Dehydrogenative silylation of alcohols catalysed by half-sandwich iron N-heterocyclic carbene complexes

Article abstract of DOI:10.1016/j.jorganchem.2014.06.005

A new series of tetramethylcyclopentadienyl-functionalised N-heterocyclic carbene complexes of iron bearing different wingtips of general type (Cp?-NHC^R)Fe(CO)I (R = ⁿBu, ⁱBu, Et, CH₂CH₂OMe, CH₂Ph) were prepared by direct reaction of Fe₃(CO)₁₂ and the corresponding imidazolium proligands. These new iron-NHC complexes have been found to be efficient catalysts for the dehydrogenative silylation of alcohols with silanes. Iron metal complexes bearing iso-butyl and n-butyl wingtips displayed slightly better catalytic performances than the related complexes (Cp?-NHC^R)Fe(CO)I (R = Et, CH₂CH₂OMe, CH₂Ph), affording quantitative yields of the corresponding silylethers in 8 h at 70 °C in acetonitrile.

Full text of DOI:10.1016/j.jorganchem.2014.06.005

Journal of Organometallic Chemistry xxx (2014) 1e5
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Journal of Organometallic Chemistry
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Dehydrogenative silylation of alcohols catalysed by half-sandwich iron
N-heterocyclic carbene complexes
*
~
Joao M.S. Cardoso, Rita Lopes, Beatriz Royo
ꢀ
Instituto de Tecnologia Qu
í
mica e Biologica da, Universidade Nova de Lisboa, Av. da República, EAN, 2780-157 Oeiras, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
A new series of tetramethylcyclopentadienyl-functionalised N-heterocyclic carbene complexes of iron
bearing different wingtips of general type (Cp*-NHCR)Fe(CO)I (R ¼ nBu, iBu, Et, CH2CH2OMe, CH2Ph) were
prepared by direct reaction of Fe3(CO)12 and the corresponding imidazolium proligands. These new iron-
NHC complexes have been found to be efficient catalysts for the dehydrogenative silylation of alcohols
with silanes. Iron metal complexes bearing iso-butyl and n-butyl wingtips displayed slightly better
catalytic performances than the related complexes (Cp*-NHCR)Fe(CO)I (R ¼ Et, CH2CH2OMe, CH2Ph),
affording quantitative yields of the corresponding silylethers in 8 h at 70 ꢀC in acetonitrile.
© 2014 Elsevier B.V. All rights reserved.
Received 3 March 2014
Received in revised form
30 May 2014
Accepted 4 June 2014
Available online xxx
Keywords:
Iron
N-Heterocyclic carbene ligands
Coupling of silanes with alcohols
Silylethers
Introduction
development of new catalytic applications of first-row transition
metal-NHC complexes with silanes [6e9,15,16], we decided to
In recent years, significant efforts have been devoted to devel-
oping catalytic applications of iron N-heterocyclic carbene (NHC)
complexes [1]. The low price and high abundance of iron, along
with the great popularity of N-heterocyclic carbene ligands in
catalysis have motivated the growing interest in this area of
research [2]. Notable examples of catalytic applications of FeeNHCs
are CeC bond formation reactions [3], polymerisation [4], CeH
activation and arene borylation [5], and reduction of functional
groups by hydrosilylation and hydrogen transfer processes [6e12].
Recently, we disclosed a simple synthetic pathway for the
preparation of iron NHC complexes by direct reaction of the cor-
responding imidazolium proligands with commercially available
Fe3(CO)12. This procedure allowed us to prepare FeeNHC com-
plexes bearing monodentate NHCs [9] and cyclopentadienyl-
explore the activity of the iron complexes containing the fragment
“(Cp*-NHCR)Fe” in the dehydrogenative silylation of alcohols. We
found that (Cp*-NHCR)Fe(CO)I complexes act as efficient catalysts
for SieO coupling.
Experimental
i
The corresponding proligands (Cp*-NHCR)I (R ¼ nBu, Bu, Et,
CH2Ph, Me) were prepared according to the method reported by us
[16,17]. All other reagents were used as received from the com-
mercial suppliers without further purification. All manipulations
were carried out under a nitrogen atmosphere using standard
Schlenk techniques, and solvents were purified from appropriate
drying agents. Deuterated solvents were degassed and stored over
molecular sieves. NMR spectra were recorded using a Bruker
Avance III 400 MHz. Elemental analyses and Electrospray mass
spectra were performed in our laboratories at ITQB.
functionalised NHC ligands, Cp-NHC (Cp ¼
h h
5-C5H4, 5-C5Me4) in
high yields [7]. This advance precludes the requirement for the
strong bases traditionally employed in the synthesis of similar
complexes [4d,13,14].
Preparation of the imidazolium salt (Cp*-NHCR)I (R ¼ CH2CH2OMe)
In this work, we extended our studies to the preparation of a
new series of iron tetramethylcyclopentadienyl-functionalised
NHCs containing different substituents in the wingtip (R) of the
imidazolium ring (Fig. 1). As part of our ongoing research into the
A procedure similar to that reported by us for the preparation of
tetramethylcyclopentadienes functionalised with imidazolium
salts was applied by using 1-bromo-2-methoxyethane [16].
1-Bromo-2-methoxyethane (0.80 mL, 8.5 mmol) was added to a
solution of Cp*-NHC (500 mg, 1.7 mmol) in 10 mL of THF with an
excess of NaI (2.5 g, 17 mmol). The mixture was stirred at 50 ꢀC for 3
* Corresponding author. Tel.: þ351 214469754; fax: þ351 214411277.
E-mail address: broyo@itqb.unl.pt (B. Royo).
http://dx.doi.org/10.1016/j.jorganchem.2014.06.005
0022-328X/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: J.M.S. Cardoso, et al., Journal of Organometallic Chemistry (2014), http://dx.doi.org/10.1016/
j.jorganchem.2014.06.005

2
J.M.S. Cardoso et al. / Journal of Organometallic Chemistry xxx (2014) 1e5
Characterisation of (Cp*-NHCEt)Fe(CO)I (3)
Yield of 3: 81%. 1H NMR (400 MHz, acetone-d6):
d
7.63e7.53 (m,
5H, CHPhenyl), 7.13 (s, 1H, CHImid), 6.48 (s, 1H, CHImid), 6.06 (d,
J ¼ 12 Hz, 1H, CHlinker), 4.35e4.23 (m, 2H, CH2Et), 3.05e2.92 (m, 2H,
CH2linker), 2.40 (s, 3H, CH3Cp*), 1.81 (s, 3H, CH3Cp*), 1.76 (s, 3H,
CH3Cp*), 1.35 (t, J ¼ 8 Hz, 3H, CH3Et). 0.96 (s, 3H, CH3Cp*). 13C{1H}
NMR (100 MHz, acetone-d6):
d 226.8 (CO), 194.8 (Fe-Ccarbene), 138.8
(CiPhenyl), 130.1 (CHPhenyl), 130.0 (CHPhenyl), 129.9 (CHPhenyl), 122.3
(CHImid), 121.3 (CHImid), 104.7 (CCp*), 91.7 (CCp*), 90.4 (CCp*), 84.1
(CCp*), 81.3 (CCp*), 67.1 (CHlinker), 46.1 (NCH2), 28.9 (CH2linker), 16.9
(CH3Et), 13.4 (CH3Cp*), 10.4 (CH3Cp*), 9.7 (CH3Cp*), 9.5 (CH3Cp*). Anal.
Calcd. for C23H27N2OFeI (530.05): C, 52.10; H, 5.13; N, 5.28. Found:
Fig. 1. Iron complexes bearing functionalised-cyclopentadienyl N-heterocyclic carbene
ligands.
days. The suspension was filtered and the filtrate was evaporated to
dryness to yield a yellow solid which was washed with diethyl
ether and hexane. Yield: 600 mg (74%). 1H NMR (400 MHz, CDCl3)
C; 52.30H, 5.29; N, 4.91. Selected IR data (KBr): n .
(CO) 1904 vs cmꢁ1
MS (ESI-TOF) m/z [MeCOeI]þ calcd. for C22H27N2Fe: 375.15; found
375.09 [MeCOeI]þ.
mixture of isomers:
d 10.40-9.92 (s, N]CHeN), 7.59-6.95 (m,
CHPhenyl, CHImid), 5.69e5.26 (m, CHlinker), 4.64e4.50 (m, NCH2),
3.81e3.70 (m, CH2OMe), 3.35, 3.33 (s, OCH3), 3.27e3.03 (m,
CH2linker), 2.78e2.33 (m, CHCp*), 1.80e1.35 (s, CH3Cp*), 1.07e0.84 (s,
Characterisation of (Cp*-NHCCH2CH2OMe)Fe(CO)I (4)
Yield of 4: 76%. 1H NMR (400 MHz, acetone-d6):
d 7.64e7.56 (m,
CH3Cp*). 13C {1H} NMR (100 MHz, CDCl3):
d 140.33 (N]CHeN),
5H, Ph), 7.05 (s, 1H, CHImid), 6.45 (s, 1H, CHImid), 6.05 (d, J ¼ 12, 1H,
CHPh-linker), 4.48e4.29 (m, 2H, NCH2), 3.69e3.67 (m, 2H, CH2OMe),
3.25 (s, 3H, OCH3), 3.02e2.97 (m, 2H, CH2linker), 2.40 (s, 3H, CH3Cp*),
1.81 (s, 3H, CH3Cp*), 1.75 (s, 3H, CH3Cp*), 0.97 (s, 3H, CH3Cp*). 13C {1H}
136.47e119.54 (CPhenyl, CImid), 70.35e64.29 (CHlinker), 59.05 (CHCp*),
53.85e49.11 (NCH2, CH2OMe), 31.94e29.71þ(CH2linker), 14.67e10.90
(OCH3, CH3Cp*). MS (ESI-TOF) m/z [MꢁI] calcd. for C23H31N2O:
351.24; found, 350.98 [MꢁI]þ.
NMR (100 MHz, acetone-d6):
d 227.0 (CO), 195.3 (Ccarbene-Fe), 138.9
(Cipso-phenyl),130.3 (CHphenyl),129.0 (CHphenyl),127.2 (CHphenyl),124.3
(CHImid), 120.9 (CHImid), 104.9 (CCp*), 92.0 (CCp*), 90.7 (CCp*), 84.3
(CCp*), 81.5 (CCp*), 72.7 (CH2OMe), 67.3 (CHlinker), 58.9 (OCH3), 51.2
(NCH2), 29.40 (CH2linker), 13.5 (CH3Cp*), 10.5 (CH3Cp*), 9.8 (CH3Cp*),
9.6 (CH3Cp*). Anal. Calcd. for C24H29N2O2FeI (560.06): C, 51.45; H,
5.22; N, 5.00. Found: C, 51.20; H, 5.31; N, 4.97. Selected IR data
General preparation of complexes 1e5
A mixture of the respective proligand (Cp*-NHCR)I (R ¼ nBu, iBu,
Et, CH2CH2OMe, CH2Ph) (1.38 mmol) and Fe3(CO)12 (0.46 mmol)
was refluxed in toluene (15 mL) overnight. Filtration and removal of
toluene under vacuum gave a green solid, which was washed with
hexane to afford the corresponding iron complexes 1e5.
(KBr):
C
n
(CO) 1902 vs cmꢁ1. MS (ESI-TOF) m/z [MꢁI]þ calcd. for
24H29N2O2Fe: 433.16; found, 432.89 [MꢁI]þ.
Characterisation of Cp*-NHCCH2Ph)Fe(CO)I (5)
Yield of 5: 74%. 1H NMR (400 MHz, acetone-d6):
5H, CHPhenyl), 7.37e7.27 (m, 5H, CHCH2Ph), 7.06 (s, 1H, CHImid), 6.54
(s, 1H, CHImid), 6.17 (d, J ¼ 1 2 Hz, 1H, CHlinker), 5.51 (dd, J ¼ 14 Hz,
2H, NCH2), 3.07e2.93 (m, 2H, CH2linker), 2.42 (s, 3H, CH3Cp*), 1.76 (s,
3H, CH3Cp*), 1.71 (s, 3H, CH3Cp*), 0.86 (s, 3H, CH3Cp*). 13C{1H} NMR
Characterisation of (Cp*-NHCnBu)Fe(CO)I (1)
d
7.65e7.51 (m,
Yield of 1: 86%. 1H NMR (400 MHz, acetone-d6):
d 7.63e7.52 (m,
5H, CHPh), 7.11 (s, 1H, CHImid), 6.47 (s, 1H, CHImid), 6.06 (d, J ¼ 12 Hz,
1H, CHPhlinker), 4.23e4.17 (m, 2H, NCH2Bu), 3.01e2.96 (m, 2H,
CH2linker, NCH2Bu), 2.40 (s, 3H, CH3Cp*), 1.80 (s, 3H, CH3Cp*), 1.80e1.75
(m, 2H, CH2nBu), 1.75 (m, 3H, CH3Cp*), 1.41e139 (m, 2H, CH3Bu), 0.95
(s, 3H, CH3Cp*), 0.94 (t, J ¼ 8, 3H, CH3nBu). 13C {1H} NMR (100 MHz,
(100 MHz, acetone-d6): d 226.6 (CO), 197.1 (Fe-Ccarbene), 138.9 (CiPh),
130.2 (CHPhenyl), 130.0 (CHPhenyl), 129.3 (CHPhenyl), 129.0 (CHPhenyl),
128.4 (CHPhenyl), 122.9 (CHImid), 122.1 (CHImid), 104.8 (CCp*), 91.7
(CCp*), 90.1 (CCp*), 84.8 (CCp*), 81.7 (CCp*), 69.2 (CHlinker), 54.4
(NCH2Ph), 28.9 (CH2linker), 13.5 (CH3Cp*), 10.4 (CH3Cp*), 9.7 (CH3Cp*),
9.6 (CH3Cp*). Anal. Calc. for C28H29N2OFeI (592.06): C, 56.78; H, 4.94;
N, 4.74. Found: C, 56.47; H, 5.08; N, 4.52. Selected IR data (KBr):
acetone-d6):
d 226.8 (CO), 194.9 (Ccarbene-Fe), 138.8 (Cipso-phenyl),
130.0 (CHphenyl), 129.9 (CHphenyl), 123.0 (CHImid), 121.0 (CHImid),
104.7 (CCp*), 91.6 (CCp*), 90.3 (CCp*), 84.2 (CCp*), 81.0 (CCp*), 67.0
(CHPh-linker), 51.1 (NCH2), 34.0 (CH2nBu), 29.8 (CH2linker), 20.4
(CH2nBu), 14.0 (CH3nBu) 13.4 (CH3Cp*), 10.4 (CH3Cp*), 9.6 (CH3Cp*), 9.5
(CO) 1904 vs cmꢁ1
.
MS (ESI-TOF): m/z [MꢁI]þ calcd. for
(CH3Cp*). Selected IR data (KBr):
25H31N2OFeI (558.08): C, 53.78; H, 5.60; N, 5.02. Found: C, 53.50;
H, 5.86; N, 5.23.
n
(CO) 1905 vs cmꢁ1. Anal. Calcd. for
n
C
28H29N2OFe: 465.16; found 464.91 [MꢁI]þ.
C
Catalytic dehydrogenative silylation of alcohols
Characterisation of (Cp*-NHCiBu)Fe(CO)I (2)
Yield of 2: 84%. 1H NMR (400 MHz, acetone-d6):
d
7.64e7.54 (m,
Acetonitrile (1 mL), (Cp*-NHCR)Fe(CO)I (1 mol%), alcohol
(3 mmol), silane (1 mmol) were charged in a vial. The vial was
tightly closed under a nitrogen atmosphere, and the mixture was
heated at 70 ꢀC for 8e16 h depending on the silane and the alcohol.
All volatiles were evaporated under vacuum, the residue was dis-
solved in CDCl3, and Ph2CH2 (1 mmol) was added to the mixture as
an internal standard. The 1H NMR was measured at room temper-
ature and the amount of the corresponding silylether produced was
evaluated by the relative intensity of the signals of the product and
internal standard. Isolation of the silylethers was carried out by
removing all the volatiles under vacuum. The residue was diluted
with hexanes (ca. 2 mL), loaded directly on to a silica gel column
and chromatographed using hexane-acetone (10:1) as eluent to
give the corresponding silylethers. The silylethers produced were
identified by comparison of the NMR data with the reported data:
5H, CHPhenyl), 7.06 (s, 1H, CHImid), 6.48 (s, 1H, CHImid), 6.08 (d,
J ¼ 12 Hz, 1H, CHlinker), 3.97 (d, J ¼ 8 Hz, 2H, NCH2); 3.02e2.97 (m,
2H, CH2linker), 2.39 (s, 3H, CH3Cp*), 2.27e2.19 (m, 1H, CHiBu, 1.80 (s,
3H, CH3Cp*), 1.75 (s, 3H, CH3Cp*), 0.95 (s, 3H, CH3Cp*), 0.94 (t, J ¼ 8,
3H, CH3nBu). 13C{1H} NMR (100 MHz, acetone-d6):
d 226.8 (CO),
195.2 (Fe-Ccarbene), 138.8 (CipsoPhenyl), 130.8 (CHPhenyl), 129.9
(CHPhenyl), 128.8 (CHPhenyl), 128.3 (CHPhenyl), 127.6 (CHPhenyl), 124.0
(CHImid), 121.7 (CHImid), 104.7 (CCp*), 91.6 (CCp*), 90.4 (CCp*), 84.3
(CCp*), 81.0 (CCp*), 67.1 (CHlinker), 58.3 (NCH2), 29.8 (CHlinker), 29.1
(CHiBu), 20.0 (CH3iBu), 19.8 (CH3iBu), 13.4 (CH3Cp*), 10.4 (CH3Cp*), 9.6
(CH3Cp*), 9.5 (CH3Cp*). Anal. Calcd. for C25H31N2OFeI (558.26): C,
53.78; H, 5.60; N, 5.02. Found: C, 53.40; H, 5.50; N, 4.87. Selected IR
data (KBr):
C
n
(CO) 1905 vs cmꢁ1. MS (ESI-TOF) m/z [MꢁI]þ calcd for
25H31N2OFe: 431.17; found 430.96 [MꢁI]þ.
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J.M.S. Cardoso et al. / Journal of Organometallic Chemistry xxx (2014) 1e5
3
With these complexes in our hands, we decided to explore their
catalytic activity in the dehydrogenative silylation of alcohols.
Silylether formation is a fundamental process in the synthesis of
functional organosilicon compounds and polymer chemistry [22].
Moreover, silylethers are widely used as protecting groups for the
hydroxyl functionality in organic synthesis [23]. The catalytic
dehydrogenative silylation of alcohols is a very convenient method
for the preparation of silylethers from an atom economy point of
view, with H2 as the only by-product formed in the coupling re-
action. Metal-catalysed dehydrogenative coupling of silanes with
alcohols has been described in the literature [24]. However, to the
best of our knowledge, iron-NHC complexes have not been suc-
cessfully employed in this coupling reaction. Dehydrogenative
coupling of thiols with hydrosilanes catalysed by CpFe(CO)Me was
recently disclosed by Nakazawa [25].
Scheme 1. Synthesis of iron complexes 1e6.
The half-sandwich iron-NHC complexes 1e7 (Fig. 2) were
applied as catalysts for the dehydrogenative silylation of alcohols.
Their catalytic activity was initially explored using ethanol and
PhSiH3 as model substrates. The reaction was carried out in
acetonitrile at 70 ꢀC in the presence of 1 mol% of catalyst (Scheme
2). All complexes 1e7 were active catalysts in the dehydrogenative
silylation of ethanol with PhSiH3, affording high yields (91e99%) of
triethoxyphenylsilane in 16 h (Table 1, entries 1e7). Complexes 1
PhSi(OEt)3, PhSi(OiPr)3 [18], Ph2Si(OEt)2 [19], PhSiH[OCH(Me)Ph]2
[20]. The obtained silylether Si(OEt)4 was compared with an
authenticate sample (commercially available).
Results and discussion
Compounds 1e5 were prepared by an extension of a method
recently reported by us [7]. Direct reaction of the iron cluster
Fe3(CO)12 with the respective proligand (Cp*-NHCR)I (R ¼ nBu, iBu,
Et, CH2CH2OMe, CH2Ph) in refluxing toluene afforded complexes
1e5 in high yields (Scheme 1). Complexes 1e5 were isolated as air
stable green crystalline solids. The identity of all compounds was
established by analytical and spectroscopic methods. The success-
ful metallation is confirmed by the appearance of the characteristic
carbene signal at 195 ppm (for complexes 1e4) and 197 ppm (for 5)
in the 13C NMR, which is the region of previously reported half-
sandwich FeeNHC complexes [6,10a,10h,13,21]. The length of the
alkyl substituents of the wingtip has no detectable impact on the
FeeCcarbene resonance frequency. Introduction of a benzyl group at
the wingtip decreases the upfield shift by 2 ppm. The formation of
the new complexes is further verified by the carbonyl signal at
227 ppm in the 13C NMR.
i
and 2, bearing the nBu and Bu wingtips respectively, displayed
slightly higher activities affording quantitative yields of the sily-
lether in 8 h (Table 1, entries 1 and 2, respectively), while longer
reaction times (12e16 h) were needed for complexes 3e6 to afford
similar yields (Table 1, entries 3e6). Interestingly, the different
substitution on the cyclopentadienyl ring (Cp vs Cp*) does not seem
to affect the catalytic performance in the dehydrogenative silyla-
tion of alcohols (Table 1, entries 6 and 7). For all catalysts, the re-
action was selective to the formation of triethoxyphenylsilane, and
no other coupling products were formed in the reaction. It seems
that variation of the wingtip substituents at R (Fig.1) does not affect
the catalytic performance.
We investigate the scope of the dehydrogenative silylation of
alcohols, using catalyst 1, EtOH and different silanes. In each case,
reactions were performed under optimised conditions heating at
70 ꢀC in MeCN. As shown in Table 1, phenylsilane, diphenylsilane,
and triethoxysilane were all very efficient substrates when reacted
with ethanol, giving high yields (85e99%) of the corresponding
silylethers in 8 h (Table 1, entries 1, 8, and 9, respectively). When
the reaction was performed with tert-butyldiphenylsilane, a 70%
The infrared spectra of 1e5 display the expected strong carbonyl
band in the region 1905e1902 cmꢁ1. The similar stretching band
frequency for all complexes 1e5 is indicative of similar donor ca-
pacity of the N-heterocyclic carbene ligands which is not influenced
by the wingtip substituents.
Fig. 2. Iron complexes applied in the catalytic dehydrogenative silylation of alcohols with silanes.
Please cite this article in press as: J.M.S. Cardoso, et al., Journal of Organometallic Chemistry (2014), http://dx.doi.org/10.1016/
j.jorganchem.2014.06.005

4
J.M.S. Cardoso et al. / Journal of Organometallic Chemistry xxx (2014) 1e5
B.R. thanks FCT for Consolidation contract IF/00346/2013. J.M.S.
Cardoso thanks FCT for a doctoral grant (SFRH/BD/66386/2009).
The NMR spectrometers are part of the National NMR Facility
supported by FCT (RECI/BBB-BQB/0230/2012). We wish to
acknowledge M. C. Almeida for providing data from the Elemental
Analyses and Mass Spectrometry Services at ITQB.
Scheme 2. Catalytic dehydrogenative silylation of ethanol with phenylsilane.
Table 1
Dehydrogenative silylation of alcohols with silanes using iron complexes 1e7 as
catalysts.a
Appendix A. Supplementary material
Entry Catalyst Silane
Alcohol
Product
Time Yieldb (%)
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2014.06.005.
1
1
2
3
4
5
6
7
1
1
1
1
1
1
1
1
1
1
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
PhSi(OEt)3
PhSi(OEt)3
PhSi(OEt)3
PhSi(OEt)3
PhSi(OEt)3
PhSi(OEt)3
PhSi(OEt)3
Ph2Si(OEt)2
Si(OEt)4
tBuPh2Si(OEt)
e
8
8
99 (92)
91 (87)
99 (93)
99 (91)
95 (87)
96 (87)
84 (79)
82 (77)
98 (91)
70 (58)
0
2
3
4
5
6
7
12
12
16
16
16
8
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Please cite this article in press as: J.M.S. Cardoso, et al., Journal of Organometallic Chemistry (2014), http://dx.doi.org/10.1016/
j.jorganchem.2014.06.005

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