Simple Coordination Complexes of Cucurbit[n]urils with Metal Ions - Supramolecular Assemblies of Cucurbit[n]urils with Metal Ions - page 17

obtained by introducing KCl into a Q[5]-ZnCl 2 system (Fig. 2.10 c) [ 14 ]. The
Zn 2 + cation coordinates to the two carbonyl oxygens of a Q[5] portal because of
its shorter ionic radius.
When a second larger metal ion is introduced to the singularly capped mononu-
clear Ln 3 + -Q[5] system, such as K + , then in particular cases fully doubly capped
heterometallic molecular capsules form. Thuéry [ 27 ] first reported a series of het-
erometallic K + /Ce 3 + , K + /Sm 3 + (2 complexes) and K + /Gd 3 + Q[5] capsules of this
type which were obtained under hydrothermal conditions in the presence of per-
rhenic acid. Five complexes were isolated with all but the fifth, a Yb 3 + complex (see
below), containing K + . Each capsule included a bound nitrate ion in its cavity (the
cavity of Q[5] is too small to include an [ReO 4 ] − anion, contrasting with Q[6] which
is able to include this anion—see discussion in Sect. 2.2 ). In the first four complexes
the lanthanide ion is nine-coordinate with a capped square antiprismatic geometry.
The [ReO 4 ] − anions are present as hydrogen bonded counter ions, although in the
case of the Sm 3 + complex this anion is also present as a monodentate ligand. The
Ce 3 + /K + complex forms linear columns along the c -crystallographic axis in which
the cerium and potassium metal centers from adjacent capsules are bridged by two
water molecules and a nitrate ion. The Sm 3 + /K + and Gd 3 + /K + complexes are effec-
tively isomorphous and differ from the Ce 3 + /K + complex in that zigzag columns are
present (involving one carbonyl and one water bridge between hetero-metal centers),
with the K + and Ln 3 + ions also bound to one and two terminal water ligands, respec-
tively. A corresponding K + /Nd 3 + capsule was subsequently reported by our group
[ 29 ] and this product was also found to include a nitrate ion.
The general arrangement of the metal centers in the heterometallic Ln 3 + -
K + -Q[5] (Ln = Ce, Nd, Sm, Gd) capsule motifs just discussed is represented
schematically in Fig. 2.11 a [ 27 , 29 ], with, for comparison, that for a related Gd 3 + /
K + complex of dimethyl-substituted Q[5}, Me 2 Q[5], given in Fig. 2.11 b [ 30 ].
Comparison of the two portals in each of these doubly capped species reveals
that the portals capped by a K + ion are larger than those capped by an Ln 3 + ion.
Reflecting this, there is a difference of ~0.3 Å in the average separation between
adjacent carbonyl oxygens rimming the pair of portals in the respective capsules
(Fig. 2.11 d, e). Our group has demonstrated that reaction of PrCl 3 with Q[5] in the
presence of CaCl 2 (Ca 2 + has a comparable ionic radius to the lanthanides) leads to
a heterometallic (Ca 2 + /Pr 3 + ) dinuclear Ca 2 + -bridged complex with the structure
shown in Fig. 2.11 c [ 29 ]. In this complex two Pr 3 + ions fully cap the respective
outer portals of the dimer while two Ca 2 + ions link and partially cap each of the
“inner” portals. As might be anticipated, in this arrangement the difference in the
respective (“outer” vs. “inner”) portal sizes (as expressed by the adjacent carbonyl
oxygen internuclear distances—see Fig. 2.11 f) show a similar trend to that for
the homo-dinuclear Ln 3 + -Q[5] complexes (see Fig. 2.8 a) containing both a fully
capped and a partially capped portal [ 18 , 26 ].
In summary, the smaller portal size of Q[5]s offers related concentrated bond-
ing sites so that a metal ion can coordinate with more portal carbonyl oxygens and
form a stable Q[5]/metal complex. Therefore, metal ions suitable for coordination
to Q[5] molecules have a wider distribution.