Mechanistic study of cationic iron(II) sandwich complexes as photoinitiators for polymerization reactions
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Light excitation of Fe(II) sandwich complex photoinitiators of the types [CpFe(ç6-arene)]Z (Cp is cyclopentadienyl and Z is the counterion), benzoylfunctionized ferrocene, and [Fe(ç6-arene)2]Z2 results in deligation of the complexes in the primary photochemical processes, leading to the formation of active but short-lived intermediates, usually in the form of the corresponding halfsandwich complex ion with added monodentate ligands. The intermediates further decompose into fully ring-deligated Fe(II) complexes in a secondary thermal process. Some of these photoproducts function as active initiating species capable of starting polymerization of monomers. Elucidation of the mechanism of initiation and polymerization of cyclohexene oxide (CHO) with [CpFebz]PF6 (bz is benzene) has been achieved by probing the photoreaction with an on-line electrospray ionization mass spectrometry (ESI-MS) technique capable of identifying intermediates with lifetimes of 4 – 400 ms. Three initiating species responsible for the polymerization of CHO have been identified: the halfsandwich intermediate, the fully ring-deligated Fe(II) complex ion, and a non-iron monocation with m/z of 140.1. The establishment of a parent-offspring relationship between the half-sandwich intermediate and the fully ring-deligated Fe(II) complex species, and the proven nature of oligomers from the three initiating species allow for the proposal of a mechanism to describe the initiation and polymerization of CHO with the photoinitiator. Products from the photodeligation of dibenzoylferrocene (DBF) in acetonitrile (AN) are characterized using the on-line ESI-MS and 1H NMR techniques. The identification of the half-sandwich species [(ç5-C5H4C(O)Ph)Fe(AN)3]+ and its decomposition product, [Fe(AN)6]2+, as well as 1-benzoyl-1,3-cyclopentadiene lead to a detailed mechanism for the photoreaction of DBF. The strong nucleophilic nature of the benzoylcyclopentadienide anion photoreleased from the deligation of DBF is reflected by its fast abstraction of a proton from the solvent environment, confirming its role in initiating anionic polymerization.