Steric Properties of N‐Heterocyclic Carbenes affect the Performance of Electronic Probes

Abstract

Electronic probes of ligands, particularly carbenes, are widely used in assessing electronic properties; the results inform the selection of a ligand for a given application. As such, it is important to ensure the data obtained is reliable and unaffected by other factors, such as the steric bulk of the ligand. The effects of such steric factors on two commonly used electronic probes (based on palladium and selenium) are investigated here, with the selenium adduct found to be particularly sensitive. It is hoped that this serves as a cautionary tale to always critically evaluate what a probe is measuring.

The Core Difference between a Mesoionic and a Normal N-Heterocyclic Carbene

“… 11 However, it is clear that these two properties can have complex interdependencies. 40 Some probe methodologies are strongly affected by the steric properties, 25 and care must be taken when assessing the data obtained using them. 24 The % V bur and steric maps both suggest that NHCs 1 and 2 are sufficiently similar such that the electronic probe results become free from steric interference.…”

“…The 77 Se NMR chemical shift for selone 1=Se is one of the highest fields of all selones reported, exceeded only by seven reported NHCs. 14 , 25 It is worth repeating that NHCs 1 and 2 are unique in that their steric properties are highly consistent, such that the 77 Se data are reliable for electronic comparisons. The reported values for sterically bulky NHCs, such as 1,3-diadamantylimidazolin-2-ylidene, are potentially not representative of the true π-electron-accepting abilities of the NHCs.…”

“…The reported values for sterically bulky NHCs, such as 1,3-diadamantylimidazolin-2-ylidene, are potentially not representative of the true π-electron-accepting abilities of the NHCs. 25 The relative 13 C– 77 Se NMR coupling constant indicates that NHC 1 has a significantly lower-energy HOMO than NHC 2 . 45 The 77 Se NMR chemical shift implies that NHC 1 has a moderately higher-energy LUMO than NHC 2 .…”

“…The reported values for sterically bulky NHCs, such as 1,3-diadamantylimidazolin-2-ylidene, are potentially not representative of the true π-electron-accepting abilities of the NHCs. 25 …”

“…We have previously compared and contrasted these techniques, including considerations around obtaining reliable data 24 and noting limitations of the methods. 25 …”