DALTON J. Chem. Soc., Dalton Trans., 1997, Pages 2201–2205 2201 Symmetry of the co-ordination sphere of di-n-butyltin(IV) in complexes with sulfanylcarboxylic acids Krisztina Gajda-Schrantz,a László Nagy,*,a Erno� Kuzmann,b Attila Vértes,b Jan Holec¡ek c and Antonín Lyc¡ka d a Department of Inorganic and Analytical Chemistry, A. József University, H-6701 Szeged, PO Box 440, Hungary b Department of Nuclear Chemistry, L. Eötvös University, Budapest, Hungary c Inorganic and General Chemistry Department, University of Pardubice, CZ-532 10 Pardubice, Czech Republic d Research Institute for Organic Syntheses, CZ-532 18 Pardubice-Rybitvi, Czech Republic Four di-n-butyltin(IV) complexes have been prepared with sulfanylacetic, 2-sulfanylpropionic, sulfanylsuccinic and 2,3-disulfanylsuccinic acid, using two different procedures.The compounds were characterised by elemental analysis, Fourier-transform, Raman, 119Sn Mössbauer, 1H, 13C and 119Sn NMR spectroscopy. The IR and Raman data indicate the presence of bidentate carboxylate groups, non-linear C]Sn]C bonds, and Sn]S bonds.The results of Mössbauer spectroscopic measurements, based on point-charge model calculations, have shown the general occurrence of trigonal-bipyramidal environments at tin(IV). The multinuclear NMR studies also suggested the {O,S} co-ordination of the di-n-butyltin(IV) fragment, within the cyclic oligomeric complexes. The investigation of the interactions between organotin(IV) mono- and di-cations and biologically active ligands (among them carboxylates and polyhydroxy compounds1,2) is of considerable importance due to the possible modification of the biological properties of the organotin(IV) group in the presence of these ligands.Therefore, efforts have been made to elaborate simple procedures for the preparation of complexes,3,4 as well as to understand the antitumour activity of the organotin(IV) cations.5 The importance of the organotin(IV) complexes is confirmed by their wide range of possible applications, particularly of those containing sulfur.Until now only few papers have been published on the structure of organotin(IV)–carboxylate compounds containing {O,S} donor sites.6–9 Continuing our work on the synthesis, equilibrium and structural characterisation of organotin(IV) complexes with carbohydrates,4,10–12 as well as carbohydrate derivatives of thiazolidine-4-carboxylic acids 13 and N-Dgluconylamino acids 14 containing {O,O} and {O,N} donors, respectively, we have synthesized four di-n-butyltin(IV) complexes with four different ligands containing {O,S} donor atoms.The symmetry of the co-ordination sphere of organotin(IV) was determined on the basis of Mössbauer spectroscopy and FTIR, Raman and 1H, 13C and 119Sn NMR measurements were performed to determine the possible binding sites. Experimental Materials Compounds L1 and L2 were from Aldrich, L3 from Sigma and L4 from Fluka.Di-n-butyltin(IV) oxide was from Fluka. Other reagents and solvents were from Reanal (Hungary). CO2H CH CH3 SH CO2H CH2 SH CO2H CH CH2 SH CO2H CO2H CH CH SH CO2H HS L3 L2 L1 L4 Preparation of the complexes Method a. The complexes were obtained by refluxing equimolar quantities of L1–L4 and di-n-butyltin(IV) oxide in methanol for 2 h [Scheme 1(a)]. Compound 1a precipitated from the mixture, while the others (2a, 3a, 4a) were obtained after removal of the solvent by rotary evaporation, then washed and/or recrystallised from methanol or chloroform.Method b. Di-n-butyltin(IV) oxide (1 g) was refluxed in methanol–n-propanol (4 :1, 250 cm3) for 5 h. The oxide reacted with n-propanol, giving di-n-butyltin(IV) propoxide and water [Scheme 1(b)]. From the solvent mixture the water distils off as an azeotrope. After its removal and cooling, an equimolar quantity of L1–L4, dissolved in methanol, was added to the solution of di-n-butyltin propoxide with vigorous stirring.Compound 1b precipitated during the stirring, 2b crystallised out,while 3b and 4b were obtained after evaporation of the solvent at room temperature and then washed or recrystallised from methanol or chloroform. Elemental analyses (C, H, S) were carried out by Ilse Beetz Microanalytical Laboratory, Kronach, Germany. The analytical data are presented in Table 1 together with other characteristic physical constants. Measurements The 119Sn Mössbauer spectra were recorded at 77 K on a conventional RANGER spectrometer in constant-acceleration mode with a source activity of 0.4 GBq.Computer evaluation was used to determine isomer shift (i.s.) and quadrupole splitting (q.s.) values. The spectra were analysed as Lorentzian lines by least-squares fitting. The reproducibility of the Mössbauer Scheme 1 m = 1 for L1–L3, 2 for L4. (i ) MeOH, 2 h reflux; (ii ) PrnOH– MeOH (1: 4), 5 h reflux; (iii ) HnL, 1 h stirring ( a ) mSnBu2O + H nL (Bu2Sn)mHn–2mL + mH2O ( b ) mSnBu2O SnBu2(OPr)2 (Bu2Sn)mHn–2mL ( i ) ( ii ) ( iii ) – mH2O – 2 mPrOH2202 J.Chem. Soc., Dalton Trans., 1997, Pages 2201–2205 parameters was found to be ±0.02 (i.s.) and 0.04 mm s21 (q.s.), respectively, in each measurement. The i.s. values are referred to that of CaSnO3. The Fourier-transform IR spectra of L1–L4 and the complexes as KBr pellets were measured on Bio-Rad Digilab Division FTS-40 and FTS-65A instruments. Liquid samples were examined as thin films on KBr plates in the range 4000–200 cm21.Fourier-transform Raman spectra were recorded using a Bio-Rad Digilab Division spectrometer from liquid and solid samples contained in glass cuvettes. Relevant vibration bands are reported in Table 2. The 119Sn (134.29), 13C (90.56) and 1H (360.13 MHz) NMR spectra of complexes containing {O,S} donor atoms were recorded on a Bruker AMX 360 spectrometer equipped with a 5 mm multinuclear tuneable probe and an X32 computer using UXNMR software.The compounds were measured in CDCl3 or (CD3)2SO in a standard manner. The 13C chemical shifts were referred to appropriate signals of the solvents and recalculated to the d scale {d(13C) 77.00 (CDCl3), 39.60 [(CD3)2SO]}. The 119Sn chemical shifts were referred to external neat SnMe4 (d 0.0) placed in a coaxial capillary. The 119Sn and 13C chemical shifts are collected in Table 4. Calculations For the determination of the steric arrangement around tin(IV) in these compounds, q.s.values were calculated on the basis of a simple but general molecular orbital model, according to the partial quadrupole splitting (p.q.s.) concept,15,16 for the possible symmetries of five-co-ordinated tin(IV) given in Fig. 3 involving binding by two butyl groups and negatively charged (deprotonated) sulfur and carboxylate oxygen atoms. It was also taken into account that the carboxylate group can co-ordinate either in a mono- or bi-dentate manner.Equations (1)–(3) were used in q.s.1 = (27Rtbe 1 4Atba 1 4Ctba 1 Btbe)/72� �� (1) q.s.2 = (22Rtba 2 5Rtbe 1 4Atbe 1 4Ctbe 2 2Btba)/132� �� (2) q.s.3 = 24Rtba 1 Atbe 1 Btbe 1 Ctbe (3) the calculation, for the general structures I–III shown in Scheme 2. The p.q.s. values of the different functional groups used in our calculations, and the calculated q.s. values for tin(IV) in different stereochemical arrangements, are given in Table 6 and Fig. 3, respectively. Results and Discussion The analytical data and the characteristic physical constants for the complexes are reported in Table 1.Results refer to the formation of compounds with 1 : 1 metal-to-ligand ratio, with the exception of di-n-butyltin(IV)–2,3-disulfanylsuccinic acid (4a, 4b) where a 2 : 1 metal-to-ligand ratio was found, reflecting the four accessible binding sites. All the complexes are soluble in Me2SO. Compounds 1a, 1b, 2a and 2b are soluble also in CHCl3, CCl4 and C6H6.Complexes 2b, 3a, 3b, 4a and 4b are soluble in methanol, too. Fourier-transform IR and Raman spectroscopy The vibration spectroscopic data (Table 2) suggest that the two kinds of preparation methods used resulted in complexes with the same structure. In the spectra of free L1–L4 a characteristband for S]H vibrations, between 2550 and 2575 Scheme 2 A C B R R R B R A C R R C A B I II III cm21, and for the CO2H group, between 1690 and 1715 cm21, are present. These bands cannot be found in the spectra of the complexes, indicating co-ordination of deprotonated thiol and carboxylate group(s) to the tin.Fig. 1 shows the IR and Raman spectra of 2,3-disulfanylsuccinic acid and its di-n-butyltin(IV) complex, as an example. It is clear that all of the four possible donor groups are deprotonated and co-ordinated in the complex, which is possible only in the case of a 2 : 1 metal-to-ligand ratio. In spite of this, in the case of 3a and 3b, one of the two carboxylate groups (probably that which is far from the thiol group) remains protonated (non-co-ordinated), as evidenced by the IR bands characteristic for the n(C]] O) stretching vibrations, slightly shifted to higher wavenumbers, compared to the free L.Thus only one extra donor group in the molecule cannot promote the formation of a higher metal-to-ligand ratio in the complex, as in the case of compounds 4a and 4b. The difference (Dn) between nasym(CO2 2) and nsym(CO2 2) compared with that for sodium salts of L1–L4 (Table 3), reflects the bridging bidentate co-ordination mode of the CO2 2 group.In the case of compounds 1a, 1b, 4a and 4b these bands are doublets due to the asymmetrical (different C]O bond lengths) co-ordination to the tin. The presence of an Sn]S absorption band in the 380– 400 cm21 region of the Raman spectra of the complexes pro- Fig. 1 Infrared and Raman spectra of 2,3-disulfanylsuccinic acid (a) and its di-n-butyltin(IV) complex (b) Table 1 Physical and analytic data for di-n-butyltin(IV) complexes of ligands containing {O,S} donor atoms Analysis (%)a Ligand Compound C H S Colour M.p./8C L1 L2 L3 L4 1ab 1bc 2ab 2bc 3ab 3bc 4ab 4bc 37.25 37.25 (37.2) 38.4 39.25 (39.20) 37.8 37.75 (37.85) 34.95 34.85 (34.9) 6.25 6.25 (6.25) 6.4 6.55 (6.6) 5.75 5.8 (5.8) 5.45 5.5 (5.35) 9.9 9.9 (9.95) 9.55 9.5 (9.5) 8.45 8.45 (8.4) 14.6 14.65 (15.5) White White Yellow White White White White White 178–181 179–182 — 99–102 111–113 110–114 250–252 247–250 a Calculated values in parentheses.b Prepared under reflux. c Propoxide method.J. Chem. Soc., Dalton Trans., 1997, Pages 2201–2205 2203 Table 2 Infrared and Raman spectral data (cm21) for free L1–L4 and their di-n-butyltin(IV) complexes IR Raman Compound n(C]] O) nasym(CO2) nsym(CO2) n(SH) n(Sn]S) nasym(Sn]C) nsym(Sn]C) L1 1a 1b L2 2a 2b L3 3a 3b L4 4a 4b 1713.1vs 1714.2vs 1697.0vs 1745.0w 1713.2m 1713.3s 1713.3 (sh) 1693.6s 1573.7s 1547.9s 1574.2s 1548.1s 1546.9s 1542.8s 1552.2s 1552.1s 1577.8 (sh) 1552.6vs 1561.4 (sh) 1553.2s 1409.0m 1393.5m 1409.3m 1394.5m 1408.0m 1409.7m 1416.7m 1417.2m 1415.7m 1389.3m 1410.9mw 1389.3mw 2576.2vs 2574.6vs 2565.2vs 2549.2m 2564.5s 2539.1ms 382.4vs 382.5vs 397.8ms 395.5ms 394.6s 394.5s 386.7vs 386.2s 598.4w 598.6w 597.3m 599.3m 596.8m 597.4m 597.3m 516.6w 548.0vs 516.0m 515.2m 516.3m 517.9m 517.8m vides support for co-ordination of sulfur to tin.The presence of two (asymmetric and symmetric) Sn]C absorption bands in the region 600–515 cm21 reveals that the R]Sn]R bond angle is less than 1808 in all compounds.Vibration bands (480–450 cm21) characteristic for Sn]O bonds also appeared. In the spectra of all complexes, bands characteristic for the n-butyl skeleton are present.17 NMR spectroscopic measurements The 119Sn and 13C NMR spectroscopic data are shown in Table 4. For compound 1b one set of chemical shifts was observed in each of the 1H and 13C NMR spectra.In the 1H NMR spectrum the SH signal and the coupling constant 3J(CH2]SH) (observed for the starting L) disappeared. These facts, a ca. 6 ppm change of d(13C) for the CH2 group with respect to the starting acid and 3J(119Sn]1H) 33.1 Hz support the formation of a Sn]S bond. The 13C NMR chemical shift of the CO2 2 group and the changed value with respect to the starting acid in CDCl3, i.e. a considerable downfield shift, indicates the bidentate character of this group.18 Taking the existence of a Sn]S bond into account, the existence of this bidentate character is only possible via co-ordination of the C]] O oxygen to another tin atom.The d(119Sn) value corresponds to a 4 1 1 type of coordination, 19 in a trigonal-bipyramidal arrangement. The value of the coupling constant 1J(119Sn]13C) corresponds to a C]Sn]C angle 20 of ca. 1308 (trigonal-bipyramidal co-ordination sphere of tin with two butyl groups in the equatorial plane).Since there is only one signal in the 119Sn NMR spectrum, five-coordination can be realised only by formation of a cyclic oligomer or a very long linear oligomer (short linear oligomers would require the existence of at least two signals for end and Table 3 The difference between nasym(CO2) and nsym(CO2) for the carboxylates and their complexes nasym(CO2) 2 nsym(CO2)/cm21 Compound Sodium salta b L1 L2 L3 L4 156.2, 181.3 190.7 198.7 203.3 138.9, 180.22 148.1 144.3 163.4 138.9, 180.8 148.3 144.1 169.9 a Complex prepared under reflux.b Complex prepared by the propoxide method. middle structural units, the relative integral intensity being dependent on the length of the chain). Meunier-Piret et al.21 published the crystal structure of di-n-butyl(thiosalicylato)- tin(IV), showing that this compound forms a centrosymmetric hexamer. This finding is in line with the above-mentioned existence of only one signal in the 119Sn NMR spectrum. The 119Sn NMR chemical shift and coupling constant of complex 1b in Me2SO have practically the same values as those in CDCl3, but the d(13C) of CO2 2 is different.This may be accounted for by the formation of a monomeric unit from a cyclic oligomer, one molecule of solvent being co-ordinated and retaining five-co-ordination of tin [Fig. 4(d )]. The asymmetric bidentate character of the CO2 group is changed to monodentate. Lockhard22 has observed such a type of monomerisation in an analogous 3-sulfanylpropionic acid derivative, based on molecular-weight determination.The solubility of product 1a is much lower than 1b in CDCl3. We cannot exclude the formation of a linear polymer under the reported experimental conditions. Compound 2b has the same character as 1b as is clear from a comparison of the data in Table 3. Two sets of 13C pairs having non-equivalent abundance have been observed in the 13C NMR spectrum for Sn]CH2 groups. The formation of a pair of signals is due to the presence of a chiral centre in the molecule (C]S carbon). Two sets of pairs can be explained by the existence of two different types of cyclic oligomers: (i ) two conformational isomers due to stereochemical reasons; (ii ) two isomers having different ring sizes.Compounds 3a, 3b, 4a and 4b are practically insoluble in CDCl3 and as a result they were measured in Me2SO. The ligand to tin ratio is 1 : 1 in 3a, 3b and 1 : 2 in 4a, 4b according to 1H NMR integrals.The interpretation of the 13C and 119Sn NMR data is the same as for 1b in Me2SO. Mössbauer measurements Mössbauer parameters determined by computer evaluation of the spectra measured at liquid-nitrogen temperature are presented in Table 5. All spectra exhibit i.s. and q.s. which clearly indicate the presence of tin(IV) species. The spectra, independently of the preparation mode (a) and (b), with ligands L1–L4, comprised only one, well developed doublet (the narrowness of the full width at half of the resonance peaks is average) (Fig. 2), which suggests the presence of completely equivalent tin environments in these compounds. For structural elucidation based on Mössbauer parameters,2204 J. Chem. Soc., Dalton Trans., 1997, Pages 2201–2205 Table 4 Tin-119 and 13C NMR data for the di-n-butyltin(IV) complexes in CDCl3 and (CD3)2SOa d(13C)[nJ(119Sn]13C)/Hz (n = 1–3)] Compound d(119Sn) C1 C2 C3 C4 CO2 2 Others L1 b 1b b 1b c L2 b 2b b L3 c 3a c L4 c 4a c — 245.0 249.8 — 263.3 — 275.0 — 265.9 — 22.65 (547.9) 21.97 (558.9) — 23.02 22.25 22.90 22.13 — 22.05 (558.9) — 22.20 (550.1) — 27.58 (23.4) 27.25 (33.9) — 27.66 — 27.31 (33.9) — 27.30 (30.2) — 26.57 (97.8) 25.94 (86.9) — 26.61 — 26.03 (86.9) — 26.13 (88.2) — 13.67 13.68 — 13.69 — 13.75 — 13.79 176.36 182.02 174.61 179.95 184.68 172.16 173.88 176.10 172.54 171.87 174.24 26.06 CH2 31.44 39.60 35.22 CH 20.33 CH3 41.37 CH 24.55 CH3 39.94 CH2 35.87 CH 42.90 42.62 44.42 CH 54.11 a Atoms C1–C4 are the carbon atoms of butyl residues in sequence from the tin atom.b In CDCl3. c In (CD3)2SO. the p.q.s. concept 15,23,24 was used. The p.q.s. values of the functional groups15,25,26 used in the calculations are listed in Table 6. All possible arrangements for five-co-ordinated tin(IV) (which was evidenced by the above IR and NMR measurements) are shown in Fig. 3. All of the structures with calculated q.s. values smaller than 2.32 mm s21 can immediately be eliminated because of the great differences from the measured values.Structures in which the R]C]R bond angle is 180 or 908 can also be eliminated on the basis of NMR measurements which had shown the presence of a ca. 1308 R]C]R bond angle. In this way three structures remained: 1, 6 and 11. According to the IR spectroscopic measurements, there is no monodentate carboxylate co-ordination so the only possible structure must be 1, with the two butyl and the thiol groups in equatorial and Fig. 2 Mössbauer spectrum of the dibutyltin(IV)–2-sulfanylpropionic acid complex Table 5 Experimental 119Sn Mössbauer parameters (mm s21) for the di-n-butyltin(IV) complexes; proposed structure TBPY Ligand Method i.s. q.s. Ref. L1 L2 L3 L4 abaababaab 1.49 1.49 1.42 1.43 1.48 1.43 1.60 1.33 1.47 1.43 3.28 3.28 3.23 3.18 3.25 3.18 3.49 3.20 3.24 3.27 This work This work 14 c This work This work This work This work 14 c This work This work a Prepared under reflux. b Propoxide.c Dimethyltin(IV) complex. the bridging bidentate carboxylic groups in axial positions (Fig. 4). However, in some cases the difference between the calculated and measured q.s. values is larger than the experimental error. This can be explained in that the calculated value is given Fig. 3 Calculated quadrupole splitting values for the tin(IV) coordination spheres in different stereochemical arrangements with trigonal-bipyramidal (TBPY) symmetry of the R2SIV cation.b = Bidentate, m = monodentate – O2Cb(m)* S – – O2Cb(m) R R R S – R b(m)CO2 – b(m)CO2 – R S – – O2Cb(m) R b(m)CO2 – TBPY 1(6) 2.98(2.46) TBPY 2(7) 1.71(1.54) TBPY 3(8) 3.52(3.28) – O2Cb(m) – O2Cb(m) S – R R R – O2Cb(m) S – R b(m)CO2 – TBPY 4(9) 2.32(1.96) TBPY 5(10) 3.07(2.55) – O2Cb S – – O2Cm R R R S – R bCO2 – mCO2 – R S – – O2Cm(b) R b(m)CO2 – TBPY 11 2.73 TBPY 12(13) 1.81(1.45) TBPY 14 3.51 – O2Cb(m) – O2Cm(b) S – R R R – O2Cm S – R bCO2 – TBPY 15(16) 2.23(2.05) TBPY 17 2.81 Table 6 Partial quadrupole splitting values (mm s21) of the functional groups used in the calculations p.q.s. (R) p.q.s.(S2) p.q.s. (CO2 2) 21.37 (t) 20.94 (tba) 21.13 (tbe) 21.03 (oc) 20.595 (tba) 20.6 (tbe) 20.56 (oc) 0.0 75 (tba)b 0.293 (tbe)b 20.1 (tba)m 0.06 (tbe)m t = Tetrahedral, oc = octahedral.J. Chem. Soc., Dalton Trans., 1997, Pages 2201–2205 2205 for the ideal structure with a 1208 C]Sn]C bond angle. On the basis of equation (3) in ref. 27 the deviation from the ideal structure can be estimated.A distortion of 5–108 results in a 0.2–0.41 mm21 greater measured q.s. value than is that calculated for the ideal structure. In case of our complexes this means a C]Sn]C angle between 125 and 1308 which agrees well with the values determined by NMR measurements. Conclusion Both synthetic procedures used resulted in the formation of complexes with 1 : 1 metal-to-ligand ratio, except for 4a and 4b where a 2 : 1 metal-to-ligand ratio was found, reflecting the four accessible binding sites.The IR, Raman and NMR spectral data for the prepared complexes indicate five-co-ordinate trigonal-bipyramidal moieties with equatorial thiol and two butyl groups and axial carboxylate groups, forming cyclic oligomers, in some cases of different sizes, or very long linear oligomers. In compounds 3a and 3b one carboxylate group is not involved in co-ordination to tin(IV). The Mössbauer measurements in combination with IR and NMR spectroscopy allowed us to determine the steric arrangement around the tin, among the seventeen theoretically possible structures.Acknowledgements This work was supported financially by the Hungarian Research Foundation (OTKA I/5 T007384) and the Grant Agency of the Czech Republic (Grant No. 203/94/0024). 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Cefalu, Mössbauer Effect Refs. Data J., 1983, 6, 69. 27 R. V. Parish, in Mössbauer Spectroscopy Applied to Inorganic Chemistry, ed. G. J. Long, Plenum, New York and London, 1984. Received 2nd December 1996; Paper 6/08136C