Chiral Inversion around a Seven-Coordinated Cobalt Center Induced by an Interaction between Sugars and Sulfate Anions

Abstract

Novel seven-coordinated cage-type cobalt(II) complexes containing N-glycosides from mannose-type aldoses and tris(2-aminoethyl)amine (tren), [Co((aldose)3tren)]X2·nH2O (1a·5H2O, aldose = d-mannose (d-Man), X = Cl-; 1b·5H2O, aldose = 6-deoxy-l-mannose (l-Rha), X = Cl-; 2a·4H2O, aldose = d-Man, X = Br-; 2b·H2O, aldose = l-Rha, X = Br-) and [Co((aldose)3tren)]SO4·nH2O (3a·4H2O, aldose = d-Man; 3b·3H2O, aldose = l-Rha), where (aldose)3tren is tris(2-(aldosylamino)ethyl)amine, were prepared and characterized by elemental analysis, electronic absorption and circular dichroism spectroscopies, and X-ray crystallography. Crystal data are as follows. 2b·2CH3OH: C26H56N4O14Br2Co, monoclinic, space group P2 1, a = 11.045(2) Å, b = 17.283(6) Å, c = 10.996(3) Å, β = 117.371(6)°, V = 1864(1) Å3, Z = 2, R = 0.072 for 2787 independent reflections. 3b·3H2O·CH3OH: C25H58N4O20SCo, orthorhombic, space group P212121, a = 14.836(2) Å, b = 22.489(2) Å, c = 12.181(3) Å, V = 4064(1) Å3, Z = 4, R = 0.077 for 2010 independent reflections. The complex cation of 2b consists of a cobalt atom coordinated by a heptadentate (l-Rha)3tren ligand to produce a mono-face-capped octahedron having pseudo-C 3 symmetry with a Λ (λ3-ob form) configuration around the metal. The complex cation of 3b has a mono-face-capped octahedron coordinated by a heptadentate (l-Rha)3tren ligand having pseudo-C 3 symmetry with a Δ (λ3-lel form) configuration. The other facial site of the complex cation is capped by the SO4 2- anion through hydrogen bonding with the hydroxy groups of l-rhamnose residues. The C 3 helical-configurational inversion around the cobalt(II) center, Δ ↔ Λ, induced by an interaction between the sugars and the SO4 2- anion, was monitored by circular dichroism spectroscopy, for which the sign of the Cotton effect of 1 and 2, having halide counteranions, is opposite to that of 3, having sulfate counteranions, and was found to proceed reversibly upon addition and removal of sulfate anions. The intensities of circular dichroism spectra for [Co((l-Rha)3tren)]2+ and [Co((d-Man)3tren)]2+ were appreciably changed upon addition of sulfate ions. The spectral changes were interpreted in terms of ion-pair formation. On the basis of electrostatic theories of ion association, the closest distance between the sulfate ion and the [Co((d-Man or l-Rha)3tren)]2+ cation was estimated as ∼5 Å, which is consistent with the Co−S distance of 4.697(6) Å observed in the crystal structure of 3b. The sulfate ion was suggested to fit into the cavity composed of the sugar hydroxyl groups through hydrogen bonds even in the solution state.

Stimuli‐Responsive Chiroptical Switching

“… (a) Helicity inversion of a Co(II) complex induced by nitrate anion; [82] (Chemdraw colour was changed from the original figure). (b) Switching of helical sense of an AIB foldamer upon formation of different boronate ester; [83] (Chemdraw colour was changed from the original figure) (c) Sulfate induced reversible helicity switching of a labile Co(II) complex; [85] Adapted with permission from Ref. [85], Copyright 1997, American Chemical Society.…”

Transition metal-based chiroptical switches for nanoscale electronics and sensors

“…Yano et al reported chiral inversion [47] induced around a seven-coordinated cobalt center by interaction between sugars and sulfate anions. New cage-type cobalt(II) complexes that consist of N-glycosides from mannose-type aldoses and tris(2-aminoethyl)amine (tren), [Co((aldose) 3 tren)]X 2 ·nH 2 O (where X = Cl − , Br − ) and [Co((aldose) 3 tren)]SO 4 ·nH 2 O exhibited C 3 helical configuration inversion around the Co(II) center.…”

“…The sulfate anion situates itself into the complex cation cavity along the C3 axis. Rha = 6-deoxy-l-mannose[47]. Copyright 1997 American Chemical Society.…”

Nickel(II) complexes with Schiff-base ligands derived from epimeric pyranose backbones as 2,3-chelators: modeling the coordination chemistry of chitosan

“…To the best of our knowledge complex [Ni(Hsal HMan)(tptz)]ClO 4 is the first transition metal complex exhibiting a non-chair conformation of a mannopyranose ring. In transition metal complexes derived from mannose or the related L L-rhamnose, containing more flexible co-ligand moieties like (aminoalkyl)amines [48][49][50][51][52][53] instead of the rigid tptz molecule or obtained from less donor-functionalized sugar moieties, 54-57 the pyranose ring generally adopts the expected chair conformation.…”