Bromination of (phosphine)gold(I) bromide complexes: stoichiometry and structure of products.
The course of the oxidative addition of elemental bromine to complexes of the type (L)AuBr is strongly influenced by the nature of the tertiary phosphine ligand L. Standard square planar gold(III) complexes (L)AuBr3 are obtained not only with L = PMe3 but also with P((I)Pro)3 for which the oxidative addition fails in the corresponding iodine system. Excess bromine is integrated into crystals of the products with the stoichiometry [(Me3P)AuBr3].(Br2) and {[(iPro)3P]AuBr3}.(Br2). Of the series of iodine analogues, an intercalate [(Me3P)AuI3]2.(I2) has been structurally characterized. [((t)Bu)3P]AuBr undergoes ligand redistribution upon treatment with bromine to give a complex reaction mixture, from which {[(tBu)3P]2Au}+(Br3)-.(Br2) could be crystallized. It contains polymeric anions [(Br5)-]n as zig-zag chains. [(o-Tol)3P]AuBr is readily brominated to give [(o-Tol)3P]AuBr3. Contrary to the situation in the gold(I) complex with its linear PAuBr unit, the square planar structure of the PAuBr3 unit causes steric hindering of the rotation of the tolyl groups about the P-C bonds as demonstrated by solution NMR studies. (The corresponding reaction with iodine is known to give only polyiodides with the oxidation state of the gold atom unchanged.) The even more severe congestion in [(Mes)3P]AuBr prevents oxidative addition not only of iodine but also of bromine. With the latter, P-Au cleavage occurs instead affording [(Mes)3PBr]+[AuBr4]-.