Solid-State ¹⁹⁹Hg and ¹¹³Cd NMR Studies of Mercury- and Cadmium-Thiolate Complexes. Spectroscopic Models for [Hg(SCys)_n] Centers in the Bacterial Mercury Resistance Proteins

¹⁹⁹Hg CP/MAS-NMR spectra have been measured for representative examples of the major coordination geometries available to monomeric mercury thiolate complexes: [Hg(SR)₂], [Hg(SR)₃]⁻, and [Hg(SR)₄]²⁻. Solid-state ¹¹³Cd NMR spectra have been measured for related [Cd(SR)₃]¹⁻ compounds. [(n-Bu)₄N] [Cd(S-2,4,6-i-Pr₃C₆H₂)₃] and [Ph₄P][Hg(S-2,3,5,6-Me₄C₆H)₃] were structurally characterized by X-ray crystallography. The metal ions in both compounds are three-coordinate with distorted Y-shaped, planar [MS₃] units. These structures are discussed in relationship to a second conformational isomer, the symmetric C_3h [M(SR)₃]¹⁻ anion. The shielding tensor, isotropic chemical shift (σ_iso), the chemical shielding anistropy (Δσ), and the asymmetry parameter (η) vary in a systematic manner as a function of coordination number and coordination geometry. ¹⁹⁹Hg and ¹¹³Cd CP/MAS-NMR data for selected compounds (σ_iso, Δσ, η) are as follows: [Hg(S-2,4,6-i-Pr₃C₆H₂)₂] (−1015, 4256, 0.16); [Ph₄P][Hg(S-2,4,6-i-Pr₃C₆H₂)₃] (−267, 1408, 0.30), [(n-Bu)₄N][Hg(SPh)₃] (−341, 1273, 0.97), [Et₄N]₂[Hg(S-2-PhC₆H₄)₄] (−433, 86.5,0.86), and [Ph₄P] [Cd(S-2,4,6-i-Pr₃C₆H₂)₃] (668, 483, 0.24). The implications of this study to solution and solid-state ¹¹⁹Hg and ¹¹³Cd NMR studies of Hg- and Cd-cysteine proteins are discussed.