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 [AuI([9]aneS2O)2]BF4·MeCN 1a, [AuII([9]aneS2O)2](BF4)2·2MeCN 2a, and [AuIII([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 AuII 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 Epa = +0.74 V and a reduction process at Epc = +0.41 V versus Fc+/Fc assigned to the two-electron AuIII/I couple and a second reduction process at Epc = +0.19 V assigned to the AuI/0 couple. This electrochemical assignment is confirmed by coulometric and UV−vis spectroelectrochemical measurements. Multifrequency EPR studies of the mononuclear AuII 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 giso = 2.0182 and Aiso = −44 × 10−4 cm−1; gxx = 2.010, gyy = 2.006, gzz = 2.037; Axx= −47 × 10−4 cm−1, Ayy = −47 × 10−4 cm−1, Azz = −47 × 10−4 cm−1; Pxx = −18 × 10−4 cm−1, Pyy = −10 × 10−4 cm−1, and Pzz = 28 × 10−4 cm−1. DFT calculations predict a singly occupied molecular orbital (SOMO) with 27.2% Au 5dxy 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, Pii (i = x, y, z) are very sensitive to the nature of the AuII 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+.