Transformation of Formazanate at Nickel(II) Centers to Give a Singly Reduced Nickel Complex with Azoiminate Radical Ligands and Its Reactivity toward Dioxygen

Reactivity toward Dioxygen

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Transformation of Formazanate at Nickel(II) Centers to Give a Singly

Reduced Nickel Complex with Azoiminate Radical Ligands and Its

Deniz Ar, Alexander F. R. Kilpatrick, Beatrice Cula, Christian Herwig, and Christian Limberg*

Cite This: https://doi.org/10.1021/acs.inorgchem.0c03761

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sı Supporting Information

ABSTRACT: The heteroleptic (formazanato)nickel bromide complex LNi(μ-

Br) NiL [LH = Mes-NH-NC(p-tol)-NN-Mes] has been prepared by

deprotonation of LH with NaH followed by reaction with NiBr (dme). Treatment

of this complex with KC led to transformation of the formazanate into azoiminate

ligands via N−N bond cleavage and the simultaneous release of aniline. At the same

time, the potentially resulting intermediate complex L′ Ni [L′ = HNC(p-tol)-N

N-Mes] was reduced by one additional electron, which is delocalized across the π

system and the metal center. The resulting reduced complex [L′ Ni]K(18-c-6) has a S

/ ground state and a square-planar structure. It reacts with dioxygen via one-

electron oxidation to give the complex L′ Ni, and the formation of superoxide was

detected spectroscopically. If oxidizable substrates are present during this process,

these are oxygenated/oxidized. Triphenylphosphine is converted to phosphine oxide,

and hydrogen atoms are abstracted from TEMPO-H and phenols. In the case of

cyclohexene, autoxidations are triggered, leading to the typical radical-chain-derived

products of cyclohexene.

INTRODUCTION

β-Diketiminatonickel(I) complexes have been shown to

reductively activate a wide range of small molecules.

commonly employed synthetic route to β-diketiminatonickel-

However, suitable precursor compounds for such inves-

tigations were not available prior to this work. Homoleptic

bis(formazanato)nickel(II) complexes with diﬀerent substitu-

■

−11

16−18

tion patterns at the ligand framework are known,

but only

(

I) species is treatment of the corresponding nickel(II) halide

one heteroleptic complex has been reported, where two

(formazanato)nickel(II) fragments are linked by two bridging

hydroxide groups. To perform a reductive dehalogenation,

,10

^d^e^rⁱ^v^a^tⁱ^v^e^wⁱ^t^h^K^C8

center, while the β-diketiminato ligand (Chart 1, left) is redox-

or sodium, which reduces the metal

for the reasons outlined above, a (formazanato)nickel(II)

halide complex is required. Here we report a representative

Chart 1. (Left) β-Diketiminate Ligand and (Right)

Formazanate Ligand

(

formazanato)nickel(II) bromide complex and its reactivity in

contact with KC , which leads to reductive cleavage of the

formazan N−N bond. This furnishes a nickel(II) complex,

which contains two azoiminate ligands plus an electron that is

delocalized across the entire system and can be removed by

dioxygen (O ) to give superoxide and the corresponding

homoleptic nickel(II) complex with two azoiminate radical

ligands.

inert. The formazanate ligand (Chart 1, right) is structurally

similar but can be reduced to a dianionic (2−) form. It

therefore, in principle, oﬀers divergent reactivity: the ligand

lowest unoccupied molecular orbital (LUMO) is accessible at

Special Issue: Advances in Small-Molecule Activation

Received: December 23, 2020

3−15

lower energy,

which may allow for reductive activations

under milder conditions, and also, in general, ligand-centered

redox processes can expand upon the intrinsic reactivity of the

metal centers. Hence, exploration of the reduction chemistry of

nickel(II) formazanate complexes warranted investigation.

XXXX The Authors. Published by

American Chemical Society

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Scheme 1. Synthesis of Complex 1

RESULTS AND DISCUSSION

■

For our studies, we chose a formazanate precursor with a tolyl

substituent in the R position and mesityl residues for Ar and

Ar , i.e., Mes-NH-NC(p-tol)-NN-Mes (LH). After

deprotonation of LH with NaH in tetrahydrofuran (THF) at

ambient temperatures and subsequent treatment with

NiBr (dme), the bromide-bridged dinuclear heteroleptic

complex LNi(μ-Br) NiL (1) was isolated as a green solid in

2% yield (Scheme 1). Elemental analysis and mass

spectrometry measurements supported the bulk purity as

well as the proposed constitution.

The structure was investigated by X-ray diﬀraction (XRD)

studies performed with suitable single crystals of 1 grown via

diﬀusion of hexane into a THF solution of 1 (Figure 1). In the

solid state, 1·3C H O contains two nickel centers in square-

planar coordination spheres. The N−Ni−N angles are

0.34(12)° and 89.60(12)°, whereas the Br−Ni−Br angles

are 82.75(18)° and 83.102(18)°, respectively. However, from

the side view (Figure1b), it becomes evident that the nickel

formazanate entity as such is not completely planar: The angle

between the idealized planes spanned by N8−N5−Ni1−Br1−

Br2 and N5−N6−C27−N7−N8 is 19.90(12)°. Likewise, the

Ni Br core is not planar but bent across the Br···Br axis

(Figure 1b). The distance between the centers of gravity of the

mesityl rings C2−C7 and C44−C49 amounts to 5.2703(4) Å

and the one between C18−C23 and C28−C33 to 4.7071(3)

Å. This is arguing against π−π stacking between the aryl rings

in the solid state.

Figure 1. (a) Molecular structure of 1·3THF as determined by single-

crystal XRD. Hydrogen atoms and solvent molecules are omitted for

clarity. Selected bond lengths [Å] and angles [deg]: Br1−Ni2

2.3557(5), Br1−Ni1 2.3632(6), Br2−Ni2 2.3703(6), Br2−Ni1

2.3793(5), Ni1−N8 1.844(3), Ni1−N5 1.854(3), Ni2−N4

The H NMR spectrum of 1 dissolved in C D (Figure S2 )

.841(3), Ni2−N1 1.853(3), N1−N2 1.301(4), N2−C1 1.337(4);

or toluene-d shows six sharp signals between δ 2 and 8 ppm,

Ni2−Br1−Ni1 92.603(19), Ni2−Br2−Ni1 91.832(19), Br1−Ni1−

Br2 82.750(18), Br1−Ni2−Br2 83.102(18), N4−Ni2−N1 90.34(12),

N8−Ni1−N5 89.60(12), N5−Ni1−Br1 176.31(9), N8−Ni1−Br2

175.35(9). (b) Side view illustrating the nonplanarity of the ligand−

nickel entities and Ni Br core.

consistent with a diamagnetic complex with C symmetry on

the NMR time scale, as one should expect from the structure

^d^e^t^e^r^mⁱⁿ^e^dⁱⁿ^t^h^e^s^o^lⁱ^d^s^t^a^t^e⁽^Fⁱ^g^u^r^e¹⁾^.^H^o^w^e^v^e^r^,^a^T^H^F^-^d8

^s^o^l^u^tⁱ^oⁿ^o^f^c^o^m^p^o^uⁿ^d¹⁽¹THF) is NMR-silent, which may

indicate a reaction with THF to yield in square-pyramidal

nickel coordination, a switch of square planar to tetrahedral

coordination, or even the formation of monomers with

tetrahedrally coordinated nickel ions. The eﬀective magnetic

and 298 K. The eﬀective magnetic moment μ_e_f_fwas found to

increase with decreasing temperature to 2.05 μ at 193 K,

suggesting that the paramagnetic species is favored at low

temperatures, which argues against the formation of monomers

as the origin of the paramagnetism. At the same time, these

observations match the ﬁndings already made decades ago for

complexes of the type NiL X (X = halide), which proved to

19,20

moment determined by the Evans method

was 1.55 μ per

nickel atom at room temperature, a value consistent with one

unpaired electron per metal center. Any paramagnetic

nickel(II) ion has two unpaired electrons, and hence the low

value for μ is suggestive of either a weak magnetic coupling in

enter into equilibria involving square-planar (Sp) and

eff

a dinuclear complex or an equilibrium between complex 1,

with the structure shown in Figure 1, and complex 1_T_H_Fwith

the nickel ions in tetrahedral (or square-pyramidal) coordina-

tion spheres. To examine this hypothesis, variable-temperature

NMR studies were carried out in 10 K increments between 193

tetrahedral (Td) structures. Solvent-dependent equilibria

between the Sp and Td forms are also known. For instance,

Mori et al. found out that their homoleptic nickel complexes

form equilibria between the Td and Sp conformations in

unpolar solvents but have a Td structure in polar solvents.

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Scheme 2. Proposed Equilibrium between the Square-Planar 1 and Tetrahedral 1_T_H_Fin THF

Consequently, we also propose for 1 a solvent- and

temperature-dependent equilibrium between Td and Sp

molecular structures, as shown in Scheme 2.

(EPR) spectrum was recorded at 77 K, which showed a signal

with corresponding g values of 2.004, 2.008, and 2.013 (Figure

S8). This supports the above hypothesis that the reduction is

With the suitable nickel(II) formazanate precursor com-

pound 1 in hand, its reduction chemistry was studied next, ﬁrst

with the aid of cyclic voltammetry (CV) studies on 1 in a THF

solution. These revealed one quasi-reversible redox event at

ligand-based. Similar observations were made using BuLi as a

mild reductant. Unfortunately, we did not succeed in growing

single crystals or isolating a pure product in a diﬀerent way, so

that further investigations were hampered. Investigations on

E ₁ _/₂ = −1.12 V versus Fc (Figure 2 ). For comparison, the

the behavior of 1 toward the stronger reductant KC brought

new observations. Upon using 2 equiv of KC a nonuniform

reaction was observed to take place, leading to unconsumed

starting material and overreduced products, so that we further

studied the conversion with excessive amounts of the reagent.

A benzene solution of 1 was stirred for 12 h in the presence

of 4 equiv of KC and 4 equiv of 18-crown-6 (18-c-6), which

resulted in a color change from green to brown. After ﬁltration

over Celite, all volatiles were evaporated and a dark-brown

microcrystalline solid was isolated, which was identiﬁed by

spectroscopic and analytical methods as well as by single-

crystal XRD (Figure 3) as [L′ Ni]K(18-c-6) [2; L′ = HN

C(p-tol)-NN-Mes] formed with a yield of 45% (Scheme 3).

Crystals suitable for XRD analysis were grown from a

concentrated benzene solution, and the solid-state molecular

structure (Figure 3) revealed that the formazanate ligands had

undergone an N−N cleavage to give ligands with an azoimine

backbone.

There is currently no synthetic procedure available for

Figure 2. CV curve (two cycles) of 1 dissolved in THF measured at a

scan rate of 100 mV·s⁻(100 mM TBAPF and 1 mM complex 1) at

uncoordinated azoimines (Chart 2). Coordinated azoimines

were reported to form through the reduction of azooximate

room temperature. The dotted line denotes the ﬁrst scan.

ligands, and they were also found to be susceptible to

reduction with a further electron to give azoiminate radical-

anion ligands (Chart 2), the free protonated forms of which

homoleptic zinc bis(formazanate) complex L Zn reported by

Otten and co-workers in 2015 undergoes two ligand-based

redox events at −1.80 and −2.30 V. Consequently, it is

tempting to assign the redox event at E = −1.12 V versus

6,28

are not synthetically accessible, either.

The presence of a [(18-c-6)K)] cation in the solid-state

structure of 2·C H , as depicted in Figure 3, requires the nickel

Fc for 1 to a Ni /Ni redox process [note that a iron(II)

complex to carry an overall formal charge of 1−. Considering

the square-planar coordination environment of the nickel

center, it is reasonable to exclude a nickel(0) oxidation state.

This leads to the inference that the associated ligands are

anionic in nature, i.e., that they correspond to azoiminates, as

shown in Scheme 3, and a third electron then either resides

within the ligand system also, so that the nickel atom is in the

oxidation state II+, or at the nickel center [→ nickel(I)]. With

the background of this hypothesis, the sterical and electronic

structure of 2 was further analyzed as described below.

bis(formazanate) complex was found to undergo metal-

centered reduction at a comparable potential]; however,

studies on other heteroleptic transition-metal formazanate

complexes have clearly shown that the LUMO of formazanate

15,24

complexes is ligand-centered.

Hence, we also assign the

process observed for 1 to a reversible ligand-centered

reduction. For comparison, the Ni /Ni event of a β-

diketiminatonickel(II) bromide complex occurs at −1.37 V

(

vs Fc/Fc ).

On this basis, subsequently the chemical reduction of 1 was

investigated. First, complex 1 was reacted with 2 equiv of

sodium naphthalenide at −78 °C in toluene for 1 h. The

product turned out to be NMR-silent because of para-

magnetism. Therefore, an electron paramagnetic resonance

Azoiminates are ﬁve-electron π systems, and indeed the

lengths of the N−N bonds in 2·C H are around 1.383(3) and

1.341(3) Å, respectively, which is typical for delocalized

systems containing N−N bonds; also the N−C distances of

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Figure 3. Molecular structure of 2·C H , as determined by single-crystal XRD. Hydrogen and solvent atoms are omitted for clarity. Selected bond

lengths [Å] and angles [deg]: Ni−N3 1.832(2), Ni−N1 1.840(2), N1−N2 1.383(3), N2−C1 1.324(3), N3−C1 1.341(3); N3−Ni−N3′ 180.0,

N3−Ni−N1′ 98.43(10), N3−Ni−N1 81.57(10), N2−C1−N3 118.4(2). (a) Polymeric structure of 2. (b) Single unit of the polymeric structure in

part a.

Scheme 3. Reduction of Complex 1 Using KC and 18-c-6, Leading to the Transformation of Formazanate to Azoiminate and

the Reduction of the Resulting Complex

Chart 2. Reduction of Azoimine by One Electron, Leading to the Five-Electron π-System Azoiminate, and by a Second

Electron to Yield the Corresponding Diamide

.324(3) and 1.341(3) Å are consistent with this interpreta-

additional unpaired electron will likely lead to a S = / ground

tion.

state, and the question is, where is the unpaired spin density

located? To further elucidate the electronic structure of 2, an

EPR spectrum was recorded for a frozen toluene solution at 77

K (Figure 4).

The counterion [(18-c-6)K] interacts via the potassium

cation with the p-tolyl residues of two neighboring anionic

units in a repetitive manner, so that altogether a coordination

polymer results with [(18-c-6)K] as the linking units. The

The X-band EPR spectrum of 2 in toluene at 77 K (Figure

mesityl substituents are perpendicular to each other, and

together with the [(18-c-6)K)] entities on both sides, they are

4) shows a signal of axial symmetry with the corresponding g

values of 2.089 and 2.009 consistent with a S = / system. On

sterically shielding the nickel center. As mentioned, the

coordination sphere of the nickel center is square-planar,

which would be untypical for a nickel ion in the oxidation state

I+: There are only a few examples of square-planar nickel(I)

complexes with neutral ligands in the literature and even fewer

the one hand, previously reported nickel(I) complexes typically

,32,33

give larger g values,

and, on the other hand, the g

anisotropy observed in the EPR spectrum of 2 is much larger

than that typically observed for hydrocarbon-based radicals.

Consequently, the unpaired spin appears to be located both at

the ligand and at the metal ion. To support this interpretation,

DFT calculations were performed (B3LYP-D3/Def2-TZVP),

which indeed conﬁrmed a doublet ground state (quartet 100.6

9,30

with anionic ligands.

However, delocalization of the

unpaired electron across the ligand π system with maintainance

of the oxidation state +II, as revealed by EPR spectroscopy and

density functional theory (DFT) calculations (vide infra),

explains the structure of 2.

−

kJ·mol higher in energy) with only 20% of the unpaired spin

density located at the nickel center (Figure 5). Still, in the EPR

spectrum, a hyperﬁne splitting caused by the interaction with

nitrogen nuclear spins was not observed, most likely because of

the combination of a small coupling constant and the signal

Two azoiminate ligands contain one unpaired electron each,

which are accommodated in the π system, and can be assumed

26,31

to be strongly coupled antiferromagnetically.

Thus, an

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for which hyperﬁne splitting caused by the nitrogen atoms was

neither observable.

On the basis of these ﬁndings, we conclude that the

structure can be viewed best as containing one monoanionic

azoiminato ligand as well as one further reduced diamide

ligand, as shown in Chart 2 on the right-hand side, with a

complete delocalization of both; that is, the extra π electron is

shared equally between the two ligands with coupling through

a Ni d orbital (on average both ligands are thus in a 1.5−

charge state). This ﬁts nicely to the ﬁndings made by

Wolczanski and co-workers for complexes with nickel in a

redox-active ligand framework showing that all redox events

were shuttling electrons in and out of the ligand scaﬀold, thus

allowing the nickel center to maintain its favorite oxidation

state II+ in a highly stabilizing square-planar coordination

sphere.

The formation of 2 from 1 in the course of reduction

formally leaves behind two “Mes-N:” fragments, which

together with four electrons and four protons could form the

corresponding aniline, and indeed Mes-NH could be detected

after the reaction via H NMR spectroscopy (Figure S4);

complex 2 itself is NMR-silent. The conversion of two

formazanate ligands into two azoiminate ligands and two

Figure 4. X-band EPR spectrum (77 K, 9.45 GHz) of compound 2 (1

mmol·L⁻in toluene) and simulation (red) for g = 2.089 and g =

−

aniline equivalents corresponds to a 6e /6H transformation,

which recently has also been observed by Teets and co-workers

in attempts to prepare platinum formazanate complexes in

ethanol with triethylamine; the latter was assumed to represent

∥

⊥

.009.

the source of the electrons. In our case, a further electron

reduces the complex, so that altogether seven reduction

equivalents are required to realize the reaction in Scheme 3.

Consequently, the reaction was repeated with 7 equiv of KC₈

and 18-c-6, and indeed higher yields (70%) of 2 were obtained.

The source of the protons in this reaction is still unclear.

Analogous reactions using C D as the solvent resulted in near-

identical product mixtures with no deuterium incorporation, as

observed by NMR spectroscopy, and therefore we assume that

solvent activation is not responsible.

We then turned our attention to the reactivity of 2. In

particular, its reactivity toward O , was of interest to us, given

the precedence for isolable nickel(II) superoxide complexes

Figure 5. Spin-density distribution calculated for the doublet ground

state of complex 2 as an anion without K(18-c-6) (B3LYP-D3/Def2-

TZVP).

after O activation.

The reaction of 2 with 1 atm of O was carried out in THF

or toluene at ambient temperatures (Scheme 4), resulting in a

color change from brown to green. After workup, L′ Ni (3)

was isolated, derived from 2 by a single-electron oxidation, as

already indicated by the absence of a cation.

Single crystals of 3 were grown by diﬀusion of pentane into a

benzene solution of 3 stored at room temperature. The

molecular structure (Figure S19 ) reveals a nickel center

broadening, which is probably caused by nuclear quadrupole

4,35

interactions.

Wieghardt and co-workers reported a

comparable nickel complex, which was generated during

spectroelectrochemical studies (EPR coupled with CV) and

Scheme 4. Oxidation of Complex 2 by O in THF at Ambient Temperatures

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surrounded by four nitrogen atoms in a square-planar fashion;

that is, the core of 2 has been retained, but there are some

diﬀerences in the bond lengths. The Ni−N bonds are shorter

in 3 than in 2, for instance, the Ni−N2 distance by 0.027 Å.

Most signiﬁcantly, the N−N distances are, on average, 0.046 Å

shorter in 3 than in 2. Furthermore, the N2−C1 bond is

elongated, whereas N3−C1 is shortened, which is reasonable

because participation of the diamide ligand (through the extra

electron) is not relevant anymore.

oxidation of 2 by O is either an outer-sphere process or the

formed encounter complex rather quickly dissociates to give 3

and potassium superoxide.

Notably, free superoxide (likely in interaction with a K

cation) could be detected by EPR. An EPR spectrum recorded

after the reaction between compound 2 and O showed a

signal with a small g anisotropy and a g tensor of 2.045, 2.009,

and 2.004, i.e., close to the value of the free electron (Figure

S9 ). These g values are very similar to those reported by

Fukuzumi and Ohkubo found for superoxide anions stabilized

The removal of an electron from 2 leaves two azoiminato

radical ligands in 3, and as judged by the diamagnetism of 3

by a variety of diﬀerent metal ions. These researchers

(

according to H NMR and EPR spectroscopy), their unpaired

postulated that the interaction with a potassium ion should

electrons are coupled strongly antiferromagnetically, as

expected (vide supra). Indeed, DFT calculations also suggested

an open-shell singlet ground state with unpaired spin density

mainly located at the nitrogen atoms binding the mesityl

residues (Figure S21 ). The CV studies of 3 in THF revealed a

increase gzz compared to the values observed for lithium (LiO

zz = 2.0546) and sodium (NaO : gzz = 2.0841) cations. As

mentioned above, after the reaction between 2 and O , the

observed signal had a gzz value of 2.045, that is, lower than one

should expect, which may be a consequence of the additional

redox event at E₁_/₂= −1.45 V versus Fc (Figure 6), which is

fully reversible, as expected, considering the stability of both 2

and 3, for which only minimal structural change occurs upon

electron transfer.

presence of 18-c-6 and a solvent. Indeed, in 2011, Buchner and

co-workers found that 18-c-6 leads to a line broadening of the

EPR spectrum of the superoxide radical anion and also changes

of the g anisotropy.

Apparently, binding of the superoxide radical anion to the

nickel center is not favorable, likely because of the relatively

stable square-planar coordination environment of the nickel-

(

II) center in 3. However, this is the basis on which many

catalysts employed industrially for oxidation reactions with O

work. In the ﬁrst step, a metal compound transfers an

electron to produce a superoxide, which reacts with the organic

substrate, and then radical-chain processes are initiated

(

autoxidations); the main function of the metal catalyst is

the initiation of the reaction and in some systems also the

splitting of the hydroperoxide intermediates. Such “homo-

lytic oxidation reactions” naturally have the disadvantage that

the selectivity is low and (because radicals determine the

proceedings) they are diﬃcult to control, so that “heterolytic

oxidation reactions”, which proceed via well-deﬁned active

oxygen−metal complexes, are clearly favorable. However, there

are only a few examples of the latter, which successfully work

catalytically with O₂, and, on the other hand, there are cases

Figure 6. Overlaid CV scans of 3 measured at a scan rate of 100 mV·

−

−1

where even homolytic oxidations can be performed with high

in THF (100 mM TBAPF and 0.05 mmol·L complex 3) at

selectivity.

room temperature. The dotted line denotes the ﬁrst scan.

To investigate the potential of 2/O to perform oxidative

conversions, the reactions of several organic substrates with 2/

In the reaction between 2 and O , electron transfer will lead

were monitored using diﬀerent analytical methods after

conﬁrmation that the substrates alone do not react with 2 or

under the same conditions (Scheme 5) and diﬀerent

initially to a superoxide and thus potentially to a nickel(II)

superoxide intermediate (if the contact occurs via the metal).

In order to gather any information on such an intermediate,

methods were used to detect the oxidation products (Table 1).

To examine whether 2 can trigger and promote autoxidation, it

was tested in the solvent-free system cyclohexene/O

Compound 2 was suspended in cyclohexene, and O was

bubbled into the solution for 30 s, followed by stirring for 16 h

at ambient temperatures. Upon the addition of O , the color of

the reaction between 2 and O was monitored by variable-

temperature UV−vis spectroscopy between −80 °C and room

temperature in THF and toluene (Figure S14 ). In all UV−vis

experiments, a new band belonging to the ﬁnal product 3 arose

around 720 nm, immediately after the addition of O . This

low-energy band is best assigned to the HOMO-to-LUMO

transition, which has a ligand-to-metal charge-transfer

character and is responsible for the strong luscious green

color of the complex. Teets and co-workers reported UV−vis

studies on various platinum azoiminate complexes, which also

showed bands at ca. 720 nm. However, the UV−vis

experiments did not reveal bands evolving/decaying before

those of the ﬁnal product 3; i.e., there was no evidence for an

intermediate with a suﬃciently long lifetime. Cryo-stopped-

ﬂow experiments performed at −40 °C in THF did not allow

for the detection of any transient species. Consequently, the

the solution changes to green. Indeed, the oxidation products

1,2-epoxycyclohexane, 2-cyclohexen-1-one, and 2-cyclohexen-

1-ol could be identiﬁed in a ratio of 1:8:7.5, employing gas

chromatography coupled to mass spectrometry (GC/MS;

TON = 8; Figure S13). On the basis of the nature of these

products, this reaction follows a radical-chain reaction

mechanism proposed by Wei et al. in 2016 and Kohantorabi

et al. in 2018. In the ﬁrst step, a cyclohexenyl radical is

produced that can be converted to a peroxy radical. This can

abstract a hydrogen atom to form ROOH (R = cyclohexenyl)

or react further to produce 2-cyclohexen-1-one and cyclo-

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Scheme 5. Reactivity of Complex 2 toward O in the Presence of Various Substrates

Table 1. Reaction Products Formed in the Oxidation of

Diﬀerent Substrates with the Combination of Complex 2

Furthermore, TEMPO-H and phenols [2,4,6-tri-tert-butyl-

phenol (TTBP), 2,6-dimethylphenol (DMP), and 2,6-di-tert-

butylphenol (DTBP)] were employed to test the potential of

and O and Their Characterization

/O₂to perform hydrogen-atom abstraction. Therefore,

reaction

−1

complex 2 (0.5 mmol·L ), in combination with O , was

substrate

conditions

product(s)

identiﬁcation

³¹P NMR =

24 ppm

−

reacted with TEMPO-H (25 mmol·L in toluene) and TTBP

triphenylphosphine

C D at rt triphenylphosphine

−1

(

12.5 mmol·L in THF), respectively, at room temperature

oxide

for 30 min. The reaction mixtures were then analyzed by EPR

spectroscopy at 77 K, which revealed the corresponding signals

typical for the radicals formed upon hydrogen-atom

abstraction from these substrates (conversion of ∼1 equiv;

cyclohexene

neat at rt 1,2-epoxycyclohexane, 2- GC/MS

cyclohexen-1-one, and

-cyclohexen-1-ol

,6-dimethylphenol

toluene,

3,3′,5,5′-tetramethyl-4,4- UV−vis

−

40 °C

diphenoquinone

45−47

to rt

Figures S10 and S11 ).

The reactions with DMP and

,6-di-tert-

toluene,

−80 °C

to rt

toluene, rt TEMPO

THF, rt TTBP radical

3,3′,5,5′-tetra-tert-butyl- UV−vis

DTBP were investigated using toluene as well as THF as

solvents at diﬀerent temperatures by UV−vis spectroscopy. In

both cases, a new band at 425 nm was observed, indicating

formation of the coupling products 3,3′,5,5′-tetra-tert-butyl-

butylphenol

4,4-diphenoquinone

TEMPO-H

EPR at 77 K

,4,6-tri-tert-

butylphenol

,4-diphenoquinone and 3,3′,5,5′-tetramethyl-4,4-diphenoqui-

none, conﬁrming that 2/O is capable of abstracting a

hydrogen atom from DMP and DTBP also (Figures S15 −

S17).

hexen-1-ol; ROOH is also decomposing to these products.

Hence, complex 2 can activate molecular oxygen for the

autoxidation of substrates under mild conditions.

CONCLUSION

■

Complex 2 also mediates oxygen-atom transfer from O , as

We have synthesized the ﬁrst heteroleptic (formazanato)-

nickel(II) bromide complex and examined its redox behavior.

demonstrated by the reaction with triphenylphosphine.

NMR spectroscopy in C D revealed that 2 equiv of PPh were

In attempts to reduce this complex with KC in the presence of

oxidized by O in the presence of 1 equiv of complex 2 to give

18-c-6, formazanate did not remain intact but underwent N−N

bond cleavage, which seems to be a potential decomposition

reaction of formazanate complexes in reduced states. As a

consequence, the formazanate ligands transformed into

azoiminato ligands, which contain an unpaired electron in

the π system and cannot be prepared, currently, in free

(protonated) form by any synthetic procedure. The resulting

triphenylphosphine oxide (Figure S1). The evolution of the

signal for OPPh started immediately after the addition of O ,

and the reaction reached completion within 15 h. Interestingly,

the EPR signal assigned to superoxide was not observed for

mixtures of 2 and O in the presence of PPh . After 15 h, the

reaction mixture was completely EPR-silent (Figure S12).

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Forum Article

complex was further reduced by one electron, yielding 2, where

the extra electron is delocalized across the entire complex

plane so that formally the two ligands are in an average 1.5−

atoms. All hydrogen atoms were added geometrically and reﬁned by

using a riding model.

Chemicals. 1,5-Dimesityl-3-(p-tolyl)formazan, NiBr dme, and

12,51,52

KC were synthesized according to literature procedures.

charge and a S = / ground state results. In contact with O ,

NaH was purchased from Sigma-Aldrich and used as received.

this electron is removed, leading to free superoxide and 3, in

which the two azoiminato radical ligands are coupled

^aⁿ^tⁱ^f^e^r^r^o^m^a^gⁿ^e^tⁱ^c^a^l^l^y^,^s^o^t^h^a^t^t^h^e^c^o^m^p^l^e^xⁱ^s^dⁱ^a^m^a^gⁿ^e^tⁱ^c^.^O2

activation at 2 can be utilized to oxidize organic and inorganic

TEMPO-H was produced according to a modiﬁed literature

53,54

procedure.

O₂was added via a balloon for the UV−vis

experiments. For all other O -involving experiments, the atmosphere

in the reaction vessel was exchanged by an O atmosphere.

substrates. In the presence of cyclohexene, autoxidation is

triggered, leading to the typical radical-chain-derived oxidation

products. Triphenylphosphine is converted into the corre-

sponding oxide, and hydrogen atoms are abstracted from

TEMPO-H and phenols.

Solvents. All solvents were degassed prior to use. Toluene, n-

hexane, pentane, and dichloromethane (DCM) were puriﬁed by

employing an MBraun Solvent Puriﬁcation System SPS, stored over a

potassium mirror in Young Schlenk tubes. Pentane and DCM were

stored over molecular sieves (3 Å). Benzene was purchased from

Sigma-Aldrich and stored over a potassium mirror. THF was

purchased from Acros Organics, distilled over sodium, and stored

over molecular sieves (3 Å).

EXPERIMENTAL SECTION

■

(

LNiBr) (1). A total of 100 mg of 1,5-dimesityl-3-(p-tolyl)formazan

General Considerations. All manipulations were carried out in a

glovebox or by using Schlenck-type techniques under a dry argon

atmosphere. Microanalyses were performed with a Leco CHNS-932

elemental analyzer. UV−vis spectra were recorded at variable

temperatures using an Agilent 8453 UV−vis spectrophotometer

equipped with a quartz dewar. Cryo-stopped-ﬂow experiments were

performed by using a SFM-2000/s stopped ﬂow mixer from Bio-Logic

Science Instruments, a high-power UV−vis ﬁber light source from

Hamamatsu, and a Tidas MMS vis/NIR from J&M Analytik AG. The

temperature was controlled by a cryostat from Huber with isopropyl

alcohol as a cooling agent. Attenuated-total-reﬂectance infrared

(

1 equiv, 0.25 mmol) was dissolved in 12 mL of THF, and NaH (6.6

mg, 1.1 equiv, 0.27 mmol) was added. After stirring overnight at room

temperature, the reaction mixture was added to a suspension of

NiBr (dme) (85.2 mg, 1.10 equiv, 0.27 mmol) in 10 mL of THF.

After reﬂuxing at 80 °C for 16 h, the resulting green solution was

ﬁltered, and all volatiles were removed under reduced pressure. The

green solid was extracted with benzene and freeze-dried under

reduced pressure (144 mg, 0.13 mmol, 92%). Elem anal. Found for

−1

C H Br N Ni (1072.72 g·mol ): H, 5.80; C, 58.28; N, 10.13.

Calcd: H, 5.45; C, 58.25; N, 10.45. H NMR (300.1 MHz, C D ): δ

.89 (d, 2H, J = 8.45 Hz), 6.97 (d, 2H, J = 8.45 Hz), 6.50 (s, 4H),

.46 (s, 12H), 2.10 (s, 6H), 2.03 (s, 3H). C{ H} NMR (75.47

(

ATR-IR) spectra were recorded with a Bruker Alpha Fourier

transform infrared spectrometer. EPR spectra were measured on an

ESR Miniscope MS5000 (Magnettech), equipped with a quartz ﬁnger

dewar. All spectra were quantiﬁed against a copper(II) standard, and

to determine the g values, spectra were simulated with Easyspin.

Electrospray ionization mass spectrometry (ESI/MS) spectra were

carried out on an Agilent Technologies 6210 time-of-ﬂight liquid

chromatography−mass spectrometry instrument. GC analysis was

carried out by using an Agilent 7890B gas chromatograph (HP5

column, 30 m) with a ﬂame-ionization detector coupled to an Agilent

MHz, C D ): δ 149.8, 137.1, 135.8, 131.3, 129.2, 124.4, 20.7, 19.0.

ESI/MS (MeCN, positive-ion mode): m/z 504.162 (LNiCNNa ).

−1

Calcd: m/z 504.17. IR (ATR-IR; cm ): ν

2967, 2946, 2916, 2855,

652, 1605, 1515, 1473, 1434, 1374, 1329, 1306, 1279, 1187, 1137,

070, 1038, 990, 952, 876, 822, 765, 660, 627, 574, 530, 517, 463,

30.

[

L′ Ni][K(18-c-6)] (2). 1 (1 equiv, 80 mg, 74.16 μmol) was dissolved

in benzene. KC (7.5 equiv, 75.7 mg, 556.2 μmol) and 18-c-6 (7.5

equiv, 147 mg, 556.2 μmol) were added to the solution. After stirring

for 12 h at room temperature, the dark-brown reaction mixture was

ﬁltered over Celite, and all volatiles were removed under reduced

pressure. The resulting brown solid was washed with n-hexane and

dried overnight in vacuo to yield 2. However, 2 thus obtained was

often contaminated somewhat by (18-c-6)KBr, which has rather

similar properties and also crystallizes together with 2. On average,

the yield of 2, determined by weighing the product and subtracting

the portion of (18-c-6)KBr according to elemental analysis, was 70%.

Crystals of 2 suitable for XRD were grown from a concentrated

benzene solution at room temperature. Elem anal. Found for a pure

977B electron impact mass spectrometry (EI-MS) spectrometer with

a triple-axis detector. The instrument was equipped with an Agilent

G4513A autoinjector (injection of approximately 10 μL). Speciﬁca-

tions for Method 5: The GC method starts with an oven temperature

of 50 °C (hold time 5 min) and includes two ramps (ramp 1, 50−190

−

−1

C, 10 °C·min ; ramp 2, 190−300 °C, 20 °C·min ), a total run time

of 24.5 min, and a solvent delay of 2.9 min. MS peaks were analyzed

and compared with the library database of NIST MS Search 2.3. All

EI-MS spectra of the detected peaks were in good agreement with the

library database of the expected substances. NMR spectra were

−

measured using a Bruker 300 MHz DPX instrument ( H NMR 300

sample of 2 (C H KN NiO ; 892.83 g·mol ): H, 6.99; C, 62.38; N,

−

MHz). Electrochemical studies were carried out using a PalmSens

EmStat Blue potentiostat under computer control. CV experiments

were performed under an argon atmosphere using a three-electrode

.85. Calcd: H, 7.00; C, 61.88; N, 9.41. ATR-IR (crystal; cm ): ν

888, 1598, 1554, 1510, 1492, 1467, 1425, 1399, 1391, 1369, 1351,

290, 1252, 1212, 1204, 1173, 1156, 1100, 1047, 1008, 960, 836, 821,

79, 741, 732, 714, 683, 657, 640, 621, 581, 522, 501, 475, 465, 428.

conﬁguration with a glassy carbon disk (7.0 mm ) as the working

electrode for complex 3 and gold as the working electrode for

complex 1, a platinum wire as the counter electrode, and a silver wire

as the pseudoreference electrode. Sample solutions were prepared by

L′ Ni (3). 2 (20 mg, 22.72 μmol) was dissolved in 2 mL of THF,

and O was bubbled into the solution. After 1 h of stirring at ambient

temperature, the reaction mixture was ﬁltered, and all of the volatiles

were removed under reduced pressure. After washing of the residue

with n-hexane and toluene, 14.5 mg of a green solid was obtained.

This consisted mainly of 2 but also contained some (18-c-6)KBr that

had been introduced into the system as an impurity of 2 and yet again

was diﬃcult to separate. Elem anal. Found for

−

dissolving the analyte (ca. 5 mmol·L ) in THF (3.0 cm ), followed

by the addition of a supporting electrolyte, [ Bu N][PF ]. The

reported midpeak potentials are referenced internally to that of the

FeCp₂⁺redox couple, which was measured by adding ferrocene (ca.

.5 mg) to the sample solution. Crystallographic data collection was

−1

performed with a Bruker D8 Venture diﬀractometer at 100 K, using

Mo Kα radiation (λ = 0.71073 Å). The data collections were

performed with a Bruker D8 VENTURE area detector with Mo Kα

radiation (λ = 0.71073 Å). Multiscan absorption corrections,

C H N Ni· / C H O KBr (781.07 g·mol ): H, 6.43; C, 61.13;

34 38 6 2 12 24 6

N, 10.32. Calcd: H, 6.45; C, 61.51; N, 10.76. Because elemental

analysis indicated a ratio of 3 to (18-c-6)KBr of 1:0.5, the yield of 3

can be determined to 11.5 mg (19.50 μmol, 85%). H NMR (300.1

implemented in SADABS, were applied to the data. The structures

MHz, CD Cl ): δ 7.48 (d, 2H, J = 8.28 Hz), 7.16 (d, 2H, J = 8.28

2 2

were solved by an intrinsic phasing method and reﬁned by full-

Hz), 7.06 (s, 2H), 6.45 (s, NH), 2.43 (s, 3H), 2.37 (s, 6H), 2.34 (s,

matrix least-squares procedures based on F with all measured

3H). C{ H} NMR (75.47 MHz, CD Cl ): δ 18.9, 21.3, 26.0, 126.3,

2 2

reﬂections with anisotropic temperature factors for all non-hydrogen

128.7, 129.7, 133.8, 137.0, 140.0. ESI/MS (MeCN, positive-ion

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

(4) Yao, S.; Bill, E.; Milsmann, C.; Wieghardt, K.; Driess, M. A

Forum Article

mode): m/z 589.41 (L′ NiH ). Calcd: m/z 589.25. ATR-IR (crystal;

−

cm ): ν

2889, 1718, 1652, 1603, 1524, 1472, 1454, 1417, 1351,

300, 1282, 1259, 1226, 1165, 1102, 1017, 961, 835, 797, 704.

“Side-on ” Superoxonickel Complex [LNi(O )] with a Square-Planar

Tetracoordinate Nickel(II) Center and its Conversion into [LNi(μ -

OH) NiL]. Angew. Chem., Int. Ed. 2008, 47, 7110−7113.

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03761.

(5) Pfirrmann, S.; Limberg, C.; Herwig, C.; Stößer, R.; Ziemer, B. A

Dinuclear Nickel(I) Dinitrogen Complex and its Reduction in Single-

Electron Steps. Angew. Chem., Int. Ed. 2009, 48, 3357 −3361.

■

(6) Horn, B.; Limberg, C.; Herwig, C.; Braun, B. Nickel(I)-mediated

transformations of carbon dioxide in closed synthetic cycles: reductive

cleavage and coupling of CO generating Ni(I)CO, Ni(II)CO and

NMR, EPR and GC measurements, UV−vis studies,

crystal data, and DFT calculations (PDF)

Ni(II)C O Ni(II) entities. Chem. Commun. 2013, 49, 10923−10925.

(7) Holze, P.; Horn, B.; Limberg, C.; Matlachowski, C.; Mebs, S.

The Activation of Sulfur Hexafluoride at Highly Reduced Low-

Coordinate Nickel Dinitrogen Complexes. Angew. Chem., Int. Ed.

Accession Codes

CCDC 2049782 −2049784 contain the supplementary crys-

tallographic data for this paper. These data can be obtained

free of charge via www.ccdc.cam.ac.uk/data_request/cif , or by

emailing data_request@ccdc.cam.ac.uk , or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

8) Duan, P.-C.; Manz, D.-H.; Dechert, S.; Demeshko, S.; Meyer, F.

014, 53, 2750−2753.

Reductive O Binding at a Dihydride Complex Leading to Redox

Interconvertible μ -1,2-Peroxo and μ -1,2-Superoxo Dinickel(II)

Intermediates. J. Am. Chem. Soc. 2018, 140, 4929−4939.

(9) Kothe, T.; Kim, U. H.; Dechert, S.; Meyer, F. Reductive Binding

of Nitro Substrates at a Masked Dinickel(I) Complex and Proton-

Coupled Conversion to Reduced Nitroso Ligands. Inorg. Chem. 2020,

AUTHOR INFORMATION

Corresponding Author

■

10) Holze, P.; Braun-Cula, B.; Mebs, S.; Limberg, C. Stabilization

9, 14207−14217.

Christian Limberg − Institut fu

Universität zu Berlin, 12489 Berlin, Germany; orcid.org/

000-0002-0751-1386 ; Phone: +4930-20937382;

Email: Christian.limberg@chemie.hu-berlin.de

r Chemie, Humboldt-

of β -Diketiminato Nickel(I) with Alkaline Metal Halide Entities for

Small Molecule Activation. Z. Anorg. Allg. Chem. 2018, 644, 973.

(

11) Horn, B.; Pfirrmann, S.; Limberg, C.; Herwig, C.; Braun, B.;

Mebs, S.; Metzinger, R. N ₂ Activation in Ni -NN-Ni units: The

Influence of Alkali Metal Cations and CO Reactivity. Z. Anorg. Allg.

Chem. 2011, 637, 1169.

Authors

Deniz Ar − Institut fu

Berlin, 12489 Berlin, Germany

Alexander F. R. Kilpatrick − Institut fu

Universität zu Berlin, 12489 Berlin, Germany; orcid.org/

000-0002-2108-2709

Beatrice Cula − Institut fu

Berlin, 12489 Berlin, Germany; orcid.org/0000-0001-

262-4925

r Chemie, Humboldt-Universität zu

(12) Chang, M.-C.; Roewen, P.; Travieso-Puente, R.; Lutz, M.;

Otten, E. Formazanate ligands as structurally versatile, redox-active

analogues of β -diketiminates in zinc chemistry. Inorg. Chem. 2015, 54,

379−388.

(13) Milocco, F.; Demeshko, S.; Meyer, F.; Otten, E. Ferrate(II)

complexes with redox-active formazanate ligands. Dalton Trans. 2018,

r Chemie, Humboldt-

r Chemie, Humboldt-Universität zu

14) Gilroy, J. B.; Otten, E. Formazanate coordination compounds:

7, 8817−8823.

synthesis, reactivity, and applications. Chem. Soc. Rev. 2020, 49, 85−

13.

15) Broere, D. L.; Mercado, B. Q.; Lukens, J. T.; Vilbert, A. C.;

Christian Herwig − Institut fur Chemie, Humboldt-

Universität zu Berlin, 12489 Berlin, Germany

(

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.0c03761

Banerjee, G.; Lant, H. M.; Lee, S. H.; Bill, E.; Sproules, S.; Lancaster,

K. M.; Holland, P. L. Reversible Ligand-Centered Reduction in Low-

Coordinate Iron Formazanate Complexes. Chem. - Eur. J. 2018, 24,

16) Tezcan, H.; Uzluk, E.; Aksu, M. L. Synthesis, spectroscopic and

417−9425.

Notes

The authors declare no competing ﬁnancial interest.

electrochemical studies on bis-[1,3-substituted (Cl, Br) phenyl-5-

phenyl formazanato]nickel(II) complexes. Spectrochim. Acta, Part A

2008, 70, 973−982.

ACKNOWLEDGMENTS

We are grateful to the Humboldt-Universität zu Berlin for

ﬁnancial support and the Deutsche Forschungsgemeinschaft

■

(17) Gilroy, J. B.; Patrick, B. O.; McDonald, R.; Hicks, R. G.

Transition metal complexes of 3-cyano-and 3-nitroformazans. Inorg.

Chem. 2008, 47, 1287−1294.

(

German Research Foundation) for funding under Germany’s

(

18) Frolova, N. A.; Vatsadze, S. Z.; Zavodnik, V. E.; Rakhimov, R.

Excellence Strategy EXC 2008/1-390540038s. We also thank

the groups of Prof. Dr. Klaus Lips and Dr. Boris Naydenov

from Helmholtz Zentrum Berlin for providing us with access to

EPR instruments.

D.; Zyk, N. V. Pyridine-containing nickel(II) bis-formazanates:

Synthesis, structure, and electrochemical study. Russ. Chem. Bull.

19) Evans, D. The determination of the paramagnetic susceptibility

006, 55, 1810−1818.

of substances in solution by nuclear magnetic resonance. J. Chem. Soc.

REFERENCES

■

1959, 2003−2005.

1) Puiu, S. C.; Warren, T. H. Three-Coordinate β -Diketiminato

(20) Schubert, E. M. Utilizing the Evans method with a

superconducting NMR spectrometer in the undergraduate laboratory.

J. Chem. Educ. 1992, 69, 62.

Nickel Nitrosyl Complexes from Nickel(I)−Lutidine and Nickel(II)−

Alkyl Precursors. Organometallics 2003, 22, 3974−3976.

2) Eckert, N. A.; Dinescu, A.; Cundari, T. R.; Holland, P. L. A T-

(21) Cotton, F. A. Advanced Inorganic Chemistry, A Comprehensive

Text, 4th ed.; Wiley: New York, 1980.

shaped three-coordinate nickel (I) carbonyl complex and the

geometric preferences of three-coordinate d complexes. Inorg.

Chem. 2005, 44, 7702−7704.

3) Kogut, E.; Wiencko, H. L.; Zhang, L.; Cordeau, D. E.; Warren,

T. H. A Terminal Ni(III)− Imide with Diverse Reactivity Pathways. J.

Am. Chem. Soc. 2005, 127, 11248−11249.

(22) Mori, Y.; Shirase, H.; Fukuda, Y. Substituent and Solvent

Effects on Square-Planar −Tetrahedral Equilibria of Bis[4-(arylimino)-

pentan-2-onato]nickel(II) and Bis[1-aryl-3-(phenylimino)butan-1-

onato]nickel(II) Complexes. Bull. Chem. Soc. Jpn. 2008, 81, 1108−

1115.

(

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

(43) Wei, Y.; Li, H.; Yue, F.; Xu, Q.; Wang, J.; Zhang, Y. Oxidation

Forum Article

(

23) Travieso-Puente, R.; Broekman, J. O. P.; Chang, M.-C.;

mechanism of molecular oxygen over cyclohexene catalyzed by a

cobalt L -glutamic acid complex. RSC Adv. 2016 , 6 , 107104 −107108.

(44) Kohantorabi, M.; Gholami, M. R. Cyclohexene oxidation

Demeshko, S.; Meyer, F.; Otten, E. Spin-Crossover in a Pseudo-

tetrahedral Bis(formazanate) Iron Complex. J. Am. Chem. Soc. 2016,

24) Kabir, E.; Wu, C.-H.; Wu, J. I. C.; Teets, T. S. Heteroleptic

38, 5503−5506.

catalyzed by flower-like core-shell Fe O @Au/metal organic frame-

3 4

Complexes of Cyclometalated Platinum with Triarylformazanate

works nanocomposite. Mater. Chem. Phys. 2018, 213, 472−481.

(45) Wind, M.-L.; Hoof, S.; Braun-Cula, B.; Herwig, C.; Limberg, C.

Routes to Heterotrinuclear Metal Siloxide Complexes for Cooperative

Ligands. Inorg. Chem. 2016, 55, 956−963.

(

25) Zimmermann, P.; Kilpatrick, A. F. R.; Ar, D.; Demeshko, S.;

Cula, B.; Limberg, C. Electron transfer within β -diketiminato nickel

Activation of O

. Inorg. Chem. 2020, 59, 6866−6875.

bromide and cobaltocene redox couples activating CO . Chem.

(46) Wind, M.-L.; Braun-Cula, B.; Schax, F.; Herwig, C.; Limberg,

Commun. 2021, 57, 875−878.

C. A Polysiloxide Complex with two Chromium(III) η

-Superoxo

26) Mu, G.; Cong, L.; Wen, Z.; Wu, J. I. C.; Kadish, K. M.; Teets,

Moieties. Isr. J. Chem. 2020, 60, 1057−1060.

T. S. Homoleptic Platinum Azo-iminate Complexes via Hydro-

(47) Pirovano, P.; Farquhar, E. R.; Swart, M.; Fitzpatrick, A. J.;

Morgan, G. G.; McDonald, A. R. Characterization and Reactivity of a

Terminal Nickel(III)−Oxygen Adduct. Chem. - Eur. J. 2015, 21,

3785−3790.

genative Cleavage of Formazans. Inorg. Chem. 2018, 57, 9468−9477.

27) Pramanik, S.; Roy, S.; Ghorui, T.; Ganguly, S.; Pramanik, K.

Iridium(III) Mediated Reductive Transformation of Closed-Shell

Azo-Oxime to Open-Shell Azo-Imine Radical Anion: Molecular and

Electronic Structure, Electron Transfer, and Optoelectronic Proper-

ties. Inorg. Chem. 2016, 55, 1461−1468.

(48) Sheldrick, G. M. SADABS; University of Go

Germany, 1996.

ttingen: Go

ttingen,

49) Sheldrick, G. M. SHELXT −Integrated space-group and crystal-

structure determination. Acta Crystallogr., Sect. A: Found. Adv. 2015,

1, 3−8.

50) Sheldrick, G. M. Crystal structure refinement with SHELXL.

Acta Crystallogr., Sect. A: Found. Adv. 2015, 71, 3−8.

51) Viculis, L. M.; Mack, J. J.; Mayer, O. M.; Hahn, H. T.; Kaner, R.

28) Pal, C. K.; Chattopadhyay, S.; Sinha, C.; Chakravorty, A. A

(

Bis(azo −imine)palladium(II) System with 10 Ligand π Electrons.

Synthesis, Structure, Serial Redox, and Relationship to Bis-

azooximates) and Other Species. Inorg. Chem. 1996, 35, 2442−2447.

29) Banerjee, S.; Sheet, D.; Sarkar, S.; Halder, P.; Paine, T. K.

B. Intercalation and exfoliation routes to graphite nanoplatelets. J.

Nickel complexes of ligands derived from (o -hydroxyphenyl)

dichalcogenide: delocalised redox states of nickel and o -chalcogeno-

phenolate ligands. Dalton Trans. 2019 , 48 , 17355 −17363.

Mater. Chem. 2005, 15, 974−978.

(52) Huang, Z.; Song, K.; Liu, F.; Long, J.; Hu, H.; Gao, H.; Wu, Q.

Synthesis and characterization of a series of 2-aminopyridine

nickel(II) complexes and their catalytic properties toward ethylene

polymerization. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 1618−

30) Lin, C.-Y.; Power, P. P. Complexes of Ni(i): a “rare ” oxidation

state of growing importance. Chem. Soc. Rev. 2017, 46, 5347−5399.

(

31) Blanchard, S.; Neese, F.; Bothe, E.; Bill, E.; Weyhermuller, T.;

Wieghardt, K. Square Planar vs Tetrahedral Coordination in

628.

Diamagnetic Complexes of Nickel(II) Containing Two Bidentate π -

Radical Monoanions. Inorg. Chem. 2005, 44, 3636−3656.

53) Wind, M.-L.; Hoof, S.; Herwig, C.; Braun-Cula, B.; Limberg, C.

The Influence of Alkali Metal Ions on the Stability and Reactivity of

Chromium(III) Superoxide Moieties Spanned by Siloxide Ligands.

Chem. - Eur. J. 2019, 25, 5743−5750.

(

32) Pfirrmann, S.; Limberg, C.; Herwig, C.; Knispel, C.; Braun, B.;

Bill, E.; Stösser, R. A Reduced β -Diketiminato-Ligated Ni H Unit

(54) Mader, E. A.; Larsen, A. S.; Mayer, J. M. Hydrogen Atom

Catalyzing H/D Exchange. J. Am. Chem. Soc. 2010, 132, 13684−

3691.

33) Rettenmeier, C.; Wadepohl, H.; Gade, L. H. Stereoselective

Transfer from Iron(II)−Tris[2,2 ′-bi(tetrahydropyrimidine)] to

TEMPO: A Negative Enthalpy of Activation Predicted by the Marcus

Equation. J. Am. Chem. Soc. 2004, 126, 8066−8067.

Hydrodehalogenation via a Radical-Based Mechanism Involving T-

Shaped Chiral Nickel(I) Pincer Complexes. Chem. - Eur. J. 2014, 20,

(

657−9665.

34) Naveed, K.-u.-R.; Wang, L.; Yu, H.; Ullah, R. S.; Haroon, M.;

Fahad, S.; Li, J.; Elshaarani, T.; Khan, R. U.; Nazir, A. Recent progress

in the electron paramagnetic resonance study of polymers. Polym.

Chem. 2018, 9, 3306−3335.

(

35) Stoll, S. Encyclopedia of Biophysics; Springer: Berlin, 2013.

36) Williams, V. A.; Hulley, E. B.; Wolczanski, P. T.; Lancaster, K.

M.; Lobkovsky, E. B. Exploring the limits of redox non-innocence:

pseudo square planar [{κ -Me C(CH N CHpy) }Ni] (n = 2+, 1+,

, − 1, − 2) favor Ni(II). Chem. Sci. 2013 , 4 , 3636 −3648.

37) Zimmermann, P.; Limberg, C. Activation of Small Molecules at

Nickel(I) Moieties. J. Am. Chem. Soc. 2017, 139, 4233−4242.

38) Stanciu, C.; Jones, M. E.; Fanwick, P. E.; Abu-Omar, M. M.

Multi-electron Activation of Dioxygen on Zirconium(IV) to Give an

Unprecedented Bisperoxo Complex. J. Am. Chem. Soc. 2007, 129,

39) Fukuzumi, S.; Ohkubo, K. Quantitative Evaluation of Lewis

2400−12401.

Acidity of Metal Ions Derived from the g Values of ESR Spectra of

Superoxide: Metal Ion Complexes in Relation to the Promoting

Effects in Electron Transfer Reactions. Chem. - Eur. J. 2000, 6, 4532−

40) Petr, A.; Kataev, V.; Buchner, B. First Direct In Situ EPR

535.

Spectroelectrochemical Evidence of the Superoxide Anion Radical. J.

Phys. Chem. B 2011, 115, 12036−12039.

(

41) Teles, J. H.; Hermans, I.; Franz, G.; Sheldon, R. A. Oxidation.

Ullmann’s Encyclopedia of Industrial Chemistry ; Wiley-VCH, 2015.

42) Sahu, S.; Goldberg, D. P. Activation of Dioxygen by Iron and

Manganese Complexes: A Heme and Nonheme Perspective. J. Am.

Chem. Soc. 2016, 138, 11410−11428.