Dis-assembly of a benzylic CF3 group mediated by a niobium(III) imido complex

Article abstract of DOI:10.1021/ja4033007

All three C-F bonds in CF₃-substituted arenes are activated by a niobium imido complex, driven by the formation of strong Nb-F bonds. The mechanism of this transformation was studied by NMR spectroscopy, which revealed the involvement of Nb(III). Attempts to extend this chemistry to nonaromatic CF₃ groups led to intramolecular reactivity.

Full text of DOI:10.1021/ja4033007

Communication
pubs.acs.org/JACS
Dis-assembly of a Benzylic CF3 Group Mediated by a Niobium(III)
Imido Complex
Thomas L. Gianetti, Robert G. Bergman,* and John Arnold*
Department of Chemistry, University of California, Berkeley, California 94720, United States
S
* Supporting Information
To probe the behavior of these mono- and bimetallic arene
ABSTRACT: All three C−F bonds in CF3-substituted
arenes are activated by a niobium imido complex, driven
by the formation of strong Nb−F bonds. The mechanism
of this transformation was studied by NMR spectroscopy,
which revealed the involvement of Nb(III). Attempts to
extend this chemistry to nonaromatic CF3 groups led to
intramolecular reactivity.
complexes further, the hydrogenolysis of complex 1 in
fluorinated solvents was studied. This unexpectedly led to a
new type of functionalization of a CF3 group in which all three
fluorines were transferred to the Nb center and a new C−N
bond was formed between the remaining organic fragment and
the tert-butylimido ligand.
The Hydrogenolysis of 1 in neat α,α,α-trifluoromethylarene
proceeds as shown in Scheme 1.
he thermodynamic stability of C−F bonds [bond
Scheme 1. Hydrogenolysis of Complex 1 in Fluorinated
Arene Solvents
−1
Tdissociation energy (BDE) = 110−130 kcal mol ],
combined with their inherent kinetic inertness, has allowed
for many technical applications of fluorocarbons in medicinal
chemistry as well as the synthesis of resistant polymers.1−3
However, such strong bonding is also troublesome in view of
the fact that these chemically inert compounds are persistent in
the environment.4,5 Therefore, in recent years, synthetic
methods for the activation and functionalization of C−F
bonds have attracted growing attention.1,2,5−16 With increasing
degree of fluorination at carbon, the C−F bond strength
increases and the C−F bond length decreases, resulting in
substantial steric shielding of the carbon site.2 Because of this
increased stability, fluorine abstraction from a CF3 moiety is
difficult and rare.6,9,11,17−20
An even more challenging transformation is the functional-
ization of CF3 groups via triple C−F activation. Recently,
significant progress has been made in regard to reduction of the
CF3 group, leading to hydrodefluorination16,21−24 and C−C
coupling.21,25 Interestingly, Ar−CF3 reduction has been
achieved with low-valent niobium Nb(0) generated in situ
from NbCl5.19 However, to our knowledge, a reaction in which
all three fluorine atoms of an organic CF3 group, as well as the
carbon fragment initially bound to them, are directly transferred
to a single metal center has not been reported.
As previously observed with benzene or toluene as the
solvent, in PhCF3 the solution quickly changed color from pale
yellow to deep red upon H2 addition at room temperature; after
a few hours, the solution lightened to an orange-brown color.
Evaporation of the solvent under reduced pressure followed by
extraction and crystallization from Et2O afforded orange blocks
1
of complex 2a in good yield (87%). Surprisingly, the H NMR
spectrum of 2a in mesitylene-d12 did not show the expected
characteristic resonances of a mono- or bimetallic arene-bound
complex at 2−4 ppm. Additionally, 19F NMR analysis revealed
two broad downfield resonances in a 2:1 ratio (+162.4 and
+92.2 ppm, respectively). Lowering the temperature to 243 K
led to a sharpening of the two resonances and better resolution,
allowing a doublet at 159.4 ppm [2JF−F = 45(2) Hz] and a
poorly resolved triplet at 85.1 ppm [2JF−F = 48(5) Hz] to be
observed. Despite the short relaxation time of these 19F
resonances (T1 = 0.8−1.1 ms), 19F−19F nuclear Overhauser
effect spectroscopy (NOESY) data obtained at both room and
low temperature showed a correlation between the two sets of
resonances. These data strongly indicated the formation of a
niobium trifluoride complex rather than a PhCF3-bound
complex.
In our effort to develop new semihydrogenation catalysts,26
we reported the efficiency of a trivalent niobium complex in
selective semihydrogenation of alkynes.27 The active catalyst in
the mechanism, a transient tricoordinated “[BDI]NbNtBu”
(BDI = 2,6-diisopropylbenzene-β-diketiminate), was trapped as
an η6-bound arene in the absence of CO. We then showed that
this η6-bound arene complex can be formed via hydrogenolysis
of the niobium complex {[BDI]Nb(NtBuN)(CH3)2} (1) in
neat benzene or toluene.28 These mono-η6-bound arene species
were found to undergo two-electron reduction chemistry and
to form the corresponding bimetallic arene-bridged complexes
via a dissociative mechanism.28
Received: April 3, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja4033007 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Crystallographic analysis of 2a confirmed the above
conclusion, revealing the presence of a diketiminate niobium
(V) trifluoride complex bearing an η2-bound imine, {[BDI]-
NbF3(tBuNCC6H5)}, having a distorted capped-octahedral
geometry with the three fluorines in the basal plane (Figure 1).
disassembly of one CF3 moiety and supports an intramolecular
activation mode.
Attempts to extend this transformation to nonaromatic CF3
groups were made by performing the hydrogenolysis of 1 in
neat 1,1,1-trifluoro-n-hexane or CH3CF3 dissolved in n-hexane.
However, the reactivity observed was completely different from
that seen with the aromatic CF3 groups and led to the isolation
of two products (Scheme 2). Upon H2 addition, the solution of
1 quickly turned red, with the formation of a purple precipitate.
After 12 h, evaporation of the solvent under reduced pressure
deposited a purple/red powder. The red material was extracted
and crystallized from hexane, affording red crystals of complex
3 in 52% yield. The remaining purple powder was extracted
with and crystallized from tetrahydrofuran, forming dark-purple
crystals of complex 4 in low yield (21%). A control experiment
in which the hydrogenolysis of complex 1 was performed in
hexane alone afforded the same product distribution. ORTEP
views of 3 and 4 are presented in Figure 2.
Figure 1. ORTEP diagram of complex 2a. H atoms and iPr groups
have been removed for clarity. Selected bond distances and angles are
presented in Table S.2 in the SI.
Figure 2. ORTEP diagrams of complexes 3 (left) and 4 (right). H
atoms in both complexes and the iPr groups in 4 have been removed
for clarity. Selected bond distances and angles are presented in Table
S.3.
The two trans fluorines show similar Nb−F distances [av
1.9095(16) Å]; the remaining fluorine is trans to one of the
BDI nitrogens ligand, resulting in a slightly longer Nb−F
distance [1.9386(16) Å]. One of the apical positions is
occupied by the moiety formed by coupling between the
imido group and the benzylic carbon of the PhCF3 reactant.
The C34−N1 bond distance of 1.261(4) Å suggests the
presence of a CN bond and therefore an imine fragment.
The Nb−C and Nb−N bond distances to the imine moiety
[Nb−C34 = 2.170(3) Å and Nb−N3 = 2.080(3) Å] are within
the range of previously observed Nb−Calkyl and Nb−Ndonor
bond lengths29,30 and are consistent with the view shown in
Scheme 1. The large angles observed within the imine moiety
[C35−C34−N3 = 131.1(3)° and C34−N3−C31 = 137.5(3)°]
imply sp2 hybridization of both C1 and N3, further supporting
the above description.
Conducting a similar experiment using 1,3-bis-
(trifluoromethyl)benzene afforded dark-yellow crystals of 2b
in very good yield (91%). Both the NMR and X-ray diffraction
analyses were analogous to those of complex 2a, showing the
formation of {[BDI]NbF3(tBuNCC6H4CF3)} [Figure S.2 in
the Supporting Information (SI)]. The 19F resonance of the
remaining CF3 group was found as a sharp singlet at −61.6
ppm. This result provides a second example of selective
Complex 3 exhibits a pseudo-square-pyramidal geometry in
which the BDI group has been transformed into a κ4-CNNC
ligand via activation of the two isopropylmethines. The Nb−C
bond distances are on average 2.260(3) Å, which is within the
range of previously reported Nb(V)−C bond lengths.29
Complex 4, on the other hand, was found to be a diamagnetic
dihydride-bridged complex, with the hydride resonance
1
observed as a broad singlet at −1.35 ppm in the H NMR
spectrum. The hydrides, which were located in the Fourier
difference map and refined isotropically, display a Nb−H
distance of 1.92(2) Å. The dimer possesses a center of
inversion about a central {Nb2(μ-H)2} core. The short Nb−Nb
bond distance of 2.7846(4) Å supports metal−metal bonding
and accounts for the observed diamagnetism of 4.31
The reactivity observed in nonaromatic solvent suggested
that (i) only benzylic CF3 groups are activated by this system,
(ii) arene coordination to Nb may be a key requirement for C−
F activation, and (iii) high-valent niobium hydrides may be
involved as intermediates during the hydrogenolysis. To
investigate these hypotheses, the hydrogenolysis of 1 in the
Scheme 2. Hydrogenolysis of Complex 1 in Nonaromatic Fluorinated Solvents
B
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Journal of the American Chemical Society
Communication
1
presence and absence of PhCF3 was followed by H and 19F
NMR spectroscopies. Since both benzene and toluene are
known to compete strongly in arene coordination28 and the use
of neat PhCF3 could interfere with the observation of
intermediates, sterically hindered mesitylene-d12 was used as
an NMR solvent.
to the final product 2a. Additionally, the formation of 2a and
intermediate C was observed when the niobium complex 5 was
stirred with 10 equiv of PhCF3 in C9D12 at room temperature
(Scheme 3 bottom).
Taken together, these experiments suggest that hydro-
genolysis proceeds via intermediate A, which is in equilibrium
with the transient tricoordinate Nb(III) species B as illustrated
in Scheme 3. In the absence of trapping ligands, A reacts further
to form 3 and 4, whereas a π-acidic ligand such as PhCF3 traps
the low-valent species B, yielding d2 arene intermediate C. This
intermediate then reacts further to activate the CF3 moiety.
While C was found experimentally to be a key intermediate
in the triple C−F activation of PhCF3, the mechanism of its
conversion to the final product 2a remained unclear. To
address this, we turned to density functional theory (DFT)
calculations for additional mechanistic information. These
preliminary calculations showed the presence of a very
exergonic overall stepwise C−F activation (ΔGC→2a = −104.9
kcal mol−1), consistent with the formation of strong Nb−F
bonds (BDENb−F = 137 kcal mol−1).33 Additionally, no
concerted transition state consistent with an oxidative addition
was found for the first C−F activation. Instead, the DFT
calculations suggested a two-step process: an initial fluorine
abstraction leading to sp2 hybridization at the benzylic carbon
(i.e., formation of a coordinated PhCF2 group) with a Nb−
First, hydrogenolysis of 1 alone resulted in the fast formation
of 3 and 4, implying that despite its aromatic character,
mesitylene-d12 is too poor a π-acidic ligand to form a persistent
1
arene-bound complex. Following the reaction by H NMR
spectroscopy showed the rapid formation and disappearance of
an intermediate complex, A, for which a broad resonance at 9.2
ppm is consistent with the presence of a Nb−H bond.32 To
gain further structural insight, soon after H2 addition the
sample was placed in a spectrometer cooled to 243 K. The
persistence of A at low temperature allowed for its character-
ization by NMR spectroscopy (see the SI). A DEPT-135 NMR
experiment showed the presence of one CH2 group, consistent
with the formation of a metallacylic niobium hydride (complex
A in Scheme 3).32
Scheme 3. Intermediates Observed by NMR Spectroscopies
C
alkyl bond (intermediate F) followed by a 1,3-shift (Scheme 4).
Scheme 4. Calculated Intermediates Involved in PhCF3
Disassembly (Free Energies and Structures of Transition
States Are Presented in Figure S.20)
In an HSQC NMR experiment, the CH2 carbon was found
to bear two inequivalent protons. Additionally, a COSY NMR
experiment showed that one of the methylene protons, H1, is
weakly correlated to the niobium hydride only, while the other
methylene proton, H2, is correlated to the methine proton.
Finally, the NOESY NMR spectrum revealed that the methine
proton is strongly correlated to H1 and the CH3 group but only
weakly to H2. These 2D NMR experiments further supported
the assignment of A and also confirmed its stereochemistry, in
which the hydride and the methine proton are cis to one
another. When performed under an atmosphere of D2, the
hydrogenolysis of 1 in hexane formed 3 and 4 in which
deuterium incorporation was observed at all positions of the
isopropyl group. Such scrambling suggests a fast equilibrium
between A and the transient “[BDI]NbNtBu” species B
(Scheme 3) and/or successive σ-bond metathesis of A.
A similar experiment was then performed in the presence of
10 equiv of PhCF3, and the same hydride intermediate A was
observed at low conversion along with a new complex C. After
10 min at room temperature, a significant amount of C was
formed; cooling the sample to 243 K allowed us to perform its
complete NMR characterization. The structure of C is
consistent with a complex containing an η6-bound PhCF3, in
which the arene protons resonate between 4 and 3 ppm in a 2:3
ratio. Meanwhile, a new singlet in the 19F NMR spectrum
appears at −62.4 ppm, which is slightly upfield from that of free
PhCF3 (−61.4 ppm; see the SI). When the solution was
allowed to warm to room temperature, C was quickly converted
Fluorine abstraction in the absence of arene coordination was
found to be almost 10 kcal mol−1 higher in free energy (see the
SI). Finally, the two remaining C−F bonds appear to be
activated stepwise. In each step, C−F bond cleavage and
nitrene transfer to the benzylic carbon take place in a concerted
manner (Scheme 4).
To conclude, the selective dis-assembly of a benzylic CF3
moiety has been observed in which three fluorine atoms and
the resulting carbene fragment are transferred to a single Nb
center, with concurrent formation of a new C−N bond.
Evidence points to the formation of an η6-arene-bound niobium
complex as a key intermediate in this process. The involvement
of arene coordination is apparently essential to stabilize the
C
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Journal of the American Chemical Society
Communication
(25) Fuchibe, K.; Mitomi, K.; Suzuki, R.; Akiyama, T. Chem.Asian
J. 2008, 3, 261.
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rate-determining transition state, since other saturated fluori-
nated substrates led to intramolecular attack upon other
ligands. DFT calculations suggested that the d2 species C
undergoes stepwise C−F activation to yield the final d0 niobium
trifluoride/η2-bound imine product.
2011, 50, 3900.
(27) Gianetti, T. L.; Tomson, N. C.; Arnold, J.; Bergman, R. G. J. Am.
Chem. Soc. 2011, 133, 14904.
(28) Gianetti, T. L.; Nocton, G.; Minasian, S. G.; Tomson, N. C.;
Kilcoyne, A. L. D.; Kozimor, S. A.; Shuh, D. K.; Tyliszczak, T.;
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ASSOCIATED CONTENT
* Supporting Information
Experimental procedures; analytical data; NMR spectra of
intermediates A and C; crystallographic data and CIFs for
complexes 2a, 2b, 3, and 4; and DFT calculation methods and
results. This material is available free of charge via the Internet
at http://pubs.acs.org.
■
S
2007, 46, 8778.
(32) Figueroa, J. S.; Piro, N. A.; Mindiola, D. J.; Fickes, M. G.;
Cummins, C. C. Organometallics 2010, 29, 5215.
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AUTHOR INFORMATION
Corresponding Author
Arnold@berkeley.edu; rbergman@berkeley.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Air Force (Grant FA9550-11-1-0008) for
financial support; Drs. A. DiPasquale and K. Durkin for
experimental assistance; and Prof. R. A. Andersen, Dr. G.
Nocton, Dr. H. S. La Pierre, and B. M. Kriegel for helpful
discussions.
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