Design of Sterically Demanding, Electron-Rich Carbene Ligands with the Perimidine Scaffold

A family of N,N‘-disubstituted perimidinium cation salts were employed as precursors to persistent
monomeric carbenes with novel molecular architectures and electronic structures. Depending on the
substituents, reaction of these 1,3-disubstituted perimidinium cations with LiN(SiMe3)2 led either to
deprotonation and generation of new carbenes or to enetetramines. In addition to spectroscopic
characterization, crystallographic analysis of C10H6(iPrN)2C (10), C10H6(iPrN)(Me3CCH2N)C (11), {C10H6[N(3,5-Me2C6H3)]2C}2 (13), and C10H6(iPrN)(3,5-Me2C6H3N)C (14) definitively confirmed the nature of
these species. The mixed benzyl/cycloheptyl-substituted carbene C10H6(cyclo-C7H13N)(p-MeC6H4CH2N)C
(17) was observed to undergo dimerization upon heating to yield both cis and trans isomers of the
enetetramine {C10H6(cyclo-C7H13N)(p-MeC6H4CH2N)C}2, (17)2. Rhodium complexes of these perimidine-based carbenes were accessed via reactions with either monomeric carbene or enetetramine. Spectroscopic
and crystallographic analysis of these rhodium−carbene complexes revealed the sterically demanding
nature of the carbene ligands, which is manifested in the observation of hindered Rh−Ccarbene bond rotation
and through %Vbur measurements, and their exceptional electron-donating ability.