The useful properties of H2O as a ligand of a hydrogenase mimic

Article abstract of DOI:10.1039/b921273f

This paper investigates the required properties of Ru-coordinated ligands of a Ni-Ru based hydrogenase mimic. A series of ligands, including MeCN, pyridine, H₂O and OH^- were coordinated to Ru, with H ₂O being the only ligand to promote H₂-activation. In addition, a tethered pyridyl moiety was synthesised and found to completely inhibit H₂-activation. We conclude, therefore, that H₂O is the ideal ligand for this mimic as a result of both its mild basicity and the availability of two lone pairs for simultaneous binding to Ru and H₂. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b921273f

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PAPER
www.rsc.org/dalton | Dalton Transactions
The useful properties of H2O as a ligand of a hydrogenase mimic†
Chunbai Zheng,a Kyoungmok Kim,a Takahiro Matsumotoa,b and Seiji Ogo*a,b
Received 12th October 2009, Accepted 11th December 2009
First published as an Advance Article on the web 12th January 2010
DOI: 10.1039/b921273f
This paper investigates the required properties of Ru-coordinated ligands of a Ni–Ru based
hydrogenase mimic. A series of ligands, including MeCN, pyridine, H2O and OH- were coordinated to
Ru, with H2O being the only ligand to promote H2-activation. In addition, a tethered pyridyl moiety
was synthesised and found to completely inhibit H2-activation. We conclude, therefore, that H2O is the
ideal ligand for this mimic as a result of both its mild basicity and the availability of two lone pairs for
simultaneous binding to Ru and H2.
Introduction
Complexes capable of inducing the heterolytic splitting of H2
into protons and hydride ions are becoming ever more important
as hydrogen may become the replacement for fossil fuels.1-7
The most effective of such complexes are found in naturally
occurring enzymes known as hydrogenases and are based around
homometallic FeFe cores or heterometallic NiFe cores and it is
here that most research has been concentrated.8-12
In this laboratory, we have recently successfully synthesised a
catalytically active hydrogenase mimic, based on a water-soluble
NiIIRuII aqua complex {[Aaqua](NO3)2}, to induce the heterolytic
splitting of H2 (Fig. 1).13a Water appears to play a crucial role
in the initial heterolytic splitting of H2.4g,4o Furthermore, we
were able to show that the catalyst proceeded through its cycle
via the following steps: initial coordination of H2; subsequent
H2O-mediated heterolytic splitting; production of a water-soluble
NiIIRuII hydride complex {[B](NO3)} with loss of H+; coordination
of a second H2 molecule with a second heterolytic splitting;
reductive elimination of both ‘hydrides’ to yield both H2 and a
unique, low-valent NiIRuI complex and, finally, oxidation of the
NiIRuI complex to return to the initial NiIIRuII stage. Over the
course of the cycle, two electrons are liberated from two molecules
of hydrogen.13
Fig. 1 A Ni–Ru based hydrogenase mimic, based on a NiIIRuII aqua
complex {[Aaqua](NO3)2}, to induce the proposed heterolytic splitting of
H2 and produce a NiIIRuII hydride complex {[B](NO3)}.
complementary proton. In short, H2O appears to play a crucial
role in the initial heterolytic splitting of H2. This conclusion is
supported by the presence of an oxygen-bearing ligand in natural
hydrogenases, which we have proposed must also be H2O. For
these reasons, we wished to study the role of the H2O ligand in
more detail and therefore investigated the possibility of replacing
water with other ligands.
We suspected that a crucial advantage of H2O in this con-
text would be its Lewis basicity, or pKa. Ligands with a
high pKa value would bind too strongly to the metal cen-
tres, whereas ligands that are too weak would be unable to
heterolytically activate H2. Accordingly, we initiated a study
of Ru-coordinated ligands, with varying basicity and binding
mode. Having previously reported experiments in the pres-
ence of the weak base MeCN,16 we extended our research
to investigate stronger bases such as pyridine and hydroxide
ion. Furthermore, we synthesised [NiII(m-SR)2RuII(Bz∧py)](BF4)2
[3](BF4)2, (m-SR)2 = N,N¢-dimethyl-N,N¢-bis(2-mercaptoethyl)-
1,3-propanediamine, Bz∧py = 2-(pentamethylbenzyl)pyridine}
to investigate the difference between inter- and intramolecular
binding of analogous ligands.
A crucial step in this mechanism is the coordination of H2 to
the metal centre with a subsequent nucleophilic attack by the
H2O ligand to heterolytically activate the H–H bond, as shown
in Fig. 1.14,15 This step results in a hydride coordinated to the
NiIIRuII centre and H3O+ as a leaving group to carry away the
aDepartment of Chemistry and Biochemistry, Graduate School of Engineer-
ing, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka, 819-0395, Japan.
E-mail: ogotcm@mail.cstm.kyushu-u.ac.jp; Fax: 81 92 802 3307; Tel: 81 92
802 2818
bCore Research for Evolutional Science and Technology (CREST), Japan
Science and Technology Agency (JST), Kawaguchi Center Building, 4-1-8
Honcho, Kawaguchi-shi, Saitama, 332-0012, Japan
† Electronic supplementary information (ESI) available: Electrospray
ionisation mass spectra of 1 (Fig. S1) and [2](BF4)2 (Fig. S3) and IR spectra
of 1 (Fig. S2), [2](BF4)2 (Fig. S4) and [3](BF4)2 (Fig. S5) as KBr disks, UV-
vis spectral change for the reaction of [Aaqua](NO3)2 with H2 (Fig. S6) and
pH titration of Bz∧py (Fig. S7). CCDC reference numbers 749758–749760.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/b921273f
2218 | Dalton Trans., 2010, 39, 2218–2225
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the aqueous layer was extracted with dichloromethane. The
combined organic layers were washed with water, dried over
MgSO4 and concentrated. The product was purified by flash
column chromatography on silica gel eluted with n-hexane–ethyl
acetate (9/1) and recrystallised from dichloromethane–n-hexane
to give pure 2-(pentamethylbenzyl)pyridine (550 mg, 90% based
on pentamethylbenzaldehyde). FT-IR (cm-1, KBr disk): 2921
Experimental
Materials and methods
All experiments were carried out under a N2 or an Ar atmo-
sphere using standard Schlenk techniques and a glovebox. The
manipulations in the acidic media were carried out with plastic-
ware (without metals). Distilled water, 0.1 M NaOH and 0.1 M
HNO3 were purchased from Wako Pure Chemical Industries, Ltd.
and D2O (99.9% D) was purchased from Cambridge Isotope
Laboratories, Inc.; these reagents were used as received. H2 gas
(99.9999%) was purchased from Taiyo Toyo Sanso Co., Ltd.
Other chemicals (highest purity available) were purchased from
Aldrich Chemicals Co. and used without further purification.
A NiII complex [NiII(SR)2] {(SR)2 = N,N¢-dimethyl-N,N¢-bis(2-
mercaptoethyl)-1,3-propanediamine},17 the NiIIRuII aqua com-
=
=
(aliphatic C–H), 1586 (aromatic C C or C N), 1566 (aromatic
=
=
=
=
C C or C N), 1474 (aromatic C C or C N), 1428 (aromatic
1
=
=
C C or C N), 749. H NMR (in CDCl3, reference to TMS,
25 ◦C): d 2.18 {s, 6H, C6(CH3)5}, 2.24 {s, 6H, C6(CH3)5}, 2.27
{s, 3H, C6(CH3)5}, 4.31 (s, 2H, -CH2-), 6.77 (d, 3J = 7.8 Hz, 1H,
NC5H4), 7.06 (m, 3J = 7.6 Hz, 1H, NC5H4), 7.47 (t, 3J = 7.4 Hz,
1H, NC5H4), 8.56 (d, 3J = 7.5 Hz, 1H, NC5H4).
plex [NiII(m-SR)2RuII(H2O)(h -C6Me6)](NO3)2 {[Aaqua](NO3)2}
6
13
[RuII(Bz∧py)Cl2] (1)
and the NiIIRuII hydride complex [NiII(m-SR)2(H2O)(m-H)RuII(h -
6
13
In a 300 mL flask were placed RuIIICl3·3H2O (1.3 g, 5.0 mmol),
molecular sieves 4A (40 g), 1,2-dichloroethane (100 mL) and
ethanol (10 mL). The solution was stirred at ambient temperature
for 30 min, and then Bz∧py was added (1.3 g, 5.5 mmol). The
reaction solution was stirred at 80 ◦C for 48 h. The reaction
solution was filtered and the solvent was removed under reduced
pressure to yield a crude product of 1 (yield 44% based on
RuIIICl3·3H2O). Orange crystals of 1 were obtained from a
dichloromethane–n-hexane solution. FT-IR (cm-1, KBr disk):
3099 (aliphatic C–H), 3068 (aliphatic C–H), 3020 (aliphatic C–
C6Me6)](NO3) {[B](NO3)} were prepared by the methods de-
scribed in the literatures.
The 1H NMR spectra were recorded on a JEOL JNM-AL
◦
1
300 spectrometer at 25 C. H NMR experiments in D2O and
CDCl3 were measured using 3-(trimethylsilyl)propionic-2,2,3,3-
d4 acid sodium salt (TSP) and tetramethylsilane (TMS) as
an internal standard, respectively. Electrospray ionisation mass
spectrometry (ESI-MS) data were obtained by a JEOL JMS-
T100LC and an API 365 triple-quadrupole mass spectrometer
(PE-Sciex). IR spectra of solid compounds in KBr disks were
recorded on a Thermo Nicolet NEXUS 8700 FT-IR instrument
from 650 to 4000 cm-1 using 2 cm-1 standard resolution. UV-
visible spectra were recorded on a JASCO V-670 UV-Visible-NIR
spectrophotometer and an Otsuka MCPD-2000 spectrometer.
The pH of the solution was adjusted by using 0.1 M HNO3/H2O
and 0.1 M NaOH/H2O. In a pH range of 2.0-11.0, the pH of the
solution was determined by a pH meter (TOA: HM-5A) equipped
with a glass electrode (TOA: GS-5015C). pD values were corrected
by adding 0.4 to the observed values (pD = pH meter reading +
0.4).18
=
=
H), 2925 (aliphatic C–H), 1600 (aromatic C C or C N), 1440
=
=
=
=
(aromatic C C or C N), 1383 (aromatic C C or C N), 1325
=
=
(aromatic C C or C N), 1250, 1194, 1161, 1070, 1055, 1024,
1005, 910, 818, 785, 769. 1H NMR (in CDCl3, reference to TMS,
25 ◦C): d 2.10 {s, 6H, C6(CH3)5}, 2.13 {s, 3H, C6(CH3)5}, 2.15
{s, 6H, C6(CH3)5}, 4.38 (s, 2H, -CH2-), 7.19 (t, 3J = 7.5 Hz, 1H,
NC5H4), 7.36 (d, 3J = 7.2 Hz, 1H, NC5H4), 7.73 (t, 3J = 7.5 Hz, 1H,
NC5H4), 8.24 (d, 3J = 7.5 Hz, 1H, NC5H4). ESI-MS (in MeOH):
m/z 376.1 {[1 - Cl]+; relative intensity (I) = 100% in the range
of m/z 100-2000}. Anal. Calcd for 1: C17H21NCl2Ru: C, 49.64; H,
5.15; N, 3.41%. Found: C, 49.82; H, 4.78; N, 3.46%.
2-(Pentamethylbenzyl)pyridine (Bz∧py)
To a solution of 2-bromopyridine (1.1 g, 7.1 mmol) in tetrahydro-
furan (10 mL) under N2 was added a solution of n-BuLi (1.6 M
in n-hexane, 3.6 mL, 5.7 mmol) dropwise over 10 min at -78 ◦C.
After stirring for 15 min, a solution of pentamethylbenzaldehyde
(500 mg, 2.8 mmol) in tetrahydrofuran (4.0 mL) was added to
the reaction mixture. The resulting solution was stirred at -78 ◦C
for 1 h and then allowed to warm to 25 ◦C. After addition of
a Na2CO3-saturated aqueous solution (20 mL), the mixture was
extracted with dichloromethane. The combined organic layers
were washed with water. The extract was dried over anhydrous
MgSO4, filtered and concentrated to yield an oil. The oil was
purified by flash column chromatography on silica gel eluted with
n-hexane–ethyl acetate (9/1). Into a 50 mL flask were charged
the oil and red phosphorus (320 mg, 10 mmol), and 47% HI
(15 mL) was added. The mixture was vigorously refluxed for 15 h.
The cooled reaction mixture was diluted with a Na2CO3-saturated
aqueous solution until basic. The deposited product was dissolved
with dichloromethane and the mixture was filtered through celite
to remove phosphorus. The organic layer was separated and
[RuII(Bz∧py)(H2O)2](BF4)2 {[2](BF4)2}
A solution of 1 (820 mg, 2.0 mmol) and AgBF4 (780 mg, 4.0 mmol)
in water (20 mL) at pH 7.0 was stirred at 25 ◦C for 6 h under N2 and
the precipitating AgCl was removed by filtration. The solvent was
evaporated to yield a yellow moisture sensitive powder of [2](BF4)2,
which was dried in vacuo (yield 95% based on 1). Brown crystals
of [2](BF4)2 were obtained from a dichloromethane–n-hexane
solution. FT-IR (cm-1, KBr disk): 3450 (O–H), 3097 (aliphatic
C–H), 3068 (aliphatic C–H), 3057 (aliphatic C–H), 3018 (aliphatic
C–H), 2966 (aliphatic C–H), 2935 (aliphatic C–H), 1605 (aromatic
=
=
=
=
C C or C N), 1560 (aromatic C C or C N), 1444 (aromatic
=
=
=
=
C C or C N), 1384 (aromatic C C or C N), 1056 (BF4), 784,
769. 1H NMR (in D2O at pD 7.0, reference to TSP, 25 ◦C): d 2.08 {s,
6H, C6(CH3)5}, 2.17 {s, 6H, C6(CH3)5}, 2.27 {s, 3H, C6(CH3)5},
3
4.73 (s, 2H, -CH2-), 7.20 (d, J = 5.1 Hz, 1H, NC5H4), 7.39 (t,
3J = 7.2 Hz, 1H, NC5H4), 7.78 (d, J = 8.1,Hz, 1H, NC5H4),
3
8.09 (t, 3J = 7.8 Hz, 1H, NC5H4). ESI-MS (in MeOH): m/z 376.6
([2 - H]+; I = 35% in the range of m/z 100-2000). Anal. Calcd for
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[2](PF6)2: C17H25NF12O2P2Ru: C, 30.64; H, 3.78; N, 2.10%. Found:
C, 30.33; H, 4.07; N, 2.12%.
molecules. Bz∧py was synthesised from pentamethylphenyl(2-
pyridyl)methanol, which was itself prepared by the reaction
of 2-bromopyridine with pentamethylbenzaldehyde (see Experi-
mental).
[NiII(l-SR)2RuII(Bz∧py)](BF4)2 {[3](BF4)2}
A solution of [2](BF4)2 (0.30 g, 0.60 mmol) in water (20 mL) at
pH 7.0 was added to a solution of the NiII complex [NiII(SR)2]
(0.17 g, 0.60 mmol) in water (20 mL) at pH 7.0. The solution
was stirred at 25 ◦C. After 4 h, the solvent was evaporated to
yield a brown powder of [3](BF4)2, which was dried in vacuo {72%
isolated yield based on [2](BF4)2}. Recrystallisation of [3](BF4)2
from a dichloromethane–n-hexane solution gave orange crystals
that were suitable for X-ray analysis. FT-IR (cm-1, KBr disk): 2933
Synthesis and structure of [RuII(Bz∧py)Cl2] (1)
A dichloro RuII complex [RuII(Bz∧py)Cl2] (1) was prepared by
the reaction of RuIIICl3·3H2O with Bz∧py in a mixed solvent
of 1,2-dichloroethane–ethanol (Fig. 2). It should be noted that
the pyridyl moiety induces a significant intramolecular templating
6
effect. Previous syntheses of analogous RuII-(h -C6Me5) complexes
required initial treatment of RuIIICl3 with p-mentha-1,5-diene to
6
=
=
produce a reactive intermediate, [RuIICl2(h -p-MeC6H4CHMe2)]2,
(aliphatic C–H), 1633 (aromatic C C or C N), 1603 (aromatic
6
=
=
=
=
that could be converted to the RuII-(h -C6Me5) complex at high
C C or C N), 1564 (aromatic C C or C N), 1460 (aromatic
=
=
=
=
temperatures.19 In this case, however, 1 could be formed directly
C C or C N), 1437 (aromatic C C or C N), 1389 (aromatic
1
=
=
in refluxing 1,2-dichloroethane–ethanol.
C C or C N), 1055 (BF4), 789, 775, 748. H NMR (in D2O at
pD 7.0, reference to TSP, 25 ◦C): d 1.67-1.89, 2.02-2.04, 2.12-
2.19, 2.42-2.44, 2.76-2.81 and 3.16-3.40 (m, 14H, -CH2-), 2.23 {s,
6H, C6(CH3)5}, 2.49 {s, 6H, C6(CH3)5}, 2.71 {s, 3H, C6(CH3)5},
3
2.86 (s, 6H, N–CH3), 4.57 (s, 2H, -CH2-), 7.16 (d, J = 7.5 Hz,
3
3
1H, NC5H4), 7.35 (t, J = 7.2 Hz, 1H, NC5H4), 7.53 (d, J =
7.8 Hz, 1H, NC5H4), 7.89 (t, 3J = 7.5 Hz, 1H, NC5H4). ESI-MS
(in MeOH): m/z 309.6 {[3]2+; I = 100% in the range of m/z 100-
2000}. Anal. Calcd for [3](BF4)2·H2O: C26H43N3B2F8NiORuS2: C,
38.50; H, 5.34; N, 5.18%. Found: C, 38.79; H, 5.24; N, 5.33%.
Fig. 2 Synthesis of 1.
Procedure for the titration of bases to a NiIIRuII aqua complex
[NiII(l-SR)2RuII(H2O)(g6-C6Me6)](NO3)2 {[Aaqua](NO3)2}
The structure of 1 was determined by X-ray analysis, as shown
in Fig. 3. Complex 1 adopts a distorted-octahedral coordination
with two perpendicular planes {the torsion angle between the
To aqueous solutions of a NiIIRuII aqua complex {[Aaqua](NO3)2}
(0.1 mM), were added increasing equivalents of MeCN and pyri-
dine, respectively. The pH of the resulting solutions were adjusted
by using 0.1 M HNO3/H2O and 0.1 M NaOH/H2O to pH 7.0.
H2 gas was bubbled through the resulting solutions for 10 min.
The yields of a NiIIRuII hydride complex [NiII(m-SR)2(H2O)(m-
◦
6
least-squares plane of h -C6Me5 and that of NC5H4 = 98.67(8) }
as a result of the formation of the expected RuII-(h -C6Me5) and
6
RuII-(NC5H4) bonds.
6
H)RuII(h -C6Me6)](NO3) {[B](NO3)} were determined by UV-vis
spectroscopy.
X-ray crystallographic analysis
Crystallographic data for 1, [2](BF4)2 and [3](BF4)2 have been
deposited with the Cambridge Crystallographic Data Center
(CCDC reference numbers 749758–749760).† Measurements were
made on a Rigaku/MSC Mercury CCD diffractometer with
˚
graphite monochromated Mo-Ka radiation (l = 0.7107 A)
and Rigaku/MSC Saturn CCD diffractometer with confocal
˚
monochromated Mo-Ka radiation (l = 0.7107 A). Data were
collected and processed using the teXsan crystallographic software
package of Molecular Structure Corporation.
Results and discussion
A new ligand, Bz∧py
Fig. 3 An ORTEP drawing of 1 with ellipsoids at 50% probability. The
hydrogen atoms are omitted for clarity. Selected interatomic distances
A new 2-(pentamethylbenzyl)pyridine (Bz∧py) ligand was de-
6
signed to prepare a RuII centre having simultaneous RuII-(h -
◦
˚
(l/A) and angles (f/ ): Ru1-Cl1 = 2.400(1), Ru1-Cl2 = 2.420(1),
C6Me5) and RuII-(NC5H4) bonds with potentially two vacant
coordination sites on the metal centre. In aqueous media, these
vacant coordination sites should be occupied by two H2O
Ru1-N1 = 2.124(2), Ru1-C1 = 2.184(2), Ru1-C2 = 2.220(2), Ru1-C3 =
2.192(2), Ru1-C4 = 2.210(2), Ru1-C5 = 2.173(2), Ru1-C6 = 2.086(2),
Cl1-Ru1-Cl2 = 90.62(2), Cl1-Ru1-N1 = 88.31(6), Cl2-Ru1-N1 = 86.88(5).
2220 | Dalton Trans., 2010, 39, 2218–2225
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1
The dichloro RuII complex 1 was characterised by H NMR
spectroscopy in CDCl3 (Fig. 4), electrospray ionisation mass
spectrometry (ESI-MS) (Fig. S1 in ESI†), IR spectroscopy (Fig. S2
in ESI†) and elemental analysis. The signals at 7.19 ppm, 7.36 ppm,
7.73 ppm and 8.24 ppm correspond to the pyridine protons of the
Bz∧py ligand; the signal observed at 4.38 ppm is derived from the
methylene group of the Bz∧py ligand and the signals at 2.10 ppm,
2.13 ppm and 2.15 ppm are derived from the C6(CH3)5 group of
the Bz∧py ligand.
Fig. 6 An ORTEP drawing of [2](BF4)2 with ellipsoids at 50% probability.
The counter anions (BF4) and hydrogen atoms of Bz∧py are omitted for
◦
˚
clarity. Selected interatomic distances (l/A) and angles (f/ ): Ru1-O1 =
2.146(2), Ru1-O2 = 2.157(2), Ru1-N1 = 2.092(3), Ru1-C1 = 2.179(3),
Ru1-C2 = 2.188(3), Ru1-C3 = 2.234(3), Ru1-C4 = 2.194(3), Ru1-C5 =
2.183(3), Ru1-C6 = 2.092(3), O1-Ru1-O2 = 79.42(9), O1-Ru1-N1 =
83.7(1), O2-Ru1-N1 = 86.5(1).
Fig. 4 1H NMR spectrum of 1 in CDCl3 at 25 ◦C. TMS: the reference
with the methyl proton resonance set at 0.00 ppm.
Synthesis and structure of [RuII(Bz∧py)(H2O)2](BF4)2 {[2](BF4)2}
Following methods developed in this laboratory,20 the chloride
ions in complex 1 were reacted with 2 equivalents of AgBF4 to yield
a diaqua RuII complex [RuII(Bz∧py)(H2O)2](BF4)2 {[2](BF4)2} in
water at pH 7.0 (Fig. 5). This complex was characterised by X-ray
analysis (Fig. 6), 1H NMR spectroscopy (Fig. 7), ESI-MS (Fig. S3
in ESI†), IR spectroscopy (Fig. S4 in ESI†) and elemental analysis.
Fig. 7 1H NMR spectrum of [2](BF4)2 in D2O at pD 7.0 at 25 ◦C. TSP:
the reference with the methyl proton resonance set at 0.00 ppm.
2.17 ppm and 2.27 ppm are derived from the C6(CH3)5 group of
the Bz∧py ligand.
Synthesis and structure of the dinuclear NiIIRuII complex
Fig. 5 Synthesis of [2](BF4)2.
[NiII(l-SR)2RuII(Bz∧py)](BF4)2 {[3](BF4)2}
In the structure determined by X-ray analysis (Fig. 6), all
hydrogen atoms were included in the least-squares calculation of
[2](BF4)2. An ORTEP drawing of [2](BF4)2 demonstrates that 2
adopts a distorted-octahedral coordination, surrounded by one
Bz∧py and two H2O ligands.
We obtained
a
water-soluble NiIIRuII complex [NiII(m-
SR)2RuII(Bz∧py)](BF4)2 {[3](BF4)2} by the reaction of the diaqua
RuII complex [2](BF4)2 with the NiII complex [NiII(SR)2] in water
at pH 7.0 (Fig. 8).
Fig. 7 shows a 1H NMR spectrum of the diaqua RuII complex
[2](BF4)2 in D2O at pD 7.0. The signals at 7.20 ppm, 7.39 ppm,
7.78 ppm and 8.09 ppm correspond to the pyridine protons of the
Bz∧py ligand; the signal observed at 4.73 ppm is derived from the
methylene group of the Bz∧py ligand and the signals at 2.08 ppm,
The structure of [3](BF4)2 was characterised by IR spectroscopy
(Fig. S5 in ESI†), X-ray analysis (Fig. 9), 1H NMR spectroscopy
(Fig. 10), ESI-MS (Fig. 11) and elemental analysis. X-ray analysis
was performed on an orange crystal of [3](BF4)2, obtained from a
dichloromethane–n-hexane solvent mixture. Fig. 9 shows that the
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Fig. 8 Synthesis of [3](BF4)2.
Fig. 10 1H NMR spectrum of [3](BF4)2 in D2O at pD 7.0 at 25 ◦C. TSP:
the reference with the methyl proton resonance set at 0.00 ppm. †: the
protons of dichloromethane which was used to recrystallise [3](BF4)2.
Fig. 9 An ORTEP drawing of [3](BF4)2 with ellipsoids at 50% probability.
The counter anions (BF4) and hydrogen atoms are omitted for clarity.
◦
˚
Selected interatomic distances (l/A) and angles (f/ ): Ru1-S1 = 2.421(1),
Ru1-S2 = 2.422(1), Ru1-N1 = 2.173(2), Ni1-S1 = 2.172(1), Ni1-S2 =
2.180(1), Ni1-N2 = 1.976(2), Ni1-N3 = 1.967(2), Ni1-S1-Ru1 = 88.90(2),
Ni1-S2-Ru1 = 88.67(2).
Fig. 11 (a) Positive-ion ESI mass spectrum of [3](BF4)2 in methanol. (b)
Signal at m/z 309.6 for [3]2+. (c) Calculated isotopic distribution for [3]2+
†: the signal of {[3 + Cl]+}. ‡: the signal of {[3 + F]+}.
.
structure of 3 contains a NiS2Ru butterfly core, in which the Ni
atom and the Ru atom are linked by two thiolato units of SR. The
Ni1-S1-Ru1 and Ni1-S2-Ru1 angles are 88.90(2) and 88.67(2)◦,
respectively. The Ni atom of 3 adopts a square planar geometry
with a SR ligand, whereas the Ru atom of 3 adopts a distorted-
octahedral coordination which is surrounded by one Bz∧py and
the metalloligand [NiII(SR)2].
The effects of changing Ru-ligand basicity
As previously reported, completely replacing H2O with MeCN to
6
form [NiII(m-SR)2RuII(MeCN)(h -C6Me6)](NO3)2 in 100% MeCN
1
Fig. 10 shows a H NMR spectrum of the NiIIRuII complex
renders the complex inactive.16 Subsequently, however, we con-
sidered that titration experiments would prove more informative
and thus we performed a series of such measurements, utilising
several different possible ligands. These experiments were intended
to investigate how changing the Ru-coordinated ligand would
enhance or inhibit the heterolytic splitting of H2 by [Aaqua](NO3)2
to form [B](NO3).
[3](BF4)2 in D2O at pD 7.0. The signals at 7.16 ppm, 7.35 ppm,
7.53 ppm and 7.89 ppm correspond to the pyridine protons of
the Bz∧py ligand; the signal observed at 4.57 ppm is derived
from the methylene group of the Bz∧py ligand; the signals at
2.23 ppm, 2.49 ppm and 2.71 ppm are derived from the C6(CH3)5
group of the Bz∧py ligand, the signal at 2.86 ppm corresponds
to the methyl protons bound to the N atoms and the signals
at 1.67-1.89 ppm, 2.02-2.04 ppm, 2.12-2.19 ppm, 2.42-2.44 ppm
and 3.16-3.40 ppm are derived from the methylene group of the
metalloligand [NiII(SR)2] of 3.
Fig. 11a shows a positive-ion ESI mass spectrum of [3](BF4)2 in
methanol. The prominent signal at m/z 309.6 {relative intensity
(I) = 100% in the range of m/z 100-2000} has a characteristic
distribution of isotopomers (Fig. 11b) that matches well with the
calculated isotopic distribution for [3]2+ (Fig. 11c).
The results of titrating [Aaqua](NO3)2 with MeCN are shown in
Fig. 12 (closed circle). Clearly, MeCN has an inhibitory effect on
the action of [Aaqua](NO3)2,—in an aqueous solution, the addition
of 100 equivalents of MeCN reduces the production of [B](NO3)
from 100% to around 45%. Whilst MeCN is capable of inhibiting
the action of [Aaqua](NO3)2, it cannot shut it down completely,
and this observation is supported by previous experiments which
showed that adding water to the MeCN-coordinated complex in
an MeCN solution revives the H2 splitting activity.
2222 | Dalton Trans., 2010, 39, 2218–2225
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Fig. 12 A correlation between the equivalent of X {MeCN (closed circle)
or pyridine (open circle) to [Aaqua](NO3)2} and the yield of [B](NO3) in
water at pH 7.0 at 25 ◦C.
Fig. 14 pH-dependent ratio of [Aaqua](NO3)2 and [Ahydroxo](NO3) in water
(closed circle). The titration of an aqueous solution of [Aaqua](NO3)2
(100 mM) with 0.1 M NaOH and 0.1 M HNO3 under an Ar atmosphere was
performed, which was monitored by UV-vis spectroscopy. pH-dependent
yields of [B](NO3) from the reaction of [Aaqua](NO3)2 with H2 in water
(open circle).
We considered the inhibitory activity of MeCN to be the result
of its ability to displace H2O as a ligand. Since MeCN has only
one lone pair available for coordination, it cannot simultaneously
bind to Ru and H2, thereby rendering the complex inert to H2. If
this is the case, N-based ligands with a higher pKa should have a
stronger inhibitory effect as they are less likely to be displaced by
H2O.
This conclusion was borne out by titration experiments with
pyridine (open circle in Fig. 12). Only 2 equivalents of pyridine
was able to reduce the yield of hydride species from 100% to 70%.
This inhibition continued steadily, reducing the yield to 15% at 10
equivalents and finally reaching 0% yield at around 60 equivalents
of pyridine. Clearly, the stronger basicity of pyridine completely
blocks the RuII centre from encroaching H2O ligand, effectively
poisoning the potential catalyst.
In order to confirm our proposals regarding the action of
pyridine, we attached a pyridine moiety directly to the hexam-
ethylbenzene ligand. This arrangement was expected to direct
the binding of the pyridine moiety exclusively to the RuII centre,
increasing the effective molarity of the pyridine whilst only
incorporating one equivalent of pyridine moiety with respect to
Ru. Indeed, this modification rendered the complex completely
inert to H2 in water in a range of pH 2.0-11.0 (Fig. 13), confirming
that coordination to Ru is responsible for inhibition, rather than
simply adding base.
completely unable to form a hydride, indeed, the titration curve
for deprotonation (closed circle in Fig. 14) is the direct inverse
of the curve for H2-activation (open circle in Fig. 14). Therefore,
we conclude that the ability to coordinate to H2 is not enough to
induce heterolytic splitting—there is also a need for a labile H3O+
ion as a leaving group.22
Complex [AX](NO3)n {X = MeCN (n = 2), pyridine (n = 2) or
OH- (n = 1)} was unreactive toward H2 in water at pH 7.0 at 25 ◦C
(Fig. 15). In contrast, the reaction of [Aaqua](NO3)2 with H2 in water
at pH 7.0 at 25 ◦C obeyed first-order kinetics (v = kobs[Aaqua]) under
a H2 atmosphere. The rate constant kobs was determined as 2.7 ¥
10-3 s-1 by UV-vis spectroscopy (Fig. S6 in ESI†).
Fig. 13 Reaction of 3 with H2 in water at pH 2.0-11.0.
Taking the series further, we deprotonated the Ru-coordinated
Fig. 15 Reactivity of [AX](NO3)n {X = MeCN (n = 2), pyridine (n = 2)
or OH- (n = 1)} and [Aaqua](NO3)2 toward H2.
H2O of [Aaqua](NO3)2 to form the hydroxo complex [NiII(m-
6
SR)2RuII(OH)(h -C6Me6)](NO3) {[Ahydroxo ](NO3)} above pH 8.4
(Fig. 14).21 This change leaves the oxygen in place, now with three
lone pairs, but it significantly increases the basicity of the ligand
and results in a concomitant reduction in lability. Subsequent
experiments showed that the hydroxo complex [Ahydroxo ](NO3) was
Fig. 16 summarises the properties of the RuII-coordinated
ligands investigated in this study with reference to the Lewis
basicity of the ligand and availability of lone pairs for binding.
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Acknowledgements
We thank Dr Yuichi Kaneko for the synthesis of the Bz∧py ligand.
This work was supported by grants-in-aid: 17350027, 17655027,
18033041 and 18065017 (Chemistry of Concerto Catalysis), the
Global COE Program, “Science for Future Molecular Systems”
from the Ministry of Education, Culture, Sports, Science and Tech-
nology, Japan and the Basic Research Programs CREST Type,
“Development of the Foundation for Nano-Interface Technology”
from JST, Japan.
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©