Beyond click chemistry: Spontaneous C-triazolyl transfer from copper to rhenium and transformation into mesoionic C-triazolylidene carbene

Article abstract of DOI:10.1039/c2cc32725b

The exceptional versatility of the 1,2,3-triazole ring in tridentate carbonyl rhenium complexes has been explored starting from a click chemistry unexpected product obtained from the spontaneous copper to rhenium transmetalation, which opens the way to successive transformation to C-triazolyl and triazolylidene mesoionic carbene.

Full text of DOI:10.1039/c2cc32725b

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COMMUNICATION
Beyond click chemistry: spontaneous C-triazolyl transfer from copper to
rhenium and transformation into mesoionic C-triazolylidene carbenew
´
Celedonio M. Alvarez, Luis A. Garcı
´
a-Escudero, Raul Garcı
´ ´
´
a-Rodrıguez and Daniel Miguel*
Received 17th April 2012, Accepted 28th May 2012
DOI: 10.1039/c2cc32725b
The exceptional versatility of the 1,2,3-triazole ring in tridentate
carbonyl rhenium complexes has been explored starting from a
‘‘click chemistry’’ unexpected product obtained from the spon-
taneous copper to rhenium transmetalation, which opens the way
to successive transformation to C-triazolyl and triazolylidene
mesoionic carbene.
In contrast to the well-known NHCs derived from imidazolium
rings,13 the mesoionic carbenes derived from 1,2,3-triazolylidene
rings exhibit better donor properties.11 Some interesting NHCs
complexes of group 7 metals have been reported recently14 but, as
far as we know, there is no previous report of complexes of group
7 metals with 1,2,3-triazolylidene MICs.
We are currently interested in the use of complexes containing
pyridine-2-carboxaldehyde as k2(N,O) ligands in metal complexes15
as convenient precursors for iminopyridine complexes via Schiff
base condensation with primary amines,16 with the aim of
developing a methodology for the application of this reaction
in the functionalization of amino acids and amino esters.17 In
the context of these studies it was expected that successive use of
Schiff reaction and CuAAC could be a convenient method for
preparation of triazolyl-substituted iminopyridines.
Copper catalyzed azide–alkyne cycloaddition (CuAAC) has
become ubiquitous in nearly all areas of chemistry. The
reaction was first reported independently by the groups of
Meldal1 and Sharpless2 in 2002, and it has been applied
successfully in many fields.3
There has been increasing interest in the use of CuAAC for
the derivatization of ligands.4,5 In most cases the triazole ring
is coordinated through one of the nitrogen atoms, with there
being only a few examples of complexes containing C-bonded
triazolyl.6 Triazolyl copper(I) intermediates have been used
recently as transmetalation reagents for iridium complexes.7
Albrecht and co-workers have recently reported that the
methylation of the triazole ring and subsequent treatment with
Ag2O in the presence of a metal-halide complex can introduce
the 1,2,3,-triazolyl-4,5-ylidene bonded through C-5 to the
metal,8 thus acting as a mesoionic carbene (MIC) pertaining
to the class of abnormal-N-Heterocyclic Carbenes (a-NHCs).9
The interest in triazolylidene ligands has expanded quickly in
the last three years and a variety of complexes with different
metals have been reported, some of which have been used in
catalysis.10 Bertrand et al. reported the preparation of stable
bis-1,2,3-triazolyl-5-ylidene (i-bitz) ligands.11 In most cases the
ligands are introduced following original Albrecht’s method,
i.e. alkylation of the triazole ring followed by treatment with
Ag2O. However, Gandelman et al. have used an alternative
route, by alkylation of a C-triazolyl ring of a pincer ligand,
previously introduced by deprotonation with a base.12
Herein we wish to report the use of CuAAC for the
derivatization of metal carbonyl compounds to afford a new
complex in which the triazolyl group acts as a bridge between
Re and Cu after transmetalation. This is, as far as we know,
the first example of a triazole ring acting as a ligand both
through C and N. Additionally, the copper can be easily
removed to give a Re triazolyl which can be converted into a
mesoionic C-triazolylidene carbene.
Complex 1 containing pyridine-2-carboxaldehyde (pyca) as
a chelate k2(N,O) ligand reacts with 3-azido-1-propylamine in
refluxing MeOH to give, after workup, orange microcrystals of
2 (Scheme 1), which was characterized by analytical and
spectroscopic methods (see ESIw for experimental details and
spectroscopic data). An X-ray determination on a crystal of 2
(see Fig. S1, ESIw) confirmed the formation of the iminopyridine
ligand arising from the Schiff base condensation of the amine
´
´
GIR-MIOMeT-IU CINQUIMA/Quımica Inorganica,
Facultad de Ciencias, Universidad de Valladolid, Prado de la Magdalena,
s/n 47005, Valladolid, Spain. E-mail: dmsj@qi.uva.es;
Fax: +34 983423013; Tel: +34 983184096
w Electronic supplementary information (ESI) available: Experimental
procedures, spectroscopic data for all compounds and crystallographic
data for 2, 5, 6 and 7. CCDC 867717–867720. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c2cc32725b
Scheme 1 Reaction schemes for the preparation of new complexes.
Chem. Commun., 2012, 48, 7209–7211 7209
c
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group with the coordinated aldehyde, and the presence of the
azide group in the pendant arm of the iminopyridine.
Treatment of 2 with phenyl acetylene in the presence of
CuSO4Á5H2O and sodium ascorbate under catalytic conditions
(15–20 mol%) in THF:water produced the expected complex 3
bearing a triazole ring from the [3+2] cycloaddition of the
azide and acetylene groups. Spectroscopic data for complex 3
are in agreement with the structure depicted in Scheme 1. The
coordination ability of the pendant triazole ring in 3 can be
exploited to obtain cationic complexes via bromide abstraction
with silver triflate, to afford complex 4.18
Fig. 1 Structure of 5 showing the atom numbering. Both anion and
cation are centrosymmetric, with only one-half of each contained in
the asymmetric unit.
In the course of the CuAAC reaction to give 3 from 2,
IR monitoring in solution revealed the presence of a small
amount of an unexpected product 5 with bands at 2017 vs.
1921 s, 1904 vs. cmÀ1 (cf. 2026 vs. 1927 s, 1903 s, cmÀ1 for 2).
It was also observed that the addition of increasing amounts of a
copper reagent led to the presence of larger amounts of 5. Indeed
when a stoichiometric amount of copper and ascorbate was
added at the beginning, the reaction afforded 5 as the only
product, which could be isolated as orange-red microcrystals.
Better yield and shorter reaction times are obtained by using a
slight excess (1.2 : 1) of Cu/ascorbate. An X-ray Àdetermination
showed that the structure of 5 consists of a CuBr2 anion and a
cation in which one copper atom is bonded to two N-triazolyl-
(CH2CH2CH2NQCPy)Re(CO)3 fragments. Both anion and
cation are centrosymmetric, with the copper atoms Cu(1) and
Cu(2) placed in the centres of symmetry of the anion and cation,
respectively. The connectivity sequences propyl–N(3)–N(4)–N(5)
and phenyl–C(4)–C(5) confirm unambiguously that the triazole
ring is C-bonded to Re and N-bonded to Cu. This situation is quite
unexpected since, as far as we know, there is no precedent of a
triazole ring acting simultaneously as C- and N-donor.
Fig. 2 Structures of 6 (left) and the cation of 7BF4 (right), showing
the atom numbering. Only one of the two independent molecules of 7
is shown. The geometrical parameters of the other molecule are not
significantly different from the one shown.
bonded to rhenium at 153.5 ppm, slightly shifted from that
of the parent 5 (153.8 ppm).
The steps leading from 1 to 2 (Schiff base condensation) and
from 2 to 5 (cycloaddition and triazolyl transfer) were per-
formed in a sequential one-pot fashion, since the isolation of 2
is not required, as the only by-product of the Schiff reaction is
a small amount of water, and the transformation of 2 into 3 or
5 is done in THF/water (see ESIw).
A rationale for the process is presented in Scheme 2: the
copper triazolyl resulting from the cycloaddition (A) transfers
the triazolyl to rhenium in exchange for Br to give B, in which
the copper atom remains bonded to the triazole ring through
one N atom, followed by recombination to give the final
product 5. Similar transmetalations from Cu carbenes have
been reported by Cazin and Albrecht.19
Upon addition of trimethyloxonium tetrafluoroborate to a
solution of 6 the IR spectra show a change in the v(CO)
absorptions from 2012 vs. 1912 s, 1899 s, cmÀ1 to 2023 vs.
1923 s, 1905 s cmÀ1, which is consistent with the formation of
the cationic complex 7 (Scheme 1). An X-ray determination
(Fig. 2) confirmed the methylation of the nitrogen in position 3,
converting the triazolyl ring into a triazolylidene. This hetero-
cyclic structure formally contains a positive and a negative
charge in the same ring and can thus be regarded as a mesoionic
carbene. The geometry of the triazole ring is not significantly
changed upon going from 6 to 7. The Re–C distance is shortened
from 2.202(6) A in 6 to 2.175(7) and 2.173(7) A in 7BF4, and the
NMR signal for the carbon bonded to Re changes from 153.5 ppm
in 6 to 162.1 ppm in 7, all of which are consistent with the
change from an anionic triazolyl to a carbene-like triazolylidene.
Treatment of triazolyl complex 6 with acid produces
instantaneous protonation of the nitrogen, which can be
reverted by treatment with Na2CO3. When 6 was heated with
a slight excess of HBr (aq.) for 2 h, it was quantitatively
converted into the bromo complex 3, according to IR and
NMR data. Similarly, heating 6 with HOTf in THF (4 h)
produced the triflate salt of the cation 4.
As shown in Fig. 1, the cation of 5 can be regarded as a
copper(I) complex having two neutral N-donor ligands. To
test whether the Re moiety would be stable on its own, it was
deemed to be of interest to liberate the ligands. A CH2Cl2
solution of 5 was stirred with aqueous concentrated ammonia
for 1 h. IR monitoring showed a slight change in the v(CO)
stretching. Subsequent workup afforded 6 as pale yellow
microcrystals. An X-ray determination (Fig. 2) confirmed that
6 is the expected mononuclear rhenium tricarbonyl complex
with the tridentate (N,N,C) triazolylpropyliminopyridine.
The 13C NMR spectrum shows the signal for the carbon
In conclusion, the successive use of Schiff base condensation
and CuAAC on the pyca complex is a convenient way to
Scheme 2 Metathesis of the intermediate copper C-triazolyl and
subsequent symmetrization to give the final product.
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7210 Chem. Commun., 2012, 48, 7209–7211
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obtain triazolyl-functionalized iminopyridines, which can be
used upon halide abstraction to produce cationic complexes
with the triazole ligand acting as (N,N,N) tridentate. When the
copper reagent used for the cycloaddition is added in a
stoichiometric amount, a triazolyl transfer produces a cationic
complex in which the triazolyl ring is C-bonded to Re and
N-bonded to Cu. This is an unprecedented coordination mode
for the triazolyl ligand. Thus, choosing stoichiometric vs.
catalytic conditions directs the C or N coordination of the
triazole ring. The copper can be extracted with aqueous
ammonia, leaving a mononuclear neutral Re complex with
the triazolylpropyliminopyridine acting as a (N,N,C) anionic
tridentate ligand. Reaction with trimethyloxonium tetrafluoro-
borate yields a cationic complex in which the triazolylidene
ring acts as a mesoionic carbene. To close the cycle, protono-
lysis of the Re–C-triazolyl produces complexes with
uncoordinated or N-coordinated triazole depending on the
counter anion used.
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