Redox Non-innocence of Thioether Macrocycles: Elucidation of the Electronic Structures of Mononuclear Complexes of Gold(II) and Silver(II)

The mononuclear +2 oxidation state metal complexes [Au([9]aneS3)2]2+ and [Ag([18]aneS6)]2+
have been synthesized and characterized crystallographically. The crystal structure of the Au(II) species
[Au([9]aneS3)2](BF4)2 shows a Jahn−Teller tetragonally distorted geometry with Au−S(1) = 2.839(5), Au−S(2) = 2.462(5), and Au−S(3) = 2.452(5) Å. The related Ag(II) complex [Ag([18]aneS6)](ClO4)2 has been
structurally characterized at both 150 and 30 K and is the first structurally characterized complex of Ag(II)
with homoleptic thioether S-coordination. The single-crystal X-ray structure of [Ag([18]aneS6)](ClO4)2 confirms
octahedral homoleptic S6-thioether coordination. At 150 K, the structure contains two independent Ag(II)−S
distances of 2.569(7) and 2.720(6) Å. At 30 K, the structure retains two independent Ag(II)−S distances of
2.615(6) and 2.620(6) Å, with the complex cation retaining 3-fold symmetry. The electronic structures of
[Au([9]aneS3)2]2+ and [Ag([18]aneS6)]2+ have been probed in depth using multifrequency EPR spectroscopy
coupled with DFT calculations. For [Au([9]aneS3)2]2+, the spectra are complex due to large quadrupole
coupling to 197Au. Simulation of the multifrequency spectra gives the principal g values, hyperfine (A) and
quadrupole (P) couplings, and furthermore reveals non-co-incidence of the principal axes of the P tensor
with respect to the A and g matrices. These results are rationalized in terms of the electronic and geometric
structure and reveal that the SOMO has ca. 30% Au 5dxy character, consistent with DFT calculations
(27% Au character). For [Ag([18]aneS6)]2+, detailed EPR spectroscopic analysis confirms that the SOMO
has ca. 26% Ag 4dxy character and DFT calculations are consistent with this result (22% Ag character).