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

Abstract

The cationic N-heterocyclic carbene (NHC) rhodium complexes [Rh(NHC)3(CO)]+ (NHC = ICy (1), IiPr2Me2 (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 1H NMR spectra for 1[PF6] and 2[PF6] are a consequence of restricted rotation associated 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, 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. 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(μ-CO)2Rh(IiPr2Me2)2 with ICy, which affords the structurally characterized mixed NHC complex [Rh(IiPr2Me2)2(ICy)(CO)][PF6] (3[PF6]).