Systematic Electronic and Structural Studies of 9-Carbene-9-Borafluorene Monoanions and Transformations into Luminescent Boron Spirocycles

Abstract

The impact of the exact spatial arrangement of the alkali metal on the electronic properties of 9-carbene-9-borafluorene monoanions is assessed, and a series of [K][9-CAAC-9-borafluorene] complexes (1–4) have been isolated (CAAC = cyclic(alkyl)(amino) carbene, (2,6-diisopropylphenyl)-4,4-diethyl-2,2-dimethyl-pyrrolidin-5-ylidene). Compound 1, which contains [B]–K(THF)3 interactions, is compared to charge-separated 2–4, which were prepared by capturing the potassium cations with 18-crown-6, 2.2.2-cryptand, or 1,10-phenanthroline. Notably, the 11B NMR spectra of charge-separated borafluorene monoanions 2–4 show distinct low-field signatures compared to 1. Theoretical calculations indicate that charge separation may be exploited to influence the nucleophilic and electron transfer properties of 9-carbene-9-borafluorene monoanions. When [K(2.2.2-cryptand)][9-CAAC-9-borafluorene] (3) is reacted with 9,10-phenanthrenequinone and 1,10-phenanthroline-5,6-dione, the carbene ligand is displaced, and new air-stable R2BO2 spirocycles are formed (5 and 6, respectively). Remarkably, compounds 5 and 6 display fluorescence under UV light in both the solid and solution phases with quantum yields of up to 20%. In addition, a drastic red-shift in the emission color is observed in 6 because of the presence of the nitrogen atoms on the phenanthroline moiety. Mechanistic insights into the formation of these spirocycles are also described based on density functional theory calculations.

Approaching Dianionic Tetraoxadiborecine Macrocycles: 10‐Membered Bora‐Crown Ethers Incorporating Borafluorenate Units

“…Herein, we report the first examples of dianionic tetraoxadiborecines (Figure 1), which feature a 10‐membered C 4 B 2 O 4 bora‐crown ether core, comprised of borafluorenate units ( 2 – 5 ) (Scheme 1). Surprisingly, the reaction of borafluorene monoanion with non‐benzenoid quinones produced large boracyclic rings, which is in contrast to the chemistry observed with benzenoid quinones [12h] . Remarkably, compounds 2 – 5 are charged species, and in terms of structure they may be viewed as the combination of a traditional crown ether and a borate.…”

“…Due to the tunable optical properties and cycloaddition chemistry, we have observed among various classes of borafluorenes, [12] we were interested in synthesizing oxaboracyclic borafluorene‐based materials with extended π‐conjugation. Recently, we reported preliminary evidence for 9‐carbene‐9‐borafluorene monoanions being used as synthons for fluorescent boron‐oxygen‐containing spirocycles by reactions with diketones [12f, h] . Therefore, we envisioned that varying the number and arrangement of annulated rings in cyclic diketones would lead to systems with tunable optical properties.…”

“…In previous studies we isolated novel R 2 BO 2 spirocycles via reactions between 9‐CAAC‐9‐borafluorene monoanion ( 1 ) [CAAC=cyclic(alkyl)(amino) carbene, (2,6‐diisopropylphenyl)‐4,4‐diethyl‐2,2‐dimethyl‐pyrrolidin‐5‐ylidene)] and benzenoid quinones [12f, h] . To understand substrate scope, we were interested in accessing additional borafluorene spirocycles through reactions with non‐benzenoid quinones.…”

Photochemically- and Thermally-Generated BN- and BNP-Doped Borafluorenate Heterocycles via Intramolecular Staudinger-Type Reactions

“…[1][2][3] Research efforts in this regard have engaged a broad array of synthetic chemists interested in fundamental chemical transformations, [4][5][6][7][8][9][10][11] as well as those concerned with uncovering new types of functional materials. 3,[12][13][14][15][16][17][18][19][20][21] Recently, our laboratory synthesized a series of borafluorene radicals, 22 cations, 23,24 and anions, [25][26][27][28][29] all of which rely on the key reaction of a neutral carbon donor ligand (Lewis base) and 9-bromo-9-borafluorene (Lewis acid). Reaction of the 9-carbene-9-borafluorene anion with carbon dioxide (CO2) resulted in the formation of a trioxaborinanone, which releases carbon monoxide (CO) to form a luminescent dioxaborinanone (Figure 1B).…”

Photochemically and Thermally Generated BN-Doped Borafluorenate Heterocycles via Intramolecular Staudinger-Type Reactions

“…Among boron-containing polycyclic aromatic hydrocarbons (PAHs), 9-borafluorenes and their derivatives have become important small-molecule platforms for the construction of higher-value boracycles with unique reactivity trends, electronic features, and photophysical properties. − Research efforts in this regard have engaged a broad array of synthetic chemists interested in fundamental chemical transformations, − as well as those concerned with uncovering new types of functional materials. ,− Recently, our laboratory synthesized a series of borafluorene radicals, cations, , and anions, − all of which rely on the key reaction of a neutral carbon donor ligand (Lewis base) and 9-bromo-9-borafluorene (Lewis acid). Reaction of the 9-carbene-9-borafluorene anion with carbon dioxide (CO 2 ) resulted in the formation of a trioxaborinanone, which releases carbon monoxide (CO) to form a luminescent dioxaborinanone (Figure A) .…”