A Novel Reversible Aryl Exchange Involving Two Organometallics:  Mechanism of the Gold(I)-Catalyzed Isomerization of trans-[PdR₂L₂] Complexes (R = Aryl, L = SC₄H₈)

[AuR 1 (tht)] ( 3a ) (R 1 = 3,5-C 6 Cl 2 F 3, tht = tetrahydrothiophene) very efficiently catalyzes the isomerization of trans -[Pd(R 1 ) 2 (tht) 2 ] ( 1a ) to cis -[Pd(R 1 ) 2 (tht) 2 ] in CDCl 3 . The 19 F NMR kinetic study leads to the first-order rate law r iso = k iso [ 1a ] = ( k spo + k cat [ 3a ])[ 1a ], where k spo = (1.50 ± 0.03) × 10 - 6 s - 1 and k cat = a /( b + [tht]) with a = (1.32 ± 0.07) × 10 - 4 s - 1 and b = (3.0 ± 0.2) × 10 - 5 mol L - 1 (at 304.4 K). The reaction of 1a and [AuR 2 (tht)] (R 2 = C 6 F 5 ) yields cis -[PdR 1 R 2 (tht) 2 ] and 3a, evidencing that the catalyzed isomerization takes place with aryl-group exchange between Pd(II) and Au(I). An associative mechanism passing through R-bridged intermediates [(tht)(R 1 ) 2 Pd(μ-R 2 )Au(tht)] and a donor−acceptor activated complex [(tht)(R 1 ) 2 (R 2 )Pd→Au(tht)] ⧧ is proposed. The results suggest that the associative displacement of tht from 1a by the nucleophilic arylgold(I) complex to give [(tht)(R 1 ) 2 Pd(μ-R 2 )Au(tht)] is the rate-determining step ( k 1 ). This is supported by the typical bimolecular activation parameters that were found: Δ H 1 ⧧ = 56.4 ± 1.6 kJ mol - 1 and Δ S 1 ⧧ = −46 ± 6 J K - 1 mol - 1 .