SciELO - Scientific Electronic Library Online

vol.20 issue8The asymmetric total synthesis of (+)- and (-)-trypargine via Noyori asymmetric transfer hydrogenationLewis acid promoted Friedländer condensation reactions between anthranilonitrile and ketones for the synthesis of tacrine and its analogues author indexsubject indexarticles search
Home Pagealphabetic serial listing  

Services on Demand




Related links


Journal of the Brazilian Chemical Society

Print version ISSN 0103-5053On-line version ISSN 1678-4790

J. Braz. Chem. Soc. vol.20 no.8 São Paulo  2009 



Different coordination modes for disulfoxides towards diorganotin(IV) dichlorides. X-ray crystal structures of 1,2-cis-bis-(phenylsulfinyl)ethene (rac-,cis-cbpse) and adducts [{Ph2SnCl2(meso-bpse)}n] and [{n-Bu2SnCl2(pdtd)}2]



Gerimário F. de SousaI, *; Javier EllenaII; Valéria R. S. MaltaIII; José D. ArdissonIV

IInstituto de Química, Universidade de Brasília, 70919-970 Brasília-DF, Brazil
IIInstituto de Física de São Carlos, Universidade de São Paulo, 13560-970 São Carlos-SP, Brazil
IIIDepartamento de Química e Biotecnologia, Universidade Federal de Alagoas, 57072-970 Alagoas-AL, Brazil
IVLaboratório de Física Aplicada, Centro de Desenvolvimento de Tecnologia Nuclear, 30123-970 Belo Horizonte-MG, Brazil




The reactions of meso-1,2-bis(phenylsulfinyl)ethane (meso-bpse) with Ph2SnCl2, 2-phenyl-1,3-dithiane trans-1-trans-3-dioxide (pdtd) with n-Bu2SnCl2 and 1,2-cis-bis-(phenylsulfinyl)ethene (rac-,cis-cbpse) with Ph2SnCl2, in 1:1 molar ratio, yielded [{Ph2SnCl2(meso-bpse)}n], [{n-Bu2SnCl2(pdtd)}2] and [{Ph2SnCl2(rac,cis-cbpse)}x] (x = 2 or n), respectively. All adducts were studied by IR, Mössbauer and 119Sn NMR spectroscopic methods, elemental analysis and single crystal X-ray diffractometry. The X-ray crystal structure of [{Ph2SnCl2(meso-bpse)}n] revealed the occurrence of infinite chains in which the tin(IV) atoms appear in a distorted octahedral geometry with Cl atoms in cis and Ph groups in trans positions. The X-ray crystal structure of [{n-Bu2SnCl2(pdtd)}2] revealed discrete centrosymmetric dimeric species in which the tin(IV) atoms possess a distorted octahedral geometry with bridging disulfoxides in cis and n-butyl moieties in trans positions. The spectroscopic data indicated that the adduct containing the rac,cis-cbpse ligand can be dimeric or polymeric. The X-ray structural analysis of the free rac-,cis-cbpse sulfoxide revealed that the crystals belong to the C2/c space group.

Keywords: Tin(IV) complexes, disulfoxide ligand, crystal structure


As reações de meso-1,2-bis(fenilsulfinil)etano (meso-bpse) com Ph2SnCl2, de 2-fenil-1,3-ditiona trans-1-trans-dióxido (pdtd) com n-Bu2SnCl2 e de 1,2-cis-bis-(phenylsulfinyl)ethene (rac-,cis-cbpse) com Ph2SnCl2, na proporção molar 1:1, levaram à formação de [{Ph2SnCl2(meso-bpse)}n], [{n-Bu2SnCl2(pdtd)}2] e [{Ph2SnCl2(rac,cis-cbpse)}x] (x = 2 ou n), respectivamente. Na investigação das propriedades estruturais dos produtos foram empregadas as espectroscopias de absorção no infravermelho, Mössbauer e RMN de 119Sn, além de análise elementar e difratometria de raios X em monocristal. O estudo de [{Ph2SnCl2(meso-bpse)}n] por difratometria de raios X revelou a ocorrência de um encadeamento infinito no qual os átomos de tin(IV) apresentam uma geometria octaédrica distorcida com os átomos de Cl em posições cis e os grupos Ph em trans. A estrutura cristalina de [{n-Bu2SnCl2(pdtd)}2] revelou a presença de espécies diméricas centrossimétricas nas quais os átomos de tin(IV) possuem geometria octaédrica distorcida com dissulfóxidos em ponte ocupando posições cis e grupos n-butila ocupando posições trans. Os dados espectroscópicos indicaram que o produto contendo o ligante rac,cis-cbpse pode ser dimérico ou polimérico. O estudo por difratometria de raios X do sulfóxido rac-,cis-cbpse livre revelou que os cristais pertencem ao grupo espacial C2/c.




Organotin(IV) compounds have been 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. The Lewis base rac-,cis-1,2-bis(phenylsulfinyl)ethene (rac-,cis-cbpse) is known to bond to different metal centres via either the O atoms, the S atoms or the olefinic (-HC=CH-) centre. An O,O-bonded chelate with this ligand has been described for a typically 'hard' Lewis acid, namely Ph3SnCl,4 whereas the 'softer' platinum(II) species interacts exclusively with the S atoms to afford an S,S-bonded 5-membered ring.5 On the other hand, the very soft platinum(0) site, interestingly, yielded yet another ligating mode, namely an h2-alkene platinum(0) linkage.6 It is thus apparent that the two factors, inherent basicity and hardness/softness, play a part in complex formation.6

Filgueiras and collaborators4 have shown that the reactions of Ph3SnCl with disulfoxides of the type meso-,rac-PhSO(CH2CH2)SOPh [1,2-bis-(phenylsulfinyl)ethane: meso-,rac-bpse] and rac-,cis-PhSO(HC=CH)SOPh [1,2-cis-bis(phenylsulfinyl)ethene: rac-,cis-cbpse], in 1:1 ratio (acid:base), afford adducts of stoichiometry [Ph3SnCl(meso-,rac-bpse)] and [Ph3SnCl(rac-,cis-cbpse)], respectively. On the other hand, Zhu et al.7 have reported the preparation and crystal structure of a bridged 2:1 (acid:base) bimetallic adduct of formula [(Ph3SnCl)2(meso-bpse)]. In contrast with these results, the X-ray structure of the adduct [{Me2SnCl2(meso-bpse)}n] has been determined and revealed the occurrence of polymeric chains along the crystallographic a axis in which the ligand and organotin(IV) moiety alternate.8



Herein, we describe the preparation and X-ray crystal structure of rac-,cis-cbpse and of the adducts [{Ph2SnCl2(meso-bpse)}n] and [{n-Bu2SnCl2(pdtd)}2], where the second compound shows an uncommon coordination mode of a disulfoxide (pdtd) towards a diorganotin(IV) dihalide, namely n-Bu2SnCl2. The spectroscopic data has shown that the adduct [{Ph2SnCl2(rac-,cis-cbpse)}x] (x = 2 or n) possesses either dimeric or polymeric molecules. In spite of being bifunctional, pdtd, meso-bpse and rac-,cis-cbpse act as bis-monodentate ligands, as a result of their peculiar stereochemistry.



Materials and methods

Solvents and diorganotin(IV) dichorides were commercially available (Aldrich) and were used as supplied. The disulfoxides pdtd, meso-bpse and rac-,cis-cbpse were kindly provided by Prof. C. Celso, who had prepared them previously.4 The adducts [Ph2SnCl2(meso-cbpse)] (m.p. 138-141 ºC), [{n-Bu2SnCl2(pdtd)}2] (m.p. 265-267 ºC) and [{Ph2SnCl2(rac-,cis-cbpse)}x] (m.p. 85-87 ºC) were synthesized by the following procedure: equal amounts (0.50 mmol) of both the diorganotin(IV) dichloride and the appropriate sulfoxide were dissolved in EtOH. After refluxing for 1 h, the solution was filtered and a clear filtrate was obtained. Slow cooling and evaporation of the first two solutions afforded an abundant crop of colorless crystals (ca. 60% yield) appropriated for X-ray diffraction analysis. [{Ph2SnCl2(meso-bpse)}n]: Found: C, 44.8; H, 3.4; S, 5.9. Calcd. for C36H34Cl4O2S2Sn2: C, 45.9, H, 3.6, S, 6.1%. [{n-Bu2SnCl2(pdtd)}2]: Found: C, 40.3; H, 5.4; S, 12.2. Calcd. for C36H60Cl4O4S4Sn2: C, 40.6, H, 5.7, S, 12.0%. [{Ph2SnCl2(rac-,cis-cbpse)}x] (x = 2 or n): Found: C, 49.5; H, 3.51; S, 9.8. Calcd. For C52H44Cl4O4S4Sn2: C, 50.3, H, 3.58, S, 10.3%.

Physical measurements

IR spectra were recorded on a Bomem MB100 FT-IR spectrophotometer in the 4000-400 cm-1 range using KBr pellets. 119Sn, 1H and 13C NMR spectra were run on a Bruker Advance DRX 400 MHz spectrophotometer using CDCl3. Chemical shifts are related to tetramethylsilane (tms) and SnMe4. 119Sn Mössbauer spectra were collected at 100 K in the transmission geometry on a constant-acceleration conventional spectrometer by using 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 microanalyses for C, H, N and S were performed in a Fissons EA 1108 elemental analyzer.

X-ray crystallography studies

Crystallographic data for rac-,cis-cbpse and adducts [{Ph2SnCl2(meso-bpse)}n] and [{n-Bu2SnCl2(pdtd)}2] are listed in Table 2. Selected bond distances and angles are presented in Table 3. Crystal data for all compounds were collected at 293(2) K on a Enraf-Nonius Kappa-CCD area detector diffractometer with graphite-monochromated MoKβ radiation (λ = 0.71073 Å). Data collections employed the COLLECT program,9 integration and scaling of the reflections were performed with the HKL Denzo-Scalepack system10 and absorption corrections were carried out using the multi-scan method.11



Results and Discussion

IR spectroscopy

Table 1 shows the selected infrared data. The IR spectra of the disulfoxides (meso-bpse, pdtd and rac-,cis-cbpse) show very strong ν(S=O) absorptions at 1037, 1044 and 1040 cm-1, respectively, which appear at 998, 981 and 965 cm-1 in the spectra of the corresponding adducts. This behavior is consistent with the fact that both S=O groups are bonded to the metal, causing the shift to lower frequency. The IR spectra of the adducts also show bands at 456 cm-1 for [{Ph2SnCl2(meso-bpse)}n], at 413 cm-1 for [{nBu2SnCl2(pdtd)}2], and at 459 cm-1 for [{Ph2SnCl2(rac-,cis-cbpse)}x], which were assigned to the ν(Sn-O) vibration.4 The Δν(S=O) value of 75 cm-1 calculated for rac-,cis-cbpse and its comparison with the 63 cm-1 shift for meso-bpse indicates that the unsaturated ligand is much more basic than the saturated one.4

NMR spectroscopy

The 1H and 13C NMR spectra for rac-,cis-cbpse in CDCl3 show a singlet resonance at δ = 6.78 and δ = 144.0 ppm attributed, respectively, to the chemically and magnetically equivalent protons and carbons present in the ethylene (-HC=CH-) moiety. Regarding the 119Sn NMR, the chemical shifts, δ(119Sn), relative to SnMe4 in six-coordinate organotin(IV) derivatives were found to lie between ca. -125 and -515 ppm,12 whereas those from five-coordinate species from +25 to -319 ppm.13 A series of five-coordinate diphenyltin(IV) complexes were found to have shifts from -62 to -276 ppm.14 Therefore, the chemical shift values of +69 and -115 ppm found for [{n-Bu2SnCl2(pdtd)}2] and [{Ph2SnCl2(rac-,cis-cbpse)}x], respectively, are indicative of considerable shielding and five-coordination of the tin(IV) atoms. In view of this, in CDCl3 solution both adducts are considerably dissociated, suggesting that the six-coordination pattern in the solid state has not remained in solution.

Mössbauer spectroscopy

119Sn Mössbauer spectral parameters of the disulfoxide derivatives are reported in Table 1, which includes parameters from literature for comparison. The similarity among the quadrupole splittings (Δ) observed for [{Ph2SnCl2(meso-bpse)}n] (3.73 mm s-1), [{Me2SnCl2(meso-bpse)}2] (3.54 mm s-1), [{n-Bu2SnCl2(pdtd)}2] (3.59 mm s-1) and [{Ph2SnCl2(rac-,cis-cbpse)}x] (3.71 mm s-1) compared to [Ph2SnCl2(pdtd)] (D = 2.85 mm s-1) may be accounted for by a greater asymmetry in the electronic density distribution around the tin(IV) in the first three adducts. These results indicate that [Ph2SnCl2(pdtd)] is five-coordinate, while the others are six-coordinate in the solid state.

The isomer shift (δ) is very sensitive to the first coordination sphere and is dependent on the nature and number of organic groups bonded to the tin(IV) nucleus. 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, and a result of this is the inverse dependence of δ with electronegativity.15 Thus, d increases on replacing the Ph by the Me groups in the coordination sphere of tin(IV), as given by the adducts [{Ph2SnCl2(meso-bpse)}n] (1.28 mm s-1) and [{Me2SnCl2(meso-bpse)}2] (1.35 mm s-1) (Table 1). The similarity in isomer shifts observed in [{n-Bu2SnCl2(pdtd)}2] (1.49 mm s-1) and in [{Ph2SnCl2(rac-,cis-cbpse)}x] (1.50 mm s-1) is noteworthy. The d value of 1.50 mm s-1 may be a consequence of the much more basic behavior of the unsaturated rac-,cis-cbpse ligand compared to the saturated meso-cbpse ligand. These results are corroborated by IR and 119Sn NMR spectroscopies. The lower value of δ found in the five-coordinate adduct [Ph2SnCl2(pdtd)] (δ = 1.32 mm s-1)16 is a consequence of rehybridization (sp3d) of tin(IV), which may be thought of as a linear combination of sp2 and pzdz2 orbitals. Thus, the equatorial Sn-C bonds will have greater s-character and smaller p-character, thereby decreasing the δ value.17

Crystal structure of rac-,cis-cbpse

Figure 1 depicts the molecular structure of rac-,cis-cbpse obtained by oxidation of 1,2-cis-bis(phenylsulfide)ethene with 30% H2O2 in HOAc. Selected bond distances and angles with their estimated standard deviations are listed in Table 3. Crystals of the rac-,cis-cbpse isomer belong to the C2/c space group. The structure of this dissulfoxide shows that the S=O sulfinyl groups are trans to each other, and the same is observed for the Ph groups, as seen from the O1-S1-C7-C7#1 and C1-S1-C7-C7#1 torsion angles of 133.7(2)º and -117.3(2)º, respectively. The S=O groups are quasi-coplanar with the their bonded Ph rings, as shown by the O1-S1-C2-C6 torsion angle of 7.15(18)º, due to the conjugation between the sulfur lone pairs and the π-systems of the Ph rings. This observation was also previously reported for other phenylsulfinyl derivative.18

Crystal structure of [{Ph2SnCl2(meso-bpse)}n]

Figure 2 shows the molecular structure of the adduct [{Ph2SnCl2(meso-bpse)}n]. Its structure determination revealed the occurrence of infinite chains along the crystallographic a axis; additionally, each meso-bpse molecule has an inversion centre at the -H2C-CH2- (ethane) group midpoint. The tin(IV) atom is octahedrally coordinated by Cl atoms at cis [Sn-Cl: 2.4591(9), 2.4648(7) Å], by Ph carbon atoms at trans [Sn-C: 2.130(3), 2.133(3) Å], and by the two O atoms from the S=O groups at cis positions [Sn-O: 2.3527(19), 2.3979(18) Å]. The ligand bonds through both O atoms and forms bridges between the tin(IV) atoms, generating chains along the n glide. This kind of chain, along with similar bond distances and angles, has also been observed when Me2SnCl2 replaces Ph2SnCl2. For example, bond distances Sn-C [2.101(4), 2.105(5 Å)], Sn-O [2.397(3), 2.404(3) Å], Sn-Cl [2.482(1), 2.486(1) Å] and bond angles Me-Sn-Me [165.2(2)º], O-Sn-O [86.03(1)º] and Cl-Sn-Cl [98.73(4)º] have been observed for [{Me2SnCl2(meso-bpse)}n].19



Crystal structure of [{n-Bu2SnCl2(pdtd)}2]

Figure 3 presents the molecular structure of the dimer [{n-Bu2SnCl2(pdtd)}2]. The X-ray crystal structure revealed a discrete centrosymmetric dimeric assembly in which the tin(IV) atoms possess a distorted octahedral geometry originated from the bis-monodentate disulfoxide bonded through the O atoms. One of the n-Bu group is disordered (C1a-C4a), while the other group (C5-C8) does not show any sign of disorder. The dimeric assembly occurs via a 12-membered ring (Sn2C2O4S4) formation and constitutes the first example of a diorganotin(IV) dichloride bonded to a disulfoxide ligand in a 1:1 molar ratio. Usually, most 1:1 adducts of diorganotin(IV) dihalides with disulfoxides form polymeric alternating chains19-21 containing six-coordinate tin(IV), but 1:1 five-coordinate adducts are rare. In view of this, it is interesting to compare our results with literature data for the five-coordinate adduct [Ph2SnCl2(pdtd)].16 The trigonal bipyramidal structure of the monomer [Ph2SnCl2(pdtd)] shows two nearly identical equatorial bond distances Sn-C [2.11(5); 2.122(5) Å] and different axial Sn-Cl [2.455(1) Å] and equatorial Sn-Cl [2.346(2) Å] bond lengths, as expected. The Sn-O distance [2.367(3) Å] is significantly shorter than the similar Sn-O [2.482(3) and 2.755(3) Å] observed for our dimeric adduct, probably due to the more acidic nature of Ph2SnCl2 compared to n-Bu2SnCl2, whereas the two S=O distances of 1.533(4) and 1.450(6) Å are significantly different, as a consequence of the former being complexed to tin(IV) and the latter remaining uncoordinated. However, in our dimeric adduct the shorter S=O distances of 1.501(3) and 1.512(3) Å indicate that the acid-base interactions are not strong enough. Perhaps, the long Sn-O1# bond distance of 2.755(3) Å is the reason the dimer dissociates in solution to the monomeric form, as suggested by spectroscopic data (Table 1). The values of the 90.35(6)º angle subtended by the two Cl atoms and of the equatorial C-Sn-C bond angle [138.6(2)º] observed in [Ph2SnCl2(pdtd)] are smaller than the equivalent angles found in our octahedral adduct, as expected (Table 3).



Structure of [{Ph2SnCl2∙(rac-,cis-cbpse)}x] (x = 2 or n)

The spectroscopic data shown in Table 1 indicate that [{Ph2SnCl2(rac-,cis-cbpse)}x] do not show any evidence of a monomeric adduct in which the tin(IV) atoms exhibit trigonal bipyramidal geometry in the solid state. However, the same spectroscopic data are not sufficient to characterize the product as being a dimeric [{Ph2SnCl2(rac-,cis-cbpse)}2], or a polymeric [{Ph2SnCl2(rac-,cis-cbpse)}n] compound containing octahedral tin(IV) centres.



The reaction between Ph2SnCl2 and the disulfoxide pdtd in 1:1 molar ratio affords a monomeric16 adduct containing a five-coordinate tin(IV) centre, while a similar reaction with n-Bu2SnCl2 affords dimeric species in which the tin(IV) atoms are six-coordinate. Surprisingly, the characterization of the adduct [{Ph2SnCl2(rac-,cis-cbpse)}x] as being a dimeric or a polymeric species was not possible using microanalysis and spectroscopic methods such as IR, NMR and 119Sn Mössbauer.



The authors are grateful to Prof. C. Celso for providing the sulfoxides used in this work and to Prof. G. M. Lima (UFMG) for the 119Sn NMR spectra. CNPq, FAPESP, FINEP (CT INFRA 0970/01) are thanked for financial support.


Supplementary Information

Crystallographic data for the compounds have been deposited at the Cambridge Crystallographic Data Center with the deposition numbers CCDC 694402 for rac-,cis-cbpse, CCDC 679894 for [{Ph2SnCl2(meso-bpse)}n] and CCDC 693796 for [{n-Bu2SnCl2(pdtd)}2]. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (Fax: +44 1223 336033 or e-mail:



1. Pellerito, L.; Naggy, L.; Chem. Rev. 2002, 224, 111.         [ Links ]

2. Gielen, M.; Appl. Organomet. Chem. 2002, 16, 481.         [ Links ]

3. Gorshkova, J. E.; Gordeliy, V. I.; Crystallogr. Rep. 2007, 52, 535.         [ Links ]

4. Filgueiras, C. A. L.; Celso, C.; Marques, E. V.; Johnson, B. F. G.; Inorg. Chim. Acta 1982, 59, 71.         [ Links ]

5. Filgueiras, C. A. L.; Holland, P. R.; Johnson, B. F. G.; Acta Crystallogr. Sect. B: Struct. Sci. 1982, 38, 954.         [ Links ]

6. De Sousa, G. F.; Filgueiras, C. A. L.; Nixon, J. F.; Hitchcock, P. B.; Inorg. Chim. Acta 1997, 261, 217.         [ Links ]

7. Zhu, F. C.; Shao, P. X.; Wang, R. J.; Wang, H. G.; Inorg. Chim. Acta 1990, 171, 85.         [ Links ]

8. Carvalho, C. C.; Francisco, R. H. P.; Gambardella, M. T. do P.; De Sousa, G. F.; Filgueiras, C. A. L.; Acta Crystallogr. Sect. C: Cryst. Struct. Commun. 1996, 52, 1629.         [ Links ]

9. Blessing, R. H.; Acta Crystallogr., Sect. A: Found. Crystallogr. 1995, 51, 33.         [ Links ]

10. Sheldrick, G. M.; SHELXS-97; Program for Automatic Solution of Crystal Structures, University of Göttingen, Germany, 1997.         [ Links ]

11. Sheldrick, G. M.; SHELXL-97; Program for Crystal Structure Refinement, University of Göttingen, Germany, 1997.         [ Links ]

12. Otera, J.; J. Organomet. Chem. 1981, 221, 57.         [ Links ]

13. Otera, J.; Hinoishi, T.; Kawabe, Y.; Okawara, R.; Chem. Lett. 1981, 273.         [ Links ]

14. De Sousa, G. F.; Deflon, V. M.; Manso, L. C. C.; Ellena, J.; Mascarenhas, Y. P.; Lang, E. S.; Gatto, C. C.; Mahieu, B.; Trans. Met. Chem. 2007, 32, 649.         [ Links ]

15. Teles, W. M.; Allain, L. R.; Filgueiras, C. A. L.; Abras, A.; Hyperfine Interact. 1994, 83, 175.         [ Links ]

16. De Sousa, G. F.; Filgueiras, C. A. L.; Nixon, J. F.; Hitchcock, P. B.; J. Braz. Chem. Soc. 1997, 8, 649.         [ Links ]

17. Filgueiras, C. A. L.; Holland, P. R.; Johnson, B. F. G.; Raithby, P. R.; Acta Crystallogr. Sect. B: Struct. Sci. 1982, 38, 2684.         [ Links ]

18. Kannan, S.; Usman, A.; Fun, H. K.; Acta Crystallogr. Sect. C: Cryst. Struct. Commun. 2003, 268.         [ Links ]

19. Carvalho, C. C.; Francisco, R. H. P.; Gambardella, M. T. do P.; De Sousa, G. F.; Filgueiras, C. A. L.; Acta Crystallogr. Sect. C: Cryst. Struct. Commun. 1996, 52, 1629.         [ Links ]

20. Carvalho, C. C.; Francisco, R. H. P.; Gambardella, M. T. do P.; De Sousa G. F.; Filgueiras, C. A. L.; Acta Crystallogr. Sect. C: Cryst. Struct. Commun. 1996, 52, 1627.         [ Links ]

21. De Sousa, G. F.; Filgueiras, C. A. L.; Abras, A.; Carvalho, C. C.; Francisco, R. H. P.; Gambardella, M. T. P.; Anais Assoc. Bras. Quim. 1995, 44, 42.         [ Links ]



Received: August 22, 2008
Web Release Date: August 14, 2009
FAPESP helped in meeting the publication costs of this article



* e-mail:

Creative Commons License All the contents of this journal, except where otherwise noted, is licensed under a Creative Commons Attribution License