Tricyanoborane‐Functionalized Anionic N‐Heterocyclic Carbenes: Adjustment of Charge and Stereo‐Electronic Properties

Abstract

The 1-methyl-3-(tricyanoborane)imidazolin-2-ylidenate anion (2) was obtained in high yield by deprotonation of the B(CN) 3 -methylimidazole adduct 1. Regarding charge and stereo-electronic properties, anion 2 closes the gap between well-known neutral NHCs and the ditopic dianionic NHC, the 1,3-bis(tricyanoborane)imidazolin-2-ylidenate dianion (IIb). The influence of the number of N-bonded tricyanoborane moieties on the σ-donating and π-accepting properties of NHCs was assessed by quantum chemical calculations and verified by experimental data on 2, IIb, and 1,3-dimeth-ylimidazolin-2-ylidene (IMe, IIa). Therefore NHC 2, which acts as a ditopic ligand via the carbene center and the cyano groups, was reacted with alkyl iodides, selenium, and [Ni-(CO) 4 ] yielding alkylated imidazoles 3 and 4, the anionic selenium adduct 5, and the anionic nickel tricarbonyl complex 8, respectively. The results of this study prove that charge, number of coordination sites, buried volume (%V bur ) and σdonor and π-acceptor abilities of NHCs can be effectively fine-tuned via the number of tricyanoborane substituents.

Boranes Paving the Way to Anionic Cyclic (Alkyl)(amino)carbenes (Ani‐cAACs)

“…The 1 J ( 77 Se, 13 C) coupling constant of 7 of 209.9 Hz was derived from the 13 C{ 1 H} NMR spectrum (Figure 5). This coupling constant is significantly smaller than values reported for selenium adducts of related neutral and anionic NHCs [228.0 Hz for the Se adduct of IMeB(CN) 3 − ( IV ); 214.5 Hz for the Se adduct of IB(CN) 3 2− ( III ); [20] 219 Hz for the Se adduct of SIMes [32] (SIMes=1,3‐bis(mesityl)imidazolidin‐2‐ylidene)]. So, Ani‐cAAC‐B(CN) 3 should be a very potent σ‐donor in agreement to the comparably high calculated HOMO energy (Figure 4).…”

“…A similar behavior was observed by us for related imidazoline-2-ylidenates with one or two B(CN) 3 groups at nitrogen, previously. [20] The σ-orbital (HOMO) is significantly raised by the C 2 F 5 BF 2 group because of the reduced Lewis acidity of this group compared to B(CN) 3 , which is also reflected by the fluoride ion affinities (FIA) of 362 and 540 kJ mol À 1 of C 2 F 5 BF 2 [30] and B(CN) 3 . [15] Thus, Ani-cAAC-B(CN) 3 is predicted to be an excellent π-acceptor, while Ani-cAAC-C 2 F 5 BF 2 (6) should be a potent σdonor.…”

“…In accordance with the relatively high energy of the πacceptor orbital of 6 (Figure 4), δ( 77 Se) of Li8 • (THF) 1.5 is comparably low (454.7 ppm in [D 8 ]THF) with respect to the Se derivatives of cAAC Me -5 and cAAC Et -6 and Ani-cAAC-B(CN) 3 . The small 1 J( 77 Se, 13 C) coupling constant of Li8 • (THF) 1.5 of 206.0 Hz underlines the strong σ-donor ability of 6 compared to Ani-cAAC-B(CN) 3 , which is 5) pm [20] and IB(CN) 3 2À (III) d(C=Se) = 184.2(4) pm. [14] The Li ion in Li8 • (THF) 2 is tetrahedrally coordinated by the O atoms of two THF ligands, the boron bonded fluorine substituent F1, and Se1.…”

Borane als Wegbereiter zu anionischen cyclischen (Alkyl)(amino)carbenen (Ani‐cAACs)

“…B. 217.0(6) pm in $${_\infty ^2 }$$ [LiIMeB(CN) 3 ⋅THF] [20] . Die Bindungslängen und ‐winkel in 6 unterscheiden sich von denen in 4 .…”

An easy-to-perform evaluation of steric properties of Lewis acids

“…During our studies on Lewis acids and Lewis acid/base adducts over the last few years 9,14,30–32 we became interested in the stereo-electronic properties of Lewis acids, which specifically include the steric properties of Lewis acids and their influence on the formation of Lewis acid/base adducts. Herein we present an easy-to-apply model we developed over the last few years to estimate the steric properties of Lewis acids without accounting for electronic factors or different electronic interaction models.…”