Isolation and crystal structure of the proposed low-valent active species in the H2 activation catalytic cycle

Article abstract of DOI:10.1002/ejic.201300049

We provide confirmation of our proposed design principles for a H ₂ activation catalytic cycle by constructing a new catalyst. We have been able to obtain a crystal structure to confirm our proposition that a reduced, dinuclear species stores electrons from hydrogen molecules in a metal-metal bond.

Full text of DOI:10.1002/ejic.201300049

FULL PAPER
DOI:10.1002/ejic.201300049
Isolation and Crystal Structure of the Proposed Low-
Valent Active Species in the H Activation Catalytic Cycle
2
CLUSTER
ISSUE
[
a]
[a,b]
Hidetaka Nakai,[a,b] and
Daisuke Inoki, Takahiro Matsumoto,
[
a,b,c]
Seiji Ogo*
Keywords: Hydrogen / Hydrides / Electron / Iridium / Hydrogen activation / Isotope exchange
We provide confirmation of our proposed design principles
for a H activation catalytic cycle by constructing a new cata-
firm our proposition that a reduced, dinuclear species stores
electrons from hydrogen molecules in a metal–metal bond.
2
lyst. We have been able to obtain a crystal structure to con-
Introduction
think it most likely that their active form is also based on
a dinuclear low-valent species, rather than a monohydride
species.
Recently, much attention has focused on developing an
understanding of the hydrogenase (H ase) class of enzymes
2
since these enzymes can extract electrons from H2.[
1–4]
Such
studies could lead to major developments in clean, green-
power cell technologies if successfully transferred to indus-
[
5]
trial processes.
In order to advance the understanding of H ases, we re-
2
ported the synthesis, structure, and mode of action of a
[6]
chemically simple [NiFe]H ase mimic. This mimic is built
2
around a [NiRu] core, and we have been able to fully repli-
cate the three chemically significant functions of natural
H ases: heterolytic activation of H , electron transfer from
2
2
the H ase mimic to another molecule, and simultaneous
2
isotope exchange between H and D O.
2
2
I
As part of our model study, we isolated a low-valent [Ni -
I
Ru ] complex and proposed it as the real active species in
[
6]
Figure 1. A catalytic cycle for the extraction of electrons from H
with metal complexes.
2
the catalytic cycle – until this point, it was generally ac-
cepted that the monohydride complex (Figure 1) was the
catalytically active species. While our proposal was sup-
ported by kinetic studies, the low-valent complex could only
be isolated as a powder and we were unable to provide a
crystal structure as further evidence. Elucidating the exact
nature of this low-valent complex is likely to be key to ad-
Using our original design principles – a [NiRu] dinuclear
core, a π-accepting ligand, a σ-donor ligand, and flexible
bridging ligands between the metal centers – we developed
vancing the understanding of the natural H ases, since we
2
an analogous H ase mimic with the aim of getting a better
2
understanding of the reduced species. This complex,
II
∧
∧
[
Ir (Cp py) (μ-SO )] {3, Cp py = (tetramethylcyclopen-
[
a] Department of Chemistry and Biochemistry, Graduate School
2 2 4
II
of Engineering, Kyushu University,
tadienyl)methylpyridine}, is based on a dinuclear Ir core,
linked by a pair of modified C Me ligands and a sulfato
bridging group. Compound 3 bears all of the crucial struc-
tural features mentioned above. Complex 3 can be prepared
7
44 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan
5
5
Fax: +81-92-802-2823
E-mail: ogotcm@mail.cstm.kyushu-u.ac.jp
Homepage: http://web.cstm.kyushu-u.ac.jp/ogo/
b] International Institute for Carbon-Neutral Energy Research
[
[
III
∧
by the reaction of a mononuclear complex [Ir (Cp py)-
(
7
WPI-I2CNER), Kyushu University,
44 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan
c] Core Research for Evolutional Science and Technology
CREST), Japan Science and Technology Agency (JST),
Kawaguchi Center Building,
-1-8 Honcho, Kawaguchi-shi, Saitama 332-0012, Japan
2+
(H
O)
]
(1) with H in water.
2
2
2
We expected 3 to have catalytic properties similar to
those of the reduced species of our original complex, and
this was found to be the case, with the added bonus that we
(
4
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have been able to prove its structure by means of X-ray lated as [2](PF ) by addition of NH PF into the aqueous
6
4
6
∧
analysis. The structure of 3 was additionally characterized solution of 2. The Cp py, sulfato, and hydrido ligands link
III
by MALDI-TOF-MS and XPS.
two Ir centers to construct the dinuclear complex, which
was characterized by X-ray analysis (Figure 4), H NMR
1
and IR spectroscopy, and ESI-MS (Figure 5). Above pH
4
.5, aqua complex 1 is reversibly deprotonated to form a
Results and Discussion
III
∧
2+
bis(μ-hydroxido) complex, [Ir (Cp py) (μ-OH) ] , that
does not react with H2. These results indicate that the
aqua ligand acts as a Lewis base to facilitate the heterolytic
cleavage of H rather than the hydroxido ligand, as expected
on the basis of the [NiRu] H ase model system.
2
2
2
[
7]
Heterolytic Activation of H with Mononuclear Aqua
2
Complex 1 in Water
2
[
6,10]
A mononuclear IrIII aqua complex [Ir (Cp py)(H O) ]-
III
∧
2
2
2
(
SO ) {[1](SO )} was prepared by a modified method for
4 4
III
∧
the synthesis of [Ir (Cp py)(H O) ](BF ) ; Ag SO was
2
2
4 2
2
4
used instead of AgBF4.[ It has been reported that a sulfato
7]
ion acts not only as a counteranion but also as a bridging
[8]
ligand to tether two metal ions, and a supporting ligand
with a flexible pyridine arm is able to act as a dinucleating
ligand to facilitate dimerization.[ The sulfato and Cp py
9]
∧
ligands, capable of constructing dinuclear complexes, allow
us to perform a model study of [NiFe]H ases containing a
2
2
Figure 3. Heterolytic activation of H by aqua complex 1 to form
monohydride complex 2 in water.
[
NiFe] dimetallic center. The structure of aqua complex 1
was unequivocally determined by X-ray crystallography by
using yellow crystals of [1](PF ) obtained from substitu-
6
2
2
–
–
tion of SO4 in [1](SO ) by PF in water/methanol. Fig-
4
6
ure 2 reveals that the Ir center has a distorted octahedral
coordination geometry with two perpendicular planes as a
III
III
result of the formation of the Ir –(C Me ) and Ir –
5
4
III
(
NC H ) bonds. The Ir center also has two aqua ligands
5 4
that play a key role in the heterolytic activation of H . The
2
distances between the Ir and carbon atoms of the C Me4
5
ring of 1 in the solid state are not equivalent: the distances
of Ir–C2 and Ir–C3 {2.169(7) and 2.185(8) Å, respectively}
trans to the pyridyl ligand are longer than those of Ir–C1,
Ir–C4, and Ir–C5 {2.129(6), 2.130(6), and 2.077(6) Å,
respectively} trans to the aqua ligands.
Figure 4. An ORTEP drawing of [2](PF
6
)·{[H–NEt
3
](PF
6
)} with el-
lipsoids at 50% probability. The counteranions (PF
6
), triethylam-
∧
3
monium ion (H–NEt ), and hydrogen atoms of Cp py are omitted
for clarity. Selected interatomic distances [Å]: Ir1···Ir2 2.8375(3),
Ir1–O1 2.127(4), Ir2–O2 2.114(4), Ir1–N2 2.132(4), Ir2–N1
2
.150(5), Ir1–C1 2.141(6), Ir1–C2 2.200(6), Ir1–C3 2.213(6), Ir1–
C4 2.146(6), Ir1–C5 2.117(5), Ir2–C16 2.126(6), Ir2–C17 2.221(6),
Ir2–C18 2.225(6), Ir2–C19 2.159(6), Ir2–C20 2.119(6).
Recrystallization of monohydride complex [2](PF ) from
6
Figure 2. An ORTEP drawing of [1](PF
0% probability. The counteranions (PF
drogen atoms of Cp py are omitted for clarity. Selected interatomic
distances [Å]: Ir1–C1 2.129(6), Ir1–C2 2.169(7), Ir1–C3 2.185(8),
6
)
2
·H
2
O with ellipsoids at
O), and hy-
a
CH Cl /triethylamine/diethyl ether solution yielded
2 2
5
6
), solvent (H
2
∧
orange crystals suitable for X-ray analysis. Figure 4 shows
the dinuclear structure with the pendant pyridine group of
∧
Ir1–C4 2.130(6), Ir1–C5 2.077(6), Ir1–N1 2.101(6), Ir1–O1 Cp py coordinated to the neighboring Ir atom. The hyd-
2.146(6), Ir1–O2 2.159(6).
rido ligand, which links the two Ir atoms, was characterized
by H NMR spectroscopy. The H NMR spectrum of 2
1
1
In the pH range of about 1.0 to 4.0, where aqua complex exhibits a characteristic signal for the hydrido ligand at
maintains its structure with aqua ligands,[ complex 1 –13.49 ppm. The flexible pendant pyridine group and the
7]
1
reacts with 0.1 MPa of H via heterolytic cleavage of H in bridging sulfato ligand provide appropriate space between
2
2
III
water to yield a dinuclear Ir
monohydride complex the Ir atoms for the hydride. A hydrido-bridged dinuclear
III
∧
+
[
Ir (Cp py) (μ-H)(μ-SO )] (2) (Figure 3), which was iso- structure is one of the candidates for a reduced form, called
2 2 4
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6
Figure 5. (a) A positive-ion ESI mass spectrum of [2](PF ) in
+
CH
Calculated isotopic distribution for [2] . (d) A positive-ion ESI
OD. The signal at m/z
3
OH. (b) The signal at m/z = 905.2 corresponding to [2] . (c)
+
mass spectrum of [D-labeled 2](PF
6
) +in CH
3
Figure 6. A proposed mechanism for the formation of the low-val-
=
906.2 corresponds to [D-labeled 2] .
ent complex 3 by the reaction of monohydride complex 2 with H
2
+
in water via dihydride intermediate P and oxidative addition of H
to 3 to generate 2.
Ni-R, in [NiFe]H ases.[
1]
The distance of Ir···Ir
2
{
2.8375(3) Å} is slightly longer than that of Ni···Ru
We have succeeded in disclosing the structure of the low-
valent dinuclear species (Figure 7) that is formed by extrac-
II
II
{2.739(3) Å} in the [Ni Ru ] hydride complex.
The vibration mode assigned to the bridging sulfato li-
tion of electrons from H in water. This reduced species
2
gand in 2 is observed by IR spectroscopy. The IR spectrum
I
I
corresponds to the low-valent [Ni Ru ] complex that is the
real active species in the catalytic cycle, which has not been
–
1
shows three bands at 1029, 1074, and 1112 cm typical of
the bridging sulfato ligand.[
11]
[6]
structurally characterized by X-ray analysis. Two Ir atoms
∧
The hydrido ligand of 2 exhibited a protic character, sim-
are linked by one sulfato and two Cp py ligands with a
II
II
[6]
ilar to the [Ni Ru ] hydride complex, which was probed
crystallographic inversion center between Ir1 and Ir1*. The
+
+
by H /D exchange as observed by ESI-MS (Figure 5). A
positive-ion ESI mass spectrum of 2 in methanol shows a
prominent signal at m/z = 905.2 and a characteristic iso-
topic distribution that matches well with the calculated iso-
topic distribution (Figures 5b and 5c). Addition of CH OD
or D O as a D source to 2 shifts the signal at m/z = 905.2
to m/z = 906.2, which corresponds to [Ir (Cp py) (μ-
D)(μ-SO )] (D-labeled 2) (Figure 5d). This monodeuteride
complex, D-labeled 2, was also prepared by using D in-
stead of H2.
length of the Ir1–Ir1* bond {2.819(1) Å} in 3 is similar to
[14]
those of the Ir–Ir single bonds reported previously,
and
it is slightly shorter than that in 2 {2.8375(3) Å}. The Ir–Ir
∧
distance is adjusted by the sulfato and Cp py ligands to
3
stabilize the dimer structure with the Ir–Ir bond. Sharp sig-
+
1
2
nals in the diamagnetic region of the H NMR spectrum of
III
∧
2
2
3 indicate the presence of an antiferromagnetic exchange
+
II
4
interaction between two Ir centers (vide infra). The unit
2
cell of 3·2{[H–THF](PF )} contains protonated THF (H–
6
THF), since this complex is isolated under acidic conditions
Heterolytic Activation of H with Dinuclear Monohydride
2
Complex 2 in Water To Form Dinuclear Low-Valent
Complex 3
Monohydride complex 2 reacted with further H2
II
(
0.1 MPa) in water to form a dinuclear low-valent Ir com-
II
∧
plex [Ir (Cp py) (μ-SO )] (3) (Figure 6). We isolated re-
2
2
4
duced species 3 and characterized it by X-ray crystallogra-
phy (Figure 7), XPS (Figure 8), and MALDI-TOF-MS
(
Figure 9). As shown in the proposed mechanism in Fig-
ure 6, 3 could be formed via reductive elimination of H2
from a dihydride intermediate P, which is supported from Figure 7. An ORTEP drawing of 3·2{[H–THF](PF )} with ellip-
6
6
a hydrogen isotope exchange reaction (vide infra). It is well soids at 50% probability. The counteranions (PF ), protonated
THF (H–THF), and hydrogen atoms are omitted for clarity. Se-
lected interatomic distances [Å]: Ir1–Ir1* 2.819(1), Ir1–O1 2.152(4),
Ir1–N1 2.135(4), Ir1–C1 2.127(5), Ir1–C2 2.214(5), Ir1–C3
known that the reductive elimination of H affords low-val-
2
ent metal complexes.[
12,13a]
Recently, dihydride species have
been proposed in the catalytic cycle of the [NiFe]-
2.219(5), Ir1–C4 2.149(5), Ir1–C5 2.124(5), S1–O1 1.516(4), S1–O2
[
6,13]
H ases.
2
1.589(4).
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and crystallized from an acetone/dichloromethane/THF/
pentane solution. Similar protonated ether complexes have
[
15]
been reported.
III
The two-electron reduction of Ir monohydride complex
II
2
(
by H to low-valent Ir complex 3 was probed by XPS
2
II
Figure 8). An XP spectrum of low-valent Ir complex 3
shows the binding energies of Ir 4f5/2 and 4f (64.0 and
7/2
III
6
1
6
1.0 eV), which are lower than those of Ir aqua complex
III
(65.1 and 62.1 eV), Ir monohydride complex 2 (64.6 and
III
III
∧
1.7 eV), and Ir trihydride complex [Ir (Cp pyH) (μ-
2
2
H) ](PF ) {[4](PF ) , vide infra} (64.9 and 61.9 eV) by
3
6 3
6 3
about 1 eV (Figure 8). These results indicate that the oxi-
dation state of the Ir center in 3 is +2.
Figure 9. (a) MALDI-TOF mass spectrum of 3 with 9-nitroanthra-
cene as matrix. (b) The signal at m/z = 903.2 corresponds to [3 –
·+
·+
H] . (c) Calculated isotopic distribution for [3 – H] .
II
character, reacts with H to yield dinuclear Ir complex 3,
2
and protonation of 3 reforms 2. This cycle reminds us of
the hydrogen isotope exchange reaction between gaseous
isotopes and medium isotopes catalyzed by water-soluble
[
6]
[NiRu] complexes.
Figure 10. (a) 1H NMR spectrum (0.5 to 2.5 ppm) of 3 in [D
acetone. The signals are assigned to the methyl protons of the
]-
6
∧
1
Cp py ligand. † indicates the signal of acetone. (b) H NMR spec-
1
trum (–13.8 to –13.2 ppm) of 3 in [D
trum (0.5 to 2.5 ppm) of 2 in [D ]acetone, which is obtained by
addition of H SO /H O to a [D ]acetone solution of 3. The signals
are assigned to the methyl protons of the Cp py ligand. † indicates
the signal of acetone. (d) H NMR spectrum (–13.8 to –13.2 ppm)
6
]acetone. (c) H NMR spec-
6
2
4
2
6
II
∧
Figure 8. XP spectra of the Ir 4f region for (a) low-valent Ir com-
III
III
1
plex 3, (b) Ir aqua complex [1](SO
4
), (c) Ir monohydride com-
III
plex [2](PF
6
), and (d) Ir trihydride complex [4](PF
6
)
3
.
of 2 in [D ]acetone, which is obtained by addition of H SO /H O
6
2
4
2
to a [D ]acetone solution of 3. The signal is assigned to the hydrido
6
III
ligand coordinated to the Ir center.
Figure 9 shows the positive-ion MALDI-TOF mass spec-
trum of 3. The prominent signal at m/z = 903.2 has a char-
acteristic isotopic distribution that matches well with the
Hydrogen Isotope Exchange Reaction between Gaseous
Isotopes and Medium Isotopes with Ir(Cp py) Catalysts
·+
calculated isotopic distribution for [3 – H] .
∧
II
Whereas low-valent Ir complex 3 was stable at ambient
+
temperature under an Ar atmosphere in the absence of H
When the reaction of D-labeled 2 (or 2) with 0.1 MPa of
III
and H , 3 was transformed into Ir monohydride complex H (or D ) was performed in D O (or H O), the generation
2
2
2
2
2
2
by protonation of the metal–metal bond in the presence of HD and D (or HD and H ), and the consumption of
2 2
+
1
of H (Figure 6), which was followed by H NMR spec- H (or D ), were followed by GC analysis (Figures 11a and
2
2
troscopy (Figure 10). When H SO /H O was added to the 11b). These results indicate that the D-labeled 2 (or 2) cata-
2
4
2
acetone solution of 3, complex 2 was formed by oxidative lyzes a hydrogen isotope exchange reaction between H and
2
+
II
II
addition of H to the Ir –Ir bond. Thus, as shown in Fig- D O (or D and H O), and the time courses indicate that
ure 6, monohydride complex 2, having H /D exchangeable HD and D (or HD and H ) are formed simultaneously.
2
2
2
+
+
2
2
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The data of the consumption of H (Figure 11a) and D2
2
(Figure 11b) were used for the first-order kinetic plots in
Figure 11c. The consumption of H and D obeyed first-
2
2
order kinetics over 3–5 half-lives; first-order rate constants
kobs(H) and kobs(D) were obtained as slopes of the linear
first-order plot shown in Figure 11c {kobs(H)
2
=
.1ϫ10 s–1 and kobs(D) = 1.3ϫ10 s }. The deuterium
–
4
–4 –1
kinetic isotope effect (KIE) value (k /k ) was determined
H
D
as 1.6 at 25 °C, which is almost same as that of the [NiRu]
[
6b]
H ase mimic.
2
The observed KIE is consistent with a
Figure 12. pD-dependent generation of HD and D
2
determined by
mechanism in which the rate-determining step involves H–
H (or D–D) bond cleavage.
GC analysis in the reaction of D-labeled 2 (0.5 mm, 1.0 mL) with
3
H
2
(2.0 cm , 86 μmol, 0.1 MPa) in D
2
O (1.0 mL) at 25 °C for
90 min.
Heterolytic Activation of H with Dinuclear Monohydride
2
Complex 2 in Water To Form Dinuclear Trihydride
Complex 4
Monohydride complex 2 reacts with 0.1 MPa of H in
2
III
water at pH 2.2 for 6 h to form dinuclear Ir trihydride
complex [Ir (Cp pyH) (μ-H) ] (4) (Figure 13). It is
III
∧
+
2
2
3
worth noting that trihydride complex 4 is also synthesized
by the reaction of 1 with H . This result suggests that trihy-
2
dride complex 4 is formed via undetected dihydride species
P. Complex 4 is gradually accumulated as a byproduct from
P for 6 h in 81% yield in the reaction cycle as shown in
Figure 6.
Figure 11. (a) Time course of the generation of HD and D
the consumption of H determined by GC analysis in the reaction
of D-labeled 2 (0.5 mm, 1.0 mL) with H
.1 MPa) in D O (1.0 mL, pD 3.0) at 25 °C. (b) Time course of the
generation of HD and H and the consumption of D determined
by GC analysis in the reaction of 2 (0.5 mm, 1.0 mL) with D
2
and
2
3
2
(2.0 cm , 86 μmol,
0
2
2
2
2
Figure 13. A proposed mechanism for the formation of trihydride
3
(
2.0 cm , 86 μmol, 0.1 MPa) in H
The first-order kinetic plots for the consumption of H
and D (in part b).
2
O (1.0 mL, pH 3.0) at 25 °C. (c)
complex 4 by the reaction of monohydride complex 2 with H
2
in
2
(in part a)
water via dihydride intermediate P.
2
The yellow crystals of [4](PF ) suitable for X-ray analy-
6
3
The hydrogen isotope exchange reaction is dependent on sis were obtained by recrystallization from a water/meth-
proton concentration. Figure 12 shows the pD-dependence anol solution (Figure 14). Three hydrido ligands, tethering
III
1
of the generation of dihydrogen isotopomers HD and D2 the two Ir centers, were characterized by H NMR spec-
1
after the reaction of D-labeled 2 with 0.1 MPa of H in troscopy. The H NMR spectrum of 4 in [D ]acetone shows
2
6
D O at 25 °C for 90 min. The rates of generation of HD a characteristic signal of the hydrido ligands at –15.08 ppm.
2
∧
and D show maxima at pD 2.6 and 2.2, respectively. This The pyridyl group of the Cp py ligand is free from the Ir
2
behavior is almost identical to that of the [NiRu] H ase center, which is occupied by the hydrido ligand instead.
2
[
17]
mimic and the natural [NiFe]H ase (Desulfovibrio vulgaris Taking into account that the pK of pyridine
is 5.2 and
2
a
Miyazaki F) with the exception of the working pH ran- [4](PF ) is isolated under acidic conditions, the pyr-
6
ges.[
16]
idylamine is suggested to be protonated. The structural pa-
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rameters around the Ir center of 4 are quite close to those Reduction of Cu2+ with Dinuclear Low-valent Complex 3 in
III
III
+ [18]
of [Ir Cp* (μ-H) ] .
{
The Ir···Ir distance in
4
Water
2
2
3
III
+
2.4617(3) Å} is comparable to that in [Ir Cp* (μ-H) ]
2.4677(4) Å} and is longer than that in [Co Cp* (μ-
H)3] {2.249(1) Å}
ionic radii of the metal centers. The isolation of mono- and
trihydride complexes 2 and 4 readily allows us to propose
the dihydride species.
2 2 3
III
Although H ase-driven catalysis is a one-electron pro-
cess, we demonstrate the reduction of Cu to Cu rather
than the reduction of Cu to Cu because Cu is less solu-
{
2
2
2
2+
0
+
[19]
as a result of the difference of the
+
0
+
II
ble in water. Low-valent Ir complex 3 stoichiometrically
reduces Cu2 to Cu in water (Figure 16). The electrons,
+
0
∧
extracted from H with Ir(Cp py) catalysts, are transferred
2
to the substrate.
Figure 16. Reduction of Cu2+ to Cu with low-valent complex 3.
0
Figure 14. An ORTEP drawing of [4](PF
0% probability. The counteranions (PF
atoms except hydrides are omitted for clarity. Selected interatomic
6
)
3
·2H
2
O with ellipsoids at
5
6
), solvents, and hydrogen
0
Under the catalytic conditions, Cu was catalytically
formed by the reaction of 1 with Cu2 and 0.1 MPa of H2
+
distances [Å]: Ir1···Ir2 2.4617(3), Ir1–C1 2.187(6), Ir1–C2 2.195(6),
Ir1–C3 2.209(6), Ir1–C4 2.195(6), Ir1–C5 2.158(5), Ir2–C16 in water at pH 2.2 (Figure 17). The formation of Cu0
2
.191(5), Ir2–C17 2.191(5), Ir2–C18 2.198(5), Ir2–C19 2.169(6), reached a constant value at around 200 min, and the turn-
Ir2–C20 2.153(5).
over number (TON, mol of product formed/mol of catalyst
) was determined as 6.5. The saturation behavior of the
1
A positive-ion ESI mass spectrum of 4 in methanol catalytic activity could be ascribed to the formation of 4,
shows the prominent signal at m/z = 811.2, which has a which is inactive for the reduction of Cu2 to Cu . The cata-
+
0
characteristic isotopic distribution that matches well with lytic activity is lower than that of the [NiRu] catalyst,[
6c]
+
the calculated isotopic distribution for [4 – 2H] (Fig- which is possibly caused by stabilization of low-valent com-
ure 15). When the synthesis of [Ir (Cp pyD) (μ-D) ]- plex 3 compared to the low-valent [Ni Ru ] complex by the
PF ) {[D-labeled 4](PF ) } is carried out by the reaction steric hindrance of the bridging sulfato and Cp py ligands
III
∧
I
I
2
2
3
∧
(
6 3 6 3
of 1 with 0.1 MPa of D in D O at pD 2.2 for 6 h at ambi- that prevent access of the Cu2 to the Ir center.
+
II
2
2
ent temperature, the ESI mass spectrum shows that the sig-
nal shifts from m/z = 811.2 to m/z = 814.2 because of iso-
topic substitution of the hydrido ligands (Figure 15d).
Figure 17. Time course of the catalytic reduction of Cu2 to Cu0
+
3
with aqua complex 1 (5.0 μmol) with H
2
(2.0 cm , 86 μmol,
0
.1 MPa) in water (1.0 mL, pH 2.2) at ambient temperature.
Figure 18 summarizes a proposed catalytic cycle, with
side reaction, for the extraction of electrons from H with
2
∧
III
use of Ir(Cp py) catalysts in water. Ir aqua complex 1
reacts with H to yield monohydride complex 2. Complex
2
Figure 15. (a) Positive-ion ESI mass spectrum of [4](PF
CH OH. (b) The signal at m/z = 811.2 corresponds to [4 – 2H] .
c) Calculated isotopic distribution for [4 – 2H] . (d) Positive-ion
ESI mass spectrum of [D-labeled 4](PF in CH OD. The signal
6 3
) in
2
reacts with further H to form the low-valent dinuclear
2
+
3
+
complex 3 by the reductive elimination of dihydrogen from
dihydride species P. To complete the catalytic cycle, complex
(
6
)
3
3
+
2+
0
at m/z = 814.2 corresponds to [D-labeled 4 – 2D] .
1 is regenerated by the reduction of Cu to Cu with 3.
Eur. J. Inorg. Chem. 2013, 3978–3986
3983
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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FULL PAPER
formed by dissolving the samples in an NMR tube (diameter =
.0 mm) with a sealed capillary tube (1.5 mm) containing 3-(tri-
methylsilyl)propionic-2,2,3,3-[D ] acid sodium salt (TSP, used as
reference with proton resonance at δ = 0.00 ppm) dissolved in D
5
4
2
O
(
(
10 mm). The chemical shifts were referenced to tetramethylsilane
TMS, δ = 0.00 ppm) in CDCl and [D ]acetone. IR spectra of solid
3
6
compounds in KBr disks were recorded with a Thermo Nicolet
–
1
–1
NEXUS 6700 FTIR instrument from 650 to 4000 cm with 2 cm
standard resolution at 25 °C. ESI-MS data were obtained with an
API 365 triple-quadrupole mass spectrometer and a JEOL JMS-
T100LC AccuTOF instrument. MALDI-TOF-MS data was re-
corded with a Bruker Autoflex III instrument, and 9-nitroanthra-
cene was used as matrix. H
a Shimadzu GC-14B and GC-8A gas chromatograph {He carrier,
0% MnCl -alumina column (model: Shinwa OGO-SP) at –196 °C
liquid N )}. A Nissin magnetic stirrer (model SW-R700) was used.
X-ray photoelectron spectroscopy experiments were performed
with a PHI 5800 XPS system equipped with an Al-K monochro-
2 2
, HD, and D gases were detected by
1
2
(
2
α
mator X-ray source operating at a power of 350 W, and the binding
energies were corrected by assuming the C 1s binding energy of the
[
20]
carbon atoms of the ligand in specimens to be 284.5 eV.
The
–
9
pressure in the test chamber was maintained below 1ϫ10 Torr
during the acquisition process. Elemental analysis data were ob-
tained with a PerkinElmer 2400II series CHNS/O analyzer.
pH Adjustment: The pH of the solution was adjusted by using 0.5 m
H
2
SO
pD of the solution was adjusted by using 0.5 m D
.0–3.0) and 0.5 m NaOD/D O (4.0–12.0). In a pH (or pD) range
4
/H
2
O (pH 1.0–3.0) and 0.5 m NaOH/H
2
O (4.0–12.0). The
2
SO /D O (pD
4
2
Figure 18. A proposed catalytic cycle, with side reaction, for the
1
2
∧
extraction of electrons from H
water.
2
with use of Ir(Cp py) catalysts in
of 1.0–12.0, the pH (or pD) values of the solutions were determined
by a pH meter (model: TOA HM20J) equipped with a pH combi-
nation electrode (model: TOA GST-5725C). Below pD 1, pD was
estimated by concentration of the solution; for example, pD values
Conclusions
of 0.1 and 1.0 m DNO
3 2
/D O were estimated to be 1 and 0, respec-
We have used the design principles learnt from our pre-
vious study to successfully construct a new H activation
tively. Values of pD were corrected by adding 0.4 to the observed
[21]
2
values (pD = pH meter reading + 0.4).
catalyst. While this particular species suffers from a side
reaction that steadily renders it inert, this catalyst provides
important confirmation of our proposed requirements for
∧
)}: A solution of [IrIII(Cp py)-
SO (0.313 g, 1.00 mmol) in
∧
[
(
IrIII(Cp py)(H
2
O)
2
](SO
4
) {[1](SO
4
Cl)
2
] (0.476 g, 1.00 mmol) and Ag
2
4
water (10 mL) at a pH value of ca. 3.0 was stirred at ambient tem-
perature for 12 h to precipitate AgCl powder, and the powder was
removed by filtration. The solvent was evaporated to yield a yellow
H activation catalysts: a reduced dinuclear core, a π-ac-
2
cepting ligand, a σ-donating ligand, and a flexible bridging
ligand. Hopefully, these findings will lead to a new class of
4
powder of [1](SO ), which was dried in vacuo (yield: 98% based on
III
∧
1
catalysts for generating energy from simple H gas.
2
[Ir (Cp py)(Cl) ]). H NMR (300 MHz, D O, reference to TSP,
2
2
pD 3.0): δ = 1.66 {s, 6 H, C
5
(CH
3
)
4
5 3 4
}, 1.81 {s, 6 H, C (CH ) }, 4.14
3
3
(
s, 2 H, -CH
2
-), 7.59 {d, JH,H = 7.8 Hz, 1 H, py}, 7.61 (t, JH,H
=
Experimental Section
3
3
6.0 Hz, 1 H, py), 8.01 (t, JH,H = 7.8 Hz, 1 H, py), 8.10 (d, JH,H
=
H
[1](SO
6.0 Hz, 1 H, py) ppm. ESI-MS (in methanol): m/z = 422.2 {[1 –
Materials and Methods: All experiments were carried out under N
2
+
3
O] , relative intensity (I) is 100 in the m/z range 100–2000}.
) [C15 22IrNO S (536.62)]: calcd. C 33.57, H 4.13, N 2.61;
or Ar by using standard vacuum line techniques or handled in a
III
III
∧
4
H
6
glovebox under N
{
2
. Ir
2
chlorido complex [Ir (Cp py)(Cl) ]
∧
found C 33.61, H 4.29, N 2.84.
Cp py = (tetramethylcyclopentadienyl)methylpyridine} was pre-
[
7]
pared by the previous method. Manipulations in acidic media
were carried out with use of plastic- and glassware (without metal).
IrIII
∧
[
2
(Cp py)
2
(μ-H)(μ-SO
4
)](PF
6
)
{[2](PF
6
)}: Complex [1](SO
4
)
(
24.1 mg, 44.9 μmol) was stirred under a H
2
atmosphere (0.1 MPa)
D
D
2
O (99.9% D), 98% D
O (99% D), 40% NaOD/D
2
SO
4
/D
2
O (99% D), concentrated DNO
3
/
in water (3.0 mL) at pH 2.2 at ambient temperature for 30 min to
give an orange solution of 2. The addition of NH PF (73.4 mg,
50 μmol) afforded an orange precipitate of [2](PF ), which was
collected by filtration and dried in vacuo {yield: 20% based on
2
2
O (99% D), CH
3
OD, and [D ]acet-
6
4
6
one were purchased from Cambridge Isotope Laboratories, Inc.;
these reagents were used as received. Distilled water was purchased
4
6
from Wako Pure Chemical Industries, Ltd. H
purchased from Taiyo Toyo Sanso Co., Ltd., D
purchased from Sumitomo Seika Chemicals Co., Ltd., and HD gas
HD 97%, H 1.8%, D 1.2%) was purchased from Isotec Inc.;
2
gas (99.9999%) was
1
[
1](SO
δ = –13.49 (s, 1 H, μ-H), 0.86 {s, 6 H, C
(CH }, 1.88 {s, 6 H, C (CH }, 1.96 {s, 6 H, C
s, 4 H, -CH -), 7.60 (t, JH,H = 6.0 Hz, 2 H, py), 7.70 (d, JH,H
4
)}. H NMR (300 MHz, in [D
6
]acetone, reference to TMS):
(CH }, 1.77 {s, 6 H,
(CH }, 4.25
2
gas (99.5%) was
5
3 4
)
C
5
3
)
4
5
3
)
4
5
3 4
)
(
2
2
3
3
(
2
=
these were also used without further purification.
3
3
7.8 Hz, 2 H, py), 7.95 (t, JH,H = 7.8 Hz, 2 H, py), 8.81 (d, JH,H
1
+
H NMR spectra were recorded with a JEOL JNM-AL300 spec-
= 6.0 Hz, 2 H, py) ppm. ESI-MS (in methanol): m/z = 905.2 {[2] ,
I = 100 in the m/z range 100–2000}. FTIR (KBr disk): ν(S–O)
trometer. 1H NMR spectroscopic experiments in D
O were per-
3984
2
Eur. J. Inorg. Chem. 2013, 3978–3986
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org
FULL PAPER
–1
=
1029, 1074, 1112 cm . [2](PF
6
) [C30
H
37
F
6
Ir
2
N
2
O
4
PS (1051.09)]: CuSO
4
·5H
2
O (0.25 g, 1 mmol) were dissolved in water (1.0 mL) at
calcd. C 34.28, H 3.55, N 2.67; found C 34.27, H 3.84, N 2.67.
2
pH 3.0. The resulting solution was stirred under a H atmosphere
III
∧
A monodeuteride complex, [Ir
labeled 2](PF )}, was prepared by the same method for the synthe-
) except for using D and D O instead of H and H O.
2
(Cp py)
2
(μ-D)(μ-SO
4
)](PF
6
) {[D-
(0.1 MPa, 86.0 μmol) at ambient temperature to gradually form
0
6
Cu , which was isolated by filtration and washed with water (3ϫ
0
sis of [2](PF
6
2
2
2
2
5 mL). The yield of Cu was determined by the weight of the ob-
0
2+
IrII
∧
tained Cu . It is confirmed that no reduction of Cu occurs in the
[
2
(Cp py)
2
(μ-SO )] (3): To a solution of [1](SO ) (24.1 mg,
4
4
absence of 1.
4
4.9 μmol) in water (3.0 mL) at pH 2.2 was added NH
4
PF
6
(
73.4 mg, 450 μmol). The resulting solution was stirred under a H
2
X-ray Crystallographic Analysis: X-ray quality crystals of [1](PF ) ·
H O, [2](PF )·{[H–NEt ](PF )}, 3·2{[H–THF](PF )}, and [4](PF ) ·
2 6 3 6 6 6 3
6
2
atmosphere (0.1 MPa) at ambient temperature for 90 min to give a
green precipitate of 3, which was recrystallized from acetone/
2H O were prepared by recrystallization from a water/methanol
2
dichloromethane/THF/pentane to afford 3·2{[H–THF](PF
lected by filtration, and dried in vacuo {yield: 20% based on
6
)}, col-
solution of [1](SO ) with NH PF , a dichloromethane/diethyl ether
4
4
6
solution of [2](PF ) with triethylamine, a THF/dichloromethane/
6
1
[
1](SO
δ = 0.69 {s, 6 H, C
(CH }, 2.17 {s, 6 H, C
H,H = 6.0 Hz, 2 H, py), 7.96 (d, JH,H = 7.8 Hz, 2 H, py), 8.17 mated Mo-K radiation (λ = 0.7107 Å). Data were collected and
4
)}. H NMR (300 MHz, in [D
(CH }, 1.91 {s, 6 H, C
(CH }, 4.40 (s, 4 H, -CH
6
]acetone, reference to TMS): acetone/pentane solution of 3·2(HPF ), and a water/methanol solu-
6
5
3
)
4
5
(CH
3
)
4
}, 1.99 {s, 6 H,
-), 7.71 (t, gaku/MSC Saturn CCD diffractometer with confocal monochro-
6 3
tion of [4](PF ) , respectively. Measurements were made with a Ri-
C
5
3
)
4
5
3
)
4
2
3
3
J
α
3
3
(
t, JH,H = 7.8 Hz, 2 H, py), 9.18 (d, JH,H = 6.0 Hz, 2 H, py) ppm.
MALDI-TOF-MS: m/z = 903.2 {[3 – H] , I = 100 in the m/z range
processed with the CrystalClear program (Rigaku). All calculations
were performed with the teXsan crystallographic software package
of Molecular Structure Corporation.
·+
–
1
2
3
3
00–2000}. FTIR (KBr disk): ν(S–O) = 1028, 1077, 1112 cm .
·2{[H–THF](PF )} [C38 S (1341.28)]: calcd. C
6
54 2 2 6 2
H F12Ir N O P
Crystallographic Data for 1: C15
24 3 2
H F12IrNO P , M = 748.51, or-
4.03, H 4.06, N 2.09; found C 33.73, H 4.27, N 1.94.
thorhombic, a = 8.843(3) Å, b = 15.048(4) Å, c = 17.962(5) Å, V =
∧
[
IrIII
2
(Cp pyH)
2
(μ-H)
3
](PF
6
)
3
{[4](PF
6
)
3
}: A solution of [1](SO
4
)
3
2
390(1) Å , space group P2
sured reflns., 5375 unique reflns. (Rint = 0.047), R = 0.0376, R
.0913, GOF = 0.928.
1 1 1
2 2 , Z = 4, T = 193.2 K, 17625 mea-
(24.1 mg, 44.9 μmol) was stirred under a H
2
atmosphere (0.1 MPa)
w
=
in water (3 mL) at pH 2.2 at ambient temperature for 6 h to give
an orange solution of 4. Addition of NH PF (22.0 mg, 135 μmol)
gave an orange powder of [4](PF , which was collected by fil-
tration and dried in vacuo {yield: 81% based on [1](SO
NMR (300 MHz, in [D ]acetone, reference to TMS): δ = –15.08 (s,
0
4
6
Crystallographic Data for 2: C36
53 2 3 4 2
H F12Ir N O P S, M = 1298.26,
6 3
)
1
monoclinic, a = 12.186(4) Å, b = 28.35(1) Å, c = 12.679(4) Å, β =
4
)}. H
3
9
4
3.598(4)°, V = 4371(2) Å , space group P2
4199 measured reflns., 9993 unique reflns. (Rint = 0.047), R =
= 0.0801, GOF = 0.855.
1
/n, Z = 4, T = 93.2 K,
6
3
H, μ-H), 2.08 {s, 12 H, C
5
(CH
3
)
4
}, 2.12 {s, 12 H, C
-), 7.62 (t, 3
JH,H = 7.2 Hz, 2 H, py), 7.93 (d,
5 3 4
(CH ) },
0.0368, R
w
4
.33 (s, 4 H, -CH
2
3
3
J
H,H = 6.3 Hz, 2 H, py), 8.53 (t, JH,H = 7.2 Hz, 2 H, py), 9.35 (d,
Crystallographic Data for 3: C38
54 2 2 6 2
H F12Ir N O P S, M = 1341.28,
3JH,H = 6.3 Hz, 2 H, py) ppm. ESI-MS (in methanol): m/z = 811.2
monoclinic, a = 20.324(9) Å, b = 13.098(6) Å, c = 16.869(8) Å, β =
+
{
[4 – 2H] , I = 100 in the m/z range 100–2000}. [4](PF
6
)
3
3
1
2
0
01.537(5)°, V = 4400(3) Å , space group C2/c, Z = 4, T = 93.2 K,
[
C
30
H
41
F
18Ir
29.07,
2
N
2
P
3
(1249.00)]: calcd. C 28.85, H 3.31, N 2.24;
3.54, 2.21. trideuteride complex,
](PF {[D-labeled 4](PF }, was prepared
by the same method for synthesis of [4](PF except for using D
and D O instead of H and H O.
4043 measured reflns., 5037 unique reflns. (Rint = 0.037), R =
.0328, R = 0.0761, GOF = 1.003.
w
found
[
C
H
N
A
III
∧
Ir
2
(Cp pyD)
2
(μ-D)
3
6
)
3
6 3
)
Crystallographic Data for 4: C30
45 2 2 2 3
H F18Ir N O P , M = 1285.03,
6
)
3
2
triclinic, a = 7.682(3) Å, b = 12.567(5) Å, c = 20.998(8) Å, α =
2
2
2
3
8
7.533(9)°, β = 83.842(9)°, γ = 86.774(7)°, V = 2010(1) Å , space
Hydrogen Isotope Exchange Reaction between Gaseous Isotopes and
Medium Isotopes Catalyzed by 2 and D-labeled 2: A 3 mL vial was
¯
group P1, Z = 2, T = 93.2 K, 21600 measured reflns., 8835 unique
reflns. (Rint = 0.038), R = 0.0285, R
CCDC-866196 (for 1), -866198 (for 2), -866197 (for 3) and -866199
for 4) contain the supplementary crystallographic data for this pa-
w
= 0.0839, GOF = 0.806.
charged with a D
2
6
O solution of [D-labeled 2](PF ) (1.0 mL,
0.5 mm, pD –1.0 to 11.0) under 0.1 MPa of H
2
(86 μmol) and was
(
capped with a septum. The vial was stirred (5000 rpm, Nissin mag-
netic stirrer Model SW-R700) vigorously at 25 °C. The gas present
per. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_requ-
est/cif.
in the vial was sampled by a gas-tight syringe and analyzed for H
HD, and D by GC. It was confirmed that no isotopic exchange
reaction between H and D O occurred in the absence of 2 (as
a blank experiment). When the experiments were performed with
2](PF ) using D and H O, the results were almost the same except
for the deuterium kinetic isotope effects, and the H /HD ratio was
/D isotope exchange
2
,
2
2
2
Acknowledgments
[
6
2
2
2
This work was supported by the World Premier International Re-
search Center Initiative (WPI) of the Ministry of Education, Cul-
ture, Sports, Science and Technology (MEXT), Japan, through
grants-in-aid 23655053, 24750058, 25620047, and 24109016 (Scien-
tific Research on Innovative Areas “Stimuli-responsive Chemical
Species”) and the Basic Research Programs CREST Type, “Devel-
opment of the Foundation for Nano-Interface Technology” of the
Japan Science and Technology Agency (JST).
+
2 2
comparable to the D /HD ratio of the H
reaction.
Quantitative Analysis of Cu0 from the Reduction of Cu2+ with 3:
Complex 3·2(H–THF·PF ) (11.7 mg, 10 μmol) and CuSO ·5H
2.5 mg, 10 μmol) were dissolved in water (1.0 mL). The resulting
6
4
2
O
(
0
solution was stirred at ambient temperature to gradually form Cu ,
which was isolated by filtration and washed with water (3ϫ 5 mL).
0
The yield of Cu , which was determined by the weight of the ob-
0
tained Cu , was 82% based on 3·2(HPF
6
). It is confirmed that no
reduction of Cu2+ occurs in the absence of 3.
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Eur. J. Inorg. Chem. 2013, 3978–3986
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FULL PAPER
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Received: January 16, 2013
Published Online: May 29, 2013
Eur. J. Inorg. Chem. 2013, 3978–3986
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© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim