Expanded-Ring N-Heterocyclic Carbenes for the Stabilization of Highly Electrophilic Gold(I) Cations

Strategies for the synthesis of highly electrophilic Au-I complexes from either hydride-or chloride-containing precursors have been investigated by employing sterically encumbered Dipp-substituted expanded-ring NHCs (Dipp = 2,6-iPr(2)C(6)H(3)). Thus, complexes of the type (NHC) AuH have been synthesised (for NHC = 6-Dipp or 7-Dipp) and shown to feature significantly more electron-rich hydrides than those based on ancillary imidazolylidene donors. This finding is consistent with the stronger s-donor character of these NHCs, and allows for protonation of the hydride ligand. Such chemistry leads to the loss of dihydrogen and to the trapping of the [(NHC)Au](+) fragment within a dinuclear gold cation containing a bridging hydride. Activation of the hydride ligand in (NHC) AuH by B(C6F5)(3), by contrast, generates a species (at low temperatures) featuring a [HB(C6F5)(3)](-) fragment with spectroscopic signatures similar to the free borate anion. Subsequent rearrangement involves B-C bond cleavage and aryl transfer to the carbophilic metal centre. Under halide abstraction conditions utilizing Na[BArf(4)] (Arf = C6H3(CF3)2-3,5), systems of the type [(NHC) AuCl] (NHC = 6-Dipp or 7-Dipp) generate dinuclear complexes [{(NHC) Au}(2)-(mu-Cl)](+) that are still electrophilic enough at gold to induce aryl abstraction from the [BAr4f](-) counterion.