Oxidation and reduction of bis(imino)pyridine iron dinitrogen complexes: Evidence for formation of a chelate trianion.

Article abstract of DOI:10.1021/ic301675t

Oxidation and reduction of the bis(imino)pyridine iron dinitrogen compound, (^iPrPDI)FeN₂ (^iPrPDI = 2,6-(2,6- ⁱPr₂-C₆H₃-N=CMe)₂C ₅H₃N) has been examined to determine whether the redox events are metal or ligand based. Treatment of (^iPrPDI)FeN ₂ with [Cp₂Fe][BAr^F₄] (BAr ^F₄ = B(3,5-(CF₃)₂-C ₆H₃)₄) in diethyl ether solution resulted in N₂ loss and isolation of [(^iPrPDI)Fe(OEt ₂)][BAr^F₄]. The electronic structure of the compound was studied by SQUID magnetometry, X-ray diffraction, EPR and zero-field ⁵⁷Fe M?ssbauer spectroscopy. These data, supported by computational studies, established that the overall quartet ground state arises from a high spin iron(II) center (S_Fe = 2) antiferromagnetically coupled to a bis(imino)pyridine radical anion (S_PDI = 1/2). Thus, the oxidation event is principally ligand based. The one electron reduction product, [Na(15-crown-5)][(^iPrPDI)FeN₂], was isolated following addition of sodium naphthalenide to (^iPrPDI)FeN₂ in THF followed by treatment with the crown ether. Magnetic, spectroscopic, and computational studies established a doublet ground state with a principally iron-centered SOMO arising from an intermediate spin iron center and a rare example of trianionic bis(imino)pyridine chelate. Reduction of the iron dinitrogen complex where the imine methyl groups have been replaced by phenyl substituents, (^iPrBPDI)Fe(N₂)₂ resulted in isolation of both the mono-and dianionic iron dinitrogen compounds, [( ^iPrBPDI)FeN₂]^- and [(^iPrBPDI) FeN₂]^2-, highlighting the ability of this class of chelate to serve as an effective electron reservoir to support neutral ligand complexes over four redox states.

Full text of DOI:10.1021/ic301675t

Article
pubs.acs.org/IC
Oxidation and Reduction of Bis(imino)pyridine Iron Dinitrogen
Complexes: Evidence for Formation of a Chelate Trianion.
Aaron M. Tondreau,† S. Chantal E. Stieber,† Carsten Milsmann,† Emil Lobkovsky,‡
Thomas Weyhermuller,§ Scott P. Semproni,† and Paul J. Chirik*,†
̈
†Department of Chemistry, Princeton University, Princeton, New Jersey, 08544, United States
‡Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York, 14853, United States
§Max-Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, D-45470 Mulheim an der Ruhr, Germany
̈
S
* Supporting Information
ABSTRACT: Oxidation and reduction of the bis(imino)-
pyridine iron dinitrogen compound, (iPrPDI)FeN2 (iPrPDI =
2,6-(2,6-iPr2−C6H3−NCMe)2C5H3N) has been examined
to determine whether the redox events are metal or ligand
based. Treatment of (iPrPDI)FeN2 with [Cp2Fe][BArF ]
4
(BArF = B(3,5-(CF3)2-C6H3)4) in diethyl ether solution
4
resulted in N2 loss and isolation of [(iPrPDI)Fe(OEt2)]-
[BArF ]. The electronic structure of the compound was
4
studied by SQUID magnetometry, X-ray diffraction, EPR and
zero-field 57Fe Mossbauer spectroscopy. These data, supported
̈
by computational studies, established that the overall quartet ground state arises from a high spin iron(II) center (SFe = 2)
antiferromagnetically coupled to a bis(imino)pyridine radical anion (SPDI = 1/2). Thus, the oxidation event is principally ligand
based. The one electron reduction product, [Na(15-crown-5)][(iPrPDI)FeN2], was isolated following addition of sodium
naphthalenide to (iPrPDI)FeN2 in THF followed by treatment with the crown ether. Magnetic, spectroscopic, and computational
studies established a doublet ground state with a principally iron-centered SOMO arising from an intermediate spin iron center
and a rare example of trianionic bis(imino)pyridine chelate. Reduction of the iron dinitrogen complex where the imine methyl
groups have been replaced by phenyl substituents, (iPrBPDI)Fe(N2)2 resulted in isolation of both the mono- and dianionic iron
dinitrogen compounds, [(iPrBPDI)FeN2]− and [(iPrBPDI)FeN2]2‑, highlighting the ability of this class of chelate to serve as an
effective electron reservoir to support neutral ligand complexes over four redox states.
dinitrogen compounds and related neutral ligand deriva-
tives.20−22 Spectroscopic and computational studies on the
five-coordinate compound, (iPrPDI)Fe(N2)2 (iPrPDI = 2,6-
(2,6-iPr2−C6H3−NCMe)2C5H3N) established a highly co-
valent molecule best described as a hybrid between Fe(0) and
Fe(II) oxidation states with resonance between the neutral and
the closed-shell singlet dianionic form of the ligand (Figure 1).
This electronic structure is similar to that established for the
corresponding iron dicarbonyl, (iPrPDI)Fe(CO)2.20,23
INTRODUCTION
■
Understanding the electronic structure of iron dinitrogen
complexes is of fundamental and long-standing interest given
the role of the base metal in industrial ammonia synthesis
catalysts, its ubiquity in the active site in the nitrogenase family
of enzymes, and recent demonstration of solution-phase N2
hydrogenation and cleavage.1,2 The emergence of bis(imino)-
pyridine iron dinitrogen compounds as precatalysts for olefin
hydrogenation,3−6 alkene3,7 and ketone hydrosilylation,8 intra-9
and intermolecular10 [2π + 2π] cycloaddition, and hydro-
genative enyne and diyne cyclization11 has prompted interest in
determining the mechanism of action of this class of
compounds. Before the fundamental steps that comprise
catalytic cycles can be understood, the electronic structure of
various types of iron dinitrogen compounds needs to be firmly
established.
Dissociation of one of the N2 ligands yields (iPrPDI)FeN2, a
compound with a distinct electronic structure from (iPrPDI)-
Fe(N2)2.21 This molecule is best described as an intermediate
spin ferrous compound antiferromagnetically coupled to a
bis(imino)pyridine triplet diradical dianion. A low-lying
paramagnetic excited state is thermally accessible at ambient
1
temperature and gives rise to dispersed H NMR chemical
shifts.21,24 Related four-coordinate compounds with principally
One notable feature of bis(imino)pyridines12 is their ability
to directly participate in the electronic structure of both
coordination and organometallic compounds, acting either as
redox noninnocent, π-acceptors or redox-active, radical-based
ligands.13−19 In iron chemistry, our laboratory has established
the electronic structures of the four- and five-coordinate
σ-donating ligands such as (iPrPDI)Fe(DMAP) (DMAP = 4-
25
N,N-(dimethylamino)pyridine) and (iPrPDI)FeNH3 are also
Received: July 30, 2012
© XXXX American Chemical Society
A
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Figure 1. Electronic structures of (iPrPDI)Fe(N2)2, (iPrPDI)FeN2 and (iPrPDI)FeNH3.
Figure 2. Electronic structures of redox series of bis(imino)pyridine cobalt dinitrogen compounds and iron carbonyl derivatives.
intermediate spin ferrous compounds antiferromagnetically
coupled to bis(imino)pyridine triplet diradical dianions.
However, mixing of the paramagnetic excited state into the
ground state by spin−orbit coupling gives rise to temperature
independent paramagnetism and different NMR spectroscopic
properties than (iPrPDI)FeN2.
Gambarotta and co-workers have independently reported
additional synthetic studies on the reduction of (iPrPDI)FeCl2
with various amounts of NaH in the presence of N2. This work
has expanded the number of known structural types in this
family of iron dinitrogen compounds.26 Addition of three
equivalents of NaH yielded an anionic iron dinitrogen
compound where the bis(imino)pyridine has been deproto-
nated and the sodium is coordinated side-on to the terminal N2
ligand. Increasing the number of equivalents of NaH used
resulted in isolation and structural characterization of different
anionic dinitrogen complexes with varying Na solvation and
degrees of bis(imino)pyridine modification.
the synthesis and characterization of a series of bis(imino)-
pyridine cobalt dinitrogen complexes27 and a family of
bis(imino)pyridine iron dicarbonyl complexes that differ by
three oxidation states (Figure 2).23
In the iron series, the presence of weaker field N2 ligands
may give rise, as in the neutral cases, to electronic structures
distinct from the iron dicarbonyl family of compounds. Outside
the realm of bis(imino)pyridine compounds, Peters and co-
workers have reported a series of trigonal bipyramidal iron
dinitrogen derivatives in three formal oxidation states (0, I, and
II) and explored N2 functionalization chemistry from these
platforms.28 Here we describe the oxidation and reduction of
(iPrPDI)FeN2 and the reduction of the phenyl-substituted
analog, (iPrBPDI)Fe(N2)2 (iPrBPDI = 2,6-(2,6-iPr2−C6H3−N
CPh)2C5H3N). The electronic structures of all of the
compounds in this study were elucidated using a combination
of spectroscopic, X-ray diffraction and computational techni-
ques, and a rare example of a trianionic bis(imino)pyridine
ligand has been identified.
The establishment of the electronic structures of the neutral
four- and five-coordinate bis(imino)pyridine iron dinitrogen
complexes, (iPrPDI)Fe(N2)2 and (iPrPDI)FeN2, raised interest
in the oxidation and reduction of these compounds. In addition
to the fundamental question as to whether the redox events are
metal or ligand based, synthetic efforts of this type may also
yield new catalyst precursors. Recently our laboratory reported
RESULTS AND DISCUSSION
■
Oxidation and Reduction of Bis(imino)pyridine Iron
Dinitrogen Compounds. Our studies commenced with the
oxidation of the neutral bis(imino)pyridine iron dinitrogen
B
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complex, (iPrPDI)FeN2. Treatment of a diethyl ether solution of
Table 1. The crystallographic data establish formation of the
bis(imino)pyridine iron diethyl ether cation where the oxygen
atom is lifted by 41° out of the metal-chelate plane. There are
no close contacts between the cation and anion, suggesting that
such interactions are not the cause of the distortion from
planarity. Infrared spectroscopy on the bulk material revealed
no strong bands assignable to N2 stretches, consistent with the
crystal being representative of the bulk product. The distortions
to the chelate are modest yet are more consistent with a one
electron reduced rather than neutral bis(imino)pyridine as
evidenced by the Nimine−Cimine distances of 1.301(4) and
1.301(4) Å and the Cimine−Cipso bond lengths of 1.441(4) and
1.448(4) Å. The Fe−Nimine distances of 2.109(2) and 2.106(3)
are elongated and are diagnostic for a high spin iron compound,
consistent with the magnetic data (vide infra). The observation
of a bis(imino)pyridine radical anion and paramagnetic, high
spin iron center are consistent with an SFe = 2 ferrous
compound.
(iPrPDI)FeN2 with [Cp2Fe][BArF ] resulted in gas evolution
4
and formation of a light green solution from which [(iPrPDI)-
Fe(OEt2)][BArF ] was isolated as light green powder in 73%
4
yield following recrystallization at −35 °C (eq 1). Note that the
iron compounds are depicted in their traditional form until the
data have been presented to establish the electronic structure of
the molecule and role of the bis(imino)pyridine ligand.
The solid-state structure of [(iPrPDI)Fe(OEt2)][BArF ] was
To further elucidate the electronic structure of [(iPrPDI)Fe-
4
(OEt2)][BArF ], the compound was studied by zero-field 57Fe
determined by single-crystal X-ray diffraction and a representa-
tion of the cationic iron portion of the molecule is presented in
Figure 3. Selected bond distances and angles are reported in
4
Mossbauer spectroscopy. The spectrum recorded in the solid
̈
state at 80 K exhibits a quadrupole doublet (Figure 4) with an
Figure 3. Molecular structure of the cation of [(iPrPDI)Fe(OEt2)]-
Figure 4. Zero-field 57Fe Mossbauer spectrum of [(iPrPDI)Fe(OEt )]-
̈
[BArF ] at 30% probability ellipsoids. Anion and hydrogen atoms
2
[BArF ] at 80 K.
4
4
omitted for clarity. A depiction of the full molecule is presented in the
Supporting Information.
isomer shift of 1.13 mm/s and ΔEQ = 2.51 mm/s, consistent
with a high spin iron(II) compound. The isomer shift is one of
the highest observed in bis(imino)pyridine iron chemistry and
highlights the weak ligand field imparted by the monoreduced
form of the chelate and the diethyl ether ligand.
Table 1. Selected Bond Distances (Å) and Angles (deg) for
[(iPrPDI)Fe(OEt2)][BArF ]
4
The magnetic properties of [(iPrPDI)Fe(OEt2)][BArF ] were
4
Fe(1)−N(1)
Fe(1)−N(2)
Fe(1)−N(3)
Fe(1)−O(1)
N(1)−C(2)
N(3)−C(8)
N(2)−C(3)
N(2)−C(7)
C(2)−C(3)
C(7)−C(8)
N(1)−Fe(1)−N(2)
N(1)−Fe(1)−N(3)
N(2)−Fe(1)−N(3)
N(2)−Fe(1)−O(1)
2.109(2)
1.954(2)
2.106(3)
2.015(2)
1.301(4)
1.301(4)
1.349(4)
1.357(4)
1.441(4)
1.448(4)
76.89(10)
145.53(10)
77.39(10)
138.12(10)
studied by variable temperature solid-state SQUID magneto-
metry (see the Supporting Information). At temperatures above
40 K, the data plateau at 4.4 μB, slightly higher than the spin
only value for three unpaired electrons, consistent with an S =
3/2 ground state and a high spin Fe(II) center antiferromag-
netically coupled to a monoreduced bis(imino)pyridine radical.
The benzene-d6 1H NMR spectrum of [(iPrPDI)Fe(OEt2)]-
[BArF ] exhibited peaks only for the anion; no resonances
4
assignable to the bis(imino)pyridine iron cation were observed.
The 19F NMR spectrum exhibited a single sharp peak centered
at 62.30 ppm suggesting little interaction of [BArF ]− with the
4
paramagnetic iron center in solution. The lack of observable
NMR resonances prompted further investigation by EPR
spectroscopy. Attempts to record the spectrum in a toluene
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glass at 77 K did not yield reliable data due to precipitation of
the iron compound. Use of a diethyl ether glass obviated these
complications. A representative EPR spectrum recorded at 77 K
along with the simulation of the data are presented in Figure 5.
are reported in Table 2. For [Na(THF)n][(iPrPDI)Fe(N2)], the
two bands likely arise from two different compounds with
Table 2. Solid-State (KBr) Infrared Stretching Frequencies
(in cm−1) of Various Bis(imino)pyridine Iron Dinitrogen
Anions
compd
ν(N2) (cm−1
)
a
(
iPrPDI)Fe(N2)
2036
[Na(THF)n][(iPrPDI)Fe(N2)]
1966, 1917
1956
[Na(15-crown-5)(THF)2][(iPrPDI)Fe(N2)]
[Na(18-crown-6)(THF)2][(iPrPDI)Fe(N2)]
[Na(THF)n][(iPrBPDI)Fe(N2)]
1949
1979, 1932
1971
[Na(18-crown-6)(THF)2][(iPrBPDI)Fe(N2)]
[Na(THF)n]2[(iPrBPDI)Fe(N2)]
[Na(18-crown-6)(THF)2]2[(iPrBPDI)Fe(N2)]
1908, 1830
1887
a
Data taken from ref 3.
different sodium coordination modes, similar to observations
reported by Gambarotta and co-workers.26 To confirm that the
bis(imino)pyridine ligand was intact and did not undergo
deprotonation,30−33 we treated the mixture of bis(imino)-
pyridine iron dinitrogen anions, in separate experiments, with
either Me3SiCl or [Cp2Fe][BPh4]. In both cases, (iPrPDI)FeN2
was obtained as the sole iron compound, suggesting that the
bis(imino)pyridine chelate was not modified during the
reduction.
Figure 5. X-band EPR spectrum of [(iPrPDI)Fe(OEt2)][BArF ]
recorded at 77 K in diethyl ether glass (black). Simulated spectrum
(red): gx = 2.11, gy = 2.22, gz = 2.03 (gavg = 2.12), D = 9.6 cm−1 and E/
D = 0.33.
4
Because of the complications of variable sodium coordination
environments, characterization of the mixture of [Na(THF)n]-
[(iPrPDI)Fe(N2)] compounds was not undertaken. Even if
these compounds were fully characterized, interaction of the
sodium cation with different portions of the bis(imino)pyridine
ligand would render interpretation of redox-activity of the
chelate ambiguous. To obviate these complications, two
different strategies were pursued. The first involved reduction
of (iPrPDI)FeN2 with magnesium activated with either
anthracene or isoprene. Both activation methods resulted in
rapid reduction of (iPrPDI)FeN2; however, multiple N2
stretches were observed by infrared spectroscopy, diagnostic
of various interactions with the bis(imino)pyridine and
dinitrogen ligands.
Because changing the nature of the reductant did not result
in an anionic iron dinitrogen complex devoid of interactions
with the cation, methods for the encapsulation of the sodium
were pursued. The anionic iron dinitrogen compound,
[Na(THF)n][(iPrPDI)Fe(N2)], was generated in THF solution,
filtered through Celite and then treated with either 15-crown-5
or 18-crown-6 to yield [Na(15-crown-5)(THF)2][(iPrPDI)Fe-
(N2)] and [Na(18-crown-6)(THF)2][(iPrPDI)Fe(N2)], respec-
tively, as dark brown solids (eq 2).
A rhombic signal, consistent with an S = 3/2 complex, was
observed. The spectrum was successfully simulated using an S =
3/2 spin Hamiltonian formalism using the large ZFS parameter
D = 9.6 cm−1 (D ≫ hν ≈ 0.3 cm−1 at X-band) consistent with
the magnetic susceptibility measurements, full rhombicity E/D
= 0.33 and gx = 2.11, gy = 2.22, gz = 2.03 (gavg = 2.12). A
distribution of the rhombicity (E/D-strain, σ(E/D) = 0.035)
was imposed to model the line broadening resulting from
microheterogeneities in the sample. The anisotropic g values
support a majority contribution of metal-centered spin density
to the total spin of the molecule which is in good agreement
with the electronic structure description as a high-spin Fe(II)
center antiferromagnetically coupled to a bis(imino)pyridine
radical anion. The analogous cobalt complex, [(iPrPDI)Co-
(OEt2)]+, was previously reported29 and is a diamagnetic
molecule resulting from antiferromagnetic coupling of a low
spin Co(II) center to a bis(imino)pyridine anion.
The successful oxidation of (iPrPDI)FeN2 prompted studies
into the corresponding reduction chemistry. In principle, cyclic
voltammetry would provide insight into both oxidative and
reductive events associated with (iPrPDI)FeN2. However,
dissolving (iPrPDI)FeN2 in THF, a common solvent for such
measurements, results in immediate N2 dissociation and
formation of (iPrPDI)Fe(THF)n.3,4 Accordingly, chemical
methods were explored exclusively. As with (iPrPDI)Fe(CO)2,23
the reduction of (iPrPDI)FeN2 proved more challenging than
the corresponding oxidation. Treatment of a THF solution of
(iPrPDI)FeN2 with 1.05 equiv of sodium naphthalenide
followed by solvent removal and washing with diethyl ether
furnished a dark red brown solid identified as [Na(THF)n]-
[(iPrPDI)Fe(N2)]. Analysis of the product mixture by solid-
state (KBr) infrared spectroscopy revealed two bands, centered
at 1966 and 1917 cm−1. All infrared stretching frequencies for
the anionic iron dinitrogen compounds prepared in this work
The syntheses of [Na(15-crown-5)(THF)2][(iPrPDI)Fe-
(N2)] and [Na(18-crown-6)(THF)2][(iPrPDI)Fe(N2)] in the
D
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presence of excess THF are notable when compared to the
corresponding neutral and cationic compounds. Oxidation of
(iPrPDI)Fe(N2)2 in diethyl ether resulted in immediate
dinitrogen dissociation to form the corresponding iron diethyl
ether complex. The neutral bis(imino)pyridine iron compound
also undergoes rapid substitution of coordinated N2 upon
dissolution in THF to form (iPrPDI)Fe(THF)2 (see the
Supporting Information).3,4 The anionic iron dinitrogen
compounds are prepared in THF and show no evidence for
N2 displacement following reduction. The reduced compounds
have a preference for coordination of π-acids, likely a
consequence of the electronic structure of the anions. Both
compounds exhibit single strong NN bands centered at 1956
(15-crown-5) and 1949 (18-crown-6) cm−1 in the solid state.
These values are reduced from the stretching frequency of 2036
cm−1 reported for the neutral four-coordinate iron dinitrogen
complex, (iPrPDI)FeN2.3,21 The solid-state magnetic suscepti-
bility of both anionic iron dinitrogen compounds were
measured using a magnetic susceptibility balance at 23 °C
and values of 1.9 μB (15-crown-5) and 1.8 μB (18-crown-6)
were obtained and are consistent with S = 1/2 compounds.
The solid-state structure of [Na(15-crown-5)(THF)2]-
[(iPrPDI)Fe(N2)] was determined by X-ray diffraction and a
representation of the anionic portion of the molecule is
presented in Figure 6. A depiction of the full molecule is
Table 3. Selected Bond Distances (Å) and Angles (deg) for
[Na(15-crown-5)(THF)2] [(iPrPDI)Fe(N2)]
Fe(1)−N(1)
Fe(1)−N(2)
Fe(1)−N(3)
Fe(1)−N(4)
N(1)−C(2)
N(3)−C(8)
N(2)−C(3)
N(2)−C(7)
C(2)−C(3)
1.8858(18)
1.8338(17)
1.8912(18)
1.784(2)
1.366(3)
1.375(3)
1.391(3)
1.389(3)
1.405(3)
1.400(3)
1.089(3)
80.82(8)
161.68(7)
80.91(8)
161.68(7)
C(7)−C(8)
N(4)−N(5)
N(1)−Fe(1)−N(2)
N(1)−Fe(1)−N(3)
N(2)−Fe(1)−N(3)
N(2)−Fe(1)−N(4)
this study are reported in Table 4. The experimentally
determined isomer shift (δ) of 0.28 mm/s is lower than the
Table 4. Zero-Field 57Fe Mossbauer Spectral Parameters for
̈
Various Anionic Bis(imino)pyridine Iron Dinitrogen
Compounds and Cationic Diethyl Ether Complex,
[(iPrPDI)Fe(OEt2)n]+; Data for Neutral (iPrPDI)Fe(N2)2,
(iPrPDI)Fe(N2), and (iPrBPDI)Fe(N2)2 are Included for
a
Comparison6,22
compd
δ (mm/s)
ΔEQ (mm/s)
[(iPrPDI)Fe(OEt2)]+
1.13
0.39
0.38
0.28
0.43
0.30
0.15
2.51
−0.53
+1.72
1.66
(
iPrPDI)Fe(N2)2
iPrPDI)FeN2
(
[(iPrPDI)Fe(N2)]−
(
iPrBPDI)Fe(N2)2
1.91
[(iPrBPDI)Fe(N2)]−
[(iPrBPDI)Fe(N2)]2−
1.93
1.49
a
All spectra were recorded at 80 K.
values of 0.38 and 0.39 mm/s previously reported for the four-
and five-coordinate neutral compounds, (iPrPDI)FeN2 and
(iPrPDI)Fe(N2)2, indicating a higher degree of covalency
resulting from a more π-acidic bis(imino)pyridine.
The X-band EPR spectrum of [Na(15-crown-5)(THF)2]-
[(iPrPDI)Fe(N2)] was recorded in toluene glass at 77 K and the
experimental data along with the simulation are presented in
Figure 7. A rhombic signal is observed that is consistent with an
S = 1/2 system. The data were simulated using a spin
Hamiltonian and g-values of 1.95, 2.15 and 2.24, gavg = 2.11.
The large g anisotropy and the strong deviation of the gavg value
from the free-electron g = 2 support a system with metal-
centered spin. The signal at g = 2 is due to a small amount of
organic radical impurity.
Figure 6. Molecular structure of the iron anion of [Na(15-crown-
5)(THF)2][(iPrPDI)Fe(N2)] at 30% probability ellipsoids. Cation and
hydrogen atoms omitted for clarity. A depiction of the full molecule is
presented in the Supporting Information.
presented in the Supporting Information. A lower-quality,
disordered structure was also obtained for [Na(18-crown-
6)(THF)2][(iPrPDI)Fe(N2)] and is reported in the Supporting
Information. For the 15-crown-5 derivative, the cation does not
exhibit any close contacts to the iron anion. The geometry of
the iron anion is best described as idealized planar, similar to
neutral four-coordinate mono(dinitrogen) compounds.21 The
metrical parameters of the bis(imino)pyridine (Table 3) signal
significant chelate distortions as the Cimine−Cipso distances are
contracted to 1.405(3) and 1.400(3) Å and the Nimine−Cimine
distances are expanded to 1.391(3) and 1.389(3) Å. These
distortions are some of the most pronounced observed in an
bis(imino)pyridine iron compound and surpass the range
typically ascribed to two electron ligand reduction.3,14
On the basis of the precedent established by Gambarotta,26
we were unsure if deprotonation of the imine methyl groups in
the bis(imino)pyridine chelate would complicate the reduction
reactions.31 Parallel studies were therefore conducted with the
iron dinitrogen complex where the imine methyl groups were
replaced by phenyl substituents, (iPrBPDI)Fe(N2)2 (iPrBPDI =
2,6-(2,6-iPr2−C6H3NCPh)2C5H3N)). Previous studies have
also shown that this chelate is electron deficient compared to
the methylated variant and may more readily accept electron
density.4,34
The electronic structure of [Na(15-crown-5)(THF)2]-
[(iPrPDI)Fe(N )] was also studied by zero-field Mossbauer
̈
2
spectroscopy at 80 K. The experimentally determined
parameters for this and all of the iron complexes examined in
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Figure 7. X-band EPR spectrum of [Na(15-crown-5)(THF)2]-
[(iPrPDI)Fe(N2)] recorded in toluene glass at 77 K (black). Simulated
spectrum (red) with g values =1.95, 2.15, and 2.24 and a gaverage = 2.11.
Figure 8. X-band EPR spectrum of [Na(15-crown-5)(THF)2]-
[(iPrBPDI)Fe(N2)] recorded in toluene glass at 77 K (black).
Simulated spectrum (red) g-values of 1.97, 2.13, and 2.24, gaverage
=
2.11.
Addition of (iPrBPDI)Fe(N2)2 to a THF solution containing
1.05 equivalents of sodium naphthalenide resulted in a color
change from red to pink. Removing the volatiles in vacuo
followed by extraction into diethyl ether and recrystallization
from pentane at −35 °C resulted in isolation of pink-red
crystals identified as [Na(THF)n][(iPrBPDI)Fe(N2)]. Similar to
the methylated compound, analysis of the crystals by solid-state
infrared spectroscopy produced two NN bands at 1979 and
1932 cm−1, likely due to different interactions of the iron
compound with the sodium cation. These values are at slightly
higher frequency than for the methylated iron dinitrogen anion,
[(iPrPDI)Fe(N2)]−, as expected for the more electron with-
drawing [iPrBPDI] chelate.3,4
extreme sensitivity and poor solubility of the compound,
reliable solution magnetic moments could not be obtained.
Single crystals of [Na(THF)2][Na(OEt2)2][(iPrBPDI)Fe-
(N2)] were obtained from a diethyl ether-THF solution at
−35 °C. A representation of the molecule is presented in
Figure 9 and metrical data are reported in the Supporting
Encapsulation of the sodium cation was accomplished by
treating the THF derivative with 18-crown-6 and yielded a
bright pink solid identified as [Na(18-crown-6)(THF)2]-
[(iPrBPDI)Fe(N2)] in 73% yield. As with the methylated
compound, [Na(18-crown-6)(THF)2][(iPrBPDI)Fe(N2)] ex-
hibits a solid state magnetic moment of 1.7 μB, consistent
with an S = 1/2 ground state. The solid state infrared stretching
frequency of 1971 cm−1 is more reduced than the neutral
dinitrogen compound but at a higher frequency than the
corresponding anion with the methylated backbone. An EPR
spectrum was recorded at 77 K in frozen toluene glass and is
presented in Figure 8. The data were simulated for an S = 1/2
system using a spin Hamiltonian with g-values of 1.97, 2.13, and
2.24. The g-values and gavg = 2.11 are similar to those of the
methylated variant, suggesting metal-centered spins and similar
electronic environments for both compounds.
Figure 9. Solid-state structure of [Na(THF)2][Na(OEt2)(THF)]-
[(iPrBPDI)Fe(N2)] at 30% probability ellispoids. Hydrogen atoms
omitted for clarity.
Perhaps somewhat surprisingly, [(iPrBPDI)Fe(N2)]− could
be reduced by an additional electron. Stirring a THF solution of
the neutral dinitrogen complex, (iPrBPDI)Fe(N2)2, with three
equivalents of sodium metal in the presence of a catalytic
amount of naphthalene furnished the bis(imino)pyridine iron
dinitrogen dianion, [Na(THF)n]2[(iPrBPDI)Fe(N2)]. Strong
N2 stretches centered at 1908 and 1830 cm−1 were observed in
the solid-state infrared spectrum and proved valuable for
routine characterization of bulk samples. Although isolated
samples are diamagnetic as judged by magnetic susceptibility
balance, NMR spectroscopy proved uninformative as only THF
resonances were observed in THF-d8 solution. Because of the
Information. We note that the crystals are extremely sensitive
and challenging to handle and isolate in reasonable quantities.
The structural data establish a pseudo four-coordinate, idealized
planar bis(imino)pyridine iron dinitrogen compound with one
of the sodium cations coordinated to the C(11)−C(12)
positions of the pyridine with two molecules of THF
completing the coordination sphere. The second sodium cation
interacts with one of the pyridine imine portions of the chelate
and is solvated by one THF and one diethyl ether molecule.
Incorporation of the diethyl ether ligands is likely a
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consequence of the recrystallization conditions. This structure,
along with the previously reported structures of Gambarotta,26
confirms the variable coordination modes for sodium cations
with bis(imino)pyridine anions and further highlights the value
in encapsulation with crown ethers. The N(4)−N(5) distance
of 1.148(2) Å is consistent with little reduction by the iron
center.
As with other compounds reported in this work,
encapsulation of the sodium cations was accomplished by
treating the THF solution of the initially formed dianion with
18-crown-6 to furnish a diamagnetic, purple solid identified as
[Na(18-crown-6)(THF)2]2[(iPrBPDI)Fe(N2)] in 70% yield.
[Na(18-crown-6)(THF)2]2[(iPrBPDI)Fe(N2)] produced a sin-
gle strong band centered at 1887 cm−1 and as expected, is the
most reduced dinitrogen ligand in the series. The zero-field
57Fe Mossbauer spectrum was recorded at 80 K and exhibits an
̈
isomer shift of 0.15 mm/sec and a quadrupole splitting of 1.49
mm/s.
Computational Studies. The oxidation and reduction
products of the bis(imino)pyridine iron dinitrogen compounds
were studied by broken symmetry DFT calculations to gain
additional insight into the electronic structure of the
compounds. In the broken symmetry notation, BS(m, n)
describes a state in which there are m unpaired spin-up
electrons and n unpaired spin-down electrons on separate
fragments.35−37 Our studies commenced with computations on
the cationic iron diethyl ether complex, [(iPrPDI)Fe(OEt2)]+ at
the B3LYP level of DFT. Although the anion was omitted from
the calculation, no additional truncations were made. The
computed ground state obtained using a simple unrestricted
quartet approach corresponds to a broken symmetry (4,1)
solution and successfully reproduces the deviation of the diethyl
ether ligand from iron-chelate plane. The computed Mossbauer
̈
parameters of δ = 1.00 mm/s and ΔEQ = 2.75 mm/s are in
acceptable agreement with the experimentally determined
values of δ = 1.13 mm/s and ΔEQ = 2.51 mm/s. A qualitative
molecular orbital diagram and the computed spin density from
the BS(4,1) solution are presented in Figure 10 and are
consistent with a high spin iron(II) center (SFe = 2)
antiferromagnetically coupled to a bis(imino)pyridine radical
anion. The deviation of the diethyl ether ligand from planarity
is likely a result of the high spin state and population of the
2
2
d
x −y -based molecular orbital. Notably, the computational
studies are fully consistent with the experimental data and
firmly establish the electronic structure of the bis(imino)-
pyridine iron cation.
Figure 10. (a) Qualitative molecular orbital diagram for [(iPrPDI)Fe-
(OEt2)]+ from a spin-unrestricted B3LYP DFT calculation. (b) Spin
density plot obtained from a Mulliken population analysis (red,
positive spin density; yellow, negative spin density).
The computational results of the bis(imino)pyridine iron
dinitrogen monoanion were less straightforward. Using the
coordinates from the higher quality X-ray crystal structure,
[Na(15-crown-5)(THF)2][(iPrPDI)Fe(N2)] as the starting
point, spontaneous symmetry breaking in an unrestricted
doublet calculation resulted in an unusual BS(1,2) solution
corresponding to a low-spin Fe(I) center (SFe = 1/2) with a
triplet bis(imino)pyridine radical dianion (SPDI = 1). Anti-
ferromagnetic coupling (S = 0.40) between the metal dyz and
iPrPDI a2 molecular orbitals accounts for the overall doublet
ground state. In this description, the SOMO of the compound
is essentially ligand based with dominant b2 PDI character. If
this electronic structure description was indeed correct, the one
electron reduction of (iPrPDI2−)FeIIN2 would be iron-based,
preserving the triplet diradical bis(imino)pyridine dianion.
Comparisons of selected computed and experimental
Table 5, and a qualitative molecular orbital diagram and spin
density plot are presented in Figure 11. The BS(1,2) solution
slightly overestimates the experimental bond distances.
Notably, the Cimine−Cipso distances are computed to be 1.423
Å, consistent with a two-electron reduced bis(imino)pyridine
ligand14,20 but longer than the experimental values of 1.400(3)
and 1.405(3) Å. This computational solution does, however,
successfully reproduce the Mossbauer isomer shift and possibly
̈
the quadrupole splitting. Recall that in a zero-field Mossbauer
̈
experiment only the absolute value of the quadrupole splitting
is measured. This description is also inconsistent with the
observed EPR spectrum of [Na(15-crown-5)(THF)2][(iPrPDI)-
selected metrical and Mossbauer parameters are presented in
̈
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Table 5. Comparison of Experimental and Computed
Metrical and Mossbauer Parameters for [(iPrPDI)Fe(N )]−
̈
2
for BS(1,2) and BS(2,1) Solutions; Bond Distances
Reported in (Å)
parameter
Fe(1)−N(1)
experimental
BS(1,2)
BS(2,1)
1.8912(18)
1.8338(17)
1.8858(18)
1.784(2)
1.375(3)
1.389(3)
1.391(3)
1.366(3)
1.400(3)
1.405(3)
1.089
1.981
1.891
1.979
1.784
1.370
1.368
1.368
1.369
1.423
1.423
1.128
0.34
1.937
1.865
1.938
1.816
1.386
1.390
1.391
1.385
1.410
1.409
1.121
0.35
Fe(1)−N(2)
Fe(1)−N(3)
Fe(1)−N(4)
N(3)−C(8)
N(2)−C(7)
N(2)−C(3)
N(1)−C(2)
C(7)−C(8)
C(2)−C(3)
N(4)−N(5)
δ (mm/s)
0.28
ΔEQ (mm/s)
|1.66|
+1.68
−1.71
Fe(N2)] which supports an iron-based SOMO, in contrast to
the principally ligand-based radical predicted by the BS(1,2)
calculation.
Repeating the DFT calculation using the xyz coordinates
from the lower quality crystal structure of [Na(18-crown-
6)(THF)2][(iPrPDI)Fe(N2)] produced a different electronic
structure description. In this case, a BS(2,1) solution was
obtained, +0.37 kcal/mol higher in energy than the BS(1,2)
outcome described above. A qualitative molecular orbit diagram
and spin density plot for the BS(2,1) solution are presented in
Figure 12. This solution was also obtained by a calculation
starting from the high-quality structure using a BS(2,1)
approach that uses the orbitals of the lowest lying quartet
state as the initial guess for the orbitals of the doublet ground
state. Such a small energy difference renders the two solutions
indistinguishable on the basis of energetic arguments alone. In
this case, the computed structure is best described as an
intermediate spin ferrous compound with a rare example of a
triply reduced bis(imino)pyridine. A three-electron-reduced
bis(imino)pyridine with lithium counterions has been pre-
viously reported by Gambarotta, Budzelaar and co-workers38
and more recently by Bart and co-workers in a uranium
cyclopentadienyl compound.39 To the best of our knowledge, a
triply reduced bis(imino)pyridine has yet to be identified in
transition metal coordination chemistry. In the BS(2,1)
solution for [(iPrPDI)Fe(N2)]−, antiferromagnetic coupling
between a dyz orbital on the iron and the singly occupied a2
PDI orbital gives rise to the overall observed doublet ground
state. The b2 orbital of the PDI ligand is doubly occupied and
accordingly the SOMO of the compound is an iron-centered
Figure 11. (a) Qualitative molecular orbital diagram for [(iPrPDI)Fe-
(N2)]− from a spin-unrestricted, broken symmetry (1,2) B3LYP DFT
calculation. (b) Spin density plot obtained from a Mulliken population
analysis (red, positive spin density; yellow, negative spin density).
2
orbital of principally dz parentage. This computed SOMO is
consistent with the experimentally observed EPR spectrum that
exhibits relatively large g anisotropy, diagnostic of an iron-
centered radical. As noted above, the distortion to the
bis(imino)pyridine chelate obtained from the X-ray crystal
structure of [Na(18-crown-6)][(iPrPDI)Fe(N2)] are also
consistent with the published values for [PDI]3−.38 This
electronic structure description more accurately reproduces the
experimentally observed Cimine−Cipso distances. The computed
values of 1.410 and 1.409 Å are within the acceptable range of
the experimental distances of 1.400(3) and 1.405(3) Å, also
supporting the BS(2,1) solution as the more accurate
description of the electronic structure. In this view, the one
electron reduction of [(iPrPDI)Fe(N2)] to [(iPrPDI)Fe(N2)]1−
is principally ligand-based and preserves the intermediate spin
(SFe = 1) ferrous center.
The electronic structure of the dianionic bis(imino)pyridine
iron dinitrogen complex, [(iPrBPDI)Fe(N2)]2− was also
investigated computationally. The inability to obtain X-ray
quality single crystals of the crown ether variant, [Na(18-
crown-6)(THF)2]2[(iPrBPDI)Fe(N2)], coupled with the dia-
magnetism of the compound limits the amount of experimental
data available to validate the computational results. As shown
from the qualitative molecular orbital diagram and spin density
plot presented in Figure 13, a BS(1,1) solution was obtained
H
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Figure 12. (a) Qualitative molecular orbital diagram for [(iPrPDI)Fe-
(N2)]− from a spin-unrestricted, broken symmetry (2,1) B3LYP DFT
calculation. (b) Spin density plot obtained from a Mulliken population
analysis (red, positive spin density; yellow, negative spin density).
Figure 13. (a) Qualitative molecular orbital diagram for [(iPrBPDI)-
Fe(N2)]2− from a spin-unrestricted, broken symmetry (1,1) B3LYP
DFT calculation. (b) Spin density plot obtained from a Mulliken
population analysis (red: positive spin density, yellow: negative spin
density).
corresponding to a low-spin Fe(I) compound and a three-
electron-reduced bis(imino)pyridine. Thus, the one electron
reduction of [(iPrBPDI3−)FeII(N2)]1− to [(iPrBPDI3−)-
FeI(N2)]2− is metal-based where the intermediate spin ferrous
center is reduced to low spin Fe(I).
Electronic Structure Summary. The electronic structures
of the series of compounds prepared over four formal oxidation
states prepared in this work are summarized in Figure 14. From
neutral (iPrPDI2−)FeIIN2, both one-electron oxidation and
reduction are ligand-based as the iron(II) ion is preserved
and the redox events occur at the bis(imino)pyridine. The
reduction chemistry provided a rare example of a bis(imino)-
pyridine trianion. With the appropriate bis(imino)pyridine, two
I
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Figure 14. Electronic structure summary of bis(imino)pyridine iron dinitrogen and diethyl ether complexes.
was used for (elements of) the simulation and analysis of magnetic
susceptibility.41
electron reduction of the neutral iron dinitrogen compound is
chemically feasible and is the first compound in the series
where a metal-based redox event occurs. In considering the
entire series, it is noteworthy that three electron transfer events
take place before the metal engages in redox chemistry.
Comparing these results to the previously reported bis-
(imino)pyridine iron carbonyl series (Figure 2),23 highlights
the impact of both coordination number and identity of the
neutral ligand on the overall electronic structure of the
compounds. Oxidation of (iPrPDI)Fe(CO)2, a highly covalent
molecule best described as hybrid between Fe(0) and Fe(II),
yielded [(iPrPDI0)FeI(CO)2]+, where the bis(imino)pyridine
adopts its neutral form. The one-electron event to generate
[(iPrPDI2‑)FeI(CO)2]− also generates an iron(I) center where
the chelate is reduced by two electrons. Substitution of the
carbonyl ligands with dinitrogen or diethyl ether results in
compounds where the ligand is more reduced. For example,
upon oxidation to the diethyl ether cation, [(iPrPDI1−)-
FeII(OEt2)]+, the bis(imino)pyridine undergoes a one electron
oxidation and maintains radical character. Likewise, reduction
of [(iPrPDI2−)FeII(N)2] to [(iPrPDI3−)FeII(N2)]− is solely
ligand based and highlights that in the absence of other π-
acid ligands, the bis(imino)pyridine mitigates the redox
chemistry in the complex.
Single crystals suitable for X-ray diffraction were coated with
polyisobutylene oil in a drybox, transferred to a nylon loop and then
quickly transferred to the goniometer head of a Bruker X8 APEX2
diffractometer equipped with a molybdenum X-ray tube (λ = 0.71073
Å). Preliminary data revealed the crystal system. A hemisphere routine
was used for data collection and determination of lattice constants.
The space group was identified and the data were processed using the
Bruker SAINT+ program and corrected for absorption using SADABS.
The structures were solved using direct methods (SHELXS)
completed by subsequent Fourier synthesis and refined by full-matrix
least-squares procedures.
X-band EPR derivative spectra were recorded on a Bruker
ELEXSYS E500 spectrometer equipped with the Bruker standard
cavity (ER4102ST) and a helium flow cryostat (Oxford Instruments
ESR 910). Microwave frequencies were calibrated with a Hewlett-
Packard frequency counter (HP5353B), and the field control was
calibrated with a Bruker NMR field probe (ER035M). The spectra
were simulated with the program GFIT by Eckhard Bill for the
calculation of powder spectra with effective g values and anisotropic
line widths (Lorentzian line shapes were used).
Zero field 57Fe Mossbauer spectra were collected on a SEE Co.
̈
Mossbauer spectrometer (MS4) with a 57Co/Rh radiation source, at
̈
80 K in constant acceleration mode. The temperature in the sample
chamber was controlled by a Janis Research Co. CCS-850 He/N2
cryostat within an accuracy of 0.3 K. The data were calibrated
relative to an α-iron at 298, with minimum experimental line widths of
0.23 mm/s. The fitting procedure to extract quantitative spectral
parameters uses a least-squares Lorentzian fitting method using the
WMOSS software developed by SEE Co.
EXPERIMENTAL SECTION
■
General Considerations. All air- and moisture-sensitive manip-
ulations were carried out using standard vacuum line, Schlenk, and
cannula techniques or in an MBraun inert atmosphere drybox
containing an atmosphere of purified nitrogen. Solvents for air- and
moisture-sensitive manipulations were initially dried and deoxygenated
using literature procedures.40 Benzene-d6 was purchased from
Cambridge Isotope Laboratories and dried over 4 Å molecular sieves.
1H NMR spectra were recorded on Varian Mercury 300, Inova 400,
500, and 600 spectrometers operating at 299.76, 399.78, 500.62, and
599.78 MHz, respectively. 13C NMR spectra were recorded on an
Quantum-chemical calculations. All DFT calculations were
performed with the ORCA program package42 as described
previously.21,23 The geometry optimizations of the complexes and
single-point calculations on the optimized geometries were carried out
at the B3LYP level of DFT. This hybrid functional often gives better
results for transition metal compounds than pure gradient-corrected
functionals, especially with regard to metal−ligand covalency.
Throughout this paper we describe our computational results by
using the broken-symmetry (BS) approach by Ginsberg and
Noodleman.35,36 Because several broken symmetry solutions to the
spin-unrestricted Kohn−Sham equations may be obtained, the general
notation BS(m,n) has been adopted, where m (n) denotes the number
of spin-up (spin-down) electrons at the two interacting fragments.36
Canonical and corresponding orbitals, as well as spin density plots
were generated with the program Molekel.
Inova 500 spectrometer operating at 125.893 MHz. All H and 13C
1
NMR chemical shifts are reported relative to SiMe4 using the 1H
(residual) and 13C chemical shifts of the solvent as a secondary
standard. Peak widths at half heights are reported for paramagnetically
broadened and shifted resonances. Solution magnetic moments were
determined by Evans method using a ferrocene standard and are the
average value of at least two independent measurements. Gouy balance
measurements were performed with a Johnson Matthey instrument
that was calibrated with HgCo(SCN)4. Infrared spectra were collected
on a Thermo Nicolet spectrometer. Elemental analyses were
performed at Robertson Microlit Laboratories, Inc., in Ledgewood,
NJ. SQUID magnetization data of crystalline powdered samples were
recorded with a SQUID magnetometer (Quantum Design) at 10 kOe
between 5 and 300 K for all samples. Values of the magnetic
susceptibility were corrected for the underlying diamagnetic increment
using tabulated Pascal constants and the effect of the blank sample
holders (gelatin capsule/straw). The program julX, written by E. Bill,
Preparation of [(iPrPDI)Fe(OEt2)][BArF ]. A 20 mL scintillation
4
vial was charged with 0.140 g (0.236 mmol) of (iPrPDI)Fe(N2)2, 0.247
g (0.236 mmol) of [Cp2Fe][BArF ] and a stir bar. Approximately 7 mL
4
of diethyl ether were added to the mixture of solids with stirring. The
reaction evolved nitrogen gas immediately and changed color from
light to dark green. After gas evolution ceased, pentane was added and
the scintillation vial was stored at −35 °C. Over time a light green
solid precipitated. The solid was collected on a glass frit and washed
with 4 x ∼ 20 mL of pentane. The solid was dried in vacuo and yielded
0.250 g (73%) of a light green powder identified as [(iPrPDI)Fe-
J
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(OEt2)][BArF ]. Single crystals were obtained from a fluorobenzene-
of 0.120 g of (iPrBPDI)Fe(N2)2 in THF was added with stirring. The
resulting reaction mixture was filtered through Celite and the volatiles
were reduced in vacuo. The residue was extracted into diethyl ether,
filtered through Celite and the volume reduced by to approximately
half. An equal volume of pentane was added and the solution was
cooled to −35 °C. The solid was collected on a glass frit and washed
with pentane and dried under vacuum and furnished 0.110 g of a
product identified as [Na(THF)n]2[(iPrBPDI)Fe(N2)]. IR (KBr):
ν(N2) = 1908, 1830 cm−1.
4
pentane mixture stored at −35 °C. Anal, for C69H65N3FeBF24O: Calcd
C, 56.19; H, 4.44; N, 2.85. Found: C, 56.34; H, 4.67; N, 2.81. Solid-
state magnetic susceptibility (23 °C): μeff = 5.2 μB. 1H NMR (benzene-
d6): δ = 1.29 (569 Hz, Et2O), 4.12 (370 Hz, Et2O), 8.44 (18 Hz), 9.67
(12 Hz). 19F NMR (benzene-d6): δ = 62.30.
Preparation of [Na(THF)n][(iPrPDI)Fe(N2)]. A scintillation vial was
charged with 0.004 g (0.174 mmol) of sodium and 0.022 g (0.182
mmol) of naphthalene followed by approximately 5 mL of THF. The
solution was stirred until the sodium was completely dissolved. Once
this occurred, a solution containing 0.100 g (0.168 mmol) of
(iPrPDI)Fe(N2)2 in THF was added. The resulting reaction mixture
was filtered through Celite and the volatiles were removed in vacuo.
The residue was washed with diethyl ether and the solid was collected
on a glass frit and washed with pentane and dried under vacuum,
yielding 0.090 g of a product identified as [Na(THF)n][(iPrPDI)Fe-
(N2)]. IR (KBr): ν(N2) = 1966, 1914 cm−1.
Preparation of [Na(18-crown-6)(THF)2]2[(iPrBPDI)Fe(N2)]. A
scintillation vial was charged with 0.020 g (0.869 mmol) of sodium
and 0.125 g (0.958 mmol) of naphthalene followed by approximately
10 mL of THF. The solution was stirred until the sodium was
completely dissolved. Once this occurred, the sodium naphthalenide
solution was added dropwise to a solution containing 0.175 g (0.216
mmol) of (iPrBPDI)FeBr2 in THF. The resulting reaction mixture was
filtered through Celite and 0.115 g (0.435 mmol) of 18-crown-6 was
added and the volatiles were removed. The residue was dissolved in
THF and the product was precipitated with excess pentane and the
dark purple powder was collected on a glass frit and washed with
pentane and dried under vacuum, yielding 0.235 g (70%) of a product
identified as [Na(18-crown-6)(THF)2]2[(iPrBPDI)Fe(N2)]. Single
crystals were obtained from a concentrated diethyl ether solution
stored at −35 °C. Anal. Calcd for C83H127FeN5Na2O16: C, 64.20; H,
8.24; N, 4.51. Found: C, 63.96; H, 8.08; N, 3.92. IR (KBr): ν(N2) =
1887 cm−1.
Preparation of [Na(15-crown-5)(THF)2][(iPrPDI)Fe(N2)]. A
scintillation vial was charged with 0.006 g (0.261 mmol) of sodium,
0.037 g (0.287 mmol) of naphthalene and approximately 10 mL of
THF. The solution was stirred until the sodium was completely
dissolved. Once this occurred, the sodium naphthalenide solution was
added dropwise to a solution containing 0.155 g (0.261 mmol) of
(iPrPDI)Fe(N2)2 in THF. The resulting reaction mixture was filtered
through Celite and 0.060 g (0.274 mmol) of 15-crown-5 was added
and the volatiles were removed. The residue was dissolved in minimal
THF and crystallized at −35 °C, yielding yielding 0.104 g (42%) of a
product identified as [Na(15-crown-5)(THF)2][(iPrPDI)Fe(N2)].
Single crystals were obtained from a THF-pentane mixture stored at
−35 °C. Anal. Calcd for C51H79FeN5NaO7: C, 64.27; H, 8.36; N, 7.35.
Found: C, 63.99; H, 8.01; N, 7.04. Solid-state magnetic susceptibility
(23 °C): μeff = 1.9 μB. IR (KBr): ν(N2) = 1956 cm−1.
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic details for [(iPrPDI)Fe(OEt2)][BArF ], [Na-
4
(15-crown-5)(THF)2][(iPrPDI)Fe(N2)], [Na(18-crown-6)-
(THF)2][(iPrPDI)Fe(N2)], and [Na(THF)2][Na(OEt2)2]-
[(iPrBPDI)Fe(N2)] in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
Preparation of [Na(18-crown-6)(THF)2][(iPrPDI)Fe(N2)]. A
scintillation vial was charged with 0.012 g (0.522 mmol) of sodium,
0.075 g (0.575 mmol) of naphthalene, and approximately 10 mL of
THF. The solution was stirred until the sodium was completely
dissolved. Once this occurred, the sodium naphthalenide solution was
added dropwise to a solution containing 0.310 g (0.522 mmol) of
(iPrPDI)Fe(N2)2 in THF. The resulting reaction mixture was filtered
through Celite and 0.140 g (0.530 mmol) of 18-crown-6 was added
and the volatiles were removed. The residue was dissolved in THF and
the product was precipitated with excess pentane. The resulting dark
brown powder was collected on a glass frit and washed with pentane
and dried under vacuum, yielding 0.390 g (75%) of a product
identified as [Na(18-crown-6)(THF)2][(iPrPDI)Fe(N2)]. Single crys-
tals were obtained from a THF-pentane mixture stored at −35 °C.
Anal. Calcd for C53H81FeN5NaO8: C, 63.84; H, 8.39; N, 7.02. Found:
C, 63.77; H, 7.94; N, 6.71. Solid-state magnetic susceptibility (23 °C):
μeff = 1.8 μB. IR (KBr): ν(N2) = 1949 cm−1.
AUTHOR INFORMATION
Corresponding Author
*E-mail: pchirik@princeton.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the U.S. National Science Foundation and Deutsche
Forschungsgemeinschaft for a Cooperative Activities in
Chemistry between U.S. and German Investigators grant.
S.C.E.S. thanks the NSF for a graduate research fellowship
(DGE-0646086) for support. C.M. is grateful to the Alexander
von Humboldt Foundation for a postdoctoral research
fellowship. S.P.S. thanks the Natural Sciences and Engineering
Research Council of Canada for a predoctoral fellowship (PGS-
D).
Preparation of [Na(THF)n][(iPrBPDI)Fe(N2)]. This molecule was
prepared in a similar manner to [Na(THF)n][(iPrPDI)Fe(N2)] with
0.004 g (0.174 mmol) of sodium, 0.022 g (0.182 mmol) of
naphthalene, and a THF solution containing 0.120 g (0.168 mmol)
of (iPrBPDI)Fe(N2)2. This procedure furnished 0.083 g of a pink
powder identified as [Na(THF)n][(iPrBPDI)Fe(N2)]. IR (KBr): ν(N2)
= 1979, 1932 cm−1.
REFERENCES
Preparation of [Na(18-crown-6)(THF)2][(iPrBPDI)Fe(N2)]. This
molecule was prepared in a similar manner to [Na(18-crown-
6)(THF)2][(iPrPDI)Fe(N2)] with 0.012 g (0.522 mmol) of sodium,
0.075 g (0.575 mmol) of naphthalene, 0.140 g (0.174 mmol) of
(iPrBPDI)FeBr2 and 0.050 g (0.190 mmol) of 18-crown-6. This
procedure yielded 0.139 g (73%) of a bright pink powder identified as
[Na(18-crown-6)(THF)2][(iPrPDI)Fe(N2)]. Anal. Calcd for
C63H87FeN5NaO8: C, 67.49; H, 7.82; N, 6.25. Found: C, 67.55; H,
7.80; N, 6.21. Solid-state magnetic susceptibility (23 °C): μeff = 1.7 μB.
IR (KBr): ν(N2) = 1971 cm−1.
■
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