New hydrophilic 3-hydroxy-4-pyridinone chelators with ether-derived substituents: Synthesis and evaluation of analytical performance in the determination of iron in waters

Article abstract of DOI:10.1016/j.poly.2018.12.005

Three new hydrophilic 3-hydroxy-4-pyridinone chelators containing ether-derived substituents were prepared by a more sustainable synthetic protocol that involves the use of microwave heating and commercially available amines, allowing the successful produ

Full text of DOI:10.1016/j.poly.2018.12.005

Polyhedron 160 (2019) 145–156
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
New hydrophilic 3-hydroxy-4-pyridinone chelators with ether-derived
substituents: Synthesis and evaluation of analytical performance in the
determination of iron in waters
Tânia Moniz a, , Luís Cunha-Silva , Raquel B.R. Mesquita , Joana L.A. Miranda , André M.N. Silva ,
⇑
a
b
b
a
a
b
a
c,
⇑
Ana M.G. Silva , António O.S.S. Rangel , Baltazar de Castro , Maria Rangel
a
REQUIMTE-LAQV, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, 4169-007 Porto, Portugal
Universidade Católica Portuguesa, CBQF – Centro de Biotecnologia e Química Fina – Laboratório Associado, Escola Superior de Biotecnologia, Rua Arquiteto Lobão Vital 172,
200-374 Porto, Portugal
b
4
c
REQUIMTE-LAQV, Instituto de Ciências Biomédicas de Abel Salazar, Universidade do Porto, 4050-313 Porto, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new hydrophilic 3-hydroxy-4-pyridinone chelators containing ether-derived substituents were
prepared by a more sustainable synthetic protocol that involves the use of microwave heating and com-
mercially available amines, allowing the successful production of highly water soluble chelators with
improved reaction yields and reaction times. Compared with parent 3-hydroxy-4-pyridinone ligands,
the new compounds were obtained faster and in higher amounts, facilitating the scale up of the synthesis,
which is crucial to produce these ligands and their respective iron(III) complexes for many biological,
analytical and agricultural applications. Ligands were fully characterized by H and C Nuclear
Magnetic Resonance, High Resolution Mass Spectrometry and Single Crystal X-ray Diffraction and their
performance in the analytical determination of iron(III) in water samples was evaluated by sequential
injection methods. The results obtained are scientifically relevant, pointing out the potential of the
new and straightforwardly synthesized ligands as analytical reagents for determination of iron(III).
Ó 2018 Elsevier Ltd. All rights reserved.
Received 16 November 2018
Accepted 7 December 2018
Available online 24 December 2018
Keywords:
3
-Hydroxy-4-pyridinone
1
13
Iron chelator
Microwave assisted synthesis
X-ray diffraction
Sequential injection
1
. Introduction
the heterocyclic ring, providing chelators with different denticity,
physicochemical properties, namely the spectroscopic properties,
Hydroxypyridinones are N-heterocyclic chelators containing
two vicinal oxygen atoms belonging to carbonyl and hydroxyl
charge and hydro-lipophilic balance (HLB), without changing their
chelating capacity [18–21]. Accordingly, 3,4-HPO have been conju-
gated with different moieties such as fluorophores [14,22,23], pho-
tosensitizers agents for photodynamic therapy [24,25], aminoacids
[26] and glycosyl units[27] to be applied in many different fields.
Regarding analytical applications, 3,4-HPO ligands can be used
as greener alternative reagents in colorimetric methods for deter-
mination of iron in natural waters. This is a topic of great interest
since conventional reagents may display high toxicity [28]. Addi-
tionally, in the environmental context, the total iron content is
not as significant as the free ionic iron content, so methods like
Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and
Atomic Absorption Spectroscopy (AAS) are not as useful as the
spectrophotometric determinations. Monitoring the iron content
in natural waters is important due to its impact in esthetic aspects
in water, namely color, and also to possible effects in structures
deployed in the water bodies.
II
III
groups, which confer high affinity towards M /M metal ions.
Among other hydroxypyridinones, namely 1-hydroxy-2-pyridi-
none and 3-hydroxy-2-pyridinone, 3-hydroxy-4-pyridinones (3,4-
HPOs) are the most studied type of ligands [1,2]. In the last dec-
ades, the chemistry and applications of 3,4-HPO have been a cen-
tral topic of research in the groups of Hider [3–8], Santos [1,9]
and Rangel [10–17] thus providing a good number of ligands and
complexes that found application in biomedical, analytical, envi-
ronmental and agriculture applications.
3
,4-HPOs are synthetically versatile allowing functionalization
by introduction of different substituents in several positions of
⇑
Corresponding authors at: Faculdade de Ciências, Universidade do Porto, 4169-
07 Porto, Portugal (T. Moniz). Instituto de Ciências Biomédicas de Abel Salazar,
0
Universidade do Porto, 4050-313 Porto, Portugal (M. Rangel).
E-mail addresses: tania.moniz@fc.up.pt (T. Moniz), mrangel@icbas.up.pt, mcran-
gel@fc.up.pt (M. Rangel).
N-Alkyl-3-hydroxy-4-pyridinones, described in this work as
‘
‘first generation chelators for analytical purposes” (Fig. 1), have
https://doi.org/10.1016/j.poly.2018.12.005
0
277-5387/Ó 2018 Elsevier Ltd. All rights reserved.

1
46
T. Moniz et al. / Polyhedron 160 (2019) 145–156
Fig. 1. Numbering and formulae of first and second generation 3,4-HPO used for analytical determination of iron in water samples.
been tested and successfully used for the spectrophotometric
sequential injection determination of iron in waters [29,30],
although we found that the analytical performance is hampered
by ligand’s solubility in aqueous media. In line with this fact, a
new 3,4-HPO ligand (PEG-HPO or MRB12 [12]) functionalized with
a hydrophilic ethylene glycol chain to improve its solubility in
water was designed and designated as ‘‘second generation 3,4-
HPO analytical applications” (Fig. 1). Both the ligand and its iron
of metal ions in the determination of iron in water samples was
also investigated for all the ligands.
2. Experimental
2.1. Materials and physical measurements
(
III) complex proved to be highly soluble in water providing a
Chemicals were obtained from Sigma–Aldrich (grade puriss,
p.a.) or Fluka (p.a.) and were used as received unless otherwise
specified. The 3-benzyloxy-2-methyl-4-pyrone (Scheme 1, A) was
synthesized in our laboratory following the procedures described
in the literature [31].
lower detection limit and higher sensitivity in flow-based methods
for iron determination in water samples [11,12]. As a disadvantage,
we found that the synthesis of MRB12 is costly and poorly efficient
presenting numerous reaction steps and a low reaction yield.
In our point of view, the vast number of applications of 3,4-HPO
chelators and in particular their promising analytical performance
justifies an effort to improve their preparation by means of more
efficient and sustainable synthetic routes. Herein, we report the
synthesis of other water soluble ‘‘second generation 3,4-HPO
ligands” containing ether groups (MRB13, MRB15 and MRB16,
Fig. 1). The use of microwave-assisted organic chemistry instead
of ‘‘traditional” reflux in oil-bath [12,15,23], allowed reduction of
both the reaction times and the possible formation of secondary
products. We considered that the use of simpler ether substituents
in the nitrogen atom of the heterocyclic ring could reduce the
number of reagents as also the number of synthetic steps and con-
sequently improve the efficiency of the synthetic approach without
compromising water solubility and the analytical performance.
Microwave-assisted reactions were carried out in a CEM Discov-
ery Labmate circular single-mode cavity instrument (300 W max
magnetron power output) from CEM Corporation, using standard
Pyrex vessels (capacity 30 mL) under sealed vessel conditions.
NMR spectra were recorded on a Bruker Avance III 400 spectrom-
1
13
eter, operating at 400.15 MHz for H and 100.62 MHz for
C
atoms, equipped with pulse gradient units, capable of producing
magnetic field pulsed gradients in the z-direction of 50.0 G/cm.
1
1
Two-dimensional H/ H correlation spectra (COSY), gradient
1
13
selected H/ C heteronuclear single quantum coherence (HSQC)
1
13
and H/ C heteronuclear multiple bond coherence (HMBC) spectra
were acquired using the standard Bruker software.
High resolution electrospray ionization mass spectra (ESI-MS)
were obtained in a Thermo Scientific LTQ-Orbitrap XL mass spec-
trometer, externally calibrated with a standard kit provided by
the manufacturer. The spectrometer was operated in the positive
ionization mode setting the capillary voltage to +3.0 kV, sheath
gas flow to 6 and the temperature of the ion transfer capillary to
275 °C. Spectra were recorded for m/z values between 250 and
2000 in the Fourier Transform (FT) mode with resolution (FWHM)
set at 60 000. Samples were prepared in water, diluted in a 50%
(v/v) water:methanol mixture immediately before analysis and
1
The structural characterization of all ligands was achieved by H
and 1 C Nuclear Magnetic Resonance, High Resolution Mass Spec-
trometry and Single Crystal X-ray Diffraction.
The 3,4-HPO ligands MRB13, MRB14, MRB15 and MRB16 were
investigated regarding their potential use in the analytical deter-
mination of iron(III) by sequential injection methods (SI) and
results were compared with those reported for PEG-HPO or
MRB12 ligand. The assessment of potential interferences of a set
3

T. Moniz et al. / Polyhedron 160 (2019) 145–156
147
Scheme 1. General approach to access N-alkoxy substituted 3-hydroxy-4-pyridinones.
directly infused into the electrospray ion source utilizing the syr-
centrated to afford compound MRB14p (0.434 g, 69% yield) as dark
ꢀ1
inge pump in the mass spectrometer at 10 ml min . NMR and Mass
spectrometry analyses were performed at Laboratório de Análise
Estrutural, Centro de Materiais da Universidade do Porto (CEMUP)
brown oil.
1
400.15 MHz H NMR (CDCl
3
, ppm): d 2.10 (s, 3H, 2-CH
), 3.49 (t, J 5.0 Hz, 2H, H2 ), 3.92 (t, J 5.0 Hz, 2H, H1 ),
3
), 3.24 (s,
0
0
3H, AOCH
3
(
Portugal).
5.15 (s, 2H, ACH
2
C
6
H
5
), 6.37 (d, J 7.5 Hz, 1H, H5), 7.28–7.30 (m,
H, H6 and 3H, Hmeta+paraACH ), 7.38–7.40 (m, 2H, Hortho-CH
, ppm): d 12.3 (2-CH ), 52.8
), 116.4 (C5),
), 137.2 (Cq,
1
C
2
C
6
H
5
2
-
1
3
6
H
5
). 100.62 MHz C NMR (CDCl
3
3
2
2
.2. Synthesis of protected 3,4-HPO ligands
0
0
(
C1 ), 58.8 (AOCH
3
), 70.9 (C2 ), 72.6 (ACH
C
2 6
H
5
1
27.6 and 127.9 (Cmeta+paraAC
6
H
5
), 128.7 (CorthoAC
6
H
5
0
.2.1. Synthesis of 1-(3 -methoxypropyl)-2-methyl-3-benzyloxy-4-
AC H
6 5
), 139.0 (C6), 141.3 (C2), 145.4 (C3), 173.0 (C4).
(
1H)-pyridinone (MRB13p)
A mixture of amine 1 (3-methoxypropylamine) (2.086 g,
.0234 mol) and protected pyrone (3-benzyloxy-2-methyl-4-pyr-
0
2
(
.2.3. Synthesis of 1-(3 -isopropoxypropyl)-2-methyl-3-benzyloxy-4-
0
1H)-pyridinone (MRB15p)
A mixture of amine 3 (3-isopropoxypropylamine) (1.645 g,
.0141 mol) and protected pyrone (3-benzyloxy-2-methyl-4-pyr-
one) (0.507 g, 0.0024 mol) dissolved in dried ethanol (2 mL) was
placed in a 30 mL reaction vial, which was then closed under argon
atmosphere and placed in the cavity of a CEM microwave reactor.
The reaction vial was irradiated to 80 °C during 4 h, using 100 W
maximum power. The reaction solvent was evaporated and the
one) (0.505 g, 0.0023 mol) dissolved in dried ethanol (2 mL) was
placed in a 30 mL reaction vial, which was then closed under argon
atmosphere and placed in the cavity of a CEM microwave reactor.
The reaction vial was irradiated to 80 °C during 4 h, using 100 W
maximum power. The reaction solvent was evaporated and the
0
2
crude oil resultant was dissolved in 30 mL of H O and the pH
adjusted to 1 with HCl 10%. The starting materials were removed
by liquid/liquid extraction with diethyl ether. The organic layer
was rejected. The pH of aqueous phase was adjusted to 10 with a
solution of NaOH 5% and the product was then extracted to the
organic layer with dichloromethane. The organic phase was con-
centrated to afford compound MRB13p (0.620 g, 92% yield) as dark
brown oil.
2
crude oil resultant was dissolved in 30 mL of H O and the pH
adjusted to 1 with HCl 10%. The starting materials were removed
by liquid/liquid extraction with diethyl ether. The organic layer
was rejected. The pH of aqueous phase was adjusted to 10 with a
solution of NaOH 5% and the product was then extracted to the
organic layer with dichloromethane. The organic phase was con-
centrated to afford compound MRB15p (0.565 g, 89% yield) as dark
brown oil.
00.15 MHz H NMR (CDCl
), 1.83 (quint, J 6.2 Hz, 2H, H2 ), 2.11 (s, 3H, 2-CH
J 5.4 Hz, 2H, H3 ), 3.51 (m, J 6.3 Hz, 1H, 4 -CH), 3.89 (t, J 7.0 Hz,
1
0
4
00.15 MHz H NMR (CDCl
H, 2-CH
.0 Hz, 2H, H1 ), 5.19 (s, 2H, ACH
.24 (d, J 7.5 Hz, 1H, H6), 7.29–7.32 (m, 3H, Hmeta+paraACH
3
, ppm): d 1.83 (m, 2H, H2 ), 2.09 (s,
0
3
7
7
7
3
), 3.26 (t, J 5.6 Hz, 2H, H3 ), 3.29 (s, 3H, AOCH
3
), 3.88 (t, J
), 6.41 (d, J 7.5 Hz, 1H, H5),
),
0
2 6 5
C H
1
0
4
3
, ppm): d 1.14 (d, J 6.3 Hz, 6H, 4 -
2 6 5
C H
0
(
CH
3
)
2
3
), 3.30 (t,
13
.38–7.40 (m, 2H, Hortho-CH
ppm): d 12.2 (2-CH
C3 ), 72.9 (ACH
2
C
6
H
5
). 100.62 MHz C NMR (CDCl
), 30.3 (C2 ), 50.4 (C1 ), 58.6 (AOCH
), 117.0 (C5), 127.9 and 128.2 (Cmeta+para
3
,
0
0
0
0
3
3
), 67.8
0
2
(
H, H1 ), 5.24 (s, 2H, ACH
C
2 6
H
5
), 6.40 (d, J 7.6 Hz, 1H, H5), 7.22
0
(
C
(
2
C H
6 5
-
2 6 5
d, J 7.6 Hz, 1H, H6), 7.28–7.42 (m, 5H, CH C H C
). 100.62 MHz 1
3
6
H
5
), 129.0 (CorthoAC
6
H
5
), 137.5 (Cq, AC ), 138.6 (C6), 141.1
6 5
H
0
0
NMR (CDCl
5
1
3
, ppm): d 12.3 (2-CH
0.5 (C1 ), 63.1 (C3 ), 71.9 (C4 ), 72.9 (ACH
27.9 and 128.2 and 129.2 (-C
3
), 22.0 (4 -(CH
3
)
2
), 30.8 (C2 ),
), 117.2 (C5),
), 138.5 (C6),
C2), 145.9 (C3), 173.2 (C4).
0
0
0
2
6
C H
5
6
H
5
), 137.7 (Cq, AC
6
H
5
0
140.7 (C2), 146.1 (C3), 173.4 (C4).
2
.2.2. Synthesis of 1-(2 -methoxyethyl)-2-methyl-3-benzyloxy-4-
(
1H)-pyridinone (MRB14p)
mixture of amine
.00928 mol) and protected pyrone (3-benzyloxy-2-methyl-4-pyr-
0
A
2
(2-methoxyethylamine) (0.691 g,
2.2.4. Synthesis of 1-(2 -ethoxyethyl)-2-methyl-3-benzyloxy-4-(1H)-
pyridinone (MRB16p)
0
one) (0.501 g; 0.00232 mol) dissolved in dried ethanol (2 mL) was
placed in a 30 mL reaction vial, which was then closed under argon
atmosphere and placed in the cavity of a CEM microwave reactor.
The reaction vial was irradiated to 90 °C during 4 h, using 100 W
maximum power. The reaction solvent was evaporated and the
A
mixture of amine
4
(2-ethoxyethylamine) (0.825 g,
0.0092 mol) and protected pyrone (3-benzyloxy-2-methyl-4-pyr-
one) (0.500 g, 0.00231 mol) dissolved in dried ethanol (2 mL) was
placed in a 30 mL reaction vial, which was then closed under argon
atmosphere and placed in the cavity of a CEM microwave reactor.
The reaction vial was irradiated to 80 °C during 5 h, using 100 W
maximum power. The reaction solvent was evaporated and the
2
crude oil resultant was dissolved in 30 mL of H O and the pH
adjusted to 1 with HCl 10%. The starting materials were removed
by liquid/liquid extraction with diethyl ether. The organic layer
was rejected. The pH of aqueous phase was adjusted to 10 with a
solution of NaOH 5% and the product was then extracted to the
organic layer with dichloromethane. The organic phase was con-
2
crude oil resultant was dissolved in 30 mL of H O and the pH
adjusted to 1 with HCl 10%. The starting materials were removed
by liquid/liquid extraction with diethyl ether. The organic layer
was rejected. The pH of aqueous phase was adjusted to 10 with a

1
48
T. Moniz et al. / Polyhedron 160 (2019) 145–156
1
3
solution of NaOH 5% and the product was then extracted to the
organic layer with dichloromethane. The organic phase was con-
centrated to afford compound MRB16p (0.576, 91% yield) as dark
4
H5), 8.19 (d, J 7.0 Hz, 1H, H6). 100.62 MHz C NMR (MeOD-d ,
0
0
0
ppm): d 13.3 (2-CH
3
), 15.2 (3 -CH
3
), 57.3 (C1 ), 67.7 (C3 ), 69.5
(C2 ), 111.4 (C5), 140.0 (C6), 143.9 (C2), 144.5 (C3), 159.7 (C4).
0
brown oil.
00.15 MHz 1H NMR (CDCl
, ppm): d 1.11 (t, J 7.0 Hz, 3H, 3 -
0
4
3
0
3
CH ), 2.11 (s, 3H, 2-CH
3
), 3.39 (q, J 7.0 Hz, 2H, H3 ), 3.55 (t, J
2.3. Single-crystal X-ray diffraction
0
0
5
6
5
1
1
.0 Hz, 2H, H2 ), 3.92 (t, J 5.0 Hz, 2H, H1 ), 5.19 (s, 2H, ACH
.39 (d, J 7.8 Hz, 1H, H5), 7.26 (d, J 7.8 Hz, 1H, H6), 7.30–7.42 (m,
2
6
C H
5
),
Suitable single crystals of the compounds MRB13-MRB16 were
manually harvested and a mounted on cryoloops using viscous
FOMBLIN Y perfluoropolyether vacuum oil (LVAC 140/13, Sigma-
Aldrich) [32]. Diffraction data were collected at on a Bruker X8
Kappa APEX II Charge-Coupled Device (CCD) area-detector diffrac-
1
3
H, CH
5.0 (3 -CH
2
0
C
6
H
5
). 100.62 MHz C NMR (CDCl
3
, ppm): d 12.7 (2-CH
3
),
),
0
0
0
3
), 53.3 (C1 ), 67.0 (C3 ), 69.1 (C2 ), 73.0 (ACH
2
6
C H
5
16.9 (C5), 127.9 and 128.2 and 129.0 (AC
6 5
H ), 137.6 (Cq,
6 5
AC H
), 139.1 (C6), 141.1 (C2), 145.8 (C3), 173.5 (C4).
tometer controlled by the APEX2 software package [33] (Mo Ka gra-
2
.2.5. Removal of benzyl protecting group of 3,4-HPO ligands
Each protected 3,4-HPO: MRB13p (0.620 g, 0.0022 mol),
phite-monochromated radiation, k = 0.71073 Å), and equipped
with an Oxford Cryosystems Series 700 cryostream monitored
remotely with the software interface Cryopad (acquisition temper-
ature of 150.0(2) K) [34]. Images were processed with the software
MRB14p (0.434 g, 0.0016 mol), MRB15p (0.565 g, 0.0018 mol)
and MRB16p (0.576 g, 0.0020 mol) was dissolved in ethanol
(
20 mL) and HCl (20 mL) and placed into a hydrogenation vessel.
The air was removed with N , a catalytic amount of 10% Pd/C
w/w) was added and the mixture was stirred at room tempera-
SAINT+ [35], and the absorption effects were corrected by the multi-
scan method implemented in SADABS [36]. The structures were
solved using SHELXT-2014 [37,38], which allow the immediate loca-
tion and identification of a considerable number of the heaviest
atoms composing the asymmetric unit. The remaining absent and
misplaced non-hydrogen atoms were located from difference Four-
ier maps from successive full-matrix least-squares refinement
cycles on F using SHELXL-v.2014 [37,39]. All the non-hydrogen
atoms were successfully refined using anisotropic displacement
parameters.
2
(
2
ture, with H at 40 psi for 6 h. The reaction mixtures were filtered,
washed with methanol and chloroform and the solvents were
evaporated in vacuum to give a brown oil product. The resulting
residues were dried under vacuum to give the hydrochloride salt
of each 3,4-HPO: MRB13 (0.477 g, 95% yield), MRB14 (0.366 g,
2
1
00% yield), MRB15 (0.457 g, 98% yield) and MRB16 (0.430, 92%
yield).
Hydrogen atoms bonded to carbon were placed at their geomet-
rical positions using the appropriate HFIX instructions (137 for the
terminal ACH , 23 for the ACH A and 43 for the aromatic groups)
3 2
and included in subsequent refinement cycles in riding-motion
approximation with isotropic thermal displacements parameters
0
1
-(3 -Methoxypropyl)-2-methyl-3-hydroxy-4-1H-pyridinone
+
3
hydrochloride (MRB13). MS: calculated for C10H16NO : 198.1125
monoisotopic molecular weight M ), found: HRMS: 198.1121.
00.15 MHz H NMR (MeOD-d
H2 ), 2.65 (s, 3H, 2-CH
H, H3 ), 4.48 (t, J 7.3 Hz, 2H, H1 ), 7.12 (d, J 6.9 Hz, 1H, H5), 8.13
d, J 6.9 Hz, 1H, H6). 100.62 MHz C NMR (MeOD-d
2.7 (2-CH
+
(
4
1
4
, ppm): d 2.11 (quint, J 6.7 Hz, 2H,
0
3
), 3.32 (s, 3H, AOCH
3
), 3.43 (t, J 5.6 Hz,
(
U
iso) fixed at 1.2 or 1.5 ꢁ Ueq of the relative atom. Furthermore,
0
0
2
(
1
1
the hydrogen atoms associated to hydroxyl groups and the crystal-
lization water molecule (only in the MRB16 structure) were mark-
edly visible in the difference Fourier maps and included in
subsequent refinement stages with the OAH distances restrained
to 0.90(2), and using a riding-motion approximation with an iso-
1
3
4
, ppm): d
), 69.5 (C3 ),
0
0
0
3
), 31.1 (C2 ), 55.4 (C1 ), 59.0 (AOCH
3
11.8 (C5), 139.5 (C6), 143.7 (C2), 145.1 (C3), 159.8 (C4).
0
1
-(2 -Methoxyethyl)-2-methyl-3-hydroxy-4-(1H)-pyridinone
tropic thermal displacement parameter fixed at 1.5 ꢁ U of the
eq
+
hydrochloride (MRB14). MS: calculated for C
monoisotopic molecular weight M ), found: HRMS: 184.0969.
00.15 MHz H NMR (MeOD-d
s, 3H, AOCH
H1 ), 7.16 (d, J 6.8 Hz, 1H, H5), 8.14 (d, J 6.8 Hz, 1H, H6).
00.62 MHz C NMR (MeOD-d
9.4 (AOCH
9
H14NO
3
: 184. 0968
oxygen atom.
+
(
4
(
Information concerning the crystallographic data collection and
structure refinement details is summarized in Table 1, while the
geometrical details about the strong hydrogen bonding interac-
tions are shown in Table 4.
1
4
, ppm): d 2.64 (s, 3H, 2-CH
), 3.78 (t, J 4.7 Hz, 2H, H2 ), 4.59 (t, J 4.7 Hz, 2H,
3
), 3.31
0
3
0
1
3
0
1
5
4
, ppm): d 13.2 (2-CH
3
), 57.2 (C1 ),
0
3
), 71.6 (C2 ), 111.4 (C5), 140.0 (C6), 143.9 (C2), 144.7
(
C3), 159.7 (C4).
2.4. Analytical procedure for iron(III) quantification
0
1
-(3 -Isopropoxypropyl)-2-methyl-3-hydroxy-4-(1H)-pyridinone
2.4.1. Reagents and solutions
+
hydrochloride (MRB15). MS: calculated for C12
monoisotopic molecular weight M ), found: HRMS: 226.1442.
00.15 MHz H NMR (MeOD-d
H20NO
3
: 226.1438
Ligand stock solutions were obtained by dissolving circa 20 mg
of each ligand in 2.0 mL of Milli-Q water (MQW), corresponding to
an approximate concentration of about 42 mmol/L. Working ligand
solutions were obtained by dilution of the stock solutions to final
concentration of about 0.6 mmol/L.
A 0.25 mol/L hydrogen carbonate buffer solution was prepared
by dissolving 1.05 g of sodium hydrogen carbonate (Merck, Ger-
many) in 50 mL of water and adjusting the pH to 10.6 with
0.5 mol/L sodium hydroxide solution A solution of 0.5 M nitric acid
was prepared from dilution of the concentrated acid (d = 1.4; 65%,
Merck, Germany).
+
(
4
1
0
4
, ppm): d 1.12 (d, J 6.3 Hz, 6H, 4 -
0
(
CH
3
)
2
), 2.10 (quint, J 6.4 Hz, 2H, H2 ), 2.66 (s, 3H, 2-CH
3
), 3.49 (t,
0
0
J 5.6 Hz, 2H, H3 ), 3.57 (m, J 6.3 Hz, 1H, 4 -CH), 4.48 (t, J 7.0 Hz,
H, H1 ), 7.10 (d, J 7.0 Hz, 1H, H5), 8.13 (d, J 7.0 Hz, 1H, H6).
00.62 MHz C NMR (MeOD-d
0
2
1
1
3
0
4
, ppm): d 12.8 (2-CH
3
), 22.3 (4 -
0
0
0
0
(
CH
3
)
2
), 31.5 (C2 ), 55.6 (C1 ), 65.0 (C3 ), 73.1 (C4 ), 111.6 (C5),
1
39.4 (C6), 143.6 (C2), 145.1 (C3), 159.6 (C4).
0
1
-(2 -Ethoxyethyl)-2-methyl-3-hydroxy-4-(1H)-pyridinone
+
hydrochloride (MRB16). MS: calculated for C10
monoisotopic molecular weight M ), found: HRMS: 198.1125.
00.15 MHz H NMR (MeOD-d
), 2.67 (s, 3H, 2-CH
H16NO
3
: 198.1125
An iron(III) stock solution of 10 mg/L (180 mM) was obtained by
dilution of the atomic absorption standard (1001 mg/L Fluka –
Sigma-Aldrich, Switzerland). This solution was used to prepare
+
(
4
CH
5
1
0
4
, ppm): d 1.10 (t, J 7.0 Hz, 3H, 3 -
0
3+
3
3
), 3.48 (q, J 7.0 Hz, 2H, H3 ), 3.85 (t, J
the Fe working standards in the dynamic range 0.1–1.0 mg/L
0
0
.0 Hz, 2H, H2 ), 4.62 (t, J 5.0 Hz, 2H, H1 ), 7.22 (d, J 7.0 Hz, 1H,
(1.8–18 mmol/L) in 0.03 M nitric acid.

T. Moniz et al. / Polyhedron 160 (2019) 145–156
149
Table 1
Crystal and structure refinement data for compounds MRB13–MRB16.
MRB13
MRB14
14ClNO
219.66
colorless prism
0.19 ꢁ 0.15 ꢁ 0.11
orthorhombic
MRB15
20ClNO
261.74
colorless block
0.14 ꢁ 0.09 ꢁ 0.06
monoclinic
MRB16
18ClNO
251.70
colorless prism
Formula
Mr
C
10
H
16ClNO
3
C
9
H
3
C
12
H
3
C
10
H
4
233.69
Crystal morphology
Crystal size (mm)
Crystal system
Space group
Unit cell dimensions
a (Å)
colorless block
0.12 ꢁ 0.04 ꢁ 0.03
monoclinic
0.20 ꢁ 0.03 ꢁ 0.02
monoclinic
P2
1
/c
P2
1
2
1
2
1
P2
1
/c
1
P2 /c
7.768(2)
13.119(3)
11.731(3)
90
108.919(10)
90
1131.0(5)
4
1.372
7.087(7)
11.804(11)
12.141(11)
90
90
90
1015.6(2)
4
1.437
11.9647(15)
8.6675(11)
13.3471(19)
90
106.863(6)
90
1324.6(3)
4
1.312
7.5041(7)
13.1185(12)
12.6117(12)
90
93.190(4)
90
1239.6(2)
4
1.349
b (Å)
c (Å)
a
(°)
b (°)
(°)
c
3
V (Å )
Z
q
Calc. (g cm 3
ꢀ
)
F(0 0 0)
l
496
0.325
464
0.357
560
0.286
536
0.308
(mmꢀ1)
h range (°)
Index ranges
3.672–25.677
ꢀ9 ꢂ h ꢂ 9
3.750–27.474
ꢀ8 ꢂ h ꢂ 9
ꢀ15 ꢂ k ꢂ 15
ꢀ15 ꢂ l ꢂ 15
8296
3.947–26.370
ꢀ13 ꢂ h ꢂ 14
ꢀ10 ꢂ k ꢂ 10
ꢀ16 ꢂ l ꢂ 16
15 993
3.60–25.025
ꢀ8 ꢂ h ꢂ 8
ꢀ15 ꢂ k ꢂ 15
ꢀ14 ꢂ l ꢂ 14
29 379
ꢀ
ꢀ
15 ꢂ k ꢂ 15
14 ꢂ l ꢂ 11
Reflections collected
Independent reflections
8788
2139
2317
2700
2165
(
R
R
int = 0.0903)
= 0.0499;
wR = 0.1008
= 0.0956;
wR = 0.1186
0.262 and ꢀ0.318
(Rint = 0.0349)
(Rint = 0.0606)
(Rint = 0.0271)
Final R indices [I > 2
r(I)]
1
R
1
= 0.0277;
wR = 0.0669
= 0.0305;
wR = 0.0690
0.202 and ꢀ0.184
R
1
= 0.0381;
wR = 0.0886
= 0.0593;
wR = 0.0980
0.305 and ꢀ0.202
R
1
= 0.0242;
wR = 0.0630
= 0.0266;
wR = 0.0645
0.216 and ꢀ0.157
2
2
2
2
Final R indices (all data)
R
1
R
1
R
1
R
1
2
2
2
2
Largest difference peak and hole (e Åꢀ3)
2
.4.2. Analytical flow analysis procedure
The analytical procedure to test the different ligands as colori-
different AR substituents were introduced by the substitution of
the oxygen by the nitrogen atom of the amine leading to 3,4-HPOs
with different N-chains and variable physicochemical properties.
The choice of the amine is determinant for the hydrophilic-lipophi-
lic balance of the new 3,4-HPO ligand. Considering these criteria
(discussed below), four alkoxyamines including 3-methoxypropy-
lamine, 2-methoxyethylamine, 3-isopropoxypropylamine and 2-
ethoxyethylamine, were selected to react with 3-benzyloxy-2-
methyl-4-pyrone using microwave-assisted synthesis (Scheme 1,
Table 2).
metric reagent for iron(III) determination has been previously
described [12]. The micro sequential injection system comprised
a MicroSIA of FIAlab (FIAlab Instruments, USA), consisting in a bi-
directional syringe pump of 2.5 mL and an eight-port selection
valve (Valco VICI Cheminert 170-0317L), connected to the central
channel of with a PVC pumping tube.
The other system components were connected with PTFE
(
Omnifit) tubing, with 0.8 mm i.d.
Detection of the colorimetric reaction was attained with an
Ocean Optics Flame – T – UV– Vis (FLMT01897) charged coupled
device (CCD) detector, equipped with a pair of FIA-P600-Fiber cable
The synthesis of ligand MRB14 was previously reported by
Hider and co-workers [41,42] nonetheless in the present work
the ligand was obtained by using microwave-assisted synthesis
instead of the traditional heating method previously described.
The compound MRB14 was included in the set of molecules con-
sidered in the present work as it is structurally related and we
are interested in the study of the influence of the chelator’s struc-
ture for the performance of the ligands and their respective iron
(III) complexes in the analytical and biological context, namely as
plant fertilizers [17].
6
00 Micron diameter (SMA terminated on one end and PEEK
sheath termination on the other end), an Ocean Optics halogen
light source HL-2000 and a FIA-Z-Cell 100 mm – Plexiglas FIA-Z
Cell with adjustable optical path (10 cm light path and 250 mL inner
volume). Data acquisition, made at 460 nm, 475 nm and 515 nm.
All the equipment was controlled on a desktop computer with FIA-
lab software installed (Lenovo, Intel Core i5).
The protocol sequence used for iron(III) determination has also
been previously detailed [12] and consisted in aspirating ligand
solution (250 mL), followed by buffer solution (20 mL) and iron stan-
dard (500 mL), and then propelling the mixture towards the detec-
tor for signal registration.
The protocol established for the synthesis of MRB12 includes
the preparation of the polyethylene glycol (PEG) functionalized
amine thus implying two additional reaction steps, which consti-
Table 2
Yields of N-alkoxy substituted 3-benzyloxy-4-pyridinones on microwave heating
a
3
. Results and discussion
under closed vessel conditions.
N-Alkoxy 3-benzyloxy-4-
pyridinones
Conditions Amine
(excess)
Yield
(%)
3.1. Design and synthesis of 3,4-HPOs
MRB13p
MRB14p
MRB15p
MRB16p
80 °C, 4 h
90 °C, 4 h
80 °C, 4 h
80 °C, 5 h
10 equiv.
4 equiv.
6 equiv.
4 equiv.
92
69
89
91
The general synthetic approach to prepare a 3-hydroxy-4-
pyridinone ligand involves the reaction of a 3-hydroxy-4-pyrone
with a primary amine [12,31,40]. The hydroxyl group in the posi-
tion 3 of the pyrone ring was previously protected with a benzyl
group, according with methods previously described [31]. Then,
a
Reactions of primary amines with 3-benzyloxy-2-methyl-4-pyrone (0.5 g,
2 mmol) were performed in dried ethanol (2 mL).

1
50
T. Moniz et al. / Polyhedron 160 (2019) 145–156
tutes a disadvantage. In the present work we chose to use commer-
cially available amines (Scheme 1, 1–4) to produce the new
ligands. This alternative allowed the reduction of the numbers of
reaction steps from 5 to 3 and significantly decreased the total time
of reaction, when compared with the method described for the
synthesis of MRB12 [12]. This choice of amines together with the
use of microwave heating instead of ‘‘traditional” reflux in oil-bath
aimed to contribute to a simpler and more sustainable synthetic
protocol for ligand production, mainly by reducing reaction time
and improve the reproducibility of the experiments.
respectively. All the chemical shift values obtained are in a good
agreement with those described in the literature for similar com-
pounds [12,40]. All spectra are available in supporting information
(Figs. S1–S22). As a representative example, the detailed discussion
1
13
of the H and C spectra of compounds MRB13p and MRB13 is
described below. For the other ligands synthetized, an extended
description of the NMR spectra is available in supporting
information.
1H and 13C spectra of MRB13p revealed multiplets in the alipha-
tic region (1.83, 3.26 and 3.88 ppm) which were assigned to pro-
0
0
0
The microwave heating process is based on the ability of a
material to heat when exposed to an alternating electric field
and depends on the dielectric properties of that material
2
tons of ACH groups (H2 , H3 and H1 , respectively) of the N-
0
0
0
substituent and their carbons, C2 , C1 and C3 , appear at 30.3,
50.4 and 67.8 ppm, respectively. Carbon C1 exhibited long range
0
(
expressed as the loss factor, tan d) [43,44]. In fact, the reactions
HMBC correlation with proton H6, allowing the assignment per-
formed. The resonance signal of the methoxyl protons in the termi-
under study were performed in ethanol, which presents a high
tan d value (tan d = 0.941) [45], meaning that it is a strong micro-
wave absorbing solvent and, in combination with reaction
reagents, should provide an efficient absorption and rapid heating
profile. The microwave-assisted synthesis of the ligands afforded:
nal group (AOCH
(AOCH ) appears at 58.6 ppm and shows HSQC correlation. Upon
the deprotection (MRB13), significantly differences in the H and
3
) appears at 3.29 ppm and the associated carbon
3
1
1
3
C spectra were detected as the shift of the protons closely to
the pyridinone ring, namely 2-CH , H5 and H6 protons, in agree-
(
a) MRB13p, MRB15p and MRB16p in good reaction yields in 4
3
or 5 h (69–92%) (Table 2) and (b) ligand MRB14p in a significantly
shorten reaction time (4 h) when compared with the ‘‘traditional”
reflux protocol (18 h) [41], a reduction of approximately 78%,
despite maintaining the reaction yield.
ment with the results previously described [47]. The most affected
0
signal of the N-substituent was H1 proton which chemical shift
1
3
varied from 3.88 to 4.48 ppm. In the C spectrum, some variations
0
were also verified, namely C1 carbon which signal shifted from
In all reactions an excess of amine was used, starting from two-
fold excess, although we found that a fourfold excess is required to
get ligands MRB14p and MRB16p. Ligand MRB15p required a
higher amount of amine and the best results were obtained using
six times more amine than the protected pyrone. Amine 1 seems
to be the less reactive and a tenfold excess of amine was required
to get the MRB13p.
50.4 to 55.4 ppm. The resonance signal at 31.1 ppm was attributed
0
0
0
to C2 carbon due to its HMBC correlation with H1 and H3 protons.
0
0
C1 (55.4 ppm) has shown correlation with the signal H6, H3 and
0
H2 , allowing its attribution.
The observed differences in the chemical shifts of 1H and 13
C
nucleus in the spectra of the protected and deprotected forms of
each compound are primarily justified by the different solvent
Purification of the protected ligands was achieved by liquid–liq-
uid extraction without need of extra purification by chromatogra-
phy, in opposition with purification of MRB12p. This fact may
justify the higher yields for MRB13p–MRB16p contrasting with
3
used for the NMR studies of protected (CDCl ) and deprotected
forms (MeOD) which as deliberated by the different solubility of
each form of the ligand. Moreover, the deprotection of the hydroxyl
group in acidic reaction conditions leads to isolate the final ligands
in the enolic form, thus justifying the variations of the chemical
shifts of the protons and carbons signals [47].
4
9% (MRB12p) [12].
In order to obtain the deprotected form of the 3,4-HPO ligand,
the benzyl-protecting group was removed under a hydrogen atmo-
sphere in the presence of Pd/C (10%) and HCl, yielding the MRB13–
MRB16 ligands hydrochloride salt in almost quantitative (92–
3.4. Solid-state structures
1
00%) yield.
Crystalline material of the four 3,4-HPO ligands (MRB13–
MRB16) revealing single-crystals suitable for X-ray diffraction
analysis were obtained by controlled recrystallization from
3.2. Selection of amine reagents and prediction of Log P values
Taking into account the objective of this work, the selection of
MeOH/CHCl
tures were solved and determined in the monoclinic space group
P2 /c, with the exception of MRB14 for which crystalized in the
orthorhombic system (space group P2 ).
3
solvent mixtures. The solid-state crystalline struc-
amines was based on the Log P values, predicted using the ACD/
Log P Software [46] (Table 3) and commercial availability.
The reaction of the selected amines with the protected 2-
methyl-3-hydroxy-4-pyrone originated the new set ligands listed
in Table 3 that have log P values in the range ꢀ0.50 to 0.50.
Compounds MRB13 and MRB14 exhibit Log P values quite sim-
ilar to that found for PEGylated 3,4-HPO (MRB12) [12]. MRB15 and
MRB16 Log P values are predicted to be slightly more lipophilic
than MRB12, a property that may be advantageous for different
applications.
1
1 1 1
2 2
The crystal structure of ligand MRB14 was firstly reported by
Hider and co-workers [42]. As it was possible to obtain crystalline
material of MRB14 revealing single-crystals suitable for X-ray
diffraction analysis, we also included the description of the results
obtained for this ligand. The information obtained constitutes
additional structural characterization data and is also useful to
compare with the results obtained for the newly described 3,4-
HPOs (MRB13, MRB15 and MRB16), in the same experimental con-
ditions. The crystal structures support unequivocally the synthesis
of the desired organic molecules, all isolated in enolic forms as
hydrochloride salts (Fig. 2). The asymmetric units (asu’s) of four
crystal structures are identical, revealing only a respective cationic
organic molecule (3,4-HPO ligand) and one chloride anion. The asu
of MRB16 comprises an additional crystallization water molecule.
The bond distances of the common 3,4-HPO ring are identical
between all the four molecules and also comparable to other pre-
viously reported [23,42,48,49]. Interestingly, the C3AO2 bond dis-
tance is slightly longer in the MRB15 and MRB16 molecules [1.336
3
.3. NMR spectroscopy
The structures of the protected and deprotected ligands in solu-
1
13
tion were established by NMR analysis ( H and C, 1D and 2D
experiments, including COSY, HSQC and HMBC spectra for
unequivocal assignment of the most characteristic proton and car-
bon chemical shifts). The assignment of the resonance signals in
1
3
C NMR spectra of the protected and deprotected compounds
1
13
1
13
was achieved by analysis of H/ C HSQC and H/ C HMBC spectra,
1
13
which provide one and multiple bond H– C connectivity,

T. Moniz et al. / Polyhedron 160 (2019) 145–156
151
Table 3
Formulae of the 3,4-HPO chelators and the predicted Log P values.
Ligand name
Formulae
MRB12 [12]
MRB13
MRB14
MRB15
MRB16
Log P (ACD lab)
ꢀ0.69
ꢀ0.38
ꢀ0.50
0.50
0.03
Table 4
Geometric information (distances in Å and angles in degrees) for the strong hydrogen bond interactions (DAHꢃ ꢃ ꢃA) of MRB13, MRB14, MRB15 and MRB16.
a
DAHꢃ ꢃ ꢃA
d (Hꢃ ꢃ ꢃA)
d (Dꢃ ꢃ ꢃA)
\ (DHA)
MRB13
MRB14
MRB15
MRB16
O1AH1ꢃ ꢃ ꢃCl1
O2AH2ꢃ ꢃ ꢃCl1
2.221(18)
2.071(12)
3.029(2)
2.954(2)
149(3)
169(3)
i
ii
O1AH1ꢃ ꢃ ꢃCl1
O2AH2ꢃ ꢃ ꢃCl1
O1AH1ꢃ ꢃ ꢃCl1
O2AH2ꢃ ꢃ ꢃCl1
2.29(2)
2.092(13)
3.037(3)
2.976(3)
141(2)
177(3)
iii
2.234(13)
2.032(10)
3.0576(14)
2.9336(13)
154(2)
178(2)
O1AH1ꢃ ꢃ ꢃO1W
O2AH2ꢃ ꢃ ꢃCl1
1.765(11)
2.072(9)
1.906(9)
2.280(9)
2.5830(13)
2.9602(10)
2.7812(13)
3.1722(11)
152.5(16)
177.1(15)
169.3(17)
177.1(14)
iv
v
O1WAH1Wꢃ ꢃ ꢃO3
O1WAH2Wꢃ ꢃ ꢃCl1
a
Symmetry transformation used to generate equivalent atoms: (i) –x ꢀ 2, ꢀy + 1, ꢀz + 1; (ii) ꢀx + 1, y ꢀ 1/2, ꢀz + 1/2; (iii) ꢀx + 1, y ꢀ 1/2, ꢀz + 3/2; (iv) ꢀx ꢀ 1, ꢀy + 1, ꢀz;
v) x, ꢀy + 1/2, z ꢀ 1/2.
(
(
(
2) and 1.335(2) Å, respectively] than that in the MRB13 [1.328
2) Å] and MRB14 [1.324(3) Å] structures.
The relative position of the chloride anions strongly influences
reinforced by an extensive network of weak non-covalent interac-
interactions
(not shown; Fig. 4). As consequence of the overall hydrogen bond-
ing network all the compounds reveal stable three-dimensional
supramolecular structures with a crystal packing considerably
dense.
tions, particularly CAHꢃ ꢃ ꢃCl, CAHꢃ ꢃ ꢃO and CAHꢃ ꢃ ꢃ
p
the crystal packing arrangement in all the compounds, due to dis-
tinct OAHꢃ ꢃ ꢃCl hydrogen bonds established with the respective
organic molecules (Fig. 3; see Table 3 for geometric details about
the strong hydrogen bonding interactions found in all the struc-
tures). In the MRB13 structure a pair of chloride anions bridges
two neighboring organic molecules positioned in anti-parallel
mode, by OAHꢃ ꢃ ꢃCl hydrogen bonds involving the hydroxyl groups
in the pyridinone ring, and originating discrete supramolecular
species (Fig. 3-MRB13).
In the structures of MRB14 and MRB15, the OAHꢃ ꢃ ꢃCl hydrogen
bonds established between the hydroxyl groups of adjacent mole-
cules and the chloride anions lead to the formation of one-dimen-
sional supramolecular entities (Fig. 3-MRB14 and -MRB15). Both
supramolecular chains show a zig–zag arrangement and extend
along the b-axis of the respective unit cell.
3.5. Analytical results
3.5.1. Features of the colorimetric determination of iron(III)
The performance of MRB13–MRB16 as colorimetric reagents for
iron(III) determination was evaluated using the previously
described sequential injection method [12] and compared with
preceding results obtained for the parent ligand MRB12.
Calibration curves were performed using iron(III) standards in
the range 1.8–18 mmol/L (0.10–1.0 mg/L) and the analytical fea-
tures displayed with the use of each ligand are summarized in
Table 5. Data related with results obtained with MRB12 was also
included for comparison.
The sensitivity was set as the calibration curve slope and the
limit of detection calculated as the concentration corresponding
to 3 times the standard deviation of the blank signal. The precision
was evaluated based on the relative standard deviation of the
9.0 mmol/L iron(III) standard.
The existence of crystallization water molecules in the structure
of MRB16 besides the chloride anions promotes an extensive
hydrogen bonding network (strong OAHꢃ ꢃ ꢃO and OAHꢃ ꢃ ꢃCl inter-
actions) involving the contiguous organic molecules and leads to
the formation of two-dimensional supramolecular structures
(
supramolecular layers) extended in the [1 0 0] direction of the
unit cell (Fig. 3-MRB16). In MRB16, contrasting with the remaining
compounds where only the hydroxyl groups of the 3-hydroxy-4-
pyridinone rings are engaged in the hydrogen bonds, the O-atoms
of the ether groups in the chains are also involved.
The extended packing of the supramolecular entities previously
described (discrete in MRB13, one-dimensional in MRB14 and
MRB15, and two-dimensional structures in MRB16) is further
From analysis of the results summarized in Table 5, we verified
that all ligands exhibited a good performance in the determination
of iron(III). The results have shown no significant differences in the
LOD and precision parameters between the ligands studied in the
present work and with the previously tested MRB12. Also, no rel-
evant differences were detected in the sensitivity of the method

1
52
T. Moniz et al. / Polyhedron 160 (2019) 145–156
Fig. 2. Crystal structures of the 3,4-HPO molecules (MRB13, MRB14, MRB15 and MRB16), showing the label scheme for all non-H-atoms which are represented as thermal
ellipsoids drawn at the 50% probability level, while H-atoms are shown as small spheres with arbitrary radius.
involving the different ligands comparing to the value determined
for MRB12 (relative deviation, RD <8%).
(ꢀ0.69 < Log P < 0.50), all the these ‘‘ether derived 3,4-HPO
ligands” exhibited similar performance in SI method analytical
determination of iron(III).
The results suggest that this new set of 3,4-HPO ligands func-
tionalized with variable ether-derived chains possess a suitable
water solubility to guarantee the high sensitivity and low detection
limit when compared to the results obtained for the non-ether
derived 3,4-HPO, Hmpp, and in a similar mode with that described
for MRB12 [12]. Despite their variable Log P values
3.5.2. Assessment of potential interferences in water monitoring
Additionally, as previously intended with MRB12, the potential
application to natural waters requires an effective selectivity in
comparison with other ions possibly present. In this context the

T. Moniz et al. / Polyhedron 160 (2019) 145–156
153
Fig. 3. OAHꢃ ꢃ ꢃCl (blue dashed lines) and OAHꢃ ꢃ ꢃO (lavender dashed lines) hydrogen bond interaction originating discrete (MRB13), one-dimensional (MRB14 and MRB15)
and two-dimensional (MRB16) supramolecular structures. (Color online.)
Fig. 4. Extended crystal packing viewed in the [1 0 0] (MRB13, MRB14 and MRB15) and [0 0 1] (MRB16) direction of the respective unit cell, with the charge balancing
chloride anions and crystallization water molecules (only in MRB16) drawn in space-filling model.
assessment of potential interferences was carried out, based upon
the expected values in natural waters, defined by UNFAO (United
Nations Food and Agriculture Organization) [50]. The percentage
of interference (IP) was calculated comparing the analytical signal
obtained for an iron(III) standard with a mixed standard of iron(III)
and the potential interfering ion. Experiments were performed

1
54
T. Moniz et al. / Polyhedron 160 (2019) 145–156
Table 5
Analytical features of the methods using different ligands for iron(III) determination considering the previously described SI method [12]. LOD – Limit of detection; RSD - Relative
standard deviation.
Ligand
Ligand concentration of 0.6 mmol/L (g/L)
Sensitivity:
LOD
Precision: RSD (%), [Fe3+] = 9.0 mmol/L
calibration curve slope (L/mmol)
(mmol/L)
MRB12 [12]
MRB13
MRB14
MRB15
MRB16
0.193
0.140
0.131
0.157
0.140
s = 12.8 ± 0.4
s = 13.0 ± 0.9
s = 13.6 ± 0.7
s = 13.8 ± 0.5
s = 13.7 ± 0.8
1.27
1.24
1.40
1.74
1.79
1
1
1
1
2
Table 6
Assessment of the influence of potential interfering ions (Mn+), for the five ligands, using the developed mSIA method, using an iron standard of 0.50 mg/L (8.95 mmol/L); values
with bold numbers represent the maximum ratio without significant interference (<10%).
Potential interfering ion (Mn+
)
UNFAO limits (mg/L)
[Mn+] (mg/L)
IP (%)
MRB12
MRB13
MRB14
MRB15
MRB16
Al3+
5
5
1
15
3
ꢀ5
ꢀ8
ꢀ0.5
11
2
11
2
24
5
15
5
13
1
12
5
13
4
ꢀ2
ꢀ14
0.3
16
ꢀ0.5
20
1
21
4
15
–
<10
ꢀ5
13
2
12
1
–
<10
–
<10
–
3
0.1
24
7
23
–
<10
1
15
–
ꢀ8
–
2
12
5
12
5
16
1
13
–
ꢀ3
–
0.9
ꢀ21
6
0
Ca2+
Co2+
Mg2+
Mn2+
Ni2+
Zn2+
Cu2+
Cd2+
Pb2+
Cr3+
15
0.1
5
0
0.1
.3
20
0
ꢀ21
–
40
ꢀ1
–
2
ꢀ4
10
–
<10
–
<10
–
<10
–
5
–
0.2
0.2
2
20
3
2
5
0
7
10
ꢀ3
15
4
16
–
3
5
1
0
1.3
–
0
<10
–
5
–
4
0.1
.5
0.1
.5
0.05
.1
0
17
3
14
7
14
–
2
–
0.5
10
–
0
ꢀ0.2
–
–
–
2
0
13
ꢀ2
<10
<10
using concentrations of the potential interfering metal ion (Mn+
)
In order to evaluate the potential of the new compounds as ana-
lytical reagents in iron(III) determination by sequential injection
methods, we tested MRB13–MRB16 ligands in the same conditions
used for MRB12. Results showed that all the ligands described are
promising molecules for this assay since they exhibit similar
results, highlighting the fact that the structural differences intro-
duced do not compromise their performance in iron(III) determi-
nation in aqueous samples. Also, results regarding the relevance
of several metal ions as potential interferences reinforce the poten-
tial application of the ligands in natural water monitoring.
In summary, the new procedure is advantageous to obtain
highly water soluble compounds that suitable for analytical appli-
cations like metal ion determination [11,12,30], biological pur-
poses, such as the treatment of several diseases like Iron
Overload [51–53], Diabetes [54–56] and in agriculture to address
Plant Iron Deficiency Chlorosis [17,57–59].
identical or greater than the UNFAO value. The standards were pre-
pared with 0.50 mg/L (8.95 mmol/L) of iron(III) and with or without
interfering ion. The tested interfering ions, with respective concen-
trations and the interference percentages are presented in Table 6.
The results shown in Table 6 are indicative that no significant
interferences are expected from metal ions existent in natural
waters. All the ligands tolerate the interference in quantities
greater than the limits defined.
4
. Conclusions
We report the synthesis and structural characterization of three
novel 3,4-HPO chelators with different ether derived substituents
in the nitrogen atom of the heterocyclic ring. The three com-
pounds, together with one previously described and revisited in
this study, provide
a
range of Log
P
values between
ꢀ0.69 < Log P < 0.50. Chelators were successfully obtained in high
Acknowledgments
reaction yields by microwave-assisted methods and using com-
mercially available amines as precursors. All the ligands were
obtained in an easier, quicker, and more efficient and sustainable
way in comparison with the methods described for the synthesis
of a highly water soluble 3,4-HPO chelator, MRB12 [12]. The reduc-
tion of overall reaction times, the reproducibility of the experi-
ments and the lower number of steps, constitutes a great
advantage considering the need of these ligands as well as their
respective metal ion complexes, to be used in many analytical, bio-
logical and agricultural applications.
This work received financial support from the European Union
(FEDER funds through COMPETE) and National Funds (FCT,
Fundação para a Ciência e Tecnologia), under the Partnership
Agreement PT2020 through project UID/QUI/50006/2013-
POCI/01/0145/FEDER/007265 (LAQV/REQUIMTE) and PTDC/
AGRPRO/3515/2014-POCI-01-0145-FEDER-016599. Programa Opera-
cional Regional do Norte (ON.2 – O Novo Norte), under the Quadro
de Referência Estratégico Nacional (QREN) and funded by Fundo
Europeu de Desenvolvimento Regional (Feder) NORTE-07-0124-

T. Moniz et al. / Polyhedron 160 (2019) 145–156
155
FEDER-000066 and NORTE-01-0145-FEDER-000024. R.B.R. Mes-
quita thanks for the grant SFRH/BDP/112032/2015. The Bruker
Avance III 400 spectrometer is part of the National NMR network
and was purchased under the framework of the National Pro-
gramme for Scientific Re-equipment, contract REDE/1517/
RMN/2005, with funds from POCI 2010 (FEDER) and (FCT).
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Vanderhelm, Synthesis, physicochemical properties, and biological evaluation
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22] S. Fakih, M. Podinovskaia, X. Kong, H.L. Collins, U.E. Schaible, R.C. Hider,
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CCDC 1845145, 1845146, 1845147 and 1845148 contains the
supplementary crystallographic data for MRB13, MRB14, MRB15
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