Mercury-199 NMR Studies of Thiacrown and Related Macrocyclic Complexes:  The Crystal Structures of [Hg(18S6)](PF₆)₂ and [Hg(9N3)₂](ClO₄)₂

We wish to report the first measurements of ¹⁹⁹Hg NMR chemical shift data for a series of homoleptic Hg(II) complexes with thiacrown ligands and related aza and mixed thia/aza macrocycles. In mercury(II) complexes containing trithiacrown through hexathiacrown ligands, we observed ¹⁹⁹Hg NMR chemical shifts in the range of −298 to −1400 ppm. Upfield chemical shifts in these NMR spectra are seen whenever (a) the number of thioether sulfur donors in the complex is decreased, (b) a thioether sulfur donor is replaced by a secondary nitrogen donor, and (c) the size of the macrocycle ring increases without a change in the nature or number of the donor atoms. Changes in noncoordinating anions, such as hexafluorophosphate and perchlorate, have little effect on the ¹⁹⁹Hg chemical shift. For several complexes, we observed ³J(¹⁹⁹Hg−¹H) coupling in the range of 50−100 Hz, the first example of proton−mercury coupling through a C−S thioether bond. Also, we obtained unusual upfield ¹³C NMR chemical shifts for methylene resonances in several of the thiacrown complexes which correspond to distortions within the five- and six-membered chelate rings bound to the mercury ion. We report the X-ray crystal structure of the complex [Hg(18S6)](PF₆)₂ (18S6 = 1,4,7,10,13,16-hexathiacyclooctadecane). The molecule crystallizes in the rare trigonal space group P3̄m1 with hexakis(thioether) coordination around the Hg(II) center confirming previous X-ray photoemission spectroscopic data on the compound. The lack of an observable ¹⁹⁹Hg NMR signal for the complex is the result of the identical length (2.689(2) Å) of all six Hg−S bonds. We additionally report the X-ray structure of the complex [Hg(9N3)₂](ClO₄)₂ (9N3 = 1,4,7-triazacyclononane) which shows hexakis(amine) coordination of the 9N3 to form a distorted trigonal prismatic structure. Solution dissociation of the one of the 9N3 ligands from the mercury ion is confirmed by multinuclear NMR experiments. For six-coordinate macrocyclic Hg(II) complexes, N6 donor sets have a preference for trigonal prisms while S6 donor sets favor octahedral structures.