Synthesis, structural and spectral studies of five- and six-coordinate adducts of organotin(IV) halides containing dibenzylsulfoxide (dbso) as ligand. The crystal structures of fac-[MeSnCl3(dbso)2] and trans-[Ph2SnCl2(dbso)2]

Abstracts

The reaction of the ambidentate ligand dibenzylsulfoxide (dbso) with Me2SnCl2 in 1:1 molar ratio leads to the formation of the five-coordinate adduct [Me2SnCl2(dbso)], whereas the same reaction conditions with Ph2SnCl2 provide the six-coordinate adduct trans-[Ph2SnCl2(dbso)2]. On the other hand, the reaction with nBu2SnCl2 forms the dimeric adduct [{nBu2SnCl2(dbso)}2], which probably possesses octahedral tin(IV) nuclei and bridging chlorides. The adducts [MeSnCl3(dbso)2] and [Ph3SnCl(dbso)] were also prepared and included in the study. All complexes were studied by microanalysis and IR, NMR (¹H, 13C and 119Sn) and Mössbauer spectroscopies to investigate their structural properties. The six-coordinate species fac-[MeSnCl3(dbso)2] and trans-[Ph2SnCl2(dbso)2] were also studied by single crystal X-ray diffractometry. These compounds crystallize in the orthorhombic, Pbcn, and monoclinic space group P2(1)/c, respectively, as discrete neutral molecules with the tin(IV) atom in a distorted octahedral geometry and the two dbso ligands in cis and trans positions, respectively.

sulfoxide complexes; organotin(IV) complexes; crystal structure analyses

A reação do ligante ambidentado dibenzilsulfóxido (dbso) com Me2SnCl2 na proporção molar 1:1 leva à formação do produto pentacoordenado [Me2SnCl2(dbso)], enquanto Ph2SnCl2 leva à formação do complexo hexacoordenado trans-[Ph2SnCl2(dbso)2], utilizando as mesmas condições de reação. Por outro lado, a reação com nBu2SnCl2 forma o produto bimetálico [{nBu2SnCl2(dbso)}2], o qual provavelmente possui núcleos de estanho(IV) octaédricos e cloretos em ponte. Os complexos [MeSnCl3(dbso)2] e [Ph3SnCl(dbso)] também foram preparados. Todos os produtos foram estudados por análise elementar e por espectroscopias no IV, RMN (¹H, 13C, 119Sn) e Mössbauer. As espécies hexacoordenadas fac-[MeSnCl3(dbso)2] e trans-[Ph2SnCl2(dbso)2] foram também estudadas por difratometria de raios X; as determinações estruturais revelaram que estes compostos cristalizam-se nos sistemas cristalinos ortorrômbico, Pbcn, e monoclínico, P2(1)/c, respectivamente. As moléculas possuem átomos de estanho(IV) numa geometria octaédrica distorcida, com os dois ligantes dbso em posições cis e trans, respectivamente.

ARTICLE

Synthesis, structural and spectral studies of five- and six-coordinate adducts of organotin(IV) halides containing dibenzylsulfoxide (dbso) as ligand. The crystal structures of fac-[MeSnCl₃(dbso)₂] and trans-[Ph₂SnCl₂(dbso)₂]

Gerimário F. de Sousa^I,*; José R. Sabino^II; Ivo Vencato^II; Carlos A. L. Filgueiras^III; José D. Ardisson^IV

^IInstituto de Química, Universidade de Brasília, 70919-970 Brasília-DF, Brazil

^IIInstituto de Física, Universidade Federal do Goiás, Campus Samambaia, 74001-970 Goiânia-GO, Brazil

^IIIInstituto de Química, Universidade Federal do Rio de Janeiro, 21495-970 Rio de Janeiro-RJ, Brazil

^IVLaboratório de Física Aplicada, Centro de Desenvolvimento de Tecnologia Nuclear, 30123-970 Belo Horizonte-MG, Brazil

ABSTRACT

The reaction of the ambidentate ligand dibenzylsulfoxide (dbso) with Me₂SnCl₂ in 1:1 molar ratio leads to the formation of the five-coordinate adduct [Me₂SnCl₂(dbso)], whereas the same reaction conditions with Ph₂SnCl₂ provide the six-coordinate adduct trans-[Ph₂SnCl₂(dbso)₂]. On the other hand, the reaction with ⁿBu₂SnCl₂ forms the dimeric adduct [{ⁿBu₂SnCl₂(dbso)}₂], which probably possesses octahedral tin(IV) nuclei and bridging chlorides. The adducts [MeSnCl₃(dbso)₂] and [Ph₃SnCl(dbso)] were also prepared and included in the study. All complexes were studied by microanalysis and IR, NMR (¹H, ¹³C and ¹¹⁹Sn) and Mössbauer spectroscopies to investigate their structural properties. The six-coordinate species fac-[MeSnCl₃(dbso)₂] and trans-[Ph₂SnCl₂(dbso)₂] were also studied by single crystal X-ray diffractometry. These compounds crystallize in the orthorhombic, Pbcn, and monoclinic space group P2₁/c, respectively, as discrete neutral molecules with the tin(IV) atom in a distorted octahedral geometry and the two dbso ligands in cis and trans positions, respectively.

Keywords: sulfoxide complexes, organotin(IV) complexes, crystal structure analyses

RESUMO

A reação do ligante ambidentado dibenzilsulfóxido (dbso) com Me₂SnCl₂ na proporção molar 1:1 leva à formação do produto pentacoordenado [Me₂SnCl₂(dbso)], enquanto Ph₂SnCl₂ leva à formação do complexo hexacoordenado trans-[Ph₂SnCl₂(dbso)₂], utilizando as mesmas condições de reação. Por outro lado, a reação com ⁿBu₂SnCl₂ forma o produto bimetálico [{ⁿBu₂SnCl₂(dbso)}₂], o qual provavelmente possui núcleos de estanho(IV) octaédricos e cloretos em ponte. Os complexos [MeSnCl₃(dbso)₂] e [Ph₃SnCl(dbso)] também foram preparados. Todos os produtos foram estudados por análise elementar e por espectroscopias no IV, RMN (¹H, ¹³C, ¹¹⁹Sn) e Mössbauer. As espécies hexacoordenadas fac-[MeSnCl₃(dbso)₂] e trans-[Ph₂SnCl₂(dbso)₂] foram também estudadas por difratometria de raios X; as determinações estruturais revelaram que estes compostos cristalizam-se nos sistemas cristalinos ortorrômbico, Pbcn, e monoclínico, P2₁/c, respectivamente. As moléculas possuem átomos de estanho(IV) numa geometria octaédrica distorcida, com os dois ligantes dbso em posições cis e trans, respectivamente.

Introduction

Organotin(IV) compounds have demonstrated to exhibit wide biological activity.^1,2 Insofar as many sulfoxides are also noteworthy for the same reasons,³ a combination of the two chemistries is an interesting line to pursue. Diorganotin(IV) dihalides, and more particularly Me₂SnCl₂, form well-known 1:2 adducts with Lewis bases.⁴ However, both Me₂SnCl₂ and Ph₂SnCl₂ form 1:1 adducts^5-7 with monodentate ligands, but reports on the isolation of both 1:1 and 1:2 adducts are scarce.⁸ The formation of 1:1 systems is curious and, surprisingly, neither the stoichiometry of the reactants nor the reaction conditions appear to exert much influence.⁴ The 1:1 adduct, [Me₂SnCl₂(dbso)], whose X-ray structure is already known,⁵ has been described as an authentic and unique five-coordinate trigonal bipyramidal compound without any intermolecular contact. The structure of dbso is shown below.

We report herein the preparation and spectroscopic investigation of five- and six-coordinate adducts involving R_4-mSnCl_m (R = Me, ⁿBu, Ph; and m = 1, 2, 3) and dibenzylsulfoxide (dbso). The X-ray diffraction study of the adducts trans-dichloro-trans-bis(dibenzylsulfoxide)-trans-diphenyltin(IV), trans-[Ph₂SnCl₂(dbso)₂], and fac-trichloro-cis-bis(dibenzylsulfoxide)methyltin-(IV), fac-[MeSnCl₃(dbso)₂], is also presented.

Experimental

Material and procedures

All starting compounds and reagents were of highest grade and were used without purification. IR spectra were recorded from KBr pellets on a Bomem MB100 FT-IR spectrometer in the 4000-400 cm^-1 region. (¹H, ¹³C, ¹¹⁹Sn) NMR spectra were run on a Varian Mercury Plus 300 MHz spectrophotometer, using CDCl₃ as solvent. Chemical shifts (ppm) are relative to TMS and Me₄Sn. ¹¹⁹Sn Mössbauer spectra were collected at 77 K in the transmission geometry, on a conventional constant-acceleration spectrometer with a CaSnO₃ source kept at room temperature. All isomer shift values reported in this work are given with respect to this source. All Mössbauer spectra were computer-fitted assuming Lorentzian line shapes and the resulting isomer shifts and quadrupole splittings are accurate to ca. ± 0.05 mm s^-1. The velocity calibration of the instrument was done at room temperature with ⁵⁷Fe enriched foil. The microanalyses were performed using a FISSONS CHNS, model EA 108 microanalizer.

Single-crystals of trans-[Ph₂SnCl₂(dbso)₂] and fac-[MeSnCl₃(dbso)₂], suitable for X-ray studies, were obtained at room temperature as colourless solids by slow evaporation from EtOH/CHCl₃ (3:1, v/v) and Me₂CO solutions, respectively. The data collection of trans-[Ph₂SnCl₂(dbso)₂] was performed using an Enraf-Nonius Kappa CCD diffractometer⁹ (Mo-K_α radiation) with cold N₂ gas cooling of the sample crystal. Diffraction data for fac-[MeSnCl₃(dbso)₂] were collected using an Enraf-Nonius CAD-4 diffractometer¹⁰ (Cu-K_α radiation) at room temperature, and data reduction was performed using the XCAD-4 program.¹¹

Synthesis

Ph₃SnCl (0.5 g, 1.3 mmol) and dbso (0.3 g, 1.3 mmol) were dissolved in EtOH (30 mL). The solution was stirred at 70 ºC for 2 h. The hot colorless solution was then left to stand at room temperature to evaporate the solvent and colorless crystals of [Ph₃SnCl(dbso)] were separated. All the other diorgorganotin(IV) adducts were obtained by slow evaporation of a CHCl₃ solution containing the appropriate diorganotin(IV) dihalide and dbso in 1:1 molar ratio; the products were then recrystallized from a suitable solvent. Attempts to prepare [Me₂SnCl₂(dbso)₂] (1:2) and [Ph₂SnCl₂(dbso)] (1:1) were unsuccessful.

dbso:¹H-NMR (300 MHz, CDCl₃): δ 7.43-7.26 (m, 10H, -Ph); 3.92 (dd, J 12 and 15 Hz, 4H, -CH₂-Ph). ¹³C-NMR (300 MHz, CDCl₃): δ 57.1 (C-Ph), 130.0 (o-C), 128.8 (p-C), 128.8 (m-C), 128.2 (q-C).

[Me₂SnCl₂(dbso)]:¹H NMR (300 MHz, CDCl₃): δ 7.43-7.26 (m, 10H, -Ph); 3.97 (dd, J 13 and 14 Hz, 4H, -CH₂-Ph); 1.06 (s, 6H, Sn-CH₃, ²J(¹¹⁹Sn-¹H) = 75.3 Hz). ¹³C NMR (300 MHz, CDCl₃): δ 130.3 (o-C), 129.0 (p-C), 128.8 (m-C), 128.7 (q-C); 55.8 (C-Ph), 9.9 (Sn-C, ¹J(¹¹⁹Sn-¹³C) = 568 Hz). Anal. calcd. for C₁₆H₂₀Cl₂OSSn: C, 42.71; H, 4.48; S, 7.12%. Found: C, 41.24; H, 4.64; S, 7.18%. Yield 90%; m.p. 117-118 ºC.

[Ph₂SnCl₂(dbso)₂]:¹H NMR (300 MHz, CDCl₃): δ 7.76-7.23 (m, 30H, -Ph); 3.89 (dd, J 15 and 18 Hz, 4H, -CH₂-Ph). ¹³C NMR (300 MHz, CDCl₃): δ 130.3 (o-C), 129.2 (p-C), 129.0 (m-C), 128.6 (q-C); 55.9 (C-Ph). Anal. calcd. for C₄₀H₃₈Cl₂O₂S₂Sn: C, 59.72; H, 4.76; S, 7.97%. Found: C, 59.61; H, 4.81; S, 8.09%. Yield 60%; m.p. 127-128 ºC.

[ⁿBu₂SnCl₂(dbso)]:¹H NMR (300 MHz, CDCl₃): δ 7.43-7.26 (m, 10H, -Ph); 3.93 (dd, J 12 and 15 Hz, 4H, -CH₂-Ph); 1.77 (m, 4H, -CH₂-Ph); 1.77 (m, 4H,-CH₂-Sn); 1.39 (m, 8H,); 0.94 (t, 6H, Sn-C-C-CH₃). ¹³C NMR (300 MHz, CDCl₃): δ 130.2 (o-C), 129.1 (p-C), 129.0 (m-C), 128.7 (q-C); 55.1 (C-Ph), Sn-C(28.9)-C(27.0)-C(26.2)-C(13.5). Anal. calcd. for C₂₂H₃₂Cl₂OSSn: C, 49.47; H, 6.04; S, 6.00%. Found: C, 48.21; H, 6.09; S, 6.21%. Yield 80%; m.p. 110-112 ºC.

[MeSnCl₃(dbso)₂]:¹H NMR (300 MHz, CDCl₃): δ 7.40-7.25 (m, 10H, -Ph); 4.02 (s, 4H, -CH₂-Ph); 1.24 (s, 3H, Sn-CH₃, ²J(¹¹⁹Sn-¹H) = 123 Hz). ¹³C NMR (300 MHz, CDCl₃): δ 130.6 (o-C), 129.1 (p-C), 129.0 (m-C), 128.0 (q-C). Anal. calcd. for C₂₉H₃₁Cl₃O₂S₂Sn: C, 49.71; H, 4.46; S, 9.15%. Found: C, 49.28; H, 4.72; S, 9.31%. Yield 70%; m.p. 147-149 ºC.

[Ph₃SnCl(dbso)]:¹H NMR (300 MHz, CDCl₃): δ 7.80 - 7.20 (m, 25H, -Ph); 3.80 (dd, J 15 and 21 Hz, 4H, -CH₂-Ph).¹³C NMR (300 MHz, CDCl₃): δ 130.1 (o-C), 129.7 (p-C), 128.9 (m-C), 128.4 (q-C); 56.6 (C-Ph). Anal. calcd. for C₃₂H₂₉ClOSSn: C, 62.42; H, 7.75; S, 5.21%. Found: C, 61.04; H, 7.81; S, 5.19%. Yield 50%; m.p. 101-103 ºC.

Crystal structure determination

The initial structure solutions were obtained by the heavy-atom method using the SHELXS-97 program.¹² The other non-H atoms positions were obtained by difference Fourier maps. Hydrogen atoms were placed in geometric positions with C-H distances in the range of 0.93-0.99 Å and refined with a riding model with their U_iso proportional to U_eq of their parent atoms. Non-H atoms were refined with anisotropic displacement parameters. For the adduct trans-[Ph₂SnCl₂(dbso)₂], no correction for absorption was applied, and in fac-[MeSnCl₃(dbso)₂] the rings were refined as idealized rigid groups with C-C bond distances of 1.39 Å. All refinements were performed with full-matrix least-squares on F², using the SHELXL-97 program.¹³ The data of fac-[MeSnCl₃(dbso)₂] were corrected for absorption using the numerical method (Gaussian), with minimum and maximum transmission coefficients of 0.103 and 0.230, respectively. The crystal data and details of data processing are given in Table 1.

Results and discussion

The structure of trans-[Ph ₂ SnCl ₂ (dbso) ₂ ]

The molecular structure with the labelling scheme is shown in Figure 2 and the selected bond distances and angles are listed in Table 2. The arrangement about tin(IV) in trans-[Ph₂SnCl₂(dbso)₂] is best described as a distorted octahedron geometry, in which the dbso molecules are well adapted for trans coordination. The coordination sphere is completed by two trans-phenyl and two trans-chloride groups and a slight distortion of the octahedron is observed. Thus, the angle C31ⁱ-Sn1-C31 is 177.3(3)º, and the other two trans bond angles O1ⁱ-Sn1-O1 and Cl1-Sn1-Cl2 are 174.0(2) and 180º, respectively. The last angle indicates that the adduct possesses crystallographic two-fold axial symmetry with tin(IV) and both Cl atoms lying on the crystallographic two-fold axis. It is interesting to compare our results with literature data for the five-coordinate adduct [Me₂SnCl₂(dbso)].⁵ In trans-[Ph₂SnCl₂(dbso)₂], the bond distance Sn1-O1 of 2.258(4) Å is significantly shorter than the equivalent bond distance value of 2.319(10) Å observed in [Me₂SnCl₂(dbso)], indicating a stronger interaction in Ph₂Sn²⁺/dbso than in Me₂Sn²⁺/dbso. On the other hand, the Sn1-C31, 2.147(6); Sn1-Cl1, 2.578(2); and Sn1-Cl2, 2.509(2) Å bond lengths are considerably longer than the Sn1-C1, 2.128(15); Sn1-C2, 2.109(16); Sn1-Cl1, 2.392(6); and Sn1-Cl2, 2.464(6) Å bond lengths found in [Me₂SnCl₂(dbso)], as expected. The arrangement of the dbso ligands differs from that observed by Nardelli and collaborators¹⁴ in cis-dichloro-cis-bis(dimethylsulfoxide)-trans-diphenyltin(IV), cis-[Ph₂SnCl₂(dmso)₂], which has the chlorine atoms and the oxygen atoms cis, and the carbon atoms trans.

A sulfoxide coordinated to a metal via the oxygen atom can be represented formally by an R₂S^δ+-O^δ- dipolar form, and coordination to an organotin(IV) species would be expected to lengthen the S-O bond. This lengthening is observed in most organotin(IV)-sulfoxide complexes.^5,15 This is corroborated by IR spectroscopy, which shows a lowering of ν(S-O) from 1033 to 969-988 cm^-1 for all complexes under study here.

The structure of fac-[MeSnCl ₃ (dbso) ₂ ]

The molecular structure with the labelling scheme is shown in Figure 1 and the selected bond distances and angles are listed in Table 2. The arrangement about tin(IV) in fac- [MeSnCl₃(dbso)₂], located in a general position of the space group P2₁/c, is a distorted octahedral geometry in which the three chlorides are arranged in a facial fashion and the dbso ligands are bonded by O atoms cis to one another. The trans position to the methyl group is occupied by a chlorine atom and not by the sulfoxide ligand, as observed in mer-[EtSnI₃(dpso)₂]¹⁶ (dpso = diphenylsulfoxide). However, the adducts mer-[MeSnCl₃(ImSOMe)], ImSOMe = 1-methyl-2-(methylsulfinylimidazole), and mer-[MeSnCl₃(PySOⁿPr)], PySOⁿPr = 2-(n-propylsulfinyl)pyridine, possess three chlorides arranged in a meridional position and the methyl group is trans to the N atom.¹⁷

In Table 2, the Sn1-Cl1, Sn1-Cl2 and Sn1-Cl3 distances are given as 2.429(2), 2.399(2) and 2.422(2) Å, respectively. Therefore, the average of the Sn1-Cl1 and Sn1-Cl3 bond lengths (Cl trans to the dbso ligand) is 2.426(2) Å, and 2.426 - 2.399 = 0.027 Å. The O-Sn-O bonds [2.203(5) and 2.241(7) Å] form an angle of 92.6(3)º at the metal and the S-O bonds lengths [1.523(8) and 1.522(5) Å] are otherwise not much different from those bonds observed in other mono-organotin(IV) adducts containing sulfoxide as ligand.^16,17 The [Sn-C 2.142(8) Å] bond distance is longer than those found in mer-[MeSnCl₃(ImSOMe)] [2.122(4) Å] and in mer-[MeSnCl₃(PySOⁿPr)] [2.110(7) Å], but it is shorter than the Sn-C [2.218(5) Å] found in mer-[EtSnI₃(dpso)₂].¹⁶

Infrared spectroscopy

Table 3 lists the most important IR data, allowing several conclusions. The dbso ligand always binds to the tin(IV) moiety through the oxygen atom, as shown by the shift of ν(S-O) to lower values as compared to the free base. The ν(Sn-O) bands appear at 483 cm^-1 for fac-[MeSnCl₃(dbso)₂], at 460 cm^-1 for trans-[Ph₂SnCl₂(dbso)₂] and at 455 cm^-1 for [Ph₃SnCl(dbso)], in accordance with reported work related to ambidentate sulfoxides bonded to tin(IV).¹⁸ However, the ν(Sn-O) absorptions in five-coordinate diorganotin(IV)-sulfoxide adducts have been reported in the 415-440 cm^-1 range.¹⁹ The ν(C=C), δ(C-H) and δ(C=C) stretching vibrations do not show any appreciable alteration, indicating that the benzyl groups are far from the coordination center.

( ¹ H, ¹³ C, ¹¹⁹ Sn) NMR spectroscopy

The assignment of the ¹H and ¹³C chemical shifts is presented in the experimental section, and coupling constants, ¹J(¹¹⁹Sn-¹³C) and ²J(¹¹⁹Sn-¹H), and chemical shifts, δ(¹¹⁹Sn), of our adducts, as well as of other compounds from the literature, are shown in Table 4. The NMR coupling constants ¹J(¹¹⁹Sn-¹³C) and ²J(¹¹⁹Sn-¹H) for compound 5, which was crystallographically characterized⁵ as being a five-coordinate tin(IV) adduct, were observed at 568 and 72 Hz, respectively. However, the adduct 6, which has higher coupling constants values of 888 and 92 Hz, is formed by six-coordinate tin(IV) atoms. In view of this, the coupling constants values for adducts 1-3 indicate that, in CDCl₃ solution, they are no longer octahedral as in the solid state, but undergo a structural change, showing five-coordinate behavior. In addition to that, the coupling constants for adduct 7 are considerably larger than those for adducts 1-5, indicating that higher ¹J(¹¹⁹Sn-¹³C) and ²J(¹¹⁹Sn-¹H) values tend to correlate with higher coordination numbers (see Table 4). This can be attributed, at least partially, to a tendency to maximize the s-character in the C-Sn-C bonds in six-coordinate derivatives.²⁰ Applying the equation ¹J(¹¹⁹Sn-¹³C) = 11.4 θ - 875, and our observed coupling constant of 568 Hz to adduct 5, whose X-ray structure is known, we find the value of 127º for the (C-Sn-C) angle θ of adduct 5, only 9º lower than the angle of 136.4(9)º reported by Ng and Rheingold⁴ (Table 4). The NMR data for the 1:1 adduct [Me₂SnCl₂(Hmimt)] [Hmimt = 1-methyl-2(3H)-imidazolinethione], whose X-ray structure shows it to be a trigonal bipyramid,²¹ present some similarity with the data for adduct 5. The reported values of ¹J(¹¹⁹Sn-¹³C), ²J(¹¹⁹Sn-¹H), δ(¹¹⁹Sn) and θ(C-Sn-C) for this compound are 491, 71 Hz, +19 ppm and 133º, respectively.

The coupling constant ²J(¹¹⁹Sn-¹H) value of 123 Hz found for fac-adduct 18 is only 8 Hz higher than that found for fac-adduct 16 and it is almost equal to that of 124 Hz found for mer-adduct 17. This observation indicates that this behavior may be due more to an electronic rather than to a structural effect, wherein the oxygen atom in BtSOMe exhibits a higher electronic donor effect than the oxygen in dbso. Thus the tin(IV) atom would be more shielded in 18 than in 16. This behaviour is also observed in ¹¹⁹Sn-NMR spectroscopy.

¹¹⁹Sn chemical shifts were found to lie between ca. -90 and -330 ppm in five-coordinate dimethyltin(IV) chelates.²² However, the values for adducts 1-5 (Table 4) are completely out of this range, indicating that the presence of chlorine atoms in the coordination sphere of the tin(IV) nucleus shifts the signal towards much more deshielded values. In diphenyltin(IV) derivatives, (adducts 8-10 and 12), the chemical shift values were found as usual, i. e., at lower frequencies than in dimethyltin(IV) adducts, in spite of the greater electron donation properties of the methyl groups compared to phenyl.²³ This observed dependence of the chemical shift of the tin(IV) atom on the nature of the diorganotin(IV) derivative can be related to the relative polarity of the Sn-C bonds. The polarity of these bonds depends on the amount of residual positive charge located on tin, as well as on the s-character of tin(IV) in these bonds.²⁴ Accordingly, more residual positive charge is localized on the central tin(IV) atom in Ph₂Sn²⁺ compared to Me₂Sn²⁺. Therefore, more shielding is expected on the tin(IV) atom in the five-coordinate diphenyltin(IV) derivative. The ¹¹⁹Sn chemical shift value of -167 ppm, observed for adduct 14, is inside the range of -125 to -515 ppm observed for six-coordinate diorganotin(IV) chelates.²²

Mössbauer spectroscopy

¹¹⁹Sn Mössbauer spectral parameters of the dbso derivatives are reported in Table 5, which includes parameters from the literature for comparison. The spectra of the adducts 2, 7, 13 and 19 are shown in Figure 3.

The isomer shift (δ) is very sensitive to the first coordination sphere and depends on the nature and number of organic groups bonded to the tin(IV) atom.^28,29 This parameter is correlated to the change of s-electron density on the metal by means of the +I inductive effect imposed by the ligands. A result of this is the inverse dependence of δ with electronegativity.³⁰ Thus δ increases on replacing the phenyl by the alkyl groups in the coordination sphere of tin(IV). This increase in δ can be seen (Table 5) starting with the adducts 17-20 derived from MeSnCl₃ (0.89-0.97 mm s^-1), including the adducts 12-16 derived from Ph₂SnCl₂ (1.26-1.39 mm s^-1), and finishing with the adducts 1-6 derived from Me₂SnCl₂ (1.35-1.47 mm s^-1). The adduct 7 derived from ⁿBu₂SnCl₂ (1.63 mm s^-1) confirms that isomer shifts increase in the sequence Ph₂Sn²⁺ < Me₂Sn²⁺ < ⁿBu₂Sn²⁺ (see Table 5).

The quadrupole splitting (Δ) values summarized in Table 5 show that this parameter can potentially throw light upon the question of which is the correct coordination number of the tin(IV) atom in adducts formed from diorganotin(IV) dichloride and monodentate ligands. The Δ value of adduct 1 (3.52 mm s^-1) is typical of dimethyltin(IV) adducts with a distorted trigonal bypyramidal geometry around the tin(IV) atom, as supported by the crystal structure determination: an authentic five-coordinate trigonal bipyramidal compound⁵ without any short intermolecular contact. The parameters δ = 1.45; Δ = 3.13 mm s^-1 and the angle θ (C-Sn-C) = 133º have been reported for the related adduct with analogous coordination geometry, namely [Me₂SnBr₂(Hmimt)].²¹ The larger Δ value of adduct 2 (3.84 mm s^-1), compared to 1 (3.52 mm s^-1), is due to the considerable intermolecular contact [S=O···Sn 2.75(1) Å], which makes this distance much shorter than the sum of the van der Waals³¹ radii of the two atoms, r_Sn = 2.2, r_o = 1.5 Å. Thus, the C-Sn-C angle is opened up to 154.7(3)º, to complete the distorted trans-C-Sn-C skeleton.¹⁶ The differences observed in Δ values ranging from 3.54-4.16 mm s^-1 for adducts 4-7, compared to adducts 1 and 3, are considerable (Table 5). Therefore, this means that these compounds (4-7) possess the same geometry around the tin(IV) atom, which corresponds to six-coordinated structures, as supported by X-ray diffraction for 4,¹⁹5,³² and 6.³³ In view of this, the Mössbauer spectra of the adducts 8-11 show a doublet with values of Δ ranging from 2.92-3.22 mm s^-1, indicating five-coordinated tin(IV) compounds, whereas the spectra of 12-16 show values ranging from 3.51-3.98 mm s^-1, which are typical of six-coordinated tin(IV) derivatives.

The geometry about tin(IV) in adducts 17 (Δ = 2.13 mm s^-1) and 18 (Δ = 2.07 mm s^-1) is that of a distorted octahedron environment, with the SnCl₃⁺ skeleton arranged in facial mode (Figure 1). On the other hand, in the octahedral adducts 19 (Δ = 1.95 mm s^-1) and 20 (Δ = 1.69 mm s^-1), the SnCl₃⁺ skeleton is organized in a meridional position with the methyl group trans to the nitrogen atom in both compounds.¹⁷ In view of this, we believe that the magnitude of Δ can be used to distinguish between fac- and mer-arrangements as more crystallographic data become available.

From the Δ value and the point-charge approach, the equation Δ = -4 [R] (1 - 0.75 sin²θ)^1/2 gives an estimate of θ, where (θ, deg) is the C-Sn-C bond angle³⁴ and [R] is the partial quadrupole splitting (pqs) of the group, arising either from the alkyl or the aryl groups. For five- and six-coordinated tin(IV), the working values used for [Me] and [Ph] were -1.13 and -0.95 mm s^-1, respectively.^34,35 The calculated bond angle values of 134º for 1 and 180º for 12 reveal trigonal bipyramidal (TBP) and octahedral (O_h) arrangements around the tin(IV) atoms, respectively.

Supplementary Information

Crystallographic data for the structural analysis of the complexes have been deposited at the Cambridge Crystallographic Data Center, with the deposition numbers CCDC 619271 for trans-[Ph₂SnCl₂(dbso)₂] and CCDC 619272 for fac-[MeSnCl₃(dbso)₂]. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB21EZ, United Kingdom; Fax: [phone], or e-mail: [e-mail].

Acknowledgements

This work was sponsored by grants from CNPq and FINEP (CT-INFRA Nº 0970/01), which are gratefully acknowledged. The author JRS is grateful to the IFSC-USP for providing access to the Kappa CCD diffractometer and cryogenic facilities.

Received: February 26, 2009

Web Release Date: August 12, 2009

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Publication Dates

• Publication in this collection
  30 Oct 2009
• Date of issue
  2009

History

• Accepted
  12 Aug 2009
• Received
  26 Feb 2009