title: Transformation of a Cp*-iridium(III) precatalyst for water oxidation when exposed to oxidative stress.
abstract: The reaction of [Cp*Ir(bzpy)NO3 ] (1; bzpy=2-benzoylpyridine, Cp*=pentamethylcyclopentadienyl anion), a competent water-oxidation catalyst, with several oxidants (H2 O2 , NaIO4 , cerium ammonium nitrate (CAN)) was studied to intercept and characterize possible intermediates of the oxidative transformation. NMR spectroscopy and ESI-MS techniques provided evidence for the formation of many species that all had the intact Ir-bzpy moiety and a gradually more oxidized Cp* ligand. Initially, an oxygen atom is trapped in between two carbon atoms of Cp* and iridium, which gives an oxygen-Ir coordinated epoxide, whereas the remaining three carbon atoms of Cp* are involved in a η(3) interaction with iridium (2 a). Formal addition of H2 O to 2 a or H2 O2 to 1 leads to 2 b, in which a double MeCOH functionalization of Cp* is present with one MeCOH engaged in an interaction with iridium. The structure of 2 b was unambiguously determined in the solid state and in solution by X-ray single-crystal diffractometry and advanced NMR spectroscopic techniques, respectively. Further oxidation led to the opening of Cp* and transformation of the diol into a diketone with one carbonyl coordinated at the metal (2 c). A η(3) interaction between the three non-oxygenated carbons of "ex-Cp*" and iridium is also present in both 2 b and 2 c. Isolated 2 b and mixtures of 2 a-c species were tested in water-oxidation catalysis by using CAN as sacrificial oxidant. They showed substantially the same activity than 1 (turnover frequency values ranged from 9 to 14 min(-1) ).
authors: Department of Chemistry, Biology and Biotechnology, University of Perugia, Via Elce di Sotto 8, 06123 Perugia ( Italy), Fax: (+39) 075-5855598.
authors: Cristiano
authors: C
authors: Zuccaccia
authors: Gianfranco
authors: G
authors: Bellachioma
authors: Olga
authors: O
authors: Bortolini
authors: Alberto
authors: A
authors: Bucci
authors: Arianna
authors: A
authors: Savini
authors: Alceo
authors: A
authors: Macchioni
keywords: NMR spectroscopy; X-ray diffraction; iridium; mass spectrometry; water chemistry