  Here we report the synthesis, structure and detailed characterisation of
three n-membered oxovanadium rings,
Na$_n$[(V=O)$_n$Na$_n$(H$_2$O)$_n$($\alpha$, $\beta$, or
$\gamma$-CD)$_2$]$m$H$_2$O (n = 6, 7, or 8), prepared by the reactions of
(V=O)SO$_4$$\cdot$$x$H$_2$O with $\alpha$, $\beta$, or $\gamma$-cyclodextrins
(CDs) and NaOH in water. Their alternating heterometallic vanadium/sodium
cyclic core structures were sandwiched between two CD moieties such that O-Na-O
groups separated neighbouring vanadyl ions. Antiferromagnetic interactions
between the $S$ = 1/2 vanadyl ions led to $S$ = 0 ground states for the
even-membered rings, but to two quasi-degenerate $S$ = 1/2 states for the
spin-frustrated heptanuclear cluster.
