Thermally triggered reductive elimination of bromine from Au(iii) as a path to Au(i)-based coordination polymers

Four new [AuBr(2)(CN)(2)](-)-based coordination polymers, Zn(pyz)(NCMe)(2)[AuBr(2)(CN)(2)](2) (1; pyz = pyrazine), Co(pyz)[AuBr(2)(CN)(2)](2)·H(2)O (2) and [M(bipy)(2)(AuBr(2)(CN)(2))][(n)Bu(4)N][AuBr(2)(CN)(2)](2) (bipy = 4,4'-bipyridine), where M = Co (5) and Zn (6), were synthesized and three of them structurally characterized. 1 forms 1-D chains connected by pyz ligands while isostructural 5 and 6 form 3-D frameworks via [AuBr(2)(CN)(2)](-) and bipy linkers. Aqueous suspensions of 2, 5 and 6 or their precursors in situ (preferred) were heated hydrothermally to 125 °C, triggering the reductive elimination of bromine from the Au(III) centres, which yielded the [Au(CN)(2)](-)-based coordination polymers M(pyz)[Au(CN)(2)](2), where M = Zn (3) or Co (4) and Zn(bipy)[Au(CN)(2)][Au{Br(0.68)(CN)(0.32)}CN] (7), or a mixture of cyanoaurate(I)-containing products in the case of 5 and 6. The structural characterization of 3 revealed a [Au(CN)(2)](-)/pyz-based framework similar to previously reported Cu(pyz)[Au(CN)(2)](2), whereas 7 formed an intricate network consisting of individual 2-D networks held together by AuAu interactions and featuring the rare [AuBrCN](-) unit. The kinetics of the thermally-induced reductive elimination of Br(2) from K[AuBr(2)(CN)(2)] in 1-BuOH yielded a t(½) of approx. 10 min to 4 h from 98 to 68 °C, and activation parameters of ΔH(‡) = 131(15) kJ mol(-1) and ΔS(‡) = 14.97(4) kJ K(-1)mol(-1), indicating that the elimination of the halogen provides the highest barrier to activation.