Ni(ll)/H2O2 reactivity in bis[(pyridin-2-yl)methyl] amine tridentate ligand system. Aromatic hydroxylation reaction by bis(μ-oxo)dinickel(lll) complex

Article abstract of DOI:10.1021/ic900059m

The nickel(ll) complexes 1^X supported by bis[(pyridin-2-yl) methyl]benzylamine tridentate ligands carrying m-substituted phenyl groups (X = OMe, Me, H, CI, NO₂) at the 6-position of pyridine donor groups (L^X, N,N-bis [(6-m

Full text of DOI:10.1021/ic900059m

Inorg. Chem. 2009, 48,
DOI:10.1021/ic900059m
4997
Ni(II)/H2O2 Reactivity in Bis[(pyridin-2-yl)methyl]amine Tridentate Ligand System.
Aromatic Hydroxylation Reaction by Bis( μ-oxo)dinickel(III) Complex
Atsushi Kunishita,‡ Yoshitaka Doi,‡ Minoru Kubo,§ Takashi Ogura,§ Hideki Sugimoto,‡ and Shinobu Itoh*,†,
†Department of Material and Life Science, Division of Advanced Science and Biotechnology, Graduate School of
Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan, ‡Department of Chemistry,
Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan,
§Research Institute of Picobiology, Graduate School of Life Science, University of Hyogo, 3-2-1 Kouto,
Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan, and Institute for Molecular Science, National Institutes of
Natural Sciences, 38 Nishigo-Naka, Myodaiji, Okazaki 444-8585, Japan
Received January 13, 2009
The nickel(II) complexes 1X supported by bis[(pyridin-2-yl)methyl]benzylamine tridentate ligands carrying
m-substituted phenyl groups (X = OMe, Me, H, Cl, NO2) at the 6-position of pyridine donor groups (LX, N,N-bis
[(6-m-substituted-phenylpyridin-2-yl)methyl]benzylamine) have been synthesized and characterized. The X-ray crystal-
lographic analyses have revealed that [NiII(LH)(CH3CN)(H2O)](ClO4)2 (1H), [NiII(LOMe)(CH3CN)(MeOH)](ClO4)2 (1OMe),
[NiII(LMe)(CH3CN)(H2O)](ClO4)2 (1Me), and [NiII(LCl)(CH3CN)(H2O)](ClO4)2 (1Cl) have a five-coordinate square
NO2
pyramidal geometry, whereas [NiII(LNO )(CH3CN)2(H2O)](ClO4)2 (1 ) exhibits a six-coordinate octahedral geometry having
2
an additional CH3CN co-ligand. 1H NMR spectra of the nickel(II) complexes 1X in CD3CN have indicated that all the
complexes have a high spin ground state. The nickel(II) complexes 1X react with hydrogen peroxide (H2O2) in acetone
to give bis(μ-oxo)dinickel(III) complexes 2X exhibiting a characteristic UV-vis absorption band at ∼420 nm. In the case
of 2H, a resonance Raman band ascribable to a Ni2O2 core vibration was observed at 611 cm-1 that shifted to 586 cm-1
upon H218O2. The bis(μ-oxo)dinickel(III) intermediates 2X undergo an efficient aromatic ligand hydroxylation reaction,
producing a mononuclear nickel(II)-phenolate complexes 4X via a putative intermediate (μ-phenoxo)(μ-hydroxo)dinickel
(II) (3X). The kinetic studies on the aromatic ligand hydroxylation process including m-substituent effects (Hammett
analysis) and kinetic deuterium isotope effects (KIE) have indicated that the reaction of 2X to 3X involves an electrophilic
aromatic substitution mechanism, where C-O bond formation and C-H bond cleavage occur in a concerted manner.
Intermediate 3H was detected by ESI-MS during the course of the reaction, and the final product 4H was characterized by
elemental analysis, ESI-MS, and X-ray crystallographic analysis.
Introduction
complex C, and (tert-butylperoxo)nickel(II) complex D have
been structurally characterized by X-ray crystallographic
analysis, and their physicochemical properties have been
explored in detail.1-4 Formation of (μ-1,2-peroxo)dinickel-
(II) complex E as well as (η1-superoxo)nickel(II) complex
F has also been demonstrated in some ligand systems.5,6
Nickel/active-oxygen complexes have attracted much
recent attention because of their relevance to the dioxygen
activation mechanism by transition metal complexes. So far,
bis(μ-oxo)dinickel(III) complex A, bis(μ-1,2-superoxo)di-
nickel(II) complex B, mononuclear (η2-superoxo)nickel(II)
(2) (a) Yao, S.; Bill, E.; Milsmann, C.; Wieghardt, K.; Driess, M. Angew.
Chem., Int. Ed. 2008, 47, 7110–7113. (b) Otsuka, S.; Nakamura, A.; Tatsuno,
Y. J. Am. Chem. Soc. 1969, 91, 6994–6999.
(3) Fujita, K.; Schenker, R.; Gu, W.; Brunold, T. C.; Cramer, S. P.;
Riordan, C. G. Inorg. Chem. 2004, 43, 3324–3326.
(4) Hikichi, S.; Okuda, H.; Ohzu, Y.; Akita, M. Angew. Chem., Int. Ed.
2009, 48, 118–191.
(5) (a) Kiber-Emmons, M. T.; Schenker, R.; Yap, G. P. A.; Brunold, T. C.;
Riordan, C. G. Angew Chem., Int. Ed. 2004, 43, 6716–6718. (b) Schenker, R.;
Kiber-Emmons, M. T.; Riordan, C. G.; Brunold, T. C. Inorg. Chem. 2005,
44, 1752–1762.
(6) Kiber-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.
*To whom correspondence should be addressed. E-mail: shinobu@
msl.eng.osaka-u.ac.jp.
(1) (a) Hikichi, S.; Yoshizawa, M.; Sasakura, Y.; Akita, M.; Moro-oka, Y.
J. Am. Chem. Soc. 1998, 120, 10567–10568. (b) Itoh, S.; Bandoh, H.;
Nagatomo, S.; Kitagawa, T.; Fukuzumi, S. J. Am. Chem. Soc. 1999, 121,
8945–8946. (c) Shiren, K.; Ogo, S.; Fujinami, S.; Hayashi, H.; Suzuki, M.;
Uehara, A.; Watanabe, Y.; Moro-oka, Y. J. Am. Chem. Soc. 2000, 122,
254–262. (d) Mandimutsira, B. S.; Yamarik, J. L.; Brunold, T. C.; Gu, W.;
Cramer, S. P.; Riordan, C. G. J. Am. Chem. Soc. 2001, 123, 9194–9195.
(e) Itoh, S.; Bandoh, H.; Nakagawa, M.; Nagatomo, S.; Kitagawa, T.;
Karolin, K. D.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 11168–11178.
(f) Schenker, R.; Mandimutsira, B. S.; Riordan, C. G.; Brunold, T. C. J. Am.
Chem. Soc. 2002, 124, 13842–13855.
©
2009 American Chemical Society
Published on Web 4/17/2009
pubs.acs.org/IC

4998 Inorg. Chem., Vol. 48, No. 11, 2009
Kunishita et al.
Scheme 1
With respect to the reactivity of the nickel/active-oxygen
complexes, aliphatic ligand hydroxylation reaction in A has
been examined most extensively to explore that the reaction
involves a hydrogen atom abstraction and oxygen rebound
mechanism.1b,1e Oxygen atom transfer reaction from C, E,
and F to PPh3 has also been examined briefly.3,5,6 The struc-
ture and reactivity of these nickel/active-oxygen complexes
have frequently been discussed in connection with those of
the related copper/active-oxygen complexes.7
Chart 1
As a part of our continuing research on dioxygen activa-
tion chemistry by copper complexes, we have recently exam-
ined H2O2-reactivity of the copper(II) complexes CuIILX
supported by the bis[(pyridin-2-yl)methyl]benzylamine tri-
dentate ligands carrying m-substituted 6-phenyl groups on
the pyridine donor groups (LX) to find the formation of a
unique 2-hydroxy-2-hydroperoxypropane (HHPP) adduct in
acetone (Scheme 1).8,9 From this HHPP-adduct, an aromatic
ligand hydroxylation reaction took place effectively to give a
copper(II)-phenolate complex via an electrophilic aromatic
substitution mechanism (Scheme 1), providing an important
insight into the oxygenation ability of the mononuclear
copper(II)-peroxo species.8,9
In this study, we have extended our research to the nickel-
(II) system using the same ligand LX to evaluate the reactivity
difference between the nickel(II) and the copper(II) com-
plexes. In the nickel system as well, a similar aromatic ligand
hydroxylation reaction took place to give a nickel(II)-phe-
nolate complex 4X. However, the reactive intermediate in this
reaction was found to be a bis(μ-oxo)dinickel(III) complex,
but not a HHPP-adduct of Ni(II). Thus, the results will
provide further insight into the oxygen activation chemistry
by transition metal complexes.
diode array spectrophotometer equipped with a Unisoku ther-
mostatted cell holder USP-203. 1H NMR spectra were recorded
on a JEOL FT-NMR GX-400 spectrometer or a Bruker
AVANCE 600 spectrometer. 2D NOESY spectra were
obtained in a phase-sensitive mode using the pulse sequence
90ꢀ - t1 - 90ꢀ - Δm - 90ꢀ and using TPPI method. The optimum
mixing times Δm cover the range 0.6-0.8 s. Other parameter
settings were as follows: SW = 10080.65 Hz in both dimensions,
2 K data block (f2), 4 scans, and 48 dummy scans in 1024
increments (f1). 1D NOESY spectra were obtained in a phase-
sensitive mode using selective refocusing with a shaped pulse.
The parameter settings were as follows: SW = 9615.38 Hz
spectral width, 64 K data points with a 350 ms shaped pulse
named Gause1.1000 by the Bruker standard version and a 3.41 s
acquisition time. In this method, measuring transient NOE
enhancements which, by the use of pulse field gradients, avoids
the need to compute difference spectra and thus give spectra
which contain no subtraction artifacts. As a result, very small
NOE enhancements can be detected readily and with much
confidence. In all measurements TMS was used as an internal
reference. Mass spectra were recorded on a JEOL JMS-700T
Tandem MS-station mass spectrometer. Resonance Raman
scattering was excited at 406.7 nm from Kr+ laser (Spectra
Physics, BeamLok 2060). Resonance Raman scattering was
dispersed by a single polychromator (Ritsu Oyo Kogaku,
MC-100) and was detected by a liquid nitrogen cooled CCD
detector (Roper Scientific, LNCCD-1100-PB). The resonance
Raman measurements were carried out using a rotated cylind-
rical cell thermostatted at -80 ꢀC or a rotating NMR tube
(outer diameter = 5 mm) thermostatted at -90 ꢀC by flashing
cold nitrogen gas. A 135o backscattering geometry was used.
Elemental analyses were recorded with a Perkin-Elmer or a
Fisons instruments EA1108 Elemental Analyzer.
Experimental Section
General Information. The reagents and the solvents used in
this study, except the ligands and the nickel complexes, were
commercial products of the highest available purity and were
further purified by the standard methods, if necessary.10
N,N-bis[(6-p-substituted-phenylpyridin-2-yl)methyl]benzylam-
ine ligands (LX) were prepared by the reported methods.11 FT-
IR spectra were recorded on a Jasco FTIR-4100, and UV-
visible spectra were taken on a Hewlett-Packard 8453 photo
Synthesis of Nickel(II) Complexes. Caution! The perchlo-
rate salts in this study are all potentially explosive and should be
handled with care.
[NiII(LH)(CH3CN)(H2O)](ClO4)2 (1H). NiII(ClO4)2 6H2O
(7) Suzuki, M. Acc. Chem. Res. 2007, 40, 609–617.
3
(145.6 mg, 0.40 mmol) was added to a CH3CN-CH3OH (1:1)
solution (5 mL) of ligand LH (176.4 mg, 0.40 mmol). After
stirring for 10 min at room temperature, insoluble material was
removed by filtration. Addition of ether (100 mL) to the filtrate
gave blue powder that was precipitated by standing the mixture
for several minutes. The supernatant was then removed by
decantation, and the remaining blue solid was washed with
(8) Kunishita, A.; Teraoka, J.; Scanlon, J. D.; Matsumoto, T.; Suzuki, M.;
Cramer, C. J.; Itoh, S. J. Am. Chem. Soc. 2007, 129, 7248–7249.
(9) Kunishita, A.; Scanlon, J. D.; Ishimaru, H.; Honda, K.; Ogura, M.;
Suzuki, M.; Cramer, C. J.; Itoh, S. Inorg. Chem. 2008, 47, 8222–8232.
(10) Armarego, W. L. F.; Perrin, D. D. In Purification of Laborator
Chemicals, 4th ed.; Butterworth-Heinemann: Oxford, 1996; pp 176, 215.
(11) Kunishita, A.; Osako, T.; Tachi, Y.; Teraoka, J.; Itoh, S. Bull. Chem.
Soc. Jpn. 2006, 79, 1729–1741.

Article
Inorg. Chem., Vol. 48, No. 11, 2009
4999
ether three times and dried to give complex 1H in 88%. FT-IR
(KBr) 3416 cm-1 (OH), 1124, 1054, and 626 cm-1 (ClO4-);
HR-MS (FAB, pos.) m/z = 499.1549 calcd for C31H27N3Ni:
499.1558; Anal. Calcd for [NiII(LH)(CH3CN)(H2O)](ClO4)2
(C33H32NiCl2N4O9): C, 52.27; H, 4.25; N, 7.39. Found:
C, 52.02; H, 4.21; N, 7.28.
from which the hydroxylated ligand LH-OH was isolated
as an oily material by silica gel column chromatography (eluent:
Hexane/AcOEt = 6: 1). 1H NMR (400 Hz, CDCl3) δ 3.78 (2 H,
s, -CH2-Ph), 3.89 (2 H, s, -N-CH2-Py), 3.92 (2 H, s, -N-CH2-Py),
6.89 (1 H, t, J = 8.0 Hz), 7.04 (1 H, d, J = 8.0 Hz), 7.20-7.25
(1 H, m), 7.27-7.37 (3 H, m), 7.39 (1 H, d, J = 8.0 Hz), 7.43
(1 H, s), 7.46 (4 H, t, J = 6.4 Hz), 7.57 (2 H, t, J = 7.4 Hz), 7.72
(1 H, d, J = 7.6 Hz), 7.74-7.81 (3 H, m), 7.98 (2 H, d,
J = 8.0 Hz).; HR-MS (FAB, pos) m/z = 458.2229, calcd for
C31H28ON3 458.2232.
[NiII(LOMe)(CH3CN)(MeOH)](ClO4)2(1OMe). This co-
mpound was synthesized in a similar manner using ligand
LOMe (100.2 mg, 0.2 mmol) instead of LH as green pow-
der in 77%. FT-IR (KBr) 3358 cm-1 (OH), 1254 cm-1(OMe),
1095, 1041, and 616 cm-1 (ClO4-); HR-MS (FAB, pos.)
m/z = 559.1770 calcd for C33H31N3NiO2: 559.1768; Anal. Calcd
The yield of hydroxylated ligand LH-OH was determined as
48% based on the Ni(II) starting material by using an integral ratio
in the 1H NMR spectrum between the benzylic proton (-CH2-) at
δ 3.92 of LH-OH and that of the original ligand LH at δ 3.96.
for [NiII(LOMe)(CH3CN)(CH3OH)](ClO4)2 H2O (C36H40NiCl2N4O12):
3
C, 50.85; H, 4.74; N, 6.59. Found: C, 51.06; H, 4.46; N, 6.51.
[NiII(LMe)(CH3CN)(H2O)](ClO4)2(1Me). This compound
was synthesized in a similar manner using ligand LMe (93.8 mg,
0.2 mmol) instead of LOMe as green powder in 67%. FT-IR
(KBr) 3387 cm-1 (OH), 1119, 1044, and 625 cm-1 (ClO4-); HR-
MS (FAB, pos.) m/z = 527.1866 calcd for C33H31N3Ni:
Product Analysis of Hydroxylated Ligand LMe-OH. The
reaction of 1Me and H2O2 was carried out under the same
conditions as described for 1H (see above). The same workup
treatment gave a yellow oily material, from which the hydro-
xylated ligand LMe-OH was isolated as oil by silica gel column
chromatography (eluent: Hexane/AcOEt = 5: 1). The detailed
1H NMR data are presented in Supporting Information,
Figure S26; HR-MS (FAB, pos) m/z = 486.2545, calcd for
C33H32ON3 486.2549.
Product Analysis of Hydroxylated Ligand LCl-OH.
The reaction of 1Cl and H2O2 was carried out under the
same conditions as described for 1H (see above). The same
workup treatment gave a yellow oily material, from which the
hydroxylated ligand LCl-OH was isolated as oil by silica gel
column chromatography (eluent: CHCl3). The detailed 1H
NMR data are presented in Supporting Information, Figures
S27 and S28; HR-MS (FAB, pos) m/z = 526.1455, calcd for
C31H26ON3Cl2 526.1453.
527.1871; Anal. Calcd for [NiII(LMe)(CH3CN)(H2O)](ClO4)2
H2O (C35H38NiCl2N4O10): C, 52.27; H, 4.76; N, 6.97. Found:
C, 52.10; H, 4.43; N, 6.58.
3
[NiII(LCl)(CH3CN)(H2O)](ClO4)2 (1Cl). This compound
was synthesized in a similar manner using ligand LCl (102.8 mg,
0.2 mmol) instead of LMe as green powder in 60%. Single
crystals of 1Cl suitable for X-ray crystallographic analysis
were obtained by vapor diffusion of ether into a CH3CN-
CH3OH (1: 1) solution of the complex. FT-IR (KBr) 3426
cm-1 (OH), 1122, 1044, and 626 cm-1 (ClO4-); HR-MS
(FAB, pos.) m/z = 567.0756 calcd for C31H25Cl2N3Ni:
567.0779; Anal. Calcd for [NiII(LCl)(CH3CN)(H2O)](ClO4)2
3
CH3CN H2O (C35H35NiCl4N5O10): C, 47.44; H, 3.98; N,
7.90. Found: C, 47.17; H, 4.10; N, 7.93.
X-ray Structure Determination. Single crystals of 1X (X =
H, OMe, Me, Cl, and NO2) and 4H suitable for X-ray crystal-
lographic analysis were obtained by vapor diffusion of ether into
an CH3CN-CH3OH (1:1) solution of the complex (in the case
of 4H, acetone solution). Each single crystal was mounted on
a glass-fiber. Data of X-ray diffraction were collected by a
Rigaku RAXIS-RAPID imaging plate two-dimensional area
detector using graphite-monochromated Mo KR radiation
3
NO2
[NiII(LNO )(CH3CN)2(H2O)](ClO4)2 (1
). This com-
2
2
pound was synthesized in a similar manner using ligand LNO
(106.3 mg, 0.2 mmol) instead of LCl as green powder in 87%.
FT-IR (KBr) 3378 cm-1 (OH), 1541 and 1358 cm-1 (NO2-),
1122, 1044, and 627 cm-1 (ClO4-); HR-MS (FAB, pos.) m/z =
589.1250 calcd for C31H25N5NiO4: 589.1260; Anal. Calcd for
[NiII(LNO )(CH3CN)2(H2O)](ClO4)2 (1/2)H2O (C35H34NiCl2N7O13.5):
C, 46.80; H, 3.82; N, 10.91. Found: C, 46.85; H, 3.95; N, 11.15.
2
˚
3
(λ = 0.71069 A). All the crystallographic calculations were
performed by using Crystal Structure software package of the
Molecular Structure Corporation [Crystal Structure: Crystal
Structure Analysis Package version 3.8.1, Molecular Structure
Corp. and Rigaku Corp. (2005)]. The structures were solved
with SIR92 and refined with CRYSTALS. All non-hydrogen
atoms and hydrogen atoms except disordered atoms and C6 and
C8 atoms of the substituent on the one pyridyl group in 1Me were
refined anisotropically. Hydrogen atoms were attached on
the atoms refined anisotropically and refined isotropically.
Atomic coordinates, thermal parameters, and intramolecular
bond distances and angles of the complexes are deposited in the
Supporting Information (CIF file format).
[NiII(LHO)](ClO4) (4H). An acetone solution (50 mL) of 1H
(75.6 mg, 0.2 mM) was cooled to -90 ꢀC using a dry ice-AcOEt
bath. Then, 30% H2O2 aqueous solution (0.5 equiv) and Et3N
(1 equiv) were added to the solution. The resulting mixture was
stirred for 1 h at -90 ꢀC and then gradually warmed up to room
temperature. After stirring for additional 30 min at room
temperature, the solvent was removed under reduced pressure
to give a brown residue, to which ether (100 mL) was added.
The resulting brown powder was precipitated by standing the
mixture for several minutes. The supernatant was then removed
by decantation, and the remaining green-brown solid was
washed with ether three times and dried to give 4H in 50%.
FT-IR (KBr) 1102 and 626 cm-1 (ClO4-); HR-MS (FAB, pos)
514.1434 calcd for C31H26N3NiO: 514.1429; Anal. Calcd for
Kinetic Measurements. The reaction of nickel(II) com-
plexes with H2O2 was performed in a 1.0 cm path length
UV-vis cell that was held in a Unisoku cryostat cell holder
USP-203. After an acetone solution containing the nickel(II)
complex (0.6 mM) and Et3N (1 equiv) was kept at a desired
temperature for several minutes, H2O2 (1 equiv) in acetone was
injected into the cell through a septum rubber cap with use of a
microsyringe. The reaction was monitored by following an
increase of the characteristic absorption band due to the nickel
active-oxygen complexes.
[NiII(LOH)](ClO4) (1/2)H2O (C31H27NiCl1N3O5.5): C, 59.70; H,
3
4.36; N, 6.74. Found: C, 60.08; H, 4.25; N, 6.92. Isotope labeling
experiment was performed using H218O2 instead of H216O2 by
the same method as described above. HRMS m/z = 516.1475
calcd for 516.1472; (C31H26NiN318O).
Product Analysis of Hydroxylated Ligand LH-OH. After
the reaction described above, the reaction mixture was acidified
to pH 1 by adding conc. HCl. Removal of the solvent by
evaporation gave an oily material, which was dissolved into
an NH3 aqueous solution. The aqueous solution was then
extracted with ether (20 mL ꢀ 5), and the combined organic
layer was dried over Na2SO4. After removal of Na2SO4
by filtration, evaporation of the solvent gave a yellow material,
Results and Discussion
Characterization of Nickel(II) Starting Materials (1X;
X = OMe, Me, H, Cl, and NO2). The nickel(II) comp-
lexes 1X were prepared by treating the ligand LX with

5000 Inorg. Chem., Vol. 48, No. 11, 2009
Kunishita et al.
Figure 1. ORTEPdrawings ofthe cationicpartsof(A)[NiII(LH)(CH3CN)(H2O)](ClO4)2 (1H) and(B)[NiII(LNO )(CH3CN)2(H2O)](ClO4)2 (1 ) showing
NO2
2
50% probability thermal ellipsoids. The hydrogen atoms are omitted for clarity.
Table 1. Summary of the X-ray Crystallographic Data of Complexes 1H and 1NO
Table 2. Selected Bond Lengths (A) and Angles (deg) of Complexes 1H and 1NO
2
2
˚
complex 1H
complex 1NO
2
Complex 1H
formula
C33H29N3NiCl2O8
725.21
C36H35.5N7.5NiCl2O13.5
918.82
Ni(1)-N(1)
Ni(1)-N(3)
Ni(1)-O(1)
2.082(5) Ni(1)-N(2)
2.034(5) Ni(1)-N(4)
2.027(5)
2.061(5)
2.019(5)
formula weight
crystal system
space group
monoclinic
C2/c (No. 15)
39.60(3)
10.404(5)
18.200(9)
90
116.311(19)
90
6722(6)
8
monoclinic
P2/a (No. 13)
17.660(12)
11.152(7)
20.295(11)
90
98.86(2)
90
3949(4)
4
˚
a, A
N(1)-Ni(1)-N(2) 165.4(2)
N(1)-Ni(1)-N(3)
N(2)-Ni(1)-N(3)
83.4(2)
82.0(2)
˚
b, A
N(1)-Ni(1)-N(4)
N(2)-Ni(1)-N(4)
O(1)-Ni(1)-N(1)
O(1)-Ni(1)-N(3)
88.7(2)
96.2(2)
˚
c, A
N(3)-Ni(1)-N(4) 104.3(2)
87.9(2)
R, deg
β, deg
γ, deg
92.99(19) O(1)-Ni(1)-N(2)
98.83(19) O(1)-Ni(1)-N(4) 156.8(2)
3
Complex 1NO
˚
2
V, A
Z
F(000)
Ni(1)-N(1)
Ni(1)-N(3)
Ni(1)-N(5)
2.155(3) Ni(1)-N(2)
2.124(2) Ni(1)-N(4)
2.058(3) Ni(1)-O(1)
2.151(3)
2.062(3)
2.100(3)
2992.00
1.433
153
1896.00
1.545
153
Dcalcd, g/cm-3
T, K
crystal size, mm
μ (MoKR), cm-1
2θmax, deg
no. of reflns measd
no. of reflns obsd
no. of variables
R a
0.20 ꢀ 0.15 ꢀ 0.10
0.50 ꢀ 0.5 ꢀ 0.4
N(1)-Ni(1)-N(2) 161.31(11) N(1)-Ni(1)-N(3)
80.84(11)
96.34(13)
88.42(13)
95.66(12)
94.51(14)
89.93(12)
7.903
55.0
31175
7.035
54.9
34938
N(1)-Ni(1)-N(4)
N(2)-Ni(1)-N(3)
93.34(12) N(1)-Ni(1)-N(5)
80.47(12) N(2)-Ni(1)-N(4)
N(2)-Ni(1)-N(5) 102.08(13) N(3)-Ni(1)-N(4)
N(3)-Ni(1)-N(5) 169.58(13) N(4)-Ni(1)-N(5)
5430 ([I > 0.20σI)])
472
0.0417
5742 ([I > 1.50σ(I)])
552
0.0577
O(1)-Ni(1)-N(1)
O(1)-Ni(1)-N(3)
O(1)-Ni(1)-N(5)
89.12(12) O(1)-Ni(1)-N(2)
86.79(11) O(1)-Ni(1)-N(4) 176.77(12)
83.12(12)
b
Rw
GOF
0.0776
1.038
0.0837
0.903
P
P
P
P
a R = ||Fo| - |Fc||/ |Fo|. b Rw = [ w(|Fo| - |Fc|)2/ wFo2]1/2
.
1
Me, and 1Cl take a similar five coordinate structure
involving H2O (CH3OH in place of H2O in 1OMe
and CH3CN as the co-ligands (Supporting Informa-
NiII(ClO4)2 6H2O in CH3CN-MeOH (1:1). The crystal
)
3
structures of [NiII(LH)(CH3CN)(H2O)](ClO4)2 (1H) and
[NiII(LNO )(CH3CN)2(H2O)](ClO4)2 (1NO ) are shown
in Figure 1 as the typical examples. The crystallographic
data and the selected bond lengths and angles of
those complexes are summarized in Tables 1 and 2,
tion, Figures S1-S3). On the other hand, 1NO shows a
2
2
2
six-coordinate octahedral geometry having additional
CH3CN co-ligand (Figure 1B). In this case, one of the
aromatic substituents connected to the 6-position of
pyridine ring is disordered. The strong electron-with-
drawing NO2 group on the aromatic substituents may
reduce the electron-donor ability of pyridine, causing an
increase of Lewis acidity of the Ni(II) center. This may
enhance the binding ability of the metal ion to the external
co-ligands, taking the six-coordinate structure. All of the
respectively, and those data of other complexes 1OMe
,
1
Me, and 1Cl are presented in Supporting Information,
Figures S1-S3, Tables S1 and S2. The nickel(II) complex
1H exhibits a five-coordinate square pyramidal geometry
(τ = 0.12),12 where two pyridine nitrogen atoms N(1) and
N(2), oxygen atom O(1) of H2O, and nitrogen atom
N(4) of CH3CN occupy each corner of the equatorial
plane and tertiary amine nitrogen atom N(3) exists at
1
Ni(II) complexes exhibit several H NMR resonances
spread over a chemical shift range of ∼60 ppm (Support-
ing Information, Figures S4-S8), indicating a high-spin
ground-state of the complexes (S = 1).
the axial position (Figure 1A). Ni(II) complexes 1OMe
,
Reaction of 1X and H2O2. Addition of 1 equiv of H2O2
into an acetone solution of the nickel(II) complex 1H
(12) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G.
C. J. Chem. Soc., Dalton Trans. 1984, 1349–1356.

Article
Inorg. Chem., Vol. 48, No. 11, 2009
5001
Figure 2. UV-vis spectra of 1H (0.6 mM) and 2H generated by the
reaction of 1H and H2O2 (0.6 mM) in the presence of NEt3 (0.6 mM) in
acetone at -90 ꢀC.
Figure 4. Spectral change for the conversion of 2H (0.6 mM) to 3H in
acetone at -60 ꢀC.
Figures S10 and S11). Thus, it can be concluded that the
reactivity of the Ni(II) complex of LX toward H2O2 is
quite different from that of the Cu(II) complex of the
same ligand.
Similar UV-vis spectra were obtained in the reac-
tions of 1Me, 1Cl, and 1NO with H2O2 under the same
2
experimental conditions (Supporting Information, Fig-
ures S12-S14), indicating that similar bis(μ-oxo)
dinickel(III) complexes 2Me, 2Cl, and 2NO , were also
2
generated in those reactions. It should be noted, however,
that the λmax and ε values of those complexes are not
exactly identical to each other: λmax = 406 nm (ε =
4600 M-1 cm-1) for 2H; 415 nm (3530 M-1 cm-1) for 2Me
;
Figure 3. Resonance Raman spectra of the reaction solution of 1H with
H126O2 (solid line) and with H218O2 (dashed line) obtained with an
excitation wavelength of 406.7 nm in acetone at -90 ꢀC: “s” denotes a
solvent Raman band.
420 nm (ε = 2590 M-1 cm-1) for 2Cl; λmax = 413 nm
(ε = 3540 M-1 cm-1) for 2NO , suggesting the existence of
2
interaction between the Ni2O2 core and the m-substituent
on the phenyl group of the ligand. In the case of 1OMe, on
the other hand, the bis(μ-oxo)dinickel(III) complex 2OMe
was not accumulated during the course of the reac-
tion because of the fast follow-up reaction as described
below.
(0.6 mM) in the presence of triethylamine (1 equiv) at a
low temperature resulted in a color change from blue to
brown. Figure 2 shows a spectral change of the reaction,
where intermediate 2H exhibiting a characteristic absorp-
tion band at 406 nm (ε = 4600 M-1 cm-1) readily
appears. The characteristic absorption band around
400 nm and its ε value are fairly close to those of the
reported bis(μ-oxo)dinickel(III) complexes supported by
similar tridentate pyridylalkylamine ligands and have
been assigned to oxo-to-metal charge transfer (LMCT)
transition.1b,1e The resonance Raman spectrum measured
at -90 ꢀC with λex = 406.7 nm exhibited a charac-
teristic peak at 611 cm-1 that shifted to 586 cm-1 upon
H128O2 substitution (Figure 3). The appearance of Raman
bands in the 600 cm-1 region and their associated isotope
shifts (Δν = 25 cm-1) are also very close to those of
the reported bis(μ-oxo)dinickel(III) complexes, where the
band has been assigned to a Ni2O2 core vibration.1 In
addition, intermediate 2H was ESR silent. All these
spectroscopic characteristics unambiguously suggest that
2H is a bis(μ-oxo)dinickel(III) complex (A in Chart 1), but
not a HHPP (2-hydroxy-2-hydroperoxypropane) adduct
of Ni(II).13 In support of this, identical UV-vis and
resonance Raman spectra were obtained in the reaction
of 1H and H2O2 in propionitrile at -90 ꢀC, where no
acetone molecule is available (Supporting Information,
Aromatic Ligand Hydroxylation from 2X. Bis(μ-oxo)
dinickel(III) complex 2H gradually decomposed, when
the temperature of the solution was raised to -60 ꢀC.
Figure 4 shows a spectral change for the conversion of 2H
to 3H, where a characteristic absorption band at 343 nm
appears with concomitant decrease of the absorption
band at 406 nm due to 2H (the shoulder around 400 nm
may be due to a contamination of the final product 4X, see
Figure 6). Similar spectral changes were obtained in the
case of 2Me, 2Cl, and 2NO as shown in Supporting
2
Information, Figures S15-S17. On the other hand,
3
OMe was generated directly from 1OMe without accumu-
lation of the bis(μ-oxo)dinickel(III) intermediate 2OMe
even at the lower temperature (-90 ꢀC) (Supporting
Information, Figure S18).
The reaction of 2X to 3X (X = Me, H, Cl, and NO2)
obeyed first-order kinetics and the first-order rate con-
stants kobs were determined as 5.6 ( 0.09 ꢀ 10-2 s-1
,
,
1.1 ( 0.001 ꢀ 10-3 s-1, 7.5 ( 0.12 ꢀ 10-4 s-1
and 5.2 ( 0.24 ꢀ 10-4 s-1 at -60 ꢀC, respectively,
from the plots of ln(A - A¥) versus time (sec). The
Hammett analysis using these rate constants and the
substituent constants σ+ gave a F value as -1.5 (Support-
ing Information, Figure S19), suggesting that the reaction
involves an electrophilic aromatic substitution mechan-
(13) Instability of 2H prevents us from getting ESI-MS date even at the
low temperature.

5002 Inorg. Chem., Vol. 48, No. 11, 2009
Kunishita et al.
Figure 6. Spectral change for the conversion of 3H to 4H in acetone at
room temperature.
Figure 5. Experimental (top) and calculated (bottom) peak envelopes
in the positive-ion electrospray mass spectra of [NiI2I(LH)(LHO)(OH)]2+
and [NiI2I(LH)2(OH)2]2+ in a 1:0.6 ratio and {[NiI2I(LH)(LHO)(OH)]
(ClO4)}+ and {[Ni2II(LH)2(OH)2](ClO4)}+ in a 1:0.6 ratio.
ism as in the case of the arene hydroxylation reaction
by (μ-η2:η2-peroxo)dicopper(II) and bis(μ-oxo)dicopper
(III) complexes.14,15
Figure 7. ORTEP drawing of the cationic part of 4H showing 50%
probability thermal ellipsoids. The hydrogen atoms are omitted for
clarity.
The reaction was then monitored by ESI-MS at low
temperature (Figure 5), where peak clusters at 515.3-
520.3 and 1129.2-1141.2 were detected. The former peak
cluster could be simulated by assuming the existence
of [NiI2I(LH)(LHO)(OH)]2+ and [NiI2I(LH)2(OH)2]2+ in a
1:0.6 ratio, where LHO denotes the deprotonated form
of a hydroxylated ligand. Another peak cluster at
1129.2-1141.2 could also be reproduced by adding the
simulation spectra of [NiI2I(LH)(LHO)(OH)](ClO4)+ and
[NiI2I(LH)2(OH)2](ClO4)+ in a 1:0.6 ratio.
When the temperature of the reaction mixture was
raised to room temperature, further spectral change was
observed as shown in Figure 6. The final spectrum
exhibiting an intense absorption band at 403 nm was
identical to the spectrum of [NiII(LHO)](ClO4) (4H),
which was isolated from the final reaction mixture in a
50% yield from a preparative scale (the corresponding
spectral changes for ligand hydroxylation of other com-
plexes are shown in Supporting Information, Figures
S20-S23). In Figure 7 is shown the crystal structure of
the isolated 4H together with the crystallographic data
and the selected bond lengths and angles listed in Tables 3
and 4, respectively. As clearly seen in Figure 7, one of the
6-phenyl substituents on the pyridine rings is hydroxy-
lated at its o-position, and the nickel(II) center takes a
square planar structure with a N3O donor set.16 The ESI-
MS of final reaction mixture with H216O2 gave a set of
peaks at 514.2 corresponding to 4H which shifted to 516.3
upon H218O2 substitution (Supporting Information,
Figure S24). The result clearly demonstrated that the
oxygen atom introduced into the hydroxylated ligand
derived from H2O2 used for the generation of 2H.
In the case of meta-substituted derivatives (X ¼ H), the
position of hydroxylation could be ortho and/or para
relative to the substituent X. To test this possibility,
detailed 1H NMR analyses were carried out on the mod-
ified ligands LMe-OH and LCl-OH isolated from the pre-
parative scale reactions of 1Me and 1Cl with H2O2,
respectively (see Experimental Section). In Supporting
Information, Figures S26-S28 are shown the detailed 1H
NMR data (chemical shift, multiplicity, coupling constant,
1D NOESY and NOESY correlations) determined by the
(14) (a) Itoh, S.; Kumei, H.; Taki, M.; Nagatomo, S.; Kitagawa, T.;
Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 6708–6709. (b) Palavicini, S.;
Granata, A.; Monzani, E.; Casella, L. J. Am. Chem. Soc. 2005, 127,
18031–18036. (c) Matsumoto, T.; Furutachi, H.; Kobino, M.; Tomii, M.;
Nagatomo, S.; Tosha, T.; Osako, T.; Fujinami, S.; Itoh, S.; Kitagawa, T.;
Suzuki, M. J. Am. Chem. Soc. 2006, 128, 3874–3875.
(15) (a) Holland, P. L.; Rodgers, K. R.; Tolman, W. B. Angew. Chem., Int.
Ed. 1999, 38, 1139–1142. (b) Mirica, L. M.; Vance, M.; Rudd, D. J.; Hedman,
B.; Hodgson, K. O; Solomon, E. I.; Stack, T. D. P. Science 2005, 308,
1890–1892.
(16) Four-coordinate Ni(II) complex 4H should be diamagnetic (low-
spin). However, the compound exhibited several 1H NMR resonances spread
over a chemical shift range from -3 to 30 ppm in CD3CN (Supporting
Information, Figure S9). This may be due to coordination of the solvent
molecule(s) to the metal center in the NMR sample solution, which makes
the complex with a five or six coordinate structure having a high-spin state.

Article
Inorg. Chem., Vol. 48, No. 11, 2009
5003
Table 3. Summary of the X-ray Crystallographic Data of Complex 4H
Scheme 2
complex 4H
formula
C30H27N3NiClO5
603.71
formula weight
crystal system
space group
monoclinic
P21/n (No. 14)
13.204(7)
11.057(7)
18.147(12)
90
90.24(2)
90
2649(3)
4
˚
a, A
˚
b, A
˚
c, A
R, deg
β, deg
γ, deg
3
˚
V, A
Z
F(000)
1252.00
1.513
153
Dcalcd, g/cm-3
T, K
crystal size, mm
μ (MoKR), cm-1
2θmax, deg
no. of reflns measd
no. of reflns obsd
no. of variables
Ra
0.15 ꢀ 0.15 ꢀ 0.10
8.803
54.9
24188
4227([I > 0.30σ(I)])
396
0.0318
b
Rw
GOF
0.0371
1.028
P
P
P
P
a R = ||Fo| - |Fc||/ |Fo|. b Rw = [ w(|Fo| - |Fc|)2/ wFo2]1/2
.
detected during the formation of 2X from 1X, the titration
experiment for the formation of 2X from 1X definitely
confirmed that the stoichiometry of 1X to H2O2 was 1:1
(Supporting Information, Figure S25). This suggests that
LXNiII-OOH is a precursor of 2X.
H
˚
Table 4. Selected Bond Lengths (A) and Angles (deg) of Complex 4
Complex 4H
Ni(1)-N(1)
Ni(1)-N(3)
1.921(3)
1.910(3)
Ni(1)-N(2)
Ni(1)-O(1)
1.854(3)
1.793(2)
At a higher temperature (-60 ꢀC), 2X gradually de-
composes to give (μ-phenoxo)(μ-hydroxo)dinickel(II)
complex [NiI2I(LX) (LXO)(OH)]2+ (3X), where one of the
ligands LX was hydroxylated at its 6-phenyl groups
to give LXO. From 3X mononuclear nickel(II) complex
4X supported by the modified ligand LXO is released
with concomitant formation of the bis(μ-hydroxo)
dinickel(II) product [NiI2I(LX)2(OH)2]2+ at room temperature.
[NiI2I(LX)(LXO)(OH)]2+ (3X) and [NiI2I(LX)2(OH)2]2+ were
detected by the ESI-MS during the course of the reaction
(Figure 5), and the formation of 4X was definitely con-
firmed by X-ray crystallographic analysis (Figure 7).
How about the mechanism of the aromatic ligand
hydroxylation process (2X to 3X)? The observed F value
(-1.5) in the Hammett analysis (Supporting Information,
Figure S19) suggests that the reaction involves an electro-
philic aromatic substitution mechanism as demonstrated
in the related arene hydroxylation reaction by (μ-η2:
η2-peroxo)dicopper(II) and bis(μ-oxo)dicopper(III)
complexes.14,15 The higher reactivity of 2OMe having
a strong electron-donating substituent (X=OMe), and
the observed regio-selectivity of the aromatic hydroxyla-
tion reaction (para-position relative to the electron-do-
nating substituent X = Me) are consistent with this
mechanism. In this case, however, appreciable amount
of KIE (kinetic deuterium isotope effect) of 1.8 was
obtained using perdeuterated ligand LH-d10 (replacing
all protons of the 6-phenyl groups) (Supporting Informa-
tion, Figure S29). This may indicate that the deprotona-
tion by another oxygen atom of the bis( μ-oxo) core is also
involved in the rate-determining step. Thus, we propose
that the C-O bond formation and deprotonation occur
in a concerted manner as illustrated in Scheme 3. This is
the first example of the aromatic hydroxylation reaction
by a bis(μ-oxo)dinickel(III) species, providing important
N(1)-Ni(1)-N(2)
N(2)-Ni(1)-N(3)
O(1)-Ni(1)-N(2)
172.79(13)
88.54(13)
95.36(12)
N(1)-Ni(1)-N(3)
O(1)-Ni(1)-N(1)
O(1)-Ni(1)-N(3)
84.63(13)
91.12(12)
171.54(12)
COSY and NOESY measurements (see Experimental
Section). As clearly demonstrated by the data, aromatic
hydroxylation in 4Me took place selectively at the para-
position relative to the methyl substituent (X), whereas
the hydroxylation occurred at both the para- and ortho-
positions in 4Cl in a 1:0.3 ratio. The higher regio-selectivity
in the X = Me system may be due to the steric effect of the
substituent. Namely, the larger methyl substituent may
prohibit the electrophilic attack to the ortho-position
by the oxo group in 2Me
.
Mechanistic Consideration. On the basis of these results,
we propose a possible reaction pathway shown in Scheme 2.
The reaction of nickel(II) starting material 1X and H2O2 in
the presence of triethylamine as the base at a very low
temperature (-90 ꢀC) provides the bis(μ-oxo)dinickel(III)
complex 2X, the formation of which has been well confirmed
by UV-vis and resonance Raman spectra (Figures 2 and 3).
This is in sharp contrast to the copper system CuIILX, where
the reaction with H2O2 provided the 2-hydroxy-2-hydro-
peroxypropane (HHPP) adduct in acetone (Scheme 1) and
the copper(II)-hydroperoxo complex (LXCuII-OOH) in
propionitrile.8,9 Thus, the reactivity of the nickel(II) com-
plex NiIILX toward H2O2 is quite different from that of the
copper(II) complex of the same ligand LX. The reason for
such a difference in the H2O2-reactivity between these two
systems is not clear at present. Accessibility of the higher
oxidation state of nickel(III) as compared to copper(III)
may enhance the O-O bond cleavage of the initially formed
hydroperoxo-adduct. Although formation of such hydro-
peroxo adduct of Ni(II) (LXNiII-OOH) could not be

5004 Inorg. Chem., Vol. 48, No. 11, 2009
Kunishita et al.
Scheme 3
Acknowledgment. The authors thank Dr. Matsumi
Doe of Osaka City University for her assistance
in obtaining the 1H NMR data of LMe-OH and
LCl-OH. This work was financially supported in part
by Grants-in-Aid for Scientific Research on Priority
Area (Nos. 19020058, 20036044, and 20037057 for S.I.)
and Global Center of Excellence (GCOE) Program
(“Picobiology: Life Science at Atomic Level” to T.O.)
from MEXT, Japan and by Grants-in-Aid for Scientific
Research (No. 20350082 for S.I.) from JSPS, Japan.
The authors also thank Asahi Glass Foundation for
the financial support and JSPS Research Fellowship
(for A.K.).
insights into the monooxygenation activity of transition-
metal active oxygen complexes.17
Supporting Information Available: Further details are given
in Figures S1-S29 and in a CIF file. This material is available
free of charge via the Internet at http://pubs.acs.org.
(17) Honda, K.; Cho, J.; Matsumoto, T.; Roh, J.; Furutachi, H.; Tosha,
T.; Kubo, M.; Fujinami, S.; Ogura, T.; Kitagawa, T.; Suzuki, M. Angew.
Chem. Int. Ed. Advanced Article.