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.
