Stereoelectronic Properties of 1,2,4-Triazole-Derived N-heterocyclic Carbenes - A Theoretical Study

Abstract

A theoretical study on stereo and electronic properties of a series of six 1,2,4-triazole-derived carbenes bearing different N4-substituents, namely isopropyl (1), benzyl (2), phenyl (3), mesityl (4), 2,6-diisopropylphenyl (5) and 1-naphthyl (6), has been carried out. Structures of the six carbenes were first optimized using Gaussian® 16 at B3LYP level. Their molecular geometries and electronic structures of the frontier orbitals were examined. The results suggest the similarity in nature of their HOMOs, which all posses s symmetry with respect to the heterocycle and essentially be the lone electron pair on the Ccarbene. Steric properties of the NHCs was also quantified using percent volume burried (%Vbur) approach. The NHC 1 with isopropyl N4-substituent was the least bulky one with %Vbur of 27.7 and the most sterically demanding carbene is 6, which has large 2,6-diisopropylphenyl substituent (%Vbur = 38.4). Interestingly, the NHCs with phenyl and 1-naphthyl N4-substituents display flexible steric hindrance due to possible rotation of the phenyl or 1-naphthyl around the N-C single bond. Beside stereoelectronic properties of the NHC, topographic steric map of their complexes with metal were also investigated. Keywords: N-heterocyclic carbene, triazolin-5-ylidene, stereoelectronic properties, percent volume burried. References [1] D. Bourissou, O. Guerret, F.P. Gabbaï, G. Bertrand, Stable Carbene, Chem. Rev. 100 (2000) 39−92. https://doi.org/10.1021/cr940472u.[2] N. Marion, S.P. Nolan, Well-Defined N-Heterocyclic Carbenes-Palladium(II) Precatalysts for Cross-Coupling Reactions, Acc. Chem. Res. 41 (2008) 1440−1449. https://doi.org/10.1021/ar800020y. [3] F.E. Hahn, M.C. Jahnke, Heterocyclic carbenes: synthesis and coordination chemistry, Angew. Chem., Int. Ed. 47 (2008) 3122−3172. http://doi. org/10.1002/anie.200703883. [4] M.N. Hopkinson, C. Richter, M. Schedler, F. 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