Oxidation and reduction of bis(imino)pyridine iron dicarbonyl complexes

Article abstract of DOI:10.1021/ic200730k

The oxidation and reduction of a redox-active aryl-substituted bis(imino)pyridine iron dicarbonyl has been explored to determine whether electron-transfer events are ligand- or metal-based or a combination of both. A series of bis(imino)pyridine iron dica

Full text of DOI:10.1021/ic200730k

FORUM ARTICLE
pubs.acs.org/IC
Oxidation and Reduction of Bis(imino)pyridine Iron
Dicarbonyl Complexes
Aaron M. Tondreau,† Carsten Milsmann,† Emil Lobkovsky,‡ and Paul J. Chirik*,†
†Department of Chemistry, Princeton University, Princeton, New Jersey 08540, United States
‡Baker Laboratory, Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States
S Supporting Information
b
ABSTRACT: The oxidation and reduction of a redox-active aryl-substituted bis(imino)pyridine iron dicarbonyl has been explored to
determine whether electron-transfer events are ligand- or metal-based or a combination of both. A series of bis(imino)pyridine iron dicarbonyl
compounds, [(iPrPDI)Fe(CO)2]ꢀ, (iPrPDI)Fe(CO)2, and [(iPrPDI)Fe(CO)2]þ [iPrPDI = 2,6-(2,6-iPr2C6H3NdCMe)2C5H3N], which
differ by three oxidation states, were prepared and the electronic structures evaluated using a combination of spectroscopic
techniques and, in two cases, [(iPrPDI)Fe(CO)2]þ and [(iPrPDI)Fe(CO)2], metrical parameters from X-ray diffraction. The data
establish that the cationic iron dicarbonyl complex is best described as a low-spin iron(I) compound (SFe = 1/2) with a neutral
bis(imino)pyridine chelate. The anionic iron dicarbonyl, [(iPrPDI)Fe(CO)2]ꢀ, is also best described as an iron(I) compound but
with a two-electron-reduced bis(imino)pyridine. The covalency of the neutral compound, (iPrPDI)Fe(CO)2, suggests that both
the oxidative and reductive events are not ligand- or metal-localized but a result of the cooperativity of both entities.
’ INTRODUCTION
bis(imino)pyridine diradical (Figure 1). These studies ex-
pand upon the seminal electrochemical work of Toma and
co-workers27,28 and the preparative and spectroscopic ex-
periments of Wieghardt and co-workers29,30 that provided
the foundation for identifying the redox activity of bis-
(imino)pyridines31ꢀ34 and implicated the importance of this
phenomenon in various reaction types including catalysis22
and group-transfer chemistry.35
The discovery that two-electron reduction is bis(imino)pyridine-
based raised the fundamental question of “how many electrons are
required to reduce the iron?” in (iPrPDI)FeLn compounds. It should
be noted that Gambarotta and co-workers have reported the ability
of the bis(imino)pyridine ligand scaffold to accept up to three
electrons,36 rendering assignment of the spectroscopic oxidation
states of subsequent reduction products a priori nonobvious. A
related question is the oxidation of (iPrPDI)FeLn compounds;
would electron loss be ligand- or metal-based? Here we describe
our first efforts toward this objective and report a combined
synthetic and spectroscopic effort aimed at understanding the
oxidation and reduction of bis(imino)pyridine iron dicarbonyl
compounds. These compounds were chosen because the carbonyl
ligands were anticipated to be substitutionally inert and offer CO
stretching frequencies as an additional diagnostic for probing the
electronic structures of the compounds.
One of the principal challenges in base metal catalysis is to enable
two-electron reaction chemistry with transition-metal ions that
have kinetically facile and thermodynamically accessible one-
electron processes.1,2 As nature has amply demonstrated in
metalloenzymes,3 radical participation with the surrounding ligands
is an effective strategy for overcoming one-electron chemistry to
promote a net two-electron transformation. In synthetic com-
pounds, redox-active metalꢀligand combinations have emerged
as useful platforms to manipulate electronic structure to enable
nontraditional redox chemistry4 and ultimately reactivity5ꢀ8 and
catalysis.4a,9,10
Bis(imino)pyridine ligands, 2,6-(R1NdCR2)2C5H3N (R1 =
alkyl, aryl, amino; R2 = H, Me), have a long-standing history in
classical Werner-type coordination chemistry with first-row
transition metals.11ꢀ16 Interest in these ligands was renewed
with Brookhart and Gibson’s introduction of sterically demand-
ing aryl substituents on the imine and demonstration of their
utility in iron- and cobalt-catalyzed ethylene polymerization.17ꢀ20
Our laboratory has since established the catalytic activity of
the reduced bis(imino)pyridine iron dinitrogen complex,
21
(iPrPDI)Fe(N2)2 [iPrPDI = 2,6-(2,6-iPr2ꢀC6H3NdCMe)2-
C5H3N] in olefin hydrogenation, hydrosilylation, [2 þ 2]
cycloaddition,22 and hydrogenative enyne and diyne cycliza-
tions.23 Significantly improved hydrogenation activity was observed
when the size of the 2,6-aryl substituents was reduced to ethyl and
methyl groups, e.g., [(RPDI)Fe]2(μ2-N2)2 (R = Me, Et).24
’ RESULTS AND DISCUSSION
Spectroscopic and computational studies on compounds with
principally σ-donating ligands such as (N,N-dimethylamino)-
pyridine,25 pyridine, or tBuNH226 established that the formal
Fe0 oxidation states are deceiving and that these molecules
are best described as intermediate-spin ferrous compounds
(SFe = 1) antiferromagnetically coupled to a triplet (SPDI = 1)
Oxidation and Reduction of the Bis(imino)pyridine Iron
Dicarbonyl Compound (iPrPDI)Fe(CO)2. The redox chemistry
of (iPrPDI)Fe(CO)2 was initially explored electrochemically to
Received: April 8, 2011
Published: June 13, 2011
r
2011 American Chemical Society
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Figure 1. Electronic structures of two-electron-reduced bis(imino)-
pyridine iron compounds bearing principally σ-donating ligands.
Figure 3. Electronic structure of (iPrPDI)Fe(CO)2 highlighting the
hybrid between the (iPrPDI2ꢀ)FeII(CO)2 and (iPrPDI0)Fe0(CO)2
resonance forms. The DFT-computed HOMO21,25 is shown to illustrate
the covalency in the molecule.
more weakly coordinating anion, [(η5-C5H5)2Fe][BArF ]
4
[BArF4 = B(C6H3-3,5-Me2)4], resulted in productive chemistry
and furnished a dark-purple-black precipitate identified as the
desired cationic bis(imino)pyridine iron dicarbonyl complex,
[(iPrPDI)Fe(CO)2][BArF ], in 98% yield (eq 1).
4
Figure 2. CV of (iPrPDI)Fe(CO)2 (glassy carbon working electrode,
0.1 M [nBu4N][PF6], scan rate 100 mV/s in THF at 295 K versus
ferrocene0/þ).
Solid-state magnetic measurements (magnetic susceptibility
balance) on [(iPrPDI)Fe(CO)2][BArF ] produced an effective
determine the chemical accessibility of the oxidized and
reduced compounds. Cyclic voltammetry on a 1 mM solution
of (iPrPDI)Fe(CO)2 with 0.1 M [nBu4N][PF6] in a tetrahy-
drofuran (THF) solution reveals clean and reversible one-
electron oxidation and reduction events (Figure 2). Scanning
anodically from a starting potential of ꢀ0.93 V (vs ferrocene),
the cyclic voltammogram (CV) shows an oxidation wave at
ꢀ0.49 V and a reduction wave at ꢀ2.46 V, indicating that both
oxidized and reduced bis(imino)pyridine iron carbonyl com-
pounds should be synthetically accessible.
4
magnetic moment of 2.0 μB at 290 K, consistent with one
unpaired electron and a doublet ground state for the molecule.
The poor solubility of the compound in common laboratory
solvents such as THF, diethyl ether, and toluene made char-
1
acterization by H NMR spectroscopy challenging. However,
dichloromethane-d2 proved to be a suitable solvent, and the 1H
NMR spectrum shows six peaks (out of a possible eight for the
C2v-symmetric compound) that are paramagnetically shifted and
broadened (see the Experimental Section).
The solid-state (KBr) IR spectrum of [(iPrPDI)Fe(CO)2]
Before the chemical oxidation and reduction chemistry is
presented, it is useful to review the electronic structure of the
neutral derivative, (iPrPDI)Fe(CO)2. Previous spectroscopic
and computational studies from our laboratory21,25 have
established that the ground state for this molecule can be
described either as a low-spin iron(II) compound with a
singlet (SPDI = 0) dianionic chelate or as a traditional Fe0 d8
complex with a neutral bis(imino)pyridine ligand (Figure 3).
The reason for the ambiguity derives from the density func-
tional theory (DFT)-computed highest occupied molecular
orbital (HOMO) of the compound, which is 68% bis-
(imino)pyridine in character with a large contribution from
the iron center. Unlike (iPrPDI)Fe(N2)2 and other derivatives
[BArF ] exhibits two strong carbonyl bands at 2028 and
4
1981 cmꢀ1. These values shift to 1982 and 1937 cmꢀ1 upon
preparation of the 13C isotopologue, [(iPrPDI)Fe(13CO)2]
[BArF ]. As presented in Table 1, these values are shifted to higher
4
frequency from the neutral analogue and are consistent with the
reduced electron density at the iron center, reducing backbonding to
the carbonyl ligands.
The solid-state structure of [(iPrPDI)Fe(CO)2][BArF ] was
4
determined by X-ray diffraction. Single crystals were obtained
from layering of a dichloromethane solution of the compound
with diethyl ether. A representation of the molecular structure of
the cationic portion of the molecule is presented in Figure 4,
while a depiction of the complete ion pair is reported in the
Supporting Information. The molecular geometry about the iron
center in the cation is best described as idealized square
pyramidal, with the bis(imino)pyridine and one carbonyl ligand
defining the basal plane. There are no obvious close contacts
between the cation and anion.
with weaker field neutral donors,26 iPrPDI)Fe(CO)2 exhibits
(
no spectroscopic evidence for the mixing of low-lying higher-
spin excited states, and therefore the electronic structure
can be considered a hybrid of the Fe0 and FeII limiting
resonance forms.
The oxidation of (iPrPDI)Fe(CO)2 was initially attempted
with [(η5-C5H5)2Fe][BPh4]. This method proved unsuccessful
because the starting bis(imino)pyridine iron dicarbonyl com-
pound was recovered. Use of the ferrocenium reagent with a
A comparison of the bond distances for the cationic iron
dicarbonyl, [(iPrPDI)Fe(CO)2][BArF ], and its neutral analo-
4
gue, (iPrPDI)Fe(CO)2, is reported in Table 2. The NimineꢀCimine
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Table 1. Solid-State (KBr) IR Stretching Frequencies for
(iPrPDI)Fe(CO)2, [(iPrPDI)Fe(CO)2][BArF ], [Na(18-crown-
4
6)(THF)2][(iPrPDI)Fe(CO)2], and 13C Isotopologues (cmꢀ1
)
compound
ν(CO)sym
ν(CO)asym
(
(
iPrPDI)Fe(CO)2
iPrPDI)Fe(13CO)2
1974
1930
2028
1982
1935
1890
1914
1870
1981
1937
1863
1819
[(iPrPDI)Fe(CO)2]þ
[(iPrPDI)Fe(13CO)2]þ
[(iPrPDI)Fe(CO)2]ꢀ
[(iPrPDI)Fe(13CO)2]ꢀ
Figure 5. Zero-field 57Fe M€ossbauer spectrum of [(iPrPDI)Fe(CO)2]-
[BArF ] recorded at 80 K.
4
Figure 4. Molecular structure of the cation of [(iPrPDI)Fe(CO)2]-
[BArF ] at 30% probability ellipsoids. The anion is omitted for clarity.
4
A representation of the full molecule is presented in the Supporting
Information.
Figure 6. X-band EPR spectrum of [(iPrPDI)Fe(CO)2][BArF ] re-
4
corded at 4.2 K in 1:1 toluene:dichloromethane glass.
Table 2. Selected Bond Distances (Å) and Angles (deg) for
[(iPrPDI)Fe(CO)2][BArF ] and (iPrPDI)Fe(CO)2
a
4
bond distances of 1.289(4) and 1.289(4) Å, and the CipsoꢀCimine
lengths of 1.462(4) and 1.459(4) Å in the cation establish that the
bis(imino)pyridine ligand is in its neutral form. By contrast, the
bis(imino)pyridine distances in the neutral iron dicarbonyl,
(iPrPDI)Fe(CO)2, exhibit significant perturbation consistent with
contributions from a closed-shell, dianionic form of the chelate. In
combination with the magnetic data, the observation of a neutral,
[(iPrPDI)Fe(CO)2][BArF
]
4
(
iPrPDI)Fe(CO)2
Fe(1)ꢀN(1)
Fe(1)ꢀN(2)
Fe(1)ꢀN(3)
Fe(1)ꢀC(34)
Fe(1)ꢀC(35)
N(1)ꢀC(2)
N(3)ꢀC(8)
N(2)ꢀC(3)
N(2)ꢀC(7)
C(2)ꢀC(3)
C(7)ꢀC(8)
C(34)ꢀO(1)
C(35)ꢀO(2)
1.984(4)
1.877(2)
1.985(4)
1.801(4)
1.811(4)
1.289(4)
1.289(4)
1.348(4)
1.350(4)
1.462(4)
1.459(4)
1.124(4)
1.128(4)
1.9500(14)
1.8488(14)
1.9622(15)
1.7823(19)
1.7809(19)
1.335(2)
redox-innocent chelate in the cation, [(iPrPDI)Fe(CO)2][BArF ],
4
suggests a low-spin iron(I) compound.
The electronic structure of [(iPrPDI)Fe(CO)2][BArF ] was
1.330(2)
4
1.376(2)
also studied by zero-field M€ossbauer spectroscopy. A representa-
tive spectrum is presented in Figure 5. An isomer shift (δ) of
0.17 mm/s and a quadrupole splitting (ΔEQ) of 0.62 mm/s were
measured. The value of δ increases from 0.03 mm/s upon
oxidation of the neutral compound, (iPrPDI)Fe(CO)2, while the
quadrupole splitting decreases from 1.17 mm/s. The changes in
both the isomer shift and quadrupole splitting are consistent with
oxidation of a highly covalent FeII T Fe0 compound to a more
metal-centered radical, low-spin, iron(I) derivative.
1.372(2)
1.423(2)
1.425(2)
1.147(2)
1.148(2)
N(1)ꢀFe(1)ꢀN(2)
N(1)ꢀFe(1)ꢀN(3)
N(2)ꢀFe(1)ꢀN(3)
N(2)ꢀFe(1)ꢀC(34)
N(2)ꢀFe(1)ꢀC(35)
C(34)ꢀFe(1)ꢀC(35)
79.49(10)
154.39(10)
78.99(10)
167.36(14)
98.51(13)
94.13(16)
79.10(6)
152.56(6)
79.32(6)
157.89(8)
105.09(8)
97.01(9)
The X-band electron paramagnetic resonance (EPR) spec-
trum of [(iPrPDI)Fe(CO)2][BArF ] was recorded in a 1:1
4
dichloromethane/toluene glass at 4.2 K. The experimental
spectrum and its simulation are presented in Figure 6. A rhombic
signal is observed, and the fit of the data yielded the following
g values: gmin = 1.994, gmid = 2.043, and gmax = 2.111, consistent
a The values for (iPrPDI)Fe(CO)2 were taken from ref 21.
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Figure 7. Zero-field 57Fe M€ossbauer spectrum of [Na-15-crown-5]-
[(iPrPDI)Fe(CO)2] recorded at 80 K.
Figure 8. X-band EPR spectrum of [Na-15-crown-5][(iPrPDI)Fe-
(CO)2] recorded at 77 K in toluene glass.
with an S = 1/2 compound. The g anisotropy indicates an iron-
rather than ligand-based radical as the singly occupied molecalur
orbital (SOMO) of the compound. The spectrum of the 13CO
The electronic structure of [Na-15-crown-5][(iPrPDI)Fe-
(CO)2] was also explored by zero-field 57Fe M€ossbauer
(Figure 7) and EPR (Figure 8) spectroscopies. The M€ossbauer
parameters (δ = ꢀ0.01 mm/s; ΔEQ = 1.19 mm/s) for the
anion are indistinguishable from those of the neutral complex
(δ = 0.03, ΔEQ = 1.19 mm/s), suggesting little perturbation to
the electronic environment of the iron nucleus upon reduction.
The X-band EPR spectrum, recorded in toluene glass at 77 K,
isotopologue, [(iPrPDI)Fe(13CO)2][BArF ], recorded under
4
the same conditions did not exhibit any detectable differences
from the natural abundance compound.
The one-electron reduction of (iPrPDI)Fe(CO)2 proved more
challenging than the corresponding oxidation, consistent with
the electrochemistry (Figure 2). Stirring (iPrPDI)Fe(CO)2 in the
presence of excess 0.5% Na(Hg) did not yield an identifiable
compound. Considerable amounts of the starting neutral iron
dicarbonyl compound were recovered. Repeating the reduction
procedure with excess 0.5% Na(Hg) in the presence of 1.2 equiv of
15-crown-5 furnished a green precipitate. Analysis of the product
mixture by solid-state (KBr) IR spectroscopy revealed six bands;
the largest two appear at 1935 and 1863 cmꢀ1 and are assigned to
the desired iron dicarbonyl dianion, [Na-15-crown-5][(iPrPDI)Fe-
(CO)2] (eq 2). These values are at lower energy than the neutral
and cationic variants, consistent with a more electron-rich iron
center. The four other CO bands observed in the spectrum are
attributed to unidentified decomposition products.
1
exhibits an axial signal consistent with an S = /2 compound.
Values obtained from the fit are gmin = 1.974, gmid = 2.004, and
gmax = 2.006. The g anisotropy (Δg = 0.032) is low and suggests
a ligand-based rather than an iron-based SOMO.
Computational Studies. The electronic structures of the
bis(imino)pyridine iron dicarbonyl cation and anion were ex-
amined by DFT. Our laboratory has demonstrated the utility of
augmenting experimentally obtained structural and spectro-
scopic data with DFT calculations to fully understand the
electronic structure of redox-active bis(imino)pyridine iron
compounds.25,37 Complete spectroscopic and computational
determination of the electron structure of the neutral compound,
(iPrPDI)Fe(CO)2, has been reported previously.25 Broken-
symmetry (BS) DFT calculations were performed at the
B3LYP level to understand the electronic structures of [(iPrPDI)-
Fe(CO)2]þ and [(iPrPDI)Fe(CO)2]ꢀ. For both the cation and
anion, the corresponding counterions [BArF ]ꢀ and [Na-15-
4
crown-5]þ were not included in the calculations, respectively.
No other truncations were performed.
The cationic bis(imino)pyridine iron dicarbonyl compound,
[(iPrPDI)Fe(CO)2]þ, was computed as a spin-unrestricted
doublet, in agreement with the experimentally established
ground state. The resulting ground state did not exhibit any
significant BS character, as is nicely shown by the expectation
value of S of 0.76 (ideally 0.75 for S = /2). The computed
structural parameters of the molecule obtained after geometry
optimization are in excellent agreement with the experimentally
observed values. A qualitative molecular orbital diagram and
Mulliken spin density plot from the ground-state solution are
presented in Figure 9. From these studies, [(iPrPDI)Fe(CO)2]þ
is best described as a low-spin iron(I) compound with three
doubly occupied principally iron-based d orbitals and a SOMO
that is primarily Fe dz2 in character. As illustrated by both the
molecular orbital diagram and the spin density plot, the bis-
(imino)pyridine ligand is in its neutral form with little direct
involvement in the overall electronic structure of the complex.
Obtaining [Na-15-crown-5][(iPrPDI)Fe(CO)2] in pure form
was challenging because the molecule proved highly sensitive and
prone to decomposition. The compound was only sparingly
soluble in toluene, THF, and diethyl ether. Chlorinated solvents
such as dichloromethane induced immediate decomposition. For
solvents where partial solubility was observed, decomposition
occurred over the course of approximately 24 h. In the solid state,
[Na-15-crown-5][(iPrPDI)Fe(CO)2] was stable for weeks if
stored in an inert atmosphere at ꢀ35 °C. To obtain clean
spectroscopic data on [Na-15-crown-5][(iPrPDI)Fe(CO)2],
the green solid isolated from the reduction reaction was washed
with copious amounts of toluene, followed by pentane, and the
resulting materials were quickly dried under reduced pressure.
Unfortunately, attempts to obtain single crystals of this com-
pound have thus far been unsuccessful.
2
1
^
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Figure 10. (a) Qualitative molecular orbital diagram for [(iPrPDI)-
Fe(CO)2]ꢀ from a spin-unrestricted B3LYP DFT calculation. S repre-
sents the spatial overlap of the antiferromagnetically coupled magnetic
orbitals. (b) Spin density plot obtained from a Mulliken population
analysis (red, positive spin density; yellow, negative spin density). The
coordinate axis is defined as the z direction, being the apical vector of an
idealized square pyramid.
Figure 9. (a) Qualitative molecular orbital diagram for [(iPrPDI)Fe-
(CO)2]þ from a spin-unrestricted B3LYP DFT calculation. (b) Spin density
plot obtained from a Mulliken population analysis (red, positive
spin density; yellow, negative spin density). The coordinate axis is
defined as the z direction, being the apical vector of an idealized
square pyramid.
This view of the electronic structure is in agreement with the
metrical parameters established from X-ray diffraction and the
dichloromethane glass EPR spectrum of the compound. To
further support the validity of the calculated electronic structure,
57Fe M€ossbauer parameters were computed. The obtained
isomer shift of 0.24 mm/s and the quadrupole splitting of
ꢀ0.85 mm/s are in good agreement with the experimentally
determined values.
The geometry-optimized structure (Table S1 in the Supporting
Information) is consistent with a strongly reduced bis(imino)-
pyridine ligand with long NimineꢀCimine bond distances of 1.362 Å
and short CipsoꢀCimine bonds of 1.414 Å. Accordingly, the ground
2
38
^
state arises from a BS (2,1) solution (S = 1.11), with the
electronic structure of the molecule best described as a low-spin
iron(I) compound with a triplet (SPDI = 1) bis(imino)pyridine
diradical. Antiferromagnetic coupling between the iron and bis-
(imino)pyridine ligand spins accounts for the overall S = 1/2 ground
state. It should be noted that the value of S of 0.81, a measure of the
The computational results for the bis(imino)pyridine iron
dicarbonyl anion, [(iPrPDI)Fe(CO)2]ꢀ, are presented in Figure 10.
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Figure 11. Electronic structure summary for bis(imino)pyridine iron dicarbonyl compounds that differ by three oxidation states.
spatial overlap between magnetic orbitals, is large. This indicates a
high degree of covalency in the molecule and suggests that the true
electronic structure is more complex than the relatively simple
[(iPrPDI2-)FeI(CO)2]ꢀ assignment. Nevertheless, the computa-
tional results clearly establish a doubly reduced bis(imino)pyridine
chelate and shed additional light on the electronic structure of
[(iPrPDI2ꢀ)FeI(CO)2]ꢀ because the instability of the molecule has
hampered structural characterization. This is also reflected in the
correct prediction of the M€ossbauer spectrum based on the
electronic structure described above. The computed parameters of
δ = 0.08 mm/s and ΔEQ = ꢀ1.39 mm/s are in reasonable
agreement with the experimental values.
A summary of the spectroscopic data and electronic structures
for the series of bis(imino)pyridine iron dicarbonyl compounds is
presented in Figure 11. Each of the electronic structures presented
in Figure 11 has been corroborated by DFT calculations. The most
oxidized compound in the series, [(iPrPDI)Fe(CO)2]þ, is a rare
example of a spectroscopically characterized bis(imino)pyridine
iron(I) compound. Interestingly, if the neutral iron dicarbonyl,
[(iPrPDI)Fe(CO)2], is viewed in its low-spin ferrous canonical
form, the overall oxidation of the molecule results in a net
reduction of the iron. A formal two-electron loss from the chelate
triggers a reduction of the iron from FeII to FeI. This phenomenon
has been observed previously with redox-active bis(imino)-
pyridine metal complexes.29
The most reduced compound in the series, the bis-
(imino)pyridine iron dicarbonyl anion, [(iPrPDI)Fe(CO)2]ꢀ,
is best described as a low-spin iron(I) compound with a triplet
bis(imino)pyridine diradical dianion. The DFT studies estab-
lished a high degree of covalency in the molecule, consistent with
the observed low M€ossbauer isomer shift, and suggest that the
[(iPrPDI2ꢀ)FeI(CO)2]ꢀ description is likely an oversimplifica-
tion. Nevertheless, the DFT results clearly establish a diradical
form of the chelate, the only compound in the series to exhibit
radical character on the ligand.
1H NMR spectra were recorded on Varian Mercury 300 and 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
1
Inova 500 spectrometer operating at 125.893 MHz. All H and 13C
1
NMR chemical shifts are reported relative to SiMe4 using the H
(residual) and 13C chemical shifts of the solvent as a secondary
standard. Peak widths at half heights are reported for paramagnetically
broadened and shifted resonances. Solution magnetic moments were
determined by the Evans method using a ferrocene standard and
are the average value of at least two independent measurements. Gouy
balance measurements were performed with a Johnson Matthey
instrument that was calibrated with HgCo(SCN)4. Elemental ana-
lyses were performed at Robertson Microlit Laboratories, Inc., in
Madison, NJ.
Single crystals suitable for X-ray diffraction were coated with
polyisobutylene oil in a drybox, transferred to a nylon loop, and then
quickly transferred to the goniometer head of a Bruker X8 APEX2
diffractometer equipped with a molybdenum X-ray tube (λ = 0.71073 Å).
Preliminary data revealed the crystal system. A hemisphere routine was
used for data collection and determination of lattice constants. The
space group was identified, and the data were processed using the Bruker
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.
M€ossbauer data were collected on an alternating constant-acceleration
spectrometer. The minimum experimental line width was 0.24 mm/s (full
width at half-height). A constant sample temperature was maintained with
an Oxford Instruments Variox or an Oxford Instruments M€ossbauer-
Spectromag 2000 cryostat. Reported isomer shifts (δ) are referenced to
iron metal at 293 K. CVs were collected in a THF solution [1 mM in
(
iPrPDI)Fe(CO)2] with 0.1 M [nBu4N][PF6] (0.1 M) electrolyte, using a
3 mm glassy carbon working electrode, platinum wire as the counter
electrode, and silver wire as the reference electrode in a drybox equipped
with electrochemical outlets. CVs were recorded using a BASi EC Epsilon
electrochemical workstation and analyzed using the BASi Epsilon-EC
software. All CVs were run at a scan rate of 100 mV/s at 295 K. Potentials
are reported versus ferrocence/ferrocenium and were obtained using the
in situ method.40
’ EXPERIMENTAL SECTION
General Considerations. All air- and moisture-sensitive mani-
pulations were carried out using standard vacuum-line, Schlenk-line, and
cannula techniques in an MBraun inert-atmosphere drybox containing
an atmosphere of purified nitrogen. Solvents for air- and moisture-
sensitive manipulations were initially dried and deoxygenated using
literature procedures.39 Benzene-d6 was purchased from Cambridge
Isotope Laboratories and dried over 4 Å molecular sieves. The neutral
bis(imino)pyridine iron dicarbonyl compound, (iPrPDI)Fe(CO)2, was
prepared according to literature methods.21 The 13C isotopologue,
Quantum-Chemical Calculations. All DFT calculations were
performed with the ORCA program package.41 The geometry optimiza-
tions of the complexes and single-point calculations on the optimized
geometries were carried out at the B3LYP level42ꢀ44 of DFT. This hybrid
functional often gives better results for transition-metal compounds
than pure gradient-corrected functionals, especially with regard to metalꢀ
ligand covalency.45 The all-electron Gaussian basis sets were those
developed by Ahlrichs' group.46ꢀ48 Triple-ζ quality basis sets def2-TZVP
with one set of polarization functions on the metals and on the atoms
directly coordinated to the metal center were used. For the carbon and
(
iPrPDI)Fe(13CO)2, was synthesized in an identical manner using 13CO.
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Inorganic Chemistry
FORUM ARTICLE
hydrogen atoms, slightly smaller polarized split-valence def2-SV(P) basis
sets were used that were of double-ζ quality in the valence region and
contained a polarizing set of d functions on the non-hydrogen atoms.
Auxiliary basis sets were chosen to match the orbital basis.49ꢀ51 The
RIJCOSX52ꢀ54 approximation was used to accelerate the calculations.
Throughout this paper we describe our computational results by
using the BS approach by Ginsberg55 and Noodleman et al.56 Because
several BS solutions to the spin-unrestricted KohnꢀSham equations
may be obtained, the general notation BS(m,n)57 has been adopted,
where m (n) denotes the number of spin-up (spin-down) electrons at
the two interacting fragments. Canonical and corresponding58 orbitals,
as well as spin density plots, were generated with the program Molekel.59
Nonrelativistic single-point calculations on the optimized geometry
were carried out to predict M€ossbauer spectral parameters (isomer shifts
and quadrupole splittings). These calculations employed the CP(PPP)
basis set for iron.60 The M€ossbauer isomer shifts were calculated from
the computed electron densities at the iron centers as previously
described.61,62
between U.S. and German Investigators grant. We also thank Jon
Darmon and Dr. Suzanne Bart for performing the electrochemical
studies.
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’ ASSOCIATED CONTENT
S
Supporting Information. Crystallographic details for
b
[(iPrPDI)Fe(CO)2][BArF ] in CIF format and DFT-computed
4
bond distances. This material is available free of charge via the
Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: pchirik@princeton.edu.
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We thank the U.S. National Science Foundation and Deutsche
Forschungsgemeinschaft for a Cooperative Activities in Chemistry
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