Solvated Nickel Complexes as Stoichiometric and Catalytic Perfluoroalkylation Agents**

Article abstract of DOI:10.1002/anie.202104559

The acetonitrile-solvated [(MeCN)Ni(C₂F₅)₃]^? was prepared in order to compare and contrast its reactivity with the known [(MeCN)Ni(CF₃)₃]^? towards organic electrophiles. Both [(MeCN)Ni(CF₃)₃]^? and [(MeCN)Ni(C₂F₅)₃]^? successfully react with aryl iodonium and diazonium salts as well as alkynyl iodonium salts to give fluoroalkylated organic products. Electrochemical analysis of [(MeCN)Ni^II(C₂F₅)₃]^? suggests that, upon electro-oxidation to [(MeCN)_nNi^III(C₂F₅)₃], reductive homolysis of a perfluoroethyl radical occurs, with the concomitant formation of [(MeCN)₂Ni^II(C₂F₅)₂]. Catalytic C?H trifluoromethylations of electron-rich arenes were successfully achieved using either [(MeCN)Ni(CF₃)₃]^? or the related [Ni(CF₃)₄]^2?. Stoichiometric reactions of the solvated nickel complexes reveal that “ligandless” nickel is exceptionally capable of serving as reservoir of CF₃ groups under catalytically relevant conditions.

Full text of DOI:10.1002/anie.202104559

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Solvated Nickel Complexes as Stoichiometric and Catalytic
Perfluoroalkylation Agents
Authors: David A. Vicic and Scott T. Shreiber
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202104559
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10.1002/anie.202104559
Angewandte Chemie International Edition
RESEARCH ARTICLE
Solvated Nickel Complexes as Stoichiometric and Catalytic
Perfluoroalkylation Agents
Scott T. Shreiber[a] and David A. Vicic*[a]
[a]
Scott T. Shreiber and David A. Vicic
Department of Chemistry
Lehigh University
6 E. Packer Avenue, Bethlehem, Pennsylvania, 18015, United States
E-mail: dav512@lehigh.edu
Supporting information for this article is given via a link at the end of the document.
Abstract: The acetonitrile-solvated [(MeCN)Ni(C2F5)3] was prepared
Previously, we found that [PPh4][2] and [PPh4]2[3] (Chart 1) could
be synthesized by the addition of one and two equivalents of
“AgCF3” to 1 in the presence of [PPh4]Cl.[29] The electronic
properties of 2 and 3 were extensively studied through the use of
density functional theory (DFT) calculations, spectroscopy-
oriented configuration interaction (SORCI) calculations, X-ray
absorption spectroscopy, and cyclic voltammetry.[29] Interestingly,
the data collectively show that 2 and 3 are better described as
physically d9 metal complexes with the additional hole delocalized
in order to compare and contrast its reactivity with the known
[(MeCN)Ni(CF3)3]
towards
organic
electrophiles.
Both
[(MeCN)Ni(CF3)3] and[(MeCN)Ni(C2F5)3] successfully react with aryl
iodonium and diazonium salts as well as alkynyl iodonium salts to give
fluoroalkylated organic products.
Electrochemical analysis of
[(MeCN)NiII(C2F5)3] suggests that, upon electro-oxidation to
[(MeCN)nNiIII(C2F5)3], reductive homolysis of a perfluoroethyl radical
occurs, with the concomitant formation of [(MeCN)NiII(C2F5)2].
Catalytic C-H trifluoromethylations of electron rich arenes were
successfully achieved using either [(MeCN)Ni(CF3)3] or the related
[Ni(CF3)4]2–. Stoichiometric reactions of the solvated nickel complexes
reveal that “ligandless” nickel is exceptionally capable of serving as
reservoir of CF3 anions or radicals under catalytically relevant
conditions.
over the multiple trifluoromethyl ligands.
The “ligandless”
complexes 2 and 3 also displayed preliminary reactivities that
warranted follow-up investigations. For example, 2 was found to
react with an aryl iodonium salt to yield trifluoromethylated arene,
presumably through a high-valent, unsupported and formally
nickel(IV) intermediate.[29] Evidence that such architectures are
accessible was provided by the synthesis and structural
characterization of [PPh4]2[Ni(CF3)4(SO4)] (4, Chart 1), the first
example of a “ligandless” trifluoromethyl nickel(IV) complex.[29]
The high-valent complex 4, while it could be crystallized, proved
to be unstable in solution. The instability of high-valent solvento
complexes was also evident in electrochemical experiments,
where 2 and 3 displayed irreversible oxidations on the timescale
of the cyclic voltammogram.[29] We believed this redox instability
could be beneficial in terms of catalysis, where high-valent
intermediates would need to irreversibly form chemical bonds or
release perfluoroalkyl radicals for chemistries that turn over.
Therefore, we sought to investigate further the oxidative
chemistries of 2 and 3 and compare the reactivities to those of a
perfluoroethyl analogue.
Introduction
Nickel is
a promising platform for developing practical
fluoroalkylation methodologies as fundamental steps relevant to
chemical bond forming reactions have been established.
Reductive eliminations involving fluoroalkyl groups have been
demonstrated at higher valent states of nickel,[1-4] the generation
of fluoroalkyl radicals via electron transfer or atom abstraction
reactions of fluoroalkyl electrophiles with nickel is known,[5-12]
fluoroalkyl-based transmetalating reagents readily react with
nickel,[13-15] and catalytic transformations involving select
fluoroalkyl groups have been reported.[6, 10-13, 16-21] Nickel is also
an earth-abundant metal with a reasonably low cost. However,
most of the aforementioned chemical transformations involving
fluoroalkyl groups required nickel to bear stabilizing ligands other
than solvent. While in many cases ligands can serve to tune and
enable the reactivity of a nickel center, in other cases they can
prevent catalysis through redistributions that affect the speciation
of an active form of the nickel catalyst. Moreover, extraneous
non-solvento ligands raise the cost of a chemical process and can
even contribute to the air-sensitivity of the system. Given these
considerations, we became interested in developing new
fluoroalkylation methods with nickel that employed solvent as the
only coordinating ligand.
Such reaction conditions have
commonly been referred to as “ligandless” conditions in other
systems.[22-28]
Chart 1. Trifluoromethyl nickel reagents previously characterized.
1
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Results and Discussion
We have shown previously that [PPh4] salts of 2 and 3 (Chart 1)
were susceptible to side reactions involving organic nucleophiles
and the phosphorus moiety,[29] so herein we describe the
preparation of tetramethylammonium salts of 2, 3, and the new
perfluoroethyl derivative 5 (Scheme 1). Such compounds were
easily prepared by the addition of “Ag-Rf” to [(dme)NiBr2] in
acetonitrile solvent in the presence of [NMe4]Cl. The 19F NMR
spectrum of 5 displays four signals with integrations consistent
with two perfluoroethyl groups cis to the acetonitrile group and
one perfluoroethyl group that is trans to the acetonitrile. The CF3
resonance that is trans to the coordinated acetonitrile appears as
a quintet from the 5JFF coupling of the other two CF2 groups, while
the resonance for the CF3 groups that are cis to the acetonitrile
3
appear as a triplet. The absence of JFF coupling is common for
perfluoroalkyl groups and is suggested to arise from the rotational
averaging of coupling constants of opposite sign between the
vicinal CF2 and CF3 groups.[30-33]
Figure 2. Cyclic voltammograms of [(MeCN)Ni(C2F5)3] (5, red) and
[(MeCN)2Ni(C2F5)2] (7, blue) in MeCN. Complex, 10 mM; electrolye, 100 mM
[NBu4][PF6]; working and counter electrode, platinum; silver psuedoreference;
scan rate, 100mV/s.
Given the accessible oxidations available to the solvated nickel
fluoroalkyl complexes, we explored their reactivity with various
oxidizing electrophiles. The reactivity of the tris-perfluoroalkyl
nickelates with iodonium salts is summarized in Scheme 2. Quite
different reactivities were observed depending on the iodonium
salt used. Reaction with the alkynyl benziodoxolone reagent 8
(Scheme 2, eq 2) led to fluoroalkylated products (56% for CF3 and
54% for C2F5). In the case of trifluoromethyl, [(MeCN)2Ni(CF3)2]
was determined to be the major identifiable nickel species after
reaction was complete. Reactions that occurred with bis(4-tert-
butylphenyl)iodonium hexafluorophosphate are shown in Scheme
2, eq 3. Interestingly, even though more elevated temperatures
were required, the byproducts [(MeCN)2Ni(CF3)2] and
[(MeCN)2Ni(C2F5)2] could be observed in higher quantities than
the reactions described in eq 2. Control experiments revealed
that reactions with reagent 8 were more sensitive to the addition
of TEMPO than reactions with the aryl iodonium salts (see
Supporting Information).
Scheme 1. Synthesis of tetramethylammonium salts of 2, 3, and 5.
The electrochemical properties of the newly prepared anionic
complex 5 were evaluated by cyclic voltammetry (Figure 2). The
oxidation of 5 is irreversible and occurs at an onset potential of
+0.21 V vs the ferrocene/ferrocenium couple. Peak potentials for
5 appear at +0.60 V and +1.26 V, which are slightly more positive
than peaks observed for the trifluoromethyl derivative 2.[29]
Interestingly, the peak potential for the second redox event occurs
at the same potential as the oxidation for the charge neutral
bisperfluoroethyl complex 7 (Figure 2), consistent with the notion
that upon oxidation to the nickel(III) species 6, a perfluoroethyl
radical is lost which generates 7 during the timescale of the
experiment (eq 1). Similar redox trends were found for the
trifluoromethyl
derivatives
[(MeCN)Ni(CF3)3]
(2)
and
[(MeCN)2Ni(CF3)2] (1),[29] suggesting that while the higher
oxidation states of solvated nickel fluoroalkyl complexes can be
easily accessed via oxidation of anionic complexes such as 2 and
5, the higher valent complexes, especially in the nickel(III) state
have limited stabilities.
2
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RESEARCH ARTICLE
Scheme 2. Reaction of [NMe4][(MeCN)Ni(Rf)3] with select iodonium salts.
Yields determined by 19F NMR spectroscopy. See reference [29] for eq 3, Rf =
CF3.
acetonitrile ligands were shorter at 1.9569(17) Å. This is
consistent with the known trans influencing properties of the
trifluoromethyl group.[34] The nickel-carbon bond lengths of
octahedral 9 in the formal +4 oxidation state were significantly
longer than the average nickel(II)-carbon bond lengths in 3
(1.930(8) Å),[29] which adopts
a distorted square planar
Next, we explored reactions of the tris-perfluoroalkyl nickelates
with diazonium salts, and the results are provided in Scheme 3.
Reactions with the diazonium salts occurred more rapidly than the
iodonium salts, and starting material was consumed after 15
minutes. Fluoroalkylated arenes were formed in fair to good
yields (41 - 70%), and similar reactivities were observed for both
electron donating and withdrawing substituents (Scheme 3).
Along with the fluorinated arenes, the bis(perfluoroalkyl)nickel
byproducts were also observed in yields ranging from 30 - 62%.
A control reaction of 3,5-dichlorophenyl diazonium with 2 in the
presence of TEMPO revealed that trifluoromethyl arene was
produced in only 19% yield, while TEMPO-CF3 was formed in
76% yield (see Supporting Information).
arrangement in the formal +2 oxidation state and also possesses
four trifluoromethyl ligands. To our knowledge, 9 represents only
the second formally nickel(IV) complex reported bearing four
trifluoromethyl ligands.[29]
Scheme 3. Reaction of [NMe4][(MeCN)Ni(Rf)3] with select diazonium salts.
Yields were determined by 19F NMR spectroscopy relative to an internal
standard.
Figure 1. ORTEP diagram of 9. Ellipsoids shown at the 40% level. Selected
bond lengths (Å): Ni1-C1 1.9569(17); Ni1-C2 2.0298(19); Ni1-N1 1.9682(15).
Selected bond angles (°): C1-Ni1-N1 174.35(7); C1-Ni1-N1A 91.62(7); N1-Ni1-
N1A 85.54(8); C1-Ni1-C2 93.32(8); Ni-Ni1C2A 87.55(7).
Diazonium compounds, like iodonium salts, are known to act as
two-electron oxidants, and it is plausible that, in both systems
explored in Schemes 2 and 3, a high-valent species like [Ar-
Ni(Rf)3(MeCN)2] is involved in reductive eliminations that
produces both fluoroalkylated arene and [(MeCN)2Ni(Rf)2] co-
products. While we were unable to identify a charge-neutral and
high valent [Ar-Ni(Rf)3(MeCN)2] species, we show here that we
can identify the related solvated species [Ni(CF3)4(MeCN)2] (9)
upon chemical oxidation of [Ni(CF3)4]2–. Reaction of [Ni(CF3)4]2–
with two equivalents of Magic Blue oxidant led to 9 in 53% yield
by 19F NMR spectroscopy (eq 4) and in 30% isolated yield. As
there are two types of trifluoromethyl groups in complex 9 (trans
to a MeCN and trans to another CF3), complex 9 exhibited two
septets in the 19F NMR spectrum in acetonitrile (δ –19.2 (sept, J
= 7.2 Hz) and –30.7 (sept, J = 7.2 Hz). X-ray quality crystals could
be grown from ether/pentane at low temperature, and an ORTEP
diagram of 9 is shown in Figure 1. The trifluoromethyl groups that
are trans to each other exhibit nickel-carbon distances of
2.0298(19) Å, while the trifluoromethyl groups that are trans to
Because the cyclic volatmmograms of the anionic fluoroalkyl
complexes of nickel revealed evidence of nickel-carbon bond
homolyses reactions, we also examined the ability of solvated
nickel complexes to perform Minisci-like C-H bond
trifluoromethylations. In 2016, Wang reported the first example of
a
nickel-catalyzed C–H trifluoromethylation of electron-rich
heteroarenes using iodotrifluoromethane as the CF3 source and
[(dppp)NiCl2] as the nickel source (dppp 1,3-
=
bis(diphenylphosphino)propane).[6] In 2019, Sanford reported
that the high-valent [TpNi(CF3)3] (10, Tp = tris(pyrazolyl)borate)
could also mediate catalytic C-H trifluoromethylations of
(hetero)arenes in the presence of Umemoto’s Reagent II (11).[17]
Mechanistic investigations of this process, both experimental and
computational, suggested that the reactions proceeded through
radical chain pathways involving nickel in variable oxidation
states.[17] Since our “ligandless” nickel system could shuttle
between the formally +2 to +4 oxidation states, we explored the
possibility of using these complexes in similar C-H
3
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10.1002/anie.202104559
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RESEARCH ARTICLE
trifluoromethylations, as eliminating the need for extraneous
supporting ligands would represent a significant advance in
practicality. Table 1 summarizes the catalyst screens for the C-H
trifluoromethylation reaction outlined in eq 5 that are based on the
conditions identified by Sanford.[17] Gratifyingly, the solvated
complex 2 and the “ligandless” complex 3 catalyzed the
trifluoromethylation of 1,3,5-trimethoxybenzene by Umemoto II
reagent (eq 5) in 78 and 96% yields, respectively, providing the
first proof-in-principle that the solvated nickel complexes can
perform catalytic trifluoromethylations. Notably, [NMe4]2[Ni(CF3)4]
(3) performed just as well as [(Tp)Ni(CF3)3] (10) under these
conditions, which was previously reported to afford product in
93% yield.[17] To test the effect of metal identity on the C-H
trifluoromethylations, a reaction with the recently prepared
[(MeCN)3Co(CF3)3] (12)[35] was examined (entry 5). Complex 12,
while displaying catalytic turnover, afforded yields that were lower
than nickel.
[NMe4]2[Ni(CF3)4] in Table 1, we then determined the scope of the
catalytic C-H trifluoromethylations (Scheme 4).
Trifluoromethylations of electron rich arenes 13a-13c proceeded
smoothly, however introduction of an aldehyde into the aromatic
scaffold (13d) lowered the yield of trifluoromethylated product
significantly. Nitrogen- and sulfur-heterocycles (13e-13g) were
also successfully trifluoromethylated.
With the optimal catalyst identified as
Scheme 4. Scope of catalytic C-H functionalizations. The * reveals site(s) of
selectivity for molecules with more than one type of aromatic C-H bond.
Combined yields of trifluoromethylations at all possible sites are provided.
Yields were determined by 19F NMR spectroscopy relative to an internal
standard.
To better understand the reactivity of [NMe4]2[Ni(CF3)4] (3) with
the Umemoto II reagent (11), a stoichiometric reaction was
performed and monitored by 19F NMR spectroscopy. Upon mixing
3 with 11 in CD3CN, the 19F NMR spectrum revealed a
transformation that was consistent with the formation of
compounds 14 and 15 (Scheme 5, eq 6). Putative compound 14
exhibits a singlet in the 19F NMR spectrum in CD3CN at δ –25.1.
The 19F NMR spectrum of compound 15 is more complicated and
consists of an apparent undecet at δ –19.6 (J = 8.2 Hz) and a
quartet at δ –28.8 (J = 8.2 Hz). Compound 15 displays similarities
in the 19F NMR spectrum to the NCCH2– (carbon bound) complex
[(NCCH2)Ni(CF3)5]2– (16), a compound that was formed in trace
amounts during the synthesis 3, fortuitously crystallized, and was
structurally characterized (see Supporting Information). The
dianionic compound 16 displays resonances at δ –24.3 (apparent
septet, J = 7.5 Hz) and –27.4 (quartet, J = 7.5 Hz) in CD3CN. In
the absence of arene substrate (Scheme 5), we propose that 15
is formed via a sequential two electron oxidation of 3 by Umemoto
II reagent as outlined in eqs 7 and 8. In catalytic reactions where
arene trapping agents are present, CF3 radicals sources may
originate both by the reaction described in eq 7 and by reductive
homolysis of 17, which would produce 2 as a byproduct. Control
reactions reveal that the catalysis described in eq 5 is completely
shut down in the presence of TEMPO. We speculate that the
formation of 14 may arise through redistribution reactions
involving trifluoromethyl anion (eq 9). The reactions described in
eq 6-9 highlight the ability of solvated nickel to form trifluoromethyl
complexes with a variety of coordination numbers, formal
oxidation states, and formal overall charges. This flexible
accommodation of trifluoromethyl groups may prove to be useful
in future methodologies where nickel is required to serve as a
reservoir of CF3 anions or radicals.
Table 1. Screening of fluoroalkyl transition metal catalysts for
trifluoromethylation of arenes.a
entry
1
catalyst
yield
[(Tp)NiIV(CF3)3] (10)
96
16
78
96
59
2
3
4
5
None
[NMe4][(MeCN)NiII(CF3)3] (2)
[NMe4]2[NiII(CF3)4] (3)
[(MeCN)3CoIII(CF3)3] (12)
[a] All reactions were run on 0.05 mmol scale with Umemoto Reagent II as the
limiting reagent, and 5 equiv. of arene. The yields were determined by 19F NMR
spectroscopy using trifluorotoluene as an internal standard.
As evident from Table 1 and reference 17, 3 and 10 provide
similar yields in the catalytic C-H functionalization reactions. The
advantage of using 3 is that it can be prepared in one step, starting
from [(dme)NiBr2]. The preparation of complex 10, on the other
hand, involves the synthesis of [(MeCN)2Ni(CF3)2] from
[(dme)NiBr2], followed by ligation with pre-made [NMe4][Tp] to
produce [NMe4][(Tp)Ni(CF3)2], followed by reaction with Umemoto
II reagent to produce 10. Moreover, "ligandless" 2 and 3 can
serve as precatalysts for rapid ligand screening reactions.
A
disadvantage of using the anionic 2 and 3 relative to 10 is that
they are air-sensitive, although we have seen no issues storing
large quantities in a glovebox at -30 °C.
4
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RESEARCH ARTICLE
Keywords: fluorine • trifluoromethylation • nickel• catalysis•
perfluoroalkylation
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A number of important precedents with regard to “ligandless”
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that both [(MeCN)Ni(CF3)3] and [(MeCN)Ni(C2F5)3] successfully
react with aryl iodonium and diazonium salts as well as alkynyl
iodonium salts to give fluoroalkylated organic products. These
results clearly show that upon oxidation of acetonitrile-supported
nickel fluoroalkyl complexes, the fluoroalkyl groups can be
transferred to organic substrates. Secondly, the electrochemical
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This electrochemical behavior shows
generality with trifluoromethyl derivatives.[29] Lastly, catalytic C-H
trifluoromethylations of electron rich arenes were successfully
achieved using either [(MeCN)Ni(CF3)3] or the related
[Ni(CF3)4]2–. From a broad perspective, the stoichiometric
reactions of the solvated nickel complexes reveal that “ligandless”
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Acknowledgements
D.A.V. thanks the Office of Basic Energy Sciences of the U. S.
Department of Energy (DE-SC0009363) for support of this work.
5
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“Ligandless” nickel was found to be exceptionally capable of acting as reservoirs of fluoroalkyl anions or radicals, and this property
was exploited to fluoroalkylate organic substrates under stoichiometric and catalytic conditions.
Institute and/or researcher Twitter usernames: @DavidVicic, @sshreiber00, @LehighChem
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