Mono- and Dinuclear Ag(I), Au(I), and Au(III) Metallamacrocycles Containing N‑Heterocyclic Dicarbene Ligands

Ag­(I) dicarbene complexes [Agm(Ln)m]­Xm (Ln = ImMe(CH2)nImMe, ImMe = N-methylimidazol-N-yl-2-ylidene; n = 3, X = PF6, m = 2; n = 6–8, 10, X = AgBr2, m = 1, 2) were prepared by reacting Ag2O with 1 equiv of the corresponding bisimidazolium salt [H2Ln]­A2 (A = PF6, Br). The dibromoargentates react with 1 equiv of AgTfO to afford [Agm(Ln)m]­(TfO)m (m = 1, 2). The room temperature transmetalation reaction of [Agm(Ln)m]­[AgBr2]m (n = 3, 5, 6–8, 10) with [AuCl­(SMe2)] and AgTfO (Ln:Au:TfO = 1:1:1) affords [Au2(μ-Ln)2]­(TfO)2 (n = 3, 5, 10), or mixtures of [Au­(κ2-Ln)]­TfO (main product for n = 7) and [Au2(μ-Ln)2]­(TfO)2 (main product for n = 6, 8). At room temperature, the equilibrium between [M2(μ-Ln)2]­(TfO)2 and [M­(κ2-Ln)]­TfO is fast for M = Ag, but slow for M = Au, in the NMR time scale. When n ≥ 7 and M = Ag or Au, the equilibrium is shifted toward the mononuclear complexes in the order 8 > 10 > 7, which proves that the (CH2)8 linker has the optimal length for trans chelation. Correspondingly, the high-temperature metalation of [H2Ln]­Br2 (n = 8, 10) with 1 equiv of [AuCl­(SMe2)] and excess of NaAcO, affords [Au­(κ2-Ln)]Br with a small amount of [Au2(μ-Ln)2]­Br2. If AgTfO is added to the reaction mixture, [Au­(κ2-L8)]2[AgBr3] is isolated instead of the desired triflate, which can be obtained by reacting the mixture of [Au­(κ2-L8)]Br and [Au2(μ-L8)2]­Br2 with AgTfO. [Au­(κ2-L10)]­TfO was isolated after thermal conversion of [Au2(μ-L10)2]­(TfO)2. [Au­(κ2-L8)]­TfO reacts with I2 to give trans-[AuI2(κ2-L8)]­TfO, which is the first Au­(III) complex containing a trans-spanning bidentate ligand. We have determined the crystal structures of complexes [Ag2(μ-L3)2]­(PF6)2, [Ag­(κ2-L7)]­TfO, [Au2(μ-L3)2]­(TfO)2, [Au­(κ2-L8)]­Br, [Au­(κ2-L8)]2[AgBr3], and trans-[AuI2(κ2-L8)]­TfO.