Generation of perfluoropolyphenylene oligomers via carbon-fluorine bond activation by Cp2Zr(C6F5)2: A dual mechanism involving a radical chain and release of tetrafluorobenzyne

Article abstract of DOI:10.1021/ja991805d

The thermal decomposition of Cp₂Zr(C₆F₅)₂ in THF results in the slow formation of Cp₂Zr(C₆F₅)F and tetrafluorobenzyne. The tetrafluorobenzyne is trapped by THF to give several products. The same reaction performed in the presence of durene or furan also results in the formation of Cp₂Zr(C₆F₅)F and the respective Diels-Alder adducts of tetrafluorobenzyne. If Cp₂Zr(C₆F₅)₂ is heated in the presence of C₆F₆, linear chains of perfluoroarenes are rapidly generated along with Cp₂Zr(C₆F₅)F. The disappearance of Cp₂Zr(C₆F₅)₂ is observed to slow dramatically after 30- 80% completion, with the extent of reaction being inversely dependent on the concentration of C₆F₆. Dual mechanisms involving a rapid radical chain- and a slower tetrafluorobenzyne-producing reaction are proposed to account for these observations.

Full text of DOI:10.1021/ja991805d

J. Am. Chem. Soc. 1999, 121, 10327-10331
10327
Generation of Perfluoropolyphenylene Oligomers via
Carbon-Fluorine Bond Activation by Cp2Zr(C6F5)2: A Dual
Mechanism Involving a Radical Chain and Release of
Tetrafluorobenzyne
Brian L. Edelbach, Bradley M. Kraft, and William D. Jones*
Contribution from the Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627
ReceiVed June 1, 1999
Abstract: The thermal decomposition of Cp2Zr(C6F5)2 in THF results in the slow formation of Cp2Zr(C6F5)F
and tetrafluorobenzyne. The tetrafluorobenzyne is trapped by THF to give several products. The same reaction
performed in the presence of durene or furan also results in the formation of Cp2Zr(C6F5)F and the respective
Diels-Alder adducts of tetrafluorobenzyne. If Cp2Zr(C6F5)2 is heated in the presence of C6F6, linear chains of
perfluoroarenes are rapidly generated along with Cp2Zr(C6F5)F. The disappearance of Cp2Zr(C6F5)2 is observed
to slow dramatically after 30-80% completion, with the extent of reaction being inversely dependent on the
concentration of C6F6. Dual mechanisms involving a rapid radical chain- and a slower tetrafluorobenzyne-
producing reaction are proposed to account for these observations.
Introduction
in which the reaction of Yb(C6F5)2 with I2 produces YbI2 and
C6F5I in addition to a small amount of C12F9I.6b They proposed
the intermediacy of tetrafluorobenzyne, which was believed to
insert into the Yb-C6F5 bond, and undergo subsequent reaction
with I2, ultimately leading to C12F9I.
The same authors also studied the thermal decomposition of
Sm(C6F5)2.6a The reaction was performed by mixing Hg(C6F5)2
with samarium metal at room temperature. Four samarium-
containing products were observed: (C6F5)SmF, SmF2, (C6F5)-
SmF2, and (C12F9)Sm as well as several polyfluorinated organic
products including C12F9H (three isomers), C18F13H, and others.
Intramolecular fluoride abstraction and concomitant formation
of tetrafluorobenzyne was proposed to account for the formation
of (C6F5)SmF, SmF2, and (C6F5)SmF2 (eq 1).
Research in the area of C-F bond activation by transition
metal complexes has increased significantly in the past decade.1
Despite this, the activation of C-F bonds by electron-deficient
d0 metal complexes has received less attention then electron-
rich late metal compounds. The proposed mechanisms of C-F
bond cleavage by early metal complexes is varied. A radical
mechanism has been reported by Weydert and co-workers in
the reaction of (C5H4Me)3U(t-Bu) with hexafluorobenzene.2 An
electron-transfer mechanism was proposed by Burk et al. in the
reaction of tetrakis(trifluoromethyl)cyclopentadienone with bis-
(cyclopentadienyl)-titanacyclobutanes.3 Sigma-bond metathesis
and/or â-fluoride elimination has been proposed to account for
C-F bond cleavage of vinyl fluoride by Cp*2ScH.4 Recently,
Kiplinger and Richmond have demonstrated that group 4
metallocenes are capable of cleaving the C-F bonds of
perfluorocycloalkanes and polyfluoroaromatics using activated
Mg or Al as the terminal reductant.5 They postulated that low
valent “zirconocene” or “titanocene” species were involved as
intermediates in the catalytic cycle, possibly cleaving the C-F
bond of the polyfluoroaromatics via formal oxidative addition.
An interesting example of intramolecular C-F activation by
a lanthanide complex was reported by Deacon and co-workers,6
(1) There are several recent reviews on C-F activation by metal reagents,
see: (a) Kiplinger, J. L.; Richmond, T. G.; Osterberg, C. E. Chem. ReV.
1994, 94, 373. (b) Burdeniuc J.; Jedlicka, B.; Crabtree, R. H. Chem. Ber./
Recl. 1997, 130, 145. (c) Richmond, T. G. In Topics in Organometallic
Chemistry; Murai, S., Ed.; Springer: New York, 1999; Vol. 3, p 243.
(2) Weydert, M.; Andersen, R. A.; Bergman, R. G. J. Am. Chem. Soc.
1993, 115, 8837.
The organic products C12F9H and C18F13H were speculated
to form by tetrafluorobenzyne insertion into polyfluoroarylsa-
marium bonds followed by hydrogen abstraction from THF (eqs
2-3). The other products were proposed to be formed by an
electron-transfer mechanism from samarium(II) species to
polyfluoroaryl groups followed by hydrogen abstraction from
THF.
We have recently reported the C-F activation of C6F6 by
[Cp2ZrH2]2 to give Cp2Zr(C6F5)F, Cp2ZrF2, and C6F5H.7 As part
of our research on C-F activation of polyfluoroaromatics by
(3) Burk, M. J.; Staley, D. L.; Tumas, W. J. J. Chem. Soc., Chem.
Commun. 1990, 809.
(4) Burger, B. J.; Ph.D. Thesis, California Institute of Technology, 1987.
(5) (a) Kiplinger, J. L.; Richmond, T. G. Chem. Commun. 1996, 1115.
(b) Kiplinger, J. L.; Richmond, T. G. J. Am. Chem. Soc. 1996, 118, 1805.
(6) (a) Deacon, G. B.; Koplick, A. J.; Raverty, W. D.; Vince, D. G. J.
Organomet. Chem. 1979, 182, 121. (b) Deacon, G. B.; Mackinnon, P. I.;
Tuong, T. D. Aust. J. Chem. 1983, 36, 43.
(7) Edelbach, B. L.; Rahman, A. K. F.; Lachicotte, R. J.; Jones, W. D.
Organometallics 1999, 18, 3170-3177.
10.1021/ja991805d CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/27/1999

10328 J. Am. Chem. Soc., Vol. 121, No. 44, 1999
Edelbach et al.
Figure 1. Plot of disappearance of Cp2Zr(C6F5)2 (0.051 M) vs time at
various concentrations of C6F6: (b) 0 M, ([) 1.54 M, (9) 0.51 M,
(2) 0.051 M in THF-d8. The reaction was performed at 85 °C.
(C6F5)2 at 125 °C in THF-d8 in the presence of furan or durene
also resulted in the formation of 1 and 2 in yields of 50 and
80%, respectively, as well as Cp2Zr(C6F5)F. The rate of
disappearance of Cp2Zr(C6F5)2 was not affected by the con-
centration of durene and was comparable to the rate without
added durene.
Cp2Zr complexes, we became interested in studying the thermal
decomposition of Cp2Zr(C6F5)2 in the presence of C6F6. In this
paper we demonstrate that the reaction of Cp2Zr(C6F5)2 with
C6F6 leads to a series of novel linear polyfluoro-polyphenylenes
and Cp2Zr(C6F5)F. Our studies indicate that dual mechanisms
involving a radical chain process and tetrafluorobenzyne forma-
tion are operating.
Thermal Reaction of Cp2Zr(C6F5)2 with C6F6. A THF-d8
solution of Cp2Zr(C6F5)2 and C6F6 (1 equiv) was heated at 85
°C in a resealable NMR tube, resulting in the rapid disappear-
ance of Cp2Zr(C6F5)2 and the concomitant formation of Cp2-
Zr(C6F5)F. The rate of this reaction was observed to slow
dramatically within a matter of hours after ∼50% completion,
and a white precipitate deposited on the bottom of the NMR
Results and Discussion
1
tube (Figure 1). H and 19F NMR spectroscopy of the liquid
Thermal Reaction of Cp2Zr(C6F5)2. A THF-d8 solution of
Cp2Zr(C6F5)2 heated at 85 °C in a resealable NMR tube resulted
in the slow formation of Cp2Zr(C6F5)F (eq 4). The rate of
showed the formation of Cp2Zr(C6F5)F and perfluorobiphenyl
as well as higher-molecular-weight linear oligomers of perfluo-
robenzene (eq 5).10 GC/MS of the liquid portion revealed
formation of Cp2Zr(C6F5)F is high during the first 1-2% of
reaction but drops substantially after only a few percent
conversion (Figure 1). Reaction continues at this slow rate over
a period of months. Mass balance would dictate the formation
of C6F4, formally tetrafluorobenzyne. When the reaction is
performed at 125 °C (or at longer times at 85 °C), several new
peaks appear in the 19F NMR spectrum, and a small amount of
an orange precipitate is observed to deposit at the bottom of
the tube. A GC/MS of the liquid portion of the reaction mixture
reveals the presence of compounds whose molecular weight is
consistent with the reaction of tetrafluorobenzyne with the THF
solvent (see Experimental Section). The reductive elimination
product, perfluorobiphenyl, was not observed.
oligomers with up to 5 aryl groups, and LCMS of the white
solid displayed oligomers of up to n ) 13. A similar reaction
performed with perfluorobiphenyl as the initial substrate (vs
C6F6) gave similar results. The complex Cp2Zr(C12F9)F was not
observed.
The rate of disappearance of Cp2Zr(C6F5)2 was monitored
with varying concentrations of added C6F6 at 85 °C. Figure 1
shows that in each experiment the initial disappearance of Cp2-
Zr(C6F5)2 was rapid. This rapid reaction was observed to slow
dramatically within a matter of hours and remained ap-
proximately constant until the Cp2Zr(C6F5)2 was depleted. An
important observation is that the greater the concentration of
C6F6, the less the reaction proceeds before the rate slows. During
reaction in the slow regime, the rate of disappearance of Cp2-
Zr(C6F5)2 is the same regardless of the concentration of C6F6.
Prior reports indicate that tetrafluorobenzyne (formed by
elimination of LiF from LiC6F5 at ∼ -20 °C) reacts with furan
and 2,3,5,6-tetramethylbenzene (durene) to give the Diels-Alder
adducts 18 and 2,9 respectfully. Heating solutions of Cp2Zr-
(8) Coe, P. L.; Stephens, R.; Tatlow, J. C. J. Chem. Soc. 1962, 3227.
(9) Brewer, P. N.; Eckhard, I. F.; Heaney, H.; Marples, B. A. J. Chem.
Soc., C 1968, 664.
(10) The oligomers were determined to be a chain of perfluoroaromatics
linked together in the para position. This was based on comparison to
literature 19F NMR spectroscopy of authentically synthesized oligomers up
to n ) 5. See: Maruo, K.; Wada, Y.; Yanagida, S. Bull. Chem. Soc. Jpn.
1992, 65, 3439. In addition, GC/MS showed only a single peak for each
mass of oligomer.

Generation of Perfluoropolyphenylene Oligomers
J. Am. Chem. Soc., Vol. 121, No. 44, 1999 10329
Figure 3. Structure of radical chain carrying intermediate/transition
state.
observation that the initial disappearance of Cp2Zr(C6F5)2 is
rapid and then slows dramatically within a matter of hours
(Figure 1). Second, radical initiators greatly enhance the rate
of the reaction and the dramatic decrease in rate is not observed
(Figure 2). Third, the rate of the reaction in the presence of an
added radical inhibitor is slow (Figure 2). Fourth, the thermal
decomposition of Cp2Zr(C6F5)2 in the absence of C6F6 generates
Cp2Zr(C6F5)F at the same slow rate as the reactions with C6F6
at later times (Figure 1). Last, the Diels-Alder adducts 1 and
2 formed slowly during the thermal decomposition of Cp2Zr-
(C6F5)2 in the presence of furan or durene, suggesting that
tetrafluorobenzyne is released in this reaction. These observa-
tions are consistent with a radical chain mechanism dominating
at early reaction times and a slower reaction involving release
of tetrafluorobenzyne occurring throughout the course of the
reaction. This latter mechanism is observed exclusively at longer
reaction times when the radical chain has terminated.
Figure 2. Plot of disappearance of Cp2Zr(C6F5)2 (0.051 M) with C6F6
(0.51 M) and radical initiators: (9) no radical initiator, (b) Na +
naphthalene, (2) 1,1′-azobis(cyclohexane-carbonitrile) in THF-d8, (∆)
radical inhibitor isopropylbenzene, C6F6 (0.051 M) in THF-d8. The
reaction was performed at 85 °C.
During the rapid portion of the reaction, the formation of
perfluorobiphenyl and higher-molecular-weight oligomers was
observed in all reactions. However, the relative amount of
higher-molecular-weight oligomers decreased with increasing
C6F6 concentration. During the slow portion of the reaction,
the formation of perfluorobiphenyl and the higher-molecular-
weight oligomers stopped. New products were observed in the
19F NMR spectra consistent with the reaction of tetrafluoroben-
zyne with THF-d8 solvent (vide supra).
Scheme 1 depicts a plausible radical chain mechanism. The
chain is initiated by a trace amount of radical initiator, for
example a minor impurity in the starting material, to give the
Zr(III) radical species, 3 (step 1). Step 2 involves fluorine atom
abstraction from C6F6 by 3 to generate Cp2Zr(C6F5)F and •C6F5
radical. This step appears to be approximately thermoneutral
based on the reported bond strength of 154 kcal/mol for the
C-F bond in C6F612 which is nearly identical to the Zr-F bond
strength estimated by Schock and Marks (∼154 kcal/mol).13 In
the third step, the •C6F5 radical combines with Cp2Zr(C6F5)2
resulting in the formation of 3 and perfluorobiphenyl. Ultimately
the chains are terminated, and the rate of reaction slows
dramatically. As depicted in Figure 1, the rate of chain
termination is inversely dependent on the concentration of C6F6,
consistent with the idea that C6F6 contains some impurity which
quenches the radical chain. There is strong evidence that a
radical chain mechanism is operating in the autoxidations of
dialkylzirconocenes of the type Cp2ZrR2. In these reactions, the
radical chain is believed to involve an SH2 type displacement
of an alkyl radical from the metal center by an alkylperoxyl
radical or di-tert-butylnitroxide.14
Effect of Radical Initiators and Inhibitors on the Reaction
of Cp2Zr(C6F5)2 with C6F6. The effect of radical initiators on
the rate of disappearance of Cp2Zr(C6F5)2 in the presence of
C6F6 was monitored. A mixture of Cp2Zr(C6F5)2 (0.051 M), C6F6
(0.51 M), sodium, and naphthalene heated at 85 °C resulted in
a moderate initial rate increase compared to the reaction without
sodium and naphthalene. In addition, the reaction with added
sodium and naphthalene proceeded to completion in ap-
proximately 1 day, compared with several months in their
absence (Figure 2). (Reduction of C6F6 has been reported to
give perfluorophenyl radicals.11) The radical initiator 1,1′-azobis-
(cyclohexanecarbonitrile) (VAZO) had a similar effect.
A THF-d8 solution of Cp2Zr(C6F5)2 (0.051 M), C6F6 (0.051
M), and the radical trap isopropylbenzene was heated at 85 °C.
In this reaction, the initial rate of disappearance of Cp2Zr(C6F5)2
was less than 1/100th that of the reaction without added
isopropylbenzene (Figure 2), and the reaction slows after only
∼15% completion (compared with 75% completion in the
absence of isopropylbenzene).
Alternative Sources of C6F4. The complex Cp2Zr(C6F5)-
CH3 was heated at 125 °C in THF-d8 in the presence of durene.
The Diels-Alder adduct 2 was formed in ∼38% yield. The
main zirconium complex formed was Cp2Zr(CH3)F. When a
THF-d8 solution of Cp2Zr(C6F5)CH3 and C6F6 was heated at
55 °C, the formation of Cp2Zr(CH3)F was initially rapid and
then slowed dramatically as the reaction proceeded. Perfluoro-
biphenyl and higher oligomers were also observed in the 19F
NMR spectrum and by GC/MS. A small amount of Cp2Zr(CH3)2
and Cp2Zr(C6F5)F were also observed, possibly as a result of
con-proportionation of Cp2Zr(C6F5)CH3 and Cp2Zr(CH3)F.
Heating Cp2Zr(C6F5)F with C6F6 at 125 °C in THF-d8 also
resulted in the formation of perfluorobiphenyl and perfluoro-
terphenyl as well as Cp2ZrF2 and products corresponding to the
reaction of THF-d8 with C6F4. A small amount of C6F5H was
also observed.
Steps 1 and 3 involve a radical attack on the zirconium-bound
perfluorophenyl group to give a phenylcyclohexadienyl radical
as shown in Figure 3.15 Facile homolytic cleavage of the Zr-
aryl bond results in compound 3 and C6F5Q (step 1) or C12F10
(step 3). Perfluorobiphenyl can now serve as a fluoride atom
source (step 2) to generate the •C12F9 radical which goes on to
form C18F14 and ultimately the higher-molecular-weight oligo-
mers. Presumably, the stability of the perfluoroarylradicals
(11) Harvey, J. N.; Schroeder, D.; Koch, W.; Danovich, D.; Shaik, S.;
Schwarz, H. Chem. Phys. Lett. 1997, 273, 164.
(12) (a) Smart, B. E. Mol. Struct. Energ. 1986, 3, 141-191.
(13) Schock, L. E.; Marks, T. J. J. Am. Chem. Soc. 1988, 110, 7701.
The Zr-F bond disruption enthalpy was estimated for Cp*ZrX2 type
complexes.
(14) (a) Brindley, P. B.; Scotton, M. J. J. Chem. Soc., Perkin Trans. 2
1981, 419. (b) J. Organomet. Chem. 1981, 222, 89.
(15) This type of radical addition is discussed by Perkins, J. M. In Free
Radicals; Kochi, J. K. Ed.; John Wiley & Sons: New York, 1973; Vol. 2,
Chapter 16.
Mechanistic Considerations. Several observations lead us
to believe that competing mechanisms are operating in the
reaction of Cp2Zr(C6F5)2 with C6F6. Most important is the

10330 J. Am. Chem. Soc., Vol. 121, No. 44, 1999
Edelbach et al.
Scheme 1
increases with the number of aryl groups in the chain. Therefore,
step 2 would become more facile with higher molecular weight
oligomers. This presumption accounts for the observation of
the higher-molecular-weight oligomers (and the white precipi-
tate) at early times in the reaction of Cp2Zr(C6F5)2 with 1 equiv
of C6F6 and is also consistent with the fact that slightly less
than half of the C6F6 still remained when Cp2Zr(C6F5)2 was
depleted. It is also worth noting that reaction of the perfluo-
rophenyl radical with C6F6 has been reported to give only traces
of perfluorobiphenyl,16 pointing toward the necessity of having
a Zr-C6F5 bond present to give the observed coupled products.
The perfluoropolyphenylene products in this reaction are
potentially interesting materials. Direct synthesis of short
oligomers (n ) 2-4) has been reported,10 and the preparation
of mixtures of meta-linked perfluoropolyphenylenes with high
molecular weights has been reported.17 Interest in the parent
protio-polyphenylenes stems from their high thermal and
thermo-oxidative stabilities18 and from their semiconductor
properties.19
Cp2Ti(C6F5)2 at 150 °C yields small quantities of Cp2Ti(C6F5)F;
however, no mention was made of tetrafluorobenzyne.20 As
mentioned earlier, the thermal decomposition of Yb(C6F5)2 and
Sm(C6F5)2 reportedly generates tetrafluorobenzyne along with
the M-F compound.6 In those reactions, it was reported that
the tetrafluorobenzyne subsequently inserted into the M-aryl
bond, although no evidence for this was seen in the reactions
described here.
Conclusions
The thermal decomposition of Cp2Zr(C6F5)2 in THF generates
Cp2Zr(C6F5)F and tetrafluorobenzyne. Evidence for tetrafluo-
robenzyne formation arises from the formation of THF-C6F4
insertion products, as well as the formation of Diels-Alder
adducts of tetrafluorobenzyne when the reaction is performed
with furan and durene. When Cp2Zr(C6F5)2 is heated in the
presence of C6F6, perfluorobiphenyl and higher-molecular-
weight linear oligomers are rapidly produced. However, this
reaction slows dramatically with no further formation of the
linear polyfluoroarenes. Added radical initiators prevent the
slowing of the reaction, while radical inhibitors slow the rate
of the reaction. These results are consistent with a rapid radical
chain mechanism which leads to the formation of Cp2Zr(C6F5)F
and the linear polyfluoroarenes. The chain is believed to be
initiated by an impurity in the Cp2Zr(C6F5)2 and is inhibited by
an impurity in the C6F6, which accounts for the observed inverse
dependence of rate on the concentration of C6F6.
Concomitant with the rapid radical chain mechanism is a
slower mechanism involving the formation of Cp2Zr(C6F5)F and
the release of tetrafluorobenzyne (eq 6). Tetrafluorobenzyne is
Experimental Section
General Considerations. All manipulations were performed under
an N2 atmosphere, either on a high-vacuum line using modified Schlenk
techniques or in a Vacuum Atmospheres Corporation Glovebox.
Tetrahydrofuran, benzene, and toluene were distilled from dark purple
solutions of benzophenone ketyl. Alkane solvents were made olefin-
free by stirring over H2SO4, washing with aqueous KMnO4 and water,
and distilling from dark purple solutions of tetraglyme/benzophenone
ketyl. Tetrahydrofuran-d8 was purchased from Cambridge Isotope Lab.
and distilled under vacuum from dark purple solutions of benzophenone
ketyl and stored in ampules with Teflon sealed vacuum line adapters.
The preparations of Cp2Zr(C6F5)2,21 Cp2ZrMeF,22 Cp2Zr(C6F5)Me,23 and
trapped by THF-d8 solvent to give several of products. Once
the radical chains are terminated, this mechanism continues to
operate at its much slower pace. The formation of Cp2Zr(C6F5)F
and tetrafluorobenzyne may result from â fluoride elimination
from one of the arylF groups on Cp2Zr(C6F5)2. This type of
unimolecular reaction is consistent with the observation that the
rate of disappearance of Cp2Zr(C6F5)2 (at longer times) does
not depend on the concentration of C6F6. â-F elimination has
been postulated to account for the formation of ethylene and
Cp*2ScF from Cp*2ScH and vinyl fluoride.4 There is literature
precedent for the formation of M-F bonds and tetrafluoroben-
zyne from M(C6F5)2 compounds. For example, the pyrolysis of
24
Cp2ZrF2 have been previously reported. Perfluorobenzene, perfluo-
robiphenyl, and 1,1′-azobis(cyclohexanecarbonitrile) (VAZO) were
(18) Jones, M. B.; Kovacic, P. In ComprehensiVe Polymer Science;
Pergamon Press: New York, 1989; Vol. 5, p 465.
(19) Encyclopedia of Chemical Technology, Supplement to 3rd ed.;
Wiley: New York, 1984; p 480.
(20) Treichel, P. M.; Chaudhari, M. A.; Stone, F. G. A. J. Organomet.
Chem. 1963, 1, 98.
(16) Oldham, P. H.; Williams, G. H.; Wilson, B. A. J. Chem. Soc., C
1971, 1094.
(17) Thrower, J.; White, M. A. Polym. Prepr. (Am. Chem. Soc., DiV.
Polym. Chem.) 1966, 7, 1077.
(21) Chaudhari, M. A.; Stone, F. G. A. J. Chem. Soc., A 1966, 838.

Generation of Perfluoropolyphenylene Oligomers
J. Am. Chem. Soc., Vol. 121, No. 44, 1999 10331
purchased from Aldrich Chemical Co. The liquids were stirred over
sieves, freeze-pump-thaw degassed three times, and vacuum distilled
prior to use.
Reaction of Cp2Zr(C6F5)2 with Various Concentrations of C6F6.
Three NMR tubes were prepared with varying concentrations of C6F6
(0.051, 0.51, and 1.53 M) in THF-d8. Each tube also contained 16 mg
(0.029 mmol, 0.051 M) of Cp2Zr(C6F5)2. The NMR tubes were heated
at 85 °C in a constant-temperature oil bath ((0.5 °C). The NMR tubes
were removed at various intervals and cooled to room temperature,
and NMR spectra were recorded. 1H and 19F NMR spectroscopy
revealed initial rapid conversion of Cp2Zr(C6F5)2 to Cp2Zr(C6F5)F, but
within a matter of hours the reaction slowed significantly (Figure 1).
In all cases perfluorobiphenyl and higher-molecular-weight oligomers
were observed. The 19F NMR spectra were consistent with the formation
of only linear oligomers, as judged by a comparison of the spectra
with those of authentically synthesized compounds in the literature.10
A GC/MS of the reaction solution displayed oligomers up to n ) 5,
with only one peak for each mass of oligomer. The relative amount of
the higher molecular weight oligomers decreased with increasing
concentration of C6F6. The reaction with 0.051 M C6F6 also generated
a white precipitate within a matter of hours. LCMS (APCI, negative
ions) of this solid revealed oligomers with chain lengths from n ) 5 to
n ) 13 (See Supporting Information).
Reaction of Cp2Zr(C6F5)2 with C6F6, Sodium, and Naphthalene.
A resealable NMR tube containing THF-d8 was charged with 16 mg
(0.029 mmol, 0.051 M) of Cp2Zr(C6F5)2, C6F6 (0.51 M), sodium (∼5
mg), and naphthalene (∼3 mg, 0.023 mmol). The solution had a slight
orange tint. The mixture was heated at 85 °C in a constant-temperature
oil bath ((0.5 °C). The NMR tube was removed at various intervals
and cooled to room temperature, and NMR spectra were recorded. 1H
and 19F NMR spectroscopy revealed rapid conversion of Cp2Zr(C6F5)2
to Cp2Zr(C6F5)F (Figure 2). The 19F NMR spectrum and GC/MS also
revealed the presence of perfluorobiphenyl and higher-molecular-weight
oligomers. The solution turned deep orange then red as the reaction
neared completion.
Reaction of Cp2Zr(C6F5)2 with C6F6, and 1,1′-Azobis(cyclohex-
anecarbonitrile)(VAZO). A resealable NMR tube containing THF-d8
was charged with 16 mg (0.029 mmol, 0.051 M) of Cp2Zr(C6F5)2, C6F6
(0.51 M), and the radical initiator 1,1′-azobis(cyclohexanecarbonitrile)
(∼2 mg, 8.2 × 10-3 mmol) and heated at 85 °C in a constant-
temperature oil bath ((0.5 °C). The NMR tube was removed at various
intervals, cooled to room temperature, and a 19F NMR spectrum
recorded. The results are similar to those with Na and naphthalene
(Figure 2).
In a preparative experiment, 80 mg Cp2Zr(C6F5)2, 18 µL C6F6, and
20 mg VAZO were dissolved in 3 mL THF in an ampule. The sample
was heated to 85 °C for 3 d over which time white solids were observed
to precipitate from the faint orange solution. The volatiles were removed
under vacuum, leaving 95 mg of solid residue (about 17 mg of which
should be perfluoropolyphenylene oligomers). The residue was washed
with THF to remove the Zr product, VAZO, and soluble oligomers,
leaving 10 mg of insoluble white oligomers of perfluoropolyphenylene.
Based on an expected yield of 1 mol benzyne/Zr, the total yield of
perfluoropolyphenylene was 80%, with about 47% yield of isolated
insoluble oligomers.
All 1H NMR and 19F NMR spectra were recorded on a Bruker
1
Avance 400 spectrometer. All H chemical shifts are reported in ppm
(δ) relative to tetramethylsilane and referenced using chemical shifts
of residual solvent resonances (THF-d8, δ 1.73). 19F NMR spectra were
referenced to external C6H5CF3 (δ 0.00 with downfield chemical shifts
taken to be positive). GC/MS was conducted on a HP 5890 Series II
gas chromatograph fitted with an HP 5970 series mass selective detector.
LCMS was conducted on a HP 1100 Series LC/MSD.
Preparation of Cp2Zr(C6F5)F. A sample of [Cp2ZrH2]2 (244 mg,
0.546 mmol) was suspended in 8 mL of THF. Perfluorobenzene (13.1
mmol, 1.5 mL) was added at room temperature, and the mixture was
heated to 65 °C. Within a few minutes vigorous gas evolution was
observed, and the suspension of [Cp2ZrH2]2 disappeared to give a
homogeneous colorless solution. The solvent and excess C6F6, C6F5H,
1
and H2 were removed to give a white powder (412 mg). A H NMR
spectrum of the sample revealed a 4:1 mixture of Cp2Zr(C6F5)F and
Cp2ZrF2. Cp2Zr(C6F5)F can be separated from Cp2ZrF2 by dissolving
the mixture in a minimum of THF and layering with hexanes. Cp2Zr-
(C6F5)F forms as a white precipitate. 1H NMR Cp2Zr(C6F5)F (THF-d8)
δ 6.405 (s, 10 H). 19F NMR Cp2Zr(C6F5)F (THF-d8) δ 168.2 (t, JF-F
20.71 Hz, Zr-F), -49.3 (m, 2 Fortho), -93.8 (t, JF-F ) 18.83 Hz,1
)
F
para), -98.8 (bs, 2 Fmeta). Calcd for C16H10F6Zr: C, 47.16; H, 2.47.
Found: C, 46.99; H, 2.37.
Thermolysis of Cp2Zr(C6F5)2 in THF-d8. A resealable NMR tube
containing THF-d8 was charged with 16 mg (0.029 mmol, 0.051 M)
of Cp2Zr(C6F5)2 and heated at 85 °C in a constant-temperature oil bath
((0.5 °C). The NMR tube was removed at various intervals and cooled
to room temperature, and NMR spectra were recorded. The disappear-
ance of Cp2Zr(C6F5)2 was monitored by 19F NMR spectroscopy (Figure
1). Cp2Zr(C6F5)2 slowly converted to Cp2Zr(C6F5)F. The 19F NMR
spectrum and GC/MS of the reaction mixture also revealed the
formation of three new compounds, consistent with the reaction of
tetrafluorobenzyne with THF-d8 or the Cp ring of the metal complexes.
Compound 1 (C6F4 + C5H6): 19F NMR (THF-d8) δ -76.2 (m, 2 F),
-80.8 (m, 2 F). MS (m/z, %) 215 (M + 1, 12); 214, (M, 100), 213,
(M - 1, 95), 195, (M - F, 52), (M - C2H2 - H, 89). Compound 2
(C6F4 + C4D8O): 19F NMR (THF-d8) δ -77.2 (m, 2 F), -79.9 (m, 2
F). MS (m/z, %) 228, (M, 68), 226, (M - 2, 15), 196, (M - C2D4, 17),
178, (M - C3D7, 100), 182 (M - C3D5, 37), 166 (M - C4D7, 57).
Compound 3 (C6F4 + C4D6O): 19F NMR (THF-d8) δ -76 (m, 2 F),
-93.1 (m, 2 F). MS (m/z, %) 224, (M, 82), 206 (M - OD, 100), 192
(M - OD - CD, 72), 173 (M - OD - CD - F, 61).
Thermolysis of Cp2Zr(C6F5)2 in the Presence of Furan. A
resealable NMR tube containing THF-d8 was charged with 14 mg (0.025
mmol) of Cp2Zr(C6F5)2 and furan (1.36 mg, 1.4 µL, 0.02 mmol) and
heated at 125 °C in an oil bath. After 5 days, 1H and 19F NMR
spectroscopy revealed that approximately three-fourths of Cp2Zr(C6F5)2
had been converted to Cp2Zr(C6F5)F. The Diels-Alder adduct 1 was
also detected by 1H, 19F, and GC/MS (∼50% yield based on 19F NMR
1
Reaction of Cp2Zr(C6F5)2 with C6F6 and Isopropylbenzene. A
resealable NMR tube containing THF-d8 was charged with 16 mg (0.029
mmol, 0.051 M) of Cp2Zr(C6F5)2, C6F6 (0.051 M), and the radical
inhibitor isopropylbenzene (0.08 mL, 0.575 mmol, 1.03 M). The tube
was heated at 85 °C in a constant-temperature oil bath ((0.5 °C). The
NMR tube was removed at various intervals and cooled to room
temperature, and a 19F NMR spectrum was recorded. The initial rate
of disappearance of Cp2Zr(C6F5)2 was over 100 times slower than the
reaction without added isopropylbenzene. A small amount of perfluo-
robiphenyl and higher-molecular-weight oligomers was observed by
19F NMR spectroscopy.
spectroscopy). H NMR 1 (THF-d8) δ 7.13 (t, JH-H ) 1.0 Hz, 2 H),
6.08 (t, JH-H ) 1.0 Hz, 2 H). 19F NMR 1 (THF-d8) δ -79.7 (m, 2 F),
-94.7 (m, 2 F). MS (m/z) 216 (M+).
Thermolysis of Cp2Zr(C6F5)2 and Cp2Zr(C6F5)Me in the Presence
of Durene. A resealable NMR tube containing THF-d8 was charged
with 13 mg (0.023 mmol) of Cp2Zr(C6F5)2 and durene (30.8 mg, 0.23
1
mmol) and heated at 125 °C in an oil bath. After 5 days, H and 19F
NMR spectroscopy revealed that approximately two-thirds of Cp2Zr-
(C6F5)2 had been converted to Cp2Zr(C6F5)F. The Diels-Alder adduct
2 was also detected by 1H, 19F, and GC/MS (∼80% yield based on 19
F
NMR spectroscopy). 1H NMR 2 (THF-d8) δ 4.58 (t, JH-F ) 1.2 Hz, 2
H), 1.79 (s, 12 H). 19F NMR 2 (THF-d8) δ - 87.4 (m, 2 F), -99.9 (m,
2 F). MS (m/z) 282 (M+). A similar reaction with Cp2Zr(C6F5)Me and
durene gave Cp2ZrMeF and 2 (∼38% yield based on 19F NMR
spectroscopy).
Acknowledgment is made to the U.S. Department of Energy,
Grant FG02-86ER13569 for their support of this work.
Supporting Information Available: LCMS spectra of the
perfluorobenzene oligomers (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
(22) Jordan, R. F. J. Organomet. Chem. 1985, 294, 321.
(23) Bochmann, M.; Sarsfield, M. J. Organometallics 1998, 17, 5908.
(24) Murphy, E. F.; Yu, P.; Dietrich, S.; Roesky, H. W.; Parisini, E.;
Noltemeyer, M. J. Chem. Soc., Dalton Trans. 1996, 1983.
JA991805D