Hybrid DFT model calculations have been performed for some cobalt complexes capable of oxidizing water. Since a very plausible mechanism for the oxygen-evolving complex involving the cuboidal Mn4Ca structure in photosystem II (PSII) has recently been established, the most important part of the present study concerns a detailed comparison between cobalt and manganese as water oxidation catalysts. One similarity found is that a M(IV)-O(•) state is the key precursor for O-O bond formation in both cases. This means that simply getting a M(IV) state is not enough; a formal M(V)═O state is required, with two oxidations on one center from M(III). For cobalt, not even that is enough. A singlet coupled state is required at this oxidation level, which is not the ground state. It is shown that there are also more fundamental differences between catalysts based on these metals. The favorable low-barrier direct coupling mechanism found for PSII is not possible for the corresponding cobalt complexes. The origin of this difference is explained. For the only oxygen-evolving cubic Co4O4 complex with a defined structure, described by Dismukes et al., the calculated results are in good agreement with experiments. For the Co4 models of the amorphous cobalt-oxo catalyst found by Nocera et al., higher barriers are found than the one obtained experimentally. The reasons for this are discussed.

In a previous detailed study of all the steps of water oxidation in photosystem II, it was surprisingly found that O2 release is as critical for the rate as O-O bond formation. A new mechanism for O2 release has now been found, which can be described as an opening followed by a closing of the interior of the oxygen evolving complex. A transition state for peroxide rotation forming a superoxide radical, missed in the previous study, and a structural change around the outside manganese are two key steps in the new mechanism. However, O2 release may still remain rate-limiting. Additionally, for the step forming the O-O bond, an alternative, experimentally suggested, mechanism was investigated. The new model calculations can rule out the precise use of that mechanism. However, a variant with a rotation of the ligands around the outer manganese by about 30° will give a low barrier, competitive with the old DFT mechanism. Both these mechanisms use an oxyl-oxo mechanism for O-O bond formation involving the same two manganese atoms and the central oxo group (O5).

Density functional calculations are used to elucidate the reaction mechanism of water oxidation catalyzed by iron tetraamido macrocyclic ligand (TAML) complexes. The oxidation of the starting TAML–Fe3+–OH2 complex by removing three electrons and two protons leads to the formation of a key intermediate, TAML·–Fe5+=O, which can undergo nucleophilic attack by either a water molecule or a nitrate ion. Both pathways involve attack on the oxo group and lead to the production of O2. The water attack is more favoured and has a total barrier of 15.4 kcal/mol. The alternative nitrate attack pathway has a barrier of 19.5 kcal/mol. Nitrate functions as a cocatalyst by first donating an oxygen atom to the oxo group to form O2 and a nitrite ion, which can then be re‐oxidized to regenerate a nitrate ion. Three possible competing pathways result in ligand modification, namely, water and nitrate attack on the ligand, as well as ligand amide oxidation. The water attack on the ligand has a low barrier of only 10.9 kcal/mol and leads to the opening of the benzene ring, which explains the observation of fast catalyst degradation. The lack of activity or lower activity of other catalysts with different substituents is also rationalized.

Link to publicationCitation for published version (APA): Li, X., Sproviero, E. M., Ryde, U., Batista, V. S., & Chen, G. (2013). Theoretical EXAFS studies of a model of the oxygen-evolving complex of photosystem II obtained with the quantum cluster approach. International Journal of Quantum Chemistry, 113(4), 474-478. DOI: 10.1002/qua.24143 General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ABSTRACTThe oxygen-evolving complex (OEC) of photosystem II is the only natural system that can form O 2 from water and sunlight and it consists of a Mn 4 Ca cluster. In a series of publications, Siegbahn has developed a model of the OEC with the quantum mechanical (QM) cluster approach that is compatible with available crystal structures, able to form O 2 with a reasonable energetic barrier, and has a significantly lower energy than alternative models. In this investigation, we present a method to restrain a QM geometry optimization towards experimental polarized EXAFS data. With this method, we show that the cluster model is compatible with the EXAFS data and we obtain a refined cluster model that is an optimum compromise between QM and polarized EXAFS data.