Enantiopure mono- and mixed-valence multinuclear cobalt complexes from amino alcohol based ligands

Abstract

Chiral amino alcohol based tridentate Schiff bases, derived from 5-OMe-, 5-H-, and 5-NO₂-substituted salicylaldehydes and (S)-(+)-2-phenylglycinol and (S)-(-)-2-amino-3-phenyl-1-propanol, were found to act as versatile ligands in their coordination behavior towards cobalt and gave a range of complexes like mononuclear low-spin Co^III complexes, mixed-valence trinuclear Co^III-Co^II-Co^III complexes, and a mixed-valence tetranuclear (Co^III)₃Co^II complex on 1:1 molar ratio reaction with Co(CH₃COO)₂·4H₂O in methanol at room temperature. The steric and electronic properties of these ligands are found to control the overall geometry and nuclearity of the resulting complexes. 5-OMe-Substituted ligands (H₂L¹ and H₂L²) gave mononuclear low-spin CoIII complexes [CoL¹(HL¹)]·0.25DMF (1) and [CoL²(HL²)]·0.25DMF (2). Single-crystal X-ray structure analysis of complex 2 revealed that the relevant complex crystallizes with four mononuclear units in the asymmetric unit (Z' = 4), and these units self-assemble through O-H···O hydrogen-bonding interactions resulting in the formation of homochiral supramolecular helices in the crystal lattice. The 5-H- and 5-NO₂-substituted ligands (H₂L³, H₂L⁴, H₂L⁵) afforded mixed-valence trinuclear Co^III-Co^II-Co^III complexes [Co(CoL³₂)₂·H₂O]·2DMF·2H₂O (3), [Co(CoL⁴₂)₂·H₂O]·2DMF·H₂O (4), and [Co(CoL⁵₂)₂·DMF]·2DMF·H₂O (5), respectively. Crystal structure analyses of complexes 3 and 5 revealed rare trinuclear geometries of these complexes, in which two terminal octahedral low-spin CoIII complexes act as chelating ligands for a central high-spin CoII center through alkoxide bridging. For steric reasons, the central cobalt adopts a highly distorted geometry in these complexes viz. distorted trigonal bipyramidal in complex 3 and distorted square planar in complex 5. The ligand H2L6 gave an interesting tetranuclear mixed-valence complex, H[Co(CoL⁶₂)₃]·H₂O (6), in which three octahedral low-spin CoIII complexes act as ligands for a central high-spin CoII center in distorted trigonal prismatic geometry. Interestingly, the three types (mononuclear: 1, 2; trinuclear: 3, 4, 5; tetranuclear: 6) of complexes showed large variations in their nuclearity and overall geometry despite the fact that the experimental conditions for all these complexes were identical. This fact highlights the influence of different substitutions on ligands in determining the final geometry of the resulting complexes in these reactions. Complexes 1-6 were characterized by elemental and routine spectral analyses. Compounds 2, 3, 5, and 6 have been unequivocally characterized by single-crystal X-ray structure determinations as well. Chiral properties of these complexes were confirmed by circular dichroism (CD) spectral studies.