Kekulé diradicaloids derived from a classical N-heterocyclic carbene

Area of Science:

• Organometallic Chemistry
• Carbene Chemistry
• Materials Science

Background:

• N-heterocyclic carbenes (NHCs) are versatile ligands in catalysis.
• Aryl halides are common substrates in cross-coupling reactions.
• Kekulé diradicaloids are fascinating molecules with unique electronic structures.

Purpose of the Study:

• To synthesize novel Kekulé diradicaloid compounds.
• To investigate the electronic properties of these diradicaloids.
• To explore the utility of NHC ligands in nickel-catalyzed carbenylation.

Main Methods:

• Nickel-catalyzed direct double carbenylation of 1,4-diiodobenzene and 4,4'-dibromobiphenyl using SIPr.
• Two-electron reduction of resulting imidazolinium salts with KC8.
• Structural characterization and DFT/CASSCF calculations.

Main Results:

• Synthesis of 1,3-imidazolinium salts [(SIPr)(C6H4)(SIPr)](I)2 and [(SIPr)(C6H4)2(SIPr)](Br)2.
• Generation of Kekulé diradicaloid compounds [(SIPr)(C6H4)(SIPr)] and [(SIPr)(C6H4)2(SIPr)].
• Computational analysis indicates closed-shell singlet ground states with varying singlet-triplet energy gaps.

Conclusions:

• Direct double carbenylation provides access to novel diradicaloid precursors.
• The synthesized diradicaloids exhibit closed-shell singlet ground states.
• The singlet-triplet energy gap is influenced by the molecular structure.