Redox behavior of novel nickel and palladium complexes supported by trianionic non-innocent ligand containing β-diketiminate and phenol groups

Article abstract of DOI:10.1142/S1088424615500248

A new type of nickel and palladium complexes with non-innocent β-diketiminate ligand having redox active phenol groups, 2,4-di-tert-butyl-6-(((1E,2E)-3-((3,5-di-tert-butyl-2-hydroxyphenyl)amino)-2-nitroallylidene)amino)phenol (LH₃, fully protonated form) have been developed, and the structure, physical properties, and reactivity of their one-electron and two-electron oxidized complexes, [M^II (L^?2-)] and [M^II(L^-)]⁺ (M = Ni^II or Pd^II) have been examined in detail. The two-electron oxidized forms of both complexes, [M^II(L^-)]⁺, exhibited hydrogen atom abstraction ability from 1,4-cyclohexadiene (CHD) comparable to its copper analog [Cu^II (L^-)]⁺ (Dalton Trans. 2013; 42: 2438-2444). The one-electron oxidized form of palladium complex, [Pd^II (L^?2-)], was also found to oxidize CHD, whereas the nickel analog, [Ni^II(L^?2-)], exhibited photo-induced oxidation ability of CHD.

Full text of DOI:10.1142/S1088424615500248

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2015; 19: 1–11
DOI: 10.1142/S1088424615500248
Published at http://www.worldscinet.com/jpp/
Redox behavior of novel nickel and palladium complexes
supported by trianionic non-innocent ligand containing
b-diketiminate and phenol groups
◊
Yuma Morimoto,June Takaichi, Shinichi Hanada, Kei Ohkubo , Hideki Sugimoto,
◊
Nobutaka Fujieda, Shunichi Fukuzumi and Shinobu Itoh*
Department of Material and Life Science, Division of Advanced Science and Biotechnology,
Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
Dedicated to Professor Shunichi Fukuzumi on the occasion of his retirement
Received 5 November 2014
Accepted 18 December 2014
ABSTRACT: A new type of nickel and palladium complexes with non-innocent b-diketiminate ligand
having redox active phenol groups, 2,4-di-tert-butyl-6-(((1E,2E)-3-((3,5-di-tert-butyl-2-hydroxyphenyl)
amino)-2-nitroallylidene)amino)phenol (LH , fully protonated form) have been developed, and the
3
structure, physical properties, and reactivity of their one-electron and two-electron oxidized complexes,
II
·2-
II
-
+
II
II
[
M (L )] and [M (L )] (M = Ni or Pd ) have been examined in detail. The two-electron oxidized forms
II
-
+
of both complexes, [M (L )] , exhibited hydrogen atom abstraction ability from 1,4-cyclohexadiene
CHD) comparable to its copper analog [Cu (L )] (Dalton Trans. 2013; 42: 2438-2444). The one-
electron oxidized form of palladium complex, [Pd (L )], was also found to oxidize CHD, whereas the
nickel analog, [Ni (L )], exhibited photo-induced oxidation ability of CHD.
II
-
+
(
II
·2-
II
·2-
KEYWORDS: nickel complex, palladium complex, b-diketiminate ligand, non-innocent ligand.
[11–14]. Galactose oxidase is a typical example of such
INTRODUCTION
enzymes, which contains a copper(II)-tyrosyl (phenoxyl)
radical as the key reactive intermediate for the oxidation
of primary alcohols to aldehydes [15–19]. In this enzyme,
the copper center is further ligated by an additional
tyrosyl residue to construct a Tyr–Cu–Tyr motif. In
order to mimic such an active site structure, salen-type
ligands consisting with two phenol groups connected by
diamine derivatives have been tested [13, 14]. In most
cases, ligand effects as well as metal ion effects on the
electronic structures of one-electron oxidized mix-valent
systems have been investigated, but little attention has
been focused on the reactivity of the phenoxyl radical
transition-metal complexes. In this respect, Stack and
co-workers have investigated the direct reaction between
Transition metal complexes supported by redox non-
innocent ligands such as porphyrin and phthalocyanine
derivatives have long been attracting much attention due
to their relevance to enzymatic catalysis and a variety of
functional materials [1, 2]. The capability of an electron
release from highly conjugated p-orbitals of porphyrin
and phthalocyanine are regarded as important factor to
facilitate multi-electron transfer such as hydroxylation of
aliphatic substrates occurring in cytochrome P450 (CYP)
and its models [3–10]. Recently, phenol-containing
ligand also have received much interest as an active site
model of metalloenzymes containing a redox-active
organic cofactor such as tyrosine or its derivatives
II
the Cu -phenoxyl radical complexes and benzyl alcohol
◊
SPP full member in good standing
to demonstrate the importance of substrate coordination
to the metal center prior to the C–H bond activation
of the substrates [20–22]. In parallel, the electronic
*
Correspondence to: Shinobu Itoh, email: shinobu@mls.eng.
osaka-u.ac.jp, tel: +81 6-6879-7932, fax: +81 6-6879-7935
Copyright © 2015 World Scientific Publishing Company

2
Y. MORIMOTO ET AL.
NO2
N
HN
OH HO
LH3
Scheme 1. Molecular structure of the ligand
structures and chemical properties of the group 10
element complexes, Ni, Pd, and Pt, supported by salen
type ligands have also been investigated in detail [23–
3
2]. In those studies, unambiguously demonstrated are
the ability of salen-type ligands modulating the multi-
electron redox processes of the metal center.
Recently, we have examined the reactivity of a discreet
II
Cu -phenoxyl radical complex generated by using a new
type of diphenol ligand consisting with a b-diketiminate
bridging moiety as shown in Scheme 1 [33]. In this study,
we have further examined the spectroscopic property and
reactivity of phenoxyl radical complexes to evaluate the
metal ion effects using Ni- and Pd-complexes supported
by the same ligand.
II
3-
II
·2-
Fig.1.ORTEPdrawingsof(a)(Et N)[Ni (L )]and(b)[Ni (L )]
4
showing 50% probability thermal ellipsoids. Hydrogen atoms,
counter cation, and solvent molecules are omitted for clarity. For
the structure of [Ni (L )], elongated bonds (more than 0.01 Å)
are indicated in blue color, and shorten bonds (more than 0.01Å)
are indicated in red color
RESULTS AND DISCUSSION
II
·2-
Synthesis and characterization
The nickel(II) and the palladium(II) complexes were
prepared by treating the neutral ligand LH (Scheme 1)
3
with an equimolar amount of nickel(II) acetate or
set of the ligand, where the torsion angles between the
O1–M1–N1 and O2–M1–N2 planes are 2.3° and 0.4°,
respectively. The bond length of Ni–O (1.841 and 1.846Å)
and Ni–N (1.855 and 1.844 Å) are slightly shorter than
palladium(II) acetate in the presence of a base (NEt OH
4
or Et N). The single crystals were obtained as described
3
in the Experimental section. In the case of Pd-complex,
+
+
II
II
2-
2-
the counter cation was exchanged from Et NH to Bu N
those of Ni -salen complex [Ni (sal )] (sal = diphenolate
3
4
to facilitate crystallization (see Experimental section).
form of salH , N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-
2
Figures 1(a) and 2(a) show the crystal structures of (Et N)
cyclohexane-(1R,2R)-diamine, Ni–O, 1.852 and 1.854 Å;
Ni–N, 1.852 and 1.860 Å) [35]. The Pd–O bond lengths
(1.997 and 1.999 Å) are slightly longer than those of
4
II
3-
II
3-
[
Ni (L )] and (Bu N)[Pd (L )], respectively (the counter
4
cations are omitted for simplicity, the crystallographic
data are presented in Supporting information, Table S1).
In both complexes, the ligand takes the tri-anionic form
II
2-
[Pd (sal )] (Pd–O, 1.970 and 1.969 Å), whereas the Pd–N
bond lengths (1.957 and 1.954 Å) are almost the same
3
-
II
2-
(
fully deprotonated form L ), since there is one counter
to those of [Pd (sal )] (Pd–N, 1.959 and 1.946 Å) [28].
Apparently, the M–O and M–N bonds of the Ni-complex
are shorter than those of the Pd-complex, which is due to
the difference in the ionic radius between the two metal
ions; Ni(II) 0.63 Å, Pd(II) 0.78 Å.
+
cation (R N ) per one complex molecule (not shown in
4
Fig. 1(a) and 2(a)), and the negative mode ESI-MS gave
an intense peak cluster at 578.30 and 626.15, respectively,
which are assignable to the molecular formula of the
II
3-
-
anionic complexes [M (L )] (M = Ni and Pd) (Fig. 3).
The peak distribution patterns are also consistent with
those expected for the respective complexes.
Redox property
II
3-
-
II
3-
-
II
3-
-
The metal centers of [Ni (L )] and [Pd (L )]
exhibited square planar geometries with the N O donor
The redox properties of [M (L )] were examined
using cyclic voltammetry (CV) as shown in Fig. 4.
2
2
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J. Porphyrins Phthalocyanines 2015; 19: 2–11

REDOX BEHAVIOR OF NOVEL NICKEL AND PALLADIUM COMPLEXES SUPPORTED BY TRIANIONIC
3
Both complexes exhibited two reversible redox couples
1
1
2
1/2
(E
and E ) due to the first and second one-electron
/2
oxidation of the anionic complexes. The redox potential
II
II
of the Ni - and Pd -complexes are summarized in Table 1
II
together with those of the Cu -complex of the same
II
3-
-
II
2-
ligand [Cu (L )] and [M (sal )] (M = Cu, Ni, and Pd)
for comparison (reference papers for the reported data
are indicated in the table footnote).
1
2
It appears that the E and E values as well as the
1/2
1/2
1
2
3-
II
DE1/2 (E – E ) values of all the L complexes (M =
1/2
1/2
1
1
Cu, Ni, and Pd) are fairly similar as E = -0.07 ~ +0.07 V,
/2
2
1
E
= +0.46 ~ +0.54 V vs Ferrocene/Ferrocenium ion (Fc/
/2
+
Fc ) and DE = 0.47 ~ 0.50 V, which are largely negative
1/2
as compared to those of the salen complexes. The largely
II
3-
-
negative shifts of the E values of [M (L )] as compared
1
/2
II
2-
to those of [M (sal )] can be attributed to the higher
HOMO level of the trianionic L as compared to that of
3-
2-
the dianionic sal [33].
In the salen system, it has been reported that the
electronic coupling between the redox active phenolate
groups correlates strongly with difference in the metal
d orbital energy and ligand MO energy [20]. In the
present ligand system, on the other hand, the degree of
electronic coupling is independent on the metal ions as
indicated by the similar E1/2 and DE1/2 values among the
three complexes (Cu, Ni, and Pd). This may be due to the
conjugation between the two phenolate groups through
II
2-
the b-diketiminate backbone. SOMO 155a of [Cu (L )]
calculated by using DFT method, which mainly consists
of p-orbitals on the ligand, clearly shows that the two
phenolate groups are electronically bridged by the
conjugated b-diketiminate moiety [33]. In the salen
system, there is no such conjugative interaction between
the two phenolate groups, since the phenolate groups
are connected by the saturated alkyl diamine moiety.
II
3-
Fig. 2. ORTEP drawings of (a) (Bu N)[Pd (L )] and
(
4
II
·2-
b) [Pd (L )] showing 50% probability thermal ellipsoids.
Hydrogen atoms, counter cation, and solvent molecules
are omitted for clarity. For the structure of [Pd (L )],
elongated bonds (more than 0.01 Å) are indicated in blue
color, and shorten bonds (more than 0.01 Å) are indicated
in red color
II
·2-
II
3-
-
II
3-
-
Fig. 3. ESI-MS (negative mode) of (a) [Ni (L )] and (b) [Pd (L )] in CH Cl . Inset: expanded spectra (EXP) and their simulation
2
2
spectra (SIM)
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J. Porphyrins Phthalocyanines 2015; 19: 3–11

4
Y. MORIMOTO ET AL.
II
3-
-
-4
II
3-
-
-4
n
Fig. 4. Cyclic voltammograms of (a) [Ni (L )] (5.0 × 10 M) and (b) [Pd (L )] (5.0 × 10 M) in CH Cl containing Bu NPF
2
2
4
6
(
0.10 M); working electrode: glassy carbon, counter electrode: Pt, reference: ferrocene, scan rate 100 mV/s
II
3-
-
Table 1. Electrochemical data of the oxidation of [M (L )] and
successfully obtained by recrystallization from CH Cl /
2 2
II
2-
[M (sal )]
n-hexane. Crystal structures of the neutral complexes
II
·2-
II
·2-
1
a
2
a
1
2
[Ni (L )] and [Pd (L )] are shown in Fig. 1(b)
and Fig. 2(b), respectively. The crystallographic data
are presented in Table S2. The crystal structure of
Complex
E
1/2, V
E
1/2, V
DE1/2 (E – E1/2), V
1/2
II
3- -b
[
[
[
[
[
Cu (L )]
-0.06
+0.04
+0.07
+0.45
+0.37
+0.45
+0.46
+0.54
+0.54
+0.65
+0.85
+0.80
0.50
0.50
0.47
0.20
0.48
0.35
II
3-
-
Ni (L )]
II
·2-
[
Ni (L )] clearly indicates that one-electron oxidation
II
3-
-
II
3-
-
Pd (L )]
of [Ni (L )] caused a distinctive structural change in the
aminophenol moiety of ringA and b-diketiminate moiety.
Namely, structures of the aminophenol moiety of rings
A and B are nearly identical in [Ni (L )] (Fig. 1(a)),
whereas contribution of a quinonoid canonical form
increases in the aminophenol moiety of ring A in the
oxidized complex [Ni (L )] (Fig. 1(b)). The distance
of Ni1–O2 (1.876 Å) increases as compared with that of
Ni1–O1 (1.838 Å). In addition, such structural changes
occurred on b-diketiminate moiety as well, e.g. the
bond distances of C4–N2 and C3–C2 are shrunk from
II
2-
c
Cu (sal )]
II
2-
d
Ni (sal )]
II
3-
-
II
2-
e
[
Pt (sal )]
a
+
b
c
d
e
vs Fc/Fc . from Ref. 15. from Ref. 19. from Ref. 20. from
II
·2-
Ref. 18.
Therefore, the electronic coupling can only be mediated
by the metal ion (Scheme 2).
1
.423 and 1.410 to 1.390 and 1.371, respectively. These
structural features of the oxidized complex are consistent
with the conclusion that the one-electron oxidation
takes place at the conjugated p-orbital consisting of
One-electron oxidation product
·2-
The one-electron oxidation products can also be
generated by the chemical oxidation using an equimolar
amount of AgSbF in CH Cl , and their single crystals
phenolate and b-diketiminate part to give L rather than
II
·2-
at the metal. One-electron oxidation product [Pd (L )]
II
·2-
showed a similar structural features to [Ni (L )] as
6
2
2
suitable for X-ray crystallographic analysis were
shown in Fig. 2(b). The notable structural change was
N
O
N
O
N
O
N
O
MII
MII
•
•
II
M (L2•–)]
II
•–
+
[
[M (sal )]
II
·-
II
·-
+
Scheme 2. Conjugation of p-orbital in [M (L )] and [M (sal )]
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REDOX BEHAVIOR OF NOVEL NICKEL AND PALLADIUM COMPLEXES SUPPORTED BY TRIANIONIC
5
II
3-
-
II
·2-
II
-
+
Fig. 5. (a) Absorption spectra of [Ni (L )] (black line), [Ni (L )] (blue line) and [Ni (L )] (red line) in CH Cl at -10°C.
2
2
II
3-
-
II
·2-
III
·2-
+
(
b) Absorption spectra of [Pd (L )] (black line), [Pd (L )] (blue line) and [Pd (L )] (red line) in CH Cl at 30°C
2
2
-
1
-1
observed at the aminophenol moiety of ring A as well as
and 540 nm (14,200 M .cm ) could also be attributed to
the MLCT transitions, and the broad absorption bands in
the NIR region (above 800 nm) are due to the phenoxyl
radical species.
the b-diketiminate moiety upon one-electron oxidation
II
3-
-
of [Pd (L )] .
In Fig. 5(a), the absorption spectrum of the one-
II
·2-
electron oxidized complex [Ni (L )] is shown in the
In frozen CH Cl at 77 K, the starting materials
[Ni (L )] and [Pd (L )] were EPR silent (data are
not shown). The H-NMR spectra of [Ni (L )] and
[Pd (L )] in CH Cl at 298 K exhibited well-resolved
2
2
II
II
3-
-
3-
-
blue line together with that of the starting material
II
3-
-
1
II
3-
-
[
Ni (L )] in the black line. The l
value of the
max
·2-
II
II
3-
-
absorption band at 429 nm of [Ni (L )] is close to
2 2
that of the strong absorption band at 414 nm of the
proton signals in the diamagnetic region (d = 0–10 ppm)
(Supporting Fig. S1). These results clearly demonstrated
that the metal(II) centers have low-spin state (S = 0) in
both complexes.
II
3-
-
starting complex [Ni (L )] , whereas the intensity of
-
1
-1
the former (e = 16,400 M .cm ) is largely reduced
-
1
-1
as compared to that of the latter (28,100 M .cm ).
Thus, the absorption band at 429 nm together with
On the other hand, the EPR measurement of the one-
-
1
-1
II
·2-
II
·2-
the shoulder around 524 nm (9,860 M .cm ) could
be attributed to metal-to-phenolate charge transfer
electron oxidation complexes [Ni (L )] and [Pd (L )]
gave relatively broad axial signals at g = 1.989 and g =
⊥
//
(
MLCT) bands as in the case of the copper complex
1.979 (g = 1.986), and g = 2.006 and g = 1.991 (g =
⊥ // // ∞
with the same ligand system [33].
2.001), respectively (Figs 6(a) and 6(c)). These signals
are attributed to the metal-coordinating phenoxyl radical
species L .
II
·2-
The reduction of the intensity of this band in [Ni (L )]
is consistent with the fact that one of the phenolate rings
is oxidized to the phenoxyl radical as suggested by the
X-ray structure shown in Fig. 2(b). Then, the broad
·2-
-
1
-1
absorption bands at 784 nm (2,180 M .cm ), 883 nm
Two-electron oxidation product
-
1
-1
-1
-1
(
[
2,090 M .cm ), and 1530 nm (1,470 M .cm ) of
II
·2-
II
·2-
Ni (L )] in the visible to near IR (vis-NIR) region could
Further oxidation of [Ni (L )] by N(Ph-Br) ·SbCl
3
6
+
be attributed to the phenoxyl radical moiety. Existence
of such a broad absorption band at 1530 nm may be
due to the expanded p-conjugation system through the
b-diketiminate moiety as indicated by the X-ray structure
(E1/2 = 0.70 V vs Fc/Fc ) [36] in CH Cl gave a two-
2
2
electron oxidized cationic complex exhibiting intense
-1
-1
absorption bands at 460 nm (11,900 M .cm ) and 494 nm
-1
-1
(11,800 M .cm ) together with a relatively weak band
-1
-1
(
see Fig. 2(b)). This electronic communication between
at 640 nm (3,800 M .cm ) and a broad NIR band
-1
-1
the phenoxyl radical and the phenolate rings through the
expanded p-conjugated system is also suggested by the
electrochemical data presented in Table 1.
around 1525 nm (2,610 M .cm ) (red line spectrum in
Fig. 5(a)). The ESI-MS measured in the positive mode
II
-
+
matches the chemical formulation of [Ni (L )] (Fig. 7).
The absorption spectrum of the one-electron oxidized
The EPR signals of the neutral complex of nickel,
[Ni (L )] (Fig. 6(a)), were significantly weakened
II
·2-
II
·2-
complex [Pd (L )] is also shown in the blue line together
II
3-
-
with that of the starting material [Pd (L )] in the black
upon the one-electron oxidation with N(Ph-Br) ·SbCl as
3
6
line in Fig. 5(b). The relatively intense absorption bands at
shown in Fig. 6(b). This clearly indicated that the main
component of two-electron oxidized cationic complex
-
1
-1
-1
-1
4
09 nm (e = 14,900 M .cm ), 503 nm (14,000 M .cm ),
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6
Y. MORIMOTO ET AL.
II
·2-
-4
II
-
+
-4
II
·2-
-4
III
·2-
+
Fig. 6. EPR spectra of (a) [Ni (L )] (5.0 × 10 M), (b) [Ni (L )] (5.0 × 10 M), (c) [Pd (L )] (5.0 × 10 M), and (d) [Pd (L )]
-
4
(
5.0 × 10 M) at 77 K
Scheme 3. Schematic energy diagram of two-electron oxidized
Pd-complex at 77 K
III
·2-
+
[
Pd (L )] (Scheme 3). Then, the small EPR signal
observed in the two-electron oxidation of the nickel system
shown in Fig. 6(b) may be due to the larger contribution of
singlet species [Ni (L )] , in which the redox locus is on
II
-
+
Fig. 7. ESI-MS (positive mode) of [Ni (L )] in CH Cl . Inset:
2
2
II
-
+
expanded spectrum (EXP) and its simulation spectrum (SIM)
the ligand moiety as discussed above [40].
II
-
was an EPR-silent diamagnetic species, [Ni (L )] (S = 0),
-
where L denotes two-electron oxidized ligand with a
quinonoid canonical form.
Reactivity of two-electron oxidized complexes
In the case of palladium complex, oxidation of the
In our previous study of the copper complexes,
reactivity toward hydrogen atom donor was examined
using 1,4-cyclohexadiene (CHD) as a model substrate
II
·2-
neutral complex [Pd (L )] using N(Ph-Br) ·SbCl gave
3
6
a relatively intense EPR signal at g = 2.003 (Fig. 6(d)).
The result suggests formation of a triplet species (S = 1)
by the oxidation of doublet neutral complex. The number
of un-paired electron of the two-electron oxidized
complex was estimated to be 1.66 using Evans method
II
-
+
[33]. The cationic complex [Cu (L )] (two-electron
oxidized complex) underwent hydrogen atom abstraction
II
·-
+
reaction from CHD to give [Cu (LH )] (protonated form
II
·2-
of [Cu (L )]) and cyclohexenyl radical intermediate, the
(
Supporting Fig. S2), which is assignable to the average
later of which further donated hydrogen atom to another
II
-
+
of singlet species (17%) and triplet species (83%)
(
molecule of [Cu (L )] to give benzene and one more
II
·-
+
Scheme 3) [37–39].
Thus the main locus of the second oxidation is thought to
[Cu (LH )] (Scheme 4). Thus, the stoichiometry of the
II
-
+
II
-
+
reaction was [Cu (L )] :CHD = 2:1, where [Cu (L )]
be the metal center in the palladium system to give mainly
acted as a one-electron oxidant (Scheme 4). Addition of
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REDOX BEHAVIOR OF NOVEL NICKEL AND PALLADIUM COMPLEXES SUPPORTED BY TRIANIONIC
7
II
[M (L2•–)]
II
[M (LH2–)]
CHD
N
N
H• transfer
N
N
2–
MII
O
+
II
MII
H
2
•–
+
O
k2
O
O
–
+
[
[
M (L )]
fast
II
•– +
M (LH )]
II
2·-
Scheme 4. Reaction of [M (L )] with 1,4-cyclohexadiene (CHD)
-
2
II
-
+
-5
Fig. 8. (a) Spectral change observed upon addition of 1,4-cyclohexadiene (2.0 × 10 M) to [Ni (L )] (5.0 × 10 M) in CH Cl at
2
2
3
0°C. Inset: first-order plot based on the time-dependent absorption change at 640 nm. (b) Plot of kobs vs. [1,4-cyclohexadiene]
a base such as triethylamine to the final reaction mixture
generated [Cu (L )].
the pseudo-first-order rate constant (k ) increased with
obs
increasing the concentration of CHD. Plot of kobs against
II
·2-
In this study, reactivity of the cationic complexes,
Ni (L )] and [Pd (L )] , was also examined under
the substrate concentration gave a straight line with a
small intercept (Fig. 8(b)), from which the second-order
II
-
+
III
·2-
+
[
the same experimental conditions. Addition of an excess
amountof1,4-cyclohexadiene(CHD)toaCH Cl solution
rate constant k and self-decomposition rate constant
2
-1
-1
-4 -1
kdec were determined as 0.15 M .s and 5.7 × 10 s as
2
2
II
-
+
-5
of [Ni (L )] (5.0 × 10 M) readily resulted in the spectral
changes as shown in Fig. 8(a). The products given by the
oxidation reaction were determined to be benzene with
GC-MS. The final spectrum of the reaction did not match
the slope and the intercept, respectively. The reactivity
III
2·-
+
of [Pd (L )] toward CHD was also examined in the
same way, and the second-order rate constant k and the
2
self-decomposition rate constant k were determined as
dec
II
·2-
-1 -1
-3 -1
the spectrum of [Ni (L )] itself, but addition of 1 equiv
0.52 M .s and 2.8 × 10 s , respectively (Fig. S4). The
self-decomposition rate constants kdec determined were
nearly equal to those directly determined by following
the decrease of the absorption bands due to the cationic
of triethylamine (base) to the final solution resulted in
II
·2-
the appearance of absorption bands due to [Ni (L )] as
III
·2-
+
shown in Fig. S3(a). [Pd (L )] was also confirmed to
II
-
+
-4 -1
show similar reactivity to that of [Ni (L )] (Fig. S3(b)).
Thus, the reaction consequences of the both nickel and
palladium complex are the same with those of the copper
complex (Scheme 4) [33]. The reaction of nickel complex
obeyed first-order kinetics in the presence of excess
amount of CHD as shown in the inset of Fig. 8(a), and
complex in the absence of the substrate (6.2 × 10 s for
II
-
+
-3 -1
III
2·-
+
[Ni (L )] and 2.4 × 10 s for [Pd (L )] ).
-1
-1
The rate constant k (0.76 M .s ) of the copper
complex [Cu (L )] is larger than those of the nickel
and palladium complexes (0.15 M .s and 0.52 M .s ),
which is not consistent with its negative value of E as
2
II
-
+
-1
-1
-1 -1
2
1
/2
Copyright © 2015 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2015; 19: 7–11

8
Y. MORIMOTO ET AL.
II
·2-
-5
Fig. 9. (a) Spectral change observed upon addition of 1,4-cyclohexadiene (0.20 M) to [Pd (L )] (5.0 × 10 M) in CH Cl at 30°C.
2
2
Inset: first-order plot based on the time-dependent absorption change at 540 nm. (b) Plot of kobs vs. [1,4-cyclohexadiene]
compared to those of the nickel and palladium complexes
of 1 equiv of triethylamine to the final reaction mixture
resulted in quantitative generation of the spectrum of
[Pd (L )] (starting material) (Fig. S5(a)). Contrary,
III
2·-
+
(
Table 1). Reactivity of [Pd (L )] is somewhat larger
II
-
+
2
II
3-
-
than that of [Ni (L )] , even though the E values
1
/2
of these complexes are identical (Table 1). These
differences in reactivity may be due to the different
electronic configuration between the cationic complexes
as discussed above.
addition of 1 equiv of 10-camphorsulfonic acid to
II
3-
-
[Pd (L )] generated spectrum identical to that of the final
II
·2-
reaction mixture of CHD and [Pd (L )] (Fig. S5(b)). The
II
·2-
reactivity of one-electron oxidized complexes [Pd (L )]
in the C–H bond activation reaction of CHD was two
orders of magnitude lower, when compared to that of
Reactivity of one-electron oxidized complexes
III
2·-
+
two-electron oxidized complex [Pd (L )] .
II
·2-
[
Cu (L )] itself showed essentially no reactivity
1
1
It is also interesting to note that E values of all
/2
toward CHD, but it disproportionated into (1/2)
1
complexes are fairly similar as E = -0.06 ~ +0.07 V,
II
3-
-
II
-
+
1/2
[
Cu (L )] and (1/2)[Cu (L )] in the treatment with
nevertheless, each one-electron oxidized complex
CHD [33]. Thus, CHD was also converted to benzene by
II
·2-
II
·2-
II
·2-
[
Cu (L )], [Ni (L )], and [Pd (L )] exhibited different
II
-
+
the reaction with generated [Cu (L )] [33]. In this case,
reaction followed second-order kinetics with respect to
the concentration of the copper complex, indicating that
the disproportionation process is the rete-determining
step.
reactivity. It is likely to be due to the energy difference
of O–H bonds of protonated aminophenol moiety [41].
EXPERIMENTAL
II
·2-
The neutral complex of nickel(II), [Ni (L )], did not
show reactivity toward CHD under ambient conditions.
In this case, photo irradiation induced the oxidation of
CHD to give benzene and the starting nickel(II) complex
Materials and methods
The reagents and solvents used in this study except
for the ligand and the complexes were commercial
products of the highest available purity and were further
purified by the standard methods, if necessary [34].
II
3-
-
[
Ni (L )] (after base treatment of the final reaction
mixture). Details of this photochemical reaction will be
reported elsewhere.
II
·2-
In the case of [Pd (L )], addition of CHD into the
The ligand LH was prepared according to the reported
3
-
5
CH Cl solution of the neutral complex (5.0 × 10 M) at
methods [33]. FT-IR spectra were recorded with a
Shimadzu FTIR-8200PC. Electronic absorption spectra
were measured using a Hewlett-Packard HP8453 diode
array spectrophotometer or a Jasco V-570 UV/VIS/NIR
spectrophotometer equipped with a Unisoku thermostated
cell holder USP-203 designed for low temperature
measurements. Mass spectra were recorded with a
JEOL JMS-700T Tandem MS station or a JEOL JMS-
700. NMR spectra were recorded on a JEOL ECP400,
a JEOL ECS400, or a Varian UNITY INOVA 600 MHz.
A concentric double tube was utilized to perform Evans
2
2
3
0°C resulted in a spectral change shown in Fig. 9(a). The
II
·2-
characteristicabsorptionbandsdueto[Pd (L )]decrease
with a concomitant increase in the new absorption bands
-1
-1
represented by the peak at 417 nm (e = 23,300 M .cm ).
The reaction obeyed first-order kinetics in the presence of
a large excess of CHD as shown in the inset of Fig. 9(a).
Plot of the first-order rate constant kobs against the
substrate concentration gave a linear line passing through
the origin, from which the second-order rate constant k
was determined as 4.4 × 10 M .s (Fig. 9(b)). Addition
2
-
3
-1 -1
Copyright © 2015 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2015; 19: 8–11

REDOX BEHAVIOR OF NOVEL NICKEL AND PALLADIUM COMPLEXES SUPPORTED BY TRIANIONIC
9
II
3-
method. EPR spectra were taken on a JEOL X-band
spectrometer (JES-RE1XE) under non-saturating micro-
wave power conditions (1.0 mW) operating at 9.2 GHz.
The magnitude of the modulation was chosen to optimize
the resolution and the signal to noise ratio (S/N) of the
observed spectrum. The g values were calibrated using
an Mn marker. Elemental analyses were recorded with a
PerkinElmer or a Fisons Instruments EA1108 Elemental
Analyzer.
(NBu )[Pd (L )]. To a CH Cl solution (3.0 mL)
4 2 2
II
containing LH (52.4 mg, 100 mmol) and Pd (CH COO)
3
3
2
(22.5 mg, 100 mmol) was added triethylamine (42 mL,
300 µmol) and the mixture was stirred for an hour at
room temperature. The reaction mixture was added
dropwise to n-hexane (60 mL) to give orange powder.
2
+
The collected powder and (Bu) NBr (322 mg, 1.0 mmol)
was dissolved in MeOH-CH Cl (v:v = 1:1), and
slow evaporation of the solvent gave orange crystals
suitable for X-ray crystallographic analysis (44.0 mg,
4
2
2
-1
X-ray structure determination
51%). IR (KBr): n, cm 2957 (C–H), 1540, 1361
1
(
(
(
NO2). H NMR (CDCl , 400 MHz): d, ppm 0.76
3
Each single crystal was mounted on a loop. Data from
X-ray diffraction were collected at -170°C by a VariMax
withRAPIDimagingplatetwo-dimensionalareadetector,
using graphite-monochromated Mo Ka radiation (l =
+
t, 12H, J = 7.2 Hz (CH CH CH CH ) N ), 1.01
3
2
2
2 4
+
m, 8H, (CH CH CH CH ) N ), 1.34 (m, 27H, CH
3
2
2
2
4
3
+
and (CH CH CH CH ) N ), 2.99 (t, 8H, J = 8.4 Hz,
3
2
2
2 4
+
(
7
CH CH CH CH ) N ), 7.08 (s, 2H, aromatic protons),
3 2 2 2 4
0
.71069 Å) to 2qmax of 55°. All of the crystallographic
-
.54 (s, 2H, aromatic protons), 9.30 (s, 2H, H). MS (ESI ):
m/z 626.15, calcd. for C H N NiO 626.22. Anal. calcd.
calculations were performed using the Crystal Structure
software package of the Molecular Structure Corporation
3
0
25
6
4
for C H N O Pd·0.5H O: C, 64.25; H, 9.06; N, 6.38.
4
7
78
4
4
2
(
Crystal Structure: Crystal Structure Analysis Package
Found C, 64.17; H, 9.29; N, 6.43.
version 3.8.1. Molecular Structure Corp. and Rigaku
Corp. (2005)). The structures were solved with SIR2008
or SHELX97 and refined with CRYSTALS. All non-
hydrogen atoms and hydrogen atoms were refined
anisotropically and isotropically, respectively.
II
·2-
II
[
Pd (L )]. (Et NH)[Pd L] (6.0 mg, 8.2 µmol) in
CH Cl (0.50 mL) was added to a CH Cl solution
0.50 mL) containing AgSbF (98%, 2.8 mg, 8.2 µmol)
with stirring at room temperature. The mixture was added
dropwise to hexane (30 mL) with vigorous stirring. After
reducing the volume of the resulting solution to 3 mL,
the precipitates were filtered off. Evaporation of the
filtrate gave dark red material, from which [Pd (L )]
was obtained as black crystals by recrystallization from
3
2
2
2
2
(
6
Synthesis
II
3-
II
·2-
(
NEt )[Ni (L )]. To an ethanol solution (1.0 mL)
4
II
containing LH (10.5 mg, 20 mmol) and Ni (CH COO) ·
H O (5.0 mg, 20 mmol) was added 1.5 M tetraethyl-
3
3
2
-1
4
hexane (mg, 49%). IR (KBr): n, cm 2952 (C–H), 1547,
1362 (NO ). Anal. calcd. for C H N O Pd: C, 59.37; H,
6.75; N, 6.70. Found C, 59.60; H, 6.84; N, 6.59.
2
ammonium hydroxide in methanol (40 ml, 60 mmol),
and the mixture was stirred for 1 h at room temperature.
Then, the solvent was removed by evaporation, and the
product was recrystallized from the resulting residue
using EtOH/n-hexane to give single crystals suitable
for X-ray crystallographic analysis (6.1 mg, 43%). IR
2
31 42
3
4
CONCLUSION
-
1
1
(
(
(
3
2
KBr): n, cm 2951 (C–H), 1531, 1361 (NO ). H NMR
In this study, a new type of Ni and Pd complexes
supported by the non-innocent b-diketiminate ligand
containing two phenol groups has been developed and
the structures, physical properties, and redox behaviors
of the nickel(II) and the palladium(II) complexes,
2
CDCl , 400 MHz): d, ppm 1.06 (t, 12H, J = 7.2 Hz,
3
+
CH CH ) N ), 1.29 (s, 18H, CH ), 1.44 (s, 18H, CH ),
3
2
4
3
3
+
.23 (q, 8H, J = 7.2 Hz, (CH CH ) N ), 6.97 (d, 2H, J =
.0 Hz, aromatic protons), 7.36 (d, 2H, J = 2.0 Hz,
aromatic protons), 9.00 (s, 2H, H). MS (ESI ): m/z
78.30, calcd. for C H N NiO 578.25. Anal. calcd. for
3 2 4
-
II
3-
-
[M (L )] , have been examined in detail. The one-
II
3-
-
II
·2-
5
electron oxidation of [Ni (L )] gave [Ni (L )], where
the supporting ligand is oxidized to provide nickel(II)-
phenoxyl radical complexes exhibiting the characteristic
absorption spectra in the visible-to-NIR region.
Further one-electron oxidation of [Ni (L )] gave the
two-electron oxidation product [Ni (L )] , where the
3
0
25
6
4
C H N O Ni·EtOH: C, 65.16; H, 9.07; N, 7.41. Found
3
9
62
4
4
C, 65.11; H, 9.31; N, 7.49.
II
·2-
II
[
Ni (L )]. (Et NH)[Ni L] (27.3 mg, 40 mmol) in
3
II
·2-
CH Cl (1.0 mL) was added to a CH Cl solution (1.0 mL)
2
2
2
2
II
-
+
containing AgSbF6 (98%, 13.5 mg, 40 mmol) with
stirring at room temperature. The mixture was added
dropwise to hexane (100 mL) with vigorous stirring.
After reducing the volume of the resulting solution to
oxidation also takes place at the ligand moiety. The one-
II
3-
-
II
·2-
and two-electron oxidation of [Pd (L )] gave [Pd (L )]
III
·2-
+
and [Pd (L )] , respectively, where the supporting
ligand is oxidized in the former, whereas the metal center
is mainly oxidized in the later.
3
mL, the precipitates were filtered off. Evaporation of
II
·2-
the filtrate gave dark red material, from which [Ni (L )]
was obtained as black crystals by recrystallization from
hexane (15.1 mg, 65%). IR (KBr): n, cm 2953 (C–H),
1
6
II
-
+
The two-electron-oxidation complexes [Ni (L )] and
-
1
III
2·-
+
[Pd (L )] exhibited hydrogen atom abstraction ability
from CHD as in the case of the copper complex with same
ligand. However, the reactivity was slightly lower than
561, 1360 (NO ). Anal. calcd. for C H N O Ni: C,
2 31 42 3 4
4.26; H, 7.31; N, 7.25. Found C, 64.12; H, 7.25; N, 7.14.
Copyright © 2015 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2015; 19: 9–11

10
Y. MORIMOTO ET AL.
that of the Cu-complex. The reaction of one-electron-
oxidation complex [Ni (L )] with CHD included
12. Jazdzewski BA and Tolman WB. Coord. Chem.
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II
·2-
photo-induced oxidation of CHD, whereas one-electron-
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·2-
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-
+
III
2·-
+
in the C–H bond activation reaction of CHD is much
higher than that of the one-electron oxidized complex
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·2-
II
·2-
[
Ni (L )] and [Pd (L )]. These results will provide
further insights into the redox functions of transition-
metal organic radical species.
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Acknowledgements
6
This work was supported by a Grant-in-Aid for
Scientific Research on Innovative Areas “Molecular
Activation Directed toward Straightforward Synthesis”
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(No. 22105007) and “Stimuli-responsive Chemical
Species” (No. 24109015) and a Grant-in-Aid for Explor-
atory Research (No. 25620044) from the Ministry of
Education, Culture, Sports, Science and Technology,
Japan. Y.M., N.F., and S.I. also express their special
thanks to the JSPS Japanese-German Graduate Externship
Program on “Environmentally Benign Bio- and Chemical
Processes” for financial support of the stay in RWTH
Aachen.
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Supporting information
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Angew. Chem., Int. Ed. 2007; 46: 5198–5201.
Crystallographic data (Tables S1 and S2) and
additional spectroscopic and kinetic data (Figs S1–S5)
are given in the supplementary material. This material is
available free of charge via the Internet at http://www.
worldscinet.com/jpp/jpp.shtml.
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