A Shape-Complementarity Approach to Inducing Short Au(III)···Au(III) Contacts in Tetracyanoaurate(III) Salts

Six sets of tetracyanoaurate(III) salts were synthesized and structurally characterized using the metal-ligand complex cations [RE(bipyO₂)₄]³⁺ (RE = Sc, Y, La; bipyO₂ = 2,2'-bipyridine-N,N'-dioxide), [Fe(bipyO₂)₃]³⁺, [Ln(dmbipyO₂)₄]³⁺ (Ln = Ce, Eu, Yb; dmbipyO₂ = 4,4'-dimethyl-2,2'-bipyridine-N,N'-dioxide), [Ca(tcmc)]²⁺ (tcmc = 1,4,7,10-tetrakis-(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane), and [Ca(12-crown-4)₂]²⁺. Noncovalent assembly of the [Au(CN)₄]^- anions tended to occur via Au···N_cyano interactions; however, rare Au(III)···Au(III) contacts between the [Au(CN)₄]^- groups - suggesting aurophilicity - could be induced when certain cation shape requirements were met. Specifically, cations with shape, size, and symmetry that allowed for packing in a complementary fashion with Au(III)···Au(III) aligned [Au(CN)₄]^- dimers or trimers - providing efficiently close-packed layers - were found to be sufficient for manifesting Au(III)···Au(III) contacts. Modifying the [RE(bipyO₂)₄]³⁺ cation with peripheral methyl groups (the [Ln(dmbipyO₂)₄]³⁺ cation) caused isoreticular replacement of a {[Au(CN)₄]₃}^3- trimer with a dumbbell-shaped {[Au(CN)₄]₂Cl}^3- tri-anion featuring an unusual Au···Cl···Au bridge - illustrating that the assembly of the anionic groups will adapt to conserve the same close packing. Au(III) aurophilicity between the [Au(CN)₄]^- groups was studied using computational methods, crystal packing of the structures was probed using Hirshfeld surface analysis, and the emission properties of compounds containing the [Eu(dmbipyO₂)₄]³⁺ luminophore were investigated, showing high quantum yields of ca. 50%.