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 mu-O bridges in the di-mu-O dimanganese complexes, [(mes-terpy)(2)Mn-2(III/IV)(mu-O)(2)(H2O)(2)](NO3)(3) (1, mes-terpy = 4'-mesityl-2,2':6',2' '-terpyridine) and [(phen)(4)Mn-2(III/IV)(mu-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 mu-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 O-18-labeled water isotope exchange in photosystem II and provide support for the existence of fully protonated terminal waters bound to Mn in the O-2-evolving complex of photosystem II.