The replacement of five-membered N-donor heterocycles from aminotrichlorogold(III) complexes. A comparison with pyridines

Author: Pitteri Bruno   Bortoluzzi Marco  

Publisher: Springer Publishing Company

ISSN: 0340-4285

Source: Transition Metal Chemistry, Vol.31, Iss.8, 2006-11, pp. : 1028-1033

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Abstract

The kinetics of the process AuCl3(nu) + Cl→ AuCl 4 + nu (nu = one of a number of five-membered N-donor heterocycles covering a wide range of basicity, namely: oxazole, 2,4,5-trimethyloxazole, thiazole, 5-methylthiazole, 4-methylthiazole, 4,5-dimethylthiazole, 2,4-dimethylthiazole, 2,4,5-trimethylthiazole, imidazole and 2-methylimidazole) have been studied in methanol at 25 °C. The reactions follow the usual two-term rate law, rate = (k 1 + k 2[Cl])[complex], observed in a square-planar substitution associative-mechanism. The second-order rate constants, k 2, indicate that the discriminating ability of Au(III) in these complexes is good and markedly influenced by the nature of the leaving group. A linear-free-energy relationship, logk 2 = 0.53pk a + constant, is observed between the rate constant and the basicity of the leaving group for its replacement by chloride. The results are compared with data from the literature regarding a series of complexes of the type AuCl3(py) (py = one of a number of pyridines) reacting with the Cl anion under the same experimental conditions. The reactivity depends not only upon ligand basicity but also upon the nature of the ligand in the order: pyridines> five-membered heterocycles. Steric factor due to the presence of a single methyl group ortho to the sp2 nitrogen atom in the ring has no influence on the rate of substitution while, surprisingly, when there are two ortho methyl groups a remarkable steric retardation effect is observable. The results are discussed in terms of reaction-profile in the associative-substitution reaction and bonding interactions in the ground and transition states.