Steric Effect on the Nucleophilic Reactivity of Nickel(III) Peroxo Complexes

Article abstract of DOI:10.1021/acs.inorgchem.5b00294

A set of nickel(III) peroxo complexes bearing tetraazamacrocyclic ligands, [Ni^III(TBDAP)(O₂)]⁺ (TBDAP = N,N′-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane) and [Ni^III(CHDAP)(O₂)]⁺ (CHDAP = N,N′-dicyclohexyl-2,11-diaza[3.3](2,6)pyridinophane), were prepared by reacting [Ni^II(TBDAP)(NO₃)(H₂O)]⁺ and [Ni^II(CHDAP)(NO₃)]⁺, respectively, with H₂O₂ in the presence of triethylamine. The mononuclear nickel(III) peroxo complexes were fully characterized by various physicochemical methods, such as UV-vis, electrospray ionization mass spectrometry, resonance Raman, electron paramagnetic resonance, and X-ray analysis. The spectroscopic and structural characterization clearly shows that the NiO₂ cores are almost identical where the peroxo ligand is bound in a side-on fashion. However, the different steric properties of the supporting ligands were confirmed by X-ray crystallography, where the CHDAP ligand gives enough space around the Ni core compared to the TBDAP ligand. The nickel(III) peroxo complexes showed reactivity in the oxidation of aldehydes. In the aldehyde deformylation reaction, the nucleophilic reactivity of the nickel(III) peroxo complexes was highly dependent on the steric properties of the macrocyclic ligands, with a reactivity order of [Ni^III(TBDAP)(O₂)]⁺ < [Ni^III(CHDAP)(O₂)]⁺. This result provides fundamental insight into the mechanism of the structure (steric)-reactivity relationship of metal peroxo intermediates. (Figure Presented).

Full text of DOI:10.1021/acs.inorgchem.5b00294

Article
pubs.acs.org/IC
Steric Effect on the Nucleophilic Reactivity of Nickel(III) Peroxo
Complexes
†
,‡
†,‡
†
†
†
§
Jalee Kim, Bongki Shin, Hyunjeong Kim, Junhyung Lee, Joongoo Kang, Sachiko Yanagisawa,
§
⊥
⊥
,†
Takashi Ogura, Hideki Masuda, Tomohiro Ozawa, and Jaeheung Cho*
†
Department of Emerging Materials Science, DGIST, Daegu 711-873, Korea
§
Picobiology Institute, Graduate School of Life Science, University of Hyogo, Hyogo 678-1297, Japan
⊥
Department of Frontier Materials, Graduate School of Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan
S Supporting Information
*
ABSTRACT: A set of nickel(III) peroxo complexes bearing
III
+
tetraazamacrocyclic ligands, [Ni (TBDAP)(O )] (TBDAP =
2
N,N′-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane) and
III
+
[
Ni (CHDAP)(O )] (CHDAP = N,N′-dicyclohexyl-2,11-
2
diaza[3.3](2,6)pyridinophane), were prepared by reacting
II
+
II
+
[
Ni (TBDAP)(NO )(H O)] and [Ni (CHDAP)(NO )] ,
3
2
3
respectively, with H O in the presence of triethylamine. The
2
2
mononuclear nickel(III) peroxo complexes were fully charac-
terized by various physicochemical methods, such as UV−vis,
electrospray ionization mass spectrometry, resonance Raman,
electron paramagnetic resonance, and X-ray analysis. The
spectroscopic and structural characterization clearly shows that
the NiO cores are almost identical where the peroxo ligand is bound in a side-on fashion. However, the different steric
2
properties of the supporting ligands were confirmed by X-ray crystallography, where the CHDAP ligand gives enough space
around the Ni core compared to the TBDAP ligand. The nickel(III) peroxo complexes showed reactivity in the oxidation of
aldehydes. In the aldehyde deformylation reaction, the nucleophilic reactivity of the nickel(III) peroxo complexes was highly
III
+
dependent on the steric properties of the macrocyclic ligands, with a reactivity order of [Ni (TBDAP)(O )]
<
2
III
+
[
Ni (CHDAP)(O )] . This result provides fundamental insight into the mechanism of the structure (steric)−reactivity
2
relationship of metal peroxo intermediates.
INTRODUCTION
Recent advances in biomimetic chemistry have a chance to
generate a series of mononuclear side-on metal(III) peroxo
complexes bearing N-tetramethylated macrocyclic ligands,
where the spectroscopic properties and reactivities of the
metal(III) peroxo species are profoundly affected by the ring-
■
Mononuclear metal−O adducts, such as metal peroxo species,
are the crucial reactive intermediates in enzymatic reactions as
well as oxidative catalytic processes. For example, manganese-
III) peroxo intermediates have been invoked as reactive
species in manganese-containing enzymes including oxalate
oxidase, catalase, and superoxide dismutase.
mononuclear iron(III) peroxo species are often observed in the
activation of dioxygen by iron enzymes (e.g., cytochromes P450
2
(
10
size effect of the supporting ligands. A notable example is the
formation of a side-on nickel(III) peroxo complex with the 12-
membered macrocycle and an end-on nickel(II) superoxo
1
−3
In addition,
11
complex bearing the 14-membered macrocycle. The former
has a nucleophilic character (e.g., aldehyde deformylation),
while the latter shows electrophilic reactivity (e.g., phosphine
oxidation) toward organic substrates. The ring-size effect in
these studies does result in significant changes in the structures
4
−6
and Rieske dioxygenases).
In synthetic model chemistry,
iron(III) peroxo complexes have been prepared and charac-
terized with various physicochemical methods, and the
reactivities of the models were investigated in biomimetic
11,12
formed and the reactivity patterns of the nickel−O species.
2
7
reactions. Some of us and others have also reported the
It is necessary to understand the role of controlling factors
related to the nature of the supporting ligand, which influences
the resulting reactivity patterns. To the best of our knowledge,
the steric effect has never been observed in the reactivity of
mononuclear metal(III) peroxo complexes toward external
synthesis and characterization of an iron(III) peroxo complex,
III
+
[
Fe (TMC)(O )] , which is capable of conducting aldehyde
2
8
deformylation, and highlighted its unique conversion
procedures; upon protonation, the iron(III) peroxo complex
is completely converted to an iron(III) hydroperoxo complex,
which readily undergoes O−O bond cleavage, affording the
Received: February 8, 2015
8a,9
formation of an iron(IV) oxo complex.
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b00294
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
substrates, although the electronic effect on the reactivity of the
13
nickel alkylperoxo complex has been reported very recently.
Here, we report a new set of nickel(III) peroxo complexes with
TBDAP (N,N′-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane)
and CHDAP (N,N′-dicyclohexyl-2,11-diaza[3.3](2,6)-
pyridinophane) ligands (Scheme 1), which give steric differ-
Scheme 1. Macrocyclic Ligands Used in This Study
+
Figure 1. ORTEP plots of (a) [Ni(TBDAP)(NO )(H O)] and (b)
3
2
+
[
Ni(CHDAP)(NO )] with thermal ellipsoids drawn at the 30%
3
probability level. Hydrogen atoms are omitted for clarity.
ences in NiO2 cores. The nickel(III) peroxo complexes
III
+
III
+
[
Ni (TBDAP)(O )] (1) and [Ni (CHDAP)(O )] (2)
2
2
were characterized by a wide range of physicochemical
methods. The intermediates 1 and 2 have been employed in
exploring the steric effect on the reactivity of the nickel(III)
peroxo species toward organic substrates in a nucleophilic
reaction.
Synthetic procedures for nickel−O2 complexes bearing
TBDAP and CHDAP ligands are illustrated in Scheme 2. 1
was prepared by adding 5 equiv of H O to a reaction solution
2
2
II
+
containing [Ni (TBDAP)(NO )(H O)] in the presence of 2
3
2
equiv of triethylamine (TEA) in CH CN at 25 °C; the color of
3
the solution changed from blue to brown. Complex 1 persisted
for several hours at 0 °C so that we were able to isolate and use
it in spectroscopic and structural characterization. The UV−vis
RESULTS AND DISCUSSION
CHDAP was prepared by a modification of the reported
■
procedure [see the Supporting Information (SI), Experimental
spectrum of 1 in CH CN at 25 °C shows two absorption bands
3
1
4
−1
−1
−1
Section]. The CHDAP ligand was characterized by elemental
analysis, electrospray ionization mass spectrometry (ESI-MS),
at ∼560 nm (ε = 15 M cm ) and 1040 nm (ε = 45 M
−1
cm ) (Figure 2a). The ESI-MS spectrum of 1 exhibits a
prominent signal at m/z 442.2 (Figure 2b), whose mass and
1
13
and H and C NMR methods (see the SI, Experimental
Section and Figures S1−S3).
isotope distribution patterns correspond to [Ni(TBDAP)-
II
+
The starting nickel complexes, [Ni (TBDAP)(NO )-
(O )] (calcd m/z 442.2). When the reaction was carried out
3
2
+
II
+
18
2
(
H O)] and [Ni (CHDAP)(NO )] , were prepared by
with isotopically labeled H O , a mass peak corresponding to
[Ni(TBDAP)( O )] appeared at m/z 446.2 (calcd m/z
2
3
2
18
+
reacting Ni(NO ) ·6H O with the corresponding TBDAP
3
2
2
2
and CHDAP ligands, respectively. The UV−vis spectrum of
446.2) (Figure 2b, inset). The shift in four mass units upon
II
+
16
18
[
Ni (TBDAP)(NO )(H O)] in CH CN exhibits three
substitution of O with O demonstrates that 1 contains two
oxygen atoms.
3
2
3
−
1
−1
absorption bands at 645 nm (ε = 10 M cm ), 823 nm (ε
−1
−1
−1
−1
=
10 M cm ), and 1066 nm (ε = 25 M cm ) (Figure 2a).
The resonance Raman spectrum of 1 collected using 442 nm
II
+
The ESI-MS spectrum of [Ni (TBDAP)(NO )(H O)] shows
three signals at mass-to-charge ratios of m/z 225.6, 246.1, and
excitation in CD CN at −20 °C shows a resonance-enhanced
3
2
3
−1
vibration at 989 cm (Figure 2c), which is tentatively assigned
to v(O−O) based on its frequency and a good correlation
between the O−O bond length and O−O stretching frequency
2
+
4
72.2 for [Ni(TBDAP)(CH CN)] (calcd m/z 225.6),
3
2
+
[
Ni(TBDAP)(CH CN) ] (calcd m/z 246.1), and [Ni-
TBDAP)(NO )] (calcd m/z 472.2), respectively (see the
3 2
+
15
(
of 1 (vide infra). This value is comparable to those reported
3
SI, Figure S4). The X-ray crystal structure of [Ni(TBDAP)-
NO )(H O)](NO ) shows a six-coordinated nickel(II) ion
for side-on nickel(III) peroxo complexes, such as [Ni(12-
+
−1
+
(
TMC)(O )] (1002 cm ) and [Ni(13-TMC)(O )] (1008
3
2
3
2
2
−
1
10c,11a
with four nitrogen atoms from the TBDAP ligand and two
oxygen atoms from a monodentate nitrate and water (Figure 1a
and Table 1). Selected bond lengths and angles are listed in the
SI, Table S1.
cm ),
indicating the peroxo character of the O unit in 1.
2
The X-band electron paramagnetic resonance (EPR)
spectrum of 1 exhibits an axial signal with g values of 2.19
1
2
z
and 2.02 (Figure 2d), which is a typical (d ) electron
II
+
16
The UV−vis spectrum of [Ni (CHDAP)(NO )] in
configuration observed for nickel(III) complexes. Spin
3
CH CN shows three absorption bands at 588 nm (ε = 15
quantification finds that the EPR signal corresponds to 89%
of the total nickel content in the sample (see the Experimental
3
−
1
−1
−1
−1
M
M
[
cm ), 835 nm (ε = 25 M cm ), and 1010 nm (ε = 45
−
1
−1
cm ) (Figure 5a). The ESI-MS spectrum of
Ni (CHDAP)(NO )] exhibits two intense ion peaks at m/
Section). The spin state of 1 in a CH CN solution was further
3
II
+
1
determined using the H NMR spectroscopy method of
Evans, and the room temperature magnetic moment of 2.3
μ clearly indicates an S = / ground state.
3
17
z 251.7 and 524.3, whose mass and isotope distribution
2
+
1
patterns correspond to [Ni(CHDAP)(CH CN)] (calcd m/z
3
B
2
+
2
51.6) and [Ni(CHDAP)(NO )] (calcd m/z 524.2),
The X-ray crystal structure of [Ni(TBDAP)(O )](ClO )-
3
2
4
respectively (see the SI, Figure S5). The molecular structure
of the cationic part for [Ni(CHDAP)(NO )](NO )(CH OH)
is shown in Figure 1b. The complex has a six-coordinated
nickel(II) ion with the CHDAP ligand and a symmetrically
coordinated bidentate nitrate anion. Selected bond lengths and
angles are listed in the SI, Table S1.
(0.5CH Cl ) (1-ClO ·0.5CH Cl ) revealed a mononuclear
2 2 4 2 2
side-on nickel−O complex in a distorted octahedral geometry
3
3
3
2
arising from the triangular NiO moiety with a small bite angle
2
of 44.19(7)° (Figure 3). The O−O bond length (1.401(2) Å)
+
of 1 is slightly longer than those of [Ni(12-TMC)(O )] (1.386
2
+
10c,11a
Å) and [Ni(13-TMC)(O )] (1.383 Å)
and lies well
2
B
DOI: 10.1021/acs.inorgchem.5b00294
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Inorganic Chemistry
Article
Table 1. Crystal Data and Structure Refinements for [Ni(TBDAP)(NO )(H O)](NO ), [Ni(CHDAP)(NO )](NO )(CH OH),
3
2
3
3
3
3
and 1-ClO ·0.5CH Cl
4
2
2
[
Ni(TBDAP)(NO )(H O)](NO )
[Ni(CHDAP)(NO )](NO )(CH OH)
1-ClO ·0.5CH Cl
3
2
3
3
3
3
4
2
2
empirical formula
fw
C H N NiO
C H N NiO
C22.5H Cl N NiO
6
2
2
34
6
7
27 40
6
7
33
2
4
553.26
100(2)
619.36
585.14
100(2)
temperature (K)
cryst syst/space group
unit cell dimens
a (Å)
100(2)
orthorhombic, Pbca
triclinic, P1
̅
monoclinic, C2/c
15.2958(5)
16.4819(6)
19.9118(8)
90
10.6798(2)
11.8168(2)
12.9559(2)
64.1040(10)
82.1330(10)
78.9810(10)
1441.11(4)
2
30.9602(6)
12.8498(2)
13.7307(2)
90.00
b (Å)
c (Å)
α (deg)
β (deg)
90
105.7590(10)
90.00
γ (deg)
volume (Å3)
90
5019.8(3)
8
1047.70(5)
8
Z
calcd density (g cm−3)
abs coeff (mm−1)
reflns collected
indep reflns [R(int)]
refinement method
data/restraints/param
GOF on F2
1.464
1.427
1.479
0.827
0.729
0.986
104898
4018 [0.0804]
full-matrix least squares on F2
24970
46826
7001 [0.0187]
full-matrix least squares on F2
6538 [0.0360]
2
full-matrix least squares on F
6538/0/327
4018/0/339
7001/0/372
1.001
1.037
1.040
final R indices [I > 2σ(I)]
R indices (all data)
R1 = 0.0324, wR2 = 0.1037
R1 = 0.0477, wR2 = 0.1239
R1 = 0.0261, wR2 = 0.0702
R1 = 0.0274, wR2 = 0.0713
R1 = 0.0341, wR2 = 0.0820
R1 = 0.0464, wR2 = 0.0874
Scheme 2. Synthetic Procedures for Mononuclear
Nickel(III) Peroxo Complexes
spectroscopic data, 1 is assigned as a side-on nickel(III) peroxo
complex.
2
was obtained by a method similar to that of 1 with the
corresponding CHDAP ligand (Scheme 2). The UV−vis
spectrum of 2 in CH CN at 25 °C shows two absorption
3
−1
−1
−1
bands at 584 nm (ε = 35 M cm ) and 950 nm (ε = 70 M
cm ) (Figure 5a), which are comparable to those of 1. The
ESI-MS spectrum of 2 shows a prominent signal at m/z 494.2
−1
(
Figure 5b), whose mass and isotope distribution patterns
+
correspond to [Ni(CHDAP)(O )] (calcd m/z 494.2). The
isotope labeling experiment with H2 O exhibited the expected
signal at m/z 498.2 for [Ni(CHDAP)( O )] (calcd m/z
2
18
2
18
+
2
4
98.2) (Figure 5b, inset). Upon 442 nm excitation at −20 °C,
16
the resonance Raman spectrum of O-labeled 2 in CD CN
3
−1
exhibits an isotopically sensitive band at 988 cm , which shifts
to 919 cm− upon O substitution (Figure 5c). The O−O
1
18
stretching vibration of 2 is very similar to that of 1. The EPR
spectrum of a frozen CH CN solution of 2 at 20 K shows an
3
axial signal with g values of 2.17 and 2.03 (Figure 5d). Spin
quantification finds that the EPR signal corresponds to 95% of
the total nickel content in the sample. The room temperature
1
7
1
magnetic moment of 2.2 μB, determined using the H NMR
1
Evans method, is consistent with an S = / ground state of 2.
2
The similarity of these spectroscopic features to those of 1 leads
us to assign 2 as a side-on nickel(III) peroxo complex.
1
7
within the metal peroxo category (∼1.4−1.5 Å). The average
Density functional theory (DFT) calculations were per-
formed to evaluate the geometric information on 2. In order to
gauge the reliability of the calculations, structural comparisons
were made between the crystal structure of 1 and that
Ni−N bond distance (2.2677 Å) is considerably longer than
axial
the average Ni−N
bond distance (1.9194 Å). This
equatorial
elongation of the axial bonds is attributable to the Jahn−Teller
effect arising from a low-spin d electron configuration (vide
supra). The two N-tert-butyl groups of the TBDAP ligand are
oriented syn to the peroxo ligand, affording steric hindrance to
7
19
calculated by DFT, where the root-mean-square deviation
(RMSD) was calculated to be 0.03 Å. Because some differences
are expected when the gas-phase calculated structures are
compared with crystal forms, the structural quality of the
calculations gave credibility to the subsequent calculations. The
protect the NiO moiety. In addition, a good correlation was
2
observed when the O−O bond length and O−O stretching
frequency of 1 was fitted in the plot of O−O stretching
frequency versus O−O bond length for side-on metal−O
NiO geometry of 2 is, as expected, of the side-on type with the
2
2
18
complexes (Figure 4). On the basis of the structural and
metal ligand atoms in an octahedral fashion, which is similar to
C
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Inorganic Chemistry
Article
Figure 3. ORTEP plot of 1 with thermal ellipsoids drawn at the 30%
probability level. Hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg): Ni−O1 1.8589(14), Ni−O2
1
1
4
.8670(14), Ni−N1 1.9195(15), Ni−N2 2.2453(16), Ni−N3
.9193(15), Ni−N4 2.2901(15), O1−O2 1.401(2); O1−Ni−O2
4.19(7), Ni−O1−O2 68.21(8), Ni−O2−O1 67.60(8).
−1
Figure 4. Plot of O−O stretching frequency (cm ) versus O−O bond
distance (Å) for side-on metal−O complexes. The gray points
2
10a,c
represent experimental and theoretical data previously reported.
II
+
The solid line represents a least-squares linear fit of the experimental
and theoretical data. 1 (red ●) is included in the diagram.
Figure 2. (a) UV−vis spectra of [Ni (TBDAP)(NO )(H O)] (black
3
2
line) and 1 (red line) in CH CN at 25 °C. (b) ESI-MS spectrum of 1
3
in CH CN at −20 °C. The minor peaks at m/z 225.7 and 246.2
3
2
+
labeled with asterisks are assignable to [Ni(TBDAP)(CH CN)] and
Ni(TBDAP)(CH CN) ] , respectively. The inset shows the
observed isotope distribution patterns for [Ni (TBDAP)( O )]
lower) and [Ni (TBDAP)( O )] (upper). (c) Resonance Raman
3
addition of 2-PPA to 1 in CH CN/CH OH (1:1) at 25 °C, the
2
+
3
3
[
3 2
characteristic UV−vis absorption bands of 1 disappeared with
pseudo-first-order decay (Figure 7a), and product analysis of
the reaction solution revealed that acetophenone (71 ± 5%)
was produced in the oxidation of 2-PPA. In addition, 1 is
converted to its nickel(II) precursor complex after the reaction
was completed, which was confirmed by ESI-MS (see the SI,
Figure S6). The pseudo-first-order rate constants, monitored at
III
16
+
2
III
18
+
(
2
spectra of 1 (32 mM) obtained upon excitation at 442 nm in CD CN
3
at −20 °C. An asterisk indicates the peak of H O . (d) X-band EPR
2
2
spectrum (red) and simulation (black) of 1 in frozen CH CN at 20 K.
3
Instrumental parameters: microwave power = 1 mW, frequency =
9
.646 GHz, sweep width = 0.15 T, modulation frequency = 100 kHz,
and modulation amplitude = 1 mT.
5
60 nm, increased proportionally with an increase of the
concentration of 2-PPA, affording a second-order rate constant
−3
−1 −1
that of 1 (Figure 6). DFT calculations show that the steric bulk
yields slightly longer metal−ligand bonds [avg. d(Ni−Naxial) =
(k ) of 7.4 × 10
M
s
(Figure 8). The activation
2
parameters for aldehyde deformylation of 1 between 278 and
⧧
−1
⧧
2
.30 Å for 1 and 2.23 Å for 2; see the SI, Table S2]. However,
308 K were determined to be ΔH = 67 kJ mol and ΔS =
1 −1
the elongated Ni−N
bonds do not significantly affect the
NiO core structures [d(O−O) = 1.36 Å for 1 and 1.36 Å for 2;
−62 J mol−
K
(Figure 7b).
axial
The reaction of 1 with para-substituted benzaldehydes, which
have electron-donating and -withdrawing substituents at the
para position of the phenyl group (p-X−Ph−CHO; X = Me, F,
H, Cl, Br), provides mechanistic insight into the nature of the
peroxo group in 1 (Table S3 in the SI). The Hammett plot of
the pseudo-first-order rate constants versus σp+ afforded a ρ
value of 4.4(3) (Figure 7c), which is consistent with the
nucleophilic character of 1 in the oxidation of aldehydes.
Product analysis of the resulting solution revealed the
2
avg. d(Ni−O) = 1.86 Å for 1 and 1.87 Å for 2], consistent with
nearly the same Raman frequencies of O [v(O−O) = 989
2
−1
−1
cm for 1 and 988 cm for 2].
It has been noted that mononuclear metal peroxo species
react with aldehydes to give the corresponding deformylated
5
a,20
products.
reactivity of metal peroxo species, we carried out the reaction of
-phenylpropionaldehyde (2-PPA) with 1 and 2. Upon the
In order to investigate the steric effect on the
2
D
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Inorganic Chemistry
Article
Figure 6. DFT-calculated structures for (a) 1 and (b) 2 (gray, C; blue,
N; red, O; green, Ni). Hydrogen atoms are omitted for clarity.
II
+
Figure 5. (a) UV−vis spectra of [Ni (CHDAP)(NO )] (black line)
3
and 2 (red line) in CH CN at 25 °C. (b) ESI-MS spectrum of 2 in
3
CH CN at −20 °C. The minor peak at m/z 251.6 labeled with an
3
2+
asterisk is assignable to [Ni(CHDAP)(CH CN)] . The inset shows
the observed isotope distribution patterns for [Ni(CHDAP)( O )]
lower) and [Ni(CHDAP)( O )] (upper). (c) Resonance Raman
3
16
+
2
18
+
(
2
spectra of 2 (32 mM) obtained upon excitation at 442 nm in CD CN
3
16
18
at −20 °C: 2 prepared with H2 O (red line) and H2 O (blue line).
An asterisk indicates the peak of H O . (d) X-band EPR spectrum
2
2
2
2
(
red) and simulation (black) of 2 in frozen CH CN at 20 K.
3
Instrumental parameters: microwave power = 1 mW, frequency =
9
.646 GHz, sweep width = 0.15 T, modulation frequency = 100 kHz,
and modulation amplitude = 1 mT.
Figure 7. Reactions of 1 with aldehydes in CH CN/CH OH (1:1).
3
3
(
a) UV−vis spectral changes of 1 (2 mM) upon the addition of 100
formation of benzoic acid (94 ± 3%) as a product, as observed
equiv of 2-PPA at 25 °C. The inset shows the time course of the
absorbance at 560 nm. (b) Plot of first-order rate constants against 1/
T to determine activation parameters. (c) Hammett plot of ln k
against σp of para-substituted benzaldehydes. The krel values were
calculated by dividing kobs of p-X−Ph−CHO (X = Me, F, H, Cl, Br) by
kobs of benzaldehyde at 25 °C.
in the oxidation of benzaldehydes by metal peroxo complex-
10a,20c
es.
It is worth noting that 1 does not react with
rel
triphenylphosphine, thioanisole, and cyclohexadiene, which
+
were used to test the electrophilic reactivity of model
1
0a
complexes.
This observation, together with the result of
the Hammett study, indicates that 1 has nucleophilic character.
Upon reaction of 2 with 2-PPA, the characteristic UV−vis
absorption bands of 2 disappeared with a pseudo-first-order
decay (see the SI, Figure S7a), and product analysis of the
reaction solutions revealed the formation of acetophenone (90
completed, 2 is reduced to its nickel(II) precursor complex,
which was confirmed by ESI-MS (see the SI, Figure S8). The
pseudo-first-order rate constants increased proportionally with
the substrate concentration (Figure 8), in which the second-
±
5%) in the oxidation of 2-PPA. After the reaction was
E
DOI: 10.1021/acs.inorgchem.5b00294
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
−
3
Upon a comparison of the kinetic data of 1 (k = 7.4 × 10
2
M− s ) and 2 (k = 6.2 × 10
1
−1
−2
M
−1 −1
s ) in the oxidation of 2-
2
PPA (Figure 8), it can be seen that the reactivity of 2 is greater
than that of 1 in the aldehyde deformylation reaction. The
differential reactivity is attributable to the structural features of
1
and 2 (Scheme 2 and Figures 3 and 6). The tert-butyl groups
of 1 provide the steric hindrance of the NiO moiety. However,
2
the cyclohexyl groups of 2 give sufficient space around the
NiO core to allow the substrate to approach the peroxo moiety
2
2
3
(
see the SI, Figure S9). These results were also elucidated by
−
1
the difference of the activation enthalphy values of 67 kJ mol
for 1 and 66 kJ mol− for 2, and the difference of ΔΔG
1
⧧
−1
Figure 8. Kinetic studies of the reactions of 1 and 2 with 2-PPA in
between 1 and 2 is 5 kJ mol . In addition, almost identical O−
−1 −1
CH CN/CH OH (1:1) at 25 °C. Plots of k against the 2-PPA
3
3
obs
O stretching frequencies of 1 (989 cm ) and 2 (988 cm )
suggest that the difference in the electronic effect of the tert-
concentration to determine second-order rate constants for the
reactions of 1 (blue ■) and 2 (red ◆).
butyl and cyclohexyl groups on the NiO cores is negligible.
2
This result is consistent with the identical ρ values from the
Hammett plots (vide supra). However, we cannot rule out a
order rate constant (k = 6.2 × 10 M−1 s−1 at 25 °C) was
−2
2
determined. The activation parameters were determined to be
ΔH = 66 kJ mol and ΔS = −48 J mol
Figure S7b). The effects of para substituents on the
benzaldehyde oxidation process were investigated, and the
Hammett plot afforded ρ = 4.3(7) (see the SI, Table S3 and
Figure S7c). We have observed the formation of benzoic acid
small electronic effect on the NiO cores by changing the tert-
⧧
−1
⧧
−1 −1
2
K
(see the SI,
24
butyl to cyclohexyl groups. Consequently, through control of
the steric factor from tert-butyl to cyclohexyl groups, the
aldehyde deformylation reaction by the nickel(III) peroxo
complexes is facilitated and the reaction rate is enhanced ∼8
times.
(
84 ± 3%) as a product in the oxidation of benzaldehyde. The
positive value is consistent with the process involving
nucleophilic character.
The cyclic voltammograms of nickel(II) chloride complexes
CONCLUSIONS
The structure and reactivity of metal−O adducts in the
■
2
oxidation of organic compounds are of crucial interest in
enzymatic activity, organic synthesis, and industrial catalysis. A
number of metal(III) peroxo complexes and their reactivity in
aldehyde deformylation have been investigated. However, the
steric effect on the reactivity of metal(III) peroxo species has
not been reported yet. In this work, we synthesized
mononuclear nickel(III) peroxo complexes bearing macrocyclic
with TBDAP and CHDAP in CH CN exhibit a quasi-reversible
3
+
redox couple at E1/2 = 0.56 and 0.57 V (vs Fc /Fc),
respectively, as shown in Figure 9 with data collected in
III
+
tetradentate N4 ligands, [Ni (TBDAP)(O )] (1) and
2
III
+
[
Ni (CHDAP)(O )] (2). The intermediates were fully
2
characterized with various physicochemical methods such as
UV−vis, ESI-MS, resonance Raman, EPR, and X-ray analysis.
Both 1 and 2 are capable of deformylating aldehydes, and the
nucleophilic character of 1 and 2 was confirmed by positive
Hammett ρ values of 4.4(3) and 4.3(7), respectively, which
were obtained in the reaction of 1 and 2 with para-substituted
benzaldehydes. A comparison of the reactivity of 1 and 2 in
aldehyde deformylation reveals that the reactivity of 2 is ∼8
times greater than that of 1. The results regarding the disparate
steric effects of the tert-butyl groups for 1 and cyclohexyl
groups for 2 are verified by spectroscopic and kinetic studies
combined with structural analyses. DFT calculations support
the experimental observations that the nucelophilic reactivity of
the nickel(III) peroxo complexes is highly dependent on the
steric properties of the supporting ligands.
Figure 9. Cyclic voltammograms of [Ni(TBDAP)Cl ] (blue) and
2
[
Ni(CHDAP)Cl ] (red) in CH CN (1 mM) containing 0.1 M
2
3
Bu NClO4 at room temperature (working electrode, Pt; counter
4
+
electrode, Pt; reference electrode, Ag/Ag ; scan rate = 100 mV).
2
1
Table 2. The redox couples correspond to the one-electron
oxidation of the Ni state to the Ni state. The data show
that the redox couples are almost identical.
EXPERIMENTAL SECTION
Materials. All chemicals obtained from Aldrich Chemical Co. were
of the best available purity and were used without further purification
unless otherwise indicated. Solvents were dried according to published
II
III
22
■
2
5
18
18
Table 2. Electrochemical Data for the Oxidation of
Nickel(II) Complexes with TBDAP and CHDAP at Room
Temperature at Scan Rate = 0.1 V
procedures and distilled under argon prior to use. H2 O (95% O-
2
enriched and 0.89% H218O in water) was purchased from ICON
2
Services Inc. (Summit, NJ). N,N′-Di-tert-butyl-2,11-diaza[3.3](2,6)-
pyridinophane (TBDAP) was prepared by reacting 2,6-bis-
(chloromethyl)pyridine with 2,6-bis[(N-tert-butylamino)methyl]-
+
complex
E1/2(II/III) (ΔE) vs Fc /Fc (V)
14
pyridine at 80 °C. The synthetic procedures of the N,N′-
II
[
[
Ni (TBDAP)Cl ]
0.56 (0.12)
0.57 (0.13)
2
dicyclohexyl-2,11-diaza[3.3](2,6)pyridinophane (CHDAP) ligand are
II
Ni (CHDAP)Cl ]
2
given in the SI, Experimental Section. [Ni(TBDAP)Cl ] was prepared
2
F
DOI: 10.1021/acs.inorgchem.5b00294
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
22
(72 μL, 90% 18O-enriched, 0.89% H218O in water) to a solution
according to the literature method. [Ni(CHDAP)Cl ] was prepared
2
2
by a modification of the reported procedure (see the SI, Experimental
Section).
containing [Ni(CHDAP)(NO )](NO )(CH OH) (4 mM) and 2
3
3
3
22
equiv of TEA in CH CN (2 mL) at ambient temperature. UV−vis
3
−1 −1
Physical Methods. UV−vis spectra were recorded on a Hewlett-
Packard 8453 diode-array spectrophotometer equipped with a
UNISOKU Scientific Instruments for low-temperature experiments
or with a circulating water bath. ESI-MS spectra were collected on a
Waters (Milford, MA) Acquity SQD quadrupole mass instrument, by
infusing samples directly into the source using a manual method. The
spray voltage was set at 2.5 kV and the capillary temperature at 80 °C.
Cold-spray-ionization time-of-flight mass spectrometry spectra were
collected on a JEOL JMS-T100CS spectrometer. Resonance Raman
spectra were obtained using a liquid-nitrogen-cooled charge-coupled
detector (CCD-1024 × 256-OPEN-1LS, Horiba Jobin Yvon) attached
to a 1-m single polychromator (MC-100DG, Ritsu Oyo Kogaku) with
[CH CN; λ , nm (ε, M cm )]: 584 (35), 950 (70). ESI-MS
(CH CN): m/z 494.2 for [Ni(CHDAP)(O )] . μ = 2.2 μB.
3 max
+
3
2
eff
X-ray Crystallography. Single crystals of [Ni(TBDAP)(NO )-
3
(H O)](NO ), [Ni(CHDAP)(NO )](NO )(CH OH), and [Ni-
2
3
3
3
3
(TBDAP)(O )](ClO )(0.5CH Cl ) (1-ClO ·0.5CH Cl ) were picked
2
4
2
2
4
2
2
from solutions by a nylon loop (Hampton Research Co.) on a
handmade copper plate mounted inside a liquid-nitrogen Dewar vessel
at ca. −40 °C and mounted on a goniometer head in a dinitrogen
cryostream. Data collections were carried out on a Bruker SMART
APEX II CCD diffractometer equipped with a monochromator in the
Mo Kα (λ = 0.71073 Å) incident beam. The CCD data were
integrated and scaled using the Bruker SAINT software package, and
−1
28
a 1200 grooves mm holographic grating. An excitation wavelength of
the structure was solved and refined using SHELXTL, version 6.12.
4
41.6 nm was provided by a helium−cadmium laser (Kimmon Koha,
Hydrogen atoms were located in the calculated positions except for
those of water in [Ni(TBDAP)(NO )(H O)](NO ). Hydrogen atoms
on the water were found from the Fourier difference map. All non-
hydrogen atoms were refined with anisotropic thermal parameters.
IK5651R-G and KR1801C), with 20 mW power at the sample point.
All measurements were carried out with a spinning cell (1000 rpm) at
3
2
3
−
20 °C. Raman shifts were calibrated with indene, and the accuracy of
−1
the peak positions of the Raman bands was ±1 cm . The effective
Crystal data for [Ni(TBDAP)(NO
)(H O)](NO ): C22H N NiO ,
3 2 3 34 6 7
1
magnetic moments were determined using the modified H NMR
orthorhombic, Pbca, Z = 8, a = 15.2958(5) Å, b = 16.4819(6) Å, c =
1
7,26,27
3
−1
method of Evans at room temperature.
A WILMAD coaxial
19.9118(8) Å, V = 5019.8(3) Å , μ = 0.827 mm , ρ
= 1.464 g
calcd
−3
insert (sealed capillary) tube containing the blank acetonitrile-d3
solvent [with 1.0% tetramethylsilane (TMS)] only was inserted into
the normal NMR tubes containing the complexes dissolved in
cm , R1 = 0.0324, wR2 = 0.1037 for 4018 unique reflections, 339
variables. Crystal data for [Ni(CHDAP)(NO )](NO )(CH OH):
NiO , triclinic, P1, Z = 2, a = 10.6798(2) Å, b =
11.8168(2) Å, c = 12.9559(2) Å, α = 64.1040(10)°, β =
82.1330(10)°, γ = 78.9810(10)°, V = 1441.11(4) Å , μ = 0.729
mm , ρcalcd = 1.427 g cm , R1 = 0.0261, wR2 = 0.0702 for 7001
unique reflections, 372 variables. Crystal data for 1-ClO ·0.5CH Cl
33Cl2N NiO , monoclinic, C2/c, Z = 8, a = 30.9602(6) Å, b =
12.8498(2) Å, c = 13.7307(2) Å, β = 105.7590(10)°, V = 5257.20(15)
Å , μ = 0.986 mm , ρcalcd = 1.479 g cm , R1 = 0.0341, wR2 = 0.0820
for 6538 unique reflections, 327 variables. The crystallographic data for
3
3
3
C
H
N
̅
27
40
6
7
acetonitrile-d (with 0.03% TMS). The chemical shift of the TMS peak
3
3
(
and/or solvent peak) in the presence of the paramagnetic metal
−1
−3
complexes was compared to that of the TMS peak (and/or solvent
peak) in the inner coaxial insert tube. The effective magnetic moment
:
2
4
2
1/2
was calculated using the equation μ = 0.0618(ΔνT/2f M) , where f is
the oscillator frequency (MHz) of the superconducting spectrometer,
T is the absolute temperature, M is the molar concentration of the
metal ion, and Δv is the difference in frequency (Hz) between the two
reference signals. Continuous-wave EPR spectra were taken at 20 K
using an X-band Bruker EMX-plus spectrometer equipped with a dual-
mode cavity (ER 4116DM). Low temperatures were achieved and
controlled using an Oxford Instruments ESR900 liquid-helium quartz
cryostat with an Oxford Instruments ITC503 temperature and gas flow
C
22.5
H
4
6
3
−1
−3
[Ni(TBDAP)(NO
(CH OH), and 1-ClO
S1 in the SI lists the selected bond distances and angles. CCDC
1060970 for [Ni(TBDAP)(NO )(H O)](NO ), 1026817 for [Ni-
(CHDAP)(NO )](NO )(CH OH), and 1026816 for 1-ClO
0.5CH Cl contain the supplementary crystallographic data for this
)(H
O)](NO
), [Ni(CHDAP)(NO
are listed in Table 1, and Table
2
)](NO )-
3
2
3
3 3
·0.5CH
Cl
3
4
2
3
2
3
·
4
3
3
3
controller. Variable amounts of CuCl (0.5−1 mM) were used as a
2
2
2
EPR standard for spin quantification. Product analysis was performed
with high-performance liquid chromatography (HPLC; Waters Pump
series P580) equipped with a variable-wavelength UV-200 detector
and a Thermo Finnigan (Austin, TX) FOCUS DSQ (dual-stage
quadrupole) mass spectrometer interfaced with a Finnigan FOCUS
gas chromatograph. Quantitative analysis was made on the basis of a
comparison of the HPLC peak integration between the products and
paper. These data can be obtained free of charge via www.ccdc.cam.ac.
uk/data_request/cif [or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax (+44) 1223-
336-033 or e-mail deposit@ccdc.cam.ac.uk].
III
+
Computational Details. The structures of [Ni (TBDAP)(O )]
2
III
+
(1) and [Ni (CHDAP)(O )] (2) in the gas phase were optimized
using a nonlocal hybrid exchange-correlation functional (HSE06).
2
29
1
13
authentic samples. H and C NMR spectra were measured with a
Bruker AVANCE III-400 spectrometer.
Spin-polarized DFT calculations were performed using the projector-
augmented-wave method30 and a plane-wave cutoff of 400 eV, as
+
30−32
Generation and Characterization of [Ni(TBDAP)(O2)] (1).
implemented in the VASP code.
Atomic structures were relaxed
−1
The treatment of [Ni(TBDAP)(NO )(H O)](NO ) (4 mM) with 5
equiv of H O in the presence of 2 equiv of TEA in CH CN (2 mL)
within 0.05 eV Å . The RMSD for the X-ray diffraction and DFT
3
2
3
+
structures of [Ni(TBDAP)(O )] was calculated to be 0.03 Å.
2
2
3
2
afforded the formation of a brown solution at 25 °C. Spectroscopic
Hydrogen atoms were not considered for structural comparison
because their atomic positions in the crystal structure cannot be
accurately determined by X-ray diffraction.
data, including UV−vis, ESI-MS, resonance Raman, and EPR, are
1
8
+
reported in Figure 2. [Ni(TBDAP)( O )] was prepared by adding 5
2
equiv of H218O (72 μL, 90% O-enriched, 0.89% H2 O in water) to
18
18
Reactivity Studies. All reactions were run in an 1 cm UV cuvette
by monitoring UV−vis spectral changes of the reaction solutions, and
rate constants were determined by fitting the changes in absorbance at
560 nm for 1 and 934 nm for 2. Reactions were run at least in
triplicate, and the data reported represent the average of these
reactions. In situ generated 1 and 2 were used in kinetic studies, such
as the oxidation of 2-PPA in CH CN/CH OH (1:1) at 25 °C. After
2
2
a solution containing [Ni(TBDAP)(NO )(H O)](NO ) (4 mM) and
3
2
3
2
equiv of TEA in CH CN (2 mL) at ambient temperature. Crystalline
3
−1
−1
yield: 0.015 g (44%). UV−vis [CH CN; λmax, nm (ε, M cm )]:
3
∼
560 (15), 1040 (45). ESI-MS (CH CN): m/z 442.2 for [Ni-
3
+
(
TBDAP)(O )] . μ = 2.3 μ . X-ray crystallographically suitable
2 eff B
crystals were obtained by the slow diffusion of Et O into a CH Cl
2
2
2
3
3
solution of 1 in the presence of NaClO . Caution! Perchlorate salts are
completion of the reactions, a pseudo-first-order fitting of the kinetic
data allowed us to determine the kobs values. Products formed in the
oxidation of 2-PPA by 1 and 2 in CH CN/CH OH (1:1) at 25 °C
4
potentially explosive and should be handled with care!
Generation and Characterization of [Ni(CHDAP)(O2)] (2).
+
3
3
The treatment of [Ni(CHDAP)(NO )](NO )(CH OH) (4 mM)
were analyzed by HPLC. Products were analyzed by injecting the
reaction mixture directly into the high-performance liquid chromato-
graph. Products were identified by comparison with authentic samples,
and product yields were determined by comparison against standard
areas prepared with authentic samples as an internal standard.
3
3
3
with 5 equiv of H O in the presence of 2 equiv of TEA in CH CN (2
2
2
3
mL) at 25 °C afforded a green solution. Spectroscopic data, including
UV−vis, ESI-MS, resonance Raman, and EPR, are reported in Figure
18
+
18
5
. [Ni(CHDAP)( O )] was prepared by adding 5 equiv of H O
2 2 2
G
DOI: 10.1021/acs.inorgchem.5b00294
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(
10) (a) Cho, J.; Sarangi, R.; Nam, W. Acc. Chem. Res. 2012, 45,
ASSOCIATED CONTENT
Supporting Information
X-ray crystallographic data in CIF format, experimental details,
Figures S1−S10, and Tables S1−S3. The Supporting
Information is available free of charge on the ACS Publications
website at DOI: 10.1021/acs.inorgchem.5b00294.
■
1
321−1330. (b) Cho, J.; Sarangi, R.; Kang, H. Y.; Lee, J. Y.; Kubo, M.;
*
S
Ogura, T.; Solomon, E. I.; Nam, W. J. Am. Chem. Soc. 2010, 132,
1
6977−16986. (c) Cho, J.; Kang, H. Y.; Liu, L. V.; Sarangi, R.;
Solomon, E. I.; Nam, W. Chem. Sci. 2013, 4, 1502−1508.
(d) Yokoyama, A.; Han, J. E.; Cho, J.; Kubo, M.; Ogura, T.; Siegler,
M. A.; Karlin, K. D.; Nam, W. J. Am. Chem. Soc. 2012, 134, 15269−
1
5272. (e) Kang, H.; Cho, J.; Cho, K.-B.; Nomura, T.; Ogura, T.;
AUTHOR INFORMATION
Corresponding Author
E-mail: jaeheung@dgist.ac.kr.
Nam, W. Chem.Eur. J. 2013, 19, 14119−14125. (f) Kim, D.; Cho, J.;
■
Lee, Y.-M.; Sarangi, R.; Nam, W. Chem.Eur. J. 2013, 19, 14112−
1
2
4118. (g) Sarangi, R.; Cho, J.; Nam, W.; Solomon, E. I. Inorg. Chem.
011, 50, 614−620.
*
Author Contributions
These authors contributed equally.
(11) (a) Cho, J.; Sarangi, R.; Annaraj, J.; Kim, S. Y.; Kubo, M.; Ogura,
T.; Solomon, E. I.; Nam, W. Nat. Chem. 2009, 1, 568−572. (b) Kieber-
Emmons, M. T.; Annaraj, J.; Seo, M. S.; Heuvelen, K. M. V.; Tosha, T.;
Kitagawa, T.; Brunold, T. C.; Nam, W.; Riordan, C. G. J. Am. Chem.
Soc. 2006, 128, 14230−14231.
‡
Notes
The authors declare no competing financial interest.
(
12) Lam, B. M. T.; Halfen, J. A.; Young, V. G., Jr.; Hagadorn, J. R.;
Holland, P. L.; Lledos, A.; Cucurull-Sanchez, L.; Novoa, J. J.; Alvarez,
S.; Tolman, W. B. Inorg. Chem. 2000, 39, 4059−4072.
13) Hikichi, S.; Kobayashi, C.; Yoshizawa, M.; Akita, M. Chem.
Asian J. 2010, 5, 2086−2092.
14) Che, C.-M.; Li, Z.-Y.; Wong, K.-Y.; Poon, C.-K.; Mak, T. C. W.;
Peng, S.-M. Polyhedron 1994, 13, 771−776.
ACKNOWLEDGMENTS
The research was supported by the NRF (Grants
013K2A2A4000610 and 2014R1A1A2056051), the Ministry
of Science, ICT and Future Planning (DGIST R&D Program
́
́
■
2
(
(
1
2
5 - B D - 0 4 0 3
&
1 5 - H R L A - 0 2 a n d K C R C
014M1A8A1049320), and the Ministry of Oceans and
15) Unfortunately, we could not observe the 18O-shifted band of 1
despite our great efforts. Because it was difficult to fully generate the
(
Fisheries (Marine Biotechnology Program 20150220) of
Korea (to J.C.). H.M. and T.O. acknowledge financial support
by the JSPS program for Accelerating Brain Circulation (R
213). We are grateful to Drs. Young Jun Park and Yong-Min
Lee (Ewha Womans University) for valuable discussions.
18
18
O-labeled 1 with 0.89% of H2 O . However, we confirmed that the
2
v(O−O) band disappeared when the solution was allowed to warm.
(16) Haines, R. I.; McAuley, A. Coord. Chem. Rev. 1981, 39, 77−119.
(17) Evans, D. F.; Jakubovic, D. A. J. Chem. Soc., Dalton Trans. 1988,
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DOI: 10.1021/acs.inorgchem.5b00294
Inorg. Chem. XXXX, XXX, XXX−XXX