The Other Face of Bunsen's Cacodyl Disulfide, Me₂As(S)‐S‐AsMe₂: The Lewis‐Base Behaviour Towards Heavy Metal Cations

Abstract

The reaction of Bunsen's cacodyl disulfide, Me2As(S)‐S‐AsMe2, with heavy metal cations in methanol produces insoluble salts (complexes) of dimethyldithioarsinic acid, Me2AsS2H, and dimethyl arsenium ion, Me2As:+. This arsenium ion prefers to react with Me2As(S)‐S‐AsMe2, when in excess, compared to AcO− or MeOH/H2O and it is also reactive towards sulfur (Sx, x = 1‐8) producing the stabilized dimethylarsino sulfenium cation, $\rm Me_{2}As- {\mathop{\rm{S}}\limits^{.}_{\ddot{}}} ^{+} \longleftrightarrow Me_{2}\mathop{\rm{A}}\limits^{+}_{} s=\mathop{\rm{S}}\limits^{.}_{\ddot{}}$. The complexes (Me2AsS2)xM (x = 1 or 2) are unstable in the presence of their own heavy metal cations decomposing to colored solids. In an attempt to prepare salts of Me2AsSH, the reactions of (Me2AsS2)xM with triphenylphosphine and trimethyl phosphite gave the metal sulfide and Me2As‐S‐AsMe2 instead.

The Lewis-base behavior of Bunsen's cacodyl disulfide, Me2As(S)‒S‒AsMe2, toward molecular halides of group 13

“…The complexation of 1 with MX 3 , observed only with BF 3 Á Et 2 O and AlCl 3 , conceivably can also be accomplished intramolecularly from 11 0 or 12 0 , Scheme 1. Decomposition of Lewis acid-base complex in the sense of 11 00 , similar to that postulated for the preparation of (Me 2 AsS 2 ) x M and for the unstable (Me 2 AsS 2 ) 3 Tl (see section 4.3 below), did not seem to have taken place [5]. GaCl 3 and InCl 3 afforded only 2 Á MX 3 without any sign of rearrangement to 1 Á MX 3 .…”

“…The product Me 2 AsS 2 Tl is a soft, pale yellow, cotton-like solid which starts decomposing at *1608C evolving the foul smelling Me 2 AsÀ ÀSÀ ÀAsMe 2 3 [1,5]. It is soluble in warm [D 6 ]DMSO giving a singlet at 1.849 ppm.…”

“…We found that the L x M complexes are not stable in the presence of their own M xþ ions, giving colored insoluble solids, and the arsenium ion Me 2 As: þ reacts preferably with 2, Equation (5), in the presence of other nucleophiles, e.g. AcO 7 , MeOH, H 2 O, found in its environment [5].…”

“…Many complexes (Me 2 AsS 2 ) x M have been prepared, generally by reacting 2 with heavy metal cations [1,5,6] or using the salt Me 2 AsS 2 Na Á 2H 2 O 7 [6,20]. Bunsen found that the reaction of 2 with CuCl 2 and AuCl 3 gave the complexes Me 2 AsS 2 M [M¼Cu(I), Au(I)] and we and others confirmed the production of Cu(I) complex starting from 2 and from 7 [5,6].…”

“…We studied this nucleophilic activity of 2 with several heavy metal cations paying attention to the required conditions for obtaining pure complexes L x M, [5] because impure ones can sometimes be formed [6]. We found that the L x M complexes are not stable in the presence of their own M xþ ions, giving colored insoluble solids, and the arsenium ion Me 2 As: þ reacts preferably with 2, Equation (5), in the presence of other nucleophiles, e.g.…”