Synthesis and reactivity of a water-soluble NiRu monohydride complex with a tethered pyridine moiety

Article abstract of DOI:10.1246/cl.151029

We report the synthesis, characterization, and reactivity of a NiRu monohydride complex with a pyridine-bound hexamethylbenzene ligand. We investigate the mechanistic insights it provides on the H₂-activation of [NiFe]hydrogenase.

Full text of DOI:10.1246/cl.151029

Received: November 6, 2015 | Accepted: December 3, 2015 | Web Released: February 5, 2016
CL-151029
Synthesis and Reactivity of a Water-soluble NiRu Monohydride
Complex with a Tethered Pyridine Moiety
Takahiro Matsumoto,1,2,3 Koji Yoshimoto,1,2,3 Chunbai Zheng,2 Yasuhito Shomura,1,4
Yoshiki Higuchi,4 Hidetaka Nakai,1,2,3 and Seiji Ogo*1,2,3
1Center for Small Molecule Energy, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395
2Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University,
744 Moto-oka, Nishi-ku, Fukuoka 819-0395
3International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University,
744 Moto-oka, Nishi-ku, Fukuoka 819-0395
4Department of Life Science, Graduate School of Life Science, University of Hyogo,
3-2-1 Koto, Kamigori-cho, Ako-gun, Hyogo 678-1297
(E-mail: ogo.seiji.872@m.kyushu-u.ac.jp)
We report the synthesis, characterization, and reactivity of
a NiRu monohydride complex with a pyridine-bound hexa-
methylbenzene ligand. We investigate the mechanistic insights
it provides on the H2-activation of [NiFe]hydrogenase.
Since then, we have been able to produce a pyridine-
¹
coordinated NiIIRuII monohydride species by using BH4
instead of H2. Here, we report the synthesis of this water-
soluble pyridine-coordinated NiIIRuII monohydride complex
[NiII(L)(μ-H)RuII(Bz^py)]+ (4, Figure 2) and its reactivity
toward H2 in water.
H2-activation by [NiFe]hydrogenase ([NiFe]H2ase) is of great
importance and interest (Figure S1 in Supporting Information) in
various fields.1,2 Although there are several studies on the mono-
hydride and dihydride species of [NiFe]H2ase (Figure S1),3-5 little
is known about their formation mechanism and reactivity.6-10
We have previously reported that an H2O-coordinated
NiIIRuII complex, [NiII(L)RuII(H2O)(η6-C6Me6)]2+ (1, L: N,N¤-
dimethyl-3,7-diazanonane-1,9-dithiolato), reacted with H2 to
give a hydrido-ligand-bridged NiIIRuII complex, [NiII(L)(H2O)-
(μ-H)RuII(η6-C6Me6)]+ (2), that further reacted with H2 to
give dihydride species (A) via heterolytic cleavage of H2
(Figure 1a).5,6 We expected the undetermined oxygen-bearing
ligand {(Ni-SIr)II, Figure S1} in the active site of H2ase to play a
crucial role as a Lewis base in the heterolytic activation of H2.
Therefore, it came as no surprise that the pyridine-coordinated
NiIIRuII complex [NiII(L)RuII(Bz^py)]2+ {3, Bz^py: 2-(penta-
methylbenzyl)pyridine}, which has no available site for hydro-
gen complexation, was unreactive toward H2 (Figure 1b).11
These hydride model studies are important because it has been
proposed that [NiFe]H2ase reacts with two consecutive mole-
cules of H2, via intermediate hydride species.4,5
Complex 3 reacted with NaBH4 in H2O at pH 7.0 and at
25 °C to yield the bridging hydride complex 4 (Figure 2). The
structure of 4 was determined by X-ray analysis (Figure 3),
ESI-MS (Figure 4), and EPR and IR spectroscopies (Figure S2
in Supporting Information).
+
2+
N
N
N
NiII
RuII
N
S
–
BH4
N
H
S
N
NiII
RuII
S
S
BH3
3
4
Figure 2. A synthetic scheme of 4 in water.
(a)
+
2+
H2O
N
N
H
S
H
H
H2
H2
N
N
NiII
RuII
NiII
RuII
S
S
NiIIRuII
S
H2O
1
2
A
(b)
2+
N
H2
N
NiII
RuII
N
Figure 3. An ORTEP drawing of [4](PF6) with ellipsoids at 50%
probability. The counter anion (PF6) and hydrogen atoms of L and
Bz^py are omitted for clarity. Selected interatomic distances (l/¡)
and angles (º/degree): Ni1£Ru1 = 2.877(1), Ru1-H1 = 1.65(4),
Ru1-S1 = 2.3911(13), Ru1-S2 = 2.3967(13), Ni1£H1 = 2.19(4),
Ni1-S1 = 2.1984(13), Ni1-S2 = 2.2028(12), Ni1-N1 = 1.983(3),
Ni1-N2 = 1.970(3), H1£N3 = 5.24(5), Ni1-S1-Ru1 = 77.52(4),
Ni1-S2-Ru1 = 77.32(4).
S
S
3
Figure 1. H2-activation by (a) NiRu(η6-C6Me6) complex and (b)
NiRu(Bz^py) complex.
Chem. Lett. 2016, 45, 197–199 | doi:10.1246/cl.151029
© 2016 The Chemical Society of Japan | 197

+
+
N
N
D+ H+
N
N
H
D
N
N
NiII
RuII
NiII
RuII
S
S
D+ H+
S
S
4
D-labeled 4
Figure 5. H+/D+ exchange of the hydrido ligand of 4.
2+
N
N
NiII
RuII
N
S
S
H2
PhCH2OH
2Fc + H+
NaBH4
3
Figure 4. (a) A positive-ion ESI mass spectrum of [4](PF6)
in methanol. (b) The signal at m/z 620.1 corresponds to [4]+.
(c) Calculated isotopic distribution for [4]+. (d) A positive-ion ESI
mass spectrum of [D-labeled 4](PF6) in methanol. The signal at
m/z 621.1 corresponds to [D-labeled 4]+.
2Fc+
PhCHO + H+
H+
Na+ + BH3
+
N
N
H
Dark-red crystals of [4](PF6) suitable for X-ray analysis
were obtained from a mixed solvent of dichloroethane/hexane.
The X-ray diffraction data were collected at SPring-8 beamline.
The residual electron density derived from the hydrido ligand
was observed and its position was determined. An ORTEP
drawing of [4](PF6) is shown in Figure 3. The Ru atom adopts
a distorted-octahedral coordination, which is surrounded by one
Bz^py, metalloligand [NiIIL], and one hydrido ligand. The Ni-
S-Ru angles {77.32(4) and 77.52(4)°} of 4 are significantly
larger than those of 2 {70.7(3) and 70.4(2)°}.6 Due to the larger
angles, the distance {2.19(4) ¡} between Ni and the hydrido
ligand in 4 is much longer than that in 2 {1.859(7) ¡}, but the
distance {1.65(4) ¡} between Ru and the hydrido ligand is
almost the same as that in 2 {1.676(8) ¡}.
N
NiII
RuII
S
S
4
Figure 6. Formation and reducing ability of 4.
H+, PhCHO, ferrocenium ion ([FeIII(η5-C5H5)2]+, Fc+), and
methylene blue (MB+) in water (Figure 6 and Figure S5 in
Supporting Information). The corresponding products, viz. H2,
PhCH2OH, ferrocene ([FeII(η5-C5H5)2], Fc), and leukomethylene
blue (MBH), respectively, were formed, which indicates that 4
is capable of performing both hydride and electron transfers.
We previously reported that the NiIIRuII hydride complex 2
reacts with H2 to form a low-valent NiIRuI complex via
reductive elimination of H2 from dihydride species A in water
under ambient conditions.5 In contrast with 2, surprisingly, the
NiIIRuII hydride complex 4 was unreactive toward H2 in water,
as confirmed by ESI-MS and UV-vis spectroscopy. This clearly
indicated that the pyridyl ligand of Bz^py prevented the
heterolytic activation of H2.
An EPR spectrum indicates paramagnetism in 4 (Figure S2).
It indicates that the hydrido ligand interacts with the Ni atom,
which causes the square pyramidal structure of the Ni atom in
4 (S = 1).
Figure 4a shows a positive-ion ESI mass spectrum of
[4](PF6) in methanol. The prominent signal at m/z 620.1
(relative intensity = 100% in the range of m/z 200-1000) has a
characteristic isotopic distribution (Figure 4b) that matches well
with the calculated isotopic distribution for [4]+ (Figure 4c).
To confirm the origin of the hydrido ligand of [4](PF6), we
synthesized [D-labeled 4](PF6) by the reaction of [3](BF4)2 with
NaBD4 in D2O at pD 7.0. In the ESI mass spectra, the signal at
m/z 620.1 shifts to 621.1 (Figure 4d). This result demonstrates
that a deuterium atom is incorporated into [D-labeled 4](PF6).
The IR spectrum of [4](PF6) shows a hydrido-derived band at
1812 cm¹1, which is slightly higher than that found for [2](NO3)
(1740 cm¹1).
The hydrido ligand of 4 exhibits protic character and
undergoes H+/D+ exchange with D+ in D2O at pD 6.0-10
(Figure 5 and Figures S3 and S4 in Supporting Information).
It was confirmed by ESI-MS that complex 4 was quite stable
below 70 °C under an Ar atmosphere at pH 6.0-11. We
investigated the pH-dependent reducing ability of 4 toward
We previously suggested that the bridging hydrido ligand of
2 is considered to easily shift to the Ni side because the Ni-
coordinated ligand is so flexible that an equatorial N-donor
moves to an axial position in order to accept the shifted hydrido
ligand (Figures S1 and S6 in Supporting Information).12 This
hydride shift is much less likely in 4, however, given the greater
¹
separation of Ni and H , meaning that the resulting species is
unreactive toward H2.
This result is significant because it informs us about the
relative importance of Ni and Ru in the second H2 reduction by
fully functioning catalyst 2. If the Ni atom played a key role in
catching and activating a second molecule of H2, complex 4
would be expected to react with H2 more quickly because the
hydrido ligand has moved away from the Ni center, leaving a
mostly vacant coordination site. Rather, a vacant Ru site seems
to be key since complex 2 is able to further react with H2;
198 | Chem. Lett. 2016, 45, 197–199 | doi:10.1246/cl.151029
© 2016 The Chemical Society of Japan

however, 4, which cannot generate a vacant Ru site by shifting
the hydrido to Ni, cannot react with H2 (Figure S7 in Supporting
Information). Given the ability of our model complexes to
faithfully reproduce the chemical behavior of [NiFe]H2ase, we
propose that, in natural enzymes, the second H2 coordination
occurs at the Fe center, rather than the Ni center.
In conclusion, we have succeeded in the isolation and
crystal structure determination of a new water-soluble NiIIRuII
monohydride complex supported by a hexamethylbenzene
ligand with a flexible pyridine arm. Its reactivity toward H2
and where H2 binds on the NiRu core were discussed. These
results should provide valuable insights into the elucidation of
the mechanism of natural H2ases.
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This work was supported by Grants-in-Aid: Nos. 26000008
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Chem. Lett. 2016, 45, 197–199 | doi:10.1246/cl.151029
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