Synthesis, electronic structure, and alkene hydrosilylation activity of terpyridine and bis(imino)pyridine iron dialkyl complexes

Article abstract of DOI:10.1021/om3004527

Iron dialkyl complexes, [N₃]Fe(CH₂SiMe ₃)₂, with three different classes of tridentate, nitrogen-based [N₃] ligands, aryl-substituted bis(imino)pyridines, terpyridine, and pyridine bis(oxazoline), have been synthesized and evaluated in the catalytic hydrosilylation of olefins with tertiary silanes. The 2,2′:6′,2″-terpyridine (terpy) complex, (terpy)Fe(CH₂SiMe₃)₂, was prepared either via alkylation of (terpy)FeCl₂ with LiCH₂SiMe₃ or by pyridine displacement from (pyridine)₂Fe(CH₂SiMe ₃)₂ by free terpyridine. The aryl-substituted bis(imino)pyridine compounds, (^RPDI)Fe(CH₂SiMe ₃)₂ (^RPDI = 2,6-(2,6-R₂-C ₆H₃N=CMe)₂C₅H₃N), with smaller 2,6-dialkyl substituents (R = Et, Me) or a 2-ⁱPr substituent (^2-iPrPDI)Fe(CH₂SiMe₃)₂ ( ^2-iPrPDI = 2,6-(2-ⁱPr-C₆H₄N=CMe) ₂C₅H₃N, are effective precursors (0.5 mol %) for the anti-Markovnikov hydrosilylation of 1-octene with (Me ₃SiO)₂MeSiH and (EtO)₃SiH over the course of 1 h at 60 °C. No hydrosilylation activity was observed with Et₃SiH. The most hindered member of the series, (^iPrPDI)Fe(CH ₂SiMe₃)₂, and the pyridine bis(oxazoline) iron compound, (R,R)-(^iPrPybox)Fe(CH₂SiMe₃) ₂ (^iPrPybox = 2,6-bis[isopropyl-2-oxazolin-2-yl]pyridine), were inactive for the hydrosilylation of 1-octene with all tertiary silanes studied. By contrast, the terpyridine precursor, (terpy)Fe(CH ₂SiMe₃)₂, reached >95% conversion at 60 °C with Et₃SiH and (Me₃SiO)₂MeSiH. In addition, the hydrosilylation of vinylcyclohexene oxide was accomplished in the presence of 1.0 mol % (terpy)Fe(CH₂SiMe₃)₂, demonstrating functional group compatibility unique to this compound that is absent from bis(imino)pyridine iron compounds. The electronic structures of all three classes of iron dialkyl compounds have been evaluated by a combination of X-ray diffraction, magnetochemistry, Moessbauer spectroscopy, and density functional theory calculations. All of the compounds are best described as high-spin iron(III) compounds with antiferromagnetic coupling to chelate radical anions.

Full text of DOI:10.1021/om3004527

Article
pubs.acs.org/Organometallics
Synthesis, Electronic Structure, and Alkene Hydrosilylation Activity
of Terpyridine and Bis(imino)pyridine Iron Dialkyl Complexes
Aaron M. Tondreau,† Crisita Carmen Hojilla Atienza,† Jonathan M. Darmon,† Carsten Milsmann,†
Helen M. Hoyt,⊥,# Keith J. Weller,‡ Susan A. Nye,‡ Kenrick M. Lewis,§ Julie Boyer,‡ Johannes G. P. Delis,∥
Emil Lobkovsky,⊥ and Paul J. Chirik*,†
†Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
‡Momentive Performance Materials Inc., 260 Hudson River Road, Waterford, New York 12188, United States
§Momentive Performance Materials Inc., 769 Old Saw Mill River Road, Tarrytown, New York 10591, United States
∥Momentive Performance Materials bv., Plasticslaan 1, 4612PX Bergen op Zoom, The Netherlands
⊥Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853, United States
S
* Supporting Information
ABSTRACT: Iron dialkyl complexes, [N3]Fe(CH2SiMe3)2,
with three different classes of tridentate, nitrogen-based “[N3]”
ligands, aryl-substituted bis(imino)pyridines, terpyridine, and
pyridine bis(oxazoline), have been synthesized and evaluated
in the catalytic hydrosilylation of olefins with tertiary silanes.
The 2,2′:6′,2″-terpyridine (terpy) complex, (terpy)Fe-
(CH2SiMe3)2, was prepared either via alkylation of (terpy)-
FeCl2 with LiCH2SiMe3 or by pyridine displacement from
(pyridine)2Fe(CH2SiMe3)2 by free terpyridine. The aryl-
substituted bis(imino)pyridine compounds, (RPDI)Fe(CH2SiMe3)2 (RPDI = 2,6-(2,6-R2-C6H3NCMe)2C5H3N), with smaller
2,6-dialkyl substituents (R = Et, Me) or a 2-iPr substituent (2‑iPrPDI)Fe(CH2SiMe3)2
(
2‑iPrPDI = 2,6-(2-iPr-C6H4N
CMe)2C5H3N, are effective precursors (0.5 mol %) for the anti-Markovnikov hydrosilylation of 1-octene with (Me3SiO)2MeSiH
and (EtO)3SiH over the course of 1 h at 60 °C. No hydrosilylation activity was observed with Et3SiH. The most hindered
member of the series, (iPrPDI)Fe(CH2SiMe3)2, and the pyridine bis(oxazoline) iron compound, (R,R)-(iPrPybox)Fe(CH2SiMe3)2
(iPrPybox = 2,6-bis[isopropyl-2-oxazolin-2-yl]pyridine), were inactive for the hydrosilylation of 1-octene with all tertiary silanes
studied. By contrast, the terpyridine precursor, (terpy)Fe(CH2SiMe3)2, reached >95% conversion at 60 °C with Et3SiH and
(Me3SiO)2MeSiH. In addition, the hydrosilylation of vinylcyclohexene oxide was accomplished in the presence of 1.0 mol %
(terpy)Fe(CH2SiMe3)2, demonstrating functional group compatibility unique to this compound that is absent from
bis(imino)pyridine iron compounds. The electronic structures of all three classes of iron dialkyl compounds have been
evaluated by a combination of X-ray diffraction, magnetochemistry, Mossbauer spectroscopy, and density functional theory
̈
calculations. All of the compounds are best described as high-spin iron(III) compounds with antiferromagnetic coupling to
chelate radical anions.
The high cost of platinum and the volatility of the precious
metals market have motivated the search for more affordable
alternatives.5 Accordingly, there has been a renewed interest in
discovering catalysts based on iron and other earth-abundant
transition metals.6,7 Early studies into iron-catalyzed alkene
hydrosilylation centered on homoleptic iron carbonyl com-
plexes, such as Fe(CO)5 and Fe2(CO)9, which required high-
temperature thermolysis or photolysis conditions for catalyst
activation.8 In addition to Si−H addition, undesired products
from dehydrogenative silylation of the alkene were also
observed depending on the specific reaction conditions.9
Most recently, Nakazawa and co-workers reported a unique
and selective dehydrogenative silylation−hydrogenation reac-
INTRODUCTION
■
Transition-metal-catalyzed alkene hydrosilylation is a key
transformation in the commercial synthesis of numerous
silicones with applications in personal care, agrochemical,
adhesive, and molding products.1,2 Most commercial silicones
are derived from the anti-Markovnikov addition of a tertiary
silane to a terminal alkene, motivating the discovery of catalysts
that exhibit exclusive terminal selectivity. For over three
decades, platinum compounds, such as Karstedt’s and Speier’s
c a t a l y s t s , P t 2 { [ ( C H 2  C H ) S i M e 2 ] 2 O } 3 a n d
H2PtCl6·6H2O/iPrOH, respectively, have been the most widely
used and studied.1,3,4 The success of these catalysts can be
traced, in part, to their preference for formation of anti-
Markovnikov products, high activity, commercial availability,
and stability.
Received: May 23, 2012
Published: June 27, 2012
© 2012 American Chemical Society
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tion with 1,3-divinyldisiloxane in the presence of catalytic
amounts of (η5-C5H5)Fe(CO)2Me.10 Ritter and co-workers
have also reported imine-pyridine-supported iron precatalysts
for the 1,4-addition of silanes, such as (EtO)3SiH to dienes.11
In 2004, our laboratory reported the synthesis of the
bis(imino)pyridine iron dinitrogen compound, (iPrPDI)Fe(N2)n
(iPrPDI = 2,6-(2,6-iPr2-C6H3NCMe)2C5H3N; n = 1, 2), and
established its utility for the hydrosilylation of terminal alkenes
with primary and secondary silanes.12 This compound is also
active for the more commercially relevant anti-Markovnikov
addition of the tertiary silanes (EtO)3SiH and
(Me3SiO)2MeSiH (MD′M) to 1-octene and allyl polyethers.13
Reducing the size of the 2,6-aryl substituents, as in the cases of
[(MePDI)Fe(N2)]2(μ2-N2) and [(EtPDI)Fe(N2)]2(μ2-N2), re-
sulted in significantly higher hydrosilylation activity, a trend
first observed in alkene hydrogenation catalysis.14 Notably, in
all of these cases, no side products arising from olefin
hydrogenation, isomerization, or dehydrogenative silylation
were observed, establishing important advantages in the
selectivity of the base metal catalysts relative to established
platinum compounds.
Scheme 1. Synthetic Routes to (terpy)Fe(CH2SiMe3)2
preferred as yields between 85% and 90% were reproducibly
obtained. This specific alkyl group was chosen due to the
commercial availability of the requisite lithio salt, stability to β-
H elimination, and established precedent in bis(imino)pyridine
iron chemistry.16,17
The limited number of synthetically accessible bis(imino)-
pyridine iron bis(dinitrogen) complexes15 and their air and
moisture sensitivity have inspired the search for alternative iron
precatalysts. Iron dialkyl complexes, (RPDI)Fe(CH2SiMe3)2,
are attractive as potential precatalysts given the number of
derivatives that can be synthesized by straightforward pyridine
displacement from (pyridine)2Fe(CH2SiMe3)2 by addition of
the free bis(imino)pyridine.16−19 Here, we describe evaluation
of terpyridine, bis(imino)pyridine, and pyridine bis(oxazoline)
iron dialkyl complexes for the hydrosilylation of alkenes. In
addition, the electronic structure of each class of compounds
has been elucidated by a combination of magnetochemistry,
structural, spectroscopic, and computational studies.
Crystals of (terpy)Fe(CH2SiMe3)2 suitable for X-ray
diffraction were obtained from a concentrated diethyl ether
solution stored at −35 °C. A representation of the molecule is
presented in Figure 1, and selected metrical parameters are
RESULTS AND DISCUSSION
■
Synthesis and Characterization of (terpy)Fe-
(CH2SiMe3)2. Iron complexes of 2,2′:6′,2″-terpyridine (terpy)
were of interest due to the conceptual relationship to
established bis(imino)pyridine compounds.13,14,20 During the
course of these investigations, Nakazawa and co-workers have
reported the hydrosilylation of 1-hexene with PhSiH3, Ph2SiH2,
and Me(Ph)SiH2 upon treatment of a catalytic amount of
various substituted (terpy)FeX2 (X = Cl, Br) derivatives with
NaBEt3H, demonstrating the catalytic competency of this class
of iron compounds.21 High activity for mono- and disilylation
of the terminal alkene was observed upon heating to 100 °C.
These mixtures were reported to be inactive for the
hydrosilylation of 1-hexene or 1-octene with tertiary silanes
and for the hydrosilylation of internal olefins.
Figure 1. Solid-state molecular structure of (terpy)Fe(CH2SiMe3)2 at
30% probability ellipsoids. Hydrogen atoms are omitted for clarity.
reported in Table 1. In the solid-state structure of (terpy)Fe-
(CH2SiMe3)2, the geometry about the iron is best described as
distorted trigonal-bipyramidal with the N(2), C(12), and
C(12I) defining the equatorial plane. The large SiMe3 groups
are directed over the plane of the terpyridine chelate, likely to
avoid steric interactions.
It is now well established that perturbations in ligand bond
distances are diagnostic of redox activity, including terpyridine
ligands.23 To probe the role of the terpyridine ligand in the
electronic structure of (terpy)Fe(CH2SiMe3)2, the bond
distances of the tridentate chelate were compared to those of
free terpyridine24 as well as a coordination complex, (terpy)-
ZnCl2,25 where the ligand is undoubtedly in its neutral form
(Table 1). Also contained in Table 1 are the bond distances for
[(terpy)2Cr]PF6, a coordination compound with an established
Attempts to prepare a terpyridine iron dinitrogen complex by
reduction of (terpy)FeX2 (X = Cl,22 Br) with excess 0.5%
sodium amalgam or 2 equiv of NaBEt3H produced an
intractable mixture of products. To avoid the complications
of reductive methods, the synthesis of iron dialkyl complexes
was pursued. Previous studies from our group have established
that such compounds are effective precatalysts for aldehyde and
ketone hydrosilylation, and the synthesis of these compounds is
general among tridentate [N3] ligands.19 The desired iron
dialkyl complex, (terpy)Fe(CH2SiMe3)2, was synthesized either
by straightforward addition of the free ligand to (pyridine)2Fe-
(CH2SiMe3)2 in pentane solution or by direct alkylation of
(terpy)FeCl2 with LiCH2SiMe3 (Scheme 1). The latter route is
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a
Table 1. Selected Bond Distances (Å) and Angles (deg) for (terpy)Fe(CH2SiMe3)2
(terpy)Fe(CH2SiMe3)2
terpyridine
(terpy)ZnCl2
[(terpy1−)2Cr]PF6
M(1)−N(1)
M(1)−N(2)
M(1)−C(12)
N(1)−C(5)
N(2)−C(6)
C(5)−C(6)
C(6)−C(7)
C(7)−C(8)
2.150(2)
2.014(2)
2.073(2)
1.353(2)
1.375(2)
1.457(2)
1.390(2)
1.386(2)
118.12(8)
75.48(3)
150.96(7)
2.196(3)
2.112(1)
2.047(3)
1.928(2)
1.343(2)
1.342(3)
1.496(3)
1.397(3)
1.381(3)
1.340(5)
1.342(4)
1.487(6)
1.385(6)
1.393(5)
1.369(6)
1.381(5)
1.448(5)
1.388(6)
1.391(7)
C(12)−Fe(1)−C(12I)
N(1)−Fe(1)−N(2)
N(1)−Fe(1)−N(1I)
74.54(9)
78.68(15)
149.09(9)
157.35(15)
Distances for free terpyridine,23 (terpy)ZnCl2,21 and [(terpy1−)2Cr]PF6 are included for comparison.
a
22
Scheme 2. Aryl-Substituted Bis(imino)pyridine Iron Dialkyl Compounds Evaluated for Catalytic Hydrosilyation of 1-Octene
Using Tertiary Silanes
terpyridine radical anion.22 The bond distances of the chelate in
Table 2. Catalytic Hydrosilylation of 1-Octene with Tertiary
(terpy)Fe(CH2SiMe3)2 are most consistent with a monore-
duced, anionic radical ligand with the most pronounced
distortions in the central pyridine ring.
Silanes in the Presence of Various Iron Dialkyl Complexes
The catalytic performance of (terpy)Fe(CH2SiMe3)2 was
assayed in the hydrosilylation of 1-octene with tertiary silanes.
Interest in the resulting alkyl silanes derives from their
application as agricultural adjuvants, and as a sensory and
performance enhancer in cosmetic formulations. Examples of
bis(imino)pyridine iron dialkyls and the pyridine bis(oxazoline)
compound, (R,R)-(iPrPybox)Fe(CH2SiMe3)2, were also in-
cluded in this study for comparison. Depictions of these
compounds and their shorthand designations are presented in
Scheme 2. Two examples, rac/meso-(2‑iPrPDI)Fe(CH2SiMe3)2
and (iPrPybox)Fe(CH2SiMe3)2, do not have synthetically
accessible iron dinitrogen compounds, and thus the iron
dialkyls represent a convenient entry point to assay the catalytic
performance of these specific iron−chelate combinations.
The results of the catalytic hydrosilylation assays of 1-octene
are summarized in Table 2. Each reaction was conducted neat
at 60 °C using 0.5 mol % of the iron complex. For the 1-octene
hydrosilylations with (terpy)Fe(CH2SiMe3)2 and the tertiary
silanes Et3SiH and MD′M, >95% conversion to the
corresponding anti-Markovnikov alkyl silane was observed
after 1 h at 60 °C. Small (<5%), yet detectable, amounts of 1-
silyl-1-octene, arising from dehydrogenative silylation, were also
observed by gas chromatography and NMR spectroscopy. No
catalytic turnover was observed with (EtO)3SiH due to
competing redistribution of the silane to (EtO)4Si (and likely
SiH4) under catalytic conditions.26,27 No catalytic activity was
observed with the pyridine bis(oxazoline) iron dialkyl complex,
(R,R)-(iPrPybox)Fe(CH2SiMe3)2.
compound
R3Si-H
Et3SiH
percent conversion
(terpy)Fe(CH2SiMe3)2
>95
>95
MD′M
a
(EtO)3SiH
Et3SiH
NR
(
iPrPDI)Fe(CH2SiMe3)2
NR
NR
NR
NR
>95
>95
NR
>95
>95
NR
>95
>95
NR
NR
NR
MD′M
(EtO)3SiH
Et3SiH
(EtPDI)Fe(CH2SiMe3)2
MD′M
(EtO)3SiH
Et3SiH
(
MePDI)Fe(CH2SiMe3)2
MD′M
(EtO)3SiH
Et3SiH
rac/meso-(2‑iPrPDI)Fe(CH2SiMe3)2
(R,R)-(iPrPybox)Fe(CH2SiMe3)2
MD′M
(EtO)3SiH
Et3SiH
MD′M
(EtO)3SiH
a
NR = no reaction defined as <5% conversion at 1 h.
substituents and the identity of the tertiary silane. As was
observed with related bis(imino)pyridine iron dinitrogen
compounds,13 Et3SiH is the least reactive silane with no
turnover being observed after prolonged periods at 60 °C. The
most substituted compound in the series, (iPrPDI)Fe-
(CH2SiMe3)2, also exhibited no catalytic hydrosilylation
activity. For the less sterically protected compounds, selective
The catalytic performance of the bis(imino)pyridine iron
dialkyl compounds was dependent on both the size of the aryl
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anti-Markovnikov hydrosilylation of 1-octene was observed
with the alkoxy silanes, MD′M and (EtO)3SiH, after 1 h at 60
°C. For (EtPDI)Fe(CH2SiMe3)2 and (MePDI)Fe(CH2SiMe3)2,
the higher temperatures required for catalytic activity are a
consequence of initiation of the iron compounds, likely through
iron−carbon bond homolysis to generate the active species.
The utility of the iron dialkyl complexes for the catalytic
hydrosilylation of 1-octene prompted exploration of the scope
of these compounds with other substrates of commercial
importance. Vinylcyclohexene oxide (VCHO, used as a mixture
of cis and trans isomers) was selected due to the utility of the
silylated product for UV-release coating applications, textile
finishes, and for preparing silicone-modified organic poly-
mers.28 Attempted hydrosilylation with the iron dinitrogen
compounds, [(RPDI)Fe(N2)]2(μ2-N2) (R = Me, Et) or the
corresponding iron dialkyl complexes, (RPDI)Fe(CH2SiMe3)2,
and MD′M resulted in decomposition of the iron compounds,
likely due to incompatibility with the epoxide functional
group.29 Similar results have been observed with Karstedt’s
catalyst where up to 50% decomposition of the platinum
compound has been reported.3 Introduction of N-heterocyclic
carbene ligands to the platinum-catalyzed reactions suppresses
this competing decomposition pathway.3 By contrast, complete
conversion to hydrosilylated product was achieved using 1 mol
% (terpy)Fe(CH2SiMe3)2 at 60 °C at 1 h. Exclusive conversion
to the anti-Markovnikov product was observed and demon-
strates the unique function of the terpyridine iron dialkyl
precatalysts in this process as compared to their bis(imino)-
pyridine iron and precious metal counterparts (eq 1).
Zero-field, 57Fe Mossbauer spectroscopic measurements
were also performed (80 K) on all three classes of iron dialkyl
complexes. The experimentally determined isomer shifts (δ)
and quadrupole splittings (ΔEQ) are reported in Table 3. Also
̈
Table 3. Experimental and Computationally Determined
a
Zero-Field, 57Fe Mossbauer Parameters
̈
compound
δ (mm·s−1
)
ΔEQ (mm·s−1
)
(terpy)Fe(CH2SiMe3)2
0.29
0.22
0.26
0.25
0.26
0.25
2.37
+2.62
2.66
(EtPDI)Fe(CH2SiMe3)2
+2.80
3.15
(R,R)-(iPrPybox)Fe(CH2SiMe3)2
+3.49
a
The experimental values were measured at 80 K. Computed values
are presented in italics. All experimental quadrupole splittings are
absolute values.
reported in Table 3 are DFT-computed (vide infra) parameters
for all three compounds. The isomer shifts for all three iron
dialkyl complexes are nearly identical, indicating the same
electronic structure.35 It is also noteworthy that the isomer
shifts are lower than the range typically accepted for high-spin
iron(II) compounds, suggesting that the traditional electronic
structure description of a ferrous compound with a redox-
neutral chelate is not an accurate representation of the true
ground states of the molecules.
Full molecule density functional theory (DFT) calculations
were carried out on all three compounds to gain additional
insight into the electronic structures of the iron dialkyl
complexes and evaluate the possibility of redox-active chelates.
The accuracy of the calculations was assayed by comparison of
experimental and computed metrical parameters for each
compound (see the Supporting Information) as well as by
comparison of computed and experimental 57Fe Mossbauer
̈
parameters (Table 3). On the basis of the magnetic data, two
possible electronic structure models were explored: a high-spin
ferrous center (SFe = 2) with a redox-innocent chelate (SL = 0)
and a high-spin ferric center (SFe = 5/2) antiferromagnetically
coupled to a chelate radical anion (SL = 1/2). These
configurations correspond to a spin-unrestricted quintet and a
broken symmetry (BS) 5,1 approach, respectively. At the
B3LYP level of density functional theory, both (EtPDI)Fe-
(CH2SiMe3)2 and (R,R)-(iPrPybox)Fe(CH2SiMe3)2 spontane-
ously broke symmetry and converged to the redox-active,
BS(5,1) solution. For (terpy)Fe(CH2SiMe3)2, both redox-
innocent and redox-active solutions were found. Comparison of
the total energies revealed that the broken-symmetry solution
was 1.9 kcal/mol lower in energy than the spin-unrestricted
solution. For all of the broken symmetry solutions, excellent
agreement between the experimental and the computed data
was observed. While the metrical parameters of (terpy)Fe-
(CH2SiMe3)2 were acceptably reproduced by the redox-
innocent, spin-unrestricted quintet model, an isomer shift of
0.45 mm·s−1 and quadrupole splitting of +2.92 mm·s−1 were
computed, which is outside the generally accepted range.36
Qualitative molecular orbital diagrams and spin density plots
derived from the BS(5,1) solutions are presented in Figures 2
and 3, respectively. The molecular orbital diagrams illustrate the
high-spin ferric centers engaged in antiferromagnetic coupling
with each of the chelate radical anions. Thus, the combined
computational and experimental data clearly establish that all
Electronic Structure of Bis(imino)pyridine and Terpyr-
idine Iron Dialkyl Complexes. The electronic structures of
the three classes of iron dialkyl complexes used in this study,
bis(imino)pyridine, terpyridine, and pyridine bis(oxazoline),
were investigated using a combination of metrical parameters
from X-ray diffraction, magnetic measurements, Mossbauer
̈
spectroscopy, and DFT calculations. The redox activity of the
bis(imino)pyridines30,31 and terpyridines22,32−34 is now well-
established, although this possibility has not been as thoroughly
investigated for the corresponding pyridine bis(oxazolines).
Electrochemical studies from our laboratory have established
that this last class of chelate is the most challenging to reduce in
the series.19a,22
Three specific compounds were selected for this study:
(EtPDI)Fe(CH2SiMe3)2, (terpy)Fe(CH2SiMe3)2, and (R,R)-
(iPrPybox)Fe(CH2SiMe3)2. Our laboratory has previously
reported that the bis(imino)pyridine and pyridine bis-
(oxazoline) iron dialkyls exhibit magnetic moments at 23 °C,
consistent with four unpaired electrons, and are S = 2
compounds.19a Solid-state measurements at 23 °C on (terpy)-
Fe(CH2SiMe3)2 produced an effective magnetic moment of 4.6
μB, also diagnostic of an S = 2 ground state and a high spin
state for the iron center.
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Figure 2. Qualitative molecular orbital diagrams of (EtPDI)Fe(CH2SiMe3)2 (left), (terpy)Fe(CH2SiMe3)2 (middle), and (R,R)-(iPrPybox)Fe-
(CH2SiMe3)2 (right).
Figure 3. Spin density plots obtained from a Mulliken population analysis of the BS(5,1) solutions for (EtPDI)Fe(CH2SiMe3)2 (left),
(terpy)Fe(CH2SiMe3)2 (middle), and (R,R)-(iPrPybox)Fe(CH2SiMe3)2 (right).
three ligand classes are redox-active in iron dialkyl chemistry.
The overlap (S) values are high for each example, ranging
between 0.73 and 0.76, and indicate a high degree of covalency
between the metal (principally dxz) and ligand radical
(principally b2) orbitals. It is also noteworthy that the spin
density plots (Figure 3) establish that the central pyridine ring
in both the terpyridine and the pyridine bis(oxazoline) iron
dialkyl complexes contains most of the radical character,
consistent with the metrical parameters determined by X-ray
diffraction. Wieghardt and co-workers have also observed a
more reduced central pyridine ring in hexacoordinate
chromium(III) coordination compounds with terpyridine
radical anions.22
monoanions. The terpyridine iron dialkyl, (terpy)Fe-
(CH2SiMe3)2, is an active precatalyst for the hydrosilylation
of 1-octene and VCHO with tertiary silanes. Successful
hydrosilylation of the latter type of substrate represents a
unique function of the terpyridine compounds. Aryl-substituted
bis(imino)pyridine iron dialkyl complexes were also found to
be effective precursors of 1-octene hydrosilylation using alkoxy-
substituted tertiary silanes. These compounds are a more
synthetically accessible class of iron precatalyst than the
previously reported dinitrogen compounds, although higher
temperatures are required to observe catalytic turnover.
EXPERIMENTAL SECTION
■
General Considerations. All air- and moisture-sensitive manip-
ulations were carried out using a standard high-vacuum line, Schlenk
or cannula techniques, or in an M. Braun inert atmosphere drybox
containing an atmosphere of purified nitrogen. The M. Braun drybox
was equipped with a cold well designed for freezing samples in liquid
nitrogen. Solvents for air- and moisture-sensitive manipulations were
CONCLUDING REMARKS
■
Iron dialkyl complexes with three different types of
meridionally coordinating [N3]-based chelates have been
synthesized and their electronic structures established as high-
spin ferric compounds with redox-active chelate radical
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dried and deoxygenated using literature procedures.37 Deuterated
solvents for NMR spectroscopy were distilled from sodium metal
under an atmosphere of argon and stored over 4 Å molecular sieves.
The iron compounds, (iPrPDI)Fe(CH2SiMe3)2, (EtPDI)Fe-
Throughout this paper, we describe our computational results by
using the broken-symmetry (BS) approach by Ginsberg52 and
Noodleman.53 Because several broken-symmetry solutions to the
spin-unrestricted Kohn−Sham equations may be obtained, the general
notation BS(m,n)54 has been adopted, where m (n) denotes the
number of spin-up (spin-down) electrons at the two interacting
fragments. Canonical and corresponding55 orbitals, as well as spin
density plots, were generated with the program Molekel.56
(CH2SiMe3)2,17
(
MePDI)Fe(CH2SiMe3)2,17 iPrPybox)Fe-
(
22
(CH2SiMe3)2,19 and (terpy)FeCl2 were prepared as described
previously. Silanes were purchased from Gelest and dried over CaH2
before use. When working with (EtO)3SiH, special precautions should
be taken.27
Nonrelativistic single-point calculations on the optimized geometry
1H NMR spectra were recorded on a Varian Inova 400
spectrometer operating at 399.860 MHz. All chemical shifts are
were carried out to predict Mossbauer spectral parameters (isomer
̈
shifts and quadrupole splittings). These calculations employed the
CP(PPP) basis set for iron.57 The Mossbauer isomer shifts were
1
̈
reported relative to SiMe4 using H (residual) chemical shifts of the
solvent as a secondary standard. 13C NMR spectra were recorded on a
Bruker 500 spectrometer operating at 125.71 MHz. 13C chemical shifts
are reported relative to SiMe4 using chemical shifts of the solvent as a
secondary standard where applicable. For paramagnetic molecules, the
1H NMR data are reported with the chemical shift, followed by the
peak width at half-height in hertz or multiplicity, followed by
integration value and, where possible, peak assignment. Solution
magnetic moments were determined by the method of Evans at 22 °C
using a ferrocene standard unless otherwise noted. Magnetic
susceptibility balance measurements were performed with a Johnson
Matthey instrument that was calibrated with HgCo(SCN)4. Elemental
analyses were performed at Robertson Microlit Laboratories, Inc., in
Ledgewood, NJ.
calculated from the computed electron densities at the iron centers as
previously described.58,59
Preparation of (terpy)Fe(CH2SiMe3)2. A round-bottom flask was
charged with 0.360 g (1.0 mmol) of (terpy)FeCl2 and approximately
10 mL of diethyl ether. The flask was chilled to −35 °C, and a solution
containing 0.188 g (2.0 mmol) of LiCH2SiMe3 and approximately 10
mL of diethyl ether was added. A color change was observed
immediately upon addition of the solution, and the resulting slurry was
stirred and warmed to ambient temperature. After 1 h, the reaction
mixture was filtered through Celite and the volatiles were removed in
vacuo. The resulting purple solid was washed with approximately 5 mL
of cold pentane, yielding 0.400 g (87%) of (terpy)Fe(CH2SiMe3)2.
Analysis for C23H33FeN3Si2 Calcd: C, 59.59; H, 7.18; N, 9.06. Found:
C, 59.19; H, 6.81; N, 9.10. Magnetic susceptibility (MSB, 23 °C): μeff
= 4.6 μB.1H NMR (benzene-d6, 20 °C): δ = 319.80 (bs, 1H), 194.68
(bs, 2H), 154.65 (bs, 2H), 115.20 (bs, 2H), 59.36 (bs, 2H), 38.56 (bs,
2H), 8.73 (bs, 18H).
GC analyses were performed using a Shimadzu GC-2010 gas
chromatograph equipped with a Shimadzu AOC-20s autosampler and
a Shimadzu SHRXI-5MS capillary column (15 m × 250 μm). The
instrument was set to an injection volume of 1 μL, an inlet split ratio of
100:1, and inlet and detector temperatures of 120 and 250 °C,
respectively. UHP-grade helium was used as carrier gas with a flow rate
of 1.12 mL/min. The temperature program used for all the analyses is
as follows: 80 °C, 1 min; 20 °C/min to 240 °C; 4 min. Quantification
was assessed externally using calibration curves of peak area versus
concentration covering the relevant concentration regime, to correct
for sample-dependent detector response, using pure samples of the
alkenes, silanes, and the hydrosilylation products. An aliquot of the
reaction mixture was removed periodically, quenched by exposure to
air and then diluted to a concentration covered within the calibration
curves and analyzed by GC-FID. All reactions were performed under
neat conditions, and an internal standard was not used to avoid
dilution of the samples.
Single crystals suitable for X-ray diffraction were coated with
polyisobutylene oil in a drybox, transferred to a nylon loop, and then
quickly transferred to the goniometer head of a Bruker X8 APEX2
diffractometer equipped with a molybdenum X-ray tube (λ = 0.71073
Å). Preliminary data revealed the crystal system. A hemisphere routine
was used for data collection and determination of lattice constants.
The space group was identified and the data were processed using the
Bruker 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.
Quantum-Chemical Calculations. All DFT calculations were
performed with the ORCA program package.38 The geometry
optimizations of the complexes and single-point calculations on the
optimized geometries were carried out at the B3LYP level39−41 of
DFT. This hybrid functional often gives better results for transition-
metal compounds than pure gradient-corrected functionals, especially
with regard to metal−ligand covalency.42 The all-electron Gaussian
basis sets were those developed by the Ahlrichs group.43−45 Triple-ζ
quality basis sets def2-TZVP with one set of polarization functions on
the metals and on the atoms directly coordinated to the metal center
were used. For the carbon and hydrogen atoms, slightly smaller
polarized split-valence def2-SV(P) basis sets were used, that were of
double-ζ quality in the valence region and contained a polarizing set of
d-functions on the non-hydrogen atoms. Auxiliary basis sets were
chosen to match the orbital basis.46−48 The RIJCOSX49−51
approximation was used to accelerate the calculations.
Preparation of rac/meso-(2‑iPrPDI)Fe(CH2SiMe3)2. A 20 mL
scintillation vial was charged with 0.443 g (1.00 mmol) of (py)4FeCl2,
and approximately 3−5 mL pentane was added. The resulting slurry
was cooled to −35 °C for approximately 30 min, after which time a
pentane solution containing 0.188 g of LiCH2SiMe3 (2.00 mmol) was
added dropwise. A color change from yellow to purple was observed
upon warming. The reaction mixture was stirred for 1−2 h at ambient
temperature. After this time, the reaction mixture was filtered through
Celite and the filtrate was transferred to a fresh scintillation vial. A
pentane slurry containing 0.397 g of 2‑iPrPDI (1.00 mmol) was added
dropwise to the stirring solution. The purple solution immediately
turned dark violet. This mixture was stirred for 1−2 h at ambient
temperature. The mixture was then cooled to −35 °C and filtered to
furnish 0.400 g (65%) of a dark purple crystalline solid identified as
(
2‑iPrPDI)Fe(CH2SiMe3)2. Analysis for C35H53N3FeSi2 Calcd: C,
62.96; H, 8.51; N, 6.69. Found: C, 62.43; H, 9.26; N, 7.83. Magnetic
1
susceptibility: μeff = 4.7 μB (benzene-d6, 23 °C). H NMR (benzene-
d6): δ = 323.97 (1H, p-py), 317.10 (1H, p-py), 51.65 (147 Hz, 2H, m-
pyr), 49.73 (147 Hz, 2H, m-py), 17.52 (136 Hz, 1H, m-py), 14.97 (64
Hz, 1H), 13.64 (147 Hz, 2H, m-py), 11.35 (295 Hz), 17.56, −18.77
(100 Hz), −151.50 (737 Hz, 6H, C(CH3)), −159.46 (737 Hz, 6H,
C(CH3)).
General Procedure for the Hydrosilylation of 1-Octene. A
scintillation vial was charged with 0.150 g (1.33 mmol) of 1-octene
and 1.33 mmol of the silane (0.295 g of MD′M; 0.220 of (EtO)3SiH;
0.155 g of Et3SiH). To this stirring solution was added 0.5 mol %
(0.007 mmol) of the iron complex (0.003 g of (terpy)Fe(CH2SiMe3)2;
0.005 g of (iPrPDI)Fe(CH2SiMe3)2; 0.004 g of (EtPDI)Fe-
(CH2SiMe3)2; 0.004 g of (MePDI)Fe(CH2SiMe3)2; 0.004 g of rac/
meso-(2‑iPrPDI)Fe(CH2SiMe3)2; 0.004 g of (S,S)-(iPrPybox)Fe-
(CH2SiMe3)2). The vial was sealed, and the reaction mixture was
stirred in an oil bath at 60 °C for 1 h. The reaction was then quenched
by exposure to air, and the resulting mixture was analyzed by NMR
spectroscopy and gas chromatography. A resonance upfield at 0.41
1
ppm in the H NMR spectrum was indicative of formation of the
desired, previously characterized products. The retention time of the
hydrosilylated products is between 7 and 8 min.
Hydrosilylation of Vinylcyclohexene Oxide (VCHO) with
MD′M Using (terpy)Fe(CH2SiMe3)2. A scintillation vial was charged
with 0.150 g (1.33 mmol) of VCHO and 0.295 g (1.33 mmol) of
MD′M. To this stirring solution was added 0.006 g (1.0 mol %) of
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(terpy)Fe(CH2SiMe3)2. The reaction was sealed and placed at the
desired temperature for 1 h. Following this time, the reaction was
quenched by opening the vessel to air and analyzed by H NMR
spectroscopy. Along with the disappearance of the Si−H and olefinic
resonances, an upfield peak observed at 0.37 ppm was indicative of
formation of the desired product.
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1
(2-(7-Oxa-bicyclo[4. 1. 0]heptan-3-yl)ethyl)bis-
1
(trimethylsiloxy)methylsilane (cis and trans). H NMR (CDCl3,
22 °C): 3.16−3.05 (m, 4H), 2.14 (t, 2H, 14.3 Hz), 2.07−1.90 (m,
2H), 1.84−1.72 (m, 1H), 1.72−1.60 (m, 1H), 1.59−1.41 (m, 3H),
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ASSOCIATED CONTENT
* Supporting Information
Crystallographic details for (terpy)Fe(CH2SiMe3)2 in CIF
format. This material is available free of charge via the Internet
at http://pubs.acs.org.
■
S
́
́
(16) Campora, J.; Naz, A. M.; Palma, P.; Alvarez, E.; Reyes, M. L.
Organometallics 2005, 24, 4878.
(17) Fernandez, I.; Trovitch, R. J.; Lobkovsky, E.; Chirik, P. J.
Organometallics 2008, 27, 109.
(18) For a review on iron-catalyzed hydrosilylation of carbonyl
compounds, see: Morris, R. H. Chem. Soc. Rev. 2009, 38, 2282.
(19) (a) Tondreau, A. M.; Darmon, J. M.; Wile, B. M.; Floyd, S. K.;
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(b) Tondreau, A. M.; Lobkovsky, E.; Chirik, P. J. Org. Lett. 2008,
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(20) Portions of this work were previously described in the following
patent applications: Delis, J. G. P.; Chirik, P. J.; Tondreau, A. M. US
2011/0009565A1 and WO 2011/006049A1.
AUTHOR INFORMATION
Corresponding Author
*E-mail: pchirik@princeton.edu.
■
Present Address
#Department of Chemistry, Knox College, Galesburg, IL 61401.
Notes
The authors declare no competing financial interest.
(21) Kamata, K.; Suzuki, A.; Nakai, Y.; Nakazawa, H. Organometallics
2012, 31, 3825.
(22) We note that the solid-state structure of (terpy)FeCl2 is actually
the ion-pair complex, [(terpy)2Fe][FeCl4]. See: Reiff, W. M.;
Erickson, N. E.; Baker, W. A., Jr. Inorg. Chem. 1969, 8, 2019.
(23) Scarborough, C. C.; Lancaster, K. M.; DeBeer, S.;
ACKNOWLEDGMENTS
■
We thank Momentive Performance Materials for financial
support and the U.S. National Science Foundation and
Deutsche Forschungsgemeinschaft for a Cooperative Activities
in Chemistry between U.S. and German Investigators grant.
C.M. acknowledges the Alexander von Humboldt Foundation
for a Feodor Lynen postdoctoral fellowship. We also thank Dr.
Marco Bouwkamp for initial experimental investigations into
the electronic structure of bis(imino)pyridine iron dialkyl
complexes.
Weyhermuller, T.; Sproules, S.; Wieghardt, K. Inorg. Chem. 2012,
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