Preparation of Complexes Bearing N-Alkylated, Anionic or Protic CAACs Through Oxidative Addition of 2-Halogenoindole Derivatives

Article abstract of DOI:10.1002/anie.202010988

CAAC precursors 2-chloro-3,3-dimethylindole 1 and 2-chloro-1-ethyl-3,3-dimethylindolium tetrafluoroborate 2BF₄ have been prepared and oxidatively added to [M(PPh₃)₄] (M=Pd, Pt). Salt 2BF₄ reacts with [Pd(PPh₃)₄] in toluene at 25 °C over 4 days to yield complex cis-[3]BF₄ featuring an N-ethyl substituted CAAC, two cis-arranged phosphines and a chloro ligand. Compound trans-[3]BF₄ was obtained from the same reaction at 80 °C over 1 day. Salt 2BF₄ reacts with [Pt(PPh₃)₄] to give cis-[4]BF₄. The neutral indole derivative 1 adds oxidatively to [Pt(PPh₃)₄] to give trans-[5] featuring a CAAC ligand with an unsubstituted ring-nitrogen atom. This nitrogen atom has been protonated with py?HBF₄ to give trans-[6]BF₄ bearing a protic CAAC ligand. The Pd^II complex trans-[7]BF₄ bearing a protic CAAC ligand was obtained in a one-pot reaction from 1 and [Pd(PPh₃)₄] in the presence of py?HBF₄.

Full text of DOI:10.1002/anie.202010988

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: Preparation of Complexes Bearing N-Alkylated, Anionic or
Protic CAACs Through Oxidative Addition of 2-Halogenoindole
Derivatives
Authors: Ekkehardt Hahn, Sebastian Termühlen, Jonas Blumenberg,
Alexander Hepp, and Constantin G. Daniliuc
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202010988
Link to VoR: https://doi.org/10.1002/anie.202010988

10.1002/anie.202010988
Angewandte Chemie International Edition
DOI: 10.1002/anie.200((will be filled in by the editorial staff))
CAAC and pCAAC Complexes
Preparation of Complexes Bearing N-Alkylated, Anionic or Protic
CAACs Through Oxidative Addition of 2-Halogenoindole Derivatives
Sebastian Termühlen, Jonas Blumenberg, Alexander Hepp, Constantin G. Daniliuc, and F. Ekkehardt
Hahn*
Abstract: CAAC precursors 2-chloro-3,3-dimethylindole 1 and 2-
chloro-1-ethyl-3,3-dimethylindolium tetrafluoroborate 2BF4 have
been prepared and oxidatively added to [M(PPh3)4] (M = Pd, Pt).
Salt 2BF4 reacts with [Pd(PPh3)4] in toluene at 25 °C over 4 d to
yield complex cis-[3]BF4 featuring an N-ethyl substituted CAAC,
two cis-arranged phosphines and a chloro ligand. Compound trans-
[3]BF4 was obtained from the same reaction at 80 °C over 1 d. Salt
2BF4 reacts with [Pt(PPh3)4] to give cis-[4]BF4. The neutral indole
derivative 1 adds oxidatively to [Pt(PPh3)4] to give trans-[5]
featuring a CAAC ligand with an unsubstituted ring-nitrogen atom.
This nitrogen atom has been protonated with pyHBF4 to give trans-
[6]BF4 bearing a protic CAAC ligand. The PdII complex trans-
[7]BF4 bearing a protic CAAC ligand was obtained in a one-pot
reaction from 1 and [Pd(PPh3)4] in the presence of pyHBF4.
group in the CAACs A make the latter ones both more nucleophilic
(-donating) and more electrophilic (-accepting). Computational
studies show that the HOMO of CAACs lies slightly higher in
energy than in NHCs and the singlettriplet energy gap in CAACs is
slightly smaller than in NHCs.[6] CAAC complexes are normally
prepared from the (in situ generated) free CAAC ligands and
suitable metal complexes.
Figure 1. CAACs A and NHCs B.
We became interested in the preparation of the currently unknown
complexes bearing CAACs with aliphatic N1-substituents or even a
proton at N1. Given the instability of free CAACs in the absence of
bulky aromatic substituents at N1, we assumed that the target N-
alkyl CAACs will not be stable in the free state and that complex
preparation must therefore proceed via the in situ generation of the
CAAC from a suitable precursor.
Various NHC complexes have been prepared by the in situ
deprotonation of azolium salts in the presence of suitable metal
complexes. Alternatively, but also without isolation of the free
carbene, NHC complexes are accessible by the oxidative addition of
the C2X bond (X = H, R, halogen) of azoles, azolium or
pyrazolium cations to low valent transition metals.[8] The oxidative
addition methodology has also been employed for the preparation of
NHC complexes bearing the freely unstable protic NHCs (pNHCs)[9]
or protic mesoionic NHCs.[10]
The preparation of cyclic (alkyl)(amino)carbenes (CAACs, A in
Figure 1) by Bertrand in 2005[1] added an interesting new derivative
to the large family of N-heterocyclic carbenes (NHCs, B in Figure
1).[2] Over the last years, various applications for CAAC ligands in
transition metal coordination chemistry, catalysis and for the
stabilization of reactive species have been described.[3] Compared to
the ubiquitous N-heterocyclic carbenes, CAACs feature special
electronic properties,[3e] which also enable the activation of small
molecules such as CO,[4] P4[5] and even H2 or NH3.[6]
Generally, CAACs of type A are obtained by C2-deprotonation of
cyclic aldiminium salts. Such salts are available from the reaction of
aldimines with epoxides[1] or 3-halogeno-2-methylpropenes[7]
followed by acid catalyzed cyclization. While these routes have led
to cyclic aldiminium salts and CAACs bearing various R and R’
groups at the quaternary carbon atom C3, the choice of the amino
substituent is restricted to aromatic, bulky, electron-withdrawing
groups such as 2,6-diisopropylphenyl. In addition, the sp3-carbon
atom C5 must be quaternary as C2-deprotonation of the aldiminium
cation would otherwise compete with deprotonation at C5. The
replacement of one electronegative and planar -donating amino
group in NHCs B for a tetrahedral - but not -donating alkyl
Encouraged by the versatility of the oxidative addition
methodology, we prepared the halogenoindole derived, annulated
[12]
CAAC precursors 1[11] and 2BF4 (Scheme 1, see the SI) for the
preparation of CAAC complexes by an oxidative addition of the
C2‒Cl bond to low-valent transition metals.
[] S. Termühlen, Dr. J. Blumenberg, Dr. A. Hepp, Prof. Dr. F. E. Hahn
Institut für Anorganische und Analytische Chemie
Westfälische Wilhelms-Universität Münster
Corrensstraße 30, 48149 Münster (Germany)
E-Mail: fehahn@uni-muenster.de
Dr. C. G. Daniliuc
Organisch-Chemisches Institute
Westfälische Wilhelms-Universität Münster
Corrensstraße 40, 48149 Münster (Germany)
Scheme 1. Synthesis of CAAC precursors 1 and 2BF4.
Supporting information and the ORCHID identification numbers for
the author(s) of this article can be found under
https://doi.org/10.102/anie.2020XXXXX.
Reaction of the 2-chlorindolium salt 2BF4 with one equivalent of
[Pd(PPh3)4] yielded the CAAC complex cis-[3]BF4 in good yield
(Scheme 2, top). The heterocyclic ligand features all characteristics
1
This article is protected by copyright. All rights reserved.

10.1002/anie.202010988
Angewandte Chemie International Edition
of the known CAAC ligands of type A except for the C4C5
annulation and the unique N1-alkyl substitution. Typical CAAC
ligand properties were also observed in the 13C NMR spectrum of
cis-[3]BF4 featuring the downfield shifted resonance for the CCAAC
carbon atom at (C2) = 242.2 (cf the CNHC resonance at (C2) 
165175 found in related NHC-Pd complexes.[9a,d,e]). The
observation of a doublet of doublets for C2 (2JCP,trans = 141.9 Hz,
2JCP,cis = 5.7 Hz) indicates the cis-disposition of the phosphine
donors in cis-[3]BF4. This cis-arrangement was confirmed by the
observation of two resonances in the 31P NMR spectrum at  = 29.8
(2JPP = 26.0 Hz, Ptrans) and  = 19.6 (2JPP = 26.0 Hz, Pcis). Some
related complexes obtained from isoindolium salts (featuring cyclic
(amino)(aryl)carbenes, CAArC)[7,13] or indazolium salts[14] have
been described. However, these carbene ligands differ significantly
from A or the CAAC ligand in cis-[3]BF4 by the aromatic character
of the carbon atom C3 which in both cases is part of a C3C4
annulated benzene ring.
and by mass spectrometry (see the SI). The 13C NMR spectrum
exhibits the resonance for the carbene carbon atom at (C2) = 232.9
as the expected doublet of doublets (2JCPtrans = 126.7 and 2JCPcis = 8.2
Hz) for a cis-diphosphine complex. The 31P NMR spectrum features
two resonances at  = 15.5 (d, 2JPP = 20.4 Hz; d, 1JPPt = 2020 Hz, Pt
1
satellites, Ptrans) and  = 11.1 (d, 2JPP = 20.4 Hz; d, JPPt = 3773 Hz,
Pt satellites, Pcis) for the two chemically different phosphorus atoms.
The HRMS spectrum (ESI, positive ions) shows the strongest peak at
m/z = 928.2363 (calcd for [4]+ = 928.2369).
The molecular structures of cis-[3]BF4, trans-[3]BF4·CH2Cl2
(Figure 2) and of cis-[4]BF4 (see the SI, Figure S1)[16] have been
determined by X-ray diffraction, confirming the composition and
coordination geometry of the complex cations. The metal atoms in
the complexes are surrounded in a slightly distorted square-planar
fashion with the CAAC plane oriented almost perpendicular to the
palladium coordination plane in both cases.
The PdC2 bond distance in trans-[3]+ is significantly shorter
than in cis-[3]+ which is attributable to the weaker trans-effect of
the chloride ligand in trans-[3]+ compared to the phosphine ligand
in cis-[3]+. Related observations were made for the PdCl bond
distances where a short separation of 2.3374(7) Å was observed for
cis-[3]+ and a longer one of 2.3512(9) Å for trans-[3]+. As expected,
the CAAC donor exerts a stronger trans-influence than the
phosphine ligand. Due to the different ligand arrangements, the
PdP separations in cis-[3]+ differ by about 0.112 Å while this
difference amounts to only 0.021 Å in trans-[3]+.
The metric parameters of the CAAC ligands are identical within
experimental error in cis-[3]+ and trans-[3]+ and compare well to
equivalent parameters observed in palladium complexes bearing
non-annulated CAAC ligands.[1] These include a C2C3 single
bond (1.518(4) Å in cis-[3]+ and 1.533(5) Å in trans-[3]+), a
significantly shorter NC2 bond (1.305(4) Å in cis-[3]+ and
1.310(5) Å in trans-[3]+) and N1C2C3 angles of 108.3(2)° in cis-
[3]+ and 108.7(3)° in trans-[3]+. Comparable metric parameters for
cis-[4]+ are very similar to those found for cis-[3]+ (see the SI,
Figure S1).
Scheme 2. Oxidative Addition of 2BF4 to [M(PPh3)4].
The oxidative addition of 2BF4 to [Pd(PPh3)4] at an elevated
temperature of 80 °C yields a complex mixture composed of trans-
[3]BF4 (major, about 60%), cis-[3]BF4 and [PdCl(PPh3)3]BF4
(Scheme 2, middle). While the components of the complex mixture
could not be separated, the individual complexes in the mixture
were identified by 31P NMR spectroscopy (see the SI). Complexes
cis-[3]BF4 and [PdCl(PPh3)3]BF4 were identified by comparison
with the 31P NMR spectra of authentical samples while trans-[3]BF4
gave the expected singulet resonance at  = 24.2. The temperature
dependent formation of cis- and trans-isomers has been previously
observed during the oxidative addition of halogenoazoles to
[M(PPh3)4] (M = Pd, Pt) complexes[9a] confirming that the CAAC
ligands behave similarly to normal NHC ligands. Related
observations have been made for palladium(II) complexes of type
[Pd(rNHC)(PPh3)2I] (rNHC = pyrazolin-4-ylidene), where the
initially formed cis-diphosphine complex slowly transforms into the
thermodynamically more stable trans-complex.[9f] The preference
for the trans-complex has been rationalized by the transphobia
effect, a term proposed for the difficulty of placing a phosphine
donor trans to a carbon donor.[9g]
Figure 2. Molecular structures of complex cation cis-[3]+ in cis-[3]BF4 (top) and
trans-[3]+ in trans-[3]BF4·CH2Cl2 (bottom). Hydrogen atoms have been omitted
for clarity and 50% probability ellipsoids are depicted. Selected bond lengths (Å)
and angles (deg) for cis-[3]+ [trans-[3]+]: Pd‒Cl 2.3374(7) [2.3512(9)], Pd‒P1
2.3956(7) [2.3721(10)], Pd–P2 2.2838(7) [2.3591(9)], Pd–C2 2.022(3)
[1.996(3)], N–C2 1.305(4) [1.310(5)], NC5 1.439(4) [1.446(5)], C2–C3 1.518(4)
[1.533(5)]; Cl–Pd–P1 84.12(2) [84.13(3)], Cl–Pd–P2 177.39(3) [85.28(3)], Cl–
Pd–C2 84.85(7) [179.61(11)], P1–Pd–P2 98.48(2) [168.75(4)], P1–Pd–C2
168.95(7) [95.83(10)], P2–Pd–C2 92.56(7) [94.79(10)], C2–N–C5 113.4(2)
[112.5(3)], N–C2–C3 108.3(2) [108.7(3)].
Finally, the oxidative addition of 2BF4 to [Pt(PPh3)4] yielded
complex cis-[4]BF4 (Scheme 2, bottom). Even at the elevated
temperature of 110 °C, only the cis-complex cation was observed in
accord with the enhanced kinetic inertness of platinum(II)
complexes.[9b] However, the reaction was completed after only 1 h
and prolonged heating might also lead to the thermodynamically
favored trans-complex cation.[9a]
Apart from azolium cations,[8b,d] neutral halogenoazoles can also
oxidatively add to low-valent transition metals to yield complexes
bearing anionic azolato ligands,[9ae] which after N-protonation yield
complexes bearing neutral protic NHC (pNHC)[9a,c‒e] ligands.[15] In
order to prepare the analogous but currently unknown complexes
Compound cis-[4]BF4 was characterized by NMR spectroscopy
2
This article is protected by copyright. All rights reserved.

10.1002/anie.202010988
Angewandte Chemie International Edition
bearing N1-protonated CAAC ligands, neutral 2-chloroindole 1 was
reacted with [Pt(PPh3)4] to give complex trans-[5] bearing an
anionic CAAC ligand with an unsubstituted ring-nitrogen atom
(Scheme 3, top). The trans-disposition of the phosphine ligands in
this case most likely results from the extended reaction time of 1 d
(compared to only 1 h for the synthesis of complex cis-[4]BF4).
Treatment of the reaction product of the oxidative addition with
pyHBF4 resulted in protonation of the ring-nitrogen atom of the
anionic CAAC ligand and formation of complex trans-[6]BF4
bearing a unique protic CAAC ligand (pCAAC, Scheme 3, bottom).
protonation of coordinated azolato ligands to give complexes with
pNHC ligands.15
Finally, 2-chloroindole 1 was oxidatively added to [Pd(PPh3)4] in
the presence of pyHBF4 to give trans-[7]BF4 directly in a one-pot
reaction (Scheme 4). Compound trans-[7]BF4 was fully
characterized by NMR spectroscopy, ESI-MS spectrometry and an
X-ray diffraction analysis (see the SI).[16] The 13C NMR spectrum
shows the resonance for the CpCAAC carbon atom at (C2) = 241.6 (t,
2JCP = 6.3 Hz) and the resonance for the N-H proton was found at
(H1) = 12.45 in the 1H NMR spectrum. Comparable metric
parameters in trans-[3]BF4 (N-ethyl substitution) and trans-[7]BF4
(N-H substitution) differ not significantly and confirm that the
pCAAC in trans-[7]+ behaves like a typical CAAC ligand.
Scheme 4. One-pot synthesis of pCAAC complex trans-[7]BF4.
Scheme 3. Synthesis of complexes trans-[5] and trans-[6]BF4.
We have prepared the novel 2-chloroindole derived CAAC
precursors 1 and 2BF4. In contrast to the classical synthesis of
CAAC complexes by deprotonation of aldiminium salts followed by
CAAC coordination to a metal center, these precursors can
oxidatively add to zero-valent transition metals. This procedure
gives access to complexes bearing CAAC ligand with N-substituents
previously unattainable. From precursor 1, the first complexes
bearing a CAAC ligand with an unsubstituted or a protonated ring-
nitrogen atom were obtained, while CAAC precursor 2BF4 yielded
complexes with N-alkyl-substituted CAAC ligands.
The 13C NMR spectrum of trans-[5] features the resonance for the
carbene carbon atom as a triplet at (C2) = 197.1 (t, JCP = 8.0 Hz),
2
indicating the trans-disposition of the phosphine donors. This
2
resonance shifts significantly downfield to (C2) = 221.4 (t, JCP
=
7.6 Hz) in trans-[6]BF4. In addition, the resonance for the N-H
1
proton was found in the H NMR spectrum of trans-[6]BF4 at  =
12.33. Generally, all resonances in the NMR spectra shift downfield
upon the transformation of trans-[5] to trans-[6]BF4 attributable to
the introduction of a positive charge in trans-[6]+. X-Ray diffraction
studies with crystals of trans-[5]CH2Cl2C6H14 and trans-
[6]BF4CH2Cl2 (Figure 3)[16] confirmed the conclusions drawn from
NMR spectroscopy.
Acknowledgements
The authors gratefully acknowledge financial support from the DFG
(SFB 858 and IRTG 2027).
Conflict of interest
The authors declare no conflict of interest.
Keywords: chloro indole • chloro indolium salts • oxidative
addition • CAAC complexes • protic CAACs
[1] V. Lavallo, Y. Canac, C. Präsang, B. Donnadieu, G. Bertrand,
Angew. Chem. Int. Ed. 2005, 44, 57055709; Angew. Chem.
2005, 117, 58515855.
[2] a) F. E. Hahn, M. C. Jahnke, Angew. Chem. Int. Ed. 2008, 47,
31223172; Angew. Chem. Int. 2008, 120, 31663216; b) F. E.
Hahn, Chem. Rev. 2018, 118, 94559456  this is a themed
issue dedicated to various aspects of NHC chemistry.
[3] a) M. Melaimi, M. Soleilhavoup, G. Bertrand, Angew. Chem.
Int. Ed. 2010, 49, 88108849; Angew. Chem. 2010, 122,
89929032; b) M. Soleilhavoup, G. Bertrand, Acc. Chem. Res.
2015, 48, 256266; c) M. Melaimi, R. Jazzar, M.
Soleilhavoup, G. Bertrand, Angew. Chem. Int. Ed. 2017, 56,
1004610068; Angew. Chem. 2017, 129, 1018010203; d) U.
S. D. Paul, U. Radius, Eur. J. Inorg. Chem. 2017, 33623375;
e) H. V. Huynh, Chem. Rev. 2018, 118, 94579492.
Figure 3.
Molecular structures of complex trans-[5] in trans-
[5]BF4CH2Cl2C6H14 (left) and of one of two essentially identical complex
cations trans-[6]+ in the asymmetric unit of trans-[6]BF4CH2Cl2 (right). Hydrogen
atoms have been omitted for clarity except for H1 at N in trans-[6]+ and 50%
probability ellipsoids are depicted. Selected bond lengths (Å) and angles (deg)
for trans-[5] [trans-[6]+]: Pt‒Cl 2.4056(5) [2.361(2)], Pt‒P1 2.3205(5)
[2.3207(14)], Pt–P2 2.3257(5) [2.332(2)], Pt–C2 1.997(2) [1.974(8)], N–C2
1.295(3) [1.302(10)], NC5 1.422(2) [1.428(10)], C2–C3 1.561(3) [1.534(11)];
Cl–Pt–P1 87.17(2) [86.10(6)], Cl–Pt–P2 88.01(2) [87.01(7)], Cl–Pt–C2
174.09(6) [172.7(2)], P1–Pt–P2 164.76(2) [165.06(10)], P1–Pt–C2 93.50(6)
[96.1(2)], P2–Pt–C2 92.77(6) [92.4(2)], C2–N–C5 108.1(2) [113.4(8)], N–C2–C3
113.1(2) [108.3(7)].
The protonation of the anionic CAAC ligand in trans-[5] to give
trans-[6]+ has only a limited impact on comparable metric
parameters. The most noticeable differences are the shrinkage of the
N1–C2–C3 angle from 113.1(2)° to 108.3(7)° and the expansion of
the C2–N1–C5 angle from 108.1(2)° to 113.4(8)° upon N-
protonation of trans-[5]. Similar changes were observed upon
[4] V. Lavallo, Y. Canac, B. Donnadieu, W. W. Schoeller, G.
Bertrand, Angew. Chem. Int. Ed. 2006, 45, 34883491; Angew.
Chem. 2006, 118, 356835721.
[5] J. D. Masuda, W. W. Schoeller, B. Donnadieu, G. Bertrand,
3
This article is protected by copyright. All rights reserved.

10.1002/anie.202010988
Angewandte Chemie International Edition
Angew. Chem. Int. Ed. 2007, 46, 70527055; Angew. Chem.
2006, 119, 71827185.
[6] G. D. Frey, V. Lavallo, B. Donnadieu, W. W. Schoeller, G.
Bertrand, Science 2007, 316, 439441.
126, 11831187; f) Y. Han, L. J. Lee, H. V. Huynh,
Organometallics 2009, 28, 27782786; g) J. Vicente, J.-A.
Abad, A. D. Frankland, M. C. Ramírez de Arellano, Chem.
Eur. J. 1999, 5, 30663075.
[7] R. Jazzar, J.-B. Bourg, R. D. Dewhurst, B. Donnadieu, G.
Bertrand, J. Org. Chem. 2007, 72, 34923499.
[10] H. Jin, T. T. Y. Tan, F. E. Hahn, Angew. Chem. Int. Ed. 2015,
54, 1381113815; Angew. Chem. 2015, 127, 1401614020.
[11] D. W. Robertson, J. H. Krushinski, D. Kau, J. Label. Compd.
Radiopharm. 1986, 23, 343354.
[12] S. Hünig, H. Balli, J. Liebigs Ann. Chem. 1957, 609, 160172.
[13] B. Rao, H. Tang, X. Zeng, L. Liu, M. Melaimi, G. Bertrand,
Angew. Chem. Int. Ed. 2015, 54, 1491514919; Angew. Chem.
2015, 127, 1512815132.
[14] a) J. C. Bernhammer, N. X. Chong, R. Jothibasu, B. Zhou, H.
V. Huynh, Organometallics 2014, 33, 36073617; b) J. C.
Bernhammer, H. Singh, H. V. Huynh, Organometallics 2014,
33, 42954301.
[15] For a recent review on complexes bearing pNHC ligands see:
S. Kuwata, F. E. Hahn, Chem. Rev. 2018, 118, 96429677.
[16] CCDC 2010431 (cis-[3]BF4), 2010432 (trans-[3]BF4·CH2Cl2),
2010433 (cis-[4]BF4), 2010434 (trans-[5]·CH2Cl2·C6H14),
2010435 (trans-[6]BF4·CH2Cl2) and 2010436 (trans-
[7]BF4·CH2Cl2) contain the crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
[8] a) P. J. Fraser, W. R. Roper, F. G. A. Stone, J. Chem. Soc.
Dalton 1974, 102105; b) D. S. McGuinness, K. J. Cavell, B.
F. Yates, B. W. Skelton, A. H. White, J. Am. Chem. Soc. 2001,
123, 83178328; c) W. H. Meyer, M. Deetlefs, M. Pohlmann,
R. Scholz, M. W. Esterhuysen, G. R. Julius, H. G.
Raubenheimer, Dalton Trans. 2004, 413420; d) A. Fürstner,
G. Seidel, D. Kremzow, C. W. Lehmann, Organometallics
2003, 22, 907909; e) M. Viciano, E. Mas-Marzá. M. Poyatos,
M. Sanaú, R. H. Crabtree, E. Peris, Angew. Chem. 2005, 117,
448451; Angew. Chem. Int. Ed. 2005, 44, 444447, f) E.
Mar-Marzá, M. Sanaú, P. Peris, Inorg. Chem. 2005, 44,
99619967; g) S. K. Schneider, G. R. Julius, C. Loschen, H.
G. Raubenheimer, G. Frenking, W. A. Herrmann, Dalton
Trans. 2006, 12261233; h) J. C. Bernhammer, H. V. Huynh,
Organometallics 2012, 31, 51215130.
[9] a) T. Kösterke, T. Pape, F. E. Hahn, J. Am. Chem. Soc. 2011,
133, 21122115; b) T. Kösterke, J. Kösters, E.-U. Würthwein,
C. Mück-Lichtenfeld, C. Schulte to Brinke, F. Lahoz, F. E.
Hahn, Chem. Eur. J. 2012, 18, 1459414598; c) D.
Brackemeyer, H. Hervé, C. Schulte to Brinke, M. C. Jahnke, F.
E. Hahn, J. Am. Chem. Soc. 2014, 136, 78417844; d) R. Das,
A. Hepp, C. G. Daniliuc, F. E. Hahn, Organometallics 2014,
33, 69756987; e) R. Das, C. G. Daniliuc, F. E. Hahn, Angew.
Chem. Int. Ed. 2014, 53, 11631166; Angew. Chem. 2014,
Manuscript received:
Accepted manuscript online:
Version of record online:
4
This article is protected by copyright. All rights reserved.

10.1002/anie.202010988
Angewandte Chemie International Edition
COMMUNICATION
S. Termühlen, J. Blumenberg, A.
Hepp, C. G. Daniliuc, F. E. Hahn
Page No. – Page No.
Preparation of Complexes
Bearing N-Alkylated, Anionic or
Protic CAACs Through
Oxidative Addition of 2-
Halogenoindole Derivatives
CAAC precursor salt 2-chloro-1-ethyl-3,3-dimethylindolium tetrafluoroborate 2BF4
reacts with [M(PPh3)4] (M = Pd, Pt) in an oxidative addition to give the MII complexes
cis/trans-[3]BF4 and cis-[4]BF4, respectively. The CAAC ligand in these complexes
bears an unusual N-alkyl substituent. The reaction of 2-chloro-3,3-dimethylindole with
[Pt(PPh3)4] yields complex trans-[5] bearing a unique anionic CAAC ligand with an
unsubstituted ring-nitrogen atom. This ring-nitrogen atom has been protonated with
pyHBF4 to give trans-[6]BF4 bearing a protic CAAC (pCAAC) ligand.
5
This article is protected by copyright. All rights reserved.