Mono‐ and Bis(imidazolidinium ethynyl) Cations and Reduction of the Latter To Give an Extended Bis‐1,4‐([3]Cumulene)‐p‐carboquinoid System

Abstract

An extended π-system containing two [3]cumulene fragments separated by a p-carboquinoid and stabilized by two capping N-heterocyclic carbenes (NHCs) has been prepared. Mono- and bis(imidazolidinium ethynyl) cations have also been synthesized from the reaction of an NHC with phenylethynyl bromide or 1,4-bis(bromoethynyl)benzene. Cyclic voltammetry coupled with synthetic and structural studies showed that the dication is readily reduced to a neutral, singlet bis-1,4-([3]cumulene)-p-carboquinoid as a result of the π-accepting properties of the capping NHCs.

Kekulé diradicaloids derived from a classical N-heterocyclic carbene

“…7 a Similar observations have been also made in many other closed-shell compounds reported recently. 11a , 14 , 23 Baumgarten 6 c addressed this issue in a very recent report that concludes that the weak intensity triplet expected for CH can be masked under even ≈0.1% of mono-radical impurity.…”

“… 12 Remarkably, during the preparation of this manuscript for submission, we came across three consecutive reports dealing with the synthesis of extended cumulenes IV supported by CAACs 13 or a classical N-heterocyclic carbene (NHC). 14 Very recently, we reported stable crystalline radicals V derived from classical NHCs. 15 While these reports nicely emphasize the relevance of singlet carbenes in the synthesis of carbon-centered π-conjugated systems and radicals, synthetic access to diradicaloids VI , which are NHC-analogues of I , remained a challenge.…”

How to Bend a Cumulene

“…There are plenty of [3]cumulenes that have been synthesized, but just a small fraction of them have been reported with a C‐C‐C angle of less than 170°, with 165° being the smallest up to now . This absence of perfect linearity can be attributed to steric repulsions in the crystal, but some theoretical calculations have shown that this slightly bent structure persists or that the bending potential is very shallow in the gas phase . This shallow potential energy surface could be attributed to other resonant structures, like the ones responsible for the bent structures of the heavier E 2 L 2 compounds.…”

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.…”