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(SiMe 3 ) 2 led either to deprotonation and generation of new carbenes or to enetetramines. In addition to spectroscopic characterization, crystallographic analysis of C 10 H 6 ( i PrN) 2 C ( 10 ), C 10 H 6 ( i PrN)(Me 3 CCH 2 N)C ( 11 ), {C 10 H 6 [N(3,5-Me 2 C 6 H 3 )] 2 C} 2 ( 13 ), and C 10 H 6 ( i PrN)(3,5-Me 2 C 6 H 3 N)C ( 14 ) definitively confirmed the nature of these species. The mixed benzyl/cycloheptyl-substituted carbene C 10 H 6 ( cyclo- C 7 H 13 N)(p-MeC 6 H 4 CH 2 N)C ( 17 ) was observed to undergo dimerization upon heating to yield both cis and trans isomers of the enetetramine {C 10 H 6 ( cyclo- C 7 H 13 N)( p- MeC 6 H 4 CH 2 N)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−C carbene bond rotation and through % V bur measurements, and their exceptional electron-donating ability.