Jahn–Teller effects on π-stacking and stereoselectivity in the phenylethaniminopyridine tris-chelates Cu(NN′)32+

Abstract

Optically pure phenylethaniminopyridine (S_C-L) tris-chelates of Fe(II) and other first row transition metal systems have previously been shown to give exclusively the fac structures in the solid state. Here it is shown by powder X-ray diffraction that the complex [CuL₃][ClO₄]₂ crystallises exclusively as the mer isomer, although – for a given absolute configuration of the ligand – of the same helicity (Δ/Λ) as that displayed by the other metal complexes. The similar ligand R_C-L^FF, which contains a peripheral ¹⁹F spin label, gave [CuL^FF₃][ClO₄]₂ which also adopts exclusively the mer structure in the crystal, but is shown by NMR spectroscopy to have a fac : mer ratio of 1 : 6 in solution at low temperature. Molecular mechanics calculations for a number of isomers and conformers are consistent with the presence of such a mixture of isomers in solution for both complexes. The origin of the difference in behaviour between Fe(II) and Cu(II) is the presence of a Jahn–Teller distortion (and the generally longer M–N bonds) in the Cu(II) complexes. This disturbs intra-ligand π-stacking, leading to the poor fac/mer stereoselectivity while leaving enantioselectivity Δ/Λ apparently unaffected.