Howson, Suzanne E., Chmel, Nikola Paul, Clarkson, Guy J., Deeth, Robert J., Simpson, D. H. (Daniel H.) and Scott, Peter, 1965 Dec. 10-. (2012) Jahn–Teller effects on π-stacking and stereoselectivity in the phenylethaniminopyridine tris-chelates Cu(NN′)32+. Dalton Transactions, Vol.41 (No.15). pp. 4477-4483. ISSN 1477-9226

Abstract

Optically pure phenylethaniminopyridine (SC-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 RC-LF, which contains a peripheral 19F spin label, gave [CuLF3][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.

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