Rock ‘n’ Roll With Gold: Synthesis, Structure, and Dynamics of a (bipyridine)AuCl3 Complex

Our previously reported microwave synthesis of (N–N)­AuCl2+ complexes (where N–N = 2,2′-bipyridine (bpy) and sterically unencumbered bpy derivatives) was used to prepare derivatives where the bpy moiety was substituted in the 6,6′-positions. Instead of the square-planar complexes, these reactions produced neutral (N–N)­AuCl3 complexes. In these, the tethered N–N ligand is bonded such that one N occupies a regular position in the square coordination plane of the Au­(III) center and the other N occupies a pseudoaxial position, interacting with Au through an elongated Au–N bond, as determined by X-ray crystallography of two complexes. Variable-temperature 1H NMR spectroscopy reveals that the two sites of the N–N ligand undergo exchange on the NMR time scale. For N–N = 6,6′-Me2bpy the activation parameters were determined to be ΔH⧧ = 8.5 ± 0.4 kcal mol–1 and ΔS⧧ = 0.7 ± 2.0 cal K–1 mol–1. The dynamic behavior of (6,6′-Me2bpy)­AuCl3 was investigated by a DFT computational study, which detailed the in-plane rocking motion seen by NMR as well as decoordination of the axially bonded N with concomitant rolling of half of the bpy moiety by rotation around the central C–C bond of the bidentate ligand.