Accessing Pincer Bis(carbene) Ni(IV) Complexes from Ni(II) via Halogen and Halogen Surrogates

Article abstract of DOI:10.1021/jacs.5b12827

This communication describes the two-electron oxidation of (^DIPPCCC)NiX (^DIPPCCC = bis(diisopropylphenyl-benzimidazol-2-ylidene)phenyl); X = Cl or Br) with halogen and halogen surrogates to form (^DIPPCCC)NiX₃. These complexes represent a rare oxidation state of nickel, as well as an unprecedented reaction pathway to access these species through Br₂ and halogen surrogate (benzyltrimethylammonium tribromide). The Ni^IV complexes have been characterized by a suite of spectroscopic techniques and can readily reduce to the Ni^II counterpart, allowing for cycling between the Ni^II/Ni^IV oxidation states.

Full text of DOI:10.1021/jacs.5b12827

Communication
pubs.acs.org/JACS
Accessing Pincer Bis(carbene) Ni(IV) Complexes from Ni(II) via
Halogen and Halogen Surrogates
Gabriel Espinosa Martinez, Cristian Ocampo, Yun Ji Park, and Alison R. Fout*
School of Chemical Sciences, University of Illinois at Urbana−Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801,
United States
S
* Supporting Information
ylidene)phenyl).15 Formation of the NiII-hydride complex,
(
DIPPCCC)NiH, was achieved by the oxidative addition of the
ABSTRACT: This communication describes the two-
electron oxidation of (DIPPCCC)NiX (DIPPCCC = bis-
(diisopropylphenyl-benzimidazol-2-ylidene)phenyl); X =
Cl or Br) with halogen and halogen surrogates to form
ligand aryl C−H bond onto a Ni0 starting material. The chloride
derivative, (DIPPCCC)NiCl (1), was synthesized in high yields
from the benzimidazolium salt of the ligand [H3(DIPPCCC)]Cl2
and subsequent addition of LiN(SiMe3)2 and NiCl2py4.15
Although 1 was previously reported by our group, the redox
chemistry of this molecule was not explored. Hypothesizing that
this highly donating pincer bis(carbene) ligand platform may
allow us to access a high-valent Ni species, electrochemical
studies were performed. Cyclic voltammetry on 1, in dichloro-
methane, depicts a single reversible oxidation wave at +0.57 V
versus Fc/Fc+, assigned to be the NiII/NiIII couple (Figure S1).
Attempts to chemically access and isolate a NiIII species by
treating 1 with Ag+, Ph3C+, or Fc+ salts were unsuccessful and
resulted in mixtures of diamagnetic products, with 1 being the
main component. In addition, salt metathesis of 1 with
noncoordinating anions was not successful. van Koten was able
to access high-valent NiIII species from the addition of CuX2 and
I2 to a NCN pincer NiII species; similar reactions did not produce
a NiIII species in our system.16
(
DIPPCCC)NiX3. These complexes represent a rare
oxidation state of nickel, as well as an unprecedented
reaction pathway to access these species through Br2 and
halogen surrogate (benzyltrimethylammonium tribro-
mide). The NiIV complexes have been characterized by a
suite of spectroscopic techniques and can readily reduce to
the NiII counterpart, allowing for cycling between the NiII/
NiIV oxidation states.
omogenous nickel catalysis has been used for the synthesis
Hof a variety of compounds including natural products,
pharmaceuticals, and polymers. Mechanistic studies on these
catalytic systems have indicated that the reactions proceed via
one- or two-electron redox events to access Ni0, NiI, NiII, and/or
NiIII intermediates.1−5 Such studies, coupled with the recent
development of well-defined PdII/PdIV catalytic cycles, which
have demonstrated complementary reactivity and selectivity to
their lower-valent counterparts,6−9 have rendered the isolation of
high-valent nickel complexes an area of great interest.1,4
Unfortunately, despite the continued development of earth-
abundant metal catalysts, isolated and well-characterized organo-
metallic NiIV complexes are relatively rare. Reports include the
isolation of octahedral NiIV species by oxidative addition of
methyl iodide onto NiII acylphenolate tris(phosphine) com-
plexes reported by Klein and co-workers,10 the pseudotetrahe-
dral bromotris(1-norbornyl)nickelIV complex isolated by Dimi-
trov and co-workers,11 and the first isolated tetraalkyl NiIV
complex reported by Turro and co-workers.12 Most recently,
Sanford and co-workers reported the isolation of octahedral NiIV
complexes featuring the tris(2-pyridyl)methane and tris-
(pyrazolyl)borate ligand platforms, capable of reductively
eliminating C−X bonds (X = O, S, N, CF3).13,14 With the
growing interest in harnessing the power of a NiII/NiIV redox
cycle for bond-forming reactions, and given our interest in the
alternate reactivity that NiIV has to offer compared to other
oxidation states, we set out to investigate the viability of accessing
complexes of nickel in the formal oxidation state +4 supported by
a monoanionic bis(carbene) pincer platform.4
Despite the failed attempts to isolate NiIII complexes ligated by
the (DIPPCCC) platform, the chemical oxidation of (DIPPCCC)-
NiX (1, X = Cl; 3, X = Br) with two-electron oxidants was
explored. Interestingly, the reaction of 1 with iodobenzene
dichloride (PhICl2) resulted in an immediate color change from
1
orange to purple (Scheme 1). Characterization by H NMR
spectroscopy of the crude purple product revealed complete
conversion of 1 to a new diamagnetic complex, 2, and the
concurrent formation of phenyl iodide, determined by 1H NMR
1
spectroscopy (7.70, 7.33 and 7.11 ppm, Figure S4). The H
NMR spectrum of 2 features two doublets, which integrate to 12
H each for the iPr methyl groups at 1.14 and 0.84 ppm (versus
Scheme 1. Synthesis of (DIPPCCC)NiCl3 (2) from
(
DIPPCCC)NiCl (1)
Previously, we reported the synthesis of NiII pincer complexes
featuring the monoanionic bis(carbene) ligand platform,
DIPPCCC (DIPPCCC = bis(diisopropylphenyl-benzimidazol-2-
Received: December 15, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b12827
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Communication
Figure 1. Molecular structures of 2, 3, and 4 shown with 50% probability ellipsoids. Solvent molecules and H atoms have been omitted for clarity.
Table 1. Selected Structural Parameters of Complexes 1−4
1 (X = Cl)
2 (X = Cl)
3 (X = Br)
4 (X = Br)
Bond Distance (Å)
Ni1−C1
Ni1−C13
Ni1−C20
Ni1−X1
1.9188(17)
1.8504(16)
1.9201(17)
2.1879(5)
1.942(3)
1.888(2)
1.936(3)
2.2682(7)
2.2192(7)
2.2106(7)
1.9284(19)
1.8528(18)
1.9328(19)
2.3365(3)
1.9703(14)
1.8994(14)
1.9788(14)
2.4102(3)
2.3511(3)
2.3675(3)
Ni1−X2
Ni1−X3
Bond Angles (deg)
C13−Ni1−X1
C1−Ni1−C20
X2−Ni−X3
175.66(5)
160.97(7)
179.13(8)
161.96(10)
171.70(3)
177.17(6)
160.57(8)
176.93(4)
160.77(6)
173.504(10)
1.21 and 0.88 ppm of 1), and one septet integrating to 4 H for the
methine protons at 2.80 ppm (versus 2.47 ppm of 1). The
diamagnetic nature of 2 could be attributed to the formation of a
dimeric complex featuring two NiIII centers antiferromagnetically
coupled; however, this formulation is unlikely due to the
bulkiness of the diisopropylphenyl flanking groups of the ligand.
The diamagnetic character of 2 could also be rationalized by a
two-electron oxidation at the nickel center to form the C2v
symmetric, octahedral complex (DIPPCCC)NiCl3, which would
be consistent with the number of resonances observed in the 1H
NMR spectrum of 2. The thermal instability and diminished
solubility at low-temperature of 2 precluded characterization by
13C NMR spectroscopy.
oxidation with elemental bromine in an effort to isolate
DIPPCCC)NiBr3. Bromine has served as a viable way to access
(
NiIII species, as shown by Zargarian17,18 and Nocera.19 In other
examples, the halide moiety only interacts with NiII halide
complexes to form trihalide anions and does not oxidize the
metal center.19 Attempts to cleanly access the precursor NiII−Br
species, (DIPPCCC)NiBr (3), by salt metathesis of 1 with
bromide salts proved to be unsuccessful; therefore, a new
synthetic route to 3 was sought. Treatment of 1 with NaSPh,
followed by removal of NaCl and protonation with HBr resulted
in the formation of 3 with no chloride impurities. As expected,
resonances in the 1H NMR spectrum of 3 were marginally shifted
from 1. The 1H NMR spectrum of 3 in THF-d8 featured the two
doublets corresponding to the iPr methyl groups at 1.23 and 0.86
ppm (versus 1.21 and 0.88 ppm of 1) and the septet
To elucidate the molecular structure of 2, crystals suitable for
X-ray analysis were grown by vapor diffusion of pentane into a
solution of 2 in benzene/THF at −35 °C (Figure 1). Solid-state
structural characterization revealed an octahedral Ni center
featuring the monoanionic pincer ligand and three chloride
ligands. Examination of the bond lengths in the ligand precludes
the possibility of a ligand-based radical (Figure S4), thus
confirming the formulation of 2 as a formal NiIV complex with the
formulation (DIPPCCC)NiCl3 (Figure 1). Comparing the
structural parameters of 2 to the NiII starting material 1 shows
elongation of all the Ni-ligand bond distances in the plane of the
pincer platform (Table 1). The Ni−CNHC distances have
elongated from 1.9188(17) and 1.9201(17) Å to 1.942(3) and
1.936(3) Å, the Ni−Caryl distance has elongated from 1.8504(16)
Å to 1.888(2) Å, and the Ni−Cl1 distance has elongated from
2.1879(5) Å to 2.2682(7) Å, likely due to the change in
coordination at the nickel center from square planar in 1 to
octahedral in 2 upon the two-electron oxidation.
i
corresponding to the Pr methine protons at 2.46 ppm (versus
2.47 ppm of 1) in addition to the appropriate number of aromatic
proton resonances, also slightly shifted from 1. Crystals of 3
suitable for X-ray analysis were grown by vapor diffusion of
diethyl ether into a concentrated solution of 3 in THF. Complex
3 features a square-planar NiII center, with structural parameters
similar to 1 and a Ni−Br distance of 2.3365(3) Å (Table 1).
Exposure of complex 3 to elemental bromine resulted in an
immediate color change from orange to dark green (Figure 2).
1
Characterization by H NMR spectroscopy of the dark green
product, 4, revealed the formation of a diamagnetic species,
featuring two doublets integrating to 12 H each for the iPr methyl
groups, and one septet integrating to four H for the iPr methine
protons, shifted to 1.19, 0.85, and 2.98 ppm, respectively, from
the starting material 3 (Figure 2). This finding is consistent with
the molecular structure of 4 being (DIPPCCC)NiBr3. This
reaction represents a unique two electron oxidation of nickel by
Encouraged by the accessibility of NiIV via the chlorine
surrogate PhICl2, we investigated the reactivity of NiII toward
B
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Journal of the American Chemical Society
Communication
Figure 2. (Left) Reaction scheme for the synthesis of (DIPPCCC)NiBr3 (4) from 3. (Right) Comparison of the 1H NMR spectra of 3 (black) and 4
(green).
bromine as previous examples only resulted in the one-electron
oxidized product.17−20
In order to explore the possible involvement of NiIII in the
oxidation of 3 to 4, complex 3 was reacted with 1/2 equiv of
BTMABr3. Monitoring of the reaction by 1H NMR spectroscopy
revealed the presence of equimolar amounts of 3 and 4 with no
observable paramagnetic species that would be consistent with
NiIII formation. This finding, as well as the clean decomposition
via thermolysis, suggests a preference for oxidizing or reducing
the nickel center by two electrons, respectively, as opposed to a
one-electron oxidation route. This result does not rule out the
possibility of the formation of a transient NiIII species, which
subsequently disproportionates to form 1/2 equiv of 4 and 1/2
equiv of 3, although such a pathway is unlikely.
Crystals of 4 suitable for X-ray analysis were grown by slow
evaporation of a benzene solution at room temperature (Figure
1). Upon refinement, an octahedral NiIV complex with the
predicted formulation (DIPPCCC)NiBr3 was noted, similar to
complex 2. In addition, the Ni−ligand bond distances in the
plane of the pincer platform have elongated (Table 1), as
observed in the case of the oxidation of 1 to 2. Finally, structural
parameters of the ligand compared in 3 and 4 indicate there is no
radical character in the ligand (Figure S9), as observed for 1 and
2. This route to access the desired NiIV complex produces
complex 4 in quantitative yield (97%).
Kraft reported the formation of a bis(carbene) palladium
complex, (NHC)2PdIVCl4, that was capable of chlorinating
alkenes and alkynes.24 Given the rather clean reduction
chemistry of 2 to 1 and 4 to 3, we investigated the ability of
complex 4 to transfer Br2 (or two ·Br) to an organic substrate.
The addition of styrene (100 equiv) to a THF solution of 4
resulted in the formation of (1,2-dibromoethyl)benzene in 87%
yield (as determined by 1H NMR spectroscopy) suggesting the
ease with which 4 can transfer halogens to a substrate (Table 2).
The bromination of cyclohexene proceeded with complete
conversion of 4 to 3. The brominated product was observed in
71% yield (assayed by 1H NMR spectroscopy) concomitant with
trace amounts of the corresponding halohydrin (GC−MS).
Independently prepared samples of the dibrominated products
confirmed the major product formation.25 Reaction of 4 with 2-
mesitylmagnesium bromide resulted in formation of mesityl
bromide and mesitylene, as well as trace homocoupled product
(GC−MS). Interestingly, the reaction of 4 with 1 equiv of
lithium hexamethyldisilazide ((Me3Si)2NLi) resulted in the
analogous formation of (Me3Si)2NBr and (Me3Si)2NH. Based
on the observed reduction of 4 to 3 and subsequent transfer of
halogen to substrates, we propose that the NiIV complexes are
undergoing a concerted reductive elimination followed by
halogenation of the alkene or other organic substrate. However,
radical reactivity cannot be ruled out and further investigations
are necessary and ongoing in our laboratory.
Following the isolation of NiIV by addition of elemental
bromine, we explored the viability of employing the bromine
surrogate benzyltrimethylammonium tribromide (BTMABr3) as
the oxidant. Upon exposure of 3 to BTMABr3, an immediate
color change from orange to dark green was noted (Figure 2) and
the formation of 4 was confirmed by 1H NMR spectroscopy. The
use of BTMABr3 is a convenient way to access complex 4 in
excellent yield (96%), as it does not require weighing elemental
bromine or preparing standard stock solutions. In addition,
PhICl2 and BTMABr3 are safer alternatives to elemental halides
and can be easily handled in a glovebox.
Interested in assessing the stability of the NiIV compounds, a
solution of 2 in THF was analyzed after 1.5 h at room
temperature by 1H NMR spectroscopy. The spectrum revealed
reduction of the trihalide to form approximately 1/3 equiv of 1.
The observation of green crystals along the walls of the NMR
tube suggested the remainder of the complex had formed the
previously reported paramagnetic compound [H3(DIPPCCC)]-
[NiCl4].15 This observation is consistent with the release of Cl2
(or two ·Cl), which, upon reacting with THF, forms HCl.21−23
Subsequent reaction of 1 with HCl results in the formation of
[H3(DIPPCCC)][NiCl4].15 Analysis of the crude 1H NMR
spectrum was consistent with the formation of this paramagnetic
species. Interestingly, it is possible to regenerate the trichloride
species from 1 present in the reduced mixture upon treatment
with additional PhICl2. In contrast, a THF solution of compound
4 was found to be stable in air for over 24 h. Heating a sample of 4
in THF-d8 at 50 °C for 6 h resulted in a 10% reduction of 4 to the
NiII species, 3 (Figure S13). After 24 h of heating only 32% of 4 to
3 was reduced, and no other products could be observed (Figure
S14).
The described results confirm the ability of the electron-rich
(
DIPPCCC) pincer ligand to stabilize higher oxidation states at
nickel. This report represents the first example of isolation of
formal NiIV organometallic complexes supported by a mono-
anionic bis(carbene) pincer ligand and obtained via oxidation
with halogen and halogen surrogates. In addition, the
coordination of halide ligands to the NiIV metal center allows
C
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for possible ligand exchange to promote bond formation via
reductive elimination pathways, featuring a NiII/NiIV redox
couple.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.5b12827.
■
S
Spectral data and synthesis for complexes 2−4 (PDF)
Selected crystallographic data for complexes 2−4 (CIF)
AUTHOR INFORMATION
Corresponding Author
*fout@illinois.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Dr. Jeffery A. Bertke for assisting with
crystallography and the NSF for financial support with a
CAREER award (13151961) to A.R.F. The authors would like
to thank Dr. Ellen Matson for help with electrochemistry and
Abdulrahman Ibrahim for insightful discussions.
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DOI: 10.1021/jacs.5b12827
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