Sterically Demanding Multidentate Ligand Tris[(2-(6-methylpyridyl))methyl]amine Slows Exchange and Enhances Solution State Ligand Proton NMR Coupling to ¹⁹⁹Hg(II)

Abstract

The solution state coordination chemistry of Hg(ClO 4 ) 2 with tris [(2-(6-methylpyridyl))methyl] amine (TLA) was investigated in acetonitrile-d 3 by proton NMR. Although Hg(II) is a d 10 metal ion commonly associated with notoriously rapid exchange between coordination environments, as many as six ligand environments were observed to be in slow exchange on the chemical shift time scale at select metal-to-ligand ratios. One of these ligand environments was associated with extensive heteronuclear coupling between protons and 199 Hg and was assigned to the complex [Hg(TLA)] 2+ . The 5 J( 1 H 199 Hg) = 8 Hz associated with this complex is the first example of five-bond coupling in a nitrogen coordination compound of Hg(II). The spectral complexity of related studies conducted in acetone-d 6 precluded analysis of coordination equilibria. Crystallographic characterization of the T-shaped complex [Hg(TLAH)(CH 2 COCH 3 )](ClO 4 ) 2 (1) in which two pyridyl rings are pendant suggested that the acidity of acetone combined with the poor coordinating abilities of the neutral solvent adds additional complexity to solution equilibria. The complex crystallizes in the triclinic space group P1 with a = 9.352(2) Å, b = 12.956(2) Å, c = 14.199(2) Å, α = 115.458(10)°, β = 90.286 (11)°, γ = 108.445(11)°, and Z = 2. The HgN amine , Hg-N pyridyl , and Hg-C bond lengths in the complex are 2.614(4), 2.159(4), and 2.080(6) Å, respectively. Relevance to development of 199 Hg NMR as a metallobioprobe is discussed.

Synthesis and crystal structure of [C6H5Hg(H2NSiMe3)][H2N{B(C6F5)3}2], a phenyl–mercury(II) cation stabilised by a non-coordinating counter-anion

“…The geometry around the nitrogen atom N(2) in the [H 2 N{B(C 6 F 5 ) 3 } 2 ] À counter-anion in 2 AE CH 2 Cl 2 resembles closely that reported for [Na( (14)°and N(2)-B(1) and N(2)-B(2) bond lengths of 1.647(2) and 1.638(2) Å , respectively. The anion is stabilised by five intramolecular N-HÁ Á ÁF hydrogen bonds in the range 1.988 (16) (3).…”

“…1. The Hg(1)-C(4) bond length of 2.044(2) Å is slightly shorter than that found in Ph 2 Hg [2.088(3) Å ] [12] [14], evidently as a consequence of the increased steric hindrance of the ligand in the latter case. The geometry around the mercury(II) atom in 2 AE CH 2 Cl 2 is almost linear, as indicated by the C(4)-Hg(1)-N(1) angle of 176.80(7)°.…”

Tripodal Tetraamine Ligands Containing Three Pyridine Units: The other Polypyridyl Ligands

“…The synthesis of such ligands is most conveniently carried out using variations of the above methods with the appropriately substituted precursors. For example, the mono‐, di‐ and trimethyl tpa ligands shown in Figure 6 can be prepared using 2‐chloromethyl‐6‐methylpyridine as the alkylating agent towards the appropriate amine42,43 while the synthesis of 6‐Metpa involving reductive amination of 6‐methyl‐2‐pyridinecarboxaldehyde has also been reported 39. Synthesis of the amino substituted ligands mapa, bapa and tapa (Figure 6) generally proceeds through hydrolysis of the appropriate pivaloylamido‐substituted species mppa, bppa and tppa, respectively 44–47…”

Solution behavior of Hg(II)-cystamine by Uv-Vis and199Hg NMR

“…Due to a large chemical shift range, stronger heteronuclear coupling and faster relaxation time, there is a considerable current interest in employing the 199 Hg nucleus as a metalloprobe and expanding the available data [20]. The spectrum for 0.5 M of 1 and 1Nu 7 were obtained in d 6 -DMSO employing 1 M HgCl 2 in DMSO as internal reference [13,14].…”