the other face of bunsen's cacodyl disulfide, me2as(s)-s-asme2: the lewis-base behaviour...

ARTICLE

DOI: 10.1002/zaac.200800391

The Other Face of Bunsen’s Cacodyl Disulfide, Me2As(S)-S-AsMe2:The Lewis-Base Behaviour Towards Heavy Metal Cations1)

Panayiotis V. Ioannou,*[a] Dimitris G. Vachliotis,[a] and Theodore D. Sideris[a]

Keywords: Cacodyl disulfide; Dimethylarsino dimethyldithioarsinate; Dimethyldithioarsinates; Dimethyl arsenium ion;Dimethylarsino sulfenium ion

Abstract. The reaction of Bunsen’s cacodyl disulfide,Me2As(S)-S-AsMe2, with heavy metal cations in methanol pro-duces insoluble salts (complexes) of dimethyldithioarsinic acid,Me2AsS2H, and dimethyl arsenium ion, Me2As:�. This arseniumion prefers to react with Me2As(S)-S-AsMe2, when in excess, com-pared to AcO� or MeOH/H2O and it is also reactive towards sulfur(Sx, x � 1-8) producing the stabilized dimethylarsino sulfenium

Introduction

By reacting cacodylic acid (dimethylarsinic acid) (1) withhydrogen sulfide in ethanol, Robert Bunsen prepared an ar-senic-containing disulfide which he called cacodyl disulfide[1]. Its structure was found to be 2 (dimethylarsino di-methyldithioarsinate [2, 3]) and not 3 (bis(dimethylarsenic)disulfide) as was thought based on the reducing propertiesof hydrogen sulfide.

Bunsen also reported on the behaviour of his disulfidetowards heat, HCl, H2SO4, HNO3 and metal salts. For thelatter, by reacting ethanolic solutions of 2 with ethanolicsolutions of AuCl3, Cu(NO3)2, Pb(AcO)2, SbCl3, andBi(NO3)3 he obtained the complexes of dimethyldithio-arsinic acid, Me2AsS2H (LH) (4): LAu, LCu, L2Pb, L3Sb,and L3Bi (erroneously written as L2Bi). However, the quan-tities of the reactants and the yields of the products werenot reported.

Organodithiophosphorus compounds have industrial,agricultural and academic interest [4]. Organodithioarseniccomplexes should be useful for comparative studies [5] and4 is a simple ligand for preparation of such compounds.Since 4 is not stable (by self-condensation gives 2 [6]), heavymetal complexes of 4 have been prepared [7] by reaction of2 [1] or LNa·2H2O [7, 8] with metal salts, and by reactionof 1/metal chlorides/HCl in MeOH and gaseous H2S [7].With L� as a ligand many complexes/salts of heavy metal

* Prof. Dr. P. V. IoannouE-Mail: [email protected]

[a] Department of ChemistryUniversity of PatrasPatras, Greece

Z. Anorg. Allg. Chem. 2009, 635, 329�336 © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 329

cation, Me2As�S..

..� ↔ Me2A

�

s � S..

.. . The complexes (Me2AsS2)xM(x � 1 or 2) are unstable in the presence of their own heavy metalcations decomposing to colored solids. In an attempt to preparesalts of Me2AsSH, the reactions of (Me2AsS2)xM with tri-phenylphosphine and trimethyl phosphite gave the metal sulfideand Me2As-S-AsMe2 instead.

Figure 1. Structures of the compounds encountered in this studyand their chemical shifts in CD3OD (TMS 0.000 ppm). Theδ values have a range of less than ±0.003 ppm of the quoted values.

cations [7, 8], organosilicon, -germanium, -tin, and -lead(summarized in [5]) and organometallic compounds [9, 10]have been prepared and studied by IR [7], magnetic meas-urements [8], and X-ray diffraction [5].

We [6] have studied the electrophilic behaviour of Bun-sen’s cacodyl disulfide 2 towards various nucleophiles ofgroup 15, e.g. Ph3P, Ph3As, (RO)3P, (RS)3P etc., and wefound that the electrophilic site of 2 was the As�S sulfur.

1) In memory of Henry B.F. Dixon, 80, of Cambridge University, adistinguished chemist and biochemist on arsenic

P. V. Ioannou, D. G. Vachliotis, T. D. SiderisARTICLESince 2 reacts with heavy metal cations means it has nucleo-philic properties as well. The reaction of 2 with M� willgive the LM and arsenium ion, Me2As:�. Arsenium ionsare a class of 6-electron powerfully electrophilic cations.They can be divided into two classes: internally stabilizedby neighbouring sulfur or nitrogen [11] and externally stabi-lized by Lewis bases D: as Me2As�:D�X�. Suitable D: areNH3 [12], pyridine [13, 14], phosphines [13, 15], and thiurea[16]. Me2As:� can also act as a ligand in transition metalcomplexes [17].

In this paper we present our results on the stoichiometryof the reaction of 2 with non-reducible cations PbII, CdII,ZnII, HgII, AgI and the reducible CuII ion. The stoichi-ometry is of importance if one wishes to indirectly studysome properties of Me2As:� in the supernatant, and be-cause the metal cation is reactive towards its LxM reactingwith it to a degree dependent of the Mx� thus giving impureLxM. Therefore, we prepared the complexes with the above-mentioned cations, the LAg and L2Hg being new ones, andglimpsed on how L2Cu is reduced to LCu. We also verifiedthe results of Bunsen on the thermal behaviour of the com-plexes LxM and we tried to desulfurize LxM with Mx�,Ph3P, and (MeO)3P to the elusive [18] thiolate Me2As-S�.In the course of our studies we also encountered the pro-duction and reactions of the new stabilized sulfenium ion,Me2As�S

..

..� ↔ Me2A

�

s�S..

..[6].

Results and Discussion

Preparation of (Me2AsS2)xM from Me2As(S)-S-AsMe2 (2)

In order to establish the stoichiometry of the reaction, inair, of 2 with non-reducible heavy metal cations, we selectedPbII because when in slight excess it immediately gives ablack solid admixed with the white product L2Pb. Then, a2:1 composition was found for PbII, HgII, CdII and ZnII

and a 1:1 for Ag�. Since these strict stoichiometries did notgive pure products we routinely used 2.5:1 and 1.25:1 moleratios, and dropwise addition of the methanolic solution ofthe metal acetate to a methanolic solution of 2 waiting forthe system to decolorize before the addition of the nextdrop. Under these conditions the yields were excellent (95-100 %) except in the case of L2Zn which had a small solu-bility in MeOH. However, dried samples of L2Zn did notgive a signal in the 1H NMR spectrum, when suspendedin CD3OD. Casey et al. [7] observed the same solubilitybehaviour in CHCl3. The reason for the excess 2 will bediscussed later on. For the reaction of 2 with CuII to giveLCu a 2:1 stoichiometry was found. In this case, excess of2 was not necessary and the yellow LCu was not contami-nated by S8 nor the gray-black Cu2S nor the black CuS.This was due to the poor reactivity of LCu towards a dilutesolution of methanolic Cu(AcO)2 ·H2O.

Studying more closely the reaction of 2 with Hg(AcO)2

and Pb(AcO)2 ·3H2O we found that the precipitated solidsadsorbed soluble species from the supernatant and also

www.zaac.wiley-vch.de © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Z. Anorg. Allg. Chem. 2009, 329�336330

more 2 was required under anaerobic conditions in orderto give pure, white L2Pb.

Studies on the Reaction of Me2As(S)-S-AsMe2 (2) withHeavy Metal Cations

We studied the reaction of 2 with Hg(AcO)2 andPb(AcO)2 ·3H2O under anaerobic and aerobic conditionsusing 2:1, 3:1, and 4:1 stoichiometries and we identifiednine methanol-soluble by-products mainly by 1H NMR.The identification of the species and their chemical shifts inCD3OD, Figures 1 and 2, are as follows. Acetic acid1.985 ppm (1.99 [19]); 1 1.920 [6]; 2 2.135 and 1.500 [6]; 31.348 [20]; 7 1.970 ppm (after isolation and X-ray determi-nation). The singlet at 1.345 ppm, Figure 2, should be acomposite one containing Me2AsIII from two differentmolecules. The first must be Me2As-OCD3 8 because it isabsent in the spectrum of 2 � Pb(AcO)2 ·3H2O (Figure 2C)due to hydrolysis [21] from the water of crystallization(compare Figure 2A). In CDCl3 (or CCl4?) Me2As-OMeresonates at 1.19 ppm [22]. The second contribution mostlikely comes from the two Me2AsIII protons of 11 because,in the anaerobic reaction of 2 � Hg(AcO)2 under the 4:1mole ratio, all of the excess of 2 reacted with the Me2As:�

present (see Discussion) producing the simple two line spec-trum, Figure 2i. Virtually at the same position (1.348 ppm)the disulfide 3 resonates, but its presence is unlikely because3 is stable (at �10 % extent) only in the presence of 2 [20]or when complexed (e.g. with AlCl3; Ioannou et al., inpreparation). The Me2AsV of 11 should resonate at1.970 ppm (Figure 2i) at the same position as 7 and, there-fore, this singlet as well is a composite one. Previously, inthe chemistry of 2 we also encountered composite singlets[6]. The singlet at 1.270 ppm is attributed to 10 and not to9 because arsinous acids R2AsOH are stable when R is anelectron withdrawing group [23] whereas with electron do-nating R groups the anhydride R2As-O-AsR2 is stable [23].Additional evidence in favour of 10 over 9 is the chromato-graphic behaviour of aqueous 9 which is best explained as10 being eluted [24], and the partial reduction of 1 withPh3P/I2 [25] in CD3OD which showed residual 1 at 1.936and 10 at 1.300 ppm, the very small differences in the shiftsbeing, most likely, due to the presence of Ph3P�O in themethanolic solution. The literature δ values of pure 10 are1.065 ppm (neat) and 1.115 ppm (in benzene) [26]. Since theproduction of 1 should come from air oxidation of 9 [27,28], we tentatively attribute the very small singlet at1.243 ppm to free dimethylarsinous acid 9.

The study of 2 with heavy metal acetates was hamperedby three facts. First, an adsorption of species by the solidwas deduced from the lower than calculated number of-CH3 groups versus the added known amount of benzeneas an internal standard. Second, the presence of some waterin CD3OD: calculated as �10 μmol HOD in 0.7 mlCD3OD compared to a 30 μmol scale of experiments. Thedanger of the reaction of the insoluble L2M with MII (M �Hg, Pb) did not allow more concentrated solutions of the

The Other Face of Bunsen’s Cacodyl Disulfide, Me2As(S)-S-AsMe2

Figure 2. The reaction of Me2As(S)-S-AsMe2 2 with Hg(AcO)2 and with Pb(AcO)2 ·3H2O in CD3OD (TMS 0.000 ppm) under variousmole ratios. The 1H NMR spectra are normalized for each row on the height of the AcOH singlet (1.985 ppm). Spectra A, a, I, and C,c, iii: 24 h reaction after the addition of metal acetates (30 μmol MII) and stirring under argon. Spectra B, b, ii, and D, d, iv: admissionof air in the previous systems and stirring for 2 h.

acetates to be used. Third, the presence of residual dioxygenin the under argon experiments. The latter two facts did notallow an unequivocal tracing of the origin of some speciesin solution.

In spite of these shortcomings certain trends can be seen(Figure 2), e.g. the more 2 is present the simpler the appar-ent composition of the supernatant and when air is admit-ted, then all AsIII compounds reacted. When the prep-aration of the L2Hg and L2Pb complexes were done in air1 and/or 7 were produced. With 2 and Cd(AcO)2 ·2H2Ounder 2:1 and 3:1 stoichiometries in air 1 and 7 were pro-duced; 1 predominating in both cases.

The AsV/AsV compound 7 was isolated as described inthe Experimental section and its identity was established by

Z. Anorg. Allg. Chem. 2009, 329�336 © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.zaac.wiley-vch.de 331

X-ray analysis. The compound 7 has been obtained (andanalysed by X-rays), along with Me2As(S)-S-As(O)Me2,from the reaction of 2 with t-butyl hydroperoxide [29]. Ourstudy confirmed the oxo-bridged nature of the AsV/AsV

compound 7, Figure 3.Compound 7 resonates as a singlet at 1.970 ± 0.003 in

CD3OD, 2.116 in D2O, 2.248 ± 0.002 in CDCl3, and2.22 ppm (probably in CDCl3) [29], and it is not stable dur-ing silica gel column chromatography, partly decomposingto S8 and other compounds which we did not identify.

Properties of the Complexes (Me2AsS2)xM, LxM

The complexes LxM adsorb compounds from the mothersolutions and, when isolated, keep tenaciously MeOH and/

P. V. Ioannou, D. G. Vachliotis, T. D. SiderisARTICLE

Figure 3. The structure of 7 in the crystal. Selected interatomicdistances /A and angles /° [the values found by the solution of thestructure in ref. [29] are given in square brackets using italicisednumbers]:

As(1)-O 1.777(3) [1.786(9)], As(1)-S(1) 2.066(2) [2.071(4)], As(1)-C(1)1.913(6) [1.90(1)], As(1)-C(2) 1.906(6) [1.93(1)]. As(2)-O 1.784(3)[1.785(8)], As(2)-S(2) 2.058(2) [2.068(4)], As(2)-C(3) 1.891(7) [1.93(1)],As(2)-C(4) 1.895(8) [1.92(1)], As(1)-O-As(2) 125.7(2) [125.0(5)], O-As(1)-S(1) 114.1(1) [112.5(3)], C(3)-As(2)-C(4) 107.8(4) [107.3(7)].

or H2O (by IR) and drying over concentrated sulphuric acid[1] or phosphorus pentoxide in vacuum was necessary forgood elemental analyses. The complexes decompose ther-mally [1] and we confirmed the evolution, at �180 °C, ofthe malodorous 5. The solid state IR spectra showed theexpected bands which have previously been assigned [7], in-dicating the presence of a 4-membered AsS2M ring.

Bunsen [1] found that the reaction of 2 with AuIII andCuII gave LAu and LCu. The reduction of the cations isnot due to 2 but to unstable L3Au and L2Cu because Caseyet al. [7] obtained LCu and S8 by reacting LNa·2H2O withCuCl2 ·2H2O. By reacting LNa·2H2O and Cu(AcO)2 ·H2O(2:1 mole ratio) in methanol in air, we confirmed the pro-duction of LCu and the co-precipitated S8 and, moreover,the evaporated supernatant contained three singlets at1.969 ppm (due to 7) and 1.913 and 1.906 ppm, one ofwhich being due to AcONa but the other is not known. Inair, the reaction of 2 with Cu(AcO)2 ·H2O (2:1 mole ratio)in methanol gave pure LCu free of S8, Cu2S or CuS. In theevaporated supernatant 7 (mainly) and 1 were present butS8 and 5 were absent.

The complexes LxM, we studied, were not inert towardsan excess of their own Mx�, reacting as suspensions inmethanol giving coloured solids indicative of sulfides at arate dependent on the metal cation. For example, in air,upon mixing L2Hg with Hg(AcO)2 the suspension becameblack while after 40 h stirring L2Cd had not totally reactedwith Cd(AcO)2 ·2H2O. The reactivity of LxM had the order:Hg > Pb > Ag > CuI > Zn � Cd. Also the nature of themethanol-soluble species differed: L2Cd gave only cacodylicacid 1, L2Zn and LAg gave only 7, LCu [withCu(AcOH)2 ·H2O] gave 1 with very little 7, while L2Hg andL2Pb, either under argon or in air, gave both 1 and 7. Inthe L2Pb/PbII, but not in the L2Hg/HgII, supernatant wedetected S8 by TLC in petroleum ether (Rf 0.80). More


complicated systems are those obtained by reacting 2 withequimolar quantities of Hg(AcO)2 and of Pb(AcO)2 ·3H2Obecause, now, Me2As:� and the excess MII can act upon thesolids L2Hg and L2Pb. The results showed that the meth-anol-soluble compounds were 1 and 7.

As noted in the Introduction, the anion Me2As-S� iselusive [18] although it was detected [6]. We, therefore, triedwithout success to isolate salts of Me2As-SH by desulfuriz-ing LxM with Ph3P and (MeO)3P. Thus, a suspension ofL2Pb in CDCl3 reacted smoothly with an equivalentamount of Ph3P to give impure (by IR) PbS and a super-natant which contained Ph3P�S (verified by TLC [6]), thesmelly 5 and other unidentified compounds. The formationof the presumably insoluble (Me2AsS)2Pb in small amountscould not be discarded. The same behaviour was observedwith L2Hg and L2Zn while the reactions of L2Cd, LAg, andLCu were too slow. Much the same was found with thebetter [30] desulfurization reagent, (MeO)3P, except in thecase of L2Hg where P(OMe)3 did not give the black HgSand 5 was not produced.

The Nucleophilic Character of Me2As(S)-S-AsMe2 (2) andSome Chemical Properties of the Arsenium Ion Me2As:�

The nucleophilic atom in the reaction of 2 with heavymetal cations is not the AsIII but either the As�S or �S�sulphur atom, Scheme 1. In either case the insoluble LxMis obtained along with the arsenium ion, Me2As:�. The in-solubility of LxM seems to be the driving force for the fis-sion of 2 as shown in Scheme 1.

The arsenium ion Me2As:� is generated during the slowaddition of a solution of heavy metal acetate into a solutionof 2 in an environment of methanol solvent, AcO�, H2O(from water of hydration of metal acetates or moisture inmethanol), (residual) dissolved O2 (from air), LxM, and 2(when in excess). Therefore, it should react with these nucle-ophiles as shown in Schemes 2 and 3.

In the supernatants we did not detect Me2As-OCOCH3

6 (1.84 and 2.47 ppm in CDCl3 [31]) because, by analogywith As(OCOCH3)3 it should be solvolysed quickly [32] to8 and in the presence of air cacodylic acid 1 is, eventually,obtained.

The reason why we did not get pure LxM under a strictx:1 stoichiometry is that the electrophilic arsenium ionMe2As:�, produced from the beginning, preferentially re-acts with 2, Scheme 3. Thus, some 2 is removed from thesystem and at the end of the addition of the metal acetatethere will be an excess of Mx� which acts on the solid LxM.When the reactions are run in air less excess of 2 is requiredbecause the arsenium ion is also removed as cacodylic acid1, Scheme 2.

The preference of Me2As:� for 2 is clearly seen in the 3:1and 4:1 cases under argon where 2 is not seen in the spectra,Figure 2, although in excess. Me2As:� should preferentiallyattack on AsV�S rather than on AsIII-S sulphur because inthe former case the intermediate 12 is resonance stabilized,Scheme 3. The intermediate 12 can have two options. One


Scheme 1.

Scheme 2.

Scheme 3.

is to capture MeOH or H2O to give the prentacoordinatedAsV compound 11 seen in the spectra, Figure 2i and 2iii.Admitting air causes 11 to decompose, because it has suit-able leaving groups [33], to 8/9 and 3. Then, 8 via 9 is


oxidized to 1, and 3 being unstable when not complexed[Ioannou et al., in preparation] should have given 2 [20]which was not detected in the spectra. Therefore, 3 musthave been oxidized by activated dioxygen to 7 because non-

P. V. Ioannou, D. G. Vachliotis, T. D. SiderisARTICLEactivated O2 has no effect on pure 2 equilibrated with 3 invarious solvents [20]. The nature of the activator is notcertain; it may be a complex of 9 ·O2 [34] acting on 3 ast-butyl hydroperoxide did [29]. The second option of 12 isto give 3 and Me2As:� via (Me2As)2S�-SAsMe2. In the lat-ter case the singlet at 1.345 ppm may contain some 3.

Properties of the (Me2AsS2)xM, LxM

The thermal decomposition of LxM to 5, equations (1)and (2) [1], most likely involves the initial production ofMe2As(S)-S-As(S)Me2. The AsV/AsV ester is not stable be-cause a very small number of such compounds is known,e.g. 7. By releasing one sulphur atom will give 2 which,then, desulfurizes to 5 [1] above its melting point (70 °C[20]).

Certain LxM complexes, e.g. M � CuII, AuIII, are notstable and give LM� (M� � CuI [1, 7] and AuI [1]). Thisbehaviour is analogous to the easy conversion of CuS andAu2S3 to Cu2S and Au2S and sulphur. The reduction can-not follow the reaction, equation (3):

because we did not detect the stable [20, 35] 5 which shouldhave been formed from Me2As-AsMe2 and S8 [36]. There-fore, the electron for the reduction of CuII in(Me2AsS2)2Cu probably comes from the nearby -S

..

..:� giving

the thiyl radical Me2As(S)-S..

... which, then, decomposes

producing S8 and 7 by an unknown radical mechanism. Thenature of the products with reducible cations like CuII de-pends on the starting compound. Thus, the reaction ofMe2AsS2Na·2H2O with Cu(AcO)2 ·H2O in non-deaeratedmethanol produces LCu � S8 � 7, while the reaction of 2with Cu(AcO)2 ·H2O produces LCu � 1 � 7, but S8 wasnot detected. In the latter case the system contains Me2As:�

as well which can give 1 as per Scheme 2 and it can alsoreact with S8, thus depleting S8 from the system and mostlikely forming the stabilized (Scheme 2) sulfenium ion Me2

A..

s�S..

..�. Therefore, it seems that the arsenium ion has an

affinity for sulphur (Sx, x � 1-8) producing a sulfenium ion.

Sulfenium ions of the type R-S..

..� [37, 38], as electrophiles

can react with MeCN, Me2S, and pyridine [39] and evencan bind dinitrogen [40]. It is likely that our sulfenium ioncan react with H2O in methanol to give more 7. We plan toprepare this sulfenium ion in a cleaner way in order to studyits behaviour.


The insoluble complexes LxM are being attacked by amethanolic solution of their own Mx� most likely on As�S sulphur atom. The intermediate obtained can decomposein various ways and at different rates giving colored solidsprobably composed of M2Sx admixed with various amountsof (Me2AsS)xM and the methanol-soluble 1 and/or 7 and insome cases S8. Evidently, the results of these heterogeneousreactions are quite difficult to generalize. Even more com-plicated pathways should follow the reaction of 2 with M2�

under 1:1 mole ratio, because the L2M, which is producedwhen half the amount of M2� has been added, can reactwith both M2� and Me2As:� electrophiles.

The attempts at the desulfurization of (Me2AsS2)xM byPh3P or (MeO)3P gave mainly the metal sulfide and 5 alongwith the Ph3P�S and (MeO)3P�S, instead of (Me2AsS)xM.A probable mechanism may involve the production of 2which, in turn, is desulfurized very rapidly by the PIII re-agents to 5 [6]. The different rates of desulfurization mayindicate the relative strengths of covalent bonding in LxM,for the more ionic the �S-M bond the more unfavourablebecomes the attack of PIII on the As�S sulphur because ahigh energy species having vicinal negative charges then re-sults.

Conclusions

Bunsen’s cacodyl disulfide 2 has electrophilic (towardsgroup 15 nucleophiles [6]) and nucleophilic (towards heavymetal cations) reactivities. In the latter case an arseniumion, Me2As:�, is co-produced which reacts with nucleo-philes in the system preferring 2 to AcO� or MeOH orH2O. This unstabilized arsenium ion also reacts with sul-phur (Sx, x � 1-8) to give the stabilized sulfenium ion,

Me2 A..

s�S..

..� ↔ Me2A

�

s�S. The complexes LxM are notstable towards their metal cations, and react with them, toa smaller or greater extent, in a non clean way. Irrespectiveof the reactants used (2 or LNa·2H2O) for the preparationof LxM, the interaction of LxM with excess Mx� shouldbe guarded against. The LxM complexes are also reactivetowards triphenylphosphine and trimethyl phosphite butthey do not give pure salts of the expected and wantedMe2As-SH. Instead, the metal sulphide is produced to-gether with Me2As-S-AsMe2 5.

Experimental Section

General

Cacodyl disulfide 2 was prepared form cacodylic acid (Serva) andhydrogen sulphide [20]. Sodium dimethyldithioarsinate dihydratewas prepared from 2, Na2S ·5H2O, and 1/8S8 [7] (Ioannou et al., inpreparation). The acetate salts and silver nitrate were AR grade.Triphenylphosphine and trimethyl phosphite were from Aldrich.Methanol, CD3OD and CDCl3 were not dried nor de-aerated. Forexperiments under argon, CD3OD (C.E. Saclay, Gif-Sur-Yvette,France), packaged under argon and containing, as stated, < 0.03 %HDO � D2O was used.


Silica gel 60 H (Merck) and silica gel H (Merck) were used for thinlayer (TLC) and column chromatography, respectively. TLCs wererun on microslides using appropriate standards when available.TLCs were made visible by iodine vapours; spraying with 35 % sul-phuric acid and charring did not produce spots. IR spectra wereobtained on a Perkin-Elmer model 16PC FT-IR spectrometer. 1HNMR spectra (400 MHz) were recorded on a Bruker DPX Avancespectrometer with internal TMS (0.000 ppm) as standard [6]. El-emental analyses were obtained through the Centre of InstrumentalAnalyses, University of Patras, Greece.

Preparation of (Me2AsS2)xM (x � 1 or 2): GeneralProcedure

To a well-stirred solution of cacodyl disulfide 2 (0.25 mmol) inmethanol (1.5 ml) was added dropwise a (dilute) solution ofM(AcO)2 ·xH2O (x � 0-3) (0.1 mmol) in methanol (5-6 ml) at sucha rate that the suspension decolorized. The addition usually lasted1 h. After stirring for another 1 h centrifugation, washing withmethanol (3x2 ml), and drying in vacuum over phosphorus pentox-ide afforded the complexes as white (for LCu yellow) powders withyields of 95-100 % (75 % for L2Zn). Darkening and evolution of 5,decomposition temperature: Pb �150, �180; Cd �180, �210; Hg>140, 187; Zn �165, >250; Ag �120, �190; CuI �185, >280 °C.C4H12S2As2Hg (Mr 538.82); C 8.83 (calc. 8.92), H 2.10 (2.24), S23.90 (23.80) %. C2H6S2AsAg (Mr 276.99); C 9.01 (calc. 8.67), H2.01 (2.18), S 23.42 (23.15) %. IR (KBr) for L2Hg: 2988 w, 2904 vw,1398 m, 1254 vw, 912 s, 872 vs, 842 m, 620 w, 596 m; for LAg:2990 w, 1386 ms, 1254 vw, 914 ms, 878 vs, 670 w, 590 m. The L2Pb[1], L2Cd [7, 8], L2Zn [7, 8], and LCu [1, 7] are known complexes.

The supernatants after evaporation and drying were also studiedby 1H NMR as described in the Results section.

Isolation and X-ray Crystallographic Study of Me2As(S)-O-As(S)Me2 (7)

The supernatants containing as main component the compound 7showing the 1.970 ppm singlet in CD3OD were combined and col-umn chromatographed, at a fast flow rate, eluting with ether. Aslightly impure fraction contained by TLC (Et2O) traces of an im-purity at Rf 0.55 and 7 at Rf 0.33 was selected. By 1H NMR(CD3OD, TMS) it contained 7 at 1.970 ppm, the impurity at1.957 ppm, and traces of 1 at 1.918 ppm. It was dissolved in di-chloromethane and 7 was crystallized by diffusing pentane. It wassoluble in CH2Cl2 and CHCl3, insoluble in Et2O, pentane, Me2CO.It is soluble in warm acetone but it does not precipitate on cooling.M. p. 159-161 °C (lit. [29] 163 °C) and its IR (KBr) showed thepublished [29] bands. 1H NMR: in CD3OD, TMS: 1.970; in D2O,DSS: 2.116; in CDCl3, TMS: 2.248 ppm (lit [29] probably inCDCl3: 2.22 ppm).

Single-crystal X-ray crystallography proved that the product 7 isthe compound Me2As(S)-O-As(S)Me2, whose structure had beendetermined [29]. The structure was solved by SHELXS-86 [41] andrefined by full-matrix least-squares techniques on F2 usingSHELXL-97 [42]. C4H12As2OS2, Mr � 290.10, monoclinic, C2/c,a � 27.603(16) A, b � 6.348(3) A, c � 12.166(6) A, β � 97.08(2)°,V � 2115.5(19) A3.


Reaction of Me2As(S)-S-AsMe2 (2) with Hg(AcO)2 andPb(AcO)2 ·3H2O Under Various Mole Ratios

Three oven-dried vials, each bearing a rubber septum and contain-ing mercuric acetate (9.6 mg, 30 μmol), were purged with argonand the salt dissolved in argon-packaged CD3OD (0.4 ml). Threeoven-dried round-bottomed flasks, having magnetic followers, se-aled by rubber septums and containing 60, 90 and 120 μmol 2,were purged with argon and the disulfide was dissolved in argon-packaged CD3OD (0.3 ml). Without delay [20] and while stirringvigorously, the solution of mercuric acetate was added via a micro-syringe during 1 h, followed by benzene (2.7 μl, 30 μmol) as an in-ternal standard. The suspension was quickly transferred to an ar-gon-purged NMR tube and the species in solution were monitoredby 1H NMR for 24 h, tumbling occasionally the tubes. Then thecontents of the tubes were stirred in air for 2 h and then their 1HNMR spectra were recorded to see the effect of dioxygen andmoisture on the species formed under argon. The precipitates werecentrifuged, washed with methanol, dried and analysed by IR.Similarly the reaction of 2 with Pb(AcO)2 ·3H2O was studied.Results are shown in Figure 2.

Reaction of (Me2AsS2)xM with Their Own Mx�

To a stirred suspension of the LxM (0.05 mmol) in methanol (1 ml)in air was added dropwise a solution of metal acetate (or AgNO3)(0.05 mmol) in methanol (2 ml) and then stirred at RT for �2 h(L2Hg and L2Pb complexes) or >40 h (L2Cd and LCu complexes).The precipitates and the supernatants were then analysed by IRand by TLC and 1H NMR, respectively. The observations are de-scribed in the Results section.

Desulfurization of (Me2AsS2)xM by Ph3P and by(MeO)3P

These reactions were run in CHCl3 or CDCl3 using equivalentamounts of reagents and followed by TLC [6] and 1H NMR. Theprecipitates and the supernants were analysed by IR and by 1HNMR, respectively. The observations are described in the Resultssection.

AcknowledgementWe thank Dr. Aris Terzis (Institute of Material Sciences, NCSR“Democritos“, Athens, Greece) and Professor S. P. Perlepes (of thisDepartment) for solving the structure of 7 by X-ray diffractionanalysis.

References[1] R. Bunsen, Justus Liebigs Ann. Chem. 1843, 46, 1.[2] N. Cameron, J. Trotter, J. Chem. Soc. 1964, 219.[3] M. W. Fricke, M. Zeller, H. Sun, V. W.-M. Lai, W. R. Cullen,

J. A. Shoemaker, M. R. Witkowski, J. T. Creed, Chem. Res.Toxicol. 2005, 18, 1821.

[4] a) I. Haiduc, D. B. Sowerby, S.-F. Lu, Polyhedron 1995, 14,3389; b) I. Haiduc, D. B. Sowerby, Polyhedron 1995, 15, 2469.

[5] I. Haiduc, J. Organomet. Chem. 2001, 623, 29.[6] P. V. Ioannou, D. G. Vachliotis, T. D. Sideris, Z. Anorg. Allg.

Chem. 2007, 663, 2077.[7] A. T. Casey, N. S. Ham, D. J. Mackey, R. L. Martin, Aust. J.

Chem. 1970, 23, 1117.

P. V. Ioannou, D. G. Vachliotis, T. D. SiderisARTICLE[8] M. Förster, H. Hertel, W. Kuchen, Angew. Chem. 1970, 82, 842;

Angew. Chem. Int. Ed. 1970, 9, 811.[9] E. Lindner, H. M. Ebinger, J. Organomet. Chem. 1974, 66, 103.[10] A. T. Casey, J. R. Thackeray, Aust. J. Chem. 1975, 28, 471.[11] a) N. Burford, B. W. Royan, P. S. White, J. Am. Chem. Soc.

1989, 111, 3746; b) C. Payrastre, Y. Madaule, J. G. Wolf, Tetra-hedron Lett. 1990, 31, 1145; c) N. Burford, T. M. Parks, B.W. Royan, J. F. Richardson, P. S. White, Can. J. Chem. 1992,70, 703.

[12] L. K. Krannich, U. Thewalt, W. J. Cook, S. R. Jain, H. H.Sisler, Inorg. Chem. 1973, 12, 2304.

[13] J. M. F. Braddock, G. E. Coates, J. Chem. Soc. 1961, 3208.[14] C. Payrastre, Y. Madaule, J. G. Wolf, T. C. Kim, M.-R.

Mazieres, R. Wolf, M. Sanchez, Heteroatom Chem. 1992, 3,157.

[15] K. A. Porter, A. C. Willis, J. Zank, S. B. Wild, Inorg. Chem.2002, 41, 6380.

[16] P. H. Javora, E. A. Meyers, R. A. Zingaro, Inorg. Chem. 1976,15, 2525.

[17] S. Burck, J. Daniels, T. Gans-Eichler, D. Gudat, K. Nättinen,M. Nieger, Z. Anorg. Allg. Chem. 2005, 631, 1403.

[18] K. Tani, S. Hanabusa, S. Kato, S. Mutoh, S. Suzuki, M. Ishida,J. Chem. Soc., Dalton Trans. 2001, 518.

[19] H. E. Gottlieb, V. Kotlyar, A. Nudelman, J. Org. Chem. 1997,62, 7512.

[20] R. A. Zingaro, K. J. Irgolic, D. H. O’Brien, L. J. Edmonson,Jr., J. Am. Chem. Soc. 1971, 93, 5677.

[21] G. Kamai, B. D. Chernokal’skii, J. Gen. Chem. USSR 1960,30, 1548.

[22] L. S. Sagan, R. A. Zingaro, K. J. Irgolic, J. Organomet. Chem.1972, 39, 301.

[23] G. O. Doak, L. D. Freedman, Organometallic Compounds ofArsenic, Antimony, and Bismuth, Wiley-Interscience, New York,1970, p. 64.

[24] J. Gailer, S. Madden, W. R. Cullen, M. B. Denton, Appl. Or-ganometal. Chem. 1999, 13, 837.


[25] P. V. Ioannou, Appl. Organometal. Chem. 2000, 14, 261.[26] H. C. Marsmann, J. R. Van Wazer, J. Am. Chem. Soc. 1970,

92, 3969.[27] G. C. Chen, R. A. Zingaro, C. R. Thompson, Carbohydrate

Res. 1975, 39, 61.[28] Z. Gong, X. Lu, W. R. Cullen, X. C. Le, J. Anal. At. Spectrom.

2001, 16, 1409.[29] L. Silaghi-Dumitrescu, S. Pascu, I. Silaghi-Dumitrescu, I. Hai-

duc, M. N. Gibbons, D. B. Sowerby, J. Organomet. Chem. 1997,549, 187.

[30] J. R. Lloyd, N. Lowther, G. Zsabo, C. D. Hall, J. Chem. Soc.,Perkin Trans. II 1985, 1813.

[31] P. Aslanidis, F. Kober, Chemiker-Ztg 1988, 112, 106.[32] G. Gattow, H. Schwank, Z. Anorg. Allg. Chem. 1971, 382, 49.[33] A. Terzis, P. V. Ioannou, Z. Anorg. Allg. Chem. 2004, 630, 278.[34] T. D. Sideris, P. V. Ioannou, Phosphorus, Sulfur, Silicon 2006,

181, 751.[35] W. T. Reichle, Inorg. Chem. 1962, 1, 650.[36] F. F. Blicke, R. A. Patelski, L. D. Powers, J. Am. Chem. Soc.

1933, 55, 1158.[37] N. Kharasch, S. J. Potempa, H. L. Wehrmeister, Chem. Rev.

1946, 39, 269.[38] D. N. Harpp, D. K. Ash, R. A. Smith, J. Org. Chem. 1979,

44, 4135.[39] G. K. Helmkamp, D. C. Owsley, W. M. Barnes, H. N. Cassey,

J. Am. Chem. Soc. 1968, 90, 1635.[40] D. C. Owsley, G. K. Helmkamp, J. Am. Chem. Soc. 1967, 89,

4558.[41] G. M. Sheldrick, SHELXS-86: Structure Solving Program.

University of Göttingen, Germany, 1986.[42] G. M. Sheldrick, SHELXS-97: Program for the Refinement of

Crystal Structures from Diffraction Data. University ofGöttingen, Germany, 1997.

Received: November 15, 2007Accepted: September 4, 2008

the other face of bunsen's cacodyl disulfide, me2as(s)-s-asme2: the lewis-base behaviour...

Documents