Reactivity of Cr(III) μ-Oxo Compounds: Catalyst Regeneration and Atom Transfer Processes

Oxidation of CpCr­[(XylNCMe)2CH] (Xyl = 2,6-Me2C6H3) with pyridine N-oxide
or air generated the μ-oxo dimer, {CpCr­[(XylNCMe)2CH]}2(μ-O). The μ-oxo dimer was converted
to paramagnetic Cr­(III) CpCr­[(XylNCMe)2CH]­(X) complexes
(X = OH, O2CPh, Cl, OTs) via protonolysis reactions. The
related Cr­(III) alkoxide complexes (X = OCMe3, OCMe2Ph) were prepared by salt metathesis and characterized by
single crystal X-ray diffraction. The interconversion of the Cr­(III)
complexes and their reduction back to Cr­(II) with Mn powder were monitored
using UV–vis spectroscopy. The related CpCr­[(DepNCMe)2CH] (Dep = 2,6-Et2C6H3) Cr­(II) complex
was studied for catalytic oxygen atom transfer reactions with PPh3 using O2 or air. Both Cr­(II) complexes reacted
with pyridine N-oxide and γ-terpinene to give
the corresponding Cr­(III) hydroxide complexes. When CpCr­[(DepNCMe)2CH] was treated with pyridine N-oxide in
benzene in the absence of hydrogen atom donors, a dimeric Cr­(III)
hydroxide product was isolated and structurally characterized, apparently
resulting from intramolecular hydrogen atom abstraction of a secondary
benzylic ligand C–H bond followed by intermolecular C–C
bond formation. The use of very bulky hexaisopropylterphenyl ligand
substituents did not preclude the formation of the analogous μ-oxo
dimer, which was characterized by X-ray diffraction. Attempts to develop
a chromium-catalyzed intermolecular hydrogen atom transfer process
based on these reactions were unsuccessful. The protonolysis and reduction
reactions of the μ-oxo dimer were used to improve the previously
reported Cr-catalyzed radical cyclization of a bromoacetal.