title: Jahn-Teller effects on π-stacking and stereoselectivity in the phenylethaniminopyridine tris-chelates Cu(NN')3(2+).
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 [CuL(3)][ClO(4)](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(F), which contains a peripheral (19)F spin label, gave [CuL(F)(3)][ClO(4)](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.
authors.affiliation: Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK.
authors.forename: Suzanne E
authors.initials: SE
authors.lastname: Howson
authors.forename: Nikola P
authors.initials: NP
authors.lastname: Chmel
authors.forename: Guy J
authors.initials: GJ
authors.lastname: Clarkson
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Deeth
authors.forename: Daniel H
authors.initials: DH
authors.lastname: Simpson
authors.forename: Peter
authors.initials: P
authors.lastname: Scott