Rhodium(III) peroxo complexes containing carbene and phosphine ligands

The Rh(I) carbene precursors [RhCl(COE)(NHC)](2), where the N-heterocyclic carbene is 1,3-bis(2,6-diisopropylphenyl) imidazol-2-ylidene (IPr) or 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), were used to synthesize the RhCl(NHC)(P-N) complexes 4 (NHC = IPr) and 5 (NHC = IMes), where P-N is P, N-chelated o-(diphenylphosphino)-N, N-dimethylaniline, and the corresponding cis-RhCl(NHC)(PPh3)(2) complexes 6 and 7. The synthesis of 4 surprisingly requires the reaction to be carried out under a hydrogen atmosphere and occurs via the intermediate dihydride RhCl(H)(2)(IPr)(P-N) (3). Complexes 4-7 in benzene readily undergo irreversible oxidative addition of O-2 to form the corresponding Rh(III) peroxide complexes 9-12. For comparative purposes, RhCl(PPh3)(P-N) (8) was synthesized from RhCl(PPh3)(3), and this also added O-2 to form a peroxo complex (13). All of the complexes were generally characterized by elemental analysis and H-1, P-31{H-1}, and C-13{H-1} NMR and IR spectroscopies and, in the cases of 9, 10, and 13, by X-ray crystallography.