A multidentate ligand based on two triazole groups from click chemistry and its copper(II) and iron(II) complexes: Synthesis, structure, and electrochemistry

Article abstract of DOI:10.1007/s11243-014-9849-2

A multidentate ligand, namely N,N-bis[[1-(phenylmethyl)-1H-1,2,3-triazol-4- yl]methyl]-2-pyridinemethaneamine (L), was synthesized through a click reaction of N,N-di-2-propyn-1-yl-2-pyridinemethanamine with benzyl azide in the presence of CuI catalyst. Treatment of L with CuCl₂ or Fe(NCS)₂ gave the complexes [CuLCl₂]·EtOH (1) and [FeL(NCS) ₂]·MeCN (2). Single-crystal X-ray studies show that in 1, the Cu(II) center has slightly distorted square pyramidal geometry resulting from the coordination of one pyridinyl nitrogen atom, one tertiary amine nitrogen atom, one triazole nitrogen atom, and two chloride atoms; in 2, the Fe(II) center has distorted octahedral geometry, coordinated by six nitrogen atoms; two each from NCS^- groups and triazole rings, one from a pyridinyl ring, and one from tertiary amine nitrogen. In addition, complexes 1 and 2 were characterized by spectroscopic and electrochemical methods.

Full text of DOI:10.1007/s11243-014-9849-2

Transition Met Chem (2014) 39:675–680
DOI 10.1007/s11243-014-9849-2
A multidentate ligand based on two triazole groups from
‘‘click chemistry’’ and its copper(II) and iron(II) complexes:
synthesis, structure, and electrochemistry
•
XiuLi You ZhenHong Wei
Received: 20 April 2014 / Accepted: 10 June 2014 / Published online: 15 July 2014
Ó Springer International Publishing Switzerland 2014
Abstract A multidentate ligand, namely N,N-bis[[1-(phen-
ylmethyl)-1H-1,2,3-triazol-4-yl]methyl]-2-pyridinemethane-
amine (L), was synthesized through a click reaction of N,N-di-
2-propyn-1-yl-2-pyridinemethanamine with benzyl azide in
the presence of CuI catalyst. Treatment of L with CuCl2 or
Fe(NCS)2 gave the complexes [CuLCl2]ÁEtOH (1) and
[FeL(NCS)2]ÁMeCN (2). Single-crystal X-ray studies show
that in 1, the Cu(II) center has slightly distorted square pyra-
midal geometry resulting from the coordination of one py-
ridinyl nitrogen atom, one tertiary amine nitrogen atom, one
triazole nitrogen atom, and two chloride atoms; in 2, the Fe(II)
center has distorted octahedral geometry, coordinated by six
nitrogen atoms; two each from NCS- groups and triazole
rings, one from a pyridinyl ring, and one from tertiary amine
nitrogen. In addition, complexes 1 and 2 were characterized by
spectroscopic and electrochemical methods.
units together [1]. One of the most popular reactions within
the click chemistry concept is the azide alkyne Huisgen
cycloaddition catalyzed by Cu(I) to generate a 1,4-substituted
1,2,3-triazole ring under mild conditions [2–9]. This reaction
is highly selective and efficient and has been intensively used
in a wide range of settings and applications [10–16]. The
cycloaddition reaction not only joins small units but also
creates a good ligation unit. The two sp2-hybridized nitrogen
atoms in the 1,2,3-triazole have the ligation ability to form
single neutral complexes [17, 18], coordination polymers [19,
20], and even multinuclear complexes with different metal
cores [21], in which the 1,2,3-triazole unit can be connected to
the metal atoms with only one sp2-hybridized nitrogen [22,
23] or both sp2-hybridized nitrogen atoms [24, 25].
Recently, we are interested in the design and synthesis of
multidentate ligands, due to their diverse potential application
in catalysis, mimics of metalloenzymes, ion recognition, and
so on [26–29]. In this paper, as a part of our continual efforts
in exploring a variety of novel multidentate ligands and their
transition metal chemistry, a ligand N,N-bis[[1-(phenyl-
methyl)-1H-1,2,3-triazol-4-yl]methyl]-2-pyridinemethane-
amine L, containing two 1,4-substituted 1,2,3-triazole units,
as well as its penta-coordinated complex [CuLCl2]ÁEtOH (1)
and hexa-coordinated complex [FeL(NCS)2]ÁMeCN (2) were
synthesized. Herein we report the synthesis, crystal structures,
and electrochemistry of complexes 1 and 2.
Introduction
‘‘Click chemistry’’ is a chemical terminology introduced by
K. Barry Sharpless in 2001 to describe a chemistry tailored to
generate substances quickly and reliably by joining small
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-014-9849-2) contains supplementary
material, which is available to authorized users.
Experimental
X. You Á Z. Wei (&)
Department of Chemistry, Nanchang University,
Nanchang 330031, China
e-mail: weizh@ncu.edu.cn
Instruments and materials
Reactions were performed under an argon atmosphere using
standard Schlenk techniques when necessary. Solvents were
freshly distilled by using appropriate drying agents prior to
X. You
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science
and Technology Normal University, Nanchang 330031, China
123

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Transition Met Chem (2014) 39:675–680
use. H NMR and 13C NMR spectra were recorded on an
AVANCE DRX 400 (Bruker) in CDCl3. Infrared data were
recorded on an FTIR spectrometer (Scimitar 2000, Varian).
UV–Vis spectra were performed on a Shimadzu UV-2550
spectrophotometer. Micro-analysis service was recorded
with a Heraeus CHN-O-Rapid. Electrochemical measure-
ments were performed on an Echo Chemie Autolab
PGESTAT 30 in dry acetonitrile with [NBu4][BF4] as the
supporting electrolyte; platinum and Ag/AgCl were used as
the working and reference electrodes, respectively. All
potentials are quoted against the ferrocenium/ferrocene
couple, Fc?/Fc. Compounds N,N-di-2-propyn-1-yl-2-pyr-
idinemethanamine [30] and benzyl azide [31] were synthe-
sized according to the literature, with little modification (see
in ESI).
(KBr, cm-1): 3,400(s), 3,021(w), 2,918(s), 2,863(w),
1,605(s), 1,488(s), 1,444(m), 1,273(s), 1,140(s), 1,106(s),
1,088(s), and 626(m). UV–Vis (DMF) [kmax/nm (e/
M-1 cm-1)]: 226 (82,050), 262 (49,300), 303 (25,600), 392
(12,304), and 800 (1,900).
1
Synthesis of complex 2
To degassed ethanol (5 mL), KSCN (0.098 g, 1 mmol) and
FeSO4Á7H2O (0.139 g, 0.5 mmol) were added to produce a
large amount of precipitate. After being stirred for 2 h, the
precipitate was filtered out, and the obtained filtrate was
added into a solution of ligand L (0.225 g, 0.5 mmol) in
ethanol (5 mL). The mixture was further stirred for 0.5 h to
produce a large amount of brown precipitate, which was
collected via filtration and washed with ethanol and dried
in vacuum. Recrystallization of the precipitate from
MeCN/Et2O gave the single crystals suitable for X-ray
diffraction analysis. Yield: 287 mg (87 %). Anal. Calcd for
C28H26FeN10S2ÁMeCN (%): C, 54.3; H, 4.4; N, 23.2.
Found: C, 54.6; H, 4.6; N, 23.6. IR (KBr, cm-1): 3,400(s),
3,011(w), 2,919(s), 2,865(w), 1,607(s), 1,489(s), 1,442(m),
1,275(s), 1,142(s), 1,108(s), 1,087(s), and 625(m). UV–Vis
Caution! In the course of preparation of benzyl azide,
sodium azide must be handled with care to avoid
explosion.
Synthesis of ligand L
To a solution of triethylamine (15 mL, 108 mmol) in THF
(100 mL), CuI (0.22 g, 1.16 mmol) was added under argon
atmosphere. The mixture was stirred for 5 min, and N,N-di-
2-propyn-1-yl-2-pyridinemethanamine (3.88 g, 21.1 mmol)
and benzyl azide (5.84 g, 43.8 mmol) were added. After
being stirred for 12 h at room temperature, the reaction
mixture was extracted with EDTA ammonium hydroxide
solution (100 mL, 1 mol L-1). The organic layer was dried
by anhydrous sodium sulfate and bleached by activated
carbon. Removal of the solvent under reduced pressure gave
an off-white solid, which was washed with diethyl ether and
used for the next reaction without further purification. Yield:
7.89 g (83 %). Anal. Calcd for C26H26N8 (%): C, 69.3; H,
Table 1 Crystal data and refinement details for complexes 1 and 2
Complexes
1
2
Chemical formula
Formula weight
Crystal color
C28H32N8OCl2Cu
631.07
C30H29FeN11S2
663.63
Blue
Brown
Crystal system
Space group
Orthorhombic
Pca21
Monoclinic
P21/c
˚
a (A)
15.736(3)
13.338(2)
13.963(2)
90
11.1178(8)
34.273(3)
8.7719(6)
98.173(1)
3,308.5(4)
28.31
˚
b (A)
1
5.8; N, 24.9. Found: C, 69.0; H, 6.0; N, 24.5. H NMR
˚
c (A)
(CDCl3, 400 MHz): 8.49 (s, 1H, Py), 7.60–7.12 (m, 15H, Py,
Ph, triazole), 5.50 (d, 4H, J = 6.8, 2PhCH2), and 3.76 (d,
6H, J = 6.4, CH2N(CH2)2). 13C NMR (CDCl3, 400 MHz):
159.1, 149.2, 144.6, 136.6, 134.1, 129.2, 128.8, 128.1,
123.7, 123.5, 122.2, 59.4, 54.2, and 47.9. IR (KBr, cm-1):
1,589 (Py), 1,569 (Py), 1497 (triazole), 1,473 (Py), 1,461
(triazole), 1,452 (triazole), and 1,435 (Py).
b (°)
3
˚
V (A )
2,930.6(8)
28.20
hmax
Z
4
4
L (Mo Ka, mm-1
)
0.965
0.621
Dcalc (g cm-3
)
1.430
1.332
F (000)
1,308
1,376
Reflections
measured
25,605
31,152
Synthesis of complex 1
Reflections unique 7,079 (Rint = 0.0409) 8,167 (Rint = 0.0448)
To a solution of ligand L (0.09 g, 0.2 mmol) in ethanol
(4 mL), CuCl2Á2H2O (0.034 g, 0.2 mmol) was added
quickly. After being stirred for 2 min, the reacted mixture
was filtered. The obtained filtrate was kept for several days
to give blue crystals of 1, which were filtered off and washed
with cold ethanol (2 mL) and dried under vacuum. Yield:
21 mg (17 %). Anal. Calcd for C26H26Cl2CuN8ÁEtOH (%):
C, 53.3; H, 5.1; N, 17.8. Found: C, 52.9; H, 4.9; N, 17.6. IR
GOF, S
1.069
1.025
R1, wR2 (obsd data) R1 = 0.0421,
wR2 = 0.1025
R1 = 0.0588,
wR2 = 0.1483
R1, wR2 (all data)
R1 = 0.0680,
wR2 = 0.1216
R1 = 0.1028,
wR2 = 0.1691
Max./min. peak (e. 1.184, -0.383
-3
0.540, -0.403
˚
A
)
123

Transition Met Chem (2014) 39:675–680
677
Scheme 1 Synthetic route for
ligand L and complexes 1–2
Fig. 1 The crystal structure of
complex 1 with hydrogen bond
between Cl(1) atom and solvent
ethanol molecule (the thermal
ellipsoids are drawn at 30 %
probability level)
(DMF) (kmax/nm (e/M-1cm-1)): 226 (12,7800), 260
(42,300), 292 (35,456), and 385 (6,546).
methods on F2 with the SHELX-97 program [32]. In crystal 1,
one benzene ring was disordered and split into two fragments
with occupancy factors of C(21)–C(26)/C(21A)–C(26A) =
0.35/0.65. In crystal 2, the solvated MeCN molecule was
disordered and split into two fragments with occupancy fac-
tors of N(11), C(29), C(30)/N(11A), C(29A), and C(30A) =
0.35/0.65. All non-hydrogen atoms were refined anisotropi-
cally. Except that the OH hydrogen atom of the ethanol
molecule in crystal 1 was added in a difference Fourier map,
X-ray single-crystal diffraction analysis
Crystallographic data for complexes 1 and 2 were collected at
296(2) K on a SMART APEX CCD diffractometer with Mo
˚
Ka radiation (k = 0.71073 A). Structures were solved by
direct methods and refined by full-matrix least-squares
123

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Transition Met Chem (2014) 39:675–680
˚
[33] in which the dialkyne was prepared from the nucleo-
philic reaction of 2-picolylamine with propargyl bromide,
and the benzyl azide was prepared from the substitution
reaction of benzyl bromide with sodium azide. Complexes
1 and 2 were synthesized by direct reaction of ligand
L with CuCl2 in open air, or fresh Fe(NCS)2 under argon
atmosphere, Scheme 1. Complex 1 was stable in solid state
and solution, while complex 2 was unstable in open air, and
its Fe(II) atom was easily oxidized into Fe(III) by dioxygen
molecules.
Table 2 Selected bond lengths (A) and angles (°) for complexes 1
and 2
Complex 1
Cu1–Cl1
2.2644(10)
2.085(3)
2.003(3)
101.98(4)
98.91(9)
92.81(8)
94.26(9)
79.38(11)
Cu1–Cl2
2.5351(10)
1.998(3)
Cu1–N1
Cu1–N2
Cu1–N4
N4–N5
1.311(4)
Cl1–Cu1–Cl2
Cl1–Cu1–N2
Cl2–Cu1–N1
Cl2–Cu1–N4
N1–Cu1–N4
Complex 2
Fe1–N1
Cl1–Cu1–N1
Cl1–Cu1–N4
Cl2–Cu1–N2
N1–Cu1–N2
N2–Cu1–N4
165.16(9)
98.10(8)
90.07(9)
82.12(11)
161.17(12)
Structural analysis of complexes 1 and 2
2.052(3)
Fe1–N2
2.096(3)
In complex 1, the coordination environment around the
Cu(II) center is best described as a distorted square pyra-
mid, with N(1), N(2), N(4), and Cl(1) forming the basal
plane and Cl(2) occupied the axial position, Fig. 1. The
atoms N(1), Cu(1), Cl(1), and Cl(2) are almost situated in
one plane which is perpendicular to the basal plane of the
square pyramid. As shown in Table 2, the bond length of
Fe1–N3
2.194(3)
Fe1–N4
2.315(2)
Fe1–N6
2.231(3)
Fe1–N9
2.224(3)
N1–Fe1–N2
N1–Fe1–N4
N1–Fe1–N9
N2–Fe1–N4
N2–Fe1–N9
N3–Fe1–N6
N6–Fe1–N9
101.23(12)
168.30(11)
108.36(11)
89.74(11)
88.87(11)
93.06(10)
149.39(10)
N1–Fe1–N3
N1–Fe1–N6
N2–Fe1–N3
N2–Fe1–N6
N3–Fe1–N4
N3–Fe1–N9
N4–Fe1–N9
92.71(11)
102.11(11)
165.46(11)
88.07(11)
76.69(10)
82.78(10)
75.59(10)
˚
Cu(1)–Cl(1) at 2.2644(10) A in the basal plane is shorter
˚
than the Cu(1)–Cl(2) bond length at 2.5351(10) A in the
axial direction due to the expected Jahn–Teller distortions
for Cu(II). The bond lengths between Cu(1) and pyridinyl
N(2) atom or triazole N(4) atom are very similar, but both
are slightly shorter than the distance of Cu(1) to the tertiary
amine N(1) atom, due to the p-back bonding in the former
two being stronger than the r-only bonding in the latter.
There is an intermolecular hydrogen bond between Cl(1)
atom and an ethanol solvate molecule with the hydrogen
other hydrogen atoms were placed in idealized positions and
treated as riding atoms. The crystal data and structure refine-
ment details for complexes 1 and 2 are listed in Table 1.
CCDC numbers 975,437–975,436 contain the supplementary
crystallographic data for complexes 1–2. These data can be
obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
˚
bond at O(1)–H(1)ÁÁÁCl(1) = 2.44(9) A.
Contrary to the Cu(II) center in complex 1, the Fe(II)-
coordination sphere in complex 2 has a slightly distorted
octahedral geometry, coordinated by six nitrogen atoms
including two thiocyanato nitrogen atoms, one pyridinyl
nitrogen atom, one tertiary amine nitrogen, and two triazole
nitrogen atoms, in which the N(1) and N(4) atoms are at the
axial positions, and N(2), N(3), N(6), and N(9) atoms are
located at the equatorial sites, Fig. 2. The two thiocyanate
Results and discussion
Synthesis and characterization
Ligand L was synthesized through an azide alkyne Huisgen
cycloaddition reaction involving dialkyne and benzyl azide
Fig. 2 The crystal structure of
complex 2 (the thermal
ellipsoids are drawn at 30 %
probability level)
123

Transition Met Chem (2014) 39:675–680
679
latter process is supported by the observation of a sharp
stripping peak at -0.65 V during the reverse scanning [34].
The peak potential separation of the first process is 0.17 V,
which is supportive of the one-electron assignment. The
ratio of the cathodic over the anodic peak currents (ipc/ipa),
0.64, suggests the reversibility of the first redox process.
Complex 2 also shows two redox processes, with the first
reduction peak at -0.042 V and the second sharp peak at
-2.08 V. The first reduction is assigned to the Fe3?/Fe2?
.
This one-electron process can be proved by the potential
separation of 0.13 V. The second irreversible redox process
is provisionally assigned to the Fe2?/Fe0 reduction, which
is also identified by the stripping peak of iron at around
-0.54 V.
Conclusions
A novel multidentate ligand with three kinds of donating
nitrogen atoms, N,N-bis[[1-(phenylmethyl)-1H-1,2,3-tria-
zol-4-yl]methyl]-2-pyridinemethaneamine, L, was synthe-
sized by azide alkyne Huisgen cycloaddition click
chemistry. Two mononuclear Cu(II), Fe(II) complexes of 1
and 2 based on ligand L were synthesized. The crystal
structures of complexes 1 and 2 showed only the N atom in
the position 3 of the 1,4-substituted 1,2,3-triazoles was
coordinated to the metals. The CVs revealed the reversible
redox processes of Cu2?/Cu? in complex 1 and Fe3?/Fe2?
in complex 2.
Fig. 3 CVs of complexes 1 and 2 in 0.1 mol L-1 [NBu4]BF4/MeCN
solution at a scanning rate of 0.1 V s-1 (the colored lines are CVs
without going through the second reduction)
Acknowledgments The authors thank the Education Department of
Jiangxi Province (Grant No.: GJJ13106) for financial support.
nitrogen atoms are cis to each other, whereas the two
coordinated triazole nitrogen atoms are trans to each other.
As shown in Table 2, the angle of \Fe(1)–N(1)–C(1) at the
axial position is 176.6(3)°, and the atoms Fe(1), N(1), C(1),
and S(1) are almost in a straight line. However, the angle of
\Fe(1)–N(2)–C(2) is only 154.1(3)°, and the atoms Fe(1),
N(2), C(2), and S(2) are not co-linear. The average bond
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