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),
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]).

阳离子型氮杂环卡宾（N-heterocyclic carbene, NHC）铑配合物[Rh(NHC)₃(CO)]⁺（其中NHC分别为ICy（1）、IiPr₂Me₂（2））可通过RhH(PPh₃)₃(CO)与游离NHC的反应分离得到。两种配合物的六氟磷酸盐（1[PF₆]与2[PF₆]）已通过X射线衍射（X-ray diffraction）完成结构表征。1[PF₆]与2[PF₆]的¹H核磁共振（Nuclear Magnetic Resonance, NMR）光谱呈现出显著的温度依赖性，该现象源于与三条Rh−CNHC键相关的旋转受限效应。针对1[PF₆]、1[BAr₄F]与2[PF₆]（其中BAr₄F = 四(3,5-双(三氟甲基)苯基)硼酸根，即B(3,5-C₆H₃(CF₃)₂)₄）的NMR谱线线型分析，分别得到了两个反位Rh−CNHC键的活化能垒，数值依次为35、38与40 kJ·mol⁻¹。脉冲梯度自旋回波（Pulsed-gradient spin-echo, PGSE）NMR测试结果表明，这些盐在二氯甲烷溶液中的离子缔合程度相对较低。¹H−¹⁹F异核Overhauser效应光谱（Heteronuclear Overhauser Effect Spectroscopy, HOESY）数据可用于确定阴离子相对于阳离子的空间排布。针对配合物1与2的形成机理初步研究表明，中性双核前驱体在反应中起到关键作用；例如(PPh₃)₂Rh(μ-CO)₂Rh(IiPr₂Me₂)₂与ICy的反应可生成经结构表征的混合NHC配合物[Rh(IiPr₂Me₂)₂(ICy)(CO)][PF₆]（3[PF₆]），这一结果佐证了上述推测。