Paola Sánchez-Portillo

Aime Hernández-Sirio

Carolina Godoy-Alcántar

Pascal G Lacroix

Vivechana Agarwal

Rosa Santillán

Victor Barba
email: vbarba@uaem.mx

Aime Hern´c Andez-Sirio

Carolina D Godoy-Alc´a

Aime Hern´ C Andez-Sirio

Vivechana A Garwal

Colorimetric metal ion (II) Sensors Based on imine boronic esters functionalized with pyridine

Keywords:

come

Introduction

Boronic acids have been widely used as building blocks for selfassembly processes [START_REF] Severin | Boronic acids as building blocks for molecular nanostructures and polymeric materials[END_REF] but also have attracted interest in sensing methods [START_REF] Nishiyabu | Boronic acid building blocks: tools for self-assembly[END_REF][START_REF] Fujita | Boronic acids in molecular self-assembly[END_REF]. Boronic acids are easily condensed with 1,2 and 1,3 diols to give rise to the formation of 5-or 6-membered cyclic boronic esters, respectively [START_REF] Severin | Boronic acids as building blocks for molecular nanostructures and polymeric materials[END_REF][START_REF] Iwasawa | Boronic esters as a system for crystallization-induced dynamic self-assembly equipped with an "on-off" switch or equilibration[END_REF][START_REF] Celis | Boron macrocycles based on multicomponent assemblies using (3-aminophenyl)boronic acid and pentaerythritol as common reagents; molecular receptors toward Lewis bases[END_REF]. One interesting characteristics for these compounds is the Lewis acid character of the tricoordinated boron atom, which can form adducts by interaction with Lewis bases [START_REF] Sheepwash | Self-assembly for arylboronate esters with pyridyl side chains[END_REF][START_REF] Cruz-Huerta | N-containing boronic esters as self-complementary building blocks for the assembly of 2D and 3D molecular networks[END_REF]. The above reactivity has been used for the construction of species of different nature, such as macrocycles [START_REF] Severin | Boronic acids as building blocks for molecular nanostructures and polymeric materials[END_REF][START_REF] Iwasawa | Boronic esters as a system for crystallization-induced dynamic self-assembly equipped with an "on-off" switch or equilibration[END_REF][START_REF] Salazar-Mendoza | Macrocycles and coordination polymers derived from self-complementary tectons based on N-containig boronic acids[END_REF][START_REF] Stephens | Crystalline polymers based on dative boron-nitrogen bonds and the quest for porosity[END_REF] and polymers [START_REF] Cruz-Huerta | N-containing boronic esters as self-complementary building blocks for the assembly of 2D and 3D molecular networks[END_REF][START_REF] Salazar-Mendoza | Macrocycles and coordination polymers derived from self-complementary tectons based on N-containig boronic acids[END_REF][START_REF] Stephens | Crystalline polymers based on dative boron-nitrogen bonds and the quest for porosity[END_REF], even, more recently the formation of metal-organic species has been possible from the combination with coordinating groups such as Schiff bases and pyridyl groups [START_REF] Wise | Large, heterometallic coordination cages based on ditopic metalloligands with 3-pyridyl donor groups[END_REF][START_REF] Jansze | Ligand aspect ratio as a decisive factor for the self-assembly of coordination cages[END_REF][START_REF] Jansze | Pd ll 2 L 4 -type coordination cage up to three nanometers in size[END_REF].

In addition, Schiff bases have been used as sensors for metal cations, mainly because of the easy formation of the N→M coordination bonds together with the chelate effect, thus, high selectivity and sensitivity for heavy and transition metal ions is obtained [START_REF] Yildirim | Synthesis of a novel fluororescent Schiff base as a possible Cu (II) ion selective sensor[END_REF][START_REF] Wang | A novel Coumarin Schiff-base as a Ni(II) ion colorimetric sensor[END_REF][START_REF] Gupta | Comparative study of colorimetric sensors based on newly synthesized Schiff bases[END_REF][START_REF] Gupta | A reversible fluorescence "off-on-off" sensor for sequential detection of Aluminum and Acetate/Fluorine ions[END_REF]. Over the last years, they have received considerable research attention because of their important applications in biological, industrial and environmental processes as chemical sensors [START_REF] Reimann | Water-soluble sulfonate schiff-base ligands as fluorescent detectors for metal ions in drinking water and biological systems[END_REF][START_REF] Luo | Synthesis and living cell imaging of a novel fluorescent sensor for selective cupric detection[END_REF][START_REF] Jiang | Simple and efficient rhodamine -derived VO 2+ and Cu 2+ -responsive colorimetric and reversible fluorescent chemosensors toward the design of multifunctional materials[END_REF][START_REF] Bhaskar | Fluorescence "turnon" sensor for highly selective recognition of Cu 2+ ion and its application to living cell imaging[END_REF][START_REF] You | Chelate-type Schiff base acting as a colorimetric sensor for iron in aqueous solution[END_REF]. Schiff bases are an important type of colorimetric chemosensors because their interaction with analytes can be detected by naked eye, without the use of expensive equipment [START_REF] Udhayakumari | Colorimetric chemosensor for multi-signaling detection of metal ions using pyrrole based Schiff bases[END_REF][START_REF] Tg | Selective colorimetric chemosensor for the detection of Hg 2+ and arsenite ions using Isatin based Schiff's bases; DFT Studies and Applications in test strips[END_REF][START_REF] Gupta | Recent advances on potentiometric membrane pharmaceutical analysis[END_REF] It is well known that for colorimetric chemosensors, the shifts at the absorption spectra are affected by the respective increase or decrease in electron density on the chromophore moiety [START_REF] Gupta | Selective naked-eye detection of Magnesium (II) ions using a coumarin-derived fluorescent probe[END_REF]. Furthermore, the color changes are more successfully observed when the interacting species includes electrostatic charges (i. e., cation-anion) than for neutral molecules [START_REF] Suksai | Chromogenic anion sensors[END_REF].

The search of metal sensors is an important topic today [START_REF] Srivastava | PVC-based 2,2,2-cryptand sensor for zinc ions[END_REF][START_REF] Srivastava | Determination of lead using a poly(ninylchloride)based crown ether membrane[END_REF][START_REF] Srivastava | Cesium PVC-crown (Dibenzo-24crown-8) based membrane sensor[END_REF][START_REF] Gupta | Cadmium (II) ion sensing through p-tert-butyl calix [6]arene based potentiometric sensor[END_REF][START_REF] Dehghani | Removal of chromium (VI) from aqueous solution using treated waste newspaper as low-cost adsorbent: kinetic modeling and isotherm studies[END_REF][START_REF] Gupta | Mercury selective potentiometric sensor based on low rim functionalized thiacalix [4]-arene as a cationic receptor[END_REF][START_REF] Gupta | Cadmium (II)-selective sensors based on dibenzo-24-crown-8 in PVC matrix[END_REF][START_REF] Jain | Copper (II)-selective electrode based on macrocyclic compounds[END_REF][START_REF] Srivastava | A PVC-based benzo-15-crown-5 membrane sensor for cadmium[END_REF], especially iron and copper metals have significant relevance because they are involved in several biological and environmental processes [START_REF] Guo | A pyrene-based dual chemosensor for colorimetric detection of Cu 2+ and fluorescent detection of Fe 3+[END_REF][START_REF] Shelaiah | Novel pyrene-and anthracene-based Schiff base derivatives as Cu 2+ and Fe 3+ fluorescence turn-on sensors and for aggregation induced emissions[END_REF]. Iron is one of the most abundant essential transition metal ion in human body, acting as cofactor for many proteins in a wide range of biochemical reactions such as electron transport and the enzyme oxidoreductase [START_REF] Cowan | Inorganic biochemistry: an introduction[END_REF]. On the other hand, copper is an essential metal found as traces in all living organisms, it helps to provide the energy needed for the biochemical processes of heme oxygenase in blood [START_REF] Tapiero | Trace elements in human physiology and pathology[END_REF].

The synthesis of ligands that selectively coordinates with metals of biological importance is still under research attention. To the best of our knowledge, until now no boronic ester coupled to the imine-pyridine fragment has been used as metal sensors. Previously, we reported the one-step synthesis of imine-boronic esters functionalized with pyridine moieties, which were found to form 2D structural arrangements by mean of intermolecular interactions [START_REF] Sánchez-Portillo | Structural analysis of pyridine-imine boronic esters involving secondary interactions on solid state[END_REF][START_REF] Sánchez-Portillo | Bis-imine boronic esters obtained by one step multicomponent reactions. Synthesis and X-ray diffraction structural analysis[END_REF]. In the present work, the synthesis of imine-pyridine-boronic ester derivatives was carried out, the compounds include pyridyl moieties in ortho position within the Schiff base, the structural 1,4 N-N' disposition allows the metal centers to form five member chelates. Complexation was monitored by naked-eye, UV-Vis and Fluorescence techniques.

Experimental

Materials and measurements

All reagents and solvents were acquired from commercial suppliers and used without further purification. The 1 H, 13 C and 11 B spectra were recorded at room temperature using a Varian VXR 400 MHz spectrometers and CDCl 3 as solvent. TMS (internal, 1 H, δ = 0.00 ppm, 13 C, δ = 0.0 ppm) and BF 3 ⋅Et 2 O (external, 11 B, δ = 0.0 ppm) were used as standard references. Mass spectra were obtained with Jeol JMS 700 equipment. Melting points were determined with a Büchi B-540 digital apparatus. The absorption and fluorescence spectra were recorded in a Thermo Scientific Genesys 10S UV-Vis and a Varian CaryEclipse in 2.00 mL quartz cells with 1 cm path lengh using methanol as solvent. All data treatment were analyzed with Origin software [START_REF]Origin(Pro)[END_REF].

General synthesis of compounds 1a -1c

The synthesis of the compounds was carried out using one-step reactions. Starting materials were added in equimolar ratio (following the order carboxylpyridine, boronic acid and aminodiol), to a flask containing the solvent mixture of methanol (25 mL)/toluene (5 mL). The mixture was heated under reflux conditions for 24 h using a Dean Stark trap to remove the condensation water. After work up, the solvent was reduced to <10 mL to induce the precipitation of compounds, the obtained solids were washed with 10 mL of diethyl ether.

Synthesis of compound 1a

Obtained from 4-(trifluoromethyl)phenylboronic acid (399.7 mg, 2.102 mmol), 2-amino-1,3-propanediol (193.2 g, 2.102 mmol) and 2pyridinecarboxaldehyde (200 μL, 2.102 mmol). 34% yield of a paleyellow solid. M.P. = 145-148 • C. IR (ATR) v: 1640 (C--N, w), 1309 (m), 1251 (m) 1103 (s), 648 (s). 1 H NMR (400 MHz, CDCl 3 ) δ = 8.64 (dd, J ortho = 4.9 Hz, J meta = 1.6 Hz, 1 H, H-8), 8.53 (s, 1H, H-3), 7.97 (dd, J ortho = 7.8 Hz, J meta = 1.6 Hz, 1 H, H-5), 7.90 (d, J = 7.8 Hz, 2 H, H-10), 7.72 (ddd, J ortho = 7.8 Hz, J ortho = 7.8 Hz, J meta = 1.6 Hz, 1 H, H-6), 7.58 (d, J = 7.8 Hz, 2 H, H-11), 7.33 (ddd, J ortho = 7.8 Hz, J ortho = 4.9 Hz, J meta = 1.6 Hz, 1 H, H-7), 4.24 (d, J = 6 Hz, 4 H, H-1), 3.93 (q, J = 6.2 Hz, 1 H, H-2), 13 C NMR (126 MHz, CDCl 3 ) δ = 164.5 (C-3), 154.0 (C-4), 149.8 (C-8), 136.9 (C-6), 134.4 (C-10), 132.6 (q, J C-F = 37.8 Hz, C-12), 125.5 (C-7), 124.4 (q, J C-F = 273 Hz, -CF 3 ), 124.4 (q, J C-F = 12 Hz, C-11), 122.0 (C-5), 66.3 (C-1), 65.1 (C-2), (B-C was not observed). 11 B NMR (128 MHz, CDCl 3 ) δ = 26, h 1/2 = 563 Hz. EI + -MS-HR for C 16 H 14 O 2 N 2 F 3 B, m/z: found 334.1113 ± 3.7, Calc. 334.1013.

Synthesis of compound 1b

Obtained from 4-(trifluoromethyl)phenylboronic acid (182.7 mg, 1.05 mmol), 2-amino-2-methyl-1,3-propanediol (110.3 g, 1.05 mmol) and 2-pyridinecarboxaldehyde (100 μL, 1.05 mmol). 44% yield of a white solid. M.P. = 153-155 • C. IR (ATR) v: 1637 (C--N, w), 1293 (s), 1251 (m), 1102 (s), 842 (m), 649 (s). 1 H NMR (400 MHz, CDCl 3 ) δ = 8.60 (dd, J ortho = 5 Hz, J meta = 1.6 Hz, 1H, H-8), 8.52 (s, 1 H, H-3), 7.98 (dd, J ortho = 7.6 Hz, J meta = 1.6 Hz, 1H, H-5), 7.88 (d, J = 7.6 Hz, 2 H, H-10), 7.68 (ddd, J ortho = 7.6 Hz, J ortho = 7.6 Hz, J meta = 1.6 Hz, 1H, H-6), 7.55 (d, J = 7.6 Hz, 2H, H-11), 7.28 (ddd, J ortho = 7.6 Hz, J ortho = 5 Hz, J meta = 1.6 Hz, 1 H, H-7), 4.32 (AB, J AB = 11 Hz, 4 H, H-1), 1.31 (s, 3 H, H-13). 13 C NMR (100 MHz, CDCl 3 ) δ = 160.9 (C-3), 153.7 (C-4), 149.9 (C-8), 137.3 (m, C-6 y C-12), 134.7 (C-10, C-12), 125.4 (C-7), 124.4 (q, J C-F = 4 Hz, C-11), 124.4 (q, J C-F = 268 Hz, -CF 3 ), 121.7 (C-5), 70.3 (C-1), 65.3 (C-2), (B-C was not observed). 11

Synthesis of compound 1c

Obtained from 4-bromo-phenylboronic acid (210.9 mg, 1.05 mmol), 2-amino-2-methyl-1,3-propanediol (110.2 g, 1.05 mmol) and 2-pyridinecarboxaldehyde (100 μL, 1.05 mmol). 83% yield of a white solid. M.P. = 201-204 • C. 1 H NMR (400 MHz, CDCl 3 ) δ = 8.60 (dd, J ortho = 4.7 Hz, J meta = 1.2 Hz, 1 H, H-8), 8.51 (s, 1 H, H-3), 7.96 (dd, J ortho = 7.8 Hz, J meta = 1.2 Hz, 1 H, H-5), 7.87 (d, J ortho = 7.8 Hz, 2 H, H-10), 7.69 (ddd, J ortho = 7.8 Hz, J ortho = 7.8 Hz, J meta = 1.2 Hz, 1 H, H-6), 7.44 (d, J ortho = 7.8 Hz, 2 H, H-11), 7.29 (ddd, J ortho = 7.8 Hz, J ortho = 4.7 Hz, J meta = 1.2 Hz, 1 H, H-7), 4.12 (AB, J AB = 10.8 Hz, 4 H, H-1), 1.30 (s, 3 H, H-13). 13 C NMR (100 MHz, CDCl 3 ) δ = 160.7 (C-3), 154.8 (C-4), 149.6 (C-8), 136.9 (C-6), 135.9 (C-10), 131.0 (C-11), 125.9 (C-12), 125.3 (C-7), 121.5 (C-5), 70.3 (C-1), [START_REF] Georgiou | Versatile self-adapting boronic acids for H-bond recognition: from discrete to polymeric supramolecules[END_REF].4 (C-2), 20.2 (C-13), (B-C was not observed). 11

X-ray measurements

Crystal intensity data were collected at T = 100 K for 1a and 1b, (293 K for 1c), using Cu-Kα radiation λ = 1.54184 Å, for 1a and 1c (Mo-K α radiation λ = 0.71073 Å for 1b), graphite monochromator on an Agilent Technologies SuperNova diffractometer equipped with the EOsS2 CCD area detector and an Oxford Instruments Cryogen cooler. Crystal data, data collection parameters and convergence results are listed in Table 1. The measured intensities were reduced to F 2 and corrected for absorption using spherical harmonics (CryAlisPro) [START_REF][END_REF]. Intensities were corrected for Lorentz and polarization effects. Structure solution, refinement, and data output were performed with the OLEX2 program package [START_REF] Dolomanov | OLEX2: a complete structure solution, refinement and analysis program[END_REF] using SHELXL-2014 [START_REF] Sheldrick | Crystal structure refinement with SHELXL[END_REF] for the refinement. Non-hydrogen atoms were refined anisotropically. All hydrogen atoms were placed in geometrically calculated positions using the riding model. Intermolecular distances were analyzed with MERCURY [START_REF] Macrae | Mercury: visualization and analysis of crystal structures[END_REF]. The Hirshfeld surfaces were calculated by CrystalExplorer software [START_REF] Turner | CrystalExplorer17[END_REF].

Computational studies

The geometries of ligands 1a (transoid) and 1b (bent "L"), as well as the complexes [Fe (1a) 3 ] 2+ , [Fe (1b) (MeOH) 4 ] 2+ and [Fe (1b) 3 ] 2+ were fully optimized using the Gaussian-09 program package [START_REF][END_REF] within the framework of the Density Functional Theory (DFT). The computed geometries of the five species are provided as supplementary materials. In any case, the computations were performed in the presence of methanol as the solvent, which was modelized by the Polarizable Continuum Model (SCRF=PCM method) [START_REF] Tomasi | Quantum mechanical continuum solvation models[END_REF]. The combination of the widely used B3LYP hybrid functional [START_REF] Becke | Density-functional thermochemistry. III. The role of exact exchange[END_REF][START_REF] Stephens | Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields[END_REF], with the double-ζ basis set 6-31G* [START_REF] Hehre | Self-consistent molecular orbital methods. XII. Further extensions of Gaussian-type basis sets for use in molecular orbital studies of organic molecules[END_REF], was employed. The UV-Vis spectra were computed by Time-Dependent Density Functional Theory (TD-DFT), using the geometries of the optimized cations. In order to select the suitable method for these computations, several functional were tested (B3LYP, B3PW91, CAM-B3LYP, and PBE0). Finally, the B3PW91 [START_REF] Becke | Density-functional thermochemistry. III. The role of exact exchange[END_REF] was chosen for its better accuracy to reproduce the experimental transition energies (<0.5 eV). The spectra were drawn by Gabedit software [START_REF] Allouche | Gabedit -a graphical user interface for computational chemistry softwares[END_REF].

Scheme 1. Synthetic strategy to obtain 1a-1c compounds. Numbering is for NMR assignments.

Results and discussion

Syntheses and characterization

The present work describes one-step reactions using 4-substituted arylboronic acids (4-(trifluoromethyl)phenylboronic acid for 1a and 1b, 4-bromophenylboronic acid for 1c), 2-amino-2-methyl-1,3-propanodiol for 1b and 1c (2-amino-1,3-propanediol for 1a), and 2-pyridine carboxaldehyde to obtain the three pyridine-imine boronic esters 1a-1c (Scheme 1). Presence of the CF 3 and Br as 4-substituent at arylboronic acids, were introduced in order to modify the acid character of the boron atom on the ester formed, and to get the formation of more structured complexes by self-assembly using N→B coordination bonds. Compounds 1b and 1c were isolated as white solids (pale yellow for 1a) which were soluble in organic solvents. Spectroscopy characterization was done using IR spectroscopy, mass spectrometry, 1 H, 13 C and 11 B NMR spectroscopy and X-ray diffraction analysis.

First evidence for the formation of compounds 1a-1c was obtained from IR spectra, the C--N imine band was observed around 1650 cm -1 , being characteristic for these type of compounds [START_REF] Severin | Boronic acids as building blocks for molecular nanostructures and polymeric materials[END_REF][START_REF] Sánchez-Portillo | Bis-imine boronic esters obtained by one step multicomponent reactions. Synthesis and X-ray diffraction structural analysis[END_REF]. Using high resolution mass spectrometry (HR-EI + technique), the m/z peak corresponding to the molecular ion [M] + was observed for all three compounds showing good agreement between the isotopic distribution calculated and experimental data [START_REF] Patiny | ChemCalc: a building block for tomorrow's chemical infrastructure[END_REF]. The 1 H NMR spectra showed the signal for the imine hydrogen shifted at δ = 8.53, 8.52 and 8.51 ppm for 1a, 1b and 1c, respectively, being in the reported range for imines [START_REF] Barba | Boron macrocycles having a calix-like shape. Synthesis, characterization, X-ray analysis, and inclusion properties[END_REF][START_REF] Barba | Direct synthesis of polymacrocyclic boron compounds: a convenient method for the synthesis of hemicarcerands[END_REF]. Hydrogen signals corresponding to aryl moieties were observed at low field δ = 7.60-8.60 ppm. Signals for hydrogens of methylene groups (CH 2 ) were observed as AB system at δ = 4.32 and 4.12 ppm, for compounds 1b and 1c respectively. For 1a, the CH 2 group was observed as doublet at δ = 4.24 ppm. The 13 C NMR spectra showed signals corresponding to imine groups at δ = 164.6, 160.9 and 160.7 ppm for 1a-1c, respectively. The 11 B NMR spectra showed signals around δ = 26 ppm, in agreement with the formation of tri-substituted boronic esters [START_REF] Nöth | In NMR-basic principles and progress[END_REF].

X-ray crystal structures

Suitable crystals for X-ray diffraction analysis were obtained from slow evaporation of the mother liquor for all three compounds. Fig. 1 shows the molecular structures for the three compounds (1a-1c). Table 1 summarizes the selected crystallographic data and Table 2 shows selected bond distances, angles and torsion angles. The X-ray diffraction confirms the formation of boronic esters and imine bonds, all compounds have a tricoordinated boron atom having angles close to 120 • , corresponding to a distorted trigonal planar geometry. Results indicated that, despite the presence of Lewis acid moieties (boronic ester) and Lewis bases (pyridine and imine moieties), the N→B dative bond was not observed, being attributed to electronic and steric effects as already reported [START_REF] Sánchez-Portillo | Structural analysis of pyridine-imine boronic esters involving secondary interactions on solid state[END_REF]. The B-O lengths are in the range of 1.358 and 1.362 Å, which are less than the sum of covalent radius because of the π retro-donating effect. It is interesting to remark that the whole conformation of compounds depends on the substituent at the C2 of the 6-membered heterocycle, thus when a hydrogen atom is present (1a) the compound acquires a linear conformation, whereas when it is replaced by a CH 3 group, a bent "L" conformation is observed. This effect has been observed in analogous compounds and mainly attributed to steric effects [START_REF] Sánchez-Portillo | Bis-imine boronic esters obtained by one step multicomponent reactions. Synthesis and X-ray diffraction structural analysis[END_REF].

An aspect that immediately strikes upon comparison of the solidstate X-ray structure of 1a and 1b is the observation of so different molecular structure for entities, which differ by a methyl group only. This rather surprising difference rises the issue of its origin, either intramolecular or intermolecular, by virtue of packing effect. At first, it seems that the magnitude of intermolecular interactions is similar in 1a and 1b with densities calculated of 1.464 and 1.459, respectively. Additionally, the number of intermolecular short contacts is equal to 19 for 1a, and 16 for 1b, which suggests no significant difference in the strength of the interactions network for both species. At the molecular level, the Gibbs free energies (G) were calculated by DFT for the two species in the linear and in the bent "L" conformations [START_REF][END_REF]. In the case of 1a, the linear form appears to be the more stable with the bent conformation located at 3.55 kcal/mol above the ground state conformation. It is worth pointing out that the stable conformation corresponds to the transoid structure depicted in Scheme 2, which is indeed Scheme 2. Transoid and cisoid conformations for 1a, and the bent "L" conformation for 1b.

that observed in the solid state. By contrast, the bent "L" conformation appears more stable in the case of 1b, with the linear transoid structure located at 1.51 kcal/mol above the ground state geometry. Altogether, these computations fully agree with the observed geometries and leads to the conclusion that the solid-state conformation in both systems are mainly governed by intramolecular effects.

Non-covalent interactions

As mentioned above, N→B dative bond formation was constrain by electronic and steric effects, in addition, the boronic ester is part of a sixmembered heterocycle, wherein the boron atom acidity is diminished in comparison with the analogous five-membered heterocycles where the N→B interaction is favored [START_REF] Sheepwash | Self-assembly for arylboronate esters with pyridyl side chains[END_REF]. Nevertheless, the imine-boronic esters 1a-1c contain several functional groups which can form non-covalent intermolecular interactions such as hydrogen bonding, π interactions and halogen interactions. The non-covalent interactions observed for all three compounds are summarized in Table 3. Mainly, hydrogen bonding C-H⋯O (2.580, 2.611 Å), C-H⋯N (2.662, 2.667 and 2.743 Å) and C-H⋯F (2.521, 2.550 and 2.609 Å) are the most common non-covalent interactions observed (Fig. S1). Additionally, C-H … π (2.643 and 3.474 Å) interactions for 1a and 1c, C-F⋅⋅⋅B (3.288 Å) donor-acceptor interaction for 1b and, C-H⋯F (3.068-3.338 Å) interactions for 1a and 1b were observed, distances are in the range already reported [START_REF] Campillo-Alvarado | Exploiting the hydrogen-bonding capacity of organoboronic acids to direct covalent bond formation in the solid state: templation and catalysis of the [2+2] photodimerization[END_REF][START_REF] Georgiou | Versatile self-adapting boronic acids for H-bond recognition: from discrete to polymeric supramolecules[END_REF][START_REF] Rodriguez | On the organizing role of water molecules in the assembly of boronic acids and 4,4'-bipyridine: 1D, 2D and 3D hydrogen-bonded architectures containing cyclophane-type motifs[END_REF]. The C-H⋯F interactions allow the formation of 2D arrangements as depicted at Fig. 2.

Hirshfeld surface analysis [START_REF] Hirshfeld | Bonded-atom fragments for describing molecular charge densities[END_REF][START_REF] Wolf | Crystal explorer version 3[END_REF][START_REF] Spackman | Hirshfeld surface analysis[END_REF] was carried out to obtain additional information on the nature of intermolecular interactions. The red areas on the surface mapped with d norm function were located on electronegative atoms which are responsible of strong hydrogen bonds. Fig. 3 shows the Hirshfeld surface for compound 1a (for compounds 1b and 1c see supplementary information, Figs. S2 and S3 respectively), showing the red areas located at oxygen atoms (boronic ester) and nitrogen atoms (pyridine groups) which participate in C-H⋯O and CH … •N hydrogen bonds. When 2D fingerprint plots are partitioned [START_REF] Becke | Density-functional thermochemistry. III. The role of exact exchange[END_REF], the H/H interaction are seen to be the most dominant interactions in all three cases, followed by F-H contacts (for compounds 1a and 1b) which are responsible of the molecular packing, in contrast, for compound 1c the second dominant interaction is the C-H … π. Table 4 summarizes the relative contribution of interactions (%) for compounds 1a-1c.

Cation sensing studies

The imine-pyridine [START_REF] Udhayakumari | Colorimetric chemosensor for multi-signaling detection of metal ions using pyrrole based Schiff bases[END_REF][START_REF] Peralta-Domínguez | A Schiff base derivative used as sensor of copper through colorimetric and surface plasmon resonance techniques[END_REF] or imine-phenol [START_REF] Maity | A bis(boronic ester)-based fluorogenic and chromogenic sensor for Fand Cu 2+[END_REF] moieties have been reported to act as chromophores for a variety of compounds since their photophysical properties can be modify by mean of metal-ligand interaction. Compounds 1a-1c showed an absorption band at 290 nm (Fig. S4). Sensitivity tests were carried out for compound 1a towards some metal cations from the first-row transition metals (M 2+ = Fe 2+ , Co 2+ , Ni 2+ , Cd 2+ , Cu 2+ and Zn 2+ ). Visible color changes were observed when methanol solutions of M 2+ (1 × 10 -3 M) were added to 1a methanol solution in a 1:1 ratio (metal:ligand). Although most of the sensory analysis are carried out in aqueous solution because of the environmental, industrial and medical applications, many related reports use methanol as medium to determinate these properties [START_REF] Gupta | Comparative study of colorimetric sensors based on newly synthesized Schiff bases[END_REF][START_REF] Gupta | A reversible fluorescence "off-on-off" sensor for sequential detection of Aluminum and Acetate/Fluorine ions[END_REF][START_REF] Gupta | Thiazole Schiff base turn-on fluorescent chemosensor for Al3+ ion[END_REF]. Herein, boronic esters decompose gradually in water solution and thus, methanol was the selected solvent for the analysis. Fig. 4 shows the colorimetric behavior observed for 1a after the addition of M 2+ solutions. The most representative change was noticed using Fe 2+ , the solution turns purple from an initial colorless solution from free 1a. By addition of Co 2+ and Cu 2+ methanol solutions, the 1a solution turns orange and green, respectively. Color changes variations were also monitored by UV-Vis titration experiments, it was observed that the spectrum of 1a shows increasing intensity of the band at 290 nm and the appearance of new bands at higher wavelengths when Fe 2+ , Co 2+ and Cu 2+ methanol solutions were gradually added (Fig. 5). For instance, UV-Vis spectrum for the solution mixture of 1a-Fe 2+ showed two new bands at 360 and 566 nm which are attributed to ligand-metal charge transfer in addition to d-d transitions respectively. Additionally, the absorption bands are maintained when the anion changes (i.e. SO 4 2-), which suggests that the contranion does not affect the formation of the complex (Fig. S5). Similar behavior was observed for the absorption spectra of Co 2+ and Cu 2+ solutions, nonetheless lower intensity bands at 360 and 462 nm (respectively) were obtained. The association constant (K a ) of 1a with Fe 2+ , Co 2+ and Cu 2+ were determined during titration experiments by the following equation [START_REF] Peralta-Dominguez | A Schiff base derivate from cinnamaldehyde for colorimetric detection of Ni2+ in water[END_REF]:

K a = A -A 0 (A max -A 0 ) [ M 2+ ]
where A and A 0 represent the absorbance of 1a (at 566, 360 and 290 nm for Fe 2+ , Co 2+ and Cu 2+ respectively) in the presence and absence of M 2+ , A max is the saturated absorbance of 1a in the presence of excess amount of M 2+ , while [M 2+ ] is the concentration of the corresponding metal ion added. The association constant K a resulted to be 5.51 × 10 3 M -1 , 3.46 × 10 3 M -1 and 3.75 × 10 3 M -1 for Fe 2+ , Co 2+ Cu 2+ respectively by 1:1 binding mode. The values show a trend of Fe 2+ >Cu 2+ >Co 2+ which is congruent with the observed spectral changes depicted at Fig. 6. Titration experiment with Fe 2+ , Co 2+ and Cu 2+ methanol solutions allowed also to determine the stoichiometric ratio (Fig. S6), the Job's graphs are depicted at Fig. 7a-c. Results showed that 1a-Fe 2+ complex Fig. 3. Hirshfeld surface of compound 1a (up) and most representative finger prints (down).

Table 4

Relative contribution of interactions (%) for compounds 1a-1c. shows the maximum absorption when the fraction/molar ratio is about 0.75, indicating a 1:3 metal:ligand stoichiometry, thus, each metal coordinates three units of 1a suggesting the hexacoordination for metals forming a [Fe (1a)] 2+ complex as depicted at Fig. 7d. In the case of 1a-Co 2+ and 1a-Cu 2+ titrations, the Job's plots show a maximum in 0.71 indicating also a stoichiometry near to 1:3 (spectral changes for each experiment are shown at Figs. S7-S9). Similar experiments were carried out for compounds 1b and 1c, these ligands show less absorbance compared with 1a. The titrations Job's experiments showed a 1:1 stoichiometry for 1b while a 2:1 stoichiometry for 1c was observed (Fig. S10). The observed stoichiometric changes are attributed to the presence of -CH 3 group which induce different ligand conformation and, consequently, major steric effect.

To determinate the use of 1a as selective sensor, a study of the possible interferences from competing ions was done. Experiments were carried out by adding simultaneously into methanol solutions of 1a, first a methanol solution of Fe 2+ followed by methanol solutions of either Co 2+ , Ni 2+ , Cu 2+ or Zn 2+ (at 1 equivalent) Fig. 8a andb shows the absorbance values at 290 and 620 nm respectively for these combinations. Only small variations were observed at 290 nm, showing that in this region the interference is minor upon the further addition of a second metal ion species. On the other hand, in the region of the ironligand complex absorption (620 nm), addition of cation Cu 2+ is revealed in the form of significant diminishing absorption to almost 50% (Fig. 8b). The last observation could be the result of the possible competitive interaction between Fe 2+ -Cu 2+ to the ligand.

Additionally, the previously reported compound 2a [START_REF] Sánchez-Portillo | Bis-imine boronic esters obtained by one step multicomponent reactions. Synthesis and X-ray diffraction structural analysis[END_REF] with analogous structure, was also tested as sensor for the metal ions described above. Compound 2a is a boronic ester bis-N-bidentate ligand and consequently, the coordination of two metal centers at the same ligand is possible forming more structured complexes (Fig. 9). The UV-Vis analysis reveals that compound 2a also showed colorimetric and spectral sensitivity in methanol (5 × 10 -4 M concentration) towards Fe 2+ , Co 2+ and Cu 2+ cations in 2:3 (metal:ligand) binding stoichiometry. Thus, each metal center interacts with three molecules of 2a and, each 2a molecule interacts with two metal centers, this stoichiometry has been observed in the formation of polyhedral species of greater complexity as for instance the tetranuclear systems depicted at Fig. 9 [START_REF] Ward | Polynuclear coordination cages[END_REF][START_REF] Riddell | Five discrete multinuclear metal-organic assemblies from one ligand: deciphering the effects of different templates[END_REF][START_REF] Riddell | Cation-and anion-exchanges induce multiple distinct rearrangements within metallosupramolecular architectures[END_REF][START_REF] Black | Generation of a dynamic system of three-dimensional tetrahedral polycatenanes[END_REF].

Due to the biological and environmental importance of iron and copper, the synthesis of new sensors detecting metals at low concentrations with the smallest detection and quantification limits (LOD and LOQ) is currently of high importance [START_REF] Gou | A new diatylethene-derived probe for colorimetric sensing of Cu(II) and fluorometric sensing of Cu(II) and Zn(II): photochromism and High Selectivity[END_REF][START_REF] Kim | A novel colorimetric chemosensor for multiple target ions in aqueous solution: simultaneous detection of Mn(II) and Fe(II)[END_REF]. For compounds 1a and 2a, LOD and LOQ values determination was done by using UV-Vis and Fluorescence spectroscopy employing standard deviation and graphic slopes [START_REF] Shirivastava | Methods for the determination of limit of detection and limit of quantification of the analytical methods[END_REF][START_REF] Gupta | Simultaneous determination of hydroxylamine, phenol and sulfite in water and waste water samples using A voltammetric nanosensor[END_REF][START_REF] Yola | A novel electro analytical nanosensor base don graphene oxide/silver nanoparticles for simultaneous determination of quercetin and morin[END_REF][START_REF] Gupta | Comparative studies of neodymium (III)-selective PVC membrane sensors[END_REF][START_REF] Goyal | Fullere-C 60 -modified edge plane pyrolytic graphite electrode for the determination of dexamethasone in pharmaceutical formulations and human biological fluids[END_REF][START_REF] Goyal | A sensitive voltammetric sensor for determination of synthetic corticosteroid triamcinolone, abused for doping[END_REF][START_REF] Maleh | A novel DNA biosensor based on a pencil graphite electrode modified with polypyrrole/ functionalized multiwalled carbon nanotubes for determination of 6-mercaptopurine anticancer drug[END_REF]. Table 5 shows the LOD and LOQ values obtained for the two compounds using both techniques (Figs. S11-S15). Data from UV-Vis spectroscopy shows that for compound 1a the sensitivity towards metal cations follows the order Fe 2+ > Cu 2+ > Co 2+ , while for 2a it follows the order Cu 2+ > Co 2+ > Fe 2+ . Differences on sensitivity can be attributed to the fact that the bis-imine boronic ester has two chelating sites per ligand and thus, the possibility to form additional metal/ligand species increases, but at the same time, the complexes stability decreases. On the other hand, using the Fluorescence technique, LOD and LOQ values showed that the sensitivity tendency remains as follow Fe 2+ >Cu 2+ >Co 2+ for both compounds, although for compound 1a the lower values observed indicate a better sensitivity. It is important to remark that the LOD values observed in this work for the cation copper (II), are lower than those reported in the literature using the fluorescence technique (1.25 mM-0.01 mM) [START_REF] Xiang | Highly sensitive and selective optical chemosensor for determination of Cu 2+ in aqueous solution[END_REF][START_REF] Carlos | A novel fluorene-derivative Schiff-base fluorescent sensor for copper (II) in organic media[END_REF][START_REF] Zhao | A rhodamine-based chromogenic and fluorescent chemosensor for copper ion in aqueous media[END_REF][START_REF] Singh | Colorimetric and ON-OFF-ON fluorescent chemosensor for the sequential detection of Cu(II) and cysteine and its application in imaging of living cells[END_REF][START_REF] Wagh | Highly selective fluorimetric sensor for Cu 2+ and Hg 2+ using a benzothiazole-based receptor in semi-aqueous media and molecular docking studies[END_REF], and even lower that the maximum allowed value in blood (15.7-23.6 μM) [START_REF] Gupta | A novel copper (II) selective sensor based on Dimethyl 4,4'(o-phenylene)bis (3.thioallophanate) in PVC matrix[END_REF] and in drinking water (<30 μM) [START_REF] Rathod | A colorimetric and flourometric investigation of Cu(II) ion in aqueous medium with a fluorescein-based chemosensor[END_REF][START_REF] Lin | Colorimetric chemosensor and test kit for detection copper(II) cations in aqueous solution with specific selectivity and high sensitivity[END_REF] (Figs. S13 andS14). As described above, the mixture 1a-Fe 2+ has an intense purple color when it is freshly prepared, nevertheless the color changes over time, indicating instability under normal environmental conditions. Color changes of the 1a-Fe 2+ solution were monitored over 96 h and are shown in Fig. 10a, during the first 2 h the color changes from purple to green, later the intensity of color diminishes being almost colorless at the final of the observed period. The UV-Vis spectral changes are shows in Fig. 10b, whereby the band at 566 nm shown a bathochromic effect (up to 630 nm), obtaining a maximum point after 2 h, later this band disappears gradually. At the end, a colorless solution was observed having a wide band around 300 nm. UV-Vis spectral changes indicate that the boron ester and imine groups have been hydrolyzed over the observed period resulting in starting materials. In fact, after 96 h, the final spectrum obtained for the mixture 1a-Fe 2+ , corresponds to the spectrum observed for the serinol/FeCl 2 mixture confirming the decomposition of the initial complex (Fig. S16).

Compounds 1a and 2a also were tested as sensors for Fe 3+ cation. Therefore, a methanol solution of FeCl 3 was added to the methanolic solutions of 1a and 2a. Results showed pale-yellow solutions in both cases being the same as for the only Fe 3+ methanol solution (Fig. 11a). Furthermore, UV-Vis spectroscopy analysis for the Fe 3+ /1a (2a) mixture solutions showed only a broad band at 350 nm which corresponds to the same observed for free FeCl 3 in methanol solution. As mentioned above, a purple color is observed when the ion Fe 2+ is used, indicating clearly the selectivity of compounds 1a and 2a towards Fe 2+ , so that these compounds can differentiate Fe 2+ from Fe 3+ ions in solution (Fig. 11b).

Computational calculations

A computational investigation has been carried out to suggest the role devoted to the methyl group in the stoichiometry observed for metal complexes built up from ligands having the same coordinating capabilities (1a, 1b). Due to the large number of possible isomers in the ML 3 complexes, the two following assumptions were introduced for the sake of simplifications: (i) the linear (form 1a) or bent (form 1b) conformation of the free ligands is maintained in the complexes. This is reasonable, if the linear ⇌ bent transformation imply C -C chemical bonds to be broken and rebuilt. (ii) the more stable geometry for the ML 3 complexes, is not C 3 in which the three ligands are oriented in the same direction, but rather the geometry in which one ligand is oriented head to tail with the respect to the other ones (C 1 ). This was confirmed in the case of the iron complexes, in which the C 3 geometry is located at an average of 1 kcal/mol above the C 1 geometry. (iii) the computations were performed on Fe II in close shell d 6 configuration but are applicable to any pseudo-octahedral metal complexes. Under these assumptions and considering methanol as the solvent, to following reactions were considered:

[Fe(MeOH) 6 ] 2+ + 1a → [Fe(1a) (MeOH) 4 ] 2+ + 2 MeOH (1) [Fe(1a) (MeOH) 4 ] 2+ + 2 1a → [Fe(1a) 3 ] 2+ + 4 MeOH (2)
The associated changes in the Gibbs free energies (ΔG) are found to be equal to -15.8 kcal/mol and -17.9 kcal/mol for (1) and (2) equations, respectively. It is worth pointing out that the conformation of 1a is the linear transoid form depicted in Scheme 2, in any cases. Interestingly, the same computations carried out for iron-1b complexes lead to ΔG values of -17.0 kcal/mol and -12.7 kcal/mol for the related (1) and (2) equations, respectively. At this stage, and although the replacement of the 6 methanol molecules seems not to be as favorable for [Fe (1b) 3 ] 2+ than for [Fe (1a) 3 ] 2+ , the overall behavior can be regarded as roughly similar for both 1a and 1b based complexes. However, [Fe (1a) 3 ] 2+ can be subjected to an additional evolution as follows:

The associated ΔG value for equation ( 3) is equal to -31.8 kcal/mol which leads to a strong stabilization of [Fe (1a) 3 ] 2+ , this effect cannot take place in [Fe (1b) 3 ] 2+ , leading to total ΔG values of -65.5 kcal/mol and -29.7 kcal/mol for the formation of [Fe (1a) 3 ] 2+ and [Fe (1b) 3 ] 2+ , respectively. Altogether, the values presented above are all negatives, which suggest that the coordination of three ligand should be possible in any case, one must take them qualitatively as a clear trend towards much better coordinating capabilities in the case of ligand 1a, as observed experimentally. The reason for such difference can be further evaluated based on the computed molecular geometries of the complexes. The averaged Fe-N bond length values are gathered in Table 6. The observation of longer Fe-N bond lengths in the proximity of the methyl group (Fe-N--C) reflects a steric hindrance in [Fe (1b) 3 ] 2+ . It leads to a distortion of the ideal octahedral symmetry of the Fe-N 6 coordination sphere, and hence to a lowering of the stabilization energy for this species.

In addition, a TD-DFT approach has been used at the B3PW91/6-31G* level to find a rational for the differences observed in the UV-Vis of spectra of 1a-Fe 2+ and 1b-Fe 2+ complexes, which can be summarized as follows: For 1a: (i) the addition of iron (II) to 1a leads to the gradual appearance of new bands at 360 nm (weak) and 566 nm (strong) (Fig. S6a); (ii) the intensity of these bands increases gradually with the concentration of iron added to the solution; (iii) the shape of the spectra remains grossly unaffected. Altogether, these observations suggest that a dominant complex is present in the mixture, the formula of which can tentatively be ascribed to [Fe (1a) 3 ] 2+ . For 1b: (i) the addition of iron (II) to 1b leads to the appearance of an intense transition at 380 nm and a far less intense transition at 535 nm (Fig. S10); (ii) on going from Fe-1b stoichiometry of 1:1 to 1:3 a rather small modification of the intensity (absorbance/concentration of metal) is observed; (iii) the shape of the spectra is grossly unaffected by the concentration of ligand. These observations suggest a dominant complex, tentatively ascribed to [Fe (1b) (MeOH) 4 ] 2+ .

The computed spectra for [Fe (1a) 3 ] 2+ , [Fe (1b) (MeOH) 4 ] 2+ , and [Fe (1b) 3 ] 2+ are gathered in Table 7 and compared with the UV-visible data. The shape of the three computed spectra is provided in Fig. S17. In the case of 1a, the shape of the computed spectrum corresponds to the experimental UV-visible spectrum. The discrepancy in energy between theory and experiment is significant at the low energy band (0.45 eV) but still acceptable for an inorganic complex subjected to potential longrange charge transfer effects. The discrepancy becomes more pronounced at higher energy transition, which prohibits a detailed analysis of the origin of the electronic effects related to these new transitions. The proposed formula ([Fe (1a) 3 ] 2+ ) appears satisfactory for this specie. In the case of 1b, where two formula may be envisioned, either [Fe (1b) 3 ] 2+ or [Fe (1b) (MeOH) 4 ] 2+ , it is interesting to observe that the associated computed spectra are significantly different, thus leading to a possible mean to discriminate between the two formulas. Indeed the more intense transition stands at low energy in [Fe (1b) 3 ] 2+ (like in [Fe (1a) 3 ] 2+ ), while the intense transition is expected at high energy for [Fe (1b) (MeOH) 4 ] 2+ . With this respect, the experimental spectrum (Fig. S10) is in better agreement with [Fe (1b) (MeOH) 4 ] 2+ , as anticipated and in agreement with the UV-Vis experimental determinations.

Conclusions

Three imine-boronic esters functionalized with pyridine were synthetized in good yield by condensation reactions. Besides the presence of donor (N) and acceptor (B) atoms, N→B dative bond formation was not observed because of electronic and steric effects influence.

Nonetheless, hydrogen bonds (C-H⋯O, C-H⋯N, C-H⋯F and C-H … π)

are observed in the solid state and support the formation of 2D molecular structure arrangements. The presence of two N-donor coordinating groups (in the compounds) was used for sensing experiments towards metal cations M 2+ . The colorimetric studies showed that compounds 1a and 2a are suitable for detection of Fe 2+ , Co 2+ and Cu 2+ ions. The association stoichiometry for ligand 1a with the metal ions (M 2+ ) was determined to be 1:3 (metal:ligand), in contrast with the stoichiometry 1:1 observed when using the ligand 1b. The above behavior was attributed to the linear conformation of the ligand 1a in contrast with the bent "L" geometry observed for ligand 1b in which the steric effects have an important effect. In addition, the ΔG calculated values showed that complex [Fe (1a) 3 ] 2+ is 35.8 kcal/mol more stable than the compared value for complex [Fe (1b) 3 ] 2 . Moderate values of LOD and LOQ were observable being for Cu 2+ less that the allowed in drinking water. Ligands 1a and 2a are suitable to real-time simple-to-use and naked-eye detection of Fe 2+ over Fe 3+ .

Fig. 1 .

 1 Fig. 1. X-ray crystal structures of compounds 1a-1c.

Fig. 2 .

 2 Fig. 2. 2D supramolecular polymer formed by C-H⋯F interactions of 1a.

Fig. 4 .

 4 Fig. 4. Colorimetric behavior of compound 1a towards the addition of metal ions in methanol solutions.

Fig. 5 .

 5 Fig. 5. UV-Vis spectral changes of 1a (1 × 10 -3 M) in methanol upon the addition of equimolar concentration of different metal ion solutions (1:1 (v/ v), ml).

Fig. 6 .

 6 Fig. 6. Absorption spectra obtained during the titration of 1a with Fe 2+ (top), Co 2+ (middle), Cu 2+ (bottom) and profile of absorbance change at 566, 360 and 290 nm respectively. Methanol solutions (1 × 10 -3 M).

Fig. 7 .

 7 Fig. 7. Job's plots from UV-Vis spectra for compound 1a with Fe 2+ (a), Co 2+ (b) and Cu 2+ (c); d) proposed interaction between ligand 1a and metal center.

Fig. 8 .

 8 Fig. 8. Absorbance changes at 290 (a) and 620 (b) nm of 1a-Fe 2+ complex by the metal cation addition in methanol (1 × 10 -3 M) to each compound. (To 620 nm, the ligand 1a no absorb).

Fig. 9 .

 9 Fig. 9. Proposed interaction between ligand 2a and metal center. a) Graphical representation of tetrahedral structure in which the ligands are in the edges and the metal centers in the vertices. b) Optimized molecular structure of tetrahedral structure by Spartan [72].

Fig. 10 .

 10 Fig. 10. a) Color changes observed for mixture 1a-Fe 2+ solution monitored by 96 h. b) UV-Vis spectra obtained at different times in methanol at 2 × 10 -3 M. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

B NMR Table 1 Selected crystallographic data of compounds 1a-1c.

	Identification code	1a	1b	1c
	Empirical formula	C 16 H 14 BF 3 N 2 O 2	C 17 H 16 BF 3 N 2 O 2	C 16 H 16 BBrN 2 O 2
	Formula weight Crystal size (mm 3 )	334.10 0.22 × 0.12 x	348.13 0.28 × 0.16 x	359.03 0.26 × 0.14 X
		0.08	0.12	0.12
	Crystal system	Triclinic	Monoclinic	Monoclinic
	Space group	P-1	P2 1 /c	P2 1 /c
	Temp. (K)	100	100	293
	Unit cell
	dimensions
	a [Å]	8.9366 (8)	6.6530 (3)	6.63819 (13)
	b [Å]	9.4794 (8)	9.8202 (5)	9.7342 (2)
	c [Å]	9.7470 (7)	24.317 (2)	24.1639 (5)
	α [ • ]	73.290 (7)	90	90
	β [ • ]	87.498 (6)	93.859 (5)	92.8341 (17)
	γ [ • ] Volume [Å 3 ]	73.545 (8) 757.81 (11)	90 1585.16 (17)	90 1559.51 (5)
	Z ρ calcd [g/cm 3 ] μ [mm ¡1 ]	2 1.464 1.039	4 1.459 0.119	4 1.525 3.654
	Collected refl.	4728	7408	5184
	Independent refl	2912	3603	3028
	(R int )
	Observed refl. [I >	2578	3024	2743
	2σ (I)]
	Parameters	344	720	724
	R [

I > 2σ (I)] R 1 / wR 2

	(128 MHz, CDCl 3 ) δ = 26, h 1/2 = 520 Hz. EI + -MS-HR for
	C 16 H 16 O 2 N 2 BrB, m/z: found 358.0493 ± 1.3, Calc. 358.0488.
		0.0424/0.1174	0.0641/0.1697	0.0386/0.1035
	R (all data) R 1 /wR 2	0.0472/0.1228	0.0745/0.1688	0.0416/0.1069
	GOOF	1.034	1.028	1.041

Table 2

 2 Selected bond distances (Å), angles ( • ) and torsion angles ( • ) of compounds 1a-1c.

		1a	1b	1c
	B1-O1	1.358 (2)	1.362 (3)	1.362 (3)
	B1-02	1.358 (2)	1.361 (3)	1.357 (3)
	B1-C10	1.571 (2)	1.578 (3)	1.568 (3)
	N1-C2	1.4639 (18)	1.487 (3)	1.486 (2)
	N1-C4	1.267 (2)	1.264 (3)	1.246 (3)
	O1-B1-C10	118.16 (13)	118.04 (19)	118.37 (17)
	O1-B1-O2	123.72 (14)	124.20 (2)	123.7 (2)
	O2-B1-C10	118.02 (13)	117.68 (19)	117.92 (17)
	C2-N1-C4	116.34 (12)	117.99 (18)	119.15 (17)
	C11-C10-B1-O1	5.9 (2)	2.6 (3)	1.9 (3)
	C10-B1-O1-C1	-173.1 (1)	179.0 (2)	179.3 (2)
	B1-O1-C1-C2	-29.6 (2)	24.8 (2)	26.1 (2)
	O1-C1-C2-N1	172.2 (1)	69.2 (2)	69.5 (2)
	C1-C2-N1-C4	100.6 (2)	-169.1 (2)	170.0 (2)
	C2-N1-C4-C5	178.3 (1)	179.6 (2)	-179.3 (2)
	N1-C4-C5-C6	-5.1 (2)	8.7 (3)	7.4 (3)

Table 3

 3 Main non-covalent interactions present in 1a-1c.

	Compound	Interaction	Distance	Distance	Angle	Symmetry
			(Å) D-X …	(Å) X … A	( • ) D-X	operation
			A		… A
	1a	C (3)-H	3.287	2.550	132.7	X,-1+y,z
		(3B)⋯F (2)
		C (8)-H (8)	3.388	2.521	155.1	X,2 + y,1 +
		… F (2)				z
		C (12)-H	3.539	2.662	157.5	-x,3-y,-z
		(12) … N
		(2)
		C (4)-H (4)	3.587	2.667	170.4	-x,4-y,-z
		… N (2)
		C (1)-H (1A)	3.440	2.580	147.8	-x,3-y,-z
		… O (1)
		C (7)-H (7)	3.502	2.643	153.9	1-x,3-y,-z
		… π
	1b	C (3)-H (3A)	3.068	2.609	109.2	-x,-1/2 +
		… F (1)				y,1/2-z
		C (12)-H	3.338	2.628	133.6	-1-x,-1/2 +
		(12) … F (3)				y,1/2-z
		C (3)-H (3A)	3.586	2.743	145.7	1-x,1-y,1-z
		… N (2)
		C (16)-F3	4.197	3.288	126.0	-1+x,y,z
		(3) … B (1)
	1c	C (15)-H	4.293	3.474	148.3	3-x,1/2 +
		(15) … π				y,1.5-z
		C (7)-H (7)	3.467	2.611	153.2	3-x,-y,2-z
		… O (1)

Table 5

 5 LOD and LOQ values determined for compounds 1a and 2a toward metal cations.

			Fe 2þ	Co 2þ	Cu 2þ
	1a	LOD (mM)	0.48	0.94	0.85
		LOQ (mM)	1.45	2.86	2.58
	2a	LOD (mM)	0.72	0.67	0.49
		LOQ (mM)	2.18	2.04	1.50
	Fluorescence
	1a	LOD (μM)	2.66	4.37	2.89
		LOQ (μM)	8.07	13.24	8.78
	2a	LOD (μM)	12.92	15.49	15.92
		LOQ (μM)	39.15	46.92	48.25

UV-Vis Spectroscopy

[Fe(1a) 3 ] 2+ (linear transoid) → [Fe(1a) 3 ] 2+ (linear cisoid)

Acknowledgements

The authors thank Consejo Nacional de Ciencia y Tecnología (CONACyT) for financial support through project No. 33602. This work has been performed within the framework of the LIA-LCMMC French-Mexican International Laboratory. The authors thank CNRS and CON-ACYT for financial support.

CRediT authorship contribution statement

Paola Sánchez-Portillo: The author certify that all . Aime Hernández-Sirio: and approved the final version of manuscript "Colorimetric Metal Ion (II) Sensor Based on Imine Boronic Esters Functionalized with Pyridine". They warrant that the article is the author's original work, hasn't received prior publication and isn't under consideration for publication elsewhere, The author certify that all , and approved the final version of manuscript "Colorimetric Metal Ion (II) Sensor Based on Imine Boronic Esters Functionalized with Pyridine". They warrant that the article is the author's original work, hasn't received prior publication and isn't under consideration for publication elsewhere, and approved the final version of manuscript "Colorimetric Metal Ion (II) Sensor Based on Imine Boronic Esters Functionalized with Pyridine". They warrant that the article is the author's original work, hasn't received prior publication and isn't under consideration for publication elsewhere, and approved the final version of manuscript The author certify that all . Rosa Santillán: The author certify that all .

Victor Barba: The author certify that all .

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.dyepig.2020.108991.