A Cationic NHC‐Supported Borole

Abstract

This work describes the synthesis and characterization of a highly reactive cationic borole. Halide abstraction with Li{Al[OC(CF3)3]4} from the NHC‐chloroborole adduct yields the first stable NHC‐supported 1‐(MeNHC)‐2,5‐(SiMe3)2‐3,4‐(Ph*)2‐borole cation. Electronically, it features both a five‐membered cyclic conjugated 4 π‐electron system and a cationic charge and thus resembles the yet elusive cyclopentadienyl cation. The borole cation was characterized crystallographically, spectroscopically (NMR, UV/Vis), by cyclovoltammetry, microanalysis and mass‐spectrometry and its electronic structure was probed computationally. The cation reacts with tolane and reversibly binds carbon monoxide. Direct comparison with the structurally related, yet neutral, 1‐mesityl borole reveals strong Lewis acidity, reduced HOMO–LUMO gaps, and increased anti‐aromatic character.

N2/CO Exchange at a Vinylidene Carbon Center: Stable Alkylidene Ketenes and Alkylidene Thioketenes from 1,2,3-Triazole Derived Diazoalkenes

“…49 The IR band is at lower wavenumbers than in free CO (2143 cm −1 ), indicating πbackbonding into the CO fragment. 13 C-labeled 4a allows to unambiguously assign the 13 C NMR signals (C α ): 13.9 ppm (4a) [C α : 12.7 ppm (6)] and C β : 149.2 ppm (4a) [C β : 144.5 ppm (6)]. The strong high field shift from 34.5 ppm (C α : 2a) to 13.9 ppm (C α : 4a) suggests a significant negative charge at C α .…”

“…The activation of carbon monoxide with main group elements is a vibrant area of research, exemplified by the recent characterization of carbonyl complexes based on silicon, , boron, − phosphorus, , or alkaline earth metals. , Carbonyl complexes of carbon, ketenes (R 2 CCO) are well explored in organic chemistry and can even be generated by the combination of electrophilic carbenes with CO. − In contrast to ketenes, the higher unsaturated homologues, alkylidene ketenes or more specifically vinylidene ketenes (R 2 CCCO) are much less investigated, presumably a result of their challenging synthetic access and high reactivity. , Carbon suboxide ( I ; Scheme A) is one of the few relatively stable compounds, which has received great interest and was suggested by Frenking and co-workers to be described as a carbon atom flanked by two neutral CO donor ligands (L → C ← L; carbone) . Following this analogy, the [CCO] fragment can be stabilized by a phosphine ligand leading to phosphoranylideneketenes ( II ). − Besides phosphacumulene ylides, which are the only structurally characterized compounds containing the XCCO fragment, alkylidene ketenes or cumulenones (X = CR 2 ) are exceedingly rare.…”

Coordination and Homologation of CO at Al(I): Mechanism and Chain Growth, Branching, Isomerization, and Reduction

“…By means of comparison, a related ETS-NOCV analysis on the mono-carbonyl adduct of a cationic borole (in which the CO ligand behaves primarily as a σ-donor) yields 26 kcal mol –1 (back-bonding to CO) and 106 kcal mol –1 (sigma donation from CO). 61 As such, the primary role of the CO donors in [(NON′)Al(CO) 2 ] − is strongly suggested to be as Z-type ligands.…”

Pentamethyl- and 1,2,4-tri(tert-butyl)cyclopentadienyl containing p-block complexes – differences and similarities

“…It is interesting that in this compound the five-membered ring of the ligand consists of four carbon and one boron atoms and the distance between Si and the five-membered ring is closer than that in Cp*. 12 Because of the excellent steric bulk, solubility, stability, and electron donor properties of Cp* ligand, it has been demonstrated versatility of the σ-(η 3/2/1 ) or π-bonding (η 5 ) in main group element chemistry. 3,4,[9][10][11]13 However, the 1,2,4-(Me 3 C) 3 4 and then treated with cAAC to isolate the liquid lowvalent monomer aluminum [η 5 -Cp′′′Al], 14 which was studied for a number of interesting reactions under mild conditions.…”