Activationof B–C Bonds in [BArF₂₄]⁻ (BArF₂₄ = Tetrakis(3,5-bis(trifluoromethyl)phenyl)borate)By Gold(I) Compounds With 1,1′-bis(phosphino)metallocene Ligands

Tetrakis(3,5-bis(trifluoromethyl)phenyl)borate ([BArF₂₄]⁻) stabilizes reactive cations; however, compounds capable of activating a B–C bond in this anion exist. We investigated the catalytic activity of gold(I) compounds with bis(phosphino)metallocene ligands, [(AuCl)₂(μ-PP)] (PP = 1,1′-bis(diphenylphosphino)ferrocene (dppf), 1,1′-bis(di<i>iso</i>-propylphosphino)ferrocene (dippf), 1,1′-bis(dicyclohexylphosphino)ferrocene (dcpf), 1,1′-bis(di<i>tert</i>-butylphosphino)ferrocene (dtbpf), 1-diphenylphosphino-1′- di<i>tert</i>-butylphosphino-ferrocene (dppdtbpf), or 1,1′-bis(diphenylphosphino)ruthenocene (dppr)). These compounds displayed minimal catalytic activity unless Na[BArF₂₄] was added, though the active species remains unidentified. Reactions of Na[BArF₂₄] with [(AuCl)₂(μ-PP)] revealed [(AuCl)₂(μ-PP)] (PP = dppf, dippf, dcpf, dtbpf, or dppdtbpf), gave single products, [(AuAr^F)₂(μ-PP)] (Ar^F = 3,5-C₆H₃(CF₃)₂), by ³¹P{¹H} NMR spectroscopy. These were characterized, and X-ray structures of three compounds (PP = dppf, dippf, or dtbpf) were determined. For dtbpf, an intermediate compound, [Au₂(μ-Ar^F)(μ-dtbpf)][BArF₂₄], was isolated and structurally characterized. The [(AuAr^F)₂(μ-PP)] and related [(AuPh)₂(μ-PP)] compounds were also prepared by the addition of RB(OH)₂ (R = Ar^F or Ph) to [(AuCl)₂(μ-PP)]. X-ray structures of [(AuPh)₂(μ-PP)] (PP = dppf or dcpf) were determined. Their electrochemistry was investigated by cyclic voltammetry. The role of the ferrocenyl backbone of the ligands was examined by performing reactions in the presence of chemical oxidants. These reactions provide suggest a combination of the ferrocenyl backbone, the substituents on the phosphorus atoms, and the [B(aryl)₄]⁻ play significant roles in this chemistry.