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
[RhI(acac)­(CO)­(L)]­PF6 (2), [RhII2(OAc)4(L)2]­2PF6 (3), and [RhIIICl4(L)2]­PF6 (4) were synthesized through
the complexation of RhI(acac)­(CO)2, RhII2(OAc)4(H2O)2, and RhIIICl3·3H2O with the phosphine-functionalized
ionic liquid (FIL) 1 ([L]­PF6, L = 1-butyl-2-diphenylphosphino-3-methylimidazolium), respectively.
The cation of L in 1 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 2–4 due to the presence of retrodonating π-binding between Rh–P
linkage. Due to the weakened reducing ability of L, the
redox reaction between L and RhCl3·3H2O during the complexation is avoided, leading to the formation
of 4, in which the Rh center is in the +3 valence state.
Single-crystal X-ray analyses show that 2–4 are all composed of a Rh-centered cation and a PF6– counteranion. The cation of 2 possesses
structural similarity to RhI(acac)­(CO)­(PPh3),
the cation of 3 with a D4h geometry possesses a structural similarity to RhII2(OAc)4(PPh3)2, and the cation of 4 exhibits an ideal RhIII-centered octahedral geometry, in which the Rh­(III) (d6) ion is six-coordinated by four chlorine atoms in the equatorial
plane and two L ligands in the axial positions. TG/DTG
analyses indicated that the thermal stabilities of 2–4 in air flow were improved dramatically in comparison to
the corresponding analogues RhI(acac)­(CO)­(PPh3), RhII2(OAc)4(PPh3)2, and RhICl­(PPh3)3. 2–4 were found to be good to excellent
catalysts for homogeneous hydroformylation of 1-octene free of any
auxiliary ligand; 3 was the best candidate. The “on
water” effect in rate acceleration was evidently observed over 2 and 4 due to their insensitivity to moisture
and oxygen.