Oxidatively Induced Aryl-CF3 Coupling at Diphosphine Nickel Complexes

Article abstract of DOI:10.1021/acs.organomet.9b00678

This communication describes the synthesis of a series of diphosphine Ni^II(Ph)(CF₃) complexes and studies of their reactivity toward oxidatively induced Ph-CF₃ bond-forming reductive elimination. Treatment of these complexes with the one-electron outer-sphere oxidant ferrocenium hexafluorophosphate (FcPF₆) affords benzotrifluoride, but the yield varies dramatically as a function of diphosphine ligand. Diphosphines with bite angles of less than 92° afforded <10% yield of PhCF₃. In contrast, those with bite angles between 95 and 102° formed PhCF₃ in yields ranging from 62 to 77%.

Full text of DOI:10.1021/acs.organomet.9b00678

Communication
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Oxidatively Induced Aryl−CF3 Coupling at Diphosphine Nickel
Complexes
James R. Bour,† Pronay Roy,† Allan J. Canty,‡ Jeff W. Kampf,† and Melanie S. Sanford*,†
†Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, United States
‡School of Natural Sciences-Chemistry, University of Tasmania, Hobart, Tasmania 7001, Australia
S
* Supporting Information
ABSTRACT: This communication describes the synthesis of a series
of diphosphine NiII(Ph)(CF3) complexes and studies of their
reactivity toward oxidatively induced Ph−CF3 bond-forming reductive
elimination. Treatment of these complexes with the one-electron
outer-sphere oxidant ferrocenium hexafluorophosphate (FcPF6)
affords benzotrifluoride, but the yield varies dramatically as a function of diphosphine ligand. Diphosphines with bite angles
of less than 92° afforded <10% yield of PhCF3. In contrast, those with bite angles between 95 and 102° formed PhCF3 in yields
ranging from 62 to 77%.
rifluoromethyl substituents have emerged as increasingly
between 95 and 102° afford the highest yields in this
Timportant moieties in pharmaceutical and agricultural transformation.
chemistry.1 As such, there has been substantial interest in the
development of metal-mediated/-catalyzed cross-coupling
methods for the introduction of CF3 groups onto (hetero)-
aromatic rings.2 In many of these transformations, aryl−CF3
bond-forming reductive elimination has been identified as a
kinetic bottleneck. Stoichiometric organometallic studies are
frequently used to identify metal/ligand combinations and
reaction conditions that facilitate this challenging step of the
catalytic cycle.3−7 Despite significant efforts in this area, metal
complexes that participate in high-yielding aryl−CF3 bond-
forming processes remain relatively rare.3−7
We initially targeted (dppe)Ni(CF3)(Ph) (1a; dppe = 1,2-
bis(diphenylphosphinoethane) due to the widespread use of
dppe as a ligand in nickel cross-coupling catalysis.9 Complex 1a
was synthesized via the route outlined in Scheme 1. The CF3
Scheme 1. Synthetic Route to Complex 1a
Recent work from our group has explored aryl−CF3
coupling at isolable high-valent nickel centers. For instance,
we have shown that the tris(pyrazolyl)borate (Tp) complexes
TpNiIV(CF3)2(Ph)6 and TpNiIII(CF3)(Ph)7 undergo Ph−CF3
bond-forming reductive elimination under mild conditions.
Similarly, van der Vlugt and Klein have recently demonstrated
oxidatively induced aryl−CF3 coupling from a NiII pincer
complex.8 However, this promising reactivity has not yet been
translated to Ni catalysis, largely due to limitations associated
with the Tp/pincer ligand scaffolds. While these tridentate
ligands are well suited for obtaining isolable high-valent Ni
complexes, they limit the open coordination sites available for
other steps of most catalytic cycles. Thus, a key objective is to
translate these promising examples of high-valent Ni-mediated
aryl−CF3 coupling to complexes bearing more catalytically
relevant ligands.
In this communication, we demonstrate that the treatment
of NiII diphosphine (P∼P) complexes of general structure
(P∼P)NiII(CF3)(aryl) with ferrocenium hexafluorophosphate
(FcPF6) leads to aryl−CF3 coupling at room temperature. A
competing unproductive decomposition pathway is identified
and mitigated. Furthermore, ligand effects are systematically
explored. These studies show that phosphines with bite angles
ligand was installed by oxidative addition of trifluoroacetic
anhydride to in situ generated Ni0(PPh3)2. Carbonyl
deinsertion afforded intermediate A,10 which underwent ligand
substitution with dppe to afford B. Finally, transmetalation
between B and 0.55 equiv of ZnPh2 afforded 1a in 65%
isolated yield.11 Filtration of a THF solution of 1a through a
plug of basic alumina proved critical to remove Lewis acidic
ZnII byproducts that can affect the reactivity of this complex.
Following recrystallization from THF/pentane, compound 1a
was characterized by 1H, 13C, 19F, and 31P NMR spectroscopy,
HRMS, and single-crystal X-ray diffraction (vide infra).
We first evaluated the reactivity of this NiII complex toward
direct Ph−CF3 coupling (Scheme 2a). Heating a solution of 1a
in acetone at 70 °C for 14 h resulted in no detectable
Received: October 11, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00678
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
Scheme 2. (a) Thermolysis of NiII Complex 1a (No Ph−CF3
Coupling) and (b) Oxidatively Induced Ph−CF3 Coupling
from 1a
difluorocarbene intermediate with adventitious water would
generate a Ni−carbonyl species (step ii), which can then
undergo 1,1-migratory insertion (step iii) and reductive
elimination to release the acid fluoride (step iv). If this
mechanism were operating, the addition of excess H2O should
accelerate the hydrolysis reaction (step ii) and also promote
carboxylic acid formation via steps v and vi in Scheme 3. To
test this proposal, we added 75 equiv of H2O to the reaction
mixture of 1b with FcPF6. Under these conditions, 4-
fluorobenzoic acid was formed in 65% yield, consistent with
the pathway outlined in Scheme 3.16
We hypothesized that the relative rates of undesired α-
fluoride elimination versus the targeted aryl−CF3 coupling
could be modulated by changing the phosphine ligand.
Previous studies of reductive elimination reactions from
group 10 metal centers have shown that increasing the bite
angle of the diphosphine is an effective strategy for accelerating
C(sp2)−X coupling.17 As such, we next evaluated a series of
(P∼P)NiII(Ph)(CF3) complexes, where P∼P = diphosphine
ligands with varied bite angles. The dppbz (2; 1,2-bis-
(diphenylphosphino)benzene, dppp (3; 1,3-bis-
(diphenylphosphino)propane), dppb (4; 1,4-bis-
(diphenylphosphino)butane), diop (5; 2,3-O-isopropylidene-
2,3-dihydroxy-1,4-bis(diphenylphosphino)butane), dppf (6;
1,1′-bis(diphenyl phosphino)ferrocene), and xantphos (7;
4,5-bis(diphenylphosphino)-9,9-dimethylxanthene) complexes
were all synthesized using the procedure outlined in Scheme 1.
31P and 19F NMR spectroscopic analyses of 2−7 show that all
but 7 adopt a cis geometry in solution.18
We next examined the reactions of 2−7 with FcPF6. The
dppbz complex 2 did not undergo productive Ph−CF3
coupling upon treatment with FcPF6. In contrast, complexes
with larger bite angle phosphines (3−7) all reacted with FcPF6
to afford PhCF3 in yields ranging from 3% (for 7) to 77% (for
6).19−21 As summarized in Table 1, ligands with bite angles
between 95 and 102° appear to be optimal for this reaction. In
contrast, modest coupling yields are observed for diphosphines
with smaller bite angles (between 86 and 92°). Finally, as
expected, the trans configuration of the xantphos complex 7
results in a low yield of PhCF3.
formation of benzotrifluoride. Instead, 1a decomposed to
generate a complex mixture of paramagnetic products.
Importantly, similar observations have been made independ-
ently by Vicic12 and Grushin13 in studies of other (P∼P)-
NiI I (aryl)(CF3 ) complexes [(P∼P)
= 1,2-bis-
( d i i s o p r o p y l p h o s p h i n o ) e t h a n e a n d 4 , 5 - b i s -
(diisopropylphosphino)-9,9-dimethylxanthene, respectively].
On the basis of our work with the NiIII model complex
TpNiIII(CF3)(Ph),7 we hypothesized that the one-electron
oxidation of 1a would induce Ph−CF3 coupling. To guide the
selection of an appropriate oxidant, cyclic voltammetry (CV)
of 1a was conducted in 0.1 M NBu4PF6 in MeCN at a scan rate
of 100 mV/s. The CV of 1a under these conditions shows an
irreversible oxidation wave with an onset potential of ∼+0.1 V
and a peak potential of +0.56 V vs Fc/Fc+ (Figure S11a).
These results suggest that ferrocenium hexafluorophosphate
(FcPF6) should be a suitable oxidant in this system. Indeed,
the treatment of 1a with 1.3 equiv of FcPF6 resulted in
complete consumption of the NiII starting material within 30
min at room temperature.14 Analysis of the crude reaction
mixture by 19F NMR spectroscopy revealed the formation of
Ph−CF3 (2% yield) and PhC(O)F (19% yield), along with a
mixture of paramagnetic Ni byproducts (Scheme 2b).
Attempts to isolate/characterize Ni species from this trans-
formation or to improve the yield of Ph−CF3 by varying the
solvent or temperature proved unsuccessful. Analogous results
were obtained with the 4-fluorophenyl analogue 1b, which
reacted with FcPF6 to afford a 1% yield of p-FC6H4CF3 and a
15% yield of the corresponding acid fluoride (eq 1).
Table 1. Oxidatively Induced Ph−CF3 Coupling as a
a
Function of Phosphine Ligand
On the basis of literature precedent with closely related
(dppe)PdII(aryl)(CF3) complexes,3d,15 we hypothesize that
PhCOF is formed via an initial α-fluoride elimination from
1a,b (Scheme 3, step i). Subsequent reaction of the transient
b
a
compound
ligand
calcd bite angle (deg)
yield Ph−CF3 (%)
c
2
1a
3
4
5
dppbz
dppe
dppp
dppb
diop
86.0
87.7 (86.8)
91.9
95.3
102
<1
2
7
d
75
Scheme 3. Proposed Pathway to Acid Fluoride and
Carboxylic Acid Products
62
77
3
6
7
dppf
xantphos
100.6 (100.2)
trans
a
Yields determined by 19F NMR spectroscopy relative to 4,4′-
b
difluorobiphenyl. Bite angles determined by DFT (see p S31 in the
Supporting Information). Experimental bite angles from the X-ray
c
crystal structures are shown in parentheses. Reaction performed in 2/
d
5 benzene/acetone due to the low solubility of 2 in acetone. Due to
the low stability of 4, this complex was generated and then reacted
with FcPF6 in situ.
B
DOI: 10.1021/acs.organomet.9b00678
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
A final set of studies focused on a detailed comparison of
complex 1a (which affords <5% yield in the oxidatively
induced PhCF3 coupling) with complex 6 (which affords the
highest yield of 77%). First, X-ray crystal structures of both 1a
and 6 were obtained, and the respective ORTEP diagrams are
shown in Figure 1. These structures reflect the anticipated
in 0.1 M NBu4PF6 in MeCN at a scan rate of 100 mV/s.24
Both CVs show irreversible oxidation waves, with nearly
identical onset potentials (at ∼+0.1 V) and similar peak
potentials (+0.56 V for 1a and +0.35 V for 6). Furthermore,
DFT calculations25 on 6+, the product of single-electron
oxidation of 6, show that the unpaired electron is localized on
nickel. This suggests that the redox chemistry occurs at the Ni
center rather than at Fe. Taken together, these data strongly
suggest that the difference in PhCF3 yield between these
complexes results from steric rather than electronic differences
between the different diphosphine ligands.
In summary, this work shows that (P∼P)NiII(CF3)(Ph)
complexes react with FcPF6 to afford Ph−CF3 coupling under
mild conditions. α-Fluoride elimination from the Ni−CF3
complex is a competing side reaction in these systems. The
highest yields are obtained with P∼P ligands that have bite
angles close to 100°, such as dppf, diop, and dppb. The ligand
effects elucidated herein could ultimately inform the develop-
ment of NiI/III-catalyzed trifluoromethylation reactions.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.9b00678.
Figure 1. (a) ORTEP diagram of 1a. Thermal ellipsoids are drawn at
50% probability. Hydrogen atoms and solvent molecules are omitted
for clarity. Selected bond lengths (Å) and angle (deg): C(1)−Ni
1.930(2), C(2)−Ni 1.918(2), P(1)−Ni 2.1633(6), P(2)−Ni
2.1963(6); C(1)−Ni−C(2) 89.6. (b) ORTEP diagram of 6. Thermal
ellipsoids are drawn at 50% probability. Hydrogen atoms and solvent
molecules omitted for clarity. Selected bond lengths (Å) and angle
(deg): C(1)−Ni 1.963(3), C(2)−Ni 1.918(3), P(1)−Ni 2.2192(7),
P(2)−Ni 2.243(7); C(1)−Ni−C(2) 83.2.
■
S
Experimental and spectral details for all new compounds
and all reactions reported (PDF)
Cartesian coordinates for the calculated structures
(PDF)
Accession Codes
increase in phosphine bite angle upon moving from dppe
(86.8°) to dppf (100.2°). This is accompanied by an increase
in the acuteness of the C(1)−Ni−C(2) angle from 89.6° in 1a
to 83.2° in 6. Literature studies have shown that this angle has
a major effect on the rates of other reductive elimination
reactions.17 With the larger bite angle phosphine, both the Ni−
CF3 and Ni−P bond distances are significantly longer (by
∼0.03 and ∼0.05 Å, respectively), likely reflecting increased
steric congestion at the Ni center with dppf relative to dppe.22
In contrast, the Ni−Ph bond distances are identical in the two
complexes (Ni−C(2) = 1.918 Å).
CCDC 1958901−1958902 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail for M.S.S.: mssanfor@umich.edu.
■
The electronic properties of 1−6 can also be compared. As
shown in Table 2, the CO stretching frequencies of the
ORCID
James R. Bour: 0000-0002-0372-7323
Pronay Roy: 0000-0002-5649-8747
Allan J. Canty: 0000-0003-4091-6040
Jeff W. Kampf: 0000-0003-1314-8541
Melanie S. Sanford: 0000-0001-9342-9436
Table 2. Comparison of CO Stretching Frequencies of
[(P∼P)Ni(CO)2] and Yields of PhCF3 Coupling from
[(P∼P)Ni(CF3)(Ph)]
23
c
ligand
νsym, νasym (cm−1
)
yield Ph−CF3 (%)
Notes
a
dppbz
dppe
dppp
dppb
dppf
2007, 1952
2004, 1945
2000, 1941
2002, 1944
<1
2
7
75
77
The authors declare no competing financial interest.
a
a
ACKNOWLEDGMENTS
■
a
This work was supported by the National Science Foundation
grant CHE-1664961 and the Australian Research Council.
J.R.B. acknowledges the National Science Foundation for a
Graduate Fellowship. Dr. Ansis Maleckis is also acknowledged
for valuable guidance and helpful discussions. X-ray
instrumentation was funded by the National Science
Foundation grant CHE-0840456.
b
2001, 1939
a
b
IR spectrum collected in benzene. IR spectrum collected in
c
CH2Cl2. 19F NMR yield from standard conditions.
zerovalent dicarbonyl complexes (P∼P)Ni(CO)2 show no
clear trends with respect to the yield of PhCF3 from the
analogous (P∼P)Ni(CF3)(Ph) species.23 For example, the
dicarbonyl analogues of compounds 2−4 have nearly identical
CO stretching frequencies, but only 4 affords a high yield of
PhCF3 upon treatment with FcPF6. In addition, cyclic
voltammetry (CV) was conducted with compounds 1 and 6
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detected. Furthermore, the addition of 75 equiv of H2O to this
transformation had minimal effect on the yield of 1-CF3-4-F-C6H4.
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phosphine ligands are not involved in this reaction and (ii) that
carbene intermediates are unlikely. See the Supporting Information
for complete details.
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the Supporting Information for details.
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D
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(24) Attempts to characterize 4 and 7 by CV were unsuccessful due
to their instability under the CV conditions.
(25) Gaussian 09 was used at the M06 level for geometry
optimization with acetone as solvent utilizing the SDD basis set on
nickel/iron and the 6-31G(d) basis set for other atoms. See the
Supporting Information for full details.
E
DOI: 10.1021/acs.organomet.9b00678
Organometallics XXXX, XXX, XXX−XXX