Unexpected lanthanide cation selectivity of bis-b-ketovinylated diaza-18-crown-6 and open-chain diamines: Cooperative effect of the second keto group

Article abstract of DOI:10.1007/s10847-013-0345-7

New diaza-18-crown-6 based ligands were prepared by the b-ketovinylation of diaza-18-crown-6 by TMS-protected ynones. The obtained N,N0-bis-(3-oxo-3- Arylpropen-1-yl)-7,16-diaza-18-crown-6 ligands (aryl = p-MeOC6H4, p-Me2NC6H4) selectively bind the Ln3? i

Full text of DOI:10.1007/s10847-013-0345-7

J Incl Phenom Macrocycl Chem
DOI 10.1007/s10847-013-0345-7
ORIGINAL ARTICLE
Unexpected lanthanide cation selectivity of bis-b-ketovinylated
diaza-18-crown-6 and open-chain diamines: cooperative effect
of the second keto group
•
Petr K. Sazonov Vasily Yu. Stolyarenko
•
•
Mikhail M. Shtern Irina P. Beletskaya
Received: 29 January 2013 / Accepted: 15 June 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract New diaza-18-crown-6 based ligands were
prepared by the b-ketovinylation of diaza-18-crown-6 by
TMS-protected ynones. The obtained N,N0-bis-(3-oxo-3-
arylpropen-1-yl)-7,16-diaza-18-crown-6 ligands (aryl =
p-MeOC6H4, p-Me2NC6H4) selectively bind the Ln3? in
preference to main group cations, showing high Ln3?/Zn2?
and Ln3?/Pb2? selectivities (from 2 to 5.5 in log bML
values determined by UV-spectrophotometric titration in
CH3OH). The Me2N-derivative is also a holmium-selective
fluoro-ionofore, as its intense emission is almost com-
pletely quenched by Ho3? salts at concentrations as low as
10-5 mol l-1. The monoazacrown analogs with only one
enaminone group complex less strongly with Ln3? and are
less selective, but bis-b-ketovinylated open-chain diamines
(N,N0-dibenzyl-3,6-dioxaoctane-1,8-diamine and N,N0-dib-
enzyldecane-1,10-diamine) show almost the same binding
constants with La3? (log bML = 3.5–3.7) and Pb2?
(log bML = 1.6) as the corresponding diaza-18-crown-6
Keywords Lanthanide cation selectivity ꢀ Azacrown ꢀ
Enaminone ꢀ Ketovinylation
Introduction
Considerable effort has been put into the development of
lanthanide-ion selective reagents [1–6] which is justified
by unique photophysical, magnetic and chemical prop-
erties of lanthanides and their numerous applications
ranging from catalysis to medical diagnostics, optical
imaging and material science [7–10]. Designing receptors
selective for specific metal cations or groups of cations is
a difficult and challenging task. Efficient coordination of
lanthanide cations (Ln3?) usually requires highly polar-
ized carbonyl (C=O) or phosphoryl (P=O) oxygen donor
centers, the anionic carboxylate group being the most
frequent choice. These donor centers are organized for
optimal metal coordination by attachment to a suitable
framework—cyclene, azacrown, calixarene or open-chain
polyamine, and the selectivity of the ligand is tuned by
the choice of the framework as well as the donor groups
[1–7, 11–16].
1
derivative. This and the changes observed in the H NMR
spectra of the azacrown enaminone ligands in the presence
of La3? indicate the exocyclic coordination of Ln3? with
the carbonyl oxygens of the side arms. Rotation around the
=C–N bond is restricted in enaminone ligands and espe-
cially in their complexes with Ln3?, which allows to
explain the observation of two sub-sets of signals in the 1H
NMR spectra of bis-b-ketovinylated diaza-18-crown-6 in
the presence of La(NO3)3 by the existence of two rotamers
of the ligands.
The donor strength of the neutral carbonyl groups is
believed to be increasing in the following order:
amide [ ketone [ aldehydic [ ester [12]. We speculated
that higher polarizability of enaminone system
N–CH=CH–C(O)R, vinylogous to amide, would lead to an
enhanced basicity of the carbonyl oxygen and increased
affinity towards Ln3?. It should be pointed out, that only
enaminones with secondary (or primary) amino groups
have been studied as ligands. They are stabilized in cis-
geometry by intramolecular hydrogen bond and are capable
of forming a chelate cycle when binding the metal cation
[17, 18]. By contrast enaminones with tertiary amino group
P. K. Sazonov (&) ꢀ V. Yu. Stolyarenko ꢀ
M. M. Shtern ꢀ I. P. Beletskaya
Chemistry Department, M.V. Lomonosov Moscow State
University, Leninskie Gory 1, Moscow 119992, Russian
Federation
e-mail: petr@elorg.chem.msu.ru
123

J Incl Phenom Macrocycl Chem
have trans-geometry and their coordination chemistry
remains unexplored.
their selectivity towards lanthanide and main group cations
(Ba2?, Zn2? and Pb2?).
A diaza-18-crown-6 modified with two Ph(CO)CH=CH–
groups at the ring nitrogens, which thus formed an enaminone
moiety (Fig. 1) did indeed show a selectivity towards rare
earth cations (Ln3? and Y3?), a selectivity quite unusual for a
Results and discussion
1
crown ether. The data of H NMR, IR- and UV-spectra all
The enaminone azacrown ligands were prepared by the
direct b-ketovinylation of the corresponding azacrown
ethers using a reaction with TMS-protected aroylacetylenes
[20, 21]. Aroylacetylenes, especially TMS-protected, as
ketovinylation agents are superior to chlorovinylketones
both because of better thermal stability and cleaner reac-
tions. The ketovinylation reaction is performed in methanol
at room temperature and proceeds with a concomitant
solvolitic removal of TMS-group (Scheme 1). However, in
case of compound 2 we have used p-anisyl-b-chlorovi-
nylketone as ketovinylating agent.
pointed to the cation coordination with the oxygen atoms of
the carbonyl groups of the ligand. This fact was reported in a
preliminary communication, however the origin of the
observed selectivity remained unclear [19]. The trans-geom-
etry of the enaminone moiety poses an obvious difficulty: it is
hard to imagine how the cation in the crown cavity could be
reached by both carbonyl oxygens. So the role of the mac-
rocycle has to be clarified as well.
In the current work we present a more detailed study of
the complexation properties of the enaminone ligands
containing one or two b-ketovinyl groups attached to
azacrown or open-chain diamine framework and compare
The enaminone system in compounds 2–7 always has
trans-geometry, this being in contrast to enaminones HN–
CH=CH–C(O)R with secondary nitrogen stabilized in cis-
geometry by intramolecular hydrogen bond [17, 18]. 13C
NMR spectra of the ligands 2–7 show evidence of
restricted rotation of ketovinyl groups around the =C–N
bond, manifested by the inequivalence in the pairs of car-
bon atoms adjacent to the nitrogen in the macrocycle (a and
a0, b and b0 on Scheme 1) and broadening of the corre-
sponding signals. In 1H NMR only signal broadening of the
CH2-groups is observed.
O
O
O
O
O
N
N
O
The metal cation selectivity of the ketovinylated aza-
crown ethers 2–7 was evaluated by UV-spectrophotometric
titration. Enaminone moiety is a strong chromophore
1
Fig. 1 N,N0-Bis-(3-oxo-3-phenylpropen-1-yl)-7,16-diaza-18-crown-6
Scheme 1 b-Ketovinylation of
azacrown ethers
b
O
O
O
O
O
O
a
O
HN
O
MeOH,
Ar
Ar
N
+
Ar
N
Ar
°
C
20
SiMe3
a'
NH
O
O
O
b'
O
or
Ar = p-MeOC6H4 (82%) 2
p-Me2NC6H4 (86%) 3
Cl
O
O
O
O
NH
O
MeOH,
O
+
Ar
°
C
20
O
N
SiMe3
O
O
O
Ar
n
n
n = 1; Ar =
n = 2; Ar =
p-MeOC6H4 (40%) 4
p-Me2NC6H4 (83%) 5
p-MeOC6H4 (91%)
p-Me2NC6H4 (83%)
6
7
123

J Incl Phenom Macrocycl Chem
free ligand 3, 4·10-6 mol·-l1
A
I
+ La(NO3)3 4·10-5 mol·-l1
0.6
500
+ Pb(ClO4)2 2·10-3 mol·-l1
+ Nd(NO3)3 8·10-5 mol·-l1
0.5
0.4
0.3
0.2
0.1
0.0
400
300
200
100
0
+ Pr(NO3)3
8·10-5 mol·-l1
+ Gd(NO3)3 ~10-3 mol·-l1
+ Er(NO3)3 ~ 10-3 mol·-l1
+ HoCl3
+ HoCl3
4·10-5 mol·-l1
9·10-4 mol·-l1
320
340
360
380
400
420
440
460
400
450
500
550
600
λ, nm
λ, nm
Fig. 2 Changes in the UV–absorption spectrum of diazacrown ligand
3 (straight line) upon the addition of La(NO3)3ꢀ6H2O (dashed line)
Fig. 4 Fluorescence spectrum of ligand 3 (in MeOH, straight line)
and its changes the presence of Ln3? and Pb2? cations
Fig. 3 Metal cation (Mn?
)
interactions with enaminone
system
H
H
H
H
h
ν
h
ν
O
N
Mn+
Mn+
O
N
O
N
O
N
H
H
H
H
Mn+
Mn+
(a) the Mn+ stabilizes the ground state
the Mn+ stabilizes the exited state
bathochromic shift
(b)
hypsochromic shift
ꢀ
ꢁ
b
Table 1 The cation binding constants M þ L ꢀ ML for the enaminone diaza- and monoazacrown ethers 1–7 and enaminone 8 in MeOH,
22 °C
O
O
n
N
Z
R = H, MeO, Me2N
R
O
n=1 Z=O,
n=2 Z=O, p-RC6H4C(O)CH=CHN-
O
Metal salt
R, n
log b
Diazacrown ligands
Monoazacrown ligands
1 H, 2
2 MeO, 2
3 Me2N, 2
4 MeO, 1
6 MeO, 2
7 Me2N, 2
8 Me2N, 0
La(NO3)3
Nd(NO3)3
Gd(NO3)3
Er(NO3)3
Yb(NO3)3
YbCl3
2.3a
2.4a
2.8
–
3.5
3.7
4.9
5.6
5.9
6.6
6.4
6.4
1.6
–
1.5
–
1.5
–
1.7
–
1.8
–
–
–
–
–
2.8a
3a
–
3.3
–
1.4
–
1.4
–
3.1
3.8
HoCl3
–
Yb(ClO4)3
Pb(ClO4)2
InCl3
–
1
–
–
–
–
–
–
–
–
1.7
–
1.0
–
B0a
0.9
–
Ba(ClO4)2
Zn(NO3)2
a
0.9
0.8
–
–
–
0.8
Data from Ref. [19]
123

J Incl Phenom Macrocycl Chem
Scheme 2 b-Ketovinylation of
open-chain diamines
X
O
+
N
N
O
PhCH2
CH2Ph
(H3C)2NC6H4
MeOH,
SiMe3
°
C
20
O
NHCH2CH2-X-CH2CH2NH
PhCH2
CH2Ph
(H3C)2N
N(CH3)2
X = -OCH2CH2O- (90%)
9
-(CH2)6- (81%) 10
a
Table 2 The metal cation induced change in 1H NMR chemical shits (Dd,ppm)
E-Et2NCH = CHC(O)C6H4NMe2 8b
of enaminone monoazacrown ether 7 and
(CH3)2N
Hm
Hm
Ho
Hβ
O
O
O
O
Ho
N
a
O
O
Hα
f
e
b
c
d
Dd (ppm)a
Ha
Hb
Ho
Hm
CH2-groups of the crown ether
Metal salt
Ha(Hb)
Hc ? Hd
He ? Hf
With ligand 7
KSCN
0.03
0.37
0.59
0.13
0.60
0.21
0.01
0.01
0.13
0.01
0.125
0.50
0.04
0.12
0.48
0.00
0.05
0.40
0.00
Pb(ClO4)2ꢀ3H2O
La(NO3)3ꢀ6H2O
With ligand 8
La(NO3)3ꢀ6H2O
a
0.02
-0.04
0.79
0.13
0.13
-0.09
0.08
–
–
Dd = d(complex) - d(crown ether)
Solvent acetone-d6, 20 °C, C * 5 9 10-3 mol l-1, 3 equivalents of metal salt
b
absorbing in near UV-region (kmax varies from 342 nm for
unsubstituted in the Ar-ring derivative 1 to 368 nm for the
p-Me2N-substituted derivatives 3, 5 and 7).
metal salts. Bathochromic shift can be explained by the
coordination of metal cation to carbonyl oxygen, while
hypsochromic shift and hypochromic effect are expected if
the cation is coordinated to the nitrogen of the enaminone
system (Fig. 3).
Addition of La(NO3)3 to a methanolic solution of the
diazacrowns 2 and 3 (C * 10-5 mol l-1) results in a
bathochromic shift and splitting of the absorption band, a
typical titration profile being shown in Fig. 2. Similarly a
bathochromic shift is observed upon the addition of other
Similarly, for monoazacrown based enaminones 4, 6, 7,
a bathochromic shift of the UV-absorption band is
observed in the presence of the metal salts (Ln3?, Pb2?),
though the changes are weaker than in case of diazacrowns
2 and 3.
rare earth metal salts. With non rare earth cations (In3?
,
Pb2?, Zn2?, Ba2?) essentially the same changes in the
spectrum occur at much higher metal salt concentration and
are less pronounced. Only a small hypochromic effect in
the spectrum of 3 is observed with Ba(ClO4)2 salt, and no
changes in the UV-spectrum can be detected with alkaline
The observed change in the absorbance Ai–A0 is directly
proportional to the molar fraction of the complex (R[MLn]/
[L]total), and binding constants can be calculated by non-
linear fitting of the metal salt concentration dependency of
123

J Incl Phenom Macrocycl Chem
this data. The simplest model of equimolar complex for-
nþ
Pb(ClO4)2 was somewhat lower (log b = 1). This strongly
suggests that the donor centres of the crown macrocycle are
not involved in the coordination with Ln3? cations.
p-Dimethylanino-substituted enaminone ligands are
intensely fluorescent and the emission of ligand 3 is almost
completely quenched by Ho3? salts at concentrations as
low as 10-5 mol l-1 (Fig. 4). The effect is selective for
holmium, since other lanthanide cations decrease the
intensity of emission only at much higher concentrations of
Ln3? (*10-3 mol l-1) and their effect is less pronounced,
notwithstanding that the ligand 3 under these conditions is
almost completely bound with Ln3?. On the other hand,
complexation appears to be a prerequisite for efficient
quenching, and the emission of enaminone 8, forming
weak complexes with Ln3? (Table 1), decreases only by
30 % in the presence of HoCl3 (*10-3 mol l-1) and even
less so with other Ln3? salts.
mation Mnþ þ L ꢀ ½MLꢁ
(Eq. 1) accurately fits the
metal salt concentration dependency of the absorbance data
(r2 [ 0.99). It is worth noting that MLn (ML2 for instance)
complexation model gives an equally good fit, however the
titration is usually performed with a large excess of the
metal, and under such conditions the formation of MLn
complexes is unlikely.1 In any case the obtained log b
values of ML complex formation (Table 1) can be used to
compare the binding strength of the ligands towards dif-
ferent metal cations.
b ꢀ ½MꢁiꢀA1
Ai ꢂ AL ¼
ð1Þ
b ꢀ ½Mꢁiþ1
Diazacrown enaminone ligands 2 and 3 form much
more stable complexes with Ln3? cations than with
cations of other metal groups (In3?, Pb2?, Zn2?, Ba2?
)
To further clarify the role of crown macrocycle we have
also prepared two open-chain bis-ketovinylated deriva-
tives, one with a dioxa-podand linker (9), mimicking half
of the diaza-18-crown-6 cycle, and the second one (10)
linked with a simple hydrocarbon C10-chain. For their
synthesis we used the same method as for the ketovinyla-
tion of azacrown ethers (Scheme 2). The starting primary
diamines were first benzylated in order to obtain enami-
nones with tertiary nitrogen and consequently trans-
geometry, to match the trans-geometry of the diaza-18-
crown-6 derivative 3.
confirming the selectivity trend previously observed for
ligand 1 (Table 1). An increase in binding constants with
all cations seen for ligands 2 and 3 in comparison with
ligand 1 is obviously due to the electron donating effect
of MeO- and Me2N-groups. This increase is moderate for
La3? but significant for Yb3? resulting in a pronounced
selectivity of Me2N-substituted ligand 3 towards heavy
lanthanides. High Ln3?/Zn2? and Ln3?/Pb2? selectivity
observed simultaneously in 3 is unusual and probably
unprecedented for a electro-neutral (non-anionic) ligand.
For instance, the ligands with amide donor groups such as
DOTAM [16, 22], and many diaza-18-crown-6 based
ligands [3, 4, 15, 22] form stronger complexes with Pb2?
than with Ln3? cations.
The measured binding constants of podand 9 with
La(NO3)3 (log b = 3.5) and Pb(ClO4)2 (log b = 1.7) were
absolutely the same as for the diazacrown lignad 3
(Table 1). And even bis-enaminone 10 without any addi-
tional oxygen donor centers in the linking chain retained
the same high Ln3?/Pb2? selectivity, while the binding
constants (log b = 3.4 for La(NO3)3, log b = 5.0 for
Yb(ClO4)3 and log b = 1.6 for Pb(ClO4)2) were but
slightly lower, than for the diazacrown lignad 3 (Table 1).
It appears that diaza-18-crown-6 moiety in 3 acts as a
spacer, and the type of the spacer in ligands 3, 9, 10 has but
Monoazacrown based enaminones 4, 6, 7 show a dif-
ferent selectivity pattern towards lanthanide and main-
group cations than their diazacrown analogs. Their com-
plexes with Ln3? cations are about two orders of magni-
tude less stable, while with Pb2? and Zn2? almost equal
binding constants are obtained for monoazacrown 7 and
diazacrown 3 ligands (Table 1). As a result the monoaza-
crown based enaminones do not show the same strong
Ln3? cation selectivity observed for diazacrowns 2 and 3.
Moreover, the size of the macrocycle in the monoazacr-
owns 5 and 7 has no effect on the complex stability with
La3?. To check the role of the macrocycle in these com-
plexation phenomena we prepared a simple model enami-
none, E-Et2NCH=CHC(O)C6H4NMe2-p (8). Surprisingly,
there was no difference in the stability of the La(NO3)3
complexes of enaminone 8 (log b = 1.8) and monoaza-
crown ligand 7, while the stability of its complex with
1
The condition of a large excess of the metal is fulfilled in the most
titration experiments except for the more stable complexes (logb [ 5)
with heavier lanthanides, when the ML2 complexes can be also
formed. However, the bML are meaningful for comparison with other
metals.
8.5
8.0
7.5
7.0
6.5
6.0
PPM
Fig. 5 The 1H NMR spectra of ligand 3 in the presence of
La(NO3)3ꢀ6H2O in acetone d6, r.t
123

J Incl Phenom Macrocycl Chem
1
a
Table 3 The change in H NMR chemical shits (Dd,ppm) of bis-enaminone ligands 2, 3, 9, 10 in the presence of La(NO3)3ꢀ6H2Ob
Dd (ppm)a
Ligand
Ha
Hb
Ho
Hm
2
0.54
0.48
0.57
0.59
0.48
0.48
0.12
0.04
0.20
0.21
-0.06
-0.07
0.05
-0.19
-0.23
3
0.03
9
From 0.49 to 0.81c
From 0.54 to 0.76c
From 0.12 to 0.36c
From 0.06 to 0.18c
From 0.09 to 0.19
From 0.12 to 0.19
-0.03
10
From -0.02 to 0.10
a
Dd = d(complex) - d(crown ether)
b
c
Solvent acetone-d6, 20 °C, C * 5 9 10-3 mol l-1, 3 equivalents of metal salt. For proton labeling see caption to Table 2
At least four doublets observed for each Ha and Hb protons
Fig. 6 Two rotamers of
ligand 3
NMe2
O
O
O
O
O
O
O
O
O
N
N
O
O
N
N
O
Me2N
NMe2
Me2N
Fig. 7 Tentative structures of
complexes formed by rotamers
of ligand 3 with Ln3?
N
O
N
O
N
O
O
N
O
O
O
O
O
O
Ln3+
O
O
3+
Ln
Me2N
NMe2
anty
C2-symmetry
syn
Cs-symmetry
NMe2
NMe2
a small effect on binding properties. However, there is a
significant effect of a second enaminone group i.e. the
difference between the ligands with one (7, 8) and with two
(3, 9, 10) enaminone groups.
MS spectrum, indicating depolymerization of the complex
under laser desorption. Upon the addition of lanthanide
salts precipitates are also formed with other bis-enaminone
ligands (2, 9, 10) but not with mono-enaminone ligands
4–8.
We did not manage to isolate the complexes of enami-
none ligands with lanthanide salts in the crystal state.
When a solution of ligand 3 (C * 0.04 M) in acetone is
mixed with equimolar amount of La(NO3)3 a glassy pre-
cipitate is immediately formed, which dissolves again upon
the addition of more La(NO3)3. The elemental analysis of
the precipitate dried in vacuo closely matches the L3M2
composition, the same as found previously for the complex
with ligand 1 [19], and corresponding to a three-dimen-
sional coordination-polymer with three CO groups coor-
dinated to one metal cation. However, a peak of a
monomeric [ML-NO3]? complex is observed in MALDI
To study the enaminone–metal cation interaction in
solution on a more molecular level and possibly to gain
further insight into the origin of Ln3? cation selectivity we
1
used the H NMR spectroscopy. The metal salt was added
in excess, to secure the complete binding of the ligand with
lanthanide cations.
1
A simple but characteristic picture is seen in the H
NMR spectra of ketovinylated monoazacrown ethers
(Table 2). With each of the three representative cations
(K?, Pb2? and La3?) some changes are observed in the
spectrum, but in each case the character of these changes is
123

J Incl Phenom Macrocycl Chem
different. The alkaline metal cation (K?) produces the
1
weakest effect in H NMR, which agrees with the lack of
complexation in methanolic solution.2 Only the CH2-pro-
tons of the crown-cycle and vinylic proton Hb show a
downfield shift, a pattern corresponding to the coordination
of the cation with the donor centres of the macrocycle only.
Group 14 cation (Pb2?) causes a larger downfield shift of
both vinylic protons signals, Hb and Ha, as well as the
protons of the CH2-groups of the crown-cycle. It can be
explained by assuming that both the crown-macrocycle and
the carbonyl oxygen of the enaminone group participate in
Pb2? coordination (cf. with the lower stability of the Pb2?
complexes of podands 8 and 10 compared to azacrown
based ligands 7 and 3). And finally, in the case of a lan-
thanide group cation (La3?) the strongest downfield shift is
observed for vinylic proton Ha, while the shifts of the CH2-
groups of the macrocycle remain almost unchanged. This
last observation indicates clearly that the crown macrocy-
cle in enaminone ligands is not involved in lanthanide
cation coordination. Besides, essentially the same shift
pattern is observed for enaminone 8 without a crown-ether
moiety. On the other hand, the stronger downfield shift of
Ha compared to Hb, is in agreement with the cation coor-
dination by the oxygen atom of the carbonyl group (as
follows from the polarization shown in Fig. 3).
Hα
Hβ
8.5
8.0
7.5
7.0
6.5
6.0
Fig. 8 The 1H NMR spectra of ligand 10 in the presence of La(NO3)3
in acetone d6, r.t
H
H
β
β
CH2CH2
CH2Ph
CH2Ph
O
N
O
N
H
H
α
α
CH2CH2
Ln3+
Ln3+
Z-isomer
E-isomer
Fig. 9 Z/E-isomerism in the complexes of N-benzyl enaminones
groups of the crown ring. The 13C NMR also confirms the
presence of two types of enaminone groups, showing a pair
of signals for each sp2-carbon, chemically inequivalent in
the free ligand.
Bis-ketovinylated diazacrowns 2 and 3 show a more
1
complex picture in the H NMR spectra upon the addition
Notably, the signals that were broad in the 13C NMR of
the free ligand 3 became narrow, indicating that the rota-
tion around the =C–N bond, occurring on the NMR time-
scale in the ligand, is much slower in the complex with
La(NO3)3 (cf. with Fig. 3b). Restricted rotation around the
=C–N bond will result in two isomeric forms of ligand 3
(Fig. 6), and consequently two isomeric complexes with
Ln3? (Fig. 7). The co-existence of such complexes in
approximately equal amounts explains the observed pat-
of La(NO3)3. Each of the vinylic protons Ha and Hb
appears now not as one doublet, but as a pair of doublets of
approximately equal integral intensity (Fig. 5, Table 3).
Similarly, twice the expected number of signals is observed
for aromatic protons. The spectra clearly manifest the
presence of two types of chemically inequivalent enami-
none groups. On the other hand, the relative shift of dif-
ferent protons upon complexation is similar to that
observed in the mono-ketovinylated azacrown 7 (Table 2),
especially if the shifts of the protons, which were chemi-
cally equivalent in the free ligand, are averaged. A similar
spectral pattern, with doubling of vinylic and aromatic
proton signals, yet spoiled by a much stronger line broad-
ening, is observed for ligand 3 upon the addition of
Y(NO3)3, Lu(NO3)3 or Pb(ClO4)2.
1
terns in H and 13C NMR spectra.3 Moreover, the stacking
of benzene rings in the complex of the syn-rotamer (Fig. 7)
allows to explain the deshielding of one set of Hm and Ho
signals upon complexation (Table 3) by the aromatic ring
current effects, which otherwise would appear rather
unexpected.
Multiple signals for the chemically equivalent protons
are also observed in the 1H NMR spectra of open-chain bis-
enaminones 9 and 10 in the presence of La(NO3)3, but in
The spectrum region corresponding to the CH2-groups
of the macrocycle in 3 also becomes more complex and is
consistent with lowering of the molecular symmetry since
two triplets of 2H integral intensity are observed. The
lowering of the symmetry is confirmed by 13C NMR
spectrum of ligand 3 in the presence of La(NO3)3, which
shows 11 signals (one of double intensity) for the CH2-
3
Alternatively, the occurrence of two subsets of signals of enami-
none groups in 1H and 13C NMR spectra can be explained by the
inequivalence of two enaminone-arms of 3 in the complex with
La(NO3)3. However, such observations as different line broadening
and nonequal (though very close) integral intensity of the signals in
the subsets make this explanation far less plausible. Moreover, the
relative integral intensity of the signals is slightly dependant on
La(NO3)3 concentartion, which can only be explained by the
equilibrium between two complex species.
2
On going from methanol to acetone the metal cation binding
constants are expected to increase by an order of magnitude or more
[22].
123

J Incl Phenom Macrocycl Chem
δ(Ho)
δ(Ho)
7,90
δ(Ho)
7,85
7,95
7,85
7,80
7,80
7,90
7,85
7,75
0,0
0,4
0,8
1,2
0,0 0,2 0,4 0,6 0,8 1,0
0,0 0,2 0,4 0,6 0,8 1,0 1,2
La(NO ) : ligand 7 molar ratio
La(NO3)3 : ligand3 molar ratio
La(NO3)3 : ligand10 molar ratio
a
b
c
3 3
Fig. 10 NMR titration curves of ligands 3 (a) and 10 (b) in methanol-d4 and ligand 7 in acetone-d6 (c) with La(NO3)3ꢀ6H2O
these cases a quadruple set of signals for vinylic protons Ha
and Hb is seen in the spectrum (Fig. 8).
large excess, when simple ML complexes are expected to
be the predominating species.
Additional duplication of signals can again result from
the restricted rotation around =C–N bond, which in the
case on N-benzyl enaminones leads to distinct Z- and
E-forms (Fig. 9). In diazacrowns 2 and 3 the substituents at
nitrogen are the same and such cis/trans-isomerism is not
possible. The strongest cation-induced downfield shift in
ligands 9 and 10 again exhibit the signals of vinylic proton
Ha (Table 3) which is only to be expected for the coordi-
nation of the cation with the carbonyl oxygen.
Conclusions
The present study confirms the selectivity of diaza-18-
crown-6 ligands with two b-ketovinyl side arms towards
lanthanide group cations, Ln3?. Measuring of the binding
constants showed that these ligands bind Ln3? more
strongly than such main group cations as In3?, Pb2?, Zn2?
and Ba2?. A high Ln3?/Zn2? and Ln3?/Pb2? selectivity,
observed simultaneously, is unusual and, to the best of our
knowledge, unprecedented for a non-anionic ligand.
p-Dimethylamino-substituted bis-enaminone ligand 3 also
shows properties of a lanthanide-selective fluoro-ionofore,
as its intense emission is almost completely quenched by
1
In addition to the H NMR spectra recorded with an
excess of metal salt (Tables 2 and 3) we performed NMR-
titrations of mono-(7) and bis-enaminone (3 and 10)
ligands with La(NO3)3, which allowed to estimate the
minimum possible metal : ligand ratio in the formed
complexes.
In all cases the titration curves showed a characteristic
sharp break at a metal : ligand ratio much lower than unity
(Fig. 10). For the two bis-enaminone ligands the break
points are almost the same, 0.30 for crown 3 and 0.27 for
podand 10 (Figs. 10a, b), corresponding to the formation
of L3M or even L4M complexes. Surprisingly, a similar
metal : ligand ratio (0.35) is observed for ligand 7 with
only one enaminone group (Fig. 10c). This raises the
question whether a bidentate coordination mode is realized
for ligands 3 and 10, or only one of two keto-groups is
coordinated to metal cation under such ligand excess
conditions. It needs pointing out, that the results of the
NMR titration should not be extended to the UV-titration
experiments in dilute solutions with metal salt present in a
Ho3? salts at concentrations as low as 10-5 mol l-1
,
making it a potential holmium sensor. It is established that
the cation binding occurs through the oxygen centers of the
carbonyl groups in b-ketovinyl side arms, but the crown
macrocycle acts only as a spacer and bis-ketovinylated
diamines 9 and 10 show the same selectivity pattern.
The second b-ketovinyl group provides a strong coopera-
tive effect, and is essential for Ln3? selectivity of bis-b-ke-
tovinyl ligands, since ligands with only one b-ketovinyl group
(monoazacrown derivatives) form less stable complexes with
Ln3? cations, while the binding constants with Pb2? and Zn2?
cations are the same for mono- and bis-b-ketovinyl ligands.
bis-b-Ketovinyl diaza-18-crown-6 ligands (2 and 3)
form two types of complexes with La(NO3)3 as indicated
123

J Incl Phenom Macrocycl Chem
1
1
by H and 13C NMR spectra. Presumably they are formed
176–178 °C. H NMR (CDCl3, 400.13 MHz), d: 3.84 (s,
by two rotameric forms of the ligand, since the rotation
around the =C–N bond becomes slow on NMR time scale
6H), 3.55–3.78 (m, 24H), 5.94 (d, J = 13 Hz, 2H), 6.94 (d,
J = 9 Hz, 4H), 7.72 (d, J = 13 Hz, 2H), 7.92 (d,
J = 9 Hz, 4H). MALDI-MS, m/z = 584 [M?H].
when the carbonyl oxygen is coordinated to La3?
.
b-Ketvinylation of azacrown ethers and open-chain
diamines
Experimental
NMR spectra were recorded using Bruker Avance spec-
trometer operating at 400.13 (1H) and 100.62 (13C) MHz
with tetramethylsilane as a reference for 1H and 13C.
MALDI MS spectra were obtained on Bruker Daltonics
Autoflex II spectrometer equipped with N2 laser (337 nm),
accelerating voltage 19 kV, in positive ion mode using
dithranol as a matrix. UV-spectrophotometric titrations
were carried out on a Hewlett-Packard 8452 and Jenway
6310 UV–Visible spectrophotometers. Fluorescence spec-
tra of ligands 3 and 8 were measured with PerkinElmer LS
55 fluorescence spectrometer at 370 nm excitation wave-
length. All spectra were measured at the Chemistry
department of the M.V. Lomonosov MSU.
The N,N0-benzylated derivatived of 3,6-dioxaoctane-
1,8-diamine and decane-1,10-diamine were prepared by the
NaBH4 reduction of the Shiff bases obtained from
benzaldehyde and the corresponding amine [23, 24]. The
TMS-protected aroylacetylenes were prepared by the
Sonogashira coupling of trimethylsilylacetylene with aroyl-
chlorides according to the published procedure [20, 21].
1-(4-Dimethylaminophenyl)-3-trimethylsilylprop-2-yne-
1-one. Pd(PPh3)2Cl2 (84 mg, 2 mol %) and CuI (42 mg,
4 mol %) were dispersed in 10 ml THF under argon, then
4-dimethylaminobenzoylchloride (1.10 g, 6 mmol), trieth-
ylamine (0.84 ml, 6 mmol) and trimethylsilylacetylene
(0.84 ml, 6 mmol) were consecutively added. After stir-
ring the reaction mixture for 2 h the solvent was removed
and the crude product purified by column chromatography
on silica gel using benzene: CH2Cl2 (1:2) as eluent. Yield
1.10 g (82 %), mp 48 °C. 1H NMR (CDCl3, 400.13 MHz),
d: 0.30 (s, 9H), 3.09 (s, 6H), 6.66 (d, J = 9 Hz, 2H), 8.03
(d, J = 9 Hz, 2H). 13C NMR (CDCl3, 100.62 MHz), d:
176.16, 154.69, 132.58, 125.62, 111.09, 102.17, 98.32,
40.56, 0.10. Elemental analysis. Calc. for C14H19NOSi
(%): C, 68.52; H, 7.80; N, 5.71. Found: C, 68.39; H, 7.76;
N, 5.51.
General procedure for the preparation of compounds 3–10.
To a solution of amine (0.5 mmol) in methanol (1 ml)
was added a solution of TMS-protected aroylacetylene
(0.55 mmol per each NH-group) in methanol (2 ml).
After stirring for 2 h the solvent was removed under
reduced pressure and the crude product purified by col-
umn chromatography on silica gel Merck 35/70. The
column was first eluted with CH2Cl2 to remove the
starting aroylacetylene and then the product was eluted by
gradually adding a small amount of CH3OH (2–10 %) to
CH2Cl2 eluent.
7,16-Bis-(3-oxo-3-(4-dimethylaminophenyl)propen-1-yl)-
diaza-18-crown-6 (3) was obtained from diaza-18-crown-6
(131 mg, 0.50 mmol) and 1-(4-dimethylaminophenyl)-3-
trimethylsilylprop-2-yne-1-one (272 mg, 1.11 mmol). Yield
261 mg (86 %), mp 195–197 °C. 1H NMR (CDCl3,
400.13 MHz), d : 3.02 (s, 12H), 3.55 (t, J = 6 Hz, 8H), 3.63
(s, 8H), 3.71 (t, J = 6 Hz, 8H), 5.82 (d, J = 13 Hz, 2H),
6.66 (d, J = 9 Hz, 4H), 7.75 (d, J = 13 Hz, 2H), 7.85 (d,
J = 9 Hz, 4H). 13C NMR (CDCl3, 100.62 MHz), d: 187.35,
152.42, 127.83 (Cquart), 152.19, 92.07 (CHvinylic), 129.41,
110.79 (CHaromatic), 70.86, 71.0 br, 67.8 br, 56.6 br, 49.9 br
(CH2), 40.15 (CH3). MALDI-MS, m/z = 609.33 [M?H].
Elemental analysis. Calc. for C34H48N4O6 (%): C, 67.08; H,
7.95; N, 9.20. Found: C, 67.06; H, 7.95; N, 9.08.
13-(3-oxo-3-(4-methoxyphenyl)propen-1-yl)-aza-15-
crown-5 (4) was obtained from aza-15-crown-5 (219 mg,
1.00 mmol) and 1-(4-methoxyphenyl)-3-trimethylsilylprop-
2-yne-1-one (240 mg, 1.03 mmol). Yield 142 mg (40 %),
1
oil. H NMR (acetone-d6, 400.13 MHz), d: 3.55–3.78 (m,
20H), 3.85 (s, 3H), 5.93 (d, 1H, J = 12 Hz), 6.95 (d, 2H,
J = 9 Hz), 7.71 (d, 1H, J = 9 Hz), 7.92 (d, 2H, J = 9 Hz).
13C NMR (CDCl3, 100.62 MHz), d: 187.65, 161.97, 133.00
(Cquart), 153.17, 92.33 (CHvinylic), 129.52, 113.28 (CHaro-
matic), 71.09, 70.36 br, 70.16 br, 70.09 br, 67.1 br, 57.8 br,
51.1 br (CH2), 55.32 (CH3). MALDI-MS, m/z = 380,
[M?H]. Elemental analysis. Calc. for C20H29NO6 (%): C,
63.31; H, 7.70; N, 3.69. Found: C, 63.12; H, 7.88; N, 3.68.
13-(3-oxo-3-(4-dimethylaminophenyl)propen-1-yl)-aza-15-
crown-5 (5) was obtained from aza-15-crown-5 (330 mg,
1.50 mmol) and 1-(4-dimethylaminophenyl)-3-trimethylsilyl-
prop-2-yne-1-one (368 mg, 1.50 mmol). Yield 460 mg (81 %),
oil. 1H NMR (CDCl3, 400.13 MHz), d: 3.02 (s, 6H); 3.51–3.77
(m, 20H); 5.84 (d, J = 13 Hz, 1H); 6.67 (d, J = 9 Hz, 2H);
7.76 (d, J = 13 Hz, 1H); 7.86 (d, J = 9 Hz, 2H). 13C NMR
7,16-Bis-(3-oxo-3-(4-methoxyphenyl)propen-1-yl)-diaza-
18-crown-6 (2) was prepared from b-chlorovinyl(p-ani-
syl)ketone (54 mg, 0.27 mmol) and diaza-18-crown-6
(28 mg, 0.108 mmol) in 0.8 ml DMF using CsF (145 mg,
1.00 mmol) as a base. After stirring for 1 h at r.t. the
reaction mixture was diluted with water and the product
was extracted with CH2Cl2 and purified by column chro-
matography on silica gel using petroleum ether: CH2Cl2 :
CH3OH (5:5:1) as eluent. Yield 52 mg (83 %), m.p.
123

J Incl Phenom Macrocycl Chem
(CDCl3, 100.62 MHz), d: 187.86, 152.86, 152.54, 129.90,
128.43, 111.26, 92.80, 71.62, 70.67, 67.69, 58.13, 51.60, 40.61.
16-(3-oxo-3-(4-methoxyphenyl)propen-1-yl)-aza-18-
crown-6 (6) was obtained from aza-18-crown-6 (132 mg,
0.50 mmol) and 1-(4-methoxyphenyl)-3-trimethylsilylprop-
2-yne-1-one (120 mg, 0.52 mmol). Yield 192 mg (91 %),
6H). 13C NMR (CDCl3, 100.62 MHz), d: 187.52, 152.42,
128.48 (Cquart), 129.44, 128.72, 127.6 br, 127.2 br, 110.76
(CHaromatic), 152.66 br, 92.6 br (CHvinylic), 70.70, 70 br, 67.7
br, 61.4 br, 55.6 br, 53.0 br, 47.7 br (CH2), 40.09 (CH3).
N,N0-Bis-(3-oxo-3-(4-dimethylaminophenyl)propen-1-yl)-
N,N0-dibenzyldecane-1,10-diamine (10) was obtained from
N,N0-dibenzyldecane-1,10-diamine (107 mg, 0.30 mmol)
and 1-(4-dimethylaminophenyl)-3-trimethylsilylprop-2-yne-
1
oil. H NMR (acetone-d6, 400.13 MHz), d: 3.59–3.75 (m,
24H), 3.84 (s, 3H), 5.95 (d, 1H, J = 12 Hz), 6.94 (d, 2H,
J = 9 Hz), 7.74 (d, 1H, J = 12 Hz), 7.93 (d, 2H, J =
9 Hz). 13C NMR (CDCl3, 100.62 MHz), d: 187.26, 161.73,
132.93 (Cquart), 153.17, 91.82 (CHvinylic), 129.28, 113.08
(CHaromatic), 70.56 br, 70.53 br, 70.48 br, 67.3 br, 56.6 br,
49.4 br (CH2), 55.14 (CH3). MALDI-MS, m/z = 424
[M?H]. Elemental analysis. Calc. for C22H33NO7 (%): C,
62.39; H, 7.85; N, 3.31. Found: C, 63.10; H, 8.14; N, 3.40.
16-(3-oxo-3-(4-dimethylaminophenyl)propen-1-yl)-aza-
18-crown-6 (7) was obtained from aza-18-crown-6
(132 mg, 0.50 mmol) and 1-(4-dimethylaminophenyl)-3-
trimethylsilylprop-2-yne-1-one (147 mg, 0.60 mmol).
Yield 181 mg (83 %), oil. 1H NMR (acetone-d6,
400.13 MHz), d: 3.02 (c, 6H); 3.57-3.63 (m, 20H); 3.73-
3.77 (m, 4H); 5.95 (d, J = 13 Hz, 1H); 6.71 (d, J = 9 Hz,
1
1-one (145 mg, 0.59 mmol). Yield 152 mg (81 %), oil. H
NMR (acetone-d6, 400.13 MHz), d: 1.31 (m, 12H), 1.62 (br.
t, 4H), 2.99 (s, 12H), 3.33 (br, 4H), 4.56 (s, 4H), 5.98 (d, 2H,
J = 13 Hz), 6.67 (d, J = 9 Hz, 4H), 7.27–7.39 (m, 10H),
7.80 (br. m, 4H), 7.82 (d, 2H, J = 13 Hz). 13C NMR
(CDCl3, 100.62 MHz), d: 187.70, 152.48, 136.5 br, 128.02
(Cquart), 129.46, 128.87, 127.8 br, 127.4 br, 110.87 (CHaro-
matic), 152.66, 92.15 br (CHvinylic), 70 br, 60.1 br, 56.2 br,
52.2 br, 48.0 br, 29.4 br, 29.3 br, 26.9 br (CH2), 40.22
(CH3). MALDI-MS, m/z = 699 [M?H].
Complex of ligand 3 with La(NO3)3 was obtained by
slowly adding a solution of La(NO3)3ꢀ6H2O (8.5 mg,
0.02 mmol) in 0.2 ml methanol to a solution of 7,16-bis-
(3-oxo-3-(4-dimethylaminophenyl)propen-1-yl)-diaza-18-
crown-6 (12 mg, 0.02 mmol) in 0.7 ml acetone: CH2Cl2
(5:2) mixture. The precipitate formed, which was sepa-
rated and dried in vacuo. Elemental analysis. Calc. for
2H); 7.68 (d, J = 13 Hz, 1H); 7.86 (d, J = 9 Hz, 2H). 13
C
NMR (CDCl3, 100.62 MHz), d: 187.16, 152.20, 127.74
(Cquart), 152.26, 91.80 (CHvinylic), 129.24, 110.60 (CHaro-
matic), 70.59, 70.53, 70.3 br, 67.4 br, 56.6 br, 49.4 br
(CH2), 39.94 (CH3). MALDI-MS, m/z = 437, [M?H]
(70 %), m/z = 435, [M-H] (100 %). Elemental analysis.
Calc. for C23H36N2O6 (%): C, 63.28; H, 8.31; N, 6.42.
Found: C, 62.95; H, 8.55; N, 6.27.
C
102H144La2N18O36 (M2L3 complex): C, 49.48; H, 5.86;
N, 10.18; La, 11.22. Found: C, 49.30; H, 5.72; N, 9.99; La,
11.32. MALDI-MS, m/z = 871.2 [ML-NO3]. The precip-
itate dissolves in acetone-d6 in the presence of excess
La(NO3)3ꢀ6H2O (2–6 equivalents) and the complex spe-
cies in the obtained solution were characterized by NMR.
In the 1H NMR spectrum two subsets of signals were
identified, using homodecoupling and NOE experiments,
belonging to two different enaminone groups (slowly
exchanging with each other). 1H NMR (acetone-d6,
400.13 MHz), d, subset a: 6.30 (d, J = 12 Hz, 1H), 6.70
(d, J = 9 Hz, 2H), 8.02 (d, J = 9 Hz, 2H), 8.15 (d,
J = 12 Hz, 1H); subset b: 5.97 (br. d, J = 12 Hz, 1H),
6.49 (d, J = 9 Hz, 2H), 7.80 (d, J = 9 Hz, 2H), 8.16 (d,
J = 12 Hz, 1H); CH2-groups: 3.81 (br. t, J = 4 Hz, 2H),
3.56–3.73 (m, 20H), 3.41 (br. t, J = 4 Hz, 2H); Me2N-
groups: 3.05 (s, 6H), 2.98 (s, 6H). 13C NMR (acetone-d6,
100.62 MHz), d: 187.71, 154.15, 153.97, 125.85, 125.78
(Cquart), 159.14, 158.27, 93.75, 92.37 (CHvinylic), 131.43,
131.38, 111.58, 111.32 (CHaromatic), 72.13, 71.71 (2C),
71.63, 71.16, 69.84, 69.81, 69.60, 59.13, 58.73, 51.59,
50.66 (CH2), 40.04, 39.99 (CH3).
3-Diethylamino-1-(4-dimethylaminophenyl)prop-2-enone
(8) was obtained from diethylamine (0.063 ml, 0.61 mmol)
and 1-(4-dimethylaminophenyl)-3-trimethylsilylprop-2-yne-
1
1-one (150 mg, 0.61 mmol). Yield 134 mg (89 %), oil. H
NMR (acetone-d6, 400.13 MHz), d: 1.21 (t, J = 7 Hz, 6H),
3.00 (s, 6H), 3.38 (q, J = 7 Hz, 4H), 5.89 (d, J = 13 Hz,
1H), 6.70 (d, J = 9 Hz, 2H), 7.64 (d, J = 13 Hz, 1H), 7.84
(d, J = 9 Hz, 2H). 13C NMR (CDCl3, 100.62 MHz), d:
187.24, 152.16, 127.93 (Cquart), 151.08, 90.87 (CHvinylic),
129.12, 110.63 (CHaromatic), 50.1 br, 42.7 br (CH2), 39.96,
14.5 br, 11.6 br, (CH3). MALDI-MS, m/z = 247 [M?H].
N,N0-Bis-(3-oxo-3-(4-dimethylaminophenyl)propen-1-yl)-
N,N0-dibenzyl-3,6-dioxaoctane-1,8-diamine, (9) wasobtained
from N,N0-dibenzyl-3,6-dioxaoctane-1,8-diamine (106 mg,
0.32 mmol) and 1-(4-dimethylaminophenyl)-3-trimethylsi-
lylprop-2-yne-1-one (158 mg, 0.65 mmol). Yield 196 mg
(90 %), oil. 1H NMR (CDCl3, 400.13 MHz), d: 3.00 (s, 12H),
3.41 (br, 4H), 3.52 (s, 4H), 3.60 (t, J = 5 Hz, 4H), 4.49 (s,
4H), 5.88 (d, J = 13 Hz, 2H), 6.66 (d, J = 9 Hz, 4H),
7.19–7.35 (m, 10H), 7.81 (br, 2H), 7.92 (d, J = 9 Hz, 4H). 1H
NMR (acetone-d6, 400.13 MHz), d: 2.99 (s, 12H), 3.50 (br,
4H), 3.60 (s, 4H), 3.69 (br, 4H), 4.60 (s, 4H), 5.99 (br), 6.65
(br. d, J = 8 Hz, 4H), 7.24-7.36 (m, 10H), 7.76–7.84 (br. m,
Cation binding constants by UV–vis spectroscopy
Working solutions were prepared in MeOH (B0.5 % H2O) in
a 10 ml measuring flask by mixing weighed amounts of the
solutions of the ligand (2–10) and the corresponding metal
123

J Incl Phenom Macrocycl Chem
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salt(s) (Ln(NO3)3ꢀ6H2O, LnCl3ꢀ6H2O, Ln(ClO4)3ꢀ6H2O,
Pb(ClO4)2ꢀ3H2O, Ba(ClO4)2, Zn(NO3)2, InCl3). Total con-
centration of the ligand (*5 9 10-5 mol l-1) was kept
constant in a series while the total concentration of the metal
saltwas variable (5 9 10-5–10-1 mol l-1). UV-spectra were
recorded at 22 °C in a 1 cm quartz cell and the obtained
absorbance values (Ai, in 380–420 nm range) were fitted to
Eq. (1) to give the concentrational metal binding constant
(b, Table 1) of equimolar complex and its absorbance (A?) as
objective variables. In cases of more stable complexes
(bML [ 104) when the approximation of [Mtotal] & [Mfree
]
was not always true, the [Mtotal] values were fitted to Eq. (2)
usingthemolarfractionoftheligand(L)boundinthecomplex
as the free variable x. The latter is derived directly from
absorbance values, Ai and A?.
12. Hancock, R.D., Martell, A.E.: Ligand design for selective com-
plexation of metal ions in aqueous solution. Chem. Rev. 89(8),
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Platas-Iglesias, C., Rodriguez-Blas, T.: Macrocyclic receptor
showing extremely high Sr(II)/Ca(II) and Pb(II)/Ca(II) selectivi-
ties with potential application in chelation treatment of Metal
intoxication. Inorg. Chem. 50(8), 3772–3784 (2011)
x
Ai ꢂ A0
A1 ꢂ A0
ð2Þ
½Mtotalꢁi¼
þ x ꢀ ½L0ꢁ
where x ¼
bML ꢀ ð1 ꢂ xÞ
14. Hay, B.P., Hancock, R.D.: The role of donor group orientation as
a factor in metal ion recognition by ligands. Coord. Chem. Rev.
212(1), 61–78 (2001)
Acknowledgments The authors gratefully acknowledge the finan-
cial support of this work by the Program of the President of the
Russian Federation for the State Support of Leading Research Schools
(Grant § HI-6965.2012.3) and thank Dr. Sci. Vladimir L. Ivanov
for obtaining the fluorescence spectra and Dr. Sci. Yu.F. Oprunenko
and Dr. Sci. A.D. Averin (Chemistry Department, M.V. Lomonosov
MSU) for helpful discussions.
15. Esteban, D., Avecilla, F., Platas-Iglesias, C., Mahia, J., de Bias,
A., Rodriguez-Blas, T.: Lead(II) complexes with macrocyclic
receptors derived from 4,13-diaza-18-crown-6. Inorg. Chem.
41(26), 7170 (2002)
16. Hancock, R.D., Maumela, H., De Sousa, A.S.: Macrocyclic
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