Hyperfine Coupling to the Bridging <sup>17</sup>O in the Di-μ-oxo Core of a Mn<sup>III</sup>−Mn<sup>IV</sup> Model Significant to the Core Electronic Structure of the O<sub>2</sub>-Evolving Complex in Photosystem II

Di-μ-oxo Mn<sup>III</sup>−Mn<sup>IV</sup> 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. <i>J. </i><i>Am. Chem. Soc.</i> <b>2006</b>, <i>128</i>, 9457−9465), we have incorporated <sup>17</sup>O into the μ-oxo cross bridges of the Mn<sup>III</sup>−Mn<sup>IV</sup> 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 <sup>17</sup>O hyperfine coupling of 12.8 ± 1.0 MHz. Narrow and highly resolved EPR features from dimers exchanged with H<sub>2</sub><sup>16</sup>O (<i>I</i> = 0) became broadened when the dimer was prepared by exchanging the cross bridging oxygens with H<sub>2</sub><sup>17</sup>O (<i>I</i> = <sup>5</sup>/<sub>2</sub>). The EPR broadening due to <sup>17</sup>O was quantitatively reproduced by a model where the dimer has two equivalent di-μ-oxo cross-bridging <sup>17</sup>Ο, and the <sup>17</sup>O 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.