Department of Chemistry, University of Fribourg, Chemin du Musée 9, 1700 Fribourg, Switzerland.
Chemistry. 2009 Sep 21;15(37):9375-86. doi: 10.1002/chem.200900249.
Sterically similar palladium dicarbene complexes have been synthesized that comprise permethylated dicarbene ligands which bind the metal center either in a normal coordination mode via C2 or abnormally via C4. Due to the strong structural analogy of the complexes, differences in reactivity patterns may be attributed to the distinct electronic impact of normal versus abnormal carbene bonding, while stereoelectronic effects are negligible. Unique reactivity patterns have been identified for the abnormal carbene complexes, specifically upon reaction with Lewis acids and in oxidative addition-reductive elimination sequences. These reactivities as well as analytical investigations using X-ray diffraction and X-ray photoelectron spectroscopy indicate that the C4 bonding mode increases the electron density at the metal center substantially, classifying such C4-bound carbene ligands amongst the most basic neutral donors known thus far. A direct application of this enhanced electron density at the metal center is demonstrated by the catalytic H(2) activation with abnormal carbene complexes under mild conditions, leading to a catalytic process for the hydrogenation of olefins.
已经合成了具有空间相似性的钯二卡宾配合物，其中包含通过 C2 以正常配位模式或通过 C4 以异常配位模式结合金属中心的全甲基化二卡宾配体。由于配合物具有很强的结构相似性，反应性模式的差异可能归因于正常和异常卡宾键合的明显电子影响，而立体电子效应可以忽略不计。异常卡宾配合物具有独特的反应性模式，特别是在与路易斯酸反应和氧化加成-还原消除序列中。这些反应性以及使用 X 射线衍射和 X 射线光电子能谱的分析研究表明，C4 键合模式大大增加了金属中心的电子密度，将这种 C4 键合的卡宾配体归类为迄今为止已知的最碱性中性供体之一。金属中心电子密度的这种增强的直接应用是通过异常卡宾配合物在温和条件下催化 H2 活化来证明的，导致烯烃加氢的催化过程。
