Distinct Mechanisms of Bridging-Oxo Exchange in Di-μ-O Dimanganese Complexes with and without Water-Binding Sites: Implications for Water Binding in the O₂-Evolving Complex of Photosystem II

Isotopic exchange between oxygens of water and μ-O bridges in the di-μ-O dimanganese complexes, [(mes−terpy)₂Mn₂^III/IV(μ-O)₂(H₂O)₂](NO₃)₃ (1, mes−terpy = 4‘-mesityl-2,2‘:6‘,2‘‘-terpyridine) and [(phen)₄Mn₂^III/IV(μ-O)₂](ClO₄)₃ (2, phen = 1,10-phenanthroline), has been investigated by a study of the kinetics of exchange. The data provide evidence for distinct mechanisms of exchange in 1 and 2 and suggest that these differences arise due to the presence and absence of terminal water-binding sites in 1 and 2, respectively. Exchange of oxygen atoms between water and μ-O bridges must involve the elementary steps of bridge protonation, deprotonation, opening, and closing. On the basis of the existing literature on these reactions in oxo-bridged metal complexes and our present data, we propose pathways of exchange in 1 and 2. The mechanism proposed for 1 involves an initial fast protonation of an oxo-bridge by water coordinated to Mn^IV, followed by a slow opening of the protonated bridge as proposed earlier for an analogous complex on the basis of DFT calculations. The mechanism proposed for 2 involves initial dissociation of phen, followed by coordination of water at the vacated sites, as observed for rearrangement of 2 to a trinuclear complex. The subsequent steps are proposed to be analogous to those for 1. Our results are discussed in the context of data on ¹⁸O-labeled water isotope exchange in photosystem II and provide support for the existence of fully protonated terminal waters bound to Mn in the O₂-evolving complex of photosystem II.