CATALYTIC OXIDATION OF SULFUR COMPOUNDS. SYNTHETIC, STRUCTURAL, AND THEORETICAL STUDIES OF MOLYBDENUM AND TUNGSTEN COMPLEXES.
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AbstractThe electronic structures and bonding interactions in the syn and anti isomers of Mo₂S₄(edt)₂²⁻ (edt = ethanedithiolato, (SCH₂CH₂S)²⁻) were examined and compared through Fenske-Hall and extended Huckel molecular orbital calculations. The electronic structures of a hypothetical closed isomer and the fragment MoS₃(edt)³⁻ were calculated as well. A molybdenum-molybdenum bonding interaction accounts for the diamagnetism of the complexes and is important in determining the relative stabilities of the isomers (syn > closed > anti). The structure of the syn isomer of (P(C₆H₅)₄)₂W₂S₄(edt)₂ was determined by X-ray crystallography. The compound crystallizes in the space group P2₁/n with a = 16.126(7)A, b = 25.03(1)A, c = 13.021(6)A, β = 101.73(4)°, and Z = 4. The calculated and observed densities are 1.755 and 1.757 g cm⁻³. Full-matrix least squares refinement of the 7414 data with Fₒ² > 3σ(Fₒ²) gave R = 0.039 and R(w) = 0.046. Each tungsten atom is ligated by five sulfur atoms in approximate tetragonal-pyramidal geometry. The W-W distance is 2.862(1)A, indicating a metal-metal bond. The average W-S(terminal) distance is 2.144(2)A, the average W-S(bridging) is 2.328(2)A, and the average W-S(edt) distance is 2.404(2)A. A variety of sulfur compounds, including hydrogen sulfide, aromatic and aliphatic thiols, cysteine, and bisulfite reduced MoO(TTP)OR, oxoalkoxotetratolylporphinatomolybdenum(V) (R = H, CH₃, C₂H₅, C₅H₁₁, MoO(TTP)), to the corresponding Mo(IV) species, MoO(TTP). The kinetics of the redox reaction between MoO(TTP)OC₂H₅ and RSH (R = C₂H₅, C₆H₅) were monitored at ambient temperature. The rate law d(Mo(IV))/dt = k₂(Mo(V))(RSH), with k₂ = 8.0 x 10⁻¹³ M⁻¹sec⁻¹ for C₂H₅SH and k₂ = 8.2 M⁻¹sec⁻¹ for C₆H₅SH describes the kinetic data. Sulfur compounds also reduced MoO₂(TTP). The kinetics of the redox reaction of the novel compound MoO(TTP)HSO₃ were monitored in chloroform solution at ambient temperature. The rate law d(Mo(IV))/dt = k₂(Mo(V))² with k₂ = 3.4 x 10² M⁻¹sec⁻¹ describes the kinetic data. Oxidation of MoO(TTP) to Mo(V) occurs in the presence of air. Oxidation catalysis by oxomolybdenum porphyrins was demonstrated for the substrates C₂H₅SH, C₆H₅SH, and HSO₃⁻. Mechanisms for these reactions are proposed.