Synthesis, structure determination and characterization by UV–Vis and IR spectroscopy of bis(diisopropylammonium) cis-dichloridobis(oxalato-κ2 O 1,O 2)stannate(IV)

In the crystal structure of the title compound, (C6H16N)2[Sn(C2O4)2Cl2], the cations are linked to the anions by N—H⋯O hydrogen bonds to generate chains along the c-axis direction. Only van der Waals interactions are observed between the chains.

The organic-inorganic title salt, (C 6 H 16 N) 2 [Sn(C 2 O 4 ) 2 Cl 2 ] or ( i Pr 2 NH 2 ) 2 -[Sn(C 2 O 4 ) 2 Cl 2 ], was obtained by reacting bis(diisopropylammonium) oxalate with tin(IV) chloride dihydrate in methanol. The Sn IV atom is coordinated by two chelating oxalate ligands and two chloride ions in cis positions, giving rise to an [Sn(C 2 O 4 ) 2 Cl 2 ] 2À anion (point group symmetry 2), with the Sn IV atom in a slightly distorted octahedral coordination. The cohesion of the crystal structure is ensured by the formation of N-HÁ Á ÁO hydrogen bonding between ( i Pr 2 NH 2 ) + cations and [SnCl 2 (C 2 O 4 ) 2 ] 2À anions. This gives rise to an infinite chain structure extending parallel to [101]. The main inter-chain interactions are van der Waals forces. The electronic spectrum of the title compound displays only one high intensity band in the UV region assignable to ligand-metal ion charge-transfer (LMCT) transitions. An IR spectrum was also recorded and is discussed.

Chemical context
As a result of their numerous applications in medicine, industry and agriculture (Kapoor et al., 2005), tin(IV) carboxylate compounds have attracted the attention of several research groups, resulting in the preparation and characterization of new compounds (Christie et al., 1979;Ng & Kumar Das, 1993;Rocamora-Reverte et al., 2012;Reichelt & Reuter, 2014). Derivatives of tin(IV) oxalate are a subclass of the tin(IV) carboxylate family and have likewise been studied extensively because oxalate ions, C 2 O 4 2-, play an important role as counter-ions or complex ligands in inorganic as well as in organometallic chemistry. One of the motivations to study these compounds is related to the rich coordinating behaviour of the oxalato ligand, which can adopt a monodentate, a bridging monodentate, a bridging bidentate, a monochelating, bidentate or a bichelating mode (Miskelly et al., 1983;Sow et al., 2012;Ś witlicka-Olszewska et al., 2014). In this context, our group has previously published the syntheses and crystal structure determinations of some tin(IV) oxalate derivatives (Sarr et al., 2013(Sarr et al., , 2018. As a continuation of this work, we have studied the interaction between bis(diisopropylammonium) oxalate with tin(IV) chloride dihydrate, which yielded the title salt (C 6 H 16 N) 2 [Sn(C 2 O 4 ) 2 Cl 2 ] or ( i Pr 2 NH 2 ) 2 [Sn(C 2 O 4 ) 2 Cl 2 ]. Its crystal structure was deter- ISSN 2056-9890 mined by single crystal X-ray diffraction and was confirmed by infrared and UV-visible spectroscopic studies.

Structural commentary
The asymmetric unit of ( i Pr 2 NH 2 ) 2 [Sn(C 2 O 4 ) 2 Cl 2 ] comprises one diisopropylammonium cation and one half of an [Sn(C 2 O 4 ) 2 Cl 2 ] 2À anion, the other half being completed by the application of twofold rotation symmetry ( Fig. 1) with the rotation axis running through the central Sn IV atom. The latter is chelated by two oxalate ligands and is additionally ligated by two Cl atoms in cis positions within a distorted octahedral coordination sphere [Cl1 i -Sn1-Cl1= 97.26 (4) ; O1-Sn1-O4 = O1 i -Sn1-O4 i = 79.27 (6) , O1-Sn1-O4 i = 90.21 (6) ; symmetry code: (i) x + 1 2 , Ày + 3 2 , z À 1 2 ]. Atoms Cl1 i , O1, O1 i and O4 define the equatorial plane [with a slight shift of Sn1 from this plane by 0.1163 (5) Å ; rms = 0.0687 Å ] while Cl1 and O4 i occupy the axial positions. The O4 i -Sn1-Cl1 angle of 168.50 (4) indicates a considerable deviation from linearity, which might be explained by the difference in size of the Cl and O atoms and by the small bite angle of 79.27 (6) between the central Sn IV atom and the chelating O1 and O4 atoms.

Figure 2
An infinite chain in the title structure, showing the N-HÁ Á ÁO hydrogen bonds as light-blue dashed lines. C-bound H atoms have been omitted for clarity.

Figure 3
The crystal packing of the title compound in a view down [001]. C-bound H atoms have been omitted for clarity.

Figure 1
The molecular entities in the organic-inorganic title salt drawn with displacement ellipsoids at the 50% probability level; hydrogen atoms are depicted as spheres of arbitrary radius

Synthesis and crystallization
The title salt was obtained by mixing bis(diisopropylammonium) oxalate ( i Pr 2 NH 2 ) 2 C 2 O 4; 0.30 g; 15.50 mmol) and tin(IV) chloride dihydrate (SnCl 2 Á2H 2 O; (0.34 g; 15.50 mmol) in a 1:1 molar ratio in methanol. The obtained yellow solution was stirred for one h and then filtered. Colourless prism-like crystals were obtained by slow evaporation of the filtrate over a period of ten days. The IR spectrum confirms the presence of oxalate and diisopropylammonium groups in the title salt. In addition, the appearance of valence vibrations (-CO 2 ) in the form of three bands shows that the oxalate ligands are not centrosymmetric, in agreement with the difference in the C-O bond lengths revealed by the X-ray study. Attributions of the vibrational bands of the title compound were made by comparison with previous studies (Sarr et al., 2018;Marinescu et al., 2002;Li et al., 2008). The vibrational bands at 3061 and 1579 cm À1 in the IR spectrum (Fig. 4) are assigned to the stretching and deformation modes (N-H) and (N-H), respectively, of -NH 2 -in the ammonium group. The bands at 1675, 1375 and 1251 cm À1 are attributed to the asymmetric and symmetric vibrations of the oxalate -CO 2 moiety while that at 795 cm À1 corresponds to the deformation vibrations (C-O) (Sarr et al., 2018;Marinescu et al., 2002;Li et al., 2008). The frequencies of stretching vibrations of the oxalate group often show slight deviations owing to the different coordination modes. The bands at 2882 cm À1 are assigned to the valence vibrations (C-H) and those at 1486 cm À1 to the deformation vibrations (C-H).
The electronic spectrum of the title compound is shown in Fig. 5. In the ultraviolet region, only one strong absorption band with a shoulder is observed. Generally, only !*, n!* and LMCT transitions can be observed in the ultraviolet-visible (UV-Vis) region. The !* transition requires an absorption of a photon with a wavelength which does not fall in the UV-Vis range. Thus, this strong absorption band at 320 nm (Fig. 5) may be assigned to ligand-metal ion charge transfer (LMCT) (Kane et al., 2016). However, as the ligandbased !* / n!* transitions absorb in the same area as the LMCT transitions, we cannot exclude a possibility of superposition of these transitions (ligand-based and LMCT) owing to the form of the absorption band (Ford & Vogler, 1993;Filho et al., 2000). In the title compound, the Sn IV atom is surrounded by electron rich ligands (chlorido and oxalato), and !* and n!* transitions may result from the nonbinding electron pairs present on chlorine atoms or oxygen atoms of oxalate (Wojciechowska et al., 2016).

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 2 The IR spectrum of the title compound.

Figure 5
The electronic spectrum of the title compound. geometrically idealized positions and constrained to ride on their parent atoms, with a N-H distance of 0.91 Å , C methyl -H = 0.98 Å and C methine = 1.0 Å , and with U iso (H) = 1.2U eq (C,N) or 1.5U eq (C methyl ). Three reflections were omitted from refinement because they were obstructed by the beam stop.

Bis(diisopropylammonium) cis-dichloridobis(oxalato-κ 2 O 1 ,O 2 )stannate(IV)
Crystal data (C 6  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.