Piperazine core-containing Schiff ligands define chemical reactivity toward divalent metal ions

Article abstract of DOI:10.1016/j.ica.2019.04.033

A series of homologous Schiff bases N,N′-bis[(4-decyloxy-salicylideneamino)-n-propyl]-piperazine](ZOPPH₂), and N,N′-bis[(4-dodecyloxy-benzylideneamino)-n-propyl]-piperazine (DBPP)(1), based on 1,4-bis(3-amino-propyl)-piperazine (APPZ), were designed and synthesized, with APPZ serving as the piperazine core, bilaterally flanked by extended alkyl chain-containing antennae. Driven by the pursuit of metallomesogenic materials bearing liquid crystalline state properties, chemical reactivity toward divalent metals Co(II)and Cu(II), in alcoholic or tetrahydrofuran/dimethylsulfoxide media, led to compounds [{Co(ZOPP)}(ClO₄)]₂.(CH₃OH).2(CH₃)₂SO(2)and [Cu(APPZ)Cl]Cl(3). All materials were characterized by elemental analysis, spectroscopic techniques (UV–Visible, FT-IR, NMR where appropriate), molar conductivity, and X-ray crystallography. Physicochemical characterization emphasizes the a)importance of N,O-containing Schiff anchors in metal ion binding, b)significance of the phenolic moiety, on the flanks of the Schiff ligands, in promoting either metal ion complexation or dissociation of the Schiff base at the azomethine moiety junction, thereby altering metal ion chemical reactivity, and c)observed oxidation of Co(II)to Co(III)upon mononuclear complex formation. Hybrid DFT calculations on DBPP and ZOPPH₂ suggest increased reactivity of the Schiff [sbnd]CH[dbnd]N[sbnd]moiety, in the absence of the phenolic moiety, and concurrent metal ion presence, thereby lending credence to the notion of metal-assisted rupture of the specific bond in DBPP and the emergence of specific metal-ligand product(s). Collectively, the data denote the significance of structural features of piperazine-core Schiff antennae ligands in a)promoting chemical reactivity toward transition metals, b)defining metal-ligand complexation and lattice architecture, and c)parameterizing such chemical reactivity into synthetic advances toward new materials with well-defined solid-state architecture, lattice and physicochemical properties.

Full text of DOI:10.1016/j.ica.2019.04.033

Accepted Manuscript
Research paper
Piperazine core-containing Schiff ligands define chemical reactivity toward di-
valent metal ions
Carmen Cretu, Liliana Cseh, Ramona Tudose, Alina Bora, Sevasti Matsia,
Antonios Hatzidimitriou, Otilia Costisor, Athanasios Salifoglou
PII:
S0020-1693(18)31081-8
https://doi.org/10.1016/j.ica.2019.04.033
ICA 18878
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
15 July 2018
10 April 2019
15 April 2019
Please cite this article as: C. Cretu, L. Cseh, R. Tudose, A. Bora, S. Matsia, A. Hatzidimitriou, O. Costisor, A.
Salifoglou, Piperazine core-containing Schiff ligands define chemical reactivity toward divalent metal ions,
Inorganica Chimica Acta (2019), doi: https://doi.org/10.1016/j.ica.2019.04.033
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1
Piperazine core-containing Schiff ligands define chemical reactivity toward divalent metal
ions
Carmen Cretu1, Liliana Cseh1*, Ramona Tudose1, Alina Bora1, Sevasti Matsia2, Antonios
Hatzidimitriou3, Otilia Costisor1 and Athanasios Salifoglou2*
1
Institute of Chemistry Timisoara of Romanian Academy, 24 Mihai Viteazul Ave, Timisoara
300223, Romania, Tel.: +40256-491818, Fax: +40256-491824, E-mail: lili_cseh@yahoo.com
2
Department of Chemical Engineering, Faculty of Engineering, Aristotle University of
Thessaloniki, Thessaloniki 54124, Greece, Tel.: +30-2310996179, Fax: +30-2310996196, E-
mail: salif@auth.gr
3
Department of Chemistry, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece,
Tel.: +30-2310997748, Fax: +30-2310997748, E-mail: hatzidim@chem.auth.gr

2
Abstract
A series of homologous Schiff bases N,N’-bis[(4-decyloxy-salicylideneamino)-n-propyl]-
piperazine] (ZOPPH2), and N,N’-bis[(4-dodecyloxy-benzylideneamino)-n-propyl]-piperazine
(DBPP)(1), based on 1,4-bis(3-amino-propyl)-piperazine (APPZ), were designed and
synthesized, with APPZ serving as the piperazine core, bilaterally flanked by extended alkyl
chain-containing antennae. Driven by the pursuit of metallomesogenic materials bearing liquid
crystalline state properties, chemical reactivity toward divalent metals Co(II) and Cu(II), in
alcoholic
or
tetrahydrofuran/dimethylsulfoxide
media,
led
to
compounds
.
[{Co(ZOPP)}(ClO4)]2 (CH3OH).2(CH3)2SO(2) and [Cu(APPZ)Cl]Cl(3). All materials were
characterized by elemental analysis, spectroscopic techniques (UV-Visible, FT-IR, NMR where
appropriate), molar conductivity, and X-ray crystallography. Physicochemical characterization
emphasizes the a) importance of N,O-containing Schiff anchors in metal ion binding, b)
significance of the phenolic moiety, on the flanks of the Schiff ligands, in promoting either metal
ion complexation or dissociation of the Schiff base at the azomethine moiety junction, thereby
altering metal ion chemical reactivity, and c) observed oxidation of Co(II) to Co(III) upon
mononuclear complex formation. Hybrid DFT calculations on DBPP and ZOPPH2 suggest
increased reactivity of the Schiff –CH=N– moiety, in the absence of the phenolic moiety, and
concurrent metal ion presence, thereby lending credence to the notion of metal-assisted rupture of
the specific bond in DBPP and the emergence of specific metal-ligand product(s). Collectively,
the data denote the significance of structural features of piperazine-core Schiff antennae ligands
in a) promoting chemical reactivity toward transition metals, b) defining metal-ligand
complexation and lattice architecture, and c) parameterizing such chemical reactivity into
synthetic advances toward new materials with well-defined solid-state architecture, lattice and
physicochemical properties.
Keywords: Schiff base ligand, coordination complex, cobalt and copper crystal structure,
molecular antennae, metal chemical reactivity.

3
1. Introduction
Schiff bases have been at the forefront of research over the past decades [1,2]. Interest in their
synthesis and emergence in complex organic molecules has stemmed from their natural source of
origin and their incipient utilization in cellular processes and pathophysiologies [3,4]. Intimately
relevant to their emergence in natural systems is their widespread use in the design and synthesis
of new and novel metal-organic materials, exemplifying commensurably new properties as a
result of the concurrent metal ion and organic ligand structural-electronic attributes. In such
interactive processes, transition metal ions can participate with different oxidation states [5],
thereby leading to discrete lattice architectures. Standing out in the family of hybrid binary
metal-organic species, comprised of metal ions and appropriately designed organic Schiff
ligands, are divalent and trivalent metal ion-Schiff base complexes, including Co(II,III) and
Cu(II), which have attracted considerable attention in the field of chemistry [6-9], biology [10],
and physics [11-13], due to their extensive practical applications. One such significant
application area involves development of liquid crystalline materials, thus drawing in a link
between metal ion coordination chemistry and appropriately configured Schiff base ligands [14].
Based on the above grounds, studies [15-17] on the coordination potential of Co(II) with
symmetrically structured Schiff bases, derived from salicylaldehyde derivatives and a variety of
diamines, have shown that the nature and length of the fragment between the two azomethine
groups affect the coordination modes to the cobalt ion [15,16]. Concurrently, increase of the
methylene chain length of the diamine moiety bestows adequate flexibility upon those
complexes, thereby switching their structure from a planar one toward a distorted or pseudo-
tetrahedral motif [8,12]. To further specify the reactivity of such distinctly configured Schiff
bases a) comprised of a common piperazine core, b) bearing well-defined aromatic salicyl
aldehyde moieties, and c) containing variable length aliphatic chains, extending out to twelve
carbon atoms, toward transition metal ions, two members of the Schiff base family of the
aforementioned organic binders were chosen as potential ligands to divalent transition metal ions
Co(II) and Cu(II). The latter starting reagents were used in the form of salts to promote chemical
reactivity in variable solvent systems.
The employed ligands exhibit long alkyl chains extending bilaterally out of the Schiff azomethine
moieties of the piperazine core, while the presence of the phenol moiety on the aromatic ancillary
group appears to play an important role in promoting metal ion coordination and chemical

4
reactivity. These structural features reflect mesomorphic phase characteristics associated with
induction of anisotropic fluidity, thereby providing perspective into the pursuit of potential
metallomesogenic materials [14,18-20]. Being driven by such a challenge, research efforts were
launched to investigate synthetically the ligand structure-specific chemistry toward Co(II) and
Cu(II), two metals with broad coordination versatility. In that respect, the collective combination
of ligand attributes (alkyl chain length, hydrophobicity, piperazine core, phenol-containing
moieties) and metal ion interactions targeted hybrid metal-organic materials exhibiting solid-state
architectures, bearing new lattice and electronic properties (e.g. luminescence). The results of the
work a) denote the distinct character of metal ion-ligand reactivity through both experimental and
theoretical data, b) suggest that metal-Schiff ligand complexation can be pursued effectively in a
ligand-structure-specific fashion, c) project the importance of the versatility of the azomethine
moiety of the Schiff ligand, seeking organizational role(s) in the coordination to the metal ion and
contributing to the arising structural architecture and lattice dimensionality of the emerging
molecular metal-ligand assemblies, and d) emphasize the role of the phenolic moiety in
modulating the sought after reactivity of N,N’-bis[(4-decyloxy-salicylideneamino)-n-propyl]-
piperazine (ZOPPH2) vs N,N’-bis[(4-dodecyloxy-benzylideneamino)-n-propyl]-piperazine
(DBPP) by facilitating (dis)assembly of the Schiff ligand and concomitant coordination of the
metal ion to the Schiff piperazine core.
2. Experimental
2.1. Materials and methods
Cobalt(II) perchlorate hexahydrate (Co(ClO4)2•6H2O), copper(II) chloride dihydrate (CuCl2
2H2O), copper(II) perchlorate hexahydrate (Cu(ClO4)2•6H2O) and the organic solvents
(tetrahydrofuran (THF), methanol, dimethylsulfoxide, N,N’-dimethylformamide (DMF)) were
purchased from Merck or Sigma Aldrich and used without further purification. Ligands ZOPPH2
and DBPP (1) were synthesized according to previously reported procedures [14,18].
Caution! Cobalt perchlorate is potentially explosive and should be handled in small quantities
with due care.
2.1.1 Physical Measurements
Infrared spectra (KBr) in the range 4000-400 cm-1 were recorded on a Cary 630 FT-IR
spectrophotometer. The simultaneous determination of carbon, hydrogen and nitrogen (%) was

5
carried out with a ThermoFinnigan Flash EA 1112 CHNS elemental analyzer. The analyzer
operation is based on the dynamic flash combustion of the sample at 1800 ºC, followed by
reduction, trapping, complete GC separation and product detection. The instrument is fully
automated and PC controlled via the Eager 300 software. Molar electrical conductivities were
measured using a Mettler Toledo FiveEasy plus (FP30) conductivity meter equipped with a Lab
conductivity sensor LE740. The solution electronic spectra of all compounds investigated (C =
10-4 M) were recorded on an Agilent Cary 60 spectrometer.
2.1.2 Photoluminescence
Fluorescence spectra were recorded in CHCl3 (for 1), DMF (for ZOPPH2 and 2), and MeOH (for
3) solutions (C = 10-4 M) using a Perkin Elmer LS-55 spectrophotometer; the excitation slit was
set at 10 and that of the emission at 7.5, while the scanning speed was 100 nm/min. All
measurements were carried out at room temperature.
2.1.3 NMR spectroscopy
1
The NMR spectra were run on a Bruker Avance III 500 spectrometer (500.13 MHz for H,
13
125.75 MHz for C) in CDCl3, at 298 K, using tetramethylsilane (TMS) as internal standard.
1
NMR assignments were carried out on the basis of H, 13C, DEPT 135 and HSQC
(HSQCEDETGPSISP) (2D) NMR spectra.
2.2. Synthesis
.
Synthesis of [{Co(ZOPP)}(ClO4)]2 (CH3OH).2(CH3)2SO (2). An ethanolic solution (20 mL) of
Co(II) perchlorate hexahydrate, Co(ClO4)26H2O, (0.61 g, 1.67 mmol) was added to a stirred
solution of ZOPPH2 (0.60 g, 0.83 mmol) in ethanol (30 mL). The resulting reaction mixture was
refluxed for 4 h. A brown solid was obtained by evaporation of the solvent at room temperature.
The solid was collected and washed with methanol and hot water. Single crystals suitable for X-
ray crystallographic work were obtained by recrystallization from DMSO. Yield: 0.59 g (86 %).
Anal. Calcd. for 2, (C93H156Cl2Co2N8O19S2 Mr 1943.21): C, 57.48; H, 8.09; N, 5.77. Found: C,
57.41; H, 8.04; N, 5.71.
Molar conductivity (DMF, 5·10-4 M) M, -1mol-1cm2: 43; IR (KBr, cm-1: 3434 (br, w), 2918 (s),
2840 (s), 1618 (vs), 1606 (vs), 1529 (m), 1488 (m), 1467 (s), 1429 (m), 1367 (m), 1338 (w), 1306
(m), 1268 (w), 1248 (m), 1232 (s), 1213 (m), 1176 (s), 1149 (m), 1117 (m), 1095 (vs), 1079 (vs),
1017 (m), 993 (m), 959 (m), 852 (w), 842 (m), 823(w), 792 (m), 780 (m), 722 (w), 661 (w), 636

6
(w), 621 (m), 577 (w), 529 (w), 496 (w), 484 (w), 466 (w), 450 (w), 434 (w); UV-Vis (DMF,
5·10-4M), (max/nm (ε/L mol-1cm-1)): 377(5240), 506(420), 578(220), 655(120).
.
Synthesis of [Cu(APPZ)Cl]Cl (3). A solution of Cu(II) chloride dihydrate, CuCl2 2H2O, (0.080
g, 0.48 mmol) in THF (4 mL) was added slowly by diffusion to a solution of APPZ (0.10 g, 0.48
mmol) in DMSO (3 mL). Single crystals suitable for X-ray crystallographic work were obtained
from the reaction mixture. Yield: 0.59 g (86 %). Anal. Calcd. for 3, (C10H24Cl2CuN4 Mr 334.78):
C, 35.88; H, 7.23; N, 16.74. Found: 35.82; H, 7.25; N, 16.69.
Molar conductivity (MeOH, 5·10-4 M) ɅM, Ω-1mol-1cm2: 112; IR (KBr, cm-1: 3260 (s), 3219 (s),
3177 (s), 3128 (s),3104 (s), 2934 (s), 2863 (s), 1602 (s), 1467 (s), 1433 (m), 1398 (m), 1346 (m),
1306 (m), 1283 (m), 1256 (m), 1213 (m), 1169 (m), 1147 (s), 1105 (m), 1079 (m), 1027 (s), 981
(m)945 (w), 919 (m), 861 (w), 834 (m), 787 (s), 675 (s), 515 (w), 478 (m), 423 (w). UV-Vis
(MeOH, 5·10-4 M), (max/nm(ε/L mol-1cm-1)): 274(6840), 619(282).
2.3. X-Ray structural determination
Single crystals of DBPP (1) were grown in the reaction mixture [14,18], selected directly from
the mother liquor under a microscope and sealed in thin-walled glass capillaries. X-ray quality
crystals of compounds 2 and 3 were grown from DMSO and THF-DMSO solutions, respectively.
For the structure determination of all compounds, single crystals of the respective compound
were mounted on a Bruker Kappa APEX II diffractometer, equipped with a triumph
monochromator at ambient temperature. Diffraction measurements were made using graphite
monochromated Mo Kα radiation. Unit cell dimensions were determined and refined by using
the angular settings of at least 50 high intensity (>20σ(I)) reflections in the range 10<2θ<40o.
Intensity data were recorded using φ and ω scan modes. The frames collected for each crystal
were integrated with the Bruker SAINT software package [21], using a narrow-frame algorithm.
Data were corrected for absorption using the numerical method (SADABS) based on crystal
dimensions. The structure was solved using the SUPERFLIP [22] package and refined by full–
matrix least-squares method on F2 using the CRYSTALS package version 14.43 and 14.61 [23].
All non-disordered non-hydrogen atoms have been refined anisotropically. For the disordered
non-hydrogen atoms, occupancy factors were first determined with fixed isotropic displacements.
Finally, all of them were isotropically refined with fixed occupancy factors.
All hydrogen atoms were found at the expected positions and refined using soft constraints. By
the end of the refinement, they were positioned using riding constraints. Crystal data and

7
structure refinement parameters of 1-3 are presented in Table 1. Illustrations were drawn by
Diamond 3.1 package [24]. The CCDC numbers for compounds are CCDC 743573 (1), CCDC
978591 (2), and CCDC 1846243 (3).
2.4. Computational studies
The molecular structures of APPZ, DBPP (1) and ZOPPH2 ligands were investigated by means of
semi-empirical quantum chemical calculations, involving Austin Model 1 (AM1) [25,26] and
Density Functional Theory (DFT), using the Hyperchem 7.52 program [27] and Jaguar module of
the Schrodinger package [28]. The AM1 geometry optimization was carried out by using the
default parameters as Polak-Ribere algorithm, with the SCF convergence set to 0.0001 kcal/mol
and the RMS gradient set to 0.001 kcal/(Åmol) [25,26]. The complete geometric optimization of
the AM1 geometries was obtained by applying hybrid DFT with the Becke, three-parameter, Lee-
Yang-Parr (B3LYP/6-311++) exchange-correlation functional and the basis set level of theory, 6-
31G(d,p) [27]. Ligand geometries were optimized without imposing any symmetry constraints.
Further, frequency calculations were done in order to check the true energy minima of the studied
ligands. The true minima were confirmed by the absence of any negative frequencies. Based on
DFT theory, several electronic properties such as, e.g. heat of formation (ΔHfθ) [29], HOMO,
LUMO, and HOMO-LUMO gap (ΔE) energies [30,31], the atomic Fukui indices [32], hardness
(η), softness (S), electronegativity (χ), chemical potential (μ), and global electrophilicity (ω) were
computed. These global electronic descriptors are used to describe the reactivity behavior of the
chemical system.
3. Results
3.1. Synthesis
Schiff bases, as starting materials, were synthesized according to optimized procedures from their
components, i.e. the core-containing piperazine and the corresponding aromatic aldehyde [14,18].
The stoichiometric reactions, under the reaction conditions leading to the synthesis of ligands
ZOPPH2 and DBPP, are shown below:

8
H2
C
NH2
H2C
H2C
H
N
C
C2H5OH
O
H2
C
H2
C
H2
C
H2
C
H2
C
+
2
CH3
C
C
C
C
O
HO
N
H2
H2
H2
H2
Reflux
H2C
H2C
NH2
C
H2
APPZ
H
H2
C
C
H2
C
H2
C
H2
C
H2
C
N
H2
C
H2C
H2C
CH3
C
C
C
C
O
HO
H2
H2
H2
H2
N
+
2 H2O
H2
C
H2
C
H2
C
H2
C
CH3
O
HO
N
H2C
H2C
C
C
C
C
C
H2
H2
H2
H2
H2
N
C
C
H
H2
ZOPPH2
H2
C
NH2
H2C
H2C
H
N
C
CH3OH, Δ
O
H2
C
H2
C
H2
C
H2
C
H2
C
H2
C
+
2
C
CH3
C
C
C
C
O
N
H2C
H2C
H2
H2
H2
H2
H2
Glacial Acetic acid
NH2
C
H2
APPZ
H
H2
C
C
H2 H2
H2
C
H2
C
H2
C
H2
C
N
H2C
H2C
C
C
C
CH3
C
C
C
C
O
H2
H2
H2
H2
H2
N
+
2 H2O
H2
C
H2
C
H2
C
H2
C
H2
C
CH3
O
N
H2C
H2C
C
C
C
C
C
C
H2
H2
H2
H2
H2
H2
N
C
C
H
H2
DBPP

9
The piperazine core-containing APPZ ligand, as shown in the above two reactivity schemes,
appears to be the key fundamental unit, based on which the two aforementioned ligands rely on
and to which the DBPP ligand dissociates upon copper reactivity (vide infra).
.
Furthermore, the Schiff base metal complex [{Co(ZOPP)}(ClO4)]2 (CH3OH).2(CH3)2SO (2) and
[Cu(APPZ)Cl]Cl (3) were synthesized through reactions between cobalt and copper salts and the
corresponding ligand, containing the available coordination sites, including the piperazine bridge,
employing variable molar ratios in an organic solvent medium. Thus, 2 was synthesized from
Co(ClO4)26H2O and ZOPPH2 in ethanol with a 2:1 molar ratio. Previous experience in the lab
[14] had shown that the hexadentate Schiff base ZOPPH2 can act as a binucleating ligand through
a two (N,O) donor atom set or a bis-tridentate ligand through two (N,N,O) donor atom sets,
depending on the involvement of the piperazine core nitrogen in metal coordination, with the
piperazine moiety being in the chair conformation. Nevertheless, the donor properties, the
relatively flexible structure, and the symmetry of similar ligands [33] also offer an opportunity to
prepare mononuclear complexes with the piperazine moiety in the boat conformation. The
mononuclear structure of the complex reported herein was supported by molar conductivity
measurements and X-ray diffraction. The value of molar conductivity suggests a 1:1 electrolyte-
type [34] complex.
In an alternative way, Cu(II) chloride reacted with APPZ in a 1:1 molar ratio, in a THF-DMSO
medium, leading to the isolation of compound 3. The isolated material was identified by
elemental analysis, FT-IR and X-Ray crystallography.
The reaction conditions and
stoichiometric reaction associated with the synthesis of 3 are given pictorially in Scheme 1:
H2
C
H2
C
+
H2C
H2C
H2C
H2C
NH2
NH2
THF
N
N
CuCl2·2H2O
Cu Cl
+
Cl
DMSO
N
N
NH2
H2C
H2C
NH2
CH2
H2C
H2C
CH2
Compound 3
APPZ

10
Scheme 1. Synthesis of compound 3
The specific chemical reactivity follows along the line of the synthesis of compound
[Cu(APPZ)(ClO4)](ClO4), which emerged from a similar reaction between Cu(ClO4)2•6H2O and
DBPP (1) in THF solution, with a 1:1 molar ratio. The overall stoichiometric reaction and
reaction conditions are presented in Scheme 2:
H2
H
C
C
H2C
H2C
H2 H2
H2
C
H2
C
H2
C
H2
C
N
C
C
C
CH3
C
C
C
C
O
H2
H2
H2
H2
THF
H2
N
Cu(ClO4)2·6H2O
+
H2
C
H2
C
H2
C
H2
C
H2
C
CH3
O
N
C
C
C
C
C
C
H2
H2
H2
H2
H2
H2
H2C
N
C
H
H2C
CH2
DBPP
+
CH2
NH2
H2C
H2C
H
C
O
N
O
O
H2 H2
H2
C
H2
C
H2
C
H2
C
-
+
Cl
2
ClO4
C
C
Cu
O
O
C
CH3
C
C
C
C
O
H2
H2
H2
H2
H2
N
NH2
H2C
H2C CH2
Scheme 2. Synthesis of [Cu(APPZ)(ClO4)](ClO4)
It proves that regardless of the solvent system employed, the final product is the same, containing
the APPZ core unit of DBPP (vide infra).
3.2. Description of Crystal Structure
(C4H8N2)[C3H6-N=CH-C6H4OC12H25]2 (DBPP) (1) crystallizes in the triclinic space group Pī.
The crystal structure (Fig. 1A) depicts a symmetrical molecule with four potential coordination
sites linked to the extended piperazine bridge. The crystal packing of the architecture in 1 is
shown in Fig. 1B. Crystal data and structure refinement parameters of DBPP are presented in

11
Table 1. All relevant bond lengths and angles with their estimated standard deviations are given
in Table 2. The distance of the derived C=N bond is 1.274(2) Å, in line with typical carbon-
nitrogen double bonds (1.279 Å). The torsion angle C(7)-N(10-C(8)-C(9) is 130.4(2)° [35]. The
interatomic distances CC and CN of the piperazine bridge of 1.500(2) and 1.463(2) Å,
respectively, are within the range expected for these single bond types of CC 1.524 and CN
1.469 Å, respectively [35,36].
The piperazine nitrogen atoms are tetrahedral, as shown by the valence angles around these
atoms, in the range of 109, corresponding to sp3 hybridization (between 106.81(14) and
112.55(14)) [37]. The piperazine ring can be thought of as composed of three moieties: plane A
composed of N(2)C(11)C(12), ring B composed of C(11)C(12)C(11)’C(12)’, and plane C
composed of N(2)’C(11)’C(12)’. The dihedral angle between planes A and B is 30.89°, equal
to that of B and C, and that of A and C is 0, thus indicating the stable chair conformation of the
piperazine ring [38]. The torsion angles CCNCexo, C(12)C(11)N(2)C(10), of 177.4(1)°
indicate that the heterocyclic N substituents are in equatorial positions. All of these arguments
support the notion that the geometry of the molecule is characterized by the “chair” conformation
of the piperazine core, with the two 4-dodecyloxy-benzylidene moieties pointing away from the
diamine bridge [39]. The benzene rings (Structure S1) of two neighboring molecules are
arranged in an off-centered parallel displaced manner, with a distance of 2.790 Å between their
mean planes. The π-π stacking distance is 5.502 Å and the angle between the centroid…centroid
vector and the normal to the ring plane is 30.68°, thus suggesting the existence of a weak
interaction between the slipped interlayers occurring as shown in Fig. 1C [40]. The
aforementioned interactions prove that the stability of the crystal package is supported by van der
Waals forces between the parallel alkyl chains, hydrogen-bonding, and π stacking interactions.
.
Compound [{Co(ZOPP)}(ClO4)]2 (CH3OH).2(CH3)2SO (2) crystallizes in the monoclinic system,
space group C2/c. Fig. 2A shows the crystal structure and the atom numbering scheme of the
complex. The position of the molecules in the unit cell can be seen in Fig. 2A. All relevant bond
lengths and angles with their estimated standard deviations are given in Table 2.
The compound consists of a well-defined complex unit [Co(ZOPP)], perchlorate counter ions,
dimethylsulfoxide and methanol solvent molecules. In view of the fact that there is one methanol
CH3OH molecule and two molecules of (CH3)2SO per two mononuclear Co(III) assemblies, the
molecular formulation of the entire compound is provided through the molecular formula cited

12
above. The actual mononuclear assembly, however, of the central metal ion remains discrete,
and it is used as such throughout the manuscript, reflecting the form of the complex unit
[Co(ZOPP)]. The central Co(III) ion is hexacoordinated and the environment can be described as
octahedral, with the N4 set arising from two imino nitrogens and two piperazine nitrogen atoms
bordering the basal plane, and two phenolic oxygens in the apical positions. The cobalt ion and
the N4 donor set are almost coplanar, with a 0.010 Å deviation of the cobalt ion from the plane.
The CoNimino distances of 1.934(2) and 1.945(2) Å denote bond lengths shorter than Co–Npip
distances of 1.981(2) and 2.004(2) Å, albeit in the range reported for similar compounds [41,42].
The two phenolic oxygen atoms occupy the apical positions, with the Co–Ophenolic distances being
between 1.893(2) and 1.898(2) Å, almost equal, shorter than those reported for similar
compounds [43,44]. Further, the angles between the apical phenolic oxygen atoms, Co(III) and
the donor nitrogen atoms in the basal plane, which should be equal to 90°, are between 87.64(9)
and 94.31(9)°. The trans angle O(1)Co(1)O(2) of 177.68(8)° is close to linearity, emphasizing
that both oxygen atoms are placed in the vertices of an almost undistorted octahedron.
The two CH2CH2 piperazine straps form a double five-membered chelate ring with the Co(II)
central ion. As a consequence, the “bite” angle N(2)Co(1)N(3) of 73.18(10)° and metal-
piperazine nitrogen distances are smaller than those reported for similar compounds [42].
Torsion angles C(12)–C(11)–N(2)–C(10)propyl of 164.8(3) – 170.3(2)° can also be noticed. These
structural results confirm the boat conformation of the piperazine moiety [45] and account for the
“reinforced” effect induced by the presence of two straps between the two piperazine nitrogen
donors [46,47].
The alkyl chains -C10H25 are disposed almost linearly with respect to the phenyl rings, with the
torsion angles C(4)–O(3)–C(25)–C(26) and C(22)–O(4)–C(35)–C(36) being 179.0(3) and
157.9(3), respectively. The perchlorate anion is also present and shows Cl–O bonds ranging
from 1.389(3) to 1.425(3) Å. The perchlorate ion is not coordinated to the Co(III) ion and
exhibits typical bond angles in the range 102.73(19)–124.8(2)°.
Molecular packing along the c axis (Fig. 2B) shows a three-dimensional open-framework
arrangement, with large ellipsoidal channels/holes of 15.930 x 12.823 Å, filled with ordered
solvent molecules. The architecture is held together by van der Waals forces and hydrogen bonds
[48,49]. Moderate intermolecular hydrogen bonding also occurs between O(11)H(494)-O(9)
(2.932 (5) Å) atoms of methanol and dimethylsulfoxide molecules. The C-H contacts, treated as

13
weak hydrogen bonds, are also considered to play a significant role in crystal packing and
molecular conformations [50].
Compound [Cu(APPZ)Cl]Cl (3) crystallizes in the monoclinic system, space group P21/n. The
crystal structure of the complex (Fig. 3A) shows that the DBPP ligand dissociates into the
fundamental core APPZ, whereupon the four N atoms of the APPZ are in the same plane
(N(1)N(2)N(3)N(4) plane), which is parallel to the c axis. The N(1)N(2)N(3)N(4) plane is the
base of the pyramid, with the pentacoordinated Cu(II) ion serving as the central metal. The Cl
atom sits at the apex of the same pyramid, 2.852 Å away from the base. The piperazine core is in
a boat conformation and separated into two dissimilar moieties. The CuNamine of 2.007(2) and
2.009(2) Å bond lengths are just shorter than the Cu–Npip bond distances of 2.057(2) and 2.062(3)
Å, but in the range reported for similar compounds [51]. The hexagonal Cu(NCH2CH2CH2N)
rings on the two opposite sides of the Cu(II) center have their central CH2 group residing on
different planes. The angles of N-Cu-N are the same for both rings and smaller than 120o (~95o),
thereby leading to deformed hexagons. The copper ion is located above the N(1)N(2)N(3)N(4)
plane at 0.391 Å.
The position of the molecules in the unit cell can be seen in Fig. 3B. Selected bond distances and
angles are given in Table 2. The molecular packing along the a axis (Fig. 3B) shows an
intercalated array of zig-zag molecules. The architecture is held together by van der Waals forces
and weak hydrogen bonds between the amine N(4) atom and the chloride Cl(2) ion at 3.258 Å,
with an angle N(4)H(103)….Cl(2) of 146.33o.
Collectively, compounds 2 and 3 have the main core APPZ fragment unit containing the
piperazine moiety in a boat conformation. The “bite” angle Npip-metal ion –Npip for all of these
compounds is around 73°, smaller than 90°, as expected for a “reinforced” effect induced by the
presence of two straps between the two piperazine nitrogen donors. This effect leads to a
distortion of the central angles, namely Npip-metal ion-Namino/imino at around 95o, smaller than
120o. The coordination geometry of the metal ions is different in the two assemblies, however
the basal plane is the same for all three complexes: the cobalt ion in 2 has an octahedral geometry
with the phenolic oxygen atoms in the apical position, whereas the copper ion in 3 has a
pyramidal geometry with the apical position occupied by a chloride ion. Moreover, the cobalt ion
in 2 is almost coplanar with the equatorial nitrogens, with a 0.010 Å deviation from the plane,
whereas the copper ion in 3 is located 0.391 Å above the plane.

14
3.3. FT-IR spectroscopy
The FT-IR spectrum of 1 shows the characteristic band of the C=N bond at 1637 cm-1 and intense
bands at 2922 and 2851 cm-1 attributed to CH2 and CH3 stretching vibrations belonging to the
dodecyloxy-(C12) alkyl chains (Fig. S1A). The bands around 2750–2850 cm-1 are assigned to the
(C-H) vibrations of the piperazine fragment [18].
The spectrum of 2 provides valuable information on the nature of the functional groups bound to
the metal ion (Fig. S1B). The strong band at about 1625 cm-1, corresponding to the (C=N)
stretching vibrations of the free ligand, is shifted to lower field values in the complex at 1608 cm-
1. The phenolic (C-O) stretching vibrations at 1228 cm-1 in the Schiff base is shifted toward
lower frequencies in the complex, at 1213 cm-1. This shift confirms the involvement of the
phenolic oxygen in the formation of the C-O-M bond. Coordination of the piperazine nitrogens
to the Co(III) ion is attested to by the absence of the (C-H) of N-CH2 (alkyl) stretching vibration
modes at about 2750 cm-1 in the IR spectrum of the complex [52]. Strong vibration bands at
1117–1079 cm-1 and a sharp medium band at 621 cm-1 can be attributed to the non-coordinated
perchlorate counter ion [33,53]. In the low frequency region, new weak bands from the complex
spectrum at 529 and 450 cm-1 can be assigned to the M–O and M–N bonds, respectively.
The spectrum of 3 shows the characteristic bands of the piperazine ligand (APPZ), with changes
in the vibration bands due to metal ion coordination (Fig. S1C). Thus, the bands in the range
3260–3104 cm-1 are assigned to the antisymmetric and symmetric ν(NH) stretching vibrations of
the NH2 group. The intense band at 1602 cm-1 is attributed to the δ(NH) vibration. The bands at
2934 and 2863 cm-1 are assigned to the ν(CH2) mode of the propylene groups. The characteristic
bands of the piperazine moiety around 2800–2700 cm-1 disappear, suggesting involvement of the
piperazine nitrogen atoms in metal ion coordination, with piperazine adopting a boat
conformation [52].
3.4. UV-Visible Spectroscopy
The electronic spectrum of the free ligand ZOPPH2 was recorded in fresh DMF solution (10-4 M).
The spectrum shows two intense bands at 277 and 302 nm, attributable to π-π* transitions, and a
medium intensity band at 385 nm, corresponding to an n–π* transition [54]. The spectrum of
DBPP (1) was recorded in chloroform (10-4 M). It shows an intense band at 270 nm and a
shoulder around 296 nm attributed to π-π* transitions. In the electronic spectrum of 2, the d-d
transitions characteristic for octahedral Co(III) ion can be observed. Thus, the bands at 506, 578

15
1
1
1
1
and 655 nm attributable to A1g → T1g, A1g →1Eg and 1A1g → A2g transitions, respectively, are
1
present. The pronounced splitting of T1g, is indicative of a trans-octahedral geometry [55,56],
which was confirmed by the X-ray crystal structure. The position and intensity of these bands
indicate an octahedral environment around the Co(III) center [57,58]. The electronic spectrum of
3 shows a broad band at 619 nm, attributed to a d-d transition, corresponding to a five-coordinate
Cu(II) center with a nitrogen-rich coordination environment [57].
3.5. NMR spectroscopy
1
The H-NMR spectra of both ligands (ZOPPH2 and 1) [14,18] reveal the distinct nature of the
emerging compounds through the presence of azomethine protons at 8.13 and 8.19 ppm for
ZOPPH2 and 1, respectively. New signals appear in the range 1.77–3.60 ppm, attributable to
protons from the bis(N-propyl)piperazine unit.
The 1H-, 13C-, 1D and 2D-NMR spectra of 2 on single crystals in CDCl3 are shown in Fig. S2A-
D). The proton and carbon assignments are presented in Table 1S. The HSQC spectrum is used
to assign each carbon atom in the aromatic region, but it is very difficult to assign exactly each
1
hydrogen atom to the corresponding carbon atom from the aliphatic chains. From the H-NMR
spectrum in solution, it can be seen that the complex molecule is distorted. Because of the
twenty hydrogen atoms of the complex there are roughly seven signals at 4.2, 3.71, 3.07-3.2,
2.83, 2.56-2.66, 2.43-2.56, 1.99 ppm, instead of four or maximum five signals (three signals from
the propyl chain and one or two from the CH2 piperazine moiety) corresponding to a symmetrical
molecule. This distortion is also confirmed through the HSQC spectrum. All twenty hydrogens
are connected to ten carbons, with eight carbons being present in the range 50-60 ppm and two
carbon atoms being present at 21.3 ppm. If the molecule were symmetrical, we would only have
four signals in the 50-60 ppm range and one signal at 21.3 ppm.
3.6. Photoluminescence
The solution fluorescence spectra (C = 10-4 M) of the free ligands ZOPPH2 and DBPP (1) show
emission peaks centered around 436 nm and 394 nm, respectively. They are both attributed to
intraligand charge-transfer (ILCT) transitions (Fig. 4) [59]. It is noted that, under the
experimental conditions employed, the spectrum of 1 exhibits enhancement of the emission
compared to ZOPPH2, thereby denoting the distinct nature of one ligand over the other.
Therefore, the presence of the phenolic moiety in the ZOPP ligand appears to cause quenching of
the emission [60,61]. The spectrum of the Co(III) complex (2) shows the same emission

16
wavelength (437 nm) as the free ligand ZOPPH2, with a slight increase in the fluorescence.
Fluorescence enhancement through complexation could be attributed to the increase in the
rigidity of the ligand, which can minimize the loss of energy through vibrational motions and
increase the emission efficiency [62]. Complex 3 shows a similar emission spectrum to the free
ligand 1, further exhibiting a decrease in the emission intensity, signifying the presence of the
APPZ core bound to the metal center (Table 2S).
3.7. Computational analysis
The X-ray crystal structure of the DBPP ligand (1) indicates that the piperazine moiety is in the
chair conformation. The same conformation was observed for the piperazine core-containing
APPZ ligand, which appears to be a key unit in the synthesis of the DBPP and ZOPPH2 ligands.
In order to identify the key structural features responsible for the preferred chair orientation, the
lability of the carbon-nitrogen double bond and global chemical reactivity of molecules, the
geometric (bond distances and bond angles, Table 3S) and electronic parameters (heat of
formation (ΔHfθ), HOMO and LUMO energies, HOMO-LUMO gap energies (ΔE), the reactivity
Fukui indices, hardness (η), softness (S), electronegativity (χ), chemical potential (μ) and global
electrophilicity (ω)) were analyzed in comparison to the experimental results (Figs. S3-S5,
Tables 4S-5S). It is well known that the conformational interconversion pathways of a six-
membered ring system are related to the nature, orientation, type, and number of substituents
attached to the system [63,64,65]. In this regard, the most stable conformation for the APPZ,
DBPP (1), and ZOPPH2 ligands was found to be the chair conformation, in accordance with the
experimental data (section 3.2). As can be seen from the results of AM1 calculations (Table 3S),
⁰
the chair conformers of all three ligands have the lowest heat of formation (ΔHf of -9.7782
kcal/mol for APPZ, -136.937 kcal/mol for DBPP and -195.9998 kcal/mol for ZOPPH2) compared
to the boat conformers; thus, the specific conformation is expected to be significantly populated
at room temperature. Moreover, the N-C and C-C bond distances, as well as bond angle values of
the piperazine moiety of the title ligands, are in very good agreement with the experimental
measurements, with a negligible error of 0.015 for the chair conformers (Table 3S). In addition,
all semi-empirical method results on the heat of formation values (as derived through AM1, RM1
and PM3 methods, Table 4S) have designated the ZOPPH2 as the most stable ligand in
comparison to the APPZ and DBPP ligands. The C=N double bond distance in the DBPP and
ZOPPH2 ligands has values of 1.280 Å and 1.265Å, respectively, which are close to the reported

17
experimental value of 1.279 Å [66-68]. The small difference between the two measured C=N
bond lengths could be ascribed to the presence of the phenolic moiety on the aromatic ring of the
ZOPPH2. On the other hand, these values could suggest potential lability of the C=N double
bond of DBPP compared to ZOPPH2 under appropriate reaction conditions. The C=N double
bond of ZOPPH2 seems to be slightly stronger than that of the DBPP ligand, requiring high
energy to break [29]. The highest occupied molecular orbital (HOMO), the lowest unoccupied
molecular orbital (LUMO) and all descriptors derived from the calculated HOMO and LUMO
energies are important stability and chemical reactivity indicators [30-32]; the values for the
orbital energies were obtained using hybrid DFT with the B3LYP/6-311++ level of theory (Table
4S). These parameters are considered the main orbitals of chemical stability, indicating the
electron-donor or electron-acceptor capacity of a molecule and implicitly its susceptibility to
nucleophilic or electrophilic attack. A high HOMO energy value corresponds to a molecule with
a strong nucleophile behavior, whereas low LUMO energy values correlate with a strong
electrophilic character [30,31]. The HOMO-LUMOs, electrostatic potential, and charge
distribution maps of ligands are drawn in Fig. 5 and Figs. S6-S7, respectively. The HOMO
orbitals of the DBPP ligand are exclusively located on the piperazine moiety, with the N atoms
being the most nucleophilic sites, whereas the LUMO orbitals are distributed around the same
fragment (the aromatic ring and the C=N double bond), with the N and C atoms being the most
electrophilic sites. In case of the ZOPPH2 ligand, the HOMO and LUMO orbitals are distributed
on the same regions (the phenolic ring and the C=N double bond), indicating the same ability to
donate or attract electrons. The charge distribution and electrostatic potential plots of the
ZOPPH2 ligand (Fig. 5c, Figs. S6-S7) indicate high accessibility of the O atom of the phenolic
ring, with an excess of negative charge (red color) around and a powerful electrostatic potential
created. The HOMO-LUMO gap energy, an important stability index, suggests that the ZOPPH2
is the most reactive one, whereas the DBPP is slightly more stable during a chemical reaction.
The same DBPP stability is evidenced by the slight increase in the hardness value (η of 4.753eV).
This DBPP stability, in turn, indicates low chemical reactivity, which could probably mean that
the interaction with other molecules would generate very weak bonds. The ZOPPH2 molecule
reactivity is also confirmed by the high value of chemical potential (µ of -3.328eV) and low
value of chemical hardness (η of 4.637eV). The latter descriptors are significant indicators of the
overall reactivity of the molecule. The highest value of electronegativity (χ of 3.328eV)

18
exhibited by the ZOPPH2 ligand is probably due to the presence of the highly electronegative O
atom attached to the aromatic ring compared to the DBPP ligand. These observations confirm
once more the superior stability of the ZOPPH2 complex with a metal ion when compared to the
DBPP one. Analysis of the Fukui indices, along with the distribution of charges (Tables 6S-8S,
Figs. S3-S5), projects a more complete image of ligand reactivity. Tables 7S-8S indicate that all
N, O, and some of the C atoms carry negative charges. This observation recommends these
atoms as the negative charge centers, which could donate electron density to the metal atoms,
thus forming coordination bonds. The values of Fukui indices, listed in Tables 7S-8S, point out
that, in both cases, the N and some C atoms are the most susceptible sites for electrophilic or
nucleophilic attacks; these sites present the highest values. The information emerging from the
theoretical study is consistent with the experimental approaches.
4. Discussion
4.1 Ligand structure-specific metal ion chemical reactivity-The nature of the ligand
Promotion of metal ion chemical reactivity toward Schiff base ligands relies heavily on the
chemical characteristics of its anchor groups, defining to a significant extent the a) reactivity
toward the metal, and b) details of the binding process in terms of thermodynamics and kinetics.
In the present case, the two major Schiff base ligands ZOPPH2 and DBPP are configured
appropriately around a basic piperazine core (APPZ) with bilateral reactive antennae. In the
ZOPPH2 ligand, the flanks to APPZ contain a basic benzene core with two substituents of notable
utility to the chemistry toward the employed metal ions: a) an alkyloxy group, with the alkyl
chain moiety in para-position with respect to the aldehyde moiety, extending to ten carbon atoms
or longer (difference between ZOPPH2 and DBPP), and b) a phenol moiety in ortho-position with
respect to the aldehyde moiety. The most significant difference between ZOPPH2 and DBPP is
the existence of the phenolic anchor group in the former species, thereby affecting profusely
metal ion chemical reactivity (vide infra).
Two metal ions, namely Co(II) and Cu(II), were employed in this work, with the products
characterized through analytical (elemental analysis, molar conductivity), spectroscopic
techniques (FT-IR, UV-Visible, NMR, Luminescence) and X-ray crystallography. Co(II) reacted
with ZOPPH2, affording the mononuclear Co(III) complex 2. ZOPPH2 has the same coordination
ability as [N,N’-bis[(2-hydroxybenzilideneamino)-propyl]-piperazine] (HBPPH2) and acts as an

19
N4O2 hexadentate ligand. The metal center resides in an octahedral environment as attested to
through the literature [68]. There, in fact, it is shown that presence of strong π-acceptor
constituents, employed in the synthesis of salen complexes, facilitates formation of octahedral
Co(III) complexes [8]. In the absence of any additional coligand and by choosing an appropriate
diamine with additional donor atoms, aside from the two nitrogens, HBPPH2 forms mononuclear
Co(II) complexes [69,70] with an octahedral geometry, exemplified by an N4O2 set of donor
atoms in the ligand. Using the same ligand HBPPH2 and involving an oxidant agent, such as
sodium perchlorate, Kuma reported the formation of a Co(III) complex [69].
4.2 Metal ion oxidation state and reaction conditions in multiparametric systems
The synthetic chemical reactivity of Co(II) and Cu(II) toward the well-defined ligands ZOPPH2
and DBPP, gave rise to materials with either the same or different oxidation state of the metal
center. To that end, the mononuclear assembly in 2 contains an oxidized metal ion center, i.e.
Co(III), with the doubly deprotonated ligand wrapped around it. The cationic charge of the
metal-organic assembly is counterbalanced by a perchlorate anion. Such events are not unusual
in the chemistry of the investigated metal ions. In fact, there are reports of Co(III) complexes
containing Schiff base ligands, starting with Co(II)-acetate and pyridine as an ancillary ligand,
synthesized and isolated under aerobic conditions [44]. Using the same Co(II) salt, Banerjee et
al. [71] obtained a trinuclear mixed-valence Co(III)-Co(II)-Co(III) complex through partial aerial
oxidation, when NH4SCN was used in the reaction. To obtain the Co(II) complex, the trinuclear
complex was allowed to react with hydrazine as a reducing agent. Hariharan et al. [16] obtained
a Co(II) complex bound to a Schiff base ligand, using CoCl2 and a nitrogen atmosphere, to avoid
oxidation of Co(II) to Co(III). It appears, therefore, that reaction conditions involve numerous
variables that could influence the fate of the reaction pathways and thus the derived product(s).
By the same token, oxidation of Cu(I) to Cu(II) was intensively investigated and can depend on
different factors, such as ligand nature, the coordinating solvent(s) or the counter ion, that allow
the metal ion to assume tetrahedral coordination when Cu(I) species are stabilized or higher
coordination number geometries preferred by Cu(II) species [72-75]. To obtain Cu(I) species, it
is also important to employ inert reaction conditions [76,77]. Collectively, the complexity of this
multiparametric system of interactions, with factors a) affecting among others the oxidation state
of the central metal ion in a complex assembly, and b) exemplified through the employment of

20
the aforementioned reaction participants, contains a wealth of information at the synthetic and
structural level, and is currently under investigation in our labs.
4.3 The importance of phenolic OH in the ligand structure. Parameters and consequences
in metal ion reactivity
Initially, the investigation was directed toward the effect of phenolic OH in the ligand, when
divalent metal ions are presented to it. To that end, when ZOPPH2 reacts with the divalent
Co(II), the integrity of the ligand is maintained upon metal ion coordination. When ZOPPH2 is
presented to Cu(II), the resulting materials are dinuclear complexes with a polymeric or discrete
structure, depending on the metal:ligand molar ratio [14]. In fact, hexadentate ZOPPH2 can act
as a binucleating ligand through a two (N,O) donor atom set or a bis-tridentate ligand through
two (N,N,O) donor atom sets, depending on the involvement of the piperazine core nitrogen in
metal coordination, with the piperazine moiety being in the chair or boat conformation in the
case of mononuclear complexes.
When DBPP (a ligand similar to ZOPPH2 yet devoid of the phenolic anchor moiety) is used with
Cu(II) (vide infra), it appears to break apart in solution, thereby resulting in the formation of the
amine constituent (essentially the core structure of the original APPZ ligand), with the latter
substrate poised to bind the metal ion in its original oxidation state. A good precedent on such
reactivity was previously reported in the literature, exemplifying interactions of Cu(II) toward a
similar ligand containing the same core, with the remainder of the structural motif containing no
phenolic OH in the o-position of the benzene ring bearing the azomethine –CH=N– moiety [52].
In contrast to that, when Co(II) is presented to that same ligand, complexation does not occur. It
appears, therefore, that the presence of the phenolic group facilitates coordination to the metal
center, thereby stabilizing that metal ion in its complex assembly (vide infra). Absence of that
moiety results in disassembly of the organic ligand in the presence of the metal ion. The above
observation is amply shown to emerge in the case of the Cu(II) system, in which the nature of the
starting reagent also appears to play a role. To that end, evidence projecting the influence of the
two parameters (phenolic moiety and starting reagent) on the chemical reactivity amounting to
changes occurring in the ligand structure, in the presence of the metal ion, can be gathered
through vibrational spectroscopy. In fact, FT-IR spectra of the materials arising from the
‒
reactivity investigation of the Cu-DBPP system, with the copper starting reagent containing NO3
or Cl‒ as a counter ion, seem to be different from those of the Cu-APPZ system (data not shown).

21
‒
On the contrary, in the case of the ClO4 anion present in the starting material, the emerging Cu-
DBPP and Cu-APPZ complexes from the two systems are similar to each other, as noted
previously [18]. Toward that end, the FT-IR spectra of the latter two cases reveal that the
characteristic bands of the free DBPP ligand are not present, indicating rupture of the imino
bonds and formation of N,N,-bis(3-aminopropyl)piperazine perchloratocopper(II) perchlorate, as
previously reported [18]. Consequently, it appears that a) upon reaction between DBPP and
Cu(ClO4)2, the product emerges different from that of the same reaction employing Cu(NO3)2 and
CuCl2 and is identified as [Cu(APPZ)(ClO4)](ClO4). The fact that the presence of the phenolic
moiety does make a difference in the chemical reactivity toward Cu(II), linked to the retention of
the integrity of the ligand as a well as the oxidation state of copper upon coordination, has been
amply exemplified through the isolation and crystal structure of [Cu9L6(μ3-
.
ClO4)2](ClO4)4 4CHCl3 (N,N’-bis[(2-hydroxybenzilideneamino)-propyl]-piperazine (H2L)), with
the coordinated ligand containing phenolic moieties on both sides of the piperazine ring [78].
Therefore, in the absence of the phenolic moiety, the nature of the Cu(II) starting material
appears to be important in the investigated chemical reactivity; b) DBPP breaks apart, at the
imino junction, into its constituents in the presence of Cu(ClO4)2. The product of the reaction is
the [Cu(APPZ)(ClO4)](ClO4) complex. The reaction product is confirmed through chemical
reactivity of Cu(ClO4)2 and APPZ; and c) the metal binding ability of the core APPZ ligand is
confirmed through the emergence of [Cu(APPZ)Cl]Cl in the reaction between CuCl2 and APPZ.
The observed behavior of the chemical reactivity of the two ligands with Cu(II), and more so in
the absence of the phenolic OH moiety, appears to be supported by DFT theoretical calculations.
In fact, quantum mechanical calculations on DBPP and ZOPPH2 ligands suggest that the
−CH=N− bond itself could exhibit a differentially distinct chemical reactivity under appropriate
reaction conditions (i.e. presence of a metal ion). Both semi-empirical AM1 and DFT methods
point to the lability of the C=N bond, a property that materializes with Cu(II) present in the
reaction medium, resulting in the isolation of the Cu-APPZ complex. The importance of that
collective theoretical and experimental work on well-defined molecular factors influencing
chemical reactivity of the Schiff ligands toward metal ions, testifies to the need of a)
identification of such structural and electronic factors guiding the relevant synthetic reactivity
toward materials with new applications (e.g. light emission, metallomesogens, liquid crystals),
and b) extending further the quest for analytical parametric description and design of organic

22
substrates-synthons, such as the Schiff ligands employed here, in the exploration of metal-
specific interactions, pertaining to other areas of research involving catalytic transformations.
Whether or not there is specificity of the chemical reactivity amounting to rupture of the C=N
bond by only Cu(II) remains to be seen. Efforts toward establishment of the same or dissimilar
reactivity in the presence of Co(II) are currently probed into in our labs.
The collective chemical reactivity in the investigated systems reveals that a) specific structural
and electronic factors are involved in the formulation of the products arising when Schiff base
ligands, such as the ones reflected into ZOPPH2, DBPP, and APPZ, are brought to react with
Cu(II) and Co(II); and b) salient structural features of both ligands and metal ions, in conjunction
with defined reaction conditions, could be factored into the reactivity patterns leading to
structurally distinct isolable crystalline products. To that end, the chemical reactivity of other
redox active metal ions is worth looking into when Schiff base ligands with the aforementioned
substituent changes in their structure are employed. The accrued knowledge from the employed
systems is expected to lay the groundwork for the development of well-defined materials
exhibiting distinctly differentiated metal coordination and lattice properties.
5. Conclusions
The synthetic pursuit of potential mesomorphic Schiff base ligands, comprised of a piperazine
core and a variably composed alkyloxy-containing salicylaldehyde moiety, led to a family of
metal ion binders (ZOPPH2, DBPP) reacting in a distinct fashion with divalent Co(II) and Cu(II).
The derived title compounds 1-3 reveal that a) in ZOPPH2 and DBPP, N and O anchors on each
side of piperazine bridge generate a symmetrical, conformationally flexible, structure poised to
promote metal ion binding; b) the abutting aromatic phenolic moiety stabilizes the arising
metallacyclic rings, playing a significant role in the reactivity pursued. In ZOPPH2, it facilitates
octahedral coordination to the oxidized Co(III) center. When absent (DBPP), the ligand breaks
apart in solution in the presence of the metal ion, thereby affording APPZ further coordinating to
the metal ion; c) the alkyl chains on the phenolic core contribute to the hydrophobicity of the
entire ligand and satisfy the requirements of these bulky substituents into optimally arranged,
ordered and oriented metal complex assemblies in the emerging lattice architecture; d) DFT
theoretical calculations on the employed ligand structures project a labile azomethine bond,
variably influenced by the presence of the metal ion, thus lending credence to the rupture upon

23
metal ion reactivity in the absence of the phenolic moiety. The specificity of that reactivity with
respect to the nature of the metal ion requires further investigation; e) the influence of the starting
reagent transition metal and thus the emerging counter ion on the chemical reactivity as well as
the (an)aerobic atmosphere conditions are variables that might contribute to the observed
reactivity. The aforementioned factors formulate the parametric investigation of the discovered
reactivity, meriting further perusal into new well-defined metal-organic materials
(metallomesogens) with specified physicochemical profile(s) and are thus currently being
investigated in our labs.
Acknowledgment
We are thankful to the Romanian Academy of Science, (Project 3.1.) for the financial support
(Program No. 3; Project No. 3.1).
Supplementary Material
CCDC 743573 (1), CCDC 978591 (2), and CCDC 1846243 (3) contain the supplementary
crystallographic data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB21EZ, UK; fax: (+44) 1223-336-033; or
deposit@ccde.cam.ac.uk).
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28
Tables

29
Table 1: Crystallographic data on compounds [(C2H4N)2(C4H7N)2(C6H4O)2(C12H25)2] (DBPP) (1),
.
[{Co(ZOPP)}(ClO4)]2 (CH3OH).2(CH3)2SO (2), and [Cu(APPZ)Cl]Cl (3)
Compound
Chemical formula
Mr
1
C48H80N4O2
745.19
2
3
C93H156Cl2Co2N8O19S2 C10H24Cl2CuN4
1943.21
334.78
Crystal system
Space group
Triclinic
Pī
Monoclinic
Monoclinic
C2/c
P21/n
Temperature (K)
295
295
295
a (Å)
b (Å)
c (Å)
5.502 (2)
6.672 (3)
32.906(16)
42.188 (2)
7.9535 (8)
13.0685 (6)
19.8524 (10)
14.3181 (15)
13.5392 (14)
α (°)
85.86 (3)
89.70 (3)
76.28 (3)
1170.4 (9)
1
90
90
β (°)
90.153 (3)
101.636 (3)
90
γ (°)
90
V (Å3)
10945.3 (10)
1510.1 (3)
4
Z
4
Radiation type
µ (mm−1)
Crystal size (mm)
Mo Kα
Mo Kα
Mo Kα
1.79
0.06
0.45
0.53 × 0.22 × 0.12
0.26 × 0.14 × 0.12
0.27×0.22×0.15