Characterization and reactivity of a terminal nickel(III)-oxygen adduct

Article abstract of DOI:10.1002/chem.201406485

High-valent terminal metal-oxygen adducts are hypothesized to be the potent oxidizing reactants in late transition metal oxidation catalysis. In particular, examples of high-valent terminal nickel-oxygen adducts are scarce, meaning there is a dearth in th

Full text of DOI:10.1002/chem.201406485

DOI: 10.1002/chem.201406485
Full Paper
&
Metal–Oxo Species
Characterization and Reactivity of a Terminal Nickel(III)–Oxygen
Adduct
Paolo Pirovano,[a] Erik R. Farquhar,[b] Marcel Swart,[c] Anthony J. Fitzpatrick,[d]
Grace G. Morgan,[d] and Aidan R. McDonald*[a]
Abstract: High-valent terminal metal–oxygen adducts are
hypothesized to be the potent oxidizing reactants in late
transition metal oxidation catalysis. In particular, examples of
high-valent terminal nickel–oxygen adducts are scarce,
meaning there is a dearth in the understanding of such oxi-
dants. A monoanionic NiII-bicarbonate complex has been
found to react in a 1:1 ratio with the one-electron oxidant
(ca. 95%). Electronic absorption, electronic paramagnetic res-
onance, and X-ray absorption spectroscopies and density
functional theory calculations confirm its description as
a low-spin (S=1= ), square planar NiIII–oxygen adduct. This
2
rare example of a high-valent terminal nickel–oxygen com-
plex performs oxidations of organic substrates, including
2,6-di-tert-butylphenol and triphenylphosphine, which are in-
dicative of hydrogen atom abstraction and oxygen atom
transfer reactivity, respectively.
tris(4-bromophenyl)ammoniumyl
hexachloroantimonate,
yielding a thermally unstable intermediate in high yield
Introduction
a dearth in the number of terminal late transition metal–
oxygen adducts (metal=Co, Ni, Cu). The groups of Ray/Nam,
and Tolman have recently made great strides in preparing the
first examples of scandium(III) oxocobalt(IV) and hydroxocop-
per(III) species, respectively.[4] However, well-characterized
high-valent terminal nickel–oxygen adducts remain elusive.
Synthetic nickel-containing complexes have been exploited
as catalysts for the oxidation of hydrocarbons[5] and water.[6]
Similarly, nickel-containing films and nanoparticles have been
employed as oxidation catalysts.[2b,7] Terminal high-valent
nickel–oxygen adducts (NiIII/IV–OX) have been postulated as the
active oxidant in these systems. However, very few such spe-
cies have been isolated and well-characterized to date.[5,8] In-
terestingly, computational studies have forecast that a NiIII=O
species would be capable of the activation of the strongest of
CꢀH bonds (CH4, bond dissociation energy (BDE)=104 kcal
molꢀ1).[9]
Nature employs Fe- and Cu-containing oxygenases to perform
hydrocarbon oxidation and Mn-containing photosystem II to
perform water oxidation.[1] Similarly, many synthetic Mn-, Fe-,
Co-, Ni-, and Cu-containing catalysts are capable of both hy-
drocarbon and water oxidation.[2] In most of these natural and
non-natural first-row transition metal-catalyzed conversions,
terminal high-valent metal–oxygen adducts, such as metal–oxo
(M=O), metal–oxyl (MꢀO·) or metal–hydroxo (MꢀOH) inter-
mediates, have been implicated as the reactive oxidants. These
oxidants are potent hydrogen atom abstraction (HAA) or
oxygen atom transfer (OAT) reagents, capable of activating
some of the most inert of substrates. Many examples of enzy-
matic and synthetic terminal oxomanganese and oxoiron spe-
cies have arisen in the past ten years;[3] however there remains
[a] P. Pirovano, Dr. A. R. McDonald
Several groups have attempted to prepare high-valent
nickel–oxygen adducts; for example Ray and co-workers re-
ported that the oxidation of [NiII(TMG3tren)(OTf)](OTf)
(TMG3tren=(tris[2-(N-tetramethylguanidyl)ethyl]amine, OTf=
trifluoromethanesulfonate) with 3-chloroperoxybenzoic acid
produced two species claimed to be NiIIIꢀO(X) complexes.[10]
However, the low yield (15%) of NiIII and identification of at
least two NiIII species in the reaction mixture hampered their
School of Chemistry and CRANN/AMBER Nanoscience Institute
The University of Dublin, Trinity College
College Green, Dublin 2 (Ireland)
E-mail: aidan.mcdonald@tcd.ie
[b] Dr. E. R. Farquhar
Case Western Reserve University Center for Synchrotron Biosciences
National Synchrotron Light Source, Brookhaven National Laboratory
Upton, NY 11973 (USA)
[c] Prof. M. Swart
´
characterization. Chmielewski and Latos-Graz˙ynski treated
Institut de Quꢀmica Computacional i Catꢁlisi
Universitat de Girona, Facultat de Ciꢂncies
Campus Montilivi, 17071 Girona, Catalunya (Spain)
a NiIIIꢀBr complex with hydroxide to yield a new species that
was claimed to be a NiIIIꢀOH complex.[11] However, this species
was only characterized by EPR spectroscopy, and no reactivity
studies were performed. Chiou and Liaw prepared NiIIIꢀOR (R=
Me, Ph) species, but did not report on their HAA or OAT reac-
tivity properties.[12] Interestingly, several m-oxo–dinickel(III)[13]
and nickel-containing heterobimetallic m-oxo complexes[14]
[d] A. J. Fitzpatrick, Dr. G. G. Morgan
School of Chemistry and Chemical Biology
University College Dublin, Science Centre
Belfield, Dublin 4 (Ireland)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406485.
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have been isolated and found to be effective HAA reagents. In
contrast, there remains a lack of suitable high-valent terminal
nickel–oxygen adducts that could help us understand further
the reactivity properties of late transition metal oxidants. With
this in mind, we set out to prepare, characterize, and investi-
gate the reactivity of such complexes. We identified Holm’s
Magic Blue) no difference in the electronic absorption features
was noted. These observations are important as they demon-
strate that solvent is not coordinating to the NiIII center in 3.
Should solvent coordinate to the NiIII center in 3, one would
anticipate different UV/Vis characteristics of the oxidized prod-
uct in different solvent media.
[NiII(OH)(pyN2 )]ꢀ (1; Scheme 1) and [NiII(OCO2H)(pyN2 )]ꢀ (2;
3 was further characterized using X-band electron paramag-
netic resonance (EPR) spectroscopy. An acetone solu-
tion of 3 was frozen in liquid nitrogen and an axial
spectrum was obtained at 113 K (Figure 2, g=2.25,
2.02). The yield of NiIII was calculated to be 95%
(ꢁ15%) by double integration of the signal of 3
against that of a radical standard (TEMPO=(2,2,6,6-
Me2
Me2
tetramethylpiperidin-1-yl)oxyl)). The obtained
g
values are consistent with an S=1= (low-spin) d7 NiIII
2
species,[21d,22] and the average g value (gav. =2.17) is
Scheme 1. Complexes 1,[15a] 2,[15a] and 3. Schematic representation of the preparation of
2 and 3.
indicative of the unpaired electron sitting on the Ni
Me2
Scheme 1) complexes (pyN2 =N,N’-bis(2,6-dimethylphenyl)-
2,6-pyridinedicarboxamidate) as excellent candidates for the
generation of high-valent terminal nickel–oxygen adducts.[15]
Using similar 2,6-pyridinedicarboxamidate ligands, thermally
stable NiIII and NiIV complexes,[16] as well as CuII–superoxide,[17]
and CuIII–OH[4c,18] entities have been isolated. Herein, we de-
scribe the oxidation of 2 (which is formed from the reaction
between 1 and CO2; Scheme 1), to yield a metastable NiIII–O(X)
species that displays the ability to perform HAA and OAT.
Results and Discussion
In acetone, at ꢀ808C, 2 was treated with the organic oxidant
tris(4-bromophenyl)ammoniumyl
hexachloroantimonate[19]
(“Magic Blue”, dissolved in CH3CN, E8’=0.70 V vs. Fc+/Fc[20]). An
instantaneous reaction took place, as evidenced by the appear-
ance of two intense features in the electronic absorption spec-
trum (lmax =520 and 790 nm; Figure 1), assigned to a novel
species 3. A titration of Magic Blue against 1 showed that the
intensity of the new features reached a maximum after the ad-
dition of one equivalent of Magic Blue (Figure 1, inset). The ad-
dition of greater than one equivalent of Magic Blue did not
cause a further change in the new features, while the presence
of excess unreacted Magic Blue in the reaction mixture was
observed in the electronic absorption spectrum (Figure S1 in
the Supporting Information). Initial indications thus suggested
that 2 had been oxidized by one electron yielding a novel spe-
cies, 3, that we postulate is a NiIIIꢀOCO2H complex.
Figure 1. UV/Vis spectrum of 2 (grey trace, 0.4 mm in acetone) that was
treated with Magic Blue at ꢀ808C to yield 3 (black trace). Inset: Titration of
Magic Blue with 2, monitored by plotting the intensity of the lmax =520 nm
feature versus equivalents of Magic Blue.
The intense absorption features in the visible and near-IR
(NIR) regions of the UV/Vis spectrum, observed upon the reac-
tion between 2 and Magic Blue to yield 3, suggest a change in
the oxidation state of the Ni center. Many of the NiIII complexes
reported to date display similarly intense chromophores in the
visible and NIR regions of their absorption spectra.[10,12,16,21] In-
termediate 3 could be generated in acetone or THF. The fea-
tures associated with 3 are the same in both solvents. Likewise,
when 3 was prepared in the presence or absence of excess
CH3CN (acetone or CH3CN can be used as the solvent for
center, rather than being a ligand-based radical. Importantly,
g ? > >g , which is typical for axially elongated octahedral
j j
complexes or square planar complexes.[21b,d,22,23]
The most likely configurations for a low-spin d7 ion in
pseudo-tetragonal symmetry involve the unpaired electron
density being located in an orbital of predominant dz2 or dxy
character. Instances where g ? > >g have been usually un-
j j
derstood to correspond to
a dz2 singly-occupied orbi-
tal.[21b,d,22,23b,d] Overall, the EPR analysis suggests that oxidation
of the square-planar precursor 2 resulted in the loss of an elec-
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X-ray absorption spectroscopy (XAS) was employed to ana-
lyze the electronic and structural properties of 3. The XAS
edge energy measured for 3 (8345 eV) lies in the expected
range for NiIII complexes.[14,24] Interestingly, there is only a very
minor shift in the K-edge energy compared to the K-edge
energy measured for NiII-containing 2 (Figure 4). This is not un-
Figure 2. Solid trace: X-band EPR spectrum of 3 in a frozen acetone solution.
Measured at 113 K; microwave power=31.6 mW; modulation amplitu-
de=0.5 mT; g ? =2.25, g =2.02, gav. =2.17. Dashed trace: simulated spec-
j j
trum for 3. The system was modelled as an S=1= electron spin with axial g
2
tensor and inhomogeneous line broadening. The greater line broadening in
the x and y directions is possibly due to unresolved hyperfine coupling with
the 14N nuclei of the ligand.
Figure 4. XANES spectra of 2 (black trace) and 3 (grey trace), along with as-
sociated first and second derivatives.
tron yielding a square-planar S=1= d7 NiIII species, 3, with
2
a metal-based dz2-like occupied orbital. Density functional
theory (DFT) Mulliken spin density calculations were per-
formed, in order to further probe the location of the unpaired
electron density in 3. These calculations supported the experi-
mental observations that the unpaired spin predominantly re-
sides in a metal-based molecular orbital. The DFT calculations
predict that either metal-based dz2- or dxy-like molecular orbi-
tals are the likely locations of the unpaired electron density
(Figure 3 displays spin density plot for dz2 occupancy). The DFT
calculations thus support the EPR-determined electronic struc-
ture of 3.
usual for Ni (and indeed Cu) K-edge analysis; several groups
have made similar observations when comparing NiII/III species
within comparable coordination environments.[24a,25] Contribu-
tions to the edge from 1s!4p absorption features distort the
edge and prevent accurate assessment of the edge shift upon
oxidation of 2 to yield 3. The derivatives of the normalized X-
ray absorption near-edge spectrum (XANES) indicate a modest
edge shift of 0.1–0.2 eV when comparing the 1s!3d pre-edge
transition of 2 to 3, in accordance with an increased oxidation
state of the Ni center in 3. Although there appears to be negli-
gible change in the edge energy between 2 and 3, the shape
of the edge and relative intensities of features in the edge are
markedly different. Such differences are indicative of the dis-
tinct electronic environments of the Ni-center in complexes 2
and 3.
Extended X-ray absorption fine structure (EXAFS) analysis of
3 yielded a disordered first coordination shell of 4 O/N donors
(see the Supporting Information, Figure S2 and Table S1). This
shell could be split into two components with two scatterers
at approximately 1.84 ꢁ and two scatterers at approximately
1.99 ꢁ giving the best fit of the experimental data obtained for
3 (Table 1). These observations suggest that a single-bonded
oxygen ligand is present in 3, that is, that the bicarbonate re-
mains intact. Attempts to fit the data containing a very short
Figure 3. Left: DFT-optimized structure of 3. Hydrogen atoms (except bicar-
bonate proton) have been excluded for clarity. Right: DFT-optimized spin
density plot of 3.
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ꢀ408C, the half-life of 3 in [H6]acetone was 5600 s, whereas in
[D6]acetone it was found to be 6800 s (kinetic isotope effect
(KIE)=1.2). The observation of such acetone-derived products
and an extended lifetime in perdeuterated solvent supports
the postulate that 3 oxidizes acetone by rate-limiting HAA
during its thermal decay. This is an important discovery, as ace-
Table 1. Nickel–ligand bond distances for 2 and 3.
NiꢀOCO2H
1.871
NiꢀN(py)
1.817
NiꢀN(amid)
2@1.895
2[a][15a]
2[b]
4@1.87
2[c]
1.90
1.96
1.80
1.84
2@1.90
2@1.99
2@1.93
3[b]
2@1.84
tone contains a very strong CꢀH bond (BDE=93 kcalmolꢀ1)[27]
,
3[c]
suggesting that 3 is a very capable oxidant.
[a] Determined using XRD; [b] determined using EXAFS; [c] determined
using DFT.
We investigated further the HAA reactivity of 3 towards ex-
ternal substrates by its reaction with molecules containing
somewhat weaker XꢀH bonds (X=C, O; we were limited in
our substrate scope by solubility issues at low temperature). At
ꢀ408C, 3 reacted with 100 equivalents of 2,6-di-tert-butylphe-
nol (DTBP), as evidenced by the disappearance (600 s) of the
electronic absorption features attributed to 3 (see the Support-
ing Information, Figure S3). This resulted in the appearance of
a new band at lmax =555 nm, which we attribute to the forma-
tion of the phenoxyl radical as a result of HAA from DTBP by
3.[28] EPR spectroscopy confirmed the formation of a phenoxyl
radical (see the Supporting Information, Figure S4). After warm-
ing to room temperature, 3,3’,5,5’-tetra-tert-butyl-[1,1’-bis(cy-
clohexane)]-2,2’,5,5’-tetraene-4,4’-dione (see the Supporting In-
formation, Scheme S1) and traces of 2,6-di-tert-butylquinone
were detected by GC-MS. These products are formed by radical
coupling or thermal decomposition, respectively, of the parent
2,6-di-tert-butylphenoxyl radical. A pseudo-first-order rate con-
stant (kobs) for this reaction was determined by plotting the
change in absorbance features for 3 against time and fitting
the resulting curve (see the Supporting Information, Figure S5).
A second-order rate constant (k2) was calculated from the
slope of a linear plot of kobs values determined under a series
of substrate concentrations (see the Supporting Information,
Figure S6). The k2 value determined for the reaction between 3
NiꢀO bond (ca. 1.65 ꢁ) resulted in very poor fits, ruling out the
possibility of 3 being a NiIII=O species. Comparison of the
EXAFS fits for 2 and 3 suggests there are little to no structural
differences in the two complexes. The fits acquired for 2
match well with the X-ray diffraction determined bond distan-
ces obtained for 2.[15a] We have employed DFT to further un-
derstand the structural properties of 3 (Table 1, Figure 3). The
DFT predictions indicate that the bicarbonate ligand in 3
would remain bound in a monodentate fashion, in good
agreement with the EXAFS analyses of a first coordination
sphere of four donors. Furthermore, the computational analy-
ses predicted that the d7 NiIII ion in 3 would remain in a square
planar coordination environment analogous to that seen for 2,
in good agreement with the EPR measurements that indicate
the NiIII ion sits in a square planar environment. DFT also pre-
dicted that the NiꢀOCO2H and NiꢀN(py) bond distances in 3
would be 1.96 ꢁ and 1.84 ꢁ (Table 1), respectively, in reasona-
bly good agreement with the EXAFS analyses showing two
O/N scatterers at approximately 1.84 ꢁ. The combination of
EXAFS and DFT predictions shows that the Ni center in 3 has
remained 4-coordinate, in a square planar environment, and
that the bicarbonate ligand is present.
and DTBP was 0.1040mꢀ1 ꢀ1, whereas for deutero-DTBP a k2
s
It is important to note that the obtained EXAFS data could
also be reasonably well fit with an OH ligand (in place of
OCO2H). Holm and co-workers demonstrated that 2 reversibly
binds CO2, and it is reasonable to suggest the affinity of 3 for
CO2 may be less than for 2.[15a,b,d] We endeavored to probe 3
using Raman spectroscopy, but failed to identify peaks that
confirmed the presence of either OH or OCO2H ligands. We
were hampered by the rich Raman spectrum of the acetone
support medium. We also failed in our efforts to obtain mass
spectrometric evidence for the molecular formula of 3, pre-
sumably as a result of the low thermal stability of 3. It is impor-
tant to note that oxidation of 1 with Magic Blue does not yield
the same spectroscopic features attributed to 3, but in fact
yields an as yet unidentified species. We therefore conclude
that 3 retains the coordinated OCO2H ligand.
value of 0.0503mꢀ1 sꢀ1 was determined, yielding a KIE value of
2.1. This KIE value is consistent with 3 performing HAA on the
DTBP OꢀH bond, and with HAA being rate-limiting.
Compound 3 was also found to react with 1-benzyl-1,4-dihy-
dronicotinamide (BNAH; CꢀH bond dissociation energy (BDE=
64 kcalmolꢀ1)) at ꢀ808C, as evidenced by a rapid disappear-
ance of the visible absorption features attributed to 3 (see the
Supporting Information, Figure S7). The product of this reac-
1
tion was identified by H NMR as 1-benzyl-1-pyridinium-3-car-
boxamide, which typically forms as a result of HAA from the
CꢀH bond of BNAH. This product formed as a result of a two-
electron oxidation of BNAH. We assume this occurs through in-
itial HAA from BNAH followed by an electron transfer from the
resultant product to another molecule of 3. In summary, 3 was
found to be a quite reactive HAA reagent capable of activating
OꢀH and CꢀH bonds at low temperatures.
Compound 3 was stable at ꢀ808C, but decayed upon warm-
ing above ꢀ408C. After thermal decay and acidic workup of
We also probed the capacity of 3 to carry out OAT. 3 was
treated with triphenylphosphine (PPh3, 50 equiv) at ꢀ808C, re-
sulting in the formation of triphenylphosphine oxide (O=PPh3,
detected by ESI-MS and 31P NMR spectroscopy) in near quanti-
tative yields (see the Supporting Information, Figure S8). OAT is
a two-electron transfer process; thus the formation of O=PPh3
from this reaction would be expected to yield a NiI product.
Me2
the reaction mixture, the protonated ligand (H2pyN2 ) was re-
covered without any indications of ligand oxidation (no evi-
dence for ligand hydroxylation or oxidative decomposition was
1
obtained by mass spectrometry or H NMR spectroscopy). In-
terestingly, ESI-MS showed the presence of trace amounts of
hydroxyacetone, pyruvic acid, and acetic acid.[26] Importantly, at
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We did not observe any NiI species, but rather only NiII prod-
ucts. Presumably, the formed NiI product reacted with another
molecule of 3 (similar to the BNAH oxidation) to yield the ob-
served NiII products. Interestingly, 3 was completely consumed
by just 0.5 equivalents of PPh3 and BNAH, confirming our sus-
picions about the mechanisms of its reductive decay. A
second-order rate constant (k2) for the reaction between 3 and
3 was immediate, as evidenced by the formation of bands in the
UV/Vis spectrum at 520 and 790 nm.
General procedure for reactivity experiments
A solution of 3 at ꢀ808C (or ꢀ408C) was prepared according to
the above procedure. Substrates were added as concentrated ace-
tone solutions. The reactions were monitored using UV/Vis spec-
troscopy, and after complete disappearance of the features corre-
sponding to 3, were allowed to warm up to room temperature.
The reaction mixtures were diluted with methanol for MS analysis,
or the solvent was removed in vacuo for NMR analysis.
PPh3 was determined (5.3mꢀ1 ꢀ1; see the Supporting Informa-
s
tion, Figures S9 and S10). This k2 value for PPh3 oxidation is
quite high compared to values determined for Fe=O and Mn=
O complexes in the same reaction.[29] We believe it is quite sig-
nificant that the Ni species displayed such a high rate constant.
3 was also found to be highly reactive towards one-electron
reductants. Ferrocene (1 equiv in acetone) reacted with 3 in-
stantaneously at ꢀ808C, as evidenced by the disappearance of
UV/Vis features attributed to 3, and the formation of new fea-
tures in the visible region that can be attributed to the ferroce-
nium cation (lmax =630 nm; see the Supporting Information,
Figure S11). The conversion of ferrocene to ferrocenium was
quantitative and the yield of ferrocenium was determined to
correspond to exactly one equivalent of 3 being reduced. 3
thus represents an exciting example of a terminal nickel–
oxygen adduct that is capable of HAA, OAT, and electron trans-
fer.
Rate constants determination
Reactions were carried out with 7 to 300 equivalents of substrate
to ensure pseudo-first-order conditions. The second-order rate con-
stant (k2) was determined from the linear dependence of the
pseudo-first-order rate constant (kobs) on substrate concentration.
Values for the observed kobs were obtained by fitting the decay of
the absorbance at 520 nm during a reaction as an exponential. The
average from repeated experiments were utilized for the determi-
nation of k2.
Additional experimental, spectroscopic, and computational infor-
mation is included in the supporting information file. This includes
[D6]acetone NMRs of 1 and 2, EPR and XAS methods and data anal-
ysis, DFT methods and optimized-structures coordinates, spectra
and kinetic data for the reactivity studies.
Conclusion
Acknowledgements
In summary, the one-electron oxidation of 2 at low tempera-
ture generates a NiIII complex, 3. EPR spectroscopy suggested
the oxidation was borne principally by the central metal atom,
yielding a low-spin d7 NiIII ion in a square planar coordination
environment. XAS analysis provided further support for this, in-
dicating that 3 contained a NiIII center coordinated by four N/O
donor ligands. DFT calculations on the structure and Mulliken
spin density further supported these experimental conclusions.
3 was capable of oxidizing organic substrates including ace-
tone, DTBP, BNAH, and PPh3. This constitutes the first example
of a terminal NiIIIꢀO(X) species that is sufficiently well-behaved
to allow thorough structural, spectroscopic, and reactivity in-
vestigations. This is an important step towards the elucidation
of the properties of these compounds and their role in oxida-
tion catalysis.
This publication has emanated from research supported in
part by the European Union (FP7–333948, AMcD), a research
grant from Science Foundation Ireland (SFI/12/RC/2278,
AMcD), and COST Action CM1305 (ECOSTBio). M.S. acknowl-
edges MINECO (CTQ2011–25086/BQU), DIUE of the Generalitat
de Catalunya (2014SGR1202), MICINN (Ministry of Science and
Innovation, Spain), the FEDER fund (UNGI08–4E-003) and
CESCA. Operation of NSLS X3B is supported by NIH grant P30-
EB-009998. The NSLS is supported by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences,
under contract no. DE-AC02–98CH10886.
Keywords: high-valent metals · metal–oxo species · nickel ·
oxidation catalysis · reactive intermediates
Experimental Section
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and they were either used directly or the compound was isolated
by crystallization. Deutero-2,6-di-tert-butylphenol was prepared ac-
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´
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Published online on && &&, 0000
&
&
Chem. Eur. J. 2015, 21, 1 – 7
www.chemeurj.org
6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
&
Metal–Oxo Species
P. Pirovano, E. R. Farquhar, M. Swart,
A. J. Fitzpatrick, G. G. Morgan,
A. R. McDonald*
&& – &&
A nickel(III)–bicarbonate complex is
obtained in high yield by the oxidation
of a nickel(II) precursor. The nickel(III)
species readily performs hydrogen atom
abstraction and oxygen atom transfer at
low temperatures, demonstrating that it
is a viable model for elusive high-valent
terminal nickel–oxygen intermediates in
oxidation catalysis.
Characterization and Reactivity of
a Terminal Nickel(III)–Oxygen Adduct
Chem. Eur. J. 2015, 21, 1 – 7
www.chemeurj.org
7
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ