A Novel Reversible Aryl Exchange Involving Two Organometallics: Mechanism of the Gold(I)-Catalyzed Isomerization of trans-[PdR2L2] Complexes (R = Aryl, L = SC4H8)
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Abstract
[AuR1(tht)] (3a) (R1 = 3,5-C6Cl2F3, tht = tetrahydrothiophene) very efficiently catalyzes the isomerization of trans-[Pd(R1)2(tht)2] (1a) to cis-[Pd(R1)2(tht)2] in CDCl3. The 19F NMR kinetic study leads to the first-order rate law riso = kiso[1a] = (kspo + kcat[3a])[1a], where kspo = (1.50 ± 0.03) × 10-6 s-1 and kcat = 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 [AuR2(tht)] (R2 = C6F5) yields cis-[PdR1R2(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)(R1)2Pd(μ-R2)Au(tht)] and a donor−acceptor activated complex [(tht)(R1)2(R2)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)(R1)2Pd(μ-R2)Au(tht)] is the rate-determining step (k1). This is supported by the typical bimolecular activation parameters that were found: ΔH1⧧ = 56.4 ± 1.6 kJ mol-1 and ΔS1⧧ = −46 ± 6 J K-1 mol-1.
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