An iron(II) complex of a diamine-bridged bis-N-heterocyclic carbene

Article abstract of DOI:10.1021/om300888q

A diamine-bridged bis-N-heterocyclic carbene ligand was synthesized as imidazolium salts with chloride or triflate counteranions. Deprotonation of the chloride salt by an iron bis-amide complex or addition of the free diamine-bis-carbene ligand to FeCl₂ led to the formation of an Fe(II) dichloride complex having a diamine-bis-carbene ligand, in which the iron atom is in a highly distorted octahedral geometry with weak Fe-N interactions. This iron complex was found to catalyze the hydrosilylation of acetophenone.

Full text of DOI:10.1021/om300888q

Communication
pubs.acs.org/Organometallics
An Iron(II) Complex of a Diamine-Bridged Bis-N-Heterocyclic Carbene
Christoph Grohmann,† Takayoshi Hashimoto,‡ Roland Frohlich,† Yasuhiro Ohki,*,‡ Kazuyuki Tatsumi,*,‡
̈
and Frank Glorius*,†
†Organisch-Chemisches Institut der Westfalischen Wilhelms-Universitat Munster, Corrensstrasse 40, 48149 Munster, Germany
̈
̈
̈
̈
‡Department of Chemistry, Graduate School of Science, and Research Center for Materials Science, Nagoya University, Furo-cho,
Chikusa-ku, Nagoya 464-8602, Japan
S
* Supporting Information
ABSTRACT: A diamine-bridged bis-N-heterocyclic carbene
ligand was synthesized as imidazolium salts with chloride or
triflate counteranions. Deprotonation of the chloride salt by an
iron bis-amide complex or addition of the free diamine-bis-
carbene ligand to FeCl2 led to the formation of an Fe(II)
dichloride complex having a diamine-bis-carbene ligand, in
which the iron atom is in a highly distorted octahedral
geometry with weak Fe-N interactions. This iron complex was
found to catalyze the hydrosilylation of acetophenone.
-heterocyclic carbenes (NHCs) have already become an
Nindispensable class of ligands for transition-metal
complexes applied in homogeneous catalysis.1,2 Their strong
σ-donating ability renders them valuable substitutes for
phosphine ligands, exhibiting more stable metal−ligand
bonding and thus increased complex robustness. In addition
to complexes of noble metals, recently Fe-NHC complexes
have also gained considerable attention, due to their possible
application as cheap, environmentally benign, and powerful
catalyst precursors.3 Well-defined4,5 and in situ generated6 Fe-
NHC complexes have been applied in homogeneous catalysis,
and most of them carry monodentate NHC ligands. NHCs
incorporated in a chelating scaffold have advantages in
preventing undesirable substitution of the NHC moiety, leading
to enhanced complex stability and to possible modulation of
reactivity. This concept has been demonstrated for iron
complexes having alkyl-bridged bidentate bis-
(imidazolylidene),7 tripodal tris(imidazolylidene)borate8 and
-amine,9 pincer type 2,6-bis(imidazolylidene)pyridine,10 and
tetradentate tetrakis(imidazolylidene)11 ligands.
using two pyridine/quinoline and amine donors; these have
been shown to afford iron complexes effective for challenging
C−H oxidation reactions.12e−i In this study, we envisioned the
replacement of the terminal nitrogen donor groups by NHCs,
such as A in Figure 1, but taking into account the possible side
reactions at the aminal moiety of A, we chose to incorporate an
aromatic ring system in place of the bridging methylene unit of
A. We also considered a sterically less demanding methyl group
as the substituent of the NHC moiety, to reduce the possible
steric congestion around the metal center.
Herein, we report the synthesis and characterization of a new
Fe-NHC complex bearing the bis(imidazo[1,5-a]pyridine-3-
ylidene) ligand BMeIPyEN, which features two amine moieties
in addition to two NHC groups. Preliminary results on the
hydrosilylation of acetophenone catalyzed by the new Fe-NHC
complex are also presented.
As shown in Scheme 1, the synthesis of the imidazolium salt
starts from the reaction of N,N′-dimethylethylenediamine
(DMEDA) with 2,6-dibromopyridine in the presence of
K2CO3, where compound 1 was obtained in 92% yield.
Subsequent formylation with n-butyllithium and dimethylfor-
mamide afforded the dialdehyde 2 in 77% yield. For the
following step to prepare the imidazolium salts, two routes were
engaged: one follows a method reported by Aron et al.,13 and
the dialdehyde 2 was reacted with methylamine hydrochloride
and formaldehyde in ethanol at 50 °C to afford the imidazolium
chloride 3a in 75% yield. Alternatively, the imine 4 was formed
by the reaction of dialdehyde 2 with methylamine in THF, and
4 was treated with a reagent mixture of silver triflate and
chloromethylpivalate to give the imidazolium triflate 3b, in
accordance with the reported procedure.14 Both routes proved
Our design of a new chelating NHC ligand was inspired by
the cis-α coordination geometry of nonheme iron(II) catalysts
with tetradentate nitrogen ligands, which provide two cis-
oriented binding sites at the iron center. Representative
examples are BPMEN12a,b and BQEN12c,d (Figure 1), each
Received: September 16, 2012
Published: November 1, 2012
Figure 1. Design of the chelating bis-carbene ligand.
© 2012 American Chemical Society
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Scheme 1. Synthesis of the Imidazolium Salts
Figure 2. Crystal structure of 6 with 50% probability ellipsoids.
Hydrogen atoms and a solvent molecule (MeCN) are omitted for
clarity. Selected bond lengths (Å): Fe(1)−C(1), 2.1380(14); Fe(1)−
C(20), 2.1172(14); Fe(1)−Cl(1), 2.4386(10); Fe(1)−Cl(2),
2.4130(10); Fe(1)−N(4), 2.5591(19); Fe(1)−N(3), 2.5803(15).
Selected bond angles (deg): C(1)−Fe(1)−C(20), 151.66(6);
Cl(1)−Fe(1)−C(20), 104.13(5); Cl(2)−Fe(1)−C(1), 101.76(7);
Cl(1)−Fe(1)−Cl(2), 98.51(2).
to be suitable for the preparation of the bis(imidazolium)salts.
Single crystals were obtained for the triflate salt 3b, and its
structure was determined by an X-ray analysis (Figure S1,
Supporting Information).
The iron complex of the new chelating NHC ligand was
synthesized via two routes (Scheme 2).4,7,10 Treatment of the
Scheme 2. Preparation of a Bis-Carbene Iron(II) Chloride
Complex from Imidazolium Salts
octahedral geometry with two NHC ligands, two chlorides, and
two weakly interacting nitrogen donors of the ligand backbone.
The Fe−C(carbene) distances of 2.1172(14) and 2.1380(14) Å
are comparable to those in the high-spin Fe(II) bis-carbene
complexes.7 Due to the rigid framework of the ligand, the C−
Fe−C angle of 151.66(6)° is smaller than that expected for an
ideal octahedral geometry. Whereas 6 can be also seen as a
highly distorted tetrahedral complex, the Fe−N(backbone)
distances of 2.5591(14) and 2.5803(15) Å display weak
interactions between the iron center, indicating more
resemblance to an octahedral geometry. Thus, complex 6
exhibits a unique structure, with a distorted geometry of iron
between octahedral and tetrahedral.18
With the new iron complex 6 in a unique and rigid
coordination geometry, we were interested in the hydro-
silylation of acetophenone. Whereas there has been consid-
erable progress in iron-catalyzed hydrosilylation over the last
years,19 only a limited number of Fe-NHC complexes have
been employed.4k,5
Table 1 summarizes the results of catalytic hydrosilylation of
acetophenone using diphenylsilane in slight excess (1.1 equiv).
Acetonitrile was used as the solvent to readily dissolve 6 in
entries 1 and 3−5. Heating the reaction mixture at 60 °C for 16
h followed by workup afforded the alcohol 7 in 56% yield
(entry 1).20 Analogous reactions in a mixed solvent of MeCN
and THF (entry 2) or at higher temperature (entry 3) were not
effective in improving the yield. Addition of sodium or silver
salts (entries 4 and 5) for removal of chlorides in 6 resulted in
lower conversion, possibly due to decreased stability of the
cationic complex. Interestingly, when methyllithium was added
to generate an Fe-methyl species (entry 6),21 the catalytic
reaction reached full conversion and gave 95% yield of alcohol
7. Among other silanes tested,22 only triethoxysilane (entry 7)
afforded alcohol 7 in good yield, whereas triethylsilane (entry
8) gave no considerable conversion. To rule out the possibility
of a base-catalyzed hydrosilylation,23 the reaction was
performed without addition of iron complex 6, showing only
15
imidazolium chloride 3a with Fe{N(SiMe3)2}2 in THF at 60
°C afforded the iron(II) dichloride complex 6 as a yellow
crystalline powder in 44% yield. Complex 6 was alternatively
prepared from the reaction of FeCl2 with free NHC ligand 5,
which was prepared by treatment of 3a with potassium tert-
butoxide in THF. Formation of the triflate analogue of 6 was
indicated by an ESI-MS analysis of the reaction mixture of 3b
with Fe{N(SiMe3)2}2; however, we have been unable to isolate
it.16
Complex 6 is moderately sensitive to air and moisture, both
in the solid state and in solution. The magnetic moment of 6
measured by SQUID magnetometry was 5.39−5.65 μB at 50−
300 K (Figure S2, Supporting Information), which is slightly
larger than the spin-only value for the S = 2 state (4.90 μB). The
S = 2 state is in accordance with the high-spin electronic
configuration of the d6 iron complex. The ESI-MS spectrum of
6 in acetonitrile displays a major signal at m/z 439.1, which can
be attributed to the cationic species [6 − Cl]+ generated by
dissociation of a chloride from 6. Thus, this cationic species
probably exists in a minor quantity in equilibrium with 6 in
acetonitrile solution. Cyclic voltammetry was measured in
acetonitrile and showed a reversible FeII/FeIII redox couple at
−0.37 V versus Ag/Ag+ (Figure S3, Supporting Information),17
which is assignable to [6]0/+
.
The molecular structure of 6 was determined by X-ray
crystallography (Figure 2). The iron atom is in a distorted-
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Communication
a
No. 25936. We thank Professors Hirofumi Yoshikawa, Michio
Matsushita, and Kunio Awaga (Nagoya University) for aiding
us with the SQUID magnetometer and Dr. Constantin Daniliuc
Table 1. Hydrosilylation of Acetophenone Catalyzed by 6
(Universitat Munster) for help with the crystallography data.
̈
̈
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ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, tables, and CIF files giving experimental
procedures and analytical data for all new compounds,
including crystallographic data, cyclic voltammograms and
SQUID measurements of compound 6. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: ohki@chem.nagoya-u.ac.jp (Y.O.); i45100a@nucc.cc.
nagoya-u.ac.jp (K.T.); glorius@uni-muenster.de (F.G.).
́
(5) Hydrosilylation catalyzed by Fe-NHCs: (a) Jiang, F.; Bezier, D.;
■
Sortais, J.-B.; Darcel, C. Adv. Synth. Catal. 2011, 353, 239. (b) Bezier,
D.; Jiang, F.; Roisnel, T.; Sortais, J.-B.; Darcel, C. Eur. J. Inorg. Chem.
2012, 1333. (c) Castro, L. C. M.; Sortais, J.-B.; Darcel, C. Chem.
Commun. 2012, 48, 151. Hydrosilylation catalyzed by in situ formed
Fe-NHCs: (d) Buitrago, E.; Zani, L.; Adolfsson, H. Appl. Organomet.
Chem. 2011, 25, 748. (e) Buitrago, E.; Tinnis, F.; Adolfsson, H. Adv.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was performed within the framework of the
International Research Training Group (IRTG) Munster/
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Research (Nos. 23000007 and 23685015) from the Ministry of
Education, Culture, Sports, Science and Technology of Japan.
Generous financial support was also provided by the European
Research Council under the European Community’s Seventh
Framework Program (FP7 2007-2013)/ERC Grant agreement
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dx.doi.org/10.1021/om300888q | Organometallics 2012, 31, 8047−8050