N-Heterocyclic carbenes via abstraction of ammonia: Normal carbenes with 'abnormal' character

Article abstract of DOI:10.1039/c2cc30611e

A new ammonia adduct of a N-heterocyclic carbene (NHC) has been isolated, which can be used as a reagent for the synthesis of transition metal carbene complexes. It represents the first example of a 1,2,3-triazolylidene with a 1,2,4-substitution pattern, thus opening a new subclass ('normal' 1,2,3-triazolylidenes) of sterically and electronically tunable NHCs. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc30611e

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N-Heterocyclic carbenes via abstraction of ammonia: ‘normal’ carbenes
with ‘abnormal’ characterw
a
a
b
a
a
¨
Lars-Arne Schaper, Karl Ofele, Renat Kadyrov, Bettina Bechlars, Markus Drees,
Mirza Cokoja,a Wolfgang A. Herrmann*a and Fritz E. Kuhn*a
¨
Received 27th January 2012, Accepted 21st February 2012
DOI: 10.1039/c2cc30611e
A new ammonia adduct of a N-heterocyclic carbene (NHC) has
been isolated, which can be used as a reagent for the synthesis of
transition metal carbene complexes. It represents the first example
of a 1,2,3-triazolylidene with a 1,2,4-substitution pattern, thus
opening a new subclass (‘normal’ 1,2,3-triazolylidenes) of sterically
and electronically tunable NHCs.
Recently, two stable carbenes were shown to split the
N–H-bond of ammonia, resulting in the compounds E and F
(Fig. 1).10 The precursor of E was additionally found to
activate H2 as well as B–H or Si–H bonds.10c Apart from these
examples requiring demanding synthetic procedures, however,
no conventional NHC could be isolated as a NH3 adduct so far.
Additional ‘adducts’ resemble some well-established storable
carbene sources, being actually NHC adducts of chloroform
(A),11 alcohol (B and C)11c,4c and pentafluorobenzene (D).5 For
carbene generation, adducts A–D offer several advantages in
comparison to more typical carbene precursors, namely azolium
salts, silver carbenes, and free carbenes. Bases are not required
for adduct based carbene formation, light is no problem in
contrast to silver mediated carbene transfer reactions, and the
carbene adducts are less sensitive than free carbenes, simplifying
the handling.
The pioneering work on the syntheses and isolation of carbenes,1a–c
and the first reports on the application of N-heterocyclic carbenes
(NHCs) as control ligands in homogeneous catalysis1d,e initiated
a rapid development of this research area. Today, carbenes,
particularly NHCs are among the most widely used ligands for
transition metal complexes, playing a very important role in
homogeneous catalysis.2 The synthesis of new NHC precursors
with different constitution and substitution pattern is of continuing
importance for the development of catalysis. Carbenes based
on 1,2,4-triazoles are widely applicable as free stable compounds
and as precursors for metal based catalysts.3–5 However, much less
is known about 1,2,3-triazolylidenes,6 although copper-catalyzed
click chemistry provides good access to this class of ligands.7
Recently, Bertrand et al. reported the first free stable 1,2,3-
triazolylidene, exhibiting a 1,3,4-substitution pattern, thus classified
as an ‘abnormal’ or mesoionic carbene (aNHC or MIC).8a,b While
the terms ‘normal’ and ‘abnormal’ were previously reserved for
C2- or C4/5 bound carbenes, respectively,8c–e this definition has
recently been broadened by assigning the term ‘normal’ carbene
to carbenes with a neutral canonical resonance structure, while
the term ‘abnormal’ was assigned to carbenes that require
additional charges in their resonance forms.8a,f Changing the
triazole substitution pattern to a 1,2,4-substitution results in the
first ‘normal’ carbene of its class, which allows exploration of
the differences between these two types of NHCs.9
In this work, a 1,2,3-triazolylidene with a novel 1,2,4-substitution
pattern is described, providing an example of a ‘normal’ 1,2,3-
triazolylidene. Additionally, the new 1,2,3-triazolylidene-based
ammonia adduct 2 and its use for carbene generation is reported.
1,2,4-Substituted 1,2,3-triazolylidenes are accessible via a ring
closing procedure using hydrazonoyl chlorides and isocyanides.12
Following this route, 1-methyl-2-phenyl-4-p-tolyl-1,2,3-triazolium
chloride (1) was obtained as starting material for carbene
generation. Conventional deprotonation routes only led to
isolation of mixtures of decomposition products or—in the
presence of suitable metal precursors—very small amounts of
metal-carbene complexes. Therefore, the more sophisticated
ammonia route was examined, where 1 reacts with NaH in
a mixture of liquid ammonia and THF (Scheme 1), being a
long-known procedure for synthesis of classical NHCs.13
a Chair of Inorganic Chemistry/Molecular Catalysis,
Catalysis Research Center, Technische Universitat Munchen,
¨
¨
Ernst-Otto-Fischer-Straße 1, D-85747 Garching, Germany.
E-mail: fritz.kuehn@ch.tum.de; wolfgangherrmann@ch.tum.de;
Fax: +49-89-289-13473; Tel: +49-89-289-13081
b Evonik Degussa GmbH, Rodenbacher Chaussee 4,
D-63457 Hanau-Wolfgang, Germany
w Electronic supplementary information (ESI) available: Experimental
procedures, characterization data for described compounds and
computational details, CIF files for compounds 2, 3, 4 and 5a. CCDC
788089, 797025, 797026 and 849440. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c2cc30611e
Fig. 1 Different carbene adducts. A–D and 2 can be applied as carbene
precursors.
c
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In the 1H-NMR spectrum, a triplet (d = 4.95 ppm, 1H,
3JH–H = 8.8 Hz) and a doublet (d = 2.05 ppm, 2H,
3JH–H = 8.9 Hz) are observed in addition to the expected
signals, being assigned to CH und CNH2 by HMQC-NMR
spectroscopy (see ESIw). Single crystals suitable for X-ray
crystallography were grown from a saturated n-hexane solution
at ꢀ30 1C. The crystal data give evidence for a non-conjugated
heterocycle, with sp3-hybridization of C1 and N1. Monitored
via 1H-NMR in C6D6 2 was found to be stable for days at RT.
It is slowly decomposing to a number of decomposition
products only when heated to 80 1C for 24 h. Hydrazonoyl
cyanide 3 (see Scheme 1 and the ESIw) could be isolated from
the resulting mixture. Interestingly, the methyl group is absent
in 3, suggesting intermediate carbene formation and subsequent
alkyl rearrangement.8a The ESI-MS of compound 2 shows only
minor amounts of 2 ([M + H]+: 267.1). The most dominant
peak can be attributed to the free carbene ([M0 + H]+: 250.3).
Based on this, it can be concluded that 2 is able to eliminate
ammonia and generate the free carbene. Therefore, the behavior
of the amine group of 2 was further examined by EXSY-NMR,
providing evidence for NH3 exchange at RT in C6D6.
Scheme 1 Synthesis and reactivity of the ammonia adduct 2. Me = CH3,
Ph = phenyl, Tol = p-Tolyl.
The results obtained in this case differ significantly from
the typical outcome obtained by applying the ammonia
route: addition of NaH does not yield the free carbene,
which would be the typical product; instead, the ammonia
adduct 2 formed, which can be safely stored in THF at
ꢀ30 1C (in a deep freezer) for several months without
decomposition. DFT calculations for the solution phase
(PCM–THF—B3LYP/6-31G*) show that with respect to
excess of NH3, the pathway to 2 via the free carbene would
be kinetically favored as the energy barrier for the deprotona-
tion of 1 yielding the free carbene (DGz = 3.4 kcal molꢀ1) is
lower than for the addition of amide, leading to adduct 2
(DGz = 10.4 kcal molꢀ1). While thermodynamic conditions
favor the formation of adduct 2, the pathway towards 2 is very
likely proceeding via the free carbene and subsequent NH3
activation. Since the first report of the NH3 route in 1998, to
the best of our knowledge none of the numerous papers using
this method reported a carbene nucleophilic enough to exist as
an ammonia adduct.
A solution of 2 in THF can be stored under argon at 0 1C
and added to [(COD)MCl]2 to yield the corresponding carbene
complexes [(COD)(NHC)MCl] (M = Rh, Ir; see ESIw). The
latter Ir-compound was treated with excess of CO producing
the corresponding carbonyl complex 4 (Scheme 1). A comparison
of the X-ray crystal structures of compounds 2 and 4 (see Fig. 2)
reveals delocalization of the NHC ligand’s p-electrons in the
latter compound. In order to classify the donor strength of the
new NHC ligand, the IR bands of the CO ligands were compared
to those of parent NHC–Ir complexes. The observed carbonyl
stretching frequencies of 4 (n(CO) = 2058 cmꢀ1 and n(CO) =
1975 cmꢀ1, n(CO)av = 2017 cmꢀ1, n(TEP) = 2044 cmꢀ1
;
The structure of 2 was established by X-ray single crystal
diffraction (Fig. 2) and NMR spectroscopy. The 13C-NMR
spectrum does not show the characteristic signal of a free carbene.
TEP = Tolman Electronic Parameter) are slightly lower than those
of the related 1,2,3-triazolylidene complexes [(NHC)Ir(CO)2Cl]
of Albrecht et al. and Bertrand et al. (n(CO)av = 2021 cmꢀ1
and n(CO)av = 2019 cmꢀ1, respectively).6,8,14
Owing to its unprecedented substitution pattern, the donor
strength of the new NHC-ligand can be classified as strongest
among 1,2,3-triazolylidenes and ‘‘normal’’ carbenes (considerably
higher than imidazol-2-ylidenes or 1,2,4-triazol-5-ylidenes)
and only marginally weaker than ‘abnormal’ NHCs, such as
imidazol-5-ylidenes or pyrazol-4-ylidenes (n(CO)av = 2003 cmꢀ1
and n(CO)av = 2002 cmꢀ1, respectively).14,15
A remaining question was, whether the in situ formation of
carbenes by liberating NH3 from 2 could be more generally
applied in carbene complex synthesis. To contribute to an
answer, a reaction was chosen where the usual Ag2O protocol
is not successful due to the occurrence of undesired anion
exchange.18 Thus, 2 was reacted with [(Me2S)AuCl] and CuI in
THF at ꢀ78 1C producing the metal carbene complexes 5a and
5b in good yields, with 5b being inaccessible via alternative
pathways.
Fig. 2 X-ray molecular structure of 2 (thermal ellipsoids are shown
at a 50% probability level). Selected bond lengths and angles: C1–N4:
1.441(2) A; C1–C2: 1.511(2) A; C1–N1: 1.483(2) A; C16–N1–C1:
112.64(15) A; torsion angle C1–N2–N1–C16: 59.11. X-Ray molecular
structure of 4. Hydrogen atoms are omitted for clarity. Thermal
ellipsoids are shown at a 50% probability level. Selected bond lengths
and angles: C1–Ir1: 2.098(8) A; C1–C2: 1.434(11) A; C1–N1: 1.364(10) A;
Ir1–C17: 1.899(9) A; C1–Ir1–C17: 178.1(3)1; C16–N1–C1: 130.0(7) A;
torsion angle C1–N2–N1–C16: 3.21. X-Ray molecular structure of 5a.
Hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown
at a 50% probability level. Selected bond lengths and angles: C1–Au1:
1.997(6) A; Au1–Cl: 2.290(1) A; C1–Au1–Cl1: 177.13(15)1; torsion
angle C1–N2–N1–C16: 3.31.
The X-ray single crystal structure of 5a is depicted in
Fig. 2. Interestingly, in the unit cell, a close Auꢁ ꢁ ꢁAu contact
of 3.499 A was observed, which is well within the range of
typical aurophilic interactions.16 The Au–Cl bond length of 5a is
2.290(1) A, comparable to bond lengths observed in the related
triazolylidene compound of Lee and Crowley (2.2940(10) A),
c
3858 Chem. Commun., 2012, 48, 3857–3859
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5 (a) G. W. Nyce, S. Csihony, R. M. Waymouth and J. L. Hedrick,
Chem.–Eur. J., 2004, 10, 4073; (b) A. P. Blum, T. Ritter and
R. H. Grubbs, Organometallics, 2007, 26, 2122; (c) A. Bittermann,
P. Harter, E. Herdtweck, S. D. Hoffmann and W. A. Herrmann,
J. Organomet. Chem., 2008, 693, 2079.
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2008, 130, 13534; (b) J. Cai, X. Yang, K. Arumugam, C. W.
Bielawski and J. L. Sessler, Organometallics, 2011, 30, 5033;
(c) J. D. Crowley, A.-L. Lee and K. J. Kilpin, Aust. J. Chem.,
2011, 64, 1118.
Scheme 2 Synthesis of thione 6.
in [Au(IPr)Cl] (2.2698(11) A), or in [Au(ICy)Cl] (2.306(3) A,
2.281(3) A) described by Nolan et al.17
7 (a) H. C. Kolb, M. G. Finn and K. B. Sharpless, Angew. Chem.,
Int. Ed., 2001, 40, 2004; (b) E. J. Yoo, M. Ahlquist, S. H. Kim,
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Angew. Chem., Int. Ed., 2010, 49, 4759; (b) J. Bouffard, B. K. Keitz,
R. Tonner, G. Guisado-Barrios, G. Frenking, R. H. Grubbs and
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and D. P. Allen, Coord. Chem. Rev., 2004, 248, 2247; (d) P. L. Arnold
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63, 105.
9 Here the canonical resonance structure would not need an additional
charge. Earlier work concerning pentacarbonylchromium(0) and
tetracarbonyliron(0) complexes with tetrazolylidene ligands showed
remarkable differences in compound characteristics between
1,4- and 1,3-dimethyltetrazol-5-ylidene substitution patterns: see
ref. 4a,b. The dipole moments of [LCr(CO)5] compounds with
In order to determine whether the abstraction of NH3 only
occurs in the presence of a metal, compound 2 was treated
with elementary sulfur allowing the isolation of thione 6, thus
further supporting the applicability of 2 as a useful carbene
precursor (see ESIw, Scheme 2).
The obtained results provide evidence that the easily accessible
and storable ammonia adduct 2 can be used as an alternative
reagent for the synthesis of carbene complexes. To the best of our
knowledge, only two other carbene ammonia-adducts have been
described so far. In both cases, the NHC–ammonia adducts are
described as stable and no reactivity studies were reported. The
novel substitution pattern of 2 allows for additional steric and
electronic fine-tuning for specific applications.19 Compound 2
provides a useful source of a carbene through an a-elimination
reaction that occurs even below room temperature providing a
solution to the difficulties arising from the use of thermally
sensitive metal precursors. As to date only ‘abnormal’ examples
of 1,2,3-triazol-5-ylidenes are known, the 1,2,4-substituted
1,2,3-triazol-5-ylidene represents the first ‘normal’ carbene of
its class, exhibiting only slightly higher donor strength than its
‘abnormal’ relatives.
L
=
1,4-dimethyl-tetrazol-5-ylidene (4.0
ꢂ
0.1 Debye) and
1,3-dimethyltetrazol-5-ylidene (8.0 ꢂ 0.1 Debye) show that the latter
exhibits a considerably lower donor strength (measured in benzene).
10 (a) G. D. Frey, V. Lavallo, B. Donnadieu, W. W. Schoeller and
G. Bertrand, Science, 2007, 316, 439; (b) T. W. Hudnall, J. P.
Moerdyk and C. W. Bielawski, Chem. Commun., 2010, 46, 4288;
(c) G. D. Frey, J. D. Masuda, B. Donnadieu and G. Bertrand,
Angew. Chem., Int. Ed., 2010, 49, 9444.
11 (a) D. J. Cardin, B. Cetinkaya, E. Cetinkaya and M. F. Lappert,
J. Chem. Soc., Dalton Trans., 1973, 514; (b) H.-W. Wanzlick and
E. Schikora, Chem. Ber., 1961, 94, 2389; (c) T. M. Trnka, J. P.
Morgan, M. S. Sanford, T. E. Wilhelm, M. Scholl, T.-L. Choi,
S. Ding, M. W. Day and R. H. Grubbs, J. Am. Chem. Soc., 2003,
125, 2546.
L.A.S. thanks the TUM Graduate School for financial
support. The authors thank Dr M. R. Buchner for help with
NMR measurements, Prof. Dr S. Schneider and Dr C. Azap
for helpful discussions.
12 (a) D. Moderhack and M. Lorke, Heterocycles, 1987, 26, 1751;
(b) D. Moderhack and A. Daoud, J. Heterocycl. Chem., 2003,
40, 625; (c) D. Moderhack, Liebigs Ann. Chem., 1989, 1271.
13 W. A. Herrmann, C. Kocher, L. J. Gooßen and G. R. J. Artus,
Chem.–Eur. J., 1996, 2, 1627–1636. Examinations towards the
deprotonation of 1 including NaH, KOtBu or Na[N(SiMe3)2] in
THF, Et2O or toluene at various temperatures did not yield the
desired results.
14 R. A. Kelly III, H. Clavier, S. Giudice, N. M. Scott, E. D. Stevens,
J. Bordner, I. Samardjiev, C. D. Hoff, L. Cavallo and S. P. Nolan,
Organometallics, 2008, 27, 202.
15 (a) G. Song, Y. Zhang and X. Li, Organometallics, 2008, 27, 1936;
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18 4 and 5a can be synthesized alternatively via the more expensive
Ag2O-route, based on H. M. J. Wang and I. J. B. Lin, Organo-
metallics, 1998, 17, 972, see ESIw. 5b could not be isolated following
this route, only mixtures of species were observed. Related problems
were described in: S. Dıez-Gonzalez, E. C. Escudero-Adan,
J. Benet-Buchholz, E. D. Stevens, A. M. Z. Slawin and S. P.
Nolan, Dalton Trans., 2010, 39, 7595.
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3857–3859 3859