Activation of aromatic, aliphatic, and olefinic carbon-fluorine bonds using Cp*2HfH2

Article abstract of DOI:10.1002/ejic.200600802

The hafnium hydride Cp*₂HfH₂ is reacted with a series of fluorocarbons to examine the scope of C-F bond activation. Aromatic, vinylic, and aliphatic C-F bonds all show some degree of reactivity, and possible mechanisms are discussed. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200600802

FULL PAPER
DOI: 10.1002/ejic.200600802
Activation of Aromatic, Aliphatic, and Olefinic Carbon–Fluorine Bonds Using
Cp*2HfH2
Ryan D. Rieth,[a] William W. Brennessel,[a] and William D. Jones*[a]
Keywords: Metallocenes / Hydrides / Reduction / Fluorocarbons
The hafnium hydride Cp*2HfH2 is reacted with a series of
fluorocarbons to examine the scope of C–F bond activation.
Aromatic, vinylic, and aliphatic C–F bonds all show some de-
gree of reactivity, and possible mechanisms are discussed.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
observed product by the route depicted in Equation (1).[7]
Similar considerations have been implicated in reactions in-
Introduction
Organometallic chemists have long been interested in the volving Ni and Rh.[8–9]
activation of typically inert C–F bonds using transition-
metal complexes. The advantages of promoting C–F bond
cleavage are both academic and practical. In one sense, the
selective and predictable disruption of these interactions
may result in greater synthetic utility of readily available
fluorocarbons. In addition, such knowledge could have en-
vironmental ramifications, eventually leading to the de-
struction of harmful chlorofluorocarbons (CFCs) in the up-
per atmosphere. While both stoichiometric[1] and catalytic[2]
C–F activation reactions have been reported using transi-
tion metals, significant efforts continue to be made to im-
prove known methodologies and to discover new ways to
cleave these strong bonds selectively with an important goal
being an understanding of the mechanisms by which these
transformations take place.
(1)
To date, a variety of mechanisms for C–F bond acti-
Electrophilic C–F bond activation has been suggested to
occur by fluoride transfer, especially when strong Lewis ac-
ids are employed. For example, fluoride transfer from an
aromatic fluorocarbon to an open coordination site in
[L2TiMe]+ systems has recently been proposed by Ziegler
and co-workers.[10] Rosenthal et al. have shown that cationic
zirconocene complexes can abstract fluoride from perfluo-
rophenyl groups to give Zr–F species.[11] In most cases
where suggested, fluoride transfer is thought to likely occur
through a cyclic transition state [Equation (2)]. Further-
more, olefinic fluorocarbons have been shown to participate
in β-fluoride elimination following insertion into metal-hy-
dride bonds, leading to the generation of a new fluorocar-
bon or small fluorine-containingmolecule.[12–15] For per-
fluoroalkyl–metal complexes, α-fluoride elimination has
been shown to be operative in several systems, many of
which feature the use of Group 9 metals.[16] A titanium per-
fluoromethyl complex has also been reported, indicating
that such species can be stable.[17]
vation by transition metals have been suggested. Some of
the more notable strategies involve electron transfer, electro-
philic pathways, homolytic cleavage, oxidative addition, and
nucleophilic routes.[3–4] While many of these mechanisms
feature intermolecular interactions, several examples in-
volve intramolecular activation,[5–6] which is oftentimes ac-
companied by the liberation of a small molecule such as
HF.
As an example of the electron-transfer mechanism, Per-
utz and co-workers have suggested that an electron-rich di-
hydride, Ru(dmpe)2H2 (dmpe = Me2PCH2CH2PMe2), is
capable of donating an electron to a heavily fluorinated aro-
matic compound, eventually leading to the formation of the
[a] Department of Chemistry, University of Rochester,
Rochester, New York 14627, USA
E-mail: jones@chem.rochester.edu
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R. D. Rieth, W. W. Brennessel, W. D. Jones
FULL PAPER
all solvents must first be dried with purple sodium/benzo-
phenone ketyl and then over green titanocene, formed from
the reaction of Cp2TiCl2 and zinc dust in the solvent of
interest.[24]
In order to obtain a general idea of how 1 reacts with
fluorinated organic compounds, the starting hafnium com-
plex was treated with several substrates, including fluori-
nated aromatics and olefinic compounds, aliphatic C–F
bonds, and fluorinated heterocycles.
(2)
In most of the reactions, two primary metal-based prod-
ucts formed, Cp*2HfHF (2) and Cp*2HfF2 (3), as a result
of either mono- or di-hydrodefluorination, respectively.
Their presence in the reaction mixtures was determined by
In addition to mechanisms involving the transfer of elec-
trons or fluoride, other mechanisms that have been iden-
tified include the involvement of radical-based species in
the C–F bond cleavage reactions of aliphatic fluorocarbons
using Cp*2ZrH2,[18] the oxidative addition of an aromatic
C–F bond to phosphane Ni0 complexes,[9,19] and the nucleo-
philic aromatic substitution on fluorinated aromatic species
using early transition-metal complexes.[1(d),6]
As an addition to the variety of metal-based systems
known to cleave C–F bonds,[3,20] the following account
summarizes C–F bond activation research done using
bis(pentamethylcyclopentadienyl)hafnium
dihydride,
Cp*2HfH2 (1). The substrate scope includes aromatic, ali-
phatic, and olefinic compounds featuring varying steric
properties and degrees of fluorination. Heterocyclic sub-
strates are also briefly considered, and mechanistic insight
into the reaction of 1 with several fluorinated alkenes is
presented.
Results and Discussion
Synthesis of Cp*2HfH2 and its Fluoro Derivatives
The synthesis of 1 was achieved by conversion of com-
[21]
mercially available Cp*2HfCl2 to Cp*2HfMe2
followed
by reaction with hydrogen at elevated temperature [Equa-
tion (3)]. Comparison of 1H NMR spectroscopic data to
known literature values for 1 confirmed that it was indeed
the expected product.[22]
Figure 1. ORTEP diagrams of: a) 2; b) 3; and c) 4. Ellipsoids are
drawn at the 50% probability level. Hydrogen atoms are omitted
for clarity. The fluoride ligand in 2 is disordered over two sites.
Table 1. Selected X-ray data for 2, 3, and 4.
Cp*2HfHF (2) Cp*2HfF2 (3) [Cp*2Hf]2O (4)
Space group
R(int.)
R(σ)
P21/c
P21/n
Pn
1.65%
3.16%
4.32%
141.7°
140.3°
N/A
3.25%
3.52%
5.02%
137.2°
139.0°
N/A
3.02%
4.06%
2.41%
130.9°
133.0°
172.2(2)°
1.991(3)
1.985(3)
(3)
R1
Cp*-Hf-Cp*
Angle[a,b,c]
Hf–O–Hf angle
Hf–F or Hf–O
bond length[b] [Å]
The long reaction time required for a reasonable degree
of conversion can be avoided using the method reported by
Bercaw, in which 1 was made directly from Cp*2HfCl2 using
n-butyllithium under hydrogen, a process that afforded the
dried, isolated product in roughly 3 d.[22] Due to the sensi-
tive nature of the starting hafnium complex,[23] great care
was taken to prevent air and moisture from being present
1.978(6)
1.998(8)
2.017(4)
2.126(4)
[a] Cp* represents the cyclopentadienyl centroid. [b] For the struc-
tures of 2 and 3, two molecules were present in the asymmetric
unit. Values for both structures are presented. [c] For purposes of
comparison, the Cp*-Hf–Cp* angle has been reported to be 144.1°
in any reaction using 1 as a starting material. To this end, for 1.[28]
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C–F Bond Activation
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Table 2. Summary of crystallographic data for 2, 3, and 4.
2
3
4
Chemical formula
Formula weight
Crystal system
Space group (Z)
a [Å]
b [Å]
c [Å]
C20H31FHf
468.94
C20H30F2Hf
486.93
C40H62Hf2O
915.88
monoclinic
Pn (2)
11.0534(7)
10.1633(6)
16.5842(10)
103.449(1)
1811.96(19)
1.679
0.50ϫ0.50ϫ0.41
100
4–60
–15ՅhՅ15
–14ՅkՅ14
–23ՅlՅ23
25483
monoclinic
P21/c (8)
15.7895(8)
15.4046(8)
16.3652(8)
109.232(1)
3758.4(3)
1.658
0.24ϫ0.20ϫ0.10
100
4–60
–19ՅhՅ30,
–15ՅkՅ15,
–15ՅlՅ16
55625
monoclinic
P21/n (8)
13.0151(5)
19.0896(8)
15.8115(7)
107.969(1)
3736.8(3)
1.731
0.47ϫ0.45ϫ0.22
100
4–60
–18ՅhՅ18
–26ՅkՅ26
–22ՅlՅ22
55828
β [deg]
V [Å]3
ρcalcd. [gcm–3]
Crystal dimensions [mm]
Temperature [K]
2θ Range [deg]
Data collected
No. of data collected
No. of unique data
10862
0.0339
9891
417
10819
0.0222
10035
435
10302
0.0302
10164
414
Agreement between equiv. data
No. of observed data [IϾ2σ(I)]
No. of parameters varied
µ [mm–1]
5.556
5.599
5.754
Absorption correction
Range of transmission factors
R1(Fo), wR2(Fo2), all data
R1(Fo), wR2(Fo2), [IϾ2σ(I)]
Goodness of fit
empirical (SADABS)
0.279–0.576
0.0409, 0.0844
0.0363, 0.0825
1.076
differential (DIRDIF)
0.0983–0.292
0.0235, 0.0519
0.0209, 0.0508
1.023
empirical (SADABS)
0.71–0.91
0.0236, 0.0579
0.0233, 0.0578
1.014
their characteristic NMR spectroscopic data. Characteriza-
Additional reactions using monofluorinated aromatic
tion of 2 was accomplished by H and 19F NMR, as well compounds were briefly pursued, but these experiments did
as by 1H-19F heteronuclear correlation spectroscopy. The not reveal the expected evidence of C–F bond cleavage.
structure of 2 was confirmed by X-ray crystallography (Fig- Specifically, 2- and 4-fluorobiphenyl were allowed to react
ure 1).[25] Complex 3 was characterized by 19F NMR spec- with 1 for over 3 d at 120 °C without displaying C–F reac-
troscopic data, which was later confirmed through indepen- tivity. In addition to reactions with monofluorinated aro-
dent synthesis of the complex.[26] Further characterization matic substrates, the activation of aliphatic C–F bonds was
1
1
was achieved through H NMR spectroscopic data[26] and studied using 1-fluorohexane. Initial conversion of 1 to 2
X-ray crystallography (Figure 1).[25] An additional hafnium was not observed by 19F NMR until the reaction mixture
complex, (Cp*2HfH)2O (4), was isolated from a crystalli- was monitored after being heated to 60 °C for 44 h. The
zation attempt of 1 and was characterized by X-ray crystal- reaction was nearly complete after 20 d at 120 °C. Further-
1
lography (Figure 1) and H NMR spectroscopic data.[23,27] more, hexane was also detected by 1H NMR spec-
This compound likely arises from side reaction with adven- troscopy[18] with the observed ratio of 2:hexane being
titious water. Selected distances and angles for complexes 0.4:1.0. Small amounts of 3 (5%) were also observed.
2, 3, and 4 are summarized in Table 1. Selected crystallo-
graphic data are compared in Table 2.
Several polyfluorinated aromatic compounds were exam-
ined. Highly symmetric substrates 1,2,4,5-tetrafluoroben-
zene and 2,2Ј,3,3Ј,5,5Ј,6,6Ј-octafluorobiphenyl did not par-
ticipate in C–F bond cleavage after days at 120 °C. How-
ever, heating for 120 h at 100 °C produced traces of 2 or 3
Reactions of 1 with Fluoroaromatics and Fluoroaliphatics
The compounds 2- and 3-fluoropyridine both were ob- in reactions involving 1,2,3,4-tetrafluorobenzene and
served to react with 1 at room temperature over the course 2,3,4,5,6-pentafluoro(allylbenzene), respectively. While
of several hours. NMR analysis of the reaction mixtures longer reaction times at elevated temperatures reduced the
revealed the presence of 2 and small amounts of 3 in both amount of substrate relative to metal-based products, the
reactions but no free pyridine in either. In both reactions, 2 products generated do not display NMR spectroscopic data
forms prior to the formation of 3, indicating a sequential that matches that of the expected organic products.[29]
reaction of the Hf–H bond. In addition, pyridine was not
Several perfluorinated aromatic compounds were also ex-
observed by GC-MS analysis of the volatile materials from amined: hexafluorobenzene, octafluoronaphthalene, and
the reactions, suggesting that it forms strong adducts with decafluorobiphenyl. NMR analysis of the reactions showed
hafnium-containing moieties in the reaction mixture. While that each reaction generated 2 upon heating, but that weeks
2-fluoropyridine reacts within a few hours, 3-fluoropyridine at 100–120 °C are required. The observed 19F NMR chemi-
reacts relatively sluggishly over the course of several days.
cal shift values of the organic products match known values
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R. D. Rieth, W. W. Brennessel, W. D. Jones
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for pentafluorobenzene, 2H-heptafluoronaphthalene, and hexafluoroisobutene produced 1,1,3,3,3-pentafluoro-2-
4H-nonafluorobiphenyl.[1(d)] Additionally, 2H-heptafluo- methyl-1-propene[14] after 50 h at room temperature [Equa-
ronaphthalene was observed by GC-MS analysis of the vol- tion (6)]. The final ratio of 2:1,1,3,3,3-pentafluoro-2-
atile materials from its respective reaction and the potential methyl-1-propene was 1.7:1.0, according to 19F NMR spec-
presence of 2H-nonafluorobiphenyl as the major product troscopic data.
from its respective reaction was discounted based on pre-
viously published NMR spectroscopic data.[30] Note that
C6F6 is substantially less reactive than the other perfluo-
roarenes.
Reactions of 1 with Fluoroolefins
Partially fluorinated olefins investigated include 3,3,3-tri-
fluoropropene, 1,1-difluoroethylene, hexafluoroisobutene,
3,4,4,4-tetrafluoro-3-trifluoromethyl-1-butene, α-(trifluoro-
methyl)styrene, and difluoromethylenecyclohexane. Reac-
tion with 3,3,3-trifluoropropene produced 2, 3, 1,1-difluo-
ropropene, and 1,1,1-trifluoropropane in a 3.7:0.5:3.4:1.0
ratio after 43 h at room temperature [Equation (4)]. How-
ever, the bulk of the conversion was observed to have oc-
curred by 19F NMR spectroscopy after roughly 3 min had
elapsed. NMR spectroscopic data from 1,1-difluoropropene
and 1,1,1-trifluoropropane match previously reported
data[14] and 1,1-difluoropropene was identified by analyzing
the volatile materials from the reaction by GC-MS.
(6)
The analogous reaction using 3,4,4,4-tetrafluoro-3-tri-
fluoromethyl-1-butene produced 1,1,1-trifluoro-2-trifluoro-
methyl-2-butene and 1,1,1-trifluoro-2-trifluoromethyl-2-bu-
tane, both of which could be confirmed by literature pre-
cedent [Equation (7)].[14] The reaction was allowed to pro-
ceed for 146 h at room temperature, although this time was
not sufficient to completely consume 1. The final ratio of
2:1,1-trifluoro-2-trifluoromethyl-2-butene: 1,1,1-trifluoro-2-
trifluoromethyl-2-butane obtained from 19F NMR spectro-
scopic data was 11.3:7.9:1.0. The olefinic organic product
was also identified by GC-MS.
(7)
(4)
The reaction of 1 with α-(trifluoromethyl)styrene pro-
duced β,β-difluoro-α-methylstyrene, the presence of which
was confirmed from previous NMR spectroscopic data
[Equation (8)].[33] First evidence of the formation of 2 was
observed after heating the reaction mixture for 18 h at
80 °C. 3 was first observed following 92 h of heating at
80 °C. The reaction was terminated after 163.5 h at 160 °C.
On the basis of 19F NMR spectroscopic data, the final ratio
of 2:3: β,β-difluoro-α-methylstyrene was 0.8:1.0: 1.5.
Reaction of 1 with 1,1-difluoroethylene produced 2 and
a species assigned as Cp*2Hf(CH2CH3)H (5) after 20 min at
room temperature [Equation (5)]. The diagnostic H NMR
1
signals for 5 [δ = 11.95 (s, Hf–H), 0.32 (t, –CH2CH3), –0.22
(q, –CH2CH3)] are close to those reported for this complex
in C6D6.[22] Furthermore, because 1-fluoroethylene[31] and
free ethylene[32] were not observed, the formation of this
complex can be rationalized by subsequent insertions and
eliminations of 1,1-difluoroethylene and fluoroethylene into
a Hf–H bond, a sequence of transformations which ulti-
mately result in ethylene formation. This ethylene then un-
dergoes Hf–H insertion to finally arrive at 5.
(8)
The reaction involving difluoromethylenecyclohexane
(5)
was carried out in the same manner, producing 2, 3, and
In order to alter the steric profile of the studied olefins, fluoromethylenecyclohexane in a 12.8:1.0: 38.0 ratio (by 19
F
bulkier alkenes were then used to contrast their reactivity NMR) after 93 h of reaction at 160 °C [Equation (9)]. Ini-
with that of the smaller compounds. The reaction of 1 and tial formation of 2 was not observed until after 22 h had
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C–F Bond Activation
FULL PAPER
elapsed at 120 °C. Literature values[14] were used to match at room temperature afforded 1H,2H-hexafluoro-cyclopen-
obtained NMR spectroscopic data with the corresponding tene as the initial major organic product. However, after an
organic product.
extended amount of time at room temperature, 1H,2H,3H-
pentafluorocyclopentene could be observed. The identity of
both fluorocyclopentene products was confirmed through
comparison to available NMR spectroscopic data.[34]
(9)
Reactions of 1 with Perfluoroolefins
Following the set of reactions using polyfluorinated al-
kenes, the perfluorinated olefins perfluoropropene and oc-
tafluorocyclopentene were used in reactions with 1. The
perfluoropropene reaction proceeded at room temperature
and yielded 2 and (E)-1,2,3,3,3-pentafluoropropene in a
1.8:1.2 ratio (using 19F NMR spectroscopic data) after
41.5 h [Equation (10)]. The identity of the organic product
was determined from previous NMR spectroscopic data[15]
and conflicted with the data available for the Z isomer.[34]
Figure 2. 19F NMR spectroscopic data for the stepwise hydrodeflu-
orination of perfluorocyclopentene. Temperatures (°C) and allowed
reaction times for the various spectra are listed in the right margin.
(a = octafluorocyclopentene; b = 1H-heptafluorocyclopentene; c =
1H,2H-hexafluorocyclopentene; d = 1H,2H,3H-pentafluorocyclo-
pentene; e = 6).
(10)
On the basis of 19F NMR spectroscopic data, the analo-
gous reaction using octafluorocyclopentene produced a
mixture of 2, 3, and 1H-heptafluorocyclopentene in a
41:1.0:27.4 ratio after 47 h at room temperature. Identity of
the organic product was confirmed by previously obtained
NMR spectroscopic data[34] and by GC-MS data obtained
from the volatile materials from the reaction. Full con-
sumption of 1 was achieved.
In an effort to understand the hydrodefluorination of oc-
tafluorocyclopentene in more detail, a stepwise C–F acti-
vation experiment was performed as follows: equimolar
amounts of 1 and octafluorocyclopentene were reacted at
room temperature, after which the volatile organic product
(1H-heptafluorocyclopentene) was vacuum-transferred
onto an additional equivalent of 1. The first step of this
reaction essentially proceeded as before, with the exception
being that a small amount of a second product isomer, 4H-
heptafluorocyclopentene, was observed by 19F NMR after
30 min of reaction at room temperature. The ratio of the
major (expected) product to the new, minor product was
5.5:1.0. After 20 h of reaction, 1 was no longer present in
the reaction mixture, and the volatile materials were vac-
uum-transferred to a second equivalent of 1. The reaction
was then periodically monitored by NMR spectroscopy
Following a period of decreased activity, the reaction
mixture was heated first to 60 °C and then to 100 °C. The
relatively high temperature drastically altered the observed
product distribution and even resulted in the formation of
a species tentatively assigned as 6 (Scheme 1). The assign-
ment of 6 is based largely on NMR integration values and
previous observation of a similar complex involving Zr.[34]
over the course of several months (Figure 2). The reaction Scheme 1.
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R. D. Rieth, W. W. Brennessel, W. D. Jones
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There appeared to be no evidence of other fluorinated cy-
clopentene derivatives following prolonged heating at
100 °C.[35] GC-MS analysis of the volatile materials from
the reaction revealed the presence of 1H,2H,3H-pentafluo-
rocyclopentene.
While the stepwise reaction of octafluorocyclopentene
shows a preference for vinylic C–F activation over cleavage
of neighboring C–F bonds, the mechanism by which the
organic products are generated remains unclear. While an
olefin insertion prior to β-fluoride elimination mechanism
seems to be operative for other non-cyclic olefins,[4,12–15]
such a process cannot be responsible for the formation of
observed products in this case, because the product pre-
dicted by the mechanism is not observed [Equation (11)].
Consequently, a new mechanism must be postulated for this
facile olefinic hydrogenolysis. At this point, preliminary cal-
culations suggest a 4-center metathesis pathway between the
Hf–H and C–F bond for the reaction.
Figure 3. Variable-temperature 19F NMR spectroscopic data for
the reaction of 3,3,3-trifluoropropene with 1. The temperatures
(°C) corresponding to the various spectra are given in the right
margin. Regions of interest are expanded for clarity. (a = 2, b = 3,
c = 7, d = 3,3,3-trifluoropropene, e = 8, f = 1,1,1-trifluoropropane,
g = 1,1-difluoropropene).
(11)
Low-Temperature Studies
Low-temperature NMR spectroscopy was used to look
for intermediates that might provide mechanistic insight
into the reaction of fluorinated olefins with 1. In particular,
reactions of 1 with 3,3,3-trifluoropropene, 1,1-difluoroeth-
(12)
Subsequent warming of the reaction mixture results in
ylene, and 1,1-difluoropropene were studied at low tem- relatively rapid disappearance of the NMR signal tenta-
perature. Reactions were initiated at –110 °C and samples tively assigned to 7 accompanied by an appreciable increase
monitored in the NMR probe as they warmed to room tem- in the signals corresponding to 2 and 1,1-difluoropropene.
perature. Of the substrates examined, the reactions involv- The β-fluoride elimination from the trifluoromethyl group
ing 3,3,3-trifluoropropene provided the most insight. Fig- is consistent with these observations. Moreover, prolonged
ure 3 shows some of the 19F NMR spectroscopic data ob- storage of the reaction mixture at room temperature pro-
tained from this reaction.
motes the growth of the signal previously assigned to 1,1,1-
The most striking aspect of the data presented in Fig- trifluoropropane. Because the hydrogenated olefin signal in-
ure 3 is the fact that some of the species observed, specifi- tensity approaches that of the resonance assigned to 8, it
cally the compounds corresponding to the peaks labeled c appears as though complex 8 may be the source of 1,1,1-
and e, seem to be present at low temperature but gradually trifluoropropane.
vanish as the temperature is increased. One way to rational-
The low-temperature experiment using 3,3,3-trifluoro-
ize these observations is summarized in Equation (12) be- propene was monitored by 1H NMR in a separate trial and
low. Assuming that insertion of the olefin into the metal- showed similar behavior to that observed by 19F NMR
hydride bond of 1 is the initial step in the reaction, one spectroscopy. Specifically, two previously unobserved sig-
could expect two different species – one with a branched nals appeared between 12–14 ppm and were tentatively as-
fluoroalkyl ligand 7 and one featuring a straight-chain fluo- signed as the Hf–H signals from 7 (δ = 13.80 ppm) and 8
roalkyl moiety 8 – to form. Because the NMR spectra taken (δ = 12.90 ppm). As expected, the signal from 7 decreased
from roughly –110 °C to –60 °C display only starting olefin, quickly upon warming and was completely absent by the
1, and two unidentified species, it seems reasonable to argue time the reaction was warmed to –10 °C. In addition, an-
that the unassigned species may be the two insertion prod- other resonance at δ = 0.50 ppm exhibited the same behav-
ucts, 7 and 8. Strengthening that assertion is the fact that ior and was assigned as the methyl group of 7 based on
the signal assigned to 8 is a sharp triplet, as would be ex- integration. The Hf–H signal from 8 along with an ad-
pected.
ditional signal at –0.03 ppm (assigned to the metal-bound
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C–F Bond Activation
FULL PAPER
itive nitrogen pressure to quench the reaction. The volatile materi-
als were removed by vacuum. Isolation of the final product was
achieved by filtering a hexane mixture of crude product through
celite, cooling the filtrate to –78 °C to induce crystallization, re-
moving the mother liquor/washing the crystals with cold hexane
via cannula transfer, and drying in vacuo (3.06 mmol, 80%). The
second step involved placing a C6D6 solution of Cp*2HfMe2
(0.06 mmol) under 15 psi of UHP-grade hydrogen gas (Airgas) at
100 °C. The reaction was periodically monitored by 1H NMR over
the course of 6 weeks, with the hydrogen being replaced many times
methylene of 8 based on integration) expectedly lingered at
room temperature.
While the reaction of 3,3,3-trifluoropropene with 1 ap-
peared to successfully demonstrate that the insertion/elimi-
nation mechanism accounts for the formation of observed
products, the same could not be said for the reactions of
1,1-difluoroethylene and 1,1-difluoropropene with 1. At the
current time, it appears as though the reaction involving
1,1-difluoroethylene proceeds too rapidly at low tempera-
ture to observe intermediates. In contrast, the reaction of with fresh hydrogen following several freeze-pump-thaw degassing
cycles (≈ 97% final conversion to 1). The reaction was repeated
on a preparative scale with the remaining Cp*2HfMe2. 1 was also
prepared in 33% yield from the reaction of Cp*2HfCl2 and BuLi,
1,1-difluoropropene has not provided results suitable for
discussion of its mechanism of reaction.
as described in ref.[22] 1H NMR (C6D6): Cp*2HfCl2, δ = 1.91, s.
.
Cp*2HfMe2, δ = 1.80, (s, 30 H), –0.70 (s, 6 H) Cp*2HfH2 (1), δ =
2.05, (s, 30 H), 15.63 (s, 2 H) ppm. H NMR (C6D12): Cp*2HfH2
Comparison with Reactions of Zirconium Dihydride
1
A comparison can be made here with the related reactiv-
ity of the zirconium analog Cp*2ZrH2. This complex has
been reported to reduce aliphatic,[36] aromatic,[1d–18] and
olefinic[14–15] C–F bonds under mild conditions. It can also
undergo β-fluoride elimination in fluorophenyl derivatives
to generate benzyne intermediates.[37] In addition, reduction
of sp3 CF2H–R has been reported to occur slowly with this
compound. The reaction mechanisms proposed vary from
free radical chain for aliphatic C–F, to nucleophilic attack
for monofluoroaromatics, to insertion/β-fluoride elimi-
nation for fluoroolefins.[4] Additionally, other electrophilic
titanocene[38] and zirconocene[39] complexes have been seen
to cleave C–F bonds in perfluoropyridine. In general, the
reactivity patterns seen with Cp*2HfH2 mimic those of
Cp*2ZrH2, except that the hafnium complex is less reactive.
Reactions proceed to give similar products either slowly, or
not at all. While fewer mechanistic studies have been under-
taken with hafnium, it is likely that the substrates react by
similar mechanisms as in the zirconium cases.
(1), δ = 2.03, (s, 30 H), 15.37 (s, 2 H) ppm.
Isolation of 2 and 3: Samples of 2 and 3 were obtained from the
non-volatile organometallic residue of the fluorocarbon reduction
reactions. X-ray quality crystals were grown by allowing a pentane
solution of the hafnium complex to evaporate into a small quantity
of toluene at –30 °C in a nitrogen glovebox. X-ray quality crystals
of 4 were obtained by allowing pentane vapor to diffuse into a
concentrated CH2Cl2 solution of 4 at room temperature in a nitro-
1
gen glovebox. For 2, H NMR (C6D12): δ = 1.95 (s, 30 H), 10.90
(d, J = 4.8 Hz, 1 H) ppm. 19F NMR: δ = 107.8 (s) ppm. For 3, H
1
NMR (C6D12): δ = 1.89 (s) ppm. 19F NMR: δ = 49.7 (s) ppm. For
1
4, H NMR (C6D12): δ = 2.00 (s, 30 H), 2.08 (s, 30 H), 9.79 (s, 2
H) ppm.
Reactions of 1 with Fluorocarbons: All experiments were run in
NMR tubes fitted with a Teflon stopper using equimolar
amounts of 1 and substrate. Unless otherwise noted, twice-dried
[D12]cyclohexane was used as solvent to dissolve mixtures of the
starting hafnium complex and solid substrates in a nitrogen
glovebox. In reactions involving liquid substrates, ca. 10 mg of 1
was first dissolved using C6D12 in a nitrogen glovebox and the pre-
viously-degassed liquid substrate was added via syringe. Gaseous
compounds were condensed into chilled reaction vessels using high
vacuum techniques.
Experimental Section
For reactions of 2- and 3-fluoropyridine, 19F data[40] showed the
loss of fluorocarbon reactant and the formation of 2 and small
amounts of 3 (2–4%) over the course of a week at room tempera-
All substrates were used as delivered without purification. Products
have been characterized and unreacted starting materials have been
identified primarily by comparing known 19F NMR chemical shift
values to those obtained during a given experiment (using external
α,α,α-trifluorotoluene in [D12]cyclohexane as a reference). Re-
ported NMR spectroscopic data not referenced to external α,α,α-
trifluorotoluene in [D12]cyclohexane were converted accordingly. In
1
ture. Pyridine was not seen by H NMR spectroscopy.[41] For reac-
tions of 2- and 4-fluorobiphenyl, no reaction was observed at
120 °C by 19F NMR spectroscopy.[42] Polyfluorinated aromatics
showed only traces of reaction if any reaction at all after extended
heating at 120 °C, as determined by monitoring the 19F NMR spec-
tra of these reactions. Substrates examined were 1,2,4,5-tetrafluo-
robenzene,[1(d)] 2,2Ј,3,3Ј,5,5Ј,6,6Ј-octafluorobiphenyl,[43] 1,2,3,4-tet-
rafluorobenzene,[1(d)] and 2,3,4,5,6-pentafluoro(allylbenzene).[44] In
contrast, perfluorinated aromatics showed some evidence for slow
reaction. 19F NMR spectroscopic examination of reactions of hexa-
fluorobenzene,[45] octafluoronaphthalene,[46] and decafluorobi-
phenyl[47] showed reduction to produce 2 after several days at 100–
120 °C. Reaction of 1 with 1-fluorohexane[48] commenced only
upon heating to 60 °C, and approached completion after 20 d at
120 °C.
1
some cases, H NMR and GC-MS data were used to further con-
firm the identity of the organic products. NMR spectra were re-
corded with Bruker Avance instruments (400 or 500 MHz). GCMS
data were recorded with a Shimadzu model QP2010 spectrometer.
A Siemens SMART CCD area detector diffractometer equipped
with an LT-2 low-temperature unit was used for X-ray crystal struc-
ture determination.
Preparation of Cp*2HfH2: From dimethyl complex: Cp*2HfCl2
(3.85 mmol; Strem) was dissolved in roughly 40 mL of diethyl ether
(previously vacuum-transferred from purple sodium/benzophenone
ketyl) in a nitrogen glovebox. On a Schlenk line, 2.1 equiv. of meth-
yllithium (8.09 mmol; 1.4  in Et2O from Aldrich) were added to 19F NMR spectroscopy was used to monitor reactivity of the par-
the reaction mixture by syringe under positive nitrogen pressure.
The reaction mixture was stirred under static nitrogen for roughly
25 h before ca. 1 mL of ACS-grade methanol was added under pos-
tially fluorinated olefins 3,3,3-trifluoropropene,[49] 1,1-difluoroeth-
ylene,[50] hexafluoroisobutene,[51] 3-trifluoromethyl-3,4,4,4-tetra-
fluoro-1-butene, α-(trifluoromethyl)styrene,[52] and difluoromethyl-
Eur. J. Inorg. Chem. 2007, 2839–2847
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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R. D. Rieth, W. W. Brennessel, W. D. Jones
FULL PAPER
enecyclohexane,[53] as described in the text. Reactions of 1 with the
perfluorinated olefins perfluoropropene[54] and octafluorocyclo-
pentene[55] were also examined by 19F NMR spectroscopy, but now
reaction occurred quickly at room temp.
8675; c) M. Aizenberg, D. Milstein, Science 1994, 94, 359–361;
d) J. Vela, J. M. Smith, Y. Yu, N. A. Ketterer, C. J. Flaschen-
riem, R. J. Lachicotte, P. L. Holland, J. Am. Chem. Soc. 2005,
127, 7857–7870; e) R. J. Young, V. V. Grushin, Organometallics
1999, 18, 294–296; f) T. Braun, R. N. Perutz, M. I. Sladek,
Chem. Commun. 2001, 2254–2255; g) Y. Ishii, N. Chatani, S.
Yorimitsu, S. Murai, Chem. Lett. 1998, 157–158.
Low-Temperature Reaction of 1 with Fluorocarbons: Compound 1
was dissolved in [D14]methylcyclohexane in a nitrogen glovebox.
The substrate of choice was measured by achieving the appropriate
pressure in a pre-calibrated glass bulb. The substrate was then con-
densed into the reaction vessel following degassing of the solution
of 1. Special effort was directed at keeping the NMR tube cold
following transfer of the substrate, which was achieved by thawing
the frozen solution of 1 in a liquid nitrogen/ethanol slush bath
(–116 °C). In most instances, the NMR probe was pre-cooled to
–110 °C.
[3] For additional discussion of these mechanisms and for exam-
ples of systems capable of C–F activation, see: J. Burdeniuc, B.
Jedlicka, R. H. Crabtree, Chem. Ber./Recueil 1997, 130, 145–
154.
[4] W. D. Jones, Dalton Trans. 2003, 3991–3995.
[5] For examples of intramolecular C–F activation, see: a) D. Hu-
ang, K. G. Caulton, J. Am. Chem. Soc. 1997, 119, 3185–3186;
b) M. Crespo, J. Granell, M. Font-Bardía, X. Solans, J. Or-
ganomet. Chem. 2004, 689, 3088–3092; c) R. P. Hughes, D. C.
Lindner, A. L. Rheingold, L. M. Liable-Sands, J. Am. Chem.
Soc. 1997, 119, 11544–11545; d) R. P. Hughes, J. M. Smith, J.
Am. Chem. Soc. 1999, 121, 6084–6085; e) P. J. Albietz, J. F.
Houlis, R. Eisenberg, Inorg. Chem. 2002, 41, 2001–2003; f)
R. P. Hughes, J. M. Smith, C. D. Incarvito, K. Lam, B. Rhati-
gan, A. L. Rheingold, Organometallics 2002, 21, 2136–2144.
[6] P. A. Deck, M. M. Konaté, B. V. Kelly, C. Slebodnick, Organo-
metallics 2004, 23, 1089–1097.
1,1-Difluoropropene was obtained through the previously men-
tioned reaction of 1 with 3,3,3-trifluoropropene, with the exception
being that the reaction was run in C6D11CD3 in instances where
1,1-difluoropropene was designed to serve as a starting material in
a subsequent low-temperature experiment. The volatile materials
from the reaction (primarily 1,1-difluoropropene, 1,1,1-trifluoro-
propane, and solvent) were vacuum-transferred from this reaction
onto another equivalent of 1 before thawing the reaction mixture
at –116 °C. Detailed descriptions of these reactions are given in the
text.
[7] M. K. Whittlesey, R. N. Perutz, M. Moore, Chem. Commun.
1996, 787–788.
[8] B. L. Edelbach, W. D. Jones, J. Am. Chem. Soc. 1997, 119,
7734–7742.
[9]
A. Steffen, M. I. Sladek, T. Braun, B. Neumann, H. G.
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T. Wondimagegn, Z. Xu, K. Vanka, T. Ziegler, Organometallics
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X-ray Structure Determinations: Single crystals of 1, 2, and 3 were
mounted under Exxon Paratone-8277TM on glass fibers and imme-
diately placed in a cold nitrogen stream at 100 K on the X-ray
diffractometer. The X-ray intensity data (1.3 hemispheres) were col-
lected over 6 h with a standard Siemens SMART APEX II CCD
Area Detector System equipped with a normal focus molybdenum-
target X-ray tube operated at 2.0 kW (50 kV, 40 mA).[56] Data were
corrected for absorption using the program SADABS.[57] Space
group assignments were made on the basis of systematic absences
and intensity statistics by using the XPREP program (Siemens,
SHELXTL 97). The structures were solved by using direct methods
and refined by full-matrix least-squares on F2.[58] For all of the
structures, the non-hydrogen atoms were refined with anisotropic
thermal parameters except for the fluoride ligand in 2, which was
disordered over two sites and refined isotropically. The Hf-bound
hydride in 2 was not included in the final refinement model. There
was nothing unusual about the solution or refinement of any of
the structures, although the fluoride and hydride ligands in 2 were
disordered (76/24 and 60/40). Compound 4 refined with a Flack
parameter of 0.055(8). Further experimental details of the X-ray
diffraction studies are provided in Table 2. CIF files for the struc-
tures 2–4 have been deposited with the CCDC.[59]
[10]
[11]
[12]
[13]
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only 2 was revealed to be a co-crystallized mixture of 2 and 3
[17]
[18]
[19]
Acknowledgments
The U. S. Department of Energy, Grant FG02-86ER13569, is
gratefully acknowledged for their support of this work.
[20]
[21]
[22]
[23]
[24]
[25]
[1] Recent examples of stoichiometric C–F activation: a) D. No-
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[2] For examples of catalytic C–F activation, see: a) S. Kuhl, R.
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2846
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Eur. J. Inorg. Chem. 2007, 2839–2847

C–F Bond Activation
FULL PAPER
(with one monoatomic ligand site having 50% occupancy of
fluorine). The data reported for 3 was isolated from a separate
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A. Herzog, F. Liu, H. W. Roesky, A. Demsar, K. Keller, M.
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NMR spectroscopic data for 3-pentafluorophenylpropan-1-ol
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[53] 1H and 19F NMR spectroscopic data for difluoromethylenecy-
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[35] NMR spectroscopic data for several fluorinated cyclopentene
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R. Fields, M. Green, M. T. Harrison, R. N. Haszeldine, A.
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[54] 19F NMR spectroscopic data for perfluoropropene: M. V. Ga-
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[56] It has been noted that the integration program SAINT pro-
duces cell constant errors that are unreasonably small, because
systematic error is not included. More reasonable errors might
be estimated at 10ϫthe listed value.
[57] The SADABS program is based on the method of Blessing:
R. H. Blessing, Acta Crystallogr., Sect. A 1995, 51, 33–38.
[58] Using the SHELX95 package, R1 = (Σ||Fo| – |Fc||)/Σ|Fo|, wR2 =
2
2 2
{Σ[w(Fo – Fc ) ]/Σ[w(Fo2)2] }1/2, where w = 1/[σ2(Fo2)+
(a·P)2 +b·P] and P = [f·(maximum of 0 or Fo2)+(1 – f)·Fc ].
2
[36] B. M. Kraft, R. J. Lachicotte, W. D. Jones, J. Am. Chem. Soc.
2000, 122, 8559–8560.
[37] B. M. Kraft, R. J. Lachicotte, W. D. Jones, Organometallics
2002, 21, 727–731.
[38] a) H. M. Hoyt, F. E. Michael, R. G. Bergman, J. Am. Chem.
Soc. 2004, 126, 1018–1019; b) I. M. Piglosiewicz, S. Kraft, R.
[59] CCDC-618856 to -618858 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Received: August 30, 2006
Published Online: January 4, 2007
Eur. J. Inorg. Chem. 2007, 2839–2847
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2847