N-heterocyclic carbenes (NHCs) have emerged as an elite class of ligands due to their strong -donation ability, straightforward synthesis, and the availability of simple approaches for fine-tuning their steric as well as electronic properties when compared to other donor containing ligand systems. The transition metal complexes of NHC ligands are well-explored and importantly, established to be efficient catalysts in diverse organic transformations.1 Even though the metal-NHC complexes have been developed to be effective catalysts, studies toward the impact of stereoelectronic tuning of the ancillary NHC ligands in their catalytic activity are relatively less-explored although they provide vital inputs for the designing of future efficient catalysts. To study such ligand tuning effect in catalysis, “hydride transfer” reaction which is known to be a crucial step in many important catalytic reactions such as hydrogenation, hydro-functionalizations, H2 storage/release, olefin isomerization etc. is an obvious choice to the researchers.2 Among late-transition metals, ruthenium(II)-based NHC complexes constitute an effective group of catalysts for various organic transformations including the hydride transfer reactions.3 Further, the cyclometalated ruthenium(II)-NHC complexes are recognized as one of the most productive class of catalysts among the ruthenium(II)−NHC complexes reported till date.4 Properties and consequently the catalytic activity of the cyclometalated ruthenium(II)-NHC complexes can be tuned by altering their stereoelectronic profiles which include changing of the chelating sites, substituents present in the NHC backbone, size of the chelating ring etc. In this direction, wingtip directed formations of cyclometalated ruthenium(II)-NHC complexes are known, which show substantial differences in their catalytic activities depending on the cyclometalation patterns.5 Further, stereoelectronic tuning of the cyclometalated ruthenium(II)-NHC complexes which are key starting materials for the synthesis of photocatalysts also help to find better catalysts for photocatalysis reactions of interest.6 In addition to the usefulness of the cyclometalated NHC complexes as effective catalysts in various functional group transformations, it is also established that this type of complexes function as intermediates in diverse catalytic conversions of the azolium salts to functionalized compounds.7 In this presentation, starting with the stereoelectronic tuning of the cyclometalated ruthenium(II)-NHC complexes followed by their catalytic application will be discussed in details. Real Money Rummy proposal and some preliminary results as well as future plans will also be presented.
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