Characterization of Oxygen Bridged Manganese Model Complexes Using Multifrequency ¹⁷O-Hyperfine EPR Spectroscopies and Density Functional Theory

Multifrequency pulsed EPR data are reported for a series of oxygen bridged (Μ-oxo/Μ-hydroxo) bimetallic manganese complexes where the oxygen is labeled with the magnetically active isotope ¹⁷O (I = 5/2). Two synthetic complexes and two biological metallocofactors are examined: a planar bis-Μ-oxo bridged complex and a bent, bis-Μ-oxo-Μ-carboxylato bridge complex; the dimanganese catalase, which catalyzes the dismutation of H₂O₂ to H₂O and O₂, and the recently identified manganese/iron cofactor of the R2lox protein, a homologue of the small subunit of the ribonuclotide reductase enzyme (class 1c). High field (W-band) hyperfine EPR spectroscopies are demonstrated to be ideal methods to characterize the ¹⁷O magnetic interactions, allowing a magnetic fingerprint for the bridging oxygen ligand to be developed. It is shown that the Μ-oxo bridge motif displays a small positive isotropic hyperfine coupling constant of about +5 to +7 MHz and an anisotropic/dipolar coupling of 9 MHz. In addition, protonation of the bridge is correlated with an increase of the hyperfine coupling constant. Broken symmetry density functional theory is evaluated as a predictive tool for estimating hyperfine coupling of bridging species. Experimental and theoretical results provide a framework for the characterization of the oxygen bridge in Mn metallocofactor systems, including the water oxidizing cofactor of photosystem II, allowing the substrate/solvent interface to be examined throughout its catalytic cycle.