Di-tert-butyltin(IV)–hydroxide–iodide, t Bu2Sn(OH)I, the last missing member in the series of pure di-tert-butyltin(IV)–hydroxide–halides

The crystal structure of di-tert-butylhydroxidoiodidotin(IV), [Sn(C4H9)2I(OH)] or t Bu2Sn(OH)I, consists of centrosymmetric dimers exhibiting the characteristic structural features of diorganotin(IV)-hydroxide-halides.

Here we present the molecular and crystal structure of the last missing member in the series of pure di-tert-butyltin hydroxide halides where Hal = I. The analogous molecule with DMSO as a hydrogen-bonded Brønsted base was formerly found as part of co-crystals with [( t Bu 2 Sn) 3 O(OH) 2 I]I (Reuter & Wilberts, 2014).

Structural commentary
The asymmetric unit of the title compound comprises one t Bu 2 Sn(OH)I moiety that dimerizes to form a centrosymmetric molecule (Fig. 1). As in all other hydroxide halides, dimerization occurs via the two hydroxyl groups that act as bridges between two trigonal-bipyramidally (tbpy) coordinated Sn IV atoms.
The anisotropic displacement parameters as well as the small isotropic displacement parameters of the hydrogen atoms (see Refinement) indicate a negligibly small rotation of the tert-butyl groups as a whole and a small rotation of the methyl groups in particular, giving rise to very precise infor-mation on bond lengths and angles. The structural features of the tert-butyl groups are characterized by C-C bond lengths in the range 1.524 (3) to 1.533 (3) Å [mean value: 1.529 (3) Å ], C-C-C angles in the range 109.5 (2) to 111.3 (2) [mean value: 110.2 (7) ], Sn-C bond lengths between 2.187 (2) and 2.193 (2) Å [mean value: 2.190 (3) Å ], and Sn-C-C angles of 107.8 (1) to 109.6 (1) [mean value: 108.8 (9) ]. All these values are more precise in comparison with those of the formerly determined di-tert-butyltin hydroxide halides (Puff et al., 1985;Di Nicola et al., 2011), especially as a result of lowtemperature measurement and high data redundancy combined with multi-scan absorption correction, but are of the same accuracy and absolute value as those of the DMSO adduct [( t Bu 2 Sn) 3 O(OH) 2 I]I [d(C-C) = 1.529 (4) Å , h(C-C-C) = 109.9 (4) , d(Sn-C) = 2.193 (10), h(Sn-C-C) = 109.4 (7) ; Reuter & Wilberts, 2014]. These data are confirmed by a redetermination of the crystal structure of the -modification of [ t Bu 2 Sn(OH)Cl)] 2 (Reuter, 2022) performed with similar experimental conditions as for the title compound and its co-crystallizate. In this context, Sn-C distances are of special interest as they belong to the longest ones observed in case of Sn in a trigonal-bipyramidal coordination. In the other hydroxide halides mentioned above, the following bond lengths have been found: d(Sn-C) mean = 2.120 (8)  Ball-and-stick model of the trigonal-bipyramidal coordination environment of the tin atom in the dimeric molecule of t Bu 2 Sn(OH)I with bond lengths (Å ) and angles ( ) characterizing the polyhedron axes. For clarity, methyl groups of the t Bu ligands are stripped down to the carbon-carbon bonds drawn as shortened sticks. Atom O1 0 is generated by symmetry code Àx, Ày + 1, Àz + 1.

Figure 1
Ball-and-stick model of the dimeric, centrosymmetric molecule found in the crystal of t Bu 2 Sn(OH)I, with atom numbering of the asymmetric unit. With the exception of the hydrogen atoms that are shown as spheres of arbitrary radius, all other atoms are drawn as displacement ellipsoids at the 40% probability level. The black dot labelled i indicates the position of the centre of symmetry.
Within the trigonal-bipyramidal coordination of the Sn IV atom (Fig. 2), both tert-butyl groups are in equatorial (eq) positions in correspondence with the predictions of the VSEPR concept. The bond angle enclosed by the two tertbutyl groups of 126.81 (8) is identical with the value [126.89 (9) ] in the co-crystal and lies in the range 122.0 (2) to 129.3 (1) of C-Sn-C angles found in the other hydroxidehalides. The iodine atom adopts an axial (ax) position and one of the bridging hydroxyl groups is in an equatorial, the other in an axial position. As a result of dimerization via the hydroxyl groups, the axis of the trigonal bipyramid strongly deviates from linearity [I ax -Sn-(OH) ax = 151.94 (4) ]. In addition, the Sn-I distance of 2.8734 (2) Å is only marginally shorter than in the co-crystal [Sn-I = 2.8852 (2) Å ], both being significantly longer than the sum (2.78 Å ) of the covalent radii (Cordero et al., 2008) of tin (1.39 Å ) and iodine (1.39 Å ) and much longer than the mean Sn-I distance of 2.661 (2) (Reuter & Pawlak, 2001) in tin(IV) iodide, SnI 4 , with tetrahedrally coordinated tin.

Supramolecular features
While the hydroxyl groups of the [ t Bu 2 Sn(OH)I] 2 molecules of the co-crystallizate (Reuter & Wilberts, 2014) are involved in OHÁ Á ÁO hydrogen-bonding to DMSO molecules, those of all other [ t Bu 2 Sn(OH)Hal] 2 molecules develop intermolecular O-HÁ Á ÁHal bonds resulting in a chain-like arrangement of the corresponding molecules in the crystal. In contrast, there are no hydrogen bonds in the crystal structure of the title compound as the voluminous iodine atoms (Fig. 4) prevent significant intermolecular OHÁ Á ÁI interactions (Table 1). Hence, only van der Waals forces exist between the individual molecules, resulting in a layer-like arrangement ( Ball-and-stick model of the four-membered, centrosymmetric {Sn-O} 2 ring in the dimeric molecule of t Bu 2 Sn(OH)I with bond lengths (Å ) and angles ( ) underlining its rhombic shape as the result of axially (ax) and equatorially (eq) bonded O atoms.  the Sn-I bonds perpendicular to the layer plane. These layers expand perpendicular to the [101] direction (Fig. 6).

Synthesis and crystallization
Yellow block-like single crystals of the title compound were obtained after several years of storage in a sample of ( t Bu 2 Sn) 2 I 2 originally prepared by the reaction of the cyclotetrastannane ( t Bu 2 Sn) 4 with I 2 in toluene at elevated temperature in a molar ratio of 1:2. As other molar ratios and temperatures result in the formation of ( t Bu 2 Sn) 4 I 2 or t Bu 2 SnI 2 (Farrar & Skinner, 1964;Adams & Drä ger, 1985;Puff et al., 1989), it seems possible that the sample was contaminated with the latter one, which reacts over the long time of storage with atmospheric moisture to give the title compound.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms were clearly identified in difference-Fourier syntheses. Those of the tertbutyl groups were refined with calculated positions (C-H = 0.98 Å ) and a common U iso (H) parameter for each of the methyl groups. The position of the H atom of the OH group was refined with a fixed O-H distance of 0.96 Å and the U iso (H) parameter refined freely. Computer programs: APEX2 and SAINT (Bruker, 2009), SHELXS (Sheldrick, 2008), SHELXL (Sheldrick, 2015), DIAMOND (Brandenburg, 2006), Mercury (Macrae et al., 2020) and publCIF (Westrip, 2010).

Figure 6
Stick-model showing the arrangement of layers with respect to the unit cell; colour code as in Fig. 4.

Di-tert-butylhydroxidoiodidotin(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.