Investigation of the mercury(ii) coordination chemistry of tris[(1-methylimidazol-2-yl)methyl]amine by X-ray crystallography and NMR

Abstract

The coordination chemistry of Hg(II) with tris[(1-methylimidazol-2-yl)methyl]amine (TMIMA) was investigated. The structures of [Hg(TMIMA)₂](ClO₄)₂ (1), [Hg(TMIMA)(NCCH₃)](ClO₄)₂ (2) and [Hg(TMIMA)Cl]₂(HgCl₄) (3) were characterized by X-ray crystallography. Complex 1 has six strong Hg–N_imidazoyl bonds ranging from 2.257(5) to 2.631(6) Å. Ligand geometry suggests the Hg–N_(NR3) distances of 2.959(6) Å in 1 reflects weak bonding interactions. This complex has a ¹⁹⁹Hg chemical shift of −1496 ppm, significantly upfield from nitrogen coordination complexes with lower coordination numbers. The five-coordinate complex 2 has Hg–N_(NR3), Hg–N_imidazoyl and Hg–N_acetonitrile bond lengths of 2.642(8), 2.198(5) and 2.264(11) Å, respectively. Complex 3 is also five coordinate, with Hg–N_(NR3), Hg–Cl and average Hg–N_imidazoyl distances in the cations of 2.758(7), 2.424(2) and 2.29(4) Å, respectively. Conditions for slow exchange on the J(HgH) coupling time-scale were found for both 1 ∶ 1 metal-to-ligand complexes in acetonitrile-d₃. Observed heteronuclear coupling constants were similar to those associated with Hg(II) substituted proteins with histidine–metal bonds. Solution and solid-state comparisons to the Hg(II) coordination chemistry of tetradenate pyridyl ligands are made. Relevance to development of ¹⁹⁹Hg NMR as a metallobioprobe is discussed.