Crystal structure of bis{μ-(E)-2-[(2-oxidophenylimino)methyl]quinolin-8-olato-κ4 O,N,N′,O′}bis[dibutyltin(IV)]

The title compound crystallizes with two independent centrosymmetric dimers in the unit cell, each featuring a typical pincer-type structure where the dianionic ligand is tetradentate, coordinating to the central tin atom through both phenolate oxygen atoms, as well as through the quinoline and imine N atoms. Each metal atom adopts a distorted pentagonal–bipyramidal SnC2N2O3 coordination arising from the N,N′,O,O′-tetradentate deprotonated Schiff base and two butyl groups in the axial sites.

Condensation of 8-hydroxyquinoline-2-carbaldehyde with 2-aminophenol gave the (E)-2-[(2-hydroxyphenylimino)methyl]quinolin-8-ol derivative that reacted with di-n-butyltin oxide with release of H 2 O to yield the chelate title complex, [Sn 2 (C 4 H 9 ) 4 (C 16 H 10 N 2 O 2 ) 2 ]. The compound crystallizes in the triclinic space group P1, with two independent centrosymmetric dimers in the unit cell. Each features a typical pincer-type structure where the dianionic ligand is tetradentate, coordinating to the central tin atom through both phenolate oxygen atoms, as well as through the quinoline and imine N atoms. Each metal atom adopts a distorted pentagonal-bipyramidal SnC 2 N 2 O 3 coordination arising from the N,N 0 ,O,O 0 -tetradentate deprotonated Schiff base, one bridging phenolate O atom of the neighbouring ligand and two butyl groups in the axial sites.

Chemical context
We are interested in the preparation of organometallic tin compounds derived from biologically active molecules. One of the aims of our research is the structural analysis, particularly their coordination modes which has influence on their biological effects. The title compound (I) includes a ligand derived from quinoline and 2-aminophenol. It has been reported that quinoline-bearing structures show broad biological activities such as antifungal (Musiol et al., 2006), antimalarial (Nasveld & Kitchener, 2005), and antitumor (Rasoul-Amini et al., 2006). The activity of bis-quinolines as antileshmanial agents has also been reported through in vitro and in vivo studies (Palit et al., 2009). More recently, it has been shown that quinoline-based thiosemicarbazones present antitumor efficacy involving an iron chelation mechanism (Serda et al., 2012). In addition, Schiff bases derived from 8-hydroxyquinoline and its derivatives are well known for their ability towards the complexation of many metals (Charles & Perrotto, 1964;Corcé et al., 2014;Albrecht et al., 2005Albrecht et al., , 2007. We report here the crystal structure of a new tin(IV) complex derived from a ligand produced from the 1:1 condensation of 8-hydroxyquinoline-2-carboxaldehyde and 2-aminophenol. The Schiff base H 2 L produced was complexed with di-n-butyltin oxide to give the title compound (I), [Sn 2 (C 4 H 9 ) 4 (C 16 H 10 N 2 O 2 ) 2 ]. ISSN 2056-9890

Structural commentary
The molecular structures of the two independent molecules in the compound (I) are shown in Fig. 1. The structure consists of an isolated homobimetallic dimer located on crystallographic inversion centres so that two independent n-Bu 2 SnL units [molecule 1: Sn1 to C126 and molecule 2: Sn2 to C226 (with the n-butyl-disordered C119-C222 atoms being slightly disordered at the terminal methyl end and the C223-C226 atoms heavily disordered with a threefold splitting)] comprise the asymmetric unit. The dimerization of these monomeric units occurs through the quinolin-8-olate group oxygen atoms, leading to a central four membered (SnO) 2 ring with a metalmetal separations Sn1Á Á Á Sn1(1 À x, 1 À y, Àz) 3.9593 (5) Å and Sn2Á Á Á Sn2(1 À x, Ày, 1 À z) = 4.0132 (5) Å .

Supramolecular features
The lozenge planes of the two independent molecules are nearly perpendicular [angle between planes = 84.20 (3) ], giving rise to several weak C-HÁ Á Á interactions between the n-butyl groups attached to the Sn atoms and the aromatic H atoms of the ligand (Table 1). These interactions are complemented by C-HÁ Á ÁO hydrogen-bonding interactions between the adjacent chains (Table 1, Fig. 2a). Such interactions generate a two-dimensional supramolecular structure parallel to the bc plane (Fig. 2b).

Database survey
There are five examples in the literature of dialkylditin(IV) compounds with bis( 2 -quinolin-8-olato) ligands (Vafaee et al., 2010;Basu Baul et al., 2009). All of these feature an octahedral coordination sphere for the tin atoms. The most The molecular structures of the two independent molecules in the title compound (I). Displacement ellipsoids are drawn at the 50% probability level. The major part of the disordered n-butyl group is shown with green bonds, while the minor components are shown with magenta and blue bonds. H atoms are omitted for clarity. curious feature of the structure of the title compound is the sevenfold coordination of each Sn atom in the binuclear core, although this coordination number is not unprecedented in the structural chemistry of tin (de Sousa et al., 2009). Only the diorganotin(IV) complexes of pyruvic acid picolinoacylhydrazone (Cui et al., 2010) share the characteristic of being formed by four rings (three five-membered rings and one fourmembered ring) and both are centrosymmetric. In pyruvic acid picolinoacylhydrazone, the four-membered Sn 2 O 2 ring shares two edges with two other five-membered rings. Meanwhile, in the title compound each ring is only fused to one another, giving rise to a more extended structure. Previously, representative elements (Sun et al., 2011), transition (Anitha et al., 2015;García-Santos et al., 2009;Yan et al., 2014) and lanthanide (Zhang et al., 2012(Zhang et al., , 2015 metal complexes of [(imino)methyl]quinolin-8-olato derivatives have been reported, and only in the case of the lanthanide complexes is the nitrogen atom of the imine group involved in the ligand coordination. So, to the best of our knowledge, the title compound is the first example of a [(imino)methyl]quinolin-8olato derivative with the ligand using the full possible denticity.

Synthesis and crystallization
3-Hydroxyquinoline-2-carboxaldehyde, 2-aminophenol, di-nbutyltin(IV) oxide and solvents were purchased from Aldrich and used without further purification. Elemental analysis were performed using an Eager 300 analyzer. The infrared spectra were recorded on Perkin Elmer 1600 FT spectrometer in the 4000-400 cm À1 range. Melting points were measured on a Fisher-Johns melting-point apparatus and are uncorrected. 119 Sn spectra were recorded with a Bruker AVANCE-II, 300 MHz NMR spectrometer operating at 111.81 MHz and using a 4mm CP-MAS probe. NMR 119 Sn chemical shift referencing is toward tetramethyltin.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were placed in calculated positions (C-H = 0.95-0.99 Å ) and treated in the riding approximation with isotropic displacement parameters set at 1.2-1.5 times the U eq value of the parent atom. The n-butyl groups in molecule 2 display some degree of orientational disorder, which was modeled into two orientations using geometrical (SADI, SIMU) and ADP (SIMU, DELU) restraints.