Position-selective intramolecular aromatic C-H bond activation of 1,2,3-triazol-5-ylidene (tzNHC) ligands in (p-cymene)ruthenium(ii) complexes

Article abstract of DOI:10.1039/c2dt32368k

A series of 1,2,3-triazol-5-ylidene (tzNHC) complexes of a (p-cymene)ruthenium system was synthesized. The reactivity of the N-bonded and C-bonded aryl groups in the tzNHC ligand was found to be significantly different with respect to intramolecular aromatic C-H bond activation.

Full text of DOI:10.1039/c2dt32368k

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ARTICLE TYPE
Position-Selective Intramolecular Aromatic C-H Bond Activation of
1,2,3-Triazol-5-ylidene (tzNHC) Ligands in (p-Cymene)Ruthenium(II)
Complexes
Kenichi Ogata, Sayuri Inomata and Shin-ichi Fukuzawa*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
system to compare the reactivity of Nꢀbonded and Cꢀbonded aryl
groups in the tzNHC ligand toward ortho metalation at the metal
50 centre. 9,10
A series of 1,2,3-triazol-5-ylidene (tzNHC) complexes of a (p-
cymene)ruthenium system was synthesized. The reactivity of
the N-bonded and C-bonded aryl groups in the tzNHC ligand
10 was found to be significantly different with respect to
intramolecular aromatic C-H bond activation.
NꢀHeterocyclic carbenes (NHCs) have emerged as an extremely
useful class of ligands for use in transition metal complexes.1 The
majority of NHC ligands are imidazolꢀ2ꢀylidenes A or 1,2,4ꢀ
15 triazolꢀ5ꢀylidenes B (Fig. 1), which are classified as normal
NHCs. In contrast, since Crabtree first described an NHC
complex in which the imidazolium moiety was coordinated at the
C4 position (C),2 alternative NHC ligands (mesoionic NHCs)
have been reported.3 In this category, following the first
20 description of the 1,2,3ꢀtriazolꢀ5ꢀylidene ligand (tzNHC, D) and
its complex by Albrecht et al.,4 several research groups have
reported the synthesis of transition metal complexes bearing the
tzNHC ligand, as well as their application to catalytic
transformations.5 Our research group has reported the synthesis
25 of palladium and copper complexes bearing the tzNHC ligand
and their application to catalytic organic reactions.6 In these
studies, the tzNHC complexes showed higher catalytic activity
than the corresponding imidazole NHC system in organic
reactions. It is known that the electron donating ability of the
30 tzNHC ligand toward the metal fragment is higher than that of the
normal imidazolꢀ2ꢀylidene ligand,4 and its electronic properties
affect its catalytic activity in transitionꢀmetalꢀcatalyzed organic
reactions. Although the catalytic merit and electronic properties
of the 1,2,3ꢀtriazolꢀ5ꢀylidene ligand have been examined, an
35 investigation of the character of the two environmentally different
substituents (Nꢀ and Cꢀbonded) of the NHC ligand, derived from
its unsymmetrical ligand structure, has yet to be developed. To
obtain information about this point, we turned our attention to an
examination of the intramolecular aromatic CꢀH bond activation
40 (ortho metalation) of a 1,2,3ꢀtriazolꢀ5ꢀylidene ligand bearing two
aryl substituents. The report for ortho metallation of tzNHC
ligand has scarcely been studied.7,8 In case of palladium
complexes, it has been described that Nꢀbonded aryl group of
tzNHC ligand is prefer to undergo a ortho metalation.8 However
45 research about this point had yet to be studied in detail.
Fig. 1 Normal NHC (A and B) and mesoionic NHC ligands (C and D)
To prepare the ruthenium complexes bearing the 1,2,3ꢀtriazolꢀ
5ꢀylidene ligands, transmetalation from the corresponding in situ
55 generated silver carbene complexes was effected using Ag2O and
tetramethylammonium chloride.11 Treatment of phenylꢀ
substituted triazolium salt 1a with 0.6 equiv of silver oxide and
tetramethylammonium chloride, followed by reaction with 0.5
equiv of [(pꢀcymene)RuCl2]2, led to the formation of (pꢀ
60 cymene)RuCl(tzNHC) complex 2a, in which the Nꢀbonded
phenyl group of the tzNHC ligand was metalated at the ortho
position by ruthenium (Scheme 1).12 This complex was obtained
as a single product; the ruthenium complex that would result from
metalation of the Cꢀbonded phenyl ring was not formed.13 In the
65 case of the reaction in THF under reflux conditions, formation of
complex 2a was also confirmed by 1H NMR, and the complex 2a
was not converted to Cꢀbonded phenyl metalated product. This
result indicated that ruthenium metal activation of the aromatic
CꢀH bond in the Nꢀbonded phenyl ring occurred more easily than
70 that of the Cꢀbonded phenyl ring. In the reaction using triazolium
salt 1b bearing an oꢀtolyl group at the 1ꢀposition instead of a
phenyl group, selective formation of the corresponding ruthenium
complex 2b also proceeded, with CꢀH bond activation of the oꢀ
1
tolyl group. These complexes were characterized by H and 13C
1
75 NMR analysis. By H NMR, the two isopropyl methyl groups of
the pꢀcymene ligand were observed as two doublet signals at 0.73
and 0.82 ppm for 1a and 0.73 and 0.80 ppm for 1b, indicating
unsymmetrical molecular structures. The 13C NMR spectra
showed characteristic signals for the carbene carbons at 166.3
80 (1a) and 167.6 ppm (1b),14 and the rutheniumꢀbonded phenyl
carbons were observed at 173.0 (1a) and 174.2 ppm (1b).
The structures of complexes 2a and 2b were also confirmed by
Xꢀray analysis (Fig. 2), which revealed the existence of covalent
bonds between the ruthenium metal and the Nꢀbonded aryl
85 substituents of the tzNHC ligands at the ortho position. The Ruꢀ
In this communication, we report the synthesis of a series of
1,2,3ꢀtriazolꢀ5ꢀylidene complexes of a (pꢀcymene)ruthenium(II)
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C(carbene) bond distance in 4c (2.0225(19) Å) was similar in
2.025(4) Å, respectively. These bond distances were within the 45 value to 2a and 2b, whereas the RuꢀC(carbene) bond distance in
C(carbene) bond distances of 2a and 2b were 2.037(3) and
normal range for previously reported 1,2,3ꢀtriazolꢀ5ꢀylidene
ruthenium(II) complexes.4 The RuꢀC( 1ꢀaryl) bond distances in
2a (2.080(2) Å) and 2b (2.068(4) Å) were similar in value to the
corresponding imidazolꢀ2ꢀylidene (pꢀcymene) ruthenium
complex involving an orthoꢀmetalated phenyl group in the
ligand.10
3c (2.102(6) Å) was longer than in the complexes involving
orthoꢀmetalated structures (2a, 2b, and 4c). In the case of 4c, the
bond distance for RuꢀC(η1ꢀphenyl) (2.0860(17) Å) was almost
the same as that in complex 2.
As described above, when the reaction site in the Nꢀbonded
aryl group of the tzNHC ligand was protected, ortho metalation
of the Cꢀbonded phenyl group could be controlled. The difference
between the orthoꢀmetalation reactivity in the Nꢀ and Cꢀbonded
aryl groups probably derived from two factors: (i) Nꢀbonded aryl
55 substituents are probably more electronꢀrich than Cꢀbonded ones
and its affect the position selective ortho metalation. (ii) For
steric reasons, the Cꢀbonded phenyl ring would appear to be
oriented perpendicular to the triazole ring due to the steric
repulsions with methyl group at the 3ꢀposition. This precludes the
60 Cꢀbonded phenyl ring from approaching the ruthenium metal
with the orientation needed for CꢀH bond activation to occur.
η
5
50
-
BF4
Ru
N
Cl
1. Ag2O, Me4NCl
N
N
R
N
N
+
R
2. [(p-cymene)RuCl2]2
N
Me
1a (R = H)
1b
Me
2a (R = H): 53% yield
2b
(R = Me)
(R = Me): 40% yield
10
Scheme 1 Synthesis of ruthenium complexes involving CꢀH bond
activation of the Nꢀbonded aryl group in the tzNHC ligand.
-
BF4
Ru Cl
1. Ag2O, Me4NCl
Cl
N
N
N
+
N
2. [(p-cymene)RuCl2]2
N
N
Me
Me
1c
3c: 80% yield
Ru Cl
K2CO3
N
THF, reflux
N
N
Fig. 2 ORTEP drawings of 2a (left) and 2b (right) with thermal
Me
4c: 75% yield
15 ellipsoids drawn at 50% probability level. Hydrogen atoms and solvents
are omitted for clarity.
Scheme 2 Synthesis of ruthenium complex bearing 1ꢀ(2,6ꢀxylyl)ꢀ4ꢀ
phenylꢀtriazolꢀ5ꢀylidene 3c, and reaction of 3c with K2CO3.
We next investigated the reaction of 1ꢀ(2,6ꢀxylyl)ꢀ4ꢀphenylꢀ
triazolꢀ5ꢀylidene for which aromatic intramolecular CꢀH bond
activation of the aryl moiety at the ortho position by the
20 ruthenium complex was not possible (Scheme 2). The reaction of
in situ generated silver tzNHC, derived from the corresponding
triazolium salt 1c, with [(pꢀcymene)RuCl2]2 resulted in the
formation of tzNHC ruthenium complex 3c in 80% yield. In spite
of the presence of aromatic CꢀH bonds at the ortho positions of
25 the Cꢀbonded phenyl group of this tzNHC ligand, metalation of
these positions did not occur under the same reaction conditions
that produced complex 2 in Scheme 1. This noteworthy result
indicated that the reactivity of the Nꢀbonded and Cꢀbonded aryl
groups of the tzNHC ligands was considerably different, and the
30 Cꢀbonded aryl group was less likely to undergo metalation than
the imidazole NHC ligand.9 To abstract the hydrogen atom from
the phenyl ring and the chloride ligand from complex 3c and
synthesize the tzNHC complex orthoꢀmetalated at the Cꢀbonded
phenyl ring, the reaction of 3c with base was carried out (Scheme
35 2). Upon treatment of 3c with K2CO3 in THF under reflux
conditions for 6 h, ortho-metalated complex 4c was obtained in
65
Fig. 3 ORTEP drawings of 3c (left) and 4c (right) with thermal ellipsoids
drawn at 50% probability level. Hydrogen atoms and solvents are omitted
for clarity.
70
Replacement of the 2,6ꢀxylyl group with a benzyl group in the
tzNHC ligand resulted in the formation of the corresponding
ruthenium complex 3d in 97% yield, which was not orthoꢀ
metalated at the Cꢀbonded phenyl group (Scheme 3). 15
1
75% yield. H NMR signals for the isopropyl methyl protons of
-
BF4
4c were observed as two doublets at 0.51 and 0.92 ppm, whereas
those of 3c were observed as one doublet at 1.16 ppm. The
40 carbene carbon of 3c was observed at 162.4 ppm by 13C NMR. In
contrast, 13C NMR measurement of 4c showed two characteristic
low field signals at 178.4 and 180.8 ppm. The structures of 3c
and 4c were also confirmed by Xꢀray analysis (Fig. 3). The Ruꢀ
Ru Cl
1. Ag2O, Me4NCl
Cl
N
N
N
+
2. [(p-cymene)RuCl2]2
N
N
N
Me
Me
1d
3d: 97% yield
2
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Scheme 3 Synthesis of ruthenium complex bearing 1ꢀbenzylꢀ4ꢀphenylꢀ
triazolꢀ5ꢀylidene 3d.
13074; K. Donnelly, M. Albrecht, XXV International Conference on
Organometalic Chemistry, Lisbon, 2012, abstract F.2.23.
R. Saravankumar, V. Ramkumar and S. Sankararaman,
Organometallics, 2011, 30, 1689.
65
70
75
80
85
90
8
9
In summary, the preparation of ruthenium(II) complexes bearing
1,2,3ꢀtriazolꢀ5ꢀylidene ligands was achieved. This system was
used to demonstrate the unique properties of the 1,2,3ꢀtriazolꢀ5ꢀ
ylidene ligands derived from their unsymmetrical structures. It
was confirmed that the reactivity of the Nꢀbonded and Cꢀbonded
aryl groups of the tzNHC ligand toward intramolecular aromatic
CꢀH bond activation (ortho metalation) was significantly
10 different. These results offer important and useful information
about the coordination chemistry of triazole NHC complexes and
their use as catalysts. Further investigation in this area is
currently ongoing in our laboratory.
Report for intramolecular aromatic CꢀH bond activation of imidazole
NHC ligand on ruthenium(II) complex, for example: P. B. Hitchcock,
M. F. Lappert, P. L. Pye and S. Thomas, J. Chem. Soc., Dalton
Trans., 1979, 1929.
5
10 Intramolecular aromatic CꢀH bond activation of imidazole NHC
ligand on (pꢀcymene)ruthenium system has been repoted: C. Zhang,
Y. Zhao, B. Li, H. Song, S. Xu and B. Wang, Dalton Trans., 2009,
5182.
11 H. M. J. Wang and I. J. B. Lin, Organometallics, 1998, 17, 972; J. C.
Garrison and W. J. Youngs, Chem. Rev., 2005, 105, 3978.
12 The (η6ꢀarene)ruthenium complexes bearing 1,2,3ꢀtriazolꢀ5ꢀylidene
which not involve aromatic CꢀH bond activation of ligand have been
reported by Albrecht et al.: A. Prades, E. Peris and M. Albrecht,
Organometallics, 2011, 30, 1162; L. Bernet, R. Lalrempuia, W.
Ghattas, H. MuellerꢀBunz, L. Vigara, A. Llobet and M. Albrecht,
Chem. Commun., 2011, 47, 8058. and ref.4.
13 In the case of NMR tube reaction between [(pꢀcycmene)RuCl2]2 and
silver salt of 1a in CD2Cl2 at room temperatre for 3 h, product 2a
was detected as single NHC complex product by 1H NMR
measurement, and unreacted starting complex [(pꢀcycmene)RuCl2]
was also confirmed. Other ruthenium complex was not detected.
14 These values are the normal range of previously reported (pꢀ
cymene)ruthenium complex bearing 1,2,3ꢀtriazoleꢀ5ꢀylidene. See ref.
9
15
This work was financially supported by a Chuo University
Grant for Special Research. We are grateful to Prof. Yoshiaki
Nishibayashi and Dr. Yoshiaki Tanabe (University of Tokyo) for
their help with Xꢀray measurements.
Notes and references
20 Department of Applied Chemistry, Institute of Science and Engineering,
Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan.
Fax: +81-3-3817-1895; Tel: +81-3-3817-1916; E-mail:
orgsynth@kc.chuo-u.ac.jp
† Electronic Supplementary Information (ESI) available:full experimantal
25 procedure and data. See DOI: 10.1039/b000000x/
15 Although the reaction of 3d with K2CO3 was carried out under THF
reflux, orthoꢀmetalated complex was not formed and 3d was
recovered.
‡ CCDC 901766 (2a), 901767 (2b), 901768 (3c) and 901934 (4c)
contains the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
30
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40
45
50
55
60
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(p-cymene)ruthenium complex bearing 1,2,3-triazol-5-ylidene (tzNHC) ligand involving position
selective intramolecular aromatic C-H bond activation were synthesized. As a result, it was found
that the reactivity between N-bonded and C-bonded aryl group in tzNHC ligand for the C-H bond
activation is significantly different.