N‑Heterocyclic Carbene Complexes of Nickel, Palladium, and Iridium Derived from Nitron: Synthesis, Structures, and Catalytic Properties

The
mesoionic compound (1,4-diphenyl-1,2,4-triazol-4-ium-3-yl)­phenylazanide,
commonly referred to as Nitron, has been employed as a “crypto-NHC”
to afford 1,2,4-triazolylidene compounds of nickel, palladium, and
iridium. Specifically, Nitron reacts with NiBr2, PdCl2, and [Ir­(COD)­Cl]2 to afford the N-heterocyclic
carbene complexes (NitronNHC)2NiBr2, (NitronNHC)2PdCl2, and (NitronNHC)­Ir­(COD)­Cl, respectively. The lattermost compound reacts
with (i) CO to afford the dicarbonyl compound (NitronNHC)­Ir­(CO)2Cl and (ii) CO, in the presence of PPh3, to afford the monocarbonyl compound (NitronNHC)­Ir­(PPh3)­(CO)­Cl. Structural studies on (NitronNHC)­Ir­(COD)­Cl
and (NitronNHC)­Ir­(CO)2Cl indicate that NitronNHC has a stronger trans influence than does
Cl; furthermore, IR spectroscopic studies on (NitronNHC)­Ir­(CO)2Cl indicate that NitronNHC is electronically
similar to the structurally related Enders carbene but is less electron
donating than imidazol-2-ylidenes with aryl substituents. Significantly,
the NitronNHC ligand affords catalytic systems, as illustrated
by the ability of (NitronNHC)­Ir­(CO)2Cl to effect
(i) the dehydrogenation of formic acid, (ii) aldehyde hydrosilylation,
(iii) dehydrocoupling of hydrosilanes and alcohols, and (iv) ketone
reduction via transfer hydrogenation.

介离子化合物（mesoionic compound）(1,4-二苯基-1,2,4-三唑-4-鎓-3-基)苯基氨基阴离子，其俗名为Nitron，已被用作潜伏型N-杂环卡宾（crypto-NHC），用以制备镍、钯与铱的1,2,4-三唑卡宾类化合物。具体而言，Nitron分别与溴化镍(II)（NiBr₂）、氯化钯(II)（PdCl₂）以及[Ir(COD)Cl]₂反应，得到N-杂环卡宾配合物(NitronNHC)₂NiBr₂、(NitronNHC)₂PdCl₂与(NitronNHC)Ir(COD)Cl。

后述配合物（即(NitronNHC)Ir(COD)Cl）可与(i)一氧化碳反应，得到二羰基配合物(NitronNHC)Ir(CO)₂Cl；(ii)在三苯基膦（PPh₃）存在下与一氧化碳反应，得到单羰基配合物(NitronNHC)Ir(PPh₃)(CO)Cl。对(NitronNHC)Ir(COD)Cl与(NitronNHC)Ir(CO)₂Cl的结构研究表明，NitronNHC的反位效应强于氯负离子；此外，对(NitronNHC)Ir(CO)₂Cl的红外光谱（IR）研究显示，NitronNHC的电子特性与结构相关的恩德斯卡宾（Enders carbene）相似，但其供电子能力弱于带有芳基取代基的咪唑-2-亚基（imidazol-2-ylidenes）。

值得注意的是，NitronNHC配体可构建高效催化体系，例如(NitronNHC)Ir(CO)₂Cl可实现以下四类反应：(i)甲酸脱氢反应、(ii)醛的硅氢化反应、(iii)氢硅烷与醇的脱氢偶联反应，以及(iv)通过转移氢化实现的酮还原反应。