Coupling of an N-heterocyclic carbene on iron with alkynes to form η5-cyclopentadienyl-diimine ligands

Article abstract of DOI:10.1002/anie.201309021

A cyclometalated N-heterocyclic carbene ligand in a half-sandwich iron complex was found to couple with alkynes, leading to a unique type of ring opening of the carbene ligand and the formation of ferrocenyl-diimine complexes. An intermediary iron complex obtained from the reaction with phenylacetylene reveals that the ring opening follows the formation of a fused heterocycle consisting of an imidazole ring and two alkynes. Iron sandwich with a side of diimines: A cyclometalated N-heterocyclic carbene ligand in a half-sandwich iron complex was found to couple with alkynes, leading to a unique type of ring opening of the carbene ligand and the formation of ferrocenyl-diimine complexes.

Full text of DOI:10.1002/anie.201309021

Angewandte
Chemie
DOI: 10.1002/anie.201309021
Iron Carbene Chemistry
Coupling of an N-Heterocyclic Carbene on Iron with Alkynes to Form
h5-Cyclopentadienyl-Diimine Ligands**
Tsubasa Hatanaka, Yasuhiro Ohki,* and Kazuyuki Tatsumi*
Abstract: A cyclometalated N-heterocyclic carbene ligand in
a half-sandwich iron complex was found to couple with
alkynes, leading to a unique type of ring opening of the carbene
ligand and the formation of ferrocenyl–diimine complexes. An
intermediary iron complex obtained from the reaction with
phenylacetylene reveals that the ring opening follows the
formation of a fused heterocycle consisting of an imidazole
ring and two alkynes.
N
-heterocyclic carbenes (NHCs) are a growing class of
ligands for transition metals,[1] and like phosphines they serve
as two-electron donors in homogeneous catalysts. An advant-
age for NHCs is their strong s-donating ability, which leads to
strong bonding to various metals. Whereas this property is
useful for preventing the decomposition of the metal com-
plexes, it has been recently demonstrated that NHCs some-
times react with other ligands within the coordination sphere
of metals.[2,3] The reported examples involve the migratory
insertion of an NHC ligand into metal–carbon bonds,[2b–e] and
the migration of alkyl or aryl groups to the carbene carbon of
NHC ligands followed by ring opening.[3] The ring opening of
NHC ligands has been also demonstrated with a dinuclear
ruthenium hydride complex[4] and with main group hydrides.[5]
The removal of an N-bound substituent of NHC ligands has
been demonstrated with Ni, Ru, and Fe complexes.[6] In this
paper, we report an unprecedented type of coupling reaction
between a cyclometalated NHC ligand on iron and alkynes at
608C, leading to the ring opening of an NHC ligand and the
formation of h5-cyclopentadienyl ligands carrying a diimine
moiety. An intermediary iron complex was also obtained from
the reaction of the iron complex with phenylacetylene at
room temperature.
Scheme 1. Reactions of 1 with alkynes.
two phenyl groups at the 2- and 4-positions of the cyclo-
pentadienyl ring, were isolated in 79% yield (Scheme 1).
Similarly, the reaction of 1 with diphenylacetylene gave an
analogous complex 2b, which was crystallized in 40% yield.
From the difference in formula between 2a and 1 + 2(phe-
nylacetylene), one can see the loss of the -CH(CH3)CH2-
group during the reaction. As described below, the lost group
was observed as propylene in the 1H NMR experiment.
Whereas complex 2a may be formed together with its
regioisomers, in terms of the positions of Ph groups on the
h5-cyclopentadienyl ring, the 1H and 13C{1H} NMR spectra of
the reaction mixture and the high yield of isolated 2a support
its selective formation. Two methyl signals for the iPr group of
the diimine moiety in 2a were observed at d = 1.18 and
1.16 ppm in the 1H NMR spectrum and at d = 24.13 and
24.09 ppm in the 13C{1H} NMR spectrum, owing to the
asymmetric structure of the h5-C5H2Ph2(diimine) ligand. The
cyclic voltammetry of 2a,b showed the reversible FeII/FeIII
redox couple of the ferrocenyl group at E1/2 = À0.11 V (2a)
and À0.15 V (2b) vs. Ag/Ag+ (in CH2Cl2), and these
potentials are similar to that of Cp*FeCp (À0.18 V).[8]
The titled reaction was discovered during the course of
our studies on the reactions of iron-NHC complexes.[7]
Treatment of complex 1[7a] bearing a cyclometalated NHC
ligand with two equivalents of phenylacetylene at 608C
resulted in the formation of a red solution after three days,
from which crystals of ferrocenyl–diimine complex 2a, having
[*] Dr. T. Hatanaka, Prof. Dr. Y. Ohki, Prof. Dr. K. Tatsumi
Department of Chemistry, Graduate School of Science, and
Research Center for Materials Science, Nagoya University
Furo-cho, Chikusa-ku, Nagoya 464-8602 (Japan)
E-mail: ohki@chem.nagoya-u.ac.jp
The molecular structures of 2a,b were determined by X-
ray crystallographic analysis. As shown on the left in Figure 1,
complex 2a reveals a typical sandwich structure, and one of
the h5-cyclopentadienyl rings carries two phenyl groups and
a diimine moiety. Whereas 2a is a new planar-chiral
ferrocene,[9,10] it crystallizes in a centrosymmetric space
group (P-1) as a result of the formation of a racemic couple.
The composition of the new h5-cyclopentadienyl ligand
suggests that the five-membered ring consists of two phenyl-
i45100a@nucc.cc.nagoya-u.ac.jp
[**] This research was financially supported by Grant-in-Aids for
Scientific Research (23000007, 23685015, 25105725, 25109522)
from the Ministry of Education, Culture, Sports, Science and
Technology (Japan).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201309021.
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Fe–C(C2–C5) interaction may be described as in between the
h4-1,3-butadiene and the bent h4-metallacyclo-3-pentene
structures, although the structure of 3 in Scheme 1 is drawn
as the latter. The C1 and N1 atoms, which are from the NHC
ligand of 1, are bound to carbon atoms originated from
phenylacetylene, C2 and C5, respectively, and the N-contain-
ing five-membered ring has the cationic imidazolium charac-
ter. Indeed, the C–N and C–C distances of the five-membered
ring are comparable to those for pentaalkyl imidazolium
cations.[12]
As the formula of 3 is the same as 2a, we assumed that 3 is
an intermediate for the synthesis of 2a. This hypothesis was
supported by the H NMR experiment, and heating a C6D6
solution of isolated 3 at 608C for three days resulted in the
formation of 2a in > 99% yield based on an internal standard
(C6Me6). On a preparative scale, 2a was isolated as crystals in
90% yield. Whereas the reaction mechanism remains uncer-
tain, the bond formation between the C1 and C5 atoms of 3
Figure 1. Molecular structures of 2a and 3, with thermal ellipsoids at
the 50% probability level. Selected bond distances (ꢀ) and angles (8):
2a, Fe–C(1–5)=2.041(2)–2.054(2), N1–C6=1.281(3), C6–C8=
1.506(3), C8–N2=1.262(3). 3, Fe–C2=2.012(3), Fe–C3=1.981(3), Fe–
C4=1.983(2), Fe–C5=2.053(2), C1–C2=1.449(3), C2–C3=1.484(4),
C3–C4=1.438(5), C4–C5=1.455(4), C5–N1=1.483(3), N1–C1=
1.336(4), C1–N2=1.361(3), N1-C1-N2=107.6(2).
1
À
À
and the cleavage of C1 N2 and C5 N1 bonds are probably
involved in this process.
acetylene molecules and the carbene carbon. The incorpo-
ration of the carbene carbon in the new cyclopentadienyl
group results in the opening of the ring of the NHC ligand,
and the remaining four atoms of the NHC ring provide
a diimine moiety. The formation of a diimine moiety is
In summary, we have found a unique type of coupling
reaction between a cyclometalated NHC ligand and alkynes,
producing h5-cyclopentadienyl ligands with a diimine moiety
through the evolution of propylene associated with C N bond
cleavage, followed by ring-opening of the NHC ligand.
À
=
indicated by the short N C distances (1.262(3) and
À
1.281(3) ꢀ) and the C6 C8 single bond (1.506(3) ꢀ). Com-
plex 2b is structurally very similar to 2a, although its Fe-C(h5-
C5Ph4(diimine)) (2.0570(14)–2.0722(15) ꢀ) and Fe-C(Cp*)
(2.0657(14)–2.0826(15) ꢀ) distances are longer than the Fe-
C(h5-C5H2Ph2(diimine)) (2.041(2)–2.054(2) ꢀ) and Fe-C-
(Cp*) (2.039(2)–2.053(2) ꢀ) distances in 2a, possibly due to
the steric congestion caused by the four phenyl groups on the
cyclopentadienyl ring.
Received: October 16, 2013
Published online: January 31, 2014
Keywords: bond cleavage · diimines · iron ·
.
N-heterocyclic carbene · sandwich complexes
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