Kinetic and mechanistic aspects of sulfur recovery from Pd2I2(mu-S)(mu-dpm)(2) using I-2 and structures of Pd(II) complexes with the chelated monosulfide of dpm

The Pd2X2(mu-S)(dpm)(2) complexes (2) (X = I, Br) react with halogens to yield PdX2(dpm) (3) and elemental sulfur. Kinetic and mechanistic studies on the X = I system in CHCl3 reveal that the reaction proceeds via addition of I-2 to give Pd2I4(dpm)(2) (4c), which then undergoes unimolecular decomposition to generate PdI2(dpm) (3c); the liberated sulfur concatenates to form elemental Ss The addition reaction is in the stopped-flow time regime and is first-order in both 2c and I-2, with Delta H-double dagger = 32 +/- 1 kJ mol-l and Delta S-double dagger = -91 +/- 3 J K-1 mol(-1). The slower decomposition reaction of 4c is first order in 4c, with Delta H-double dagger. = 80 +/- kJ mol(-1) and Delta S-double dagger = -26 +/- 3 J K-1 mol(-1). Byproduct PdX2(dpm(S)) (5) [dpm(S) = Ph2PCH2P(S)Ph-2] also forms under some conditions via reaction of 3 with an S, species (n < 8). Complexes 5 (X = Cl (a), Br (b), I(c)) were also synthesized directly, and the structure of the 5c species, as well as of [Pd(dpm(S))(2)]Cl-2, were determined by X-ray analyses that reveal the envelope configuration of the five-membered Pd-PPh2CH2P(S)Ph-2 chelate ring.