Generation and Reactivity of a NiIII2(μ-1,2-peroxo) Complex

Article abstract of DOI:10.1021/jacs.0c10958

High-valent transition metal-oxo, -peroxo, and -superoxo complexes are crucial intermediates in both biological and synthetic oxidation of organic substrates, water oxidation, and oxygen reduction. While high-valent oxygenated complexes of Mn, Fe, Co, and Cu are increasingly well-known, high-valent oxygenated Ni complexes are comparatively rarer. Herein we report the isolation of such an unusual high-valent species in a thermally unstable NiIII2(μ-1,2-peroxo) complex, which has been characterized using single-crystal X-ray diffraction and X-ray absorption, NMR, and UV-vis spectroscopies. Reactivity studies show that this complex is stable toward dissociation of oxygen but reacts with simple nucleophiles and electrophiles.

Full text of DOI:10.1021/jacs.0c10958

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Generation and Reactivity of a NiIII (μ-1,2-peroxo) Complex
2
Norman Zhao, Alexander S. Filatov, Jiaze Xie, Ethan A. Hill, Andrey Yu. Rogachev,
and John S. Anderson*
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ABSTRACT: High-valent transition metal−oxo, −peroxo, and −superoxo complexes are crucial intermediates in both biological
and synthetic oxidation of organic substrates, water oxidation, and oxygen reduction. While high-valent oxygenated complexes of
Mn, Fe, Co, and Cu are increasingly well-known, high-valent oxygenated Ni complexes are comparatively rarer. Herein we report the
isolation of such an unusual high-valent species in a thermally unstable NiIII2(μ-1,2-peroxo) complex, which has been characterized
using single-crystal X-ray diffraction and X-ray absorption, NMR, and UV−vis spectroscopies. Reactivity studies show that this
complex is stable toward dissociation of oxygen but reacts with simple nucleophiles and electrophiles.
igh-valent oxygenated transition metal species, such as
using a variety of spectroscopic techniques. Reactivity studies
Hoxo, peroxo, and superoxo complexes, are ubiquitous show that 2 is comparatively stable toward oxygen dissociation,
C−H activation, and O atom transfer but reacts rapidly with
both nucleophiles and electrophiles at low temperature. These
results demonstrate that high-valent bridging Ni−peroxo
intermediates are viable and provide insight into their
properties and reactivity.
intermediates in oxidative reactivity. In particular, oxygenated
first-row transition metal complexes are prominent in both
biological and synthetic systems for the oxidation of organic
substrates,1,2 water oxidation,3,4 and oxygen reduction.5,6
Despite the generality of these proposed intermediates, the
high reactivity of oxygenated transition metal complexes can
make isolation and characterization challenging. Nevertheless,
understanding their structure, properties, and viability is
essential to elucidating or improving many processes.7
The synthesis of the Ni−chloride precursor [PhB(tBuIm)3]-
NiCl (1) was recently reported by our group.66 As 1 shows no
reactivity under an atmosphere of oxygen for several days, we
screened common halide abstractors such as Na+, Ag+, and Tl+
salts to encourage reactivity. While addition of AgOTf or
TlOTf led to intractable mixtures of diamagnetic products,
While numerous examples of high-valent oxygenated
complexes of Mn,8−14 Fe,11,12,14−18 Co,13,19−24 and Cu25−33
have been isolated and studied, significantly fewer oxygenated
Ni complexes have been reported,13,34−49 despite the fact that
high-valent Ni−oxo, −peroxo, and −superoxo complexes have
been invoked in water oxidation,50,51 biological superoxide
dismutation,52 oxygen reduction,53 and organic oxidation
catalysis.35,40,54 No well-defined terminal Ni−oxo complexes
are known, and Ni−superoxo species are still rare.40,41,55,56 In
the case of Ni−peroxo complexes, several examples of
mononuclear NiIII species have been reported,57−60 and
some dinuclear complexes have been transiently observed.61,62
treatment of 1 with NaBArF in dichloromethane (DCM)
4
caused the solution to change from dull chartreuse green to
dark emerald green, indicative of the formation of a new
species, 1-Na (Scheme 1 and Figure 1). A similar color change
was observed in other noncoordinating solvents such as 1,3-
difluorobenzene. However, 1-Na is extremely sensitive to even
small amounts of coordinating impurities such as ethers or
variations in preparation conditions, precluding detailed
characterization of this species (Figure S1). While we do not
have concrete characterization data on this complex, we
tentatively propose an intermediate structurally similar to 1
with a weak interaction between Na+ ions and the chloride
ligand. This proposed structure is also supported by a
One NiII (μ-1,2-peroxo) complex has recently been structur-
2
ally characterized,63 but no high-valent bridging Ni−peroxo
complexes are known, despite the potential importance of
these species in, for example, oxo coupling mechanisms for
water oxidation by Ni-based layered double hydroxides.50,51,64
Previously, tris(NHC)phenylborate (NHC = N-heterocyclic
carbene) ligands have been used to stabilize unusual CoIII−oxo
and FeIII−oxo complexes.19,65 We rationalized that this system
might also aid in the stabilization of high-valent Ni complexes
with oxygen-based ligands. Herein we report the use of
1
comparison of the paramagnetic H NMR spectra of 1 and
1-Na, which show shifted but similar overall patterns of
resonances. Furthermore, treatment of 1-Na with 12-crown-4
ether to sequester Na+ ions regenerates 1 as determined by 1H
NMR spectroscopy (Figure S2). On the basis of these data and
Received: October 16, 2020
PhB(tBuIm)3 to isolate the first example of a NiIII2(μ-1,2-
−
peroxo) complex, {[PhB(t BuIm)3 ]Ni−O−O−Ni-
[(tBuIm)3BPh]}{BArF }2 (2) (BArF = tetrakis(3,5-bis-
4
4
(trifluoromethyl)phenyl)borate). Complex 2 has been struc-
turally characterized, and its properties have been examined
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Scheme 1. Synthesis of 1-Na and 2
Figure 2. SXRD structure of 2. Ni atoms are shown in green, O atoms
in red, C atoms in gray, N atoms in blue, and B atoms in tan.
Ellipsoids are shown at 50% probability. For clarity, solvent molecules,
counterions, and H atoms have been omitted, and parts of the ligand
scaffold are shown as wireframe.
Figure 1. UV−vis spectra of 1, 1-Na (at RT), and 2 (at −78 °C) in
DCM. The left y axis is for 1 and 1-Na, and the right y axis is for 2.
similar species previously reported, we tentatively propose that
1-Na is a dimer as depicted in Scheme 1.67
comparison, the O−O bond lengths in two previously isolated
Ni2(μ-1,2-superoxo) complexes are 1.33 and 1.35 Å.34,63 The
spin density of 2 calculated by DFT is primarily localized on
Ni, which further supports the formulation of this complex as a
NiIII2(μ-1,2-peroxo) complex (Figure S5). The solid-state
structure of 2 shows that the NiIII centers adopt a seesaw
geometry, with a Ni−O bond length of 1.796(9) Å and a Ni−
O−O−Ni dihedral angle of 161.8(5)°. The Ni−O bond
Treatment of 1-Na in DCM with dry oxygen at room
temperature results in an intractable brown mixture of
diamagnetic products as ascertained by 1H NMR spectroscopy.
However, at −78 °C addition of either stoichiometric or excess
oxygen to 1-Na in DCM results in a nearly instantaneous color
change from emerald green to dark purple (Figures S3 and
S4). We assign this new purple species as the dimeric NiIII2(μ-
1,2-peroxo) complex {[PhB(tBuIm)3]Ni−O−O−Ni-
lengths are shorter than those in Meyer’s NiII (μ-1,2-peroxo)
2
complex, which has Ni−O bond lengths of 1.834(2) Å and a
dihedral angle of 89.9(2)°.63 The shorter Ni−O distances are
consistent with the higher oxidation state of NiIII in 2.
Similar solution and solid-state structures of 2 are supported
[(tBuIm)3BPh]}{BArF }2 (2) (Scheme 1). The distinct color
4
change is reflected in the UV−vis spectrum of 2 (Figure 1),
which displays features at 410 nm (740 M−1 cm−1) and 550
nm (970 M−1 cm−1) that are dramatically different from those
in the spectrum of 1 or 1-Na. Time-dependent density
functional theory (TD-DFT) calculations provide a reasonable
reproduction of the electronic absorption spectrum, assigning
the main feature at 550 nm to a mixed ligand-to-metal charge-
transfer transition (Figure S5).
1
by NMR spectroscopic data. The H NMR spectrum of 2
collected at −78 °C has broadened and shifted resonances,
consistent with a paramagnetic species (Figure S6). In
1
compound 1, the H NMR resonances (CD2Cl2) at 106 and
16 ppm have been assigned to the hydrogens of the imidazol-2-
ylidene backbone. A similar pattern is seen in 2, but with a
doubling of these signals (118, 106, −13, −17 ppm),
suggestive of an asymmetric dimer at −78 °C. Additionally,
two large resonances corresponding to the tert-butyl groups are
visible at 15 and 17 ppm. We propose that this pattern arises
from a combination of the seesaw geometry about the nickel
centers and slow isomerization about the B−Ni−O vector at
−78 °C, as observed in a Ni−methyl complex supported by
this ligand scaffold.66 The same 1H NMR spectra are observed
for samples of crystalline 2 dissolved in CD2Cl2 and samples of
2 generated in situ by addition of O2 to 1-Na, confirming that
complex 2 is formed in situ in a relatively clean manner.
The effective magnetic moment of 2 was measured by
Evans’ method at −78 °C to be μeff = 3.2(1) μB. This value is
higher than would be expected for two weakly coupled S = 1/2
Fortunately, dark-purple crystals of 2 could be grown over
several days at −78 °C. Single-crystal X-ray diffraction (SXRD)
confirmed the formation of a NiIII2(μ-1,2-peroxo) complex
(Figure 2). While the quality and resolution of the data set is
limited because the crystal contained large numbers of solvent
molecules and severe disorder of BArF − counterions, the
4
structure of 2 nevertheless provides important information.
The O−O bond length is 1.42(2) Å, which falls between the
O−O bond lengths observed in Meyer’s β-diketiminato
NiII (μ-1,2-peroxo) complex (1.465(2) Å) and Nam’s
2
mononuclear η2 12-TMC and 13-TMC NiIII−peroxo com-
plexes (12-TMC = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclo-
dodecane, 13-TMC = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacy-
clotridecane) (1.386(4) and 1.383(4) Å, respectively).57,63 For
B
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Communication
centers (μS.O. = 2.45 μB) and is more consistent with
ferromagnetic coupling to give an S = 1 ground state (μS.O.
=
2.82 μB). DFT calculations support this assignment, predicting
a ferromagnetically coupled system (J = 36 cm−1) and showing
spin density consistent with ferromagnetic exchange (Figure
S5). Additionally, the X-band electron paramagnetic resonance
spectrum of a solution of 2 in DCM at 15 K is nearly silent,
with only a weak signal centered around g = 2 that accounts for
<10% of the Ni in the sample by spin integration. While metal
complexes bridged by dioxygen ligands are commonly
antiferromagnetically coupled, there is a recent example of a
ferromagnetically coupled copper system.68 While we were
unable to perform solid-state magnetometry on 2 because of its
thermal instability, we did perform variable-temperature Evans’
method measurements. An accurate determination of the J
coupling constant is not possible because of the limited
temperature range from the solvent freezing point and
compound stability, but a persistent moment of ∼3 μB further
supports the assigned S = 1 ground state (Figure S7).
To further interrogate the solution-state structure of 2 and
probe the assigned oxidation states of Ni, we turned to Ni K-
edge X-ray absorption spectroscopy (XAS). The Ni K-edge of
2 (8346.1 eV, THF frozen solution, −135 °C) occurs at higher
energy than that of 1 (8345.4 eV, powder, room temperature)
(Figures 3A, S8, and S9 and Table S1). The difference in the
Ni K-edge between 1 and 2 is outside of error ( 0.4 eV), but it
should be noted that one-electron oxidation from NiII to NiIII
in synthetic complexes produces shifts ranging from 0 to 1.8
eV and care must therefore be taken in interpreting this shift as
an indicator of a change in oxidation state.35,57,69,70 Analysis of
the extended X-ray absorption fine structure (EXAFS) region
of 2 suggests a reasonable fit with a simple model containing
one oxygen and three carbon atoms in the first shell, consistent
with the structure obtained by SXRD (Figures 3B and S10 and
Table S2). Taken together, the observed bond lengths,
magnetic properties, and shift in the Ni K-edge of 2 support
the assignment of a NiIII2(μ-1,2-peroxo) complex.
Figure 3. (A) Ni K-edge X-ray absorption spectra of 1 and 2, showing
the normalized energies of the X-ray absorption near-edge structure
(XANES) region. (B) EXAFS (black) and fit (red) in R space at the
Ni K-edge absorption of 2.
We synthesized the 18O2 isotopologue of 2 to further
characterize the O−O bond through vibrational spectroscopy.
However, we were unable to assign a vibrational band to the
O−O stretch by either infrared or Raman spectroscopy of 2 at
low temperatures, likely because of either Raman laser
photodegradation of 2 or weak resonance enhancement of
the peroxo vibration (Figure S11).37
With complex 2 in hand, we sought to examine its reactivity,
particularly the possible reversibility of dioxygen binding, as
this process has relevance to oxygen evolution. Subjecting 2 to
several freeze−pump−thaw cycles in DCM resulted in no
change in the UV−vis spectrum of the solution. This
observation excludes an equilibrium process for O2 binding.
Additionally, 2 does not react with dihydroanthracene or
phosphines at low temperature. However, addition of 10 equiv
of tetrabutylammonium chloride (TBACl) to 2 in DCM at
−78 °C produced a color change from dark purple to green
upon warming to −50 °C, and 87% recovery of 1 was observed
by 1H NMR spectroscopy (Figure S12 and S13). The balanced
reaction requires the formal release of O2, but we were not able
to observe dioxygen release into the headspace by GC analysis
under the reaction conditions. We hypothesize that a short-
lived reactive oxygen species (ROS) or Ni complex must be
generated that reacts further in solution. However, we were
unable to definitively intercept any such species using a variety
of reagents, including the singlet oxygen trap 9,10-dimethylan-
thracene or the ROS trap 1,3-diphenylisobenzofuran.71
To investigate alternative reactivity, we next treated 2 with
trimethylsilyl chloride (TMSCl), hypothesizing that oxygen
could be released in the form of bis(trimethylsilyl) peroxide
(TMS2O2). Instead, addition of 10 equiv of TMSCl to 2 at
−78 °C yielded bis(trimethylsilyl) ether (TMS2O) in good
yield as determined by 1H NMR and GC−MS analysis.
Complex 1 was not observed in this reaction mixture. Addition
of TMSCl to 18O-labeled 2 followed by GC−MS analysis
confirmed that the oxygen in the resultant TMS2O arose from
2 (Figure S14). We propose that any TMS2O2 formed in the
reaction is decomposed by reactive Ni-containing products.
Indeed, TMS2O2 that was added concomitantly with TMSCl
to 2 under the same reaction conditions decomposed. Despite
these complicated product distributions, the observed
reactivity suggests that this NiIII2(μ-1,2-peroxo) complex
does not release oxygen dissociatively and instead reacts with
other species such as nucleophiles or electrophiles.
In summary, we have generated and characterized an
unusual NiIII2(μ-1,2-peroxo) complex, 2. The SXRD structure
1
and X-ray absorption, H NMR, and UV−vis spectra of 2
confirm the assignment of a NiIII2(μ-1,2-peroxo) moiety.
Furthermore, this complex is reactive toward both nucleo-
C
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philes, such as Cl−, and electrophiles, such as TMSCl. Taken
together, these data demonstrate that NiIII2(μ-1,2-peroxo)
species are synthetically accessible and provide insights into
their reactivity.
Thomas Brunold (UW-Madison) for helpful discussions
about vibrational spectroscopy and Prof. Jonathan Rittle for
helpful discussions on the reactivity of the Ni−peroxo
complex.
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AUTHOR INFORMATION
Corresponding Author
John S. Anderson − Department of Chemistry, University of
Chicago, Chicago, Illinois 60637, United States;
orcid.org/0000-0002-0730-3018; Email: jsanderson@
uchicago.edu
■
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Alexander S. Filatov − Department of Chemistry, University of
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ACKNOWLEDGMENTS
■
This work was supported by an NSF CAREER Award (Grant
1654144). J.S.A. also gratefully acknowledges support from the
Sloan Research Foundation (FG-2019-11497) and 3M
Corporation through an NTFA. N.Z. was supported by a
National Science Foundation Graduate Research Fellowship
(DGE-1746045). Financial support from the ACS Petroleum
Research Fund to A.Yu.R. (60838-ND6) is gratefully acknowl-
edged. MRCAT operations are supported by the U.S.
Department of Energy (DOE) and the MRCAT member
institutions. This research used resources of the Advanced
Photon Source, a DOE Office of Science User Facility
operated for the DOE Office of Science by Argonne National
Laboratory under Contract DE-AC02-06CH11357. We thank
Dr. Mark Warren, Dr. Joshua Wright, and Dr. John Katsoudas
for assistance with XAS data acquisition at beamline 10-BM-
A,B. We also thank John Hack (UChicago), Ida DiMucci
(Cornell), Prof. Kyle Lancaster (Cornell), Ryan Hall (UW-
Madison), Laura Elmendorf (UW-Madison), and Prof.
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