Host–guest
solution chemistry with a wide range of organic
hosts is an important and established research area, while the use
of inorganic hosts is a more nascent area of research. In the recent
past in a few cases, Keplerate-type molybdenum oxide-based porous,
spherical clusters, shorthand notation {Mo
132
}, have been
used as hosts for organic guests. Here, we demonstrate the synthetically
controlled encapsulation of first-row transition metals (M = Mn, Fe,
and Co) within a Keplerate cluster that was lined on the inner core
with phosphate anions, {Mo
132
PO
4
}. The resulting
M
2+
x
⊂{Mo
132
PO
4
} host–guest complexes were characterized by
31
P NMR and ENDOR spectroscopy that substantiated the encapsulation
of the first-row transition metal guest. Magnetic susceptibility measurements
showed that the encapsulation of up to 10 equiv showed little magnetic
interaction between the encapsulated metals, which indicated that
each guest atom occupied a single site. Visualization of the capsules
and differentiation of the Mo atoms of the capsule framework and the
encapsulated transition metal were possible using spherical and chromatic
double aberration-corrected electron microscopy combined with energy-filtered
TEM (EFTEM) elemental maps. In addition, use of visible light-induced
XPS for chemically resolved electrical measurements (CREM) confirmed
the successful encapsulation of M within {Mo
132
PO
4
} and furthermore showed photoinduced electron transfer from M to
Mo. In the future, such targeted electron transfer between host {Mo
132
} and a transition metal guest could be used as photoinitiated
switches using inorganic compounds and for single site photocatalytic
reactions in confined space.