Syntheses and crystal structures of two self-assembled dizinc(II) helicates with novel hydrazone linked polypyridyl ligands

Article abstract of DOI:10.1007/s11164-016-2426-x

The rationally designed polydentate ligands, L¹ and L², based on the pyridinyl moiety and the hydrazone fragment have been synthesized to coordinate zinc(II) ions. We utilize pyridine as a rigid core connecting two bipyridines as ligand building blocks with a hydrozone linker for the L¹. The L² has a reversed design in which a bipyridine was used as a hinging-available building block of the ligand core, connecting two pyridazine arms with a hydrazone linker. Two novel helical dizinc(II) complexes were obtained by the reaction of L¹ and L² with zinc(II) perchlorate in acetonitrile. The structures of both helicates were confirmed by X-ray diffractometry. Single-stranded helicate Zn₂L¹ contains two zinc ions bridged by an oxygen atom. Except for the L² ligand, no other bridging species were found between the two zinc ions in the double-stranded helicate Zn₂L₂ ². The self-assembling process of helicate Zn₂L¹ in solution state was studied by UV–Vis spectrometric titration experiments. The stepwise formation constants imply a slightly positive cooperative behavior for the formation of helicates.

Full text of DOI:10.1007/s11164-016-2426-x

Res Chem Intermed
DOI 10.1007/s11164-016-2426-x
Syntheses and crystal structures of two self-assembled
dizinc(II) helicates with novel hydrazone linked
polypyridyl ligands
Hsien-Chang Kao1 Yi-Chun Wang1
•
•
Wen-Jwu Wang1
Received: 18 December 2014 / Accepted: 18 February 2015
Ó Springer Science+Business Media Dordrecht 2016
Abstract The rationally designed polydentate ligands, L1 and L2, based on the
pyridinyl moiety and the hydrazone fragment have been synthesized to coordinate
zinc(II) ions. We utilize pyridine as a rigid core connecting two bipyridines as
ligand building blocks with a hydrozone linker for the L1. The L2 has a reversed
design in which a bipyridine was used as a hinging-available building block of the
ligand core, connecting two pyridazine arms with a hydrazone linker. Two novel
helical dizinc(II) complexes were obtained by the reaction of L1 and L2 with zin-
c(II) perchlorate in acetonitrile. The structures of both helicates were confirmed by
X-ray diffractometry. Single-stranded helicate Zn2L1 contains two zinc ions bridged
by an oxygen atom. Except for the L2 ligand, no other bridging species were found
between the two zinc ions in the double-stranded helicate Zn2L22. The self-assem-
bling process of helicate Zn2L1 in solution state was studied by UV–Vis spectro-
metric titration experiments. The stepwise formation constants imply a slightly
positive cooperative behavior for the formation of helicates.
Keywords Supramolecular chemistry Á Double-strand Á Helicates Á Dizinc
complex
Introduction
Self-assembly supramolecules have received considerable attention during the last
few decades [1–7] because of the important role played by supramolecules for
applications of catalysts, medicines, sensors, devices, and functional materials [8–
11]. The design and synthesis of inorganic helical complexes involving non-
covalent interactions, such as metal–ligand coordination bonding, hydrogen
&
Wen-Jwu Wang
wjw@mail.tku.edu.tw
1
Department of Chemistry, Tamkang University, Tamsui, New Taipei City 25137, Taiwan
123

H.-C. Kao et al.
bonding, charge transfer interaction, hydrophobic interaction, and metal–metal
interaction has attracted extensive interest. The stereochemistry of metallo-
supramolecules can be built up through rational design of characteristic ligands
as well as metal ions with preferred stereochemistry [12–15]. Based on the choice of
the coordination sphere with variation of the donor ligand, various single- and
double-stranded helical architectures can be feasible.
The selection of suitable organic ligands is crucial for constructing extended
metal complexes. N-heterocyclic ligands have been frequently employed due to
their ability to engage in diverse coordinations. Two N-heterocycles linked by
suitable spacers have been proven to lead to a great structural diversity among
complexes [16–19]. Various spacers have been used to connect polypyridinyl and
other components, which is the key factor to control the resulting structure of the
complex [20, 21]. Furthermore, the preparation and modification of those ligands in
high yield is remarkable. In spite of the dipyridinyl ligand and its analogue being
studied by a number of researchers, studies of the hydrazone group as linkers are
relatively rare. The hydrazone group has a twistable N–N single bond that is
employed to change between a cisnoid and a transnoid form. The polypyridinyl–
hydrazone compounds are versatile N-donor ligands since the flexible nature of the
ligands frequently fitted diverse requirements.
From the other point of view, transition metal ions play a key role in the
formation of coordination compounds with a suitable coordination number and
geometry [22, 23]. Zn(II) ions can assume multiple coordination geometries ranging
from regular or distorted tetrahedral to trigonal bipyramidal, square pyramidal, and
octahedral geometries of different coordination numbers. Meanwhile, the kinetic
lability of coordinated ligands is also responsible for their wide utility within
biological and photochemical systems [24–27].
Following these criteria, novel a multi-dentate ligand Py-Me-(Bpy)2 (L1) and a
Bpy-(Me-Pz)2 (L2) were designed and synthesized. L1 has a central pyridine core
and two bipyridyl arms. L2, on the other hand, has a bipyridyl core and two pyrazine
arms. The core and the arms are connected by a hydrozone linker on each.
Previously, we have shown that the ligand L2 formed a double helix structure with
an octahedral geometry to metal ions such as Ni(II) and Cu(II), through a self-
assembly process. We now turn our focus to Zn(II), which is one of the most
interesting transition metals due to its variable coordination ability. We report
herein the synthesis, crystal structure, and spectroscopic characterization of the self-
assembled zinc helicate complexes.
Experimental
All commercially available chemicals were of reagent grade and used without
further purification, unless otherwise stated. Diethyl ether was distilled from sodium
benzophenone ketyl immediately prior to use. Dichloromethane was distilled over
calcium hydride. All solvents were of HPLC grade. The electrospray ionization
mass spectral (ESI–MS) measurements were carried out on a LCQ (Finnegan MAT,
USA) using methanol as the mobile phase. The samples for ESI–MS measurements
were prepared by redissolving the final isolated compounds in methanol. 1H NMR
123

Syntheses and crystal structures of two self-assembled…
spectra were measured in CDCl3 with a Bruker AC300 300 MHz spectrometer at
room temperature. Spectrophotometric titrations were performed with a Shimadzu
UV-1601 UV–visible spectrophotometer connected to an external computer.
Synthesis of the ligand and complex
1-Methyl-2,20-bipyridinium Iodide, (MBPI)
A 14 mL methyl iodide solution was added to a 35 mL nitrobenzene solution of
2,20-bipyridine (127.99 mmol, 19.99 g). The mixture was placed at room temper-
ature for 2 weeks and the yellow crystals were generated, which were then collected
by filtration and air-dried. The yellow crystals were recrystallized by n-hexane to
give 30 g (120.3 mmol) of the product.
1-Methyl-2,20-bipyridine-6-one, (MBPO)
A 10 mL aqua solution of MBPI (4.01 g, 13.00 mmol) and a 9 mL aqua solution of
sodium hydroxide (5.01 g, 125.25 mmol) were added dropwise into 26 mL
potassium hexacyanoferrate(III) (10.49 g, 30.99 mmol) in an ice bath. The deep
brown mixture was stirred for 1 h, then extracted by dichloromethane. The product
was treated using a column of silica gel. The yellow solution was concentrated and
dried to get the brown product (0.97 g, 5.2 mmol).
6-Chloro-2,20-bipyridine, (CBP)
A 9.8 mL phosphoryl chloride solution was added into a mixture of MBPO (0.99 g,
5.32 mmol) and phosphorus pentachloride (0.93 g, 4.46 mmol). The brown mixture
was refluxed for 1 day and became a deep brown color. The unreacted phosphoryl
chloride was removed and then put into an ice bath. Then 100 mL of ice water was
added and the precipitate was filtered out. The solution was slowly added into
ammonium hydroxide until the pH reached 7. The white precipitate was generated
and then collected and recrystallized by dichloromethane in a 76 % yield.
1H NMR (300 MHz, CDCl3) d (ppm) 8.66 (d, 1H, J = 2.3 Hz, Cl-Bipy-H), 8.39
(d, 1H, J = 4.0 Hz, Cl-Bipy-H), 8.34 (d, 1H, J = 3.8 Hz, Bipy-H), 7.82 (t, 1H,
J = 3.8 Hz, Bipy-H), 7.77 (t, 1H, J = 3.8 Hz, Bipy-H), 7.35 (d, 1H, J = 3.8 Hz,
Bipy-H), 7.31 (t, 1H, J = 7.7 Hz, 1H, Cl-Bipy-H).
6-Hydrazine-2,20-bipyridine, (HzBpy)
Hydrazine monohydrate (20 mL) and a few drops of methanol were added to CPB
(2.01 g, 10.52 mmol), then set to reflux for 1 day under dry nitrogen with vigorous
stirring. An additional 15 mL of hydrazine monohydrate was added after the
solution became clear. The solution was then refluxed for one more day, after which
the mixture was then extracted and recrystallized by dichloromethane to give 1.72 g
123

H.-C. Kao et al.
(88 % yield) of the product. Anal. Calcd for C10H10N4: C, 64.50; H, 5.41; N, 30.09
Found. C 64.48, H 5.46, N 30.11.
1H NMR (300 MHz, CDCl3) d (ppm) 8.66 (d, 1H, J = 2.3 Hz, Bipy-H), 8.29 (d,
1H, J = 3.0 Hz, Bipy-H), 7.80 (d, 1H, J = 2.2 Hz, Bipy-H), 7.75 (t, 2H,
J = 3.3 Hz, Bipy-H), 7.62 (t, 1H, J = 3.8 Hz, NH-NH2-Bipy-H), 7.26 (t, 1H,
J = 3.8 Hz, NH-NH2-Bipy-H), 6.75 (d, 1H, J = 4.0 Hz, NH-NH2-Bipy-H), 3.68 (s,
1H, Bipy-NH).
2,6-Diacetylpyridine bis(2,20-bipyridyl-6-hydrazone), (Py-Me-(Bpy)2, L1)
A 30 mL methanol solution of HzBpy (1.80 g, 9.67 mmol) was added to a 15 mL
methanol solution of 2,6-diacetyl–pyridine (0.80 g, 4.9 mmol). The solution was set
to reflux for 1 day, and the light yellow precipitate was generated. The precipitate
was collected by filtration and washed with methanol and diethyl ether, then dried
under vacuum to give 2.23 g (4.41 mmol, 90 % yield) of the product. Anal. Calcd
for C29H25N9: C 69.72, H 5.04, N 25.23. Found. C 69.68, H 5.11, N 25.27.
1H NMR (300 MHz, CDCl3) d (ppm) 8.70 (d, 1H, J = 2.1 Hz, Bipy-H), 8.28 (d,
1H, J = 3.9 Hz, Py-H), 8.10 (d, 1H, J = 3.9 Hz, C=N–NH-Bipy-H), 7.84 (m, 2.5
H, Py-H, Bipy-H, C=N–NH-Bipy-H), 7.72 (t, 1H, J = 3.9 Hz, Bipy-H), 7.52 (d, 1H,
J = 3.6 Hz, C=N–NH-Bipy-H), 7.28 (dd, 1H, J = 4.5, 3.2 Hz, Bipy-H), 2.54 (s,
3H, CH3).
[Zn2(Py-Me-(Bpy)2)(OH)(CH3CN)(H2O)](ClO4)3, (Zn2L1)
A 10 mL acetonitrile solution of Zn(ClO4)2Á6 H2O (14.89 mg, 0.04 mmol) was
added slowly into a 10 mL acetonitrile solution of L1 (10 mg, 0.02 mmol). The
solution immediately turned light yellow. Toluene was diffused into the mixture and
the yellow crystals were collected by filtration to give a product in 55 % yield. Anal.
Calcd for C44Cl4H40N20O16Zn2: C 38.36, H 2.93, N 20.33. Found. C 38.19, H 3.06,
N 20.32.
2,20-Bipyridyl-6,60-bis(2-acetylpyrazinohydrazone), (Bpy-(Me-Pz)2, L2)
The ligand L2 was synthesized following the procedure described in our previous
paper [28, 29].
[Zn2(Bpy-(Me-Pz)2)](ClO4)4, (Zn2L22)
A 15 mL acetonitrile solution of Zn(ClO4)2Á6 H2O (18.61 mg, 0.05 mmol) was
added slowly into a 15 mL acetonitrile solution of L2 (19.89 mg, 0.05 mmol). The
solution turned light yellow immediately. Toluene was diffused into the mixture,
and the yellow crystals were collected by filtration to give a product in 55 % yield.
123

Syntheses and crystal structures of two self-assembled…
Anal. Calcd for C44Cl4H40N20O16Zn2: C 38.36, H 2.93, N 20.33. Found. C 38.19, H
3.06, N 20.32.
Crystal structure determination of complex Zn2L1, Zn2L22
Data were collected at 120 K on a Nonius KappaCCD area detector situated at the
˚
window of a rotating anode (kMo-ka = 0.71073 A). The structure was solved by
direct methods, SHELXS-97, and refined using SHELXL-97 [30]. Hydrogen atoms
NaOH
MeI/PhNO2
K3[Fe(CN6)]
N
N
N
N
N
N
O
BP
MBPI
MBPO
POCl3/PCl5
N
O
O
N2H4/
N
N
N
N
N
N
NH
N
HN
NHNH2
Cl
HzBpy
CBP
N
N
N
N
Py-Me-(Bpy)2, L1
O
N
acetic acid 100%
/H2O2
POCl3/PCl5
N
N
N
N
N
O
Cl
Cl
BP
DCBP
DBPO
N2H4/
N
O
N
N
N
HN
NH
N
N
N
N
N
N
N
H2NHN
NHNH2
N
Bpy-(Me-Pz)2, L2
Scheme 1 Synthesis of ligands L1 and L2
DHzBpy
123

H.-C. Kao et al.
were included in the refinement, but thermal parameters and geometry were
constrained to ride on the atom to which they were bonded. The data were corrected
for absorption effects using SADABS [31]. The perchlorate anions exhibited
crystallographic and dynamic disorder in both Zn2L1 and Zn2L22. Zn2L1:
C33H38Cl3N11O16Zn2, Monoclinic, P21/n, a = 17.3644(4), b = 10.3116(2),
a = 90°,
˚
c = 24.4661(8) A,
b = 92.7430(10)°,
c = 90°,
volume =
3
3
˚
4375.75(19) A , Z = 4, Dc = 1.642 Mg/m , hmax = 25.03°, 26,971 measured,
7644 unique (Rint = 0.0844) and 7644 (I [ 2r(I)) reflections, R1 (obs.) = 0.0610
-3
Zn2L22:
C48H48Cl4N22O17Zn2, Monoclinic, P21/n, a = 12.0256(13), b = 21.274(2),
˚
and wR2 (all data) = 0.2023,
q
max/qmin = 1.122/-0.874 eA
.
3
˚
˚
c = 24.059(3) A, a = 90°, b = 103.280(2)°, c = 90°, volume = 5990.7(11) A ,
Z = 4, Dc = 1.636 Mg/m3, hmax = 26.03°, 33,578 measured, 11,766 unique
(Rint = 0.0457) and 11,766 (I [ 2r(I)) reflections, R1 (obs.) = 0.0767 and wR2
-3
˚
(all data) = 0.1086, qmax/qmin = 0.465/-0.508 eA
.
Determination of formation constant
In a typical experiment, 100 mL of ligand L1 in acetonitrile/dichloromethane (1:1,
v/v) solution (0.101 mM) was titrated with a concentration solution (ca. 4
equivalent) of the zinc(II) perchlorate hexahydrate in CH3CN. Each addition of
the salt solution (25 mL increments at 5 min intervals), was carried out by using a
precision micropipette at 25 °C. The absorbance was recorded using a 1 cm quartz
cell and then transferred to the computer. The spectral data were analyzed with the
9410190-1 #1-209 RT: 0.00-5.00 AV: 209 NL: 2.41E7
T: + c ESI Full ms [ 150.00-2000.00]
562.5
100
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
500.5
564.4
566.3
501.6
644.2
642.2
567.3
568.3
377.4
378.4
396.4
398.5
743.9
1127.1
648.1
663.9
666.9
1227.0
1228.0
741.9
502.6
545.5
1123.1
747.9
748.9
1128.1
315.5
1223.0
1162.7
351.4
350
614.1
600
1060.9
1050
1130.1
1231.0
845.7
850
740.1
499.7
857.6
825.9
800
457.0
450
892.8 945.9
900 950
995.8
1000
1122.4
1100
1264.4
0
300
400
500
550
650
700
750
1150
1200
1250
m/z
Fig. 1 ESI–MS spectrum of Zn2L1 (0.2 mM) in methanol solution
123

Syntheses and crystal structures of two self-assembled…
Specfit program [32–35], by which the absorption and the stability constants of each
species forming in equilibrium processes were obtained. The Specfit program uses
factor analysis (which confirms the number of different absorbing species) to reduce
the absorbance matrix and to extract the eigenvalue prior to the multiwavelength fit
of the reduced data set according to the Marquard algorithm [36].
Results and discussion
Synthesis
The synthesis of the precursor HzBpy is the key step for approving the ligand,
according to Scheme 1. The ligand L1, pyridinyl as the central core and two
bipyridinyls as the arms, were synthesized by the condensation of HzBpy and 2,6-
diacetyl-pyridine. The ligand L2 was synthesized by a modified literature method.
The stepwise addition of hydrazine to the reaction vessel is the key step for
approving high yield to prepare the ligand L2. The free ligands L1 and L2 were then
added into one equivalent Zn(ClO4)2Á6H2O in CH3CN leading to the dinuclear
single-stranded helical complex Zn2L1 and dinuclear double-stranded helical
complex Zn2L22. The complex Zn2L1 was characterized by ESI-Mass spectroscopy
(Fig. 1), and the structure of Zn2L1 and Zn2L22 were characterized by X-ray
crystallography.
Table 1 Selected bond lengths
1
Zn2L1
˚
(A) and angles[°] for Zn2L and
Zn2L22
Bond length
Zn(1)–O(1)
Zn(1)–N(10)
Zn(1)–N(2)
Zn(1)–N(1)
Zn(1)–N(4)
Zn(2)–N(5)
Bond angle
1.909(4)
2.031(6)
2.040(5)
2.163(5)
2.273(5)
2.335(5)
Zn(2)–O(1)
Zn(2)–O(2)
Zn(2)–N(8)
Zn(2)–N(6)
Zn(2)–N(9)
1.984(4)
2.065(4)
2.102(5)
2.185(5)
2.285(5)
Zn(1)–O(1)–Zn(2) 126.5(2)
O(1)–Zn(2)–N(5) 87.39(16)
N(5)–Zn(2)–N(9)
N(6)–Zn(2)–N(8)
N(5)–Zn(2)–N(6)
Zn2L22
141.15(18) O(1)–Zn(2)–N(8) 102.66(17)
74.1(2)
O(2)–Zn(2)–N(5) 78.23(17)
O(2)–Zn(2)–N(8) 106.80(19)
71.12(18)
Bond length
Zn(1)–N(18)
Zn(1)–N(5)
2.118(3)
2.168(2)
2.239(3)
2.115(3)
2.160(2)
2.213(2)
Zn(1)–N(3)
Zn(1)–N(16)
Zn(1)–N(1)
Zn(2)–N(8)
Zn(2)–N(6)
Zn(2)–N(11)
2.133(2)
2.180(3)
2.264(2)
2.129(3)
2.177(2)
2.246(3)
Zn(1)–N(20)
Zn(2)–N(13)
Zn(2)–N(15)
Zn(2)–N(10)
123

H.-C. Kao et al.
The ESI–MS spectrum of ZnL1 shows peaks at m/z = 1227, 1127, 743, 644, 562,
and 500, which may be attributed to species [Zn2L12 ? ClO4]?, [Zn2L21]?,
[Zn2L1 ? ClO4 ? OH]?, [Zn2L1 ? OH]?, [ZnL1]? and [L1 ? H]?. The peak at
Fig. 2 The structure of the
double helical complex cation of
Zn2L1, a ORTEP diagram,
b representation of the helical
structure, c representation of the
intermolecular p–p interaction,
-
ClO4 ions, solvent and
hydrogen atoms are omitted for
clarity
123

Syntheses and crystal structures of two self-assembled…
m/z = 1227 has been double checked to confirm a singly charged species, which
indicates the presence of dinuclear complexes [Zn2L12 ? ClO4]? in gaseous phase.
The peak at m/z = 1127, was assigned to [Zn2L12]? resulting from loss of an anion
Fig. 3 The structure of the
double helical complex cation of
Zn2L22, a ORTEP diagram,
b representation of the double
helical structure,
c representation of the
intermolecular and
intramolecular p–p interaction,
the ClO4 ions, solvent, and
-
hydrogen atoms are omitted for
clarity
123

H.-C. Kao et al.
Fig. 4 a The changes in UV/Vis spectra of 10-4 M L1 CH CN solution observed upon addition of
c
3
Zn(ClO4)2 to 10 equiv mole ratio. b Calculated spectra, c species distribution for complexation of Zn(II)
with L1 by Specfit
and deprotonation of [Zn2L12 ? ClO4]? species. The ms/ms analysis shows that the
signal of [ZnL1]? was derived from the decomposition of [Zn2L1]?. The single
positive charge species results from the reduction of the charges by deprotonation of
the NH groups. The simulated isotopic pattern is in good agreement with the
measured spectrum. These results indicate the presence of dinuclear complexes. The
intermediate species observed in the mass spectrum are consistent with those
obtained in UV–Vis spectrometric titration modeling by the Specfit program.
X-ray structure
Zn2L1
Straw yellow crystals of Zn2L1 were obtained by diffusion of toluene into a solution
of the complex in CH3CN. Selected bond lengths and angles are listed in Table 1.
Crystallographic analysis reveals that a single-stranded helix structure is constructed
by a twist ligand L1 surrounding the metal axis (Fig. 2). In addition, two zinc ions
are bound to a bridge oxygen atom as a Zn–O–Zn structure. The helical structure is
separated into two planes in a hinge-type, Zn(1) and one bipyridine in one plane,
and Zn(2), pyridine, and another bipyridine in the second plane. The dihedral angle
of these two planes is 74.18°. The Zn(1) ion bonds to two nitrogen atoms of
bipyridine, acetonitrile, and oxygen in a trigonal bipyramidal coordination
geometry. The Zn(2) ion occupies a distorted octahedral coordination geometry
with two oxygen atoms in trans position and the four nitrogen atoms (Nbipyridine
Nhydrazone, and Npyridine) in a plane. The distance between Zn(1) and Zn(2) was
,
˚
3.4 A, the angle of Zn(1)–O(1)–Zn(2) was 126.5°.
The distance between Zn(1) to the nitrogen atoms was around 1.909(4)–
˚
2.273(5) A while Zn(2) to the nitrogen atoms was around 1.984(4)–2.335(5) A. A
˚
˚
distance of 3.7 A was found, which implies that the intermolecular p–p stacking
interaction between two bipyridine rings is from different helical units.
Zn2L22
Straw yellow crystals of Zn2L22 were obtained by diffusion of toluene into a solution
of the complex in CH3CN. Selected bond lengths and angles are listed in Table 1.
Crystallographic analysis revealed a double-stranded helix structure, which was
formed with two zinc centers and two ligands (Fig. 3). The ligand wraps along the
helical axis, which is formed by zinc ions. Each ligand provides three coordination
sites for zinc in a twisting octahedral geometry by one pyridine of bipyridine unit,
one hydrazone unit, and one pyrazine unit. Each pyridine of bipyridine unit is
coordinated to a different zinc ion interlocking into a double helix structure. The
dihedral angle between two pyridine rings of bipyridine is around 78°–85°. The Zn–
˚
Zn internal distance is 4.7 A and implied no internal contact between the zinc(II)
123

Syntheses and crystal structures of two self-assembled…
123

H.-C. Kao et al.
ions. The distances between pyridine nitrogen atoms and zinc ions are around
˚
2.160(2) A (Zn(2)–N(15)) and 2.180(3) A (Zn(1)–N(16)). The distances between
˚
˚
the hydrazone nitrogen atoms and zinc ions are around 2.115(3) A (Zn(2)–N(13))
˚
and 2.133(2) A (Zn(1)–N(3)). The distances between pyrazine nitrogen atoms and
˚
˚
copper ions are around 2.213(2) A (Zn(2)–N(10)) and 2.264(2) A (Zn(1)–N(1)).
The Npy–Zn–Nhyz angles are around 146.02(9)° and 148.33(1)° when Npy and Nhyz
are from the same strand, which is smaller when the Npy and Nhyz belong to
different strands (around 86.12(9)° and 99.87(9)°). The distance between pyridine
˚
rings from one to the other ligand of about 3.5 A suggests that there is
intramolecular p–p stacking interaction between the two ligand strands. Similarly,
˚
a distance of about 3.7 A between the pyrazine rings from different helical
structures implied a weak intermolecular p–p stacking interaction between helical
units (Fig. 3c) The height of the cylinder defined by the helical strand in the
˚
complex is about 11 A, which was smaller than the dinickel(II) and dicopper(II)
complexes in our previous report. No intermolecular interactions between
perochlorate ions and helix strands were found, implying that the stacking
interaction plays a key role in maintaining the helical conformation.
Spectrometric titration studies
The self-assembling process of L1 with Zn(ClO4)2 in CH3CN was studied by UV–
visible spectrometric titration (Fig. 4). The L1 absorption maxima occur at 245 and
338 nm (p–p*). Upon addition of Zn2? to L1 to 0.5:1 ratio, the 338 nm peak
decreases, and a new band appears at 385 nm, which can be attributed to
coordination effects in the hydrazone moiety of L1. An isosbestic point at 363 nm
was observed. Subsequent titration to 1:1 ratio of the two new additional isosbestic
points at 231 and 271 nm were observed. These results revealed that L1 chelates to
Zn2? to form a stable [ZnL1]2? species. But the succeeding equilibrium formation
of the double helical [Zn2L12]4? species cannot be ruled out. Then, we propose a
formation processes including [Zn2L12]4? for an analysis of the stepwise charac-
teristic in the formation of [Zn2L1]2?. The possibility for the formation of [M3L12]6?
can be eliminated due to all coordination sites of the ligand being occupied by metal
ions in its precursor, as found in solid state structure.
The SPECFIT/32 Global Analysis System, which performs a nonlinear best fit of
the spectrometric data, was employed to obtain the thermodynamic parameters of
the helicate formation processes. Figure 4 is the simulated UV–Vis spectra and
partition curves of constitutions. The stability constants bml for each MmLl species in
the proposed model are shown in Table 2. The calculated results highly conform to
experimental data and satisfy the proposed model.
The calculated bml values and the overall stability constant of the complex Zn2L1
are consistent with the similar process in analogous zinc complexes [37–39]. The
presence of equilibrium intermediates fit well with those observed in the ESI–MS
spectra in which no species containing more than two metal ions were found. The
stepwise formation constants imply a slightly positive cooperative behavior for the
formation of helicate.
123

Syntheses and crystal structures of two self-assembled…
Table 2 Stepwise formation
Stepwise formation equation
Formation constant
equation and calculated
formation constant
Zn2? ? L1 ꢀ [ZnL1]2?
log b11 = 4.99
[ZnL1]2? ? Zn2? ꢀ [Zn2L1]4?
[Zn2L1]2? ? L1 ꢀ [Zn2L12]4?
log b12 = 10.04 0.41
log b22 = 14.64 0.60
Conclusion
The aim of this study was to obtain the self-assembled helicates between zinc(II)
and polydentate ligands. We synthesized two ligands L1 and L2 based on the
pyridinyl and hydrazone moieties, and which were designed with an opposite
strategy. L1 has a relatively rigid structure design with the pyridine as a ligand core.
L2 has a hinging-available design with the application of a bipyridine as a ligand
core. The self-assembled dizinc(II) complexes were obtained by reaction of L1 and
L2 with zinc(II) perchlorate in acetonitrile. The solid-state structure of single-
stranded helicate Zn2L1 and double-stranded helicate Zn2L22 were confirmed by
X-ray diffractometry. The mass spectrum and the spectrometric titration results of
L1 implied the double-stranded helicate could be found at the solution state. The
spectrometric titration results and the estimate of formation constants for the
complexes also indicated that the formation of the helicate is driven by positive
cooperativity.
Supporting information
Supplementary data of selected 1H and 13C NMR data associated with this article
are available; the crystallographic data in CIF format have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publication numbers
CCDC reference number CCDC 1037076 for Zn2L1 and 1032478 for Zn2L22.
Copies of the data can be obtained, free of charge, on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK [fax: ?44(0) 1223 336033 or email:
deposit@ccdc.cam.ac.uk].
Acknowledgments This work was financially supported by Tamkang University and the Ministry of
Science and Technology (NSC 97-2113-M-032-006). The authors thank Mr. Pierre LaPorte for reading
the article.
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