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atoms of a phosphonate/sulfonate linker will bridge two metal ions maximum. The
large bridging numbers are unattainable with other commonly employed classes
of ligands such as carboxylates [
69
]. Noticeably, whereas the ligating directional-
ity of a carboxylate group is confined to a plane, the spherical ligating shape of
the electron density that encompasses a phosphonate/sulfonate group allows metal
coordination to an additional dimension, which serves to further increase the con-
nectivity of the network and favor the formation of a robust structure. This coordi-
native flexibility in terms of bridging modes, combined with the roughly spherical
shape of the PO
3
/SO
3
unit, has led us to draw the analogy between a phosphonate/
sulfonate group and a “Ball of Velcro.” It should be noted that the chelation capac-
ity of the organic linkages to metal ions usually follows the sequence of phospho-
nates > carboxylates > sulfonates [
39
,
64
]. Therefore, the predisposition of simple
metal phosphonates to a dense layered motif makes forming high surface area
materials a challenge [
70
]. Numerous methodologies have thus been developed to
incorporate considerable mesoporosity in the non-siliceous hybrid framework.
2.3 General Strategies of Incorporating Organic Groups
2.3.1 Surface Functionalization
Surface modification can have a significant influence on the materials behavior at
the nanoscale and can lead to nano-/mesostructures with novel properties. Post-
synthetic functionalization or grafting refers to the subsequent modification of the
inner surfaces of mesostructured inorganic phases with organic groups (Fig.
2.7
).
Fig. 2.7
Surface modification of mesoporous inorganic frameworks, R represents organic functional
group
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