Abstract
The direct double carbenylation of 1,4-diiodobenzene and 4,4'-dibromobiphenyl with a classical N-heterocyclic carbene, SIPr (1) (SIPr = :C{N(2,6-iPr2C6H3)}2CH2CH2), by means of nickel catalysis gives rise to 1,3-imidazolinium salts [(SIPr)(C6H4)(SIPr)](I)2 (2) and [(SIPr)(C6H4)2(SIPr)](Br)2 (3) as off-white solids. Two-electron reduction of 2 and 3 with KC8 cleanly yields Kekulé diradicaloid compounds [(SIPr)(C6H4)(SIPr)] (4) and [(SIPr)(C6H4)2(SIPr)] (5), respectively, as crystalline solids. Structural parameters and DFT as well as CASSCF calculations suggest the closed-shell singlet ground state for 4 and 5. 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.
Highlights
IntroductionMolecules containing two unpaired electrons in two nearly degenerate molecular orbitals are called diradicals, which may have parallel (triplet) or antiparallel (singlet) spins. Depending on the interaction between unpaired electrons, singlet diradicals are further classi ed as open-shell (OS) and closed-shell (CS) singlets
Molecules containing two unpaired electrons in two nearly degenerate molecular orbitals are called diradicals, which may have parallel or antiparallel spins.1 Depending on the interaction between unpaired electrons, singlet diradicals are further classi ed as open-shell (OS) and closed-shell (CS) singlets
We reported stable crystalline radicals V derived from classical N-heterocyclic carbene (NHC)
Summary
Molecules containing two unpaired electrons in two nearly degenerate molecular orbitals are called diradicals, which may have parallel (triplet) or antiparallel (singlet) spins. Depending on the interaction between unpaired electrons, singlet diradicals are further classi ed as open-shell (OS) and closed-shell (CS) singlets. Diradicaloids are molecules with partial singlet diradical nature in their ground state.. Diradicaloids are molecules with partial singlet diradical nature in their ground state.2 In this context, Thiele's (TH) and Chichibabin's (CH) hydrocarbons I (Fig. 1), reported shortly a er Gomberg's discovery of the Ph3Cc radical in 1900, are noteworthy examples. Thiele's (TH) and Chichibabin's (CH) hydrocarbons I (Fig. 1), reported shortly a er Gomberg's discovery of the Ph3Cc radical in 1900, are noteworthy examples They can be described either as an OS diradical (Ia) featuring a phenylene or diphenylene linker or a CS p-quinodimethane (Ib). Since their rst isolation, the ground state spin state of I has been a subject of intense experimental and theoretical investigations, leading to a very controversial discussion, the so-called “diradical paradox”.7. CH is generally described as a diradicaloid (the resonance hybrid of 1a and 1b) with a signi cant diradical character, whereas TH is considered as a pquinodimethane
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