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

Abstract

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( i PrN)2C (10), C10H6( i PrN)(Me3CCH2N)C (11), {C10H6[N(3,5-Me2C6H3)]2C}2 (13), and C10H6( i PrN)(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 %V bur measurements, and their exceptional electron-donating ability.

Quinobis(imidazolylidene): Synthesis and Study of an Electron‐Configurable Bis(N‐Heterocyclic Carbene) and Its Bimetallic Complexes

“…The Rh1 À C1 distance of 2.033(2) in 4 a is within the range of rhodium-NHC bond lengths observed in analogous BBI complexes (2.003-2.073 ) [20] and monometallic NHCsupported variants (2.00-2.09 ). [22,57,59,90,92,[94][95][96] In addition, the average cis and trans rhodium-olefin distances in 4 a (2.115 and 2.211 , respectively) are comparable to those observed in the BBI-based variants (2.128-2.190 and 2.110-2.207 for cis and trans positions, respectively) and consistent with the Rh-cod bond lengths in monometallic NHCsupported analogues (2.09-2.23 ). [22,57,59,90,92,[94][95][96] Furthermore, the bite angle of the cod ligand in 4 a (86.…”

“…www.chemeurj.org well to the range of Rh À CO (1.84-1.88 and 1.90-1.91 for the cis and trans positions, respectively) and C À O (1.065(9) and 1.113 (8) , respectively) distances observed in similar complexes supported by NHCs. [95,96] Given that the structural features of [RhA C H T U N G T R E N N U N G (cod)Cl] or [Rh(CO) 2 Cl] fragments coordinated to 1 agree well with those bound by other NHCs, we conclude that the Rh-NHC interactions are comparable. Collectively, the spectral and structural similarities between 4 and 5 as well as other NHC-supported [M-A C H T U N G T R E N N U N G (cod)Cl] and [M(CO) 2 Cl] complexes indicate that the quinone linker does not perturb the fundamental coordination chemistry of the two imidazolylidene units.…”

“…The Rh1ÀC1 distance in 5 a (2.067 (4) ) is within the range of values reported for other NHC-supported [Rh(CO) 2 Cl] complexes (2.06-2.09 ). [95,96] Furthermore, the rhodium-carbonyl and C À O bond lengths measured cis (1.787(7) and 1.065(9) , respectively) and trans (1.890(5) and 1.113 (8) , respectively) to the NHCs in 5 a compare (7), N2ÀC1 1.351(6), S1ÀC5 1.706(6), C1ÀC5 1.486 (7); N1-C1-N2 108.0(4), S1-C5-N3 136.5(4), N1-C1-C5-S1 88.7(6), C25-C24-N3-C5 16.5(10). www.chemeurj.org well to the range of Rh À CO (1.84-1.88 and 1.90-1.91 for the cis and trans positions, respectively) and C À O (1.065(9) and 1.113 (8) , respectively) distances observed in similar complexes supported by NHCs.…”

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

“…E. Hahn and M. C. Jahnke Reviews 3146 www.angewandte.org (Scheme 63). [224] The first stable carbene with a six-membered heterocycle 128 a was prepared by Alder et al according to Scheme 63. [83a] Richeson et al described a similar carbene with a six-membered heterocycle 128 b, [225] which is derived from diaminonaphthalene. The carbenes 128 a and 128 b do not dimerize to the entetraamines under standard conditions.…”

Synthesis of New C2-Symmetric Six-Membered NHCs and Their Application for the Asymmetric Diethylzinc Addition of Arylaldehydes

“…To date, most of the research has employed five-membered NHCs based on imidazole or imidazoline. The so-called "expanded-ring" NHCs with six- [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] and seven- [38][39][40][41][42][43][44][45][46][47][48][49] heterocyclic rings have recently attracted attention, as these "non-standard" NHCs show quite different properties, such as stronger basicity (nucleophilicity) and greater steric demand [50]. Structurally, the larger ring sizes of these unusual NHCs will lead to a comparatively large N-CNHC-N angle and consequential smaller C NHC -N-C R angle, which in turn results in better protection of metal centers and subsequently better performance in catalysis.…”