Iron(II) complexes bearing chelating cyclopentadienyl-N-heterocyclic carbene ligands as catalysts for hydrosilylation and hydrogen transfer reactions

Article abstract of DOI:10.1021/om100246j

A series of piano-stool iron(II) complexes bearing bidentate cyclopentadienyl-functionalized N-heterocyclic carbene ligands (Cp-NHC)Fe(CO)I (Cp = substituted and unsubstituted cyclopentadienyl) have been prepared upon reaction with Fe(CO)₄I₂ and characterized by spectroscopic and crystallographic methods. The 16-electron half-sandwich compound (Cp*-NHC)FeCl (Cp* = η⁵-C₅Me₄) has been synthesized by using FeCl₂ as precursor material. The new iron complexes displayed good catalytic activity in catalytic transfer hydrogenation of ketones and hydrosilylation reactions.

Full text of DOI:10.1021/om100246j

Organometallics 2010, 29, 2777–2782 2777
DOI: 10.1021/om100246j
Iron(II) Complexes Bearing Chelating Cyclopentadienyl-N-Heterocyclic
Carbene Ligands as Catalysts for Hydrosilylation and Hydrogen Transfer
Reactions
†
V. V. Krishna Mohan Kandepi, Joao M. S. Cardoso, Eduardo Peris, and Beatriz Royo*
†
‡
,†
~
†
´
Instituto de Tecnologıa Quımica e Biologica, Universidade Nova de Lisboa, Avenida da Repuꢀblica, EAN,
2780-157 Oeiras, Portugal, and Departamento de Quımica Inorganica y Organica, Universitat Jaume I,
´
ꢀ
‡
´
ꢀ
ꢀ
Avenida Vicente Sos Baynat s/n, 12071 Castelloꢀn, Spain
Received March 30, 2010
A series of piano-stool iron(II) complexes bearing bidentate cyclopentadienyl-functionalized N-het-
erocyclic carbene ligands (Cp-NHC)Fe(CO)I (Cp = substituted and unsubstituted cyclopentadienyl)
have been prepared upon reaction with Fe(CO)4I2 and characterized by spectroscopic and crystallo-
graphic methods. The 16-electron half-sandwich compound (Cp*-NHC)FeCl (Cp* = η5-C5Me4) has
been synthesized by using FeCl2 as precursor material. The new iron complexes displayed good catalytic
activity in catalytic transfer hydrogenation of ketones and hydrosilylation reactions.
Introduction
in organic synthesis have demonstrated the versatility of this
metal.6 However, catalytic applications of iron complexes con-
taining NHC ligands are scarce.7 The first use of NHCs in
homogeneous iron catalysis was reported by Grubbs and co-
workers in 2000.8 They described the catalytic activity of well-
defined [(NHC)2FeX2] complexes in atom transfer radical poly-
merization (ATRP) of styrene and methyl methacrylate. Latter
on, Gibson, Shen, and co-workers developed several NHC-
based iron catalysts for polymerization reactions.9,10 Other
catalytic applications of Fe-NHCs embrace C-C bond forming
and cyclization reactions.11 Furthermore, stable NHCs can
mediate unusual transformations of organometallic iron com-
plexes.12 The use of Fe-NHCs in homogeneous catalysis is an
emerging field that represents an opportunity to develop new
sustainable catalytic systems.
We recently disclosed a simple synthetic pathway for the
preparation of cyclopentadienyl-functionalized N-heterocyclic
carbene (NHC) ligands that allowed us to independently vary
their structural components, namely, the substitution of the
cyclopentadienyl ring, the spacer, and the NHC fragment.1
Following this method, we prepared a series of unsubstituted
and sterically demanding cyclopentadienyl-NHCs that we
coordinated to a variety of metal centers, e.g., Ir,1 Rh,2 Ru,3 and
Mo.4 As a part of our ongoing research on cyclopentadienyl-
functionalized N-heterocyclic carbene ligands, we sought to syn-
thesize iron complexes of general formula (Cp-NHC)Fe(CO)X
and (Cp-NHC)FeX in an attempt to extend the poorly devel-
oped NHC-iron chemistry. Iron offers significant advantages
compared to transition metals typically used in catalysis; iron is
cheap, nontoxic, environmentally friendly, and abundant.5 During
the last years an increasing number of catalytic applications
Half-sandwich iron complexes containing NHCs are
known.13-15 Recently, Tatsumi and co-workers described
the activation of C-H bonds of thiophenes, furans, and
pyridine, promoted by a coordinatively unsaturated half-
sandwich iron complex containing a metallacycle derived
*Corresponding author. E-mail: broyo@itqb.unl.pt.
ꢀ
(1) da Costa, A. P.; Viciano, M.; Sanau, M.; Merino, S.; Tejeda, J.;
Peris, E.; Royo, B. Organometallics 2008, 27, 1305.
ꢀ
(2) da Costa, A. P.; Sanau,M.;Peris,E.;Royo,B.Dalton Trans. 2009, 6960.
(3) da Costa, A. P.; Mata, J.; Royo, B.; Peris, E. Organometallics
(7) For a recent review on NHCs in late transition metal catalysis:
ꢀ
Dıez-Gonzalez, S.; Marion, N.; Nolan, S. P. Chem. Rev. 2009, 109, 3612.
´
2010, DOI: 10.1021/om100090c
(4) Kandepi, V.V. K. M.; da Costa, A. P.; Peris, E.; Royo, B.
Organometallics 2009, 28, 4544.
(8) Louie, J.; Grubbs, R. H. Chem. Commun. 2000, 1479.
(9) McGuinness, D. S.; Gibson, V. C.; Steed, J. W. Organometallics
2004, 23, 6288.
(10) Chen, M.-Z.; Sun, H.-M.; Li, W.-F.; Wang, Z.-G.; Shen, Q.;
(5) Ritter, S. K. Sci. Technol. 2008, 86, 53.
(6) For reviews on iron catalysis: (a) Bolm, C.; Legros, J.; Le Paih, J.;
Zani, J. Chem. Rev. 2004, 104, 6217. (b) Morris, R. H. Chem. Soc. Rev.
2009, 38, 2282. (c) Iron Catalysis in Organic Chemistry; Plietker, B., Ed.;
Wiley-VCH: Weinheim, 2008. (d) Enthaler, S.; Junge, K.; Beller, M. Angew.
Chem., Int. Ed. 2008, 47, 3317. Selected reports on iron catalysis: (e) Driller,
K. M.; Klein, H.; Jackstell, R.; Beller, M. Angew. Chem., Int. Ed. 2009, 48,
6041. (f) Suzuki, K.; Oldenburg, P. D.; Que, L., Jr. Angew. Chem., Int. Ed.
2008, 47, 1887. (g) Gelalcha, F. G.; Bitterlich, B.; Anilkumar, G.; Tse, M. K.;
Beller, M. Angew. Chem., Int. Ed. 2007, 46, 7293. (h) Zhou, S.; Junge, K.;
Addis, D.; Das, S.; Beller, M. Angew. Chem., Int. Ed. 2009, 48, 1. (i) Zhou,
S.; Addis, D.; Das, S.; Junge, K.; Beller, M. Chem. Commun. 2009, 4883. (j)
Shaikh, N. S.; Enthaler, S.; Beller, M. Angew. Chem., Int. Ed. 2008, 47,
2497. (k) Oldenburg, P. D.; Shteinman, A. A.; Que, L., Jr. J. Am. Chem. Soc.
2005, 127, 15672. (l) F€urstner, A. Angew. Chem., Int. Ed. 2009, 48, 1364.
Zhang, Y. J. Organomet. Chem. 2006, 691, 2489.
(11) (a) Bedford, R. B.; Betham, M.; Bruce, D. W.; Danopoulos, A. A.;
Frost, R. M.; Hird, M. J. Org. Chem. 2006, 71, 1104. (b) Hatakeyama, T.;
Nakamura, M. J. Am. Chem. Soc. 2007, 129, 9844. (c) Yamagami, T.;
Shintani, R.; Shirakawa, E.; Hayashi, T. Org. Lett. 2007, 9, 1045. (d) Plietker,
€
B.; Dieskau, A.; Mows, K.; Jatsch, A. Angew. Chem., Int. Ed. 2008, 47,
198.
(12) Lavallo, V.; Grubbs, R. H. Science 2009, 326, 559.
(13) Buchgraber, P.; Toupet, L.; Guerchais, V. Organometallics 2003,
22, 5144.
(14) Mercs, L.; Labat, G.; Neels, A.; Ehlers, A.; Albrecht, M.
Organometallics 2006, 25, 5648.
(15) Ohki, Y.; Hatanaka, T.; Tatsumi, K. J. Am. Chem. Soc. 2008,
130, 17174.
r
2010 American Chemical Society
Published on Web 05/28/2010
pubs.acs.org/Organometallics

2778 Organometallics, Vol. 29, No. 12, 2010
Kandepi et al.
Scheme 1
Scheme 2
from an NHC.15 We believe that the presence of a tethered
Cp-NHC system may help to stabilize species not reach-
able by combination of other monodentate ligands and
also provide interesting catalytic applications. In this
work, we have prepared a series of piano-stool iron(II)
complexes bearing bidentate cyclopentadienyl-functiona-
lized N-heterocyclic carbene ligands of general type (Cp-
NHC)Fe(CO)I, and we have explored their catalytic appli-
cations in the reduction of carbonyl groups, namely, in
hydrosilylation of aldehydes and hydrogen transfer of
ketones.
pound, which displayed a CO stretching band at 1911
cm-1 in its IR spectrum, Scheme 2.
The crystal structures of 5 and 6 were unequivocally
determined by single-crystal X-ray diffraction analysis.
Quality crystals were obtained by cooling ether solutions
to -10 °C. The molecular structures and selected bond
lengths and angles are depicted in Figure 1 (for 5) and
Figure 2 (for 6). The structures can be described as a
distorted four-legged piano stool with the cyclopentadie-
nyl-NHC ligand chelating the iron center. The cyclopenta-
dienyl ligands are bonded in pentahapto fashion with metal
ring carbon distances ranging from 2.107(4) to 2.69(4) A for
5 and 2.149(4) to 2.050(5) A for 6. The Fe-Ccarbene bond
distances of 1.942(4) A (for both compounds 5 and 6) are in
good agreement with the Fe-Ccarbene bond distances re-
ported for related complexes, which are in the range
1.98-1.968 A.13-15
For complex 6, three of the phenyls of the benzyl groups
are situated above the cyclopentadienyl ring and directed
away from the metal center, while the other phenyl group of
the fourth benzyl group is situated below the cyclopentadie-
nyl ring and approaches the metal center atom. The same
disposition of the phenyl rings about the metal center has
been observed in other metal complexes containing the η5-
C5Bz5 ligand.17,18
Preliminary studies of the catalytic activity of the novel
iron complexes 4-7 were performed to explore their poten-
tial in the reduction of organic molecules. We studied the
hydrosilylation of carbonyl groups and hydrogen transfer
hydrogenation.
The use of well-defined organometallic and coordina-
tion complexes of iron as catalysts for the reduction of
carbonyl groups has been recently explored by several
groups. Casey and co-workers, inspired by the bifunc-
tional, ionic hydrogenation catalysis known for ruthe-
nium, recently disclosed the catalytic activity of related
iron-cyclopentadienyl complexes.19,20 Their catalysts
showed high activity in the hydrogenation of several
Results and Discussion
The iron(II) carbonyl compounds (Cpx-NHC)Fe(CO)I
[Cpx = Cp*(η5-C5Me4) (4); Cp (η5-C5H4) (5); CpBz (η5-
C5(CH2Ph)4) (6)] were synthesized as shown in Scheme 1.
The in situ reaction at low temperature (-60 °C) of proli-
gands 1-3 with two equivalents of n-butyllithium followed
by addition of FeI2(CO)4 affords complexes 4-6 as green
crystalline solids in high yield. All complexes are stable in the
solid state and can be handled in air. Complexes 4-6
exhibited characteristic 13C NMR shifts at δ 195 (for 4), δ
188 (for 5), and δ 193 (for 6) for the carbene carbon,
confirming the coordination of NHC fragments to the metal
center. These resonances appeared in the same range of
related Fe-NHC complexes.13,15
The IR spectra of 4-6 show the CO stretching bands at
1906 (4), 1949 (5), and 1920 cm-1 (6). The shift to lower
frequency in compound 4 is a consequence of the stronger
donor capacity of the tetramethylcyclopentadienyl ring com-
pared to the unsubstituted-Cp and tetrabenzyl-cyclopenta-
dienyl ligand. The less electron-donating capacity of the
benzyl-substituted cyclopentadienyl ligand compared to
the Cp* ligand has been already observed in complexes
containing the [Fe(C5Bz5)] fragment.16
The coordinatively and electronically unsaturated iron(II)
species (Cp*-NHC)FeCl (7) was prepared by addition of
FeCl2 to a THF solution of proligand 1, previously treated
with two equivalents of n-butyllithium, as shown in Scheme 2.
Complex 7 was isolated in 67% yield as a crystalline red
solid. This complex is highly sensitive to air and moisture and
needs to be handled under inert conditions. Compound 7 is
1
paramagnetic, and its H NMR spectrum displays broad
signals. The high-resolution mass spectrum of 7 gave
the molecular peak corresponding to [M - Cl]þ at m/z =
396.1056. Complex 7 was converted under CO atmosphere in
dichloromethane into the corresponding carbonyl com-
(17) Chambers, J. W.; Baskar, A. J.; Bott, S. G.; Atwood, J. L.;
Rausch, M. D. Organometallics 1986, 5, 1635.
(18) Song, L.-C.; Zhang, L.-Y.; Hu, Q.-M.; Huang, X.-X. Inorg.
Chim. Acta 1995, 230, 127.
(19) Casey, C. P.; Guan, H. J. Am. Chem. Soc. 2007, 129, 5816.
(20) Casey, C. P.; Guan, H. J. Am. Chem. Soc. 2009, 131, 2499.
(16) Delville-Desbois, M.-H.; Mross, S.; Astruc, D. Organometallics
1996, 15, 5604.

Article
Organometallics, Vol. 29, No. 12, 2010 2779
Figure 2. Molecular diagram of compound 6. All hydrogen
atoms have been omitted for clarity. Selected bond distances (A)
and angles (deg): Fe(1)-C(3) 1.942(5), Fe(1)-I(1) 2.6208(8),
Figure 1. Molecular diagram of compound 5. Selected bond
distances (A) and angles (deg): Fe(1)-C(3) 1.942(4), Fe(1)-I(1)
2.6452(6), Fe(1)-C(5) 1.759(5), C(5)-O(1) 1.134(5), Fe(1)-
Cpcentroid 2.0972, C(3)-Fe(1)-I(1) 90.69(11), C(3)-Fe(1)-C(5)
97.46(18), C(5)-Fe(1)-I(1) 94.06(14), Cpcentroid-Fe(1)-C(3)
120.538, Cpcentroid-Fe(1)-I(1) 118.462, Cpcentroid-Fe(1)-C(5)
117.06.
Fe(1)-C(7) 1750(5), C(7)-O(1) 1.148(6), Fe(1)-CpBz
centroid
2.0992, C(3)-Fe(1)-I(1) 88.64(15), C(3)-Fe(1)-C(7) 100.1(2),
C(7)-Fe(1)-I(1) 89.62(17), CpBzcentroid-Fe(1)-C(3) 116.24,
CpBzcentroid-Fe(1)-I(1) 122.29, CpBzcentroid-Fe(1)-C(7)
119.86.
reaction was monitored by 1H NMR. As shown in Table 1,
the hydrosilylation reaction proceeds faster in acetonitrile
(quantitative conversion in 2 h, Table 1, entry 1) than in
benzene and tetrahydrofuran (24 h was needed to achieve
quantitative conversions). No reaction takes place in chlori-
nated solvents such as dichloromethane and chloroform.
Dimethylphenylsilane was ineffective as stoichiometric re-
ductant, while PhSiH3 and Ph2SiH2 worked well in the
reduction of aldehyhdes, although a mixture of the corre-
sponding silylated ether and alcohol was obtained for Ph2-
SiH2 (Table 1, entry 7). Benzaldehyde derivatives containing
functional groups such as halo (Table 1, entries 9 and 10),
cyano (Table 1, entry 8), nitro (Table 1, entry 1), and ester
(Table 1, entry 11) were well tolerated. Aliphatic alde-
hydes and ketones were not reduced under the conditions
used in these reactions. When the catalytic hydrosilylation
reaction is performed at room temperature, longer reac-
tion times are needed to achieve quantitative conversions
(30 h was required for the hydrosilylation of 4-nitrobenzal-
dehyde with (EtO)2MeSiH at 25 °C). The carbonyl com-
plexes of general formula (Cpx-NHC)Fe(CO)I were inactive
with respect to catalyzing the hydrosilylation of carbonyl
groups.
The catalytic transfer hydrogenation of ketones was car-
ried out in 2-propanol, serving as a hydrogen source. All
complexes 4-7 catalyzed the hydrogenation of CdO via
hydrogen transfer from i-PrOH/KOH at 80 °C. From the
data shown in Table 2, aryl ketones were converted into the
corresponding alcohols in good yields with a catalyst loading
of 1 mol %. Under these conditions, the reactions were
almost complete in 6 h. As shown in Table 2, the catalytic
activity of complexes 4-7 is very similar. Interestingly, the
different substitution on the cyclopentadienyl ring does not
seem to affect the catalytic performance in the hydrogen
transfer reaction (Table 2, entries 1, 4, and 5). Furthermore,
ketones, aldehydes, and imines using H2 as a reducing
agent and also displayed activity in transfer hydrogena-
tion reaction using 2-propanol as hydrogen source. Re-
cently, Morris and co-workers reported the first version of
an enantioselective reduction of prochiral ketones in the
presence of iron complexes containing P,N,P,N-tetraden-
tate ligands.21-24,6b In addition, Chirik and co-workers
developed iron catalysts with bis(imino)pyridine ligands
that are active in the hydrosilylation of aldehydes and
ketones.25 They also recently described enantiopure pyr-
idine bis(oxazoline) and bis(oxazoline) iron dialkyl com-
plexes as efficient catalysts for hydrosilylation of various
ketones.26 So far, no reports on a [(NHC)Fe]-based catalyst
for hydrogen transfer and hydrosilylation of carbonyl
groups are found in the literature. On the basis of our find-
ings on the use of (Cp-NHC)IrX2 as catalyst for hydrogen
transfer reactions,1 we decided to explore the capability of
related iron complexes as catalysts for reduction of carbo-
nyl groups.
We have investigated the catalytic activity of (Cp*-
NHC)FeCl (7) in the hydrosilylation of aldehydes using
a catalyst loading of 1 mol % with 1.2 equiv (based on
substrate) of (EtO)2MeSiH at 80 °C. The progress of each
(21) Sui-Seng, C.; Freutel, F.; Lough, A. J.; Morris, R. H. Angew.
Chem., Int. Ed. 2008, 47, 940.
€
(22) Sui-Seng, C.; Haque, F. N.; Hadzovic, A.; Putz, A.-M.; Reuiss,
V.; Meyer, N.; Lough, A. J.; Zimmer-De Iuliis, M.; Morris, R. H. Inorg.
Chem. 2009, 48, 735.
(23) Mikhailine, A.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc.
2009, 131, 1394.
(24) Meyer, N.; Lough, A. J.; Morris, R. H. Chem.;Eur. J. 2009, 15,
5605.
(25) Tondreau, A. M.; Lobkovsky, E.; Chirik, P. J. Org. Lett. 2008,
10, 2789.
(26) Tondreau, A. M.; Darmon, J. M.; Bradley, M. W.; Floy, S. K.;
Lobkovsky, E.; Chirik, P. J. Organometallics 2009, 28, 3928.

2780 Organometallics, Vol. 29, No. 12, 2010
Table 1. Hydrosilylation of Aldehydes Catalyzed by 7a
Kandepi et al.
a All reactions were carried out with 1.0 equiv of aldehyde, 1.2 equiv of silane using 1 mol % of catalyst. b Yield determined by 1H NMR spectroscopy.
c The corresponding silylated ether is obtained accompanied by desilylated alcohol; yield indicated in parentheses.
the coordinatively unsaturated species (Cp*-NHC)FeCl
displayed comparable activity to the corresponding carbo-
nyl complex 4 (Table 2, entries 1 and 6). We therefore
investigated the scope of the catalytic transfer hydrogena-
tion of ketones with catalyst 4, which proved to be fairly
stable under air atmosphere, since these reactions could be
carried out without any special attention to keep inert
conditions. The conversion rates in hydrogen transfer of
complex 4 are comparable with those of our related (Cp*-
NHC)IrI2 complex, although the amount of catalyst 4
(1 mol %) used in the reaction was higher compared to
the iridium complex (0.1 mol %); decreasing the amount of
catalyst to 0.5 mol % diminished the yield of the corre-
sponding alcohol (75%, Table 2, entry 6). However, the
potentially low cost of the iron catalyst is a great
advantage.6c

Article
Organometallics, Vol. 29, No. 12, 2010 2781
Table 2. Catalytic Transfer Hydrogenation Catalyzed by Com-
plexes 4, 6, and 7a
the volatiles were evaporated. The crude oil was purified by flash
chromatography (hexane/ethyl acetate, 1:4), affording a yellow
oil of Cp-CPh2-CHPh-NHC (3.67 g, 60%). Iodomethane (353
mg, 1.2 mmol) was added to a solution of Cp-CPh2-CHPh-NHC
(467 mg, 1.2 mmol) in 5 mL of acetone. The reaction was stirred
at room temperature for 12 h, and all the volatiles were eva-
porated, affording a white solid, which was washed several times
with dried diethyl ether to yield the title compound 2 in quan-
titative yield. 1H NMR (400 MHz, CDCl3): δ 9.30 (s, 1H,
NCHN), 7.39-7.16 (m, 16H, CHPhþCHImid), 6.71 (s, 1H,
CHImid), 6.32 (m, 2H, CHCp), 6.25 (s, 1H, CHPhlinker), 6.01 (m,
2H, CHCp), 3.87 (s, 3H, NCH3), 3.04 (s, 1H, CHCp). 13C{1H}
NMR (100 MHz, CDCl3,): δ 147.9 (Cphenyl), 140.2 (Cphenyl), 139.7
entry
substrate
catalyst
t (h) yield (%)b
1
2
3
4
5
6
7
acetophenone
cyclohexanone (Cp*-NHC)Fe(CO)I (4)
benzophenone (Cp*-NHC)Fe(CO)I (4)
acetophenone
acetophenone
acetophenone
acetophenone
(Cp*-NHC)Fe(CO)I (4)
6
2
18
6
6
6
86
100
85
85
84
(Cp-NHC)Fe(CO)I (5)
(CpBz-NHC)Fe(CO)I(6)
(Cp*-NHC)FeCl (7)
(Cphenyl), 135.0 (CPh2linker), 132.8 (NCN), 128.9-127.6 (CHPh
CHCp), 122.8 (CHImid), 122.1 (CHImid), 69.4 (CHCp), 36.9
þ
80
75
(Cp*-NHC)Fe(CO)I (4)c
6
(NCH3). MS (ESI): m/z 403 (100, M þ Hþ).
a Reactions carried out with 2 mmol of ketone, KOH (10 mL, 0.2 M in
i-PrOH), and 1 mol % of catalyst. Temperature 80 °C. b Yield deter-
mined by 1H NMR spectroscopy. c Reaction carried out with 0.5 mol %
of catalyst.
Synthesis of 4. A solution of 1 (400 mg, 0.93 mmol) in THF
(30 mL) was treated with two equivalents of n-BuLi (1.16 mL of
1.6 M in hexane, 1.86 mmol) at -60 °C. The mixture was stirred
for 15 min followed by warming to room temperature. To the
resulting mixture was added at once a solution of FeI2(CO)4
(0.39 g, 0.93 mmol) in THF (5 mL), and the reaction mixture was
stirred overnight. All volatiles were then removed under va-
cuum, and the remaining solid was extracted with diethyl ether,
yielding the iron complex 4 (369 mg, 0.71 mmol, 77%) as a
green solid. 1H NMR (400 MHz, acetone-d6): δ 7.62-7.51 (m,
5H, Ph), 7.13 (s, 1H, CHImid), 6.44 (s, 1H, CHImid), 6.00-5.97
(dd, 1H, CHPhlinker), 3.79 (s, 3H, NCH3), 2.98-2.96 (m, 2H,
CH2linker), 2.36 (s, 3H, CH3Cp*), 1.81 (s, 3H, CH3Cp*), 1.74 (s,
3H, CH3Cp*), 0.96 (s, 3H, CH3Cp*). 13C{1H} NMR (100 MHz,
acetone-d6): δ 226.5 (CO), 195.5 (Ccarbene-Fe), 138.3 (Cipsophenyl),
129.7-128.7 (CHphenyl), 123.8 (CHImid), 120.3 (CHImid), 103.7
(CCp*), 91.8 (CCp*), 90.4 (CCp*), 83.0 (CCp*), 81.6 (CCp*), 66.6
(CHPhlinker), 38.7 (NCH3), 28.9 (CH2linker), 12.9 (CH3Cp*), 10.2
(CH3Cp*), 9.5 (CH3Cp*), 9.2 (CH3Cp*). HRMS (ESI-TOF): m/z
[M - I þ NCMe]þ calcd for C22H25N2OFeNCCH3, 430.1588;
found, 430.1582; [M - I]þ calcd for C22H25N2OFe, 389.1317;
Conclusion
We synthesized new Fe(II) complexes containing cyclo-
pentadienyl-functionalized N-heterocyclic carbene ligands
of the general type (Cpx-NHC)Fe(CO)I. The molecular
structures of (Cp-NHC)Fe(CO)I and (CpBz-NHC)Fe(CO)I
showed the cyclopentadienyl-NHC ligand chelating the iron
center in a four-legged piano-stool geometry. The catalytic
activity of the new complexes was tested in hydrosilylation
and hydrogen transfer reactions, revealing the efficient ac-
tivity of (Cp*-NHC)FeCl in the hydrosilylation of aldehydes
and the good activity of (Cpx-NHC)Fe(CO)I complexes in
catalytic transfer hydrogenation of ketones using 2-propanol
as hydrogen source.
Experimental Section
found, 389.1323. IR (KBr): ν(Fe-CO) 1906 (vs) cm-1
.
General Procedures. Ligands 11 and 34 were synthesized
according to the method described by us. Ligand 2 has been
synthesized following a similar method to that described for
Synthesis of 5. A solution of 2 (150 mg, 2.8 mmol) in THF
(15 mL) was treated with two equivalents of n-BuLi (3.50 mL of
1.6 M in hexane, 5.60 mmol) at -60 °C. The mixture was stirred
for 15 min followed by warming to room temperature. To the
resulting mixture was added at once a solution of FeI2(CO)4
(118 mg, 2.8 mmol) in THF (15 mL), and the reaction mixture
was stirred overnight. All volatiles were then removed under
vacuum, and the remaining solid was extracted with diethyl
ether, yielding the iron complex 5 (616 mg, 1.0 mmol, 36%) as a
27
1 and 3 (see Experimental Section). FeI2(CO)4 and 6,6-di-
phenylfulvene28 were prepared following literature procedures.
FeCl2 was used as purchased. All other reagents were used as
received from commercial suppliers without further purifica-
tion. 1H and 13C NMR spectra were recorded on a Bruker
Avance III 400 MHz. Infrared (IR) spectra were recorded on
samples as KBr pellets using a Mattson 7000 FT-IR spectro-
meter. Electrospray mass spectra (ESI-MS) were recorded on a
Micromass Quatro LC instrument; nitrogen was employed as
drying and nebulizing gas. A QTOF I (quadrupole-hexapole-
TOF) mass spectrometer with an orthogonal Z-spray-electro-
spray interface (Micromass, Manchester, UK) was used for
high-resolution mass spectrometry (HRMS). The drying gas
as well as nebulizing gas was nitrogen at a flow of 400 and
80 L/h, respectively. Complexes 4-7 did not give reproducible
elemental analyses due to the retention of fractional amounts of
solvent. In lieu of acceptable elemental analyses for compounds
4-7 HRMS (ESI-TOF) were recorded.
1
green solid. H NMR (400 MHz, acetone-d6): δ 8.01 (s, 1H,
CHImid), 7.56 (s, 1H, CHImid), 7.37-6.85 (m, 15H, Ph), 5.28 (s,
1H, CHCp), 4.93 (s, 1H, CHCp), 4.55 (s, 2H, CHCp) 4.42 (s, 1H,
CHPhlinker), 3.76 (s, 3H, NCH3). 13C{1H} NMR (100 MHz,
acetone-d6): δ 224.3 (CO), 188.2 (Ccarbene-Fe), 146.5 (Cphenyl),
146.4 (Cphenyl) 139.5 (Cphenyl), 132.0-125.2 (CHphenyl
þ
CHImid), 105.2 (CCp), 95.6 (CHCp), 85.8 (CHCp), 78.2 (CHCp),
72.1 (CHCp), 65.9 (CHPhlinker), 41.0 (NCH3), 26.4 (CPh2linker).
HRMS (ESI-TOF): m/z [M - I]þ calcd for C30H25N2OFe,
485.1317; found, 485.1320; [M - I þ NCCH3]þ calcd for
C30H25N2OFeNCCH3, 526.1582; found, 526.1581. IR (KBr):
ν(Fe-CO) 1949 cm-1
.
Synthesis of [Cp-CPh2-CHPh-NHCMe]I (2). A hexane solu-
tion of n-BuLi (11.86 mL of 1.6 M in hexane, 18.97 mmol) was
added dropwise to a solution of benzylimidazole (2.50 g, 15.80
mmol) in dried THF (20 mL) at -60 °C. The solution was stirred
for 20 min, 6,6-diphenylfulvene (3.60 mg, 15.80 mmol) was
added, and the reaction mixture was allowed to reach room
temperature and stirred for 1 h. Methanol was then added, and
Synthesis of 6. A solution of 3 (450 mg, 0.55 mmol) in 20 mL of
THF was was treated with two equivalents of n-BuLi (0.69 mL
of 1.6 M in hexane, 1.11 mmol) at -60 °C. The mixture was
stirred for 15 min followed by warming to room temperature. To
the resulting mixture was added at once a solution of FeI2(CO)4
(230 mg, 0.55 mmol) in THF (5 mL), and the reaction mixture
was stirred overnight. All volatiles were then removed under
vacuum, and the remaining solid was extracted with diethyl
ether, yielding the iron complex 6 (409 mg, 0.45 mmol, 83%) as a
green solid. 1H NMR (400 MHz, acetone-d6): δ 7.22-6.28
(m, 33H), 4.54 (d, J = 11.5 Hz, 1H), 4.41 (d, J = 5.7 Hz, 1H), 3.9
(27) Hieber, W.; Wirsching, A. Z. Anorg. Allg. Chem. 1940, 245, 35.
(28) Kice, L. J.; Parham, F. M. J. Am. Chem. Soc. 1958, 80, 3792.

2782 Organometallics, Vol. 29, No. 12, 2010
Kandepi et al.
(s, 3H), 3.66 (s, 1H), 3.48 (d, J = 7.4 Hz, 3H), 3.32 (d, J = 3.3 Hz,
1H), 3.26 (d, J=3.7 Hz, 1H), 2.42 (d, J=16.2 Hz, 1H). 13C{1H}
NMR (100 MHz, acetone-d6): δ 225.9, 193, 140.6, 140.0, 139.9,
139.1, 138.4, 137.6, 130.49-125.35, 124.2, 122, 105.2, 96.5, 94.8,
88.2, 87.7, 69.0, 45.1, 39.32, 36.1, 32.1, 31.7, 30.9. HRMS (ESI-
TOF): m/z [M]þ calcd for C52H45N2FeO, 769.2883; found,
tube with 0.5 mL of CDCl3. The evolution of the reaction was
determined by integration.
X-ray Diffraction Studies. Data collection was carried out in a
Bruker Appex-II CCD diffractometer at 100 K. Space group
assignment was based on systematic absences, E statistics, and
successful refinement of the structures. The structures were solved
by direct methods with the aid of successive difference Fourier
maps and were refined using the SHELXTL 6.1 software package
(Sheldrick, G. M. SHELXTL, version 6.1; Bruker AXS, Inc.:
Madison, WI, 2000). All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were assigned to ideal positions
and refined using a riding model. Details of the data collection,
cell dimensions, and structure refinement are given in Table 3
(SupportingInformation). Thediffractionframeswereintegrated
using the SAINT package (SAINT, version 5.0; Bruker Analy-
tical X-ray System: Madison, WI, 1998).
769.2878. IR (KBr): ν(Fe-CO) 1920 (vs) cm-1
.
Synthesis of 7. A solution of 1 (260 mg, 0.61 mmol) in THF
(15 mL) was treated with two equivalents of n-BuLi (0.76 mL
of 1.6 M in hexane, 1.23 mmol) at -60 °C. The mixture was
stirred for 15 min followed by warming to room temperature.
To the resulting mixture was added at once a solution of FeCl2
(700 mg, 0.61 mmol) in THF (5 mL), and the reaction mixture
was stirred overnight. All volatiles were then removed under
vacuum, and the remaining solid was washed with diethyl ether
and extracted with dichloromethane, yielding the iron complex 7
(176 mg, 0.40 mmol, 67%) as a red solid. HRMS (ESI-TOF):
m/z [M - Cl]þ calcd for C21H25N2FeCl, 396.1068; found,
396.1056. UV-vis (CH2Cl2): absorption 360 nm (Supporting
Information).
Hydrosilylation of Aldehydes. A typical procedure was per-
formed as follows. A dried J. Young tube equipped with a Teflon
screw cap was flushed with nitrogen and charged with catalyst
(0.010 mmol). The appropriate solvent (0.4 mL) followed by the
corresponding aldehyde (1.01 mmol) and neat silane (1.24 mmol)
were added. The samples were monitored periodically by 1H
NMR. Reaction times and temperatures are indicated in
Table 1.
Acknowledgment. We gratefully acknowledge finan-
cial support from FCT of Portugal, POCI 2010, FEDER
through project PTDC/QUI/64458/2006, and the MEC
of Spain (CTQ2008-04460). V.V.K.M.K. and J.M.S.C.
thank FCT for grants SFRH/BPD/26944/2006 and
SFRH/BD/66386/2009, respectively. We thank FCT for
REDE/1517/RMN/2005 and REDE/1504/REM/2005.
We thank Dr. A. L. Llamas-Saiz from University of
Santiago de Compostela, Spain, for the X-ray diffraction
studies.
Hydrogen Transfer. A mixture of the appropriate ketone (2
mmol), KOH (10 mL, 0.2 M in i-PrOH), and catalyst was
refluxed. The reaction was monitored by 1H NMR spectroscopy
by introducing aliquots of the reacting solution inside an NMR
Supporting Information Available: Full crystallographic data
as CIF files and NMR and HRMS spectra of compounds are
available free of charge via the Internet at http://pubs.acs.org.