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.