Activation of Dioxygen at a Lewis Acidic Nickel(II) Complex: Characterization of a Metastable Organoperoxide Complex

Article abstract of DOI:10.1002/anie.201609526

In metal-mediated O₂activation, nickel(II) compounds hardly play a role, but recently it has been shown that enzymes can use nickel(II) for O₂activation. Now a low-coordinate Lewis acidic nickel(II) complex has been synthesized that reacts with O₂to give a nickel(II) organoperoxide, as proposed for the enzymatic system. Its formation was studied further by UV/Vis absorption spectroscopy, leading to the observation of a short-lived intermediate that proved to be reactive in both oxygen atom transfer and hydrogen abstraction reactions, while the peroxide efficiently transfers O atoms. Both for the enzyme and for the functional model, the key to O₂activation is proposed to represent a concomitant electron shift from the substrate/co-ligand.

Full text of DOI:10.1002/anie.201609526

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International Edition: DOI: 10.1002/anie.201609526
German Edition: DOI: 10.1002/ange.201609526
Dioxygen Activation
Activation of Dioxygen at a Lewis Acidic Nickel(II) Complex:
Characterization of a Metastable Organoperoxide Complex
Patrick Holze, Teresa Corona, Nicolas Frank, Beatrice Braun-Cula, Christian Herwig,
Anna Company, and Christian Limberg*
Dedicated to Professor Herbert W. Roesky on occasion of his 80th birthday
Abstract: In metal-mediated O2 activation, nickel(II) com-
pounds hardly play a role, but recently it has been shown that
enzymes can use nickel(II) for O2 activation. Now a low-
coordinate Lewis acidic nickel(II) complex has been synthe-
sized that reacts with O2 to give a nickel(II) organoperoxide, as
proposed for the enzymatic system. Its formation was studied
further by UV/Vis absorption spectroscopy, leading to the
observation of a short-lived intermediate that proved to be
reactive in both oxygen atom transfer and hydrogen abstrac-
tion reactions, while the peroxide efficiently transfers O atoms.
Both for the enzyme and for the functional model, the key to
O2 activation is proposed to represent a concomitant electron
shift from the substrate/co-ligand.
intensifying research on O2 activation with nickel(II). Herein
we report the synthesis of a low-coordinated nickel(II)
complex that reacts with O2 to generate a nickel(II) organo-
peroxide moiety, a transformation that to some extent
parallels the step that has been postulated in the mechanism
of the nickel quercetinase to follow O2 activation (Scheme 1).
O
wing to its triplet ground state, the utilization of O2 for
Scheme 1. Proposed mechanism for the reactivity of the quercetin 2,4-
oxidation reactions typically requires a prior activation at
a metal center.[1] While iron(II)[2] and copper(I) ions[3] have
rather favorable properties in this context and are thus found
also in many enzymes and homogeneous catalysts, nickel(II)
complex units are not usually considered to show potential for
O2 activation, unless the substrate is activated for O2
reactivity.[4] There have been some sporadic publications
reporting nickel(II) complexes that actually showed reactivity
towards O2 via unknown mechanisms, but the nature of the
initial products and the metal-based final products remained
very vague.[5] O2 activation by nickel complexes with clear-cut
product identification and structural information has so far
been restricted to a few examples containing the metal in the
oxidation state + 1 or 0.[6,7] In this context, the recent
observation that a quercetin-2,4-dioxygenase (quercetinase)
activates O2 at a [(NHis)3NiII] site to yield a Ni–O2 species with
unknown electronic structure (suggested to represent a nickel-
(II) superoxide, however)[8] is a fascinating stimulus for
dioxygenase.[8]
We wondered whether Lewis acidity might be the key for
nickel(II) to attract O2 and prepare it for the conversion of
a simultaneously bound ligand. Striving for Lewis-acidic
nickel(II) complexes, we chose the b-diketiminate ligand
[LtBu] (LtBu = [HC(C(tBu)NC6H3(iPr)2)2]À), as in the past this
has been shown to guarantee low-coordination numbers for
nickel centers, which is one prerequisite.[9–12] Beyond that, the
donor strengths of the co-ligands should be low, so that
a maximum of positive charge remains at nickel. In this
context we have observed that molecular alkali metal halide
units (MX) in the coordination spheres of b-diketiminato-
bound nickel(I) centers are rather labile and are even
replaced by N2.[13] This is remarkable, as N2 in combination
with Ni had been shown to be a rather weakly coordinating
ligand already,[11] giving way to very inert molecules such
sulfur hexafluoride and carbon dioxide.[9,10] Hence, the fact
that N2 in turn eliminates MX, underlines the rather high
reactivity of complexes with bound MX.[13] Consequently, we
[*] Dr. P. Holze, N. Frank, Dr. B. Braun-Cula, Dr. C. Herwig,
Prof. Dr. C. Limberg
envisioned treatment of [LtBuNiCl][12] with Na[BArF ] (ArF =
Humboldt-Universitꢀt zu Berlin, Institut fꢁr Chemie
Brook-Taylor-Strasse 2, 12489 Berlin (Germany)
E-mail: Christian.limberg@chemie.hu-berlin.de
4
3,5-Bis(trifluormethyl)phenyl) that was supposed to generate
NaCl; NaCl was then conceivable to either remain in the
coordination sphere of the nickel(II) ion or to precipitate
forming a salt of [LtBuNi]+ with the weakly coordinating anion
T. Corona, Dr. A. Company
Grup de Quꢂmica Bioinspirada, Supramolecular i Catꢃlisi (QBIS-
CAT), Institut de Quꢂmica Computacional i Catꢃlisi (IQCC)
Departament de Quꢂmica, Universitat de Girona
17003 Girona, Catalonia (Spain)
[BArF ]À. Any option was expected to produce a highly
4
reactive nickel(II) complex.
First [LtBuNiCl] (1; accessed readily from [LtBuNiF], 2) was
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201609526.
treated with only 0.5 equiv of anhydrous Na[BArF ] in
4
dichloromethane as the solvent. Work-up led to a green
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powder that could be recrystallized from dichloromethane/
hexane. An X-ray diffraction analysis identified this product
assigned, and as anticipated a ratio [LtBu]/[BArF ] of 2:1 could
4
be derived. Beyond this 3 was characterized by H1H COSY
1
as [(LtBuNiCl)2(m-Na)][BArF ], 3, (Scheme 2) and its molec-
and 19F NMR, UV/Vis spectroscopy, an Evans measurement,
and elemental analysis (see the Supporting Information).
To further increase the Lewis acidity, 1 was reacted with
4
ular structure is shown in Figure 1. The cation of 3 consists of
two [LtBuNiCl] units, which are connected by a Na+ ion, and
a whole equivalent of anhydrous Na[BArF ], and after work-
4
1
up a green powder was again isolated. A H NMR spectrum
showed a signal pattern very similar to the one observed for 3,
but the individual signals appeared shifted by up to 3 ppm,
and the [LtBu]/[BArF ] ratio amounted to 1:1. From that we
4
conclude that the new product has a cation that is derived
from the one of 3 by the addition of a further Na+ ion to the
core and thus has the composition [(LtBuNiCl)(m-Na)][BArF ],
4
as confirmed by elemental analysis; further characterization
1
includes H–1H COSY and 19F NMR, UV/Vis spectroscopy,
CV, and an Evans measurement (see the Supporting Infor-
mation). Based on the findings for 3 and previous experience
with the structural arrangement of neutral [(LRNiD)2(M)2]
(M = alkali metal, D = donor) compounds,[11,13] a dimeric
structure [(LtBuNiCl)(m-Na)]2[BArF ]2 (4) as shown in
4
Scheme 2 is reasonable, which was also confirmed by DFT
calculations, predicting the structure shown in Figure 2 for
a dinuclear compound (B3LYP/Def2-SVP; see the Support-
ing Information).
Scheme 2. Syntheses of Lewis acidic low-coordinate nickel(II) com-
plexes.
Figure 2. Structure of 4 (triplet ground state), as optimized at the
B3LYP/Def2-SVP level. All hydrogen atoms are omitted for clarity.
Selected bond lengths [ꢄ]: Ni–N 1.823 to 1.902, Ni–Cl 2.263 and
2.290.
Figure 1. Molecular structure of 3.[23] Protons, the anion [BArF ]À, and
4
half a solvent molecule n-hexane are omitted for clarity. Selected bond
lengths [ꢄ] and angles [8]: Ni1–N1 1.769(4), Ni1–N2 1.849(3), Ni1–Cl1
2.1962(13), Ni1–C24 2.356(4), Cl1–Na1 2.671(2), Cl2–Na1 2.662(2),
Na1–C15 2.855(5), Na1–C16 2.934(5), Na1–C61 2.967(4), Na1–C62
2.825(4), Na1–C63 3.023(4); N1-Ni1-N2 96.94(16), N1-Ni1-Cl1 148.77-
(12), N2-Ni1-Cl1 114.28(11).
Notably, 4 was found to be reactive towards O2: Treatment
of a dichloromethane solution of 4 with O2 at room temper-
ature led to an immediate color change from green to red–
brown, and removing of all volatiles yielded a brown solid
that was unstable at room temperature, converting it into an
insoluble oil upon storage.
F
À
À
a [BAr ] anion is compensating its charge. The Ni N bond
When exposure of 4 to O2 was performed at À808C, the
color of the resulting brown solutions did not change over
days, so that attempts to crystallize the product could be
made, which finally were successful by applying a special
overlaying technique (see the Supporting Information) in the
presence of O2 at À808C. Thus two types of orange crystals
were obtained that were suitable for X-ray diffraction
analysis. They turned out to belong to two structural isomers
of an organoperoxide nickel(II) complex [(LtBuOONiIICl)(m-
4
À
lengths (1.769(4) ꢀ, 1.856(3) ꢀ) are the shortest Ni N bonds
of all known [LtBu] nickel(II) complexes.This already hints to
a pronounced Lewis acidity, and a further indication is that, as
judged by the short distances between each Ni atom and one
of the C atoms that link the aryl rings to the N atoms (Ni1-C24
2.356(4) ꢀ, Ni2-C47 2.299(4) ꢀ), the nickel ions undergo
weak interactions with the aryl rings of the b-diketiminate,
whereas in [LtBuNiCl] (1) hardly any interaction is found (Ni
À
C 2.535 ꢀ).[12] As should be expected, the contact with the
Na(OEt2))]x[BArF ]x
(LtBuOO = C(OOÀ)(C(tBu)N(dipp))2,
4
À
À
sodium ion elongates the Ni Cl bonds (Ni1 Cl1 2.1962-
dipp = 2,6-diisopropylphenyl) derived from the product
(13) ꢀ, Ni2 Cl2 2.2080(13) ꢀ compared to Ni Cl 2.137(2) ꢀ
in 1).[12] The 1H NMR resonances observed for CD2Cl2
solutions of 3 were paramagnetically shifted but could be
[(LtBuOONiIICl)(m-Na)]x[BArF ]x, 5, forming in the pure solvent
À
À
4
dichloromethane by the addition of an OEt2 molecule to the
Na+ ion in the course of crystallization. One of the isomers is
2
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a
dimer {[LtBuOONiIICl][Na(OEt2)]}2[BArF ]2, 5(OEt2)
a dichloromethane solution of 4 at À508C were monitored by
4
(Figure 3), while the other, {[LtBuOONiIICl][Na(OEt2)]}x-
UV/Vis spectroscopy (Figure 4). Compound 4 exhibits an
absorption maximum at 783 nm (e(4) = 3900 LmolÀ1 cmÀ1),
the intensity of which quickly and continuously decreases
upon O2 addition, in agreement with the sensitivity observed
[BArF ]x, 5(OEt2)’, is polymeric. In dinuclear 5(OEt2), the
4
planar [C3N2Ni] motif that is usually observed in nickel(II)
complexes with b-diketiminate ligands[9,12,14] has given way to
Figure 4. UV/Vis spectral changes of the reaction of 4 with O2. a) Time
traces of the extinction at 783 nm (absorption maximum of 4) and
518 nm (absorption maximum of 5). Spectrum A corresponds to 4,
spectrum B to an intermediate observed after 502 s, spectrum C to the
decay product of the intermediate, and D to the spectrum of 5.
b) Initial spectroscopic changes (782 s) after exposure of 4 to O2. After
502 s, a short-lived intermediate formed. c) Formation of the broad
absorption maximum of 5.
Figure 3. Crystal structure of 5(OEt2).[23] Protons, disordered fluorine
atoms, and the non-coordinating ArF groups of the anion [BArF ]À are
4
omitted for clarity. Selected bond lengths [ꢄ] and angles [8]: Ni1–O2
1.864(5), Ni1–N1 1.986(5), Ni1–N2 2.004(5), Ni1–Cl1 2.239(2), Cl1–
Na1 2.739(4), Na1–O2 2.224(6), Na1–O3 2.901(6), O2–O3 1.432(7);
N1-Ni1-N2 97.2(2), O2-Ni1-Cl1 140.86(16), N1-Ni1-Cl1 122.98(17),
N2-Ni1-Cl1 105.51(17), O3-O2-Ni1 115.6(3).
for 4 (Figure 4b). After 502 s, two new absorption maxima are
observed at 610 (e = 800 LmolÀ1 cmÀ1) and 710 nm (e =
800 LmolÀ1 cmÀ1), which cannot be detected anymore after
782 s, and after 1752 s a rather broad band at 518 nm starts to
become visible, which reaches its maximum after 3002 s
(e518 = 1020 LmolÀ1 cmÀ1; Figure 4c) and then also vanishes.
Through an independent measurement with crystalline 5-
(OEt2), it could be shown that the broad band at 518 nm
belongs to 5, which thus is the final product of the reaction
sequence (Scheme 3).
On the way to 5, obviously an intermediate species, 6, with
a short but definite lifetime occurs, which was also supported
by an NMR monitoring (see the Supporting Information), but
all attempts to obtain more information on its electronic
structure failed owing to its sensitivity.
To investigate the potential of this primary product to
perform oxidation reactions, the reaction of 4 with O2 was also
monitored in the presence of organic substrates by UV/Vis
spectroscopy, after confirming that the substrates alone do not
react with 4 under these conditions (as shown exemplarily for
PPh3). While no changes were observed in the presence of
thioanisole or sterically encumbered phenols such as 2,4,6-tri-
tert-butylphenol, it was found that reaction of 4 with O2
occurred much faster upon addition of substrates such as
triphenylphosphine and tri-tert-butylphosphine (to test for
oxygen atom transfer, OAT) and 9,10-dihydroanthracene
(dha) or xanthene (to test for hydrogen atom transfer, HAT),
as indicated by the rate of the decay of the 783 nm band,
which proved to depend linearly on the substrate concen-
tration (see the Supporting Information). These results can be
explained assuming that 6 forms as part of an equilibrium
with 4 and O2 and then reacts with the mentioned substrates,
a boat conformation, as the O2 reaction has converted it into
a tridentate [LtBuOO] ligand. That is, bidentate [LtBu] has
transferred two electrons onto the O2 moiety with formation
À
of a C O bond to the a-carbon atom of the ligand. Hence, the
a-carbon atom in [LtBuOO] is sp3-hybridized and links two
uncharged imine functions, while the distal O atom carries
a negative charge and binds to nickel. There are a few
examples of structurally characterized compounds reported
that feature a NiOOR structural motif, but all of them were
prepared setting out from peroxides,[7,15–17] and its generation
using dioxygen is without precedent;[19] it is important to note
that the O2 reaction does not require light,[18] as the reaction
and in situ crystallization has been carried out in the dark.
À
À
The O O distance (O2 O3 1.432(7) ꢀ) is at the lower end of
1
À
the O O bond range previously observed for h -nickel
[7,16,17,20,21]
À
peroxide complexes (O O 1.440(7)–1.505(3) ꢀ).
The sodium atom in 5(OEt2) interacts with the peroxide
moiety of [LtBuOO], a chloride ion, a fluorine atom of the
[BArF ] anion, and a diethyl ether molecule, which explains
4
the necessity to have diethyl ether in the solvent mixture for
a successful crystallization. Owing to low resolution as well as
to disorder, the data quality of 5(OEt2)’ only allowed us to
depict a model of its structure (see the Supporting Informa-
tion). Both products, 5(OEt2) and 5(OEt2)’, are metastable at
À808C, allowing crystallization, but with rising temperatures
both solutions and the crystalline solids become increasingly
unstable, so that they cannot even be stored at À508C (see
below).
To obtain more information about the course of the
reaction, about the formation of potential intermediates and
about product stability, the proceedings after addition of O2 to
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Scheme 3. The reaction of 4 with O2 to yield 5, which in contact with Et2O was crystallized at low temperatures, forming 5(OEt2) and 5(OEt2)’.
thus accelerating the decay of 4 through a shift in the
equilibrium (see the Supporting Information). Analysis of the
organic products at the end of the reaction confirmed that
oxidation of the substrates had occurred. Triphenylphosphine
oxide and tri-tert-butylphosphine oxide were detected by
31P NMR analysis, and formation of anthracene was con-
firmed by UV/Vis after the reaction with dha.
Contemplating an intermediate that forms on the way
between 4 and 5, a b-diketiminyl nickel(II) superoxide
complex similar to that postulated by Dobbek and co-workers
(Scheme 1)[8] offers itself. However, it is difficult to identify it
on the basis of its signature in the UV/Vis spectrum as
reported data for comparison is scarce and the features
published so far reveal no fingerprint, which is probably due
to different binding modes and positioning on the continuum
between a nickel(II) superoxide and a nickel(III) peroxide.[22]
Hence, lacking any further information, the nature of the
short-lived intermediate has to remain speculative.
For comparison, the reactivity of the final product 5 was
also investigated exemplarily for some representative sub-
strates (see the Supporting Information). While the reaction
rate with PPh3 to give OPPh3 (0.32 LmolÀ1 sÀ1) indicates
a potential as an electrophilic oxidant, which is also under-
lined by its reaction with cyclooctene (0.0017 LmolÀ1 sÀ1), 5 is
not competent to perform hydrogen-atom abstraction reac-
tions (no reaction is observed with xanthene or 9,10-
dihydroanthracene). On the other hand, measurement of
the reaction rate with a series of para-substituted benzalde-
hydes afforded a Hammett plot with a positive slope (1 =
+ 1.1), demonstrating that 5 is also potent as a nucleophilic
oxidant. Interestingly, such an ambivalent behavior was also
observed for the only other nickel(II)organoperoxide that can
be compared, [(TpiPr)Ni(OOtBu)],[17] so that this might be the
intrinsic nature of such compounds.
In conclusion, both the results on the quercetinase by
Dobbek and co-workers and the findings made herein
(Scheme 4) show that O2 activation at nickel(II) complex
units is possible, if a redox-active ligand is bound that on
contact of the nickel center with O2 can provide an electron so
that a nickel(III) state can be avoided.[8] A primary Ni–O2
product has been identified in the investigations on the
quercitinase, and also in the work described here a reactive
intermediate was observed, but we can only speculate on its
nature. The enzymatic Ni–O2 adduct was suggested to
subsequently react by combination of the distal O atom
Scheme 4. Comparison of results obtained investigating O2 activation
at nickel(II)–ligand units within a nickel–quercitinase and a b-
diketiminatonickel(II) complex.
characterized in the present study, providing the first clear-cut
identification including structure determination of a product
formed through O2 activation at a nickel(II) complex frag-
ment. Although the reaction leading to the peroxide complex
is not biomimetic, as here the co-ligand at the same time
served as the substrate, the results demonstrate facile addition
of O2 across a comparable nickel-bound p system and also
confirmed that such species are rather reactive and undergo
further transformations, as proposed for the biological
system.
Acknowledgements
We are grateful to the Humboldt Universitꢁt zu Berlin for
financial support and the cluster of excellence UniCat for
fruitful discussions. We thank Dr. Kallol Ray, Dr. Michael
Haumann, and Dr. Stefan Mebs for attempts to collect XAS
data. We are also thankful to the European Commission
(FP7-PEOPLE-2011-CIG-303522 to A.C.) and the MINECO
of Spain (CTQ2013-43012-P and RYC-2011–08683 to A.C.)
for financial support.
Conflict of interest
The authors declare no conflict of interest.
Keywords: dioxygen activation · nickel · oxidation · oxygenases ·
with the electron hole created in the ligand. This would
peroxide
II
À
À
produce a C OO Ni unit (remaining hypothetical in the
biological investigation), and an example of such has been
4
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Manuscript received: September 28, 2016
Revised: October 31, 2016
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Dioxygen Activation
P. Holze, T. Corona, N. Frank,
B. Braun-Cula, C. Herwig, A. Company,
C. Limberg*
&&& — &&&
Activation of Dioxygen at a Lewis Acidic
Nickel(II) Complex: Characterization of
a Metastable Organoperoxide Complex
Nickel(II) can do it too: O2 activation can
occur even at low temperature if nickel(II)
cooperates with a redox-active ligand, as
recently also suggested for the nickel-
dependent quercitinase. First precedence
is provided that a nickel(II)organoperoxo
moiety can be generated when setting out
from a nickel(II) complex and O2 (partial
structure of crystallized metastable per-
oxide shown).
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!