Photochemical hydrogen production from 3d transition-metal complexes bearing o-phenylenediamine ligands

Article abstract of DOI:10.1016/j.jphotochem.2015.05.028

A series of 3d transition-metal complexes bearing o-phenylenediamine (opda) ligands [M(opda)₃]²⁺ (M = Mn²⁺, Co²⁺, Ni²⁺, and Zn²⁺) were systematically synthesized as perchlorate salts and th

Full text of DOI:10.1016/j.jphotochem.2015.05.028

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JPC 9916 No. of Pages 8
Journal of Photochemistry and Photobiology A: Chemistry xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Photochemical hydrogen production from 3d transition-metal
complexes bearing o-phenylenediamine ligands
Masaki Yoshida , Show Ueno , Yuka Okano , Akane Usui , Atsushi Kobayashia,b,
a
a
a
a
a,
Masako Kato *
a
Department of Chemistry, Faculty of Science, Hokkaido University, North-10 West-8, Kita-ku, Sapporo 060-0810, Japan
PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
b
A R T I C L E I N F O
A B S T R A C T
2
+
Article history:
A series of 3d transition-metal complexes bearing o-phenylenediamine (opda) ligands [M(opda) ]
(
3
Received 22 March 2015
Received in revised form 14 May 2015
Accepted 21 May 2015
Available online xxx
2+
2+
2+
2+
M = Mn , Co , Ni , and Zn ) were systematically synthesized as perchlorate salts and their structures
were determined. All the opda complexes exhibited hydrogen-evolving activities under photoirradiation
conditions as in the case of previously reported [Fe(opda) ] , whereas replacement of the opda ligands
3
with other aromatic amine ligands caused a dramatic decrease of hydrogen-evolving activities. These
2+
results support a ligand-based reaction mechanism for photochemical hydrogen evolution from
Keywords:
2+
, which involves 1pp* excitation of the opda ligand followed by homolytic N ꢀꢀ H bond
3
[M(opda) ]
3
d Transition-metal complexes
cleavage. In contrast to the non-catalytic reactions, an obvious metal-ion dependency was observed for
the catalytic systems including hydroquinone as a proton/electron donor. The results of this work
demonstrate a strategy for the design of a molecular photochemical hydrogen-production system under
ambient temperature and pressure.
o-Phenylenediamine
N ꢀꢀ H bond homolysis
Non-innocent ligands
ã2015 Elsevier B.V. All rights reserved.
1. Introduction
remains still a great challenge to develop such systems driven at
low temperature and without precious-metal catalysts [20,21].
Hydrogen (H
2
) gas is considered to be one of the most
In this context, the authors have developed an interest in the
photochemically assisted H -evolving reaction (HER) from organic
important candidates for clean energy carriers because
H
2
2
produces only water upon combustion without any greenhouse
gas emission [1,2]. From the viewpoint of renewable energy, photo
induced H production from water has attracted great attention
2
because of potential applications in a possible hydrogen society in
the future [3–6]. In addition, the storage and transfer of hydrogen
are also important issues to be addressed by the current
hydrides under ambient temperature. We have focused on metal
complexes bearing o-phenylenediamine (opda) ligands as a new
class of hydrogen production/storage materials because of the
following attractive properties: (i) the aromatic amines, including
opda, undergo homolytic N ꢀꢀ H bond cleavage by photoexcitation
(Scheme 1a) [22–26], which is considered to be applicable for the
technology because H
energy density [7,8]. The development of reversible H
storage materials has been intensively studied by using a variety of
materials [9], such as metals and alloys [10], clathrates [11], porous
materials [12,13], inorganic hydrides (e.g., ammonia borane)
2
gas is explosive with a poor volumetric
production/
2
photochemical H production under mild conditions, and (ii) opda
2
complexes are widely known to possess reversible multielectron/
multiproton pooling capability (Scheme 1b) by multistep redox
processes between opda, o-semibenzoquinodiimine (s-bqdi), and
o-benzoquinodiimine (bqdi) [27–31], which is advantageous for
[
14–16], and organic hydrides [17–19]. Among them, organic
hydrides are considered to be promising candidates for H
production/storage materials due to their excellent gravimetric
reversible H
developed a photochemical H
opda complex [Fe(opda) ](ClO
this system, H was photochemically liberated with concomitant
2
production/storage. Indeed, we have previously
production system from iron(II)–
([Fe-opda] in Scheme 2) [32]. In
2
2
3
4 2
)
ꢀ3
capacities (e.g., 7.1 wt%, 64.8 kg m
for benzene/cyclohexane
2
conversion) [9]. However, most of these systems require relatively
high temperatures and precious-metal catalysts, therefore it
dehydrogenation of the opda ligands and the photocatalytic
reaction proceeded in the presence of hydroquinone which served
as a H /e donor.
+
ꢀ
To create more efficient systems, we attempted to reveal what is
the most critical factor in this reaction process. Herein, the
synthesis, structural characterization, and photo induced HERs of a
*
Corresponding author. Tel.: +81 11 706 3817; fax: +81 11 706 3447.
E-mail address: mkato@sci.hokudai.ac.jp (M. Kato).
http://dx.doi.org/10.1016/j.jphotochem.2015.05.028
010-6030/ã 2015 Elsevier B.V. All rights reserved.
1
Please cite this article in press as: M. Yoshida, et al., Photochemical hydrogen production from 3d transition-metal complexes bearing o-
phenylenediamine ligands, J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.05.028

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JPC 9916 No. of Pages 8
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M. Yoshida et al. / Journal of Photochemistry and Photobiology A: Chemistry xxx (2015) xxx–xxx
be regarded as potentially explosive and should therefore be
handled with care.
(
a) Photochemistry of aromatic amines (including opda)
NH2
NH
hν
3 4 2
2.2. Synthesis of [Mn(opda) ](ClO ) ([Mn-opda])
Ar
Ar
+
H•
n-Hexane/THF (2:1 v/v; 30 mL) was slowly layered onto a
solution of Mn(ClO 6H O (999 mg, 2.76 mmol) in THF (10 mL).
Then, a solution of opda (896 mg, 8.29 mmol) in n-hexane/THF
9:10 v/v; 19 mL) was also layered onto the Mn-containing
aromatic amines
N-H bond homolysis
4
)
2
ꢁ
2
(
b) Redox-chemistry of opda complexes
(
H
H
N
solution. Storage of the mixture for five weeks at 255 K afforded
single crystals suitable for X-ray crystallographic analysis. The
2
+
2
+
H
N
–
H
–H
N
–
–
–
e
–e
M
M
M
colorless crystals of [Mn-opda]
washed with THF (three times), and dried in vacuo. Yield: 306 mg
(0.471 mmol, 17%). Elemental analysis calculated for
MnN 8.6 ([Mn-opda] 0.6THF): C, 39.42; H, 4.67; N,
300K = 4.58 emu K
). UV–vis absorption (THF): max, nm (
ꢁ
THF were collected by filtration,
+
+
N
H
+H
N
H
+H
N
H
–
–
+e
+e
2
opda complex
s-bqdi complex
bqdi complex
C
H
28.8Cl
O
ꢁ
20.4
2
6
13.52; found: C, 39.29; H, 4.68; N, 13.22.
x
M
T
Scheme 1. Photochemical and redox properties of opda.
ꢀ
1
mol
M
(meff = 6.05
m
B
l
e,
ꢀ1
ꢀ1
3
cm ) 299 (3.98 ꢂ 10 ).
ClO4
2
ClO4
2
)
(
)
(
2
.3. Synthesis of [Co(opda) ](ClO ) ([Co-opda])
3
4 2
H N NH2
N
NH2
2
n-Hexane/THF (2:1 v/v; 15 mL) was slowly layered onto a
solution of Co(ClO 6H O (991 mg, 2.71 mmol) in THF (10 mL).
Then, a solution of opda (878 mg, 8.12 mmol) in n-hexane/THF
(9:10 v/v; 19 mL) was also layered onto the Co solution. Storage of
the mixture for two weeks at 255 K afforded single crystals suitable
H N
M
NH2
H N
M
N
N
4
)
ꢁ
2
2
2
2
N
H
N
N
H
H
2
2
2
for X-ray crystallographic analysis. The pink crystals of [Co-opda]
THF were collected by filtration, washed with THF (three times),
and dried in vacuo. Yield: 744 mg (1.14 mmol, 42%). Elemental
ꢁ
2
+
M = Ni2+: [Ni-8aq]
M = Mn : [Mn-opda]
2
+
Fe : [Fe-opda]
2
+
Co : [Co-opda]
2
+
analysis
THF 0.8H
.85; N, 12.51.
absorption (THF, MeCN):
calculated
for
C
H
33.6Cl
CoN
O
([Co-opda]
ꢁ
Ni : [Ni-opda]
22
2
6
9.8
2
+
Zn : [Zn-opda]
ꢁ
2
O): C, 39.51; H, 5.06; N, 12.56, found: C, 39.51; H,
ꢀ
1
4
x
M
T300K = 3.51 emu K mol
(m
eff = 5.30
m
B
). UV–vis
Scheme 2. Structures of complexes studied in this paper.
ꢀ1
ꢀ1
3
l
max, nm (
e
, M cm ) 299 (4.80 ꢂ10 ),
2
2
5
52 (2.08 ꢂ 10 ), 755 (1.75 ꢂ10 ).
series of 3d transition-metal complexes bearing opda ligands,
3 4 2
M(opda) ](ClO )
([M-opda], M = Mn2 , Fe , Co , Ni , and Zn ),
+
2+
2+
2+
2+
[
2
.4. Synthesis of [Ni(opda) ](ClO ) ([Ni-opda])
3
4 2
are reported. Although these opda complexes were reported
previously [33–35], they were discussed mainly from the
viewpoint of metal-centered spectroscopic properties (e.g., d ꢀꢀ d
transitions) and photochemical reactions have not been investi-
gated. We found that these 3d metal complexes have the
photochemical hydrogen production abilities. We also investigated
the photo induced HER from a Ni2 complex bearing 8-amino-
quinoline (8aq), fac-[Ni(8aq) ](ClO ) ([Ni-8aq]) [36] to compare
3 4 2
the photochemical reactivity with that of the corresponding opda
complex.
n-Hexane/THF (2:1 v/v; 15 mL) was slowly layered onto a
solution of Ni(ClO 6H O (551 mg, 1.51 mmol) in THF (10 mL).
Then, a solution of opda (453 mg, 4.19 mmol) in n-hexane/THF
9:10 v/v; 19 mL) was also layered onto the Ni solution. Storage of
the mixture for 2 weeks at 255 K afforded single crystals suitable
for X-ray crystallographic analysis. The pale-purple crystals of
4
)
2
ꢁ
2
(
+
[
(
Ni-opda]
three times), and dried in vacuo. Yield: 711 mg (1.09 mmol, 72%).
NiO9.6 ([Ni-opda]
O): C, 39.73; H, 5.03; N, 12.64, found: C, 39.88; H, 4.95;
ꢁ
THF were collected by filtration, washed with THF
Elemental analysis calculated for C22
THF 0.6H
N, 12.45. UV–vis absorption (THF, MeCN):
H33.2Cl
2
N
6
ꢁ
ꢁ
2
ꢀ1
ꢀ1
l
max, nm (
e
, M cm
)
2. Experimental
3
2
98 (3.12 ꢂ10 ), 552 (7.5), 774 (10).
2.1. General
2
.5. Synthesis of [Zn(opda) ](ClO ([Zn-opda])
3
4 2
)
All manipulations were performed under an N
2
atmosphere by
techniques. Mn(ClO 6H O,
2
O, Ni(ClO 6H O, and 8-amino-
THF (5 mL) and n-hexane/THF (2:1 v/v; 15 mL) were slowly
layered onto a solution of Zn(ClO 6H O (398 mg, 1.07 mmol) in
THF (10 mL). Then, a solution of opda (347 mg, 3.21 mmol) in
n-hexane/THF (9:10 v/v; 19 mL) was also layered onto the Zn
solution. Storage of the mixture for 2 weeks at 255 K afforded
single crystals suitable for X-ray crystallographic analysis. The
using
Fe(ClO
standard
Schlenk
6H
4
)
2
ꢁ
2
4
)
2
ꢁ
2
4
)
2
ꢁ
6H O, Co(ClO
2
4
)
2
ꢁ
2
4
)
2
ꢁ
quinoline (8aq) were purchased from Aldrich Chemical Co., Inc.,
o-Phenylenediamine (opda) and n-hexane were purchased from
Tokyo Chemical Industry Co., Inc., HQ and 4 Å molecular sieves
(
4AMS) were purchased from Wako Pure Chemical Industries, Ltd.
colorless crystals of [Zn-opda]
washed with THF (three times), and dried in vacuo. Yield: 489 mg
0.740 mmol, 69%). Elemental analysis calculated for
ZnN 8.9 ([Zn-opda] 0.9THF): C, 39.69; H, 4.81;
N, 12.86, found: C, 39.53; H, 4.73; N, 12.67. H NMR (270 MHz,
THF-d ): = 7.06–7.00 (m, 2H), 6.96–6.91 (m, 2H), 4.84 ppm (br s,
H). UV–vis absorption (THF):
ꢁ
THF were collected by filtration,
Zn(ClO 6H O and acetonitrile were purchased from Kanto
4
)
2
ꢁ
2
Chemical Co., Inc., THF was purchased from Junsei Chemical Co.,
Inc. All of the solvents were distilled then degassed in at least three
freeze-pump-thaw cycles just prior to use. 4AMS were activated by
(
C
21.6
H31.2Cl
2
6
O
ꢁ
1
heating in vacuo prior to use. [Fe-opda] THF was prepared
ꢁ
8
d
according to the published method [32]. Caution!: Although we
experienced no difficulties with the perchlorate salts, they should
ꢀ1
ꢀ1
4
(
lmax, nm (e, M cm ) 299
3
4.55 ꢂ10 ).
Please cite this article in press as: M. Yoshida, et al., Photochemical hydrogen production from 3d transition-metal complexes bearing o-
phenylenediamine ligands, J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.05.028

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JPC 9916 No. of Pages 8
M. Yoshida et al. / Journal of Photochemistry and Photobiology A: Chemistry xxx (2015) xxx–xxx
3
2.6. Synthesis of fac-[Ni(8aq)
3
](ClO
)
4 2
([Ni-8aq])
2 column (2 m; Shinwa Chemical Industries Ltd.) and a recorder
Shimadzu C-R8A Chromatopac Data Processor).
(
A solution of Ni(ClO
5 mL) was added dropwise to a solution of 8aq (360 mg,
.50 mmol) in THF (25 mL). The pale-red precipitate was collected
by filtration and washed with n-hexane. Yield: 449 mg (crude,
4
)
2
ꢁ
6H
2
O (184 mg, 0.502 mmol) in THF
(
2
2.9. Photochemical HERs
For photochemical hydrogen evolution, each sample was
0
0
.65 mmol as [Ni-8aq]). This crude product (149 mg, ca.
.217 mmol) was dissolved in MeOH (25 mL) and Et O vapor
prepared in situ in a handmade Schlenk flask, which was
ꢀ1
2
connected to the quartz tube [32]. Each ligand (2.39 ꢂ10 mmol;
opda or 8aq) was dissolved in a solution of M(ClO 6H
(7.98 ꢂ 10 mmol; M = Mn , Fe , Co , Ni , or Zn ) in THF (4 mL)
in the presence of ground 4AMS (250 mg) under an N atmosphere.
was slowly diffused into this solution at 283 K to afford single
crystals suitable for X-ray crystallographic analysis. The pale-red
4
)
2
ꢁ
2
O
ꢀ
2
2+
2+
2+
2+
2+
crystals of [Ni-8aq]
in vacuo. Yield: 88 mg (0.12 mmol). Elemental analysis calculated
for C28 NiO10 ([Ni-8aq] MeOH O): C, 45.44; H, 4.09; N,
1.35; found: C, 45.35; H, 4.02; N, 11.25. UV–vis absorption (THF,
ꢁ
2MeOH were collected by filtration and dried
2
Each sample flask was doubly sealed with rubber septa. Before
irradiation, gas (0.3 mL) was collected from the headspace via a
gastight syringe (Tokyo Garasu Kikai Co., Ltd.) and analyzed by GC
H30Cl
2
N
6
ꢁ
ꢁH
2
1
ꢀ
1
ꢀ1
4
4
MeCN):
l
max, nm (
e
, M cm ) 303 (1.29 ꢂ10 ), 316 (1.13 ꢂ10 ),
to confirm the successful N
irradiated by means of the 300 W xenon lamp in a water bath at
93 K. The gas samples were collected from the headspace at each
2
purge. Then, the samples were
5
33 (10).
.7. Measurements
Elemental analyses were performed at the analysis center at
2
2
analytical timepoint and the amount of H evolved was deter-
2
mined as a function of the irradiation time.
2
.10. Photochemical HERs with HQ
For photochemical H evolution from a suspension of [M-opda]
Hokkaido University. UV–vis absorption spectra in solution were
recorded with a Shimadzu MultiSpec-1500 spectrophotometer or a
Shimadzu UV-2400PC spectrophotometer over the range
2
with excess HQ as a reducing agent, the same reaction systems
described above were used, but the sample preparation procedures
l
= 250–800 nm at 293 K. Infrared spectra were recorded with a
JASCO FT-IR 660 spectrometer equipped with a Smart-Orbit (ZnSe)
ATR accessory. Magnetic susceptibility of the crystalline samples
was measured by using a Quantum Design MPMS-XL SQUID
magnetometer with
corrections were made based on Pascal’s constants [37].
NMR spectra were recorded with a JEOL EX-270 spectrometer. The
ꢀ
2
were different. A solution of M(ClO
4
)
2
ꢁ
6H
2
O (7.98 ꢂ 10 mmol)
ꢀ1
and opda (2.39 ꢂ10 mmol) in THF (2 mL) was stirred for 30 s at
room temperature in the presence of ground 4AMS (250 mg) under
a magnetic field of 1 T. Diamagnetic
ꢀ1
1
an N
2
atmosphere. A solution of HQ (7.98 ꢂ 10 mmol) in THF
H
(
2 mL) was added via cannula. After the sample preparation,
photochemical HERs were investigated by similar methods to
those described above.
signals from the residual solvent protons of THF-d
were used as internal standards [38].
8
(d= 3.58 ppm)
2
.11. Single-crystal X-ray diffraction measurements
2.8. Equipment used for the photochemical HERs
Crystallographic measurements were performed with a Rigaku
Light irradiation was performed with a xenon lamp (MAX-303,
Asahi Spectra Co., Ltd.) equipped with a UV mirror module
AFC-7R diffractometer equipped with a Mercury CCD area detector,
and graphite-monochromated Mo K radiation ( = 0.71069 Å).
a
l
(
250–385 nm; Asahi Spectra Co. Ltd.) with a quartz light guide
5 ꢂ1000 L, Asahi Spectra Co., Ltd.). The amount of photochemi-
was determined by using a GC instrument
Shimadzu GC-14B, Ar carrier) equipped with a ShinCarbon ST
Specimens of suitable size and quality were selected under paraffin
oilandmountedonMicroMounts(MiTeGen).Thecrystalwascooled
(F
cally evolved H
(
2
by using a N -flow-type temperature controller. The structures
2
were solved by direct methods (SIR2004 or SHELXS-97) [39,40],
Table 1
Crystal parameters and refinement data.
[
C
Mn-opda]
32Cl MnN
[Co-opda]
32Cl CoN
[Ni-opda]
32Cl
[Zn-opda]
32Cl
[Ni-8aq]
2 6 9
28Cl N NiO
Formula
22
H
2
6
O
9
C
22
H
2
6
O
9
C
22
H
2
N
6
NiO
9
C
22
H
2
N
6
O
9
Zn
C
28
H
M
w
650.37
Monoclinic
P2 /n
17.925(3)
8.3299(14)
19.083(3)
100.717(2)
2799.6(8)
4
654.37
Monoclinic
P2 /n
17.811(3)
8.3163(9)
18.898(3)
100.5632(19)
2751.9(6)
4
654.14
Monoclinic
P2 /n
17.860(4)
8.3408(15)
18.791(4)
100.395(2)
2753.3(9)
4
660.82
Monoclinic
P2 /n
17.839(4)
8.3654(15)
18.913(4)
100.391(3)
2776.0(9)
4
722.17
Monoclinic
P2/n
15.432(3)
12.224(2)
17.253(3)
99.103(2)
3213.6(10)
4
Crystal system
Space group
a, Å
b, Å
c, Å
1
1
1
1
b
, deg
3
V, Å
Z
ꢀ
3
r
calc, g cm
1.543
0.722
1.579
0.879
1.578
0.96
1.581
1.138
1.542
0.831
v(MoK
a
), mm-1
T,K
150
150
150
150
150
Reflns measured
Uniquereflns
22344
6400
22167
6206
21025
6300
21224
6182
23871
7165
R(int)
0.0217
0.0389
0.1062
1.053
0.044
0.0349
0.0718
0.1896
1.127
0.0446
0.06
0.1609
1.073
0.0273
0.0634
0.1916
1.03
a
R
wR
1
(I>2
s
(I))
0.0596
0.1717
1.049
b
2
(all data)
GOF
a
R
wR
1
=
S
= {[
(|F
S
o
| ꢀ |F
c o
|)/|F |.
b
2
2 2
2)2]}1/2, w = 1/[s2(F
2
2
2
2)/3 ([Mn-opda]: m = 0.0552, n = 1.2598; [Co-opda]: m = 0.0000, n = 0.0000;
2
w(F
o
ꢀ F
c
) ]/[
S
w(F
o
o
) + (mP) + nP], in which P = (F
o
+ 2F
c
[
Ni-opda]: m = 0.1014, n = 4.5981; [Zn-opda]: m = 0.0926, n = 2.8747; [Ni-8aq]: m = 0.1093, n = 5.5125).
Please cite this article in press as: M. Yoshida, et al., Photochemical hydrogen production from 3d transition-metal complexes bearing o-
phenylenediamine ligands, J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.05.028

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JPC 9916 No. of Pages 8
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M. Yoshida et al. / Journal of Photochemistry and Photobiology A: Chemistry xxx (2015) xxx–xxx
THF H O
THF
2
Hb
Ha
NHc2
NH2
2
H
2H
4H
Hc
Ha Hb
Hb
H
c2
Ha
N
Zn
2
H
2H
4H
Hc
N
H
2
3
Ha Hb
1
0
8
6
4
2
δ / ppm
Fig. 1. 1H NMR spectra of opda (top) and [Zn-opda] (bottom) in THF-d
(270 MHz).
8
which successfully located nonhydrogen atoms. All calculations
were performed with the Crystalstructure crystallographic soft-
ware package [41], except for refinement calculations, which were
performed with SHELXL-97 [40]. The crystallographic data for the
complexes are summarized in Table 1.
2.12. Powder X-ray diffraction measurements
Powder X-ray diffraction (PXRD) measurements were con-
ducted with a Bruker D8 Advance diffractometer equipped with a
graphite monochrometer, by using Cu K
dimensional LinxEye detector.
a radiation and a one-
2.13. DFT calculations
Calculations were carried out by using the DFT method
implemented in the Gaussian 03 series of programs [42]. The
structure was fully optimized by using the B3LYP method, which
uses the hybrid Becke’s three-parameter exchange functional [43]
with the correlation energy functional of Lee Yang, and Parr [44].
Calculations were performed by using the 6–31G(d) basis set
[45–47] implemented in Gaussian 03, without any extra polariza-
tion or diffuse function. To address solvation effects, the
polarizable continuum model (PCM) was used for the ground
and excited states [48]. The excited states were calculated by the
time-dependent DFT (TD-DFT) method within the Tamm–Dancoff
approximation as implemented in Gaussian 03 [49–51]. The basis
set was replaced by 6–31 + G(d) in TD-DFT calculations to account
for the Rydberg states [45–47].
3. Results and discussion
3.1. Syntheses and spectroscopic properties
A series of opda complexes were successfully obtained by the
2 2
) 6H
O (M = Mn2 , Co , Ni , and Zn ) with
+
2+
2+
2+
reaction of M(ClO
4
ꢁ
opda (3 equiv) in a degassed THF/n-hexane mixture. The IR spectra
of all the opda complexes were almost identical (Fig. S1 in the
Supporting information), which supports that all the opda
complexes form tris(opda) structure similar to [Fe-opda] [32].
The effective magnetic moments of [Mn-opda] and [Co-opda] at
3
00 K (
ment with those reported for [Mn(opda)
opda) ](SeO O ( eff = 6.06 and 5.22 , respectively) [33,35],
and indicated the high-spin nature of these opda complexes.
m
eff = 6.05 and 5.30
m
B
, respectively) showed good agree-
3
](picrate) and [Co
2
(
3
4
)
ꢁH
2
m
m
B
Fig. 2. Molecular structures of the cationic parts of (a) [Mn-opda], (b) [Co-opda],
c) [Ni-opda], (d) [Zn-opda], and (e) [Ni-8aq], showing the atom-labeling scheme.
Thermal ellipsoids are displayed at the 50% probability level. The crystallographic
data are summarized in Table 1.
(
Although all the opda complexes are fairly stable under a dry N
atmosphere, they are gradually oxidized in the presence of O
2
2
2
+
[27–32]. A Ni complex bearing 8aq ligands [Ni-8aq] was also
prepared by the reaction of Ni(ClO 6H O with 8aq (3 equiv) in
4
)
2
ꢁ
2
Please cite this article in press as: M. Yoshida, et al., Photochemical hydrogen production from 3d transition-metal complexes bearing o-
phenylenediamine ligands, J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.05.028

G Model
JPC 9916 No. of Pages 8
M. Yoshida et al. / Journal of Photochemistry and Photobiology A: Chemistry xxx (2015) xxx–xxx
5
Table 2
Photochemical H
production from [M-opda] (7.98 ꢂ 10ꢀ mmol) or free opda ligand (2.39 ꢂ10 mmol) without any metal ion in THF (4 mL) under an N
2
ꢀ1
atmosphere at 20 C
ꢄ
2
2
ꢀ1
for 70 h in the presence of 4AMS (irradiation with 300 W Xe lamp,
corresponding M(ClO
l
= 250–385 nm). Each [M-opda] complex was prepared in situ from opda (2.39 ꢂ10 mmol) and
O (7.98 ꢂ 10ꢀ mmol).
2
4
2 2
) ꢁ6H
[Mn-opda]
[Fe-opda]
[Co-opda]
[Ni-opda]
[Zn-opda]
Free opda
evolved (mmol)a
(mol)/M(mol)
1.0 ꢂ10
ꢀ1
8.7 ꢂ10
ꢀ2
8.4 ꢂ10
ꢀ2
7.0 ꢂ10
ꢀ2
7.3 ꢂ10
ꢀ2
2.2 ꢂ10
ꢀ2
H
H
2
2
1.3
1.1
1.1
0.88
0.92
–
a
Note: the present irradiation wavelength range (l= 250–385 nm) was narrower than the previously used conditions (without any band-path filters or mirror modules).
Therefore the amount of H
2
evolved decreased in comparison with the previous one [32].
degassed THF. The compositions of the complexes were confirmed
by elemental analysis and X-ray crystallography (vide infra). The
powder X-ray diffraction (PXRD) pattern of [Zn-opda] also
suggested that the predominant species of powder samples of
opda (Table S1 in the Supporting information) [57], which suggests
that oxidation of the opda ligands into bqdi or s-bqdi did not take
place.
The structure of [Ni-8aq] was also successfully determined
(Fig. 2e). The coordination geometry is a distorted octahedron
composed of three bidentate 8aq ligands in a facial fashion. This
facial configuration is considered to be caused by the strong trans
influence of the quinoline group (Nqui) relative to the amino group
(Nam) [58]. The PXRD pattern of [Ni-8aq] was almost consistent
with the simulated pattern of the crystal of [Ni-8aq] (Fig. S3b in the
Supporting information), which also suggests the predominant
species of [Ni-8aq] form the facial configuration. The Ni–Nam
distances (2.108(4)–2.129(4) Å) are comparable to the Ni–N
distances in [Ni-opda], whereas the Ni–Nqui distances (2.081
[
Zn-opda] should form a tris(opda) structure (Fig. S3a in the
1
Supporting information). A H NMR spectrum of [Zn-opda] clearly
exhibited only three identical proton signals corresponding to
opda ligands (Fig. 1), indicating that the tris(opda) structure was
maintained in a THF solution.
[Fe-opda] [32] and the other opda complexes displayed an
intense absorption band in the UV region around = 298 nm,
l
which corresponds to the opda-based pp* transition (Fig. S2a in
the Supporting information). On the other hand, [Ni-8aq] displays
intense absorption bands in the UV region around
l= 303 nm with
(
3)–2.102(3) Å) are slightly shorter due to the strong coordination
a
broad shoulder around = 330–400 nm (Fig. S2b in the
l
of quinoline.
Supporting information), which correspond to the 8aq-based
pp* transitions. These pp* absorption bands of [Ni-8aq] were
found to be redshifted relative to those of [Ni-opda] because of the
3
.3. Photochemical HERs
expanded
p orbitals of [Ni-8aq]. In addition to these ligand-
The photochemical HER for each complex was investigated. A
centered absorption bands, two weak absorption bands were
observed for [Co-opda], [Ni-opda], and [Ni-8aq] (Figs. S2c and d in
the Supporting Information) in the visible-to-NIR region, which
correspond to the metal-centered d–d transitions. The ligand-field
ꢀ
2
suspension of each opda complex (7.98 ꢂ 10 mmol) in THF
4 mL), prepared in situ from M(ClO 6H O and opda (3 equiv),
was irradiated by a xenon lamp (
(
4
)
2
ꢁ
2
l
= 250–385 nm) under an N
atmosphere in the presence of 4AMS at 20 C. All the opda
complexes exhibit photochemical HER activities with production
2
4
4
ꢄ
absorptions of [Co-opda] were assigned to the
T
1g(F) ! T1g(P)
4
4
(l
= 552 nm) and
T
1g(F) ! T2g(F) (
l= 755 nm) transitions [52].
2
of almost 1 equivalent of H gas (Table 2), as was observed for [Fe-
The absorptions of [Ni-opda] and [Ni-8aq] were also assigned to
3
3
3
3
opda] [32]. For the detailed discussion of the metal ion
dependency, however, the data under the present conditions
would be unsatisfied ones because the reactions have been done in
suspended solutions based on the low and different solubilities of
the complexes. However, the HERs were not observed at all in the
dark for all the systems, which suggests that the HERs were driven
by photochemical reaction. In addition, considering that the solid
samples are much more stable against light than the solute ones,
HER based on the solid sample in suspended solutions could be
the
A
2g ! T1g(F) ( ’ 550 nm) and
A
2g ! T2g(F) (
l
’ 800 nm)
transitions [35,36]. These metal-centered absorption bands were
consistent with previous reports [35,36].
3.2. Crystal structures
The X-ray crystal structures of the cations of [Mn-opda],
[Co-opda], [Ni-opda], and [Zn-opda] are shown in Fig. 2a–d. A
1
excluded. As shown in Fig. 1, the H NMR spectrum of [Zn-opda]
summary of the crystallographic data is given in Table 1. In all
structures, the coordination geometry at each metal atom is a
distorted octahedron composed of three bidentate opda ligands.
The structures of all the opda complexes are similar to that of
indicates that the complex is stable in the THF solution in the dark.
2
[Fe-opda], although the M–N(opda) distances are slightly different
(Figs. S4–S8 in the Supporting information). The Mn–N distances in
[Mn-opda] are in the range of 2.270(2)–2.337(2) Å and are slightly
1
.8
0
1
.5
1
[Ni-opda]
longer than the Fe–N distances in [Fe-opda] (2.204(2)–2.269(2))
32], whereas the M–N distances in the other opda complexes are
slightly shorter than those of [Fe-opda] (2.163(2)–2.225(2) for
Co-opda], 2.113(3)–2.172(3) for [Ni-opda], and 2.166(3)–2.270(3)
[
0.6
0.4
0.2
0
[
for [Zn-opda]). The order of the M–N distances (Mn > Fe > Co ꢃ
Zn > Ni) is consistent with the order of the ionic radius of each
divalent metal ion [53]. Additionally, the M–N distances in
[Ni-8aq]
0
.5
0
free 8aq
[Mn-opda] and [Co-opda] are quite similar to those reported
0
10 20 30 40 50 60 70 80
Time / h
for other high-spin Mn(II) and Co(II) complexes [54–56], which
clearly indicates the high-spin nature of [Mn-opda] and
2
+
ꢀ2
[Co-opda], also observed for [Fe-opda]. In all structures of the
Fig. 3. Photochemical H
THF (4 mL) under an N
with 300 W Xe lamp,
from opda or 8aq (2.39 ꢂ10 mmol) and Ni(ClO
2
production from each Ni complex (7.98 ꢂ 10 mmol) in
ꢄ
2
atmosphere at 20 C in the presence of 4AMS (irradiation
= 250–385 nm). The Ni complexes were prepared in situ
opda complexes, the N ꢀꢀ C and C ꢀꢀ C bond distances of the opda
2
+
l
ligands are almost similar to those reported for noncoordinating
ꢀ1
ꢀ2
4
)
2
ꢁ
6H
2
O (7.98 ꢂ 10 mmol).
Please cite this article in press as: M. Yoshida, et al., Photochemical hydrogen production from 3d transition-metal complexes bearing o-
phenylenediamine ligands, J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.05.028

G Model
JPC 9916 No. of Pages 8
6
M. Yoshida et al. / Journal of Photochemistry and Photobiology A: Chemistry xxx (2015) xxx–xxx
∗
∗
H
H
H
N
H
N
H
N
H N NH
2
2
2
2
N
l
hν
MII
MII
MII
MII
H2
H N
+
NH
Internal
Conversion
N
H
N
H
N-H bond
Homolysis
2
N
H
N
H
2
n
2
n
2
2
n-
1
N
H
N
2
k
H
n ≤ 3
Ground State
m
∗
∗
ππ Excited State
πσ Excited State
(
k + l + m ≤ 3)
Photodehydrogenated
Product
Scheme 3. Plausible mechanism for photochemical HER from the [M-opda] complex.
We confirmed that the hydrogen evolution occurred also from the
solution prepared by using the isolated complex [Zn-opda] (i.e., ex
situ sample) on photoirradiation in the same conditions. On the
other hand, the control experiment for free opda without metal
ions showed less HER activity, which indicates the importance of
the metal ion backbone under the present conditions.
activity probably because the lowest 1pp* excited state of the
complex with the extended
p-conjugated ligand was stabilized
relative to that of the opda ligand in addition to the redox inactive
feature of the 8aq ligand. Preliminary TD-DFT calculation for 8aq
1
showed that the lowest pp* excited state (Ecalcd = 3.3844 eV,
f = 0.0691, HOMO ! LUMO; see Table S3 and Scheme S1) was
To confirm the importance of the opda ligand, we also
stabilized relative to that of the opda ligand because of the
1
investigated photochemical H
aq ligand has an extended
2
production from [Ni-8aq]. The
system with no reversible redox
extended
p
-conjugated system of 8aq. Therefore, the ps* excited
8
p
state (Ecalcd = 4.8807 eV, f = 0.0084, HOMO ! LUMO + 2 and
HOMO ! LUMO + 4; see Table S3 and Scheme S1) could not be
accessible by internal conversion in the case of 8aq, which caused
decreased HER activity.
couples in the oxidative region even though it is also an aromatic
amine. In contrast to the H -evolving activity of [Ni-opda], only a
small amount of H was detected after a long induction period
about 18 h) under photoirradiation (Fig. 3). The amount of H
2
2
(
2
We also examined catalytic hydrogen production from the
evolved from [Ni-8aq] was lower than that observed for the control
experiment without any Ni2 ions under the same conditions.
These results indicate that the HER activities were regulated by the
electronic structure of aromatic amine ligands rather than by the
central metal ion.
[
1
M-opda] complexes in the presence of hydroquinone (HQ;
0 equiv) as a sacrificial electron/proton donor. Interestingly,
+
two different trends were obtained for the catalytic hydrogen
production (Table 3): [Mn-opda] showed catalytic HER activity
similar to that of [Fe-opda], whereas the other complexes
produced much less amounts of hydrogen gas, even in the
presence of excess HQ. The difference could be caused by the
stability of the photodehydrogenated form of each complex. In the
case of 3d late-transition-metal complexes (number of d electrons
ꢅ7), the strong ligand field of s-bqdi or bqdi in the photo-
dehydrogenated intermediates could lead to the drastic structural
change and the ligand dissociation as in the air-oxidation of [Co
As shown in Scheme 3, we previously proposed the photo-
1
chemical HER mechanism of [Fe-opda] as follows: (i) a pp
*
excited state was formed by photoirradiation at around
ii) N ꢀꢀ H bond homolysis proceeded, coupled with hydrogen-atom
abstraction, followed by H elimination [32]. After this process, the
l= 298 nm,
(
2
opda ligands in the complex were partially oxidized to bqdi or
s-bqdi ligands. This mechanism is based on the well-established
photochemical N ꢀꢀ H bond activation of aromatic amines. Accord-
ing to the theoretical study and the ultrafast spectroscopy of
aminobenzene [22], the photochemical N ꢀꢀ H fission could occur
2
+
2+
(
opda)
3
]
3
[34], although the stable oxidized form, [Fe(bqdi) ]
2
+
can be formed easily by the oxidation of [Fe(opda)
3
]
(Scheme S2).
In addition to such difference in the stability of the intermediates,
their reactivities toward HQ might also cause the difference in the
catalytic HER activities. However, further investigation would be
necessary for the elucidation of the mechanism.
1
by the photoexcitation to higher-lying pp* level followed by the
1
formation of the ps* state. For the complex, however, we found
1
that the access to the ps* state should become possible from
1
slightly lower-lying pp* state [32].
The present study supports this ligand-based mechanism
because the HER activity observed depends strongly on the ligand
4. Conclusion
(
2
Fig. 3). It is notable that a similar amount of H evolution was
observed even in the [Zn-opda] system including the redox-
inactive metal ion. On the other hand, [Ni-8aq] exhibited much less
We have demonstrated the photo induced HERs from 3d
2
+
2+
transition metal complexes with opda, [M-opda] (M = Mn , Co
,
2
+
2+
Ni
, and Zn ). All the complexes analyzed by the X-ray
crystallography adopt deformed tris-chelated structures similar
to that found for [Fe-opda]. The photo induced HER activities of the
complexes depended weakly on the central metal ion and strongly
on the ligand. The results obtained in this study support the
Table 3
Photochemical
2
H
2
production from [M-opda] (7.98 ꢂ 10ꢀ mmol) with HQ
ꢀ1
ꢄ
(
7.98 ꢂ 10 mmol) in THF (4 mL) under an N
2
atmosphere at 20 C for 190 h in
the presence of 4AMS (irradiation with 300 W Xe lamp,
l= 250–385 nm). Each
ꢀ1
[
M-opda] complex was prepared in situ from opda (2.39 ꢂ10 mmol) and
previously suggested ligand-based HER mechanism, which
O (7.98 ꢂ 10ꢀ mmol).
2
corresponding M(ClO
4
)
2
ꢁ
6H
2
involves 1pp* excitation of the opda ligand followed by homolytic
N ꢀꢀ H bond cleavage. On the other hand, a distinct metal ion
[
Mn-
[Fe-
opda]
[Co-
opda]
[Ni-
opda]
[Zn-opda]
2
opda]
dependency was observed for the catalytic HERs of the 3opda/M
+
7.9 ꢂ10ꢀ2 4.1 ꢂ10ꢀ2 5.9 ꢂ10ꢀ2
/THF/HQ systems: The catalytic HERs were observed in the system
H
2
evolved
mmol)a
0.44
0.23
2+
2+
(
including Mn as well as Fe . Although further investigation
would be necessary for the elucidation of the mechanism, this
study should provide useful information for the development of
new systems for photochemically assisted HERs from opda
complexes at ambient temperature.
TON
5.5
2.9
0.99
0.51
0.73
a
Note: the present irradiation wavelength range (
than the previously used condition (without any band-path filters or mirror
modules). Therefore the amount of H
l= 250–385 nm) was narrower
2
evolved decreased in comparison with the
previous one [32].
Please cite this article in press as: M. Yoshida, et al., Photochemical hydrogen production from 3d transition-metal complexes bearing o-
phenylenediamine ligands, J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.05.028

G Model
JPC 9916 No. of Pages 8
M. Yoshida et al. / Journal of Photochemistry and Photobiology A: Chemistry xxx (2015) xxx–xxx
7
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