Click-triazole: Coordination of 2-(1,2,3-triazol-4-yl)-pyridine to cations of traditional tetrahedral geometry (Cu(i), Ag(i))

Article abstract of DOI:10.1039/c0cc02899a

Coordination studies of the pyridine-triazole diad to copper(i) and silver(i) reveal the potential and conditions for the solution- and solid-state self-assembly of supramolecular architectures based on this motif.

Full text of DOI:10.1039/c0cc02899a

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Click-triazole: coordination of 2-(1,2,3-triazol-4-yl)-pyridine to cations
of traditional tetrahedral geometry (Cu(I), Ag(I))w
Olivier Fleischel, Nan Wu and Anne Petitjean*
Received 29th July 2010, Accepted 15th September 2010
DOI: 10.1039/c0cc02899a
Coordination studies of the pyridine-triazole diad to copper(I)
and silver(I) reveal the potential and conditions for the solution-
and solid-state self-assembly of supramolecular architectures
based on this motif.
Copper(I) complexes have been elegantly used in the
3
self-assembly and selection of helicates
f–g
3i
and grids due to
the resulting predictable tetrahedral organization of binding
sites. Chelation of Cu(I) by 1,2,3-triazole-based ligands has
been reported in the solid state to occur through the N2
0
5a
Due to its high efficiency, functional group tolerance, simple
purification procedure and compatibility with various solvents
including aqueous media, the copper-catalyzed azide–alkyne
cycloaddition (CuAAC) has received tremendous attention and
nitrogen atoms in 4,4 -bis-triazole and through both N2
5b
and N3 nitrogen atoms in tris-(benzyltriazolylmethyl)amine.
Herein, we investigate Copper (I) coordination to pyridine-
triazole ligands in solution (stoichiometry, binding constants,
1
found many applications. In addition to its role as a connector,
redox) and in the solid state.
1
the resulting 1,2,3-triazole has also recently been highlighted as
3 3
H NMR titration of L1 with Cu(I) in CDCl /CD CNw
2
a
2b–e
2f–k
a ligand for metal ions in sensing,
luminescence
Historically, polypyridine ligands such as
,2 -bipyridines have been essential in the development of
and
points to the involvement of both the pyridine and triazole
2l
magnetism.
rings: the proton signals of the pyridine, of the triazole and of
0
2
2
the CH unit shift and broaden upon Cu(I) addition due to
3a–e
3f–g
3h
self-assembled architectures
such as helicates,
cages
C–C bond rotation and nitrogen coordination. The resulting
solutions are bright yellow, and UV-vis titrations agree with
the contribution of two types of complexes (Fig. 1).
3i
and grids. However, their synthesis and functionalization
may be somewhat cumbersome, involving toxic reagents and
3c
extensive purification. As a result, replacing a pyridine with a
triazole becomes attractive because of the easy access to starting
materials and the facile functionalization via the N1 nitrogen
At low concentrations (Fig. 1a), a first band around 320 nm
arises as Cu(I) is added, whereas at higher concentrations, a
second band around 370 nm is distinctly present (Fig. 1b).
These two bands were assigned to MLCT transitions in the
(
Scheme 1 for numbering). Surprisingly, a systematic study of
+
2
+
2
the coordination of the triazole-pyridine diad has not yet been
reported, particularly in solution where self-assembly expresses
its full potential (e.g., error-correction, switching). For the
success of dynamic self-assembly, it is essential to understand
the conditions (e.g., concentration range, solvents) where the
desired species are present. Herein, we report the binding
properties of 2-(1,2,3-triazol-4-yl)-pyridine to cations of
CuL1(CH CN)
3
and Cu(L1)
complexes respectively. DFT
calculations indeed support a metal-based HOMO and a
ligand-based LUMO in each case (Fig. 1). Within the
+
CuL
2
complexes, the HOMO-LUMO energy gap is larger
0
than in its 2,2 -bipyridine counterpart, as the increase in
destabilization due to the presence of the triazole is greater
for the LUMO.w This was also observed in related triazole-
+
+
2f,i,j,4a
traditional tetrahedral geometry (Cu , Ag ) and the properties
+
derived octahedral Ru(II) and Fe(II) complexes.
of the derived complexes (Cu ).
4e
Based on the UV-vis data, the determined binding constants
0
4
a–d
Ligands L1
and L2 w (Scheme 1) were synthesized by
conventional CuAAC (or ‘‘Click’’) microwave assisted
were compared to those of Cu(I) binding to 2,2 -bipyridine
0
0
(Bpy) and 6,6 -dimethyl-2,2 -bipyridine (Mebpy) (Table 1).w
As evidenced by the binding constants, the replacement of one
pyridine by a triazole leads to significantly less stable 1 : 1 and
reactions.
2
: 1 L/M complexes. A possible explanation lies in the
Scheme 1 2-(1,2,3-Triazol-4-yl)-pyridine ligands used in this study.
Department of Chemistry, Chernoff Hall, Queen’s University,
90 Bader Lane, Kingston, ON, K7L 3N6, Canada.
E-mail: anne.petitjean@chem.queensu.ca; Fax: +1 613 533 6669;
Tel: +1 613 533 6587
w Electronic supplementary information (ESI) available: Synthesis
and characterization of L1 and L2, NMR and UV-vis titrations,
simulations of species distribution, mass spectrometry, CVs and
electrochemistry data, DFT and X-ray details. CCDC 787040. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c0cc02899a
Fig. 1 UV-vis titrations of L1 with Cu(CH CN)
3
4
ꢂ3
ꢀBF
4
in 1 : 1 degassed
ꢂ5
CHCl /CH
3
3
CN at 6 ꢁ 10 M (a) and 1 ꢁ 10 M (b). Insets: results
from DFT calculations (based on L2; isovalue 0.07 (a) and 0.07 (b)).
8
454 Chem. Commun., 2010, 46, 8454–8456
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Table 1 Compared binding constants (1 : 1 CHCl
3 3
/CH CN, error
expressed as 3s), susceptibility to O quantified by half-lives t1/2
2
+
2
(0.2 mM, CHCl
SCE, 0.1 M nBu
3
), and redox potentials of Cu(L)
, 25 1C, 100 mV s
(E1/2 in V vs.
ꢂ1
4
NꢀPF
6
)
Mebpy
Bpy
L1
L2
a
Log(b1)
Log(b2)
5.69 ꢃ 0.01
10.74 ꢃ 0.03
N/A
1.04
0.82
5.20 ꢃ 0.03
9.9 ꢃ 0.1
B1.5
0.33
0.23
3.06 ꢃ 0.01
5.09 ꢃ 0.06
50
0.56
0.20
a
t
1/2 (min)
15
0.59w
0.18w
E
E
1/2(CH
1/2 (THF)
2
Cl
2
)
Scheme 2 Geometrical changes upon oxidation/reduction of the
pyridine-triazole copper complex (note the relative orientation of the
R groups).
a
3 3
L2 was not soluble enough in CHCl /CH CN to obtain reliable data.
stability probably stems from the combined steric protection
+
+
of Cu from O by the four methyl groups in Cu(Mebpy)
,
+
2
2
2
and the steric destabilization of the corresponding Cu
complex (towards a more ‘‘square planar’’ conformation).
Interestingly, despite the open access to the copper center
(
no substituent on the pyridine 6 positions, unhindered
+
+
triazole N2, Scheme 2), Cu(L1)
2
and Cu(L2)
2
are both
5
a
reasonably stable towards oxidation. In the pyridine-triazole
series, sterics do play a role in tuning the stability of the
complex, as L1 is more resistant than L2 (Table 1). Although
the benzyl groups would not overly destabilize the corres-
Fig. 2 (a) Single crystal X-ray structure of Cu(L1)
2
ꢀBF
4
(H atoms
2
+
˚
ponding Cu complex (see Scheme 2), it is possible that more
and anion omitted for clarity);z selected bond distances (A): Cu–N7:
steric hindrance limits O diffusion towards Cu(I). As in the
2
2
.067(3), Cu–N3: 2.006(3), Cu–N8: 2.017(3), Cu–N4: 2.051(3);
+
(b) compared CVs in CH Cl
2
2
(E1/2 in V vs. Fc /Fc, 0.1 M
bipyridine system, position 6 of the pyridine may also be
substituted to further stabilize the pyridine-triazole-derived
Cu(I) complexes.
ꢂ1
N(nBu)
4
ꢀPF
6
, 100 mV s ).
+
+
2
electron-withdrawing nature of the triazole which results in
poorer s donation by the connected pyridine, and is weakly
compensated by enhanced p back-donation. As a result, in
order to reach saturation of Cu(I) with triazole-pyridine units
in this medium, submolar concentrations must be used.w
In the solid-state, however, the 2 : 1 complex readily formsz
and crystallizes (Fig. 2a). The distorted tetrahedral environ-
ment around the copper center is very reminiscent of that in
The oxidation of Cu(L1)
2
and Cu(L2)
Cl and THF. As shown
is reversibly oxidized in CH
potential intermediate between those of Cu(Mebpy)
was further
studied via cyclic voltametry in CH
2
2
+
in Fig. 2b, Cu(L1)
2
2
Cl
2
+
at a
2
and
+
2
+
Cu(Bpy)
(Table 1). The more facile oxidation of Cu(L1)
2
+
compared to Cu(Mebpy)
2
may be related to the more easily
7
2+
accommodated Cu square planar arrangement (Scheme 2),
whereby the pyridine H6 proton interacts with the triazole
7
nitrogen N2, as displayed in the solid state.
+
5c
Cu(Mebpy)2
1.25 and 81.921 (81.66 and 82.211 in Cu(Mebpy)2 ) and the
inter-ligand angles (N7–Cu–N3, N4–Cu–N8) are 115.18 and
:
The bite angles (N7–Cu–N8, N3–Cu–N4) are
+
2/1+
2
8
Interestingly, the redox potentials of Cu(L1)
/1+
and
2
Cu(L2)2
are fairly sensitive to the environment: indeed,
+
1
31.601 (115.95 and 130.331 in Cu(Mebpy)
bond distances are also comparable: ranging from 2.017 A
2
). Pyridine–Cu
they drop by nearly 0.4 V in THF (Table S1, ESIw). Again, the
openness of the triazole ring to incoming ligands makes such
2
f
˚
+
pyridine-triazole Cu complexes appealing for redox-based
˚
N8–Cu) to 2.051 A (N4–Cu) in the pyridine-triazole diad, and
(
+
˚
˚
from 2.018 A to 2.052 A in Cu(Mebpy)
bond (N3–Cu, Fig. 2a) is significantly shorter than in other
2
. One triazole N–Cu
sensing.** Note that the redox potential does not seem to vary
much with the nature of the (alkyl-based) triazole substituent
(compare L1- and L2-derived complexes), which makes such
system a robust scaffold onto which sensors may be built.
Finally, as substituted bipyridines and phenanthrolines have
5
a,b
reported Cu(I) complexes bearing chelating triazoles.
gas phase, the ESI-MS analysisw shows exclusive formation of
Cu(L1) complexes in the +1 and the +2 states.y
In the
2
6c
As a matter of fact, susceptibility of Cu(I) complexes to
been elegantly used to control molecular motions, the
novelty of the pyridine-triazole copper complex seems worth
highlighting in the context of molecular machines. As
suggested in Scheme 2, upon redox switching, the geometrical
distribution of the triazole substituents (R) is entirely
controlled, from a nearly perpendicular (+1 state) to an
antiparallel (+2 state) arrangement. This is different from
disymmetrically substituted bipyridines or phenanthrolines,
oxidation by O is of interest: much of the lack of enthusiasm
2
for Cu(I) coordination chemistry comes from its facile reaction
+
2
with oxygen. The stability of Cu(L)
gated in (non-degassed) CHCl
solvent ensures that 2 + 1 L/M mixtures exclusively yield
complexes was investi-
3
. Such a non-coordinating
+
the Cu(L)
2
complexes.8 The decay of the MLCT absorbance
as a function of time was monitored and fit.w The resulting
half-lives at submillimolar concentrations are compared in
2
+
where the orientation of substituents in the Cu
state is
+
Table 1. Whereas Cu(Mebpy)
no oxidation at 25 1C in CHCl
is fully oxidized within minutes. Such a drastic difference in
2
is quite stable to O
2
mostly random. As a result, the pyridine-triazole and the
2/1+
6
+
2
(
3
within a few days), Cu(Bpy)
reversible commutation of its Cu
applications in molecular motion.
complexes may also find
This journal is c The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 8454–8456 8455

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z CCDC 787040.
As confirmed by the exclusive corresponding 2 : 1 MLCT
absorption.
* In this context, the significant stabilizing effect of the supporting
8
*
electrolyte is noteworthy (no oxidation in CH
week).
ww (i) Binding constants are too large to be determined by H NMR,
2
Cl
2
over more than a
1
and absorption by CH
3
NO
2
prevents the use of UV-vis methods;
ii) the ESI-MS analysis of the 1 + 1 mixture required the introduction
(
of a competing solvent (CH
which likely induces the partial dissociation Ag(L1)
3
CN) in order to favour vaporisation,
+
2
+
into Ag(L1) .w
1
Fig. 3 (a) H NMR titration (400 MHz, 25 1C) of L1 (40 mM) with
Ag(OTf) in 1 : 1 CDCl /CD NO (lines are only connecting the experi-
1
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3
3
2
mental points; no data fitting); (b) Job plot in the same conditions.
Silver(I) complexes: The coordination of oligo-pyridyl triazole
+
ligands to Ag has recently been examined towards metallo-
4c,d,f
supramolecular architectures
and displays interesting
patterns at least in the solid state, where coordination of the
triazole N2 and sometimes N3 nitrogens occurs. In solution and
2
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in the gas phase, mixtures were observed.
Since the prepara-
tion of most of pyridyl-triazole-based silver complexes reported
4c,d,f
up to now involved precipitation or crystallization
(which do
not discriminate between binding strengths) we investigated the
(
2
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1
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8
NMR titrations were performed in 1 : 1 CDCl /CD NO . In
3
3
2
this non-competitive solvent, very strong binding occurs to form
a 2 : 1 L/M complex as evidenced by a very sharp turning point
in chemical shifts followed by an apparent plateau at 0.5
¨
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+
equivalent of Ag (Fig. 3a).
+
Consistent with a most dominant Ag(L1)
2
¨
H. Gorls, E. Altuntas, L. Gonzalez and U. S. Schubert, J. Org.
´
complex, the Job
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3
with the titration, these solution studies point to a very strong
+
2
: 1 Ag(L1)
2
complex under these conditions.ww Such formula-
4d
tion has indeed crystallized as a distorted tetrahedral complex.
Overall, this study provides a better understanding of the
+ +
appropriate conditions for Cu and Ag induced self-
assembly of pyridine-triazole units, and some of the resulting
+
+
properties. Both Cu and Ag form well-defined 2 : 1 L/M
complexes through the coordination of the pyridine and
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4
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(
e.g. CH
2
Cl
2
, CHCl
3
/CH
3
NO
2
) or (ii) at high concentrationsw
CN). The corresponding
+ 4b
in competing solvents (e.g. CH
3
(
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+
Cu(L1)
2
(this study) and Ag(L1)
2
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solid state into distorted tetrahedral complexes. Finally,
electrochemistry reveals the potential of the copper complexes
in the field of sensing and molecular motion.
The authors acknowledge Dr N. J. Mosey (DFT), Prof
S. Wang, Dr J.-P. Collin and Dr A. Marquis-Rigault for
helpful discussions, and financial support from the Natural
Sciences and Engineering Council of Canada (NSERC-USRA
to NW), the Canadian Institute for Health Research, Queen’s
University, the Canadian Foundation for Innovation and the
Ontario Ministry of Research and Innovation.
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Notes and references
z The same crystal structure was obtained for 1 + 1 and 2 + 1 L/M
+
mixtures.
J. Kosmrlj, Inorg. Chem., 2010, 49, 4820.
ˇ
8 A. Marquis, J.-P. Kintzinger, R. Graff, P. N. W. Baxter and
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y [Cu(L1)
2
]
was oxidized during ESI-MS analysis; same results for
1
+ 1 and 2 + 1 L/M mixtures.
8
456 Chem. Commun., 2010, 46, 8454–8456
This journal is c The Royal Society of Chemistry 2010