1978 1431Reactions in Mixed Non-aqueous Systems containing Sulphur Dioxide.Part I. The Dissolution of Main-group Metals in the Binary MixtureDimethyl Sulphoxide-Sulphur DioxideBy W. David Harrison, J. Bernard Gill, and David C. Goodall," Department of Inorganic and StructuralChemistry, The University, Leeds LS2 9JTThe metals M = Mg, Al, In, or Sn react with the mixed non-aqueous solvent dimethyl sulphoxide (dmso)-sulphurdioxide to form the metal disulphates, and Sr, Ba, or Pb react with the same system to form the metal sulphates.Other metals (Li, Na, Be, Ca, Ga, TI, Sb, or Bi) dissolve in the mixed solvent, but so far it has not been possible tocharacterise the products. Phase studies and a Raman spectroscopic investigation indicate the existence of a 1 : Iadduct of dmso and SO, which is considered responsible for the reaction of metals with the system.A likelymechanism for the oxidative process is discussed.A RECENT communication reported the reaction ofmetals with the mixed non-aqueous system dimethylsulphoxide-sulphur dioxide. The metals M = Mg, Al, In,or Sn dissolve in the mixed solvent to form crystallinemetal disulphates M,(S,O,),~zdmso, but Sr, Ba, or Pbdissolve to form only the metal sulphates. The metalsLi, Na, Be, Ca, Ga, T1, Sb, or Bi dissolve in the mixedsolvent, but no pure products have yet been isolated.dmso is at 1 053 cm-l. In the complexes, if the bands a t1000-900 cm-l are assigned to the S-0 stretchingfrequencies, there is a shift of 50-150 cm-l when dmsoco-ordinates to the metals.The shift in each case is toa lower frequency compared to the free solvent, andindicates co-ordination to the metal through oxygen.Comparisons of the spectra of the disulphates with thespectrum of anhydrous potassium disulphate and theMg(dmso) 6 (s207)430s575s582s695s835s907m945s960s970 (sh)1005s1015s1057s1160s1240s1315mTABLE 1Infrared bands (cm-l) of metal disulphatesIZ(s207) 3 1n2(dmso)12(S207)3 Sn (dmso) 1 3 ( ~ 2 O 7 ) 2415s 432 (sh)445s 457s 488m }576s 580w580s 586s 592w 1 644s 655w835s 840s 835m.912s 900s955s 950s462 (sh) 495s942 (sh)950s } 984s 985m1010s 1010s1026s 1030s1050s 1056s 1056s1125s 1 130 (sh)1 140s1163s 1 155s1 226s1235s1320m 1320m 1325mThe metals dissolve neither in dmso nor in SO, separ-ately, and the mixed solvent is required for dissolutionof metal to occur and for the oxidation of S I V to SvI.The products were characterised by elemental analysisfor metal, sulphur, carbon, and hydrogen, thermogravi-metric analysis, and i.r.spectroscopy. Some metalsreact completely with the mixed solvent within a fewhours, others react more slowly. The state of divisionof the metal has a marked effect on the rate of reaction,and some metals, particularly Sr, Ba, and Pb, becomepassive with a coating of insoluble sulphate. Thesolvated disulphates are hygroscopic solids. They dis-solve readily in water, but, as in the case of the alkali-metal disulphates, the disulphate ion is probablyhydrolysed to the [HSO,]- ion.They are quite solublein dmso. The i.r. bands of the solvated disulphates areshown in Table 1. The S-0 stretching frequency ofreported spectrum of sodium disulphate clearly indicatethe presence of the disulphate ion. The thermal decom-position of the solvated metal disulphates has beenstudied up to 1 000 "C. The compound Mg(dmSO)6(S,O7)begins to lose dimethyl sulphoxide at 120 "C and form-ation of an intermediate solvate, Mg(S,O,)*dmso, wasobserved at 260-350 "C. Further heating results in theloss of sulphur trioxide and dmso, and Mg[SO,] existsfrom 430 "C. The compound Al,(drnso),,(S,O,), beginsto lose dmso at 100 "C, and at 440 "C Al,(SO,), is ob-tained. No evidence was obtained for any intermediatedimethyl sulphoxide complexes.Further heating resultsin the loss of SO,, and Al,O, exists from 600 "C. Thecompound Sn(drns~)~(S~O,), begins to lose dmso a t1 W. D. Harrison, J. B. Gill, and D. C. Goodall, J.C.S. Ckem.A. Simon and H. Wagner, 2. anorg. Chem., 1961,311, 102.Comm., 1976, 5401432 J.C.S. Dalton180 "C, and at 470 "C Sn(S04), is obtained. No evidencewas obtained for any intermediate dimethyl sulphoxidecomplexes. Further heating results in the loss of SO,,and SnO, exists from 950 "C.It is important to consider the steps involved in theoxidation of metal to metal disulphate. Dimethylsulphide forms during the reaction and was collected(b.p. 37 "C). It is clear that dimethyl sulphoxide isacting as an oxidising species. It is not clear a t thisstage whether all the oxygen for the oxidation arisesfrom dmso.Phase studies indicate the existence of adiscrete 1 : 1 adduct (m.p. -38 "C) of dmso and SO,.The adduct cannot be prepared from SMe, and SO,.Raman spectroscopic studies support the formation ofsuch an adduct. The bands are shown in Table 2.TABLE 2Bands (cm-1) in the Raman spectra of dmso, SO,, anddmso-SO,SO, dmso dmso-SO,310 300335 335390 385625 535670 670700 7009 60 9501050 10101145 11401310 132013351420 1420Bandshift(cm-') inmixedsolvent- 100- 5 + '0"0- 10- 40-5- 150AssignmentC-S-C deform.asym. C-S-0 deform.sym. C-S-0 deform.Y, (SO,) bendsym. C-S str.C-S str.S-0 str.vl(S02) str.C-H deform.v3(S02) asym.str.C-H deform.There is a shift of 40 cm-l in the S-0 stretching fre-quency of dmso and a shift of 15 cm-l in the asymmetricstretching frequency (v,) of SO, when adduct formationoccurs. The lowering of the S-0 stretching frequencyof dmso in the adduct indicates co-ordination of dimethylsulphoxide to SO, through oxygen. However, the smallshift of 40 cm-1 suggests that either dmso is not verystrongly bound to SO, or there is possibly some back-co-ordination from the oxygen of sulphur dioxide to thesulphur of dimethyl sulphoxide. This tends to favourthe adduct, as does the situation with respect to thecharges carried by the various oxygen and sulphuratoms. A diagnostic pattern can be established, usingA(vl - v,),, from Table 2, for sulphur dioxide in variousadducts.For example, in the adduct SbF,*SO,, wheresulphur dioxide bonds through oxygen, A = 220 cm-l.In the adduct NMe,H*SO,, where sulphur dioxide bondsthrough sulphur, A = 125 cm-l, and in the adducts ofa M. C . R. Symons and R. Atkins, ' The Structure of InorganicR. Maylor, J. B. Gill, and D. C . Goodall, J.C.S. Dalton, 1973,Radicals,' Elsevier, Amsterdam, 1967, p. 146.634.sulphur dioxide with alkali-metal halides, where SO, alsobonds through sulphur, A = 180 cm-l.From the Raman spectrum of the dimethyl sulphoxide-sulphur dioxide adduct, A = 180 cm-l, indicating weakbonding through the sulphur of SO,, or perhaps bondingthrough sulphur and oxygen, as shown earlier. In thelatter case, bonding through oxygen and sulphur willtend to cancel out their different effects on A and give avalue close to that for pure SO, (190 cm-l).A possible mechanism for the oxidation, involving theformation of metal sulphite as one of the intermediates,has already been out1ined.l However, although somemetal sulphites undergo conversion into sulphates anddisulphates in the presence of dmso and SO,, there is noevidence to show that, when metals react directly withdmso-SO, to form metal disulphates, a metal sulphiteintermediate is involved.In the case of the morereactive metals (M = Li, Na, or Sn) formation of metaldithionite has been observed. This is considered tooccur as a result of dimerisation of the [SO,]- radical ion, p4 0dit hionitedmso -SO2___jdisul phi t e2 (d m so -sop) 1disu lphateformed when the metal reacts with sulphur dioxide[equations (1) and (2)].Formation of the [SO,]- ionhl + xS0, ---t Mz+ + x[SO,]- (1)2[SO,]- =+= [S,O,I2- (2)had been observed when sodium dissolves in a mixtureof dmso and SO,.,The metal dithionites are oxidised to metal disulphatesby the mixed solvent dmso-SO,. It appears that neithersingle component of the mixed solvent will bring aboutthe oxidation. It is not yet clear exactly how thedmso-SO, adduct releases its oxygen in the oxidativeprocess. Rinker and Lynn had observed the formationof a very reactive form of dithionite, which they con-sidered to be the form containing an S-0-S link,[SO,*SO,]2-. Although in the solid state dithioniteshave the S-S link, [0,S*S0,]2-, the form with the S-0-Slink may well be the reactive intermediate in theoxidative process.This intermediate is particularlyattractive, in that it can be oxidised by dmso-SO, todisulphate without any rearrangement, and a transi-R. G. Rinker and S. Lynn, Ind. and Eng. Chem. (ProductRes. and Develofiment), 1969, 8, 3381978 1433tionary disulphite having an S-0-S link may form, as isconsidered to be the case when transifion-metal sulphitesare converted into disulphates:EXPERIMENTALAll the operations were carried out in a closed systemunder dry nitrogen. Nitrogen and sulphur dioxide weredried by passing the gases separately through concentratedsulphuric acid and phosphorus(v) oxide. Filtrations werecarried out under a nitrogen stream, using a sinter tube.Diethyl ether was distilled and dried with sodium.P~e~arations.-Mg(dmso),(S,O,).Finely divided mag-nesium was added to freshly distilled dimethyl sulphoxide(10 cm3) and the mixture was saturated with sulphurdioxide. The mixture was retained in a closed containerand left until no more metal dissolved. The solution wasfiltered, and diethyl ether added to the filtrate. A colour-less crystalline solid precipitated, was filtered off, washedwith diethyl ether, and pumped for 12 h (Found: C, 21.8;H, 5.55; S, 38.45. Calc. for C1,H3,MgOl3S8: C, 21.55; H,5.40; s, 38.35%).A1,(dmso),,(S,0,), (Found: C, 19.1; H, 4.85; Al, 3.55.Calc. for C24H,2A1,0,3S,8: C, 18.95; H, 4.80; Al, 3.55%)and 1n2(dmso),,(S,0,), (Found: C, 16.9; H, 4.20; S, 34.15.Calc.for C,,H,,In,0,3S,8: C, 17.0; H, 4.25; S, 34.0%)were prepared similarly as colourless crystalline solids.l Sn(dmso),(S,O,),. The method described above for themagnesium complex was followed, with slight modification.After the initial fairly rapid reaction a white solid formed,which was allowed to stand for 1 week until most of it haddissolved to form a colourless solution. The solution wasfiltered, and diethyl ether added to the filtrate. A colour-less crystalline solid precipitated, was filtered off, washedwith diethyl ether, and pumped for 12 h (Found: C, 14.8;H, 3.60; S, 33.75. Calc. for Cl,H3,0.$,,Sn: C, 15.3; H,3.85; S, 34.1%).Physical Measurements.-The melting point of the di-methyl sulphoxide-sulphur dioxide adduct was determinedfrom the phase diagram, obtained by plotting melting pointagainst mol fraction of SO, in dmso. Temperatures weremeasured to k l . 0 "C by an accurate alcohol thermometer,immersed in the samples contained in a refrigerated bath ofsilicone fluid.Infrared spectra were recorded on a Perkin-Elmer 457grating spectrophotometer as Nujol mulls. Raman spectraThe phase diagram is shown in the Figure.Mol fraction of SO,Phase diagram of melting point against composition fordmso-SO,were obtained using a Coherent Radiation model 52tunable argon-ion laser, operating at 488 nm. Scatteredradiation was analysed by a double-grating Coderg (modeltype PHO) spectrometer.Thermogravimetric curves were obtained using a Stantonthermobalance. Aluminium was determined gravimetric-ally as aluminium oxide. C, H, and S were determined byMr. A. Hedley of this department.[8/143 Received, 27th Januavy, 1978