Solution and solid-state NMR studies of MeHgII and RSHgII (R = Me, Et) complexes with pyrazolyl-containing ligands

The solid state and solution NMR spectra of the tetrakis(pyrazol-1-yl) borate (L(a)) and tris(3,5-dimethylpyrazol-1-yl) hydroborate (L(b)) ligands and their complexes with MeHg(II) and RSHg(II) (R = Me, Et) are reported. The solid state C-13 and N-15 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 Hg-199 resonance is too large in these T-shaped complexes and does not allow the observation of the Hg-199 resonance under cross polarisation-magic angle spinning (CPMAS) experimental mode. On the other hand Hg-199 CPMAS NMR spectra were obtained for the complexes with the L(b) ligand. The reduced Hg-199 CSA value together with the information gained from the C-13 and N-15 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 HgSCH3CH3 moieties) in the low temperature NMR limiting spectra.