Reversible Interconversion Between a Monomeric Iridium Hydroxo and a Dinuclear Iridium μ‑Oxo Complex

Treatment of the (PCsp2P)­IrICl complexes 2R (R = iPr, tBu) with cesium hydroxide in THF leads to the corresponding monomeric Ir­(I) hydroxo complexes 5R in good to excellent yields of 70% (R = iPr) and 92% (R = tBu). The compounds are green in color and while they exhibit very similar 31P NMR data to the chlorides 2, the 1H NMR spectrum of each features a triplet (3JHP = 3.8 Hz) at 4.22 (R = tBu) and 4.31 (R = iPr) ppm that broadens in the presence of excess water and exchanges deuterium with D2O. Bands at 3642 and 3625 cm–1 are observed in the IR spectrum for the νOH stretch. In the case of R = iPr, a second product is observed in the crude reaction mixture and dominates when 5iPr is heated under vacuum and H2O is removed. This product is deep blue in color, and an X-ray crystal structure analysis reveals it to be the S4 symmetric dinuclear (PCsp2P)­Ir–O–Ir­(PCsp2P) complex 6iPr, 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 6iPr with water, monomeric hydroxo complex 5iPr is quantitatively regenerated. Further, reaction of 6iPr with an excess of phenol smoothly yields the previously prepared (PCsp2P)­IrOPh complex 3iPr. Kinetic studies of the reaction indicated that it is first order in both [6iPr] and [HOPh] and exhibits a kH/kD of 1.9 when DOPh is employed. Eyring analysis is consistent with the bimolecular nature of the reaction, with ΔH = 13.1(5) kcal mol–1 and ΔS = −13(2) cal K–1. Finally, kobs is observed to increase when electron-withdrawing groups are incorporated in the para 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.