Synthesis and characterization of novel Rh-NHC complexes: addition of phenylboronic acid to aldehydes, DFT/TD-DFT, and QTAIM studies

Herein, three new Rh(I)-NHC complexes (2a-c) were synthesized by reacting benzimidazolium salts with the [Rh(OMe)(COD)]₂ compound. The structures of the synthesized compounds were elucidated by various spectroscopic methods such as ¹H, ¹³C NMR, FT-IR, and UV–vis spectroscopies, and the elemental analysis technique. A thorough theoretical investigation was conducted to explore the electronic structure, reactivity, donor-acceptor interactions, and thermodynamic properties of newly synthesized Rh-NHC complexes. To this end, Density Functional Theory (DFT) calculations were utilized. The findings reveal that the 3,4,5-trimethoxybenzyl substitution contributes to the formation of a more stable and rigid structure in the Rh complex. Additionally, TD-DFT calculations were performed to assess the potential electronic transitions of the complexes 2a-c within a chloroform environment. It is possible to interpret the observed peaks of the three complexes as predominantly associated with intra-ligand π → π* transitions and metal-ligand charge-transfer (MLCT) transitions. The quantum theory of atoms in molecules (QTAIM) was employed to facilitate comprehension of bond characteristics and intermolecular interaction strengths in 2a-c complexes. Furthermore, the complexes were evaluated in the reaction of phenylboronic acid to aldehydes, and satisfactory yields were obtained for the desired products.