Synthesis, Interconversions, and Structural Characterization of the [(S₄)₂MoS]²⁻, [(S₄)₂MoO]²⁻, (Mo₂S₁₀)²⁻, and (Mo₂S₁₂)²⁻ Anions

The MoS₄²⁻ anion reacts with elemental sulfur and “active” sulfur reagents such as organic trisulfides or ammonium sulfides to afford binary molybdenum sulfides. The successful isolation of these sulfides, which appear to be components of a complex equilibrium system, depends on the solvent system and the nature of the counterions present in solution. With (C₂H₅)₄N⁺ as the counterion the [(S₄)₂MoS]²⁻ anion, I, can be isolated from either CH₃CN or DMF solutions. The hydrolysis of I in DMF or CH₃CN affords the [(S₄)₂MoO]²⁻ anion, II. In the presence of the (C₆H₅)₄P⁺ cation the (Mo₂S₁₀)²− (III) and (Mo₂S₁₂)²⁻ (IV) anions can be isolated from DMF solutions as mixed-anion (C₆H₅)₄P⁺ salts. Both I and II crystallize in the orthorhombic space group Ibca with eight molecules in the unit cell. The cell dimensions are a = 15.594 (3) Å, b = 13.264 (4) Å, and c = 27.577 (5) Å for I and a = 15.470 (1) Å, b = 13.224 (2) Å, and c = 27.425 (3) Å for II. In the structure of III/IV, both anions occupy the same position in the crystal lattice, with III being the major component (72%). III/IV crystallizes in the triclinic space group P1̄ with two molecules per unit cell. The cell dimensions are a = 22.288 (4) Å, b = 11.724 (4) Å, c = 10.512 (2) Å, α = 78.06 (4)°, β = 86.00 (3)°, and γ = 76.10 (3)°. Intensity data for all three structures were collected with a four-circle computer-controlled diffractometer by the θ−2θ scan technique. For I and II, all non-hydrogen atoms were refined with anisotropic thermal parameters. For III/IV, the S₂²⁻ ligand of Mo₂S₁₀²⁻ and the S₄²⁻ ligand of Mo₂S₁₂²⁻, as well as the DMF molecule of crystallization, were refined with isotropic thermal parameters. The remaining non-hydrogen atoms were refined with anisotropic thermal parameters. Refinement by a full-matrix least-squares procedure, of 145 parameters on 2146 data for I, 145 parameters on 1175 data for II, and 585 parameters on 7022 data for III/IV, gave final R values of 0.025, 0.047, and 0.065, respectively. In I and II, the Mo(IV) ion is coordinated by two bidentate S₄²⁻ chelates and a terminal sulfur or oxygen atom in a distorted-square-pyramidal arrangement. The molybdenum is situated above the basal sulfur plane by 0.72 Å for I and 0.76 Å for II. The Mo₂S₁₀²⁻ and Mo₂S₁₂²⁻ anions have in common the (Mo₂S₄)²⁺ core as well as the tetrasulfide ligand attached to Mo(1). Mo(2) is coordinated by either a persulfido group in III or a tetrasulfido unit in compound IV. The core contains two (Mo^v=S)³⁺ units bridged asymmetrically by two sulfide ligands in the syn configuration, with a Mo—Mo distance of 2.846 (1) Å. In all three structures, an alternation of the S-S bond lengths in the S₄²⁻ chelate rings is observed, and the Mo-S₄ ring is in the “puckered” or envelope configuration. The coordinated sulfur atoms of the S₄²⁻ ligand are asymmetrically bound to the molybdenum. The MoS₄ ring conformation and the consequent effects on Mo-S bonding are attributed to intraligand sulfur electron lone-pair repulsions.