A time-resolved mass spectrometric technique has been used for the determination of rates of exchange of μ-O atoms with water for the complexes [(mes-terpy)2Mn2III/IV(μ-O) 2(H2O)2](NO3)3 (1, mes-terpy = 4′-mesityl-2,2′:6′,2″-terpyridine), [(bpy)4Mn2III/IV(μ-O)2](CIO 4)3 (2, bpy = 2,2′-bipyridine), [(phen) 4Mn2III/IV(μ-O)2](CIO 4)3 (3, phen = 1,10-phenanthroline), [(bpea) 2Mn2III/IV(μ-O)2(μ-OAc)] (CIO4)2 (4, bpea = bis(2-pyridyl)ethylamine), [(bpea) 2Mn2IV/IV(μO)2(μ-OAc)] (CIO4)3 (4ox), [(terpy)4Mn 4IV/IV/IV/IV(μ-O)5(H2O) 2]-(CIO4)6 (5, terpy = 2,2′:6′, 2″-terpyridine), and [(tacn)4Mn4 IV/IV/IV/IV(μ-O)6]Br3.5(OH) 0.5·6H2O (6, tacn = 1,4,7-triazacyclononane). The rate of exchange of μ-OAc bridges with free acetate in solution has been measured for complexes 4 and 4ox. These are the first measurements of rates of ligand exchange on biologically relevant high-valent Mn complexes. The data analysis method developed here is of general utility in the quantitation of isotope exchange processes by mass spectrometry. We find that the presence of labile coordination sites on Mn increases μ-O exchange rates, and that all-MnIV states are more inert toward exchange than mixed Mn III-MnIV states. The rates of μ-O exchange obtained in this work for a di-μ-oxo Mn2III/IV dimer with labile coordination sites are compared with the oxygen isotope incorporation rates from substrate water to evolved dioxygen measured in different S states of the oxygen evolving complex (OEC) of photosystem II (PSII). On the basis of this comparison, we propose that both substrate waters are not bound as μ-O bridges between Mn atoms in the S2 and S3 states of the OEC.
ASJC Scopus subject areas
- Colloid and Surface Chemistry