Synthesis and catalytic application of [PPP]-pincer iron, nickel and cobalt complexes for the hydrosilylation of aldehydes and ketones

Article abstract of DOI:10.1039/c8nj03919d

A new synthetic strategy for the novel diphosphine-phosphine oxide ligand (1) (Ph₂P-(C₆H₄))₂P(O)H was designed. A series of [PPP]-pincer Fe, Ni, and Co complexes were prepared. All of them were formed by chelate-assisted P-H activation. Two metal hydrides [(Ph₂P-(C₆H₄))₂P(O)]Fe(H)(PMe₃)₂ (2) and [(Ph₂P-(C₆H₄))₂P(O)]Ni(H)(PMe₃) (3) were obtained at room temperature. The combination of ligand 1 with Co(PMe₃)₄Me or Co(PMe₃)₄ afforded the same Co(i) complex [(Ph₂P-(C₆H₄))₂P(O)]Co(PMe₃)₂ (4) via P-H bond activation. The catalytic performance of the Fe, Ni, and Co complexes for the hydrosilylation of aldehydes and ketones was explored. At a catalyst loading of 2 mol%, complex 2 displayed the best catalytic activity for the hydrosilylation by using (EtO)₃SiH as the hydrogen source under mild conditions. Complexes 2, 3, and 4 were characterized by spectroscopic methods and X-ray diffraction analysis.

Full text of DOI:10.1039/c8nj03919d

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DOI: 10.1039/C8NJ03919D
Synthesis and catalytic application of [PPP]-pincer iron, nickel and
cobalt complexes for hydrosilylation of aldehydes and ketones
Xinghao Qi,a Hua Zhao,a Hongjian Sun,a Xiaoyan Li,*a Olaf Fuhrb and Dieter
Fenskeb
Abstract
A new synthetic strategy of the novel diphosphine–phosphine oxide ligand (1)
(Ph2P-(C6H4))2P(O)H was designed. A series of [PPP]-pincer Fe, Ni, and Co
complexes were prepared. All of them were formed by chelate-assisted P-H
activation. Two metal hydrides [(Ph2P-(C6H4))2P(O)]Fe(H)(PMe3)2 (2) and
[(Ph2P-(C6H4))2P(O)]Ni(H)(PMe3) (3) were obtained at room temperature. The
combination of ligand 1 with Co(PMe3)4Me or Co(PMe3)4 afforded the same
Co(I) complex [(Ph2P-(C6H4))2P(O)]Co(PMe3)2 (4) via P-H bond activation. The
catalytic performance of Fe, Ni, and Co complexes for the hydrosilylation of
aldehydes and ketones was explored. With a catalyst loading of 2 mol%,
complex 2 displayed the best catalytic activity for the hydrosilylation by using
(EtO)3SiH as the hydrogen source under mild conditions. Complexes 2, 3, and
4 were characterized by spectroscopic methods and X-ray diffraction analysis.
_______________________________
aSchool of Chemistry and Chemical Engineering, Key Laboratory of Special
Functional Aggregated Materials, Ministry of Education, Shandong University,
Shanda Nanlu 27, 250199 Jinan, PR China. E-mail: xli63@sdu.edu.cn
bInstitut für Nanotechnologie (INT) und Karlsruher Nano-Micro-Facility (KNMF),
Karlsruher Institut für Technologie (KIT), Hermann-von-Helmholtz-Platz 1, 76344
Eggenstein-Leopoldshafen, Germany
†Electronic supplementary information (ESI) available: The table of selected
1
31
13
crystallographic data and original IR, H NMR, P NMR, and C NMR spectra of
complexes 2 – 4 and catalytic products. For ESI and crystallographic data in CIF or
other electronic format see DOI: XXXXXXX
1

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Introduction
Hydrido metal complexes have been widely used in various fields of
organometallic chemistry.1 Activity of these complexes is mainly controlled by
designing different (pre)ligands. Until now, metal hydrides supported by pincer-type
ligands, such as [PCP], [NCN], [SiCSi] and [PNP]-type, have been explored with
dramatic catalytic development.2-4 The focus on the design of the tridentate ligands is
triggered by the construction of metal hydrides through chelate-assisted E-H (E = C, Si,
P, S etc) bond activation. For instance, the selective C–H bond cleavage of
hydrocarbons is one of the most popular methods.5-7 Successful C–H activation induced
by transition metal compounds has been reported for preparation of a large number of
mononuclear late transition metal complexes, i.e. [PCP]Ir(H),8-10 [PCP]Rh(H)11,12 and
[PCP]Ru(H),13 featuring a pincer-type ligand.
We also want to know if the transition-metal complexes with a monoanionic
[PPP]-pincer ligand exhibit rich coordination chemistry because there are surprisingly
few reports on this subject.14-17 Mazzeo applied a monoanionic [PPHP]-pincer ligand in
Ca or Mg-catalyzed ring-opening polymerization of ɛ-caprolactone18 and in
Pd-catalyzed allylation of aldehydes.19 Togni developed a novel chiral ferrocenyl
tridentate phosphine ligand for asymmetric hydroamination of acrylonitrile
derivatives.20-23 Recently, Zhang and co-workers reported Rh(II) complexes ligated
with a novel secondary phosphine oxide [PPH(O)P] which exhibited good catalytic
activity toward carbonyl reduction.24 Although transition-metal complexes with
triphosphorus ligands have good catalytic activity for some chemical transformations,
2

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the development of structurally novel [PPH(O)P] ligands and their complexes with
inexpensive metals, especially various metal hydrides with high reactivity, is still
desired.
Our group has explored the construction of metal hydrides via C-H, Si-H, N-H or
S-H activation. It has been confirmed that most of the metal hydrides have good
catalytic activity.25-29 In order to design novel metal hydrides via the activation of the
P-H bond, herein, we report a new synthetic strategy of the novel tridentate secondary
phosphine oxide ligand 1 and the related coordination chemistry of ligand 1 toward iron,
nickel and cobalt. The synthesis of iron(II) hydride 2 and nickel(II) hydride 3 were
achieved while the related cobalt(II) hydride 4a was not stable and converted to a more
stable cobalt(I) 4. The catalytic properties of metal complexes 2, 3, 4 and 4a for
hydrosilylation reactions of aldehydes and ketones were investigated. It was found that
iron(II) hydride 2 showed the best catalytic effect.
Results and Discussion
Reaction of Fe(PMe3)4 with (Ph2P-(C6H4))2P(O)H (1)
Similar to Bourissou’s strategy,30 the diphosphine–phosphine oxide ligand 1,
[PPH(O)P], was synthesized by the hydrolysis of ligand (Ph2P-(C6H4))2PCl in the
yield of 60% (Eq. (1)).
3

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After combination of ligand 1 with Fe(PMe3)4 in tetrahydrofuran (Eq. (2)), the
color of the solution gradually changed from brown to red. Hydrido iron(II) complex
2, [PP(O)P]Fe(H)(PMe3)2, as a P-H bond activation product was isolated from the
diethyl ether extract as a red powder at 0 ℃. Complex 2 as red crystals could be
o
obtained from a mixed solution of THF and n-pentane after 3 days at - 20 C.
Complex 2 is very air-sensitive.
The stretching vibration absorption peak of the (Fe-H) bond in complex 2
1
appears at 1848 cm-1 in the IR spectrum of complex 2. In the H NMR spectrum of
complex 2, the hydrido H signal is located at -17.27 ppm as a m peak caused by the
31
multiple P-H couplings. In the P NMR spectrum of complex 2, the signals of Ph2P
groups, P(O) and two kinds of PMe3 ligands were observed at 102.6, 94.3, 21.9 and
18.9 ppm, respectively, with an integral ratio of 2 : 1 : 1 : 1.
Fig. 1. Molecular structure of complex 2. The thermal ellipsoids are displayed at the
4

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50% probability level, and most of the hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angles [°]: Fe1-P2 2.232(7), Fe1-P1 2.243(7), Fe1-P8
2.248 (7), Fe1-P10 2.251(7), Fe1-P3 2.251(7), Fe1-H1 1.50(3); P1-Fe1-P8 85.46(3)，
P2-Fe1-P10 108.26(3), P8-Fe1-P3 95.48(3), P10-Fe1-P3 95.29(3), P2-Fe1-H1
172.7(1), P1-Fe1-P10 83.47(2).
The molecular structure of complex 2 was further confirmed by single crystal
X-ray diffraction structure analysis (Figure 1). In complex 2, the Fe atom is centered
in a distorted octahedral coordination geometry. The [PPP]-pincer ligand is in a
fac-coordination mode. The H1-Fe1-P2 is located in the axial position with a bond
angle of 172.7(13)°. The axial H1-Fe1-P2 is basically perpendicular to the equatorial
plane determined by [P1, P3, P8 and P10]. Therefore, the sum of the coordination
bond angles in the equatorial plane is 359.7 °, close to 360 °. The bond length of
Fe1-H1 (1.50(3) Å) is slightly longer than a normal Fe-H bond (1.42 Å).25
Reaction of Ni(PMe3)4 with (Ph2P-(C6H4))2P(O)H (1)
Hydrido nickel(II) complex 3, [PP(O)P]Ni(H)(PMe3), was also obtained by the
reaction of ligand 1 with Ni(PMe3)4 in THF (Eq. (3)). Unlike hexa-coordinate iron(II)
hydride 2, the penta-coordinate nickel(II) complex 3 with an 18-electron configuration
is stable. The typical υ(Ni-H) stretching band was found at 1837 cm-1 in the IR
5

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1
spectrum of complex 3. In the H NMR spectrum of complex 3, a dq peak of the
hydrido H appears at -10.81 ppm with the 31P−1H coupling constants (2JH,P = 153 Hz,
2JH,P = 39 Hz). There is a doublet observed at 0.93 ppm for the PMe3 ligand. In the 31P
NMR spectrum of complex 3, three types of signals appear at 139.6, 59.4 and -10.0
ppm with a relative integral ratio of 1 : 2 : 1, corresponding to P(=O), 2PPh2 and
PMe3 groups.
Fig. 2. Molecular structure of complex 3. The thermal ellipsoids are displayed at the
50% probability level, and most of the hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angles [°]: Ni1–P4 2.152(1), Ni1–P2 2.134(1), Ni1–P3
2.171(1), Ni1–P1 2.177(1), Ni1–H50 1.53(4); P3–Ni1–H50 174.8(13), P1–Ni1–P2
122.07(2), P2–Ni1–P4 125.26(2), P1–Ni1–P4 112.57(2).
The hydrido nickel(II) complex 3 as yellow crystals suitable for X-ray diffraction
structure analysis was isolated from its diethyl ether solution. In the molecular
structure of complex 3, the nickel atom is coordinated with four P atoms and one H
atom in trigonal-bipyramidal coordination fashion (Figure 2). The axial position of
6

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P3-Ni1-H50 has a bond angle of 174.8(13) °, deviated from 180 °. The axial direction
is substantially perpendicular to the trigonal plane composed of [P1, P2, P4 and Ni1].
1
The dq coupling pattern of the hydrido ligand in the H NMR spectrum could be
2
explained by this coordination mode. The coupling constant JP3-H50 (156 Hz) is
2
2
2
significantly larger than the coupling constants JP1-H50, JP2-H50 and JP4-H50 (42 Hz)
because the former has a bond angle of 180o and the latter has a bond angle of 90o.
The Ni1–H50 bond distance in 3 is 1.53(4) Å within the normal range of the known
Ni–H bonds in the literature.25
Reaction of Co(PMe3)4 and Co(PMe3)4Me with (Ph2P-(C6H4))2P(O)H (1)
Encouraged by the successful preparation of the iron and nickel hydrides 2 and 3,
ligand 1 was treated with 1 equivalent of Co(PMe3)4 in THF at −78 °C (Eq. (4)). A
dark red solution was formed when the reaction mixture was warmed to the room
temperature. Upon cooling the solution to 0 °C over 24 hours, dark red powder was
obtained in 40% yield. Complex 4 [PP(O)P]Co(PMe3)2 is a [PPP]-pincer Co(I)
complex. In the 31P NMR spectrum of complex 4, the signals of -P(=O), -PPh2 groups
and two kinds of PMe3 ligands appear at 155.6, 68.1, 15.7 and 0.1 ppm, respectively,
1
with the integral ratio of 1 : 2 : 1 : 1. In H NMR spectrum of 4, the two doublets at
7

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0.84 and 1.58 ppm are the hydrogen signals of the two PMe3 ligands. From the above
spectroscopic information, it can be concluded that complex 4 should be a
penta-coordinate cobalt(I) complex formed through P-H bond cleavage. It is
considered that the hydrido cobalt(II) complex 4a produced via (P-H) bond activation
is an intermediate in the formation of complex 4. This conjecture has been confirmed
by our experiments. In the IR characterization of the reaction solution, there is a
strong typical υ(Co-H) stretching band signal at 1889 cm-1. This indicates that the
reaction may undergo a process with hydrido cobalt(II) intermediate 4a as an
intermediate. It is regrettable that the attempt to isolate intermediate 4a has failed
because it is not stable. In addition, 4a was also verified by in situ 1H NMR spectrum
(Supporting Information). The hydrido resonance of 4a was recorded at −15.38 ppm.
Complex 4 is formed via one-electron reductive elimination of 4a with the escape of
H2, confirmed by GC (Supporting Information).
Fig. 3. Molecular structure of complex 4. The thermal ellipsoids are displayed at the
50% probability level, and hydrogen atoms are omitted for clarity. Selected bond
lengths (Å) and angles [°]: Co1–P1 2.191(9), Co1–P3 2.193(9), Co1–P4 2.230(1),
Co1–P5 2.220(9), Co1–P6 2.222(9); P4–Co1–P5 177.15(4), P1–Co1–P3 116.83(3),
8

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P3–Co1–P6 118.81(4), P1–Co1–P6 121.28(4).
The single crystals of complex 4 suitable for structural elucidation by X-ray
diffraction analysis were obtained from a mixture of n-pentane and tetrahydrofuran at
-20 °C over one week. The cobalt atom is centered in a slightly distorted
trigonal-bipyramidal geometry (Figure 3). The axial P4-Co1-P5 (177.15(4)o) is almost
perpendicular to the trigonal plane formed by [Co1P1P3P6]. The sum of the
coordination bond angles in the trigonal plane is 356.92°, deviated from 360°. This
coordination form explains the 31P NMR of 4 very well because the chemical
environments of the two -PPh2 groups are same and the chemical environments of the
two PMe3 ligands are different.
The reaction of ligand 1 with Co(PMe3)4Me in tetrahydrofuran at room
temperature also resulted in the formation of a dark red powder. After the IR and
NMR characterization, we found that complex 4 is also the product of reaction (5).
This result indicates that the P-H bond activation of ligand 1 could also be realized by
the release of methane with the substitution of two PMe3 ligands by two Ph2P−
groups.
Catalytic activities of complexes 2, 3, 4a and 4 for hydrosilylation
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Hydrido metal complexes often act as important intermediates in many DcOaIt:a1l0y.1t0i3c9/C8NJ03919D
cycles. The catalytic systems with hydrido metal complexes have been established for
the processes of hydrogenation and hydrosilylation of unsaturated compounds.
Therefore, complexes 2, 3, 4a (1 + Co(PMe3)4) and 4 as catalysts could be used in the
hydrosilylation reduction of carbonyl compounds. With benzaldehyde as the template
substrate we screened the catalytic conditions, including catalyst loading, reaction
time, reaction temperature, silane and solvent (Table 1). The catalytic reaction did not
occur without catalyst and silane (Table1, entries 1 and 2). It was also necessary to
complete the reaction with a catalyst loading of 2 mol% complex 2 (Table1, entries 3
- 5). It must be noted that iron hydride 2 has the strongest catalytic activity in
comparison with complexes 3, 4 and 4a (Table1, entries 6 - 8). (EtO)3SiH was more
suitable for this catalytic system compared to Ph3SiH, Et3SiH and MePh2SiH (Table1,
o
entries 3, 15 - 17). When the temperature rose to 80 C, the conversion declined
sharply (Table1, entry 20). We guess that complex 2 decomposed at this temperature
because a grey precipitate appeared in the solution. Because of the solubility of
catalysts, polar solvents seemed to be more suitable for this system. Comparing the
catalytic results in different solvents, tetrahydrofuran was the best solvent among
them with the yield of 84% for the catalytic reaction (Table 1, entries 3, 21-23).
Through the screening of reaction time, it was found that the catalytic efficiency
reached the limit of reaction at 24 h (Table 1, entries 10 - 14). From Table 1 the
optimized conditions are summarized as follows: 2 mol% complex 2, 1.2 equivalents
(EtO)3SiH, 60 oC, THF as solvent and reaction time 24 h.
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Table 1. Optimization of Catalytic Conditions for Hydrosilylation of PhCHOa
Loading
T(℃
Entry Catalyst
Silane
Solvent
Time(h) Conversion(%)b
(mol%)
)
1
2
0
2
1
2
4
2
2
2
2
2
2
2
2
2
2
2
(EtO)3SiH
none
THF 60
THF 60
THF 60
THF 60
THF 60
THF 60
THF 60
THF 60
THF 60
THF 60
THF 60
THF 60
THF 60
THF 60
THF 60
THF 60
24
24
24
24
24
24
24
24
12
16
18
20
22
36
24
24
0
2
2
0
3
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
Et3SiH
56
2
4
84(81)d
87(83)d
< 5
2
5
2
6
3
7
50(42)d
54
4
8
4ac
2
9
61(55)d
67
10
11
12
13
14
15
16
2
71
2
73
2
76
2
86
2
21
2
Ph2MeSiH
<10
2
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17
18
19
20
21
22
23
2
2
2
2
2
2
2
Ph2SiH2
THF 60
THF 30
THF 50
THF 80
24
24
24
24
12
12
12
24
31
2
2
2
2
2
2
2
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
73(71)d
20
(EtO)3SiH Dioxane 60
42
(EtO)3SiH Toluene
(EtO)3SiH DMF
60
60
57
61
a PhCHO (1.0 mmol), silane (1.2 mmol) and 2 mL solvent.
b Determined by GC with n-dodecane as an internal standard.
c in situ [1 + Co(PMe3)4].
d Isolated yields.
Under the optimized conditions, the scope of the substrates was expanded to
explore the catalytic hydrosilylation reactions (Table 2). It was found that the aromatic
aldehyde substrates with the electron-withdrawing groups had higher yields than those
with electron-donating groups (Table 2, 6b - 6d and 6h). The dihalogeno aldehydes
could be also converted to the corresponding alcohols in moderate yields (Table 2, 6e
and 6f). Furthermore, even aliphatic as well as other aromatic substrates could
complete this transformation in better yields for this catalytic system (Table 2, 6i –
6k). Additionally, we found that 2 exhibited excellent selectivity for the carbonyl
reduction of α,β-unsaturated aldehydes with a longer reaction time (Table 2, 6l - 6p).
Table 2. Catalytic Hydrosilylation of Aldehydes with 2 as a Catalysta,b
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6a, 81%
6d, 74%
6b, 86%
6e, 75%
6c, 78%
6f, 73%
6g, 58%
6j, 80%
6h, 54%
6k, 78%
6n, 68% c
6i, 79%
6l, 73% c
6o, 65% c
6m, 72% c
6p, 77% c
a 5 (1.0 mmol), (EtO)3SiH (1.2 mmol) and 2 mL THF, 60 oC, 24 h.
b Isolated yield.
c 36 h.
We also investigated the catalytic performances of complex 2 for the
hydrosilylation reaction of various aromatic ketones (Table 3). Although we extended
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the reaction time (36h) and increased the catalyst loading (5 mol%), the yields were
still unsatisfactory because the ketones were not as active as the aldehydes. As for the
aldehydes (Table 2), the electron-withdrawing groups of the ketone substrates were
favorable for the hydrosilylation while the electron-donating groups of the ketone
substrates were unfavorable for the catalytic reaction. Up to date, there are only a few
reports on iron hydride as catalyst for hydrosilylation of carbonyl compounds, such as
the work by Nikonov,31 Guan32 and Darcel.33 Moreover, Guan34 and Thomas35 used
the pincer-type iron complexes with [POCOP] and [ONO] pincer ligands to achieve
the reduction of carbonyl derivatives. We have also published the synthesis of several
pincer-type hydrido iron complexes chelated with [PCP]26, [POCOP]36 and [PSiP]25
pincer-ligands, which could be used as catalysts for efficient hydrosilylation of
carbonyl compounds. In comparison with the results of these reports, complex 2 is
less active for this catalytic system. We consider the reason is that the weak
σ-donating ability of the pentavalent phosphorus atom makes the electronic density at
the Fe center lower.
Table 3. Catalytic Hydrosilylation of Ketones with 2 as a Catalysta,b
8a, 54%
8b, 60%
8c, 68%
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8d, 61%
8g, 33%
8e, 36%
8h, 35%
8f, 25%
a 7 (1.0 mmol), (EtO)3SiH (1.2 mmol) and 2 mL THF, 60 oC, 36 h.
b Isolated yield.
To gain further mechanistic insight into this efficient iron hydride catalysis, we
conducted the stoichiometric reaction of complex 2 with benzaldehyde or (EtO)3SiH
respectively. However, there were no appreciable reactions observed like the reported
mechanistms.37-39 Additionally, a stoichiometric reaction of complex 2, benzaldehyde
and (EtO)3SiH (1:1:1) was also explored. The disappearance of signal of ν(H–Fe) was
obviously observed in the infrared spectrum. Our attempts to isolate an active
intermediate have failed. The operando IR spectroscopy was utilized to track the
reaction process between p-fluorobenzaldehyde and 20 mol% of complex 2 in THF
(Figure 4). At 60 oC, the system established an equilibrium of reaction. When silane
was added at 25 min, the signal of C=O bond at 1726 cm-1 weakened rapidly and the
signal of C-O at 1155 cm-1 became stronger at the same time.
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Fig. 4. The change trend of IR absorption with time
Combined with our previous work,28,40,41 we propose a possible catalytic cycle
for this hydrosilylation reaction (Scheme 1). At first, the C=O bond of carbonyl
compound inserts into the Fe-H bond to form intermediate 2b containing a C-O bond.
This process achieves an equilibrium in a relatively short time. With the addition of
silane, 2b rapidly converts to the corresponding silicon ether via intermediate 2c to
promote the equilibrium movement, closing the catalytic cycle. Silane plays two roles
in this process: first, it acts as a hydrogen source; second, as a catalyst promoter it
combines with alkoxy group via reductive elimination to form silyl ether to promote
the shift of reaction equilibrium.
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Scheme 1. Possible mechanism of hydrido iron-catalyzed hydrosilylation reaction
Conclusion
The reactivity of (O=)PV-H bond was utilized to synthesize
diphosphine–phosphine oxide ligand 1 via P-H bond activation. The hydrido iron(II)
and nickel(II) complexes 2 and 3 were prepared by the reactions of ligand 1 with
Fe(PMe3)4 and Ni(PMe3)4 while the reaction of Co(PMe3)4 or Co(PMe3)4Me with
ligand 1 gave rise to cobalt(I) complex 4 without Co-H bond because the hydrido
cobalt(II) complex 4a is not stable. The catalytic properties of pincer metal complexes
2, 3, 4 and 4a were explored for the hydrosilylation of aldehydes and ketones. The
hydrido iron(II) complex 2 showed the best catalytic activity for hydrosilylation of
aldehydes and ketones. In addition, the ,-unsaturated aldehydes could also be
selectively reduced to the ,-unsaturated alcohols.
Experimental section
General procedures and materials
All air-sensitive reactions were carried out under anhydrous and nitrogen atmosphere
conditions by standard Schlenk technique. Solvents were dried by standard methods
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44
and freshly distilled prior use. (Ph2P(C6H4))2PCl,42 Fe(PMe3)4,43 Ni(PMe3)4,
45
Co(PMe3)4 and Co(PMe3)4Me45 were prepared by known methods. Infrared spectra
(4000-400 cm-1) were obtained from Nujol mulls between KBr disks and recorded on
1
a Bruker ALPHA FT-IR instrument. H, 13C{1H} and 31P{1H} NMR spectra were
recorded on Bruker Avance 300 MHz, 400 MHz and 600 MHz. GC was determined
with n-dodecane as an internal standard. Elemental analyses were carried out on
Elementar Vario ELIII instrument.
Synthesis of ligand 1
Ligand Ph2P(C6H4))2PCl (0.97 g, 1.64 mmol) was slowly added to a mixture of 20 ml
H2O and 40 ml THF at room temperature. After the mixture was stirred overnight, the
product was extracted with hexyl acetate and dried with Na2SO4. After removal of all
the solvents, the ligand 1 was purified by column chromatography over silica gel with
hexyl acetate. Yield: 0.56 g (60%). m.p.: 167 – 168oC. IR (Nujol mull, KBr, cm-1):
1
3050 ν(H-C); 2386 ν(P-H); 1585 ν(C=C). H NMR (300 MHz, C6D6, 298 K, ppm):
2
7.09 - 7.34 (m, 26H, Ar-H); 7.81-7.90 (m, 2H, Ar-H), 9.36 (dt, 1H, JP,H = 516 Hz,
2JP,H = 3 Hz, P-H). 31P NMR (121 MHz, C6D6, 298 K, ppm): 11.4 (t, 2JP,P = 73 Hz, 1P,
P(O)H), -18.4 (d, 2JP,P = 85 Hz, 2P, PPh2). 13C NMR (151 MHz, CDCl3, 298 K, ppm):
140.2 (dd, JP,C = 17 Hz, JP,C = 11 Hz, Carom), 138.0 (dd, JP,C = 103 Hz, JP,C = 33 Hz,
Carom), 135.8 (dd, JP,C = 120 Hz, JP,C = 9 Hz, Carom), 135.1 (d, JP,C = 10 Hz, Carom),
133.9 (m, Carom), 133.5 (dd, JP,C = 27, JP,C = 20 Hz, Carom), 131.9 (s, Carom), 129.1 (d,
JP,C = 12 Hz, Carom), 128.7 (d, JP,C = 6 Hz, Carom), 128.4 (m, Carom).
Synthesis of complex 2
Solution of ligand 1 (570 mg, 1 mmol) in 40 mL of THF was slowly added to a
solution Fe(PMe3)4 (396 mg, 1.1 mmol) in 30 mL of THF at room temperature,
forming a red reaction mixture. The reaction mixture was stirred overnight, and all
volatiles were removed in vacuo. The residue was extracted with 60 ml Et2O and 30
mL THF, and filtered. The combined organic solutions were concentrated slowly in
vacuo until small red crystals formed in ether at 0 ℃. Yield: 505 mg (65%). m.p.: 160
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o
– 161 C. C42H47FeOP5(C4H8O)2 (922.70 g/mol): calcd. C 65.08, H 6.88, O, 5.20;
found C 65.37, H 6.99, O, 5.36. IR (Nujol mull, KBr, cm-1): 3049 ν(H-C); 1848
ν(Fe-H); 1590 ν(C=C); 939 ρ(PCH3). 1H NMR (300 MHz, C6D6, 298 K, ppm): -17.27
2
2
(m, 1H, Fe-H); 0.80 (d, 9H, JP,H = 6 Hz, PCH3); 0.86 (d, 9H, JP,H =6 Hz, PCH3);
31
6.94-8.90 (m, 28H, Ar-H ). P NMR (121 MHz, C6D6, 298 K, ppm): 102.6 (m, 2P,
PPh2); 94.3 (m, 1P, OP); 21.9 (m, 1P, PMe3); 18.9 (m, 1P, PMe3). 13C NMR (151
MHz, C6D6, 298 K, ppm): 21.9 (d, 2JP,C = 30 Hz, PCH3), 25.6 (d, 2JP,C = 15 Hz, PCH3),
127.3 (Carom), 127.9 (Carom), 128.1 (Carom), 128.2 (Carom), 128.4 (Carom), 129.0 (Carom),
131.2 (Carom), 132.8 (Carom), 133.2 (Carom), 133.5 (Carom).
Synthesis of complex 3
Solution of ligand 1 (570 mg, 1 mmol) in 40 mL of THF was slowly added to a
solution Ni(PMe3)4 (400 mg, 1.1 mmol) in 30 mL of THF at -78 ℃, forming an
orange reaction mixture. The reaction mixture was stirred overnight, and all volatiles
were removed in vacuo. The residue was extracted with 60 ml Et2O and filtered. The
combined organic solutions were concentrated slowly in vacuo until small yellow
crystals formed in ether at 0 ℃. Yield: 420 mg (60%). m.p.: 152 - 153℃.
C39H38NiOP4(C4H10O) (779.40 g/mol): calcd. C 66.26, H 6.21, O, 4.10; found C
66.56, H 6.17, O, 4.35. IR (Nujol mull, KBr, cm-1): 3053 ν(H-C); 1836 ν(Ni-H); 1584
1
2
ν(C=C); 950 ρ(PCH3). H NMR (300Mz, C6D6, 298K, ppm)：-10.9 (dq, 1H, JP,H
=
2
155 HZ, JP,H = 42 Hz, Ni-H ); 0.93 (d, 9H, 2JP,H =6 Hz, PCH3 ); 6.83 - 8.83 (m, 28H,
31
Ar-H ). P NMR (121 MHz, C6D6, 298 K, ppm): 139.6 (m, 1P, OP); 59.4 (dd, 2JP,P =
121 Hz, 24 Hz, 2P, Ph2P); -10.0 (td, 2JP,P = 133 Hz, 48 Hz, 1P, PMe3). 13C NMR (151
MHz, C6D6, 298 K, ppm): 18.5 (dd, 2JP,C = 24 Hz, 2JP,C = 5 Hz, PCH3), 128.1 (Carom),
128.2 (Carom), 128.6 (Carom), 129.4 (Carom), 130.2 (Carom), 130.3 (Carom), 131.7 (Carom),
131.9 (Carom), 133.1 (Carom), 139.3 (Carom).
Synthesis of complex 4
(a) A Solution of ligand 1 (570 mg, 1 mmol) in 40 mL of THF was slowly added to a
o
solution Co(PMe3)4Me (412 mg, 1.1 mmol) in 30 mL of THF at -78 C, forming an
19

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orange reaction mixture. The color of the mixture solution changed to dark red when
warmed to room temperature. The reaction mixture was stirred overnight, and all
volatiles were removed in vacuo. The residue was extracted with 60 ml Et2O and 30
mL THF, and filtered. The combined organic solutions were concentrated slowly in
vacuo until small red crystals formed in ether at 0 oC. Yield: 470 mg (60%).
(b) A Solution of ligand 1 (570 mg, 1 mmol) in 40 mL of THF was slowly added to a
solution Co(PMe3)4 (399 mg, 1.1 mmol) in 30 mL of THF at -78 oC, forming a brown
reaction mixture. The color of the mixture solution changed to dark red when warmed
to room temperature. The reaction mixture was stirred overnight, and all volatiles
were removed in vacuo. The residue was extracted with 60 ml Et2O and 30 mL THF,
and filtered. The combined organic solutions were concentrated slowly in vacuo until
o
small red crystals formed in ether at 0 C. Yield: 312 mg (40%). m.p.: 133 - 134℃.
C42H46CoOP5 (C4H8O)2 (924.77 g/mol): calcd. C 64.93, H 6.76, O, 5.19; found C
65.31, H 6.50, O, 5.01. IR (Nujol mull, KBr, cm-1): 3049 ν(H-C); 1582 ν(C=C); 941
1
2
ρ(PCH3). H NMR (300Mz, C6D6, 298K, ppm)： 0.84 (d, JP,H = 6 Hz, 9H, PCH3);
2
31
1.58 (d, JP,H =6 Hz, 9H, PCH3); 6.86 - 8.28 (m, 28H, Ar-H). P NMR (121 MHz,
C6D6, 298 K, ppm): 155.6 (m, 1P, OP); 68.1 (m, 2P, PPh2); 15.7 (m, 1P, PMe3); 0.1
2
(m, 1P, PMe3). 13C NMR (151 MHz, C6D6, 298 K, ppm): 23.1 (d, JP,C = 21 Hz,
2
2
PCH3), 23.8 (dd, JP,C = 21 Hz, JP,C = 6.0 Hz, PCH3), 127.1 (Carom), 127.9 (Carom),
128.2 (Carom), 128.2 (Carom),128.6 (Carom), 130.5 (Carom), 130.6 (Carom), 132.1 (Carom),
133.1 (Carom), 140.0 (Carom).
General Procedure for the Catalytic Hydrosilylation
Under a nitrogen atmosphere, 1 mmol aldehyde or ketone substrate was weighed into
a 20 mL Schlenk tube containing a magnetic stirring. Later, 1.2 mmol (EtO)3SiH and
2 mmol% iron hydride 2 were added to the tube. The system was then heated at 60 ℃
for 24 h. The conversion was determined by GC with n-dodecane as an internal
standard. After cooling to room tempurature, the mixture was quenched with MeOH
(2 mL) and a 10% aqueous solution of NaOH (2 mL) with vigorous stirring at 50 ℃
for about 24 h. The product alcohol was extracted with 60 mL diethyl ether for three
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times and dried over Na2SO4. After filtration, volatile materials were evaporated in
vacuo. The crude product was purified by column chromatography over silica gel with
a mixture of petroleum ether and ethyl acetate (10 : 1) as eluent to provide the
product.
X-ray Structure Determination
Intensity data were collected on a Bruker APEX-II (2, 3) or Rigaku R-AXIS SPIDER
IP (4) diffractometer. Using Olex2,46 the structure was solved with the ShelXS47
structure solution program using Direct Methods and refined with the ShelXL48
refinement package using Least Squares minimization. Hydrogen atoms were
included in calculated positions and were refined using a riding model. A summary of
crystal data, data collection parameters, and structure refinement details is given in the
Supporting Information. CCDC-1557137 (2), CCDC-1400375 (3) and
CCDC-1561097 (4) contain supplementary crystallographic data for this paper.
Copies of the data can be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK (fax: (+44)1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk).
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the NSF China (no. 21572119 and 21372143).
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TOC
The catalytic performance of [PPP]-pincer hydrido Fe, Ni and Co complexes for the
hydrosilylation of aldehydes and ketones was explored.
24