Cationic iron(II) complexes of the mixed cyclopentadienyl (Cp) and the N-heterocyclic carbene (NHC) ligands as effective precatalysts for the hydrosilylation of carbonyl compounds

Article abstract of DOI:10.1016/j.jorganchem.2014.03.016

A series of iron(II) complexes of N-heterocyclic carbene ligands was synthesized and fully structurally characterized. Specifically, the benzimidazole based {Cp[1,3-di-R-benzimidazol-2-ylidene]-Fe(CO)₂}I [R = Et (1b), i-Pr (2b) and n-Bu (3b)] a

Full text of DOI:10.1016/j.jorganchem.2014.03.016

Accepted Manuscript
Cationic Iron(II) Complexes of the Mixed Cyclopentadienyl (Cp) and the N-
heterocyclic Carbene (NHC) Ligands as Effective Precatalysts for the Hydrosilylation
of Carbonyl Compounds
Dharmendra Kumar, A.P. Prakasham, Linus Paulin Bheeter, Jean-Baptiste Sortais,
Manoj Gangwar, Thierry Roisnel, Alok Kalita, Christophe Darcel, Prasenjit Ghosh
PII:
S0022-328X(14)00139-9
10.1016/j.jorganchem.2014.03.016
JOM 18521
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 13 January 2014
Revised Date: 7 March 2014
Accepted Date: 19 March 2014
Please cite this article as: D. Kumar, A.P. Prakasham, L.P. Bheeter, J.-B. Sortais, M. Gangwar, T.
Roisnel, A. Kalita, C. Darcel, P. Ghosh, Cationic Iron(II) Complexes of the Mixed Cyclopentadienyl
(Cp) and the N-heterocyclic Carbene (NHC) Ligands as Effective Precatalysts for the
Hydrosilylation of Carbonyl Compounds, Journal of Organometallic Chemistry (2014), doi: 10.1016/
j.jorganchem.2014.03.016.
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ACCEPTED MANUSCRIPT
Cationic Iron(II) Complexes of the Mixed Cyclopentadienyl (Cp) and the N-
heterocyclic Carbene (NHC) Ligands as Effective Precatalysts for the
Hydrosilylation of Carbonyl Compounds
a
a
b
b
Dharmendra Kumar, A. P. Prakasham, Linus Paulin Bheeter, Jean-Baptiste Sortais, Manoj
a
c
a
b
a
Gangwar, Thierry Roisnel, Alok Kalita, Christophe Darcel* and Prasenjit Ghosh*
a
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai, 400 076, India;
b
UMR 6226 CNRS-Université de Rennes 1 “Institut des Sciences Chimiques de Rennes”, Team “Organometallics: Materials and Catalysis” - Centre
for Catalysis and Green Chemistry, Campus de Beaulieu, 35042, Rennes, France
c
UMR 6226 CNRS-Université de Rennes 1 “Institut des Sciences Chimiques de Rennes”, Team “Centre de diffraction X” - Campus de Beaulieu,
3
5042, Rennes, France
*
Corresponding authors: P. Ghosh (Fax+91 22 2572 3480; E-mail: pghosh@chem.iitb.ac.in) and C. Darcel (Fax+33 2 23 23 69 39; E-mail:
christophe.darcel@univ-rennes1.fr)
Keywords: Iron / NHC ligands / benzimidazole / Imidazole / hydrosilylation
Abstract
A series of iron(II) complexes of N-heterocyclic carbene ligands were synthesized and fully structurally characterized. Specifically, the
benzimidazole based {Cp[1,3-di-R-benzimidazol-2-ylidene]-Fe(CO) }I [R = Et (1b), i-Pr (2b) and n-Bu (3b)] and the imidazole based
2
{
Cp[1-benzyl-3-R-imidazol-2-ylidene]Fe(CO) }PF [R = Me (4b) and Et (5b)] type of complexes were synthesized from their respective
2 6
benzimidazolium iodide (1−3)a and their imidazolium hexafluorophosphate (4−5)a salts by the reaction with CpFe(CO) I in the presence
2
of MN(SiMe ) (M = Li or K) as a base. The molecular structures of the (1−5)b complexes reveal that the metal center display a
3
2
conventional piano stool structure. More importantly, the (1−5)b complexes, when irradiated with visible light, effectively catalyzed the
hydrosilylation reaction of carbonyl compounds namely, of the aldehyde and ketone substrates, using organosilane reagents. Specifically,
the (1−5)b complexes performed the hydrosilylation of a representative benzaldehyde substrate using phenylsilane in ambient conditions
at 30 °C while that of the representative acetophenone substrate at a more elevated temperature of 70 °C. The benzimidazole derived
complexes (1−3)b displayed superior activity than the imidazole derived (4−5)b complexes.
1. Introduction
[
1]
Iron, being the second most abundant metal and also the fourth most abundant element on the earth's crust, the need for its gainful
[
2]
utilization in various commercial and non-commercial application purposes is easily discernible. The metal, for its non-toxic nature,
ready availability and economic viability, arises enormous interest in the academic and industrial worlds alike as potential catalysts for
many useful transformations of contemporary organic synthesis. Consequently, in the recent years, a rich chemistry of iron catalysis has
[
3]
[4]
[5]
[6]
been developed in C−H bond activation reactions, reductions, in selective carboxylic acid derivatives hydrosilylation, in C−C and
[
7]
[8]
[9]
[10]
[11]
C−O cross-coupling reactions, olefin isomerization reactions and etc. including a few in the asymmetric synthesis.
[
12]
[13,14]
Owing to our long standing interest in the catalytic applications of iron along with that of the N-heterocyclic carbenes (NHC),
we hypothesized that the iron complexes of the much famed and also catalytically relevant N-heterocyclic carbene ligands would be of
[
15,16]
considerable interest.
In light of the fact that a few cyclopentadienyl ligand derived iron complexes of the N-heterocyclic carbene
1

ACCEPTED MANUSCRIPT
[
17]
ligands have indeed been catalytically active for the borylation of heteroarenes,
the hydrosilylation and the hydrogen transfer
[
18]
reactions,
heterocyclic carbene (NHC) ligands, would thus be worth examining further for their catalytic potentials. More interestingly so, we
we rationalized the related type {Cp(NHC)Fe(CO) }X (X = anion), bearing both cyclopentadienyl (Cp) and the N-
2
[
19]
observed that of the limited examples of the {Cp(NHC)Fe(CO) }X (X = anion) type of iron complexes that exist in the literature, only a
2
[
20]
handful for them have been structurally characterized, and because of which we became interested in developing the chemistry of these
type of complexes. In this regard it is worth mentioning that for the hydrosilylation reaction as catalyzed by the molecularly defined
[
21]
[22]
[23]
Cp−NHC iron complexes, the reduction of aldehydes, ketones, imines and amides were performed using organosilane reagents
like the phenylsilane and diphenylsilane.
Based on these findings, which suggest that the heteroleptic iron complexes of the mixed NHC and the Cp ligands form efficient
catalysts for the hydrosilylation reaction, herein, we report a series of iron complexes of the type {Cp(NHC)Fe(CO) }X (X = I, PF ) that
2
6
are supported over benzimidazole (1−3)b and imidazole (4−5)b derived unsymmetrical N-heterocyclic carbene ligands (Figure 1) for
application in the catalysis of the hydrosilylation reaction.
2. Results and Discussion
2
.1. Synthesis and characterization of the iron-NHC complexes
A series of new iron complexes of two different families of the N-heterocyclic carbene ligands were synthesized (Figure 1).
Figure 1. Prepared iron complexes (1-5)b.
In particular, the benzimidazole derived N-heterocyclic carbene complexes, {Cp[1,3-di-R-benzimidazol-2-ylidene]Fe(CO) }I [R = Et
2
(
1b), i-Pr (2b) and n-Bu (3b)], were synthesized in moderate yields from the respective benzimidazolium iodide (1−3)a salts, which were
deprotonated at 0 °C in the presence of LiN(SiMe ) as a base, by the reaction with CpFe(CO) I at room temperature for 16 hours
3
2
2
(Scheme 1).
Scheme 1. Synthesis of the complexes (1-3)b
The unsymmetrical imidazole based N-heterocyclic carbene complexes, {Cp[1-benzyl-3-R-imidazol-2-ylidene]Fe(CO) }PF [R = Me
2
6
(
4b) and Et (5b)], were similarly synthesized from the corresponding imidazolium hexafluorophosphate (4−5)a salts, which were
deprotonated at -20 °C in the presence of KN(SiMe ) , by the reaction with CpFe(CO) I in the presence of 1.1 equiv. of AgPF at room
3
2
2
6
temperature for 10 hours (Scheme 2).
2

ACCEPTED MANUSCRIPT
Scheme 2. Synthesis of the complexes (4-5)b
All of the (1−5)b complexes are diamagnetic and hence have been appropriately characterized by various NMR spectroscopies. For
1
example, the characteristic cyclopentadienyl (Cp) resonance appeared as a singlet in the range 5.39−5.62 ppm in H NMR and in the range
13
8
7.3−88.1 ppm in the C NMR spectrum. As expected, the infrared spectrum of the (1−5)b complexes exhibited two carbonyl stretching
-
1
-1
frequencies (νCO) at 2037−2039 cm and 1985−1993 cm and is consistent with the presence of two CO ligands attached to the iron center,
[
18-19]
which are in the range of the νCO of the previous described NHC cyclopentadienyl complexes.
These carbonyl moieties appeared at its
13
1
characteristic highly downfield region 210.7−211.1 ppm in the C{ H} NMR spectrum. Furthermore, in concurrence with the values
reported in the literature, the characteristic carbene resonance of the NHC ligand for benzimidazole derived (1-3)b complexes appeared in
13
1
[18a]
the range 178.7−180.5 ppm in the C{ H} NMR
while that for the imidazole based (4-5)b complexes appeared at 165.1 and 164.4
[
18b,19b]
-
ppm respectively.
Additionally, the presence of PF6 counter anion in the (4−5)b complexes were confirmed by the observation of a
31
19
1
septet at ca. −144.3 ppm in P NMR and a corresponding doublet at ca. −72.6 ppm in F NMR exhibiting a J coupling of ca 711 Hz.
FP
The molecular structures of all of the (1−5)b complexes as determined by the X-ray single crystal diffraction studies reveal that the iron
center adopt a conventional “piano stool” geometry with the cyclopentadienyl (Cp) moiety representing the seat of the stool and the N-
heterocyclic carbene ligands along with two CO ligands forming its legs (Figures 2−3 and Tables 1−2). Quite interestingly, in the (1−5)b
complexes, the Cp ligand was found to be located relatively more close to the iron center than its other ligands, as observed in a shorter
Cp(centroid)•••Fe distance [1.7200(3)−1.7331(7) Å] in comparison to the Fe−NHC bond [1.965(5)−1.9777(4) Å] and the other two Fe−CO
-
bond [1.765(5)−1.783(5) Å and 1.770(5)−1.780(5) Å] distances. The iodide and the PF6 counter anions were seen to be non-coordinating
in the (1−3)b and (4−5)b complexes respectively.
O1
C17
O2
C20
Fe1
N1
Fe1
I1
N2
C1
C19
C18
C1
I1
N2
O1
N1
O2
1
b
2b
3b
o
Figure 2. ORTEP views of the complexes 1b, 2b and 3b. For 1b: Selected bond lengths (Å) and angles ( ): Fe1-C18 1.765(5), Fe1-C17 1.780(5), Fe1-C1
1
.972(5), O1-C17 1.143(5), O2-C18 1.165(5), N1-C1 1.375(5), N2-C1 1.363(5), C18-Fe1-C17 91.0(2), C18-Fe1-C1 96.28(17), C17-Fe1-C1 95.90(19), N2-
o
C1-Fe1 127.4(3), N1-C1-Fe1 126.4(3), N2-C1-N1 105.8(3). For 2b: Selected bond lengths (Å) and angles ( ): Fe1-C1 1.972(4), Fe1-C19 1.780(4), Fe1-C20
1
.770(4), C1-N2 1.371(4), C1-N1 1.353(5), O2-C20 1.147(4), O1-C19 1.145(4), C20-Fe1-C19 89.2(16), C20-Fe1-C1 92.93(16), C19-Fe1-C1 95.75(16),
2
N1-C1-N2 106.7(3), N1-C1-Fe1 127.8(2), N2-C1-Fe1 125.6(3). For 3b: (hydrogen atoms and one molecule of H O are omitted for clarity); Selected bond
o
lengths (Å) and angles ( ): Fe1-C1 1.976(4), Fe1-C22 1.765(5), Fe1-C21 1.762(6), O2-C21 1.149(7), O1-C22 1.153(6), N1-C1 1.361(6), N2-C1 1.354(6),
C21-Fe1-C22 90.3(2), C21-Fe1-C1 95.1(2), C22-Fe1-C1 97.1(2), N1-C1-N2 106.4(4), N2-C1-Fe1 126.8(3), N1-C1-Fe1 126.4(3)
3

ACCEPTED MANUSCRIPT
Table 1. X-ray crystallographic data (1−3)b
compound
lattice
formula
formula weight
space group
a/Å
1b
2b
3b
Monoclinic
Triclinic
Monoclinic
C
18
H21FeIN
2
O
3
C
20
H25FeIN
2
O
4
C
22
H
27FeIN
552.22
P2 /c
2 3
O
496.12
P 21
540.17
P ‒1
1
9.23(2)
10.80(7)
9.95(2)
90.00
107.16(4)
90.00
948(3)
2
9.711(7)
9.931(7)
12.229(7)
92.058(9)
90.989(14)
18.436(6)
10.789(3)
25.125(9)
90.00
b/Å
c/Å
α/°
β/°
109.085(5)
90.00
γ/°
110.688(14)
1102.0(13)
2
3
V/Å
4723(3)
8
Z
temperature (K)
radiation (λ,Å)
100(2)
0.71075
1.738
2.442
25.33
3413
100(2)
0.71075
1.628
150(2)
0.71075
1.553
-3
ρ(calcd.), g cm
-
1
µ(Mo Kα), mm
θ max, deg.
no. of data
2.112
1.970
25.36
25.00
3937
8283
no. of parameters
235
262
523
R
1
0.0288
0.0503
1.007
0.0364
0.0861
1.062
0.0525
0.1217
1.167
wR
2
GOF
Fe1
O2
O1
C1
N2
N1
F1
F6
F2
P1
F3
F5
F4
4
b
5b
o
Figure 3. ORTEP views of the complexes 4b and 5b. (hydrogen atoms are omitted for clarity) For 4b: Selected bond lengths (Å) and angles ( ): Fe1-C21
1
9
.9753(19), Fe1-C1 1.777(2), Fe1-C3 1.777(2), C1-O2 1.136(3), C3-O4 1.140(3), C23-C24 1.336(3), C21-N22 1.360(2), C21-N25 1.361(2), C21-Fe1-C1
4.19(9), C21-Fe1-C3 97.57(8), C1-Fe1-C3 93.86(9), N22-C21-N25 103.57(16), N22-C21-Fe1 125.96(14), N25-C21-Fe1 130.40(14). For 5b: Selected
o
bond lengths (Å) and angles ( ): Fe1-C1 1.977(4), Fe1-C18 1.782(4), Fe1-C19 1.777(4), C18-O1 1.141(5), C19-O2 1.138(5), C2-C3 1.339(5), C1-N1
1
1
.363(4), C1-N2 1.367(5), C1-Fe1-C18 96.51(16), C1-Fe1-C19 92.16(16), C18-Fe1-C19 90.67(18), N1-C1-N2 104.1(3), N1-C1-Fe1 129.6(3), N2-C1-Fe1
26.2(3)
4

ACCEPTED MANUSCRIPT
Table 2. X-ray crystallographic data (4−5)b
compound
lattice
formula
formula weight
space group
a/Å
4b
5b
Monoclinic
Monoclinic
C
18
H17FeN
2
O
2
PF
6
19 2 2 6
C H19FeN O PF
494.16
P2 /c
508.18
P2 /c
1
1
12.1755(10)
10.4387(6)
15.5426(12)
90.00
13.584(6)
10.517(4)
15.681(8)
90.00
b/Å
c/Å
α/°
β/°
97.841(3)
90.00
114.645(7)
90.00
γ/°
3
V/Å
1956.9(2)
4
2036.2(16)
4
Z
temperature (K)
radiation (λ,Å)
150(2)
0.71073
1.677
100(2)
0.71073
1.658
-3
ρ(calcd.), g cm
-
1
µ(Mo Kα), mm
θ max, deg.
no. of data
0.926
0.893
27.48
25.35
4437
3736
no. of parameters
272
281
R
1
0.0345
0.0911
1.029
0.0458
0.1239
1.182
wR
2
GOF
2
.2. Catalytic evaluation of the prepared NHC-Fe complexes
The catalytic activities of these new molecularly defined iron NHC complexes (1-5)b were evaluated on hydrosilylation of carbonyl
derivatives such as of the aldehydes and ketones. Based on our previous observations with the {Cp[1,3-di-R-imidazol-2-ylidene]Fe(CO) }I
2
[
21b]
[
R = Mes and i-Pr]
complexes, that required activation by visible light, we chose to study the hydrosilylation of benzaldehyde (6) and
acetophenone (7) as two representative benchmark test reactions for the aldehyde and ketone substrates using the newly synthesized (1-5)b
complexes under analogous photo-activation conditions with the aim of comparing their catalytic activities. The catalysis results along
with the reaction conditions are reported in the Table 3
For example, in case of the reduction of benzaldehyde, the hydrosilylation reaction was performed at 30 °C for 3 hours in the presence of
1
6
.2 equiv. of phenylsilane and 2 mol% of a Fe−NHC complex as the pre-catalyst. The benzimidazole derived iron complex 1b yielded
8% conversion after 3 hours of reaction time, whereas only 15% and 6% conversions were observed in case of the complexes 2b and 3b,
thus showing the crucial influence of substituents on the benzimidazole moiety (Entries 1, 4 and 7). Nevertheless, by extending the
reaction time to 17 hours, full conversions were achieved in each case (Entries 2, 5 and 8).
In case of the imidazole based iron complexes, 4b led to 90% conversion after 3 hours at 30 °C, whereas 5b gave only 44% conversion
(Entries 10 and 13). Notably, using 1 mol% of 4b under similar conditions, the same conversion was observed. Benzyl substituents on the
imidazole-based NHC ligand seems to have a positive influence on the catalytic activity than the i-propyl moiety in our earlier reported
complex {Cp[1,3-di-(i-Pr)-imidazol-2-ylidene]Fe(CO) }I that showed 28% conversion after 3 hours at 30 °C, while the best performance
2
[
21b]
was seen the mesityl substituted imidazole-based Fe−NHC complex {Cp[1,3-di-(IMes)-imidazol-2-ylidene]Fe(CO) }I
that resulted full
2
conversion in 3 hours at 30 °C. However, after a prolonged reaction time of 17 hours at 30 °C, the 4b and 5b complexes resulted in full
conversions (entries 11 and 14). Notably, when the reaction were performed in solvent free conditions for 3 h at 30 °C using 2 mol% of the
complexes (1-5)b, full conversions were obtained (Entries 3, 6, 9, 12 and 15).
5

ACCEPTED MANUSCRIPT
Table 3. Hydrosilylation of benzaldehyde and acetophenone as catalyzed by the Fe−NHC complexes (1−5)b
Entry
Catalyst
2 mol%)
Benzaldehyde
Conditions Conv. (%)
THF, 30 °C, 3 h
Acetophenone
Conditions Conv. (%)
neat, 70 °C, 17 h 90
(
[b]
[b]
1
2
3
1b
2b
3b
4b
5b
68
17 h
> 98
> 98
Neat, 30 °C, 3 h
4
5
6
THF, 30 °C, 3 h
17 h
15
neat, 70 °C, 17 h
neat, 70 °C, 17 h
neat, 70 °C, 17 h
neat, 70 °C, 17 h
75
> 98
> 98
Neat, 30 °C, 3 h
7
8
9
THF, 30 °C, 3 h
17 h
6
> 98
78
90
Neat, 30 °C, 3 h
> 98
[c]
1
1
1
0
1
2
THF, 30 °C, 3 h
17h
90(90 )
> 98
> 98
Neat, 30 °C, 3 h
1
1
1
3
4
5
THF, 30 °C, 3 h
17 h
44
72
> 98
> 98
Neat, 30 °C, 3 h
[
2
a]Aldehyde or ketone (1 mmol), phenylsilane (1.2 mmol), [CpFe(CO) (NHC)][X] (2 mol%), THF (4 mL) or neat, 30-70 °C, 17 h under visible light
1
irradiation. (ii) quench with NaOH 2M (2 mL) and MeOH (2 mL) at rt for 2 h. [b] Conversion determined by H-NMR analysis. [c] Reaction performed
with 1 mol% of the complex 4b in THF at 30 °C for 3 h.
In case of the hydrosilylation of acetophenone by the (1-5)b complexes, it was necessary to heat the reaction at 70 °C under solvent free
conditions in order to obtain good conversions (72-98%). Notably, the benzimidazole-derived Fe−NHC complexes 1b and 3b gave
encouraging 90−98% conversions after 17 hours of reaction time, whereas the remaining three 2b, 4b and 5b complexes gave satisfactory
7
2−78% conversions (Entries 1, 7 versus 4, 10 and 13). Thus, overall, the benzimidazole-based Fe−NHC complexes (1-3)b seem to be
interesting precatalysts for the hydrosilylation of carbonyl compounds. Additionally, the complexes 1b and 4b were able to quantitatively
transform an activated acetophenone, namely 4-trifluoromethylacetophenone into the corresponding alcohol at 70 °C for 17 hours.
3. Conclusions
In summary, several ionic {Cp(NHC)Fe(CO) }X (X = I, PF ) type of iron N-heterocyclic carbene complexes derived from the
2
6
benzimidazole (1−3)b and the imidazole (4−5)b motifs were synthesized and fully structurally characterized. These iron complexes were
found to be active catalyst precursors for the hydrosilylation of carbonyl derivatives. Further investigations on the reactivity of such
benzimidazole-derived iron complexes are under investigations in both of our groups.
4. Experimental Section
4
.1. General Procedures.
All manipulations were carried out using a combination of a glovebox and standard Schlenk techniques. Solvents were purified and
degassed by standard procedures. KN(SiMe ) , LiN(SiMe ) and AgPF , were purchased from Sigma Aldrich (USA) and used without any
3
2
3 2
6
[
24]
[25]
[26]
[27]
[28]
[29]
1
further purification. 1a, 2a, 3a, 4a, 5a and CpFe(CO)2I were synthesized according to modified literature procedures. H,
6

ACCEPTED MANUSCRIPT
13
1
19
1
1
C{ H} and F{ H} NMR spectra were recorded on a Varian and Bruker 400 MHz NMR spectrometer. H NMR peaks are labeled as
singlet (s), doublet (d), triplet (t), and septet (sept). Infrared spectra were recorded on a Perkin Elmer Spectrum One FT-IR spectrometer.
Mass spectrometry measurements were done on a Micromass Q-T of spectrometer. Elemental Analysis was carried out on Thermo Quest
FLASH 1112 SERIES (CHNS) Elemental Analyzer. X-ray diffraction data for compounds (1−3)b were collected on a Rigaku Hg 724+
diffractometer and for the compounds (4−5)b were collected on APEXII, Bruker-AXS diffractometer equipped with a CCD detector, using
graphite-monochromated Mo Ka radiation (k = 0.71073 Å) at T = 150(2) K, and crystal data collection and refinement parameters are
summarized in Table 1. The structures were solved using direct methods and standard difference map techniques, and were refined by full-
2
[30]
matrix least-squares procedures on F with SHELXTL (Version 6.10).
4
4
.2. Synthesis of the NHC-Fe comples
.2.1. Synthesis of {Cp[1,3-di-ethyl-benzimidazol-2-ylidene]Fe(CO) }I (1b)
2
To a solution of 1,3-di-ethyl-benzimidazolium iodide (1a) (0.501 g, 1.66 mmol) in THF (ca. 10 mL), a solution of LiN(SiMe ) (0.552 g,
3
2
o
3
.30 mmol) in THF (ca. 10 mL) was added dropwise and the resulting mixture was stirred for 1 hour at 0 C under N atmosphere. A
2
solution of CpFe(CO) I (0.861 g, 2.83 mmol) in THF (ca. 10 mL) was added to it and the resultant mixture was stirred for another 16
2
hours at room temperature. All volatiles were then removed under vacuum to give the crude product as a brown residue which was purified
by column chromatography using 230−400 mesh silica gel as a stationary phase and by eluting it with a CHCl /MeOH mixture (95 : 5 v/v)
3
1
o
to give the product 1b as a brown solid (0.293 g, 37%). H NMR (CDCl , 400 MHz, 25 C): δ 7.53–7.52 (m, 2H, C H N ), 7.4–17.39 (m,
3
7
4
2
3
13
1
2
2
H, C H N ), 5.59 (s, 5H, C H ), 4.71 (q, 4H, J = 7 Hz, CH CH ), 1.64–1.58 (m, 6H, CH CH ). C{ H} NMR (CDCl , 100 MHz,
7 4 2 5 5 HH 2 3 2 3 3
5 °C): δ 210.8 (CO), 178.9 (Fe–NCN of C H N ), 135.4 (C H N ), 124.2 (C H N ), 111.2 (C H N ), 87.9 (C H ), 45.6 (CH CH ), 14.7
7
4
2
7
4
2
7
4
2
7
4
2
5
5
2
3
−
1
+
(
CH CH ). IR data (cm ) KBr pellet (ν ): 2039 (s) and 1993 (s). HRMS (ES): m/z 351.0793 [M−I] , calcd 351.0796. Anal. Calcd. for
2 3 CO
C H FeIN O : C, 45.22; H, 4.01; N, 5.86; Found: C, 44.52; H, 4.39; N, 5.68%.
18
19
2
2
4
.2.2. Synthesis of {Cp[1,3-di-i-propyl-benzimidazol-2-ylidene]Fe(CO) }I (2b) To a solution of 1,3-di-i-propyl-benzimidazolium iodide
2
(
2a) (0.401 g, 1.21 mmol) in THF (ca. 10 mL), a solution of LiN(SiMe ) (0.482 g, 2.88 mmol) in THF (ca. 10 mL) was added dropwise
3 2
o
and the resulting mixture was stirred for 1 hour at 0 C under N atmosphere. A solution of CpFe(CO) I (0.738 g, 2.42 mmol) in THF (ca.
2
2
1
0 mL) was added and the reaction mixture was allowed to stir for another 16 hours at room temperature. All volatiles were then removed
under vacuum to give the crude product as a brown residue which was purified by column chromatography using 230−400 mesh silica gel
1
as a stationary phase and by eluting it with a CHCl /MeOH mixture (94 : 6 v/v) to give the product 2b as a yellow solid (0.251 g, 41%). H
3
o
3
NMR (CDCl , 400 MHz, 25 C): δ 7.99-7.97 (m, 2H, C H N ), 7.37–7.33 (m, 2H, C H N ), 5.62 (s, 5H, C H ), 5.53 (sept, 2H, J = 7
3
7
4
2
7
4
2
5
5
HH
3
13
1
Hz, CH[CH ] ), 1.82 (d, 12H, J = 7 Hz, CH[CH3]2). C{ H} NMR (DMSO-d , 100 MHz, 25 °C): δ 211.0 (CO), 180.5 (Fe–NCN of
3
2
HH
6
−1
C H N ), 134.3 (C H N ), 122.8 (C H N ), 113.2 (C H N ), 87.9 (C H ) 54.3 (CH[CH ] ), 20.1 (CH[CH ] ). IR data (cm ) KBr pellet
7
4
2
7
4
2
7
4
2
7
4
2
5
5
3 2
3 2
+
(
νCO): 2039 (s) and 1994 (s). HRMS (ES): m/z 379.1102 [M−I] , calcd 399.1104. Anal. calcd. for C H FeIN O •1/3H O: C, 46.90; H,
20 23 2 2 2
4
.66; N, 5.47; Found: C, 47.56; H, 4.41; N, 5.35%.
4
.2.3. Synthesis of {Cp[1,3-di-n-butyl-benzimidazol-2-ylidene]Fe(CO) }I (3b)
2
To a solution of 1,3-di-n-butyl-benzimidazolium iodide (3a) (0.674 g, 1.88 mmol) in THF (ca. 10 mL), a solution of LiN(SiMe ) (0.610 g,
3
2
o
3
.65 mmol) in THF (ca. 10 mL) was added dropwise and the resulting mixture was stirred for 1 hour at 0 C under N atmosphere. A
2
solution of CpFe(CO) I (1.00 g, 3.29 mmol) in THF (ca. 10 mL) was added to it and the resultant mixture was stirred for another 16 hours
2
at room temperature. All volatiles were then removed under vacuum to give the crude product as a brown residue which was purified by
column chromatography using 230−400 mesh silica gel as a stationary phase and by eluting it with a CHCl /MeOH mixture (95 : 5 v/v) to
3
1
o
give the product 1b as a brown solid (0.278 g, 28%). H NMR (CDCl , 400 MHz, 25 C): δ 7.50–7.48 (m, 2H, C H N ), 7.40–7.38 (m, 2H,
3
7
4
2
3
C H N ), 5.58 (s, 5H, C H ), 4.58–4.53 (m, 4H, [CH ] CH ), 1.88 (br, 4H, [CH ] CH ), 1.65 (m, 4H, ([CH ] CH ), 1.06 (t, 6H, J = 7
7
4
2
5
5
2 3
3
2 3
3
2 3
3
HH
13
1
Hz, [CH ] CH ). C{ H} NMR (CDCl , 100 MHz, 25 °C): δ 210.7 (CO), 178.7 (Fe–NCN of C H N ), 135.6 (C H N ), 124.2 (C H N ),
2
3
3
3
7
4
2
7
4
2
7
4
2
−1
1
11.3 (C H N ), 88.1 (C H ) 50.3 ([CH ] CH ), 31.8 ([CH ] CH ), 20.2 ([CH ] CH ), 13.9 ([CH ] CH ). IR data (cm ) KBr pellet (νCO):
7
4
2
5
5
2 3
3
2 3
3
2 3
3
2 3
3
+
2
039 (s) and 1986 (s). HRMS (ES): m/z 407.1416 [M−I] , calcd 407.1422. Anal. Calcd. for C H FeIN O : C, 49.46; H, 5.09; N, 5.24;
22 27 2 2
Found: C, 49.89, H, 5.24, N, 4.80%.
4
.2.4. Synthesis of {Cp[1-benzyl-3-methyl-imidazol-2-ylidene]Fe(CO) }PF (4b)
2 6
7

ACCEPTED MANUSCRIPT
To a solution of 1-benzyl-3-methyl-imidazolium hexafluorophosphate (4a) (0.300 g, 0.943 mmol) in dry THF (ca. 10 mL) at −20 °C, a
solution of KN(SiMe ) (0.376 g, 1.89 mmol) was added dropwise while stirring. The resultant yellow solution was vigorously stirred for
3
2
1
hour at −20 °C and then allowed to attain room temperature while stirring. The reaction mixture was again cooled to −20 °C, and to
which a solution of CpFe(CO) I (0.487 g, 1.60 mmol) in dry THF (ca. 10 mL) was added dropwise. The reaction mixture was stirred for 1
2
hour and AgPF (0.446 g, 1.76 mmol) was added and further stirred at room temperature for another 10 hours. The solvent was then
6
removed under vacuum and the residue was purified by 230−400 mesh silica gel column chromatography by elution with MeOH/CHCl3
1
(
1:9 v/v) mixed medium to get the product (4b) as a dark green solid (0.184 g, 39% yield). H NMR (CDCl , 400 MHz, 25 °C): δ ppm,
3
7
3
1
.43–7.37 (m, 4H, C H ), 7.19–7.17 (m, 2H, C H and NCHCHN), 7.11 (s, 1H, NCHCHN), 5.49 (s, 2H, CH ), 5.42 (s, 5H, C H ), 4.05 (s,
6 5 6 5 2 5 5
13
1
H, CH3). C{ H} NMR (CDCl , 100 MHz, 25 °C): δ ppm, 211.1 2(CO), 165.1 (NC(2)N), 134.9 ipso-(C H ), 129.2 (C H ), 128.5 (C H ),
3
6
5
6
5
6
5
31
1
27.6 (NCHCHN), 127.1 (C H ), 125.9 (NCHCHN), 87.3 (C H ), 55.5 (CH ), 41.0 (CH ). P{ H} NMR (CDCl , 162 MHz, 25 °C): δ
6
5
5
5
2
3
3
1
19
1
1
−1
ppm, −144.4 (sept, J = 712 Hz.) F{ H} NMR (CDCl , 376 MHz, 25 °C): δ ppm, –72.6 (d, J = 711 Hz). IR data (cm ) KBr pellet
PF
3
FP
+
(
νCO): 2038 (s) and 1989 (s). HRMS (ES): m/z 349.0634 [M-PF ] , calcd 349.0639. Anal. calcd. for C H FeN O PF : C, 43.75; H, 3.47;
6 18 17 2 2 6
N, 5.67; Found: C, 43.60; H, 3.70; N, 5.55%.
4
.2.5. Synthesis of {Cp[1-benzyl-3-ethyl-imidazol-2-ylidene]Fe(CO) }PF (5b)
2 6
To a solution of 1-benzyl-3-ethyl-imidazolium hexafluorophosphate (5a) (0.330 g, 0.993 mmol) in dry THF (ca. 10 mL) at −20 °C, a
solution of KN(SiMe ) (0.396 g, 1.99 mmol) was added dropwise while stirring. The resultant yellow solution was vigorously stirred for
3
2
1
hour at −20 °C and then allowed to attain room temperature while stirring. The reaction mixture was again cooled to −20 °C, and to
which a solution of CpFe(CO) I (0.528 g, 1.74 mmol) in dry THF (ca. 10 mL) was added dropwise. The reaction mixture was stirred for 1
2
hour and AgPF (0.483 g, 1.91 mmol) was added and further stirred at room temperature for another 10 hours. The solvent was then
6
removed under vacuum and the residue was purified by 230−400 mesh silica gel column chromatography by elution with MeOH/CHCl3
1
(
1:9 v/v) mixed medium to get the product (5b) as a dark green solid (0.144 g, 29% yield). H NMR (CDCl , 400 MHz, 25 °C): δ ppm,
3
3
7
.42–7.34 (m, 5H, C H ), 7.18 (s, 1H, NCHCHN), 7.15 (s, 1H, NCHCHN), 5.50 (s, 2H, CH ), 5.39 (s, 5H, C H ), 4.40 (q, 2H, J = 7 Hz,
6
5
2
5
5
HH
3
13
1
CH CH ), 1.55 (t, 3H, J = 7 Hz, CH CH ). C{ H} NMR (CDCl , 100 MHz, 25 °C): δ ppm, 211.1 1(CO), 164.4 (NC(2)N), 134.9 ipso-
2
3
HH
2
3
3
(
C H ), 129.3 (C H ), 128.5 (C H ), 127.2 (C H ), 126.7 (NCHCHN), 124.8 (NCHCHN), 87.4 (C H ), 55.6 (CH ), 47.6 (CH CH ), 16.2
6 5 6 5 6 5 6 5 5 5 2 2 3
31
1
1
19
1
(
CH3). P{ H} NMR (CDCl , 162 MHz, 25 °C): δ ppm, −144.2 (sept, J = 711 Hz). F{ H} NMR (CDCl , 376 MHz, 25 °C): δ ppm, –
3 PF 3
1
−1
+
7
2.61 (d, J = 726 Hz). IR data (cm ) KBr pellet (ν ): 2037 (s) and 1987 (s). HRMS (ES): m/z 363.0796 [M-PF ] , calcd 363.0791.
FP CO 6
Anal. calcd. for C H FeN O PF : C, 44.91; H, 3.77; N, 5.51; Found: C, 44.57; H, 3.68; N, 6.43%.
19
19
2
2
6
4
.3. General procedure for the hydrosilylation of benzaldehyde and acetophenone catalyzed by the Fe−NHC complexes
A 10 mL oven dried Schlenk tube containing a stirring bar, was charged with one of the (1-5)b, 8 or 9 complex (0.01 mmol, 2 mol%).
After purging with argon (argon-vacuum three cycles) benzaldehyde or acetophenone (0.5 mmol) was added followed by PhSiH (68 ꢀL,
3
0
1
.6 mmol, 1.2 equiv.). The reaction mixture was stirred in a preheated oil bath at 30 ºC (for benzaldehyde) or 70 °C (for acetophenone) for
7 hours under light irradiation (24 W fluocompact bulb-light). Then 1 mL of MeOH was added followed by 1 mL of 2M NaOH aqueous
solution with vigorous stirring. The reaction mixture was further stirred for 1 hour at room temperature and was extracted with diethyl
ether (2×10 mL). The combined organic layers were washed with brine (3×10 mL), dried over anhydrous MgSO4, filtered and
1
concentrated under vacuum. The conversion was determined by H NMR. The residue can be then purified by silica gel column
chromatography using ethyl acetate-petroleum ether mixture (10 to 50%) to afford the desired product
Acknowledgments. We thank Indo-French Centre for the Promotion of Advanced Research – IFCPAR (Project No: 4605-1), New
Delhi, for financial support of this research.
PG is grateful to the Sophisticated Analytical Instrument Facility at IIT Bombay, India, for the characterization data and thanks Prof. R.
Murugavel for the use of his Single Crystal X-ray Diffraction Facility established through a DAE-SRC Outstanding Investigator Award.
DK thanks CSIR-UGC, New Delhi, for research fellowship.
CD and JBS thank the French "Ministère de l’Enseignement Supérieur et de la Recherche" and the CNRS for financial support. LPB
thanks IFCPAR for a PhD grant.
8

ACCEPTED MANUSCRIPT
Supplementary Material Available: CCDC-940393 (for 1b), CCDC-940172 (for 2b), CCDC-935844 (for 3b), CCDC-910399 (for 4b) and CCDC-926830
for 5b) contain the supplementary crystallographic data for this paper. The data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html or [from the Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: (internat.)
(
1
13
1
19
31
+
44-1223/336-033; E-mail: deposit@ccdc.cam.ac.uk H NMR, C{ H} NMR, F NMR,, P NMR, IR, HRMS, elemental analysis and the CIF file giving
X-ray crystallographic data of (1−5)b are available free of charge via the internet at doi:XXXXXXXXXXX.
9

ACCEPTED MANUSCRIPT
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Table of Contents
A series of cationic iron(II) complexes of the mixed cyclopentadienyl (Cp) and the benzimidazole- and imidazole-derived N-heterocyclic
carbene (NHC) ligands effectively catalyze the hydrosilylation of aldehyde and ketone compounds.
1
1

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Cationic Iron(II) Complexes of the Mixed Cyclopentadienyl (Cp) and the N-
heterocyclic Carbene (NHC) Ligands as Effective Precatalysts for the
Hydrosilylation of Carbonyl Compounds
a
a
b
b
Dharmendra Kumar, A. P. Prakasham, Linus Paulin Bheeter, Jean-Baptiste Sortais,
a
c
a
b
Manoj Gangwar, Thierry Roisnel, Alok Kalita, Christophe Darcel* and Prasenjit
a
Ghosh*
Research Highlights
Three new {Cp[1,3-di-R-benzimidazol-2-ylidene]-Fe(CO) }I complexes were prepared
2
Two new {Cp[1-benzyl-3-R-imidazol-2-ylidene]Fe(CO) }PF complexes were synthesized
2
6
Full structural characterizations including X-ray of these 5 complexes were done.
Evaluation of their activities was performed in hydrosilylation of carbonyl compounds
Benzimidazole based complexes displayed superior activity than the imidazole ones

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Supplementary Material Available
Cationic Iron(II) Complexes of the Mixed Cyclopentadienyl (Cp) and
the N-heterocyclic Carbene (NHC) Ligands as Effective Precatalysts for
the Hydrosilylation of Carbonyl Compounds
a
a
b
b
Dharmendra Kumar, A. P. Prakasham, Linus Paulin Bheeter, Jean-Baptiste Sortais,
a
c
a
,b
Manoj Gangwar, Thierry Roisnel, Alok Kalita, Christophe Darcel* and Prasenjit
,
a
Ghosh*
a
Department of Chemistry
Indian Institute of Technology Bombay,
Powai, Mumbai 400 076, India.
b
UMR 6226 CNRS-Université de Rennes 1 “Institut des Sciences Chimiques de Rennes”,
Team “Organometallics: Matrials and Catalysis” - Centre for Catalysis and Green
Chemistry
Campus de Beaulieu, 35042 – Rennes, France
c
UMR 6226 CNRS-Université de Rennes 1 “Institut des Sciences Chimiques de Rennes”,
Team “Centre de diffraction X” - Campus de Beaulieu, 35042, Rennes, France
Email: pghosh@chem.iitb.ac.in, christophe.darcel@univ-rennes1.fr,
Fax: +91 22 2572 3480, +33 2 23 23 69 39
1

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Figure S1. H NMR spectrum of 1b in CDCl .
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Figure S2. Expanded H NMR spectrum of 1b in CDCl .
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Figure S3. C{ H} NMR spectrum of 1b in CDCl .
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Figure S4. IR spectrum of 1b in KBr pellet.
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Figure S5. High resolution mass spectrum of 1b.
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Figure S6. Elemental analysis data of 1b.
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Figure S7. H NMR spectrum of 2b in CDCl .
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Figure S8. Expanded H NMR spectrum of 2b in CDCl .
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Figure S9. C{ H} NMR spectrum of 2b in DMSO-d .
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Figure S10. IR spectrum of 2b in KBr pellet.
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Figure S11. High resolution mass spectrum of 2b..
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Figure S12. Elemental analysis data of 2b.
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Figure S13. H NMR spectrum of 3b in CDCl ..
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Figure S14. Expanded H NMR spectrum of 3b in CDCl .
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Figure S15. C{ H} NMR spectrum of 3b in CDCl .
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Figure S16. IR spectrum of 3b in KBr pellet.
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