Reversible Ligand-Centered Reduction in Low-Coordinate Iron Formazanate Complexes

Article abstract of DOI:10.1002/chem.201801298

Coordination of redox-active ligands to metals is a compelling strategy for making reduced complexes more accessible. In this work, we explore the use of redox-active formazanate ligands in low-coordinate iron chemistry. Reduction of an iron(II) precursor occurs at milder potentials than analogous non-redox-active β-diketiminate complexes, and the reduced three-coordinate formazanate-iron compound is characterized in detail. Structural, spectroscopic, and computational analysis show that the formazanate ligand undergoes reversible ligand-centered reduction to form a formazanate radical dianion in the reduced species. The less negative reduction potential of the reduced low-coordinate iron formazanate complex leads to distinctive reactivity with formation of a new N?I bond that is not seen with the β-diketiminate analogue. Thus, the storage of an electron on the supporting ligand changes the redox potential and enhances certain reactivity.

Full text of DOI:10.1002/chem.201801298

A Journal of
Accepted Article
Title: Reversible Ligand-Centered Reduction in Low- Coordinate Iron
Formazanate Complexes
Authors: Daniel Broere, Brandon Mercado, James Lukens, Avery
Vilbert, Gourab Banerjee, Hannah Lant, Shin Hee Lee,
Eckhard Bill, Kyle Lancaster, Stephen Sproules, and Patrick
Holland
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To be cited as: Chem. Eur. J. 10.1002/chem.201801298
Link to VoR: http://dx.doi.org/10.1002/chem.201801298
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Reversible Ligand-Centered Reduction in Low-Coordinate Iron
Formazanate Complexes
Daniel L. J. Broere,*,[a] Brandon Q. Mercado,[a] James T. Lukens,[b] Avery C. Vilbert,[b] Gourab
Banerjee,[a] Hannah M. C. Lant,[a] Shin Hee Lee,[a] Eckhard Bill,[c] Stephen Sproules,ꢀdꢁ Kyle M.
Lancaster,[b] and Patrick L. Holland*,[a]
Abstract: Coordination of redox-active ligands to metals is
a
accessible reduced states: as shown on the right of Figure 1, the
ligand orbital can be lower in energy, moderating the redox
potential. Accessible ligand-centered redox processes can
expand upon the intrinsic reactivity of metal centers.4 Redox-
active ligands have enabled multi-electron processes in systems
where the transition metal changes oxidation states by less than
two electrons, thereby facilitating elementary reactions such as
oxidative addition and reductive elimination.5 Alternatively, redox-
compelling strategy for making reduced complexes more accessible.
In this work, we explore the use of redox-active formazanate ligands
in low-coordinate iron chemistry. Reduction of an iron(II) precursor
occurs at milder potentials than analogous non-redox-active β-
diketiminate complexes, and the reduced three-coordinate
formazanate-iron compound is characterized in detail. Structural,
spectroscopic and computational analysis show that the formazanate
ligand undergoes reversible ligand-centered reduction to form a
formazanate radical dianion in the reduced species. The less negative
reduction potential of the reduced low-coordinate iron formazanate
complex leads to distinctive reactivity with formation of a new N-I bond
that is not seen with the β-diketiminate analogue. Thus, the storage
of an electron on the supporting ligand changes the redox potential
and enhances certain reactivity.
active ligands can enable single-electron transformations without
6
changing metal oxidation state.
A recent computational
investigation by Hu and Chen 7 demonstrated the utility of
multiconfigurational calculations for characterizing the redox-
active bisiminopyridine ligands, which enable facile reductive
elimination in iron-catalyzed [2+2] cycloadditions developed by
Chirik and coworkers.8 Given the wide use of β-diketiminates,
there is motivation for exploring a redox-active ligand that is
structurally similar, but can readily accept electrons, enabling
one-electron reduction at less negative potentials.
Introduction
Recent studies have shown that β-diketiminates can
undergo ligand-centered oxidation in homoleptic complexes of
zinc, 9 cobalt, 10 and nickel, 11 , 12 but isolated transition metal
complexes that display ligand-centered reduction have not been
reported.13 Here, we use a formazanate14 as a redox-active ligand
that is structurally similar to β-diketiminates, but which can accept
electrons. By placing a low-energy unoccupied ligand orbital near
the metal, they could make the complex easier to reduce (Figure
1, right). Otten has shown that formazanates undergo facile
ligand-centered reduction when bound to boron and to zinc.15,16
To our knowledge, reduced formazanates on a redox-active
transition metal have not yet been isolated, though they have
been spectroscopically observed.17 Moreover, a recently reported
homoleptic formazanate iron(II) complex adopted a low-spin
iron(I) configuration upon one-electron reduction rather than
generating a ligand-centered radical,18 highlighting the need for
further study.
β-Diketiminate ligands have found extensive use as auxiliary
ligands to stabilize unusual metal complexes throughout the
periodic table.1 In particular, highly reduced low-coordinate base
metal complexes featuring β-diketiminate ligands are able to
activate a number of challenging chemical bonds.2 Reductive
processes in β-diketiminate complexes are normally metal-
centered due to the very negative potentials required for ligand-
centered reduction.3 However, the negative potential required for
metal-centered reduction (placing electrons in the LUMO at the
left of Figure 1) often leads to the use of harsh reductants such as
Na or KC8, which are often incompatible with exogenous
substrates for catalytic transformations. Moreover, catalytic
turnover is often not feasible because the reduced states are too
high in energy.
The ability of redox-active ligands to reversibly accept and
donate electrons offers an appealing way to generate more
[a]
Dr. Daniёl L. J. Broere, Dr. Brandon Q. Mercado, Gourab Banerjee,
Hannah M. C. Lant, Shin Hee Lee and Prof. Dr. Patrick L Holland
Department of Chemistry, Yale University, New Haven, Connecticut
06520, United States
E-mail: daniel.broere@yale.edu / patrick.holland@yale.edu
James T. Lukens, Avery C. Vilbert and Prof. Dr. Kyle M. Lancaster
Department of Chemistry and Chemical Biology, Baker Laboratory,
Cornell University, Ithaca New York 14853.
[b]
[c]
[d]
Dr. Eckhard Bill
Max Planck Institute for Chemical Energy Conversion, Stiftstrasse
34-36, D-45470 Mülheim an der Ruhr, Germany
Dr. Stephen Sproules
WestCHEM, School of Chemistry, University of Glasgow, Glasgow
G12 8QQ, United Kingdom
Figure 1. Conceptual representation of how the redox-active formazanate
ligand can provide access to a more accessible reduced state.
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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Herein, we describe the synthesis, characterization and
Cooling a solution of 2 or 3 in pentane yielded crystals
suitable for single crystal X-ray diffraction. In 2, two
crystallographically independent molecules were found in the
asymmetric unit, which share a π–π interaction between the tolyl
groups in the formazanate ligands (Figure S37). Each molecule
displays a planar, three-coordinate geometry at iron where the
sum of the three angles is 359.8 ± 0.5 ° (Figure 2, left). A notable
feature in the solid state structure of 2 is the small dihedral angle
between the formazanate and N-phenyl groups (1.6 – 28.3°),
which is very different from analogous three-coordinate Fe
complexes containing a N(SiMe3) and bulky β-diketiminate
ligands (65 – 90°).22 We attribute this difference to the lack of
sterically demanding ortho-substituents on the N-phenyl groups
in complex 2. The Fe–N bond lengths to the formazanates (1.95
– 1.97 Å) are shorter than those in analogous three-coordinate Fe
complexes containing a N(SiMe3) and bulky β-diketiminate
reactivity of some new low-coordinate iron formazanate
complexes and their one-electron reduction products.
Spectroscopic and computational data are used to characterize
the frontier orbitals. These studies reveal that the iron-
formazanate complexes undergo ligand-centered reduction at
relatively mild potentials, and demonstrate an isolable example of
a redox-active metal with a formazanate radical dianion. In order
to illustrate the new opportunities coming from redox activity at
the formazanate ligand, we demonstrate how the formazanate
complexes activate chemical bonds like low-coordinate β-
diketiminate iron complexes, but in contrast the reactions are
mediated by ligand-centered redox changes. In addition, we show
that the less electron donating formazanate ligand affects the
stability of higher metal oxidation states through an unusual N-I
elimination reaction.
Results and Discussion
Synthesis and characterization. The dropwise addition of a
suspension of the known19 formazan 1 to a stirred solution of
Fe[N(SiMe3)2]2 in pentane gives the three-coordinate formazanate
complex 2 in near quantitative yield (Scheme 1). The 1H NMR
spectrum of complex 2 in C6D6 at 298 K shows the expected
number of paramagnetically shifted resonances between +82 and
-40 ppm, which are tentatively assigned based on their relative
integration and distance from the metal center (Figure S1). The
presence of a single equivalent of THF in the synthesis of complex
2 results in the exclusive formation of the four-coordinate complex
3 (Scheme 1).20 Hence, care must be taken to avoid the presence
of THF in the solvent and starting materials.21 We observed that
p-tol
N
N
N
HN
1
Fe[N(SiMe3)2]2
Fe[N(SiMe3)2]2(THF)
p-tol
98%
93%
p-tol
N
N
N
N
N
N
N
1 THF
>99%
N
Figure 2. Displacement ellipsoid plots (50% probability) of complex 2 and 3.
Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and
angles (°) for 2: Fe2–N18 1.903(3); Fe2–N15 1.964(4); Fe2–N45 1.966(4);
N18–Fe2–N15 132.5(2); N18–Fe2–N45 135.2(2); N15–Fe2–N45 91.7(1). For
3: Fe2–N18 1.937(2); Fe2–N15 1.979(2); Fe2–N45 1.984(2); Fe2–O20
2.145(1); N18–Fe2–N15 130.40(7); N18–Fe2–N45 127.99(6); N15–Fe2–N45
91.59(6).
Fe
N
Fe
O
N
Me3Si
2
3
SiMe3
Me3Si SiMe3
Scheme 1. Synthesis of complexes 2 and 3.
the resonance for the SiMe3 protons shifts >40 ppm upfield upon
the addition of one equiv of THF to complex 2. Due to fast
exchange on the NMR time scale and an equilibrium constant >
5000 M-1, the chemical shift can be used as an indication of the
THF content of the sample (Figure S27).
ligands (1.99 – 2.03 Å).22 The N–N bond lengths (1.319(5) –
1.327(5) Å) are slightly longer than commonly observed for
formazanate complexes (1.30−1.31 Å)15e,16, which was also
observed for the reported homoleptic iron formazanate complex.18
A notable difference is the significantly longer Fe–N bonds
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(1.955(4) – 1.966(4) Å) in 2 when compared with the homoleptic
bis(formazanate)iron(II) species (1.817(2) − 1.833(2) Å).18 The
solid state structure of complex 3 (Figure 2, right) shows that the
iron center adopts a distorted trigonal pyramidal geometry (τ4 =
0.70 and 0.72)23 for both molecules in the asymmetric unit. As
expected, coordination of an additional ligand causes all Fe–N
bond lengths in 3 to be elongated (1.979(2) – 2.000(2) Å)
compared to 2. Interestingly, the bulky amide forces the bound
THF into a geometry wherein the C atoms lie over the
formazanate ligand.
solution for days under inert atmosphere. Single crystals suitable
for X-ray structure determination were obtained by cooling a THF
solution of 5. The solid state structure reveals that the Na is
sandwiched between two crown ethers, and the resulting cation
is separated from the anionic formazanate complex (Figure 3).
Although the anion in 5 is isostructural to neutral complex 2, there
are some remarkable differences in the bond lengths and angles.
The iron amide bond (Fe1–N14) is longer in complex 5 by ca. 0.05
Å, suggestive of a more electron-rich Fe center. The N–N bonds
within the formazanate are elongated (1.377(6) and 1.345(6) Å)
and the Fe–N (formazanate) bonds (1.951(5) and 1.958(5) Å) are
shortened compared to 2 (Figure 2), which is consistent with
either formazanate-centered reduction or significant π-
backdonation into the formazanate ligand.
Cyclic voltammetry of complex 3 in THF revealed two
red1
reversible one-electron reduction events at E
= -1.65 V and
½
E
½
red2 = -2.47 V vs Fc+/Fc (Figure S29). The first reduction occurs
at a potential ~0.8 V more positive than the corresponding three
coordinate β-diketiminate complexes (E
red
= -2.4 to -2.6 V vs
½
Fc+/Fc).24 The nature of this reduction will be discussed below.
The addition of CoCp*2 to a solution of 2 or 3 in hexanes gave the
one-electron reduced species 4, which precipitated as a green
solid (Scheme 2). The 1H NMR spectrum of 4 in THF-d8 at 298 K
shows the expected number of paramagnetically shifted
resonances between +105 and –25 ppm consistent with a single
C2 symmetric species (Figure S9). Intraligand bond lengths in
v
redox-active ligands can often be used to determine the locus of
a redox event.4-6,16, 25 Unfortunately, we were unable to grow
crystals of complex 4 that were suitable for X-ray structure
determination. Although complex 4 is stable in the solid state for
months, it decomposes in THF solution at room temperature
within hours to form a mixture of species, concomitant with a color
change from dark green to brown. X-ray diffraction of crystals
obtained from this mixture afforded a low quality structure
revealing a complex wherein the formazanate has undergone N–
N bond scission (Figure S39).26 However, we were unable to
isolate sufficient amounts of this unusual product for full
characterization, and thus we sought a more stable analogue.
Since the C-H bonds in CoCp*2 and [CoCp*2]+ are susceptible to
deprotonation or C-H activation, 27 and because of potential
stabilizing effects of alkali cations,28 we used an equivalent of
elemental Na to reduce complex 3 in THF, which resulted in a
green solution similar in color to 4. Addition of 2 equiv of 12-
crown-4 and cooling the THF mixture gave 5 as a green crystalline
Figure 3. Displacement ellipsoid plot (50% probability) of complex 5. Hydrogen
atoms and a THF molecule have been omitted. Selected bond lengths (Å) and
angles (°): Fe1–N11 1.935(5); Fe1–N14 1.951(5); Fe1–N41 1.958(5); N11–N21
1.377(6); N31–N41 1.345(6); N11–Fe1–N14 134.9(2); N11–Fe1–N41 96.5(2);
N14–Fe1–N41 128.6(2).
Magnetism and EPR spectroscopy. The temperature
dependence of the magnetic susceptibility of solid samples of
complexes 2–5 was studied in the range 2–290 K using a SQUID
magnetometer, and at room temperature in solution using the
Evans method. The room temperature magnetic moments of
complex 2 and 3 in C6D6 solution were 4.8 ± 0.1 µB and 4.7 ± 0.1
1
solid in 67% yield (Scheme 2). The H NMR (Figure S15) and
zero-field Mӧssbauer spectra (vide infra) of 5 are nearly identical
to those for complex 4. However, unlike 4, complex 5 is stable in
µB, respectively, in agreement with
a high-spin iron(II)
configuration for both complexes (S = 2, spin-only value µs.o. = 4.9
µB). In the solid state, three-coordinate 2 and four-coordinate 3
have nearly identical, temperature independent magnetic
moments of 5.1 μB over the range 30 – 290 K (Figures S4 and S8),
which is near the solution values and consistent with a high-spin
iron(II) (S = 2) ion in each complex. Below 30 K, the μeff for 2
decreases under the influence of zero-field splitting. The sign and
magnitude of the zero-field parameters were determined from
simulations of μeff(T) in combination with isofield magnetization
measurements at 1, 4, and 7 T (Figure S4 inset). Global fitting of
the magnetic data yielded D = –3.8 cm–1 and rhombicity E/D =
0.19. The temperature profile is different for 3, where there is a
slight rise in μeff below 30 K, presumably due to intermolecular
interactions. The best fit was obtained for D = 3.1 cm–1 and E/D =
0.30, where the sign of D is opposite to that for three-coordinate
2, though the sign loses meaning so close to full rhombicity. The
similarity in the magnitude of these spin-Hamiltonian parameters
2
3
or
Na
2 12-c-4
CoCp*2
97%
67%
p-tol
p-tol
O
O
O
O
O
O
Co+
N
N
N
N
N
N
N
N
Na
O
O
Fe
Fe
N
N
Me3Si
Me3Si
SiMe3
SiMe3
4
5
Scheme 2. Synthesis of complexes 4 and 5.
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for 2 and 3 reveal only a very minor change to the electronic
structure upon THF coordination.
Mӧssbauer spectroscopy. The zero-field Mӧssbauer spectra of
solid samples of 2 and 3 at 80 K (Figure 5) each showed a
quadrupole doublet, with 2 at δ = 0.63 mm s–1 and |ΔEQ| = 2.48
mm s–1, and 3 at δ = 0.71 mm s–1 and |ΔEQ| = 1.46 mm s–1. These
parameters are within the range previously observed for three-29
and four-coordinate30 high-spin iron(II) diketiminate complexes.
The smaller quadrupole splitting and larger isomer shift of 3 in
comparison to 2 are consistent with the increase in coordination
number.31 The zero-field Mӧssbauer spectrum of a solid sample
of 4 at 80 K shows an asymmetric doublet with δ = 0.48 mm s–1
and |ΔEQ| = 1.25 mm s–1 (Figure 5, bottom), which shows no
significant changes upon warming to 173 K. The zero-field
Mӧssbauer spectrum of a solid sample of 5 at 80 K is nearly
identical to that of 4, with δ = 0.49 mm s–1 and |ΔEQ| = 1.30 mm
s–1 (Figure S16). The asymmetric broadening of the doublets is
common for Kramers systems as a result of intermediate rates of
spin relaxation.32 Interestingly, the change in isomer shift between
2/3 and 4/5 is comparable, but opposite in direction from the
reduction of the homoleptic iron formazanate complex reported by
Otten and co-workers,18 which undergoes metal-centered
reduction.
Room temperature magnetic moments of 3.8 ± 0.1 µB and 3.9
± 0.1 µB were determined from THF solutions of 4 and 5,
respectively, suggesting S = 3/2 ground states for both compounds
(µs.o. = 3.9 µB). Both 4 and 5 exhibited temperature independent
effective magnetic moments of 4.0 μB over the range 50 – 290 K
(Figures S14 and S18), consistent with these values. Much like 2,
4 shows a low-temperature downturn in μeff due to zero-field
splitting but the more gentle decrease is modelled with a larger D
= –16.8 cm–1 and large rhombicity E/D = 0.29. For 5, on the other
hand, the downturn in μeff is more abrupt, suggesting
intermolecular interactions like those observed in 3. Nevertheless
an excellent fit was obtained from modelling the combined μeff(T)
and isofield magnetization plots (including an intermolecular
interaction approximation), which gave D = -10.1 cm–1 and E/D =
0.26. These solid state measurements were augmented with an
EPR analysis of the odd-electron complexes 4 and 5. Frozen
solution spectra recorded in THF at 10 K highlighted the
aforementioned solution instability of 4. EPR spectra of 4 (Figure
S12) show a rhombic S = 3/2 signal, and a strong rhombic S = 1/2
signal that originates from [(formazanate)2FeI]–,18 which is one of
the decomposition products of 4 (see SI for more details). The S
3
= /2 signal has peaks at resonant field positions consistent with
3
an S = /2 species having g = (2.17, 2.24, 1.87), D = -5.7 cm–1,
and E/D = 0.19 (Figure S12). The greater solution stability
displayed by 5 yielded an EPR spectrum (Figure 4) wholly
consisting of an S = 3/2 signal with geff = (2.68, 2.03, 2.34), D = 16
cm–1 and E/D = 0.33.
Figure 4. X-band EPR spectrum of 5 in THF at 10 K. Experimental data are
represented by the black line; simulation is depicted by the red trace. Asterisk
denotes trace impurity contributing <0.5% to the total signal. Experimental
conditions: frequency 9.3701 GHz; power 0.2 mW; modulation 2.0 mT.
Minor deviations in g and E/D may be due to structural differences
of solid and solution samples (D was taken from magnetic data
because the second Kramers doublet could not be determined
3
due to relaxation issues). The increased quality of the S =
/
2
signal in the sample of 5 is consistent with the greater solution
stability of this compound. Our assignment of 5 is bolstered by the
agreement of g values and zfs parameters between EPR and
magnetometry.
Figure 5. Zero-field Mössbauer spectrum of a solid samples of complex 2 (top),
3 (middle), 4 (bottom) at 80 K. The black circles are experimental data with the
fit represented by the black line. The thin grey trace is the residual of the fit.
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Though it is tempting to use the shift in Mössbauer parameters to
assign the metal oxidation state, previous results advise caution.
In earlier work we described a di(μ-hydrido)diiron(II) complex33
with a high-spin electronic configuration (S = 2) supported by
Mӧssbauer and EPR spectrum and magnetism of 5, the
absorption spectrum supports an electronic configuration where a
ligand-centered radical (SL = 1/2) is strongly antiferromagnetically
coupled to a high-spin iron(II) center (SFe = 2) to give an overall
Stotal = 3/2 ground state.
bulky β-diketiminate ligand that gave a mononuclear three-
coordinate iron(I) hydride complex with an S =
3
/
2
electronic
The Fe K-edge X-ray absorption spectra of 2 – 4 are
overlaid in Figure 7. They are characterized by electric dipole
forbidden but quadrupole-allowed 1s → 3d pre-edge transitions
that appear at the base of the dipole-allowed 1s → np rising edge
that dominates a K-edge spectrum. Since the ligand field typically
increases with increasing oxidation state for a given ligand set,
the pre-edge energy often provides a useful marker for oxidation
state. The centers of mass for pre-edge peaks in spectra obtained
for 2 – 4 are identical within experimental error at 7112.2 ± 0.1 eV.
As a ~1 eV increase in energy is expected per unit of oxidation,
the similar edge positions indicate that the physical Fe oxidation
state is the same for the Fe ion in neutral 2 and 3, and the
monoanion in 4. Moreover, it is noteworthy that the pre-edge peak
is of the same intensity for each complex. Electric quadrupole
allowed 1s → 3d pre-edge transitions gain intensity through 4p
mixing into the 3d orbitals and therefore are sensitive to the
symmetry of the complex.38 Here, the uniform peak height is
consistent with the dominant trigonal symmetry in this series
where the inclusion of a THF ligand in 3 has little effect on its
electronic structure (vide supra). In neutral 2 and 3, there is a
shoulder to higher energy at ~7113.2 eV. In contrast, the shoulder
in 4 lies to lower energy at 7111.5 eV, and is tentatively assigned
as transition to an orbital with significant ligand character.39
Although the rising K-edge is frequently used as an
additional metric of physical oxidation state in first-row metals, the
rising-edge energies are also influenced by coordination
geometry and ligand identity.40 In the present case, similar edge
positions are displayed by three-coordinate 2 and 4 of 7117.0 and
7117.4 eV, respectively, while the edge is shifted by 0.7 eV to
higher energy at 7118.1 eV for four-coordinate 3. Thus, these do
not change the conclusion from the pre-edge features, that all the
complexes have the same oxidation state at iron.
configuration.34 The change in observed isomer shift in the zero-
field Mӧssbauer spectra of solid samples of the di(μ-
hydrido)diiron(II) complex (δ = 0.59 mm s–1) and mononuclear
three-coordinate iron(I) hydride (δ = 0.40 mm s–1) at 80 K is
comparable to the decrease in isomer shift between 2/3 and 4/5.
On the other hand, we have also observed similar changes in
isomer shift from changes in ligand oxidation state. Namely,
oxidation of a four-coordinate high-spin iron(II) complex (S = 2)
with a dianionic redox-active tetrazene ligand (δ = 0.81 mm s–1)
gives a complex where a tetrazene radical anion (S = 1/2) is
strongly antiferromagnetically coupled to a high-spin iron(II)
center (S = 2) to give an overall S = 3/2 electronic configuration (δ
= 0.69 mm s–1).35 Consequently, the observed changes in isomer
shift between 2/3 and 4/5 could be attributed to either ligand- or
metal-centered reduction.
UV-Vis and X-ray absorption spectroscopy, and
computations. The iron(II) complexes 2 and 3 in hexane solution
each show an intense absorption in the visible region at 524 nm
(19 × 103 cm-1), which we assign as the formazanate *
transition by analogy to intense bands at similar energies
uncoordinated formazans.36 The absorption in these compounds
is red-shifted by ca. 30 nm (~1 × 103 cm-1) relative to the
protonated formazanate ligand.15e In contrast, the electronic
spectrum of 5 recorded at in THF at ambient temperature (Figure
6) exhibits prominent absorption bands at 414 and 696 nm (ε = 25
× 103 and 10 × 103 M-1 cm-1). Absorption bands with comparable
intensity and position have been observed in reduced triaryl-
formazanates bound to redox-inert atoms (carbon:37 λ = 398 and
721 nm, ε = 7.5 × 103 and 3.5 × 103 M-1 cm-1; boron:15a λ = 454
and 716 nm, ε = 15 × 103 and 7.5 × 103M-1 cm-1; zinc:15a λ = 462
and 755 nm, ε = 20 × 103 and 10 × 103 M-1 cm-1), and are
diagnostic of a formazanate radical dianion. Together with the
Figure 6. Experimental (solid) and calculated (dotted, SORCI) absorption
spectra for complex 3 (pink) and 5 (green) in THF solution.
Figure 7. Overlay of the normalized Fe K-edge XAS spectra of 2 (red), 3 (blue),
and 4 (green). Inset shows an expansion of the pre-edge region with a
comparison of the experimental spectra above the corresponding calculated
spectra obtained from B3LYP TD-DFT calculations. Calculated intensity in
arbitrary units.
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CFG. 1
(27%)
CFG. 2
(25%)
FeI
Density functional theory calculations were carried out to
facilitate spectroscopic and electronic structure interpretations. In
particular, time-dependent DFT (TDDFT) calculations are known
to reproduce metal K-edge XAS with remarkable fidelity after
accounting for inaccurate modeling of the 1s core potential with
either a scalar or linear energy shift.41 Time-dependent (TD) DFT
calculations (B3LYP hybrid density functional and CP(PPP) basis
set on Fe with ZORA-def2-TZVP(-f) on all remaining atoms) were
performed on the crystallographic coordinates of each complex.
For 2 and 3, these reproduce the pre-edge region with good
fidelity (Fig 7 inset), but the TD-DFT calculation for 4 produces a
pre-edge with an energy maximum that is underestimated by 1
eV. Inspection of the frontier orbitals calculated for 4 using this
level of theory revealed a straightforward unrestricted Kohn-Sham
solution consistent with (S = 3/2) iron(I). Given the consistency of
the spectroscopic data described above with an iron(II)
configuration, we evaluated a broken-symmetry (BS) option
where the four electrons from high-spin iron(II) are allowed to
FeII – L•(AF)
143: b2
0.0 % Fe 3d
0.0 % TMS N
40.1 % Fz N
142: b1
79.6 % Fe 3dxz
3.7 % TMS N (π*)
7.5 % Fz N (*)
141: a2
20.1 % Fe 3dxy
0.0 % TMS N
63.4 % Fz N (π*)
140: b2
95.8 % Fe 3dyz
0.4 % TMS N
1.2 % Fz N
1
participate in a magnetic interaction with an S = /2 formazanate
radical anion (BS 4,1). While this approach reproduces the Fe K
pre-edge with remarkable fidelity, the BS calculation converges to
a ferromagnetic S = 5/2 solution, which conflicts with the magnetic
measurements. Thus, the single-reference DFT methods fail to
acceptably reproduce physical/spectroscopic properties of 4.
We hypothesized that the ground states of the complex ions
139: a1
95.1 % Fe 3dx
2
2
–y
0.2 % TMS N
1.3 % Fz N
138: a2
77.7 % Fe 3dxy
0.0 % TMS N
in
4 and 5 are multiconfigurational, explaining the poor
performance of DFT. Thus, we carried out a multireference
configuration interaction (MRCI) calculation using the
spectroscopy oriented configuration interaction (SORCI)
method.42 We employed an active space of 11 electrons and 9
orbitals [CAS(11,9)] chosen to include five Fe 3d orbitals and four
electrons from the formazanate π system. Five quartet, doublet,
and sextet states were calculated. The chosen active space gave
≥ 90% reference weights for all states. The resulting state
energies are given in Table S1. SORCI appropriately predicts a
ground-state quartet, with the lowest spin-forbidden excited
state–a sextet–occurring at ca. 4000 cm–1 to higher energy. The
UV-vis absorption spectrum calculated by SORCI reproduces the
intense absorption at 14,000 cm–1.
19.6 % Fz N (π)
137: a1
83.7 % Fe 3dz
2
3.2 % TMS N
2.2 % Fz N
136: b2
2.6 % Fe 3dyz
0.0 % TMS N
43.5 % Fz N
135: b1
16.7 % Fe 3dxz
7.5 % TMS N (π)
59.7% Fz N ()
Inspection of the quartet ground state exposes substantial
multiconfigurational character. Five configurations have greater
than 5% participation in the ground state, with the two leading
configurations contributing ca. 25% each. Details of these two
principal leading configurations are shown in Figure 8. Evaluating
the composition of each configuration reveals that the ground
state is best described as a nearly equal mix of configurations
falling into two electronic structure categories. In one category,
high-spin iron(II) participates in antiferromagnetic coupling with
the unpaired electron residing in an a2-symmetry formazanate π*
MO. The second category represents high-spin iron(I) without
ligand participation.
In further agreement with our assignment, the homoleptic
formazanate iron complex reported by Otten and co-workers,18
which is proposed to undergo metal-centered reduction, lacks the
characteristic absorption bands for ligand-centered reduction that
5 displays at 414 and 696 nm. Overall, the electronic absorption
spectra show that the one-electron reduction of the formazanate
complexes involves substantial ligand participation.
Figure 8. Calculated SORCI averaged atomic natural orbitals (AANOs)
comprising the CAS(11,9) reference as well as the two leading configurations
comprising the ground-state quartet of the complex ion in 4. AANOs comprising
principally Fe 3d character (the Fe ligand field) are boxed, and the
antiferromagnetic (AF) interaction between Fe 3dxy and the a2 formazanate (Fz)
π* is indicated with a red bracket. Orbitals are plotted at an isovalue of 0.03 au.
Reactivity of the reduced complexes. No differences are found
in the 1H NMR spectra of complexes 4 and 5, except for
resonances attributable to the countercation. Yet, complex 4
decomposes within hours in solution whereas no signs of
decomposition were observed for solutions of 5 over the course
of several days (vide supra). To investigate whether this was from
reaction between the anion and CoCp*2 or from sodium
stabilization that slows decomposition, NaBArF was added to a
4
solution of complex 4 and [CoCp*2]PF6 was added to a solution of
5. In both cases decomposition was observed to the same mixture
of unidentified species, showing that the decomposition of the
one-electron reduced complex is brought about by the presence
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10.1002/chem.201801298
Chemistry - A European Journal
FULL PAPER
of the CoCp*2 cation. Though we have not been able to determine
the fate of the CoCp*2 cation, it is possible that decomposition
occurs through a pathway involving C-H activation of one of the
Cp* ligands or N-N cleavage in the formazanate.27
pronounced at higher NaI concentration. We propose that this
intermediate is a five-coordinate Fe(III) species that is formed
from 7 and NaI. This five-coordinate species either reductively
eliminates I-N(SiMe3)2 in analogy to the reaction known for a
46
47
Reduced low-coordinate β-diketiminate complexes often
abstract halide atoms from sp2 and sp3 carbon atoms.
Ni(IV) complex,
or loses •N(SiMe3)2
as observed for
43
lanthanide complexes.48 In the latter scenario the formation of I-
N(SiMe3)2 and H-N(SiMe3)2 can be explained by abstraction of an
I atom from Fe, and H atom from the THF by •N(SiMe3)2,
respectively. In the scenario that I-N(SiMe3)2 is reductively
eliminated from Fe(III), the formation of H-N(SiMe3)2 arises from
H atom abstraction by •N(SiMe3)2 , which is formed by reaction of
I-N(SiMe3)2 with Fe or due its decomposition at room
temperature.45
Considering the significantly less negative reduction potential of
5, we wanted to evaluate its competence in the activation of C-X
(X = halogen bonds). Addition of alkyl and aryl iodides to solutions
of 5 results in rapid formation of iron(II) iodide-amide complex 6,
which can also be prepared by reaction of 5 with 0.5 equiv of I2 or
by addition of 1 equiv NaI to solutions of 2 or 3 (Scheme 3).
Addition of trityl chloride to a solution of 5 showed formation of
triphenylmethyl radical (observed as Gomberg's dimer)44 and the
chloride analogue of 6.
Accessing the ferric oxidation state was possible by a
reaction of 2 or 3 with 0.5 equiv of I2 giving complex 7, which has
a high-spin iron(III) (S = 5/2) configuration (see Supporting
Information). Accordingly, we expected that treatment of 5 with
one equiv of I2, or 6 with 0.5 equiv of I2 in THF, would also give 7.
However, these reactions result in the formation of complex 8,
which still has a high-spin iron(II) (S = 2) center, in 82% isolated
yield. The only additional reaction product in the oxidations of 5
and 6 is NaI. To test our hypothesis that complex 7 is formed in
In contrast, the β-diketiminate analogue of complex 7 (see
Supplementary Information) does not react with stoichiometric or
excess NaI. This difference suggests that the formazanate ligand
is beneficial for N-I elimination. Both of the potential pathways,
reductive elimination and Fe-N homolysis, are thermodynamically
more favorable when the lower oxidation state is favored,
indicating that the substitution of diketiminate for formazanate
successfully modulates the electron density of the iron center in a
way that enables new reactivity.
these reactions but undergoes
a decomposition pathway
Conclusions
involving NaI we modified the reaction conditions to enable
monitoring by 1H NMR spectroscopy. Indeed, the addition of NaI
to a THF-d8 solution of complex 7 gave 8 in 65% spectroscopic
yield together with 6. Similarly, the addition of I2 to a THF-d8
solution of complex 5 gave 8 in 70 % spectroscopic yield together
with 6. In addition, formation of varying amounts of I-N(SiMe3)2
and H-N(SiMe3)2 were observed in both reactions. The presence
of both compounds was confirmed by spiking the mixture with
commercial H-N(SiMe3)2 and independently synthesized I-
N(SiMe3)2.45
This manuscript has described the synthesis of low-coordinate
iron formazanate complexes that can reversibly undergo
reduction at a less negative potential compared to analogous iron
β-diketiminate species. Distinguishing the high-spin iron(I)
description from the antiferromagnetically coupled iron(II)-ligand
radical description is challenging, and the distinctive absorption
spectra support the latter model. Moreover, multiconfigurational
calculations support the presence of a substantial character of
iron(II) with a formazanate radical dianion. Thus, in contrast with
the earlier reported bis(formazanate) complexes that have
predominant iron(I) character,18 these highly electron-rich amido
complexes have electron density on the supporting ligand. To our
knowledge, 5 is the first well-characterized complex of a redox-
active metal with a formazanate radical dianion. Despite the less
negative reduction potential, 5 activates carbon halogen bonds
mimicking the reactivity of low valent β-diketiminate iron
complexes. In an important difference, the less-electron donating
formazanate does not stabilize the higher ferric oxidation state,
and therefore a different reaction pathway is followed, leading to
N-I elimination.
p-tol
p-tol
SiMe3
Na
Na
I
N
N
N
N
N
N
N
Fe-
N
N
SiMe3
SiMe3
N
SiMe3
I2
Fe
H
I
I
N
5
8
Me3Si
SiMe3
- R
1
/
2 I2
NaI
or
I
R
p-tol
p-tol
Na
N
N
N
N
N
N
1
2 I2
/
N
N
Fe
Fe
I
- NaI
Acknowledgements
Me3Si
Me3Si
I
N
N
6
7
This work was supported by The Netherlands Organization for
Scientific Research (Rubicon Postdoctoral Fellowship 680-50-
1517 to D.L.J.B.) and the National Institutes of Health (Grant GM-
065313 to P.L.H.). K.M. L. thanks the A. P. Sloan Foundation and
the National Science Foundation (CHE-1454455) for funding.
P.L.H. thanks the Alexander von Humboldt Foundation for
support. We thank Prof. Gary Brudvig for access to an EPR
spectrometer.
SiMe3
SiMe3
1
I
/2 I2
Na
2 or 3
Scheme 3. Reactivity of complex 5 and 7, and independent synthesis of
reaction products.
In the reaction of 7 with NaI an intermediate with very broad
resonances is observed, and the apparent concentration is more
Conflict of interest
The authors declare no competing financial interest.
This article is protected by copyright. All rights reserved.

10.1002/chem.201801298
Chemistry - A European Journal
FULL PAPER
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10.1002/chem.201801298
Chemistry - A European Journal
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Title
Low-coordinate iron formazanate complexes undergo reversible ligand-centered
reduction, which is possible at milder potentials than related iron complexes with
non-redox-active β-diketiminate ligands. The ligand-centered radical avoids iron(I),
yet retains the capability to abstract halide atoms.
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