Hyperfine Coupling to the Bridging ¹⁷O in the Di-μ-oxo Core of a Mn^III−Mn^IV Model Significant to the Core Electronic Structure of the O₂-Evolving Complex in Photosystem II

Di-μ-oxo Mn^III−Mn^IV dimers are models for coupled, mixed-valence manganese in the oxygen-evolving centers of green plants. Using a recently reported method of exchanging water oxygen into the di-μ-oxo cross-bridges (Tagore, R.; Chen, H.; Crabtree, R. H.; Brudvig, G. W. J. Am. Chem. Soc. 2006, 128, 9457−9465), we have incorporated ¹⁷O into the μ-oxo cross bridges of the Mn^III−Mn^IV bipyridyl dimer for study of oxygen electron-spin hyperfine couplings by electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR). The ENDOR evidence was for a di-μ-oxo ¹⁷O hyperfine coupling of 12.8 ± 1.0 MHz. Narrow and highly resolved EPR features from dimers exchanged with H₂¹⁶O (I = 0) became broadened when the dimer was prepared by exchanging the cross bridging oxygens with H₂¹⁷O (I = ⁵/₂). The EPR broadening due to ¹⁷O was quantitatively reproduced by a model where the dimer has two equivalent di-μ-oxo cross-bridging ¹⁷Ο, and the ¹⁷O hyperfine coupling was highly consistent with that determined by ENDOR. This work explicitly points out evidence for covalent spin transfer to the cross-bridging di-μ-oxo oxygens which provide chemical bonds and antiferromagentic coupling between the mixed-valence manganese centers.