Redox and Photochemical Reactivity of Cerium(IV) Carbonate and Carboxylate Complexes Supported by a Tripodal Oxygen Ligand.

Abstract

Listen

Translate article

The protonolysis and redox reactivity of a Ce(IV) carbonate complex supported by the Kläui tripodal ligand [(η5-C5H5)Co{P(O)(OEt)2}3]- (LOEt-) have been studied. Whereas treatment of [Ce(LOEt)2(CO3)] (1) with RCO2H afforded [Ce(LOEt)2(RCO2)2] (R = Me (2), Ph (3), 2-NO2C6H3 (4)), the reaction of 1 with PhCH2CO2H resulted in formation of a mixture of Ce(IV) (5) and Ce(III) (6) carboxylate species. In benzene in the dark, 5 was slowly converted into 6 via Ce(IV)-O(carboxylate) homolysis. Recrystallization of a mixture of 5 and 6 from hexane led to isolation of yellow crystals of 6 that were identified as [Ce(LOEt)2(PhCH2CO2)]. Treatment of 1 with sulfamic acid, trifluoroacetamide, and trifluoromethanesulfonamide gave [Ce(LOEt)2(SO3NH)2] (7), [Ce(LOEt)2(CF3CONH)2] (8), and [Ce(LOEt)2(CF3SO2NH)2] (9), respectively. The crystal structures of 2, 4, and 6-8 have been determined. H atom transfer (HAT) of 2,6-di-tert-butylphenol and 9,10-dihydroanthracene (DHA) with 1 afforded 3,3',5,5'-tetra-tert-butyldiphenoquinone and anthracene, respectively. The oxidation of DHA with 1 under air yielded anthracene and anthraquinone. While 1 is stable in acetonitrile, it is readily reduced to a Ce(III) species in tetrahydrofuran. In air, 1 reacted with tetrahydrofuran to produce tetrahydrofuran hydroperoxide that can reduce the Ce(IV) carbonate rapidly. Upon irradiation with blue LED light, the Ce(IV)-LOEt carboxylate complexes underwent facile decarboxylation via Ce-O homolysis. 1 proved to be an efficient catalyst precursor for decarboxylative oxygenation of arylacetic acids. For example, irradiation of phenylacetic acid with blue LED light in the presence of 5 mol % of 1 under air afforded benzyl alcohol and benzaldehyde in 10 and 90% yield, respectively.