CH3F. The transition state for the following
step, insertion of CH2 into an aliphatic C–H bond of toluene
forming ethylbenzene and Cp2CeF, has a free energy barrier of
Fig. 6 The transition state in the reaction of Cp2CeCH2Ph and CH3F,
distances in A and angles in degrees.
˚
This pathway is also examined for the 4-methylbenzyl complex.
Remarkably the free energy barrier is 34.3 kcal mol-1, only 1 kcal
mol-1 higher than that calculated for the benzyl complex.
Discussion
Comparison of the free energy profiles of the three pathways
allows us to eliminate the s-bond metathesis as a pathway
for the formation of ethylbenzene since the free energy barrier
is significantly higher than the other two calculated pathways
(Fig. 7). The present result generalizes the results found for the H
for X exchange reactions between Cp2CeH and CH3X, for which
the s-bond metathesis pathway is also energetically unfavorable,
that is, the kite-shaped transition state is a high energy process
when a methyl group is in the b-position, regardless of the nature
of the chemical groups at the a-positions.
Fig. 5 Transition states for proton transfer for the reaction of, left,
Cp2CeCH2Ph and CH3F and, right, Cp2CeCH3 and CH3F.
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6648–6660 | 6653
©

for either pathway but the experiments show that the methyl
group is transferred intact, which is only consistent with the IA
mechanism.
Experimentally, the reactions between Cp¢2CeCH2Ph and CH3X
are complicated by the equilibrium between the benzyl derivative
and the metallacycle and toluene, since the metallacycle also reacts
with CH3X, Scheme 1. The relative rates of reaction of the benzyl
with CH3X versus the formation of metallacycle and toluene can
be estimated by the relative amount of CH3CH2Ph and CH3Ph
formed. This is reliable, however, only when X = F, since the
reaction of this halide is clean and relatively rapid with the benzyl
and metallacycle. At 20 ◦C, the ratio of CH3CH2Ph to CH3Ph is
1 : 4, showing that the C–C bond forming reaction is slower than
the elimination of toluene. The ratio of CH3CH2Ph to p-xylene
in the reaction of Cp¢2Ce(4-methylbenzyl) with CH3F is similar,
implying no appreciable substituent effect on the rate of reaction,
consistent with the calculations. However, the rate of reaction
of the benzyl derivative is much faster than that of the phenyl,
presumably because a benzyl group is a better nucleophile than
a phenyl group. The experimental studies give only qualitative
mechanistic information about the C–C bond forming reaction,
however, the computational studies show that the benzyl group
is indeed behaving as a nucleophile in the transition state for the
PhCH2 for F group exchange reaction. In the transition state,
the benzyl group is attached at only one point, the para carbon
of the benzene ring. An NBO analysis of the individual atoms
in the benzyl group in the ground state and the transition state
shows that: (i) the variation in the charges at the ipso, ortho
and meta carbon are small, (ii) the negative charge on the para-
carbon increases by almost 0.2 e and (iii) the negative charge on
the methylene carbon decreases by 0.30 e (Fig. 8). The charge
redistribution can be understood by considering three of the
valence bond structures that represent the benzyl anion (Fig. 8,
bottom). Their relative weights are determined by the position of
the Cp2Ce fragment. In the ground state, resonance structures A
and B dominate while in the transition state, resonance structures
A and C dominate. Thus, from the ground to the transition
state, the haptotropic shift of the Cp2Ce fragment is energetically
facilitated by the continuous interaction with the electron density
on the benzyl anion as it moves to the para-carbon. It should be
noted that the net charge on the benzyl anion is almost constant
during this haptotropic shift; what changes is the localization of
the sites of the density. As mentioned earlier, it is the large positive
charge on the Cp2Ce fragment that controls and guides the charge
redistribution as the molecules reach the transition state in the
exoergic reaction.
Fig. 7 Free energy profiles in kcal mol-1 for the reaction of Cp2CeCH2Ph
and CH3F. [Ce] represents the Cp2Ce fragment. The IA process is in green,
the two-step process (proton transfer, CH2 insertion) is in black and the
s-bond metathesis is in red.
The free energy barrier of the proton transfer step in the reaction
of Cp2CeR with CH3F increases from 18 kcal mol-1 to 22 and to
32 kcal mol-1 when R is H, CH3 and benzyl, respectively. The
proton transfer has the lowest energy barrier when R is H, since
this ligand concentrates a large negative charge in a spherical
orbital. In the methyl complex, the negative charge is localized
in a hybrid orbital carrying the density on the negatively charged
methyl group, i.e. the overlap with the proton is less. As the bond
between the methyl group and the incoming proton develops, the
Ce–Me bond distorts significantly as the methyl group tilts in order
to share its electron density with the proton (Fig. 5). In the case
of benzyl, two factors contribute to the increase in the free energy
barrier for proton transfer; (i) the negative charge of the benzyl is
dispersed over the whole group and its CH2 group forms a weaker
C ◊ ◊ ◊ H interaction (this is also shown by the difference between the
deprotonation enthalpies of CH4 and toluene, which are 417 and
374 kcal mol-1, respectively15) and (ii), the distortion that occurs
in the methyl group cannot be as strong in the benzyl group since
the distortion will force the benzene ring and the Cp2Ce fragment
into close contact. As a result, the energy barrier for the proton
transfer step between Cp2CeCH2Ph and CH3F is higher than in
Cp2CeMe.
The calculated free energy barriers for the IA mechanism and
the proton transfer pathways are similar (Fig. 7). The relative
energy of these transition states are influenced presumably by the
presence of the six bulky Me3C groups on the two cyclopentadienyl
ligands in the experimental systems. The Me3C groups not only
create steric effects in the ground and transition states but they
modify the angle between the two cyclopentadienyl rings that
influences the interaction between the metal with the other
ligands. The computational model does not indicate a preference
Several X-ray crystal structures of metal-benzyl compounds are
informative models for the transition state of the Cp¢2CeCH2Ph
and CH3F reaction. The solid state structure of the ion-pair,
[(C5Me5)Zr(CH2Ph)2]+[PhCH2B(C6F5)3]-, shows that the two ben-
3
7
zyl groups in the cation are bonded to zirconium in an h - and h -
fashion; in connection to the present article, the geometry of the
7
h -bonded benzyl group is of particular interest, since the ortho-
and meta-carbons are coplanar while the para- and ipso-carbons
are out of the plane by 13◦ and 15◦, respectively, and the CH2
fragment is out of the plane defined by the ortho- and ipso-carbons
by 21◦. Thus, the benzene ring is in a “boat conformation” and
the C–C distances are consistent with the dominant influence of
resonance structure C.16 Two recent solid state crystal structures
6654 | Dalton Trans., 2010, 39, 6648–6660
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The Royal Society of Chemistry 2010
©

in the experimental studies, CH3CD2C6D5 without scrambling the
hydrogens of the CH3 group. In this mechanism, the metal allows
the CH2 fragment of the benzyl group to behave as a nucleophile
towards the CH3F molecule, which is itself activated by the metal
to behave as an electrophile.
Experimental details
General
All manipulations were performed under an inert atmosphere
using standard Schlenk and dry box techniques. All solvents
were dried and distilled from sodium or sodium benzophenone
ketyl. Anhydrous methyl fluoride, methyl chloride, and methyl
bromide were used without further purification. Methyl iodide
was obtained commercially and purified by distillation onto
˚
activated 4 A molecular sieves. NMR spectra were recorded on
◦
Bruker AV-300 or AV-400 spectrometers at 20 C in the solvent
specified. 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-butylcyclopentadienyl ligand. Unless otherwise
specified, samples for GC-MS were prepared by adding a drop
of nitrogen-purged H2O, agitating, and allowing the samples
to stand closed for 10 min. The samples were then dried over
magnesium sulfate, filtered, and diluted ten-fold with pentane. A
1 mL sample was injected into a HP6890 GC system with a J&W
DB-XLB universal non-polar column, attached to an HP5973
Mass Selective Detector.
Fig. 8 NBO charges on the benzyl ring in, left, Cp2CeCH2Ph and right,
the IA transition state. The position of the Cp2Ce fragment is qualitatively
reproduced by its projection on the benzyl plane and is indicated by [Ce].
of thorium complexes with benzyl groups are noteworthy, since
-
the benzyl group in the anion in both of them, PhCH2B(C6F5)3 ,
6
is attached to a ThIV cation in an h -fashion. In both anions, the
CH2 fragment bonded to the B(C6F5)3 group is out of the plane of
the benzene ring by 10–12◦ and the Th–C distances to the benzene
ring vary in the order C(para) < C(meta) < C(ortho) ꢀ C(ipso).17,18
The possibility of multihapto interactions between a positive
ion, Li+, Na+, K+, Rb+, Cs+, and a benzyl anion has been shown
with MP2 calculations.19 The size of the metal ion is important
Cp¢2CeCD2C6D5. C6D5CD2MgCl was prepared by slowly
adding a solution of benzyl chloride-d7 (1 g, 7.5 mmol) in 10 mL
of diethyl ether to magnesium turnings (0.18 g, 7.4 mmol) in
10 mL of diethyl ether and heating the resulting pale blue solution
at reflux for 1 h. The solution was filtered and titrated with
a standard 0.1 N aqueous HCl solution; the concentration of
C6D5CD2MgCl was determined to be 0.25 M. Cp¢2CeOTf·0.5
hexane2 (3.44 g, 4.3 mmol) was dissolved in 30 mL of diethyl
ether and C6D5CD2MgCl solution (17 mL, 0.25 M in diethyl
ether, 4.25 mmol) was added via syringe. The solution immediately
changed from yellow to red, and became cloudy within 5 min.
After 5 min the solvent was removed under reduced pressure,
yielding a red powder. The 1H NMR spectrum of the crude
product contained resonances corresponding to Cp¢2CeCl20 and
CMe3 resonances identical to those of Cp¢2CeCH2C6H5;2 the two
species were present in approximately a 1 : 1 ratio. The red solid
was extracted with 25 mL of pentane to isolate Cp¢2CeCD2C6D5,
and the yellow solid residue was extracted further with 25 mL of
toluene to recover the remaining Cp¢2CeCl. The volume of the
pentane solution was reduced to 10 mL and cooled to -10 ◦C,
3
7
since there is a preference for h -benzyl for Li and Na and h -
benzyl for the larger ions. In addition, calculations show that in the
3
5
case of Li+, a haptotropic shift from h to h -benzyl occurs with
a very low energy barrier. The crystal structures and calculated
ground state structures show that the interaction between a benzyl
1
anion and a positively charged metal fragment ranges from h to
7
h . In these structures the CH2 group is close to the metal ion
fragment and the classification of the bond type is determined
by the distance between the metal ion and the carbon atoms
of the arene. The calculated transition state for the carbon–
carbon bond forming reaction shows that the CH2 group is not
attached to the metal and therefore the electron density is available
for a nucleophilic attack on the methyl group of CH3F, whose
electrophilicity is enhanced by the coordination of the fluorine
atom to Cp2Ce.
Conclusion
1
giving red blocks. Yield, 0.78 g (1.1 mmol, 26%). The H NMR
The net reaction between Cp¢2CeCH2Ph and CH3X (X = F, Cl,
Br, I) yields Cp¢2CeX and CH3CH2Ph. Calculations carried out for
X = F show that the mechanism of the benzyl for fluoride exchange
reaction does not proceed by way of a simple four-center transition
state, since a lower barrier process in which the benzyl group is
attached at the para position to the Cp2Ce fragment leaves the
CH2 group free to act as a nucleophile forming the C–C bond with
CH3F. This yields ethylbenzene or, when Cp¢2CeCD2C6D5 is used
spectrum contained resonances identical to the CMe3 and ring C–
H resonances observed in Cp¢2CeCH2C6H5, H NMR (C6D6): d
1
50.63 (2H, n1/2 = 245 Hz), 13.25 (2H, n1/2 = 245 Hz), -0.53 (18H,
n1/2 = 190 Hz), -1.80 (18H, n1/2 = 195 Hz), -13.19 (18H, n1/2
=
45 Hz). The resonance at 50.63 ppm was incorrectly reported as a
fold-over resonance at -32.62 ppm in a previous publication.2
The 2H NMR spectrum contained resonances consistent with
1
the aromatic benzyl group resonances observed in the H NMR
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6648–6660 | 6655
©

spectrum of Cp¢2CeCH2C6H5,2 2H NMR (C6D6): d 4.18 (2H, n1/2
=
at 2.29 ppm, and the single broad CMe3 resonance at 1.12 ppm in
a 3 : 9 : 16 : 1 : 16 area ratio.
50 Hz), 2.32 (1H, n1/2 = 80 Hz); the other resonances were not
observed.
The sample in C6D6 was heated at 60 ◦C for 2 days. Three
1
Cp¢H-CMe3-resonances were present in the H NMR spectrum,
Cp¢2Ce(4-methylbenzyl). Method A: Cp¢2CeOTf·0.5 hexane2
(1 g, 1.2 mmol) was dissolved in 50 mL of pentane and
4-methylbenzylmagnesium chloride solution21 (1.8 mL, 0.69 M
in diethyl ether, 1.2 mmol) was added via syringe. The solution
immediately changed from yellow to red. After 2 min the solvent
was removed under reduced pressure, yielding a red-orange
powder. The red solid was extracted with 10 mL of pentane, and
the yellow solid residue was extracted further with 25 mL toluene
to recover Cp¢2CeCl. The volume of the pentane solution was
reduced to 7 mL and cooled to -10 ◦C, giving red needles. Yield,
0.48 g (0.68 mmol, 56%). MP 119–122 ◦C (sample turned purple
on melting). 1H NMR (C6D12): d 40.65 (2H, n1/2 = 300 Hz), 13.60
(2H, n1/2 = 250 Hz), 0.27 (3H, 5 Hz), -0.57 (36H, n1/2 = 40 Hz),
-10.91 (18H, n1/2 = 150 Hz). Anal. Calcd. for C42H67Ce: C, 70.8;
H, 9.48. Found C, 70.9; H, 9.41.
d 1.26, 1.18, 1.08 ppm in a 9 : 6.5 : 9 area ratio. A broadened
peak at 1.16 ppm was also observed, presumably due to CMe3-
dx groups. The 13C NMR contained three resonances due to the
CMe3 groups, d 32.58, 30.83, and 30.12 ppm. A 1 : 1 : 1 pattern
at 30.10 ppm (JCD = 3.6 Hz) was consistent with the presence
of CMe2CDH2 groups. GC-MS analysis showed the presence
of partially deuterated toluene and partially deuterated Cp¢H.
Molecular ion isotope pattern for toluene: (M–1 or 2)+ m/z
(relative abundance): 92 (3), 93 (6), 94 (12), 95 (74), 96 (100),
97 (96), 98 (24), 99 (2). For Cp¢H, (M)+ m/z (calculated relative
abundance for C17H30/found): 234 (100/100), 235 (19/76), 236
(2/61), 237 (0.1/13), 238 (0.004/2), giving the observed ratio of
Cp¢H, Cp¢H-d1, and Cp¢H-d2 as 35 : 20 : 17.
2
NMR tube equilibration of Cp¢2Ce(CH2C6H5) isomers in
toluene-h8
Method B: Cp¢2CeCH2C6H5 was dissolved in p-xylene in an
NMR tube and heated at 60 ◦C for 2 days. The red solution was
taken to dryness and the red solid residue was dissolved in C6D12.
◦
Cp¢2Ce(CH2Ph) was dissolved in toluene-h8 and heated at 60 C
1
The H NMR spectrum was identical to that of the red crystals
◦
for 1 day, then allowed to stand at 19 C for one day, yielding a
obtained via Method A.
deep red solution. The 1H NMR spectrum contained resonances
due to Cp¢2Ce(CH2Ph) and Cp¢[(Me3C)2C5H2C(Me)2CH2]Ce in
a 12 : 1 area ratio, as well as four new paramagnetic resonances,
1H NMR (C7H8, 400 MHz): d -1.585 (n1/2 = 40 Hz), -1.771
(n1/2 = 40 Hz), -9.716 (n1/2 = 40 Hz), -10.216 (n1/2 = 40 Hz),
in approximately a 1 : 3 : 1.5 : 1 area ratio. Assuming that the
resonances at -9.716 and -10.216 correspond to the unique CMe3
resonances of two different Cp¢2Ce-R complexes, the ratio of
Cp¢2Ce(CH2Ph), Cp¢[(Me3C)2C5H2C(Me)2CH2]C and the two new
species was 12 : 1 : 3 : 2. The sample was hydrolyzed with D2O and
NMR tube equilibration of Cp¢2Ce(4-methylbenzyl) in C6D12
Cp¢2Ce(4-methylbenzyl) was dissolved in C6D12 in an NMR
tube and allowed to stand. After one day at 19 ◦C, the red
solution had turned purple and resonances due to the metallacycle
Cp¢[(Me3C)2C5H2C(Me)2CH2]Ce2 and p-xylene had appeared in
the 1H NMR spectrum. The ratio of Cp¢2Ce(4-methylbenzyl) and
Cp¢[(Me3C)2C5H2C(Me)2CH2]Ce was approximately 1 : 1. After
two days, the ratio was 1 : 2.5. After 5 days, the ratio was 1 : 4.
The ratio did not change upon further standing.
2
filtered. The H NMR spectrum contained two Cp¢D ring C–D
resonances at 3.115 (d, JHD = 3 Hz) and 2.966 (s) in a 3 : 4 area
ratio, and resonances due to toluene-d1 at 7.174, 7.095, 7.067, and
2.16 (t, JHD = 2.1 Hz) in a 2.5 : 1 : 1 : 6 area ratio.
Solid state H-for-D exchange in Cp¢2CeCD2C6D5
A crystalline sample of Cp¢2CeCD2C6D5 was allowed to stand
at 20–25 ◦C for 1430 days. A small amount of the sample was
NMR tube equilibration of (Cp¢-d27)2Ce(CD2C6D5) isomers in
toluene-d8
1
dissolved in C6D6, and the H NMR spectrum was as described
2
above for the fresh sample. The H NMR spectrum contained
the aromatic benzyl group resonances at d 4.18 and 2.32, and
also a resonance at d -13.15 (n1/2 = 100 Hz) with an integrated
intensity equal to roughly two deuterons relative to the benzyl
ligand resonances. The sample was hydrolyzed and filtered. The
1H NMR spectrum of the hydrolysate contained a sharp singlet
at 2.120 ppm and a weak 1 : 1 : 1 pattern (JHD = 1.8 Hz) at
2.105 ppm due to C6D5CH3 and C6D5CDH2, respectively. No
resonances due to the aromatic toluene CH-groups were observed.
Six Cp¢H-CMe3 resonances were also present in the spectrum, d
1.38, 1.32, 1.27, 1.19, 1.15, and 1.09 ppm in a 1 : 1 : 2 : 1.5 : 1 : 2 area
Cp¢2Ce(CH2Ph) was dissolved in C6D6 and heated at 60 ◦C for
4 days to perdeuterate the CMe3 groups. The sample was taken
to dryness and the solid residue was dissolved in fresh C6D6. The
sample was heated for an additional 4 days, then taken to dryness,
and the solid residue was dissolved in toluene-d8. The sample was
heated at 60 ◦C for 1 day, and then allowed to stand at 19 ◦C for
one day. The sample was taken to dryness and the solid residue was
2
dissolved in cyclohexane-d12. The H NMR spectrum contained
C(CD3)3 resonances due to (Cp¢-d27)2Ce(CD2C6D5) and the two
new species observed in the previous experiment in a 2 : 1 : 1 area
ratio. The sample was hydrolyzed with H2O and filtered. The 1H
NMR spectrum contained multiple Cp¢H ring resonances, as well
as resonances due to isomers of toluene-d7 at 7.108, 7.038, and
2.246. Subtracting the area of the residual toluene peaks in the
cyclohexane-d12 solution before hydrolysis relative to an internal
standard indicated that the resonances had increased in intensity
after hydrolysis in an approximate 1 : 1 : 3 area ratio.
2
ratio, arising from two Cp¢H isomers.12 The H NMR spectrum
contained a triplet at 2.05 ppm (JHD = 2.1 Hz) due to C6D5CDH2
and a single broadened CMe3-dx resonance at 1.13 ppm (n1/2
=
4 Hz). Aromatic toluene 2H resonances were masked by the solvent
peak. Another sample of aged Cp¢2CeCD2C6D5 was dissolved in
2
C6H12 and hydrolyzed. The H NMR spectrum contained three
aromatic resonances at 7.27, 7.16, and 7.09 ppm, the CDH2 triplet
6656 | Dalton Trans., 2010, 39, 6648–6660
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©

NMR tube reaction of CH3F and Cp¢2Ce(CH2C6H5) in benzene-d6
NMR tube reaction of CH3F and (Cp¢-d27)2Ce(C6D5) in benzene-d6
Cp¢2Ce(CH2C6H5) was dissolved in benzene-d6 and heated at
Cp¢2Ce(CH2C6H5) was dissolved in C6D6 in an NMR tube. The
tube was cooled in a liquid nitrogen isopropanol bath, the head
space was evacuated, and replaced with CH3F (1 atm). The tube
was warmed to 19 ◦C and allowed to stand. After 1 day, resonances
in the 1H NMR spectrum due to Cp¢2CeF had appeared; the ratio
of Cp¢2Ce(CH2C6H5) to Cp¢2CeF was 2 : 1. Resonances due to
Cp¢¢Cp¢CeF had also appeared (Cp¢¢ is Cp¢+CH2),1,4 as well as
CH3C6H5 and CH3CH2C6H5 in a 4 : 1 area ratio. After 2 days, all
Cp¢2Ce(CH2C6H5) resonances had disappeared from the 1H NMR
spectrum. Integration of the CMe3 signal intensities relative to
the residual solvent proton signal indicated that slightly less than
half an equivalent of Cp¢2CeF had formed relative to the starting
material.
◦
60 C for 2 days. The sample was taken to dryness, dissolved in
fresh benzene-d6, and heated at 60 ◦C for 2 days. This procedure
was repeated two more times, with the sample heated for 8 days
after the last addition of benzene-d6, yielding a solution of (Cp¢-
d27)2Ce(C6D5). The sample was taken to dryness, and dissolved
in fresh benzene-d6. The tube was cooled in a liquid nitrogen
isopropanol bath, the head space was evacuated, and replaced
◦
with CH3F (1 atm). The tube was warmed to 19 C and allowed
to stand. After 3 days, resonances in the 2H NMR spectrum
matching those observed in the 1H NMR spectrum of the previous
experiment, presumably due to (Cp¢-d27)2CeF and (Cp¢¢-d27)(Cp¢-
d27)CeF had appeared; the ratio of (Cp¢-d27)2Ce(C6D5) to the two
fluoride species was 4 : 1. A diamagnetic singlet had appeared at
1
2.10 ppm in the H NMR, suggesting the formation of toluene.
NMR tube reaction of CH3F and Cp¢2Ce(CD2C6D5) in benzene-d6
After 2 additional days, the ratio was 6 : 5. After 11 days, the ratio
of (Cp¢-d27)2Ce(C6D5) to the two fluoride species was 1 : 1. After
Cp¢2Ce(CD2C6D5) was dissolved in C6D6 in an NMR tube. The
tube was cooled in a liquid nitrogen isopropanol bath, the head
space was evacuated, and replaced with CH3F (1 atm). The tube
was warmed to 19 ◦C and allowed to stand. After 1 day, resonances
in the 1H NMR spectrum due to Cp¢2CeF had appeared; the ratio
of Cp¢2Ce(CD2C6D5) to Cp¢2CeF was 1 : 2. Resonances due to
Cp¢¢Cp¢CeF had also appeared, along with a singlet at 1.05 ppm
and a broad hump at 2.4 ppm presumably corresponding to the
CH3 and residual protons in the CD2 group of CH3CD2C6D5,
respectively. After 2 days, all Cp¢2Ce(CD2C6D5) resonances had
disappeared from the 1H NMR spectrum. Integration of the CMe3
signal intensities relative to the residual solvent proton signal
indicated that slightly less than half an equivalent of Cp¢2CeF had
2
42 days, only the two fluoride species remained in the H NMR.
GC-MS analysis indicated the presence of toluene-d5, (M-1)+
m/z 96. (Cp¢-d27)H and (Cp¢¢-d27)H were the only other major
components besides C6D6.
NMR tube reaction of CH3Cl, CH3Br, and CH3I with
Cp¢2Ce(CH2C6H5) in cyclohexane-d12
Cp¢2Ce(CH2C6H5) was dissolved in cyclohexane-d12 in an NMR
tube. The tube was cooled in a liquid nitrogen isopropanol bath,
and the head space was evacuated. In the case of CH3Cl and
CH3Br, the head space was filled with the halomethane gas (1 atm).
In the case of CH3I, an excess was added by vacuum transfer and
the head space was backfilled with N2. The sample was warmed
to 19 ◦C and allowed to stand. In the case of CH3Cl, after
10 min, resonances of the same intermediate complex observed
in the reaction of CH3Cl with Cp¢[(Me3C)2C5H2C(Me)2CH2]Ce,
Cp¢2CeCH2Cl4 had appeared in the 1H NMR spectrum; the ratio
of Cp¢2Ce(CH2C6H5) and Cp¢2CeCH2Cl was approximately 32 : 1.
After three days, only resonances due to Cp¢2CeCl remained in the
1H NMR spectrum, and yellow crystals of Cp¢2CeCl had formed.
Diamagnetic resonances due to CH3C6H5 and CH3CH2C6H5 had
also appeared. In the case of CH3Br, after 19 h, resonances due
Cp¢2CeBr and Cp¢2CeCH2Br4 had appeared in the 1H NMR spec-
trum; the ratio of Cp¢2Ce(CH2C6H5), Cp¢2CeBr, and Cp¢2CeCH2Br
was approximately 1.5 : 7 : 1. Diamagnetic resonances due to
CH3C6H5 and CH3CH2C6H5 had also appeared. After five days,
only resonances due to Cp¢2CeBr remained in the 1H NMR
spectrum, and orange crystals of Cp¢2CeBr had formed. In the
case of CH3I, after three hours, resonances due Cp¢2CeI and
Cp¢2CeCH2I4 had appeared in the 1H NMR spectrum; the ratio of
Cp¢2Ce(CH2C6H5), Cp¢2CeI, and Cp¢2CeCH2I was approximately
75 : 10 : 1. After 24 h, resonances due to Cp¢2Ce(CH2C6H5) had
disappeared from the 1H NMR spectrum. Paramagnetic reso-
nances due to Cp¢2CeI and Cp¢2CeCH2I were present in a 3 : 1 ratio,
and diamagnetic resonances due to CH3C6H5 and CH3CH2C6H5
had also appeared. After five days, the ratio was 24 : 1. After
11 days, orange crystals had formed, and only resonances due
to Cp¢2CeI remained in the spectrum. The GC-MS analysis of all
three samples showed three principle components in addition to
2
formed relative to the starting material. The H NMR spectrum
contained resonances due to C6D6, CHD2C6D5, and a multiplet
at 2.35 ppm presumably corresponding to the CD2 group of
CH3CD2C6D5. No signal was observed at 1.05 ppm in the2H NMR
spectrum. GC-MS analysis showed three principle components
in addition to Cp¢H, with (M-2)+ m/z 97 (CHD2C6D5), 113
(CH3CD2C6D5), and (M)+ 248 (Cp¢¢H) in an approximate ratio
of 1 : 1 : 6.
NMR tube reaction of CH3F and Cp¢2Ce(C6H5) in benzene-h6
Cp¢2Ce(CH2C6H5) was dissolved in cyclohexane-d12 and
heated at 60 ◦C for 12 h, yielding
a
solution of
Cp¢[(Me3C)2C5H2C(Me)2CH2]Ce. The sample was ta◦ken to dry-
ness, dissolved in cyclohexane-d12, and heated at 60 C for 12 h
to remove residual toluene. The sample was taken to dryness,
◦
dissolved in benzene-h6, and heated at 60 C for 12 h, yielding a
very deep red solution of Cp¢2Ce(C6H5).2 The tube was cooled in
a liquid nitrogen isopropanol bath, the head space was evacuated,
and replaced with CH3F (1 atm). The tube was warmed to 19 ◦C
and allowed to stand. After 1 h, resonances in the 1H NMR
spectrum due to Cp¢2CeF and Cp¢¢Cp¢CeF had appeared; the
ratio of Cp¢2Ce(C6H5) to the two fluoride species was 30 : 1. After
2 days, the ratio was 6 : 5, and a diamagnetic singlet had appeared
at 2.10 ppm, suggesting the formation of toluene. After 5 days, the
ratio was 1 : 4. GC-MS analysis indicated the presence of toluene.
Cp¢H and Cp¢¢H were the only other major components besides
C6H6.
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Dalton Trans., 2010, 39, 6648–6660 | 6657
©

Cp¢H, with (M-1)+ m/z 91 (CH3C6H5), 106 (CH2C6H5+CH3), and
(M)+ 248 (Cp¢¢H).
2.52 (2H, q), 2.24 (3H, s), and 1.17 (3H, t), apparently due to 4-
ethyltoluene. After 2 days, all Cp¢2Ce(4-methylbenzyl) resonances
had disappeared from the 1H NMR spectrum. Integration of the
CMe3 signal intensities relative to the residual solvent proton
signal indicated that approximately 70% of the starting material
had been converted to Cp¢2CeF. The GC-MS analysis showed three
principle components in addition to Cp¢H, with (M-1)+ m/z 106
(CH3C6H4CH3), 120 (CH3CH2C6H4CH3), and (M)+ 248 (Cp¢¢H).
NMR tube reaction of CD3Br or CD3I and Cp¢2Ce(CH2C6H5) in
benzene-d6
Cp¢2Ce(CH2C6H5) was dissolved in benzene-d6 in an NMR tube.
The tube was cooled in a liquid nitrogen isopropanol bath, and the
head space was evacuated. In the case of CD3Br, the head space
was filled with the gas (1 atm); in the case of CD3I, an excess was
added by vacuum transfer, and th◦e head space was backfilled with
N2. The tube was warmed to 19 C and allowed to stand. In the
case of CD3Br, after 24 h, resonances due to Cp¢2Ce(CH2C6H5)
NMR tube reaction of CH3Cl, CH3Br, or CH3I and
Cp¢2Ce(4-methylbenzyl) in C6D12
Cp¢2Ce(4-methylbenzyl) was dissolved in C6D12 in an NMR tube.
The tube was cooled in a liquid nitrogen isopropanol bath,
and the head space was evacuated. In the case of CH3Cl and
CH3Br, the head space was refilled with the halomethane gas
(1 atm). In the case of CH3I, an excess was added by vacuum
transfer, and the head space was backfilled with N2. In the case of
CH3Cl, after 3 days, all Cp¢2Ce(4-methylbenzyl) resonances had
1
had disappeared from the H NMR spectrum and resonances
due to Cp¢2CeBr had appeared. In the case of CD3I, after 24 h,
resonances due to Cp¢2CeI had appeared in the 1H NMR spectrum,
and the ratio of Cp¢2Ce(CH2C6H5) to Cp¢2CeI was 1 : 18. After
2 days, resonances due to Cp¢2Ce(CH2C6H5) had disappeared
1
from the H NMR spectrum. In both cases, a broad multiplet
at 1.05 ppm presumably corresponding to residual protons in the
CD3 group and a singlet at 2.4 ppm presumably corresponding
to the CH2 group of CD3CH2C6H5 had also appeared in a
1 : 2 ratio. Integration of the CMe3 signal intensities relative to
the residual solvent proton signal indicated approximately 70%
conversion of Cp¢2Ce(CH2C6H5) to Cp¢2CeBr and 85% conversion
1
disappeared from the H NMR spectrum, and orange crystals
of Cp¢2CeCl had formed. In the case of CH3Br, after 30 min,
resonances due to Cp¢2CeCH2Br and Cp¢2CeBr had appeared
1
in the H NMR spectrum; the ratio of Cp¢2Ce(4-methylbenzyl)
to Cp¢2CeCH2Br and Cp¢2CeBr was 20 : 1 : 1. After 17 h, the
ratio was 2.5 : 1 : 75. After 2 days, all Cp¢2Ce(4-methylbenzyl) and
2
of Cp¢2Ce(CH2C6H5) to Cp¢2CeI. The H NMR spectra in both
1
Cp¢2CeCH2Br resonances had disappeared from the H NMR
cases contained resonances due to C6D6, CD3Br or CD3I, and
a multiplet at 0.98 ppm presumably corresponding to the CD3
group of CD3CH2C6H5. No signal was observed at 2.4 ppm due to
deuteria in the benzylic sites in either 2H NMR spectrum. GC-MS
analysis showed three principle components in addition to Cp¢H,
with (M-1)+ m/z 91 (CH3C6H5), 108 (CH2C6H5+CD3), and (M)+
251 (Cp¢H+CD2).
spectrum, and orange crystals of Cp¢2CeBr had formed. In the
case of CH3I, after 30 min, resonances due to Cp¢2CeCH2I
1
had appeared in the H NMR spectrum; the ratio of Cp¢2Ce(4-
methylbenzyl)toCp¢2CeCH2I was 21 : 1. After 2 days, all Cp¢2Ce(4-
1
methylbenzyl) resonances had disappeared from the H NMR
spectrum and resonances due to Cp¢2CeI had appeared. The ratio
of Cp¢2CeCH2I to Cp¢2CeI was 1 : 6. In all cases, diamagnetic
resonances due to p-xylene and 4-ethyltoluene also appeared
in the 1H NMR spectrum, and GC-MS analysis showed four
principle components in addition to Cp¢H, with (M-1)+ m/z 106
(CH3C6H4CH3), 120 (CH3CH2C6H4CH3), and (M)+ 248 (Cp¢¢H).
NMR tube reaction of CD3Br and Cp¢2Ce(CH2C6H5) in benzene-h6
Cp¢2Ce(CH2C6H5) was dissolved in C6H6 in an NMR tube. The
tube was cooled in a liquid nitrogen isopropanol bath, the head
space was evacuated, and replaced with CD3Br (1 atm). The tube
was warmed to 19 ◦C and allowed to stand. After 24 h, the sample
was ◦cooled in a liquid nitrogen isopropanol bath maintained at
-20 C, the head space was evacuated, the tube cap was closed,
and the sample was warmed to 19 ◦C. This freeze–pump–thaw
procedure was performed two more times to remove residual
CD3Br. A yellow precipitate formed, and the yellow solution was
filtered into a clean tube. The 2H NMR spectrum of the solution
contained resonances due to CD3Br and a singlet at 1.06 ppm
presumably corresponding to the CD3 group of CD3CH2C6H5.
No signal was observed at 2.4 ppm in the 2H NMR spectrum.
NMR tube reaction of CD3Br and Cp¢2Ce(4-methylbenzyl) in
C6H12
Cp¢2Ce(4-methylbenzyl) was dissolved in C6H12 in an NMR tube.
The tube was cooled in a liquid nitrogen isopropanol bath, the
head space was evacuated, the sample was warmed to 19 ◦C, and
the head space was refilled with CD3Br (1 atm). After 20 min,
resonances due to Cp¢2CeCD2Br and Cp¢2CeBr had appeared
1
in the H NMR spectrum; the ratio of Cp¢2Ce(4-methylbenzyl)
to Cp¢2CeCD2Br and Cp¢2CeBr was 100 : 1 : 4. After 2 days,
all Cp¢2Ce(4-methylbenzyl) and Cp¢2CeCD2Br resonances had
1
disappeared from the H NMR spectrum, and orange crystals
NMR tube reaction of CH3F and Cp¢2Ce(4-methylbenzyl) in C6D12
of Cp¢2CeBr had formed. Diamagnetic resonances at 6.92 and
6.95 ppm corresponding to the aromatic protons of p-xylene and
4-ethyltoluene had also appeared. A diamagnetic resonance had
Cp¢2Ce(4-methylbenzyl) was dissolved in C6D12 in an NMR tube.
The tube was cooled in a liquid nitrogen isopropanol bath, the
head space was evacuated, and replaced with CH3F (1 atm).
The tube was warmed to 19 ◦C and allowed to stand. After
1 day, resonances in the 1H NMR spectrum due to Cp¢2CeF
had appeared; the ratio of Cp¢2Ce(4-methylbenzyl) to Cp¢2CeF
was 1 : 1. Resonances due to Cp¢¢Cp¢CeF and p-xylene had also
appeared, along with new diamagnetic resonances at 6.96 (4H, s),
2
appeared in the H NMR spectrum at d 1.17 corresponding to
the -CH2CD3 group of 4-ethyltoluene; the intense resonance due
to excess CD3Br masked the region where a resonance due to
deuterium bound to the secondary carbon of the ethyl group
would have been observed. The GC-MS analysis showed four
principle components in addition to Cp¢H, with (M-1)+ m/z 106
6658 | Dalton Trans., 2010, 39, 6648–6660
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The Royal Society of Chemistry 2010
©

(CH3C6H4CH3), 122 (CH3C6H4CH3+CD2), and (M)+ m/z 251
(Cp¢H+CD2).
Table 1 Crystallographic data
Cp¢2Ce(4-methylbenzyl)·
Compound
Cp¢2CeCH2C6H5 0.5(pentane)
NMR tube reaction of CD3I and Cp¢2Ce(4-methylbenzyl) in C6D6
Empirical Formula
Formula Weight
T/K
C41H65Ce
698.05
179(2)
Triclinic
¯
P1
13.228(1)
16.080(1)
18.499(1)
96.377(1)
105.388(1)
96.173(1)
3731.57(14)
4
C44.25H72.5Ce
744.64
120(2)
Triclinic
¯
P1
10.528(2)
12.032(3)
17.562(4)
84.069(4)
79.816(4)
66.418(3)
2005.4(8)
2
Cp¢2Ce(4-methylbenzyl) was dissolved in C6D6 in an NMR tube.
The tube was cooled in a liquid nitrogen isopropanol bath, the
head space was evacuated, the sample was warmed to 19 ◦C,
an excess of CD3I was added by vacuum transfer, and the
head space was backfilled with N2. After 1 day, all Cp¢2Ce(4-
methylbenzyl) resonances had disappeared and resonances due to
Cp¢2CeCD2I and Cp¢2CeI had appeared in the 1H NMR spectrum
from the 1H NMR spectrum. Diamagnetic resonances at 6.92 and
6.95 ppm corresponding to the aromatic protons of p-xylene and
4-ethyltoluene had also appeared. A diamagnetic resonance had
appeared in the 2H NMR spectrum at d 1.02 corresponding to the
-CH2CD3 group of 4-ethyltoluene. No resonance was observed at
2.55 ppm which would have indicated deuteration at the secondary
position in the ethyl group. The GC-MS analysis showed four
principle components in addition to Cp¢H, with (M-1)+ m/z 106
(CH3C6H4CH3), 122 (CH3C6H4CH3+CD2), and (M)+ m/z 251
(Cp¢H+CD2).
Crystal System
Space Group
˚
a/A
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
V/A
Z
3
˚
Unique Reflections (Rint
)
12450 (0.0229)
0.0361, 0.0707
7273 (0.0821)
0.0493, 0.0933
a
R1, wR2
a R1 based on selected data with I > 2s(I); wR2 based on all data.
followed to confirm that transition states connect to reactants and
products. The zero point energy (ZPE) and entropic contribution
have been estimated within the harmonic potential approximation.
The Gibbs free energy, G, was calculated for T = 298.15 K and
1 atm. The NBO analysis34 was carried out replacing Ce by La
because of the technical requirement to have an even number
of f electrons for the calculations. This method has been used
successfully in previous studies.1,2,34b
Crystallographic studies of Cp¢2Ce(CH2C6H5) and
Cp¢2Ce(4-methylbenzyl)
Single crystals of appropriate dimension were mounted on glass
fibers or Kapton loops using Paratone N hydrocarbon oil. All
measurements were made on a SMART 100022 diffractometer
with a CCD area detector and graphite monochromated Mo-Ka
radiation. Data were collected at low temperature using 10 s w or
w and f scans. Frame data were integrated using SAINT23 and
empirical absorption corrections were applied using SADABS.24
The data were also corrected for Lorentz-polarisation effects.
The structures were solved using direct methods25 and expanded
using Fourier techniques.26 Non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were included in calculated
positions but not refined. All calculations for Cp¢2Ce(CH2C6H5)
were performed using the teXsan27 crystallographic software
package of Molecular Structure Corporation. All calculations for
Cp¢2Ce(4-methylbenzyl) were performed using the SHELXTL28
crystallographic software package of Bruker Analytical X-ray
Systems Inc. Crystallographic data are given in Table 1 and full
crystallographic details are included in the ESI.†
Acknowledgements
This work was supported by the Director, Office of Science, Office
of Basic Energy Sciences (OBES), of the U.S. Department of
Energy (DOE) under Contract No. DE-AC02-05CH11231. We
thank F. J. Hollander and A. G. DiPasquale at CHEXRAY,
the U.C. Berkeley X-ray diffraction facility, for help with the
crystallography. L.M thanks the CINES and CALMIP for a
generous grant of computing time. L.M. is also a junior member
of the Institut Universitaire de France, L.M. and O.E thank
the CNRS and Ministe`re de l’Enseignement Supe´rieur et de la
Recherche for funding.
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6660 | Dalton Trans., 2010, 39, 6648–6660
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