Supramolecular rectangles and ladders constructed from Ni(II), Cu(II) and Zn(II) mononuclear complexes with bicompartmental ligands and 4-aminopyridine as tectons

Article abstract of DOI:10.1016/j.ica.2017.06.058

By reacting [M^II(valpn)] or [M^II(valdmpn)] mononuclear complexes with 4-aminopyridine in different molar ratios, five [M^II(valXpn)(4-NH₂-py)] and one [Ni^II(valdmpn)(4-NH₂-py)₂] complexes were obtained and structurally characterized (M^II = Ni^II, Cu^II, Zn^II; valpnH₂ and valdmpnH₂ are the Schiff bases resulted from the condensation of o-vanillin with 1,3-propylenediamine and 2,2-dimethyl-1,3-propylenediamine respectively). In the [Cu(valpn)(4-NH₂-py)], [Ni(valpn)(4-NH₂-py)], [Zn(valpn)(4-NH₂-py)] and [Ni(valdmpn)(4-NH₂-py)] complexes the metal ions are pentacoordinated with a square pyramidal geometry. The specific hydrogen bond interactions of the amino group belonging to the 4-aminopyridine with the free O₂O′₂ compartment of the Schiff base ligands led to various supramolecular architectures: rectangles, chains, and ladders. The spectroscopic properties of the compounds were also investigated.

Full text of DOI:10.1016/j.ica.2017.06.058

ꢀꢁꢁꢂꢃꢄꢂꢅꢆ ꢇꢈꢉꢊꢁꢋꢌꢃꢄ

Supramolecular rectangles and ladders constructed from Ni(II), Cu(II) and
Zn(II) mononuclear complexes with bicompartmental ligands and 4-
aminopyridine as tectons
Augustin M. Madalana,*, Cristian D. Eneb
aInorganic Chemistry Department, Faculty of Chemistry, University of Bucharest, Str.
Dumbrava
Rosie
23,
020464-Bucharest,
Romania,
e-mail:
augustin.madalan@chimie.unibuc.ro
b”Ilie Murgulescu” Institute of Physical Chemistry of the Romanian Academy, Coordination
and Supramolecular Chemistry Laboratory, Splaiul Independenței 202, 060021-Bucharest,
Romania
Dedicated to Professor Ionel Haiduc on the occasion of his 80th anniversary
Abstract
By reacting [MII(valpn)] or [MII(valdmpn)] mononuclear complexes with 4-aminopyridine in
different molar ratios, five [MII(valXpn)(4-NH2-py)] and one [NiII(valdmpn)(4-NH2-py)2]
complexes were obtained and structurally characterized (MII = NiII, CuII, ZnII; valpnH2 and
valdmpnH2 are the Schiff bases resulted from the condensation of o-vanillin with 1,3-
propylenediamine and 2,2-dimethyl-1,3-propylenediamine respectively). In the [Cu(valpn)(4-
NH2-py)], [Ni(valpn)(4-NH2-py)], [Zn(valpn)(4-NH2-py)] and [Ni(valdmpn)(4-NH2-py)]
complexes the metal ions are pentacoordianated with a square pyramidal geometry. The
specific hydrogen bond interactions of the amino group belonging to the 4-aminopyridine
with the free O2O’2 compartment of the Schiff base ligands led to various supramolecular
architectures: rectangles, chains, and ladders. The spectroscopic properties of the compounds
were also investigated.
Keywords: hydrogen bonding, supramolecular rectangles, supramolecular ladders,
bicompartmental ligands, 4-aminopyridine, photoluminescence
Introduction
1

In metallosupramolecular chemistry, the construction of molecular objects originates
from the processing of the structural information stored within the ligands by the metal ions
which operate through their coordination algorithm [1]. Depending on morphology of the
ligands and stereochemical preferences of the metal ions, the self-assembly processes
involving these species can lead to various structural motifs such as grids [2], helicates [3],
cubes [4], prisms or octahedrons [5]. From the specific interactions of the metal ions with the
ligands result interesting redox [6], optical [7], and magnetic properties [8].
Schiff-bases are easily accessible ligands which offer the possibility of controlling the
nuclearity of their complexes by accommodating each metal ion into a well-defined
coordination environment (compartment). Two types of bicompartmental ligands will be
emphasised here: one derived from 2,6-diformyl-p-cresol and the other from o-vanillin. Thus,
the macrocyclic and end-off ligands resulted from the condensation of 2,6-diformyl-p-cresol
with various diamine (Scheme 1a) can generate binuclear complexes with divalent
transitional metal ions (NiII, CuII, ZnII). The preference of the CuII and ZnII metal ions for
coordination number 5 was exploited for synthesizing molecular rectangles by connecting the
binuclear nodes with exo-bidentate ligands: 4,4’-bipyridine, bis(4-pyridyl)ethane, bis(4-
pyridyl)ethylene or isonicotinato anion [9].
Schiff-bases derived from o-vanillin and diamines usually behave as bicompartmental
ligands, allowing the synthesis of heterobinuclear 3d-4f complexes [10] or 3d-3d’ complexes
[10c, 11]. In the 3d-4f complexes, the 3d metal ion is hosted into the N2O2 compartment,
while the oxophilic lanthanide cation occupies the larger, open O2O’2 compartment. In the
binuclear complexes, the phenoxo oxygen atoms act as bridges between the two metal ions.
In mononuclear complexes with the metal ion hosted into the N2O2 compartment, the larger
vacant O2O’2 compartment can act as a receptor not only toward other metal ions, but also
toward hydrogen bond donors (e.g. the aqua ligand arising from another complex, identical
[12] or different [13] – Scheme 1b). Ammonium cation [14] or bis-protonated diamines [15]
can be also used as guest, directing the crystalline organisation of such mononuclear
complexes by co-crystallisation.
In this paper, we report on the syntheses, structural characterization, and optical
properties of new supramolecular architectures based on [MII(valpn)] or [MII(valdmpn)]
mononuclear complexes and 4-aminopyridine as tectons (where MII = NiII, CuII, ZnII; and
valpnH2 and valdmpnH2 are the Schiff bases resulted from the condensation of o-vanillin
with 1,3-propylenediamine and 2,2-dimethyl-1,3-propylenediamine respectively, in a 2:1
molar ratio).
2

Scheme 1 (a) Formation of molecular rectangles by connecting symmetrical binuclear CuII or ZnII complexes
with exo-bidentate ligands; (b) supramolecular recognition by hydrogen interactions of coordinated water
molecules (donor) into the external O2O’2 free compartment of [MII(valXn)] mononuclear complexes (acceptor)
2. Experimental part
2.1. Syntheses
The [Cu(valpn)], [Ni(valpn)], [Zn(valpn)] and [Ni(valdmpn)] precursors were synthesized
following the same general procedure: 5 mmol of Cu(NO3)2·3H2O, Zn(NO3)2·6H2O or
Ni(NO3)2·6H2O, were added to an ethanolic solution (20 mL) of 3-methoxysalycilaldeyde (10
mmol), 1,3-propylenediamine or 2,2-dimethyl-1,3-propylenediamine respectively (5 mmol),
and triethylamine (20-30 mmol). The mixtures were boiled for 15-30 min. The precipitates
resulted by cooling were filtered off and washed with ethanol and diethyl ether.
2.1.1. [Cu(valpn)(4-NH2-py)]·0.33CH3CN – 1
0.1 mmol of [Cu(valpn)] and 0.15 mmol of 4-aminopyridine were dissolved in 30 ml of
acetonitrile. The slow evaporation at room temperature of the resulting mixture affords dark
green single crystals after 4-5 days. Yield: 75%. As a general rule, we collect crystals prior
total evaporation of the solvents in order to avoid contamination with the excess of 4-
aminopyridine. All the crystals were washed with a 1:1 mixture of ethanol-diethyl ether.
3

Selected IR data (KBr pellet, cm-1): 3437(m), 3237(w-br), 3095(w-br), 2998(w), 2930(w),
2841(w), 1629(vs), 1603(s), 1552(w), 1510(w), 1476(s), 1453(s), 1331(m), 1227(vs),
1079(m), 991(w), 823(w), 739(s), 631(w).
2.1.2. [Ni(valpn)(4-NH2-py)]·0.33CH3CN – 2
0.1 mmol of [Ni(valpn)] and 0.15 mmol of 4-aminopyridine were dissolved in 40 ml of
acetonitrile-chloroform (1:1) mixture. The slow evaporation at room temperature of the
resulting mixture affords dark green single crystals. Yield: 65%. Selected IR data (KBr pellet,
cm-1): 3362(m-w), 3228(m), 3052(w), 2921(w), 2851(w), 1662(m), 1619(vs), 1541(m-w),
1521(w), 1474(s), 1449(s), 1406(m-w), 1335(m), 1241(m), 1222(vs), 1075(m-w), 973(w),
821(w), 734(m), 626(w).
2.1.3. [Ni(valpn)(4-NH2-py)] – 3
0.1 mmol of [Ni(valpn)] and 0.15 mmol of 4-aminopyridine were dissolved in 40 ml of
acetonitrile-methanol (1:1) mixture. The slow evaporation at room temperature of the
resulting mixture affords green single crystals. Yield: 70%. Selected IR data (KBr pellet, cm-
1): 3380(m-w), 3329(m), 3236(m), 3048(w), 2930(m-w), 2827(w), 1656(s), 1614(vs),
1540(m-w), 1529(m-w), 1477(s), 1451(s), 1409(m-w), 1340(m), 1240(s), 1221(vs), 1070(m-
w), 1019(m-w), 734(s), 647(w).
2.1.4. [Zn(valpn)(4-NH2-py)]·xCH2Cl2 – 4
0.1 mmol of [Zn(valpn)] and 0.12 mmol of 4-aminopyridine were dissolved in 40 ml of
dichloromethane. The slow evaporation at room temperature of the resulting mixture affords
yellow single crystals. Yield: 60%. Selected IR data (KBr pellet, cm-1): 3352(m), 3223(m),
3053(w), 2923(w), 2849(w), 1665(m), 1620(vs), 1543(m), 1522(m-w), 1477(s), 1451(s),
1408(m), 1335(m), 1240(s), 1219(vs), 1076(m), 1021(m), 855(w), 735(s), 625(w).
2.1.5. [Ni(valdmpn)(4-NH2-py)] – 5
0.1 mmol of [Ni(valdmpn)] and 0.15 mmol of 4-aminopyridine were dissolved in 30 ml of
acetonitrile. The slow evaporation at room temperature of the resulting mixture affords dark
green single crystals. . Yield: 60%. Selected IR data (KBr pellet, cm-1): 3377(m), 3332(m-w),
3192(m-w), 2999(w), 2951(w), 2835(w), 1662(m), 1612(vs), 1545(m-w), 1522(m-w),
1475(s), 1450(s), 1412(m), 1334(m), 1220(vs), 1069(m), 1020(m-w), 856(w), 736(m),
641(w).
4

2.1.6. [Ni(valdmpn)(4-NH2-py)2]·0.5CH3CN – 6
0.1 mmol of [Ni(valdmpn)] and 0.25 mmol of 4-aminopyridine were dissolved in 40 ml of
acetonitrile-chloroform (1:1) mixture. The slow evaporation at room temperature of the
resulting mixture affords light green single crystals. Yield: 70%. Selected IR data (KBr pellet,
cm-1): 3344(m), 3236(m), 3051(w), 2953(w), 2895(w), 2831(w), 1615(vs), 1563(w),
1541(w), 1516(w), 1473(s), 1450(s), 1335(m), 1218(vs), 1068(m), 1008(m-w), 855(w),
736(m), 639(w).
2.2. Physical measurements
The IR spectra (KBr pellets) were collected on a Bruker Tensor 37 spectrophotometer in the
4000-400 cm-1 range. The UV-Vis-NIR spectra (diffuse reflectance technique) were recorded
on a JASCO V-670 spectrophotometer. The fluorescence spectra of solid samples were
collected on a JASCO FP-8300 spectrofluorometer.
2.4. X-ray crystallography
Single crystal X-ray diffraction measurements were performed on a STOE IPDS II
diffractometer operating with Mo-Kα (λ = 0.71073 Å) X-ray tube with graphite
monochromator. The structures were solved by direct methods and refined by full-matrix
least squares techniques based on F2. The non-H atoms were refined with anisotropic
displacement parameters. In the case of the compound 4, we could not find a good model to
refine the disordered dichloromethane solvent molecules and the SQUEEZE procedure was
used for the final refinement. The solvent accessible volume found was 618 Å3 with a count
of 131 electrons per unit cell. Calculations were performed using SHELX-2014
crystallographic software package. A summary of the crystallographic data and the structure
refinement for crystals 1 - 6 is given in Table 1. CCDC reference numbers: 1547885-
1547890.
The X-ray powder diffraction measurements (XRPD) included in the Supplementary
Information were carried out on a Proto AXRD Benchtop using the Cu-Kα radiation with a
wavelength of 1.540593 Å in the range 5-35º 2θ.
5

Table 1. Crystallographic data, details of data collection and structure refinement parameters for compounds 1-6
Compound
1
2
3
Chemical formula
M (g mol-1)
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
C24.67H27CuN4.33O4
511.71
173(2)
0.71073
Trigonal
R-3
C24.67H27NiN4.33O4
506.88
173(2)
0.71073
Trigonal
R-3
C24H26N4NiO4
493.20
173(2)
0.71073
Orthorhombic
Pnma
22.6015(7)
22.6015(7)
23.8497(7)
90
22.642(2)
22.642(2)
23.799(3)
90
8.3646(17)
19.495(4)
13.261(3)
90
90
90
90
β (°)
120
10550.9(7)
18
1.450
0.972
120
10566(2)
18
1.434
0.866
90
2162.4(8)
4
1.515
0.938
γ (°)
V (Å3)
Z
Dc (g cm-3)
µ (mm-1)
F(000)
4794
1.039
0.0262, 0.0694
0.0344, 0.0731
4776
0.927
0.0795, 0.1575
0.1891, 0.2006
1032
0.948
0.0711, 0.1628
0.1289, 0.2014
Goodness-of-fit on F2
Final R1, wR2 [I>2σ(I)]
R1, wR2 (all data)
Largest difference in peak
and hole (e Å-3)
0.307, -0.399
0.765, -0.535
1.267, -0.539
Compound
4
5
6
Chemical formula
M (g mol-1)
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
C24H26N4O4Zn
499.86
173(2)
0.71073
Trigonal
R-3
22.7562(16)
22.7562(16)
23.8969(17)
90
C26H30N4NiO4
521.25
293(2)
0.71073
Monoclinic
P21/c
11.0981(12)
17.7273(13)
13.3789(17)
90
C32H37.50N6.50NiO4
635.89
173(2)
0.71073
Monoclinic
P21/c
14.9173(7)
16.0180(6)
26.7592(14)
90
90
120
10717.0(17)
18
1.394
107.636(9)
90
2508.4(5)
4
1.380
0.813
90.288(4)
90
6393.9(5)
8
1.321
2680
β (°)
γ (°)
V (Å3)
Z
Dc (g cm-3)
µ (mm-1)
1.068
F(000)
4680
0.660
0.0439, 0.1033
0.1158, 0.1366
1096
0.778
0.0651, 0.1647
0.1015, 0.1723
2680
1.066
0.0852, 0.2101
0.1447, 0.2336
Goodness-of-fit on F2
Final R1, wR2 [I>2σ(I)]
R1, wR2 (all data)
Largest difference in peak
and hole (e Å-3)
0.342, -0.506
0.928, -0.342
0.741, -0.490
6

3. Results and discussion
3.1. Description of crystal structures
4-aminopyridine molecule, by its functional groups, has a dual behaviour:
coordinating ligand and hydrogen bonds donor and/or acceptor. For the compounds reported
herein, both features were exploited to build supramolecular architectures. The pyridyl
nitrogen atom can coordinate to a metal ion, while the amino group can act as a hydrogen
bonds donor. In [MII(valpn)] or [MII(valdmpn)] mononuclear complexes (MII = NiII, CuII or
ZnII), the axial positions of the metal ion are available for coordination and the larger O2O’2
free compartment is a potential receptor for hydrogen bond donors. By combining these two
tectons, a number of supramolecular architectures can be imagined, depending on the molar
ratio between the mononuclear complexes and 4-aminopyridine (Scheme 2). For a 1:1 molar
ratio supramolecular rectangles or supramolecular zigzag chains can be obtained, while for a
1:2 stoichiometry one possible arrangement is represented by a supramolecular ladder.
Scheme 2 Formation of supramolecular rectangles, zigzag chains or ladders by reacting [MII(valXn)]
mononuclear complexes with 4-aminopyridine in different molar ratios
The reactions of [Cu(valpn)] or [Ni(valpn)] with 4-aminopyridine (4-NH2-py) in the
same complex-to-ligand molar ratio (1:1.5), but different solvents (acetonitrile for the copper
derivative and an acetonitrile-chloroform mixture for the nickel derivative) afforded the
isostructural mononuclear compounds [Cu(valpn)(4-NH2-py)]·0.33CH3CN
[Ni(valpn)(4-NH2-py)]·0.33CH3CN – 2, respectively.
–
1
and
7

Further we will describe the crystal structure of compound 1 mentioning the
differences in 2 when present. The metal ion displays a square-pyramidal geometry (Figure
1a). The basal plane is formed by the two imino nitrogen atoms and the two phenoxo oxygen
atoms of the Schiff base ligand. In the apical position is coordinated the pyridyl nitrogen
atom of 4-aminopyridine. The basal bond lengths are Cu1-N1 = 2.0216(13), Cu1-N2 =
2.0042(14), Cu1-O2 = 1.9575(11), and Cu1-O3 = 1.9660(11) Å, shorter than the apical one,
Cu1-N3 = 2.2101(13) Å. By contrast, the apical bond length in compound 2 is shorter, Ni1-
N3 = 2.004(8) Å, and very similar to the basal ones, Ni1-N1 = 2.034(7), Ni1-N2 = 2.047(8),
Ni1-O2 = 1.994(6), and Ni1-O3 = 2.002(6) Å.
The neutral mononuclear complexes generate supramolecular dimers through
hydrogen bond interactions established between the amino groups of the 4-aminopyridine and
the phenoxo and methoxy oxygen atoms of the free compartment of the Schiff base ligands
(Figure 1b). The hydrogen interactions are donating bifurcated and the N4’···O1-4 distances
are 3.024(3), 2.992(2), 2.949(3), and 2.982(3) Å respectively (’ = 2-x, 1-y, -z).
Figure 1. Perspective views of the mononuclear complexes [Cu(valpn)(4-NH2-py)] (a) and of the
supramolecular dimers (b) in the crystal structure of 1 (symmetry code: ’ = 2-x, 1-y, -z)
In the lattice of crystal 1, six supramolecular dimers delimit cages hosting disordered
acetonitrile solvent molecules. Each cage contains two solvent molecules disordered on three
crystallographic positions (Figure 2).
8

Figure 2. (a) Top (along the crystallographic c axis) and (b) side views of the cages formed by six
supramolecular dimers in crystal 1
By using [Ni(valpn)] and 4-aminopyridine in the same molar ratio (1:1.5), but with a
different solvent combination, i.e. acetonitrile-methanol (1:1), another type of crystals was
obtained, [Ni(valpn)(4-NH2-py)] – 3. The structural investigation of the compound 3 by X-
ray diffraction on single crystal reveals also the presence of mononuclear complexes
[Ni(valpn)(4-NH2-py)], but a completely different lattice organization. The nickel ion is
placed in a square-pyramidal environment also with the apical bond length very close to the
basal ones, Ni1-N2 = 2.027(6), Ni1-N1 = 2.055(5), and Ni1-O2 = 2.004(4) Å (Figure 3a).
Each mononuclear complex is involved in hydrogen bonding with other two complexes
through the free compartment O1O2O2’O1’ of the Schiff base (acceptor) and the amino
group of the 4-aminopyridine (donor), generating in this way a supramolecular zigzag
(castellated) chain (Figure 3b). The N3”···O1 and N3”···O2 distances are 3.090(5) and
2.869(7) Å respectively (” = 0.5+x, 0.5-y, -0.5-z).
Figure 3. Perspective views of the mononuclear complexes [Ni(valpn)(4-NH2-py)] (a) and of a supramolecular
chain (b) in the crystal structure of 3 (symmetry codes: ’ = x, 0.5-y, z; ” = 0.5+x, 0.5-y, -0.5-z)
9

The supramolecular chains are parallel, running along the crystallographic a axis
(Figure 4). In the ac plane, the CH moieties of the iminic groups establish weak CH···π
interactions with the neighbouring chains. The non-covalent interactions between chains
extend along the crystallographic b axis through π−π contacts (3.46-3.65 Å) involving the
phenyl rings of the Schiff base ligands.
Figure 4. Packing diagram in crystal 3, view along the crystallographic b axis
The reaction of [Zn(valpn)] with 4-aminopyridine in 1:1.2 molar ratio, in
dichloromethane, gave the compound [Zn(valpn)(4-NH2-py)]·xCH2Cl2 – 4, isomorphous with
the compounds 1 and 2. The stereochemistry of the zinc ion is similar to that of the nickel ion
in compound 2 with the apical bond length very close to the basal ones: Zn1-N3 = 2.038(4) ,
Zn1-N1 = 2.127(3), Zn1-N2 = 2.122(5), Zn1-O2 = 2.007(3) and Zn1-O3 = 2.019(3) Å (we
used the same labelling scheme for the organic ligands in the compounds 1, 2, and 4).
Compound 4 crystallizes with dichloromethane solvent molecules in the lattice (replacing the
acetonitrile from the compounds 1 and 2), but we could not find a good model to refine the
disordered dichloromethane solvent molecules. SQUEEZE procedure was used for the final
refinement of the crystal structure. The count of 131 electrons per unit cell found by
SQUEEZE procedure is consistent with about 0.17 molecules of dichloromethane per
mononuclear complex of Zn(II).
10

Another pentacoordinated Ni(II) mononuclear complex, [Ni(valdmpn)(4-NH2-py)] –
5, was also obtained by reacting [Ni(valdmpn)] with 4-aminopyridine in molar ratios 1:1.5 in
acetonitrile (Figure 5a). The bond lengths for the coordination surrounding of the Ni(II) ions
are Ni1-N1 = 2.064(4), Ni-N2 = 2.050(4), Ni-N3 = 2.036(4), Ni1-O2 = 2.011(3) and Ni1-O3
= 1.997(3) Å. Supramolecular rectangles are formed by donating bifurcated hydrogen
interactions between the amino groups and the phenoxo and methoxy oxygen atoms of the
bicompartmental ligands (Figure 5b). The N4’···O1-4 distances are 2.999(6), 2.972(7),
2.952(7), and 2.995(7) Å respectively (’ = -x, 1-y, 1-z). In compound 5, the supramolecular
dimers are also reinforced by π−π interactions established between the two 4-aminopyridine
ligands (3.36-3.42 Å). In the compounds 1-4 this type of interactions were not evidenced
between the 4-aminopyridine ligands.
Figure 5. Perspective views of the mononuclear complexes [Ni(valdmpn)(4-NH2-py)] (a) and of the
supramolecular dimers (b) in the crystal structure of 5 (symmetry code: ’ = -x, 1-y, 1-z)
By increasing the molar ratio in reaction between the 4-aminopyridine and
[Ni(valdmpn)] to 2.5:1, we obtained [Ni(valdmpn)(4-NH2-py)2]·0.5CH3CN (6) in which the
two 4-aminopyridine ligands occupy the axial positions of the metal ions. Compound 6
crystallizes in the monoclinic P21/c space group with two crystallographically independent
mononuclear complexes in the asymmetric unit. In both mononuclear complexes, the nickel
ions exhibit an elongated octahedral stereochemistry with the donor atoms of the Schiff base
ligand placed in the equatorial plane and the pyridyl nitrogen atoms of the 4-aminopyridine
molecules coordinated in the axial positions (Figure 6). The bond lengths defining the
equatorial planes are: Ni1-N1 = 2.070(7), Ni1-N2 = 2.089(6), Ni1-O2 = 2.051(4), Ni1-O3 =
2.046(5), Ni2-N7 = 2.107(6), Ni2-N8 = 2.060(7), Ni2-O6 = 2.030(5), and Ni2-O7 = 2.049(5)
Å. The Ni-N bond lengths for the axial positions are: Ni1-N3 = 2.147(6), Ni1-N5 = 2.186(6),
Ni2-N9 = 2.171(6), and Ni2-N11 = 2.133(6) Å respectively.
11

Figure 6. Perspective view of the two crystallographically independent mononuclear complexes
[Ni(valdmpn)(4-NH2-py)2] in the crystal structure of compound 6
The amino groups of the 4-aminopyridine ligands from the two crystallographically
independent mononuclear complexes establish hydrogen bonds with O2O’2 free
compartments of adjacent complexes. These hydrogen bonds generate supramolecular ladder-
like chains (Figure 8), each O2O’2 free compartment of the [Ni(valdmpn)] moieties
interacting with two 4-aminopyridine ligands from two different complexes. The rungs are
formed by the [Ni(valdmpn)] fragments and the side rails by the 4-aminopyridine ligands.
The chains are parallel, running along the crystallographic b axis. The N···O distances for the
N-H···O hydrogen interactions range between 2.96 and 3.40 Å. The 4-aminopyridine
molecules within the ladders are also involved in π−π interactions (3.35-3.53 Å).
Figure 7. View of the supramolecular ladders generated by hydrogen bonding between [Ni(valdmpn)(4-NH2-
py)2] mononuclear complexes in the crystal structure of compound 6
12

3.2. Spectroscopic studies
The electronic spectra of compounds 1-6 were acquired over a wavelength range
spanning UV to NIR (from 200 to 1800 nm). As the Ni(II) complexes form the majority of
the compounds presented herein, let us first analyse their spectra. Considering the general
3
case of octahedral nickel(II) species, three spin allowed transition originating from the A2g
3
3
ground state are expected: to T2g(F) (7000-13000 cm-1), to T1g(F) (11000-20000 cm-1), and
3
to T1g(P) (19000-27000 cm-1). In addition, a spin forbidden transition can usually be seized
3
1
near the second spin allowed one and corresponds to the transition from A2g to Eg(D) [16].
Only compound 6, [Ni(valdmpn)(4-NH2-py)2]·0.5CH3CN, contains a hexacoordinated Ni(II)
ion and its geometry is a tetragonally distorted octahedron (D4h) as the basal Ni-O and Ni-N
bonds are virtually equal. It is obvious then that the spectrum appearance of compound 6
should not strongly differ from the one expected for Oh symmetry. Thus, two bands of low
intensity can be distinguished at higher wavelengths: one with a maximum at around 920 nm
(10870 cm-1) and a shoulder at around 790 nm (12658 cm-1) and the other one with a
maximum at around 610 nm (16393 cm-1) (Figure 8). Taking into account that the ground
3
3
term A2g becomes B1g and, at the same time, the degeneracy of the excited triplet states is
lifted when Oh changes to D4h symmetry, the corresponding transitions are presumptively
3
3
3
3
1
1
3
from B1g to Eg and B2g [both derived from T2g(F)], to A1g and/or B2g(1Eg), and to A2g
3
and/or Eg [3T1g(F)], respectively [17]. As for the higher energy transitions [i.e. from 3B1g to
3
3
3A2g and Eg terms resulting from T1g(P)], they are most probably embedded into the
intraligand and/or CT intense band with a maximum at around 380 nm (26316 cm-1).
Actually, this latter band together with the split one having a similar intensity and ranging
between 200 and 300 nm can be found in the spectra of all the nickel(II) and copper(II)
derivatives gathered herein and, therefore, are more likely to originate from intraligand
transitions within both ligands.
The other Ni(II) compounds are very similar: crystals 2 and 3 are pseudopolymorphs,
whereas the complex unit in 5 differs from the former two only by the diamine residue
included into the Schiff base ligand. The Ni(II) ion is pentacoordinated in all these three
complexes and its geometry is square pyramidal. Actually, this polyhedron can be obtained
by the removal of one 4-aminopyridine from the octahedral array specific to compound 6. As
the nickel - donor atom bonds are almost equal in the basal plane, the symmetry can be
assumed as C4v rather than C2v. Consequently, the corresponding spectral terms derive from
those specific to the tetragonally distorted octahedron (D4h) by slightly lowering this
symmetry [18]. The spectra of compounds 2, 3, and 5 display the same contour with four
13

evident bands arising from d-d transitions. Thus, the large band spreading towards the NIR
3
3
end corresponds to the transition from B1 to E [3T2g(F)] (1690, 1650, and 1740 nm for
compounds 2, 3, and 5, respectively). The two closely disposed maxima at around 800 and
3
3
870 nm are due to the transitions from 3B1 to B2 and to A2, but their order cannot be stated.
The band at 650 (for complexes 2 and 3) or at 635 nm (for complex 5) is assigned to the
3
3
transition from B1 to E [3T1g(F)]. The intraligand bands at around 380 nm should mask the
higher energy d-d transitions.
Figure 8. Electronic spectra of nickel(II) and copper(II) complexes
As for the copper(II) complex (1), the d-d transitions build up the absorption envelope
between 540 and 1400 nm with a maximum at around 645 nm and a shoulder at around 920
nm (this asymmetric shape of the d-d transitions band with a shoulder in the lower energy
region was frequently stated as characteristic for the square pyramidal stereochemistry of
Cu2+ ion). The intense bands at 235, 275, and 375 nm are probably due to intraligand
transitions. However, the shoulder at 460 nm is specific to this complex and can originate
from a LMCT transition.
The room temperature photoluminescence of compound [Zn(valpn)(4-NH2-
py)]·xCH2Cl2 (4) was investigated using different wavelengths for excitation in the 350-430
nm range. The resulting emission spectrum displays an asymmetric band (tailed to the lower
energy side) with a maximum at 470 nm. The highest intensity of this band is obtained when
λex = 420 nm. The corresponding excitation spectrum (λem = 470 nm) reveals a complex band
within the 240-460 nm domain (Figure 9).
14

Figure 9. Emission (right) and excitation (left) spectra for the compound [Zn(valpn)(4-NH2-py)]·xCH2Cl2 – 4
The examples presented herein show that various supramolecular objects or
architectures can be obtained by combining bimodal tectons which are able to establish both
coordinative bonds and non-covalent interactions. The mononuclear complexes belonging to
the [M(valXn)] family have the axial positions of the metal ion available for coordination and
they can effectively act as receptors towards a large variety of hydrogen bond donors.
Reacting [M(valXn)] complexes with 4-aminopyridine, we obtained supramolecular
rectangles, zigzag chains or ladders depending on stoichiometry and reaction conditions. In
the [MII(valXpn)(4-NH2-py)] complexes, the metal ions exhibit a square pyramidal geometry,
coordination number five being still rare for the nickel (II) ion.
Acknowledgements. Financial support from University of Bucharest (project 72/2013) is
highly acknowledged.
15

[1] (a) J.-M. Lehn, Supramolecular Chemistry - Concepts and Perspectives, VCH, Weinheim
(1995); (b) J.-M. Lehn, Chem.–Eur. J. 6 (2000) 2097; (c) M. Fujita, Chem. Soc. Rev. 27
(1998) 417; (d) B. Champin, P. Mobian, J.-P. Sauvage, Chem. Soc. Rev. 36 (2007) 358.
[2] (a) M. Ruben, J. Rojo, F.J. Romero-Salguero, L.H. Uppadine, J.-M. Lehn, Angew.
Chem., Int. Ed. 43 (2004) 3644; (b) K. Petukhov, M.S. Alam, H. Rupp, S. Strömsdörfer,
P. Müller, A. Scheurer, R.W. Saalfrank, J. Kortus, A. Postnikov, M. Ruben, L.K.
Thompson, J.-M. Lehn, Coord. Chem. Rev. 253 (2009) 2398; (c) L.N. Dave, T.S.M.
Abedin, L.K. Thompson, Dalton Trans. (2008) 1661.
[3] (a) J.-M. Lehn, Angew. Chem., Int. Ed. 29 (1990) 1304; (b) C. Piguet, G. Bernardinelli,
G. Hopfgartner, Chem. Rev. 97 (1997) 2005; (c) M. Albrecht, Chem. Rev. 101 (2001)
3457.
[4] S. Roche, C. Haslam, H. Adams, S.L. Heath, J.A. Thomas, Chem. Commun. (1998)
1681.
[5] M. Fujita, M. Tominaga, A. Hori, B. Therrien, Acc. Chem. Res. 38 (2005) 371.
[6] (a) L.H. Uppadine, P. Gisselbrecht, J.-M. Lehn, Chem. Commun. (2004) 718; (b) D.M.
Bassani, J.-M. Lehn, S. Serroni, F. Puntoriero, S. Campagna, Chem. Eur. J. 9 (2003)
5936; (c) M. Boiocchi, L. Fabbrizzi, Chem. Soc. Rev. 43 (2014) 1835.
[7] (a) A.-S. Chauvin, S. Comby, M. Baud, C. De Piano, C. Duhot, J.-C.G. Bünzli, Inorg.
Chem. 48 (2009) 10687; (b) N. André, T.B. Jensen, R. Scopelliti, D. Imbert, M. Elhabiri,
G. Hopfgartner, C. Piguet, J.-C.G. Bünzli, Inorg. Chem. 43 (2004) 515; (c) P. Cucos, F.
Tuna, L. Sorace, I. Matei, C. Maxim, S. Shova, R. Gheorghe, A. Caneschi, M.
Hillebrand, M. Andruh, Inorg. Chem. 53 (2014) 7738.
[8] (a) L.N. Dawe, L.K. Thompson, Angew. Chem., Int. Ed. 46 (2007) 7440; (b) M. U.
Anwar, L.K. Thompson, L.N. Dawe, F. Habib, M. Murugesu, Chem. Commun. 48
(2012) 4576; (c) A.M. Madalan, X.-Y. Cao, G. Rogez, J.-M. Lehn, Inorg. Chem. 53
(2014) 4275; (d) P. Cucos, M. Pascu, R. Sessoli, N. Avarvari, F. Pointillart, M. Andruh,
Inorg. Chem. 45 (2006) 7035; (e) A.K. Mondal, H.S. Jena, A. Malviya, S. Konar, Inorg.
Chem. 55 (2016) 5237.
[9] (a) D. Visinescu, M. Andruh, A. Müller, M. Schmidtmann, Y. Journaux, Inorg. Chem.
Commun. 5 (2002) 42; (b) D. Visinescu, G.I. Pascu, M. Andruh, J. Magull, H.W.
Roesky, Inorg. Chim. Acta 340 (2002) 201; (c) D. Visinescu, A.M. Madalan, V.
Kravtsov, Yu. A. Simonov, M. Schmidtmann, A. Müller, M. Andruh, Polyhedron 22
(2003) 1385; (d) G. S. Papaefstathiou, Z. Zhong, L. Geng, L. R. MacGillivray, J. Am.
16

Chem. Soc. 126 (2004) 9158; (e) G. Marinescu, G. Marin, A.M. Madalan, A. Vezeanu,
C. Tiseanu, M. Andruh, Cryst. Growth Des. 10 (2010) 2096.
[10] (a) J.-P. Costes, F. Dahan, A. Dupuis, J.-P. Laurent, Inorg. Chem., 35 (1996) 2400; (b)
M. Andruh, Chem. Commun., (2007) 2565; (c) M. Andruh, Chem. Commun., 47 (2011)
3025.
[11] (a) D.G. Branzea, A. Guerri, O. Fabelo, C. Ruiz-Pérez, L.-M. Chamoreau, C.
Sangregorio, A. Caneschi, M. Andruh, Cryst. Growth Des. 8 (2008) 941; (b) D. G.
Branzea, L. Sorace, C. Maxim, M. Andruh, A. Caneschi, Inorg. Chem. 47 (2008) 6590;
(c) M. Nayak, R. Koner, H.-H. Lin, U. Flörke, H.-H. Wei, S. Mohanta, Inorg. Chem. 45
(2006) 10764.
[12](a) S. Nastase, F. Tuna, C. Maxim, C.A. Muryn, N. Avarvari, R.E.P. Winpenny, M.
Andruh, Cryst. Growth Des. 7 (2007) 1825; (b) M.Á. Vázquez-Fernández, M.I.
Fernández-García, A.M. González-Noya, M. Maneiro, M.R. Bermejo, M.J. Rodríguez-
Doutón, Polyhedron 31(2012) 379; (c) C.D. Ene, S. Nastase, C. Maxim, A.M. Madalan,
F. Tuna, M. Andruh, Inorg. Chim. Acta 363 (2010) 4247.
[13] (a) A.M. Madalan, N. Avarvari, M. Andruh, New J. Chem. 30 (2006) 521; (b) S. Harza,
R. Koner, M. Nayak, H.A. Sparkes, J.A.K. Howard, S. Mohanta, Cryst. Growth Des. 9
(2009) 3603.
[14] A. Cucos, A. Ursu, A.M. Madalan, C. Duhayon, J.-P. Sutter, M. Andruh,
CrystEngComm 13 (2011) 3756.
[15](a) M. Nayak, S. Sarkar, P. Lemoine, S. Sasmal, R. Koner, H.A. Sparkes, J.A.K.
Howard, S. Mohanta, Eur. J. Inorg. Chem. (2010) 744; (b) S. Sarkar, M. Fleck, S.
Mohanta, J. Mol. Struct. 1021 (2012) 174.
[16] E.I. Solomon, A.B.P. Lever, Inorganic Electronic Structure and Spectroscopy, John
Wiley & Sons, Inc., New York (1999).
[17] C.E. Taylor, A.E.Underhill, J. Chem. Soc. A (1969) 368.
[18] M. Gerloch, J. Kohl, J. Lewis, W. Urland, J. Chem. Soc. A (1970) 3269.
17

-
-
-
-
Six Ni(II), Cu(II) and Zn(II) mononuclear complexes with bicompartmental Schiff
base and 4-aminopydine ligands were synthesised;
The crystal structure of the Ni(II), Cu(II) and Zn(II) mononuclear complexes were
determined;
Hydrogen bond interactions generate supramolecular rectangles, zigzag chains and
ladders;
The spectroscopic properties of the compounds were investigated.
18

19