[N′-(4-Decyloxy-2-oxidobenzylidene)-3-hydroxy-2-naphthohydrazidato-κ3 N,O,O′]dimethyltin(IV): crystal structure and Hirshfeld surface analysis

A highly distorted pentacoordinated C2NO2 geometry is observed for the Sn atom owing to a tridentate mode of coordination of the Schiff base ligand and the restricted bite angles it subtends. In the crystal, supramolecular layers sustained by C—H⋯O, π–π, C—H⋯π(arene) and C—H⋯π(chelate ring) interactions are formed.

The title diorganotin compound, [Sn(CH 3 ) 2 (C 28 H 32 N 2 O 4 )], features a distorted SnC 2 NO 2 coordination geometry almost intermediate between ideal trigonalbipyramidal and square-pyramidal. The dianionic Schiff base ligand coordinates in a tridentate fashion via two alkoxide O and hydrazinyl N atoms; an intramolecular hydroxy-O-HÁ Á ÁN(hydrazinyl) hydrogen bond is noted. The alkoxy chain has an all-trans conformation, and to the first approximation, the molecule has local mirror symmetry relating the two Sn-bound methyl groups. Supramolecular layers sustained by imine-C-HÁ Á ÁO(hydroxy), -[between decyloxy-substituted benzene rings with an inter-centroid separation of 3.7724 (13) Å ], C-HÁ Á Á(arene) and C-HÁ Á Á(chelate ring) interactions are formed in the crystal; layers stack along the c axis with no directional interactions between them. The presence of C-HÁ Á Á(chelate ring) interactions in the crystal is clearly evident from an analysis of the calculated Hirshfeld surface.

Chemical context
Organotin(IV) compounds with Schiff base ligands have been actively studied because of their versatile chemistry, e.g. solution versus solid-state structures, and their potential as biologically active compounds such as in anti-cancer and antimicrobial applications (Davies et al., 2008;Nath & Saini, 2011). Among these Schiff base ligands, those derived from 3-hydroxy-2-napthoic hydrazide have long been known to have promising anti-microbial (Dogan et al., 1998b) and anticonvulsant activities (Dogan et al., 1998a). Subsequently, various organotin compounds derived from these Schiff base ligands have been prepared and their anti-cancer potential explored (Lee et al., 2012(Lee et al., , 2013. These studies have revealed interesting biological activities and often correlations were possible with their solid-state structures (Lee et al., 2009(Lee et al., , 2010. Complementary studies on vanadium complexes with these Schiff base ligands focused upon their urease inhibitory activities (You et al., 2012). In addition, the catalytic properties of vanadium (Hosseini-Monfared et al., 2010, cerium (Jiao et al., 2014) and palladium complexes (Arumugam et al., 2015) have been explored. Further, structural data for copper (Liu et al., 2012), molybdenum (Miao, 2012) and vanadium (Kurup et al., 2010) complexes are available. As part of our ongoing work with these ONO tridentate ligands (Lee et al., ISSN 2056-9890 2013), we hereby describe the crystal and molecular structures of the title compound, (I), as well as a detailed analysis of the intermolecular associations through a Hirshfeld surface analysis.

Structural commentary
The tin(IV) atom in (I), Fig. 1, is complexed by a di-anionic, tridentate Schiff base ligand noteworthy for the appended fused-ring system and for the long alkoxy chain substituent. The five-coordinate geometry is completed by two Sn-bound methyl groups, Table 1. The resulting C 2 NO 2 coordination geometry is highly distorted with the value of being 0.52, i.e. almost exactly intermediate between ideal square-pyramidal ( = 0) and trigonal-bipyramidal ( = 1.0) (Addison et al., 1984). The widest angle at the tin atom is subtended by the two alkoxide-O atoms, i.e. 157.14 (6) , with the other angles ranging from an acute 73.16 (6) , for O1- to 125.89 (9) , being subtended by the two Sn-bound methyl groups.
The five-membered, SnON 2 C chelate ring is almost planar with a r.m.s. deviation of 0.0222 Å and in the same way, the sixmembered, SnONC 3 ring is close to planar with a r.m.s. deviation of 0.0155 Å ; the dihedral angle between the chelate rings is small, being 2.90 (4) . The bond lengths involving the nitrogen atoms comprising the backbone of the chelate rings suggest some conjugation, i.e. N1-C1, N1-N2 and N2-C12 are 1.317 (3), 1.397 (2) and 1.303 (3) Å , respectively. The 10 atoms of the fused-ring system appended to the fivemembered chelate ring make a dihedral angle of 2.01 (3) with the chelate ring, a conformation allowing the formation of an intramolecular hydroxy-O-HÁ Á ÁN(hydrazinyl) hydrogen bond to close an S(6) loop, Table 2. The dihedral angle between the six-membered and fused benzene rings is 1.12 (5) , indicating a strictly co-planar relationship. Significant planarity in the molecule is indicated by the dihedral angle of 5.84 (4) between the appended fused-ring system at C1 and the fused benzene ring. In addition, the decyloxy side chain has an all-trans conformation with the range of torsion angles being À174.96 (18) , for C21-C22-C23-C24, to 179.79 (19) , for C25-C26-C27-C28. Indeed, the r.m.s. deviation for the least-squares plane through all non-hydrogen atoms except the Sn-bound methyl groups is relatively small at 0.1179 Å , with maximum deviations being for the terminal methyl group of the alkoxy chain, i.e. 0.296 (2) Å , and a central methylene-C22 atom, i.e. 0.194 (2) Å . Hence, to a first approximation, the molecule has mirror symmetry, relating the two Sn-bound methyl groups.

Supramolecular features
Aside from participating in an intramolecular hydroxy-O-HÁ Á ÁN(hydrazinyl) hydrogen bond, the hydroxy-O atom accepts an interaction from a centrosymmetrically-related imine-H atom, Table 2. This has the result that a 16-membered {Á Á ÁOC 3 N 2 CH} 2 synthon is formed, which encapsulates two six-membered {Á Á ÁHOC 3 N} synthons formed by the intramolecular hydroxy-O-HÁ Á ÁN(hydrazinyl) hydrogen bonding mentioned above, Fig. 2a. Centrosymmetrically related dimeric aggregates are linked viainteractions between decyloxy-substituted benzene rings [inter-centroid separation = 3.7724 (13) Å for symmetry operation: 1 À x, 1 À y, 1 À z]. The remaining interactions are of the type C-HÁ Á Á and involve methylene-C-H exclusively. While two of the interactions have benzene rings as acceptors, the other two have chelate rings as acceptors, i.e. are of the type C-HÁ Á Á(chelate), a phenomenon gaining increasing attention (Tiekink, 2017); Table 2. Taken alone, the C-HÁ Á Á interactions lead to supramolecular chains as illustrated in Fig. 2b. The result of all of the identified intermolecular interactions is the formation of supramolecular layers that stack along the c axis with no directional interactions between them, Fig. 2c. The molecular structure of (I), showing the atom-labelling scheme and displacement ellipsoids at the 70% probability level.  Hydrogen-bond geometry (Å , ).

Hirshfeld surface analysis
The Hirshfeld surface analysis for (I) was performed as described in a recent publication of a related organotin structure (Mohamad et al., 2017). From the view of the Hirshfeld surface mapped over d norm , in the range À0.053 to + 1.621 au, Fig. 3, the bright-red spots appearing near the hy-droxy-O2 and imine-H12 atoms represent the acceptor and donor of the intermolecular C-HÁ Á ÁO interaction forming the {Á Á ÁOC 3 N 2 CH} 2 synthon as discussed in the previous section; these are also viewed as blue and red regions near the H and O atoms on the Hirshfeld surface mapped over electrostatic potential (over the range AE 0.075 au), Fig

Figure 3
Hirshfeld surface for (I), mapped over d norm in the range À0.053 to 1.621 au.

Figure 4
A view of Hirshfeld surface for (I), mapped over the electrostatic potential in the range AE0.075 au.
sponding to positive and negative potentials, respectively. In the absence of more conventional hydrogen bonds in the packing of (I), the structure contains two types of C-HÁ Á Á interactions. The donors and acceptors of the C-HÁ Á Á(arene) contacts are also viewed as respective light-blue and red regions on the Hirshfeld surface mapped over electrostatic potential, Fig. 4. In Fig. 5, the bright-orange spots enclosed within the circles around chelate (blue circle) and benzene (red) rings on the d e mapped Hirshfeld surface, Fig. 5, illustrate all acceptors of the C-HÁ Á Á contacts. The immediate environment about a reference molecule within the Hirshfeld surface mapped with the shape-index property is illustrated in Fig. 6. The C-HÁ Á Á(chelate) and C19-H19AÁ Á Á(C13-C18) contacts at 1 À x, Ày, 1 À z and their reciprocal contacts, i.e. Á Á ÁH-C, are represented with blue and white dotted lines, respectively, in Fig. 6a. The other C-HÁ Á Á contacts involving benzene rings andstacking interactions at 1 À x, 1 À y, 1 À z are illustrated in Fig. 6b. The overall two-dimensional fingerprint plot and those delineated into HÁ Á ÁH, CÁ Á ÁH/HÁ Á ÁC, OÁ Á ÁH/HÁ Á ÁO, NÁ Á ÁH/  Table 3. The most notable observation from the Hirshfeld surface analysis of the structure of (I) is that hydrogen atoms are involved in the overwhelming majority of surface contacts, i.e. 97.0%. Two views of the Hirshfeld surface for (I) mapped over d e , showing intermolecular C-HÁ Á Á interactions involving the chelate and benzene rings of a reference molecule highlighted with blue and red circles, respectively. Refer to Table 2 for designations of rings 1-4. Ring 5 comprises the (C13-C18) atoms.

Figure 6
Two views of Hirshfeld surface for (I) mapped with shape-index property about a reference molecule. The C-HÁ Á Á and Á Á ÁH-C interactions in both (a) and (b) are indicated with blue and white dotted lines, respectively. The yellow dotted lines in (b) indicatestacking between benzene (C13-C18) rings.  Fig. 7b, is due to a short interatomic contact between benzene-H18 and methylene-H25A atoms, Table 4. The involvement of methylene-H atoms in C-HÁ Á Á interactions with the arene and chelate rings results in the second largest contribution to the overall Hirshfeld surface, i.e. 20.9%, in the form of CÁ Á ÁH/ HÁ Á ÁC contacts, Fig. 7c. The short interatomic CÁ Á ÁH/HÁ Á ÁC contact between the ring-C18 and methylene-H19A atoms, Table 4, accounts for the presence of an interaction between these atoms. Another short interatomic CÁ Á ÁH/HÁ Á ÁC contact, namely C10Á Á ÁH18 (Table 4), is merged in the corresponding plot of Fig. 7c. The presence of two C-HÁ Á Á(chelate) interactions, Table 2, can be easily recognized from the fingerprint plots delineated into CÁ Á ÁH/HÁ Á ÁC and NÁ Á ÁH/ HÁ Á ÁN contacts, Fig. 7c and e, as their ring centroids (Cg1 and Cg2; Table 2) are close to the N and C atoms of the chelate rings and so provide discernible contributions to the Hirshfeld surface. A recent study also confirmed the impact of C-HÁ Á Á(chelate) interactions upon the Hirshfeld surface of a metal-organic compound (Jotani et al., 2016). A pair of short spikes with tips at d e + d i $2.5 Å on the parabolic distribution of points around d e + $ 2.7 Å shown by a pair of red arcs in Fig. 7d are the result of C-HÁ Á ÁO and short interatomic OÁ Á ÁH/HÁ Á ÁO contacts, Table 4. A small but recognizable contribution, i.e. 1.8%, from CÁ Á ÁC contacts to the Hirshfeld surface is assigned tostacking interactions between symmetry-related (C13-C18) benzene rings, and appears as an arrow-like distribution of points around d e = $1.9 Å in Fig. 7f. The other contacts, having low percentage contribution to the surface, are likely to have a negligible effect on the molecular packing.

Database survey
According to a search of the crystallographic literature (Groom et al., 2016), there are approximately 100 diorganotin structures with Schiff base ligands having an O-C N-N C-C . . . C-O backbone, as in (I). Of these, 13 have the 3hydroxynaphthalene residue, reflecting the biological interest in these compounds (see Chemical context). Two dimethyltin structures are available with identical ligands apart from having a substituent in the 5-position, i.e. chloride (Lee et al., 2009) and bromide (Lee et al., 2010), rather than in the 4position as for (I); the two halide structures are isostructural. An overlap diagram of (I) and the two 5-halide derivatives is shown in Fig. 8, which highlights the similarity between the structures. This borne out by the values of (Addison et al., 1984), i.e. 0.47 and 0.46 for the chloride and bromide structures, respectively, cf. 0.52 for (I).

Synthesis and crystallization
All chemicals and solvents were used as purchased without purification, and all reactions were carried out under ambient conditions. The melting point was determined using an Electrothermal digital melting point apparatus and was uncor- Overlap diagram of (I), red image, the 5-Cl analogue (green) and the 5-Br analogue (blue). The molecules have been arranged so that the fivemembered chelate rings are superimposed.   rected. The IR spectrum was obtained on a Perkin Elmer Spectrum 400 FT Mid-IR/Far-IR spectrophotometer from 4000 to 400 cm À1 . The 1 H NMR spectrum was recorded at room temperature in DMSO-d 6 solution on a Jeol ECA 400 MHz FT-NMR spectrometer. N-(4-Decoxy-2-oxidobenzylidene)-3-hydroxy-2-napthohydrazide (1.0 mmol, 0.463 g) and triethylamine (1.0 mmol, 0.14 ml) in ethyl acetate (25 ml) were added to dimethyltin dichloride (1.0 mmol, 0.220 g) in ethyl acetate (10 ml). The resulting mixture was stirred and refluxed for 3 h. The filtrate was evaporated until a precipitate was obtained. The precipitate was recrystallized from dichloromethane:dimethylformamide (1:1), and yellow prismatic crystals suitable for X-ray crystallographic studies were obtained from the slow evaporation of the filtrate. Yield