Enhancing Diradical Character of Chichibabin's Hydrocarbon through Fluoride Substitution

Abstract

In this work, 5-SIDipp [SIDipp=1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene] (1) derived Chichibabin's hydrocarbon with an octafluorobiphenylene spacer (3) has been reported. The addition of two equivalents of 5-SIDipp with decafluorobiphenyl in presence of BF₃ gives the double C-F bond activated imidazolium salt with two tetrafluoroborate anions, 2. Further reduction of 2 gives the fluorine substituted 5-SIDipp based Chichibabin's hydrocarbon, 3. Quantum chemical calculations suggested a singlet state of 3 with a singlet-triplet energy gap (ΔE_S-T ) of 3.7 kcal mol^-1 , which is substantially lower with respect to the hydrogen substituted NHC-based Chichibabin's hydrocarbons (10.7 kcal mol^-1 , B3LYP). As a result, the diradical character (y) of 3 (y=0.62) is also noticeably higher than the hydrogen substituted CHs (y=0.41-0.43). The ▵E_S-T was found to be higher in CASSCF (22.24 kcal mol^-1 ) and CASPT2 (11.17 kcal mol^-1 ) for 3 and the diradical character (d) is 44.6 %.

Keywords: Chichibabin′s Hydrocarbon; C−F Bond Activation; DFT; N-Heterocyclic Carbene.