The transition metal complexes of N-heterocyclic carbenes (NHCs) have gained increased momentum since their first use in the design of homogeneous catalysts.1a The unique strong σ-donor nature and the ease of synthesis along with various topological modifications through back bone make them new genre of ancillary ligands.1b Recently, 1,2,3-triazol-5-ylidenes (mesoionic carbenes, MICs) have emerged as a new subclass of abnormal carbenes (aNHCs) which are easily accessible from the triazolium salts formed via click reaction between alkyne and azide.2 It is well known that the chelated ligands provide very high thermal stability as well as appropriate rigidity to their metal complexes which possibly enhance the activity in catalysis. In this regard, metal complexes comprising of chelated N-heterocyclic carbene ligands such as classical (bis)NHC, ImNHC with secondary heteroatom donors and the pincer analogues are well exploited for their potential catalytic activity in diverse organic transformations.2b Recently, research towards the synthesis of special class of chelating ligands incorporating two different NHC donors, possessing two distinct unique -donor/-acceptor properties, on a single platform has been emerged. These heteroditopic NHC ligands offer easy tunability of the electronic properties due to their distinctly different donor strengths as well as the steric properties because of numerous possibilities for the alteration of substitution patterns. It has been observed previously that the metal complexes derived from these ligands exhibit tunable activity in diverse hydride transfer reactions.2 These observations suggest that analogous metal complexes will be successful catalysts for other functional group transformations also. The metal catalyzed transformation of alcohols, cheap and highly abundant from diverse sustainable resources, into numerous value-added chemicals via borrowing hydrogen (BH) reactions have gained significant attentions over the last decade. In this direction, owing to their remarkable activity, metal-NHC complexes catalyzed BH reactions have become a fast-developing research area.3 In this presentation, synthesis and different approaches for fine-tuning the stereoelectronic properties of the M-NHC complexes towards the formation of various C-C and C-N bonds via BH methodology2c,4 will be discussed along with the supported mechanistic insights.
References:
1. (a) Herrmann, W. A.; Elison, M.; Fischer, J.; Kocher, C.; Artus, G. R. J. Angew. Chem. Int. Ed., 1995, 34, 2371. (b) Nelson, D. J.; Nolan, S. P. Chem. Soc. Rev. 2013, 42, 6723.
2. (a) Sluijter, S. N.; Elsevier, C. J. Organometallics 2014, 33, 6389. (b) Illam, P. M.; Donthireddy, S. N. R.; Chakrabartty S.; Rit, A. Organometallics 2019, 38, 2610. (c) Donthireddy, S. N. R.; Illam P. M.; Rit, A. Inorg. Chem. 2020, 59, 1835.
3. Huang, M.; Liu, J.; Li, Y.; Lan, X.-B.; Su, P.; Zhao C.; Ke, Z. Catal. Today 2021, 370, 114.
4. Donthireddy, S. N. R.; Pandey, V. K.; Rit, A. J. Org. Chem. 2021, 86, 6994.