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

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]⁻ (L_OEt^–) have been studied. Whereas treatment of [Ce­(L_OEt)₂(CO₃)] (<b>1</b>) with RCO₂H afforded [Ce­(L_OEt)₂(RCO₂)₂] (<i>R</i> = Me (<b>2</b>), Ph (<b>3</b>), 2-NO₂C₆H₃ (<b>4</b>)), the reaction of <b>1</b> with PhCH₂CO₂H resulted in formation of a mixture of Ce­(IV) (<b>5</b>) and Ce­(III) (<b>6</b>) carboxylate species. In benzene in the dark, <b>5</b> was slowly converted into <b>6</b> via Ce­(IV)-O­(carboxylate) homolysis. Recrystallization of a mixture of <b>5</b> and <b>6</b> from hexane led to isolation of yellow crystals of <b>6</b> that were identified as [Ce­(L_OEt)₂(PhCH₂CO₂)]. Treatment of <b>1</b> with sulfamic acid, trifluoroacetamide, and trifluoromethanesulfonamide gave [Ce­(L_OEt)₂(SO₃NH)₂] (<b>7</b>), [Ce­(L_OEt)₂(CF₃CONH)₂] (<b>8</b>), and [Ce­(L_OEt)₂(CF₃SO₂NH)₂] (<b>9</b>), respectively. The crystal structures of <b>2</b>, <b>4</b>, and <b>6–8</b> have been determined. H atom transfer (HAT) of 2,6-di-<i>tert</i>-butylphenol and 9,10-dihydroanthracene (DHA) with <b>1</b> afforded 3,3′,5,5′-tetra-<i>tert</i>-butyldiphenoquinone and anthracene, respectively. The oxidation of DHA with <b>1</b> under air yielded anthracene and anthraquinone. While <b>1</b> is stable in acetonitrile, it is readily reduced to a Ce­(III) species in tetrahydrofuran. In air, <b>1</b> reacted with tetrahydrofuran to produce tetrahydrofuran hydroperoxide that can reduce the Ce­(IV) carbonate rapidly. Upon irradiation with blue LED light, the Ce­(IV)-L_OEt carboxylate complexes underwent facile decarboxylation via Ce–O homolysis. <b>1</b> 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 <b>1</b> under air afforded benzyl alcohol and benzaldehyde in 10 and 90% yield, respectively.