An Iodido-Bridged Dimer of Cubane-Type RuIr3S4 Cluster: Structural Rearrangement to New Octanuclear Core and Catalytic Reduction of Hydrazine

Article abstract of DOI:10.1002/ejic.201901146

Nitrogen-fixing enzymes contain octanuclear metal–sulfur clusters at the active site, which are constructed on the basis of combined two cubic M₄S₃C skeletons. In this study, the dimer of cubane-type RuIr₃S₄ cluster [{(Cp*Ir)₃(μ₃-S)₄Ru}₂(μ₂-I)₃]I (2: Cp* = η⁵-C₅Me₅) was synthesized via oxidation of [(Cp*Ir)₃(μ₃-S)₄(CymRu)] (Cym = η⁶-p-iPrC₆H₄Me) with I₂ followed by ligand exchange. Two cubane cores are bridged by three iodido ligands in 2, while these cubes are fused into a unique Ru₂Ir₆S₈ framework by 2e-reductuion to give [(Cp*Ir)₆Ru₂(μ₃-S)₈][I]₂. Addition of excess PhNHNH₂ to 2 cleaved the dimer structure to form the hydrazine adduct of single cubane [(Cp*Ir)₃(μ₃-S)₄{RuI(NH₂NHPh)₂}]I. Reduction of N₂H₄ with Cp₂Co and [HNEt₃][BF₄] was catalyzed by 2 in much higher rate than disproportionation of N₂H₄. The molecular structures of all new cluster compounds were characterized by X-ray diffraction studies.

Full text of DOI:10.1002/ejic.201901146

A Journal of
Accepted Article
Title: An Iodido-Bridged Dimer of Cubane-type RuIr3S4 Cluster:
Structural Rearrangement to New Octanuclear Core and Catalytic
Reduction of Hydrazine
Authors: Hidetake Seino, Keiichi Hirata, Yusuke Arai, Risa Jojo, and
Masaaki Okazaki
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201901146
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10.1002/ejic.201901146
European Journal of Inorganic Chemistry
FULL PAPER
An Iodido-Bridged Dimer of Cubane-type RuIr3S4 Cluster:
Structural Rearrangement to New Octanuclear Core and Catalytic
Reduction of Hydrazine
Hidetake Seino,*[a] Keiichi Hirata,[b] Yusuke Arai,[a] Risa Jojo,[a] and Masaaki Okazaki[c]
Abstract: Nitrogen-fixing enzymes contain octanuclear metal–sulfur
clusters at the active site, which are constructed on the basis of
combined two cubic M4S3C skeletons. In this study, the dimer of
cubane-type RuIr3S4 cluster [{(Cp*Ir)3(μ3-S)4Ru}2(μ2-I)3]I (2: Cp* = η5-
C5Me5) was synthesized via oxidation of [(Cp*Ir)3(μ3-S)4(CymRu)]
(Cym = η6-p-iPrC6H4Me) with I2 followed by ligand exchange. Two
cubane cores are bridged by three iodido ligands in 2, while these
cubes are fused into a unique Ru2Ir6S8 framework by 2e-reductuion
to give [(Cp*Ir)6Ru2(μ3-S)8][I]2. Addition of excess PhNHNH2 to 2
cleaved the dimer structure to form the hydrazine adduct of single
cubane [(Cp*Ir)3(μ3-S)4{RuI(NH2NHPh)2}]I. Reduction of N2H4 with
Cp2Co and [HNEt3][BF4] was catalyzed by 2 in much higher rate than
disproportionation of N2H4. The molecular structures of all new
cluster compounds were characterized by X-ray diffraction studies.
nitrogenase substrates such as hydrazine and ethyne.[5] Among
all the sulfido clusters except for the protein-embedded
nitrogenase cofactors, coordination and reduction of molecular
N2 have been attained only with cubane-type clusters containing
biological and non-biological transition metals.
O
O
O–
O–
O–
O–
H
N
H
N
O
O
O
O
O
O
N
N
O
O
His
His
Mo
S
V
S
S
S
S
S
Fe
Fe
Fe
Fe
Fe
C
Fe
C
O
C
O
O
S
S
S
S
S
Fe
Fe
Introduction
Fe
Fe
S
Fe
Fe
S
S
S
S
S
Biological nitrogen fixation that reduces N2 gas into ammonia is
accomplished by nitrogenase enzymes under atmospheric
pressure and ambient temperature.[1] Their active site contains a
large metal–sulfur cluster called FeMo-cofactor, which consists
of seven iron, one molybdenum, nine sulfur, and one carbon
atoms, in the case of Mo-nitrogenase at its resting state (Figure
1).[2] Presence of a closely related cluster structure has been
confirmed for the V-nitrogenase, where the molybdenum center
and one of three μ2-sulfides are replaced by a vanadium atom
and a μ2-carbonate, respectively.[3] The geometry of these
clusters is constructed from two cubane-type M4S3C skeletons
sharing one interstitial carbon atom, and such basic frameworks
are preserved not only in resting state but also in substrate
bound structures.[4] Synthetic Fe4S4 and (Mo/V)Fe3S4 single-
cubane clusters have been studied since 1970s as model
compounds of the above enzyme cofactors, and some of them
have been demonstrated to catalyze reduction of some
Fe
Fe
S
S
Cys
Cys
Figure 1. Structures of FeMo-cofactor and FeV-cofactor in nitrogenases.
However, only limited numbers of sulfido clusters have
been discovered to activate molecular N2 at present (Figure 2)
The first example bearing N2 ligand has been synthesized by
Mizobe and co-workers using a cubane-type RuIr3S4 core.[6]
Stable coordination of N2 to the RuII corner is attributed to strong
back donation from this metal center, which is directly bound to
three sulfido donors and surrounded by three coordinatively
saturated IrIII centers. However, the terminal N2 ligand is not
sufficiently activated for conversion to any nitrogen compounds.
Very recently, Ohki group has isolated the double-cubane
clusters, in which two TiMo3S4 cubes are bridged by an end-on
N2 ligand bound to the Ti vertices.[7] In accordance with high
degree of N2 activation indicated by crystallographic and
spectroscopic data, the N2 ligand is protonated with HCl or H2O
to produce NH3 and N2H4 in sub-stoichiometric yields. Further,
photocatalytic reduction of N2 into NH3 has been demonstrated
in Kanatzidis group by using a series of chalcogenide aerogels
composed of the Fe4S4 and MoFe3S4 building blocks linked by
anionic tin sulfides.[8]
[a]
Prof. Dr. H. Seino, Y. Arai, R. Jojo
Faculty of Education and Human Studies, Akita University
Tegata-Gakuenmachi 1-1, Akita 010-8502, Japan
E-mail: seino@gipc.akita-u.ac.jp
[b]
[c]
K. Hirata
Institute of Industrial Science, The University of Tokyo
Komaba 4-6-1, Meguro-ku, Tokyo 153-8505, Japan
Prof. Dr. M. Okazaki
Graduate School of Science and Technology, Hirosaki University
Bunkyo-cho 3, Hirosaki 036-8561, Japan
Supporting information and ORCID(s) from the author(s) for this
article are available on the WWW under https://doi.org/10.1002/ejic.
This article is protected by copyright. All rights reserved.

10.1002/ejic.201901146
European Journal of Inorganic Chemistry
FULL PAPER
THF THF
K
N
N
even at room temperature in CH2Cl2 to yield a single species.
The product was isolated as brown crystals and determined by
single crystal X-ray diffraction study to be the iodido-bridged
double-cubane cluster [{(Cp*Ir)3(μ3-S)4Ru}2(μ2-I)3]I (2) (Scheme
1). In contrast to the reactivity of 1[I]2, the PF6– salt of 12+ is fairly
stable in CH2Cl2, while substitution of its Cym ligand smoothly
takes place in MeCN, probably triggered by coordination of the
solvent to the Ru center.
Me
S
S
Me
S
Ir
S
N
Ru
S
Mo
Mo
Mo
Mo
Ti N N Ti
N
Me
Ir
S
S
S
S
S
Me
S
S
Ir
K
Mo
Mo
THF THF
Ph
S
Ph
S
3–
Cl
Cl
2–
[I]2
Cl
Cl
S
S
Cl
Fe
S
Fe Fe
Fe Fe
Mo
Mo
S
S
Ir
S
S
Ir
S
+ [Sn2S6]4– +
S
S
S
S
Ru
S
S
Ru
S
S
S
S
S
S
Ph
I2
Fe
Fe
Fe
S
Fe
Cl
Fe
Cl
Ir
Ir
Cl
Cl
Ir
Ir
Cl
THF
Chalcogels capable of photochemical N2 reduction
1
1[I]2
Figure 2. Activation of molecular N2 by sulfido clusters. Moderate activation by
coordination (up-left), sub-stoichiometric reduction to ammonia (up-right), and
photocatalytic conversion to ammonia (down).
I
I
I
S
S
Ir
2 moles
Ir
Ir
Ir
Because the assemblies of two M4S4 cores fulfill the same
nuclearity of FeMo-cofactor and another cluster in nitrogenase
(P cluster), a number of approaches have been examined to
synthesize biomimetic octanuclear cores from cubane-type
Fe4S4 and MoFe3S4 building blocks.[5a,9,10] Accumulation of
cluster units can also be an effective strategy in general catalyst
developments, since unique catalysis of the M4S4 clusters
Ru
Ru
S
S
CH2Cl2
30 °C
S
S
S
S
I
Ir
Ir
( – p-iPrC6H4Me )
2
[I]2
containing
noble
metals
has
been
recognized
increasingly.[5c,11,12] The cubane-type RuIr3S4 core is a member
capable of N2 activation, and its constitution abundant in valence
electrons is distinct from more common base metal clusters. In
this study, a dimeric form of the RuIr3S4 cubane was synthesized
and fused into a different Ru2Ir6S8 core. It was also revealed that
the cubane dimer was readily cleaved into single cubes and able
to catalyze reduction of hydrazine.
Ir
S
S
2 Cp2Co
THF
Ir
Ir
S
S
Ru
Ru
S
S
S
Ir
S
( –2 [Cp2Co]I )
Ir
Ir
3
Scheme 1. Redox-involving conversion of cubane-type RuIr3S4 clusters.
Results and Discussion
The structure of the metal–sulfur framework of 2 is shown
in Figure 3(up). The cation consists of two cubane-type RuIr3S4
cores, which are triply bridged by μ2-iodido ligands connecting
two Ru corners. Analogous double-cubane structures have been
found in [{(RS)3Fe3(μ3-S)4M}2(μ2-ER')3]3– systems (M = Mo, W, V,
Nb; ER' = SR, OMe)[13–15] and [{(Cp*Mo)3(μ3-S)4M}2(μ2-Cl)3] (M =
V, Cr).[16] Each Ru atom in 2 has a distorted octahedral
geometry, in which three bridging I atoms are located at the
trans positions of three S atoms (I–Ru–S angles at 167.08(9)–
171.96(9)°) to form a facial configuration. The distance between
two Ru atoms is 3.595(2) Å, indicating absence of direct bonding
interactions between two RuIr3S4 units. In each cubane core,
one intermetallic bond is found between Ru and one of Ir atoms,
corresponding to 70-electron count for the single cube (d(Ru–Ir)
= 2.819(1), 2.836(1) Å). With respect to the bridging iodides, the
Synthesis and Characterization of Double-Cubane and
Fused Double-Cubane Clusters
We have previously reported that the cubane-type cluster
[(Cp*Ir)3(μ3-S)4(CymRu)] (1: Cym = η6-p- iPrC6H4Me; Cp* = η5-
C5Me5) serves as a precursor of various RuIr3S4 derivatives.[6a]
Two-electron oxidation of 1 induces dramatic weakening of the
Ru–Cym bond, which enables coordination of a wide variety of
σ-donors to the Ru atom. Oxidation of 1 was performed with two
equiv of [Cp2Fe][PF6] in the previous work, giving the salt 1[PF6]2.
In this work, 1 was treated with equimolar amount of I2 in THF,
and the NMR measurement of the resulting solid product
supported formation of the same dication 12+. However, it was
observed that the iodide salt 1[I]2 slowly released p-cymene
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10.1002/ejic.201901146
European Journal of Inorganic Chemistry
FULL PAPER
Ru–I bonds vertical to the Ru–Ir bond axes are slightly longer
than others (2.828(2) and 2.807(2) Å vs. 2.752(2)–2.782(1) Å).
The solution H NMR spectrum measured at 22 °C exhibits one
interactions (d(Ru–Ru) = 2.860(1); d(Ru–Ir) = 2.776(1)–2.836(1);
d(Ir–Ir) = 2.7911(5) Å), and one Ru–Ir bond (2.8073(8) Å) is
observed in the RuIr3S4 cube. Presence of seven metal–metal
bonds is consistent with a 130-electron Ru2Ir6 core. Therefore,
all metal centers are connected through eight μ3-sulfides and
linked through metal–metal bonds except for Ir(2) and Ir(3)
atoms. If Ru–Ir bonds are ignored, coordination of five S atoms
to the Ru(1) center in the cubane core forms a distorted tbp
geometry, and the Ru(1)–Ir(1) bond axis virtually lies on its
equatorial plane. With respect to the other Ru(2) center, two
RuS2 triangles each defined in RuIr2S2 and Ru2IrS2 tbp units
exhibit a dihedral angle of 75°. Although the solid-state structure
has a C1 symmetry, the 1H NMR spectrum of 3 in CD3CN
showed only three signals of Cp* ligands in a 3:2:1 ratio at room
temperature. This indicates presence of fluxional behaviors that
exchange three Ir sites in the RuIr3S4 cube each other and make
two Ir centers in the RuIr2S2 tbp unit equivalent. The dynamic
motion may be explained by the combination of Ru(1)–Ir bond
migration inside the cubane core and pseudo-rotation around
the penta-coordinate Ru(1) center.
1
broad signal for the Cp* ligands at δ = 1.91, and this probably
indicates dynamic migration of the Ru–Ir bond in the cubane
core as observed also for a single cubane.[6a] While the iodide
counter anion is not directly interacting with the cationic part in
solid state of 2, incorporation of three CH2Cl2 molecules around
the juncture of double-cubane is found (Figure 3(down)). Each
solvated CH2Cl2 is adjacent to two μ2-I atoms and their C–H
bonds are directed toward the S atoms of different RuIr3S4 units
facing each other.
Figure 3. Crystal structure of 2·3(CH2Cl2): views of the cluster core (up) and
the full cationic part with solvated CH2Cl2 molecules (down). Thermal ellipsoids
are drawn at 50% probability and hydrogen atoms are omitted for clarity.
Figure 4. Crystal structure of the dicationic part of 3. Thermal ellipsoids are
drawn at 50% probability and hydrogen atoms of the Cp* ligands are omitted
for clarity.
Because
oxidation state can coordinate molecular N2 as mentioned
above,[6] we examined reduction of formal [RuIVIrIII
(or
a ]
cubane-type RuIr3S4 cluster at [RuIIIrIII
3
]
3
[RuIIIIrIVIrIII2]) cores of 2. Treatment with excess (> 4 equiv)
cobaltocene (Cp2Co) in THF under 1 atm N2 followed by
recrystallization from MeCN/Et2O afforded black crystals of new
cluster compound (Scheme 1). Single crystal X-ray analysis
revealed novel octanuclear structure of the dicationic cluster
[(Cp*Ir)6Ru2(μ3-S)8][I]2 (3) as shown in Figure 4. The cluster
framework contains a cubane-type RuIr3S4 core and a trigonal
bipyramidal (tbp) RuIr2S2 unit, and two Ru atoms in these
substructures are bridged by additional IrS2 fragment (Ir(4)) to
form another tbp architecture of Ru2IrS2. All intermetallic
distances in each tbp M3S2 unit indicate presence of bonding
The conversion of
2 into 3 involves a two-electron
reduction process: the oxidation state of cluster core changes
from {[RuIr3S4]5+}2 to [Ru2Ir6S8]8+. The reduction is accompanied
by shape shift of one of two RuIr3S4 cores, while the other
cubane-type framework is retained with minimal alteration of
coordination environment around the Ru center. This core
rearrangement is regarded as that two M2S2 squares
constructing
a M4S4 cube slip each other. Such kind of
reorganization has been observed in a Re2IrPdS4 system, in
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10.1002/ejic.201901146
European Journal of Inorganic Chemistry
FULL PAPER
which an internal redox process triggers shift into a different
shape.[17] Some cluster frameworks related to the structure of 3
has been reported to date. The hexanuclear cluster [MoFe5(μ3-
S)6(CO)6(PEt3)3] is composed of a cubane-type MoFe3S4 and a
tbp MoFe2S2 units by sharing the Mo vertex.[18] In the
Table 1. Catalytic disproportionation of N2H4 by cubane-type M4S4 clusters.
[b]
[b]
catalyst
N2H4/catalyst[a]
temperature,
time
NH3
N2
Reference
nonanuclear
cluster
[{(Cp*Mo)3(μ3-S)4Ru}2(μ3-
[c]
[c]
S)2{Pt(PPh3)2}][PF6]2, two RuMo3S4 cubes are connected by a
PtS2 moiety, which bridges the two Ru atoms to form a tbp
PtRu2S2 moiety.[19]
MoFe3-1
MoFe3-2
RuMo3-1
RuMo3-2
RuMo3-3
2
10
10
20
20
20
100
r.t., 1 h
8.6
0.7
6.6
–
–
21a
21a
24
r.t., 1 h
The FeIr3S4 congeners [(Cp*Ir)3(μ3-S)4(FeCl)]0/+ have been
previously synthesized and characterized at two different
oxidation states.[20] The single-cubane structure, in which the Fe
corner binds a chlorido ligand to configure a distorted tetrahedral
geometry, is preserved at both [FeIr3S4]4+ and [FeIr3S4]5+ states.
Dimer formation of formal [RuIr3S4]4+ (3) and [RuIr3S4]5+ (2) cores
may be attributed to that Ru and I atoms have large radii as well
as that Ru prefers relatively high coordination numbers.
60 °C, 18 h
60 °C, 18 h
60 °C, 18 h
r.t., 22 h
1.6
4.7
2.0
16
20.0
6.9
50
24
24
This work
[a] Charged amount of N2H4 per single cubane unit. [b] Equivalents of formed
NH3 and N2 per single cubane unit. [c] Not reported.
+
2–
Reactions with Hydrazines
Ln
L
Cl
S
S
Mo
S
Fe
S
Reduction of hydrazine compounds have been investigated by
using wide variety of cubane-type cluster catalysts having
Cl
R3P Ru
O
Mo
O
S
S
Cl
Mo
Fe
Cl
Cl
MoFe3S4,[21] VFe3S4,[21a,22] Mo2M2S4 (M
=
Rh, Ir),[23] and
S
S
Mo
Fe
Cl
[24]
RuMo3S4
cores. In the absence of reducing agent, double-
Cl
cubane cluster 2 showed moderate activity in disproportionation
MoFe3-1: L = MeCN
MoFe3-2: L = PEt3
RuMo3-1: R = Ph, Ln = (H)2
RuMo3-2: R = cHex, Ln = (H)2
RuMo3-3: R = Ph, Ln = (NH3)
of N2H4, in which N2H4 serves as both oxidant and reductant.
When
2 was reacted with 200 equiv of N2H4 at room
temperature, 90 equiv of N2H4 was converted after 22 h
(Scheme 2). The ratio of the yields of NH3 and N2 (100 and 32
equiv to 2, respectively) was close to the stoichiometric value of
4/1. Similar activity has been found for some MoFe3S4 clusters
(Table 1). This cluster core binds N2H4 at the Mo atom, and
coordination of substantially inert ligand such as PEt3 to the Mo
site significantly decreases catalytic activity (MoFe3-1 vs MoFe3-
2). Because the Ir atoms in 2 are capped by the Cp* ligands, it is
probable that the Ru center binds N2H4 via dissociation of the
bridging iodido ligands. Catalytic disproportionation of N2H4 has
also been performed previously by using [(Cp*Mo)3(μ3-
S)4{RuH2(PR3)}][PF6] (R = Ph, cyclohexyl) at 60 °C (Table 1).[24]
Although these clusters are less active at room temperature
(TON < 4 after 18 h) than 2, some cluster products, whose Ru
In spite of repeated trials using 2, we could not obtain
cluster species that bind N2H4 or other ligands derived from N2H4.
On the other hand, reaction of 2 with 10 equiv of PhNHNH2 was
found to form the adduct between single cubane and PhNHNH2,
[(Cp*Ir)3(μ3-S)4{RuI(NH2NHPh)2}]I (4, Scheme 3). The molecular
structure of 4 has been characterized by X-ray crystallography
as shown in Figure 5. The Ru center in the cubane-type core
binds one terminal iodido ligand and two PhNHNH2 molecules to
compose a distorted octahedral geometry along with three μ3-
sulfido ligands. In agreement with a 70-electron configuration,
short interatomic distance of 2.7813(7) Å is observed at the
Ru(1)–Ir(1) vector, which is mutually vertical to the Ru(1)–N(3)
bond. With respect to each PhNHNH2, the NH2 end bound to the
Ru center is likely to exhibit hydrogen bonding interactions with
the iodide counteranion (d(N···I) = 3.752(5), 3.567(5) Å) and the
solvated THF molecule (d(N···O) = 3.167(6), 3.035(7) Å),
although these hydrogen atoms have not been located
successfully. Solution NMR measurements of 4 have suggested
that coordination of PhNHNH2 is quite labile. When an
analytically pure sample of 4 was dissolved in THF-d8, the 1H
NMR spectra showed many Cp* signals. Simple spectra
consistent with the crystal structure were obtained in the
presence of excess PhNHNH2 (~15 equiv). In the Cp* signal
region, one sharp peak is assigned to the Cp* located syn to the
iodido ligand (bound to Ir(3)), and the other two Cp* appear as a
–
–
site coordinates NH3 or bridging NH2 and N2H3 , have been
successfully isolated from the reaction mixtures.
Stoichiometry:
2 (0.5 mol%)
3 N2H4
4 NH3 + N2
N2H4
NH3
+
N2
+
H2 (+ N2H4)
THF, r.t., 22 h
1.1×102
2.0×102
1.0×102 32
25%
trace
molar ratio to 2
58%
16%
N-based yield*
* Calculated based on nitrogen atoms.
Scheme 2. Disproportionation of N2H4 catalyzed by 2.
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10.1002/ejic.201901146
European Journal of Inorganic Chemistry
FULL PAPER
broadened signal probably due to Ru–Ir bond migration faster
than the NMR time scale. Formation of 4 confirms coordination
of hydrazine derivatives to the Ru site and suggests that the
iodido-bridged double-cubane structure of 2 is easily cleaved
into single cubes.[25]
fold amount of 2,6-lutidinium chloride (Lut·HCl) is added as the
proton source (Table 2). The produced NH3 is protonated by
excess Lut·HCl, and the resulting NH4+ causes precipitation with
the anionic MoFe3S4 catalyst to gradually drop reaction rates.[21]
Conversion rates of N2H4 in these catalytic systems are faster
than disproportionation by 10 times or more, indicating that
reduction by N2H4 is much slower than that by Cp2Co. It has
been elucidated that N2H4 molecule is activated by only one
MoFe3S4 or VFe3S4 cubane core via terminal coordination to the
Mo or V site.[21,26]
I
Ph
NH
PhNHNH2
(10 equiv)
H2N
S
Ir
S
2
2
H2N Ru
I
S
THF
Cp2Co (2 equiv)
[Cp2Co][BF4]
NEt3
Ir
Ph NH
S
Ir
[HNEt3][BF4] (2 equiv)
catalyst time yield
1 h 87%
2
3
N2H4
2 NH3
24 h
9%
4
catalyst (0.5 mol%)
THF, r.t.
24 h 11%
none
Scheme 3. Formation of the single cubane coordinating PhNHNH2.
Scheme 4. Reduction of N2H4 with combination of Cp2Co and [HNEt3][BF4] in
the presence or absence of cluster catalysts.
Table 2. Catalytic reduction of N2H4 by some representative cubane-type M4S4
clusters in the presence of Cp2Co (2 equiv) and proton source.[a]
catalyst
proton source
solvent
MeCN
MeCN
MeCN
MeCN
THF
NH3 yield[b]
Reference
21e
MoFe3-1
MoFe3-3
MoFe3-4
Lut·HCl (4 equiv)
Lut·HCl (4 equiv)
Lut·HCl (4 equiv)
Lut·HCl (4 equiv)
[HNEt3][BF4] (2 equiv)
38
96
66
60
87
21e
21d
[c]
VFe3
22
2
This work
[a] Reactions were conducted at ambient temperature with the molar ratio of
N2H4/(single cubane unit) = 100, except where noted. [b] Conversion (%) after
1 h. [c] N2H4/VFe3 = 40.
Figure 5. Crystal structure of 4·THF. Thermal ellipsoids are drawn at 50%
probability and hydrogen atoms are omitted for clarity.
3–
O
Cl
O
S
Fe
S
HOOC
O
Mo
O
4–
S
O
O
Fe
O
Reduction of N2H4 with cluster catalysts was also
investigated in the presence of Cp2Co and [HNEt3][BF4] as
external electron and proton sources. Reaction of N2H4, Cp2Co,
[HNEt3][BF4], and 2 in a 200/400/400/1 molar ratio at room
temperature attained 87% conversion to NH3 in 1 h (Scheme 4).
This reaction rate corresponds to that nearly 90 molecules of
N2H4 per one cubane core are reduced within 1 h and is much
faster than the above mentioned disproportionation reaction. It is
interesting to note that essentially no catalytic activity has been
observed for 3, even though 3 can be generated by the action of
2 with Cp2Co (vide supra). Series of MoFe3S4 and VFe3S4
catalysts have been intensively studied in similar reaction
systems using 2 equiv of Cp2Co as reducing agent, while four-
S
Fe
O
Cl
Cl
O
S
Fe
S
Cl
S
Mo
S
MoFe3-3
Cl
S
Mo
S
S
Fe
Fe
S
S
Fe
–
Cl
DMF
S
Fe
Cl
Cl
S
Fe
Fe
DMF
DMF
V
S
Cl
Cl
S
Fe
MoFe3-4
S
Fe
Cl
Cl
VFe3
(DMF = HCONMe2)
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10.1002/ejic.201901146
European Journal of Inorganic Chemistry
FULL PAPER
(Cp* ligand), 3035, 1267, 727, 694 (solvated CH2Cl2). 1H NMR data for
1[I]2 (CD2Cl2): δ = 1.47 (br d, 6H, CHMe2), 1.84 (br, Cp*, overlapping with
the Cp* signal of 2), 2.53 (br s, 3H, C6H4Me), 3.00 (m, 1H, CHMe2), 6.11
(vbr, 4H, C6H4).
Conclusions
Synthesis of [(Cp*Ir)6Ru2(μ3-S)8][I]2 (3): A suspension of 2·3(CH2Cl2)
(63.2 mg, 19.8 μmol) and Cp2Co (16.5 mg, 87.2 μmol) in THF (5 mL) was
stirred at room temperature for 24 h. The brown precipitate was collected
by filtration, and washed with THF to remove unreacted Cp2Co. The
residual solid was dissolved in MeCN (5 mL), and diethyl ether (15 mL)
was layered. The crystals grown on diffusion were filtered after 18 days,
and black prismatic crystals of 3 (38.0 mg, 72% yield) were manually
separated from yellow crystals of [Cp2Co]I. Although X-ray diffraction
study indicated that 3 crystallized with MeCN and Et2O solvates, they
were lost after vacuum drying. 1H NMR (CD3CN): δ = 1.76 (vbr, 45H,
Cp*), 2.25 (s, 30H, Cp*), 2.26 (s, 15H, Cp*). IR (KBr): 2976, 2910, 1466,
1376, 1029 (Cp* ligand). Anal. Calcd for C60H90I2Ir6Ru2S8 (2677.11): C,
26.92; H, 3.39; found C, 27.17; H, 3.46; N, 0.08.
In this work, we have derivatized the cubane-type RuIr3S4
precursor 1 and obtained a dimer of [RuIr3S4]5+ cores 2, whose
Ru centers are bridged each other by three iodido ligands. Since
a [RuIr3S4]3+ cluster binding molecular N2 at the Ru center is
precedented (Figure 2, up-left),[6] we have envisaged that
reduction of 2 leads to coordination of N2 to single cubane core,
or rather between two cubane cores. However, we have
observed
a
unique fusion of two formal [RuIr3S4]4+ cores
accompanied by deformation of one cubane skeleton to give 3
having no suitable coordination site for N2. This result indicates
that some appropriate supporting ligand on the Ru site (such as
Me2NCH2CH2NMe2 chelate) is necessary for preventing direct
bond formation between RuIr3S4 cores. In contrast, a cubane-
type TiMo3S4 structure endures reduction from formal
[TiMo3S4]5+ to [TiMo3S4]+ state and can capture N2 at the
tetrahedral Ti center (Figure 2, up-right).[7] On the other hand, 2
has been found to catalyze reduction of N2H4 in a system using
external electron and proton sources. The catalytic cycle
Synthesis of [(Cp*Ir)3(μ3-S)4{RuI(NH2NHPh)2}]I (4): A suspension of
2·3(CH2Cl2) (79.6 mg, 25.0 μmol) and phenylhydrazine (25 μL, 250 μmol)
in THF (5 mL) was stirred at room temperature for 24 h. GLC analysis of
liquid phase confirmed formation of 33 μmol of aniline. The resulting dark
brown solution was filtered, and the filtrate was concentrated to 3 mL
followed by addition of hexane (17 mL). Black prismatic crystals were
collected by filtration to give 4·THF (66 mg, 75% yield). 1H NMR (THF-
d8): δ = 1.79 (s, 15H, Cp*), 1.82 (vbr, 30H, Cp*); measured in the
presence of 15-molar excess PhNHNH2, which hampered assignment of
the coordinated PhNHNH2 signals. IR (KBr): 3301, 3185 (NH st), 1601,
1496 (aromatic C–C), 754, 685 (aromatic CH δ oop and C–C γ), 2975,
2910, 1450, 1375, 1029 (Cp* ligand). Anal. Calcd for C46H69I2Ir3N4ORuS4
(1753.86): C, 31.50; H, 3.97; N, 3.19; found C, 31.95; H, 3.95; N, 2.87.
probably proceeds with
a single-cubane species, because
addition of PhNHNH2 has been found to cleave the iodido-
bridged structure of 2.
Experimental Section
Disproportionation of Hydrazine: In
a dry 20-mL Schlenk tube
General Methods: All manipulations were carried out under N2 using
standard Schlenk techniques. Solvents were dried by common methods
and distilled under N2 before use. Cluster 1[6a] and [HNEt3][BF4][27] were
prepared according to the literature methods, while other chemicals were
obtained commercially and used as received. 1H NMR spectra were
measured on a JEOL alpha-400 (at 400 MHz) or a JEOL ECA500 (at 500
MHz) spectrometer at 22 °C. Chemical shifts were referred to those of
isotopic impurities (CD2Cl2 at 5.32, CD3CN at 1.93, THF-d8 at 1.73 ppm).
IR spectra were recorded on a Thermo Scientific Nicolet iS50 FT-IR
spectrometer by KBr method. Gas phase analyses were carried out on a
equipped with a magnetic stirring bar, THF (5 mL) and N2H4 (16 μL, 0.51
mmol) was added, and the mixture was frozen in a liquid nitrogen bath.
Under a counterflow of nitrogen gas, solid of 2·3(CH2Cl2) (8.4 mg, 2.6
μmol) was added, and the reaction vessel was evacuated. The mixture
was warmed to room temperature and stirred for 22 h. The reaction
vessel was backfilled to atmospheric pressure with pure argon gas, and
the gas phase was analyzed by gas chromatography (three
measurements using each 100 μL sample gas). Then, the mixture was
frozen at –196 °C, and the vessel was connected to a 50-mL Schlenk
tube containing aqueous H2SO4 (0.5 mol/L, 10 mL) through a U-shape
glass adaptor. The volatile materials in the reaction mixture were
transferred to the frozen aqueous H2SO4 by trap-to-trap distillation under
vacuum. The aqueous solution was subjected to quantitative
determination of NH3 and N2H4 by calorimetry using indophenol[28] and
azo-dye[29] methods.
Shimadzu GC-14B gas chromatograph equipped with
a TCD and
stainless columns filled with molecular sieves 5A (mesh 60–80, 3.0
mm×2.0 m) by using argon as the carrier gas. Elemental analyses were
performed with a Perkin-Elmer 2400 series II CHN analyzer or a System
Engineering CE-440 M CHN analyzer.
Synthesis of [{(Cp*Ir)3(μ3-S)4Ru}2(μ2-I)3]I (2): Cluster 1 (604 mg, 0.449
mmol) and iodine (114 mg, 0.449 mmol) were charged in a 50-mL
Schlenk tube equipped with a magnetic stirring bar. The reaction vessel
was cooled to –70 °C, and THF (40 mL) was added via syringe with
stirring. The mixture was gradually warmed to room temperature and
stirred for 18 h. Brown solid was collected by filtration, washed
repeatedly with THF, and dried under vacuum for 2 h. This crude solid of
1[I]2 (689 mg) was dissolved in CH2Cl2 (20 mL), and the solution was
stirred at 30 °C for 24 h. The resultant dark red solution was filtered, and
the concentrated filtrate (15 mL) was layered with hexane (45 mL).
Brown microcrystals were filtered, washed with hexane, and dried under
vacuum to give 2·3(CH2Cl2) (583 mg, 82% yield). 1H NMR (CD2Cl2): δ =
1.91 (br s, Cp*). Anal. Calcd for C63H96Cl6I4Ir6Ru2S8 (3185.70): C, 23.75;
H, 3.04; found C, 23.79; H, 2.74. IR (KBr): 2976, 2912, 1449, 1375, 1029
Catalytic Reduction of Hydrazine: A standard method is as follows. In
a dry 20-mL Schlenk tube equipped with a magnetic stirring bar, Cp2Co
(1.0 mmol) and [HNEt3][BF4] (1.0 mmol) were placed. After addition of
THF (5 mL) and N2H4 (0.50 mmol), the mixture was frozen in a liquid
nitrogen bath. Under a counterflow of nitrogen gas, solid of 2·3(CH2Cl2)
(2.5 μmol) was added, and the reaction vessel was evacuated. The
mixture was warmed to room temperature and stirred for 1 h. The
resultant mixture was frozen again at liquid nitrogen temperature, and the
Schlenk tube was connected through a U-shape glass adaptor to a 50-
mL Schlenk tube containing aqueous H2SO4 (0.5 mol/L, 10 mL). The
volatile materials in the reaction mixture were transferred to the frozen
aqueous H2SO4 by trap-to-trap distillation under vacuum. The aqueous
solution was subjected to quantitative determination of NH3 and N2H4 by
calorimetry as described above.
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10.1002/ejic.201901146
European Journal of Inorganic Chemistry
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X-Ray Crystallography: Single crystals were mounted on CryoLoops or
Micromounts using Paratone-N in a cold flow of N2 for measurements.
Diffraction experiments were performed on a Rigaku Mercury CCD
diffractometer (2·3(CH2Cl2) and 4·THF) or a Rigaku RAXIS RAPID
imaging plate diffractometer (3·0.75MeCN·0.75Et2O) equipped with a
graphite monochromatized Mo Kα source (λ = 0.71070 Å). Data were
processed using the CrystalClear program package[30] and were
corrected for absorption. Structure solution and refinements were carried
out using the CrystalStructure program package.[31] The positions of the
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This work was supported by JSPS KAKENHI Grant Number JP
21350033, 16K05765 and 18K05136. A part of this work has
been done in the laboratory of the late Professor Yasushi
Mizobe at Institute of Industrial Science, The University of Tokyo.
Keywords: Transition metal sulfido clusters • Cubane-type
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European Journal of Inorganic Chemistry
FULL PAPER
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[25] Although electronic spectra of the RuIr3S4 clusters reported here are
poor in features, the spectral line shape of a mixture of 2 and N2H4 has
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Figure S8). This may indicate that the dominant structure of the cluster
species under catalytic disproportionation conditions is relevant to the
PhNHNH2 adduct 4.
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10.1002/ejic.201901146
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
FULL PAPER
Cubane-type RuIr3S4 cluster forms a
dimer in which two Ru centers are
triply bridged by iodido ligands each
other. Two-electron reduction induces
fusion of two RuIr3S4 units
Metal–Sulfido Clusters
Hidetake Seino,* Keiichi Hirata, Yusuke
Arai, Risa Jojo, Masaaki Okazaki
Page No. – Page No.
accompanied by deformation of one
cubane core to give a unique sulfido-
bridged octanuclear framework. The
cubane dimer is catalytically active in
reduction of N2H4 with Cp2Co and
[HNEt3][BF4].
An Iodido-Bridged Dimer of Cubane-
type RuIr3S4 Cluster: Structural
Rearrangement to New Octanuclear
Core and Catalytic Reduction of
Hydrazine
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