Stable Ionic Rh(I,II,III) Complexes Ligated by an
Imidazolium-Substituted Phosphine with π‑Acceptor Character:
Synthesis, Characterization,
and Application to Hydroformylation
- Publication date
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Abstract
The stable ionic Rh(I,II,III) complexes
[Rh<sup>I</sup>(acac)(CO)(<b>L</b>)]PF<sub>6</sub> (<b>2</b>), [Rh<sup>II</sup><sub>2</sub>(OAc)<sub>4</sub>(<b>L</b>)<sub>2</sub>]2PF<sub>6</sub> (<b>3</b>), and [Rh<sup>III</sup>Cl<sub>4</sub>(<b>L</b>)<sub>2</sub>]PF<sub>6</sub> (<b>4</b>) were synthesized through
the complexation of Rh<sup>I</sup>(acac)(CO)<sub>2</sub>, Rh<sup>II</sup><sub>2</sub>(OAc)<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>, and Rh<sup>III</sup>Cl<sub>3</sub>·3H<sub>2</sub>O with the phosphine-functionalized
ionic liquid (FIL) <b>1</b> ([<b>L</b>]PF<sub>6</sub>, <b>L</b> = 1-butyl-2-diphenylphosphino-3-methylimidazolium), respectively.
The cation of <b>L</b> in <b>1</b> is an imidazolium-substituted
phosphine with a positive charge vicinal to the P(III) atom, which
acts as an electron-deficient donor with π-acceptor character
to afford the stable complexes <b>2</b>–<b>4</b> due to the presence of retrodonating π-binding between Rh–P
linkage. Due to the weakened reducing ability of <b>L</b>, the
redox reaction between <b>L</b> and RhCl<sub>3</sub>·3H<sub>2</sub>O during the complexation is avoided, leading to the formation
of <b>4</b>, in which the Rh center is in the +3 valence state.
Single-crystal X-ray analyses show that <b>2</b>–<b>4</b> are all composed of a Rh-centered cation and a PF<sub>6</sub><sup>–</sup> counteranion. The cation of <b>2</b> possesses
structural similarity to Rh<sup>I</sup>(acac)(CO)(PPh<sub>3</sub>),
the cation of <b>3</b> with a <i>D</i><sub>4<i>h</i></sub> geometry possesses a structural similarity to Rh<sup>II</sup><sub>2</sub>(OAc)<sub>4</sub>(PPh<sub>3</sub>)<sub>2</sub>, and the cation of <b>4</b> exhibits an ideal Rh<sup>III</sup>-centered octahedral geometry, in which the Rh(III) (d<sup>6</sup>) ion is six-coordinated by four chlorine atoms in the equatorial
plane and two <b>L</b> ligands in the axial positions. TG/DTG
analyses indicated that the thermal stabilities of <b>2</b>–<b>4</b> in air flow were improved dramatically in comparison to
the corresponding analogues Rh<sup>I</sup>(acac)(CO)(PPh<sub>3</sub>), Rh<sup>II</sup><sub>2</sub>(OAc)<sub>4</sub>(PPh<sub>3</sub>)<sub>2</sub>, and Rh<sup>I</sup>Cl(PPh<sub>3</sub>)<sub>3</sub>. <b>2</b>–<b>4</b> were found to be good to excellent
catalysts for homogeneous hydroformylation of 1-octene free of any
auxiliary ligand; <b>3</b> was the best candidate. The “on
water” effect in rate acceleration was evidently observed over <b>2</b> and <b>4</b> due to their insensitivity to moisture
and oxygen
