Chemistry Reference
In-Depth Information
3.1.3.2 UV-Vis spectroscopy
UV-vis spectra were recorded on a Shimadzu UV-2401PC spectrophotometer in
transmission mode with quartz cuvettes (1.0 cm optical path length). Samples (
1
)-(
4
)
and (
11
)+(
12
) were recorded in CH
2
Cl
2
, samples (
5
)-(
10
) were recorded in toluene as
solvent. The Keggin cluster dominates the absorption features and shows characteristic
absorption peaks at around 228 and 258 nm which are assigned to the intramolecular
(O
d
→ W and O
b,c
→ W) ligand to metal charge transfer (LMCT) transitions.
[64, 65]
In
addition absorption peaks in the visible spectrum occur for all clusters except for iron
due to the incorporated hetero-element which is sixfold coordinated to oxygen (five oxo
ligands by the Keggin cluster and one oxygen bound water ligand) which suggests a
comparison with the well known hexaqua complexes of the respective heteroelement.
In case of the iron(III) d
5
-compounds, where d
Æ
d excitations are spin forbidden, the
clusters yellow color is caused by a batochromic shift of the LMCT absorption due to the
irons charge of +3 (compared e.g. to the isoelectronic Mn(II) analogues).
[66]
The assignment of all peaks was done with the help of similar data from literature (see
footnote of Table 3 or with the corresponding Tanabe-Sugano diagrams, which were
particularly useful for additionally assigning spin-forbidden intercombination transitions
where they occurred. A detailed overview of the UV-Vis data is given below in Table 3:
Search WWH ::
Custom Search