Nitrogenase model complexes [Cp*Fe(μ-SR1) 2(μ-η2-R2N=NH)FeCp*] (R 1 = Me, Et; R2 = Me, Ph; Cp* = η5- C5Me5): Synthesis, structure, and catalytic N-N bond cleavage of hydrazines on diiron centers

Article abstract of DOI:10.1021/ja805025w

The reactions of [Cp*Fe(μ-SR¹)₃FeCp*] (Cp* = η⁵-C₅Me₅; R¹ = Et, Me) with 1.5 equiv R²NHNH² (R²= Ph, Me) give the μ-η²-diazene diiron thiolate-bridged complexes [Cp*Fe(μ-SR¹)₂(μ-η²-R²N=NH)FeCp*], along with the formation of PhNH₂ and NH₃. These μ-η²-diazene diiron thiolate-bridged complexes exhibit excellent catalytic N-N bond cleavage of hydrazines under ambient conditions. Copyright

Full text of DOI:10.1021/ja805025w

Published on Web 10/28/2008
Nitrogenase Model Complexes [Cp*Fe(µ-SR1)2(µ-η2-R2NdNH)FeCp*] (R1 ) Me,
Et; R2 ) Me, Ph; Cp* ) η5-C5Me5): Synthesis, Structure, and Catalytic N-N
Bond Cleavage of Hydrazines on Diiron Centers
Yanhui Chen, Yuhan Zhou, Pingping Chen, Yinsong Tao, Yang Li, and Jingping Qu*
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian UniVersity of Technology,
Dalian 116012, People’s Republic of China
Received July 1, 2008; E-mail: Qujp@chem.dlut.edu.cn
Scheme 1
The research on the structure and function of nitrogenases
constitutes a major task in chemistry and biochemistry, because of
their remarkable roles on biological transformation of N2 to NH3
under ambient conditions.1 Although the structure of FeMoco has
been elucidated by single-crystal X-ray diffraction analysis in recent
years,2 precisely where and how the N2 interacts with the FeMoco
is still unclear. Two essentially different assumptions on the active
site for N2 conversion have been proposed: one is at single
molybdenum or iron center,3 and the other is at di(multi)-iron
centers. However, compared with the enormous studies on N2,
diazenes and hydrazines at the single atom center,4 those at the
di(multi)-iron centers, especially iron sulfur clusters, are relatively
rare, mainly because of the difficulties in obtaining stable inter-
mediate model complexes bearing N2, diazenes, and hydrazines.5
Thus, the reactions of N2, diazenes, and hydrazines mediated by
the di(multi)-iron centers bearing sulfur ligands are of particular
interest.
Recently, Holland et al. reported a sulfide-bridged diiron complex
with a bridging phenylhydrazido ligand, and found the cleavage of
N-N single bond of phenylhydrazine.6 Sellmann et al. previously
reported some diazene diiron sulfur complexes in which the
HNdNH ligand bridges the two isolate monoiron sulfur units.7
However, the diiron sulfur (thiolate)-bridged complex with diazenes
ligand and its roles in catalytic cleaving N-N bond of nitrogenase
substrates, such as diazenes and hydrazines, have never been
explored. Here, we report the synthesis and characterization of a
class of new nitrogenases model complexes [Cp*Fe(µ-SR1)2(µ-η2-
R2NdNH)FeCp*] (Cp* ) η5-C5Me5; 2a, R1 ) Et, R2 ) Ph; 2b,
R1 ) Me, R2 ) Ph; 2c, R1 ) Et, R2 ) Me), together with their
excellent catalytic N-N bond cleavage of hydrazines on diiron
centers under ambient conditions.
Å, Fe2-N2 ) 1.83 Å). The N-N and Fe-Fe bonds are essentially
coplanar, with N1-Fe1-Fe2-N2 torsion angle of 1.78°. The
N1-N2 bond length of 1.33 Å is in the range of the NdN double
bond.10b,11 The solid-state structure of 2c shows an analogous
structure of 2a except a symmetrical mirror plane through S1, S2
atom, and the center of N1 and N2 atoms causing a disorder in
crystallography (Figure S20). Such a coordination geometry with
the bidentate NdN group bonding to two iron atoms in iron sulfur
cluster suggests a new nitrogenase model. In the FeMoco, the six
“belt” iron atoms appear to be distorted from tetrahedral toward a
trigonal pyramidal geometry with the average pyramidalization
parameter τ ) 0.46 ( 0.03,6a the iron atoms in 1a, 2a, 2b, and 2c
are also pyramidalized with the approximative τ values in the range
from 0.57 to 0.66 (Table S4).
The formation of PhNH2 and NH3 is the clear evidence of
cleaving N-N bond of PhNHNH2 by the thiolate-bridged diiron
complexes. Such a cleavage is well-known for metals in groups
4-6, but the reactivity of multinuclear complexes, especially iron
sulfur clusters, is relatively unexplored.6a,12
These nitrogenase model complexes promote us to investigate
catalytic cleaving N-N bond of hydrazines (eq 1). Treatment of
2a with excess PhNHNH2 can not produce PhNH2, NH3, and N2
under the ambient conditions, even at 60 °C, while the catalytic
reaction proceeds smoothly in the presence of appropriate reductive
and protonic acid. The catalytic reactions of N-N cleavages of
hydrazines with 2a are investigated (Table 1). The results show
that 2a exhibits an excellent catalytic activity. In the entries 3 and
4, the yields of PhNH2 are up to 93% and 94%, while in the
The reaction of complex [Cp*Fe(µ-SEt)3FeCp*] (1a)8 with 1.5
equiv PhNHNH2 in THF at 60 °C for 36 h gives a µ-η2-
phenyldiazene diiron thiolate-bridged 2a, along with the formation
of PhNH2 and NH3. Complex 2a is isolated as brown microcrys-
talline solid in 86% yield. Analogous complexes, 2b and 2c, are
obtained similarly (Scheme 1). These complexes have been
characterized by 1H NMR9 and IR spectroscopy, element analysis,
and single-crystal X-ray diffraction.
The 1H NMR spectrum of 2a exhibits a singlet with the intensity
of 1 H in low field (δ ) 13.19), which is the characteristic of proton
attached to the η1-N atom in phenyldiazene ligand.10 The IR
spectrum of 2a (KBr) shows the ν(N-H) band at 3216 cm-1 10a
.
1H NMR spectra of complexes of 2b and 2c (Supporting Informa-
tion, Figures S3 and S4) are also consistent with their structures.
The solid-state structure of 2a is shown in Figure 1.
Complex 2a consists of a di(µ-thiolate)diiron unit Cp*Fe(µ-
SEt)2FeCp* bridged by a bidentate HNdNPh group, which is
σ-bonded to the Fe2 through two nitrogen atoms (Fe1-N1 ) 1.89
Figure 1. ORTEP (ellipsoids at 30% probability) diagram of 2a.
9
15250 J. AM. CHEM. SOC. 2008, 130, 15250–15251
10.1021/ja805025w CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Catalytic Cleaving N-N Bonds of Hydrazines on
Diiron-Sulfur Cluster 2aa
cleavage of PhNHNH2, with the formation of CO complex
[Cp*Fe(µ-SEt)CO]2, which implies that the catalyst transform to
the intermediate [Cp*Fe(µ-SEt)]2 by the cleavage of NdN double
bond of phenyldiazene on the diiron centers.
cat 2a
R2NHNHR3 + 2H+ + 2e.
8 R2NH2 + R3NH2 (1)
THF, rt, 12 h
In summary, the new nitrogenase model complexes as well as
their excellent catalytic properties of cleaving N-N bond of
hydrazines on diiron centers under ambient conditions are demon-
strated. These results suggest that some steps of the biological N2
reduction could take place at diiron sites. Further studies are under
way to clarify the catalytic reactivity of the diazene complexes
reported herein.
yield (%)
PhNH2
proton
source
reducing
agent
entry
substrate
NH3
1
PhNHNH2
PhNHNH2
PhNHNH2
PhNHNH2
PhNHNH2
PhNHNH2
PhNHNH2
PhNHNH2
PhNHNH2
MeNHNH2
NH2NH2
none
none
trace trace
trace trace
2b
3
Lut · HBPh4
Lut · HBPh4
Lut · HBF4
Lut · HBF4
Lut · HBF4
Lut · TsOH
Lut · HCl
Lut · HBF4
Lut · HBPh4
Lut · HBPh4
Cp2Cr
Cp2Cr
Cp2Cr
Cp2Cr
Cp2Cr
Cp2Cr
Cp2Cr
Cp2Co
Cp2Co
Cp2Co
Cp2Cr
Cp2Cr
19
23
28
10
93
94
89
63
8
4
5c
6d
7
Acknowledgment. This work was financially supported by the
National Natural Science Foundation of China (No. 20572011) and
the Program for Changjiang Scholars and Innovative Research Team
in University (No. IRT0711).
trace
8
9
trace trace
trace 22
10e
11e
12
13
93
73
71 (MeNH2)
none
Supporting Information Available: Synthesis, characterization,
structure, catalytic experiment, and the spectroscopic data (CIF). This
material is available free of charge via the Internet at http://pubs.acs.org.
PhNHNHPh Lut · HBPh4
PhN)NPh Lut · HBPh4
none 45
none 36
a Reaction conditions: substrate (0.2 mmol), 2a (10 µmol, 5.0 mol%),
proton source (0.4 mmol), reducing agent (0.4 mmol), THF (10 mL),
12 h at room temp, Lut ) 2,6-Lutidine. PhNH2 is analyzed by HPLC,
and the yield was obtained by integration against an integral standard of
m-toluidine according to a calibration curve. The yields of NH3 and
MeNH2 are obtained by 1H NMR analysis.4b,13 b The blank experiment.
c 2a (4.0 µmol, 2.0 mol%). d 2a (2.0 µmol, 1.0 mol%). e Substrate (1.0
mmol).
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JA805025W
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J. AM. CHEM. SOC. VOL. 130, NO. 46, 2008 15251