Elaboration of multimodal ligands based on BOX units: Toward full zinc release in Zn(II) complexes by external stimulation

Article abstract of DOI:10.1016/j.dyepig.2021.109476

The development of molecular switches such as photochromic and electrochromic systems still represents a major concern in numerous application fields. If numerous pure organic switch families have been explored so far, concerning the coordination compounds, the choice seems more limited. In this context, we are exploring the possibility to elaborate a new addressable ligand family based on benzazolo-oxazolidine (BOX) moiety, a poorly investigated switch family, known for its acido-, electro- and photochromism properties. As proof of concept, two ligands bearing a BOX moiety were prepared using a pyridine as metal binding site after what corresponding Zn(II) complexes were synthesized and characterized. The acido-, electro- and photochemical behaviors of the free ligand as well as complexes were studied. This study reveals a subtle and interesting tautomeric equilibrium between oxazolidine ring opening and pyridine protonation whatever the nature of the stimulation. More interestingly, it conducts to observe the possibility to catch and release zinc ion on demand by using two external and orthogonal stimulations.

Full text of DOI:10.1016/j.dyepig.2021.109476

Dyes and Pigments 193 (2021) 109476
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Elaboration of multimodal ligands based on BOX units: Toward full zinc
release in Zn(II) complexes by external stimulation
Youssef Aidibi, Magali Allain, Abdelkrim El-Ghayoury, Philippe Leriche, Lionel Sanguinet*
Univ Angers, CNRS, MOLTECH – Anjou, SFR Matrix, F-49000, Angers, France
A R T I C L E I N F O
A B S T R A C T
Keywords:
The development of molecular switches such as photochromic and electrochromic systems still represents a
major concern in numerous application fields. If numerous pure organic switch families have been explored so
far, concerning the coordination compounds, the choice seems more limited. In this context, we are exploring the
possibility to elaborate a new addressable ligand family based on benzazolo-oxazolidine (BOX) moiety, a poorly
investigated switch family, known for its acido-, electro- and photochromism properties. As proof of concept, two
ligands bearing a BOX moiety were prepared using a pyridine as metal binding site after what corresponding Zn
(II) complexes were synthesized and characterized. The acido-, electro- and photochemical behaviors of the free
ligand as well as complexes were studied. This study reveals a subtle and interesting tautomeric equilibrium
between oxazolidine ring opening and pyridine protonation whatever the nature of the stimulation. More
interestingly, it conducts to observe the possibility to catch and release zinc ion on demand by using two external
and orthogonal stimulations.
Photoresponsiveness
Electrochomism
Halochromism
Zinc complex
1. Introduction
reviews were dedicated to multi-chromophoric molecular systems
especially based on DAE unit [6,10,11]. In these multi-photochromic
The elaboration of molecular systems allowing a modulation of their
molecular properties as response to an external stimulation continues to
rise a growing attention from a large scientific community. Depending of
(i) the nature of the modulated molecular properties and (ii) the kind of
stimulus, such systems can find applications in numerous fields such as
optical data storage and molecular logic gate[1], medical imaging[2],
magnetism [3], and chemical sensors to name a few. Among these last,
the elaboration of photochromic systems, able to undergo a reversible
transformation under light irradiation has attracted much attention.
Thus, amongst the numberless pure organic photochromic materials
elaborated and studied [4], the diarylethene (DAE) [5,6], azobenzene
[7] and spiropyrans [8,9] based systems represent certainly the most
three widespread photochromic families. Allowing the modulation be-
tween only two discrete levels of properties (referenced generally as On
and Off states), several of these motifs may be associated in order to
elaborate molecular systems exhibiting a higher degree of complexity
[10]. However, as the multistep synthesis of organic compounds asso-
ciating n switchable units appeared often tricky and hazardous, the
resulting structures did not necessary allow the awaited expansion of the
number of metastable states (theoretically up to 2n). In fact, several
systems, different behaviors can be observed ranging from the perfect
selectivity of the switch addressability to the complete inhibition of the
photochromic properties of the DAE as function of the status of the other
commutable unit (generally referenced as gated photochromism) [11,
12].
In this context, the coordination chemistry offers an interesting
approach to circumvent some of these issues. First, it provides an effi-
cient and convenient way to elaborate multi-chromophoric systems as
exemplified by the large variety of DAE multimers using a metallic
center as connector[10]. Second, the resulting complexes could lead to
the combination of several functionalities by gathering, for example, an
electroactive organometallic moiety and a photoresponsive ligand[13,
14], or, as more recently shown by Crassous and Rigaut teams, by
connecting two ligands with complementary properties merging
photochromic and chiroptical properties[15]. To obtain such systems,
the classical approach consists in functionalizing
a
classical
nitrogen-based binding unit (pyridine, bi-pyridine, ter-pyridine or phe-
nanthroline), in most cases, by a DAE as photochromic unit or, more
rarely,
by
spiropyran
and
spiroxazine
moieties[16,17].
Photo-responsive metal complexes featuring an azo-aryl function were
* Corresponding author.
E-mail address: lionel.sanguinet@univ-angers.fr (L. Sanguinet).
https://doi.org/10.1016/j.dyepig.2021.109476
Received 18 February 2021; Received in revised form 12 May 2021; Accepted 13 May 2021
Available online 26 May 2021
0143-7208/© 2021 Elsevier Ltd. All rights reserved.

Y. Aidibi et al.
Dyes and Pigments 193 (2021) 109476
also reported. In these cases, the photochromic and electrochemical
properties were strongly affected by the strength of interactions between
azo and metal units and, then, the obtainment of the multi-functional
system was not assured[17–21]. From these observation, it may be
concluded that the elaboration of photo-addressable metal complexes
may lay on a limited choice of structures. In addition, a similar situation
is also observed from systems incorporating electro-active ligands.
Despite systems including “non-innocent ligand” [22] for which the
electrostimulation conducts to redox state change of the metal center,
most of the electro-responsive complexes are elaborated from ligands
incorporating a tetrathiafulvalene (TTF) as redox active unit [23,24].
In this context, we report here on our efforts to develop a new family
of ligands based on indolino-oxazoline derivatives (referenced later as
BOX), a poorly investigated family of multi-modal addressable units, on
their coordination behavior as well as on properties of corresponding
complex. Indeed, the opening and closure of the oxazolidine ring can be
induced using indifferently three different kinds of stimulation and re-
sults to nice photo-, electro- and acido-chromic performances [25]. Up
to now, restricted to the elaboration of pure organic molecular systems
mainly dedicated to the development of nonlinear optical (NLO)
switches, this promising unit was, to the best of our knowledge, never
involved in supramolecular assembly neither in metallic complexes. To
fulfill this gap, we have successfully prepared two dyads incorporating a
BOX unit associated to a pyridine moiety as the simplest nitrogen-based
binding unit and studied their coordination behavior toward Zinc (II) as
well as the switching properties of the resulting system.
2.2. Synthesis
As previously reported [27], both trimethyl indolino[2,1,b]oxazoli-
dine derivatives were prepared according to the following procedure:
General procedure: A mixture of indolenine derivative (86.9 mmol)
and 2-iodoethanol (130.4 mmol, 1.5 eq.) in toluene (135 mL) was
refluxed overnight. After cooling to room temperature, the precipitate
was filtered off, washed with cold Et2O (3 × 30ml) and acetone (3 ×
30ml) and dried to afford corresponding indoleninium iodide salt which
was used in the next step without further purification.
To a suspension of indoleninium iodide salt (70.5 mmol) in water
(500 mL), 250 mL of aqueous NaOH solution (2 M) were added under
vigorous agitation at room temperature. After 30 min, the crude mate-
rial was extracted with Et2O (3 × 1000 mL). The organic phases were
combined, washed with water (2 times), brine (1 time) and dried over
MgSO4. The solvent was removed under reduced pressure to afford the
corresponding trimethyl indolino [2,1-b]oxazolidine derivative as a
brown oil which was used without further purification.
2,3,3-trimethylindolino[2,1-b]oxazoline (3a): Product was iso-
lated as a brown oil (93%). 1H NMR (300 MHz, CDCl3) δ (ppm) 7.14 (td,
J = 7.7, 1.2 Hz, 1H), 7.08 (d, J = 7.4 Hz, 1H), 6.93 (td, J = 7.4, 0.8 Hz,
1H), 6.77 (d, J = 7.8 Hz, 1H), 3.90–3.43 (m, 4H), 1.43 (s, 3H), 1.39 (s,
3H), 1.18 (s, 3H).
2,3,3,5-tetramethylindolino[2,1-b]oxazoline (3b): Product was
isolated as a brown oil (91%). 1H NMR (300 MHz, CDCl3) δ (ppm) 6.93
(d, J = 7.9 Hz, 1H), 6.89 (s, 1H), 6.66 (d, J = 7.9 Hz, 1H), 3.91–3.42 (m,
4H), 2.30 (s, 3H), 1.41 (s, 3H), 1.38 (s, 3H), 1.17 (s, 3H).
General procedure for the elaboration of the ligands: A mixture
of 4-pyridinecarboxaldehyde (2.3 mmol) and corresponding indolino-
oxazolidine (2.8 mmol) were dissolved in a little amount of DCM. 1 g
of technical grade silica was suspended and the solvent was removed
under reduced pressure. The resulting reaction mixture was heated
under stirring at 100 ◦C during 10 min. After cooling down to room
temperature, the crude material was directly purified by flash chroma-
tography (PE/EtOAc, 3/7).
2. Experimental
2.1. General
Flash chromatography was performed with analytical grade solvents
using Aldrich silica gel (technical grade, pore size 60 Å, 230–400 mesh
particle size). Flexible plates ALUGRAM® Xtra SIL G UV254 from
MACHEREY-NAGEL were used for TLC. All chemical reagents were used
as received form commercial suppliers (Sigma-Aldrich or Acros).
The NMR spectra were recorded with a Bruker AVANCE III 300 (1H,
300 MHz and 13C, 75 MHz) using CDCl3 as solvent. Chemical shifts are
reported in ppm relative to the solvent residual value: δ = 7.26 for 1H
NMR and δ = 77.16 for 13C NMR. Coupling constants are reported in Hz
and rounded to the nearest 0.1 Hz.
(E)-9,9-Dimethyl-9a-(2-(pyridin-4-yl)vinyl)-2,3,9,9a-
tetrahydrooxazolo[3,2-a]indole; 4a: product was isolated as a yellow
solid (496 mg, 73%). m.p.: 115 ◦C
1H NMR (300 MHz, CDCl3) δ (ppm): 8.58 (dd, J = 4.5, 1.6 Hz, 2H),
7.31 (dd, J = 4.6, 1.5 Hz, 2H), 7.22–7.15 (m, 1H), 7.09 (dd, J = 7.4, 0.9
Hz, 1H), 6.96 (td, J = 7.4, 1.0 Hz, 1H), 6.83 (dd, J = 11.9, 6.6 Hz, 2H),
6.53 (d, J = 15.9 Hz, 1H), 3.85–3.37 (m, 5H), 1.46 (s, 3H), 1.17 (s, 3H).
13C NMR (75 MHz, CDCl3) δ (ppm): 150.3, 150.2, 143.8, 139.5, 131.5,
130.1, 127.8, 122.4, 122.0, 121.3, 112.1, 109.5, 63.8, 50.2, 48.2, 28.4,
Infrared spectra were recorded on a Bruker Vertex 70 spectrometer.
UV–Vis spectra were performed with a PerkinElmer 950 spectrometer
from solutions in ACN (HPLC grade from VWR chemicals) in presence of
Chlorobenzene (99+% form ACROS). In agreement with our previous
studies, titrations were performed using either daily fresh prepared
NOSbF6 (Sigma-Aldrich) solution in ACN or a HCl solution in ACN
prepared by successive dilution from brand new aqueous solution of HCl
(37% from VWR chemicals). UV Irradiation were performed directly in
spectroscopy cells by using 254 nm lamp (6 W) suitable for thin layer
chromatography. The Mass spectra were recorded using BIFLEX III
Bruker Daltonics spectrometer (MALDI-TOF, dithranol matrix) and
Elemental analysis (EA) was performed on a Thermo Scientific flash
2000 elemental analyzer.
20.5. IR
υ
‾ = 2962, 2889, 1595, 1476, 1454, 1292, 1149, 975, 750 cmꢀ 1
Anal. calcd for C19H20N2O: C, 78.05; H, 6.89; N, 9.58, found: C, 77.80;
H, 6.75; N, 9.45. cm ꢀ 1. MS (MALDI-TOF): m/z calcd. For C19H20N2O:
292.1576 [M+H]; found: 292.1574. Anal. calcd for C19H22N2O: C,
78.05; H, 6.89; N, 9.58, found: C, 77.80; H, 6.75 N, 9.45.
(E)-7,9,9-trimethyl-9a-(2-(pyridin-4-yl)vinyl)-2,3,9,9a-tetrahy-
drooxazolo[3,2-a]indole; 4b: Product was isolated as a yellow solid
(627 mg, 88%). m.p.: 114–115 ◦C. 1H NMR (300 MHz, CDCl3) δ (ppm)
8.57 (d, J = 6.0 Hz, 2H), 7.30 (d, J = 6.1 Hz, 2H), 6.99 (d, J = 7.9 Hz,
1H), 6.90 (s, 1H), 6.83 (d, J = 15.9 Hz, 1H), 6.71 (d, J = 7.9 Hz, 1H),
6.53 (d, J = 15.9 Hz, 1H), 3.83–3.34 (m, 4H), 2.32 (s, 3H), 1.45 (s, 3H),
1.16 (s, 3H). 13C NMR (75 MHz, CDCl3) δ (ppm) 150.3, 148.0, 143.8,
139.6, 131.6, 131.4, 130.0, 128.3, 123.16, 121.3, 111.9, 109.8, 63.8,
The electrochemistry studies were performed on dichloromethane
solutions (HPLC grade) with tetra-n-butylammonium hexa-
fluorophosphate (TBAP, electrochemical grade from Fluka) as the sup-
port electrolyte (0.1 M) under a dry and oxygen-free atmosphere (H2O
< 1 ppm, O2 < 1 ppm). Cyclic voltammetry (CV) was performed in a
three-electrode cell equipped with a platinum millielectrode as working
electrode, a platinum wire counter-electrode and a silver wire used as a
quasi-reference electrode. The voltammograms were recorded on a
potentiostat/galvanostat (BioLogic – SP150) driven by the EC-Lab soft-
ware with positive feedback compensation. All of them were calibrated
versus Ferrocene/Ferrocenium oxidation potential (+0.405 V vs SCE or
+0.425 V vs Ag/AgCl) [26].
50.4, 48.2, 28.4, 21.1, 20.4. IR
υ
‾ = 2967, 2882, 1737, 1413 cmꢀ 1. MS
(MALDI-TOF): m/z calcd. For C20H22N2O: 306.1732 [M+H]; found:
306.1728. Anal. calcd for C20H22N2O: C, 78.40; H, 7.24; N, 9.14, found:
C, 78.13; H, 7.04; N, 9.3.
General procedure for the elaboration of Zinc complexes: In a
test tube, a solution of desired ligand (0.147 mmol) in CH2Cl2 (5 mL)
was mixed with a solution of ZnCl2 (0.073 mmol) in CH3CN (5 mL) and
ultra-sonicated for 2 min. On top of the resulting solution a layer of
2

Y. Aidibi et al.
Dyes and Pigments 193 (2021) 109476
diethyl ether was added, which led to a precipitate after one week. The
desired compound was filtered off, washed with diethyl ether and dried
under vacuum.
catalysis, solvent-free conditions) [25,29,30]. Surprisingly, the associ-
ation of BOX with an aromatic nitrogen heterocycle was remained un-
explored up to now. Only few examples using heterocycles were
reported and, to the best of our knowledge, are limited to thiophene and
furan derivatives. For such reason, the condensation reaction with
pyridinecarboxaldehyde was carried out using a silica-mediated pro-
cedure known to improve the reactivity of the trimethylindolino [2,1-b]
oxazolidine derivative [30]. Starting from an equimolar mixture of 3a
(3b) and 4-pyridinecarboxaldehyde, the formation of the desired ligands
4a (4b) was well noticed after heating during 10 min. Unfortunately, the
full conversion of the aldehyde could not be reached even under pro-
longed heating times and its presence in the reaction mixture led to
inextricable purification difficulties. By using a slight excess of BOX (1.2
equivalent) the reaction equilibrium can be displaced and the purifica-
tion of target molecules was greatly facilitated allowing to obtain li-
gands 4a and 4b in excellent yields (73 and 88% respectively).
In order to prepare a first series of photo-responsive metal complexes
based on BOX, our choice fell on zinc(II) as metallic ion. Indeed, this
metal choice presents several advantages such as a low cost, no stereo-
chemistry preference arising from ligand field stabilization effects, and
the preparation of corresponding complex is easy and well documented
especially from pyridine based ligands [31]. More important, the
absence of low-energy metal-to-ligand or metal d–d electronic transi-
tions, its lack of redox activity and finally its diamagnetic properties
[32] present some important advantage for the study of the electro- and
photo-induced transformations of our multi-responsive ligand especially
by NMR spectroscopy [33]. Using classical experimental conditions with
ZnCl2 [34], the two different zinc complexes 5a and 5b were obtained in
good yields (69 and 78% respectively) without particular difficulties.
Their complete and straightforward syntheses are illustrated on Scheme
1.
Dichlorobis-[9,9-dimethyl-9a-(2-(pyridin-4-yl)vinyl)-2,3,9,9a-
tetrahydrooxazolo[3,2-a]indole]zinc(II), 5a: product was isolated as
yellow solid (69%)
1H NMR (300 MHz, CD3CN) δ (ppm): 8.66 (dd, J = 5.0, 1.5 Hz, 2H),
7.74 (dd, J = 5.0, 1.6 Hz, 2H), 7.24–7.17 (m, 1H), 7.15 (dd, J = 7.4, 0.8
Hz, 1H), 6.96 (ddd, J = 10.6, 3.5, 2.1 Hz, 2H), 6.89 (d, J = 7.8 Hz, 1H),
6.78 (d, J = 15.9 Hz, 1H), 3.82–3.36 (m, 4H), 1.44 (s, 3H), 1.15 (s, 3H).
IR
υ
‾
= 2960, 1612, 1477, 1275, 749 cmꢀ 1. Anal. calcd for
C
40H44N4O2Cl2Zn: C, 63.30; H, 5.59; N, 7.77, found: C, 62.92; H, 5.44;
N, 7.35.
Dichlorobis-[7,9,9-trimethyl-9a-(2-(pyridin-4-yl)vinyl)-2,3,9,9a-
tetrahydrooxazolo[3,2-a]indole]zinc(II) 5b: product was isolated as
yellow single crystals (78%)
1H NMR (300 MHz, CD3CN) δ (ppm): 8.61 (d, J = 6.5 Hz, 2H), 7.68
(d, J = 6.5 Hz, 2H), 6.97 (d, J = 7.9 Hz, 1H), 6.94 (s, 1H), 6.90 (d, J =
16.1 Hz, 1H), 6.73 (dd, J = 11.9, 4.0 Hz, 2H), 3.77–3.28 (m, 6H), 2.27 (s,
3H), 1.38 (s, 3H), 1.09 (s, 3H). 13C NMR (75 MHz, CD3CN) δ
(ppm):149.7, 149.3, 149.0, 140.5, 136.4, 132.0, 129.6, 129.0, 124.0,
112.8, 110.5, 64.4, 50.8, 48.8, 28.5, 20.9, 20.6. IR υ‾ = 2958, 1613,
1489, 811 cmꢀ 1. Anal. calcd for C40H44N4O2Cl2Zn: C, 64.13; H, 5.92; N,
7.48, found: C, 62.15; H, 5.75; N, 7.14.
2.3. Crystal data collection and processing
X-ray single-crystal diffraction data for free ligand, 4b, and its zinc
(II) complex, 5b, were collected on an Agilent SuperNova diffractometer
equipped with Atlas CCD detector and mirror monochromated micro-
focus Cu-K
α radiation (λ = 1.54184 Å). The two structures were
3.2. Switching properties of free ligands
solved by direct methods, expanded and refined on F2 by full matrix
least-squares techniques using SHELX97 programs (G.M. Sheldrick,
1998). All non-H atoms were refined anisotropically and the H atoms
were included in the calculation without refinement. Multi-scan
empirical absorption was corrected using the CrysAlisPro program
(CrysAlisPro, Agilent Technologies, V1.171.37.35 g, 2014). Crystallo-
graphic data for the structural analysis have been deposited at the
Cambridge Crystallographic Data Centre, CCDC 2040158 for ligand 4b
and CCDC 2040159 for the zinc complex 5b.
Before investigating properties of transition metal complexes, the
multimodal commutabilities of the free ligands have been characterized.
Indeed, due to the presence of a pyridine moiety acting as a weak base
˜
(pKa 5,23), their properties, especially their acidochromic properties
could strongly be affected. Thus, depending on the open/closed status of
the BOX and on the pyridine protonation (Scheme 2), 4 different states
should be envisioned for each ligand.
Acidochromic properties. To monitor the switching properties of
such system, UV–Visible spectroscopy represents certainly the most
convenient technique as the opening of the oxazolidine ring comes with
an enhancement of the conjugation as the hybridation of the carbon in
position 2 changes from sp3 to sp2 [25]. Under their closed form (C),
compounds 4a and 4b are characterized by an absorption band centered
at 243 and 245 nm respectively highlighting the poor influence of the
indoline substituent in position 5 in agreement with previous studies on
BOX derivatives[35]. As expected, a strong modification of the UV–Vi-
sible absorption spectra is observed in both cases as soon as the stimu-
lation of the system starts by addition of increasing hydrochloric acid
aliquots (Fig. 1).
2.4. Quantum chemical calculations
Full geometry optimizations of compound 4b and corresponding zinc
complex 5b were performed based on density functional theory (DFT)
using the classic B3LYP functional with the 6-311G(d) basis set. All
optimized structures correspond to stationary points on the potential
energy surfaces, as confirmed by only real vibrational frequencies.
Linear optical properties were determined at the time-dependent DFT
level using the B3LYP functional with the larger 6-311 + G(d) basis set.
Solvent (acetonitrile) effects were included in all calculations by means
of the polarizable continuum model in its integral equation formalism
(IEF-PCM) [28].
Indeed, one can observe that, due to the gradual increase of a unique
intense band centered on 381 nm (Fig. 1, left) upon the acidification
with HCl, the 4b solution turns deep yellow. As a continuous increase of
the band at 381 nm up to 2 equivalents is observed, it is important to
notice the absence of any isosbestic point. It can be then presumed that
the stimulation of the system under its closed form is not selective and
leads randomly to the BOX opening and pyridine protonation. As a
consequence, this stimulation leads to a complex mixture of four
different states of the compound (C, CP, O and OP). At the opposite, the
small 14 nm bathochromic shift (λmax = 395 nm) observed upon addi-
tion of a large excess of acid (Fig. 1, right) could translate the full con-
version of the molecular system to its OP form. This hypothesis is
supported by theoretical study based on the density functional theory
3. Results and discussion
3.1. Synthesis
To prepare the photo-active ligands based on BOX unit, a classical
convergent synthesis strategy was envisioned. Corner stone of this
strategy, the condensation reaction between a trimethylindolino [2,1-b]
oxazolidine derivative and some aromatic aldehyde is well-documented
and was reported for numerous aromatic hydrocarbon in various
experimental conditions (protic and aprotic solvents, acid or basic
3

Y. Aidibi et al.
Dyes and Pigments 193 (2021) 109476
Scheme 1. Representation of the synthetic pathways to obtain the different Zn(II) metal complexes.
Scheme 2. Representation of the 4 different states of the dyad BOX-Pyridine depending on the open (O) or closed (C) status of the BOX unit and the protonation (P)
of the pyridine moiety.
Fig. 1. Evolution of the UV–visible absorption spectrum of 4b in acetonitrile (0.05 mM) upon addition of HCl aliquots.
(DFT) and by its time dependent extension (TD-DFT) using B3LYP-6.311
G(d) as basis with IEFPCM solvation model. Fig. 2 gathers the simulated
spectra for all 4 different forms of ligand 4b.
As expected, theoretical calculations indicate that only forms
exhibiting an open BOX (O and OP forms) should exhibit an absorption
band in the visible range. Indeed, the protonation of the pyridine moiety
Compound
4b
Wavelength
(nm)
f
C
40000
CP
O
272
261
0.195
0.612
C
CP
O
OP
30000
20000
10000
0
291
270
0.789
0.039
427
313
292
0.833
0.257
0.145
480
301
281
0.724
0.310
0.109
200
300
400
500
600
700
OP
Wavelength / nm
Fig. 2. Left: Simulated UV–visible absorption spectra. Right: most intense transitions (f > 0.1) computed by B3LYP-6.311 + G(d) methodology for free ligand 4b
under its C, CP, O and OP forms.
4

Y. Aidibi et al.
Dyes and Pigments 193 (2021) 109476
(CP form) should only undergo a slight bathochromic (30 nm) shift from
the C form. Similar behavior seems occurring once the BOX is opened as
a 60 nm bathochromic shift between O and OP forms is predicted. As a
consequence, it can be assumed that the coloration changes of the so-
lution upon the acidification result from the formation of both open
forms (O and OP) which should be in equilibrium with the two closed
forms (C and CP).
can then confirm that the stimulation of the free ligands under their
closed form by pH changes leads primarily to a mixture CP and O forms
in equilibrium which is displaced by further stimulation to OP formation
(Scheme 3).
Photochemical properties. On the basis of our previous works on
BOX derivatives [35], solution of free ligands in acetonitrile have been
irradiated with 254 nm light in the presence of chlorobenzene (PhCl,
10%). As expected, the UV irradiation leads to the strong coloration of
the solution reaching a photostationary state after few minutes. As
example, the UV–Visible spectrum changes of a 4b solution under UV
irradiation is presented on Fig. 4. The strong similarity of the UV–visible
spectra obtained by either irradiation or acidic titration allow us to
conclude that both stimulations trend at the end to the same state (OP
form) and then, confirms the multi-modal switching abilities for this
kind of ligands.
Nevertheless, as the pyridine already presents a weak basicity, this
later is known to decrease when substituted with an electron with-
drawing group [36]. Thus, in these conditions, it is reasonable to assume
that this weakness may justify the large excess of acid necessary to
displace the equilibrium toward the full conversion of C to OP form
translated on the UV–Visible spectra by a bathochromic shift. To sum-
marize, we can presume that the addition of HCl to a solution of ligands
under their closed form leads in a first place to a mixture of C, CP and O
forms in equilibrium, which is displaced to OP formation when the
addition is pushed further as shown on Scheme 3.
Importantly, one can observe on Fig. 4 (right) two stages in the
evolution of the UV–Visible spectrum changes under UV irradiation. For
short irradiation time (t < 270s), the appearance of a new broad ab-
sorption band centered at 381 nm is observed which was assigned to the
generation of the open form (O). Under prolonged irradiation time,
additional hyper- and bathochromic shifts of the visible absorption band
(395 nm) are observed and, as under acid stimulation, can be assigned to
the generation of the OP state.
In this context, the NMR spectroscopy has been used to unambigu-
ously confirm the unselective competition between pyridine protonation
and BOX opening under pH changes. Requiring higher concentration
and not limited by solvent wavelength cut-off value, acetonitrile was
advantageously replaced by DMSO to carry this study due to some sol-
ubility issues. As exemplified for compound 4b on Fig. 3, the 1H NMR
spectra of both free ligands present typical signature of BOX derivatives
under their closed forms such as: (i) two doublets (6.66 and 6.79 ppm)
with a 16 Hz vicinal constant coupling confirming the trans isomery of
the compounds; (ii) three singlets (1.05, 1.36 and 2.23 ppm) assigned to
each methyl group and (iii) some multiplets between 3.2 and 3.8 ppm
arising from the methylenic protons of the constrained oxazolidine
cycle. These last are completed by two doublets (7.53 and 8.52 ppm for
compound 4b, Fig. 3) assigned to the pyridine unit.
To explain the generation of OP form under UV irradiation, one may
first consider that the action of chlorobenzene as photosensitizer for
BOX opening is known since the first report of their photochemical
properties [38,39]. If the photochemical process is still not fully eluci-
dated, it is admitted that a photo-induced electron transfer between
compounds results in the opening of the oxazolidine ring and the gen-
eration of the corresponding zwitterionic form which is instantaneously
converted in O form by trapping a proton from surrounding media
(especially water traces). Photochemically inactive, this species seems
involved in a tautomeric equilibrium with the CP form which can again
undergo a similar photochemical process with photo-excited chloro-
benzene. The proposed route leading to the conversion of C to OP form
under UV irradiation is illustrated in Scheme 4.
As expected, the appearance of a new set of signals as soon as the
addition of acid aliquots starts (Fig. 3) demonstrates the opening of the
BOX unit and the formation of O form. This latter is clearly identified by
the large downfield shift of the two doublets assigned to the ethylenic
bridge from 6.66 to 6.79 ppm to 7.97 and 8.35 ppm. Concurrently, the
characteristic peak-intensities of C form signals decrease and a gradual
displacement of the corresponding aromatic signals to higher chemical
shifts is noticed. As example, both doublets assigned to pyridine protons
initially at 8.52 and 7.53 ppm are low field shifted to 8.70 and 7.92 ppm,
respectively, after addition of one equivalent of HCl. As already
observed for other pyridine derivatives [37], such behavior translates a
fast equilibrium at the NMR time scale between the closed (C) and the
protonated closed (CP) forms. Up to one equivalent, the signals position
assigned to the O form are not affected by the addition of acid. It is
necessary to push further the addition of acid to observe their shift to
lower field. As a consequence, it can be concluded that the pyr-
idine/pyridinium equilibrium is still effective once the oxazolidine ring
is opened but exhibits a lower pKa value due to the generation of an
indoleninium acting as a strong withdrawing group. As we can see on
Figs. 3 and 3.4 equivalents of acid are required to reach a stationary
point corresponding to the full conversion of the compound to its pro-
tonated open form (OP). As suggested by UV–Visible spectroscopy, we
Based on this hypothesis in which each individual step acts as a
pseudo first order reaction, it is possible to describe the variation of each
species as function of 4 kinetic constants.
d[C]
= ꢀ k1[C]
(1)
(2)
(3)
(4)
dt
d[O]
= k1[C] + kꢀ 2[CP] ꢀ k2[O]
dt
d[CP]
= k2[O] ꢀ (k3 + kꢀ 2)[CP]
= k3[CP]
dt
d[OP]
dt
Equation (1) is easily solved by well-known first order integrated rate
law: [C] = [C]0eꢀ k t. Moreover, if one considers CP as an intermediate, it
1
is possible to apply quasi-stationary state approximation. As a
Scheme 3. Representation of the responsiveness of the ligand under stimulation by acid.
5

Y. Aidibi et al.
Dyes and Pigments 193 (2021) 109476
Fig. 3. 1H NMR spectra upon titration of 4b (14 mM) by HCl in DMSO‑d6 at 20 ◦C up to 3.4 equivalents.
Fig. 4. Left: UV–Visible spectrum changes of a solution of 4b (0.05 mM) under its closed form in a mixture ACN/PhCl (90/10) upon irradiation at 254 nm. Right:
Corresponding evolution of the absorbance at different wavelengths.
consequence, equations (3) and (4) become
⎛
(
)
1t
k1[C]0eꢀ k
⎜
⎝
d[O]
kꢀ 2
(k3 + kꢀ 2)
1t
= k1[C]0eꢀ k + k2[O]
ꢀ 1
(5)
(6)
[OP] =
k1(kꢀ 2 + k3) ꢀ k2k3
dt
⎞
d[OP]
k2k3
(k3 + kꢀ 2)
k
k
+k ꢀ k
k
(
)
3
+k
3
)
ꢀ k1(kꢀ 2 + k3)e(
t + ek1t(k1(kꢀ 2 + k ) ꢀ k k ) ꢀ k k
(8)
1
ꢀ 2
2
3
⎟
⎠
=
[O]
k
ꢀ 2
3
2
3
2 3
dt
After resolution of both differential equations, the concentration of O
and OP can be expressed as function of time and the different kinetic as
following:
As only O and OP forms should absorb in near-visible and visible
spectral range (vide infra), the evolution of the spectrum under UV
irradiation was fitted in this region by expression (9) in order to support
our hypothesis and estimate the different kinetic constants.
⎛
⎞
k1(kꢀ 2 + k3)[C] eꢀ k t
1
k
(
k
+k ꢀ k k
)
ꢀ 2 3 2 3
(
)
1
⎜
⎝
⎟
t ꢀ 1
⎠
k1(kꢀ 2 + k3) ꢀ 0k2k3
k
+k
3
ꢀ 2
[O] =
e
(7)
Abs(λ) = ελO [O] + εOλ P[OP]
(9)
As one can notice (Fig. 5), the experimental evolution of the ab-
sorption spectrum is nicely reproduced allowing to estimate the two
kinetic constants of photoinduced ring opening processes to 3.10ꢀ 3 and
6

Y. Aidibi et al.
Dyes and Pigments 193 (2021) 109476
Scheme 4. Representation of the responsiveness of the ligand under stimulation by UV light including two irreversible photochemical processes and one equilibrium.
Fig. 5. Left: Fitting of the evolution the absorption spectra (350–470 nm) by equation (9). Right: Resulting simulation of the evolution of [C]; [O] and [OP] as
function of the irradiation time.
7.10ꢀ 6 sꢀ 1 for k1 and k3 respectively. The drastic decrease of photo-
reactivity between C and CP can be explained by the reinforcement of
the electron withdrawing ability of the pyridine by its protonation which
reduces the stability of the corresponding open form. The equilibrium
constant between O and CP forms, k2/k-2, is estimated around ~250. It
suggests then the quick and efficient transformation of the photo-
generated specie O issued from the first opening to CP form which is
available to undergo a second photochemical process leading to final OP
form. In this context, despite a difference by almost 3 order of magnitude
between both kinetic constants, the switching of free ligand under light
stimulation occurs in a stepwise manner but unfortunately with an
insufficient selectivity as illustrated by the simulated evolution of C, O
and OP forms as function of irradiation time (Fig. 5).
context, the electrochemical behavior of ligands 4b and 5b has been
investigated by cyclic voltammetry (CV) and their voltammograms are
presented in Fig. 6.
As expected, compound 4b exhibits a non-reversible oxidation pro-
cess at 0.74 V vs Fc/Fc+ which generates a new system as translated by
the appearance of a non-reversible reduction process (ꢀ 0.88 V) on the
scan back. In agreement with our previous studies[35,40], this typical
electrochemical behavior indicates that the oxidation of the C form is
mainly localized on the indoline unit leading to corresponding radical
cation which undergoes a chemical process leading to the opening of the
oxazolidine ring. In similar way, the reduction process seems assigned to
the electrochemical closing from O to C form. Interestingly, in the pre-
sent case, an important difference of charge exchanged between
oxidation (C to O) and reduction (O to C) processes is observed. Once
again, the tautomeric equilibrium between O and CP forms can explain
this behavior. Indeed, it can be suggested that the oxidation of the C
form leads to the generation of the O form which is partially converted
Electrochromic properties. As mentioned earlier, the opening of
the oxazolidine ring can be also induced by an electrochemical process.
Noteworthy, this later requires the presence of a substituent in position 5
of the BOX to avoid undesirable C–C oxidative coupling[40]. In this
7

Y. Aidibi et al.
Dyes and Pigments 193 (2021) 109476
Fig. 6. Cyclic voltammograms of derivatives 4b (0.62 mM) and 5b (0.57 mM) in 0.1 M TBAPF6/CH3CN solution at 100 mV sꢀ 1
.
to CP form due to the tautomeric equilibrium (vide supra). Acting
independently from the pyridinium unit when the oxazolidine ring is
closed, the BOX moieties should be also able to undergo an oxidation
process at a close oxidation potential, leading the corresponding radical
cation and, at the end, to the OP formation. In fact, this hypothesis is
supported by theoretical calculations.
from 381 to 395 nm is noticed when a slight excess of oxidant is added.
Assigned to the full conversion from C to OP form, it translates that, as
under acidic conditions, an overstimulation is requisite to counteract the
tautomerism between O and CP.
3.3. Switching properties of corresponding zinc complexes
As represented on Fig. 7, the HOMO of compound 4b is mainly
localized on the BOX unit for both C and CP forms. In addition, the
variation of HOMO energy level due to the pyridine protonation appears
quite limited (0.14eV) and not sufficient to induce an important varia-
tion of oxidation potential. As a consequence, the electro-switching of
system occurs in two simultaneous steps: the C to O transformation and
the conversion from CP to OP happen then at close potentials. Thus,
contrary to other molecular systems including several BOX units [41]
which allow observation of two well resolved oxidation processes, this
poor difference leads to an unselective wide oxidation wave including
both processes. This assumption is confirmed by the titration of com-
pound 4b using NOSbF6 as an oxidizing reagent.
Structure of the zinc complexes. If the introduction of a transition
metal ion in the close vicinity of a multimodal switchable unit such as
BOX could open new prospects, the elaboration of such complexes stays,
to the best of our knowledge, unreported. Since the coordination unit
used here is a monodentate ligand (pyridine), the use of Zn(II) metal
cation shouldn’t affect the molecular structure of the BOX based ligand.
In order to support this hypothesis, comparative X-ray diffraction
studies between free ligand 4b and corresponding zinc(II) complex 5b
were carried out. For both, selected parameters of the X-ray diffraction
data collection and refinement are provided in SI, as well as selected
bond lengths and angles.
As one can observe on Fig. 8, the UV–visible spectra obtained by
NOSbF6 or acidic titration appear very similar, then translating that both
stimulation leads to the same transformation, and then, confirming the
multi-modal switching abilities of these molecular systems. More
important, a bathochromic shift of the maximum absorption wavelength
As expected, the coordination of the ligand by the zinc(II) induces
only slight modifications of its structure. Except different space group
(P21 21 21 and C 2/c for 4b and 5b respectively), free ligand and complex
exhibit one independent molecule in the unit cell. With a stoichiometry
of 2:1, the zinc ion is tetracoordinated presenting a distorted tetrahedral
Fig. 7. Molecular orbitals and energies of compound 4b under its C (left) and CP (right) forms calculated at the B3LYP/6-311G(d) level with IEFPCM (ACN) sol-
vation model.
8

Y. Aidibi et al.
Dyes and Pigments 193 (2021) 109476
Fig. 8. UV–Visible spectrum changes of a solution of 4b in ACN (0.05 mM) upon addition of NOSbF6.
coordination sphere formed by two pyridine nitrogen atoms and two
chlorine atoms.
preserved once the complex formed. This assumption is perfectly sup-
ported by ab initio quantum calculations (B3LYP/6.311G(d)). As
example, complex 5b exhibits quasi-degenerated HOMO and LUMO
orbitals, similar in energy and localization to that observed for free
ligand 4b (see SI).
As suggested by theoretical calculations, the ligand is not planar.
Changing from 86.67 (2)◦ in free ligand to 74.77 (2)◦ and 69.41 (2)◦ in
complex, metal binding has only a moderate effect on the dihedral angle
between the phenyl and pyridine planes. In similar way, bond lengths
within the pyridine and phenyl units do not differ substantially from one
species to the other. However, a disorder of carbon atoms can be high-
lighted in the complex as seen in Fig. 9. To explain this difference, it can
be suggested that the molecular conformation of free ligand is stabilized
by strong intramolecular C–H⋯O and C–H⋯N, (C6–H6⋯O1 2.415 (0) Å
and C13–H13A⋅⋅⋅O1 2.379, C7–H7⋯N2 2.717) hydrogen bonds (Fig. 9).
Furthermore, each molecule interacts with adjacent molecules by one
non-conventional hydrogen bonds C–H⋯N, (C2–H2⋯N1, 2.544 (0) Å),
In this context, it is not surprising to observe only a slight variation of
the optical properties between the free ligand and corresponding com-
plexes. As shown on Fig. 12 for 4b and 5b, the zinc complexation doesn’t
induce any strong variation of the absorption maximum wavelength
(245 nm) but logically leads to twice the extinction coefficient (28.7 103
and 56.3 103 L molꢀ 1 cmꢀ 1 for compound 4b and 5b respectively).
Noteworthy, an hyperchromic effect was observed in the 260–290 nm
range with the complexation. By following the absorption at 280 nm of a
solution of 4b upon the addition of ZnCl2 aliquots, it is possible to
confirm the 2:1 stoichiometry of the complex and its stability in diluted
conditions.
and two hydrogen bonds C–H⋅⋅⋅
π
from phenyl ring and from pyridine
fragment respectively (C4–H4⋅⋅⋅
π
(2.712 (0) Å, C13–H13B⋅⋅⋅ (2.853 (0)
π
Å), as illustrated in Fig. 10. These interactions lead to the formation of
3D supramolecular layers parallel with the bc crystallographic plane
(Fig. 3).
As performed with free ligands, the switching abilities of corre-
sponding zinc(II) complexes were investigated by monitoring the evo-
lution of the system by UV–Visible spectroscopy when stimulated by
acid, light and oxidant.
Concerning corresponding zinc(II) complex 5b, RX diffraction of
does not reveal any intra- or inter-molecular hydrogen bond involving
Acidochromic properties. As observed with free ligand, a strong
modification of the UV–Visible absorption spectra is observed as soon
some acid aliquots are added. As presented for compound 5b in Fig. 13,
acidification of the solution leads to formation of an intense broad band
centered at 381 nm. Based on previous study on free ligand, this
coloration is assigned to the BOX opening (vide infra). Analogously to
4b, starting from corresponding zinc complex, adding 4 eq. of acid is
necessary to attain stabilized spectroscopic properties. Here also, this
excess of acid induces a slight hyperchromic and bathochromic shift of
the absorption maximum wavelength.
the pyridine moiety. In place, C–H⋅⋅⋅
π intermolecular interactions
(H40⋅⋅⋅centroid 2.819 (2) Å and H20B⋅⋅⋅centroid 2.640 (2) Å) along the a
axes and short intermolecular C–H⋯Cl contacts (C10–H10⋯Cl1 = 2.691
(2) Å) are evidenced, resulting in a supramolecular chain parallel to the
crystallographic c axis (Fig. 11).
More important, X-Ray studies confirm that BOX units are not
involved inside the coordination sphere of the metallic ion and that their
own structure are not affected by the metal binding. On this basis, it can
be then presumed that the switching abilities of the ligand will be
Fig. 9. Crystal structure and atom numbering of free ligand 4b (left) and its corresponding zinc(II) complex 5b (right).
9

Y. Aidibi et al.
Dyes and Pigments 193 (2021) 109476
Fig. 10. Perspective view of 4a ligand in the ac (left) and bc (right) planes showing the two types of interactions C–H⋯N hydrogen bonds (orange dashed lines),
C–H⋅⋅⋅ interactions, from phenyl ring (purple dashed lines) and from pyridine fragment (green dashed lines).
π
Fig. 11. Perspective of the supramolecular chain generated by hydrogen bonding C–H⋯Cl (green-cyan dashed lines) in complex 5b in the ab plane (top), and those
reinforced by C–H⋅⋅⋅ interactions (blue dashed line, bottom).
π
Fig. 12. (a) UV visible spectra of free ligand 4b and corresponding zinc complex 5b in the ACN. (b) Variation of UV–Visible spectrum of 4b (0.03 mM) in ACN upon
the addition of ZnCl2 aliquots. (c) Evolution of the absorbance at 280 nm upon addition of ZnCl2.
10

Y. Aidibi et al.
Dyes and Pigments 193 (2021) 109476
Fig. 13. Evolution of the UV–visible absorption spectrum of 5b in acetonitrile (0.02 mM) upon addition of HCl aliquots.
As, no isosbestic point is observed during this addition, these simi-
As represented on Fig. 14 for compound 5b, both stimulation leads to
strong yellow coloration of the solution and to the appearance of a
unique band centered at 381 nm. In addition, a typical slight hyper and
bathochromic shift is noticed for longer irradiation time or more than 2
eq. of chemical oxidant is added. As expected, a perfect overlap of
UV–Visible spectra is observed whatever the nature of the stimulation. It
can be then concluded that this molecular system reaches an identical
final state in all cases. Assigned to the formation of OP form, it implies
the zinc ion release under acido-, photo- and electro-stimulation.
Concerning the electro-stimulation, the presence of two redox active
ligands in close vicinity raises the question of their possible selective
addressability and the possibility to form mixed valences species. For
this reason, the complex 5b was studied by CV. According to Fig. 6, the
cyclic voltammogram of 5b exhibits one pseudo non-reversible oxida-
tion wave at 0,72 V very close to the free ligand oxidation process at
0.74 V what underlines an equivalent environment. More important, it
was not possible to observe any splitting of the oxidation peaks, which
translates the perfect electronic equivalence of both BOX units and then
their independent opening.
larities let presume (i) a complete independence of both BOX units
grafted on the metal center, and (ii) the presence in solution of more
than two species in equilibrium. It could suggest that, again, a tauto-
meric equilibrium between an opened and a protonated closed form
occurs. Thus, it is worth to suppose that, whatever the external stimu-
lation, the opening of the BOX is accompanied by the pyridine proton-
ation and, then, by the complex dissociation and zinc cation release.
To clarify this point, 1H NMR titrations were performed in d3 ACN.
The gradual addition of acid leads to observe i) the decrease of initial
closed form signals and concomitant appearance of new resonances
assigned to the open form of the complex and ii) the formation of a
precipitate suggesting the formation of CP and, as consequence, the
complex dissociation. To support this hypothesis, it is important to note
that the partial acidic stimulation of the free ligand 4b also leads to
observe similar precipitate formation in ACN. Furthermore, when an
excess ZnCl2 is added to a mixture of O and CP forms of 4b in equilib-
rium, the precipitate disappears and 1H NMR spectroscopy confirms the
formation of corresponding zinc(II) complex under its O form (see S.I.)
Finally, the addition of an excess of acid induces the complete dissoci-
ation of 5b leading to the exclusive formation of free 4b under its OP
form (Scheme 5).
In comparison to the free ligand, the observation of a full non-
reversible oxidation process requires to decrease the scan rate. Based
on these results, we can assume that the kinetic rate leading to the
opening of the BOX was decreased with the complexation of the Zn
which probably also slows down the tautomeric equilibrium leading to
the CP form.
To conclude, the metal binding did not affect strongly the acid-
ochromic properties of the ligand. As a consequence, the tautomeric
equilibrium between O and CP forms is still present and leads to observe
the Zn release as soon as the complex is under acidic stimulation.
Release of Zinc ion under photo- or electro-stimulation. The
multimode switchable ability of the BOX suggests the possibility to
release zinc ion under milder stimulation than addition of acid. In order
to verify this assumption, the light (254 nm) and electrochemical
(NOSbF6) stimulations of zinc complexes were carried out and moni-
tored by UV–Visible spectroscopy.
4. Conclusion
In summary, we have demonstrated that the successful combination
of a BOX unit with a nitrogen-containing heteroarene, as simple as a
pyridine, leads to a new class of multi-addressable ligands exhibiting
multimodal switching abilities. Based on this, a first set of BOX based
Scheme 5. Proposed route leading to the zinc ion release from BOX based complexes under an acidic stimulation.
11

Y. Aidibi et al.
Dyes and Pigments 193 (2021) 109476
Fig. 14. UV–Visible spectrum changes of a solution of complex 5b in ACN (0.02 mM) upon irradiation at 254 nm in presence of PhCl 10% (left) and NOSbF6 (right).
coordination complexes 5a-b have been synthesized and characterized
allowing to demonstrate that the acido-; electro- and photoswitching
properties of the BOX moiety are perfectly maintained after metal-
binding. Nevertheless, the moderate basicity of pyridine moiety con-
ducts to observe a subtle and interesting tautomeric equilibrium be-
tween the open O form and the closed pyridinium protonated form CP of
the system whatever the nature of the stimulation. More important, both
of them lead to the same opened protonated OP form when stronger
stimulation is applied. Due to some lability, this tautomeric equilibrium
is not suppressed by metal binding. Therefore, upon all types of stimu-
lation (photon, proton or electron), the BOX of complex 5 are opened
and the pyridine motifs protonated leading to the obtainment of the OP
form accompanied by the destruction of the complex and therefore
release of zinc ion. At the opposite, a simple basic treatment of the
mixture allows to restore the initial state what leads to the regeneration
of the zinc complex. This possibility to catch and release the metal on
demand by using two external and orthogonal stimulations represents a
promising approach especially for studying the numerous biological
processes involving zinc ions and, as consequence, requires further
investigations.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2021.109476.
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