A Dual NMR Probe Approach to Understanding the Electronic Properties of N‐Heterocyclic Carbenes

Abstract

The electronic properties of a series of imidazolium-derived N-heterocyclic carbenes systematically substituted at the 4,5-positions were investigated through NMR analysis of their palladium and selenone derivatives. This combined approach provided information on both the σ-donating ability and the π-accepting ability of the carbenes, allowing correlation with the electronic nature of the substituents on the carbene and providing an understanding of the structural features that affect these properties; in principle, this allows design of related ligands with known electronic parameters. Along with a greater understanding of the electronic properties of this series of carbenes, the work allows a comparison of the utility of both NMR probes based on the information gained, and the differences in requirements for preparation and acquisition. With this in mind, the selenone probe is proposed as being able to provide a wide range of electronic information, along with being relatively easily prepared.

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

“… NHC 1 is a stronger electron donor than its analogue NHC 2 (TEP, alane, p K a ). NHC 1 is a stronger σ-electron donor than NHC 2 ; indeed, it is stronger than any 1,3-dimesitylimidazolin-2-ylidene reported 14 , 24 and is only exceeded by less bulky 1,3-diisopropyl- and 1,3-dimethylimidazolin-2-ylidenes 14 , 43 (HEP and 13 C– 77 Se coupling constant). NHC 1 is significantly less π-electron-accepting than NHC 2 .…”

“… b Data taken from ref ( 39 ). c Data reported in ref ( 24 ). d Of the corresponding azolium salt, data from refs ( 35 ) and ( 46 ).…”

“…The HEP has been used to investigate the electronic properties of a wide variety of NHCs. The preparation and characterization of the palladium complex 2[Pd] have been previously reported, 24 while complex 1[Pd] was prepared using the same method with spectroscopic data that matched that in the literature, 30 once the solvent referencing was considered. The SCXRD structure ( Figure 6 ) shows the expected trans - 1[Pd] complex.…”

“… 23 The second technique assesses the π-electron-accepting ability of the NHC in an NHC-selenium adduct by measuring the 77 Se NMR chemical shift of the selenium nuclei. We have previously compared and contrasted these techniques, including considerations around obtaining reliable data 24 and noting limitations of the methods. 25 …”

“… j On the spread that we have determined for imidazolin-2-ylidene-derived NHCs, 24 the span for 77 Se NMR chemical shifts is ca . 140 ppm, and 13 C– 77 Se NMR coupling is ca .…”