Synthesis, electronic structure, and catalytic activity of reduced bis(aldimino)pyridine iron compounds: Experimental evidence for ligand participation

Article abstract of DOI:10.1021/ic102186q

The two-electron reduction chemistry of the aryl-substituted bis(aldimino)pyridine iron dibromide, (^iPrPDAI)FeBr₂ (^iPrPDAI = 2,6-(2,6-ⁱPr₂-C₆H ₃-N=CH)₂C₅H

Full text of DOI:10.1021/ic102186q

ARTICLE
pubs.acs.org/IC
Synthesis, Electronic Structure, and Catalytic Activity of Reduced
Bis(aldimino)pyridine Iron Compounds: Experimental Evidence for
Ligand Participation
Sarah K. Russell,†,§ Carsten Milsmann,†,§ Emil Lobkovsky,† Thomas Weyherm€uller,‡ and Paul J. Chirik*,†,§
†Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853, United States
‡Max-Planck Institute for Bioinorganic Chemistry, Stiftstrasse 34-36, D-45470 M€ulheim an der Ruhr, Germany
S Supporting Information
b
ABSTRACT: The two-electron reduction chemistry of the aryl-
substituted bis(aldimino)pyridine iron dibromide, (iPrPDAI)FeBr2
(iPrPDAI = 2,6-(2,6-iPr2-C6H3-NdCH)2C5H3N), was explored
with the goal of generating catalytically active iron compounds and
comparing the electronic structure of the resulting compounds to
the more well studied ketimine derivatives. Reduction of
(iPrPDAI)FeBr2 with excess 0.5% Na(Hg) in toluene solution
under an N2 atmosphere furnished the η6-arene complex,
(iPrPDAI)Fe(η6-C7H8) rather than a dinitrogen derivative. Over
time in pentane or diethyl ether solution, (iPrPDAI)Fe(η6-C7H8)
underwent loss of arene and furnished the dimeric iron compound,
[(iPrPDAI)Fe]2. Crystallographic characterization established a diiron compound bridged through an η2-πinteraction with an imine arm on
an adjacent chelate. Superconducting quantum interference device (SQUID) magnetometry established two high spin ferrous centers each
coupled to a triplet dianionic bis(aldimino)pyridine chelate. The data were modeled with two strongly antiferromagnetically coupled, high
spin iron(II) centers each with an S = 1 [PDAI]2- chelate. Two electron reduction of (iPrPDAI)FeBr2 in the presence of 1,3-butadiene
furnished (iPrPDAI)Fe(η4-C4H6), which serves as a precatalyst for olefin hydrogenation with modest turnover frequencies and catalyst
lifetimes. Substitution of the trans-coordinated 1,3-butadiene ligand was accomplished with carbon monoxide and N,N-4-dimethylamino-
pyridine (DMAP) and furnished (iPrPDAI)Fe(CO)2 and (iPrPDAI)Fe(DMAP), respectively. The molecular and electronic structures of
these compounds were established by X-ray diffraction, NMR and M€ossbauer spectroscopy, and the results compared to the previously
studied ketimine variants.
’ INTRODUCTION
hydrosilylation,4 the [2π þ 2π] cycloisomerization of dienes,9
and the hydrogenative cyclization of enynes and diynes.10
One other notable feature of the aryl-substituted bis-
(imino)pyridine iron dinitrogen and related neutral ligand com-
pounds is their electronic structures.11,12 The redox activity of the
bis(imino)pyridine chelate,13-16 the ability to engage in rever-
sible transfer of 1-3 electrons with the metal center,17,18 often
produces metal complexes whose formal oxidation state assign-
ment is deceiving. For example, the neutral ligand compounds,
Replacing precious metals with more sustainable, abundant,
and potentially nontoxic iron compounds in homogeneous
catalysis is an area of widespread interest, and the number of
reactions that proceed with high activity and selectivity continues
to grow.1 Much of this recent activity has been inspired by
Brookhart and Gibson’s independent reports of ethylene and
R-olefin polymerization upon activation of aryl-substituted
bis(imino)pyridine iron dihalides, (RPDI)FeCl2 (RPDI = 2,6-
(ArNdCMe)2C5H3N; Ar = 2,6-R2-C6H3; 2,4,6-Me3-C6H2,
etc.), with methylaluminoxane (MAO).2,3 Our laboratory has
reported that two electron reduction of these compounds with
excess 0.5% sodium amalgam under an N2 atmosphere furnished
the corresponding iron dinitrogen complexes, (iPrPDI)Fe(N2)2,4
(iPrBPDI)Fe(N2)2,5 and [(RPDI)Fe(N2)]2(μ2-N2) (R = Et; R =
Me; R = Me, iPr) (Figure 1).6 These compounds serve as efficient
precatalysts for the hydrogenation of olefins4,7 with the less
sterically protected, dimeric precursors, [(RPDI)Fe(N2)]2(μ2-
N2), exhibiting the higher turnover frequencies.6 In addition
to olefin hydrogenation,8 (iPrPDI)Fe(N2)2 has been shown to
be an effective precatalyst for alkyne hydrogenation,4 olefin
(iPrPDI)Fe-L (L = N2, DMAP,11 NH3,19,20 NH2 Bu,12 etc.), are
t
best described as ferrous compounds with a two-electron reduced
bis(imino)pyridine rather than the formal iron(0) alternative.
This concept appears general as reduced bis(imino)pyridine
ligands have been also been identified in reduced cobalt
chemistry.21-24
In a continuing effort to improve catalyst performance and to
determine the role of redox activity during turnover, our labora-
tory continues to explore modified tridentate ligand architec-
tures. The presence of ligand centered radicals on the
Received: October 29, 2010
Published: March 11, 2011
r
2011 American Chemical Society
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Figure 1. Bis(imino)pyridine iron dinitrogen complexes and their shorthand designations.
bis(imino)pyridine suggests that modifications in the plane of
the chelate may alter reactivity and ultimately catalytic perfor-
mance. Previous studies from our laboratory have demonstrated
that replacing the imine methyl groups with phenyl substituents,
for example, (iPrPDI)Fe(N2)2 versus (iPrBPDI)Fe(N2)2, resulted
in a more active 1-hexene hydrogenation catalyst. However, the
increased reactivity of the phenylated compound also resulted in
shorter catalyst lifetimes as η6-coordination of the imine aryl
groups and the phenyl rings were identified as irreversible
deactivation pathways.5 In ethylene polymerization catalysis,
lower activities and molecular weights have been observed when
bis(aldimino)pyridine iron dichlorides, for example, (iPrPDAI)-
FeCl2 (iPrPDAI = 2,6-(ArNdCH)2C5H3N; Ar = 2,6-iPr2-C6H3)
are used in place of the corresponding bis(imino)pyridine
compounds.2a,3a,25,26 Here we describe the differences in the
two-electron reduction chemistry between aryl-substituted bis-
(aldimino)pyridine and bis(imino)pyridine iron dihalide com-
pounds and evaluation of redox activity between the two types of
chelates. Notable differences in the isolated iron products and
their catalytic performances were found and magnetochemistry
on a diiron product provided experimental evidence for the redox
activity and triplet diradical character of the bis(aldimino)pyridine
supporting ligand.
’ RESULTS AND DISCUSSION
Reduction of the Bis(aldimino)pyridine Iron Dibromide,
Attempts to recrystallize (iPrPDAI)Fe(η6-C6H6) or (iPrPDAI)-
Fe(η6-C7H8) from diethyl ether, pentane, or a toluene/pentane
mixture at -35 °C furnished brown crystals identified as the
dimeric iron compound, [(iPrPDAI)Fe]2, in 43% yield (eq 2).
Paramagnetic [(iPrPDAI)Fe]2 was also prepared by allowing the
arene compounds to stand in nonaromatic solvent for 24 h at
23 °C or by direct reduction of the corresponding iron dibromide,
(iPrPDAI)FeBr2, with excess sodium naphthalenide in THF
followed by recrystallization from diethyl ether.
(
iPrPDAI)FeBr2. The synthetic method used to prepare the
ketimine-based bis(imino)pyridine iron dinitrogen complex,
(iPrPDI)Fe(N2)2, was initially explored as a route to the related
aldimine-substituted compound. Stirring (iPrPDAI)FeBr2 with
excess 0.5% sodium amalgam under a dinitrogen atmosphere did
not yield the target iron dinitrogen complex as judged by the
absence of NtN bands in the solution and solid state IR spectra.
Analysis of the resulting solid, obtained following filtration and
solvent removal, by 1H NMR spectroscopy in benzene-d6
established formation of a Cs symmetric molecule with an η6-
benzene ligand and a κ2-bis(aldimino)pyridine chelate (eq 1).
Berry and co-workers have reported a related structure in
bis(imino)pyridine ruthenium chemistry with the 2,6-dimethyl
aryl-variant of the ligand.27 In the iron chemistry, our laboratory
has reported similar 18 electron arene complexes, (iPrDI)Fe(η6-
C6D6) and (iPrDI)Fe(η6-C7H8) (iPrDI = (ArNdC(Me)(Me)
CdNAr)2; Ar = 2,6-iPr2-C6H3).28 In the case of the bis-
(aldimino)pyridine iron compond, 1 equiv of free toluene was
observed in the benzene-d6 1H NMR spectrum of (iPrPDAI)
Fe(η6-C6D6), suggesting that the initial product was the η6-
toluene compound, (iPrPDAI)Fe(η6-C7H8).
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Table 1. Selected Bond Distances (Å) for [(iPrPDAI)Fe]2
[(iPrPDAI)Fe]2
Fe(1)-N(1)
Fe(1)-N(2)
Fe(1)-N(3)
Fe(1)-N(6)
Fe(1)-C(38)
Fe(2)-N(4)
Fe(2)-N(5)
Fe(2)-N(6)
Fe(2)-N(3)
Fe(2)-C(7)
2.127(2)
1.967(2)
2.272(2)
1.984(2)
2.085(2)
2.149(2)
1.967(2)
2.264(2)
1.983(2)
2.076(2)
N(1)-C(1)
N(3)-C(7)
N(4)-C(32)
N(6)-C(38)
1.326(3)
1.392(3)
1.322(3)
1.392(3)
C(1)-C(2)
1.410(3)
1.454(4)
1.412(3)
1.447(3)
C(6)-C(7)
C(32)-C(33)
C(37)-C(38)
Fe(1)-Fe(2) distance
2.805(5)
found in [(iPrPDAI)Fe]2 to those of the free chelate or other
bis(imino)pyridine compounds as the η2-CdN imine interaction
introduces perturbations not associated with these other molecules.
The values in the unperturbed portion of the chelate, N(1)-C(1)
and N(4)-C(32), are elongated to 1.326(3) and 1.322(3) Å,
respectively while the C(1)-C(2) and C(32)-C(33) bond lengths
are contracted to 1.410(3) and 1.412(3) Å, respectively. These
values are typically associated with one electron reduction.33 The
imine portion of the bis(aldimino)pyridine chelate involved in
bonding to the adjacent iron centers exhibit much larger bond
perturbations from the free ligand because of backbonding from the
second metal. For example, the N(3)-C(7) and N(6)-C(38)
distances are 1.392(3) Å in both cases.
Figure 2. Solid state structure of [(iPrPDAI)Fe]2 at 30% probability
ellipsoids. Hydrogen atoms and two aryl groups omitted for clarity (top).
Bottom view: All aryl groups removed to highlight the core of the molecule.
The identity of [(iPrPDAI)Fe]2 was established by X-ray
diffraction, and a representation of the solid state structure is
presented in Figure 2. The unit cell contains a disordered
molecule of diethyl ether that reduces the overall quality of
the model. Selected bond distances and angles for this com-
pound are reported in Table 1. The crystallographic data for
[(iPrPDAI)Fe]2 establish a bimetallic diiron compound with a
metal-metal distance of 2.8047(5) Å. An idealized C2 axis of
symmetry passes through the midpoint of the two iron atoms
and relates each monomeric subunit of the dimer. The orienta-
tion of the isopropyl aryl substituents is responsible for the
deviation from idealized symmetry. Each iron atom is five
coordinate, ligated by a κ3-bis(aldimino)chelate and an η2-
CdN imine interaction from the adjacent iron-bis-
(aldimino)pyridine subunit. The π-interaction with the imine
results in deviation of the metal from the idealized κ3-chelate
plane by approximately 20°.
The benzene-d6 1H NMR spectrum of [(iPrPDAI)Fe]2 ex-
hibits the number of peaks consistent with a C2 symmetric
molecule observed over a 280 ppm chemical shift window. The
structure of the dimer is preserved in benzene-d6 solution as 21 of
the expected 23 peaks of the C2 symmetric dimer were observed.
The asymmetry of the bis(aldimino)pyridine ligand in each
monomeric subunit gives rise to the larger number of peaks. If
the dimer dissociated into a putative three-coordinate iron
monomer, benzene coordination would likely be observed;
however, no evidence for diamagnetic (iPrPDAI)Fe(η6-C6D6)
was obtained by solution NMR spectroscopy at 23 °C.
The electronic structure of [(iPrPDAI)Fe]2 was also studied by
zero-field M€ossbauer spectroscopy at 80 K. A representative
spectrum is presented in Figure 3. Fitting the experimental data
yielded an isomer shift (δ) of 0.84 mm/s and a quadrupole
splitting (ΔEQ) of 1.93 mm/s. The isomer shift is comparable to
the value of 0.90 mm/s (δ) measured for (iPrPDAI)FeBr2 and is
consistent with a high spin iron(II) compound. Thus, both iron
centers in [(iPrPDAI)Fe]2 are ferrous, suggesting overall two
electron reduction of the chelate, one in the unperturbed portion
of the chelate and the other in the η2-imine.
As is well established with bis(imino)pyridines11,17 and in chem-
istry of other redox-active ligands,29-32 bond distortions in the ligand
signal non-innocence and participation in the electronic structure.
For comparison and reference, the crystal structure of the free ligand,
iPrPDAI, was obtained. A representation of the molecular structure is
presented in the Supporting Information, Figure S1. The reference
Caldimine-Naldimine distances are 1.265(2) and 1.265(2) Å while
those for the Caldimine-Cipso are 1.475(2) and 1.476(2) Å. Caution
should be exercised when comparing the ligand bond distances
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g = 2.15, all attempts to fit the temperature dependence in the low
temperature region produced simulation parameters that were
physically meaningless. The best fit resulted in a very small
isotropic coupling constant, J, of -1.315 cm-1, which is likely
too small for a doubly bridged molecule, while the obtained zero
field splitting parameter, |D| = 104 cm-1, is much larger than
reported values for other high spin Fe(II) compounds. To our
knowledge, the largest axial zero-field splitting for a high-spin
ferrous complex was reported by M€unck, Holland, and co-
workers for a three-coordinate β-diketiminate iron methyl com-
plex with a value of |D|= 50 cm-1 34
Chang, Long, and co-
.
workers have reported similar values for a family of trigonal
pyramidal iron(II) complexes.35 Furthermore, the large axial
zero-field splitting parameters in these compounds can be
explained by the particular symmetry of the complexes, which
leads to an almost orbitally degenerate ground state with
unquenched orbital angular momentum. In contrast, the sym-
metry around the iron centers in [(iPrPDAI)Fe]2 is significantly
lower. Therefore, the unusual parameters led us to investigate a
more complicated spin system for the molecule.
Figure 3. Zero-field 57Fe M€ossbauer spectrum of [(iPrPDAI)Fe]2
at 80 K.
The M€ossbauer data and metrical parameters from the solid
state structure establish two equivalent high spin Fe(II) centers
and two [iPrPDAI]2- chelates, respectively. It has been well
established11 that bis(imino)pyridine ligands in their doubly
reduced, dianionic form possess nearly degenerate singlet (S = 0)
and triplet (S = 1) states, which have both been identified in
coordination compounds.11,23 Therefore, each high spin Fe(II)
center (SFe = 2) and each [iPrPDAI]2- ligand (SPDAI = 1) were
treated as separate entities, yielding a total of 4 spin centers. The
coupling scheme for such a four-spin system in the given
geometry and symmetry is presented in the inset in Figure 4.
While all coupling pathways are in principle antiferromagnetic in
nature, metal-ligand (J1 and J2) and metal-metal (J3) coupling
represent competing pathways for the alignment of the spins on
the iron centers. Because of the symmetry of the system, the
number of independent parameters was significantly reduced by
assuming equivalent g and D values for the two iron centers. To
further limit the number of fit parameters, g-anisotropy and
zero-field splitting were neglected for the ligands (gPDAI = 2.00;
|DPDAI| = 0 cm-1), which is in good agreement with the small
deviations from the g value of the free electron and the small
zero-field splitting parameters typically observed in organic
triplet diradicals.36,37 The best fit obtained by this model agreed
well with the experimental data over the whole temperature
Figure 4. Variable temperature SQUID magnetic data for
[(iPrPDAI)Fe]2.
As indicated by its benzene-d6 1H NMR spectrum,
[(iPrPDAI)Fe]2 is paramagnetic. A solution (benzene-d6, method
of Evans, 293 K) magnetic moment of 4.7 μB was measured
for the molecule. A slightly lower value of 4.2 μB was determined
in the solid state by magnetic susceptibility balance (296 K). The
unusual structural motif observed in the solid state structure
of [(iPrPDAI)Fe]2 prompted a more detailed investigation of
the magnetochemistry to determine the degree of electronic
communication between two iron centers bearing redox-active
ligands.
range and produced simulation parameters of |DFe| = 10.0 cm-1
gFe = 2.11, J1 = -1000 cm-1, J2 = -147 cm-1, J3 = -103 cm-1
,
.
To further substantiate our model, multiple-field variable-
temperature measurements were performed at 1, 4, and 7 T.
Reassuringly, the obtained isofield magnetization curves were
successfully modeled using the above-mentioned parameters
(Supporting Information, Figure S6). However, it should be
noted that satisfactory fits to the magnetic susceptibility data
were also obtained with different sets of simulation parameters.
An analysis of the error surface upon variation of J2 vs J3 (all other
parameters constant) revealed that even under these strongly
constrained conditions no unique solution can be identified,
because multiple minima of similar goodness of fit can be found
(Supporting Information, Figure S7). Somewhat surprisingly,
the values for J2 and J3 are also strongly dependent on the
magnitude of J1 (increase in J1 f increase in J2 and J3) despite the
strength of the antiferromagnetic interaction between the iron
and the tridentate ligand in each half of the dimer that precludes
The solid state magnetic behavior of [(iPrPDAI)Fe]2 was
studied by superconducting quantum interference device
(SQUID) magnetometry, and the temperature dependence of
the effective magnetic moment, μeff, is presented in Figure 4. In
the range from 100-300 K μeff is almost temperature indepen-
dent and reaches a value of 4.3 μB at 300 K, consistent with the
magnetic moment of 4.2 μB measured at 296 K by magnetic
susceptibility balance. Below 100 K, μeff decreases monotonically
from 4.1 μB at 100 K to 1.5 μB at 2 K. The room temperature
value of μeff is very close to the spin only value of 4.0 μB for two
weakly interacting spins of S = 1. On the basis of this information,
we modeled the data initially with each [(iPrPDAI)Fe] subunit as
a separate S = 1 system. While this model readily reproduced the
high-temperature region of the data using electronic g values of
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an experimental determination of J1 by magnetic susceptibility
measurements in the accessible temperature range. Although this
increases the error of the other variables, the qualitative model
remains intact. Very strong antiferromagnetic coupling between
each of the high-spin Fe(II) ions with its chelating [iPrPDAI]2-
ligand, in agreement with previous results for the ketimine-
substituted analogs,11,23 results in two essentially uncoupled S
= 1 [(iPrPDAI)Fe] subunits at high temperatures (100-300 K).
At lower temperatures, weaker antiferromagnetic coupling be-
tween the two subunits gives rise to lower magnetic moments
and accounts for the temperature dependence of μeff. In contrast
to J1 and J2, which describe the direct magnetic interactions
between paramagnetic metal (S = 2) and ligand (S = 1)
fragments, the metal-metal interaction J3 is mediated via a
superexchange pathway. Nevertheless, J3 is essential within the
proposed four-spin coupling model. All simulations employing
only the direct interactions J1 and J2 resulted in slightly lower fit
quality and required large axial zero-field splitting parameters
(>50 cm-1) for the iron centers in addition to ferromagnetic
coupling constants J2. Note that because of the competing
coupling pathways J2 and J3 the effective coupling between the
two halves of the dimer is weak, and the compound is para-
magnetic over the whole temperature range, despite the sizable
antiferromagnetic coupling constants >100 cm-1. In combina-
tion with the failure of the simple two spin model to reproduce
the low temperature region of the data within reasonable
parameters, this model provides strong experimental evidence
for the presence of a diradical bis(imino)pyridine ligand coordi-
nated in its triplet state.
Figure 5. Solid state structure of (iPrPDAI)Fe(η4-C4H6) at 30%
probability ellipsoids. Hydrogen atoms omitted for clarity.
Table 2. Selected Bond Distances (Å) and Angles (deg) for
(
iPrPDAI)Fe(η4-C4H6) and (iPrPDI)Fe(η4-C4H6)a
(
iPrPDAI)Fe(η4-C4H6)
(
iPrPDI)Fe(η4-C4H6)
Fe-Npy
1.852(2)
1.841(7)
Fe-Nimine
Fe-CCH2
Fe-CCH
1.966(2), 1.979(2)
2.129(2), 2.240(1)
2.041(3), 2.055(3)
2.005(1), 1.966(1)
2.179(2), 2.196(1)
2.050(1), 2.057(1)
Nimine-Cimine
Cimine-Cipso
1.329(3), 1.333(3)
1.410(3), 1.402(3)
1.332(1), 1.372(1)
1.422(1), 1.414(1)
Preparation of (iPrPDAI)Fe(η4-C4H6) and (iPrPDAI)Fe-
(DMAP). To directly compare the electronic structures and ulti-
mately the catalytic activities of reduced bis(aldimino)pyridine
iron compounds with their more studied bis(imino)pyridine
iron counterparts, compounds with identical metal-ligand
combinations were targeted. In bis(imino)pyridine iron chem-
istry, reduction of the iron dihalide precursor in the presence of
1,3-butadiene has proven to be a reliable and straightforward
method for synthesis of precatalysts for iron-catalyzed olefin
hydrogenation9 even when the corresponding dinitrogen com-
plex is synthetically inaccessible. In addition, the solid state
structure of (iPrPDI)Fe(η4-C4H6) established the trans geometry
of the butadiene ligand.9,38 Synthesis of the target (iPrPDAI)Fe-
(η4-C4H6) compound was accomplished by stirring a pentane
slurry of (iPrPDAI)FeBr2 with excesses of both 0.5% Na(Hg)
(∼3 equiv) and 1,3-butadiene (∼10 equiv). Filtration and re-
crystallization from diethyl ether at -35 °C yielded a brown solid
in 70% isolated yield (eq 3).
a Data taken from reference 9.
compound to temperatures below -20 °C resulted in a sharpen-
ing of the bis(imino)pyridine resonances while those for the
butadiene remain slightly broadened (see Supporting Informa-
tion, Figure S4, for a stack plot), suggesting that the dynamic
process involves dissociation and recoordination of the diene.
Warming the sample above 40 °C produced the number of
resonances consistent with a C2v symmetric molecule with only
two observable butadiene peaks, demonstrating that the ex-
change process is faster than the NMR time scale.
The solution dynamic behavior of the η4-butadiene ligand
in (iPrPDAI)Fe(η4-C4H6) prompted a similar investigation
of the previously reported ketimine-substituted compound for
1
comparison.9 The benzene-d6 solution H NMR spectrum of
(iPrPDI)Fe(η4-C4H6) at 20 °C exhibits the number of reso-
nances consistent with a molecule of C2 molecular symmetry.
Importantly, these resonances are sharper than those of the
corresponding aldimine complex, (iPrPDAI)Fe(η4-C4H6), under
identical conditions, suggesting a higher barrier for the dynamic
process involving the butadiene ligand. Warming the solution to
40 °C resulted in a broadening of the bis(imino)pyridine and
diene resonances, further supporting a higher barrier for dis-
sociation and recoordination in the ketimine substituted com-
pound as compared to its aldimine counterpart.
Single crystals of (iPrPDAI)Fe(η4-C4H6) suitable for X-ray
diffraction were obtained from a concentrated hexane solution
cooled to -35 °C. A representation of the solid state molecular
structure is presented in Figure 5 and selected bond distances are
reported in Table 2. Also presented in Table 2 are the distances
for the analogous bis(imino)pyridine iron butadiene compound,
(iPrPDI)Fe(η4-C4H6). The overall molecular geometry of
The benzene-d6 1H NMR spectrum of (iPrPDAI)Fe(η4-
C4H6) at 20 °C exhibits the number of resonances consistent
with a C2 symmetric molecule. Notably, all of the resonances are
broad, suggesting a dynamic process on the time scale of the
spectroscopic experiment. Cooling a toluene-d8 solution of the
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(iPrPDI)Fe(η4-C4H6) is similar to (iPrPDAI)Fe(η4-C4H6) with
both compounds containing a relatively unusual trans-butadiene
ligand.39,40 The bond distances in the bis(aldimino)chelate are
consistent with two electron reduction and indicate a ferrous
oxidation state. The distortions in Nimine-Cimine and Cimine
-
Cipso bond distances are comparable to the bis(imino)pyridine
variant with a slightly more pronounced Cimine-Cipso contrac-
tion for the aldimino example. The butadiene ligand of the
bis(aldimino)pyridine complex is less symmetrically bound to
the iron center with one shorter and one longer Fe-CCH2 bond
in comparison to the bis(imino)pyridine complex which has two
very similar Fe-CCH2 bond distances.
To further explore electronic structure differences between
reduced aryl-substituted bis(aldimino)pyridine and bis(imino)-
pyridine iron compounds, other neutral ligand derivatives of
[(iPrPDAI)Fe] were targeted. Two specific examples, (iPrPDAI)-
Fe(CO)2 and (iPrPDAI)Fe(DMAP) (DMAP = N,N-4-dimethyl-
aminopyridine), were selected because of the extensive spectro-
scopic and computational studies carried out on the correspond-
ing bis(imino)pyridine derivatives.11,12 Synthesisofthe dicarbonyl
compound, (iPrPDAI)Fe(CO)2, was accomplished by addition
of one atmosphere of carbon monoxide to the iron butadiene
compound (iPrPDAI)Fe(η4-C4H6) and furnished the desired
product as a green solid in 92% yield (eq 4). This molecule was
also prepared from direct sodium amalgam reduction of (iPrPDAI)-
FeBr2 in the presence of excess carbon monoxide.
Figure 6. Solid state structure of (iPrPDAI)Fe(DMAP) at 30% prob-
ability ellipsoids. Hydrogen atoms omitted for clarity.
Table 3. Selected Bond Distances (Å) and Angles (deg) for
(
iPrPDAI)Fe(DMAP) and (iPrPDI)Fe(DMAP)a
(
iPrPDAI)Fe(DMAP)
(
iPrPDI)Fe(DMAP)
Fe-Npy
1.836(2)
1.821(3)
1.833(2)
Fe-Nimine
Fe-NDMAP
Nimine-Cimine
Cimine-Cipso
Cipso-Npy
1.904(2), 1.922(2)
1.889(2), 1.917(2)
1.978(2)
1.908(3), 1.943(3)
1.979(3)
1.985(2)
1.339(3), 1.355(3)
1.349(3), 1.343(3)
1.407(4), 1.406(4)
1.406(3), 1.406(3)
1.379(3), 1.373(3)
1.382(3), 1.376(3)
1.350(5), 1.358(5)
1.414(5), 1.406(5)
1.390(5), 1.387(5)
a Data taken from reference 11.
Diamagnetic (iPrPDAI)Fe(CO)2 was characterized by multi-
nuclear NMR, solution IR, and solid state zero-field M€ossbauer
spectroscopies. The benzene-d6 1H and 13C NMR spectra exhibit
the number of peaks expected for a C2v symmetric molecule,
consistent with rapid exchange between the carbonyl ligands on
the NMR time scale. The pentane solution infrared spectrum of
(iPrPDAI)Fe(CO)2 exhibits two strong carbonyl bands centered
at 1981 and 1925 cm-1. These values are at modestly higher
energy than those (νCO = 1974, 1914 cm-1) for (iPrPDI)
Fe(CO)24 demonstrating that the ketimine-substituted complex
has a slightly more electron rich iron center than the correspond-
ing aldimine compound.
The synthesis of the other targeted neutral ligand derivative,
(iPrPDAI)Fe(DMAP), was also accomplished from the correspond-
ing butadiene compound. Addition of 1 equiv of DMAP to a diethyl
ether solution of (iPrPDI)Fe(η4-C4H6) furnished a brown solid
identified as (iPrPDAI)Fe(DMAP) in 95% isolated yield (eq 5).
The electronic structure of (iPrPDI)Fe(DMAP) was determined
previously and is best described as an intermediate spin ferrous
compound antiferromagnetically coupled to a bis(imino)pyridine
diradical dianion.11 The benzene-d6 1H and 13C NMR spectra of
diamagnetic (iPrPDI)Fe(DMAP) exhibit unusual shifts of the in-
plane chelate resonances that signal temperature independent
paramagnetism arising from mixing of singlet and triplet states
via spin orbit coupling. For example, in the 1H NMR spectrum, the
imine methyl groups are shifted upfield to -5.85 ppm while the
meta- and para-pyridine hydrogens were located at 12.42 and 9.04
ppm, respectively. Importantly, these resonances do not shift
dramatically as a function of temperature supporting TIP rather
than contributions from thermal population of paramagnetic
excited states or from contamination by paramagnetic iron impu-
rities. The benzene-d6 1H NMR spectrum of the aldimine variant,
(iPrPDAI)Fe(DMAP), exhibits the number of peaks for an idea-
lized C2 symmetric molecule. Notable features include a substan-
tially downfield shifted aldimine hydrogen centered at 17.16 ppm
and the meta- and para-pyridine hydrogens at 12.66 and
8.50 ppm, respectively. For context, the aldimine hydrogen appears
at 8.54 ppm in the free ligand and at 8.20 ppm in the diamagnetic
iron dicarbonyl compound, (iPrPDAI)Fe(CO)2. Accordingly, the
meta-pyridine hydrogens appear at 8.30 and 7.62 ppm in the free
ligand and the dicarbonyl compound, respectively. The
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Figure 7. Representative zero-field M€ossbauer spectra for (iPrPDAI)Fe(η4-C4H6), and (iPrPDAI)Fe(CO)2 and (iPrPDAI)Fe(DMAP) recorded at 80 K.
corresponding para-pyridine values are 7.19 and 7.16 ppm, respec-
tively. Variable temperature NMR experiments in toluene-d8
solution produce little change in the chemical shift of these
resonances. For example, the aldimine hydrogen moves approxi-
mately 4 ppm over a 120 °C temperature range. At -40 °C, this
peak is located at 18.95 ppm and shifts upfield to 14.68 ppm upon
warming to 80 °C. This shift is larger than what would be expected
from a pure diamagnetic compound (without mixing of excited
states) yet smaller than that anticipated for a simple paramagnetic
compound obeying the Curie-Weiss law.
Table 4. Zero-Field Mo€ssbauer Parameters for
iPrPDAI)Fe(η4-C4H6) and (iPrPDAI)Fe(DMAP)
Determined at 80 Ka
(
δ (mm/s)
ΔEQ (mm/s)
(
(
iPrPDAI)Fe(η4-C4H6)
iPrPDI)Fe(η4-C4H6)
0.37
0.38
0.73
0.38
(
(
iPrPDAI)Fe(CO)2
iPrPDI)Fe(CO)2
0.01
0.03
1.15
1.17
(
(
iPrPDAI)Fe(DMAP)
iPrPDI)Fe(DMAP)
0.30
0.31
2.04
1.94
Single crystals of (iPrPDAI)Fe(DMAP) were obtained from a
concentrated diethyl ether solution cooled to -35 °C. Two
molecules are present in the asymmetric unit and differ by the
direction of rotation of the DMAP ligand with respect to the iron-
chelate plane. A representation of the solid state structure of one
of the molecules is presented in Figure 6, and selected bond
distances and angles for both are reported in Table 3. Also
included in Table 3 are the corresponding values for
(iPrPDI)Fe(DMAP) for comparison. The overall molecular geo-
metry of the iron in (iPrPDAI)Fe(DMAP) is best described as
idealized square planar with the plane of the N,N-dimethylami-
nopyridine ligand canted at 53.78(6)° and 55.36(6)° with respect
to the iron chelate plane. This value is 50.36(10)° in the ketimine
compound, (iPrPDI)Fe(DMAP).
a Data is also included for the analogous bis(imino)pyridine iron com-
pounds for comparison. For (iPrPDI)Fe(CO)2 and (iPrPDI)Fe(DMAP)
the data were taken from ref 11.
environments at each iron center. For the butadiene and DMAP
derivatives, the isomer shifts are consistent with intermediate spin
ferrous compounds11 while for the carbonyl compounds the lower
value of δ indicates a higher degree of covalency and contributions
from Fe(0).
Evaluation in Catalytic Olefin Hydrogenation. The synthe-
sis of reduced bis(aldimino)pyridine iron compounds prompted
evaluation of their catalytic performance. The analogous bis-
(imino)pyridine iron dinitrogen complexes are highly active
catalysts for olefin hydrogenation,4-7 hydrosilylation,4,5 and
cycloisomerization reactions.9,10 In a recent study with the
ketimine-substituted compounds, the hydrogenation of ethyl-
3-methylbut-2-enoate was used as a representative model
substrate to evaluate the effect of changes in catalyst structure
on the efficiency and lifetime of turnover. This substrate was
also used in this study to determine differences between
aldimine- and ketimine-substituted iron precatalysts (eq 6). It
should be noted that the originally reported iron bis-
(dinitrogen) compound, (iPrPDI)Fe(N2)2, reaches only 65%
conversion to product after 72 h after which time the iron is
deactivated by irreversible C-O bond cleavage.7,41 Full con-
version is obtained when the aryl groups are reduced to ethyl or
methyl substituents. In each of the precatalyst screening
experiments described in this study, standard conditions were
employed where a 0.92 M solution of substrate was prepared in
benzene-d6 along with 5 mol % of the iron precatalyst and four
atmospheres of dihydrogen. The progress of the reaction was
then evaluated using 1H NMR spectroscopy.
The distortions to the chelate in the solid state structure of
(iPrPDAI)Fe(DMAP) are consistent with two electron reduction
and hence an intermediate spin ferrous compound, analogous to the
electronic structure previously determined for (iPrPDI)
Fe(DMAP).11 In one of the molecules, the Nimine-Cimine bonds
are elongated to 1.339(3) and 1.355(3) Å while in the other these
values are 1.349(3), 1.343(3) Å, longer than the values in
[(iPrPDAI)Fe]2, where one electron reduction of the unperturbed
portion of the chelate was observed. Likewise, the Cimine-Cipso bond
distances are contracted to 1.407(4) and 1.406(4) Å in one molecule
and to 1.406(3) and 1.406(3) Å in the other. These distortions are
comparable and in some instances more pronounced that in the
ketimine compound. The iron-nitrogen bond distances in
(iPrPDAI)Fe(DMAP) also establish an intermediate rather than
high spin ferrous compound. Short Fe-Npyr distances of 1.836(2)
and 1.833(2) Å are observed along with contracted Fe-Nimine
bonds of 1.904(2), 1.922(2), 1.889(2), and 1.917(2) Å. In analogous
high spin compounds, the latter values are typically over 2.0 Å.
The electronic structures of (iPrPDAI)Fe(η4-C4H6), (iPrPDAI)
Fe(CO)2, and (iPrPDAI)Fe(DMAP) were also investigated by zero-
field 57Fe M€ossbauer spectroscopy. Representative spectra are
reported in Figure 7, and the experimental parameters for these
andthecorrespondingketiminecompoundsarepresentedinTable4.
For each compound, there is little difference in the isomer shift (δ)
and quadrupole splitting (ΔEQ) between the aldimine- and the
ketimine-substituted compounds, indicating similar electronic
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Attempts to hydrogenate ethyl-3-methylbut-2-enoate with
[(iPrPDAI)Fe]2 under standard conditions outlined in the pre-
ceding paragraph produced no turnover after days at 23 °C. In an
attempt to observe catalytic turnover, a similar experiment was
performed with cyclohexene as the substrate. Previous studies
with the ketimine precatalyst, (iPrPDI)Fe(N2)2, established
much more rapid hydrogenation of the unactivated alkene. Once
again, no turnover was observed after 2 days, demonstrating that
[(iPrPDAI)Fe]2 is not an effective hydrogenation precatalyst nor
does it dissociate in solution to a catalytically active species. As an
alternative, (iPrPDAI)Fe(η4-C4H6) was explored with the goal
of hydrogenating the coordinated olefin to generate an active
iron catalyst. For comparison, the corresponding bis(imino)-
pyridine iron butadiene complex, (iPrPDI)Fe(η4-C4H6),9 was
also studied. Each catalytic hydrogenation of ethyl-3-methylbut-
2-enoate was carried out for 24 h and the conversion to
By comparison, the ketimine-substituted iron butadiene com-
pound (iPrPDI)Fe(η4-C6H6) undergoes complete conversion to
the corresponding dihydrogen compound (iPrPDI)Fe(η2-H2)4
overnight. Therefore, the difference in hydrogenation activity
between the two complexes can be traced, in part, to the slower
initiation of the aldimine iron butadiene compound relative to its
ketimine counterpart and also to the catalyst deactivation by
coordination of the arene solvent. As a result, the relatively more
open coordination sphere around the iron in the aldimine
complex does not improve catalysis but rather inhibits it by
slowing the likely dissociative precatalyst activation processes
and enabling competing deactivation pathways.
’ CONCLUDING REMARKS
The chemistry of reduced aryl-substituted bis(aldimino)-
pyridine complexes has been explored and compared to their
more studied ketimine counterparts. Two electron reduction of
the iron dibromide precursor in aromatic solvents such as
benzene or toluene furnished substitutionally labile bis-
(aldimino)pyridine arene complexes. Over time in aliphatic
hydrocarbon or ethereal solvents, these compounds converted
to the diiron complex, [(iPrPDAI)Fe]2, where each iron is
bridged via an η2-interaction with an imine group on an adjacent
chelate. M€ossbauer spectroscopy established two high spin
ferrous centers, and modeling the variable temperature magne-
tochemical data provided experimental evidence for diradical
character on the bis(aldimino)chelate. Both arene coordination
and dimerization in lieu of dinitrogen coordination contrast
the known chemistry for analogous ketimine-substituted bis-
(imino)pyridine compounds and suggest that the iron center in
the aldimine variants is less sterically protected. Performing the
reduction of the iron dibromide in the presence of 1,3-butadiene
yielded the diamagnetic diene compound, (iPrPDAI)Fe(η4-C4H6),
which serves as catalyst precursor for olefin hydrogenation
as well as a synthetic precursor for the iron dicarbonyl,
1
product determined by H NMR spectroscopy. The ketimine-
substituted precatalyst, (iPrPDI)Fe(η4-C4H6), produced 60%
conversion after this time interval, comparable to the 65%
conversion reported for the iron bis(dinitrogen) precatalyst,
(iPrPDI)Fe(N2)2.6,7 By comparison, the aldimine-substituted
iron butadiene compound, (iPrPDAI)Fe(η4-C4H6), reached only
20% conversion after 24 h. Running the reaction for 48 h
improved the conversion to 35% but no additional turnover
was observed at longer reaction times. With cyclohexene as the
substrate, the ketimine complex, (iPrPDI)Fe(η4-C4H6) reached
>99% conversion after 1 h while the aldimine compound,
(iPrPDAI)Fe(η4-C4H6), produced only 28% conversion after
4 h, finally reaching >99% conversion after 24 h.
Additional studies were conducted to determine the origin of
the poor catalytic performance of the aldimine-substituted iron
butadiene compound. Treatment of a benzene-d6 solution of
(iPrPDAI)Fe(η4-C4H6) with 4 atm of dihydrogen resulted in loss
of butane over the course of days at 23 °C with concomitant
formation of the iron benzene complex, (iPrPDAI)Fe(η6-C6D6)
(eq 7). Accordingly, analysis of the iron compound following
catalytic hydrogenation of cyclohexene by 1H NMR spectrosco-
py established formation of (iPrPDAI)Fe(η6-C6D6) along with
other unidentified (both diamagnetic and paramagnetic) iron
compounds. No evidence for formation of [(iPrPDAI)Fe]2 was
obtained by 1H NMR spectroscopy following these experiments
or hydrogenations conducted in aliphatic hydrocarbon solvent.
Catalyst recycling experiments were also conducted whereby the
hydrogenation of cyclohexene was conducted in benzene-d6 with
5 mol % of (iPrPDAI)Fe(η4-C4H6) and run to >95% conversion.
The reaction was then recharged with another 20 equiv of
cyclohexene (per original amount of (iPrPDAI)Fe(η4-C4H6))
and 4 atm of H2 added. After 16 h, 40% conversion to alkane was
observed. For comparison, 82% conversion was observed with
the original charge after 16 h, suggesting competing catalyst
deactivation.
(
iPrPDAI)Fe(CO)2, and 4-N,N-dimethylaminopyridine,
(iPrPDAI)Fe(DMAP), derivatives. The carbonyl stretching fre-
quencies of (iPrPDAI)Fe(CO)2 establish a slightly less electron
rich iron center than the corresponding ketimine-substituted
bis(imino)pyridine compound, but M€ossbauer spectroscopy on
all compounds established little change in the overall electronic
structure. The relatively poor catalytic performance of the
aldimine precursor was traced to the more open coordination
environment around the iron center, resulting in slower pre-
catalyst activation reactions and enabling catalyst deactivation
events.
’ 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 contain-
ing an atmosphere of purified nitrogen. Solvents for air- and moisture-
sensitive manipulations were dried and deoxygenated using literature
procedures.42 Benzene-d6 was purchased from Cambridge Isotope
Laboratories, distilled from Na metal, 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 Inova
1
500 spectrometer operating at 125.893 MHz. All H and 13C NMR
chemical shifts are reported relative to SiMe4 using the 1H (residual) and
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13C chemical shifts of the solvent as a secondary standard. Infrared
spectra were collected on a Thermo Nicolet spectrometer. Elemental
analyses were performed at Robertson Microlit Laboratories, Inc., in
Madison, NJ.
Preparation of (iPrPDAI)Fe(η6-C6H6). This compound was
prepared in a similar manner to (iPrPDAI)Fe(η6-C7H8) with 0.500 g
(0.747 mmol) of (iPrPDAI), 0.45 g Na in 9.0 g of Hg and approximately
15 mL of benzene. This procedure yielded 0.341 g (78%) of a brown
solid identified as (iPrPDAI)Fe(η6-C6H6). 1H NMR (benzene-d6): δ =
0.93 (d, 6.8 Hz, 6H, CH(CH3)2), 1.32 (d, 6.8 Hz, 12H, CH(CH3)2),
1.37 (d, 6.8 Hz, 6H, CH(CH3)2), 3.44 (q, 6.8 Hz, 2H, CH(CH3)2), 3.55
(q, 6.8 Hz, 2H, CH(CH3)2), 6.57 (d, 8.0 Hz, 1H, m-py), 6.76 (t, 8.0 Hz,
1H, p-py), 7.21 (d, 2H, 7.6 Hz, m-Ar), 7.26 (d, 2H, 7.6 Hz, m-Ar), 7.32 (t,
2H, 7.6 Hz, p-Ar), 7.74 (d, 8.0 Hz, 1H, m-py), 9.69 (s, 1H, NdCH), one
NdCH peak under residual benzene peak. 13C NMR (benzene-d6): δ =
23.43 (CH(CH3)2), 24.34 (CH(CH3)2), 27.29 (CH(CH3)2), 28.32
(CH(CH3)2), 28.83 (CH(CH3)2), 118.30 (m-py), 120.72 (m-py),
123.56 (Ar), 124.16 (Ar), 125.48 (Ar), 127.17 (Ar), 127.73 (p-py),
128.92 (Ar), 138.72, 141.73, 141.87, 148.90, 150.18, 155.41, 160.77
(quaternary carbons), 164.93 (NdCH), one NdCH peak coincident
with the benzene peak.
Single crystals suitable for X-ray diffraction were coated with poly-
isobutylene oil in a drybox, transferred to a nylon loop, and then quickly
transferred to the goniometer head of a Bruker X8 APEX2 diffract-
ometer 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
SAINTþ 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.
Zero-field 57Fe M€ossbauer spectra were recorded on a SEE Co.
M€ossbauer spectrometer (MS4) at 80 K in constant acceleration mode.
57Co/Rh was used as the radiation source. WMOSS software was used
for the quantitative evaluation of the spectral parameters (least-squares
fitting to Lorentzian peaks). The minimum experimental line widths
were 0.23 mm s-1. The temperature of the sample was controlled
by a Janis Research Co. CCS-850 He/N2 cryostat within an accuracy of
(0.3 K. Isomer shifts were determined relative to R-iron at 298 K.
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 suscept-
ibility were corrected for the underlying diamagnetic increment by using
tabulated Pascal constants and the effect of the blank sample holders
(gelatin capsule/straw). Samples used for magnetization measurement
were recrystallized multiple times and checked for chemical composition
by 1H NMR and M€ossbauer spectroscopy. The program julX written by
E. Bill was used for the simulation and analysis of magnetic susceptibility
data.43
Preparation of [(iPrPDAI)Fe]2. The arene complex, (iPrPDAI)Fe-
(η6-C7H8), was generated as described above with 0.500 g (0.747
mmol) of (iPrPDAI)FeBr2, 0.045 g (1.96 mmol) of sodium metal, and
9.00 g (44.9 mmol) of mercury. The resulting crude solid was dissolved
in diethyl ether and stored at -35 °C overnight. The solution turned
green-brown, and a crystalline solid formed. The crystals were collected
and dried and yielded 0.165 g (43%) of a green-brown solid identified as
[(iPrPDAI)Fe]2. Crystals suitable for X-ray diffraction were grown from
a diethyl ether solution. Analysis for C62H78N6Fe2: Calcd C, 73.08;
H, 7.71; N, 8.25. Found C, 73.16; H, 7.76; N, 7.83. Magnetic suscept-
ibility: μeff = 4.2 μB (Gouy balance, 296 K), μeff = 4.3 μB (SQUID,
300K). 1H NMR (benzene-d6): δ = -63.59 (50 Hz), -15.43 (15 Hz),
-9.82 (42 Hz), -9.76 (26 Hz), -8.58 (13 Hz), -8.18 (20 Hz), -2.25
(17 Hz), -1.86 (17 Hz), -1.60 (22 Hz), 3.93 (19 Hz), 5.01 (16 Hz),
7.55 (21 Hz), 16.62 (15 Hz), 18.26 (106 Hz), 25.86 (25 Hz), 28.03
(18 Hz), 38.95 (25 Hz), 103.76 (27 Hz), 113.62 (35 Hz), 187.33 (158 Hz),
209.28 (61 Hz).
Preparation of (iPrPDAI)FeBr2. A 100 mL round-bottom flask
was charged with 1.55 g (3.42 mmol) of iPrPDAI and 0.736 g (3.41
mmol) of FeBr2. Approximately 20 mL of tetrahydrofuran (THF) was
added, and the resulting reaction mixture was stirred for 16 h producing
a green solution. Pentane (50 mL) was then added to the flask, and blue
solid precipitated. The solid was collected by filtration and yielded 2.12 g
(93%) of a dark blue powder identified as (iPrPDAI)FeBr2. Analysis for
C31H39N3FeBr2: Calcd C, 55.63; H, 5.87; N, 6.28. Found C, 55.51; H,
5.90; N, 6.16. 1H NMR (CD2Cl2): δ = -8.07 (22 Hz), -0.67 (156 Hz,
24H, CH(CH3)2), 1.55 (9 Hz) 1.82 (14 Hz), 3.69 (14 Hz), 14.71 (24
Hz), 59.88 (44 Hz).
Preparation of (iPrPDAI)Fe(η4-C4H6). A thick-walled glass vessel
wascharged with 15.00g (74.78 mmol) of mercury, approximately 30 mL
of pentane, and a stir bar. Sodium metal (0.075 g, 3.26 mmol) was added
to the vessel. The resulting slurry was stirred for 20 min to amalgamate,
then 0.750 g (1.12 mmol) of solid (iPrPDAI)FeBr2 was added to the
vessel. The vessel was transferred out of the drybox and submerged in
liquid nitrogen. On the high vacuum line, the vessel was evacuated, and
2000 mmHg (10.77 mmol, ∼10 equiv, in 5 ꢀ 400 mm portions) of
butadiene was added via calibrated gas bulb. The reaction mixture was
stirred for 24 h at room temperature during which time the solution
changed from blue to orange. After 24 h the excess butadiene was
removed on the high vacuum line, and the vessel was brought back into
the drybox. The orange solution was decanted away from the amalgam
and filtered through Celite. The remaining product was extracted into
diethyl ether and also filtered through Celite. Recrystallization from a
diethyl ether solution at -35 °C yielded 0.440 g (70%) of a brown solid
identified as (iPrPDAI)Fe(η4-C4H6). Crystals suitable for X-ray diffrac-
tion were grown from a hexane solution. Analysis for C35H45N3Fe: Calcd
C, 74.59; H, 8.05; N, 7.46. Found C, 74.19; H, 7.92; N, 7.18. 1H NMR
(benzene-d6): δ = 0.80 (bs, 6H, CH(CH3)2), 0.88 (bs, 6H, CH(CH3)2),
1.12 (bs, 12H, CH(CH3)2), 1.86 (bs, 2H, CH(CH3)2), 2.54 (bs, 2H,
CH(CH3)2), 2.94 (bs, 2H, C4H6 CH2), 3.54 (bs, 2H, C4H6 CH2), 4.59
(bs, 2H, C4H6 CH), 6.99 (bs, 6H, m- and p- Ar), 7.48 (bs, 1H, p-py), 7.95
(bs, 2H, m-py), 8.43 (bs, 2H, NdCH). 1H NMR(toluene-d8, -40 °C): δ
= 0.80 (d, 6.5 Hz, 6H, CH(CH3)2), 0.91 (d, 6.5 Hz, 6H, CH(CH3)2),
1.09 (d, 6.5 Hz, 6H, CH(CH3)2), 1.14 (d, 6.5 Hz, 6H, CH(CH3)2), 1.82
(sept, 6.5 Hz, 2H, CH(CH3)2), 2.52 (sept, 6.5 Hz, 2H, CH(CH3)2), 2.78
(m, 2H, C4H6 CH2), 3.39 (d, 12 Hz, 2H, C4H6 CH2), 4.49 (m, 2H, C4H6
CH), 6.92 (d, 7.0 Hz, 2H, m-Ar), 6.97 (d, 7.0 Hz, 2H m-Ar), 7.11 (t, 2H,
7.0 Hz, p-Ar), 7.43 (t, 7.5 Hz, 1H, p-py), 7.95 (d, 7.5 Hz, 2H, m-py), 8.35
Preparation of (iPrPDAI)Fe(η6-C7H8). A 100 mL round-bottom
flask was charged with 9.0 g (45 mmol) of mercury and approximately
15 mL of toluene. Sodium metal (0.045 g, 1.96 mmol) was added, and
the slurry was amalgamated for 10 min. Solid (iPrPDAI)FeBr2 (0.500 g,
0.747 mmol) was added followed by approximately 5 mL of diethyl
ether. Within 10 min the color of the solution changed from blue to
green to brown. Approximately 2 min after the color had reached a
constant brown color, the solution was decanted from the amalgam and
then filtered through Celite. The amalgam was washed with pentane,
and the wash filtered through Celite. Removal of all solvent and
collection of the crude material yielded 0.365 g (81%) of a brown solid
identified as (iPrPDAI)Fe(η6-C7H8). Dissolution in C6D6 resulted in
immediate displacement of the η6-bound toluene ligand with a molecule
of C6D6 and formation of (iPrPDAI)Fe(η6-C6H6) and 1 equiv of free
toluene. 1H NMR (toluene-d8): δ = 0.92 (d, 6.8 Hz, 6H, CH(CH3)2),
1.32 (d, 6.8 Hz, 12H, CH(CH3)2), 1.41 (d, 6.8 Hz, 6H, CH(CH3)2),
3.40 (q, 6.8 Hz, 2H, CH(CH3)2), 3.55 (q, 6.8 Hz, 2H, CH(CH3)2), 6.60
(d, 8.0 Hz, 1H, m-py), 6.71 (t, 8.0 Hz, 1H, p-py), 7.08 (s, 1H, NdCH),
7.18 (d, 2H, 7.6 Hz, m-Ar), 7.22 (d, 2H, 7.6 Hz, m-Ar), 7.32 (t, 2H, 7.6
Hz, p-Ar), 7.70 (d, 8.0 Hz, 1H, m-py), 9.70 (s, 1H, NdCH).
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ARTICLE
(s, 2H, NdCH). 13C NMR (benzene-d6): δ = 22.79 (CH(CH3)2), 27.02
(CH(CH3)2), 27.44 (CH(CH3)2), 28.10 (CH(CH3)2), 28.65
(CH(CH3)2), 64.69 (C4H6 CH2), 104.54 (C4H6 CH), 118.19 (m-py),
123.80 (m-Ar), 127.14 (p-Ar), 139.00, 147.89, 149.93 (quaternary
carbons), 151.07 (N=CH), p-py not located.
thank Dr. Eckhard Bill for insightful discussions about the
magnetic data.
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room temperature, and an immediate color change from red-brown to
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’ AUTHOR INFORMATION
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Corresponding Author
*E-mail: pchirik@princeton.edu.
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Present Addresses
§Department of Chemistry, Princeton University, Princeton,
New Jersey 08544, United States.
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We thank U.S. National Science Foundation and Deutsche
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Inorganic Chemistry
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