Centrosymmetric Binuclear Boron Compounds Derived from Dithiooxamides : Synthesis , Characterization , and Their Photophysical Properties

Autonomous University of Nuevo Leon, Faculty of Chemical Sciences, University City, San Nicolás de los Garza, 66451 Nuevo León, Mexico Department of Chemistry, Center for Research and Advanced Studies of the IPN, A. P. 14-740, 07000 Mexico City, Mexico Autonomous University of Nuevo León, Faculty of Medicine, Francisco I. Madero and Dr. Eduardo Aguirre Pequeño, Mitras Centro, 64460 Monterrey, Nuevo León, Mexico


Introduction
Boron compounds are important in biological systems, and they result from the interaction with hydroxyl and amine groups [1].Boron has a high affinity for oxygen-forming borates that are involved in enzyme inhibition.
e isoelectronic nature of C�C and B-N bonding increases the use of boron in organic synthesis [2].Boron compounds are widely studied due to their various applications such as luminescent materials [3,4], lasers [5][6][7][8][9][10], OLEDs [11][12][13], materials for nonlinear optics [14],and chemical materials used in fluorescent tests [15,16], among others.e development of ladder-type π-conjugated molecules with fully ring-fused structures gives rise to a set of desired properties such as intense luminescence, good thermal stability, and high carrier mobility.
ese properties are important in terms of their applications in optoelectronics, organic fieldeffect transistors, and lasers [17][18][19].Representative elements have been introduced to the π-conjugated skeleton to modulate the electronic structures and different properties like optical properties [20][21][22][23][24][25][26].Some compounds have been reported with π-conjugated diboron ladders.Zhang and coworkers reported a series of ladder-type π-conjugated diboron complexes I-IV with high thermal stabilities (Figure 1) [27].e same research group synthesized four novel diboron-containing π-conjugated ladders (V-VIII); skeletons were modified by introducing electron-withdrawing and electron-donating groups into different sites of the backbones.ese materials present good thermal stability, high fluorescence quantum yields, and strong electron affinity [28].Recently, Jacquemin and coworkers reported first-principle simulations of the excited states' properties of a large series of ladder-type π-conjugated organic molecules containing heteroatoms (Si, S, B, O, and N).ey observed the phenyl rings that bonded to the boron atom do not play any role in the optical transition in compounds with similar core in this report [29].Another research group has studied diboron BNheterocycles; they suggested that the location of BN units, the steric congestion, and the linker unit within the π-conjugated backbone can greatly a ect the electronic structure of these molecules as well as their photophysical/photochemical properties [30].It is therefore important to study the properties and characteristics of new compounds and to understand the impact when modifying the structures.Fluorescent sensors have received attention because they have advantages in terms of sensitivity, selectivity, and the easy detection of the uorescence changes of the systems [31].Due to this, we are interested in the synthesis of new materials with the ability to sense di erent stimuli.In the present work, we reported the synthesis and characterization of two new boron compounds derived from dithiooxamides.ey present greater stability in the solid state than in solution; however compound 2 was unstable in solution, obtaining an X-ray structure for a mononuclear species.

Material and Equipment.
All starting materials were purchased from the Aldrich Chemical Company.Solvents were used without further puri cation.Melting points were performed on an Electrothermal Mel-Temp apparatus and were uncorrected.A high-resolution mass spectrum was obtained by LC/MSD TOF, on an Agilent Technologies instrument, with APCI as the ionization source.UV-vis spectra were obtained with a PerkinElmer Lambda 356 UV/VIS spectrophotometer, and emission measurements were performed on a Fluorolog-3 spectro uorometer. 1H and 13 C spectra were recorded on a Bruker avance DPX 400.Chemical shifts (ppm) were relative to (CH 3 ) 4 Si for 1 H and 13 C.
e crystal was mounted on a Lindeman tube.e structures were solved by direct methods using SHELXS-97 [32] and re ned against F 2 on all data by full-matrix least-squares with SHELXL-97 [33].All of the software manipulations were done under the WIN-GX environment program set [34].All heavier atoms were found by Fourier map di erence and re ned anisotropically.Some hydrogen atoms were found by Fourier map di erences and re ned isotropically.

Absorbance, Emission, and Luminescence Quantum
Yields.UV-vis absorption spectra were measured on a PerkinElmer Lambda 365 spectrophotometer (a solution of 1 mg of the compound in 50 ml of solvent was prepared to determine the photophysical properties).Optical band gap (Eg) was determined from the intercept with the X axis of the tangent of the absorption spectrum drawn at absorbance of 0.1 [35].
e emission spectra have been recorded with a Fluorolog-3 spectrofluorometer, by exciting 10 nm below the longer wavelength absorption band.e molar extinction coefficients were calculated by the rearranged Beer-Lambert equation.Fluorescence quantum yields in solution were determined according to the procedure reported in literature [36,37] and using quinine sulphate in H 2 SO 4 0.1 M as the standard (quinine sulphate Journal of Chemistry λ ext � 365 nm, Φ � 0.546 at room temperature) [38].ree solutions with absorbance at the excitation wavelength lower than 0.1 were analyzed for each sample, and the quantum yield was averaged.

Results and Discussion
3.1.Synthesis.e ligands (1 and 3) were synthesized by condensation reactions of two equivalents of aldehyde with dithiooxamide in DMF under reflux for 24 h, according to the procedure previously reported [39,40].ey were fully characterized by NMR ( 1 H and 13 C), UV-vis, and mass spectrometry.
e boron compounds were obtained in moderate yields (60-73%) by excess of Et 2 O•BF 3 with the ligand under reflux in THF (Scheme 1).e resulting boron compounds are soluble in organic solvents compounds like THF and DMSO.e compounds were characterized by NMR ( 1 H and 13 C), UV-vis, and mass spectrometry.e binuclear compound 2 is unstable in solution; we tried to obtain crystals suitable for X-ray diffraction but only crystallize the complex with one boron atom.e compound 2a crystallized by slow evaporation of THF/hexane (1/9).Suitable single crystals for X-ray analysis were obtained, and their ORTEP drawing of molecular structure is shown in Figure 2, while refinement parameters are available in Table 1.
e complex mononuclear 2a crystallized in the P2 (1) /n space group, the molecules were monoclinic and crystallized as a yellow prism.e crystal structure of 2a shows the one boron atom tetracoordinated with the ligand and the formation of four heterocycles of five and six members.Boron atoms adopt typical tetrahedral geometry, bond lengths B-O 1.418(5) Å and B-N 1.582(4) Å which are characteristic for tetracoordinated boron complex, compared with molecules reported [41,42].e crystal has a rigid π-conjugated between the rings 2, 3, and 4; however, the ring 5 is slightly outside of the plane of the thiazolothiazole skeleton caused by the steric effect of the substituent groups tert-butyl (Figure 3).e structure of the compound shows intermolecular interactions with S2 and the proton of the tert-butyl group of other molecules (2.923-2.995Å) and parallelly displaced π-π interactions in the range from 3.905 to 4.111 Å. e intramolecular interactions shown are (H15-S2, 2.616), (N2-H of OH, 1.931), (H13 of tBu-O2, 2.336-2.278),(S1-H9, 2.741), and (H6 of tBu-O1, 2.380-2.314)Å (Figure 4).e spectra 1 H NMR for the ligands (1) and (3) show the downfield H-bonded phenolic proton at 11.83 and 11.08 ppm; when the boron atom is coordinated, this signal disappears and gives the first indication for the formation of compound 4.For complex 2, it was difficult to obtain a clean spectrum (Figure S4); it shows a mixture of signals between the ligand (1) and the possible compound 2, which gives us a possible indication of instability.e proposed structure of the boron compounds was confirmed by the mass spectra of boron derivatives; it showed the base peak corresponding to the molecular ion.
e compound 2 shows a pattern of fragmentation due to the loss of one boron atom, obtaining the mononuclear compound (2a), followed by the loss of the second boron atom to obtain the ligand (Scheme 2).e isotopic distribution of parent ions in the spectra demonstrated the presence of two atoms of boron in the compound 2. e comparison of the predicted theoretical and experimental isotopic distributions of spectra for the compound is given in Figure 5.However, compound 4 does not present the fragmentation pattern corresponding to the species [M-BF2]; mainly, the ligand 3 fragment is observed (427.0677m/z, 15.8%) (Figure S11).

Photophysical Characterization.
e boron complexes are more emissive in the solid state than in solution.e UV-vis absorption and emission spectra of 2 and 4 in THF are shown in Figure 6 and the date in Table 2. Interestingly, the absorption spectra of complex 2 show an intense peak at 436 nm.
e first absorptions are mostly attributed to the transitions from HOMO to LUMO, excited states involve possibly, mainly, the π-conjugated core [29].
e fluorescence spectra show maximum wavelengths between 465 and 509 nm for the compound 4 is more intense the emission.Compound 4 in solution (solvent THF) absorbs at 428 nm and emits at 509 nm; however in the solid state, it shows a clear red color, this difference in color is due to the fact that the compound in solution shows solvatochromism [43,44].
is behavior was possibly caused by the introduction of the fluorine atoms at the boron atom, similar to that reported by Matsui research group, they studied pyrazine-boron complexes with lower molar extinction coefficient values of (ε) lower when the boron atom presents substituents as in comparison with phenyl substituents [45].However, the molar extinction coefficients (ε) of 2 and 4 were less than the binuclear species with different central cores [46].e optical band gap values (2.75-2.80eV) are slightly lower than similar compounds reported by Wang [30].Fluorescence quantum yields in solution were determined according to the procedure reported in literature [36,37], the boron compound exhibits lower values of fluorescence quantum yields of 2.9 to 4.1 than similar compounds reported as V-VIII [28], these low values of Φ F can be due to the presence of electro-attractor atoms like fluorine and the promotion of nonradiative processes, and this behavior is similar to that reported by Kubota et al. [47].Compound 2 shows a gradual decomposition in THF because it shows a slightly orange coloration, and after being exposed for one day at room temperature, the color of the solution changes to light yellow similar to the ligand; in addition, the absorption spectra of the solution at room temperature for one day is similar to the ligand (Figure S12).e compounds present greater luminescence in the solid state and, as a result of this, they were ground for two minutes on a mortar.It is important to mention the boron compounds respond to mechanical stimuli such as friction observing.Compound 2 decreases the fluorescence intensity, similar to other molecules [48], while compound 4 slightly changes its coloration taking a bright red color (Figure 7); this may indicate a slight change in molecular arrangements by the application of the force [49].
It is important to study the ability to sense oxygen in cells because the deficit and excess oxygen is associated with health problems or diseases [50,51].Recently, a boron compound with the ability to sense oxygen at the cellular level has been reported [52].erefore, the effect of dissolved oxygen concentration on the emission intensity of compound 4 was evaluated in the degassed THF at 25 bar and with oxygen pressure for 60 s, followed by the addition of air for 60 s in two cycles (Figure 8).In the first cycle, the compound 4 shows a decrease in the fluorescence intensity, and in the second cycle a slight shift towards the blue is observed.When applying air in the second cycle, the compound decreases even more than the intensity of emission, the behavior similar to that reported in the literature, a consequence of paramagnetic behavior of oxygen molecules as well as generating the triplet state [53].However, compound 4 in the presence of nitrogen for 60 seconds exhibited an increase in the emission, which may be a consequence of oxidation of the compound, and due to absorption spectra, the compound shows a hypsochromic shift.

Conclusions
In summary, we describe the synthesis, characterization in solution, and the solid state of two boron compounds, observing greater stability in the solid state; it was possible to obtain a structure of x-rays for a mononuclear compound.e effect of oxygen and dissolved air in compound 4 was studied observing a decrease in the intensity of emission upon addition of oxygen and increase in this emission when it is substituted by nitrogen, and in the

Figure 5 :
Figure 5: Comparison of theoretical and experimental isotopic distributions of spectra of the [M-F] + of compound 2. e spectra clearly indicate the presence of two boron atoms.

Figure 8 :
Figure 8: Emission spectra of compound 4 in THF degassed with oxygen, air, and nitrogen.

Figure 7 :
Figure 7: Fluorescence images of compounds 2 and 4. (a, c) Under UV light at 25 °C.(b, d) After grinding under UV light at 25 °C.

Table 1 :
Crystal data of compound 2a.

Table 2 :
Photophysical data of compounds in THF.