Phosphane-pyridine iron complexes: Synthesis, characterization and application in reductive amination through the hydrosilylation reaction

Article abstract of DOI:10.1002/ejic.201200550

A series of 6 cyclopentadienyl phosphanyl-pyridine piano-stool iron complexes was prepared in good yields, characterized, and studied in the catalytic reductive amination of benzaldehyde derivatives through hydrosilylation reactions by using polymethylhydrosiloxane as the hydrosilane source and in dimethylcarbonate as the solvent at 40 °C. Single-crystal X-ray structural analyses were performed for all the complexes. A series of 6 cyclopentadienyl phosphanyl-pyridine piano-stool iron complexes was prepared, characterized, and studied in the catalytic reductive amination of benzaldehyde derivatives in dimethylcarbonate through hydrosilylation reactions by using polymethylhydrosiloxane as the hydrosilane source. Copyright

Full text of DOI:10.1002/ejic.201200550

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SHORT COMMUNICATION
DOI: 10.1002/ejic.201200550
Phosphane-Pyridine Iron Complexes: Synthesis, Characterization and
Application in Reductive Amination through the Hydrosilylation Reaction
Hassen Jaafar,[a] Haoquan Li,[a] Luis C. Misal Castro,[a] Jianxia Zheng,[a]
Thierry Roisnel,[b] Vincent Dorcet,[b] Jean-Baptiste Sortais,*[a] and Christophe Darcel*[a]
Keywords: Iron / Homogeneous catalysis / Hydrosilylation / Amination
A series of 6 cyclopentadienyl phosphanyl-pyridine piano-
stool iron complexes was prepared in good yields, charac-
terized, and studied in the catalytic reductive amination of
benzaldehyde derivatives through hydrosilylation reactions
by using polymethylhydrosiloxane as the hydrosilane source
and in dimethylcarbonate as the solvent at 40 °C. Single-
crystal X-ray structural analyses were performed for all the
complexes.
gen transfer,[6] or hydrosilylation[7] of the in situ formed
imines is a simple way in molecular synthesis to obtain
amines. Up to now, iron catalysts were used only in scarce
examples to perform such a reaction.[6k,8]
Introduction
The current interest in the design of mixed donor phos-
phane–pyridine complexes is considerable because these li-
gands have a potential importance in catalysis and coordi-
nation chemistry. Because of the soft and hard nature of
the phosphorus and nitrogen donor atoms, respectively,
transition metal complexes bearing the phosphane-pyridine
ligand have been extensively studied during the past dec-
ades.[1,2] One of the less-explored areas of such phosphanyl-
pyridine coordination chemistry is the reactivity of iron
complexes.[3] Meanwhile, the recent and special significance
for homogeneous catalysis for the use of cheap and environ-
mentally benign metals such as iron is becoming a highly
attractive and challenging area of research.[4] In that regard,
the investigation of the synthesis and reactivity of new mo-
lecular-defined phosphane-pyridine iron complexes can po-
tentially provide guidelines for the rational design of ef-
ficient iron-based catalysts.
On the basis of these results and our background in iron-
catalyzed reactions,[9] and especially in hydrosilylation of
C=O[10] and C=N bonds,[11] we pursue our efforts to de-
velop environmentally friendly, molecular defined iron sys-
tems. We have recently reported the efficiency of a series of
[CpFe(CO)2(PR3)][X] (X = I, BF4, PF6) complexes to cata-
lyze the hydrosilylation of aldehydes and ketones[10e] and
esters.[12] We report herein the comparative study of pen-
dant and bidentate cyclopentadienyl iron phosphanyl
methyl-pyridine complexes 1–6 for reductive amination of
aldehydes under hydrosilylation conditions.
Results and Discussion
According to the established methodology described for
the preparation of the monophosphane [CpFe(CO)2(PR3)]-
[X],[10e,13] the complexes [CpFe(CO)2(Ph2PCH2Pyr)][X] (1:
X = PF6) and (2: X = BF4) were obtained in one step by
reaction of 1 equiv. of the (diphenylphosphanyl)methyl-pyr-
idine with 1 equiv. of the precursor [CpFe(CO)2(THF)][X]
in CH2Cl2 at room temperature in the dark for 30 min
(Scheme 1).
On another hand, direct reductive amination of carbonyl
compounds with amines is an elegant and efficient method
for the preparation of substituted amines. It is usually car-
ried out by using borohydride reducing reagents.[5] In this
area, the catalytic reductive amination reaction of aldehydes
through transition-metal-catalyzed hydrogenation, hydro-
[a] UMR 6226 CNRS – Université de Rennes 1 “Institut des
Sciences Chimiques de Rennes” “Organometallics: Materials
and Catalysis” – Campus de Beaulieu,
Bat 10C, 263 avenue du Général Leclerc 35042 Rennes Cedex,
France
E-mail: jean-baptiste.sortais@univ-rennes1.fr
christophe.darcel@univ-rennes1.fr
[b] UMR 6226 CNRS – Université de Rennes 1 “Institut des br/
>Sciences Chimiques de Rennes” “Centre de Diffractométrie X
” – Campus de Beaulieu,
Bat 10B, 263 avenue du Général Leclerc 35042 Rennes Cedex,
France
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201200550.
Scheme 1. Preparation of the Cp-(P-pyr) iron complexes 1,2, 4, and
5.
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J.-B. Sortais, C. Darcel et al.
SHORT COMMUNICATION
The corresponding complexes 1 and 2 were obtained in the range of those for complexes bearing a monodentate
good isolated yields (80 and 68%, respectively) after phosphane (2.21–2.26 Å).[10e] The IR spectra of both com-
recrystallization. Single crystals of the new complexes 1 and plexes 1 and 2 show the CO stretching bands at 2015, 2056
2 were obtained, and their molecular structures were con- and 2015, 2058 cm–1, respectively.
firmed by X-ray crystallography (Figure 1, Table 1, and
Supporting Information).
When complexes 1 and 2 were stirred under visible light
irradiation in dry and degassed dichloromethane at room
temperature for 7 h, the corresponding P,N-bidentate
[CpFe(CO) (Ph2PCH2Pyr)][X] (4: X = PF6) and (5: X =
BF4) complexes were obtained quantitatively. When the
pyridine arm is coordinated to the metal center, the iron is
in a pseudo-tetrahedral geometry, with four different sub-
stituents and becomes a stereogenic center:[14] characteristic
diastereotopic protons of the methylene bridge appear in
1H NMR spectra as two broad signals, (e.g. for 4 at δ =
3.84 and 4.53 ppm). Furthermore, the two phenyl rings on
the phosphorus atom are also diastereotopic, which lead to
anisochronous 13C signals.
Figure 1. Ortep view of complex 2, drawn at the 50% probability
level. Hydrogen atoms and BF4 are omitted for clarity.
The iodo analogous compound 3 was prepared by a one-
pot procedure by reaction of [CpFe(CO)2(I)] with 1 equiv.
of (diphenylphosphanyl)methyl-2-pyridine in toluene
heated overnight at reflux in a 45% isolated yield. Single
crystals of the three new P,N-bidentate iron complexes 3–5
were obtained, and their molecular structures were con-
firmed by X-ray crystallography (Figure 2, Table 1, and
Supporting Information). In the molecular structures of 3–
5, typically four-legged piano-stool geometries were ob-
served, and no significant structural difference was ob-
served between the complexes. The bond lengths are fairly
constant and have the following mean values [range in
bracket] (Å): Fe–P 2.216 [2.2137–2.2192], Fe–CO 1.772
[1.767–1.780], and Fe–N 1.998 [1.992–2.004]. The N–Fe–P
bond angles are also constant 82.50° [82.47–82.52]; such
small values are the result of the geometric demands im-
parted by the five-membered chelate rings. The complexes
display a CO stretching band at 1980–1982 cm–1 in their IR
spectra. These lower ν(CO) frequencies translate the anti-
stretching effect induced by P–Fe–N coordination bonds.
Table 1. Selected bond lengths [Å] and angles [°] for complexes 1–
6.
Complex 1
Fe–P
2.2336(6)
1.721
1.784(2)
1.778(2)
P–Fe–CO
P–Fe–CO
OC–Fe–CO
92.02(7)
94.85(7)
94.80(10)
Fe–Cp
Fe–CO
Fe–CO
Complex 2
Fe–P
2.2327(6)
1.715
1.781(2)
1.780 (2)
P–Fe–CO
P–Fe–CO
OC–Fe–CO
89.21(7)
97.91(8)
92.93(10)
Fe–Cp
Fe–CO
Fe–CO
Complex 3
Fe–P
2.2151(7)
1.716
1.769(3)
1.992(2)
P–Fe–CO
P–Fe–N
N–Fe–CO
94.06(9)
82.52(6)
92.14(11)
Fe–Cp
Fe–CO
Fe–N
Complex 4
Fe–P
2.2137(5)
1.714
1.7674(19)
1.9987(15)
P–Fe–CO
P–Fe–N
N–Fe–CO
94.50(6)
82.47(4)
92.57(7)
Fe–Cp
Fe–CO
Fe–N
Complex 5
Fe–P
2.2192(9)
1.718
1.780(3)
2.004(3)
P–Fe–CO
P–Fe–N
N–Fe–CO
94.04(11)
82.50(8)
92.98(13)
Fe–Cp
Fe–CO
Fe–N
Complex 6
Fe–P
2.2100(8)
1.719
1.773(3)
2.6337(4)
P–Fe–CO
P–Fe–I
P–Fe–CO
94.01(9)
96.08(2)
94.01(9)
Figure 2. Ortep view of complex 5, drawn at the 50% probability
Fe–Cp
Fe–CO
Fe–I
level. Hydrogen atoms and BF4 are omitted for clarity.
Interestingly, when the same reaction was performed
with 1 equiv. of the ligand (diphenylphosphanyl)methyl-2-
The molecular structures of 1 and 2 closely resemble quinoline in toluene heated overnight at reflux, the corre-
those of the analogous monophosphane Fe complexes sponding P,N-bidentate iron complex was not obtained.
[CpFe(CO)2(PR3)][X] reported previously.[10e] Typically, Whereas, when the reaction was performed under visible
four-legged piano-stool geometries were observed, and no light irradiation in dry and degassed toluene overnight at
significant structural difference was observed between both room temperature, the corresponding [CpFe(CO)2-
complexes: for example, the Fe–P distance of 2.23 Å is in (Ph2PCH2Quinoline)][I] complex 6 was obtained in an 87%
2
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Phosphane -Pyridine Iron Complexes
isolated yield. After recrystallization by diffusion of pent- PF6), similar results were obtained and conversions of up
ane into a solution of 6 in dichloromethane, single crystals to 79% were observed (Table 2, Entries 4 and 5). Interest-
of the new complex 6 suitable for X-ray diffraction studies ingly, by varying the nature of the counterion and by using
were obtained, and their molecular structures were con- the tetrafluoroborate complex 5, the conversion was im-
firmed by X-ray crystallography (Figure 3 and Table 1).
proved to 94% (Table 2, Entry 6). Again, the beneficial ef-
fect of light irradiation was observed because in the dark,
the conversion decreased to 72% (Table 2, Entry 7). Using
the neutral phosphorus-ligated [CpFe(CO)(I)(Ph2P–CH2–
Quin)] complex 6, a conversion of 83% was obtained
(Table 2, Entry 8).
Table 2. Optimization for the reductive amination of benzaldehyde
under hydrosilylation conditions with catalysts 1–9.[a]
Entry
Catalyst
(mol-%)
Silane
(equiv.)
Solvent
Time Conv.
[h]
[%][b]
Figure 3. Ortep view of complex 6, drawn at the 50% probability
1
1 (5)
2 (5)
2 (5)
3 (5)
4 (5)
5 (5)
5 (5)
6 (5)
7 (5)
8 (5)
9 (5)
5 (5)
5 (5)
5 (5)
5 (5)
5 (5)
5 (5)
5 (5)
PhSiH3 (1)
PhSiH3 (1)
PhSiH3 (1)
PhSiH3 (1)
PhSiH3 (1)
PhSiH3 (1)
PhSiH3 (1)
PhSiH3 (1)
PhSiH3 (1)
PhSiH3 (1)
PhSiH3 (1)
PMHS (4)
PMHS (4)
PMHS (4)
PMHS (4)
PMHS (4)
PMHS (4)
PMHS (4)
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
EtOAc
CPME
2-Me-THF 24
DMC
DMC
DMC
CH2Cl2
1
1
1
1
1
1
1
1
1
1
1
24
24
24
51
68
56
50
79
94
72
83
35
41
58
95
68
50
55
80
91
74
68
level. Hydrogen atoms are omitted for clarity.
2
3[c]
4
Preliminary studies of the catalytic activity of the novel
iron complexes 1–6 and their monodentate analogs 7–9
were performed to explore their potential in the reductive
amination of aldehydes under hydrosilylation conditions
(Scheme 2).
5
6
7[c]
8
9
10
11
12
13
14
15
16
17
18[c]
19
24
24[d]
24[d]
24
FeCl2 (5) PMHS (4)
[a] Typical conditions: benzaldehyde (0.5 mmol), amine (0.5 mmol),
complex 1–9 (0.025 mmol, 5 mol-%), silane (1–4 equiv.), in 1 mL
of solvent at 30 °C, under visible light irradiation. [b] Conversion
determined by GC. [c] In the absence of light irradiation. [d] 40 °C,
24 h, dimethylcarbonate (DMC), cyclopentylmethylether (CPME).
Scheme 2. Reductive amination of benzaldehyde under hydro-
silylation conditions.
The screening of the prepared iron complexes 1–9 was
performed on the reaction of benzaldehyde (0.5 mmol) with
Encouraged by the good conversion obtained, the amin-
di(n-propyl)amine (0.5 mmol, 1 equiv.) in dichloromethane ation of benzaldehyde was studied by using cheap PMHS
in the presence of 5 mol-% of the complex and 1 equiv. of (polymethylhydrosiloxane) as the reducing agent. When
phenylsilane as the reducing agent at 30 °C under visible performing the reaction at 30 °C in dichloromethane for
light activation for 1 h.
24 h, a full conversion was observed (95%) (Table 2, Entry
As illustrated in Table 2, by using the phosphane ligated 12). The nature of the solvent was also studied. By replac-
complexes [CpFe(CO)2(Ph2P–CH2–Pyr)][X] (1: X = PF6) ing dichloromethane by organic solvents such as EtOAc, 2-
and (2: X = BF4), moderate conversions were obtained, 51 methyltetrahydrofuran (2-Me-THF), or cyclopentylmethyl-
and 68%, respectively (Table 2, Entries 1 and 2). It must be ether (CPME), the reaction slowed down and reduced con-
underlined that without light irradiation, a slight decrease versions were observed (68, 55, and 50%, respectively)
in the conversion was observed (56% vs. 68%, Entries 3 vs. (Table 2, Entries 13–15). Interestingly, the use of a greener
2), which shows that light has a beneficial and notable effect solvent such as dimethylcarbonate[15] (DMC) permitted an
on the catalytic reduction. Notably, when performing the encouraging conversion at 30 °C for 24 h to be obtained
reaction with complexes [CpFe(CO)2(PR3)][BF4] (7–9) (80%, Table 2, Entry 16). By performing the reaction at
bearing simple phosphanes such as PCy3, PPh3, or 40 °C, a conversion of 91% was reached under light irradia-
PPhMe2, similar level of conversions were observed (35– tion and 74% in the absence of light (Table 2, Entries 17
58%) (Table 2, Entries 9–11), which shows that these cat- and 18). Notably, when iron salts such as FeCl2 were used,
ionic bis(carbonyl) iron complexes are modest catalysts for a moderate conversion of 68% was obtained after 24 h at
such transformations. When the reaction was performed 30 °C, which shows the beneficial effect of the prepared
with
bidentate
diphenylphosphanyl-methyl-pyridine complexes on the reactivity of the reductive amination
[CpFe(CO)(Ph2P–CH2–Pyr)][X] (3: X = I) and (4: X = (Table 2, Entry 19).
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J.-B. Sortais, C. Darcel et al.
SHORT COMMUNICATION
The optimized conditions [aldehyde/amine/PMHS (1:1:4) respectively (Table 3, Entries 6 and 7). Interestingly, with
in the presence of 5 mol-% of complex 5 in 1 mL of dimeth- electron-withdrawing para substituents, slightly lower con-
ylcarbonate at 40 °C for 24–48 h under visible light irradia- versions were observed (71–84%), and the corresponding
tion] were used to study the scope of this reaction tertiary amines were produced in moderate yields (53–72%)
(Scheme 3). Starting from benzaldehyde, several secondary (Table 3, Entries 8–12).
amines, such as di-n-propyl-, allyl-, and benzylamine were
Of main interest is the chemoselectivity of this reductive
tested, and the corresponding tertiary amines were obtained amination as no dehalogenation by-product was obtained
with good isolated yields (73–77%) (Table 3, Entries 1–3). (Table 3, Entries 8 and 9), and the reaction tolerated func-
Notably, more hindered secondary amines such as diiso- tional groups such as cyano, diethylamino, ester, and ketone
propylamine did not lead to a full reaction, as only 19% (Table 3, Entries 10–13). By contrast, when performing the
conversion was observed (Table 3, Entry 4). Similarly, with transformation with p-hydroxybenzaldehyde, no reaction
a less reactive secondary amine such as diphenylamine, only occurred, which shows the inhibiting effect of the hydroxy
a low conversion was obtained (Table 3, Entry 5). With group (Table 3, Entry 14).
these limitations in hand, we decided to investigate the in-
fluence of the nature of the aldehydes on the reaction. With
Conclusions
classical electron-donating groups such as methyl or meth-
oxy in the para position of benzaldehyde, the corresponding
amines were obtained in good isolated yields, 85 and 80%,
We have developed a range of piano-stool iron complexes
imparting heterodifunctional phosphanes such as 2-(2-di-
phenylphosphanylmethyl)pyridine. From spectral and struc-
tural studies, it has been established that the coordinated
Ph2P–CH2–Pyr acts as both a unidentate and chelating bi-
dentate ligand. Furthermore, it has been shown that P–N
bidentate iron complexes catalyze the reductive amination
of aromatic aldehydes through a hydrosilylation process. It
must be noted that they are more powerful than the mono-
dentate complexes in such a reaction. It must be pointed
out that the reactions were performed (i) under mild condi-
tions (30–40 °C) and (ii) by using dimethylcarbonate as a
green solvent and PMHS (polymethoxyhydrosilane) as a
cheap silane source.
Scheme 3. Reductive amination of benzaldehydes under hydro-
silylation conditions.
Table 3. Scope of iron-catalyzed reductive amination of aldehydes
with complex 5.[a]
Experimental Section
Typical Procedure for Cationic P,N-Bidentate Complex 5 Synthesis:
In a Schlenk tube, [CpFe(CO)2(THF)]BF4 (402 mg, 1.2 mmol) and
(PPh2CH2Pyr) (334 mg, 1.2 mmol) were added under argon into
dried and degassed CH2Cl2 (20 mL). The Schlenk tube was
wrapped with aluminum foil, and the solution was stirred for
30 min at room temperature. After the solution was concentrated,
the final complex precipitated in excess diethyl ether, which led to
the formation of a beige yellow powder (440 mg, 68% yield). The
obtained complex 2 (308 mg, 0.6 mmol) was then added under ar-
gon to dried and degassed CH2Cl2 (20 mL). The solution was
stirred at room temperature for 7 h under light irradiation. The
color of the reaction solution turned to orange. After the solution
was concentrated in vacuo, the complex precipitated in diethyl
ether. The orange powder was further washed with diethyl ether
(3ϫ 20 mL), dried, and stored under argon (298 mg, 97% yield).
Typical Procedure for Iron-Catalyzed Reductive Amination of Benz-
aldehyde Derivatives: A 10-mL oven-dried Schlenk tube containing
a stirring bar was charged with iron complex 5 (0.025 mmol) and
dimethyl carbonate (1 mL). After purging with argon (argon-vac-
uum three cycles), the aldehyde (0.5 mmol) and amine (0.5 mmol)
were added followed by PMHS (2.0 mmol). The reaction mixture
was stirred in a preheated oil bath at 40 °C for 24 h under light
irradiation (24 W compact fluorescent light bulb). MeOH (1 mL)
was then added followed by NaOH (1 mL of a 2 m aqueous solu-
tion) with vigorous stirring. The reaction mixture was further
stirred for 1 h at room temperature and was extracted with diethyl
[a] Typical conditions: aldehyde (0.5 mmol), amine (0.5 mmol),
complex 5 (0.025 mmol, 5 mol-%), PMHS (2 mmol, 4 equiv.), in
1 mL of dimethylcarbonate (DMC) at 40 °C, 24 h, under visible
light irradiation. [b] Conversion determined by 1H NMR spec-
troscopy. [c] Isolated yield after purification by column chromatog-
raphy. [d] 48 h.
4
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Phosphane -Pyridine Iron Complexes
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Table 3 by H NMR spectroscopy. The product was then purified
by silica gel column chromatography by using pentane/ethyl acetate
mixture (0 to 10%) to achieve the desired product.
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Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures, descriptions, characterization and
1
crystallographic data, and H and 13C NMR spectra of complexes
1–6 and the obtained amines are presented.
Acknowledgments
We are grateful to the Université de Rennes 1, Centre National de
la Recherche Scientifique (CNRS), Rennes Métropole and Min-
istère de lЈEnseignement Supérieur et de la Recherche for support,
the “Région Bretagne” for a grant “CREATE” to H. J., Rennes 1
Foundation for a grant to H. L., the Ministère des Affaires
Etrangères and the Fundación Gran Mariscal de Ayacucho for a
grant to L. C. M. C., and Axa Research Fund for a grant to J. Z.
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Received: May 22, 2012
Published Online: ᭿
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Job/Unit: I20550
/KAP1
Date: 21-06-12 15:46:36
Pages: 6
J.-B. Sortais, C. Darcel et al.
SHORT COMMUNICATION
Homogeneous Iron Catalysis
A series of 6 cyclopentadienyl phosphanyl-
pyridine piano-stool iron complexes was
prepared, characterized, and studied in the
catalytic reductive amination of benzal-
dehyde derivatives in dimethylcarbonate
through hydrosilylation reactions by using
polymethylhydrosiloxane as the hydrosilane
source.
H. Jaafar, H. Li, L. C. Misal Castro,
J. Zheng, T. Roisnel, V. Dorcet,
J.-B. Sortais,* C. Darcel* ................. 1–6
Phosphane-Pyridine Iron Complexes: Syn-
thesis, Characterization and Application in
Reductive Amination through the Hydro-
silylation Reaction
Keywords: Iron / Homogeneous catalysis /
Hydrosilylation / Amination
6
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