Crystal structure of (μ-trans-1,2-bis{2-[(2-oxidophenyl)methylidene]hydrazin-1-ylidene}ethane-1,2-diolato-κ3 O,O′,N)bis[di-tert-butyltin(IV)]

In a binuclear complex containing two Sn4+ ions, connected by the doubly N-deprotonated oxalylbis[(2-oxidobenzylidene)hydrazide] ligands, and with each Sn4+ is linked to two tert-butyl groups, the coordination sphere of each Sn atom is best described as a trigonal bipyramid.

The binuclear complex, [Sn 2 (C 4 H 9 ) 4 (C 16 H 10 N 4 O 4 )], contains two Sn 4+ ions, connected by doubly N-deprotonated oxalylbis[(2-oxidobenzylidene)hydrazide] ligands, and each Sn 4+ ion is linked to two tert-butyl groups. The coordination sphere of each Sn atom is best described as a distorted trigonal bipyramid. Each stannic ion in the complex is in a C 2 O 2 N environment. The two homologous parts of the doubly deprotonated ligand are located in trans positions with respect to the C-C bond of the oxalamide group. The oxalamide group exhibits an asymmetric coordination geometry, as seen by the slight difference between the C-O and C-N bond lengths. The three-dimensional network is a multilayer of complex molecules with no strong supramolecular interactions.

Chemical context
Stannic Schiff base complexes formed using a salicylaldehyde derivative as a keto precursor have been widely studied in recent decades (Reisi et al., 2010;Kumar & Nath, 2018;Tan et al., 2017;Paul et al., 2014;Pé rez-Pé rez et al., 2016). These Schiff bases may have both hard-atom donors, such as nitrogen or oxygen (Stadler et al., 2009;Rehman et al., 2008;Yin et al., 2008), and/or soft-atom donors, such as sulfur (Hong et al., 2010), which allow them to bind to different types of metal ions, yielding complexes with interesting properties. Due to the ability of the Sn 4+ ion to form very stable complexes with Schiff bases or carbanions, many studies have been carried out with regard to their potential applications in medicine (Beltrá n et al., 2007), catalysis (Orita et al., 1999) and biotechnology (Pellerito & Nagy, 2002). Schiff bases with O and N hard-donor sites, which can generate five-and sixmembered rings upon coordination to metal ions, can be obtained from the condensation of a salicylaldehyde derivative and hydrazides (Pellerito & Nagy, 2002). Many research groups have designed hydrazone ligands to prepare metal complexes with particular properties. Thus, organotin(IV) complexes were synthesized from ligands having a hydrazone moiety. The antibacterial (Rehman et al., 2016), antifungal (Ö ztaş et al., 2009) and antitumour (Lee et al., 2015) properties of these complexes have been studied. The structures of these organotin(IV) complexes and their properties can be diverse depending on the number of alkyl groups linked to Sn 4+ (Lima et al., 2015;Luna-García et al., 2009). In this context, we have synthesized a symmetric ligand by a condensation reaction between salicylaldehyde and oxalohydrazide. This ligand was ISSN 2056-9890 used to synthesize the organostannic(IV) complex, the structure of which is described herein.

Structural commentary
The structure of the title complex is shown in Fig. 1 (Reichelt & Reuter, 2013 for each of the two Sn atoms can be characterized by the trigonality parameter = ( À )/60, with and being the two largest angles around Sn (Addison et al., 1984). The value of is 1 in the case of a trigonal bipyramidal geometry, whereas = 0 for a perfect square-based pyramid. In the case of our complex, the values of (0.44 for Sn1 and 0.41 for Sn2) indicate intermediate geometries between the two perfect environments. For the two Sn atoms, the comparison of the values of the angles found in the coordination sphere with the ideal values of the angles for trigonal bipyramidal geometry indicates that the environment around the Sn atoms is best described as a strongly distorted trigonal bipyramid. The bond angles between the tert-butyl groups around Sn [C-Sn-C = 128.35 (12) for Sn1 and 130.02 (12) for Sn2] result in compression of the bond angles with the third atom which forms the equatorial plane with the two tert-butyl groups [N-Sn-C = 113.85 (10) and 117.79 (10) for Sn1, and 113.63 (11) and 116.29 (10) for Sn2]. The sum of the angles in the basal planes are, respectively, 359.99 for Sn1 and 359.94 for Sn2. The O atoms occupy the apical positions with comparable angles of 154.61 (7) for Sn1 and 154.73 (7) for Sn2. The angles between the apical O atoms and the atoms in the basal plane are in the range 72.35 (7)-97.12 (11) for Sn1 and between 72.39 (6) and 96.48 (9) for Sn2. The ligand, which acts in a tridentate fashion, forms two rings upon coordination with the tin centres, i.e. a five-membered OCNNSn ring and a six-membered OCCCNSn ring, sharing atom N1 for Sn1 and N4 for Sn2. The angles resulting from the five-membered ring [N1-Sn1-O2 = 72.35 (7) and N4-Sn2-O3 = 72.39 (6) ] are much smaller than the angles resulting from the sixmembered ring [N1-Sn1-O1 = 82.32 (8) and N4-Sn2-O4 = 82.39 (7) ]. The better flexibility of the six-membered ring can explain this observed difference in values. The five-and six-membered rings obtained after coordination of the ligand are not planar, as indicated by the torsion angles for the two Sn atoms in the complex: Sn1-N1-N2-C8 0.6, Sn1-O2-C8-N2 0.5, Sn1-O1-C1-C6 6.3, Sn1-N1-C7-C6 À 2, Sn2-N4-N3-C9 2.1, Sn2-O3-C9-N3 À 1.2, Sn2-O4-C16-C11 À 3.7 and Sn2-N4-C10-C11 À 0.5 . For all four t Bu groups, the angles around the central C atom (Sn-C-C and C-C-C) vary in the range from 106.0 (3) to 112.3 (4) and indicate a tetrahedral environment around the central C atom. Both t Bu groups reveal an eclipsed conformation regarding the methyl groups. The C-C bond lengths are in the range 1.81 (5)-1.542 (9) Å and are comparable to the values found in the literature (Reichelt & Reuter, 2013  The molecular structure of the title compound, showing the atomnumbering scheme. Displacement ellipsoids are plotted at the 50% probability level.

Supramolecular features
The overall structure is a complex three-dimensional network which is constructed from neutral quasi-centrosymmetric complexes disposed in different orientations onto intersecting multilayers (Fig. 2). The complex molecules display no strong supramolecular interactions and there are no hydrogenbonding contacts in the crystal. This may be a consequence of a steric hindrance generated by the tert-butyl groups which could keep the complex molecules distant from each other.

Database survey
No information was found in the databases for this ligand.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were geometrically optimized and refined as riding, with U iso (H) = 1.2U eq (C) (1.5 for CH 3 groups).

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.