Cationic tris N-heterocyclic carbene rhodium carbonyl complexes: Molecular structures and solution NMR studies

The cationic N-heterocyclic carbene (NHC) rhodium complexes [Rh(NHC)(3)(CO)](+) (NHC = ICy (1), (IPr2Me2)-Pr-i (2)) have been isolated from the reactions of RhH(PPh3)(3)(CO) with the free NHCs. The hexafluorophosphate salts of both compounds, 1[PF6] and 2[PF6], have been characterized by X-ray diffraction. The observed temperature dependences of the H-1 NMR spectra for 1[PF6] and 2[PF6] are a consequence of restricted rotation associated with the three Ph-C-NHC bonds. Line shape analyses from the NMR studies on 1[PF6], 1[BAr4F], and 2[PF6] (BAr4F = B(3,5-C6H3(CF3)(2))(4)) afford activation barriers for the two trans-positioned Rh-C-NHC bonds of 35, 38, and 40 kJ mol(-1), respectively. Pulsed-gradient spin-echo (PGSE) NMR measurements show that there is only a relatively small amount of ion pairing for these salts in dichloromethane solution. H-1-F-19 HOESY data help to place the anions relative to the cations. Preliminary mechanistic studies on the formation of 1 and 2 suggest a role for neutral dinuclear precursors, as revealed by the reaction of (PPh3)(2)Rh(mu-CO)(2)Rh((IPr2Me2)-Pr-i)(2) with ICy, which affords the structurally characterized mixed NHC complex [Rh((IPr2Me2)-Pr-i)(1)(ICy)(CO)][PF6] (3[PF6]).