Water Oxidation DOI: 10.1002/anie.200801132 Sustained Water Oxidation Photocatalysis by a Bioinspired Manganese Cluster** Robin Brimblecombe, Gerhard F. Swiegers,* G. Charles Dismukes,* and Leone Spiccia* The creation of efficient catalysts for splitting water into H 2 and O 2 is one of the greatest challenges for chemists working on the production of renewable fuel. [1,2] The water oxidizing center (WOC) within photosynthetic organisms is the only natural system able to efficiently photooxidize water using visible light, and is thus a blueprint for catalyst design. One of the atomic structural models of the WOC derived from X-ray diffraction involves a “cubelike” core comprised of a {CaMn 3 O 4 } unit tethered to a fourth manganese atom through one or two bridging oxo units. [3] A few nonbiological tetramanganese complex mimics of this site have been prepared that contain an incomplete or distorted cubic {Mn 4 O x } core [4–7] or are part of a larger Mn x –oxo lattice. [4] However, none of these have shown activity towards water oxidation. We have previously synthesized a prototypical molecular manganese–oxo cube [Mn 4 O 4 ] n+ in a family of “cubane” complexes [Mn 4 O 4 L 6 ],whereL  is a diarylphosphinate ligand (p-R-C 6 H 4 ) 2 PO 2  (R = H, alkyl, OMe). [6,8] The diphenylphos- phinate complex (1,R = H, Figure 1) assembles spontane- ously from manganese(II) and permanganate salts in high yield in non-aqueous solvents. [9] The release of O 2 by the {Mn 4 O 4 } 6+ core in 1 was shown to be possible on thermody- namic grounds, but cannot take place because of the rigidity of the core arising from the six diarylphosphinate ligands, which bridge pairs of manganese atoms on the six cube faces. The assembly of 1 is also driven by intramolecular van der Waals forces that attract three aryl rings from adjacent phosphinate ligands. The cubic core in 1 is a much stronger oxidant than any known dimanganese complex with {Mn 2 O 2 } 3+ cores. Cubane 1 abstracts hydrogen atoms from various organic substrates by breaking O  H and N  H bonds with dissociation energies greater than 390 kJmol 1 . [10] Titrations of 1 against com- pounds containing either amine or phenol groups reach an end point after the abstraction of four successive hydrogen atoms, yielding two water molecules (from corner oxo groups) plus [L 6 Mn 4 O 2 ], the so-called “pinned butterfly” complex 2 (Scheme 1). [11] {Mn 4 O 4 } cubane complexes are unique in releasing an O 2 molecule upon photoexcitation of the Mn ! O charge transfer band, which reaches a maximum at 350 nm. [12,13] This process, which occurs with high quantum efficiency only in the gas phase, involves the core oxygen atoms and is triggered by ejection of one phosphinate ligand, thereby generating the [L 5 Mn 4 O 2 ] + “butterfly” complex 3 (Scheme 1). In contrast, noncuboidal manganese molecular complexes possessing {Mn 2 O}, {Mn 2 O 2 }, and {Mn 3 O 6 } cores in the Mn 3+ or Mn 4+ oxidation states fail to release O 2 , but instead photodecompose into multiple fragments. [13] Thus, O 2 release is favored by complexes with a {Mn 4 O 4 } cubane core. The composition of the butterfly complexes 2 and 3 differs only by one phosphinate ligand (Scheme 1). This finding suggests the possibility of creating a catalytic cycle that could oxidize two water molecules bound to 2 along the reverse pathway in Scheme 1 (1-3H ! 1-2H ! 1-H ! 1), eventually forming 3 by photochemical release of O 2 and a phosphinate ligand. Thus far it has proved impossible to realize a catalytic cycle, as in Scheme 1, because O 2 is not photodissociated from 1 or 1 + (the one-electron oxidized cubane) in condensed phases. [12–14] This was attributed to a large activation barrier for O 2 release when all the phosphinate ligands remain ligated or re-ligate by fast geminate recombination. Figure 1. X-ray crystal structure of 1. [6] [*] Dr. G. F. Swiegers Division of Molecular and Health Technologies Commonwealth Scientific and Industrial Research Organisation (CSIRO) Clayton, Victoria, 3169 (Australia) Fax: (+ 61)3-9545-2446 E-mail: leone.spiccia@sci.monash.edu.au Prof. Dr. G. C. Dismukes Department of Chemistry & Princeton Environmental Institute Princeton University Princeton, NJ 08544 (U.S.A.) Fax: (+ 1)609-258-1980 R. Brimblecombe, Prof. Dr. L. Spiccia School of Chemistry, Monash University Victoria, 3800 (Australia) Fax: (+ 61)3-9905-4597 E-mail: dismukes@princeton.edu [**] This work was supported by the Australian Research Council, the US National Institutes of Health, a Lemberg Fellowship, an Australian Postgraduate Award, and a Fulbright Postgraduate Award. We thank A. M. Bond, N. Fay, R. J. S Morrison, J. Scarino, J. Sheats, and G. Felton for their assistance and comments. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200801132. Angewandte Chemie 7335 Angew. Chem. Int. Ed. 2008, 47, 7335 –7338 # 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim