Iron–molybdenum-oxo complexes as initiators for olefin autoxidation with O2

Abstract

The reaction between [(TPA)Fe(MeCN)₂](OTf)₂ and [nBu₄N](Cp*MoO₃) yields the novel tetranuclear complex [(TPA)Fe(μ-Cp*MoO₃)]₂(OTf)₂, 1, with a rectangular [Mo–O–Fe–O–]₂ core containing high-spin iron(II) centres. 1 proved to be an efficient initiator/(pre)catalyst for the autoxidation of cis-cyclooctene with O₂ to give cyclooctene epoxide. To test, which features of 1 are essential in this regard, analogues with zinc(II) and cobalt(II) central atoms, namely [(TPA)Zn(Cp*MoO₃)](OTf), 3, and [(TPA)Co(Cp*MoO₃)](OTf), 4, were prepared, which proved to be inactive. The precursor compounds of 1, [(TPA)Fe(MeCN)₂](OTf)₂ and [nBu₄N](Cp*MoO₃) as well as Cp₂*Mo₂O₅, were found to be inactive, too. Reactivity studies in the absence of cyclooctene revealed that 1 reacts both with O₂ and PhIO via loss of the Cp* ligands to give the triflate salt 2 of the known cation [((TPA)Fe)₂(μ-O)(μ-MoO₄)]²⁺. The cobalt analogue 4 reacts with O₂ in a different way yielding [((TPA)Co)₂(μ-Mo₂O₈)](OTf)₂, 5, featuring a Mo₂O₈⁴⁻ structural unit which is novel in coordination chemistry. The compound [(TPA)Fe(μ-MoO₄)]₂, 6, being related to 1, but lacking Cp* ligands failed to trigger autoxidation of cyclooctene. However, initiation of autoxidation by Cp* radicals was excluded via experiments including thermal dissociation of Cp₂*.