Ammonia formation by a thiolate-bridged diiron amide complex as a nitrogenase mimic

Article abstract of DOI:10.1038/nchem.1594

Although nitrogenase enzymes routinely convert molecular nitrogen into ammonia under ambient temperature and pressure, this reaction is currently carried out industrially using the Haber-Bosch process, which requires extreme temperatures and pressures to activate dinitrogen. Biological fixation occurs through dinitrogen and reduced N_xH_y species at multi-iron centres of compounds bearing sulfur ligands, but it is difficult to elucidate the mechanistic details and to obtain stable model intermediate complexes for further investigation. Metal-based synthetic models have been applied to reveal partial details, although most models involve a mononuclear system. Here, we report a diiron complex bridged by a bidentate thiolate ligand that can accommodate HN=NH. Following reductions and protonations, HN=NH is converted to NH 3 through pivotal intermediate complexes bridged by N₂H ₃^- and NH₂^- species. Notably, the final ammonia release was effected with water as the proton source. Density functional theory calculations were carried out, and a pathway of biological nitrogen fixation is proposed.

Full text of DOI:10.1038/nchem.1594

ARTICLES
PUBLISHED ONLINE: 17 MARCH 2013 | DOI: 10.1038/NCHEM.1594
Ammonia formation by a thiolate-bridged diiron
amide complex as a nitrogenase mimic
Yang Li†, Ying Li†, Baomin Wang, Yi Luo, Dawei Yang, Peng Tong, Jinfeng Zhao, Lun Luo, Yuhan Zhou,
Si Chen, Fang Cheng and Jingping Qu
*
Although nitrogenase enzymes routinely convert molecular nitrogen into ammonia under ambient temperature and
pressure, this reaction is currently carried out industrially using the Haber–Bosch process, which requires extreme
temperatures and pressures to activate dinitrogen. Biological fixation occurs through dinitrogen and reduced NxHy species
at multi-iron centres of compounds bearing sulfur ligands, but it is difficult to elucidate the mechanistic details and to
obtain stable model intermediate complexes for further investigation. Metal-based synthetic models have been applied to
reveal partial details, although most models involve a mononuclear system. Here, we report a diiron complex bridged by a
bidentate thiolate ligand that can accommodate HN5NH. Following reductions and protonations, HN5NH is converted to
–
NH3 through pivotal intermediate complexes bridged by N2H3– and NH2 species. Notably, the final ammonia release was
effected with water as the proton source. Density functional theory calculations were carried out, and a pathway of
biological nitrogen fixation is proposed.
itrogen fixation has been the subject of extensive investigation model complexes with two or more iron centres have been described
for the past century. A successful artificial fixation known as to date. Peters and co-workers successfully synthesized a number of
N
the Haber–Bosch process is routinely practised on a large NxHy bridged diiron complexes supported by Fe[PhBPR3] (R ¼ Ph
scale, producing about 160 million tonnes of ammonia per year, or CH2Cy) scaffolds20,21. More recently, the Holland group reported
which is essentially used as fertilizer for one-third of Earth’s popu- a unique tetra-iron nitride complex with b-diketiminate auxiliary
lation. The extreme conditions necessary for this process (200– ligands, which can be reduced by dihydrogen to release
400 atm and 400–600 8C) make it very energy-consuming; indeed, ammonia22. Additionally, some thiolate- and sulfido-bridged
it consumes over 1% of the world’s annual energy supply. In con- diiron model complexes bearing N2H2 and PhNNH2 ligands have
trast, nitrogenases in some microorganisms produce ammonia also been reported by Sellmann23,24, Holland25,26 and colleagues.
from dinitrogen at ambient temperature and pressure.
Structural and functional mimicking of nitrogenase with syn-
Understanding the fixation mechanisms of the FeMo-cofactor, the thetic diiron models bearing thiolate ligands has been one of the
active site of these enzymes, would provide insights into designing fields researched by our group. To this end, a series of diazene
and optimizing artificial fixation.
bridged diiron complexes supported by the monodentate thiolate
Elucidating the detailed mechanism of enzymatic nitrogen fix- ligand [Cp‡Fe(m-SEt)2(m-h1:h1-HN¼NH)FeCp‡]nþ (Cp‡ ¼ h5-C5Me5,
ation has long been a challenge for scientists1,2. Based on the h5-C5Me4H, n ¼ 0, 1), were prepared and spectroscopically charac-
known [Fe7MoS9C] core structure of the FeMo-cofactor3,4, as well terized clearly27. Catalytic cleavage of the N–N bond of hydrazine
as experimental and theoretical studies, two general pathways for was realized using the phenylhydrazine complex [Cp*Fe(m-SEt)2
dinitrogen reduction to ammonia have been formalized by (m-h1:h1-PhN¼NH)FeCp*] as catalyst28,29
.
Hoffman and Dean: the distal (that is, Mn–N;N ꢀ Mnþ3;N þ NH3
In this Article, the bidentate benzene-1,2-dithiolate (bdt) ligand
ꢀ Mn þ NH3) and alternating (Mn–N;N ꢀ Mn–NH¼NH ꢀ was used in constructing diiron model complexes bearing NxHy
Mn–NH2–NH2 ꢀ Mn þ 2 NH3) mechanisms5,6. Two main species (Fig. 1b). The choice of the bdt ligand was based on the
assumptions have been proposed for dinitrogen binding and following considerations: (1) its redox non-innocent character30,31
reduction on the FeMo-cofactor: one involves dinitrogen binding (that is, bdt can act as either a dianionic or a p-radical monoanionic
to the molybdenum atom7–12, and the other suggests the partici- ligand), and (2) it supports variable coordination geometries32–36
.
pation of one or more iron atoms of the metallocluster13,14
.
These properties mean that the bdt ligand can potentially provide
Recent site-directed mutagenesis studies have suggested a high variable Fe–S scaffolds in one complex, allowing this complex to
probability that the ‘belt’ iron atoms of the FeMo-cofactor are the support the binding and transformation of different NxHy species.
site of dinitrogen binding and reduction6,15–17. In addition, some Here, we report the synthesis, interconversion and ammonia-releas-
density functional theory (DFT) calculations have proposed that ing reaction of bdt-bridged diiron NxHy complexes, through which a
the transformation of dinitrogen to ammonia is mediated by the reduction sequence of the NxHy species was established.
two ‘belt’ iron atoms through a series of NxHy bridged inter-
mediates18,19 (Fig. 1a). Although the coordination chemistry of
Results and discussion
dinitrogen and reduced dinitrogen species with iron has seen a
growth spurt in the past decade13,14, most iron-based synthetic
models involve the single iron system. Only a few examples of
Synthesis, characterization and interconversion of diiron NxHy
complexes. The diiron precursor [Cp*Fe(m-h2:h4-bdt)FeCp*] (1)
was prepared in a similar manner as the recently reported triply
State Key Laboratory of Fine Chemicals, School of Pharmaceutical Science and Technology, Faculty of Chemical, Environmental and Biological
Science and Technology, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China, †These authors contributed equally to this work.
e-mail: qujp@dlut.edu.cn
*
320
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1594
ARTICLES
resulted in the production of the dark green [Cp*Fe(m-h2:h2-bdt)
(m-h1:h1-HN¼NH)FeCp*] (2a) and the release of ammonia.
Crystals of 2a suitable for X-ray diffraction analysis were not
obtained owing to its high air sensitivity and good solubility in
most solvents. One resonance for Cp* methyl protons (d ¼ 1.27 ppm)
and two for bdt protons (d ¼ 6.31 and 6.77 ppm) are shown in
a
b
S
O
S
S
S
S
R
Fe
Fe
Fe
Fe
O
S
C
R'
S
O
S
S
Mo
Fe SCys
HisN
Fe
NxHy
Fe
Fe
Fe
S
1
SH
the H NMR spectrum (in deuterated THF-d8) of 2a, indicating
NxHy
the symmetrical geometry of 2a. A N2H2 resonance is noted at
15.29 ppm, which splits into an apparent doublet (1JNH ¼ 68.4 Hz)
when 2a is prepared using 15N2H4. The broadness of the doublets
precludes the resolution of higher-order N–H and H–H coupling.
The 15N NMR spectrum (Fig. 3) shows an AA′XX′ splitting pattern
centred at 349.87 ppm, indicative of two equivalent sp2-hybridized
nitrogen atoms. This is similar to that observed in the related
N2
NH3
N2
NH3
Figure 1 | Hypothetical diiron sites of ammonia production from dinitrogen.
a,b, Possible routes are shown for the FeMo-cofactor (a) and our diiron
model complex (b).
thiolate-bridged diiron complexes [Cp*Fe(m-SR)3FeCp*] (R ¼ Et, side-on N2H2 species, cis-[Fe(DMeOPrPE)2(h2-N2H2)]38. The
Ph)37. Treatment of [Cp*FeCl]2 with 1 equiv. of Li2(bdt) in infrared spectrum of 2a shows the n (N–H) band at 3,242 cm21
tetrahydrofuran (THF) from 278 8C to room temperature which shifts to 3,236 cm21 in the sample of 15N-2a.
,
produced 1 as red-black crystals. The solid-state structure of 1 is
To obtain more insight into the coordination geometry of the
shown in Supplementary Fig. S1. The most distinctive structural N2H2 ligand, one-electron oxidation of 2a was performed with
.
feature of 1 is that it is not symmetrical; the bdt ligand leans Fc PF6 (Fc ¼ Ferrocene). The solid-state structure of [Cp*Fe(m-
towards one iron centre, and binds to the two iron centres in a h2:h2-bdt)(m-h1:h1-HN¼NH)FeCp*][PF6] (2a[PF6]) is shown in
m-h2:h4 manner: to one iron centre through the two thiolates (in Fig. 4a. The N2H2 ligand coordinates the [Fe2S2] scaffold in a
an h2 fashion) and to the other through the two thiolates and their cis fashion. The long Fe Fe distance of 3.176(2) Å is indicative of
...
two adjacent carbons (h4). The distances C23–C24 and C25–C26 the absence of a bonding interaction between the two iron
in the benzene ring are found to be noticeably shorter than the centres. The N–N bond distance of 1.312(4) Å is similar to those
others (Supplementary Table S15). This is characteristic of the found in the previously reported diiron complexes bearing an
p-radical monoanionic bdt ligand30, indicating an electron transfer end-on bridging N2H2 ligand (1.28–1.37 Å)20,21,23,24,27,39. This dis-
from the bdt ligand to the metal centres in order to stabilize the tance is closer to the value of an N(sp2)–N(sp2) double bond.
highly electron-deficient diiron core. We assume that this electron- The shortened Fe–N bond distances of 1.827(3) Å and 1.826(3) Å
deficient nature of 1 would make it a good candidate as a in 2a[PF6] suggest a strong interaction between the iron orbitals
nitrogenase model complex, so as to accommodate the electron- and the system of diazene. Unlike complex 1, the carbon–carbon
donating NxHy species (Fig. 2).
distances of the benzene ring in 2a[PF6] are fairly consistent, indi-
The room-temperature addition of 8 equiv. of hydrazine cating a dianionic character of the bdt ligand. The Mo¨ssbauer spec-
to a red-purple THF solution of [Cp*Fe(m-h2:h4-bdt)FeCp*] (1) trum of 2a[PF6] was recorded with isomer shift and quadrupole
BPh4
H
H
S
S
N
S
N
S
N
S
S
(iv)
(ii)
Fe
Fe
Fe
R
Fe
H
Fe
Fe
N
R
2a, R = H
3a[BPh4], R = H
3b[BPh4], R = Me
1
2b, R = Me
(v)
(iii)
(i)
PF6
BPh4
Cl
Cl
Fe
Fe
S
N
S
N
S
S
Fe
R
Fe
H
Fe
Fe
N
H
R
2a[PF6], R = H
4a[BPh4], R = H
4b[BPh4], R = Me
2b[PF6], R = Me
Figure 2 | Synthesis of diiron NxHy complexes and their methyl-substituted analogues. Reagents and conditions: (i) 1 equiv. Li2(bdt), THF, 278 8C to room
temperature, 45%; (ii) for 2a, 8 equiv. anhydrous hydrazine, THF, room temperature, 2 h, 88%; for 2b, 8 equiv. methyl hydrazine, THF, room temperature,
.
.
3 h, 68%; (iii) 1 equiv. Fc PF6, CH2Cl2, room temperature, 1 h, 85% for 2a[PF6] and 83% for 2b[PF6]; (iv) for 3a[BPh4], 1 equiv. Lut HBPh4 (Lut ¼ 2,6-lutidine),
.
.
THF, 220 8C, 3 h, 89%; for 3b[BPh4], 1 equiv. Lut HBPh4, CH2Cl2, 250 8C to room temperature, 3 h, 71%; (v) 2 equiv. CoCp2, 2 equiv. Lut HBPh4, THF,
278 8C to room temperature, 85% for 4a[BPh4] and 56% for 4b[BPh4].
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1594
ARTICLES
δ = 349.87
diazene ligand, the formal valences of the two iron atoms rise from
a
1JNH = 68.4 Hz
FeII–FeII in 2 to FeIII–FeIII in 3þ. Both 3a[BArF ] and 3a[BPh4] are
15N-2a
4
stable in the solid state, but convert to 2aþ and 4aþ, probably
15N
through an intermolecular proton transfer in solution at room
1
temperature, as evidenced by H NMR spectroscopy. The solid-
2
state structure of 3a[BArF ] reveals that a m-h1:h2 N2H3 ligand
4
bridges the diiron centre (Fig. 4b). Comparatively, the coordination
manner of the bdt ligand changes from m-h2:h2 in 2 to m-h1:h2 in
3a[BArF ]. Meanwhile, the Fe–Fe distance of 3a[BArF ] is pulled
15N{1H}
4
4
close to 2.6990(11) Å and falls into the range of a bonding inter-
action. We believe that it is the binding flexibility of the bdt
ligand, including the apparent synergy between its binding mode
and the Fe–Fe interaction, that allows for the observed transform-
ation. The N–N bond length of 1.402(5) Å is longer than that in
2a[PF6] (1.312(4) Å) after protonation, within the range of the
N–N single bond lengths. This distance is close to the N–N
366
362
358
354
350
ppm
346
342
338
334
b
δ = 111.58
δ = 33.05
15N-3a[BPh4]
1JNH = 80.0 Hz
1JNN = 9.4 Hz
1JNH = 84.0 Hz
1JNN = 9.4 Hz
lengths observed in hydrazido–borane complexes [(SiPR )Fe(h1-
3
N2H3B(C6F5)3)] (R ¼ Ph and i-Pr)41 and slightly longer than that
15N
2
of the mono-iron N2H3 complex [(PhBPPh3)Fe(h2-N2H3)(CO)]39.
The Fe–N distances (Fe2–N1 1.892(5) Å, Fe1–N1 1.907(4) Å,
Fe2–N2 1.937(5) Å) indicate the sp3-hybridization of nitrogen
2
atoms in the N2H3 ligand.
To locate the N2H32 protons, 15N-3a[BPh4] was synthesized by
15N{1H}
protonation of 15N-2a. The H NMR spectrum (250 8C, THF-d8)
1
of 3a[BPh4] shows three distinct protons (d ¼ 10.24, 5.51 and
2
4.10 ppm) for the N2H3 ligand. They all split into doublets
1
(1JNH ¼ 80 Hz for d ¼ 10.24 ppm; JNH ¼ 84 Hz for d ¼ 5.51 and
4.10 ppm) following 15N labelling. The 15N{ H} NMR spectrum
1
124
116
108
44
38
32
26
(250 8C, THF-d8, Fig. 3b) reveals two resonances at 111.58 and
33.05 ppm, which split into a doublet (1JNH ¼ 80.0 Hz) and triplet
(1JNH ¼ 84.0 Hz), respectively, on turning the proton decoupler
off, implicating the presence of an –NH nitrogen atom and an
–NH2 nitrogen atom. N–N coupling is also observed (1JNN ¼ 9.4 Hz).
Correlated information from the 1H–15N heteronuclear single
quantum correlation (HSQC) spectrum (250 8C, THF-d8) suggests
that the proton assigned at 10.24 ppm attaches to the nitrogen
assigned at 111.58 ppm, while protons assigned at 5.51 and
4.10 ppm connect the nitrogen assigned at 33.05 ppm
(Supplementary Fig. S19). All these NMR experiments confirm
ppm
c
δ = 76.38
15N-4a[BPh4]
1JNH = 72.0 Hz
15N
15N{1H}
2
that complex 3a[BPh4] contains an N2H3 ligand. Complexes
3a[BPh4], 3a[BArF ] and their 15N-labelled congeners were
4
further characterized by ESI-HRMS and infrared spectrometry.
85
81
77
73
69
65
Although N2H32 is believed to be an important intermediate in
ppm
2
the reduction of N2 to NH3, iron N2H3 complexes were not syn-
Figure 3 | Characterization by 15N NMR spectra. Spectra obtained in
THF-d8 for a, 15N-labelled 2a; b, 3a[BPh4]; c, 4a[BPh4].
thesized until recently. Tyler and co-workers prepared the first
2
iron(II) N2H3 complex [Fe(DMeOPrPE)2(h2-N2H3)]þ by either
protonation of [Fe(DMeOPrPE)2(h2-N2H2)] or deprotonation of
splitting to be 0.29 and 0.74 mm s21, respectively (Supplementary [Fe(DMeOPrPE)2(h2-N2H4)]2þ (ref. 40). The first iron N2H3
Fig. S58). Complexes 2a[PF6] and 15N-2a[PF6] were further charac- complexes characterized by X-ray crystal diffraction analysis were
2
terized by electrospray ionization high-resolution mass spectrometry the zwitterionic iron(II) hydrazido–borane complexes [(SiPR )Fe(h1-
3
(ESI-HRMS) and infrared spectrometry.
N2H3B(C6F5)3)] (R ¼ Ph and i-Pr)41. Another N2H32 iron complex
According to the spectroscopic data discussed above, we assign [(PhBPPh3)Fe(h2-N2H3)(CO)], reported by the Peters group, was
the N2H2 ligand in 2a and 2a[PF6] to the diazene ligand, rather achieved by addition of hydrazine to [(PhBPPh3)Fe(Me)] at low
than the hydrazido(2–) ligand presented in some mono-iron side- temperature, followed by treatment with CO (ref. 39). These
on N2H2 complexes38,40. Considering the multimetal structure of complexes show two different coordination geometries of the
the FeMo-cofactor, the end-on cis bridging diazene species, N2H32 ligand, h1 and h2, on the single iron centres. In contrast,
corresponding to 2a and 2a[PF6], may be a possible intermediate the case we report here represents the coordination of the N2H32
in biological nitrogen fixation.
ligand on diiron centres. This m-h1:h2 coordination of the N2H32
F
The addition of the proton source Lut HBAr4 (Lut ¼ 2,6-luti- ligand in 3aþ offers a new possible mode for N2H32 binding to
.
dine, BArF ¼ tetrakis(3,5-bis(trifluoromethyl)phenyl)borate anion) the FeMo-cofactor.
2
4
.
and Lut HBPh4, to the THF solution of 2a at 230 8C and
Further reduction and protonation of 3a[BPh4] was carried out.
220 8C, respectively, resulted in the N2H3 complexes [Cp*Fe(m- The addition of 2 equiv. of CoCp2 to the THF solution of 3a[BPh4]
h1:h2-bdt)(m-h1:h2-NH2–NH)FeCp*][BArF ] (3a[BArF ]) and at 278 8C, followed by the addition of 2 equiv. of Lut HBPh4,
.
4
4
[Cp*Fe(m-h1:h2-bdt)(m-h1:h2-NH2–NH)FeCp*][BPh4] (3a[BPh4]). afforded
brown
[Cp*Fe(m-h2:h2-bdt)(m-NH2)FeCp*][BPh4]
Protonation-induced electron transfer occurs in this reaction. By (4a[BPh4]) with the release of ammonia. The released ammonia
1
transferring two electrons from the diiron centre to the protonated was collected and quantified, showing an 80% yield by H NMR
322
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1594
ARTICLES
a
b
c
S2
S1
S1
N2
S2
Fe1
S1
S2
Fe2
Fe1
Fe2
Fe1
N2
N1
Fe2
N1
N1
2a[PF6]
3a[BArF4
]
4a[BPh4]
d
e
f
S2
S2
Fe1
S1
Fe2
S1
N1
S1
S2
N2
Fe1
Fe1
N1
Fe1A
N1A
Fe2
N1
C27
C17
C17A
C37
4b[BPh4]
2b
3b[BPh4]
Figure 4 | Oak Ridge thermal ellipsoid plot (ORTEP) diagrams, with the thermal ellipsoids shown at a 50% probability level. a, 2a[PF6]. b, 3a[BArF4].
c, 4a[BPh4]. d, 2b. e, 3b[BPh4]. f, 4b[BPh4]. For each panel, anions, one THF molecule and hydrogen atoms on carbons are omitted for clarity.
See Supplementary Information for more details.
in DMSO-d6 as its NH4Cl salt after vacuum transfer of the reaction corresponding to [Cp*Fe(m-bdt)(N2H4)FeCp*]þ and [Cp*Fe(m-
volatiles into an ether solution of HCl. The crystal structure of bdt)(NH)FeCp*]þ were observed by the ESI-HRMS spectroscopy.
2
4a[BPh4] indicates that a bridging NH2 ligand binds to the We posited that the steps involved in the conversion of 3a[BPh4]
diiron centres (Fig. 4c). The Fe–N distances of 1.924(3) and to 4a[BPh4] are energetically downhill with low activation energy
1.932(3) Å are as expected for coordination of sp3-hybridized nitro- barriers. As a result, it is difficult to capture the intermediates
2
gen to iron, and similar to those observed in the two bridging NH2
diiron complexes synthesized to date, 2.049(8) and 2.034(9) Å
along the reaction until the formation of 4a[BPh4].
To access the whole conversion process from 3a[BPh4] to
for [{(PhBPPh3)Fe}2(m-NH2)2]20 and 1.996(2) Å for [{(Ar)2C6H3} 4a[BPh4], DFT model calculations were performed. The computed
Fe}2(m-NH2)2]42. The Fe–Fe distance of 2.4423(6) Å suggests the profile of the free energy in solution and gas phase is shown in Fig. 5
interaction of the two iron atoms. The 1H NMR spectrum (see Supplementary section ‘Experimental’ for computational details). In
(THF-d8) reveals a solution structure similar to that observed in the calculations, the Cp* ligand was modelled by Cp (Cp ¼ h5-C5H5),
2
the solid state. An NH2 resonance is noted at 8.84 ppm and which is generally adopted in computations of the metallocene
splits into a doublet upon 15N labelling. Its 15N NMR spectrum system for the sake of CPU processing time. The cationic species
(THF-d8, Fig. 3c) contains a triplet at 76.38 ppm (1JNH ¼ 72.0 Hz) [CpFe(m-h1:h2-bdt)(m-h1:h2-NH2–NH)FeCp]þ (3amþ) and
that becomes singlet upon decoupling. The correlation of the [CpFe(m-h2:h2-bdt)(m-NH2)FeCp]þ (4amþ) were taken as model
proton resonance and the 15N resonance in the 1H–15N HSQC spec- compounds for the cationic parts of 3a[BPh4] and 4a[BPh4],
2
trum confirms the presence of the NH2 ligand (Supplementary respectively. In the following and Fig. 5, the superscripts in the lab-
Fig. S26). Complexes 4a[BPh4] and 15N-4a[BPh4] were further elling of stationary points indicate the charge and spin multiplicity
characterized by ESI-HRMS and infrared spectrometry.
of corresponding species. For example, ‘3am[2]’ denotes the neutral
In the conversion of 3a[BPh4] to 4a[BPh4], the coordination species 3am with spin multiplicity of 2, and labelling ‘3amþ[1]
’
of the bdt ligand shifts from m-h1:h2 back to m-h2:h2. Thus, represents species 3am with charge of þ1 and multiplicity of 1.
three coordination manners for the bdt ligand, including m-h2:h4 As shown in Fig. 5, the reduction of starting model complex
p-radical monoanionic for 1, m-h2:h2 dianionic for 2a, 2a[PF6] 3amþ[1] (S ¼ 0) is exergonic by 9 kcal mol21 in solution, leading
and 4a[BPh4], and m-h1:h2 dianionic for 3a[BPh4] and to 3am[2] (S ¼ 1/2).
3a[BArF ], are observed during the conversion of 1 to 4a[BPh4].
The protonation of 3amþ[1], which leads to dicationic species
4
...
It follows that the bdt ligand can shift the coordination according with either the NH NH3 or NH2–NH2 moiety, is computed to be
to the nature of the NxHy species, which, to some extent, mimics endergonic by more than 68 kcal mol21 in the gas phase and
the flexibility of the FeMo-cofactor.
more than 21 kcal mol21 in solution (Supplementary Fig. S62).
This suggests an energetically unfavourable protonation of
DFT investigations of the reaction from 3a1 to 4a1. Evidently, two 3amþ[1] in comparison with its reduction leading to 3am[2]. The
reductive protonation steps, as well as the release of one ammonia protonation of 3am[2] favours 6þ[2] (S ¼ 1/2) with an NH NH3
...
molecule, are involved in the transformation of 3a[BPh4] to moiety rather than 91[2] with an H2N–NH2 moiety, because the
4a[BPh4]. We reduced the amount of reductant or proton source former is significantly lower in free energy than the latter (by
in an attempt to analyse the details of this multistep reaction. 20 kcal mol21 in solution). The conversion of 3am[2] to 6þ[2] is
However, complicated products were obtained, and no signals exergonic by 14 kcal mol21 in solution and 29 kcal mol21 in the
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1594
ARTICLES
+
+
NH2
FeCp
NH2
S
S
S
S
S
S
Reduction
Protonation
NH3
FeCp
CpFe
CpFe
FeCp
CpFe
Favourable pathway
ΔΔGR = –13.52 (–28.84)
ΔΔGR = –8.84 (–2.12)
N
N
N
H
H
H
3am+[1]
3am[2]
6+[2]
ΔGR = 0.00 (0.00)
ΔGR = –8.84 (–2.12)
ΔGR = –22.36 (–30.96)
ΔΔGR
= –1.37 (2.88)
Unfavourable
pathway
Protonation
S
S
NH3
+
S
S
S
S
2e–/2H+
NH3
+
CpFe
FeCp
NH2
FeCp
Overall
reaction
S
S
N
H
CpFe
N
7+[2]
ΔGR = –23.73 (–28.08)
H2
9+[2]
ΔGR = –1.72 (–2.23)
ΔΔGR
= –19.56 (–10.86)
Reduction
FeCp
+
+
S
S
S
S
S
S
Isomerization
Protonation
CpFe
CpFe
FeCp
CpFe
FeCp
ΔΔGR = –2.31 (–3.51)
ΔΔGR = –10.50 (–21.24)
N
N
N
H
H2
H2
4am+[1]
ΔGR = –56.10 (–63.69)
8+[1]
7[1]
ΔGR = –43.29 (–38.94)
ΔGR = –53.79 (–60.18)
Figure 5 | Computed minimum free-energy pathway for the transformation of 3am1 to 4am1. TPSSTPSS/TZVP relative free energies in solution (DGR) are
in kcal mol21. The data in parentheses are the corresponding relative free energies in the gas phase. The energy shown is the reaction free energy including
that of all species involved in the corresponding reaction. See Supplementary section ‘Experimental’ for computational details and Figs S61–63 for a more
detailed energy profile.
gas phase (see the DDGR of this conversion in Fig. 5). The release of (m-h1:h1-MeN¼NH)FeCp* (2b) can be obtained in a similar
NH3 from 6þ[2] yields 7þ[2] and the isolated NH3 molecule. This manner by the addition of methyl hydrazine into the THF solution
step is almost isoenergetic (DDGR ¼ 21 kcal mol21 in solution) of 1. Oxidation of 2b with Fc PF6 afforded [Cp*Fe(m-h2:h2-
.
due to the very weak binding between the released NH3 and the bdt)(m-h1:h1-MeN¼NH)FeCp*][PF6] (2b[PF6]). They show the
NH moiety in 6þ[2]. These results suggest that the cleavage of the same cis coordination manner as the diazene analogues, with N–N
N–N bond to liberate NH3 most probably occurs through bond distances of 1.292(10) and 1.305(4) Å for 2b (Fig. 4d) and
...
HN NH3 rather than the H2N–NH2 moiety.
2b[PF6], respectively. Protonation of 2b gives [Cp*Fe(m-h1:h2-
The 7þ[2] could undergo reduction and subsequent protonation bdt)(m-h1:h2-NH2–NMe)FeCp*][BPh4] (3b[BPh4], Fig. 4e). The
to give 8þ[1] (S ¼ 0) with a relative free energy in solution of methyl group attaches to the bridging nitrogen atom and two
–54 kcal mol21. The reduction of 7þ[2] leads to neutral species hydrogen atoms located at the terminal nitrogen atom. This
7
[1]. This step is exergonic by 20 kcal mol21 in solution and implies that the protonation might take place at the ¼NH nitrogen
11 kcal mol21 in the gas phase. The subsequent protonation of atom rather than at the ¼NMe nitrogen atom of the MeN¼NH
7[1] is also exergonic by 11 kcal mol21 and 21 kcal mol21 in sol- ligand in 2b. The N–N bond distance of 1.447(4) Å is ꢁ0.04 Å
ution and the gas phase, respectively. The 81[3] (S ¼ 1) species has longer than that of 3a[BArF ]. Taken together, the unchanged
4
almost the same energy (–53 kcal mol21) as that of 8þ[1] (S ¼ 0) coordination pattern, similar geometry and spectroscopic data of
(Supplementary Fig. S59). The isomerization of 8þ[1] gives the 2b, 2b[PF6] and 3b[BPh4], compared with their unsubstituted
final product 4amþ[1] (S ¼ 0). Such an isomerization is exergonic analogues, suggest that steric hindrance does not significantly
(DDGR ¼ 22 kcal mol21 in solution and 24 kcal mol21 in the influence the coordination and reactivity of the nitrogenous ligands.
gas phase) and occurs through a transition state with a free energy Thus, it is reasonable to believe that the reductive protonation of
barrier of 6 kcal mol21 in solution (Supplementary Fig. S59), 3b[BPh4] proceeds through the same pathway as 3a[BPh4].
suggesting a feasible process. As shown in Fig. 5, the whole
The addition of 2 equiv. of CoCp2 to the THF solution of
process (transformation of 3amþ[1] to 4amþ[1]) was computed to 3b[BPh4] at 278 8C, followed by the addition of 2 equiv. of
be exergonic by 56 kcal mol21 in solution and 64 kcal mol21 in Lut HBPh4, resulted in a mixture of 4a[BPh4] and the NHMe
2
.
gas phase. The current DFT results suggest that the ammonia complex [Cp*Fe(m-h2:h2-bdt)(m-NHMe)FeCp*][BPh4] (4b[BPh4]),
release was mainly derived from the unbridging NH2 of 3amþ.
with a ratio of 1:4. A relatively pure sample of 4b[BPh4] was obtained
in 56% yield (Fig. 4f). This offers further experimental evidence that
Methyl hydrazine experiment. To corroborate the reaction the ammonia released in the reaction of 3a[BPh4] to 4a[BPh4] was
2
pathway from 3a[BPh4] to 4a[BPh4] proposed by the DFT mainly derived from the unbridging NH2 of the N2H3 ligand,
calculation, we studied the reaction of 1 with methyl hydrazine, in which is consistent with the DFT calculation results.
which the two nitrogen atoms are differentiated by different
substitutions, together with the subsequent reductive protonation Ammonia release from complex 4a[BPh4] in water. In the studies
process (Fig. 2). The methyl diazene complex Cp*Fe(m-h2:h2-bdt) that have been reported to date, some well-defined iron dinitrogen
324
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1594
ARTICLES
1 was found to be very effective in stabilizing nitrogenous donors,
due in part to the binding flexibility of the bdt ligand, as evidenced
by the fact that a series of diiron NxHy species containing cis-h1:h1-
diazene, h1:h2-hydrazido(1–) and amide bridges have been syn-
thesized and characterized. Following stepwise protonation and
Table 1 | Ammonia production from complex 4a[BPh4] in
the presence of reducing agent and proton source.
Entry Reducing agent
E1/2 (V)* Proton source
Yield of
NH3 (%)
2
.
1
CoCp2 (2 equiv.)
CoCp2 (6 equiv.)
CoCp2 (2 equiv.)
CrCp2 (2 equiv.)
CoCp*2 (2 equiv.) 21.87
CrCp*2 (2 equiv.) 21.35
CoCp2 (2 equiv.)
None
21.15
Lut HBPh4 (2 equiv.) 46
reduction, the transformation of coordinated N2H2 to NH2
2
.
Lut HBPh4 (6 equiv.) 51
2
3
4
5
6
7
8
9
(through an N2H3 intermediate followed by the final release of
H2O (50 equiv.)
H2O (50 equiv.)
H2O (50 equiv.)
H2O (50 equiv.)
CH3OH (50 equiv.)
H2O (50 equiv.)
None
98
8
96
93
15
ammonia) was realized at diiron sites. DFT calculations were
carried out to gain insight into the mechanism of the reaction
3aþ to 4aþ. We assume that the first reductive protonation occurs
20.88
2
at the unbridging NH2 of the N2H3 ligand. After releasing NH3,
2
the resulting NH2 bridged complex is converted to the NH2
Trace
4
bridged complex through the second reductive protonation. These
results provide effective evidence for a diiron mechanism for bio-
CoCp2 (2 equiv.)
þ
Reaction conditions: 4a[BPh4] (0.2 mmol), THF (6 ml), room temperature, 12 h. The NH4 was
quantified by integration of the NH4þ resonance with respect to an internal reference of ferrocene
(see Supplementary section ‘Experimental’ for details). *Electrochemical data (E1/2) of reductant
in MeCN versus Ag/Agþ in 0.1 M AgNO3 (ref. 50).
2
2
logical nitrogen fixation and show that N2H2, N2H3 and NH2
are very probably the species produced during the N2 reduction
process within natural enzymes. In addition,
a reduction
route different from both alternating and distal mechanisms,
HN¼NH ꢀ HN–NH2 ꢀ NH(þNH3) ꢀ NH2 ꢀ NH3, is pre-
sented. Further studies are under way to investigate the reactivity
of this kind of [Fe2S2] scaffold towards N2 and to elucidate how
N2 is converted to ammonia at diiron sites.
and nitride complexes have shown the ability to produce ammonia
by the addition of a hydrogen atom source and/or reductant22,43–49
.
Although no example of ammonia release from an iron amide
complex has been reported, it is believed to be the intermediate at
the late stage of the dinitrogen reduction route. Studies were
therefore carried out to examine the competence of 4a[BPh4] in
producing ammonia.
Received 23 July 2012; accepted 4 February 2013;
published online 17 March 2013
.
The addition of 2 equiv. of CoCp2 and 2 equiv. of Lut HBPh4 to
the THF solution of 4a[BPh4] (0.2 mmol) produced NH3 (46%) and
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Acknowledgements
This work was supported by the National Natural Science Foundation of China (nos
21231003, 21174023, 21104008, 21076037, 21028001, 20972022) and the ‘111’ project of
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Author contributions
J.Q. and B.W. supervised the project. J.Q. and Ya.L. conceived and designed the
experiments. Ya.L., Yi.L., D.Y. and P.T. performed the experiments. Y.Lu., L.L. and S.C.
performed the computational studies. J.Q., Ya.L., B.W. and F.C. co-wrote the paper. J.Z. and
Y.Z. analysed the data. All authors discussed the results and commented on the manuscript.
36. Julia, F., Jones, P. G. & Gonzalez-Herrero, P. Synthesis and photophysical
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Additional information
Supplementary information and chemical compound information are available in the
online version of the paper. Reprints and permissions information is available online at
www.nature.com/reprints. Correspondence and requests for materials should
be addressed to J.Q.
38. Field, L. D., Li, H. L., Dalgarno, S. J. & Turner, P. The first side-on bound metal
complex of diazene, HN¼NH. Chem. Commun. 1680–1682 (2008).
Competing financial interests
The authors declare no competing financial interests.
326
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