Solution and solid-state NMR studies of MeHg^II and RSHg^II (R = Me, Et) complexes with pyrazolyl-containing ligands

The solid state and solution NMR spectra of the tetrakis(pyrazol-l-yl) borate (L^a) and tris(3,5-dimethylpyrazol-l-yl) hydroborate (L^b) ligands and their complexes with MeHg^II and RSHg^II (R = Me, Et) are reported. The solid state ¹³C and ¹⁵N solid state NMR spectra of the MeHgL^a complex are consistent with the reported X-ray structure. The close similarity of the spectral data shown by MeHgL^a and MeSHgL^a suggests that the two species have the same structure. In order to confirm this hypothesis, the structure of MeSHgL^a has been determined by single crystal X-ray diffraction. The chemical shift anisotropy (CSA) of ¹⁹⁹Hg resonance is too large in these T-shaped complexes and does not allow the observation of the ¹⁹⁹Hg resonance under cross polarisation-magic angle spinning (CPMAS) experimental mode. On the other hand ¹⁹⁹Hg CPMAS NMR spectra were obtained for the complexes with the L^b ligand. The reduced ¹⁹⁹Hg CSA value together with the information gained from the ¹³C and ¹⁵N CPMAS NMR spectra suggest a tetra coordination around mercury in these complexes. The tight co-ordination mode shown by complexes with the L^b ligand is reflected in a decrease in the thiol intermolecular exchange rate in solution. This decreased ligand lability allows the detection of ³J_Hg-H couplings (inside HgSCH₃ and HgSCH₂CH₃ moieties) in the low temperature NMR limiting spectra.