Organotin ( IV ) Derivatives of 2-Acetylpyridine-N ( 4 )-Phenylthiosemicarbazone , HAP 4 P , and 2-Hydroxyacetophenone-N ( 4 )-Phenylthiosemicarbazone , H 2 DAP 4 P . Crystal and Molecular Structure of [ SnMe 2 ( DAP 4 P ) ] and [ SnBu 2 ( DAP 4 P ) ]

As reações de 2-acetilpiridina-N(4)-feniltiosemicarbazona, HAP4P, e 2-hydroxiacetofenonaN(4)-feniltiosemicarbazona, H 2DAP4P, com R4-mSnXm (m = 2, 3; R = Me, nBu, Ph e X = Cl, Br) levaram à formação de complexos organoestânicos hexae penta-coordenados, que foram estudados por análise elementar, espectroscopias no IV, RMN de 1H e Mössbauer. As estruturas moleculares dos complexos [SnMe 2(DAP4P)] e [Sn Bu2(DAP4P)] foram determinadas por análises de difração de raios X. Nos compostos [SnClMe 2(AP4P)] e [SnBrMe2(AP4P)], o ligante desprotonado AP4P -


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][2][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,Stridentate ligand and coordinates to the metal through both the nitrogen and the thiolate sulphur atoms.This coordination mode was observed in [SnX 3 (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 [SnCl 2 R 2 (HFPT)] 6 , where the thiosemicarbazone molecule acts as a neutral N(azomethine),S-bidentate ligand.Finally, a third coordination mode was found in [SnClMe 2 (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,Sand 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 uncertain 7 .
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-hydroxyacetophenone-N(4)-phenylthiosemicarbazone, H 2 DAP4P, with organotin(IV) compounds.The structures of these ligands are shown below.
[SnCl 2 Me(AP4P)] (1 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 1 H NMR spectra only for 3, 4 and 8, in CDCl 3 , 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 BaSnO 3 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 α Mo radiation (λ = 0.71073 Å), obtained in a fine focus sealed tube 9 .
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 software 10 .The structures were solved using the heavy-atom method 11 .The SHEL97 software 12 was used for refinement by full-matrix least-squares calculations.

Syntheses
HAP4P and H 2 DAP4P 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; H 2 DAP4P: mp 120-122 ºC) which were filtered and dried under vacuum over CaCl 2 .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 previously 8 .The microanalyses were performed using a HERAEUS CHN rapid elemental analyzer, giving the following results for C, H and N:

Crystal structures of [SnMe 2 (DAP4P)](8) and [Sn n Bu 2 (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)    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: A 13  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 thiosemicarbazones are not very different.In both structures there are intermolecular interactions involving N(3)-HΛO.The geometric features of these interactions are respectively: N(3)-H 0.85 Å, N(3)-HΛO (1/2 + x, 1/2y, 1 -z) 3.033 Å, N(3)-HΛO 179.1º for compound 8 and N(3)-H 0.86 Å, N(3)-HΛO 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 ring 8 .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.

H NMR spectroscopy
The 1 H NMR spectrum of complex 8 showed two singlets at δ 2.7 and 0.7 [ 2 J( 119 Sn-CH 3 ) 73 Hz], due to two magnetically non-equivalent methyl groups assigned to N=C-CH 3 and Sn-CH 3 , respectively.According to the literature, the use of the Lockhart-Manders 16 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.

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 H 2 DAP4P 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 H 2 DAP4P 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 ν(NN-H), and retain the absorption bands found in the free ligands at 3301 and 3305 cm -1 , attributed to ν(PhN-H) vibrations, which are shifted to lower frequencies.Thus, the bands in the 3426-3237 cm -1 range are assigned to the ν(NN-H) stretching frequencies.
The ν(C=N) absorptions at 1523 and 1521 cm -1 for HAP4P and H 2 DAP4P, 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 ion 18 .The same trend is exhibited by the bands at 1360 cm -1 for HAP4P and at 1366 cm -1 for H 2 DAP4P.
The bands at 1189 and 782 cm -1 in HAP4P and at 1197 and 757 cm -1 in H 2 DAP4P, which have a significant contribution from ν(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 ν(Sn-O) mode.

Mössbauer spectroscopy
119 Sn 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 (δ) are lower than those of the parent acids 15  n Bu 2 ] (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 (1.09mm 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 δ values correspond to the complexes containing electronegative groups (Cl ou Ph), whereas alkyl groups (Me, n Bu), of low electronegativity, lead to higher δ values.
Quadrupole splitting (∆) values, presented in Table 4, are not sufficient in themselves to characterize a Sn(IV) complex as either tetra-, penta-, hexa-, or heptacoordinated 20 .However, ∆ has been useful in distinguishing between cis-, trans-[SnX 4 R 2 ] and [SnX 3 R 2 ] configurations in complexes with octahedral and trigonal bipyramidal geometries, respectively.Thus, a complex with a trans-[SnX 4 R 2 ] configuration usually has ∆ 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-SnX 4 R 2 configuration.Sham and Bancroft 21 , also using point charge calculations, have shown that the quadrupole splittings (∆) for trans-[SnX 4 R 2 ] compounds decrease smoothly from the value (4 mm s -1 ) for a regular octahedral geometry (θ = 180º) as the structure becomes more distorted, i.e. the q angle (R-Sn-R) becomes less than 180º.
The quadrupole splitting (∆) values, shown in Table 4, indicate that complexes 2-5 adopt a trans-[SnX 4 R 2 ] arrangement and the octahedral diphenyl compound (complex 2) is the most distorted among all of them.The values between 2.03-2.85mm 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 (∆) value for complex 10 (0.0 mm s -1 ) indicates that the charge distribution around the Sn(IV) nucleus is highly symmetrical 5 and the geometry is considerably non distorted.

Table 1 .
Crystallographic data and experimental parameters.

Table 3 .
Main IR bands (cm -1 ) for the ligands HAP4P and H 2 DAP4P and their complexes