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

Abstract

Four new [AuBr₂(CN)₂]⁻-based coordination polymers, Zn(pyz)(NCMe)₂[AuBr₂(CN)₂]₂ (1; pyz = pyrazine), Co(pyz)[AuBr₂(CN)₂]₂·H₂O (2) and [M(bipy)₂(AuBr₂(CN)₂)][ⁿBu₄N][AuBr₂(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 [AuBr₂(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 Au⋯Au interactions and featuring the rare [AuBrCN]⁻ unit. The kinetics of the thermally-induced reductive elimination of Br₂ from K[AuBr₂(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⁻¹mol⁻¹, indicating that the elimination of the halogen provides the highest barrier to activation.