Carbohydrates as ligands: coordination equilibria and structure of the metal complexes

The stereochemistry of chiral-at-metal complexes is much more abundant, albeit complicated, than chiral-at-carbon compounds, but how to make use of stereolabile metal-centers remains a formidable challenge due to the highly versatile coordination geometry of metal ions and racemization/epimerization problem. We demonstrate herein a stepwise assembly of configurationally stable [Pd6(FeL3)8]28+ (Δ/Λ-MOCs-42) homochiral octahedral cages from unstable D3-symmetry tris-chelate-Fe type metalloligands via strong face-directed stereochemical coupling and facile chiral-induced resolution processes based on stereodifferentiating host–guest dynamics. Kinetic studies reveal that the dissociation rate of MOC-42 cages is 100-fold slower than that of Fe-metalloligands and the racemization is effectively inhibited, making the cages retain their chirality over extended periods of time (>5 months) at room temperature. Recyclable enantioseparation of atropisomeric compounds has been successfully achieved, giving up to 88% ee.

Design and Enantioresolution of Homochiral Fe(II)-Pd(II) Coordination Cages from Stereolabile Metalloligands: Stereochemical Stability and Enantioselective Separation.

Stereoselectivity in electron transfer reactions involving metal ion complexes

The iron-inulin complex, which could be applied as a parenteral hematinic agent, is studied by physical techniques. Electron microscopy shows that the complex contains very small particles that are approximately spherical and 10–30 nm in diameter. The examination reveals the presence of an electron-transparent sheath, probably consisting of linked inulin oligomers with M
 w ∼ 3000 g mol−1, around the electron-dense, ironcontaining core of the FeOOH particles. FTIR spectroscopy suggests the existence of stronger hydrogen bonds, which are formed in the complexes between FeOOH and oligosaccharides, compared to the hydrogen bond in the initial compounds. A gel filtration technique on a Sephadex column showed conclusively that a large proportion of combined inulin oligomers with M
 w ∼ 1500 g mol−1 as secondary fractions are present in complexes. These low-molecular-weight inulin fractions have a significant influence on the complex stability.

Physical investigation of the colloidal iron-inulin complex

6.1 – Cobalt

The stereoselectivities in the electron-transfer reactions of [Co(en)3]2+ with eight Δ anionic Co(III) complexes in aqueous solution have been determined by observing the enantiomeric excesses of the [Co(en)3]3+ product. The results are compared with the ion-pairing stereoselectivity between [Co(en)3]3+ and the anionic complexes. Even the Δ anionic complexes which form favorable pairs with Δ-[Co(en)3]3+ yielded the excesses of Λ-[Co(en)3]3+ product.

Stereoselectivity in electron-transfer reactions of tris(ethylenediamine)-cobalt(II) with several anionic cobalt(III) complexes

Shape selectivity in outer-sphere electron transfer reactions

The removal of various mono-, di-, oligo-, and poly-saccharides from aqueous solutions by adsorptive binding to precipitated iron(III) hydroxide was very selective in respect of their stereo-chemistry and chain length.

Tsuyoshi Mitani

Papers

Stereoselectivity in electron-transfer reactions of tris(ethylenediamine)-cobalt(II) with several anionic cobalt(III) complexes

Shape selectivity in outer-sphere electron transfer reactions

Selective Removal of Sugars from Aqueous Solutions by Adsorption on Precipitated Iron(III) Hydroxide

Conformational effects on stereoselectivities in electron-transfer and ion-association reactions between metal complexes