Hydrogen-bonding interactions, geometrical selectivity and spectroscopic properties of cobalt(III) complexes with unsymmetrical tridentate amine-amidato-phenolato type ligands

Four cobalt(III) complexes with the formula of [Co(Ln)2]− bearing tridentate amine-amidato-phenolato-type ligands (Ln: n = 1–4) were synthesized. All of the complexes were characterized by 1H NMR spectroscopy and X-ray analysis. The geometrical selectivity was found to depend on the flexibility of the amine-amidato chelate in combination with the planar 2-oxybenzamido 6-membered chelate; that is, the amine-amidato 5-membered chelate took the mer-type geometry, and the 6-membered chelate took the fac-type geometry. In most of the mer-type complexes, intermolecular double hydrogen bonds via amidato(O) and amino group were selectively formed between their enantiomeric pairs of mononuclear complexes. In the case of two chiral ligands {L22– = 2-amino-1-(2-oxybenzamido)propane; L32− = trans-1-amino-2-(2-oxybenzamido)cyclohexane}, [Co(L3)2]− showed diastereoselectivity while [Co(L2)2]− did not. Furthermore, PPh4[Co(L1)2] (L12− = 2-amino-1-(2-oxybenzamido)-2-methylpropane) showed an apparent solvatochromic behavior in several solvents. Although the molecular structures of [Co(L2 or L3)2]− are quite similar to that of [Co(L1)2]−, these complexes did not exhibit such a solvatochromic behavior.

Four cobalt(III) complexes with the formula of [Co(Ln)2]− bearing tridentate amine-amidato-phenolato-type ligands (Ln: n = 1–4) were synthesized. One of the complexes having a trans-cyclohexylene backbone, mer-[Co(L3)2]− {L32– = trans-1-amino-2-(2-oxybenzamido)cyclohexane}, afforded violet crystals consisting of a hydrogen-bonded dimer of [Co(RR-L3)(SS-L3)]−; however, in solution the complex slowly isomerized to the corresponding C2-symmetrical species ([Co(RR-L3)2]− and [Co(SS-L3)2]−). The preferential formation of the heterochiral isomer in the crystals would result from the intermolecular double hydrogen bonds between the amino and amidato groups.Figure optionsDownload as PowerPoint slideHighlights
► Four cobalt(III) complexes bearing tridentate amine-amidato-phenolato-type ligands were synthesized.
► The geometrical selectivity of the complexes was examined.
► The effect of the hydrogen-bonding interactions on diastereoselectivity was studied.
► An apparent solvatochromic behavior was observed for one of the complexes.