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

Treatment of the (PC_sp²P)­Ir^ICl complexes 2^R (R = ⁱPr, ^tBu) with cesium hydroxide in THF leads to the corresponding monomeric Ir­(I) hydroxo complexes 5^R 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 (³J_HP = 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 ν_OH stretch. In the case of R = ⁱPr, a second product is observed in the crude reaction mixture and dominates when 5^iPr 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 (PC_sp²P)­Ir–O–Ir­(PC_sp²P) complex 6^iPr, 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 6^iPr with water, monomeric hydroxo complex 5^iPr is quantitatively regenerated. Further, reaction of 6^iPr with an excess of phenol smoothly yields the previously prepared (PC_sp²P)­IrOPh complex 3^iPr. Kinetic studies of the reaction indicated that it is first order in both [6^iPr] and [HOPh] and exhibits a k_H/k_D 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, k_obs 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.