Formation of Dinuclear Au^II and Au^I/Au^III Mixed-Valence Complexes is Directed by Structural Constraints Imposed by Cyclophane-NHC Ligands

The oxidation of dinuclear Au^I complexes containing cyclophane-bis(NHC) ligands by SOCl₂, Br₂ and I₂ has been investigated. The outcome of the oxidation was dependent on the nature of the cyclophane-NHC framework. The new complexes were fully characterised by X-ray diffraction and ¹H and ¹³C NMR spectroscopy. When the Au^I complex contained an ortho-cyclophane framework (dictating short intra-cation Au···Au distances), oxidation resulted in dinuclear Au^II complexes of the form X–Au^II(NHC)₂–Au^II(NHC)₂–X (X = Cl, Br, I), the Au···Au distance being in the range 2.6055(3)–2.6501(1) Å. These Au^II complexes are strongly luminescent. When the Au^I complex contained an meta-cyclophane framework (dictating longer intra-cation Au···Au distances), oxidation with SOCl₂ or Br₂ resulted in dinuclear complexes containing Au^I and Au^III centres of the form X–Au^III(NHC)₂–X–Au^I(NHC)₂. NMR studies showed that interconversion of the Au^I and Au^III sites within the complexes occurred at rates comparable to the NMR timescale.