Cationic Tris N-Heterocyclic Carbene Rhodium Carbonyl Complexes: Molecular Structures and Solution NMR Studies

The cationic N-heterocyclic carbene (NHC) Rh complexes [Rh(NHC)3(CO)]+ (NHC = ICy (1), IiPr2Me2 (2)) were isolated from the reactions of RhH(PPh3)3(CO) with the free NHCs. The hexafluorophosphate salts of both compds., 1[PF6] and 2[PF6], were characterized by x-ray diffraction. The obsd. temp. dependences of the 1H NMR spectra for 1[PF6] and 2[PF6] are a consequence of restricted rotation assocd. with the three Rh-CNHC 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-CNHC bonds of 35, 38, and 40 kJ mol-1, resp. Pulsed-gradient spin-echo (PGSE) NMR measurements show that there is only a relatively small amt. of ion pairing for these salts in CH2Cl2 soln. 1H-19F 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)2Rh(micro -CO)2Rh(IiPr2Me2)2 with ICy, which affords the structurally characterized mixed NHC complex [Rh(IiPr2Me2)2(ICy)(CO)][PF6] (3[PF6]).