Structural diversity of complexes between Cu(ii) or Ni(ii) and endocyclic oxygen- or nitrogen-containing ligands: synthesis, X-ray structure determinations and circular dichroism spectra

Abstract

Complexes between Cu(II) or Ni(II) and O- or N-containing ligands, pyridine-2,6-dicarboxylic acid (H₂L¹), 4-pyranone-2,6-dicarboxylic acid (H₂L²), 4H-pyran-2,6-dicarboxylic acid (H₂L³), (S)-tetrahydro-2-furoic acid (S-HL⁴), 2,2′-bipyridine (L⁵), 1,10-phenanthroline (L⁶) or bis(2-pyridylmethyl)amine (L⁷), have been synthesised and characterised by X-ray crystallography. In the ionic complex [Cu(H₂L¹)(L⁵)(OH₂)]·[Cu(L¹)₂]·3H₂O (1), one L¹ group and L⁵ are close to parallel, which is not the case for the dimeric complex [Cu₂(HL¹)₂(L¹)(L⁶)]·5.5H₂O (2), in which the HL¹ groups on each Cu(II) centre are bent inwards at an angle of 11°. The complex {[Cu(HL³)(L³)_0.5(OH₂)]·1.75H₂O}_n (3) shows a paddle-wheel arrangement. In {[Cu(S-L⁴)(L⁷)]ClO₄}_n (S-4), coordination occurs via the endocyclic O atom and the pendant carboxylic acid group and supports binding modes posited previously for complexes formed in solution between Cu(II) and N-acetylneuraminic acid (sialic acid). The CD spectrum of S-4 (670 nm, −4.8 mdeg; 550 nm, +1.4 mdeg) is equal and opposite to its enantiomer {[Cu(R-L⁴)(L⁷)]ClO₄}_n (R-4). Complexes between Cu(II) or Ni(II) and H₂L² show a 1-dimensional coordination polymer in [Cu(L²)(OH₂)₄]_n (5) and a discrete complex in [Ni(HL²)₂(OH₂)₄] (6). Complexes 1–6 exhibited a range of coordination geometries, nuclearities, chiralities and polymeric architectures, which suggest that the interactions between Cu(II) or Ni(II) and endocyclic O- or N-containing bioligands in chemical biology settings will be varied.