New p-aminophenol-based dendritic melamines. Iterative synthesis, structure, and electrochemical characterisation

Article abstract of DOI:10.1016/j.crci.2016.07.002

Avoiding any protective–deprotective step, the synthesis of new (G-0, -1, -2) dendritic melamines is reported. Their construction consisted of chemoselective S_N2–Ar amination of cyanuric chloride with p-aminophenol (peripheral unit) and piperazine or 4,4'-bipiperidine (linkers). This novel class of amino-s-triazines is primarily investigated by DFT calculations (optimal geometry and electronic structure) in tandem with (VT) ¹H NMR spectroscopy providing details of the rotational diastereomerism about the C(s-triazine)-N(exocyclic) partial double bonds, solvation effects and conformation of the linkers. These data are subsequently exploited in electrochemical investigations (cyclic voltammetry on the Pt electrode/DMSO, 0.1 M KCl). Two reversible electron-transfer phenomena have been observed. Thus, depending on the variable π-deficiency strength of the s-triazine ring acting as the EWG on the adjacent NH group and the ability of the latter to undergo redox processes in tandem with the phenolic p-HO group, two electrochemical pathways are proposed, namely the p-benzoquinonimine route and the electropolymerization route.

Full text of DOI:10.1016/j.crci.2016.07.002

C. R. Chimie xxx (2016) 1e13
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ꢀ
Full paper/Memoire
New p-aminophenol-based dendritic melamines. Iterative
synthesis, structure, and electrochemical characterisation
Cristina Morar a, Graziella Liana Turdean b, Attila Bende c, Pedro Lameiras d,
,
, *
Cyril Antheaume e, Liana Maria Muresan b **, Mircea Darabantu a
a
ꢀ
Babes-Bolyai University, Department of Chemistry, 11 Arany Janos St., 400028 Cluj-Napoca, Romania
b
ꢀ
Babes-Bolyai University, Department of Chemical Engineering, 11 Arany Janos St., 400028 Cluj-Napoca, Romania
c National Institute for Research and Development of Isotopic and Molecular Technologies, Molecular and Biomolecular Physics
Department, 65-103, Donath St., PO Box 700, 400293 Cluj-Napoca 5, Romania
d University of Reims Champagne-Ardenne, ICMR, UMR 7312, BP 1039, 51687 Reims, France
e University of Strasbourg, Faculty of Pharmacy, 74, route du Rhin, BP 60024, 67401 Illkirch cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Avoiding any protectiveedeprotective step, the synthesis of new (G-0, -1, -2) dendritic
melamines is reported. Their construction consisted of chemoselective SN2eAr amination
of cyanuric chloride with p-aminophenol (peripheral unit) and piperazine or 4,40-bipi-
peridine (linkers). This novel class of amino-s-triazines is primarily investigated by DFT
calculations (optimal geometry and electronic structure) in tandem with (VT) 1H NMR
spectroscopy providing details of the rotational diastereomerism about the C(s-triazine)-
N(exocyclic) partial double bonds, solvation effects and conformation of the linkers. These
data are subsequently exploited in electrochemical investigations (cyclic voltammetry on
the Pt electrode/DMSO, 0.1 M KCl). Two reversible electron-transfer phenomena have been
Received 2 May 2016
Accepted 11 July 2016
Available online xxxx
Keywords:
p-Aminophenol
Cyclic voltammetry
Dendrimers
DFT
observed. Thus, depending on the variable
p-deficiency strength of the s-triazine ring
Melamine
acting as the EWG on the adjacent NH group and the ability of the latter to undergo redox
processes in tandem with the phenolic p-HO group, two electrochemical pathways are
proposed, namely the p-benzoquinonimine route and the electropolymerization route.
© 2016 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
Later on, the patented applications of these macromolecules
as new plasticizers promoted subsequent findings in the
There are multidisciplinary approaches nowadays
focused on N-substituted amino-s-triazines with hydroxy-
phenyl units [1e4]. Actually, the use of p-aminophenol in the
classic SN2-Ar amination of cyanuric chloride, yielding m-
trivalent branched melamine (2,4,6-triamino-1,3,5-
triazine)-polycarbonates, has been known since 1987 [1a].
domain of organic materials, for example thermoplastic
compounds [1b] and ingredients for lubricants or lubricant
compositions [1c,1d]. More recently, the synthesis and
structural properties of arborescent architectures built
around trimesic(1,3,5-tricarbonylbenzene) [2a] or s-triazine
[2b] cores by means of p-aminophenol units were also
described. The first synthesis of a (G-1) dendritic melamine
comprising p-aminophenol as an internal linker and 2-
aminopyridine as a peripheral unit, reported by Gamez and
co-workers in 2002, targeted a novel polydentate and pol-
ynucleating N-donor ligand [3]. On the other hand, it is also
worth mentioning the bioimpact of some N-substituted
* Corresponding author.
** Corresponding author.
E-mail addresses: limur@chem.ubbcluj.ro (L.M. Muresan), darab@
chem.ubbcluj.ro (M. Darabantu).
http://dx.doi.org/10.1016/j.crci.2016.07.002
1631-0748/© 2016 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Please cite this article in press as: C. Morar, et al., New p-aminophenol-based dendritic melamines. Iterative synthesis, structure,
and electrochemical characterisation, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.07.002

2
C. Morar et al. / C. R. Chimie xxx (2016) 1e13
melamines with (O-protected)hydroxyphenyl motifs conju-
gating 4-aminoquinoline as antimalarial agents [4].
In the above multi-facet context, we consider of interest
to extend our expertise in the field of dendritic melamines'
preparation and structure [5] (including electrochemical
approach [5e]) by using a versatile-redox and bidentate
nucleophile, p-aminophenol. The last one was designed to
play the role of a peripheral unit, in an iterative-convergent
type synthesis, a strategy previously developed by Sima-
nek's group [6]. Next, cyclic voltammetry has been employed
to study the electrochemical behaviour of the first (G-2)
dendrimer encompassing p-aminophenol as the peripheral
unit and of its precursors on the Pt electrode in DMSO.
To our knowledge, no similar investigation has been
reported so far.
synthesis of amino-s-triazines (THF, 1,4-dioxane, acetone
etc.) [6], most likely because of its high polarity. That is, in
order to adopt a protectiveedeprotective strategy, 1b was
diacetylated (Table 1, entry 4) in high yield (90%) with
complete O,O0-chemoselectivity. Unfortunately, the
resulting O,O0-diacetyl derivative 2 was inappropriate for
our envisaged iterative syntheses (Scheme 1). Thus, the
reaction between 2 and piperazine (not depicted in
Scheme 1), besides the expected amination of the s-
triazine chlorine, consisted as well of the partial transfer
of the acetyl group (O-phenol / N-piperazine) affording a
multicomponent reaction mixture. By contrast, compound
2
was of crucial relevance in electrochemical in-
vestigations (Section 3.1.) To conclude, for the present
report, we had to manage our strategy limited to (G-0)
chlorodendron 1b.
Indeed,
dendrimers
containing
redox
active
functionalities are especially important because of their
interesting electron-transfer properties [7]. Thus, the pe-
ripheral redox active units, undergoing multiple electron
transfer and redox properties, can be modulated by the size
and nature of the dendritic branches [8]. Aminophenols are
well-documented electrochemically active compounds since
they have two groups (eNH2 and eOH), which could be
oxidized [9]. Moreover, the electropolymerization of p-ami-
nophenolitselfonaPtelectrodeinorganicsolvents,providing
a soluble electroactive polymer, was also reported [9a].
Based on our earlier published methodology [5aed],
the chemoselective mono-attachment of piperazine, as
the first linker, to 1b (Table 1, entry 5) gave the (G-0)
melamine 3a. Thus, during the portionwise addition of
1b to a 300% molar excess of piperazine, the TLC moni-
toring of the amination revealed a very clean evolution
towards 3a only, i.e. the complete absence of the dimeric
melamine 4a as a side product (Scheme 1). As shown in
Table 1 (entry 5), it was the single situation, that of 3a, in
which both techniques for its purification (direct crys-
tallisation or column chromatography on partially deac-
tivated silica gel) provided good and comparable
quantitative results. Our attempt to prepare compound
3a by applying the method of Bath and co-workers [4c]
(equimolar ratio 1b: piperazine, 82% claimed yield of
3a) failed (Table 1, entry 6). The resulting mixture 3a
(minor) and 4a (major) could be successfully separated
by column chromatography.
2. Results and discussion
2.1. Synthesis
The chemistry we followed is resumed in Scheme 1 and
the reaction conditions are presented in Table 1.
First, we tested the chemoselectivity in the amination
of cyanuric chloride by p-aminophenol (p-AP) in the
synthesis of melamine 1a (Table 1, entry 1) versus that of
chlorodiamino-s-triazine 1b (Table 1, entries 2 and 3).
Although the preparation of 1a was of applied interest for
several authors [1aed,3], in our hands only the procedure
of Negoro and Kawata [1c,1d] gave satisfaction as excel-
lent and reproducible yield, 91% [10]. Under similar but
milder conditions (Table 1, entry 2), amination of cyanuric
chloride performed with two molar equiv. of p-amino-
phenol yielded compound 1b contaminated with traces of
melamine 1a. Hence, 1b required purification by column
chromatography when an important loss of the material
due to its relative retention on silica gel was observed,
affecting the yield. Our option of choice was then the
method of Bhat and co-workers [4b,4c] (Table 1, entry 3)
which produced 1b with good yield, 73%, in an expedi-
tious manner.1 Compound 1b exhibited moderate solubi-
lity in organic solvents usually recommended for the
The protocol for the non-symmetric anchorage of the
4,40-bipiperidine linker to 1b (Table 1, entry 7) producing
melamine 3b was the same as in the case of piperazine
against 1b but the amination reached completion in a much
longer time. The yield of 3b was critically influenced by the
work-up used for its isolation as a pure analytical sample,
86% (direct crystallisation from boiling ethanol) versus 45%
(column chromatography on a partially deactivated silica
gel). Our tentative explanation relates, once again, to the
high relative retention on column chromatography of 3b
(see also the next examples).
The symmetric analogue 4b of 3b was also prepared
from the latter in reaction with 1b (Table 1, entry 8).
As expected, the one-pot synthesis of (G-1) chloroden-
drons 5a and 5b (Table 1, entries 9 and 10 respectively)
required mild conditions for the first step amination. How-
ever, the second step was mandatory to taxing reaction pa-
rameters (36 h in refluxing 1,4-dioxane), suggesting the low
reactivity of the intermediates due to their strong solvation
in the depicted solvents. Except for DMF and DMSO, both
compounds 5a and 5b manifested low solubility in common
organic volatile solvents. Therefore, in order to avoid the loss
of the material on column chromatography (i.e. compound
5b, 47% yield), purifications were successfully realised by
crystallisations from boiling ethanol (86% optimised yield of
5b). Even so, the (G-1) chlorodendron 5b showed an unex-
pected weak reactivity in subsequent aminations attempted
1
Our analytical sample 1b was different with respect to that of Bath
and co-workers [4b,4c]. Our diagnosis concerned the aspect (white
powder in our case against reported black crystals), melting point
(343.7e344.1 ꢀC in our case, against reported 251e252 ꢀC) and the
appropriate solubility for NMR analysis (only DMSO-d6 in our case,
against reported CDCl3); see Figs. SI-4-7 in SI (Supporting information).
Similar observations apply for compound 3a with respect to the sample of
Bath and co-workers [4c]; see Figs. SI-12-14.
Please cite this article in press as: C. Morar, et al., New p-aminophenol-based dendritic melamines. Iterative synthesis, structure,
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C. Morar et al. / C. R. Chimie xxx (2016) 1e13
3
R1
R2 =
HO
NH
N
N
N
piperazine
N
N
N R1
HN
N
NH
N
C3N3Cl3
HO
NH
HO
NH2
3a: R1 = H; 4a: R1 = R2
4-Aminophenol
OR2
OR2
(4-AP)
R2 =
NH
1a: R1 = 4-HO-Ph-NH, R2 = H
HO
1b: R1 = Cl, R2 = H
Ac2O
N
N
2: R1 = Cl, R2 = Ac
4,4'-bipiperidine
N
N
N R1
HO
NH
3b: R1 = H
4b: R1 = R2
1b
H
N
N
Cl
N
N
N
N
R1 =
R1 =
R2
N
N
R2
N
N
C3N3Cl3
HO
H
N
piperazine
H
N
3a
N
N
N
N
R2 =
R2 =
N
N
N
N
6
HO
5a
Cl
R1
C3N3Cl3
R1
N
N
R3
N
R3
N
R2
N
R1 =
N
N
N
R2 =
= R3
C3N3Cl3
HO
H
N
N
N
3b
N
N
N
N
5b
R1
N
R1 =
N
N
R2
H
N
N
N
R2 =
N
N
7
HO
R1
Scheme 1.
with piperazine or 4,40-bipiperidine. Therefore, for the pre-
sent work, we had to limit the study by using henceforth (G-
1) chlorodendron 5a only.
In so doing, we effected, in accordance with the same
methodology as that in the case of (G-0) melamine 3a, the
selective anchorage of the next piperazine linker on 5a
(Table 1, entry 11) and obtained (G-1) melamine 6. We
detected no dimeric melamine as a side product and iso-
lated 6 by column chromatography on a partially deacti-
vated silica gel or, as an optimal decision, by crystallisation
(88% yield).
with good global yield (85%). Compound 7, the first (G-2)
dendrimer comprising p-aminophenol as the peripheral
unit, could not be eluted on TLC due to its low solubility in
common organic solvents. Its identity was fully confirmed
by spectral methods.
2.2. Preliminary structural assignments
As early as 1971 [11a], it was established that the C(s-
triazine)-N(exocyclic) linkages in amino-s-triazines have a
partial double bond character due to the p(N)/
conjugation between the lone pair of the exocyclic N-atom
in conjunction with the high -deficient s-triazine ring
p(C]N)
The final installation of the trivalent s-triazine core on 6
(Table 1, entry 12) was performed in a one-pot procedure
p
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4
C. Morar et al. / C. R. Chimie xxx (2016) 1e13
Table 1
Reaction conditions and results in the synthesis of compounds 1e7 (see Scheme 1).
Entry
Reaction
Conditions
Products, yields (%) and isolation: direct
crystallisation (d.c.), column chromatography
on silica gel (c.c.)a
1
2
3
p-AP / 1a
p-AP / 1b
p-AP / 1b
i) 0.30 equiv C3N3Cl3, butanone/0e5 ꢀC
(30 min); ii) 1.00 equiv AcONa, H2O/5e9 ꢀC
(30 min); iii) reflux (3 h)
1a: 91 (d.c.)
1b: 56 (c.c.)
1b: 73 (d.c.)b
i) 0.50 equiv C3N3Cl3, butanone/0e5 ꢀC
(30 min); ii) 1.00 equiv AcONa, H2O/5e9 ꢀC
(30 min); iii) r.t. (24 h)
i) 0.50 equiv C3N3Cl3, acetone/0e5 ꢀC (2 h); ii)
1.00 equiv NaHCO3, H2O/5e9 ꢀC; iii) 45 ꢀC (3 h);
r.t. (15 h)
4
5
6
7
8
9
1b / 2
2.23 equiv Ac2O, 1.00 equiv K2CO3, THF/r.t.
(4 h); reflux (10 h)
4.00 equiv piperazine, 1.00 equiv K2CO3, THF/r.t.
(15e20 h)
1.00 equiv piperazine, 1.00 equiv K2CO3, 1,4-
dioxane/reflux (8 h)
4.00 equiv 4,40-bipiperidine, 1.00 equiv K2CO3,
THF/r.t. (36e42 h)
2: 90 (d.c.)
1b / 3a
1b / 3a, 4a
1b / 3b
3b / 4b
3a / 5a
3a: 83 (c.c.); 85 (d.c.)
3a: 17 (c.c.); 4a 83 (c.c.)c
3b: 45 (c.c.); 86 (d.c.)
4b: 64 (d.c.)
1.00 equiv 1b, 1.00 equiv K2CO3, 1,4-dioxane/
reflux (13 h)
i) 1.00 equiv 3a, 1.00 equiv K2CO3, THF/ꢁ15 ꢀC
(20e24 h); ii) 1.00 equiv 3a, 1.00 equiv K2CO3,
THF/r.t. (20e24 h); iii) 1,4-dioxane, reflux (36 h)
i) 1.00 equiv 3b, 1.00 equiv K2CO3, THF/ꢁ15 ꢀC
(22 h); ii) 1.00 equiv 3b, 1.00 equiv K2CO3, THF/
r.t. (24 h); iii) 1,4-dioxane, reflux (36 h)
i) 4.00 equiv piperazine, 1.00 equiv K2CO3, THF/
r.t. (20e30 h); ii) reflux (12 h)
5a: 90 (d.c.)
10
3b / 5b
5b: 47 (c.c.); 86 (d.c.)
11
12
5a / 6
6 / 7
6: 55 (c.c.); 88 (d.c.)
7: 85 (d.c.)
i) 0.30 equiv C3N3Cl3, 1.00 equiv K2CO3, 1.4-
dioxane/r.t. (24 h); ii) reflux (48 h)
a
On partially deactivated silica gel (eluent EtOH: aq. NH3 25%) for compounds 3a, 4a, 3b and 6.
67% Yield according to Refs. [4b] and [4c].
Partial conversions of 1b into the depicted compounds, calculated based on effective amounts isolated by column chromatography. Full details of these
b
c
syntheses are given in the SI (Supporting information).
(ii) The increasing order of
3
energies (Table 2)
[11]. There is a restricted rotation thus induced. Variable
temperature NMR spectroscopy is an appropriate tech-
nique for monitoring this stereodynamism [12], including
examples of arylamino-s-triazines [12def,12h]. As for other
amino-s-triazines, in the case of monomeric (G-0) den-
drons 1a, 1b, 2, 3a and 3b, at room temperature, a topo-
logical idealised model [5aed,12g] predicted, for 1a, a two
term rotational diastereomerism (asymmetric $ propeller)
and three terms (synesyn $ antiesyn $ antieanti) for 1b,
2, 3a and 3b (Scheme 2).
HOMO
correlated with the decreasing strength of the EWG
N(O)-substituting the p-(R2O)phenylamino units
(Scheme 1), namely acetyl and variable
triazine ring, i.e. 2 < paracetamol ~ 1b. However, an
p-deficient s-
HO
HO
H
H
N
N
HO
OH
OH
Therefore, before any electrochemical investigation, a
study by means of DFT [13e20] calculations in solution
(DMSO) was implemented for compounds 1a, 1b, 2, 3a, 3b
and7, selectedastherepresentative. Theresultsare shownin
Tables 2 and 3 and Fig. 1 together with those referring to the
starting p-aminophenol (p-AP) and its N-acetyl derivative,
the well-known anti-inflammatory drug paracetamol (PA).
They deserved the below comments:
H
N
H
N
N
N
H
H
1a-asymmetric
1a-propeller
OH
R1
R1
R1
OR2
R2O
OR2
H
H
N
H
N
N
N
N
H
N
H
H
OR2
anti-syn*
OR2 OR2
anti-anti
(i) The optimised geometry of melamine 1a was found to
be of type asymmetric (Table SI-1, Supporting infor-
mation); meanwhile antieanti was the preferred
rotameric arrangement adopted by the p-(R2O)phenyl
units in compounds 1b, 2, 3a and 3b in DMSO (Table
SI-2, Table 2). The same antieanti stereochemistry
was local at the periphery of dendrimer 7 which dis-
closed a vaulted global shape [21] (Fig. 1).
syn-syn
1,3,5-triazin-2,4,6-triyl
1,4-phenylene
1b: R1 = Cl, R2 = H; 2: R1 = Cl, R2 = Ac;
3a: R1 = piperazin-1-yl, R2 = H; 3b: R1 = 4,4'-bipiperidin-1-yl, R2 = H
*R1 and p-(R2O)phenyl groups are the reference ligands for descriptors syn and anti
Scheme 2.
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and electrochemical characterisation, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.07.002

C. Morar et al. / C. R. Chimie xxx (2016) 1e13
5
Table 2
Full geometry optimisation and calculated
3
3
in p-aminophenol (p-AP), paracetamol (PA) and of (G-0) dendrons 1a, 1b, 2, 3a and 3b in DMSO.a
HOMO, LUMO
No.
HOMO
LUMO
3
(eV)
3
(eV)
Gap (eV)
6.92
HOMO
LUMO
p-AP
ꢁ6.24
þ0.68
PA
ꢁ7.00
ꢁ6.81
þ0.50
þ0.31
7.50
7.12
1a
1b
ꢁ6.99
ꢁ0.03
6.96
2
ꢁ7.47
ꢁ0.23
7.24
3a
ꢁ6.80
þ0.35
7.15
3b
ꢁ6.81
þ0.35
7.16
a
The full geometry optimisation has been carried out at the DFT level of theory considering the M06-2X [13] exchange-correlation functional together
with the def2-TZVP [14] basis set in the presence of a solvent environment implemented in the Gaussian 09 [15] program package. The solvent effects have
been taken into account via the Polarizable Continuum Model (PCM) using the integral equation formalism variant (IEFPCM) [16] considering the DMSO
(
3
¼ 46.826) as the solvent environment.
equalisation of
3
energies in the melamine series
values ranging between 1.20 and 1.25, anyhow much
greater than those of connexions C(Ph)eNH. The
calculated Bond Lengths (BLs) fully supported their
inverted dependence with respect to the corresponding
BO indexes (Table 3).
HOMO
1a ~ 3a ~ 3b resulted from calculation. Obviously, the
highest level was that of p-aminophenol.
3
HOMO
(iii) The partial double bond nature of connexions C(s-
triazine)-N(exocyclic) was disclosed by Bond Order (BO)
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6
C. Morar et al. / C. R. Chimie xxx (2016) 1e13
Table 3
Indicative Bond Order (BO) indexes, Bond Lengths (BLs) and Lone Pair (LP) orbital populations in p-aminophenol (p-AP), paracetamol (PA) and (G-0) dendrons
1a, 1b, 2, 3a and 3b in DMSO.
No.
C(s-triazine)-N(H)
C(Ph)-N(H)
C(s-triazine)-N<
Lone Pair (LP) orbital populations (e)a
N(H)
O(H, Ac)
-N<
s-Triazine
N(1, 3, 5)
BOb
BL (Å)c
BO
BL (Å)
BO
BL (Å)
p-AP
PA
1a
1b
2
d
1.15
1.05
1.04
1.03
1.04
1.04
1.04
1.375
1.410
1.411
1.410
1.406
1.407
1.407
d
1.82
1.64
1.68
1.64
1.64
1.68
1.68
3.88
3.86
3.86
3.86
3.76
3.86
3.86
d
d
d
d
d
d
1.21
1.25
1.24
1.20
1.20
1.353
1.351
1.343
1.360
1.361
d
d
1.90
1.90
1.90
1.90e1.91
1.90e1.91
d
d
d
d
3a
3b
1.23
1.23
1.353
1.352
1.67
1.67
a
Obtained by considering the NBO electron population analysis at the M06-2X/def2-TZVP level of theory where e is the elementary electric charge carried
by a single electron.
b
Calculated at the M06-2X/def2-TZVP level of theory considering the Wiberg definitions of bond order (see Ref. [19]) implemented in the NBO analysis
module.
c
Calculated at the M06-2X/def2-TZVP level of theory.
population of the phenolic oxygen was less diminished
(3.76e3.88) as
a
consequence of the LPO(H,
(C]O) delocalisation.
Ac) / (Ph) or LPO /
p
p
Furthermore, data issued from (VT) 1H NMR in-
vestigations (Table 4) completed those provided by theo-
retical anticipations.
Unsurprisingly, the most
p-deficient diamino-chloro-s-
triazines 1b and 2 displayed, at room temperature, the most
convincing 1H NMR spectral appearance, consistent with the
nearly frozen nature of their rotational equilibria (Scheme 2)
on the NMR time scale (Figs. SI-4 and SI-8). In contrast, under
the same conditions, the rotamerism of all melamines (G-0,
-1, -2) was identified by NMR as a classically entitled “slow
exchange status between (un)equally populated sites” [22].
Upon heating to 80e90 ꢀC (Table 4), all compounds became
single mediated structures, in a fast freely rotating status
about all C(s-triazine)-N(exocyclic) bonds.
Fig. 1. The optimised geometry structure of compound 7 obtained at M06-
2X/def2-TZV level of theory [14] (the H atoms were omitted for reasons of
simplicity).
(iv) The lowest nitrogen Lone Pair (LP) orbital populations
Temperature Gradients (TGs) of the NH protons were
also indicative because they revealed the influence of
different types of N-ligands on solvation. Although this
(1.64e1.68) were found in LPN(H) /
cetamol) and LPN(exocyclic) / (s-triazine) conju-
gated compounds 1a, 1b, 2, 3a and 3b. The LP orbital
p(C]O) (para-
p
Table 4
Relevant (VT) 1H and 2D-1H-DOSY-NMR data (500 MHz, DMSO-d6) of compounds 1a, 1b, 2, 3e5a, 3e5b, 6 and 7.
No.
Relevant dH values (ppm) (298 K)
Relevant dH values (ppm) (363 K)
Temperature gradients (TGs)
D (m
m2/s)
dHb (nm)
as (DdNH/D
Τ)ꢂ103 (ppb/K)a
NH
OH
NH
OH
1a
1b
8.74
9.70
9.83
9.91c
10.18
10.34c
8.71
8.81
8.72
8.72
8.83
8.72
8.78
8.79
9.06
9.27
8.37
9.46
8.74
8.91
ꢁ5.69
ꢁ3.69
ꢁ5.69
ꢁ6.92
ꢁ3.82
ꢁ6.73
ꢁ5.69
ꢁ6.15
ꢁ6.00
ꢁ6.55
ꢁ4.46
ꢁ7.27
ꢁ4.77
ꢁ6.15
152
218
1.44
1.00
2
d
9.97d
d
230
0.95
3a
4a
3b
4b
5a
5b
6
9.01
9.04
9.02
9.01
9.04
9.01
9.04
9.05
8.34
8.41
8.33
8.36d
8.54
8.32d
8.47
8.39
8.68
8.69
8.66
8.70
8.79
8.66
8.78
8.70
152
137
131
125
90
72
82
70
1.44
1.59
1.67
1.75
2.43
3.03
2.66
3.12
7
a
Calculated as [(dH298K
e
dTHK)/(298 KeT K)] ꢂ 103 < 0.
b
dH (Hydrodynamic diameter) issued from D [diffusion coefficient observed in 2D-1H-DOSY NMR charts in 5 mM DMSO-d6
(
h
, dynamic viscosity
2.00 ꢂ 10ꢁ3 kg mꢁ1 sꢁ1) at 298 K] by applying the StokeseEinstein equation.
c
Multiple dH values due to more than one (antieanti) species found in a frozen rotational equilibrium in agreement with the highest
p-deficiency of the s-
triazine ring in the analysed series (Scheme 2, Figs. SI-4 and SI-8).
d
At 353 K.
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C. Morar et al. / C. R. Chimie xxx (2016) 1e13
7
parameter is appropriate for amide eN(H)eC(]
O)e ↔ eNþ(H)]C(eOꢁ)e protons in peptides and proteins
[23], it can also be applied to amino-s-triazines, eN(H)e
C(]Ne)e ↔ eNþ(H)]C(eNꢁe)e as established by Sima-
nek and Moreno [21b]. Following this extrapolation, if TG
values of “amide-like” protons in amino-s-triazines are
more negative than ꢁ4 ppb/K in strong hydrogen bond
acceptor solvents, such as DMSO-d6 [12g], the NH groups
are exposed to the solvent rather than forming intra-
molecular hydrogen bonds. On the other hand, a TG value
less negative than ꢁ4 ppb/K indicates that the NH group
preferentially forms intramolecular hydrogen bonds at
room temperature. If so, besides the normal aptitude for
binding of p-HO phenolic functionalities, the contribution
to solvation of NH groups, i.e. Me2S]O/H/N<, was also
significant. Indeed, as one can see, almost all TGs in Table 4
were more negative than ꢁ4 ppb/K in conjunction with an
antieanti arrangement of phenolic (peripheral) units
(Scheme 2, Table 2) in all types of compounds. Consistent
with some of our previous data [5b], when the rotamerism
was 1H NMR detected as a mixture of stereoisomers
(compounds 1b and 2, Table 4), this situation disclosed the
major rotamer antieanti (Table 2) being the most solvated
as well (TGs as ꢁ6.92 ppb/K and ꢁ6.73 ppb/K, respectively,
Figs. SI-4 and SI-8).
N
N
N
N
N
N
N
N
N
N
chair-chair flipping
piperazin-1,4-diyl linker
N
N
N
N
4a, 5a
N
N
N
N
N
N
N
N
N
N
anancomeric 4,4'-biperidin-1,1'-diyl linker: 4b, 5b
Scheme 3.
produced during electrochemical oxidation. We note that
the reduction peak C3, observed even in anhydrous DMSO
0.1 M KCl ultrasonicated solution in the absence of any
organic compound, was related to the activity of the Pt
electrode [24] and will be not discussed here. The potential
values Ep corresponding to peaks A1, C1 and C2, issued
from the voltammetric response, are listed in Table 5.
Since the A1 anodic potentials values were indicative of
the electron donating strength of the compounds, we
considered that these potentials also reflect their oxidation
easiness, i.e. increasing with the decrease of the oxidation
potential value.
Our attention was primarily focused on the pair of peaks
A1/C1. Thus, by comparing voltammograms of the bis(p-
aminophenol)-based diamino-chloro-s-triazine 1b with its
O,O0-diaceyl analogue 2 (Fig. 2A), the disappearance of peak
A1, around 0.020 V versus Ag/AgCl, KClsat, proved that this
was due to absence of the free p-HO-phenolic group in 1b,
involved in the first step of oxidation. That is, the replace-
ment of p-HO (from 1b) by the p-AcO group (in 2) leads to
peak A1 vanishing, i.e. the single NH group in 2 was unable
to trigger any oxidation process. We note that, in the case of
paracetamol, containing just one p-aminophenol unit, a
recent well-documented N-acetyl-p-benzoquinonimine
oxidation product is mentioned in the literature as a result
Surprisingly, the different conformational nature of the
internal linker, anancomeric2 [22b] (rigid, 4,40-bipiperidin-
1,10-diyl) against chair $ chair flipping (piperazine-1,4-
diyl) determined lower TGs in 4b (ꢁ6.55 ppb/K) and 5b
(ꢁ7.27 ppb/K) with respect to 4a (ꢁ6.15 ppb/K) and 5a
(ꢁ4.46 ppb/K) (Scheme 3).
Dimeric (G-0) melamines 4a and 4b were more Me2S]
O/H/N< solvated in comparison with their monomeric
precursors 3a (TG as ꢁ5.69 ppb/K) and 3b (TG as
ꢁ6.00 ppb/K), respectively.
To what extent the above structural findings have a
corresponding electrochemical impact, we will discuss
hereafter.
2.3. Electrochemical characterisation
of
a reversible A1/C1 two-electron transfer process
2.3.1. Cyclic voltammetry
(Scheme 4) [25aec].
Next, the electrochemical behaviour of our p-amino-
phenol-based amino-s-triazines was investigated by means
of cyclic voltammetry. The cyclic voltammograms recorded
on the Pt electrode in DMSO, 0.1 M KCl solution are pre-
sented in Fig. 2.
Except for compounds 2 and 7, all voltammograms
revealed the presence of a single well-defined anodic
oxidation peak A1 closely located around 0 V versus Ag/
AgCl, KClsat. By contrast, 1e3 cathodic reduction peaks, C1-
3, were disclosed as C1 and C3 (paracetamol and com-
pounds 1b, 3b, and 6), C2 and C3 (compound 1a) or C1, C2
and C3 (compounds 3a, 4a, 4b, 5a and 5b). The variable
incidence of the reduction peaks C1 and/or C2 was
attributed to the successive reduction of the species
The above redox pathway occurs in spite of the low
3
level (Table 2) and low LPN(H) orbital population of
HOMO
paracetamol (Table 3). As shown in Table 5, under our
conditions, the peak A1 of paracetamol was located at a
negative value, ꢁ0.027 V versus Ag/AgCl, KClsat
.
Therefore, concerning the oxidation stage, we postu-
lated a similarity between the EW influence of the variable
p
-deficient s-triazine ring linked to the NH group in our
compounds and that of the EW Ac group in paracetamol. If
so, as for paracetamol, the pair of peaks A1/C1 also dis-
closed an overall and reversible two-electron transfer
process/p-aminophenol unit even for compounds 1b and
3a exhibiting positive Ep A1 values (Table 5). Thus, in a
two-step oxidation process, a novel extended
pep-con-
jugated system of type (s-triazinyl)-p-benzoquinonimine
was formed (Scheme 5).
One should observe that the decreasing order of the A1
peak potentials (Table 5), for example in the (G-0) dendron
2
According to the definition from Ref. [22b]: “Fixed in
a single
conformation either by geometric constraints or because of an over-
whelmingly on-sided conformational equilibrium.”
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8
C. Morar et al. / C. R. Chimie xxx (2016) 1e13
(A)
(B)
(C)
Fig. 2. Cyclic voltammograms of 10ꢁ3 M paracetamol (PA), 1a, 1b, 2 (2A), 3a, 3b, 4a, 4b (2B) and 5a, 5b, 6, 7 (2C). Experimental conditions: electrolyte, anhydrous
and ultrasonicated DMSO/0.1 M KClsat solution deaerated with Ar; starting potential ꢁ1 V versus Ag/AgCl, KClsat; scan rate, 0.050 Vꢁ1; the 25th cycle is shown.
series as 1b > 1a > 3a > 3b, was parallel to the decreasing
adjacent NH group ability for oxidation. Therefore, we
assumed the p-benzoquinonimine route (Scheme 5) to be
valid for all derivatives exhibiting the pair of peaks A1/C1
(Fig. 2) and the unique electronic transfer for compounds
strength of the
expressed as
p
-deficiency of the EWG s-triazine
values (Table 2) or LPN(H) orbital
3
HOMO
populations (Table 3). They crucially determined the
Table 5
Values of A1, C1, C2 peaks potential of compounds paracetamol (PA), 1a, 1b, 2, 3ae5a, 3be5b, 6 and 7 (for Experimental conditions, see Fig. 2).
Compound Ep (V) vs. Ag/AgCl, KClsat
1b
1a
3a
4a
5a
5b
PA
6
3b
4b
2
7
A1
C1
C2
0.020
ꢁ0.270
d
0.015
d
0.013
ꢁ0.015
ꢁ0.180
ꢁ0.415
ꢁ0.021
ꢁ0.245
ꢁ0.430
ꢁ0.024
ꢁ0.210
ꢁ0.420
ꢁ0.027
ꢁ0.275
d
ꢁ0.048
ꢁ0.205
d
ꢁ0.052
ꢁ0.220
d
ꢁ0.090
ꢁ0.275
ꢁ0.460
d
d
d
da
ꢁ0.230
ꢁ0.240
ꢁ0.440
ꢁ0.400
ꢁ0.440
a
Too weak to be measured.
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C. Morar et al. / C. R. Chimie xxx (2016) 1e13
9
Similar concepts applied in the case of angularly built
(G-1) dendrons 5a, 5b and 6.
O
O
The oxidation potential values corresponding to peak A1
in these compounds revealed that 5a, 5b and 6 (Fig. 2C)
behave similarly in what the easiness of the oxidation was
concerned. However, the oxidation current intensity was
much smaller in the case of (G-1) melamine 6 with respect
to its (G-1) chlorodendron precursor 5a, presumably
because of the blocking of the electrode surface by
adsorption of the compound 6 oxidation products.
It is important to notice that, for compound 7 (Fig. 2C),
the oxidation peak A1 almost disappeared, suggesting that
the dendrimer was much more difficult to oxidize than all
its precursors. This behaviour could be motivated by the
vaulted spatial arrangement (Fig. 1) of 7, which impeded
the contact between the redox centres of the dendrimer
and the Pt electrode surface.
HN
N
O
A1
-2e-, -2H+
C1
+2e-, +2H+
OH
Paracetamol
N-acetyl-
p-benzoquinonimine
Scheme 4.
1b, 3b and 6 (Table 5). Nevertheless, to what amount this
route involved, simultaneously, all p-aminophenol pe-
ripheral units could not be established because of the
iterative nature of the oxidation process.
Furthermore, the influence of several other factors
concerning the redox process A1/C1 (Scheme 5) was also
observed. Thus, (G-0) dimeric melamines 4a and 4b dis-
closed significantly lower anodic oxidation potentials
against (G-0) monomeric melamines 3a and 3b, respec-
tively, most probably because of a better NH-solvation in
DMSO expressed by TG values (Table 4).
2.3.2. Influence of the starting potential
In order to investigate the influence of the starting po-
tential on the electrochemical response of compounds,
dimeric G-0 dendron 4a was selected as a typical example
(Fig. 3).
As expected, the width of the scanned potential domain
was proven to be of great importance. Starting from an
enough negative value of potential (e.g., at least ꢁ0.7 V
versus Ag/AgCl, KClsat), the appearance of the oxidation
peak A1 and of the subsequent reduction processes
responsible for reduction peaks C1 and C2 was noticed.
When we limited the potential range to a narrow domain
(e.g., ꢁ0.525 V to þ0.300 V versus Ag/AgCl, KClsat), no
characteristic oxido-reduction peaks appeared. Generally,
the starting potential for the electrochemical measure-
ments was ꢁ1 V versus Ag/AgCl, KClsat. If the experiment
was done within a large potential window (ꢁ1.0 V to
þ0.6 V), reversing the scan direction (e.g., starting from
þ0.6 V versus Ag/AgCl, KClsat), no influence on the shape of
the voltammogram was observed (Fig. 3).
A comparison between the CVs of (G-0) melamines 3a
and 3b (Fig. 2B) showed that the replacement of the
piperazine-1-yl ligand with the more electron-donor 4,4-
bipiperidin-1-yl dramatically decreased the anodic oxida-
tion potential, i.e. from þ0.013 V to ꢁ0.052 V versus Ag/
AgCl, KClsat. We associated this behaviour with the higher
basicity of 3b versus 3a together with the different
conformational nature of their diaza-linkers, flipping in 3a
but anancomeric in 3b (Scheme 3). Consequently, the
adsorption on the electrode surface of 3b was more inti-
mate in comparison with that of 3a. Presumably for the
same conformational reason, a similar fluctuation was
observed in the case of dimeric (G-0) melamines 4a
(ꢁ0.015 V) and 4b (ꢁ0.090 V). Mutatis-mutandis, the CVs of
the (G-0) melamine 3a against its dimer 4a from one hand
or of 3b against 4b on the other hand (Fig. 2B) exhibited a
comparable enhancement of the oxidation current in the
case of dimers due, very likely, to the doubling of the
number of peripheral phenolic groups and to the symmetry
of the molecule.
2.3.3. Reproducibility
The reproducibility of the voltammetric measurements
was investigated by recording 3 successive voltammo-
grams for the same electrode with cleaning of its surface
between measurements. The reproducibility of cyclic
The A1/C1 redox process: the p-benzoquinonimine route
Tz
Tz
Tz
HN
HN
N
A1 fast
-1e-, -1H+
A1 slow
-1e-, -1H+
•
C1
C1
OH
+1e-, +1H+
O
O
+1e-, +1H+
N-(s-triazinyl)
p-benzoquinonimines
1b, 3a-5a
3b-5b, 6
Tz : s-Triazine-4,6-diyl environments
Scheme 5.
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C. Morar et al. / C. R. Chimie xxx (2016) 1e13
(A)
Fig. 3. Cyclic voltammogram of 10ꢁ3 M 4a at different values of starting
potential. Experimental conditions: electrolyte, anhydrous and ultra-
sonicated DMSO/0.1
M KClsat solution deaerated with Ar; scan rate,
(B)
0.050 V sꢁ1; the 7th cycle is shown.
voltammetric measurements for compound 4a was good.
Thus, for a mean value of the peak current intensity of
9.48 ꢂ 10ꢁ6 A, for 3 successive measurements the relative
standard deviation (RSD) was 11.9% (results not shown).
2.3.4. Influence of repetitive potential cycling
Dimeric (G-0) melamine dendrons 4a and 4b, having a
low s-triazine p-deficiency, were selected in order to study
the influence of repetitive potential cycling on the elec-
trochemical response under potentiodynamic conditions
by continuous cycling of the electrode potential (25 cycles)
in the potential range of ꢁ1 V ÷ þ0.6 V versus Ag/AgCl,
KClsat, in DMSO solution, on the Pt electrode. The results are
shown in Fig. 4.
A progressive increase of the peak current was observed
suggesting a possible polymerization taking place on the Pt
electrode surface. We note that the electropolymerization
of p-aminophenol on the Pt electrode in organic media
leading to a soluble and electroactive polymer was reported
by Taj et al. [9a] as early as 1992. As proposed in Scheme 6,
in our case, the electrochemical pathway should be (i)
similar to that of p-aminophenol [9h], irrespective of the
presence of the s-triazinyl fragment and (ii) related to that
of paracetamol [25d].
Fig. 4. Cyclic voltammograms of 10ꢁ3 M 4a (4A) and 4b (4B) on the Pt
electrode. Experimental conditions: electrolyte, anhydrous and ultra-
sonicated DMSO/0.1 M KClsat solution deaerated with Ar; starting potential,
ꢁ1 V versus Ag/AgCl, KClsat; scan rate, 0.050 V sꢁ1.
Thus, the electrochemical significance of peak A1 was
almost the same as that in Scheme 5, fast p-HO phenolic
against slow NH oxidation, the last one being the most
sensitive to the EWG s-triazine vicinity. As a result, a radical
o,o0-coupling, similar to that previously reported for para-
cetamol [25d], occurred to produce dimeric and then linear
tetrameric species, respectively, with a s-trans arrangement
of the s-triazine bulky motifs. One must observe that the
cathodic peak potential C2 (Table 5) was located at much
more negative values in comparison with C1, in agreement
with the decreasing order of stability against reduction of
the involved pep-conjugated system, aromatic (C2) > p-
benzoquinonimine (C1).
Besides 4aand 4b, the electropolymerization routecould be
extrapolated to all melamines exhibiting the reduction peak
C2 namely (G-0) 1a and 3a, (G-1) 5a and 5b. The electro-
polymerization route appeared to be the single one in the case
of (G-0) melamine 1a (Fig. 2A); however it was not observed
for melamines (G-0) 3b (Fig. 2B) and (G-1) 6 (Fig. 2C).
In order to perform a quantitative evaluation of the poly-
mer matrix permeability, cyclic voltammograms of com-
pounds 4a and 4b were recorded for different potential scan
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C. Morar et al. / C. R. Chimie xxx (2016) 1e13
11
The A1/C2 redox process: the electropolymerization route
Tz
Tz
HO
NH
Tz NH
o,o'-radical
dimerisation
NH
A1 fast
-1e-, -1H+
A1 slow
-1e-, -1H+
•
C1
+1e-, +1H+
C2
+1e-, +1H+
HN Tz
OH
O
OH
1a, 3a, 4a,
4b, 5a, 5b
Tz NH
HO
Tz NH
HO Tz-HN
HO
o,o'-radical
dimerisation
etc.
NH-Tz
•
OH
N Tz
OH
HN Tz
OH
Tz : s-Triazine-4,6-diyl environments
Scheme 6.
rates on the Pt/4a electrode, immersed in a 5 mM K3[Fe(CN)6]
solution, containing 0.1 M KCl (results not shown).
The voltammetric wave corresponding to the anodic
and cathodic peak currents of the [Fe(CN)6]3ꢁ/[Fe(CN)6]4ꢁ
couple obeys Equation (1):
dendritic melamines comprising p-aminophenol (periph-
eral unit) and piperazine or 4,40-bipiperidine (linkers) was
reported. The chemoselective amination of cyanuric chlo-
ride performed with the above amino-nucleophiles was
more accessible if piperazine was the internal linker of
choice. Overall, the low solubility of these amino-s-triazines
in common volatile organic solvents and their high relative
retention on column chromatography were the two main
obstacles to be overcome, successfully, with good yields.
À
Á
Ip ¼ 2:69 ꢂ 105
n
2=3AD1=2v1=2Co
(1)
where n is the number of electrons, A is the active surface
area (cm2), D is the diffusion coefficient (cm2/s), v is the
scan rate and Co is the concentration in bulk solution (mol/
cm3).
3.2. Structure
Using Equation (1), the K3[Fe(CN)6] diffusion coefficient
at the Pt electrode was found to be equal to 3.3 ꢂ 10ꢁ6 cm2/s
which is in concordance with those reported in the litera-
ture [26]. Assuming this value for the K3[Fe(CN)6] diffusion
coefficient, the area of the modified electrode Pt/4a was
estimated to be 0.2226 cm2, which is greater with a factor of
1.13 than the geometrical surface of the electrode
(0.19625 cm2), confirming that during the 25 cycling process
an immobilization of compound 4a on the Pt surface
occurred.
DFT calculations in DMSO (with global energy minima,
indicative bond order indexes, bond lengths and lone pair
orbital populations) helpfully established and discrimi-
nated the rotational diastereomerism of p-aminophenol
peripheral units about the partial double bonds C(s-
triazine)-N(exocyclic) as being of type asymmetric or anti-
eanti. The last one was also localised at the periphery of the
first p-aminophenol-based (G-2) dendritic melamine
which adopted a vaulted global shape. Calculation revealed
a lower N-lone pair orbital population in PheNH units in
comparison with that of the phenolic oxygen atom. (VT) 1H
NMR spectroscopy disclosed, by means of temperature
gradients, our p-aminophenol-based amino-s-triazines
being not only OH but also strongly >N/H/O]SMe2
solvated molecules. Solvation was higher if the internal
linker was anancomeric (4,40-bipiperidin-1,10-diyl) against
flipping (piperazine-1,4-diyl); however it was less depen-
dent on the molecular dimension (dH).
In addition, the relative increase of the charge transfer
on the electrode surface can be estimated by the Equation
(2):
Q ¼ ðQ25=Q2 ꢁ 1Þ ꢂ 100
(2)
where Q25 is the electric charge calculated from the under
surface area of the oxidation peak after 25 cycles and Q2 is
the same parameter for the second cycle. In the case of Pt/
4a and Pt/4b, the value of Q was about 6.54% and 12%,
respectively, after 25 continuous cycles, indicating that an
activation of the surface took place.
3.3. Electrochemical characterisation
Almost all the above structural assignments had an
impact on electrochemical characterisation of our amino-s-
triazines by cyclic voltammetry (Pt electrode/DMSO). A two-
electron transfer process was observed. Depending on the
3. Conclusions
3.1. Synthesis
variable strength of p-deficiency of the s-triazine ring acting
Avoiding any protectiveedeprotective step, the iterative
synthesis of a new 10 member family of (G-0, -1, -2)
as the EWG on the adjacent NH group and the modulation
ability of the latter to undergo redox processes in tandem
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12
C. Morar et al. / C. R. Chimie xxx (2016) 1e13
with the phenolic p-HO group, two electrochemical path-
ways, similar to those already reported for paracetamol and
p-aminophenol were proposed, (i) the p-benzoquinonimine
route and (ii) the electropolymerization route.
0.1 M KCl (Reactivul Bucuresti). The reagents were of
analytical grade and used as received.
4.4. Electrochemical measurements
4. Experimental
Cyclic voltammetry was performed on a PC-controlled
electrochemical analyser Autolab-PGSTAT 10, EcoChemie,
The Netherlands. Electrochemical experiments were car-
ried out using an undivided cell equipped with three-
electrodes: a platinum electrode as the working electrode,
a platinum wire as the counter electrode and Ag/AgCl,
KClsat as the reference electrode, respectively. The electro-
lyte solution was deoxygenated by bubbling argon for
15 min before measurements.
4.1. General
Melting points were carried out on an ELECTRO-
THERMAL instrument. NMR spectra were recorded on a
Bruker AM 400, 500 or 600 instrument operating at 400,
500 or 600 and 100, 125 or 150 MHz for 1H and 13C nuclei,
respectively. All chemical shifts (d values) are given in parts
per million (ppm); all homocoupling patterns (nJH,H values)
are given in Hertz. In the NMR descriptions, some specific
abbreviations were used: “br s” (broad singlet), “br d”
(broad doublet), “dd app. t” (doublet of doublets appearance
as a triplet), “ddd app. td” (doublet of doublets of doublets
appearance as a triplet of doublets), Pip (Piperazine linker)
and Bipip (4,40-Bipiperidine linker). TLC monitoring was
performed by using aluminium sheets with silica gel 60
Acknowledgement
The financial support from a Grant provided by the
Research Council Romania (Project PN-II-ID-PCE-2011-3-
0128) is gratefully acknowledged. A.B. acknowledges the
financial support from the Romanian National Authority for
Scientific Research and Innovation (ANCSI) through the
Core Program 2015.
F254 (Merck) (visualisation in UV at
l
¼ 254 nm). Column
chromatography was conducted on Silica gel Si 60
(40e63 mm, Merck). IR spectra were recorded on a JASCO
FT-IR 6100 Spectrometer. Only relevant absorption maxima
are listed, throughout, in cmꢁ1: s (strong), m (medium) and
w (weak). Microanalyses were performed on a Carlo Erba
CHNOS 1160 apparatus. Mass spectra were carried out on a
LTQ ORBITRAP XL (Thermo Scientific) instrument which
was externally calibrated using the manufacturer's ESI(þ)
calibration mix. The samples were introduced into the
spectrometer by direct infusion.
Appendix A. Supporting information
Supporting information related to this article can be
found at http://dx.doi.org/10.1016/j.crci.2016.07.002.
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