Comment

Organotin(IV) complexes with O,N,O'-tridentate Schiff base ligands have received attention due to their biological properties, particularly their possible antitumor activity (Nath et al., 1997; Basu Baul et al., 2001; Al-Allaf et al., 2003; Zamudio-Rivera et al., 2005; Tian et al., 2006; Beltrán et al., 2007; Kobakhidze et al., 2010). Diorganotin complexes of N-(2-hydroxyarylidene)--amino acids have been prepared by several groups (Dakternieks et al., 1998; Basu Baul et al., 1999, 2005; Wang et al., 1992; Smith et al., 1992; Beltrán et al., 2003; Yin et al., 2004). We have synthesized such complexes in connection with our investigation of the syntheses and structural properties of chiral silicon and tin complexes with O,N,O'-tridentate ligands (Warncke et al., 2012; Böhme et al., 2006).

Dimethyl{N-[(2-oxidonaphthalen-1-yl-O)methylidene]valinato-²N,O}tin(IV) was prepared from enantiomerically pure (S)-N-[(2-hydroxynaphthalen-1-yl)methylidene]valine and dichloridodimethyltin in the presence of triethylamine. After work-up of the reaction mixture and recrystallization (see Experimental), two different crystal types were observed under the microscope. Pale-yellow needles were characterized as enantiomerically pure crystals, (I), in the space group P3₂. Some yellow prisms were found between the surface of the solvent and the glass wall of the Schlenk tube. These were identified as the racemate, (II), of the same compound in the space group P. The valinate ligand used for the synthesis was the pure S enantiomer. Therefore, the formation of enantiomerically pure compound (I) was expected. The bulk material of the tin complex shows a large value for the optical rotation (see Experimental) which hints at the formation of an enantiomerically pure product. The formation of the racemate was surprising. Only careful examination of the crystalline product under a microscope allowed the identification of the racemate (estimated to be less than 5% of the overal yield of crystalline product). We assume that a small portion of the ligand was racemized during the complex formation. The mechanism of such a racemization reaction has been investigated recently (Warncke et al., 2012).

Fig. 1 shows the molecular structure of (I) and selected bond lengths and angles are listed in Table 1. The presence of the S form of the enantiomerically pure compound was established by anomalous dispersion effects in diffraction measurements on the crystal. The Flack (1983) parameter of the final refinement is -0.02 (1). The bond lengths at the Sn atom are comparable with the bond lengths of the only known tin complex with an N-(hydroxynapthylidene)amino acid ligand (Smith et al., 1996). The Sn atom in (I) is pentacoordinated, with three bonds to the O,N,O'-tridentate valinate ligand and two bonds to methyl groups. The coordination geometry about the Sn atom is characterized by the Addison parameter (Addison et al., 1984). The value of is 0.44, which is almost halfway between the values which define a square pyramid and a trigonal bipyramid. An alternative description of a pentacoordinated geometry is provided by Holmes (1984) who uses an idealized trans-basal angle of 150° for a square pyramid. According to this description, the Sn atom in (I) is 34.4% along the Berry pseudorotation coordinate from a trigonal bipyramid to a square pyramid.

R and S isomers are present in centrosymmetric crystal structure (II). The same atom-labelling scheme as in (I) has been used for the atoms of (II). Selected bond lengths and angles are listed in Table 2. The main difference from the structure of (I) is the formation of dimers in the crystal lattice. Fig. 2 shows the dimer of (II) which is formed between two Sn1-O2 units. A crystallographic inversion centre is located at the centre of the dimeric unit [Sn1O2ⁱ = 2.8077 (18) Å; symmetry code: (i) -x + 1, -y + 2, -z]. The formation of the dimer implies several structural changes compared with (I). These are mainly an enlarged C17-Sn1-C18 angle and elongated bonds involving the Sn1 atom. These geometrical changes lead to a lower value of 0.25, i.e. the square-pyramidal character is far more pronounced. Alternatively, one could consider the coordination geometry as a distorted hexacoordinated tin complex with atom O2ⁱ at a very long distance. The calculated density of (II) is greater than that of (I) (1.647 versus 1.536 Mg m^-1). This means that the crystal structures under investigation obey Wallach's rule (Wallach, 1895). However, this rule has been critically assessed recently (Brock et al., 1991). The formation of dimers might offer an explanation for the more dense packing in (II). This is quantified by the packing coefficients 0.705 for (II) and 0.661 for (I). The different densities and packing coefficients, as well as the mere occurrence of the racemic by-product and its higher melting point, hint at a greater stability of the racemic crystals of (II) in comparison with their chiral counterpart, (I).

Figure 1 The molecular structure of (I), in the space group P32, drawn with 50% probability displacement ellipsoids.

Figure 2 A view of the dimer of (II), in the space group P, drawn with 50% probability displacement ellipsoids. The same atom numbering as in (I) has been used. [Symmetry code: (i) -x + 1, -y + 2, -z.]

Experimental

(S)-N-[(2-Hydroxynaphthalen-1-yl)methylidene]valine was prepared from L-valine and 2-hydroxy-1-naphthaldehyde according to a literature method (Nitta et al., 1992). The preparation of the tin complex was performed in Schlenk tubes under argon with dry and air-free solvents.

The yellow suspension of (S)-N-[(2-hydroxynaphthalen-1-yl)methylidene]valine (1.26 g, 4.64 mmol) and triethylamine (1.22 g, 12.06 mmol, 30% excess) in tetrahydrofuran (50 ml) was stirred at 273 K. A solution of dichloridodimethyltin (1.02 g, 4.64 mmol) in tetrahydrofuran (10 ml) was added dropwise. A white precipitate formed and the resulting suspension was stirred for 30 min at 273 K and then for 4 d at room temperature. The triethylamine hydrochloride was filtered off and washed with tetrahydrofuran (4 × 5 ml). The volatiles were removed completely from the filtrate under reduced pressure and the residue was extracted with hot chloroform (10 ml). This CHCl₃ solution was evaporated to dryness in a vacuum and the residue was recrystallized from absolute methanol (12 ml) as yellow crystals (yield 1.13 g, 58.2%). The crystals used for X-ray structure determination of (I) and (II) were taken from the bulk material. Analysis calculated for C₁₈H₂₁NO₃Sn: C 51.71, H 5.06, N 3.35%; found: C 51.30, H 5.11, N 3.38%.

¹¹⁹Sn NMR (CDCl₃): -158.3; ¹H NMR (CDCl₃): 0.59 (s, 3H, Sn-CH₃), 1.00 (s, 3H, Sn-CH₃), 1.08 (d, 3H, CH-CH₃, ³J_HH = 6.8 Hz), 1.11 (d, 3H, CH-CH₃, ³J_HH = 6.8 Hz), 2.39 [septet, 1H, CH(CH₃)₂, ³J_HH = 6.8 Hz], 3.97 (d, 1H, CH-COO, ³J_HH = 4,7 Hz), 6.93 (m, 1H, H_ar), 7.37 (m, 1H, H_ar), 7.57 (m, 1H, H_ar), 7.73 (m, 1H, H_ar), 7,86 (m, 1H, H_ar), 7.91 (m, 1H, H_ar), 9.03 (s, 1H, CH=N). ¹³C NMR (CDCl₃): -1.2, 1.7 (Sn-CH₃), 18.3, 19.1 [CH(CH₃)₂], 34.3 [CH(CH₃)₂], 74.7 (CH-COO), 108.4 (C_ar-CH=N), 118.4, 124.0, 124.6, 127.2, 129.0, 129.6, 133.9, 139.5 (8 × C_ar), 166.3 (CH=N), 172.4 (C_ar-O), 173.2 (COO). []_D²⁰ = - 406.2° (c = 1 g per 100 ml CHCl₃). UV-Vis (c = 2.153 × 10 ^-4 mol l^-1, solvent CHCl₃): _max (nm) (, l mol^-1 cm^-1) 411 (8568), 333 (5668), 257 (17215).

In (I), similarity and rigid-bond restraints were imposed on the anisotropic displacement parameters of atoms C5 and C6. Together with the floating origin restraint for the polar space group, a total of eight restraints was used. All H atoms in both structures were positioned geometrically and refined using a riding model (including free rotation about the C-C bond for the methyl groups). The aromatic H atoms and the azomethine H atom on C11 were constrained to an ideal geometry, with C-H = 0.95 Å and U_iso(H) = 1.2U_eq(C), the H atoms on tertiary C atoms C12 and C14 with C-H = 1.00 Å and U_iso(H) = 1.2U_eq(C), and the methyl H atoms with C-H = 0.98 Å and U_iso(H) = 1.5U_eq(C).

For both compounds, data collection: X-AREA (Stoe & Cie, 2009); cell refinement: X-AREA; data reduction: X-RED (Stoe & Cie, 2009); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: SHELXL97.

Supplementary data for this paper are available from the IUCr electronic archives (Reference: EG3106 ). Services for accessing these data are described at the back of the journal.