ARSENIC SULFUR HETEROCYCLE FORMATION VIA METAL COORDINATION - SYNTHESIS AND MOLECULAR-STRUCTURE OF CYCLO-(CH3ASS)3, CYCLO-(CH3ASS)4, [(CO)3MO][ETA-3-CYCLO-(CH3AS)6S3], AND THE TRIPLE-DECKER SANDWICH COMPLEX [ETA-5-(C5H5)2MO2(ETA-2,MU-AS3)(ETA-2,MU-ASS)]

Molybdenum hexacarbonyl facilitates the ordered formation of crystalline methylarsathianes (methylarsenic sulfides), cyclo-(CH3AsS)n, n = 3 or 4, from elemental sulfur and pentamethyl-cyclo-pentaarsine, cyclo-(CH3As)5. In the absence of Mo(CO)6, an array of oligomeric products is formed, many containing S–S or As–As bonds instead of the perfect S–As alternation of the arsathianes. Both heterocycles have been structurally characterized: (CH3AsS)3, triclinic, {formula-omited},a = 8.856 (3) Å, b = 10.189 (2) Å, c = 12.925 (4) Å, α = 75.25 (2)°, β = 70.09 (2)°, γ = 89.54°, V = 1056.4 (5), Å3, Z = 4, R(F) = 4.97%;, (CH3AsS)4, orthorhombic, Pn21a, a = 11.495 (3) Å, b = 30.624 (5) Å, c = 8.276 (2) Å, V = 2913.2 (10) Å3, Z = 8, R(F) = 7.68%. Both of the arsathiane cyclooligomers crystallize with two independent, but chemically similar, molecules in the asymmetric unit. The rings adopt the conformations of their cycloalkane isolobal analogues, i.e., cyclohexane and cyclooctane. Possible pathways for the formation of the heterocycles involve, repetitively, initial coordination of the homoarsenic precursor, followed by oxidation by sulfur to form a pentavalent species, and release from the metal and rearrangement by insertion of S into an As-As bond. In support of this pathway is the isolation of a complex containing a partially sulfurated arsenic ring, [(CO)3Mo][cyclo-(CH3As)6S3], which contains a nine-membered ring of three (CH3As)2S units coordinated exclusively through As to a fac-Mo(CO)3 group. The complex has been structurally characterized: monoclinic, P21/c, a = 10.551 (3) Å, b = 10.819 (3) Å, c = 22.407 (6) Å, β = 90.33°, V = 2557.7 (10) Å3, Z = 4, R(F) = 8.43%. Unlike the case for arsaoxanes, (RAsO)n which have demonstrated ligand properties, no evidence was found for the formation of coordinated arsathianes in reactions with either Mo(CO)6 or [CpMo(CO)3]2 (Cp =η5-C5H5). However, reactions of the arsathianes with [CpMo(CO)3]2 lead to the formation of several naked arsenic clusters, only one of which contains sulfur: [CpMo(CO)2]2(η2,µ-As2) and [CpMo(CO)2]2(η2,µ-As2)2 are obtained in low yields. The sulfur-containing product, Cp2Mo2(η2,µ-As3)(η2,µ-AsS), is a triple-decker sandwich possessing a planar middle deck composed of an allylic As3 group and an {formula-omited} group: monoclinic, C2/c, a = 1 1.668 (2) Å, b = 1 1.273 (3) Å, c = 12.881 (4) Å, β = 119.63 (2)°, V = 1472.7 (4) Å3, Z = 4, R(F) = 2.86%. © 1990, American Chemical Society. All rights reserved.