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 PBu 3 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): PCy 3 = −30.9 ± 1.9, PBu 3 = −28.9 ± 0.3, PMe 3 = −27.1 ± 0.4, PMe 2 Ph = −26.0 ± 0.5, PMePh 2 = −23.8 ± 0.3, PPh 3 = −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 R 3 P S bond strength, estimates for which range from 88 to 98 kcal/mol. The enthalpies of S atom transfer to PPh 3 by S AsPPh 3 and S SbPh 3 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 PCy 3, yielding BzSSBz and S PCy 3, 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 SbPPh 3 to S PCy 3 .