N-Heterocyclic carbene stabilized parent sulfenyl, selenenyl, and tellurenyl cations (XH⁺, X = S, Se, Te)

Abstract

NHC-stabilized parent sulfenyl (H-S), selenenyl (H-Se) and tellurenyl (H-Te) cations have been achieved by treatment of NHC chalcogen adducts with trifluoromethanesulfonic acid. Computational investigations show that most of the positive charges are localized at chalcogen atoms and carbene carbons with strong donor acceptor interactions between the lone pair of chalcogen atoms and the vacant orbital of the carbene centre, accounting for its unexpected stability.

Nature of Bonding in Donor–Acceptor Interactions Exemplified by Complexes of N‐Heterocyclic Carbenes with 1,2,5‐Telluradiazoles

“…[7] All other relevant compoundsi ncluding [RTe(NHC)] + (4a 2.091(8) ; [8] 4c 2.117(5) ; [9] Scheme 2), [(NHC) 2 Te] 2 + (2.136(8), 2.138(5) ), [6] (NHC)Te X 2 (X = Cl, 2.103(3) ; [10] X = I, 2.106(5) [11a] ), and (NHC)[TeN(OÀBF 3 ) = C(H)(C 6 H 4 )] (2.179(9) ) [11b] possessm uch shorter CÀTe bond lengths. The CÀTe bond lengths of 1a·2 (2.343(4) )a nd 1a·3 (2.524(2)a nd 2.530(2) ;t wo independent conformers) are significantly longert han the sum of single-bond covalent radii for Te and C( 2.12 ).…”

“…Bond lengths imply that resonance form I prevails for 1a·2 and 1a·3,w hereas in [RTe(NHC)] + (4a,c) [8,9] ac loser and strongerb ond with as horter bond length implies the dominanceo fr esonance form II (Scheme 2). Bond lengths imply that resonance form I prevails for 1a·2 and 1a·3,w hereas in [RTe(NHC)] + (4a,c) [8,9] ac loser and strongerb ond with as horter bond length implies the dominanceo fr esonance form II (Scheme 2).…”

“…[8,9] Scheme3.Valence isoelectronic six-p-electron Hückel aromatics. [8,9] Scheme3.Valence isoelectronic six-p-electron Hückel aromatics.…”

Stable Boron Dithiolene Radicals

“…The four‐coordinate boron atom in 4 exhibits a 11 B NMR resonance at δ =18.9 ppm, which is comparable to the values reported for the zwitterions containing dioxoborocyclic moieties ( δ =10.2–11.1 ppm) . While comparing well to those [1.732(4)– 1.737(2) Å] in carbene‐stabilized sulfenyl cations, the C(1)−S(1) bond in 4 [1.7256(18) Å] is about 0.09 Å longer than that in 3 . [1.633(9) Å], but approximately 0.11 Å shorter than the C(40)−S(1) single bond in 4 [1.834(4) Å].…”

“…The C(1) and S(1) atoms bear positive charges of +0.21 and +0.23, respectively. The Wiberg bond index (1.12) of the C(1)−S(1) bond in 4 is only marginally smaller than that (1.17) of the carbene‐stabilized parent sulfenyl cation (HS + ), thereby suggesting modest electron back‐donation from the S(1) atom to the empty p orbital of the C(1) atom. Consequently, two resonance structures ( 4 a and 4 b in Scheme ) may be proposed for 4 .…”