Phosphinite-iminopyridine iron catalysts for chemoselective alkene hydrosilylation

Article abstract of DOI:10.1021/ja404963f

A series of new pincer iron complexes with electron-donating phosphinite-iminopyridine (PNN) ligands has been prepared and characterized. These iron compounds are efficient and selective catalysts for the anti-Markovnikov alkene hydrosilylation of primary, secondary, and tertiary silanes. More importantly, the system exhibits unprecedented functional group tolerance with reactive groups such as ketones, esters, and amides. Furthermore, the iron-catalyzed alkene hydrosilylation was successfully applied to the synthesis of a valuable insecticide, silafluofen. The electronic properties and structures of the iron complexes have been studied by spectroscopies and computational methods. Overall, the iron catalysts may provide a low-cost and environmentally benign alternative to currently employed precious metal systems for alkene hydrosilylation.

Full text of DOI:10.1021/ja404963f

Article
pubs.acs.org/JACS
Phosphinite-Iminopyridine Iron Catalysts for Chemoselective Alkene
Hydrosilylation
Dongjie Peng,† Yanlu Zhang,† Xiaoyong Du,† Lei Zhang,† Xuebing Leng,† Marc D. Walter,*,‡
and Zheng Huang*,†
†State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Science, 345
Lingling Road, Shanghai 200032, P.R. China
‡Institut fur Anorganische und Analytische Chemie, Technische Universitat Braunschweig, Hagenring 30, 38106 Braunschweig,
̈
̈
Germany
S
* Supporting Information
ABSTRACT: A series of new pincer iron complexes with
electron-donating phosphinite-iminopyridine (PNN) ligands
has been prepared and characterized. These iron compounds
are efficient and selective catalysts for the anti-Markovnikov
alkene hydrosilylation of primary, secondary, and tertiary
silanes. More importantly, the system exhibits unprecedented
functional group tolerance with reactive groups such as
ketones, esters, and amides. Furthermore, the iron-catalyzed alkene hydrosilylation was successfully applied to the synthesis
of a valuable insecticide, silafluofen. The electronic properties and structures of the iron complexes have been studied by
spectroscopies and computational methods. Overall, the iron catalysts may provide a low-cost and environmentally benign
alternative to currently employed precious metal systems for alkene hydrosilylation.
Chart 1. PDI, PNP, and PNN Iron Complexes
1. INTRODUCTION
Alkene hydrosilylation is one of the largest volume reactions
conducted with homogeneous catalysts in chemical industry.1
Alkylsilanes produced from alkene hydrosilylations are widely
used as raw materials in manufacturing silicon rubbers, molding
implants, releasing coatings, and adhesives.2 To date, alkene
hydrosilylation has been dominated by the use of Pt catalysts
such as Speier’s and Karstedt’s complexes.3 However, platinum
is an extremely rare metal. The low abundance, high cost, and
environmental issues concerning heavy metals has motivated
the investigation of safer and inexpensive alternatives.4 In this
respect, catalysts based on earth-abundant and environmentally
benign Fe are highly attractive for alkene hydrosilylation.
In fact, iron carbonyl complexes-catalyzed alkene hydro-
silylations have been known for half a century; however, these
systems often require photolysis to generate the active catalyst,5
and undesired side reactions, such as dehydrogenative
silylation, compete with alkene hydrosilylation.5a,6 A key
breakthrough was the report by the Chirik group of iron
complexes with redox−active bis(imino)pyridine (PDI) ligands
(Chart 1). The (PDI)Fe systems are remarkably efficient for
selective anti-Markovnikov alkene hydrosilylation with various
silanes and it is also compatible with functionalized alkenes
including N,N-dimethylallylamine and allyl polyethers,7,8 and
the redox-activity of the noninnocent bis(imino)pyridine
ligands was believed to play an essential role in the catalysis.8b,9
Broad functional group compatibility is of paramount
importance for the general synthetic applications of alkene
hydrosilylation in organic synthesis. Despite significant
improvements in terms of catalytic efficiency employing Fe
complexes with bis(imino)pyridine ligands, the functionalized
alkene substrates have been limited to amino-, polyether-, and
epoxide-substituted olefins.7b,8b In comparison, Pt and Rh
catalysts have also been used for hydrosilylation of alkenes with
amides, ketones, esters and amines functionalities.3g,10 These
transformations incorporate silicon to highly functionalized
systems, allowing facile synthesis of biologically active silicon-
containing peptides10a and insecticides,11 as well as surface-
functionalized luminescent silicon nanoparticles.12 In contrast,
iron-catalyzed chemoselective hydrosilylation of alkenes con-
taining carbonyl group has remained unknown. Since the first
row transition metals are in general more oxophilic than the
second and third row late transition metals, iron-catalyzed
ester,13 amide,14 or ketone15 hydrosilylation can compete
favorably with alkene hydrosilylation. Indeed, in the reactions
of olefin-substituted carbonyl compounds, all previously
reported iron catalysts effected chemoselective ketone16 and
Received: May 17, 2013
© XXXX American Chemical Society
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ester13 hydrosilylation, but no alkene hydrosilylation (Scheme
1).17
Scheme 2. Synthesis of (RPNNR′)FeX2 Complexes 5a−h
Scheme 1. Chemoselectivity in Hydrosilylation of Olefin-
Substituted Ketone and Ester
To fully replace the Pt, Rh, and other precious metals with Fe
in catalytic alkene hydrosilylation, the functional group
compatibility of the latter must be improved. We envisioned
that the utilization of electron-donating pincer phosphine
ligands might result in an electron-rich Fe center, and thus
reduce the oxophilicity on the metal and potentially improve its
tolerance toward functional groups. In fact, bis-
(dialkylphosphino)pyridine18 (Chart 1), a redox-inactive
PNP-type ligand, has proven to be more electron-donating
than the bis(imino)pyridine ligands.18a Unfortunately, the PNP
iron complex exhibited no activity in alkene hydrosilylation
because of facile catalyst decomposition via ligand dissociatio-
n.18a
salts. These iron complexes are paramagnetic, high-spin FeII
species, as indicated by magnetic susceptibility measurements
(see Experimental Section for details). Their 1H NMR
resonances are broadened and paramagnetically shifted.
Complexes (tBuPNNiPr)FeCl2 (5a) with the most bulky
ligand and (iPrPNNH)FeBr2 (5h) with the least bulky ligand
were also characterized by X-ray diffraction analyses. The solid
structures of 5a and 5h reveal a distorted square-pyramidal
geometry at the mononuclear iron site (Figure 1). Regardless of
the ortho-aryl and phosphino substituents, the planes of the
Guided by these precedents, we sought to develop less
oxophilic iron complexes with improved stability using
electron-donating PNN-type ligands. A couple of PNN pincer
iron complexes have been reported,18b and very recently, we
have shown that an iron complex bearing Milstein’s electron-
donating bipyridyl-based phosphine pincer ligand catalyzes
alkene hydroboration.19 Herein, we report on a series of new
(PNN)Fe complexes using novel phosphinite-iminopyridine
ligands (Chart 1). These complexes are efficient catalyst
precursors for anti-Markovnikov alkene hydrosilylation with
primary, secondary, and tertiary silanes. More importantly, the
iron catalysts tolerate a wide range of organic functional groups,
and for the first time, iron-catalyzed chemoselective hydro-
silylation of alkenes containing amide, ester, and ketone
functionalities has been achieved.
2. RESULTS AND DISCUSSION
2.1. Preparation of the PNN Ligands and Iron Dihalide
Complexes. The synthesis of the RPNNR′ ligands and the
corresponding iron complexes is outlined in Scheme 2.
Deprotection of the methoxy group of 2-acetyl-6-methoxypyr-
idine 1 with HCl formed 6-acetyl-2(1H)-pyridinone 2, which
undergoes Schiff-base condensation with arylamines bearing
various substituents at the 2,6-aryl positions to give the
respective imines 3a−d in high yields. Deprotonation with
NaH, followed by the addition of dialkylchlorophosphine,
generated the PNN phosphinite-iminopyridines ligands 4a−h
in 87−99% yield. The neutral Fe(II) dihalide complexes
(RPNNR′)FeX2 (R = tBu, R′ = iPr, 5a; R = tBu, R′ = Et, 5b; R =
tBu, R′ = Me, 5c; R = tBu, R′ = H, 5d; R = iPr, R′ = iPr, 5e; R
= iPr, R′ = Et, 5f; R = iPr, R′ = Me, 5g; R = iPr, R′ = H, 5h)
were prepared by the addition of ligands to the anhydrous iron
Figure 1. ORTEP diagrams of complexes 5a and 5h. Thermal
ellipsoids shown at 50% probability. Selected bond distances (Å) and
angles (deg) for 5a: Fe1−N1, 2.203(6); Fe1−N2, 2.185(6); Fe1−P1
2.571(2); Fe1−Cl1, 2.297(2); Fe1−Cl2, 2.316(2); C6−N2, 1.292(9);
C5−C6, 1.490(10); N1−Fe1−N2, 73.2(2); N1−Fe1−P1, 72.55(16);
P1−Fe1−N2, 138.46(16); Cl1−Fe1−Cl2, 113.57(9). For 5h: Fe1−
N1, 2.187(7); Fe1−N2, 2.178(8); Fe1−P1 2.528(3); Fe1−Br1,
2.4383(18); Fe1−Br2, 2.4634(17); C6−N2, 1.301(12); C5−C6,
1.471(13); N1−Fe1−N2 274.0(3); N1−Fe1−P1, 73.4(2); P1−Fe1−
N2, 147.0(2); Br1−Fe1−Br2, 106.37(6).
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aryl rings are essentially orthogonal to the plane defined by the
two nitrogen and one phosphorus atoms.
product was obtained from the reaction of 1-octene with
Ph2SiH2 using precatalyst 5a; instead dehydrogenative hydro-
silylation occurred to form allylsilane (38%) and (E)-vinylsilane
(12%) (Table 1). In contrast, reducing the steric demand of the
2,6-aryl substituents has an advantageous effect on the
hydrosilylation with Ph2SiH2. Reactions with complexes 5b,c
afforded the desired product 7 after 3 h in ∼50% yields, but
moderate amounts of products attributed to dehydrogenative
hydrosilylation were also detected (Table 1). Notably, iPr-for-
tBu substitution on the P atom of the PNN ligands apparently
has a more favorable effect than the variation of the ortho-aryl
substituents. While hydrosilylation with (iPrPNNEt)FeBr2 (5f)
gave 75% of 7 and 7% of allylsilane, reactions with precatalysts
5e, 5g, and 5h exclusively formed the hydrosilylation product 7
in nearly quantitative yields (98−99%).
2.2. (PNN)Fe-Catalyzed Hydrosilylation of 1-Octene
with Primary, Secondary, and Tertiary Silanes. Com-
plexes 5a−h were tested as precatalysts for the hydrosilylation
of a nonpolar olefin with primary, secondary, and tertiary
silanes, respectively. Our studies commenced with the reaction
of 1-octene with the primary silane PhSiH3, and these results
are summarized in Table 1. The active catalysts were generated
Table 1. (PNN)Fe Precatalysts 5a−h for Alkene
Hydrosilylation with Primary, Secondary, and Tertiary
a
Silanes
Generally tertiary silanes are less reactive than primary and
secondary silanes for alkene hydrosilylation.7b,8b As expected,
no alkene hydrosilylation was observed with (EtO)3SiH and the
most sterically demanding complexes 5a and 5e. Nevertheless
5b and 5c exhibited moderate activity for the addition of
(EtO)3SiH, but dehydrogenative hydrosilylation also occurred
to give allylsilane as a side product. Gratifyingly, complexes with
reduced steric crowding (5d, and 5f−h) are efficient for the
hydrosilylation with (EtO)3SiH, giving the anti-Markovnikov
product 8 exclusively in excellent isolated yields (85−95%,
Table 1).22
In addition, hydrosilylation of 1-octene with the industrially
important (Me3SiO)2MeSiH (MD′M) using precatalysts 5f
(87%), 5g (72%), and 5h (82%) proceeded smoothly at
ambient temperature to give the desired product 9 in high
yields (Table 1). Complexes 5a−e exhibit no activity for the
hydrosilylation with MD′M.
precat.
silane
% yield
94
precat.
silane
% yield
5a
PhSiH3
5e
PhSiH3
91
b
Ph2SiH2
(EtO)3SiH
MD′M
−:38:12
NR
Ph2SiH2
(EtO)3SiH
MD′M
99
NR
NR
NR
b
5b
5c
5d
PhSiH3
89
5f
PhSiH3
38
b
b
Ph2SiH2
(EtO)3SiH
MD′M
53:6:4
Ph2SiH2
(EtO)3SiH
MD′M
75:7:−
95
b
b
52:10:−
NR
91
b
PhSiH3
99
5g
5h
PhSiH3
24
b
Ph2SiH2
(EtO)3SiH
MD′M
50:20:5
42:13:−
NR
Ph2SiH2
(EtO)3SiH
MD′M
98
91
72
0
b
PhSiH3
15:12:2
55:18:3
85
PhSiH3
b
Ph2SiH2
(EtO)3SiH
MD′M
Ph2SiH2
(EtO)3SiH
MD′M
99
90
82
2.3. (PNN)Fe-Catalyzed Hydrosilylation of Function-
alized Alkenes. A large number of functionalized alkenes
underwent hydrosilylation with silanes in high yield at ambient
temperature using the PNN iron system. These results are
summarized in Scheme 3. Using complex 5a as the precatalyst,
styrene and its derivatives bearing electron-donating or
electron-withdrawing substituents were efficiently hydrosily-
lated with PhSiH3 to form the linear products 10a−f in 72−
89% isolated yields. γ-Phenylpropene was also hydrosilylated in
high yield (10g, 87%). Furthermore, the hydrosilylation of 4-
vinyl-cyclohexene occurred selectively at the terminal olefin to
give 10h in 98% isolated yield. The reaction of allyltrimethylsi-
lane afforded the product 10i in 96% yield. Aliphatic alkenes
containing protecting groups, such as a silylether (TBDPS, 10j,
99%) and a tosylate (10k, 92%), also underwent hydrosilylation
in high yields. In addition, ketals are also tolerated as
demonstrated by the isolation of 10l in 83% yield.
Ether functionalities including an allyl ether moiety are
tolerated as shown by isolation of hydrosilylation products
10m−o in 80−97% yields. Furthermore, these iron catalysts are
also compatible with amino-substituted alkenes. Hydrosilyla-
tion of N-allyl-N-phenylbenzylamine and 9-vinyl-carbazole
formed the desired products 10p and 10q in 81 and 67%
isolated yields, respectively.
NR
a
For PhSiH3: 1 mol % Fe precatalyst and 2 mol % NaBHEt3; For
Ph2SiH2 and (EtO)3SiH: 2 mol % Fe precatalyst and 4 mol %
NaBHEt3; For MD′M: 5 mol % Fe precatalyst and 10 mol %
NaBHEt3. Reported yields are isolated yields (average of three runs)
b
unless otherwise noted. Ratio of hydrosilylation product: allylsilane:
(E)-vinylsilane. Yields were determined by H NMR with an internal
standard.
1
in situ upon the addition of NaBHEt3 (2 equiv) to the
precursors 5a−h.20 Using the relatively bulky complexes
(
tBuPNNiPr)FeCl2 (5a), (tBuPNNEt)FeBr2 (5b), (tBuPNNMe)-
FeBr2 (5c), and (iPrPNNiPr)FeBr2 (5e), the reactions proceeded
efficiently at 23 °C to give the anti-Markovnikov product 6 in
89−99% isolated yields after 3 h.21 In contrast, the reactions
with less sterically hindered complexes gave low yields of 6 due
to competing side reactions. With 5d as the precatalyst,
dehydrogenative hydrosilylation occurred to give 12%
allylsilane and 2% (E)-vinylsilane. Using 5f−h as the
precatalysts, varying amounts of Ph2SiH2 were formed from
the redistribution of PhSiH3 (see Supporting Information (SI)
for product distributions). In an extreme case, the reaction with
the least hindered precatalyst 5h, no hydrosilylation product
was detected; instead, Ph2SiH2 (12%) was formed.
Overall, the PNN iron system significantly enhances the
synthetic utility of hydrosilylation by catalyzing addition of
silanes to alkenes containing carbonyl groups. Reaction of
amide- and ester-substituted olefins with PhSiH3 underwent
chemoselective alkene hydrosilylation with 1 mol % iron
complex 5a to form products 10f, 10r, and 10s in 72, 73, and
64% isolated yields, respectively. For comparison, the same
Iron-catalyzed alkene hydrosilylation with secondary silane,
Ph2SiH2, is typically slower than the corresponding reaction
with PhSiH3 and often proceeds with only poor conversion.7a,8a
Whereas complex (tBuPNNiPr)FeCl2 5a is highly efficient for the
hydrosilylation of 1-octene with Ph3SiH, no hydrosilylation
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Scheme 3. (PNN)Fe-Catalyzed Hydrosilylation of
Functionalized Alkenes
Scheme 4. Opposite Chemoselectivity in Hydrosilylation of
5-Hexen-2-one with Ph2SiH2 Catalyzed by Tilley’s, Chirik’s,
and PNN Iron Complexes
a
a
Reported yields are isolated yields. b The values in parentheses are 1H
NMR yields for the reactions using 1 mol % (iPrPDI)Fe(N2)2 as the
catalyst; c 2 mol % 5a and 4 mol % NaBHEt3. d 4 mol % 5a and 8 mol
% NaBHEt3.
reactions using the related bis(imido)pyridine iron complex
(iPrPDI)Fe(N2)2 as the catalyst gave the products 10f (5%) and
10r (45%) in much lower yields (see Scheme 3). Reaction of
these olefins with the secondary silane Ph2SiH2 using 5h also
occurred to give, after 3 h, 10t (70%) and 10u (79%) in
synthetically useful yields.
yield) (Scheme 4). Varying amounts of the ketone hydro-
silylation product 12 were observed in the reactions with the
three (PDI)Fe dinitrogen catalysts (Scheme 4).
2.4. (PNN)Fe-Catalyzed Hydrosilylation of Ketone-
Functionalized Alkenes. Next, we investigated iron-catalyzed
hydrosilylation of alkenes bearing ketone functionalities. In
general, ketones are highly reactive groups and can readily
undergo hydrosilylation in the presence of various iron
catalysts.15,16 Thus, selective alkene hydrosilylation in the
presence of ketone functionalities is very challenging. Indeed,
Tilley et al. reported that the iron amide catalyst [Fe{N-
(SiMe3)2}2]2 is selective for ketone hydrosilylation of 5-hexen-
2-one (11) with Ph2SiH2 to give the silyl ether 12 in 95% yield
(Scheme 4).16a Using the bis(imino)pyridine Fe catalyst
(iPrPDI)Fe(CH2SiMe3)2, Chirik et al. have shown that 11
underwent selective ketone reduction with Ph2SiH2 to form 5-
hydroxy-1-hexene (13) in 75% yield with no evidence for
alkene reduction (Scheme 4).16b Since bis(imino)pyridine Fe
complexes with smaller aryl substituents might be superior to
(iPrPDI)Fe(N2)2 in alkene hydrosilylations,7b we also carried
out the hydrosilylation of 11 with Ph2SiH2 using Chirik’s iron
dinitrogen complexes with varying steric demand. Consistent
with Chirik’s report, (iPrPDI)Fe(N2)2 is inactive for alkene
hydrosilylation of 11. Whereas the reaction with [(EtPDI)Fe-
(N2)]2(μ-N2) also gave negligible amounts of the alkene
hydrosilylation product 14, the less sterically encumbered
derivative [(MePDI)Fe(N2)]2(μ-N2) gave 50% of 14 (1H NMR
Given the high efficiency of 5g and 5h in the hydrosilylation
of 1-octene with Ph2SiH2 (vide supra) we used these complexes
as precatalysts for the hydrosilylation of 5-hexen-2-one (11). It
is noteworthy that the reaction of 11 with Ph2SiH2 using 5g at
23 °C afforded 84% of the alkene hydrosilylation product 14
(1H NMR yield) after 5.5 h. When complex 5h was employed,
11 underwent exclusively alkene hydrosilylation in quantitative
1
yield as determined by H NMR spectroscopy (86% isolated
yield) (Scheme 4). In both reactions, no carbonyl reduction
product 12 was detected. Similarly, the reaction of 2-allyl-
cyclohexanone 15 with Ph2SiH2 catalyzed by 5h exclusively
formed the desired product 16 in 80% isolated yield (Scheme
4).
2.5. Synthesis of the Insecticide Silafluofen by Iron-
Catalyzed Alkene Hydrosilylation . To demonstrate the
synthetic value of the PNN iron-catalyzed alkene hydro-
silylation, 5h was applied to the synthesis of the novel
pyrethroid-like insecticide, silafluofen. Silafluofen is an
attractive agricultural insecticide because of its high insecticidal
activity, low toxicity, and good stability. The key step in the
industrial preparation of silafluofen requires the expensive
Speier’s platinum catalyst for alkene hydrosilylation under
rather harsh reaction condition (130 °C).11 To our delight,
with 5h (10 mol %) as the precatalyst, the reaction of 3-(4-
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fluoro-3-phenoxyphenyl)-1-propene (17) and (4-
methoxyphenyl)dimethylsilane (18) occurred at 23 °C to
form the desired product silafluofen (19) in 55% isolated yield
(eq 1).
2.6. Unreactive Substrates in (PNN)Fe-Catalyzed
Hydrosilylation. A list of unreactive olefin substrates is
provided in Figure 2. Internal olefins, such as 2-hexene and
We also conducted electrochemical measurements of the free
tBuPNNiPr ligand 4a and the iron carbonyl complex 21 to study
the electronic properties of the (PNN)Fe fragment using the
same conditions as reported for the PDI system (see SI). For
comparison, the reduction potentials of the PNN system and
Table 2. Cyclic Voltammetry Data for Ligands and Iron
Dicarbonyl Complexes
ligand and cmpd
tBuPNNiPr
iPrPDI
redn (V vs Fc/Fc+)
oxidn (V vs Fc/Fc+)
−2.74
−2.62
−2.54
−2.46
−
−
−0.65
−0.49
a
(
(
tBuPNNiPr)Fe(CO)2
a
iPrPDI)Fe(CO)2
a
Reference 24.
Figure 2. Unreactive hydrosilylation substrates.
the related iPrPDI system24 are included in Table 2. The
tBuPNNiPr ligand 4a exhibits a reduction wave at a more
negative reduction potential than the iPrPDI ligand (120 mV
difference), implying that the tBuPNNiPr ligand is more electron-
donating than the iPrPDI system. The carbonyl complex
cyclohexene, are unreactive for (PNN)Fe-catalyzed hydro-
silylation with PhSiH3 or Ph2SiH2. Reactions of allyl chloride
(20c) and alkenes bearing unprotected alcohols (20d and 20e)
and amide with an NH group (20f) also yielded no
hydrosilylation product. Furthermore, no reaction of styrene
derivatives containing nitro (20g), nitrile (20h), or pyridine
(20i) units was observed, and allyl acetoacetate (20j) likely
poisons the catalyst by metal chelation.
(
tBuPNNiPr)Fe(CO)2 (21) exhibits a reversible oxidation
potential and an irreversible reduction potential. Although we
currently cannot determine the electronic structure of the
oxidized and reduced species, the cyclic voltammetry data show
that complex 21 is more readily oxidized by 160 mV and less
easily reduced by 80 mV than the (iPrPDI)Fe(CO)2 (22),
highlighting the electron-richness of the (tBuPNNiPr)Fe frag-
ment.
2.7. Electronic Properties and Electronic Structure of
the PNN Iron Fragment. The results described above clearly
show that the (PNN)Fe catalysts are tolerant of a variety of
functional groups and chemoselective for alkene hydro-
silylations in the presence of these functionalities. We presumed
that the excellent functional group compatibility results from
the less oxophilic nature of the (PNN)Fe complex. To confirm
this hypothesis, it is of interest to study the electronic
properties and electronic structure of the PNN iron fragment.
2.7.1. Electronic Properties of the PNN Iron Fragment
Investigated by IR Spectroscopis and Electrochemical
Measurements. A useful tool to evaluate the electron-donating
ability of the phosphinite-iminopyridine ligands is the νCO
stretching frequencies of the respective carbonyl complexes.
Carbonyl complex (tBuPNNiPr)Fe(CO)2 (21) was obtained in
51% yield by the reaction of CO with 5a in the presence of 2
equiv of NaBHEt3 (eq 2). The νCO stretching frequencies of 21
in pentane solution (1902 and 1956 cm−1) are red-shifted from
those of the related iron bis(imino)pyridine complex (iPrPDI)-
Fe(CO)2 (22) (1914 and 1974 cm−1).23 The data are
consistent with a more electron-rich (tBuPNNiPr)Fe fragment
as compared to the (iPrPDI)Fe fragment, resulting in increased
backbonding to the CO ligands in 21.
2.7.2. Electronic Structure of the (tBuPNNiPr)Fe(CO)2
Complex Fragment Investigated by X-ray Analysis and VT
NMR Studies. In contrast to 5a−h, the dark-blue complex
(
tBuPNNiPr)Fe(CO)2 21 displays narrow, well-resolved NMR
resonances, and the chemical shifts are within the expected
region for a diamagnetic molecule. We also succeeded in
growing crystals of 21 suitable for an X-ray diffraction
experiment, and in the solid state 21 adopts a pseudosquare
planar-pyramidal geometry (Figure 3). Interestingly, the
CimineNimine bond distance in 21 (1.344(6) Å) is significantly
longer than that in the dihalide complexes 5a (1.292(9) Å) and
5h (1.301(12) Å), which might suggest that the phosphinite-
iminopyridine ligand is reduced to a radical anion in 21.
Consistent with this hypothesis, the CimineCpyridine bond
distance in 21 (1.404(7) Å) is shorter than those in 5a
(1.490(10) Å) and 5h (1.471(13) Å). On the basis of solution
magnetic susceptibility studies (see Experimental Section) the
ground state of 5 can unambiguously be assigned as a high-spin
E
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Figure 3. ORTEP diagram of complex 21. Thermal ellipsoids shown at
50% probability. Selected bond distances (Å) and angles (deg): Fe1−
N2, 1.913(4); Fe1−N1, 1.923(5); Fe1−P1 2.1768(18); Fe1−C1,
1.757(5); Fe1−C2, 1.787(7); C16−N1, 1.344(6); C16−C17,
1.404(7), C1−O1, 1.167(6); C2−O2, 1.156(7); N1−Fe1−N2,
79.18(18); N2−Fe1−P1, 79.80(15); P1−Fe1−N1, 148.40(14); C1−
Fe1−C2, 95.7(3); N2−Fe1−C1, 158.9(2).
(SFe = 2) ferrous center coordinated to the neutral PNN ligand.
However, previous work by Lu and Wieghardt showed that α-
iminopyridines are redox non-innocent ligands, and one
notable structural feature of α-iminopyridyl radical anions is a
significantly elongated CimineNimine bond and shortened
CimineCpyridine bond distances consistent with an electron
transfer into the LUMO of the α-iminopyridine ligand.25 This
raises the question regarding the electronic structure of 21.
Two pictures consistent with a singlet ground state (S = 0) may
be developed, (a) a neutral PNN ligand is bound to a d8-Fe(0)
Fe(CO)2 fragment with considerable π-backbonding into the
LUMO of the α-iminopyridine system (closed-shell config-
uration, 21), or alternatively (b) an open-shell case in which the
Figure 4. Comparisons between the X-ray structural data and
calculated geometric parameters for both complexes 5a and 21.
Note: the crystal structure of 5a contains two molecules in the
asymmetric unit.
S
Fe = 1/2 metal center and the PNN ligand radical anion (SPNN
= 1/2) (21′, see eq 2). Related arguments were developed with
respect to the electronic ground state of (iPrPDI)Fe(CO)2
(22).26
To gain insight in the electronic structure of 21 variable-
temperature (VT) 1H NMR studies have been undertaken, and
the δ vs T−1 plots are shown in SI. No significant temperature
1
dependence of the H NMR chemical shifts was observed,
consistent with a diamagnetic molecule. Furthermore, the
temperature dependence of the 1H NMR resonances is
unchanged when the Me-group is exchanged with an H-atom
in the α-iminopyridine moiety (see SI for details), arguing
against the presence of unpaired spin density in these
positions.27
2.7.3. Electronic Structure of the (PNN)Fe Complexes
Fragment Investigated by DFT Calculations. To further
elaborate this electronic structures of the (PNN)Fe complexes,
DFT calculations were performed on 5a and 21 at the B3LYP
level of theory. In Figure 4, the experimental and calculated
geometric parameters for both complexes 5a and 21 are
compared. For 5a a reasonabe agreement between experimental
and calculated geometry was obtained (see Figure 4).
For complex 21, the molecular structure of the closed-shell
system was computed, and the calculated metric parameters are
in good agreement with the experimental ones (Figure 4). The
corresponding molecular orbital diagram obtained for 21 is
presented in Figure 5. These representations display significant
backbonding from the filled Fe d-orbitals to the π* orbitals of
the two CO ligands (HOMO−1, HOMO−2, and HOMO−3).
Figure 5. The molecular orbital diagram of complex 21 with a closed-
shell singlet state spin configuration.
strong π-backbonding that gives rise to the observed
contraction of the Cimine−Cpyridine bond distance.
The geometry of the triplet state for 21 was also computed,
and the most notable deviation between the calculated and
2
In addition, the HOMO is composed of the Fed orbital and
z
the LUMO of the PNN ligand, which can be explained by a
F
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experimental structure was found for the Fe−Npyridine, Fe−
N
imine, and Fe−P bonds (Figure 4). Interestingly, enforcing an
open-shell singlet state starting from the triplet state led to a
geometry that is very similar to that of the closed-shell isomer
(Figure 4).
The energy for the spin-unrestricted (open-shell singlet)
calculation is slightly lower in energy than the closed-shell one
(Figure 6). However, the electronic structure of complex 21 is
Figure 6. Energy differences between the triplet state, the closed-shell
singlet state, and the open-shell singlet state for complex 21.
probably best described as a delocalized (covalent) system with
a charge distribution such as Fe(+I)−(PNN)1−, but without
significant diradical character consistent with the VT-NMR data
(vide supra).28 Further electronic structure studies are in
progress and will be reported in due course.
2.8. Probing the Binding Affinities of Olefin Vs
Carbonyl Functionalities toward the (PNN)Fe- and
(PDI)Fe-Fragment. As demonstrated experimentally the
(PNN)Fe systems exhibit unprecedented high chemoselectivity
for hydrosilylation of alkenes bearing reactive functionalities,
such as carbonyl groups. This selectivity has a steric component
(as shown in section 2.4), but the electrochemical data also
suggest electronic reasons, e.g. the (PNN)Fe complexes are
more electron-rich than the related (PDI)Fe complexes. The
elaboration of the hydrosilylation mechanism and therefore the
origin of the chemoselectivity are beyond the scope of this
manuscript and will require more detailed mechanistic and
computational studies. Nevertheless, (iPrPNNH)Fe(olefin) and
(iPrPDI)Fe(olefin) species might act as potential catalytic
intermediates in the Fe-catalyzed alkene hydrosilylation,7a,29
and therefore we sought to compare at this point the relative
ground state stabilization energies of alkene versus carbonyl
binding for 5-hexen-2-one 11 to the (iPrPNNH)Fe-fragment30
and the (iPrPDI)Fe-fragment.
The enhanced electron-richness of the (PNN)Fe fragment
(vide supra) should increase the binding affinity of the olefin
functionality because of more favorable π-backbonding
interactions. Similar to 21, several spin configurations were
considered, and their energies are compared in Figure 7. The
differences between the (iPrPNNH)Fe and (iPrPDI)Fe systems
are striking. We find a dramatic stabilization of the carbonyl-
bound species in the (PDI)Fe system, which is 11.9 kcal/mol
more stable than the lowest olefin-bound isomer. In contrast,
for the (PNN)Fe complex a clear preference for the olefin-
bound isomer is observed, which is 4.1 kcal/mol more stable
than the carbonyl-bound one (Figure 7). Overall, these results
are consistent with the electrochemical observations, but steric
effects will also effect the binding preferences to a certain extent
and a comparison between the (iPrPNNH)Fe and (tBuPNNiPr)Fe
can be found in the SI.
Figure 7. Free energy differences between the olefin-bound vs O-
bound structures for the (iPrPNNH)Fe-fragment (top) and (iPrPDI)Fe-
fragment (bottom).
3. CONCLUDING REMARKS
We have prepared a series of iron pincer complexes ligated by
phosphinite-iminopyridine ligands. Electrochemical measure-
ments and IR spectroscopy indicate that these complexes are
more electron-rich than the related (PDI)Fe complexes.
Selective anti-Markovnikov alkene hydrosilylations with
primary, secondary, and tertiary silanes have been achieved
with these (PNN)Fe catalysts. Iron precursors with large
substituents at both the P atom and 2,6-aryl positions are more
effective than those with smaller substituents for alkene
hydrosilylation with primary silanes, whereas the less sterically
crowded iron precursors are highly active and selective for
alkene hydrosilylation with secondary and tertiary silanes.
Although the (PNN)Fe catalysts are less efficient than
Chirik’s (PDI)Fe catalysts in hydrosilylation of simple α-olefins,
successful hydrosilylations of alkenes bearing various function-
alities, such as amide, ester, and ketone groups, highlight the
remarkable chemoselectivity of the phosphinite-iminopyridine
iron catalysts in alkenes hydrosilylation. These cost-effective
and environmentally friendly iron hydrosilylation catalysts that
are compatible with reactive functionalities provide distinct
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concentrated under dynamic vacuum. The residue was purified by flash
chromatography, eluting with petrol ether to afford 124.0 mg of the
advantages over traditional precious metal catalysts and
previously reported iron catalysts for preparing functionalized
alkylsilanes. Further investigations regarding the mechanistic
implications of these observations are ongoing and will be
reported in due course.
1
desired product octyl(phenyl)silane 6 (0.56 mmol, 94%). H NMR
(400 MHz, CDCl3) δ 7.55 (m, 2H), 7.39−7.33 (m, 3H), 4.27 (t, 3JH,H
= 3.6 Hz, 2H, SiH2), 1.47−1.43 (m, 2H, CH2), 1.27−1.25 (m, 10H,
3
CH2), 0.96−0.93(m, 2H, SiCH2), 0.86 (t, JH,H = 6.8 Hz, 3H,
CH2Me); 13C{1H} NMR (101 MHz, CDCl3) δ 135.4, 133.0, 129.6,
128.1, 33.0 (Ar-CH2), 32.1 (CH2), 29.4 (CH2), 29.3 (CH2), 25.2
(CH2), 22.8 (CH2), 14.3 (CH2Me), 10.2 (SiCH2). These spectroscopic
data agree with the reported data.44
4. EXPERIMENTAL SECTION
General Considerations. All manipulations were performed
under an argon or nitrogen atmosphere by using standard Schlenk
techniques. All solvents were purified and dried according to standard
methods prior to use. Diphenylsilane (98.0%), triethoxylsilane
(97.0%), and (Me3SiO)2MeSiH (98.0% MD′M) were purchased
from TCI, and phenylsilane (98.0%) was purchased from J&K
Scientific Ltd. All the silanes were used without further purification.
Sodium hydride was purchased from Tianjin Beidouxing Fine
Chemical Co., Ltd. and washed with pentane and dried under vacuum
prior to use. Di-tert-butylchlorophosphine (96%) and chlorodiisopro-
pylphosphine (96%) were purchased from Acros and used as received.
Anhydrous iron(II) dichloride (98%) and iron(II) dibromide (98%)
were purchased from Adamas and Aldrich, respectively, and used as
received. NaHBEt3 (1 M in toluene) was purchased from Aldrich and
used as received. Hex-2-ene (85.0%) and pent-4-en-ol (98%) were
purchased from Aldrich and used as received. 1-nitro-4-vinylbenzene
(>95%) was purchased from J&K and used as received. 3-chloroprop-
1-ene (>98%) was purchased from Shanghai Tianlian Fine Chemical
Co., Ltd. and distilled from calcium hydride prior to use. 4-
vinylbenzonitrile (98%) was purchased from Alfa Aesar and used as
received. Cyclohexene (>99%) was purchased from Sinopharm
chemical Reagent Co., Ltd. and distilled from calcium hydride prior
to use. n-Octene (≥98.0%) and 4-vinylpyridine (>96%) was purchased
from Aladdin and distilled from calcium hydride prior to use. Styrene
(99.0%), 4-methylstyrene (96.0%), 4-methoxystyrene (95.0%), 4-
fluorostyrene (98.0%), 4-chlorostyrene (98.0%), but-3-en-1-ol (98%),
allyl 3-oxobutanoate (>98%), 2-allylcyclohexanone (≥98%), 4-vinyl-
cyclohex-1-ene (95%), and 4-vinylphenyl acetate (98.0%) were
purchased from TCI and used as received. Allyltrimethylsilane
(≥97%), allylbenzene (≥98%) and 9-vinylcarbazole (≥98%) were
purchased from Adamas and used as received. Other alkenes including
(1-(hex-5-en-1-yloxy)-2,2-dimethylpropane-1,1-diyl)dibenzene,31 hex-
5-en-1-yl 4-methylbenzenesulfonate,32 N-allyl-N-benzylaniline,33
((pent-4-en-1-yloxy)methyl)benzene,34 ((but-3-en-1-yloxy)methyl)-
benzene,35 ((allyloxy)methyl)benzene,36 2-(but-3-en-1-yl)-2-methyl-
1,3-dioxolane,37 hex-5-en-1-yl benzoate,38 and N,N-dimethylpent-4-
enamide,39 were synthesized according to the literature procedures.
Compounds 1-(6-methoxypyridin-2-yl)ethanone,40 3-(4-fluoro-3-phe-
noxyphenyl)-1-propene,41 (4-methoxyphenyl)dimethylsilane,42 and N-
allylbenzamide43 were prepared according to the reported procedure.
1H NMR spectra of 5a−h were recorded on a Mercury (300 MHz)
instrument. 1H, 13C, and 31P NMR spectra of all the other compounds
were recorded on Varian and Agilent instruments (400 MHz, 101
MHz, and 162 MHz respectively). The variable-temperature 1H NMR
6-Acetylpyridin-2(1H)-one (2). In a 500 mL, single-neck flask was
added 1-(6-methoxypyridin-2-yl)ethanone (10.0 g, 66.2 mmol, 1
equiv), 4 M HCl solution (90 mL, 396.9 mmol, 6 equiv), and 1,4-
dioxane (200 mL). The reaction mixture was stirred at 80 °C for 13 h.
Then the solution was concentrated under reduced pressure. The
residue was purified by flash chromatography on silica gel, eluting with
dichloromethane/methanol 20:1 (v/v) to afford the title compound 2
as an off-white solid (9.1 g, 93% yield).
3
3
1H NMR (400 MHz, D2O) δ 7.73 (dd, JH,H = 9.2, JH,H = 6.9 Hz,
3
3
1H, 2-H), 7.27 (dd, JH,H = 6.9, JH,H = 0.9 Hz, 1H, 1- or 3-H), 6.80
3
3
(dd, JH,H = 9.2, JH,H = 0.9 Hz, 1H, 1- or 3-H), 2.55 (s, 3H, COMe);
13C{1H} NMR (101 MHz, D2O) δ 194.0 (C6), 163.9 (C5), 142.4 (C1,
C2 or C3), 138.6 (C4), 125.7 (C1, C2 or C3), 113.2 (C1, C2 or C3),
24.5 (COMe). Anal. Calcd for C7H7NO2: C, 61.31, H, 5.14, N, 10.21.
Found: C, 61.16, H, 5.22, N, 10.27.
(E)-6-(1-((2,6-diisopropylphenyl)imino)ethyl)pyridin-2(1H)-one
(3a). In a 100 mL single-neck flask was added 1-(6-hydroxypyridin-2-
yl)ethanone (2) (2.0 g, 14.6 mmol, 1 equiv), 2,6-diisopropylaniline
(3.9 g, 21.9 mmol, 1.5 equiv), p-tosyl acid (30.0 mg, 150 μmol, 1 mol
%) and n-butanol (40 mL). After heating at reflux for 24 h with
azeotropic removal of water using a Dean−Stark trap, the reaction
mixture was concentrated under reduced pressure. The residue was
purified by flash chromatography on silica gel eluting with dichloro-
methane/methanol 40:1 (v/v) to afford the title compound 3a as a
yellow solid (3.8 g, 89% yield).
1H NMR (400 MHz, CDCl3) δ 10.34 (s, 1H, NH), 7.50 (dd, 3JH,H
=
9.3, 3JH,H = 6.8 Hz, 1H, 2-H), 7.18−7.11 (m, 3H, Ar-H), 6.77 (d, 3JH,H
3
1
= 9.3 Hz, 1H, 1- or 3-H), 6.66 (d, JH,H = 6.8 Hz, 1H, 1- or 3-H),
were recorded on Agilent instrument (600 MHz). H NMR chemical
3
2.62−2.51 (m, 2H, CHMe), 2.01 (s, 3H, NCMe), 1.12 (d, JH,H
=
shifts were internally referenced to TMS (tetramethylsilane) signal or
solvent residual signals. 13C NMR chemical shifts were internally
referenced to solvent signals. 31P NMR chemical shifts were referenced
to an external 85% H3PO4 standard. The 1H NMR data of
paramagnetic complexes are reported with the chemical shift, the
peak width at half-height in Hertz, and integration value. Elemental
analysis, infrared spectra, and high resolution mass spectra (HRMS)
were collected by Analytical Laboratory of Shanghai Institute of
Organic Chemistry (CAS).
Typical Procedure for Catalytic Hydrosilylation of Alkenes
with Silanes. In a N2 glovebox, a vial (10 mL) was charged with 1-
octene (94 μL, 0.6 mmol), phenylsilane (74 μL, 0.6 mmol), dry
toluene (1 mL), and complexes 5a (6.0 μmol, 1 mol %). The reaction
mixture was cooled to −34 °C, and NaBHEt3 (12 μmol, 2 mol %, 1 M
in toluene) was then added to the reaction mixture. The resulting
mixture was stirred for 3 h at room temperature. After that, the vial
was removed from the glovebox, and the reaction mixture was
6.9 Hz, 12H, CHMe2); 13C{1H} NMR (101 MHz, CDCl3) δ 162.3 (N
= C), 157.9 (CO), 144.1 (Ar-Cip), 140.5 (C4), 140.1 (C1, C2 or
C3), 136.0 (Ar-Co), 124.9 (Ar-Cp), 124.7 (Ar-Cm), 123.1 (C1, C2 or
C3), 106.9 (C1, C2 or C3), 28.3 (CHMe2), 23.3 (CHMe2), 22.8
(CHMe2), 15.8 (NCMe). Anal. Calcd for C19H24N2O: C, 76.99, H,
8.16, N, 9.45. Found: C, 76.75, H, 7.99, N, 9.31.
(E)-6-(1-((2,6-Diethylphenyl)imino)ethyl)pyridin-2(1H)-one (3b).
In a 100 mL, single-neck flask was added 1-(6-hydroxypyridin-2-
yl)ethanone (2) (2.0 g, 14.6 mmol, 1 equiv), 2,6-diethylaniline (3.3 g,
21.9 mmol, 1.5 equiv), p-tosylate acid (30.0 mg, 150 μmol, 1 mol %),
and n-butanol (60 mL). After heating at reflux for 24 h with azeotropic
removal of water using a Dean−Stark trap, the reaction mixture was
concentrated under reduced pressure. The residue was purified by
flash chromatography on silica gel, eluting with dichloromethane/
methanol 40:1 (v/v) to afford the title compound 3b as a yellow solid
(3.7 g, 95% yield). 1H NMR (400 MHz, CDCl3) δ 10.34 (s, 1H), 7.49
H
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3
3
(dd, JH,H = 9.3, JH,H = 6.8 Hz, 1H, 2-H), 7.12−7.05 (m, 3H, Ar-H),
Celite under Ar. The solvent was removed under reduced
pressure to afford a yellow solid 4a (2.2 g, 95% yield).
1H NMR (400 MHz, CDCl3) δ 8.00 (dd, 3JH,H = 7.5, 4JP,H = 0.7 Hz,
3
4
3
6.76 (dd, JH,H = 9.2, JH,H = 1.3 Hz, 1H, 1- or 3-H), 6.66 (d, JH,H
=
6.8 Hz, 1H, 1- or 3-H), 2.35−2.20 (m, 4H, CH2CH3), 2.00 (s, 3H,
NCMe), 1.10 (t, 3JH,H = 7.5 Hz, 6H, CH2Me2); 13C{1H} NMR (101
MHz, CDCl3) δ 162.4 (CN), 158.0 (CO), 145.8 (Ar-Cip), 140.8
(C4), 140.1 (C1, C2 or C3), 131.6 (Ar-Co), 126.3 (Ar-Cp), 125.2 (Ar-
Cm), 124.5 (C1, C2, or C3), 106.9 (C1, C2, or C3), 24.7 (CH2CH3),
15.8 (N = CMe), 14.0 (CH2CH3). Anal. Calcd for C17H20N2O: C,
76.09, H, 7.51, N, 10.44. Found: C, 76.31, H, 7.66, N, 10.49.
3
1H, Py-Hm), 7.71 (t, JH,H = 7.8 Hz, 1H, Py-Hp), 7.16−7.06 (m, 3H,
3
Ar-H), 6.97 (d, JH,H = 7.7 Hz, 1H, Py-Hm3′), 2.79−2.68 (m, 2H,
CHMe2), 2.16 (s, 3H, NCMe), 1.24 (d, JP,H = 11.6 Hz, 18H,
3
CMe3), 1.13 (d, JH,H = 6.9 Hz, 12H, CHMe2); 31P{1H} NMR (162
MHz, CDCl3) δ 155.9 (s); 13C{1H} NMR (101 MHz, CDCl3) δ 167.1
2
(N = C), 164.0 (d, JP,C = 7.6 Hz, Py-Co), 154.7 (Py-Co′), 146.7 (Ar-
(E)-6-(1-((2,6-Dimethylphenyl)imino)ethyl)pyridin-2(1H)-one (3c).
In a 100 mL, single-neck flask was added 1-(6-hydroxypyridin-2-
yl)ethanone (2) (2.0 g, 14.6 mmol, 1 equiv), 2,6-dimethylaniline (2.7
g, 21.9 mmol, 1.5 equiv), p-tosylate acid (30.0 mg, 150 μmol, 1 mol
%), and n-butanol (50 mL). After heating at reflux for 24 h with
azeotropic removal of water using a Dean−Stark trap, the reaction
mixture was concentrated under reduced pressure. The residue was
purified by flash chromatography on silica gel, eluting with
dichloromethane/methanol 40:1 (v/v) to afford the title compound
3c as a yellow solid (3.6 g, 99% yield). 1H NMR (400 MHz, CDCl3) δ
Cip), 139.3 (Py-Cp), 136.0 (Ar-Co), 123.5 (Ar-Cp), 123.0 (Ar-Cm),
3
1
115.2 (Py-Cm′), 113.8 (d, JP,C = 2.6 Hz, Py-Cm), 35.8 (d, JP,C = 26.9
2
Hz, CMe3), 28.3 (CHMe2), 27.8 (d, JP,C = 15.6 Hz, CMe3), 23.3
(CHMe2), 23.0 (CHMe2), 17.6 (NCMe). HRMS (EI), m/z Calcd
for C27H41N2OP (M+) 440.2957, found: 440.2968.
tBuPNNEt Ligand, (E)-N-(1-(6-((Di-tert-butylphosphino)oxy)-
pyridin-2-yl)ethylidene)-2,6-diethylaniline (4b). Under an atmos-
phere of argon, NaH (93.0 mg, 3.7 mmol, 1.2 equiv) and THF (10
mL) were added to a 100 mL Schlenk tube. The solution of (E)-6-(1-
((2,6-diethylphenyl)imino)ethyl)pyridin-2-ol (3b) (840.0 mg, 3.1
mmol, 1 equiv) in THF (15 mL) was added dropwise to the Schlenk
tube at room temperature. After that the mixture was stirred for 10
min, tBu2PCl (618.0 mg, 3.4 mmol, 1.1 equiv) was added, and the
resulting mixture was stirred for 3 h. The solvent was removed under
vacuo, hexane (30 mL) was added, and the dark-yellow mixture was
filtered through a pad of Celite under Ar. The solvent was removed
under reduced pressure to afford a yellow solid 4b (1.2 g, 93% yield).
3
3
10.35 (s, 1H, NH), 7.49 (dd, JH,H = 9.3, JH,H = 6.8 Hz, 1H, 2-H),
7.08−6.96 (m, 3H, Ar-H), 6.76 (dd, 3JH,H = 9.3, 4JH,H = 1.4 Hz, 1H, 1-
3
or 3-H), 6.66 (d, JH,H = 6.8 Hz, 1H, 1- or 3-H), 1.99 (s, 3H, N
CMe), 1.97 (s, 6H, Ar-Me); 13C{1H} NMR (101 MHz, CDCl3) δ
162.3 (CN), 157.9 (CO), 146.8 (Ar-Cip), 140.7 (C4), 140.1 (C1,
C2 or C3), 128.1 (Ar-Co), 125.6 (Ar-Cp), 125.1 (Ar-Cm), 124.0 (C1,
C2 or C3), 106.9 (C1, C2 or C3), 18.0 (Ar-Me), 15.0 (NCMe);
Anal. Calcd for C15H16N2O: C, 74.97, H, 6.71, N, 11.66. Found: C,
74.75, H, 6.72, N, 11.52.
3
4
1H NMR (400 MHz, CDCl3) δ 8.00 (dd, JH,H = 7.5, JP,H = 0.7 Hz,
3
1H, Py-Hm), 7.70 (t, JH,H = 7.8 Hz, 1H, Py-Hp), 7.10−7.00 (m, 3H,
3
Ar-H), 6.96 (d, JH,H = 8 Hz, 1H, Py-Hm′3), 2.44−2.26 (m, 4H,
(E)-6-(1-(Phenylimino)ethyl)pyridin-2(1H)-one (3d). In a 100 mL,
single-neck flask was added 1-(6-hydroxypyridin-2-yl)ethanone (2)
(2.0 g, 14.6 mmol, 1 equiv), aniline (2.0 mL, 21.9 mmol, 1.5 equiv), p-
tosylate acid (30.0 mg, 150 μmol, 1 mol %), and n-butanol (50 mL).
After heating at reflux for 24 h with azeotropic removal of water using
a Dean−Stark trap, the reaction mixture was concentrated under
reduced pressure. The residue was purified by flash chromatography
on silica gel, eluting with dichloromethane/methanol 30:1 (v/v) to
CH2Me), 2.14 (s, 3H, NCMe), 1.24 (d, JP,H = 11.6 Hz, 18H,
CMe3), 1.13 (t, JH,H = 7.5 Hz, 6H, CH2Me); 31P{1H} NMR (162
3
MHz, CDCl3) δ 156.0 (s); 13C{1H} NMR (101 MHz, CDCl3) δ 167.1
2
(N = C), 164.0 (d, JP,C = 7.6 Hz, Py-Co), 154.7 (Py-Co′), 148.0 (Ar-
Cip), 139.3 (Py-Cp), 131.4 (Ar-Co), 125.9 (Ar-Cm), 123.3 (Ar-Cp),
3
1
115.2 (Py-Cm′), 113.8 (d, JP,C = 3.0 Hz, Py-Cm), 35.7 (d, JP,C = 27.0
Hz, CMe3), 27.8 (d, 2JP,C = 15.6 Hz, CMe3), 24.7 (CH2Me), 17.3 (N
C-Me), 13.8 (CH2Me). HRMS (ESI), m/z Calcd for C25H38N2OP (M
+ H)+ 413.2722, found: 413.2730.
1
afford the title compound 3d as a brown solid (3.1 g, 87% yield). H
3
NMR (400 MHz, CDCl3) δ 10.28 (s, 1H, NH), 7.47 (dd, JH,H = 9.2,
tBuPNNMe Ligand, (E)-N-(1-(6-((Di-tert-butylphosphino)oxy)-
pyridin-2-yl)ethylidene)-2,6-dimethylaniline (4c). Under an atmos-
phere of argon, NaH (113.0 mg, 4.7 mmol, 1.1 equiv) and THF (10
mL) were added to a 100 mL Schlenk tube. The solution of (E)-6-(1-
((2,6-dimethylphenyl)imino)ethyl)pyridin-2-ol (3c) (1.0 g, 4.3 mmol,
1 equiv) in THF (20 mL) was added dropwise to the Schlenk tube at
room temperature. After that the mixture was stirred for 10 min,
tBu2PCl (851.0 mg, 4.7 mmol, 1.1 equiv) was added, and the resulting
mixture was stirred for 3 h. The solvent was removed under vacuo,
hexane (30 mL) was added, and the dark-yellow mixture was filtered
through a pad of Celite under Ar. The solvent was removed under
3JH,H = 6.8 Hz, 1H, 2-H), 7.38 (t, 3JH,H = 7.9 Hz, 2H, Ar-Hm), 7.17 (t,
3
3JH,H = 7.5 Hz, 1H, Ar-Hp), 6.79 (d, JH,H = 7.3 Hz, 2H, Ar-Ho), 6.72
(d, 3JH,H = 9.1 Hz, 1H, 1- or 3-H), 6.66 (d, J = 6.8 Hz, 1H, 1- or 3-H),
2.18 (s, 3H, NCMe); 13C{1H} NMR (101 MHz, CDCl3) δ 162.3
(N = C), 157.3 (CO), 148.7 (Ar-Cip), 141.3 (C4), 140.1 (C1, C2 or
C3), 129.1 (Ar-Co), 124.8 (Ar-Cp), 119.8 (Ar-Cm and C1, C2, or C3),
107.0 (C1, C2, or C3), 15.2 (NCMe). Anal. Calcd for C13H12N2O:
C, 73.56, H, 5.70, N, 13.20. Found: C, 73.47, H, 5.65, N, 13.23.
tBuPNNiPr Ligand, (E)-N-(1-(6-((Di-tert-butylphosphino)oxy)-
pyridin-2-yl)ethylidene)-2,6-diisopropylaniline (4a).
1
reduced pressure to afford a yellow solid 4c (1.6 g, 97% yield). H
NMR (400 MHz, CDCl3) δ 8.03 (d, 3JH,H = 7.5 Hz, 1H, Py-Hm), 7.71
(t, 3JH,H = 7.8 Hz, 1H, Py-Hp), 7.07−6.91 (m, 4H, Ar-H, Py-Hm′), 2.15
(s, 3H, NCMe), 2.04 (s, 6H, Ar-Me), 1.25 (d, J = 11.6 Hz, 18H,
CMe3); 31P{1H} NMR (162 MHz, CDCl3) δ 155.8 (s); 13C{1H}
2
NMR (101 MHz, CDCl3) δ 167.4 (N = C), 163.9 (d, JP,C = 7.7 Hz,
Py-Co), 154.7 (Py-Co′), 148.9 (Ar-Cip), 139.4 (Py-Cp), 127.9 (Ar-Co),
125.6 (Ar-Cp), 123.0 (Ar-Cm), 115.2 (Py-Cm′), 113.8 (d, 3JP,C = 2.5 Hz,
1
2
Py-Cm), 35.7 (d, JP,C = 26.8 Hz, CMe3), 27.7 (d, JP,C = 15.6 Hz,
CMe3), 18.1 (Ar-Me), 17.0 (NCMe). HRMS (EI), m/z Calcd for
C23H33N2OP (M+) 384.2331, found: 384.2327.
Under an atmosphere of argon, NaH (158.0 mg, 6.3 mmol, 1.2
equiv) and THF (30 mL) were added to a 100 mL Schlenk
tube. The solution of (E)-6-(1-((2,6-diisopropylphenyl)imino)-
ethyl)pyridin-2-ol (3a) (1.6 g, 5.3 mmol, 1 equiv) in THF (20
mL) was added dropwise to the Schlenk tube at room
temperature.After that the mixture was stirred for 10 min,
tBu2PCl (1.0 g, 5.5 mmol, 1.1 equiv) was added, and the
resulting mixture was stirred for 3 h. The solvent was removed
under vacuo, and then hexane (40 mL) was added. The
resulting dark-yellow mixture was filtered through a pad of
tBuPNNH Ligand, (E)-N-(1-(6-((Di-tert-butylphosphino)oxy)pyridin-
2-yl)ethylidene)aniline (4d). Under an atmosphere of argon, NaH
(130.0 mg, 5.4 mmol, 1.1 equiv) and THF (10 mL) were added to a
100 mL Schlenk tube. The solution of (E)-6-(1-(phenylimino)ethyl)-
pyridin-2-ol (3d) (1.0 g, 4.9 mmol, 1 equiv) in THF (20 mL) was
added dropwise to the Schlenk tube at room temperature. After that
the mixture was stirred for 10 min, tBu2PCl (972.0 mg, 5.4 mmol, 1.1
equiv) was added, and the resulting mixture was stirred for 3 h. The
solvent was removed under vacuo, hexane (30 mL) was added, and the
brown mixture was filtered through a pad of Celite under Ar. The
I
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Journal of the American Chemical Society
Article
solvent was removed under reduced pressure to afford an orange oil
4d (1.5 g, 88% yield). NMR spectra: H NMR (400 MHz, CDCl3) δ
mixture was stirred for 3 h. The solvent was removed under vacuo,
hexane (40 mL) was added, and the dark-yellow mixture was filtered
through a pad of Celite under Ar. The solvent was removed under
1
7.89 (dd, 3JH,H = 7.5 Hz, 4JP,H = 0.7 Hz, 1H, Py-Hm), 7.60 (t, 3JH,H = 8
Hz, 1H, Py-Hp), 7.37−7.26 (m, 2H, Ar-Ho), 7.12−7.07 (m, 1H, Ar-
Hp), 6.94 (d, 3JH,H = 8.1 Hz, 1H, Py-Hm′), 6.82−6.79 (m, 2H, Ar-Hm),
2.31 (s, 3H, NCMe), 1.23 (d, 3JP,H = 11.7 Hz, 18H, CMe3); 31P{1H}
NMR (162 MHz, CDCl3) δ 155.4 (s); 13C{1H} NMR (101 MHz,
CDCl3) δ 167.7 (N = C), 163.9 (d, 2JP,C = 7.6 Hz, Py-Co), 155.1 (Py-
Co′), 151.5 (Ar-Cip), 139.3 (Py-Cp), 129.0 ((Ar-Co)), 123.5 (Ar-Cp),
1
reduced pressure to afford a yellow solid 4g (1.8 g, 99% yield). H
NMR (400 MHz, CDCl3) δ 8.00 (d, 3JH,H = 7.5 Hz, 1H, Py-Hm), 7.69
(t, 3JH,H = 7.8 Hz, 1H, Py-Hp), 7.06−6.90 (m, 4H, Py-Hm′, Ar-H), 2.15
(s, 3H, NCMe), 2.02 (s, 6H, Ar-Me), 1.16−1.07 (m, 14H, PCH,
CHMe2); 31P{1H} NMR (162 MHz, CDCl3) δ 146.7 (s); 13C{1H}
2
NMR (101 MHz, CDCl3) δ 167.2 (N = C), 163.3 (d, JP,C = 6.1 Hz,
Py-Co), 154.5 (d, 4JP,C = 1.2 Hz, Py-Co′), 148.9 (Ar-Cip), 139.3 (d, 4JP,C
= 1.1 Hz, Py-Cp), 127.9 (Ar-Co), 125.6 (Ar-Cp), 123.0 (Ar-Cm), 115.1
3
119.4 (Ar-Cm), 115.3 (Py-Cm′), 113.8 (d, JP,C = 2.3 Hz, Py-Cm), 35.7
(d, 1JP,C = 27.0 Hz, CMe3), 27.7 (d, 2JP,C = 15.7 Hz, CMe3), 16.8 (N
CMe). HRMS (EI), m/z Calcd for C21H29N2OP (M+) 356.2018,
found: 356.2023.
3
1
(Py-Cm′), 113.6 (d, JP,C = 1.9 Hz, Py-Cm), 28.0 (d, JP,C = 18.8 Hz,
CHMe2), 18.1 (d, 2JP,C = 20.6 Hz, PCH), 18.0 (Ar-Me), 17.5 (d, 2JP,C
=
iPrPNNiPr Ligand, (E)-N-(1-(6-((Diisopropylphosphino)oxy)pyridin-
2-yl)ethylidene)-2,6-diisopropylaniline (4e). Under an atmosphere of
argon, NaH (71.0 mg, 3.0 mmol, 1.1 equiv) and THF (15 mL) were
added to a 100 mL Schlenk tube. The solution of (E)-6-(1-((2,6-
diisopropylphenyl)imino)ethyl)pyridin-2-ol (3a) (0.8 g, 2.7 mmol, 1
equiv) in THF (10 mL) was added dropwise to the Schlenk tube at
room temperature. After that the mixture was stirred for 10 min,
iPr2PCl (450 mg, 3.0 mmol, 1.1 equiv) was added, and the resulting
mixture was stirred for 3 h. The solvent was removed under vacuo,
hexane (30 mL) was added, and the dark-yellow mixture was filtered
through a pad of Celite under Ar. The solvent was removed under
9.3 Hz, CHMe2), 16.9 (NCMe). HRMS (EI), m/z Calcd for
C21H29N2OP (M+) 356.2018, found: 356.2021.
iPrPNNH Ligand, (E)-N-(1-(6-((Diisopropylphosphino)oxy)pyridin-
2-yl)ethylidene)aniline (4h). Under an atmosphere of argon, NaH
(84.0 mg.0, 3.5 mmol, 1.1 equiv) and THF (10 mL) were added to a
100 mL Schlenk tube. The solution of (E)-6-(1-(phenylimino)ethyl)-
pyridin-2-ol (3d) (676.0 mg, 3.2 mmol, 1 equiv) in THF (20 mL) was
added dropwise to the Schlenk tube at room temperature. After that
the mixture was stirred for 10 min, iPr2PCl (534.0 mg, 3.5 mmol, 1.1
equiv) was added, and the resulting mixture was stirred for 3 h. The
solvent was removed under vacuo, hexane (25 mL) was added, and the
brown mixture was filtered through a pad of Celite under Ar. The
solvent was removed under reduced pressure to afford an orange oil
1
reduced pressure to afford a yellow solid 4e (1.0 g, 92% yield). H
3
4
NMR (400 MHz, CDCl3) δ 7.99 (dd, JH,H = 7.5 Hz, JP,H = 0.7 Hz,
1
3
4h (910.0 mg, 87% yield). H NMR (400 MHz, CDCl3) δ 7.89 (d,
1H, Py-Hm), 7.70 (t, JH,H = 7.8 Hz, 1H, Py-Hp), 7.16−7.06 (m, 3H,
Ar-H), 6.92 (d, JH,H = 8.1 Hz, 1H, Py-Hm′), 2.79−2.69 (m, 2H,
3
3JH,H = 7.5 Hz, 1H, Py-Hm), 7.67 (t, JH,H = 7.8 Hz, 1H, Py-Hp), 7.35
3
3
3
CHMe2), 2.18 (s, 3H, NCMe), 2.10−1.99 (m, 2H, CHMe2), 1.24−
1.10 (m, 24H, CHMe2); 31P{1H} NMR (162 MHz, CDCl3) δ 146.8
(s); 13C{1H} NMR (101 MHz, CDCl3) δ 166.9 (N = C), 163.4 (d,
2JP,C = 6.1 Hz, Py-Co), 154.6 (d, 4JP,C = 1.1 Hz, Py-Co′), 146.7 (Ar-Cip),
(t, JH,H = 7.8 Hz, 2H, Ar-Hm), 7.09 (t, JH,H = 7.4 Hz, 1H, Ar-Hp),
3
3
6.89 (d, JH,H = 8.1 Hz, 1H, Py-Hm′), 6.80 (d, JH,H = 7.6 Hz, 2H, Ar-
Ho), 2.32 (s, 3H, NCMe), 2.13−1.87 (m, 2H, CHMe2), 1.22−1.13
(m, 12H, CHMe2); 31P{1H} NMR (162 MHz, CDCl3) δ 146.5 (s);
13C{1H} NMR (101 MHz, CDCl3) δ 167.3 (N = C), 163.2 (d, 2JP,C
=
3
139.3 (d, JP,C = 1.9 Hz, Py-Cm), 136.0 (Ar-Co), 123.6 (Ar-Cp), 123.1
(Ar-Cm), 115.1 (Py-Cm′), 113.5 (d, JP,C = 0.8 Hz, Py-Cp), 28.3
6.1 Hz, Py-Co), 154.9 (d, 4JP,C = 1.1 Hz, Py-Co′), 151.5 (Ar-Cip), 139.3
(Py-Cp), 129.0 (Ar-Co), 123.5 (Ar-Cp), 119.3 (Ar-Cm), 115.2 (Py-Cm′),
4
1
(CHMe2), 28.1 (d, JP,C = 19.1 Hz, PCH), 23.3 (CHMe2), 23.0
2
3
1
(CHMe2), 18.1 (d, JP,C = 20.6 Hz, PCHMe2), 17.6 (NCMe), 17.5
113.5 (d, JP,C = 1.6 Hz, Py-Cm), 28.0 (d, JP,C = 19.1 Hz, CHMe2),
2
2
2
(d, JP,C = 9.7 Hz, PCHMe2). HRMS (ESI), m/z Calcd for
18.1 (d, JP,C = 20.5 Hz, CHMe2), 17.5 (d, JP,C = 9.3 Hz, CHMe2),
16.7 (NCMe). HRMS (EI), m/z Calcd for C19H25N2OP (M+)
328.1705, found: 328.1708.
C25H37N2OP (M + H)+ 413.2722, found: 413.2716.
iPrPNNEt Ligand, (E)-N-(1-(6-((Diisopropylphosphino)oxy)pyridin-
2-yl)ethylidene)-2,6-diethylaniline, (4f). Under an atmosphere of
argon, NaH (120.0 mg, 5.0 mmol, 1.1 equiv) and THF (20 mL) were
added to a 100 mL Schlenk tube. The solution of (E)-6-(1-((2,6-
diethylphenyl)imino)ethyl)pyridin-2-ol (3b) (1.2 g, 4.6 mmol, 1
equiv) in THF (30 mL) was added dropwise to the Schlenk tube at
room temperature. After that the mixture was stirred for 10 min,
iPr2PCl (763.0 mg, 5.0 mmol, 1.1 equiv) was added, and the resulting
mixture was stirred for 3 h. The solvent was removed under vacuo,
hexane (30 mL) was added, and the dark-yellow mixture was filtered
through a pad of Celite under Ar. The solvent was removed under
(
tBuPNNiPr)FeCl2 (5a). In a N2-filled glovebox, ligand 4a (1.2 g, 2.8
mmol) was added to a solution of FeCl2 (350.0 mg, 2.8 mmol) in
THF (60 mL) at room temperature with vigorous stirring. The
colorless solution turned to a dark-blue suspension immediately. After
being stirred at room temperature for 24 h, the solution was reduced
to about 10 mL via evaporation under vacuo. Then the suspension was
filtered, and the resulting solid was washed with 20 mL ether and dried
under vacuo. The product was obtained as a dark-blue powder (1.4 g,
90% yield). The complex 5a was dissolved in a mixture of
dichloromethane and tetrahydrogenfuran. The solvent evaporated
slowly at room temperature in the N2-filled glovebox. After a few days,
dark-green crystals of complex 5a were obtained for X-ray analysis. 1H
NMR (300 MHz, CD2Cl2) δ 87.5 (81.9, 1H), 59.1 (78.4, 1H), 58.0
(49. 7, 1H), 11.2 (118.0, 18H, CMe3), 8.1 (34.0, 3H, NCMe), 3.1
(22.0, 12H, CHMe2), −5.93 (7.0, 1H), −10.9 (65.2, 2H, CHMe2),
−12.0 (31.0, 1H), −12.5 (34.6, 1H). Anal. Calcd for
C27H41Cl2FeN2OP: C, 57.16, H, 7.28, N, 4.94. Found: C, 57.32, H,
7.42, N, 6.68. μeff (Evan’s method, CDCl3, 25 °C) = 5.3 μB.
1
reduced pressure to afford a yellow solid 4f (1.5 g, 87% yield). H
3
NMR (400 MHz, CDCl3) δ 7.99 (d, JH,H =7.5 Hz, 1H, Py-Hm),
7.69(t, 3JH,H = 7.8 Hz, 1H, Py-Hp), 7.11−6.99 (m, 3H, Ar−H), 6.92 (d,
3JH,H = 8.1 Hz, 1H, Py-Hm′), 2.44−2.27 (m, 4H, CH2Me), 2.16 (s, 3H,
NCMe), 2.10−1.98 (m, 2H, CHMe2), 1.23−1.11 (m, 18H, CH2Me,
CHMe2); 31P{1H} NMR (162 MHz, CDCl3) δ 146.9 (s); 13C{1H}
2
NMR (101 MHz, CDCl3) δ 166.9 (N = C), 163.4 (d, JP,C = 6.1 Hz,
Py-Co), 154.6 (d, 4JP,C = 1.2 Hz, Py-Co′), 148.0 (Ar-Cip), 139.3 (d, 4JP,C
= 1.2 Hz, Py-Cp), 131.4 (Ar-Co), 126.0 (Ar-Cp), 123.3 (Ar-Cm), 115.1
(
tBuPNNEt)FeBr2 (5b). In a N2-filled glovebox, ligand 4b (654.0 mg,
3
1
1.6 mmol, 1 equiv) was added to a solution of FeBr2 (342.0 mg, 1.6
mmol, 1 equiv) in THF (50 mL) at room temperature with vigorous
stirring. The orange solution turned to dark blue immediately. Then
the reaction flask was sealed and heated in an oil bath. After being
stirred at 80 °C for 16 h, the reaction solution was cooled to room
temperature. The solvent was removed under vacuo. Then 10 mL
ether was added, and the resulting suspension was filtered. The solid
was washed with 10 mL ether and dried in vacuo. The product was
(Py-Cm′), 113.5 (d, JP,C = 1.9 Hz, Py-Cm), 28.1 (d, JP,C = 19.1 Hz,
2
PCH), 24.7 (CH2Me), 18.2 (d, JP,C = 20.4 Hz, CHMe2), 17.6 (M =
CMe), 17.4 (d, 2JP,C = 21.2 Hz, CHMe2), 13.8 (CH2Me). HRMS (EI),
m/z Calcd for C21H29N2OP (M+) 356.2018, found: 356.2021.
iPrPNNMe Ligand, (E)-N-(1-(6-((Diisopropylphosphino)oxy)pyridin-
2-yl)ethylidene)-2,6-dimethylaniline (4g). Under an atmosphere of
argon, NaH (144.0 mg, 6.0 mmol, 1.1 equiv) and THF (20 mL) were
added to a 100 mL Schlenk tube. The solution of (E)-6-(1-((2,6-
dimethylphenyl)imino)ethyl)pyridin-2-ol (3c) (1.3 g, 5.5 mmol, 1
equiv) in THF (30 mL) was added dropwise to the Schlenk tube at
room temperature. After that the mixture was stirred for 10 min,
iPr2PCl (920.0 mg, 6.0 mmol, 1.1 equiv) was added, and the resulting
1
obtained as a gray powder (880.0 mg, 88% yield). H NMR (300
MHz, CD2Cl2) δ 85.2 (8.9, 1H), 61.6 (72.3, 2H), 55.3 (45.4, 1H),
13.6 (114.1, 18H, CMe3), 8.7 (27.2, 3H, NCMe), 1.2 (17.1, 4H,
CH2Me), −1.0 (34.0, 6H, CH2Me), −13.6 (24.5, 1H), −15.2 (38.5,
J
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Journal of the American Chemical Society
Article
1H). Anal. Calcd for C25H37Br2FeN2OP: C, 47.80, H, 5.94, N, 4.46.
Found: C, 47.90, H, 6.02, N, 4.50. μeff (Evan’s method, CDCl3, 25 °C)
= 5.3 μB.
to about 5 mL via evaporation under vacuo. Then the suspension was
filtered, and the resulting solid was washed with 15 mL ether and dried
under vacuo. The product was obtained as a dark-blue powder (1.4 g,
tBuPNNMe)FeBr2 (5c). In a N2-filled glovebox, ligand 4c (638.0 mg,
1
(
95% yield). H NMR (300 MHz, CD2Cl2) δ 133.4 (161.7, 1H), 82.9
1.7 mmol, 1 equiv) was added to a solution of FeBr2 (358.0 mg, 1.7
mmol, 1 equiv) in THF (100 mL) at room temperature with vigorous
stirring. The orange solution turned to dark blue immediately. Then
the reaction flask was sealed and heated in an oil bath. After being
stirred at 80 °C for 17 h, the reaction solution was cooled to room
temperature. The solvent was removed under vacuo. Then ether (10
mL) was added, and the resulting suspension was filtered. The solid
was washed with 10 mL ether and dried in vacuo. The product was
(68.6, 2H), 68.3 (74.8, 2H), 53.5 (35.3, 1H), 13.9 (107.1, 6H,
CHMe2), 10.8 (20.5, 3H, NCMe), 10.3 (93.4, 6H, CHMe2), 4.56
(78.6, 6H, Ar-Me), −12.9 (18.5, 1H), −15.4 (30.7, 1H). Anal. Calcd
for C21H29Br2FeN2OP: C, 44.09, H, 5.11, N, 4.90. Found: C, 44.11, H,
5.18, N, 4.9. μeff (Evan’s method, CDCl3, 25 °C) = 5.6 μB.
(iPrPNNH)FeBr2 (5h). In a N2-filled glovebox, ligand 4h (720.0 mg,
2.2 mmol, 1 equiv) was added to the solution of FeBr2 (470.0 mg, 2.2
mmol, 1 equiv) in THF (50 mL) at room temperature with vigorous
stirring. The orange solution turned to dark blue immediately. Then
the reaction flask was sealed and heated in an oil bath. After being
stirred at 80 °C for 18 h, the reaction solution was cooled to room
temperature. The solvent was removed in vacuo. Then ether (15 mL)
was added, and the resulting suspension was filtered. The solid was
washed with 10 mL ether and dried in vacuo. The product was
obtained as a brown powder (1.1 g, 92% yield). The complex 5h was
dissolved in dichloromethane and the solvent evaporated slowly at
room temperature in the N2-filled glovebox. After a few days, dark-red
crystals of 5h were obtained suitable for X-ray analysis. 1H NMR (300
MHz, CD2Cl2) δ 137.2 (172.2, 1H), 76.3 (53.7, 2H), 56.1 (28.7, 1H),
47.8 (65.1, 2H, CHMe2), 14.3 (92.0, 6H, CHMe2), 14.0 (22.6, 3H,
NCMe), 7.9 (74.1, 6H, CHMe2), −1.7 (149.3, 2H), −8.7 (17.3,
1H), −27.2 (26.3, 1H). Anal. Calcd for C19H25Br2FeN2OP: C, 41.95,
H, 4.63, N, 5.15. Found: C, 41.86, H, 4.92, N, 5.05. μeff (Evan’s
method, CDCl3, 25 °C) = 5.1 μB.
1
obtained as a brown powder (860.0 mg, 86% yield). H NMR (300
MHz, CD2Cl2) δ 84.8 (63.8, 1H), 60.8 (60.7, 1H), 55.8 (36.2, 1H),
14.0 (103.5, 18H, CMe3), 11.8 (108.6, 3H, NCMe), 9.3 (22.8, 1H),
4.1 (60.5, 3H, Ar-Me), 2.0 (18.6, 3H, Ar-Me), −13.9 (21.7, 1H), −16.4
(25.3, 1H). Anal. Calcd for C23H33Br2FeN2OP: C, 46.03, H, 5.54, N,
4.67. Found: C, 45.76, H, 5.62, N, 4.73. μeff (Evan’s method, CDCl3,
25 °C) = 5.6 μB.
(
tBuPNNH)FeBr2 (5d). In a N2-filled glovebox, ligand 4d (808.0 mg,
2.3 mmol, 1 equiv) was added to a solution of FeBr2 (488.0 mg, 2.3
mmol, 1 equiv) in THF (100 mL) at room temperature with vigorous
stirring. The orange solution turned to dark blue immediately. Then
the reaction flask was sealed and heated in an oil bath. After being
stirred at 80 °C for 15 h, the reaction solution was cooled to room
temperature. The solvent was removed under vacuo. Then ether (10
mL) was added, and the resulting suspension was filtered. The solid
was washed with 10 mL ether and dried under vacuo. The product was
1
obtained as a brown powder (1.1 g, 85% yield). H NMR (300 MHz,
CD2Cl2) δ 81.8 (56.9, 2H), 57.6 (29.9, 1H), 42.0 (55.9, 2H), 15.8
(106.8, 18H, CMe3), 13.0 (22.7, 3H, NCMe), −6.8 (128.8, 1H),
−10. Nine (20.7, 1H), −22.7 (28.6, 1H). Anal. Calcd for
C21H29Br2FeN2OP: C, 44.09, H, 5.11, N, 4.90. Found: C, 43.98, H,
5.39, N, 5.03. μeff (Evan’s method, CDCl3, 25 °C) = 5.6 μB.
Silafluofen (19). In N2-filled glovebox, a vial (10 mL) was charged
with 4-allyl-1-fluoro-2-phenoxybenzene (17) (114.0 mg, 0.5 mmol),
dimethyl(4-propylphenyl)silane (18) (106.9 mg, 0.6 mmol), complex
5h (50.0 μmol, 10 mol %), and dry toluene (0.9 mL). The reaction
mixture was cooled to −34 °C and NaBHEt3 (100 μL, 20 mol %, 1 M
in toluene) was then added to the mixture. The reaction mixture was
stirred at room temperature for 5 d. The vial was then removed from
the glovebox. The mixture was filtered through a pad of silica gel, and
the solvent was removed under vacuo. The desired product was
obtained as colorless oil by flash chromatography on silica gel, eluting
with petrol ether/ethyl acetate (v/v = 100:1) (113.0 mg, 55% yield).
1H NMR (300 MHz, CDCl3) δ 7.39−7.29 (m, 4H), 7.11−7.03 (m,
(iPrPNNiPr)FeBr2 (5e). In a N2-filled glovebox, ligand 4e (850.0 mg,
2.1 mmol, 1 equiv) was added to a solution of FeBr2 (440.0 mg, 2.1
mmol, 1 equiv) in THF (100 mL) at room temperature with vigorous
stirring. The colorless solution turned to dark-blue suspension
immediately. After being stirred at room temperature for 24 h, the
volume of solution was reduced to about 15 mL via evaporation under
vacuo. Then the suspension was filtered, and the resulting solid was
washed with 15 mL ether and dried under vacuo. The product was
obtained as gray powder (1.2 g, 93% yield). The 1H NMR spectrum of
complex 5e could not be obtained due to its poor solubility in
common organic solvents. Anal. Calcd for C25H37Br2FeN2OP: C,
47.80, H, 5.94, N, 4.46. Found: C, 47.95, H, 6.16, N, 4.28.
2H), 7.00 (d, 3JH,H = 8.1 Hz, 2H), 6.90−6.83(m, 4H), 4.03 (q, 3JH,H
=
3
7.0 Hz, 2H, OCH2Me), 2.54 (t, JH,H = 7.5 Hz, 2H, CH2), 1.63−1.52
3
(m, 2H, CH2), 1.42 (t, JH,H = 7.0 Hz, 3H, OCH2Me), 0.74−0.68 (m,
2H, CH2), 0.22 (s, 6H, SiMe2); 19F NMR (282 MHz, CDCl3) δ
−135.7 (m); 13C NMR (101 MHz, CDCl3) δ 159.8, 157.7, 152.7 (d,
1JF,C = 246.1 Hz), 143.0 (d, 2JF,C = 11.6 Hz), 139.5 (d, 3JF,C = 3.8 Hz),
135.1, 129.9, 129.8, 124.9 (d, 3JF,C = 6.6 Hz), 123.0, 122.1, 117.1, 116.7
(d, 2JF,C = 18.1 Hz), 114.2, 63.3 (OCH2Me), 39.0 (CH2), 26.0 (CH2),
15.7, 15.0, −2.8 (SiMe2). These spectroscopic data agree with the
reported data.45
(iPrPNNEt)FeBr2 (5f). In a N2-filled glovebox, ligand 4f (1.0 g, 2.6
mmol, 1 equiv) was added to the solution of FeBr2 (560.0 mg, 2.6
mmol, 1 equiv) in THF (100 mL) at room temperature with vigorous
stirring. The orange solution turned to dark blue immediately. Then
the reaction flask was sealed and heated in an oil bath. After being
stirred at 80 °C for 25 h, the reaction solution was cooled to room
temperature. The solvent was removed in vacuo. Then ether (20 mL)
was added, and the resulting suspension was filtered. The solid was
washed with 10 mL ether and dried under vacuo. The product was
(
tBuPNNiPr)Fe(CO)2 (21). In a N2-filled glovebox, a thick-walled
reaction vessel (100 mL) was charged with complex 5a (307 mg, 0.54
mmol) and dry toluene (50 mL). The reaction mixture was cooled to
−34 °C, and NaBHEt3 (1 mL, 1.0 mmol, 1 M in toluene) was added
to the mixture. The mixture was stirred for about 5 min. Then the
vessel was transferred from the glovebox, and the reaction mixture was
frozen to liquid nitrogen temperature. The vessel was evacuated, and
CO was added. The resulting mixture was warmed to room
temperature and stirred for 7 h. The solvent was removed under
vacuo. Pentane (50 mL) was then added, and the solution was filtered
through Celite. The solvent was removed under vacuo, and the
resulting dark-green solid was recrystallized in pentane to yield 153.0
mg (51%) of 21. The complex 21 was dissolved in ether. The solvent
evaporated slowly at room temperature in the N2-filled glovebox. After
a few days, dark-green crystals of complex 21 were obtained suitable
for X-ray analysis.
1
obtained as a brown powder (1.3 g, 83% yield). H NMR (300 MHz,
CD2Cl2) δ 130.6 (154.6, 2H), 83.2 (63.4, 1H), 69.1 (66.7, 2H,
CHMe2), 53.1 (38.2, 1H), 16.7 (136.0, 3H, NCMe), 10.2 (28.4,
12H, CHMe2), 7.1 (166.7, 1H), 3.6 (77.7, 4H, CH2Me), −0.5 (26.8,
6H, CH2Me), −12.4 (17.2, 1H), −13.0 (26.9, 1H). Anal. Calcd for
C23H33Br2FeN2OP: C, 46.03, H, 5.54, N, 4.67. Found: C, 45.99, H,
5.69, N, 4.68. μeff (Evan’s method, CDCl3, 25 °C) = 5.6 μB.
(iPrPNNMe)FeBr2 (5g). In a N2-filled glovebox, ligand 4g (886.0 mg,
2.5 mmol, 1 equiv) was added to the solution of FeBr2 (536.0 mg, 2.5
mmol, 1 equiv) in THF (100 mL) at room temperature with vigorous
stirring. The colorless solution turned to dark blue immediately. Then
the reaction flask was sealed and heated in an oil bath. After being
stirred at 80 °C for 1 h, dark-blue solid precipitated from the solution.
After being stirred for another 18h at 80 °C, the reaction mixture was
cooled to room temperature, and the volume of solution was reduced
K
dx.doi.org/10.1021/ja404963f | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
(d) Hartwig, J. F. Organotransition Metal Chemistry; University of
Science Books: Sausalito, CA, 2010; pp 677−691.
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Voronkov, M. Perspectives of Hydrosilylation; Institute for Organic
Synthesis: Riga, Lativia, 1992; (b) Marciniec, B. Comprehensive
Handbook on Hydrosilylation; Pergamon: Oxford, U.K., 1992;
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G. A., Robert, W., Eds.; Academic Press: New York, 1979; Vol. 17, pp
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3
1H NMR (400 MHz, C6D6) δ 7.25 (s, 3H, Ar-H), 6.87 (d, JH,H
=
8.3 Hz, 1H, Py-Hm), 6.72 (t, 3JH,H = 7.7 Hz, 1H, Py-Hp), 6.22 (d, 3JH,H
= 7.0 Hz, 1H, Py-Hm′), 3.05−2.95 (m, 2H, CHMe2), 1.82 (s, 3H, N
CMe), 1.52 (d, 3JH,H = 6.8 Hz, 6H, CH(Me)2-1 or -2), 1.16 (d, 3JP,H
=
14.0 Hz, 18H, CMe3), 1.09 (d, 3JH,H = 6.8 Hz, 6H, CH(Me)2-1 or -2);
31P NMR (162 MHz, C6D6) δ 261.9 (s); 13C NMR (101 MHz,
CDCl3) δ 217.4 (CO), 217.3 (CO), 166.1 (N = C), 151.4 (Py-Co′),
148.2 (Ar-Cip), 144.51 (d, 2JP,C = 6.0 Hz, Py-Co), 140.8 (Py-Cp), 126.5
W. Angew. Chem., Int. Ed. 1991, 30, 438. (g) Marko,
Buisine, O.; Mignani, G.; Branlard, P.; Tinant, B.; Declercq, J.-P.
Science 2002, 298, 204. (h) Buisine, O.; Berthon-Gelloz, G.; Briere, J.-
F.; Sterin, S.; Mignani, G.; Branlard, P.; Tinant, B.; Declercq, J.-P.;
Marko, I. E. Chem. Commun. 2005, 3856.
́ ́
I. E.; Sterin, S.;
̀
́
(Ar-Co), 125.9 (Ar-Cp), 123.8 (Ar-Cm), 117.2 (Py-Cm′), 92.4 (d, 3JP,C
=
́
1
2
(4) Catalysis Without Precious Metals; Bullock, R. M., Ed.; Wiley-
4.0 Hz, Py-Cm), 42.3 (d, JP,C = 10.1 Hz, CMe3), 27.7 (d, JP,C = 5.9
Hz, CMe3), 25.3 (CHMe2), 24.7 (CH(Me)2-1 and -2), 16.1 (N
CMe); Anal. Calcd for C29H41FeN2O3P: C, 63.05, H, 7.48, N, 5.07.
Found: C, 62.73, H, 7.41, N, 5.15; IR (pentane): 1956.0, 1902.3 cm−1;
IR (KBr): 1930.9, 1882.5 cm−1.
Computational Details. All computations were performed using
the DFT functional method B3LYP, as implemented in the Gaussian
09 program.46 No symmetry restrictions were imposed (C1). Fe, P, O,
N, C, and H were represented by an all-electron 6-311G(d,p) basis set.
The nature of the extrema (minima) was established with analytical
frequencies calculations. The zero point vibration energy (ZPE) and
entropic contributions were estimated within the harmonic potential
approximation. The Gibbs free energy, ΔG, was calculated for T =
298.15 K and 1 atm. Geometrical parameters were reported within an
accuracy of 10−3 Å and 10−1 degrees.
VCH: Weinheim, 2010, pp 83−142.
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134, 804.
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126, 13794. (b) Tondreau, A. M.; Atienza, C. C. H.; Weller, K. J.; Nye,
S. A.; Lewis, K. M.; Delis, J. G. P.; Chirik, P. J. Science 2012, 335, 567.
(c) Hojilla Atienza, C. C.; Tondreau, A. M.; Weller, K. J.; Lewis, K. M.;
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(8) The Nakazawa and Chirik groups independently reported related
terpyridine iron complexes for alkene hydrosilylation. See: (a) Kamata,
K.; Suzuki, A.; Nakai, Y.; Nakazawa, H. Organometallics 2012, 31,
3825. (b) Tondreau, A. M.; Atienza, C. C. H.; Darmon, J. M.;
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, characterization data, and crystallo-
graphic data (CIF) for complexes 5a, 5h, and 21, computa-
tional details, and a complete list of authors for the Gaussian 09
program noted in reference 46. This material is available free of
charge via the Internet at http://pubs.acs.org.
(9) Blanchard, S.; Derat, E.; Desage-El Murr, M.; Fensterbank, L.;
Malacria, M.; Mouries
(10) (a) Min, G. K.; Hernan
2013, 46, 457. (b) Sabourault, N.; Mignani, G.; Wagner, A.;
Mioskowski, C. Org. Lett. 2002, 4, 2117.
(11) Schubert, H. H. (Hoechst Aktiengesellschaft). U.S. Patent
̀
-Mansuy, V. Eur. J. Inorg. Chem. 2012, 2012, 376.
dez, D.; Skrydstrup, T. Acc. Chem. Res.
AUTHOR INFORMATION
■
́
Corresponding Authors
huangzh@sioc.ac.cn
mwalter@tu-bs.de
4,920,212, 1990.
Notes
(12) (a) Warner, J. H.; Hoshino, A.; Yamamoto, K.; Tilley, R. D.
Angew. Chem., Int. Ed. 2005, 44, 4550. (b) Rosso-Vasic, M.; Spruijt, E.;
Popovic, Z.; Overgaag, K.; van Lagen, B.; Grandidier, B.;
Vanmaekelbergh, D.; Dominguez-Gutierrez, D.; De Cola, L.;
Zuilhof, H. J. Mater. Chem. 2009, 19, 5926.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the financial support from the “1000
Youth Talents Plan”, the National Natural Sciences Foundation
of China (No. 21121062), and the Chinese Academy of
Sciences. Prof. Maurice Brookhart is greatly acknowledged for
helpful comments. We thank the reviewers for their
constructive comments and suggestions.
́
(13) Bezier, D.; Venkanna, G. T.; Castro, L. C. M.; Zheng, J.; Roisnel,
T.; Sortais, J.-B.; Darcel, C. Adv. Synth. Cat. 2012, 354, 1879.
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Chem., Int. Ed. 2009, 48, 9507. (b) Sunada, Y.; Kawakami, H.; Imaoka,
T.; Motoyama, Y.; Nagashima, H. Angew. Chem., Int. Ed. 2009, 48,
9511. (c) Tsutsumi, H.; Sunada, Y.; Nagashima, H. Chem. Commun.
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2011, 47, 6581. (d) Bezier, D.; Venkanna, G. T.; Sortais, J.-B.; Darcel,
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