Hydrogen for fluorine exchange in C6F6 and C 6F5H by monomeric [1,3,4-(Me3C) 3C5H2]2CeH: Experimental and computational studies

Article abstract of DOI:10.1021/ja0451012

The net reaction of monomeric Cp′₂CeH [Cp′ = 1,3,4-(Me₃C)₃(C₅H₂)] in C ₆D₆ with C₆F₆ is Cp′₂-CeF, H₂, and tetrafluorobenzyne. The pentafluorophenylmetallocene, Cp′₂Ce(C₆F ₅), is formed as an intermediate that decomposes slowly to Cp′₂CeF and C₆F₄ (tetrafluorobenzyne), and the latter is trapped by the solvent C₆D₆ as a [2+4] cycloadduct. In C₆F₅H, the final products are also Cp′₂CeF and H₂, which are formed from the intermediates Cp′₂Ce(C₆F₅) and Cp′₂Ce(2,3,5,6-C₆F₄H) and from an unidentified metallocene of cerium and the [2+4] cycloadducts of tetra- and trifluorobenzyne with C₆D₆. The hydride, fluoride, and pentafluorophenylmetallocenes are isolated and characterized by X-ray crystallography. DFT-(B3PW91) calculations have been used to explore the pathways leading to the observed products of the exergonic reactions. A key step is a H/F exchange reaction which transforms C₆F₆ and the cerium hydride into C₆F₅H and Cp′₂CeF. This reaction starts by an η¹-F-C₆F₅ interaction, which serves as a hook. The reaction proceeds via a σ bond metathesis where the fluorine ortho to the hook migrates toward H with a relatively low activation energy. All products observed experimentally are accommodated by pathways that involve C-F and C-H bond cleavages.

Full text of DOI:10.1021/ja0451012

Published on Web 12/07/2004
Hydrogen for Fluorine Exchange in C6F6 and C6F5H by
Monomeric [1,3,4-(Me3C)3C5H2]2CeH: Experimental and
Computational Studies
Laurent Maron,*,† Evan L. Werkema,‡ Lionel Perrin,§ Odile Eisenstein,*,§ and
Richard A. Andersen*,‡
Contribution from the Laboratoire de Physique Quantique, UMR 5626, IRSAMC, UniVersite´
Paul Sabatier, 118 Route de Narbonne, 31064 Toulouse Cedex 4, France, Chemistry Department
and Chemical Sciences DiVision of Lawrence Berkeley National Laboratory, UniVersity of
California, Berkeley, California 94720, and LSDSMS, UMR 5636 UniVersite´ Montpellier 2,
34095 Montpellier Cedex 5, France
Received August 13, 2004; E-mail: laurent.maron@irsamc.ups-tlse.fr; odile.eisenstein@univ-montp2.fr; raandersen@lbl.gov
Abstract: The net reaction of monomeric Cp′2CeH [Cp′ ) 1,3,4-(Me3C)3(C5H2)] in C6D6 with C6F6 is Cp′2-
CeF, H2, and tetrafluorobenzyne. The pentafluorophenylmetallocene, Cp′2Ce(C6F5), is formed as an
intermediate that decomposes slowly to Cp′2CeF and C6F4 (tetrafluorobenzyne), and the latter is trapped
by the solvent C6D6 as a [2+4] cycloadduct. In C6F5H, the final products are also Cp′2CeF and H2, which
are formed from the intermediates Cp′2Ce(C6F5) and Cp′2Ce(2,3,5,6-C6F4H) and from an unidentified
metallocene of cerium and the [2+4] cycloadducts of tetra- and trifluorobenzyne with C6D6. The hydride,
fluoride, and pentafluorophenylmetallocenes are isolated and characterized by X-ray crystallography. DFT-
(B3PW91) calculations have been used to explore the pathways leading to the observed products of the
exergonic reactions. A key step is a H/F exchange reaction which transforms C6F6 and the cerium hydride
into C6F5H and Cp′2CeF. This reaction starts by an η1-F-C6F5 interaction, which serves as a hook. The
reaction proceeds via a σ bond metathesis where the fluorine ortho to the hook migrates toward H with a
relatively low activation energy. All products observed experimentally are accommodated by pathways
that involve C-F and C-H bond cleavages.
Introduction
by the large H-F bond disruption enthalpy, which in general
implies that when H2 is replaced by M-H, the reaction will be
exothermic when the M-F bond enthalpy is larger than the
M-H bond enthalpy by about 30 kcal mol-1. This condition is
met by the f-block and early d-transition metals. Lanthanide
fluoride bonds are strong; the averaged Ce-F bond enthalpy
of CeF3(g) is 153 kcal mol-1,6 which implies that the H/F
exchange will be exothermic, provided that the lanthanide-
The hydrogen for fluorine exchange in fluorocarbons is a
thermodynamically favorable reaction that does not occur in
absence of a catalyst.1 For example, passing a mixture of C6F6
and dihydrogen over Pd/C or Pt/C at 300 °C gives a mixture of
aromatic hydrofluorocarbons; Pt/C at 58% conversion gives
C6F5H and mixed isomers of C6F4H2 and C6F3H3 in a 4:3:1
ratio.2 The exchange reaction is exothermic when the sum of
the C-F and H-H bond enthalpies is less than those of C-H
and H-F in eq 1, where the bond dissociation enthalpies are in
hydride bond enthalpy is less than about 125 kcal mol-1
.
Although the averaged bond enthalpy of M-H in MH3(g) is
unknown, it is unlikely to be greater than 100 kcal mol-1 since
parentheses in units of kcal mol-1
.
the M-H BDE in Cp*2LaH and Cp*2LuH is 67 kcal mol-1 7
.
The rate, of course, is unknown.
C6F5-F
C6F5H
(x)
+ H-H f
+ HF
(135)4,5
Lanthanide metals in gas phase will defluorinate fluorocar-
(154)3
(105)4
(1)
bons.8 For example, Ce+(g) defluorinates C6F6 giving CeF+-
•
(g), the radical C6F5 , CeF2+(g), and C6F4 (a benzyne).9 In
Although the bond disruption enthalpy of C6F5-H is unknown,
the reaction will be exothermic if the C-H bond enthalpy is
greater than 125 kcal mol-1, which is reasonable since the bond
dissociation energy (BDE) of C6H4F-H is 126 kcal mol-1 3
(3) Smart, B. E. In Chemistry of Functional Groups; Patai, S., Rappoport, Z.,
Eds.; Wiley: New York, 1983; Suppl. D, Chapter 14.
(4) Dasent, W. E. Inorganic Energetics, 2nd ed.; Cambridge University Press:
Cambridge, U.K., 1982; p 103.
.
(5) Slayden, S. W.; Liebman, J. F.; Mallard, W. G. In Chemistry of Functional
Groups. The chemistry of halides, pseudohalides and azides; Patai, S.,
Rappoport, Z., Eds.; Wiley: Chichester, U.K., 1995; Suppl D2, p 387.
(6) Pankratz, L. B. Thermodynamic Properties of Halides, Bulletin 674, Bureau
of Mines, 1984.
Thus, the exothermicity of a H/F exchange reaction is driven
† Universite´ Paul Sabatier.
‡ University of California.
§ Universite´ Montpellier 2.
(7) Nolan, S. P.; Stern, D.; Hedden, D.; Marks, T. J. Bonding Energetics in
Organometallic Chemistry; ACS Symposium Series 428; American Chemi-
cal Society: Washington, DC, 1990; pp 159-174.
(1) Hudlicky, M. J. Fluorine Chem. 1989, 44, 345.
(2) Florin, R. E.; Pummer, W. J.; Wall, L. A. J. Res. Natl. Bur. Stand. 1959,
62, 119.
(8) Mazurek, U.; Schwarz, H. Chem. Commun. 2003, 1321.
9
10.1021/ja0451012 CCC: $30.25 © 2005 American Chemical Society
J. AM. CHEM. SOC. 2005, 127, 279-292
279

A R T I C L E S
Maron et al.
addition, the metallocene (Me5C5)2Yb abstracts a fluorine atom
from C6F6 giving [(Me5C5)2Yb]2(µ-F),10 and (MeC5H4)3UCMe3
exchanges the CMe3 with F in C6F6.11 The general subject of
intermolecular C-F activation has been extensively studied and
several reviews are available, largely within the framework of
oxidative addition reactions.8,12 Computational studies have
shown that the C-F activation may be exothermic but that a
high activation barrier is usually observed with late transition-
metal systems.13 In early transition metals, the activation barrier
is generally lower and several cases of H/F exchange have been
observed.14 The reactions of bare metal centers have been
analyzed by computational methods, showing that electron
transfer as well as oxidative insertion paths are possible.15
In this paper, we show that a monomeric metallocene hydride
of cerium undergoes H/F exchange with fluoroaromatics and
DFT calculations provide details about the relative activation
barriers for the reaction pathways.
Results
Figure 1. ORTEP diagram of Cp′2CeH (50% probability ellipsoids). The
hydrogen atoms on the Cp′ ligands were placed in calculated positions;
H(59), the hydride ligand, was found in the difference Fourier map and
refined with isotropic thermal parameters. Ce-C (ave) ) 2.81(2) Å, Ce-
Cp(ring centroid) (ave) ) 2.53 Å, Ce-H ) 1.90(5) Å, Cp(ring centroid)-
Ce-Cp(ring centroid) ) 155°, Cp(ring centroid)-Ce-H (ave) ) 101°.
General Strategy. The DFT calculations of the possible
reaction pathways for H/F exchange are carried out on a
monomeric (C5H5)2MH (Cp2MH) reactant and a (C5H5)2MF
(Cp2MF) product in the gas phase. It is essential that the
synthetic studies begin and end with a monomeric metallocene
hydride and fluoride derivative, respectively, in order that the
experimental and calculational results track each other as closely
as possible. Since no monomeric metallocene lanthanide hy-
drides or fluorides are known, synthetic studies were initiated
with the goal of making them. In general, the base-free
metallocene hydrides are dimeric in the solid state and in
solution.16 The dimeric cerium hydrides, (C5Me5)4Ce2(µ-H)2 and
[1,3-(Me3C)2C5H3]4Ce2((µ-H)2, are known.17,18 A tiny number
of metallocene lanthanide fluorides are known10,19,20 and only
one, [1,3-(SiMe3)2C5H3]4Sm2(µ-F)2, contains symmetrical bridg-
ing fluorides.21 Our focus is on the metallocene derivatives of
cerium since the 4f1 electron configuration simplifies the
interpretation of their magnetic properties. In addition, one of
our long-term interests is comparison of the chemical and
physical properties between the 4f and 5f block metals with
identical radii, that is, Ce(III) and U(III). The 1,3-di-tert-
butylcyclopentadienyl ligand yields the dimeric cerium hydride,
[1,3-(Me3C)2C5H3]4Ce2(µ-H)]2,18 suggesting that increasing the
number of Me3C-groups on the cyclopentadienyl ring will yield
a monomeric hydride, a deduction that is correct.
(9) Cornehl, H. H.; Hornung, G.; Schwarz, H. J. Am. Chem. Soc. 1996, 118,
9960.
Synthetic Studies. The synthetic sequence begins with the
synthesis of the bis (1,3,4-tri-tert-butylcyclopentadienyl) cerium
triflate, Cp′2CeOTf, then replacing the triflate by a benzyl group;
the synthesis details are in the Experimental Section. Addition
of dihydrogen to the benzyl derivative in pentane gives the deep
purple hydride, Cp′2CeH, mp 152-155 °C, which may be
crystallized from pentane. The hydride gives a (M-2)+ molecular
ion in the mass spectrum, but the Ce-H stretching frequency
could not be identified in the infrared spectrum, even when
directly compared with the deuteride (prepared by using D2
rather than H2), nor could the Ce-H resonance be observed in
the H NMR spectrum. The H NMR spectrum clearly shows
the paramagnetic isotropic shifts for the two types of Me3C-
groups in a 2:1 area ratio and the equivalent ring methyne
resonances; the details are given in the Experimental Section.
Figure 1 shows an ORTEP diagram of the hydride; important
structural parameters are given in the figure caption. The solid-
state structure clearly shows that the hydride is a monomer.
(10) Burns, C. J.; Andersen, R. A. J. Chem. Soc., Chem. Commun. 1989, 136.
(11) Weydert, M.; Andersen, R. A.; Bergman, R. G. J. Am. Chem. Soc. 1993,
115, 8837.
(12) Kiplinger, J. L.; Richmond, T. G.; Osterberg, C. E. Chem. ReV. 1994, 94,
373. Richmond, T. G. Top. Organomet. Chem. 1999, 3, 243. Burdeniuc,
J.; Jedlicka, B.; Crabtree, R. H. Chem. Ber./Recueil 1997, 130, 145. Murphy,
E. F.; Murugavel, R.; Roesky, H. W. Chem. ReV. 1997, 97, 3425. Alonso,
F.; Beletskaya, I. P.; Yus, M. Chem. ReV. 2002, 102, 4009. Braun, T.;
Perutz, R. N. Chem. Commun. 2002, 2749. Jones, W. D. Dalton Trans.
2003, 3991.
(13) Su, M. D.; Chu, S.-Y. J. Am. Chem. Soc. 1997, 119, 10178. Bosque, R.;
Fantacci, S.; Clot, E.; Maseras, F.; Eisenstein, O.; Perutz, R. N.; Renkema,
K. B.; Caulton, K. G. J. Am. Chem. Soc. 1998, 120, 12634. Ge´rard, H.;
Davidson, E. R.; Eisenstein, O. Mol. Phys. 2002, 100, 533. Reinhold, M.;
McGrady, J. E.; Perutz, R. N. J. Am. Chem. Soc. 2004, 126, 5268.
(14) Edelbach, B. L.; Rahman, A. K. F.; Lachicotte, R.; Jones, W. D.
Organometallics 1999, 18, 3170. Edelbach, B.; Kraft, B. M.; Jones, W. D.
J. Am. Chem. Soc. 1999, 121, 10327. Kraft, B. M.; Lachicotte, R. J.; Jones,
W. D. J. Am. Chem. Soc. 2000, 122, 8559. Kraft, B. M.; Jones, W. D. J.
Organomet. Chem. 2002, 658, 132. Watson, L. A.; Yandulov, D. V.;
Caulton, K. G. J. Am. Chem. Soc. 2001, 123, 603. Yang, H.; Gao, H.;
Angelici, R. J. Organometallics 1999, 18, 2285. Turculet, L.; Tilley, T. D.
Organometallics 2002, 21, 3961. Clot, E.; Me´gret, C.; Kraft, B. M.;
Eisenstein, O.; Jones, W. D. J. Am. Chem. Soc. 2004, 126, 5647.
(15) Harvey, J. N.; Schro¨der, D.; Koch, W.; Danovich, D.; Shaik, S.; Schwarz,
H. Chem. Phys. Lett. 1997, 278, 391. Chen, Q.; Freiser, B. S. J. Phys.
Chem. A 1998, 102, 3343. Zhang, D.; Liu, C.; Bi, S. J. Phys. Chem. A
2002, 106, 4153.
1
1
The fluoride is prepared by allowing Cp′2CeCH2Ph to react
with BF3‚OEt2 in pentane, a method used previously.22 The
fluoride, mp 164-167 °C, crystallizes from pentane as orange
crystals that yield a monomeric molecular ion in the gas-phase
(16) Ephritikhine, M. Chem. ReV. 1997, 97, 2193. Schaverien, C. J. AdV.
Organomet. Chem. 1994, 36, 283. Jeske, G.; Lauke, H.; Mauermann, H.;
Swepston, P. N.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985,
107, 8091.
(17) Heeres, H. J.; Renkema, J.; Booij, M.; Meetsma, A.; Teuben, J. H.
Organometallics 1988, 7, 2495.
(18) Gunko, Y. K.; Bulychev, B. M.; Soloveichik, G. L.; Belsky, V. K. J.
Organomet. Chem. 1992, 424, 289.
(21) Xie, Z. W.; Liu, Z. X.; Xue, F.; Mak, T. C. W. J. Organomet. Chem. 1997,
539, 127.
(22) Lukens, W. W.; Beshouri, S. M.; Blosch, L. L.; Stuart, A. L.; Andersen,
R. A. Organometallics 1999, 18, 1235.
(19) Burns, C. J.; Berg, D. J.; Andersen, R. A. J. Chem. Soc., Chem. Commun.
1987, 272.
(20) Watson, P. L.; Tulip, T. H.; Williams, I. Organometallics 1990, 9, 1999.
9
280 J. AM. CHEM. SOC. VOL. 127, NO. 1, 2005

Hydrogen for Fluorine Exchange in C6F6 and C6F5H
A R T I C L E S
Scheme 1
benzyl derivative though stable at 25 °C in the solid state, gives
a (M-PhCH3)+ molecular ion in the mass spectrum. In C6D6
1
solution, the CMe3 resonances slowly disappear from the H
NMR spectrum as the intensity of the C6H6-xDx resonances
increase relative to an internal standard as the resonances due
to PhMe appear. Over the course of a week at 65 °C, all the
CMe3 resonances disappear. Replacing essentially all of the
C6D6 by C6H6 regenerates Me3C resonances with chemical shifts
different from those of the benzyl derivative. The new reso-
nances have intensities and a coupling pattern (the meta and
para resonances appear as doublet and triplet, respectively)
consistent with those expected for Cp′2CePh. Thus, the CMe3
groups undergo reversible H/D exchange with C6D6 or C6H6,
with formation of a Cp′2CeC6H5 or Cp′2CeC6D5 species. In
C6D12 solvent, the resonances due to toluene appear after 3 h
at 65 °C in addition to another set of rather broadened
resonances that appears as five equal area resonances and two
additional resonances of about one-third their intensity. These
observations suggest that this species is the chiral metallacycle
[(Me3C)3C5H2)][Me3C)2C5H2(CMe2CH2)]Ce, a deduction con-
sistent with the reactions shown in Scheme 1; experimental
details are in the Experimental Section. A related metallacyle
has been observed by heating (Me5C5)2CeCH(SiMe3)2 in C6D12,
which is a tetramer in the solid state.23
Figure 2. ORTEP diagram of Cp′2CeF (50% probability ellipsoids). All
the non-hydrogen atoms were refined anisotropically and the hydrogen atoms
were placed in calculated positions. Ce-C (ave) ) 2.82(2) Å, Ce-Cp-
(ring centroid) (ave) ) 2.55 Å, Ce-F ) 2.165(2) Å, Cp(ring centroid)-
Ce-Cp(ring centroid) ) 149°, Cp(ring centroid)-Ce-F (ave) ) 105(2)°.
mass spectrum. The 1H NMR spectrum at 20 °C shows
resonances for the two inequivalent types of Me3C groups in a
2:1 ratio and a single-ring methyne resonance. The fluoride
sublimes at 140 °C in a vacuum and crystals suitable for an
X-ray study were grown in this way. An ORTEP diagram of
the monomeric fluoride is shown in Figure 2 and some bond
parameters are given in the figure caption.
On a synthetic scale (ca. 1 g), warming Cp′CeCH2Ph in
pentane for 12 h gives a deep purple solution, which yields a
purple glassy solid that cannot be persuaded to crystallize. The
1H NMR spectrum in C6D12 is identical to that observed in the
NMR tube experiment mentioned above. Refluxing a solution
of Cp′2CeCH2Ph in C6H6 for 3 days gives a red solution, which
yields a red powder whose elemental analysis indicates that the
1
stoichiometry is Cp′2CeC6H5. The H NMR spectrum in C6D6
The hydride and fluoride derivatives are stable in C6D6 and
C6D12 solution for an indefinite period of time. In contrast, the
(23) Booij, M.; Meetsma, A.; Teuben, J. H. Organometallics 1991, 10, 3246.
9
J. AM. CHEM. SOC. VOL. 127, NO. 1, 2005 281

A R T I C L E S
Maron et al.
Figure 4. Plot of chemical shift (δ) vs 1/T for Cp′2CeC6F5 in PhMe-d8.
ortho-F, and the other two resonances are a doublet and triplet
at δ -161 and δ -158 in area ratio 2:1, respectively, J ) 18
Hz. Cooling shifts the resonances and the coupling between
meta-F, and para-F disappears as the resonances broaden but
do not decoalesce to -50 °C. The 2:1 pattern of the CMe3
resonances changing to a 1:1:1 pattern at low temperature is
consistent with a metallocene whose idealized symmetry
changes from C2V to Cs, where the mirror plane is coplanar with
the C6F5 ring. Several physical processes can account for loss
of the time-averaged C2-axis and mirror plane, two of which
are slowing the librational motion of the Cp′-rings about their
pseudo-C5 axis and slowing the rocking motion of the C6F5 ring
in the mirror plane. If the C6F5 ring rocking is slowed by a
Ce‚‚‚F interaction, as observed in the crystal structure, and if
the Cp′ rings undergo libration, then the CMe3 groups will
appear as a 1:1:1 pattern and the F nuclei in the C6F5 ring may
appear as distinct resonances or as broadened features depending
upon how much the chemical shift changes as a function of
temperature. In this context, the librational motion of the Cp′
rings in Cp′2CeH and Cp′2CeF is not slowed, that is, the 2:1
pattern is observed at -90 °C. Further studies are needed to
define the physical process responsible for the fluxional
processes.
Solution NMR Mixing Experiments, H/F Exchange Reac-
tions. C6F6. When an excess of C6F6 is added to Cp′2CeH in
C6D6 in an NMR tube, the purple color of the hydride
instantaneously turns orange and a gas is evolved. Examination
of the 1H NMR spectrum shows that the resonances due to Cp′2-
CeH are absent and new sets of resonances due to Cp′2CeF,
Cp′2CeC6F5, and dihydrogen appear. Over time, the resonances
due to Cp′2CeF increase in intensity at the expense of those
due to Cp′2CeC6F5. These events are symbolized by the
unbalanced eqs 2 and 3.
Figure 3. ORTEP diagram of Cp′2Ce(C6F5) (50% probability ellipsoids).
All the non-hydrogen atoms were refined anisotropically and the hydrogen
atoms were placed in calculated positions. Ce-C (ave) ) 2.81(5) Å, Ce-
Cp(ring centroid) (ave) ) 2.54 Å, Ce-C(35) ) 2.621(4) Å, Ce‚‚‚F(5) )
2.682(2), Cp(ring centroid)-Ce-Cp(ring centroid) ) 147°, Cp(ring cen-
troid)-Ce-C(35) ) 107(1)°, Cp(ring centroid)-Ce-F(5) (ave) ) 101(1)°,
Ce-C(35)-C(36) ) 149.7(3)°, Ce-C(35)-C(40) ) 97.5(3)°.
is identical to that observed in the NMR tube experiment
described above. Although neither metallacycle nor phenyl
derivative is obtained in crystalline form, the sequence of
reactions in Scheme 1 is consistent with the formulation given.
Further proof that the metallacycle formulation is correct is
derived by dissolving it in a mixture of C6F5H and C6D12 and
observing a new set of Me3C-resonances in a 1:2 ratio. On a
synthetic scale, dissolving the metallacycle in an excess of
C6F5H in pentane yields an orange solution from which orange
crystals may be isolated, whose 1H NMR spectrum is identical
to that observed in the mixing experiment. The crystals
decompose rather violently on attempted melting in a sealed
melting point capillary at about 125 °C; extracting the residue
into C6D6 and examining the 1H NMR spectrum shows
resonances due to Cp′2CeF, as well as several unidentified
resonances. The thermal behavior is presumably the reason for
the incomprehensible mass spectrum. Fortunately, single crystals
of Cp′2CeC6F5 suitable for X-ray studies are obtained from
pentane and an ORTEP diagram is shown in Figure 3. As can
been seen, the carbon atom of the C6F5 group C(35) is located
close to the idealized C2 axis but the Ce-C(35)-C(36,40)
angles are very anisotropic because of a short Ce‚‚‚F-C(ortho)
contact of 2.682(2) Å. A pair of close ortho-F‚‚‚Sm contacts of
2.531(8) Å and 2.539(7) Å were found in dimeric (C5Me5)4-
Sm2(C6F5)2 and in monomeric (C5Me5)Yb(C6F5)(thf)3 where the
ortho-F‚‚‚Yb distance is 3.162 Å.24 In solution at 25 °C, the 1H
and 19F NMR spectra are not consistent with the solid-state
crystal structure, where the idealized symmetry is C1 and all of
Cp′2CeH + C6F6 f Cp′2CeF + Cp′2CeC6F5 + H2 (2)
Cp′2CeC6F5 f Cp′2CeF + C6F4
(3)
Examination of the 19F NMR spectrum of this mixture shows
resonances due to Cp′2CeC6F5 and an AA′BB′ pattern whose
chemical shifts and coupling pattern are identical to the Diels-
Alder adduct 1, X ) D, between tetrafluorobenzyne and
benzene-d6.25 Hydrolysis of the mixture with H2O and injection
into a GCMS gives the M+ for 1, X ) D. Repeating the
experiment, but replacing the C6D6 with C6H6 followed by
hydrolysis with H2O and injection into a GCMS gives M+ for
1, X ) H, Y ) F. Thus, Cp′2CeC6F5 decomposes by forming
Cp′2CeF and tetrafluorobenzyne that is trapped by the solvent
1
the CMe3 groups and F atoms are inequivalent. The H NMR
absorptions change as a function of temperature and the CMe3
resonance of area 2 decoalesces into two equal area resonances
while the other CMe3 resonance of area 1 has a normal
temperature dependence (Figure 4), giving a ∆G#TC ) 10 kcal
mol-1. The 25 °C 19F NMR spectrum consists of three
resonances, two of area 2 and one of area 1. One of the area 2
resonances is broad (ν1/2 ) 480 Hz), probably because of the
(25) Fruchier, A.; Claramunt, R. M.; Elguero, J.; Carmona, D.; Esteban, M.
Bull Soc. Chim. Belg. 1992, 101, 697. Brewer, J. P. N.; Eckhard, I. F.;
Heaney, H.; Marples, B. A. J. Chem. Soc. C 1968, 664.
(24) Castillo, I.; Tilley, T. D. J. Am. Chem. Soc. 2001, 123, 10526. Deacon G.
B.; Forsyth, C. M. Organometallics 2003, 22, 1349.
9
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Hydrogen for Fluorine Exchange in C6F6 and C6F5H
A R T I C L E S
benzene, a common decomposition pattern of pentafluorophen-
ylmetal compounds.26
C6F5H. Adding an excess of C6F5H to Cp′2CeH dissolved in
C6D6 in an NMR tube changes the purple solution to an orange
1
one with liberation of a gas. The H NMR spectrum shows
resonances due to Cp′2CeF and Cp′2CeC6F5, and two new sets
of paramagnetic resonances in a ratio of 6:2:4:1, assuming the
resonances are all due to CMe3 groups and H2. Integration of
the initial and final CMe3 resonances relative to an internal
standard shows that the conversion is quantitative. Over time,
all of the resonances disappear, except those due to Cp′2CeF,
which is the only cerium containing metallocene at the end of
the reaction. The second most abundant set of resonances is
identical to those formed by addition of the metallacycle to
1,2,4,5-tetrafluorobenzene (Scheme 1) in C6D12 implying that
its identity is Cp′2Ce(2,3,5,6-C6F4H). When the latter solution
is evaporated to dryness, dissolved in C6D12, and heated to 65
°C for 12 h, resonances due to Cp′2CeF form. The liberated
C6F3H is trapped by the Cp′ ring, since the 19F NMR spectrum
shows a new ABCX (2, X ) H) pattern as well as resonances
The origin of dihydrogen is less readily explained, though
the net reactions symbolized in eq 4a-d may be suggested.
Equation 4a is an intermolecular C-F activation, 4c is a C-H
activation, and both yield Cp′2CeC6F5, which ultimately yields
Cp′2CeF. Equation 4b and 4d destroys the HF that is formed
producing Cp′2CeF. No intermediates are detected, other than
Cp′2CeC6F5, and only the reaction in 4c can be easily tested,
since we do not have facilities to deal with anhydrous HF.
1
in the H NMR spectrum due to 2, X ) H. Hydrolysis with
H2O and injection into a GCMS gives a M+ due to 2, X ) H.
When the trifluorobenzyne is trapped with C6D6, the NMR
spectra and GCMS are consistent with 1, X ) D, Y ) H.29
The remaining set of new resonances in the original reaction of
Cp′2CeH and C6F5H is due to an unknown cerium-containing
product. This set of resonances forms exclusively when a
solution of the metallacycle is added to Cp′2Ce(2,3,5,6-C6F4H)
in C6D12, suggesting that the unknown compound is Cp′2Ce-
(1,4-C6F4)CeCp′2. Hydrolysis with H2O gives 1,2,4,5-tetrafluo-
robenzene as the only substance detected by 19F NMR spec-
troscopy consistent with this deduction. The net reactions that
account for the products conclusively identified in the NMR
experiments may be written in eq 5a-f.
Cp′2CeH + C6F6 f Cp′2CeC6F5 + HF
Cp′2CeC6F5 + HF f Cp′2CeF + C6F5H
Cp′2CeH + C6F5H f Cp′2CeC6F5 + H2
Cp′2CeH + HF f Cp′2CeF + H2
(4a)
(4b)
(4c)
(4d)
Neither dihydroanthracene nor addition of H2 or D2 alters the
reaction, and no anthracene or hydrogenated products are
detected. However, changing the solvent from C6D6 to C6D12
removes the trap for tetrafluorobenzyne, and the Cp′-ring is now
the trap. In C6D12, Cp′2CeC6F5 decomposes to Cp′2CeF and a
new organic product over 12 h at 60 °C. Hydrolysis with D2O
or H2O and injection into the GCMS shows one major
fluorinated organic compound with M+ ) 382 along with Cp′D
or Cp′H. This organic product is likely to be 2, X ) F, the
[2+4] cycloaddition product of C6F4, and the substituted
cyclopentadienyl ring. The 19F NMR spectrum consists of an
Cp′2CeH + C6F5H f Cp′2CeC6F5 + H2
Cp′2CeH + C6F5H f Cp′2CeC6F4H + HF
Cp′2CeH + C6F4H2 f Cp′2CeC6F4H + H2
Cp′2CeC6F5 + HF f Cp′2CeF + C6F5H
Cp′2CeC6F4H + HF f Cp′2CeF + C6F4H2
Cp′2CeH + HF f Cp′2CeF + H2
(5a)
(5b)
(5c)
(5d)
(5e)
(5f)
1
AA′BB′ pattern similar to that found in 1, and the H NMR
spectrum shows two CMe3 resonances in a 2:1 ratio. Other
isomers of 2 could be formed, since low intensity absorptions
1
are visible in the H and 19F NMR spectra, but 2 is the major
isomer formed. When deutero metallocene, {[C(CD3)3]3C5H2}2-
CeC6F5, is allowed to decompose in C6D12 and the residue
hydrolyzed with H2O and analyzed by GCMS, the M+ at 410
is observed, implying that the source of the “other” hydrogen
is a ring -CMe3. Close inspection of the 19F NMR spectrum
resulting from decomposition of Cp′2CeC6F5 in C6D6 shows that
the 2, X ) F is also present to the extent of about 10%,
suggesting that the Cp′-ring can compete with C6D6 as a trap
for tetrafluorobenzyne. Cyclopentadiene and its anion are
capable of trapping benzyne27 and furan and other dienes trap
tetrafluorobenzyne28 so the suggestion that Cp′ traps tetrafluo-
robenzyne is not without precedent.
The reaction symbolized in 5a and 5b are intermolecular C-H
and C-F activations, respectively, while those in the remaining
equations result from secondary reactions. Since the reactions
between Cp′2CeH and C6F6-xHx (x ) 0, 1) are rapid and the
only intermediate that is detected is either Cp′2CeC6F5 or Cp′2-
CeC6F4H, only the net reaction symbolized by eqs 4 and 5 can
be written with confidence. The mechanism of the H/F chemical
exchange in Cp′2CeH is unknown from the experiments
described. In the next section, DFT calculations provide
guidance about the relative thermodynamic quantities and kinetic
barriers for the H/F exchange processes.
Computational Studies
(26) Banks, R. E. Fluorocarbons and their DeriVatiVes, 2nd ed.; MacDonald &
Co: London, 1970; Chapter 5. Cohen, S. C.; Massey, A. G. AdV. Fluorine
Chem. 1970, 6, 83.
(27) Hoffmann, R. W. Dehydrobenzene and cycloalkynes; Academic Press: New
York, 1967. Buske, G. R.; Ford, W. T. J. Org. Chem. 1976, 41, 1995.
(28) Wenk, H. H.; Sander, W. Chem. Eur. J. 2001, 7, 1837.
Reactant, Intermediate, and Product. The reaction path-
ways that could account for the observed products of the reaction
(29) Harrison, R.; Heaney, H. J. Chem. Soc. C 1968, 889.
9
J. AM. CHEM. SOC. VOL. 127, NO. 1, 2005 283

A R T I C L E S
Maron et al.
between Cp′2CeH, modeled by Cp2LaH, 3, and C6F6 and C6F5H
were analyzed with DFT calculations with methodology that
has been successful for the reaction of metallocene hydrides
with H2, CH4, and CF4 for the entire family of lanthanide
elements.30-34 These studies show no dependence of the nature
of the lanthanide on the geometry of the metallocene except
that due to the lanthanide contraction. They also show a minor
dependence of the activation barrier of about 1 kcal mol-1
depending on the specific metal chosen. Therefore, the choice
of using lanthanum, which lies to the immediate left of cerium
and thus has a similar ionic radius, will not affect the calculated
differences in energy to any significant extent. The numerical
values calculated for La can be transferred to the case of Ce
with an expected change of less than 1-2 kcal mol-1. Modeling
of Cp′ by C5H5 is a more severe approximation because of the
large difference in steric size between these cyclopentadienyl
ligands, especially when some significant geometrical reorga-
nization can occur.34-36 For this reason, two key structures were
calculated with the QM/MM methodology using ONIOM
(B3PW91:UFF). Calculated free energies, G, estimated at 298.15
K, are given in kcal mol-1 with respect to separated reactants
Cp2LaH and C6F6 or C6F5H. The entropy calculated in gas phase
is exaggerated by around 50% with respect to what it is in the
condensed phase at least for the particular case of a reaction in
water.37 Neglecting the translational part of the entropy as
suggested by Sakaki could lead to an underestimation of the
energy barriers for a bimolecular process.38 Calculating the
change in energy ∆E and in free energy ∆G for the model
reactions gives similar conclusions and free energies are used
throughout this paper.
Initially, the calculated geometries of reactant Cp2LaH, 3,
and product Cp2LaF, 4, and an intermediate Cp2LaC6F5, 5, were
optimized and compared to the solid-state structures of Cp′2-
CeH, Cp′2CeF, and Cp′2Ce-C6F5 (Figure 5). In 3, the La-H
bond length of 2.14 Å is reasonably close to the Ce-H distance
of 1.9 Å, given the uncertainty of this datum. The Cp(centroid)-
La distance of 2.56 Å is close to the experimental values of
2.55 and 2.51 Å. The smaller Cp(centroid)-La-Cp(centroid)
angle of 134° relative to Cp′(centroid)-Ce-Cp′(centroid) angle
of 155° is clearly due to the smaller size of C5H5 relative to
1,3,4-(Me3C)3C5H2. Similar trends were noted when comparing
C5H5 and Me5C5 lanthanides derivatives.34-36 In 4, the La-F
distance of 2.18 Å is slightly longer than the experimental Ce-F
distance of 2.165(2) Å. The Cp(centroid)-La distance of 2.58
Å differs slightly from the experimental values in the Ce
metallocene to ring distance of 2.57 and 2.53 Å, but the
calculated Cp(centroid)-La-Cp(centroid) angle of 131° is again
much smaller than the Cp′(centroid)-Ce-Cp′(centroid) angle
of 149°. In the metallocene-pentafluorophenyl complex, 5, the
bidentate nature of the interaction between C6F5 and the metal
is reproduced with La-C and La-F distances equal to 2.61
Figure 5. Optimized geometry (distance in Å, angles in degrees) of Cp2-
LaH, 3, Cp2LaF, 4, and Cp2La(C6F5), 5; small white circle ) H, medium
white circle ) C, large size white circle ) La, medium black circle ) F.
and 2.78 Å, respectively, compared to the experimental Ce-C
and Ce-F distances of 2.621(4) and 2.682(2) Å. As before,
the ring (centroid)-metal distances are comparable, 2.55 Å for
Cp compared to 2.54 Å for Cp′, but the angle between the two
rings is smaller with Cp (132°) than with Cp′ (147°). The Cp′-
(centroid)-Ce-Cp′(centroid) angle gets smaller on going from
H to F (155° and 149°, respectively); a smaller variation in the
same direction is found in the calculated systems (134° for the
hydride compared to 131° for the fluoride) because the Cp-
(centroid)-La-Cp(centroid) angle is already very small.
Direct H/F Exchange. The first mechanism for H/F exchange
tested by computational methods is to allow a fluorine atom in
C6F6 to approach the metallocene hydride and then to allow a
LaH‚‚‚C(C6F6) and a La‚‚‚F interaction to develop, that is, a σ
bond metathesis transition state.31,32,34,39,40 The transition state
will look like 6 and involve a carbon at the â position of the
kite-shaped transition state. This transition state looks attractive
since the geometry around each atom is acceptable; in particular,
the geometry of the carbon in the â position is pseudotetrahedral.
Indeed, a molecular orbital calculation has suggested such a
transition state41 for which there is no experimental evidence.40
Such a transition state looks like a planar cyclohexadienyl
fragment with the positively charged metal center bonded to H
and F. Despite considerable effort, no such transition state could
be located on the potential energy surface for either C6F6 or
C6F5H. The kite-shaped transition state geometry is clearly
energetically unfavorable with carbon at the â position even
when it is not hypervalent. One can speculate that the absence
of such a transition state could be due to an electronic charge
(30) Maron, L.; Eisenstein, O. J. Am. Chem. Soc. 2001, 123, 1036.
(31) Maron, L.; Perrin, L.; Eisenstein, O. J. Chem. Soc., Dalton Trans. 2002,
534.
(32) Perrin, L.; Maron, L.; Eisenstein, O. Dalton Trans. 2003, 4313.
(33) Perrin L.; Maron L.; Eisenstein O. ActiVation and Functionalization of C-H
Bonds; ACS Symposium Series 885; Goldman, A., Goldberg, K., Eds.;
American Chemical Society: Washington, DC, 2004; pp 116-135.
(34) Scherer, E. C.; Cramer, C. J. Organometallics 2003, 22, 1682.
(35) Perrin, L.; Maron, L.; Eisenstein, O.; Schwartz, D. J.; Burns, C. J.; Andersen,
R. A. Organometallics 2003, 22, 5447.
(36) Perrin, L.; Maron, L.; Eisenstein, O. New J. Chem. 2004, 28, 1255.
(37) Cooper J.; Ziegler, T. Inorg. Chem. 2002, 41, 6614.
(38) Sakaki, S.; Tatsunori, T.; Michinori, S.; Sugimoto, M. J. Am. Chem. Soc.
2004, 126, 3332.
(39) Steigerwald, M. L.; Goddard, W. A., III. J. Am. Chem. Soc. 1984, 106,
308. Rappe´, A. K.; Upton, T. H. J. Am. Chem. Soc. 1992, 114, 7507. Rappe´,
A. K. Organometallics 1990, 9, 466. Ziegler, T.; Folga, E.; Berces, A. J.
Am. Chem. Soc. 1993, 115, 636. Ziegler, T.; Folga, E. J. Organomet. Chem.
1994, 478, 57.
(40) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Berger, B. J.; Nolan, M. C.;
Santasierio, B. D.; Schaefer, W. D.; Bercaw, J. E. J. Am. Chem. Soc. 1987,
109, 203.
(41) Deelman, B.-J.; Teuben, J. H.; Macgregor, S. A.; Eisenstein, O. New J.
Chem. 1995, 19, 691.
9
284 J. AM. CHEM. SOC. VOL. 127, NO. 1, 2005

Hydrogen for Fluorine Exchange in C6F6 and C6F5H
A R T I C L E S
transition state, 8, 16.4 kcal mol-1 above the separated reactants
is reached. This transition state places the hydrogen only 1.96
Å from the ring carbon atom that is not yet significantly
rehybridized, as the carbon atom is part of an essentially planar
C6F6 ring. This transition state then connects to a cyclohexa-
dienyl adduct, 9, with the metal bonded almost equally to the
five carbons of the pentadienyl fragment (2.9-3.0 Å) and 3.51
Å from the rehybridized carbon. The C-H bond is almost in
the plane of the benzene ring while the C-F bond is almost
perpendicular to it. The C(sp3)-F bond (1.435 Å) is longer than
that calculated in CH3F (1.382 Å), consistent with some
delocalization of the π density into the σ*CF orbital. Structure
9 is calculated to be 16.1 kcal mol-1 below the reactants.
However, no path could be located that allows the Cp2La
fragment to abstract a fluorine. Migration of the metal fragment
to a position between H and F is not possible since a transition
state or a minimum of type 6 could not be located. Dissociation
into separated metallocene ion and cyclohexadienyl anion is a
most unlikely event, especially in a nonpolar solvent such as
benzene. Therefore, the path 7 f 8 f 9 can only account for
hydride transfer between the metal and the C6F6 ring. A
reversible hydride transfer related to the calculated one has been
observed in [(Ar*N))(Ar*NH)Ta(H)(OSO2CF3)] (Ar*N ) 2,6-
(2,4,6 Me3C6H2)2C6H3N).42 The bulkiness of the Cp′ ring also
disfavors the transfer of the hydride since it requires the
fluoroarene to be perpendicular to the plane bisecting the Cp-
La-Cp angle in the adduct 7, transition-state 8, and cyclohexa-
dienyl adduct 9. To evaluate the destabilizing effect of the tert-
butyl substituent, the adduct between C6F6 and Cp′2LaH was
calculated with the ONIOM method (Figure 7). The bulky Cp′
groups prevent the close approach of C6F6 to the metal as
illustrated by the shortest distance La‚‚‚C(C6F6) that increases
from 3.48 to 5.15 Å upon replacing Cp by Cp′. In addition,
whereas the C6F6 is approximately parallel to the La-H
direction with Cp2La-H, this is not the case with the Cp′ ligand.
As a result, the attractive interaction between C6F6 and the
Figure 6. Two views of the optimized geometries (distances in Å) of the
structures associated with H transfer to C6F6. The value in parentheses is
the number of imaginary frequencies, 0 for a minimum, 1 for a transition
state.
distribution poorly adapted for good stabilization. It is reasonable
to assume that considerable density would be accumulated in
the π system of the ring. However, the metal, which would be
located nearby the C(sp3)-H and C(sp3)-F bonds, would be
far from the part that should carry the negative charge and thus
would have no stabilizing influence. This point of view is further
supported by the location of a minimum for a structure with
the metal bonded to the π system of the cyclohexadienyl ring
as discussed in the following section.
metallocene hydride has decreased by 10.6 kcal mol-1 43
This
.
pathway is disfavored by steric hindrance and involves a
nonproductive hydride transfer and is therefore not considered
further.
F/H Exchange via HF Formation. Alternative pathways
therefore had to be developed and explored by calculations.
Figure 7 shows that C6F6 can slide between the two cyclopen-
tadienyl rings to point a fluorine atom toward the metal center
even in the presence of the bulky tert-butyl groups. Thus, the
free energy of the σ adduct of C6F6 and Cp2La-H, 10, is 8.2
kcal mol-1 below the separated reactants. The CMe3 groups
destabilize the σ adduct by only 6.8 kcal mol-1, that is, by less
than for the π adduct.43 In addition, the calculated La‚‚‚F
distance increases only moderately (from 2.746 to 3.084 Å)
when replacing Cp by Cp′. The stability of the σ adduct
illustrated by a free energy of dissociation of 8.2 kcal mol-1 is
significant and shows that a fluoroarene can bind to an
electrophilic metal center by using a fluorine lone pair, a
coordination mode observed for C6F6 with a cationic zir-
conocene.44 The calculated La‚‚‚F distance in 10 is also similar
Hydride Addition to the Arene. An alternative mechanism
is one in which the metallocene hydride adds to the six-
membered ring since the electronegative fluorine makes the
aromatic ring more of an electrophile. The result is adduct, 7,
in which the aromatic ring is perpendicular to the plane bisecting
the Cp-La-Cp angle (Figure 6) and a C6F6(centroid)-La
distance of 3.29 Å. The free energy of this π adduct 7 is 10.6
kcal mol-1 above the separated reactants. From this adduct, a
(42) Gavenonis, J.; Tilley, T. D. Organometallics 2002, 21, 5549.
(43) This value is based on the differences in ∆E and not in ∆G because of the
way frequencies are treated in the ONIOM method.
(44) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10015.
9
J. AM. CHEM. SOC. VOL. 127, NO. 1, 2005 285

A R T I C L E S
Maron et al.
Figure 9. Optimized geometries of the extrema for the F/H exchange
reaction via HF formation. The free energies are in kcal mol-1. Large white
circle ) La, middle size white circle ) C, small white circle ) H, black
circle ) F. In parentheses, the number of imaginary frequencies, 0 for a
minimum, 1 for a transition state.
bisecting the Cp-La-Cp angle forms Cp2La(C6F5)(FH), 12,
61.8 kcal mol-1 below reactants. In 12, the C6F5 group is bonded
to La via C and F(2) as in Cp2LaC6F5, 5, but the La‚‚‚F(2)
distance is significantly longer because FH remains in the
coordination sphere of the metal. This F-H adduct transforms,
almost without activation energy via transition-state 13, into
Cp2La-F and C6F5H with a total gain of free energy of 90.5
kcal mol-1 relative to reactants. This last step, which is formally
a σ bond metathesis since it involves an interchange between
the F-H and La-C bonds, is perhaps better viewed as a proton
transfer from H-F to the negatively charged σ aryl group.
During the proton transfer, the carbon of the fluoroarene moves
away from the metal (2.67 and 2.78 Å in 12 and 13,
respectively), but this is compensated by a shortening of the
La‚‚‚F(2) bond (3.08 and 2.70 Å, respectively). This clearly
emphasizes the key role of F(2) serving as a hook between the
Cp2La fragment and the arene so as to keep them in close
proximity and lower the total free energy.
Figure 7. Space-filling and ball-and-sticks representation of the optimized
geometries of the π and σ adducts of C6F6 and Cp′2LaH by the ONIOM
(B3PW91:UFF) method. The distance given corresponds to the shortest
one.
We have thus identified a pathway that transforms Cp2La-H
and C6F6 into Cp2LaF and C6F5H, with one high but still
accessible energy barrier associated with the C-F bond cleav-
age. At transition-state 11, the carbon that loses its fluorine is
not yet bonded to La (3.7 Å), while the hydride is (La-H )
2.4 Å), and C, F(1), and H are aligned. The alignment of C,
F(1), and H allows the carbon to maintain some bonding
interaction with F. The distance La‚‚‚F(1) (2.52 Å) is rather
short suggesting some bonding interaction. A remarkable aspect
of this transition state is the reorganization of electronic charge
that occurs on the way to 12. In the reactant, 10, the polarity of
the σ bonds is C(1)δ+-F(1)δ- and Laδ+-Hδ- but in the product,
12, the polarity is C(1)δ--Laδ+ and F(1)δ--Hδ+. Stated in
another way, F(1) and La exchange partners, which demands
that C(1) and H must also exchange their charges! In the initial
adduct, 10, the charge distribution is not very favorable because
Figure 8. Free-energy profile ∆G (kcal mol-1 at 298.15 K) for the reaction
of F/H exchange via HF formation.
to that calculated in Cp2La-C6F5 illustrating the significant
binding interaction.
Figure 8 shows the free-energy profile for the H/F exchange
between Cp2La-H and C6F6 starting with structure 10, which
is without direct participation of the arene π orbitals, and Figure
9 shows the structures of all extrema. In 10, F(1) is only 1.84
Å from the hydride. A transition state, 11, in which the C-F(1)
bond is cleaved, is located 29.8 kcal mol-1 above separated
reactants. From 11, a rotation of the F‚‚‚H atom pair in the plane
9
286 J. AM. CHEM. SOC. VOL. 127, NO. 1, 2005

Hydrogen for Fluorine Exchange in C6F6 and C6F5H
A R T I C L E S
Figure 11. Free-energy profile (kcal mol-1 at 298.15 K) for the reaction
Cp2LaH + C6F5H f Cp2LaF + C6F4 + H2.
Figure 10. NBO charges on selected atoms in (a) the reactant 10 and in
(b) the transition-state 11.
LaH with C6F5H where the F/H exchange occurs at the position
para to the existing C-H bond. This forms 1,2,4,5-tetrafluo-
robenzene and Cp2La-F. However, we did not look at the
energy profile for F/H exchange at the ortho or meta position
with respect to the existing C-H bond, since there is no
experimental evidence for these two isomers.
the negatively charged F(1) is relatively close to the hydride
that also carries a negative charge and C(1) carries a positive
charge. At this point, the change in NBO charges from the
reactant 10 to the transition-state 11 shows the magnitude of
the electron reshuffling (Figure 10). The relatively large positive
charge at the metal remains almost constant from reactant
(+2.34) to the transition state (+2.41) as do the charges on
C(2) and F(2). Hence, all of the changes in the electron density
occur between the hydride and the C(1)-F(1) bond that is
cleaved. In the reactant, the hydride is strongly negatively
charged (-0.64) while the charge on carbon is +0.31 and the
fluorine is -0.29. The larger charge on the hydride relative to
that on F(1) and even on F(2) illustrates the strong nucleophilic
character of the hydride ligand. At the transition state, the
hydride charge is only -0.11 while the charge on F(1) increases
to -0.51 and the carbon charge decreases to nearly zero. The
0.5 electron density lost by the hydride is distributed to the
carbon (0.3 e- gained) and to the fluorine (0.2 e- gained). This
is consistent with a net transfer of density from the nucleophilic
hydride to the σ*C-F weakening the C-F bond. This electron
flow is necessary to form the final product as the hydride
becomes protonic to bind to the fluoride. In this reorganization,
the La‚‚‚F(1) and La‚‚‚C interactions develop. The electron
redistribution is the reason for the high-energy barrier,45 and
any interaction that hinders the charge redistribution will raise
the barrier.
The free-energy profile is shown in Figure 11 and the
geometry of the extrema is shown in Figure 12. The pathway
involving C-H activation starts with the formation of an adduct
14, in which an ortho F is coordinated to La and where the
C-H bond points toward the hydride. This leads to the
formation of a σ complex between Cp2La-H and C6F5H, which
has a geometry identical to 10. From 14, a transfer of hydrogen
from carbon-forming Cp2La(C6F5)(H2) occurs in a moderately
exergonic step (∆G ) -15.5 kcal mol-1 below separated
reactants), with a small activation barrier (∆G# ) 8.2 kcal mol-1
above 14) as it crosses the transition-state 15. The transition
state is clearly a proton transfer from C6F5-H to the hydride
as shown by the alignment of the carbon, the transferring H,
and the hydride. At the transition state, the carbon is not yet
bonded to La (La‚‚‚C ) 3.04 Å). The transition-state 15 leads
to a dihydrogen complex, Cp2La(C6F5)(H2), 16, where H2 hardly
perturbes the geometry of free Cp2La(C6F5). Dihydrogen loss
from 16 occurs without change in free energy. From Cp2La-
(C6F5), 5, C-F activation occurs with a significant activation
barrier (23.8 kcal mol-1 above separated reactants and 38.6 kcal
mol-1 above Cp2La(C6F5)). The transition-state 17 produces
C6F4 and Cp2LaF. The overall transformation is exergonic
compared to isolated Cp2LaH and C6F5H even if trapping of
C6F4 by C6H6 (C6D6 in the experiments) is not included in the
modeling. However, the last elementary step (5 f 17 f Cp2-
LaF + C6F4) is endergonic in the absence of trapping. When
the trapping reaction is included, the free energy of the final
species is -53.0 kcal mol-1 compared to the path initiated by
the C-F cleavage. The transition-state 17 for formation of C6F4
and Cp2La-F shows an unusual orientation of the tetrafluo-
robenzyne ligand. Unlike all other ligands in this study, it does
not lie in the plane bisecting the Cp-La-Cp angle but is
significantly twisted out of it. This could be associated with a
need for the very strongly electron deficient fluorinated benzyne
to interact with the electron density of the Cp ring.
The large thermodynamic driving force in the reaction is
already apparent in the formation of 12 and is mostly associated
with the formation of HF. However, formation of La-C6F5 bond
also plays a role. The calculated ∆G of +22 kcal mol-1 for the
isodesmic reaction 6 illustrates this point.
Cp2La-C6F5 + H-C6H5 f Cp2La-C6H5 + H-C6F5 (6)
The H-C bond energy increases with F substitution on the arene
but the energy of the M-C bond increases even more especially
when there is a strong ionic character in the M-C bond,46 which
is the case for lanthanide species. There is thus an energy
preference for the reactant side in eq 6.
Reaction of Cp2LaH with C6F5H. In C6F6, all pathways
necessarily involve a C-F activation step, which is not the case
for C6F5H, where there is a choice between C-H and C-F
activation. A path essentially identical to that calculated for C6F6
with an almost identical energy pattern (difference of about 1
kcal mol-1 in the free-energy profiles and essentially identical
geometries for the extrema) is found for the reaction of Cp2-
Interpretation of Experiments from Calculations. The
experiments described show that the net H/F exchange reaction
that results in formation of a Ce-F bond from a Ce-H bond is
exothermic and rapid. The first resonances detected on mixing
Cp′2CeH and C6F6 in the 1H NMR spectra are due to Cp′2CeF,
Cp′CeC6F5, and H2. The dihydrogen is derived from the reaction
of the metallocene hydride and C6F5H confirmed by independent
experiment, which implies that the net rate constants for
(45) Shaik, S.; Shurki, A. Angew. Chem., Int. Ed. 1999, 38, 586.
(46) Clot, E.; Besora, M.; Maseras, F.; Eisenstein, O.; Oelckers, B.; Perutz, R.
N. Chem. Commun. 2003, 490.
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A R T I C L E S
Maron et al.
Figure 12. Optimized geometries of the extrema for the reaction of Cp2LaH and C6F5H via successive C-H and C-F activation for formation of Cp2LaF,
C6F4, and H2. The structures of reactant Cp2La-H, C6F6, and products Cp2La-F and C6F4 are not shown. The free energies are in kcal mol-1. Large white
circle ) La, middle size white circle ) C, small white circle ) H, black circle ) F. In parentheses, the number of imaginary frequencies, 0 for a minimum,
1 for a transition state.
formation of Cp′2CeF from Cp′2CeH and C6F6 or C6F5H are
comparable since Cp′2CeC6F5 and Cp′2CeF are formed in
comparable amounts even though the individual rate constants
for intermolecular C-H and C-F activation are different.
Calculations show comparable energy barriers for the rate-
determining steps, which in both cases is cleavage of the C-F
bond, although by different mechanisms. The calculated barriers
of the reaction of Cp2LaH and C6F6 or C6F5H indicate that the
C-F activation barrier is about 4 times that for C-H activation.
The calculated barrier for intramolecular C-F cleavage in going
from Cp2LaC6F5 to Cp2LaF and C6F4 is comparable to that of
the intermolecular C-F cleavage in the reaction of Cp2LaH and
C6F6. Once Cp′2CeC6F5 is formed, two pathways are available
for further reaction; in the presence of HF, a barrierless path to
Cp′2CeF and C6F5H, or in absence of HF, a high barrier path to
Cp′2CeF and tetrafluorobenzyne. The latter is then trapped by
C6H6 or by Cp′ in a [2+4] cycloaddition that is Woodward-
Hoffmann allowed. The slow rate for elimination of tetrafluo-
robenzyne is consistent with experiment, since Cp′2CeC6F5 is
an isolable compound and it decomposes to Cp′2CeF and the
Diels-Alder adduct, which is spectroscopically detected.
The calculated pathways provide a qualitative rational for the
observed products of the reaction between Cp′2CeH and C6F6
or C6F5H. The C6F6 can only react via C-F cleavage and the
calculations show that C6F5H and the cerium fluoride complex,
which is observed, are formed in a step that has a significant
but presumably accessible barrier. As soon as C6F5H is formed,
it can easily react via the C-H bond to form Cp′2Ce-C6F5
complex and H2, both of which are observed. The reverse
reaction is not thermodynamically favored in agreement with
experiment. The decomposition of the aryl complex to C6F4 and
the cerium fluoride requires a high barrier, in agreement with
the isolation of Cp′2Ce(C6F5) and observation of its slow
elimination of C6F4 that is trapped, which lowers the net free
energy of the reaction. Figures 8 and 11 show that HF and H2
are produced during the reaction but experimentally only H2 is
observed. This is no contradiction. The bonding between H2
and the metal center is extremely weak (the calculations give
no variation in ∆G upon loss of H2), so H2 is released as soon
as it forms. The reaction of H2 with the metallocene aryls is
not expected to produce the metallocene hydride and the arene
since this reaction is thermodynamically unfavorable. For
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A R T I C L E S
10 min. The samples were then dried over magnesium sulfate, filtered,
and diluted 10 fold with hexane. A 1-µL sample was injected into an
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′D and the fluorobenzyne to benzene
cycloaddition products. The abbreviation Cp′ is used for the 1,3,4-tri-
tert-butylcyclopentadienyl ligand.
example, the transformation of Cp2LaH and C6F5H into Cp2-
LaC6F5 and H2 is exergonic by 14.8 kcal mol-1. In 1,2,4,5-
tetrafluorobenzene, the reaction is even more exergonic (∆G
) -21.9 kcal mol-1). Liberation of H2 is thus expected and
observed. In contrast, HF is more strongly bound and release
of HF is energetically unfavorable. For example, the loss of
HF from Cp2La(C6F5)(HF) is endergonic by 18 kcal mol-1
.
Cp′2CeOTf‚0.5 Hexane. Cerium triflate47 (15.0 g, 25.5 mmol) and
Cp′2Mg48 (12.0 g, 24.5 mmol) were stirred at reflux in a mixture of
pyridine (10 mL) and toluene for 24 h. The dark brown suspension
was taken to dryness under reduced pressure. The residue was extracted
with hexane (200 mL). The volume of the solution was reduced until
precipitation occurred, warmed to dissolve the precipitate, and then
cooled to -15 °C. The large brown crystals were recrystallized five
times from hexane, until the crystals obtained were bright yellow in
color. Yield, 11 g (14 mmol, 55%). MP 300-302 °C. 1H NMR
(C7D8): δ 4.16 (18H, ν1/2 ) 75 Hz), 1.25 (m, hexane), 0.88 (t, hexane,
the integrated intensity of these two absorptions indicated about 0.5
hexane per metallocene), -5.36 (18H, ν1/2 ) 60 Hz), -13.66 (18H,
ν1/2 ) 60 Hz). IR: 1340(s), 1240(m), 1220(m), 1210(m), 1190(m),
1180(m), 1170(w), 1020(s), 960(w), 830(m), 780(w), 690(w), 680(w),
640(s), 600(w), 550(w), 520(w), 450(w), 370(w) cm-1. MS (M)+ m/z
(calcd, found) 755 (100, 100) 756 (41, 40) 757 (26, 25) 758 (8,8).
Anal. Calcd for C35H58CeF3O3S + 0.5 hexane: C, 57.1; H, 8.20. Found
C, 57.2; H, 8.32.
Further, the addition of HF across the metal-carbon occurs with
a low activation barrier so the HF formed is rapidly consumed
to make Cp2LaF and an arene with one less fluorine. Finally,
the reaction of Cp2LaH and HF leads first to Cp2LaH(FH) 1.5
kcal mol-1 below separated reactants, which transforms into
Cp2LaF and H2 by going over a transition state that is 10.3 kcal
mol-1 above the separated reactants. The final compounds Cp2-
LaF and H2 are 62.8 kcal mol-1 below Cp2LaH and HF.
Therefore, any HF that is formed is consumed in production of
the metal fluoride.
Epilogue
The calculations provide a reasonable framework to guide
our understanding of the hills and valleys that the strongly
exergonic H/F exchange reaction must traverse, without oxida-
tive addition, reductive elimination sequences. The key points
that emerge are (i) the activation barrier for C-H activation is
lower than for C-F activation but the final product distribution
is not determined by the relative barriers of these two processes.
The calculated pathways show that the energy barriers for C-F
cleavage from Cp2LaC6F5 and for the reaction of Cp2LaH and
C6F6 are comparable. (ii) The F/H exchange occurs by initial
coordination of the fluoroarene by the fluorine lone pairs in a
σ type complex. The π orbitals of the arene are not directly
involved in the F/H exchange. (iii) The formal bond metathesis
that occurs between M-H and C-F to form M-C and F-H
requires a strong electronic rearrangement at the transition state.
During this process, the hydrogen atom in Ce-H that initially
is hydridic becomes protonic, while the carbon that loses the F
atom is initially positively charged and then neutral and then
carbanionic. The electron flow that is associated with this
electronic reorganization is different from that occurring in a
typical σ bond metathesis and is the principal reason for the
high barrier. (iv) Throughout the exchange reaction, the lone
pair on the ortho-F is hooked to the electropositive metal center
maintaining the translational entropy nearly constant. (v) The
net result is that the thermodynamic favorable state is reached
in a formal H/F exchange.
Cp′2CeCH2C6H5. The triflate, Cp′2CeOTf‚0.5 hexane (4.0 g, 5.0
mmol), was dissolved in 30 mL of diethyl ether and C6H5CH2MgCl
(6.7 mL, 0.75 M in diethyl ether, 5.0 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 and the red solid was extracted into 20 mL of pentane.
The volume of the solution was reduced to 10 mL and cooled to -15
°C, giving red blocks. Yield, 2.1 g (3.0 mmol, 60%). MP 111-113
1
°C. H NMR (C6D6): δ 13.25 (2H, ν1/2 ) 245 Hz), 4.29 (2H, ν1/2
)
20 Hz), 2.47 (1H, ν1/2 ) 14 Hz), -0.53 (18H, ν1/2 ) 190 Hz), -1.80
(18H, ν1/2 ) 195 Hz), -13.19 (18H, ν1/2 ) 45 Hz), -32.62 (2H, ν1/2
) 280 Hz). IR: 1590(s), 1490(s), 1370(s), 1360(s), 1240(s), 1220(m),
1210(w), 1170(w), 1160(w), 1030(w), 1000(m), 960(w), 930(m), 890-
(w), 860(w), 820(m), 810(s), 790(m), 730(s), 720(w), 700(m), 690-
(m), 680(m), 510(w), 440(w), 360(w) cm-1. Anal. Calcd for C41H65-
Ce: C, 70.5; H, 9.39. Found C, 70.3; H, 9.32. MS: no (M)+ was
observed but (M-PhCH3)+ was found m/z (calcd, found) 605 (100, 100)
606 (39, 43) 607 (17, 21) 608 (6, 6).
Cp′2CeH. The benzyl, Cp′2CeCH2C6H5 (1.0 g, 1.4 mmol), was
dissolved in 10 mL of pentane. The headspace in the Schlenk tube
was evacuated and replaced with H2 (1 atm). The red solution turned
purple over 30 min. After 2 h, the volume of the solution was reduced
until precipitation occurred and then warmed to dissolve the precipitate.
Cooling to -15 °C yielded purple crystals. Yield, 0.76 g (1.2 mmol,
1
85%). MP 152-155 °C. H NMR (C6D6): δ 31.86 (4H, ν1/2 ) 220
Experimental Details
Hz), -3.44 (36H, ν1/2 ) 45 Hz), -12.45 (18H, ν1/2 ) 45 Hz). Neither
the resonances nor the stretching frequencies of Ce-H were conclu-
sively identified. IR: 2160(m), 1360(s), 1250(s), 1200(s), 1170(m),
1020(s), 1000(s), 960(m), 930(w), 920(w), 870(w), 840(w), 810(s), 800-
(m), 790(w), 780(w), 680(s), 670(s), 600(m), 520(s), 480(w), 440(m),
360(s) cm-1. MS: no (M)+ was observed but (M-2)+ was found m/z
(calcd, found) 605 (100, 100) 606 (39, 43) 607 (17, 21) 608 (6, 6).
Anal. Calcd for C34H59Ce: C, 67.2; H, 9.78. Found C, 67.5; H, 10.11.
Cp′2CeF. The benzyl, Cp′2CeCH2C6H5 (1.0 g, 1.4 mmol), was
dissolved in 10 mL of pentane and BF3‚OEt2 (0.09 mL, 0.032 g, 0.23
mmol) was added via syringe. The red solution immediately turned
orange. The solution volume was reduced until precipitation occurred
and then warmed to dissolve the precipitate. Cooling to -15 °C yielded
General. All manipulations were performed under an inert atmo-
sphere using standard Schenk and drybox techniques. All solvents were
dried and distilled from sodium or sodium benzophenone ketyl. Fluoro-
and hydrofluorobenzenes were dried and vacuum transferred from
calcium hydride. Infrared spectra were recorded on a Perkin-Elmer 283
spectrometer as Nujol mulls between CsI plates. NMR spectra were
recorded on Bruker AMX-300 or AMX-400 spectrometers at 19 °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. Melting points were measured on a Thomas-Hoover
melting point apparatus in sealed capillaries. Electron impact mass
spectrometry and elemental analyses were performed by the microana-
lytical facility at the University of California, Berkeley. Samples for
GC-MS were prepared from NMR reaction samples by adding a drop
of D2O or H2O, agitating, and allowing the sample to stand closed for
(47) Sofield, C. D.; Andersen, R. A. J. Organomet. Chem. 1995, 501, 271.
(48) Weber, F.; Sitzmann, H.; Schultz, M.; Sofield, C. D.; Andersen, R. A.
Organometallics 2002, 21, 3139.
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A R T I C L E S
Maron et al.
an orange powder. Yield, 0.44 g (0.70 mmol, 50%). MP 164-167 °C.
1H NMR (C6D6): δ 20.00 (4H, ν1/2 ) 70 Hz), -2.50 (36H, ν1/2 ) 10
Hz), -6.81 (18H, ν1/2 ) 10 Hz). IR: 2380(w), 2280(w), 2180(w), 1630-
(w), 1590(w), 1350(s), 1270(w), 1260(m), 1240(s), 1200(s), 1170(s),
1110(m), 1090(m), 1020(m), 1000(s), 960(s), 930(w), 920(w), 870-
(w), 810(s), 780(m), 700(m), 690(m), 680(s), 640(w) cm-1. MS (M)+
m/z ) (calcd, found) 625 (100, 100), 626 (39, 40) 627 (20, 20) 628 (6,
5). Anal. Calcd for C34H58CeF: C, 65.2; H, 9.34. Found C, 65.3; H,
9.46.
Cp′((Me3C)2C5H2C(Me2)CH2)Ce. Cp′2Ce(CH2C6H5) (0.7 g, 1.0
mmol) was heated in pentane (10 mL) for 12 h. The red solution turned
deep purple. The solvent was removed yielding a glassy solid. 1H NMR
(C6D12) δ 35.83 (1H, ν1/2 ) 200 Hz), 33.71 (1H, ν1/2 ) 158 Hz), 16.24
(3H, ν1/2 ) 30 Hz), 5.97 (3H, ν1/2 ) 50 Hz), -3.40 (9H, ν1/2 ) 50
Hz), -5.63 (9H, ν1/2 ) 45 Hz), -7.56 (9H, ν1/2 ) 50 Hz), -10.10
Ce. The tube was cooled in a liquid nitrogen 2-propanol bath, and the
headspace was evacuated and replaced with CF3H (1 atm). The tube
was warmed to 19 °C and agitated. The H NMR showed resonances
whose integrated intensities indicated quantitative formation of Cp′2-
CeH.
1
Cp′2Ce(C6F5). Cp′((Me3C)2C5H2C(Me2)CH2)Ce (0.6 g. 1 mmol) was
dissolved in pentane (10 mL) and C6F5H (0.18 mL, 1 mmol) was added
via syringe. The purple solution immediately turned orange. The
solution volume was reduced to 5 mL and the solution was cooled to
-10 °C, yielding orange crystals. The low yield was due to the high
solubility of the compound. Yield: 0.15 g (0.19 mmol), 19%. 1H NMR
(C6D12) δ -1.77 (36H, ν1/2 ) 190 Hz), -10.29 (18H, ν1/2 ) 55 Hz),
19F NMR (C6D12) δ -157.64 (1F, t, J ) 18 Hz), -160.97 (2F, d, J )
18 Hz), -210.4 (2F, ν1/2 ) 482 Hz). The solid material decomposed
rapidly above 135 °C, which precludes analysis by EI-MS.
Cp′2Ce(p-C6F4H). Cp′2Ce(CH2C6H5) was dissolved in C6D12 and
the solution was heated at 60 °C for 12 h yielding Cp′((Me3C)2C5H2C-
(Me2)CH2)Ce. Three drops of 1,2,4,5-tetrafluorobenzene (an excess)
were added to a clean NMR tube and the solution of Cp′((Me3C)2C5H2C-
(Me2)CH2)Ce was slowly added with agitation. The solution turned
from purple to orange. The 1H and 19F NMR spectra showed that Cp′2-
Ce(p-C6F4H) formed quantitatively. 1H NMR (C6D12) δ 3.20 (1H, ν1/2
) 18 Hz), -1.84 (36H, ν1/2 ) 130 Hz), -9.80 (18H, ν1/2 ) 50 Hz),
19F NMR(C6D12) -140.57 (2F, ν1/2 ) 30 Hz), -241.5 (2F, ν1/2 ) 410
Hz).
(9H, ν1/2 ) 10 Hz), -16.13 (9H, ν1/2 ) 45 Hz), -29.25 (1H, ν1/2
)
110 Hz).
Cp′2CeC6H5. Cp′2Ce(CH2C6H5) (1 g, 1.4 mmol) was heated at reflux
in C6H6 (20 mL) for 3 days. The solution turned a deeper red and a
small amount of yellow precipitate formed. The solvent was removed
under reduced pressure and pentane (15 mL) was added. The solution
was filtered, the volume was reduced to 10 mL, and the solution was
cooled to -15 °C, giving a deep red powder. Yield, 123 mg (0.2 mmol,
14%). 1H NMR (C6D6): δ 7.88 (1H, t), 6.12 (2H, d), -1.73 (36H, ν1/2
) 8 Hz), -10.48 (18H, ν1/2 ) 11 Hz); the ortho proton resonance was
not observed. Anal. Calcd for C40H63Ce: C, 70.2; H, 9.28. Found C,
70.1; H, 9.31.
{[C(CD3)3]3C5H2}{[C(CD3)3]2C5H2[C(CD3)2CD2]}Ce. The benzyl,
Cp′2CeCH2C6H5, was dissolved in C6D6 in an NMR tube. The sample
was heated at 60 °C. After 6 days, the solution was taken to dryness.
Fresh C6D6 was added, and the sample was heated at 60 °C for another
6 days. The solution was taken to dryness, and the deep red solid residue
was dissolved in C6D12. The sample was heated for 1 day at 60 °C to
generate the metallacycle. To determine the degree of deuteration of
the Cp′-rings, a drop of degassed D2O was added. GC MS analysis
showed a mixture of Cp′D-d28, Cp′D-d27, and Cp′D-d26 in a 40:8:1 ratio,
(M)+ m/z (calcd, found) 260 (1, 1) 261 (18, 18) 262 (100, 100) 263
(19, 17) 264 (2, 2).
Cp′2Ce(1,4-C6F4)CeCp′2. Two NMR tubes containing equal amounts
of concentrated solutions of Cp′2Ce(CH2C6H5) in C6D12 were heated
at 60 °C for 12 h yielding solutions of Cp′((Me3C)2C5H2C(Me2)CH2)-
Ce. Three drops of 1,2,4,5-tetrafluorobenzene (an excess) were added
to a clean NMR tube and one of the solutions of Cp′((Me3C)2C5H2C-
(Me2)CH2)Ce was slowly added with agitation. The orange solution
was taken to dryness to remove excess 1,2,4,5-tetrafluorobenzene, and
the second solution of Cp′((Me3C)2C5H2C(Me2)CH2)Ce was added. 1H
NMR(C6D12) -1.98 (36H, ν1/2 ) 200 Hz), -9.15 (18H, ν1/2 ) 65 Hz),
19F NMR (C6D12) -217.06 (2F, broad s). After 10 min, a yellow
precipitate began to form. The solution was stored at room temperature
for 1 day and then filtered. The insoluble yellow precipitate was
suspended in C6D6 and heated at 60 °C for 1 day, yielding a clear
1
yellow/orange solution. The H NMR revealed resonances consistent
NMR Tube Reaction of {[C(CD3)3]3C5H2}2CeC6F5 in C6D12. A
drop of C6F5H was added to the solution of the perdeuterated
with Cp′2CeF.
2
metallacyle. The deep purple solution rapidly turned orange. The H
Decomposition of Cp′2Ce(1,4-C6F4)CeCp′2 with H2O. Cp′2Ce(1,4-
C6F4)CeCp′2 was suspended in C6D12 in an NMR tube, and a drop of
degassed H2O was added. The tube was agitated vigorously and then
allowed to stand for 10 min. The solution was dried over MgSO4 and
filtered. The 1H and 19F NMR showed the presence of 1,2,4,5-
tetrafluorobenzene.
and 19F NMR spectrum showed resonances consistent with Cp′2CeC6F5,
and the 1H spectrum showed only traces of undeuterated tert-butyl
resonances. The solution was taken to dryness to remove excess C6F5H,
and the orange residue was dissolved in C6D12. The sample was heated
at 60 °C for 1 day and then hydrolyzed with a drop of degassed D2O.
The 19F NMR spectrum indicated the formation of a new fluorine-
containing species whose spectrum was not perturbed by the addition
of D2O. Analysis by GCMS showed one major component in addition
to Cp′D-d28, with (M)+ m/z 410. This is believed to be the symmetric
isomer of the [2+4] cycloaddition of tetrafluorobenzyne and Cp′D-
d28, 2. Characterization of cycloaddition product: 19F NMR (C6D12) δ
-145.46 (2F, m), -158.78 (2F, m). GC MS analysis suggests a mixture
of 2-d28 and 2-d27 in a 3:1 ratio, (M)+ m/z (calcd, found) 409 (34, 32)
410 (100, 100) 411 (25, 33).
Reaction of Cp′2Mg and BrMgC6F5. Cp′2Mg (0.35 g, 0.71 mmol)
was dissolved in cyclohexane (5 mL) and BrMgC6F5 (1.6 mL, 0.22 M
in diethyl ether, 0.35 mmol) was added.49 The cloudy solution was
stirred at reflux for 12 h, by which time the solution had turned bright
pink. A drop of degassed water was added to an aliquot of the solution
(1 mL). The complicated 19F NMR spectrum contained six major
signals, which appeared to correspond to two fluorine-containing
compounds in a 1:1.5 ratio. GC MS analysis showed two primary
components in addition to Cp′H, one with (M)+ m/z 382 and one with
(M-57)+ m/z 325, in a 1:1.5 ratio. These are believed to be the two
isomers of the [2+4] cycloaddition product of tetrafluorobenzyne and
Cp′H, one symmetric and the other asymmetric, with the asymmetric
isomer readily eliminating a t-Bu group. Characterization of symmetric
isomer: 1H NMR 4.19 (2H, s), 1.10 (36H, s), 0.45 (18H, s), 19F NMR
(C6D6) δ -145.32 (2F, D), -158.65 (2F, m). GC MS (M)+ m/z (calcd,
found) 382 (100, 100) 383 (26, 26) 384 (3, 3). Asymmetric isomer:
1H NMR (C6D6) 3.99 (1H, s), 2.04 (1H, s), 0.92 (18H, s), 0.45 (18H,
s), 19F NMR (C6D6) δ -130.98 (1F, m), -147.20 (1F, m), -157.33
NMR Tube Reaction of Cp′((Me3C)2C5H2C(Me2)CH2)Ce and
C6H6. Cp′2Ce(CH2C6H5) was dissolved in C6D12 and the solution was
heated at 60 °C for 12 h yielding Cp′((Me3C)2C5H2C(Me2)CH2)Ce. The
solution was taken to dryness and the solid residue was dissolved in
C6H6. The sample was heated at 60 °C for 3 days and then taken to
dryness and the solid residue dissolved in C6D6. The 1H NMR showed
resonances characteristic of Cp′2CeC6H5 and the integrated intensities
indicated quantitative conversion.
NMR Tube Reaction of Cp′((Me3C)2C5H2C(Me2)CH2)Ce and H2
in C6D12. Cp′2Ce(CH2C6H5) was dissolved in C6D12 and the solution
was heated at 60 °C for 12 h yielding Cp′((Me3C)2C5H2C(Me2)CH2)-
(49) Nield, E.; Stephens, R.; Tatlow, J. C. J. Chem. Soc. 1959, 166.
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A R T I C L E S
(1F, m), -158.90 (1F, m). GC MS (M-57)+ m/z (calcd, found) 325
(100, 100) 326 (21,23) 327 (2, 2).
1,4-methanonaphthalene-d6: 1H NMR(C6D6) δ 6.06 (1H, m) 19F NMR
δ -126.74 (1F, m) -139.48 (1F, m) -152.68 (1F, m). GC-MS (M)+
m/z (calcd, found) 214 (100, 100) 215 (14,14).
NMR Tube Reaction of Cp′2CeC6F5 in C6D12. Cp′2CeC6F5 was
dissolved in C6D12 and the solution was heated at 60 °C for 12 h. The
1H and 19F NMR spectra indicated the formation of Cp′2CeF and a
new fluorine-containing species whose spectrum was not perturbed by
the addition of a drop of D2O. GC MS analysis showed four primary
components in addition to Cp′H, one with (M)+ m/z 382 and three
with (M-57)+ m/z 325, in a 13:5:1:2 ratio. These are believed to be
isomers of the [2+4] cycloaddition product of tetrafluorobenzyne and
Cp′H, one symmetric and the others asymmetric, with the asymmetric
isomers readily eliminating a t-Bu group. Characterization of the
symmetric isomer: 1H NMR (C6D12) 4.23 (2H, s), 1.22 (36H, s), 0.61
(18H, s),19F NMR (C6D12) δ -145.37 (2F, m), -158.72 (2F, m). GC
MS (M)+ m/z (calcd, found) 382 (100, 100) 383 (26, 28) 384 (3, 3).
NMR Tube Reaction of Cp′2CeC6F5 in C6H6. Cp′2CeC6F5 was
dissolved in C6H6 and the solution was heated at 60 °C for 12 h. The
1H and 19F NMR spectra indicated the formation of Cp′2CeF, the
symmetric [4+2] cycloaddition product of Cp′H and tetrafluorobenzyne,
and 5,6,7,8-tetrafluoro-1,4-dihydro-1,4-methanonaphthalene-d0, the cy-
cloaddition product of tetrafluorobenzyne to C6H6(Fruchier697). The
latter two products were present in a 1:20 ratio. Characterization of
5,6,7,8-tetrafluoro-1,4-dihydro-1,4-methanonaphthalene-d0: 1H NMR
(C6D6) δ 6.45 (4H, s) 4.77 (2H, s) 19F NMR δ -149.33 (2F, m),
-161.91 (2F, m). GC MS (M)+ m/z (calcd, found) 226 (100, 100) 227
(13, 12).
NMR Tube Reaction of Cp′2CeC6F5 in C6D6. Cp′2CeC6F5 was
dissolved in C6D6 and the solution was heated at 60 °C for 12 h. The
1H and 19F NMR spectra indicated the formation of Cp′2CeF, the
symmetric [4+2] cycloaddition product of Cp′H and tetrafluorobenzyne,
and 5,6,7,8-tetrafluoro-1,4-dihydro-1,4-methanonaphthalene-d6, the cy-
cloaddition product of tetrafluorobenzyne to C6D6.25 The latter two
products were present in a 1:10 ratio. Characterization of 5,6,7,8-
tetrafluoro-1,4-dihydro-1,4-methanonaphthalene-d6: 19F NMR (C6D6)
δ -149.32 (2F, m), -161.96 (2F, m). GC MS (M)+ m/z (calcd, found)
232 (100, 100) 233 (14, 13).
NMR Tube Reaction of Cp′2CeH and C6F6 in C6D6. Cp′2CeH was
dissolved in C6D6 and a drop of C6F6 was added. The solution
immediately turned from purple to orange and gas bubbles were
1
evolved. The H and 19F NMR spectra showed resonances indicative
of Cp′2CeF and Cp′2Ce(C6F5), C6F5H, and H2. The cerium-containing
species were present in a 3:2 ratio and accounted quantitatively for all
of the Cp′2CeH starting material. The sample was stored at room
temperature for 7 days. The 1H and 19F NMR spectra showed resonances
indicative of Cp′2CeF and 5,6,7,8-tetrafluoro-1,4-dihydro-1,4-metha-
nonaphthalene-d6, the [2+4] cycloaddition product of tetrafluorobenzyne
and C6D6.
NMR Tube Reaction of Cp′2CeH and C6F5H in C6D6. Cp′2CeH
was dissolved in C6D6 and a drop of C6F5H was added. The solution
immediately turned from purple to orange and gas bubbles were
1
evolved. The H and 19F NMR spectra showed resonances indicative
of Cp′2CeF, Cp′2Ce(C6F5), Cp′2Ce(p-C6F4H), Cp′2Ce(p-C6F4)CeCp′2,
and 1,2,4,5-tetrafluorobenzene. The cerium-containing species were
present in a 3.5:2.5:1:4 ratio and accounted quantitatively for all of
the Cp′2CeH starting material. The sample was stored at room
temperature for 7 days. The 1H and 19F NMR spectra showed resonances
indicative of Cp′2CeF, 5,6,7,8-tetrafluoro-1,4-dihydro-1,4-methanon-
aphthalene-d6, and 5,7,8-trifluoro-1,4,6-trihydro-1,4-methanonaphtha-
lene-d0, the [2+4] cycloaddition products of tetrafluorobenzyne and
2,3,5-trifluorobenzyne to C6D6, respectively.
Computational Details
The Stuttgart-Dresden-Bonn relativistic large effective core po-
tential (RECP)50 has been used to represent the inner shells of La. The
associated basis set augmented by an f polarization function (R ) 1.000)
has been used to represent the valence orbitals. F has also been
represented by an RECP51 with the associated basis set augmented by
two contracted d polarization Gaussian functions (R1 ) 3.3505-
(0.357851), R2 ) 0.9924(0.795561)).52 C and H have been represented
by an all-electron 6-31G(d, p) basis set.53 The two-layer ONIOM
calculations were carried out at the B3PW91:UFF level.54 Calculations
have been carried out at the DFT(B3PW91) level55 with Gaussian 98.56
The nature of the extrema (minimum or transition state) has been
established with analytical frequency calculations and the intrinsic
reaction coordinate (IRC) has been 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. Following the tradition, we report geometrical parameters
NMR Tube Reaction of Cp′2Ce(p-C6F4H) in C6H6. Cp′2Ce-
(CH2C6H5) was dissolved in C6D12 and the solution was heated at 60
°C for 12 h yielding Cp′((Me3C)2C5H2C(Me2)CH2)Ce. Three drops of
1,2,4,5-tetrafluorobenzene were added to a clean NMR tube and the
solution of Cp′((Me3C)2C5H2C(Me2)CH2)Ce was slowly added with
1
agitation. The solution turned from purple to orange. The H and 19F
NMR spectra were consistent with quantitative formation of Cp′2Ce-
(p-C6F4H). The solution was taken to dryness to remove excess 1,2,4,5-
tetrafluorobenzene and the solid residue was dissolved in C6H6. The
sample was heated to 60 °C for 12 h. The solution was taken to dryness
and dissolved in C6D6. The 1H and 19F NMR spectra indicated the
formation of Cp′2CeF and 5,7,8-trifluoro-1,4,6-trihydro-1,4-methanon-
aphthalene-d0, the cycloaddition product of 2,3,5-trifluorobenzyne to
C6H6.29 Characterization of 5,7,8-trifluoro-1,4,6-trihydro-1,4-metha-
nonaphthalene-d0: 1H NMR δ 6.40 (4H, m) 6.08 (1H, m) 4.89 (2H,
m) 19F NMR δ -126.74 (1F, m) -139.48 (1F, m) -152.68 (1F, m).
GC-MS (M)+ m/z (calcd, found) 208 (100, 100) 209 (13,13).
(50) Dolg, M.; Stoll, H.; Savin, A.; Preuss, H. Theor. Chim. Acta 1989, 75,
173. Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1993, 85, 441.
(51) Igel-Mann, H.; Stoll, H.; Preuss, H. Mol. Phys. 1988, 65, 1321.
(52) Maron, L.; Teichteil,C. Chem. Phys. 1998, 237, 105.
(53) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213.
(54) Svensson, M.; Humbel, S.; Froese, R. D. J.; Matsubara, T.; Sieber, S.;
Morokuma, K. J. Phys. Chem. 1996, 100, 19357. UFF stands for universal
force field.
(55) Perdew, J. J. P.; Wang, Y. Phys. ReV. B 1992, 82, 284. Becke, A. D. J.
Chem. Phys. 1993, 98, 5648. Burke, K.; Perdew, J. P.; Yang, W. In
Electronic Density Functional Theory: Recent Progress and New Direc-
tions; Dobson, J. F., Vignale, G., Das, M. P., Eds.; Plenum: New York,
1998.
NMR Tube Reaction of Cp′2Ce(1,4-C6F4H) in C6D6. Cp′2Ce-
(CH2C6H5) was dissolved in C6D12 and the solution was heated at 60
°C for 12 h yielding Cp′((Me3C)2C5H2C(Me2)CH2)Ce. Three drops of
1,2,4,5-tetrafluorobenzene were added to a clean NMR tube and the
solution of Cp′((Me3C)2C5H2C(Me2)CH2)Ce was slowly added with
(56) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Gomperts, G.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.: Pople, J. A. Gaussian 98, Revision A.9; Gaussian,
Inc.: Pittsburgh, PA, 1998.
1
agitation. The solution turned from purple to orange. The H and 19F
NMR spectra were consistent with quantitative formation of Cp′2Ce-
(1,4-C6F4H). The solution was taken to dryness to remove excess
1,2,4,5-tetrafluorobenzene and the solid residue was dissolved in C6D6.
The sample was heated to 60 °C for 12 h. The 1H and 19F NMR spectra
indicated the formation of Cp′2CeF and 5,7,8-trifluoro-1,4,6-trihydro-
1,4-methanonaphthalene-d6, the cycloaddition product of 2,3,5-trifluo-
robenzyne to C6D6.29 Characterization of 5,7,8-trifluoro-1,4,6-trihydro-
9
J. AM. CHEM. SOC. VOL. 127, NO. 1, 2005 291

A R T I C L E S
Maron et al.
with an accuracy of 10-3 Å and angles with an accuracy of 10-1 degrees
although we often discuss the geometrical parameters with lesser
accuracy because of the many approximations made in the modeling
and in the level of calculation.
Supporting Information Available: Crystallographic data
(also deposited with the Cambridge Crystallographic Data
Centre; copies of the data (CCDC247161-247163) can be
obtained free of 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, UK; fax +44 1223 336033); labeling
diagrams, tables giving atomic positions and anisotropic thermal
parameters, bond distances, and angles, and least-squares planes
for each structure, and list of energies and free energies for all
calculated species. This material is available free of charge via
the Internet at http://pubs.acs.org. Structure factor tables are
available from the authors.
Acknowledgment. This work was partially supported by the
Director Office of Energy Research Office of Basic Energy
Sciences, Chemical Sciences Division of the U.S. Department
of Energy under Contract No DE-AC03-76SF00098. Calcula-
tions were in part carried out on the national computing center
CINES and CALMIP (France). O. E. thanks the Miller Institute
for a Visiting Miller Professorship at U.C. Berkeley. We thank
Dr. Fred Hollander for assistance with the crystallography (at
CHEXRAY the U.C. Berkeley X-ray diffraction facility).
JA0451012
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292 J. AM. CHEM. SOC. VOL. 127, NO. 1, 2005