FULL PAPER DOI: 10.1002/ejic.201100954 An NMR Study of the Oxidative Degradation of Cp*Ir Catalysts for Water Oxidation: Evidence for a Preliminary Attack on the Quaternary Carbon Atom of the –C–CH 3 Moiety Cristiano Zuccaccia, [a] Gianfranco Bellachioma, [a] Sandra Bolaño, [b] Luca Rocchigiani, [a] Arianna Savini, [a] and Alceo Macchioni* [a] Keywords: Water oxidation / NMR spectroscopy / Hydrogen peroxide / Iridium / Cyclopentadienyl ligands The reaction between H 2 O 2 and two water oxidation cata- lysts {[Cp*Ir(H 2 O) 3 ](NO 3 ) 2 (1, Cp* = pentamethylcyclopen- tadienyl) and [Cp*Ir(bzpy)(NO 3 )] (2, bzpy = 2-benzoyl- pyridine)} was studied by means of in situ 1D- and 2D-NMR experiments in order to elucidate if catalyst degradation pro- ceeds through the initial functionalization of a quaternary carbon atom (C-attack) or by hydrogen abstraction (H-at- tack) of the Cp* –C–CH 3 moiety. It was shown that 1 under- went double functionalization of the –C–CH 3 moiety of Cp* leading to the formation of –C(OR)–CH 2 OR (R = H or OH) in Introduction The oxidation of water to give molecular oxygen is an endergonic and kinetically difficult process whose imple- mentation requires external energy and the use of an appro- priate catalyst. [1,2] Due to the importance of water oxidation for hydrogen generation through the photochemical split- ting of water, [3–11] many efforts have been devoted to the development of new and efficient catalysts. [12–17] Focusing on molecular catalysts, the harsh experimental conditions required for catalysis severely limits the typolog- ies of the ancillary ligands that can be used to support the active metal center. As a consequence until a few years ago, coordination complexes based on N- and O-ligands have been almost exclusively used as molecular catalysts. In 2008, Bernhard and co-workers showed that catalysts featuring C-ligands, i.e. organometallic catalysts, are also stable enough to be used as water oxidation catalysts. [18] This find- ing provided the organometallic community with the ex- traordinary opportunity to enter fully into the fascinating field of artificial photosynthesis. [19] Results were not long in coming, and several efficient and robust organometallic [a] Department of Chemistry, University of Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy Fax: +39-075-585-5598 E-mail: alceo@unipg.it [b] Departamento de Química Inorgánica, Universidade de Vigo, Campus Universitario, 36310 Vigo, Spain Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.201100954. © 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2012, 1462–1468 1462 a strictly analogous manner to that previously observed for the reaction of 1 with Ce 4+ . On the contrary, two new inter- mediates associated with the oxidative degradation of 2, which are functionalized only at the quaternary carbon atom(s) of the –C–CH 3 moiety [–C(OR)–CH 3 ], were inter- cepted and characterized by NMR spectroscopy. This indi- cates that the oxidative degradation of water oxidation cata- lysts, featuring the –Cp* ancillary ligand, likely starts with preferential C-attack. catalysts have been recently developed. [20–26] Most of them are iridium(III) organometallic complexes based on Cp* li- gands, and show remarkable performance with TOF (turn- over frequency) values  50 min –1 and TON (turnover number) values  4000. Although their performance is no- table, [Cp*IrL n ] (L = generic ligand) catalysts slowly un- dergo Cp* oxidation under strongly oxidative [i.e. in the presence of cerium ammonium nitrate (CAN), (NH 4 ) 2 Ce- (NO 3 ) 6 ] and acidic (i.e. in solution acidified to pH = 1 with HNO 3 ) conditions, ultimately leading to a large-scale for- mation of CH 3 COOH and, most likely, iridium acetates or oxides. [27] By directly monitoring the catalysis within an NMR tube, it has been shown that the degradation of the catalysts involves the double oxidative functionalization of both the methyl and quaternary carbon atoms of the –C– CH 3 moiety of Cp*. [27a] Based on the results of an inte- grated experimental and computational study, it has been proposed that the reaction originates from intramolecular attack by the superoxide O 2 ligand at the carbon (C-attack) and hydrogen (H-attack) [28] positions of the –C–CH 3 moi- ety in the Cp*Ir III L n (η 1 -O 2 · ) species. This reaction leads to the formation of a –C(OR)–CH 2 OR fragment (R = H or OH). [27a] Several intermediates associated with the oxidative degradation were intercepted, and a degradation pathway was delineated. [27a] Unfortunately, it was not possible to understand which of the two processes occurs first when CAN acts as a sacrificial oxidant. In contrast to our pre- vious work, [27a] we decided to use H 2 O 2 (instead of CAN) as sacrificial oxidant in the hope of slowing down the oxi-