Ferrocenyl-Imidazolylidene Ligand for Redox-Switchable Gold-Based Catalysis. A Detailed Study on the Redox-Switching Abilities of the Ligand

An
imidazolium salt with a fused benzoferrocenyl was synthesized.
This compound was used as
an N-heterocyclic carbene (NHC) precursor, and the related ferrocenyl-imidazolylidene
complexes Fc-NHC-MLn (MLn = IrCl­(COD), IrCl­(CO)2, AuCl) were synthesized and fully characterized, including
the crystallographic characterization of some of the key structures.
The oxidation of the iridium carbonyl compound Fc-NHC-IrCl­(CO)2 with acetylferrocenium tetrafluoroborate afforded the oxidized
ferrocenium-NHC-IrCl­(CO)2 (Fe­(III)) species; however the
isolated product contained a byproduct resulting from the protonation
of the starting Fe­(II) complex. The analysis of the electron-donating
character of the neutral ligand and the ligand resulting from the
oxidation was carried out by studying the variation of the ν­(CO)
stretching frequencies of Fc-NHC-IrCl­(CO)2 and its oxidized
analogue, revealing that this shift is 2.9 cm–1,
which is consistent with a decrease of the electron-donating character
of the ligand produced by the generation of a positively charged metal
complex. DFT calculations were carried out in order to rationalize
these results. The effects of the oxidation of the ligand in homogeneous
catalysis were tested by using a related ferrocenyl-imidazolylidene-gold­(I)
complex. In the hydroamination of terminal alkynes, the results indicated
that the oxidation of the ligand produced a moderate increase of the
activity of the gold catalyst. In the cyclization of alkynes with
furans, the neutral complex was not active, while the product resulting
from its oxidation produced moderate to good yields in the formation
of the final products.

本研究合成了一种带有稠合苯并二茂铁基的咪唑鎓盐。该化合物被用作氮杂环卡宾（N-heterocyclic carbene, NHC）前驱体，进而合成了相关二茂铁基咪唑亚基配合物Fc-NHC-MLn（其中MLn = IrCl(COD)、IrCl(CO)₂、AuCl），并对其进行了全面表征，部分关键结构完成了晶体学表征。
采用四氟硼酸乙酰二茂铁鎓对羰基铱配合物Fc-NHC-IrCl(CO)₂进行氧化，得到了氧化态二茂铁鎓-NHC-IrCl(CO)₂（Fe(III)）物种，但分离得到的产物中含有由起始Fe(II)配合物质子化产生的副产物。通过研究Fc-NHC-IrCl(CO)₂及其氧化态类似物的CO伸缩振动频率（ν(CO)）变化，对中性配体与氧化后配体的给电子能力进行了分析，结果显示频率偏移量为2.9 cm⁻¹，这与带正电金属配合物生成导致配体给电子能力下降的结论相符。
为合理化上述实验结果，本研究开展了密度泛函理论（DFT）计算。通过使用相关二茂铁基咪唑亚基金(I)配合物，考察了配体氧化对均相催化的影响。在端炔的氢胺化反应中，结果表明配体氧化可使金催化剂的活性适度提升；在炔烃与呋喃的环化反应中，中性配合物未表现出催化活性，而其氧化产物则可使目标产物获得中等至良好的收率。