PH-Dependent isotope exchange and hydrogenation catalysed by water-soluble NiRu complexes as functional models for [NiFe]hydrogenases

Article abstract of DOI:10.1039/b807555g

The pH-dependent hydrogen isotope exchange reaction between gaseous isotopes and medium isotopes and hydrogenation of the carbonyl compounds have been investigated with water-soluble bis(μ-thiolate)(μ-hydride)NiRu complexes, Ni^II(μ-SR)₂(μ-H)Ru^II {(μ-SR)₂ = N,N′-dimethyl-N,N′-bis(2-mercaptoethyl)-1, 3-propanediamine}, as functional models for [NiFe]hydrogenases. In acidic media (at pH 4-6), the μ-H ligand of the Ni^II(μ-SR) ₂(μ-H)Ru^II complexes has H⁺ properties, and the complexes catalyse the hydrogen isotope exchange reaction between gaseous isotopes and medium isotopes. A mechanism of the hydrogen isotope exchange reaction between gaseous isotopes and medium isotopes through a low-valent Ni^I(μ-SR)₂Ru^I complex is proposed. In contrast, in neutral-basic media (at pH 7-10), the μ-H ligand of the Ni ^II(μ-SR)₂(μ-H)Ru^II complexes acts as H^-, and the complexes catalyse the hydrogenation of carbonyl compounds.

Full text of DOI:10.1039/b807555g

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pH-Dependent isotope exchange and hydrogenation catalysed by
water-soluble NiRu complexes as functional models for [NiFe]hydrogenases†
Bunsho Kure,a Takahiro Matsumoto,a Koji Ichikawa,a Shunichi Fukuzumi,b Yoshiki Higuchi,c Tatsuhiko Yagid
and Seiji Ogo*a
Received 6th May 2008, Accepted 16th July 2008
First published as an Advance Article on the web 30th July 2008
DOI: 10.1039/b807555g
The pH-dependent hydrogen isotope exchange reaction between gaseous isotopes and medium isotopes
and hydrogenation of the carbonyl compounds have been investigated with water-soluble
bis(l-thiolate)(l-hydride)NiRu complexes, NiII(l-SR)2(l-H)RuII {(l-SR)2 =
N,Nꢀ-dimethyl-N,Nꢀ-bis(2-mercaptoethyl)-1,3-propanediamine}, as functional models for
[NiFe]hydrogenases. In acidic media (at pH 4–6), the l-H ligand of the NiII(l-SR)2(l-H)RuII complexes
has H+ properties, and the complexes catalyse the hydrogen isotope exchange reaction between gaseous
isotopes and medium isotopes. A mechanism of the hydrogen isotope exchange reaction between
gaseous isotopes and medium isotopes through a low-valent NiI(l-SR)2RuI complex is proposed. In
contrast, in neutral–basic media (at pH 7–10), the l-H ligand of the NiII(l-SR)2(l-H)RuII complexes
acts as H−, and the complexes catalyse the hydrogenation of carbonyl compounds.
Introduction
Hydrogenases (H2ases) are enzymes that catalyse the reversible
interconversion of H2 into two protons and two electrons under
ambient conditions.1,2 The enzymes also catalyse hydride transfer
and electronic reduction of the electron carriers (e.g., NAD+ and
cytochrome c3) with H2.3–10
H2ases are classified into two major families on the basis of
the metal content of their respective dinuclear active sites, i.e.,
[NiFe]H2ases11–13 and [FeFe]H2ases.14,15 X-Ray analysis, spectro-
scopic techniques and theoretical methods on the structures of the
H2ases have shown that the active sites of both types of H2ases have
characteristic bimetallic units with bis(l-thiolato) ligands, M(l-
SRꢀ)2M {M = Ni or Fe, (l-SRꢀ)2 = two bridging cysteine residues,
1,3-propanedithiolato or di(thiomethyl)amine}, and unidentified
ligands (depicted as X in Fig. 1).11–16 The unidentified ligands are
proposed to be oxygen ligands such as H2O, OH− or O2− in the
resting state, and might be a hydride (H−) ligand in the active
state.11,14–16
Fig. 1 Active site structures of resting forms of [NiFe]H2ases (a) and
[FeFe]H2ases (b) (X = H2O, OH− or O2−, Y = CH2 or NH).
(H2, HD and D2) and medium isotopes (H+ and D+)18–30 catalysed
by H2ases (eqn (1)), i.e., H2 bound to the enzyme (E) is split
heterolytically to form H+ and an enzyme hydride (EH−) (eqn
(2)).18–21 The backward reaction in eqn (2) regenerates H2. In D2O,
EH− should react with D+ to provide E with evolution of HD
(eqn (3)). In such a case, H2 must be initially converted to HD as a
single exchange product and then D2 as a double exchange product
in D2O, i.e., the generation of HD and D2 must be sequential.18
However, all experiments of hydrogen isotope exchange reaction
between gaseous isotopes and medium isotopes with H2ases
performed till now have shown that the single exchange product
(HD) and the double exchange products (D2 or H2) were formed
simultaneously.18–30 Thus, the detail of the mechanism of the
hydrogen isotope exchange reaction between gaseous isotopes and
medium isotopes has so far been the subject of controversy.
Important parts of our understanding of the H2-activation
mechanism come from studies on protein film voltammetry17 and
the hydrogen isotope exchange reaction between gaseous isotopes
aCenter for Future Chemistry, Kyushu University, 744 Moto-oka, Nishi-
ku, Fukuoka, 819-0395, Japan. E-mail: ogo-tcm@mbox.nc.kyushu-u.ac.jp;
Fax: +81 92 802 3308; Tel: +81 92 802 3295
bDepartment of Material and Life Science, Division of Advanced Science
and Biotechnology, Graduate School of Engineering, Osaka University,
Solution Oriented Research for Science and Technology, Japan Science and
Technology Agency (JST), Suita, Osaka, 565-0871, Japan
(1)
H2 + E ꢀ H+ + EH−
D+ + EH− ꢀ HD + E
(2)
(3)
cDepartment of Life Science, Graduate School of Life Science, University of
Hyogo, Koto, Kamigori, Hyogo, 678-1297, Japan
dShizuoka University, 836 Oya, Shizuoka, 422-8529, Japan
There are a few studies of the hydrogen isotope exchange
reaction between gaseous isotopes and medium isotopes and
hydrogenation of unsaturated compounds catalysed by the
M(l-Z)2M (M = metal ions, Z = thiolato, sulfido or hydrosul-
fido ligands) complexes (Table 1).31–40 However, time-dependent
† Electronic supplementary information (ESI) available: Electrospray
ionisation mass spectra of 2 (Fig. S1), IR spectra of 2 (Fig. S2), 1H NMR
spectrum of 4 in H2O (Fig. S3), pH-dependent profile of TOFs for the
hydrogenation of glyoxylic acid (Fig. S4) and XPS of 1, 2 and 3 (Fig. S5).
See DOI: 10.1039/b807555g
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Table 1 Hydrogen isotope exchange reaction between gaseous isotopes (H2, HD and D2) and medium isotopes (H+ and D+) and hydrogenation of
substrates catalysed by M(l-Z)2M (M = metal ions, Z = thiolato, sulfido or hydrosulfido ligands) complexes
Isotope exchange
Hydrogenation
Entry
1a
Catalyst
System
Time-dependent generation of isotopes
Substrate/product
Solvent
H2/D+
D2/H+
NDe
Ethylene/ethane
Dichloromethane
2b
H2/D+
NDe
Sulfur/hydrogen sulfidef
Chloroform-d1
3c
—
—
Cyclohexene/cyclohexane
Toluene
4d
—
—
1-Octyne/octane, 1-octene
and 2-octene
Acetone
5
This work
H2/D+
D2/H+
Simultaneous
Benzaldehyde/benzyl
alcohol
Water
a Ref. 32–35. b Ref. 36. c Ref. 39. d Ref. 40. e ND: not determined. f Hydrogenation of b-bromostyrene and azobenzene was also reported by similar
complexes (ref. 37 and 38).
hydrogen isotope exchange reaction between gaseous isotopes and
medium isotopes has never been carried out with the M(l-Z)2M
complexes (indicated as ND in Table 1).41–50 All hydrogenation
with the M(l-Z)2M complexes proceeds in organic solvents but
not in water in which the H2ases operate.
Frontier works on model studies for [NiFe]H2ases have been
carried out by Rauchfuss and co-workers, Artero and Fontecave
and co-workers and DuBois and co-workers.51,52
We recently reported53 the synthesis and structures of water-
6
soluble [NiII(l-SR)2RuII(OH2)(g -C6Me6)](OTf)2 {[1](OTf)2,
(l-SR)2 = N,Nꢀ-dimethyl-N,Nꢀ-bis(2-mercaptoethyl)-1,3-propa-
nediamine, OTf = CF3SO3} and [NiII(OH2)(l-SR)2(l-H)RuII(g -
6
Fig. 2 Water-soluble Ni(l-SR)2Ru {(l-SR)2 = N,Nꢀ-dimethyl-N,Nꢀ-bis-
(2-mercaptoethyl)-1,3-propanediamine} complexes 1–4.
C6Me6)](NO3) {[2](NO3)} as functional models for the resting
form (E) and the active form (EH−) of [NiFe]H2ases, respectively
(Fig. 2).54 The structures of 1 and 2 were determined by X-ray
and neutron diffraction analyses.
We report herein the pH-dependent hydrogen isotope exchange
reaction between gaseous isotopes and medium isotopes at pH 4–
6
6 catalysed by 2 via a low-valent intermediate [NiI(l-SR)2RuI(g -
C6Me6)] (3), and hydrogenation of the carbonyl compounds at
6
pH 7–10 catalysed by [NiII(OH)(l-SR)2(l-H)RuII(g -C6Me6)] (4),
which is a deprotonated species of 2 whose pKa value is 6.5 (Fig. 3).
Fig. 3 pH-Dependent hydrogen isotope exchange reaction between
gaseous isotopes and medium isotopes and hydrogenation of the carbonyl
compounds catalysed by the Ni(l-H)Ru complexes.
Experimental
Materials and methods
6
[NiII(l-SR)2RuII(OH2)(g -C6Me6)](OTf)2 {[1](OTf)2, (l-SR)2
=
OTf = CF3SO3} was prepared by the method described
N,Nꢀ -dimethyl-N,Nꢀ -bis(2-mercaptoethyl)-1,3-propanediamine,
4748 | Dalton Trans., 2008, 4747–4755
in the literature.53 [NiII(OH2)(l-SR)2(l-H)RuII(g -C6Me6)](NO3)
6
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{[2](NO3)} was synthesised from the reaction of 1 with H2
the range of m/z 100–2000}. FT-IR (cm−1, as a KBr disk): 1248
◦
(0.1 MPa) in the range of pH 4–7 at 25 C.53 The manipulations
(Ni–D–Ru).
in the acidic media were carried out with plastic- and glass-
ware (without metals). Distilled water, 0.1 M NaOH/H2O and
0.1 M HNO3/H2O were purchased from Wako Pure Chemical
Industries, Ltd., 65% DNO3/D2O (99% D) was purchased from
Isotec Inc., K2DPO4 (99% D) was purchased from CDN Isotopes,
CH3COOD (99% D) was purchased from Aldrich Co., and D2O
(99.9% D), 40% NaOD/D2O (99% D) and KD2PO4 (98% D)
were purchased from Cambridge Isotope Laboratories, Inc.; these
reagents were used as received. H2 gas (99.9999%) was purchased
from Taiyo Toyo Sanso Co., Ltd., D2 gas (99.5%) was purchased
from Sumitomo Seika Chemicals Co., Ltd., and HD gas (HD 97%,
H2 1.8%, D2 1.2%) was purchased from Isotec Inc.; these were used
without further purification.
Matrix-assisted laser desorption/ionisation time-of-flight mass
spectrometry (MALDI-TOF-MS) was recorded on ultraflex
TOF/TOF (Bruker Daltonics), where dithranol was used as a
matrix. Electrospray ionisation mass spectrometry (ESI-MS) data
were obtained by an API 365 triple-quadrupole mass spectrometer
(PE-Sciex) in the positive detection mode, equipped with an ion
spray interface. The sprayer was held at a potential of +5.0 kV,
and compressed N2 was employed to assist liquid nebulisation. 1H
NMR spectra were recorded on a JEOL JNM-AL300 spectrome-
ter at 23 ◦C. H2, HD and D2 gases were determined using Shimadzu
GC-14B and GC-8A (He carrier) wi◦th a MnCl2–alumina column
(model: Shinwa OGO-SP) at −196 C (liquid N2) and equipped
with a thermal conductivity detector. IR spectra were recorded
on a Thermo Nicolet NEXUS 8700 FT-IR instrument from
650 to 4000 cm−1 using 2 cm−1 standard resolution at ambient
temperature. X-Ray photoelectron spectra (XPS) were measured
on a VG Scientific ESCALAB MK II electron spectrometer by use
of Mg-Ka radiation, and the binding energies were corrected by
assuming a C 1s binding energy of the carbon atoms of the ligand
in the specimens as 284.5 eV.55 A magnetic stirrer (model: Nissin
SW-R700) was used.
[NiI(l-SR)2RuI(g6-C6Me6)] (3)
Complex [2](NO3) (15.0 mg, 24.1 lmol) was dissolved in H2O
(2.0 mL) at pH 4 (25 mM sodium acetate buffer), and H2 gas was
bubbled through the solution at 23 ◦C. After 1 h, the solution
was evaporated to give a dark red crude oil, which was extracted
with chloroform (1 × 3 mL) and the resulting solution was
filtered and evaporated under reduced pressure to give a dark
1
red powder of 3 {yield: 81% based on [2](NO3)}. H NMR (in
◦
6
CDCl3, reference to TMS, 23 C): d 2.12 (s, 18H, g -C6Me6),
1.80–3.08 {m, 20H, (l-SR)2}. Anal. calc. for 3·CH3COONa:
C23H41N2NaNiO2RuS2: C, 44.24; H, 6.62; N, 4.49%. Found: C,
1
43.92; H, 6.99; N, 4.20%. It was confirmed by H NMR that
the sample for the elemental analysis included one equivalent of
sodium acetate, which could be derived from the buffer solution
of CH3COOH/CH3COONa. MALDI-TOF-MS: m/z 542.1 ([3]•+;
I = 100% in the range of m/z 200–1000). FT-IR (cm−1, as a
KBr disk): 2957 (aliphatic C–H), 2924 (aliphatic C–H), 2854
=
(aliphatic C–H), 1720, 1576 (aromatic C C), 1457 (aromatic
=
C C), 1263, 1197, 1178, 1150, 1111, 1072, 1017, 969, 872, 862,
833, 776, 749. XPS: 852.7 eV (Ni 2p3/2 region), 279.4 eV (Ru 3d5/2
region).
[NiII(OH)(l-SR)2(l-H)RuII(g6-C6Me6)] (4)
Complex 4 can be prepared from complex 1 (method A) or
complex 2 (method B) as starting materials as follows.
Method A. The aqua complex [1](NO3)2 (125 mg, 0.20 mmol)
was added to an H2O solution (5 mL, pH 9) of Na3PO4·12H2O
(95 mg, 0.25 mmol). H2 was bubbled through the solution at 25 ◦C
to gradually precipitate dark-red solids of 4. After 3 h, the solids
were collected by filtration, and dried in vacuo. Further solid 4
was obtained by concentrating the filtrate to ca. 1 mL below 30 ◦C
{total yield: 72% based on [1](NO3)2}.
The pH of the solution was adjusted by using 0.1 M HNO3/H2O
(pH 1–3), 25 mM CH3COOH/CH3COONa (pH 4–6), 25 mM
Na2HPO4/KH2PO4 (pH 7–8) and 0.1 M NaOH/H2O (pH 9–10)
solutions. In a pH range of 1.0–11.0, the pH of the solution was
determined by a pH meter (model: TOA HM-5A) equipped with a
glass electrode (model TOA GS-5015C). pD Values were corrected
by adding 0.4 to the observed values (pD = pH meter reading +
0.4).56,57 In the biphasic media, the pH value of the aqueous phase
is adopted.
Method B. Complex [2](NO3) (10.0 mg, 16.0 lmol) was added
to H2O and the pH of the resulting solution was adjusted to
pH 9 by using 1 mM NaOH/H2O. The solvent was evaporated
to precipitate a dark-red powder of 4. The powder was collected
by filtration, and dried in vacuo {yield: 72% based on [2](NO3)}.
Anal. calc. for 4·NaNO3: C21H40N3NaNiO4RuS2: C, 39.08;H, 6.25;
N, 6.51%. Found: C, 39.54; H, 6.68; N, 6.59%. The sample for the
−
elemental analysis was prepared by method B. Na+ and NO3
[NiII(OH2)(l-SR)2(l-D)RuII(g6-C6Me6)](NO3) {[D-labeled
2](NO3)}
in the sample could be derived from the NaOH/H2O solution
used for adjusting the pH of the solution and the counteranion of
[2](NO3), respectively. The existence of NO3− was confirmed by IR
measurement, i.e., a characteristic absorption of uncoordinating
The aqua complex [1](NO3)2 (125 mg, 0.2 mmol) was added to a
D2O solution (5 mL) of K2DPO4/KD2PO4 (25 mM) at pD 5. D2
(0.1 MPa) was bubbled through the solution at 23 ◦C to gradually
precipitate dark-red crystals of [D-labeled 2](NO3). After 3 h of
D2 bubbling, the crystals were isolated by filtration. Further solid
[D-labeled 2](NO3) was obtained by evaporating the filtrate to
ca. 1 mL below 30 ◦C {yield: 80% based on [1](NO3)2}. ESI-
MS analysis of the filtrate has shown a prominent signal at m/z
544.2 {[D-labeled 2 − H2O]+; relative intensity (I) = 100% in
NO3 was observed at 1384 cm−1. ESI-MS (in CH3CN) m/z
−
543.2 ([4 − OH]+; I = 100% in the range m/z 100–2000). FT-
IR {cm−1, as a KBr disk}: 3433 (O–H), 1743 (Ni–H–Ru), 1570
=
=
=
(aromatic C C), 1473 (aromatic C C), 1465 (aromatic C C),
1384 (NO3−), 1328, 1290, 1262, 1207, 1110, 1097, 1067, 1044, 1025,
972, 951, 803. XPS: 853.9 eV (Ni 2p3/2 region), 280.5 eV (Ru 3d5/2
region).
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Typical procedure for hydrogen isotope exchange reaction between
gaseous isotopes and medium isotopes catalysed by [2](NO3)
A 3 mL vial was charged with 1.0 lmol of [2](NO3) and a stir
bar under H2, and was capped with a septum. After injecting
1 mL of Ar-bubbled D2O into the vial, 1 mL of H2 gas was
removed immediately. The vial was stirred (5000 rpm, Nissin
magnetic stirrer Model SW-R700) vigorously at 60 ◦C for 1 h.
The gas present in the vial was sampled using a gas-tight syringe
and analysed for H2, HD and D2 gases by GC. Isotope ratios
for each of the identical runs were averaged. It was confirmed
that no isotope exchange reactions between gaseous isotopes and
medium isotopes occurred in the absence of 1, 2 or 3 (as blank
experiments). When the experiments were performed using D2
and H2O, the results were the same except for the isotope effects,
and the H2/HD ratio was comparable to the D2/HD ratio of the
H2/D+ isotope exchange reaction.
Fig. 4 pD-Dependent H+/D+ exchange of the hydrido ligand of [2](NO3)
(1.0 lmol) in D2O (1 mL) at pD 1–10 at 60 ◦C for 15 min.
for 1 h. The remaining red oil was extracted with chloroform and
the resulting solution was filtered and evaporated under vacuum
to yield a dark red powder of 3. It is important to note that the low-
valent complex 3 was not obtained from the reaction of 2 with H2
in neutral–basic media at pH 7–10. Fig. 5a shows a MALDI-TOF
mass spectrum of a signal at m/z 542.1 {relative intensity (I) =
100% in the range of m/z 200–1000}. The envelope at m/z 542.1
has a characteristic distribution of isotopomers that matches well
with the calculated isotopic distribution for [3]•+ (Fig. 5b and 5c).
X-Ray photoelectron spectra (XPS) of 3 show that the binding
energies of Ni 2p3/2 and Ru 3d5/2 are 852.7 eV and 279.4 eV, which
correspond to Ni(I) and Ru(I), respectively (Fig. 6).59 The values
of the binding energies of Ni 2p3/2 and Ru 3d5/2 in 3 are lower than
those of [1](NO3)2 (Ni 2p3/2: 853.9 eV, Ru 3d5/2: 280.3 eV). 1H NMR
measurement indicates diamagnetism in 3, i.e., the signals of the
Typical procedure for the hydrogenation of carbonyl compounds
catalysed by 4
Benzaldehyde (250 lmol) and 4 (3.1 mg, 5.0 lmol) were dissolved
in H2O (2 mL) at pH 7–10. The solution was stirred under H2
at a pressure of 0.1–0.7 MPa at 60 ◦C. After 1 h, the solution
was cooled to 0 ◦C, and the resulting mixture was extracted by
CDCl3. The yield of benzyl alcohol in the CDCl3 solution was
determined by 1H NMR with 1,4-dioxane as an internal standard.
It was confirmed that no hydrogenation occurred in the absence
of 1 or 4 (as blank experiments).
Results and discussion
Behavior of Ni(l-H)Ru complexes in acidic media (at pH 4–6)
Only crystals of 2 {Ni(l-H)Ru} and the D-labeled 2 {Ni(l-D)Ru}
available for X-ray analysis were used for all experiments in this
study. It is very important to note that the hydride complex 2 does
not react with H+ to evolve H2 in acidic media at pH 4–6,58 i.e.,
complex 2 does not decompose to the aqua complex 1 with the
evolution of H2 in acidic media at pH 4–6. It was confirmed by ESI-
MS (Fig. S1 in ESI†) and IR (Fig. S2 in ESI†) that the structure
of 2 was preserved in acidic media at pH 4–6. The hydrido ligand
of 2 exhibits a protic character and undergoes an H+/D+ exchange
with D+ in D2O at pD 4–6 (eqn (4)).
Fig. 5 (a) MALDI-TOF mass spectrum of 3. Dithranol was used as a
matrix. The signal at m/z 542.1 corresponds to [3]•+. (b) The signal at m/z
542.1. (c) Calculated isotopic distribution for [3]•+
.
(4)
Fig. 4 shows the result of pD-dependent H+/D+ exchange for
15 min at 60 ◦C determined by ESI-MS, which indicates a
maximum around pD 4. At pD 4–6, the lower pD of the solution,
the faster is the rate of the H+/D+ exchange, i.e., the rate of the
H+/D+ exchange is dependent on D+ concentration.
A low-valent complex NiI(l-SR)2RuI
Complex [2](NO3) was dissolved in◦H2O at pH 4–6. H2 gas was
bubbled through the solution at 23 C, to evaporate the solvent,
Fig. 6 (a) XPS of Ni 2p region for 3. (b) XPS of Ru 3d region for 3.
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6
protons of g -C6Me6 and (l-SR)2 of 3 were observed at 2.12 and
1.80–3.08 ppm, respectively. The reason for diamagnetism should
be explained on the basis of electronic coupling between the two
metal ions (NiI and RuI).
Hydrogen isotope exchange reaction between gaseous isotopes and
medium isotopes catalysed by the D-labeled 2 in acidic media
(at pH 4–6)
The D-labeled 2 catalyses a pD-dependent hydrogen isotope
exchange reaction between H2 and D2O as shown in Fig. 7.
The pD-dependence of the hydrogen isotope exchange reaction
between gaseous isotopes and medium isotopes (Fig. 7) is similar
to the pD-dependence in the H+/D+ exchange (Fig. 4). The
evolution of HD and D2 was determined by GC.
Fig. 7 pD-Dependent generation of HD and D2 in the reaction of
[2](NO3) (1.0 lmol) with H2 (2.0 cm3, 84 lmol, 0.1 MPa) in D2O (1 mL)
at 60 ◦C for 1 h.
The time course of the hydrogen isotope exchange reaction
between H2 and D2O (or between D2 and H2O) catalysed by the
D-labeled 2 (or 2) shows that HD and D2 (or HD and H2) are
formed simultaneously as shown in Fig. 8a (or Fig. 8b), which
agrees with the time-dependent profile in the hydrogen isotope
exchange reaction between gaseous isotopes and medium isotopes
by H2ases.18–30 The data of the consumption of H2 (Fig. 8a) and
D2 (Fig. 8b) were used for the first-order kinetic plots in Fig. 8c.
The consumption of H2 (or D2) obeyed the first-order kinetics
over 3–5 half-lives. In this case, a kinetic isotope effect {KIE =
1.7, kobs(H) = 1.03 × 10−4 s−1 and kobs(D) = 6.11 × 10−5 s−1} was
observed (Fig. 8c). The presence of the KIE is consistent with a
mechanism in which the rate-determining step involves H–H (or
D–D) bond cleavage.
Fig. 8 (a) Time course of the generation of HD and D2 and the
consumption of H2 determined by GC analysis in the reaction of [D-labeled
2](NO3) (1.0 lmol) with H2 (2.0 cm3, 84 lmol, 0.1 MPa) in D2O (1 mL,
pD 4.0) at 60 ◦C. (b) Time course of the generation of HD and H2 and the
consumption of D2 determined by GC analysis in the reaction of [2](NO3)
(1.0 lmol) with D2 (2.0 cm3, 84 lmol, 0.1 MPa) in H2O (1 mL, pH 4.0) at
60 ◦C. (c) The first-order kinetic plots for the consumption of H2 (Fig. 8a)
and D2 (Fig. 8b).
Q (HD hydride) via heterolytic activation of H2. This may be a
rate-determining step for the hydrogen isotope exchange reaction
between gaseous isotopes and medium isotopes judging from the
observed KIE as shown in Fig. 8c. The H+/D+ exchange of
Q with D+ in D2O occurs quickly to give a double D-labeled
dihydride species R (DD hydride). Then, intramolecular reductive
elimination of HD from Q and D2 from R occurs to give the low-
valent complex 3. This is the reason why HD and D2 are produced
simultaneously (but not sequentially). To complete the catalytic
cycle, the D-labeled 2 is regenerated by deuteration of 3. This is
totally different from the previously proposed mechanism of the
hydrogen isotope exchange reactions between gaseous isotopes
(H2) and medium isotopes (D2O), where the hydrido ligand of
the metal hydride species (M–H) acts as H− and reacts with
medium D+ to form HD as the first step in the reaction.43,47,49
It was confirmed that the low-valent complex 3 also catalysed
the hydrogen isotope exchange reaction between H2 and D2O (or
between D2 and H2O).
The pH-dependence and time course of the hydrogen isotope
exchange reaction between gaseous isotopes and medium isotopes
catalysed by 1 and 2 are similar to those catalysed by the D-
labeled 2.
Mechanism of the hydrogen isotope exchange reaction between
gaseous isotopes and medium isotopes in acidic media
Based on the results obtained, we propose a mechanism of the
hydrogen isotope exchange reaction between gaseous isotopes and
medium isotopes. In Fig. 9, D-labeled 2 is used as a starting
complex whose l-D ligand has a D+ character, but not a D−
character at pD 4–6 (vide supra). First, the reaction of the D-
labeled 2 with H2 may produce a single D-labeled dihydride species
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Fig. 10 pH-Dependent UV-vis spectra of 2 (4.7 × 10−5 M) in the
pH range 4.5–9.5. The inset gives a plot of absorbance (l = 296 nm)
versus pH. Experiments were performed by the titration of 2 with 0.1 M
NaOH/H2O at 23 ◦C.
Fig. 11 shows the pH-dependent yield (based on 4) of the o-
fluorobenzyl alcohol that was obtained by a reaction of 4 with
o-fluorobenzaldehyde under N2 in the absence of H2 at 60 ◦C for
15 min, i.e., the reduction takes place in the neutral–basic media
at pH 7–10.
Fig. 9 A proposed mechanism of the hydrogen isotope exchange reaction
between gaseous isotopes and medium isotopes. D-Labeled 2 is used as a
starting complex with H2 in D2O at pD 4–6.
Behavior of Ni(l-H)Ru complexes in neutral-basic media
(at pH 7–10)
Complex 2 is reversibly deprotonated to form 4, which is a
deprotonated species of 2, in neutral–basic media at pH 7–10
(eqn (5)).60
(5)
Fig. 11 pH-Dependent yield (based on 4) of o-fluorobenzyl alcohol from
the reaction of o-fluorobenzaldehyde (500 lmol) with 4 (5 lmol) in H2O
(2 mL) at 60 ◦C for 15 min under N2 in the absence of H2.
UV-vis titration experiments revealed that the pKa value of 2 was
6.5 (Fig. 10). It is well known that aqua complexes can deprotonate
to form hydroxo complexes in neutral–basic media.61–65 1H NMR
measurements indicate paramagnetism in 4 (Fig. S3 in ESI†). A
similar character was observed in 2.53 In an IR spectrum of 4 in
the 650–4000 cm−1 region as a KBr disk, a peak at 3433 cm−1 was
assigned to m(O–H) that shifts to 2688 cm−1 by isotopic substitution
of a hydrogen atom in the hydroxo ligand (OD),66 although the O–
H stretching observed by IR is not direct evidence for the existence
of 4.67
Catalytic hydrogenation of carbonyl compounds with 4 in
neutral–basic media at pH 7–10 (with H2)
On the other hand, under catalytic conditions (0.5 MPa of
H2), benzaldehyde (50 equivalents) is reduced to benzyl alcohol
catalysed by 4 in H2O at pH 8 at 60 ◦C for 12 h (yield: 98%,
eqn (7)).
Stoichiometric hydrogenation of carbonyl compounds with 4 in
neutral-basic media at pH 7–10 (without H2)
(7)
Complex 4 has a reducing ability toward carbonyl compounds
such as benzaldehyde at pH 7–10 under stoichiometric conditions
(4 : carbonyl compound = 1 : 1) in the absence of H2 (eqn (6)),
To the best of our knowledge, this is the first example of the
hydrogenation of substrates catalysed by M(l-Z)2M (M = metal
ions, Z = thiolato, sulfido or hydrosulfido ligands) complexes
in aqueous media.54 1H NMR experiments revealed that the
D-labeled hydrogen atoms were incorporated into the carbonyl
compounds when D2 was used as the hydrogen donor in the
hydrogenation (eqn (8)).
(6)
e.g., benzaldehyde is reduced to benzyl alcohol (yield: 6%) with 4
at pH 8 at 60 ◦C for 1 h.
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(8)
The pH-dependence on the catalytic reduction of glyoxylic acid
under H2 (0.5 MPa) is similar to the pH-dependence on the
reduction under the stoichiometric conditions described above
(Fig. S4 in ESI†).
Fig. 12 reveals temperature-dependent turnover frequencies
{TOFs = (mol of product formed/mol of catalyst) per h} in the
hydrogenation of o-fluorobenzaldehyde catalysed by 4 in H2O at
pH 8 at 0.5 MPa of H2. The TOFs of the hydrogenation were
drastically increased above 40 ◦C. The catalytic reactions were
carried out at 60 ◦C, and it was confirmed by ESI-MS that
complex 4 was quite stable at 60 ◦C under Ar at pH 7–10 in
the absence of reducible aldehydes. Fig. 13 shows the dependence
of TOFs upon concentrations of o-fluorobenzaldehyde ◦used in
the hydrogenation catalysed by 4 in H2O at pH 8 at 60 C. The
TOFs of the hydrogenation of aldehydes examined in this study
saturate at a 1 : 50 ratio of 4 : aldehydes. As shown in Fig. 14,
the TOFs of the hydrogenati◦on of o-fluorobenzaldehyde catalysed
by 4 in H2O at pH 8 at 60 C are dependent on the pressure of
H2 in the range from 0.1 to 0.5 MPa, and saturate at 0.5 MPa.
Fig. 15 shows the time course of the turnover numbers (= TONs,
mol of product formed/mol of catalyst) in the hydrogenation
of o-fluorobenzaldehyde under the optimised catalytic conditions
Fig. 14 H2 Pressure-dependent TOFs {(mol of product formed/mol of
catalyst) per h} for the hydrogenation of o-fluorobenzaldehyde (250 lmol)
with 4 (5 lmol) in H2O (2 mL) at pH 8 at 60 ◦C.
Fig. 15 Time course of the TONs of the hydrogenation of o-fluoro-
benzaldehyde (250 lmol) with 4 (5 lmol) in H2O (2 mL) at 60 ◦C at
0.5 MPa of H2.
(pH 8, 60 ◦C, 0.5 MPa of H2, 1 : 50 ratio of 4 : aldehydes). In
addition, it was confirmed that the catalyst 4 could be reused at
least three times; i.e., upon addition of more substrates after the
reaction, the catalytic cycle was resumed (Fig. 16), though the
catalyst 4 was gradually deactivated.
Fig. 12 Temperature-dependent profile of TOFs {(mol of product
formed/mol of catalyst) per h} of the hydrogenation of o-fluorobenzal-
dehyde (250 lmol) with 4 (5 lmol) in H2O (2 mL) at pH 8 at 0.5 MPa
of H2.
Fig. 16 Resumption of the catalytic hydrogenation of o-fluoro-
benzaldehyde (250 lmol) with 4 (5 lmol) in H2O (2 mL) at 60 ◦C at
0.5 MPa of H2.
Table 2 summarises the hydrogenation of aldehydes catalysed
by 4 under the optimised catalytic conditions. The water-soluble
aldehyde (glyoxylic acid) is converted to the corresponding alcohol
much more efficiently than water-insoluble aldehydes (benzalde-
hyde and o-fluorobenzaldehyde). The TOF of the hydrogenation
of a benzaldehyde derivative containing an electron-withdrawing
Fig. 13 TOFs {(mol of product formed/mol of catalyst) per h} depending
upon the number of moles of substrates for the hydrogenation of
o-fluorobenzaldehyde with 4 (5 lmol) in H2O (2 mL) at pH 8 at 60 ◦C
at 0.5 MPa of H2.
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Table 2 Hydrogenation of water-soluble aldehydes (entries 1 and 2) in
water and water-insoluble aldehydes (entries 3 and 4) in biphasic media
catalysed by 4 at pH 8a
catalysed by 2 at pH 4–6 and hydrogenation of carbonyl com-
pounds catalysed by 4 at pH 7–10. In the hydrogen isotope
exchange reaction between gaseous isotopes and medium isotopes,
the generation of HD and D2 (or HD and H2) is simultaneous and
pH-dependent, which is similar to H2ases. A low-valent NiI(l-
SR)2RuI complex 3 has been isolated by the reaction of 2 with
H2 in acidic media at pH 4–6. The formation of 3 by reductive
elimination of HD and D2 (or HD and H2) from the dihydride
species produced by the reaction of 2 with H2 (or D2) is the key
step for the simultaneous generation of HD and D2 (or HD and
H2). In the hydrogenation, complex 4 can reduce the aldehydes
to the corresponding alcohols in H2O. Thus, the l-H ligand of
the NiII(l-SR)2(l-H)RuII complexes has both protic character (at
pH 4–6) and hydridic character (at pH 7–10) depending on pH.
Entry Substrate
1
Product
TOFb
3
t/h
Yield (%)
44
T/◦C
24
23
2
3
24
6
4
98
98
60
60
12
4
11
9
99
60
Acknowledgements
a In the case of biphasic media (entries 3 and 4), the pH value of the aqueous
phase is adopted. b Turnover frequency: (mol of product formed/mol of
4) per h.
This work was supported by a grant in aids: 17350027, 17655027,
18033041, and 18065017 (Chemistry of Concerto Catalysis),
and the Global COE Program, “Science for Future Molecular
Systems” from the Ministry of Education, Culture, Sports, Science
and Technology, Japan.
group (o-fluorobenzaldehyde) is higher than the TOF of the
hydrogenation of benzaldehyde at pH 8 at 60 ◦C.
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