Preparation, Characterization, and Catalytic Reactions of NCN Pincer Iron Complexes Containing Stannyl, Silyl, Methyl, and Phenyl Ligands

Article abstract of DOI:10.1021/acs.organomet.5b00082

Preparation and reactivity of chiral and achiral NCN pincer Fe complexes containing bis(oxazolinyl)phenyl (abbreviated as phebox) ligands with SnMe₃, SiMe₃, Me, and Ph ligands were investigated. Irradiation of (phebox)SnMe₃ (2) with 1 equiv of Fe(CO)₅ led to oxidative addition to give NCN pincer stannyl complex (phebox)Fe(CO)₂(SnMe₃) (3). Similarly, oxidative addition of (phebox)SiMe₃ (4) with Fe(CO)₅ resulted in the formation of silyl complex (phebox)Fe(CO)₂SiMe₃ (5). Me and Ph complexes (phebox)Fe(CO)₂R (7, R = Me; 8, R = Ph) were synthesized by transmetalation of the bromide complex (phebox)Fe(CO)₂Br (1) with ZnMe₂ and ZnPh₂, respectively. These phebox Fe complexes served as catalysts for hydrosilylation of a ketone and C-H silylation of N-methylindole. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.organomet.5b00082

Article
pubs.acs.org/Organometallics
Preparation, Characterization, and Catalytic Reactions of NCN
Pincer Iron Complexes Containing Stannyl, Silyl, Methyl,
and Phenyl Ligands
Jun-ichi Ito,* Satomi Hosokawa, Hairuzana Binti Khalid, and Hisao Nishiyama*
Department Applied Chemistry, Graduate School of Engineering, Nagoya University, Chikusa, Nagoya 464-8603, Japan
S
* Supporting Information
ABSTRACT: Preparation and reactivity of chiral and achiral NCN pincer Fe
complexes containing bis(oxazolinyl)phenyl (abbreviated as phebox) ligands
with SnMe3, SiMe3, Me, and Ph ligands were investigated. Irradiation of
(phebox)SnMe3 (2) with 1 equiv of Fe(CO)5 led to oxidative addition to
give NCN pincer stannyl complex (phebox)Fe(CO)2(SnMe3) (3). Similarly,
oxidative addition of (phebox)SiMe3 (4) with Fe(CO)5 resulted in the forma-
tion of silyl complex (phebox)Fe(CO)2SiMe3 (5). Me and Ph complexes
(phebox)Fe(CO)2R (7, R = Me; 8, R = Ph) were synthesized by transmetalation of the bromide complex (phebox)Fe(CO)2Br
(1) with ZnMe2 and ZnPh2, respectively. These phebox Fe complexes served as catalysts for hydrosilylation of a ketone and
C−H silylation of N-methylindole.
developed.15 Concurrently, Si−H and Sn−H oxidative addition
INTRODUCTION
■
and salt metathesis between an anionic complex and a silyl
Transition metal complexes containing meridional-type ligands
halide or a silyllithium and a metal halide were reported as
have been applied to catalytic and stoichiometric reactions of
organic and inorganic molecules.1 Since nonprecious metal
common synthetic strategies for the preparation of silyl and
stannyl complexes.10 In contrast, oxidative addition of Si−C
catalysts are considered to be a significant research area toward
environmentally friendly processes,2 Fe complexes containing
pyridine-based meridional ligands and related nitrogen ligands
have been extensively developed as efficient catalysts in
polymerization, oligomerization, cycloaddition, hydrosilylation,
and hydrogenation, as well as asymmetric transformations.3−7
In contrast to neutral meridional pyridine ligands, NCN and
PCP pincer ligands based on phenyl and alkyl scaffolds are
expected to exhibit different properties arising from the anionic
ligand framework with a Fe−C σ-bond. In addition, the pres-
and Sn−C is rare in Fe complexes, and examples are limited to
reactions of cyclosilabutane or Ph3Sn(CH2)2PPh2 with Fe(0)
carbonyl complexes.16
Recently, we described chiral NCN-Fe complexes (phebox)-
Fe(CO)2Br (1) and their application to asymmetric hydro-
silylation of ketones.9a In the catalysis, the addition of Na(acac)
(acac = acetylacetonate) enhanced reactivity and enantiose-
lectivity, suggesting that the exchange of the Br ligand afforded
a catalytically active species. We hypothesized that introduction
of group 14 elements into the phebox-Fe framework might
modify reactivity. In this context, oxidative addition of Si−C
and Sn−C bonds of the phebox ligand precursor (phebox)-
MMe3 (M = Si, Sn) could be a versatile method.
Here, we report the preparation of NCN pincer Fe stannyl
and silyl complexes, which was achieved by oxidative addition
of SiMe3- and SnMe3-substituted ligand precursors. Structurally
related alkyl and phenyl complexes are also described that were
synthesized by transmetalation of the bromide complexes with
organozinc reagents. Catalytic activities of these Fe complexes
were evaluated in the hydrosilylation of ketones and C−H
silylation of indole derivatives.
ence of two metallacycles contributes to the robust structure.
Since the early studies of [C6H3(CH2NMe2)2]FeCl2
8a,b
and
{[C6H3(CH2PMe2)2]FeCl2}n,8c NCN and PCP pincer Fe
complexes have been developed by several research groups
for stoichiometric and catalytic reactions.8,9 In particular, C−H
bond activation of PCP ligand precursors with Fe(PMe3)4
and Fe(Me)2(PMe3)4 was used to form unique Fe hydride
complexes, which served as efficient catalysts in hydrosilylation
of carbonyl compounds and dehydrogenation of ammonia
borane.8d−i However, examples of chiral and achiral NCN-Fe com-
plexes remain to be explored compared to second- and third-row
metals, such as Rh and Pd, likely due to their instability.
Reactivity of metal complexes is also affected by other ligands
besides the spectator ancillary ligand. In that sense, group 14
elements, such as silyl, stannyl, alkyl, and aryl ligands, are
considered to be important species for transition metal com-
plexes.10−12 The characteristic aspects of silyl and stannyl
complexes would be their strong trans effect and σ-donating
characters.13 Since CpFe(CO)2SiMe3 was reported,14 various
silyl transfer reactions mediated by Fe silyl species have been
RESULTS AND DISCUSSION
Following the preparation of (phebox-dm)Fe(CO)2Br (1),9a
we examined oxidative addition of the stannyl precursor
■
Received: January 29, 2015
Published: April 1, 2015
© 2015 American Chemical Society
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(phebox-dm)SnMe3 (2a)17 to Fe(0) carbonyl complexes. When
thermal reaction of 2a with Fe2(CO)9 was carried out in THF
at 50 °C for 12 h, the desired stannyl Fe complex (phebox-dm)-
Fe(CO)2SnMe3 (3a) was obtained in 21% yield after
purification by silica gel column chromatography (eq 1).
Figure 1. ORTEP diagram of 3a at the 50% probability level.
Hydrogen atoms have been omitted for clarity. Selected bond lengths
(Å): Fe1−C1 = 1.9226(19), Fe1−C10 = 1.806(2), Fe1−C11 =
1.790(2), Fe1−N1 = 2.0088(12), Fe1−Sn1 = 2.5930(3), C10−O2 =
1.137(3), C11−O3 = 1.153(3), Sn1−C12 = 2.1657(19), Sn1−C13 =
2.172(3). Selected angles (deg): N1−Fe1−N1 = 156.42(7), C1−Fe1−
C11 164.54(9), Sn1−Fe1−C10 = 177.02(6), C1−Fe1−Sn1 =
78.55(6).
However, the formation of unidentified byproducts decreased
the yield of 3a. In contrast, the reaction of 2a with Fe(CO)5 did
not proceed under heating at 50 °C. Oxidative addition requires
a coordinatively unsaturated precursor Fe(CO)4, which can be
generated by the fragmentation of Fe2(CO)9.18 Thus, we
examined the irradiation reaction of Fe(CO)5 to generate a
Fe(CO)4 species. As a result, irradiation of a THF solution
of 2a and Fe(CO)5 by a Xe lamp (250−385 nm) at room
temperature for 12 h resulted in the formation of 3a in 57%
yield (eq 1). The use of a similar procedure provided the chiral
complex (phebox-ip)Fe(CO)2SnMe3 (3b) in 62% yield.
Notably, 3a and 3b are stable in air and light in the solid
state. When a C6D6 solution of 3b was irradiated by a Xe lamp
for 15 h, decomposition of 3b was not observed.
[1.790(2) Å] due to the trans influence of the SnMe3 ligand.
Another structural feature is that the equatorial CO ligand
deviates significantly from linearity with the C1−Fe1−C11
bond angle of 164.54(9)°, compared to that of 1a (175.82°).
Next, we studied oxidative addition of a C−Si bond of the
organosilane precursor (phebox-dm)SiMe3 (4a)20 to synthesize
a silyl complex (eq 2). The irradiation reaction of 4a with
1
The H NMR spectrum of 3a measured in C6D6 showed a
signal from the SnMe3 group at δ −0.08 ppm with satellite
peaks (JSnH = 38 Hz). The methyl groups on the oxazolines
were observed as two singlet peaks at δ 0.96 and 0.97 in the
integral ratio of 6H:6H. This spectral feature suggests that 3a
has Cs symmetric geometry. In the 13C NMR spectrum, the
signal of the SnMe3 group appeared at δ −7.1 ppm with
satellite peaks (JSnC = 169 Hz). The IR spectrum showed CO
stretching vibrations at 1988 and 1907 cm−1. These peaks of 3a
were shifted to lower frequency compared to those of 1a (2020
and 1969 cm−1), suggesting enhanced back-donation. In the 1H
NMR spectrum of 3b, signals of the iPr groups on the
oxazolines were observed as four independent peaks due to an
Fe(CO)5 for 24 h led to the formation of the silyl complex
(phebox-dm)Fe(CO)2(SiMe3) (5a). The purification of the
crude product by silica gel column chromatography afforded
5a in 60% yield. Similarly, chiral complexes 5b−d, containing
isopropyl, phenyl, and benzyl groups on the oxazolines, were
also obtained in 70−76% yields in a similar manner. Oxidative
addition of the C−Si bond took longer compared to that of the
C−Sn bond. As mentioned earlier, a C−Si bond cleavage by
metal complexes was promoted by coordination of a phosphine
ligand or the ring opening of distorted cyclosilabutane.16,21 In
our case, coordination of the nitrogen atom on the oxazoline
ring could act as a directing group for the C−Si bond cleavage.
1
unsymmetrical structure. The H and 13C NMR spectra of 3b
exhibited signals of the SnMe3 at δ −0.02 and −7.1 ppm,
respectively.
Crystals of 3a were obtained by cooling a CH3CN solution at
−20 °C. The ORTEP diagram obtained by X-ray analysis
is shown in Figure 1. The structure around the Fe center
is described as a pseudo-octahedron with a meridionally co-
ordinated phebox ligand. The Fe1−C1 bond length of
1.9226(19) Å is comparable to those of neutral NCN Fe
complexes, 1a [1.930(2) Å],9a (phebox)Fe(CO)(CNtBu)Br
[1.923(7) Å],9b and [C6H3(CH2NMe2)2]FeCl2 [1.937(2) Å],8a
and shorter than those of the PCP-Fe complexes [C6H3(OPiPr2)2]-
Fe(H)L2 [L = PMe3, CO; Fe−C = 1.995(2)−2.001(2) Å].8h,i
The SnMe3 ligand is attached at a position vertical to the NCN
ligand plane. This feature is a consequence of oxidative addition
of the C−Sn bond of 2a to the Fe(0) center. The Sn−Fe bond
length of 2.5930(3) Å is slightly longer than in CpFe(CO)-
(SnMe3)2(H) [2.558(1), 2.569(1) Å].19 The Fe1−C10 bond
length [1.806(2) Å] is longer than the Fe1−C11 bond length
1
Complexes 5a−d were identified on the basis of H and 13C
1
NMR, IR spectra, and elemental analysis. In the H NMR
spectrum of 5a, two singlet signals of methyl groups on the
oxazolines were observed at δ 0.97 and 1.02 ppm due to the Cs
symmetry. The methyl signal of the SiMe3 group appeared at δ
0.04 ppm and 3.9 in the 1H and 13C NMR spectra, respectively.
In the IR spectrum, two CO absorptions were observed at 1982
and 1902 cm−1, which were similar to those of 3a.
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Crystals of 5a and 5b were obtained by cooling a pentane
solution to −20 °C. The ORTEP diagrams are shown in
Figures 2 and 3. The complexes 5a and 5b were described as
transmetalation of 1a and 1b with several organometallic
reagents. As a result, reaction of 1a with 2 equiv of ZnMe2 at
room temperature for 15 min resulted in the formation of
methyl complex 7a, which was isolated in 84% yield by column
chromatography (eq 3). Similarly, reaction of 1b with ZnMe2
Figure 2. ORTEP diagram of 5a at the 50% probability level.
Hydrogen atoms have been omitted for clarity. Selected bond lengths
(Å): Fe1−C1 = 1.920(2), Fe1−N1 = 2.0121(13), Fe1−Si1 =
2.4006(10), Fe1−C10 = 1.823(2), Fe1−C11 = 1.789(2), C10−O2 =
1.134(3), C11−O3 = 1.156(3). Selected angles (deg): N1−Fe1−N1 =
156.87(7), C1−Fe1−C11 = 167.79(8), Si1−Fe1−C10 = 175.12(6).
afforded the methyl complex 7b in 55% yield. On the other
hand, the use of MeLi and MeMgBr resulted in decomposi-
tion with a free phebox ligand. Unfortunately, irradiation of
(phebox-dm)Me with Fe(CO)5 in THF-d8 did not afford 7a.
In the 1H NMR spectrum of 7a, two singlets assigned to the
methyl groups on the oxazolines were observed at δ 0.91 and
0.98 ppm. The characteristic singlet signal of the methyl ligand
appeared at δ −0.02 ppm. This chemical shift was in the range
of other iron methyl complexes.24 The 13C NMR spectrum of
7a showed a signal for the methyl ligand at δ 13.6 ppm. The IR
spectrum of 7a revealed two CO absorptions at 1982 and
1927 cm−1, which were shifted to higher wave numbers com-
pared to 3a and 5a.
The phenyl complex 8 was also synthesized by treatment of
1a with an excess amount of ZnPh2 in 52% yield (eq 3). The
13C NMR spectrum of 8 showed the signal assigned to the ipso-
carbon of the phenyl ligand at δ 165.0 ppm.
Crystals of 7a and 8 were obtained by cooling a pentane
solution to −20 °C. The geometry around the Fe atom is
pseudo-octahedral, and the methyl and phenyl ligands are
coordinated to the vertical position of the phebox plane
(Figures 4 and 5). In 7a, the Fe−Me bond length (Fe1−C17 =
2.071(3) Å) is in the range of other Fe methyl complexes (Fe−
Me = 2.00−2.18 Å).25 The Fe−Ph bond in 8 of 2.0619(19) Å
was also found to be similar to those of other iron phenyl
complexes (Fe−Ph = 1.99−2.09 Å).26
Figure 3. ORTEP diagram of 5b at the 50% probability level.
Hydrogen atoms have been omitted for clarity. Selected bond lengths
(Å): F1−C1 = 1.9145(13), Fe1−C19 = 1.8176(14), Fe1−C20 =
1.7847(15), Fe1−N1 = 1.9915(11), Fe1−N2 = 2.0115(11), Fe1−Si1 =
2.4237(5), C19−O3 = 1.1414(17), C20−O4 = 1.1516(18), N1−Fe1−
N2 = 156.90(5), C1−Fe1−C20 = 165.78(6), Si1−Fe1−C19 =
176.92(4).
In comparing structures of 3a, 5a, 7a, and 8, the Fe−CO
bond trans to the stannyl, silyl, methyl, and phenyl ligands was
significantly affected by these ligands. The longest distance of
the Fe−CO bond was observed in the silyl complex 5a. This
trend likely follows the strength of the trans influence of these
ligands.13
In order to evaluate the catalytic activity of this series of
phebox Fe complexes with stannyl, silyl, and alkyl ligands,
asymmetric hydrosilylation of ketone 9 was examined.7a−e,27
The catalytic reaction of 9 with 1.5 equiv of 11a was performed
in the presence of 2 mol % of the Fe complexes at 50 °C for
24 h (Table 1). The silyl complex 5b was found to show
the highest reactivity among 3b, 5b, and 7b to give the
corresponding (R)-alcohol 10a in 98% yield with 32% ee
(entries 1−3). Low reactivity of the stannyl complex 3b was
probably due to its high stability.28 In the case of 7b, de-
composition of the phebox-Fe framework might lead to the
decrease in yield and enantioselectivity. We previously reported
pseudo-octahedrons with meridional coordination of the
phebox ligand and cis arrangement of the CO ligands. The
SiMe3 ligand was perpendicularly coordinated to the phebox
plane as described in the SnMe3 ligand of 3a. In 5a, the
Fe1−C1 bond length [1.920(2) Å] is comparable with those of
other phebox Fe complexes.9 The Fe1−Si1 bond length of
2.4006(10) Å is slightly longer than those of (C5H5)Fe
and (C5Me5)Fe silyl complexes (2.26−2.34 Å)15g,22a and
(CO)2(dppe)Fe(H)SiMe3 [2.360(2) Å].22b The Fe1−C10
bond length of 1.823(2) Å is slightly longer than the Fe1−
C11 bond length of 1.789(2) Å, suggesting a trans influence of
the silyl group as observed in the stannyl complex 3a.
We were interested in comparing the methyl and phenyl
complexes to the stannyl and silyl complexes. Thus far,
CpFe(CO)2R has been prepared by oxidative addition of
MeI to [CpFe(CO)2]− or transmetalation of CpFe(CO)2X
with organometallic reagents.23 In this regard, we examined
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a
Table 1. Asymmetric Hydrosilylation of Ketones
b
entry
cat.
ketone
HSiR3
solvent
yield (%)
ee (%)
1
2
3
4
5
6
3b
5b
7b
5b
5b
5b
5c
5d
5b
5b
5b
3b
5b
5b
5b
5b
5b
9a
9a
9a
9a
9a
9a
9a
9a
9a
9a
9a
9a
9a
9b
9c
9d
9e
11a
11a
11a
11a
11a
11a
11a
11a
11b
11c
11d
11a
11a
11a
11a
11a
11a
hexane
hexane
hexane
toluene
THF
8
98
56
99
17
0
4
32
12
34
8
Figure 4. ORTEP diagram of 7a at the 50% probability level.
Hydrogen atoms have been omitted for clarity. Selected bond lengths
(Å): Fe1−C1 = 1.924(2), Fe1−N1 = 2.007(2), Fe1−N2 = 2.004(2),
Fe1−C17 = 2.071(3), Fe1−C18 = 1.802(3), Fe1−C19 = 1.789(3),
C18−O3 = 1.143(3), C19−O4 = 1.149(3). Selected angles (deg):
N1−Fe1−N2 = 156.84(8), C1−Fe1−C19 = 169.158(11).
MeCN
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
c
7
90
99
85
66
2
21
3
c
8
9
15
6
10
11
0
d
12
29
31
90
81
82
88
2
d
13
8
14
15
16
17
18
16
9
10
a
Reaction condition: 9 (0.5 mmol), 11 (0.75 mmol), Fe catalyst
b
(0.01 mmol), 50 °C, 24 h. (EtO)2MeSiH (11a), PhSiH3 (11b),
Ph2SiH2 (11c), (Me3SiO)2MeSiH (11d). 48 h. Irradiation by Xe
c
d
lamp at room temperature for 24 h.
Scheme 1
Figure 5. ORTEP diagram of 8 at the 50% probability level. Hydrogen
atoms have been omitted for clarity. Selected bond lengths (Å): Fe1−
C1 = 1.9236(18), Fe1−N1 = 2.0046(16), Fe1−N2 = 2.0059(16),
Fe1−C17 = 2.0619(19), Fe1−C23 = 1.8031(19), Fe1−C24 =
1.802(2), C23−O3 = 1.138(2), C24−O4 = 1.149(2). Selected angles
(deg): N1−Fe2−N2 = 156.85(6), C1−Fe1−C24 = 175.32(8).
that the bromide complex 1b required a base, Na(acac), to
facilitate the hydrosilylation of 9.9a In contrast, the silyl complex
5b underwent the catalytic reaction in the absence of bases,
indicating that 5b served as an efficient precursor. The use of
nonpolar solvents promoted the catalytic reaction, while THF
and MeCN resulted in a decrease in the yield (entries 4−6).
Silyl complexes 5c and 5d with phenyl and benzyl groups
required longer times to complete the reaction probably due to
the presence of bulky substituents (entries 7 and 8). Regarding
the hydrosilanes, 11a was found to be the best reducing agent
(entries 4, 9−11). Catalytic reaction under irradiation of a Xe
lamp at room temperature resulted in low yields (entries 12, 13).
Other aromatic ketones 9b−e gave high yields, but modest
enantioselectivity (entries 14−17). Notably, the absolute con-
figuration of 10a was the same as that obtained by 1b.9a
A proposed mechanism is shown in Scheme 1, following the
mechanism of Rh-catalyzed hydrosilylation of ketones proposed
by Ojima and co-workers.29 We propose the silyl intermediate
A as the active species. Insertion of a CO bond into the
Fe−Si bond of B forms a silyloxyalkyl intermediate C.30 Reac-
tion of C with hydrosilane gives the product and regenerates A.
In this regard, oxidative addition of a Si−H bond and reduc-
tive elimination of the product might proceed following the
proposed mechanism of the formation of CpFe(CO)2SiR3 from
CpFe(CO)2Me.15e,f,n,31 We previously reported that the
coordination number of the phebox ligand was flexible.9b
A
change in coordination between NCN-tridentate and NC-
bidentate modes might promote the final exchange process.
We also examined the silyl transfer reaction using the phebox
Fe complexes. In this context, direct C−H silylation of aromatic
compounds with hydrosilane is an attractive transformation to
silyl compounds.32 Recently, developments of C−H silylation
of indole derivatives with hydrosilane catalyzed by Ir, Ru, and
Fe have been reported.33 When the mixture of 10 equiv of
N-methylindole (12) and Ph2MeSiH (11e) in the presence of
the silyl complex 5a (0.2 equiv) was heated at 60 °C for 3 days,
the corresponding β-silylated indole 13 was obtained in 46%
yield (eq 4).34 In this reaction, the α-silylated product was
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not detected as reported by Ru and Fe catalysts,33c,d nor was
incorporation of the SiMe3 group observed. We are tentatively
assuming that a Fe-SiPh2Me intermediate as described in A
of Scheme 1 was formed by reaction of 5a and Ph2MeSiH.35
Electrophilic substitution with N-methylindole then produced
the silylated compound 13.
Reaction of (Phebox)SiMe3 with Fe(CO)5. To a THF solution
(6 mL) of (phebox-dm)SiMe3 (4a) (103 mg, 0.30 mmol) was added
Fe(CO)5 (45 μL, 0.33 mmol) under an argon atmosphere. The
mixture was stirred under irradiation for 24 h. After removal of the
solvent, the crude product was purified by column chromatography on
silica gel with ethyl acetate/hexane (1:5) to give (phebox-dm)Fe-
(CO)2SiMe3 (5a) (83 mg, 0.18 mmol, 60%). A similar procedure
using (phebox-ip)SiMe3 (4b) (74.5 mg, 0.20 mmol), 4c (44.1 mg,
0.10 mmol), and 4d (46.9 mg, 0.10 mmol) gave (phebox-ip)-
Fe(CO)2SiMe3 (5b) (74.3 mg, 0.152 mmol, 76%), 5c (38.7 mg,
0.070 mmol, 70%), and 5d (43.5 mg, 0.075 mmol, 75%), respectively.
5a: 1H NMR (300 MHz, C6D6, rt): δ 0.04 (s, 9H, SiMe3), 0.97
(s, 6H), 1.02 (s, 6H), 3.69 (s, 4H), 7.03 (t, J = 7.5 Hz, 1H), 7.72 (d,
J = 7.5 Hz, 2H). 13C NMR (75 MHz, C6D6, rt): δ 3.9, 26.4, 29.0, 66.2,
82.1, 121.6, 124.3, 131.8, 168.1, 208.2, 216.6, 219.2. IR (KBr): ν 2969,
2903, 1982 (νCO), 1902 (νCO), 1606, 1543, 1486, 1396, 1337, 1204,
978, 828, 736 cm−1. Anal. Calcd for C21H28FeN2O4Si: C, 55.27; H,
6.18; N, 6.14. Found: C, 55.28; H, 6.24; N, 6.15. 5b: 1H NMR
(300 MHz, C6D6, rt): δ −0.03 (s, 9H, SiMe3), 0.49 (d, J = 7.2 Hz, 6H),
0.55 (d, J = 6.6 Hz, 3H), 0.62 (d, J = 6.9 Hz, 3H), 2.20−2.35 (m, 2H),
3.24−3.38 (m, 2H), 3.72−3.98 (m, 4H), 7.01 (t, J = 7.8 Hz, 1H), 7.70
(d, J = 7.8 Hz, 1H), 7.73 (d, J = 7.8 Hz, 1H). 13C NMR (75 MHz,
C6D6, rt): δ 4.0, 14.5, 16.1, 19.3, 20.8, 29.3, 30.1, 70.1, 71.1, 71.4, 71.7,
121.4, 124.6, 124.8, 130.0, 131.7, 169.5, 170.8, 208.1, 217.8, 218.2. IR
(KBr): ν 2952, 1979 (νCO), 1901 (νCO), 1604, 1489, 1387, 1335, 1145,
966, 827, 732 cm−1. Anal. Calcd for C23H32FeN2O4Si: C, 57.02; H,
6.66; N, 5.78. Found: C, 57.13; H, 6.80; N, 5.76. 5c: 1H NMR
(300 MHz, C6D6, rt): δ 0.29 (s, 9H, SiMe3), 3.93−4.26 (m, 6H),
6.96−7.16 (m, 11H), 7.81 (d, J = 7.5 Hz, 2H). 13C NMR (75 MHz,
C6D6, rt): δ 3.6, 70.0, 70.8, 77.5, 78.7, 121.3, 124.6, 124.7, 128.7, 129.0,
129.1, 129.5, 130.3, 131.5, 137.9, 140.4, 169.7, 170.1, 206.9, 216.3,
220.3. IR (KBr): 2962, 1981 (νCO), 1923 (νCO), 1483, 1390, 1261,
1142, 1089, 971 cm−1. Anal. Calcd for C29H28FeN2O4Si: C, 63.05; H,
CONCLUSION
■
We described the preparation and characterization of a series
of NCN pincer Fe complexes containing stannyl, silyl, methyl,
and phenyl ligands. The silyl and stannyl complexes were suc-
cessfully obtained by oxidative addition of C−Si and C−Sn
bonds in the ligand precursors by Fe(CO)5 under irradiation
conditions. The methyl and phenyl complexes were synthesized
by transmetalation of the bromide Fe complex with ZnMe2
and ZnPh2, respectively. The phebox Fe complexes adopted
similar pseudo-octahedral geometry, containing the meridio-
nally NCN-coordinated phebox ligand with the η1-stannyl, silyl,
methyl, and phenyl ligands at the apical position. The catalytic
activity of pincer Fe complexes was evaluated in the hydro-
silylation of a ketone, where the silyl complex was found to
be the most suitable catalyst. The silyl complex also catalyzed
the C−H silylation of N-methylindole with hydrosilane to give
the β-silylated indole compound.
EXPERIMENTAL SECTION
General Procedures. All air- and moisture-sensitive compounds
■
were manipulated using standard Schlenk and vacuum line techniques
1
under an argon atmosphere. H and 13C NMR spectra were obtained
1
at 25 °C on a Varian Mercury 300 spectrometer. H NMR chemical
1
shifts are reported in δ units, in ppm, relative to the singlet at 7.26 ppm
for CDCl3 and 7.16 ppm for C6D6. 13C NMR spectra are reported in
terms of chemical shifts relative to the triplet at 77.0 ppm for CDCl3
and 128.0 ppm for C6D6. Infrared spectra were recorded on a JASCO
FT/IR-230 spectrometer. Elemental analyses were recorded on a
YANACO MT-6. Photoreaction was performed by using an ASAHI
SPECTRA MAX-303 with a xenon lamp. Column chromatography
was performed with a silica gel column (Kanto Kagaku Silica gel 60N).
5.11; N, 5.07. Found: C, 63.20; H, 4.93; N, 4.92. 5d: H NMR (300
MHz, C6D6, rt): δ 0.00 (s, 9H, SiMe3), 2.17 (t, J = 12.8 Hz, 1H), 2.38
(t, J = 12.8 Hz, 1H), 3.66−3.94 (m, 8H), 6.79−7.07 (m, 11H), 7.74
(d, J = 7.5 Hz, 2H). 13C NMR (75 MHz, C6D6, rt): δ 4.2, 41.7, 41.9,
67.1, 68.0, 75.8, 76.7, 121.7, 124.8, 124.9, 127.0, 127.1, 129.06, 129.1,
129.4, 130.6, 131.3, 137.25, 137.27, 170.0, 170.3, 207.3, 218.3, 218.6.
IR (KBr): 2960, 1980 (νCO), 1912 (νCO), 1485, 1392, 1260, 1090, 970,
801, 732 cm−1. Anal. Calcd for C31H32FeN2O4Si: C, 64.14; H, 5.56; N,
4.83. Found: C, 64.08; H, 5.57; N, 4.68.
17
20
(Phebox)SnMe3 and (Phebox)SiMe3 were prepared by the
literature methods.
Reaction of 1a with ZnMe2. To a toluene solution (10 mL) of
(phebox-dm)Fe(CO)2Br (1a) (231 mg, 0.5 mmol) was added a
hexane solution of ZnMe2 (1 M, 1 mL, 1 mmol) under an argon atmo-
sphere. The mixture was stirred at room temperature for 15 min. After
removal of the solvent, the crude product was purified by column
chromatography on silica gel with ethyl acetate/hexane (1:6) to give
(phebox-dm)Fe(CO)2Me (7a) (170.6 mg, 0.42 mmol, 84%). A similar
procedure using (phebox-ip)Fe(CO)2Br (1b) (245 mg, 0.50 mmol)
gave (phebox-ip)Fe(CO)2Me3 (7b) (117.7 mg, 0.28 mmol, 55%). 7a:
1H NMR (300 MHz, C6D6, rt): δ −0.02 (s, 3H, Me), 0.91 (s, 6H),
0.98 (s, 6H), 3.56 (d, J = 8.3 Hz, 2H), 3.63 (d, J = 8.3 Hz, 2H), 7.06 (t,
J = 7.5 Hz, 1H), 7.71 (d, J = 7.5 Hz, 2H). 13C NMR (75 MHz, C6D6,
rt): δ 13.6 (Me), 27.3, 27.9, 65.7, 82.1, 122.0, 124.9, 131.1, 168.5,
207.7, 219.9, 226.1. IR (KBr): ν 2971, 2931, 2870, 1982 (νCO), 1927
(νCO), 1610, 1485, 1394, 1338, 1206, 1140, 979, 734 cm−1. Anal. Calcd
for C19H22FeN2O4: C, 57.30; H, 5.57; N, 7.03. Found: C, 56.59; H,
5.80; N, 6.85. Correct elemental analysis could not be obtained after
Reaction of (Phebox)SnMe3 with Fe(CO)5. To a THF solution
(6 mL) of (phebox-dm)SnMe3 (2a) (130 mg, 0.30 mmol) was added
Fe(CO)5 (45 μL, 0.33 mmol) under an argon atmosphere. The
mixture was stirred under irradiation for 12 h. After removal of the
solvent, the crude product was purified by column chromatography on
silica gel with ethyl acetate/hexane (1:5) to give (phebox-dm)Fe-
(CO)2SnMe3 (3a) (94.7 mg, 0.17 mmol, 57%). A similar procedure
using (phebox-ip)SnMe3 (2b) (139 mg, 0.30 mmol) gave (phebox-ip)-
Fe(CO)2SnMe3 (3b) (106.8 mg, 0.19 mmol, 62%). 3a: 1H NMR
(300 MHz, C6D6, rt): δ −0.08 (JSnH = 38 Hz, 9H, SnMe3), 0.96
(s, 6H), 0.97 (s, 6H), 3.66 (d, J = 8.4 Hz, 2H), 3.73 (d, J = 8.4 Hz,
2H), 7.03 (t, J = 7.4 Hz, 1H), 7.73 (d, J = 7.4 Hz, 2H). 13C NMR
(75 MHz, C6D6, rt): δ −7.1 (JSnC = 169 Hz, SnMe3), 27.0, 28.6, 65.8,
82.2, 121.7, 124.8, 131.2, 168.1, 208.6, 214.7, 219.4. IR (KBr): ν 2973,
2901, 1988 (νCO), 1907 (νCO), 1605, 1541, 1484, 1397, 1335, 981,
728 cm−1. Anal. Calcd for C21H28FeN2O4Sn: C, 46.11; H, 5.16; N,
1
5.12. Found: C, 46.02; H, 5.27; N, 5.01. 3b: H NMR (300 MHz,
1
several attempts. 7b: H NMR (300 MHz, C6D6, rt): δ 0.09 (br, 3H,
C6D6, rt): δ −0.02 (JSnH = 38 Hz, 9H, SnMe3), 0.42 (d, J = 7.2 Hz,
3H), 0.46 (d, J = 6.9 Hz, 3H), 0.54 (d, J = 6.6 Hz, 3H), 0.58 (d, J =
6.6 Hz, 3H), 2.21−2.32 (m, 2H), 3.22−3.32 (m, 2H), 3.69−4.01
(m, 4H), 7.01 (t, J = 7.6 Hz, 1H), 7.72 (d, J = 7.6 Hz, 1H), 7.76 (d, J =
7.8 Hz, 1H). 13C NMR (75 MHz, C6D6, rt): δ −7.1 (JSnC = 170 Hz,
SnMe3), 14.4, 15.2, 19.2, 20.3, 29.4, 30.2, 70.1, 71.0, 71.2, 71.4, 121.6,
125.0, 125.2, 130.1, 131.3, 169.7, 170.5, 208.7, 216.0, 218.2. IR (KBr):
ν 2960, 2909, 1981 (νCO), 1903 (νCO), 1603, 1543, 1487, 1388, 1334,
1144, 968, 734 cm−1. Anal. Calcd for C23H32FeN2O4Sn: C, 48.04; H,
5.61; N, 4.87. Found: C, 48.45z; H, 5.88; N, 4.87.
Me), 0.44 (d, J = 6.9 Hz, 3H), 0.46 (d, J = 7.2 Hz, 3H), 0.55 (d, J = 6.3
Hz, 3H), 0.59 (d, J = 7.2 Hz, 3H), 2.19 (br, 2H), 3.19 (br, 2H), 3.60−
3.90 (br, 4H), 7.03 (br, 1H), 7.68 (br, 2H). 13C NMR (75 MHz, C6D6,
rt): δ 12.2 (Me), 14.5, 15.0, 19.2, 19.8, 29.5, 29.9, 68.7, 71.0, 71.1, 71.4,
122.0, 125.1, 125.2, 130.4, 130.5, 170.1, 170.5, 207.7, 218.3, 226.2. IR
(KBr): ν 2961, 2873, 1987 (νCO), 1921 (νCO), 1651, 1611, 1556, 1487,
1386, 1335, 1250, 1145, 968, 733 cm−1. Anal. Calcd for C21H26FeN2O4:
C, 59.17; H, 6.15; N, 6.57. Found: C, 58.01; H, 6.33; N, 6.29. Correct
elemental analysis could not be obtained after several attempts.
1381
DOI: 10.1021/acs.organomet.5b00082
Organometallics 2015, 34, 1377−1383

Organometallics
Article
Reaction of 1a with ZnPh2. To (phebox-dm)Fe(CO)2Br (1a)
(185 mg, 0.4 mmol) and ZnPh2 (351 mg, 1.6 mmol) was added
toluene (8 mL) under an argon atmosphere. After being stirred at
room temperature for 1 min, the reaction mixture was quickly purified
by column chromatography on silica gel with ethyl acetate/hexane
(1:6) to give (phebox-dm)Fe(CO)2Ph (8) (96.6 mg, 20.8 mmol,
52%). 8: 1H NMR (300 MHz, C6D6, rt): δ 0.85 (s, 6H), 0.92 (s, 6H),
3.42 (d, J = 8.4 Hz, 2H), 3.55 (d, J = 8.4 Hz, 2H), 6.94 (t, J = 6.6 Hz,
1H), 7.01−7.07 (m, 4H), 7.41 (d, J = 7.2 Hz, 2H), 7.65−7.72
(m, 2H). 13C NMR (75 MHz, C6D6, rt): δ 27.1, 28.3, 66.2, 81.9, 122.5,
122.9, 125.6, 126.5, 130.2, 132.0, 132.3, 139.6, 165.0 (Fe-Cipso), 169.5,
209.2, 216.1, 221.1. IR (KBr): ν 2963, 2005 (νCO), 1935 (νCO), 1611,
1549, 1486, 1397, 1261, 1094, 1022, 803 cm−1. Anal. Calcd for
C24H24FeN2O4: C, 62.62; H, 5.26; N, 6.09. Found: C, 62.09; H, 5.29;
N, 5.81. Correct elemental analysis could not be obtained after several
attempts.
Hydrosilylation of 9a. To a solution of Fe catalyst (0.01 mmol)
and 9a (0.5 mmol) was added hydrosilane (0.75 mmol) by a syringe
under an argon atmosphere. After being stirred at 50 °C for 24 h, a
HCl solution was added at 0 °C. The mixture was extracted with
EtOAc. The extract was washed with brine and saturated NaHCO3
and dried over Na2SO4. After removal of the solvent, the residue was
purified by silica gel column chromatography with ethyl acetate/
hexane to give 10a.
Reaction of 12 with Ph2MeSiH. To a solution of Fe catalyst
(0.06 mmol) and 12 (3 mmol) was added Ph2MeSiH (0.3 mmol) by a
syringe under an argon atmosphere. The mixture was stirred at 60 °C
for 3 days. The crude product was purified by column chromatography
on silica gel to give 13.
X-ray Diffraction. The diffraction data for 3a, 5a, 5b, 7a, and 8
were collected on a Bruker SMART APEX CCD diffractometer
with graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å). An
empirical absorption correction was applied by using SADABS. The
structure was solved by direct methods and refined by full-matrix least-
squares on F2 using SHELXTL. All non-hydrogen atoms were refined
with anisotropic displacement parameters. All hydrogen atoms were
located on calculated positions and refined as rigid groups. Crystallo-
graphic data are summarized in the Supporting Information.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedure and characterization of 13, crystallo-
graphic data and CIF files for 3a, 5a, 5b, 7a, and 8, and NMR
spectra. This material is available free of charge via the Internet
at http://pubs.acs.org.
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H.; Furuta, A. Chem. Commun. 2007, 760−762. (d) Tondreau, A. M.;
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: jito@apchem.nagoya-u.ac.jp (J. Ito).
*E-mail: hnishi@apchem.nagoya-u.ac.jp (H. Nishiyama).
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by a Grant-in-Aid for Scientific
Research from the Japan Society for the Promotion of Science
(Nos. 22245014, 26410114).
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