Completing the series of boron-nucleophilic cyanoborates: boryl anions of type NHC–B(CN)2 –

The novel boryl anions NHC–B(CN)2 – complete a series of cyanoborates with continuously increasing boron-centred nucleophilicity.


Synthetic details and procedures
General Remarks: All procedures involving air-or moisture-sensitive compounds were performed in a glove box (MBraun 200B) under dry argon atmosphere or using Schlenk techniques. Solvents were purified using a Solvent Purification System (Braun) and stored over molecular sieve (3-4 Å). All commercially available compounds (TCI, abcr, Sigma Aldrich) were used without further purification if not otherwise stated. The compounds B(SEt) 3 [1] , Me 3 SiCN [2] , 1,3-dimethyl-benzimidazolium-iodide (BAC•HI) [3] , B (IMes) [4] and A [IMe(Me) 2 ] [5] were prepared according to literature methods. Abbreviations used: 2 IMes BAC Compound 7A crystallizes as a dichloromethane solvate; both residues display crystallographic mirror symmetry. Compound 7B crystallizes as a pseudomerohedral twin because its  angle is close to 90°; the relative volume of the smaller twin component refined to 0.4131 (4). The asymmetric unit contains three molecules related approximately by the translation c/3; however, the separation is not exact. Each molecule displays approximate mirror symmetry (r.m.s deviations 0.08, 0.05, 0.03 Å). The molecule of 7C displays crystallographic mirror symmetry. For K-8B, regions of badly disordered THF were removed with the program SQUEEZE; [13] the quoted composition, formula mass etc. involve an idealized content of 4 THF per asymmetric unit. The compound [K(18-cr-6)]-8C crystallizes as an inversion-symmetric dimer. Compound 11 crystallizes with two independent molecules, each with mirror symmetry; the dichloromethane molecule also has mirror symmetry.

IMes-B(SEt) 3 (1B).
B(SEt) 3 (1.28 g, 6.57 mmol) was added to a solution of IMes (2.00 g, 6.57 mmol) in toluene (25 mL). After stirring for 5 min, pentane (40 mL) was added to complete the precipitation of a crystalline solid. After filtration, washing with pentane (40 mL) and drying in vacuo the product (2.16 g, 4.34 mmol, 66%) was obtained as colorless crystalline material. Crystals suitable for X-ray crystallography were obtained by slow diffusion of pentane into a solution of the product in THF. 1
The solid residue was recrystallized by slow diffusion of pentane into a highly concentrated
Crystals suitable for X-ray crystallography were obtained by slow diffusion of pentane into a solution of the product in THF.

IMes-B(CN) 2 Me (9).
Methyl iodide (0.05 mL, 0.80 mmol) was added dropwise to a solution of K-8B (300 mg, 0.74 mmol) in THF (15 mL). A colorless precipitate formed immediately. After stirring for 10 min at r.t. the precipitate was removed by filtration. The filtrate was evaporated to dryness

IMes-B(CN) 2 Au(PMe 2 Ph) (10).
AuCl(PMe 2 Ph) (187 mg, 0.51 mmol) in THF (4 mL) was added to a solution of K-8B (207 mg, 0.51 mmol) in THF (20 mL). After stirring for 2 h, the mixture was filtered, and the filtrate was evaporated to dryness in vacuo. The product was obtained as a red solid (190 mg, 0.27 mmol, 53%). Analytically pure samples were obtained by repeated (three times) slow diffusion of pentane into a solution of the crude product in THF to afford a colorless crystalline material.

BAC-B(CN) 2 Me (11).
Methyl iodide (0.2 mL, 3.2 mmol) was added to a suspension of K-8C (400 mg, 1.6 mmol) in THF (5 mL). After 10 min the reaction mixture was dried in vacuo, the residue was suspended in H 2 O and the solution extracted three times with CH 2 Cl 2 . The combined organic phases were dried over Na 2 SO 4 and the solvent was removed in vacuo. The residue was dissolved in

Alternative preparation of K-8B and K-8C by reduction with K in liquid NH 3 .
In a typical reaction a portion of potassium (234 mg, 6.00 mmol) was dissolved in liquid NH 3
[10] Solvent effects were taken into account at the polarizable continuum model level, using COSMO (Conductor-Like Screening Model) [11] implemented in ORCA, with parameters for tetrahydrofuran. Numerical frequency calculations revealed the optimized structures of 8B and 8C to be stationary points on the energy surface. xyz-Data of the optimized structures are given in Tables 5−9.  Table 4.