Stable Ionic Rh(I,II,III) Complexes Ligated by an Imidazolium-Substituted Phosphine with π‑Acceptor Character: Synthesis, Characterization, and Application to Hydroformylation

The stable ionic Rh­(I,II,III) complexes [Rh^I(acac)­(CO)­(<b>L</b>)]­PF₆ (<b>2</b>), [Rh^II₂(OAc)₄(<b>L</b>)₂]­2PF₆ (<b>3</b>), and [Rh^IIICl₄(<b>L</b>)₂]­PF₆ (<b>4</b>) were synthesized through the complexation of Rh^I(acac)­(CO)₂, Rh^II₂(OAc)₄(H₂O)₂, and Rh^IIICl₃·3H₂O with the phosphine-functionalized ionic liquid (FIL) <b>1</b> ([<b>L</b>]­PF₆, <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 RhCl₃·3H₂O 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 PF₆^– counteranion. The cation of <b>2</b> possesses structural similarity to Rh^I(acac)­(CO)­(PPh₃), the cation of <b>3</b> with a <i>D</i>_4<i>h</i> geometry possesses a structural similarity to Rh^II₂(OAc)₄(PPh₃)₂, and the cation of <b>4</b> exhibits an ideal Rh^III-centered octahedral geometry, in which the Rh­(III) (d⁶) 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^I(acac)­(CO)­(PPh₃), Rh^II₂(OAc)₄(PPh₃)₂, and Rh^ICl­(PPh₃)₃. <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

Stable Ionic Rh(I,II,III) Complexes Ligated by an Imidazolium-Substituted Phosphine with π‑Acceptor Character: Synthesis, Characterization, and Application to Hydroformylation

The stable ionic Rh­(I,II,III) complexes [Rh^I(acac)­(CO)­(<b>L</b>)]­PF₆ (<b>2</b>), [Rh^II₂(OAc)₄(<b>L</b>)₂]­2PF₆ (<b>3</b>), and [Rh^IIICl₄(<b>L</b>)₂]­PF₆ (<b>4</b>) were synthesized through the complexation of Rh^I(acac)­(CO)₂, Rh^II₂(OAc)₄(H₂O)₂, and Rh^IIICl₃·3H₂O with the phosphine-functionalized ionic liquid (FIL) <b>1</b> ([<b>L</b>]­PF₆, <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 RhCl₃·3H₂O 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 PF₆^– counteranion. The cation of <b>2</b> possesses structural similarity to Rh^I(acac)­(CO)­(PPh₃), the cation of <b>3</b> with a <i>D</i>_4<i>h</i> geometry possesses a structural similarity to Rh^II₂(OAc)₄(PPh₃)₂, and the cation of <b>4</b> exhibits an ideal Rh^III-centered octahedral geometry, in which the Rh­(III) (d⁶) 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^I(acac)­(CO)­(PPh₃), Rh^II₂(OAc)₄(PPh₃)₂, and Rh^ICl­(PPh₃)₃. <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