Synthesis of silyl iron hydride: Via Si-H activation and its dual catalytic application in the hydrosilylation of carbonyl compounds and dehydration of benzamides

Article abstract of DOI:10.1039/c8dt00289d

The hydrido silyl iron complex (o-Ph₂PC₆H₄SiMe₂)Fe(PMe₃)₃H (2) was obtained via the activation of the Si-H bond of the bidentate silyl ligand o-Ph₂P(C₆H₄)SiMe₂H (1) by Fe(PMe₃)₄. 2 showed good to excellent catalytic activity in both the reduction of aldehydes/ketones and the dehydration of benzamide. In addition, with complex 2 as a catalyst, α,β-unsaturated carbonyls could be selectively reduced to the corresponding α,β-unsaturated alcohols. The mechanisms of the formation of 2 and the catalytic dehydration process are proposed and partly experimentally verified.

Full text of DOI:10.1039/c8dt00289d

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10.1039/C8DT00289D.
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Synthesis of Silyl Iron Hydride via Si-H Activation and its Double Catalytic
Application in Hydrosilylation of Carbonyl Compounds and Dehydration of
Benzamides
Shishuai Ren,+[a] Shangqing Xie,+[a] Tingting Zheng,+[a,b] Yangyang Wang,+[a,c] Shilu
Xu,[a] Benjing Xue,[a] Xiaoyan Li,[a] Hongjian Sun,*[a] Olaf Fuhr,[d] and Dieter
Fenske[d]
Abstract: Hydrido silyl iron complex
(2) was obtained
via the activation of Si-H bond of bidentate silyl ligand o-Ph2P(C6H4)SiMe2H (1) by
Fe(PMe3)4. 2 showed excellent to good catalytic activity in both the reduction of
aldehydes / ketones and the dehydration of benzamide. In addition, with complex 2 as
a catalyst α,β-unsaturated carbonyls could be selectively reduced to the corresponding
α,β-unsaturated alcohols. The mechanisms of the formation of 2 and the catalytic
dehydration process are proposed and partly-experimentally verified.
_____________________________________________________________________
[+] These authors contributed equally to this work.
[a] S. Ren, S. Xie,T.Zheng, B. Xue, S. Xu, Prof. Dr. X. Li, Prof. Dr. H. Sun
School of Chemistry and Chemical Engineering
Key Laboratory of Special Functional Aggregated Materials
Ministry of Education
Shandong University
Shanda Nanlu 27, 250199 Jinan (People’s Republic of China)
E-mail: hjsun@sdu.edu.cn
[b] T.Zheng, Department of Chemistry, Capital Normal University, 100037 Beijing,
People’s Republic of China
[c] College of Chemistry and Chemical Engineering, Northwest Normal University,
730070 Lanzhou, PR China
[d] Dr. O. Fuhr, Prof. Dr. D. Fenske
Institut für Nanotechnologie (INT) und Karlsruher Nano-Micro-Facility (KNMF)
Karlsruher Institut für Technologie (KIT)
Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen (Germany)
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† CCDC reference number 1430351 (2b). Copies of the data can be obtaineDdOfI:r1e0e.10o3V9fi/eCw8ADrtTic0le0O28nl9inDe
charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax:
(+44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk). For crystallographic data in
CIF or other electronic format see DOI: xx.xxxx/xxxxxxxxxx.
Introduction
In recent decade, tridentate pincer complexes have been widely explored in new
ligands design and catalytic application.1-2 Some silyl pincer transition metal
complexes were studied due to the strong trans-effect of silicon atom.3-8 Meanwhile,
the tetradentate silyl ligands were studied by Peters and Nakazawa.9-14 The
tetradentate ligands with three anchoring groups showed excellent ability in fixing H2
and N2 to form the related stable transition metal products.15-17 However, the catalytic
activity of transition-metal complexes with a tetradentate ligand is poor because of the
steric effect.18-20 Therefore, chemists are want to know if transition metal complexes
with stable bidentate silyl ligands show good catalytic activity. The Sabo-Etienne’s
group studied the reactions of [N, Si] bidentate ligands with ruthenium and obtained a
series of complexes with Si-H bond coordination to the metal center.21-22 Smith et al.
reported that the Ir complexes with a [P, Si] ligand are effective catalysts for
ortho-borylation directed by a range of different functional groups. Furthermore, he
found that the [N, Si] precursor was also highly active.23 The electron-rich framework
of the bidentate [P, Si] ligand played an important role in the catalytic cycle. In
addition, it was of interest to study complexes with cheaper transition metals.
Recently, we reported the synthesis and characterization of a series of Ni, Co,
and Fe complexes bearing
a
tridentate bis(phosphino)silyl ligand
(κ3-(2-Ph2PC6H4)2SiMeH, [PSiP]-H)24 and a tetradentate tris(phosphino)silyl ligand
((o-(Ph2P)C6H4)3SiH)19. The hydrido iron(II) complex [PSiP]Fe(H)(PMe3)2 was found
to be an excellent catalyst for hydrosilylation of aldehydes and ketones under mild
condition.24 In order to expand our exploration from tridentate and tetradentate to
bidentate ligands in this field, we reported the synthesis of a silyl [P, Si]-chelate cobalt
hydride, CoHCl(PMe3)2(1-Si(Me)2-2-(PPh2)C6H4), through the reaction of bidentate
ligand o-HSi(Me)2(PPh2)C6H4 (1) with CoCl(PMe3)3.25 We found that the silyl [P,
2

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Si]-chelate cobalt hydride is catalytically more active for the Kumada coupling
reaction than the [PSiP]-pincer cobalt hydride26. Although some irone-based
complexes were prepared and used as catalysts for hydrosilylation of aldehydes and
ketones,27 generally there are two disadvantages of these catalytic systems: (a)
Syntheses of the iron complexes are more laborious; (b) It is not easy to handle these
iron complexes in catalysis because they are air-sensitive. Therefore, more practical
iron complexes as catalysts should be further developed. As continuation of the study
in this field, in this paper hydrido iron complex 2 was obtained via the activation of
Si-H bond of bidentate silyl ligand 1 by Fe(PMe3)4. In comparison with syntheses of
pincer iron complexes, the preparation of chelate iron complex 2 is easier. The
experiments indicate that 2 not only has excellent catalytic activity in the reduction of
aldehydes and ketones but also exhibits good catalytic results for the dehydration of
benzamide.
Results and discussion
Reaction of 1 with Fe(PMe3)4
When bidentate ligand 1 was combined with Fe(PMe3)4 in n-pentane at 0oC, the
deep yellow solution turned light. After stirring at room temperature for 16 h, the
solution turned turbid and a large amount of yellow powder formed. After evaporation
of the volatiles, the residue was extracted with n-pentane and diethyl ether. The Fe(II)
product 2 was obtained as deep yellow crystals in a yield of 82%. Complex 2 is stable
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DOI: 10.1039/C8DT00289D
in solid state in the air.
In the IR spectrum of 2, the typical vibration of the Si-H bond of 1 at 2111 cm-1
disappeared. Instead, the Fe-H signal at 1963 cm-1, comparable to that (1959 cm-1) of
hydrido pincer PCP iron(II) complex,28 suggests that the Si-H bond was activated.
1
Meanwhile, the in-plane bending (rocking) of PMe3 appeared at 939 cm-1. In the H
NMR of 2, a multiple peak for the hydrido hydrogen was found at -14.91 ppm (Fig. 1,
left). Due to the coupling effect with four P atoms (3 x PPMe3 and 1 x PPPh2), the peak
splits into a dddd signal, with the coupling constants being 32.7, 20.6, 13.3 and 4.8 Hz.
The largest coupling (32.7 Hz) is the J(HPa) because <H-Fe-Pa is the largest angle.
J(HPc) and J(HPd) are similar (20.6 and 13.3 Hz, or 13.3 and 20.6 Hz) because –PdPh2
and PcMe3 groups have similar chemical environments. The last coupling is J(HPb)
31
(4.8 Hz) because PbMe3 is situated at the special position. In the P NMR of 2, four
sets of signals were identified at 21.1, 24.2, 30.0 and 109.2 ppm, respectively,
indicating that the four P atoms are in different chemical environments. In addition, it
was found that the isomer 2’ of complex 2 was also formed as a byproduct during the
reaction. The hydrido signal of 2’ can be explained by the structure of isomer 2’ (Fig.
1, right).26
4

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Fig. 1 1H NMR Signals of Fe-H in 2 (left) and 2’ (right)
Scheme 1 Proposed mechanism for the formation of 2
The formation mechanism of 2 is proposed in Scheme 1. At first, one ligand of
PMe3 in Fe(PMe3)4 is replaced by ligand 1 to form a new Fe(0) intermediate 2a. With
the support of the phosphor atom of the -PPh2 group Si-H bond coordinates to iron
center to form η2-(Si-H) intermediate 2b29 accompanied by the dissociation of one
another PMe3 ligand. In the last step, the Si-H bond is cleaved by the electron-rich
iron(0) center and hydrido Fe(II) product 2 is obtained.
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Fig. 2 ORTEP plot of complex 2b at the 50% probability level (hydrogen atoms are
omitted for clarity). Selected bond lengths (Å) and angles (deg): Si8-H51 1.80(6),
Fe2-P1 2.168(2), Fe2-P4 2.199(2), Fe2-P2 2.199(1), Fe2-Si8 2.254(2), Fe2-H51
1.36(8); P1-Fe2-P4 106.48(7), P1-Fe2-P2 106.47(6), P4-Fe2-P2 97.93(7), P1-Fe2-Si8
92.39(6), P4-Fe2-Si8 123.50(6), P2-Fe2-Si8 127.37(8), P1-Fe2-H51 82(3),
P4-Fe2-H51 171(3), P2-Fe2-H51 81(2), Si8-Fe2-H51 53(2)
We did in situ IR spectrum to verify the existence of intermediate 2b. A pentane
solution of 1 was combined with Fe(PMe3)4 at 0oC. After stirring at room temperature
for 3 h, a vibration at 2062 cm-1 in the in situ IR spectrum was found to be the
possible signal of the η2-(Si-H) bond of intermediate 2b (See Fig S5, Supporting
Information). Fortunately, we accidentally got the structure of compound 2b when
trying to determine the structure of compound 2. Single crystal X-ray diffraction
confirmed that 2b has a distorted tetrahedral coordination geometry with the iron
atom in the center (Fig. 2). The bond length of Fe2-H51 (1.36(8) Å) is obviously
shorter than typical values of terminal Fe-H bonds (1.53-1.61 Å)24 and comparable to
the Fe-H distance (1.35(3) Å) in a similar iron complex.30 However, in a stable
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tripodal silyl hydrido iron complex the Fe-H bond distance is 1.42D(3O)I: 10Å.10.3199/C8DT00289D
Furthermore, the bond length of Si8-H51 (1.80(6) Å) in complex 2b is also
comparable to that (1.88(3) Å) of a similar iron complex31 and much longer than that
(1.5 Å) in free silane. This information suggests that the Si-H bond in intermediate
2b was partly activated and complex 2b is an η2-(Si-H) coordinate Fe(0) complex.
The bond length of Fe2-Si8 (2.254(2) Å) is comparable with those reported in the
literatures.19,24,32
Catalytic reduction of aldehydes and ketones by complex 2
In recent years, various chelate iron complexes have been synthesized and used as
catalysts for many reactions.31-40 It was confirmed that some iron hydrides showed
highly catalytic activity.24,28,41-43 Based on the previous work in our laboratory,24,44-46
the catalytic reduction of aldehydes by 2 with silanes as hydrogen source was
explored. It can be seen from Table 1 that (EtO)3SiH showed the best catalytic results
for the reduction of benzaldehyde. The conversion of benzaldehyde was 96% (entry 1,
87% yield) in one hour at 50oC using (EtO)3SiH as hydrogen source. The other silanes,
such as Et3SiH, Ph3SiH and Me2PhSiH, demonstrated almost no effect on this
catalytic system (entries 2 - 4). Meanwhile, a secondary silane, such as Ph2SiH2, was
also proved to be effective for 90% conversion (entry 5, 83% yield) in two hours.
From Table 1 it can be concluded that moderate yields could also be achieved when
the catalytic reactions were performed for a longer time even if the substrates have
electron-withdrawing groups such as -F, -Br, and –Cl (entries 6 - 9). For the substrate
with a larger steric effect, such as 1-naphthaldehyde, good yield could also be
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obtained (entry 12). Furthermore, this catalytic system is also suitableDOfoI: r10.1t0h3e9/C8DT00289D
reduction of aliphatic aldehyde, such as benzylaldehyde (entry 13).
Table 1 The hydrosilylation of aldehydes catalyzed by 2a
entry
aldehyde
Alcohol
time (h)
1
yield (%) b
87
Si-H
1
2
3
4
5
6
(EtO)3SiH
Et3SiH
2
2
2
2
<1
<1
<1
83
Ph3SiH
Me2PhSiH
Ph2SiH2
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
4
4
5
2
1
91
95
83
89
93
7
8
9
Cl
10
CH OH
2
(EtO)3SiH
(EtO)3SiH
Cl
11
3
91
8

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12
13
DOI: 10.1039/C8DT00289D
4
86
(EtO)3SiH
(EtO)3SiH
1
85
a: 1 mol% 2, THF, 50 oC.
b: Isolated yield.
The temperature was elevated to 60oC for the catalytic reduction of ketones by 2
due to the low activity of the substrates (Table 2). From Table 2 we know that the
aromatic ketones with less bulky substituents give higher yields (entries, 1 - 3 and 5)
than those with bulky group (entries 4 and 6). The aliphatic cyclic ketones could also
be reduced by 2 in moderate yields (entries 7 - 8).
Table 2 The hydrosilylation of ketones catalyzed by 2a
entry
ketone
Alcohol
time (h)
4
yield (%) b
77
OH
1
OH
2
2
4
8
96
95
60
F
3
4
OH
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5
6
7
DOI: 10.1039/C8DT00289D
4
8
4
94
OH
67
68c
76c
8
4
a: 1 mol% 2, THF, 60oC
b: Isolated yield.
c: GC yield.
Recently, we reported the synthesis of hydrido iron(II) complexes
24
(o-Ph2PC6H4)2SiMeFe(H)(PMe3)2
bearing
a
tridentate
ligand
and
(o-(Ph2P)C6H4)3SiFeH(PMe3)19 bearing a tetradentate ligand. The former has catalytic
activity and the latter has no catalytic activity for hydrosilylation of aldehydes /
ketones. This might be caused by the steric effect of the chelate ligand. Under similar
catalytic conditions, complex 2 bearing a bidentate ligand shows a similar catalytic
activity with (o-Ph2PC6H4)2SiMeFe(H)(PMe3)2 bearing a tridentate ligand.
Furthermore, some α,β-unsaturated aldehydes as substrates were also tested for
the hydrosilylation reaction (Table 3). Excitedly, all of the substrates were selectively
reduced to the corresponding α,β-unsaturated alcohols. It is worth noting that the
α-Me substitutional cinnamyl alcohol was obtained in the highest yield. With
α-bromocinnamaldehyde as substrate, 3-phenyl-2-propyn-1-ol was isolated in the
yield of 70 %. This is the result of the elimination of HBr from the corresponding
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cinnamyl alcohol product, 2-bromo-3-phenyl-prop-2-en-1-ol. ,-Unsaturated
ketones are also applicable to this catalytic system by increasing the catalyst loading
to 2%, extending the reaction time to 8 h and raising the temperature to 60°C (Table
3).
Table 3 Selective reduction of α,β-unsaturated carbonyls catalyzed by 2.a,b
OH
OH
OH
H
H
83%
88%
70%
OH
OH
H
H
(
)
5
Cl
68%
71%
OH
OH
90% c
72%
a: cinnamaldehyde (1.0 mmol), (EtO)3SiH (1.2 mmol),
2 (0.01 mmol) in 2 ml THF, 50℃, 4h.
b: Isolated Yield.
c: GC Yield.
Catalytic dehydration of amides by complex 2
We reported that hydrido thiophenolato iron(II) complexes could be used as
efficient catalysts to realize the dehydration of primary amides to nitriles under
hydrosilylation conditions.47 Based on the high catalytic ability of 2 in the reduction
of aldehydes and ketones, we will ask whether the catalytic system is also suitable for
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reduction of amides. Beller’s group reported the dehydration and reduction oDfOaI:m10i.1d0e39s/C8DT00289D
by iron complexes.48-50 Adolfsson studied the catalytic reductions of tertiary amides to
amines by an iron carbene complex.51 Nagashima disclosed a similar catalytic
reduction
Me3SiO(SiMeHO)nSiMe3 as hydrogen source.52
Table 4 Attempt to catalytic reduction of amides by 2a
by
iron
carbonyl
complexes
using
HN(SiMe2H)2
and
entry
amides
t (h)
conversion
(%/GC)
< 1
< 1
1
2
32
32
3
4
24
24
70
> 99
a: Amide (1.0 mmol), (EtO)3SiH (3.0 mmol),
2 (0.02 mmol), 2 ml THF, 60℃.
At the beginning, the catalytic reduction of various amides by 2 were employed
using (EtO)3SiH as reducing agent (Table 4). It was verified that secondary and
tertiary amides such as acetanilide and N, N-diethyl benzamide showed no conversion
(entries 1 - 2). In contrast, the primary amides such as benzamide and 2,6-difluoro
benzamide showed good activity (conversions: 70% and 99% respectively) in 24
hours at 60oC with 2% catalyst loading. NMR confirmed that the catalytic products
are cyanobenzene and 2,6-difluorocyanobenzene as dehydration products, instead of
the expected amines.
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Table 5 Catalytic dehydration of benzamide by complex 2
entry
1
Silane
2 (mol %)
Solvent
THF
t (h)
24
24
24
48
24
24
24
24
24
24
24
24
24
24
Yield (%)b
1
2
3
2
2
2
2
2
2
2
2
2
2
2
48
70
59
66
88
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
Et3SiH
2
THF
3
THF
4
THF
5a
6
THF
THF
-
-
7
THF
Ph3SiH
8
THF
Me2PhSiH
-
9
THF
Ph2SiH2
-
10
11
12
13
14
(EtO)3SiH
toluene
58
63
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
benzene
dioxane
acetonitrile
n-butyl
50
53
24
ether
a: 5 eq. silane; b: GC yield.
Using benzamide as a model substrate, the further study indicates that, as the
reduction of aldehydes (Table 1), no conversion of the substrate could be reached
when the hydrogen sources were Et3SiH, Ph3SiH, Me2PhSiH and even Ph2SiH2
(Table 5, entries 6 - 9). It was found that the best catalyst loading was 2% (Table 5,
entries 1 - 2). When the catalyst loading was increased (Table 5, entry 3) or the
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reaction time was extended (Table 5, entry 4), the yield became even lDoOwI:e10r..10I3n9/C8DT00289D
addition, it was verified that the yield was much higher if much more (EtO)3SiH was
used (Table 5, entry 5). Different solvents were also employed for the catalytic
system (Table 5, entry 2 and entries 10 - 14 ). It was found that best yield was reached
in THF (Table 5, entry 2).
On the basis of the results in Tables 5, the optimized catalytic reaction
conditions for the dehydration of primary amides are selected as follows: amide (1.0
mmol), (EtO)3SiH (3.0 mmol), and 2 (0.02 mmol) in 2.0 mL of THF, 60℃, 24 h. The
dehydration of primary amides with different functionalities was catalyzed by
complex 2 under the optimized conditions. The corresponding nitriles were obtained
in good yields (Table 6). In comparison with our reported results,46 we can conclude
that complex 2 shows almost the similar activity and selectivity as hydrido
thiophenolato iron(II) complexes for the dehydration of primary amides to nitriles.
The dehydration of primary amides to nitriles by complexes 2 is also suitable for a
wide scope of substrates.
Table 6 Reductive dehydration of primary amides to nitriles catalyzed by 2.a,b
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Me
CN
CN
CN
CN
Cl
O2N
89%
69%
86%
71%
I
CN
CN
CN
CN
N
Me
MeO
Me
73%
70%
84%
69%
Cl
CN
CN
CN
88%c
75%
79%
O
O
OH
CN
CN
NH2
81%c
77%c, d
a: amide (1.0 mmol), (EtO)3SiH (3.0 mmol),
2 (0.02 mmol) in 2 ml THF, 60℃, 24 h.
b: Isolated Yield.
c: GC Yield.
d: (EtO)3SiH (4.0 mmol)
Scheme 2 Proposed mechanism for catalytic dehydration of benzamide by 2
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In order to better understand the reaction mechanism, we did stoichiometric
reactions of complex 2 with benzamide and triethoxysilane respectively under the
catalytic conditions. To our disappointment, the reaction of complex 2 (1 equiv) with
(EtO)3SiH (1 equiv) did not work. The stoichiometric reaction of complex 2 with
benzamide resulted in release of dihydrogen detected by GC. However, we could not
isolate the expected product due to its instability. On the basis of the above
experimental results and the possible reaction mechanism reported by Nagashima53
and Beller,49 a plausible mechanism for the catalytic dehydration of benzamide by 2 is
proposed in Scheme 2. At the first step, the reaction of benzamide with iron hydride 2
generates intermediate A with the release of dihydrogen, detected by GC. With the
addition of (EtO)3SiH into the reaction system, (EtO)3SiH coordinates to the Fe center
after PMe3 dissociation to form B with a four-membered ring. In the presence of
PMe3, intermediate C is formed with the regeneration of 2. Intermediate C reacts with
iron hydride 2 to give rise to intermediate D with the second N-H bond activation and
the release of the second dihydrogen molecule, detected by GC. D reacts with another
equiv. of (EtO)3SiH to provide E with a new four-membered ring. In the presence of
PMe3, F is produced with the recovery of 2. The elimination of F affords the final
product cyanobenzene and the byproduct (EtO)3SiOSi(EtO)3. The byproduct
(EtO)3SiOSi(OEt)3 was confirmed by LCMS and GC.
Conclusion
[P, Si]-chelate silyl hydrido iron(II) complex 2 was synthesized via Si-H bond
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activation. Complex 2 was proved to be a highly active catalyst for the reduction of
aldehydes and ketones. Furthermore, the dehydration reaction of amides using 2 as
catalyst was also explored. Meanwhile, a mechanism for this catalytic dehydration
process was proposed and partly-experimentally verified. The results showed that
silyl hydrido iron(II) complex 2 has double catalytic activity for both hydrosilylation
of aldehydes / ketones and dehydration of amides. In addition, with complex 2 as a
catalyst α,β-unsaturated carbonyls could be selectively reduced to the corresponding
α,β-unsaturated alcohols.
Experimental section
General Procedures and Materials. Standard vacuum techniques were used in the
manipulations of volatiles and air-sensitive materials. Solvents were dried by metal
sodium and distilled under nitrogen before use. Fe(PMe3)4 was prepared according to
literature procedures.54 Infrared spectra (4000-400 cm-1), as obtained from Nujol
mulls between KBr disks, were recorded on a Bruker ALPHA FT-IR instrument.
NMR spectra were recorded using Bruker Avance 300 MHz spectrometer. Melting
points were measured in capillaries sealed under N2 and were uncorrected. Elemental
analyses were carried out on an Elementar Vario ELIII instrument.
Synthesis of o-Ph2P(C6H4)SiMe2H (1). Compound 1 was prepared by modified
procedure of the literature.55 To a solution of o-Ph2PC6H4Br (6.2 g, 18.2 mmol) in 100
mL of THF was slowly added 8.5 mL of n-BuLi (2.5 M in hexane) at -78oC. The
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temperature was maintained for 30 min and Me2ClSiH (2.14 g, 22.7 mmol) wDaOsI:a1d0.d10e3d9/C8DT00289D
in one portion. After stirred at -78 oC for 15 min, the mixture was allowed to warm to
room temperature overnight. The volatiles were evaporated to dryness under vacuum,
and the residue was purified by flash column chromatography using n-hexane as an
eluent. The title compound was obtained as white crystals in a yield of 87% (5.08 g,
15.8 mmol).
Synthesis of o-Ph2P(C6H4)Si(Me)2Fe(H)(PMe3)3 (2). 1 (0.48 g, 1.5 mmol) in 30 mL
of n-pentane was combined with Fe(PMe3)4 (0.65 g, 1.8 mmol) in 10 mL of n-pentane
at 0oC. After stirred at room temperature for 16 h, the turbid solution was evaporated
under reduced pressure and the resulting yellow powder was extracted with n-pentane
and diethyl ether. Complex 2 (0.74 g, 1.23 mmol) was obtained as deep yellow
crystals in a yield of 82%. m.p.: 162-164 °C . IR (Nujol mull, 4000 - 400 cm-1): 3046
ν(ArH), 1963 ν(Fe-H), 1576 ν(C=C), 939 ρ(PMe3). 1H NMR: (300 MHz, benzene-d6,
300 K, δ): -14.91 (dddd, J = 32.7, 20.6, 13.3, 4.8 Hz, FeH, 1H), 0.54 (s, SiMe, 3H),
1.07 (t, PMe3 18H, SiMe, 3H, overlap；total 21H), 1.50 (d, J = 6.2 Hz, PMe3 9H), 7.1 -
7.22 (m, Ar, 7H), 7.32 - 7.47 (m, Ar, 4H), 7.98 (d, J = 7.4 Hz, Ar, 1H), 78.31 (t, Ar,
2H); 31P NMR (121 MHz, benzene-d6, 300 K, δ): 21.1 (m, PMe3, 1P), 24.2 (m, PMe3,
13
1P), 31.0 (m, PMe3,1P), 109.2 (m, PPh2, 1P) ppm. C NMR (75 MHz, benzene-d6,
300 K, δ): 12.0, 16.3, 18.9, 24.7, 29.1, 29.3, 29.5, 31.6, 31.9, 31.9, 32.2, 36.5, 128.2,
129.3, 132.0, 133.2, 133.4, 134.4, 134.5, 135.6, 135.9, 136.0, 137.9 ppm. Anal. Calcd
for C29H48FeP4Si (604.52 g/mol): C, 57.62; H, 8.00. Found: C, 57.20; H, 8.34.
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DOI: 10.1039/C8DT00289D
General procedure for the catalytic reduction of aldehydes and ketones by
complex 2. To a 25 mL Schlenk tube containing a yellow solution of 2 (0.01 mmol) in
3 mL of THF was added benzaldehyde (1.0 mmol) and (EtO)3SiH (1.2 mmol). The
reaction mixture was stirred at 50 °C and monitored by TLC and GC. After a period
of time, 3 mL of 10% NaOH and 1 mL MeOH was added and stirred at 60°C for 28h.
The mixture was extracted with 50 mL of diethyl ether for three times. The organic
phase was dried by Na2SO4 and purified by flash column chromatography.
General procedure for the catalytic dehydration of amides by complex 2. To a 25
mL Schlenk tube containing a yellow solution of 2 (0.02 mmol) in 3 mL of THF was
added benzamide (1.0 mmol) and (EtO)3SiH (3 mmol). The reaction mixture was
stirred at 60 °C and monitored by TLC and GC. After a period of time, 3 mL of 10%
HCl was added and stirred at room temperature for 2 h. The mixture was extracted
with 50 mL of diethyl ether for three times. The organic phase was dried by Na2SO4
and purified by flash column chromatography.
X-ray structure determination. Diffraction data were collected on a Bruker SMART
ApexII CCD diffractometer equipped with graphite-monochromated Mo-Kα radiation
(λ = 0.71073 Å). The structures were resolved by direct or Patterson methods with the
SHELXS-97 program and were refined on F2 with SHELXTL. Hydrogen atoms were
included in calculated positions and were refined using a riding model. To improve
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the refinement of 2b, the SQUEEZE routine of PLATON was applied to treat the
diffuse electron density. A summary of crystal data, data collection parameters, and
structure refinement details, is given in the Supplementary Information.
CCDC-1430351 (2b) contain the supplementary crystallographic data for this paper.
Acknowledgements.
We gratefully acknowledge the financial support by NSF China No.
21572119/21272138.
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