Unusual reaction of (E)-2-[(benzo[d]thiazol-2-ylimino)methyl]-5-(diethylamino)phenol with triphenylborane: crystal structures and optical properties

The molecular and crystal structure of (E)-2-[(benzo[d]thiazol-2-ylimino)methyl]-5-(diethylamino)phenol and its reaction product with triphenylborane are described. In compound Et2N-Bz, one of the ethyl groups and the benzothiazole ring are disordered over two sets of atomic sites with major occupancy components of 0.822 (5) and 0.843 (2), respectively.


Chemical context
Recently, boron complexes have gained increasing attention in fluorescent materials because they have many potential applications in the field of photoelectric devices, fluorescent sensors and probes (Li et al., 2013;Shi et al., 2020).Among them, boranils, i.e. boron complexes using salicylaldimine as a ligand, have emerged as promising materials due to their excellent optical properties, ICT (intermolecular charge transfer), high Stokes shift and simple synthesis (Vidyasagar et al., 2019).An additional advantage of boranils is that their emission characteristics can be adjusted in a flexible way through structural changes such as extending the �-conjugation system, adding donor/acceptor substituents, increasing molecular rigidity and flattening the structures (Frath et al., 2014;Zhao et al., 2019;Mace ´et al., 2021;Al-Sharif et al., 2020).These complexes can be synthesized on a multi-gram scale with a two-step process, including synthesis of a Schiff-base ligand via a condensation reaction between an amine and an hydroxyaldehyde, and complexation with commercial boron compounds (Massue et al., 2021).In addition, Schiff bases containing the benzothiazole component have a wide range of bioapplications (Shinde & Waghamode, 2017;Ceramella et al., 2022;Bhat et al., 2017), but their optical potential does not seem to have received much attention.Recently, several studies have shown that these derivatives can be used as fluorescent chemosensors in living cells (Khan et al., 2021), aggregation-induced emission (AIE) active materials (Kachwal et al., 2018) and potential non-linear optical materials (Muhammad et al., 2018).
In this study, we intended to design a new boron(III) complex by replacing the amine component in the structure of boranils with 2-aminobenzothiazole to extend their �-conjugated system.From this idea, (E)-2- [(benzo[d]thiazol-2-ylimino)methyl]-5-(diethylamino)phenol (compound Et 2 N-Bz) was synthesized with high efficiency via a condensation reaction between 2-aminobenzothiazole and 4-(diethylamino)-2hydroxybenzaldehyde (Fig. 1).As planned, boron complex (II) would be formed by reaction between ligand Et 2 N-Bz and BPh 3 (triphenyl borane).In the expected complex, boron would coordinate with the ligand through the oxygen atom of the hydroxyl group and the nitrogen atom of the imine group.But more surprisingly, the results of NMR and SC-XRD analysis indicated that the product obtained had structure (I) instead of the expected structure (II).This phenomenon can be explained by the fact that due to the simultaneous presence of Lewis acid BPh 3 in the CHCl 3 solvent, ligand Et 2 N-Bz is hydrolyzed and the boron atom is cyclized with the two oxygen atoms.To further elucidate this assumption, the interaction of 4-(diethylamino)-2-hydroxybenzaldehyde (Et 2 N-CHO) with BPh 3 has been tested under similar experimental conditions.However, the TLC analysis results showed that no compounds were formed.The crystal structures and photophysical properties of the ligand Et 2 N-Bz and complex (I) are presented in this work.

Structural commentary
Compound Et 2 N-Bz crystallizes in the monoclinic space group P2 1 /n with one molecule in the asymmetric unit (Fig. 2).One of the ethyl groups (C20-C21) and the benzothiazole ring are disordered over two sets of atomic sites with major occupancy components of 0.822 (5) and 0.843 (2), respectively.The Schiff base displays an E configuration with respect to the C11 N10 double bond.The benzothiazole ring is planar [maximum deviation = 0.010 (3) A ˚for N3] and subtends a dihedral angle of 5.08 (7) � with phenyl ring C12-C17.The hydroxyl group O18-H18 is involved in an intramolecular hydrogen bond with the imino nitrogen atom N10 (Fig. 2, Table 1).One of the orientations of the benzothiazole group shows a short intramolecular contact (H11� � �S1B = 2.46 A ˚).
Complex (I) crystallizes in the triclinic space group P1 with two molecules in the asymmetric unit (Fig. 3).The r.m.s.deviation for the best fit (with inversion) of the two molecules is 0.849 A ˚.The orientations of the ethyl groups differ in molecules A (containing atom B1) and B (containing atom B2).In molecule A, the ethyl groups are on a different side of the ring to which the diethylamino group is attached, whereas in molecule B both ethyl groups are on the same side.The 1,3dioxa-2-borata-1,2,3,4-tetrahydronaphthalene ring shows a slight envelope conformation with the boron atom as the flap.For molecule A, the deviation of atom B1 from the best plane through the ring is 0.315 (3) A ˚, for molecule B the deviation for B2 is 0.301 (3) A ˚.For molecule A, this boron-containing plane makes dihedral angles of 84.96 ( 14) and 81.09 (12) � with phenyl rings C8-C13 and C14-C19, respectively.For molecule

Figure 3
The molecular structure of molecules A and B in the asymmetric unit of (I) showing the atom-labeling scheme and displacement ellipsoids at the 30% probability level.
For compound (I), both molecules A and B are linked by a C24-H24� � �O2 hydrogen bond (Table 2).In addition, molecules A and B interact further through C1-H1� � �O4 i hydrogen bonds (see Table 2 for details).This builds a chain of alternating A and B molecules running in the b-axis direction (Fig. 6).Within this chain and between neighboring chains research communications

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.44, update of September 2023; Groom et al., 2016) for the benzothiazole fragment shown in Fig. 7a gave 39 hits.
For the majority of the hits (32 out of 50 values) the S-C-N C torsion angle averages around �180 � (�ap or trans), while for 17 entries this torsion angle is close to 0 � (�sp or cis; see Fig. 7b).For one entry (refcode UXIRIE; Sovic ´et al., 2016), the unusual value of 121.0 � (+ac) is caused by the incorporation of the terminal C-C bond of the search fragment into an indole ring.For Et 2 N-Bz this torsion angle is 177.55 (15) � for the major component of the benzothiazole ring and À 2.6 (4) � for the minor component.

Photophysical properties
The UV-vis absorption and emission properties of the compounds Et 2 N-CHO, Et 2 N-Bz, and complex (I) at 10 mM in chloroform solvent are shown in Fig. 8 and Table 3. Accordingly, it can be seen that Et 2 N-Bz absorbs at 436 nm, while complex (I) shows absorption at 347 nm, which is a small shift from that of Et 2 N-CHO (343 nm).The absorption peaks (343 nm and 347 nm) are attributed to the �-�* transition of the aromatic ring.Under a UV lamp with a 365 nm excitation wavelength, a solution of Et 2 N-Bz fluoresces green, while a solution of complex (I) shifts towards blue.In addition, this complex exhibits a longer emission wavelength and greater fluorescence intensity than that of Et 2 N-CHO, demonstrating that complexation with boron can improve fluorescence properties compared to the free ligand.
To investigate the AIE (aggregation-induced emission) properties of Et 2 N-Bz and (I), we recorded the emission spectra of their 10 mM solutions in different fractions of water in a MeOH-water mixture.The results show that only compound Et 2 N-Bz is present as AIE active material (Fig. 9 and Fig. S1).The fluorescence color change from 0% to 99% water in the MeOH-water mixture from green to yellow is easily observed under a 365 nm UV lamp.The � em of Et 2 N-Bz in AIE spectra increases as the water fraction increases.This phenomenon can be explained by the fact that the solubility of Et 2 N-Bz decreased when the water ratio increased, which   shortened the distance between molecules and �-� stacking interactions appeared (Fig. 4), which affected the electron density in the molecule, thus the emission wavelength and the emission intensity also changed (Hong et al., 2009).
In other solvents: The experiments in other solvents such as toluene, THF, ethanol were conducted under the same conditions as in chloroform.The course of reaction was monitored by TLC analysis.The results indicated that no new products were formed after 24 h of reaction.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4.All H atoms bonded to C atoms were placed in idealized positions and refined using a riding model with C-H distances of 0.93 (aromatic), 0.97 (CH 2 ) and 0.96 A ˚(CH 3 ).Non-hydrogen atoms were refined anisotropically and hydrogen atoms with isotropic temperature factors fixed at 1.2 times U eq of the parent atoms (1.5 for methyl groups).For the O-H group in Et 2 N-Bz, the SHELXL command AFIX 148 was used in combination with U(H) = 1.2U eq (O).One of the ethyl groups in Et 2 N-Bz is disordered over two sets of sites with refined occupancies of 0.822 (5) and 0.178 (5).Also the benzothiazole group is disordered over two positions by a rotation of 180 � resulting in refined occupancies of 0.843 (2) and 0.157 (2) for atoms S1 and N3.

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.122.1 (3) N2-C45-H45A 108.9 C22-N1-C20 114.9 (3) N2-C45-H45B 108.9

Figure 1
Figure 1 Synthesis of compounds Et 2 N-Bz and (I).

Figure 2
Figure 2The molecular structure of Et 2 N-Bz showing the atom-labeling scheme and displacement ellipsoids at the 30% probability level.The intramolecular O-H� � �N hydrogen bond is shown as a dashed line.Minor disorder components are shown in orange (ethyl group) and red (benzothiazole ring).

Figure 5
Figure 5 Chain of molecules running in the a-axis direction in the crystal packing of Et 2 N-Bz.The O-H� � �N and C-H� � �O hydrogen bonds are shown as blue and gray dashed lines, respectively.Only major disorder components are shown.Symmetry codes: (i) 1 + x, y, z; (ii) À 1 + x, y, z.

Figure 6
Figure 6 Chain of molecules running in the b-axis direction in the crystal packing of (I).The C-H� � �O and C-H� � �� hydrogen bonds are shown as gray dashed lines.Symmetry codes: (i) x, À 1 + y, z; (ii) x, 1 + y, z.Cg4 and Cg9 are the centroids of rings C14-C19 and C37-C42, respectively.

Figure 7 (
Figure 7 (a) Search fragment used in Conquest to perform the CSD survey.(b) The distribution of the torsion angle S-C-N-C in the search fragment shown as a histogram.

Figure 9 (
Figure 9 (a) Photoluminescence spectra and (b) fluorescent color change of compound Et 2 N-Bz at 10 mM in different fractions of water in a MeOHwater mixture.

Table 4
Experimental details.