A novel reversible aryl exchange involving two organometallics:: Mechanism of the gold(I)-Catalyzed isomerization of trans-[PdR2L2] complexes (R = aryl, L = SC4H8)

[AuR1(tht)] (3a) (R-1 = 3,5-C6Cl2F3, 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 CDCl3. The F-19 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) x 10(-6) s(-1) and k(cat) = a/(b + [tht]) with a = (1.32 +/- 0/07) x 10(-4) s(-1) and b = (3.0 +/- 0.2) x 10(-5) mol L-1 (at 304.4 K). The reaction of 1a and [AuR2(tht)] (R-2 = C6F5) yields cis-[(PdRR2)-R-1(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(mu-R-2)Au(tht)] and a donor-acceptor activated complex [(tht)(R-1)(2)(R-2)Pd-->Au(tht)]double dagger is proposed. The results suggest that the associative displacement of tht from la by the nucleophilic arylgold(I) complex to give [(tht)(R-1)(2)Pd(mu-R-2)Au(tht)] is the rate-determining step (k(1)). This is supported by the typical bimolecular activation parameters that were found: Delta H(1)double dagger = 56.4 +/- 1.6 kJ mol(-1) and Delta S(1)double dagger = -46 +/- 6 J K-1 mol(-1).