Potentiometric and AM1d studies of the equilibria between silver(I) and diaza-15-crown and diaza-18-crown ethers with nitrogen in different positions in various solvents

Article abstract of DOI:10.1080/00958972.2012.751099

Complex formation and stability constants between typical and atypical diaza-15-crown and diaza-18-crown ethers with silver(I) were determined in methanol, acetonitrile and propylene carbonate by the potentiometric method. In two of the diaza crown ethers, AA-diaza-15 and AA-diaza-18-crown, two nitrogens in the macrocyclic ring replaced two consecutive oxygens instead of two opposite ones in the two other diaza crown ethers. It was found that complexes of 1:1 and 1:2 metal-to-ligand stoichiometry were formed. The solvent composition and cavity size of crown ethers significantly influences the stability constants of complexes. AA-diaza-15 and AA-diaza-18-crown ethers were examined for comparison with diaza-15-crown and diaza-18-crown ethers. AA-diaza crown ethers formed less stable 1:1 metal-to-ligand complexes with silver(I) than typical diaza crown ethers but their ability to form 1:2 metal-to-ligand complexes was stronger. The energetically most favorable structures of the 1:1 metal-to-ligand complexes were calculated and visualized by the AM1d method at the semiempirical level of theory.

Full text of DOI:10.1080/00958972.2012.751099

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Potentiometric and AM1d studies of
the equilibria between silver(I) and
diaza-15-crown and diaza-18-crown
ethers with nitrogen in different
positions in various solvents
Jaromir Kira a , Pawel Niedzialkowski a & Tadeusz Ossowski a
a Chemistry , University of Gdansk , Gdansk , Poland
Accepted author version posted online: 19 Nov 2012.Published
online: 19 Dec 2012.
To cite this article: Jaromir Kira , Pawel Niedzialkowski & Tadeusz Ossowski (2013) Potentiometric
and AM1d studies of the equilibria between silver(I) and diaza-15-crown and diaza-18-crown ethers
with nitrogen in different positions in various solvents, Journal of Coordination Chemistry, 66:1,
180-190, DOI: 10.1080/00958972.2012.751099
To link to this article: http://dx.doi.org/10.1080/00958972.2012.751099
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Journal of Coordination Chemistry
Vol. 66, No. 1, 10 January 2013, 180–190
Potentiometric and AM1d studies of the equilibria between
silver(I) and diaza-15-crown and diaza-18-crown ethers with
nitrogen in different positions in various solvents
JAROMIR KIRA*, PAWEL NIEDZIALKOWSKI and TADEUSZ OSSOWSKI
Faculty of Chemistry, University of Gdansk, Gdansk, Poland
(Received 13 June 2012; in final form 1 October 2012)
Complex formation and stability constants between typical and atypical diaza-15-crown and
diaza-18-crown ethers with silver(I) were determined in methanol, acetonitrile, and propylene
carbonate by the potentiometric method. In two of the diaza-crown ethers, AA-diaza-15 and
AA-diaza-18-crown, two nitrogens in the macrocyclic ring replaced two consecutive oxygens
instead of two opposite ones in the two other diaza-crown ethers. It was found that complexes of
1 : 1 and 1 : 2 metal-to-ligand stoichiometry were formed. The solvent composition and cavity size
of crown ethers significantly influences the stability constants of complexes. AA-diaza-15 and
AA-diaza-18-crown ethers were examined for comparison with diaza-15-crown and diaza-18-crown
ethers. AA-diaza-crown ethers formed less stable 1 : 1 metal-to-ligand complexes with silver(I) than
typical diaza-crown ethers, but their ability to form 1 : 2 metal-to-ligand complexes was stronger.
The energetically most favorable structures of the 1 : 1 metal-to-ligand complexes were calculated
and visualized by the AM1d method at the semiempirical level of theory.
Keywords: Azacrown ethers; Equilibrium; Silver(I)
1. Introduction
Crown ethers are efficient complexing agents for many metal ions. Numerous thermody-
namic and kinetic data indicate that complexes of 1:1 metal-to-ligand stoichiometry are
predominantly formed [1]. However, complexes of 1:1 metal-to-ligand stoichiometry for
selected transition and alkali metal ions have also been reported [2–4]. The interactions
between the macrocyclic ring and a metal ion in solution are usually analyzed in terms of
the interactions with a positively charged ion that leads to the 1 : 1 inclusion complex. This
is indicated by the strong effect of the macrocycle on stability constants in comparison
with aliphatic amines [5]. The formation of such complexes does not necessarily imply
that the metal ion is located precisely within the macrocycle cavity. For 1 : 2 type com-
plexes, a sandwich structure in which the metal ion is located between two coronand mole-
cules has been suggested as the most probable one [6, 7].
Interactions of aza-crown ethers with metal ions that possess a high affinity for nitrogen,
such as silver(I), indicate that the number of nitrogens within the macrocyclic ring plays a
*Corresponding author. Email: jaromir@chem.univ.gda.pl
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online Ó 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.751099

Potentiometric
181
major role in stabilization of AgL+ complexes [7–9]. For monoaza-crown ethers, AgL+, the
Ag(I) is located within the macrocycle cavity which partially shields the metal ion from the
+
solvent [5]. In AgL2 , Ag(I) forms sandwich-like structures with monoaza-crown ethers. In
this case, the metal ion is completely shielded from the solvent [5]. The formation of 1 : 2
metal-to-ligand complex ions by Ag(I) and aza-crown ethers with two or more nitrogens in
the macrocyclic ring is probably more complicated. Ag(I) forms linear 1 : 2 metal-to-ligand
complex ions with aliphatic amines [10]. In diaza-crown ethers and AA-diaza-crown ethers,
two nitrogens potentially participate in coordination with silver(I). The AA means that the
two nitrogens in the macrocycle ring are separated by only one ethylene unit. Unfortunately,
no crystallographic data are available for the structures of these complexes.
2. Experimental
2.1. Materials and syntheses
Silver(I) perchlorate (Fluka) and tetraethylammonium perchlorate (TEAP; Fluka) were
purified as described previously [11]. 1,4,10-Trioxa-7,13-diazacyclopentadecane (A215C5,
Kryptofix® 21, Merck) and 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (A218C6,
Kryptofix® 22, Merck) were used as purchased (figure 1). Propylene carbonate (PC,
Merck), methanol (MeOH, Aldrich), and acetonitrile (AN, Aldrich) were of the highest
purity. 2-(2-Aminoethylamino) ethanol (Aldrich) and 2-chloroethanol (Aldrich) were
previously distilled. Silica gel 60 (<0.03 mm, Merck) and aluminum oxide 90 (Merck)
were used for chromatography.
The syntheses of 1,4,7-trioxa-10,13-diazacyclopentadecane (AA15C5) and 1,4,7,10-tetra-
oxa-13,16-diazacyclopentadecane (AA18C6) were performed by a modified version of
intramolecular cyclization (figure 2) [12]. To a stirred suspension of 2-(2-aminoethylamino)
ethanol (208 g, 0.2 M) was added 2-chloroethanol (16.1 g, 0.2 M) and powdered sodium
carbonate (16.96 g, 0.16 M). The mixture was stirred at 135 °C for 25 h. After this had
been cooled to room temperature, 300 mL of MeOH was added and the mixture was
filtered; the resulting solution was evaporated to yield a crude product (yellow oil), which
was recrystallized twice from THF over charcoal to give 3 12.05 g (40.7%) TLC (SiO2)
CH2Cl2: MeOH 2,5 : 2,5 Rf = 0.4. 1H NMR: (δ, CDCl3) 400 MHz: 2.78–2.84 (m; 8H),
3.66 (t; 4H). To obtain AA15C5 (4.29 g, 0.028 M), 3 (2-[2]ethanol) was dissolved in a
solution of (4.40 g, 0.11 M) potassium metal in 224 mL t-butyl alcohol. To this stirred
Figure 1. The structures of the diaza-crown ethers studied.

182
J. Kira et al.
H
H
N
N
OH
Cl
+
HO
N
H2N
OH
OH
H
135 C
1
2
3
O
n
H
N
O
O
OH
TsO
OTs
+
HO
N
O
n
H
NH HN
n=1, 2
3
4, 5 n=1, 2
6,7
Figure 2. Synthetic scheme of AA15C5 and AA18C6.
solution, diethylene glycol bis(p-toluenesulphonate) (11.60 g, 0.028 M) in freshly distilled
dioxane (84 mL) was added dropwise at 40 °C during 4 h, after which the reaction was
continued for 20 h. The resulting white reaction mixture was filtered. The precipitate was
rinsed with dichloromethane (3 ꢀ 100 mL) and the solvent then evaporated from the pooled
rinsings. To the residue, 60 mL of water was added to the residue and the solution was
extracted twice with hexane to remove by-products. It was then extracted with dichloro-
methane (5 ꢀ 100 mL). The dichloromethane extracts were combined and the solvent evap-
orated. The product was dissolved in water purified on an ion-exchange column (Dowex
50X2–100). The eluant was evaporated under reduced pressure and dried under vacuum.
The crude product 6 was finally distilled in a Kugelrohr apparatus at 200 °C (0.01 hPa) to
yield 3.601 g (57%) as a clear oil. TLC (SiO2) CH2Cl2: MeOH 2,5 : 2,5 Rf = 0.2. 1H
NMR: (δ, CDCl3) 500 MHz: 3.04 (s; 2H), 3.48–3.58 (m; 8H), 3.58–3.75 (m; 12H). 7
AA18C6 was obtained in a similar manner from 5 with a yield of 41% TLC (SiO2)
1
CH2Cl2: MeOH 2,5 : 2,5 Rf = 0.2. H NMR: (δ, CDCl3) 500 MHz: 2.81 (s; 2H), 3.28–3.44
(m; 8H), 3.48–3.65 (m; 16H).
2.2. Potentiometric measurements
Potentiometric titrations were performed at 25 °C using an OP-205 Radelkis pH-meter.
Silver(I) solutions in PC, MeOH, and AN were prepared from the perchlorate salt; the
concentration range was 6–9 ꢀ 10ꢁ4 M dmꢁ3. Ligand concentrations were in the range
3.5-9.5 ꢀ 10ꢁ3 mol dmꢁ3. Measurements were performed using a 0.5 mL Hamilton syringe
equipped with a Gage 30 Teflon tube; the half-cells were connected by a salt bridge filled
with 0.1 M dmꢁ3 TEAP dissolved in PC, MeOH, or AN. The silver concentration during
the titrations was determined using silver-wire electrodes dipped in the solution. The
equilibrium constants and the simulations were calculated using the STOICHIO program
[13–15] based on the nonlinear least-squares Gauss–Newton–Marquardt algorithm [16].
2.3. Semiempirical calculations
AM1d calculations were performed using the Win Mopac 2007 program at the semiempiri-
cal level (Cache Work System Pro Version 7.5.085, Fujitsu) [17,18]. The conformers of

Potentiometric
183
the diaza-crown ethers and their 1 : 1 metal-to-ligand complexes were searched for
simultaneously by molecular dynamics and CONFLEX® experiments. The CONFLEX®
program systematically and exhaustively generates low-energy conformers of a molecule
of any shape [19]. The energetically most favorable structures of the 1 : 1 metal-to-ligand
complexes were found in the collection of several thousand structures optimized by the
AM1d semiempirical method [20–25].
3. Results and discussion
3.1. Potentiometric studies of formation of diaza-crown ether complexes
On the potentiometric titration curves, there is a single potential jump for AA15C5 and
AA18C6 in PC, MeOH, and AN. The largest changes in potential were observed in PC
(ꢁ650 mV for AA15C5; ꢁ600 mV for AA18C6), medium in MeOH (ꢁ250 mV;
ꢁ300 mV), and the lowest in AN (ꢁ150 mV; ꢁ200 mV). Therefore, the data for PC
(figure 3) were used to carry out a graphical analysis of the potential of the ligands
[26,27]. With the large number of experimental data, an advanced graphical and statistical
analysis can be performed [26,27]. The fitting of theoretical points to experimental points
does not lead to a simple equilibrium of 1 : 1 stoichiometry. It suggests that, apart from the
equilibrium resulting in formation of AgL+, other equilibria are also likely. Attempts to fit
the curves to data points have shown that variations in the potential as a function of the
titrant added are best represented by two equilibria:
Figure 3. Graphical fitting for models 1o (-) and 2o (…) and experimental points (o) of silver(I) (6.00 ꢀ 10ꢁ4
M dmꢁ3
)
from potentiometric titration with AA15C5 (4.00 ꢀ 10ꢁ3 M dmꢁ3
;
left) and AA18C6 (4.00 ꢀ
10ꢁ3 M dmꢁ3; right) in PC at 25 °C.

184
J. Kira et al.
Agþ þ L ¼ AgLþ
K1
K1
ð1Þ
ð2Þ
AgLþ þ L ¼ AgLþ2
For A215C5 and A218C6, there was a single potential in PC, MeOH, and AN. The
largest changes in potential were observed in PC (ꢁ600 mV for A215C5; ꢁ750 mV for
A218C6), medium in MeOH (ꢁ300 mV; ꢁ400 mV), and the lowest in AN (ꢁ200 mV;
ꢁ300 mV). Therefore, the data for PC (figure 4) were used to illustrate the graphical anal-
ysis of the potentials of the ligands [26,27]. From a graphical analysis of the potential
changes, one may infer that the two equilibria (1) and (2) best explain the changes in the
+
potential, with the exception of A218C6 in PC. We have no evidence for AgL2 forming
in PC for A218C6. In this work, we have presented experimental data for A215C5 and
A218C6, especially that there are no separate data for equilibrium (2) for A215C5 and
A218C6 in MeOH and AN in the literature (with the exception of A215C5 [28]).
Additionally, in table 1, we compare the values of ω(x) and σE obtained from the experi-
ment and calculated for each model. These values are lowest for model 2o which involves
two equilibria leading to formation of AgL+ and AgL2 . Since ω(x) and σE for model 2o
+
are small, there is no need to further analyze the equilibria [26,27]. Model 2o is suitable
for all the ligands and every solvent. The stability constants of all the ligands are presented
in table 2 in the form of log K1 and log K2.
Both diaza and AA-diaza-crown ethers have two nitrogens in the macrocyclic ring. The
structural differences in these macrocycles could lead to a different way of complexing the
Figure 4. Graphꢁic3al fitting for models 1o (-) and 2o (…) and experimental points (o) of silver(I)
(6.00 ꢀ 10ꢁ4 M dm
)
from potentiometric titration with A215C5 (8.00 ꢀ 10ꢁ3 M dmꢁ3
; left) and A218C6
(9.00 ꢀ 10ꢁ3 M dmꢁ3; right) in PC at 25 °C.

Potentiometric
185
Table 1. Comparison of statistical values for AA15C5, AA18C6, A215C5, and A218C6 with experimental points
for models 1o and 2o; ω(x) – the sum of the squares of the differences between measured and calculated
potentials and concentrations; σE – standard deviation of the potential.
Model
of eq.
ω(x)
σE
ω(x)
σE
ω(x)
σE
Species
PC
MeOH
AN
AA15C5
1o
2o
AgL+
7.41e + 04
5.26e + 03
0.057
0.011
1.02e + 04
1.73e + 02
AA18C6
4.73e + 03
3.11e + 02
A215C5
2.80e + 02
1.71e + 02
0.010
0.002
1.62e + 04
6.21e + 02
0.014
0.003
AgL+, AgL2
+
+
+
+
1o
2o
AgL+
3.69e + 04
7.75e + 03
0.038
0.013
0.008
0.005
2.03e + 03
7.63e + 02
0.005
0.004
AgL+, AgL2
1o
2o
AgL+
3.32e + 04
9.36e + 03
0.029
0.015
0.003
0.002
5.87e + 02
3.43e + 02
0.003
0.002
AgL+, AgL2
A218C6
1.26e+03
2.62e+02
1o
2o
AgL+
3.94e+03
0.009
–
0.008
0.004
5.45e+02
3.97e+02
0.004
0.003
AgL+, AgL2
–
metal ion in the macrocycle cavity (figure 1). The stability constants of formation of AgL+
for diaza and AA-diaza-crown ethers (table 2) are larger than for monoaza-crown ethers
[5]. This suggests that Ag(I) interacts with both nitrogens in the ring of these macrocycles.
It is known from previous work [5] that Ag(I) fits well in the ring cavity of monoaza-15-
crown and monoaza-18-crown ethers. The difference between the log K1 and log K2 values
of typical diaza and AA-diaza-crown ethers shows that AgL+ were predominantly formed
Table 2. Stability constants given in the form of log K1 and log K2 for AA15C5, AA18C6, A215C5, and
A218C6 obtained from potentiometry in PC, MeOH, and AN at 25 °C.
Ligand
log K1
log K2
PC
AA15C5
AA18C6
A215C5
11.58 0.06
12.27 0.06
5.09 0.11
4.34 0.10
3.17 0.09 [26]
3.11 0.10 [7]
3.51 0.07 [8]
–
13.12 0.05⁄ [26]
13.15 0.05 [8,9,26]
A218C6
15.41 0.06
14.81 0.03 [7]
15.57 0.01 [8]
MeOH
AA15C5
AA18C6
A215C5
6.26 0.08
4.82 0.13
7.42 0.08
4.16 0.14
7.52 0.05⁄⁄ [28]
7.49 0.01 [7,9]
9.88 0.06
2.38 0.08⁄⁄ [28]
A218C6
2.60 0.10
9.74 0.08 [7]
AN
AA15C5
AA18C6
A215C5
5.33 0.07
4.14 0.11
5.72 0.06
3.63 0.11
6.72 0.04⁄⁄ [28]
6.43 0.01 [7,29]
7.96 0.05
2.33 0.09⁄⁄ [28]
A218C6
1.82 0.11
7.75 0.02 [7]
⁄Misprint in 26 ⁄⁄also presented by us in 28.

186
J. Kira et al.
and that both types of ligands are efficient complexing agents for silver(I). The difference
in log K1 between A218C6 and A215C5 is greater (2.29 in PC, 2.36 in MeOH, 1.24 in
AN) than between AA18C6 and AA15C5 (0.69 in PC, 1.16 in MeOH, 0.39 in AN). This
implies that the difference in the size of the macrocyclic ring influences the formation of
AgL+ more strongly in diaza-crown ethers than in AA-diaza-crown ethers. On the other
hand, the two nitrogens replacing two adjacent oxygens in the macrocyclic ring weaken
this influence. The data suggest that the two nitrogens, located opposite to one another in
A218C6, cooperate with each other much more strongly than in A215C5.
The difference in log K1 between A215C5 and AA15C5 is 1.54 in PC, 1.26 in MeOH,
and 1.39 in AN. For A218C6 and AA18C6, the difference in log K1 is 3.14 in PC, 2.46 in
MeOH, and 2.24 in AN. The difference in log K1 between diaza and AA-diaza-crown
ethers is much greater for A218C6 which binds silver(I) more strongly. Two nitrogens
replacing two adjacent oxygens in the macrocyclic ring weaken formation of AgL+ in
diaza-crown ethers.
+
3.2. Formation of AgL2 complexes
+
Both diaza and AA-diaza-crown ethers form AgL2 complex ions (table 1). Ag(I) ions
+
form linear AgL2 complexes with aliphatic amines [5,10]. Previous work [5] indicates
that Ag(I) ions form sandwich-type complexes with monoaza-crown ethers with a 1 : 2
metal-to-ligand stoichiometry. The formation of linear or sandwich-type complexes has
characteristic K1/K2 values. If the value is <4 (as opposed to crown ethers), then it is char-
acteristic of a statistically purely linear coordination [5]. The K1/K2 values for crown ethers
investigated in PC, MeOH, and AN are shown in table 3. These values are >4 in all cases,
+
which confirms that the AgL2 complex ion has a sandwich-type structure in all the inves-
tigated ligands in different organic solvents.
The log K2 values of AA-diaza-crown ethers are greater than those of diaza-crown
+
ethers (table 2), suggesting that the mechanism of formation of AgL2 between AA-diaza-
crown ethers and Ag(I) differs from that of typical diaza-crown ethers. In the case of AA-
diaza-crown ethers, log K2 changes from 5.09 for AA15C5 in PC to 3.63 for AA18C6 in
AN. At the same time, the log K1 values are lower than those of typical diaza-crown
ethers. This observation suggests that the lower tendency of AA-diaza-crown ethers to
form AgL+ inclusion-type complex ions is suppressed by their greater tendency to form
+
AgL2 . The differences of the log K2 values are generally larger in MeOH and AN which
solvate Ag(I) ions more strongly than PC. It is known from previous work on monoaza-
crown ethers [5] that Ag(I) in AgL+ is best shielded from solvent by the monoaza-18-
+
crown. In AgL2 sandwich-type complex ion, Ag(I) is well shielded from solvent by all
Table 3. Values of K1/K2 for silver(I) complexes with diaza and AA-diaza-crown ethers in different organic
solvents.
Ligand
K1/K2
PC
MeOH
AN
AA15C5
AA18C6
A215C5
A218C6
3.09 ꢀ 106
8.51 ꢀ107
8.91 ꢀ109
–
2.75 ꢀ 101
1.82 ꢀ 103
1.38 ꢀ 105
1.90 ꢀ 107
1.55 ꢀ 101
1.23 ꢀ 102
2.45 ꢀ 104
1.38 ꢀ 106

Potentiometric
187
monoaza-crown ethers, irrespective of the size of the macrocycle. This is due both to the
high log K1 value for monoaza-18-crown and to the log β2 values (from log K1 + log K2)
of all monoaza-crown ethers [5]. Similarly, we obtained high log K1 values for the investi-
gated ligands (table 2) and likewise, high log K1 + log K2 values. The log K1 of 15.41 for
+
A218C6 is the highest value in PC; we have no evidence that AgL2 complex ion is
formed in PC with this ligand. We suggest that the differences in log K1 values and log K2
+
values are due to the different extents to which the Ag(I) is shielded in AgL+ and AgL2
and are solvent-dependent.
3.3. Discussion of structure of complexes
Application of the Born–Haber cycle to the complex formation reactions equations (1) and
(2) in two solvents (PC; S = MeOH, AN) leads to the following relationships between
stability constants and free energies of transfer of the species involved in the equilibrium
[30,31]:
ꢁ2:303RT log½K1ðSÞ=K1ðPCÞꢂ ¼ DGtrðAgLþÞ ꢁ DGtrðLþÞ ꢁ DGtrðAgþÞ
ꢁ2:303RT log½b2ðSÞ=b2ðPCÞꢂ ¼ DGtrðAgLþ2 Þ ꢁ 2DGtrðLþÞ ꢁ DGtrðAgþÞ
ð3Þ
ð4Þ
Substitution of the free energy of transfer of Ag+ into equations (3) and (4) gives the
difference in the free energy of transfer of complexes and the corresponding ligands; the
values obtained are presented in table 4. The free energy of transfer ΔGtr of Ag(I) from
PC to the other organic solvents presented changes in the order AN (ꢁ42.0 kJ Mꢁ1
)
< MeOH (ꢁ12.2 kJ Mꢁ1) [32]. The more the value of ΔGtr(AgL+)ꢁΔGtr(L) from equation
+
(3) and of ΔGtr(AgL2 )ꢁ2ΔGtr(L) from equation (4) approaches zero, the weaker the
access of solvent to Ag(I) ion in AgL+ or AgL2 [5]. Literature data show that for the
+
AgL+ inclusion type complex with cryptates Ag(I) is well shielded from the solvent [7,
30, 31] like the sandwich-type AgL2 with monoaza-crown ethers [5]. For the AgL+ inclu-
+
sion type complex with monoaza-crown ethers Ag(I) is shielded more strongly by the
monoaza-18-crown ether than by the monoaza-15-crown ether [5].
For typical diaaza and AA-diaza-crown ethers, the values of ΔGtr(AgL+)ꢁΔGtr(L)
obtained in AN as compared to PC are low and for A218C6 close to zero. This suggests
that Ag(I) ions are not completely shielded from interaction with solvent in AgL+, except
for the complex with A218C6 in which Ag(I) ions are well shielded. Therefore, we have
Table 4. Differences in the free energies of Ag+ transfer from PC to MeOH and AN between Ag+ complexes
and ligand and log β2 values (from equation. log K1 + log K2), with the exception of A218C6 in PC, for
comparison at 25 °C.
ΔGtr(AgL2+)ꢁ
Ligand
ΔGtr(AgL+)ꢁΔGtr(L)
2ΔGtr(L)
log β2
AN
MeOH
AN
AN
MeOH
AN
AA15C5
AA18C6
A215C5
A218C6
ꢁ6.4
ꢁ4.6
ꢁ5.5
0.5
18.2
15.5
19.7
19.3
ꢁ0.9
ꢁ0.6
ꢁ0.7
–
19.7
16.5
24.3
–
16.67
16.61
16.29
–
11.08
11.58
9.90
9.47
9.35
9.05
9.78
12.48

188
J. Kira et al.
Table 5. Changes in enthalpy of formation (kcal/M) of diaza-crown ethers and their complexes with Ag+ cations
calculated by the AM1d method (WinMopac 2007).
Species
Hf (kcal Mꢁ1
)
ΔHf⁄
AA15C5
ꢁ146.884
95.830
AA15C5, Ag+ uncomplexed
AA15C5, Ag+ complexed
AA18C6
11.822
ꢁ84.008
ꢁ88.668
ꢁ84.746
ꢁ89.664
ꢁ194.561
AA18C6, Ag+ uncomplexed
AA18C6, Ag+ complexed
A215C5
48.153
ꢁ40.515
ꢁ147.379
95.335
A215C5, Ag+ uncomplexed
A215C5, Ag+ complexed
A218C6
10.589
ꢁ194.523
A218C6, Ag+ uncomplexed
A218C6, Ag+ complexed
48.191
ꢁ41.473
⁄ΔHf = Hf complexedꢁHf uncomplexed.
Figure 5. The most favorable structures of the 1 : 1 metal-to-ligand complexes with Ag+ cations and diaza-crown
ethers.

Potentiometric
189
no evidence that AgL2 forms in A218C6 in PC. The values of ΔGtr(AgL+)ꢁΔGtr(L) are
lower than those for monoaza crown ethers (ꢁ11.5 for monoaza-15-crown; ꢁ3.5 for
monoaza-18-crown 5). This means that diaza and AA-diaza-crown ethers shield silver(I) in
+
AgL+ better than monoaza crown ethers. For AgL2 ΔGtr(AgL2 )ꢁ2ΔGtr(L) is close to
+
+
zero. This means that Ag(I) ions are well shielded from interactions with solvent; this also
+
suggests a sandwich-like structure of AgL2 complex ions.
MeOH solutions are more complex. The values of ΔGtr(AgL+)ꢁΔGtr(L) and
+
ΔGtr(AgL2 )ꢁ2ΔGtr(L) are positive for all the ligands associated with ligand protonation
or hydrogen bonding by MeOH. This could be a reason for the major decrease in the
stability constants of AgL+ and AgL2 .
+
3.4. Theoretical models of AgL+ complexes
Changes of the enthalpies of formation (ΔHf) of the most favorable AgL+ complexes of
diaza-crown ethers with Ag+ and of uncomplexed molecules calculated by the AM1d
method [17,18] are listed in table 5. The conformers were selected from molecular dynam-
ics and CONFLEX® [19] experiments and are believed to be the global minimum energy
conformers in the AM1d semiempirical method. In the AA15C5-Ag+ and A215C5-Ag+
structures, the silver is located in the middle of the macrocyclic ring and is shielded from
further planar interactions (figure 5). For AA18C6-Ag+ and A218C8-Ag+ structures, it is
located in the middle of the macrocyclic ring but this is twisted over the ion which is then
+
shielded even better. Structures of AgL2 were not considered because the CONFLEX®’s
algorithm does not rotate one ring over another. The theoretical structures of the AgL+
complexes verify the conclusions drawn from experimental data.
4. Conclusion
The results obtained in this work show that controlling the size and the arrangement of
nitrogens in the macrocyclic ring has a significant influence on designing chemical
molecules for specific purposes. With regard to monoaza-crown ethers [5], diaza-crown
ethers, and AA-diaza-crown ethers, one observes an increase in stability constants of the
1 : 1 metal-to-ligand complexes with Ag+ in the series of monoaza-crown ethers < AA-
diaza-crown ethers < diaza-crown ethers. A similar effect is observed in the series
AA15C5 < AA18C6 and A215C5 < A218C6. Knowledge of how a metal ion is shielded by
a molecule interacting with it is a key to understanding the role of a complexing center in
larger supramolecules. The diaza-crown ethers studied here are N-substituted ligands which
provide the opportunity to study more complicated interactions [27,28] between silver(I)
and these specific ligands as a model of interactions in supramolecules.
Acknowledgment
The authors gratefully acknowledge the financial support from Grant DS/8210-4-0177-12
from the University of Gdansk and KBN NN204 122640.

190
J. Kira et al.
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