Quantifying the Electronic and Steric Properties of 1,3-Imidazole-Based Mesoionic Carbenes (iMICs)
Arne Merschel, Dennis Rottschäfer, Beate Neumann, Hans‐Georg Stammler, Rajendra S. Ghadwal
Abstract
1,3-Imidazole-based mesoionic carbenes (iMICs) are a very promising class of carbon-donor ligands in synthesis and catalysis. However, a systematic study of the quantification of stereoelectronic properties of iMICs is lacking. This is most likely due to the absence of rational synthetic routes to suitable iMIC compounds. Herein, we report the synthesis of the appropriate phosphinidene (iMICAr)PPh (iMICAr = :CCH(NDipp)2CAr; Ar = Ph (5a), DMP (5b); DMP = 4-Me2NC6H4; Dipp = 2,6-iPr2C6H3), selenium (iMICAr)Se (Ar = Ph (6a), DMP (6b)), and nickel carbonyl compounds (iMICAr)Ni(CO)3 (Ar = Ph (7a), DMP (7b)). The π-accepting property of iMICAr (Ar = Ph (2a), DMP (2b)) has been evaluated by 31P and 77Se NMR spectroscopic analyses of 5 and 6, respectively. The overall donor property of 2 has been assessed by determining the Tolman electronic parameter (TEP) of 7. Studies suggest that iMICAr (2) are stronger σ donors and weaker π acceptors compared to classical N-heterocyclic carbenes (NHCs). The steric profile of iMICAr (2) has been obtained by calculating the percentage buried volume (%Vbur) for (iMICAr)CuX (Ar = Ph, X = I (8a); Ar = DMP, X = Br (8b)).