Kinetic and Mechanistic Aspects of Sulfur Recovery from Pd₂I₂(μ-S)(μ-dpm)₂Using I₂and Structures of Pd(II) Complexes with the Chelated Monosulfide of dpm

Abstract

The Pd(2)X(2)(&mgr;-S)(dpm)(2) complexes (2) (X = I, Br) react with halogens to yield PdX(2)(dpm) (3) and elemental sulfur. Kinetic and mechanistic studies on the X = I system in CHCl(3) reveal that the reaction proceeds via addition of I(2) to give Pd(2)I(4)(dpm)(2) (4c), which then undergoes unimolecular decomposition to generate PdI(2)(dpm) (3c); the liberated sulfur concatenates to form elemental S(8). The addition reaction is in the stopped-flow time regime and is first-order in both 2c and I(2), with DeltaH() = 32 +/- 1 kJ mol(-)(1) and DeltaS() = -91 +/- 3 J K(-)(1) mol(-)(1). The slower decomposition reaction of 4c is first order in 4c, with DeltaH() = 80 +/-1 kJ mol(-)(1) and DeltaS() = -26 +/- 3 J K(-)(1) mol(-)(1). Byproduct PdX(2)(dpm(S)) (5) [dpm(S) = Ph(2)PCH(2)P(S)Ph(2)] also forms under some conditions via reaction of 3 with an S(n)() 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 chelate ring.