A DFT/B3LYP study of the mechanisms of the O-2 formation reaction catalyzed by the [(terpy) (H2O)Mn-III (O)(2)Mn-IV(OH2) (terpy)] (NO3)(3) complex: A paradigm for photosystem II

摘要

We present a theoretical study of the reaction pathway for dioxygen molecular formation catalyzed by the [(terpy)(H2O)Mn-III(O)(2)Mn-IV(OH2) (terpy)](NO3)(3) (terpy = 2,2 ':6 ',2 ''-terpyridine) complex based on DFT-B3LYP calculations. In the initial state of the reaction, a partial oxido radical (0.44 spins) is formed ligated to Mn. This radical is involved in a nucleophylic attack by bulk water in the O-O bond reaction formation step, in which the oxido fractional unpaired electron is delocalized toward the outermost Mn of the mu-oxo bridge, instead of the ligated Mn center. The reaction then follows with a series of proton-coupled electron transfer steps, in which the oxidation state, as well as the bond strength of the OO moiety increase, while the OO-Mn(1) bond gets weaker until O-2 is released. In this model, basic acetate ions from the buffer solution capture protons in the proton-transfer steps. In each step there is reduction of the OO-Mn(1) binding strength, with concomitant increase of the O-O bond strength, which culminates with the release of O-2 in the last step. This last step is entropy driven, while formation of hydroperoxide and superoxide moieties is enthalpy driven. According with experiments, the rate-limiting step is the double oxidation of Mn(IV,III) or peroxymonosulfate binding, which occur prior to the O-O bond formation step. This supports our findings that the barriers of all intermediate steps are below the experimental barrier of 19-21 kcal/mol. The implications of these findings for understanding photosynthetic water-splitting catalysis are also discussed. (c) 2017 Elsevier Inc. All rights reserved.