Crystallographic, Electrochemical, and Electronic Structure Studies of the Mononuclear Complexes of Au(I)/(II)/(III) with [9]aneS₂O ([9]aneS₂O = 1-oxa-4,7-dithiacyclononane)

The mononuclear macrocyclic complexes [Au^I([9]aneS₂O)₂]BF₄·MeCN 1a, [Au^II([9]aneS₂O)₂](BF₄)₂·2MeCN 2a, and [Au^III([9]aneS₂O)₂](ClO₄)₆(H₅O₂)(H₃O)₂ 3 ([9]aneS₂O = 1-oxa-4,7-dithiacyclononane) have been prepared and structurally characterized by single crystal X-ray crystallography. The oxidation of [Au([9]aneS₂O)₂]⁺ to [Au([9]aneS₂O)₂]²⁺ involves a significant reorganization of the co-ordination sphere from a distorted tetrahedral geometry in [Au([9]aneS₂O)₂]⁺ [Au−S 2.3363(12), 2.3877(12), 2.6630(11), 2.7597(13) Å] to a distorted square-planar co-ordination geometry in [Au([9]aneS₂O)₂]²⁺. The O-donors in [Au([9]aneS₂O)₂]²⁺ occupy the axial positions about the Au^II center [Au···O = 2.718(2) Å] with the S-donors occupying the equatorial plane [Au−S 2.428(8) and 2.484(8) Å]. [Au([9]aneS₂O)₂]³⁺ shows a co-ordination sphere similar to that of [Au([9]aneS₂O)₂]²⁺ but with significantly shorter axial Au···O interactions [2.688(2) Å] and equatorial Au−S bond lengths [2.340(4) and 2.355(6) Å]. The cyclic voltammogram of 1 in MeCN (0.2 M NBu₄PF₆, 253 K) at a scan rate of 100 mV s⁻¹ shows an oxidation process at E_p^a = +0.74 V and a reduction process at E_p^c = +0.41 V versus Fc⁺/Fc assigned to the two-electron Au^III/I couple and a second reduction process at E_p^c = +0.19 V assigned to the Au^I/0 couple. This electrochemical assignment is confirmed by coulometric and UV−vis spectroelectrochemical measurements. Multifrequency EPR studies of the mononuclear Au^II complex [Au([9]aneS₂O)₂]²⁺ in a fluid solution at X-band and as frozen solutions at L-, S-, X-, K-, and Q-band reveal g_iso = 2.0182 and A_iso = −44 × 10⁻⁴ cm⁻¹; g_xx = 2.010, g_yy = 2.006, g_zz = 2.037; A_xx= −47 × 10⁻⁴ cm⁻¹, A_yy = −47 × 10⁻⁴ cm⁻¹, A_zz = −47 × 10⁻⁴ cm⁻¹; P_xx = −18 × 10⁻⁴ cm⁻¹, P_yy = −10 × 10⁻⁴ cm⁻¹, and P_zz = 28 × 10⁻⁴ cm⁻¹. DFT calculations predict a singly occupied molecular orbital (SOMO) with 27.2% Au 5d_xy character, consistent with the upper limit derived from the uncertainties in the ¹⁹⁷Au hyperfine parameters. Comparison with [Au([9]aneS₃)₂]²⁺ reveals that the nuclear quadrupole parameters, P_ii (i = x, y, z) are very sensitive to the nature of the Au^II co-ordination sphere in these macrocyclic complexes. The observed geometries and bond lengths for the cations [Au([9]aneS₂O)₂]^+/2+/3+ reflect the preferred stereochemistries of d¹⁰, d⁹, and d⁸ metal ions, respectively, with the higher oxidation state centers being generated at higher anodic potentials compared to the related complexes [Au([9]aneS₃)₂]^+/2+/3+.