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

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.