Crystal structure of 2-(methylamino)tropone

The title compound crystallizes in the monoclinic space group P21/c with three independent molecules in the asymmetric unit. Two types of hydrogen-bonding interactions, C—H⋯O and N—H⋯O, are observed, as well as bifurcation of these interactions. The N—H⋯O interactions link molecules to form infinite chains. The packing of molecules in the unit cell shows a pattern of overlapping aromatic rings, forming column-like formations. π–π interactions are observed between the overlapping aromatic rings.

Tropolone and its derivatives are versatile ligands used in inorganic and organometallic chemistry (Roesky, 2000;Dias et al., 1995;Nozoe et al., 1997;Schutte et al., 2010;Steyl et al., 2010). The carbonyl oxygen and vicinal coordinating substituent, specifically nitrogen in this study, impart a metalchelating ability to these types of ligands. The complexes of these ligands with first and second row transition elements have increased over the past few decades. The ligands of importance in this study and future work, namely 2-(alkylamino)tropones and aminotroponimines, are N,O and N,N 0 bidentate, monoanionic ligands containing a ten -electron backbone (Roesky, 2000). The -conjugated backbone is characteristic of these ligands (Nishinaga et al., 2010). Considering the above-mentioned characteristics, tropolone could be considered analogous to the O-donor 2 -O,O 0 acetylacetonate ligand (acac-O,O 0 ). The tropolone bidentate ligand differs from the acac-O,O 0 ligand in a few ways. Of importance to our study is the larger aromatic delocalization, which could afford greater polarizability. Tropolone is also ISSN 2056-9890 more acidic than the acac-O,O 0 ligand. The acidity of the O,N and N,N 0 bidentate ligands used in our study and the effect thereof on the chelating ability could be compared to these O,O 0 bidentate ligands described in the literature. The ligandmetal-ligand angle, better known as the 'bite angle', would also be smaller for a tropolone-derived complex, since it would form a five-membered metallocycle instead of a sixmembered one as with acac-O,O 0 (Bhalla et al., 2005). This could show interesting steric and electronic influences at the metal centre and could be further compared to the steric and electronic studies conveyed on -diketone moieties in similar metal complexes by Manicum et al. (2018). These ligands, including the title compound, 2-(methylamino)tropone, will form part of the synthesis of water-soluble complexes of rhenium(I) tricarbonyl, gallium(III) and copper(II). Rhenium(I) , gallium(III) (Green & Welch, 1989) and copper(II) (Boschi et al., 2018) are highly utilized radioisotopes in the radiopharmaceutical industry.
When designing diagnostic or therapeutic radiopharmaceuticals, certain mechanistic aspects are very important, as it is the basis on which some predictions are made regarding the in vivo behaviour. Kinetic studies, utilizing different techniques, are executed to determine the reaction mechanisms by which the proposed radiopharmaceutical complexes will form and react. Results of such studies are important in nuclear medicine as it gives indications regarding the in vivo stability, uptake and excretion as well as the pharmacokinetics of the compounds. Kinetic investigations by Schutte et al. (2011Schutte et al. ( , 2012, Schutte-Smith et al. (2019) and Manicum et al. (2019) were done on rhenium(I) tricarbonyl tropolonato complexes with satisfying results and conclusions. In the study, methanol substitution was studied using entering nucleophiles in fac-[Re(Trop)(CO) 3 (MeOH)]. The kinetic study performed at high pressure indicated positive volumes of activation for all of the reactions studied. This was a clear indication towards a dissociative interchange mechanism.
The application of these ligands in coordination chemistry could be further increased by adding electron donating or withdrawing moieties to the nitrogen atom.

Structural commentary
2-(Methylamino)tropone crystallizes in the monoclinic P2 1 /c space group with three independent molecules, A, B and C, in the asymmetric unit (Fig. 1). The bond distances and angles of the three molecules agree well with each other and with those in similar structures (Barret et al., 2014;Dwivedi et (12) ] are slightly larger than the usual 120 for trigonal-planar bond angles, because of the steric influence of the methyl group. These angles are close to the same angle in 2-(benzylamino)tropone [125.09 (12) ; Barret et al., 2014]. This could be compared to the large angle in 2-(t-butylamino)tropone [131.9 (2) ; Siwatch et al., 2011], which deviates even more from 120 due to the highly steric tertiary butyl group. A plane fitted through the seven ring carbon atoms of the three molecules in the asymmetric unit indicates that the molecules are planar. The root-mean-square deviations of molecules A, B and C from the planes are 0.0141 (12), 0.0261 (11) and 0.0345 (11) Å , respectively. The C8-N1-C2-C3 torsion angle, which involves the methyl group, differs for molecule A [À0.8 (2)  The molecular structure of 2-(methylamino)tropone, indicating the numbering scheme, with displacement ellipsoids drawn at the 50% probability level.

Supramolecular features
Nine hydrogen-bonding interactions, three C-HÁ Á ÁO and six N-HÁ Á ÁO, are observed (Table 1 and Fig. 2). Infinite chains are formed along [001]. These supramolecular chains are formed through N-HÁ Á ÁO interactions linking the molecules together. As in the crystal structure of tropolone (Shimanouchi & Sasada, 1973), bifurcation of the hydrogen bonds take place. Bifurcation, also known as the over-coordination of a hydrogen bond, creates both inter-and intramolecular branches, which might contribute to the stability of the structures. This is an interesting phenomenon seen in the orientation of water molecules, where the distribution of acceptor hydrogen bonds, terminating at the lone pairs of the oxygen, is higher (Markovitch & Agmon, 2008). This forms over-coordinated oxygens and could also be seen in this crystal structure (Fig. 2). These interactions clearly contribute to the array of the molecules in the asymmetric unit (Fig. 2). The molecules show an interesting packing format in the unit cell. 'Column'-like structures are formed by molecule B packing in a head-to-tail pattern with the aromatic rings overlapping (Fig. 3). A -interaction is observed, with a perpendicular distance of 3.4462 (19) Å between the overlapping aromatic rings of two inversion-related B molecules (Fig. 4). These - Packing of molecules viewed perpendicular to the ac plane. Table 1 Hydrogen-bond geometry (Å , ). (17) (2)  121 Symmetry codes: (i) x; Ày þ 3 2 ; z À 1 2 ; (ii) x; y À 1; z.

Figure 2
Hydrogen-bonding interactions (Table 1) and infinite chains along [001] in the unit cell.
format of the molecules in the unit cell, but could also assist in the formation of one-dimensional infinite chains, as Wong et al. (2018) have found in water-soluble platinum (II) salts.

Synthesis and crystallization
Tropolone (505 mg, 4.132 mmol) was dissolved in 20 mL of a 40% methylamine solution. The reaction mixture was stirred at room temperature for 7 d. The product was extracted three times with 30 mL of chloroform, and the organic layer was washed with 50 mL of water.

2-(Methylamino)cyclohepta-2,4,6-trien-1-one
Crystal data Extinction correction: SHELXL2018 (Sheldrick, 2015) Extinction coefficient: 0.0113 (9) Special details Experimental. The intensity data was collected on a Bruker X8 ApexII 4K Kappa CCD diffractometer using an exposure time of 10 seconds/frame. A total of 1436 frames was collected with a frame width of 0.5° covering up to θ = 27.99° with 99.7% completeness accomplished. 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.