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

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posted on 03.04.2020, 18:11 by Patrick Bazinet, Tiow-Gan Ong, Julie S. O'Brien, Nathalie Lavoie, Eleanor Bell, Glenn P. A. Yap, Ilia Korobkov, Darrin S. Richeson
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.