NHC-phosphane rhodium complexes and their reaction with oxygen

Abstract

The new homoleptic square planar Rh+I complexes [Rh(NHC-PPh2)2][X] (cis-2[X] and trans-2[X], X = Cl, PF6) with chelating phosphane N-heterocyclic carbene ligands were synthesized by reaction of [Rh(μ-Cl)(COD)]2 with the free ligand {1-[2-(diphenylphosphino)ethyl)]-3-ethylimidazol-2-ylidene)}. The ligand precursor was obtained by deprotonation of the imidazolium salts 1 {1-[2-(diphenylphosphino)ethyl)]-3-ethyl-imidazoliumiodide} 1[I] or hexafluorophosphate 1[PF6], respectively, with KN(SiMe3)2 in THF. Cis-2[Cl] and trans-2[Cl] coexist in a dynamic equilibrium which has been studied by temperature-dependent 31P NMR spectroscopy. cis-2[Cl] and trans-2[Cl] react with O2 at ambient temperature to afford the new cis and trans peroxo Rh+III complexes [Rh(η2-O2)(NHC-PPh2)2]Cl (cis-3[Cl] and trans-3b[Cl]). 31P NMR spectra prove that the bonding of O2 to cis-2[Cl] is reversible. In solution, cis-3[Cl] shows a fluxional behaviour as evidenced by 31P NMR spectra. The fluxionality is attributed to the flexibility of the chelating rings formed by the ligand and the metal centre. Cis-2[PF6] reacts with S8 to give the novel Rh+III complex [Rh(η2-S2)(NHC-PPh2)2][PF6] (cis-4[PF6]). Contrary to the analogous peroxo complex cis-3[Cl], no fluxionality could be observed in this case. All complexes have been characterised by 31P, 1H, 13C NMR spectroscopy and single-crystal X-ray analysis.