Bis(μ-benzoato-κ2 O:O′)bis(benzoato-κO)octabutyldi-μ3-oxido-tetratin(IV)

The asymmetric unit of [{Sn(C4H9)2(C6H5COO)}2O]2 consists of two independent centrosymmetric half formula units. Both molecules adopt the ladder structure typical for this class of dimeric tetraorganodistannoxane dicarboxylates. A nearly linear very short C—H⋯O contact gives rise to a chain-like arrangement of molecules in the [111] direction

The asymmetric unit of the title compound, [{Sn(C 4 H 9 ) 2 (C 6 H 5 COO)} 2 O] 2 , consists of two half molecules, completed by application of inversion symmetry. Both molecules adopt a ladder structure typical for this class of dimeric tetraorganodistannoxane dicarboxylates characterized by a centrosymmetric four-membered (Sn-O) 2 ring of rhomboidal shape that is extended on both sides by folded six-membered Sn-O-C rings. To a first approximation, both kinds of Sn atoms (Sn i and Sn o ) are trigonal-bipyramidally coordinated. The bond angles between the n-butyl groups are widened [135.64 (7)-146.20 (7) ] in comparison with an ideal trigonal bipyramid. Sn-O bond lengths within the {R 2 SnO 3 } coordination sphere depend strongly on the position of the corresponding O atom -axial (ax) or equatorial (eq) -as well as on the functionality of the carboxylate groups which exhibit 2 (-COO i ) and 1 (-COO o ) coordination modes, respectively. In summary, the following sequence of distances [ (12) Å ]. The n-butyl groups adopt an anti-anti conformation with exception of two disordered outer n-butyl groups of the second molecule which exhibit gaucheanti and anti-gauche conformations. Weak intramolecular SnÁ Á ÁO interactions between the different O atoms of the outer carboxyl groups with the inner, as well as outer, Sn atoms give rise to a strongly distorted octahedral coordination at these Sn atoms. Intermolecular interactions between the individual molecules are restricted to van der Waals and OÁ Á ÁH-C interactions of which a nearly linear very short C-HÁ Á ÁO contact between the H atom of the phenyl group of one of the molecules with the outer non-coordinating C O group of the other molecule is the most prominent. It gives rise to a chain-like arrangement of the molecules along [111]. The two n-butyl groups attached to the outer Sn atom of one molecule are disordered over two sets of sites with occupancies of 0.806 (3)/ 0.194 (3) and 0.702 (3)/0.298 (3).

Chemical context
Our focus on organotin(IV) carboxylates is due to the variety of architectures and the diverse applications displayed by those compounds (Davies, 1997;Chandrasekhar et al., 2008). In our search of new structures displayed by these compounds and their derivatives, we recently reported the structure of monomeric di-n-butyltin (IV) dibenzoate, nBu 2 Sn(OOCPh) 2 (Reuter & Okio, 2016), with the tin atom sixfold coordinated via intramolecular complexation. While that compound has been synthesized by the reaction of di-n-butyltin(IV) oxide, nBu 2 SnO, with benzoic acid, PhCOOH, in a molar ratio of 1:2, we herein present the structure of [{nBu 2 Sn(OOCPh)} 2 O] 2 obtained from the same reactants using a molar ratio of 1:1. ISSN 2056-9890

Structural commentary
The title compound ( Fig. 1) crystallizes with two formula units [{nBu 2 Sn(OOCPh)} 2 O] 2 in space group P1. The asymmetric unit consists of one formula unit composed of two half molecules, completed by application of inversion symmetry (Fig. 1). With the exception of both n-butyl groups attached to the outer Sn atom (Sn4) of the second molecule, all parts of the structure are well ordered. The disorder of the two n-butyl groups was managed by a split model with site occupancies of 0.806 (3)/0.194 (3) and 0.702 (3)/0.298 (3). No further consideration will be made for the structural parameters of those nbutyl groups. It is noteworthy, however, that this disorder is caused by the conformational flexibility of the n-butyl group which adopts -in the case of the major/minor components -a gauche-anti/anti-gauche and anti-gauche/anti-gauche confor-

Figure 2
Main types of conformations adopted by the n-butyl groups of the title compound: (a) anti-anti, (b) anti-gauche and (c) gauche-anti. Displacement ellipsoids of the non-H atoms are drawn at the 50% probability level and bonds to Sn atoms are indicated as short sticks. mation with respect to the Sn-C -C -C and C -C -C -C torsion angles (Fig. 2b,c). This conformation of the disordered n-butyl groups is in contrast to the conformation of all other n-butyl groups of both dimers, which show exclusively an anti-anti conformation (Fig. 2a). Structural parameters (Table 1) within the ordered n-butyl groups follow the general trends: d(C-C) mean = 1.521 (6) Å , h(C-C / -C) mean = 112.6 (11) while bond angles at C range from 112.5 (1) to 123.1 (1) . Sn-C distances are in the range of 2.127 (2)-2.134 (2) Å , mean value 2.130 (3) Å . The Sn-O framework of both molecules adopts the ladder structure typical for this class of tetraorganodistannoxane dicarboxylates (Chandrasekhar et al., 2008). This ladder-type structure is characterized by a central, four-membered (Sn-O) 2 ring on both sides extended by six-membered Sn-O-C rings. Its two inner (Sn i ) and two outer (Sn o ) tin atoms areto a first approximation -fivefold, trigonal-bipyramidally coordinated and linked together via two 2 -coordinating oxygen atoms (O i ) and two chelating (-COO i ) carboxylate groups. The structure is completed by two monodentate carboxylate groups (-COO o ) attached to the outer tin atoms (Fig. 3).  (2)], in accordance with a delocalized -system. In the two benzoate ligands, the carboxylate groups and the phenyl groups are not co-planar, but are inclined to each other at angles of 15.1 (2), 14.8 (3)/3.9 (3) and 17.3 (1) .
Another characteristic feature of the molecular structure comprises some additional, very weak interactions [d(SnÁ Á ÁO) = 2.7857 (2)/2.7141 (2) Å ] of the 1 -O atoms (O21/O41) of the outer carboxylate groups with the inner tin atoms (Sn1/Sn3), while those of the 0 -O atoms (O22/O42) of the outer carboxylate groups with the outer tin atoms (Sn2/Sn4) are still longer [2.8901 (2)/2.9883 (2) Å ]. Taking these weak interactions into account, both kinds of Sn atoms adopt a strongly distorted octahedral coordination. All bonding features (except the last ones) of the two molecules are summarized in Fig. 5, which also visualizes the major structural differences between the molecules as a result of the different orientations of the n-butyl groups relative to the Sn-O framework. Location of axial (ax) and equatorial (eq) bonds with respect to the trigonal-bipyramidal coordination at the inner (Sn i ) and outer (Sn o ) Sn atoms and of the outer (-COO o ) 1 -and inner (-COO i ) 2 -carboxylate groups.

Figure 4
Folded conformation of the outer six-membered Sn-O-C rings of molecule 1 as an example. Displacement ellipsoids are drawn at the 50% probability level and bonds to C atoms are indicated as short sticks. contact between the hydrogen atom H26 of a phenyl ring of molecule 1 and the non-coordinating oxygen atom O42 of the second molecule attracts attention as it leads to a chain-like arrangement of the two molecules along [111] (Fig. 6). All other C-HÁ Á ÁO C contacts are longer than 2.73 Å .

Database survey
Tetraorganodistannoxane dicarboxylates, [R 2 Sn(OOCR 0 )] 2 O, have been extensively structurally characterized. The Cambridge Structural Database (Groom et al., 2016) quotes as many as 214 entries (date: 10.01.2017). The majority of organic moieties attached to tin are found to be n-butyl (145) while for the dicarboxylates benzoic acid derivatives (90) are the most studied. Even for the combination of R = nBu and R 0 = benzoic acid derivatives not less than 67 structures are described, but from the parent compound with R 0 = PhCOO À , only the structure of the methyl compound (R = Me) has been completely characterized (Amini et al., 2002).   The short, nearly linear, C-HÁ Á ÁO C interactions (dashed sticks, blue) between two different neighbouring molecules responsible for the chain-like arrangement along the [111] direction. 0.147 g (1.2 mmol) of benzoic acid in ethanol under reflux for 3.5 h. After removal of the solvent, single crystals were obtained by recrystallization of the solid from ethanol/nhexane.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Most of the hydrogen atoms were clearly identified in difference Fourier syntheses. Their posi-tions were calculated assuming idealized geometries and allowed to ride on the carbon atoms with C-H = 0.98 Å (-CH 3 ), 0.99 Å (-CH 2 -), and 0.95 Å (C-H arom ) using one common isotropic displacement parameter for each n-butyl and phenyl group. Disorder of both n-butyl groups at the outer Sn atom (Sn4) of the second molecule was refined using a split model with site occupancies of 0.806 (3)/0.194 (3) and 0.702 (3)/0.298 (3). In order for the structural model to be chemically meaningful, the atomic positions of the minor components were restrained to a target value for the C-C distance [d(C-C) = 1.526 (3) Å ] and displacement parameters were taken from the chemically equivalent C atoms of the major occupancy component.   program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2006) and Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Bis(µ-benzoato-κ 2 O:O′)bis(benzoato-κO)octabutyldi-µ 3 -oxido-tetratin(IV)
Crystal data 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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )