A combined experimental and computational study of unexpected C-F bond activation intermediates and selectivity in the reaction of pentafluorobenzene with a (PEt3)2Ni synthon

Article abstract of DOI:10.1021/om900176v

The reaction of the anthracene adduct (PEt₃)₂Ni(η ²-C₁₄H₁₀) with C₆F₅H or C₆F₆ provides a facile route to the isolable dinuclear adducts [(PEt₃)<

Full text of DOI:10.1021/om900176v

3842 Organometallics 2009, 28, 3842–3855
DOI: 10.1021/om900176v
A Combined Experimental and Computational Study of Unexpected
C-F Bond Activation Intermediates and Selectivity in the Reaction
of Pentafluorobenzene with a (PEt3)2Ni Synthon
Samuel A. Johnson,* Erin T. Taylor, and Sean J. Cruise
Department of Chemistry & Biochemistry, University of Windsor, Windsor, Ontario, Canada, N9B 3P4
Received March 5, 2009
The reaction of the anthracene adduct (PEt3)2Ni(η2-C14H10) with C6F5H or C6F6 provides a facile
route to the isolable dinuclear adducts [(PEt3)2Ni]2(μ-η2:η2-C6F5H) and [(PEt3)2Ni]2(μ-η2:η2-C6F6).
The Ni(PEt3)2 moieties reside on opposite faces of the fluoroarene ring, and bond lengths indicate
considerable loss of aromaticity. Solutions of [(PEt3)2Ni]2(μ-η2:η2-C6F5H) with added C6F5H
convert with >97% selectivity to the ortho-F activated compound trans-(PEt3)2NiF(2,3,4,5-
C6F4H). The addition of C6F5H to solutions of [(PEt3)2Ni]2(μ-η2:η2-C6F5H) also provided equili-
brium amounts of the mononuclear complex (PEt3)2Ni(η2-C6F5H) and the C-H activation product
trans-(PEt3)2NiH(C6F5) as intermediates. Increased C6F5H concentrations accelerate the rate of
C-F bond activation. The reaction of (PEt3)4Ni with C6F5H under similar conditions also provided
spectroscopic evidence for the presence of the binuclear adduct [(PEt3)2Ni]2(μ-η2:η2-C6F5H), the
mononuclear adduct (PEt3)2Ni(η2-C6F5H), and the C-H activation product trans-(PEt3)2NiH-
(C6F5) in the reaction mixture; however, unlike the C-F bond activation observed from pure
[(PEt3)2Ni]2(μ-η2:η2-C6F5H) and C6F5H, the reaction of Ni(PEt3)4 and C6F5H yields a mixture of
C-F activation products that included the para-F and meta-F activation products as major products.
This indicates that alternate mechanisms of C-F activation dominate with this source of the Ni
(PEt3)2 moiety under these conditions, suggestive of a radical mechanism. Density functional theory
calculations were carried out to provide insight into the 19F NMR spectra of these complexes, as well
as to gain understanding of the importance of phosphine donor choice in the thermodynamics of the
interconversion of the dinuclear complexes, mononuclear complexes, and C-H activation products.
Introduction
alternate synthetic route to these species,5,6 with the potential
to prepare compounds with previously unobtainable pat-
terns of fluorination.
Many modern pharmaceuticals and agrochemicals con-
tain partially fluorinated substituents,1,2 which are com-
monly accessed using selective fluorinating reagents.3 The
use of transition metal complexes in the catalytic activation4
and functionalization of C-F or C-H bonds in partially
fluorinated organics has been envisioned as an attractive
Zero-valent nickel phosphine complexes are attractive for
these transformations, not only due to the relatively low cost
of nickel compared to the heavier platinum group metals and
the commercial availability of the phosphine donor ligands,
but also because DFT calculations have predicted that these
nickel complexes should exhibit a decreased thermodynamic
propensity for C-H bond activation compared to their
heavier analogues,7 and thus might be expected to undergo
selective C-F bond activation even in the presence of more
reactive C-H bonds.8-10 Although there are a plethora of
examples of the selective C-F bond activation of perfluori-
nated arenes,11 only a few examples of the selective activation
*To whom correspondence should be addressed. E-mail: sjohnson@
uwindsor.ca. Fax: (519) 973-7098.
(1) Mueller, K.; Faeh, C.; Diederich, F. Science 2007, 317, 1881–1886.
(2) Brooke, G. M. J. Fluorine Chem. 1997, 86, 1–76.
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992–1012. (b) Prakash, G. K. S.; Chacko, S. Curr. Opin. Drug Discovery
Dev. 2008, 11, 793–802. (c) Shimizu, M.; Hiyama, T. Angew. Chem., Int.
Ed. 2005, 44, 214–231.
(4) (a) Schaub, T.; Backes, M.; Radius, U. J. Am. Chem. Soc. 2006,
128, 15964–15965. (b) Bohm, V. P. W.; Gstottmayr, C. W. K.; Wes-
kamp, T.; Herrmann, W. A. Angew. Chem., Int. Ed. 2001, 40, 3387–3389.
(c) Young, R. J.Jr; Grushin, V. V. Organometallics 1999, 18, 294–296.
(5) (a) Kiplinger, J. L.; Richmond, T. G.; Osterberg, C. E. Chem. Rev.
1994, 94, 373–431. (b) Burdeniuc, J.; Jedlicka, B.; Crabtree, R. H. Chem.
Ber. 1997, 130, 145–154. (c) Braun, T.; Perutz, R. N. Chem. Commun. 2002,
2749–2757. (d) Braun, T.; Perutz, R. N. Spec. Publ.-R. Soc. Chem. 2003,
287, 136–151. (e) Mazurek, U.; Schwarz, H. Chem. Commun. 2003, 1321–
1326. (f) Richmond, T. G. Angew. Chem., Int. Ed. 2000, 39, 3241–3244.
(6) Perutz, R. N. In Comprehensive Organometallic Chemsitry III;
Crabtree, R. H., Mingos, M. P., Eds.; Elsevier: New York, 2007, ; Vol. 1,
pp 725-756.
(7) Reinhold, M.; McGrady, J. E.; Perutz, R. N. J. Am. Chem. Soc.
2004, 126, 5268–5276.
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N. Dalton Trans. 2003, 4065–4074. (b) Clot, E.; Besora, M.; Maseras, F.;
Megret, C.; Eisenstein, O.; Oelckers, B.; Perutz Robin, N. Chem.
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Chem. Soc. 2008, 130, 17278–17280.
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r
pubs.acs.org/Organometallics
Published on Web 05/19/2009
2009 American Chemical Society

Article
Organometallics, Vol. 28, No. 13, 2009 3843
of the more synthetically useful partially fluorinated arenes
have been reported.12,13 For example, the oxidative addition
of C6F6 to a Ni(0) phosphine complex, obtained from
the reaction of PEt3 with Ni(COD)2, was reported as early
as 1977,14 albeit with difluorophosphoranes (F2PR3) as
unwanted byproducts when excess fluoroarene is used.15
However, the reaction of C6F5H with Ni(PEt3)4 was reported
to be unselective and produced a mixture of all three possible
C-F activation products, along with phosphoranes and
1,2,4,5-tetrafluorobenzene.15
The observation of difluorophosphoranes in the reaction of
Ni(PEt3)4 with C6F6 suggests that excess phosphine may
interfere with the reaction in these systems. The presence of
excess phosphine may also decrease the equilibrium con-
centration of the species Ni(PEt3)2 and Ni(PEt3)3 in favor of
the 18-electron complex Ni(PEt3)4. This may explain the lack
of selectivity in the reaction of pentafluorobenzene with
Ni(PEt3)4; this coordinatively saturated species is probably
only suited to oxidative addition reactions by a mechanism
that involves electron transfer.16 Herein we use the anthracene
adduct (PEt3)2Ni(η2-C14H10),17 which provides an opportunity
to examine the reaction of fluorinated arenes with a source of
the (PEt3)2Ni moiety in the absence of excess phosphine.
Figure 1. ORTEP depiction of the solid-state molecular struc-
ture of 1 as determined by X-ray crystallography. Hydrogen
atoms are omitted for clarity, and the 2-fold disorder of
the hydrogen atom associated with the aromatic ring is not
shown. The 30% probability ellipsoids are shown. Selected
˚
bond lengths (A): Ni(1)-C(1), 1.935(3); Ni(1)-C(2), 1.957(3);
Ni(1)-P(2), 2.1732(9); Ni(1)-P(1), 2.1790(9); Ni(2)-C(4),
1.933(3); Ni(2)-C(3), 1.952(3); Ni(2)-P(3), 2.1765(9); Ni(2)-
P(4), 2.1803(9); C(1)-C(2), 1.437(4); C(1)-C(6), 1.444(5);
C(2)-C(3), 1.460(4); C(3)-C(4), 1.431(4); C(4)-C(5), 1.447
(5); C(5)-C(6), 1.322(5); F(1)-C(1), 1.310(5); F(2)-C(2),
1.408(3); F(3)-C(3), 1.411(3); F(4)-C(4), 1.335(4); F(5)-C
(5), 1.365(4); F(6)-C(6), 1.360(4).
Results and Discussion
Synthesis and Characterization of a Dinuclear Ni(PEt3)2
Adduct of C6F5H. The reaction of a red solution of the
(PEt3)2Ni-anthracene adduct (PEt3)2Ni(η2-C14H10)17 with
∼4 equiv of pentafluorobenzene in pentane results in an
immediate precipitation of anthracene from the solution and
a color change to yellow. Analysis of the crude solution by
NMR spectroscopy reveals the adduct [(PEt3)2Ni]2(μ-η2:η2-
C6F5H) (1) as the only fluoroaromatic-containing product,
as shown in eq 1. Complex 1 was readily isolated by filtering
off the anthracene produced, followed by crystallization
from the pentane solution at -40 °C. A moderate excess of
C6F5H is required to drive this reaction to completion,
indicative of rapid equilibrium. This was verified by the
addition of anthracene to the isolated complex, which results
in a slight back reaction to produce the red (PEt3)2Ni-
Table 1. Crystallographic Data for Compounds 1, 2, and 5
1
2
5
empirical formula
C
30H61F5-
Ni2P4
C18H31F5-
NiP2
463.08
C30H60F6-
Ni2P4
775.56
fw
cryst syst
757.56
monoclinic
19.195(3)
10.0767(16)
41.554(6)
90.
105.297(6)
90.
7753(2)
P2/c
8
1.298
1.178
173(2)
55.0
83 977
orthorhombic monoclinic
˚
a (A)
9.224(3)
9.411(3)
25.879(8)
90.
90.
90.
2246.4(12)
P2(1)2(1)2(1) P2/c
4
1.369
1.046
173(2)
50.0
19.3710(19)
10.0468(10)
41.310(4)
90.
105.496(4)
90.
7747.4(13)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
space group
1
anthracene adduct and free C6F5H, as verified by H, 19F,
Z value
8
1.33
1.185
173(2)
55.0
84 942
17 524; 0.037
0.94-0.79
13 992
781
Dcalc (g/cm3)
μ(Mo KR) (mm-1
temperature (K)
2θmax (deg)
and 31P{1H} NMR spectroscopy.
)
total no. of reflns
21 391
no. unique reflns; Rint
transmn factors
no. with I g 2σ(I)
no. variables
17 559; 0.033 3950; 0.046
0.81-0.66
14 997
781
0.90-0.73
3763
241
reflns/params
R; wR2 (all data)
GOF
19.2
0.060; 0.12
1.147
15.6
0.053; 0.14
1.108
17.9
0.053; 0.098
0.976
-
3
˚
residual density (e /A ) 1.135; -0.555 1.032; -0.350 0.882; -0.839
ORTEP depiction of the solid-state molecular structure is
shown in Figure 1. X-ray crystallographic collection and
refinement parameters are included in Table 1. The structure
of 1 features two molecules in the asymmetric unit that have
identical connectivities. A pair of (PEt3)2Ni fragments are
bound to opposite faces of the C6F5H ring. The structure
features a disorder of the aromatic hydrogen atom. This
was modeled as a 2-fold disorder, with a 50% occupancy of
the hydrogen atom on both C(1) and C(4), although it
is impossible to rule out smaller partial occupancies at other
sites.
Single crystals suitable for structural analysis by X-ray
crystallography were obtained from this synthesis, and an
(12) (a) Whittlesey, M. K.; Perutz, R. N.; Moore, M. H. Chem.
Commun. 1996, 787–788. (b) Edelbach, B. L.; Jones, W. D. J. Am.
Chem. Soc. 1997, 119, 7734–7742.
(13) Schaub, T.; Fischer, P.; Steffen, A.; Braun, T.; Radius, U.; Mix,
A. J. Am. Chem. Soc. 2008, 130, 9304–9317.
(14) Fahey, D. R.; Mahan, J. E. J. Am. Chem. Soc. 1977, 99, 2501–2508.
(15) Cronin, L.; Higgitt, C. L.; Karch, R.; Perutz, R. N. Organo-
metallics 1997, 16, 4920–4928.
(16) Tsou, T. T.; Kochi, J. K. J. Am. Chem. Soc. 1979, 101, 6319–6332.
(17) Stanger, A.; Boese, R. J. Organomet. Chem. 1992, 430, 235–243.

3844 Organometallics, Vol. 28, No. 13, 2009
Johnson et al.
The structure of 1 features significant differences in the
Ni-C distances; the Ni(1)-C(1) and Ni(2)-C(4) bond
lengths of 1.935(3) and 1.933(3) A, respectively, are slightly
˚
shorter than the Ni(1)-C(2) and Ni(2)-C(3) bond lengths,
which are 1.957(3) and 1.952(3) A, respectively. The back-
˚
bonding of the Ni(PEt3)2 moieties to the arene ring results in
significant elongation of the coordinated double bonds from
typical aromatic values, with C(1)-C(2) and C(3)-C(4)
˚
distances of 1.437(4) and 1.431(4) A, respectively, as well
as considerable nonplanarity of the F-substituents. The
remaining C-C bond lengths of the C6F5H moiety are also
significantly perturbed by the back-donation. The C(5)-
˚
C(6) distance of 1.322(5) A is close to that expected for a
double bond, whereas the C(2)-C(3), C(4)-C(5), and C(6)-
C(1) distances of 1.460(4), 1.447(5), and 1.444(5) A, respec-
˚
tively, are closer to the values expected for single bonds.
There appears to be little precedent for such dinuclear
complexes bearing two metal centers η2-coordinated to a
single aromatic ring18 in the C-F activation literature,
despite related examples relevant to C-H activation.19
Alternate structures were suggested for related dinuclear
nickel complexes of hexakis(trifluoromethyl)benzene, but
were based solely on NMR data in the absence of X-ray
structural data, and it seems likely that the structures of
these complexes may be analogous to 1.20 The bonding in 1
is similar to that previously observed in the complex
[(tBu2PCH2CH2PtBu2)2Ni]2(μ-η2:η2-C6H6); however, it is
notable that the reaction of this complex with C6F6 did not
generate the dinuclear analogue [(tBu2PCH2CH2PtBu2)2-
Ni]2(μ-η2:η2-C6F6), but rather the mononuclear adduct
(tBu2PCH2CH2PtBu2)2Ni(η2-C6F6).21
Figure 2. Experimental (top) and modeled (bottom) variable-
temperature 19F NMR spectra for compound 1 obtained
at 282 MHz.
Scheme 1
1
The room-temperature 300 MHz H NMR spectrum of
1 in C6D6 features an upfield-shifted broad multiplet for
the hydrogen bound to the aromatic moiety at δ 3.12 and
resonances at δ 1.01 and 1.61 for the PEt3 ligands. The
1:36:24 integration of these peaks is consistent with the
formulation [(PEt3)2Ni]2(μ-η2:η2-C6F5H) assigned from
X-ray crystallography. The 282 MHz 19F{1H} NMR spec-
trum at room temperature displays five broad featureless
peaks at δ -134.9, -144.0, -155.2, -158.7, and -185.4 with
line widths varying from 107 to 153 Hz. The broadness of
these peaks is indicative of a fluxional process, but the
presence of five, rather than three, fluorine environments
indicates that rapid exchange via repeated concerted 1,2-shifts
of both Ni(PEt3)2 groups is not occurring at room tempera-
ture. A variable-temperature 19F NMR study was performed
to gain further insight into the nature of this fluxional process,
and the experimental spectra are shown at the top of Figure 2.
Upon cooling below room temperature, four of the reso-
nances exhibit temperature-dependent chemical shifts, but
of these four, only the peaks at -144.0, -155.2, and -185.4
broaden significantly. Further cooling to 200 K does not lead
to any observed decoalescence, but rather the sharpening of
these resonances into unresolved multiplets.
provides insight into the nature of the chemical exchange
in this compound. It was hypothesized that the energy
difference between the three possible isomers of [(PEt3)2Ni]2-
( μ-η2:η2-C6F5H) might strongly favor one isomer, but allow
for the conversion to an equilibrium amount of a second low-
energy isomer at higher temperatures. Density functional
theory (DFT) calculations using the B3LYP functional and
TZVP basis set were performed on the model complexes
[(PH3)2Ni]2(μ-η2:η2-C6F5H) 1a-cH and [(PMe3)2Ni]2-
( μ-η2:η2-C6F5H) 1a-cMe, to estimate the energy differences
between the three isomers of the model complexes, shown in
Scheme 1. Calculations with both PH3 and PMe3 as the
model phosphines agree with the crystallographic observa-
tion that the lowest energy isomer is 1a, which has the nickel
centers at the 1,2- and 3,4-positions of the pentafluorophenyl
ring, where the aromatic carbon bearing the hydrogen
substituent is considered the 1-position. Conversion to the
isomers with the nickel centers at the 2,3- and 4,5-positions,
which are labeled 1bH and 1bMe for the PH3- and PMe3-
containing model complexes, is predicted to have a ΔG298 K
The observation of line broadening without any appa-
rent decoalescence is atypical for fluxional processes and
(18) Beck, V.; O’Hare, D. J. Organomet. Chem. 2004, 689, 3920–3938.
(19) Chin, R. M.; Dong, L.; Duckett, S. B.; Jones, W. D. Organome-
tallics 1992, 11, 871–876.
(20) (a) Stone, F. G. A.; Browning, J.; Cundy, C. S.; Green, M.
J. Chem. Soc. A 1971, 448–452. (b) Browning, J.; Green, M.; Spencer, J.
L.; Stone, F. G. A. J. Chem. Soc., Dalton Trans. 1974, 97–101.
(21) Bach, I.; Poerschke, K.-R.; Goddard, R.; Kopiske, C.; Krueger,
C.; Rufinska, A.; Seevogel, K. Organometallics 1996, 15, 4959–4966.
of 3.7 and 4.6 kcal mol-1, respectively. The isomer with the
3
nickel centers at the 1,2- and 5,6-positions, labeled 1c, is
predicted to be slightly more favorable than 1b, but still

Article
Organometallics, Vol. 28, No. 13, 2009 3845
higher energy than 1a. The conversion of 1aH to 1cH is
Table 2. Experimental 1a and Calculated 1cMe NMR Shifts in
a
predicted to have a ΔG298 K of 1.3 kcal mol-1, and the
ppm versus CFCl3
3
conversion of 1aMe to 1cMe is predicted to have a ΔG298 K of
1cMe 19F change in 19F 1cMe 19F change in 19F
chemical chemical shifts chemical chemical shifts
2.0 kcal mol-1. The differences in Gibbs free energies for the
3
label 1a 19
in 1a shifts
F
shifts via
1,2-shift
via
1,2-shift
shifts via
1,3-shift
via
1,3-shifts
PMe3 model complexes can be used to estimate the equilibria
between the analogous isomers bearing PEt3. At 298 K, it is
anticipated that ∼97% of the sample will exist as isomer
F2 -136.4 -178.7
F6 -146.3 -134.2
F3 -154.6 -159.4
F5 -163.3 -187.7
F4 -182.0 -156.8
42.3
-12.1
4.8
24.4
-25.2
-134.2
-178.7
-187.7
-159.4
-156.8
-2.2
32.4
33.1
-3.9
-25.2
1a, the remaining 3% as 1c. The 4.6 kcal mol-1 free energy
3
difference for 1b indicates that it will be a negligible 0.04% of
the sample at 298 K. At 200 K, the ratios change consider-
ably, and the predicted percentages of 1a and 1c are 99.4 and
0.6, respectively.
a Bolded changes in chemical shift indicate resonances that are
predicted to be most shifted compared to 1a by the proposed exchange
mechanism.
To create a model to fit the observed variable-temperature
19F NMR spectra, chemical shifts for the 1c isomer are
required, but were not available experimentally; however,
it was found that DFT could provide an accurate estimate of
the 19F NMR chemical shifts for this class of complexes. The
calculated chemical shifts for the model complexes [(PMe3)2-
Ni]2(μ-η2:η2-C6F5H), 1aMe, are δ -136.5, -146.3, -154.6, -
163.3, and -182.0, which has a maximum error of 4.6 ppm
and a root-mean-square error of 3.1 ppm compared to the
experimentally observed spectrum for 1. This accuracy in
predicted shifts described further confirms that 1a is the
predominant isomer in solution. It also allows assignment of
the experimentally observed resonances at δ -134.9, -144.0,
-155.2, -158.7, and -185.4 as the 2, 6, 3, 5, and 4 sites of
the C6F5H moiety, respectively. The addition of C6F5D to 1
(vide infra) generated an equilibrium amount of [(PEt3)2-
Ni]2(μ-η2:η2-C6F5D), 1-d1, which allowed experimental
verification that the peaks at δ -134.9 and -144.0 in 1 are
ortho to the C-H/D bond, due to the observable 0.3 ppm
isotope shift22 of these resonances compared to 1-d1. In
comparison, the alternate isomer 1b is predicted to exhibit
19F NMR resonances at δ -149.3, -159.3, -160.8, -169.6,
and -189.6 for the 6, 4, 3, 2, and 5 sites of the aromatic ring.
The 2c isomer has predicted 19F NMR resonances at
δ -134.2, -156.8, -159.4, -178.7, and -187.6 for the 6, 4,
3, 2, and 5 sites of the aromatic ring. That 1b and 1c are not
the predominant isomers in solution for 1 is confirmed by the
large difference between the observed and predicted shifts;
the maximum errors in predicted shifts are 15.3 and 20 ppm,
and the root-mean-square errors are 10.0 and 11.1 ppm,
respectively.
Two plausible intramolecular mechanisms can be envi-
sioned for the exchange process observed in the 19F NMR
of 1a. One possibility is that both metals do a simultaneous
1,2-shift in the same direction around the ring, which is
shown at the top of Scheme 2. The simultaneous clockwise or
counterclockwise 1,2-shifts starting from 1a can produce
either isomer 1b or 1c, but further shifts are not consistent
with the observed variable-temperature 19F spectra, because
this would allow for the exchange of the pairs of ortho- and
meta-fluorine environments and result in only three fluorine
environments in the high-temperature spectra. The require-
ment that only certain 1,2-shifts can occur is counterintui-
tive, particularly because an additional counterclockwise
1,2-shift of both Ni fragments in 1c would generate the
lowest energy isomer 1a.
Scheme 2
could occur via an η4-bound intermediate.17 This mechanism
of exchange is shown at the bottom of Scheme 2. The
localized formal double and single bonds of the fluorinated
rings are maintained despite the 1,3-shifts. Thus even
repeated 1,3-shifts to generate both the 1b and 1c iso-
mers do not exchange the 2,6-ortho and 3,5-meta fluorine
environments.
These two mechanisms exchange different 19F environ-
ments and thus should be differentiable using the energies
and 19F NMR chemical shifts determined by DFT calcula-
tions. The simultaneous 1,2-shifts convert the F2, F3, F4, F5,
and F6 positions of the major isomer 1a to the F6, F5, F4, F3,
and F2 positions of the predicted minor isomer 1c, whereas
the consecutive 1,3-shifts convert the F2-F6 positions of the
major isomer 1a to the identically numbered F2-F6 positions
of the 1c isomer. Table 2 summarizes the calculated 19F NMR
chemical shifts for 1a, the chemical shifts they exchange
with in 1c via either mechanism, and the differences in shifts.
Only the mechanism that involves 1,3-shifts of the Ni-
(PEt3)2 moieties in 1a is consistent with the observed vari-
able-temperature 19F NMR spectra, because it predicts that
an exchange with the thermally accessible 1c isomer should
lead to the most broadening and shifting of the resonances
This limitation is not encountered with an alternate
mechanism of exchange between the three isomers that
involves 1,3-shifts of only one Ni moiety at a time, which
(22) Keen, A. L.; Doster, M.; Johnson, S. A. J. Am. Chem. Soc. 2007,
129, 810–819.

3846 Organometallics, Vol. 28, No. 13, 2009
Johnson et al.
associated with the F6 (δ -146.3), F3 (δ -154.6), and F4
(δ -182.0) nuclei of 1a, as labeled in Scheme 2, due to the
large chemical shift differences of 32.4, 33.1, and -25.2 ppm
between these two isomers. The model not only correctly
predicts which resonances should broaden at higher tem-
peratures but also correctly predicts the direction in which
the chemical shifts should change. The F2 and F5 nuclei have
negligibly different shifts in the two isomers if the exchange
occurs via 1,3-shifts, which is consistent with the fast-
exchange limit being observed even at low temperatures.
A quantitative model of the variable-temperature 19F
NMR spectra from 208 to 298 K was performed using the
experimentally observed low-temperature chemical shifts for
1 and using the calculated shifts of 1c for the second lowest
energy isomer. Attempts to model the spectrum using the
Figure 3. Variable-temperature 31P{1H} NMR spectra for
complex 1 (left), obtained at 121.5 MHz, and modeled spectra
(right).
phosphorus environments. A 243 K 31P-COSY spectrum
confirms that the phosphine environments in exchange are
on different Ni centers; cross-peaks are observed between the
31P resonances at δ 24.7 and 14.0, as well the resonances at δ
20.6 and 17.5, not the phosphorus environments in exchange
form 263 to 293 K. Thus the exchange process observed
in the 31P{1H} NMR spectra between 263 to 293 K
must involve multiple 1,3-shifts that exchange the identities
of the Nia and Nib centers shown in Scheme 2. The only
mechanism that is viable for the exchange of pairs of
phosphorus environments is shown in Scheme 2. The Ni-
(PEt3)2 fragments must make three consecutive 1,3-shifts to
exchange the locations of the Nia and Nib in the 1a isomers
on the top and the bottom of the scheme. As drawn, these
shifts allow for the exchange of the PEt3 moieties labeled
La and Lc as well as Lb and Ld. An alternate motion where the
Ni(PEt3)2 moieties spin as they undergo shifts, related exam-
ples of which have been observed in mononuclear aromatic
adducts of related mononuclear Ni complexes23 and calcu-
lated for mononuclear C6F6 adducts,7 is not viable; the
exchange in the clockwise and counterclockwise directions
depicted in Scheme 2 would result in the exchange of different
pairs of phosphorus nuclei, but microscopic reversibility
requires that both pathways have the same barrier. The
observation that the barrier of 31P environment exchange is
greater than that of the barrier of 19F exchange is consistent
with the fact that the 31P exchange requires the intermediacy
of the highest energy isomer, 1b, the activation barrier to
which is presumably greater than that between 1a and 1c.
Selective Ortho C-F bond Activation of C6F5H. In the
presence of added C6F5H, solutions of 1 in a variety of
hydrocarbon or ethereal solvents convert to give primarily
(>97%) the complex derived from C-F bond activation
at the 1-position, trans-(PEt3)2NiF(2,3,4,5-C6F4H) (2), as
shown in eq 2. The rate of C-F bond activation is strongly
dependent on the conditions (vide infra), but is slower than
the formation of 1 shown in eq 1.
DFT-calculated 2 kcal mol-1 energy difference between
3
1a and 1c failed; the best fit to the experimental data was
obtained by using a 1.5 kcal mol-1 energy difference. These
3
model spectra are shown at the bottom of Figure 2 and
correctly predict the main features of the observed spectrum.
The rate constant data from this analysis predicts an
Arrhenius activation energy of 10 kcal mol-1
.
3
At higher temperatures an alternate exchange pathway
appears to be accessible, and the pair of meta resonances
observed at -155.2, -158.7 in the room-temperature spec-
trum coalesce at 328 K. Similarly, the ortho resonance at
δ -134.9, -144.0 in the room-temperature spectrum broad-
en significantly and near coalescence at 338 K. These high-
temperature NMR spectra can be modeled using the
Arrhenius equation to estimate a 19.5 kcal mol-1 activation
3
energy for this process or the Eyring equation to estimate a
ΔHq of 18.8 kcal mol-1 and a ΔSq of 7.3 kcal K-1 mol-1
.
3
3
3
The room-temperature 121.5 MHz 31P{1H}NMR spec-
trum of 1 in C6D6 features a broad asymmetric multiplet at
δ 18, with a peak width at half height of 870 Hz. A variable-
temperature 31P{1H} NMR study was performed in C7D8
and is shown in Figure 3. At 243 K a slow exchange limit
spectrum is obtained, with four distinct muliplets, which is
consistent with the solid-state structure. No further change is
observed on cooling as low as 223 K. Upon warming, these
peaks broaden and coalesce near 293 K. These spectra are
readily modeled from 263 to 293 K by assuming only two
pairs of 31P environments are in exchange, namely, those at
δ 24.7 and 17.5 as well as the pair of shifts at δ 20.6 and 14.0.
This simple model predicts that two resonances should be
observed at higher temperatures; however, the nature of this
exchange is clearly more complicated, with additional
exchange pathways available at higher temperatures that
exchange all four phosphorus environments. Thus, rather
than sharpening at higher temperatures, the signals all
coalesce to give a broad resonance at 303 K, which could
not be modeled. At higher temperatures, decomposition by
rapid C-F bond activation occurred, as evidenced by the
appearance of a doublet at δ 12.5. This decomposition
prevented the collection of a fast exchange limit spectrum.
The rates of exchange modeled from 263 to 293 K can
be used to estimate an Arrhenius activation energy of
13.3 kcal mol-1 or plotted using the Eyring equation
3
to estimate a ΔHq of 12.7 kcal mol-1 and a ΔSq of -
3
13.2 kcal K-1 mol-1. These values are significantly larger
Crystals suitable for analysis by X-ray crystallography
were obtained from slow evaporation of a pentane solution,
3
3
than the Arrhenius activation energy of 10.0 kcal mol-1
3
predicted for the conversion of 1a and 1c from the variable-
temperature 19F NMR spectra, which implies that the
conversion between 1a and 1c does not exchange any
(23) Atesin, T. A.; Li, T.; Lachaize, S.; Garcia, J. J.; Jones, W. D.
Organometallics 2008, 27, 3811–3817.

Article
Organometallics, Vol. 28, No. 13, 2009 3847
activation in pentafluorobenzene, all occur in the para-
position.12,13,26 A recent example of catalytic hydrodefluor-
ination of pentafluorobenzene to produce 1,2,4,5-tetrafluor-
obenzene has been reported, but is proposed to occur via
aryne formation from a pentafluorophenyl complex formed
by C-H bond activation.27 Regardless of the differing
mechanism, this suggests that an initial interaction of the
C-H bond with the Ni(PEt3)2 fragment could conceivably
be responsible for the unexpected ortho C-F bond selectivity
in the formation of 2, and thus attempts were made to gain
insight into the mechanism of this reaction.
The mechanism of C-F bond oxidative addition in fluori-
nated aromatics commonly occurs via an intermediate where
the arene complex is η2-coordinated to the metal complex,28
although alternate mechanisms that bypass this inter-
mediate9 or involve nucleophilic attack or electron transfer
are well-documented.6,12 The hexafluorobenzene and octa-
fluoronaphthalene adducts (tBu2PCH2CH2PtBu2)2Ni(η2-
C6F6)21 and (PEt3)2Ni(η2-C10F8)29 are known, and both
undergo subsequent slow C-F bond activation, which sup-
ports the presence of arene-coordinated intermediates in
the reaction of Ni(0) phosphine adducts with fluorinated
aromatics. However, recent evidence suggests that even this
traditional mechanism may feature surprising subtleties,
such as phosphine assistance30 in the C-F bond clea-
vage step and rapid, reversible C-H bond activation prior
to C-F bond activation.10
A mechanism that involves a mononuclear intermediate is
still feasible for the conversion of 1 to 2, if the mononuclear
species 3 is generated in equilibrium with 1 and C6F5H
prior to the C-F activation step, as shown in Scheme 3 as
mechanism A. We previously observed that the reaction of
the phenanthrene adduct (PEt3)2Ni(η2-C10H14) with 1,2,4,5-
tetrafluorobenzene produces an equilibrium amount of
the C-H activation product trans-(PEt3)2NiH(2,3,5,6-
C6F4H).10 Thus it seems likely that if the mononuclear
adduct 3 is an intermediate, hydride 4 could also be present
in equilibrium, as shown in Scheme 3, mechanism A. Either
3 or 4 could conceivably convert to 2.
An alternate pathway, labeled mechanism B in Scheme 3,
where initial C-F bond activation occurs directly from the
binuclear adduct 1, is also plausible. This intermediate could
then react with C6F5H to generate the C-F activation
product 2 and half an equivalent of 1. This latter mechanism
could explain the unusual selectivity of C-F bond activa-
tion. More complicated mechanisms that involve radical
intermediates that arise from electron transfer cannot be
excluded, but are omitted here due to lack of supporting
evidence.
The addition of C6F5H (0.5-90 equiv) to a solution of 1 in
C6D6 produced solutions that still contain 1, as determined
by 1H, 19F, and 31P{1H} NMR spectroscopy; however, two
other species were also observed in equilibrium amounts.
The highest concentration equilibrium species is assignable
Figure 4. ORTEP depiction of the solid-state molecular struc-
ture of 2 as determined by X-ray crystallography. Hydrogen
atoms are omitted for clarity, and 30% probability ellipsoids are
˚
shown. Selected bond lengths (A): Ni(1)-C(1), 1.858(4); Ni(1)-
F(1), 1.916(3); Ni(1)-P(2), 2.1899(14); Ni(1)-P(1), 2.1907(14).
Selected bond angles (deg): P(2)-Ni(1)-P(1), 171.41(6); C(1)-
Ni(1)-F(1), 178.0(2); P(1)-Ni(1)-C(1), 92.62(14); P(2)-Ni-
(1)-C(1), 93.35(14); Ni(1)-C(1)-C(2), 121.67(35); Ni(1)-
C(1)-C(6), 121.35(36); C(2)-C(1)-C(6), 117.0(4).
and an ORTEP depiction of the solid-state molecular
structure is shown in Figure 4. The connectivity in 2 demon-
strates that it is produced from the activation of the fluorine
substituent ortho to hydrogen in C6F5H. The structural
features of square-planar 2 resemble those of the known
analogue trans-(PEt3)2NiF(C6F5).15 The Ni-C(1) bond
˚
˚
length in 2 of 1.858(4) A is only 0.022(6) A longer than that
of trans-(PEt3)2NiF(C6F5), whereas the Ni(1)-F(1) distance
˚
˚
of 1.916(3) A is longer by a more significant 0.080(6) A. The
most notable difference in angles is the C(2)-C(1)-C(6)
angle of 117.0(4)°, which is significantly larger than the
previously reported analogous angle in trans-(PEt3)2NiF-
(C6F5) of 112.4(7)°.15
The connectivity of 2 was confirmed using 19F NMR
spectroscopy, where four aromatic F multiplets are observed
at δ -116.9, -142.7, -160.2, and -166.1 and the Ni fluoride
resonance at δ -380.7 is a doublet of triplets with a 4JFF of
11.0 Hz to a single ortho-F and a 47.6 Hz coupling to two
identical phosphorus nuclei. The 31P{1H} NMR spectrum
consists of a doublet of multiplets centered at δ -12.5, with a
47.6 Hz coupling to the terminal fluoride and unresolved
coupling to the remaining aromatic fluorine nuclei. The 1H
NMR features a complex multiplet at δ 6.89, corresponding
to the aromatic hydrogen.
Equilibrium Mononuclear Adduct Formation and C-H
Bond Activation. The selectivity of the Ni(PEt3)2 moiety for
C-F bond activation at the ortho-F is atypical for penta-
fluorobenzene C-F activation. Nucleophilic substitution at
C6F5H typically occurs at the 3-position, para with respect to
the hydrogen substituent, and in some cases in the 2-position,
meta with respect to the H substituent. Nucleophilic sub-
stitution at the 1-position is only rarely observed, and never
with high selectivity.2,24 Unselective ortho activation
products are known,15,25 but of the few previous examples
of transition metal complexes capable of selective C-F bond
(27) Reade, S. P.; Mahon, M. F.; Whittlesey, M. K. J. Am. Chem.
Soc. 2009, 131, 1847–1861.
(28) (a) Harman, W. D. Top. Organomet. Chem. 2004; 7, 95-127.(b)
Harman, W. D. Coord. Chem. Rev. 2004, 248, 853–866. (c) Begum, R.
A.; Sharp, P. R. Organometallics 2005, 24, 2670–2678.
(29) Braun, T.; Cronin, L.; Higgitt, C. L.; McGrady, J. E.; Perutz, R.
N.; Reinhold, M. New J. Chem. 2001, 25, 19–21.
(30) (a) Erhardt, S.; Macgregor, S. A. J. Am. Chem. Soc. 2008, 130,
15490–15498. (b) Nova, A.; Erhardt, S.; Jasim, N. A.; Perutz, R. N.;
Macgregor, S. A.; McGrady, J. E.; Whitwood, A. C. J. Am. Chem. Soc.
2008, 130, 15499–15511.
(24) (a) Burdon, J. Tetrahedron 1965, 21, 3373–3380. (b) Brooke, G.
M.; Burdon, J.; Tatlow, J. C. J. Chem. Soc. 1962, 3253–3255. (c) Castle,
M. D.; Plevey, R. G. J. Fluorine Chem. 1973, 2, 431–433.
(25) Kraft, B. M.; Jones, W. D. J. Organomet. Chem. 2002, 658,
132–140.
(26) (a) Edelbach, B. L.; Rahman, A. K. F.; Lachicotte, R. J.; Jones,
W. D. Organometallics 1999, 18, 3170–3177. (b) Doster, M. E.; Johnson,
S. A. Angew. Chem., Int. Ed. 2009, 48, 2185–2187.

3848 Organometallics, Vol. 28, No. 13, 2009
Johnson et al.
Scheme 3
as the mononuclear adduct (PEt3)2Ni(η2-C6F5H), 3. This
complex displays a 31P{1H} NMR resonance at δ 15.9 and
three resonances in the 19F NMR spectrum at δ -138.8,
-164.9, and -171.3, with relative integrals of 2:2:1. The
resonance at δ -138.8 has a 3JFH of 11.5 Hz, consistent with
its identity as the ortho-F, which identifies the resonances at
δ -164.9 and -171.3 as the meta- and para-F resonances,
respectively. The presence of only three 19F resonances for a
solution of 3 in C7D8, even as low as 223 K, implies a rapidly
fluxional compound where the (PEt3)2Ni moiety readily
moves between η2-coordination of adjacent carbon atoms
of the π-system with a negligible barrier, consistent with both
calculations7 and experimental precedent23 for related mo-
Scheme 4
1
lecules. The H NMR spectrum is also consistent with this
fluxionality and displays a multiplet resonance at δ 5.16 that
is a triplet of triplets, with JHF values of 11.4 and 4.9 Hz.
Signals for the PEt3 groups observed at 0.85 and 1.39 were
also attributable to mononuclear complex 3.
in the predicted shifts are 3.0 and 3.1 ppm for 3aMe and 3cMe
,
respectively. These results support the assignment of the 19F
chemical shifts of 3, but do not help the assignment if isomer
a or c is predominant in the solution.
DFT calculations were performed on the η2-isomers of the
model complexes (PH3)2Ni(η2-C6F5H), 3a-cH, and
(PMe3)2Ni(η2-C6F5H), 3a-cMe, shown in Scheme 4. These
calculations indicate that the isomers a and c, where the Ni-
(PR3)2 moiety is bound to the 1,2 and 3,4 sites of the aromatic
ring, respectively (where the hydrogen bearing carbon is
labeled the 1 site), are very close in energy and should both
be present at room temperature, whereas the 2,3 isomer
b should not be present in significant amounts. The calculated
19F NMR shifts for 3aMe, obtained by averaging the pairs of
ortho and meta environments, are δ -142.6 for the ortho-
fluorines, δ -168.3 for the meta-fluorines, and δ -169.9 for the
para-fluorine. Surprisingly, the calculated shifts for the 3cMe
isomer are nearly identical. The ortho, meta, and para fluo-
rine substituents are predicted to display chemical shifts of
δ-143.4, -166.7, and -169.2. Both model isomers are in good
agreement with the observed shifts. Model 3aMe has a max-
imum error in chemical shifts of 3.9 ppm, whereas model 3cMe
has a maximum error of 4.6 ppm, compared to the experimen-
tally observed spectrum for 3. The root-mean-square errors
The second species observed in the mixtures of 1 and
C6F5H was an equilibrium amount of hydride 4, the presence
of which was verified by the observation of a broad, barely
1
resolvable triplet resonance in the H NMR spectrum at
δ -14.92. In C7D8 solutions of 1 and C6F5H, this signal
sharpens upon cooling to 243 K to reveal a triplet of triplets
of triplets with coupling to two equivalent 31P nuclei with a
2JPH value of 68.4 Hz, two equivalent ortho-F nuclei with a
5
4JFH value of 10.5 Hz, and a JFH value of 4.7 Hz. This
resonance is comparable to the known analogue trans-
(PEt3)2NiH(2,3,5,6-C6F4H),10 which displays a hydride
2
1H NMR resonance at δ -14.3, with a JPH value of 67.7
4
5
Hz as well as JFH and JFH constants of 9.5 and 4.2 Hz,
respectively. Resonances for hydride 4 were observed in the
19F{1H} NMR spectrum at δ -114.1, -162.5, and -163.9
and are comparable to those observed for the known struc-
turally related fluoride trans-(PEt3)2NiF(C6F5), which
displays aromatic resonances at δ -115.4, -161.1, and
-163.8.15 Further evidence that the 19F signals observed

Article
Organometallics, Vol. 28, No. 13, 2009 3849
Scheme 5
are indeed hydride 4 was obtained by comparing the 19F{1H}
and 19F NMR spectra. The ortho-F multiplet at δ -114.1
displays a JFH of 10.5 Hz, and the meta-F resonance at
4
δ -163.9 displays a 5JFH of 4.8 Hz, which are both consistent
with the coupling constants of the hydride signal observed in
1
the H NMR of 4. The para-F resonance at δ -162.5 is
a triplet of triplets with JFF values of 20.0 and 3.6 Hz, but no
resolved coupling to the hydride. The 31P{1H} resonance
associated with 4 was observed at δ 23.4, and its identity was
verified by the observation of a 2JPH value of ∼70 Hz in the
1H-coupled 31P NMR spectrum.
Figure 5. Left: Concentration of 1 and C6F5H versus time in the
reaction of 1.2 equiv of C6F5D with 1 at 303 K. Experimentally
determined concentration of 1 and C6F5H are shown as boxes
and circles, respectively. Lines show the simulated kinetic data.
Right: Experimentally determined concentration of 3 versus
time. Experimental data points are shown as diamonds, with
lines showing simulated kinetics data. Simulated data for the
concentration of 3-d1 are included for comparison.
These data can be used to estimate an equilibrium constant
for the conversion of 3 to 4 of 0.25(3), which corresponds to
a ΔG°303 K value of +0.8(1) kcal mol-1 for this conversion.
3
By monitoring reagent concentrations versus the internal
standard Ph3SiF, it was possible to determine an equilibrium
constant of 0.011(4) for the equilibrium conversion of the
dinuclear complex 1 and C6F5H to 2 equiv of the mono-
nuclear complex 2 shown in Scheme 1. The large variation in
this number reflects the sensitivity of this equilibrium to the
changing C6F5H concentration. The equilibrium constant
The exact rate law for the conversion of 1 to 2 via
mechanism A is complicated due to the side-equilibrium
with hydride 4 and the potential for either the first step, the
formation of mononuclear 3, or the second step, the conver-
sion of 3 to 2, to be rate determining. It proved possible to
examine the rate of the initial equilibrium formation of the
mononuclear complex 3 via the reaction of 1 with C6F5D, as
shown in Scheme 5.
corresponds to a ΔG°303 K value of +2.7(2) kcal mol-1 for
3
this conversion. It follows that the ΔG°303 K value for the
conversion of the dinuclear complex 1 and C6F5H to 2 equiv
of hydride 4 is +4.3(3) kcal mol-1
.
3
An experiment where 20 equiv of C6F5H was added to
a C7D8 solution of 20 mg of 1 at low temperatures and then
transferred into a NMR probe precooled to 243 K provided
evidence that C-H activation is slow at this temperature.
Only trace amounts of 4 were observed, and other than 1 and
C6F5H, the only other significant 19F and 31P{1H} NMR
resonances observed were those associated with 3. Warming
to 273 K and monitoring by 19F and 31P{1H} NMR spec-
troscopy, the resonances associated with hydride 4 are
observed, which indicates that C-H bond activation occurs
rapidly well below room temperature.
This equilibration was studied using a 0.028 M solution of
1 in C6D6, with 1.2 equiv of added C6F5D. The progress of
the reaction was followed by monitoring the decrease in the
concentration of 1 and the appearance of C6F5H by 1H
NMR spectroscopy at 303 K. Equilibrium was reached
within 40 min. An initial model for the rate of equilibration
was performed with the rates of all four permutations of the
conversion between 1/1-d1 and C6F5H/C6F5D to 3/3-d1
assumed identical. The equilibrium constant for the reaction
of 0.07 was determined from the 1H NMR spectrum and was
assumed to be unchanged by secondary isotope effects.
The rates k1 and k-1 were determined to be 0.070 and
The observation of species 3 and 4 does not necessarily
rule out the possibility of mechanism B as the lowest energy
pathway for C-F bond activation, but these mechanisms
should be distinguishable by monitoring the effect of added
C6F5H on the rate of conversion of 1 to 2, because the rate
law derived from mechanism B has no dependence on
C6F5H concentration. Qualitatively, it is evident that the
addition of C6F5H accelerates the reaction. In the presence
of ∼90 equiv of C6F5H, ∼25 mM solutions of 1 in C6D6
converts to 2 over the course of approximately 5.5 h at
303 K. Under identical conditions, solutions with ∼20 and
9 equiv of C6F5H take ∼17 and 33 h to go to completion,
respectively. A solution with 2.8 equiv of C6F5H barely
reached two half-lives over a 48 h period. It is therefore
unlikely that mechanism B contributes significantly to
the C-F bond activation pathway, whereas mechanism
A appears plausible.
10 M-1 s-1, respectively, from this preliminary analysis.
3
This model provided an excellent fit for the concentration
of 1 and C6F5H versus time, but failed to accurately predict
the concentration of 3 versus time. With these rate constants,
it can be shown that the initial concentrations of 3 and 3-d1
should be nearly equivalent and increase steadily for ∼100 s
according to this model. The slight conversion to equilibrium
concentrations of 3 and 3-d1 then occurs gradually. The
experimental concentration of 3 versus time is shown on
the right of Figure 5 and does not fit this model, as it
continually increases as C6F5H is liberated. A second model
was tested with an additional equilibrium reaction that could
interconvert 3 and C6F5D to 3-d1 and C6F5H. If this reaction
was treated as being significantly faster than the conversions

3850 Organometallics, Vol. 28, No. 13, 2009
Johnson et al.
of 1/1-d1 and C6F5H/C6F5D to 3/3-d1, the rate constants k1
,
and k-1 were determined to be 0.04 and 5.7 M-1 s-1
respectively. The lowest rate constant for the conversion
3
of 3 and C6F5D to 3-d1 and C6F5H that was found to fit the
observed concentration of 3 was 5 M-1 s-1, with the reverse
3
reaction assumed to occur at the same rate; a larger rate
constant cannot be ruled out, but does not significantly
affect the result of the simulation. The simulated lines31
and experimental data points from this model are shown in
Figure 5.
The product distribution after conversion of this mixture
to 2 and 2-d1 provided further insight into the C-F activa-
tion reaction mechanism. The ratio of 2 to 2-d1 formed
can be determined from integration of the 19F NMR signal
of the F nucleus in the 5-position of the aromatic ring,
adjacent to the H/D nucleus. The resonance at -142.7 for
2 is accompanied by a resonance at -143.0 for 2-d1, which
are similar in intensity. A mechanism where the hydride 4 is
an immediate precursor to the C-F bond activation transi-
tion state therefore seems unlikely, as a primary kinetic
isotope effect should significantly favor 4 over 4-d1.10 How-
ever, it is impossible from this data to rule out a smaller
secondary isotope effect in this reaction.
The C-F bond activation that forms 2 is considerably
slower than the rate of formation of 3, so the formation of the
mononuclear complex is not rate determining under most
reaction conditions. However, the rapid rate at which both 1
and 3 transfer to Ni(PEt3)2 fragments to C6F5H raises
questions regarding the nature of the reaction mechanism.
It is not clear whether the expected mechanism, in which
3 converts from a π-bound ground state to a transition state
with the ortho C-F bond σ-bound to the Ni(PEt3)2 moiety,
must occur in an intramolecular fashion7 or if both intra-
molecular and intermolecular pathways are accessible to
produce the intermediate to the C-F activation product 2.
Attempts to model the reaction kinetics of the conversion
of 1 to 2 in the presence of excess C6F5H were made, but the
observed reaction kinetics proved poorly reproducible,
with spectroscopically identical samples of 1 resulting in
moderately different reaction rates.
Figure 6. ORTEP depiction of the solid-state molecular struc-
ture of 5 as determined by X-ray crystallography. Hydrogen
atoms are omitted for clarity. The 30% probability ellipsoids are
˚
shown. Selected bond lengths (A): C(1)-C(2) 1.438(3); C(2)-
C(3), 1.460(3); C(3)-C(4), 1.436(3); C(4)-C(5), 1.451(4); C(5)-
C(6), 1.323(4); C(6)-C(1), 1.448(4); Ni(1)-C(1), 1.910(2); Ni
(1)-C(2), 1.953(2); Ni(1)-P(2), 2.1827(7); Ni(1)-P(1), 2.1853
(7); Ni(2)-C(4), 1.907(2); Ni(2)-C(3), 1.953(2);Ni(2)-P(3),
2.1791(7); Ni(2)-P(4), 2.1821(7). Selected angles (deg):
P(2)-Ni(1)-P(1) 113.87(3); P(3)-Ni(2)-P(4), 110.36(3).
Crystals suitable for characterization by X-ray crystal-
lography were obtained by cooling a pentane solution of 5 to
-40 °C, and an ORTEP depiction of the solid-state mole-
cular structure is shown in Figure 6. The unit cell parameters
for 5 are nearly identical to those observed for 1, and the
gross structural details are also similar. The structure of
5 features greater distortions in the Ni bonding to the
aromatic rings than was observed in 1. The Ni(1)-C(2)
˚
and Ni(2)-C(3) bond lengths in 5 are both 1.953(2) A,
similar to the analogous bond lengths in 1; however, the
Ni(1)-C(1) and Ni(2)-C(4) bond lengths of 1.910(2) and
1.907(2) A, respectively, are significantly shorter than the
˚
˚
related bond lengths in 1 of 1.935(3) and 1.957(3) A. The
C-C bond distances of the C6F6 ring are altered from their
aromatic lengths in a manner similar to that observed in 1,
˚
Synthesis of a Dinuclear Ni(PEt3)2 Adduct of C6F6. Pre-
vious reports of the activation of C6F6 with Ni(PEt3)2 pre-
cursors failed to observe any intermediates.14,15 However,
similar to the synthesis of 1, the reaction of the anthracene
adduct (PEt3)2Ni(η2-C14H10) with C6F6 in pentane proceeds
immediately, with a color change from red to yellow and
the precipitation of anthracene to produce [(PEt3)2Ni]2-
(μ-η2:η2-C6F6), 5, as shown in eq 3. Unlike the reaction
of (PEt3)2Ni(η2-C14H10) with C6F5H, there is no indication
that this reaction is in equilibrium, presumably due to the
enhanced electron-accepting ability of C6F6, and this reaction
occurs readily with a stoichiometric quantity of C6F6.
with the C(5)-C(6) bond length of 1.323(4) A indicative of a
double bond.
Variable-Temperature 19F and 31P NMR Spectroscopy.
Complex 5 exhibits a very broad pair of overlapping peaks
in the 282.5 MHz room-temperature 19F NMR spectrum.
These broad 19F resonances span from approximately δ -60
to -130, indicative of fluxionality. The experimental vari-
able-temperature 19F NMR spectra from 190 to 330 K are
shown in Figure 7. The slow-exchange limit spectrum ob-
tained at 190 K displays three fluorine environments, as
anticipated for a C2 symmetric species, with two peaks nearly
coincidental at δ -158.0 and a third peak at δ -195.0. The
nearly coincident peaks coalesce upon warming to 220 K, but
coalescence of the third resonance at δ -195.0 does not occur
until room temperature. This fluxional process is presum-
ably analogous to that proposed for complex 1 and thus
involves 1,3-shifts of the (PEt3)2Ni moieties around the C6F6
ring. Unlike complex 1, these shifts exchange all the 19F
nuclei environments, and modeled variable-temperature
spectra are shown on the right-hand side of Figure 7. The
activation barrier can be estimated as 12.3 kcal mol-1 using
3
the Arrhenius equation, or alternatively, the temperature
dependence of the rate of exchange can be fit using the Eyring
(31) Hinsberg, W.; Houle, F.; Allen, F.; Yoon, E. Chemical Kinetics
Simulator 1.01; International Business Machines Corporation, 1996.
equation, which provides a ΔHq value of 11.8 kcal mol-1
3

Article
Organometallics, Vol. 28, No. 13, 2009 3851
Figure 8. Experimental (left) and modeled (right) variable-
temperature 31P{1H} NMR spectra for complex 5 obtained
at 121.5 MHz.
Figure 7. Experimental (left) and modeled (right) variable-
temperature 19F NMR spectra for complex 5, obtained at
282.4 MHz.
δ 12.2 (2JPF = 48 Hz). In the presence of added C6F6, a single
new resonance was observed in the 19F NMR spectra, which
could be attributed to a small equilibrium amount of the
fluxional mononuclear complex (PEt3)2Ni(η2-C6F6), 6, at
δ -167.4, and a resonance in the 31P{1H} NMR was
observed at δ 17.0. The latter appeared to be a multiplet,
but the coupling could not be adequately resolved to un-
ambiguously assign this complex.
Intermediates in the Reaction of C6F6 and C6F5H with Ni
(PEt3)4. Previous reports14,15 of C-F bond activation in
both C6F6 using Ni(PEt3)4 or (PEt3)2Ni(1,5-cyclooctadiene)
gave no indication of intermediates via NMR analysis;
however, in consideration of the broad peaks observed in
the 19F and 31P{1H} spectra of 5 we decided to re-examine
this reaction. Immediately upon mixing a purple C6D6
solution of Ni(PEt3)4 with C6F6, the solution turned yellow,
which implies a reaction has taken place, although the C-F
bond activation reaction takes weeks at room temperature.
The 19F and 31P{1H} NMR spectra both reveal the reso-
nances observed for 5.
with a small contribution from ΔSq of 1.0 cal mol-1. These
3
values are slightly larger than the barrier of 10 kcal mol-1
determined for the interconversion of 1a and 1c.
3
To further test our previous assumption that DFT calcu-
lations could be used to determine the 19F NMR shifts in
these dinuclear adducts, attempts were made to model the
shifts observed in 5. The calculated 19F chemical shifts for
5 using the model complex [(PH3)2Ni]2(μ-η2:η2-C6F6) were
found to be δ -156.4, -161.4, and -187.8, and changing the
phosphine donor to the significantly stronger PMe3 donor in
the model complex [(PMe3)2Ni]2(μ-η2:η2-C6F6) resulted in
only slightly different predicted shifts of δ -156.9, -160.7,
and -191.0, which is in excellent agreement with the experi-
mentally observed shifts of δ -158.0, -158.0 (nearly coin-
cidental) and -195.0, with a maximum error of 4.0 ppm and
a root-mean-square error of 2.9 ppm. This prediction also
allows the identification of the resonance at δ -195.0 in 5 as
corresponding to F(1) and F(4), from the labeling scheme
used in Figure 6.
Variable-temperature 121.5 MHz 31P{1H} NMR spectra
were also obtained for 5 and are shown in Figure 8. Complex
5 displays a single broad resonance in the room-temperature
31P{1H} NMR spectrum at δ 18.9. At 350 K a fast-exchange
limit 31P{1H} NMR spectrum could be obtained, despite the
fact that 5 rapidly converts to the C-F activation product,
a metallic nickel precipitate, and other unidentified pro-
ducts at this temperature. The resonance for 5 is a septet at
δ 18.3 due to coupling of the exchange-averaged phosphorus
environments to six equivalent fluorine nuclei, with an
observed JFP of 8.8 Hz. Cooling a room-temperature solu-
tion leads to decoalescence at 260 K, and two multiplets are
observed at 210 K. These spectra were modeled, as shown on
the right-hand side of Figure 8, and the rates of exchange
determined from these spectra were used to estimate the
barrier to phosphine exchange. The Arrhenius activation
The analogous addition of C6F5H to Ni(PEt3)4 resulted in
a color change from purple to yellow and the observation of
the adduct 1 by 19F and 31P{1H} NMR spectroscopy. The
resonances associated with 3 and 4 were also observable in
the 19F spectrum. This reaction was previously reported to
give a mixture that included multiple C-F bond activation
products, presumed to be due to unselective activation of the
ortho-, meta-, and para-F environments. Under these condi-
tions, the reaction proceeded to provide four C-F activa-
tion products, as shown in Scheme 6. Unexpectedly, the
major product (65%) is not the ortho activation product 2,
but rather the para activation product (PEt3)2NiF(2,3,5,6-
C6F4H) (7), with a 31P{1H} NMR resonance at δ 13.6 (d, JPF
= 45.6 Hz) and 19F NMR resonances at δ -117.5, -141.9,
and -387.3, which is consistent with previous characteriza-
tion.10 The second most prevalent C-F activation product
(22%) is due to activation at the meta site, (PEt3)2NiF-
(2,3,4,6-C6F4H) (8), and displays 19F NMR resonances at
δ -91.0, -110.1, -142.1, -168.4, and -387.6 and a 31P{1H}
resonance at δ 13.2 (d, JPF = 47.4 Hz). Confirmation of this
assignment was aided by the prediction of the 19F NMR
shifts for the model complex (PH3)2NiF(2,3,4,6-C6F4H), 9H,
using DFT calculations, which yielded calculated shifts of
δ -97.0, -112.9, -141.2, and -169.4 for the 6, 2, 4, and
3 sites, respectively. The ortho-F activation product 2 was a
minor product (12%) in this mixture, with 19F NMR reso-
nances at δ -116.6, -142.4, -159.8, -165.7, and -380.3 and
energy of 11.5 kcal mol-1 is similar to the value determined
3
from the variable-temperature 19F NMR spectra. Likewise
the Eyring equation can be used to calculate a ΔHq value -
of 10.9 kcal mol-1 with a small contribution from ΔSq of
3
-2.6 cal mol-1
.
3
C-F Bond Activation in Complex 5. Similar to complex 1,
complex 5 undergoes C-F bond activation at room tem-
perature, but in the absence of added C6F6 this reaction
proceeds with a dark precipitate of metallic nickel and
numerous impurities. In the presence of added C6F6,
this reaction occurs cleanly to yield the known C-F
bond activation product trans-(PEt3)2NiF(C6F5),14,15 which
is observed as a doublet in the 31P{1H} NMR spectrum at
a
31P{1H} NMR resonance at δ 12.5 (d, JPF = 44.7 Hz).
Trace amounts of the previously reported product of C6F6

3852 Organometallics, Vol. 28, No. 13, 2009
Johnson et al.
Scheme 6
Scheme 7
activation (2%), (PEt3)2NiF(C6F5) (9), were also observed
at δ -115.5, -161.2, and -163.9, along with some
1,2,4,5-tetrafluorobenzene at δ -139.5. These trace products
formally represent a hydrodefluorination reaction, as we
have previously reported with 1,2,4,5-tetrafluorobenzene.
Although it is possible that this reaction occurs by a trans-
metalation reaction involving the intermediate hydride 4, we
cannot rule out alternate mechanisms, such as electron
transfer16 from Ni(PEt3)4 to provide an intermediate anionic
fluoroaromatic radical, C6F5H•-, which could lose F- to
generate the C6F4H• radical. This radical could then abstract
H• from C6F5H to provide 1,2,4,5-tetrafluorobenzene and
the C6F•5 fragment, which could then combine with the Ni(I)
species generated from electron transfer and the F- moiety
to form (PEt3)2NiF(C6F5). Irrespective of the mechanisms
involved in the production of these various C-F activation
products, it is apparent that the C-F bond activation of
C6F5H with Ni(PEt3)4 does not proceed in a selective man-
ner, despite the observation of the same intermediates
observed in the selective conversion of 1 to 2.
An improvement in selectivity using Ni(PEt3)4 could be
achieved by using an excess of C6F5H. With 20 equiv the
major C-F activation product was found to be 2 (67%), with
7 (27%) and 8 (6%) observed as minor products. This
observation is consistent with more than one operating
reaction mechanism in this system, with the activation from
the mechanism adopted by 1 increasingly prevalent with
added C6F5H.
DFT Calculations. These experimental results demonstrate
that in the presence of excess arene the antifacial dinuclear
bis-(PEt3)2Ni adducts of both C6F5H and C6F6 convert to
mononuclear adducts, as well as a C-H activation product
in the case of C6F5H. We sought to investigate these inter-
conversions by DFT, to gauge the importance of phosphine
donor and fluoroaromatic in the thermodynamic preference
for dinuclear adduct, monononuclear adduct, or C-H
activation in these systems.
Scheme 8
PMe3, and PEt3 donors, respectively. The Gibbs free energy
changes are all slightly lower that the enthalpy changes,
though the ΔG298 K value for PMe3 is inexplicably lower
than that of PEt3.
The lower transformation in Scheme 7 displays the analo-
gous equilibrium between [(PR3)2Ni]2(μ-η2:η2-C6F5H) and
C6F5H to produce the mononuclear complexes (PR3)2Ni-
(η2-C6F5H)C6F5H, which was examined for phosphine do-
nors PH3 (1aH, 3cH), PMe3 (1aMe, 3cMe), and PEt3 (1, 3).
A similar trend is observed with the enthalpy changes, which
are 9.5, 8.1, and 5.3 kcal mol-1 for PH3, PMe3, and PEt3
3
donors, respectively; however, the Gibbs free energy change
for the conversion also drops steadily in this series from
7.7 kcal mol-1 for conversion of 1aH to 3cH to 3.6 kcal mol-1
3
3
for the conversion of 1a to 3c. This latter calculated Gibbs free
energy change for these species in the gas phase is close to that
estimated from the experimentally determined value in C6D6
.
Calculations of the influence of phosphine donor on the
of +2.7 kcal mol-1
3
thermodynamics of the C-H bond activation step that
converts the mononuclear complex (PR3)2Ni(η2-C6F5H)
3c to hydride 4 were also carried out, using the PH3, PMe3,
and PEt3 donors and the same labeling convention, as shown
in Scheme 8. The donor ability of the phosphine has a large
influence on this reaction, with the oxidative addition en-
thalpically disfavored by 5.6 kcal mol-1 for the weaker PH3
3
The reaction of the dinuclear C6F6 adducts [(PR3)2Ni]2-
(μ-η2:η2-C6F6) with C6F6 to produce the mononuclear
complexes (PR3)2Ni(η2-C6F6) was examined for phosphine
donors PH3 (5H, 6H), PMe3 (5Me, 6Me), and PEt3 (5, 6), as
shown in Scheme 7. The enthalpy change for these reactions
is slightly unfavorable; however the value decreases with
the larger and more electron-donating phosphines, with
donor and enthalpically favored by -1.2 and -1.3 kcal
3
mol-1 for the complexes with the stronger PMe3 and PEt3
donors, respectively. A similar trend is observed for the
Gibbs free energy values for these transformations. This
reflects the propensity for a more electron rich metal center
to undergo oxidative addition. Surprisingly, the differences
between the values calculated for the PMe3 and PEt3 donors
are negligible. The calculated value for the conversion of 3 to
enthalpy changes of 10.1, 8.0, and 7.1 kcal mol-1 for PH3,
3

Article
Organometallics, Vol. 28, No. 13, 2009 3853
4 of -2.0 kcal mol-1 is slightly in error compared to the
protons (C6D5H, δ 7.15; C7D7H, δ 2.09) with respect to
tetramethylsilane at δ 0.00. 31P{1H} NMR spectra were refer-
enced to external 85% H3PO4 at δ 0.0. 13C{1H} spectra were
referenced relative to solvent resonances (C6D6, δ 128.0;
C7D8, δ 20.4). Elemental analyses were performed by the
Centre for Catalysis and Materials Research (CCMR), Wind-
3
experimentally determined value of 0.8 kcal mol-1, which
3
may be due to the increased entropy of 3 due to the presence
of alternate isomers with nearly identical energies, but may
also reflect the errors associated with gas-phase rather than
solution DFT calculations.
sor, Ontario, Canada. The compounds (PEt3)2Ni(η2-C14H10)17
34
and Ni(PEt3)4
were prepared by literature procedures.
Conclusions
The compounds C6F5H, C6F6, and C6F5Br were purchased
from Aldrich, degassed, and dried over molecular sieves prior
to use.
Although the C-F bond activation of C6F6 by Ni(PEt3)4
has been known for over thirty years,14 little insight has
been obtained into the mechanism of this reaction or of the
analogous previously reported unselective activation of
C6F5H.15 Despite the apparent similarity as a source of the
(PEt3)2Ni moiety, the reaction of the anthracene adduct
(PEt3)2Ni(η2-C14H10) with pentafluorobenzene produces
an isolable dinuclear adduct, which is slightly thermodyna-
mically favored over the mononuclear adduct. Reaction with
excess C6F5H provided an equilibrium amount of the mono-
nuclear arene adduct as well as the C-H activation product.
These mixtures ultimately undergo C-F activation at the
ortho-F with >97% selectivity. This selectivity is unique to
this system, which may prove useful in the functionalization
of partially fluorinated aromatic compounds. Mechanistic
studies show that hopping of the Ni(PEt3)2 moiety between
C6F5H rings is more rapid than C-F bond activation and
that the commonly accepted intramolecular mechanism of
C-F bond activation may not be true.
A reinvestigation of the reaction of C6F5H with Ni(PEt3)4
revealed the same dinuclear, mononuclear, and C-H activa-
tion intermediates observed with (PEt3)2Ni(η2-C14H10), but
further reaction proceeded to a mixture of C-F activation
products, with activation at the para- and meta-F providing
the major products. Radical intermediates appear likely in
these reactions and may explain some previously reported
byproduct in the activation of 1,2,4,5-tetrafluorobenzene.10
These mechanistic studies point to a significantly more
complex mechanism of C-F activation in these reactions
than has previously been anticipated and also demonstrate
the viability of nickel phosphine complexes in selective
activation and functionalization of both C-F and C-H
bonds10,32 of fluorinated aromatics.
Synthesis of [(PEt3)2Ni]2(μ-η2:η2-C6F5H) (1). To a stirred
solution of the anthracene adduct (PEt3)2Ni(η2-C14H10)
(1.00 g, 2.11 mmol) in 5 mL of pentane was added 0.5 mL (4.5
mmol) of C6F5H. The reaction mixture immediately turned
from dark red to orange, and anthracene precipitated
from solution over the course of 5 min. The mixture was then
immediately filtered through Celite and cooled to -40 °C; if
the solution is left for an excessive time at room temperature,
2 is produced as a byproduct. The resultant yellow crystals
were isolated by filtration and dried under vacuum (yield
0.56 g, 70%). 1H NMR (C6D6, 300 MHz, 298 K): δ 1.01
(m, 36H, PCH2CH3), 1.61 (m, 24H, PCH2CH3), 3.12 (m, 1H,
C6F5H). 19F NMR (C6D6, 282.4 MHz, 298 K): δ -134.9 (br, 1F,
W1/2 = 114 Hz), -144.0 (br, 1F 1F, W1/2 = 132 Hz), -155.2
(br, 1F, W1/2 = 153 Hz), -158.7 (br, 1F, W1/2 = 107 Hz),
-185.4 (br, 1F, W1/2 = 115 Hz). 31P{1H} NMR (C6D6,
121.5 MHz, 298 K): δ 18.5 (br and asymmetric, W1/2
=
870 Hz). Anal. Calcd for C30H61F5Ni2P4 (MW 758.08):
C, 47.53; H, 8.11. Found: C, 47.63; H, 8.09.
Synthesis of (PEt3)2NiF(2,3,4,5-C6F4H) (2). A pentane solu-
tion of [(PEt3)2Ni]2(μ-η2:η2-C6F5H) (1.02 g, 1.34 mmol) with
added C6F5H (0.5 mL, 4.5 mmol) was left at room temperature
for 1 week. The sample was crystallized by slow evaporation and
dried under vacuum (yield 1.14 g, 91%). The assignments of the
19F NMR coupling constants were made with the aid of the 19F-
1
{1H} spectrum and Gaussian enhancement. H NMR (C6D6,
300 MHz, 298 K): δ 0.96 (m, 18H, PCH2CH3), 1.08 (m, 12H,
PCH2CH3), 6.89 (m, 1H, C6F5H ) . 19F NMR (C6D6, 282.4
MHz, 298 K): δ -116.9 (ddddt, 1F, JFF = 31.2, 14.2, 11.0,
0.9 Hz, JPF = 2.6 Hz, ortho-2-F ), -142.7 (dddt, 1F, JFF =20.3,
14.2 Hz, JFH = 9.7 Hz, JPF = 2.5 Hz, ortho to H 5-F ), -160.2
(ddddt, 1F, JFF = 31.2 19.2, 1.6 Hz, JFH = 2.7 Hz, JPF
=
2.8 Hz, 3-F ), -166.1 (ddddt, 1F, JFF = 20.3, 19.2, 1.0 Hz, JFH
= 8.6 Hz, JPF = 4.1 Hz, 4-F ), -380.7 (dtm, 1F, JFF = 11.0 Hz,
J
PF = 47.6 Hz, Ni-F ). 31P{1H} NMR (C6D6, 121.5 MHz, 298
K): δ 12.5 (dm, 2JPF = 47.6 Hz). Anal. Calcd for C18H31F5NiP2
(MW 463.07): C, 56.44; H, 7.64. Found: C, 56.10; H, 7.87.
NMR Spectroscopic Characterization of (PEt3)2Ni(η2-
C6F5H) (3). To solutions of 1 in C6D6 were added 0.5 to
90 equiv of C6F5H. In each case an equilibrium amount of 3
Experimental Section
General Procedures. Unless otherwise stated, all manipula-
tions were performed under an inert atmosphere of nitrogen
using either standard Schlenk techniques or an MBraun glove-
box. Dry, oxygen-free solvents were employed throughout.
Anhydrous pentane was purchased from Aldrich, sparged with
dinitrogen, and passed through activated alumina under a
positive pressure of nitrogen gas and further deoxygenated using
Ridox catalyst columns.33 Deuterated benzene was dried by
heating at reflux with sodium/potassium alloy in a sealed vessel
under partial pressure, then trap-to-trap distilled, and freeze-
pump-thaw degassed three times. Deuterated toluene was
purified in an analogous manner by heating at reflux over Na.
NMR spectra were recorded on a Bruker AMX (300 MHz) or
Bruker AMX (500 MHz) spectrometer. All chemical shifts
are reported in ppm, and all coupling constants are in Hz. For
19F{1H} NMR spectra, CFCl3 in CDCl3 was used as the external
reference at δ 0.00. 1H NMR spectra were referenced to residual
1
was produced. H NMR (C6D6, 300 MHz, 298 K): δ 0.85 (m,
18H, PCH2CH3), 1.39 (m, 12H, PCH2CH3), 5.16 (tt, 1H, JHF =
11.4, 4.9 Hz, C6F5H ). 19F NMR (C6D6, 282.4 MHz, 298 K):
3
δ -138.8 (d of second order m, 2F, JFH =11.5 Hz, ortho-F ),
-164.9 (second order m, 2F, meta-F ), and -171.3 (m, 1F,
para-F ). 31P{1H} NMR (C6D6, 121.5 MHz, 298 K): δ 15.9 (s).
NMR Spectroscopic Characterization of (PEt3)2Ni(η2-
C6F5H) (4). To solutions of 1 in C6D6 were added 0.5 to
90 equiv of C6F5H. In each case an equilibrium amount of 4
1
was produced. H NMR (C6D6, 300 MHz, 298 K): δ -14.92
(br, Ni-H ). 1H NMR (C6D6, 300 MHz, 243 K): δ -14.92
(ttt, 2JPH = 68.4 Hz, 4JFH = 10.5 Hz, 5JFH = 4.7 Hz, Ni-H ).
19F NMR (C6D6, 282.4 MHz, 298 K): δ -114.1 (d of second
order m, 2F, 4JFH = 10.5 Hz, ortho-F ), -162.5 (m, 1F, para-F ),
and -163.9 (second order m, 2F, 5JFH = 4.8 Hz meta-F ). 31P
{1H} NMR (C6D6, 121.5 MHz, 243 K): δ 23.4 (s). 31P NMR
(C6D6, 121.5 MHz, 243 K): δ 23.4 (d, 2J ≈ 70 Hz).
(32) Nakao, Y.; Kashihara, N.; Kanyiva, K. S.; Hiyama, T. J. Am.
Chem. Soc. 2008, 130, 16170–16171.
(33) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518–1520.
(34) Cundy, C. S. J. Organomet. Chem. 1974, 69, 305–310.

3854 Organometallics, Vol. 28, No. 13, 2009
Johnson et al.
Synthesis of C6F5D. A three-neck round-bottom flask
equipped with a water-cooled condenser was charged with Mg
(2.734 g, 0.1125 mol, 1.5 equiv) and 80 mL of ether. Neat C6F5Br
(10.0 mL, 22.0 g, 0.0750 mol) was added in an initial 3 mL
aliquot to initiate the reaction, followed by slow dropwise
addition via syringe. The solution was stirred for 1 h and then
quenched by addition of D2O (7 mL, 5 equiv). The ether layer
was separated and dried with MgSO4 in the ambient atmo-
sphere, and the ether was removed on a benchtop rotary
evaporator. The remaining oil was distilled under N2 at a
pressure slightly higher than 1 atm at 98-102 °C. Yield (9.7 g,
data collection, and structure refinement are listed in Table 1.
Data reductions were performed using the SAINT36 software,
and the data were corrected for absorption using SADABS.37
The structures were solved by direct methods using SIR9738
and refined by full-matrix least-squares on F2 with anisotropic
displacement parameters for the non-H atoms using SHEL-
XL-9739 and the WinGX40 software package, and thermal
ellipsoid plots were produced using ORTEP32.41
Calculations. Ab initio DFT calculations were performed
using the hybrid functional B3LYP42 method with the Gaussian
03 package.43 The basis functions used were the TZVP set,
provided in the Gaussian 03 program. Calculated 19F NMR
shielding tensors were predicted using the gauge-independent
atomic orbital method that is the default of the Gaussian
03 program. The isotropic shielding values were converted to
chemical shifts using a linear fit44 to the experimental chemical
shifts, given in parentheses, for a set of molecules, CFCl3 (δ 0.0),
hexafluorobenzene (δ -164.9), pentafluorobenzene (δ -162.3,
-154.0, -139.0), SiF4 (δ -163.3), CF4 (δ -62.3), CFH3 (δ
-271.9), and fluorobenzene (δ -113.5). A plot of the calculated
isotropic shielding values versus experimental shift was fit by
a linear model with a slope of -1.11 and an intercept of -158.7.
Further details are available in the Supporting Information.
For these molecules a variety of conformers are possible due
to rotation around the Ni-P bonds, which complicates finding
true energy minima. The different potential conformers of 5H
were tested in C2 symmetry, with all four permutations of initial
P-Ni-P-H dihedral angles of 0° and 180° that define the PH3
substituent starting conformations used as starting geometries.
All the optimizations resulted in the same conformer with P-
Ni-P-H closest to the 0° starting conformer. A similar test of
four starting conformers of 5Me with varied P-Ni-P-C dihe-
dral angles of C2 symmetry resulted in two local minima, with
the lowest energy conformer with four P-Ni-P-C dihedral
angles closest to the 0° dihedral angle starting conformer and the
1
76%). The product had no H NMR resonances. 19F NMR
(C6D6, 282.4 MHz, 298 K): δ -139.8 (second order m, 2F, ortho-
F), -154.5 (t, JFF = 20.8 Hz, 1F, para-F), -162.7 (second order
m, 2F, meta-F).
Reaction of 1 with C6F5D. A C6D6 solution of 1 (0.0255 M)
and C6F5D (0.0305 M) was transferred frozen into a NMR
probe preheated to 303 K. The rate of exchange was monitored
by integration of the resonances associated with C6F5H, 1, and
3 using 1H NMR spectroscopy.
Synthesis of [(PEt3)2Ni]2(μ-η2:η2-C6F6) (5). To a stirred solu-
tion of the anthracene adduct (PEt3)2Ni(η2-C14H10) (1.00 g,
2.11 mmol) in 5 mL of pentane was added C6F6 (0.12 mL,
0.5 equiv). The reaction mixture immediately turned from dark
red to yellow and anthracene precipitated from solution. The
mixture was filtered through Celite and cooled to -40 °C. The
resultant yellow crystals were isolated by filtration and dried
under vacuum (yield 0.69 g, 84%). 1H NMR (C6D6, 300 MHz,
298 K): δ 0.99 (m, 36H, PCH2CH3), 1.61 (m, 24H, PCH2CH3).
19F NMR (C6D6, 282.4 MHz, 298 K): δ -170 (br and asym-
metric, W1/2 = ∼6000 Hz). 31P{1H} NMR (C6D6, 121.5 MHz,
298 K): δ 18.8 (br s). 19F NMR (C6D6, 282.4 MHz, 220 K):
δ -156.1, -156.2 (second order AB m), -196.5 (m). 31P{1H}
NMR (C6D6, 121.5 MHz, 190 K): δ 15.2 (m), 21.3 (dd, 3JPP
=
3
32 Hz, JFP = 32 Hz). Anal. Calcd for C30H60F6Ni2P4
(MW 776.07): C, 46.43; H, 7.79. Found: C, 46.52; H, 7.81.
NMR-Scale Reaction of Ni(PEt3)4 with C6F6. To a solution of
Ni(PEt3)4 (42 mg, 0.079 mmol) in C6D6 was added C6F6 (15 mg,
0.080 mmol). The mixture immediately turned from purple to
yellow. The reaction was monitored immediately by 19F NMR
spectroscopy, which revealed the presence of 5 and C6F6.
NMR-Scale Reaction of Ni(PEt3)4 with C6F5H. To a solution
of Ni(PEt3)4 (42 mg, 0.0790 mmol) in C6D6 was added C6F6
(14 mg, 0.083 mmol). The reaction was monitored immediately
by 19F NMR spectroscopy, which revealed the presence of 1 and
C6F5H, along with trace equilibrium amounts of 3 and 4. The
reaction proceeded to provide a mixture of C-F activation
products over the course of weeks. 19F NMR (C6D6, 282.4
MHz, 298 K): major product (7) δ -117.5 (m, 2F, ortho-F), -
141.9 (m, 2F, meta-F), -387.3 (m, 1F, Ni-F). 31P{1H} NMR
alternate conformer only 0.5 kcal mol-1 higher in energy. For 5,
3
crystallographic evidence revealed more than one conformer in
the two molecules in the asymmetric unit, which suggests that
the energy difference between these conformers is negligible.
Thus, only the conformer with the 0° P-Ni-P-C dihedral
angles as a starting point was optimized. The conformation of
(36) SAINTPlus, Data reduction and correction program; Bruker
AXS Inc.: Madison, WI, 2001, .
(37) SADABS, An empirical absorption correction program; Bruker
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(38) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.;
Spagna, R. J. Appl. Crystallogr. 1999, 32, 115–119.
(39) Sheldrick, G. M.
Gottingen, 1997, .
SHELXL-97; Universitat Gottingen:
(40) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837–838.
(41) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.
(42) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(C6D6, 121.5 MHz, 298 K): major product (7) δ 13.6 (d, JPF
=
45.6 Hz). 19F NMR (C6D6, 282.4 MHz, 298 K): minor product
(8) δ -91.0 (m, 1F, 6-F), -110.1 (m, 1F, 2-F), -142.1 (m, 1F,
4-F), -168.4 (m, 1F, 3-F), and -387.6 (tm, 1F, Ni-F). 31P{1H}
NMR (C6D6, 121.5 MHz, 298 K): minor product (8) δ 13.2
(d, JPF = 47.4 Hz). Trace amounts of the previously reported
product of C6F6 activation (2%), (PEt3)2NiF(C6F5) (9), were
also observed at δ -115.5, -161.2, and -163.9, along with some
1,2,4,5-tetrafluorobenzene at δ -139.5.
(43) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken,
V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
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Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.01; Gaussian, Inc.:
Wallingford, CT, 2004, .
X-ray Crystallography. The X-ray structures were obtained
at low temperature, with the crystals covered in Paratone and
placed rapidly into the cold N2 stream of the Kryo-Flex low-
temperature device. The data were collected using the
SMART35 software on a Bruker APEX CCD diffractometer
using a graphite monochromator with Mo KR radiation
˚
(λ = 0.71073 A). A hemisphere of data was collected using
a counting time of 10-30 s per frame. Details of crystal data,
(44) (a) Fukaya, H.; Ono, T. J. Comput. Chem. 2003, 25, 51–60.
(b) Sanders, L. K.; Oldfield, E. J. Phys. Chem. A 2001, 105, 8098–8104.
(c) Chan, J. C. C.; Eckert, H. THEOCHEM 2001, 535, 1–8.
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Article
Organometallics, Vol. 28, No. 13, 2009 3855
the P-C-C bond angles of the ethyl substituents was taken
from the crystal structure as a starting point in this DFT
optimization. The optimized geometry has C2 symmetry.
Three different starting conformations of the mononuclear
adduct 6H were optimized to local minima, with two having Cs
symmetry and identical starting P-Ni-P-H dihedral angles of
0° and 180°, and one conformer having C1 symmetry and
different starting P-Ni-P-H dihedral angles of 0° and 180°.
These provided three local minima, with the lowest energy
conformer corresponding to the Cs symmetric species with opti-
mized P-Ni-P-H angles that started at 0°. The other confor-
The dinuclear adducts of C6F5H, 1a-cH, 1a-cMe, and 1a, as
well as the mononuclear adducts 2a-cH, 2a-cMe, and 3a,c were
optimized starting from the geometries obtained with the C6F6
analogues; all these species have local minima with C1 symme-
try. The hydrides 3H, 3Me, and 3 were all found to have local
minima with C2 symmetry.
Acknowledgment is made to the National Sciences and
Engineering Council (NSERC) of Canada and the Shared
Hierarchical Academic Research Computing Network
(SHARCNET) for their support.
mers were a maximum of 0.4 kcal mol-1 higher in energy. Three
3
different starting conformations for 6Me were also investigated.
The lowest energy conformer was a Cs symmetric species
optimized from P-Ni-P-C angles starting at 0°. The other
Supporting Information Available: Full details of ref 41;
crystallographic information in CIF format for 1, 2, and 5;
select experimental NMR spectra, optimized coordinates, and
energies for DFT calculations. This material is available free of
charge via the Internet at http://pubs.acs.org.
conformers were a maximum of 0.2 kcal mol-1 higher in energy.
3
The Cs symmetric structure for the mononuclear Ni(PEt3)2
adduct, 6, was found to have an imaginary frequency. A mini-
mum energy C1 symmetric structure was located.