A Bis(μ-oxido)dinickel(III) Complex with a Triplet Ground State

Article abstract of DOI:10.1002/anie.201802779

A bis(μ-oxido)dinickel(III) complex was synthesized and characterized by single crystal X-ray diffraction, resonance Raman, and ESI-mass measurements. Magnetic susceptibility measurements by SQUID and EPR spectroscopy reveal that the complex has a triplet ground state, which is unprecedented for high-valent metal (M) complexes with [M₂(μ-O)₂] diamond core. DFT studies indicate ferromagnetic coupling of the nickel(III) centers. The complex exhibits hydrogen abstraction reactivity and oxygenation reactivity toward external substrates.

Full text of DOI:10.1002/anie.201802779

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Title: A Bis(µ-oxido)dinickel(III) Complex with a Triplet Ground State
Authors: Yuma Morimoto, Yusaku Takagi, Toru Saito, Takehiro Ohta,
Takashi Ogura, Norimitsu Tohnai, Motohiro Nakano, and
Shinobu Itoh
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A Bis(µ-oxido)dinickel(III) Complex with a Triplet Ground State
Yuma Morimoto,[a]* Yusaku Takagi,[a] Toru Saito,[b] Takehiro Ohta,[c] Takashi Ogura,[c] Norimitsu
Tohnai,[a] Motohiro Nakano,[d] Shinobu Itoh[a]*
Abstract: A bis(µ-oxido)dinickel(III) complex is synthesized and
Scheme 1. Formation of [NiIII(dpema)2(O)2]2+ (2) by the oxidation of
[NiII(dpema)2(OH)2]2+ (1) with H2O2
characterized by single crystal X-ray diffraction, resonance Raman,
and ESI-mass spectroscopies. Magnetic susceptibility
measurements by SQUID and EPR reveal that the complex has a
triplet ground state, which is unprecedented for high-valent metal (M)
complexes with [M2(µ-O)2] diamond core. DFT studies indicate
ferromagnetic coupling of the nickel(III) centers. The complex
exhibits hydrogen abstraction reactivity and oxygenation reactivity
toward external substrates.
High-valent metal-oxide species are frequently invoked as the
key reactive intermediates in the oxidation reactions of a wide
range of homogeneous and heterogeneous systems as well as
of metalloenzymes in biological systems.[1–3] This type of
reactive intermediates conducts variety of oxidation reactions, i.e.
hydroxylation of aliphatic and aromatic compounds, and
oxygenation of olefins and heteroatoms. Inorganic chemists
have dedicated to create new transition-metal active-oxygen
complexes to elucidate the factors that determine their reactivity
toward external substrates.[4,5] In addition to the general factors
such as reduction potential and steric hindrance at the second-
coordination sphere, exchange interactions between the reactive
species and substrates are attracting much recent attentions in
bioinorganic chemistry. For example, Shaik and co-workers
have computationally demonstrated that the activation barrier in
hydrogen atom abstraction reactions by mononuclear oxido-
iron(IV) complexes in a quintet (S = 2) state tends to be lower
Figure 1.
Crystal structures of (a) [NiII(dpema)2(OH)2]2+ (1) and (b)
[NiIII(dpema)2(O)2]2+ (2). In the crystal structures, hydrogen atoms (except
hydroxide) and counter anions are omitted for simplicity.
than that by mononuclear oxido-iron(IV) complexes in a triplet (S
= 1) state.[6] This tendency is ascribed to the larger exchange
interaction between the substrate and the oxidant in a quintet
state. Total spin of the dimetal complexes are also affected by
the manner of magnetic coupling of metal centers. Dinuclear
complexes bridged by active oxygen ligands tend to be
antiferromagnetic,[7] nonetheless Que and co-workers reported a
rare example of the ferromagnetically coupled diiron(IV) complex
bridged by a µ-oxide ligand with its Mössbauer and EXAFS
spectra, and density functional theory (DFT) studies.[8]
Reactivities of such dinuclear high valent metal oxide species,
especially in ferromagnetic states, have been scarcely studied
due to a paucity of the suitable examples. Careful inspections on
the inter-metal spin-spin interaction must be necessary to
elucidate importance of multinuclear-metal core structures
involved in biological systems such as soluble methane
monooxygenase with iron-dinuclear core and oxygen evolving
center occurring in photosystem II with manganese-tetranuclear
core.[3e,9] DFT calculations have contributed to the understanding
of magnetic interactions in transition metal complexes. However,
the theoretical prediction without concrete experimental data is
still a challenge, as energy splitting between different spin states
depends on the chosen exchange-correlation functionals.[10] We
herein report the first example of bis(µ-oxido)dinickel(III)
complex in a triplet ground state caused by ferromagnetic
coupling of their unpaired electrons.
[a]
Dr. Y. Morimoto, Y. Takagi, Prof. Dr. N. Tohnai, Prof. Dr. S. Itoh
Department of Material and Life Science,
Graduate School of Engineering, Osaka University
2-1 Yamadaoka, Suita, Osaka 565-0871, Japan.
E-mail: y-morimoto@mls.eng.osaka-u.ac.jp,
shinobu@mls.eng.osaka-u.ac.jp
[b]
[c]
[d]
Dr. T. Saito
Department of Biomedical Information Sciences,
Graduate School of Information Sciences, Hiroshima City University
3-4-1 Ozuka-Higashi, Asa-Minami-Ku, Hiroshima 731-3194, Japan.
Dr. T. Ohta, Prof. Dr. T. Ogura
Picobiology Institute, Graduate School of Life Science
University of Hyogo, RSC-UH LP Center
1-1-1 Koto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan.
Prof. Dr. M. Nakano
Research Center for Structural Thermodynamics,
Graduate School of Science, Osaka University
1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan.
[***] This work is supported by MEXT (Grant-in-Aid for Young Scientist
(B) JP16K17878 to Y.M and JP16K17856 to T.S., and Grants-in-Aid
for Scientific Research on Innovative Areas “3D Active-Site Science”
(No.JP26105012) to T.S.), and financial support for young scientist
from Osaka University. S.I. thanks JST (the CREST program,
161052502) and
Research (JP16K13963) from JSPS.
a
Grant-in-Aid for challenging Exploratory
Bis(µ-hydroxido)dinickel(II) complex supported by dpema
ligand (N,N-(di-[2-pyridine-2-yl]ethyl)methylamine) (1) was
obtained by treating nickel(II) perchlorate with the ligand in a
moderate yield (45 %). As shown in Figure 1a, the complex has
Supporting information for this article is given via a link at the end of
the document.
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a center of symmetry in the Ni2(OH)2 core and the nickel centers
show square pyramidal geometry (5 = 0.11)[11] with the amine
nitrogen atoms, N(1) and N*(1), at the apical positions (Table
S1). Temperature dependence of its magnetic susceptibility
showed that the nickel(II) centers are antiferromagnetically
coupled in the ground state (J = –58.8 cm–1), but its triplet state
and quintet state with rather small zero-field splitting (D = 11.3
cm–1) are enough accessible at room temperature (Figure S1, in
Supporting Information (SI)). This is consistent with its solution
1H-NMR at room temperature showing broad peaks in a
paramagnetic region (Figure S2).
Solution EPR and 1H-NMR spectra of 2 were taken at 116 K
and 223 K, respectively (Figure 3 and Figure S2). The EPR
spectrum of 2 exhibited half-field signals around
g = 4,
suggesting that its spin ground state is a triplet. Broad NMR
signals observed at a paramagnetic region were consistent with
the ferromagnetic coupling of nickel(III) centers (d7) of 2.
An addition of hydrogen peroxide into an acetone solution
of 1 at –80 ºC resulted in generation of a new absorption band at
405 nm ( = 5.06 × 103 M–1 cm–1) (Figure 2). An equimolar
amount of hydrogen peroxide accomplished full conversion of 1
to the new species (Figure 2, inset).[12] The resulting spectrum is
Figure 3. X-band EPR spectrum of 2 (1.0 mM) measured at 116 K (top) and
the simulated spectrum (bottom).
To get further insight into the electronic structure of 2,
magnetic susceptibility of its crystalline sample was measured
with SQUID in a temperature range from 2 K to 200 K, above
which the solid sample decomposes (Figure 4). The complex
shows
a certain paramagnetism over the wide range of
temperature, which can be interpreted as the result of
ferromagnetic coupling in the dinickel cores as suggested by the
solution EPR. The temperature dependence of effective Bohr
magneton number was nicely fitted from 2 K to 70 K with a spin
Hamiltonian (H) given in eq 1 consisting of a superexchange
term (–2JS1·S2), an antisymmetric exchange term (d·S1×S2)
(Dzialoshinsky–Moriya interaction),[15] and the Zeeman term. The
discontinuity observed at around 77 K may be due to
Figure 2. Spectral changes observed upon the additions of H2O2 into an
acetone solution of [NiII2(dpema)2(OH)2]2+ (1: 2.3 × 10–4 M) at –80 °C. Inset:
Plot of absorbance observed at 405 nm vs molar ratio of added H2O2 against 1.
H = –2 J S1 · S2
+
d·S1 × S2
+
µB(S1 + S2)·g·B
(1)
nearly identical to those of the reported bis(µ-oxido)dinickel(III)
complexes (Table S2).[5a,5c] Cold-spray-ionization mass
spectroscopy (CSI-MS) gave a molecular ion peak cluster at
729.1, which can be assigned to [NiIII2(dpema)2(O)2](ClO4)+
(Figure S3). Resonance Raman measurement of the resulting
a structural phase transition changing the geometry of the Ni2O2
core, even though the detail is not clear. From the fitting, the
exchange coupling constant, the DM coupling constant, and the
g value were obtained as J = 14.7 cm–1, |d| = –41.5 cm–1, and g
= 2.43.[16] Based on the simulated parameters in the SQUID
measurement, its EPR signals were also simulated as shown
solution by 405 nm laser excitation shows
a resonance
enhanced Raman band at 614 cm–1 that can be assigned to a
totally symmetric Ni2O2 diamond core vibration (Figure S4, Table
S3).[13] All these results confirmed formation of
a bis(µ-
oxido)dinickel(III) complex 2 (Scheme 1).
The formation of dinickel(III) complex 2 was also endorsed
by single crystal X-ray crystallography (Figure 1b and S5, Table
S1). In this case, four positions are assignable to two sets of
oxide ligands due to their disorder, and one of which is shown in
Figure 1b.[14] In the oxidation of NiII (OH)2 core to NiIII2(O)2, the
2
alkyl amine nitrogen atoms, N(1) and N(1*), on the apical
positions in 1 were displaced by the pyridine nitrogen atoms,
N(2) and N(3*), in 2. The Ni–Ni distance (2.863 Å) was close to
those of other bis(µ-oxido)dinickel(III) complexes so far been
reported, however Ni–O bond lengths are deviated from the
reported values due to large motions of oxygen atoms causing
the disorder of the Ni2O2 core (Table S4).
Figure 4. Temperature dependence of effective Bohr magneton number of
[NiIII2(dpema)2(O)2]2+ (2) under magnetic field of 1.00 T.
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in Figure 3. Antisymmetric exchange has not been reported for
neither bis(µ-oxido)dinickel(III) nor the other dinuclear active
oxygen-transition metal complexes, except for peroxido-
dicopper(II) complex reported by Meyer and co-workers.[17]
(Table S10). These results indicate the bis(µ-oxido)dinickel(III)
complex prefers hydrogen atom abstraction reactions to oxygen
atom transfer reactions, nonetheless we are still investigating its
origin.
The geometry optimizations of [NiIII2(dpema)2(O)2]2+ at the
UDFT-D3 level were conducted using the crystal structure of 2
as the initial structure. We choose the UPBE0-D3BJ/def2-TZVP
model[18] on the basis of preliminary calculations using several
exchange-correlation functionals with the smaller 6-311G* basis
set (Table S5). The resulting J value evaluated by Yamaguchi
formula correctly reproduces the ferromagnetic interaction (J =
Table 1. Oxidation of External Substrates by 2.
substrate/product
DTB/DTB2
yield, %
45
T, °C
–60
–60
–60
–20
k, M–1s–1
13
xanthene/xanthone
PPh3/P(O)Ph3
PhSMe/PhS(O)Me
32
7.3 × 10–1
1.1 × 10–2
2.3 × 10–3
36
30 cm–1).[19] Besides the J value for [NiII (dpema)2(OH)2]2+
2
computed by this model is in qualitatively agreement with the
experimental result (–105.2 vs –58.8 cm–1). In the optimized
structure, the distances and angles around nickel centers and
Ni–Ni distance are consistent with those of the X-ray structure,
and Ni–O distances converged into typical values around 1.8 Å
(Table S6). Vibration analysis confirmed that the Raman band
around 614 cm–1 (calc. 648 cm–1) is a breathing mode of the
Ni2O2 core. Mulliken spin densities of 2 indicate that the formal
charges of two Ni ions are +3 in both spin states, and the
magnitude of the spin densities on the bridging oxide ligands in
the triplet state are larger than those in the singlet state (Table
S7). Locus of the spins of 2 was also scrutinized with
correspond molecular orbital (CMO) analysis, which correlates
the  and  orbitals with the maximum overlap.[20] As the
computed orbital overlap between CMOs is far smaller than 1.0
(0.114), the dz2 orbitals of those nickel cores do not have
significant overlaps (Figure 5). Such orthogonality of dz2 orbitals
may provide triplet ground state of 2.
28
In summary, we have succeeded to isolate
a bis(µ-
oxido)dinickel(III) complex, [NiIII2(dpema)2(O)2]2+ (2) from the
reaction of the corresponding bis(µ-hydroxido)dinickel(II)
complex, [NiII (dpema)2(OH)2]2+ (1) and hydrogen peroxide. The
2
complex 2 shows a sufficient stability under low temperature
conditions, allowing full characterization of the complex including
X-ray crystal structure. The spin-ground state of 2 has been
determined to be a triplet by EPR and SQUID measurements,
which is the first example of a ferromagnetically coupled high-
valent metal complex with an [M2(µ-O)2] diamond core. DFT
studies have demonstrated the nature of two spin centers in an
orthogonal relation resulting ferromagnetic coupling of the spin
centers. The complex 2 was found to show reactivity toward
organic substrates. Notably, this system is the first example to
show an oxygenation reactivity of a bis(µ-oxido)dinickel(III)
complex toward an external hydrocarbon substrate. This system
will become a bridgehead to investigate the effect of spin-spin
interactions of multi-nuclear reactive species on its reactivities.
Acknowledgements
We thank Dr. M. Hendrich for his kind distribution and help
about EPR simulation software, SpinCount.
Keywords: reactive intermediate • bis(µ-oxido) complex • nickel
• magnetic property • oxidation reaction
Figure 5. Correspond molecular orbitals (CMO), 165 (a) and 165 (b) of 2
calculated at the UPBE0-D3BJ/def2-TZVP level.
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Finally, reactivity of 2 toward external substrates was
examined. The characteristic absorption band of 2 readily
decreased upon an addition of an excess amount of 2,4-di-tert-
buthylphenol (DTB) or xanthene to form 3,5,3′,5′-tetra-tert-butyl-
biphenyl-2,2′-diol (DTB2) or xanthone, respectively. The second-
order-rate constants (k) of these reactions were determined by
[2]
the
plot
of
pseudo-first-order-rate
constant
against
concentrations of the substrates (Table 1, Figure S6, S7).
Complex 2 also shows reactivity toward triphenyl phosphine
(PPh3) and thioanisole (PhSMe) to give the corresponding
phosphine oxide (P(O)Ph3) and sulfoxide (PhS(O)Me),
respectively, although k values were smaller than those of afore
mentioned reactions (Table 1, Figure S8, S9). The present
complex shows relatively high reactivity toward external
substrates as compared to the other bis(µ-oxido)dinickel(III)
complexes and the related bis(µ-oxido)dicopper(III) complexes
[3]
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[13]
We have tried to measure resonance Raman spectrum and CSI-
MS of the bis(µ-18O) dinickel(III) complex generated using H218O2.
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Entry for the Table of Contents
Layout 2:
COMMUNICATION
Yuma Morimoto,* Yusaku Takagi,
Toru Saito, Takehiro Ohta, Takashi
Ogura, Norimitsu Tohnai, Motohiro
Nakano, Shinobu Itoh*
Page No. – Page No.
Title: A Di-nickel(III) Bis-µ-oxide
Complex in High-spin State
A ferromagnetically coupled bis(µ-oxide) dimetal complex is characterized by X-ray
crystallography, EPR, SQUID, and so on. DFT calculation explained the origin of
triplet ground state as the orthogonality of dz2 orbitals storing unpaired electrons.
The complex exhibited reactivities toward external substrates.
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