Oxidative addition of carbon-carbon bonds with a redox-active Bis(imino)pyridine iron complex

Article abstract of DOI:10.1021/ja306526d

Addition of biphenylene to the bis(imino)pyridine iron dinitrogen complexes, (^iPrPDI)Fe(N₂)₂ and [( ^MePDI)Fe(N₂)]₂(μ₂-N₂) (^RPDI = 2,6-(2,6-R₂-₆H₃=CMe) ₂C₅H₃N; R = Me, ⁱPr), resulted in oxidative addition of a C- bond at ambient temperature to yield the corresponding iron biphenyl compounds, (^RPDI)Fe(biphenyl). The molecular structures of the resulting bis(imino)pyridine iron metallacycles were established by X-ray diffraction and revealed idealized square pyramidal geometries. The electronic structures of the compounds were studied by Moessbauer spectroscopy, NMR spectroscopy, magnetochemistry, and X-ray absorption and X-ray emission spectroscopies. The experimental data, in combination with broken-symmetry density functional theory calculations, established spin crossover (low to intermediate spin) ferric compounds antiferromagnetically coupled to bis(imino)pyridine radical anions. Thus, the overall oxidation reaction involves cooperative electron loss from both the iron center and the redox-active bis(imino)pyridine ligand.

Full text of DOI:10.1021/ja306526d

Article
pubs.acs.org/JACS
Oxidative Addition of Carbon−Carbon Bonds with a Redox-Active
Bis(imino)pyridine Iron Complex
Jonathan M. Darmon,†,∥ S. Chantal E. Stieber,∥ Kevin T. Sylvester,† Ignacio Fernandez,‡ Emil Lobkovsky,†
́
Scott P. Semproni,∥ Eckhard Bill,§ Karl Wieghardt,*,§ Serena DeBeer,*,†,§ and Paul J. Chirik*,∥
†Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14850, United States
‡
́
́
Area de Química Organica, Universidad de Almería, Carretera de Sacramento s/n, 04120, Almería, Spain
§Max-Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, D-45470 Mulheim an der Ruhr, Germany
∥Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
̈
S
* Supporting Information
ABSTRACT: Addition of biphenylene to the bis(imino)-
pyridine iron dinitrogen complexes, (iPrPDI)Fe(N2)2 and
[(MePDI)Fe(N2)]2(μ2-N2) (RPDI = 2,6-(2,6-R2C6H3
NCMe)2C5H3N; R = Me, iPr), resulted in oxidative addition
of a CC bond at ambient temperature to yield the
corresponding iron biphenyl compounds, (RPDI)Fe-
(biphenyl). The molecular structures of the resulting bis-
(imino)pyridine iron metallacycles were established by X-ray
diffraction and revealed idealized square pyramidal geometries.
The electronic structures of the compounds were studied by Mossbauer spectroscopy, NMR spectroscopy, magnetochemistry,
̈
and X-ray absorption and X-ray emission spectroscopies. The experimental data, in combination with broken-symmetry density
functional theory calculations, established spin crossover (low to intermediate spin) ferric compounds antiferromagnetically
coupled to bis(imino)pyridine radical anions. Thus, the overall oxidation reaction involves cooperative electron loss from both
the iron center and the redox-active bis(imino)pyridine ligand.
ligands rather than the metal (Scheme 1). Heyduk and co-
workers have provided an illustrative example of this concept
with the formal addition of chlorine to a bis(amido)phenolate
zirconium(IV) compound to furnish the corresponding
dichloride and convincingly demonstrated that electron loss
occurs at each of the chelating ligands.11
INTRODUCTION
■
Oxidative addition is a fundamental transformation in organo-
metallic chemistry that is often a key bond activation step
responsible for introducing substrates into the metal coordina-
tion sphere during catalytic turnover.1 As exemplified by square
planar, d8 metal compounds such as Vaska’s complex,
(Ph3P)2Ir(CO)Cl, the traditional version of the reaction
involves a formal two-electron oxidation at the metal (e.g., d8
to d6; Scheme 1).2
The potential economic and environmental advantages of
iron have renewed interest in developing base metal catalysts as
alternatives to precious metals.12−18 Aryl-substituted bis-
(imino)pyridine iron dinitrogen compounds, (RPDI)Fe(N2)2
and [(RPDI)Fe(N2)]2(μ2-N2) (ArPDI = 2,6-(2,6-R2C6H3
Scheme 1. Oxidative Addition Reactions
i
NCMe)2C5H3N; R = Pr, Et, Me), have emerged as an
effective and versatile class of catalyst precursors for olefin and
alkyne hydrogenation,19−23 intra-24,25 and intermolecular
cyclization26 of unsaturated olefins as well as the regioselective,
anti-Markovnikov hydrosilylation of olefins with tertiary
silanes.27−29 Despite the high activity of these compounds in
olefin hydrogenation and hydrosilylation, many of the
mechanistic details surrounding catalytic turnover have yet to
be elucidated. The now well-established redox activity of the
bis(imino)pyridine chelate30,31 renders such studies more
challenging as fundamental transformations such as oxidative
Metal complexes with redox-active ligands, those where the
ligands participate directly in the electronic structures of the
molecules,3,4 have received renewed attention due to their
interesting electronic structures, role in metalloenzymes,5,6
ability to promote unusual group transfer reactivity,7,8 and
importance in base metal catalysis.9,10 Enabling redox chemistry
at the supporting ligands potentially changes the paradigm for
oxidative addition; electron loss could occur at the supporting
Received: July 10, 2012
Published: October 8, 2012
© 2012 American Chemical Society
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Scheme 2. Treatment of (iPrPDI)Fe(N2)2 with D2 and PhSiH3
Scheme 3. C−X Oxidative Addition to (iPrPDI)FeN2 and (MePDI)CoN2
addition and reductive elimination likely involve ligand-based
redox events.32
metric chemistry, addition of allyl ether to (iPrPDI)Fe(N2)2
produced an equimolar mixture of the iron allyl and iron
alkoxide complexes, (iPrPDI)Fe(η3-CH2CHCH2) and (iPrPDI)-
Fe(OCH2CHCH2), respectively, demonstrating that oxida-
tive chemistry can occur over two iron complexes (Scheme 3).
Extension of this chemistry to alkyl and vinyl halides allowed
synthesis of various bis(imino)pyridine iron alkyl complexes,
including those with β-hydrogens.34 Elucidation of the
electronic structures of both (iPrPDI1−)FeIIR (R = alkyl) and
(iPrPDI1−)FeIIX (X = Cl, Br) classes of compounds established
ferrous compounds with monoreduced, bis(imino)pyridine
radical anions.
Attempts to study the oxidative addition of catalytically
relevant substrates such as dihydrogen or PhSiH3 to one of the
most well-studied bis(imino)pyridine iron precatalysts,
(iPrPDI)Fe(N2)2, furnished the iron σ-complexes, (iPrPDI)Fe-
(η2-H2) and (iPrPDI)Fe(η2-PhSiH3)2, offering little direct
information about the oxidative addition process.19 Repeating
the reaction with D2 yielded (iPrPDI)Fe(η2-D2) with con-
comitant deuterium incorporation into the isopropyl methyl
positions of the 2,6-aryl substituents, suggesting that a net
oxidative addition−reductive elimination of C−H bonds was
operative (Scheme 2).
Formal oxidative addition of carbon−heteroatom bonds has
been observed with bis(imino)pyridine iron and cobalt
dinitrogen complexes. Carbon−oxygen bond cleavage in diallyl
ether and various esters has been identified as a catalyst
deactivation pathway during iron-catalyzed [2π + 2π] cyclo-
addition23 and olefin hydrogenation,33 respectively. In stoichio-
Studies into the electronic structures of (iPrPDI)Fe(N2)2 and
(iPrPDI)FeN2, two compounds in equilibrium in solution, have
established redox non-innocent and redox-active bis(imino)-
pyridine chelates, respectively.35,36 The five-coordinate com-
plex, (iPrPDI)Fe(N2)2, is a highly covalent molecule similar to
(iPrPDI)Fe(CO)2, where the oxidation state of the iron is best
described as a resonance hybrid between Fe(0) and Fe(II) with
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the bis(imino)pyridine ligand acting as a classical π-acceptor.
This electronic structure description is also applicable to the
dimeric bis(imino)pyridine iron dinitrogen compounds,
[(RPDI)Fe(N2)]2(μ2-N2), for which there is no evidence for
dissociation of N2 or formation of monomeric compounds in
solution or the solid state.22 In this bonding situation, the
bis(imino)pyridine is termed “redox non-innocent” in accord
with Jørgensen’s original definition of a case where the
covalency of the metal−ligand interaction creates ambiguity
over the metal oxidation state.37
pyridine chelate (to participate or not) to smoothly adjust to
the electronic requirements of a metal complex and a specific
redox process.
Although these studies are informative for understanding
electron transfer processes in reduced iron and cobalt
complexes with redox-active ligands, little is known about
oxidative addition reactions relevant to catalytic hydrogenation,
hydrosilylation, and hydrogenative cyclization reactions that
occur at a single iron center. To circumvent complications from
formation of σ-complexes, products with strong metal−carbon
bonds were targeted as a method to study oxidative addition
For the four-coordinate compound, (iPrPDI)FeN2, the iron is
best described as an intermediate spin iron(II) center
antiferromagnetically coupled to a triplet bis(imino)pyridine
diradical dianion. Unlike (iPrPDI)Fe(N2)2, the oxidation state of
the iron in (iPrPDI)FeN2 is unambiguously intermediate spin
ferrous. Accordingly, we use the term “redox active” to
distinguish this electronic structure from the description of
(iPrPDI)Fe(N2)2, as the oxidation state of the iron in the four-
coordinate compound is unambiguous.33 As the four-
coordinate compound, (iPrPDI2−)FeIIN2, predominates in
solution, the formal oxidative addition of alkyl halides may be
viewed as a ligand based process. The triplet diradical
bis(imino)pyridine in (iPrPDI2−)FeIIN2 undergoes a one-
electron oxidation to yield (iPrPDI1−)FeIIR and (iPrPDI1−)FeIIX
while the iron maintains the ferrous oxidation state.
In related chemistry with cobalt, Zhu and Budzelaar have
reported the oxidative addition of aryl halides with (MePDI)-
CoN2 to yield (MePDI)CoAr (Ar = substituted aryl) and
(MePDI)CoX (X = Cl, Br, I)38 and have presented evidence for
radical pathways.39 The electronic structure of the (RPDI)CoN2
family of compounds has been established as low-spin, d8
cobalt(I) derivatives with bis(imino)pyridine radical anions, e.g.
(MePDI1−)CoIN2, with the SOMO of the complex essentially
chelate-based.40 Because both (MePDI1−)CoIIAr and (MePDI1−)-
CoIIX are known Co(II) compounds antiferromagnetically
coupled to bis(imino)pyridine radical anions,41 the formal
oxidative addition process is solely metal based. This behavior is
opposite the oxidative addition process in iron chemistry as
electron loss occurs solely at the cobalt center and
demonstrates the flexibility of the redox-active bis(imino)-
reactions with (iPrPDI)Fe(N2)2 and related precatalysts. As
amply demonstrated by Jones and co-workers,42 biphenylene is
an attractive substrate for this purpose due to the presence of a
relatively weak C−C bond (BDE = 65.4 kcal/mol) coupled to
the formation of two strong metal−phenyl bonds. Accordingly,
well-defined C−C oxidative events have been reported for
iron,43 platinum,44 rhodium,45 and nickel.46 Here we describe
the oxidative addition of the C−C bond of biphenylene to
bis(imino)pyridine iron dinitrogen compounds to yield the
corresponding iron metallacycles. These studies highlight the
flexibility of the redox-active bis(imino)pyridine chelate to
enable catalysis by mitigation of otherwise likely high energy
Fe(IV) intermediates.
RESULTS AND DISCUSSION
■
Synthesis and Structure of Bis(imino)pyridine Iron
Biphenyl Compounds. Addition of 1 equiv of biphenylene
(per iron center) to a saturated pentane solution containing
either (iPrPDI)Fe(N2)2 or [(MePDI)Fe(N2)]2(μ2-N2) at am-
bient temperature resulted in C−C bond cleavage and yielded
Figure 1. Solid state structures for (iPrPDI)Fe(biphenyl) (left) and (MePDI)Fe(biphenyl) (right) at 30% ellipsoids. Only one representative molecule
(of two) from the asymmetric unit of the crystal containing (MePDI)Fe(biphenyl) is shown. All solvent molecules and hydrogen atoms are omitted
for clarity. Data for (iPrPDI)Fe(biphenyl) collected at 173 K; data for (MePDI)Fe(biphenyl) collected at 100 K.
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the corresponding bis(imino)pyridine iron biphenyl com-
pounds, (RPDI)Fe(biphenyl), as green powders (eq 1).
Monomeric iron dinitrogen complexes are depicted in eq 1
for clarity, and all bis(imino)pyridine iron complexes are drawn
in their traditional (formal oxidation state notation) form until
the electronic structure determinations are presented.
group through the iron-chelate plane at 23 °C. Similar behavior
has been reported for five-coordinate compounds with neutral
ligands such as (iPrPDI)Fe(N2)2 and (iPrPDI)Fe(CO)2 where
C2v symmetry is observed by NMR spectroscopy although
distorted square pyramidal structures are observed in the solid
state.19 Cooling a toluene-d8 solution of (MePDI)Fe(biphenyl)
to 0 °C resulted in a sharpening and increase in the number of
the observed resonances. Under these conditions, (MePDI)Fe-
(biphenyl) exhibited the number of resonances consistent with
a molecule of Cs symmetry over a range of 85 ppm,
demonstrating the static limit, analogous to the 23 °C spectrum
of (iPrPDI)Fe(biphenyl). Variable temperature 1H NMR
experiments for both compounds are reported in the
Supporting Information (Figures S1 and S2).
The solid state structures of both (iPrPDI)Fe(biphenyl) and
(MePDI)Fe(biphenyl) were determined by X-ray diffraction and
are presented in Figure 1. Selected bond distances and angles
are reported in Table 1. In both compounds, the overall
Table 1. Selected Bond Distances (Å) and Angles (deg) for
(iPrPDI)Fe(biphenyl) and (MePDI)Fe(biphenyl)
(
iPrPDI)Fe(biphenyl)
(
MePDI)Fe(biphenyl)
Kinetic Stability of (RPDI)Fe(biphenyl) Compounds.
The kinetic stability of the (RPDI)Fe(biphenyl) compounds
was investigated as it impacts the ability to isolate and handle
pure material used for subsequent spectroscopic studies.
Benzene-d6 solutions of (MePDI)Fe(biphenyl) proved to be
indefinitely stable at 23 °C under an inert atmosphere. By
contrast, allowing a benzene-d6 solution of (iPrPDI)Fe-
(biphenyl) to stand for 30 h at 23 °C resulted in liberation
of a stoichiometric quantity of biphenyl along with formation of
the previously reported bis(imino)pyridine iron intramolecular
olefin complex arising from dehydrogenation of the isopropyl
aryl substituents (eq 2).48
Fe(1)−N(1)
Fe(1)−N(2)
Fe(1)−N(3)
Fe(1)−Cbasal
Fe(1)−Capical
N(1)−C(2)
N(3)−C(8)
C(2)−C(3)
C(7)−C(8)
1.959(2)
1.852(2)
1.988(2)
1.965(3)
1.943(3)
1.315(4)
1.317(4)
1.438(4)
1.433(4)
1.9402(18)
1.8569(16)
1.9503(18)
1.9656(19)
1.9436(19)
1.318(2)
1.320(2)
1.452(3)
1.447(3)
N(1)−Fe(1)−N(2)
N(1)−Fe(1)−N(3)
N(1)−Fe(1)−Cbasal
N(1)−Fe(1)−Capical
N(2)−Fe(1)−N(3)
N(2)−Fe(1)−Cbasal
N(2)−Fe(1)−Capical
N(3)−Fe(1)−Cbasal
N(3)−Fe(1)−Capical
79.41(9)
79.80(7)
157.38(7)
103.06(7)
93.75(7)
80.11(7)
175.93(8)
92.47(7)
97.57(7)
97.39(7)
154.78(9)
99.76(10)
99.70(11)
79.46(10)
175.09(12)
92.47(12)
104.21(10)
94.81(11)
molecular geometry is best described as idealized square
pyramidal with the three nitrogen atoms of the bis(imino)-
pyridine and one of the carbon atoms of the biphenyldiyl ligand
defining the basal plane. The other carbon of the metallacycle
defines the apical position. The iron−carbon bond distances of
Fe(1)−C(34) and Fe(1)−C(45) of 1.965(3) and 1.943(3) Å,
respectively, in (iPrPDI)Fe(biphenyl) are shorter than the iron−
carbon bond lengths of 2.062(3) and 2.054(3) Å, respectively,
previously reported for (iPrPDI)Fe(CH2SiMe3)2, a molecule
with an S = 2 ground state.27,47 In both (iPrPDI)Fe(biphenyl)
and (MePDI)Fe(biphenyl), the bond distances of the bis-
(imino)pyridine chelate are diagnostic of redox activity and key
for indirectly assigning the metal oxidation state.29,39 In both
complexes, the iron−nitrogen and iron−carbon bond lengths
are sufficiently contracted to eliminate a high spin state, and the
elongation of the Nimine−Cimine and contraction of Cimine−Cipso
distances are consistent with one-electron reduction and hence,
a ferric oxidation state assignment.
The transfer dehydrogenation reaction, previously observed
in the context of bis(imino)pyridine iron-catalyzed enyne and
diyne cyclizations,24 was also confirmed by deuterium labeling
experiments. Allowing (iPrPDI*)Fe(biphenyl) (* indicates
deuteration of the isopropyl methyl groups) to stand in
benzene-d6 yielded 2-d1-biphenyl along with isotopologues of 1.
Analysis of the liberated biphenyl by 1H and 2H NMR
spectroscopy established exclusive deuterium incorporation
into the 2-position of the arene. Complete conversion to
products required 96 h at 23 °C, consistent with a normal
primary kinetic isotope effect on transfer dehydrogenation,
similar to observations in related enyne chemistry.24 Because
the iron product, 1, is NMR silent and has a triplet ground
state, its formation, which is kinetically competitive during
preparation of (iPrPDI)Fe(biphenyl), must be taken into
account when conducting spectroscopic or magnetic measure-
ments on the starting iron compound.
Magnetochemistry. The observation of paramagnetic
compounds by NMR spectroscopy prompted more detailed
investigations into the magnetic behavior of (iPrPDI)Fe-
(biphenyl) and (MePDI)Fe(biphenyl). Ambient temperature
(23 °C) measurements using a magnetic susceptibility balance
yielded effective magnetic moments of 1.5 and 2.0 μB for
(iPrPDI)Fe(biphenyl) and (MePDI)Fe(biphenyl), consistent
with S = 1 molecules. Similarly, effective moments of 1.1
((iPrPDI)Fe(biphenyl)) and 1.7 μB ((MePDI)Fe(biphenyl))
were determined by the method of Evans in benzene-d6
solution at 23 °C.
NMR Spectroscopic Studies. The benzene-d6 1H NMR
spectra of (iPrPDI)Fe(biphenyl) and (MePDI)Fe(biphenyl) at
23 °C exhibit a number of resonances consistent with
paramagnetic Cs and C2v symmetric iron complexes, respec-
tively. The resonances for each spectrum are shifted over an 86
i
(R = Pr) and 33 ppm (R = Me) range. The significantly
broader resonances of (MePDI)Fe(biphenyl) compared to
(iPrPDI)Fe(biphenyl), in combination with the higher C2v
symmetry, suggest that the less sterically congested bis(imino)-
pyridine iron complex allows for rapid rocking of the biphenyl
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Variable temperature measurements were also conducted on
both compounds using SQUID magnetometry. The temper-
ature dependence of the magnetic moment for (MePDI)Fe-
(biphenyl) is presented in Figure 2. This compound was
Representative data are reported in the Supporting Informa-
tion.
̈
Mossbauer Spectroscopic Studies. The electronic
structures of (iPrPDI)Fe(biphenyl) and (MePDI)Fe(biphenyl)
were also studied using zero- and applied-field 57Fe Mossbauer
̈
spectroscopy. Experimentally determined isomer shifts (δ) and
quadrupole splittings (ΔEQ) are reported in Table 2;
Table 2. Zero-Field 57Fe Mossbauer Spectroscopic
̈
Parameters for Bis(imino)pyridine Iron Compounds
Relevant to This Study (Data Collected at 80 K)
a
compound
δ (mm/s)
ΔEQ (mm/s)
(
(
iPrPDI)Fe(biphenyl)
MePDI)Fe(biphenyl)
0.07
0.05
+3.58
3.69
(
(
(
(
(
(
(
iPrPDI)Fe(N2)2
iPrPDI)FeN2
iPrPDI)Fe(DMAP)
iPrPDI)FeNDipp
iPrPDI)FeN1Ad
iPrPDI)Fe(CH2SiMe3)2
iPrPDI)Fe(CO)2
0.39
0.38
0.31
0.30
0.04
0.27
0.03
−0.53
+1.72
b
b
+1.94
c
Figure 2. Variable temperature SQUID magnetization data for
(
+1.08
MePDI)Fe(biphenyl) from 4 to 300 K. Data (black dots) are
c
−2.38
2.65
corrected for underlying diamagnetism, and the simulation (blue line)
depicts a phase transition from an S = 0 to S = 1 state (as fit using the
Sorai and Seki domain model)49,50 with a transition temperature of
352 K and enthalpy, nΔH = 805 cm−1. The model also contains 0.5%
of a paramagnetic impurity with θ = −5 K.
d
b
1.17
a
Unless a sign is reported, all values of ΔEQ are absolute values.
Values taken from ref 29. Values taken from ref 30. Values taken
from ref 27.
b
c
d
selected for initial study due to its kinetic stability. At
temperatures below 120 K, the magnetic moment plateaued
at 0.4 μB. The low temperature data were modeled for a
diamagnetic iron compound with 200 × 10−6 cm3/mol of
temperature independent paramagnetism containing 0.5% of an
unknown S = 5/2 paramagnetic impurity. At temperatures
above 120 K, a sharp rise in the magnetic moment is observed,
reaching a value of 1.6 μB at 300 K. The overall temperature
dependence of the magnetic moment is consistent with a spin
crossover (SCO) from an S = 0 to S = 1 ground state and was
modeled using the Sorai and Seki domain model, where the
high spin fraction is determined by x = 1/[1 + exp{(nΔH/
R)(1/T − 1/Tc)}].49,50 Although the S = 1 state is not fully
resolved, the high transition temperature of 352 K (transition
width/enthalpy, nΔH = 805 cm−1) resulting from the model
supports an incomplete population of the higher spin state at
300 K. The low temperature increase in magnetic moment
(below 20 K) was modeled with a Weiss temperature (θ) = −5
K in the paramagnetic impurity and likely results from the
influence of zero field splitting and magnetization saturation.
Because the high spin state is experimentally not fully resolved,
zero field splitting (D) = 0 and g = 2 were constrained as they
cannot be properly determined. However, a change in the
magnitude of either of these parameters would not alter the
conclusions regarding the initial and final spin states:
(MePDI)Fe(biphenyl) is an S = 0 to S = 1 spin crossover
compound. Similar SCO behavior has been observed with
bis(imino)pyridine iron imido (S = 0 to S = 1)51 and
bis(chelate) bis(imino)pyridine cobalt (S = 1/2 to S = 3/2)
compounds.52 The variable temperature magnetic data
observed for (iPrPDI)Fe(biphenyl) displays the same general
trends as for (MePDI)Fe(biphenyl) (see Supporting Informa-
tion) but the instability of the compound (vide supra) made
collection of reliable magnetic data more challenging. In an
attempt to circumvent these issues and to evaluate the influence
representative spectra are presented in Figure 3. Also included
in Table 2 are the values of δ and ΔEQ for several other
bis(imino)pyridine iron compounds for comparison. The
diamagnetic iron dinitrogen complexes, (iPrPDI)Fe(N2) and
(iPrPDI)Fe(DMAP),31 were chosen as intermediate spin ferrous
(SFe = 1) compounds with two-electron reduced bis(imino)-
pyridine chelates (SPDI = 1). The iron imido, (iPrPDI)Fe
N(Dipp) (Dipp = 2,6-iPr2−C6H3),32 and dialkyl, (iPrPDI)Fe-
(CH2SiMe3)2,28 complexes were chosen as molecules with
Fe(III) oxidation states and one-electron reduced chelates. It
should be noted that the imido was characterized as an
intermediate spin Fe(III) compound, (SFe = 3/2), while the
dialkyl was determined to be high spin (SFe = 5/2).
The isomer shifts of the (RPDI)Fe(biphenyl) compounds,
0.07 (R = iPr) and 0.05 mm/s (R = Me), are lower than those
of the known intermediate spin ferrous derivatives, (iPrPDI)-
Fe(N2) and (iPrPDI)Fe(DMAP), and the intermediate and high
spin ferric species, (iPrPDI)FeN(Dipp) and (iPrPDI)Fe-
(CH2SiMe3)2, respectively. The values of δ are comparable to
that measured for (iPrPDI)Fe(CO)2, a highly covalent molecule
that is best described by (iPrPDI2−)FeII(CO)2 and (iPrPDI0)-
Fe0(CO)2 resonance forms and the alkyl-substituted iron imido
complex, (iPrPDI)FeN1Ad, an established spin crossover
compound.51
57Fe Mossbauer spectroscopy has been applied to detect spin
̈
crossover behavior in iron coordination compounds.53 The
most well-studied examples are octahedral complexes of
iron(II) where the spectral parameters of the low- and high-
spin states are different and the time scale of the experiment
(10−7 s) allows detection of the two different spin isomers.49,53
In bis(imino)pyridine chemistry, Mossbauer spectroscopy
̈
distinguished the low- and high-spin states of (iPrPDI)FeN2Ad
and allowed determination of the energetics of the spin
transition.51 To probe whether such behavior could be detected
in organometallic iron complexes, variable temperature data
were collected on both (iPrPDI)Fe(biphenyl) and (MePDI)Fe-
of impurities, all magnetic samples were analyzed by Mossbauer
̈
spectroscopy prior to SQUID, MSB, or Evans measurements.
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Figure 3. Zero-field 57Fe Mossbauer spectra of (iPrPDI)Fe(biphenyl) (left) and (MePDI)Fe(biphenyl) (right). Data collected at 80 K.
̈
(biphenyl). For the former compound, spectra were obtained at
10, 80, 150, 200, and 250 K. The parameters obtained at 250 K
(δ = 0.01 mm/s and ΔEQ = 3.51 mm/s) are indistiguishable
from the values (δ = 0.07 mm/s and ΔEQ = 3.58 mm/s)
obtained at 80 K. Similar behavior was observed with
(MePDI)Fe(biphenyl), where parameters of δ = −0.02 mm/s
and ΔEQ = 3.43 mm/s were measured at 295 K indistiguishable
from the values of δ = 0.05 mm/s and ΔEQ = 3.69 mm/s at 80
K. Representative spectra for both compounds are presented in
the Supporting Information.
Although SCO behavior has been detected in Fe(III)
compounds previously,55 the differences in isomer shift and
quadrupole splitting between the low and high spin forms may
be sufficiently small to be differentiated by the experiment. As
will be described in the Computational Studies section, the
predicted parameters for the S = 1 state of (iPrPDI)Fe-
(biphenyl) are δ = 0.18 mm/s and ΔEQ = 3.70 mm/s. These
values, when considering the accepted errors in computed
Mossbauer parameters,56 are indistinguishable from the values
̈
predicted (and observed) at 80 K.
To determine the sign of the quadrupole splitting and gain
additional insight into the low temperature magnetic ground
state, applied field Mossbauer measurements were made from 1
̈
to 7 T at 4.2 and 120 K using (iPrPDI)Fe(biphenyl) as a
representative example. The spectra exhibit characteristic
magnetic splitting upon the application of an external field
(Figure 4) and were modeled using an S = 0 ground state and
confirmed the diamagnetism of the molecule at low temper-
atures. Simulation of the data yielded an isomer shift of 0.07
mm/s, a positive quadrupole splitting, ΔEQ = +3.58(2) mm/s,
and an asymmetry parameter for the electric field gradient, η =
0.45(5). The large and positive quadrupole splittings for
(RPDI)Fe(biphenyl) (the value for (MePDI)Fe(biphenyl) is
likely also positive) indicate a large asymmetric field gradient
along the z-axis.53
X-ray Absorption and Emission Studies. To gain
additional insight into the oxidation and spin state of the
iron, (iPrPDI)Fe(biphenyl) was also studied by X-ray
Absorption Spectroscopy (XAS). The pre-edge region of an
XAS spectrum (∼7110−7116 eV) is a well-known indicator of
oxidation state, with shifts of ∼1 eV occurring for each one-
electron oxidation event at the iron center,57,58 and this
technique has been previously used to aid in the understanding
of the electronic structure of bis(imino)pyridine com-
Figure 4. Applied field 57Fe Mossbauer spectra of (iPrPDI)Fe-
(biphenyl) recorded at 1 T, 4.2 K (bottom); 4 T, 4.2 K; 7 T, 4.2 K;
7 T, 120 K (top).
̈
pounds.36,51 Here, at low (10 K) and high (298 K)
temperatures, XAS is also applied to provide a second
experimental probe of the SCO behavior which was
demonstrated by the SQUID data but was invisible to the
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Figure 5. Comparison of the normalized Fe K-edge XAS spectra for (iPrPDI)FeCl2 and (iPrPDI)Fe(biphenyl). The data for the latter were collected
at 10 (blue) and 298 (red) K. The data for (iPrPDI)FeCl2 were taken from ref 36. See Supporting Information for figures detailing the full set of
experiments.
Figure 6. Variable temperature XES spectra of (iPrPDI)Fe(biphenyl) highlighting the main line (left) and V2C (right) at 100 K (blue) and 298 K
(red).
zero-field Mossbauer experiment. As XAS is a molecular
consistent with a ferric oxidation state. The pre-edge of
(iPrPDI)Fe(biphenyl) is 0.7−1 eV lower in energy than known
Fe(IV) compounds,57,65 suggesting that oxidative addition of
biphenylene to (iPrPDI2−)FeIIN2 was not exclusively iron-based
with retention of the dianionic chelate. Instead, the rising edge
(at ∼7121.5 eV) of (iPrPDI)Fe(biphenyl) is also consistent with
a ferric rather than ferrous oxidation state and confirms
participation of the bis(imino)pyridine chelate in the overall
electronic structure and in the course of the oxidative addition
reaction.
̈
technique, it potentially eliminates the detection of bulk
magnetic effects and is less sensitive to impurities than
traditional magnetic measurements. Previous studies of
light,59 magnetic field,60 and temperature61−63 induced SCO
events in iron compounds have demonstrated the sensitivity of
XAS for detecting small electronic structural differences in SCO
compounds. It is generally observed that, upon increasing spin
state, the rising edge shifts to lower energy due to longer
metal−ligand bonds, which make the iron center appear “more
reduced.” In the pre-edge region, a decrease in pre-edge
intensity is observed for the higher spin state, consistent with
longer metal−ligand bonds for the higher spin state, which
results in a decrease in covalently mediated metal 3d−4p
mixing.62−64
Variable temperature X-ray emission spectroscopy (XES)
studies were also performed to study the spin crossover
behavior of (iPrPDI)Fe(biphenyl). X-ray emission spectroscopy
detects emission from the relaxation of a 3p electron into a 1s
hole created by excitation of the core electron and thus allows
different electronic states to be accessed as compared to XAS.
XES is of particular interest for SCO compounds because it is a
molecular technique that is sensitive to the metal spin state.
Additionally, XES of SCO compounds has not been widely
XAS data for (iPrPDI)Fe(biphenyl) were obtained at 10, 35−
43, 57−66, and 298 K (Figure 5). The spectra up to 66 K all
overlay, and the data are presented together with the Fe K-edge
spectrum of (iPrPDI0)FeIICl2, an established five-coordinate,
high spin ferrous compound.30 At 10 K, the 1s to 3d pre-edge
feature appears at 7111.8 eV for (iPrPDI)FeIICl2 while
(iPrPDI)Fe(biphenyl) exhibits two pre-edge features with
similar intensities at 7111.5 and 7112.5 eV (Figure 5b),
explored,59 62,66 and further studies are necessary to establish
a,
frameworks such that this method could be used for unknown
systems, or systems for which not enough material can be
isolated for traditional magnetic measurements. (iPrPDI)Fe-
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(biphenyl) offers a unique system for applying XES, in terms of
both exploring the electronic structure of the compound and
understanding what effect spin state changes in SCO
compounds have upon spectral features in XES.
program with the B3LYP functional as with other bis(imino)-
pyridine metal complexes.36,61 For computational expediency,
XAS and XES calculations were performed using BP86. This
functional has also shown better agreement with experimental
spectra (as compared to B3LYP) and improves computational
efficiency.71,77 Unrestricted Kohn−Sham (UKS) calculations
were performed for singlet and triplet ground states as well as
the corresponding broken symmetry possibilities, BS(1,1) and
BS(3,1). 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.72−74 Both
the singlet and triplet calculations converged to the respective
BS(1,1) and BS(3,1) broken symmetry solutions. A Restricted
Kohn−Sham (RKS) solution was also evaluated.
The main line (7030−7080 eV) of the spectrum, which
includes the Kβ′ (∼7040−7050 eV) and Kβ1,3 (∼7055−7065
eV) features, contains the most information about the metal
spin state (Figure 6, left).62,67 In low-spin compounds, a Kβ′
feature is absent but is present in high-spin derivatives
(∼7040−7050 eV).62,67 Similarly, the Kβ1,3 transition (7055−
7065 eV) shifts 1−2 eV higher in energy for high-spin
compounds and is dominated by 3p−3d exchange interactions
with smaller contributions from 3p spin−orbit coupling.62,67
The energy splitting between the Kβ′ and Kβ1,3 is a result of the
exchange interaction between 3p and 3d electrons and is
therefore sensitive to the d-orbital occupancy and metal
oxidation state.68−70 The Kβ2,5 and Kβ′′ feature, or valence
to core region (V2C, 7080−7120 eV), results from a ligand np
or ns to metal 1s transition and is predominantly sensitive to
ligand identity. The V2C has also been shown to have a lower
intensity for high-spin complexes as compared to low-spin
complexes, and these trends have been confirmed in bis-
(imino)pyridine iron compounds.36
The BS(3,1) solution was found to be the lowest in energy,
whereas the BS(1,1) solution was 7.9 kcal higher in energy, and
the RKS solution was the highest in energy by 12.5 kcal (Table
3). Although these results are opposite the experimental data,
Table 3. Experimental and Computed 57Fe Mossbauer
̈
Spectroscopic Parameters and Relative Computed Ground
State Energies for (iPrPDI)Fe(biphenyl)
exptl
80 K
exptl
250 K
As shown in Figure 6 (left), the spectrum of (iPrPDI)Fe-
(biphenyl) at 100 K exhibits essentially no Kβ′ feature and is
consistent with a low spin iron center at this temperature. This
is in agreement with established parameters for diamagnetic
bis(imino)pyridine iron compounds.34 Increasing the temper-
ature to 298 K produced no significant change in the Kβ′
region. The Kβ1,3 shifts by ∼0.4 eV to higher energy upon
increasing temperature (from 7058.2 eV at 10 K to 7058.6 eV
at 298 K), consistent with the presence of some higher spin
state component at higher temperatures. Typical energies for
the Kβ1,3 for low spin Fe(III) compounds are from 7057.8 to
7059.1 eV, and those for high spin Fe(III) compounds fall
between 7060.0 and 7060.4 eV.67 The Kβ1,3 energies at both 10
and 298 K fall within the energy regime for low spin Fe(III)
compounds and are clearly outside the range of high spin
Fe(III) compounds. For bis(imino)pyridine Fe(II) compounds
the Kβ1,3 energy difference for low spin versus intermediate
spin compounds (not SCO) is 0.6−0.8 eV, suggesting
approximately 50% conversion from low spin to intermediate
spin (iPrPDI)Fe(biphenyl) at 298 K.
BS(3,1) BS(1,1)
RKS
relative calcd energy
(kcal)
na
na
0.00
+7.91
+12.47
δ (mm/s)
ΔEQ (mm/s)
η
0.07
0.01
3.51

0.18
0.08
0.14
+3.58
0.45
+3.70
0.42
+2.64
0.26
−2.98
0.88
where the S = 0 ground state is preferred, DFT methods are
known to erroneously favor higher spin states.75 It has been
suggested that computed energy differences of HS − LS = −6
to 0 kcal or less are indicative of spin-crossover compounds.75
While our data are slightly outside this range, we believe the
energy differences are most consistent with a spin-crossover
compound where the DFT favors the higher spin state.
The BS(1,1) geometry optimization reproduced experimen-
tal crystallographic parameters, although the metal−ligand
bond distances are slightly elongated, as is common when using
the B3LYP functional (Table 4).76 A qualitative molecular
orbital (MO) diagram and spin density plot are presented in
The V2C shows a slight shift (0.3 eV) to higher energy at
298 K but displays no significant changes in area. In a simple
picture, one would expect a decrease in the V2C XES area upon
increasing spin state due to longer metal−ligand bond lengths.
Previous studies67 have shown that high-spin ferric complexes
have approximately twice the intensity of low spin ferric
complexes. However, in this case a smaller change is predicted,
as only an intermediate spin ferric state is accessed. The
observed shift and similar areas for the low spin and
intermediate spin V2C XES spectra are further supported by
calculations (vide infra). Hence the XES data are consistent
with a diamagnetic ground state for (iPrPDI)Fe(biphenyl)
which crosses to a higher spin state at higher temperatures (but
is not fully accessed at 298 K). The data presented here indicate
that the Kβ1,3 is sensitive to spin state changes of 0.4−0.6 μB.
Computational Studies. The electronic structures of the
high and low spin forms of (iPrPDI)Fe(biphenyl) were
investigated with full molecule DFT calculations. Geometry
optimizations were initiated from the experimental solid-state
structures, and calculations were performed with the ORCA
Table 4. Experimental and Computed Bond Lengths (Å) and
Angles (deg) for (iPrPDI)Fe(biphenyl)
exptl
BS(3,1)
BS(1,1)
RKS
Fe(1)−N(1)
Fe(1)−N(2)
Fe(1)−N(3)
Fe(1)−C(34)
Fe(1)−C(45)
1.959(2)
1.852(2)
1.988(2)
1.965(3)
1.943(3)
2.188
1.916
2.179
1.986
2.032
2.057
1.889
2.050
1.987
1.951
2.048
1.879
2.047
1.978
1.961
N(1)−C(2)
N(3)−C(8)
N(2)−C(3)
N(2)−C(7)
C(2)−C(3)
C(7)−C(8)
1.315(4)
1.317(4)
1.360(4)
1.362(4)
1.438(4)
1.433(4)
1.311
1.313
1.369
1.369
1.457
1.456
1.316
1.318
1.363
1.364
1.457
1.455
1.307
1.307
1.355
1.355
1.465
1.465
N(2)−Fe(1)−C(34)
N(2)−Fe(1)−C(45)
175.09(12)
92.47(12)
178.6
98.9
179.3
96.4
176.7
92.0
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Figure 7. Qualitative molecular orbital diagram (left) and spin density plot (right) obtained from the BS(1,1) solution for (iPrPDI)Fe(biphenyl).
Spin density plot obtained from a Lowdin population analysis of the BS(1,1) solution for (iPrPDI)Fe(biphenyl) (red = positive spin density, yellow =
̈
negative spin density). Total electron densities are Fe = +0.85, PDI = −0.78, biphenyl = −0.07.
Figure 8. Qualitative molecular orbital diagram (left) and spin density plot (right) obtained from the BS(3,1) solution for (iPrPDI)Fe(biphenyl).
Spin density plot obtained from a Lowdin population analysis of the BS(3,1) solution for (iPrPDI)Fe(biphenyl) (red = positive spin density, yellow =
̈
negative spin density). Total electron densities are Fe = +2.65, PDI = −0.57, biphenyl = −0.08.
Figure 7. The BS(1,1) solution establishes a low spin iron(III)
center that is antiferromagnetically coupled to a bis(imino)-
pyridine radical giving rise to a diamagnetic ground state. The
The BS(3,1) solution also reproduces the experimentally
determined crystallographic parameters within computational
error, but with a slightly larger deviation from experiment than
the BS(1,1) solution (Table 4). Figure 8 depicts a qualitative
MO diagram and spin density plot for the BS(3,1) solution
described by an intermediate spin iron(III) center antiferro-
magnetically coupled to a bis(imino)pyridine radical (S = 0.68)
giving rise to the overall S = 1 spin state. Analogous to the
BS(1,1) solution, the dyz iron orbital mediates antiferromag-
netic coupling to the bis(imino)pyridine ligand. Upon
increasing the spin state, an electron is promoted from the
computed Mossbauer parameters of δ = 0.08 mm/s, ΔE
=
̈
Q
+2.64 mm/s, and η = 0.26 are in good agreement with the 80 K
experimental values of 0.08, +3.58, and 0.45, respectively,
thereby validating the computational results. For bis(imino)-
pyridine iron compounds, computational quadrupole splittings
often do not agree completely with experiment and are
especially challenging for SCO compounds (errors of 1.00
mm/s are considered reasonable).55
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2
dxz orbital into the previously unoccupied dz orbital. The
computed Mossbauer parameters of δ = 0.18 mm/s, ΔE
=
̈
Q
+3.70 mm/s, η = 0.42 are in excellent agreement with the
experimental values of δ = 0.01 mm/s, ΔEQ = 3.51 mm/s
determined at 250 K.
The computational results for (MePDI)Fe(biphenyl) were
analogous to those obtained for (iPrPDI)Fe(biphenyl). The
BS(3,1) solution was once again computed to be lowest in
energy. The diamagnetic BS(1,1) solution was +7.2 kcal higher
in energy, and the RKS solution was highest in energy by +13.1
kcal (see Supporting Information for bond distances and
angles, MO diagrams, and spin density plots). The low spin
calculation similarly converged to the BS(1,1) solution, and the
intermediate spin calculation converged to the BS(3,1)
solution. The computed Mossbauer parameters displayed
̈
similar correlations to experimental parameters, as in (iPrPDI)-
Fe(biphenyl).
The XAS pre-edge was calculated using time-dependent DFT
(TDDFT) with the BP86 functional on the geometry-
optimized coordinates.36 This functional offers improved
accuracy and speed over B3LYP and has proven successful
for XAS calculations for coordination compounds, metal-
loenzymes, and bis(imino)pyridine compounds.36,58,77,78
Although experimentally observed transitions occur between
states, simplified DFT models based on MO theory have been
found to correlate well to experimentally observed features. The
BP86 solution was compared to the B3LYP solution to ensure
that the same electronic structure description was obtained. For
the broken symmetry solutions, BP86 produced a more
covalent solution, but the overall electronic structure
description remained the same. The calculated XAS spectra
for (iPrPDI)Fe(biphenyl) are presented in Figure 9. The
BS(1,1) calculation contains two pre-edge features at 7111.1
and 7112.4 eV, which are in agreement with the experimentally
observed pre-edge features at 7111.5 and 7112.5 eV. In a
simplified molecular orbital model, the first feature is due to a
transition to the dyz orbital, and the second feature is due to a
Figure 9. Experimental (top) and calculated (bottom) XAS spectra of
(iPrPDI)Fe(biphenyl) using TD-DFT. A shift of 181.25 eV and
broadening of 1.5 eV have been applied to the computed spectra.
2
transition to the dz orbital. Both features have significant
bis(imino)pyridine contributions. The BS(3,1) calculation only
displays one pre-edge feature at 7112.6 eV which has
2
contributions from several transitions with dxz, dyz, and dz
Fe(intermediate spin III)(biphenyl) S = 1 state at more elevated
temperatures.
contributions. An increase in iron spin state from BS(1,1) to
BS(3,1) is in agreement with the experimental data in which
the intensity of the 7111.5 eV pre-edge feature decreases
relative to the feature at 7112.5 eV.
The elucidation of the electronic structure of the (RPDI)-
Fe(biphenyl) compounds by spectroscopic, magnetochemical,
structural, and computational studies allows for an accounting
of the electron flow in the C−C oxidative addition from the
corresponding iron dinitrogen precursors. As highlighted in
Scheme 4, the oxidative addition to (iPrPDI)Fe(N2) is therefore
comprised of both metal and ligand redox events. Both the iron
and bis(imino)pyridine undergo concomitant one-electron
oxidations resulting in overall two-electron cleavage of the
C−C bond. Similar metal−ligand cooperativity in two-electron
oxidation events are well-established in metalloenzyme
chemistry, viz. cytochrome P-450.6 We note that intermediates,
such as arene π-complexes,42 may also form on the reaction
coordinate for oxidative addition and therefore influence the
pathway of electron flow. However, we have not observed such
intermediates and accordingly not deduced their electronic
structures. Thus, the electron flow associated with oxidative
addition in this work concerns only the transformation from
reactants to products.
XES calculations were performed using a simple one-electron
approximation, as has been previously described and applied to
a variety of systems.36 The calculated BS(1,1) and BS(3,1) XES
spectra show similar intensities and features (see Supporting
Information) which derive from various bis(imino)pyridine
orbital combinations, although the BS(3,1) solution is shifted
to slightly higher energy by 0.6 eV. The experimentally
observed shift of 0.3 eV (between 100 and 298 K) suggests that
approximately 50% conversion to the high spin state
(intermediate spin Fe(III)) has been reached and supports
the lack of a decrease in intensity upon accessing the higher
spin state (see Supporting Information). The computed
electronic structure descriptions for three different spectro-
scopic methods (Mossbauer, XAS, XES) are in agreement with
̈
experimentally observed spectra, lending support for the
electronic structure description of a (iPrPDI−1)-
Fe(low spin III)(biphenyl) S = 0 state at low temperatures which
undergoes
a spin transition to a (
i P r PDI− 1 )-
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Scheme 4. Electron Flow in C−C Activation by a Redox-Active Bis(imino)pyridine Iron Dinitrogen Compound
N, 7.05. Magnetic susceptibility (Evans): μeff = 1.7 μB (benzene-d6, 23
°C). Magnetic susceptibility balance (22 °C) μeff = 2.0 μB. H NMR
(benzene-d6, 22 °C) δ = 51.94 (36 Hz, 1H), 37.98 (55 Hz, 6H), 5.32
(2,500 Hz), 3.72 (14 Hz, 6H), −19.11 (37 Hz, 2H).
CONCLUDING REMARKS
■
1
Carbon−carbon oxidative addition of biphenylene has been
observed at ambient temperature with bis(imino)pyridine iron
dinitrogen compounds. The redox activity of the bis(imino)-
pyridine chelate raises the question as to whether the formal
electron transfer events are metal- or ligand-based. Character-
ization of the resulting (RPDI)Fe(biphenyl) compounds by X-
ASSOCIATED CONTENT
* Supporting Information
■
S
Additional experimental procedures, including general consid-
erations, spectroscopic and computational details. Crystallo-
graphic data for (iPrPDI)Fe(biphenyl) and (MePDI)Fe-
(biphenyl) in cif format. This material is available free of
charge via the Internet at http://pubs.acs.org.
ray diffraction; zero-field and applied field 57Fe Mossbauer,
̈
XAS, and XES spectroscopies; and variable temperature
SQUID magnetometry established spin crossover compounds
arising from ferric centers antiferromagnetically coupled to
ligand radical anions. At low temperature, diamagnetic ground
states are observed and slightly higher energy S = 1 states are
populated at increased temperatures. Accounting for the
electron flow in the net two-electron C−C cleavage reaction
involves cooperative one-electron oxidation at both the metal
and bis(imino)pyridine ligand and avoids formation of
presumably high energy iron(IV) compounds. Gaining a
more detailed understanding of the nature of oxidative addition
reactions with redox-active iron complexes also provides
important insight into a fundamental transformation that likely
constitutes a key substrate activation step in catalytic cycles and
provides an important starting point for understanding the
mechanism of turnover.
AUTHOR INFORMATION
Corresponding Author
pchirik@princeton.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
P.J.C. and K.W. thank the U.S. National Science Foundation
and Deutsche Forschungsgemeinschaft for a Cooperative
Activities in Chemistry between U.S. and German Investigators
grant. S.D. acknowledges the Department of Chemistry and
Chemical Biology at Cornell University, the Max Planck
Society, the Sloan Foundation, and an ACS Petroleum
Research Fund Grant (50270-DN13) for generous financial
support. J.M.D. also acknowledges support from the National
Institute of General Medical Sciences (Award Number
T32GM008500). S.C.E.S. thanks the NSF for a graduate
research fellowship (DGE-0646086) for support. S.P.S. thanks
the Natural Sciences and Engineering Research Council of
Canada for a predoctoral fellowship (PGS-D). This work is
based, in part, upon research conducted at the Cornell High
Energy Synchrotron Source (CHESS), which is supported by
the National Science Foundation and the National Institutes of
Health/National Institute of General Medical Sciences under
NSF Award DMR-0936384. Portions of the research were
carried out at the Stanford Synchrotron Radiation Lightsource,
a national user facility operated by Stanford University on
behalf of the DOE, BES. The SSRL SMB Program is supported
by DOE, BER and NIH, NCRR, BMTP. We also thank
Daigoro Hirai and Robert J. Cava for help in collecting
additional SQUID measurements.
EXPERIMENTAL SECTION
■
Preparation of (iPrPDI)Fe(biphenyl). Biphenylene (0.061 g, 0.40
mmol) was added to a 20 mL pentane solution containing 0.232 g of
(iPrPDI)Fe(N2)2 (0.39 mmol) in pentane. The resulting slurry was
stirred for 14 h after which time a green precipitate formed. The solid
(0.110 g, 0.16 mmol) was collected by filtration. The supernatant was
reduced to approximately half the original volume, stored at −35 °C,
and yielded an additional 0.059 g (0.086 mmol) of a green powder.
The combined mass of the isolated green solid was 0.169 g (62%
yield) and was identified as (iPrPDI)Fe(biphenyl). Anal. Calcd for
C45H51FeN3: C, 78.36; H, 7.45; N, 6.09. Found: C, 77.92; H, 7.28; N,
6.00. Magnetic susceptibility (Evans): μeff = 1.1 μB (benzene-d6, 23
1
°C). Magnetic susceptibility balance (22 °C): μeff = 1.5 μB. H NMR
(benzene-d6) δ = 46.94 (40 Hz, 1H, p-pyr), 33.86 (43 Hz, 1H), 26.45
(24 Hz, 6H), 9.56 (20 Hz, 1H), 8.50 (22 Hz, 2H), 5.74 (22 Hz, 1H),
5.23 (19 Hz, 2H), 4.71 (110 Hz, 1H), 4.57 (23 Hz, 2H), 2.79 (18 Hz,
6H, CH(CH3)2), 2.16 (21 Hz, 6H, CH(CH3)2), 1.62 (6 Hz, 6H,
CH(CH3)2), 0.57 (23 Hz, 6H, CH(CH3)2), −1.30 (24 Hz, 1H), −3.91
(30 Hz, 3H), −14.03 (38 Hz, 3H, C(CH3),), −15.21 (71 Hz, 1H),
−21.79 (40 Hz, 1H), −39.02 (93 Hz, 1H).
Preparation of (MePDI)Fe(biphenyl). A 20 mL scintillation vial
was charged with biphenylene (0.096 g, 0.64 mmol), [(MePDI)Fe-
(N2)]2(μ2-N2) (0.292 g, 0.312 mmol), and approximately 10 mL of
pentane. The mixture was stirred for 18 h after which time a green
precipitate formed. The mixture was cooled to −35 °C, and the solid
was collected by filtration. The product was recrystallized from a
mixture of toluene and diethyl ether at −35 °C to yield 0.199 g (55%
yield) of a green solid identified as (MePDI)Fe(biphenyl). Anal. Calcd
for C45H51FeN3: C, 76.95; H, 6.11; N, 7.28. Found: C, 76.71; H, 5.72;
REFERENCES
■
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