Synthesis, structure, circular dichroism of a Δ(−)546-di-μ-hydroxo-tetrakis(S-prolinato)dicobalt(iii) complex and NMR study of its interaction with chiral and non-chiral probes in solutions†
Abstract
Slow oxidation of cobalt(II) ions by aerial oxygen in the presence of S-prolinate led to isolation of a new μ-OH bridged binuclear chiral complex of cobalt(III): Co2(μ-OH)2(S-Pro)4. The X-ray structure of this compound was studied. CoIII ions are located in N2O4 donor sets containing N and O atoms of S-prolinates at trans(N), cis(O)- or cis(N), trans(O)-positions. Optical properties of Co2(μ-OH)2(S-Pro)4 were investigated by means of electronic and circular dichroism spectroscopy providing data for comparison with the corresponding mononuclear complex of S-prolinate, cis-(−)-Co(S-Pro)3, as well as with other previously reported μ-OH bridged cobalt(III) complexes. CD spectra also confirmed that no racemization of S-prolinate occurred during synthesis. The solution behaviour of Co2(μ-OH)2(S-Pro)4 in the presence of various organic acids was studied by 1H and 13C NMR techniques evidencing specific interactions between carboxylic groups of the substrate and hydrogen-bonding groups of the Co2(μ-OH)2(S-Pro)4 molecule. Relying on the NMR spectroscopy results, the most probable models of binding between Co2(μ-OH)2(S-Pro)4 and S-proline as well as N-benzoylated α-amino acids were discussed.
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