Thermochemistry of Sulfur Atom Transfer. Enthalpies of Reaction of Phosphines with Sulfur, Selenium, and Tellurium, and of Desulfurization of Triphenylarsenic Sulfide, Triphenylantimony Sulfide, and Benzyl Trisulfide

The enthalpies of reaction of neat PBu₃ with solid sulfur (-27.1 ± 0.5 kcal/mol), selenium (-20.0 ± 0.6 kcal/ mol), and tellurium (-4.9 ± 0.6 kcal/mol) have been measured by solution calorimetry. The enthalpies of reaction of a series of phosphines with sulfur in toluene solution have been measured as follows (values in kcal/mol): PCy₃ = -30.9 ± 1.9, PBu₃ = -28.9 ± 0.3, PMe₃ = -27.1 ± 0.4, PMe₂Ph = -26.0 ± 0.5, PMePh₂ = -23.8 ± 0.3. PPh₃ = -21.5 ± 0.3. These values correlate with literature data for enthalpies of protonation and indicate that P to S σ donation is probably the dominant factor in determining the R₃P=S bond strength, estimates for which range from 88 to 98 kcal/mol. The enthalpies of S atom transfer to PPh₃ by S=AsPPh₃ and S=SbPh₃ in toluene solution are -17.7 ± 1.2 and -21.5 ± 1.0 kcal/mol, respectively. The enthalpy of removal of the central S atom from BzSSSBz by PCy₃, yielding BzSSBz and S=PCy₃, is -29.0 ± 1.8 kcal/mol. These data are used to establish a range of enthalpies of S atom transfer in these compounds which spans 31 kcal/mol from S=SbPPh₃ to S=PCy₃.