Dioxygenase-like reactivity of an isolable superoxo-nickel(II) complex

Article abstract of DOI:10.1002/chem.201001138

Although O₂ activation by metals such as iron and copper has been a matter of intensive research in the last decades, this type of chemistry for nickel systems is still in its infancy. Moreover, studies regarding the oxidizing ability of the resulting "Ni_n-O₂" species towards exogenous substrates are scarce. In this work, we report on the reactivity of an isolable and thermally stable mononuclear superoxo-nickel compound [Ni^II(β-diketiminato)( O₂)] (1) towards different types of organic substrates. In addition, we have been able to prove that the β-diketiminato ligand can undergo partial intramolecular oxidation due to close proximity between the isopropyl groups of the β-diketiminato- aryl and the superoxo subunits. Compound 1 performs hydrogen-atom abstraction from O-H and N-H groups and most importantly it shows an unprecedented dioxygenase- like reactivity in the oxidation of 2,4,6-tri-tert-butylphenol. The latter reaction most likely occurs through the mediation of a putative [Ni ^III-oxo] intermediate, affording an unprecedented oxidation product of the phenol that incorporates two oxygen atoms from a single O₂ subunit. Results presented herein provide evidence of the striking oxidizing ability of dioxygen-nickel species and further support the viability to use such systems as oxidation catalysts analogous to its heavy metal congener, palladium.

Full text of DOI:10.1002/chem.201001138

FULL PAPER
DOI: 10.1002/chem.201001138
Dioxygenase-Like Reactivity of an Isolable Superoxo–Nickel(II) Complex
Anna Company,[a] Shenglai Yao,[a] Kallol Ray,[b] and Matthias Driess*[a]
Abstract: Although O2 activation by
metals such as iron and copper has
been a matter of intensive research in
the last decades, this type of chemistry
for nickel systems is still in its infancy.
Moreover, studies regarding the oxidiz-
ing ability of the resulting “Nin–O2”
species towards exogenous substrates
are scarce. In this work, we report on
the reactivity of an isolable and ther-
mally stable mononuclear superoxo–
nickel compound [NiII(b-diketimina-
to)(O2)] (1) towards different types of
organic substrates. In addition, we have
been able to prove that the b-diketimi-
nato ligand can undergo partial intra-
molecular oxidation due to close prox-
imity between the isopropyl groups of
the b-diketiminato-aryl and the super-
oxo subunits. Compound 1 performs
hydrogen-atom abstraction from O-H
and N-H groups and most importantly
it shows an unprecedented dioxyge-
nase-like reactivity in the oxidation of
2,4,6-tri-tert-butylphenol. The latter re-
action most likely occurs through the
mediation of a putative [NiIII-oxo] in-
termediate, affording an unprecedented
oxidation product of the phenol that
incorporates two oxygen atoms from a
single O2 subunit. Results presented
herein provide evidence of the striking
oxidizing ability of dioxygen–nickel
species and further support the viabili-
ty to use such systems as oxidation cat-
alysts analogous to its heavy metal con-
gener, palladium.
Keywords: dioxygenases · H-atom
À
abstraction · nickel · O O activa-
tion · phenols
Introduction
cal relevance of such reactions but also to the potential tech-
nological applicability that these processes may have. In the
last decades, studies of synthetic systems that activate
oxygen have been essentially focused on iron and copper, in
part due to the prevalence of these two metals in O2-activat-
ing metalloenzymes.[3–6] A wide range of species derived
from the interaction of such metals with dioxygen (or its ac-
tivated forms) have been synthesized and structurally char-
acterized. Some of these structures, which range from multi-
nuclear configurations to mononuclear metal–superoxo or
high-valent metal–oxo species, are biologically relevant and
they have found to be capable of performing bioinspired ef-
ficient and selective oxidation of organic substrates. Despite
some advances in the last years[7] and the recent discovery
of two nickel-based enzymes involved in processes using
O2,[8–11] studies based on the activation of molecular O2 by
synthetic nickel compounds are comparatively scarce. This is
especially surprising if one takes into account that for de-
cades complexes based on palladium, the second-row transi-
tion-metal counterpart of nickel, have been applied as oxi-
dation catalysts in important industrial transformations, such
as the Wacker process.[12,13]
One of the most important roles played by dioxygen in
aerobic organisms is its implication in the synthesis of im-
portant biomolecules, usually involving the energetically fa-
[1]
À
À
vorable (exothermic) formation of C O and O H bonds.
However, direct reaction of O2 with most organic substrates
does not occur due to its inherent electronic structure. In
order to overcome the high kinetic barrier associated to the
reactions of triplet O2 with organic (singlet) molecules, in
biological systems dioxygen is reductively activated to super-
oxo (O2 ), peroxo (O22À) or oxo (2O2À) forms by a metal
À
center found in oxidase and oxygenase enzymes.[2]
Understanding the mechanisms by which molecular O2
can be activated and used as oxidant in oxidative transfor-
mations is of crucial importance, not only due to the biologi-
[a] Dr. A. Company, Dr. S. Yao, Prof. Dr. M. Driess
Institute of Chemistry: Metalorganics and Inorganic Materials
Technische Universitꢀt Berlin, Strasse des 17. Juni 135
Sekr. C2, 10623 Berlin (Germany)
E-mail: matthias.driess@tu-berlin.de
[b] Dr. K. Ray
In the last years some nickel complexes capable of acti-
vating O2 have been characterized. The most common ap-
proach to synthesize such complexes involves the reaction
of nickel(II) with peroxo compounds (that can be seen as a
Institut fꢁr Chemie. Humboldt-Universitꢀt zu Berlin
Brook-Taylor Strasse 2, 12489 Berlin (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001138.
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9669

2eÀ reduced version of dioxygen) as most nickel(II) species
are inert towards O2. These configurations include dinuclear
while for (alkylperoxo)nickel(II) some electrophilic charac-
ter (oxidation of triphenylphosphine and CO) has also been
observed.[19] Finally, little is known about the chemistry re-
lated to mononuclear (superoxo)nickel(II) compounds. Up
to date, the three reported examples of this type have
proven to be suitable to oxidize triphenylphosphine to its
phosphine oxide,[18,21,22] but nothing is known about their
chemistry with other organic substrates.
Encouraged by the few reactivity studies reported for this
type of species and having in hand the stable [NiII(b-diketi-
minato)(O2)] superoxide (1),[22] in this paper we wish to
report the behavior of this species in the presence of differ-
ent types of organic molecules. In particular, we report for
the first time on its surprising reactivity in the oxidation of
phenolic substrates.
(III),[14–16] bis(m-alkylper-
systems such as bisACHTUNGTRENNUNG(m-oxo)dinickelACHTUNGTRENNUGN
oxo)dinickel(II)[17] and (trans-m-1,2-peroxo)dinickel(II)[18] or
mononuclear systems like (alkylperoxo)nickel(II)[19] or
(peroxo)nickelACHTUNGTRENNUNG
(III) compounds.[20] However, particularly de-
signed ligand structures allow the synthesis of nickel(I) com-
plexes (by reduction of the starting nickel(II) compounds)
that can directly interact with O2. Using this strategy three
different mononuclear (superoxo)nickel(II) species resulting
from the 1eÀ reduction of dioxygen by the nickel(I) center
have been characterized. Riordan et al. were the first to
demonstrate the viability of such species with the synthesis
of two different mononuclear nickel-superoxo com-
pounds.[18,21] However, both systems were found to be rather
thermolabile and they could only be generated at low tem-
perature, which prevented isolation and limited the study of
their reactivity. On the contrary, we recently reported the
straightforward synthesis of an isolable and strikingly stable
Results and Discussion
[NiII(b-diketiminato)(O2)]
superoxo
complex
1
As previously reported by some of us,[22] compound 1 can be
obtained by reaction of the nickel(I) precursor [(NiI(b-dike-
timinato))2(m-h3:h3-C6H5Me)] with dry O2 in toluene and iso-
lated from the reaction mixture as thermally stable green
crystals. Compound 1 was fully characterized by several
(Scheme 1)[22] that can be crystallized and handled at room
1
spectroscopic techniques (X-ray, FT-IR, EPR, H NMR, EI-
MS). The collected spectroscopic data pointed towards an
electronic configuration fully consistent with a NiII–superoxo
species, a formulation also supported by DFT calculations.
In the previous work, we already reported the ability of 1 to
perform the oxidation of PPh3, but the oxidation of other or-
ganic substrates was not probed.
In order to determine the oxidation properties of 1, we
studied its reactivity towards different types of substrates
(Scheme 1). At first, we learned that 1 is inert towards al-
kenes, sulfides and aldehydes. Moreover, it remained also
À
unchanged towards substrates bearing weak C H bonds
such as 9,10-dihydroanthracene or triphenylmethane. How-
ever, reaction of 1 with cyclohexanol afforded cyclohexa-
none in 18% yield with respect to the amount of the starting
nickel complex. A similar oxidation process occurred upon
reaction with 1-phenylethanol giving acetophenone as oxi-
dized product in 22% yield. Moreover, reaction with 2,4-di-
tert-butylphenol (DTBP) gave the corresponding 2,6-di-tert-
butyl-1,4-benzoquinone (DTQ) in ~50% yield. The ability
Scheme 1. Reactivity of 1 towards selected exogenous substrates. R=2,6-
iPr2C6H3.
temperature. Its reactivity with other metals has already
been reported by some of us.[23]
Despite the advances in the synthesis of several nickel–di-
oxygen configurations, little information has been gathered
regarding their ability to oxidize exogenous organic mole-
cules. One important drawback may be related to the strong
oxidants generated during the oxygen activation process,
which have a kinetic preference to perform intramolecular
ligand hydroxylation rather than a process involving the in-
termolecular interaction with an exogenous substrate.[24,25]
This phenomenon has been observed for selected dinuclear
systems described above. For example, the well-defined bis-
À
of 1 to activate N H bonds was probed by reaction with 1,2-
diphenylhydrazine as a substrate, which afforded the expect-
ed azobenzene as the oxidized product. Most likely, these
oxidation reactions take place through an initial hydrogen-
À
À
atom abstraction from the O H or N H bond. Indeed, sub-
stituted phenols are usually the substrates of choice in order
to probe the H-atom abstracting ability of transition-metal
complexes.[26,27] Overall, from our initial substrate screening,
it seems clear that 1 cannot perform oxygen-atom transfer
to alkenes, sulfides or alkanes but it readily activates and
ACHTUNGTRENNUNG(m-oxo)dinickelACHTUNGTRENNUNG(III) configuration can decompose through
aromatic hydroxylation,[15] aliphatic hydroxylation[16] or N-
dealkylation[14] of the supporting ligand. In the case of the
À
À
mononuclear (peroxo)nickel
(III) configuration reactivity
oxidizes O H and N H bonds.
The oxidation of DTBP is an especially interesting trans-
formation as it involves the incorporation of a new oxygen
studies with exogenous organic substrates indicate that it be-
haves as a nucleophile in the deformylation of aldehydes[20]
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A Superoxo–Nickel(II) Complex
FULL PAPER
atom into the oxidized DTQ product. A deeper analysis of
this transformation indicated that when 18O-labeled 1 (gen-
erated by reaction of the NiI precursor with 18O2) was used,
approximately 85% 18O-atom incorporation occurred in the
resulting DTQ, so that the oxygen atom incorporated into
the oxidized product originates from 1. Thus, in this particu-
lar case, compound 1 does not act only as an oxidant but
also as an oxygenation reagent. This transformation may be
explained by the mechanism depicted in Scheme 2 (R=H),
Figure 1. EI-MS spectrum of 2tBu obtained after reaction of 1 equiv tri-
tert-butylphenoxyl radical with 1 (left), 18O-labeled 1 (middle) and a mix-
ture of 1 and 18O-labeled 1 (right).
yield. This observation further confirms that during phenol
oxidation 1 equiv of 1 is consumed in an initial hydrogen-
atom abstraction event. Most remarkably, the oxidized prod-
uct derived from the reaction between 18O-labeled 1 and
TTBP or tri-tert-butylphenoxyl radical showed a shift of four
mass units (m/z 298) indicative that two 18O-atoms were in-
corporated into the oxidized product (Figure 1, middle).
UV/Vis monitoring of the reaction between 2,4,6-di-tert-bu-
tylphenol (TTBP) and 1 at 258C clearly showed the de-
crease of the characteristic band of 1 (lmax =980 nm) follow-
ing a single exponential decay (Figure 2). The reaction was
Scheme 2. Schematic representation of the mechanism of phenol oxida-
tion by 1.
which is reminiscent to the reaction path proposed for se-
lected cobalt–superoxo systems.[28,29] Reaction starts with a
À
hydrogen-atom abstraction from the O H bond by the (su-
peroxo)nickel(II) (1) to generate a phenoxyl radical which
rapidly interacts with a second molecule of 1 to form a
metal–arylperoxo compound (B in Scheme 2). After hydro-
À
gen migration and O O bond cleavage, DTQ incorporating
a new oxygen atom coming from the superoxo moiety is
formed. Remarkably, the oxidation of DTBP requires the
consumption of two equivalents of 1, as a sacrificial (super-
oxo)nickel(II) unit is necessary to perform the initial H-
atom abstraction event. This stoichiometry agrees with the
experimental observation that DTQ is obtained in ~50%
yield with respect to the initial amount of 1.
The use of alkyl para-substituted-2,6-di-tert-butylphenols
(R=Me, tBu in Scheme 2) allowed us to get deeper insight
into the mechanism of phenol oxidation by 1. A priori, as
the hydrogen migration step is prevented for para-substitut-
ed-2,6-di-tert-butylphenols, isolation of peroxy-p-quinol
(RPPQ) as the oxidized product would be expected. In fact,
Figure 2. UV/Vis absorption spectra of the reaction of 1 with TTBP at
258C in toluene ([1]=1 mm, [TTBP]=0.18m). Inset: time course of the
decay of the 980 nm band (g) and the first-order fit (c).
found to be first-order with respect to substrate concentra-
tion and a second-order rate constant of k2 =0.0032mÀ1 sÀ1
was obtained. Moreover, with [D1]-TTBP as the substrate, a
kinetic isotope effect (KIE) of 2.1 was measured, indicating
À
that the initial cleavage of the O H bond is involved in the
rate-determining step. Similar KIE values for the oxidation
of TTBP have been measured in other metal systems in
which a hydrogen-atom abstraction mechanism is postulat-
ed.[27,30]
reaction of
1 with 2 equiv of 2,4,6-di-tert-butylphenol
(TTBP) gave an oxidation product with a mass value of m/z
294 in 40% yield with respect to 1, which at first glance
seems fully consistent with the formation of tBuPPQ
(Figure 1, left). On the other hand, direct reaction of 1 with
1 equiv of the stable tri-tert-butylphenoxyl radical (A if R=
tBu, Scheme 2) afforded the same oxidation product in 90%
Surprisingly, 1H NMR analysis of the oxidized product
from the reaction with TTBP failed to show the characteris-
tic singlet at d=6.7 ppm expected for tBuPPQ,[31] which sug-
gests the formation of a structurally different oxidized spe-
1
cies. By means of 1D and 2D H and 13C NMR spectroscopy,
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M. Driess et al.
we could determine that, indeed, the oxidized species corre-
sponds to compound 2tBu (Figure 3), which is a structural
isomer of tBuPPQ. Its structure was finally confirmed by X-
ray diffraction analysis (Figure 3).[32] FT-IR analysis of 2tBu
noxyl radicals. More interestingly, as far as we know, the re-
activity with alkyl para-substituted-2,6-di-tert-butylphenols
exhibited by 1 finds no precedent in the literature. For in-
stance, oxygenation of this type of substrates by cobalt(II)
complexes in the presence of O2 gives the corresponding
RPPQ.[38,39] Mechanistic studies point towards the mediation
of a transient cobalt–superoxo species in these systems.
Moreover,
reaction
of
the
isolated
superoxo
[CoIII(CN)5O2]3À with tri-tert-butylphenoxyl radical also
leads to the same product.[40] On the other hand, reported
reactivity of selected Cu–superoxo complexes with TTBP af-
fords exclusively DTQ.[33,34] Remarkably, Tolman et al. re-
ported the synthesis of the copper–superoxo analogue of 1,
namely [CuII(b-diketiminato)(O2)],[41] but this compound be-
haves as a poor oxidant and it is inert toward phenols.[42]
Thus, although phenol oxidation seems a common feature
for most metal–superoxo complexes, only in the case of 1 it
has been observed a dioxygenase behavior that leads to the
formation of compound 2tBu through an intriguing mecha-
nism.
Figure 3. Left: Schematic representation of 2tBu. Right: X-ray structure of
2
tBu. Hydrogen atoms have been omitted for clarity.
(Figure S1) showed two intense features at n˜ =1721 and
1685 cmÀ1 corresponding to the stretching frequencies of the
two ketone subunits. The lower energy band is assigned to
the a,b-unsaturated ketone, while the higher energy feature
corresponds to the acyclic ketone subunit, which downshifts
to n˜ =1689 cmÀ1 when 18O-labeled 1 is used. Another iso-
Nishinaga et al. demonstrated that compounds 2R can be
formed by direct decomposition of RPPQ under strong
acidic conditions (Scheme 3).[43] Heterolysis of the O O
À
tope-sensitive band is found at 1120 cmÀ1 (D
ACHTUNGTRENNUNG
À17 cmÀ1) and it is assigned to the characteristic nas
ACHTUNGTRENNUNG
vibration from the ether functionality. Thus, the two oxygen
atoms incorporated during phenol oxidation end up in the
acyclic ketone and in the ether group of 2tBu. In order to de-
termine if these two oxygen atoms came from the same NiII-
superoxo unit, we analyzed the oxidation of 1 equiv of tri-
tert-butylphenoxyl radical with 0.5 equiv of 1 and 0.5 equiv
of 18O-labeled 1 (Figure 1, right). No mass peak at m/z 296
of the putative mixed-labeled product was detected, and in-
stead only non-labeled (m/z 294) and doubly 18O-labeled
2tBu (m/z 298) were detected, proving that the two oxygen
atoms incorporated in 2tBu origin from the same (superoxo)-
nickel(II) unit, and thus from the same O2 molecule. The
same reactivity pattern was observed for the reaction of 1
with 4-methyl-2,6-di-tert-butylphenol (R=Me in Scheme 2)
affording compound 2Me as the oxidized species. These re-
sults clearly indicate that compound 1 can be considered as
a dioxygenase-type of system in the oxidation of para-substi-
tuted-2,6-di-tert-butylphenols because the two oxygen atoms
from a single dioxygen molecule end up into the substrate
Scheme 3. Mechanism of decomposition of RPPQ under strong acidic
conditions as reported by Nishinaga et al.[43]
À
in a process in which the O O bond is fully cleaved.
Although the reactivity of Ni–superoxo compounds versus
phenols had never been reported before (in part, because of
the few reported examples of this type of structures), other
metal–superoxo species are known to be capable of oxidiz-
ing phenols. In particular and analogously to 1, reaction of
Cu–superoxo[33,34] and Co–superoxo[28,35,36] compounds with
DTBP leads to the formation of DTQ. Instead, for the
single reported Fe–superoxo species,[37] the same reaction
does not result in oxygen incorporation into the substrate,
but only in the formation of the coupling product derived
from direct intermolecular reaction of two generated phe-
bond assisted by a proton gives a quinoxy cation, which suf-
fers a ring expansion and a final rearrangement to form 2R.
Interestingly, in the same work it is reported that compound
2tBu could not be generated during the acidic decomposition
of tBuPPQ and instead DTQ was detected. This observation
was reasoned on the basis of the susceptibility of the tert-
butyl group towards b-scission due to the relative stability of
the resulting tert-butyl cation. Nevertheless, in order to test
if 2R could be generated during our work-up procedure, we
subjected directly synthesized MePPQ[31] to a silica column,
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A Superoxo–Nickel(II) Complex
FULL PAPER
following the same procedure as in our previous reactivity
experiments with 1. We found that MePPQ remained un-
changed under our experimental conditions and no trace of
the corresponding 2Me was detected.
Putting it all together, both our experimental results with
1 and previously reported reactivities of other metal–super-
oxo species, oxidation of TTPB by such compounds could
be explained by the mechanistic scenario depicted in
Scheme 4. We propose that the key intermediate in the oxi-
E that rearranges to the final isolated 2tBu. Although NiIII-
oxo compounds have never been directly detected, they
have been proposed as key intermediates in some oxidation
processes.[45–47] In fact, the appearance of transient 3 in oxi-
dative transformations was also previously proposed by us
for the oxidation of triphenylphosphine by 1.[22] Moreover, it
has been computationally calculated that (b-diketiminato)-
NiIII–oxo species could be good candidates for facile oxida-
tion of methane.[48] In fact, the active oxidizing character of
the Ni–oxo unit has been directly experimentally proved in
gas-phase reactions, where the oxidizing ability of this spe-
cies in methane oxidation surpasses that exhibited by the
other first-row metal–oxo compounds probed.[49]
We also checked the stability of 1 in solution at room
temperature in order to determine if intramolecular oxida-
tion processes took place when no substrate was present.
The green-colored compound 1 slowly decomposed in tolu-
ene to give a brown mixture after five days. Ligand extrac-
tion by acidic work-up indicated the partial oxidation of the
b-diketiminato ligand (m/z 418) to give a dehydrogenated
product (m/z 416) in approximate 10% yield as ascertained
by GC-MS. We suggest that the formation of this product
derives partially from the dehydration under our work-up
conditions of an initially generated hydroxylated b-diketimi-
nato ligand. In fact, ESI-MS analysis of a solution of 1 in
toluene showed the presence of a peak fully consistent with
the monohydroxylated b-diketiminato unit ([M+H]+
À
435.3). Most likely, hydroxylation occurs in the tertiary C H
bond of one of the ligand isopropyl groups situated in close
proximity to the superoxo subunit. The oxygen atom incor-
porated into this hydroxylated species originates from the
Ni–superoxo unit as ascertained by the use of 18O-labeled 1
([M+H]+ 437.3). As previously observed for selected cobalt
and nickel-based species derived from O2 activation, dehy-
drogenation of the isopropyl group may also be directly ef-
fected by metal–dioxygen species.[50–53] Indeed, in the present
case, the presence of dehydrogenated ligand is also observed
in the ESI-MS spectrum of the decomposition species de-
rived from 1 prior to the acidic work-up but it appears not
to be the major reaction pathway given the low intensity of
the corresponding peak.
Remarkably, analysis of the reaction mixture after reac-
tion of 1 with phenols indicated that similar oxidation pro-
cesses of the b-diketiminato ligand took place. It is impor-
tant to highlight here that Karlin et al.[34] recently reported a
system in which intramolecular ligand hydroxylation oc-
curred through a putative hydroperoxo–copper(II) species
formed by reaction of a superoxo–copper(II) with a hydro-
gen-atom donor such as phenols. Analogously to the system
described by Karlin, in the present case it is feasible the in-
volvement of a transient hydroperoxo–nickel(II) species
generated by hydrogen-atom abstraction (from phenol or al-
ternatively from the solvent when no hydrogen-atom donor
is present) by 1. However, we do not have any definitive
proof to rule out the superoxo–nickel(II) compound 1 as the
active species in such intramolecular oxidations. It seems
clear, though, that the bulkiness of the b-diketiminato
Scheme 4. Proposed mechanism of oxidation of TTBP by different metal-
superoxo compounds: Ni–superoxo (1) versus Co–superoxo and Cu–su-
peroxo.
dation process is a common metal–arylperoxo compound B
formed after initial hydrogen abstraction from TTPB and
posterior coupling of the newly formed tri-tert-butylphenox-
yl radical A with another metal–superoxo unit. The different
reactivity patterns exhibited by cobalt and copper versus
À
nickel suggest significant differences in the relative O O
À
and M O bond strengths of the respective intermediates of
type B. Formation of DTQ from Cu–superoxo species can
À
be reasoned by a heterolytic O O bond cleavage in B gen-
erating a quinoxy cation that suffers a b-scission to give a
tert-butyl cation, following a mechanism similar to the de-
composition of tBuPPQ described by Nishinaga et al.[43] In
the case of cobalt-superoxo species, the isolation of tBuPPQ
À
in the reaction with TTBP may be due to a direct Co O
bond cleavage in B. Finally, the unexpected generation of
2tBu in the reaction of 1 with TTBP can be explained by the
À
homolysis of the O O bond in B leading to a quinoxyl radi-
II
C
cal and a putative nickel
U
pound. Ring expansion of the quinoxyl radical[44] and final
oxygen rebound with the newly generated 3 gives compound
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M. Driess et al.
was washed with ethyl acetate. The solvent from the filtrates was re-
moved under vacuum and the resulting residue was dissolved in toluene
(12 mL). The yield of azobenzene (75%) was determined spectrophoto-
metrically by measuring the intensity of its characteristic absorption band
at l=442 nm and correlation with a calibration curve.
ligand may be understood as playing two roles: on one hand
it stabilizes the Ni–superoxo unit by sterically protecting it,
but on the other hand, it constitutes a potential nearby sub-
strate susceptible to be attacked that may partially hamper
the interaction with exogenous substrates. Intramolecular
ligand oxidation by well-defined dinuclear nickel–dioxygen
species has been reported in a few cases[14–16] and it has also
been observed for putative mononuclear nickel-dioxygen in-
termediates.[24,54]
Reaction with 2,4,6-tri-tert-butylphenol (TTBP): Compound 1 (19.0 mg,
37 mmol) was dissolved in toluene (20 mL) and 2 equiv of TTBP
(19.6 mg, 74 mmol) were added at once. After stirring for 24 h the initial
green solution turned brown. The solvent was removed under vacuum
and the resulting brown residue was passed through a silica column. Ini-
tial elution with CH2Cl2 allowed the removal of excess TTBP. Elution
with ethyl acetate afforded the oxidized product (2tBu) together with b-di-
ketiminato ligand. The solvent from these last eluates was removed
under reduced pressure to yield a pale-yellow solid which was analyzed
by 1H NMR and GC-MS. Relative integration of the 1H NMR signals of
Conclusion
2tBu with respect to the b-diketiminato ligand indicated 40% yield of 2tBu
.
GC-MS analysis showed a mass value of m/z 294 for 2tBu. A similar ex-
perimental procedure was followed for the oxidation of 4-methyl-2,6-di-
tert-butylphenol, which afforded 2Me as ascertained by comparison of its
1H NMR spectrum with the previously reported spectroscopic data for
this compound.[43]
In this work we have shown the distinct reactivity of a
stable superoxo–nickel(II) compound 1 in the oxidation of
À
À
O H and N H groups from exogenous substrates. In addi-
tion, we have been able to prove that the b-diketiminato
ligand can undergo partial intramolecular oxidation due to
close proximity between the isopropyl groups of the b-dike-
timinato-aryl and the superoxo subunits. Nevertheless, 1
shows dioxygenase-like activity when exposed to para-sub-
stituted-2,6-di-tert-butylphenols, affording the unprecedent-
ed oxidation product (2R) that incorporates two oxygen
atoms from a single O2 subunit. The mechanism of this
transformation is proposed to occur through the mediation
of a NiIII-oxo species, which has already been suggested as a
key oxidizing species in several oxidation processes, includ-
ing the previously reported oxidation of triphenylphosphine
by 1. Results presented here are furthermore striking be-
cause unlike the reported reactivity of analogous superoxo–
metal species containing cobalt, iron and copper, 1 shows di-
Reaction with tri-tert-butylphenoxyl radical: Compound
1 (62.4 mg,
0.12 mmol) was dissolved in toluene (60 mL) and 1 equiv of tri-tert-butyl-
phenoxyl radical (32.0 mg, 0.12 mmol) was added. After stirring for 3 h,
the initial turquoise solution turned brown. The solvent was removed
under vacuum and the resulting brown residue was passed through a
silica column. After initial elution with CH2Cl2, the column was eluted
with ethyl acetate. The solvent from these last eluates was removed
under reduced pressure to yield a pale-yellow solid (88 mg) consisting of
a mixture of oxidized product (2tBu) and b-diketiminato ligand as ascer-
1
1
tained by H NMR and GC-MS. Relative integration of the H NMR sig-
nals of 2tBu with respect to the b-diketiminato ligand indicated 90% yield
of 2tBu. GC-MS analysis indicated a mass value of m/z 294 for 2tBu
.
18O-Labeled experiments: Reactivity experiments were performed fol-
lowing exactly the same experimental procedure as described above but
using 18O-labeled 1 (generated by reaction of the NiI precursor with
18O2).
À
oxygenase-like activity with full O O bond cleavage. Over-
Determination of the kinetic isotope effect (KIE): Kinetic studies were
performed by adding appropriate amounts of TTBP or [D1]-TTBP to
1 mm solutions of 1 in toluene at 258C. Spectral changes were directly
monitored by UV/Vis spectroscopy. Rate constants, k1, were determined
by pseudo-first-order fitting of the decay of the absorption band at
980 nm. Plots of k1 versus substrate concentration afforded the second
order rate constant, k2 (Figure S2). All reactions were performed under
complete anaerobic conditions.
all, reactivity studies on nickel species derived from the in-
teraction with O2 offer novel and interesting perspectives in
understanding and designing metal-mediated dioxygen acti-
vation and selective oxygenation of organic substrates.
Characterization of 2tBu: As stated above, reaction between 1 and tri-tert-
butylphenoxyl radical afforded a mixture of 2tBu and b-diketiminato
ligand. Isolation of 2tBu was achieved by treatment of this mixture with
acetonitrile, filtration and removal of the solvent from the filtrates. The
resulting solid was essentially 2tBu free of b-diketiminato ligand which
could be fully characterized by NMR, EI-MS and FT-IR. Crystals of 2tBu
were grown by slow evaporation of a saturated hexane solution of the
compound. 1H NMR (CDCl3, 400 MHz, 298 K): d = 7.76 (s, 1H, CH),
3.37 (d, J=15.9 Hz, 1H, CH2), 2.90 (d, J=15.9 Hz, 1H, CH2), 1.25 (s,
9H, tBuCH3), 1.11 (s, 9H, tBuCH3), 0.98 ppm (s, 9H, tBuCH3); 13C NMR
(CDCl3, 100 MHz, 298 K): d = 209.8 (C=O), 205.7 (C=O), 170.1 (CH),
129.3 (Cq), 91.0 (Cq), 44.4 (tBuCq), 38.6 (CH2), 37.6 (tBuCq), 29.6 (tBuCq),
28.2 (tBuCH3), 26.0 (tBuCH3), 24.5 (tBuCH3); EI-MS: m/z: 294.218 [M]+;
FT-IR (KBr): n˜ = 3082 (m), 2958 (s), 2906 (m), 2869 (m), 1721 (s), 1685
(s), 1610 (s), 1478 (m), 1463 (m), 1394 (w), 1362 (s), 1345 (m), 1336 (m),
1225 (w), 1120 (s), 1075 (w), 1049 cmÀ1 (w).
Experimental Section
Reactivity studies: Reaction with cyclohexanol, 1-phenylethanol, 2,6-di-
tert-butylphenol (DTBP), benzaldehyde, styrene, triphenylmethane, 9,10-
dihydroanthracene or thioanisole: In
a typical experiment, 1 (4 mg,
7.9 mmol) was dissolved in toluene under N2 and 10 equiv of the specific
substrate were added. After stirring for 24 h at room temperature, bi-
phenyl (internal standard) was added and the reaction mixture was
passed through a short silica path (in order to remove the nickel com-
plex), which was washed with ethyl acetate. The resulting filtrates con-
taining the organic products were analyzed by GC-MS. The oxidized
products, if formed, were identified by comparison of their GC retention
times and GC-MS spectrum with those of authentic compounds and they
were quantified by calibration curves using biphenyl as internal standard.
Reaction with 1,2-diphenylhydrazine: Compound 1 (4.5 mg, 8.8 mmol)
was dissolved in toluene under N2 and 2 equiv of 1,2-diphenylhydrazine
(0.34 mL of a 52 mm solution in toluene) were added. An immediate
color change occurred upon substrate addition and the initial green solu-
tion turned orange-brown. After stirring for 30 min at room temperature,
the solvent from the reaction mixture was removed under reduced pres-
sure. The resulting brown residue was dissolved in CH2Cl2 and passed
through a short silica pad (in order to remove the nickel complex), which
Intramolecular ligand oxidation: Compound 1 (22.6 mg, 44 mmol) was
dissolved in toluene (20 mL) and the green solution was stirred at room
temperature for 5 days. The solvent from the resulting brown mixture
was removed under vacuum and the residue was treated with concentrat-
ed HCl (8 mL) and vigorously stirred for 1 hour. After extraction with
CH2Cl2 (3ꢃ10 mL), the organic phases were combined, dried over
MgSO4, filtered and the solvent removed under vacuum. The resulting
9674
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ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 9669 – 9675

A Superoxo–Nickel(II) Complex
FULL PAPER
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Received: April 29, 2010
Published online: July 19, 2010
Chem. Eur. J. 2010, 16, 9669 – 9675
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9675