The air stable NHC IPrBr is reported. A stereoelectronic study of IPrBr and its similarly stable relative IMesBr demonstrates metal complex specific changes in NHC donicity versus the ubiquitous IPr and IMes. Application to a Suzuki coupling and an iridium transfer hydrogenation gives superior outcomes using IPrBr and IMesBr.

The preparation of a 1,3-bis(2,6-terphenyl)imidazol-2-ylidene, IDitop (Ditop = 2,6-bis(4-tolyl)phenyl-) is reported. Several late transition metal complexes as well as the selenium adduct of IDitop were prepared to study and compare the stereoelectronic properties of IDitop to other well-known N-heterocyclic carbenes (NHCs). A study of the steric character of IDitop taken from crystal structure data of the complexes and adducts reveals a high degree of flexibility. Preliminary application to challenging Suzuki coupling reactions was undertaken.