Electronic effects in 4-substituted bis(imino)pyridines and the corresponding reduced iron compounds

Article abstract of DOI:10.1021/om201212m

A family of 4-substituted bis(imino)pyridines, 4-X-^iPrPDI (4-X-^iPrPDI = 2,6-(2,6-ⁱPr₂-C₆H ₃N=CMe)₂-4-X-C₅H₂N; X = CF ₃, ^tBu, Bn, NMe₂), has been synthesized and the iron coordination chemistry studied. Sodium amalgam reduction of the iron dihalides (4-X-^iPrPDI)FeX₂ (X = Cl, Br) in the presence of excess carbon monoxide furnished the corresponding iron dicarbonyl compounds (4-X-^iPrPDI)Fe(CO)₂. Equilibrium mixtures of the four- and five-coordinate iron dinitrogen compounds (4-X-^iPrPDI)FeN ₂ and (4-X-^iPrPDI)Fe(N₂)₂ were prepared by performing the sodium amalgam reduction of the iron dihalides under a dinitrogen atmosphere. Electrochemical and spectroscopic measurements were conducted on the free ligands and the iron derivatives to systematically evaluate the influence of each para pyridine substituent on the electronic structure of the compound.

Full text of DOI:10.1021/om201212m

Article
pubs.acs.org/Organometallics
Electronic Effects in 4-Substituted Bis(imino)pyridines and the
Corresponding Reduced Iron Compounds
Jonathan M. Darmon,†,‡ Zoe R. Turner,† Emil Lobkovsky,‡ and Paul J. Chirik*,†
̈
†Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
‡Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States
S
* Supporting Information
ABSTRACT: A family of 4-substituted bis(imino)pyridines,
4-X-iPrPDI (4-X-iPrPDI = 2,6-(2,6-iPr2-C6H3NCMe)2-4-X-
t
C5H2N; X = CF3, Bu, Bn, NMe2), has been synthesized and
the iron coordination chemistry studied. Sodium amalgam
reduction of the iron dihalides (4-X-iPrPDI)FeX2 (X = Cl, Br)
in the presence of excess carbon monoxide furnished the
corresponding iron dicarbonyl compounds (4-X-iPrPDI)Fe-
(CO)2. Equilibrium mixtures of the four- and five-coordinate
iron dinitrogen compounds (4-X-iPrPDI)FeN2 and (4-
X-iPrPDI)Fe(N2)2 were prepared by performing the sodium
amalgam reduction of the iron dihalides under a dinitrogen atmosphere. Electrochemical and spectroscopic measurements were
conducted on the free ligands and the iron derivatives to systematically evaluate the influence of each para pyridine substituent
on the electronic structure of the compound.
[(iPrPDI2−)FeII] and [(iPrPDI0)Fe0] resonance forms, analogous
INTRODUCTION
Iron is an attractive precious-metal surrogate due to its natural
■
to the case for the dicarbonyl derivative (iPrPDI)Fe(CO)2.24 In
abundance, low cost, and biocompatibility.1 Aryl-substituted
this bonding regime, the bis(imino)pyridine is classified as
“redox non-innocent”, a term coined by Jørgensen in the
bis(imino)pyridine iron compounds have emerged as an
important class of catalysts. Interest in these compounds
intensified with the discovery of high-activity ethylene
context of nitrosyl ligands to signal ambiguity in metal
oxidation state assignment.25
polymerization upon activation of (ArPDI)FeCl2 with methyl-
The electronic structure of the four-coordinate bis(imino)-
pyridine iron dinitrogen compound (iPrPDI)FeN2 differs from
that of its five-coordinate counterpart. Variable-temperature
aluminoxane (MAO).2,3 Numerous modifications have been
made to the bis(imino)pyridine ligand scaffold to alter the
degree of oligomerization or polymerization,4 while inves-
tigations into the nature of the propagating species have
established the efficacy of high-spin iron(II) alkyl cations with a
neutral bis(imino)pyridine chelate.5,6
NMR, Mossbauer, X-ray absorption, and X-ray emission
̈
spectroscopy, in combination with DFT calculations, support
an intermediate-spin ferrous compound antiferromagnetically
coupled to a bis(imino)pyridine triplet diradical.26 Population
of a thermally accessible paramagnetic excited state gives rise to
Inspired by the thermal7−10 and photochemical11 activity of
Fe(CO)5 in unactivated olefin hydrogenation and hydro-
silylation reactions,12 our laboratory sought to prepare a
modular mimic of the proposed [Fe(CO)3] active species13−20
by two-electron reduction of (ArPDI)FeX2 (X = halide)
derivatives. Stirring (iPrPDI)FeBr2 (iPrPDI = 2,6-(2,6-iPr2-
C6H3NCMe)2C5H3N) with excess 0.5% sodium amalgam
under an N2 atmosphere yielded, following filtration and
recrystallization, the five-coordinate bis(imino)pyridine iron
dinitrogen compound (iPrPDI)Fe(N2)2. This compound is in
equilibrium with the four-coordinate iron mono(dinitrogen)
derivative (iPrPDI)FeN2 where the relative concentrations of
each species depend on the N2 pressure. Investigations into the
electronic structures of the two compounds established
participation of the bis(imino)pyridine chelate in both cases,
a well-established phenomenon with this ligand class.21−23 The
five-coordinate compound (iPrPDI)Fe(N2)2 is a highly covalent
compound best described as a hybrid structure between
1
temperature-dependent H NMR chemical shifts, a phenom-
enon attributed to poor overlap of an iron SOMO with the
ligand-centered radicals. This behavior is distinct from that of
(iPrPDI)Fe(DMAP) and other related four-coordinate iron
compounds with principally σ-donating ligands, where a triplet
excited state mixes into the diamagnetic ground state via spin−
orbit coupling, giving rise to temperature-independent para-
magnetism.23a,27 The ground states of this class of compounds
are also best described as intermediate-spin ferrous derivatives
antiferromagnetically coupled to bis(imino)pyridine triplet
diradicals. In both (iPrPDI)FeN2 and (iPrPDI)Fe(DMAP), we
refer to the bis(imino)pyridine chelate as “redox active” to
signify direct participation in the electronic structure and the
presence of ligand-centered radicals. This terminology is used
Received: December 5, 2011
Published: March 12, 2012
© 2012 American Chemical Society
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Figure 1. 4-Substituted bis(imino)pyridines used in this study and associated shorthand designations.
to distinguish this situation, where the metal oxidation state is
unambiguously iron(II), from the “redox non-innocent”
examples such as (iPrPDI)Fe(N2)2 and (iPrPDI)Fe(CO)2,
where a single oxidation state formalism is inadequate.
The equilibrium mixture of the four- and five-coordinate
bis(imino)pyridine iron dinitrogen compounds serves as an
effective precatalyst for alkene hydrogenation,28−31 hydro-
silylation,28,29,32 and [2 + 2] cycloaddition reactions.33 Since
the initial preparation and discovery of the catalytic perform-
ance of (iPrPDI)Fe(N2)n (n = 1, 2), our laboratory has been
engaged in efforts to improve catalytic activity and substrate
scope through systematic modification of the bis(imino)-
pyridine architecture. Replacement of the imine methyl groups
with phenyl substituents yielded an iron dinitrogen compound,
are likely compatible with the reducing methods used to
prepare the corresponding iron dinitrogen complexes. The two
alkyl-substituted bis(imino)pyridines, 4-Bn-iPrPDI and 4-tBu-
PDI, were synthesized according to the previously reported
methods of Cam
́
pora38 and Burger,39 respectively.
The CF3 and NMe2 derivatives, 4-CF3-iPrPDI and 4-
Me2N-iPrPDI, were prepared as outlined in Scheme 1. Both
syntheses began with chlorination of chelidamic acid with
SOCl2. Introduction of the NMe2 substituent was accomplished
by initial hydrolysis to the diacid chloride followed by treatment
with aqueous dimethylamine at 160 °C.40 With this material in
hand, conversion to the 4-NMe2-substituted diacetylpyridine
and ultimately the bis(imino)pyridine proceeded via standard
methods.41
For the CF3 example, esterification of the trichloro
compound with methanol at 45 °C furnished the 4-chloro
methyl ester, which was then treated with NaI in the presence
of CH3C(O)Cl in acetonitrile to yield the corresponding
iodide.42 Introduction of the CF3 group was accomplished with
FSO2CF2CO2CH3 in the presence of 5 equiv of CuI and 5%
(dppf)PdCl2 at 100 °C, following the procedure of Johnson
utilized to prepare 4-substituted pyridine bis(oxazolines).43 As
with the NMe2 derivative, conversion to diacetylpyridine and
the bis(imino)pyridine was accomplished using standard
methods.41
(
iPrBPDI)Fe(N2)2
(
iPrBPDI = 2,6-(2,6-iPr2-C6H3N
CPh)2C5H3N), that exhibited increased initial hydrogenation
activity for α-olefins such as 1-hexene, but rapid intramolecular
coordination of phenyl or 2,6-diisopropyl aryl groups resulted
in catalyst deactivation and shorter lifetimes. Reducing the size
of the 2,6-aryl substituents from isopropyl to ethyl or methyl
resulted in dimeric iron dinitrogen complexes, [(RPDI)Fe-
(N2)]2(μ2-N2) (RPDI = 2,6-(2,6-R2-C6H3NCMe)2C5H3N; R
= Me, Et), that exhibited significantly increased activity for the
hydrogenation of ethyl 3-methylbut-2-enoate.34 However,
replacing the aryl imino substituents with alkyl groups35 or
the bis(imino)pyridine with related α-diimines36 is deleterious,
as catalytically inactive bis(ligand) iron or arene complexes are
formed.
Synthesis of the iron dihalide complexes of each of the 4-
substituted bis(imino)pyridine ligands was accomplished in a
straightforward manner2,3 by stirring the free ligand with either
FeCl2 or FeBr2 in THF for 12−24 h followed by precipitation
with pentane and collection by filtration (eq 1). Each of the
Given the now well-established redox activity of bis(imino)-
pyridines,23 in-plane electronic modifications may impact the
electronic structure and ultimately the reactivity/catalytic
activity of the resulting iron complex. Zhu and Budzelaar
have evaluated substituent effects on the σ-donating and π-
accepting properties of aryl-substituted bis(imino)pyridines and
other related tridentate chelates.37 Variations in the 4-positions
of the pyridine ring were found to change both the donating
and accepting properties of the chelate significantly and have
inspired preparation of several new iron dinitrogen compounds.
Here we describe our efforts toward this objective with the
synthesis, characterization, and systematic evaluation of the
electronic effects of 4-substituted bis(imino)pyridine iron
dinitrogen complexes.
bis(imino)pyridine iron dihalides was isolated as a blue or blue-
green powder in high yield (88−98%) and exhibits an S = 2
ground state, as determined by magnetic susceptibility balance
measurements. Two examples, (4-Me2N-iPrPDI)FeCl2 and
(CF3-iPrPDI)FeCl2, were characterized by X-ray diffraction,
and representations of the solid-state structures are presented
RESULTS AND DISCUSSION
■
Synthesis of 4-Substituted Bis(imino)pyridine Com-
pounds. The series of 4-substituted bis(imino)pyridine ligands
targeted for this study is presented in Figure 1. In each case, the
2,6-diisopropyl aryl imine variant was used such that the steric
environment of the resulting iron series remains, to a first
approximation, constant. Substituents were also selected that
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Scheme 1. Synthesis of 4-Me2N-iPrPDI and 4-CF3-iPrPDI
a
Ligands
1
was routinely characterized by H and 13C NMR spectroscopy,
solution infrared spectroscopy, and combustion analysis.
The solid-state structure of one example, (4-Me2N-iPrPDI)-
Fe(CO)2, was determined by X-ray diffraction, and a
representation of the molecular structure is presented in Figure
3. Selected metrical parameters are reported in Table 2. As with
the structure of (iPrPDI)Fe(CO)2,28 an idealized square-
pyramidal geometry is observed with apical and basal carbonyl
ligands. The observation of a C2v-symmetric molecule by
benzene-d6 1H and 13C NMR spectroscopy at 23 °C indicates
rapid exchange between these positions under these conditions.
The [NMe2] group is nearly coplanar with the pyridine ring,
with a dihedral angle of 10.3(3)° between the two idealized
planes. The bond distances in the bis(imino)pyridine are
statistically indistinguishable from those in (iPrPDI)Fe(CO)2
(Table 2) and support back-bonding between the iron center
and the terdentate chelate.
Given their rich catalytic and stoichiometric chemistry, the
synthesis of the corresponding bis(imino)pyridine iron
dinitrogen complexes was also explored. Inspired by the
conditions used to prepare (iPrPDI)Fe(N2)2,28 each of the
bis(imino)pyridine iron dihalide complexes was stirred with an
excess of 0.5% sodium amalgam in toluene under a dinitrogen
atmosphere. In one case, with (4-Me2N-iPrPDI)FeCl2, an equal
volume of THF was added to the mixture to improve solubility
and facilitate the reduction. Filtration and recrystallization from
pentane or pentane−diethyl ether mixtures at −35 °C
furnished green and green-brown solids identified as the
desired iron dinitrogen compounds (4-X-iPrPDI)Fe(N2)2, in
modest to good yields (39−65%, eq 3).
a
Reagents and conditions: (i) SOCl2; (ii) H2O, 0−45 °C; (iii)
Me2NH (aq, 35%), 160 °C; (iv) SOCl2, MeOH; (v) NaOEt, EtOAc;
HCl; (vi) 2,6-diisopropylaniline, MeOH, H+ (cat); (vii) MeOH, 45
°C; (viii) NaI, CH3C(O)Cl, CH3CN; (ix) FSO2CF2CO2CH3, 5 equiv
of CuI, 5% (dppf)PdCl2·CH2Cl2, DMF, 100 °C.
in Figure 2. Selected metrical parameters, along with those for
(iPrPDI)FeCl2,2b are presented in Table 1.
One exception was (4-CF3-iPrPDI)FeCl2. Reduction of this
compound yielded an intractable mixture of products, mostly
comprised of free bis(imino)pyridine ligand. The solid-state
(KBr) infrared spectrum of the residue isolated following
reduction did not contain any strong bands assignable to N2
stretches, suggesting that no iron dinitrogen complex was
present in the mixture. More careful monitoring of the
reduction reaction by analyzing aliquots by 1H NMR
spectroscopy revealed a smooth one-electron reduction to
yield a paramagnetic intermediate, (4-CF3-iPrPDI)FeCl.44
To probe the electronic effects of each of the 4-substituents,
the corresponding series of bis(imino)pyridine iron dicarbonyl
complexes was prepared. Sodium amalgam reduction of each of
the iron dihalide complexes in toluene solution under 1 atm of
carbon monoxide furnished the corresponding bis(imino)-
pyridine iron dicarbonyl complexes (eq 2). Each (4-X-iPrPDI)-
Fe(CO)2 compound was isolated as a diamagnetic green or
yellow-brown crystalline solid in moderate (63−69%) yield and
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Figure 2. Molecular structures of (4-CF3-iPrPDI)FeCl2 (left) and (4-Me2N-iPrPDI)FeCl2 (right) with 30% probability ellipsoids. In both cases, only
one molecule from the asymmetric unit is shown. Hydrogen atoms are omitted for clarity.
Table 1. Bond Distances (Å) and Angles (deg) for (4-
CF3-iPrPDI)FeCl2, (4-Me2N-iPrPDI)FeCl2, and (iPrPDI)FeCl2
(4-CF3-iPrPDI) (4-Me2N-iPrPDI)
(
iPrPDI)
a
FeCl2
FeCl2
FeCl2
Fe(1)−N(1)
Fe(1)−N(2)
Fe(1)−N(3)
Fe(1)−Cl(1)
Fe(1)−Cl(2)
2.177(3)
2.058(3)
2.184(3)
2.2535(9)
2.2940(10)
2.284(3)
2.100(2)
2.253(3)
2.3339(10)
2.2828(9)
2.222(4)
2.091(4)
2.225(5)
2.2627(17)
2.3173(19)
N(1)−C(2)
N(3)−C(8)
1.291(4)
1.285(4)
1.286(4)
1.284(4)
1.301(7)
1.295(7)
C(2)−C(3)
C(7)−C(8)
1.479(4)
1.480(5)
1.497(4)
1.497(4)
1.466(8)
1.482(8)
N(1)−Fe(1)−N(2)
N(1)−Fe(1)−Cl(1)
N(1)−Fe(1)−Cl(2)
N(2)−Fe(1)−Cl(1)
N(2)−Fe(1)−Cl(2)
N(2)−Fe(1)−N(3)
N(3)−Fe(1)−Cl(1)
N(3)−Fe(1)−Cl(2)
Cl(1)−Fe(1)−Cl(2)
73.92(10)
96.31(7)
103.81(7)
149.11(8)
93.92(8)
73.01(10)
101.68(7)
98.22(7)
116.95(4)
71.90(10)
100.22(8)
104.19(8)
96.77(8)
152.67(8)
72.92(9)
101.28(7)
98.78(7)
110.47(4)
73.67(16)
98.14(12)
100.57(12)
147.90(13)
94.52(13)
72.59(16)
99.28(12)
102.47(12)
117.58(7)
Figure 3. Solid-state structure of (4-Me2N-iPrPDI)Fe(CO)2 with 30%
probability ellipsoids. Hydrogen atoms are omitted for clarity.
observation time scales such as infrared and Mossbauer
̈
spectroscopy (vide infra).
Two of the new bis(imino)pyridine iron bis(dinitrogen)
compounds, (4-Me2N-iPrPDI)Fe(N2)2 and (4-tBu-iPrPDI)Fe-
(N2)2, were also characterized by single-crystal X-ray
diffraction. Representations of the solid-state structures are
presented in Figure 4, and selected bond distances and angles
are presented in Table 3. For (4-tBu-iPrPDI)Fe(N2)2, two
independent molecules were located in the asymmetric unit
that were statistically indistinguishable. Only the values for one
of these molecules are presented in Table 3. Rapid loss of
dinitrogen in the solid state made isolation and handling of the
crystals of both samples challenging; specifically, how each
sample was individually handled produced varying amounts of
the four- and five-coordinate compounds.
As is frequently observed in five-coordinate bis(imino)-
pyridine iron compounds bearing two neutral ligands, near-
idealized square-pyramidal geometries are observed with N2
ligands in the apical and basal positions. For (4-Me2N-iPrPDI)-
Fe(N2)2, the [NMe2] group is essentially coplanar with the
pyridine ring, with a dihedral angle between the two idealized
planes of approximately 3°. All three crystallographically
characterized dinitrogen compounds, (iPrPDI)Fe(N2)2,
(4-tBu-iPrPDI)Fe(N2)2, and (4-Me2N-iPrPDI)Fe(N2)2, exhibit
a
Values taken from ref 2b.
However, continued reduction resulted in consumption of this
intermediate, and no new iron compounds were identified.
Each of the (4-X-iPrPDI)Fe(N2)2 complexes was routinely
1
characterized by H and 13C NMR infrared and zero-field
Mossbauer spectroscopy. As has been observed previously,28
̈
the five-coordinate iron bis(dinitrogen) complexes were
isolated upon recrystallization but reverted to a mixture of
(4-X-iPrPDI)Fe(N2)2 and (4-X-iPrPDI)Fe(N2) compounds in
solution. Benzene-d6 solution infrared spectra of two examples,
(4-tBu-iPrPDI)Fe(N2)n and (4-Me2N-iPrPDI)Fe(N2)n (n = 1, 2),
are presented in the Supporting Information. The H and 13C
1
NMR spectra exhibit the appropriate number of resonances for
a single C2v-symmetric compound, demonstrating rapid
interconversion between the four- and five-coordinate com-
pounds in solution on the NMR time scale. The presence of
both species can be detected by techniques with shorter
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Table 2. Metrical Parameters for (4-Me2N-iPrPDI)Fe(CO)2
and (iPrPDI)Fe(CO)2
Table 3. Metrical Parameters for (4-Me2N-iPrPDI)Fe(N2)2,
(4-tBu-iPrPDI)Fe(N2)2, and (iPrPDI)Fe(N2)2
a
(4-Me2N-iPrPDI)Fe(CO)2
(
iPrPDI)Fe(CO)2
(4-Me2N-iPrPDI)
Fe(N2)2
(4-tBu-iPrPDI)
Fe(N2)2
(
iPrPDI)
a
Fe(N2)2
Fe(1)−N(1)
Fe(1)−N(2)
Fe(1)−N(3)
Fe(1)−C(36)
Fe(1)−C(37)
1.9540(14)
1.8648(14)
1.9582(14)
1.7806(18)
1.7839(18)
1.9622(15)
1.8488(14)
1.9500(14)
1.7809(19)
1.7823(19)
Fe(1)−N(1)
Fe(1)−N(2)
Fe(1)−N(3)
Fe(1)−N(4)
Fe(1)−N(6)
1.9738(14)
1.8500(14)
1.9134(14)
1.8386(16)
1.8670(15)
1.9444(16)
1.8415(16)
1.9444(16)
1.9013(19)
1.8347(18)
1.9473(16)
1.8362(14)
1.9452(16)
1.8800(19)
1.8341(16)
N(1)−C(2)
N(3)−C(8)
1.330(2)
1.327(2)
1.330(2)
1.335(2)
N(1)−C(2)
N(3)−C(8)
1.332(2)
1.362(2)
1.342(2)
1.334(3)
1.333(2)
1.333(2)
C(2)−C(3)
C(7)−C(8)
1.427(2)
1.438(2)
1.425(2)
1.423(2)
C(2)−C(3)
C(7)−C(8)
1.464(3)
1.419(3)
1.423(3)
1.425(3)
1.427(2)
1.428(3)
N(1)−Fe(1)−N(2)
N(1)−Fe(1)−N(3)
N(1)−Fe(1)−C(36)
N(1)−Fe(1)−C(37)
N(2)−Fe(1)−N(3)
N(3)−Fe(1)−C(36)
N(3)−Fe(1)−C(37)
C(36)−Fe(1)−C(37)
78.49(6)
148.83(6)
103.98(7)
97.41(7)
78.55(6)
102.33(7)
97.35(7)
93.56(8)
79.10(6)
152.56(6)
101.54(7)
96.36(7)
79.32(6)
100.30(7)
97.27(7)
97.01(9)
N(1)−Fe(1)−N(2)
N(1)−Fe(1)−N(4)
N(1)−Fe(1)−N(6)
N(2)−Fe(1)−N(4)
N(2)−Fe(1)−N(6)
N(2)−Fe(1)−N(3)
N(3)−Fe(1)−N(4)
N(3)−Fe(1)−N(6)
N(4)−Fe(1)−N(6)
80.42(6)
104.73(6)
94.17(6)
97.49(7)
161.58(7)
79.31(6)
94.78(6)
99.71(6)
100.91(7)
79.42(7)
99.33(7)
97.00(7)
103.16(7)
159.45(8)
79.64(7)
99.06(7)
97.65(7)
97.38(8)
79.90(6)
98.92(7)
97.41(7)
159.09(8)
79.49(6)
99.74(7)
96.65(7)
98.02(8)
a
Data taken from ref 28.
a
Data taken from ref 28.
near-identical metrical parameters, suggesting an iron(0)−
iron(II) hybrid for all three compounds. Addition of a 4-
substituent does not significantly alter the steric environment
imparted by the bis(imino)pyridine ligand.
where the aryl substituents have been replaced by cyclohexyl
groups.35
Each bis(imino)pyridine exhibits a reversible reduction wave
in THF solution with [Bu4N][PF6] as an electrolyte, silver wire
as the reference electrode, and glassy carbon as the working
electrode. No other reversible oxidation or reduction waves
were observed in this solvent window. As expected, 4-
CF3-iPrPDI exhibits the least negative reduction potential
(most easily reduced) followed by the phenylated derivative
iPrBPDI. The difference in reduction potentials between these
two compounds is 200 mV. Replacing the 4-[CF3] substituent
by hydrogen changes the reduction potential by 270 mV, while
introduction of either a [Bn] or [tBu] group in the 4-position of
the ligand results in 330 and 380 mV shifts, respectively, as
compared to (4-CF3-iPrPDI). The most electron donating
member of the series, (4-Me2N-iPrPDI), is as expected the most
Evaluation of the Electronic Effects of the 4-
Substituent. A series of electrochemical and spectroscopic
measurements were made on each class of compounds (free
ligands and iron dicarbonyl and iron dinitrogen derivatives) to
assay the electronic influence of each of the 4-substituents.
Electrochemical studies were carried out on the free bis-
(imino)pyridines, and representative cyclic voltammograms are
presented in Figure 5. The measured reduction potentials
(versus ferrocene/ferrocenium and SCE) are reported in Table
4. Also included in Table 4 are iPrBPDI, a 2,6-diisopropyl-
substituted bis(imino)pyridine known to be electron with-
drawing relative to iPrPDI,31 and CyAPDI, a bis(imino)pyridine
Figure 4. Solid-state structures of (4-tBu-iPrPDI)Fe(N2)2 (left) and (4-Me2N-iPrPDI)Fe(N2)2 (right) with 30% probability ellipsoids. Hydrogen
atoms are omitted for clarity.
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Figure 5. Cathodic CV scans of 4-substituted bis(imino)pyridine
ligands: 4-CF3-iPrPDI (red), iPrPDI (orange), 4-Bn-iPrPDI (green),
4-tBu-iPrPDI (blue), and 4-Me2N-iPrPDI (purple).
Figure 6. CV scans of 4-substituted bis(imino)pyridine iron
dicarbonyl complexes: (4-CF3-iPrPDI)Fe(CO)2 (red), (iPrPDI)Fe-
(CO)2 (orange), (4-Bn-iPrPDI)Fe(CO)2 (green), (4-tBu-iPrPDI)Fe-
(CO)2 (blue), and (4-Me2N-iPrPDI)Fe(CO)2 (purple). The scans
were initiated anodically.
Table 4. Reduction Potentials of Various
Bis(imino)pyridines
Table 5. Cyclic Voltammetry Data for (4-X-iPrPDI)Fe(CO)2
Complexes
a
ligand
redn potential (V vs Fc/Fc+) redn potential (V vs SCE)
4-CF3-iPrPDI
−2.35
−2.55
−2.62
−2.72
−2.76
−2.82
−2.83
−1.79
−1.99
−2.06
−2.16
−2.20
−2.26
−2.27
iPrBPDI
oxidn (V vs
Fc/Fc+)
redn (V vs oxidn (V vs redn (V vs
a a
compd
Fc/Fc+)
SCE)
SCE)
iPrPDI
(4-CF3-iPrPDI)
Fe(CO)2
−0.27
−0.49
−0.53
−0.56
−0.86
−2.27
0.29
−1.71
−1.90
−1.91
−1.97
−1.97
4-Bn-iPrPDI
4-tBu-iPrPDI
4-Me2N-iPrPDI
(
iPrPDI)
−2.46
−2.47
−2.53
−2.53
0.07
b
Fe(CO)2
b
CyPDI
(4-Bn-iPrPDI)
Fe(CO)2
0.03
a
Formal potential for the ferrocene+/0 couple vs SCE with [Bu4N]-
(4-tBu-iPrPDI)
Fe(CO)2
0.00
b
[PF6] taken from ref 45. Data taken from ref 46.
(4-Me2N-iPrPDI)
Fe(CO)2
−0.30
challenging to reduce, with a reduction potential 200 mV more
negative than that of iPrPDI. It is noteworthy that the reduction
potential of the series of bis(imino)pyridine ligands can be
tuned by 470 mV by simply altering the substituents from
[CF3] to [NMe2] in the 4-position of the pyridine ring,
highlighting the modular electronic nature of this chelate class.
Electrochemical studies were also carried out on the series of
4-substituted bis(imino)pyridine iron dicarbonyl complexes, (4-
X-iPrPDI)Fe(CO)2, using the same conditions as for the free
ligands. Representative cyclic voltammagrams are presented in
Figure 6, and the oxidation and reduction potentials for each
compound are reported in Table 5. As was reported previously
for (iPrPDI)Fe(CO)2,24 each of the 4-substituted bis(imino)-
pyridine iron dicarbonyl complexes exhibits clean and reversible
one-electron oxidation and reduction waves. On the basis of
chemical oxidation and reduction and isolation of the
corresponding cations and anions, respectively, the electronic
structures of these species have been elucidated. The oxidation
event is principally ligand-based to form an iron(I) compound
with a neutral bis(imino)pyridine chelate, [(iPrPDI0)-
FeI(CO)2]+. In contrast, the reduction event is primarily
metal-based and also yielded an iron(I) compound, but with a
dianionic chelate, [(iPrPDI2‑)FeI(CO)2]−.24 Computational
studies established that these formalisms are slight over-
simplifications, as contributions from covalency complicate
oxidation state assignment and suggest redox non-innocence.
For the series of 4-substituted compounds (4-X-iPrPDI)Fe-
(CO)2 (Figure 6, Table 5), the reduction potentials follow the
same overall trend as the free bis(imino)pyridines. The [CF3]-
a
Formal potential for the ferrocene+/0 couple vs SCE with [Bu4N]-
b
[PF6] taken from ref 45. Data taken from ref 24.
substituted compound has the least negative reduction
potential, followed by the parent compound, (iPrPDI)Fe(CO)2.
The difference of 190 mV is smaller than the 270 mV difference
measured for free ligands. A similar trend is observed in
oxidation potentials where the parent compound, (iPrPDI)Fe-
(CO)2, is more readily oxidized than (4-CF3-iPrPDI)Fe(CO)2
by 220 mV. The oxidations of the alkyl-substituted compounds,
(4-Bn-iPrPDI)Fe(CO)2 and (4-tBu-iPrPDI)Fe(CO)2, are essen-
tially indistinguishable, while the reduction events differ slightly.
The most electron rich member of the series, (4-Me2N-iPrPDI)-
Fe(CO)2, is more readily oxidized by 370 mV than the parent
compound, while the reduction potential differs by only 7 mV
and is much less sensitive than the free ligand. A plot of the
potential of the (4-X-iPrPDI)Fe(CO)2 redox event versus
σp gives a linear relationship (Figure 7).
+/0
47
The carbonyl stretching frequencies of each of the (4-
X-iPrPDI)Fe(CO)2 compounds were measured in pentane
solution by infrared spectroscopy, and the values are reported
in Table 6. The carbonyl stretching frequencies are sensitive to
the identity of the 4-substituent, with the most electron poor
member of the series, (4-CF3-iPrPDI)Fe(CO)2, exhibiting the
highest stretching frequencies while the most electron rich
member, (4-Me2N-iPrPDI)Fe(CO)2, has the lowest values.
There is little difference in the stretching frequencies of the
parent compound (iPrPDI)Fe(CO)2 and the alkyl-substituted
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Table 8. Dinitrogen Stretching Frequencies for (4-
X-iPrPDI)Fe(N2)n (n = 1, 2) Compounds Recorded in
Pentane Solution
ν(NN) (cm−1
)
complex
n = 1
n = 2
a
(
iPrPDI)Fe(N2)n
2046
2044
2041
2034
2132, 2073
2129, 2071
2128, 2067
2117, 2055
(4-Bn-iPrPDI)Fe(N2)n
(4-tBu-iPrPDI)Fe(N2)n
(4-Me2N-iPrPDI)Fe(N2)n
b
a
b
Data from ref 28. Values were recorded in toluene at −30 °C.
Introduction of a [Me2N] substituent reduces the N2 stretching
frequency by 12 cm−1 from (iPrPDI)Fe(N2)2, also consistent
with a more electron rich metal center. Similar effects are
observed within the series of five-coordinate bis(dinitrogen)
compounds, with the 4-alkyl-substituted compounds exhibiting
a slightly more electron rich iron center than the parent
compound and the [NMe2] derivative being the most
significant.
Figure 7. Plot of the (4-X-iPrPDI)Fe(CO)2+/0 redox couple vs σp value
(y = 0.432x − 0.496; R2 = 0.999).
Table 6. Carbonyl Stretching Frequencies of (4-
X-iPrPDI)Fe(CO)2 Measured by Infrared Spectroscopy
Analogous to the carbonyl derivatives, the Mossbauer
̈
parameters within the series of (4-X-iPrPDI)Fe(N2) and (4-
X-iPrPDI)Fe(N2)2 compounds, respectively, are essentially
indistinguishable and suggest a similar electronic structure for
each class of molecule (Table 9). Representative spectra are
complex
(4-CF3-iPrPDI)Fe(CO)2
iPrBPDI)Fe(CO)2
iPrPDI)Fe(CO)2
ν(CO) (cm−1
)
1925, 1983
a
(
1921, 1979
a
(
1914, 1974
(4-Bn-iPrPDI)Fe(CO)2
(4-tBu-iPrPDI)Fe(CO)2
(4-Me2N-iPrPDI)Fe(CO)2
1913, 1972
1911, 1971
1906, 1965
Table 9. Zero-Field 57Fe Mossbauer Parameters for (4-
̈
X-iPrPDI)Fe(N2)n Compounds Recorded at 80 K
a
Data from ref 31; all values recorded in pentane solution.
compd
δ (mm/s)
ΔEQ (mm/s)
a
(
(
iPrPDI)Fe(N2)2
0.39
0.38
0.39
0.36
0.37
0.36
0.37
0.35
0.53
1.72
0.51
1.81
0.51
1.74
0.44
1.79
derivatives (4-Bn-iPrPDI)Fe(CO)2 and (4-tBu-iPrPDI)Fe(CO)2,
similar to the results of the electrochemical measurements.
To probe whether introduction of the 4-substituents induced
a gross change in electronic structure, zero-field 57Fe
a
iPrPDI)Fe(N2)
(4-Bn-iPrPDI)Fe(N2)2
(4-Bn-iPrPDI)Fe(N2)
(4-tBu-iPrPDI)Fe(N2)2
(4-tBu-iPrPDI)Fe(N2)
(4-Me2N-iPrPDI)Fe(N2)2
(4-Me2N-iPrPDI)Fe(N2)
Mossbauer spectra were collected on each (4-X-iPrPDI)Fe-
̈
(CO) compound in the solid state at 80 K. The Mossbauer
parameters, δ and ΔEQ, are reported in Table 7. In each case,
̈
2
a
Data from ref 27.
Table 7. Zero-Field 57Fe Mossbauer Parameters for (4-
̈
X-iPrPDI)Fe(CO)2
presented in Figure 8, and each exhibits a mixture of four- and
five-coordinate compounds, the ratio of which was largely
determined by how the sample was manipulated. The isomer
shifts are consistent with iron(II) compounds in all cases. As
has been observed previously,27 the four-coordinate com-
pounds exhibit a larger quadrupole splitting than the
corresponding five-coordinate derivatives, likely due to a
change in electric field gradient in the two coordination
geometries.
compd
δ (mm/s)
ΔEQ (mm/s)
(4-CF3-iPrPDI)Fe(CO)2
0.02
0.03
0.02
0.03
0.04
1.14
1.17
1.09
1.08
1.23
a
(
iPrPDI)Fe(CO)2
(4-Bn-iPrPDI)Fe(CO)2
(4-tBu-iPrPDI)Fe(CO)2
(4-Me2N-iPrPDI)Fe(CO)2
a
Data from ref 23a.
both parameters for each (4-X-iPrPDI)Fe(CO)2 compound are
essentially invariant and indicate no change in the gross
electronic structure of the molecules as a function of the 4-
substituent.
Similar spectroscopic measurements were carried out on the
bis(imino)pyridine iron dinitrogen complexes. The N2
stretching frequencies of the four- and five-coordinate iron
dinitrogen compounds as determined by pentane solution
infrared spectroscopy are reported in Table 8. Within the series
of four-coordinate iron dinitrogen compounds, the alkyl-
substituted derivatives exhibit slightly more reduced N2
stretching frequencies as compared to those of the parent
compound, signaling a modestly more electron rich iron center.
CONCLUDING REMARKS
■
A family of 4-substituted bis(imino)pyridine ligands and their
corresponding iron dihalide, dicarbonyl, and dinitrogen
complexes have been synthesized. The influence of the 4-
substituent on the electronic properties of the free ligand and
the resulting iron compounds has been evaluated using
electrochemical, vibrational, and Mossbauer spectroscopy.
̈
While the introduction of electron-withdrawing or electron-
donating substituents does not influence the redox activity of
the bis(imino)pyridine ligand and hence the overall electronic
structure of the corresponding iron compounds, the remote
substituents have a measurable influence on the electronic
properties of the metal. Electrochemical measurements on the
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Figure 8. Zero-field 57Fe Mossbauer spectra of (4-X-iPrPDI)Fe(N ) (n = 1, 2): (4-Bn-iPrPDI)Fe(N ) (left), (4-tBu-iPrPDI)Fe(N ) (middle), and
̈
2
n
2
n
2 n
(4-Me2N-iPrPDI)Fe(N2)n (right).
equipped with a molybdenum X-ray tube (λ = 0.710 73 Å). The space
group was identified, and the data were processed using the Bruker
SAINT+ program and corrected for absorption using SADABS. The
structures were solved using direct methods (SHELXS) completed by
subsequent Fourier synthesis and refined by full-matrix least-squares
procedures. A Bruker APEX2 Duo diffractometer equipped with
molybdenum and copper X-ray tubes (λ = 0.710 73 and 1.541 78 Å,
respectively) was used to collect data for (4-CF3-iPrPDI)FeCl2 (Cu
source), (4-Me2N-iPrPDI)FeCl2 (Cu source), (4-tBu-iPrPDI)Fe(N2)2
(Cu source), and (4-Me2N-iPrPDI)Fe(CO)2 (Cu source). The space
groups were identified, and the data were processed using the Bruker
SAINT+ program and corrected for absorption using SADABS. The
structures were solved using direct methods (SIR92) completed by
subsequent Fourier synthesis and refined by full-matrix least-squares
procedures.
free bis(imino)pyridines appear to be the most sensitive to 4-
subsitution. The influence of these effects on the catalytic
performance of these compounds will be reported in due
course.
EXPERIMENTAL SECTION
■
General Considerations. All air- and moisture-sensitive manip-
ulations were carried out using standard vacuum line, Schlenk, and
cannula techniques or in an MBraun inert-atmosphere drybox
containing an atmosphere of purified nitrogen. Solvents for air- and
moisture-sensitive manipulations were initially dried and deoxygenated
using literature procedures.48 Benzene-d6 was purchased from
Cambridge Isotope Laboratories and dried over 4 Å molecular sieves.
The following compounds were prepared according to literature
procedures: 4-Bn-iPrPDI,38 4-tBu-iPrPDI,39 dimethyl 4-
(trifluoromethyl)pyridine-2,6-dicarboxylate, and dimethyl 4-
(dimethylamino)pyridine-2,6-dicarboxylate.40
57Fe Mossbauer spectra were recorded on a SEE Co. Mossbauer
̈
̈
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 temperature of the samples was controlled
by a Janis Research Co. CCS-850 He/N2 cryostat within an accuracy
of 1 K. Isomer shifts were determined relative to α-iron at 298 K.
Preparation of 4-Dimethylamino-2,6-diacetylpyridine. A 250
mL round-bottom flask was charged with dimethyl 4-
(dimethylamino)pyridine-2,6-dicarboxylate (5.00 g, 21.0 mmol) and
sodium ethoxide (6.14 g, 90.2 mmol, 4.3 equiv). To this mixture was
added 50 mL of ethyl acetate with rapid stirring. After 1 h, the reaction
mixture was brought to reflux, and an additional 75 mL of ethyl acetate
was added once the reaction mixture thickened. After 6 h, the mixture
was cooled to room temperature and 20 mL of concentrated HCl was
added dropwise. Refluxing was resumed for an additional 12 h, after
which the reaction was cooled to room temperature and water was
added until the salt was dissolved. The resulting mixture was
neutralized with sodium bicarbonate and extracted three times with
dichloromethane. The organic extracts were dried with magnesium
sulfate, filtered, and concentrated in vacuo to give 3.67 g (84% yield)
of a light brown solid identified as 4-dimethylamino-2,6-diacetylpyr-
idine. HR-MS (+ESI): calcd for C11H15N2O2, [M + H]+, m/z 207.112
80; found, m/z 207.112 84. 1H NMR (chloroform-d, 22 °C): δ 2.78 (s,
6H, CCH3), 3.17 (s, 6H, N(CH3)2), 7.46 (s, 2H, m-pyr). 13C NMR
(chloroform-d, 22 °C): δ = 25.9 (C(O)CH3), 39.6 (N(CH3)2), 106.8
(m-pyr), 153.4 (o-pyr), 155.6 (p-pyr), 201.2 (C(O)CH3).
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
Inova 500 spectrometer operating at 125.893 MHz. All H and 13C
1
NMR chemical shifts are reported relative to SiMe4 using the 1H
(residual) and 13C chemical shifts of the solvent as a secondary
standard. For diamagnetic complexes, many assignments were made
on the basis of COSY and HSQC NMR experiments. Solution
magnetic moments were determined by the Evans method49 using a
ferrocene standard and are the average value of at least two
independent measurements. Magnetic susceptibility balance measure-
ments were performed with a Johnson Matthey instrument that was
calibrated with HgCo(SCN)4. All solid-state values were recorded at
21 °C unless otherwise noted. Peak widths at half-heights are reported
for paramagnetically broadened and shifted resonances. Infrared
spectra were collected on a Thermo Nicolet spectrometer. High-
resolution mass spectra were collected on an Agilent 6220 Accurate-
Mass Time-of-Flight LC/MS. The mass spectrometer was calibrated
externally before each use with purine and the Agilent ES-TOF tuning
mix (part number G1969-85000). These compounds were assigned
(M + H)+ m/z ratios of 121.050 873 and 922.009 798, respectively.
Elemental analyses were performed by Robertson Microlit Labo-
ratories, Inc., in Ledgewood, NJ.
Cyclic voltammograms (CVs) were collected in THF solution (1
mM in compound) with [nBu4N][PF6] (0.1 M), using a 3 mm glassy-
carbon working electrode, platinum wire as the counter electrode, and
silver wire as the reference 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 ferrocene/ferrocenium and were obtained using the in situ
method.50
Preparation of 4-Me2N-iPrPDI. A 100 mL round-bottom flask was
charged with 2.0 g (9.7 mmol) of 4-dimethylamino-2,6-diacetylpyr-
idine, 4.3 g (24 mmol) of 2,6-diisopropylaniline, 2 drops of formic
acid, and 25 mL of methanol. The reaction mixture was brought to
reflux and stirred for 72 h. After this time, the reaction mixture was
cooled to room temperature and then placed in an ice bath. Filtration
of the resulting solid gave 3.2 g (63% yield) of a pale yellow solid
identified as 4-Me2N-iPrPDI. HR-MS (+ESI): calcd for C35H49N4, [M
+ H]+, m/z 525.395 17; found, m/z 525.395 21. 1H NMR (benzene-d6,
22 °C): δ 1.19 (d, 7 Hz, 12H, CH(CH3)2), 1.25 (d, 7 Hz, 12H,
CH(CH3)2), 2.38 (s, 6H, N(CH3)2 or CCH3), 2.43 (s, 6H, N(CH3)2
or CCH3), 3.05 (spt, 4H, CH(CH3)2), 7.16−7.23 (m, 6H, m-, p-aryl),
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. Data for (4-Me2N-iPrPDI)-
Fe(N2)2 were collected using a Bruker X8 APEX2 diffractometer
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8.04 (s, 2H, m-pyr). 13C NMR (benzene-d6, 22 °C): δ 17.7 (C(N)
CH3), 23.1 (CH(CH3)2), 23.5 (CH(CH3)2), 28.8 (CH(CH3)2), 38.6
(N(CH3)2), 105.3 (m-pyr), 123.6 (aryl), 124.1 (aryl), 136.1 (aryl),
147.4 (aryl), 155.6 (o-pyr), 156.1 (p-pyr), 168.1 (C(N)CH3).
mmol) of 4-CF3-iPrPDI and 0.090 g (0.71 mmol) of FeCl2. Filtration
from THF/pentane and washing with diethyl ether gave 0.47 g (98%
yield) of analytically pure blue-green solid identified as (4-CF3-iPrPDI)-
FeCl2. Anal. Calcd for C34H42F3N3FeCl2: C, 60.37; H, 6.26; N, 6.21.
Found: C, 60.32; H, 6.54; N, 5.92. Solid-state magnetic susceptibility:
Preparation of 4-CF3-iPrPDI. 4-Trifluoromethyl-2,6-diacetylpyr-
idine was first prepared in a manner similar to that for 4-
dimethylamino-2,6-diacetylpyridine with 1.76 g (6.65 mmol) of
dimethyl 4-(trifluoromethyl)pyridine-2,6-dicarboxylate, 1.96 g (36.3
mmol) of sodium ethoxide, and 15 mL of ethyl acetate. An additional
50 mL of ethyl acetate was added during the course of the reaction.
The crude, golden oil (1.50 g) isolated from the reaction, identified as
4-trifluoromethyl-2,6-diacetylpyridine, was used without purification
1
μeff = 4.5 μB. H NMR (benzene-d6, 22 °C): δ −75.1 (120 Hz, 6H,
CCH3), −13.5 (26 Hz, 2H, p-aryl), −3.6 (191 Hz, 12H, CH(CH3)2),
−3.3 (75 Hz, 16H, CH(CH3)2 and m-aryl), 16.2 (44 Hz, 4H,
CH(CH3)2), 67.9 (98 Hz, 2H, m-pyr).
Preparation of (4-Me2N-iPrPDI)Fe(CO)2. A thick-walled vessel
was charged with 8.8 g of mercury, approximately 10 mL of toluene,
and a stir bar. Sodium (0.044 g, 1.9 mmol) was cut into small pieces
and added slowly to the rapidly stirred slurry. The resulting amalgam
was stirred for an additional 30 min to ensure complete dissolution. A
slurry of (4-Me2N-iPrPDI)FeCl2 (0.25 g, 0.38 mmol) in 10 mL of
toluene was added to the reaction vessel, which was then sealed. The
resulting mixture was brought to −196 °C, and the vessel was
evacuated. One atmosphere of CO was introduced, and the reaction
mixture was stirred for 24 h. The resulting green-yellow mixture was
then decanted away from the amalgam and filtered through a pad of
Celite. The solvent was removed in vacuo and the residue
recrystallized from pentane/ether at −35 °C to give 0.16 g (65%
yield) of an olive green solid identified as (4-Me2N-iPrPDI)Fe(CO)2.
Dissolving the compound in benzene gave a dark yellow-brown
solution. Anal. Calcd for C37H48N4O2Fe: C, 69.80; H, 7.60; N, 8.80.
1
for the synthesis of 4-CF3-iPrPDI. H NMR (chloroform-d, 22 °C): δ
2.82 (s, 6H, CCH3), 8.43 (s, 2H, m-pyr). 19F NMR (benzene-d6, 22
°C): δ −63.5. 4-CF3-iPrPDI was prepared in a manner similar to that
for 4-Me2N-iPrPDI with 1.00 g (4.32 mmol) of 4-trifluoromethyl-2,6-
diacetylpyridine, 2.30 g (12.9 mmol) of 2,6-diisopropylaniline, and two
drops of formic acid in 15 mL of methanol. Following filtration, 0.78 g
(33% yield) of 4-CF3-iPrPDI was isolated as a light yellow solid. HR-
MS (+ESI): calcd for C10H9F3NO2, [M + H]+, m/z 550.340 36; found,
m/z 550.340 72. 1H NMR (benzene-d6, 22 °C): δ 1.11 (d, 4 Hz, 12H,
CH(CH3)2), 1.13 (d, 4 Hz, 12H, CH(CH3)2), 2.18 (s, 6H, CCH3),
2.82 (m, 4H, CH(CH3)2), 7.12−7.18 (m, 6H, m-, p-aryl), 8.88 (s, 2H,
m-pyr). 13C{1H} NMR (benzene-d6, 22 °C): δ 17.2 (C(N)CH3), 23.0
(CH(CH3)2), 23.4 (CH(CH3)2), 28.9 (CH(CH3)2), 118.0 (3 Hz, m-
pyr), 123.1 (aryl), 123.5 (273 Hz, CF3), 123.6 (aryl), 135.8 (aryl),
140.1 (q, 34 Hz, p-pyr), 146.4 (aryl), 157.1 (o-pyr), 166.1
(C(N)CH3). 19F{1H} NMR (benzene-d6, 22 °C): δ −63.3.
1
Found: C, 69.46; H, 7.57; N, 8.66. H NMR (benzene-d6, 22 °C): δ
1.04 (d, 7 Hz, 12H, CH(CH3)2), 1.47 (d, 7 Hz, 12H, CH(CH3)2),
2.10 (s, 6H, CCH3 or N(CH3)2), 2.65 (s, 6H, CCH3 or N(CH3)2),
2.98 (spt, 7 Hz, 4H, CH(CH3)2), 7.16−7.17 (m, 6H, m-, p-aryl), 7.33
(s, 2H, m-pyr). 13C{1H} NMR (benzene-d6, 22 °C): δ 16.5 (C(N)
CH3), 24.8 (CH(CH3)2), 25.4 (CH(CH3)2), 27.9 (CH(CH3)2), 40.6
(N(CH3)2), 106.6, 123.9, 126.6, 140.47, 146.9, 150.5, 153.3, 184.3
(aryl, pyr, imine), 215.2 (CO). IR (pentane): νCO 1905, 1965 cm−1.
Preparation of (4-tBu-iPrPDI)Fe(CO)2. The compound was
prepared in a manner similar to that for (4-Me2N-iPrPDI)Fe(CO)2
with 0.25 g (0.32 mmol) of (4-tBu-iPrPDI)FeBr2, 10.9 g of mercury,
and 0.053 g (2.3 mmol) of sodium. Recrystallization from pentane/
ether at −35 °C gave 0.15 g (69% yield) of a yellow-brown solid
identified as (4-tBu-iPrPDI)Fe(CO)2. Anal. Calcd for C39H53FeN3O2:
Preparation of (4-Me2N-iPrPDI)FeCl2. A 20 mL scintillation vial
was charged with 0.083 g (0.65 mmol) of FeCl2 and 2 mL of THF. A
solution of 0.36 g (0.68 mmol) of 4-Me2N-iPrPDI in 5 mL of THF was
then added to the slurry with rapid stirring, causing an immediate
color change to dark blue-green. After 12 h, 10 mL of pentane was
added, causing the product to precipitate from solution. Filtration
from THF/pentane and washing with diethyl ether gave 0.42 g (98%
yield) of analytically pure blue-green solid identified as (4-
Me2N-iPrPDI)FeCl2. Anal. Calcd for C35H48N4FeCl2: C, 64.52; H,
7.43; N, 8.60. Found: C, 64.58; H, 7.69; N, 8.34. Solid-state magnetic
1
susceptibility: μeff = 4.5 μB. H NMR (dichloromethane-d2, 22 °C): δ
1
−22.4 (200 Hz, 4H, CH(CH3)2), −7.6 (22 Hz, 2H, p-aryl), −4.7 (42
Hz, 6H, CCH3), −4.6 (66 Hz, 12H, CH(CH3)2), −3.7 (21 Hz, 12H,
CH(CH3)2), 15.2 (21 Hz, 4H, m-aryl), 27.8 (22 Hz, 6H, N(CH3)2),
91.9 (45 Hz, 2H, m-pyr).
C, 71.88; H, 8.20; N, 6.45. Found: C, 72.03; H, 7.96; N, 6.55. H
NMR (benzene-d6, 22 °C): δ 0.97 (d, 7 Hz, 12H, CH(CH3)2), 1.37 (s,
9H, C(Me)3), 1.41 (d, 7 Hz, 12H, CH(CH3)2), 2.19 (s, 6H, CMe),
2.84 (spt, 7 Hz, 4 Hz, CH(CH3)2), 7.10−7.18 (m, p, m-aryl), 8.04 (s,
m-pyr, 2H). 13C{1H} NMR (benzene-d6, 22 °C): δ 16.4 (C(N)CH3),
24.8 (CH(CH3)2), 25.2 (CH(CH3)2), 27.9 (CH(CH3)2), 31.5
(C(CH3)3), 35.1 (C(CH3)3), 118.5, 123.9, 126.8, 140.5, 142.0,
145.9, 150.1, 155.4 (aryl, pyr, imine), 214.8 (CO). IR (pentane):
νCO 1911, 1971 cm−1.
Preparation of (4-tBu-iPrPDI)FeBr2. The compound was prepared
in a manner similar to that for (4-Me2N-iPrPDI)FeCl2 with 1.50 g
(2.79 mmol) of 4-tBu-iPrPDI and 0.57 g (2.6 mmol) of FeBr2.
Filtration from THF/pentane and washing with diethyl ether gave 1.93
g (97% yield) of analytically pure, dark blue solid identified as
(4-tBu-iPrPDI)FeBr2. Anal. Calcd for C37H51N3FeBr2: C, 58.98; H,
6.82; N, 5.58. Found: C, 59.11; H, 6.71; N, 5.29. Magnetic
susceptibility (magnetic susceptibility balance): μeff = 4.5 μB. 1H
NMR (dichloromethane-d2, 22 °C): δ −23.7 (30 Hz, 6H, CCH3),
−14.2 (290 Hz, 4H, CH(CH3)2), −10.6 (18 Hz, 2H, p-aryl), −5.2 (15
Hz, 12H, CH(CH3)2), −1.9 (66 Hz, 12H, CH(CH3)2), 4.8 (7 Hz, 9H,
C(CH3)3), 14.8 (16 Hz, 4H, m-aryl), 78.0 (44 Hz, 2H, m-pyr).
Preparation of (4-Bn-iPrPDI)FeCl2. The compound was prepared
in a manner similar to that for (4-Me2N-iPrPDI)FeCl2 with 0.40 g
(0.70 mmol) of 4-Bn-iPrPDI and 0.084 g (0.66 mmol) of FeCl2.
Filtration from THF/pentane and washing with diethyl ether gave 0.41
g (88% yield) of analytically pure, blue solid identified as (4-
Bn-iPrPDI)FeCl2. Anal. Calcd for C40H49N3FeCl2: C, 68.77; H, 7.07;
N, 6.02. Found: C, 68.75; H, 6.71; N, 5.82. Magnetic susceptibility
Preparation of (4-Bn-iPrPDI)Fe(CO)2. The compound was
prepared in a manner similar to that for (4-Me2N-iPrPDI)Fe(CO)2
with 0.15 g (0.21 mmol) of (4-Bn-iPrPDI)FeCl2, 7.0 g of mercury and
0.035 g (1.5 mmol) of sodium. Recrystallization from pentane/ether at
−35 °C gave 0.10 g (68% yield) of a yellow-brown solid identified as
(4-Bn-iPrPDI)Fe(CO)2. Anal. Calcd for C42H49N3O2Fe: C, 73.78; H,
1
7.22; N, 6.15. Found: C, 73.68; H, 7.48; N, 6.05. H NMR (benzene-
d6, 22 °C): δ 0.99 (d, 7 Hz, 12H, CH(CH3)2), 1.45 (d, 7 Hz, 12H,
CH(CH3)2), 2.08 (s, 6H, CCH3), 2.85 (spt, 4H, CH(CH3)2), 4.06 (s,
2H, CH2Ph), 7.10−7.28 (m, 11H, CH2Ph, m-, p-aryl), 7.76 (s, 2H, m-
pyr). 13C{1H} NMR (benzene-d6, 22 °C): δ 16.4 (C(N)CH3), 24.8
(CH(CH3)2), 25.2 (CH(CH3)2), 27.9 (CH(CH3)2), 42.4
(CH2(C6H5)), 122.0, 123.9, 126.8, 128.1, 129.1, 129.5, 131.7, 140.4,
140.6, 150.0, 155.5, 198.0 (aryl, phenyl, pyr, imine), 228.1 (CO). IR
(pentane): νCO 1913, 1972 cm−1.
1
(magnetic susceptibility balance): μeff = 4.6 μB. H NMR (dichloro-
methane-d2, 22 °C): δ −35.1 (19 Hz, 2H, p-Bn), −32.8 (39 Hz, 6H,
CCH3), −25.2 (279 Hz, 4H, CH(CH3)2), −10.6 (21 Hz, 2H, p-aryl),
−6.8 (19 Hz, 12H, CH(CH)3), −5.6 (70 Hz, 12H, CH(CH3)2), 9.3
(20 Hz, 1H, p-Bn), 9.7 (23 Hz, 2H, p-Bn), 13.2 (20 Hz, 2H, p-Bn),
14.4 (23 Hz, 4H, m-aryl), 83.5 (49 Hz, 2H, m-pyr).
Preparation of (4-CF3-iPrPDI)Fe(CO)2. The compound was
prepared in a manner similar to that for (4-Me2N-iPrPDI)Fe(CO)2
with 0.11 g (0.20 mmol) of (4-CF3-iPrPDI)FeCl2, 3.67 g of mercury,
and 0.018 g (0.78 mmol) of sodium. Recrystallization from pentane/
ether at −35 °C gave 0.083 g (63% yield) of a yellow-brown solid
identified as (4-CF3-iPrPDI)Fe(CO)2. Anal. Calcd for
C36H42F3N3O2Fe: C, 65.36; H, 6.40; N, 6.35. Found: C, 65.09; H,
Preparation of (4-CF3-iPrPDI)FeCl2. The compound was prepared
in a manner similar to that for (4-Me2N-iPrPDI)FeX2 with 0.40 g (0.73
2283
dx.doi.org/10.1021/om201212m | Organometallics 2012, 31, 2275−2285

Organometallics
Article
1
6.58; N, 6.30. H NMR (benzene-d6, 22 °C): δ 0.91 (d, 7 Hz, 12H,
(aryl), 154.4 (phenyl), 181 (p-pyr, located by HMBC), one resonance
not located.
CH(CH3)2), 1.36 (d, 7 Hz, 12H, CH(CH3)2), 1.87 (s, 6H, CCH3),
2.61 (spt, 4H, CH(CH3)2), 7.06−7.20 (m, 6H, m-, p-aryl), 8.02 (s, 2H,
m-pyr). 13C NMR (benzene-d6, 22 °C): δ 16.2 (C(N)CH3), 24.8
(CH(CH3)2), 25.0 (CH(CH3)2), 27.9 (CH(CH3)2), 117.4, 124.1,
127.2, 138.2, 140.2, 144.4, 149.4, 158.4 (aryl, pyr, CF3, imine). 19F
NMR (benzene-d6, 22 °C): δ −59.9. IR (pentane): νCO 1925, 1983
cm−1.
ASSOCIATED CONTENT
* Supporting Information
■
S
Figures and CIF files giving additional spectroscopic data and
complete crystallographic data for (4-CF3-iPrPDI)FeCl2, (4-
Me2N-iPrPDI)Fe(CO)2, and (4-Me2N-iPrPDI)FeN2. This ma-
terial is available free of charge via the Internet at http://pubs.
acs.org.
Preparation of (4-Me2N-iPrPDI)Fe(N2)2. A round-bottom flask
was charged with 17.6 g of mercury, approximately 10 mL of toluene,
and a stir bar. Sodium (0.088 g, 3.8 mmol) was cut into small pieces
and added slowly to the rapidly stirred slurry. The resulting amalgam
was stirred for an additional 30 min to ensure complete dissolution. A
solution containing 0.50 g (0.77 mmol) of (4-Me2N-iPrPDI)FeCl2 in
10 mL of toluene and 10 mL of THF was added to the vessel. The
reaction mixture was stirred vigorously, forming a brown solution.
After 3 h, the reaction mixture was decanted from the amalgam and
concentrated in vacuo to remove the THF. The residue was extracted
with pentane and yielded a brown-green mixture that was filtered
through a pad of Celite and concentrated to ∼15 mL. The
concentrated solution was stored at −35 °C and deposited 0.19 g
(39%) of a green-brown solid identified as (4-Me2N-iPrPDI)Fe(N2)2.
1H NMR (benzene-d6, 22 °C): δ −0.17 (d, 7 Hz, 12H, CH(CH3)2),
1.28 (d, 7 Hz, 12 H, CH(CH3)2), 1.98 (spt, 7 Hz, 4H, CH(CH3)2),
2.95 (s, Δν1/2 = 11 Hz, 6H, N(CH3)2), 4.23 (bs, 6H, CCH3), 7.56 (d,
7 Hz, 4H, m-aryl), 7.63 (m, 2H, p-aryl), 9.13 (s, 2H, m-pyr). 13C{1H}
NMR (benzene-d6, 22 °C): δ 13.7 (C(N)CH3), 24.0 (CH(CH3)2),
27.5 (CH(CH3)2), 34.6 (CH(CH3)2), 40.8 (N(CH3)2), 80.5 (m-pyr),
125.3 (aryl), 125.6 (aryl), 145.6 (aryl), 151.2 (aryl), 162.4 (pyr or
C(N)CH3), 173.0 (pyr or C(N)CH3), 190.5 (pyr or C(N)CH3). In
solution, under a dinitrogen atmosphere, the compound is in
equilibrium with (4-Me2N-iPrPDI)Fe(N2). IR (pentane, 23 °C): νNN
2034 cm−1.
AUTHOR INFORMATION
Corresponding Author
*pchirik@princeton.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the U.S. National Science Foundation and the
Deutsche Forschungsgemeinschaft for a Cooperative Activities
in Chemistry between U.S. and German Investigators grant.
J.M.D. also acknowledges support from the National Institute
of General Medical Sciences (Award No. T32GM008500). The
content is solely the responsibility of the authors and does not
necessarily represent the official views of the National Institute
of General Medical Sciences or the National Institutes of
Health.
REFERENCES
■
Preparation of (4-tBu-iPrPDI)Fe(N2)2. The compound was
prepared in a manner similar to that for (4-Me2N-iPrPDI)Fe(N2)2
with 0.25 g (0.33 mmol) of (4-tBu-iPrPDI)FeBr2, 10.7 g of mercury,
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(benzene-d6, 22 °C): δ −0.26 (d, 5 Hz, 12H, CH(CH3)2), 1.08−1.20
(m, 16H, CH(CH3)2 and CH(CH3)2), 1.66 (s, 9H, C(CH3)3), 7.59−
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prepared in a manner similar to that for (4-Me2N-iPrPDI)Fe(N2)2
with 0.25 g (0.36 mmol) of (4-Bn-iPrPDI)FeCl2, 8.4 g of mercury,
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