Synthesis of the gold analogue of the elusive doubly protonated water molecule

SINGLY coordinated metal cations of the type (L)M⁺ (where M is Cu, Ag or Au, and L is a donor ligand) are, in terms of their valence orbital characteristics, analogous to the proton¹. The analogy is particularly pronounced for the gold cation, in which relativistic effects strongly contract the 6s valence orbital², thereby permitting the incorporation of these proton analogues into stable molecular and ionic species having very short intramolecular bonds. Structural and stoichiometric parallels between gold cations and protons are illustrated by the species CH₄ and C(AuL)₄, NH₄⁺ and N(AuL)₄⁺ and OH₃⁺ and O(AuL)₃⁺ (refs 3-5), as well as by pairs of hypercoordinated cations such as CH₅⁺ and C(AuL)₅⁺ (refs 6, 7). We have recently synthesized the stable dicationic gold species [(LAu)₆C]²⁺ and [(LAu)₅N]²⁺ (refs 8, 9); their hydrogen counterparts, CH₆²⁺ and NH₅₂₊, have never been observed and are expected to be intrinsically unstable¹⁰. Here we describe the preparation of the first stable four-coordinated dicationic oxygen compound, [(LAu)₄O]²⁺, whose analogue, the doubly protonated water molecule [H₄O]²⁺, has been predicted to be stable only in the gas phase, or to appear as a transient state during proton transfer in super acid media^11,12.