A multi-iron system capable of rapid N2 formation and N 2 cleavage

Article abstract of DOI:10.1021/ja505193z

The six-electron oxidation of two nitrides to N₂ is a key step of ammonia synthesis and decomposition reactions on surfaces. In molecular complexes, nitride coupling has been observed with terminal nitrides, but not with bridging nitride complexes that more closely resemble catalytically important surface species. Further, nitride coupling has not been reported in systems where the nitrides are derived from N₂. Here, we show that a molecular diiron(II) diiron(III) bis(nitride) complex reacts with Lewis bases, leading to the rapid six-electron oxidation of two bridging nitrides to form N₂. Surprisingly, these mild reagents generate high yields of iron(I) products from the iron(II/III) starting material. This is the first molecular system that both breaks and forms the triple bond of N₂ at room temperature. These results highlight the ability of multi-iron species to decrease the energy barriers associated with the activation of strong bonds.

Full text of DOI:10.1021/ja505193z

Communication
pubs.acs.org/JACS
A Multi-iron System Capable of Rapid N2 Formation and N2 Cleavage
K. Cory MacLeod, David J. Vinyard, and Patrick L. Holland*
Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520, United States
S
* Supporting Information
the iron(I) oxidation level. In the presence of 1 atm N2, the
ABSTRACT: The six-electron oxidation of two nitrides
N−N triple bond of N2 was cleaved to give two bridging
to N2 is a key step of ammonia synthesis and
decomposition reactions on surfaces. In molecular
complexes, nitride coupling has been observed with
terminal nitrides, but not with bridging nitride complexes
that more closely resemble catalytically important surface
species. Further, nitride coupling has not been reported in
systems where the nitrides are derived from N2. Here, we
show that a molecular diiron(II) diiron(III) bis(nitride)
complex reacts with Lewis bases, leading to the rapid six-
electron oxidation of two bridging nitrides to form N2.
Surprisingly, these mild reagents generate high yields of
iron(I) products from the iron(II/III) starting material.
This is the first molecular system that both breaks and
forms the triple bond of N2 at room temperature. These
results highlight the ability of multi-iron species to
decrease the energy barriers associated with the activation
of strong bonds.
nitrides in a tetranuclear complex that has two iron(II) ions and
two iron(III) ions (1 in Scheme 1). Herein, we report the
Scheme 1. Reactions of Compound 1 That Transfer Six
Electrons from Nitrides to Iron
eactive nitrido complexes of the late transition metals have
R
aroused much interest, particularly in the past few years.1
One exciting reaction of these complexes is nitride coupling to
form N2, a six-electron reaction that makes and breaks many
bonds. In these reactions, N2 formation is driven by the
reduction of two metal nitrides, and thus it is seen with
complexes of metals having high formal oxidation states.
Nitride coupling is the microscopic reverse of N2 cleavage, and
thus it is mechanistically relevant to the industrial production of
ammonia in the iron-catalyzed Haber−Bosch process. Dis-
sociation of the N−N bond is the rate-limiting step in the
Haber−Bosch process,2 and likewise assembly of the N−N
triple bond is the challenging step for NH3 decomposition
catalysts (which are of interest for NH3 fuel cells).3
In molecular systems, nitride coupling has been seen with
terminal M−N multiple bonds.1 However, complexes with
bridging nitrides would be more relevant to the bridging modes
of the nitrides in heterogeneous catalysts, particularly the
proposed nitride intermediates in catalytic ammonia synthesis
and decomposition.2 Additionally, reported examples of nitride
coupling are limited to studies of terminal nitrides that are
prepared from nitride/nitrene sources rather than N2.1
Studying nitride coupling in systems where the nitrides come
from N2 would be most relevant, given the reversible N2
cleavage in the Haber−Bosch systems.
Figure 1. ORTEP diagrams of the X-ray crystal structures of
LFe(CNXyl)3 (3, left) and LFe(CO)3 (4, right) using 50% thermal
ellipsoids. The xylyl groups on the isocyanides in 3 are omitted for
clarity.
To our knowledge, there is only one reported iron system
where a nitride complex is generated from N2.4 Our group
described a reaction sequence that started with reduction of
LFeCl (L = bulky β-diketiminate ligand, shown in Figure 1) to
Received: May 23, 2014
© XXXX American Chemical Society
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selective formation of N−N bonds in the bis(nitride) complex
to release the two bridging nitride ligands as N2 in high yield.
This is the first demonstration of N−N triple bond cleavage
and N−N triple bond formation by a single homogeneous
system. Both reactions occur at or below room temperature,
indicating that the multi-iron active site greatly lowers the
kinetic barriers for N2 formation and cleavage.
We previously reported a tetrairon−bis(nitride) complex (1
in Scheme 1), in which the iron sites were shown to be two
iron(II) and two iron(III).4 Crystalline samples of 1 are stable
under N2 or Ar for 1 month at −40 °C but decompose slowly
in benzene solution at 25 °C with a half-life of 8 h. The major
decomposition product under these conditions has now been
identified as the iron(I) complex LFe(η6-benzene) (2) in 56%
spectroscopic yield (Scheme 1).
Compound 2 was independently synthesized by reduction of
[LFeCl]2 with 2 molar equivalents of KC8 in thf/benzene (4:1)
under an argon atmosphere. The solid-state molecular structure
of 2 (Figure S-20) showed the η6 binding mode of the benzene
ligand, with Fe−C distances ranging from 2.128(2) to 2.147(3)
Å. The solution magnetic moment (1.7 μB) and rhombic EPR
signal (g = 2.182, 2.010, 1.979) indicate that 2 has low-spin
iron(I) with an S = 1/2 electronic configuration. The zero-field
57Fe Mossbauer spectrum of solid 2 at 80 K has a single
quadrupole doublet with δ = 0.68 mm/s and |ΔEQ| = 0.69 mm/
s (Figure S-1).
The rate of conversion of 1 to 2 increased in the presence of
pyridine. Upon treatment of 1 in benzene with 4 equiv of
̈
Figure 2. Zero-field Mossbauer spectra at 80 K. The black circles are
̈
the data, and the red lines are simulations. (Top) Mossbauer spectrum
̈
of a frozen benzene solution of the reaction mixture of 1 with CNXyl
(12 equiv). The blue line represents the major product of the reaction
(3) with δ = 0.15 mm/s and |ΔEQ| = 0.79 mm/s accounting for 94% of
the sample. The green line represents an unknown byproduct (6% of
the sample) with δ = 0.83 mm/s and |ΔEQ| = 2.16 mm/s. (Bottom)
pyridine, 1 was completely consumed within 10 min to produce
1
2 in 72% yield (determined by H NMR and Mossbauer
̈
spectroscopies on the crude reaction mixtures; see Figures S-3
and S-11). Additionally, a sub-stoichiometric amount of
pyridine (0.22 equiv compared to 1) also produced 2 in 70%
yield (determined by 1H NMR spectroscopy), with the reaction
requiring only 3 h to reach completion. The absence of pyridine
in the reaction product, as well as the observed rate dependence
on pyridine concentration, suggests that pyridine is a catalyst
for decomposition of the bis(nitride) 1. This indicates that
Lewis bases may act to break up the tetrairon complex during
formation of the iron(I) product, and thus subsequent research
used slender Lewis bases that could access the metal sites more
easily.
Mossbauer spectrum of independently synthesized LFe(CNXyl) (3),
̈
3
with a fit having δ = 0.17 mm/s and |ΔEQ| = 0.81 mm/s. Analogous
spectra for the CO reaction are shown in the Supporting Information.
at 80 K showed a quadrupole doublet with δ = 0.17 mm/s and
|ΔEQ| = 0.81 mm/s (Figure 2, bottom), which are the same as
the parameters observed for the product of the reaction of 1
with CNXyl. These results indicate that the reaction of the
diiron(II)diiron(III) complex 1 with isocyanide yields 94%
crude yield of the iron(I) product 3.
The tricarbonyl analogue of compound 3 was also prepared
independently by reaction of the iron(I) compound 2 with CO
(1 atm), which resulted in a rapid color change to green. The
solid-state molecular structure confirmed a five-coordinate
square-pyramidal iron species of the formula LFe(CO)3 (4,
Figure 1, right). The IR spectrum of 4 contained three strong
bands (2023, 1952, and 1938 cm−1) in the C−O stretching
region. Consistent with a square-pyramidal low-spin iron(I)
electronic configuration, 4 had a solution magnetic moment of
1.8 μB, and a nearly axial EPR signal (g = 2.043, 2.038, 2.000).
Treatment of a benzene solution of 1 with 12 equiv of
CNXyl (2,6-dimethylphenyl isocyanide) caused a rapid color
change from red-brown to green. Mossbauer analysis of a
̈
frozen solution (80 K) of the crude reaction mixture showed
conversion to a single product (94%) with δ = 0.15 mm/s and
|ΔEQ| = 0.79 mm/s (Figure 2, top). On the basis of its
1
Mossbauer parameters and H NMR spectrum (Figure S-12),
̈
the predominant (94%) reaction product was identified as a
tris(isocyanide) compound, LFe(CNXyl)3 (3).
The zero-field Mossbauer spectrum of solid 4 at 80 K had δ =
̈
The independent synthesis of 3 was achieved by treating the
iron(I) compound 2 with CNXyl. Crystallization from pentane
provided a 5-coordinate tris(isocyanide) compound LFe-
(CNXyl)3 (3) with a square pyramidal geometry at the iron
center (Figure 1, left). The IR spectrum of 3 contained two
bands at 2075 and 1977 cm−1 (the latter of which is composed
of two overlapping bands), which correspond to the isocyanide
C−N stretching vibrations. Consistent with the solid-state
structure, a frozen solution of 3 had an axial EPR signal with g⊥
= 2.052 and g|| = 2.002, and a solution magnetic moment of 1.6
μB, indicating a low-spin (S = 1/2) iron(I) electronic
0.12 mm/s and |ΔEQ| = 0.77 mm/s accounting for 97% of the
iron, and a second minor component (3%) with δ = 0.18 mm/s
and |ΔEQ| = 2.04 mm/s, which is assigned to a 4-coordinate
dicarbonyl species LFe(CO)2.5,6
Treatment of a benzene solution of the bis(nitride) 1 with
CO (1 atm) caused a rapid color change from red-brown to
green. The zero-field Mossbauer spectrum of a frozen solution
̈
(80 K) of the crude reaction mixture contained signals
corresponding to the tricarbonyl 4 and dicarbonyl analogue
LFe(CO)2 accounting for a combined 94% of the iron
containing products (Figure S-4).1j In summary, addition of
either isocyanide or CO to the Fe2+2Fe3+2(N3−)2 complex 1
configuration. The zero-field Mossbauer spectrum of solid 3
̈
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Scheme 2. Key N2-Cleaving and N2-Forming Reactions in the Multi-iron System
gives a high yield of an iron(I) product, suggesting that all four
iron(II)/iron(III) sites are reduced by a total of six electrons.
This, in turn, suggests the possible oxidation of the two N3−
ligands by a total of six electrons to give N2.
by formation of the reduced iron(I) compound LFe(CNXyl)3
(3).
Though the mechanism of the N−N bond formation is not
known in detail, mechanistic information was gathered using
isotopically labeled 1-15N2. A benzene solution containing a 1:1
mixture of 15N-labeled 1 and unlabeled 1 was treated with 13
equiv of CNXyl. The headspace contained exclusively 14N2 (m/
z = 28) and 15N2 (m/z = 30). The lack of mixed-label 14N15N at
m/z = 29 indicates that N2 formation proceeds by an
intramolecular N−N bond formation process, and that the
nitride ligands do not exchange between complexes. Addition-
ally, the fact that 15N-labeled 1 can be handled and stored
under N2 atmosphere without loss of the 15N-labeled nitride
ligands indicates that the nitride ligands do not exchange with
atmospheric N2 in either solution or the solid state prior to the
addition of the π-acidic ligands.
Consistent with this hypothesis, compound 1 was observed
to release a gas upon reaction with CNXyl or pyridine.
Monitoring the pressure change upon addition of 13 equiv of
CNXyl to a solution of 1 showed the production of 1.00 0.04
mol of gas per mol of 1. Analysis of the resulting gas by GC-MS
under an Ar atmosphere confirmed that N2 was formed in the
reactions (Figure 3). A sample of 1 labeled with 15N gave 15N2
that was observed in the mass spectrum at m/z = 30. These
results demonstrate the quantitative release of N2 from
bis(nitride) 1 upon addition of CNXyl, which is accompanied
In a previous report, we described the reduction of [LFeCl]2
with 1 equiv of potassium per iron atom, which cleaves the
N−N bond of dinitrogen at room temperature to form 1
(Scheme 2, left).4 We proposed that the iron(II) complex is
first reduced by the potassium to form a transient intermediate
with formally iron(I) ions, which react cooperatively with a
single N2 molecule.7 Notably, the ability of four iron centers to
arrange around a single molecule of N2 provided compound 1,
which has two iron(II) and two iron(III) metal centers.
Therefore, the six-electron reduction of the N2 unit was
accomplished using four metal centers, thereby avoiding the
need to access a high oxidation state of iron. In an equivalent
manner, the N−N bond formation described herein is a six-
electron oxidation of nitrides to N2 with quantitative reduction
of iron centers to iron(I) in compounds 2, 3, and 4 (Scheme 2,
right).
A number of well-defined molecular systems are known to
cleave the triple bond in dinitrogen.8 These systems use highly
reducing metal centers, typically early transition metals in low
oxidation states, and are driven by the oxidation of the metal
and formation of a very strong metal−ligand multiple bond.9
Conversely, a number of reports have described the reverse
reaction, N2 formation as a result of metal−nitride coupling.1
The N2-forming systems use middle to late transition metals in
unusually high oxidation states, and are driven by the reduction
of the metal. Thus, the known reactions in each direction are
brought about by a large thermodynamic driving force, which is
unlike the reversible cleavage/formation of N2 that is
characteristic of the heterogeneous catalyst. To our knowledge,
no homogeneous system has been reported to both cleave and
form the N−N triple bond of dinitrogen, and it is distinctive
that this multi-iron system is poised at the brink of N2
activation. However, the N−N cleavage/formation here is not
Figure 3. Mass spectra of the gas evolved during reactions with CNXyl
under Ar (m/z = 40) with 1 (a), 15N-labeled 1 (b), and a mixture of
15N-labeled 1 and unlabeled 1 (c). The spectra indicate the formation
of 14N2 and 15N2 with m/z = 28 and 30, respectively. The smaller peak
at m/z = 28 in (b) resulted from unavoidable contamination by 14N2 in
air.
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diketiminate iron(I) carbonyl compounds in the literature (Table S-
1).6
(6) Smith, J. M.; Sadique, A. R.; Cundari, T. R.; Rodgers, K. R.;
Lukat-Rodgers, G.; Lachicotte, R. J.; Flaschenriem, C. J.; Vela, J.;
Holland, P. L. J. Am. Chem. Soc. 2006, 128, 756.
truly reversible, because the iron(I) species are not the same in
both reactions. Driving forces for the new N2-forming reaction
likely include the stabilization of iron(I) by the π-accepting
ligands in 2, 3, and 4, and also the precipitation of KCl as a
byproduct.
(7) Figg, T. M.; Holland, P. L.; Cundari, T. R. Inorg. Chem. 2012, 51,
However, the kinetic aspects are most significant. This work
suggests that the ability to place multiple iron centers (≥3)
around a single molecule of N2 gives fast rates for N−N bond
cleavage/formation. As a testament to the low barriers for these
reactions, both the N−N bond cleavage and formation
reactions in this system occur rapidly at room temperature.
This indicates that cooperation between several iron centers
facilitates multi-electron reactions of difficult substrates like N2,
identifying this as a key strategy for accomplishing homoge-
neous reactions with this inexpensive metal. At the same time, it
provides key insight into the bond cleaving and forming steps
in the iron-catalyzed Haber−Bosch process, by showing that
four iron atoms can cooperate to accomplish N−N bond
transformations observed on heterogeneous iron catalysts.10
7546.
(8) (a) Hidai, M.; Mizobe, Y. Chem. Rev. 1995, 95, 1115.
(b) Cummins, C. C. Chem. Commun. 1998, 1777. (c) MacKay, B.
A.; Fryzuk, M. D. Chem. Rev. 2004, 104, 385. (d) MacLeod, K. C.;
Holland, P. L. Nat. Chem. 2013, 5, 559.
(9) Cherry, J.-P. F.; Johnson, A. R.; Baraldo, L. M.; Tsai, Y.-C.;
Cummins, C. C.; Kryatov, S. V.; Rybak-Akimova, E. V.; Capps, K. B.;
Hoff, C. D.; Haar, C. M.; Nolan, S. P. J. Am. Chem. Soc. 2001, 123,
7271.
(10) Theoretical studies on iron surfaces suggest that surface steps or
“C7” sites could be the site of N2 cleavage: (a) Falicov, L. M.;
Somorjai, G. A. Proc. Natl. Acad. Sci. U.S.A. 1985, 82, 2207.
(b) Mortensen, J. J.; Hansen, L. B.; Hammer, B.; Nørskov, J. K. J.
Catal. 1999, 182, 479. We note that these defects offer sites at which a
single N2 molecule could interact simultaneously with several surface
iron atoms.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
patrick.holland@yale.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was funded by the National Institutes of Health
(GM065313). We thank William W. Brennessel for collecting
X-ray diffraction data. We also thank Prof. Gary Brudvig for
access to EPR instrumentation.
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D
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