2,6-Bis(1-benzyl-1H-1,2,3-triazol-4-yl)pyridine and its octahedral copper complex

Article abstract of DOI:10.1107/S010827011004223X

In the tridentate ligand 2,6-bis(1-benzyl-1H-1,2,3-triazol-4-yl)pyridine, C₂₃H₁₉N₇, both sets of triazole N atoms are anti with respect to the pyridine N atom, while in the copper complex aqua-[2,6-bis(1-benzyl-1H-1,2,3-tr

Full text of DOI:10.1107/S010827011004223X

metal-organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
promote the cycloaddition of internal alkynes. Mechanistic
studies have demonstrated that these reactions involve term-
inal copper acetylides and proceed via a stepwise non-
concerted process (Tornoe et al., 2002). We prepared benzyl-
triazole from benzyl azide and alkynes using a ‘click reaction’
(Horne et al., 2004), which gave 1,4-disubstituted 1,2,3-tri-
azoles. 1,2,3-Triazoles can be used for the preparation of bi-
and tridentate ligands by appropriate choice of the alkynes
that are used for the cycloaddition reaction. The tridentate
triazole ligand 2,6-bis(1-benzyl-1H-1,2,3-triazol-4-yl)pyridine,
(I), was prepared from 1,5-diethynylpyridine. The first
published work on (I), abbreviated as BTP, was by Meudtner
et al. (2007). The tridentate triazole coordination to a metal
ion leads to conformational changes in the ligand. The
conformational sensitivity of BTP can be suitably modulated
to design nanoswitches (Piot et al., 2009) sensitive to metal
ions or pH changes.
ISSN 0108-2701
2,6-Bis(1-benzyl-1H-1,2,3-triazol-4-yl)-
pyridine and its octahedral copper
complex
Paulraj Danielraj,a Babu Vargheseb and S. Sankararamana*
aDepartment of Chemistry, Indian Institute of Technology Madras, Chennai 600 036,
India, and bSophisticated Analytical Instruments Facility, Indian Institute of
Technology Madras, Chennai 600 036, India
Correspondence e-mail: sanka@iitm.ac.in
Received 17 September 2010
Accepted 18 October 2010
Online 6 November 2010
In the tridentate ligand 2,6-bis(1-benzyl-1H-1,2,3-triazol-4-yl)-
pyridine, C23H19N7, both sets of triazole N atoms are anti with
respect to the pyridine N atom, while in the copper complex
aqua[2,6-bis(1-benzyl-1H-1,2,3-triazol-4-yl)pyridine](pyridine)-
(tetrafluoroborato)copper(II) tetrafluoroborate, [Cu(BF4)-
(C5H5N)(C23H19N7)(H2O)]BF4, the triazole N atoms are in
the syn–syn conformation. The coordination of the CuII atom
is distorted octahedral. The ligand structure is stabilized
through intermolecular C—Hꢀ ꢀ ꢀN interactions, while the
crystal structure of the Cu complex is stabilized through
water- and BF4-mediated hydrogen bonds. Photoluminiscence
studies of the ligand and complex show that the ligand is
fluorescent due to triazole–pyridine conjugation, but that the
fluorescence is quenched on complexation.
Comment
Huisgen’s dipolar cycloaddition of organic azides with alkynes
is the most direct route for the synthesis of 1,2,3-triazoles
(Huisgen et al., 1967). These are nitrogen heteroarenes which
have found numerous applications in organic (Karthikeyan &
Sankararaman, 2008), organometallic (Karthikeyan
&
Sankararaman, 2009) and medicinal chemistry (Shia et al.,
2002), as well as in materials chemistry (Crowley & Bandeen,
2010). 1,2,3-Triazole-based materials have advantageous
properties for high-performance metal coatings and adhesives
(Zhu et al., 2006). However, there are major problems
commonly associated with Huisgen’s dipolar cycloaddition
methodology, including the need for long reaction times and
high temperatures, as well as the formation of regioisomeric
mixtures of products when using unsymmetrical alkynes. It
was found that cycloadditions of terminal alkynes with alkyl
azides catalysed by CuI can be conducted at room temperature
and are highly regioselective (Rostovtsev et al., 2002). The
cycloaddition of alkynes with azides under CuI-catalysed
conditions leads exclusively to 1,4-disubstituted 1,2,3-triazoles
in high yields. This type of copper catalysis, however, does not
A view of ligand (I), with its ‘horseshoe’ conformation and
the atom-numbering scheme, is shown in Fig. 1. The torsion
angles N7—C14—C15—N4 [156.2 (3)ꢁ, antiperiplanar] and
N7—C10—C9—N3 [ꢂ171.8 (3)ꢁ, antiperiplanar] show that
the triazole moieties are positioned anti–anti wth respect to
the N atom of the pyridine ring. The anti–anti conformation is
preferred by (I) due to electrostatic repulsion between the
lone pairs of atoms N5 and N2 and N7. Protonation of these N
atoms or coordination with metals can remove this electro-
static interaction and make the conformation between triazole
and pyridine syn–syn. Density functional theory calculations
(Meudtner et al., 2007) on a model system predict the stabi-
lization energy for the syn–syn phase to be 6.4 kcal molꢂ1
(1 kcal molꢂ1 = 4.184 kJ molꢂ1) more than that for the anti–
m366 # 2010 International Union of Crystallography
doi:10.1107/S010827011004223X
Acta Cryst. (2010). C66, m366–m370

metal-organic compounds
Figure 3
Figure 1
The molecular structure of ligand (I), showing the atom-labelling scheme.
Displacement ellipsoids are drawn at the 40% probability level.
The molecular structure of (II), showing the atom-labelling scheme.
Displacement ellipsoids are drawn at the 50% probability level. Only one
of the two disordered orientations for the C18–C23 ring is shown.
Fig. 3. The coordination geometry around the CuII ion is
distorted octahedral. The two bite angles of the ligand with the
metal atom are N4—Cu1—N7 = 78.94 (11)ꢁ and N7—Cu1—
N3 = 78.54 (11)ꢁ. Atoms N3, N4, N7 and N8 form a near-
regular plane, with the metal atom deviating slightly out of the
mean plane in the direction of the water molecule
˚
[0.0755 (2) A]. The water molecule is almost normal to the
equatorial plane. The axial bond lengths [Cu1—O1 =
˚
˚
2.271 (3) A and Cu1—F7 = 2.464 (4) A] are considerably
elongated compared with the four Cu—N coordination
˚
distances [Cu—N = 1.948 (3)–2.067 (3) A]. This is common
among octahedrally coordinated copper compounds (Silver-
side et al., 2007).
As would be expected, there is considerable conformational
change between free ligand (I) and the coordinated ligand in
complex (II). The pyridine and triazole moieties are nearly in
the same plane, the dihedral angle between the two triazole
moieties being 7.5 (2)ꢁ [the corresponding value in the ligand
is 24.7 (2)ꢁ]. The triazole rings have almost rotated through
180ꢁ about the respective pyridine–triazole bonds, to bring
both triazole N atoms into a syn–syn conformation with
respect to the pyridine N atoms. The relevant torsion angles in
the complex (with corresponding values in the ligand in square
brackets) are N7—C10—C9—N3 = 0.1 (5)ꢁ [ꢂ171.8 (3)ꢁ] and
N7—C14—C15—N4 = 2.2 (5)ꢁ [156.2 (3)ꢁ]. Similar confor-
mational changes during complexation were noticed in 4-(2-
pyridyl)-1,2,3-triazole (Meudtner et al., 2007).
In the crystal structure of complex (II), the cations and
anions are linked by O—Hꢀ ꢀ ꢀF hydrogen bonds to generate
chains which extend in the a direction; details are given in
Table 2 and Fig. 4. In addition to the O—Hꢀ ꢀ ꢀF hydrogen
bonds, geometry calculations show other significant inter-
actions, including C—Hꢀ ꢀ ꢀF and C—Hꢀ ꢀ ꢀO contacts and a
C13—H13ꢀ ꢀ ꢀꢀ interaction with the centroid of the C1–C6
phenyl ring at (1 ꢂ x, ꢂy, 2 ꢂ z); details are given in Table 2.
Figure 2
The packing of (I) in the unit cell. Intermolecular interactions are shown
as dashed lines. [Symmetry codes: (i) x ꢂ 1, y, z; (ii) 1 ꢂ x, 1 ꢂ y, 1 ꢂ z.]
anti phase. The dihedral angles of the N1–N3/C9/C8 and N4–
N6/C16/C15 triazole rings with the pyridine ring are 7.5 (3)
and 22.5 (2)ꢁ, respectively. The C1–C6 and C18–C23 benzyl
rings are inclined to each other at an angle of 14.3 (2)ꢁ.
Analysis of the shortest intermolecular contacts shows that
chains of molecules are developed along the a direction by
C—Hꢀ ꢀ ꢀN interactions (Table 1 and Fig. 2). The inversion-
related N1–N3/C8/C9 five-membered rings at (x, y, z) and
(1 ꢂ x, 1 ꢂ y, 1 ꢂ z) overlap significantly, with a centroid–
˚
centroid separation of 3.537 (2) A, leading to an N2ꢀ ꢀ ꢀN3-
˚
(1 ꢂ x, 1 ꢂ y, 1 ꢂ z) separation of 3.576 (5) A.
A view of the copper complex with (I), namely aqua[2,6-
bis(1-benzyl-1H-1,2,3-triazol-4-yl)pyridine](pyridine)(tetra-
fluoroborato)copper(II) tetrafluoroborate, (II), is shown in
ꢃ
Acta Cryst. (2010). C66, m366–m370
Danielraj et al.
C23H19N7 and [Cu(BF4)(C5H5N)(C23H19N7)(H2O)]BF4 m367

metal-organic compounds
Figure 6
The fluorescence emission spectra of (I) in acetonitrile (5 ꢄ 10ꢂ5 M) with
(A) 1 ꢄ 10ꢂ5 M copper tetrafluoroborate, (B) 2 ꢄ 10ꢂ5 M copper
tetrafluoroborate and (C) 3 ꢄ 10ꢂ5 M copper tetrafluoroborate.
The C1–C6 phenyl ring and its equivalent at (ꢂx, ꢂ1 ꢂ y,
˚
2 ꢂ z) have a centroid–centroid separation of 3.837 (2) A but
˚
with a slippage of 1.95 A; the distance between the centroid of
one ring and the plane of the inversion-related ring is
˚
3.302 (2) A.
Figure 4
A view of the hydrogen-bonded chain extending along the a direction in
(II). For symmetry codes, see Table 2.
Ligand (I) is photoluminiscent and Fig. 5 shows the exci-
tation and emission spectra. There are two excitation bands
centred at 237 and 301 nm, and an emission band at 339 nm.
The luminiscence is quenched on complexation or protonation
of the ligand; Fig. 6 shows the luminiscence being quenched
when copper tetrafluoroborate is added to the ligand solution.
The conformational change of the pyridine–triazole moiety,
from anti–anti in the ligand to syn–syn in the complex, is the
cause of the quenching (Choi et al., 2006). The ligand could
thus be potentially useful as a pH or metal ion sensor.
Experimental
The Sonagashira coupling reaction (Sonogashira et al., 1975) of 2,6-
dibromopyridine with 2-methylbut-3-yn-2-ol gave 4,40-(pyridine-2,6-
diyl)bis(2-methylbut-3-yn-2-ol) in good yield. 2,6-Diethynylpyridine
was prepared from 4,40-(pyridine-2,6-diyl)bis(2-methylbut-3-yn-2-ol)
by treating it with KOH in toluene at 353 K in 70% yield (Shinohara
et al., 2001). 2,6-Diethynylpyridine (0.25 g, 2 mmol) was dissolved in
tBuOH–H2O (1:1 v/v, 4.0 ml), and benzyl azide (0.53 g, 4 mmol),
CuSO4 (20 mg) and sodium ascorbate (40 mg) were added. The
resulting mixture was stirred at room temperature overnight, diluted
with saturated aqueous NaCl (50 ml) and extracted with dichloro-
methane. The organic layer was dried over Na2SO4 and the solvent
evaporated under reduced pressure. The crude products were puri-
fied by flash chromatography on silica gel eluting with CH2Cl2–
EtOAc, to yield the corresponding ligand, (I). The compound (m.p.
475 K) was crystallized from chloroform by slow evaporation of the
solvent at room temperature in an open tube.
Figure 5
The fluorescence spectra of (I) in acetonitrile (5 ꢄ 10ꢂ5 M). Key: (A)
excitation spectrum of (I) (ꢁemission = 339 nm), (B) emission spectrum of
(I) (ꢁexcitation = 237 nm), and (C) emission spectrum of (I) (ꢁexcitation
301 nm).
=
There are also ꢀ–ꢀ interactions, with an almost complete
overlap of the coordinated pyridine rings at (x, y, z) and (1 ꢂ x,
˚
ꢂy, 2 ꢂ z), and a centroid–centroid separation of 3.815 (2) A
˚
and a slippage of only 0.60 A (slippage is the distance between
For the preparation of complex (II), (I) (0.1 g, 0.03 mmol) and
copper(II) tetrafluoroborate (0.08 g, 0.03 mmol) were dissolved in
acetonitrile. A drop of pyridine was added and the mixture warmed
the centroid of one ring and the perpendicular projection of
the centroid of the second ring on the plane of the first ring).
ꢃ
m368 Danielraj et al.
C23H19N7 and [Cu(BF4)(C5H5N)(C23H19N7)(H2O)]BF4
Acta Cryst. (2010). C66, m366–m370

metal-organic compounds
to give a clear solution. The clear solution was allowed to stand in an
open tube for crystallization of (II) by slow evaporation.
Table 1
Hydrogen-bond geometry (A, ) for (I).
ꢁ
˚
D—Hꢀ ꢀ ꢀA
C7—H7Bꢀ ꢀ ꢀN3i
D—H
Hꢀ ꢀ ꢀA
Dꢀ ꢀ ꢀA
D—Hꢀ ꢀ ꢀA
Compound (I)
0.97
2.45
3.411 (5)
171
Symmetry code: (i) x ꢂ 1; y; z.
Crystal data
C23H19N7
ꢄ = 104.61 (3)ꢁ
3
˚
Mr = 393.45
Triclinic, P1
a = 5.8800 (12) A
V = 994.6 (3) A
Z = 2
Table 2
Hydrogen-bond geometry (A, ) for (II).
ꢁ
˚
˚
Mo Kꢂ radiation
ꢅ = 0.08 mmꢂ1
T = 298 K
0.25 ꢄ 0.22 ꢄ 0.20 mm
˚
b = 11.460 (2) A
Cg is the centroid of the C1–C6 phenyl ring.
˚
c = 16.140 (3) A
ꢂ = 103.53 (3)ꢁ
ꢃ = 99.31 (3)ꢁ
D—Hꢀ ꢀ ꢀA
D—H
Hꢀ ꢀ ꢀA
Dꢀ ꢀ ꢀA
D—Hꢀ ꢀ ꢀA
O1—H2Aꢀ ꢀ ꢀF4
O1—H2Aꢀ ꢀ ꢀF1
O1—H1Aꢀ ꢀ ꢀF8i
C5—H5ꢀ ꢀ ꢀF5i
0.79 (5)
0.79 (5)
0.81 (5)
0.95
0.95
0.95
2.34 (5)
2.19 (5)
1.94 (5)
2.50
2.35
2.37
3.040 (5)
2.901 (5)
2.718 (5)
3.379 (6)
3.051 (5)
3.213 (4)
3.366 (4)
3.331 (4)
150 (4)
150 (4)
162 (4)
154
130
147
Data collection
Bruker Kappa-APEXII CCD area-
detector diffractometer
Absorption correction: multi-scan
(SADABS; Bruker, 1999)
Tmin = 0.934, Tmax = 0.973
10767 measured reflections
3417 independent reflections
1811 reflections with I > 2ꢆ(I)
Rint = 0.059
C24—H24ꢀ ꢀ ꢀF5i
C22—H22ꢀ ꢀ ꢀF5ii
C11—H11ꢀ ꢀ ꢀO1iii
C13—H13ꢀ ꢀ ꢀCgiii
0.95
0.95
2.55
2.55
144
140
Symmetry codes: (i) x ꢂ 1; y; z; (ii) x; y þ 1; z; (iii) ꢂx þ 1; ꢂy; ꢂz þ 2.
Refinement
R[F2 > 2ꢆ(F2)] = 0.054
wR(F2) = 0.185
S = 1.01
3417 reflections
272 parameters
1 restraint
H-atom parameteꢂrs3 constrained
˚
Áꢇmax = 0.33 e A
and hence not refined in the last set of cycles. The Uiso(H) values of
these H atoms were fixed at 1.2Ueq(C17). Water H atoms were
located in a difference map and refined isotropically without
restraints. In both compounds, the remaining H atoms bound to C
ꢂ3
˚
Áꢇmin = ꢂ0.25 e A
˚
atoms were constrained as riding, with C—H = 0.95 A for aromatic
˚
CH and 0.97 A for secondary CH2 groups, with Uiso(H) = 1.2Ueq(C).
Compound (II)
Crystal data
Thermal motion analysis of BTP (refined without restraints)
showed the difference between the mean-square displacements of
[Cu(BF4)(C5H5N)(C23H19N7)-
(H2O)]BF4
Mr = 727.73
Triclinic, P1
a = 8.2990 (17) A
ꢃ = 84.76 (3)ꢁ
ꢄ = 87.36 (3)ꢁ
V = 1552.7 (5) A
2
˚
atoms N7 and C10 in the direction of the N7—C10 bond as 0.0205 A
3
˚
2
and the standard uncertainty as 0.0027 A . To avoid this apparent
˚
Z = 2
˚
inconsistency, a rigid-bond restraint was set between N7 and C10. In
CUBTP, pseudo-isotropic restraints were applied to the C atoms of
the disordered phenyl ring to remove minor inconsistencies among
displacement parameters caused by disorder.
Mo Kꢂ radiation
ꢅ = 0.79 mmꢂ1
T = 173 K
0.41 ꢄ 0.22 ꢄ 0.20 mm
˚
b = 12.956 (3) A
˚
c = 14.520 (3) A
ꢂ = 88.56 (3)ꢁ
For both compounds, data collection: APEX2 (Bruker, 2004); cell
refinement: APEX2 and SAINT (Bruker, 2004); data reduction:
SAINT and XPREP (Bruker, 2004); program(s) used to solve
structure: SIR92 (Altomare et al., 1993); program(s) used to refine
structure: SHELXL97 (Sheldrick, 2008); molecular graphics:
ORTEP-3 (Farrugia, 1997) and PLATON (Spek, 2009); software
used to prepare material for publication: SHELXL97.
Data collection
Bruker Kappa-APEXII CCD area-
detector diffractometer
Absorption correction: multi-scan
(SADABS; Bruker, 1999)
Tmin = 0.761, Tmax = 0.903
20211 measured reflections
7222 independent reflections
4381 reflections with I > 2ꢆ(I)
Rint = 0.047
Refinement
R[F2 > 2ꢆ(F2)] = 0.056
wR(F2) = 0.149
S = 1.02
7222 reflections
459 parameters
72 restraints
H atoms treated by a mixture of
independent and constrained
refinement
Supplementary data for this paper are available from the IUCr electronic
archives (Reference: FG3196). Services for accessing these data are
described at the back of the journal.
ꢂ3
˚
Áꢇmax = 0.58 e A
ꢂ3
˚
Áꢇmin = ꢂ0.61 e A
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C23H19N7 and [Cu(BF4)(C5H5N)(C23H19N7)(H2O)]BF4
Acta Cryst. (2010). C66, m366–m370