Electron spray ionization mass study on dioxygenation process in the reaction of catecholatoiron(III) complexes with molecular oxygen

The oxygenation reactions of two catecholatoiron(III) complexes, [Fe(TPA)(3,5-di-tert-butylcatecholate)]BPh4 (1) and [Fe(TPA)(4-chlorocatecholate)]BPh4 (2) (TPA = tris(pyridin-2-ylmethyl)amine), with O2 have been investigated by means of ESI-MS spectrometry to elucidate the detailed mechanisms of oxygen atom insertion from O2 into the catecholate ligands promoted by iron(III) complexes. Both 1 and 2 gave products formed by incorporation of two oxygen atoms into the catecholate ligands; 2,4-di-tert-butylmuconolactone for 1 and cis-dienelactone for 2. ESI-MS spectra of the products formed by the reaction with 18O2 revealed the following points: (1) Two oxygen atoms of 2,4-di-tert-butylmuconolactone are mostly derived from 18O2. (2) cis-Dienelactone is obtained as a mixture of mono- and double-18O-labeled species with a ratio of 50:50. These results suggest that the second oxygen atom is incorporated into muconic anhydride through nucleophilic attack on the carbonyl groups of muconic anhydride by the 18O-oxo-group of the metal center, and that this process competes with dissociation of muconic anhydride from the metal center.