Crystalline Anions Based on Classical N‐Heterocyclic Carbenes

Abstract

Herein, the first stable anions K[SIPr Bp ] (4 a-K) and K[IPr Bp ] (4 b-K) (SIPr Bp = BpC{N(Dipp)CH 2 } 2 , IPr Bp = BpC{N(Dipp)CH} 2 ; Bp = 4-PhC 6 H 4 ; Dipp = 2,6-iPr 2 C 6 H 3 ) derived from classical N-heterocyclic carbenes (NHCs) (i.e. SIPr and IPr) have been isolated as violet crystalline solids. 4 a-K and 4 b-K are prepared by KC 8 reduction of the neutral radicals [SIPr Bp ] (3 a) and [IPr Bp ] (3 b), respectively. The radicals 3 a and 3 b as well as [Me-IPr Bp ] 3 c (Me-IPr Bp = BpC{N(Dipp)CMe} 2 ) are accessible as crystalline solids on treatment of the respective 1,3-imidazoli(ni)um bromides (SIPr Bp )Br (2 a), (IPr Bp )Br (2 b), and (MeÀ IPr Bp )Br (2 c) with KC 8 . The cyclic voltammograms of 2 a-2 c exhibit two oneelectron reversible redox processes in À 0.5 to À 2.5 V region that correspond to the radicals 3 a-3 c and the anions (4 a-4 c) À . Computational calculations suggest a closed-shell singlet ground state for (4 a-4 c) À with the singlet-triplet energy gap of 17-24 kcal mol À 1 .

1,3‐Imidazol‐basierte mesoionische Carbene und anionische Dicarbene: Erweiterung der Grenzen klassischer N‐heterocyclischer Carbene

“…Bemerkenswert ist, dass die Anzahl der stabilen Redox-Zustände sowie die E 1/2 -Werte von 2 a und verwandten Spezies durch die Variation der Art von NHC-und C2-Aryl-Gruppen weiter verändert werden können. [36] Die CVs der Bis-Imidazoli(ni)um-Salze 4 a-c (Schema 1) weisen zwei reversible Ein-Elektronen-Redox-Prozesse auf, die den Radikal-Kationen und Diradikalen entsprechen. Diese Verbindungen sind durch die Reduktion von 4 a-c als kristalline Festkörper zugänglich.…”

“…Chemische Reduktionen von 2 a – c liefern neutrale Radikale (NHC Ar )⋅ ( 8 a ) als kristalline Feststoffe. Bemerkenswert ist, dass die Anzahl der stabilen Redox‐Zustände sowie die E 1/2 ‐Werte von 2 a und verwandten Spezies durch die Variation der Art von NHC‐ und C2‐Aryl‐Gruppen weiter verändert werden können [36] . Die CVs der Bis‐Imidazoli(ni)um‐Salze 4 a – c (Schema 1) weisen zwei reversible Ein‐Elektronen‐Redox‐Prozesse auf, die den Radikal‐Kationen und Diradikalen entsprechen.…”

Ring‐Opening of 1,3‐Imidazole Based Mesoionic Carbenes (iMICs) and Ring‐Closing Clicks: Facile Access to iMIC‐Compounds

“…(IPrÀ Ar)X (IPrÀ Ar = 1-Ar = ArC{N(2,6-iPr 2 (C 6 H 3 ))CH} 2 ; Ar = C 6 H 5 (Ph), 4-NMe 2 C 6 H 4 (DMP), 4-PhC 6 H 5 (Bp); X = Cl or Br), [20,40] (7-Ar)X (7-Ar = ArC {N(2,6-iPr 2 (C 6 H 3 ))} 2 CHCSiMe 3 ), [26] [(Allyl)PdCl] 2 , [41] [Ir(cod)Cl] 2 , [42] and (Et 2 O) 2 BeBr 2 were prepared according to the reported methods. [43] Synthesis of N-Ethynylformimidamide (6-Ph): (1-Ph)Br (2.06 g, 3.78 mmol) was suspended in 15 mL THF and nBuLi (2.5 M in nhexane, 3.47 ml, 8.69 mmol) was added at room temperature (rt).…”

1,3‐Imidazole‐Based Mesoionic Carbenes and Anionic Dicarbenes: Pushing the Limit of Classical N‐Heterocyclic Carbenes

“…Remarkably, the number of stable redox states as well as the values of E 1/2 of 2 a and related species can be further manipulated by varying the type of NHCs and C2-aryl groups. [36] The CVs of bisimidazoli(ni)um salts 4 a-c (Scheme 1) show two oneelectron reversible redox processes correspond to radical cations and diradicals. These species are accessible as crystalline solids on reductions of 4 a-c. [26] Thus, the bis-iMICs derived from 4 a have a substantial potential as ditopic bridging ligands [37] for bimetallic complexes as well as promising building-blocks for advanced functional materials.…”

“…Chemical reductions of 2 a cleanly afford neutral radicals (NHC Ar )⋅ ( 8 a ) as crystalline solids. Remarkably, the number of stable redox states as well as the values of E 1/2 of 2 a and related species can be further manipulated by varying the type of NHCs and C2‐aryl groups [36] . The CVs of bis‐imidazoli(ni)um salts 4 a – c (Scheme 1) show two one‐electron reversible redox processes correspond to radical cations and diradicals.…”