N-N bond cleavage of hydrazines with a multiproton-responsive pincer-type iron complex

Article abstract of DOI:10.1021/ja3122944

N-N bond cleavage of hydrazines on transition metals is of considerable importance in understanding the mechanism of biological nitrogen fixation under ambient conditions. We found that a metal-ligand-bifunctional complex of iron with a pincer-type ligand bearing two proton-responsive pyrazole arms catalyzes the disproportionation of hydrazine into ammonia and dinitrogen. The NH groups in the pyrazole ligands and hydrazines are crucial for the reaction, which most likely occurs through multiple and bidirectional proton-coupled electron transfer between the iron complex and hydrazine. The multiproton-responsive pincer-type ligand also stabilizes the intermediate diazene complex through a hydrogen-bonding network, as revealed by structural characterization of a κ¹N-phenylhydrazine complex.

Full text of DOI:10.1021/ja3122944

Communication
pubs.acs.org/JACS
N−N Bond Cleavage of Hydrazines with a Multiproton-Responsive
Pincer-Type Iron Complex
Kazuki Umehara, Shigeki Kuwata,* and Takao Ikariya*
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, O-okayama,
Meguro-ku, Tokyo 152-8552, Japan
S
* Supporting Information
Scheme 1. Synthesis of Protic Pincer-Type Iron Complexes 2a
and 3
a
ABSTRACT: N−N bond cleavage of hydrazines on
transition metals is of considerable importance in under-
standing the mechanism of biological nitrogen fixation
under ambient conditions. We found that a metal−ligand-
bifunctional complex of iron with a pincer-type ligand
bearing two proton-responsive pyrazole arms catalyzes the
disproportionation of hydrazine into ammonia and
dinitrogen. The NH groups in the pyrazole ligands and
hydrazines are crucial for the reaction, which most likely
occurs through multiple and bidirectional proton-coupled
electron transfer between the iron complex and hydrazine.
The multiproton-responsive pincer-type ligand also
stabilizes the intermediate diazene complex through a
hydrogen-bonding network, as revealed by structural
characterization of a κ1N-phenylhydrazine complex.
roton-coupled electron transfer (PCET) plays pivotal roles
Pin a broad range of biological transformations and energy
conversions with notable efficiency.1 A representative example is
enzymatic nitrogen fixation, which involves multiple and
alternating flows of protons and electrons to the N2 substrate
without the formation of high-energy intermediates.2 We
envisioned that such multiple PCET may proceed smoothly at
a redox-active metal center ligated by multiple proton-delivering
functional groups. In such a bifunctional platform, transfer of the
accumulated protons to the coordinated substrate would render
the functional ligands more electron-donating to facilitate
coupled metal-to-substrate electron transfer. This strategy has
rarely been explored in functional modeling of nitrogenase
enzymes,3,4 although reductive transformations of dinitrogen5,6
and related nitrogenous substrates5,7−10 on transition-metal
complexes have been extensively studied. Within this context,
and in relation to our continuing study of metal−ligand
cooperating bifunctional catalysts,11−13 we recently reported
stepwise and reversible deprotonation of a ruthenium complex
bearing a multiproton-responsive pincer-type bis(pyrazole)
ligand, 2,6-bis(5-tert-butyl-1H-pyrazol-3-yl)pyridine (LH2 in
Scheme 1), as well as N2 coordination therein.12 Here we
describe multiple and bidirectional PCET in a related metal−
ligand-bifunctional complex of iron with ligand LH2, leading to
facile reduction and oxidation of hydrazine, which is a
nitrogenase substrate.14
a
Reagents and conditions: (a) MeOH, rt. (b) NaOTf (excess), PMe3
(2.3 equiv), MeCN, rt. (c) NH3 (excess), CH2Cl2, rt. X = OTf
(OSO2CF3).
ray analysis (Figure S1).15 Ligand exchange of 1 with PMe3 and
solvent acetonitrile, with addition of NaOTf to provide triflate
counterions, led to the formation of the diamagnetic iron
complex 2a. The pyrazole NH signal at δ 12.40 in the 1H NMR
spectrum of 2a was identified by exchange with added D2O. The
Brønsted acidic nature of the pyrazole protons is further
suggested by the crystal structure of 2a depicted in Figure 1, in
which the pyrazole NH groups point to the triflate counteranions
with hydrogen-bonding interactions (mean NH···O distance =
1.97 Å). While 2a was inert toward dinitrogen, the nitrile ligand
in 2a was cleanly replaced by ammonia to give ammine complex
3, whose 1H NMR spectrum shows a resonance ascribed to the
ammine ligand at δ 3.09.
The pincer-type complex 2a having two ionizable NH groups
around the labile nitrile ligand proved to effect the catalytic
disproportionation of hydrazine into ammonia and dinitrogen.
Treatment of 2a with 5 equiv of hydrazine led to complete
consumption of the hydrazine and the clean formation of
ammonia and dinitrogen along with ammine complex 3.16 No
dihydrogen evolution was observed. The product yields
summarized in Table 1 clearly indicate that 2a catalyzes the
disproportionation of hydrazine according to the equation
The reaction of iron(II) chloride with the pincer ligand LH2 in
methanol afforded chlorido−methanol complex 1 (Scheme 1).
The high-spin complex 1 (μeff = 5.0μB) was characterized by X-
Received: December 17, 2012
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja3122944 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
In this reaction, complex 3 was derived from 2a and free
ammonia. When the reaction was carried out under a slightly
reduced pressure to suppress the ammonia coordination, the
isolated yield of 4a increased to 63%. The overall reaction can
thus be described as disproportionation of two molecules of
phenylhydrazine to give phenyldiazene, aniline, and ammonia. X-
ray analysis of 4a revealed the κ1N coordination of a cis-PhN
NH ligand, which is almost coplanar with the pincer ligand
(Figure 2). The diazene ligand is involved in the hydrogen-
Figure 1. Structure of nitrile complex 2a. H atoms except for the
pyrazole NH hydrogens have been omitted for clarity. Ellipsoids are
drawn at the 30% probability level.
Table 1. Catalytic Disproportionation of Hydrazine with
Protic Pincer-Type Iron Complexes
Figure 2. Structure of phenyldiazene complex 4a. H atoms except for
the NH hydrogens have been omitted for clarity. Ellipsoids are drawn at
the 30% probability level.
b
yields
a
a
c
catalyst
conv. of N2H4 (%)
NH3
N2
1.5
d
de
,
d
bonding network with the protic pincer ligand and triflate anions
[O(1)···H(3), 2.35(3) Å;17 N(7)···H(2), 2.32 Å]. The Fe−
Nproximal and NN distances of 1.875(3) and 1.270(3) Å,
respectively, are comparable to those found in related diazene
2a
2b
2c
>98
5.5 (6.1)
2.8
54
0.79
0.52
37
0.34
a
b
Determined by colorimetry. In moles per mole of Fe complex.
complexes.4,7,8,18 The H NMR spectrum of the diamagnetic
1
c
d
e
Determined by GC. Average value from three runs. The value in
parentheses includes the amount of ammine complex 3 estimated by
1H NMR analysis. For details, see the Supporting Information.
complex 4a exhibited a singlet at extremely low field (δ 17.46)
that was assigned to the diazene proton on the basis of the
observation that it split into a doublet upon 15N labeling (1JNH
=
67.0 Hz). The 15N{1H} NMR resonances of 4a-15N2 appeared at
δ 414.1 and 466.3. These spectroscopic parameters agree with
the diazene formulation.4,7,8,18−20 The two pyrazole NH protons
at δ 12.27 were found to be equivalent even at −80 °C, indicating
a rapid rotation about the iron−diazene bond in solution despite
the presence of the hydrogen-bonding network. These NH
signals disappeared upon treatment with D2O, consistent with
the expected Brønsted acidic nature of these protons.
On the other hand, the reaction of 2a with 1,1-diphenylhy-
drazine afforded diphenylamine and an almost equimolar
amount of hydrazinophosphonium triflate 5 along with ammine
complex 3 and other paramagnetic iron complexes (eq 2):
shown therein with a reasonable material balance. Notably, the
reactions with the N-methylated analogues 2b and 2c15 were
much slower and more complicated, suggesting that the
importance of the NH groups in the catalysis.
Although disproportionation of hydrazine itself is catalyzed by
a number of transition-metal complexes, including iron-based
ones,7−9 little is known about the detailed mechanism of N−N
bond cleavage. To elucidate the mechanism of the catalytic
disproportionation of hydrazine with 2a, we next examined the
reaction using substituted hydrazines. When 2a was treated with
phenylhydrazine, the N−N bond cleavage occurred in a
substoichiometric manner to give the deep-violet phenyldiazene
complex 4a, aniline, and ammine complex 3 in a molar ratio of
approximately 1:1:1 (eq 1):
1
Triflate salt 5 was unambiguously characterized by H and
31P{1H} NMR spectroscopy, X-ray analysis (Figure S2), and an
alternative synthesis of 5 from an iodophosphonium salt and 1,1-
diphenylhydrazine.15 Formation of the phosphorus(V) salt 5
strongly suggests that a high-valent iron−hydrazido species is
involved in the early stage of the reaction of 2a with hydrazines.
B
dx.doi.org/10.1021/ja3122944 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
The isolation of phenyldiazene complex 4a and hydrazino-
phosphonium salt 5 along with the low catalytic activity of the N-
methylated complexes 2b and 2c suggests a possible mechanism
for the catalytic disproportionation of hydrazine (Scheme 2).
pyrazolato moiety occurs, affording iron(II) diazene complex
4, which is isolable for R = Ph (4a). Since such a PCET is not
possible in the diphenylhydrazido(1−) analogue (R = R′ = Ph),
reductive elimination of the hydrazido(1−) ligand and
trimethylphosphine from the iron(IV) center would instead
take place, generating phosphonium salt 5. Finally, the labile
HNNH ligand in 4b would be replaced20 by hydrazine to
regenerate hydrazine complex A and free diazene, the latter of
which immediately undergoes bimolecular disproportionation to
yield dinitrogen and hydrazine.24
Scheme 2. Proposed Mechanism for N−N Bond Cleavage of
a
Hydrazines
In summary, we have demonstrated that the metal−ligand-
bifunctional iron complex 2a bearing a pincer-type bis(pyrazole)
ligand cleaves the N−N bond of hydrazines. Mechanistic
investigations using N-substituted pyrazoles and hydrazines led
us to conclude that the reaction occurs through multiple proton
and electron shuttling between hydrazine and the iron complex
mediated by the two proton-responsive pyrazoles appropriately
placed in the rigid pincer framework. It would be worth
mentioning that the μ-sulfido ligands in the FeMo-cofactor of
nitrogenase are proposed to mediate consecutive PCET to
coordinated dinitrogen via the protonated hydrosulfido form.25
Furthermore, the proton-responsive pincer ligand stabilizes the
diazene intermediate through multiple hydrogen bonds. Efforts
will be directed toward the application of multiproton-responsive
ligands to more the challenging conversion of dinitrogen under
mild conditions as well as other important multielectron
reduction/oxidation processes such as carbon dioxide reduction
and water oxidation.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and X-ray crystallographic data for 1,
2a, 4a, and 5 (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
skuwata@apc.titech.ac.jp; tikariya@apc.titech.ac.jp
■
a
Fe = Fe(PMe3)22+. The total charges of the Fe complexes (2+) and
Notes
the tert-butyl groups in the pincer ligands have been omitted.
The authors declare no competing financial interest.
First, a hydrazine binds to the iron through the less-hindered
nitrogen atom to give hydrazine complex A. Hydrogen bonding
between the pyrazole NH group and the uncoordinated nitrogen
atom in the hydrazine ligand promotes reductive N−N bond
cleavage, leading to the formation of iron(IV)−amido complex
B.21 In this step, hydrazine acts as an oxidant.22 We note some
similarity between this PCET process and the heterolytic O−O
bond cleavage of hydrogen peroxide in heme peroxidase and
catalases, in which distal peptide residues serve as acid−base
catalytic groups.23 In addition, we previously demonstrated that a
protic N-heterocyclic carbene ligand promotes C−O bond
cleavage of allyl alcohol through analogous PCET.13 The NH2
ligand in intermediate B then undergoes a proton transfer from
the remaining pyrazole NH group and is converted to the more
labile ammine ligand in C. Subsequent ligand substitution by a
second molecule of hydrazine and a proton shift from the
coordinated hydrazine to the pyrazolato arm give iron(IV)−
hydrazido(1−) complex D and ammonia. This acid−base
catalysis of the second pyrazole NH group rationalizes the
lower reactivity of 2b without this NH group. When a hydrogen
atom on the distal nitrogen is available in D (R′ = H), subsequent
PCET from the hydrazido(1−) ligand to the iron(IV)−
ACKNOWLEDGMENTS
■
This work was supported by a Grant-in-Aid for Scientific
Research (S) (22225004) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan. Prof. Hiroyuki
Kawaguchi, Prof. Hiroharu Suzuki, and Mr. Kyo Namura (Tokyo
Institute of Technology) are gratefully acknowledged for helping
us to measure the magnetic susceptibility.
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dx.doi.org/10.1021/ja3122944 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX