Metallosupramolecular helices constructed from nickel(II) and multidentate “click” triazole ligands

Article abstract of DOI:10.1016/j.poly.2020.114805

A series of binuclear metallosupramolecular helices [Ni₂L₃]⁴⁺ has been synthesized from Ni(II) salts and bis(bidentate) pyridyltriazole or aminomethyltriazole ligands, separated by ortho- (o-xpt and o-xamt, respectively),

Full text of DOI:10.1016/j.poly.2020.114805

Polyhedron 191 (2020) 114805
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Metallosupramolecular helices constructed from nickel(II)
and multidentate ‘‘click” triazole ligands
Uttam R. Pokharel a,b, Jordan C. Theriot b,1, Frank R. Fronczek b, Andrew W. Maverick b,
⇑
a Department of Chemistry and Physical Sciences, Nicholls State University, Thibodaux, LA 70301, USA
b Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA
a r t i c l e i n f o
a b s t r a c t
A series of binuclear metallosupramolecular helices [Ni2L3]4+ has been synthesized from Ni(II) salts and
bis(bidentate) pyridyltriazole or aminomethyltriazole ligands, separated by ortho- (o-xpt and o-xamt,
respectively), meta- (m-xpt, m-xamt), and para-xylylene (p-xpt, p-xamt) spacers. All complexes were
characterized by elemental analysis, ESI-MS, and in four cases, the structures confirmed by X-ray crystal-
lography. The xpt and xamt ligands, derived by ‘‘click” reactions, exhibit similar bonding behavior regard-
less of modifications in their coordination pockets and spacer groups, forming triple-helical binuclear
complexes with pseudo-octahedral geometry at Ni. The crystallographically characterized complexes
Article history:
Received 20 June 2020
Accepted 15 September 2020
Available online 19 September 2020
Keywords:
Supramolecular coordination
Binuclear triple helicate
Pyridyltriazole
Aminomethyltriazole
Nickel(II)
show shorter NiꢀꢀꢀNi separations with the ortho spacer ([Ni2(o-xpt)3]4+, 9.332(2); [Ni2(o-xamt)3]4+
,
8.9718(10) Å) than with the para spacer ([Ni2(p-xpt)3]4+, 11.574(2); [Ni2(p-xamt)3]4+, 11.899(2), 11.924
(2) Å).
Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction
time consuming [14–16]. As such, there is still a need for similar
binucleating ligands that can be prepared by mild and modular
Metal-directed self-assembly of transition metal ions and poly-
topic ligands has been an emerging field of coordination chem-
istry: unusual shapes of molecules such as helicates, boxes, rings,
catenates, and cages can be synthesized from relatively simple
starting materials [1–7]. In particular, tetracationic, triple-helical
binuclear transition-metal supramolecules are an attractive area
in recent years owing to their potential applications as therapeutic
agents in the treatment of cancer [8–12] and Alzheimer’s disease
[13]. These helices are designed considering the structure of the
bridging groups, the coordination pockets of the ligand, stereo-
chemical preferences of the metal ions, and non-covalent interac-
tions. Careful choice of the linker group and the coordination
behavior of the metal ion can control the overall dimensions of
supramolecular aggregates [5]. While many different metal–ligand
combinations have been studied for the self-assembly of
supramolecular triple-helical complexes, the most common is the
nickel(II)-pyridine complexation. These helical polynuclear com-
plexes can be assembled from Ni(II) and oligobipyridine or quater-
pyridine ligands; however, synthesis of the ligands is tedious and
methods. As an analog of bipyridines, pyridyltriazoles have
received tremendous attention owing to their high efficiency of
synthesis, ease of purification, functional group tolerance, and
compatibility with various solvents. Pyridyltriazoles are readily
synthesized by the copper-catalyzed Huisgen azide-alkyne cycliza-
tion (a ‘‘click” reaction) [17,18].
Our research group has long been involved [19–21] in designing
discrete macrocyclic metal–organic compounds based on Cu(II)
and multifunctional organic linkers. We have recently studied
the self-assembly of metallosupramolecular cages from tetraden-
tate bis(pyridyltriazole) ligands, separated with aromatic spacers,
and Cu(II) to capture bifunctional Lewis-basic guests [22,23]. The
most common coordination numbers in copper(II) complexes are
4 and 5. Thus, when Cu(II) reacts with a tetradentate binucleating
ligand LL, the most stable products have a metal:ligand ratio of 2:2,
i.e. [Cu2(LL)2] (Fig. 1a), with approximately square-planar coordi-
nation [24] as well as square-pyramidal species such as [Cu2(LL)2-
X2]2+ (Fig. 1b) [22]. As an extension of the work, we were also
interested in studying metallosupramolecular cages with the
[Ni2(LL)3]n+ stoichiometry. Although the complexation of pyridyl-
triazole ligands with transition metal ions has been studied exten-
sively [25], aminomethyltriazole ligands are still relatively new.
There are limited examples of 1:1 complexation of aminomethyl-
triazoles with Pd(II), Pt(II), Tc(I), and Re(I) [26–28].
⇑
Corresponding author.
E-mail address: maverick@lsu.edu (A.W. Maverick).
Present address: Department of Chemistry and Biochemistry, University of
1
Colorado, Boulder, CO, USA.
https://doi.org/10.1016/j.poly.2020.114805
0277-5387/Ó 2020 Elsevier Ltd. All rights reserved.

U.R. Pokharel et al.
Polyhedron 191 (2020) 114805
Fig. 1. Supramolecular assembly of tetradentate ligands with Cu(II) often produces (a) four-coordinate or (b) five-coordinate complexes.
When a pyridyltriazole ligand coordinates to metal ions that
have square-planar geometrical preferences, the coordination
sometimes deviates slightly from ideal square-planar geometry,
e.g. by twisting of the ligands [22]. This might be due to steric hin-
drance between the H atom of a pyridyl group and the triazole ring
of another ligand (Fig. 2a).
When the pyridyl moieties are replaced by aminomethyl
groups, the hydrogen atoms of the amino groups lie out of the
coordination plane (Fig. 2b), potentially reducing inter-ligand
repulsions. This kind of comparison has been made in other fami-
lies of ligands, such as with 2,20-bipyridine and 2-(aminomethyl)
pyridine. In addition, the presence of amino groups in the ligands
can improve the solubility of complexes in protic media due to
hydrogen bonding with the solvent.
Fig. 2. (a) A trans-bis(pyridyltriazole) complex showing unfavorable triazoleꢀꢀꢀH(C)
interactions that could lead to structural distortion; (b) a trans-bis(aminomethyl-
triazole) complex which avoids such repulsive interactions by replacing pyridyl
moieties with aminomethyl groups.
Mononuclear complexes ML3 with bidentate chelating ligands
generally have idealized D3 symmetry, with enantiomers desig-
(a)
(c)
(b)
(d)
nated P (or
D) and M (or K) [29–32]. Analogous dimeric complexes
constructed from bis(bidentate) ligands LL, i.e. [M2(LL)3]n+, can be
either homochiral (with P,P and M,M enantiomers, each having ide-
alized D3 symmetry) or achiral (with one metal P and the other M,
and idealized C3h symmetry) [29,30,33]. The homochiral diastere-
omers are called helicates, because the screw sense is the same at
both metals; the P,M diastereomer is called a mesocate. The heli-
cate and mesocate diastereomers of [M2(LL)3]n+ are illustrated
schematically in Fig. 3.
Our current focus is to investigate the self-assembly of flexible
tetradentate bis(pyridyltriazole) or bis(aminomethyltriazole)
ligands, separated with ortho-, meta-, and para-xylylene spacers
(Chart 1), and Ni(II) to see the effects of change in the aromatic core
as well as coordination pockets in determining the final topology of
the supramolecular architecture. Although complexes of ortho-
[17,34], meta- [17,22,35–37], and para- [17,29,30,35,37–39] xylyle-
nebis(pyridyltriazole) ligands have been reported for different
transition metals, as far as we are aware, no metal complexes of
tetradentate bis(aminomethyltriazole) ligands have been reported.
Herein we report the synthesis and characterization, including
molecular structures, of triple-helical Ni(II) complexes constructed
with the three isomeric xylylenebis(pyridyltriazole) and xylylene-
bis(aminomethyltriazole) ligands and Ni(II) salts.
Fig. 3. Schematic illustration of the two enantiomers of a mononuclear complex
[ML3]n+, with colored planes representing chelating ligands, and their assembly into
helicate and mesocate forms [M2(LL)3]n+. (a) The M enantiomer of a mononuclear
complex [ML3]n+; (b) the P enantiomer of [ML3]n+, represented by different ligand
colors; (c) the M,M diastereomer of a binuclear complex [M2(LL)3]n+, a helicate, with
the same chirality (and the same color scheme) at both metals; and (d) the M,P
diastereomer of [M2(LL)3]n+, a mesocate, with opposite chirality (and different color
schemes) at the two metals, and a mirror plane (– – – –) in the center.
2. Experimental section
2.1. Materials and analyses
boc-protected meta-, and para-xylylenebis(aminomethyltriazole)
were synthesized by following the procedure reported by the Hari-
das group [40] and the procedure was extended to synthesize boc-
protected ortho-xylylenebis(aminomethyltriazole), as described
below. Caution: Organic azides may be explosive under certain con-
ditions. We did not experience any difficulties in working with the
xylylenebis(azides) required for the aminomethyltriazole prepara-
tions. However, we did not prepare them in quantities larger than
All chemicals were reagent grade obtained from commercial
sources and used without further purification. Microanalyses (C,
H, N) were performed by M-H-W laboratories, Phoenix, Arizona.
1H NMR and 13C NMR spectra were recorded on a Varian Inova
spectrometer (1H, 500 MHz; 13C, 125 MHz) at ca. 22 °C and were
referenced to residual solvent peaks. The ESI-MS (electrospray
mass spectra) were acquired on an Agilent 6210 instrument. The
2

U.R. Pokharel et al.
Polyhedron 191 (2020) 114805
2.3.2. m-xamtꢀ2HCl
Yield 91%; 1H NMR (D2O): d 4.32 (s, 4H, CH2), 5.63 (s, 4H, CH2),
7.25 (s, 1H, Ar), 7.32 (d, 2H, 3J = 7.5 Hz, Ar), 7.44 (t, 1H, 3J = 7.5 Hz,
Ar), 8.14 (s, 2H, triazole CH). ESI-MS: m/z 299.1729 [M – 2HCl + H]+
(calcd 299.1733), 597.3378 [2 M – 4HCl + H]+ (calcd 597.3387). The
salt (200 mg, 0.54 mmol) was dissolved in 6 M aqueous NaOH
(10 mL) and extracted with chloroform (2 ꢁ 20 mL). The organic
layer was dried with MgSO4, filtered, and the filtrate evaporated
to dryness to give m-xamt (139 mg, 86%) as a white solid, mp:
82 °C. 1H NMR (CDCl3): d 1.64 (br, 4H, NH2), 3.96 (s, 4H, CH2),
5.48 (s, 4H, CH2), 7.16 (s, 1H, Ar), 7.24 (d, 2H, 3J = 8.2 Hz, Ar),
7.35 – 7.38 (m, 3H, 1 Ar + 2 triazole CH). 13C NMR (CDCl3): d
37.9, 53.8, 105.2, 120.8, 127.7, 128.5, 136.1, 150.3.
Chart 1. Xylylenebis(pyridyltriazole) and xylylenebis(aminomethyltriazole)
ligands.
2.3.3. p-xamtꢀ2HCl
Yield 75%; 1H NMR (D2O): d 4.33 (s, 4H, CH2), 5.65 (s, 4H, CH2),
7.36 (s, 4H, Ar), 8.15 (s, 2H, triazole CH). ESI-MS: m/z 299.1703 [M –
2HCl + H]+ (calcd 299.1733), 597.3315 [2 M – 4HCl + H]+ (calcd
597.3387). The product (200 mg, 0.54 mmol) was dissolved in
10% aqueous NaOH (10 mL) and extracted with chloroform
(2 ꢁ 20 mL). The organic layer was dried with MgSO4, filtered,
and the filtrate evaporated to dryness to give p-xamt (152 mg,
94%) as a white solid, mp 162 °C. 1H NMR (CDCl3): d 3.96 (s, 4H,
CH2), 5.49 (s, 4H, CH2), 7.27 (s, 4H, Ar), 7.35 (2H, triazole CH). 13C
NMR (CDCl3): d 37.8, 53.8, 120.6, 120.9, 128.8, 129.1, 135.6.
that given here, and we handled them only behind a hood sash or a
safety shield.
2.2. Synthesis of o-xylylenebis(boc-aminomethyltriazole)
To a stirred solution of boc-propargylamine (2.53 g, 16.4 mmol)
in dry acetonitrile (25 mL) at 0 °C, diisopropylethylamine (DIEA)
(3.13 mL, 18.8 mmol) and o-xylylenebis(azide) [41] (1.40 g,
7.45 mmol) were added under nitrogen. CuI (146 mg, 0.77 mmol)
was added and the reaction mixture stirred for 16 h at room tem-
perature. A white suspension formed. The volatiles were removed
in vacuo, and the solid mass was dissolved in chloroform and
extracted with a solution (100 mL) prepared from concentrated
NH3(aq) (10.0 mL) and ammonium chloride (1.00 g) in water.
The organic layer was collected and washed successively with
H2SO4 (4 M), saturated NaHCO3(aq), and water. The organic layer
was dried with anhydrous MgSO4, filtered, and the filtrate evapo-
rated to dryness. The product was purified by trituration with cold
diethyl ether to give o-xylylenebis(boc-aminomethyltriazole)
(3.42 g, 92%) as a white solid, mp: 185 °C. 1H NMR (CDCl3): d
1.39 (s, 18H, (CH3)3), 4.34 (d, 4H, 3J = 6.0 Hz, NHCH2), 5.28 (br,
2H, NHCH2), 5.58 (s, 4H, CH2), 7.27 (d, 3J = 7.5 Hz, 2H, Ar), 7.29
(d, 3J = 7.5 Hz, 2H, Ar), 7.45 (s, 4H, Ar). 13C NMR (CDCl3): d 28.5,
36.3, 51.4, 79.9, 122.0, 129.9, 130.6, 133.4, 146.5, 156.1.
2.4. Synthesis of Ni(II) xpt complexes 1a–3a
To a stirred solution of Ni(BF4)2ꢀ6H2O (58 mg, 0.17 mmol) in
methanol (20 mL), the xpt ligand (100 mg, 0.25 mmol) dissolved
in chloroform (20 mL) was added dropwise. The reaction mixture
was stirred for 3 h at room temperature. The precipitate was sep-
arated by filtration and washed with chloroform and methanol to
give a pink powder. Vapor diffusion of ethyl ether into solutions
of 1a and 3a in DMF gave pink crystalline solids suitable for anal-
ysis by X-ray crystallography.
2.4.1. [Ni2(o-xpt)3](BF4)4 (1a)
Yield 70%; ESI-MS: m/z 1557.3829 [Ni2(o-xpt)3(BF4)3]+ (calcd
1557.3825), 735.1881 [Ni2(o-xpt)3(BF4)2]2+ (calcd 735.1898),
461.4566 [Ni2(o-xpt)3(BF4)]3+ (calcd 461.4573), 324.5919 [Ni2(o-
xpt)3]4+ (calcd 324.5912). Anal. Calcd for C66H54B4F16N24Ni2-
ꢀ3CHCl3: C, 41.31, H 2.86, N 16.76. Found: C 41.28, H 2.82, N 16.24.
2.3. General method of preparation of the xylylenebis
(aminomethyltriazoles) o-xamt, m-xamt, and p-xamt via their HCl
salts
2.4.2. [Ni2(m-xpt)3](BF4)4 (2a)
Yield 77%; ESI-MS: m/z 324.5928 [Ni2(m-xpt)3]4+ (calcd
324.5912). Anal. Calcd. for
45.82, H 3.63, N 19.03. Found: C 46.53, H 3.78, N 19.01.
C
66H54B4F16N24Ni2ꢀCHCl3ꢀ2DMF:
C
The boc-protected xylylenebis(aminomethyltriazole) (2.0 g,
4.0 mmol) was dissolved in a mixture of conc. HCl(aq) and metha-
nol (1:5 v/v) (20 mL), and heated to reflux for 12 h. A white suspen-
sion was formed. The reaction mixture was cooled to room
temperature and filtered. The residue was washed with cold
methanol and dried.
2.4.3. [Ni2(p-xpt)3](BF4)4 (3a)
Yield 78%; ESI-MS: 735.1749 [Ni2(p-xpt)3(BF4)2]2+ (calcd
735.1898), 461.4552 [Ni2(p-xpt)3(BF4)]3+ (calcd 461.4573),
324.5935 [Ni2(p-xpt)3]4+ (calcd 324.5912). Anal. Calcd. for C66H54
-
B4F16N24Ni2ꢀ2CHCl3ꢀ2DMF: C 43.72, H 3.47, N 17.91. Found: C
43.64, H 3.43, N 18.17.
2.3.1. o-xamtꢀ2HCl
Yield 87%; 1H NMR (D2O): d 4.27 (s, 4H, CH2), 5.72 (s, 4H, CH2),
7.40 (d, 3J = 7.6 Hz, 2H, Ar), 7.51 (d, 2H, 3J = 7.6 Hz, Ar), 8.00 (s, 2H,
triazole-H). This product (500 mg, 1.35 mmol) was dissolved in 6 M
NaOH(aq) (10 mL) and extracted with dichloromethane
(3 ꢁ 50 mL). The organic layer was dried with MgSO4, filtered,
and the filtrate evaporated to dryness to give o-xamt (102 mg,
25%) as a white solid, mp 59 °C. 1H NMR (DMSO d6): d 3.74 (br,
4H, CH2), 5.76 (s, 4H, CH2), 7.14 (d, 3J = 7.5 Hz, 2H, Ar), 7.34 (d,
3J = 7.5 Hz, 2H, Ar), 7.85 (s, 2H, triazole CH). 13C NMR (DMSO d6):
d 37.2, 49.7, 122.1, 128.7, 129.1, 134.3, 150.2.
2.5. Synthesis of complexes 1b and 2b
To a stirred solution of o- or m-xamtꢀ2HCl (100 mg, 0.27 mmol)
in water (5 mL), Ni(BF4)2ꢀ6H2O (61 mg, 0.17 mmol) and triethy-
lamine (2.0 mL) was added. The reaction mixture was stirred for
3 h at room temperature and filtered, and excess NH4PF6 was
added to the filtrate. The resulting precipitate was collected and
rinsed with water to give a pink powder. Vapor diffusion of ethyl
ether into a solution of 1b in DMF or MeCN gave pink crystalline
solid suitable for analysis for X-ray crystallography.
3

U.R. Pokharel et al.
Polyhedron 191 (2020) 114805
Table 1
Crystal data and structure refinement for compounds 1a, 3a, 1b and 3b.
2.5.1. [Ni2(o-xamt)3](PF6)4 (1b)
Yield 68%. Anal. Calcd. for C42H54F24N24Ni2P4ꢀ2CH3CN: C 33.00,
H 3.61, N 21.75. Found: C 32.86, H 3.70, N 21.36.
Compound
1a
3a
[Ni2(o-xpt)3]
(BF4)4ꢀCHCl3
CCDC deposition number 2010484
[Ni2(p-xpt)3]
(BF4)4ꢀ3DMF
2010486
C75H75B4F16N27Ni2O3
1867.28
Monoclinic
P21/c
2.5.2. [Ni2(m-xamt)3](PF6)4 (2b)
Yield 61%; ESI-MS: m/z 1445.2544 [Ni2(m-xamt)3](PF6)3]+
(Calcd. 1445.2590), 650.1389 [Ni2(m-xamt)3](PF6)2]2+ (calcd
650.1471), Anal. Calcd. for C42H54F24N24Ni2P4ꢀ2H2O: C 31.68, H
3.42, N 21.11. Found C 31.56, H 3.40, N 21.01.
formula
mw
crystal system
space group
C67H55B4Cl3F16N24Ni2
1767.36
Monoclinic
P21/n
a/Å
b/Å
c/Å
b/deg
V/Å3
Z
12.5605(7)
16.6264(10)
47.126(3)
95.049(4)
9803.4(10)
4
90.0(5)
1.197
0.05 ꢁ 0.25 ꢁ 0.40
3.735–60.063
32785/14007/11894
25.3000(14)
15.0624(8)
24.5532(14)
111.075(3)
8730.8(9)
2.5.3. Synthesis of [Ni2(p-xamt)3](NO3)4 (3b)
To a stirred solution of Ni(NO3)2ꢀ6H2O (64 mg, 0.22 mmol) in
methanol (10 mL), p-xamt (100 mg, 0.33 mmol) in chloroform
(10 mL) was added slowly. The reaction mixture was stirred for
3 h, and the resulting precipitate was collected by filtration to give
3b (92 mg, 66%) as a light pink solid. The product was crystallized
from its solution in DMF by vapor diffusion of diethyl ether. ESI-
MS: m/z 1196.3255 [Ni2(p-xamt)3](NO3)3]+ (Calcd 1196.3299).
Anal. Calcd. for C54H82N32Ni2O16ꢀ4DMF: C 41.77, H 5.32, N 28.86.
Found: C 41.68, H 5.34, N 28.79.
4
T/K
90.0(5)
1.421
0.08 ꢁ 0.12 ꢁ 0.17
3.481–62.626
55904/13804/9255
D
calc/g cmꢂ3
cryst dimen/mm
h limits, deg
reflns, measd/unique/
obsd
F(000)
3584
3832
l
/mmꢂ1
1.901
0.0402
0.1189
0.3317
1.041
1.376
0.0836
0.0829
0.2395
1.033
Rint
R[I > 2r(I)]
Rw (all data)
GOF
2.6. X-ray analyses
Intensity data were collected at low temperature on a Bruker
Kappa Apex-II DUO CCD diffractometer fitted with an Oxford Cryo-
stream chiller. Radiation was from a CuKa source (IlS microfocus
Compound
1b
3b
[Ni2(o-xamt)3]
(PF6)4ꢀ5CH3CN
[Ni2(p-xamt)3]
(NO3)4ꢀ1.5DMFꢀ0.76H2O
tube with QUAZAR multilayer optics). Data reduction included
absorption corrections by the multiscan method, with SADABS
[42]. The structures were determined by direct methods and differ-
ence Fourier techniques and refined by full-matrix least squares,
using SHELXL [43]. All non-hydrogen atoms were refined
anisotropically, except for a disordered phenyl group and a disor-
dered BFꢂ4 ion in 1a and partially populated water molecules in
3b. Disordered solvent molecules were removed using the
SQUEEZE procedure [44] for all compounds except 1b; in 3b, rea-
sonable models for four of the eight independent nitrate ions could
not be refined, so they were also removed with SQUEEZE. In the
final model for 1a, the disordered phenyl had two conformations
with occupancies 0.557(16)/0.443(16), and one BFꢂ4 ion was disor-
dered in two orientations with 0.533(10)/0.467(10) occupancies by
rotation about a B-F vector. In 1b, two PFꢂ6 anions were similarly
disordered by rotation about F-P-F lines. In one, the two occupan-
cies were 0.815(9)/0.185(9), and in the other they were 0.639
(12)/0.361(12). In 3b, one DMF molecule was disordered into two
orientations with occupancies 0.595(11)/0.405(11), and this disor-
der is correlated with two partially populated water molecules. All
H atoms were placed in idealized positions, except for those in
water molecules in 3b, which could not be located. In 1b, the Ni
dimer lies on an approximate twofold axis, and the structure is
pseudosymmetric to C2/c. It is an inversion twin with population
of the two components 0.646(18)/0.354(18). For additional details,
see Table 1.
CCDC deposition
number
formula
mw
crystal system
space group
a/Å
b/Å
c/Å
b/deg
V/Å3
Z
2010485
2010487
C52H69F24N29Ni2P4
1797.66
Monoclinic
Cc
28.3349(16)
12.3518(7)
22.6035(12)
109.023(2)
7478.9(7)
C46.5H66.02N29.5Ni2O14.26
1383.69
Monoclinic
P21/n
30.0989(19)
12.7391(17)
34.642(2)
109.292(3)
12537.0(13)
8
90.0(5)
1.593
0.04 ꢁ 0.12 ꢁ 0.39
2.374–66.645
56208/21540/15704
4
T/K
90.0(5)
1.597
0.27 ꢁ 0.20 ꢁ 0.14
3.30–67.57
23950/10751/10541
D
calc/g cmꢂ3
cryst dimen/mm
h limits, deg
reflns, measd/
unique/obsd
F(000)
3664
6248
l
/mmꢂ1
2.499
0.0260
0.0407
0.1160
1.029
1.613
0.0429
0.0628
0.1921
1.027
Rint
R[I > 2r(I)]
Rw (all data)
GOF
sponding diazides by reaction with NaN3 in DMF/H2O at room tem-
perature, followed by the ‘‘click” reaction between the diazides and
boc-propargylamine in the presence of CuI/DIEA [40]. The boc-pro-
tected meta- and para-xylylenebis(aminomethyltriazole) were
synthesized by following the procedure reported by the Haridas
group [40]; we extended the method to the analogous ortho isomer
(92% yield). Deprotection was performed by heating the boc-pro-
tected amines in the presence of HCl (Scheme 1). The xylylenebis
(aminomethyltriazole) ligands were isolated as hydrochloride salts
xamtꢀ2HCl. ESI-MS analysis of these hydrochloride salts shows
peaks corresponding to the protonated free amine, xamtH+ (m/
z = 299.18), and its dimer (m/z = 597.33). Water solubility of these
hydrochloride salts varies in the order ortho ꢃ meta > para. Depro-
tonation with NaOH(aq), and extraction into organic solvents,
afforded the free bases o-, m-, and p-xamt.
3. Results and discussion
3.1. Synthesis of ligands
The tetradentate N-donating ligands used in the current studies
are depicted in Chart 1. The pyridyltriazole ligands, o-xpt, m-xpt
and p-xpt, were synthesized by the [3 + 2] cycloaddition reaction
of 2-ethynylpyridine with in situ generated diazides by the cop-
per-catalyzed azide-alkyne cycloaddition (CuAAC; ‘‘click” reaction)
[17]. We attempted to use this method to synthesize boc-protected
xylylenebis(aminomethyltriazoles), but it gave very poor yields.
Therefore, we first converted the xylylene dibromides to the corre-
4

U.R. Pokharel et al.
Polyhedron 191 (2020) 114805
3b was synthesized by first isolating the free amine, p-xamt. The
coordination of p-xamt was achieved by stirring with nickel(II)
nitrate (3:2 ratio) in methanol. The microanalyses of all products
were consistent with the formation of binuclear complexes
[Ni2L3]4+ and the results were further supported by ESI mass anal-
ysis. Unlike Ni2(xpt)3(BF4)4 complexes, the ESI-MS analysis of Ni2(-
xamt)3(PF6)4 complexes showed peaks corresponding to only
monocationic ([Ni2(xamt)3](PF6)+3; m/z 1445.25) and dicationic
([Ni2(xamt)3](PF6)22+; m/z 650.14) ions.
3.3. X-ray crystallographic studies
Scheme 1. Synthesis of xylylenebis(aminomethyltriazole) dihydrochloride salts
and free bases.
X-ray quality crystals were isolated for the complexes 1a, 3a, 1b
and 3b. In all cases, the crystals contained the expected nickel(II)
metallosupramolecular helicates [Ni2L3]4+. In all complexes the
two metal centers are crystallographically non-equivalent. The
complex 3a has also been crystallographically characterized previ-
ously as its BF4 salt by Petitjean and co-workers [30]. Although we
followed similar conditions for reaction and crystallization as the
3.2. Synthesis of the complexes
The Ni(II) complexes of o-, m- and p-xpt (1a–3a) were synthe-
sized by slow addition of ligands (3 equivalents) in chloroform to
stirred solutions of Ni(II) salts (2 equivalents) in methanol at room
temperature (Scheme 2a). The compounds were isolated in 70 –
80% yield as pink solids. [Ni2(o-xpt)3](BF4)4 (1a) and [Ni2(p-xpt)3]
(BF4)4 (3a) are readily soluble in DMF, whereas [Ni2(m-xpt)3]
(BF4)4 (2a) is less soluble. ESI-MS analysis of the isomeric salts
showed weak signals corresponding to [Ni2(xpt)3](BF4)+3 (m/z
1557.38), while the [Ni2(xpt)3](BF4)22+ (m/z 735.19), [Ni2(xpt)3]
(BF4)3+ (m/z 461.46), and [Ni2(xpt)3]4+ (m/z 324.59) signals were
fairly strong. The observed isotopic distributions were in good
agreement with calculated values. Additional characterization
was performed by microanalysis, and in the case of 1a and 3a, by
X-ray structure determination.
To modify the physical and coordination properties of the bis-
chelating ‘‘click” ligands, we used the aminomethyl group as a sub-
stitute for the pyridyl group. The Ni(II) complexes of o- and m-
xamt were obtained by the treatment of Ni(BF4)2ꢀ6H2O with the
ligand hydrochloride salts (3:2 ratio) in the presence of excess tri-
ethylamine in aqueous medium. To improve the solubility of the
complexes in organic solvents, the BF4 salts were metathesized
with ammonium hexafluorophosphate to give pink solids (1b and
2b) in 60-70% overall yield. Because of the poor solubility of p-
xamtꢀ2HCl in water as well as in organic solvents, the complex
Petitjean group, the previously reported complex crystallizes in
—
the R 3 c space group while our complex 3a crystallizes in the mon-
oclinic space group P21/c. The asymmetric unit of complex 3b
includes two crystallographically independent molecules. We were
unable to obtain crystals suitable for X-ray crystallography from
the meta ligands (complexes 2a and 2b) despite multiple attempts.
The crystal data and structure refinement parameters for the com-
pounds are presented in Table 1.
The cations in the structures are similar (Fig. 4) and confirm the
formation of the binuclear triple-helical architectures regardless of
change in coordination pockets of the ligands. In each structure,
the nickel(II) centers are bound to three pyridyltriazole (1a and
3a) or aminomethytriazole (1b and 3b) units from three different
ligand strands in the fac configuration attaining a pseudo-octahe-
dral coordination geometry. In the pyridyltriazole structures (1a
and 3a), the triazole rings are approximately coplanar with the
adjacent pyridyl units: the torsion angles between the two donor
N atoms and bridging C atoms of the pyridyltriazole units are in
the range of 3 – 6°. In contrast, the aminomethyltriazole ligands
are more flexible, leading to a wider range of analogous torsion
angles (2 – 33°).
Scheme 2. Synthesis of nickel(II) metallosupramolecular cages [Ni2(LL)3]X4. Reagents: (a) (i) Ni(BF4)2ꢀ6H2O; (b) (ii) Ni(BF4)2ꢀ6H2O followed by NH4PF6; or Ni(NO3)2ꢀ6H2O.
5

U.R. Pokharel et al.
Polyhedron 191 (2020) 114805
Fig. 4. ORTEP diagrams of the solid state structures of nickel(II) metallosupramolecular helices: 1a, [Ni2(o-xpt)3]4+ 3a, [Ni2(p-xpt)3]4+ 1b, [Ni2(o-xamt)3]4+
; ; ; 3b,
[Ni2(p-xamt)3]4+. Thermal ellipsoids are shown at the 50% probability level. Anions, hydrogen atoms, and solvent molecules have been removed for clarity.
The stereochemistry of the self-assembly process can be influ-
enced by the number of carbon atoms in the spacer groups Y
between the two ML3 moieties [45]. With an odd number of carbon
atoms in the spacer units, the complexes usually form as an achiral
mesocate (with opposite chirality, P,M, at the two metal atoms, and
idealized mirror symmetry. On the other hand, spacer units with
an even number of carbon atoms tend to produce helicates, as
racemic mixtures of the (P,P) and (M,M) enantiomers. Consistent
with the even number of carbon atoms in both o- and p-xylylene
groups, all four complexes (1a, 3a, 1b, and 3b) crystallize as race-
mic mixtures of the (P,P) and (M,M) helicates. Despite numerous
attempts, we were unable to produce suitable crystals of the com-
plexes from m-xylylene linkers (2a and 2b). However, the pub-
lished example of [Fe2(m-xpt)3]4+ crystallizes as a (P,M)-mesocate
[39].
efficient packing of the two ML3 moieties. As far as we are aware,
the present structures of [Ni2(o-xpt)3]4+, [Ni2(o-xamt)3]4+ and
[Ni2(p-xamt)3]4+ are the first of these xylylene-linked M2(LL)3 to
be reported.
The average Ni-N(pyridine) and Ni-N(triazole) distances, and N
(pyridine)-Ni-N(triazole) angles in the [Ni2(xpt)3]4+ structures are
2.106(14) and 2.054(14) Å, and 78.7(5)°, respectively. These are
very similar to those for analogous structures in the literature:
2.10(5) (5791 hits in 1340 Ni(py)n structures) and 2.07(3) Å (33
hits in 11 Ni(triazole)n structures), and 78.8(4)° (19 hits in 8 Ni
(pyridyltriazole)n structures) [46].
The folding of the ligands in these supramolecular structures
depends on the nature of the xylylene spacer involved. The com-
A summary of metal–metal distances and structure types for o-,
m-, and p-xylylene-linked pyridyltriazole, aminomethyltriazole,
and related complexes is presented in Table 2. The p-linked com-
plexes show longer metal–metal distances, as expected. The MꢀꢀꢀM
distances for the two m-linked complexes are longer still. This
order of MꢀꢀꢀM distances (o < p < m) is reproduced by simple molec-
ular modeling calculations (Hyperchem). Models of the helicates
suggest that, because the adjacent complexes have the same chi-
rality, the bridging groups can continue the same screw sense as
the ligating moieties. This leads to efficient packing and relatively
small MꢀꢀꢀM distances. In contrast, in the mesocates, the chirality
changes from M to P at the m-xylylene bridging group, so it is
approximately parallel to the MꢀꢀꢀM axis, leading to a larger sepa-
ration between the metal atoms. This is seen qualitatively in
Fig. 3: the change in chirality in the mesocate interferes with
Table 2
Structure types and metal–metal distances for [M2(xpt)3]4+ and [M2(xamt)3]4+ and
related species.
Compound
Type
MꢀꢀꢀM (Å)
Ref
[Ni2(o-xpt)3](BF4)4 (1a)
[Ni2(o-xamt)3](PF6)4 (1b)
[Fe2(m-xpt)3](BF4)4
[Fe2(2,6-pmpt)3](BF4)4
[Ni2(p-xpt)3](BF4)4
[Ni2(p-xpt)3](BF4)4 (3a)
[Fe2(p-xpt)3](BF4)4
[Ru2(p-xpt)3](BF4)4
Helicate
Helicate
Mesocate
Mesocate
Helicate
Helicate
Helicate
Helicate
Helicate
9.332(2)
8.9718(10)
12.182(3)
12.234(1)
11.4647(12)
11.574(2)
11.3912(11)
11.6368(9)
11.899(2), 11.924(2)
This work
This work
[39]
[39]
[30]
This work
[30]
[35]
a
[Ni2(p-xamt)3](NO3)4 (3b)
This work
a
This complex is identical to [Fe2(m-xpt)3](BF4)4, except that the central aro-
matic rings in the 2,6-pmpt ligands are 2,6-disubstituted pyridine rather than m-
disubstituted benzene.
6

U.R. Pokharel et al.
Polyhedron 191 (2020) 114805
Fig. 5. Packing diagram for [Ni2(o-xpt)3]4+ (1a), showing the
p-p interactions
between a phenylene ring of one complex and a pyridyl ring of an adjacent
complex. Anions, hydrogen atoms, and solvent molecules have been removed for
clarity.
pounds containing ortho-xylylene spacers (1a and 1b) exhibit a
similar folding of their ligands (Figs. 4 and 6). In both structures,
one of the phenylene rings twists almost perpendicular to the
NiꢀꢀꢀNi axis, while the other two rings are oriented along the axial
direction. In the crystal structure of 1a, adjacent molecules show
Fig. 6. Space-filling models of the molecular structures of the triple helicates.
Different colors are used to distinguish the three different ligands in each structure.
Anions, hydrogen atoms, and solvent molecules have been removed for clarity. 1a,
[Ni2(o-xpt)3]4+; 3a, [Ni2(p-xpt)3]4+; 1b, [Ni2(o-xamt)3]4+; 3b, [Ni2(p-xamt)3]4+
.
a pyridyl-phenylene
p
interaction (centroidꢀꢀꢀcentroid 4.38 Å)
(Fig. 5). In the crystal structure of 1b, the two phenylene groups
lying along the internuclear axis interact with a nearby triazole
ring with CHꢀꢀꢀ interactions (Hꢀꢀꢀcentroid distance ca. 2.7 Å). These
p
intra- and intermolecular non-covalent interactions may con-
tribute to the almost spherical shapes of the complexes (approxi-
mate overall diameter of 15 Å) (to H) with axial distance ranging
from ~ 13 to 18 Å (Fig. 6).
The triple helical complexes with para-xylylene spacers (3a and
3b) were found to be more cylindrical than their ortho counterparts
(Fig. 6). The overall length of the molecules with p-xylylene spacers
ranges from ca. 16 Å (3b) to 20 Å (3a) (HꢀꢀꢀꢀH) while the diameters
of the circumscribed cylinders in both structures are nearly 10 Å.
The orientation of the three p-xylylene spacers is orthogonal in
complex 3a while two of them are in almost parallel orientation
(with an inter-centroid distance of ca. 5.2 Å) in complex 3b. The
pyridyltriazole units between two adjacent helicates are associated
in a head-to-tail arrangement (the pyridyl group of one molecule
stacks over the triazole group of the other molecule) in 3a with
an average interplanar distance of ca. 4.2 Å. In addition, the hydro-
gen atom attached to C64 of pyridyl moiety of one helicate makes a
close approach (3.763 Å) to the triazole N11 of another helicate.
These intermolecular contacts are illustrated in Fig. 7. Petitjean
and co-workers have reported such interactions in the structure
Fig. 7. Packing diagram for [Ni2(p-xpt)3](BF4)4 (3a), showing intermolecular
interactions. Anions, hydrogen atoms, and solvent molecules have been removed
for clarity.
p-p
other hand, the pyridyl groups in 1a and 3a show CAHꢀꢀꢀN non-
bonded distances of 2.77
0.09 Å, approximately equal to the
sum of the H and N van der Waals radii (2.65–2.75 Å). These con-
tacts could lead to differences in N-Ni-N angles for the two ligands.
However, the average N(pyridyl)-Ni-N(pyridyl) angles in 1a and 3a
(95.7 2.3°) are not significantly larger than those for N(amino-
methyl)-Ni-N(aminomethyl) in 1b and 3b (94.6 2.2°). Similarly,
of [Fe2(p-xpt)3](BF4)4, but the distances are shorter (p – p stacking,
3.52 Å; HpyꢀꢀꢀN(triazole), 2.66 Å) [30] than in the current complex.
Because complexes 1b and 3b are the first aminomethyltria-
zole-based helicates to be reported, we were interested in any
structural differences between them and the pyridyltriazole com-
plexes (1a and 3a). The pyridyl groups (in 1a and 3a) and the ami-
nomethyl groups (in 1b and 3b) are fac-oriented, so they could
interfere sterically. In the aminomethyl complexes (1b and 3b),
the closest nonbonded contacts between the –CH2NH2 groups are
NHꢀꢀꢀHN, which average 2.72 0.17 Å. This is larger than twice
the H atom van der Waals radius (2.2–2.4 Å); thus, there are few
or no repulsive interactions among the –CH2NH2 groups. On the
Ni-N(triazole) distances (2.061
are shorter than either Ni-N(pyridyl) or Ni-N(aminomethyl), but
there is little difference between the latter two (2.108 0.016
0.016 Å in all four structures)
and 2.115 0.016 Å, respectively). This agrees with distances found
in 6 previous examples of [Ni(2-aminomethylpyridine)3]2+ struc-
tures: Ni-N(pyridyl), 2.099
0.019 Å; Ni-N(aminomethyl),
2.115 0.017 Å [46]. Therefore, despite their chemical differences,
the aminomethyl and pyridyl ligating groups provide similar coor-
dination environments in these helicates.
7

U.R. Pokharel et al.
Polyhedron 191 (2020) 114805
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In order to understand the roles of spacer groups and chelating
pockets in determining coordination behavior of ligands and
dimensions of the overall complexes, we studied the complexation
of Ni(II) with tetradentate N-donating ligands separated by xyly-
lene spacers by altering two parameters: 1) the structure of the
xylylene core (o-, m-, p-), and 2) the nature of the chelating groups.
Pyridyltriazoles have been used extensively as coordination pock-
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CRediT authorship contribution statement
Uttam R. Pokharel: Conceptualization, Methodology, Investiga-
tion, Visualization, Writing - original draft, Writing - review & edit-
ing. Jordan C. Theriot: Methodology, Investigation. Frank R.
Fronczek: Investigation, Formal analysis, Resources, Validation,
Writing - review & editing. Andrew W. Maverick: Funding acqui-
sition, Conceptualization, Visualization, Writing - review & editing.
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Synthesis, structure and characterization of novel nickel(II) and iron(II)
Declaration of Competing Interest
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The authors declare that they have no known competing finan-
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to influence the work reported in this paper.
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Acknowledgments
This research was supported in part by the Louisiana Board of
Regents Support Fund (through the Louisiana EPSCoR project,
NSF award number EPS-1003897) and Albemarle Corporation,
and by the LSU Philip W. West Professorship. J. C. T. is grateful
for partial support through the California Institute of Technology
SURF program. The sponsors had no role in the design, execution,
or publication of this work.
Appendix A. Supplementary data
CCDC 2010484, 2010486, 2010485, and 2010487 contain the
supplementary crystallographic data for structures 1a, 3a, 1b,
and 3b, respectively. These data can be obtained free of charge
via https://www.ccdc.cam.ac.uk/structures/ or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.poly.2020.114805.
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