Chemistry Reference
In-Depth Information
enable the formation of a multitude of different supramolecular structures.
[40]
One main
goal for synthetic chemists is a deeper understanding of this self-organizing process to
ideally direct it to the incorporation of certain well known structural elements.
POM formation is best illustrated using the archetypal Keggin anion [XM
12
O
40
]
n-
(X = B,
Si, P, etc.; M = W, Mo). POMs in general, and the Keggin anion in particular are prepared
by acidifying an aqueous solution of an oxometalate precursor such as Na
2
MO
4
(M = W,
Mo). The decrease of pH results in an expansion of the metal coordination shell of the
starting materials from a tetrahedral to an octahedral coordination sphere leading to an
increased polarization of the M=O double bonds. Afterwards the terminal oxygen atoms
can be protonated more easily, initiating the onset of consecutive condensation
reactions between the [MO
6
] units which are hereby linked by one or two bridging μ-
oxo-ligands respectively and react in corner- and edge-sharing modes (see Scheme 1).
Larger polynuclear building blocks are formed from highly reactive hydrated complexes
such as O[MO(OH)
4
]
2
3-
.
[41]
Scheme 1
Condensation reaction of [WO
6
]-octahedra to larger polynuclear [W
x
O
y
]
m-
building blocks.
These unstable intermediates can immediately react further and aggregate to highly
symmetric (C3
v
) edge-sharing triads [W
3
O
13
]. The final framework is obtained by further
fusion of the unstable hydrated polynuclear metal-oxide intermediates. Hereby the
cluster growth is essentially influenced by empty d-orbitals of the high oxidation state
metals that enable the formation of strong π - bonds, thus decreasing basicity and
nucleophilicity of the terminal M=O double bonds. Consequently further spontaneous
condensation reactions are prevented and the cluster growth comes to an end - the
final cluster structure is completed.
Reasons for this phenomenon and the enormous variety of compounds that can be
formed by linking together metal-oxide building blocks are the easy change of
Search WWH ::
Custom Search