Synthesis of Complexes with Abnormal "protic" N-Heterocyclic Carbenes

Article abstract of DOI:10.1002/anie.201507206

Neutral 4-iodo-N-ethylimidazole 3 oxidatively adds to [Pt(PPh₃)₄] to give, in the presence of different tetraalkylammonium salts, complexes trans-[4], trans-[5], and trans-[6] containing an anionic C4-bound heterocycle with an unsubstituted ring-nitrogen atom. Complex trans-[4] reacts with the proton source NH₄I under protonation of the ring-nitrogen atom to produce complex trans-[7]I which bears an NH,NR-substituted aNHC ligand. The reaction of trans-[4] with CH₃I yields the complex trans-[8]I which has a classical aNHC ligand with two alkylated ring-nitrogen atoms.

Full text of DOI:10.1002/anie.201507206

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201507206
German Edition: DOI: 10.1002/ange.201507206
Abnormal N-Heterocyclic Carbenes
Synthesis of Complexes with Abnormal “Protic” N-Heterocyclic
Carbenes
Hanpeng Jin, Tristan Tsai Yuan Tan, and F. Ekkehardt Hahn*
Abstract: Neutral 4-iodo-N-ethylimidazole 3 oxidatively adds
to [Pt(PPh3)4] to give, in the presence of different tetraalkyl-
ammonium salts, complexes trans-[4], trans-[5], and trans-[6]
containing an anionic C4-bound heterocycle with an unsub-
stituted ring-nitrogen atom. Complex trans-[4] reacts with the
proton source NH4I under protonation of the ring-nitrogen
atom to produce complex trans-[7]I which bears an NH,NR-
substituted aNHC ligand. The reaction of trans-[4] with CH3I
yields the complex trans-[8]I which has a classical aNHC
ligand with two alkylated ring-nitrogen atoms.
bearing related potentially mesoionic remote NHCs (rNHCs)
have also been described.[9]
The number of complexes bearing aNHC ligands is still
limited and mainly two methods for their synthesis have
emerged over the last years. The deprotonation method
employs rather strong bases for the removal of the C4 or C5
proton from imidazolium salts to yield the aNHC in situ,
which can then coordinate to a suitable metal center.[5,6]
Given the high acidity of the C2 proton of imidazolium
salts, deprotonation at this position constitutes a competing
reaction to the C4/C5 deprotonation. The undesired depro-
tonation at the N-CH-N position is normally prevented by
a protection group R2 at the C2 position[5,8] (such as in D) or
by the selection of the R1 and R3 substituents of the
imidazolium salt.[10] In selected cases, the anion YÀ present
in the imidazolium salt also influences the ratio of C2 to C4/
C5 deprotonation.[11] For the in situ generation of aNHC
ligands, the base for the deprotonation can also be a ligand of
the metal precursor to which the aNHC is subsequently
supposed to coordinate.[12] An alternative method is the
synthesis via oxidative addition of imidazolium salts. Low-
valent Group 10 element (mostly Pd0 and Pt0) precursors have
R
esearch on metal complexes bearing N-heterocyclic car-
bene ligands (NHCs) A (Figure 1) has experienced an
explosive development over the last two decades.[1] A vast
number of differently substituted NHCs and their metal
complexes have been described.[2] Due to the facile tunability
of the excellent s-donor properties of the NHC ligands and
the rather high stability of their complexes, NHC complexes
have found multiple applications in homogeneous catalysis[3]
and other areas.[4]
À
been used to activate the C4/C5 H bond of C2-protected
imidazolium salts via an oxidative addition with formation of
aNHC complexes.[13] If the imidazolium starting material
bears both a donor substituent at one of the ring-nitrogen
atoms and a C5–X bond (X = Br, I), protection of the C2
position is not necessary and exclusively oxidative addition of
À
the C5 X bond to give the aNHC/N-donor chelate complex
Figure 1. NHC and aNHC complexes.
was observed.[14]
Recently, complexes bearing protic NHC ligands E have
been described (Scheme 1). Instead of possessing two alky-
lated or arylated nitrogen atoms, protic NHC ligands feature
an NH,NR or NH,NH substitution pattern on the heterocycle.
Complexes with protic NHCs can be obtained by the
template-controlled cyclization of b-functionalized isocya-
nides[15] or more conveniently by the oxidative addition of 2-
halogenoazoles to transition metals in a low oxidation state
(Scheme 1).[16] The latter method also allows the convenient
C8-metalation of biomolecules such as caffeine and ade-
nine.[17] Complexes bearing the related protic aNHC ligand
such as F (featuring a protonated ring-nitrogen atom and a C2
Less common than complexes bearing normal NHCs are
those bearing an abnormal NHC (aNHC, also called meso-
ionic carbenes, MICs)[5] ligand B (Figure 1) featuring a C4- or
C5-metalated heterocycle. The first example of an aNHC
complex, C, was described in 2001.[6] It has been demonstrated
that aNHC ligands exhibit stronger s-donor and weaker p-
acceptor properties than their NHC congeners[2b, 7] and even
the stable free aNHC ligand D featuring protection groups at
the C2 and C5 positions of the heterocycle has been isolated.[8]
The superb s-donor properties of aNHC ligands make their
complexes an attractive synthetic target and some complexes
[*] H. Jin, T. T. Y. Tan, Prof. F. E. Hahn
Institut für Anorganische und Analytische Chemie
Westfälische Wilhelms-Universität Münster
Corrensstrassee 30, 48149 Münster (Germany)
E-mail: fehahn@uni-muenster.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201507206.
Scheme 1. Preparation of complexes with protic NHC ligands.
Angew. Chem. Int. Ed. 2015, 54, 13811 –13815
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
13811

.
Angewandte
Communications
proton) are not known yet. In our quest to prepare such
complexes we studied the oxidative addition of 4-halogeno-N-
alkylazoles to Pt0 complexes. Here we describe this oxidative
addition which in the presence of a proton source under mild
conditions yields PtII complexes of type F bearing a protic
aNHC ligand.
Compound 4-iodo-N-ethylimidazole 3 was obtained from
4,5-diiodoimidazole 1 by N-alkylation to give 2 followed by
reductive removal of one iodo substituent using a published
procedure for related compounds[18] (Scheme 2, see the
and trans-[6] feature very similar resonances for the H2 and
H5 protons. The 13C{1H} NMR spectrum in addition to the
heteronuclear multiple-bond correlation spectrum (HMBC,
recorded due to signal overlap in the 13C{1H} spectra) show
the signal of the C4 carbon atom of complex trans-[4] at d =
130.9 ppm. The equivalent resonances for trans-[5] (d =
126.2 ppm) and trans-[6] (d = 124.0 ppm) were found at
similar chemical shifts. Compared to the starting material 3
(d(C4) = 81.3 ppm in CDCl3) the signals for the C4 carbon
atom in complexes trans-[4]—trans-[6] experience a signifi-
cant downfield shift upon metalation with PtII. The degree of
the downfield shift for the C4 resonances corresponds to the
donor effect of the halogens in the order I > Br> Cl.[7b,19]
Only one singlet was detected in the 31P{1H} NMR spectra of
trans-[4]—trans-[6] (trans-[4] d = 20.0 ppm, 1JPt,P = 2930 Hz;
trans-[5] d = 22.1 ppm, 1JPt,P = 2942 Hz; trans-[6] d =
23.1 ppm, 1JPt,P = 2987 Hz) and the chemical shift of this
resonance also depends on the halogeno ligand present.[19]
These singlet resonances indicate the coupling of two chemi-
cally equivalent phosphorus atoms to the platinum atom and
this implies a trans arrangement of the phosphorus donors in
square–planar complexes. While no platinum complexes of
type trans-[4] have been described yet, the C4 13C{1H}
resonances observed for trans-[4]—trans-[6] compare well
with the C(aNHC) resonances observed for platinum com-
plexes with conventional (both ring-nitrogen atoms alky-
lated) aNHC ligands (d(aNHC) = 124.4 and 126.3 ppm;[13]
d(aNHC) = 119.0–126.0 ppm[12]). Formation of complexes
trans-[4]—trans-[6] was also confirmed by mass spectrometry.
The HR-ESI mass spectra (positive ions) show for all three
complexes the peak with highest intensity for the complex
cations [4 + H]+, [5 + H]+, and [6 + H]+, each with the correct
isotope distribution (see the Supporting Information).
Scheme 2. Oxidative addition of 4-iodo-N-ethylimidazole to [Pt0-
(PPh3)4].
The composition and coordination geometry of trans-[4]
was unequivocally established by an X-ray diffraction study.
Crystals of composition trans-[4]·2CH3OH were obtained by
the slow diffusion of pentane into a saturated methanol
solution of trans-[4] at ambient temperature. The structure
analysis (Figure 2)[20] confirms the trans arrangement of the
Supporting Information). Compound 3 reacts with [Pt-
(PPh3)4] in acetonitrile at 608C over 20 h to afford complex
trans-[4] in an excellent yield of 78% (Scheme 2). The
oxidative addition proceeds under rather mild conditions
À
without affecting the C2 H bond. In the presence of an excess
of tetrabutylammonium bromide or tetrabutylammonium
chloride, complexes trans-[5] and trans-[6] were obtained in
79% and 75% yield, respectively (see the Supporting
Information). Complex trans-[4] and its derivatives can be
considered to contain the heterocycle bonded to platinum(II)
via a carbanionic C4 atom (trans-[4]) or, alternatively, as
a complex bearing an anionic aNHC ligand (trans-[4’])
featuring an unsubstituted N3 ring-nitrogen atom.
Complexes trans-[4]—trans-[6] were fully characterized
by NMR spectroscopy. While the complexes are stable
towards air and moisture, they are only poorly soluble in
nonpolar solvents. Thus, a mixture of CD3OD and CD2Cl2
(10:1, v/v) was used for the NMR measurements. The
1H NMR spectrum of trans-[4] (see the Supporting Informa-
tion) features the resonances for the H2 and H5 protons as
singlets at d = 6.71 ppm and d = 5.51 ppm, respectively. The
resonance for the H2 proton disappeared after 12 h which is
indicative of enhanced acidity and an H/D exchange at the C2
position after C4 metalation. The 1H NMR spectra of trans-[5]
Figure 2. Molecular structure of trans-[4] in trans-[4]·2CH3OH. Hydro-
gen atoms have been omitted for clarity. Selected bond lengths [Š] and
angles [8]: Pt-I 2.6858(3), Pt-P1 2.3055(9), Pt-P2 2.3048(9), Pt-C4
2.005(4), N1-C2 1.338(7), N1-C5 1.375(6), N3-C2 1.330(6), N3-C4
1.395(5), C4-C5 1.383(6); I-Pt-P1 92.23(2), I-Pt-P2 89.82(2), I-Pt-C4
174.81(11), P1-Pt-P2 177.63(4), P1-Pt-C4 87.41(10), P2-Pt-C4 90.44(10),
C2-N1-C5 106.4(4), C2-N3-C4 105.5(4), N1-C2-N3 112.9(4), N3-C4-C5
107.8(4), N1-C5-C4 107.5(4).
13812 www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 13811 –13815

Angewandte
Chemie
two triphenylphosphine donors. The coordination geometry
around the platinum atom is slightly distorted square–planar
with bond angles of P1-Pt-P2 177.63(4)8 and I-Pt-C4 174.81-
(11)8. The plane of the aNHC ligand is oriented almost
perpendicular to the platinum coordination plane. The Pt–C4
bond length of 2.005(4) Š falls in the range observed
previously for conventional platinum aNHC complexes.[12]
Both the classical NHC bearing (type A)[21] and the protic
NHC bearing (type E)[16c,d] analogues of trans-[4] have been
described. A comparison of the Pt-C2/4 bond lengths in these
complexes reveals that the N,N-dimethylimidazolylidene
complex (type A) features a Pt–C2 bond length of 1.995(5) Š,
which is identical within experimental error to the Pt–C4
bond length observed for trans-[4]. For the complex bearing
the protic NH,NH-NHC ligand (type E) a shorter Pt–C2 bond
length of 1.978(5) Š was found which might be due to sterics
as the protic NH,NH-NHC ligand lacks sterically demanding
N,N’ substituents. No indications for the enhanced s-donor
capability of the aNHC ligand in trans-[4] can be derived from
[D6]DMSO). In addition, the H2 and H5 protons are detected
at d = 7.95 ppm and d = 6.27 ppm, respectively, more than
1 ppm downfield relative to the analogous signals of trans-[4].
The 13C{1H} NMR spectrum (in [D6]DMSO) features the
resonance for the C4 carbon atom as a triplet at d = 126.3 ppm
(2JC,P = 10.6 Hz). In spite of the protonation, this resonance is
shifted slightly upfield compared to the C4 resonance for
trans-[4] (d = 130.9 ppm). Only one singlet was observed in
the 31P{1H} NMR spectrum (d = 17.2 ppm, 1JPt,P = 2658 Hz)
confirming the trans arrangement of the two PPh3 ligands.
An X-ray diffraction analysis with crystals of the compo-
sition trans-[7]I, obtained by slow diffusion of diethyl ether
into a concentrated solution of trans-[7]I in dichloromethane
at ambient temperature, confirms the conclusions drawn from
the NMR spectra. Most of the metric parameters found in
trans-[7]I (Figure 3)[20] fall in the range observed for the
neutral complex trans-[4]. For example, the protonation does
À
À
this comparison of the Pt C bond lengths. The Pt I bond
length in trans-[4] (2.6858 (3) Š), however, is significantly
longer ( ꢀ 0.04 Š) than the equivalent distance in the
NMe,NMe-NHC complex (type A, 2.6449(5) Š)[21] and the
complex bearing the protic NH,NH-NHC ligand (type E,
2.6472(5) Š).[16c] This may, however, be due to the negative
charge of the heterocycle in trans-[4].
An inspection of the bond lengths within the heterocycle
reveals only small variations thereby giving some additional
weight to resonance structure trans-[4’] (Scheme 2). A local-
ization of the negative charge of the heterocycle at atom C4
should have resulted in a shortening of the N3–C4 separation
based on electrostatic interactions but contrary to this, the
N3–C4 bond length of 1.395(5) Š is the longest within the
heterocycle.
Complex trans-[4] contains an anionic heterocycle with an
unsubstituted ring-nitrogen atom. The metalation enhances
the basicity of this nitrogen atom over that of the starting
material 3. Consequently, trans-[4] reacts with the weak acid
NH4I at 08C under protonation of the N3 ring-nitrogen atom
to give complex trans-[7]I in 90% yield. With both ring-
nitrogen atoms bearing substituents, the heterocycle can now
be considered an aNHC although it is the first example of
a protic aNHC. The protonation reaction is reversible[22] and
the reaction of trans-[7]I with tBuOK results in N3-deproto-
nation and reformation of complex trans-[4] in 89% yield.
(Scheme 3, see the Supporting Information). Similar behavior
has been observed for the deprotonation of protic NHC
ligands in complexes of type E.
Figure 3. Molecular structure of trans-[7]+ in trans-[7]I (hydrogen atoms
except for H3 have been omitted for clarity). Selected bond lengths [Š]
and angles [8]: Pt-I 2.6647(3), Pt-P1 2.3157(12), Pt-P2 2.3067(12), Pt-C4
2.000(4), N1-C2 1.327(7), N1-C5 1.384(6), N3-C2 1.335(6), N3-C4
1.379(6), C4-C5 1.366(6); I-Pt-P1 91.20(3), I-Pt-P2 91.82(3), I-Pt-C4
176.91(14), P1-Pt-P2 175.49(4), P1-Pt-C4 87.07(14), P2-Pt-C4 89.75(14),
C2-N1-C5 108.4(4), C2-N3-C4 110.6(4), N1-C2-N3 107.9(4), N3-C4-C5
104.8(4), N1-C5-C4 108.2(4).
not significantly change the Pt–C4 bond lengths (2.005(4) Š
for trans-[4]; 2.000(5) Š for trans-[7]I). An exception is the
Pt–I separation in trans-[7]I (2.6647(3) Š) which is about
0.2 Š shorter than in trans-[4]. This may be attributed to the
overall positive charge of trans-[7]+ compared to the overall
neutral charge found in trans-[4].
The protonation of the imidazole scaffold in trans-[4] to
give the protic aNHC ligand in trans-[7]I causes some
geometrical changes within the heterocycle. The differences
Complex trans-[7]I is stable towards air and moisture in
the solid state. It is soluble in CH2Cl2 and DMSO. The
1H NMR spectrum of trans-[7]I features the resonance for the
strongly deshielded N3-H proton at d = 12.34 ppm (in
À
in the N C bond lengths become smaller in accord with the
formation of a delocalized 6p electron aromatic system. The
C2-N3-C4 angle of the heterocycle in the nonprotonated
complex trans-[4] (105.5(4)8) expands significantly upon
protonation and measures 110.6(4)8 in trans-[7]I.
Complex trans-4 also reacts with methyl iodide at room
temperature in toluene to give the platinum(II) complex
trans-[8]I with a conventional aNHC ligand in 89% yield.
Complex trans-[8]I reacts further with AgBF4 in acetonitrile
to give compound trans-[9](BF4)2 in 90% yield (Scheme 4).
Scheme 3. Reversible protonation of the heterocycle in trans-[4].
Angew. Chem. Int. Ed. 2015, 54, 13811 –13815
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 13813

.
Angewandte
Communications
Scheme 4. N-Alkylation of trans-[4] and ligand exchange in trans-[7]I.
Compounds trans-[8]I and trans-[9](BF4)2 are stable
towards air and moisture in the solid state. They are soluble
in CH2Cl2 or DMSO. 13C{1H} NMR spectra show the
resonance for the C4 carbon atom at d = 134.4 ppm (in
CD2Cl2) for complex trans-[8]I and at d = 119.6 ppm (in
CD2Cl2) for complex trans-[9](BF4)2, respectively. The 31P{1H}
NMR spectra of the complexes feature resonances at d = 16.5
(1JPt,P = 2577 Hz) for (trans-[8]I and 16.7 (1JPt,P = 2478 Hz) for
trans-[9](BF4)2 confirming the trans arrangement of the two
chemically equivalent PPh3 ligands (see the Supporting
Information).
Figure 4. Heterobimetallic NHC complexes and metalation sites at
aNHC complexes.
the C2 position of trans-[8]I (Figure 4) and we are currently
investigating the preparation of heterobimetallic complexes
starting from trans-[4].
Acknowledgements
We have developed a new straightforward synthetic
method for the preparation of platinum(II) complexes bear-
ing different types of aNHC ligands. The oxidative addition of
neutral 4-iodo-N-ethylimidazole to [Pt(PPh3)4] yields the
complex trans-[4] which can be regarded as a complex bearing
an aNHC ligand with an unsubstituted N3 ring-nitrogen atom.
Complex trans-[4] can be reversibly protonated at the N3
ring-nitrogen atom to yield complexes trans-[7]I bearing
a unique protic aNHC ligand. The N-methylation of the
heterocycle in trans-[4] to give complex trans-[8]I with
a conventional aNHC ligand is also possible. We assume
that it will be possible to extend the scope of the oxidative
addition to other low-valent transition metals.
Apart from their novelty, complexes trans-[4] and trans-
[8]I are potentially useful starting materials for the prepara-
tion of novel heterobimetallic complexes with an imidazole
scaffold. NHCs have been used as spectator ligands in
catalytically active transition metal complexes.[3a,b] Recent
interest has focused on heterobimetallic complexes derived
from poly-NHC scaffolds which have already found applica-
tions in cooperative catalysis.[3c] Most heterobimetallic NHC
complexes have been prepared by the stepwise metalation of
suitable ligand precursors leading to complexes of types G[23]
or H[24] (Figure 4), but even the single-step, site-selective
metalation of a tricarbene ligand leading to complex I has
been reported[25] in addition to other heterobimetallic com-
plexes featuring NHC donors.[26] While complexes G–I
feature a rather large separation of the metal centers, the
N3-metalation of trans-[4] (instead of N3-protonation or N3-
alkylation) would generate a heterobimetallic complex with
a short Pt–M separation. Alternatively, heterobimetallc com-
plexes would result from the deprotonation and metalation of
We thank the Deutsche Forschungsgemeinschaft (SFB 858
and IRTG 2027) for financial support.
Keywords: alkylation · N-heterocyclic carbenes ·
oxidative addition · platinum · protonation
How to cite: Angew. Chem. Int. Ed. 2015, 54, 13811–13815
Angew. Chem. 2015, 127, 14016–14020
[1] a) M. N. Hopkinson, C. Richter, M. Schedler, F. Glorius, Nature
2014, 510, 485 – 496; b) M. C. Jahnke, F. E. Hahn, Top. Organo-
met. Chem. 2010, 30, 95 – 129; c) P. G. Edwards, F. E. Hahn,
Dalton Trans. 2011, 40, 10278 – 10288; d) F. E. Hahn, M. C.
Jahnke, Angew. Chem. Int. Ed. 2008, 47, 3122 – 3172; Angew.
Chem. 2008, 120, 3166 – 3216.
[2] a) T. Drçge, F. Glorius, Angew. Chem. Int. Ed. 2010, 49, 6940 –
6952; Angew. Chem. 2010, 122, 7094 – 7107; b) M. Melaimi, M.
Soleilhavoup, G. Bertrand, Angew. Chem. Int. Ed. 2010, 49,
8810 – 8849; Angew. Chem. 2010, 122, 8992 – 9032; c) O. Schus-
ter, L. Yang, H. G. Raubenheimer, M. Albrecht, Chem. Rev.
2009, 109, 3445 – 3478; d) M. C. Jahnke, F. E. Hahn, Chem. Lett.
2015, 44, 226 – 237.
[3] a) S. Díez-Gonzµlez, N. Marion, S. P. Nolan, Chem. Rev. 2009,
109, 3612 – 3676; b) M. Poyatos, J. A. Mata, E. Peris, Chem. Rev.
2009, 109, 3677 – 3707; c) J. A. Mata, F. E. Hahn, E. Peris, Chem.
Sci. 2014, 5, 1723 – 1732.
[4] a) L. Mercs, M. Albrecht, Chem. Soc. Rev. 2010, 39, 1903 – 1912;
b) W. Liu, R. Gust, Chem. Soc. Rev. 2013, 42, 755 – 773; c) K. M.
Hindi, M. J. Panzner, C. A. Tessier, C. L. Cannon, W. J. Youngs,
Chem. Rev. 2009, 109, 3859 – 3884; d) F. E. Hahn, ChemCatChem
2013, 5, 419 – 430; e) A. Grossmann, D. Enders, Angew. Chem.
Int. Ed. 2012, 51, 314 – 325; Angew. Chem. 2012, 124, 320 – 332;
f) M. Schmidtendorf, T. Pape, F. E. Hahn, Angew. Chem. Int. Ed.
2012, 51, 2195 – 2198; Angew. Chem. 2012, 124, 2238 – 2241;
13814 www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 13811 –13815

Angewandte
Chemie
g) Y.-F. Han, G.-X. Jin, F. E. Hahn, J. Am. Chem. Soc. 2013, 135,
9263 – 9266.
[5] a) M. Viciano, M. Feliz, R. Corberµn, J. A. Mata, E. Clot, E.
Peris, Organometallics 2007, 26, 5304 – 5314; b) P. L. Arnold, S.
Pearson, Coord. Chem. Rev. 2007, 251, 596 – 609; M. Albrecht,
Chem. Commun. 2008, 3601 – 3610.
Hahn, Angew. Chem. Int. Ed. 2014, 53, 1163 – 1166; Angew.
Chem. 2014, 126, 1183 – 1187; d) R. Das, A. Hepp, C. G.
Daniliuc, F. E. Hahn, Organometallics 2014, 33, 6975 – 6987.
[17] D. Brackemeyer, A. HervØ, C. Schulte to Brinke, M. C. Jahnke,
F. E. Hahn, J. Am. Chem. Soc. 2014, 136, 7841 – 7844.
[18] X. Yang, P. Knochel, Chem. Commun. 2006, 2170 – 2172.
[19] H. V. Huynh, S. Guo, W. Wu, Organometallics 2013, 32, 4591 –
4600.
[6] S. Gründemann, A. Kovacevic, M. Albrecht, J. W. Faller, R. H.
Crabtree, Chem. Commun. 2001, 2274 – 2275.
[7] a) A. R. Chianese, A. Kovacevic, B. M. Zeglis, J. W. Faller, R. H.
Crabtree, Organometallics 2004, 23, 2461 – 2468; b) H. V.
Huynh, Y. Han, R. Jothibasu, J. A. Yang, Organometallics
2009, 28, 5395 – 5404.
[20] CCDC 1415499 (trans-[4]·2 MeOH) and CCDC 1415500 (trans-
[7]I contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre.
[8] E. Aldeco-Perez, A. J. Rosenthal, B. Donnadieu, P. Parames-
waran, G. Frenking, G. Bertrand, Science 2009, 326, 556 – 559.
[9] a) Y. Han, H. V. Huynh, Dalton Trans. 2011, 40, 2141 – 2147;
b) H. V. Huynh, H. L. Ong, J. C. Bernhammer, G. Frison, Eur. J.
Inorg. Chem. 2013, 4654 – 4661.
[10] H. Lebel, M. K. Janes, A. B. Charette, S. P. Nolan, J. Am. Chem.
Soc. 2004, 126, 5046 – 5047.
[11] M. Baya, B. Eguillor, M. A. Esteruelas, M. Olivµn, E. OÇate,
Organometallics 2007, 26, 6556 – 6563.
[21] D. S. McGuinness, K. J. Cavell, B. F. Yates, B. W. Skelton, A. H.
White, J. Am. Chem. Soc. 2001, 123, 8317 – 8328.
[22] B. Crociani, F. Dibianca, A. Giovenco, J. Organomet. Chem.
1985, 291, 259 – 272.
[23] a) A. Zanardi, R. Corberµn, J. A. Mata, E. Peris, Organometal-
lics 2008, 27, 3570 – 3576; b) A. Zanardi, J. A. Mata, E. Peris, J.
Am. Chem. Soc. 2009, 131, 14531 – 14537; c) S. Sabater, J. A.
Mata, E. Peris, Organometallics 2012, 31, 6450 – 6456; d) S.
Sabater, J. A. Mata, E. Peris, Nat. Commun. 2013, 4, 2553.
[24] S. Gonell, M. Poyatos, J. A. Mata, E. Peris, Organometallics
2012, 31, 5606 – 5614.
[12] V. Khlebnikov, M. Heckenroth, H. Müller-Bunz, M. Albrecht,
Dalton Trans. 2013, 42, 4197 – 4207.
[13] D. Bacciu, K. J. Cavell, I. A. Fallis, L.-l. Ooi, Angew. Chem. Int.
Ed. 2005, 44, 5282 – 5284; Angew. Chem. 2005, 117, 5416 – 5418.
[14] E. Kluser, A. Neels, M. Albrecht, Chem. Commun. 2006, 4495 –
4497.
[15] a) F. E. Hahn, V. Langenhahn, N. Meier, T. Lügger, W. P.
Fehlhammer, Chem. Eur. J. 2003, 9, 704 – 712; b) A. Flores-
Figueroa, T. Pape, K.-O. Feldmann, F. E. Hahn, Chem. Commun.
2010, 46, 324 – 326.
[25] R. Maity, H. Koppetz, A. Hepp, F. E. Hahn, J. Am. Chem. Soc.
2013, 135, 4966 – 4969.
[26] a) D. Mendoza-Espinosa, B. Donnadieu, G. Bertrand, Chem.
Asian J. 2011, 6, 1099 – 1103; b) M. T. Zamora, M. J. Ferguson,
M. Cowie, Organometallics 2012, 31, 5384 – 5395; c) M. T.
Zamora, M. J. Ferguson, R. McDonald, M. Cowie, Organo-
metallics 2012, 31, 5463 – 5477; d) C. Schulte to Brinke, F. E.
Hahn, Eur. J. Inorg. Chem. 2015, 3227 – 3231.
[16] a) T. Kçsterke, T. Pape, F. E. Hahn, J. Am. Chem. Soc. 2011, 133,
2112 – 2115; b) T. Kçsterke, J. Kçsters, E. Würthwein, C. Mück-
Lichtenfeld, C. Schulte to Brinke, F. Lahoz, F. E. Hahn, Chem.
Eur. J. 2012, 18, 14594 – 14598; c) R. Das, C. G. Daniliuc, F. E.
Received: August 3, 2015
Published online: September 25, 2015
Angew. Chem. Int. Ed. 2015, 54, 13811 –13815
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 13815