Structure and Properties of 1,3,3-Trimethyl-6′-chlorospiro[indoline-2,2′-2H-chromene]

Indoline spiropyran containing an σ-acceptor chlorine atom in 6′ position of the 2H-chromene part of the molecule was synthesized and studied. The use of 1,2,3,3-tetramethyl-3H-indolium perchlorate as a starting compound made it possible to achieve higher product yields as compared to previous studies. The molecular structure of the compound was established by single crystal X-ray diffraction analysis. The features of the crystal structure and intermolecular interactions were investigated using CrystalExplorer17 software package. The photochromic behavior in acetonitrile solution was studied for the first time. It was found that the merocyanine form of spiropyran is characterized by an absorption maximum at 592 nm, which is 37 nm closer to the range of the “biological window” in comparison with the nitro-substituted analog.

DOI: 10.1134/S1070363221070069 Spiropyrans represent one of the most interesting classes of organic compounds, the molecules of which can undergo reversible isomerization between the colorless spirocyclic and brightly colored merocyanine forms under the influence of various external factors. This process, in the case of influence of light radiation with various spectral composition on spiropyrans, is called photochromism [1]. Photochromic spiropyrans can be used as easily tunable molecular switches in various advanced industry of science and technology [2][3][4].
It is known that the introduction of electron-withdrawing substituents into the 2H-chromene moiety of the molecule facilitates the processes of photoinitiated cleavage of the C spiro -O bond [5][6][7]. Therefore, many studies of the photochromic behavior of spiropyrans have been focused on spiropyrans containing a nitro group in the para-position to the oxygen atom of the 2H-chromene fragment. But, the presence of a nitro group in a spiropyran molecule leads to the involvement of low-lying triple excited states of the molecule in the photophysical mechanism of photoinduced transformations and determines their tendency to photodegradation [8]. Consequently, the search and study of spirocyclic structures modified with electronwithdrawing substituents other than nitro group is a promising area of investigations [9]. This article is devoted to the study of indoline spiropyran 1 containing a chlorine atom as an σ-acceptor substituent in 6′ position. As is known, spiropyrans with halogen substituents can be used as precursors for crosscoupling reactions [10][11][12][13].
The structure of compound 1 was confirmed by the data of IR and NMR spectroscopy with two-dimensional methods. In the IR spectrum of compound 1, in addition to the absorption bands of C=C (1608 cm -1 ), C Ar -N (1261 cm -1 ), and C spiro -O (926 cm -1 ) characteristic of spirocyclic compounds [17], there is an absorption band of the С-Сl bond (746 cm -1 ).
The investigation of the compound 1 structure by 1 H NMR showed that, due to the intense magnetic nonequivalent gem-dimethyl groups of the hetarene part, their hydrogen atoms are appeared as two three-proton singlet signals (1.16 and 1.29 ppm). The characteristic signals of protons in 3′ and 4′ positions are observed at 5.73 and 6.78 ppm in the form of doublet signals with a spin-spin coupling constant (J-coupling constant) 10.2 Hz, which indicates the cis-configuration of the vinyl fragment C 3′ =C 4′ . In the 1 H NMR spectrum, the three-proton singlet signal of the NCH 3 group is present at 2.71 ppm. Aromatic proton signals were assigned using the 1 H-1 H COSY NMR method. It was established that the multiplet signal at 6.99-7.04 ppm refers to the hydrogen atoms in the 5′ and 8′ positions of the 2H-chromene moiety, and the hydrogen atom in the 7′ position is represented as a doublet signal at 6.63 ppm with a J-coupling constant 9.3 Hz. 13 C NMR spectrum contains signals of all carbon atoms. Their assignment was carried out using the 1 H-13 C HMBC and 1 H-13 C HSQC methods. The 1 H-13 C HSQC NMR spectrum made it possible to determinate carbon atoms bound with hydrogen atoms. Thus, the signals of carbon atoms at 25.12 and 25.83 ppm correspond to gemmethyl groups, and at 28.87 ppm-to the methyl group at the nitrogen atom. The signals of carbon atoms in 3′ and 4′ positions are located at 120.73 and 128.43 ppm, respectively. The aromatic carbon atoms of the indoline fragment are found at 106.83, 119.27, 121.48, and 127.64 ppm, and at 129.31, 126.13, 120.04 and 116.33 ppm for the 2H-chromene fragment. Quaternary carbon atoms were assigned with 1 H-13 C HMBC.
In 15 N NMR spectrum, a signal of the nitrogen atom is observed at 91.14 ppm. A study by the 1 H-15 N HMBC method allows to find a correlation between the signal of the nitrogen atom and the signals of aromatic hydrogen atoms at 3′, 4′, 4, 6, and 7 positions, as well as with the three-proton signal of the N-methyl group at 2.71 ppm.
The molecular structure of spiropyran 1 was refined by X-ray diffraction analysis and is shown in Fig. 1. Crystallographic data are given in Table 1. The independent part of the unit cell contains two molecules, the spirocyclic center of both is realized in the R position. Representing in Fig. 1, two independent molecules aligned to the left fragment in Fig. 1, while in the right part of the molecule deviate from each other at an angle 23.9° for one molecule and 25.3° for the second (the atoms numbers of the second molecule are increased by 20) in different directions relative to the line mentally connecting the concurrent (in Fig. 1) atoms O 1′ (O 21A ) and C 3′ (C 23B ). Classical intermolecular hydrogen bonds are not formed. In both cases, nitrogen atoms are close to plane, the sum of the angles at the N 1 atom is 349.91°, and at the N 21A atom -351.52°. The values of the lengths of the main bonds, angles and torsion angles are shown in Table 2.
To detect intermolecular interactions in the crystal of compound 1, Hirschfeld surfaces were calculated and obtained. Shape-index is sensitive to very slight changes in surface shape (Fig. 2a). Red continuous areas on the surface, close to triangular and rectangular shape, denote concave areas and represent the atoms involved in the π-π interaction of molecules above it. Blue continuous areas on the surface, close to triangular and rectangular shape, denote convex areas indicating the atoms involved in the π-π interaction in this molecule. A large plane area outlined with a blue contour in the surface curvedness image (Fig. 2b) also indicates the π-π interaction in molecules [18]. In Fig. 2c, the distance (3.554 Å) is shown between the planes drawn through the C 5′ -C 10′ atoms involved in the π-π interaction, and the angle between the  planes is 0°, ie, they are parallel. Consequently, paralleldisplaced π-π stacking is observed in the crystal [18,19].
Donor and acceptor groups in crystal packing can be classified according to the Hirschfeld surface plotted according to the electrostatic potential [19]. On this surface, donors and acceptors of hydrogen bond are represented by blue and red sections on the Hirschfeld surface, respectively (Fig. 3a). From Fig. 3a, it can be seen that the carbon atoms C 6 -C 8 of the indoline π-system, oxygen and chlorine atoms, and also carbon of the methyl group at the nitrogen atom can act as acceptors.
The hydrogen bond donors can be H 4 -H 7 , H 3′ , H 7′ , and H 8′ atoms, as well as some hydrogen atoms of the gemmethyl groups. The Hirschfeld surface (Fig. 3b), plotted using the normalized contact distance d norm , shows the atoms of one of the molecules in the crystal involved in intermolecular hydrogen bonds (marked with red dots). For this molecule, it was found that the hydrogen atom of the gem-methyl group H 11C is donors of hydrogen bonds, and it forms a bond with the chlorine atom Cl 1′ of another molecule, as well as the hydrogen atom H 8′ , which implements the bifurcate type of interaction with the carbon atoms C 4 and C 9 of another molecule. The C 7′ carbon atom is an acceptor in the nonclassical intermolecular hydrogen bond with H 7′ , as the C 5 and C 6 carbon atoms, which form nonclassical intermolecular hydrogen bond with the H 3′ atom.
Investigating other molecules in the crystal, other intermolecular interactions were also found (Fig. 4). It is interesting that the methyl groups at the nitrogen atom interact with each other, forming nonclassical intermolecular hydrogen bonds of the C-H···H type. In addition to these interactions, the molecules in the crystal are kept with 22 shortened contacts. The discovered intermolecular hydrogen bonds and π-π interactions lead to the fact that molecules in the compound 1 crystal are packed into layers (Fig. 5).
The obtained compound 1 exists predominantly in the spirocyclic form A in acetonitrile solution, characterized by three bands with maxima at 205, 222, and 297 nm. UV irradiation effect (λ irr 313 nm) to the spiropyran solution leads to the C spiro -O bond cleavage and the transformation of the molecule into the merocyanine isomer B (Scheme 2). Due to the photoisomerization under the  119.07 (17) influence of UV irradiation, a new absorption band appears in the visible region of the spectrum with a maximum at 592 nm, which is attributed to to the photoinduced isomer B (Fig. 6). After the termination of the irradiation, as a result of the thermal recyclization reaction, the solution was bleached. The isomer B lifetime (τ B ) at room temperature was 19.6 s. The spectral-kinetic characteristics of isomers A and B are given below.
EXPERIMENTAL NMR spectra were recorded on a Bruker AVANCE-600 (600 MHz) spectrometer in CDCl 3 . IR spectrum was recorded on a Varian Excalibrum 3100 FT-I device with incomplete internal reflection method. Electronic absorption spectra were recorded on an Agilent 8453 spectrophotometer with a sample thermostating gadget. Photolysis of solutions was performed using a Newport system equipped with a 200 W mercury lamp with a set of interference filters. Spectral purity acetonitrile (Aldrich) was used to prepare solutions. Elemental analysis was performed by the classical microanalysis method [21]. Melting points were determined using a Fisher-Jones apparatus (ThermoFisher Scientific, United States).
The unit cell parameters of the crystal and the threedimensional set of intensities were obtained on an Xcalibur, Eos autodiffractometer (MoK α -irradiation, graphite monochromator). An empirical account of absorption was carried out using the Multiscan procedure. The structure was solved by a direct method and refined by the full-matrix least squares method (LS) according to F 2 with the SHELXTL program in the anisotropic approximation for non-hydrogen atoms. In the crystal structure, most H atoms are localized in the Fourier synthesis of the difference electron density, and then the coordinates and isotropic thermal parameters of all H atoms were calculated in the LS procedure using the riding model [22]. The crystallographic data has been deposited in the Cambridge Crystallographic Data Center (CCDC 2079252) and can be freely obtained from: www. ccdc.cam.ac.uk/data_request/cif. Hirschfeld surfaces were calculated by the MP2 method using the 6-311G(d, p) basis set in the CrystalExplorer v17.5 program [23].  triethylamine was added dropwise. The reaction mixture was refluxed with stirring for 25 min, and then cooled, 15 mL of distilled water was added, and the mixture was extracted with chloroform. The extract was dried with anhydrous Na 2 SO 4 and the solvent was evaporated.

FUNDING
The study was financially supported by the Ministry of Science and Higher Education of the Russian Federation within the framework of the state assignment in the field of scientific activity no. 0852-2020-00-19. X-ray diffraction study was performed in accordance with the state assignment, state registration no. АААА-А19-119092390076-7 (V. V. Tkachev, S. M. Aldoshin).