10.1021/ja412650s.s001 Richard J. Burford Richard J. Burford Warren E. Piers Warren E. Piers Daniel H. Ess Daniel H. Ess Masood Parvez Masood Parvez Reversible Interconversion Between a Monomeric Iridium Hydroxo and a Dinuclear Iridium μ‑Oxo Complex American Chemical Society 2014 phenol substrate metal hydroxo ligands ν OH stretch THF D 2O Bands form metal oxo moieties crude reaction mixture 1 H NMR spectrum PCsp 2P 31 P NMR data HOMO PCsp 2P complexes 2 R Monomeric Iridium Hydroxo DFT 5 iPr JHP 6 iPr IR M 06 density 2014-02-26 00:00:00 Dataset https://acs.figshare.com/articles/dataset/Reversible_Interconversion_Between_a_Monomeric_Iridium_Hydroxo_and_a_Dinuclear_Iridium_Oxo_Complex/2320039 Treatment of the (PC<sub>sp<sup>2</sup></sub>P)­Ir<sup>I</sup>Cl complexes <b>2</b><sup><b>R</b></sup> (R = <sup><i>i</i></sup>Pr, <sup><i>t</i></sup>Bu) with cesium hydroxide in THF leads to the corresponding monomeric Ir­(I) hydroxo complexes <b>5</b><sup><b>R</b></sup> in good to excellent yields of 70% (R = <sup><i>i</i></sup>Pr) and 92% (R = <sup><i>t</i></sup>Bu). The compounds are green in color and while they exhibit very similar <sup>31</sup>P NMR data to the chlorides <b>2</b>, the <sup>1</sup>H NMR spectrum of each features a triplet (<sup>3</sup><i>J</i><sub>HP</sub> = 3.8 Hz) at 4.22 (R = <sup><i>t</i></sup>Bu) and 4.31 (R = <sup><i>i</i></sup>Pr) ppm that broadens in the presence of excess water and exchanges deuterium with D<sub>2</sub>O. Bands at 3642 and 3625 cm<sup>–1</sup> are observed in the IR spectrum for the ν<sub>OH</sub> stretch. In the case of R = <sup><i>i</i></sup>Pr, a second product is observed in the crude reaction mixture and dominates when <b>5</b><sup><b><sup><i>i</i></sup>Pr</b></sup> is heated under vacuum and H<sub>2</sub>O is removed. This product is deep blue in color, and an X-ray crystal structure analysis reveals it to be the S<sub>4</sub> symmetric dinuclear (PC<sub>sp<sup>2</sup></sub>P)­Ir–O–Ir­(PC<sub>sp<sup>2</sup></sub>P) complex <b>6</b><sup><b><sup><i>i</i></sup>Pr</b></sup>, which features a μ-oxo ligand along an allene-like molecular core. Time-dependent DFT calculations with the M06 density functional show that a metal-to-ligand HOMO–LUMO excitation is mainly responsible for the blue color. Upon reaction of <b>6</b><sup><b><sup><i>i</i></sup>Pr</b></sup> with water, monomeric hydroxo complex <b>5</b><sup><b><sup><i>i</i></sup>Pr</b></sup> is quantitatively regenerated. Further, reaction of <b>6</b><sup><b><sup><i>i</i></sup>Pr</b></sup> with an excess of phenol smoothly yields the previously prepared (PC<sub>sp<sup>2</sup></sub>P)­IrOPh complex <b>3</b><sup><b><sup><i>i</i></sup>Pr</b></sup>. Kinetic studies of the reaction indicated that it is first order in both [<b>6</b><sup><b><sup><i>i</i></sup>Pr</b></sup>] and [HOPh] and exhibits a <i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> of 1.9 when DOPh is employed. Eyring analysis is consistent with the bimolecular nature of the reaction, with Δ<i>H</i><sup>⧧</sup> = 13.1(5) kcal mol<sup>–1</sup> and Δ<i>S</i><sup>⧧</sup> = −13(2) cal K<sup>–1</sup>. Finally, <i>k</i><sub>obs</sub> is observed to increase when electron-withdrawing groups are incorporated in the <i>para</i> position of the phenol substrate and decrease when electron-donating groups are employed. These observations suggest that the rate-limiting step in this reaction is protonation of the μ-oxo ligand by the phenol substrate. This reaction serves as a model system for the reversible condensation of metal hydroxo ligands to form metal oxo moieties.