Experimental and quantum-chemical studies of anabasine complexes with copper(II) and zinc(II) ions

Article abstract of DOI:10.1016/j.poly.2014.10.008

Anabasine copper(II) and zinc(II) complexes have been studied using experimental methods and quantum-chemical studies. Preferred coordination centers and possible structures of the complexes have been determined. It has been revealed, that the main coordi

Full text of DOI:10.1016/j.poly.2014.10.008

Polyhedron 85 (2015) 841–848
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Experimental and quantum-chemical studies of anabasine complexes
with copper(II) and zinc(II) ions
⇑
Romualda Bregier-Jarze˛bowska, Karolina Malczewska-Jaskóła, Wojciech Jankowski, Beata Jasiewicz ,
Marcin Hoffmann, Anna Ga˛sowska, Renata Jastrza˛b
Adam Mickiewicz University, Faculty of Chemistry, 61-614 Poznan, Umultowska 89b, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Anabasine copper(II) and zinc(II) complexes have been studied using experimental methods and quan-
tum-chemical studies. Preferred coordination centers and possible structures of the complexes have been
determined. It has been revealed, that the main coordination center for anabasine complexes with both
Cu(II) and Zn(II) ions is the piperidine nitrogen atom, while in zinc complexes also the nitrogen atom
from the pyridine residue of anabasine is engaged to some small degree. In both systems investigated
preferable formation of hydroxo-complexes was observed.
Received 23 July 2014
Accepted 11 October 2014
Available online 30 October 2014
Keywords:
Anabasine
Cu(II) and Zn(II) complexes
Potentiometry
Ó 2014 Elsevier Ltd. All rights reserved.
Spectroscopic methods
Quantum-mechanical calculations
1. Introduction
the core of numerous biological processes. Several studies on the
preparation and characterization of metal complexes with nicotine
Anabasine (1) (also called neonicotine, Fig. 1), is one of the sev-
eral minor tobacco alkaloids found in wild tree tobacco Nicotiana-
glauca Graham (solanaceae), or in Anabasis aphylla L.
(chenopodiaceae), poisonous plants. It has been established to be
have been reported but examples of anabasine complexes and its
coordination to metal ions present in living organisms are lacking
in the literature. Recently, we have obtained N-methylanabasine
complex with zinc salts. Its X-ray structure shows that two N-
methylanabasine units and two bromide anions form a tetrahedral
coordination around the Zn2+ cation [13]. Herein we report the
complexation reaction of anabasine with Cu((II) and Zn(II) salts
in aqueous solution. The potentiometric, spectroscopic as well as
theoretical methods were used to determine the composition, sta-
bility constants and coordination mode of anabasine complexes
with Cu(II) and Zn(II). Moreover, the synthesis and spectroscopic
properties of anabasine-ZnCl2 complex in solid state is also
reported.
selective
a7-nAChR (neuronal nicotinic acetylcholine receptor)
agonist in an animal model with low toxicity for the potential
treatment of schizophrenia [1–3]. As a nAChR agonist, anabasine
blocks the function of nAChRs via desensitization in a mechanism
similar to that of nicotine. Compound 1 possess a wide spectrum
of biological activity [4–7] and is used as precursors for drug design
and pesticide preparation [8–10]. Anabasine hydrochloride is
employed in medicine as an antismoking agent, anabasine phos-
phorylates are compounds showing antitumor, antituberculosis,
hepatoprotective, antiviral, antibacterial, antifungal, anticholines-
terase, and other activities [11]. Dithiophosphinates of anabasine
are potential drug candidates and precursors for the synthesis of
pharmaceutically important compounds [12]. Anabasine is impor-
tant also due to its role in smoking. Together with nicotine and
nornicotine it is reported to provide a physiological stimulus,
which make the use of tobacco product active.
In this paper we can additionally verify the hypothesis that dif-
ferent ways of coordination of metal ions by nicotine and anaba-
sine can be reflected in their biological activity.
2. Experimental
Nowadays, there has been a growing interest in the study of
interactions of metal cations with organic and bioorganic
compounds. Complexes of metal and biological compounds are at
2.1. Materials and synthesis
Copper(II) nitrate(V) trihydrate (purity 99%) from POCh–Poland
was purified by recrystallisation from water. Zinc (II) nitrate(V)
hydrate (purity 99.99%), (ꢀ)-anabasine (purity P 97%) and ZnCl2
(purity 98%) were commercial products of Aldrich.
⇑
Corresponding author.
http://dx.doi.org/10.1016/j.poly.2014.10.008
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

842
R. Bregier-Jarze˛bowska et al. / Polyhedron 85 (2015) 841–848
5'
the correct choice of a model are given in Ref. [16]. The modes of
4'
interactions were determined on the basis of the spectroscopic
investigations in the pH range in which particular complexes dom-
inate, as established on the basis of the equilibrium study. Distri-
bution of particular forms was determined by the HALTAFALL
computer program [17].
3'
2'
6'
HN
4
1'
5
6
3
2
N
2.3. Spectral measurements
1
Fig. 1. The structure of anabasine.
2.3.1. NMR spectroscopy
The samples for 13C NMR investigation (in solution) were pre-
pared by dissolving appropriate amounts of the species Cu(II) or
Zn(II) ions and anabasine in D2O. DNO3 and NaOD were used to
adjust pD of solutions, correcting pH-readings (obtained with a
pH meter N517 made by Mera-Tronik) according to the formula:
pD = pHreadings + 0.40 [18]. The concentrations of the ligands was
0.1 M, at the concentration rate of Cu(II) to the ligand of
M:L = 1:100 and Zn(II) to the ligand of 1:5. 13C NMR spectra were
recorded on an NMR Gemini 300VT Varian spectrometer using
dioxane as an internal standard. The positions of 13C NMR signals
were converted to the TMS scale. 1H NMR and 13C NMR spectra
of the complex obtained in solid state were recorded using CD3OD
as a solvents and TMS as an internal standard.
Melting points were determined on Melt-Temp II apparatus
(Laboratory Devices Inc.) Elemental analysis was carried out by
means of a Perkin-Elmer 2400 CHN automatic device.
The concentration of Cu(II) and Zn(II) ions were determined by
the method of inductively coupled plasma optical mass spectros-
copy (ICP MS) on Spectrometer Mass Varian.
Anabasine (162 mg, 1 mmol) and ZnCl2 (136 mg, 1 mmol) were
dissolved in 20 ml of methanol. The solution was evaporated until
the product started to precipitate. The resulting precipitate was fil-
tered off and recrystallized from ethanol. The purity of all the com-
plexes was checked by TLC.
[(C10H14N2)ZnCl2-H2O] White crystals. M.p.>250 °C. Yield: 78%.
Anal. Calc. for C10H17N2ZnO2Cl2: C, 37.97; H, 5.06; N, 8.86. Found:
C, 37.14; H, 5.21; N, 8.73%.
2.3.2. Vis spectroscopy
Vis spectra were taken on a UV/Vis Thermo Fisher Scientific
Evolution 300 Spectrophotometer. The samples were prepared in
H2O for the same ligand and metal concentration as in samples
for potentiometric titrations using a Plastibrand PMMA cell with
1 cm path length.
2.2. Potentiometric measurements
Potentiometric studies were performed on a Methrom 702 SM
Titrino with an autoburette.
A
glass electrode Methrom
6.0233.100 was calibrated in terms of hydrogen ions concentration
[14] with a preliminary use of phthalate (pH = 4.002) and borax
(pH = 9.225) as a standard buffers. The concentration of the ligand
in the titrated systems was 2ꢁ ꢂ 10ꢀ3 M. The concentration ratio in
the Cu:Ab and Zn:Ab systems were 1:1, 1:2 and 1:4. Potentiometric
2.3.3. EPR spectroscopy
The EPR spectra were recorded on an SE/X 2547 Radiopan Spec-
trophotometer at 77 K in glass capillary tubes of 130-ll capacity.
Samples were made a water:glycol mixture (3:1), and the Cu(II)
to anabasine ratio was 1:2. The concentration of the Cu(II) ions
was 0.005 M.
titrations were performed at the constant ionic strength
l = 0.1 M
(KNO3), at 20 1 °C under neutral gas atmosphere (helium), using
as a titrant CO2-free NaOH solution (0.21053 M). The selection of
the models and determination of the stability constants of the
complexes were made using the HYPERQUAD [15] computer pro-
gram. Five titrations were performed for each particular systems
and 100–350 points for each titrating curve were used for com-
puter analyses. The HYPERQUAD program use the nonlinear
method of least squares to minimize the sum (S) of squares of
residuals between the observed quantities (fobs) and those calcu-
lated on the basis of the model (fcalc).
2.3.4. EI MS spectroscopy
EI MS mass spectrum was recorded on AMD-Intectra (Harp-
stedt, Germany) D-27243 model 402 double-focusing mass spec-
trometer], ionizing voltage 70 eV, accelerating voltage 8 kV, mass
resolution 10000 at 10% valley. Samples were introduced by a
direct insertion probe at a source temperature of ꢃ150 °C. The ele-
mental compositions of the ions were determined by peak match-
ing method relative to perfluorokerosene.
S ¼ Rni¼1wiðfobs ꢀ fci alcÞ2
i
2.4. Quantum–mechanical calculations
where n is the number of measurements and wi is the statistical
weight.
The testing usually begins with the simplest hypothesis and
then in the next steps the models are expanded to include progres-
sively more species, and the results are scrutinized to eliminate
those species rejected in the refinement process. The criteria of
Calculations were performed, within DFT framework at M06/
SDD level [19,20]. The M06 method was selected based on the
results from extensive comparative studies, it is also recommended
for calculations for organometallic and inorganometallic com-
pounds and for noncovalent interactions [19]. To assess basis set
Table 1
Overall stability constants (log b), and equilibrium constants (log Ke) of complex formation in Cu(II)/Ab and Zn(II)/Ab systems.
System
Species
Equilibrium
log b
log Ke
Cu(II)/Ab
Cu(Ab)(OH)
Cu(Ab)2
Cu(Ab)2(OH)2
Zn(Ab)
Zn(Ab)2OH
Zn(Ab)2(OH)2
Cu + Ab + H2O ¡ Cu(Ab)(OH) + H+
Cu + 2Ab ¡ Cu(Ab)2
ꢀ1.75(7)
10.54(9)
ꢀ7.53(3)
4.54(7)
0.19(8)
ꢀ9.42 (8)
8.22
Cu(Ab)(OH) + Ab + H2O ¡ Cu(Ab)2(OH)2 + H+
Zn+Ab ¡ Zn(Ab)
Zn(II)/Ab
4.54
9.65
4.39
Zn(Ab) + Ab + H2O ¡ Zn(Ab)2OH + H+
Zn(Ab)2OH + H2O ¡ Zn(Ab)2(OH)2 + H+
Equilibrium constant of complex formation for Zn(Ab)2OH KZn(Ab)2OH = [Zn(Ab)2(OH)]/{[ZnAb][Ab][H2O]} = log bZn(Ab)2OH ꢀ log bZn(Ab) + 14 = 9.65.

R. Bregier-Jarze˛bowska et al. / Polyhedron 85 (2015) 841–848
843
effects on the results further calculations were performed with
composite basis set. For the hydrogen, oxygen, nitrogen, and car-
bon atoms we used 6–31 + G(d) [21] basis set, while for the zinc
and cooper atom we used SDD basis set in which zinc atom with
electrons core was described by pseudopotential. Vibrational anal-
yses for the optimized structures of isolated molecules were per-
formed to verify if a given structure corresponds to potential
energy minima and to calculate zero point vibrational energies,
entropies, and thermal corrections to Gibbs free energies. Finally,
to take into account the effect of aqueous solvent we used the
Polarizable Continuum Model (PCM) [22] with water chosen as sol-
vent as in experimental studies.
and the stepwise protonation constant of the nitrogen atom from
pyridine ring H+ + HAb = H2Ab+ (logbH2Ab ꢀ logbHAb = 3.51) is much
lower. In the further parts of publication the charges are omitted for
clarity. The protonation constants determined are consistent with
literature data, piperidine nitrogen is more basic than pyridine
nitrogen atom [23–25].
3.2. Potentiometric and spectral investigation of Cu(II)/anabasine and
Zn(II)/anabasine systems
In all systems investigated, at the molar ratio metal:ligand of
1:1, 1:2 and 1:4 (titration was performer from pH close to 2.5–
10.5) no precipitation formed. Computer analysis of potentiometric
titration data have shown the formation of: Cu(Ab)(OH), Cu(Ab)2
and Cu(Ab)2(OH)2 complexes in Cu(II)/anabasine system and
Zn(Ab), Zn(Ab)2(OH), Zn(Ab)2(OH)2 complexes in Zn(II)/anabasine
system (Table 1). Unexpectedly, in the Cu(II)/Ab system no forma-
tion of Cu(Ab) complex was implied by any calculations.
3. Results and discussion
3.1. Protonation constants of anabasine (Ab)
The protonation constants of anabasine: 2H+ + Abꢀ = H2Ab+ and
H+ + Abꢀ = HAb, logbH2Ab = 12.68(1); and logbHAb = 9.17(1) were
determined by the potentiometric method on the basis of computer
analysis of potentiometric data collected in the pH range 2.5–10.5.
The protonation constant logbHAb = 9.17 corresponding to attach-
ment of proton to nitrogen atom the piperidine residue of alkaloid,
The convergence of the theoretically calculated and experimen-
tal titration curves (Fig. 2a and b) of the Cu(II)/anabasine and
Zn(II)/anabasine systems indicates that the coordination model
proposed has been chosen correctly.
The theoretical curves of potentiometric titration were computer
generated taking into account the proper forms of the ligand and the
Fig. 2. Experimental (dotted line) and simulated (solid line) titration curve for the Cu(II)/Ab (a) and Zn(II)/Ab (b) systems, CAb = 2 ꢂ 10ꢀ3 mol/dm3, CCu2+ = 1 ꢂ 10ꢀ3 mol/dm3,
CZn2+ = 1 ꢂ 10ꢀ3 mol/dm3.
10
8
10
8
(a)
(
b
)
6
6
4
4
2
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
NaOH [cm3]
NaOH [cm3]
Fig. 3. Titration curves: (a) Cu(II)/anabasine system; (b) Zn(II)/anabasine system; CAb = 2 ꢂ 10ꢀ3 mol/dm3, CCu2+ = 1 ꢂ 10ꢀ3mol/dm3, CZn2+ = 1 ꢂ 10ꢀ3 mol/dm3; (dotted line –
free anabasine, experimental curve; solid line – anabasine with Cu(II) or Zn(II) ions – complexes were taken into account).

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R. Bregier-Jarze˛bowska et al. / Polyhedron 85 (2015) 841–848
(a) 100
(b)
100
Zn(Ab)2OH
HAb
HAb
Cu(Ab)2(OH)2
80
80
60
40
Zn2+
60
40
Cu(Ab)(OH)
Cu2+
H2Ab
Cu(Ab)2
Zn(Ab)2OH2
H2Ab
Zn(Ab)
20
0
20
0
Ab
Ab
3
4
5
6
7
8
9
10
3
4
5
6
7
8
9
10
pH
pH
Fig. 4. Distribution diagrams for the: (a) Cu(II)/Ab and (b) Zn(II)/Ab systems, CAb = 2 ꢂ 10ꢀ3 mol/dm3, CCu = CZn = 1 ꢂ 10ꢀ3 mol/dm3; percentage of the species refers to total
metal and percentage of the ligand forms refers to total free ligand.
possible coordination complexes (Table 1). A comparison of the
experimental metal/ligand titration curves with the experimental
curves of titration of ligand alone (Fig. 3a,b) shows that in the system
with Cu(II) and in those with Zn(II) ions, the complexation starts
from pH close to 5.0.
It is in agreement with the distribution of complex forms in
both systems (Fig. 4a and b) and also confirms the correctness of
the model assumed.
observed (Fig. 4a), confirm the presence of two different species in
the system. The values of g|| = 2.314 and A|| = 147 correspond to the
coordination with participation of one nitrogen atom in the Cu(A-
b)(OH) complex, while the values g|| = 2.255 and A|| = 172 corre-
spond to the coordination with two nitrogen atoms in Cu(Ab)2
complex. AtpH 10.5, atwhichonlyCu(Ab)2(OH)2 is formed, thevalue
of kmax is 665 nm, whileg|| and A|| are 2.248 and 184(values similarto
those for Cu(Ab)2 species, Fig. 5), which suggests a coordination with
involvement of two nitrogen atoms [27,28].
In order to identify which of the nitrogen atoms in anabasine acts
as the centre of coordination in the complexes formed, the 13C NMR
study was performed. In order to minimize the NMR signal broaden-
ing caused by the paramagnetic character of Cu(II) ions, the spectra
were recorded at low concentrations of the ions (the pH ranges of the
complexes dominance in the distribution curves of the species are
practically the same as for systems of higher concentrations of the
metal ions and the ligands). Significant changes in the chemical
shifts were only observed in the pH ranges in which the occurrence
of the complexes was deduced on the basis of the potentiometric
measurements. The NMR method has been earlier applied to study
similar systems [29–31]. Results of 13C NMR studied are presented
in Table 3, while an exemplary NMR spectrum is shown in supple-
mentary materials (Fig. S1).
3.2.1. Cu(II)/anabasine system
In Cu(II)/anabasine system, the complexes Cu(Ab)(OH) and
Cu(Ab)2 start forming from pH at about 5.0 (Fig. 4a). Although
the nitrogen atom from pyridine residue is available for coordina-
tion in the whole pH range studied (at pH 4.35 about 50% of pyri-
dine residue atoms undergo deprotonation), the formation of
complexes is parallel to deprotonation of the nitrogen atom from
piperidine residue, which means that the centre of interaction
between copper(II) ions and anabasine is the nitrogen atom from
piperidine and not the one from pyridine residue. The maximum
concentration of Cu(Ab)2 is observed at pH close to 7.5 at which
this species binds about 40% of copper ions. The hydroxo-complex
Cu(Ab)(OH) binds maximum about 60% of copper ions at pH about
8.5 (Fig. 4a).
The Cu(Ab)2(OH)2 complex starts forming at pH of about 8.0,
and its concentration increases with increasing pH and at
pH ꢄ 10.5 this complex binds about 90% of the Cu(II) ions. Unex-
pectedly, no Cu(Ab) species was detected in the system discussed,
at least in detectable concentration. In the earlier studied system of
Cu(II)/nicotine, in which the main centre of coordination was the
nitrogen atom from pyridine residue, the species CuH(Nic) and
Cu(Nic) were observed to form already at low pH [26]. The results
of spectroscopic analysis (Vis and EPR) of the system discussed are
given in Table 2.
In the NMR spectrum of Cu(II)/Ab at pH 8.0, the position of
signals assigned to the carbon atoms from piperidine residue in
A||
g||
g
The EPR parameters obtained from the EPR spectra recorded at
pH 8, at which parallel formation of Cu(Ab)(OH) and Cu(Ab)2 is
Table 2
Vis and EPR spectra data for Cu(II)/Ab system.
Species
pH
kmax [nm] EPR
g||
A|| [10ꢀ4 cmꢀ1
]
g\ [10ꢀ4 cmꢀ1
]
Cu(Ab)(OH)
Cu(Ab)2
Cu(Ab)2(OH)2 10.5 665
8.0 690
8.0 690
2.314 147
2.255 172
2.248 184
2.046
2.046
2.042
320
340
240
260
280
300
[mT]
Fig. 5. EPR spectrum of Cu(Ab)2(OH)2 species, pH 10.5.

R. Bregier-Jarze˛bowska et al. / Polyhedron 85 (2015) 841–848
845
Table 3
13C chemical shifts for the anabasine in Cu(II)/Ab and Zn(II)/Ab systems in relations to the free anabasine (in parentheses); [ppm].
pH
Species
Anabasine
C2
C3
C4
C5
C6
C20
C30
C40
C50
C60
8.0 Cu(Ab)(OH) and
Cu(Ab)2
10.5 Cu(Ab)2(OH)2
148.50
(0.04)
148.07
(0.00)
148.49
(0.03)
134.44
(0.02)
136.44
(0.04)
134.40
(0.06)
136.88
(0.07)
140.90
(0.00)
136.85
(0.04)
125.54
(0.02)
125.09
(0.03)
125.56
(0.04)
150.31
(0.05)
148.31
(0.04)
150.32
(0.06)
59.15
(0.13)
59.33
(0.14)
59.15
(0.13)
30.21
(0.09)
33.60
(0.06)
30.20
(0.08)
22.63
(0.02)
25.14
(0.06)
22.66
(0.05)
23.21
(0.11)
25.26
(0.04)
23.22
(0.12)
46.56
(0.18)
47.20
(0.19)
46.53
(0.15)
7.0 Zn(Ab)
anabasine change, which means that these atoms are involved in
the coordination. The signals of C20 and C60 are shifted by 0.130
and 0.180 ppm, while the signals of C2 and C6 from the pyridine
residue are shifted only by 0.040 and 0.050 ppm, respectively. A
similar character of the shifts is observed in the spectrum recorded
at pH = 10.5. The signals of C20 and C60 from the piperidine ring are
shifted by 0.140 and 0.190 ppm relative to their positions in the
spectrum of free ligand, while the signal of C6 from the pyridine
residue is shifted only by 0.040 ppm, and the signal of C2 is
unchanged. Thus, in the Cu(Ab)(OH) complex, in which one nitro-
gen atom was found to take part in the coordination, this nitrogen
atom comes from piperidine residue of anabasine. In the species
Cu(Ab)2 and Cu(Ab)2(OH)2, in which two nitrogen atoms are
involved in the coordination, the centre of interaction is the nitro-
gen from piperidine residue from each of anabasine molecule. It is
in contrast to the earlier studied complexes of copper(II) with nic-
otine, in which the centre of coordination was the nitrogen atom
from the pyridinium residue [26]. Calculated chemical shifts of
13C NMR spectra for Cu complexes are in Table S1 (supplementary
materials).
due are shifted only by 0.030 and 0.060 ppm, which means that in
the complexes with diamagnetic zinc ion suggests a small contri-
bution of the nitrogen atom from pyridine residue of anabasine.
In the 13C NMR spectrum of the complex obtained in solid state
(Fig. S2, supplementary materials) the greatest changes in the
chemical shifts of carbon atoms were observed in piperidine ring,
indicating that the free electron pair located on the piperidine
nitrogen atom is involved in coordination process. The chemical
shifts of carbon atoms C20, C30, C40, C50and C60 are upfield shifted
from 2.90 to 4.38 ppm (see Table 4) in relation to the chemical
shifts of anabasine [33]. The signals of aromatic ring of anabasine
in the complex are generally slightly shifted upfield in comparison
with the appropriate signal in the spectrum of free base. The differ-
ences are smaller and amount to 0.31–1.38 ppm. The pyridine ring
protons are observed in the range 7–9 ppm in the 1H NMR spec-
trum. The protons of piperidine ring are located in the range 1.6–
4.2 ppm. Due to the complexation, the resonance signal of H2 is
shifted by about 0.50 ppm towards increasing magnetic field. Cal-
culated chemical shifts of 13C NMR spectra for Zn complexes are in
Table S1 (supplementary materials).
Anabasine complex with ZnCl2 showed fragmentation in the EI
mass spectra. The base peak in this complex is the one correspond-
ing to the even-electron fragment ion at m/z = 84 [C5H10N]+. This
ion is obtained by cleavage of the bond between the C3 atom of
the pyridine ring and the C20 atom of the piperidine ring of anaba-
sine molecule. The next most abundant ions were those attributed
to [C7H7N]+ (m/z105), [C10H14N2]+ (m/z 162), [C8H9N2]+ (m/z 133)
and [C7H7N2]+ (m/z 119) (see Fig. 6). Because anabasine readily
forms zinc complexes in alkaline solution, in the EI MS spectrum
we can see key fragment consist of molecule of anabasine, zinc
and hydroxide ions at m/z = 243 [Zn(Ab)(OH)]+. The detailed ESI
MS analysis of complex discussed above was described in [34].
3.2.2. Zn(II)/anabasine system
The process of complexation in the system Zn(II)/Ab starts from
pH close to 5 with formation of Zn(Ab), which reaches maximum
concentration at pH close to 7.5, binding about 40% of zinc ions
(Table 1, Fig. 4b). Starting from pH near 7, the hydroxo-complex
Zn(Ab)2(OH) is formed whose maximum concentration is found
at pH close to 9 at which it binds about 90% of zinc ions. Another
hydroxo-complex Zn(Ab)2(OH)2 is formed in the system starting
from pH close to 9 (Fig. 4b). The calculated equilibrium constants
of formation of Zn(Ab), Zn(Ab)2(OH) and Zn(Ab)2(OH)2 are 4.54,
9.65 and 4.39 (Table 1). The value of log Ke = 4.54 for the reaction
Zn + Ab ¡ Zn(Ab) corresponds to coordination of one nitrogen
atom to zinc ion [32]. Coordination of another nitrogen atom (from
another molecule of anabasine) and the hydroxyl group OH- to the
complex Zn(Ab) with formation of Zn(Ab)2(OH) is described by
Ke = 9.65 (Table 1). In order to establish the mode of interaction
of zinc ions with anabasine, the 13C NMR spectrum of Zn(II)/Ab
was recorded at pH 7.0, at which the complex Zn(Ab) forms. Anal-
ysis of the shifts of carbon signals from anabasine in the system
with zinc ions suggests that the main centre of coordination is
the nitrogen atom from piperidine residue. The shifts of the signals
assigned to C20 and C60, that neighbor the nitrogen atom from
piperidine ring of anabasine are 0.130 and 0.150 ppm (Table 3),
while the signals assigned to C2 and C6 from the pyridinium resi-
3.3. Theoretical studies
Three interactions schemes for both central atoms as suggested
by experimental measurements were subjected to computational
studies. For zinc ion these were (i) a complex consisted of one mol-
ecule of anabasine and two nitrate ions originating from zinc salt
used in experimental research and one molecule of water from sol-
vent, (ii) a complex consisted of two molecules of anabasine, one
hydroxide ion and one nitrate ion, and (iii) a complex composed
of two molecules of anabasine and two hydroxide ions. For copper
ion the interaction schemes were (i) a complex consisted of two
molecules of anabasine and two nitrate ions originating from cop-
Table 4
13C chemical shifts for the anabasine and its ZnCl2 complex obtained in solid state.
C2
C3
C4
C5
C6
C20
C30
C40
C50
C60
Anabasine d [ppm]
149.00
149.38
+0.38
142.14
141.25
ꢀ0.89
136.66
135.35
ꢀ1.31
125.31
123.93
ꢀ1.38
148.87
148.56
ꢀ0.31
60.61
57.56
ꢀ3.05
35.10
30.72
ꢀ4.38
26.12
22.37
ꢀ3.75
26.32
22.93
ꢀ3.39
48.26
45.36
ꢀ2.90
AnabasineꢀZnCl2 d [ppm]
D
d[ppm]

846
R. Bregier-Jarze˛bowska et al. / Polyhedron 85 (2015) 841–848
Fig. 6. EI MS spectrum of anabasine complex with ZnCl2.
per salt used in experimental research, (ii) a complex consisted of
two molecules of anabasine and two hydroxide ions, and (iii) a
complex consisted of one molecule of anabasine, one hydroxide
ion, one nitrate ion and one water molecule from solvent. All these
structures possessed neither negative nor positive charge which
was consistent with the experimental studies (see Fig. 7).
All geometry optimizations of zinc complexes have led to a tet-
rahedral structure of the central Zn cation indicating energetic
preference towards tetrahedral structure of Zn(II) complexes with
anabasine. Contrary the geometry optimizations of copper com-
plexes led mainly to a flat square structure of the central Cu cation
with minor deviations of angles. This deviations may be caused by
possibility of existence complexes consisting of five ligands or
complexes in which nitrate ions acts as a bidentate ligand. Those
complexes can have structure of tetragonal pyramid and this struc-
Fig. 7. The optimized structures of the modeled zinc complexes with anabasine.
Fig. 8. The optimized structures of the modeled cooper complexes with anabasine.

R. Bregier-Jarze˛bowska et al. / Polyhedron 85 (2015) 841–848
847
Table 5
Comparison of the relative energies of zinc complexes obtained in M06 calculations, energy in vacuum with thermal and solvation corrections obtained in composite basis set,
and population of obtained conformers.
Species Nitrogen ring
Relative Energy in
vacuum [kcal/mol]
Entropy [cal/
mol⁄K]
Thermal correction to
G [hartree]
Solvation correction to
G [hartree]
Relative
[kcal/mol]
D
G
Population of
conformers
M06/GEN
M06/GEN
M06/GEN
M06/GEN
M06/GEN
X [%]
d
i
N-Piperidine
N-Pyridine
N-Piperidine
N-Pyridine
N-Piperidine and
Pyridine
N-Piperidine
N-Pyridine
0.0a
3.8
155.6
161.4
187.9
203.2
197.8
0.237458
0.234182
0.430794
0.423603
0.426263
ꢀ0.039727
ꢀ0.039723
ꢀ0.040052
ꢀ0.034650
ꢀ0.038802
0.0
95.3
4.7
25.0
0.1
1.8
0.7
4.1
ii
0.0b
4.5
e
1.4
0.0
74.9
f
iii
2.4
2.8
0.0c
182.5
185.0
181.2
0.427645
0.424926
0.428279
ꢀ0.035154
ꢀ0.029572
ꢀ0.029405
0.0
2.2
1.6
92.0
2.2
5.9
N-Piperidine and
Pyridine
Absolute energy baselines [in hartree]:
a
ꢀ1362.676330.
b
ꢀ1580.509692.
c
ꢀ1376.090340.
d
ꢀ1362.478599.
e
ꢀ1580.119989.
f
ꢀ1375.694076.
ture was obtained for complex in which both molecules of anaba-
sine connect with copper via piperidine ring nitrogen atom and one
of nitrate ions acts as a bidentate ligand. Our efforts to obtain com-
plexes with octahedral structure were unsuccessful as these struc-
tures after optimization converged to the flat square geometry (see
Fig. 8).
Based on the obtained Gibbs free energies for zinc and copper
complexes from each scheme of interactions [35] we calculated
populations of conformers using a standard Boltzmann formalism.
The percentage of a conformer X is given as:
was more favorable from about 0.7 kcal/mol to 4.1 kcal/mol. In
the first scheme of interaction the energy difference favoring
coordination mode to zinc via piperidine ring nitrogen atom over
the pyridine ring nitrogen was 1.8 kcal/mol and as we can see
calculated populations of conformers suggests that in this
scheme of interaction practically only the structure with anaba-
sine connected via piperidine ring nitrogen atom is likely to be
observed. In the case of interaction scheme (ii) the lowest energy
zinc complexes have one anabasine molecule connected via
piperidine nitrogen atom and the other anabasine molecule con-
nected via pyridine ring nitrogen atom. In interaction scheme
(iii) the lowest energy zinc complexes have both molecules of
anabasine connected via piperidine ring nitrogen atom. As we
can see calculated populations of conformers suggest that for
(ii) we are also likely to observe the complex with both anaba-
sine molecules connected via piperidine ring nitrogen atom.
Energy differences between the lowest energy complex and
other complexes are respectively for the second scheme: 0.7
and 4.1 kcal/mol, for the third scheme: 1.6 and 2.2 kcal/mol
(see Table 5).
GX0
expðꢀ RT
Þ
%X ¼
ꢂ 100%
Gi0
Riexpðꢀ RTÞ
where G0X is the relative energy of a conformer X and i runs over all
conformers.
In all interaction schemes the most energetically preferred
structure has one or both anabasine molecules connected to cen-
tral zinc atom via piperidine ring nitrogen atom. The Gibbs free
energy of such complex in comparison to the other structures
Table 6
Comparison of the relative energies of copper complexes obtained in M06 calculations, energy in vacuum with thermal and solvation corrections obtained in composite basis set,
and population of obtained conformers.
Species Nitrogen ring
Relative Energy in
vacuum [kcal/mol]
Entropy [cal/
mol⁄K]
Thermal correction to
G [hartree]
Solvation correction to
G [hartree]
Relative
[kcal/mol]
D
G
Population of
conformers
M06/GEN
M06/GEN
M06/GEN
M06/GEN
M06/GEN
X [%]
i
N-Piperidine
N-Pyridine
N-Piperidine and
Pyridine
N-Piperidine
N-Pyridine
N-Piperidine and
Pyridine
0.0a
8.3
10.2
198.0
211.2
207.8
0.435262
0.426528
0.428598
ꢀ0.074740
ꢀ0.078093
ꢀ0.081858
0.0d
0.7
1.5
72.0
22.2
5.7
ii
iii
0.0b
9.9
2.3
181.8
187.2
182.2
0.428482
0.424833
0.428136
ꢀ0.026870
ꢀ0.028329
ꢀ0.027644
0.0e
6.6
2.9
93.2
<0.1
6.7
N-Piperidine
N-Pyridine
0.0c
13.8
147.6
152.1
0.236311
0.232186
ꢀ0.025667
ꢀ0.059300
<0.1
0.0f
49.3
50.7
Absolute energy baselines [in hartree]:
a
ꢀ1755.049346.
b
ꢀ1346.259220.
c
ꢀ1128.418450.
d
ꢀ1754.688825.
e
ꢀ1345.857608.
f
ꢀ1128.223550.

848
R. Bregier-Jarze˛bowska et al. / Polyhedron 85 (2015) 841–848
The lowest energy structures of copper complexes obtained with
the most energetically favorable structure has anabasine molecule
or both anabasine molecules connected via piperidine ring nitro-
gen atom, the only exception are Cu(II) complex within interaction
scheme (iii) for which both structures with anabasine coordinating
via piperidine and pyridine nitrogen atoms are likely to exist in
equilibrium (49%:51%).
(i) and (ii) schemes of interactions were the structures with both
anabasine molecules connected via piperidine ring nitrogen atom.
Energy of such structure was lower than other obtained complexes
from 0.7 kcal/mol to even 6.6 kcal/mol. In interaction scheme (iii)
the lowest energy structure is the one with anabasine molecule
connected via pyridine ring nitrogen atom. Yet the difference
between the composite Gibbs free energies of these structures in
the interaction (iii) was very small. In the interaction schemes (i)
and (ii) the differences between the lowest energy structure and
structure with coordination mode to copper via the pyridine ring
nitrogen atom of both anabasine molecules or one anabasine mol-
ecule connected via piperidine nitrogen atom and one anabasine
molecule connected via pyridine ring nitrogen atom were for the
(i): 0.7 and 1.5 kcal/mol; for the (ii): 6.6 and 2.9 kcal/mol. Calcu-
lated populations of conformers suggests that in the first and the
second interaction schemes complexes with both anabasine mole-
cules connected via piperidine ring nitrogen atom are likely to be
observed. In the interaction (iii) the difference in energy between
complex with anabasine molecule connected via piperidine ring
nitrogen atom and complex with anabasine molecule connected
via pyridine ring nitrogen atom was lower than 0.1 kcal/mol. Popu-
lation of conformers in the interaction (iii) suggest that both struc-
tures (with anabasine coordinating via nitrogen atom of piperidine
ring or pyridine ring) are likely to be observed (see Table 6).
In all the structures of Cu(II) complexes copper central atom
possessed larger than 0.6 spin density (calculated according to
the Mulliken scheme) suggesting, as expected, strong localization
of the unpaired electron on central Cu(II) ion. Some of the com-
plexes obtained had hydrogen bonds in their structure (Supple-
mentary information). These bonds were depicted in the Figs. 7
and 8.
Acknowledgement
This research was supported in part by PL-Grid Infrastructure.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2014.10.008.
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