Organotin(IV) Derivatives of 2-Acetylpyridine-N(4)-Phenylthiosemicarbazone, HAP4P, and 2-Hydroxyacetophenone-N(4)-Phenylthiosemicarbazone, H2DAP4P: Crystal and Molecular Structure of [SnMe2(DAP4P)] and [SnBu2(DAP4P)]

Abstracts

The reactions of 2-acetylpyridine-N(4)-phenylthiosemicarbazone, HAP4P, and 2-hydroxyacetophenone-N(4)-phenylthiosemicarbazone, H2DAP4P, with R4-mSnXm (m = 2, 3; R = Me, nBu, Ph and X = Cl, Br) led to the formation of hexa- and penta-coordinated organotin(IV) complexes, which were studied by microanalysis, IR, ¹H-NMR and Mössbauer spectroscopies. The molecular structures of [SnMe2(DAP4P)] and [Sn nBu2(DAP4P)] were determined by single-crystal X-ray diffraction studies. In the compounds [SnClMe2(AP4P)] and [SnBrMe2(AP4P)], the deprotonated ligand AP4P- is N,N,S-bonded to the Sn(IV) atoms, which exhibit strongly distorted octahedral coordination. The structures of [SnMe2(DAP4P)] and [Sn nBu2(DAP4P)] revealed that the DAP4P2- anion acts as a O,N,S-tridentate ligand. In these cases, the Sn(IV) atoms adopt a strongly distorted trigonal bipyramidal configuration where the azomethine N and the two C atoms are on the equatorial plane while the O and the S atoms occupy the axial positions.

thiosemicarbazone complexes; organotin(VI) complexes; crystal structure analyses

As reações de 2-acetilpiridina-N(4)-feniltiosemicarbazona, HAP4P, e 2-hydroxiacetofenona-N(4)-feniltiosemicarbazona, H2DAP4P, com R4-mSnXm (m = 2, 3; R = Me, nBu, Ph e X = Cl, Br) levaram à formação de complexos organoestânicos hexa- e penta-coordenados, que foram estudados por análise elementar, espectroscopias no IV, RMN de ¹H e Mössbauer. As estruturas moleculares dos complexos [SnMe2(DAP4P)] e [Sn nBu2(DAP4P)] foram determinadas por análises de difração de raios X. Nos compostos [SnClMe2(AP4P)] e [SnBrMe2(AP4P)], o ligante desprotonado AP4P- está N,N,S-ligado aos átomos de Sn(IV) que exibem cordenação octaédrica fortemente distorcida. As estruturas dos complexos [SnMe2(DAP4P)] e [Sn nBu2(DAP4P)] revelaram que o ânion DAP4P2- age como um ligante O,N,S-tridentado. Nestes casos os átomos de Sn(IV) adotam coordenação com geometria bipiramidal trigonal fortemente distorcida, com o átomo de N e os dois átomos de C no plano equatorial, enquanto que os átomos de O e S ocupam as posições axiais.

Article

Organotin(IV) Derivatives of 2-Acetylpyridine-N(4)-Phenylthiosemicarbazone, HAP4P, and 2-Hydroxyacetophenone-N(4)-Phenylthiosemicarbazone, H₂DAP4P. Crystal and Molecular Structure of [SnMe₂(DAP4P)] and [SnBu₂(DAP4P)]

Gerimário F. de Sousa^a*, Regina H. P. Francisco^b, M. Teresa do P. Gambardella^b, Regina H. de A. Santos^b and Anuar Abras^c

^a Instituto de Química, Universidade de Brasília, 70919-970 Brasília - DF, Brazil

^b Instituto de Química de São Carlos, Universidade de São Paulo, 13560-970 São Carlos - SP, Brazil

^c Departamento de Física, Universidade Federal de Minas Gerais, 30123-970 Belo Horizonte - MG, Brazil

As reações de 2-acetilpiridina-N(4)-feniltiosemicarbazona, HAP4P, e 2-hydroxiacetofenona-N(4)-feniltiosemicarbazona, H₂DAP4P, com R_4-mSnX_m (m = 2, 3; R = Me, ⁿBu, Ph e X = Cl, Br) levaram à formação de complexos organoestânicos hexa- e penta-coordenados, que foram estudados por análise elementar, espectroscopias no IV, RMN de ¹H e Mössbauer. As estruturas moleculares dos complexos [SnMe₂(DAP4P)] e [SnⁿBu₂(DAP4P)] foram determinadas por análises de difração de raios X. Nos compostos [SnClMe₂(AP4P)] e [SnBrMe₂(AP4P)], o ligante desprotonado AP4P^- está N,N,S-ligado aos átomos de Sn(IV) que exibem cordenação octaédrica fortemente distorcida. As estruturas dos complexos [SnMe₂(DAP4P)] e [SnⁿBu₂(DAP4P)] revelaram que o ânion DAP4P^2- age como um ligante O,N,S-tridentado. Nestes casos os átomos de Sn(IV) adotam coordenação com geometria bipiramidal trigonal fortemente distorcida, com o átomo de N e os dois átomos de C no plano equatorial, enquanto que os átomos de O e S ocupam as posições axiais.

The reactions of 2-acetylpyridine-N(4)-phenylthiosemicarbazone, HAP4P, and 2-hydroxyacetophenone-N(4)-phenylthiosemicarbazone, H₂DAP4P, with R_4-mSnX_m (m = 2, 3; R = Me, ⁿBu, Ph and X = Cl, Br) led to the formation of hexa- and penta-coordinated organotin(IV) complexes, which were studied by microanalysis, IR, ¹H-NMR and Mössbauer spectroscopies. The molecular structures of [SnMe₂(DAP4P)] and [SnⁿBu₂(DAP4P)] were determined by single-crystal X-ray diffraction studies. In the compounds [SnClMe₂(AP4P)] and [SnBrMe₂(AP4P)], the deprotonated ligand AP4P^- is N,N,S-bonded to the Sn(IV) atoms, which exhibit strongly distorted octahedral coordination. The structures of [SnMe₂(DAP4P)] and [SnⁿBu₂(DAP4P)] revealed that the DAP4P^2- anion acts as a O,N,S-tridentate ligand. In these cases, the Sn(IV) atoms adopt a strongly distorted trigonal bipyramidal configuration where the azomethine N and the two C atoms are on the equatorial plane while the O and the S atoms occupy the axial positions.

Keywords: thiosemicarbazone complexes, organotin(VI) complexes, crystal structure analyses

Introduction

N,N,S- and O,N,S-tridentate thiosemicarbazones derived from 2-formyl- and 2-acetylpyridine form two important classes of compounds possessing biological activity^1-3. In this context, a number of complexes of first-row transition elements have been extensively studied by X-ray structural analysis and a number of other spectroscopic techniques^2,4. However organotin(IV) complexes of thiosemicarbazones have received less attention.

The chelating behaviour of N,N,S-tridentate thiosemicarbazones has been investigated and three different complexation modes have been identified. In the most common one, the compound acts as a mononegative N,N,S-tridentate ligand and coordinates to the metal through both the nitrogen and the thiolate sulphur atoms. This coordination mode was observed in [SnX₃(FPT)] (X = Cl, Br, I and HFPT = 2-formylpyridinethiosemicarbazone)⁵. A second coordination mode was reported for a series of octahedral 1:1 adducts of the general formula [SnCl₂R₂(HFPT)]⁶, where the thiosemicarbazone molecule acts as a neutral N(azomethine),S-bidentate ligand. Finally, a third coordination mode was found in [SnClMe₂(FPT)]². In this trigonal bipyramidal 1:1 complex, the ligand acts as a mononegative N,S-bidentate ligand, while the pyridine nitrogen atom remains uncoordinated.

A number of organotin(IV) complexes containing O,N,S- and O,N,O-tridentate ligands and exhibiting biological or pharmacological activity have been studied, but structural features which may affect the antitumor activity and cytotoxicity of organotin(IV) are still uncertain⁷.

These observations have increased our interest in the structural properties of these kinds of ligands and have motivated us to study the reactions of 2-acetylpyridine-N(4)-phenylthiosemicarbazone, HP4P, and 2-hydroxy-acetophenone-N(4)-phenylthiosemicarbazone, H₂DAP4P, with organotin(IV) compounds. The structures of these ligands are shown below.

Experimental

Syntheses

HAP4P and H₂DAP4P were prepared from a 1:1 molar ratio of N(4)-phenylthiosemicabazide and the appropriate ketone (2-acetylpyridine or 2-acetylphenol) in boiling EtOH (15 mL) for 30 min. After cooling, the compounds were obtained as pale-yellow needles (HAP4P: mp 171-174 °C; H₂DAP4P: mp 120-122 ºC) which were filtered and dried under vacuum over CaCl₂. The organotin(IV) complexes were obtained by the following procedure: 0.20 mmol of the appropriate ligand were dissolved by refluxing in dry MeOH for 5 min. To this solution were added 0.21 mmol of one organotin(IV) species in 5 mL of MeOH, and the resulting mixture was refluxed for 1 h and filtered to give a clear solution. Cooling the solution and slowly evaporating the solvent led to crystalline products with yields on the order of 70%. Single crystals suitable for X-ray diffraction were isolated only for complexes 2, 3, 7 and 8. The structures of 2 and 3 were solved and described previously⁸. The microanalyses were performed using a HERAEUS CHN rapid elemental analyzer, giving the following results for C, H and N:

[SnCl₂Me(AP4P)] (1). Anal. Calc. for C₁₅H₁₇Cl₂N₄ SSn: C, 37.95; H, 3.61; N, 11.80. Found: C, 38.81; H, 3.56; N, 12.81%.

[SnClPh₂(AP4P)] (2). Anal. Calc. for C₂₆H₂₃ClN₄SSn: C, 54.06; H, 4.02; N, 9.70. Found: C, 52.48; H, 4.22; N, 11.10%.

[SnClMe₂(AP4P)] (3). Anal. Calc. for C₁₆H₁₉ClN₄SSn: C, 42.37; H, 4.22; N, 12.35. Found: C, 42.31; H, 4.19; N, 12.25%.

[SnBrMe₂(AP4P)] (4). Anal. Calc. for C₁₆H₁₉BrN₄SSn: C, 38.59; H, 3.85; N, 11.25. Found: C, 37.07; H, 3.55; N, 11.12%.

[SnClⁿBu₂ (AP4P)] (5). Anal. Calc. for C₂₂H₃₁ClN₄SSn: C, 49.14; H, 5.81; N, 10.42. Found: C, 48.82; H, 5.75; N, 10.36%.

[SnCl₂Me(HDAP4P)] (6). Anal. Calc. for C₁₆H₁₇Cl₂N₃ SSn: C, 39.29; H, 3.48; N, 8.59. Found: C, 41.89; H, 4.00; N, 8.69%.

[SnPh₂(DAP4P)] (7). Anal. Calc. for C₂₇H₂₃N₃OSSn: C, 58.30; H, 4.17; N, 7.55. Found: C, 59.56; H, 4.40; N, 7.75%.

[SnMe₂(DAP4P)] (8). Anal. Calc. for C₁₇H₁₉N₃OSSn: C, 47.25; H, 4.43; N, 9.72. Found: C, 47.13; H, 4.29; N, 9.71%.

[SnⁿBu₂ (DAP4P)] (9). Anal. Calc. for C₂₃H₃₁N₃OSSn: C, 53.51; H, 6.05; N, 8.14. Found: C, 53.38; H, 5.82; N, 8.13%.

Infrared spectra were recorded on a Nicolet 5ZDX-FT spectrophotometer in the 4000-400 cm^-1 range using KBr pellets. Due to the poor solubility of some of the complexes, it was possible to obtain ¹H NMR spectra only for 3, 4 and 8, in CDCl₃, using a 250 MHz Bruker spectrometer. Chemical shifts are relative to internal tetramethylsilane. ¹¹⁹Sn Mössbauer spectra were measured using a Model AM-1 Mössbauer efect spectrometer, moving a 15-mCi BaSnO₃ source at room temperature. The isomer shift values are given with respect to this source. The samples were measured at liquid nitrogen temperature and all spectra were computer fitted assuming Lorentzian line shapes.

Crystal structure determinations

Single-crystal X-ray diffraction data were collected on an Enraf-Nonius CAD-4 automatic diffractometer, with a graphite monochromated K_aMo radiation (l = 0.71073 Å), obtained in a fine focus sealed tube⁹.

Experimental parameters and crystallographic data for both complexes are shown in Table 1. In both cases, the data reductions were carried out with the XCAD-4 software¹⁰. The structures were solved using the heavy-atom method¹¹. The SHEL97 software¹² was used for refinement by full-matrix least-squares calculations.

Results and Discussion

Crystal structures of [SnMe₂(DAP4P)](8) and[Sn ⁿBu₂(DAP4P)] (9).

The structure determinations of compounds 8 and 9 revealed similar molecular structures with the occurrence of pentacordinated Sn(IV) complexes, with a trigonal bipyramidal (TBP) geometry. In the coordination polyhedron of the two compounds, two carbons (from alkyl groups) and the N(1) atom (from the thiosemicarbazone) occupy the equatorial plane. Atoms S and O occupy axial positions. Figures 1 and 2 show the molecules along with the labeled atoms.

In both complexes, the largest bond angle involving the Sn(IV) atom was O–Sn–S: 151.32(6) and 156.96(15)°, respectively, indicating strong deviations from the ideal value of 180°. Dihedral angles of 88 and 87° were observed between the planes through Sn, C(16), C(17), N(1) and Sn, O, S (8) and Sn, C(16), C(20), N(1) and Sn, O, S (9), respectively. Selected bond distances and angles of the two complexes are shown in Table 2.

The structures of the complexes [SnPh₂(Hdaptsc)]Cl¹³ (A), H₂daptsc = 2,6-diacetylpyridinebis(thiosemicarbazone), [SnMe₂(Hdapf)]₂[SnCl₄ Me₂]¹⁴ (B), H₂ dapf = 2,6-diacetyl-pyridinebis(2-furoylhydrazone) and [SnMe₂(Hdapt)] Br.H₂O¹⁵ (C), H₂dapt = 2,6-diacetylpyridinebis(2-tenoyl-hidrazone) showed that one proton from the azomethine group of the ligand molecules has been eliminated upon coordination. The proton on the other azomethine group remained bonded to the nitrogen after coordination and the ligand showed one S atom as a thione (or O as keto) and the other S atom as a thiolate (or O as enolate), as evidenced by the differences observed in bond distances between the sulfur (or oxygen) and Sn(IV) atoms and between the sulfur (or oxygen) and the carbon atoms. Thus, the following bond distances were observed in the compounds mentioned: A¹³, Sn–S(1) (thiolate form) 2.592(1), S(1)–C(13) 1.731(5), Sn–S(2) (thione form) 2.703(1), and S(2)–C(23) 1.694(6) Å; in complex B¹⁴, the distances are: Sn(2)–O(1) (enolate form) 2.227(4), O(1)–C(4) 1.276(8), Sn(2)–O(2) (keto form) 2.474(4) and O(2)–C(14) 1.242(7) Å; for C¹⁵ the values are: Sn–O(1) (keto) 2.493(4), O(1)–C(3) 1.241(7), Sn–O(2) (enolate) 2.209(4) and O(2)–C(13) 1.286(7) Å.

In the cases, of compounds 8 and 9, part of the thiosemicarbazone derivative molecule is planar. The observed dihedral angle between the phenyl rings was 51° for complex 8 and 57° for complex 9. The packing mode is determined by the nature of the alkyl ligands. The presence of butyl groups in 9, larger and with larger displacement parameters, prevents the orthorhombic symmetry once the average distances between the thio-semicarbazones are not very different. In both structures there are intermolecular interactions involving N(3)–HLO. The geometric features of these interactions are respectively: N(3)–H 0.85 Å, N(3)–HLO (1/2 + x, 1/2 - y, 1 - z) 3.033 Å, N(3)–HLO 179.1º for compound 8 and N(3)–H 0.86 Å, N(3)–HLO 174.6º for compound 9.

In complexes 3 and 4, whose structures are nearly identical, except for the presence of a chloride ligand in 3 and a bromide ligand in 4, the ligand molecule bears only one substituent on the pyridine ring⁸. The observed distances were: Sn–S 2.4728(8), S–C(10) 1.750(3) Å for the chloride complex and Sn–S 2.4743(12), S–C(10) 1.739(5) Å for the bromide complex, indicating that the S atom is coordinated in the thiolate form.

¹H NMR spectroscopy

The ¹H NMR spectrum of complex 8 showed two singlets at d 2.7 and 0.7 [²J(¹¹⁹Sn–CH₃ ) 73 Hz], due to two magnetically non-equivalent methyl groups assigned to N=C–CH₃ and Sn–CH₃, respectively. According to the literature, the use of the Lockhart-Manders¹⁶ equation shown below and of the observed coupling constant of 73 Hz yields a C–Sn–C angle of 123º , in excellent agreement with the angle of 122.50(13)º observed in the solid state. This data suggests that the basic structural features of the solid-state phase remain in solution.

q = 0.0161{²J(¹¹⁹Sn–CH ₃)}² -

- 1.32{²J(¹¹⁹Sn–CH₃ )] + 133.4

A similar result has been reported for a trigonal bipyramidal (TBP) complex with an analogous coordination geometry, [SnMe₂(L)]¹⁷ (H₂L = salicyl-aldeydethiosemicarbazone). In this case, the ²J(¹¹⁹Sn–CH₃ ) was 70 Hz.

Infrared spectroscopy

Table 3 shows the assignment of the main IR absorption bands for the ligands and their complexes. In the 3463-1168 cm^-1 region the HAP4P and H₂DAP4P ligands show bands attributed to hydrogen-bonded O–H and N–H overlapping with the C–H stretching absorptions of phenyl and pyridyl rings. As a result of the monodeprotonation of the HAP4P ligand and double deprotonation of the H₂DAP4P ligand (except in complex 6), the spectra of compounds 1-5 and 7-9 lack bands located at 3241 and 3168 cm^-1, attributed to n(NN–H), and retain the absorption bands found in the free ligands at 3301 and 3305 cm^-1, attributed to n(PhN–H) vibrations, which are shifted to lower frequencies. Thus, the bands in the 3426-3237 cm^-1 range are assigned to the n(NN–H) stretching frequencies.

The n(C=N) absorptions at 1523 and 1521 cm^-1 for HAP4P and H₂DAP4P, respectively, are shifted to higher frequencies by ca. 15-30 cm^-1 in the spectra of the complexes, indicating coordination of the azomethine nitrogen N(2) to the metal ion¹⁸. The same trend is exhibited by the bands at 1360 cm^-1 for HAP4P and at 1366 cm^-1 for H₂DAP4P.

The bands at 1189 and 782 cm^-1 in HAP4P and at 1197 and 757 cm^-1 in H₂DAP4P, which have a significant contribution from n(C=S) stretching vibrations, are shifted to lower frequencies in the spectra of the complexes, suggesting coordination through the sulfur atom^18,19. The far IR bands observed in the 509-504 cm^-1 range were tentatively assigned to the n(Sn–O) mode.

Mössbauer spectroscopy

¹¹⁹Sn Mössbauer spectroscopy was performed on all nine complexes, giving the results shown in Table 4, which include parameters from the literature for comparison.

Complexes 1-6 exhibit octahedral coordination while complexes 7-9 adopt a strongly distorted trigonal bipyramidal (TBP) configuration where the anions AP4P^- and HDAP4P^- act as N,N,S- and O,N,S-tridentate ligands, respectively. Their isomer shifts (d) are lower than those of the parent acids¹⁵ [SnCl₃Me] (1.20 mm s^-1), [SnCl₂Ph₂](1.32 mm s^-1), [SnCl₂Me₂](1.49 mm s^-1), [SnBr₂Me₂](1.59 mm s^-1) and [SnCl₂ⁿBu₂](1.75 mm s^-1).

Isomer shifts always decrease upon adduct formation, as a result of rehybridization to higher coordination for the Sn(IV) atoms in the complexes in which s orbital participation is less than 25%. The reduction in the isomer shift in complex 6 (0.90 mm s^-1), compared to 1 (0.98 mm s^-1), is due to the structural differences between AP4P^- and HDAP4P^-. The former ligand has one ligating pyridine N atom, whereas the latter one has a phenol O atom. Therefore, AP4P^- is a much less electronegative ligand than HDAP4P^-, which accounts for the lower value of the isomer shift in complex 6.

In complexes 2 to 5, the ligand remains unchanged, but the phenyl groups in complex 2 (1.26 mm s^-1) were replaced by methyl groups in 3 (1.38 mm s^-1), the chloride in complex 3 was replaced by bromide in 4 (1.42 mm s^-1) and the methyl groups in complex 4 were replaced by n-butyl groups in 5 (1.54 mm s^-1). Again, a similar effect is observed as before, and the isomer shifts decrease on going from complex 5 to 2. Similar results were observed for complexes 7 (1.09 mm s^-1), 8 (1.18 mm s^-1) and 9 (1.31 mm s^-1).

A somewhat analogous trend can be seen in literature examples, as shown in Table 4. Although the ligands are not the same in all cases, they are related, and one can see that lower d values correspond to the complexes containing electronegative groups (Cl ou Ph), whereas alkyl groups (Me, ⁿBu), of low electronegativity, lead to higher d values.

Quadrupole splitting (D) values, presented in Table 4, are not sufficient in themselves to characterize a Sn(IV) complex as either tetra-, penta-, hexa-, or hepta-coordinated²⁰. However, D has been useful in distinguishing between cis-, trans-[SnX₄R₂]and [SnX₃R₂]configurations in complexes with octahedral and trigonal bipyramidal geometries, respectively. Thus, a complex with a trans-[SnX₄R₂] configuration usually has D values of ca. 4 mm s^-1, while a value of ca. 2 mm s^-1 would be expected for a complex with a cis-SnX₄R₂ configuration. Sham and Bancroft²¹, also using point charge calculations, have shown that the quadrupole splittings (D) for trans-[SnX₄R₂]compounds decrease smoothly from the value (4 mm s^-1) for a regular octahedral geometry (q = 180º) as the structure becomes more distorted, i.e. the q angle (R-Sn-R) becomes less than 180º.

The quadrupole splitting (D) values, shown in Table 4, indicate that complexes 2-5 adopt a trans-[SnX₄R₂] arrangement and the octahedral diphenyl compound (complex 2) is the most distorted among all of them. The values between 2.03-2.85 mm s^-1 for complexes 7-9 and 13-15 are typical of diorganotin(IV) complexes in a distorted trigonal bipyramidal (TBP)^17,22 environment. The lower quadrupole splitting (D) value for complex 10 (0.0 mm s^-1) indicates that the charge distribution around the Sn(IV) nucleus is highly symmetrical⁵ and the geometry is considerably non distorted.

Acknowledgments

The authors are grateful for financial support from CNPq, CAPES, FINEP, FAPEMIG and FAPESP in Brazil. The authors are grateful also to Dr. Julio Zukerman-Schpector for data collection, carried out on a CAD-4 at the Instituto de Química-USP.

Supplementary Information

Crystallographic data (excluding structure factors) for the structures have been deposited with the Cambridge Crystallographic Data Center as supplementary publication nos. 149653 and 149654. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, (fax: [phone] or e-mail: [e-mail]).

Received: November 6, 2000

Published on the web: July 29, 2001

FAPESP helped in meeting the publication costs of this article.

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Publication Dates

• Publication in this collection
  15 Apr 2002
• Date of issue
  Dec 2001

History

• Received
  06 Nov 2000
• Accepted
  29 July 2001