The Huynh’s Electronic Parameter (HEP)
The HEP provides information on the electronic influence of a trans-ligand L on the 13Ccarbene NMR signal of the iPr2-bimy reporter ligand. A stronger donor would lead to a relatively weaker palladium(II)-NHC bond, which in turn results in a downfield shift. Conversely, a weaker donor induces a highfield shift. The palladium(II) metal centre is insufficiently electron-rich to engage in notable backdonation to bound ligands. Therefore comparison of HEP values (ppm) allows for a ranking of ligands with regards to their sigma-donating ability. Another unique feature of the HEP is that comparison can be made between classical Werner-type ligands (N, O, S, halides) and organometallic ligands (carbenes, phosphines, etc) on a unified scale. Moreover, HEP has also been extended to the more challenging evaluation of bidentate ligands, i.e. HEP2. Currently, HEP values of ~70 monodentate and ~20 bidentate ligands have been reported in literature. It is our hope that the HEP can be used as a tool-box to help chemists understand and explain reactivity trends, which are often ligand-based. Research in our laboratory is on-going to further explore the scopes and also limitations of our parameter. [Read on]
Functionalization of NHCs
Stereoelectronic properties of NHCs can be fine-tuned by the choice of the N-substituents. An even greater structural and functional diversity can be achieved by introducing additional functions at one or both nitrogen atoms of NHC ligands. Traditionally, this can be done by functionalization of azolium salts as the most commonly used NHC precursors. These individually-modified salts are then subjected to metallation to give the desired functionalized NHC complex. This pre-functionalization approach is suitable for target-specific complexes, where the suitable functional group has already been identified. For the development of complex libraries, however, this conventional route is too time-consuming, too labour-intensive and overall noneconomic.
Instead, the divergent post-functionalization approach should be explored. Here, only one functionalized salt and its respective (parent) complex is prepared. A variety of functional groups can then be introduced on the same parent complex using simple organic transformations, e.g. nucleophilic substitutions, click chemistry, etc. to give a first “daughter” generation of functionalized NHC complexes. This alternative and sustainable approach has been demonstrated for up to three generations.
N-heterocyclic carbenes have made major impact as ancillary ligands in transition-mediated catalysis. Due to their nucleophilic character NHCs are also often directly employed as organocatalysts. We have employed some of complexes as catalyst precursors for classical C–C coupling reactions such as Mizoroki-Heck, Suzuki-Miyaura, Kumada-Corriu and Sonogashira couplings. Here it is of interest to use less hazardous or low cost solvents such as water or ionic liquids in line with green chemistry principles.
More challenging transformations such as C–H activation, hydroelementation and alcohol activation are also being pursued with the ultimate aim to improve carbon economy and sustainability.
Beyond classical NHCs
Most NHCs are derived from imidazole, benzimidazole, triazole and imidazoline. These are therefore referred to as classical NHCs. In recent years, more and more species derived from other heterocycles have been prepared that can be collectively called non-classical carbenes. For some of these, it is not even clear if they are still carbenes, and terms like mesoionic/abnormal and remote have been introduced to set them apart from their classical and well-understood counterparts. Nevertheless, study of these compounds will ensure new discoveries and create new knowledge.
The detailed study of NHC complexes and their structural preferences has led to a better understanding of certain intramolecular interactions. These include C–H···M anagostic (not agostic!) interactions, inter-ligand donation and transphobia effect. These weak interactions are somewhat new to the majority of chemists. Our interest in them is fundamentally grounded and driven by our curiosity driven search for new knowledge.