Results and Discussion - New Synthetic Routes to Polyoxometalate Containing Ionic Liquids - page 74

are based on organic ligands that are prone to oxidation or insertion reactions,
quenching high oxidation states of the iron. Clusters as all inorganic materials could play
a helpful role in this field of research. For this reason it was intended to first bind an
azide ligand on the iron(III) substituted Keggin cluster and afterwards photolyze it to
obtain the corresponding nitrido complex, which would correspond to an oxidation by
two states yielding the iron(V)-nitrido complex. Making use of the ionic liquid medium is
crucial as it excludes water as solvent which would compete with the azide for the free
coordination position or quench higher iron oxidation states. Therefore great care was
taken to fully exclude external water and all reaction steps were carried out using
schlenk technique and dried solvents.
The reaction with N 3 - was carried out by stirring a solution of 8 in dry chloroform over
an excess of NaN 3 for 4 h which resulted in a light color change from brownish yellow to
an orange color ( 8b ). After filtering off not reacted NaN 3 from the reaction mixture was
one part of the solution evaporated to dryness. An IR-spectrum was obtained from this
product in a KBr pellet prepared under glovebox atmosphere (IR-1). The other (still
dissolved) part was transferred into a quartz tube and irradiated for 24 h using a 150 W
medium-pressure mercury light source (Heraeus TQ-150, UV Consulting Peschl
O max = 365 nm equipped with a pyrex deep-UV cutoff filter O cutoff = 320 nm) which
resulted in a color change from orange/yellow to deep blue ( 8c ). An IR-spectrum was
prepared in the same manner (IR-2). The blue colored compound is shave-stable under
exclusion from air and starts to precipitate slowly over several days. Contact with air
leads to a very fast reaction yielding again a compound of yellow color within around 5
seconds.
IR-1 shows an additional strong peak at a wavenumber of 2060 cm -1 compared to the
precursor 8 , which is a good indicator that the azide ligand could have coordinated to
the iron centre, particularly with respect to the additional color change of the reaction
solution. After the photolyzation this peak has completely vanished in IR-2 and no peak
is seen in the region from 2300-1800 cm -1 .
The color change of all steps was monitored by UV-Vis spectroscopy. Starting compound
8 shows a LMCT transition at 285 nm and a d Æ d transition falling together with the
boarders of this peak (see Figure 40 left spectrum). After the reaction with NaN 3 a