Low temperature reaction of [Fe(TPA)(CH3CN)2]2+ with excess 3-chloroperoxybenzoic acid in semi-frozen acetonitrile; EPR detection of an acylperoxo iron(III) adduct

Treatment of [(TPA)FeII(CH3CN)2]2+ (TPA = tris(2-pyridylmethyl)amine) with excess (≥ 2 Eq) of 3-chloroperoxybenzoic acid (mCPBA) in semi-frozen acetonitrile in liquid N2 vapour generates a rhombic EPR signal assigned to the S = ½ low spin acylperoxoiron(III) species [(TPA)Fe(O3CC6H4-3Cl)(CH3CN)]2+5; the elusive precursor on the pathway to [(TPA)FeIII(5-chlorosalicylate)]+4 via putative [(TPA)FeV(O)(O2CC6H4-3Cl)]2+. Formation of cyclohexene epoxide in the presence of added cyclohexene at low temperatures (<−40 °C) suggests that intermolecular alkene epoxidation competes successfully with the intramolecular reactions involving 5 to generate 4.

Graphical abstractTreatment of [(TPA)Fe(CH3CN)2]2+1 with excess (≥ 2 Eq) of 3-chloroperoxybenzoic acid (mCPBA) in semi-frozen acetonitrile generates an S = ½ EPR signal assigned to the low spin acylperoxo iron(III) species; [(TPA)Fe(O3CC6H4-3Cl)(CH3CN)]2+5; the elusive precursor to putative ‘(TPA)FeV(O)’ on the pathway to [(TPA)FeIII(5-chlorosalicylate)]+. Solutions of 5 are also active in promoting cyclohexene epoxidation.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Mixing [(TPA)FeII(CH3CN)2]2+ with ≥2eq mCPBA in semi-frozen acetonitrile forms [(TPA)Fe(O3CC6H4-3Cl)(CH3CN)]2+5. ► Detection (by EPR) of 5 for the first time is as a result of mixing the reagents in the semi-frozen acetonitrile medium. ► 5 is the direct precursor of putative ‘(TPA)FeV(O)2+’ on the pathway to [(TPA)FeIII(5-Cl-salicylate)]+, the final product. ► Generation of cyclohexene epoxide shows that intermolecular alkene epoxidation competes with intramolecular CH O-insertion. ► The findings cannot rule out the possibility that acylperoxoiron(III) complexes can themselves promote alkene epoxidation.