Effects of structures of HgX2 complexes (X = CF3SO3 and Cl) with chiral bidentate ligands on circular dichroism spectra

Abstract

The effects of the structures of HgX₂ complexes (X = CF₃SO₃ and Cl) with chiral bidentate ligands [(R)-(−)-1-(1-naphthyl)-N,N-bis(pyridin-4-ylmethyl)ethanamine, (R)-(+)-1-phenyl-N,N-bis(pyridin-4-ylmethyl)ethanamine, (R)-(−)-1-(1-naphthyl)-N,N-bis(pyridin-3-ylmethyl)ethanamine, and (R)-(+)-1-phenyl-N,N-bis(pyridin-4-ylmethyl)ethanamine] on circular dichroism (CD) spectra are reported. The CD spectra of free (R)-(−)-1a and (R)-(+)-1b, which have 4-pyridyl groups, are similar to those of (R)-(−)-2a and (R)-(+)-2b, which have 3-pyridyl groups. When equimolar amounts of Hg(CF₃SO₃)₂ are added to these ligands, a decrease in the CD intensity of (R)-(−)-2a is observed, and significant spectral changes (intensity and shape) are observed in (R)-(−)-1a, (R)-(+)-1b, and (R)-(+)-2b. X-ray crystal structures suggest that the (R)-(−)-2a/Hg(CF₃SO₃)₂ complex is a coordination polymer with no bridging CF₃SO₃ anions, whereas the (R)-(−)-1a/Hg(CF₃SO₃)₂ and (R)-(+)-1b/Hg(CF₃SO₃)₂ complexes are coordination polymers with CF₃SO₃ anion bridges, and the (R)-(+)-2b/Hg(CF₃SO₃)₂ complex has a tricyclic structure bridged by CF₃SO₃ anions. Two different conformers, which have clockwise and counter-clockwise screw senses between the naphthalene axis and the pyridine axis, are observed in the ligand components of the (R)-(−)-2a/Hg(CF₃SO₃)₂ complex. In contrast, the X-ray structures of the (R)-(−)-2a/HgCl₂ and (R)-(+)-2b/HgCl₂ complexes are one-dimensional coordination polymers without bridging Cl anions. The significant CD spectral changes in the HgX₂ complexes with different chiral bidentate ligands require formation of rigid structures and a single-directional screw sense between chromophores.