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

Abstract

The synthesis of a SIPr [1,3‐bis(2,6‐diisopropylphenyl)‐imidazolin‐2‐ylidene] derived Kekulé diradicaloid with a tetrafluorophenylene spacer (3) has been described. Two synthetic routes have been reported to access 3. The cleavage of C−F bond of C6F6 by SIPr in the presence of BF3 led to double C−F activated compound with two tetrafluoro borate counter anions (2), which upon reduction by lithium metal afforded 3. Alternatively, 3 can be directly accessed in one step by reacting SIPr with C6F6 in presence of Mg metal. Compounds 2 and 3 were well characterized spectroscopically and by single‐crystal X‐ray diffraction studies. Experimental and computational studies support the cumulenic closed‐shell singlet state of 3 with a singlet‐triplet energy gap (ΔES–T) of 23.7 kcal mol−1.

Bonding and Stability of C6F4 Bridged by Bis‐Carbenes: EDA‐NOCV Analysis of (L)2C6F4 [L = SNHCDip, cAACMe]

“…This deviation is slightly more pronounced in (SNHC Dip ) 2 C 6 F 4 ( 1 ). The geometrical parameters of (SNHC Dip ) 2 C 6 F 4 ( 1 ) are similar to the reported crystal structure and B3LYP/def2‐SVP level optimized geometry [15] …”

“…Theoretical calculations suggest that the singlet state is more stable than triplet state (Figure S1) by 22–26 kcal/mol. The singlet triplet energy gap (Δ E S‐T ) of complex ( 1 ) (26.1 kcal/mol) calculated at BP86‐D3(BJ)/Def2‐TZVPP is slightly higher than previously reported 23.7 kcal/mol [15] . The C L −C 1 /C 4 (Figure 1) distance of 1.391–1.397 Å of complexes ( 1 ) and ( 2 ) indicates double character of C L −C 1 and C L −C 4 bonds [15] .…”

“…The C−C bond distances inside the C 6 F 4 ring range from 1.355–1.446 Å. The lengthening of C−N bond lengths (1.377–1.393 Å) of ligands in both complexes, compared to free cAAC (1.320 Å) [3] shows the possibility of π‐backdonation [13–16] . While the N’−C’−C”−N” torsion angle 175.5° (Figure 1) in (SNHC Dip ) 2 C 6 F 4 ( 1 ) indicates that SNHC Dip ligands are almost in the same plane, the N’−C’−C”−N” torsion angle of 0° in (cAAC Me ) 2 C 6 F 4 ( 2 ) shows that the two cAAC Me ligands are oriented trans to each other.…”

“…Very recently the theoretical bonding analysis of a series of donor ligands [(L) 2 C 3 ; L = cAAC, SNHC, PPh 3 ] stabilized C 3 species has shown that cAAC prefers to form all electron sharing σ‐bond and π‐bond while other ligands L = NHC, PPh 3 prefer to form both electron sharing σ‐bond and a dative σ‐bond [14] . The finding of such bonding scenarios has made us curious about the bonding analysis of previously reported (SNHC Dip ) 2 C 6 F 4 ( 1 ) which has been isolated in the laboratory [15] . Herein, we report the theoretical investigation on bonding scenarios of 1 and its cAAC analogue (cAAC Me ) 2 C 6 F 4 ( 2 ) [SNHC Dip = saturated N‐heterocyclic carbene and cAAC Me = cyclic alkyl(amino) carbene] (Scheme 1) since there has been a spike in the syntheses of non‐Kekulé type cAAC‐stabilized organic molecules with diradical character with different spacer in between two cAAC ligands [16]…”

Activation of C–F, Si–F, and S–F Bonds by N-Heterocyclic Carbenes and Their Isoelectronic Analogues

“…A related mesoionic product was obtained by Sen and co-workers when a saturated NHC 10 was used as a nucleophile (Scheme 11, bottom). 58,59 In this case, the NHC backbone underwent deprotonation under reaction conditions. In the presence of BF 3 and 2 equivalents of 10, double substitution occurred.…”