Comment

There have been several reports dealing with the impact of organotin chemistry in the biosphere (Gielen, 1994; Ng et al., 1991). Exploration of the structure-activity relationships of such systems has led to numerous reports in recent years (Gielen, 1994; Selvaratnam et al., 1994; McManus et al., 1994). The structural chemistry of organotin carboxylic acid esters has been extensively explored because of the rich diversity of structural motifs (Tiekink, 1994). Among organotin carboxylates, dimeric distannoxanes comprise the most interesting class with respect to their structural chemistry. Reports on crystallographic studies show that these compounds may adopt a variety of structural modes depending on the nature of organic substituents at the Sn atom or carboxylate ligand (Danish et al., 1996). There have been numerous crystallographic reports on these compounds describing their dimeric nature and there are at least five distinct structural types known for them (Tiekink, 1991). In continuation of our studies of organotin(IV) carboxylates (Sadiq-ur-Rehman et al., 2006), we have synthesized the title compound (I), the crystal structure of which is reported here.

The structure of (I) is composed of a centrosymmetric dimer of oxoditin units (Fig. 1). The endocyclic Sn-O distance in the central core [Sn1-O7 of 2.286 (2) Å] and the endocyclic distance [Sn1-O1 = 2.188 (2) Å] are quite similar to those observed in the tetrabutylbis(N-phthaloylglycinato)distannoxane dimer (Parvez et al., 2000) and the tetrabutylbis(N-phthaloylphenylalaninato)distannoxane dimer (Hans et al., 2002). Both independent Sn atoms in (I) are in a five-coordinate O₃C₂Sn distorted trigonal-bipyramidal geometry. The carboxylate ligand shows different modes of coordination with Sn. Firstly, it acts as monodentate, coordinated to Sn1 via O1; the Sn1O2 distance is 2.883 (2) Å, i.e. too long to be considered bonding [likewise, the Sn2O1 distance of 2.913 (2) Å]. In the other coordination mode, the ligand bridges two Sn atoms in a bidentate fashion, thus resulting in a six-membered Sn1-O7-C8-O8-Sn2ⁱ-O13 ring [symmetry code: (i) -x, -y, -z]. The Sn-C distances lie in the very narrow range 2.119 (3)-2.135 (3) Å, while the Sn-O distances range between 2.037 (2) and 2.286 (2) Å (Table 1).

Figure 1 Structure of (I). Displacement ellipsoids are drawn at the 50% probability level. The atoms labelled with A are at the symmetry position (-x, -y, -z). H atoms have been omitted. For clarity, only one component of the disordered butyl group (C15-C18) is shown.

Experimental

A mixture of 3,5-dinitro-4-chlorobenzoic acid (2 g, 1 mmol) and di-n-butyltin oxide (2.02 g, 1 mmol) was refluxed in dry toluene (150 ml) for 5-6 h using a Dean and Stark trap. The reaction mixture was cooled to room temperature and solvent was evaporated under reduced pressure. The solid product was recrystallized from chloroform, resulting in rod-shaped crystals of (I) (yield 85%, m.p. 513-514 K).

Crystal data

[Sn4O2(C7H2ClN2O6)4(C4H9)8] Mr = 1945.88 Triclinic, a = 12.1784 (11) Å b = 12.5143 (12) Å c = 13.5682 (12) Å = 104.597 (1)° = 110.770 (1)° = 95.960 (1)° V = 1827.9 (3) Å3 Z = 1 Dx = 1.768 Mg m-3 Mo K radiation = 1.58 mm-1 T = 100 (2) K Rod, colourless 0.40 × 0.20 × 0.20 mm

Data collection

Bruker SMART CCD area-detector diffractometer and scans Absorption correction: multi-scan (SADABS; Bruker, 2001) Tmin = 0.571, Tmax = 0.743 14580 measured reflections 7352 independent reflections 6497 reflections with I > 2(I) Rint = 0.030 max = 26.5°

Refinement

Refinement on F2 R[F2 > 2(F2)] = 0.032 wR(F2) = 0.086 S = 1.02 7352 reflections 482 parameters H-atom parameters constrained w = 1/[2(Fo2) + (0.0514P)2 + 0.862P] where P = (Fo2 + 2Fc2)/3 (/)max = 0.001 max = 1.61 e Å-3 min = -1.47 e Å-3

Table 1 Selected geometric parameters (Å, °) Sn1-O13 2.0391 (19) Sn1-C19 2.124 (3) Sn1-C15 2.135 (3) Sn1-O1 2.188 (2) Sn1-O7 2.286 (2) Sn2-O13i 2.0372 (19) Sn2-C23 2.119 (3) Sn2-C27 2.125 (3) Sn2-O13 2.1708 (19) Sn2-O8i 2.276 (2) Sn2Sn2i 3.2908 (5) O13-Sn1-C19 106.03 (10) O13-Sn1-C15 108.78 (10) C19-Sn1-C15 144.19 (12) O13-Sn1-O1 83.08 (8) C19-Sn1-O1 99.15 (10) C15-Sn1-O1 93.09 (10) O13-Sn1-O7 89.89 (8) C19-Sn1-O7 86.80 (10) C15-Sn1-O7 85.19 (11) O1-Sn1-O7 171.81 (8) O13i-Sn2-C23 108.48 (10) O13i-Sn2-C27 114.33 (10) C23-Sn2-C27 136.69 (12) O13i-Sn2-O13 77.16 (8) C23-Sn2-O13 97.19 (10) C27-Sn2-O13 98.24 (10) O13i-Sn2-O8i 91.86 (8) C23-Sn2-O8i 89.20 (10) C27-Sn2-O8i 83.31 (10) O13-Sn2-O8i 168.58 (8) Symmetry code: (i) -x, -y, -z.

H atoms were included in calculated positions using the riding model, with C-H = 0.95-0.99 Å and with U_iso(H) = 1.2U_eq(C), or 1.5U_eq(C) for methyl groups. In one of the butyl groups (C15-C18), atoms C16 and C17 are disordered over two sites (C16/C17 and C16A/C17A); the occupancies refined to 0.496 (15) and 0.504 (15), respectively, and were fixed at 0.5 for the final cycles of refinement. The highest residual density peak is located 0.95 Å from atom Sn2 and the deepest hole is located 0.98 Å from atom Sn1.

Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2002); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Bruker, 2001); software used to prepare material for publication: SHELXTL.