A Synthetic Oxygen Atom Transfer Photocycle from a Diruthenium Oxyanion Complex

Three
new diruthenium oxyanion complexes have been prepared, crystallographically
characterized, and screened for their potential to photochemically
unmask a reactive RuRuO intermediate. The most promising
candidate, Ru2(chp)4ONO2 (4, chp = 6-chloro-2-hydroxypyridinate), displays a set of signals
centered around m/z = 733 amu in
its MALDI-TOF mass spectrum, consistent with the formation of the
[Ru2(chp)4O]+ ([6]+) ion. These signals shift to 735 amu in 4*,
which contains an 18O-labeled nitrate. EPR spectroscopy
and headspace GC-MS analysis indicate that NO2• is released upon photolysis of 4, also consistent with
the formation of 6. Photolysis of 4 in CH2Cl2 at room temperature in the presence of excess
PPh3 yields OPPh3 in 173% yield; control experiments
implicate 6, NO2•, and free
NO3– as the active oxidants. Notably,
Ru2(chp)4Cl (3) is recovered after
photolysis. Since 3 is the direct precursor to 4, the results described herein constitute the first example
of a synthetic cycle for oxygen atom transfer that makes use of light
to generate a putative metal oxo intermediate.