A boron-centered radical: a potassium-crown ether stabilized boryl radical anion

Abstract

A boron radical contact ion-pair MesB{4-(3,5-dimethylpyridinyl)}K(18-crown-6)(THF) (1K) has been isolated and characterized by electron paramagnetic resonance (EPR) spectroscopy, UV-vis absorption spectroscopy and single crystal X-ray diffraction. The geometry, bonding and spin density distribution are shown to be affected by the NK interaction. The unpaired electron resides mainly on the boron atom and falls between those of triarylboron radical anions and neutral boron radicals. The work provides a novel boron-centered radical intermediate, connecting anionic and neutral boryl radicals.

N‐Heterocyclic Carbene Stabilized Boryl Radicals

“…For 2 and 3 , similar A ( 11 B) value of 7.9 G and 7.3 G were reported, while significantly smaller couplings were established for the radicals 1 and 5 – 7 , for which strong localization of the unpaired electron on the carbene fragment was discerned . Larger values were reported only for anionic boryl radicals such as [B(Mes) 3 ] .− (9.87–10.32 G), whereas the recently published Mes 2 B{4‐(3,5‐dimethylpyridinyl)}K(18‐crown‐6)(THF) afforded A ( 11 B)=7.90 G which resembles the situation found for the radicals 11 a – c . Additional hyperfine coupling constants were determined to the hydrogen atoms of the aromatic substituents, while coupling to the nitrogen atoms of the NHC is negligible.…”

“…As imilar route was employed for the preparation of the cationic precursors of 2 and 7, [14,29] and the activation of P À Cl bonds by silylimidazolium salts was also reported. [27][28][29] Larger values were reported only for anionic boryl radicals such as [B(Mes) 3 ]C À (9.87-10.32 G), [2] whereas the recently published Mes 2 B{4-(3,5-dimethylpyridinyl)}K(18-crown-6)(THF) afforded A-( 11 B) = 7.90 Gw hich resembles the situation found for the radicals 11 a-c. [48] Additional hyperfine coupling constants were determined to the hydrogen atoms of the aromatic substituents,while coupling to the nitrogen atoms of the NHC is negligible. 1.5-1.6 and 2.4-2.6 ppm for the hydrogen atoms of the tBu and Me groups, respectively.I nt he 13 CNMR spectra, broad signals (FWHM = 15-22 Hz) for the carbene carbon atoms were detected at 148.1 ppm (10 a)a nd 145.5 ppm (10 c), while the corresponding signal for 10 b was not found.…”

Durch N‐heterocyclische Carbene stabilisierte Borylradikale

“…Für 2 und 3 wurden ähnliche A ( 11 B)‐Werte von 7.9 G bis 7.3 G berichtet, während wesentlich kleinere Kopplungen für die Radikale 1 sowie 5 – 7 festgestellt wurden, wodurch einen starke Delokalisierung des ungepaarten Elektrons über das Carbenfragment offenbart wird . Größere Werte sind lediglich für anionische Borylradikale, wie [B(Mes) 3 ] .− (9.87–10.32 G), bekannt, während das kürzlich publizierte Mes 2 B{4‐(3,5‐dimethylpyridinyl)}K(18‐Krone‐6)(THF) mit A (11B)=7.90 G eine ähnliche elektronische Situation wie 11 a – c aufweist . Zusätzliche Hyperfeinkopplungskonstanten wurden zu den Wasserstoffatomen der aromatischen Substituenten bestimmt, wohingegen die Kopplung zu den Stickstoffatomen des NHCs vernachlässigbar ist.…”

Phenylpyridyl‐Fused Boroles: A Unique Coordination Mode and Weak B−N Coordination‐Induced Dual Fluorescence

“…[15] Compared to benzene,p yridine is electron withdrawing due to the higher electronegativity of Nvs. C. Using pyridine as one of the substituents in triarylboranes enhances the acceptor strength of boron, as demonstrated by the reduction potentials of pyridine-containing triarylboranes, [16] 9-borylated acridine [17] and boron-doped polycyclic aromatic hydrocarbons (PAHs) with pyridine bound to boron. [18] The coordination ability of pyridine can be used for further functionalization.…”