Structure and spectroscopic properties of N,S-coordinating 2-methylsulfanyl-N-[(1H-pyrrol-2-yl)methylidene]aniline methanol monosolvate

The title Schiff base, 2-methylsulfanyl-N-[(1H-pyrrol-2-yl)methylidene]aniline, crystallizes in the presence of a methanol molecule and features three distinct hydrogen bonds to each heteroatom in the molecule. The crystal lattice exhibits an array of methanol molecules sandwiched between the title compound exhibiting weak supramolecular interactions.


Chemical context
Compounds that contain N-and S-donor atoms have exhibited biomedical activities such as antibacterial properties. In addition, such N,S-compounds can be useful ligands to form transition metal complexes which we have been investigating for their use as biomimetic models for Cu enzyme models (Alberto et al., 2013;Cross et al., 2011). Recently, we have reported the synthesis and structure of xylylene-bridged bis-[ortho-aminothiophenols] for the design of binuclear transition metal complexes (Alberto et al., 2013;Cross et al., 2011). Copper complexes of these N 2 S 2 -ligands are studied as small biomimetic metal models for the analysis of non-blue/type-II copper enzymes such as peptidylglycine -hydroxylating monooxygenase (PHM), which is one of the two non-coupled copper ion domains of the bifunctional peptidylglycineamidating monooxygenase (PAM, EC 1.14.17.3) (Klinman, 2006;McIntyre et al., 2009). Recently, we reported the X-ray structure of a trinuclear palladium(II) complex containing N,S-coordinating 2-(benzylsulfanyl)anilinide and 1,3-benzothioazole-2-thiolate ligands (Cross et al., 2014). The 2-aminothiophenol group can be used as a synthetic building motif for the preparation of benzothiazolines (Chou et al., 2008), thioethers (Ham et al., 2006;Schwindt et al., 1976a) and polyurethanes (Schwindt et al., 1976b), and has medical applications in antitrypanosomal, antileishmanial and antimalarial treatments (Parveen et al., 2005).
Herein, we report the X-ray structure of 2-methylsulfanyl-N-[(1H-pyrrol-2-yl)methylidene]aniline methanol mono- ISSN 2056-9890 solvate (1) which features an aryl methyl thioether group and an imino-2-pyrrole motif. The imine pendant prevents the reversible formation of the benzothiazoline, a transformation that was evident in the structure we reported previously that featured a free amino group and was bonded to a palladium centre (Setifi et al., 2014). Li and co-workers first described the synthesis of (1) in 16% isolated yield (He et al., 2009). The 1 H and 13 C NMR data were reported and are congruent with our data (Basuli et al., 1996). Compound (1) was complexed with CrCl 3 (thf) 3 (He et al., 2009) and with VCl 3 (thf) 3 (Mu et al., 2011) as ethylene polymerization catalysts (He et al., 2009). We now provide additional compound data such as HRMS, UV-vis and FT-IR. Fig. 1 shows the molecular structure of the yellow title compound. The imino group is coplanar with the pyrrole group, and the dihedral angle between the plane of the combined (pyrrol-2-yl)imino moiety and that of the benzene ring carbons is 42.71 (5) . The imino group N1-C8 bond distance [1.2829 (17) Å ] is normal. The sulfur and imino nitrogen atoms are very nearly coplanar with the benzene ring atoms [S is 0.0595 (18) Å and N1 is 0.0620 (19) Å out of plane], while the methyl carbon C7 is 0.310 (3) Å out of the benzene ring plane.

Structural commentary
The three heteroatoms of the main molecule of (1) are each involved in hydrogen-bonding interactions with the adjacent co-crystallized solvent methanol molecule ( Fig. 2 and Table 1). The closest interaction is between the protonated nitrogen of the pyrrol-2-yl group and the methanol oxygen [N2Á Á ÁO1S   Illustration of hydrogen-bonded interactions (dotted lines) between the 2-methylsulfanyl-N-[(1H-pyrrol-2-yl)methylidene]aniline molecule and a nearby solvent methanol molecule.

Supramolecular features
As shown in Fig. 3, the methanol molecules are sandwiched between the main molecules of (1) in such a manner as to precludestacking interactions between aromatic rings of adjacent molecules. The hydrogen-bonded methanol-main molecule units are linked by pairs of weakC-HÁ Á ÁO methanol interactions, forming inversion dimers consisting of two main molecules and two solvent molecules (Table 1). In summary, 2-methylsulfanyl-N-[(1H-pyrrol-2-yl)methylidene]aniline is a conjugated imine that exhibits three hydrogen-bonding interactions to methanol within the crystal packing which would make the compound effective for tridentate N,N,S metal chelation, particularly in the case where the N-hydrogen of the pyrrol-2-yl group is deprotonated to form an anionic species.
A closely related compound that features a pyridyl group instead of a pyrrole has been extensively reported in metal complexes and whether the thioether bonds to the metal centre varies, which sheds perspective on the binding nature of compound (1). For example, the thioether of the pyridyl ligand does not initially bind to the metal centre of a manganese carbonyl complex unless in the presence of oxygen (Lumsden et al., 2014). When reacted with a rhenium carbonyl complex (Jana et al., 2013), the thioether does not participate in bonding, and in contrast, the thioether binds to iron in its respective carbonyl complex (Muthiah et al., 2015).
This variance in thioether bonding is also found when reacting the pyridyl ligand with various copper complexes (Addison et al., 1984;Schnö dt et al., 2011;Patra et al., 2011b;Chatterjee et al., 2012;Balamurugan et al., 2006) where copper is our target metal centre for (1) and for our other NNS ligands. Addison and co-workers have reported a systematic study on the properties of various copper-thioether interactions (Addison et al., 1984). In the study, they considered the presence of a nitrogen donor in an equatorial plane to the thioether, strong donor solvents, and the redox chemistry of the resultant metal complexes, which would affect the displacement of the thioether group.
The methanol molecule present in the X-ray structure of (1) does illustrate the three heteroatoms that could bond to a metal centre, though perspective can be gained from the metal complexes formed with the pyridyl ligand relative. The reported molecular structures with the pyridyl metal complexes all feature distorted octahedral geometry. In the case of (1), the thioether group sits above the neighbouring benzene ring, which would contribute to the formation of a distorted octahedral complex and remove the direct equatorial interaction of the sulfur to the donating nitrogen of the imine group. In addition, the pyrrole substituent is relatively less basic than pyridine, hence deprotonation of the pyrrole ligand must occur to elicit coordination of a metal centre.

Synthesis and crystallization
All chemicals were purchased from commercial sources (Fisher Scientific and Sigma-Aldrich) and used without further purification. A colorless solution containing 0.683 g (7.18 mmol) of 2-pyrrolecarboxaldehyde dissolved in 15 mL of MeOH was added drop-wise to a light-green solution containing 1.00 g (7.18 mmol) of 2-(methylsulfanyl)aniline in 5 mL of MeOH with stirring. After refluxing the light-green solution overnight, the solution changed color to olive green. The solution was cooled to ambient temperature, and the solvent was removed under reduced pressure. The olive-green residue was dissolved in 10 mL of MeOH, and was placed in the freezer at 263 K with a needle-punctured rubber septum. Crystals formed from the solution, and, after several days, were collected by vacuum filtration and washed with cold hexanes. 0.780 g (50%) of yellow crystals were isolated.

Spectroscopic investigations
NMR spectra were recorded on a Bruker Avance II 400 MHz spectrometer operating at 400.17 MHz for 1 H and 100.6 MHz for 13 C, and were referenced to tetramethylsilane ( = 0 p.p.m.). High-resolution MS data were obtained using a Waters XevoG2 QToF instrument in positive electrospray ionization mode. Theoretical m/z values are reported for an abundance greater than 10% of base signal. UV-Vis spectra were recorded in quartz cuvettes on a Varian Cary 100 Bio UV-Vis spectrometer. FT-IR spectra were recorded on a Thermo Nicolet 6700 FT-IR Spectrometer as KBr pellet (approximately 1.5 mg compound in 300 mg anhydrous KBr) in the 4,000 cm À1 to 400 cm À1 range with 2 cm À1 resolution.
Spectroscopic measurements confirmed the structure of (1). High-resolution mass spectrometry gave an [M] + ion of 217.0833 m/z, close to the calculated mass of 217.0755 m/z and the IR spectrum of (1) exhibited an imine stretch of 1611 cm À1 that is characteristic for aniline-based imines. Absorbances located at 290 nm and 300 nm in the UV spectrum are characteristic of the -* transition of pyrrole and C N bonds, respectively. The -* transition of benzene is also present with an absorbance around 350 nm.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 2. Hydrogen atoms attached to carbons were assigned positions based on the sp 2 or sp 3 hybridization geometries of their attached atoms. Hydrogens attached to sp 2 -hybridized carbons were given isotropic displacement parameters U iso (H) = 1.2U iso (C) for their attached atoms, while methyl-group hydrogens were given isotropic displacement parameters U iso (H) = 1.5U eq (C) for their attached carbons. The coordinates and displacement parameters for the hydrogens attached to N2 and O1S were allowed to refine freely. Computer programs: APEX2 and SAINT (Bruker, 2013), SHELXD (Schneider & Sheldrick, 2002), SHELXL2014 (Sheldrick, 2015), SHELXTL (Sheldrick, 2008) and Mercury (Macrae et al., 2006).