Dinickel(II) complexes with pyridine-substituted bis(triazolylmethyl)amine ligands: Structures and magnetic properties

Article abstract of DOI:10.1016/j.poly.2020.114813

The coordination behavior of the newly prepared bis(triazolylmethyl)amine-type ligands bis[(1-R-1H-1,2,3-triazol-4-yl)methyl]amine [R = CH₂Ph (L1), CH₂-2-pyridyl (L2)] and 1-(1-benzyl-1H-1,2,3-triazol-4-yl)-N-((1-(pyridin-2-ylmethyl)

Full text of DOI:10.1016/j.poly.2020.114813

Polyhedron 191 (2020) 114813
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Dinickel(II) complexes with pyridine-substituted bis(triazolylmethyl)
amine ligands: Structures and magnetic properties
Jeeranun Inthong a, Vasut Nakarajouyphon a, Kwanchanok Udomsasporn b, Khamphee Phomphrai b,
Nobuto Yoshinari c, Takumi Konno c, Preeyanuch Sangtrirutnugul a,
⇑
a Center of Excellence for Innovation in Chemistry (PERCH-CIC), Department of Chemistry, Faculty of Science, Mahidol University, Bangkok 10400, Thailand
b Department of Materials Science and Engineering, School of Molecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology, Rayong 21210, Thailand
c Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 July 2020
Accepted 16 September 2020
Available online 24 September 2020
The coordination behavior of the newly prepared bis(triazolylmethyl)amine-type ligands bis[(1-R-1H-
1,2,3-triazol-4-yl)methyl]amine [R = CH2Ph (L1), CH2-2-pyridyl (L2)] and 1-(1-benzyl-1H-1,2,3-triazol-
4-yl)-N-((1-(pyridin-2-ylmethyl)-1H-1,2,3-triazol-4-yl)methyl)methanamine (L3) toward NiII ions was
investigated. The reaction of NiCl2ꢀ6H2O with the benzyl-substituted bis(triazolylmethyl)amine ligand
L1 afforded the mononuclear NiII complex Ni(L1)2Cl2 (1). On the other hand, dinuclear NiII complexes,
[Ni2(L2)2Cl2]Cl2 (2) and [Ni2(L3)2Cl2]Cl2 (3), were obtained when NiCl2ꢀ6H2O was reacted with the pyri-
dine-substituted ligands L2 and L3, respectively. Single-crystal X-ray analysis revealed that the two octa-
hedral NiII centers in 2 and 3 are bridged by two triazole rings through nitrogen donors to generate the
dinuclear complexes with a syn arrangement. Meanwhile, treatment of Ni(NO3)2 with L2 afforded the
related dinickel(II) complex [Ni2(L2)2(NO3)2](NO3)2 (20), in which two non-bridging triazole rings and
two nitrate ions are positioned in an anti arrangement. Complexes 2 and 3 showed antiferromagnetic
coupling between the two NiII centers (S = 1), with J values following the trend: 20 > 2 > 3.
Ó 2020 Published by Elsevier Ltd.
Keywords:
Amine–triazole ligands
Bridging triazole
Dinickel complex
Antiferromagnetism
1. Introduction
as a multidentate ligand to enhance the rates of Cu-catalyzed
azide-alkyne cycloaddition (CuAAC or ‘‘click” reaction) [14]. On
The use of bridging ligands to construct dinuclear transition
metal complexes containing unpaired d electrons allows some con-
trols over the intramolecular metal–metal distances and spin–spin
interactions, which directly influence the properties of the result-
ing complexes, including magnetism and catalytic activities
[1–3]. Among this class of compounds, dinuclear nickel(II)
complexes are of particular interest due to their interesting
ferro- and antiferromagnetic properties [4–6], as well as their
unique catalytic activities [7].
the contrary, the related, but more flexible tridentate ligand bis
(benzyltriazolylmethyl)amine (BBTA), which contains two triazole
rings connected by a secondary amine group, has been much less
explored. Although the metal binding affinity of BBTA is expected
to be lower than that of TBTA, thermodynamic studies have shown
that the binding constants of TBTA and BBTA to CuII and NiII centers
are not significantly different. Specifically, the binding constant
ratios of TBTA to BBTA are 2.2 for CuII and 4.7 for NiII ions [15].
Despite its structural similarity to TBTA, BBTA has been much less
explored in coordination chemistry as well as in catalytic applica-
tions, with only two reports involving Cu-catalyzed azide-alkyne
cycloaddition (CuAAC) [16] and Ru-catalyzed hydrogenation of
ketones to secondary alcohols via outer-sphere hydrogenation
(i.e. the NAH and Ru–H moieties participate in ketone coordina-
tion) [17]. Also surprisingly, no crystal structures of metal com-
plexes supported by bis(triazolyl)amine-type ligands have
previously been reported, although a nickel complex with a related
triazole–2° amine–pyridyl ligand has been crystallographically
characterized [18]. Given the good binding constants with more
structural flexibility, bis(triazolylmethyl)amine is an attractive
Amine–1,2,3-triazole-based chelate ligands have gained
considerable interest in a wide range of research fields, including
coordination chemistry, material science and catalysis [8–13]. For
example, tris(benzyltriazolylmethyl)amine (TBTA), which is
a
well-known amine–triazole featuring a tertiary amine and three
1,2,3-triazole functional groups, has been shown to bind various
transition metal centers, typically in a tetradentate tripodal fash-
ion. Based on its unique coordination behavior, TBTA has been used
⇑
Corresponding author.
E-mail address: preeyanuch.san@mahidol.ac.th (P. Sangtrirutnugul).
https://doi.org/10.1016/j.poly.2020.114813
0277-5387/Ó 2020 Published by Elsevier Ltd.

J. Inthong et al.
Polyhedron 191 (2020) 114813
ligand, in addition to tris(triazolylmethyl)amine, based on their
potential applications in molecular magnetism and catalysis.
In this work, we prepared a series of bis(triazolylmethyl)amine
ligands bearing benzyl and 2-pyridylmethyl substituents on the
triazole rings (L1–L3; Scheme 1) and investigated their coordina-
tion behavior to NiII ions. For the symmetric L2 and the unsymmet-
ric L3 ligands, the pyridyl substituent(s) was incorporated into the
ligand framework to promote the binding to a NiII center and to
generate a dinuclear NiII structure. The dinickel(II) complexes
obtained in this study were all characterized by X-ray crystallogra-
phy, representing the first structurally characterized multinuclear
NiII complexes bridged by bis(triazolyl)amine-based ligands (L2
and L3). Furthermore, it was found that different NiII substrates,
NiCl2ꢀ6H2O and Ni(NO3)2, afforded dinickel(II) structures with
different geometrical configurations (syn vs. anti). The magnetic
properties of these dinickel(II) complexes were also investigated
in order to evaluate the magnetic interactions between the two NiII
centers in the solid state.
2.3. Magnetic measurements.
The crystalline sample was packed into a hand-made aluminum
sample holder and loaded into a plastic straw. The plastic straw
was injected into the SQUID instrument under a helium atmo-
sphere. The temperature dependency of the magnetic susceptibil-
ity was recorded at temperatures ranging from 2 to 300 K with
0.5 T. The field dependency of the magnetization was recorded at
2 K from 0.01 T to 7 T. After applying diamagnetic corrections using
Pascal’s constant, the data were analyzed using the PHI program
[22].
2.4. X-ray crystallography.
Single-crystal X-ray crystallography was performed on a Bruker
D8 venture. Diffraction measurements were made on a Photon II
detector using
I
l
S
3.0 Microfocus source Mo
K
radiation
a
(k = 0.71073 Å) at 100 K. The structures of the complexes were
solved by direct methods using intrinsic phasing (XT program)
and refined by full-matrix least squares against F2 using the XL pro-
gram. The intensity data were corrected using SADABS and data
integration was acquired using SAINT.
2. Experimental section
2.1. General.
Air-sensitive experiments were carried out under dry N2 using
standard Schlenk techniques. Unless otherwise noted, all chemi-
cals were obtained from commercial sources and used as received
and all reactions were carried out under standard air atmosphere.
Benzyl azide [19], 2-(azidomethyl)pyridine [20] and tris[(1-benzyl-
1H-1,2,3-triazol-4-yl)methyl]amine (TBTA) [21] were prepared
according to the literature methods. All solvents were of analytical
grade and used without further purification.
3. Synthesis of the ligands
3.1. Bis[(1-R-1H-1,2,3-triazol-4-yl)methyl]amine [R = CH2Ph (L1),
CH2-2-pyridyl (L2)]
Both ligands were prepared according to the synthetic proce-
dures of the previously reported methods, although the procedure
for L1 was slightly modified [16]. To a 40 mL 1:1 CH2Cl2:H2O solu-
tion of dipropargylamine (0.47 g, 5.0 mmol) were added RN3,
sodium ascorbate (0.030 g, 0.15 mmol, 3.0 mol%), tris(benzyltria-
zolylmethyl)amine (TBTA; 0.027 g, 0.050 mmol, 1.0 mol%) and
CuSO4ꢀ5H2O (0.012 g, 0.050 mmol, 1.0 mol%). The reaction mixture
was left to stir at room temperature for 24 h, after which an aque-
ous solution of EDTA/NH4OH (50 mL) was added and the resulting
mixture was allowed to stir for 3 h. The reaction mixture was then
extracted with CH2Cl2 (3 ꢂ 70 mL) and the organic layer was dried
over anhydrous Na2SO4, filtered and dried in vacuo to afford the
product, which was used in the next step without further
purification.
2.2. Characterization.
Nuclear magnetic resonance (NMR) spectra were recorded
using a 400 MHz Bruker Avance spectrometer for 1H (400 MHz)
and 13C{1H} (100 MHz) nuclei. Chemical shifts are reported in
ppm (part per million) using residual solvent peaks as references.
Fourier transform infrared spectroscopy (FT-IR) (400–4000 cmꢁ1
)
were collected on a Bruker Alpha instrument (Bruker Optics GmbH,
Ettlingen, Germany). Elemental analyses for carbon, hydrogen and
nitrogen were performed on a Yanaco MT-6 CHN Corder, whereas
high-resolution mass spectra were acquired from a Bruker micrO-
TOF II-OS mass spectrometer (ESI mode). Magnetic properties were
measured on a Quantum Design MPMS2 superconducting quan-
tum interference device magnetometer (SQUID). Electron spin res-
onance (ESR) spectra were obtained from a JEOL JES-FA200
spectrometer at 103 K using a JEOL ES-13060DVT5 temperature
control device. Powder X-ray diffraction patterns were recorded
at room temperature in the reflection mode [2h range of 5–40°
with a step size of 0.02°] by Malvern Panalytical Aeris XRD,
L1. RN3 = PhCH2N3 (1.5 g, 11.0 mmol) was used. The product
was obtained as a pale yellow solid in 80% yield (1.4 g, 4.0 mmol).
1H NMR (400 MHz, CDCl3) d, ppm: 7.39 (s, 2H, trzH), 7.34–7.27 (m,
6H, ArH), 7.21 (dd J = 7.3, 2.3 Hz, 4H, ArH), 5.44 (s, 4H, CH2), 3.85 (s,
4H, CH2). 13C{1H} NMR (101 MHz, CDCl3) d, ppm: 128.1, 127.7,
127.1, 120.8 (aromatic Cs), 53.1, 42.9 (CH2). HRMS: calcd for
C
20H22N7 [M + H]+: 360.1937; found: 360.1946.
L2. RN3 = 2-pyCH2N3 (1.4 g, 10.3 mmol) was used. The product
was obtained as a yellow brown oil in 82% yield (1.5 g, 4.1 mmol).
1H NMR (400 MHz, DMSO d6) d, ppm: 8.54 (d J = 4.8 Hz, 2H, pyH),
8.35 (s, 2H, trzH), 7.83 (td J = 7.7, 1.8 Hz, 2H, pyH), 7.40–7.26 (m,
4H, pyH), 5.77 (s, 4H, CH2), 4.28 (s, 4H, CH2). 13C{1H} NMR
equipped with a PIXcel1D-Medipix3 detector and Cu Ka radiation
(k = 1.5419 Å), operating at a tube voltage of 40.0 kV and tube cur-
rent of 15 mA.
Scheme 1. Structures of the ligands L1–L3.
2

J. Inthong et al.
Polyhedron 191 (2020) 114813
(100 MHz, DMSO d6) d, ppm: 154.8, 149.5, 139.0, 137.5, 126.4,
123.4, 122.4 (aromatic Cs), 54.5, 41.0 (CH2). HRMS: calcd for
C
5.1 Hz, 7H, ArH), 5.73 (s, 2H, CH2), 5.62 (s, 2H, CH2), 4.13 (d
J = 4.8 Hz, 4H, CH2). 13C{1H} NMR (101 MHz, DMSO d6) d, ppm:
154.9, 150.0, 141.1, 141.0, 137.5, 136.0, 128.9, 128.3, 128.1,
125.7, 125.01, 123.5, 122.5 (aromatic Cs), 54.6, 53.0, 41.8 (CH2).
HRMS: calcd for C19H21N8 [M + H]+: 361.1889; found: 361.1881.
18H20N9 [M + H]+: 362.1842; found: 362.1817.
(1-Benzyl-1H-1,2,3-triazol-4-yl)methanamine (I). Compound
I was prepared following the synthetic procedure of L1. Propargy-
lamine (0.55 g, 10 mmol), PhCH2N3 (1.5 g, 11 mmol), TBTA (0.050 g,
0.10 mmol, 1.0 mol%), sodium ascorbate (0.060 g, 0.30 mmol,
3.0 mol%) and CuSO4ꢀ5H2O (0.030 g, 0.10 mmol, 1.0 mol%) were
mixed in a 40 mL 1:1 CH2Cl2:H2O solution. The product was
obtained as a pale yellow solid in 79% yield (1.5 g, 7.9 mmol). 1H
NMR (400 MHz, DMSO d6) d, ppm: 7.94 (s, 1H, trzH), 7.33 (dd
J = 11.2, 7.3 Hz, 5H, ArH), 5.56 (s, 2H, CH2), 3.76 (s, 2H, CH2). 13C
{1H} NMR (100 MHz, DMSO d6) d, ppm: 149.8, 136.2, 128.7,
128.1, 128.0, 122.0 (aromatic Cs), 52.8, 37.1 (CH2). HRMS: calcd
for C10H13N4 [M + H]+: 189.1140; found: 189.1134.
Tert-butyl-((1-benzyl-1H-1,2,3-triazol-4-yl)methyl)carba-
mate (II). To a 40 mL THF solution of I (0.94 g, 5.0 mmol) was
added di-tert-butyl bicarbonate (1.3 g, 6.0 mmol) and the reaction
mixture was left stirring at room temperature. After 24 h, the mix-
ture was extracted with CH2Cl2 (3 ꢂ 70 mL), after which the
organic layer was dried over anhydrous Na2SO4 and filtered.
Finally, all volatiles were removed under reduced pressure to
afford a yellow powder in 78% yield (1.1 g, 3.9 mmol). 1H NMR
(400 MHz, CDCl3) d, ppm: 7.42 (s, 1H, trzH), 7.36–7.13 (m, 5H,
ArH), 5.45 (s, 2H, CH2), 5.25 (s, 1H, NH), 4.32 (d J = 6.0 Hz, 2H,
CH2), 1.37 (s, 9H, (CH3)3). 13C{1H} NMR (101 MHz, CDCl3) d, ppm:
155.9, 145.9, 134.6, 129.0, 128.7, 128.0, 121.9 (aromatic Cs), 79.5,
4. Synthesis of the nickel(II) complexes
[Ni(L1)2Cl2] (1). To a solid mixture of NiCl2ꢀ6H2O (0.24 g,
1.00 mmol) and L1 (0.72 g, 2.0 mmol) was added 6 mL of CH3OH.
After 24 h of stirring at room temperature, all volatiles were
removed under reduced pressure. Layer diffusion of dimethylac-
etamide (DMAc) onto the diethyl ether solution of 1 at –18 °C pro-
vided a pink solid in 53% yield (0.45 g, 0.53 mmol). Anal. Calcd For
Ni(L1)2Cl2ꢀ2H2O0.5CH3OH (MW = 900.52 g/mol), C40.5H48Cl2N14
-
NiO2.5: C, 54.02; H, 5.37; N, 21.78; found: C, 54.11; H, 5.28; N,
21.89. ESI-MS: calcd for [Ni(L1)2Cl]+, C40H42ClN14Ni: 811.2759;
found: 811.2828. FT-IR (cmꢁ1): 3363 (OAH), 3216 (NAH).
syn-[Ni2(L2)2Cl2][Cl]2 (2). The synthesis of 2 followed that of 1
but one equiv. of L2 (0.36 g, 1.0 mmol) was used instead of L1.
Crystallization by layer diffusion of tBuOH onto the CH3OH solution
of 2 at room temperature afforded a violet crystalline solid, suit-
able for X-ray crystallographic analysis, in 39% yield (0.19 g,
0.20 mmol). Anal. CalcdForNi2(L2)2Cl4ꢀ9H2O (MW= 1144.17g/mol),
C36H56Cl4N18O9Ni2: C, 37.79; H, 4.93; N, 22.04; found: C, 37.75; H,
4.89; N, 21.83. ESI-MS: calcd for [Ni2(L2)2Cl3]+, C36H38Cl3N18Ni2:
945.1254; found: 945.1370. FT-IR (cmꢁ1): 3365 (OAH), ca. 3200
(NAH).
54.1, 36.1, 28.3 (aliphatic Cs). HRMS: calcd for
C15H21N4O2
[M + H]+: 289.1665; found: 289.1614.
anti-[Ni2(L2)2(NO3)2][NO3]2 (20). To a solid mixture of Ni
(NO3)2ꢀ6H2O (0.29 g, 1.00 mmol) and L2 (0.36 g, 1.0 mmol) was
added 6 mL of CH3OH. After 24 h of stirring at room temperature,
purple precipitates were generated and collected by filtration.
Layer diffusion of tBuOH onto an aqueous solution of these precip-
itates at room temperature afforded a purple crystalline solid, suit-
able for X-ray crystallographic analysis, in 58% yield (0.34 g,
0.29 mmol). Calcd For Ni2(L2)2(NO3)4ꢀ3H2O (MW = 1142.31 g/mol),
Tert-butyl((1-benzyl-1H-1,2,3-triazol-4-yl)methyl)(prop-2-
yn-1-yl)carbamate (III). Under N2, compound II (0.87 g, 3.0 mmol)
and 60% of NaH in mineral oil (0.13 g, 3.3 mmol) were stirred in
DMF at 0 °C for 30 min. At this temperature, propargyl bromide
80% in toluene (0.30 mL, 6.0 mmol) was added slowly to the reac-
tion flask, after which the reaction mixture was allowed to stir at
room temperature overnight. Solvent evaporation under vacuum
afforded a red brown oil in 81% yield (0.70 g, 2.4 mmol). 1H NMR
(400 MHz, CDCl3) d, ppm: 7.45 (s, 1H, trzH), 7.36–7.20 (m, 5H,
ArH), 5.49 (s, 2H, CH2), 4.56 (s, 2H, CH2), 4.04 (s, 2H, CH2), 2.12
(s, 1H, „CH), 1.42 (s, 9H, (CH3)3). 13C{1H} NMR (101 MHz, CDCl3)
d, ppm: 154.8, 134.8, 131.0, 129.2, 128.9, 128.2 (aromatic Cs),
80.9, 79.5, 72.5, 54.3, 41.6, 36.5, 30.7, 28.4 (aliphatic Cs). HRMS:
calcd for C18H23N4O2 [M + H]+: 327.1821; found: 327.1813.
Tert-butyl((1-benzyl-1H-1,2,3-triazol-4-yl)methyl)((1-(pyri-
C
36H44N22O15Ni2: C, 37.85; H, 3.92; N, 26.98; found: C, 37.68; H,
3.83; N, 26.79. ESI-MS: calcd for [Ni2(L2)2(NO3)3]+, C36H38N21O9-
Ni2: 1024.187; found: 1024.174; calcd for [Ni(L2)2(NO3)]+,
C
36H38N19O3Ni: 842.276; found: 842.269. FT-IR (cmꢁ1): ca. 3300
(OAH), 3210 (NAH).
syn-[Ni2(L3)2Cl2][Cl]2 (3). The synthesis of 3 followed that of 2
but one equiv. of L3 (0.36 g, 1.0 mmol) was used instead of L2.
t
Crystallization by layer diffusion of BuOH onto a CH3OH solution
din-2-ylmethyl)-1H-1,2,3-triazol-4-yl)methyl)carbamate
(IV).
of 3 at room temperature afforded a violet crystalline solid, suit-
able for X-ray crystallographic analysis, in 41% yield (0.20 g, 0.20
mmol). Anal. Calcd. For Ni2(L3)2Cl4ꢀ4.5H2O (MW = 1061.11 g/mol),
Compound IV was synthesized following the procedure of L1,
except with the use of compound III (1.3 g, 4.0 mmol) and 2-(azi-
domethyl)pyridine (0.59 g, 4.4 mmol) as alkyne and azide sub-
strates. The reaction gave a dark brown solid in 53% yield (0.98 g,
2.1 mmol). 1H NMR (400 MHz, CDCl3) d, ppm: 8.50–8.45 (m, 1H,
pyH), 7.63–7.56 (m, 2H, trzH), 7.25 (t J = 6.1 Hz, 4H, ArH), 7.19–
7.12 (m, 4H, pyH), 5.52 (s, 2H, CH2), 5.40 (s, 2H, CH2), 4.44 (d
J = 7.4 Hz, 4H, CH2), 1.28 (s, 9H, (CH3)3). 13C{1H} NMR (101 MHz,
CDCl3) d, ppm: 155.2, 154.4, 149.6, 148.6, 137.7, 137.3, 134.7
129.0, 128.6, 128.0, 123.3, 122.3 (aromatic Cs), 80.4 (C(CH3)3),
55.5, 54.0, 31.2 (CH2), 28.29 (CH3). HRMS: calcd for C24H28N8O2Na
[M + Na]+: 483.2233; found: 483.2232.
C
38H49Cl4N16O4.5Ni2: C, 43.01; H, 4.65; N, 21.12; found: C, 43.10; H,
4.72; N, 21.02. ESI-MS: calcd for [Ni2(L3)2Cl3]+, C38H40Cl3N16Ni2:
943.1349; found: 943.1620. FT-IR (cmꢁ1): 3365 (OAH), 3245
(NAH).
5. Results and discussion
5.1. Synthesis of the ligands (L1, L2 and L3)
1-(1-Benzyl-1H-1,2,3-triazol-4-yl)-N-((1-(pyridin-2-
ylmethyl)-1H-1,2,3-triazol-4-yl)methyl)methanamine (L3).
Symmetric, multidentate bis(triazolylmethyl)amine ligands
containing benzyl (for L1) and 2-pyridylmethyl substituents (for
L2) on the triazole rings were prepared in high yields (ca. 80%)
from CuAAC of dipropargylamine and the corresponding azides,
A
mixture of IV (2.3 g, 5.0 mmol) and trifluoroacetic acid (3.8 mL,
50 mmol) in 100 mL of CH2Cl2 was stirred overnight at room tem-
perature. The reaction was then washed with a saturated aqueous
solution of NaHCO3. Solvent evaporation afforded a brown solid in
67% yield (1.2 g, 3.4 mmol). 1H NMR (400 MHz, DMSO d6) d, ppm:
8.52 (dd J = 4.9, 1.6 Hz, 1H, pyH), 8.25 (s, 1H, trzH)), 8.22 (s, 1H,
trzH), 7.81 (td J = 7.7, 1.8 Hz, 1H, ArH), 7.33 (ddd J = 15.2, 8.2,
catalyzed by CuSO4ꢀ5H2O/TBTA (TBTA
= tris(triazolylmethyl)
amine). Meanwhile, the unsymmetric amine–triazole-based ligand
L3, featuring both benzyl and 2-pyridylmethyl substituents, was
synthesized via a multi-step synthesis (Scheme 2). First, the 1°
amine group of (1-benzyl-1H-1,2,3-triazol-4-yl)methanamine (I)
3

J. Inthong et al.
Polyhedron 191 (2020) 114813
Scheme 2. Synthetic pathway for L3.
was protected by the tert-butyloxycarbonyl (Boc) group to give the
product II. Subsequent nucleophilic addition of propargyl bromide
followed by CuAAC with 2-pyridylmethylazide yielded the com-
pounds III and IV, respectively. Finally, selective cleavage of N-
Boc using trifluoroacetic acid afforded the unsymmetric ligand L3
in approximately 22% total yield.
5.2. Synthesis and characterization of the nickel(II) complexes (1, 2, 20
and 3)
Complexation of bis(triazolylmethyl)amine ligands (L1, L2, L3)
to NiII centers was carried out in CH3OH at room temperature.
The reaction of NiCl2ꢀ6H2O with two equiv. of the symmetric bis
Fig. 1. ORTEP structure of syn-[(l-L3)2Ni2Cl2][Cl]2 (3). Thermal ellipsoids are shown at 30% probability. Solvent molecules and H atoms, with the exception of NAH, are
omitted for clarity. Selected bond distances (Å): Ni(1)–Cl(1) 2.409(1); Ni(1)–N(1) 2.097(4); Ni(1)–N(4) 2.145(4); Ni(1)–N(5) 2.092(3); Ni(1)–N(14) 2.075(4); Ni(1)–N(16)
2.119(3). Selected bond angles (°): N(1)–Ni(1)–N(5) 90.8(1); N(1)–Ni(1)–N(4) 80.2(1); N(1)–Ni(1)–N(16) 85.4(1); N(5)–Ni(1)–N(4) 77.3(1); N(14)–Ni(1)–N(16) 87.9(1); Cl(1)–
Ni(1)–N(1) 167.8(1); N(4)–Ni(1)–N(14) 168.0(1); N(5)–Ni(1)–N(16) 175.6(1).
4

J. Inthong et al.
Polyhedron 191 (2020) 114813
(benzyltriazolylmethyl)amine L1 gave a pink solution. Crystalliza-
tion by layer diffusion of dimethylacetamide (DMAc) onto the
diethyl ether solution of the product at ꢁ18 °C resulted in pink pre-
cipitates (1). Although X-ray quality crystals could not be obtained
for complex 1, elemental analysis data are consistent with the
empirical formula of Ni(L1)2Cl2ꢀ2H2Oꢀ0.5CH3OH. The ESI-MS spec-
trum of 1 exhibits only a cluster of signals at m/z = 811.28, of which
the isotope pattern matches well with the simulation pattern for
[Ni(L1)2Cl]+ (Fig. S17). Thus, it is most likely that 1 has a mononu-
clear structure of the type Ni(L1)2Cl2, in which two L1 ligands pos-
sibly coordinate to an octahedral NiII center in a tridentate mode
using an amine N and two triazole N donors.
Similar reactions between NiCl2ꢀ6H2O with one equiv. of the
symmetric L2 and the unsymmetric L3 ligands, containing 2-
pyridylmethyl substituent(s) on the triazole ring, also produced
pink solutions. Recrystallization of the solids by layer diffusion of
tBuOH into the CH3OH solution afforded crystalline solids of 2
and 3.
Single crystal X-ray crystallographic analysis of 2 gave large R
values (e.g., R1 = 15.0%) due to major disorders of the outer sphere
Clꢁ ions as well as the poor crystal quality. Thus, only the atom
connectivity of its overall structure was determined (Fig. S21). For-
tunately, we were able to obtain crystal data for 3, which possesses
in a structure closely related to that of 2. As shown in Fig. 1, the
two NiII centers in 3 are linked by two 1,2,3-triazole rings through
the medial and proximal N donors to form a dinickel(II) structure
in [Ni2(l
-L3)2Cl2]2+. Both NiII centers are almost co-planar with
the linking triazole rings, with small deviations of 0.002–0.246 Å.
In addition, each L3 ligand with a pyridine substituent connects
two NiII centers in a
l-
j5 coordination mode. In particular, the
L3 ligand facially chelates to one NiII center through the N3trz
,
Namine and N30trz donors and to the other NiII center via the N2trz
and Npy donors. As a result, a six-membered, boat-shaped metalla-
cyclic with a NiꢀꢀꢀNi separation of 4.1262(8) Å is formed in 3. Nota-
bly, two Clꢁ ions, each of which completes the octahedral NiII
environment featuring an N5Cl donor set, coordinate to the two NiII
centers in a syn arrangement. There exist two crystallographically
independent Clꢁ counter ions that weakly interact with coordi-
nated amine groups with ClꢀꢀꢀNamine distances of 3.217(4) and
3.428(4) Å [23]. In addition, inner-sphere and outer-sphere Clꢁ
ions weakly interact with lattice H2O molecules (Clꢀ ꢀ ꢀOw = 3.058
(7)–3.280(6) Å) and triazolyl protons (Clꢀ ꢀ ꢀCtrz = 3.420(4) Å)
(Fig. S22).
Fig. 2. ORTEP structure of anti-[(
l
-L2)2Ni2(NO3)2][NO3]2 (20). Thermal ellipsoids are
shown at 30% probability. A water molecule, two nitrate ions and H atoms, with the
exception of NAH, are omitted for clarity. Selected bond distances (Å): Ni(1)–O(1)
2.0818(9); Ni(1)–N(1) 2.095(1); Ni(1)–N(3) 2.052(1); Ni(1)–N(4) 2.060(1); Ni(1)–N
(5) 2.137(1); Ni(1)–N(6) 2.087(1). Selected bond angles (°): N(1)–Ni(1)–N(3) 89.08
(5); N(4)–Ni(1)–N(5) 80.75(5); N(5)–Ni(1)–N(6) 78.84(5); N(1)–Ni(1)–N(4) 170.10
(5); N(3)–Ni(1)–N(5) 171.60(5); O(1)–Ni(1)–N(6) 164.68(5).
To obtain good quality crystals with the ligand L2, Ni(NO3)2 was
employed as a substrate, instead of NiCl2ꢀ6H2O. Fortunately, this
reaction afforded X-ray quality purple crystals (20). As shown in
Fig. 2, the entire complex cation features a dinickel(II) structure
broad absorption bands at ca. 3200 cmꢁ1, assignable to 2° NAH
stretching vibrations (Fig. S13–S16). In addition, the elemental
analyses of 2, 20 and 3 are consistent with the formulas obtained
from the X-ray data. With some water molecules present (vide
supra).
of the type [Ni2(
linked by two triazole rings from two L2 ligands, each adopting a
l
-L2)2(NO3)2]2+, in which two NiII centers are
l-
j5 coordination mode. While this dinickel(II) structure in 20 is
It should be noted that only a few dinuclear metal complexes,
whose metal centers are bridged by an amine–1,2,3-triazole che-
late through two N donors, have been reported and structurally
characterized, likely due to the relatively weak electron-donating
ability of triazolyl N donors. Pioneering work by P. S. Donnelly
and co-workers demonstrated dinuclear CuI and AgI structures
with bridging TBTA ligands by single-crystal X-ray analysis
[12,26], whereas no crystal structure of nickel complexes bridged
by an amine–triazole chelate via both triazole N2, N3 donors has
been reported. The successful synthesis and structurally character-
ization of 2, 20 and 3 are most likely due to the presence of the
2-pyridylmethyl substituent in L2 and L3, which enhances the abil-
ity of the ligand to bridge two metal centers. This hypothesis is also
consistent with the formation of the mononuclear NiII complex Ni
(L1)2Cl2 (1) generated in the presence of the benzyl-substituted
ligand L1. In addition, based on the crystal structures of 2 and 3,
only one pyridyl substituent in the bis(triazolyl)amine ligands
very similar to that observed in 2, two NOꢁ3 ions coordinate to both
NiII centers in an anti arrangement to form a centrosymmetric
structure with an inversion center at the center of the six-mem-
bered Ni2(l-L2)2 ring. In addition, the Ni2(l-L2)2 ring adopts a
chair configuration in 20, rather than the boat configuration as
observed in 2. Compared to 3, the NiII centers in 20 are much more
deviated from the mean plane of the triazole rings (0.177 Å,
0.677 Å), with a slightly shorter NiꢀꢀꢀNi separation of 4.0698(5) Å.
The packing structure of 3, shown in Fig. 3, reveals short contacts
between the coordinated NOꢁ3 ions and the triazolyl proton (H7;
O3ꢀ ꢀ ꢀC7 = 3.221(2) Å) as well as the methylene hydrogen atom of
trzCH2Ph (H6A; O1ꢀ ꢀ ꢀC6 = 3.328(2) Å), compared to the typical
Cꢀ ꢀ ꢀO van der Waals distance of 3.7 Å [24]. Meanwhile, the amine
group forms a relatively strong hydrogen bond with the outer-
sphere NOꢁ3 ion, with an Nꢀ ꢀ ꢀO distance of 2.82(2) Å [25]. For all
four NiII complexes, the infrared spectra contain moderate-to-weak
5

J. Inthong et al.
Polyhedron 191 (2020) 114813
Fig. 3. A view from the b-axis showing intermolecular hydrogen bonding in 20. All solvent molecules, one nitrate ion, and some H atoms are omitted for clarity. H-bonding
interactions are shown as blue dotted lines.
participates in the metal coordination. In fact, coordination of the
other pendent pyridylmethyl group in 2 to an adjacent dinickel
(II) moiety appears to be prevented due to steric reasons.
temperature-dependent magnetic features of all three complexes
in the form of MT vs. T plots are shown in Fig. 4 (red plots). The
observed
v
v
MT values at 300 K of 2, 20 and 3 are 2.56, 2.38 and
To investigate the effect of NOꢁ3 ions on the dinickel(II) struc-
ture, the chloride-containing complex 2 was treated with 4 equiv.
of AgNO3 in CH3OH at room temperature, immediately resulting in
grey precipitates. After 24 h of stirring, the mixture was filtered to
remove insoluble materials. Layer diffusion of CH3OH onto the fil-
trate afforded purple crystals (24% yield). The powder X-ray
diffraction pattern of this product was identical with that of 20
(Fig. S23). This result demonstrates not only that Clꢁ ions are
replaced by NOꢁ3 , but the structural conversion from the syn to anti
configuration also occurs. In other words, the dinickel(II) structure
is fluxional in solution and the syn vs. anti configuration of the
complex is dependent of the auxiliary Clꢁ vs. NO3ꢁ ligand. The ESI
mass spectra of 2, 20 and 3 commonly feature a fragmented signal
corresponding to [Ni(L2 or L3)2(Cl or NO3)]+, in addition to a paren-
tal signal corresponding to [Ni2(L2 or L3)2(Cl or NO3)3]+ (Fig. S18–
20). This suggests that the dinickel(II) structures in 2, 20 and 3 are
in equilibrium with their respective mononuclear structures in
solution, which allow for the syn-to-anti conversion, accompanied
by the replacement of coordinated Clꢁ ions in 2 by NO3ꢁ ions.
2.44 cm3 molꢁ1 K, respectively, indicative of S = 1 for this
dinickel(II) system (the theoretical value is 2.00 cm3 molꢁ1 K for
giso = 2). Upon cooling, the
antiferromagnetically coupled spins. Below ca. 40 K, the observed
MT values lower sharply and vanish when the temperatures are
vMT values decrease steadily, suggesting
v
close to zero. Based on the vM curves, the antiferromagnetic cou-
plings of 2, 20 and 3 are confirmed to be maximum at ca. 20 K
(Fig. 4).
The temperature dependency of the vMT values (2–300 K, 0.5 T)
and the field dependency of the magnetization (M) curves (0.01–
7 T, 2 K) were fitted to a theoretical curve based on the Hamilto-
2
nian, H = -2JS1ꢀS2 + gbH(S1 + S2) + 2D[Sz ꢁ 13 SðS þ 1Þ], where g is
the Landé’s factor, J is the isotropic exchange constant between
the two NiII centers, D is the axial single-ion zero-field splitting
parameter. The temperature independent paramagnetism (T.I.P.)
b
b
values and the fraction of monomeric magnetic impurity (
also considered. We could not reproduce the M vs. H plot without
considering or D. The best fitted parameters are summarized in
q) were
q
Table 1. The negative J values of all the three complexes further
confirm the intramolecular antiferromagnetic coupling between
the two NiII centers, whereas their magnitudes decrease in the
order 20 (–7.8 cmꢁ1) > 2 (–5.8 cmꢁ1) > 3 (–4.8 cmꢁ1). The highest
J value of 20 is attributed to better overlap between the magnetic
orbitals of dx2-y2 (Ni) and those of the bridging triazolyl ligands
[27]. This is explained by the dihedral angle (20.39°) between the
plane generated from N3–Ni1–N4 (Fig. 2) and the mean plane of
the bridging triazolyl rings, which is slightly smaller than the
5.3. Magnetic studies of the dinickel(II) complexes (2, 20 and 3)
Since 2, 20 and 3 are the first examples of amine–triazolyl-
bridged dinickel(II) complexes, an investigation of their magnetic
behaviors should provide insight into the effect of the bridging
triazolyl group and the coordinated inorganic anions (NO3 vs. Cl)
on the magnetic interactions between the two NiII centers. The
6

J. Inthong et al.
Polyhedron 191 (2020) 114813
Fig. 4. Plots of
v
MT vs. T (red, left), vM vs. T (blue, left) and M vs. H (green, right) of the dinickel(II) complexes 2 (a), 3 (b) and 20 (c). Black lines indicate the fitted curves.
7

J. Inthong et al.
Polyhedron 191 (2020) 114813
Table 1
The best fitted parameters for the magnetic data of 2, 3 and 20.
Compound
G
J/cmꢁ1
D/cmꢁ1
T.I.P./cm3molꢁ1
q
2
3
20
2.2687(5)
2.1714(4)
2.2049(6)
ꢁ5.774(2)
ꢁ4.8406(10)
ꢁ7.857(7)
+4.31(2)
+4.250(16)
+9.97(5)
4.10(6) ꢂ 10ꢁ4
5.71(5) ꢂ 10ꢁ4
5.87(6) ꢂ 10ꢁ4
0.019
0.021
0.077
dihedral angles observed in 3 (22.70°–23.69°). It should be noted
that the magnetic properties of the present dinickel(II) complexes
are comparable to those of the related pyrazolate-bridged dinickel
(II) complexes reported by Mukherjee and co-workers [27]. How-
nugul: Conceptualization, Writing - review & editing, Supervision,
Funding acquisition.
Declaration of Competing Interest
ever, the more planar Ni2(
features a much smaller dihedral angle (5.53(17)°) between the
planes of the pyrazolate ring of the {Ni1( -L’)} unit and
{Ni2(
-L’)}, which leads to the higher J value of –10.3 cmꢁ1. Thus,
l
-L0)2 core (L0 = 3-(2-pyridyl)pyrazole)
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
l
l
the lower dihedral angle, together with the NiꢀꢀꢀNi separation that
is shorter than that in 3 by ca. 0.06 Å, most likely accounts for the
improved intramolecular antiferromagnetic coupling for 20. The
non-zero D values for 2, 3 and 20 indicate the presence of a mag-
netic anisotropy for each NiII center, which is in line with the silent
X-band EPR spectra of these complexes at 103 K (Fig. S24).
Acknowledgements
This research is funded by the Thailand Research Fund (Grant
No. RSA6080082) and through the Royal Golden Jubilee Ph.D. Pro-
gram, Thailand Grant No. PHD/0220/2557 (for V. N.). We also
gratefully acknowledge financial support from the Center for
Excellence for Innovation in Chemistry (PERCH-CIC), Ministry of
Higher Education, Science, Research and Innovation, CIF Grant, Fac-
ulty of Science, Mahidol University and Mahidol University under
the National Research Universities Initiative.
6. Conclusion
In this work, two new bridging amine–triazole ligands, bis[(1-
(pyridin-2-ylmethyl)-1H-1,2,3-triazol-4-yl)methyl]amine (L2) and
1-(1-benzyl-1H-1,2,3-triazol-4-yl)-N-((1-(pyridin-2-ylmethyl)-1H-
1,2,3-triazol-4-yl)methyl)methanamine (L3), were prepared by
incorporating 2-pyridylmethyl substituents into the 1,2,3-triazole
ring. The reactions of L2 and L3 with NiII ions afforded the
Appendix A. Supplementary data
Characterization data of ligands L1–L3, their intermediates and
the nickel(II) complexes 1, 2, 2’ and 3, including multinuclear NMR
spectra, FT-IR and ESI-MS. CCDC-2008186 (2’) and CCDC-2008185
(3) contain the supplementary crystallographic data for this paper
and can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.poly.2020.114813.
dinickel(II) complexes [Ni2(
l-L2)2Cl2]Cl2 (2), [Ni2(l-L2)2(NO3)2]
(NO3)2 (20), and [Ni2(
l-L3)2Cl2]Cl2 (3). On the other hand, the
mononuclear NiII complex [Ni(L1)2]Cl2 (1) was produced when
bis[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine (L1), which does
not have pyridyl substituents, was employed for the reaction. This
is indicative of the importance of triazolyl substituents in the for-
mation of multinuclear metal complexes. Notably, the replacement
of coordinated Clꢁ ions in 2 by NOꢁ3 ions was found to result in a
syn-to-anti structural transformation to give 20, accompanied by
the boat-to-chair conformational change of the six-membered
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