Crystalline Anions Based on Classical N‐Heterocyclic Carbenes
Arne Merschel, Dennis Rottschäfer, Beate Neumann, Hans‐Georg Stammler, Mark R. Ringenberg, Maurice van Gastel, T. Ilgin Demirer, Diego M. Andrada, Rajendra S. Ghadwal
Abstract
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‐ i Pr 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 (Me−IPr Bp )Br ( 2 c ) with KC 8 . The cyclic voltammograms of 2 a–2 c exhibit two one‐electron reversible redox processes in −0.5 to −2.5 V region that correspond to the radicals 3 a–3 c and the anions ( 4 a–4 c ) − . Computational calculations suggest a closed‐shell singlet ground state for ( 4 a–4 c ) − with the singlet‐triplet energy gap of 17–24 kcal mol −1 .