Synthesis and electronic structure of bis(imino)pyridine iron metallacyclic intermediates in iron-catalyzed cyclization reactions

Article abstract of DOI:10.1021/ja400895j

The bis(imino)pyridine iron dinitrogen compound, (^iPr(TB)PDI) Fe(N₂)₂ (^iPr(TB)PDI = 2,6-(2,6- ⁱPr₂-C₆H₃-N=C-(CH₂) ₃)₂(C₅H₁N)) is an effective precatalyst for the [2π + 2π] cycloaddition of diallyl amines as well as the hydrogenative cyclization of N-tosylated enynes and diynes. Addition of stoichiometric quantities of amino-substituted enyne and diyne substrates to (^iPr(TB)PDI)Fe(N₂)₂ resulted in isolation of catalytically competent bis(imino)pyridine iron metallacycle intermediates. A combination of magnetochemistry, X-ray diffraction, and Moessbauer spectroscopic and computational studies established S = 1 iron compounds that are best described as intermediate-spin iron(III) (S_Fe = 3/2) antiferromagnetically coupled to a chelate radical anion (S_PDI = 1/2). Catalytically competent bis(imino)pyridine iron diene and metallacycles relevant to the [2π + 2π] cycloaddition were also isolated and structurally characterized. The combined magnetic, structural, spectroscopic, and computational data support an Fe(I)-Fe(III) catalytic cycle where the bis(imino)pyridine chelate remains in its one-electron reduced radical anion form. These studies revise a previous mechanistic proposal involving exclusively ferrous intermediates and highlight the importance of the redox-active bis(imino)pyridine chelate for enabling catalytic cyclization chemistry with iron.

Full text of DOI:10.1021/ja400895j

Article
pubs.acs.org/JACS
Synthesis and Electronic Structure of Bis(imino)pyridine Iron
Metallacyclic Intermediates in Iron-Catalyzed Cyclization Reactions
Jordan M. Hoyt,† Kevin T. Sylvester,‡ Scott P. Semproni,† and Paul J. Chirik*,†
†Department of Chemistry, Princeton University, Princeton, New Jersey, United States 08544
‡Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York, United States 14850
S
* Supporting Information
ABSTRACT: The bis(imino)pyridine iron dinitrogen com-
pound, (iPr(TB)PDI)Fe(N2)2 (iPr(TB)PDI = 2,6-(2,6-iPr2-C6H3-
NC-(CH2)3)2(C5H1N)) is an effective precatalyst for the
[2π + 2π] cycloaddition of diallyl amines as well as the
hydrogenative cyclization of N-tosylated enynes and diynes.
Addition of stoichiometric quantities of amino-substituted
enyne and diyne substrates to (iPr(TB)PDI)Fe(N2)2 resulted in
isolation of catalytically competent bis(imino)pyridine iron metallacycle intermediates. A combination of magnetochemistry, X-
ray diffraction, and Mossbauer spectroscopic and computational studies established S = 1 iron compounds that are best described
̈
as intermediate-spin iron(III) (SFe = 3/2) antiferromagnetically coupled to a chelate radical anion (SPDI = 1/2). Catalytically
competent bis(imino)pyridine iron diene and metallacycles relevant to the [2π + 2π] cycloaddition were also isolated and
structurally characterized. The combined magnetic, structural, spectroscopic, and computational data support an Fe(I)−Fe(III)
catalytic cycle where the bis(imino)pyridine chelate remains in its one-electron reduced radical anion form. These studies revise a
previous mechanistic proposal involving exclusively ferrous intermediates and highlight the importance of the redox-active
bis(imino)pyridine chelate for enabling catalytic cyclization chemistry with iron.
Takacs and co-workers demonstrated the utility of reduced
iron compounds in formal [4π + 4π] ene-type reactions26 as
well as enediene carbocyclizations for the stereoselective
formation of bicycles.27 The latter has been applied to the
enantioselective synthesis of (−)-protoemetinol and related
INTRODUCTION
■
Metal-catalyzed cyclization reactions that combine multiple π-
fragments have emerged as a powerful, atom economical and
often selective method for the synthesis of various hetero- and
carbocycles encountered in target-directed synthesis.1−7 Exem-
plary substrates include α,ω-dienes, enynes, and diynes that can
participate in metal-catalyzed cycloisomerization4 or undergo
additional coupling with other unsaturates including CO,
alkynes, olefins, and nitriles.8−10 A pervasive mechanistic
theme invokes coordination of two unsaturated moieties to a
reduced metal center followed by oxidative coupling to form a
metallacycle which enables the critical C−C bond-forming
event and often dictates the chemo-, regio-, or even
enantioselectivity of the overall transformation.11−14 This
metallacyclic intermediate is also susceptible to a range of
organometallic transformations including hydrogenation, metal-
mediated atom transfer, reductive elimination, β-hydrogen
elimination, or various migratory insertion processes that can
be used for further functionalization.15−17 An alternative
mechanistic pathway has also been identified and involves
formation of metal allylic intermediates arising from alkene
carbo- or hydrometalation and is frequently invoked in
palladium-catalyzed transformations.18−20
29
alkaloids.28 Simple iron salts such as FeCl3 or FeCl2 treated
30
t
with excess BuMgCl have been reported to promote the
cyclization of enynes. Furstner and co-workers have described
̈
the rich catalytic chemistry associated with [Li(TMEDA)[(η5-
C5H5)Fe(C2H4)2] including enyne and diyne skeletal rear-
rangements, [4π + 2π], [5π + 2π], [2π + 2π + 2π], and Alder-
ene-type cyclizations.31
The bis(imino)pyridine iron dinitrogen complexes, (iPrPDI)-
Fe(N2)2 and [(MePDI)Fe(N2)]2(μ2-N2),33 are highly active
32
and often highly selective precatalysts for a range of
transformations including olefin hydrogenation34 and hydro-
silylation.35 For 1,6-enynes and diynes, hydrogenation with
catalytic quantities of (iPrPDI)Fe(N2)2 furnished the corre-
sponding 3,4-disubstituted five-membered ring compounds
with little evidence for linear hydrogenation products.36 The
turnover frequencies of the iron compounds are comparable to
rhodium-catalyzed hydrogenative cyclization reactions pio-
neered by Krische and co-workers.37−39 The iron-catalyzed
reactions, unlike the rhodium variants, do not require
exogeneous base implicating homo- rather than heterolytic
H2 splitting. Stoichiometric experiments and deuterium labeling
The continued motivation to develop sustainable synthetic
methods has renewed interest in the discovery of catalytic
methods using earth-abundant elements in lieu of precious
metals.21−23 Due to its relatively low cost and high terrestrial
abundance, catalysts based on iron have received considerable
attention in cyclization catalysis.22−25 Seminal studies by
Received: January 26, 2013
© XXXX American Chemical Society
A
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studies on cyclization reactions promoted by (iPrPDI)Fe(N2)2
established cyclometalation of the isopropyl methyl substitu-
ents from an aryl substituent on the bis(imino)pyridine ligand
as operative during catalytic turnover.36
pyridine iron dinitrogen complex. Previous studies from our
laboratory have shown that C−C reductive elimination from a
formally 18-electron, bis(imino)pyridine iron allyl alkyl
compound is induced by strong field ligands such as carbon
monoxide and 1,3-butadiene,41 suggesting that imine dissoci-
ation may be operative during catalytic turnover. To
experimentally explore this possibility, a bis(imino)pyridine
variant was targeted where the imine backbones are tethered to
the meta-position of the pyridine ring. This approach was
inspired by Goldberg’s study probing chelate opening prior to
C−C reductive elimination from six-coordinate platinum(IV)
complexes.49
In chemistry more unique to reduced bis(imino)pyridine
iron compounds, (iPrPDI)Fe(N2)2 promotes an intramolecular
[2π + 2π] cycloaddition of α,ω-dienes to furnish the
corresponding [3.2.0]bicycloheptanes in high yield.40 An
intermolecular variant of this reaction has also been discovered
whereby both (iPrPDI)Fe(N2)2 and [(MePDI)Fe(N2)]2(μ2-N2)
serve as effective precatalysts for the [2π + 2π] cycloaddition of
ethylene and butadiene to form vinylcyclobutane.41 Notably, no
cyclohexene, arising from thermally allowed [4π + 2π] Diels−
Alder chemistry, was observed, highlighting the selectivity of
the iron catalysts for cyclobutane formation. Examples of
thermal, intramolecular [2π + 2π] cycloaddition have been
reported previously but typically require activated π-systems
such as allenes, allenenes, or alleynes to induce turnover.42
The ability of bis(imino)pyridine iron complexes to promote
unique [2π + 2π] cycloaddition chemistry and compete with
precious metals in hydrogenative cyclizations has inspired
efforts to understand the mechanisms of these catalytic
reactions. We were particularly interested in understanding
the features of the bis(imino)pyridine iron compounds that
enable cyclobutane formation. The redox-activity of the
bis(imino)pyridine chelate,43−46 the ability to engage in
reversible electron transfer with the metal center,47 raises the
intriguing possibility that this feature of the iron catalyst enables
the unique cycloaddition chemistry. In our initial communica-
tion40 we postulated, on the basis of model compounds that
were not catalytically competent, that the redox-activity of the
bis(imino)pyridine chelate maintains iron(II) compounds
throughout the catalytic cycle and prevents formation of
iron(0) species that could be deleterious for catalyst stability.48
Here we describe a new bis(imino)pyridine iron dinitrogen
precatalyst for the intramolecular [2π + 2π] cycloaddition of
α,ω-dienes as well as the hydrogenative cyclization of enynes
and diynes. For each type of catalytic cyclization reaction, a
catalytically competent iron metallacycle has been isolated and
the electronic structure established by a combination of X-ray
diffraction, spectroscopic, and computational studies. Based on
these findings, new mechanistic insights on iron-catalyzed
cyclizations and the role of the redox-active bis(imino)pyridine
chelate have been elucidated.
The requisite bis(imino)pyridine iron dichloride precursor,
(
iPr(TB)PDI)FeCl2
(
iPr(TB)PDI = 2,6-(2,6-iPr2-C6H3-NC-
(CH2)3)2(C5H1N)) was previously reported by Kim and co-
workers.50 Sodium amalgam reduction of (iPr(TB)PDI)FeCl2
with excess 0.5% sodium amalgam under a dinitrogen
atmosphere followed by filtration and recrystallization furnished
a dark green solid identified as (iPr(TB)PDI)Fe(N2)2 in 61%
isolated yield (eq 1).
Diamagnetic (iPr(TB)PDI)Fe(N2)2 was characterized by H
1
NMR, infrared, and zero-field 57Fe Mossbauer spectroscopies.
̈
The solid-state structure was determined by single crystal X-ray
diffraction, and a representation of the molecular structure is
presented in Figure 1. Selected bond distances and angles are
reported in Table 1 as well as the corresponding metrical
parameters for (iPrPDI)Fe(N2)2 for comparison. The overall
molecular geometry of (iPr(TB)PDI)Fe(N2)2 is best described as
RESULTS AND DISCUSSION
■
Figure 1. Solid-state structure for (iPr(TB)PDI)Fe(N2)2 at 30%
ellipsoids. Isopropyl substituents on one aryl ring and hydrogen
atoms omitted for clarity.
Synthesis of Bis(imino)pyridine Iron Dinitrogen
Complex Bearing a Tetrahydroacridyl Ligand. Our
studies commenced with the synthesis of a new bis(imino)-
B
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Table 1. Selected Bond Distances (Å) and Angles (deg) for
(
Table 2. Zero-Field 57Fe Mossbauer Spectroscopic
̈
a
iPr(TB)PDI)Fe(N2)2 and (iPrPDI)Fe(N2)2
Parameters for Bis(imino)pyridine Iron Compounds
a
Relevant to This Study
(
iPr(TB)PDI)Fe(N2)2
(
iPrPDI)Fe(N2)2
b
compound
iPr(TB)PDI)Fe(N2)2
δ (mm/s)
ΔEQ (mm/s)
Fe(1)−N(1)
Fe(1)−N(2)
Fe(1)−N(3)
Fe(1)−N(4)
Fe(1)−N(6)
N(1)−C(1)
N(3)−C(7)
N(2)−C(2)
N(2)−C(6)
C(1)−C(2)
C(6)−C(7)
N(1)−Fe(1)−N(2)
N(1)−Fe(1)−N(3)
N(2)−Fe(1)−N(6)
N(4)−Fe(1)−N(6)
N(3)−C(7)−C(6)
N(1)−C(1)−C(2)
1.986(2)
1.828(2)
1.982(2)
1.860(2)
1.833(2)
1.332(3)
1.330(3)
1.369(3)
1.364(2)
1.414(2)
1.418(3)
79.58(7)
156.30(7)
150.77(8)
102.70(8)
113.4(2)
113.3(2)
1.947(2)
1.836(1)
1.945(2)
1.880(2)
1.834(1)
1.334(2)
1.333(2)
1.377(2)
1.379(2)
1.427(2)
1.428(3)
79.90(7)
154.77(7)
159.09(8)
98.02(8)
112.6(2)
113.5(2)
(
0.45
0.38
0.39
0.38
0.07
0.05
0.17
0.15
0.22
0.23
0.16
0.27
0.63
0.65
0.30
0.24
0.30
0.73
0.83
0.62
0.83
RKS
0.75
(
(
(
(
iPrPDI)Fe(N2)2
iPrPDI)FeN2
iPrPDI)Fe(biphenyl)
MePDI)Fe(biphenyl)
−0.53
c
,
f
+1.72
+3.58
3.69
d
1
3.05
e
BS(3,1)
+2.59
2.79
(
iPrPDI)Fe(diyne-C)
2
2.22
e
BS(3,1)
+2.16
2.65
(
iPrPDI)Fe(enyne-D)
3
2.47
e
e
BS(3,1)
+2.20
2.66
4
BS(3,1)
+2.38
2.58
a
The data for (iPrPDI)Fe(N2)2 are taken from ref 32.
5
6
1.81
e
BS(3,1)
+1.47
2.58
square pyramidal with the bis(imino)pyridine chelate and one
of the dinitrogen ligands defining the basal plane. As reported
by Kim and co-workers,50 the rigidity of the chelate backbone
opens the iron coordination sphere and results in elongated
iron−imine bonds. In (iPr(TB)PDI)Fe(N2)2, the Fe−Nimine
bonds are 1.986(2) and 1.982(2) Å and are longer than the
values of 1.947(2) and 1.945(2) Å in (iPrPDI)Fe(N2)2. The
Nimine−Cimine distances are elongated to 1.332(3) and 1.330(3)
Å and the Cimine−Cipso distances contracted to 1.414(2) and
1.418(3) Å, consistent with a redox noninnocent, π-accepting
chelate as established for (iPrPDI)Fe(N2)2.51,52
7
a
Data were collected at 80 K, and values reported in italics are
computed values. Unless a sign is reported, all values of ΔEQ are
absolute values. Values taken from refs 51 and 53. Values taken from
b
c
d
e
ref 54. Broken symmetry solution resulting where the iron−imine
distances are fixed based on the values obtained from the
experimentally determined crystal structures. Computed values from
geometry-optimized solutions where an imine ligand has undergone
f
dissociation are reported in the Supporting Information. Sign
determined by applied field measurement.
The solution behavior of (iPr(TB)PDI)Fe(N2)2 was inves-
tigated by infrared spectroscopy, as previous studies have
established an equilbrium between four- and five-coordinate
bis(imino)pyridine iron dinitrogen complexes with each
possessing a distinct electronic structure.51 In pentane solution,
three bands were observed. Two, centered at 2124 and 2061
cm−1, are assigned to (iPr(TB)PDI)Fe(N2)2 and are more
reduced than the values of 2132 and 2073 cm−1 reported for
(iPrPDI)Fe(N2)2. A single band was also located at 2025 cm−1
and assigned to the four-coordinate compound, (iPr(TB)PDI)-
FeN2. This value is also significantly reduced from the band
observed at 2046 cm−1 reported for (iPrPDI)FeN2, consistent
with a more electron-donating ligand upon introduction of alkyl
groups used to tether the imine groups to the central pyridine
ring.
Evaluation of Catalytic Competency. With a new
bis(imino)pyridine iron dinitrogen complex in hand, we
explored its catalytic performance in intramolecular [2π +
2π] cycloaddition as well as hydrogenative enyne and diyne
cyclization. Stirring a 0.10 M benzene-d6 solution of either
diallyl-tert-butylamine or diallylaniline with 10 mol % of
(
iPr(TB)PDI)Fe(N2)2 resulted in rapid cyclization, furnishing
the corresponding azobicyclo[3.2.0]heptane in near quantita-
tive yield (Scheme 1). For these substrates, the activity of
(
iPr(TB)PDI)Fe(N2)2 is similar but slightly reduced as compared
to the previously reported (iPrPDI)Fe(N2)2 precatalyst.40
Although relatively high iron loadings of 10 mol % were used
for initial catalyst screening, effective turnover was achieved at
reduced catalyst concentrations of 1 mol %.
The electronic structure of (iPr(TB)PDI)Fe(N2)2 was also
(
iPr(TB)PDI)Fe(N2)2 also proved to be an effective precatalyst
studied by zero-field 57Fe Mossbauer spectroscopy at 80 K. A
for the hydrogenative cyclization of both enynes and diynes.
Exposure of an 0.14 M benzene-d6 solution of N-allyl-N-(but-2-
ynyl)-4-methylbenzenesulfonamide (A) to 4 atm of H2 in the
presence of 10 mol % of (iPr(TB)PDI)Fe(N2)2 furnished a
mixture of saturated and unsaturated pyrrolidines (Scheme 1).
At shorter reaction times (30 min), the unsaturated pyrrolidine
predominates, constituting 81% of the product mixture, with
the balance of the material being the saturated heterocycle. The
saturated compound arises from hydrogenation of the
unsaturated pyrrolidine and was observed as a 52:48 mixture
of syn and anti diastereomers. Continuing the hydrogenation
for 3 h resulted in exclusive formation of the saturated product
with no change in the diastereoselectivity.
̈
doublet was observed in the solid state, consistent with
exclusive isolation of the five-coordinate complex, (iPr(TB)PDI)-
Fe(N2)2. The isomer shift of 0.45 mm/s is slightly higher than
the value of 0.39 mm/s reported for (iPrPDI)Fe(N2)2 (Table 2),
consistent with the elongated iron−imine bonds observed in
the solid-state structure. Full molecule, density functional
theory calculations were also performed, and a restricted
Kohn−Sham (RKS) solution successfully reproduced the
experimentally observed Mossbauer parameters (δ = 0.38
̈
mm/s; ΔEQ = 0.75 mm/s), consistent with a covalent iron
compound that is adquately described as a resonance hybrid
between Fe(0) and Fe(II) canonical forms.51
C
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spectroscopies established spin crossover behavior from low to
intermediate-spin ferric as a function of temperature. Although
(iPrPDI)Fe(biphenyl) was kinetically unstable to transfer
dehydrogenation from an isopropyl aryl substituent, isolation
of these compounds suggested that iron metallacycles related to
diyne cyclization may also be possible.
Scheme 1. Iron-Catalyzed [2π + 2π] and Hydrogenative
Cyclization Reactions Promoted by (iPr(TB)PDI)Fe(N2)2
Addition of a hexane solution of B to a hexane slurry of
(
iPr(TB)PDI)Fe(N2)2 at 23 °C resulted in precipitation of a red
solid identified as the desired iron metallacyclopentadiene, 1 in
1
84% yield (eq 2). The benzene-d6 H NMR spectrum of 1 at 23
°C was featureless and not useful for characterization of the
compound. Similar spectroscopic properties were observed for
(iPrPDI)Fe(biphenyl).54 A solid sample of 1 was analyzed by
zero-field 57Fe Mossbauer spectroscopy and exhibited a
̈
quadrupole doublet with an isomer shift of 0.17 mm/s and a
quadrupole splitting (ΔEQ) of 3.05 mm/s. Parameters in this
range are consistent with an iron(III) compound but are by no
means exclusive for this oxidation state assignment.55 As
reported in Table 2 however, the Mossbauer parameters for 1
̈
are similar to those reported for both (iPrPDI)Fe(biphenyl) and
(MePDI)Fe(biphenyl).
The magnetic ground state of 1 was determined in both
solution and the solid state. In benzene-d6 at 23 °C a moment
of 2.8 μB was determined by the method of Evans,56 consistent
with the spin only value for two unpaired electrons and an S = 1
molecule. Because (iPrPDI)Fe(biphenyl) and (MePDI)Fe-
(biphenyl) exhibit spin crossover, the solid-state magnetic
susceptibility of 1 was measured by SQUID magnetometry
from 2 to 300 K (Figure S2, Supporting Information). The data
revealed simple paramagnetic behavior with no evidence for
SCO. The magnetic moment plateaued at 3.0 μB, consistent
with the solution measurements and an S = 1 ground state.
Single crystals of 1 were obtained from a concentrated
diethyl ether solution stored at −35 °C. A representation of the
solid-state molecular structure is presented in Figure 2. Selected
bond distances and angles for all iron metallacycles structurally
characterized in this work are presented in Table 3. The
crystallographic data were a three-component twin that was
successfully modeled and confirmed the identity of 1 as the
metallacyclopentadiene. One molecule of diethyl ether was also
present in the unit cell. The geometry about the metal can be
described as distorted square pyramidal with one of the carbon
substituents occupying the apical position. The C(39)−C(40)
distance of 1.420(7) Å is diagnostic of C−C bond formation
and the C(38)−C(39) and C(40)−C(41) bond lengths of
1.357(7) and 1.360(5) Å are as expected for a metal-
lacyclopentadiene.
The hydrogenative cyclization of N,N-dibut-2-ynyl-4-meth-
ylbenzenesulfonamide (B) was accomplished under the same
conditions as the catalytic enyne chemistry (Scheme 1) and
yielded 54% of the 3,4-dialkenyl pyrrolidine as the (E,E) isomer
after 30 min. At this time interval, 9% of the saturated
pyrrolidine was observed along with 30% of linear products
arising from hydrogenation of the alkyne without cyclization.
The linear products are a mixture of amino-alkenes and alkanes.
Formation of such a quantity of linear products is a
consequence of the increased hydrogenation activity of
(
iPr(TB)PDI)Fe(N2)2 relative to (iPrPDI)Fe(N2)2. For the latter
iron precatalyst, almost exclusive cyclization was observed. The
catalytic chemistry summarized in Scheme 1 clearly establishes
the competency of (iPr(TB)PDI)Fe(N2)2 in established iron-
catalyzed cyclization reactions.
Synthesis and Electronic Structure of Bis(imino)-
pyridine Iron Metallacycles: Hydrogenative Diyne and
Enyne Cyclization. To isolate, characterize, and understand
the electronic structure of catalytically relevant iron metalla-
cycles, diyne substrates were initially explored. Our laboratory
has recently reported the synthesis and characterization of iron
metallacycles resulting from the C−C oxidative addition of
biphenylene to (iPrPDI)Fe(N2)2 and [(MePDI)Fe(N2)]2(μ2-
N2).54 Both (PDI)Fe(biphenyl) derivatives were determined to
be iron(III) compounds with bis(imino)pyridine radical anions.
Magnetic studies along with X-ray absorption and emission
As is well established for redox-active bis(imino)-
pyridines,45,46 distortions to the bond distances of the chelate
signal participation in the electronic structure of the compound
D
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1.421(7) Å are consistent with the established values for one
electron reduction. The iron−imine bond lengths of 2.073(2)
and 2.069(4) Å are also relatively long, a result of the
constrained geometry imparted by tethering the imine carbon
to the meta position of the pyridine ring. The metrical data
from the solid-state structure of 1 in combination with
magnetochemistry and the Mossbauer parameters clearly
̈
establish an overall S = 1 compound derived from an
intermediate-spin ferrous center antiferromagneticaly coupled
to a bis(imino)pyridine radical anion. Compound 1 is
structurally similar to (iPrPDI)Fe(biphenyl) with the exception
of elongated Fe−Nimine (2.073(2) and 2.069(4) Å as compared
to 1.959(2) and 1.988(2) Å) and Fe−C bond lengths
(2.004(5) and 1.996(6) Å as compared to 1.965(3) and
1.943(3) Å) likely due to the more open bis(imino)pyridine
chelate and the fully populated S = 1 state at the temperature of
the X-ray data collection.
Full molecule DFT studies were also conducted using the
ORCA program with the B3LYP functional.57 This approach
has proven successful for the electronic structure determination
of other redox-active bis(imino)pyridine iron and cobalt
complexes.46,51,53,54,58−60 Geometry optimizations with 1
resulted in dissociation of the iron−imine bonds so the Fe−
Nimine distances were constrained to the experimentally
determined distances and the remainder of the molecule
optimized. Subsequent calculations were performed in a similar
manner. Bond distances and relative energies for all calculations
can be found in the Supporting Information (Table S7).
Unrestricted Kohn−Sham (UKS) calculations were performed
for the S = 1 ground state along with various broken symmetry
possibilities. In broken symmetry notation, BS(m,n) describes a
state where there are m unpaired spin up electrons and n
unpaired spin down electrons on separate fragments.61−63 All S
= 1 inputs converge to the same BS(3,1) solution, which
emerged as the lowest energy of all of the computational
Figure 2. Solid-state structure for 1 at 30% ellipsoids. Isopropyl
substituents on one aryl ring and hydrogen atoms omitted for clarity.
Table 3. Selected Bond Distances (Å) and Angles (deg) for
1−4
1
2
3
4
Fe(1)−N(1)
Fe(1)−N(2)
Fe(1)−N(3)
Fe(1)−C(38)
Fe(1)−C(41)
N(1)−C(2)
N(3)−C(8)
C(2)−C(3)
C(7)−C(8)
C(38)−C(39)
C(39)−C(40)
C(40)−C(41)
N(1)−Fe(1)−N(2)
N(1)−Fe(1)−N(3)
N(2)−Fe(1)−N(3)
C(38)−Fe(1)−C(41)
C(38)−Fe(1)−N(1)
C(38)−Fe(1)−N(2)
C(38)−Fe(1)−N(3)
C(41)−Fe(1)−N(1)
C(41)−Fe(1)−N(2)
C(41)−Fe(1)−N(3)
2.073(2)
1.862(5)
2.069(4)
2.004(5)
1.996(6)
1.327(7)
1.327(8)
1.424(6)
1.421(7)
1.357(7)
1.420(7)
1.360(5)
78.5(2)
2.044(2)
1.906(2)
2.066(2)
2.002(2)
2.027(2)
1.328(3)
1.325(3)
1.421(4)
1.420(3)
1.320(4)
1.504(3)
1.548(3)
77.57(7)
143.40(7)
77.45(7)
84.81(9)
105.38(8)
105.08(8)
106.66(8)
99.20(8)
170.07(7)
100.83(8)
2.738(2)
2.012(2)
2.040(2)
2.092(2)
2.090(2)
1.286(3)
1.321(3)
1.470(3)
1.431(3)
1.394(3)
2.778(2)
1.397(3)
68.31(6)
148.84(6)
80.54(7)
150.67(8)
82.49(7)
98.09(8)
103.45(8)
83.91(7)
100.81(7)
101.64(7)
2.079(3)
1.910(3)
2.123(3)
2.046(4)
2.012(4)
1.320(5)
1.319(5)
1.429(6)
1.435(6)
1.528(6)
1.544(6)
1.530(6)
77.5(1)
solutions examined. The computed Mossbauer parameters (δ =
̈
0.21 mm/s, ΔEQ = +3.05 mm/s) from the BS(3,1) output
successfully reproduce the experimental values (Table 2). A
qualitative molecular orbital diagram and corresponding spin
density plot for this solution are presented in Figure 3. This
solution corresponds to an intermediate-spin Fe(III) center
antiferromagnetically coupled to a bis(imino)pyridine radical
anion, consistent with all of the experimental data. The two
SOMOs accounting for the S = 1 state are almost exclusively
metal based, corresponding to dz2 and dxz orbitals on iron.
To determine if 1 was catalytically competent, the hydro-
genative cyclization of substrate B was studied. Exposure of a
benzene-d6 solution of 0.14 M B and 10 mol % 1 to 4 atm H2,
identical conditions to those used with (iPr(TB)PDI)Fe(N2)2,
resulted in >98% conversion to products after 30 min at 23 °C.
143.4(1)
78.0(2)
144.2(1)
77.5(1)
85.3(2)
84.8(2)
103.9(2)
96.4(2)
106.6(1)
105.8(1)
104.6(1)
98.1(1)
106.2(2)
100.8(2)
178.3(2)
101.9(2)
169.2(2)
101.9(1)
and possible radical character. The elongation of the Nimine
Cimine bonds in 1 to 1.327(7) and 1.327(8) Å and the
contraction of the Cimine−Cipso distances to 1.424(6) and
−
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Figure 3. Qualitative molecular orbital diagram (left) and spin density plot (right) for the BS(3,1) solution for 1.
As with (iPr(TB)PDI)Fe(N2)2, 50% of the product mixture was
the (E,E)-isomer of the 3,4-dialkenyl pyrrolidine with the
remainder of the material identified as the saturated 3,4-diethyl
pyrrolidine (10%) and open chain olefin isomerization and
alkane products (32%) (Scheme 1). Thus, 1 is catalytically
competent and a likely intermediate in iron-catalyzed diyne
hydrogenative cyclization with no detectable change in activity
or selectivity from the iron dinitrogen precursor.
The successful isolation and electronic structure determi-
nation of 1 prompted synthesis of bis(imino)pyridine iron
metallacycles relevant to the hydrogenative cyclization of
enynes. In our initial communication, we reported observation
of the iron metallacycle resulting from addition of enyne A to
(iPrPDI)Fe(N2)2.36 Because the metallacycle underwent transfer
dehydrogenation with isopropyl aryl substituents and was
therefore kinetically unstable, characterization data was limited
to solution magnetochemistry and NMR spectroscopy. No
structural or Mossbauer data were obtained, prohibiting
̈
Figure 4. Solid-state structure for 2 at 30% ellipsoids. Hydrogen atoms
and isopropyl substituents on one aryl ring omitted for clarity.
definitive assignment of metal and ligand oxidation state.
With (iPr(TB)PDI)Fe(N2)2, addition of a hexane solution of
N,N-dibut-2-ynyl-4-methylbenzenesulfonamide (A) to a hexane
slurry of the iron dinitrogen compound furnished a red solid
identified as the bis(imino)pyridine iron metallacycle 2 in 68%
yield (eq 3).
methylated carbon, C(38) defining the apical position. The
C(39)−C(40) distance of 1.504(3) Å confirms C−C bond
formation. The C(38)−C(39) distance of 1.320(4) Å is
consistent with an alkene while the C(40)−C(41) bond length
is extended to 1.548(3) Å, signaling a carbon−carbon single
bond. The metrical parameters of the metallacycle are
consistent with formation of an iron cyclopentene. The bond
distances of the bis(imino)pyridine chelate are statistically
indistinguishable from those in 1 and are consistent with one-
electron reduction.43,45,46 Thus, the metrical data, in combina-
Unlike 1, the benzene-d6 1H NMR spectrum of 2 at 23 °C
exhibits 27 paramagnetically shifted resonances as anticipated
for a Cs symmetric iron complex. A solution magnetic moment
(Evans) of 2.8 μB was measured in benzene-d6 at 23 °C,
consistent with two unpaired electrons and an overall S = 1
ground state. The zero-field 57Fe Mossbauer spectrum of 2 was
̈
recorded at 80 K and exhibits a quadrupole doublet with an
isomer shift of 0.23 mm/s and a quadrupole splitting of 2.22
mm/s. The isomer shift for 2 is similar to 1 and is consistent
with a similar iron oxidation state.
tion with the Mossbauer parameters and magnetic measure-
̈
ments support an intermediate-spin ferric compound anti-
ferromagnetically coupled to a bis(imino)pyridine radical anion.
Full molecule, broken symmetry DFT calculations (B3LYP)
were performed on 2 and produced a similar BS(3,1) solution
as for 1. A qualitative molecular orbital diagram and a spin
density plot for this solution are presented in Figure 5. The
The molecular structure of 2 was also determined by single
crystal X-ray diffraction (Figure 4). Minor positional disorder
was present in the backbone of the ligand and in the
metallacyclopentene that was successfully modeled. Selected
bond distances and angles are reported in Table 3. As with 1, an
idealized square pyramidal iron complex was observed with the
computed Mossbauer parameters (δ = 0.12 mm/s, ΔE = 2.08
̈
Q
mm/s) are in good agreement with the experimental values
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Figure 5. Qualitative molecular orbital diagram (left) and spin density plot (right) for the BS(3,1) solution for 2.
(Table 2). The SOMOs, as with 1, are iron-based with the
unpaired electrons residing in principally dxz and dz2 orbitals.
The combined magnetic, spectroscopic, metrical, and computa-
tional data clearly establish 2 as an intermediate-spin Fe(III)
compound antiferromagnetically coupled to a bis(imino)-
pyridine radical anion.
The successful isolation and characterization of 1 and 2
prompted reinvestigation of metallacycle synthesis with the
original bis(imino)pyridine iron dinitrogen precatalyst,
(iPrPDI)Fe(N2)2 (Scheme 2). Specifically, we sought to employ
susceptibility measurements on both compounds were
consistent with S = 1 molecules, in agreement with the ground
states of 1 and 2. In addition, the 57Fe Mossbauer parameters
̈
(Table 2) for both (iPrPDI)Fe(diyne-C) and (iPrPDI)Fe(enyne-
D) are similar to the analogous metallacycles with the
tetrahydroacridyl-substituted bis(imino)pyridine iron com-
plexes. Thus, all of the spectroscopic data support identical
electronic structures between iron metallacycles with two
different bis(imino)pyridine ligands.
Synthesis and Electronic Structure of Bis(imino)-
pyridine Iron Metallacycles: [2π + 2π] Cycloadditions.
The relative insolubility of enyne and diyne metallacycles
derived from (iPr(TB)PDI)Fe(N2)2 motivated exploration of the
putative intermediates from a [2π + 2π] α,ω-diene cyclo-
addition. To complete the series, a hexane slurry of
Scheme 2. Synthesis of [(iPrPDI)Fe] Metallacycles Relevant
to Diyne and Enyne Hydrogenative Cyclization
(
iPr(TB)PDI)Fe(N2)2 was treated with a hexane solution of
diallyl tosyl amine and furnished a paramagnetic red solid, 3 (eq
4).
Single crystals of 3 were obtained from a hexane solution
cooled to −35 °C, and X-ray diffraction established the identity
of the compound not as the iron metallacyclopentane but
rather the diolefin complex. The backbone methylene groups
were disordered (∼75:25) over two positions as was the iron
center (90:10). Both disorders were successfully modeled and
one orientation of each is presented in the depiction of the
structure in Figure 6. The C(39)−C(40) distance of 2.778(2)
Å is longer than those observed in 1 and 2. Accordingly, the
C(40)−C(41)/C(38)−C(39) distances of 1.394(3) and
1.397(3) Å are consistent with coordinated alkenes. Notably,
the Fe(1)−N(1) distance of 2.738 Å is outside the range of a
typical iron−imine dative bond and indicates dissociation of
one arm of the chelate. Formation of a κ2-bis(imino)pyridine
ligand complicates correlation of the bond length distortions to
the redox activity and ultimately metal and ligand oxidation
state. As expected, for the portion of the ligand dissociated from
the metal, Nimine−Cimine distance of 1.286(3) Å along with the
Cimine−Cipso bond length of 1.470(3) are hallmark values for a
chelate in its neutral form. However, the portion of the ligand
the precipitation strategy that proved successful with
(
iPr(TB)PDI)Fe(N2)2. Accordingly, diyne C and enyne D with
phenyl substituents were chosen to reduce the solubility of the
resulting iron metallacycle in hydrocarbon solvents thereby
facilitating isolation and potentially inhibiting deleterious
transfer hydrogenation chemistry. Addition of 1 equiv of
diyne C or enyne D to a hexane solution of (iPrPDI)Fe(N2)2
resulted in precipitation of dark purple solids identified as the
iron metallacycles, (iPrPDI)Fe(diyne-C) or (iPrPDI)Fe(enyne-
D) in 84 and 67% yields, respectively (Scheme 2). Magnetic
bound to the metal center demonstrates bond lengths, Nimine
−
Cimine distance of 1.321(3) Å and Cimine−Cipso bond length of
1.431(3), that are indicative of a monoreduced chelate in both
the bis(imino)pyridine and mono(imino)pyridine complexes.64
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antiferromagnetically coupled to a bis(imino)pyridine radical
anion (SPDI = 1/2). The difference in iron oxidation state,
intermediate-spin Fe(III) versus high-spin Fe(I), accounts for
the variance in observed isomer shift.
The catalytic competency of 3 was evaluated by allowing a
benzene-d6 solution of the compound to stand at 23 °C for
1
several hours. Monitoring the reaction by H NMR spectros-
copy revealed little formation of the [3.2.0]azobicyloheptane;
rather several new, paramagnetic iron compounds were
observed. One of these was characterized by single crystal X-
ray diffraction (see Supporting Information) and identified as
the bis(imino)pyridine iron amide compound, (iPr(TB)PDI)-
FeN(Ts)(C3H5). This observed lack of reactivity with diallyl
tosyl amine stands in contrast to the rapid cyclization of this
substrate observed with (iPrPDI)Fe(N2)2 which proceeded to
complete conversion in less than 1 h.
Figure 6. Solid-state structure for 3 at 30% ellipsoids. Hydrogen atoms
and isopropyl substituents on one aryl ring omitted for clarity.
The isolation of diene complex 3 raised the question whether
iron olefin complexes or metallacycles are formed with more
catalytically competent substrates. In the presence of 10 mol %
of (iPr(TB)PDI)Fe(N2)2 in benzene-d6 solution, N,N-diallylani-
line underwent quantitative cyclization and furnished the
corresponding [3.2.0]azobicyloheptane in 3 h at 23 °C.
Addition of 1 equiv of N,N-diallylaniline to a slurry of
The electronic structure of 3 was further studied by solid-
state 57Fe Mossbauer spectroscopy and broken-symmetry DFT
̈
calculations. An isomer shift of 0.63 mm/s was measured at 80
K, a value significantly higher than those measured for the iron
diyne and enyne metallacycles. Open shell DFT calculations
converged to a BS(3,1) solution; this solution successfully
(
iPr(TB)PDI)Fe(N2)2 in hexane resulted in rapid precipitation
reproduced the experimentally observed Mossbauer isomer
̈
of a purple powder identified as the iron metallacyclopentane,
4, in 55% yield (eq 5).
As with the other metallacycles prepared in this work, 4 is
shift and quadrupole splitting (Table 2). A qualitative molecular
orbital diagram and spin density plot from the BS(3,1) solution
is presented in Figure 7. The experimental and computational
data are best described as a high-spin Fe(I) center (SFe = 3/2)
paramagnetic with a solution magnetic moment of 2.9 μB
Figure 7. Qualitative molecular orbital diagram (left) and spin density plot (right) for the BS(3,1) solution for 3.
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(benzene, 23 °C). The solid-state magnetic behavior was
studied by SQUID magnetometry between 2 K and 300 K and
established simple paramagnetic behavior (Figure S7). The
benzene-d6 NMR spectrum exhibited several broad peaks
The discrepancy between isolation of iron diene complexes
and metallacycles was explored with additional substrate
modifications (Figure 10). Introduction of a 4-methoxysub-
stituent into N,N-diallylaniline and addition to (iPr(TB)PDI)Fe-
(N2)2 furnished the iron metallacycle, 5 as determined by X-ray
between −12 and 12 ppm. The zero-field 57Fe Mossbauer
̈
spectrum exhibits an isomer shift of 0.30 mm/s and a
quadrupole splitting of 2.66 mm/s. Within the series of the
three metallacycles, the isomer shift steadily increases from 1 to
2 to 4 and is likely a result of the decreased s-character and
hence reduced field strength when moving from two sp2 to two
sp3 hydrocarbyl ligands.
diffraction and Mossbauer spectroscopy. Because of the
̈
similarity between 4 and 5, details for the latter compound
are relegated to the Supporting Information.
An additional objective of these studies was to determine the
influence of bis(imino)pyridine hapticity on the electronic
structure of the iron diene compound. Because 3 exhibited a κ2-
bis(imino)pyridine ligand, an iron diene complex was targeted
where coordination of all three nitrogens was retained.
Addition of 1,5-hexadiene to a hexane slurry of (iPr(TB)PDI)-
Fe(N2)2 furnished a dark green solid identified as (iPr(TB)PDI)-
Fe(η2, η2-C6H10) (6) in 83% yield (Figure 10). A magnetic
moment of 2.6 μB was measured in benzene-d6 solution at 23
°C, diagnostic of an S = 1 ground state similar to other iron
metallacycle and diene complexes isolated in this study. The
The solid-state structure of 4 was determined by X-ray
diffraction. The crystallographic data definitively establish
formation of the bis(imino)pyridine iron metallacycle. The
overall geometry of the metal center is best described as
distorted square pyramidal with one of the carbon atoms of the
metallacycle defining the apical position. The iron atom lies
outside the plane of the chelate, and the carbon atoms that
define the metallacycle are essentially perpendicular to the
idealized iron-chelate plane. The C(39)−C(40) distance of
1.544(6) Å establishes carbon−carbon bond formation (Table
3).
Mossbauer isomer shift of 0.73 mm/s (Table 2) is also
̈
consistent with a high-spin iron compound and is slightly
higher than the value of 0.63 mm/s observed with 3.
The solid-state structure of 6 was determined by X-ray
diffraction and a representation of the molecular structure is
presented in Figure 11. Selected bond distances and angles are
reported in the Supporting Information. The coordinated diene
was disordered over two positions and successfully modeled.
Only one orientation is shown in Figure 11. While the iron−
imine bond distances are elongated (2.320(4) and 2.372(4) Å),
the hapticity of the bis(imino)pyridine chelate is best described
as κ3. We note that all of the iron−ligand bonds are elongated,
likely a consequence of the high spin state of the metal center.
There is little perturbation to the C−C distances (1.39(1) and
1.36(1) Å) of the coordinated olefins, consistent with little
backbonding from the high-spin iron center. The distortions to
the bis(imino)pyridine (Nimine−Cimine = 1.300(6), 1.313(6) Å;
Cimine−Cipso = 1.442(7), 1.445(6) Å) chelate signal formation of
one-electron-reduced radical anion. The combined magnetic,
spectroscopic, and structural data are therefore consistent with
a high-spin iron(I) center antiferromagnetically coupled to a
bis(imino)pyridine radical anion. The electronic structure of 6
was also examined with broken symmetry DFT calculations.
Similar to 3, a BS(3,1) was obtained (Figure S15, Supporting
Information). The only notable difference being that the spin
density is more symmetrically distributed over the ligand likely
arising from the more symmetric Fe−Nimine distances, a
consequence of the κ3 coordination of the chelate.
Figure 8. Solid-state structure for 4 at 30% ellipsoids. Isopropyl
substituents on one aryl ring and hydrogen atoms omitted for clarity.
As with the other metallacycles and the diene compound, the
electronic structure of 4 was examined with broken symmetry
DFT calculations. The computed Mossbauer parameters are in
̈
good agreement with the experimental values and validate the
accuracy of the optimized BS(3,1) solution. From this solution,
a molecular orbital diagram and spin density plot were
generated and are presented in Figure 9. As with the other
metallacycles, the BS(3,1) solution corresponds to an
intermediate-spin iron(III) compound with a bis(imino)-
pyridine radical anion.
One final diene, diallylfluorene, was selected because it
undergoes catalytic [2π + 2π] cyclization at 45 °C in the
presence of either (iPrPDI)Fe(N2)2 or (iPr(TB)PDI)Fe(N2)2. The
absence of a central nitrogen atom obviates potential
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Figure 9. Qualitative molecular orbital diagram (left) and spin density plot (right) for the BS(3,1) solution for 4.
Figure 10. Additional bis(imino)pyridine iron metallacycle and diene complexes prepared in this study.
mm/s reported for 3 and 6, respectively, resulting in
formulation as an iron diene complex rather than the
metallacyclic alternative.
Comparison to Bis(imino)pyridine Iron Dialkyl Com-
plexes. Our group has previously reported the synthesis of
bis(imino)pyridine iron dialkyl complexes, (RPDI)Fe-
(CH2SiMe3)2,65,66 each with an S = 2 ground state. Subsequent
structural, spectroscopic, and computational studies established
that these compounds are best described as high-spin Fe(III)
derivatives antiferromagnetically coupled to a bis(imino)-
pyridine radical anion.67 The high-spin ferric center in these
compounds contrasts the intermediate-spin Fe(III) centers
observed in metallacycles 1, 2, (iPrPDI)Fe(diyne-C), (iPrPDI)-
Fe(enyne-D), 4, and 5. What is the origin of this difference?
Because the bis(imino)pyridine chelates were different
between the iron dialkyl and the metallacycles structurally
characterized in this work, the synthesis of (iPr(TB)PDI)Fe-
(CH2SiMe3)2 was targeted. Dialkylation of (iPr(TB)PDI)FeCl2
Figure 11. Solid-state structure for 6 at 30% probability ellipsoids.
Isopropyl substituents on one aryl ring and hydrogen atoms omitted
for clarity.
65
with 2 equiv of LiCH2SiMe3 in diethyl ether followed by
filtration and two recrystallizations from pentane at −35 °C
furnished dark purple crystals in 24% yield identified as
decomposition by iron amide formation. Addition of
diallylfluorene to a hexane slurry of (iPr(TB)PDI)Fe(N2)2
furnished dark green solid identified as the corresponding
iron diene complex 7 in 45% yield (Figure 10). The benzene-d6
solution magnetic moment of 2.9 μB recorded at 23 °C is
(
iPr(TB)PDI)Fe(CH2SiMe3)2 (eq 6).
The zero-field 57Fe Mossbauer spectrum of (iPr(TB)PDI)Fe-
̈
(CH2SiMe3)2 was obtained at 80 K and established an isomer
shift of 0.22 mm/s and a quadrupole splitting of 2.82 mm/s.
These parameters are similar to those previously reported for
(EtPDI)Fe(CH2SiMe3)2 (δ = 0.26 mm/s; ΔEQ = 2.66 mm/s),
consistent with an S = 1 compound. The Mossbauer isomer
̈
shift of 0.62 mm/s is comparable to the values of 0.63 and 0.73
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values ascribed to a high-spin ferric center antiferromagnetically
coupled to a bis(imino)pyridine radical anion.67 An unidenti-
Table 4. Selected Bond Distances (Å) and Angles (deg) for
(
65
iPr(TB)PDI)Fe(CH2SiMe3)2 and (iPrPDI)Fe(CH2SiMe3)2
fied second iron compound was detected in the Mossbauer
̈
(
iPr(TB)PDI)
(
iPrPDI)
spectrum (δ = 0.77 mm/s; ΔEQ = 1.40 mm/s) that constituted
up to 20% of the product mixture. A solution magnetic moment
(Evans method) of 4.4 μB was measured in benzene at 23 °C,
slightly lower than the value expected for four unpaired
electrons and an S = 2 ground state. The low value reflects the
presence of the second unidentified iron species.
The solid-state structure was determined by single crystal X-
ray diffraction and a representation of the molecule is shown in
Figure 12. Selected metrical parameters for the compound
Fe(CH2SiMe3)2
Fe(CH2SiMe3)2
Fe(1)−N(1)
Fe(1)−N(2)
Fe(1)−N(3)
Fe(1)−C(10)
Fe(1)−C(14)
N(1)−C(2)
N(3)−C(8)
C(2)−C(3)
2.208(4)
2.012(3)
2.287(4)
2.070(3)
2.068(4)
1.304(6)
1.306(6)
1.433(7)
1.445(6)
74.7(1)
2.203(2)
2.013(2)
2.263(3)
2.062(3)
2.054(3)
1.302(3)
1.301(3)
1.448(3)
1.454(3)
74.04(7)
141.07(7)
72.88(7)
112.0(1)
103.37(9)
107.93(9)
105.86(9)
98.2(1)
C(7)−C(8)
N(1)−Fe(1)−N(2)
N(1)−Fe(1)−N(3)
N(2)−Fe(1)−N(3)
C(10)−Fe(1)−C(14)
C(10)−Fe(1)−N(1)
C(10)−Fe(1)−N(2)
C(10)−Fe(1)−N(3)
C(14)−Fe(1)−N(1)
C(14)−Fe(1)−N(2)
C(14)−Fe(1)−N(3)
144.8(1)
73.2(1)
117.6(2)
101.5(1)
107.2(1)
101.5(1)
98.6(1)
135.1(1)
93.8(1)
140.0(1)
94.1(1)
The electronic structure of (iPr(TB)PDI)Fe(CH2SiMe3)2 was
also examined by full molecule DFT calculations. A BS(5,1)
solution, corresponding to a high-spin iron(III) center
antiferromagnetically coupled to a bis(imino)pyridine radical
anion, successfully reproduced both the metrical and
Figure 12. Solid-state structure of (iPr(TB)PDI)Fe(CH2SiMe3)2 at 30%
probability ellipsoids. Isopropyl substituents on one aryl ring and
hydrogen atoms omitted for clarity.
Mossbauer parameters. A qualitative molecular orbital diagram
̈
and spin density plot from this solution are reported in the
Supporting Information (Figure S16). The combined struc-
tural, spectroscopic, and computational data on (iPr(TB)PDI)-
Fe(CH2SiMe3)2 are most consistent with an overall S = 2
complex derived from a high-spin ferric center antiferromag-
netically coupled to a bis(imino)pyridine radical anion. This
electronic structure is analogous to all other (RPDI)Fe-
(CH2SiMe3)2 compounds prepared to date, suggesting that
the identity of the specific bis(imino)pyridine has little
influence on the overall electronic structure of the compound.
What then is the origin of the difference in spin state
between the iron metallacycles (intermediate-spin ferric) and
the dialkyl complexes (high-spin ferric)? To probe this issue,
DFT calculations were performed on the hypothetical bis-
(imino)pyridine iron dimethyl complex, (iPr(TB)PDI)Fe(CH3)2.
Square pyramidal and trigonal bipyramidal geometries were
considered using the iron metallacycle, 4, and the iron dialkyl,
along with those previously reported for (iPrPDI)Fe-
(CH2SiMe3)2 are reported in Table 4. One notable structural
feature is the transition of the overall molecular geometry to a
more trigonal bipyramidal arrangement. In this view, the two
alkyl ligands and the pyridine define the equatorial plane and
the imine donors, albeit distorted due to the constraints of the
chelate, occupy the axial positions. The angle between the two
alkyl ligands, C(10)−Fe(1)−C(14), of 117.7(2)° is more open
than the value of 84.8(2)° measured in the iron metallacycle, 4
which adopts a more square pyramidal geometry. The metrical
parameters of the bis(imino)pyridine chelate in (iPr(TB)PDI)-
Fe(CH2SiMe3)2 are comparable to those previously reported
for (iPrPDI)Fe(CH2SiMe3)2 and are consistent with one-
electron reduction. In addition, the iron−nitrogen bonds
from both imines (2.208(4) and 2.287(4) Å) and the central
pyridine donor (2.012(3) Å) are elongated, consistent with a
high-spin metal center.
(
iPr(TB)PDI)Fe(CH2SiMe3)2, metrical parameters as initiating
points. For each geometry, BS(5,1) and BS(3,1) possibilities
were evaluated, corresponding to high and intermediate-spin
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Scheme 3. Proposed Catalytic Cycle for the Hydrogenative Cyclization of Enynes (illustrated) and Diynes (implied) with
Redox-Active Bis(imino)pyridine Iron Complexes
Scheme 4. Proposed Catalytic Cycle for the [2π + 2π] Cyclization of α,ω-Dienes with Redox-Active Bis(imino)pyridine Iron
Complexes
ferric centers, respectively. For both spin states, the square
pyramidal geometry was energetically favored considerably (see
Table S8 in Supporting Information for details) over the
trigonal bipyramidal alternative. Between the BS(3,1) and
BS(5,1) solutions in the square pyramidal regime, the S = 1,
BS(3,1) possibility was favored by ∼13 kcal/mol. This result is
anticipated as the BS(3,1) solution corresponds to the
preferred electronic structure description of the input molecule.
Following optimization from the square pyramidal metrical
parameters, both spin states converged, producing C−Fe−C
angles of 93.9° (BS(3,1)) and 92.2° (BS(5,1)), close to the
value observed experimentally in 4. In the case of the S = 1,
BS(3,1) input, optimization from the trigonal bipyramidal
metrical parameters converged to a solution closer to those
expected for a square pyramid (C−Fe−C angle of 91.3°). On
the basis of these findings, we conclude that the BS(3,1)
solution and hence S = 1 ground state in a square pyramidal
geometry is the preferred electronic and steric arrangement for
bis(imino)pyridine iron dialkyl and metallacycles. The
exceptions are the (RPDI)Fe(CH2SiMe3)2 compounds which
are S = 2. The steric pressure resulting from the large alkyl
groups favor a more idealized trigonal bipyramidal geometry
with elongated Fe−C bonds and hence an overall weaker ligand
field. Unfortunately, we66 and others68 have failed to synthesize
other bis(imino)pyridine iron dialkyl complex with different
hydrocarbyl substituents.
L
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Article
Electronic Structure Summary and Implications for
Catalytic Turnover. The isolation, structural characterization,
and elucidation of the electronic structure of bis(imino)-
pyridine iron diene and various metallacycle complexes
provides new insight into the mechanism of turnover and the
flow of electrons during the critical bond-forming events that
comprise the catalytic cycles. A mechanism accounting for the
electronic structures of the iron metallacycles formed during
hydrogenative enyne cyclization is presented in Scheme 3. The
pathway for diyne cyclization would be analogous and is not
explicitly illustrated. Recent studies on the electronic structure
of the parent iron precatalyst, (iPrPDI)FeN2, established an
intermediate-spin iron(II) compound (SFe = 1) with a triplet
diradical bis(imino)pyridine dianion.51 Substitution of the
dinitrogen ligand by the substrate results in metallacycle
formation, including construction of the C−C bond. The enyne
cyclization event forms an S = 1 iron compound with formal
one-electron oxidation events occurring both at the bis(imino)-
nickelocyclopentanes and found that stoichiometric cyclo-
butane formation was favored when the nickel metallacycle
was four coordinate. Phosphine dissociation to form three-
coordinate derivatives opened a coordinate site for sequential
β-hydrogen and C−H reductive eliminations resulting in
formation of linear butenes. At higher phosphine concen-
trations, five-coordinate Ni(II) complexes persisted and
resulted in exclusive formation of ethylene arising from
cycloreversion of the metallacycle. More recently, Xu and
Bernskoetter73 have investigated ethane formation from the
low-spin, six-coordinate Co(III) dimethyl complex, cis,mer-
(Me3P)3CoMe2I, in chemistry related to that initially described
by Yamamoto.74 A combination of kinetic studies and crossover
and isotopic labeling experiments coupled with solvent effects
and phosphine inhibition studies all supported a concerted C−
C bond-forming event from an unobserved five-coordinate
Co(III) intermediate, similar to established C−C reductive
elimination pathways in Pt(IV) chemistry. In the cobalt case,
however, the generation of an S = 1 four-coordinate cobalt
product necessitates a spin-state change along the reaction
coordinate.
pyridine (e.g., [PDI]2− → [PDI]1−) and iron (e.g., Fe2+
→
Fe3+). Turnover-limiting hydrogenation36 forms the bis-
(imino)pyridine iron alkyl (or alkenyl) hydride, which by
analogy to iron dialkyls67 is likely an iron(III) compound with a
one-electron reduced chelate. Because the S = 2 ground state of
the iron dialkyls is anomalous and results from the steric
pressure imparted by large hydrocarbyl ligands, it is likely that
the bis(imino)pyridine iron alkyl hydride intermediate has an S
= 1 ground state. Thus, at high concentrations of substrate
where the formation of the iron dinitrogen complex is
minimized, the catalytic cycle involves ferric intermediates
with a bis(imino)pyridine radical anion where an S = 1 spin
state is maintained.
More germane to this study is Kochi and co-workers’75
seminal report of the relative rates and favored pathways of
sp3−sp3 C−C reductive elimination as a function of iron
oxidation state. Variants of Yamamoto’s (bipy)2FeR2 (R = Me,
Et, nPr, nBu, cyclo-(CH2)4) complexes,76 where the alkyl groups
are in a cis configuration, formed the basis for this study. For
the neutral, formally ferrous compound, (bipy)2FeEt2, ethylene
and ethane were the principal organic products, consistent with
a pathway involving κ2−κ1 interconversion of the bipy ligand,
opening a coordination site for β-hydrogen elimination.
Reductive elimination of a C−H bond accounts for the
observed CH3−CH3 product. Electrochemical or chemical
oxidation of the (bipy)2FeR2 compounds furnished low-spin
Fe(III) derivatives that decomposed principally by iron−carbon
bond homolysis. For the ferracyclopentane example, (bipy)2Fe-
(cyclo-(CH2)4, cyclobutane was the dominant product of the
radical pathway. Continued oxidation to the Fe(IV) dications,
[(bipy)2FeR2]2+, by electrochemical methods cleanly generated
coupled alkyl products from exclusive sp3−sp3 C−C reductive
elimination. While mechanistic studies were limited by the
transient nature of the iron dications, a concerted pathway was
favored. Kochi also noted that the relative rates of
decomposition markedly increased as a function of formal
iron oxidation state, where Fe(II) < Fe(III) < Fe(IV).
The isolation and elucidation of the electronic structures of 4
and 5 provides important insight into the catalytic competency
of the sp3−sp3 C−C reductive elimination and the role and
importance of the redox-active bis(imino)pyridine ligand. On
the basis of the work of Kochi, it is likely that if the
bis(imino)pyridine iron metallacycles maintained their formal
Fe(II) oxidation state where the chelate was in a redox-innocent,
neutral form, cyclobutane formation would be inhibited.
Because the iron complexes are five coordinate and have
open coordination sites, it is also likely with a lower metal
oxidation state, β-hydrogen elimination would become
competitive with C−C reductive elimination and the products
of the reaction would likely be altered. The redox-active
bis(imino)pyridine is therefore an important contributor to
catalytic activity, as it enables formation of an intermediate-spin
ferric complex which facilitates C−C reductive elimination. It is
also plausible that this pathway proceeds by iron−carbon bond
homolysis, and additional experimentation is required to
A proposed mechanism for the thermal bis(imino)pyridine
iron-catalyzed [2π + 2π] cycloaddition of α,ω-dienes is
presented in Scheme 4. In this case, both iron diene and
metallacycle complexes were isolated and fully characterized,
providing more complete insight into the electronic structure of
iron compounds formed along the catalytic reaction coordinate.
Iron diene complexes with both κ2 and κ3 bis(imino)pyridine
ligands were synthesized and show no variance in electronic
structure, demonstrating that if imine dissociation is indeed
operative, it does not change the physical oxidation state of the
iron or the bis(imino)pyridine chelate. Unlike our initial
proposal suggesting that ferrous oxidation state was maintained
throughout the catalytic cycle,40 the electronic structure data
presented in this report clearly support an Fe(I)−Fe(III)
couple for catalytic [2π + 2π] cycloaddition. We note that a
similar redox couple has also been invoked in iron-catalyzed
cross-coupling reactions.69,70 The oxidative event that promotes
iron metallacycle and hence C−C bond formation is metal
based, as the high-spin Fe(I) diene complex converts to
intermediate-spin Fe(III). Notably, the oxidation state of the
chelate is maintained as the bis(imino)pyridine remains in the
radical anion form through the catalytic cycle.
The most intriguing step of the catalytic cycle shown in
Scheme 4 is the sp3−sp3 reductive elimination reaction. It is the
facility of this step that enables cyclobutane formation and gives
rise to the unique catalytic activity observed with the
bis(imino)pyridine iron catalysts. Previous studies have focused
on the mechanism of stoichiometric sp3−sp3 C−C reductive
elimination from first row transition metals and provide a
foundation for the bis(imino)pyridine iron compounds
described here.71 Grubbs and co-workers72 reported C−C
reductive elimination from low spin, Ni(II) phosphine
M
dx.doi.org/10.1021/ja400895j | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
was stirred for 5 min and then filtered. A red solid was collected and
was washed with hexanes (2 × 1 mL), yielding 0.056 g (84%) of the
desired product as a dark red solid. Anal. for C52H64N4O2SFe: Calcd
C, 72.20; H, 7.46; N, 6.48. Found: C, 71.96; H, 7.27; N, 6.22.
Magnetic susceptibility (Evans): μeff = 2.8 μB (benzene-d6, 23 °C).
Preparation of (iPr(TB)PDI)Fe(N-Allyl-N-(but-2-ynyl)-4-methyl-
benzenesulfonamide) (2). This compound was prepared in a
fashion similar to that for (iPr(TB)PDI)Fe(N,N-dibut-2-ynyl-4-methyl-
benzenesulfonamide) using 0.050 g (0.077 mmol) of (iPr(TB)PDI)Fe-
(N2)2 and 0.021 g (0.081 mmol) of N-allyl-N-(but-2-ynyl)-4-
methylbenzenesulfonamide in approximately 2 mL of hexanes. This
procedure yielded 0.045 g (68%) of the desired iron metallacycle as a
evaluate such a possibility. We note that in our initial
communication,40 deuterium-labeling experiments were re-
ported that supported retention of stereochemistry during
catalytic [2π + 2π] cycloaddition, suggesting that if Fe−C
homolysis is indeed operative, ring closure and cyclobutane
formation is faster than C−C bond rotation.
All of the experimental data support an S = 1 spin surface for
iron-catalyzed [2π + 2π] cycloaddition. If the catalytic reaction
is completed in the presence of excess dinitrogen, S = 0
bis(imino)pyridine iron dinitrogen complexes can be isolated
that maintain their catalytic performance, suggesting that the
iron complexes can pass from S = 0 to S = 1 reaction surfaces
with impunity. During turnover, once product has been
released, the bis(imino)pyridine maintains the radical anion
oxidation state and diene coordination occurs at a high-spin
Fe(I) center. This oxidation state is likely beneficial as Fe(0)
complexes may be subject to decomposition by chelate
dissociation and formation of metallic iron. Such a pathway
inhibits catalytic turnover in the Grubbs nickel compounds.72
Thus, the redox-active chelates serve multiple purposes that
enable the unique catalytic [2π + 2π] cycloaddition observed
with bis(imino)pyridine iron compounds. By maintaining their
radical anion form, these versatile ligands inhibit decomposition
by deposition of metallic iron as well as enable iron(III)
metallacycles that favor sp3−sp3 C−C reductive elimination
over other competing processes such as β-hydrogen elimi-
nation. These insights should prove useful for future catalyst
and reaction development.
1
dark red solid. H NMR (benzene-d6, 20 °C): δ = −102.07, −18.29,
−16.79, −3.17, −0.02, 0.18, 2.38, 3.43, 4.08, 4.48, 6.58, 7.43, 9.52,
9.91, 10.20, 13.48, 13.98, 20.74, 26.94, 29.55, 30.18, 33.58, 36.55,
51.06, 53.69, 127.48, 241.11 ppm. Anal. for C51H64N4O2SFe: Calcd C,
71.81; H, 7.56; N, 6.57. Found: C, 71.71; H, 7.29; N, 6.33. Magnetic
susceptibility (Evans): μeff = 2.8 μB (benzene-d6, 23 °C).
Preparation of (iPr(TB)PDI)Fe(N,N-diallylaniline) (4). This
compound was prepared in a similar fashion to (iPr(TB)PDI)Fe(N,N-
dibut-2-ynyl-4-methylbenzenesulfonamide) using 0.075 g (0.116
mmol) of (iPr(TB)PDI)Fe(N2)2 and 0.060 g (0.348 mmol) of N,N-
diallylaniline in approximately 2 mL of hexanes. This procedure
yielded 0.048 g (55% yield) of a dark purple solid identified as the
1
desired iron metallacycle. H NMR (benzene-d6, 20 °C): δ = −10.41,
−5.03, −1.16, 2.50, 3.67, 5.22, 8.62, 9.27 ppm. Anal. for C49H62N4Fe:
Calcd C, 77.14; H, 8.19; N, 7.34. Found: C, 76.85; H, 8.23; N, 6.97.
Magnetic susceptibility (Evans): μeff = 2.9 μB (benzene-d6, 23 °C).
Preparation of (iPr(TB)PDI)Fe(CH2SiMe3)2. A 20 mL scintillation
vial was charged with 0.450 g (0.454 mmol) of (iPr(TB)PDI)FeCl2 and
approximately 10 mL of diethyl ether, and the resulting slurry was
frozen in a liquid nitrogen-cooled cold well. A second 20 mL
scintillation vial was charged with 0.088 g (0.930 mmol) of
LiCH2SiMe3 and approximately 2 mL of diethyl ether. This solution
was added dropwise to the frozen contents of the first vial. The
reaction mixture was then stirred at room temperature for 6 h, slowly
forming a dark purple solution. The solvent was then removed in
vacuo, reconstituted in approximately 10 mL of toluene, and filtered
through Celite. The solvent was again removed in vacuo, and the
resulting purple solid was twice recrystallized from small portions of
diethyl ether, yielding 0.174 g (24% yield) of a dark purple solid
CONCLUDING REMARKS
■
A family of bis(imino)pyridine iron metallacycles has been
synthesized, isolated, and structurally characterized, allowing
elucidation of the electronic structure of important inter-
mediates in iron-catalyzed hydrogenative cyclization and
cycloaddition reactions. All of the experimental and computa-
tional data support a monoreduced bis(imino)pyridine radical
anion throughout catalytic turnover and that an Fe(I)−Fe(III)
cycle is operative for C−C bond formation. These features
enable a sufficiently high oxidation state iron complex to enable
facile sp3−sp3 C−C reductive elimination and avoid reduced
iron compounds that would result in catalyst decomposition by
metal deposition.
identified as predominately (iPr(TB)PDI)Fe(CH SiMe ) by Mossbauer
̈
2
3 2
spectroscopy.
ASSOCIATED CONTENT
* Supporting Information
■
S
Additional experimental procedures, including general consid-
erations and spectroscopic and computational details. Repre-
EXPERIMENTAL SECTION77
sentative zero-field 57Fe Mossbauer spectra and crystallographic
■
̈
Preparation of (iPr(TB)PDI)Fe(N2)2. A 100 mL round-bottom flask
was charged with 0.070 g (3.03 mmol) of sodium metal and
approximately 50 mL of toluene. With stirring, 14.0 g (69.3 mmol) of
mercury was added to the flask followed by 0.400 g (0.606 mmol) of
data in cif format for 1, 2, 3, 4, (iPr(TB)PDI)FeN(Ts)(Allyl), 6,
and (iPr(TB)PDI)Fe(CH2SiMe3)2. This material is available free
of charge via the Internet at http://pubs.acs.org.
(
iPr(TB)PDI)FeCl2. The reaction mixture was stirred for 14 h after
AUTHOR INFORMATION
Corresponding Author
pchirik@princeton.edu
which time the resulting dark green solution was filtered through
Celite, and the toluene was removed in vacuo. The resulting green/
brown solid was recrystallized from pentane at −35 °C to yield 0.240 g
(61%) of (iPr(TB)PDI)Fe(N2)2 as a dark green solid. 1H NMR
(benzene-d6, 20 °C): δ = 0.64 (bs, 8H, NCCH2CH2CH2), 0.85 (d,
12H, CH(CH3)2), 1.73 (bs, 4H, NCCH2), 2.68 (bs, 4H,
CH(CH3)2), 7.36−7.46 (m, 6H, m and p-Ar), p-pyr and CH(CH3)2
not located. Anal. for C37H47N7Fe: Calcd C, 68.83; H, 7.34; N, 15.19.
Found: C, 68.95; H, 7.16; N, 14.79. IR (pentane): ν (N2) = 2124 and
2061 cm−1 (five coordinate); 2025 cm−1 (four coordinate).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the U.S. National Science Foundation and Deutsche
Forschungsgemeinschaft for a Cooperative Activities in
Chemistry between U.S. and German Investigators grant
(CHE-1026084). We also thank Dr. Daigoro Hirai and Prof.
Robert Cava for assistance with the SQUID measurements.
S.P.S. thanks the Natural Sciences and Engineering Research
Council of Canada for a predoctoral fellowship.
Preparation of (iPr(TB)PDI)Fe(N,N-Dibut-2-ynyl-4-methylben-
zenesulfonamide) (1). A 20 mL scintillation vial was charged with
0.050 g (0.077 mmol) of (iPr(TB)PDI)Fe(N2)2 and approximately 2 mL
of hexanes. The slurry was stirred for 5 min before 0.022 g (0.081
mmol) of N,N-dibut-2-ynyl-4-methylbenzenesulfonamide in approx-
imately 2 mL of hexanes was added. The resulting reaction mixture
N
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Journal of the American Chemical Society
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