NH3 formation from N2 and H2 mediated by molecular tri-iron complexes

Article abstract of DOI:10.1038/s41557-020-0483-7

Living systems carry out the reduction of N₂ to ammonia (NH₃) through a series of protonation and electron transfer steps under ambient conditions using the enzyme nitrogenase. In the chemical industry, the Haber–Bosch process hydrogenates N₂ but requires high temperatures and pressures. Both processes rely on iron-based catalysts, but molecular iron complexes that promote the formation of NH₃ on addition of H₂ to N₂ have remained difficult to devise. Here, we isolate the tri(iron)bis(nitrido) complex [(Cp′Fe)₃(μ₃-N)₂] (in which Cp′ = η⁵-1,2,4-(Me₃C)₃C₅H₂), which is prepared by reduction of [Cp′Fe(μ-I)]₂ under an N₂ atmosphere and comprises three iron centres bridged by two μ₃-nitrido ligands. In solution, this complex reacts with H₂ at ambient temperature (22 °C) and low pressure (1 or 4 bar) to form NH₃. In the solid state, it is converted into the tri(iron)bis(imido) species, [(Cp′Fe)₃(μ₃-NH)₂], by addition of H₂ (10 bar) through an unusual solid–gas, single-crystal-to-single-crystal transformation. In solution, [(Cp′Fe)₃(μ₃-NH)₂] further reacts with H₂ or H⁺ to form NH₃. [Figure not available: see fulltext.].

Full text of DOI:10.1038/s41557-020-0483-7

Articles
https://doi.org/10.1038/s41557-020-0483-7
NH formation from N and H mediated by
3
2
2
molecular tri-iron complexes
1
1
1
1
1
Matthias Reiners , Dirk Baabe , Katharina Münster , Marc-Kevin Zaretzke , Matthias Freytag ,
1
2
2
3
3
Peter G. Jones , Yannick Coppelꢀ ꢀ , Sébastien Bontemps , Iker del Rosal , Laurent Maronꢀ ꢀ and
Marc D. Walterꢀ ꢀ1
ꢀ✉
Living systems carry out the reduction of N to ammonia (NH ) through a series of protonation and electron transfer steps
2
3
under ambient conditions using the enzyme nitrogenase. In the chemical industry, the Haber–Bosch process hydrogenates N
2
but requires high temperatures and pressures. Both processes rely on iron-based catalysts, but molecular iron complexes that
promote the formation of NH on addition of H to N have remained difficult to devise. Here, we isolate the tri(iron)bis(nitrido)
3
2
2
5
complex [(Cp′Fe) (μ -N) ] (in which Cp′ = η -1,2,4-(Me C) C H ), which is prepared by reduction of [Cp′Fe(μ-I)] under an N
3
3
2
3
3
5
2
2
2
atmosphere and comprises three iron centres bridged by two μ -nitrido ligands. In solution, this complex reacts with H at ambi-
3
2
ent temperature (22ꢀ°C) and low pressure (1 or 4ꢀbar) to form NH . In the solid state, it is converted into the tri(iron)bis(imido)
3
species, [(Cp′Fe) (μ -NH) ], by addition of H (10ꢀbar) through an unusual solid–gas, single-crystal-to-single-crystal transfor-
3
3
2
2
+
mation. In solution, [(Cp′Fe) (μ -NH) ] further reacts with H or H to form NH .
3
3
2
2
3
he activation and functionalization of dinitrogen (N ) as the after recrystallization the tri(iron)bis(nitrido) compound
2
ultimate source of environmental nitrogen is a challenging but [(Cp′Fe) (μ -N) ] (1) (Fig. 1), which was completely characterized
3
3
2
T
indispensable process for life on our planet. Two leading strate- by various spectroscopic measurements and X-ray crystallogra-
gies have been devised for this purpose. The industrial Haber–Bosch phy. The synergistic interplay of the three iron sites appears to be
process uses a heterogeneous iron-based catalyst operating at high necessary for the complete cleavage of N . This is consistent with
2
22,23,25
temperatures and elevated pressures for the conversion of H and N
previous observations
, and is further substantiated by the reac-
2
2
1–3
to NH (ref. ), whereas the iron-containing metalloenzyme nitroge- tion of a related complex [Cp′FeI(IiPr Me )] (in which IiPr Me =
3
2
2
2
2
27
nase accomplishes this transformation by alternating protonation and 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) . Upon reduc-
4–7
reduction steps at ambient pressure and temperature . Both systems tion, this complex activates N but leads only to the monomeric
2
1
have been studied extensively with exploration of various molecular
N
adduct [Cp′Fe(IiPr Me )(η -N )]. The molecular structure
2
2
2
2
8
,9
10
model systems based on a variety of elements ranging from boron , of 1 is shown in Fig. 2 and relevant bond distances are provided
11
12
13,14
15–17
iron , cobalt and molybdenum
to uranium . For the early in the Supplementary Information (Supplementary Fig. 13 and
18–21
transition metals , examples have been established, but an iron Supplementary Table 2).
nitrido complex that produces NH upon addition of H , similar to
The long N···N bond distance of 2.267(2)Å in complex 1 con-
3
2
the industrial Haber–Bosch process, is still lacking. For example, the firms a complete reduction of N to two nitrido ligands that occupy
2
first iron nitrido species derived from N splitting has been described the axial positions of a trigonal bipyramid; the three Cp′Fe frag-
2
2
2,23
only recently , but formation of NH required subsequent addition ments lie in the equatorial plane. The atoms N1 and N2 lie 1.15Å
3
24
of protons and reducing reagents . Another example of a tri(iron) and 1.12Å, respectively, out of the plane defined by the Fe1, Fe2
complex capable of N activation under reducing conditions is and Fe3 atoms, whereas the uniform Fe–Fe bond distances rang-
2
[
Fe Br L], in which L is a cyclophane bridged by three β-diketiminate ing from 2.4727(4)Å to 2.4734(4)Å are consistent with three
3
3
25
arms; however, the NH release also required protonation to occur . equivalent Fe sites. Another iron complex containing a μ -bridged
3
3
tbs
tbs
6−
Similarly, terminally coordinated N ligands may be transformed cata- nitrido ligand is [( L)Fe (μ -N)] (in which ( L) = [1,3,5-C H
2
3
3
6
9
26
t
6−
lytically to NH in the presence of reductants and a proton source .
(NPh-o-NSi BuMe ) ] ), which was isolated from the reaction of
3
2 3
tbs
Here, we describe the preparation of [(Cp′Fe) (μ -N) ] (1) (in [( L)Fe (μ -N)(μ -Br) ] with KHB(sec-Bu) under an N atmo-
3
3
2
3
3
2
2
3
2
5
28
tbs
which Cp′ = η -1,2,4-(Me C) C H ) by N cleavage, and its reac- sphere . By contrast, when the precursor [( L)Fe (μ -Br)(μ -Br)
3
3
5
2
2
3
2
3
tivity towards H in solution and in the solid state. In the latter it (Br)] was reduced with KC under N , the iron imido complex
2
8
2
tbs
forms exclusively the H addition product [(Cp′Fe) (μ -NH) ] (2) in [( L)Fe (μ -NH) ] was isolated (presumably as a result of proton-
2
3
3
2
3
2
3
25
a single-crystal-to-single-crystal (SCSC) transformation. Moreover, ation) . Overall, the structural motif with μ -bound N atoms is a
3
+
NH can be generated from 1 and 2 on addition of either H or H
molecular model related to the Fe(110) surface encountered in the
3
2
9–31
in solution.
Haber–Bosch process2
.
5
7
Results
Electronic ground-state properties of complex 1. Zero-field Fe
Synthesis and structural characterization of complex 1. Mössbauer measurements on 1 (Fig. 3a and Supplementary Fig. 24)
Treatment of [Cp′Fe(μ-I)] (A) with KC in THF under N yields show a single doublet of Lorentzian lines and therefore confirm
2
8
2
1
2
Technische Universität Braunschweig, Institut für Anorganische und Analytische Chemie, Braunschweig, Germany. CNRS, LCC (Laboratoire de Chimie
3
de Coordination), Université de Toulouse, UPS, INPT, Toulouse, France. Université de Toulouse, INSA-UPS-LPCNO and CNRS-LPCNO, Toulouse, France.
✉
e-mail: mwalter@tu-bs.de
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Organometallic residue:
[
(Cp'Fe)3(µ3-NH)2] (2)
Cp'Fe(µ2-N)]2 (3)
[
Volatile material:
H2
NH3 (3-7%), HCp' (volatile material)
(
1 or 4 bar)
Me3C
CMe3
CMe3
Solution
Fe
N
(hexane)
CMe3
Fe
Me3C
Me3C
I
CMe3
Me3C
I
2 KC8, N2, THF
2 KI
N
Fe
Fe
Me3C
Me3C
CMe3
–
CMe3
Fe
CMe3
Solid state
CMe3
CMe3
CMe3
H2
10 bar)
Fe
CMe3
(
Me3C
Me3C
HN
CMe3
NH
Fe
CMe3
1
Fe
CMe3
Me3C
CMe3
2
Fig. 1 | Synthesis and reactivity of compound 1 towards H in solution and in the solid state, producing compounds 2, 3 and NH . N splitting occurs
2
3
2
under reducing conditions to form the tri(iron)bis(nitrido) complex 1. In solid state, 1 reacts with H to form the tri(iron)bis(imido) species 2, whereas in
2
solution with H it forms the tri(iron)bis(imido) species 2, the bis(iron)bis(nitride) 3 and NH .
2
3
Fe3
Fe3
N2
N1
N1
N2
Fe1
Fe2
Fe2
Fe1
1
2
Fig. 2 | Molecular structures of compound 1 and compound 2. For both complexes, the structures show the splitting of the N molecule and its relative
2
arrangement within a core of three iron atoms (thermal ellipsoids drawn at the 50% probability level). Hydrogen atoms are removed for clarity except
those connected to N1 and N2 in complex 2.
−1
that all iron sites in 1 are virtually equivalent, a finding that is also of J=−1.36cm ) between the individual iron centres to form a
derived from our X-ray crystallographic data (see above). The val- ground state at T=3K with μ =2.33μ , corresponding to a total
eff
B
−
1
ues obtained at T=20K for the isomer shift (δ =0.454(1)mms ) spin per molecule of S =1/2 (see Section 6 in the Supplementary
iso
t
−
1
and the quadrupole splitting (ΔE =0.669(1)mms ) are consis- Information for details). This conclusion is corroborated further by
Q
tent with those found for other iron complexes in the +III oxida- an X-band electron paramagnetic resonance (EPR) spectrum that
tion state (Supplementary Table 5 and references therein). However, was recorded at T=5K in frozen solution (Supplementary Fig. 15).
an unambiguous assignment of the spin state based solely on these
Mössbauer parameters is difficult. Hence, solid-state magnetic sus- Reactivity of complex 1 towards H in solution and in the solid
2
ceptibility measurements on compound 1 were conducted between state. Exposure of 1 to HCl produces 70–76% NH Cl, which was
4
temperatures of T=3 and 300K. The effective magnetic moment quantified by the indophenol protocol (Supplementary Table 1 and
32
(
μ ) above approximately T=100K shows only small variations Supplementary Fig. 1) . This is similar to the previously reported
eff
with temperature (Supplementary Fig. 17), and the observed value of nacnac-stabilized iron complex [{(nacnac)Fe} (μ -N)(μ -N)
3
3
2
μ =3.22 μ at T=300K is close to the spin-only value of μ =3.00 (KCl) (Fe(nacnac))] (in which nacnac = MeC[C(Me)N(2,6-Me
eff
B
eff
2
22–24,33,34
μ for three (paramagnetic) S=1/2 spin centres per molecule,
C H )] )
. Moreover, whereas complex 1 is stable under an
B
2
6
3
2
which supports the presence of three low-spin (S=1/2) iron(ꢀꢀꢀ) N and Ar atmosphere, it reacts in hexane solution with H (about
2
2
centres in complex 1. Furthermore, a decreasing μ value when the 4bar) to form NH with a yield of about 3–7%, and the NH yield can
eff
3
3
temperature is below approximately T=50K implies weak (intra- further be increased to about 10% when this hydrogenation reac-
molecular) antiferromagnetic coupling (with coupling strength tion is carried out in THF (Supplementary Table 1). Nevertheless,
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a
the relatively low NH yield is attributed to the instability of 1 in
3
the presence of NH , which induces decomposition to form Cp′H
3
3
5
0 h
100% complex 1)
and [Cp′Fe(μ-N)] ; this was independently verified by addition
2
(
15
of NH to 1. Isotopic labelling was performed using N-labelled
3
2
1
and D (where D represents deuterium or H), and we detected
2
1
5
2
ND and DCp′ in the H NMR spectrum (Supplementary Fig. 4).
3
Two organometallic species, the mixed-valent tri(iron)bis(imido)
15
15
species [(Cp′Fe) (μ - ND) ] (2- N,D), and the dimeric iron(ꢀꢁ)
3
3
2
1
5
15
35
nitrido complex [Cp′Fe(μ- N)] (3- N) are formed in the ratio
2
1
1
:2 (as determined by H NMR spectroscopy). Owing to their very
6
h
similar solubilities, only minor quantities of both species could be
isolated, preventing a closer examination of their intrinsic physi-
cochemical properties. However, 3 can be prepared independently
from the reaction of [Cp′Fe(μ-I)] with NaN and exhibits no reac-
(22.4% complex 2)
× 1.50
2
3
3
5
tivity towards H even at elevated pressures . Therefore, 3 is not
2
involved in formation of NH , which leaves the tris(iron)bis(imido)
3
1
2 h
complex 2 as a possible intermediate in this process.
(41.1% complex 2)
Synthesis and structural characterization of complex 2. Every
attempt to obtain complex 2 from complex 1 using standard
H-atom donors in solution failed, but exposure of 1 to H in the
2
solid state resulted in a clean conversion to 2. Furthermore, no NH
3
2
4 h
could be detected in the gas phase. The iron imido species 2 (whose
molecular structure is shown in Fig. 2 and Supplementary Fig. 14)
can be recrystallized from diethyl ether. The metrical parameters
of complex 2 differ from those of complex 1 only slightly, but a
few notable features should be mentioned. The Fe–Fe distances
can be grouped into two slightly shorter bonds of 2.5195(4)Å and
(
78.9% complex 2)
T = 20 K
–
3
–2
–1
0
1
2
3
2
.5284(4)Å for Fe1–Fe2 and Fe2–Fe3, respectively, and a slightly
–
1
Velocity (mm s
)
longer bond of 2.5408(4)Å for Fe1–Fe3, implying that two differ-
ent iron sites are present in a 2:1 ratio. The N1···N2 distance is also
significantly increased from 2.267(2)Å to 2.352(3)Å on addition of
b
1
00
Complex 2
H . The overall structural feature of a trigonal bipyramid with three
2
Cp′Fe moieties occupying the equatorial plane is conserved. We
also carried out this transformation as an SCSC process, which was
experimentally supported by the minor changes in the cell dimen-
sions of 1 and 2 (Supplementary Table 2). This is a rare example
80
60
40
20
0
36–40
of an SCSC reaction in organometallic compounds ; this sys-
tem thereby resembles the heterogeneous Haber–Bosch process, in
which surface-bound iron imido species are formed on addition of
H . However, when the reduction of [Cp′Fe(μ-I)] with KC in THF
2
2
8
is carried out under a mixture of H and N , only the di(iron) tetra-
2
2
Complex 1
4
1,42
hydride [Cp′Fe(μ-H) ] is isolated , preventing direct formation
2
2
of NH from H and N .
3
2
2
Experimental determination of the barrier to H addition to com-
2
0
50
100
150
200
250
300
350
plex 1 in the solid state. The process of H addition can be moni-
Time of H2 exposure (h)
2
57
tored further by zero-field Fe Mössbauer (Fig. 3) and solid-state
1
5
Fig. 3 | Conversion of complex 1 into complex 2 upon different H exposure
N Hahn-echo magic angle spinning (MAS) NMR spectroscopy
Fig. 4). As a starting point, when the reduction of [Cp′Fe(μ-I)]
2
times followed by Mössbauer spectroscopy. a, Selected zero-field
Mössbauer spectra for the conversion of 1 (green line for three equivalent
Fe(iii) sites) into 2 (orange line for two Fe(ii) sites and red line for one Fe(iii)
site) after different H (10ꢀbar) reaction times recorded at Tꢀ=ꢀ20ꢀK. A small,
but so far unidentified, impurity with a volume fraction ranging between
(
2
15
15
is carried out in a N atmosphere, the N-labelled tri(iron)
2
1
5
bis(nitrido) species (1- N) is isolated. Two resonances at δ=1,520
15
and 1,400ppm can be found in the solid-state N NMR spectrum,
reflecting the different chemical environments of the two N atoms
2
2
and 4% was visible in all H -treated compounds (10ꢀbar). However, for
(
Fig. 4c and Supplementary Figs. 8, 10, 11). Note that fast paramag-
2
15
clarity, the sub-spectrum of this impurity is not shown in the figure (see
Supplementary Fig. 25 for further details, including the sub-spectrum of
the impurity). Grey circles, experimental data; lines, fit with doublets of
Lorentzian lines adapting a model described in the text. b, Consumption
netic relaxation prevents N NMR spectra from being recorded
1
5
in solution. Exposure of 1- N to H results in a slow decrease of
2
1
5
15
N NMR resonance lines attributed to 1- N, whereas a new reso-
nance line appears at δ=−256ppm that is attributed to the tri(iron)
1
5
and formation of 1 and 2 plotted against H exposure time. Respective
bis(imido) species (2- N) (Fig. 4b and 4a and Supplementary
2
1
5
15
volume fractions of 1 (green circles) and 2 (orange circles) were determined
by Mössbauer spectroscopy at Tꢀ=ꢀ20ꢀK (Supplementary Table 4). Black and
grey stars indicate volume fractions for a sample that was obtained after an
H exposure time of 24 h and with an applied H pressure of 80ꢀbar. Lines
are fits with an exponential decay (green) or an exponential growth law
red) with rate constants of 0.079(14) and 0.078(10)ꢀh , respectively.
Figs. 10–12). As with 1- N, no N NMR resonance was detected
1
5
for 2- N when these measurements were carried out in solution.
To obtain further insights into the kinetics of this transformation
from 1 to 2, zero-field Mössbauer measurements were executed
2
2
on a series of samples with different H exposure times between
2
−1
(
1
5 min and 14 d. The line shape of the Mössbauer spectra changes
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a
SSB
SSB
b
c
1,900 1,700 1,500 1,300 1,100
900
700
500
300
100
–100
–300
–500
–700
–900
F1 (ppm)
Fig. 4 | Complete conversion of 15N-labelled 1 into 2 as monitored by 15N Hahn-echo MꢁS NMR spectra. a, After 10ꢀd with 10ꢀbar of H (full conversion
2
1
5
15
15
15
15
15
to [(Cp′Fe) (μ - NH) ] (2- N)). b, After 24ꢀh (mixture of [(Cp′Fe) (μ - N) ] (1- N) and [(Cp′Fe) (μ - NH) ]) (2- N)). c, Before addition of H to
3
3
5
2
15
3
3
2
3
3
2
2
1
[
(Cp′Fe) (μ - N) ] (1- N). Spinning rate is 16ꢀkHz; relaxation delay is 1ꢀs; SSB, spinning side band.
3
3
2
systematically over the exposure time, as shown by representative Fe(ꢀꢀꢀ), S=1/2 spin centres (that is, S =1−(1−1/2)=1/2), in which
t
spectra in Fig. 3a. However, the quantitative analysis is complicated one electron is delocalized between two iron atoms (see Section 8
because of the very similar Mössbauer parameters obtained for in the Supplementary Information for details). Consistent with this,
−
1
complexes 1 and 2 (Supplementary Table 4). Therefore, we postu- the quadrupole splitting value of ΔE =0.384(11)mms associ-
Q
lated that (1) the sub-spectrum representing residual amounts of 1 ated with the two Fe(ꢀꢀ) sites in compound 2 is very small, which
in the reaction mixture can be described by the same Mössbauer also suggests substantial covalent contributions to the electric field
57
45
parameters as for pure compound 1, (2) the binding of two H atoms gradient at the Fe nucleus site . To provide further experimental
in 2 results in two Fe(ꢀꢀ) centres whereas one iron site remains in support for these spin- and oxidation-state assignments, solid-state
the +III oxidation state, (3) both Fe(ꢀꢀ) sites are equivalent, (4) the magnetic susceptibility measurements were conducted on com-
relative signal intensities of the Fe(ꢀꢀ) and Fe(ꢀꢀꢀ) centres in complex pound 2 (obtained after 10d of H (10bar) exposure) at tempera-
2
2
adopt a 2:1 ratio, and (5) the sub-spectra of complex 2 in the reac- tures between T=3 and 300K. Considering three uncoupled spins at
tion mixture can be described by the same Mössbauer parameters as ambient temperature, the observed value of μ =5.47 μ at T=300K
eff
B
for pure compound 2. The result of this analysis is shown in Fig. 3 is in line with the spin-only value of μ =4.36 μ for two S=1 and
eff
B
and Supplementary Table 4. The experimental and theoretical data one S=1/2 spin centres. The larger experimentally observed value
are consistent with the model advanced above. Based on the relative is ascribed to low-lying unquenched orbital contributions attrib-
amounts of complexes 1 and 2 as a function of different H exposure uted to the two paramagnetic Fe(ꢀꢀ) sites. Furthermore, a marked
2
times (Fig. 3b), and assuming pseudo-first-order kinetics and the decrease in μ values is observed with decreasing temperature
eff
4
3
‡
Eyring equation , the barrier to H addition is estimated to be ΔG
(Supplementary Fig. 20), and suggests substantial (intramolecular)
2
−
1
(
295K)=23.7(2)kcalmol (ΔG is the change in Gibbs free energy antiferromagnetic coupling between the individual spin centres of 2
and ‡ denotes a transition state).
and/or the presence of zero-field splitting (ZFS) caused by the Fe(ꢀꢀ)
intermediate-spin (S=1) sites. An analysis revealed an estimation
−1
Electronic ground-state properties of complex 2. The two sub- for the ZFS parameters (D=136cm , E/D=0) and the coupling
−1
spectra in the Mössbauer spectrum that is associated with complex 2 strength (J=−3.5 and −0.7cm ) acting between the individual
Fig. 3a) are attributed to the presence of two iron sites with dif- spin centres (see Section 6 in the Supplementary Information for
ferent oxidation and spin states. The first iron site is described by details). Similarly to complex 1 (see above), the effective magnetic
(
−
1
an isomer shift of δ =0.369(5)mms and quadrupole splitting of moment of μ =2.36μ at T=3K and the recorded X-band EPR
iso
eff
B
−
1
ΔE =0.563(8)mms , which are close to the values found for com- spectrum of complex 2 in frozen solution at T=5K (Supplementary
Q
plex 1, implying an Fe(ꢀꢀꢀ) site and an S=1/2 spin state. For the sec- Fig. 16) are consistent with the formation of a ground state with a
−
1
ond iron site, a more positive isomer shift of δ =0.565(3)mms
total spin per molecule of S =1/2.
iso
t
was determined, which is consistent with those found for rare Fe(ꢀꢀ)
44
intermediate-spin (S=1) complexes . The presence of two Fe(ꢀꢀ) Reactivity of complex 2 towards H in solid state and solution.
2
atoms in compound 2, which adopt an intermediate spin of S=1, is No H/D exchange occurs when D is added to 2 in the solid state.
2
also corroborated by density functional theory (DFT) and complete As with complex 1 (see above), the addition of HCl to 2 forms
active space self-consistent field (CASSCF) computations, indicat- NH Cl in a similar 75–80% yield (Supplementary Table 1). When
4
ing a complex coupling scenario between two Fe(ꢀꢀ), S=1 and one hexane solutions of 2 were exposed to H (about 4bar) and the
2
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‡
‡
‡
1
5.4
∆r
r
G
∆ H)
(7.7)
(
–1
(
kcal mol )
8
.5
TSAB
(
4.8)
TSE2
3
.0
(
–0.4)
0
.0
A
–2.7
–8.0)
B
–1.4
–6.9)
(
TSCD
1
+ H2
(
D
C
–
16.8
–16.4
(–22.0)
E
E
A
(–20.9)
–19.5
–21.9)
B
(
2
–
35.1
(–45.7)
D
C
2
Fig. 5 | Computed enthalpy profile at room temperature for the reaction of complex 1 with H to yield compound 2. The Gibbs free energy (ΔG) values
2
for the intermediates (A–ꢄ), transition states (labelled with ‡ in the text) and final product F (complex 2) are given in brackets. Iron, pale red; nitrogen, pale
blue; carbon, light grey; hydrogen atoms in the H molecule undergoing heterolytic cleavage, red and orange. Heterolytic H cleavage promoted by complex
2
2
1
yields the tris(iron)(hydrido)(imido)(nitrido) intermediate B, which readily rearranges to intermediate C featuring a (μ -NH, μ -H) coordination mode.
2
2
Hydride transfer to the Cp′ ring via the transition state ꢂSCD results in species D, which converts to the slightly favoured isomer ꢄ. In the last step the Cp′
ligand delivers a proton to the second nitrido functionality via transition state ꢂSꢄ2 to yield the final product 2.
volatile material was vacuum-transferred at ambient tempera- direct activation of H by the μ -bridging N atom increases the bar-
2
3
‡
−1
‡
−1
ture, a minor quantity of NH (about 0.5%) was detected, which rier to Δ H =42.6kcalmol (Δ G =48kcalmol , Supplementary
3
r
r
increased to about 5%, when the residue was heated. This feature Fig. 26), which contradicts the experimentally observed reaction
‡
−1
implies that iron-containing species are formed that incorpo- barrier of ΔG (295K)=23.7(2)kcalmol (see above). Furthermore,
rate coordinated NH , which requires elevated temperatures to be it also illustrates the mobility of the bridging N atoms above the
3
released (Supplementary Table 1). Nevertheless, our results estab- plane formed by the three Cp′Fe fragments. The trinuclear arrange-
lish that both compounds 1 and 2 can form NH on addition of ment of 1 facilitates this rearrangement and allows it to occur in the
3
H , but complex 1 is much more reactive than 2 and the pathways solid state. Following the reaction coordinate leads to a tris(iron)
2
may be different.
(hydrido)(imido)(nitrido) intermediate, B, that readily forms C
with a (μ -NH, μ -H) coordination mode. To account for the forma-
2
2
48
Computational studies. To gain additional insights into the tion of 2, a ‘carambole’ mechanism , involving the transient migra-
solid-state reactivity of 1 with H and therefore to propose a plausi- tion of a hydrogen to the Cp′ ring, was investigated computationally
2
ble mechanism for the formation of 2, DFT (B3PW91; Fe, SDDALL; starting from intermediate C. The feasibility of proton transfer to a
5
other atoms, 6-31G(d,p)) computations were performed (Fig. 5). Cp ring was also established previously for [(η -C Me ) Co] in the
5
5 2
This functional has proven its ability to properly describe geom- context of catalytic N -to-NH formation, in which the metallocene
2
3
4
9,50
etry, electronic structure and reaction mechanisms involving iron facilitates a process of proton-coupled electron transfer (PCET)
.
35,46
complexes . Several processes were computed and found to be More recent studies also established the feasibility of an H-atom
5
1
not kinetically viable (see Supplementary Fig. 26 for details). We transfer reaction in iron indenyl half-sandwich complexes . Hence,
also probed whether the two H atoms originate from one molecule we computed a mechanism that involves a hydride-to-proton
of H and whether the Cp′ ligand is actively involved in the H-atom ‘umpolung’ reaction. The hydride transfer to the Cp′ ligand and
2
transfer process by acting as a proton transfer relay. The first step of therefore the conversion of C to D proceeds via transition state
‡
−1
the reaction involves heterolytic H cleavage by one nitrido func- TSCD and has a low activation barrier of Δ H =12.9kcalmol ,
2
r
4
8
tionality in 1. This process is associated with a low activation barrier which is in agreement with the results of a previous study . After
‡
−1
‡
−1
(
Δ H =4.8kcalmol ; Δ G =8.5kcalmol ), which is consistent minor rearrangements, intermediate D converts to the slightly more
r
r
with a distinct nucleophilicity of the nitrido moiety. Furthermore, stable isomer E. In the last step, intermediate E transforms into the
this reactivity is reminiscent of that observed for {([PhBP3] final product 2, and the Cp′ ligand delivers the proton directly to
Fe) (μ -N)}{Na(THF) } (in which [PhBP3] = PhB(CH PPh ) ) the second nitrido via transition state TSE2. Therefore, the pres-
2
2
5
2
2 3
47
yielding {([PhBP3]Fe) (μ -NH)(μ -H)}{Na(THF) } . To accom- ence of the Cp′ ligand is crucial for the H-atom transfer process, and
2
2
2
5
modate this reaction, the nitrido N atom in 1 undergoes a μ - to otherwise the reaction stops at intermediate C. The barrier for the
3
‡
−1
μ -(edge) migration at the first transition state (TS). In contrast, a last step is the highest in the reaction profile (Δ H =29.6kcalmol ;
2
r
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Articles
Nature Chemistry
‡
−1
16. Falcone, M. & Mazzanti, M. Four-electron reduction and functionalization of
Δ G =34.9kcalmol ), and agrees with the experimentally deter-
r
‡
−1
2
N by a uranium(ꢀꢀꢀ) bridging nitride. Chimia 72, 199–202 (2018).
mined barrier of ΔG (295K)=23.7(2)kcalmol (see above). Note
1
7. Falcone, M., Poon, L. N., Fadaei Tirani, F. & Mazzanti, M. Reversible
dihydrogen activation and hydride transfer by a uranium nitride complex.
Angew. Chem. Int. Ed. 57, 3697–3700 (2018).
that in this last step the nitrido again alternates between a μ -
3
and a μ -bridging mode. The overall reaction is very exothermic
2
−
1
(
Δ H=−45.7kcalmol ), which is consistent with the lack of H/D 18. Pool, J. A., Bernskoetter, W. H. & Chirik, P. J. On the origin of dinitrogen
r
5
2
2
hydrogenation promoted by [(η -C
5
Me
4
H)
2
Zr]
2
(μ
2
2
,η ,η -N ). J. Am. Chem. Soc.
exchange when the complex is exposed to D (see above).
2
1
26, 14326–14327 (2004).
1
9. Pool, J. A., Lobkovsky, E. & Chirik, P. J. Hydrogenation and cleavage
of dinitrogen to ammonia with a zirconium complex. Nature 427,
Summary
We present a molecular tri(iron)bis(nitrido) complex that reacts
5
27–530 (2004).
with H to form NH . The initial step of H addition, which converts 20. Ohki, Y. & Fryzuk, M. D. Dinitrogen activation by group 4 metal complexes.
2
3
2
Angew. Chem. Int. Ed. 46, 3180–3183 (2007).
the tri(iron)bis(nitrido) complex into the mixed-valent tri(iron)
bis(imido) species, occurs in a rare SCSC transformation. This pro-
cess was followed by NMR and Mössbauer spectroscopy, allowing
2
1. Fryzuk, M. D. Side-on end-on bound dinitrogen: an activated bonding mode
that facilitates functionalizing molecular nitrogen. Acc. Chem. Res. 42,
1
27–133 (2009).
the barrier of the addition of H to be evaluated experimentally. For
2
2
22. Rodriguez, M. M., Bill, E., Brennessel, W. W. & Holland, P. L. N reduction
the solid-state conversion of complex 1 to complex 2, additional
and hydrogenation to ammonia by a molecular iron-potassium complex.
Science 334, 780–783 (2011).
DFT studies were conducted. Some key features that emerge from
2
3. MacLeod, K. C. & Holland, P. L. Recent developments in the homogeneous
reduction of dinitrogen by molybdenum and iron. Nat. Chem. 5, 559 (2013).
4. MacLeod, K. C., McWilliams, S. F., Mercado, B. Q. & Holland, P. L. Stepwise
these computations are: (1) H splitting occurs along an Fe–N edge;
2
(
2) the flexibility of the N atoms is essential for a μ - to μ -(edge)
3
2
2
migration above the (Cp′Fe) moiety, significantly lowering the bar-
3
2
N–H bond formation from N -derived iron nitride, imide and amide
rier of this process to form an intermediate with a (μ -NH, μ -H)
intermediates to ammonia. Chem. Sci. 7, 5736–5746 (2016).
5. Lee, Y. et al. Dinitrogen activation upon reduction of a triiron(ꢀꢀ) complex.
Angew. Chem. Int. Ed. 54, 1499–1503 (2015).
2
2
2
2
2
coordination mode; and (3) the Cp′ ligand is not a spectator but
actively participates as a proton relay in the H-atom transfer to the
6. Anderson, J. S., Rittle, J. & Peters, J. C. Catalytic conversion of nitrogen to
ammonia by an iron model complex. Nature 501, 84–87 (2013).
7. Reiners, M., Baabe, D., Zaretzke, M.-K., Freytag, M. & Walter, M. D.
Reversible dinitrogen binding to [Cp′Fe(NHC)] associated with an
μ -bound N atom by a ‘carambole’ mechanism.
3
Online content
Any methods, additional references, Nature Research report-
ing summaries, source data, extended data, supplementary infor-
mation, acknowledgements, peer review information; details of
author contributions and competing interests; and statements of
2
N -induced spin state change. Chem. Commun. 53, 7274–7277 (2017).
2
2
8. Ferreira, R. B. et al. Catalytic silylation of dinitrogen by a family of triiron
complexes. ACS Catal. 8, 7208–7212 (2018).
9. Bozso, F., Ertl, G. & Weiss, M. Interaction of nitrogen with iron surfaces: II.
Fe(110). J. Catal. 50, 519–529 (1977).
data and code availability are available at https://doi.org/10.1038/ 30. Somorjai, G. A. & Materer, N. Surface structures in ammonia synthesis.
s41557-020-0483-7.
Top. Catal. 1, 215–231 (1994).
3
1. Spencer, N. D., Schoonmaker, R. C. & Somorjai, G. A. Iron single crystals
as ammonia synthesis catalysts: eꢄect of surface structure on catalyst activity.
J. Catal. 74, 129–135 (1982).
Received: 23 September 2019; Accepted: 5 May 2020;
Published: xx xx xxxx
3
3
2. Weatherburn, M. W. Phenol-hypochlorite reaction for determination of
ammonia. Anal. Chem. 39, 971–974 (1967).
3. Chiang, K. P., Bellows, S. M., Brennessel, W. W. & Holland, P. L.
Multimetallic cooperativity in activation of dinitrogen at iron–potassium
sites. Chem. Sci. 5, 267–274 (2014).
References
1
2
3
.
.
.
Haber, F. ꢂe production of ammonia from nitrogen and hydrogen.
Naturwissenschafen 10, 1041–1049 (1922).
3
3
3
4. Bhutto, S. M. & Holland, P. L. Dinitrogen activation and functionalization
Schlögl, R. Catalytic synthesis of ammonia—a ‘never-ending story’? Angew.
Chem. Int. Ed. 42, 2004–2008 (2003).
using β-diketiminate iron complexes. Eur. J. Inorg. Chem. 2019,
1
861–1869 (2019).
Kandemir, T., Schuster, M. E., Senyshyn, A., Behrens, M. & Schlögl, R.
ꢂe Haber–Bosch process revisited: on the real structure and stability of
5. Reiners, M. et al. Reactivity studies on [Cp′Fe(μ-I)]
2
: nitrido-, sulꢃdo- and
diselenide iron complexes derived from pseudohalide activation. Chem. Sci. 8,
‘
ammonia iron’ under working conditions. Angew. Chem. Int. Ed. 52,
4
108–4122 (2017).
1
2723–12726 (2013).
Tuczek, F. Synthetic vs biological nitrogen ꢃxation. Nachr. Chem. 54,
190–1194 (2006).
6. Huang, Z., White, P. S. & Brookhart, M. Ligand exchanges and selective
catalytic hydrogenation in molecular single crystals. Nature 465,
4.
5.
6.
7.
1
5
98–601 (2010).
Lancaster, K. M. et al. X-ray emission spectroscopy evidences a central carbon
in the nitrogenase iron–molybdenum cofactor. Science 334, 974–977 (2011).
Spatzal, T. et al. Evidence for interstitial carbon in nitrogenase FeMo cofactor.
Science 334, 940 (2011).
3
3
7. Pike, S. D. et al. Synthesis and characterization of a rhodium(ꢀ) σ-alkane
complex in the solid state. Science 337, 1648–1651 (2012).
8. Pike, S. D. et al. Solid-state synthesis and characterization of σ-alkane
2
2
Hoꢄman, B. M., Lukoyanov, D., Yang, Z.-Y., Dean, D. R. & Seefeldt, L. C.
Mechanism of nitrogen ꢃxation by nitrogenase: the next stage. Chem. Rev.
7 4 2
complexes, [Rh(L2)(η ,η -C H12)][BArF ] (L = bidentate chelating
phosphine). J. Am. Chem. Soc. 137, 820–833 (2015).
1
14, 4041–4062 (2014).
39. Chadwick, F. M. et al. Selective C–H activation at a molecular rhodium
sigma-alkane complex by solid/gas single-crystal to single-crystal H/D
exchange. J. Am. Chem. Soc. 138, 13369–13378 (2016).
8.
Walter, M. D. in Advances in Organometallic Chemistry Vol. 65 (ed. Pérez, P. J.)
Ch. 5 (Academic Press, 2016).
9
1
.
Nishibayashi, Y. (ed.) Transition Metal–Dinitrogen Complexes (Wiley, 2019).
40. Chadwick, F. M. et al. A rhodium-pentane sigma-alkane complex:
characterization in the solid state by experimental and computational
techniques. Angew. Chem. Int. Ed. 55, 3677–3681 (2016).
0. Légaré, M.-A. et al. Nitrogen ꢃxation and reduction at boron. Science 359,
896–900 (2018).
11. Del Castillo, T. J., ꢂompson, N. B. & Peters, J. C. A synthetic single-site Fe
41. Walter, M. D., Grunenberg, J. & White, P. S. Reactivity studies on [Cp′FeI]
2
:
5
7
from iron hydrides to P -activation. Chem. Sci. 2, 2120–2130 (2011).
nitrogenase: high turnover, freeze–quench Fe Mössbauer data, and a hydride
resting state. J. Am. Chem. Soc. 138, 5341–5350 (2016).
4
2
42. Reiners, M. et al. Monomeric Fe(ꢀꢀꢀ) half-sandwich complexes [Cp′FeX ]
1
2. Kuriyama, S. et al. Direct transformation of molecular dinitrogen into
ammonia catalyzed by cobalt dinitrogen complexes bearing anionic PNP
pincer ligands. Angew. Chem. Int. Ed. 55, 14291–14295 (2016).
3. Yandulov, D. V. & Schrock, R. R. Catalytic reduction of dinitrogen to
ammonia at a single molybdenum center. Science 301, 76–78 (2003).
4. Ashida, Y., Arashiba, K., Nakajima, K. & Nishibayashi, Y.
Molybdenum-catalysed ammonia production with samarium diiodide and
alcohols or water. Nature 568, 536–540 (2019).
- synthesis, properties and electronic structure. Dalton Trans. 47,
10517–10526 (2018).
43. Moore, W. J. & Hummel, D. O. (eds) Physikalische Chemie 4th edn
(Walter de Gruyter, 1986).
1
1
44. Argouarch, G., Hamon, P., Toupet, L., Hamon, J.-R. & Lapinte, C.
5
[(η -C
5
Me
5
)Fe(Ph2PCH
2
CH
2
CH
2
PPh2)][SO
3
CF
3
], a stable 16-electron
complex with a coordinating counteranion and without agostic interaction:
the dramatic role of a trivial methylene group. Organometallics 21,
1341–1348 (2002).
1
5. Falcone, M., Chatelain, L., Scopelliti, R., Zivkovic, I. & Mazzanti, M.
Nitrogen reduction and functionalization by a multimetallic uranium nitride
complex. Nature 547, 332–335 (2017).
45. Gütlich, P., Bill, E. & Trautwein, A. X. (eds) Mössbauer Spectroscopy and
Transition Metal Chemistry (Springer, 2011).
NꢁꢂꢃRꢄ CHꢄMISꢂRY | www.nature.com/naturechemistry

Nature Chemistry
Articles
4
4
4
4
6. Spasyuk, D. M. et al. Facile hydrogen atom transfer to iron(ꢀꢀꢀ) imido radical
50. Chalkley, M. J., Oyala, P. H. & Peters, J. C. Cp* noninnocence leads to a
complexes supported by a dianionic pentadentate ligand. Chem. Sci. 7,
remarkably weak C–H bond via metallocene protonation. J. Am. Chem. Soc.
141, 4721–4729 (2019).
51. Drover, M. W., Schild, D. J., Oyala, P. H. & Peters, J. C. Snapshots of a
migrating H-atom: characterization of a reactive iron(ꢀꢀꢀ) indenide hydride
and its nearly isoenergetic ring-protonated iron(ꢀ) isomer. Angew. Chem. Int.
Ed. 58, 15504–15511 (2019).
5939–5944 (2016).
7. Brown, S. D., Mehn, M. P. & Peters, J. C. Heterolytic H
2
activation mediated
by low-coordinate L
3
Fe-(μ-N)-FeL complexes to generate Fe(μ-NH)(μ-H)Fe
3
species. J. Am. Chem. Soc. 127, 13146–13147 (2005).
8. Kefalidis, C. E. et al. Can a pentamethylcyclopentadienyl ligand act as a
proton-relay in ꢀ-element chemistry? Insights from a joint experimental/
theoretical study. Dalton Trans. 44, 2575–2587 (2015).
9. Chalkley, M. J., Del Castillo, T. J., Matson, B. D., Roddy, J. P. & Peters, J. C.
Catalytic N -to-NH conversion by Fe at lower driving force: a proposed role
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
2
3
for metallocene-mediated PCET. ACS Cent. Sci. 3, 217–223 (2017).
© The Author(s), under exclusive licence to Springer Nature Limited 2020
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were analysed by least squares fitting with doublets of Lorentzian lines using the
Methods
5
9
software package NORMOS .
General considerations. All synthetic and spectroscopic manipulations were
2
carried out under an atmosphere of puriꢃed N or Ar, in either a Schlenk apparatus
or a glovebox. Solvents were dried and deoxygenated either by distillation under
a nitrogen or argon atmosphere from sodium benzophenone ketyl (for THF,
pentane, hexane, diethyl ether) or by an MBraun GmbH solvent puriꢃcation
system (for all other solvents). Deuterated solvents were dried with sodium under
Computational studies. All DFT calculations were carried out with the Gaussian
60
16 suite of programs . Geometries were fully optimized in the gas phase without
61,62
symmetry constraints using the B3PW91 functional . The nature of the extrema
was verified by analytical frequency calculations. The calculations of electronic
energies and enthalpies of the extrema of the potential energy surface (minima
and transition states) were performed at the same level of theory as the geometry
optimizations. Intrinsic reaction coordinate (IRC) calculations were performed to
confirm the connections of the optimized transition states. Iron atoms were treated
with a Stuttgart effective core potential augmented with a polarization function
an N
and stored under N
and gas chromatographic analysis with an elementar varioMICRO or elementar
varioMICRO CUBE instrument. [Cp′FeI] and KC were prepared according to
2
or an Ar atmosphere, vacuum-transferred, freeze-pump-thaw degassed
2
or Ar. Elemental analyses were performed by combustion
2
8
5
2,53
procedures reported in the literature
.
63,64
(
ζ =2,462) . For the other elements (H, C and N), Pople’s double-ζ basis set
f
65,66
NMR studies. Quick pressure valve NMR tubes were used for the reactions
with H in solution. NMR spectra were collected on Bruker machines (QV500
6-31G(d,p) was used . The electronic charges (at the DFT level) were computed
67
using the natural population analysis technique .
2
and AV400). Solid-state NMR experiments were recorded at the Laboratoire de
Chimie de Coordination (Toulouse) on a Bruker Avance III HD 400 spectrometer
equipped with a 3.2mm MAS probe at ambient temperature. Samples were spun
Synthesis of [(Cp′Fe) (μ -N )] (1). KC (272mg, 2.00mmol, 2 eq.) and THF
3
3
2
8
(10ml) were added to a bomb flask equipped with a magnetic stirring bar. The
bomb vessel was cooled in a liquid nitrogen bath (77K), and when the suspension
1
2
13
2
between 10kHz and 16kHz at the magic angle using ZrO rotors. H, H and C
MAS spectra were recorded with recycle delays of 1s, 0.5s and 0.2s, respectively.
was completely frozen, a solution of [Cp′FeI] (832mg, 1.00mmol, 1 eq.) in
2
1
5
N Hahn-echo experiments were synchronized with the spinning rate (16kHz)
THF (10ml) was added slowly to the bomb flask under a stream of N . The flask
2
1
5
and with a recycle delay of 1s. N CP and HETCOR MAS experiments were
performed with a recycle delay of 1.5s and contact times of 3ms and 1ms,
was sealed, removed from the cold bath and allowed to slowly warm to room
temperature while the mixture was stirred. During this time the colour of the
suspension changed from light brown to red and finally to olive green. After the
2
54
respectively. The H spectrum was fitted using the DMFIT software . To study
the addition of H
2
to complex 1 in the solid state, a ZrO
2
rotor was charged with
suspension was stirred at ambient temperature for 30min, the N pressure was
2
[
(Cp′Fe) (μ -N )] (1) (about 80mg) and the initial solid-state NMR spectra were
3
3
2
released slowly to the Schlenk line and the solvent was removed under dynamic
recorded. The rotor was then opened and exposed to H
2
(10bar) for 1d and then
oil pump vacuum. The green-brown residue was extracted with hexane (4 ×10ml)
for an additional 9d. The reaction progress of the identical sample was monitored
by solid-state NMR spectroscopy.
and filtered. The solvent was again removed under dynamic oil pump vacuum,
and the green-brown solid was re-dissolved in a minimum amount of Et O
2
(
about 10ml) and stored for crystallization at −30°C to yield fine dark green
Crystallographic studies. Crystals were mounted on glass fibres in inert
oil and transferred to the cold gas stream of an Oxford Diffraction Nova A
diffractometer. Intensity data were recorded at 100K using mirror-focussed
needles. The yield was 53% (310mg, 0.35mmol). When the reaction was carried
out at room temperature under 1atm of N , complex 1 was isolated with
about 20% yield (besides significant quantities of [Cp′ Fe]). Complex 1 was
stable under an N and Ar atmosphere for several months without any sign
of decomposition. Mp: 174°C (dec.). Elemental analysis (%) for C51
calculated, C 68.38, H 9.79, N 3.13; found, C 67.89, H 9.75, N 3.14. The EI
mass spectrum showed a molecular ion at m/z=895amu. UV/Vis (n-hexane,
2
2
Cu K
α
radiation (λ=1.54184Å). Absorption corrections were based on
2
2
multi-scans. Structures were refined anisotropically on F using the program
3 2
H87Fe N :
5
5
SHELXL-2017 (ref. ). The NH hydrogen atoms were refined freely but
with a common isotropic temperature factor. Other hydrogens were
included using rigid idealized methyl groups or a riding model. For
further refinement details see Section 4 (‘Crystallographic details’) in the
Supplementary Information.
−
1
−1
−1
−1
22°C, nm): 216 (ɛ = 38,800lmol m ), 252 (ɛ = 34,000lmol cm ),
−1
−1
−1
−1
282 (ɛ = 33,800lmol cm ), 315 (ɛ = 40,800lmol cm ), 375 (sh,
−1
−1
−1
−1
1
ɛ = 16,200lmol cm ), and 667 (ɛ = 1,770lmol cm ). H NMR (C D12, 298K):
6
δ=7.37 (s, br, 6H, ring-CH, ν1/2 =360Hz), 0.20 (s, br, 54H, tBu-H, ν1/2 =100Hz),
1
5
X-band EPR studies. X-band EPR spectra were recorded on a Bruker EMX
spectrometer with an OXFORD ESR900 continuous-flow cryostat. Frozen toluene
and methylcyclohexane solutions of complexes 1 and 2, respectively, were prepared
in quartz tubes (707-SQ-250M, Wilmad-LabGlass). The experimental spectra were
−1.68 (s, br, 27H, tBu-H, ν1/2 =60Hz) ppm. N Hahn-echo MAS NMR: δ=1,520,
1,403ppm. For further details see Section 3 (‘Solid-state NMR studies’)
in the Supplementary Information.
When complex 1 was exposed to small amounts of H
O, we observed the
2
5
6
formation of [{Cp’Fe(μ -OH)} ]68 besides considerable decomposition. This
simulated with EasySpin 5.2.2 (ref. ).
2
3
2 2
contrasts with the behaviour of 1 when exposed to Ar, N or H , during which the
formation of [{Cp′Fe(μ -OH)} ] was not observed.
Solid-state magnetic susceptibility studies. Solid-state magnetic susceptibility
studies were performed on a Cryogenic closed-cycle SQUID magnetometer
between T=2.6K and 300K with an externally applied magnetic field of
2
3
Synthesis of [(Cp′Fe)
-N
3
(µ
3
2
-NH) ] (2). Either a powder or crystals of
57
H
ext =1kOe. The samples were prepared in quartz tubes as described previously ,
[(Cp′Fe)
3
(μ
3
2
)] (100mg, 0.11mmol) were pressurized with H
2
(10bar) for
and the diamagnetic background signal of an empty sample holder including
quartz wool was determined experimentally and subtracted from the raw
magnetization data. The experimental data were also corrected for the overall
14d at ambient temperature. When the reaction was performed on crystalline
material, an SCSC transformation occurred. Complex 2 was stable under an N
2
and Ar atmosphere for several months without any sign of decomposition. Mp:
58
diamagnetism of the investigated molecules using tabulated Pascal constants .
197°C (dec.). The EI mass spectrum showed a molecular ion at m/z=897amu.
−1
−1
To ensure the validity of the Curie law approximation for an applied magnetic
field of Hext =1kOe, supplementary measurements at T=2.6K with applied
magnetic fields between Hext =0.05kOe and 10kOe were also executed
UV/Vis (n-hexane, 22°C, nm): 222 (ɛ = 35,200lmol cm ), 275 (sh, ɛ =
−1 −1 −1 −1 −1
−1
22,000lmol cm ), 360 (ɛ = 19,000lmol cm ), 399 (ɛ = 20,900lmol cm ),
−1
−1
1
and 680 (sh, ɛ = 1,050lmol cm ). H NMR (C
D12, 298K): δ=1.00 (s,
6
(
Supplementary Figs. 19,22).
br, ν1/2 =30Hz), 0.59 (s, br, ν1/2 =50Hz), 0.48 (s, br, ν1/2 =70Hz), 0.27 (s, br,
1
5
ν
1/2 =70Hz), 0.02 (s, br, ν1/2 =50Hz), and −0.60 (s, br, ν1/2 =30Hz) ppm. N
5
7
Fe Mössbauer studies. Zero-field 57Fe Mössbauer measurements were performed
Hahn-echo MAS NMR: δ=−256.2ppm. For further details see Section 3
on a standard Halder and WissEl transmission spectrometer with sinusoidal
velocity sweep. Velocities were calibrated using an α-Fe foil at T=300K and
confirmed by supplementary room-temperature measurements with powders
of sodium nitroprusside (Na
(
(‘Solid-state NMR studies’) in the Supplementary Information.
General procedure for the reaction of compounds 1 and 2 with H . A solution
2
of 1 (or 2) (40mg, 0.045mmol) in hexane (10ml) was loaded in a bomb flask and
2
[Fe(CN)
5
NO]∙2H
2
O) or potassium ferrocyanide
K
4
[Fe(CN) ]∙3H O) at T=300 K. Polycrystalline powders were prepared
6
2
cooled in liquid nitrogen (77K). The bomb flask was then evacuated, refilled with
5
7
−2
with an area density corresponding to about 0.25–0.34mg Fecm and were
stored in sample containers made of Teflon or polyether ether ketone (PEEK).
The temperature-dependent measurements were conducted on a CryoVac
continuous-flow cryostat (CFC) or a Janis closed-cycle cryostat (CCC) with helium
exchange gas (adjusted to approximately 100mbar during the measurement). The
temperature was recorded with a calibrated Si diode located close to the sample
container, and indicated variations of less than 0.1K during the experiment. The
minimum experimental line width (full width at half maximum, or FWHM) was
2
H (1bar), and closed tightly. After the flask had thawed to ambient temperature,
the H
at room temperature. All volatiles (solvent, hydrolysed ligand and NH
carefully transferred in vacuum onto a frozen solution of HCl in Et O (2.0M).
After the solution was thawed, the solvent was removed by oil pump vacuum.
The remaining NH Cl was dissolved in H O (1,000µl). Afterwards, 20µl (or 40µl)
aliquots were used for each titration of the indophenolic titration of NH
2
pressure had increased to about 4bar and the reaction was stirred for 24h
3
) were
2
4
2
3
2
3
. All
reactions were carried out three times with the same batch of compounds 1 and 2,
and therefore three different entries are listed in Supplementary Table 1. To ensure
reproducibility, three independent runs were performed with the same batch.
−
1
−1
<
0.24mms (in the case of the CryoVac CFC setup) or <0.27mms (in the case
of the Janis CCC setup). The Mössbauer source, with nominal activity of about
5
7
5
0mCi of Co in a rhodium matrix, was stored at ambient temperature during the
measurement. Isomer shifts (δiso) are quoted relative to metallic iron at T=295K
General procedure for the reaction of compounds 1 and 2 with HCl. In the first
method, a solution of 1 (or 2) (40mg, 0.045mmol) in hexane (or THF) (10ml) was
but were not corrected in terms of the second-order Doppler shift. The spectra
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loaded in a bomb flask, cooled in liquid nitrogen, treated with 1ml of a solution
of HCl in Et O (2.0M) and closed tightly. After thawing to ambient temperature,
2
58. Bain, G. A. & Berry, J. F. Diamagnetic corrections and Pascal’s constants.
J. Chem. Educ. 85, 532–536 (2008).
the mixture was stirred for 1h. All volatiles (solvent, hydrolysed ligand and NH
were carefully transferred in vacuum onto a frozen solution of HCl in Et O (2.0M),
Cl.
3
)
59. Brand, R. A. WinNormos-for-Igor v. 3.0 (Wissenschaꢅliche Elektronik
GmbH, 2009).
2
and the residue was heated extensively (>450°C) to decompose the formed NH
4
60. Gaussian 16, Revisions B.01v and B.01 (Gaussian Inc., 2016).
61. Becke, A. D. Density-functional thermochemistry. III. ꢂe role of exact
exchange. J. Chem. Phys. 98, 5648–5652 (1993).
After the mixture was thawed to ambient temperature, the solvent was removed
by oil pump vacuum. The remaining NH Cl was dissolved in H O (1,000µl).
4
2
Afterwards, 40µl aliquots were used for each titration of the indophenolic
62. Burke, K., Perdew, J. P. & Wang, Y. in Electronic Density Functional ꢁeory:
Recent Progress and New Directions (eds Dobson, J. F., Vignale, G. et al.)
Ch. II (Plenum, 1997).
3
2
3
titration of NH (ref. ). All reactions were carried out three times with the same
batch of compounds 1 and 2, and therefore three different entries are listed in
Supplementary Table 1. To ensure reproducibility, three independent runs were
performed with the same batch.
63. Dolg, M., Wedig, U., Stoll, H. & Preuss, H. Energy‐adjusted ab initio
pseudopotentials for the ꢃrst row transition elements. J. Chem. Phys. 86,
866–872 (1987).
In the second method, a solution of 1 (or 2) (40mg, 0.045mmol) in hexane
(
10ml) was loaded in a bomb flask, cooled in liquid nitrogen, treated with 1ml
64. Ehlers, A. W. et al. A set of f-polarization functions for pseudo-potential basis
sets of the transition metals Sc-Cu, Y-Ag and La-Au. Chem. Phys. Lett. 208,
111–114 (1993).
2
of a solution of HCl in Et O (2.0M) and closed tightly. After thawing to ambient
temperature, the mixture was stirred for 1h. Afterwards, 40µl aliquots were used
for each titration of the indophenolic titration of NH directly from the reaction
65. Hariharan, P. C. & Pople, J. A. ꢂe inꢆuence of polarization functions
on molecular orbital hydrogenation energies. ꢁeor. Chem. Acc. 28,
213–222 (1973).
3
3
2
mixture . The two methods delivered the same yields of ammonia during the
reaction with HCl. All reactions were carried out three times with the same
batch of compounds 1 and 2, and therefore three different entries are listed in
Supplementary Table 1. To ensure reproducibility, three independent runs were
performed with the same batch. Yields for [(Cp′Fe) (μ -N )] (1) after addition
66. Hehre, W. J., Ditchꢃeld, R. & Pople, J. A. Self-consistent molecular
orbital methods. XII. Further extensions of Gaussian-type basis sets
for use in molecular orbital studies of organic molecules. J. Chem. Phys. 56,
2257–2261 (1972).
3
3
2
of H
2
were 3–10% and after addition of HCl were 70–76% (see Supplementary
Table 1 for details). Yields for [(Cp′Fe)
3
(µ
3
-NH)
2
] (2) after addition of H
2
67. Reed, A. E., Curtiss, L. A. & Weinhold, F. Intermolecular interactions
from a natural bond orbital, donor–acceptor viewpoint. Chem. Rev. 88,
899–926 (1988).
(
about 5%) and after addition of HCl were 75–80% (see Supplementary
Table 1 for details).
6
8. Walter, M. D. & White, P. S. [{Cp′Fe(μ-OH)}
3
: the synthesis of a unique
organometallic iron hydroxide. Dalton Trans. 41, 8506–8508 (2012).
Data availability
The authors declare that all of the data supporting the findings of this study are
available within the paper and the Supplementary Information, and also from
the corresponding authors upon reasonable request. This includes experimental
details, NMR studies in solution, solid-state NMR studies, crystallographic
details, X-band EPR studies, solid-state magnetic susceptibility studies, zero-field
ꢁcknowledgements
We thank P. Schweyen for EPR data collection on complex 1, and M. Bröring and
F. Jochen Litterst for providing access to the SQUID magnetometer and 57Fe Mössbauer
spectrometer. We acknowledge the help of S. Schneider and J. Abbenseth in the
5
7
15
2
Fe Mössbauer studies, and computational details. Crystallographic data for
studies of solution N and H NMR. This work was supported by the Emmy
Noether (WA 2513/2) and Heisenberg (WA 2513/6-8) programs of the Deutsche
Forschungsgemeinschaft (DFG) (for M.D.W.); a fellowship for experienced researchers
of the Humboldt Foundation and the Chinese Academy of Science (L.M.);
a computational grant from CalMip; and CNRS (S.B.). This contribution is
dedicated to the memory of Richard A. Andersen.
the structures reported in this article have been deposited at the Cambridge
Crystallographic Data Centre under deposition numbers 1939746 (1) and
1
939747 (2). Copies of the data can be obtained free of charge at https://www.ccdc.
cam.ac.uk/structures.
References
ꢁ
uthor contributions
5
2
2. Walter, M. D. & White, P. S. [Cp′FeI] as convenient entry into iron-modiꢃed
M.R., D.B., Y.C., S.B. and M.D.W. conceived the experiments; M.R., D.B., K.M., M.-K.Z.,
M.F., P.G.J., Y.C., S.B. and M.D.W. designed and performed experiments and analysed
the data; I.d.R. and L.M. performed computational studies; M.R., D.B., S.B., L.M. and
M.D.W. wrote the manuscript.
6
1
pincer complexes: bimetallic η ,κ -POCOP-pincer iron iridium compounds.
New J. Chem. 35, 1842–1854 (2011).
5
3. Schwindt, M. A., Lejon, T. & Hegedus, L. S. Improved synthesis of
(
aminocarbene)chromium(0) complexes with use of C8K-generated
2−
Cr(CO)
5
. Multivariant optimization of an organometallic reaction.
Organometallics 9, 2814–2819 (1990).
4. Massiot, D. et al. Modelling one- and two-dimensional solid-state NMR
spectra. Magn. Reson. Chem. 40, 70–76 (2002).
Competing interests
5
5
5
5
The authors declare no competing interests.
5. Sheldrick, G. Crystal structure reꢃnement with SHELXL. Acta Crystallogr. C
ꢁ
dditional information
7
1, 3–8 (2015).
Supplementary information is available for this paper at https://doi.org/10.1038/
6. Stoll, S. & Schweiger, A. EasySpin, a comprehensive soꢅware package for
spectral simulation and analysis in EPR. J. Magn. Reson. 178, 42–55 (2006).
7. Walter, M. D., Schultz, M. & Andersen, R. A. Weak paramagnetism in
s41557-020-0483-7.
Correspondence and requests for materials should be addressed to M.D.W.
Reprints and permissions information is available at www.nature.com/reprints.
2
compounds of the type Cp′ Yb(bipy). New J. Chem. 30, 238–246 (2006).
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