Syntheses of ester and amide derivatives of calix[6]arene and their complexation affinities towards La3+, Eu3+, and Yb3+

Article abstract of DOI:10.1080/10610278.2019.1650179

One hexaester (1) and three hexaamide derivatives (2–4) of calix[6]arenes were prepared for the first time in one synthetic step, under microwave radiation. Compounds 2 and 3 were novel ones, while ester calix[6]arene derivative 1 and amide derivative 4 were prepared previously, by conventional syntheses. Microwave-assisted syntheses shortened the reaction times from 48 to 2 h. The binding properties of calix[6]arenes towards selected lanthanide cations (La³⁺, Eu³⁺, Yb³⁺) were studied by spectroscopic and mass spectrometric techniques. No complexation was observed with the ester derivative, while compounds 2–4 formed 1:1 complexes. Based on spectrophotometric titrations, the stability constants of resulting complexes could only be estimated (lg K ≥ 6). Calixarene derivatives, as well as their complexes, were analysed by ESI MS and MS/MS spectrometry. Corresponding fragmentation pathways were proposed, and in some cases confirmed by MS³ experiments. The results obtained by different techniques were in accordance.

Full text of DOI:10.1080/10610278.2019.1650179

Supramolecular Chemistry
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Syntheses of ester and amide derivatives of
calix[6]arene and their complexation affinities
towards La3+, Eu3+, and Yb3+
Marina Tranfić Bakić, David Klarić, Maria Soledad Espinosa, Saša Kazazić, Leo
Frkanec, Paola Alejandra Babay & Nives Galić
To cite this article: Marina Tranfić Bakić, David Klarić, Maria Soledad Espinosa, Saša Kazazić,
Leo Frkanec, Paola Alejandra Babay & Nives Galić (2019): Syntheses of ester and amide
derivatives of calix[6]arene and their complexation affinities towards La3+, Eu3+, and Yb3+,
Supramolecular Chemistry, DOI: 10.1080/10610278.2019.1650179
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SUPRAMOLECULAR CHEMISTRY
https://doi.org/10.1080/10610278.2019.1650179
ARTICLE
Syntheses of ester and amide derivatives of calix[6]arene and their
complexation affinities towards La3+, Eu3+, and Yb3+
Marina Tranfić Bakić a, David Klarićb, Maria Soledad Espinosac, Saša Kazazić d, Leo Frkanece,
Paola Alejandra Babayc and Nives Galićb
aDepartment of Chemistry and Biochemistry, Faculty of Food Technology and Biotechnology, University of Zagreb, Zagreb, Croatia;
c
bDepartment of Chemistry, Faculty of Science, University of Zagreb, Zagreb, Croatia; Department of Analytical Chemistry, National
Commission of Atomic Energy, Buenos Aires, Argentina; dDivision of Physical Chemistry, Ruđer Bošković Institute, Zagreb, Croatia; eDivision
of Organic Chemistry and Biochemistry, Ruđer Bošković Institute, Zagreb, Croatia
ABSTRACT
ARTICLE HISTORY
Received 23 April 2019
Accepted 17 July 2019
One hexaester (1) and three hexaamide derivatives (2–4) of calix[6]arenes were prepared for the
first time in one synthetic step, under microwave radiation. Compounds 2 and 3 were novel ones,
while ester calix[6]arene derivative 1 and amide derivative 4 were prepared previously, by conven-
tional syntheses. Microwave-assisted syntheses shortened the reaction times from 48 to 2 h. The
binding properties of calix[6]arenes towards selected lanthanide cations (La3+, Eu3+, Yb3+) were
studied by spectroscopic and mass spectrometric techniques. No complexation was observed with
the ester derivative, while compounds 2–4 formed 1:1 complexes. Based on spectrophotometric
titrations, the stability constants of resulting complexes could only be estimated (lg K ≥ 6).
Calixarene derivatives, as well as their complexes, were analysed by ESI MS and MS/MS spectro-
metry. Corresponding fragmentation pathways were proposed, and in some cases confirmed by
MS3 experiments. The results obtained by different techniques were in accordance.
KEYWORDS
calix[6]arene; lanthanides;
complexation; UV
spectroscopy; mass
spectrometry (MS)
Introduction
separation from nuclear waste and transmutation into
short-lived isotopes is currently the preferred strategy in
nuclear waste management optimization. Within this pro-
cess, intra- and inter-group separation of lanthanide (Ln)
and actinide (An) families of elements is of significant inter-
est for their reuse as fuel for new generation reactors.
Difficulties arise from similar ionic radii and very similar
chemical behaviour of these cations, and therefore
a quest for ligands that could efficiently and selectively
bind and separate them continues (3, 4).
Nuclear radioactive waste management is an important
and quite challenging step in the development of sustain-
able and environmentally safe nuclear fuel cycles. Today,
the standard for used nuclear fuel processing is PUREX
(Plutonium Uranium Refining by Extraction) process,
a large-scale actinide solvent extraction separation, which
recovers uranium and plutonium for reuse in the produc-
tion of new nuclear fuel (1, 2). The by-product of this
process is radiotoxic waste consisting of nitric acid solutions
containing transplutonium long-lived radionuclides. Their
Calixarenes, a family of macrocycles well known for
their sensitive and selective cation binding properties
CONTACT Marina Tranfić Bakić
University of Zagreb, Zagreb, Croatia; Nives Galić
mtranfic@pbf.hr
Department of Chemistry and Biochemistry, Faculty of Food Technology and Biotechnology,
ngalic@chme.pmf.hr Department of Chemistry, Faculty of Science, University of Zagreb, Zagreb,
Croatia
Supplemental data for this article can be accessed here
© 2019 Informa UK Limited, trading as Taylor & Francis Group

2
M. TRANFIĆ BAKIĆ ET AL.
(5), emerged as an obvious choice for extraction of radio-
nuclides from nuclear waste. Furthermore, relatively easy
functionalization of the lower rim of the calixarene annu-
lus provides means to obtain a wide range of scaffolds
which makes them perfect three-dimensional platforms
for molecular design (5, 6). This was the reason why their
utilization in nuclear waste treatments was proposed
some decades ago by Commissariat à l’énergie atomique
et aux énergies alternatives, France (CEA) (3, 7). Ever since,
calixarene-based supramolecular ligands received much
attention as potential extractants to be used in nuclear
waste partitioning (8). Amongst the first calixarene deri-
vatives studied for this purpose were those incorporating
phosphonic groups (9–12), which were proved to be good
extractants for separation of lanthanides and actinides in
the frame of nuclear reprocessing. Unfortunately, these
ligands cannot be completely burned, since they incorpo-
rate phosphorous atoms, which is their significant disad-
vantage. On the other hand, utilization of derivatives
consisting only of C, H, O, and N atoms would enable
their incineration to gaseous products after use without
an increase of secondary solid waste. Having in mind that
amide derivatives of calixarenes have been known to have
a large coordination affinity towards transition and inner
transition metal cations, it is not surprising that they pre-
sent an interesting alternative (13–15). Although several
of such ligands showed good results in respect to com-
plexation and extractions of selected actinides, a quest for
more efficient, selective, and sensitive ligands of desired
chemical composition and properties is still ongoing.
Since 1990s, when it became more available and more
widely used in chemical laboratories, microwave heating
technique has emerged as an alternative approach in
organic synthesis, offering several advantages over conven-
tional heating, most importantly spectacular acceleration of
reactions, higher yields under milder reaction conditions,
and higher product purities (16–18). In calixarene chemis-
try, microwave-assisted chemistry is dominantly used for
the synthesis of p-alkylcalix[4]arenes by condensation reac-
tion of p-alkylated phenols in the presence of formalde-
hyde, which was shown to be much faster and more
efficient compared to conventional procedure (19, 20).
Nevertheless, only a few papers describing modifications
of the lower calixarene rim utilizing microwave radiation
can be found in the literature (21, 22). Research presented
by S. K. Nayak and M. K. Choudhary describes previously
mentioned advantages of this approach when applied to
syntheses of 1,3-dialkyl ethers of calix[4]arene form parent
p-tert-butylcalix[4]arene. Furthermore, it was shown that
microwave-assisted synthesis does not require the utiliza-
tion of anhydrous reaction conditions, which is of tremen-
dous importance in conventional synthesis, and the
obtained calixarene derivatives attained exclusively the
cone conformation, which is also a very significant advan-
tage (23). To the best of our knowledge, the only example
thus far of microwave-assisted modification of the lower
rim of calix[6]arenes was described by Galán et al., utilizing
NaH as a strong base in DMF (24). Herein, we propose
a different microwave-based methodology for modification
of the calix[6]arene lower rim by using K2CO3, amild base, in
acetonitrile.
UV spectroscopy is a very useful tool for studies of
described cation-ligand complexations. On the other
hand, mass spectrometry (MS) offers some advantages in
studies of non-covalent interactions occurring during com-
plexations, such as specificity, sensitivity, and speed, as well
as the possibility of stoichiometry determination (11).
Recently, we have reported the results of studies on
binding properties of some peptidocalixarenes as well
as ester and amide derivatives of calix[4]arenes towards
alkali-metal and selected lanthanide cations (La3+, Ce3+,
Eu3+, Yb3+) (25, 26).
In this paper, we present syntheses, UV-Vis spectroscopic
and MS investigations of ester and amide derivatives of
calix[6]arenes, namely 5,11,17,23,29,35-hexa-tert-butyl
-37,38,39,40,41,42-hexakis-[(ethoxycarbonyl)methoxy]calix
[6]arene
(1),
5,11,17,23,29,35-hexa-tert-butyl-37,38,
39,40,41,42-hexakis-(N-ethyl-carbamoylmethoxy)calix[6]
arene (2), 5,11,17,23,29,35-hexa-tert-butyl-37,38,39,40,41,
42-hexakis-(N-ethyl-N-methyl-carbamoylmethoxy)calix[6]
arene (3), and 5,11,17,23,29,35-hexa-tert-butyl-37,38,39,
40,41,42-hexakis-(N,N-diethyl-carbamoylmethoxy)calix[6]
arene (4) (Scheme 1). Compounds 1, 3, and 4 were pre-
pared with use of microwave radiation, while synthesis of 2
did not require heating and was performed at room tem-
perature. Coordination properties of the prepared ligands
towards selected lanthanide cations (La3+, Eu3+, Yb3+) were
studied.
Materials and methods
Chemicals
All reagents used in the syntheses were of the best grade
commercially available and were used without further
purification. Solvents were purified by standard proce-
dures. Microwave-assisted syntheses were performed in
a Milestone START S Microwave Labstation for Synthesis
microwave reactor. The power of the microwave radiation
ranged from 0 to 500 W and was allowed to vary in order
to maintain the set temperature. Reaction course and
purity of the products were checked by thin-layer chro-
matography (TLC) on Merck, DC-Alufolien Kieselgel 60
1
F254. H NMR and 13C NMR spectra were recorded on
a Bruker Avance AV300 or AV600 MHz spectrometer with
(CH3)4Si as an internal standard.

SUPRAMOLECULAR CHEMISTRY
3
Scheme 1. Structures of the studied calix[6]arene derivatives and corresponding fragmentation pathways.
The salts used in the investigations of complexation of
synthesized ligands were La(NO3)3 × 6H2O, Eu(NO3)3 × 6H2
O, Eu(CF3SO3)3 98%, Yb(NO3)3 × xH2O, (Fluka, p.a. and
Sigma Aldrich, 99.99%). The solvents, acetonitrile (Merck,
Uvasol) and methanol (Merck, Uvasol), were used without
further purification.
71.4, 73.4, 126.9, 128.5, 131.7, 132.2, 132.8, 132.9, 133.3,
145.5, 145.8, 146.0, 146.5, 150.9, 151.9, 152.1, 152.8, 153.3,
154.2, 167.4, 167.7, 168.1, 168.2, 168.6; 1H NMR (DMSO-d6,
300 MHz, 100°C), δ/ppm: 0.70–1.61 (m, 72H, C(CH3)3 and
NHCH2CH3), 2.55–4.83 (m, 36H, ArCH2Ar,OCH2CO, and
NHCH2CH3), 6.47–8.44 (m, 18H, ArH and NH). MS(MALDI
TOF/TOF): m/z = 1483.9524 [M + H]+. (C90H126N6O12, exact
mass = 1483.9512).
Syntheses
Microwave-assisted synthesis of 5,11,17,23,29,35-hexa-
tert-butyl-37,38,39,40,41,42-hexakis-(N-ethyl-N-methyl-car
bamoylmethoxy)calix[6]arene (3)
Microwave-assisted synthesis of 5,11,17,23,29,35-hexa-tert
-butyl-37,38,39,40,41,42-hexakis-[(ethoxycarbonyl)meth-
oxy]calix[6]arene (1)
In 5 cm3 of acetonitrile, 0.25 g of 4-tert-butylcalix[6]
arene (5,11,17,23,29,35-hexa-tert-butylcalix[6]arene-37,
38,39,40,41,42-hexol) (0.25 mmol), 680 μL of 2-bromo-
N-ethyl-N-methylacetamide (3.75 mmol), and 1.04 g of
potassium carbonate (7.50 mmol) were suspended and
stirred for 2 h at 94°C while exposed to microwave
radiation. The organic solvent was evaporated and the
reaction mixture was then extracted with dichloro-
methane/water. The dichloromethane layer was col-
lected and evaporated, and the residue was
recrystallized from acetonitrile to give 282 mg of com-
pound 3 (72% yield).
In 5 cm3 of acetonitrile, 0.39 g of 4-tert-butylcalix[6]arene
(5,11,17,23,29,35-hexa-tert-butylcalix[6]arene-
37,38,39,40,41,42-hexol) (0.40 mmol), 1.01 g of 2-bro-
moethyl acetate (6.03 mmol), and 1.67 g of potassium
carbonate (12.06 mmol) were suspended and stirred for 2
h at 92°C while exposed to microwave radiation. The
organic solvent was evaporated and the reaction mixture
was then extracted with dichloromethane/water. The
dichloromethane layer was collected and evaporated, and
the residue was recrystallized from absolute ethanol to give
560 mg of compound 1 (94% yield). NMR and IR spectra
correspond to the literature data (27).
IR (KBr, ν/cm−1): 2957, 1659, 1481; 1H NMR (CDCl3, 300
MHz, 25°C), δ/ppm: 0.71–1.42 (m, 54H, C(CH3)3), 2.38–4.95
(m, 54H, ArCH2Ar, OCH2CO, NCH2CH3, and NHCH2CH3),
3.32 (s, 18H, NCH3), 6.40–6.76 (m, 4H, ArH), 7.23–7.76 (m,
8H, ArH); 13C NMR (CDCl3, 300 MHz), δ/ppm: 12.4, 12.6,
12.6, 12.6, 13.1, 14.0, 14.0, 14.1, 14.1, 14.2, 31.4, 31.5, 31.6,
31.7, 31.8, 32.6, 32.7, 32.8, 34.0, 34.2, 34.2, 34.3, 34.3, 34.4,
34.4, 34.5, 41.8, 42.2, 42.3, 42.3, 42.4, 43.3, 43.4, 43.6, 43.8,
43.9, 43.9, 70.3, 71.1, 71.9, 123.5, 128.1, 128.4, 128.7, 129.1,
132.3, 132.4, 132.9, 133.3, 133.5, 145.3, 145.7, 146.3, 154.1,
154.9, 167.2, 167.6; 1H NMR (DMSO-d6, 300 MHz, 100°C), δ/
ppm: 0.70–1.61 (m, 54H, C(CH3)3), 2.55–4.83 (m, 60H,
ArCH2Ar, NCH3, NCH2CH3, and NHCH2CH3), 4.23–4.94 (bs,
12H, OCH2CO), 6.19–7.94 (m, 12H, ArH). MS (MALDI TOF/
TOF): m/z = 1569.0530 [M + H]+. (C96H138N6O12, exact
mass = 1569.0529)
5,11,17,23,29,35-hexa-tert-butyl-37,38,39,40,41,42-hexa
kis-(N-ethyl-carbamoylmethoxy)calix[6]arene (2)
In a flask with a stopper, 1.0 g (0.87 mmol) of
55,11,17,23,29,35-hexa-tert-butyl-37,38,39,40,41,42-hexak
is-[(ethoxycarbonyl)methoxy]calix[6]arene (1) was dis-
solved in 7 mL of ethylamine. The resulting solution was
left in a sealed flask at room temperature until TLC analy-
sis showed the disappearance of starting hexaethyl ester
1 (usually requiring 10–14 days). The reaction mixture was
evaporated to dryness, and the residue was triturated
with ethanol, filtered and dried. Compound 2 (983 mg)
was obtained as a white solid (74% yield).
IR (KBr, ν/cm−1): 2959, 1666, 1533, 1475; 1H NMR (CDCl3,
300 MHz, 25°C), δ/ppm: 0.75–1.46 (m, 72H, C(CH3)3 and
NHCH2CH3), 2.28–4.90 (m, 36H, ArCH2Ar,OCH2CO, and
NHCH2CH3), 5.95–8.50 (m, 18H, ArH and NH); 13C NMR
(CDCl3, 300 MHz), δ/ppm: 12.4, 14.5, 14.8, 15.1, 15.4, 29.2,
29.5, 31.2, 31.3, 31.4, 31.5, 31.6, 31.7, 31.8, 34.0, 34.4, 63.3,
Microwave-assisted synthesis of 5,11,17,23,29,35-hexa-
tert-butyl-37,38,39,40,41,42-hexakis-(N,N-diethyl-carbamo
ylmethoxy)calix[6]arene (4)

4
M. TRANFIĆ BAKIĆ ET AL.
In 5 cm3 of acetonitrile, 0.60 g of 4-tert-butylcalix
The stock solutions of ligands and metal salts
(c = 1 × 10–3 and 5 × 10–3 mol dm–3) were prepared in
acetonitrile. The molar ratio Ln3+: ligand were 1:1, 5:1 and
10:1 in solutions measured by QqQ. The ligand concentra-
tions in solutions measured by IT amounted to 1 × 10–6 mol
dm–3, while the metal ion concentration was c(Ln3+) = 5
× 10–5 mol dm–3.
[6]arene (5,11,17,23,29,35-hexa-tert-butylcalix[6]arene-
37,38,39,40,41,42-hexol) (0.62 mmol), 1.79 g of 2-bromo-
N,N-diethylacetamide (9.24 mmol), and 2.55 g of potas-
sium carbonate (18.48 mmol) were suspended and stirred
for 2 h at 94°C while exposed to microwave radiation.
Organic solvent was evaporated and the reaction mixture
was then extracted with dichloromethane/water. The
dichloromethane layer was collected and evaporated,
and the residue was recrystallized from methanol to give
777 mg of compound 4 (76% yield).
Results and discussion
Synthesis
NMR and IR spectra correspond to the literature
data (28).
The studied compounds were prepared in one synthetic
step from p-tert-butylcalix[6]arene, under microwave radia-
tion. Ester and amide subunits were introduced into the
lower rim of calix[6]arene by alkylation of phenolic groups
with electrophiles. The conventional synthesis of com-
pound 1 was previously described by Arnaud-Neu et al.
(27) and synthesis of compound 4 by Casnati et al. (28).
Nevertheless, microwave-assisted syntheses presented in
this paper showed some advantages compared to conven-
tional syntheses. The dominant positive effect was signifi-
cantly shorter reaction times. More precisely, while
conventional syntheses were performed under reflux for
48 h, reaction mixtures subjected to microwave radiation
provided the desired products in only 2 h with similar
reaction yields. Another very important advantage was
the absence of necessity for anhydrous reaction conditions,
facilitating the performing of the reaction.
Spectrophotometry and spectrofluorimetry
UV titrations were performed using a Perkin-Elmer λ
25double-beam spectrophotometer, whereas fluorimetric
measurements were carried out using a PerkinElmer LS-55
spectrofluorimeter, both equipped with a thermostatting
device. UV and fluorescence spectra were obtained at
(25.0
0.1) °C using 1 cm optical path length quartz
cells. Spectral changes of solutions of ligands were
recorded upon stepwise additions of a lanthanide salt
solution directly into the measuring cell. Absorbances
were collected with an integration time of 0.2 s, whereas
fluorescence intensities were sampled with scanning
speed of 600 nm min–1, both with a resolution of 0.5
nm. Titrations for each M3+/L system (M3+ stands for
a lanthanide cation and L denotes a calix[6]arene ligand)
were done at least in triplicate. The obtained data were
processed using the HYPERQUAD program (29) and
OriginPro 2016. In the course of determinations of stabi-
lity constants of the complexes, absorption of the nitrate
anion was taken into account.
The obtained calixarene derivatives were characterized
by spectroscopic methods, elemental analysis, and high-
1
resolution mass spectrometry. The H NMR spectra of all
four studied calixarene derivatives in CDCl3 at 25 °C
showed very complex signal patterns, containing a large
number of peaks, which can be ascribed to structural flex-
ibility of calix[6]arenes (30). Since the exchange between
conformations is slow on the NMR timescale, sets of peaks
originating from each separate conformer appear in the
spectrum. The exception was of compound 1, whose
1H NMR spectrum consisted of five broad signals, as
described by Arnaud-Neu (27). In this case, a rise in tem-
perature caused faster exchange between conformers and
thus sharpening of the signals. Similar was observed for
Mass spectrometry
Mass spectra were acquired by Triple Quadrupole 6420
(Agilent Technologies) and Ion trap (Amazon ETD, Bruker
Daltonik, Bremen, Germany) instruments. The working con-
ditions for QqQ were as follows: capillary potential = 3.5 kV,
fragmentor voltage = 135 V, gas flow (N2) = 8 dm3 min–1
and temperature 300 °C. Collision energies used were
10–50 V. Collision gas was N2. The spectra were acquired
in positive ion mode from m/z 100 to 2000. The Ion trap
operated at the following conditions: capillary potential =
4.5 kV, drying gas flow rate: 5 dm3 min-1; drying gas tem-
perature: 250 °C. Collision energy used for CID fragmenta-
tion was in the range 0.2–1.0 V. Helium was used as the
collision gas. The spectra were recorded in the m/z range
50–2500.
1
hexaamide diethyl derivative 4 (28). H NMR spectra of
newly synthesized calixarene derivatives 2 and 3 were
also recorded in DMSO-d6 at 100°C. In their case, on the
contrary, increase in temperature led to merging of peaks
into broad signals, indicating that conversion between the
conformers, although it did become faster, still was not fast
enough to result with a single average peak. This effect was
more pronounced in the case of tertiary methyl-ethyl
amide derivative 3 than for secondary amide derivative 2.

SUPRAMOLECULAR CHEMISTRY
5
The composition of the mixture of conformers was not
further investigated. All proceeding measurements were
performed with mixtures of conformational isomers as
such at 25 °C.
of the complexes with 1:1 stoichiometry and rather strong
binding. In all cases, stability constants of the resulting
complexes were too high to enable their spectrophoto-
metric determination, and could be only estimated at lg
K ≥ 6. As examples, titrations of these ligands with La3+ are
presented in Figures 1, 2, and 3, and their titrations with
Yb(NO3)3, Eu(NO3)3, and Eu(CF3SO3)3 in SM, Figs. S1.5 –
S1.13. The same behaviour was observed for complexation
of the corresponding calix[4]arene derivatives (ester, sec-
ondary ethylamide and tertiary diethylamide) with the
same cations (26).
It is known that calix[6]arene derivatives with tertiary
amide subunits at the lower rim, such as diethylamine
derivative, strongly bind sodium and potassium cations
(28, 31), as well as strontium cation (32), resulting with
formation of either 1:1 or 1:2 complexes, i.e. two cations
occupying the calixarene cavity. The formation of
a bimetallic complex was also reported for hexamethoxy-
calix[6]arene with titanium(IV)cation (33). On the contrary,
we found that La3+, Yb3+, and Eu3+, which have comparable
ionic radii, form only 1:1 complexes with ligands 2–4. The
reason could be mutual repulsion of triply charged ions in
close proximity inside a limited space offered by the calix-
arene cavity, whereas in the previous example, titanium
cations are incorporated into calixarene cavity in the form
of (CI3TiOTiCl2) units, rather than ‘naked’ Ti4+ cations.
During titrations of ligands 2–4 with Eu(NO3)3, after the
n(M+)/n(L) = 1 break in the titration curve, absorption
increased significantly, as shown in Figures S1.5, S1.8,
and S1.11 in SM. This effect was the result of absorbance
of nitrate ions from Eu(NO3)3 salt as can be seen in Fig.
Spectrophotometric titrations and
spectrofluorimetry
To investigate the affinity of the studied calixarene deriva-
tives towards selected lanthanide cations, spectrophoto-
metric titrations were performed. The stepwise addition of
the acetonitrile solution of La(NO3)3 to the solution of
ligand 1 caused a linear increase in the absorbance (Fig.
S1.1 in Supplemental material, SM). The same trend was
observed for the complexation of ligand 1 with Yb(NO3)3
and Eu(NO3)3 (Figs. S1.2 and S1.3 in SM). On the other hand,
upon addition of the solution of europium triflate, insignif-
icant changes in the absorbance spectrum of ligand 1
occurred (Fig. S1.4 in SM). All this pointed towards the
conclusion that ligand 1 does not bind the studied cations,
and any observed increase in the absorbance can be
accounted for by the absorbance of the nitrate anion,
which absorbs in the same spectral region as the studied
calix[6]arene ligand 1.
Unlike the ester derivative, amide derivatives 2, 3, and 4
showed strong affinity towards the complexation of the
studied lanthanide cations. During the corresponding titra-
tions, a sharp break in titration curves at n(M+)/n(L) = 1
(where M3+ denotes any of the studied cations, and
L calixarene ligands) appeared, suggesting the formation
Figure 1. Spectrophotometric titration of 2 (c = 6.97 × 10–5 mol dm–3) with La3+ (c (La(NO3)3 × 6H2O) = 5.02 × 10–4 mol dm–3) in
acetonitrile. l = 1 cm; ϑ = (25.0 0.1) °C; the spectra are corrected for dilution. Inset: Dependence of absorbance at 271 nm on La3+
concentration.

6
M. TRANFIĆ BAKIĆ ET AL.
Figure 2. Spectrophotometric titration of 3 (c = 7.23 × 10–5 mol dm–3) with La3+ (c (La(NO3)3 × 6H2O) = 5.00 × 10–4 mol dm–3) in
acetonitrile. l = 1 cm; ϑ = (25.0 0.1) °C; the spectra are corrected for dilution. Inset: Dependence of absorbance at 271 nm on La3+
concentration.
Figure 3. Spectrophotometric titration of 4 (c = 6.51 × 10–5 mol dm–3) with La3+ (c (La(NO3)3 × 6H2O) = 5.02 × 10–4 mol dm–3) in
acetonitrile. l = 1 cm; ϑ = (25.0 0.1) °C; the spectra are corrected for dilution. Inset: Dependence of absorbance at 276 nm on La3+
concentration.
S1.5c. The other nitrates (Yb(NO3)3 and La(NO3)3) also
absorb in this particular region of the spectra but with
much lower molar absorptivities. Also, the above men-
tioned increase in absorbance was not noticed during the
titrations with europium triflate.
fluorescence, and the changes upon addition of the
cations were too small to provide any insight into the
complexation process.
Finally, the emission spectra of the ligands and their
complexes in acetonitrile were also recorded. The wave-
length corresponding to absorption maxima (deter-
mined from the UV-Vis spectra) was used as excitation
wavelength for spectrofluorimetric measurements.
Unfortunately, free ligands did not show any significant
Mass spectrometry
MS and MS/MS spectra of compounds 1-4
To get more insight into the complexation reaction of
calixarene derivatives, the mass spectrometry was used.
The MS spectra of compounds 1–4 in acetonitrile are

SUPRAMOLECULAR CHEMISTRY
7
shown in SM (Tables S2.1–S2.5, Fig. S1 in SM). Depending
on the ligand and instrument used, the most intense
signals were assigned to [M + 2H]2+, [M + H + K]2+ or [M
+ Na]+ (M stands for the calixarene molecule). The peaks
corresponding to adducts with impurities already present
Higher fragmentation potential (1.2 V) was applied
for CID experiments on protonated ligand 3 (Fig. S2.8 in
SM). Although the cleavage of calixarene ring and the
loss of four subunits were noticed, as well as the part of
subunit, the signal of parent ion was still the most
intense one, indicating the high stability of compound
3 in the gas phase. This was confirmed by acquiring
MS/MS spectrum on triple quadrupole at 70 V (Fig. S2.9
in SM). Proposed fragmentation scheme is given in Fig.
S2.10 in SM.
The cleavage of bonds in α- and β- position to
heteroatom in subunits was observed during fragmen-
tation of [4 + Na]+ ion resulting with loss of parts of
subunit: C5H10NO (–100 Da), C6H12NO (–114 Da) and C6
H12NO2 (–130 Da) (Fig. S2.11 and S2.12 in SM).
in the system (Na+, K+, NH4 ) are usually observed in MS
+
spectra of calixarene derivatives (11, 26).
Tandem mass spectrometry was used to determine
fragmentation pathways of selected ions: [M + H]+, [M +
2H]2+, [M + Na]+, [M + K]+ and [M + H + K]2+ whenever it
was possible to isolate them. Previously, we have shown
that both homolytic and heterolytic cleavage can occur
during the collision-induced dissociation of calixarenes
(26). Similar cleavage was recorded during fragmentation
of compound 1, heterolytic for [1 + H]+ (loss of 28 Da) and
homolytic for adducts [1 + Na]+, [1 + K]+ and [1+ H + K]2+.
As an example, the fragmentation scheme of [1+ Na]+ is
given in Figure 4, while MS/MS spectra are shown in Figs
S2.2–S2.4 in SM. During adducts fragmentation, loss of
part and whole subunit were noticed. In MS/MS spectrum
of doubly charged [1 + H + K]2+ ion, the formation of
singly charged ions and corresponding signals at higher
masses was observed.
The fragmentation of protonated amide derivative
2 included the loss of C2H6N (–45 Da), C3H6NO (–73
Da) and C4H8NO group (–85 Da). The MS/MS spec-
trum at 0.8 V and fragmentation scheme are given in
Figs. S2.5 and S2.6 in SM. More energy (1.0 V) was
needed to fragment sodiated adduct (Fig. S2.7
in SM).
The fragmentation of protonated amide derivative 2
included the loss of C2H6N (–45 Da), C3H6NO (–73 Da)
and C4H8NO group (–85 Da). The MS/MS spectrum at
0.8 V and fragmentation scheme are given in Figs. S2.5
and S2.6 in SM. More energy (1.0 V) was needed to
fragment sodiated adduct (Fig. S2.7 in SM).
MS and MS/MS spectra of lanthanide complexes
The MS spectra of complexes recorded by QqQ or IT differ
due to different ligand concentrations and Ln3+:ligand
molar ratio. However, some general conclusions can be
pointed out. In MS spectra acquired by QqQ, dominant
signals were those assigned to complexes with Ln3+ like
[M + Ln]3+, while in MS spectra recorded with IT, the
Figure 4. Proposed fragmentation pathway of [1+ Na]+ (m/z 1512) at 0.8V.

8
M. TRANFIĆ BAKIĆ ET AL.
Figure 5. MS2 (a) and MS3 (b) spectra of [2+ Yb(NO3)]2+ (m/z = 857.5) at 0.20 V.
intense signals were assigned to [M + Ln(NO3)2]2+ or even
[M + H + K]2+ ions (Tables S2.6–2.20 in SM). Due to large
cavity in calix[6]arene derivatives, the signals assigned to
compounds were not described in the literature previously.
The coordination properties of calix[6]arenes towards
selected lanthanide cations (La3+, Eu3+, Yb3+) were investi-
gated by spectroscopic and mass spectrometric measure-
ments. No complexation of the cations by the hexaester
derivative was observed, while hexaamide derivatives
strongly bound lanthanide cations, and can, therefore, be
potentially used as good extracting agents.
complexes
with
two
metal
salts
([M
+
Ln(NO3)+Ln(NO3)2]3+ or [M + 2× Ln(NO3)2]2+) were
noticed, which was not the case in MS spectra of analogue
complexes with derivatives of calix[4]arenes.17 However,
the corresponding intensities were low, except in the case
of complexes with Eu3+, indicating that with this particular
ion not only 1:1 complex was formed in solution. Among
studied ligands, the compound 2 can form intramolecular
H-bonds which should be broken upon complexation. As
a consequence, the intensities of the signals assigned to
[M + Ln]3+ complexes were less intense than those
observed in MS spectra of complexes with compounds 3
and 4, indicating a stronger binding of metal ions by these
ligands. The complexation of lanthanides by ester deriva-
tive was not observed at all, which was in agreement with
results obtained by UV titrations.
Acknowledgments
This work has been supported by the Croatian Science
Foundation (projects: IP-2014-09-4841, IP-2014-09-7309, IP-
2018-01-6910, and IP-11-2013-7387) and by Agencia
Nacional de Promoción Científica y Tecnológica (ANPCyT),
Argentina, (project: PICT-2014-1997).
Disclosure statement
No potential conflict of interest was reported by the authors.
MS/MS measurements were performed on [M + Ln]3+
and [M + Ln(NO3)2]2+. As example, the MS2 and MS3
spectra of ytterbium complex with compound 2 are
given on Figure 5, and all other spectra on Figs. S2.16–
S2.34 in SM. As can be seen, the loss of nitro group (–63
Da) followed by loss of C4H8NO group (–86 Da) was
recorded, and confirmed by MS3 experiment. The clea-
vage of the bonds next to phenyl oxygen, and loss of
adequate fragments of calixarene subunits was recorded
for other complexes as well. In general, the complexes
were quite stable, and the increase from ~0.2 to 1 V was
needed to completely fragment the isolated ions.
ORCID
Marina Tranfić Bakić
http://orcid.org/0000-0002-0315-5900
Saša Kazazić
http://orcid.org/0000-0002-6185-2229
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