Unexpected photo-initiated oxidation of antimony in (tetra-tert-butyl)phthalocyaninatoantimony(III) complex in the presence of singlet oxygen acceptors

The title complex cation, [Sb(tbpc)]+ (where tbpc denotes tetra(tert-butyl)phthalocyaninate, C48H48N82−), was gradually bleached under irradiation with visible light (900 > λ/nm > 600) in aerated nonaqueous solvents, such as CH2Cl2, CHCl3, benzene, chlorobenzene. On the other hand, irradiation under the same conditions but in the presence of 1,3-diphenylisobenzofuran (DPBF) caused slow oxidation of [Sb(tbpc)]+ to the corresponding antimony(V) derivatives, [Sb(tbpc)LL′]+, (L and L′ denote monoanionic axial ligands containing oxygen as the donor atom), which was monitored by optical absorption and ESI-MS spectra of the photolyzed solutions. Both the photobleaching and oxidation have efficiently been inhibited by preliminary addition of β-carotene in photolyzed solutions, indicating that photosensitization of singlet oxygen (1O2) by [Sb(tbpc)]+ itself is involved. The use of furans (except dibenzofuran) or oxazoles instead of DPBF gave rise to similar oxidation but not in the case of the other type 1O2-acceptors, such as 9,10-diphenylanthracene, tetraphenylcyclopentadienone, or 2,3-dimethyl-2-butene, indicating that formation of ozonide-type endo-peroxide through 1,4-cycloaddition with 1O2 is essential. Peroxides, such as EtOOH and H2O2, generated through nucleophilic attack to the ozonide by EtOH and H2O (present in the solvent) respectively, are considered to oxidize [Sb(tbpc)]+ to the antimony(V) species. Photochemistry of antimony–phthalocyanine complex has been studied for the first time of pnicogen derivatives of phthalocyanine.