Dimeric Copper and Lithium Thiolates: Comparison of Copper Thiolates with Their Lithium Congeners

The direct reactions of the large terphenyl thiols HSAr^iPr4 (Ar^iPr4= -C₆H₃-2,6-(C₆H₃-2,6-iPr₂)₂) and HSAr^iPr6 (Ar^iPr6= -C₆H₃-2,6-(C₆H₂-2,4,6-iPr₃)₂) with stoichiometric amounts of mesitylcopper(I) in THF at ca. 80 °C afforded the first well-characterized dimeric copper thiolato species {CuSAr^iPr4}₂ (1) and {CuSAr^iPr6}₂ (2) with elimination of mesitylene. The complexes 1 and 2 were characterized by NMR and electronic spectroscopy as well as by X-ray crystallography. They have dimeric Cu₂S₂ core structures in which the two copper atoms are bridged by the sulfurs from the thiolato ligands and feature short Cu--Cu distances near 2.4 Å as well as a weak copper-flanking aryl ring interaction from a terphenyl substituent. The structures of the planar Cu₂S₂ cores bear a resemblance to the Cu_A site in nitrous oxide reductase in which two cysteines also bridge two copper atoms. The related dimeric Li₂S₂ structural motif was also observed in the lithium congeners {LiSAr^iPr4}₂ (3) and {LiSAr^iPr6}₂ (4) which were synthesized directly from the thiols and n-BuLi in hexanes. However, despite the very similar effective ionic radii of the Li⁺ (0.59 Å) and Cu⁺ (0.60 Å) ions, the Li--Li structures display very much longer (by more than ca. 0.5 Å) separations than the corresponding Cu--Cu distances in 1 and 2, which may be due to weaker dispersion interactions.