Gaggioli Carlo Alberto, Bistoni Giovanni, Ciancaleoni Gianluca, Tarantelli Francesco, Belpassi Leonardo, Belanzoni Paola
Dipartimento di Chimica, Biologia e Biotecnologie, Università degli Studi di Perugia, Via Elce di Sotto 8, 06123, Perugia, Italy.
Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, 45470, Mülheim an der Ruhr, Germany.
Chemistry. 2017 Jun 1;23(31):7558-7569. doi: 10.1002/chem.201700638. Epub 2017 May 15.
In view of their intensive use as ligands in many reactions catalyzed by transition-metal complexes, modulation of the bonding properties of N-heterocyclic carbenes (NHCs) on a rational basis is highly desirable, which should enable optimization of current applications or even promote new functions. In this paper, we provide a quantitative analysis of the chemical bond between a metal fragment AuCl and a series of 29 different NHCs in [(NHC)AuCl] complexes. NHCs electronic properties are modified through: i) variation of the groups attached to the NHC nitrogen atoms or backbone; ii) change of unsaturation/size of the NHC ring; iii) inclusion of paracyclophane moieties; or iv) heteroatom substitution on the NHC ring. For evaluating the donation and back-donation components of the Dewar-Chatt-Duncanson (DCD) model in the NHC-AuCl bond, we apply the charge-displacement (CD) analysis within the NOCV (natural orbitals for chemical valence) framework, a methodology that avoids the constraint of using symmetrized structures. We show that modulation of the NHC bonding properties requires substantial modification of their structure, such as, for instance, insertion of two ketone groups into the NHC backbone (which enhances the π back-donation bond component and introduces an effective electronic communication within the NHC ring) or replacement of a nitrogen atom in the ring with an sp or sp carbon atom (which increases and decreases the π back-donation bond component, respectively). We extend our investigation by quantitatively comparing the NHC electronic structures for a subset of 13 NHCs in [(NHC)PPh] adducts, the P NMR chemical shift values of which are experimentally available. The latter have been considered as a suitable tool for measuring the NHCs π acceptor properties [Bertrand et al., Angew. Chem. Int. Ed. 2013, 52, 2939-2943]. We show that information obtained using the metal fragment can be transferred to the PPh moiety and vice versa. However, the P NMR chemical shift values only qualitatively correlate with the π acceptor properties of the NHCs, with the stronger π acidic carbenes as the most outliners.
鉴于氮杂环卡宾（NHCs）在许多由过渡金属配合物催化的反应中被大量用作配体，基于合理的基础对其键合性质进行调控是非常必要的，这将有助于优化当前的应用，甚至促进新功能的开发。在本文中，我们对[(NHC)AuCl]配合物中金属片段AuCl与一系列29种不同的NHCs之间的化学键进行了定量分析。通过以下方式改变NHCs的电子性质：i）连接在NHC氮原子或主链上的基团的变化；ii）NHC环的不饱和程度/大小的改变；iii）对环番部分的引入；或iv）NHC环上的杂原子取代。为了评估NHC-AuCl键中Dewar-Chatt-Duncanson（DCD）模型的给予和反馈给予成分，我们在NOCV（化学价自然轨道）框架内应用电荷位移（CD）分析，这是一种避免使用对称结构限制的方法。我们表明，对NHC键合性质的调控需要对其结构进行实质性的修改，例如，在NHC主链中插入两个酮基（这增强了π反馈给予键成分，并在NHC环内引入了有效的电子通讯）或用一个sp或sp碳原子取代环中的一个氮原子（这分别增加和减少了π反馈给予键成分）。我们通过定量比较[(NHC)PPh]加合物中13种NHCs子集的NHC电子结构来扩展我们的研究，其磷核磁共振化学位移值可通过实验获得。后者被认为是测量NHCs π受体性质的合适工具[Bertrand等人，《德国应用化学》2013年，52卷，2939 - 2943页]。我们表明，使用金属片段获得的信息可以转移到PPh部分，反之亦然。然而，磷核磁共振化学位移值仅与NHCs的π受体性质存在定性关联，其中较强的π酸性卡宾最为突出。
