Kekulé diradicaloids derived from a classical N-heterocyclic carbene

Abstract

Two-electron reduction of bis(1,3-imidazolinium) salts 2 and 3 with KC8 gives rise to stable diradicaloids 4 and 5, respectively. Calculations reveal a very low singlet–triplet energy gap ΔES–T for 5 (10.7 kcal mol–1), while ΔES–T for 4 (29.1 kcal mol–1) is rather large.

Saturated N‐Heterocyclic Carbene Based Thiele's Hydrocarbon with a Tetrafluorophenylene Linker

“…In fact, the c AAC moieties are excellent for stabilizing a variety of organic, main‐group and transition‐metal radical species, as demonstrated from the extensive studies by the groups of Bertrand, Roesky, Braunschweig, and others . In contrast, radicals derived from N‐heterocyclic carbenes (NHCs) are astoundingly small, although very recently NHC derivatives of Kekulé diradicaloids have been realized by Ghadwal and co‐workers –…”

Impact of the macrocyclic structure and dynamic solvent effect on the reactivity of a localised singlet diradicaloid with π-single bonding character

“…For example, the isolation and fundamental understanding of carbenes enable their practical application in chemical synthesis. [1][2][3][4][5][6][7][8][9][10][11][12][13] Although much effort was devoted to proving the existence and exploring the role of singlet diradicals, key intermediates in homolytic bond cleavage, [14][15][16][17][18][19][20][21] in chemical reactions over the last century, important aspects remain unexplored. Diradicaloids, singlet diradicals with closed-shell character, 22 have attracted signicant attention not only because of their reactivity, but also due to their unique properties, which stem from the borderline character between open-and closed-shell molecules.…”

Carbene derived diradicaloids – building blocks for singlet fission?

“…Further examples of cumulenes connected by two NHCs with saturated 39 , 40 and unsaturated 41 backbones were very recently reported. Equally, heterocyclic carbenes can stabilize cumulenes and carbon allotropes, 42 , 43 which show unique optoelectronic properties.…”