Synthesis of sulfur- and nitrogen-bridged diiron complexes and catalytic behavior toward hydrazines

Article abstract of DOI:10.1021/om300134t

Novel thiolate-bridged diiron complexes bearing diazenido and diazene ligands in a side-on manner have been prepared and characterized by X-ray analysis. These sulfur-bridged diiron complexes work as effective catalysts toward the reduction of hydrazines into amines and ammonia via a sequential process of protonation and reduction on the sulfur-bridged diiron skeleton, as is likely observed at the active site of nitrogenase.

Full text of DOI:10.1021/om300134t

Communication
pubs.acs.org/Organometallics
Synthesis of Sulfur- and Nitrogen-Bridged Diiron Complexes and
Catalytic Behavior toward Hydrazines
Masahiro Yuki, Yoshihiro Miyake, and Yoshiaki Nishibayashi*
Institute of Engineering Innovation, School of Engineering, The University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan
S
* Supporting Information
ABSTRACT: Novel thiolate-bridged diiron complexes bearing diazenido
and diazene ligands in a side-on manner have been prepared and
characterized by X-ray analysis. These sulfur-bridged diiron complexes
work as effective catalysts toward the reduction of hydrazines into amines
and ammonia via a sequential process of protonation and reduction on the
sulfur-bridged diiron skeleton, as is likely observed at the active site of
nitrogenase.
he development of nitrogen fixation under mild reaction
conditions is one of the most important subjects in
[(Cp*Fe)2(μ-SAr)] (Ar = 2-(trimethylsilyl)phenyl).8 During
this study, diazenido- and diazene-bridged diiron complexes
were isolated as reactive intermediates from the reaction of the
thiolate-bridged diiron complex with methylhydrazine and
further protonation. Interestingly, the diiron complexes work as
effective catalysts toward the catalytic reduction of hydrazines
into ammonia via nitrogen−nitrogen bond cleavage of
hydrazines. We believe that the properties of these complexes
give useful information to elucidate the function of the
nitrogenase enzyme. Herein, we describe the preparation and
reactivity of the thiolate-bridged diiron complexes.
T
chemistry. Industrially, ammonia is produced from molecular
dinitrogen and dihydrogen by the use of iron-based
heterogeneous catalysts. In this reaction system, extreme
reaction conditions such as high reaction temperatures and
high pressures are required in order to activate molecular
dinitrogen. In sharp contrast to the Haber−Bosch process,
biological nitrogen fixation by the nitrogenase enzyme is well-
known to occur at ambient temperature and pressure. A recent
X-ray structural study has revealed that the active site of
nitrogenase contains an Fe7MoS9C cluster core.1 Although a
detailed reaction mechanism of the conversion of molecular
dinitrogen into ammonia by the nitrogenase enzyme has not
yet been disclosed, recent theoretical studies have proposed
reactive intermediates that contain nitrogenous species bound
to two iron centers in the cluster.2 Therefore, sulfur-bridged
transition-metal cluster complexes have received much
attention as being possibly relevant to the active site of
nitrogenase.3 Especially, studies on the function of the
reduction of nitrogenous species by diiron complexes bearing
sulfur ligands are limited to only a few cases.4
The reaction of [Cp*FeCl(tmeda)] (tmeda = N,N,N′,N′-
tetramethylethylenediamine) with 1 equiv of KSAr in THF at
room temperature for 24 h gave the doubly thiolate bridged
diiron complex [Cp*Fe(μ-SAr)]2 (1) in 77% yield (Scheme 1).
Scheme 1. Preparation of the Doubly Thiolate Bridged
Diiron Complex [Cp*Fe(μ-SAr)]2 (1)
Toward the goal of achievement of a novel nitrogen fixation
system under mild reaction conditions, we have quite recently
found the molybdenum-catalyzed reduction of molecular
dinitrogen into ammonia under mild reaction conditions by
using a dinitrogen-bridged dimolybdenum complex bearing a
tridentate PNP-type pincer ligand as a catalyst.5 In this reaction
system, molecular dinitrogen under atmospheric pressure was
catalytically converted into ammonia in the presence of both
electron source and proton source, and up to 23 equiv of
ammonia was produced on the basis of the catalyst (12 equiv of
ammonia was produced on the basis of the molybdenum atom
of the catalyst). This is another successful example of the
catalytic and direct conversion of molecular dinitrogen into
ammonia under ambient conditions.6
The molecular structure of 1 was confirmed by X-ray analysis.
An ORTEP drawing of 1 is shown in Figure S1 (Supporting
Information). The bond distance between the two iron atoms is
revealed to be 2.7429(4) Å. This result indicates the presence
of a direct interaction between the two iron atoms in 1.
Although the preparation and reactions of the doubly thiolate
bridged diruthenium complexes [Cp*RuII(μ-SR)]2 have been
reported for the last two decades,9 this is the first example of
the preparation of the analogous iron complex [Cp*FeII(μ-
SR)]2.
As an extension of our study on the development of a novel
nitrogen fixation system under mild reaction conditions,7 we
have synthesized the thiolate-bridged diiron complex
Received: February 17, 2012
Published: March 23, 2012
© 2012 American Chemical Society
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Communication
The reaction of 1 with 1.25 equiv of methylhydrazine in THF
at room temperature for 20 h gave the methyldiazenido-bridged
diiron complex [(Cp*Fe)2(μ-SAr)(μ-η2:η2-NNMe)] (2) in
64% yield and triply thiolate bridged diiron complex [Cp*Fe-
(μ-SAr)3FeCp*] (3) in 36% yield together with the formation
of methylamine in 57% yield and ammonia in 57% yield
(Scheme 2). In this reaction, all yields of the complexes and
Scheme 3. (a) Disproportionation of Methylhydrazine and
(b) Reduction of Methylhydrazine with [Fe(II)−Fe(II)] (1)
Scheme 2. Reaction of [Cp*Fe(μ-SAr)]2 (1) with 1.25 equiv
of Methylhydrazine
(Scheme 3a). Although a detailed reaction mechanism of the
oxidation of 1 with methylhydrazine has not yet been
determined, the hydrazido complex [(Cp*Fe)2(μ-SAr)(μ-
NHNHMe)] (4) may be formed by ligand exchange of the
coordinated thiolate with 1 equiv of methylhydrazine as the first
step (eq 1). A similar hydrazido complex was previously
products are estimated on the basis of 1. The molecular
structures of 2 and 3 were confirmed by X-ray analysis. ORTEP
drawings of 2 and 3 are shown in Figure 1a and Figure S3
isolated and proposed as a reactive intermediate in the
reductive process of molecular dinitrogen at the active site of
nitrogenase.13 Then, the produced hydrazido complex 4 is
oxidized with another 1 equiv of methylhydrazine to afford 2
together with methylamine and ammonia (eq 1). On the other
hand, 3 is formed by the reaction of 1 with 0.5 equiv of
methylhydrazine and ArSH, where methylhydrazine oxidizes
one of the two iron atoms Fe(II)−Fe(II) in 1 to Fe(II)−
Fe(III) in 3 together with methylamine and ammonia (Scheme
3b). It is noteworthy that methylhydrazine works as both a
reducing and an oxidizing reagent.
Next, we examined the catalytic activity of the diiron
complexes toward reduction of hydrazines into amines and
ammonia via nitrogen−nitrogen bond cleavage of hydrazines.
Typical results are shown in Table 1. The reaction of
methylhydrazine with 2 equiv of lutidinum tetraphenylborate
([LutH]BPh4) as a proton source and cobaltocene as a
reductant in the presence of 5 mol % of 2 in THF at room
temperature for 1 h under an atmospheric pressure of argon
gave methylamine and ammonia in 93% and 80% yields,
respectively, without the formation of molecular dihydrogen
and dinitrogen (Table 1, run 1). Although only small amounts
of methylamine and ammonia were observed even in the
absence of catalyst (Table 1, run 3), the reduction of
methylhydrazine proceeded smoothly in the presence of only
0.5 mol % of 2 to produce lower amounts of methylamine and
ammonia (Table 1, run 2). No reaction occurred at all in the
absence of [LutH]BPh4 and cobaltocene (Table 1, run 4). This
result indicates that no disproportionation of methylhydrazine
takes place under the same reaction conditions. Separately, we
confirmed that no reaction occurred at all when the catalytic
reduction was carried out under an atmospheric pressure of
Figure 1. ORTEP drawings of (a) [(Cp*Fe)2(μ-SAr)(μ-η2:η2-
NNMe)] (2) and (b) [(Cp*Fe)2(μ-SAr)(μ-η2:η2-NHNMe)]OTf (6).
(Supporting Information). The X-ray analysis of 2 reveals that
two iron atoms in 2 are bridged by thiolate and
methyldiazenido ligands, respectively. The bond distance
between two nitrogen atoms in the methyldiazenido ligand
with side-on coordination is 1.331(3) Å. This bond distance is
similar to that of the diazenido ligand in the dicobalt complex
[Co2(CO)4(μ-dppm)(μ-η2:η2-NNTol)]SbF6 (dppm = bis-
(diphenylphosphino)methane; Tol = p-CH3C6H4).10 Complex
2 is the first example of a diiron complex bearing a
methyldiazenido ligand with side-on coordination.11 The core
skeleton, including the bond distance between the two iron
atoms and the bond angles between iron and sulfur atoms in 2,
is similar to that of the Fe7MoS9C cluster core in nitrogenase.
On the other hand, the molecular structure of 3 is almost the
same as that of the reported triply thiolate bridged diiron
complexes [Cp*Fe(μ-SR)3FeCp*] (R = Me, Et, Ph).12
The result of the formation of 2 and 3 in Scheme 2 suggests
the existence of the two competitive reactions shown in
Scheme 3. With regard to the formation of 2 from the reaction
of 1 with 2 equiv of methylhydrazine, 2 equiv of
methylhydrazine disproportionates into methylamine and
ammonia as reduced products and a bridging methyldiazenido
ligand as an oxidized product together with free thiol (ArSH)
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Communication
a
Table 1. Iron-Catalyzed Reduction of Hydrazines
Scheme 4. Proposed Reaction Pathway for Reduction of
Methylhydrazine
+
−
MeNHNH + 2H + 2e
2
5 mol % catalyst
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ MeNH + NH
2
3
THF, room temp, 1 h
yield (%)
NH3
run
cat.
MeNH2
H2
1
2
3
4
5
6
7
8
9
2
93
32
10
0
80
26
7
0
0
b
2
c
58
0
d
2
2
1
3
5
6
2
2
2
2
de
,
0
0
72
53
32
76
70
55
26
68
>99
59
51
6
20
44
0
methylhydrazine, as shown in Scheme 3a. Thus, methylhy-
drazine is reduced into methylamine and ammonia and the
bridging hydrazido ligand in 4 is oxidized into the bridging
methyldiazenido ligand in 2. It is noteworthy that the
disproportionation of hydrazines is considered to be involved
in the catalytic reduction of hydrazines. However, we have not
yet excluded other reaction pathways involving key reactive
intermediates such as 2.
Unfortunately, we have not yet been successful in isolating
the proposed hydrazido complex 4, although a similar
hydrazido complex was isolated by another research group.13
However, we could isolate the methyldiazene-bridged diiron
complex [(Cp*Fe)2(μ-SAr)(μ−η2:η2-NHNMe)]OTf (6) from
the reaction of 2 with 1 equiv of [LutH]OTf (Scheme 5). The
f
10
11
12
0
g
i
h
56
54
5
j
15
a
All reactions of methylhydrazine (0.60 mmol) with 2 equiv of
[LuH]BPh4 (1.20 mmol) and Cp2Co (1.20 mmol) in the presence of
5 mol % of catalyst (0.03 mmol) were carried out in THF (10 mL) at
room temperature for 1 h under an atmospheric pressure of argon. 2
(0.5 mol %) was used. In the absence of catalyst. In the absence of
both [LuH]BPh4 and Cp2Co. Under an atmospheric pressure of H2.
Hydrazine was used in place of methylhydrazine. Phenylhydrazine
was used in place of methylhydrazine. Phenylamine was formed. 1,1-
b
c
d
e
f
g
h
i
j
Dimethylhydrazine was used in place of methylhydrazine. Dimethyl-
amine was formed.
dihydrogen in the absence of [LutH]BPh4 and cobaltocene
(Table 1, run 5). These results indicate that molecular
dihydrogen did not work as a proton or electron source. At
present, we consider that the reduction of methylhydrazine
proceeds via a sequential process of protonation and reduction
on the sulfur-bridged diiron skeleton of 2.
Scheme 5. Preparation and Reactivity of 6
When 1 was used in place of 2 as a catalyst, both
methylamine and ammonia were obtained in 72% and 70%
yields, respectively, together with a small amount of dihydrogen
(Table 1, run 6). When either 3 or the thiolate-bridged
diruthenium complex [Cp*Ru(μ-SAr)]2 (5)15 was used as a
catalyst, the selectivity for methylamine and ammonia was low
and a larger amount of dihydrogen was produced (Table 1,
runs 7 and 8). These results indicate that 2 works as the most
effective catalyst in the reduction of methylhydrazine.
Interestingly, 2 works as an effective catalyst toward the
catalytic reduction of hydrazine, where ammonia was formed
quantitatively (Table 1, run 10). Other hydrazine derivatives
such as phenylhydrazine and 1,1-dimethylhydrazine can be
reduced into the corresponding amines and ammonia under the
same reaction conditions (Table 1, runs 11 and 12). Results of
the stoichiometric and catalytic reactions indicate that 2 works
as a key reactive intermediate in the catalytic reduction of
hydrazines.
On the basis of experimental results of the stoichiometric and
catalytic reactions, a proposed reaction pathway for the catalytic
reduction using 2 as a catalyst is shown in Scheme 4. The initial
step is the formation of the hydrazido complex 4 by the
protonation and reduction of 2. Then, 4 is oxidized with 1
equiv of methylhydrazine to give the starting complex 2
together with methylamine and ammonia as reduced products.
This step is quite similar to the reaction of 1 with
molecular structure of 6 was confirmed by X-ray analysis. An
ORTEP drawing of 6 is shown in Figure 1b. The bond distance
between two nitrogen atoms in the methyldiazene ligand with
side-on coordination is 1.373(4) Å, which is slightly longer than
that in the methyldiazenido ligand in 2. The reaction of 6 with
4 equiv of [LutH]BPh4 and 4 equiv of cobaltocene in THF at
room temperature for 1 h gave methylamine and ammonia in
51% and 67% yields, respectively. In addition, we confirmed
that 6 has a similar catalytic activity toward the reduction of
methylhydrazine (Table 1, run 9). These results indicate that 6
can be considered one of the reactive intermediates in the
catalytic reduction of hydrazines.
Qu and co-workers have prepared related doubly thiolate
bridged diiron complexes bearing a bridging diazene ligand with
end-on coordination and investigated their catalytic activity
toward the reduction of hydrazines, where the diiron complexes
worked as effective catalysts.4b In sharp contrast to the end-on
mode of the diazene ligand for the diiron complexes,4b the side-
on modes of the diazenido ligand for 2 and diazene ligand for 6
were observed in our case due to the presence of only one
sterically demanding substituent such as the 2-trimethylsilyl-
benzenthiolate group. Unfortunately, the use of a more
sterically demanding substituent such as 2,6-bis(trimethylsilyl)-
benzenthiolate group did not afford the corresponding thiolate-
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Communication
(4) (a) Sellmann, D.; Hille, A.; Rosler, A.; Heinemann, F. W.; Moll,
M.; Brehm, G.; Schneider, S.; Reiher, M.; Hess, B. A.; Bauer, W. Chem.
Eur. J. 2004, 10, 819. (b) Chen, Y.; Zhou, Y.; Chen, P.; Tao, Y.; Li, Y.;
̈
bridged diiron complexes but rather a mononuclear iron
complex bearing a π-arenethiolate ligand, [Cp*Fe(η5-SAr′)]
(Ar′ = 2,6-bis(trimethyl)phenyl) (7).16 As described in Scheme
4, the reaction pathway for the reduction of hydrazines using 2
as a catalyst is considered to be quite different from that
proposed by Qu and co-workers.4b The results described in this
paper provide another successful example of the catalytic
reduction of hydrazines by using sulfur-bridged diiron
complexes as catalysts, although other transition-metal
complexes bearing bridging sulfur ligands were previously
reported to work as catalysts.17
In summary, we have newly prepared thiolate-bridged diiron
complexes bearing diazenido and diazene ligands with side-on
coordination. These sulfur-bridged diiron complexes have been
revealed to work as effective catalysts toward the reduction of
hydrazines into amines and ammonia. The reduction of
hydrazine is considered to proceed via a sequential process of
protonation and reduction on the sulfur-bridged diiron
skeleton. On the basis of experimental results, a novel reaction
pathway for the reduction of hydrazines has been proposed,
where the disproportionation of hydrazines is considered to be
one of the key steps. We believe that the result described in this
paper provides the ability of two iron atoms to cooperatively
bind and transform a nitrogenous substrate, which is a
capability that may be shared by the Fe7MoS9C cluster core
in nitrogenase.18 Further work is currently in progress to
develop the iron-catalyzed reduction of dinitrogen into
ammonia under mild reaction conditions.19
Qu, J. J. Am. Chem. Soc. 2008, 130, 15250. (c) Saouma, C. T.; Muller,
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P.; Peters, J. C. J. Am. Chem. Soc. 2009, 131, 10358. (d) Lee, Y.;
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2011, 133, 8440. (g) Saouma, C. T.; Kinney, R. A.; Hoffman, B. M.;
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DOI: 10.1039/C2DT30105A.
(8) Recently, we have developed novel catalytic transformation of
propargylic alcohols by using thiolate-bridged diruthenium complexes
as catalysts. (a) For a recent example, see: Ikeda, M.; Miyake, Y.;
Nishibayashi, Y. Angew. Chem., Int. Ed. 2010, 49, 7289 and references
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(9) (a) Hidai, M.; Mizobe, Y. Can. J. Chem. 2005, 83, 358 and
references therein. (b) Qu and co-workers reported the preparation of
analogous iron complexes such as [Cp*Fe(μ-SR)3FeCp*] (Fe(II)−
Fe(III) complex) and [Cp*FeX(μ-SR)]2 (Fe(III)−Fe(III) com-
plex),4b,e,12 but there has been no report on the preparation of
[Cp*Fe(μ-SR)]2 (Fe(II)−Fe(II) complex) as shown in the present
paper.
ASSOCIATED CONTENT
* Supporting Information
Figures, tables, text, and CIF files giving experimental
procedures, spectroscopic data, and X-ray data. This material
is available free of charge via the Internet at http://pubs.acs.org.
■
S
(10) DeBlois, R. E.; Rheingold, A. L.; Samkoff, D. E. Inorg. Chem.
1988, 27, 3506.
(11) Sutton, D. Chem. Rev. 1993, 93, 995.
(12) Chen, Y.; Zhou, Y.; Qu, J. Organometallics 2008, 27, 666.
AUTHOR INFORMATION
Corresponding Author
*E-mail: ynishiba@sogo.t.u-tokyo.ac.jp.
Notes
■
(13) (a) Vela, J.; Stoian, S.; Flaschenriem, C. J.; Munck, E.; Holland,
̈
P. L. J. Am. Chem. Soc. 2004, 126, 4522. (b) Lees, N. S.; McNaughton,
R. L.; Gregory, W. V.; Holland, P. L.; Hoffman, B. M. J. Am. Chem. Soc.
2008, 130, 546.
The authors declare no competing financial interest.
(14) See the Supporting Information for experimental details.
(15) The complex 5 was prepared and characterized by X-ray
crystallography. See the Supporting Information for experimental
details.
(16) (a) See the Supporting Information for experimental details.
(b) A similar mononuclear ruthenium complex bearing a π-
arenethiolate ligand, [Cp*Ru(η5-SAr′)], has already been reported:
Yuki, M.; Miyake, Y.; Nishibayashi, Y. Organometallics 2010, 29, 4148.
(17) (a) Malinak, S. M.; Demadis, K. D.; Coucouvanis, D. J. Am.
Chem. Soc. 1995, 117, 3126. (b) Demadis, K. D.; Coucouvanis, D.
Inorg. Chem. 1995, 34, 3658. (c) Demadis, K. D.; Malinak, S. M.;
Coucouvanis, D. Inorg. Chem. 1996, 35, 4038. (d) Grand, N. L.; Muir,
K. W.; Petillon, F. Y.; Pickett, C. J.; Schollhammer, P.; Talarmin, J.
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Chem. Eur. J. 2002, 8, 3115.
(18) Lukoyanov, D.; Dikanov, S. A.; Yang, Z.-Y.; Barney, B. M.;
Samoilova, R. I.; Narasimhulu, K. V.; Dean, D. R.; Seefeldt, L. C.;
Hoffman, B. M. J. Am. Chem. Soc. 2011, 133, 11655 and references
therein.
ACKNOWLEDGMENTS
■
This work was supported by the Funding Program for Next
Generation World-Leading Researchers (GR025). Financial
support from Toyota Motor Corp. is also gratefully acknowl-
edged. We thank the Research Hub for Advanced Nano
Characterization at The University of Tokyo for X-ray analysis.
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